Intra-prefrontal 8-OH-DPAT and M100907 improve visuospatial attention and decrease impulsivity on the five-choice serial reaction time task in rats

Interleukin-6 attenuates serotonin 2a receptor signaling by activating the JAK-STAT pathway

The serotonin 2A (5-HT2A) receptor and the proinflammatory cytokine, interleukin-6 (IL-6), have both been implicated in psychiatric disorders. Previously, we demonstrated that these molecules both facilitate cognitive flexibility, a prefrontal cortex-mediated executive function impaired in multiple mental illnesses. In this study, we tested the hypothesis that IL-6 influences 5-HT2A receptor signaling, providing a potential mechanism by which this cytokine may influence behavior. We first demonstrated that 5-HT2A receptors and IL-6-mediated STAT3 phosphorylation colocalize in cells of the prefrontal cortex, providing the neuroanatomical substrate for a potential interaction. In the neuronally derived A1A1 cell line, which expresses both IL-6 and 5-HT2A receptors, we found that IL-6 attenuates inositol phosphate (IP) accumulation in response to the 5-HT2 agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI), suggesting that IL-6 can regulate 5-HT2A receptor function. To identify the signaling pathway(s) that mediate this effect, we measured DOI-mediated IP accumulation in the presence of IL-6 and either the JAK-STAT inhibitor 124 [(9β,10α,16α,23E)-2,16,20,25-tetrahydroxy-9-methyl-19-norlanosta-1,5,23-triene-3,11,22-trione], JSI-124, or the extracellular signal-regulated kinase inhibitor, 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD-98059). The IL-6 effect was blocked by JSI-124 but not PD-98059. Furthermore, silencing RNA knockdown of either JAK or STAT blocked the IL-6 effect, suggesting that IL-6-induced JAK-STAT activation can regulate 5-HT2A receptor signaling. Finally, to determine if IL-6 specifically regulates the 5-HT2A receptor system, we measured IP production mediated by another Gq-coupled receptor, bradykinin B2. IL-6 had no effect on bradykinin-mediated IP accumulation, suggesting that regulation may occur at the 5-HT2A receptor. These results may provide clues to the pathologic mechanisms underlying certain psychiatric disorders and may suggest novel therapeutic strategies for their treatment.

The effect of developmental vitamin D deficiency in male and female Sprague-Dawley rats on decision-making using a rodent gambling task

Developmental vitamin D (DVD) deficiency is a plausible risk factor for schizophrenia that has been associated with behavioural alterations including disruptions in latent inhibition and response inhibition. The rodent gambling task (rGT) assesses risk-based decision-making, which is a key cognitive deficit observed in schizophrenia patients. The primary aim of this study was to examine risk-based decision-making in DVD-deficient and control rats on the rGT. We also evaluated the performance of female Sprague-Dawley rats on the rGT for the first time. Adult male and female Sprague-Dawley rats from control and vitamin D deficient dams were trained to perform the rGT in standard operant chambers and their performance and choice-preferences were assessed. Female rats were significantly faster to reach rGT training criteria compared with male rats and DVD-deficient rats were faster to reach training criteria than control animals. After reaching stable performance on the rGT DVD-deficient and control rats showed a significant preference for the optimal choice-option in the rGT, but there were no significant effects of sex or diet on these responses. DVD deficiency did not alter the decision-making processes on the rGT because no significant changes in choice-preferences were evident. This is the first study to demonstrate that once established, the performance of females is comparable to male Sprague-Dawley rats on the rGT.

Greater sensitivity to novelty in rats is associated with increased motor impulsivity following repeated exposure to a stimulating environment: implications for the etiology of impulse control deficits

Heightened motor impulsivity and increased novelty-seeking commonly co-occur in psychiatric disorders, including drug addiction. However, the relationship between these two phenomena remains unclear. One-time tests of novelty sensitivity commonly used in preclinical experiments, such as the open-field or novel-object test, fail to capture the fact that novelty-seekers repeatedly experience novel, stimulating situations. The present study therefore investigated whether repeated exposure to a novel, stimulating environment (SE) altered impulsive action. Male Long-Evans rats were trained to perform the five-choice serial reaction time task (5CSRTT) which measures motor impulsivity in the form of premature responding as well as attention and motivation. Animals were then exposed to a novel SE (1 h/day for 16 days) immediately prior to the 5CSRTT. Significant increases in premature responding were observed in a subgroup of reactive animals termed high responders (HR-SE). These rats were not more impulsive at baseline, and levels of impulsivity normalised once exposure to the SE was discontinued. No other aspect of 5CSRTT performance was affected by the SE challenge. We also determined that HR-SE rats were hyperactive in a novel environment. Biochemical analyses revealed changes in gene and protein expression within the dorsal hippocampus of HR-SE rats, including decreases in mRNA encoding the dopamine D1 receptor and brain-derived neurotrophic factor. These results indicate a novel mechanism by which impulsivity and novelty-reactivity interact that may enhance addiction vulnerability synergistically. Furthermore, studying such context-induced impulsivity may provide insight into the process by which environmental load precipitates psychiatric symptoms in impulse control disorders.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Genetic and pharmacological modulation of the steroid sulfatase axis improves response control; comparison with drugs used in ADHD

Maladaptive response control is a feature of many neuropsychiatric conditions, including attention deficit hyperactivity disorder (ADHD). As ADHD is more commonly diagnosed in males than females, a pathogenic role for sex-linked genes has been suggested. Deletion or point mutation of the X-linked STS gene, encoding the enzyme steroid sulfatase (STS) influences risk for ADHD. We examined whether deletion of the Sts gene in the 39,X(Y*)O mouse model, or pharmacological manipulation of the STS axis, via administration of the enzyme substrate dehydroepiandrosterone sulfate or the enzyme inhibitor COUMATE, influenced behavior in a novel murine analog of the stop-signal reaction time task used to detect inhibitory deficits in individuals with ADHD. Unexpectedly, both the genetic and pharmacological treatments resulted in enhanced response control, manifest as highly specific effects in the ability to cancel a prepotent action. For all three manipulations, the effect size was comparable to that seen with the commonly used ADHD therapeutics methylphenidate and atomoxetine. Hence, converging genetic and pharmacological evidence indicates that the STS axis is involved in inhibitory processes and can be manipulated to give rise to improvements in response control. While the precise neurobiological mechanism(s) underlying the effects remain to be established, there is the potential for exploiting this pathway in the treatment of disorders where failures in behavioral inhibition are prominent.

