Disruption of dopaminergic neurotransmission in nucleus accumbens core inhibits the locomotor stimulant effects of nicotine and D-amphetamine in rats

Sexual satiety modifies methamphetamine-induced locomotor and rewarding effects and dopamine-related protein levels in the striatum of male rats

RATIONALE

Drug and natural rewarding stimuli activate the mesolimbic dopaminergic system. Both methamphetamine (Meth) and copulation to satiety importantly increase dopamine (DA) release in the nucleus accumbens (NAc), but with differences in magnitude. This paper analyzes the interaction between Meth administration and the intense sexual activity associated with sexual satiety.

OBJECTIVES

To evaluate possible changes in Meth-induced behavioral effects and striatal DA-related protein expression due to sexual satiety.

METHODS

Meth-induced locomotor activity and conditioned place preference (CPP) were tested in sexually experienced male rats that copulated to satiety (S-S) or ejaculated once (1E) the day before or displayed no sexual activity (control group; C). DA receptors and DA transporter expression were determined by western blot in the striatum of animals of all sexual conditions treated with specific Meth doses.

RESULTS

Meth's locomotor and rewarding effects were exacerbated in S-S animals, while in 1E rats, only locomotor effects were enhanced. Sexual activity, by itself, modified DA-related protein expression in the NAc core and in the caudate-putamen (CPu), while Meth treatment alone changed their expression only in the NAc shell. Meth-induced changes in the NAc shell turned in the opposite direction when animals had sexual activity, and additional changes appeared in the NAc core and CPu of S-S rats.

CONCLUSION

Sexual satiety sensitizes rats to Meth's behavioral effects and the Meth-induced striatal DA-related protein adaptations are modified by sexual activity, evidencing cross-sensitization between both stimuli.

The Effects of Intravermis Cerebellar Microinjections of Dopaminergic Agents in Motor Learning and Aversive Memory Acquisition in Mice

The cerebellum receives dopaminergic innervation and expresses the five types of described dopaminergic receptors. The cerebellar function involves both motor movement and cognition, but the role of cerebellar dopaminergic system on these processes remain unclear. The present study explores the behavioral responses to intracerebellar microinjection of dopaminergic agents in motor and emotional memory. For this, naïve Swiss mice had their cerebellar vermis implanted with a guide canula, received a intravermis microinjection of Dopamine, D1-like antagonist SCH-23390 or D2-like antagonist Eticlopride, and underwent a behavioral analysis of motor learning (by a Rotarod and balance beam learning protocol) or aversive memory acquisition (by the inhibitory avoidance task). The mixed-effects analysis was used to evaluate groups performance, followed by Tukey’s post hoc when appropriated. In this study, Dopamine, SCH-23390 and Eticlopride at the doses used did not affected motor control and motor learning. In addition, the administration of Dopamine and SCH-233390 had no effects on emotional memory acquisition, but the animals that received the highest dose of Eticlopride had an improvement in aversive memory acquisition, shown by a suppression of its innate preference for the dark compartment of the inhibitory avoidance apparatus following an exposure to a foot shock. We propose that cerebellar dopaminergic D2 receptors seem to participate on the modulation of aversive memory processes, without influencing motor performance at the doses used in this study.

Potential beneficial effects of caffeine administration in the neonatal period of an animal model of schizophrenia

Obstetric complications, like maternal hypertension and neonatal hypoxia, disrupt brain development, leading to psychiatry disorders later in life, like schizophrenia. The exact mechanisms behind this risk are not yet well known. Spontaneously hypertensive rats (SHR) are a well-established model to study neurodevelopment of schizophrenia since they exhibit behavioral alterations mimicking schizophrenia that can be improved with antipsychotic drugs. SHR mothers are hypertensive, and the SHR offspring develop in preeclampsia-like conditions. Hypoxic conditions increase levels of adenosine, which play an important role in brain development. The enhanced levels of adenosine at birth could be related to the future development of schizophrenia. To investigate this hypothesis adenosine levels of brain neonatal Wistar rats and SHR were quantified. After that, caffeine, an antagonist of adenosinergic system, was administrated on PND (postnatal day) 7 (neurodevelopmental age similar to a human at delivery) and rats were observed at adolescent and adult ages. We also investigated the acute effects of caffeine at adolescent and adult ages. SHR control adolescent and adult groups presented behavioral deficits like hyperlocomotion, deficit in social interaction (SI), and contextual fear conditioning (CFC). In SHR, neonatal caffeine treatment on PND 7 normalized hyperlocomotion, improved SI, and CFC observed at adolescent period and adult ages, showing a beneficial effect on schizophrenia-like behaviors. Wistar rats neonatally treated with caffeine exhibited hyperlocomotion, deficit in SI and CFC when observed at adolescent and adult ages. Acutely caffeine treatment administrated at adolescent and adult ages increased locomotion and decreased SI time of Wistar rats and impair CFC in adult Wistars. No effects were observed in SHR. In conclusion, caffeine can be suggested as a useful drug to prevent behavioral deficits observed in this animal model of prenatal hypoxia-induced schizophrenia profile when specifically administered on PND 7.

Development of a peptide targeting dopamine transporter to improve ADHD-like deficits

Attention-deficit hyperactivity disorder (ADHD) is a neurocognitive disorder characterized by hyperactivity, inattention, working memory deficits and impulsivity. Its worldwide prevalence is estimated to be 3–5% in children and adolescents. The mainstay treatment for ADHD is stimulant medications (e.g. methylphenidate), which increase synaptic dopamine by directly blocking dopamine transporter (DAT). Although these pharmacological agents are effective, they are often associated with various side effects including risks for future substance use disorders in ADHD patients. Here, we investigated an interaction between DAT and dopamine D2 receptor (D2R) as a novel target to develop potential therapeutics for the treatment of ADHD by using an interfering peptide (TAT-DAT_NT) to dissociate this protein complex. We found that TAT-DAT_NT promotes locomotor behavior in Sprague-Dawley rats. Furthermore, using in vivo microdialysis and high-performance liquid chromatography, we found that the disruption of D2R-DAT elevates extracellular dopamine level. More importantly, the interfering peptide, TAT-DAT_NT, attenuates hyperactivity and improves spontaneous alternation behavior in spontaneously hypertensive rats (SHR) ------ a common animal model of ADHD. This work presents a different means (i.e. other than direct blockade by a DAT inhibitor) to regulate the activity of DAT and dopaminergic neurotransmission, and a potential target site for future development of ADHD treatments.

