Serotonergic Innervations of the Orbitofrontal and Medial-prefrontal Cortices are Differentially Involved in Visual Discrimination and Reversal Learning in Rats.

Cross-species studies have identified an evolutionarily conserved role for serotonin in flexible behavior including reversal learning. The aim of the current study was to investigate the contribution of serotonin within the orbitofrontal cortex (OFC) and medial prefrontal cortex (mPFC) to visual discrimination and reversal learning. Male Lister Hooded rats were trained to discriminate between a rewarded (A+) and a nonrewarded (B-) visual stimulus to receive sucrose rewards in touchscreen operant chambers. Serotonin was depleted using surgical infusions of 5,7-dihydroxytryptamine (5,7-DHT), either globally by intracebroventricular (i.c.v.) infusions or locally by microinfusions into the OFC or mPFC. Rats that received i.c.v. infusions of 5,7-DHT before initial training were significantly impaired during both visual discrimination and subsequent reversal learning during which the stimulus-reward contingencies were changed (A- vs. B+). Local serotonin depletion from the OFC impaired reversal learning without affecting initial discrimination. After mPFC depletion, rats were unimpaired during reversal learning but slower to respond at the stimuli during all the stages; the mPFC group was also slower to learn during discrimination than the OFC group. These findings extend our understanding of serotonin in cognitive flexibility by revealing differential effects within two subregions of the prefrontal cortex in visual discrimination and reversal learning.

Reconsolidation and extinction are dissociable and mutually exclusive processes: behavioral and molecular evidence.

Memory persistence is critically influenced by retrieval. In rats, a single presentation of a conditioned fear stimulus induces memory reconsolidation and fear memory persistence, while repeated fear cue presentations result in loss of fear through extinction. These two opposite behavioral outcomes are operationally linked by the number of cue presentations at memory retrieval. However, the behavioral properties and mechanistic determinants of the transition have not yet been explored; in particular, whether reconsolidation and extinction processes coexist or are mutually exclusive, depending on the exposure to non-reinforced retrieval events. We characterized both behaviorally and molecularly the transition from reconsolidation to extinction of conditioned fear and showed that an increase in calcineurin (CaN) in the basolateral amygdala (BLA) supports the shift from fear maintenance to fear inhibition. Gradually increasing the extent of retrieval induces a gradual decrease in freezing responses to the conditioned stimulus and a gradual increase in amygdala CaN level. This newly synthesized CaN is required for the extinction, but not the reconsolidation, of conditioned fear. During the transition from reconsolidation to extinction, we have revealed an insensitive state of the fear memory where NMDA-type glutamate receptor agonist and antagonist drugs are unable either to modulate CaN levels in the BLA or alter the reconsolidation or extinction processes. Together, our data indicate both that reconsolidation and extinction are mutually exclusive processes and also reveal the presence of a transitional, or "limbo," state of the original memory between these two alternative outcomes of fear memory retrieval, when neither process is engaged.

The dopamine D2/D3 receptor agonist quinpirole increases checking-like behaviour in an operant observing response task with uncertain reinforcement: a novel possible model of OCD.

Excessive checking is a common, debilitating symptom of obsessive-compulsive disorder (OCD). In an established rodent model of OCD checking behaviour, quinpirole (dopamine D2/3-receptor agonist) increased checking in open-field tests, indicating dopaminergic modulation of checking-like behaviours. We designed a novel operant paradigm for rats (observing response task (ORT)) to further examine cognitive processes underpinning checking behaviour and clarify how and why checking develops. We investigated i) how quinpirole increases checking, ii) dependence of these effects on D2/3 receptor function (following treatment with D2/3 receptor antagonist sulpiride) and iii) effects of reward uncertainty. In the ORT, rats pressed an 'observing' lever for information about the location of an 'active' lever that provided food reinforcement. High- and low-checkers (defined from baseline observing) received quinpirole (0.5mg/kg, 10 treatments) or vehicle. Parametric task manipulations assessed observing/checking under increasing task demands relating to reinforcement uncertainty (variable response requirement and active-lever location switching). Treatment with sulpiride further probed the pharmacological basis of long-term behavioural changes. Quinpirole selectively increased checking, both functional observing lever presses (OLPs) and non-functional extra OLPs (EOLPs). The increase in OLPs and EOLPs was long-lasting, without further quinpirole administration. Quinpirole did not affect the immediate ability to use information from checking. Vehicle and quinpirole-treated rats (VEH and QNP respectively) were selectively sensitive to different forms of uncertainty. Sulpiride reduced non-functional EOLPs in QNP rats but had no effect on functional OLPs. These data have implications for treatment of compulsive checking in OCD, particularly for serotonin-reuptake-inhibitor treatment-refractory cases, where supplementation with dopamine receptor antagonists may be beneficial.