Activity-dependent serotonergic excitation of callosal projection neurons in the mouse prefrontal cortex

Layer 5 pyramidal neurons (L5PNs) in the mouse prefrontal cortex respond to serotonin (5-HT) according to their long-distance axonal projections; 5-HT_1A (1A) receptors mediate inhibitory responses in corticopontine (CPn) L5PNs, while 5-HT_2A (2A) receptors can enhance action potential (AP) output in callosal/commissural (COM) L5PNs, either directly (in “COM-excited” neurons), or following brief 1A-mediated inhibition (in “COM-biphasic” neurons). Here we compare the impact of 5-HT on the excitability of CPn and COM L5PNs experiencing variable excitatory drive produced by current injection (DC current or simulated synaptic current) or with exogenous glutamate. 5-HT delivered at resting membrane potentials, or paired with subthreshold depolarizing input, hyperpolarized CPn and COM-biphasic L5PNs and failed to promote AP generation in COM-excited L5PNs. Conversely, when paired with suprathreshold excitatory drive generating multiple APs, 5-HT suppressed AP output in CPn L5PNs, enhanced AP generation in COM-excited L5PNs, and generated variable responses in COM-biphasic L5PNs. While COM-excited neurons failed to respond to 5-HT in the presence of a 2A receptor antagonist, 32% of CPn neurons exhibited 2A-dependent excitation following blockade of 1A receptors. The presence of pharmacologically revealed 2A receptors in CPn L5PNs was correlated with the duration of 1A-mediated inhibition, yet biphasic excitatory responses to 5-HT were never observed, even when 5-HT was paired with strong excitatory drive. Our results suggest that 2A receptors selectively amplify the output of COM L5PNs experiencing suprathreshold excitatory drive, while shaping the duration of 1A-mediated inhibition in a subset of CPn L5PNs. Activity-dependent serotonergic excitation of COM L5PNs, combined with 1A-mediated inhibition of CPn and COM-biphasic L5PNs, may facilitate executive function by focusing network activity within cortical circuits subserving the most appropriate behavioral output.

Performance of compulsive behavior in rats is not a unitary phenomenon - validation of separate functional components in compulsive checking behavior

A previous analysis of the quinpirole sensitisation rat model of obsessive-compulsive disorder revealed that the behavioral phenotype of compulsive checking consists of three constitutive components – vigor of checking performance, focus on the task of checking, and satiety following a bout of checking. As confirmation of this analysis, the aim of the present study was to reconstitute, without quinpirole treatment, each of the putative components, with the expectation that these would self-assemble into compulsive checking. To reconstitute vigor and satiety, the employed treatment was a bilateral lesion of the nucleus accumbens core (NAc), as this treatment was shown previously to exaggerate these components. To reconstitute focus, the employed treatment was a low dose of the serotonin-1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin hydrochloride (DPAT) (0.0625 mg/kg), as high doses of this drug induce compulsive behavior and exacerbate focus. Results showed that injection of DPAT to NAc lesion rats did yield compulsive checking. Neither the drug alone nor the NAc lesion by itself produced compulsive checking. The demonstrated synthesis of compulsive checking by the combined treatment of low-dose DPAT and NAc lesion strengthened the previous fractionation of the model obsessive-compulsive disorder phenotype into three constitutive components, and suggested a role for serotonin-1A receptors outside the NAc in enhanced focus on the task of checking.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Subcircuit-specific neuromodulation in the prefrontal cortex

During goal-directed behavior, the prefrontal cortex (PFC) exerts top-down control over numerous cortical and subcortical regions. PFC dysfunction has been linked to many disorders that involve deficits in cognitive performance, attention, motivation, and/or impulse control. A common theme among these disorders is that neuromodulation of the PFC is disrupted. Anatomically, the PFC is reciprocally connected with virtually all neuromodulatory centers. Recent studies of PFC neurons, both in vivo and ex vivo, have found that subpopulations of prefrontal projection neurons can be segregated into distinct subcircuits based on their long-range projection targets. These subpopulations differ in their connectivity, intrinsic properties, and responses to neuromodulators. In this review we outline the evidence for subcircuit-specific neuromodulation in the PFC, and describe some of the functional consequences of selective neuromodulation on behavioral states during goal-directed behavior.

Dissociable contributions of anterior cingulate cortex and basolateral amygdala on a rodent cost/benefit decision-making task of cognitive effort

Personal success often requires the choice to expend greater effort for larger rewards, and deficits in such effortful decision making accompany a number of illnesses including depression, schizophrenia, and attention-deficit/hyperactivity disorder. Animal models have implicated brain regions such as the basolateral amygdala (BLA) and anterior cingulate cortex (ACC) in physical effort-based choice, but disentangling the unique contributions of these two regions has proven difficult, and effort demands in industrialized society are predominantly cognitive in nature. Here we utilize the rodent cognitive effort task (rCET), a modification of the five-choice serial reaction-time task, wherein animals can choose to expend greater visuospatial attention to obtain larger sucrose rewards. Temporary inactivation (via baclofen-muscimol) of BLA and ACC showed dissociable effects: BLA inactivation caused hard-working rats to 'slack off' and 'slacker' rats to work harder, whereas ACC inactivation caused all animals to reduce willingness to expend mental effort. Furthermore, BLA inactivation increased the time needed to make choices, whereas ACC inactivation increased motor impulsivity. These data illuminate unique contributions of BLA and ACC to effort-based decision making, and imply overlapping yet distinct circuitry for cognitive vs physical effort. Our understanding of effortful decision making may therefore require expanding our models beyond purely physical costs.