Cortical high gamma network oscillations and connectivity: a translational index for antipsychotics to normalize aberrant neurophysiological activity

Oscillatory activity in the gamma frequency range is a critical mechanism, which integrates neural networks within and across brain structures during cognitive processes. In schizophrenia, abnormalities in high gamma oscillations are ubiquitous and most likely reflect dysfunction in neuronal networks. In conscious rats, disturbed network oscillations associated with positive symptoms and cognitive deficits were modeled in different cortical areas by the dopaminergic agonist (amphetamine) and the N-methyl-D-aspartate (NMDA) receptor antagonists (PCP and MK801). Subsequently, the efficacies of marketed atypical antipsychotics (olanzapine, risperidone, and clozapine) to normalize dysfunctional oscillations and network connectivity were examined. Acute NMDA antagonists elicited aberrant synchrony in the gamma frequency oscillations. In addition, coherent slow alpha network activity was observed with MK801 and amphetamine, both of whose oscillatory rhythms were correlated with pronounced locomotor activity. All antipsychotics commonly decreased slow alpha and high gamma network oscillations in different cortical regions as well as motion behavior. In the combined treatments, antipsychotics attenuated NMDA antagonist-induced abnormalities in functional network oscillations and connectivity, whose effects on motor behavior is mechanistically related. These results suggest that pharmacologically induced disruption of cortical gamma oscillations and network connectivity in rats is a candidate model to study dysfunctional oscillatory patterns described in positive and negative symptoms of schizophrenia. The efficacy of antipsychotics to rescue cortical network oscillatory patterns is in line with the idea that glutamatergic and dopaminergic systems play a role in maintaining the integrity of cortical circuits. Thus, gamma oscillations could provide a powerful translational index to assess the integrity of neural networks and to evaluate the efficacy of drugs with potential antipsychotic properties.

Interference of norepinephrine transporter trafficking motif attenuates amphetamine-induced locomotor hyperactivity and conditioned place preference

Amphetamine (AMPH)-mediated norepinephrine transporter (NET) downregulation requires NET-T258/S259 trafficking motif. The present study utilizes cell permeable NET-T258/S259 motif interfering peptide, which blocks AMPH-induced NET downregulation, to explore the role of this form of NET regulation in AMPH-mediated behaviors. In rats receiving intra-accumbal microinjections of TAT-conjugated peptides encompassing NET-T258/S259 motif, acute systemic AMPH failed to inhibit NE transport in the TAT-NET-T258/S259 wild-type (WT) peptide injected hemisphere but not in the vehicle or scrambled peptide injected hemisphere. Acute AMPH-induced hyperactivity was significantly reduced in rats receiving intra-accumbal TAT-NET-T258/S259 WT peptide compared to those receiving intra-accumbal vehicle or TAT-NET-T258A/S259A mutant peptide or corresponding TAT-conjugated scrambled peptide. Basal locomotor activity was not altered by peptide infusions alone. Similarly AMPH-induced locomotor sensitization was significantly reduced in rats receiving intra-accumbal TAT-NET-T258/S259 WT peptide prior to AMPH challenge and not in rats receiving the mutant or scrambled peptide. In conditioned place preference (CPP) paradigm, a single bilateral intra-accumbal microinjection of TAT-NET-T258/S259 WT peptide prior to CPP testing significantly reduced AMPH-induced CPP expression. Likewise, a single bilateral intra-accumbal microinjection of TAT-NET-T258/S259 WT peptide prior to drug-challenge significantly attenuated AMPH-primed CPP reinstatement. On the other hand, bilateral intra-accumbal microinjection of scrambled peptide did not affect AMPH-induced CPP expression or reinstatement. These data demonstrate a role for T258/S259-dependent NET regulation in AMPH-induced hyperactivity and sensitization as well as AMPH-induced CPP expression and reinstatement.

Role of dopamine projections from ventral tegmental area to nucleus accumbens and medial prefrontal cortex in reinforcement behaviors assessed using optogenetic manipulation

Dopamine (DA) neurons in the ventral tegmental area (VTA) are predicted to play important roles in reward. In pharmacological studies, the rewarding effects of methamphetamine are mediated by DA neurons localized in the VTA. The nucleus accumbens (NAc) and medial prefrontal cortices (mPFC) are the main projections from the VTA. However, the role of these projections remains unclear, particularly the mPFC projections. In the present study, DAT-Cre transgenic mice received an injection of adeno-associated viral vectors encoding channelrhodopsin2 (ChR2) or control vector into the VTA resulting in the selective expression of these opsins in DA neurons. Then, we stimulated the VTA, NAc (core and shell) or mPFC (prelimbic cortex (PL) and infralimbic cortex (IL)) via an optical fiber. The mice with ChR2 learned instrumental responses corresponding to the delivery of photostimulation into the VTA. The projections to the NAc core and shell from the VTA and stimulation of the NAc subregion both induced reinforcement. For projections to the mPFC (IL and PL), we verified that stimulation of the IL induced reinforcement dependent on DA from the VTA but not the PL. Furthermore, micro-infusion of methamphetamine into the NAc core and NAc shell also induced hyper-locomotion in a dose-dependent manner with a slight tendency of increased excitation of the IL but not PL. Taken together, excitation of the projection into the NAc core, NAc shell and IL elicited positive behavior during reward.

Chronic lithium treatment rectifies maladaptive dopamine release in the nucleus accumbens

Chronic lithium treatment effectively reduces behavioral phenotypes of mania in humans and rodents. The mechanisms by which lithium exerts these actions are poorly understood. Pre-clinical and clinical evidence have implicated increased mesolimbic dopamine (DA) neurotransmission with mania. We used fast-scan cyclic voltammetry to characterize changes in extracellular DA concentrations in the nucleus accumbens (NAc) core evoked by 20 and 60 Hz electrical stimulation of the ventral tegmental area (VTA) in C57BL6/J mice treated either acutely or chronically with lithium. The effects of chronic lithium treatment on the availability of DA for release were assessed by depleting readily releasable DA using short inter-train intervals, or administering d-amphetamine acutely to mobilize readily releasable DA. Chronic, but not acute, lithium treatment decreased the amplitude of DA responses in the NAc following 60 Hz pulse train stimulation. Neither lithium treatment altered the kinetics of DA release or reuptake. Chronic treatment did not impact the progressive reduction in the amplitude of DA responses when, using 20 or 60 Hz pulse trains, the VTA was stimulated every 6 s to deplete DA. Specifically, the amplitude of DA responses to 60 Hz pulse trains was initially reduced compared to control mice, but by the fifth pulse train, there was no longer a treatment effect. However, chronic lithium treatment attenuated d-amphetamine-induced increases in DA responses to 20 Hz pulse trains stimulation. Our data suggest that long-term administration of lithium may ameliorate mania phenotypes by normalizing the readily releasable DA pool in VTA axon terminals in the NAc. Read the Editorial Highlight for this article on Page 520.

Essential Role of Mesolimbic Brain-Derived Neurotrophic Factor in Chronic Social Stress-Induced Depressive Behaviors

BACKGROUND

Previous work has shown that chronic social defeat stress (CSDS) induces increased phasic firing of ventral tegmental area (VTA) dopamine (DA) neurons that project to the nucleus accumbens (NAc) selectively in mice that are susceptible to the deleterious effects of the stress. In addition, acute optogenetic phasic stimulation of these neurons promotes susceptibility in animals exposed to acute defeat stress. These findings are paradoxical, as increased DA signaling in NAc normally promotes motivation and reward, and the influence of chronic phasic VTA firing in the face of chronic stress is unknown.

METHODS

We used CSDS with repeated optogenetic activation and pharmacologic manipulations of the mesolimbic VTA-NAc pathway to examine the role of brain-derived neurotrophic factor (BDNF) and DA signaling in depressive-like behaviors. We measured BDNF protein expression and DA release in this model.

RESULTS

Pharmacologic blockade of BDNF-tyrosine receptor kinase B (TrkB) signaling, but not DA signaling, in NAc prevented CSDS-induced behavioral abnormalities. Chronic optogenetic phasic stimulation of the VTA-NAc circuit during CSDS exacerbated the defeat-induced behavioral symptoms, and these aggravated symptoms were also normalized by BDNF-TrkB blockade in NAc. The aggravated behavioral deficits induced by phasic stimulation of the VTA-NAc pathway were blocked as well by local knockdown of BDNF in VTA.