Approaches for drug delivery with intracortical probes.

Intracortical microprobes allow the precise monitoring of electrical and chemical signaling and are widely used in neuroscience. Microelectromechanical system (MEMS) technologies have greatly enhanced the integration of multifunctional probes by facilitating the combination of multiple recording electrodes and drug delivery channels in a single probe. Depending on the neuroscientific application, various assembly strategies are required in addition to the microprobe fabrication itself. This paper summarizes recent advances in the fabrication and assembly of micromachined silicon probes for drug delivery achieved within the EU-funded research project NeuroProbes. The described fabrication process combines a two-wafer silicon bonding process with deep reactive ion etching, wafer grinding, and thin film patterning and offers a maximum in design flexibility. By applying this process, three general comb-like microprobe designs featuring up to four 8-mm-long shafts, cross sections from 150×200 to 250×250 µm², and different electrode and fluidic channel configurations are realized. Furthermore, we discuss the development and application of different probe assemblies for acute, semichronic, and chronic applications, including comb and array assemblies, floating microprobe arrays, as well as the complete drug delivery system NeuroMedicator for small animal research.

Gamma aminobutyric acidergic and neuronal structural markers in the nucleus accumbens core underlie trait-like impulsive behavior.

BACKGROUND: Pathological forms of impulsivity are manifest in a number of psychiatric disorders listed in DSM-5, including attention-deficit/hyperactivity disorder and substance use disorder. However, the molecular and cellular substrates of impulsivity are poorly understood. Here, we investigated a specific form of motor impulsivity in rats, namely premature responding, on a five-choice serial reaction time task. METHODS: We used in vivo voxel-based magnetic resonance imaging and ex vivo Western blot analyses to investigate putative structural, neuronal, and glial protein markers in low-impulsive (LI) and high-impulsive rats. We also investigated whether messenger RNA interference targeting glutamate decarboxylase 65/67 (GAD65/67) gene expression in the nucleus accumbens core (NAcbC) is sufficient to increase impulsivity in LI rats. RESULTS: We identified structural and molecular abnormalities in the NAcbC associated with motor impulsivity in rats. We report a reduction in gray matter density in the left NAcbC of high-impulsive rats, with corresponding reductions in this region of glutamate decarboxylase (GAD65/67) and markers of dendritic spines and microtubules. We further demonstrate that the experimental reduction of de novo of GAD65/67 expression bilaterally in the NAcbC is sufficient to increase impulsivity in LI rats. CONCLUSIONS: These results reveal a novel mechanism of impulsivity in rats involving gamma aminobutyric acidergic and structural abnormalities in the NAcbC with potential relevance to the etiology and treatment of attention-deficit/hyperactivity disorder and related disorders.

An intra-cerebral drug delivery system for freely moving animals.

