Differential contributions of dopamine and serotonin to orbitofrontal cortex function in the marmoset

Tryptophan hydroxylase(2) gene polymorphisms predict brain serotonin synthesis in the orbitofrontal cortex in humans

Brain regional serotonin synthesis can be estimated in vivo using positron emission tomography (PET) and α-[((11))C]methyl-L-tryptophan ((11)C-AMT) trapping (K*) as a proxy. Recently, we reported evidence of lower normalized (11)C-AMT trapping in the orbitofrontal cortex (OBFC) of subjects meeting the criteria for an impulsive and/or aggressive behavioral phenotype. In this study, we examined whether part of the variance in OBFC serotonin synthesis is related to polymorphisms of the gene that encodes for the indoleamine's rate-limiting enzyme in the brain, tryptophan hydroxylase-2 (TPH(2)). In all, 46 healthy controls had PET (11)C-AMT scans and were genotyped for 11 single-nucleotide polymorphisms (SNPs) distributed across the TPH(2) gene and its 5' upstream region. Several TPH(2) SNPs were associated with lower normalized blood-to-brain clearance of (11)C-AMT in the OBFC. Dose-effect relationships were found for two variants (rs6582071 and rs4641527, respectively, located in the 5' upstream region and intron 1) that have previously been associated with suicide. Associations in the OBFC remained statistically significant in a mixed larger sample of patients and controls. These results suggest that in humans, genetic factors might partly account for variations in serotonin synthesis in the OBFC.

Serotonin selectively modulates reward value in human decision-making

Establishing a function for the neuromodulator serotonin in human decision-making has proved remarkably difficult because if its complex role in reward and punishment processing. In a novel choice task where actions led concurrently and independently to the stochastic delivery of both money and pain, we studied the impact of decreased brain serotonin induced by acute dietary tryptophan depletion. Depletion selectively impaired both behavioral and neural representations of reward outcome value, and hence the effective exchange rate by which rewards and punishments were compared. This effect was computationally and anatomically distinct from a separate effect on increasing outcome-independent choice perseveration. Our results provide evidence for a surprising role for serotonin in reward processing, while illustrating its complex and multifarious effects.

Prefrontal dopaminergic receptor abnormalities and executive functions in Parkinson's disease

The main pattern of cognitive impairments seen in early to moderate stages of Parkinson's disease (PD) includes deficits of executive functions. These nonmotor complications have a significant impact on the quality of life and day-to-day activities of PD patients and are not effectively managed by current therapies, a problem which is almost certainly due to the fact that the disease extends beyond the nigrostriatal system. To investigate the role of extrastriatal dopamine in executive function in PD, PD patients and a control group were studied with positron-emission-tomography using a high-affinity dopamine D2/D3 receptor tracer, [(11) C]FLB-457. All participants were scanned twice while performing an executive task and a control task. Patients were off medication for at least 12 h. The imaging analysis revealed that parkinsonian patients had lower [(11) C]FLB-457 binding than control group independently of task conditions across different brain regions. Cognitive assessment measures were positively correlated with [(11) C]FLB-457 binding in the bilateral dorsolateral prefrontal cortex and anterior cingulate cortex only in control group, but not in PD patients. Within the control group, during the executive task (as compared to control task), there was evidence of reduced [(11) C]FLB-457 binding (indicative of increased dopamine release) in the right orbitofrontal cortex. In contrast, PD patients did not show any reduction in binding during the executive task (as compared with control task). These findings suggest that PD patients present significant abnormalities in extrastriatal dopamine associated with executive processing. These observations provide important insights on the pathophysiology of cognitive dysfunction in PD.

Serotonin at the level of the amygdala and orbitofrontal cortex modulates distinct aspects of positive emotion in primates

