Vol.:(0123456789) Clinical Drug Investigation (2025) 45:701–742 https://doi.org/10.1007/s40261-025-01473-4 REVIEW ARTICLE Revisiting the Role of Serotonin in Attention‑Deficit Hyperactivity Disorder: New Insights from Preclinical and Clinical Studies Matia B. Solomon1 · Brittney Yegla2 · Jeffrey H. Newcorn3 · Vladimir Maletic4 · Jonathan Rubin2 · Trevor W. Robbins5,6 Accepted: 29 July 2025 / Published online: 3 September 2025 © The Author(s) 2025 Abstract Attention-deficit hyperactivity disorder (ADHD) is characterized by core symptoms of inattention, hyperactivity, and impul- sivity. Aberrant dopaminergic and noradrenergic neurotransmission are often implicated in the pathogenesis of these symp- toms because ADHD treatments increase synaptic levels of these neurotransmitters in brain regions associated with attention and impulse control. However, some ADHD treatments also enhance serotonergic neurotransmission in these regions, which could contribute to their efficacy. Here, we review preclinical and clinical data highlighting functional interactions between the serotonergic and catecholaminergic systems in mediating ADHD phenotypes and responses to treatment. The potential utility of serotonergic compounds for treating distinct behavioral features and psychiatric comorbidities (e.g., depression) is also discussed. Overall, preclinical and clinical studies underscore important neuromodulatory effects of serotonin on the catecholaminergic system in mediating distinct ADHD behavioral phenotypes, notably hyperactivity–impulsivity and emotional dysregulation. Incorporating a basic understanding of dynamic monoaminergic interactions and their contribu- tions to ADHD symptoms may identify new targets for treatment. Beyond ADHD core symptoms, emotional dysregulation, which is closely linked to serotonergic dysfunction, is common in ADHD and significantly contributes to negative outcomes across the lifespan. Therefore, an expanded conceptualization of ADHD that includes emotional dysregulation may facilitate insight into ADHD pathology and treatment. 1  Introduction Attention-deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, with reported prevalence rates of 10% in children (6–11 years), 13% in adolescents (12–17 years) [1], and 5% in adults [2]. At its core, ADHD is defined by excessive levels of inatten- tion, hyperactivity, and impulsivity in multiple environments (e.g., school, work, home). These behavioral phenotypes are largely attributed to structural and functional abnormalities in cortico-striatal-limbic circuits and hindbrain regions sub- serving attention, cognitive and motor inhibition, and emo- tional regulation. Specifically, these brain regions include the dorsolateral prefrontal cortex (PFC) [3, 4], medial pre- frontal cortex (mPFC) [5–7], orbitofrontal cortex [8, 9], Matia B. Solomon and Brittney Yegla: Denotes co-first authors. With great sadness, the authors wish to acknowledge the passing of Dr. Vladimir Maletic, and recognize his substantial contributions and counsel in drafting the initial manuscript. * Brittney Yegla byegla@supernus.com Matia B. Solomon matia.solomon@uc.edu Jeffrey H. Newcorn jeffrey.newcorn@mssm.edu Vladimir Maletic vmaletic@icloud.com Jonathan Rubin jrubin@supernus.com Trevor W. Robbins twr2@cam.ac.uk 1 Department of Psychology and Neuroscience Graduate Program, University of Cincinnati, Cincinnati, OH, USA 2 Supernus Pharmaceuticals Inc., Rockville, MD, USA 3 Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York, NY, USA 4 Department of Psychiatry/Behavioral Science, University of South Carolina School of Medicine, Greenville, SC, USA 5 Department of Psychology, University of Cambridge, Cambridge, UK 6 Behavioural and Clinical Neuroscience Institute, University of Cambridge, Cambridge, UK http://crossmark.crossref.org/dialog/?doi=10.1007/s40261-025-01473-4&domain=pdf 702 M. B. Solomon et al. Key Points  Impulsivity and emotional dysregulation, which are linked to serotonin (5-HT) deficiency, may be improved with distinct 5-HT receptor-targeting agents. Corticostriatal 5-HT and its interactions with catechola- mines modulate attention-deficit hyperactivity disorder (ADHD) behaviors and responses to standard treatments. Some standard ADHD treatments increase 5-HT in ADHD-relevant brain networks. anterior cingulate [10–14], striatum (i.e., caudate, putamen, globus pallidus) [12–15], amygdala [16], and cerebellum [17–19] (Fig. 1). While the emergence of ADHD symptoms typically occurs during childhood, for many people it is a life-long condition [20] that often includes impairment in a variety of associated domains, such as executive dysfunction [21], emotional dysregulation [22, 23], and comorbid con- ditions, such as mood [2, 24–26], anxiety [2, 27], and sub- stance use disorders [28, 29]. Notably, it has been proposed that timely and successful treatment of ADHD may prevent the onset of some of these comorbidities [30, 31]. The toll of untreated ADHD cannot be overstated, as it is linked with a sixfold increased risk for mortality, particularly before the age of 30 years [32, 33]. The etiology of ADHD has yet to be fully elucidated; however, both genetic and environmental risk factors are implicated [34–38]. In addition to these risk factors, aberrant signaling in inflammatory pathways [39–41] and multiple neurotransmitter systems [42–44] are noted as key factors. The catecholamines dopamine (DA) and norepinephrine (NE) are the most well-studied neurotransmitters impli- cated in ADHD pathogenesis, primarily because effective ADHD treatments (both stimulants and nonstimulants) increase these neurotransmitters in brain regions such as the PFC and striatum, which are critical for impulse control, attention, and emotional regulation [45, 46]. In addition to the supportive evidence provided by the efficacy of these compounds, the centrality of the catecholamine systems in ADHD has also been established through brain imag- ing studies in patients with ADHD that consistently show DA and NE system disruptions, and preclinical experiments using genetic and pharmacological manipulations [47–49]. While the subject of several theoretical and systematic reviews [50–52], the role of serotonin (5-HT) in ADHD has been overshadowed by research centering on DA and NE. This largely results from the well-documented actions of approved treatments on catecholaminergic systems, while selective serotonin reuptake inhibitors (SSRIs) are report- edly ineffective for ADHD [53, 54]. Yet, there is consid- erable evidence from preclinical and some clinical studies indicating a role for 5-HT in certain ADHD behavioral phe- notypes (e.g., behavioral inhibition) and stimulant-mediated effects. Given the ubiquity of monoamines (DA, NE, and 5-HT) and their importance in central nervous system func- tion, understanding their dynamic interplay may be critical for advancing our understanding of the etiology, manifesta- tions, and treatment of ADHD. Therefore, a reexamination of the role of 5-HT in ADHD is warranted. We review evidence from preclinical and clinical stud- ies investigating potential links between the serotonergic system and ADHD phenotypes. We discuss the necessity and sufficiency of serotonergic neurotransmission based on studies involving tryptophan modulation, genetic manipula- tions (i.e., knockout [KO] rodents), and drugs with known serotonergic activity on ADHD behavioral phenotypes. Serotonergic effects on brain function and behavior are vast and include interactions with multiple neurotransmitter systems and neuropeptides across brain networks. For this review, we focus on its intricate connection and functional interactions with the catecholaminergic systems in ADHD behavioral phenotypes and stimulant-mediated effects in pre- clinical species and patients with ADHD. Although 5-HT is broadly linked with some ADHD phenotypes, the potential role of 5-HT receptor subtypes in mediating clinical effects is unclear. Thus, we describe preclinical studies, highlight- ing their effects in key cortico-striatal-limbic regions (i.e., PFC, orbitofrontal cortex, and nucleus accumbens). Given the renewed interest in emotional dysregulation (i.e., emo- tional impulsivity and reactive aggression) as an inherent feature in ADHD [22, 23] and its link to the serotonergic system [55–57], we also discuss 5-HT within the context Fig. 1   Brain regions implicated in attention deficit/hyperactivity dis- order. Bs brain stem, Cb cerebellum, CC cingulate cortex, HPC hip- pocampus, NAc nucleus accumbens, OFC orbitofrontal cortex, PFC prefrontal cortex, Str striatum. Created with BioRender.com 703Serotonin in ADHD: Preclinical and Clinical Insights of emotional dysregulation. As our conceptualization of ADHD evolves so does our understanding of the putative role of 5-HT as an important component in its etiology and treatment. To identify appropriate articles for the review, PubMed and GoogleScholar were utilized in a non-time-restricted fashion, with search terms focused on 5-HT, 5-HT receptor subtypes, 5-HT genes, and ADHD-relevant terms, such as impulsivity, attention, hyperactivity, emotional dysregula- tion, and aggression in both preclinical and clinical publica- tions. A variety of preclinical techniques, including but not limited to microdialysis, optogenetics, electrophysiology, behavioral assays, pharmacological agents, toxins, viral vec- tors, and genetically modified rodents, were included in the search with the purpose of identifying interactions between 5-HT and other monoamines for neurochemical interactions and behavioral effects. 2 � Serotonin (5‑HT) The rate-limiting step in 5-HT synthesis is conversion of tryptophan to 5-hydroxytryptamine (5-HTP) by tryptophan hydroxylase (TPH), which has two isoforms, TPH1 and TPH2 [58, 59]; however, TPH2 is the only isoform produced by serotonergic neurons in the brain [59]. In addition to its rate of synthesis, 5-HT concentrations in the brain are regu- lated by (1) 5-HT transporters (SERT) [60]; (2) 5-HT auto- receptors (i.e., 5-HT1A and 5-HT1B receptors located on the presynaptic membrane) [61–67], both of which decrease its concentration in the synapse; as well as (3) monoamine oxidase [68], an enzyme that causes cytosolic degradation of all monoamines. Broadly, 5-HT regulates a variety of behav- iors, including emotion [69], cognition [70], motor output [71], feeding [72], and sleep [73]. Serotonergic efferents from the dorsal and median raphe nuclei widely project to cortico-striatal-limbic regions associated with many of these behaviors, including the cingulate cortex, mPFC, basal gan- glia, amygdala, and hippocampus [74–76] (Fig. 2). Proper serotonergic signaling is critical for prefrontal and somatosensory cortical development and function [77–80]. These brain regions are associated with delayed maturation or reduced gray matter in individuals with ADHD [81–84]. The integral role for 5-HT in the development and func- tion of these brain regions is noteworthy, given the well- documented involvement of the PFC in executive function, including behavioral inhibition and emotional regulation, as well as sensory dysregulation issues that are apparent in some individuals with ADHD [85]. Disruption in seroton- ergic neurotransmission during development also impacts the catecholaminergic systems and their projections to the PFC [86]. Together, these findings highlight a critical role for 5-HT in the development and function of ADHD-relevant brain networks, including interconnections with the catecho- laminergic systems (Fig. 2), through which it may influence the onset of ADHD symptoms and perhaps other closely related psychiatric comorbidities. The diverse effects of 5-HT in the brain are due to the diversity of presynaptic and postsynaptic 5-HT receptors in multiple brain regions, their expression on multiple cell types, their dimerization with other serotonergic receptors, and their diverse secondary messenger signaling. Seroto- nin also functions as a neuromodulator that can regulate the activity of other neurotransmitters, including DA, NE [87, 88], and the principal inhibitory (γ-aminobutyric acid, GABA) and excitatory (glutamate) neurotransmitters in the brain [89]. Serotonin mediates its biological effects through at least 14 different receptor subtypes: 5-HT1 (1A, 1B, 1D, 1F), 5-HT2 (2A, 2B, 2C), 5-HT3, 5-HT4, 5-HT5 (5A, 5B), 5-HT6, and 5-HT7 (Table 1). Apart from the 5-HT3 receptor, which functions as a ligand-gated ion channel, most 5-HT receptor subtypes are classified as metabotropic (G-protein coupled) receptors that activate distinct second messenger systems to mediate either excitatory or inhibi- tory neurotransmission [90]. Delineating the role of these distinct receptor subtypes is critical to understanding the role for 5-HT in brain function and behavior; however, for this review we primarily highlight select receptor subtypes, including 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, and 5-HT3, which have been implicated in the core behavioral features Fig. 2   Monoaminergic signaling in the human brain. The mono- aminergic systems highly overlap across brain regions implicated in ADHD, especially the cortex, midbrain, and brainstem. DA dopa- mine, NE norepinephrine, 5-HT serotonin. Created with BioRender. com 704 M. B. Solomon et al. of ADHD, emotional dysregulation, and stimulant-mediated efficacy. 3 � Serotonergic Mechanisms in Preclinical Studies Relevant to ADHD The following sections will discuss preclinical evidence of the role of 5-HT in ADHD-relevant behaviors, specifically hyperactivity, impulsivity, emotional dysregulation, inat- tention, and cognitive inflexibility using healthy rodents, as well as preclinical models of ADHD. Tables 2 and 3 describe these preclinical assays and their corresponding clinical tasks. Tables 4 and 5 summarize the impact of spe- cific 5-HT receptor subtypes on each of these phenotypes, as well as their involvement in stimulant-related effects on these measures. 3.1 � Role of 5‑HT in Hyperactivity Hyperactivity can be easily monitored in the laboratory by assessing locomotor activity in various environments. Locomotor activity is often associated with activation of the dopaminergic system [91, 92]. Serotonin also plays a major role in locomotor activity [93], likely via interactions with several neurotransmitter systems including the mesocorti- colimbic dopaminergic pathway [94], and can be impacted by factors such as age and environment. For instance, neo- natal 5-HT depletion methods in healthy rodents induce hypolocomotion [95–97]. However, factors such as nov- elty of the environment or age of the rodent when 5-HT is depleted (i.e., adulthood) can conversely induce hyperactiv- ity or have no effect, respectively [94, 95, 98]. These varied serotonergic effects on locomotion could result from engage- ment of diverse 5-HT receptor subtypes and their down- stream effects on interconnected neurotransmitter systems, including the dopaminergic system. For example, in healthy rodents, activation of 5-HT receptor subtypes that facilitate DA release in the mesocorticolimbic dopaminergic pathway (i.e., 5-HT1B and 5-HT2A receptors) increase locomotor activity, while activation of 5-HT receptors that inhibit this pathway (i.e., 5-HT2C receptors) tend to decrease locomo- tor activity [87, 99–101]. For a thorough review on 5-HT receptor subtype expression and modulation of dopaminer- gic signaling in various brain regions, see [102]. Preclinical models of ADHD have been used to assess hyperactivity in rodents, including neonatal 6-hydroxydo- pamine (6-OHDA) lesions, spontaneously hypertensive rats (SHRs), and DA transporter (DAT) KOs. Each model displays some of the core behavioral features of ADHD, Table 1   Impact of agonism of select serotonin receptor subtypes on the dopaminergic system across multiple brain regions Impact of engaging various serotonin receptors subtypes on dopamine, which could include changes in dopamine levels, metabolism, impulse- or depolarization-dependent release, and/or electrical activity of dopaminergic neurons. De Deurwaerdère and colleagues’ 2021 review [102] served as the foundation for this table, in addition to a few select references mentioned in the footnotes a For 5-HT7, little to no articles have been published on the effects of selective 5-HT7 agonists on dopamine-related measures. The selective 5-HT7 antagonist SB-269970 has been shown to have no effect on MK-801-induced dopamine levels in rat prefrontal cortex (via microdialysis; [459]) but does inhibit neuronal firing of dopamine neurons following amphetamine treatment in the ventral tegmental area, though not the sub- stantia nigra pars compacta[460] Agonism Effect on dopamine Brain Regions Impacted 5-HT1A Increase Prefrontal cortex, substantia nigra (locally) Decrease Striatum, nucleus accumbens 5-HT1B Increase Striatum, nucleus accumbens, substantia nigra (locally) 5-HT1D Unclear Selective agonists have not been examined for effects on dopamine 5-HT1F Unclear Selective agonists have not been examined for effects on dopamine 5-HT2A Increase Prefrontal cortex, striatum, nucleus accumbens 5-HT2B Increase Nucleus accumbens Decrease Prefrontal cortex 5-HT2C Decrease Prefrontal cortex, striatum, nucleus accumbens 5-HT3 Increase Prefrontal cortex, striatum, nucleus accumbens 5-HT4 Increase Striatum 5-HT5 Unknown, no selective agonists confirmed 5-HT6 Mixed Highest dose of WAY-181187 decreased dopamine in the prefrontal cortex and striatum and was reversed by 5-HT6 antagonist for striatal dopamine [457] ST1936 increased dopamine in the prefrontal cortex and nucleus accumbens shell [458] 5-HT7 Uncleara 705Serotonin in ADHD: Preclinical and Clinical Insights Ta bl e  2   P re cl in ic al a nd c lin ic al a ss ay s o f a tte nt io n, im pu ls iv ity , a nd c og ni tiv e fle xi bi lit y Fr eq ue nt ly u se d pr ec lin ic al b eh av io ra l a ss es sm en ts a nd th ei r r el at iv el y eq ui va le nt c lin ic al e va lu at io ns fo r m ea su rin g im pu ls iv ity , a tte nt io n, a nd c og ni tiv e fle xi bi lit y in a tte nt io n- de fic it hy pe ra c- tiv ity d is or de r r es ea rc h 5C SR TT ​ fi ve -c ho ic e se ria l r ea ct io n tim e ta sk , C AN TA B C am br id ge N eu ro ps yc ho lo gi ca l T es t A ut om at ed B at te ry , C PT c on tin uo us p er fo rm an ce ta sk , D D T de la y di sc ou nt in g ta sk , S SR TT ​ s to p si gn al re ac tio n tim e ta sk , T AP te st of a tte nt io na l p er fo rm an ce D im en si on Pr ec lin ic al C lin ic al B rie f d es cr ip tio n/ be ha vi or al m ea su re Im pu lsi vi ty Im pu ls iv e ac tio n an d su st ai ne d at te nt io n fiv e- ch oi ce se ria l r ea ct io n tim e ta sk (5 C SR TT ) 5C SR TT ​ O pe ra nt ta sk th at m ea su re s a cc ur ac y of id en tif yi ng si gn al tr ia ls (% h it; % c or re ct ) a nd a bi lit y to w ith ho ld re sp on di ng (p re m at ur e re sp on se s) fiv e- ch oi ce c on tin uo us p er fo rm an ce ta sk (C PT ) C PT O pe ra nt ta sk th at m ea su re s a cc ur ac y of d et ec tin g si gn al ev en ts (% h it; % c or re ct ) i n co nt ra st to n on -s ig na l ev en ts a nd th e ab ili ty to w ith ho ld re sp on di ng (p re m a- tu re re sp on se s) Im pu ls iv e ac tio n G o/ no -g o G o/ no -g o O pe ra nt ta sk in cl ud in g go (a ct io n) a nd n o- go (a ct io n re str ai nt ) t ria ls , m ea su rin g im pu ls iv e ac tio n th ro ug h w ith ho ld in g of a re sp on se o n th e no -g o tri al s St op pi ng St op si gn al re ac tio n tim e ta sk (S SR TT ) St op si gn al O pe ra nt ta sk a ss es si ng a ct io n ca nc el la tio n, fo llo w in g si gn al a nd st op c ue s ( i.e ., tim e to st op a n in iti at ed m ot or re sp on se ) Im pu ls iv e ch oi ce D el ay d is co un tin g ta sk (D D T) D el ay d is co un tin g M ea su re s s el ec tio n be tw ee n an im m ed ia te , s m al le r re w ar d (i. e. , m or e im pu ls iv e) a nd a d el ay ed , l ar ge r re w ar d (i. e. , l es s i m pu ls iv e) A tte nt io n/ co gn iti ve fl ex ib ili ty A tte nt io n Te st of a tte nt io na l p er fo rm an ce (T A P) A b at te ry o f t es ts e va lu at in g ac cu ra cy o f r es po ns es (% co rr ec t), re ac tio n tim es , a nd e ye m ov em en ts w he n re sp on di ng se le ct iv el y to st im ul i; it ga ug es a le rtn es s, vi gi la nc e, w or ki ng m em or y, im pu ls iv ity (g o/ no -g o) , co gn iti ve fl ex ib ili ty (a tte nt io na l s et -s hi fti ng ta sk ), an d su st ai ne d an d di vi de d at te nt io n A tte nt io na l s et sh ift in g Ex tra di m en si on al se t-s hi fti ng ; i nt ra di m en si on al se t sh ift in g CA N TA B In tra /e xt ra di m en si on al se t s hi ft; W is co ns in C ar d So rti ng T as k Ta sk th at m ea su re s t he a bi lit y to sw itc h w ith in (i nt ra - di m en si on al ) a nd b et w ee n (e xt ra -d im en si on al ) le ar ne d at te nt io na l s et s i n a go al -o rie nt ed fa sh io n. C og ni tiv e fle xi bi lit y is a ss es se d as th e nu m be r o f tri al s r eq ui re d to a cq ui re th e ne w ru le a nd th e nu m be r of e rr or s m ad e Re ve rs al le ar ni ng Re ve rs al le ar ni ng W is co ns in C ar d So rti ng T as k; In str um en ta l r ev er sa l le ar ni ng ; P av lo vi an re ve rs al ; p ro ba bi lis tic re ve rs al le ar ni ng Ta sk th at m ea su re s t he a bi lit y to re ve rs e th e va lu e as so - ci at ed w ith a p ai r o f s tim ul i ( i.e ., pr ev io us ly ir re le va nt sti m ul us b ec om es re le va nt ). C og ni tiv e fle xi bi lit y is as se ss ed a s t he n um be r o f t ria ls re qu ire d to a cq ui re th e ne w ru le a nd th e nu m be r o f e rr or s m ad e 706 M. B. Solomon et al. including hyperactivity, impulsivity, inattention (see review by Fan X and colleagues [103]), or reactive aggression [104, 105]; most of these behaviors are attenuated by methylpheni- date or amphetamine. Thus, these models demonstrate face and predictive validity. Moreover, these models exhibit sig- nificant dopaminergic disruption with corresponding altera- tions in serotonergic measures, emphasizing the dynamic interaction of these two monoaminergic systems as dis- cussed in [103]. 3.1.1 � 6‑OHDA‑Lesioned Model In neonatal rats, a combination of intracranial 6-OHDA and systemic desipramine infusions, to lesion central dopaminer- gic neurons but preserve noradrenergic neurons, respectively, causes paradoxical motor hyperactivity [106]. Serotonergic agents (citalopram, fluvoxamine, fenfluramine, quipazine, ketanserin, and mianserin) and compounds that primarily target the catecholaminergic system (amphetamine, methyl- phenidate, nisoxetine, and desipramine) reduced hyperactiv- ity in this rodent model [107–110]. Notably, both d-meth- ylphenidate and selective DAT inhibitors, amfonelic acid, and GBR-12909, induced hyperactivity in controls, whereas methylphenidate but not the selective DAT inhibitors atten- uated hyperactivity in rats with neonatal 6-OHDA lesions [108], suggesting that inhibition of the NE transporter (NET) or SERT is critical for these therapeutic effects. However, this model exhibits compensatory mechanisms after 6-OHDA lesions that may contribute to its manifes- tation of hyperactivity, such as dopamine D1 receptor hypersensitivity, altered 5-HT2 transcript levels, increased striatal 5-HT and 5-HIAA (5-HT metabolite) levels, and enhanced serotonergic innervation of the striatum, result- ing in increased serotonergic tone [111–115]. Hyperactivity in neonatal 6-OHDA-lesioned mice was normalized when striatal 5-HT levels were decreased using para-chloropheny- lalanine (PCPA), suggesting that enhanced 5-HT signaling in this region after 6-OHDA lesioning caused an imbalance in motor control [115]. Moreover, functional interactions between striatal D1 and 5-HT2A receptors may mediate hyperactivity in this model; 5-HT2A receptor antagonism (ketanserin and M100907) but not 5-HT2C receptor antago- nism attenuated the D1 supersensitivity and corresponding hyperactivity in adult rats [114–117]. These findings dem- onstrate that 5-HT2A receptor antagonists, via inhibitory effects on the dopaminergic system, are beneficial in miti- gating the hyperactive phenotype in this preclinical model of ADHD. 3.1.2 � SHR Model The SHR model is characterized by disruptions in mono- aminergic systems, with reports of reduced presynaptic serotonergic function, enhanced dopaminergic presyn- aptic activity in the accumbens (D2 autoreceptors and DA reuptake via DAT), lower evoked DA release in the striatum and accumbens, and increased D1 and D2 recep- tor expression in the frontal cortex, dorsal striatum, and nucleus accumbens [118–122]. Discrepant findings of hyper- versus hypo-dopaminergic tone in SHRs are likely brain region dependent or due to comparisons with vary- ing controls (Wistar Kyoto, Wistar, or Sprague Dawley rats). For instance, SHRs and Wistar Kyoto rats showed no difference in 5-HT in the striatum [123, 124], but SHRs had decreased serotonergic activity [125] and 5-HT turnover (5-HIAA/5-HT) in other catecholaminergic brain regions, such as the ventral tegmental area (i.e., dopamin- ergic neurons) and locus coeruleus (i.e., noradrenergic neurons) [124]. Table 3   Preclinical and clinical assays of emotional dysregulation A variety of preclinical and clinical assessments frequently used to evaluate emotional dysregulation in animal models and individuals with attention-deficit hyperactivity disorder a Latency to attack and aggressive behaviors (i.e., frequency and duration of chasing, biting, attacking, dominant postures/sideway threats) are assessed after the initial stimulus/condition Reactive aggression model Brief description Preclinicala Isolation Rodents have prolonged social isolation prior to exposure to conspecific Alcohol Rodents are given alcohol prior to exposure to conspecific Discontinuation of scheduled reward Rodents are assessed for their interactions with a conspecific following an extinction protocol (i.e., omission of a scheduled reward) Social instigation Intruding conspecifics are placed into the home cage (resident–intruder paradigm) Clinical Point subtraction game Participants compete for points or money against fictitious opponent, often with low or high provocation components (measure “stealing” of points); can also be paired with alcohol Taylor competitive reaction test Participants exposed to a fictitious interpersonal scenario to compete on a reaction time task; aggression is measured as the number and severity of noxious stimuli (e.g., noise blasts) participant delivers to the opponent 707Serotonin in ADHD: Preclinical and Clinical Insights Table 4   Impact of selective 5-HT receptor ligands on ADHD-relevant behaviors in preclinical species Target Activity Ligand Administra- tion Hyperactiv- ity in ADHD models Impulsive action Impulsive choice Aggression Attention Cognitive flexibility TPH Inhibitor PCPA ↓ [115] 5-HT1A Agonist 8-OHDPAT Systemica ↑ [154] ↑ [160, 161] ↓ [153] Systemic ↓ [191, 193] Intra-OFC ↓ [162] Intra-PFC ↑ [153] Intra-nucleus accumbens ↑ [160] Genetic overexpression on serotonin neurons a ↑ [189] Lower expression of tran- scripts and protein ↑ [195] 5-HT1B Agonist Anpirtoline Systemica ↓ [55] CP-94,253 Intra-DRa ↓ [195] CP-94,253 Intra-PFCa ↑ [195] Antagonist SB 224289 Systemic ↓ [141] 5-HT1B KO mice ↑ [205] 5-HT2 Agonist DOI Systemic, intra-striatal ↑ [116] ↑ [163] Quipazineb Systemic ↓ [109] ↓ [210] m-Chlorophe- nylpipera- zine Systemic ↓ [197] 5-HT2A Antagonist M100917 Intra-PFC Systemic; intra- nucleus accumbens ↓ [142] [151] ↓ [158] ↑ [153] ↓ [153] Reduced transcripts in the frontal cortex ↓ [227] 5-HT2A/C Antagonist Ketanserin Systemic; intra-PFC ↓ [110] ↓ [152, 164, 325] ↑ [164, 326] ↓ [199] Mianserinb Systemic ↓ [110] Ritanserinb Intra-striatal (hyperactiv- ity) Systemic (aggression) ↓ [114, 117] ↓ [199] 708 M. B. Solomon et al. Compounds that enhance 5-HT neurotransmission, including serotonin–norepinephrine reuptake inhibitors (SNRIs; venlafaxine, duloxetine, and milnacipran), attenu- ate hyperactivity in SHRs but do not impact controls [126]. While these compounds increase prefrontal monoamine concentrations (5-HT, NE, and DA), the improvement in hyperactivity may be due to enhanced noradrenergic and dopaminergic neurotransmission; similar effects were not observed with the SSRI citalopram. Furthermore, methyl- phenidate and the norepinephrine reuptake inhibitors (NRIs) atomoxetine and reboxetine, which increased prefrontal DA and NE but not 5-HT, decreased hyperactivity in this model. Thus, it appears that compounds that directly target NET or DAT consistently decrease hyperactivity in SHRs. Relative to the catecholaminergic systems, there are far fewer studies that have investigated the role of the serotonergic system on hyperactivity in SHRs. Because of the inconsistent find- ings with SSRIs (fluvoxamine, citalopram) on hyperactivity [126–128], some question the role of the serotonergic system in the etiology of this phenotype in SHRs, as discussed in [129]. 