Nucleus Accumbens Shell Of Rats Research Articles

Prokineticin-2 Participates in Chronic Constriction Injury-Triggered Neuropathic Pain and Anxiety via Regulated by NF-κB in Nucleus Accumbens Shell in Rats.

Neuropathic pain (NP) is an intractable pain that results from primary nervous system injury and dysfunction. Herein, we demonstrated in animal models that peripheral nerve injury induced enhanced pain perception and anxiety-like behaviors. According to previous reports, nucleus accumbens (NAc) shell is required for complete expression of neuropathic pain behaviors and mood alternations, we found the elevated mRNA and protein level of Prokineticin-2 (Prok2) in the NAc shell after Chronic Constriction Injury (CCI). Prok2 knockdown in the NAc shell reversed NP and anxiety-like behaviors in rats, indicating that Prok2 might play a fundamental role in NP and anxiety co-morbidity. CCI significantly enhanced Prok2 co-expression with NF-κB P-p65 in comparison with control animals. In addition to reversing the established nociceptive hypersensitivities and anxiety simultaneously, NAc microinjection of NF-κB siRNA or specific inhibitor PDTC reversed Prok2 upregulation. Besides, Prok2 was significantly decreased in vitro when co-transfected with si-NF-κB. Dual-Luciferase assay showed NF-κB directly activated Prok2 gene transcriptional activity. Overall, these findings provide new insights into the neurobiological mechanisms behind NP and comorbid anxiety. The NF-κB/Prok2 pathway could be a potential therapeutic target for NP and anxiety disorders.

Intravenous Injection of GluR2-3Y Inhibits Repeated Morphine-Primed Reinstatement of Drug Seeking in Rats

Studies have demonstrated that the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor is essential to drug addiction. In this study, we explored the influence of GluR2-3Y, an interfering peptide to prevent the endocytosis of AMPA receptors containing the GluR2 subunit, on morphine-seeking behavior in the rat self-administration model. After self-administration was established, the rats received intravenous injections of GluR2-3Y during the extinction sessions. There were no significant differences in both active and inactive pokes compared to the control group of rats that received GluR2-3S, indicating that GluR2-3Y has no significant influences on the extinction of morphine self-administration. The other two groups of rats were trained, extinguished, and reinstated by repeated morphine priming (respectively, called Prime 1, Prime 2, and Prime 3). Only one intravenous injection of GluR2-3Y was performed before Prime 1. Compared to the control group, GluR2-3Y did not affect Prime 1, but significantly attenuated the morphine-seeking behavior during repeated morphine-primed reinstatement, indicating an inhibitory after effect of GluR2-3Y on morphine-seeking behavior in rats. The long-term depression (LTD) in the nucleus accumbens (NAc) shell was also assessed. Pretreatment with GluR2-3Y altered the ability of LTD induction to the level of that in the naive group, while pretreatment with GluR2-3S had no effects on LTD. Our results demonstrated that the intravenous injection of GluR2-3Y, to block the endocytosis of AMPA receptors, inhibited the reinstatement of morphine-seeking behavior, which may be induced by modulating the neuronal plasticity in the NAc shell of rats.

Oxytocin receptors mediate oxytocin potentiation of methylphenidate-induced stimulation of accumbens dopamine in rats.