Impaired top-down regulation of the amygdala, and its modulation by serotonin (5-HT), is strongly implicated in the dysregulation of negative emotion that characterizes a number of affective disorders. However, the contribution of these mechanisms to the regulation of positive emotion is not well understood. This study investigated the role of 5-HT within the amygdala and the orbitofrontal cortex (OFC), on the expression of appetitive Pavlovian conditioned emotional responses and their reversal in a primate, the common marmoset. Its effects were compared to those of the amygdala itself. Having developed conditioned autonomic and behavioural responses to an appetitive cue prior to surgery, marmosets with excitotoxic amygdala lesions failed to display such conditioned autonomic arousal at retention, but still displayed intact cue-directed conditioned behaviours. In contrast, 5,7-DHT infusions into the amygdala, reducing extracellular 5-HT levels, selectively enhanced the expression of appetitive conditioned behaviour at retention. Similar infusions into the OFC, producing marked reductions in post-mortem 5-HT tissue levels, had no overall effect on autonomic or behavioural responses, either at retention or during reversal learning, but caused an uncoupling of these responses, thereby fractionating emotional output. These data demonstrate the critical role of the amygdala in the expression of appetitive autonomic conditioning, and the region-selective contribution of 5-HT in the amygdala and OFC, respectively, to the expression of conditioned behaviour and the overall coordination of the emotional response. They provide insight into the neurochemical mechanisms underlying the regulation of positive emotional responses, advancing our understanding of the neural basis of pathologically dysregulated emotion.

Abnormally rapid reversal learning and reduced response to antipsychotic drugs following ovariectomy in female rats

Epidemiological and clinical life cycle studies indicate that favorable illness course and better response to antipsychotic drugs (APDs) in women with schizophrenia are positively correlated with estrogen levels. Accordingly, the estrogen hypothesis of schizophrenia proposes a neuroprotective role of estrogen in women vulnerable to schizophrenia. Previously we demonstrated in the rat that low levels of estrogen induced by ovariectomy led to disruption of latent inhibition (LI) reflecting impairment of selective attention, a core deficit of schizophrenia. LI disruption was reversed by 17β-estradiol and the atypical APD clozapine, whereas the typical APD haloperidol was ineffective unless co-administered with 17β-estradiol. Here we aimed to extend these findings by testing ovariectomized rats in another selective attention task, discrimination reversal. Ovariectomy led to a loss of selective attention as manifested in abnormally rapid reversal. The latter was normalized by high dose of 17β-estradiol (150 μg/kg) and clozapine (2.5mg/kg), but not by haloperidol (0.1mg/kg) or lower doses of 17β-estradiol (10 and 50 μg/kg). However, co-administration of haloperidol with 17β-estradiol (50 μg/kg) was effective. In sham rats low 17β-estradiol (10 μg/kg) produced rapid reversal, while high 17β-estradiol (150 μg/kg), haloperidol alone, or haloperidol-17β-estradiol combination reduced reversal speed. Clozapine did not affect reversal speed in sham rats. These results strengthen our previous results in suggesting that schizophrenia-like attentional abnormalities as well as reduced response to APDs in female rats are associated with low level of gonadal hormones. In addition, they support the possibility that estrogen may have an antipsychotic-like action in animal models.

Translational approaches to obsessive-compulsive disorder: from animal models to clinical treatment

Obsessive-compulsive disorder (OCD) is characterized by obsessions (intrusive thoughts) and compulsions (repetitive ritualistic behaviours) leading to functional impairment. Accumulating evidence links these conditions with underlying dysregulation of fronto-striatal circuitry and monoamine systems. These abnormalities represent key targets for existing and novel treatment interventions. However, the brain bases of these conditions and treatment mechanisms are still not fully elucidated. Animal models simulating the behavioural and clinical manifestations of the disorder show great potential for augmenting our understanding of the pathophysiology and treatment of OCD. This paper provides an overview of what is known about OCD from several perspectives. We begin by describing the clinical features of OCD and the criteria used to assess the validity of animal models of symptomatology; namely, face validity (phenomenological similarity between inducing conditions and specific symptoms of the human phenomenon), predictive validity (similarity in response to treatment) and construct validity (similarity in underlying physiological or psychological mechanisms). We then survey animal models of OC spectrum conditions within this framework, focusing on (i) ethological models; (ii) genetic and pharmacological models; and (iii) neurobehavioural models. We also discuss their advantages and shortcomings in relation to their capacity to identify potentially efficacious new compounds. It is of interest that there has been rather little evidence of 'false alarms' for therapeutic drug effects in OCD models which actually fail in the clinic. While it is more difficult to model obsessive cognition than compulsive behaviour in experimental animals, it is feasible to infer cognitive inflexibility in certain animal paradigms. Finally, key future neurobiological and treatment research areas are highlighted.

Functional neuroimaging in obsessive-compulsive disorder

BACKGROUND AND AIM

Obsessive-compulsive disorder (OCD) is a severe, highly prevalent and chronically disabling psychiatric disorder that usually emerges during childhood or adolescence. This paper aims to review the literature on functional neuroimaging in OCD, analysing the reported dysfunctional connectivity in the corticostriatothalamocortical circuitry.