Dissecting impulsivity and its relationships to drug addictions

Addictions are often characterized as forms of impulsive behavior. That said, it is often noted that impulsivity is a multidimensional construct, spanning several psychological domains. This review describes the relationship between varieties of impulsivity and addiction-related behaviors, the nature of the causal relationship between the two, and the underlying neurobiological mechanisms that promote impulsive behaviors. We conclude that the available data strongly support the notion that impulsivity is both a risk factor for, and a consequence of, drug and alcohol consumption. While the evidence indicating that subtypes of impulsive behavior are uniquely informative--either biologically or with respect to their relationships to addictions--is convincing, multiple lines of study link distinct subtypes of impulsivity to low dopamine D2 receptor function and perturbed serotonergic transmission, revealing shared mechanisms between the subtypes. Therefore, a common biological framework involving monoaminergic transmitters in key frontostriatal circuits may link multiple forms of impulsivity to drug self-administration and addiction-related behaviors. Further dissection of these relationships is needed before the next phase of genetic and genomic discovery will be able to reveal the biological sources of the vulnerability for addiction indexed by impulsivity.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Differential Effects of Single-Dose Escitalopram on Cognitive and Affective Interference during Stroop Task

BACKGROUND AND OBJECTIVE

Our aim was to study the regulatory role of serotonin [(5-hydroxytryptamine (5-HT)] on two key nodes in the cognitive control networks - the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC). We hypothesized that increasing the levels of 5-HT would preferentially modulate the activity in ACC during cognitive control during interference by negative affects compared to cognitive control during interference by a superimposed cognitive task.

METHODS

We performed a functional magnetic resonance imaging investigation on 11 healthy individuals, comparing the effects of the selective 5-HT reuptake inhibitor escitalopram on brain oxygenation level dependent signals in the ACC and the DLPFC using affective and cognitive counting Stroop paradigms (aStroop and cStroop).

RESULTS

Escitalopram significantly decreased the activity in rostral ACC during aStroop compared to cStroop (p < 0.05). In the absence of escitalopram, both aStroop and cStroop significantly activated ACC and DLPFC (Z ≥ 2.3, p < 0.05).

CONCLUSION

We conclude that escitalopram in a region and task specific manner modified the cognitive control networks and preferentially decreased activity induced by affective interference in the ACC.

Pharmacological Interventions for the MATRICS Cognitive Domains in Schizophrenia: What's the Evidence?

Schizophrenia is a disabling, chronic psychiatric disorder with a prevalence rate of 0.5-1% in the general population. Symptoms include positive (e.g., delusions, hallucinations), negative (e.g., blunted affect, social withdrawal), as well as cognitive symptoms (e.g., memory and attention problems). Although 75-85% of patients with schizophrenia report cognitive impairments, the underlying neuropharmacological mechanisms are not well understood and currently no effective treatment is available for these impairments. This has led to the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative, which established seven cognitive domains that are fundamentally impaired in schizophrenia. These domains include verbal learning and memory, visual learning and memory, working memory, attention and vigilance, processing speed, reasoning and problem solving, and social cognition. Recently, a growing number of studies have been conducted trying to identify the underlying neuropharmacological mechanisms of cognitive impairments in schizophrenia patients. Specific cognitive impairments seem to arise from different underlying neuropharmacological mechanisms. However, most review articles describe cognition in general and an overview of the mechanisms involved in these seven separate cognitive domains is currently lacking. Therefore, we reviewed the underlying neuropharmacological mechanisms focusing on the domains as established by the MATRICS initiative which are considered most crucial in schizophrenia.

Role of medial prefrontal cortex serotonin 2A receptors in the control of retrieval of recognition memory in rats

Often, retrieval cues are not uniquely related to one specific memory, which could lead to memory interference. Controlling interference is particularly important during episodic memory retrieval or when remembering specific events in a spatiotemporal context. Despite a clear involvement of prefrontal cortex (PFC) in episodic memory in human studies, information regarding the mechanisms and neurotransmitter systems in PFC involved in memory is scarce. Although the serotoninergic system has been linked to PFC functionality and modulation, its role in memory processing is poorly understood. We hypothesized that the serotoninergic system in PFC, in particular the 5-HT2A receptor (5-HT2AR) could have a role in the control of memory retrieval. In this work we used different versions of the object recognition task in rats to study the role of the serotoninergic modulation in the medial PFC (mPFC) in memory retrieval. We found that blockade of 5-HT2AR in mPFC affects retrieval of an object in context memory in a spontaneous novelty preference task, while sparing single-item recognition memory. We also determined that 5-HT2ARs in mPFC are required for hippocampal-mPFC interaction during retrieval of this type of memory, suggesting that the mPFC controls the expression of memory traces stored in the hippocampus biasing retrieval to the most relevant one.

Expression of α(1)-adrenergic receptors in rat prefrontal cortex: cellular co-localization with 5-HT(2A) receptors

The prefrontal cortex (PFC) is involved in behavioural control and cognitive processes that are altered in schizophrenia. The brainstem monoaminergic systems control PFC function, yet the cells/networks involved are not fully known. Serotonin (5-HT) and norepinephrine (NE) increase PFC neuronal activity through the activation of α(1)-adrenergic receptors (α(1)ARs) and 5-HT(2A) receptors (5-HT(2A)Rs), respectively. Neurochemical and behavioural interactions between these receptors have been reported. Further, classical and atypical antipsychotic drugs share nm in vitro affinity for α(1)ARs while having preferential affinity for D(2) and 5-HT(2A)Rs, respectively. Using double in situ hybridization we examined the cellular expression of α(1)ARs in pyramidal (vGluT1-positive) and GABAergic (GAD(65/67)-positive) neurons in rat PFC and their co-localization with 5-HT(2A)Rs. α(1)ARs are expressed by a high proportion of pyramidal (59-85%) and GABAergic (52-79%) neurons. The expression in pyramidal neurons exhibited a dorsoventral gradient, with a lower percentage of α(1)AR-positive neurons in infralimbic cortex compared to anterior cingulate and prelimbic cortex. The expression of α(1A), α(1B) and α(1D) adrenergic receptors was segregated in different layers and subdivisions. In all them there is a high co-expression with 5-HT(2A)Rs (∼80%). These observations indicate that NE controls the activity of most PFC pyramidal neurons via α(1)ARs, either directly or indirectly, via GABAergic interneurons. Antipsychotic drugs can thus modulate the activity of PFC via α(1)AR blockade. The high co-expression with 5-HT(2A)Rs indicates a convergence of excitatory serotonergic and noradrenergic inputs onto the same neuronal populations. Moreover, atypical antipsychotics may exert a more powerful control of PFC function through the simultaneous blockade of α(1)ARs and 5-HT(2A)Rs.