While the illicit use and misuse of stimulants like cocaine and methylphenidate (MP) has increased, there remains no FDA-approved treatments for psychostimulant use disorders (PSUD). Oxytocin (OT) has shown promise as a potential pharmacotherapy for PSUD. Dopamine (DA) neurotransmission plays a significant role in PSUD. We have recently shown that OT blunts the reinforcing effects of MP but, surprisingly, enhanced MP-induced stimulation of DA levels. Such effects have been suggested as a result of activation of OT receptors or, alternatively, could be mediated by direct actions of OT on MP blockade of the DA transporter. Here, we employed fast scan cyclic voltammetry (FSCV) to investigate the effects of systemic OT on MP-induced changes in the dynamics of DA, phasic release and uptake, in the nucleus accumbens shell (NAS) of Sprague-Dawley rats. We also tested the systemic effects of an antagonist of OT receptors, atosiban, to counteract the OT enhancement of dopaminergic effects of MP under microdialysis procedures in the NAS in rats. Administration of OT alone (2mg/kg; i.p.) did not significantly modify evoked NAS DA dynamics measured by FSCV, and when administered 10min before MP (0.1, 0.3, 1.0mg/kg; i.v.), OT did not potentiate MP-induced increases in phasic DA release and did not alter DA clearance rate, suggesting no direct interactions of OT with the MP-induced blockade of DA uptake. Also, OT alone did not elicit significant changes in tonic, extracellular NAS DA levels measured by microdialysis. However, consistent with previous studies, we observed that OT pretreatments (2mg/kg; i.p.) potentiated MP-induced (0.1, 0.3, 1.0mg/kg; i.v.) efflux of extracellular NAS DA levels. This effect was abolished when rats were pretreated with atosiban (2mg/kg; i.p.), suggesting that OT receptors mediate this OT action. Overall, our results suggest that OT receptors mediated OT potentiation of MP-induced stimulation of extracellular NAS DA levels, likely driven by modulation of DA receptor signaling pathways, without affecting MP blockade of DAT.

Maternal high-dense diet programs interferon type I signaling and microglia complexity in the nucleus accumbens shell of rats showing food addiction-like behavior.

This study aimed to characterize the molecular immune networks and microglia reactivity in the nucleus accumbens (NAc) shell affected by fetal nutritional programming leading to addiction-like behavior in the offspring of Wistar rats. Fetal nutritional programming by energy-dense foods leads to addiction-like behavior in the offspring. Exposure to energy-dense foods also activates systemic and central inflammation in the offspring. Females Wistar rats were exposed to cafeteria (CAF) diet or control diet for 9 weeks (prepregnancy, pregnancy and lactation), and male offspring at 2 months of age were diagnosed with food addiction-like behavior using operant conditioning. Global microarray analysis, RTqPCR, proinflammatory plasma profile and microglia immunostaining were performed in the NAc shell of male rats. SIM-A9 microglia cells were stimulated with IFN-α and palmitic acid, and microglia activation and phagocytosis were determined by RTqPCR and incubation of green-fluorescent latex beads, respectively. Microarray analysis in the NAc shell of the male offspring exposed to CAF during development and diagnosed with addiction-like behavior showed increasing in the type I interferon-inducible gene, Ift1 , gene network. Genomic and cellular characterization also confirmed microglia hyperreactivity and upregulation of the Ifit1 in the NAc shell of animals with addiction-like behavior. In-vitro models demonstrated that microglia do respond to IFN-α promoting a time-dependent genomic expression of Ift1, IL-1β and IL-6 followed by increased phagocytosis. Prenatal exposure to energy-dense foods primes the IFN type I signaling and microglia complexity in the NAc shell of rats diagnosed with food addiction-like behavior.

A ghrelin receptor antagonist reduces the ability of ghrelin, alcohol or amphetamine to induce a dopamine release in the ventral tegmental area and in nucleus accumbens shell in rats

The orexigenic peptide ghrelin increases the release of dopamine in the nucleus accumbens (NAc) shell via central ghrelin receptors, especially those located in the ventral tegmental area (VTA). The activity of the VTA dopamine neurons projecting to NAc shell, involves somatodendritic dopamine release within the VTA. However, the effects of ghrelin on the concomitant dopamine release in the VTA and NAc shell is unknown. It is further unknown whether addictive drugs, such as alcohol and amphetamine, enhance the dopamine levels in both these areas via ghrelin receptor dependent mechanisms. Thus, the effects of a ghrelin receptor antagonist, JMV2959, on the ability of i) central ghrelin ii) systemic alcohol or iii) systemic amphetamine to increase the dopamine release in the VTA and in the NAc shell in rats by using in vivo microdialysis was explored. We showed that systemic administration of JMV2959 blocks the ability of central ghrelin to increases dopamine release in the VTA and the NAc shell, and reduces the alcohol- and amphetamine-induced dopamine release in both these areas. Locomotor activity studies was then conducted in an attempt to correlate the ghrelin-induced dopamine release in the VTA to a behavioural outcome. These revealed that local infusion of a dopamine D1 receptor antagonist into the VTA blocks the ability of central ghrelin to cause a locomotor stimulation in mice. Collectively, this study adds to the growing body of evidence indicating that ghrelin signalling modulates the ability of ghrelin, and addictive drugs, to activate the mesoaccumbal dopamine pathway.