Microinfusions of drugs directly into the central nervous system of awake animals represent a widely used means of unravelling brain functions related to behaviour. However, current approaches generally use tethered liquid infusion systems and a syringe pump to deliver drugs into the brain, which often interfere with behaviour. We address this shortfall with a miniaturised electronically-controlled drug delivery system (20 × 17.5 × 5 mm³) designed to be skull-mounted in rats. The device features a micropump connected to two 8-mm-long silicon microprobes with a cross section of 250 × 250 µm² and integrated fluid microchannels. Using an external electronic control unit, the device allows infusion of 16 metered doses (0.25 µL each, 8 per silicon shaft). Each dosage requires 3.375 Ws of electrical power making the device additionally compatible with state-of-the-art wireless headstages. A dosage precision of 0.25 ± 0.01 µL was determined in vitro before in vivo tests were carried out in awake rats. No passive leakage from the loaded devices into the brain could be detected using methylene blue dye. Finally, the device was used to investigate the effects of the NMDA-receptor antagonist 3-((R)-2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid, (R)-CPP, administered directly into the prefrontal cortex of rats during performance on a task to assess visual attention and impulsivity. In agreement with previous findings using conventional tethered infusion systems, acute (R)-CPP administration produced a marked increase in impulsivity.

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.

Impulsive behaviour induced by both NMDA receptor antagonism and GABAA receptor activation in rat ventromedial prefrontal cortex.

RATIONALE: Previous work has demonstrated a profound effect of N-methyl-D: -aspartic acid receptor (NMDAR) antagonism in the infralimbic cortex (IL) to selectively elevate impulsive responding in a rodent reaction time paradigm. However, the mechanism underlying this effect is unclear. OBJECTIVES: This series of experiments investigated the pharmacological basis of this effect in terms of excitatory and inhibitory neurotransmission. We tested several pharmacological mechanisms that might produce the effect of NMDAR antagonism via disruption or dampening of IL output. METHODS: Drugs known to affect brain GABA or glutamate function were tested in rats pre-trained on a five-choice serial reaction time task (5-CSRTT) following either their systemic administration or direct administration into the IL. RESULTS: Systemic lamotrigine administration (15 mg/kg), which attenuates excess glutamate release, did not counteract the ability of the intra-IL NMDAR antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-L: -phosphonic acid ((R)-CPP) to increase premature responding on the 5-CSRTT. Putative elevation of local extracellular glutamate via intra-IL infusions of the selective glutamate reuptake inhibitor DL: -threo-ß-benzyloxyaspartate as well as local α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor antagonism also had no effect on this task. However, intra-IL infusions of the GABA(A) receptor agonist muscimol produced qualitatively but not quantitatively comparable increases in impulsive responding to those elicited by (R)-CPP. Moreover, the GABA(A) receptor antagonist bicuculline blocked the increase in impulsivity produced by (R)-CPP when infused in the IL. CONCLUSIONS: These findings implicate glutamatergic and GABAergic mechanisms in the IL in the expression of impulsivity and suggest that excessive glutamate release may not underlie increased impulsivity induced by local NMDA receptor antagonism.

Modulation of high impulsivity and attentional performance in rats by selective direct and indirect dopaminergic and noradrenergic receptor agonists.

RATIONALE: Impulsivity is associated with a number of psychiatric disorders, most notably attention deficit/hyperactivity disorder (ADHD). Drugs that augment catecholamine function (e.g. methylphenidate and the selective noradrenaline reuptake inhibitor atomoxetine) have clinical efficacy in ADHD, but their precise mechanism of action is unclear. OBJECTIVE: The objective of this study is to investigate the relative contribution of dopamine (DA) and noradrenaline (NA) to the therapeutic effects of clinically effective drugs in ADHD using rats selected for high impulsivity on the five-choice serial reaction time task (5CSRTT). METHODS: We examined the effects of direct and indirect DA and NA receptor agonists and selective DA and NA reuptake inhibitors in rats showing high and low levels of impulsivity on the 5CSRTT (designated high impulsive 'HI' and low impulsive 'LI', respectively). Drugs were administered by systemic injection in a randomized, counterbalanced manner. RESULTS: Low doses of quinpirole (a D2/D3 agonist) and sumanirole (a D2 agonist) selectively reduced impulsivity on the 5CSRTT, whilst higher doses resulted in increased omissions and slower response latencies. The NA reuptake inhibitor, atomoxetine, and the alpha-2 adrenoreceptor agonist, guanfacine, dose dependently decreased premature responding. The dopaminergic reuptake inhibitor GBR-12909 increased impulsivity, whereas the nonselective DA and NA reuptake inhibitor methylphenidate had no significant effect on impulsive responses in HI and LI rats. CONCLUSIONS: These findings indicate that high impulsivity can be ameliorated in rats by drugs that mimic the effects of DA and NA, just as in ADHD, and that activation of D2/3 receptors selectively decreases high impulsivity on the 5CSRTT.