3.1.3 � DAT KO Mouse Model DAT KO mice have increased extracellular DA levels in the striatum [130, 131] and nucleus accumbens [132] but not the frontal cortex, which has low DAT expression under normal physiological conditions [133]. Relative to controls, DAT The table illustrates serotonergic receptors and the 5-HT synthesizing enzymes that have been targeted pharmacologically with agonists, antag- onists, or inhibitors in preclinical species or were genetically altered (e.g., knockdown) to evaluate their impact on phenotypes observed in ADHD, including hyperactivity, impulsivity (impulsive action and impulsive choice), inattention, cognitive inflexibility, and emotional dysregu- lation (i.e., aggression). Indicators represent the corresponding changes in behavior after modulation of the receptor/enzyme: ↑ = increased behavior; ↓ = decreased behavior; = no effect on behavior ADHD attention-deficit hyperactivity disorder, DR dorsal raphe, KO knockout, OFC orbitofrontal cortex, PCPA para-chlorophenylalanine, PFC prefrontal cortex, TPH tryptophan hydroxylase, VGV valine glycine valine a Posited to engage the presynaptic receptor b A few compounds are nonselective for their designated target. For completeness, these compounds also engage the following targets: S32212 (5-HT2A antagonist, alpha2 adrenergic antagonist); mianserin (histamine 1, α2A, α2C, 5-HT2 antagonist); quipazine (5-HT2, 5-HT3 agonist); ritanserin (5HT2, α1A, α1B, α1D, 5-HT5, 5-HT6, 5-HT7 antagonist) Table 4   (continued) Target Activity Ligand Administra- tion Hyperactiv- ity in ADHD models Impulsive action Impulsive choice Aggression Attention Cognitive flexibility 5-HT2C Antagonist SB 242084 Systemic; intra- nucleus accumbens ↑ [150, 151] Intra-PFC [151] SER-082 Systemic ↓ [164] Inverse ago- nist S32212b Systemic ↓ [198] Lower expression of tran- scripts and protein ↑ [195] VGV isoform expression ↑ [196] 5-HT3 Antagonist Granisetron, ondansetron Systemic ↓ [165] ↓ [201] MDL-7222 but not tropisetron Systemic ↓ [200] Higher receptor density ↑ [204] 5-HT6 Antagonist SB-270146A Systemic [164] 709Serotonin in ADHD: Preclinical and Clinical Insights KO mice have disturbances in their catecholaminergic sys- tems, including reduced D1 and D2 receptor expression in the substantia nigra and ventral tegmental area [134, 135] and increased NET expression in the PFC [136], which is likely because of the capacity of NET to take up both NE and DA from the synapse [137]. However, there are con- flicting reports regarding the impact of DAT KO or DAT knockdown on the serotonergic system. One study reported no effect on 5-HT efflux in the PFC, striatum, and nucleus accumbens [133], while Fox and colleagues [138] reported enhanced serotonergic tone and sensitivity in the striatum. Despite these differing research findings, serotonergic targeting agents attenuate hyperactivity in this model. For example, Gainetdinov et al. [139] showed hyperactivity was reduced by methylphenidate and treatments that increase 5-HT levels, such as l-tryptophan loading and systemic fluoxetine (SSRI). In contrast, infusion of fluoxetine directly into the PFC did not affect hyperactivity [136], suggesting that serotonergic agents are likely active at sites outside of the PFC to induce this normalizing locomotor effect [139]. Increasing prefrontal DA levels is a primary mechanism by which hyperactivity can be normalized in this model; intra- PFC infusions of amphetamine, desipramine (NRI), and nepicastat, which blocks synthesis of NE and thus results in depleted NE and increased DA levels, reversed hyperactiv- ity [136] (though see [139] for conflicting evidence on NRI nisoxetine). One potential mechanism by which 5-HT may reduce hyperactivity in this model is via 5-HT1B receptors (Table 4; Fig. 3). 5-HT and 5-HT1B receptor agonism can potentiate DA release in the nucleus accumbens [102] and enhance methylphenidate-induced hyperactivity in healthy rodents [140]. In contrast, 5-HT1B receptor antagonism, as well as reduced 5-HT1B receptor expression (i.e., 5-HT1B Het/ DAT KOs), normalize locomotor activity levels in DAT KOs [141]. 5-HT2A receptors also are implicated, as 5-HT2A receptor antagonism using M100907 attenuates hyperactiv- ity in DAT KO mice but is ineffective in control mice [142]. Interestingly, the normalizing effects of amphetamine, meth- ylphenidate, fluoxetine, quipazine (5-HT2 receptor agonist), and l-tryptophan were blocked by N-methyl-d-aspartate (NMDA) antagonists in the DAT KO model [143], empha- sizing the integrated nature of glutamate, 5-HT, and DA interactions in regulating locomotor activity. Summary While dysregulation in the dopaminergic system is a primary feature of these preclinical ADHD models, the findings suggest that imbalances in the inter- play of serotonergic and dopaminergic neurotransmission can affect hyperactivity in these models. These preclini- cal findings support a role for monoamine imbalance in the etiology of ADHD rather than deficits within a sin- gle neurotransmitter system. The data also suggest that the efficacy of some serotonergic targeting agents may be dependent upon the endogenous tone of the catecholamin- ergic and serotonergic systems in each model. 3.2 � Role of 5‑HT in Impulsive Action and Impulsive Choice Impulsivity is a multi-faceted construct that can be meas- ured in a number of ways in experimental animals and humans [144] (Table 2). Basic divisions of impulsivity have been proposed: impulsive action which includes inability to “wait,” impulsive choice (i.e., impulsive deci- sion making), and impulsivity related to the inability to “stop,” which are assessed by different paradigms [145, 146]. Impulsivity can also manifest as emotional impul- sivity, though this will be discussed later. In rodents, the five-choice serial reaction time task (5CSRTT) includes a measure of premature, maladaptive responding (e.g., impulsive action). The go/no-go task assesses the capac- ity to withhold a response during infrequent designated periods (e.g., impulsive action). The temporal discount- ing of reward paradigm examines impulsive choice of an immediate, but less rewarding outcome. The stop signal reaction time (SSRT) task measures the ability to inhibit a motor response after it has been initiated [145]. Although these test paradigms, which have human equivalents [144] and are relevant to ADHD [147], index some aspects of impulsivity, these impulsivity measures have been shown to be distinct constructs dependent on different neural substrates. Moreover, manipulations of the 5-HT system differentially affect performance in these test settings. It should be noted that, although SHRs have been included in some of these impulsivity assays, most utilized healthy rodents or those behaviorally characterized as more or less impulsive. 3.2.1 � Role of 5‑HT in Impulsive Action Regarding impulsive action, profound depletion of central 5-HT using intracerebroventricular (i.c.v.) 5,7-dihydroxy- tryptamine (5,7-DHT) produced significant increases in the number of premature responses in the 5CSRTT in rats, which depends mainly on dorsal raphe 5-HT projections [148–150]. This impulsivity was exacerbated with a 5-HT2C receptor antagonist but was attenuated by a 5-HT2A recep- tor antagonist, consistent with apparent opposing actions of these receptors [150]. These opposing effects have also been observed in non-lesioned rats after microinfusions of these receptor antagonists directly into the nucleus accum- bens, but not the PFC [151]. By contrast, the selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) reduced premature responses when infused into rat mPFC [152, 153]. These effects were likely 710 M. B. Solomon et al. driven by engagement of the postsynaptic 5-HT1A receptor because activation of presynaptic 5-HT1A receptors, which decreases 5-HT signaling, impaired 5-HT function and enhanced impulsivity [154]. Conversely, general increases in 5-HT at the synapse, via the SSRI citalopram, decreased impulsive action in rats [155]. 3.2.2 � Role of 5‑HT in Impulsive Choice The effects of similar 5-HT manipulations on impulsive choice, measured through the temporal discounting of the reward paradigm, are less clear. In alignment with the impact of 5-HT on impulsive action, rat studies demonstrated that 5,7-DHT lesions to the serotonergic dorsal and median raphe nuclei, which produced selective and substantial 5-HT deple- tion in the parietal cortex, hippocampus, amygdala, nucleus accumbens, and hypothalamus, induced a significant bias Fig. 3   Serotonergic modulation of dopamine signaling. Serotonin (5-HT) exerts modulatory effects, commonly associated with inhibi- tory action, on the dopaminergic system, as reviewed in [102]. Illus- trated here are a few examples of how specific 5-HT receptors may modulate dopamine (DA) signaling in relation to ADHD phenotypes. There are significant reciprocal innervations and crosstalk between these two monoaminergic systems at the site of their primary nuclei, including the dorsal raphe (DR; 5-HT) and ventral tegmental area (VTA; DA; bottom right inset). Serotonergic projections modulate fir- ing activity of dopaminergic neurons in the VTA via multiple recep- tors, including 5-HT2C receptors, which exhibit inhibitory effects [441–446]. 5HT2C receptors are expressed on GABAergic interneu- rons, which, upon activation, inhibit firing of ventral tegmental neu- rons. Upon systemic application of SSRIs, DA firing in the ventral tegmental area has been observed to subtly decrease [447, 448], whereas lesions of the dorsal raphe enhanced DA activity in the ven- tral tegmental area [449]. Low doses of a 5-HT1A receptor agonist (8-OHDPAT), which hypothetically engages the autoreceptor and thus decreases 5-HT activity in the dorsal raphe, have been shown to increase DA firing rate and DA release in the ventral tegmental area under basal and stimulated conditions [450–456]. Neurons from the ventral tegmental area and substantia nigra project to the striatum and nucleus accumbens (top right inset) and target medium spiny neurons, which release γ-aminobutyric acid (GABA). These medium spiny neurons also receive projections from glutamatergic neurons in the prefrontal cortex (as well as other areas), which appear to be modu- lated by serotonergic receptors, such as 5-HT1B. Engagement of these 5-HT1B receptors is implicated in altering ADHD-relevant phe- notypes such as hyperactivity in preclinical models [140, 141]. Lastly, 5-HT2A receptors are heavily expressed in the cortex and have been shown to modulate activity of the nucleus accumbens, as well as send feedback signals to the dorsal raphe and ventral tegmental area, reviewed in [102]. Glu, glutamate. Created with BioRender.com 711Serotonin in ADHD: Preclinical and Clinical Insights towards the immediate, smaller reward in the acquisition of a temporal discounting two-choice task [156]. This result suggests increased impulsive choice due to sensitivity of delay. In addition to replicating the effect of 5-HT depletion on selection of the immediate small reward, Bizot and col- leagues [157] also observed that the SSRIs fluoxetine and fluvoxamine reduced impulsive choice; though Baarendse and Vanderschuren [155] showed contrasting results for cit- alopram and paroxetine. Despite an i.c.v. 5,7-DHT treatment that achieved similar levels of 5-HT depletion, Winstanley and colleagues [158–160] found no effect of 5-HT deple- tion on impulsive choice, either on acquisition or established performance. However, 5-HT depletion significantly blocked the anti-impulsive effects of amphetamine [159], which may indicate that 5-HT modulates how this first-line treatment in ADHD interacts with the circuitry underlying impulsive choice. Serotonin receptor subtypes elicit distinct effects on impulsive choice (Table 4). 5-HT1A receptors are expressed both pre- and post-synaptically, and it is posited that these subpopulations, as well as regional differences in expression, result in divergent effects on impulsive choice. Systemic administration of the 5-HT1A receptor agonist, 8-OHDPAT, presumably acting on presynaptic autoreceptors to reduce 5-HT release, increased impulsive choice [160, 161], and this effect was dependent on DA in the nucleus accumbens [160]. However, when it was infused into the orbitofrontal cortex, 8-OHDPAT decreased impulsive choice, which may be due to postsynaptic receptor engagement or regional, cir- cuit-level differences [162]. Similar to its effects on impul- sive action, 5-HT2A receptor agonism with 2,5-dimethoxy- 4-iodoamphetamine (DOI) increased impulsive choice upon systemic administration [163], although the 5-HT2A/C receptor antagonist ketanserin had no effect [164]. Con- versely, the 5-HT2C receptor antagonist SER-082 reduced impulsive choice while the 5-HT6 receptor antagonist SB- 270146-A had no effect [164]. Finally, Mori and colleagues [165] reported that the 5-HT3 receptor antagonists grani- setron and ondansetron reduced impulsive choice in mice. These 5-HT3 receptor-mediated effects on impulsive choice may be due to their suppressant effects on the mesolimbic DA system and are consistent with their ability to reduce some DA-dependent behaviors, including hyperactivity and the rewarding properties of drugs of abuse [166, 167]. Hence, the role of 5-HT in impulsive choice depends on the integrated effects of specific 5-HT receptors in different neural locations. In general, measures of impulsivity in the 5CSRTT and delayed discounting task frequently dissociate in their responses to 5-HT (and catecholamine) manipulations, which may reflect the differential neurobiology underly- ing these distinct constructs of impulsivity, as well as the diverse functions of 5-HT. Serotonin is often implicated in behavioral inhibition, as originally proposed by Sou- brié [168]. Miyazaki et al. [169] showed that infusions of 8-OHDPAT directly into the dorsal raphe reduced “patience” of rats waiting for delayed rewards, with obvious relevance to both the 5CSRTT and the delayed discounting procedures. In addition, 5-HT neurons in the dorsal raphe increased tonic firing during the “waiting” epochs, as rats awaited reinforc- ing stimuli, showing a direct relationship between the wait- ing period and 5-HT neuronal firing rate. Serotonin is sug- gested to enhance the behavioral responses to rewards. For example, similar 5-HT treatments increased the breakpoint in progressive ratios of reinforcement (for either food or cocaine reinforcement), suggestive of a general disinhibi- tory action on motor function related to reward [170]. 3.2.3 � Role of 5‑HT in Action Cancellation Another commonly used paradigm for measuring impulsiv- ity is the SSRT procedure, in which a stimulus signals a halt to the completion of an already initiated motor response; impulsivity is exhibited by a prolongation of the SSRT (Table 2). This task has been frequently used in the assess- ment of individuals with ADHD [147]. Eagle and colleagues [171] showed that 5-HT depletion via i.c.v. 5,7-DHT, at a concentration sufficient to produce high levels of impulsivity on other assays (i.e., premature responding in the 5CSRTT, impaired go/no-go acquisition and performance [172]), had no effect on SSRT performance. Moreover, systemic citalo- pram had no effects [173]. This relative lack of effect might derive from the consideration that the SSRT task does not involve anticipatory responses to reward, as do the other impulsivity procedures. There has been less systematic anal- ysis of the possible role of 5-HT in stop-signal inhibition in experimental animals. However, there are some indications from human studies that 5-HT agents, such as citalopram and escitalopram, may have significant actions in healthy participants and those with clinical disorders [174, 175]. Thus, it may be premature to conclude at this stage a lack of 5-HT involvement in stop-signal inhibition. Summary The findings suggest that 5-HT has task- dependent effects on impulsive behaviors. While reduction in 5-HT is generally associated with increased impulsivity, it is more consistently linked with increased impulsive action as measured in the 5CSRTT. Similar to the reported findings with hyperactivity, the overall inhibitory effects of 5-HT on impulsivity may be due to modulatory effects on DA sign- aling. For example, serotonergic compounds that decrease DA signaling in the mesolimbic pathway (i.e., 5-HT2A and 5-HT3 receptor antagonists) attenuate impulsivity on some tasks (premature responding and impulsive choice), while those that increase DA signaling (i.e., 5-HT2C receptor antagonist) exacerbate impulsivity (premature responding). These findings suggest that 5-HT and DA jointly regulate 712 M. B. Solomon et al. this form of impulsive behavior, and the nucleus accumbens, mPFC, and orbitofrontal cortex are critical nodes for mediat- ing some of these behavioral effects. 3.3 � Role of 5‑HT in Emotional Dysregulation/ Impulsive Aggression Emotional dysregulation (i.e., emotional impulsivity, reac- tive aggression) is viewed by many as an inherent feature of ADHD [23]. Here, we describe studies investigating the impact of 5-HT on reactive aggression as one form of emo- tional dysregulation and exclude studies involving instru- mental aggression (i.e., predatory; Table 3). Emotional reactivity can also encompass additional behaviors repre- sentative of pathological expression of an emotion (e.g., laughing or crying); however, a larger expanse of studies has assessed impulsive aggression and thus permit evaluation of the underlying role of 5-HT. It is also worth noting that even though many view emotional dysregulation as an inherent feature of ADHD [22, 23], increased cognitive, behavioral, and emotional impulsivity are not directly linked, as dis- cussed in [22]. These findings suggest distinct neurocircuitry and/or neurochemical substrates may regulate these types of impulsivities. Serotonin deficiency, whether induced by genetic varia- tion, diet (i.e., tryptophan depletion), or pharmacologically, induces aggression in rodents [176–182]. Consistent with these findings, TPH2 KO rodents and TPH knock-in mice expressing a mutation in the TPH2 gene had lower levels of 5-HT and displayed increased impulsivity, compulsiv- ity, and aggression than healthy controls [183–188]. Con- ditional overexpression of 5-HT1A receptors located on serotonergic neurons to reduce serotonergic firing increased aggression, strengthening the argument for a role of 5-HT in impulsivity and aggression [189]. Corroborating these findings, increased serotonergic tone observed in SERT KO rats resulted in reduced aggression and improved impulsivity [190]. Similarly, acute treatment with the 5-HT precursor, 5-hydroxytryptophan (5-HTP), some SSRIs (i.e., sertraline, fluoxetine, and fluvoxamine; but not citalopram or parox- etine), and the 5-HT releasing agent fenfluramine attenu- ated aggression in the isolation-induced aggressive rat model [191]. However, the effects of chronic SSRI treatment on aggression in rats are complex; Peeters and colleagues [192] found that chronic citalopram reduced aggression in some Long Evans rats but increased it in others. Baseline aggres- sion, 5-HT1A receptor density, anxiety measures, and cue responsivity did not predict the interindividual differences in response to citalopram on aggression; thus, the precise role of the 5-HT system in these chronic effects of citalopram is unclear. 3.3.1 � Role of 5‑HT Receptors in Impulsive Aggression In terms of specific receptor subtypes, 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, and 5-HT3 receptors are implicated in aggression (Table  4). The 5-HT1A receptor agonist 8-OHDPAT consistently reduced aggression in rodents [191, 193], with its effect potentiated by 5-HT depletion (via PCPA), which suggests that it elicited these effects pri- marily through postsynaptic 5-HT1A receptors. Emphasiz- ing the integrated nature of 5-HT receptors on behaviors, blockade of other 5-HT receptors modulated this 5-HT1A receptor agonist effect. The 5-HT2A/5-HT2C receptor antagonist ketanserin potentiated the anti-aggressive effects of 8-OHDPAT, while (-)-penbutolol (a 5-HT1A/5-HT1B receptor antagonist) weakened its effects [191]. Transcript and protein expression of the 5-HT1A and 5-HT2C receptors vary according to trait aggression in rats, with a lower level of 5-HT1A and 5-HT2C mRNA and functional state recep- tors observed in the cortico-limbic circuitry of aggressive rats compared with tame counterparts [194, 195]. Moreover, variations in 5-HT2C receptor expression and activity via alternative RNA splicing/editing (VGV isoform) increased aggression in mice [196]. 5-HT2C receptor agonism is pur- portedly linked with anti-aggressive behaviors in Syrian hamsters (although the authors reached this conclusion with the nonselective 5-HT receptor agonist m-chlorophenylpip- erazine (mCPP) [197]). S32212, a compound with 5-HT2C receptor inverse agonism (i.e., blocking constitutive activ- ity of the 5-HT2C receptor), 5-HT2A receptor antagonism, and alpha2 adrenoreceptor antagonism, reduced aggres- sive behavior in mice [198]. The anti-aggressive effects of S32212 may be due to its 5-HT2A or 5-HT2C receptor properties because the 5-HT2A/5-HT2C receptor antagonist ritanserin also decreased aggression in isolation-reared mice [199]. 5-HT3 receptor antagonists (MDL-7222, odansetron) also reduced aggression in rats and mice [200, 201], which is consistent with their capacity to attenuate dopamine-medi- ated behaviors including amphetamine-induced hyperactiv- ity [166] and impulsive choice [165]. While compounds that target 5-HT3 receptors modulate aggressive behavior, their ability may depend on the methods used to induce aggression (isolation versus apomorphine versus alcohol) [200–203]. In addition, its expression varied by baseline levels of aggression; highly aggressive hamsters displayed increased 5-HT3 receptor density relative to those with low levels of aggression [204]. In alignment with the view that 5-HT reduces aggression, 5-HT1B KO mice displayed increased aggression [205]. Some of these receptors reportedly act as presynaptic ter- minal receptors, where they regulate not only the release of 5-HT but also other neurotransmitters including GABA, glutamate, and DA [206]. Through these means, 5-HT1B receptors likely influence both inhibitory and excitatory 713Serotonin in ADHD: Preclinical and Clinical Insights signaling in the brain. Consistent with the data from KO mice, pharmacological evidence demonstrated that systemic administration of the 5-HT1B receptor agonist anpirtoline reduced “frustration” after omission of a scheduled reward in mice [55]. Selective infusions of the 5-HT1B receptor ago- nist CP-94,253 into the dorsal raphe versus PFC produced contrasting effects on aggression in the alcohol drinking- induced aggression model (reviewed in [195]), demonstrat- ing regional differences in how the 5-HT1B receptor affects aggressive behavior. These divergent effects may arise from serotonergic modulation of other neurotransmitter systems in these brain regions. Agonists of 5-HT1B and 5-HT1A receptors reduce aggres- sion in rodents, but it is a challenge to identify their exact mechanism given that these targets include autoreceptors and postsynaptic receptors. De Boer and Koolhaas [207] proposed that 5-HT1A and 5-HT1B receptor agonists reduced social aggression (comparatively to pathological aggression) via presynaptic action at inhibitory somatodendritic receptors (5-HT1A) or inhibitory terminal receptors (5-HT1B), suggest- ing that a decrease in serotonergic tone reduced this type of aggression. These findings challenge the viewpoint that 5-HT deficiency is solely linked to aggression, as discussed in [207], and suggest that, consistent with its modulatory role, aberrant serotonergic signaling (too low or too high) may induce certain types of aggressive behavior. 3.3.2 � Serotonin and Dopamine Interactions in Impulsive Aggression Several findings suggest that the serotonergic and dopamin- ergic systems interact and jointly regulate hyperactivity- impulsivity [208] and impulsive aggression [209]. Whereas serotonergic deficiency is associated with increased aggres- sion, dopaminergic hyperactivity is associated with increased aggression, suggesting opposing roles for these systems in mediating aggressive phenotypes. Seo and colleagues [209] proposed that dysfunction in 5-HT-DA signaling at the level of the ventral PFC may be important in understanding how these two systems interact to influence impulsive aggression and other comorbid disorders. Loss of inhibitory control of the PFC over subcortical regions such as the nucleus accumbens and amygdala is linked to increased impulsivity, as well as depression and anxiety, which are common comorbid condi- tions in ADHD. In this vein, deficits in serotonergic neuro- transmission in the PFC may increase risk for cognitive and aggressive impulsivity, likely owing to altered interactions with the mesocorticolimbic dopaminergic system and the ensuing impact on downstream targets that regulate motor inhibition, reward, and emotional processing (nucleus accum- bens, amygdala). Summary While the data overwhelmingly indicate that deficits in 5-HT neurotransmission, such as lower 5-HT1A and 5-HT2C receptor expression levels, increase impulsive aggression, while 5-HT1A, 5-HT1B, and 5-HT2A receptor agonism decrease impulsive aggression, there is some phar- macological evidence that challenges this viewpoint, poten- tially owing to utilization of different models of aggression and the diverse impact of specific brain regions (PFC versus dorsal raphe nucleus) and serotonergic receptors on aggressive behavior. Overall, these findings suggest that aberrant seroton- ergic signaling contributes to emotional dysregulation. These data also highlight functional interactions between the seroton- ergic and dopaminergic systems, as well as glutamatergic and GABAergic neurons, and support hypotheses of how these two neurotransmitter systems interact to mediate certain indices of emotional dysregulation. 3.4 � Role of 5‑HT in Executive Function 3.4.1 � Role of 5‑HT in Attention There have been surprisingly few studies on the role of 5-HT in attentional processes. Profound 5-HT depletion by i.c.v. 5,7-DHT had no major effect on the ability of rats to detect brief visual signals of reward, despite pro- ducing excessive impulsive responding in the 5CSRTT [148]. Selective 5-HT depletion of the median raphe had no effect on detection accuracy; however, 5-HT depletion of structures innervated by the dorsal raphe (primarily neocortex and striatum) generated a transient improve- ment in accuracy [149], supporting the concept that serotonergic inputs to corticostriatal circuits may impact attention. Microinfusion of the 5-HT1A receptor agonist 8-OHDPAT or the selective 5-HT2A receptor antagonist M100917 into rat mPFC also enhanced attentional accu- racy [153]. In contrast to the selective 5-HT2A receptor antagonist M100917, the 5-HT2/5-HT3 receptor agonist quipazine selectively impaired attentional accuracy [210]. Neither acute treatment with the SSRI citalopram nor the 5-HT releasing agent fenfluramine had major effects on attentional accuracy in rats in the 5CSRTT [155], demon- strating that general increases in serotonergic tone do not alter sustained attention. These data suggest that increased 5-HT does not enhance attention per se, but circuit- and receptor-specific alterations of the 5-HT system can impact attentional capacities. Some of these brain region-specific effects of 5-HT modulation on attention may be due to interactions between 5-HT and distinct neuronal subtypes. For exam- ple, the impairments on attentional accuracy, as well as impulsive responding, produced by the NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phos- phonic acid (CPP) could be reversed by treatment with either 8-OHDPAT or the 5-HT2A receptor antagonist M-100907 when infused into rat mPFC [211, 212]. Thus, 714 M. B. Solomon et al. it appears that 5-HT can modulate some of the “top-down” PFC mechanisms influencing attention. In addition to circuit-specific effects, 5-HT may impact attention in a performance-dependent manner. A recent study in rhesus monkeys showed that the 5-HT precursor 5-HTP modu- lated attention directed towards social images in a base- line-dependent manner. 