CONCLUSIONS

These findings show that BDNF-TrkB signaling, rather than DA signaling, in the VTA-NAc circuit is crucial for facilitating depressive-like outcomes after CSDS and they establish BDNF-TrkB signaling as a pathologic mechanism during periods of chronic stress.

Caffeine alters the behavioural and body temperature responses to mephedrone without causing long-term neurotoxicity in rats

Administration of caffeine with 3,4-methylenedioxymethamphetamine (MDMA) alters the pharmacological properties of MDMA in rats. The current study examined whether caffeine alters the behavioural and neurochemical effects of mephedrone, which has similar psychoactive effects to MDMA. Rats received either saline, mephedrone (10 mg/kg), caffeine (10 mg/kg) or combined caffeine and mephedrone intraperitoneally twice weekly on consecutive days for three weeks. Locomotor activity (days 1 and 16), novel object discrimination (NOD, day 2), elevated plus maze (EPM) exploration (day 8), rectal temperature changes (day 9) and pre-pulse inhibition (PPI) of acoustic startle response (day 15) were assessed. Seven days after the final injection, brain regions were collected for the measurement of 5-hydroxytryptamine (5-HT), dopamine and their metabolites. Combined caffeine and mephedrone further enhanced the locomotor response observed following either drug administered alone, and converted mephedrone-induced hypothermia to hyperthermia. Co-administration also abolished mephedrone-induced anxiogenic response on the EPM, but had no effect on NOD or PPI. Importantly, no long-term neurotoxicity was detected following repeated mephedrone alone or when co-administered with caffeine. In conclusion, the study suggests a potentially dangerous effect of concomitant caffeine and mephedrone, and highlights the importance of taking polydrug use into consideration when investigating the acute adverse effect profile of popular recreational drugs.

Dopaminergic and cholinergic learning mechanisms in nicotine addiction

Nicotine addiction drives tobacco use by one billion people worldwide, causing nearly six million deaths a year. Nicotine binds to nicotinic acetylcholine receptors that are normally activated by the endogenous neurotransmitter acetylcholine. The widespread expression of nicotinic receptors throughout the nervous system accounts for the diverse physiological effects triggered by nicotine. A crucial influence of nicotine is on the synaptic mechanisms underlying learning that contribute to the addiction process. Here, we focus on the acquisition phase of smoking addiction and review animal model studies on how nicotine modifies dopaminergic and cholinergic signaling in key nodes of the reinforcement circuitry: ventral tegmental area, nucleus accumbens (NAc), amygdala, and hippocampus. Capitalizing on mechanisms that subserve natural rewards, nicotine activates midbrain dopamine neurons directly and indirectly, and nicotine causes dopamine release in very broad target areas throughout the brain, including the NAc, amygdala, and hippocampus. In addition, nicotine orchestrates local changes within those target structures, alters the release of virtually all major neurotransmitters, and primes the nervous system to the influence of other addictive drugs. Hence, understanding how nicotine affects the circuitry for synaptic plasticity and learning may aid in developing reasoned therapies to treat nicotine addiction.

The role of mesoaccumbens dopamine in nicotine dependence

There is abundant evidence that the dopamine (DA) neurons that project to the nucleus accumbens play a central role in neurobiological mechanisms underpinning drug dependence. This chapter considers the ways in which these projections facilitate the addiction to nicotine and tobacco. It focuses on the complimentary roles of the two principal subdivisions of the nucleus accumbens, the accumbal core and shell, in the acquisition and maintenance of nicotine-seeking behavior. The ways in which tonic and phasic firing of the neurons contributes to the ways in which the accumbens mediate the behavioral responses to nicotine are also considered. Experimental studies suggest that nicotine has relatively weak addictive properties which are insufficient to explain the powerful addictive properties of tobacco smoke. This chapter discusses hypotheses that seek to explain this conundrum. They implicate both discrete sensory stimuli closely paired with the delivery of tobacco smoke and contextual stimuli habitually associated with the delivery of the drug. The mechanisms by which each type of stimulus influence tobacco dependence are hypothesized to depend upon the increased DA release and overflow, respectively, in the two subdivisions of the accumbens. It is suggested that a majority of pharmacotherapies for tobacco dependence are not more successful because they fail to address this important aspect of the dependence.

A tobacco extract containing alkaloids induces distinct effects compared to pure nicotine on dopamine release in the rat

It has been suggested that minor alkaloids in plants play a role in the biological and neuronal actions of nicotine. We hypothesized that these molecules modulate the effect of nicotine on the activity of central dopamine (DA) neurons, one of the main cellular targets in addiction to drugs. In this study the effect of a single intraperitoneal injection of either nicotine or an alkaloid extract of the tobacco plant (0.5 mg/kg) on the efflux of DA were investigated. DA was measured in vivo by intracerebral microdialysis in the nucleus accumbens and the striatum of freely-moving rats. Results show that nicotine enhanced accumbal and striatal DA extracellular levels (+47 and 20% above baseline, respectively). The extract also evoked a significant increase in DA extracellular levels in both regions (+33 and +38% above baseline). However, this effect was significantly higher compared to nicotine in the striatum only. In conclusion, the tobacco extract enhanced the neurochemical effect of nicotine alone in the striatum, a response that could underlie the higher propensity of developing addictive-like behavior using nicotine with tobacco alkaloids.

Development of behavioral preferences for the optimal choice following unexpected reward omission is mediated by a reduction of D2-like receptor tone in the nucleus accumbens

To survive in a dynamic environment, animals must identify changes in resource availability and rapidly apply adaptive strategies to obtain resources that promote survival. We have utilised a behavioral paradigm to assess differences in foraging strategy when resource (reward) availability unexpectedly changes. When reward magnitude was reduced by 50% (receive one reward pellet instead of two), male and female rats developed a preference for the optimal choice by the second session. However, when an expected reward was omitted (receive no reward pellets instead of one), subjects displayed a robust preference for the optimal choice during the very first session. Previous research shows that, when an expected reward is omitted, dopamine neurons phasically decrease their firing rate, which is hypothesised to decrease dopamine release preferentially affecting D2-like receptors. As robust changes in behavioral preference were specific to reward omission, we tested this hypothesis and the functional role of D1- and D2-like receptors in the nucleus accumbens in mediating the rapid development of a behavioral preference for the rewarded option during reward omission in male rats. Blockade of both receptor types had no effect on this behavior; however, holding D2-like, but not D1-like, receptor tone via infusion of dopamine receptor agonists prevented the development of the preference for the rewarded option during reward omission. These results demonstrate that avoiding an outcome that has been tagged with aversive motivational properties is facilitated through decreased dopamine transmission and subsequent functional disruption of D2-like, but not D1-like, receptor tone in the nucleus accumbens.