Prefrontal and monoaminergic contributions to stop-signal task performance in rats.

Defining the neural and neurochemical substrates of response inhibition is of crucial importance for the study and treatment of pathologies characterized by impulsivity such as attention-deficit/hyperactivity disorder and addiction. The stop-signal task (SST) is one of the most popular paradigms used to study the speed and efficacy of inhibitory processes in humans and other animals. Here we investigated the effect of temporarily inactivating different prefrontal subregions in the rat by means of muscimol microinfusions on SST performance. We found that dorsomedial prefrontal cortical areas are important for inhibiting an already initiated response. We also investigated the possible neural substrates of the selective noradrenaline reuptake inhibitor atomoxetine via its local microinfusion into different subregions of the rat prefrontal cortex. Our results show that both orbitofrontal and dorsal prelimbic cortices mediate the beneficial effects of atomoxetine on SST performance. To assess the neurochemical specificity of these effects, we infused the α2-adrenergic agonist guanfacine and the D(1)/D(2) antagonist α-flupenthixol in dorsal prelimbic cortex to interfere with noradrenergic and dopaminergic neurotransmission, respectively. Guanfacine, which modulates noradrenergic neurotransmission, selectively impaired stopping, whereas blocking dopaminergic receptors by α-flupenthixol infusion prolonged go reaction time only, confirming the important role of noradrenergic neurotransmission in response inhibition. These results show that, similar to humans, distinct networks play important roles during SST performance in the rat and that they are differentially modulated by noradrenergic and dopaminergic neurotransmission. This study advances our understanding of the neuroanatomical and neurochemical determinants of impulsivity, which are relevant for a range of psychiatric disorders.

Contrasting roles for dopamine D1 and D2 receptor subtypes in the dorsomedial striatum but not the nucleus accumbens core during behavioral inhibition in the stop-signal task in rats.

Dopamine and dopamine-receptor function are often implicated in behavioral inhibition, and deficiencies within behavioral inhibition processes linked to attention deficit/hyperactivity disorder (ADHD), schizophrenia, obsessive-compulsive disorder, and drug addiction. In the stop-signal task, which measures the speed of the process of inhibition [stop-signal reaction time (SSRT)], psychostimulant-related improvement of SSRT in ADHD is linked with dopamine function. However, the precise nature of dopaminergic control over SSRT remains unclear. This study examined region- and receptor-specific modulation of SSRT in the rat using direct infusions of the dopamine D1 receptor (DRD1) antagonist SCH 23390 or dopamine D2 receptor (DRD2) antagonist sulpiride into the dorsomedial striatum (DMStr) or nucleus accumbens core (NAcbC). DRD1 and DRD2 antagonists had contrasting effects on SSRT that were specific to the DMStr. SCH 23390 decreased SSRT with little effect on the go response. Conversely, sulpiride increased SSRT but also increased go-trial reaction time and reduced trial completion at the highest doses. These results suggest that DRD1 and DRD2 function within the DMStr, but not the NAcbC, may act to balance behavioral inhibition in a manner that is independent of behavioral activation.

Dissociable effects of lesions to orbitofrontal cortex subregions on impulsive choice in the rat.