The effects of prenatal and postnatal environmental interaction: prenatal environmental adaptation hypothesis

Adverse antenatal maternal environments during pregnancy influence fetal development that consequently increases risks of mental health problems including psychiatric disorders in offspring. Therefore, behavioral and brain alterations caused by adverse prenatal environmental conditions are generally considered as deficits. In this article, we propose a novel hypothesis, along with summarizing a body of literatures supporting it, that fetal neurodevelopmental alterations, particularly synaptic network changes occurring in the prefrontal cortex, associated with adverse prenatal environmental conditions may be adaptation to cope with expected severe postnatal environments, and therefore, psychiatric disorders may be able to be understood as adaptive strategies against severe environmental conditions through evolution. It is hoped that the hypothesis presented in this article stimulates and opens a new venue on research toward understanding of biological mechanisms and therapeutic treatments of psychiatric disorders.

Serotonin modulation of cortical neurons and networks

The serotonergic pathways originating in the dorsal and median raphe nuclei (DR and MnR, respectively) are critically involved in cortical function. Serotonin (5-HT), acting on postsynaptic and presynaptic receptors, is involved in cognition, mood, impulse control and motor functions by (1) modulating the activity of different neuronal types, and (2) varying the release of other neurotransmitters, such as glutamate, GABA, acetylcholine and dopamine. Also, 5-HT seems to play an important role in cortical development. Of all cortical regions, the frontal lobe is the area most enriched in serotonergic axons and 5-HT receptors. 5-HT and selective receptor agonists modulate the excitability of cortical neurons and their discharge rate through the activation of several receptor subtypes, of which the 5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₃ subtypes play a major role. Little is known, however, on the role of other excitatory receptors moderately expressed in cortical areas, such as 5-HT_2C, 5-HT₄, 5-HT₆, and 5-HT₇. In vitro and in vivo studies suggest that 5-HT_1A and 5-HT_2A receptors are key players and exert opposite effects on the activity of pyramidal neurons in the medial prefrontal cortex (mPFC). The activation of 5-HT_1A receptors in mPFC hyperpolarizes pyramidal neurons whereas that of 5-HT_2A receptors results in neuronal depolarization, reduction of the afterhyperpolarization and increase of excitatory postsynaptic currents (EPSCs) and of discharge rate. 5-HT can also stimulate excitatory (5-HT_2A and 5-HT₃) and inhibitory (5-HT_1A) receptors in GABA interneurons to modulate synaptic GABA inputs onto pyramidal neurons. Likewise, the pharmacological manipulation of various 5-HT receptors alters oscillatory activity in PFC, suggesting that 5-HT is also involved in the control of cortical network activity. A better understanding of the actions of 5-HT in PFC may help to develop treatments for mood and cognitive disorders associated with an abnormal function of the frontal lobe.

The role of zebrafish (Danio rerio) in dissecting the genetics and neural circuits of executive function

Zebrafish have great potential to contribute to our understanding of behavioral genetics and thus to contribute to our understanding of the etiology of psychiatric disease. However, progress is dependent upon the rate at which behavioral assays addressing complex behavioral phenotypes are designed, reported and validated. Here we critically review existing behavioral assays with particular focus on the use of adult zebrafish to explore executive processes and phenotypes associated with human psychiatric disease. We outline the case for using zebrafish as models to study impulse control and attention, discussing the validity of applying extant rodent assays to zebrafish and evidence for the conservation of relevant neural circuits.

Peptide inhibitors disrupt the serotonin 5-HT2C receptor interaction with phosphatase and tensin homolog to allosterically modulate cellular signaling and behavior

Serotonin (5-hydroxytryptamine; 5-HT) signaling through the 5-HT(2C) receptor (5-HT(2C)R) is essential in normal physiology, whereas aberrant 5-HT(2C)R function is thought to contribute to the pathogenesis of multiple neural disorders. The 5-HT(2C)R interacts with specific protein partners, but the impact of such interactions on 5-HT(2C)R function is poorly understood. Here, we report convergent cellular and behavioral data that the interaction between the 5-HT(2C)R and protein phosphatase and tensin homolog (PTEN) serves as a regulatory mechanism to control 5-HT(2C)R-mediated biology but not that of the closely homologous 5-HT(2A)R. A peptide derived from the third intracellular loop of the human 5-HT(2C)R [3L4F (third loop, fourth fragment)] disrupted the association, allosterically augmented 5-HT(2C)R-mediated signaling in live cells, and acted as a positive allosteric modulator in rats in vivo. We identified the critical residues within an 8 aa fragment of the 3L4F peptide that maintained efficacy (within the picomolar range) in live cells similar to that of the 3L4F peptide. Last, molecular modeling identified key structural features and potential interaction sites of the active 3L4F peptides against PTEN. These compelling data demonstrate the specificity and importance of this protein assembly in cellular events and behaviors mediated by 5-HT(2C)R signaling and provide a chemical guidepost to the future development of drug-like peptide or small-molecule inhibitors as neuroprobes to study 5-HT(2C)R allostery and therapeutics for 5-HT(2C)R-mediated disorders.

The role of impulsivity in relapse vulnerability

Drug dependence in humans is often accompanied by behavioral disturbances such as maladaptive levels of impulsivity. In turn, there is accumulating evidence from preclinical laboratory animal and clinical studies indicating that impulsive behavior might be causally linked to several distinct processes in drug addiction, including the onset, maintenance and relapsing nature of drug use. This leads to the question as to whether pharmacological or behavioral approaches aimed at ameliorating impulsivity might prove effective therapeutic interventions in human drug dependence. This paper reviews evidence for an important role of impulsivity as a determinant of drug dependence with a particular focus on relapse vulnerability and addresses the implications of these findings for the clinical management of relapse prevention.

Nothing tastes as good as skinny feels: the neurobiology of anorexia nervosa

Individuals with anorexia nervosa (AN) engage in relentless restrictive eating and often become severely emaciated. Because there are no proven treatments, AN has high rates of relapse, chronicity, and death. Those with AN tend to have childhood temperament and personality traits, such as anxiety, obsessions, and perfectionism, which may reflect neurobiological risk factors for developing AN. Restricted eating may be a means of reducing negative mood caused by skewed interactions between serotonin aversive or inhibitory and dopamine reward systems. Brain imaging studies suggest that altered eating is a consequence of dysregulated reward and/or awareness of homeostatic needs, perhaps related to enhanced executive ability to inhibit incentive motivational drives. An understanding of the neurobiology of this disorder is likely to be important for developing more effective treatments.