Nicotine-motivated behavior in Caenorhabditis elegans requires the nicotinic acetylcholine receptor subunits acr-5 and acr-15

Signaling at nicotinic acetylcholine receptors in Caenorhabditis elegans controls many behaviors, including egg-laying and locomotor activity. Here, we show that C. elegans approaches a point source of nicotine in a time-, concentration- and age-dependent manner. Additionally, nicotine paired with butanone under starvation conditions prevented the reduced approach to butanone that is observed when butanone is paired with starvation alone and pairing with nicotine generates a preference for the tastes of either sodium or chloride over baseline. These results suggest nicotine acts as a rewarding substance in C. elegans. Furthermore, the nicotinic receptor antagonist mecamylamine, the smoking cessation pharmacotherapy varenicline, mutation of the dop-1 and dop-2 dopamine receptors, and mutations of either acr-5 or acr-15, two nicotinic receptor subunit genes with sequence homology to the mammalian α7 subunit, all reduced the nicotine approach behavior. These two mutants also were defective at associating the presence of nicotine with butanone under starvation conditions and acr-5 mutation could obviate the effect of pairing nicotine with salts. Furthermore, the approach deficit in acr-15 mutants was rescued by selective re-expression in a subset of neurons, but not in muscle. Caenorhabditis elegans may therefore serve as a useful model organism for nicotine-motivated behaviors that could aid in the identification of novel nicotine motivational molecular pathways and consequently the development of novel cessation aids.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

Distinct effects of enriched environment on dopamine clearance in nucleus accumbens shell and core following systemic nicotine administration

Environmental enrichment during development may reduce drug abuse liability by modulating dopamine transporter (DAT) function. Nucleus accumbens (NAc) shell and core respond differentially to regulate the rewarding properties and locomotor stimulant effects of psychostimulants. The current study evaluated dopamine (DA) clearance (CL(DA) ) in the NAc shell and core using in vivo voltammetry in rats raised in an enriched condition (EC) or an impoverished condition (IC) and determined the effect of nicotine (0.4 mg/kg) on CL(DA) . Baseline CL(DA) in NAc shell and core was not different between EC and IC rats. In the saline control group, CL(DA) in NAc shell was greater across time in IC when compared with EC rats, whereas CL(DA) in NAc core was greater in EC rats when compared with IC rats. Consistent with these findings, opposite effects of enrichment on DA clearance in shell and core were obtained following acute nicotine administration. In NAc shell, nicotine increased CL(DA) in EC rats, but not in IC rats. Conversely, in NAc core, nicotine increased CL(DA) in IC rats, but not in EC rats. The current results demonstrate that environmental enrichment differentially regulates the response to nicotine in NAc shell and core via alterations in DAT function, which may explain how environmental enrichment reduces the behavioral response to nicotine.

BDNF is a negative modulator of morphine action

Brain-derived neurotrophic factor (BDNF) is a key positive regulator of neural plasticity, promoting, for example, the actions of stimulant drugs of abuse such as cocaine. We discovered a surprising opposite role for BDNF in countering responses to chronic morphine exposure. The suppression of BDNF in the ventral tegmental area (VTA) enhanced the ability of morphine to increase dopamine (DA) neuron excitability and promote reward. In contrast, optical stimulation of VTA DA terminals in nucleus accumbens (NAc) completely reversed the suppressive effect of BDNF on morphine reward. Furthermore, we identified numerous genes in the NAc, a major target region of VTA DA neurons, whose regulation by BDNF in the context of chronic morphine exposure mediated this counteractive function. These findings provide insight into the molecular basis of morphine-induced neuroadaptations in the brain's reward circuitry.

Caffeine increases the motivation to obtain non-drug reinforcers in rats

BACKGROUND

Caffeine is widely considered to be a reinforcer in humans, but this effect is difficult to measure in non-human animals. We hypothesized that caffeine may have dual reinforcing effects comparable to nicotine--limited primary reinforcing effects, but potent reinforcement enhancing effects. The present studies tested this hypothesis by investigating the effect of caffeine on responding for non-drug rewards.

METHODS

In two experiments, rats were shaped to respond on a progressive ratio (PR) schedule for sucrose solution (20%, w/v; experiment 1) or a fixed ratio 2 (FR2) schedule for a moderately reinforcing visual stimulus (VS; experiment 2). Pretreatment with various doses of caffeine (0-50 mg/kg, intraperitoneal injection) were administered prior to tests over successive week days (M-F). In experiment 1, acute administration of low-moderate caffeine doses (6.25-25 mg/kg) increased responding for sucrose under the PR schedule. This effect of caffeine declined over the initial 15 test days. In experiment 2, only acute pretreatment with 12.5mg/kg caffeine increased responding for the visual stimulus and complete tolerance to this effect of caffeine was observed over the 15 days of testing. In follow up tests we found that abstinence periods of 4 and 8 days resulted in incomplete recovery of the enhancing effects of caffeine.

CONCLUSION

The findings suggest that caffeine enhances the reinforcing effects of non-drug stimuli, but that the pharmacological profile of these effects may differ from other psychomotor stimulants.

Preclinical evidence that activation of mesolimbic alpha 6 subunit containing nicotinic acetylcholine receptors supports nicotine addiction phenotype

INTRODUCTION

Nicotine is a major psychoactive ingredient in tobacco yet very few individuals quit smoking with the aid of nicotine replacement therapy. Targeted therapies with more selective action at nicotinic acetylcholine receptors (nAChRs) that contain a β2 subunit (β2*nAChRs; *denotes assembly with other subunits) have enjoyed significantly greater success, but exhibit potential for unwanted cardiac, gastrointestinal, and emotive side effects.

DISCUSSION

This literature review focuses on the preclinical evidence that suggests that subclasses of β2*nAChRs that assemble with the α6 subunit may provide an effective target for tobacco cessation. α6β2*nAChRs have a highly selective pattern of neuroanatomical expression in catecholaminergic nuclei including the ventral tegmental area and its projection regions. α6β2*nAChRs promote dopamine (DA) neuron activity and DA release in the mesolimbic dopamine system, a brain circuitry that is well-studied for its contributions to addiction behavior. A combination of genetic and pharmacological studies indicates that activation of α6β2*nAChRs is necessary and sufficient for nicotine psychostimulant effects and nicotine self-administration. α6β2*nAChRs support maintenance of nicotine use, support the conditioned reinforcing effects of drug-associated cues, and regulate nicotine withdrawal.

CONCLUSIONS

These data suggest that α6β2*nAChRs represent a critical pool of high affinity β2*nAChRs that regulates nicotine dependence phenotype and suggest that inhibition of these receptors may provide an effective strategy for tobacco cessation therapy.

The role of estrogen and testosterone in female rats in behavioral models of relevance to schizophrenia

RATIONALE

The sex steroid hormone, estrogen, may play a protective role in schizophrenia. We previously found that estrogen treatment inhibited serotonin-1A (5-HT(1A)) and dopamine D(2) receptor-mediated disruptions of prepulse inhibition (PPI), a measure of sensorimotor gating which is deficient in schizophrenia.

OBJECTIVES

The present study aimed to further explore the role of sex steroid hormones in schizophrenia. Part 1 of this study examined whether estrogen could inhibit PPI disruption induced by the N-methyl-D: -aspartate (NMDA) receptor antagonist, MK-801. Part 2 investigated whether the functionally protective effect of estrogen occurs in another animal model of schizophrenia, amphetamine-induced locomotor hyperactivity. Part 3 compared our previous PPI findings in estrogen-treated rats, to treatment with testosterone.