Electrical high frequency stimulation of the nucleus accumbens shell does not modulate depressive-like behavior in rats

Deep brain stimulation (DBS) is an effective tool for treatment-resistant depression, though it is still unclear which brain area to target in order to get robust results. While research suggests that the nucleus accumbens (NAc) plays an important role in depression, studies using NAc DBS to improve depressive behavior have not been able to fully explain underlying molecular mechanisms. We therefore used unilateral high frequency stimulation of the NAc shell in rats to verify its effectiveness in treating depression and study involved neurotransmitter systems. Animals underwent social isolation and food deprivation to induce depressive-like symptoms, and performed the forced swim test (FST) to see possible changes in depressive behavior due to NAc shell stimulation. Neurotransmitter levels were measured using in-vivo microdialysis to detect DBS-induced changes. Non-treated control rats showed no significant difference between swimming and floating during FST, verifying that our depression model induced depressive-like symptoms in rats. Furthermore, stimulated and sham-operated animals showed a significant increase in mobility during FST compared to control rats, suggesting an improvement of depressive-like symptoms. However, stimulated rats did not differ from sham-operated rats in their behavior during FST, nor did neurotransmitter levels significantly changed in stimulated rats compared to control rats. Our data suggest that NAc shell stimulation did not alter depressive behavior in rats and had no effects on the molecular level. However, depressive behavior was positively altered when stimulation electrode and microdialysis probe were inserted simultaneously into the NAc shell, causing significant tissue damage and therefore possibly altering the glutamatergic system.

Ghrelin signalling within the rat nucleus accumbens and skilled reach foraging

Motivation alters behaviour in a complex manner and nucleus accumbens (NAc) shell has been implied as a key structure regulating such behaviour. Recent studies show that acute ghrelin signalling enhances motivation when assessed in a simple motor task. The aim of the present study was to define the role of ghrelin signalling on motivation in a more complex motor behaviour. Rats were tested in the Montoya staircase, an animal model of skilled reach foraging assessed by the number of sucrose pellets consumed. Electrophysiological recordings were conducted to explore the neurophysiological correlates of ghrelin signalling. The initial electrophysiological results displayed that ex vivo administration of ghrelin increased NAc shell output in brain slices from drug- and training-naïve rats. In rats with an acquired skilled reach performance, acute as well as repeated treatment with a ghrelin receptor (GHSR-1 A) antagonist (JMV2959) decreased the number of sucrose pellets consumed. Moreover, infusion of JMV2959 into NAc shell reduced this consumption. Sub-chronic, during ten days, JMV2959 treatment during training on the Montoya staircase reduced the number of pellets consumed, whereas ghrelin improved this behaviour. In addition, field potential and whole cell recordings were conducted in NAc shell of rats that had been treated with ghrelin or GHSR-1 A antagonist during training on the Montoya staircase. Sub-chronic administration of ghrelin during motor-skill learning selectively increased the frequency of inhibitory transmission in the NAc shell, resulting in a net suppression of accumbal output. Collectively these data suggest that ghrelin signalling in NAc shell enhances skilled reached foraging tentatively by increasing the motivation.

Dnmt3a2 in the Nucleus Accumbens Shell Is Required for Reinstatement of Cocaine Seeking.