Highly impulsive rats: modelling an endophenotype to determine the neurobiological, genetic and environmental mechanisms of addiction

Impulsivity describes the tendency of an individual to act prematurely without foresight and is associated with a number of neuropsychiatric co-morbidities, including drug addiction. As such, there is increasing interest in the neurobiological mechanisms of impulsivity, as well as the genetic and environmental influences that govern the expression of this behaviour. Tests used on rodent models of impulsivity share strong parallels with tasks used to assess this trait in humans, and studies in both suggest a crucial role of monoaminergic corticostriatal systems in the expression of this behavioural trait. Furthermore, rodent models have enabled investigation of the causal relationship between drug abuse and impulsivity. Here, we review the use of rodent models of impulsivity for investigating the mechanisms involved in this trait, and how these mechanisms could contribute to the pathogenesis of addiction.

Synergism between a serotonin 5-HT2A receptor (5-HT2AR) antagonist and 5-HT2CR agonist suggests new pharmacotherapeutics for cocaine addiction

Relapse to cocaine dependence, even after extended abstinence, involves a number of liability factors including impulsivity (predisposition toward rapid, unplanned reactions to stimuli without regard to negative consequences) and cue reactivity (sensitivity to cues associated with cocaine-taking which can promote cocaine-seeking). These factors have been mechanistically linked to serotonin (5-hydroxytryptamine, 5-HT) signaling through the 5-HT(2A) receptor (5-HT(2A)R) and 5-HT(2C)R; either a selective 5-HT(2A)R antagonist or a 5-HT(2C)R agonist suppresses impulsivity and cocaine-seeking in preclinical models. We conducted proof-of-concept analyses to evaluate whether a combination of 5-HT(2A)R antagonist plus 5-HT(2C)R agonist would have synergistic effects over these liability factors for relapse as measured in a 1-choice serial reaction time task and cocaine self-administration/reinstatement assay. Combined administration of a dose of the selective 5-HT(2A)R antagonist M100907 plus the 5-HT(2C)R agonist WAY163909, each ineffective alone, synergistically suppressed cocaine-induced hyperactivity, inherent and cocaine-evoked impulsive action, as well as cue- and cocaine-primed reinstatement of cocaine-seeking behavior. The identification of synergism between a 5-HT(2A)R antagonist plus a 5-HT(2C)R agonist to attenuate these factors important in relapse indicates the promise of a bifunctional ligand as an anti-addiction pharmacotherapeutic, setting the stage to develop new ligands with improved efficacy, potency, selectivity, and in vivo profiles over the individual molecules.

Milnacipran enhances the control of impulsive action by activating D₁-like receptors in the infralimbic cortex

RATIONALE

Elevated impulsivity is often observed in patients with depression. We recently found that milnacipran, an antidepressant and a serotonin/noradrenaline reuptake inhibitor, could enhance impulse control in rats. However, the neural mechanisms underlying the effects of milnacipran on impulsive action remain unclear. Milnacipran increases not only extracellular serotonin and noradrenaline but also dopamine specifically in the medial prefrontal cortex, which is one of the brain regions responsible for impulsive action.

OBJECTIVES

Our goal was to identify whether D(1)- and/or D(2)-like receptors in the infralimbic cortex (IL), the ventral portion of the medial prefrontal cortex, mediates the milnacipran-enhanced impulse control in a three-choice serial reaction time task.

METHODS

The rats were bilaterally injected with SCH23390, a selective D(1)-like receptor antagonist (0.3 or 3 ng/side) or eticlopride, a selective D(2)-like receptor antagonist (0.3 or 1 μg/side) into the IL after acute intraperitoneal administration of milnacipran (10 mg/kg).

RESULTS

Intra-IL SCH23390 injections reversed the milnacipran-enhanced impulse control, whereas injections of eticlopride into the IL failed to block the effects of milnacipran on impulsive action.

CONCLUSIONS

This is the first report that demonstrates a critical role for D(1)-like receptors of the IL in milnacipran-enhanced control of impulsive action.

The selective serotonin reuptake inhibitor, escitalopram, enhances inhibition of prepotent responding and spatial reversal learning

Previous findings indicate treatment with a selective serotonin reuptake inhibitor (SSRI) facilitates behavioral flexibility when conditions require inhibition of a learned response pattern. The present experiment investigated whether acute treatment with the SSRI, escitalopram, affects behavioral flexibility when conditions require inhibition of a naturally biased response pattern (elevated conflict test) and/or reversal of a learned response pattern (spatial reversal learning). An additional experiment was carried out to determine whether escitalopram, at doses that affected behavioral flexibility, also reduced anxiety as tested in the elevated plus-maze. In each experiment, Long-Evans rats received an intraperitoneal injection of either saline or escitalopram (0.03, 0.3 or 1.0 mg/kg) 30 min prior to behavioral testing. Escitalopram, at all doses tested, enhanced acquisition in the elevated conflict test, but did not affect performance in the elevated plus-maze. Escitalopram (0.3 and 1.0 mg/kg) did not alter acquisition of the spatial discrimination, but facilitated reversal learning. In the elevated conflict and spatial reversal learning test, escitalopram enhanced the ability to maintain the relevant strategy after being initially selected. The present findings suggest that enhancing serotonin transmission with an SSRI facilitates inhibitory processes when conditions require a shift away from either a naturally biased response pattern or a learned choice pattern.

Current status of functional imaging in eating disorders

Eating Disorders are complex psychiatric problems that involve biologic and psychological factors. Brain imaging studies provide insights about how functionally connected brain networks may contribute to disturbed eating behavior, resulting in food refusal and altered body weight, but also body preoccupations and heightened anxiety. In this article, we review the current state of brain imaging in eating disorders, and how such techniques may help identify pathways that could be important in the treatment of those often detrimental disorders.