METHOD

This study included papers published in peer-reviewed journals dealing with functional imaging in OCD.

RESULTS

Striatal dysfunction, mainly of the caudate nucleus, leads to inefficient thalamic gating, resulting in hyperactivity within the orbitofrontal cortex (intrusive thoughts) and the anterior cingulate cortex (non-specific anxiety). Compulsions consist of ritualistic behaviours performed to recruit the inefficient striatum and neutralise unwanted thoughts and anxiety. Functional neuroimaging findings are discussed against the background of specific cognitive impairments, mainly regarding visuospatial processing, executive functioning and motor speed. Cognitive deficits are partial and specific, matching imaging data.

CONCLUSIONS

Several studies have targeted brain regions hypothesised to be involved in the pathogenesis of OCD, showing the existence of dysfunctional connectivity in the corticostriatothalamocortical circuitry. Improvements in spatial resolution of neuroimaging techniques may contribute to a better understanding of the neurocircuitry of OCD and other anxiety disorders.

Corticolimbic function in impulsive aggressive behavior

Building on animal and human lesion evidence, neuroimaging studies are increasingly identifying abnormalities in corticolimbic circuits mediating aggressive behavior. This review focuses on three neural systems involved in impulsive/reactive aggression: 1) subcortical neural systems that support the production of aggressive impulses; 2) decision-making circuits and social-emotional information processing circuits that evaluate the consequences of aggressing or not aggressing; and 3) frontoparietal regions that are involved in regulating emotions and impulsive motivational urges. We review psychiatric disorders, including borderline personality disorder and antisocial personality disorder, characterized by elevated reactive aggression, focusing on abnormalities in these three neural systems.

The importance of serotonin for orbitofrontal function

The orbitofrontal cortex (OFC) receives a dense serotonin (5-hydroxytryptamine, or 5-HT) innervation from the dorsal raphe nucleus, with a smaller contribution from the median raphe nucleus. The reciprocal innervation from the OFC enables the OFC to regulate not only its own 5-HT input but the 5-HT input to the rest of the forebrain. This article reviews the evidence from studies in rodents and primates that implicate 5-HT in the OFC in the ability of animals to adapt their responding to changes in reward contingencies in the environment. A consensus is emerging that reductions in orbitofrontal 5-HT, whether the result of localized infusions of 5,7-dihydroxytryptamine (5,7-DHT), peripheral treatment with parachloroamphetamine (PCA) or para-chlorophenylalanine (PCPA), or chronic cold stress impairs this ability. Genetic variation in the 5-HT transporter can also affect this ability. An explanation regarding insensitivity to reward loss is ruled out by the finding that marmosets with 5-HT reductions in the OFC display a decline of responding as rapid as that of control animals when reward is withheld during extinction of a two-pattern discrimination task. The failure of these same animals to explore alternative stimuli during extinction, along with the recent electrophysiological evidence that dorsal raphe nucleus neurons encode future motivational outcomes, implicates orbitofrontal 5-HT in the process by which animals either exploit current resources or explore alternative resources based on current reward expectations.

Catecholamine influences on dorsolateral prefrontal cortical networks

The symptoms of attention-deficit/hyperactivity disorder (ADHD) involve impairments in prefrontal cortical top-down regulation of attention and behavior. All current pharmacological treatments for ADHD facilitate catecholamine transmission, and basic research suggests that these compounds have prominent actions in the prefrontal cortex (PFC). The dorsolateral PFC is especially sensitive to levels of norepinephrine and dopamine, whereby either too little or too much markedly impairs PFC function. Recent physiological studies have shown that norepinephrine strengthens PFC network connectivity and maintains persistent firing during a working memory task through stimulation of postsynaptic α(2A)-adrenoceptors on PFC neurons. Conversely, dopamine acts at D1 receptors to narrow spatial tuning, sculpting network inputs to decrease noise (i.e., stabilization of the representation). The stimulant medications and atomoxetine appear to enhance PFC function by indirectly increasing these catecholamine actions through blockade of norepinephrine and/or dopamine transporters. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.