Epigenetic mechanisms have gained increasing attention as regulators of synaptic plasticity and responsiveness to drugs of abuse. In particular, it has been shown that the activity of the DNA methyltransferase 3a (Dnmt3a) mediates certain long-lasting effects of cocaine. Here we examined the role of the Dnmt isoforms, Dnmt3a1 and Dnmt3a2, within the nucleus accumbens (NAc) on transcriptional activity of immediate early genes (IEGs) and acute and long-lasting responsiveness to cocaine and cocaine conditioned cues. Using primary striatal cultures, we show that transcription of Dnmt3a2, but not that of Dnmt3a1, is activated by dopamine D1 receptor signaling and that knockdown of Dnmt3a2 using viral vector-mediated expression of Dnmt3a2-specific shRNAs impairs induction of the IEGs, Arc, FosB, and Egr2 Acute cocaine administration increases expression of Dnmt3a2 but not that of Dnmt3a1 in the NAc shell. In contrast, in the NAc core, expression of Dnmt3a1 and Dnmt3a2 was unaffected by cocaine administration. shRNA-mediated knockdown of Dnmt3a2 in vivo impairs the induction of IEGs, including Egr2 and FosB indicating that Dnmt3a2 regulates cocaine-dependent expression of plasticity genes in the rat NAc shell. Cocaine self-administration experiments in rats revealed that Dnmt3a2 regulates drug cue memories that drive reinstatement of cocaine seeking as well as incubation of this phenomenon within the NAc shell. Dnmt3a2 does not influence the primary reinforcing effects of cocaine. Thus, Dnmt3a2 mediates long-lasting cocaine cue memories within the NAc shell. Targeting Dnmt3a2 expression or function may interfere with cocaine craving and relapse.SIGNIFICANCE STATEMENT In humans, drug craving can occur in response to conditioned cues, even after extended periods of abstinence. In rats, cue-induced cocaine seeking has been shown to increase progressively during the first 2 months of abstinence from drug self-administration. This phenomenon, referred to as incubation of cocaine seeking, is consistent with the hypothesis that in humans craving increases over time and remains high following prolonged abstinence. Those long-lasting behavioral changes are likely to be mediated by epigenetic effects and neuroplastic changes within the mesolimbic brain reward system. Here we show that a specific isoform of DNA-methyltransferases in the NAc shell regulates drug cue memories that drive reinstatement of cocaine seeking after both early abstinence and incubation of cocaine craving.

Cocaine- and amphetamine-regulated transcript peptide in the nucleus accumbens shell inhibits cocaine-induced locomotor sensitization to transient over-expression of α-Ca2+ /calmodulin-dependent protein kinase II

Cocaine- and amphetamine-regulated transcript (CART) peptide is a widely distributed neurotransmitter that attenuates cocaine-induced locomotor activity when injected into the nucleus accumbens (NAc). Our previous work first confirmed that the inhibitory mechanism of the CART peptide on cocaine-induced locomotor activity is related to a reduction in cocaine-enhanced phosphorylated Ca2+ /calmodulin-dependent protein kinaseIIα (pCaMKIIα) and the enhancement of cocaine-induced D3R function. This study investigated whether CART peptide inhibited cocaine-induced locomotor activity via inhibition of interactions between pCaMKIIα and the D3 dopamine receptor (D3R). We demonstrated that lentivirus-mediated gene transfer transiently increased pCaMKIIα expression, which peaked at 10days after microinjection into the rat NAc shell, and induced a significant increase in Ca2+ influx along with greater behavioral sensitivity in the open field test after intraperitoneal injections of cocaine (15mg/kg). However, western blot analysis and coimmunoprecipitation demonstrated that CART peptide treatment in lentivirus-transfected CaMKIIα-over-expressing NAc rat tissues or cells prior to cocaine administration inhibited the cocaine-induced Ca2+ influx and attenuated the cocaine-increased pCaMKIIα expression in lentivirus-transfected CaMKIIα-over-expressing cells. CART peptide decreased the cocaine-enhanced phosphorylated cAMP response element binding protein (pCREB) expression via inhibition of the pCaMKIIα-D3R interaction, which may account for the prolonged locomotor sensitization induced by repeated cocaine treatment in lentivirus-transfected CaMKIIα-over-expressing cells. These results provide strong evidence for the inhibitory modulation of CART peptide in cocaine-induced locomotor sensitization. Cover Image for this issue: doi: 10.1111/jnc.14187.

The effects of antipsychotics on the density of cannabinoid receptors in selected brain regions of male and female adolescent juvenile rats