The orbitofrontal cortex (OFC) is implicated in a variety of adaptive decision-making processes. Human studies suggest that there is a functional dissociation between medial and lateral OFC (mOFC and lOFC, respectively) subregions when performing certain choice procedures. However, little work has examined the functional consequences of manipulations of OFC subregions on decision making in rodents. In the present experiments, impulsive choice was assessed by evaluating intolerance to delayed, but economically optimal, reward options using a delay-discounting paradigm. Following initial delay-discounting training, rats received bilateral neurotoxic or sham lesions targeting whole OFC (wOFC) or restricted to either mOFC or lOFC subregions. A transient flattening of delay-discounting curves was observed in wOFC-lesioned animals relative to shams--differences that disappeared with further training. Stable, dissociable effects were found when lesions were restricted to OFC subregions; mOFC-lesioned rats showed increased, whereas lOFC-lesioned rats showed decreased, preference for the larger-delayed reward relative to sham-controls--a pattern that remained significant during retraining after all delays were removed. When locations of levers leading to small-immediate versus large-delayed rewards were reversed, wOFC- and lOFC-lesioned rats showed retarded, whereas mOFC-lesioned rats showed accelerated, trajectories for reversal of lever preference. These results provide the first direct evidence for dissociable functional roles of the mOFC and lOFC for impulsive choice in rodents. The findings are consistent with recent human functional imaging studies and suggest that functions of mOFC and lOFC subregions may be evolutionarily conserved and contribute differentially to decision-making processes.

High impulsivity predicting vulnerability to cocaine addiction in rats: some relationship with novelty preference but not novelty reactivity, anxiety or stress.

RATIONALE: Impulsivity is a vulnerability marker for drug addiction in which other behavioural traits such as anxiety and novelty seeking ('sensation seeking') are also widely present. However, inter-relationships between impulsivity, novelty seeking and anxiety traits are poorly understood. OBJECTIVE: The objective of this paper was to investigate the contribution of novelty seeking and anxiety traits to the expression of behavioural impulsivity in rats. METHODS: Rats were screened on the five-choice serial reaction time task (5-CSRTT) for spontaneously high impulsivity (SHI) and low impulsivity (SLI) and subsequently tested for novelty reactivity and preference, assessed by open-field locomotor activity (OF), novelty place preference (NPP), and novel object recognition (OR). Anxiety was assessed on the elevated plus maze (EPM) both prior to and following the administration of the anxiolytic drug diazepam, and by blood corticosterone levels following forced novelty exposure. Finally, the effects of diazepam on impulsivity and visual attention were assessed in SHI and SLI rats. RESULTS: SHI rats were significantly faster to enter an open arm on the EPM and exhibited preference for novelty in the OR and NPP tests, unlike SLI rats. However, there was no dimensional relationship between impulsivity and either novelty-seeking behaviour, anxiety levels, OF activity or novelty-induced changes in blood corticosterone levels. By contrast, diazepam (0.3-3 mg/kg), whilst not significantly increasing or decreasing impulsivity in SHI and SLI rats, did reduce the contrast in impulsivity between these two groups of animals. CONCLUSIONS: This investigation indicates that behavioural impulsivity in rats on the 5-CSRTT, which predicts vulnerability for cocaine addiction, is distinct from anxiety, novelty reactivity and novelty-induced stress responses, and thus has relevance for the aetiology of drug addiction.

Yohimbine increases impulsivity through activation of cAMP response element binding in the orbitofrontal cortex.

BACKGROUND: Stress can increase impulsivity and has a negative impact on psychiatric outcome. Norepinephrine is heavily implicated in responses to stress, and the alpha(2) antagonist yohimbine is used clinically to study this aspect of the stress response. Yohimbine induces mild anxiety and increases impulsivity in healthy volunteers but has more detrimental effects in some psychiatric populations, triggering mania in bipolar patients and drug craving in substance-dependent individuals. Understanding the mechanism by which yohimbine affects brain function could provide insight into the heightened reaction to stress in these patients. METHODS: Yohimbine's effects were assessed in rats using the five-choice serial reaction time test of attention and impulse control. We then examined whether yohimbine altered activity of cyclic adenosine monophosphate response element binding (CREB) protein-a transcription factor implicated in the stress response-in brain areas that regulate impulsivity. The behavioral consequences of any changes in CREB activity were subsequently assessed using viral-mediated gene transfer to regionally overexpress CREB or the dominant negative antagonist mCREB. RESULTS: Yohimbine increased impulsive responding in rats and selectively increased CREB phosphorylation within the orbitofrontal cortex but not medial prefrontal cortex or nucleus accumbens. Overexpressing mCREB within the orbitofrontal cortex blocked yohimbine's effects on impulsivity, whereas overexpressing CREB in this region increased impulsive responding and potentiated the proimpulsive actions of yohimbine. DISCUSSION: These data suggest a novel molecular mechanism contributing to impulsivity that may be sensitive to stress. Such findings may improve our understanding of the neurobiological pathways linking the response to stress and impulsivity in both healthy and psychiatric populations.