Dopamine, but not serotonin, regulates reversal learning in the marmoset caudate nucleus

Studies of visual discrimination reversal learning have revealed striking neurochemical dissociations at the level of the orbitofrontal cortex (OFC) with serotoninergic, but not dopaminergic, integrity being important for successful reversal learning. These findings have considerable implications for disorders such as obsessive compulsive disorder and schizophrenia, in which reversal learning is impaired, and which are primarily treated with drugs targeting the dopaminergic and serotoninergic systems. Dysfunction in such disorders however, is not limited to the OFC and extends subcortically to other structures implicated in reversal learning, such as the medial caudate nucleus. Therefore, because the roles of the serotonin and dopamine within the caudate nucleus are poorly understood, this study compared the effects of selective serotoninergic or selective dopaminergic depletions of the marmoset medial caudate nucleus on serial discrimination reversal learning. All monkeys were able to learn novel stimulus-reward associations but, unlike control monkeys and monkeys with selective serotoninergic medial caudate depletions, dopamine-depleted monkeys were markedly impaired in their ability to reverse this association. This impairment was not perseverative in nature. These findings are the opposite of those seen in the OFC and provide evidence for a neurochemical double dissociation between the OFC and medial caudate in the regulation of reversal learning. Although the specific contributions of these monoamines within the OFC-striatal circuit remain to be elucidated, these findings have profound implications for the development of drugs designed to remediate some of the cognitive processes underlying impaired reversal learning.

Modulation of physical understanding by common marmosets (Callithrix jacchus)

The understanding of physical causality in common marmosets was tested using support problems in which a pair of sheets was presented to determine whether subjects would choose the sheet that had a food item on it (i.e., the sheet was supporting the food item). In two experiments, the conditions were manipulated in terms of the length of the sheet, the distance between the sheet and the food item, the presence of a gap separating the two sheets, and the size of the food item. In Experiment 1, the marmosets had difficulty rejecting an irretrievable food item when it was located closer to them than a retrievable item. Although their performance was strongly affected by the size of the irretrievable food item, they quickly learned to reject that alternative. In contrast, no improvement was found when one sheet was divided into two pieces such that the food item could not be retrieved when its near side was pulled. A similar response tendency was observed in Experiment 2, in which the effects of the large food item were examined in three different conditions. Thus, common marmosets were influenced by the perceptual features of the food in solving the support problems, as are other non-human primates. In addition, they consistently failed to appreciate the presence of a gap and, therefore, failed to reject the distracter alternative. However, all animals rapidly learned that the size of the food item was an irrelevant variable, and some showed an elementary conceptual understanding of support. These findings suggest that marmosets' physical understanding may improve with experience.

Methylphenidate and atomoxetine enhance prefrontal function through α2-adrenergic and dopamine D1 receptors

OBJECTIVE

This study examined the effects of the attention-deficit/hyperactivity disorder treatments, methylphenidate (MPH) and atomoxetine (ATM), on prefrontal cortex (PFC) function in monkeys and explored the receptor mechanisms underlying enhancement of PFC function at the behavioral and cellular levels.

METHOD

Monkeys performed a working memory task after administration of a wide range of MPH or ATM doses. The optimal doses were challenged with the α(2)-adrenoceptor antagonist, idazoxan, or the D(1) dopamine receptor antagonist, SCH23390 (SCH). In a parallel physiology study, neurons were recorded from the dorsolateral PFC of a monkey performing a working memory task. ATM, SCH, or the α(2) antagonist, yohimbine, were applied to the neurons by iontophoresis.

RESULTS

MPH and ATM generally produced inverted-U dose-response curves, with improvement occurring at moderate doses, but not at higher doses. The beneficial effects of these drugs were blocked by idazoxan or SCH. At the cellular level, ATM produced an inverted-U dose-response effect on memory-related firing, enhancing firing for preferred directions (increasing "signals") and decreasing firing for the nonpreferred directions (decreasing "noise"). The increase in persistent firing for the preferred direction was blocked by yohimbine, whereas the suppression of firing for the nonpreferred directions was blocked by SCH.

CONCLUSIONS

Optimal doses of MPH or ATM improved PFC cognitive function in monkeys. These enhancing effects appeared to involve indirect stimulation of α(2) adrenoceptors and D(1) dopamine receptors in the PFC. These receptor actions likely contribute to their therapeutic effects in the treatment of attention-deficit/hyperactivity disorder.