METHODS

Female Sprague-Dawley rats were ovariectomized (OVX) or sham-operated. Some OVX rats received silastic implants filled with either a low (E20) or high dose (E100) of estradiol, or a low (T5) or high dose (T20) of testosterone, for at least 2 weeks before behavioral testing.

RESULTS

The disruption of PPI caused by MK-801 (0.1 mg/kg) was significantly reduced by treatment with estradiol (E20 and E100). However, estradiol treatment did not alter amphetamine-induced (0.25 and 0.5 mg/kg) locomotor hyperactivity, in terms of distance traveled, ambulation, or vertical counts. In contrast to estrogen, testosterone treatment did not affect disruption of PPI after administration of 8-OH-DPAT (0.5 mg/kg) or apomorphine (0.3 mg/kg). Testosterone treatment significantly enhanced the MK-801-induced (0.1 mg/kg) PPI disruption.

CONCLUSIONS

Estrogen is functionally protective against 5-HT(1A)-, dopamine D(2)-, and NMDA receptor-induced PPI disruptions, while testosterone treatment enhances NMDA receptor-mediated PPI disruptions.

Catecholaminergic depletion in nucleus accumbens enhances trace conditioning

PURPOSE

To examine the effect of dopamine depletion in nucleus accumbens on trace conditioning; to distinguish the role of core and shell sub-regions, as far as possible.

MATERIAL/METHODS

6-hydroxydopamine was used to lesion dopamine terminals within the core and shell accumbens. Experiment 1 assessed conditioning to a tone conditioned stimulus that had previously been paired with footshock (unconditioned stimulus) at a 30s trace interval. Experiment 2 subsequently assessed contiguous conditioning (at 0s trace) using a light conditioned stimulus directly followed by the unconditioned stimulus.

RESULTS

Both sham and shell-lesioned animals showed the normal trace effect of reduced conditioning to the trace conditioned stimulus but 6-hydroxydopamine injections targeted on the core subregion of the nucleus accumbens abolished this effect and enhanced conditioning to the trace conditioned stimulus. However, the depletion produced by this lesion placement extended to the shell. In Experiment 2 (at 0s trace), there was no effect of either lesion placement as all animals showed comparable levels of conditioning to the light conditioned stimulus. Neurochemical analysis across core, shell and comparison regions showed some effects on noradrenalin as well as dopamine.

CONCLUSIONS

The pattern of changes in noradrenalin did not systematically relate to the observed behavioural changes after core injections. The pattern of changes in dopamine suggested that depletion in core mediated the increased conditioning to the trace conditioned stimulus seen in the present study. However, the comparison depletion restricted to the shell subregion was less substantial, and a role for secondarily affected brain regions cannot be excluded.

Dissociable roles of dopamine within the core and medial shell of the nucleus accumbens in memory for objects and place

There is increasing focus on the role of the nucleus accumbens (NAc) in learning and memory, but there is little consensus as to how the core and medial shell subregions of the NAc contribute to these processes. In the current experiments, we used spontaneous object recognition to test rats with 6-hydroxydopamine lesions targeted at the core or medial shell of the NAc on a familiarity discrimination task and a location discrimination task. In the object recognition variant, control animals were able to discriminate the novel object at both 24-hr and 5-min delay. However, in the lesion groups, performance was systematically related to dopamine (DA) levels in the core but not the shell. In the location recognition task, sham-operated animals readily detected the object displacement at test. In the lesion groups, performance impairment was systematically related to DA levels in the shell but not the core. These results suggest that dopamine function within distinct subregions of the NAc plays dissociable roles in the modulation of memory for objects and place.

Blockade of mGLUR5 receptors differentially alters amphetamine-induced enhancement of locomotor activity and of brain stimulation reward

This study was aimed at determining the role of mGLUR5 glutamate receptors on amphetamine-induced enhancement of locomotion and of brain stimulation reward (BSR). The effect of different doses of the mGLUR5 antagonist, MPEP (0, 1, 3 and 9 mg/kg, i.p.), was assessed on reward induced by electrical stimulation of the lateral hypothalamus, and on the enhancement of reward by amphetamine (1 mg/kg, i.p.) in adult male Long Evans rats. The effect of a single dose of MPEP (0 and 9 mg/kg) on amphetamine-induced increase in locomotor activity was also assessed. Systemic injection of MPEP alone did not alter reward threshold and maximum rate of responding. Amphetamine produced a 25-30% decrease in reward threshold, an effect not altered by the highest dose of MPEP. At this dose, MPEP produced a weak inhibition of spontaneous locomotion and a significant attenuation of the enhanced locomotor activity induced by amphetamine. These findings show that mGLUR5 glutamate receptors are unlikely to constitute important elements of the reward-relevant pathway, and do not intervene in the enhancement effect of amphetamine. They also show, however, that these glutamate receptors play a key role in amphetamine-induced increased locomotor activity, providing additional evidence for a dissociation between the substrates that mediate these two behaviours.

Reduced dopamine function within the medial shell of the nucleus accumbens enhances latent inhibition

Latent inhibition (LI) manifests as poorer conditioning to a CS that has previously been presented without consequence. There is some evidence that LI can be potentiated by reduced mesoaccumbal dopamine (DA) function but the locus within the nucleus accumbens of this effect is as yet not firmly established. Experiment 1 tested whether 6-hydroxydopamine (6-OHDA)-induced lesions of DA terminals within the core and medial shell subregions of the nucleus accumbens (NAc) would enhance LI under conditions that normally disrupt LI in controls (weak pre-exposure). LI was measured in a thirst motivated conditioned emotional response procedure with 10 pre-exposures (to a noise CS) and 2 conditioning trials. The vehicle-injected and core-lesioned animals did not show LI and conditioned to the pre-exposed CS at comparable levels to the non-pre-exposed controls. 6-OHDA lesions to the medial shell, however, produced potentiation of LI, demonstrated across two extinction tests. In a subsequent experiment, haloperidol microinjected into the medial shell prior to conditioning similarly enhanced LI. These results underscore the dissociable roles of core and shell subregions of the NAc in mediating the expression of LI and indicate that reduced DA function within the medial shell leads to enhanced LI.

Chronic forced exercise during adolescence decreases cocaine conditioned place preference in Lewis rats

Chronic physical activity (exercise) may be beneficial in the prevention of substance use disorders; however, the extent to which physical activity can interfere with the reinforcing effects of drugs during the adolescent period, which is one of great vulnerability for drug experimentation, has not been fully evaluated. Here, we assess the effects of chronic forced exercise during adolescence on preference for cocaine using the conditioned place preference (CPP) paradigm in male and female Lewis rats. The group of rats exposed to exercise ran on a treadmill for 6 weeks on a progressive time-increased schedule for up to 1h of exercise per day, while the groups of sedentary rats remained in their home cage. Following the 6 weeks of exercise exposure, rats were tested for cocaine CPP. Results showed that chronic exercise significantly attenuated cocaine CPP in both males and females compared to a sedentary environment. Furthermore, male exercise rats failed to show significant cocaine CPP. In contrast, female exercise rats still showed cocaine CPP but it was significantly reduced compared to the female sedentary rats. Females also exhibited greater cocaine CPP than males overall. These findings suggest that strategies to promote physical activity during adolescence may be protective against cocaine abuse in both males and females, and these findings merit further investigation. We also corroborate a gender-specific sensitivity to the reinforcing effects of cocaine, highlighting the need to consider gender-tailored exercise interventions for drug abuse prevention.