Antipsychotic drugs have been increasingly prescribed to children and adolescents for treating various mental disorders, such as childhood-onset schizophrenia. The abnormality of endocannabinoid system is involved in the pathophysiology of these disorders in juveniles. This study investigated the effect of antipsychotics on the cannabinoid (CB) receptors in the brain of both male and female juvenile rats. The postnatal rats (PD23±1) were administered aripiprazole (1 mg/kg), olanzapine (1 mg/kg), risperidone (0.3 mg/kg) or vehicle (control) for 3 weeks. Quantitative autoradiography was used to investigate the binding densities of [3H]CP-55940 (an agonist for CB1R and CB2R) and [3H]SR141716A (a selective CB1R antagonist) in the rat brains. Risperidone significantly upregulated the [3H]CP55940 and [3H]SR141716A bindings in the prefrontal cortex (PFC), nucleus accumbens core (NAcC), nucleus accumbens shell (NAcS), cingulate cortex (Cg), and the caudate putamen (CPu) in male rats. Moreover, aripiprazole significantly elevated the [3H]SR141716A binding in the Cg and NAcS of female rats. Furthermore, there is an overall higher [3H]SR141716A binding level in the brain of female rats than male rats. Therefore, treatment with aripiprazole, olanzapine and risperidone could induce differential and gender specific effects on the binding density of cannabinoid receptors in the selected brain regions of childhood/adolescent rats.

Systemic Oxytocin Attenuates Methylphenidate Self‐Administration and Potentiates Its Effects on Dopamine in the Nucleus Accumbens Shell in Rats

Clinical and preclinical reports have shown that the hypothalamic neurohormone oxytocin plays a role in social bonding, a behavior that is also controlled by the neurotransmitter dopamine. Dopamine also plays an important role in both natural and pathologic behaviors related to reward and reinforcement, including substance use disorders. Dopamine is especially relevant for psychostimulant used is orders, and early reports have shown that oxytocin treatments might interfere with psychostimulant behavioral effects. Several recent studies have focused on oxytocin effects in models of reward or drug abuse in order to understand whether this neuropeptide interferes with pathological compulsive behaviors. Thus, oxytocin has been tested as a potential therapy for alcohol and psychostimulant use disorders. In the present study, the effects of oxytocin were assessed in Sprague‐Dawley rats: 1) trained to self‐administer methylphenidate intravenously (1 mg/kg, fixed ratio 5) during single daily 1‐hr sessions, 2) implanted with microdialysis probes to assess methylphenidate(0.1–1 mg/kg, i.v.) effects on extracellular levels of dopamine in the nucleus accumbens, a brain area related to reward and reinforced behavior, and 3)placed in infrared activity monitors to measure behavioral activation.Injections of oxytocin (0.1–2mg/kg, i.p.) 10 min before the sessions dose‐dependently decreased maximal self‐administration of methylphenidate (0.03–1.0 mg/kg/injection, i.v.). Though, oxytocin (0.2–2 mg/kg, i.p.) had little to no significant effects on extracellular levels of dopamine in the accumbens shell, oxytocin dose‐dependently enhanced the dopamine levels stimulated by intravenous methylphenidate (0.1–1 mg/kg, i.v.). Notably, this enhancement of dopamine levels did not result in stimulation of behavioral activities at levels higher than those produced by methylphenidate alone.The present results suggest that oxytocin attenuates the psychostimulant‐like reinforcing effects of methylphenidate, likely acting through altered dopamine neurotransmission in the terminals of the mesolimbic system. Thus, these results confirm and extend the potential translational therapeutic value of oxytocin for psychostimulant use disorders.Support or Funding InformationResearch Funded by:NIDA‐IRP, NIH/DHHS; Bench‐to‐Bedside Grant by the OBSSR/NIH; Intramural Grant ZIAAA000218‐01 (CPN Section) DICBR/NIAAA/NIH/DHHS.

Mu-opioid receptor activation in the medial shell of nucleus accumbens promotes alcohol consumption, self-administration and cue-induced reinstatement

Endogenous opioid signaling in ventral cortico-striatal-pallidal circuitry is implicated in elevated alcohol consumption and relapse to alcohol seeking. Mu-opioid receptor activation in the medial shell of the nucleus accumbens (NAc), a region implicated in multiple aspects of reward processing, elevates alcohol consumption while NAc opioid antagonists reduce it. However, the precise nature of the increases in alcohol consumption, and the effects of mu-opioid agonists on alcohol seeking and relapse are not clear. Here, we tested the effects of the mu-opioid agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) in rat NAc shell on lick microstructure in a free-drinking test, alcohol seeking during operant self-administration, extinction learning and expression, and cue-reinforced reinstatement of alcohol seeking. DAMGO enhanced the number, but not the size of drinking bouts. DAMGO also enhanced operant alcohol self-administration and cue-induced reinstatement, but did not affect extinction learning or elicit reinstatement in the absence of cues. Our results suggest that mu-opioid agonism in NAc shell elevates alcohol consumption, seeking and conditioned reinforcement primarily by enhancing the incentive motivational properties of alcohol and alcohol-paired cues, rather than by modulating palatability, satiety, or reinforcement.