Orbitofrontal dopaminergic dysfunction causes age-related impairment of reversal learning in rats

Reversal learning is a domain that involves cognitive flexibility and is defined as the ability to rapidly alter established patterns of behavior when confronted with changing circumstances. This function depends critically on the orbitofrontal cortex (OFC) in the prefrontal cortical (PFC) structure, which is among the most sensitive to the influences of aging, and impaired reversal learning is a common functional disturbance of aged brain. The present study was designed to clarify the precisely neurochemical basis of this impaired learning in rats. For this purpose, we first examined reversal learning in young (3-month-old) and aged (24-month-old) rats using a T-maze discrimination task. The ability of aged rats to learn initially a reward rule for a T-maze discrimination task was almost equal to that of young rats, suggesting that simple discrimination ability was normal in aged rats. However, the ability to learn a reversed rule in a subsequent task was markedly impaired in aged rats. In addition, aged rats had reduced dopaminergic transmission concomitant with attenuated tyrosine hydroxylase (TH) activity in the OFC. Moreover, age-related impairment of reversal learning was improved by an intra-OFC infusion of 30 ng, but not 10 ng, of the D1 receptor agonist SKF 81297. Increasing dose of SKF 81297 to 100 ng also improved the impairment, but this effect was weaker than that of 30 ng, indicating that the SKF 81297 response was an inverted "U" pattern. The maximum SKF 81297 response (30 ng) was abolished by the D1 receptor antagonist SCH 23390. Thus, age-related impairment of reversal learning was due to a D1 receptor-mediated hypodopaminergic mechanism in the OFC. This finding provides direct evidence showing the involvement of OFC dopaminergic dysfunction in the development of cognitive inflexibility during the normal aging process.

Selective lesions of the dorsomedial striatum impair serial spatial reversal learning in rats

Impairments in reversal learning have been attributed to orbitofrontal cortex (OFC) dysfunction in many species. However, the role of subcortical areas interconnected with the OFC such as the striatum remains poorly understood. This study directly evaluated the contribution of core and shell sub-regions of the nucleus accumbens (NAc), dorsomedial (DMS) and dorsolateral (DLS) striatum to reversal learning of an instrumental two-lever spatial discrimination task in rats. Selective NAc core, DMS and DLS lesions were achieved with microinjections of quinolinic acid and NAc shell lesions with ibotenic acid. Damage to NAc core or shell did not affect retention of a previously acquired instrumental spatial discrimination. In contrast, DLS and DMS lesions produced changes in aspects of discrimination performance such as the latency to collect earned food pellets. Neither NAc core or shell lesions nor DLS lesions affected the main indices of reversal performance. Conversely, DMS lesion rats showed a significant impairment in reversal learning. DMS damage increased the number of errors to reach criteria that were perseverative in nature. The deficit in reversal learning in DMS lesion rats was not associated with an impairment to extinguish instrumental responding. There were no effects on spontaneous locomotor activity. Our data are in agreement with recent work showing that lesions of the medial striatum in marmoset monkeys produce perseverative impairments during a serial visual discrimination reversal task and support the hypothesis that dorsomedial striatal dysfunction contributes to pathological perseveration, which is a common feature of many psychiatric disorders.

Probing compulsive and impulsive behaviors, from animal models to endophenotypes: a narrative review

Failures in cortical control of fronto-striatal neural circuits may underpin impulsive and compulsive acts. In this narrative review, we explore these behaviors from the perspective of neural processes and consider how these behaviors and neural processes contribute to mental disorders such as obsessive-compulsive disorder (OCD), obsessive-compulsive personality disorder, and impulse-control disorders such as trichotillomania and pathological gambling. We present findings from a broad range of data, comprising translational and human endophenotypes research and clinical treatment trials, focussing on the parallel, functionally segregated, cortico-striatal neural projections, from orbitofrontal cortex (OFC) to medial striatum (caudate nucleus), proposed to drive compulsive activity, and from the anterior cingulate/ventromedial prefrontal cortex to the ventral striatum (nucleus accumbens shell), proposed to drive impulsive activity, and the interaction between them. We suggest that impulsivity and compulsivity each seem to be multidimensional. Impulsive or compulsive behaviors are mediated by overlapping as well as distinct neural substrates. Trichotillomania may stand apart as a disorder of motor-impulse control, whereas pathological gambling involves abnormal ventral reward circuitry that identifies it more closely with substance addiction. OCD shows motor impulsivity and compulsivity, probably mediated through disruption of OFC-caudate circuitry, as well as other frontal, cingulate, and parietal connections. Serotonin and dopamine interact across these circuits to modulate aspects of both impulsive and compulsive responding and as yet unidentified brain-based systems may also have important functions. Targeted application of neurocognitive tasks, receptor-specific neurochemical probes, and brain systems neuroimaging techniques have potential for future research in this field.