Norepinephrine and dopamine modulate impulsivity on the five-choice serial reaction time task through opponent actions in the shell and core sub-regions of the nucleus accumbens

Impulsive behavior is a hallmark of several neuropsychiatric disorders (eg, attention-deficit/hyperactivity disorder, ADHD). Although dopamine (DA) and norepinephrine (NE) have a significant role in the modulation of impulsivity their neural loci of action is not well understood. Here, we investigated the effects of the selective NE re-uptake inhibitor atomoxetine (ATO) and the mixed DA/NE re-uptake inhibitor methylphenidate (MPH), both with proven clinical efficacy in ADHD, on the number of premature responses on a five-choice serial reaction time task, an operational measure of impulsivity. Microinfusions of ATO into the shell, but not the core, sub-region of the nucleus accumbens (NAcb) significantly decreased premature responding whereas infusions of MPH in the core, but not the shell, sub-region significantly increased premature responding. However, neither ATO nor MPH significantly altered impulsive behavior when infused into the prelimbic or infralimbic cortices. The opposing effects of ATO and MPH in the NAcb core and shell on impulsivity were unlikely mediated by ancillary effects on behavioral activation as locomotor activity was either unaffected, as in the case of ATO infusions in the core and shell, or increased when MPH was infused into either the core and shell sub-region. These findings indicate an apparently 'opponent' modulation of premature responses by NE and DA in the NAcb shell or core, respectively, and suggest that the symptom clusters of hyperactive-impulsive type ADHD may have distinct neural and neurochemical substrates.

Sensitivity to cognitive effort mediates psychostimulant effects on a novel rodent cost/benefit decision-making task

Amotivational states and insufficient recruitment of mental effort have been observed in a variety of clinical populations, including depression, traumatic brain injury, post-traumatic stress disorder, and attention deficit hyperactivity disorder. Previous rodent models of effort-based decision making have utilized physical costs whereas human studies of effort are primarily cognitive in nature, and it is unclear whether the two types of effortful decision making are underpinned by the same neurobiological processes. We therefore designed a novel rat cognitive effort task (rCET) based on the 5-choice serial reaction time task, a well-validated measure of attention and impulsivity. Within each trial of the rCET, rats are given the choice between an easy or hard visuospatial discrimination, and successful hard trials are rewarded with double the number of sugar pellets. Similar to previous human studies, stable individual variation in choice behavior was observed, with 'workers' choosing hard trials significantly more than their 'slacker' counterparts. Whereas workers 'slacked off' in response to administration of amphetamine and caffeine, slackers 'worked harder' under amphetamine, but not caffeine. Conversely, these stimulants increased motor impulsivity in all animals. Ethanol did not affect animals' choice but invigorated behavior. In sum, we have shown for the first time that rats are differentially sensitive to cognitive effort when making decisions, independent of other processes such as impulsivity, and these baseline differences can influence the cognitive response to psychostimulants. Such findings could inform our understanding of impairments in effort-based decision making and contribute to treatment development.

Comparative effects of different test day challenges on performance in the 5-choice serial reaction time task

The 5-choice serial reaction time task (5-CSRTT) is a valuable cognitive test that permits the simultaneous assessment of several different cognitive modalities, including attention, impulse control, processing speed, and cognitive flexibility. Increasing task difficulty on test days through various challenges can further enhance the versatility of this test by selectively enhancing the cognitive load on different aspects of the task. Systematic comparisons of the effects of different test day challenges on 5-CSRTT performance are essential to verify how these challenges affect different task measures and which manipulations are best suited for future studies of different aspects of cognition. We trained Wistar rats in the 5-CSRTT under standard conditions, then challenged them on the test days by (1) decreasing the duration of the stimulus to be detected, (2) increasing the time interval between trials (intertrial interval, ITI), (3) randomly varying the ITI, or (4) adding a flashing light distractor. All test day challenges produced distinct profiles of performance disruption that reflected differential effects on different cognitive modalities. Decreased stimulus duration selectively impaired attentional performance, while increased ITI increased impulsive-like premature responses and decreased trials completed. Variable ITI induced only mild, nonsignificant disruptions in response inhibition and processing speed, while the flashing light distractor produced comprehensive impairment affecting multiple aspects of 5-CSRTT performance, including disrupted attention and increased premature and timeout responses. This improved understanding of the effects of different test day challenges in the 5-CSRTT will allow researchers to use these manipulations of a valuable cognitive test to their full potential.

Trait impulsive choice predicts resistance to extinction and propensity to relapse to cocaine seeking: a bidirectional investigation

Despite the strong association between impulsivity and addiction in humans, it is still a matter of debate whether impulsive choice predisposes to, or results from, drug dependence. Furthermore, it is unknown whether treating impulsivity can protect against relapse propensity. Therefore, this study explored the bidirectional relationship between impulsive choice and cocaine taking and seeking in rat behavioral models. In experiment 1, to determine whether impulsive choice predisposes to cocaine taking or seeking, rats were selected based on trait impulsivity in a delayed reward task and subsequently compared on various stages of cocaine self-administration (SA). To examine the consequence of cocaine intake on impulsive choice, impulsivity was monitored once a week throughout various stages of cocaine SA. To determine whether treating impulsive choice can protect against relapse propensity, in experiment 2, impulsive choice was manipulated by pharmacological interventions and cocaine-associated contextual cues. Trait impulsive choice as determined in experiment 1 predicted high extinction resistance and enhanced propensity to context-induced relapse in the cocaine SA model, whereas cocaine intake did not alter impulsive choice. Furthermore, acute changes in impulsive choice were not related to rates of context-induced relapse. Taken together, the current data indicate that trait impulsive choice predicts persistent cocaine seeking during extinction and enhanced propensity to relapse, whereas acute manipulations of impulsive choice had no favorable outcomes on relapse measures. These observations suggest that trait impulsivity can be used as a predictive factor for addiction liability, but treating this impulsivity does not necessarily protect against relapse.

The role of 5-HT1A receptors in phencyclidine (PCP)-induced novel object recognition (NOR) deficit in rats

RATIONALE

Atypical antipsychotic drugs (APDs), many of which are direct or indirect serotonin (5-HT)(1A) agonists, and tandospirone, a 5-HT(1A) partial agonist, have been reported to improve cognition in schizophrenia.