Ghrelin and endocannabinoids participation in morphine-induced effects in the rat nucleus accumbens.

In addition to dopamine, endocannabinoids are thought to participate in neural reward mechanisms of opioids. Number of recent studies suggests crucial involvement of ghrelin in some addictive drugs effects. Our previous results showed that ghrelin participates in morphine-induced changes in the mesolimbic dopaminergic system associated with reward processing. The goal of the present study was to test whether the growth hormone secretagogue receptor (GHS-R1A) antagonist JMV2959 was able to influence morphine-induced effects on anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) in the nucleus accumbens shell (NACSh). We used in vivo microdialysis to determine changes in levels of AEA and 2-AG in the NACSh in rats following (i) an acute morphine dose (5, 10 mg/kg s.c.) with and without JMV2959 pretreatment (3, 6 mg/kg i.p.) or (ii) a morphine challenge dose (5 mg/kg s.c.) with and without JMV2959 (3, 6 mg/kg i.p.) pretreatment, administered during abstinence following repeated doses of morphine (5 days, 10-40 mg/kg). Co-administration of ghrelin (40 ug/kg i.p.) was used to verify the ghrelin mechanisms involvement. Pretreatment with JMV2959 significantly and dose-dependently reversed morphine-induced anandamide increases in the NACSh in both the acute and longer-term models, resulting in a significant AEA decrease. JMV2959 significantly intensified acute morphine-induced decreases in accumbens 2-AG levels and attenuated morphine challenge-induced 2-AG decreases. JMV2959 pretreatment significantly reduced concurrent morphine challenge-induced behavioral sensitization. JMV2959 pretreatment effects were abolished by co-administration of ghrelin. Our results indicate significant involvement of ghrelin signaling in morphine-induced endocannabinoid changes in the NACSh.

D1 and D2 antagonists reverse the effects of appetite suppressants on weight loss, food intake, locomotion, and rebalance spiking inhibition in the rat NAc shell.

Obesity is a worldwide health problem that has reached epidemic proportions. To ameliorate this problem, one approach is the use of appetite suppressants. These compounds are frequently amphetamine congeners such as diethylpropion (DEP), phentermine (PHEN), and bupropion (BUP), whose effects are mediated through serotonin, norepinephrine, and dopaminergic pathways. The nucleus accumbens (NAc) shell receives dopaminergic inputs and is involved in feeding and motor activity. However, little is known about how appetite suppressants modulate its activity. Therefore, we characterized behavioral and neuronal NAc shell responses to short-term treatments of DEP, PHEN, and BUP. These compounds caused a transient decrease in weight and food intake while increasing locomotion, stereotypy, and insomnia. They evoked a large inhibitory imbalance in NAc shell spiking activity that correlated with the onset of locomotion and stereotypy. Analysis of the local field potentials (LFPs) showed that all three drugs modulated beta, theta, and delta oscillations. These oscillations do not reflect an aversive-malaise brain state, as ascertained from taste aversion experiments, but tracked both the initial decrease in weight and food intake and the subsequent tolerance to these drugs. Importantly, the appetite suppressant-induced weight loss and locomotion were markedly reduced by intragastric (and intra-NAc shell) infusions of dopamine antagonists SCH-23390 (D1 receptor) or raclopride (D2 receptor). Furthermore, both antagonists attenuated appetite suppressant-induced LFP oscillations and partially restored the imbalance in NAc shell activity. These data reveal that appetite suppressant-induced behavioral and neuronal activity recorded in the NAc shell depend, to various extents, on dopaminergic activation and thus point to an important role for D1/D2-like receptors (in the NAc shell) in the mechanism of action for these anorexic compounds.