5-HTP supplementation improved attention in those with low baseline attention levels, with cerebrospinal fluid concentrations of 5-HTP across all rhe- sus monkeys positively correlating with looking duration and thus attention [213]. This baseline-dependency might conceivably be relevant to the ADHD phenotype. 3.4.2 � Role of 5‑HT in Cognitive Flexibility Several studies in rats and monkeys have demonstrated the importance of 5-HT to various capacities categorized under executive function, including cognitive flexibility (Table 2). Global 5-HT depletion [214] and local depletion in the mPFC or orbitofrontal cortex [215–218] impaired reversal learning, a measure of cognitive flexibility. Nota- bly, vortioxetine, a serotonergic targeting compound that enhances 5-HT neurotransmission in the mPFC, prevented these deficits in reversal learning [219–221]. Importantly, vortioxetine not only enhanced 5-HT neurotransmission but also indirectly increased DA, NE, and acetylcholine, which could contribute to its improved effects on reversal learning [220–222]. Serotonergic contributions to cognitive flexibil- ity are specific to reversal learning; serotonergic lesions do not induce deficits in another measure of cognitive flexibil- ity, attentional set-shifting [216], or a spatial self-ordered sequencing task [223]. Utilizing more translational means for evaluating the effect of lower 5-HT levels on ADHD- related behaviors, diet-induced tryptophan depletion has been applied in rodents and nonhuman primates. In contrast to 5-HT depletion studies, Van der Plasse and Feenstra [224] did not observe a significant impact of tryptophan depletion on spatial reversal learning, which the authors suggested may be indicative of a differential role of 5-HT on visual versus spatial reversal learning. However, they did not verify the degree of 5-HT reduction in their rats. Based on Merchán and colleagues’ findings [225], a more sustained protocol of tryptophan depletion may be necessary to induce sufficient reductions in 5-HT, and thus, an observable effect on spatial reversal learning. Initial findings demonstrated that rats with inferior per- formance on spatial reversal learning due to perseveration had lower orbitofrontal levels of 5-HT and its metabolite 5-HIAA, as well as decreased expression of 5-HT2A recep- tors [226]. Their behavior was remediated by acute cit- alopram treatment. In addition, rats characterized as more compulsive exhibited reversal learning deficits, as well as reduced expression of 5-HT2A mRNA transcripts in the frontal cortex [227], suggesting that dysfunction in cortical 5-HT2A receptors may underlie these deficits in reversal learning. Moreover, individual variability in basal impul- sive or compulsive behaviors, as well as species differences, may mediate sensitivity to serotonergic effects. For instance, reduced 5-HT levels via chronic tryptophan depletion increased cognitive inflexibility (compulsive behaviors) in Wistar rats that exhibited higher basal compulsivity but not those with low basal compulsivity [225]. Lister hooded rats, however, displayed normal levels of compulsivity, despite hyperactivity, with tryptophan depletion. The 5-HT-medi- ated effects on cognitive flexibility are receptor subtype- (e.g., 5-HT2A, 5-HT2C receptors) and brain-region specific (e.g., lateral versus medial orbitofrontal cortex, mPFC, stria- tum). Refer to the following review for the contribution of specific 5-HT receptors on behavioral flexibility [228]. Summary Overall, there is some evidence that 5-HT plays a role in the prominent cognitive symptoms of ADHD, including inattention, but these findings are more consistent in measures of impulsivity and cognitive inflexibility. More- over, the data underscore a critical role for balanced 5-HT neurotransmission and serotonergic interactions with other neurotransmitter systems in cortico-striatal-limbic regions, including the mPFC and orbitofrontal cortex, in attenuating these ADHD behavioral phenotypes. Lastly, inter-individual variability in basal levels of specific phenotypes (i.e., cogni- tive inflexibility) can mediate the effect of 5-HT manipula- tion on these behaviors. 4 � Stimulant‑Mediated Effects on the Serotonergic System in Preclinical Models 4.1 � Stimulant Effects on DA and NE For decades, stimulants (amphetamine and methylphe- nidate) have been the first-line treatment for ADHD. The main mechanism of action of stimulants is attributed to their ability to enhance catecholaminergic neurotransmis- sion in ADHD-relevant brain networks, which improves inattention, hyperactivity–impulsivity, and other indices of executive function in individuals with ADHD. This stimu- lant-mediated effect occurs in several ways, with a primary mechanism including binding to catecholaminergic trans- porters, DAT and NET, and ultimately increasing synaptic catecholamine concentrations; see review by Faraone [45]. Compounds that target monoamine transporters are typi- cally categorized as reuptake inhibitors or neurotransmitter 715Serotonin in ADHD: Preclinical and Clinical Insights releasers (Fig. 4). Methylphenidate acts as a reuptake inhibi- tor by preventing NE and DA from cycling back into the presynaptic terminal via NET and DAT, as reviewed in [45]. Stimulants that act as releasers, such as ampheta- mine, increase extracellular concentrations of monoamines by stimulating their efflux through reverse transport of the transporter, inducing channel-like activity at the transporter, or disrupting monoamine storage in presynaptic vesicles via interactions with the vesicular membrane associated trans- porter 2 (VMAT2) [229–231]. To this end, amphetamine enhances DA release by causing reversed efflux of the DAT, leading to transport of DA into the synapse [232]. Amphet- amine is also a potent releaser of NE in vitro [233, 234] and in vivo [235]. A more novel mechanism of action of amphetamine is attributed to its interaction with trace amine associated receptor 1 (TAAR1) [236], which plays a criti- cal role in regulating the monoamine balance in the brain, particularly DA and 5-HT. In addition, amphetamine inhibits monoamine oxidase [237, 238]. Although both methylpheni- date and amphetamine ultimately enhance DA and NE activ- ity, they do so via distinct mechanisms (for further reviews, see [45, 239]). 4.2 � Stimulant Effects on 5‑HT While the primary mechanism of action of stimulants is attributed to effects on dopaminergic and noradrenergic neurotransmission, amphetamine may also have relevant serotonergic effects. Amphetamine stimulates the release of [3H]-5-HT from rat synaptosomes [234, 240] and rat cortical [241] and hippocampal slices [242]. Notably, a therapeuti- cally relevant dose of amphetamine also increases 5-HT in the basal ganglia (caudate-putamen, accumbens) in rodents [243, 244] and in the frontal cortex, hippocampus, caudate- putamen, and thalamus in nonhuman primates [245]. In healthy human volunteers, an acute oral amphetamine chal- lenge increased extracellular 5-HT release throughout the cortex (frontal, parietal, temporal) but not in the cerebellum [246], suggesting brain-region specific effects. Because it is only a weak inhibitor of the human and murine SERT [247], amphetamine is presumed to enhance serotonergic neuro- transmission via interaction with VMAT2, TAAR1, and/or the MAO enzyme. Overall, these findings support that clini- cally relevant doses of amphetamine serve to stimulate 5-HT Fig. 4   In vitro pharmacology of select monoaminergic transporter inhibitors. Heatmap represents pKi values (i.e., negative log of the inhibition constant of drugs that inhibit monoaminergic transporters and their selectivity between the three monoamine transporters. Bind- ing affinity ranked by color. For the heatmap of pKi values, higher binding affinities (i.e., high pKi values) are shown in red and lower binding affinities (i.e., low pKi values) are shown in blue. Data were acquired from the International Union of Basic and Clinical Phar- macology/British Pharmacological Society (IUPHAR/BPS) Guide to Pharmacology, except for viloxazine [413] and vortioxetine [nor- epinephrine transporter (NET) and dopamine transporter (DAT) data acquired from Trintellix’s label]. Data for atomoxetine are dissocia- tion constant (Kd) rather than inhibition constant (Ki) values. All data were acquired using human transporters, except for the pKi value of imipramine at DAT, which used rat transporters 716 M. B. Solomon et al. release in ADHD-relevant brain regions across species and experimental modalities. In contrast to amphetamine, methylphenidate exhibits selective inhibition of NET and DAT but not SERT. In a microdialysis study, methylphenidate did not increase extra- cellular levels of 5-HT in rat striatum or hippocampus even at high systemic doses [248], consistent with earlier data on the drug showing low affinity for SERT and minimal effects on 5-HT reuptake in vitro [249–251]. However, a recent study in rats demonstrated that therapeutically rel- evant doses of methylphenidate increased 5-HT levels in the mPFC (using high-performance liquid chromatography) and was associated with improved memory [252]. These lim- ited findings suggest that the impact of methylphenidate on serotonergic neurotransmission may be brain region-specific and/or sensitive to different experimental methodologies. Summary Undoubtedly, catecholamines are the primary target for stimulants, but the aforementioned studies also indicate a stimulatory role of amphetamine and methylphe- nidate on serotonergic neurotransmission in ADHD-relevant brain networks. However, relative to methylphenidate, the impact of amphetamine on serotonergic neurotransmission appeared more robust. It is worth noting that on average the effect size for amphetamine in ADHD clinical trials is reportedly greater than for methylphenidate [253–256], but whether serotonergic effects are a contributing factor remains to be determined. 5 � Does Serotonergic Manipulation Affect Stimulant Efficacy in ADHD‑Relevant Preclinical Models? In this section we set out to answer: Does 5-HT have a mod- ulatory effect on ADHD treatment? We discuss evidence of stimulant-mediated effects that are altered in the presence of serotonergic drugs (Table 5), as well as under condi- tions associated with serotonergic dysfunction (i.e., dietary, genetic, or molecular). 5.1 � Serotonin Modulation of Stimulant Effects on Hyperactivity Overall, there has been little evidence that amphetamine and methylphenidate elicit behavioral effects through direct 5-HT actions. However, 5-HT does modulate behavioral responses to stimulants. Depletion of 5-HT, following treat- ment with PCPA or the neurotoxin 5,7-DHT, potentiates methylphenidate- or amphetamine-induced hyperactivity [257–259]. Conversely, enhanced 5-HT neurotransmission with pargyline (monoamine oxidase B inhibitor) blocks amphetamine-mediated hyperactivity [260]. These behav- ioral patterns are consistent with the concept of DA-5-HT opponency observed in some brain regions [259, 261–264]. Although not viewed as a model of hyperactivity given its normal basal activity levels, the TPH2 knockout mouse displayed enhanced sensitivity to amphetamine-induced hyperactivity [265]. Interestingly, this serotonergic effect on amphetamine-induced locomotion was mediated by a noradrenergic mechanism, as evidenced by normalization of motor activity and striatal NE release separately by the 5-HT precursor, 5-HTP, and the NE precursor, DOPS [265]. Thus, the integral signaling across monoaminergic systems is critical to efficacious responsivity to ADHD treatments in reducing hyperactivity. The neonatal 6-OHDA lesioned rodent, a model of ADHD, has provided evidence that 5-HT mechanisms modulate stimulant-mediated effects on hyperactivity (with the caveat that this model exhibits altered seroton- ergic innervation and content). Heffner and Seiden [109] observed that methysergide (a nonselective 5-HT receptor antagonist and 5-HT2B receptor partial agonist) was able to block the hyperactivity-reducing effects of ampheta- mine, whereas drugs that antagonized DA, NE, muscarinic, or opiate receptors could not. In contrast, Avale and col- leagues [115] showed that 5-HT depletion (via PCPA) reduced 6-OHDA-induced hyperactivity but did not impact the effects of amphetamine. However, these results could be due to floor effects. Overall, these findings (1) confirm the oppositional relationship between DA and 5-HT and (2) suggest that selective 5-HT receptor engagement may medi- ate the therapeutic effects of stimulants in an ADHD model. 5.2 � Serotonin Modulation of Stimulant Effects on Impulsivity Amphetamine effectively reduces impulsivity in the tempo- ral discounting of reward paradigm [159], a well-validated, highly translational assay assessing impulsive choice [147], though its effects have been found to vary based on experimen- tal parameters (e.g., delay length, schedule of reinforcement, visual cue signaling the reward), baseline impulsivity, and treatment duration [159, 266–269]. 5-HT depletion via i.c.v. 5,7-DHT did not impact performance in the assay, but it did block amphetamine-induced effects, mainly at the highest dose of amphetamine and in rats with high basal levels of impulsiv- ity [159]. Cis-flupenthixol, a mixed DA receptor antagonist, blocked the behavioral effects of lower doses of amphetamine but only in the presence of 5-HT depletion. This experiment suggests a contributory role for 5-HT and DA in mediating amphetamine effects on impulsivity. Similarly, a series of 717Serotonin in ADHD: Preclinical and Clinical Insights pharmacological manipulations further demonstrated an essen- tial role for 5-HT in mediating amphetamine effects [160]. At a dose that did not affect temporal discounting of reward on its own, intra-accumbal 8-OHDPAT (5-HT1A receptor ago- nist) blocked the ability of amphetamine to reduce impulsive choice in intact rats, and this inhibitory effect of 8-OHDPAT was attenuated in rats with a 6-OHDA lesion. Administration of the 5-HT1A receptor antagonist WAY100635 potentiated the effect of a large dose of amphetamine to reduce impulsive choice. In contrast, high doses of the 5-HT1A receptor agonist 8-OHDPAT increased impulsive choice, but this was blocked by a 6-OHDA lesion of the nucleus accumbens but not an i.c.v. 5,7-DHT lesion. Moreover, 6-OHDA infusion into the nucleus accumbens, which lesioned DA and NE neurons, had no impact on amphetamine-mediated reductions in impulsivity [160]. These results suggest that the ability of amphetamine to decrease impulsive choice is not solely dependent on cat- echolaminergic function in the accumbens but also involves engagement of accumbal 5-HT1A receptors and potentially other neurotransmitters in other brain regions. This interpreta- tion is consistent with observations that systemic administra- tion of 8-OHDPAT inhibited the increase in DA in the nucleus accumbens, dorsal striatum, and frontal cortex produced by amphetamine [270, 271]. These findings imply that the effects of amphetamine on this form of impulsive responding depend on both DA and 5-HT, but the precise neural mechanisms underlying these interactions are unclear. 5-HT2A and 5-HT2C receptors may also modulate impul- sivity induced by stimulant drugs in rodents. Amphetamine increased impulsivity (i.e., premature responding) in the Table 5   Selective serotonin receptor modulation of stimulant effects in preclinical species Involvement of 5-HT receptor subtypes in stimulant-mediated effects for attention-deficit hyperactivity disorder-relevant behaviors, including hyperactivity and impulsivity. See [102] for full review on 5-HT receptor subtype expression and their modulation of dopaminergic signaling in subregions of the brain. Indicators represent the corresponding changes in behavior following modulation of the receptor/transporter/enzyme: ↑ = potentiates the effect; ↓ = attenuates the effect; = no effect; X = blocks or cancels the effect 5,7-DHT 5,7-dihydroxytryptamine, 5-HT serotonin, 6-OHDA 6-hydroxydopamine, AMPH amphetamine, MPH methylphenidate, MAO B mono- amine oxidase B, NAc nucleus accumbens, PCPA para-chlorophenylalanine, SERT serotonin transporter, TPH tryptophan hydroxylase, VTA ven- tral tegmental area Target Activity Ligand Administration Mediation of stimulant effect 5-HT neurons Lesion 5,7-DHT Intracerebroventricular ↑ AMPH-induced hyperactivity [260] X AMPH-induced reductions in impulsive choice [159] TPH Inhibitor PCPA ↑ MPH- and AMPH-induced hyperactivity [257–260] ↓ Hyperactivity in 6-OHDA model [115] AMPH-induced decrease in hyperactivity in neonatal 6-OHDA model [115] TPH knockout mouse ↑ Sensitivity to AMPH-induced hyperactivity [265] MAO B Inhibitor Pargyline Systemic ↓ AMPH-induced hyperactivity [257, 260] SERT Inhibitor Fluoxetine Systemic ↑ MPH-induced hyperactivity [140] 5-HT recep- tors, nonse- lective Antagonist Methysergide Systemic X AMPH-induced decrease in hyperactivity in neonatal 6-OHDA model [109] 5-HT1A Agonist 8-OHDPAT Intra-accumbal X AMPH-induced reductions in impulsive choice [160] ↑ Impulsive choice, which was blocked by 6-OHDA lesions [160] Antagonist WAY100635 Systemic ↑ AMPH-induced effects on impulsive choice [160] 5-HT1B Agonist CP 94253/ CP 93129 Systemic ↑ MPH-induced hyperactivity [140] Antagonist GR 55562 Systemic ↓ Meth-AMPH- and fluoxetine-potentiated MPH hyperac- tivity [140] 5-HT2A Antagonist MDL 100,907, amperozide Systemic X AMPH-induced hyperactivity [278, 461] X AMPH-induced increases in impulsive action [272] SR46349B Systemic, intra VTA (not intra-PFC) X AMPH-induced hyperactivity [279] Antagonist Ritanserin Intra-striatal Fluoxetine-potentiated MPH hyperactivity [140] ↓ D1 hypersensitivity in neonatal 6-OHDA lesioned mice [114, 117] 5-HT2C Agonist Ro60-0175 Systemic X AMPH-induced increases in impulsive action [272] 5HT3 Antagonist Ondansetron Systemic, intra-NAc ↓ AMPH-induced hyperactivity [166] 718 M. B. Solomon et al. 5CSRTT [272]. Stimulant-mediated impulsive responses were blocked by both a 5-HT2C receptor agonist and a 5-HT2A receptor antagonist. 5-HT2C receptors may medi- ate these effects through modulation of dopaminergic path- ways (mesolimbic, mesocortical, and nigrostriatal), where they colocalize with dopaminergic and GABAergic neurons [273]. For instance, activation of 5-HT2C receptors local- ized on GABAergic interneurons in the ventral tegmental area (VTA) may inhibit DA neurotransmission in afferent regions (e.g., mPFC, nucleus accumbens, VTA) [273, 274]. The 5-HT2 receptor agonist DOI potentiated amphetamine- induced DA increases in the striatum [275]. Conversely, systemic 5-HT2A receptor antagonists attenuated amphet- amine-induced DA increases in the mPFC, striatum, and nucleus accumbens [271, 275–279]. Interestingly, only when the 5-HT2A receptor antagonist SR46349B was infused sys- temically or into the VTA (but not the PFC) did it reduce these amphetamine-mediated effects on DA release [279]. These findings demonstrate that 5-HT receptors moderate dopaminergic signaling and behaviors (e.g., impulsivity, locomotion, compulsivity); however, there is no evidence that amphetamine is able to directly or indirectly (through its release of 5-HT) engage these 5-HT receptors. Summary Based on these preclinical studies, 5-HT can modify some ADHD-relevant actions of stimulants. In gen- eral, activation of 5-HT receptors that increase DA neuro- transmission (e.g., 5-HT2A receptors) potentiate stimulant- mediated effects, while activation of those that decrease DA neurotransmission (e.g., 5-HT2C receptors) attenuate stimulant-mediated effects. Collectively, the experiments show that 5-HT and DA (and likely NE) interact to influence behavioral responses of stimulants, most notably hyperactiv- ity and impulsivity. 6 � Clinical Studies The initial link between 5-HT and behavioral manifesta- tions of ADHD was proposed by Coleman [280]. Coleman reported that lower 5-HT platelet levels were associated with hyperactivity, while increased levels were associated with improved attention span. Although limited by small sample size, correlative analyses, and uncertain diagnostic criteria, the findings fostered the hypothesis that serotonergic dys- function is linked to core ADHD behavioral features. Fol- lowing this seminal study, several candidate gene studies have reported associations between ADHD presentation and single-nucleotide polymorphisms (SNPs) in genes related to 5-HT synthesis (TPH2), reuptake (5-HTT), degrada- tion (MAOA, MAOB), and receptor subtypes (5-HT1B, 5-HT2A, 5-HT2C) [281–290], which may be indicative of serotonergic dysfunction in some populations with ADHD. Owing to small sample sizes and a lack of replicability of these findings across studies, as well as a failure to identify SNPs in serotonin-related genes significantly associated with ADHD in recent large scale genome-wide association stud- ies [291–294], these findings from candidate gene studies should be interpreted with caution. However, these results also do not preclude the possibility that dysfunction in sero- tonergic genes increase the risk for ADHD in subsets of the population. Whereas the technical bandwidth for manipulating the serotonergic system is broader in preclinical experiments, clinical and neuroimaging studies are primarily limited to the use of acute diet-induced tryptophan depletion or acute or chronic treatment with serotonergic targeting compounds. These studies have evaluated the role of 5-HT in ADHD-rel- evant behaviors in healthy controls, ADHD populations, and individuals presenting with ADHD-associated symptoms (e.g., impulsive aggression) or comorbidities (e.g., depres- sion). Because of the limited number of published studies investigating the role of serotonergic neurotransmission in ADHD, when relevant, we extrapolate from findings where serotonergic function was evaluated in conditions with closely related behavioral and neuroanatomical phenotypes (e.g., conduct disorder, depression) (Table 6). 6.1 � Role of 5‑HT in Impulsivity in ADHD and Non‑ADHD Populations Many of the behavioral tasks designed to gauge impul- sivity in preclinical studies have human-equivalent ver- sions (Table 2). For example, behavioral tests that assess impulsive action and impulsive choice in humans include go/no-go, four-choice serial reaction time task (4CSRTT), continuous performance test, the SSRT task [145], and delayed discounting of reward [295]. There is some genetic evidence suggesting that serotonergic deficiency is linked with impulsive behaviors. The functional TPH2 gene poly- morphism (G-703T; rs4570625) and DNA methylation in the 5′ untranslated region of TPH2 (TPH2-5′UTR) are asso- ciated with altered neural processing in the 4CSRTT [296]. Some groups hypothesized that this SNP may be associated with lower enzyme activity, and thus, decreased serotoner- gic neurotransmission, as briefly discussed in [282, 297]. A previous study reported that DNA methylation in the TPH2- 5′UTR and the SNP G-703T (rs4570625) led to decreased TPH2 mRNA expression and even decreased DNA-protein interactions for the SNP [298], which would theoretically disrupt serotonergic neurotransmission; though see [299]. Notably, on the 4CSRTT, higher levels of DNA methyla- tion correlated with more premature responses (i.e., impul- sive action) in participants with ADHD, but not in healthy controls, suggesting that individuals with ADHD may be particularly more sensitive to lower levels of 5-HT in the manifestation of impulsivity. However, for a more thorough 719Serotonin in ADHD: Preclinical and Clinical Insights Table 6   Clinical studies evaluating the effects of 5-HT-targeting agents on behavioral outcomes relevant to ADHD phenotypes Target/activity Ligand Population Hyperactivity Impulsive action Impulsive choice Aggression Attention Cognitive flexibility 5-HT1A agonist (presynaptic) / partial agonist (postsynaptic) Buspirone ADHD ↓ [321] ↓ [321] ↑ [321] 5-HT2A antago- nist Pipamperone Intellectual dis- abilities ↓ [350] Quetiapinea Borderline per- sonality ↓ [322] ↓ [322] 5-HT2C agonist Lorcaserin Intermittent explosive disorder ↓ [351] 5-HT2A/5-HT2C antagonist Ketanserin Healthy ↓ [324] SERT inhibitor Paroxetine Healthy ↓ [357] Conduct disorder ↓ [341] ↓ [341] Depression [341] ADHD [53] [53] [53] Fluoxetine Healthy ↓ [357] Personality disorder ↓ [342] Depression [363] ADHD + depres- sion [54] [54] [54] Escitalopram Citalopram Healthy ↓ [357, 358] Depression [364, 365] Sertraline Depression [363–365] Venlafaxine ER Depression [363–365] ADHD ↓ [51] ↓ [51] ↑ [51] SERT inhibitor; 5-HT2 agonist Fenfluramine Healthy [315] Conduct disorder ↓ [314, 315] ↓ [314] SERT inhibitor; 5-HT receptor modulator Vortioxetine Depression ↑ [367, 368] ADHD [418] [418] [418] NET/DAT/SERT inhibitor Desipramine Imipramine ADHD ↓ [406] ↓ [406] ↑ [406] NET inhibitor; 5-HT receptor modulator Viloxazine ER ADHD ↓ [417] ↓ [417] ↑ [417] 720 M. B. Solomon et al. evaluation of a putative role of 5-HT in impulsive pheno- types, several studies have experimentally altered sero- tonergic neurotransmission via tryptophan modulation in diverse clinical populations and healthy participants; these are reviewed below. 6.1.1 � Tryptophan Studies Two studies have investigated the impact of acute tryptophan depletion on impulsivity in an ADHD population [300, 301]. In a randomized controlled trial of adults with ADHD and healthy controls, neither tryptophan loading nor tryptophan depletion impacted performance on a version of a go/no-go task, delay discounting task, or Iowa gambling task [300]. In another study, adolescent males with high trait aggression and ADHD committed more inhibition errors on a go/no-go task following acute tryptophan depletion relative to those given placebo. In contrast, adolescent males with low trait aggression and ADHD had fewer inhibition errors following acute tryptophan depletion relative to those given placebo. These findings imply that the impact of 5-HT neurotrans- mission on impulsivity in ADHD is dependent upon trait aggression. If intact serotonergic neurotransmission is criti- cal for impulse control, then its disruption could induce this phenotype in otherwise “healthy” subjects. Several studies indicate that disruption in serotonergic signaling via acute tryptophan depletion increases impulsivity in neurotypical participants [302–308]; however, see [309, 310] for null effects of tryptophan depletion. Similar to preclinical stud- ies, 5-HT is more closely linked with impulsive action than impulsive choice, with the majority of studies reporting no effect of tryptophan depletion on impulsive choice [303, 307, 310] and one study reporting that low 5-HT increased delayed discounting of reward [311]. Enhancing serotonergic neurotransmission via l-trypto- phan loading or pharmacological manipulations has yielded mixed results on impulsivity. These differences may be due to the nature of the assessments (e.g., questionnaire versus laboratory-based tests), differences in the mechanism of action of the serotonergic compounds, or duration of treat- ment with serotonergic compounds (acute versus chronic). Dougherty and colleagues [304] found that acute tryptophan depletion increased impulsivity from baseline performance while l-tryptophan loading had no significant effects on impulsivity in the immediate memory task (a modified ver- sion of the continuous performance test). The lack of an effect with tryptophan loading could be because of the fact that these were otherwise healthy individuals; these effects might have been more pronounced in vulnerable popula- tions, perhaps with lower baseline levels of 5-HT, as sug- gested by [312, 313]. Consistent with this, fenfluramine reduced impulsive choice (delay discounting) in men with a history of conduct disorder [314] but not in those without the disorder [315]. Conversely, in healthy individuals, acute treatment with the SSRI escitalopram improved impulsivity on a SSRT task [316]. 