Reward and the serotonergic system

Anhedonia, as a failure to experience rewarding stimuli, is a key characteristic of many psychiatric disorders including depression and schizophrenia. Investigations on the neurobiological correlates of reward and hedonia/anhedonia have been a growing subject of research demonstrating several neuromodulators to mediate different aspects of reward processing. Whereas the majority of research on reward mainly focused on the dopamine and opioid systems, a serotonergic mechanism has been neglected. However, recent promising results strengthen the pivotal role of serotonin in reward processing. Evidence includes electrophysical and pharmacological as well as genetic and imaging studies. Primate research using single-unit recording of neurons within the dorsal raphe nucleus argues for a serotonergic mediation of reward value, whereas studies using intracranial self-stimulation point to an important contribution of serotonin in modulating motivational aspects of rewarding brain stimulation. Pharmacological studies using agonists and antagonists of serotonergic receptor subtypes and approaches investigating an increase or decrease of the extracellular level of serotonin offer strong evidence for a serotonergic mediation, ranging from aversion to pleasure. This review provides an argument for serotonin as a fundamental mediator of emotional, motivational and cognitive aspects of reward representation, which makes it possibly as important as dopamine for reward processing.

Enhancement of spatial reversal learning by 5-HT2C receptor antagonism is neuroanatomically specific

We have recently demonstrated that systemic administration of 5-HT(2C) and 5-HT(2A) receptor antagonists significantly enhanced and impaired spatial reversal learning, respectively. In this study, the role of 5-HT(2C) and 5-HT(2A) receptor subtypes in the mediation of these opposing effects was further investigated with respect to neuroanatomical specificity. The roles of 5-HT(2C) and 5-HT(2A) receptors were examined within some of the brain regions implicated in cognitive flexibility, namely the orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), and nucleus accumbens (NAc), by means of targeted infusions of selective 5-HT(2C) and 5-HT(2A) receptor antagonists (SB 242084 and M100907, respectively). Intra-OFC 5-HT(2C) receptor antagonism produced dose-dependent effects similar to those of systemic administration, i.e., improved spatial reversal learning by reducing the number of trials (all doses: 0.1, 0.3, and 1.0 microg) and perseverative errors to criterion (0.3 and 1.0 microg) compared with controls. However, the highest dose (1.0 microg) showed a nonselective effect, as it also affected retention preceding the reversal phase and decreased learning errors. Intracerebral infusions of SB 242084 into the mPFC or NAc produced no significant effects on any behavioral measures. Similarly, no significant differences were observed with intra-OFC, -mPFC, or -NAc infusions of M100907. These data suggest that the improved performance in reversal learning observed after 5-HT(2C) receptor antagonism is mediated within the OFC. These data also bear on the issue of whether 5-HT(2C) receptor antagonism within the OFC might help elucidate the biological substrate of obsessive-compulsive disorder, offering the potential for therapeutic application.

Opposing effects of 5,7-DHT infusions into the orbitofrontal cortex and amygdala on flexible responding

Central serotonin is implicated in a variety of emotional and behavioral control processes. Serotonin depletion can lead to exaggerated aversive processing and deficient response inhibition, effects that have been linked to serotonin's actions in the amygdala and orbitofrontal cortex (OFC), respectively. However, a direct comparison of serotonin manipulations within the OFC and amygdala in the same experimental context has not been undertaken. This study compared the effects of infusing the serotonin neurotoxin, 5,7-dihydroxytryptamine into the OFC and amygdala of marmosets performing an appetitive test of response inhibition. Marmosets had to learn to inhibit a prepotent response tendency to choose a box containing high-incentive food and instead choose a box containing low-incentive food, to obtain reward. OFC infusions caused long-lasting reductions in serotonin tissue levels, as revealed at postmortem, and exaggerated prepotent responses. In contrast, the significantly reduced prepotent responses following amygdala infusions occurred at a time when serotonin tissue levels had undergone considerable recovery, but there remained residual reductions in extracellular serotonin, in vivo. These opposing behavioral effects of serotonin manipulations in the same experimental context may be understood in terms of the top-down regulatory control of the amygdala by the OFC.