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.

Serotonin modulates sensitivity to reward and negative feedback in a probabilistic reversal learning task in rats.

Depressed patients show cognitive deficits that may depend on an abnormal reaction to positive and negative feedback. The precise neurochemical mechanisms responsible for such cognitive abnormalities have not yet been clearly characterized, although serotoninergic dysfunction is frequently associated with depression. In three experiments described here, we investigated the effects of different manipulations of central serotonin (5-hydroxytryptamine, 5-HT) levels in rats performing a probabilistic reversal learning task that measures response to feedback. Increasing or decreasing 5-HT tone differentially affected behavioral indices of cognitive flexibility (reversals completed), reward sensitivity (win-stay), and reaction to negative feedback (lose-shift). A single low dose of the selective serotonin reuptake inhibitor citalopram (1 mg/kg) resulted in fewer reversals completed and increased lose-shift behavior. By contrast, a single higher dose of citalopram (10 mg/kg) exerted the opposite effect on both measures. Repeated (5 mg/kg, daily, 7 days) and subchronic (10 mg/kg, b.i.d., 5 days) administration of citalopram increased the number of reversals completed by the animals and increased the frequency of win-stay behavior, whereas global 5-HT depletion had the opposite effect on both indices. These results show that boosting 5-HT neurotransmission decreases negative feedback sensitivity and increases reward (positive feedback) sensitivity, whereas reducing it has the opposite effect. However, these effects depend on the nature of the manipulation used: acute manipulations of the 5-HT system modulate negative feedback sensitivity, whereas long-lasting treatments specifically affect reward sensitivity. These results parallel some of the findings in humans on effects of 5-HT manipulations and are relevant to hypotheses of altered response to feedback in depression.

Environmental enrichment produces a behavioral phenotype mediated by low cyclic adenosine monophosphate response element binding (CREB) activity in the nucleus accumbens.

BACKGROUND: Previous research has shown that rats reared in an enriched condition (EC) are more sensitive to the acute effects of amphetamine than rats reared in an isolated condition (IC); yet, EC rats self-administer less amphetamine than IC rats. The present study used cocaine to further explore this environmental enrichment behavioral phenotype, as well as the underlying molecular mechanisms involved. METHODS: Enriched condition and IC rats were studied in a broad battery of behavioral tests, including cocaine conditioned place preference (CPP) and self-administration and several measures of anxiety- and depression-related behavior. The involvement of the transcription factor, cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), in mediating EC versus IC differences was investigated. RESULTS: Enriched condition rats exhibited less cocaine self-administration, despite showing enhanced cocaine CPP. Enriched condition rats also displayed less depression-like behavior but higher levels of anxiety-like behavior. This behavioral phenotype is consistent with low CREB activity in the nucleus accumbens, a key brain reward region. Indeed, EC rats have less phospho-CREB (the transcriptionally active form of the protein) in the nucleus accumbens than IC rats, and a selective knockdown of CREB in this brain region of normally reared rats, by use of a novel viral vector expressing a short hairpin RNA (shRNA) directed against CREB, reproduced the EC behavioral phenotype. CONCLUSIONS: These studies identify a potential molecular mechanism for how rearing environment-a nonpharmacological, nonsurgical manipulation-can modify a wide range of complex emotional behaviors.

Loss of the imprinted snoRNA mbii-52 leads to increased 5htr2c pre-RNA editing and altered 5HT2CR-mediated behaviour.