6.1.2 � Serotonin Receptor Subtypes A handful of clinical studies have evaluated the contribu- tion of 5-HT receptor subtypes on impulsive behaviors in humans, but few have included participants with ADHD. Studies have utilized compounds targeting specific 5-HT receptor subtypes in individuals with borderline personal- ity (a condition characterized by impulsivity and high-risk taking behaviors) or people with a history of cocaine use, which may be of relevance to the defining characteristic of impulsivity. ADHD is often comorbid with substance abuse disorder [28, 29] and risk-taking behaviors [317, 318], and decreased 5-HT signaling is associated with these comor- bidities [319, 320]. Most clinical studies have primarily focused on the role of 5-HT1A and 5-HT2A receptors in impulsive behaviors; however, the overall evidence is preliminary. For example, research on the 5HT1A receptor is limited to a small double- blind study of buspirone in individuals with ADHD, sug- gesting 5-HT1A receptor agonism may improve symptoms including impulsivity [321]; buspirone is characterized as a full agonist at 5-HT1A presynaptic receptors and partial agonist on 5-HT1A postsynaptic receptors (Table 6). With regard to 5-HT2A receptors, Van den Eynde and colleagues [322] reported that quetiapine, a 5-HT2A and D2 recep- tor antagonist, decreased impulsivity (Stroop Color Word Task) and risky decision making (Iowa Gambling Task) in Table 6   (continued) The table displays clinical studies evaluating ADHD-relevant phenotypes, such as hyperactivity, impulsivity (impulsive action and impulsive choice), attention, cognitive inflexibility, and emotional dysregulation (i.e., aggression), after administration of compounds that pharmacologi- cally engage serotonergic targets. These studies were conducted in different populations, including individuals denoted as healthy and those diagnosed with ADHD, conduct disorder, depression, intellectual disabilities, intermittent explosive disorder, or personality disorder. Indicators represent the corresponding changes in behavior following modulation of the receptor/transporter: ↑= increased behavior; ↓ = decreased behav- ior; = no effect on behavior 5-HT serotonin, ADHD attention-deficit hyperactivity disorder, DAT dopamine transporter, ER extended release, NET norepinephrine trans- porter, SERT serotonin transporter a Quetiapine has a complex pharmacological profile, but one of its primary actions is antagonistic effects at 5-HT2A receptors [323] 721Serotonin in ADHD: Preclinical and Clinical Insights individuals with borderline personality disorder. It is impor- tant to note that quetiapine has a complex pharmacological profile, but one of its primary actions is its 5-HT2A receptor antagonism [323]. Consistent with the decrease in impulsive and risk-taking behaviors with quetiapine administration, blockade of 5-HT2A/2C receptors with ketanserin increased risk aversion in a gambling task in healthy volunteers [324]. These clinical findings are consistent with preclinical studies reporting that systemic blockade of 5-HT2A/2C receptors with ketanserin decreased premature responding (impulsiv- ity) in the 5CSRTT [325] and risky decision making in rats [326]. Moreover, these data underscore the potential utility of compounds targeting the 5-HT2A receptor for conditions that are associated with increased impulsivity and risk-tak- ing behaviors. 6.2 � Role of 5‑HT in Emotional Dysregulation in ADHD and Non‑ADHD Populations Emotional dysregulation (e.g., frustration, reactive aggres- sion, emotional impulsivity) occurs in approximately 25–50% of children and 30–70% of adults with ADHD [22, 327–329] and significantly contributes to poorer clinical outcomes across the lifespan [330]. Aggression is classi- fied into premeditated (predatory) or impulsive (affective) aggression; here we focus on the latter (Table 3). 6.2.1 � Tryptophan Studies Given the association between serotonergic dysfunction and emotional dysregulation, several studies have evaluated the impact of acute tryptophan depletion on reactive aggres- sion in ADHD populations [301, 331–335]. These studies consistently demonstrated an inverse relationship between 5-HT and reactive aggression in adolescents and adults with ADHD, which was dependent upon baseline levels of impul- sivity or trait aggression. Similar to clinical populations, acute tryptophan depletion increased aggressive responding in healthy males [336] and in men with high levels of base- line aggression [337–339]. However, the study by Cleare and Bond [339] did not find marked differences in impul- sive aggression in response to acute tryptophan depletion in people with low trait impulsivity, suggesting that individu- als with a predisposition for high aggression may be more sensitive to alterations in serotonergic neurotransmission. Women have exhibited enhanced sensitivity to serotoner- gic modulation as well, though it was dependent upon basal plasma tryptophan levels [340]. For example, behavioral responses to tryptophan depletion or tryptophan loading, including increased and decreased aggression, respectively, were more evident in women with higher baseline levels of plasma tryptophan. 6.2.2 � Serotonin Receptor Subtypes In alignment with the theory that deficits in serotonergic neurotransmission increase aggression in some individu- als, fenfluramine and the SSRIs paroxetine and fluoxetine reportedly decreased aggression in individuals with condi- tions characterized by emotional dysregulation, including conduct and personality disorders [314, 315, 341, 342]. Although larger, more robust studies are needed, genetic and pharmacological studies have implicated 5-HT1B, 5-HT2A, and 5-HT2C receptors in emotional dysregulation (reactive aggression). Based on a meta-analysis evaluat- ing associations between 5-HT1B, 5-HT2A, and 5-HT2C genetic variants and ADHD, the SNP rs6296 in the 5-HT1B gene may increase risk for ADHD [52] and in a candidate gene study (N = 967) was significantly associated with child- hood aggression and hostility [343]. Preclinical studies also report enhanced aggression in 5-HT1B receptor knockout mice [205]. These genetic findings align with pharmaco- logical evidence demonstrating that the 5-HT1B/1D recep- tor agonist zolmitriptan and the 5-HT1B receptor agonist CP-94,253 decreased alcohol-induced aggression in indi- viduals reporting modest alcohol consumption [344]. These anti-aggressive effects with 5-HT1B receptor agonism may be due to enhanced serotonergic signaling in the orbitofron- tal cortex [345], a critical brain region for the inhibitory con- trol of emotion. Moreover, the orbitofrontal cortex appears to play an important modulatory role in reactive, but not instrumental, aggression [346]. Polymorphisms in the 5-HT2A gene (rs7322347, A-1438G) may predispose individuals to aggressive [347] and impulsive behaviors [348]. Likewise, in two candidate gene studies focused on serotonin-related genes, SNPs in the 5-HT2C gene (rs6318, rs518147) may increase suscep- tibility to violent behavior/criminal impulsivity [349] and ADHD [289]. 5-HT2A receptor antagonism with pipam- perone [350] and 5-HT2C receptor agonism with lorcaserin [351] decreased aggressive responding in individuals with intellectual disabilities and intermittent explosive disorder, respectively, indicating that these compounds can decrease aggressive responding in diverse populations (Table 6). These effects on aggressive behavior are consistent with preclinical findings demonstrating opposing roles of these receptor subtypes in motor, cognitive, and emotional impul- sivities [197, 199]. 6.3 � Role of 5‑HT in Attention in ADHD and Non‑ADHD Populations 6.3.1 � Tryptophan Studies Two published studies have evaluated the impact of acute tryptophan depletion on attention in ADHD populations 722 M. B. Solomon et al. [352, 353], and these showed contrasting effects. Zepf and colleagues conducted an intra-subject cross-over study of male children and adolescents with ADHD. Tryptophan depletion improved lapses of attention relative to sham treat- ment but had no effect on phasic alertness on a test battery of attentional performance [352]. Conversely, Mette and col- leagues conducted an intra-subject crossover study of adult males with and without ADHD, showing that tryptophan depletion worsened attention on the continuous performance test in adults with ADHD but not in controls [353]. These findings suggest that effects of acute tryptophan depletion may be age- or task-dependent; however, additional studies are needed, involving broader populations (e.g. including females and pediatric controls) and comparable measures of attention. Unlike the consistent findings with impulsivity and emo- tional dysregulation, acute tryptophan depletion did not induce prominent effects on different aspects of attention, including sustained attention or attentional set-shifting, in healthy subjects (see [354] for a systematic review). Trypto- phan depletion impaired Pavlovian and instrumental reversal learning, which is consistent with preclinical studies [355], but it did not impact probabilistic reversal learning [356], indicating task-specific effects. While acute tryptophan depletion did not strongly impact attentional processes in healthy individuals, there is evidence to suggest that vulner- able populations (e.g., depressed) may be more sensitive to its impact on sustained attention and executive function, as discussed in [354]. 6.3.2 � Serotonin Receptor Subtypes Few clinical studies have evaluated the role of specific 5-HT receptor subtypes in attention-related processes. SSRIs are known to impair divided attention and sustained attention in healthy individuals [357]. In a four-arm study, Wingen and colleagues [358] evaluated the effects of placebo, escit- alopram alone, or escitalopram co-administered with either the 5-HT2A receptor antagonist ketanserin or the 5-HT1A receptor antagonist pindolol on divided and sustained atten- tion in healthy participants. When given alone or in combi- nation with ketanserin or pindolol, escitalopram impaired divided and sustained attention relative to placebo treat- ment [358]. Neither ketanserin nor pindolol potentiated or attenuated the effect of escitalopram on divided attention but the combination of escitalopram and ketanserin induced greater deficits on sustained attention. The authors attributed this potentiating effect of 5-HT2A receptor antagonism to increased inhibitory effects on the mesocorticolimbic dopa- minergic pathway [359–361], which is consistent with the notion that diminished dopaminergic neurotransmission is linked with attention deficits [362]. 6.3.3 � SSRIs and Vortioxetine SSRIs, including escitalopram, sertraline, fluoxetine, and paroxetine, have been shown to worsen or have no effect on various attentional processes in healthy individuals [357] or individuals with depression (comorbid with ADHD) [363–365]. Treatment duration may impact these effects. For instance, acute treatment of escitalopram in a healthy population impaired learning on a probabilistic reversal learning task, as well as extradimensional set shifting and reversal performance [316]; however, chronic administration of escitalopram had no impact on these measures, though a reduction in reinforcement sensitivity was observed [366]. While SSRIs induce little, or impairing, effects on atten- tion, there is some evidence that newer serotonergic target- ing agents may be beneficial (Table 6). For example, vor- tioxetine, a SSRI with 5-HT1A receptor agonism, 5-HT1B receptor partial agonism, and 5-HT3, 5-HT7, and 5-HT1D receptor antagonism, improved selective and sustained atten- tion and other indices of executive dysfunction in individuals with depression [367, 368]. The contrasting effectiveness of vortioxetine versus other SSRIs on cognitive impairment may reflect differences in their engagement of distinct 5-HT receptors (Fig. 5), indirect effects on other neurotransmitter systems, or differences in disorder-specific effects (depres- sion versus ADHD) [220, 221, 369]. Taken together, the findings suggest that therapeutic effects on attention may depend on engagement of specific 5-HT receptors rather than a general increase in 5-HT. The data also highlight some divergent effects of these compounds on attention based on the disease status of the individual (healthy versus depressed), which may be due to altered brain neurochem- istry mediating treatment response and treatment duration. Summary Although several candidate gene studies iden- tified associations between polymorphisms within the serotonergic system and manifestations of distinct ADHD behavioral phenotypes, including impulsivity and emotional dysregulation, limitations within these studies, such as small sample sizes, inability to control for covariates, and a gen- eral lack of replication, lend to cautious interpretation of the results. Moreover, the associations with serotonergic genes identified in candidate gene studies have not been observed in larger genome-wide association study reports. Though limited, available studies using tryptophan depletion show differential (e.g., task-dependent) effects on attention and increased impulsivity in individuals with ADHD. In healthy individuals, acute tryptophan depletion is more frequently linked to impulsivity, with little impact on attention; how- ever, various compounds that facilitate serotonergic neuro- transmission (SSRIs) appear to impair attention in healthy individuals. A newer serotonergic targeting agent (vortiox- etine) appears to improve attention and executive deficits in individuals with depression. The differences in efficacy 723Serotonin in ADHD: Preclinical and Clinical Insights of serotonergic targeting compounds on behavioral features core to ADHD versus those expressed in psychiatric condi- tions comorbid with ADHD may depend upon heterogeneity of brain function across individuals. In both neurotypical and ADHD populations, 5-HT deficiency via acute trypto- phan depletion is consistently linked with increased reactive aggression, underscoring the importance of intact serotoner- gic neurotransmission to emotion regulation. Taken together, these data suggest a role for 5-HT in the pathogenesis of some core ADHD behavioral features and may implicate the utilization of compounds with specific effects on the serotonergic system (e.g., 5-HT1B, 5-HT2A, and 5-HT2C receptors) for ADHD treatment on the basis of presentation subtype (hyperactivity–impulsivity versus inattention) or co- expression of emotional dysregulation. 7 � Role of 5‑HT on Brain Networks in ADHD and Non‑ADHD Populations Much of the knowledge regarding the neural circuitry regu- lating attention, behavioral inhibition, and emotional dys- regulation is driven by preclinical studies, yet there is cross- species convergence in implicated brain regions. One of the most consistently studied networks in ADHD is the default mode network (DMN), including the mPFC, anterior and posterior cingulate gyri, ventral precuneus, parietal cortex, and hippocampus [370, 371]. While there are no univer- sally accepted biomarkers for ADHD, altered function of the DMN has been advanced as a putative neuroanatomical biomarker for this disorder [372, 373]. Theoretically, ADHD is characterized by hyperactivity of the DMN and visual attention network, which may manifest as “daydreaming” or hypersensitivity to visual stimuli leading to disruption of goal-oriented activities [374]. Thus, in a neurotypical brain the DMN is upregulated when the mind is wandering but downregulated during attention-demanding tasks, and con- versely the task positive network (TPN), which includes the dorsolateral PFC and anterior cingulate, increases in activity. This divergent synchronization between the TPN and DMN during attention-demanding tasks is believed to be disrupted in ADHD. For example, increased connectivity between the DMN and TPN while performing a go/no-go task was linked with impaired impulse control [375]. Notably, stimulants increased task-related suppression of DMN brain regions, potentially restoring the TPN/DMN balance in ADHD [376–378]. Given the impact of stimulants on monoaminer- gic neurotransmission, these findings implicate the potential for monoaminergic modulation of the DMN to improve its functionality in ADHD. Interestingly, a recent study using a machine-based ADHD classification model combining PET imaging and genetic screening suggested that altered SERT binding in DMN nodes, including the precuneus and poste- rior cingulate gyrus, and SNPs in the 5-HT1B (rs130058) and 5-HT2A (rs1328684) genes confer risk for ADHD [379]. There are some limitations to this study, including lack of external validation, but it implies that serotonergic dysfunc- tion in ADHD-relevant brain networks may also be an under- lying factor in the etiology of the condition. 7.1 � Tryptophan Studies and SSRIs Relative to healthy individuals, there are limited stud- ies investigating serotonergic neurotransmission on DMN activity in ADHD populations. Collectively, these studies indicate that the DMN is sensitive to serotonergic manipula- tion (i.e., acute tryptophan depletion and SSRIs); however, the functional consequences of these brain changes are not consistently explored across studies. In adolescent boys with ADHD, acute tryptophan depletion decreased functional connectivity of the right superior premotor cortex with the DMN during resting state, which may be of relevance to Fig. 5   Binding affinity of select FDA-approved drugs at serotoner- gic receptors. Heatmap represents data as a ratio of binding affinity for the specific target in relation to its primary target [norepineph- rine transporter or serotonin transporter; i.e., pKi (negative log of the inhibition constant) of 5-HT2A/pKi of serotonin transporter for vortioxetine]. These data were acquired using human receptor iso- forms, except for fluoxetine, which utilized rat receptors. The ratio of binding affinity is ranked by color, with greater activity shown in dark-red and less activity shown with white. Data were acquired from the International Union of Basic and Clinical Pharmacology/British Pharmacological Society (IUPHAR/BPS) Guide to Pharmacology, except for viloxazine [413] 724 M. B. Solomon et al. motor planning [380]. In neurotypical individuals, acute tryptophan depletion consistently reduced functional con- nectivity of the precuneus nucleus (self-referential process- ing) with the DMN during resting state [381, 382]. It also modulated the functional activity of the orbitofrontal cor- tex, which was associated with more depressive mood, and the superior parietal lobe, which correlated with increased anger–hostility [381, 382]. Acute tryptophan depletion in healthy individuals reduced sensorimotor network activity and increased DMN activity [78]. In contrast, either single or short-term administration of the SSRIs escitalopram or sertraline, or the SNRI duloxetine, decreased intra-DMN functional connectivity with the precuneus or anterior and posterior cingulate gyri [383–386] and connectivity within the TPN [384] during resting state. Moreover, based on modeling of resting-state networks, the primary nuclei of the serotonergic and dopaminergic systems have opposing effects on the sensorimotor network and DMN, which could have implications not only for ADHD but also other psychi- atric conditions [78]. It is important to note that the referenced studies investi- gating the impact of 5-HT on DMN activity in neurotypical individuals utilized acute tryptophan depletion or short-term drug administration, so these changes in brain activity may not be evident with long-term use, which is more relevant to clinical populations. Therefore, neuroimaging studies that employ clinically relevant drug regimens combined with various behavioral assays (e.g., impulsivity) would provide greater insight regarding the contributions of 5-HT to the functional coherence of the DMN and its putative role in the etiology of ADHD. 7.2 � Role of 5‑HT on ADHD‑Related Brain Networks Relevant to Impulsivity Only one neuroimaging study evaluated the impact of a serotonergic targeting agent on impulsive behaviors in indi- viduals with ADHD. This functional magnetic resonance imaging (fMRI) study revealed that an acute dose of fluox- etine normalized hypoactivity of the orbitofrontal cortex and dorsal striatum (caudate) in boys with ADHD in response to a task designed to gauge motor inhibition. Interestingly in this study, boys with autism spectrum disorder exhibited hyperactivation in the right frontal cortex, and fluoxetine normalized these functional measures as well. Although there were no effects of fluoxetine on overall performance in the task, these changes in brain activation correlated with inhibitory responses [387], suggesting that targeting the 5-HT system may modulate ADHD-relevant functions. The authors proposed that the differential effects of fluoxetine in these disorders might be due to baseline differences in 5-HT levels, with autism spectrum disorder associated with ele- vated 5-HT and ADHD associated with lower levels of 5-HT [388–390]. While they did not assess 5-HT levels in this study, there are reports that baseline levels of 5-HT impact the sensitivity to sertraline (SSRI) treatment [391]. There are several limitations to this study, including acute administra- tion of fluoxetine and a small sample size; however, it may provide a conceptual framework to better understand how 5-HT signaling in cortico-striatal-limbic regions modulates some impulsive behaviors in diverse populations. 7.3 � Role of 5‑HT on ADHD‑Related Brain Networks Relevant to Emotional Dysregulation No neuroimaging studies investigating the impact of sero- tonergic targeting agents on emotion dysregulation in indi- viduals with ADHD were identified. However, the few neu- roimaging studies of relevance supported the involvement of dysfunctional 5-HT signaling in various cortical regions to impulsive aggression. For example, relative to healthy con- trols, adults with impulsive aggression displayed decreased metabolic responses in the left medial orbitofrontal cortex and the ventral medial and anterior cingulate in response to mCPP, a non-selective 5-HT agonist [392], and in response to fenfluramine [393]. In addition, individuals with impul- sive aggression have reduced SERT availability in the ante- rior cingulate [394]. Collectively, these findings suggest that altered serotonergic activity in brain regions implicated in behavioral inhibition and emotional regulation may contrib- ute to increased impulsive aggression in some individuals. It is important to note that individuals can display impulsivity and emotional dysregulation independent of having ADHD. Nonetheless, these findings provide some neural underpin- nings for serotonergic influence on impulsive aggression, which may be of relevance for individuals with ADHD pre- senting with symptoms of emotional dysregulation. 7.4 � Role of 5‑HT on ADHD‑Related Brain Networks Relevant to Attention At this time, no neuroimaging studies were identified exam- ining the effects of serotonergic targeting agents on various cognitive processes (e.g., sustained, selective, divided atten- tion) or executive function in relation to attention networks in individuals with ADHD. Therefore, studies are needed to explore the putative involvement of 5-HT on these endpoints via modulation of distinct ADHD-relevant brain networks (e.g., frontoparietal). Broadly, 5-HT is hypothesized to mod- ulate the excitation/inhibition balance of neural circuitry to serve as a “thresholding” mechanism, which impacts execu- tive function [78, 395]. The impact of 5-HT on attentional processes appears to exhibit an inverted “U” shape and is likely influenced by the interaction of 5-HT and catechola- mines in these brain regions. 725Serotonin in ADHD: Preclinical and Clinical Insights Summary Overall, the findings demonstrate that 5-HT modulates ADHD-relevant brain networks and suggest that some of these neural adaptations may drive changes in distinct ADHD behaviors (e.g., impulsivity and emotional regulation). More clinical studies are needed to evaluate the functional consequences of these brain changes in ADHD populations. The highlighted studies indicate that the impact of serotonergic neurotransmission on core behavioral fea- tures of ADHD and associated brain networks is nuanced and likely dependent upon several factors (not discussed in detail above), including age [396, 397], basal 5-HT levels [387], symptom constellation (predominant inattentive ver- sus hyperactivity–impulsivity) [398], presentation of other comorbidities [302], gene–environment interaction [399], treatment duration, and sex [400, 401]. 8 � Does 5‑HT Play a Role in ADHD Treatment? Despite significant preclinical and clinical evidence link- ing the serotonergic system to core behavioral features of ADHD and emotional dysregulation, the potential role of 5-HT in ADHD treatment has been largely discounted for two main reasons: (1) SSRIs as monotherapy are regarded as ineffective [53, 54] and, as discussed in [402], (2) the primary mechanism of action of standard ADHD treatments is attributed to their potent ability to enhance DA and NE neurotransmission in critical cortico-striatal-limbic circuits. However, we have provided evidence that stimulants also increase 5-HT neurotransmission in similar brain regions across species [234, 240–246, 252]. Thus, the serotonergic system could also contribute to stimulant-mediated effects. Preclinical data support this postulate; various methods that manipulate the serotonergic system (i.e., pharmacological agents, lesions, TPH2 KO) modulate behavioral responses to stimulants in healthy rodents and in some preclinical models of ADHD [109, 159, 257–260, 265, 270–272, 275]. Although there are limited studies, clinical evidence further supports these preclinical data. For example, polymorphisms in the genes for TPH2 [403] and SERT [404] not only increase risk for ADHD but may also mediate the behavio- ral responses to methylphenidate in individuals with ADHD. Furthermore, in children and adolescents with ADHD and comorbidities (dysthymic symptoms, oppositional defiant disorder, conduct disorder, anxiety) who initially experi- enced inadequate treatment responses to methylphenidate alone displayed improvements in inattention, hyperactivity, impulsivity, depression, and anxiety scores when fluoxetine (8-week treatment) was combined with methylphenidate [405]. Together, these data implicate dysfunction in the serotonergic system as a risk factor for ADHD and support a facilitatory role for 5-HT in ADHD treatment. Moreover, these findings highlight the utility of treatment regimens that target both the catecholaminergic and serotonergic systems in ADHD comorbid with depression and anxiety [405]. 