OBJECTIVES AND METHODS

We tested the effect of 5-HT(1A) agonism, alone, and in combination with other psychotropic agents, including the atypical APD, lurasidone, in reversing the deficit in novel object recognition (NOR) induced by subchronic treatment with the non-competitive NMDA receptor antagonist, phencyclidine (PCP) (2 mg/kg, b.i.d., for 7 days).

RESULTS

Subchronic treatment with PCP induced a persistent NOR deficit. Lurasidone (0.1 mg/kg), a potent 5-HT(1A) partial agonist, 5-HT(2A) antagonist, and weaker D(2) antagonist, tandospirone (0.6 mg/kg), and the selective post-synaptic 5-HT(1A) agonist, F15599 (0.16 mg/kg), ameliorated the subchronic PCP-induced-NOR deficit. The 5-HT(1A) antagonist, WAY100635 (0.6 mg/kg), blocked the ameliorating effects of tandospirone and lurasidone. The combination of sub-effective doses of tandospirone (0.2 mg/kg) and lurasidone (0.03 mg/kg) also reversed the PCP-induced NOR-deficit. Buspirone, a less potent partial 5-HT(1A) agonist than tandospirone, was less effective. Co-administration of tandospirone (0.2 mg/kg) and pimavanserin (3 mg/kg), a relatively selective 5-HT(2A) receptor inverse agonist, did not reverse the effect of sub-chronic PCP on NOR. The D(2) antagonist, haloperidol, blocked the ameliorating effect of tandospirone on the PCP-induced deficit in NOR.

CONCLUSIONS

These results indicate that 5-HT(1A) agonism is adequate to ameliorate the PCP-induced impairment in NOR and suggest further study of utilizing the combination of a 5-HT(1A) agonist and an atypical APD to ameliorate some types of cognitive impairment in schizophrenia.

Attentional processing in C57BL/6J mice exposed to developmental vitamin D deficiency

Epidemiological evidence suggests that Developmental Vitamin D (DVD) deficiency is associated with an increased risk of schizophrenia. DVD deficiency in mice is associated with altered behaviour, however there has been no detailed investigation of cognitive behaviours in DVD-deficient mice. The aim of this study was to determine the effect of DVD deficiency on a range of cognitive tasks assessing attentional processing in C57BL/6J mice. DVD deficiency was established by feeding female C57BL/6J mice a vitamin D-deficient diet from four weeks of age. After six weeks on the diet, vitamin D-deficient and control females were mated with vitamin D-normal males and upon birth of the pups, all dams were returned to a diet containing vitamin D. The adult offspring were tested on a range of cognitive behavioural tests, including the five-choice serial reaction task (5C-SRT) and five-choice continuous performance test (5C-CPT), as well as latent inhibition using a fear conditioning paradigm. DVD deficiency was not associated with altered attentional performance on the 5C-SRT. In the 5C-CPT DVD-deficient male mice exhibited an impairment in inhibiting repetitive responses by making more perseverative responses, with no changes in premature or false alarm responding. DVD deficiency did not affect the acquisition or retention of cued fear conditioning, nor did it affect the expression of latent inhibition using a fear conditioning paradigm. DVD-deficient mice exhibited no major impairments in any of the cognitive domains tested. However, impairments in perseverative responding in DVD-deficient mice may indicate that these animals have specific alterations in systems governing compulsive or reward-seeking behaviour.

The utility of rat models of impulsivity in developing pharmacotherapies for impulse control disorders

High levels of impulsive behaviours are a clinically significant symptom in a range of psychiatric disorders, such as attention deficit hyperactivity disorder, bipolar disorder, personality disorders, pathological gambling and substance abuse. Although often measured using questionnaire assessments, levels of different types of impulsivity can also be determined using behavioural tests. Rodent analogues of these paradigms have been developed, and similar neural circuitry has been implicated in their performance in both humans and rats. In the current review, the methodology underlying the measurement of different aspects of impulsive action and choice are considered from the viewpoint of drug development, with a focus on the continuous performance task (CPT), stop-signal task (SST), go/no-go and delay-discounting paradigms. Current issues impeding translation between animal and human studies are identified, and comparisons drawn between the acute effects of dopaminergic, noradrenergic and serotonergic compounds across species. Although the field could benefit from a more systematic determination of different pharmacological agents across paradigms, there are signs of strong concordance between the animal and human data. However, the type of impulsivity measured appears to play a significant role, with the SST and delay discounting providing more consistent effects for dopaminergic drugs, while the CPT and SST show better predictive validity so far for serotonergic and noradrenergic compounds. Based on the available data, it would appear that these impulsivity models could be used more widely to identify potential pharmacotherapies for impulse control disorders. Novel targets within the glutamatergic and serotonergic system are also suggested.

Rats tested after a washout period from sub-chronic PCP administration exhibited impaired performance in the 5-Choice Continuous Performance Test (5C-CPT) when the attentional load was increased

It is well documented that schizophrenia patients exhibit dysfunction in various cognitive domains, including attention/vigilance, as demonstrated by impaired performance in the myriad of Continuous Performance Tests (CPTs). NMDA receptor antagonists provide a pharmacological model in animals of the cognitive disruption presented in the disorder. We therefore examined the effects of a sub-chronic PCP treatment regimen (5.0mg/kg 7-days bi-daily) in the recently developed rodent test of vigilance, the 5-Choice Continuous Performance Test (5C-CPT). We assessed the effects of this regimen after at least a 7-day washout period on both baseline performance and when the attentional load was increased. Sub-chronic PCP treatment impaired 5C-CPT performance in a manner consistent with impaired vigilance in patients with schizophrenia, with reduced hit rate and impaired signal sensitivity. These effects were only evident when performance was challenged following parameter manipulations. These data demonstrate that attention/vigilance is sensitive to disruption following sub-chronic PCP treatment in a pre-clinical task that may demonstrate increased analogy to human vigilance tasks. Although the PCP-induced attentional deficits are not as large as those deficits observed in other domains, these data provide evidence that this pharmacological model can affect multiple cognitive domains and may be useful for assessing putative pro-cognitive therapeutics for schizophrenia.