A combined histological and MRI brain atlas of the common marmoset monkey, Callithrix jacchus

The common marmoset, Callithrix jacchus, is of growing importance for research in neuroscience and related fields. In the present work, we describe a combined histological and magnetic resonance imaging (MRI) atlas constructed from the brains of two adult female marmosets. Histological sections were processed from Nissl staining and digitized to produce an atlas in a large format that facilitates visualization of structures with significant detail. Naming of identifiable brain structures was performed utilizing current terminology. The histological sections and a simplified schematic atlas are available online at http://udn.nichd.nih.gov/brainatlas_home.html.

Response disengagement on a spatial self-ordered sequencing task: effects of regionally selective excitotoxic lesions and serotonin depletion within the prefrontal cortex

Prefrontal cortex (PFC) is critical for self-ordered response sequencing. Patients with frontal lobe damage are impaired on response sequencing tasks, and increased blood flow has been reported in ventrolateral and dorsolateral PFC in subjects performing such tasks. Previously, we have shown that large excitotoxic lesions of the lateral PFC (LPFC) and orbitofrontal cortex FC (OFC), but not global prefrontal dopamine depletion, markedly impaired marmoset performance on a spatial self-ordered sequencing task (SSOST). To determine whether LPFC or OFC was responsible for the previously observed impairments and whether the underlying neural mechanism was modulated by serotonin, the present study compared the effects of selective LPFC and OFC excitotoxic lesions and 5,7-DHT-induced PFC serotonin depletions in marmosets on SSOST performance. Severe and long-lasting impairments in SSOST performance, including robust perseverative responding, followed LPFC but not OFC lesions. The deficit was ameliorated by task manipulations that precluded perseveration. Depletions of serotonin within LPFC and OFC had no effect, despite impairing performance on a visual discrimination reversal task, thus providing further evidence for differential monaminergic regulation of prefrontal function. In the light of the proposed attentional control functions of ventrolateral PFC and the failure of LPFC-lesioned animals to disengage from the immediately preceding response, it is proposed that this deficit may be due to a failure to attend to and register that a response has been made and thus should not be repeated. However, 5-HT does not appear to be implicated in this response inhibitory capacity.

The neuropsychopharmacology of fronto-executive function: monoaminergic modulation

We review the modulatory effects of the catecholamine neurotransmitters noradrenaline and dopamine on prefrontal cortical function. The effects of pharmacologic manipulations of these systems, sometimes in comparison with the indoleamine serotonin (5-HT), on performance on a variety of tasks that tap working memory, attentional-set formation and shifting, reversal learning, and response inhibition are compared in rodents, nonhuman primates, and humans using, in a behavioral context, several techniques ranging from microiontophoresis and single-cell electrophysiological recording to pharmacologic functional magnetic resonance imaging. Dissociable effects of drugs and neurotoxins affecting these monoamine systems suggest new ways of conceptualizing state-dependent fronto-executive functions, with implications for understanding the molecular genetic basis of mental illness and its treatment.

Lesions of the medial striatum in monkeys produce perseverative impairments during reversal learning similar to those produced by lesions of the orbitofrontal cortex

The ability to switch responding between two visual stimuli based on their changing relationship with reward is dependent on the orbitofrontal cortex (OFC). OFC lesions in humans, monkeys, and rats disrupt performance on a common test of this ability, the visual serial discrimination reversal task. This finding is of particular significance to our understanding of psychiatric disorders such as obsessive-compulsive disorder (OCD) and schizophrenia, in which behavioral inflexibility is a prominent symptom. Although OFC dysfunction can occur in these disorders, there is considerable evidence for more widespread dysfunction within frontostriatal and frontoamygdalar circuitry. Because the contribution of these subcortical structures to behavioral flexibility is poorly understood, the present study compared the effects of excitotoxic lesions of the medial striatum (MS), amygdala, and OFC in the marmoset monkey on performance of the serial reversal task. All monkeys were able to learn a novel stimulus-reward association but, compared with both control and amygdala-lesioned monkeys, those with MS or OFC lesions showed a perseverative impairment in their ability to reverse this association. However, whereas both MS and OFC groups showed insensitivity to negative feedback, only OFC-lesioned monkeys showed insensitivity to positive feedback. These findings suggest that, for different reasons, both the MS and OFC support behavioral flexibility after changes in reward contingencies, and are consistent with the hypothesis that striatal and OFC dysfunction can contribute to pathological perseveration.