8.1 � Role of 5‑HT in Nonstimulant‑Mediated Effects We can further speculate on a putative role for 5-HT in ADHD treatment by evaluating the efficacy of treatment options on the basis of their pharmacological activity on ser- otonergic and catecholaminergic systems (Figs. 4, 5). Tricy- clic antidepressants with known inhibitory effects on SERT, NET, and DAT, such as imipramine and desipramine, and newer monoamine transporter inhibitors, including daso- traline and centanafadine, demonstrated efficacy in ADHD treatment [406–408]. However, it is difficult to determine if their serotonergic properties contributed to their mechanism of action, because imipramine, desipramine, dasotraline, and centanafadine have strong binding affinity to NET or DAT (Fig. 4; [407, 409, 410]), which may drive these changes in ADHD treatment efficacy independent of their effects on SERT. Venlafaxine, an SNRI with predominant inhibition of SERT and modest NET inhibition, has demonstrated efficacy in a few small studies (Fig. 4; Table 6). It is suggested that at lower therapeutic doses venlafaxine primarily engages SERT [411], suggesting that the serotonergic properties could con- tribute to its mechanism of action. Completion of preclinical studies that manipulate SERT activity via pharmacological/ genetic techniques would allow a more thorough evaluation of the contribution of 5-HT to venlafaxine’s efficacy. One caveat with the venlafaxine findings is that there are limited controlled clinical trials (N = 2), as several of the reported findings were in open-label studies with small sample sizes; reviewed in [51]. Nonetheless, these findings suggest that 5-HT may play a role in ADHD treatment. Other nonstimulant options with demonstrated efficacy in ADHD and pharmacological activity on catecholaminergic and serotonergic systems include viloxazine and atomoxe- tine. Even though both drugs are classified as NRIs, there are differences in their pharmacological profiles towards sero- tonergic targets, which could result in differences in treat- ment efficacy (Figs. 4, 5). Atomoxetine has moderate bind- ing at SERT [412], while viloxazine has negligible binding affinity for SERT [413] but unique effects at 5-HT receptors that are not observed with atomoxetine and may theoretically contribute to its mechanism of action. For instance, based on preclinical in vitro and in vivo studies, viloxazine exhib- ited antagonistic effects on 5-HT2B receptors and agonistic effects at 5-HT2C receptors [413]. Moreover, in microdi- alysis studies, viloxazine increased 5-HT in addition to DA and NE in rat mPFC at therapeutically relevant concentra- tions [413, 414]; this enhanced prefrontal 5-HT release has not been reported with atomoxetine treatment [369]. It is tempting to speculate that these unique serotonergic recep- tor modulating properties could potentially contribute to 726 M. B. Solomon et al. the mechanism of action of viloxazine, especially given the role of 5-HT2C receptors in hyperactivity–impulsivity and emotional dysregulation in both rodents and humans. However, as yet, there are no mechanistic studies to spe- cifically support or refute this contention. Far less has been reported on 5-HT2B receptors with regard to the etiology of and treatment for ADHD. However, there are some reports that 5-HT2B receptors are linked with distinct behavioral phenotypes (i.e., impulsive aggression) and comorbidities (depression) in mice and humans [195, 415] and are required for the antidepressant effects of SSRIs [416]. A recent nonrandomized study by Price and Price [417], conducted as a step-down treatment from atomoxetine to viloxazine (extended-release formulation) in a small sam- ple of patients with ADHD, suggested greater improvement from baseline ADHD-RS-5 and Adult ADHD Investigator Symptom Rating Scale (AISRS) mean scores in inattention and hyperactivity–impulsivity and faster speed of onset in adolescents and adults with extended-release viloxazine treatment compared with the initial atomoxetine treat- ment. One potential implication of these findings is that the unique 5-HT receptor modulating properties of viloxazine may theoretically contribute to these differences in treat- ment efficacy, though well-controlled studies are necessary to evaluate this assertion. Given the unique 5-HT receptor modulating properties of viloxazine (i.e., 5-HT2C recep- tors), this compound could be advantageous for individuals with ADHD and other behavioral features sensitive to 5-HT modulation (i.e., emotional dysregulation and depression); however, there are no clinical studies that have directly tested this hypothesis. As previously mentioned, a prevailing viewpoint in the field is that the efficacy of ADHD treatments, including stimulants and nonstimulants, is because of their potent abil- ity to enhance DA and NE neurotransmission in cortico- striatal and limbic circuits, notably in the mPFC. However, vortioxetine, a multi-targeting serotonergic compound, also increased DA and NE in the mPFC [220, 221] but was inef- fective in ADHD [418]. It is likely that vortioxetine (with negligible binding to DAT or NET) indirectly enhanced DA and NE neurotransmission via interactions with specific 5-HT receptors (e.g., 5-HT1A, 5-HT1B heteroreceptors) that modulate catecholaminergic neurotransmission [419]. Simi- larly, fluoxetine increased prefrontal DA and NE neurotrans- mission [420], likely via 5-HT2C receptors, yet was also ineffective in ADHD as monotherapy. The lack of effect with vortioxetine and fluoxetine in ADHD may be because these compounds do not enhance prefrontal catecholamine levels to the same degree as stimulant and nonstimulant ADHD treatments (i.e., viloxazine extended-release, atomoxetine). Conversely, approved stimulants and nonstimulants may alter catecholamine levels in brain areas beyond the PFC that are important for treatment efficacy. These findings suggest that indirectly enhancing DA and NE neurotransmission in the mPFC may not be sufficient for optimal ADHD treatment efficacy. The data suggest that successful treatment of core ADHD behavioral features requires direct engagement of at least one of the catecholaminergic systems. 8.2 � Role of 5‑HT in ADHD Comorbidities Approximately 50–80% of people with ADHD are diagnosed with another psychiatric condition [2, 421–423], most com- monly mood and anxiety disorders [424]. In fact, relative to the general population, mood and anxiety disorders are disproportionately more common in people with ADHD [2, 425]. High genetic correlation between ADHD symptoms and other constructs (e.g., autism spectrum disorder traits, cognitive phenotypes, and externalizing symptoms), as well as common genetic variants shared between ADHD and depression, lend support to the high prevalence of comor- bidities in ADHD [426]. It is unclear if untreated ADHD increases risk for depression and anxiety or vice versa, but ADHD, comorbid with anxiety or depression, significantly impairs daily functioning and is associated with poorer long-term outcomes [330, 427]. Thus, effective treatment for these comorbidities is essential for long-term success in individuals with this disorder; however, most ADHD clini- cal trials exclude patients with symptomatic comorbidities. While SSRIs as monotherapy appear to be relatively ineffective in treating the core behavioral features of ADHD (inattention, hyperactivity, impulsivity), they are mainstay treatments for anxiety and depression across age groups [428–430]. Although limited in number, available studies support their utility in combination with ADHD treatments [53, 54, 405] and suggest that compounds tar- geting both the catecholaminergic and serotonergic sys- tems are necessary for treating both core ADHD symp- toms and its psychiatric comorbidities. In some instances, ADHD treatments that primarily target the catecholamin- ergic system (methylphenidate, amphetamine, and ato- moxetine) successfully treated emotional dysregulation and the core behavioral features of inattention and hyper- activity–impulsivity [431]. However, these treatments were more effective at attenuating core ADHD symptoms compared with emotional lability, suggesting the need for adjunctive treatments to mitigate this behavioral feature in some individuals with ADHD. In this vein, citalopram adjunctive to psychostimulant treatment mitigated emo- tional dysregulation in children and adolescents with severe mood dysregulation who had been unresponsive to stimulant treatment alone [432], demonstrating beneficial effects of serotonergic targeting agents. In double-blind trials, atomoxetine showed positive effects in treating comorbid anxiety with ADHD (gen- eralized anxiety disorder in children and social anxiety 727Serotonin in ADHD: Preclinical and Clinical Insights disorder in adults) but has not shown a clear signal in treat- ing depression [433, 434]. The only well-controlled study of atomoxetine for depression in ADHD showed improve- ment in ADHD but not depression. A second double-blind pediatric trial showed significant improvement from base- line in ADHD, anxiety, and depression when atomoxetine was combined with fluoxetine [435]. The fluoxetine group showed greater responsivity of depressive symptoms; how- ever, owing to the study design (no placebo only control group), it is unclear whether improvements in depression were due to the addition of atomoxetine or other factors. To fully evaluate the utility of serotonergic targeting com- pounds in treating psychiatric comorbidities, studies that include individuals with ADHD comorbid with psychiatric disorders are needed. Prior to showing efficacy as a treatment for ADHD, viloxazine had been approved as a treatment for depression in Europe. The unique serotonergic targeting properties of viloxazine may be connected to its reported antidepres- sant efficacy [436], which could provide an avenue to treat both conditions with a single medication. A decentralized clinical trial enrolling adults with a primary diagnosis of ADHD as well as anxiety and/or depression symptoms (NCT06185985) showed that participants treated with vilox- azine extended-release experienced improvement in ADHD, anxiety, and depression symptoms [437]. Despite overlapping features, ADHD, depression, and anxiety are distinct conditions with different brain neuro- chemistry that likely impacts treatment response. Nonethe- less, based on the presented preclinical and clinical data, treatment strategies that target select 5-HT receptor subtypes may be particularly advantageous for individuals with dis- tinct comorbidities. For example, compounds targeting 5-HT1B, 5-HT2C, and 5-HT3 receptors may be useful for individuals with impulsive aggression [55, 197, 200, 201, 205, 343, 351, 415], whereas those targeting 5-HT2B and 5-HT7 receptors may be useful for ADHD with comorbid depression or anxiety [416, 438, 439]. Given the abundance of preclinical data showing serotonergic contributions to ADHD behaviors, additional well-controlled clinical studies are warranted to further understand the role of serotonergic medications adjunctive with ADHD-approved treatments or as monotherapies in treating ADHD comorbid with certain psychiatric conditions. 9 � Conclusions Both preclinical and clinical studies support the notion that disruption in 5-HT neurotransmission can induce behaviors consistent with ADHD phenotypes, most notably impulsive action and emotional dysregulation. We presented findings demonstrating that ADHD treatments, including stimulants and nonstimulants, impact serotonergic neurotransmission in brain regions implicated in behavioral inhibition and emo- tional regulation, which could contribute to their efficacy. We also presented compelling preclinical data demonstrating that various 5-HT receptor subtypes modulate behavioral responses to stimulants, likely through interactions with catecholamine systems but also by affecting other neuro- transmitters (e.g., glutamate, GABA). In totality, the data suggest that monoamine imbalance, rather than deficien- cies in any one neurotransmitter system, contributes to the behavioral manifestations of ADHD. Moreover, based on preclinical data and current therapeutic treatments, direct engagement of at least one of the catecholaminergic appears to be important for medication efficacy in treating the suite of core ADHD symptoms; however, this may be expanded as new therapeutic targets are identified. Meanwhile, com- pounds that predominately target the serotonergic systems are effective in treating mood and anxiety disorders, which is particularly notable given that most individuals with ADHD have at least one psychiatric comorbidity, with depression and anxiety having the highest frequency. Thus, compounds that target both serotonergic and catecholaminergic systems may be necessary for comprehensive treatment of individu- als with complex ADHD. Indeed, more studies are war- ranted to test these suppositions more thoroughly. 10 � Future Directions Throughout this review, we emphasize the critical ques- tions that remain unanswered and provide some potential future directions for preclinical and, more substantially, clinical studies. Additional clinical studies are required to determine the extent to which stimulant-mediated effects are dependent upon serotonergic activity. Clinical stud- ies evaluating preferential treatment response show that ADHD medications are not therapeutically interchange- able and suggest that mechanistic differences are important to individual responses. Additional studies may help to isolate the degree to which serotonergic effects contribute to differences in efficacy. Given that the serotonergic sys- tem is tightly linked with emotion regulation, more studies should also investigate the potential benefit of seroton- ergic targeting agents to mitigate this behavioral feature in ADHD. In addition, consideration of how real-world factors (e.g., stress, normal or pathological brain aging, comorbidities, and sex) impact 5-HT signaling in ADHD- relevant brain networks is important. Of note, given the reported sex differences in monoaminergic functioning, it is likely that sex is an important factor with regard to the manifestation of core behavioral features and treat- ment efficacy (see [440] for a review). Lastly, gaining greater insight into the dynamic interactions between the 728 M. B. Solomon et al. catecholaminergic and serotonergic systems may be criti- cal to further advance our understanding of the etiology of ADHD. This knowledge may assist in the development of novel treatment strategies or repurposing of existing compounds that target multiple monoaminergic systems to more effectively treat ADHD based on ADHD subtype (hyperactivity–impulsivity versus inattention) and comor- bid symptom constellations. Funding  This work was funded by Supernus Pharmaceuticals Inc. Authors employed by Supernus were involved in the design and writing of this manuscript. Editorial support, funded by Supernus, was provided by Lisa M. Pitchford, PhD, ISMPP CMPP™ of JB Ashtin. Supernus funded open access for this publication. Declarations  Conflict of interest  M.B.S. was formerly employed by Supernus Phar- maceuticals Inc. B.Y. and J.R. are currently employed by Supernus Pharmaceuticals Inc. T.R., J.N., and V.M. are consultants and/or advi- sory board members for Supernus Pharmaceuticals Inc. J.N. is also a consultant for Hippo T&C, Lumos, MindTension, NFL, and OnDosis; an advisory board member for Medice, Mind Tension, OnDosis, Ot- suka; and receives research support from Otsuka. V.M. is also a con- sultant for AbbVie/Allergan, Acadia Pharmaceuticals, Inc. Alfasigma, USA, Inc., AlkernesInc., Axsome, Eisai, Ironshore, Intra-Cellular Therapies, Janssen, H. Lundbeck A/S, Jazz Pharmaceuticals, Nov- enPharmaceuticals Inc., Otsuka America Pharmaceutical, Inc., Sage Pharmaceuticals, Sunovion Pharmaceuticals, and Takeda Pharma- ceutical Company Limited. T.R. is also a consultant with Cambridge Cognition, has a research grant with Shionogi, and receives editorial honoraria from Springer-Nature and Elsevier. Ethics approval  Not applicable. Consent to participate  Not applicable. Consent for publication  Not applicable. Availability of data and material  Not applicable. Code availability  Not applicable. Author contributions  J.R., M.B.S., and B.Y. conceptualized and designed the review. M.B.S., B.Y., and T.W.R. conducted the litera- ture search. M.B.S., B.Y., T.W.R., and V.M. drafted the initial manu- script. V.M. was not able to review or approve the final version of the manuscript, as he sadly passed away prior to the revision stage. His inclusion as a co-author was approved by his family in recognition of his contributions to the original submission. All remaining authors critically reviewed the manuscript, read and approved the final submit- ted manuscript, and agree to be accountable for the work. Open Access  This article is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, which permits any non-commercial use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc/4.0/. References 1. Benard V, Cottencin O, Guardia D, Vaiva G, Rolland B. The impact of discontinuing methylphenidate on weight and eating behavior. Int J Eat Disord. 2015;48(3):345–8. 2. Kessler RC, Adler L, Barkley R, Biederman J, Conners CK, Demler O, et al. The prevalence and correlates of adult ADHD in the United States: results from the National Comorbidity Survey Replication. Am J Psychiatry. 2006;163(4):716–23. 3. Seidman LJ, Valera EM, Makris N, Monuteaux MC, Boriel DL, Kelkar K, et al. Dorsolateral prefrontal and anterior cingulate cortex volumetric abnormalities in adults with attention-deficit/ hyperactivity disorder identified by magnetic resonance imaging. Biol Psychiatry. 2006;60(10):1071–80. 4. Wu T, Liu X, Cheng F, Wang S, Li C, Zhou D, et al. Dorso- lateral prefrontal cortex dysfunction caused by a go/no-go task in children with attention-deficit hyperactivity disorder: a functional near-infrared spectroscopy study. Front Neurosci. 2023;17:1145485. 5. Bos DJ, Oranje B, Achterberg M, Vlaskamp C, Ambrosino S, de Reus MA, et al. Structural and functional connectivity in children and adolescents with and without attention deficit/hyperactivity disorder. J Child Psychol Psychiatry. 2017;58(7):810–8. 6. Chantiluke K, Barrett N, Giampietro V, Brammer M, Simmons A, Murphy DG, et al. Inverse effect of fluoxetine on medial pre- frontal cortex activation during reward reversal in ADHD and autism. Cereb Cortex. 2015;25(7):1757–70. 7. Salavert J, Ramos-Quiroga JA, Moreno-Alcazar A, Caseras X, Palomar G, Radua J, et al. Functional imaging changes in the medial prefrontal cortex in adult ADHD. J Atten Disord. 2018;22(7):679–93. 8. Connaughton M, O’Hanlon E, Silk TJ, Paterson J, O’Neill A, Anderson V, et al. The limbic system in children and adolescents with attention-deficit/hyperactivity disorder: a longitudinal struc- tural magnetic resonance imaging analysis. Biol Psychiatry Glob Open Sci. 2024;4(1):385–93. 9. Liao W, Cao L, Leng L, Wang S, He X, Dong Y, et al. Lack of functional brain connectivity was associated with poor inhibi- tion in children with attention-deficit/hyperactivity disorder using near-infrared spectroscopy. Front Psychiatry. 2023;14:1221242. 10. Bayard F, Nymberg Thunell C, Abe C, Almeida R, Banaschewski T, Barker G, et  al. Distinct brain structure and behavior related to ADHD and conduct disorder traits. Mol Psychiatry. 2020;25(11):3020–33. 11. Bledsoe JC, Semrud-Clikeman M, Pliszka SR. Anterior cingulate cortex and symptom severity in attention-deficit/hyperactivity disorder. J Abnorm Psychol. 2013;122(2):558–65. 12. Konrad K, Neufang S, Hanisch C, Fink GR, Herpertz-Dahlmann B. Dysfunctional attentional networks in children with attention deficit/hyperactivity disorder: evidence from an event-related functional magnetic resonance imaging study. Biol Psychiatry. 2006;59(7):643–51. 13. Nakao T, Radua J, Rubia K, Mataix-Cols D. Gray matter volume abnormalities in ADHD: voxel-based meta-analysis exploring the effects of age and stimulant medication. Am J Psychiatry. 2011;168(11):1154–63. 14. Vaidya CJ, Bunge SA, Dudukovic NM, Zalecki CA, Elliott GR, Gabrieli JD. Altered neural substrates of cognitive control in http://creativecommons.org/licenses/by-nc/4.0/ 729Serotonin in ADHD: Preclinical and Clinical Insights childhood ADHD: evidence from functional magnetic resonance imaging. Am J Psychiatry. 2005;162(9):1605–13. 15. Booth JR, Burman DD, Meyer JR, Lei Z, Trommer BL, Dav- enport ND, et al. Larger deficits in brain networks for response inhibition than for visual selective attention in attention deficit hyperactivity disorder (ADHD). J Child Psychol Psychiatry. 2005;46(1):94–111. 16. Chen MH, Lin HM, Sue YR, Yu YC, Yeh PY. Meta-analysis reveals a reduced surface area of the amygdala in individuals with attention deficit/hyperactivity disorder. Psychophysiology. 2023;60(9): e14308. 17. Castellanos FX, Giedd JN, Berquin PC, Walter JM, Sharp W, Tran T, et al. Quantitative brain magnetic resonance imaging in girls with attention-deficit/hyperactivity disorder. Arch Gen Psychiatry. 2001;58(3):289–95. 18. Castellanos FX, Giedd JN, Marsh WL, Hamburger SD, Vaituzis AC, Dickstein DP, et al. Quantitative brain magnetic resonance imaging in attention-deficit hyperactivity disorder. Arch Gen Psychiatry. 1996;53(7):607–16. 19. Durston S, Hulshoff Pol HE, Schnack HG, Buitelaar JK, Steen- huis MP, Minderaa RB, et al. Magnetic resonance imaging of boys with attention-deficit/hyperactivity disorder and their unaffected siblings. J Am Acad Child Adolesc Psychiatry. 2004;43(3):332–40. 20. Gentile JP, Atiq R, Gillig PM. Adult ADHD: diagnosis, dif- ferential diagnosis, and medication management. Psychiatry (Edgmont). 2006;3(8):25–30. 21. Boonstra AM, Oosterlaan J, Sergeant JA, Buitelaar JK. Execu- tive functioning in adult ADHD: a meta-analytic review. Psy- chol Med. 2005;35(8):1097–108. 22. Shaw P, Stringaris A, Nigg J, Leibenluft E. Emotion dysregula- tion in attention deficit hyperactivity disorder. Am J Psychiatry. 2014;171(3):276–93. 23. Soler-Gutiérrez AM, Perez-Gonzalez JC, Mayas J. Evidence of emotion dysregulation as a core symptom of adult ADHD: a systematic review. PLoS ONE. 2023;18(1): e0280131. 24. Das D, Cherbuin N, Butterworth P, Anstey KJ, Easteal S. A population-based study of attention deficit/hyperactivity dis- order symptoms and associated impairment in middle-aged adults. PLoS ONE. 2012;7(2): e31500. 25. Biederman J, Faraone SV, Spencer T, Wilens T, Mick E, Lapey KA. Gender differences in a sample of adults with attention deficit hyperactivity disorder. Psychiatry Res. 1994;53(1):13–29. 26. Biederman J, Ball SW, Monuteaux MC, Mick E, Spencer TJ, Mc CM, et al. New insights into the comorbidity between ADHD and major depression in adolescent and young adult females. J Am Acad Child Adolesc Psychiatry. 2008;47(4):426–34. 27. Philipsen A, Graf E, Jans T, Matthies S, Borel P, Colla M, et al. A randomized controlled multicenter trial on the multimodal treat- ment of adult attention-deficit hyperactivity disorder: enrollment and characteristics of the study sample. ADHD Atten Deficit Hyperact Disord. 2014;6(1):35–47. 28. Levin FR, Evans SM, Kleber HD. Practical guidelines for the treatment of substance abusers with adult attention-deficit hyper- activity disorder. Psychiatr Serv. 1999;50(8):1001–3. 29. Wilens TE. Impact of ADHD and its treatment on substance abuse in adults. J Clin Psychiatry. 2004;65(Suppl 3):38–45. 30. Biederman J, Monuteaux MC, Spencer T, Wilens TE, Far- aone SV. Do stimulants protect against psychiatric disorders in youth with ADHD? A 10-year follow-up study. Pediatrics. 2009;124(1):71–8. 31. Biederman J, Wilens T, Mick E, Spencer T, Faraone SV. Phar- macotherapy of attention-deficit/hyperactivity disorder reduces risk for substance use disorder. Pediatrics. 1999;104(2): e20. 32. Schiavone N, Virta M, Leppamaki S, Launes J, Vanninen R, Tuulio-Henriksson A, et al. Mortality in individuals with childhood ADHD or subthreshold symptoms—a prospective perinatal risk cohort study over 40 years. BMC Psychiatry. 2022;22(1):325. 33. Kosheleff AR, Mason O, Jain R, Koch J, Rubin J. Func- tional impairments associated with ADHD in adulthood and the impact of pharmacological treatment. J Atten Disord. 2023;27(7):669–97. 34. Faraone SV, Larsson H. Genetics of attention deficit hyperactivity disorder. Mol Psychiatry. 2019;24(4):562–75. 35. Asarnow RF, Newman N, Weiss RE, Su E. Association of attention-deficit/hyperactivity disorder diagnoses with pedi- atric traumatic brain injury: a meta-analysis. JAMA Pediatr. 2021;175(10):1009–16. 36. Wang Q, Zhao HH, Chen JW, Gu KD, Zhang YZ, Zhu YX, et  al. Adverse health effects of lead exposure on children and exploration to internal lead indicator. Sci Total Environ. 2009;407(23):5986–92. 37. Nigg JT, Knottnerus GM, Martel MM, Nikolas M, Cavanagh K, Karmaus W, et al. Low blood lead levels associated with clinically diagnosed attention-deficit/hyperactivity disor- der and mediated by weak cognitive control. Biol Psychiatry. 2008;63(3):325–31. 38. Braun JM, Kahn RS, Froehlich T, Auinger P, Lanphear BP. Exposures to environmental toxicants and attention deficit hyperactivity disorder in U.S. children. Environ Health Perspect. 2006;114(12):1904–9. 39. Lasky-Su J, Anney RJ, Neale BM, Franke B, Zhou K, Maller JB, et al. Genome-wide association scan of the time to onset of attention deficit hyperactivity disorder. Am J Med Genet B Neuropsychiatr Genet. 2008;147B(8):1355–8. 40. Smith TF, Anastopoulos AD, Garrett ME, Arias-Vasquez A, Franke B, Oades RD, et  al. Angiogenic, neurotrophic, and inflammatory system SNPs moderate the association between birth weight and ADHD symptom severity. Am J Med Genet B Neuropsychiatr Genet. 2014;165B(8):691–704. 41. Hariri M, Djazayery A, Djalali M, Saedisomeolia A, Rahimi A, Abdolahian E. Effect of n-3 supplementation on hyperac- tivity, oxidative stress and inflammatory mediators in children with attention-deficit-hyperactivity disorder. Malays J Nutr. 2012;18(3):329–35. 42. Edden RA, Crocetti D, Zhu H, Gilbert DL, Mostofsky SH. Reduced GABA concentration in attention-deficit/hyperactivity disorder. Arch Gen Psychiatry. 2012;69(7):750–3. 43. Silveri MM, Sneider JT, Crowley DJ, Covell MJ, Acharya D, Rosso IM, et al. Frontal lobe gamma-aminobutyric acid levels during adolescence: associations with impulsivity and response inhibition. Biol Psychiatry. 2013;74(4):296–304. 44. MacMaster FP, Carrey N, Sparkes S, Kusumakar V. Proton spec- troscopy in medication-free pediatric attention-deficit/hyperac- tivity disorder. Biol Psychiatry. 2003;53(2):184–7. 45. Faraone SV. The pharmacology of amphetamine and methylphe- nidate: relevance to the neurobiology of attention-deficit/hyper- activity disorder and other psychiatric comorbidities. Neurosci Biobehav Rev. 2018;87:255–70. 46. Berridge CW, Devilbiss DM. Psychostimulants as cogni- tive enhancers: the prefrontal cortex, catecholamines, and attention-deficit/hyperactivity disorder. Biol Psychiatry. 2011;69(12):e101–11. 47. Arnsten AF. Catecholamine influences on dorsolateral prefrontal cortical networks. Biol Psychiatry. 2011;69(12):e89-99. 48. Del Campo N, Chamberlain SR, Sahakian BJ, Robbins TW. The roles of dopamine and noradrenaline in the pathophysiology and treatment of attention-deficit/hyperactivity disorder. Biol Psy- chiatry. 2011;69(12):e145–57. 730 M. B. Solomon et al. 49. Madras BK, Miller GM, Fischman AJ. The dopamine transporter and attention-deficit/hyperactivity disorder. Biol Psychiatry. 2005;57(11):1397–409. 50. Oades RD. Role of the serotonin system in ADHD: treatment implications. Expert Rev Neurother. 2007;7(10):1357–74. 51. Banerjee E, Nandagopal K. Does serotonin deficit mediate sus- ceptibility to ADHD? Neurochem Int. 2015;82:52–68. 52. Hou YW, Xiong P, Gu X, Huang X, Wang M, Wu J. Associa- tion of serotonin receptors with attention deficit hyperactivity disorder: a systematic review and meta-analysis. Curr Med Sci. 2018;38(3):538–51. 53. Weiss M, Hechtman L, Adult ARG. A randomized double-blind trial of paroxetine and/or dextroamphetamine and problem- focused therapy for attention-deficit/hyperactivity disorder in adults. J Clin Psychiatry. 2006;67(4):611–9. 54. Findling RL. Open-label treatment of comorbid depression and attentional disorders with co-administration of serotonin reup- take inhibitors and psychostimulants in children, adolescents, and adults: a case series. J Child Adolesc Psychopharmacol. 1996;6(3):165–75. 55. de Almeida RM, Miczek KA. Aggression escalated by social instigation or by discontinuation of reinforcement (“frustration”) in mice: inhibition by anpirtoline: a 5-HT1B receptor agonist. Neuropsychopharmacology. 2002;27(2):171–81. 56. Krakowski M. Violence and serotonin: influence of impulse con- trol, affect regulation, and social functioning. J Neuropsychiatry Clin Neurosci. 2003;15(3):294–305. 57. Puig Pérez S. Serotonin and emotional decision-making [Inter- net]. In: Ying Q, editor. Serotonin: Intech Open; 2019. 58. Walther DJ, Bader M. A unique central tryptophan hydroxylase isoform. Biochem Pharmacol. 2003;66(9):1673–80. 59. Walther DJ, Peter JU, Bashammakh S, Hortnagl H, Voits M, Fink H, et al. Synthesis of serotonin by a second tryptophan hydroxy- lase isoform. Science. 2003;299(5603):76. 60. Yang D, Gouaux E. Illumination of serotonin transporter mechanism and role of the allosteric site. Sci Adv. 2021;7(49): eabl3857. 61. Bortolozzi A, Amargós-Bosch M, Toth M, Artigas F, Adell A. In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice. J Neurochem. 2004;88(6):1373–9. 62. Knobelman DA, Hen R, Lucki I. Genetic regulation of extracellular serotonin by 5-hydroxytryptamine(1A) and 5-hydroxytryptamine(1B) autoreceptors in different brain regions of the mouse. J Pharmacol Exp Ther. 2001;298(3):1083–91. 63. Liu RJ, Lambe EK, Aghajanian GK. Somatodendritic autorecep- tor regulation of serotonergic neurons: dependence on L-tryp- tophan and tryptophan hydroxylase-activating kinases. Eur J Neurosci. 2005;21(4):945–58. 64. Malagié I, Trillat AC, Bourin M, Jacquot C, Hen R, Gardier AM. 5-HT1B autoreceptors limit the effects of selective serotonin re- uptake inhibitors in mouse hippocampus and frontal cortex. J Neurochem. 2001;76(3):865–71. 65. Richardson-Jones JW, Craige CP, Nguyen TH, Kung HF, Gardier AM, Dranovsky A, et al. Serotonin-1A autoreceptors are neces- sary and sufficient for the normal formation of circuits underlying innate anxiety. J Neurosci. 2011;31(16):6008–18. 66. Romero L, Artigas F. Preferential potentiation of the effects of serotonin uptake inhibitors by 5-HT1A receptor antagonists in the dorsal raphe pathway: role of somatodendritic autoreceptors. J Neurochem. 1997;68(6):2593–603. 67. Trillat AC, Malagie I, Scearce K, Pons D, Anmella MC, Jac- quot C, et  al. Regulation of serotonin release in the frontal cortex and ventral hippocampus of homozygous mice lacking 5-HT1B receptors: in vivo microdialysis studies. J Neurochem. 