Nucleus accumbens lesions modulate the effects of methylphenidate

The psychostimulant methylphenidate (MPD, Ritalin) is the prescribed drug of choice for treatment of ADHD. In recent years, the diagnosis rate of ADHD has increased dramatically, as have the number of MPD prescriptions. Repeated exposure to psychostimulants produces behavioral sensitization in rats, an experimental indicator of a drug's potential liability. In studies on cocaine and amphetamine, this effect has been reported to involve the nucleus accumbens (NAc), one of the nuclei belonging to the motive circuit. The aim of this study was to investigate the role of the NAc on the expression of behavioral sensitization as a response to MPD exposure. In the present study, 20 male Sprague-Dawley rats were divided randomly into three groups: an intact control group, a sham-operated group, and a NAc bilateral electrical lesion group. Locomotor activity was assessed for the first 2h following 2.5mg/kg MPD injection, using open field monitoring systems. Recordings were made during 6 days of continuous MPD administration, and then upon re-challenge with the same dose following 3 days of washout. Acute MPD exposure elicited an increase in locomotor activity in all three groups. However, the NAc lesion group exhibited significantly increased locomotor activity in comparison to sham and control groups. Chronic MPD did not elicit sensitization in the NAc lesion group, while both sham and control groups did exhibit behavioral sensitization to repetitive MPD administration. These findings suggest that the NAc plays a significant role in eliciting locomotor activity as an acute effect of MPD, and in the expression of sensitization due to chronic MPD exposure.

Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors in the nucleus accumbens shell regulate progressive ratio responding maintained by nicotine

Beta2 subunit containing nicotinic acetylcholine receptors (beta2(*)nAChRs; asterisk ((*)) denotes assembly with other subunits) are critical for nicotine self-administration and nicotine-associated dopamine (DA) release that supports nicotine reinforcement. The alpha6 subunit assembles with beta2 on DA neurons where alpha6beta2(*)nAChRs regulate nicotine-stimulated DA release at neuron terminals. Using local infusion of alpha-conotoxin MII (alpha-CTX MII), an antagonist with selectivity for alpha6beta2(*)nAChRs, the purpose of these experiments was to determine if alpha6beta2(*)nAChRs in the nucleus accumbens (NAc) shell are required for motivation to self-administer nicotine. Long-Evans rats lever-pressed for 0.03 mg/kg, i.v., nicotine accompanied by light+tone cues (NIC) or for light+tone cues unaccompanied by nicotine (CUEonly). Following extensive training, animals were tested under a progressive ratio (PR) schedule that required an increasing number of lever presses for each nicotine infusion and/or cue delivery. Immediately before each PR session, rats received microinfusions of alpha-CTX MII (0, 1, 5, or 10 pmol per side) into the NAc shell or the overlying anterior cingulate cortex. alpha-CTX MII dose dependently decreased break points and number of infusions earned by NIC rats following infusion into the NAc shell but not the anterior cingulate cortex. Concentrations of alpha-CTX MII that were capable of attenuating nicotine self-administration did not disrupt locomotor activity. There was no effect of infusion on lever pressing in CUEonly animals and NAc infusion alpha-CTX MII did not affect locomotor activity in an open field. These data suggest that alpha6beta2(*)nAChRs in the NAc shell regulate motivational aspects of nicotine reinforcement but not nicotine-associated locomotor activation.

Electrophysiological evidence of mediolateral functional dichotomy in the rat accumbens during cocaine self-administration: tonic firing patterns

Given the increasing research emphasis on putative accumbal functional compartmentation, we sought to determine whether neurons that demonstrate changes in tonic firing rate during cocaine self-administration are differentially distributed across subregions of the NAcc. Rats were implanted with jugular catheters and microwire arrays targeting NAcc subregions (core, dorsal shell, ventromedial shell, ventrolateral shell and rostral pole shell). Recordings were obtained after acquisition of stable cocaine self-administration (0.77 mg/kg/0.2mL infusion; fixed-ratio 1 schedule of reinforcement; 6-h daily sessions). During the self-administration phase of the experiment, neurons demonstrated either: (i) tonic suppression (or decrease); (ii) tonic activation (or increase); or (iii) no tonic change in firing rate with respect to rates of firing during pre- and post-drug phases. Consistent with earlier observations, tonic decrease was the predominant firing pattern observed. Differences in the prevalence of tonic increase firing were observed between the core and the dorsal shell and dorsal shell-core border regions, with the latter two areas exhibiting a virtual absence of tonic increases. Tonic suppression was exhibited to a greater extent by the dorsal shell-core border region relative to the core. These differences could reflect distinct subregional afferent processing and/or differential sensitivity of subpopulations of NAcc neurons to cocaine. Ventrolateral shell firing topographies resembled those of core neurons. Taken together, these observations are consistent with an emerging body of literature that differentiates the accumbens mediolaterally and further advances the likelihood that distinct functions are subserved by NAcc subregions in appetitive processing.

Electrolytic lesions of the nucleus accumbens enhance locomotor sensitization to nicotine in rats

Electrolytic lesions of the medial core of the nucleus accumbens (NAc) in male Long-Evans rats increased spontaneous locomotion, enhanced the locomotor stimulating effect of acute 5.0 mg/kg cocaine, enhanced the development and subsequent expression of locomotor sensitization produced by repeated injections of 0.4 mg/kg nicotine but not 7.5 mg/kg cocaine, and enhanced the expression of conditioned locomotion. Given that 6-hydroxydopamine lesions of the NAc typically have effects on locomotor-related processes that are opposite of those produced by electrolytic and excitotoxic lesions, these data are consistent with a hypothesis that the NAc output, especially from the core, inhibits a variety of such processes and that the DA input to the NAc enhances these processes by inhibiting this inhibitory output.

Simultaneous anhedonia and exaggerated locomotor activation in an animal model of depression

RATIONALE

Anhedonia, or hyposensitivity to normally pleasurable stimuli, is a cardinal symptom of depression. As such, reward circuitry may comprise a substrate with relevance to this symptom of depression.

OBJECTIVES

Our aim was to characterize in the rat changes in the rewarding properties of a pharmacological and a natural stimulus following olfactory bulbectomy (OBX), a pre-clinical animal model of depression.

METHODS

We measured amphetamine enhancement of brain stimulation reward, changes in sucrose intake, as well as striatal cAMP response element binding protein (CREB) activity, a molecular index previously associated with depressant-like behavior. Moreover, since alteration of psychomotor activity is also a common symptom of depression, and psychostimulant reward and locomotion are thought to share common neurobiology, we used the same treatment schedule of amphetamine to probe for changes in locomotion.

RESULTS

Our findings show that OBX produces a behavioral phenotype characterized by both anhedonia and exaggerated locomotor activation. Thus, we observed a blunted response to the rewarding properties of amphetamine (1 mg/kg, 21 days post-lesion), a long-lasting reduction in sucrose intake and increased striatal CREB activity. In addition, the same dose of amphetamine, at a coincident time post-lesion, triggered an exaggerated response to its locomotor-stimulant actions.

CONCLUSIONS

These paradoxical findings are not consistent with the notion that reward and locomotion are mediated by a common substrate; this dissociation may be useful in modeling psychiatric disorders such as mixed depressive states. In addition, our findings suggest that central reward circuitry may constitute a possible target for rationally designed therapeutics for depression.