The Prader-Willi syndrome (PWS) genetic interval contains several brain-expressed small nucleolar (sno)RNA species that are subject to genomic imprinting. In vitro studies have shown that one of these snoRNA molecules, h/mbii-52, negatively regulates editing and alternative splicing of the serotonin 2C receptor (5htr2c) pre-RNA. However, the functional consequences of loss of h/mbii-52 and subsequent increased post-transcriptional modification of 5htr2c are unknown. 5HT2CRs are important in controlling aspects of cognition and the cessation of feeding, and disruption of their function may underlie some of the psychiatric and feeding abnormalities seen in PWS. In a mouse model for PWS lacking expression of mbii-52 (PWS-IC+/-), we show an increase in editing, but not alternative splicing, of the 5htr2c pre-RNA. This change in post-transcriptional modification is associated with alterations in a number of 5HT2CR-related behaviours, including impulsive responding, locomotor activity and reactivity to palatable foodstuffs. In a non-5HT2CR-related behaviour, marble burying, loss of mbii-52 was without effect. The specificity of the behavioural effects to changes in 5HT2CR function was further confirmed using drug challenges. These data illustrate, for the first time, the physiological consequences of altered RNA editing of 5htr2c linked to mbii-52 loss that may underlie specific aspects of the complex PWS phenotype and point to an important functional role for this imprinted snoRNA.

DeltaFosB induction in orbitofrontal cortex potentiates locomotor sensitization despite attenuating the cognitive dysfunction caused by cocaine.

The effects of addictive drugs change with repeated use: many individuals become tolerant of their pleasurable effects but also more sensitive to negative sequelae (e.g., anxiety, paranoia, and drug craving). Understanding the mechanisms underlying such tolerance and sensitization may provide valuable insight into the basis of drug dependency and addiction. We have recently shown that chronic cocaine administration reduces the ability of an acute injection of cocaine to affect impulsivity in rats. However, animals become more impulsive during withdrawal from cocaine self-administration. We have also shown that chronic administration of cocaine increases expression of the transcription factor DeltaFosB in the orbitofrontal cortex (OFC). Mimicking this drug-induced elevation in OFC DeltaFosB through viral-mediated gene transfer mimics these behavioural changes: DeltaFosB over-expression in OFC induces tolerance to the effects of an acute cocaine challenge but sensitizes rats to the cognitive sequelae of withdrawal. Here we report novel data demonstrating that increasing DeltaFosB in the OFC also sensitizes animals to the locomotor-stimulant properties of cocaine. Analysis of nucleus accumbens tissue taken from rats over-expressing DeltaFosB in the OFC and treated chronically with saline or cocaine does not provide support for the hypothesis that increasing OFC DeltaFosB potentiates sensitization via the nucleus accumbens. These data suggest that both tolerance and sensitization to cocaine's many effects, although seemingly opposing processes, can be induced in parallel via the same biological mechanism within the same brain region, and that drug-induced changes in gene expression within the OFC play an important role in multiple aspects of addiction.

Serotonin depletion impairs waiting but not stop-signal reaction time in rats: implications for theories of the role of 5-HT in behavioral inhibition.

Central serotonin (5-HT) function is thought to be a critical component of behavioral inhibition and impulse control. However, in recent clinical studies, 5-HT manipulations failed to affect stop-signal reaction time (SSRT), which is a fundamental process in behavioral inhibition. We investigated the effect of central 5-HT depletion (intracerebroventricular 5,7-dihydroxytryptamine) in rats on two aspects of behavioral inhibition, SSRT and 'waiting', using the stop-signal task. 5-HT depletion had no effects on SSRT or any other primary measure on the stop-signal task. However, within the same task, there was a deficit in 'waiting' in 5-HT-depleted rats when they were required to withhold from responding in the terminal element of the stop-signal task for an extended period. D-Amphetamine had dose-dependent, but not 5-HT-dependent effects on SSRT. Conversely, the dose that tended to improve, or decrease, SSRT (0.3 mg/kg) impaired the ability to wait, again independently of 5-HT manipulation. These findings suggest that SSRT and 'waiting' are distinct measures of behavioral inhibition, and show that 5-HT is critical for some forms of behavioral inhibition but not others. This has significant implications for the treatment of conditions such as attention deficit and hyperactivity disorder, substance abuse, and affective disorders, in which inhibitory and impulse-control deficits are common.