1997;69(5):2019–25. 68. Prah A, Purg M, Stare J, Vianello R, Mavri J. How mono- amine oxidase A decomposes serotonin: an empirical valence bond simulation of the reactive step. J Phys Chem B. 2020;124(38):8259–65. 69. Meneses A, Liy-Salmeron G. Serotonin and emotion, learning and memory. Rev Neurosci. 2012;23(5–6):543–53. 70. Švob Štrac D, Pivac N, Muck-Seler D. The serotonergic system and cognitive function. Transl Neurosci. 2016;7(1):35–49. 71. Kawashima T. The role of the serotonergic system in motor control. Neurosci Res. 2018;129:32–9. 72. Voigt JP, Fink H. Serotonin controlling feeding and satiety. Behav Brain Res. 2015;277:14–31. 73. Ursin R. Serotonin and sleep. Sleep Med Rev. 2002;6(1):55–69. 74. Azmitia EC, Segal M. An autoradiographic analysis of the dif- ferential ascending projections of the dorsal and median raphe nuclei in the rat. J Comp Neurol. 1978;179(3):641–67. 75. Del Cid-Pellitero E, Garzon M. Medial prefrontal cortex receives input from dorsal raphe nucleus neurons targeted by hypocretin1/orexinA-containing axons. Neuroscience. 2011;172:30–43. 76. Luchetti A, Bota A, Weitemier A, Mizuta K, Sato M, Islam T, et al. Two functionally distinct serotonergic projections into hip- pocampus. J Neurosci. 2020;40(25):4936–44. 77. Bennett-Clarke CA, Leslie MJ, Lane RD, Rhoades RW. Effect of serotonin depletion on vibrissa-related patterns of tha- lamic afferents in the rat’s somatosensory cortex. J Neurosci. 1994;14(12):7594–607. 78. Conio B, Martino M, Magioncalda P, Escelsior A, Inglese M, Amore M, et al. Opposite effects of dopamine and serotonin on resting-state networks: review and implications for psychiatric disorders. Mol Psychiatry. 2020;25(1):82–93. 79. Puig MV, Gulledge AT. Serotonin and prefrontal cortex function: neurons, networks, and circuits. Mol Neurobiol. 2011;44(3):449–64. 80. Teissier A, Soiza-Reilly M, Gaspar P. Refining the role of 5-HT in postnatal development of brain circuits. Front Cell Neurosci. 2017;11:139. 81. Berger I, Slobodin O, Aboud M, Melamed J, Cassuto H. Matura- tional delay in ADHD: evidence from CPT. Front Hum Neurosci. 2013;7: 691. 82. Carmona S, Vilarroya O, Bielsa A, Tremols V, Soliva JC, Rovira M, et al. Global and regional gray matter reductions in ADHD: a voxel-based morphometric study. Neurosci Lett. 2005;389(2):88–93. 83. Shaw P, Eckstrand K, Sharp W, Blumenthal J, Lerch JP, Green- stein D, et al. Attention-deficit/hyperactivity disorder is char- acterized by a delay in cortical maturation. Proc Natl Acad Sci USA. 2007;104(49):19649–54. 84. Hoogman M, Bralten J, Hibar DP, Mennes M, Zwiers MP, Schw- eren LSJ, et al. Subcortical brain volume differences in partici- pants with attention deficit hyperactivity disorder in children and adults: a cross-sectional mega-analysis. Lancet Psychiatry. 2017;4(4):310–9. 85. Cascio CJ. Somatosensory processing in neurodevelopmental disorders. J Neurodev Disord. 2010;2(2):62–9. 86. Garcia LP, Witteveen JS, Middelman A, van Hulten JA, Mar- tens GJM, Homberg JR, et al. Perturbed developmental serotonin signaling affects prefrontal catecholaminergic innervation and cortical integrity. Mol Neurobiol. 2019;56(2):1405–20. 87. Alex KD, Pehek EA. Pharmacologic mechanisms of serotoner- gic regulation of dopamine neurotransmission. Pharmacol Ther. 2007;113(2):296–320. 88. Blier P. Crosstalk between the norepinephrine and serotonin systems and its role in the antidepressant response. J Psychiatry Neurosci. 2001;26(Suppl(Suppl)):S3-10. 731Serotonin in ADHD: Preclinical and Clinical Insights 89. Ciranna L. Serotonin as a modulator of glutamate- and GABA- mediated neurotransmission: implications in physiological func- tions and in pathology. Curr Neuropharmacol. 2006;4(2):101–14. 90. Sharp T, Barnes NM. Central 5-HT receptors and their function; present and future. Neuropharmacology. 2020;177: 108155. 91. Ryczko D, Dubuc R. Dopamine and the brainstem locomotor networks: from lamprey to human. Front Neurosci. 2017;11: 295. 92. Alttoa A, Seeman P, Koiv K, Eller M, Harro J. Rats with per- sistently high exploratory activity have both higher extracellular dopamine levels and higher proportion of D(2) (High) receptors in the striatum. Synapse. 2009;63(5):443–6. 93. Flaive A, Fougere M, van der Zouwen CI, Ryczko D. Seroton- ergic modulation of locomotor activity from basal vertebrates to mammals. Front Neural Circuits. 2020;14: 590299. 94. Brus R, Nowak P, Szkilnik R, Mikolajun U, Kostrzewa RM. Serotoninergics attenuate hyperlocomotor activity in rats. Poten- tial new therapeutic strategy for hyperactivity. Neurotox Res. 2004;6(4):317–25. 95. Erinoff L, Snodgrass SR. Effects of adult or neonatal treatment with 6-hydroxydopamine or 5,7-dihydroxytryptamine on loco- motor activity, monoamine levels, and response to caffeine. Phar- macol Biochem Behav. 1986;24(4):1039–45. 96. Marsden CA, Curzon G. Studies on the behavioural effects of tryptophan and rho-chlorophenylalanine. Neuropharmacology. 1976;15(3):165–71. 97. Dringenberg HC, Hargreaves EL, Baker GB, Cooley RK, Van- derwolf CH. P-chlorophenylalanine-induced serotonin depletion: reduction in exploratory locomotion but no obvious sensory- motor deficits. Behav Brain Res. 1995;68(2):229–37. 98. Kostrzewa RM, Brus R, Kalbfleisch JH, Perry KW, Fuller RW. Proposed animal model of attention deficit hyperactivity disor- der. Brain Res Bull. 1994;34(2):161–7. 99. O’Reilly KC, Connor M, Pierson J, Shuffrey LC, Blakely RD, Ahmari SE, et al. Serotonin 5-HT(1B) receptor-mediated behav- ior and binding in mice with the overactive and dysregulated serotonin transporter Ala56 variant. Psychopharmacology. 2021;238(4):1111–20. 100. Halberstadt AL, van der Heijden I, Ruderman MA, Risbrough VB, Gingrich JA, Geyer MA, et al. 5-HT(2A) and 5-HT(2C) receptors exert opposing effects on locomotor activity in mice. Neuropsychopharmacology. 2009;34(8):1958–67. 101. Boulenguez P, Rawlins JN, Chauveau J, Joseph MH, Mitchell SN, Gray JA. Modulation of dopamine release in the nucleus accum- bens by 5-HT1B agonists: involvement of the hippocampo- accumbens pathway. Neuropharmacology. 1996;35(11):1521–9. 102. De Deurwaerdère P, Chagraoui A, Di Giovanni G. Serotonin/ dopamine interaction: electrophysiological and neurochemical evidence. Prog Brain Res. 2021;261:161–264. 103. Fan X, Bruno KJ, Hess EJ. Rodent models of ADHD. Curr Top Behav Neurosci. 2012;9:273–300. 104. Rodriguiz RM, Chu R, Caron MG, Wetsel WC. Aberrant responses in social interaction of dopamine transporter knockout mice. Behav Brain Res. 2004;148(1–2):185–98. 105. Bouchatta O, Manouze H, Bouali-Benazzouz R, Kerekes N, Ba- M’hamed S, Fossat P, et al. Neonatal 6-OHDA lesion model in mouse induces attention-deficit/ hyperactivity disorder (ADHD)- like behaviour. Sci Rep. 2018;8(1):15349. 106. van der Kooij MA, Glennon JC. Animal models concerning the role of dopamine in attention-deficit hyperactivity disorder. Neu- rosci Biobehav Rev. 2007;31(4):597–618. 107. Shaywitz BA, Klopper JH, Gordon JW. Methylphenidate in 6-hydroxydopamine-treated developing rat pups. Effects on activity and maze performance. Arch Neurol. 1978;35(7):463–9. 108. Davids E, Zhang K, Kula NS, Tarazi FI, Baldessarini RJ. Effects of norepinephrine and serotonin transporter inhibitors on hyperactivity induced by neonatal 6-hydroxydopamine lesioning in rats. J Pharmacol Exp Ther. 2002;301(3):1097–102. 109. Heffner TG, Seiden LS. Possible involvement of serotonergic neurons in the reduction of locomotor hyperactivity caused by amphetamine in neonatal rats depleted of brain dopamine. Brain Res. 1982;244(1):81–90. 110. Luthman J, Fredriksson A, Plaznik A, Archer T. Ketanserin and mianserin treatment reverses hyperactivity in neonatally dopa- mine-lesioned rats. J Psychopharmacol. 1991;5(4):418–25. 111. Shaywitz BA, Yager RD, Klopper JH. Selective brain dopa- mine depletion in developing rats: an experimental model of minimal brain dysfunction. Science. 1976;191(4224):305–8. 112. Zhang K, Tarazi FI, Baldessarini RJ. Role of dopamine D(4) receptors in motor hyperactivity induced by neonatal 6-hydroxydopamine lesions in rats. Neuropsychopharmacol- ogy. 2001;25(5):624–32. 113. Towle AC, Criswell HE, Maynard EH, Lauder JM, Joh TH, Mueller RA, et al. Serotonergic innervation of the rat caudate following a neonatal 6-hydroxydopamine lesion: an anatomi- cal, biochemical and pharmacological study. Pharmacol Bio- chem Behav. 1989;34(2):367–74. 114. Bishop C, Kamdar DP, Walker PD. Intrastriatal serotonin 5-HT2 receptors mediate dopamine D1-induced hyperlo- comotion in 6-hydroxydopamine-lesioned rats. Synapse. 2003;50(2):164–70. 115. Avale ME, Nemirovsky SI, Raisman-Vozari R, Rubinstein M. Elevated serotonin is involved in hyperactivity but not in the paradoxical effect of amphetamine in mice neonatally lesioned with 6-hydroxydopamine. J Neurosci Res. 2004;78(2):289–96. 116. Bishop C, Tessmer JL, Ullrich T, Rice KC, Walker PD. Sero- tonin 5-HT2A receptors underlie increased motor behaviors induced in dopamine-depleted rats by intrastriatal 5-HT2A/2C agonism. J Pharmacol Exp Ther. 2004;310(2):687–94. 117. Bishop C, Walker PD. Combined intrastriatal dopamine D1 and serotonin 5-HT2 receptor stimulation reveals a mechanism for hyperlocomotion in 6-hydroxydopamine-lesioned rats. Neuro- science. 2003;121(3):649–57. 118. Linthorst AC, Van den Buuse M, De Jong W, Versteeg DH. Electrically stimulated [3H]dopamine and [14C]acetylcho- line release from nucleus caudatus slices: differences between spontaneously hypertensive rats and Wistar-Kyoto rats. Brain Res. 1990;509(2):266–72. 119. Akiyama K, Yabe K, Sutoo D. Quantitative immunohistochem- ical distributions of tyrosine hydroxylase and calmodulin in the brains of spontaneously hypertensive rats. Kitasato Arch Exp Med. 1992;65(4):199–208. 120. Linthorst AC, De Lang H, De Jong W, Versteeg DH. Effect of the dopamine D2 receptor agonist quinpirole on the in vivo release of dopamine in the caudate nucleus of hypertensive rats. Eur J Pharmacol. 1991;201(2–3):125–33. 121. Russell V, de Villiers A, Sagvolden T, Lamm M, Taljaard J. Differences between electrically-, ritalin- and D-amphetamine- stimulated release of [3H]dopamine from brain slices suggest impaired vesicular storage of dopamine in an animal model of attention-deficit hyperactivity disorder. Behav Brain Res. 1998;94(1):163–71. 122. Fujita S, Okutsu H, Yamaguchi H, Nakamura S, Adachi K, Saigusa T, et al. Altered pre- and postsynaptic dopamine recep- tor functions in spontaneously hypertensive rat: an animal model of attention-deficit hyperactivity disorder. J Oral Sci. 2003;45(2):75–83. 123. Fuller RW, Hemrick-Luecke SK, Wong DT, Pearson D, Threl- keld PG, Hynes MD 3rd. Altered behavioral response to a D2 agonist, LY141865, in spontaneously hypertensive rats exhibit- ing biochemical and endocrine responses similar to those in normotensive rats. J Pharmacol Exp Ther. 1983;227(2):354–9. 732 M. B. Solomon et al. 124. de Villiers AS, Russell VA, Sagvolden T, Searson A, Jaffer A, Taljaard JJ. Alpha 2-adrenoceptor mediated inhibition of [3H] dopamine release from nucleus accumbens slices and mono- amine levels in a rat model for attention-deficit hyperactivity disorder. Neurochem Res. 1995;20(4):427–33. 125. Stocker SD, Muldoon MF, Sved AF. Blunted fenfluramine- evoked prolactin secretion in hypertensive rats. Hypertension. 2003;42(4):719–24. 126. Umehara M, Ago Y, Fujita K, Hiramatsu N, Takuma K, Matsuda T. Effects of serotonin-norepinephrine reuptake inhibitors on locomotion and prefrontal monoamine release in spontaneously hypertensive rats. Eur J Pharmacol. 2013;702(1–3):250–7. 127. Pollier F, Sarre S, Aguerre S, Ebinger G, Mormede P, Michotte Y, et al. Serotonin reuptake inhibition by citalopram in rat strains differing for their emotionality. Neuropsychopharmacology. 2000;22(1):64–76. 128. Hiraide S, Ueno K, Yamaguchi T, Matsumoto M, Yanagawa Y, Yoshioka M, et al. Behavioural effects of monoamine reuptake inhibitors on symptomatic domains in an animal model of atten- tion-deficit/hyperactivity disorder. Pharmacol Biochem Behav. 2013;105:89–97. 129. Russell VA, Sagvolden T, Johansen EB. Animal models of atten- tion-deficit hyperactivity disorder. Behav Brain Funct. 2005;1: 9. 130. Gainetdinov RR, Jones SR, Fumagalli F, Wightman RM, Caron MG. Re-evaluation of the role of the dopamine transporter in dopamine system homeostasis. Brain Res Brain Res Rev. 1998;26(2–3):148–53. 131. Jones SR, Gainetdinov RR, Jaber M, Giros B, Wightman RM, Caron MG. Profound neuronal plasticity in response to inacti- vation of the dopamine transporter. Proc Natl Acad Sci USA. 1998;95(7):4029–34. 132. Carboni E, Silvagni A. Dopamine reuptake by norepi- nephrine neurons: exception or rule? Crit Rev Neurobiol. 2004;16(1–2):121–8. 133. Shen HW, Hagino Y, Kobayashi H, Shinohara-Tanaka K, Ikeda K, Yamamoto H, et al. Regional differences in extracellular dopa- mine and serotonin assessed by in vivo microdialysis in mice lacking dopamine and/or serotonin transporters. Neuropsychop- harmacology. 2004;29(10):1790–9. 134. Giros B, Jaber M, Jones SR, Wightman RM, Caron MG. Hyperlocomotion and indifference to cocaine and ampheta- mine in mice lacking the dopamine transporter. Nature. 1996;379(6566):606–12. 135. Jones SR, Joseph JD, Barak LS, Caron MG, Wightman RM. Dopamine neuronal transport kinetics and effects of ampheta- mine. J Neurochem. 1999;73(6):2406–14. 136. Harris SS, Green SM, Kumar M, Urs NM. A role for cortical dopamine in the paradoxical calming effects of psychostimulants. Sci Rep. 2022;12(1):3129. 137. Morón JA, Brockington A, Wise RA, Rocha BA, Hope BT. Dopamine uptake through the norepinephrine transporter in brain regions with low levels of the dopamine transporter: evidence from knock-out mouse lines. J Neurosci. 2002;22(2):389–95. 138. Fox MA, Panessiti MG, Hall FS, Uhl GR, Murphy DL. An evaluation of the serotonin system and perseverative, com- pulsive, stereotypical, and hyperactive behaviors in dopamine transporter (DAT) knockout mice. Psychopharmacology. 2013;227(4):685–95. 139. Gainetdinov RR, Wetsel WC, Jones SR, Levin ED, Jaber M, Caron MG. Role of serotonin in the paradoxical calm- ing effect of psychostimulants on hyperactivity. Science. 1999;283(5400):397–401. 140. Borycz J, Zapata A, Quiroz C, Volkow ND, Ferre S. 5-HT 1B receptor-mediated serotoninergic modulation of methylphenidate-induced locomotor activation in rats. Neuropsy- chopharmacology. 2008;33(3):619–26. 141. Hall FS, Sora I, Hen R, Uhl GR. Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout. PLoS ONE. 2014;9(12): e115009. 142. Barr AM, Lehmann-Masten V, Paulus M, Gainetdinov RR, Caron MG, Geyer MA. The selective serotonin-2A receptor antagonist M100907 reverses behavioral deficits in dopamine transporter knockout mice. Neuropsychopharmacology. 2004;29(2):221–8. 143. Gainetdinov RR, Mohn AR, Bohn LM, Caron MG. Glutamater- gic modulation of hyperactivity in mice lacking the dopamine transporter. Proc Natl Acad Sci USA. 2001;98(20):11047–54. 144. Dalley JW, Robbins TW. Fractionating impulsivity: neuropsy- chiatric implications. Nat Rev Neurosci. 2017;18(3):158–71. 145. Esteves M, Moreira PS, Sousa N, Leite-Almeida H. Assessing impulsivity in humans and rodents: taking the translational road. Front Behav Neurosci. 2021;15: 647922. 146. Winstanley CA, Eagle DM, Robbins TW. Behavioral models of impulsivity in relation to ADHD: translation between clinical and preclinical studies. Clin Psychol Rev. 2006;26(4):379–95. 147. Solanto MV, Abikoff H, Sonuga-Barke E, Schachar R, Logan GD, Wigal T, et al. The ecological validity of delay aversion and response inhibition as measures of impulsivity in AD/HD: a supplement to the NIMH multimodal treatment study of AD/ HD. J Abnorm Child Psychol. 2001;29(3):215–28. 148. Harrison AA, Everitt BJ, Robbins TW. Central 5-HT depletion enhances impulsive responding without affecting the accuracy of attentional performance: interactions with dopaminergic mecha- nisms. Psychopharmacology. 1997;133(4):329–42. 149. Harrison AA, Everitt BJ, Robbins TW. Doubly dissociable effects of median- and dorsal-raphe lesions on the performance of the five-choice serial reaction time test of attention in rats. Behav Brain Res. 1997;89(1–2):135–49. 150. Winstanley CA, Theobald DE, Dalley JW, Glennon JC, Robbins TW. 5-HT2A and 5-HT2C receptor antagonists have opposing effects on a measure of impulsivity: interactions with global 5-HT depletion. Psychopharmacology. 2004;176(3–4):376–85. 151. Robinson ES, Dalley JW, Theobald DE, Glennon JC, Pezze MA, Murphy ER, et al. Opposing roles for 5-HT2A and 5-HT2C receptors in the nucleus accumbens on inhibitory response con- trol in the 5-choice serial reaction time task. Neuropsychophar- macology. 2008;33(10):2398–406. 152. Passetti F, Dalley JW, Robbins TW. Double dissociation of sero- tonergic and dopaminergic mechanisms on attentional perfor- mance using a rodent five-choice reaction time task. Psychop- harmacology. 2003;165(2):136–45. 153. Winstanley CA, Chudasama Y, Dalley JW, Theobald DE, Glen- non JC, Robbins TW. Intra-prefrontal 8-OH-DPAT and M100907 improve visuospatial attention and decrease impulsivity on the five-choice serial reaction time task in rats. Psychopharmacology. 2003;167(3):304–14. 154. Carli M, Samanin R. The 5-HT(1A) receptor agonist 8-OH- DPAT reduces rats’ accuracy of attentional performance and enhances impulsive responding in a five-choice serial reaction time task: role of presynaptic 5-HT(1A) receptors. Psychophar- macology. 2000;149(3):259–68. 155. Baarendse PJ, Vanderschuren LJ. Dissociable effects of monoam- ine reuptake inhibitors on distinct forms of impulsive behavior in rats. Psychopharmacology. 2012;219(2):313–26. 156. Mobini S, Chiang TJ, Ho MY, Bradshaw CM, Szabadi E. Effects of central 5-hydroxytryptamine depletion on sensitivity to delayed and probabilistic reinforcement. Psychopharmacology. 2000;152(4):390–7. 733Serotonin in ADHD: Preclinical and Clinical Insights 157. Bizot J, Le Bihan C, Puech AJ, Hamon M, Thiébot M. Serotonin and tolerance to delay of reward in rats. Psychopharmacology. 1999;146(4):400–12. 158. Winstanley CA, Dalley JW, Theobald DE, Robbins TW. Fraction- ating impulsivity: contrasting effects of central 5-HT depletion on different measures of impulsive behavior. Neuropsychophar- macology. 2004;29(7):1331–43. 159. Winstanley CA, Dalley JW, Theobald DEH, Robbins TW. Global 5-HT depletion attenuates the ability of amphetamine to decrease impulsive choice on a delay-discounting task in rats. Psychop- harmacology. 2003;170(3):320–31. 160. Winstanley CA, Theobald DE, Dalley JW, Robbins TW. Interac- tions between serotonin and dopamine in the control of impulsive choice in rats: therapeutic implications for impulse control dis- orders. Neuropsychopharmacology. 2005;30(4):669–82. 161. Blasio A, Narayan AR, Kaminski BJ, Steardo L, Sabino V, Cottone P. A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT(2)A/C and 5-HT(1)A receptor agonists. Psychop- harmacology. 2012;219(2):377–86. 162. Yates JR, Perry JL, Meyer AC, Gipson CD, Charnigo R, Bardo MT. Role of medial prefrontal and orbitofrontal monoamine transporters and receptors in performance in an adjusting delay discounting procedure. Brain Res. 2014;1574:26–36. 163. Evenden JL, Ryan CN. The pharmacology of impulsive behav- iour in rats VI: the effects of ethanol and selective serotonergic drugs on response choice with varying delays of reinforcement. Psychopharmacology. 1999;146(4):413–21. 164. Talpos JC, Wilkinson LS, Robbins TW. A comparison of mul- tiple 5-HT receptors in two tasks measuring impulsivity. J Psy- chopharmacol. 2006;20(1):47–58. 165. Mori M, Tsutsui-Kimura I, Mimura M, Tanaka KF. 5-HT(3) antagonists decrease discounting rate without affecting sensitiv- ity to reward magnitude in the delay discounting task in mice. Psychopharmacology. 2018;235(9):2619–29. 166. Costall B, Domeney AM, Naylor RJ, Tyers MB. Effects of the 5-HT3 receptor antagonist, GR38032F, on raised dopaminergic activity in the mesolimbic system of the rat and marmoset brain. Br J Pharmacol. 1987;92(4):881–94. 167. Engleman EA, Rodd ZA, Bell RL, Murphy JM. The role of 5-HT3 receptors in drug abuse and as a target for pharmaco- therapy. CNS Neurol Disord Drug Targets. 2008;7(5):454–67. 168. Soubrié P. Reconciling the role of central serotonin neurons in human and animal behavior. Behav Brain Sci. 1986;9(2):319–35. 169. Miyazaki KW, Miyazaki K, Doya K. Activation of dorsal raphe serotonin neurons is necessary for waiting for delayed rewards. J Neurosci. 2012;32(31):10451–7. 170. Roberts DC, Loh EA, Baker GB, Vickers G. Lesions of cen- tral serotonin systems affect responding on a progressive ratio schedule reinforced either by intravenous cocaine or by food. Pharmacol Biochem Behav. 1994;49(1):177–82. 171. Eagle DM, Lehmann O, Theobald DE, Pena Y, Zakaria R, Ghosh R, et al. Serotonin depletion impairs waiting but not stop-sig- nal reaction time in rats: implications for theories of the role of 5-HT in behavioral inhibition. Neuropsychopharmacology. 2009;34(5):1311–21. 172. Harrison AA, Everitt BJ, Robbins TW. Central serotonin deple- tion impairs both the acquisition and performance of a symmet- rically reinforced go/no-go conditional visual discrimination. Behav Brain Res. 1999;100(1–2):99–112. 173. Bari A, Eagle DM, Mar AC, Robinson ES, Robbins TW. Disso- ciable effects of noradrenaline, dopamine, and serotonin uptake blockade on stop task performance in rats. Psychopharmacology. 2009;205(2):273–83. 174. Hughes LE, Rittman T, Regenthal R, Robbins TW, Rowe JB. Improving response inhibition systems in frontotemporal demen- tia with citalopram. Brain. 2015;138(Pt 7):1961–75. 175. Ye Z, Altena E, Nombela C, Housden CR, Maxwell H, Ritt- man T, et al. Selective serotonin reuptake inhibition modulates response inhibition in Parkinson’s disease. Brain. 2014;137(Pt 4):1145–55. 176. de Boer SF, Lesourd M, Mocaer E, Koolhaas JM. Selective antiaggressive effects of alnespirone in resident-intruder test are mediated via 5-hydroxytryptamine1A receptors: a compar- ative pharmacological study with 8-hydroxy-2-dipropylamino- tetralin, ipsapirone, buspirone, eltoprazine, and WAY-100635. J Pharmacol Exp Ther. 1999;288(3):1125–33. 177. Kantak KM, Hegstrand LR, Eichelman B. Dietary tryptophan modulation and aggressive behavior in mice. Pharmacol Bio- chem Behav. 1980;12(5):675–9. 178. Kästner N, Richter SH, Urbanik S, Kunert J, Waider J, Lesch KP, et al. Brain serotonin deficiency affects female aggression. Sci Rep. 2019;9(1): 1366. 179. Lasley SM, Thurmond JB. Interaction of dietary trypto- phan and social isolation on territorial aggression, motor activity, and neurochemistry in mice. Psychopharmacology. 1985;87(3):313–21. 180. Mosienko V, Bert B, Beis D, Matthes S, Fink H, Bader M, et al. Exaggerated aggression and decreased anxiety in mice deficient in brain serotonin. Transl Psychiatry. 2012;2(5):e122. 181. Olivier B, Mos J, van Oorschot R, Hen R. Serotonin receptors and animal models of aggressive behavior. Pharmacopsychiatry. 1995;28(Suppl 2):80–90. 182. Vergnes M, Depaulis A, Boehrer A. Parachlorophenylalanine- induced serotonin depletion increases offensive but not defensive aggression in male rats. Physiol Behav. 1986;36(4):653–8. 183. Lesch KP, Merschdorf U. Impulsivity, aggression, and seroto- nin: a molecular psychobiological perspective. Behav Sci Law. 2000;18(5):581–604. 184. Sachs BD, Rodriguiz RM, Siesser WB, Kenan A, Royer EL, Jacobsen JP, et al. The effects of brain serotonin deficiency on behavioural disinhibition and anxiety-like behaviour fol- lowing mild early life stress. Int J Neuropsychopharmacol. 2013;16(9):2081–94. 185. Angoa-Pérez M, Kane MJ, Briggs DI, Sykes CE, Shah MM, Francescutti DM, et al. Genetic depletion of brain 5HT reveals a common molecular pathway mediating compulsivity and impul- sivity. J Neurochem. 2012;121(6):974–84. 186. Kane MJ, Angoa-Perez M, Briggs DI, Sykes CE, Francescutti DM, Rosenberg DR, et al. Mice genetically depleted of brain serotonin display social impairments, communication deficits and repetitive behaviors: possible relevance to autism. PLoS ONE. 2012;7(11): e48975. 187. Alonso L, Peeva P, Stasko S, Bader M, Alenina N, Winter Y, et al. Constitutive depletion of brain serotonin differentially affects rats’ social and cognitive abilities. iScience. 2023;26(2): 105998. 188. Meng X, Grandjean J, Sbrini G, Schipper P, Hofwijks N, Stoop J, et al. Tryptophan hydroxylase 2 knockout male rats exhibit a strengthened oxytocin system, are aggressive, and are less anx- ious. ACS Chem Neurosci. 2022;13(20):2974–81. 189. Audero E, Mlinar B, Baccini G, Skachokova ZK, Corradetti R, Gross C. Suppression of serotonin neuron firing increases aggres- sion in mice. J Neurosci. 2013;33(20):8678–88. 190. Homberg JR, Pattij T, Janssen MC, Ronken E, De Boer SF, Schoffelmeer AN, et al. Serotonin transporter deficiency in rats improves inhibitory control but not behavioural flexibility. Eur J Neurosci. 2007;26(7):2066–73. 734 M. B. Solomon et al. 191. Sánchez C, Meier E. Behavioral profiles of SSRIs in animal mod- els of depression, anxiety and aggression. Are they all alike? Psychopharmacology. 1997;129(3):197–205. 192. Peeters D, Rietdijk J, Gerrits D, Rijpkema M, de Boer SF, Verkes RJ, et al. Searching for neural and behavioral parameters that predict anti-aggressive effects of chronic SSRI treatment in rats. Neuropharmacology. 2018;143:339–48. 193. Sánchez C, Hyttel J. Isolation-induced aggression in mice: effects of 5-hydroxytryptamine uptake inhibitors and involve- ment of postsynaptic 5-HT1A receptors. Eur J Pharmacol. 1994;264(3):241–7. 194. Popova NK, Naumenko VS, Kozhemyakina RV, Plyusnina IZ. Functional characteristics of serotonin 5-HT2A and 5-HT2C receptors in the brain and the expression of the 5-HT2A and 5-HT2C receptor genes in aggressive and non-aggressive rats. Neurosci Behav Physiol. 2010;40(4):357–61. 195. Popova NK, Tsybko AS, Naumenko VS. The implication of 5-HT receptor family members in aggression, depression and suicide: similarity and difference. Int J Mol Sci. 2022. https://​ doi.​org/​10.​3390/​ijms2​31588​14. 196. Martin CB, Ramond F, Farrington DT, Aguiar AS Jr., Chevarin C, Berthiau AS, et al. RNA splicing and editing modulation of 5-HT(2C) receptor function: relevance to anxiety and aggres- sion in VGV mice. Mol Psychiatry. 2013;18(6):656–65. 197. Harvey ML, Swallows CL, Cooper MA. A double dissociation in the effects of 5-HT2A and 5-HT2C receptors on the acquisi- tion and expression of conditioned defeat in Syrian hamsters. Behav Neurosci. 2012;126(4):530–7. 198. Dekeyne A, Brocco M, Loiseau F, Gobert A, Rivet JM, Di Cara B, et  al. S32212, a novel serotonin type 2C receptor inverse agonist/alpha2-adrenoceptor antagonist and potential antidepressant: II. A behavioral, neurochemical, and elec- trophysiological characterization. J Pharmacol Exp Ther. 2012;340(3):765–80. 199. Sakaue M, Ago Y, Sowa C, Sakamoto Y, Nishihara B, Koyama Y, et al. Modulation by 5-hT2A receptors of aggressive behavior in isolated mice. Jpn J Pharmacol. 2002;89(1):89–92. 200. Rudissaar R, Pruus K, Skrebuhhova T, Allikmets L, Matto V. Modulatory role of 5-HT3 receptors in mediation of apomor- phine-induced aggressive behaviour in male rats. Behav Brain Res. 1999;106(1–2):91–6. 201. McKenzie-Quirk SD, Girasa KA, Allan AM, Miczek KA. 5-HT(3) receptors, alcohol and aggressive behavior in mice. Behav Pharmacol. 2005;16(3):163–9. 202. White SM, Kucharik RF, Moyer JA. Effects of serotonergic agents on isolation-induced aggression. Pharmacol Biochem Behav. 1991;39(3):729–36. 203. Shimizu K, Kurosawa N, Seki K. The role of the AMPA receptor and 5-HT(3) receptor on aggressive behavior and depressive-like symptoms in chronic social isolation-reared mice. Physiol Behav. 2016;153:70–83. 204. Cervantes MC, Delville Y. Serotonin 5-HT1A and 5-HT3 recep- tors in an impulsive-aggressive phenotype. Behav Neurosci. 2009;123(3):589–98. 205. Saudou F, Amara DA, Dierich A, LeMeur M, Ramboz S, Segu L, et al. Enhanced aggressive behavior in mice lacking 5-HT1B receptor. Science. 1994;265(5180):1875–8. 206. Sari Y. Serotonin1B receptors: from protein to physiological func- tion and behavior. Neurosci Biobehav Rev. 2004;28(6):565–82. 207. de Boer SF, Koolhaas JM. 5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis. Eur J Pharmacol. 2005;526(1–3):125–39. 208. Dalley JW, Roiser JP. Dopamine, serotonin and impulsivity. Neu- roscience. 2012;215:42–58. 209. Seo D, Patrick CJ, Kennealy PJ. Role of serotonin and dopamine system interactions in the neurobiology of impulsive aggression and its comorbidity with other clinical disorders. Aggress Violent Behav. 2008;13(5):383–95. 