Expression of c-fos mRNA in the basal ganglia associated with contingent tolerance to amphetamine-induced hypophagia

Tolerance to the hypophagic effect of psychostimulants is contingent on having access to food while intoxicated. Rats given chronic injections of such drugs with access to food learn to suppress stereotyped movements, which interfere with feeding. In contrast, controls given the drug after food access do not learn to suppress stereotypy and, therefore, do not become tolerant. To determine the role of the basal ganglia in this phenomenon, we used in situ hybridization to measure the expression of c-fos mRNA, a marker for neural activation, in the brains of tolerant and nontolerant rats. Rats given chronic amphetamine injections prior to food access learned to suppress stereotyped movements, whereas yoked controls given the drug after feeding did not. Following an acute injection of amphetamine, both of these groups had higher levels of c-fos mRNA than saline-treated controls throughout the striatum, in the nucleus accumbens core, the ventral pallidum and layers V-VI of the motor cortex. In contrast, tolerant rats, which had learned to suppress stereotypy, had higher levels of c-fos mRNA than both amphetamine- and saline-treated controls in the entopeduncular nucleus, globus pallidus, subthalamic nucleus, pedunculopontine nucleus, nucleus accumbens shell, olfactory tubercle, somatosensory cortex, and layers II-IV of motor cortex. These data suggest that the learned suppression of amphetamine-induced stereotypy involves the activation of dorsal striatal pathways previously implicated in response selection as well as the ventral striatum, long implicated in appetitive motivation and reinforcement.

Dopamine signaling through D1-like versus D2-like receptors in the nucleus accumbens core versus shell differentially modulates nicotine reward sensitivity

Considerable evidence implicates the mesolimbic dopamine (DA) system in the processing of nicotine's reinforcing properties, specifically the ventral tegmental area (VTA) and the terminal fields of VTA DAergic projections to the "core" (NAcore) and "shell" (NAshell) subdivisions of the nucleus accumbens (NAc). However, the specific roles of DA D(1)-like and D(2)-like receptor subtypes in nicotine reward processing within these NAc subregions have not been elucidated. We report that microinfusions of DA D(1)-like or D(2)-like receptor-specific antagonists into NAcore or NAshell double dissociate the rewarding and aversive properties of systemic or intra-VTA nicotine, and differentially regulate sensitivity to the rewarding properties as well as the motivational valence of either intra-VTA or systemic nicotine administration. Using a place conditioning procedure, NAshell infusions of a D(2)-like receptor antagonist switched the motivational valence of intra-VTA nicotine from aversive to rewarding and potentiated nicotine reward sensitivity to sub-reward threshold intra-VTA nicotine doses. In contrast, NAcore infusions of a D(1)-like receptor antagonist switched intra-VTA nicotine aversion to reward, and potentiated reward sensitivity to sub-reward threshold nicotine doses. Thus, D(1)-like versus D(2)-like receptors in NAcore versus NAshell subdivisions play functionally dissociable roles in modulating systemic or intra-VTA nicotine motivational processing.

The microinjection of AMPA receptor antagonist into the accumbens shell failed to change food intake, but reduced fear-motivated behaviour in free-feeding female rats

This study investigated the effect of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2.5 and 5.0 nmol/side) microinjected into the core and shell sub-regions of the accumbens (Acb) nucleus, on food intake and the level of anxiety in female rats. Bilateral microinjections of CNQX (5.0 nmol/side) into the Acb shell (AP, +1.08 to +2.04), but not into the Acb core, induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since the animals exhibited low level of SAP in the feeding test. The anxiolytic-like effect induced by 5.0 nmol CNQX microinjection into the Acb shell may not be ascribed to changes in the motor activity of the animals, because the frequency of locomotion, rearing and grooming remained unchanged after the drug microinjection. However, neither Acb shell nor Acb core CNQX microinjections were able to change the animals food intake along 1h feeding behaviour evaluation. Food intake remained unchanged 24h after the drug microinjections either into the Acb shell or into the Acb core. The data suggest that AMPA receptor blockade in the Acb nucleus may differentially change the ingestive and defensive behaviours in female rats.

N,N'-Alkane-diyl-bis-3-picoliniums as nicotinic receptor antagonists: inhibition of nicotine-evoked dopamine release and hyperactivity

The current study evaluated a new series of N,N'-alkane-diyl-bis-3-picolinium (bAPi) analogs with C6-C12 methylene linkers as nicotinic acetylcholine receptor (nAChR) antagonists, for nicotine-evoked [3H]dopamine (DA) overflow, for blood-brain barrier choline transporter affinity, and for attenuation of discriminative stimulus and locomotor stimulant effects of nicotine. bAPi analogs exhibited little affinity for alpha4beta2* (* indicates putative nAChR subtype assignment) and alpha7* high-affinity ligand binding sites and exhibited no inhibition of DA transporter function. With the exception of C6, all analogs inhibited nicotine-evoked [3H]DA overflow (IC50 = 2 nM-6 microM; Imax = 54-64%), with N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB; C12) being most potent. bPiDDB did not inhibit electrically evoked [3H]DA overflow, suggesting specific nAChR inhibitory effects and a lack of toxicity to DA neurons. Schild analysis suggested that bPiDDB interacts in an orthosteric manner at nAChRs mediating nicotine-evoked [3H]DA overflow. To determine whether bPiDDB interacts with alpha-conotoxin MII-sensitive alpha6beta2-containing nAChRs, slices were exposed concomitantly to maximally effective concentrations of bPiDDB (10 nM) and alpha-conotoxin MII (1 nM). Inhibition of nicotine-evoked [3H]DA overflow was not different with the combination compared with either antagonist alone, suggesting that bPiDDB interacts with alpha6beta2-containing nAChRs. C7, C8, C10, and C12 analogs exhibited high affinity for the blood-brain barrier choline transporter in vivo, suggesting brain bioavailability. Although none of the analogs altered the discriminative stimulus effect of nicotine, C8, C9, C10, and C12 analogs decreased nicotine-induced hyperactivity in nicotine-sensitized rats, without reducing spontaneous activity. Further development of nAChR antagonists that inhibit nicotine-evoked DA release and penetrate brain to antagonize DA-mediated locomotor stimulant effects of nicotine as novel treatments for nicotine addiction is warranted.

Increased expression of 5-HT6 receptors in the nucleus accumbens blocks the rewarding but not psychomotor activating properties of cocaine

BACKGROUND

Repeated exposure to cocaine produces enduring forms of drug experience-dependent behavioral plasticity, including conditioned place preference (CPP) and psychomotor sensitization, a progressive and persistent increase in cocaine's psychomotor activating effects. Although serotonin-6 receptors (5-HT6Rs) are abundantly expressed in the brain regions thought to underlie these phenomena, such as the nucleus accumbens (NAc), surprisingly little is known about the role of 5-HT6Rs in the rewarding and psychomotor activating effects of cocaine.

METHODS

Viral-mediated gene transfer was used to selectively increase 5-HT6R expression in the NAc of rats. The effects of 5-HT6R overexpression and the selective 5-HT6R antagonist Ro4368554 on CPP and psychomotor sensitization were examined.