The role of serotonin in the regulation of patience and impulsivity

Classic theories suggest that central serotonergic neurons are involved in the behavioral inhibition that is associated with the prediction of negative rewards or punishment. Failed behavioral inhibition can cause impulsive behaviors. However, the behavioral inhibition that results from predicting punishment is not sufficient to explain some forms of impulsive behavior. In this article, we propose that the forebrain serotonergic system is involved in “waiting to avoid punishment” for future punishments and “waiting to obtain reward” for future rewards. Recently, we have found that serotonergic neurons increase their tonic firing rate when rats await food and water rewards and conditioned reinforcer tones. The rate of tonic firing during the delay period was significantly higher when rats were waiting for rewards than for tones, and rats were unable to wait as long for tones as for rewards. These results suggest that increased serotonergic neuronal firing facilitates waiting behavior when there is the prospect of a forthcoming reward and that serotonergic activation contributes to the patience that allows rats to wait longer. We propose a working hypothesis to explain how the serotonergic system regulates patience while waiting for future rewards.

Lithium, but not valproic acid or carbamazepine, suppresses impulsive-like action in rats

RATIONALE

Higher impulsivity is a pathological symptom in several psychiatric disorders, including bipolar disorder, and is a risk factor for suicide.

OBJECTIVES

Our goal was to determine whether major mood-stabilizing drugs used for the treatment of bipolar disorder could suppress impulsive-like action in the three-choice serial reaction time task (3-CSRTT).

METHODS

Following training for the 3-CSRTT, rats were acutely administered lithium chloride (LiCl; 0, 3.2, 10, and 32 mg/kg, i.p.), valproic acid (0, 10, 32, and 100 mg/kg, i.p.), or carbamazepine (0, 10, 20, and 30 mg/kg, i.p.). To assess the anorexic effects of lithium, a simple food consumption test was conducted.

RESULTS

LiCl dose-dependently decreased the number of premature responses, an index of impulsive-like action. A high dose of LiCl (32 mg/kg) decreased food consumption, but its anorexic effects were not correlated with the effects of LiCl on premature responses. A moderate dose of LiCl (20 mg/kg) significantly reduced the number of premature responses without affecting motivation-related measures in the 3-CSRTT or the amount of food consumption. Although carbamazepine prolonged reward latency, an index of motivation for food, neither valproic acid nor carbamazepine significantly affected premature responses.

CONCLUSION

It is likely that lithium has a suppressive effect on impulsive action independent of the anorexic effect. Lithium may suppress impulsive behavior and thereby decrease the risk of suicide. The present results could provide an explanation for the antisuicidal effects of lithium and suggest that lithium could be a beneficial treatment for impulsivity-related disorders.

Dorsal-striatal 5-HT₂A and 5-HT₂C receptors control impulsivity and perseverative responding in the 5-choice serial reaction time task

RATIONAL

Prefrontal cortex (PFC) and dorsal striatum are part of the neural circuit critical for executive attention. The relationship between 5-HT and aspects of attention and executive control is complex depending on experimental conditions and the level of activation of different 5-HT receptors within the nuclei of corticostriatal circuitry.

OBJECTIVE

The present study investigated which 5-HT(2A) and 5-HT(2C) receptors in the dorsomedial-striatum (dm-STR) contribute to executive attention deficit induced by blockade of NMDA receptors in the PFC.

MATERIALS AND RESULTS

Executive attention was assessed by the five-choice serial reaction time task (5-CSRTT), which provides indices of attention (accuracy) and those of executive control over performance such as premature (an index of impulsivity) and perseverative responding. The effects of targeted infusion in dm-STR of 100 and 300 ng/μl doses of the selective 5-HT(2A) antagonist M100907 and 1 and 3 μg/μl doses of 5-HT(2C) agonist Ro60-0175 was examined in animals injected with 50 ng/μl dose of a competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-phosphonic acid (CPP) in the mPFC. Blockade of NMDA receptors impaired accuracy as well as executive control as shown by increased premature and perseverative responding. The CPP-induced premature and perseverative over-responding were dose-dependently prevented by both M100907 and Ro60-0175. Both drugs partially removed the CPP-induced accuracy deficit but only at the highest dose tested.

CONCLUSIONS

It is suggested that in the dorsal striatum, 5-HT by an action on 5-HT(2A) and 5-HT(2C) receptors may integrate the glutamate corticostriatal inputs critical for different aspects of the 5-CSRT task performance.

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT₂A/C and 5-HT₁A receptor agonists

RATIONALE

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.

OBJECTIVES

This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats.

METHODS

Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT(2A/C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI] and the 5-HT(1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(±)-8-OH-DPAT] on impulsive choice were then evaluated.

RESULTS

Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task.

CONCLUSIONS

We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.

Pre-treatment with the mGlu2/3 receptor agonist LY379268 attenuates DOI-induced impulsive responding and regional c-Fos protein expression

RATIONALE

Overactivation of serotonin (5-hydroxytryptamine, 5-HT)(2A) receptors causes impulsivity and attentional deficits. Since 5-HT(2A) receptors are known to entertain antagonistic interactions with metabotropic glutamate (mGlu)2/3 receptors, this interaction may provide an alternative target for a novel class of antipsychotics.

OBJECTIVES/METHODS

The study characterizes interactions between 5-HT(2A) and mGlu2/3 receptors implicated in impulse control. Hooded Lister rats were trained in a 5-choice serial reaction time task (5-CSRTT) and treated with the 5-HT(2A/2C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI, 0.1 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg). In addition, associated drug-induced changes in neuronal activity were assessed via c-Fos immunoreactivity (Fos IR), and co-localization of c-Fos and GABAergic markers was detected using double immunofluorescence labeling.

RESULTS

Systemic DOI caused impulsive overresponding that was attenuated in animals pre-treated with LY379268. LY379268 itself had no significant effect on the rats' performance in the 5-CSRTT. DOI enhanced Fos IR within fronto-cortical and limbic brain structures, and this effect was blocked by LY379268 pre-treatment. Double immunofluorescence labeling showed a specific co-localization of DOI-elicited Fos IR with GABAergic (GAD(67)-positive) cells lacking the calcium-binding protein parvalbumin while LY379268 increased Fos IR in GABAergic and non-GABAergic cells.

CONCLUSION

Our results suggest that impulsivity is possibly due to a primary increase in Glu transmission mediated via 5-HT(2A) receptor activation. Thus, mGlu2/3 receptor agonists might have some potential for treating motor impulsivity-related impairments while their cognitive enhancing effects were not confirmed in this study.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.