210. Carli M, Samanin R. Serotonin2 receptor agonists and seroton- ergic anorectic drugs affect rats’ performance differently in a five-choice serial reaction time task. Psychopharmacology. 1992;106(2):228–34. 211. Mirjana C, Baviera M, Invernizzi RW, Balducci C. The sero- tonin 5-HT2A receptors antagonist M100907 prevents impair- ment in attentional performance by NMDA receptor block- ade in the rat prefrontal cortex. Neuropsychopharmacology. 2004;29(9):1637–47. 212. Carli M, Baviera M, Invernizzi RW, Balducci C. Dissociable contribution of 5-HT1A and 5-HT2A receptors in the medial prefrontal cortex to different aspects of executive control such as impulsivity and compulsive perseveration in rats. Neuropsy- chopharmacology. 2006;31(4):757–67. 213. Weinberg-Wolf H, Fagan NA, Anderson GM, Tringides M, Dal Monte O, Chang SWC. The effects of 5-hydroxytryptophan on attention and central serotonin neurochemistry in the rhesus macaque. Neuropsychopharmacology. 2018;43(7):1589–98. 214. Lapiz-Bluhm MD, Soto-Pina AE, Hensler JG, Morilak DA. Chronic intermittent cold stress and serotonin depletion induce deficits of reversal learning in an attentional set-shifting test in rats. Psychopharmacology. 2009;202(1–3):329–41. 215. Clarke HF, Dalley JW, Crofts HS, Robbins TW, Roberts AC. Cognitive inflexibility after prefrontal serotonin depletion. Sci- ence. 2004;304(5672):878–80. 216. Clarke HF, Walker SC, Crofts HS, Dalley JW, Robbins TW, Rob- erts AC. Prefrontal serotonin depletion affects reversal learning but not attentional set shifting. J Neurosci. 2005;25(2):532–8. 217. Clarke HF, Walker SC, Dalley JW, Robbins TW, Roberts AC. Cognitive inflexibility after prefrontal serotonin depletion is behaviorally and neurochemically specific. Cereb Cortex. 2007;17(1):18–27. 218. Walker SC, Robbins TW, Roberts AC. Differential contributions of dopamine and serotonin to orbitofrontal cortex function in the marmoset. Cereb Cortex. 2009;19(4):889–98. 219. Wallace A, Pehrson AL, Sanchez C, Morilak DA. Vortioxetine restores reversal learning impaired by 5-HT depletion or chronic intermittent cold stress in rats. Int J Neuropsychopharmacol. 2014;17(10):1695–706. 220. Mørk A, Pehrson A, Brennum LT, Nielsen SM, Zhong H, Lassen AB, et al. Pharmacological effects of Lu AA21004: a novel mul- timodal compound for the treatment of major depressive disorder. J Pharmacol Exp Ther. 2012;340(3):666–75. 221. Pehrson AL, Cremers T, Betry C, van der Hart MG, Jorgensen L, Madsen M, et al. Lu AA21004, a novel multimodal antide- pressant, produces regionally selective increases of multiple neurotransmitters—a rat microdialysis and electrophysiology study. Eur Neuropsychopharmacol. 2013;23(2):133–45. 222. Mørk A, Montezinho LP, Miller S, Trippodi-Murphy C, Plath N, Li Y, et al. Vortioxetine (Lu AA21004), a novel multimodal antidepressant, enhances memory in rats. Pharmacol Biochem Behav. 2013;105:41–50. 223. Walker SC, Robbins TW, Roberts AC. Response disengagement on a spatial self-ordered sequencing task: effects of regionally selective excitotoxic lesions and serotonin depletion within the prefrontal cortex. J Neurosci. 2009;29(18):6033–41. 224. van der Plasse G, Feenstra MG. Serial reversal learning and acute tryptophan depletion. Behav Brain Res. 2008;186(1):23–31. 225. Merchan A, Navarro SV, Klein AB, Aznar S, Campa L, Sunol C, et al. Tryptophan depletion affects compulsive behaviour in rats: strain dependent effects and associated neuromechanisms. Psychopharmacology. 2017;234(8):1223–36. 226. Barlow RL, Alsiö J, Jupp B, Rabinovich R, Shrestha S, Roberts AC, et al. Markers of serotonergic function in the orbitofrontal https://doi.org/10.3390/ijms23158814 https://doi.org/10.3390/ijms23158814 735Serotonin in ADHD: Preclinical and Clinical Insights cortex and dorsal raphe nucleus predict individual variation in spatial-discrimination serial reversal learning. Neuropsychophar- macology. 2015;40(7):1619–30. 227. Prados-Pardo Á, Martin-González E, Mora S, Martin C, Olmedo-Córdoba M, Pérez-Fernandez C, et al. Reduced expres- sion of the Htr2a, Grin1, and Bdnf genes and cognitive inflex- ibility in a model of high compulsive rats. Mol Neurobiol. 2023;60(12):6975–91. 228. Alvarez BD, Morales CA, Amodeo DA. Impact of specific sero- tonin receptor modulation on behavioral flexibility. Pharmacol Biochem Behav. 2021;209: 173243. 229. Rudnick G, Clark J. From synapse to vesicle: the reuptake and storage of biogenic amine neurotransmitters. Biochim Biophys Acta. 1993;1144(3):249–63. 230. Sulzer D, Edwards RH. Antidepressants and the monoamine masquerade. Neuron. 2005;46(1):1–2. 231. Kahlig KM, Binda F, Khoshbouei H, Blakely RD, McMahon DG, Javitch JA, et al. Amphetamine induces dopamine efflux through a dopamine transporter channel. Proc Natl Acad Sci USA. 2005;102(9):3495–500. 232. Robertson SD, Matthies HJ, Galli A. A closer look at amphetamine-induced reverse transport and trafficking of the dopamine and norepinephrine transporters. Mol Neurobiol. 2009;39(2):73–80. 233. Heikkila RE, Orlansky H, Mytilineou C, Cohen G. Ampheta- mine: evaluation of d- and l-isomers as releasing agents and uptake inhibitors for 3H-dopamine and 3H-norepinephrine in slices of rat neostriatum and cerebral cortex. J Pharmacol Exp Ther. 1975;194(1):47–56. 234. Rothman RB, Baumann MH, Dersch CM, Romero DV, Rice KC, Carroll FI, et al. Amphetamine-type central nervous sys- tem stimulants release norepinephrine more potently than they release dopamine and serotonin. Synapse. 2001;39(1):32–41. 235. Kuczenski R, Segal DS, Cho AK, Melega W. Hippocampus norepinephrine, caudate dopamine and serotonin, and behav- ioral responses to the stereoisomers of amphetamine and meth- amphetamine. J Neurosci. 1995;15(2):1308–17. 236. Underhill SM, Hullihen PD, Chen J, Fenollar-Ferrer C, Rizzo MA, Ingram SL, et al. Amphetamines signal through intracellular TAAR1 receptors coupled to Galpha(13) and Galpha(S) in discrete subcellular domains. Mol Psychiatry. 2021;26(4):1208–23. 237. Miller HH, Shore PA, Clarke DE. In vivo monoamine oxi- dase inhibition by d-amphetamine. Biochem Pharmacol. 1980;29(10):1347–54. 238. Robinson JB. Stereoselectivity and isoenzyme selectivity of monoamine oxidase inhibitors. Enantiomers of amphetamine, N-methylamphetamine and deprenyl. Biochem Pharmacol. 1985;34(23):4105–8. 239. Riddle EL, Hanson GR, Fleckenstein AE. Therapeutic doses of amphetamine and methylphenidate selectively redistrib- ute the vesicular monoamine transporter-2. Eur J Pharmacol. 2007;571(1):25–8. 240. Holmes JC, Rutledge CO. Effects of the d- and l-isomers of amphetamine on uptake, release and catabolism of norepineph- rine, dopamine and 5-hydroxytryptamine in several regions of rat brain. Biochem Pharmacol. 1976;25(4):447–51. 241. Heal DJ, Cheetham SC, Prow MR, Martin KF, Buckett WR. A comparison of the effects on central 5-HT function of sibutramine hydrochloride and other weight-modifying agents. Br J Pharmacol. 1998;125(2):301–8. 242. Balfour DJ, Iyaniwura TT. An investigation of amphetamine- induced release of 5-HT from rat hippocampal slices. Eur J Phar- macol. 1985;109(3):395–9. 243. Hernandez L, Lee F, Hoebel BG. Simultaneous microdialysis and amphetamine infusion in the nucleus accumbens and striatum of freely moving rats: increase in extracellular dopamine and serotonin. Brain Res Bull. 1987;19(6):623–8. 244. Kuczenski R, Segal DS. In vivo measures of monoamines during amphetamine-induced behaviors in rats. Prog Neuropsychophar- macol Biol Psychiatry. 1990;14(Suppl):S37-50. 245. Yang KC, Takano A, Halldin C, Farde L, Finnema SJ. Serotonin concentration enhancers at clinically relevant doses reduce [(11) C]AZ10419369 binding to the 5-HT(1B) receptors in the nonhu- man primate brain. Transl Psychiatry. 2018;8(1):132. 246. Erritzoe D, Ashok AH, Searle GE, Colasanti A, Turton S, Lewis Y, et al. Serotonin release measured in the human brain: a PET study with [(11)C]CIMBI-36 and d-amphetamine challenge. Neuropsychopharmacology. 2020;45(5):804–10. 247. Han DD, Gu HH. Comparison of the monoamine transporters from human and mouse in their sensitivities to psychostimulant drugs. BMC Pharmacol. 2006;6:6. 248. Kuczenski R, Segal DS. Effects of methylphenidate on extracel- lular dopamine, serotonin, and norepinephrine: comparison with amphetamine. J Neurochem. 1997;68(5):2032–7. 249. Andersen PH. The dopamine inhibitor GBR 12909: selectiv- ity and molecular mechanism of action. Eur J Pharmacol. 1989;166(3):493–504. 250. Wall SC, Gu H, Rudnick G. Biogenic amine flux mediated by cloned transporters stably expressed in cultured cell lines: amphetamine specificity for inhibition and efflux. Mol Pharma- col. 1995;47(3):544–50. 251. Gatley SJ, Pan D, Chen R, Chaturvedi G, Ding YS. Affinities of methylphenidate derivatives for dopamine, norepinephrine and serotonin transporters. Life Sci. 1996;58(12):231–9. 252. Salman T, Afroz R, Nawaz S, Mahmood K, Haleem DJ, Zarina S. Differential effects of memory enhancing and impairing doses of methylphenidate on serotonin metabolism and 5-HT1A, GABA, glutamate receptor expression in the rat prefrontal cortex. Bio- chimie. 2021;191:51–61. 253. Faraone SV, Buitelaar J. Comparing the efficacy of stimulants for ADHD in children and adolescents using meta-analysis. Eur Child Adolesc Psychiatry. 2010;19(4):353–64. 254. Joseph A, Ayyagari R, Xie M, Cai S, Xie J, Huss M, et al. Com- parative efficacy and safety of attention-deficit/hyperactivity dis- order pharmacotherapies, including guanfacine extended release: a mixed treatment comparison. Eur Child Adolesc Psychiatry. 2017;26(8):875–97. 255. Stuhec M, Munda B, Svab V, Locatelli I. Comparative efficacy and acceptability of atomoxetine, lisdexamfetamine, bupropion and methylphenidate in treatment of attention deficit hyperactiv- ity disorder in children and adolescents: a meta-analysis with focus on bupropion. J Affect Disord. 2015;178:149–59. 256. Cortese S, Adamo N, Del Giovane C, Mohr-Jensen C, Hayes AJ, Carucci S, et al. Comparative efficacy and tolerability of medications for attention-deficit hyperactivity disorder in chil- dren, adolescents, and adults: a systematic review and network meta-analysis. Lancet Psychiatry. 2018;5(9):727–38. 257. Breese GR, Cooper BR, Hollister AS. Involvement of brain monoamines in the stimulant and paradoxical inhibitory effects of methylphenidate. Psychopharmacologia. 1975;44(1):5–10. 258. Hollister AS, Breese GR, Kuhn CM, Cooper BR, Schanberg SM. An inhibitory role for brain serotonin-containing systems in the locomotor effects of d-amphetamine. J Pharmacol Exp Ther. 1976;198(1):12–22. 259. Mabry PD, Campbell BA. Serotonergic inhibition of catechola- mine-induced behavioral arousal. Brain Res. 1973;49(2):381–91. 260. Breese GR, Cooper BR, Mueller RA. Evidence for involvement of 5-hydroxytryptamine in the actions of amphetamine. Br J Pharmacol. 1974;52(2):307–14. 736 M. B. Solomon et al. 261. Mabry PD, Campbell BA. Ontogeny of serotonergic inhibi- tion of behavioral arousal in the rat. J Comp Physiol Psychol. 1974;86(2):193–201. 262. Kuczenski R. Effects of para-chlorophenylalanine on ampheta- mine and haloperidol-induced changes in striatal dopamine turnover. Brain Res. 1979;164:217–25. 263. Lyness WH, Friedle NM, Moore KE. Increased self-administra- tion of d-amphetamine after destruction of 5-hydroxytryptamin- ergic neurons. Pharmacol Biochem Behav. 1980;12(6):937–41. 264. Leccese AP, Lyness WH. The effects of putative 5-hydroxy- tryptamine receptor active agents on d-amphetamine self- administration in controls and rats with 5,7-dihydroxytryptamine median forebrain bundle lesions. Brain Res. 1984;303(1):153–62. 265. Carli M, Kostoula C, Sacchetti G, Mainolfi P, Anastasia A, Vil- lani C, et al. Tph2 gene deletion enhances amphetamine-induced hypermotility: effect of 5-HT restoration and role of striatal noradrenaline release. J Neurochem. 2015;135(4):674–85. 266. Yates JR, Day HA, Evans KE, Igwe HO, Kappesser JL, Miller AL, et al. Effects of d-amphetamine and MK-801 on impulsive choice: modulation by schedule of reinforcement and delay length. Behav Brain Res. 2019;376: 112228. 267. Higgins GA, Brown M, MacMillan C, Silenieks LB, Thevarkun- nel S. Contrasting effects of d-amphetamine and atomoxetine on measures of impulsive action and choice. Pharmacol Biochem Behav. 2021;207: 173220. 268. Belles L, Arrondeau C, Uruena-Mendez G, Ginovart N. Concur- rent measures of impulsive action and choice are partially related and differentially modulated by dopamine D(1)- and D(2)-like receptors in a rat model of impulsivity. Pharmacol Biochem Behav. 2023;222: 173508. 269. Cardinal RN, Robbins TW, Everitt BJ. The effects of d-amphetamine, chlordiazepoxide, alpha-flupenthixol and behavioural manipulations on choice of signalled and unsig- nalled delayed reinforcement in rats. Psychopharmacology. 2000;152(4):362–75. 270. Ichikawa J, Kuroki T, Kitchen MT, Meltzer HY. R(+)-8-OH- DPAT, a 5-HT1A receptor agonist, inhibits amphetamine- induced dopamine release in rat striatum and nucleus accumbens. Eur J Pharmacol. 1995;287(2):179–84. 271. Kuroki T, Ichikawa J, Dai J, Meltzer HY. R(+)-8-OH-DPAT, a 5-HT1A receptor agonist, inhibits amphetamine-induced seroto- nin and dopamine release in rat medial prefrontal cortex. Brain Res. 1996;743(1–2):357–61. 272. Fletcher PJ, Rizos Z, Noble K, Higgins GA. Impulsive action induced by amphetamine, cocaine and MK801 is reduced by 5-HT(2C) receptor stimulation and 5-HT(2A) receptor blockade. Neuropharmacology. 2011;61(3):468–77. 273. Bubar MJ, Cunningham KA. Distribution of serotonin 5-HT2C receptors in the ventral tegmental area. Neuroscience. 2007;146(1):286–97. 274. Howell LL, Cunningham KA. Serotonin 5-HT2 receptor interac- tions with dopamine function: implications for therapeutics in cocaine use disorder. Pharmacol Rev. 2015;67(1):176–97. 275. Ichikawa J, Meltzer HY. DOI, a 5-HT2A/2C receptor agonist, potentiates amphetamine-induced dopamine release in rat stria- tum. Brain Res. 1995;698(1–2):204–8. 276. Ichikawa J, Meltzer HY. Amperozide, a novel antipsychotic drug, inhibits the ability of d-amphetamine to increase dopa- mine release in vivo in rat striatum and nucleus accumbens. J Neurochem. 1992;58(6):2285–91. 277. Porras G, Di Matteo V, Fracasso C, Lucas G, De Deurwaerdere P, Caccia S, et al. 5-HT2A and 5-HT2C/2B receptor subtypes modulate dopamine release induced in vivo by amphetamine and morphine in both the rat nucleus accumbens and striatum. Neuropsychopharmacology. 2002;26(3):311–24. 278. Sorensen SM, Kehne JH, Fadayel GM, Humphreys TM, Ketteler HJ, Sullivan CK, et al. Characterization of the 5-HT2 receptor antagonist MDL 100907 as a putative atypical antipsychotic: behavioral, electrophysiological and neurochemical studies. J Pharmacol Exp Ther. 1993;266(2):684–91. 279. Auclair A, Blanc G, Glowinski J, Tassin JP. Role of serotonin 2A receptors in the D-amphetamine-induced release of dopamine: comparison with previous data on alpha1b-adrenergic receptors. J Neurochem. 2004;91(2):318–26. 280. Coleman M. Serotonin concentrations in whole blood of hyperac- tive children. J Pediatr. 1971;78(6):985–90. 281. Sheehan K, Lowe N, Kirley A, Mullins C, Fitzgerald M, Gill M, et al. Tryptophan hydroxylase 2 (TPH2) gene variants associated with ADHD. Mol Psychiatry. 2005;10(10):944–9. 282. Walitza S, Renner TJ, Dempfle A, Konrad K, Wewetzer C, Hal- bach A, et al. Transmission disequilibrium of polymorphic vari- ants in the tryptophan hydroxylase-2 gene in attention-deficit/ hyperactivity disorder. Mol Psychiatry. 2005;10(12):1126–32. 283. Zoroğlu SS, Erdal ME, Alasehirli B, Erdal N, Sivasli E, Tutkun H, et al. Significance of serotonin transporter gene 5-HTTLPR and variable number of tandem repeat polymorphism in atten- tion deficit hyperactivity disorder. Neuropsychobiology. 2002;45(4):176–81. 284. Karmakar A, Maitra S, Chakraborti B, Verma D, Sinha S, Mohanakumar KP, et al. Monoamine oxidase B gene variants associated with attention deficit hyperactivity disorder in the Indo-Caucasoid population from West Bengal. BMC Genet. 2016;17(1):92. 285. Quist JF, Barr CL, Schachar R, Roberts W, Malone M, Tannock R, et al. Evidence for the serotonin HTR2A receptor gene as a susceptibility factor in attention deficit hyperactivity disorder (ADHD). Mol Psychiatry. 2000;5(5):537–41. 286. Quist JF, Barr CL, Schachar R, Roberts W, Malone M, Tannock R, et al. The serotonin 5-HT1B receptor gene and attention deficit hyperactivity disorder. Mol Psychiatry. 2003;8(1):98–102. 287. Hawi Z, Dring M, Kirley A, Foley D, Kent L, Craddock N, et al. Serotonergic system and attention deficit hyperactivity disorder (ADHD): a potential susceptibility locus at the 5-HT(1B) recep- tor gene in 273 nuclear families from a multi-centre sample. Mol Psychiatry. 2002;7(7):718–25. 288. Ribasés M, Ramos-Quiroga JA, Hervas A, Bosch R, Bielsa A, Gastaminza X, et al. Exploration of 19 serotoninergic candidate genes in adults and children with attention-deficit/hyperactivity disorder identifies association for 5HT2A, DDC and MAOB. Mol Psychiatry. 2009;14(1):71–85. 289. Xu X, Brookes K, Sun B, Ilott N, Asherson P. Investigation of the serotonin 2C receptor gene in attention deficit hyperactivity disorder in UK samples. BMC Res Notes. 2009;2:71. 290. Li J, Wang Y, Zhou R, Zhang H, Yang L, Wang B, et al. Associa- tion between polymorphisms in serotonin 2C receptor gene and attention-deficit/hyperactivity disorder in Han Chinese subjects. Neurosci Lett. 2006;407(2):107–11. 291. Neale BM, Lasky-Su J, Anney R, Franke B, Zhou K, Maller JB, et al. Genome-wide association scan of attention deficit hyper- activity disorder. Am J Med Genet B Neuropsychiatr Genet. 2008;147B(8):1337–44. 292. Ebejer JL, Duffy DL, van der Werf J, Wright MJ, Montgomery G, Gillespie NA, et al. Genome-wide association study of inat- tention and hyperactivity-impulsivity measured as quantitative traits. Twin Res Hum Genet. 2013;16(2):560–74. 293. Demontis D, Walters GB, Athanasiadis G, Walters R, Therrien K, Nielsen TT, et al. Genome-wide analyses of ADHD identify 27 risk loci, refine the genetic architecture and implicate several cognitive domains. Nat Genet. 2023;55(2):198–208. 294. Demontis D, Walters RK, Martin J, Mattheisen M, Als TD, Agerbo E, et al. Discovery of the first genome-wide significant 737Serotonin in ADHD: Preclinical and Clinical Insights risk loci for attention deficit/hyperactivity disorder. Nat Genet. 2019;51(1):63–75. 295. Vanderveldt A, Oliveira L, Green L. Delay discounting: Pigeon, rat, human–does it matter? J Exp Psychol Anim Learn Cogn. 2016;42(2):141–62. 296. Akhrif A, Romanos M, Peters K, Furtmann AK, Caspers J, Lesch KP, et al. Serotonergic modulation of normal and abnormal brain dynamics: the genetic influence of the TPH2 G-703T genotype and DNA methylation on wavelet variance in children and ado- lescents with and without ADHD. PLoS ONE. 2023;18(4): e0282813. 297. Latsko MS, Gilman TL, Matt LM, Nylocks KM, Coifman KG, Jasnow AM. A novel interaction between tryptophan hydroxylase 2 (TPH2) gene polymorphism (rs4570625) and BDNF Val66Met predicts a high-risk emotional phenotype in healthy subjects. PLoS ONE. 2016;11(10): e0162585. 298. Zhang Y, Chang Z, Chen J, Ling Y, Liu X, Feng Z, et al. Meth- ylation of the tryptophan hydroxylase-2 gene is associated with mRNA expression in patients with major depression with suicide attempts. Mol Med Rep. 2015;12(2):3184–90. 299. Scheuch K, Lautenschlager M, Grohmann M, Stahlberg S, Kirchheiner J, Zill P, et al. Characterization of a functional promoter polymorphism of the human tryptophan hydroxy- lase 2 gene in serotonergic raphe neurons. Biol Psychiatry. 2007;62(11):1288–94. 300. Dinu LM, Singh SN, Baker NS, Georgescu AL, Overton PG, Dommett EJ. The effects of tryptophan loading on attention defi- cit hyperactivity in adults: a remote double blind randomised controlled trial. PLoS ONE. 2023;18(11): e0294911. 301. Zepf FD, Holtmann M, Stadler C, Demisch L, Schmitt M, Wockel L, et al. Diminished serotonergic functioning in hostile children with ADHD: tryptophan depletion increases behavioural inhibition. Pharmacopsychiatry. 2008;41(2):60–5. 302. LeMarquand DG, Benkelfat C, Pihl RO, Palmour RM, Young SN. Behavioral disinhibition induced by tryptophan depletion in nonalcoholic young men with multigenerational family histories of paternal alcoholism. Am J Psychiatry. 1999;156(11):1771–9. 303. Crean J, Richards JB, de Wit H. Effect of tryptophan depletion on impulsive behavior in men with or without a family history of alcoholism. Behav Brain Res. 2002;136(2):349–57. 304. Dougherty DM, Marsh DM, Mathias CW, Dawes MA, Brad- ley DM, Morgan CJ, et al. The effects of alcohol on laboratory- measured impulsivity after L: -tryptophan depletion or loading. Psychopharmacology. 2007;193(1):137–50. 305. Dougherty DM, Richard DM, James LM, Mathias CW. Effects of acute tryptophan depletion on three different types of behavioral impulsivity. Int J Tryptophan Res. 2010;3:99–111. 306. Walderhaug E, Lunde H, Nordvik JE, Landro NI, Refsum H, Magnusson A. Lowering of serotonin by rapid tryptophan deple- tion increases impulsiveness in normal individuals. Psychophar- macology. 2002;164(4):385–91. 307. Worbe Y, Savulich G, Voon V, Fernandez-Egea E, Robbins TW. Serotonin depletion induces “waiting impulsivity” on the human four-choice serial reaction time task: cross-species translational significance. Neuropsychopharmacology. 2014;39(6):1519–26. 308. Neufang S, Akhrif A, Herrmann CG, Drepper C, Homola GA, Nowak J, et al. Serotonergic modulation of “waiting impulsiv- ity” is mediated by the impulsivity phenotype in humans. Transl Psychiatry. 2016;6(11): e940. 309. LeMarquand DG, Pihl RO, Young SN, Tremblay RE, Seguin JR, Palmour RM, et al. Tryptophan depletion, executive functions, and disinhibition in aggressive, adolescent males. Neuropsychop- harmacology. 1998;19(4):333–41. 310. Dougherty DM, Mullen J, Hill-Kapturczak N, Liang Y, Karns TE, Lake SL, et  al. Effects of tryptophan depletion and a simulated alcohol binge on impulsivity. Exp Clin Psychophar- macol. 2015;23(2):109–21. 311. Schweighofer N, Bertin M, Shishida K, Okamoto Y, Tanaka SC, Yamawaki S, et al. Low-serotonin levels increase delayed reward discounting in humans. J Neurosci. 2008;28(17):4528–32. 312. Silber BY, Schmitt JA. Effects of tryptophan loading on human cognition, mood, and sleep. Neurosci Biobehav Rev. 2010;34(3):387–407. 313. Richard DM, Dawes MA, Mathias CW, Acheson A, Hill- Kapturczak N, Dougherty DM. L-Tryptophan: basic metabolic functions, behavioral research and therapeutic indications. Int J Tryptophan Res. 2009;2:45–60. 314. Cherek DR, Lane SD. Effects of d,l-fenfluramine on aggressive and impulsive responding in adult males with a history of con- duct disorder. Psychopharmacology. 1999;146(4):473–81. 315. Cherek DR, Lane SD. Fenfluramine effects on impulsivity in a sample of adults with and without history of conduct disorder. Psychopharmacology. 2000;152(2):149–56. 316. Skandali N, Rowe JB, Voon V, Deakin JB, Cardinal RN, Cor- mack F, et al. Dissociable effects of acute SSRI (escitalopram) on executive, learning and emotional functions in healthy humans. Neuropsychopharmacology. 2018;43(13):2645–51. 317. Pollak Y, Dekkers TJ, Shoham R, Huizenga HM. Risk-taking behavior in attention deficit/hyperactivity disorder (ADHD): a review of potential underlying mechanisms and of interventions. Curr Psychiatry Rep. 2019;21(5):33. 318. Spiegel T, Pollak Y. Attention deficit/hyperactivity disorder and increased engagement in sexual risk-taking behavior: the role of benefit perception. Front Psychol. 2019;10:1043. 319. Kirby LG, Zeeb FD, Winstanley CA. Contributions of serotonin in addiction vulnerability. Neuropharmacology. 2011;61(3):421–32. 320. Long AB, Kuhn CM, Platt ML. Serotonin shapes risky decision making in monkeys. Soc Cogn Affect Neurosci. 2009;4(4):346–56. 321. Davari-Ashtiani R, Shahrbabaki ME, Razjouyan K, Amini H, Mazhabdar H. Buspirone versus methylphenidate in the treatment of attention deficit hyperactivity disorder: a double-blind and ran- domized trial. Child Psychiatry Hum Dev. 2010;41(6):641–8. 322. Van den Eynde F, Senturk V, Naudts K, Vogels C, Bernagie K, Thas O, et al. Efficacy of quetiapine for impulsivity and affective symptoms in borderline personality disorder. J Clin Psychophar- macol. 2008;28(2):147–55. 323. Maan JS, Ershadi M, Khan I, Saadabadi A. Quetiapine. StatPearls [Internet]. Treasure Island: StatPearls Publishing; 2023. 324. Macoveanu J, Rowe JB, Hornboll B, Elliott R, Paulson OB, Knudsen GM, et al. Serotonin 2A receptors contribute to the regulation of risk-averse decisions. Neuroimage. 2013;83:35–44. 325. Fletcher PJ, Tampakeras M, Sinyard J, Higgins GA. Opposing effects of 5-HT(2A) and 5-HT(2C) receptor antagonists in the rat and mouse on premature responding in the five-choice serial reaction time test. Psychopharmacology. 2007;195(2):223–34. 326. Persons AL, Tedford SE, Celeste T. Mirtazapine and ketan- serin alter preference for gambling-like schedules of reinforce- ment in rats. Prog Neuropsychopharmacol Biol Psychiatry. 2017;77:178–84. 327. Beheshti A, Chavanon ML, Christiansen H. Emotion dysregu- lation in adults with attention deficit hyperactivity disorder: a meta-analysis. BMC Psychiatry. 2020;20(1):120. 328. Graziano PA, Garcia A. Attention-deficit hyperactivity disorder and children’s emotion dysregulation: a meta-analysis. Clin Psy- chol Rev. 2016;46:106–23. 329. Bunford N, Evans SW, Wymbs F. ADHD and emotion dysregu- lation among children and adolescents. Clin Child Fam Psychol Rev. 2015;18(3):185–217. 738 M. B. Solomon et al. 330. Faraone SV, Rostain AL, Blader J, Busch B, Childress AC, Con- nor DF, et al. Practitioner Review: emotional dysregulation in attention-deficit/hyperactivity disorder—implications for clini- cal recognition and intervention. J Child Psychol Psychiatry. 2019;60(2):133–50. 331. Kötting WF, Bubenzer S, Helmbold K, Eisert A, Gaber TJ, Zepf FD. Effects of tryptophan depletion on reactive aggression and aggressive decision-making in young people with ADHD. Acta Psychiatr Scand. 2013;128(2):114–23. 332. Stadler C, Zepf FD, Demisch L, Schmitt M, Landgraf M, Poustka F. Influence of rapid tryptophan depletion on laboratory-pro- voked aggression in children with ADHD. Neuropsychobiology. 2007;56(2–3):104–10. 333. von Polier GG, Biskup CS, Kotting WF, Bubenzer S, Helmbold K, Eisert A, et al. Change in electrodermal activity after acute tryptophan depletion associated with aggression in young people with attention deficit hyperactivity disorder (ADHD). J Neural Transm (Vienna). 2014;121(4):451–5. 334. Zepf FD, Stadler C, Demisch L, Schmitt M, Landgraf M, Poustka F. Serotonergic functioning and trait-impulsivity in attention- deficit/hyperactivity-disordered boys (ADHD): influence of rapid tryptophan depletion. Hum Psychopharmacol. 2008;23(1):43–51. 335. Zimmermann M, Grabemann M, Mette C, Abdel-Hamid M, Uekermann J, Kraemer M, et al. The effects of acute tryptophan depletion on reactive aggression in adults with attention-deficit/ hyperactivity disorder (ADHD) and healthy controls. PLoS ONE. 2012;7(3): e32023. 336. Moeller FG, Dougherty DM, Swann AC, Collins D, Davis CM, Cherek DR. Tryptophan depletion and aggressive responding in healthy males. Psychopharmacology. 1996;126(2):97–103. 337. Bjork JM, Dougherty DM, Moeller FG, Swann AC. Differential behavioral effects of plasma tryptophan depletion and loading in aggressive and nonaggressive men. Neuropsychopharmacology. 2000;22(4):357–69. 338. Bjork JM, Dougherty DM, Moeller FG, Cherek DR, Swann AC. The effects of tryptophan depletion and loading on laboratory aggression in men: time course and a food-restricted control. Psychopharmacology. 1999;142(1):24–30. 339. Cleare AJ, Bond AJ. The effect of tryptophan depletion and enhancement on subjective and behavioural aggression in normal male subjects. Psychopharmacology. 1995;118(1):72–81. 340. Marsh DM, Dougherty DM, Moeller FG, Swann AC, Spiga R. Laboratory-measured aggressive behavior of women: acute tryp- tophan depletion and augmentation. Neuropsychopharmacology. 2002;26(5):660–71. 341. Cherek DR, Lane SD, Pietras CJ, Steinberg JL. Effects of chronic paroxetine administration on measures of aggressive and impul- sive responses of adult males with a history of conduct disorder. Psychopharmacology. 2002;159(3):266–74. 342. Coccaro EF, Kavoussi RJ. Fluoxetine and impulsive aggressive behavior in personality-disordered subjects. Arch Gen Psychia- try. 1997;54(12):1081–8. 343. Hakulinen C, Jokela M, Hintsanen M, Merjonen P, Pulkki- Raback L, Seppala I, et al. Serotonin receptor 1B genotype and hostility, anger and aggressive behavior through the lifespan: the Young Finns study. J Behav Med. 2013;36(6):583–90. 