RESULTS

Increased expression of 5-HT6Rs in the NAc blocks a CPP to cocaine but has no effect on either the acute locomotor response to cocaine or on the development of cocaine-induced locomotor sensitization. Furthermore, antagonism of 5-HT6Rs facilitates the acquisition of a CPP to cocaine but has no effect on cocaine-induced stereotypy.

CONCLUSIONS

These results demonstrate that 5-HT6Rs in the NAc can selectively modulate drug reward, possibly through facilitation of reward learning.

Behavioral functions of the mesolimbic dopaminergic system: an affective neuroethological perspective

The mesolimbic dopaminergic (ML-DA) system has been recognized for its central role in motivated behaviors, various types of reward, and, more recently, in cognitive processes. Functional theories have emphasized DA's involvement in the orchestration of goal-directed behaviors and in the promotion and reinforcement of learning. The affective neuroethological perspective presented here views the ML-DA system in terms of its ability to activate an instinctual emotional appetitive state (SEEKING) evolved to induce organisms to search for all varieties of life-supporting stimuli and to avoid harms. A description of the anatomical framework in which the ML system is embedded is followed by the argument that the SEEKING disposition emerges through functional integration of ventral basal ganglia (BG) into thalamocortical activities. Filtering cortical and limbic input that spreads into BG, DA transmission promotes the "release" of neural activity patterns that induce active SEEKING behaviors when expressed at the motor level. Reverberation of these patterns constitutes a neurodynamic process for the inclusion of cognitive and perceptual representations within the extended networks of the SEEKING urge. In this way, the SEEKING disposition influences attention, incentive salience, associative learning, and anticipatory predictions. In our view, the rewarding properties of drugs of abuse are, in part, caused by the activation of the SEEKING disposition, ranging from appetitive drive to persistent craving depending on the intensity of the affect. The implications of such a view for understanding addiction are considered, with particular emphasis on factors predisposing individuals to develop compulsive drug seeking behaviors.

Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex

Anatomical and functional refinements of the meso-limbic dopamine system of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas the anteromedial VTA has few if any projections to the ventral striatum, and the lateral VTA largely projects to the ventrolateral striatum (accumbens core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into the anterior VTA. A review of the literature suggests that (1) the midbrain has corresponding zones for the accumbens core and medial shell; (2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and (3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed learning than the lateral projection system, as described in the variation-selection hypothesis of striatal functional organization.

The microinjection of AMPA receptor antagonist into the accumbens shell, but not into the accumbens core, induces anxiolysis in an animal model of anxiety

This study investigated the effect of the AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) microinjected into the core and shell sub-regions of the accumbens nucleus (Acb), on the level of fear/anxiety and emotional learning, in female rats submitted to the elevated plus-maze (EPM), an animal model of anxiety. Bilateral microinjections of DNQX (330 and 660 ng) into the Acb shell (AP, +1.08 to +2.16) induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since there was an increased percentage of entries in the open arms of the maze. The 660 ng DNQX microinjection into the Acb shell also increased the percentage of entries into the open arms in relation to 660 ng DNQX microinjection into the Acb core. Prior DNQX microinjections in both core and shell sub-regions of the Acb failed to impair the emotional learning, since the animals exhibited an increase of the open arm avoidance on EPM Trial 2 in relation to EPM trial 1. DNQX microinjections into both sub-regions of the Acb did not change the number of entries into the enclosed arms, either in the EPM Trial 1 or in the EPM Trial 2, which indicates an absence of drug-induced locomotor impairment. Similarly, DNQX microinjections into both sub-regions of the Acb failed to alter the total arm entries, rearing, grooming and head-dipping frequency. The anxiolytic-like effect induced by DNQX suggests that the AMPA receptor in the Acb shell, but not in the Acb core, may underlie anxiety regulation in the EPM.

Lesions of the nucleus accumbens shell can reduce activity in the elevated plus-maze

Across different behavioural tasks, nucleus accumbens (n.acc) lesions have generated conflicting effects on locomotor activity and in particular, the relative roles of the n.acc shell and core subfields in this have been controversial. To date there is only one study examining effects of lesions to the medial n.acc on elevated plus-maze (EPM) behaviour; these lesions were shown to increase both locomotor and exploratory activity. Given the well-documented distinction between shell and core, the present study sought to extend previous research by testing lesions selective to each n.acc subfield in the EPM. Results showed no statistical differences between core lesioned and sham-operated animals on any measure. In contrast, shell lesions consistently reduced locomotion and exploratory activity. This direction of effects is opposite to that previously observed after medial n.acc. lesions. In conclusion, locomotion and exploratory activity were clearly reduced by shell but not core lesions, consistent with other evidence for the functional heterogeneity of n.acc shell and core.

Involvement of dopamine D1 and D2 receptors in the nucleus accumbens core and shell in inhibitory response control

RATIONALE

Impaired inhibitory control over behavior is a key feature in various psychiatric disorders, and recent studies indicated an important role for dopamine D(1) and D(2) receptors and the nucleus accumbens (Acb) in this respect.

OBJECTIVE

The present experiments were designed to study the role of dopamine D(1) and D(2) receptors in the Acb in inhibitory response control.

METHODS

Rats were trained in a five-choice serial reaction time task and received bilateral infusions into the Acb core or shell of either SCH 23390 or eticlopride (representing selective dopamine D(1) and D(2) receptor antagonists, respectively). Subsequently, the effects of systemic amphetamine on inhibitory response control were examined.

RESULTS

Eticlopride into either the Acb core or shell did not affect premature responding, a measure for inhibitory response control, but increased reaction time and errors of omission. In contrast, SCH 23390 into both regions reduced premature responding, slightly improved attentional performance in the core and increased errors of omission in the shell. Amphetamine robustly increased premature responding which was dose-dependently blocked by eticlopride in the Acb core and attenuated by eticlopride in the shell. In addition, amphetamine slightly decreased accuracy and reaction time, and these effects were inhibited by eticlopride in both regions. SCH 23390 infusion into the Acb core or shell did not alter amphetamine's effects.

CONCLUSION

Our data provide evidence for the involvement of dopamine D(1) and D(2) receptors in the Acb core and shell in inhibitory response control and attentional performance.

Dorsal raphe vs. median raphe serotonergic antagonism. Anatomical, physiological, behavioral, neuroendocrinological, neuropharmacological and clinical evidences: relevance for neuropharmacological therapy

Monoaminergic neurons located in the central nervous system (CNS) are organized into complex circuits which include noradrenergic (NA), adrenergic (Ad), dopaminergic (DA), serotonergic (5-HT), histaminergic (H), GABA-ergic and glutamatergic systems. Most of these circuits are composed of more than one and often several types of the above neurons. Such physiologically flexible circuits respond appropriately to both external and internal stimuli which, if not modulated adequately, can trigger pathophysiologic responses. A great deal of research has been devoted to mapping the multiple functions of the CNS circuitry, thereby forming the basis for effective neuropharmacological therapeutic approaches. Such lineal strategies that seek to normalize complex and mixed physiological disorders, however, meet only partial therapeutic success and are often followed by undesirable side effects and/or total failure. In light of these, we have worked to develop possible models of CNS circuitry that are less affected by physiological interaction using the models to design more effective therapeutic approaches. In the present review, we cite and present evidence supporting the dorsal raphe versus median raphe serotonergic circuitry as one model of a reliable paradigm, necessary to the clear understanding and therapy of many psychiatric and even non-psychiatric disturbances.