344. Gowin JL, Swann AC, Moeller FG, Lane SD. Zolmitriptan and human aggression: interaction with alcohol. Psychopharmacol- ogy. 2010;210(4):521–31. 345. De Almeida RM, Rosa MM, Santos DM, Saft DM, Benini Q, Miczek KA. 5-HT(1B) receptors, ventral orbitofrontal cor- tex, and aggressive behavior in mice. Psychopharmacology. 2006;185(4):441–50. 346. Blair RJ. The roles of orbital frontal cortex in the modulation of antisocial behavior. Brain Cogn. 2004;55(1):198–208. 347. Banlaki Z, Elek Z, Nanasi T, Szekely A, Nemoda Z, Sasvari- Szekely M, et al. Polymorphism in the serotonin receptor 2a (HTR2A) gene as possible predisposal factor for aggressive traits. PLoS ONE. 2015;10(2): e0117792. 348. Nomura M, Kusumi I, Kaneko M, Masui T, Daiguji M, Ueno T, et  al. Involvement of a polymorphism in the 5-HT2A receptor gene in impulsive behavior. Psychopharmacology. 2006;187(1):30–5. 349. Toshchakova VA, Bakhtiari Y, Kulikov AV, Gusev SI, Trofimova MV, Fedorenko OY, et al. Association of polymorphisms of sero- tonin transporter (5HTTLPR) and 5-HT2C receptor genes with criminal behavior in Russian criminal offenders. Neuropsycho- biology. 2017;75(4):200–10. 350. van Hemert JC. Pipamperone (Dipiperon, R3345) in trouble- some mental retardates: a double-blind placebo controlled cross-over study with long-term follow-up. Acta Psychiatr Scand. 1975;52(4):237–45. 351. Coccaro EF, Lee RJ. 5-HT(2c) agonist, lorcaserin, reduces aggressive responding in intermittent explosive disorder: a pilot study. Hum Psychopharmacol. 2019;34(6): e2714. 352. Zepf FD, Gaber TJ, Baurmann D, Bubenzer S, Konrad K, Her- pertz-Dahlmann B, et al. Serotonergic neurotransmission and lapses of attention in children and adolescents with attention deficit hyperactivity disorder: availability of tryptophan influ- ences attentional performance. Int J Neuropsychopharmacol. 2010;13(7):933–41. 353. Mette C, Zimmermann M, Grabemann M, Abdel-Hamid M, Uekermann J, Biskup CS, et al. The impact of acute trypto- phan depletion on attentional performance in adult patients with ADHD. Acta Psychiatr Scand. 2013;128(2):124–32. 354. Mendelsohn D, Riedel WJ, Sambeth A. Effects of acute trypto- phan depletion on memory, attention and executive functions: a systematic review. Neurosci Biobehav Rev. 2009;33(6):926–52. 355. Kanen JW, Apergis-Schoute AM, Yellowlees R, Arntz FE, van der Flier FE, Price A, et al. Serotonin depletion impairs both Pavlovian and instrumental reversal learning in healthy humans. Mol Psychiatry. 2021;26(12):7200–10. 356. Kanen JW, Arntz FE, Yellowlees R, Cardinal RN, Price A, Christmas DM, et al. Probabilistic reversal learning under acute tryptophan depletion in healthy humans: a conventional analysis. J Psychopharmacol. 2020;34(5):580–3. 357. Knorr U, Madsen JM, Kessing LV. The effect of selective serotonin reuptake inhibitors in healthy subjects revisited: a systematic review of the literature. Exp Clin Psychopharmacol. 2019;27(5):413–32. 358. Wingen M, Kuypers KP, Ramaekers JG. The role of 5-HT1a and 5-HT2a receptors in attention and motor control: a mech- anistic study in healthy volunteers. Psychopharmacology. 2007;190(3):391–400. 359. Nash JF. Ketanserin pretreatment attenuates MDMA-induced dopamine release in the striatum as measured by in vivo micro- dialysis. Life Sci. 1990;47(26):2401–8. 360. Broderick PA, Olabisi OA, Rahni DN, Zhou Y. Cocaine acts on accumbens monoamines and locomotor behavior via a 5-HT2A/2C receptor mechanism as shown by ketanserin: 24-h follow-up studies. Prog Neuropsychopharmacol Biol Psychia- try. 2004;28(3):547–57. 361. Pehek EA, McFarlane HG, Maguschak K, Price B, Pluto CP. M100,907, a selective 5-HT(2A) antagonist, attenuates dopa- mine release in the rat medial prefrontal cortex. Brain Res. 2001;888(1):51–9. 362. Volkow ND, Wang GJ, Kollins SH, Wigal TL, Newcorn JH, Telang F, et al. Evaluating dopamine reward pathway in ADHD: clinical implications. JAMA. 2009;302(10):1084–91. 739Serotonin in ADHD: Preclinical and Clinical Insights 363. Luo LL, Chen X, Chai Y, Li JH, Zhang M, Zhang JN. A distinct pattern of memory and attention deficiency in patients with depression. Chin Med J (Engl). 2013;126(6):1144–9. 364. Shilyansky C, Williams LM, Gyurak A, Harris A, Usherwood T, Etkin A. Effect of antidepressant treatment on cognitive impairments associated with depression: a randomised longi- tudinal study. Lancet Psychiatry. 2016;3(5):425–35. 365. Gyurak A, Patenaude B, Korgaonkar MS, Grieve SM, Wil- liams LM, Etkin A. Frontoparietal activation during response inhibition predicts remission to antidepressants in patients with major depression. Biol Psychiatry. 2016;79(4):274–81. 366. Langley C, Armand S, Luo Q, Savulich G, Segerberg T, Son- dergaard A, et al. Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: a double- blind, placebo-controlled semi-randomised study. Neuropsy- chopharmacology. 2023;48(4):664–70. 367. McIntyre RS, Florea I, Tonnoir B, Loft H, Lam RW, Chris- tensen MC. Efficacy of vortioxetine on cognitive functioning in working patients with major depressive disorder. J Clin Psy- chiatry. 2017;78(1):115–21. 368. Mahableshwarkar AR, Zajecka J, Jacobson W, Chen Y, Keefe RS. A randomized, placebo-controlled, active-reference, dou- ble-blind, flexible-dose study of the efficacy of vortioxetine on cognitive function in major depressive disorder. Neuropsychop- harmacology. 2015;40(8):2025–37. 369. Bymaster FP, Katner JS, Nelson DL, Hemrick-Luecke SK, Threlkeld PG, Heiligenstein JH, et al. Atomoxetine increases extracellular levels of norepinephrine and dopamine in prefron- tal cortex of rat: a potential mechanism for efficacy in atten- tion deficit/hyperactivity disorder. Neuropsychopharmacology. 2002;27(5):699–711. 370. Utevsky AV, Smith DV, Huettel SA. Precuneus is a func- tional core of the default-mode network. J Neurosci. 2014;34(3):932–40. 371. Alves PN, Foulon C, Karolis V, Bzdok D, Margulies DS, Volle E, et al. An improved neuroanatomical model of the default-mode network reconciles previous neuroimaging and neuropathological findings. Commun Biol. 2019;2:370. 372. Thome J, Ehlis AC, Fallgatter AJ, Krauel K, Lange KW, Riederer P, et al. Biomarkers for attention-deficit/hyperactivity disorder (ADHD). A consensus report of the WFSBP task force on bio- logical markers and the World Federation of ADHD. World J Biol Psychiatry. 2012;13(5):379–400. 373. Chen H, Yang Y, Odisho D, Wu S, Yi C, Oliver BG. Can bio- markers be used to diagnose attention deficit hyperactivity dis- order? Front Psychiatry. 2023;14:1026616. 374. Sonuga-Barke EJ, Castellanos FX. Spontaneous attentional f luctuations in impaired states and pathological condi- tions: a neurobiological hypothesis. Neurosci Biobehav Rev. 2007;31(7):977–86. 375. Duffy KA, Rosch KS, Nebel MB, Seymour KE, Lindquist MA, Pekar JJ, et al. Increased integration between default mode and task-relevant networks in children with ADHD is associated with impaired response control. Dev Cogn Neurosci. 2021;50: 100980. 376. Silberstein RB, Pipingas A, Farrow M, Levy F, Stough CK. Dopaminergic modulation of default mode network brain func- tional connectivity in attention deficit hyperactivity disorder. Brain Behav. 2016;6(12): e00582. 377. Peterson BS, Potenza MN, Wang Z, Zhu H, Martin A, Marsh R, et al. An FMRI study of the effects of psychostimulants on default-mode processing during Stroop task performance in youths with ADHD. Am J Psychiatry. 2009;166(11):1286–94. 378. Querne L, Fall S, Le Moing AG, Bourel-Ponchel E, Delignieres A, Simonnot A, et al. Effects of methylphenidate on default-mode network/task-positive network synchronization in children with ADHD. J Atten Disord. 2017;21(14):1208–20. 379. Kautzky A, Vanicek T, Philippe C, Kranz GS, Wadsak W, Mit- terhauser M, et al. Machine learning classification of ADHD and HC by multimodal serotonergic data. Transl Psychiatry. 2020;10(1):104. 380. Biskup CS, Helmbold K, Baurmann D, Klasen M, Gaber TJ, Bubenzer-Busch S, et al. Resting state default mode network con- nectivity in children and adolescents with ADHD after acute tryp- tophan depletion. Acta Psychiatr Scand. 2016;134(2):161–71. 381. Helmbold K, Zvyagintsev M, Dahmen B, Biskup CS, Bubenzer- Busch S, Gaber TJ, et al. Serotonergic modulation of resting state default mode network connectivity in healthy women. Amino Acids. 2016;48(4):1109–20. 382. Kunisato Y, Okamoto Y, Okada G, Aoyama S, Demoto Y, Munakata A, et al. Modulation of default-mode network activity by acute tryptophan depletion is associated with mood change: a resting state functional magnetic resonance imaging study. Neu- rosci Res. 2011;69(2):129–34. 383. van de Ven V, Wingen M, Kuypers KP, Ramaekers JG, Formis- ano E. Escitalopram decreases cross-regional functional connec- tivity within the default-mode network. PLoS ONE. 2013;8(6): e68355. 384. van Wingen GA, Tendolkar I, Urner M, van Marle HJ, Denys D, Verkes RJ, et al. Short-term antidepressant administration reduces default mode and task-positive network connectivity in healthy individuals during rest. Neuroimage. 2014;88:47–53. 385. Klaassens BL, Rombouts SA, Winkler AM, van Gorsel HC, van der Grond J, van Gerven JM. Time related effects on functional brain connectivity after serotonergic and cholinergic neuromodu- lation. Hum Brain Mapp. 2017;38(1):308–25. 386. Klaassens BL, van Gorsel HC, Khalili-Mahani N, van der Grond J, Wyman BT, Whitcher B, et al. Single-dose serotonergic stimu- lation shows widespread effects on functional brain connectivity. Neuroimage. 2015;122:440–50. 387. Chantiluke K, Barrett N, Giampietro V, Santosh P, Brammer M, Simmons A, et al. Inverse fluoxetine effects on inhibitory brain activation in non-comorbid boys with ADHD and with ASD. Psychopharmacology. 2015;232(12):2071–82. 388. Mulder EJ, Anderson GM, Kema IP, de Bildt A, van Lang ND, den Boer JA, et al. Platelet serotonin levels in pervasive devel- opmental disorders and mental retardation: diagnostic group dif- ferences, within-group distribution, and behavioral correlates. J Am Acad Child Adolesc Psychiatry. 2004;43(4):491–9. 389. Piven J, Tsai GC, Nehme E, Coyle JT, Chase GA, Folstein SE. Platelet serotonin, a possible marker for familial autism. J Autism Dev Disord. 1991;21(1):51–9. 390. Spivak B, Vered Y, Yoran-Hegesh R, Averbuch E, Mester R, Graf E, et al. Circulatory levels of catecholamines, serotonin and lipids in attention deficit hyperactivity disorder. Acta Psychiatr Scand. 1999;99(4):300–4. 391. Holck A, Wolkowitz OM, Mellon SH, Reus VI, Nelson JC, Westrin A, et al. Plasma serotonin levels are associated with anti- depressant response to SSRIs. J Affect Disord. 2019;250:65–70. 392. New AS, Hazlett EA, Buchsbaum MS, Goodman M, Reynolds D, Mitropoulou V, et al. Blunted prefrontal cortical 18fluorodeoxy- glucose positron emission tomography response to meta-chloro- phenylpiperazine in impulsive aggression. Arch Gen Psychiatry. 2002;59(7):621–9. 393. Siever LJ, Buchsbaum MS, New AS, Spiegel-Cohen J, Wei T, Hazlett EA, et al. d,l-fenfluramine response in impulsive personality disorder assessed with [18F]fluorodeoxyglucose positron emission tomography. Neuropsychopharmacology. 1999;20(5):413–23. 394. Frankle WG, Lombardo I, New AS, Goodman M, Talbot PS, Huang Y, et  al. Brain serotonin transporter distribution in 740 M. B. Solomon et al. subjects with impulsive aggressivity: a positron emission study with [11C]McN 5652. Am J Psychiatry. 2005;162(5):915–23. 395. Aznar S, Hervig MS. The 5-HT2A serotonin receptor in execu- tive function: implications for neuropsychiatric and neurodegen- erative diseases. Neurosci Biobehav Rev. 2016;64:63–82. 396. Sargin D, Jeoung HS, Goodfellow NM, Lambe EK. Serotonin regulation of the prefrontal cortex: cognitive relevance and the impact of developmental perturbation. ACS Chem Neurosci. 2019;10(7):3078–93. 397. Brummelte S, Mc Glanaghy E, Bonnin A, Oberlander TF. Developmental changes in serotonin signaling: implications for early brain function, behavior and adaptation. Neuroscience. 2017;342:212–31. 398. Smoller JW, Biederman J, Arbeitman L, Doyle AE, Fagerness J, Perlis RH, et al. Association between the 5HT1B receptor gene (HTR1B) and the inattentive subtype of ADHD. Biol Psychiatry. 2006;59(5):460–7. 399. van der Meer D, Hartman CA, Richards J, Bralten JB, Franke B, Oosterlaan J, et al. The serotonin transporter gene polymor- phism 5-HTTLPR moderates the effects of stress on attention- deficit/hyperactivity disorder. J Child Psychol Psychiatry. 2014;55(12):1363–71. 400. Cadoret RJ, Langbehn D, Caspers K, Troughton EP, Yucuis R, Sandhu HK, et al. Associations of the serotonin transporter promoter polymorphism with aggressivity, attention deficit, and conduct disorder in an adoptee population. Compr Psychiatry. 2003;44(2):88–101. 401. Walderhaug E, Magnusson A, Neumeister A, Lappalainen J, Lunde H, Refsum H, et  al. Interactive effects of sex and 5-HTTLPR on mood and impulsivity during tryptophan deple- tion in healthy people. Biol Psychiatry. 2007;62(6):593–9. 402. Riley TB, Overton PG. Enhancing the efficacy of 5-HT uptake inhibitors in the treatment of attention deficit hyperactivity dis- order. Med Hypotheses. 2019;133: 109407. 403. Manor I, Laiba E, Eisenberg J, Meidad S, Lerer E, Israel S, et al. Association between tryptophan hydroxylase 2, performance on a continuance performance test and response to methylphenidate in ADHD participants. Am J Med Genet B Neuropsychiatr Genet. 2008;147B(8):1501–8. 404. Thakur GA, Grizenko N, Sengupta SM, Schmitz N, Joober R. The 5-HTTLPR polymorphism of the serotonin transporter gene and short term behavioral response to methylphenidate in chil- dren with ADHD. BMC Psychiatry. 2010;10:50. 405. Gammon GD, Brown TE. Fluoxetine and methylphenidate in combination for treatment of attention deficit disorder and comorbid depressive disorder. J Child Adolesc Psychopharmacol. 1993;3(1):1–10. 406. Budur K, Mathews M, Adetunji B, Mathews M, Mahmud J. Non- stimulant treatment for attention deficit hyperactivity disorder. Psychiatry (Edgmont). 2005;2(7):44–8. 407. Koblan KS, Hopkins SC, Sarma K, Jin F, Goldman R, Kol- lins SH, et al. Dasotraline for the treatment of attention-deficit/ hyperactivity disorder: a randomized, double-blind, placebo- controlled, proof-of-concept trial in adults. Neuropsychophar- macology. 2015;40(12):2745–52. 408. Adler LA, Adams J, Madera-McDonough J, Kohegyi E, Hobart M, Chang D, et al. Efficacy, safety, and tolerability of centana- fadine sustained-release tablets in adults with attention-deficit/ hyperactivity disorder: results of 2 phase 3, randomized, double- blind, multicenter, placebo-controlled trials. J Clin Psychophar- macol. 2022;42(5):429–39. 409. Schreiber R, Campbell U, Quinton MS, Hardy LW, Fang QK, Lew R. In vitro and in vivo pharmacological characterization of dasotraline, a dual dopamine and norepinephrine transporter inhibitor in vivo. Biomed Pharmacother. 2022;153: 113359. 410. Matuskey D, Gallezot JD, Nabulsi N, Henry S, Torres K, Dias M, et al. Neurotransmitter transporter occupancy following adminis- tration of centanafadine sustained-release tablets: a phase 1 study in healthy male adults. J Psychopharmacol. 2023;37(2):164–71. 411. Aldosary F, Norris S, Tremblay P, James JS, Ritchie JC, Blier P. Differential potency of venlafaxine, paroxetine, and atomox- etine to inhibit serotonin and norepinephrine reuptake in patients with major depressive disorder. Int J Neuropsychopharmacol. 2022;25(4):283–92. 412. Ding YS, Naganawa M, Gallezot JD, Nabulsi N, Lin SF, Ropchan J, et al. Clinical doses of atomoxetine significantly occupy both norepinephrine and serotonin transports: implications on treat- ment of depression and ADHD. Neuroimage. 2014;86:164–71. 413. Yu C, Garcia-Olivares J, Candler S, Schwabe S, Maletic V. New insights into the mechanism of action of viloxazine: serotonin and norepinephrine modulating properties. J Exp Pharmacol. 2020;12:285–300. 414. Garcia-Olivares J, Yegla B, Bymaster FP, Earnest J, Koch J, Yu C, et al. Viloxazine increases extracellular concentrations of nor- epinephrine, dopamine, and serotonin in the rat prefrontal cortex at doses relevant for the treatment of attention-deficit/hyperactiv- ity disorder. J Exp Pharmacol. 2024;16:13–24. 415. Tikkanen R, Tiihonen J, Rautiainen MR, Paunio T, Bevilacqua L, Panarsky R, et al. Impulsive alcohol-related risk-behavior and emotional dysregulation among individuals with a serotonin 2B receptor stop codon. Transl Psychiatry. 2015;5(11): e681. 416. Diaz SL, Doly S, Narboux-Neme N, Fernandez S, Mazot P, Banas SM, et al. 5-HT(2B) receptors are required for serotonin-selective antidepressant actions. Mol Psychiatry. 2012;17(2):154–63. 417. Price MZ, Price RL. Extended-release viloxazine compared with atomoxetine for attention deficit hyperactivity disorder. CNS Drugs. 2023;37(7):655–60. 418. Biederman J, Lindsten A, Sluth LB, Petersen ML, Ettrup A, Erik- sen HF, et al. Vortioxetine for attention deficit hyperactivity dis- order in adults: a randomized, double-blind, placebo-controlled, proof-of-concept study. J Psychopharmacol. 2019;33(4):511–21. 419. Stahl SM. Modes and nodes explain the mechanism of action of vortioxetine, a multimodal agent (MMA): actions at serotonin receptors may enhance downstream release of four pro-cognitive neurotransmitters. CNS Spectr. 2015;20(6):515–9. 420. Bymaster FP, Zhang W, Carter PA, Shaw J, Chernet E, Phebus L, et al. Fluoxetine, but not other selective serotonin uptake inhibi- tors, increases norepinephrine and dopamine extracellular levels in prefrontal cortex. Psychopharmacology. 2002;160(4):353–61. 421. Sobanski E, Bruggemann D, Alm B, Kern S, Deschner M, Schu- bert T, et al. Psychiatric comorbidity and functional impairment in a clinically referred sample of adults with attention-deficit/ hyperactivity disorder (ADHD). Eur Arch Psychiatry Clin Neu- rosci. 2007;257(7):371–7. 422. Daviss WB. A review of co-morbid depression in pediatric ADHD: etiology, phenomenology, and treatment. J Child Ado- lesc Psychopharmacol. 2008;18(6):565–71. 423. Danielson ML, Bitsko RH, Ghandour RM, Holbrook JR, Kogan MD, Blumberg SJ. Prevalence of parent-reported ADHD diagno- sis and associated treatment among US children and adolescents, 2016. J Clin Child Adolesc Psychol. 2018;47(2):199–212. 424. Katzman MA, Bilkey TS, Chokka PR, Fallu A, Klassen LJ. Adult ADHD and comorbid disorders: clinical implications of a dimen- sional approach. BMC Psychiatry. 2017;17(1):302. 425. Larson K, Russ SA, Kahn RS, Halfon N. Patterns of comorbidity, functioning, and service use for US children with ADHD, 2007. Pediatrics. 2011;127(3):462–70. 426. Ribasés M, Mitjans M, Hartman CA, Soler Artigas M, Demontis D, Larsson H, et al. Genetic architecture of ADHD and overlap with other psychiatric disorders and cognition-related pheno- types. Neurosci Biobehav Rev. 2023;153: 105313. 741Serotonin in ADHD: Preclinical and Clinical Insights 427. Sun S, Kuja-Halkola R, Faraone SV, D’Onofrio BM, Dalsgaard S, Chang Z, et  al. Association of psychiatric comorbidity with the risk of premature death among children and adults with attention-deficit/hyperactivity Disorder. JAMA Psychiat. 2019;76(11):1141–9. 428. Jakubovski E, Johnson JA, Nasir M, Muller-Vahl K, Bloch MH. Systematic review and meta-analysis: dose-response curve of SSRIs and SNRIs in anxiety disorders. Depress Anxiety. 2019;36(3):198–212. 429. Zhou X, Teng T, Zhang Y, Del Giovane C, Furukawa TA, Weisz JR, et al. Comparative efficacy and acceptability of antidepres- sants, psychotherapies, and their combination for acute treat- ment of children and adolescents with depressive disorder: a systematic review and network meta-analysis. Lancet Psychiatry. 2020;7(7):581–601. 430. Cipriani A, Furukawa TA, Salanti G, Chaimani A, Atkinson LZ, Ogawa Y, et al. Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-anal- ysis. Lancet. 2018;391(10128):1357–66. 431. Surman CBH, Walsh DM. Do treatments for adult ADHD improve emotional behavior? A systematic review and analysis. J Atten Disord. 2022;26(14):1822–32. 432. Towbin K, Vidal-Ribas P, Brotman MA, Pickles A, Miller KV, Kaiser A, et al. A double-blind randomized placebo-controlled trial of citalopram adjunctive to stimulant medication in youth with chronic severe irritability. J Am Acad Child Adolesc Psy- chiatry. 2020;59(3):350–61. 433. Childress AC. A critical appraisal of atomoxetine in the manage- ment of ADHD. Ther Clin Risk Manag. 2016;12:27–39. 434. Michelson D, Adler LA, Amsterdam JD, Dunner DL, Nier- enberg AA, Reimherr FW, et al. Addition of atomoxetine for depression incompletely responsive to sertraline: a randomized, double-blind, placebo-controlled study. J Clin Psychiatry. 2007;68(4):582–7. 435. Kratochvil CJ, Newcorn JH, Arnold LE, Duesenberg D, Emslie GJ, Quintana H, et al. Atomoxetine alone or combined with fluoxetine for treating ADHD with comorbid depressive or anxiety symptoms. J Am Acad Child Adolesc Psychiatry. 2005;44(9):915–24. 436. Findling RL, Candler SA, Nasser AF, Schwabe S, Yu C, Gar- cia-Olivares J, et al. Viloxazine in the management of CNS disorders: a historical overview and current status. CNS Drugs. 2021;35(6):643–53. 437. Adler L, Lieberman V, Yarullina I, Brijbasi L, Rubin J. Viloxa- zine ER for adults with attention-deficit/hyperactivity disorder and mood symptoms: results of a decentralized, open-label, phase IV trial. Poster presented at American Psychiatric Asso- ciation Annual Meeting; May 17-21, 2025; Los Angeles, CA. 438. Stahl SM. The serotonin-7 receptor as a novel therapeutic target. J Clin Psychiatry. 2010;71(11):1414–5. 439. Quintero-Villegas A, Valdes-Ferrer SI. Central nervous system effects of 5-HT(7) receptors: a potential target for neurodegenera- tive diseases. Mol Med. 2022;28(1):70. 440. Leiser SC, Pehrson AL, Robichaud PJ, Sanchez C. Multimodal antidepressant vortioxetine increases frontal cortical oscillations unlike escitalopram and duloxetine—a quantitative EEG study in rats. Br J Pharmacol. 2014;171(18):4255–72. 441. Di Giovanni G, Di Matteo V, Di Mascio M, Esposito E. Prefer- ential modulation of mesolimbic vs. nigrostriatal dopaminergic function by serotonin(2C/2B) receptor agonists: a combined in vivo electrophysiological and microdialysis study. Synapse. 2000;35(1):53–61. 442. Di Matteo V, Di Giovanni G, Di Mascio M, Esposito E. Biochem- ical and electrophysiological evidence that RO 60-0175 inhibits mesolimbic dopaminergic function through serotonin(2C) recep- tors. Brain Res. 2000;865(1):85–90. 443. Gobert A, Rivet JM, Lejeune F, Newman-Tancredi A, Adhumeau- Auclair A, Nicolas JP, et al. Serotonin(2C) receptors tonically suppress the activity of mesocortical dopaminergic and adrener- gic, but not serotonergic, pathways: a combined dialysis and elec- trophysiological analysis in the rat. Synapse. 2000;36(3):205–21. 444. Millan MJ, Lejeune F, Gobert A. Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents. J Psychopharmacol. 2000;14(2):114–38. 445. Pessia M, Jiang ZG, North RA, Johnson SW. Actions of 5-hydroxytryptamine on ventral tegmental area neurons of the rat in vitro. Brain Res. 1994;654(2):324–30. 446. Ugedo L, Grenhoff J, Svensson TH. Ritanserin, a 5-HT2 receptor antagonist, activates midbrain dopamine neurons by blocking serotonergic inhibition. Psychopharmacology. 1989;98(1):45–50. 447. Di Mascio M, Di Giovanni G, Di Matteo V, Prisco S, Esposito E. Selective serotonin reuptake inhibitors reduce the spontaneous activity of dopaminergic neurons in the ventral tegmental area. Brain Res Bull. 1998;46(6):547–54. 448. Prisco S, Esposito E. Differential effects of acute and chronic fluoxetine administration on the spontaneous activity of dopa- minergic neurones in the ventral tegmental area. Br J Pharmacol. 1995;116(2):1923–31. 449. Guiard BP, El Mansari M, Merali Z, Blier P. Functional interac- tions between dopamine, serotonin and norepinephrine neurons: an in-vivo electrophysiological study in rats with monoaminergic lesions. Int J Neuropsychopharmacol. 2008;11(5):625–39. 450. Arborelius L, Chergui K, Murase S, Nomikos GG, Höök BB, Chouvet G, et al. The 5-HT1A receptor selective ligands, (R)- 8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons. Naunyn Schmiedebergs Arch Pharmacol. 1993;347(4):353–62. 451. Chen NH, Reith ME. Monoamine interactions measured by microdialysis in the ventral tegmental area of rats treated sys- temically with (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin. J Neurochem. 1995;64(4):1585–97. 452. Lejeune F, Millan MJ. Induction of burst firing in ventral teg- mental area dopaminergic neurons by activation of serotonin (5-HT)1A receptors: WAY 100,635-reversible actions of the highly selective ligands, flesinoxan and S 15535. Synapse. 1998;30(2):172–80. 453. Lejeune F, Newman-Tancredi A, Audinot V, Millan MJ. Interac- tions of (+)- and (-)-8- and 7-hydroxy-2-(di-n-propylamino)tetra- lin at human (h)D3, hD2 and h serotonin1A receptors and their modulation of the activity of serotoninergic and dopaminergic neurones in rats. J Pharmacol Exp Ther. 1997;280(3):1241–9. 454. Prisco S, Pagannone S, Esposito E. Serotonin-dopamine inter- action in the rat ventral tegmental area: an electrophysiological study in vivo. J Pharmacol Exp Ther. 1994;271(1):83–90. 455. Rasmusson AM, Goldstein LE, Deutch AY, Bunney BS, Roth RH. 5-HT1a agonist +/-8-OH-DPAT modulates basal and stress- induced changes in medial prefrontal cortical dopamine. Syn- apse. 1994;18(3):218–24. 456. Tanda G, Carboni E, Frau R, Di Chiara G. Increase of extracellu- lar dopamine in the prefrontal cortex: a trait of drugs with antide- pressant potential? Psychopharmacology. 1994;115(1–2):285–8. 457. Schechter LE, Lin Q, Smith DL, Zhang G, Shan Q, Platt B, et al. Neuropharmacological profile of novel and selective 5-HT6 receptor agonists: WAY-181187 and WAY-208466. Neuropsy- chopharmacology. 2008;33(6):1323–35. 458. Valentini V, Frau R, Bordi F, Borsini F, Di Chiara G. A micro- dialysis study of ST1936, a novel 5-HT6 receptor agonist. Neu- ropharmacology. 2011;60(4):602–8. 742 M. B. Solomon et al. 459. Bonaventure P, Aluisio L, Shoblock J, Boggs JD, Fraser IC, Lord B, et al. Pharmacological blockade of serotonin 5-HT(7) receptor reverses working memory deficits in rats by normalizing cortical glutamate neurotransmission. PLoS ONE. 2011;6(6): e20210. 460. Mnie-Filali O, Dahan L, Zimmer L, Haddjeri N. Effects of the serotonin 5-HT(7) receptor antagonist SB-269970 on the inhibi- tion of dopamine neuronal firing induced by amphetamine. Eur J Pharmacol. 2007;570(1–3):72–6. 461. Moser PC, Moran PM, Frank RA, Kehne JH. Reversal of amphet- amine-induced behaviours by MDL 100,907, a selective 5-HT2A antagonist. Behav Brain Res. 1996;73(1–2):163–7. Revisiting the Role of Serotonin in Attention-Deficit Hyperactivity Disorder: New Insights from Preclinical and Clinical Studies Abstract 1 Introduction 2 Serotonin (5-HT) 3 Serotonergic Mechanisms in Preclinical Studies Relevant to ADHD 3.1 Role of 5-HT in Hyperactivity 3.1.1 6-OHDA-Lesioned Model 3.1.2 SHR Model 3.1.3 DAT KO Mouse Model 3.2 Role of 5-HT in Impulsive Action and Impulsive Choice 3.2.1 Role of 5-HT in Impulsive Action 3.2.2 Role of 5-HT in Impulsive Choice 3.2.3 Role of 5-HT in Action Cancellation 3.3 Role of 5-HT in Emotional DysregulationImpulsive Aggression 3.3.1 Role of 5-HT Receptors in Impulsive Aggression 3.3.2 Serotonin and Dopamine Interactions in Impulsive Aggression 3.4 Role of 5-HT in Executive Function 3.4.1 Role of 5-HT in Attention 3.4.2 Role of 5-HT in Cognitive Flexibility 4 Stimulant-Mediated Effects on the Serotonergic System in Preclinical Models 4.1 Stimulant Effects on DA and NE 4.2 Stimulant Effects on 5-HT 5 Does Serotonergic Manipulation Affect Stimulant Efficacy in ADHD-Relevant Preclinical Models? 5.1 Serotonin Modulation of Stimulant Effects on Hyperactivity 5.2 Serotonin Modulation of Stimulant Effects on Impulsivity 6 Clinical Studies 6.1 Role of 5-HT in Impulsivity in ADHD and Non-ADHD Populations 6.1.1 Tryptophan Studies 6.1.2 Serotonin Receptor Subtypes 6.2 Role of 5-HT in Emotional Dysregulation in ADHD and Non-ADHD Populations 6.2.1 Tryptophan Studies 6.2.2 Serotonin Receptor Subtypes 6.3 Role of 5-HT in Attention in ADHD and Non-ADHD Populations 6.3.1 Tryptophan Studies 6.3.2 Serotonin Receptor Subtypes 6.3.3 SSRIs and Vortioxetine 7 Role of 5-HT on Brain Networks in ADHD and Non-ADHD Populations 7.1 Tryptophan Studies and SSRIs 7.2 Role of 5-HT on ADHD-Related Brain Networks Relevant to Impulsivity 7.3 Role of 5-HT on ADHD-Related Brain Networks Relevant to Emotional Dysregulation 7.4 Role of 5-HT on ADHD-Related Brain Networks Relevant to Attention 8 Does 5-HT Play a Role in ADHD Treatment? 8.1 Role of 5-HT in Nonstimulant-Mediated Effects 8.2 Role of 5-HT in ADHD Comorbidities 9 Conclusions 10 Future Directions References