Monitoring dopamine transmission in the rat nucleus accumbens shell and core during acquisition of nose-poking for sucrose

On the basis of between subjects monitoring of in vivo dopamine (DA) transmission in the rat nucleus accumbens (NAc) shell and core during response-contingent and non-contingent sucrose feeding we have hypothesized that long term, daily exposure to sucrose feeding results in the acquisition of conditioned/discriminative stimuli capable of activating accumbens shell DA transmission in a non-habituating fashion. In order to verify this hypothesis we have now monitored within the same subject the changes in accumbens shell and core DA during acquisition of fixed ratio 1 (FR1) nose-poking for sucrose pellets. Once full training was obtained, dialysate DA was monitored in the same rat on three different sessions: responding for sucrose, extinction and non-contingent sucrose presentation. Dialysate DA steadily increased in the shell during operant sessions as training progressed but was activated in the core only early and transiently in training (5th session). After full training, reinforced as well as non-reinforced responding for sucrose activated DA selectively in the NAc shell. Non-contingent sucrose feeding activated DA in the shell and in the core. No habituation of shell DA responsiveness was observed under contingent and non-contingent sucrose feeding. These observations are consistent with the hypothesis that learning of FR1 nose-poking for sucrose involves acquisition of conditioned activation of DA transmission in the shell and active suppression in the core and that loss of habituation of shell DA responsiveness is related to change from primary-rewarding to conditioned/discriminative as driving stimuli of DA transmission in this area.

MGluR5 in the nucleus accumbens shell regulates morphine-associated contextual memory through reactive oxygen species signaling.

Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) critically modulates drug and drug-related behaviors. However, the role of mGluR5 in the opiate-induced contextual memory remains unclear. Here, we found that microinfusion of the mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) into the nucleus accumbens (NAc) shell, but not into the core, significantly attenuated the expression of morphine conditioned place preference (CPP) in rats. Following the expression of morphine CPP, the protein level of membrane mGluR5 was selectively increased in the NAc shell. In primary striatal neurons, we observed that treatment with the mGluR5 agonist CHPG increased the phosphorylation level of extracellular signal-regulated kinase (ERK), which was dependent on the mGluR5-inositol-1,4,5-trisphosphate-reactive oxygen species (ROS) pathway. Moreover, the microinjection of the ROS scavenger Tempol into the NAc shell of rats blocked the expression of morphine CPP. Further, the administration of t-BOOH, a ROS donor, into the NAc shell rescued the retrieval impairment of morphine CPP produced by MTEP. Our previous study demonstrated that the expression of morphine CPP increased the phosphorylation of ERK selectively in the NAc shell. Thus, results of the present study suggest that mGluR5 in the NAc shell, but not in the core, is essential for the retrieval of morphine contextual memory, which is mediated at least in part, through the ROS/ERK signaling pathway. Uncovering the molecular basis of opiate contextual memory will benefit the development of new therapeutic approaches for the treatment of opiate addiction.

Blockade of mGluR5 in the nucleus accumbens shell but not core attenuates heroin seeking behavior in rats.

Glutamatergic neurotransmission in the nucleus accumbens (NAc) is crucial for the relapse to heroin seeking. The aim of this study was to determine whether mGluR5 in the NAc core or shell involved in heroin seeking behavior in rats. Male SD rats were self-administered heroin under a fixed-ratio 1 (FR1) reinforcement schedule for 14 d, and subsequently withdrawn for 2 weeks. The selective mGluR5 antagonist 2-methyl-6-phenylethynyl-pyridine (MPEP, 5, 15 and 50 nmol per side) was then microinjected into the NAc core or shell 10 min before a heroin-seeking test induced by context, cues or heroin priming. Microinjection of MPEP into the NAc shell dose-dependently decreased the heroin seeking induced by context, cues or heroin priming. In contrast, microinjection of MPEP into the NAc core did not alter the heroin seeking induced by cues or heroin priming. In addition, microinjection with MPEP (15 nmol per side) in the NAc shell reversed both the percentage of open arms entries (OE%) and the percentage of time spent in open arms (OT%) after heroin withdrawal. Microinjection of MPEP (50 nmol per side) in the striatum as a control location did not affect the heroin seeking behavior. Microinjection of MPEP in the 3 locations did not change the locomotion activities. Blockade of mGluR5 in NAc shell in rats specifically suppresses the relapse to heroin-seeking and anxiety-like behavior, suggesting that mGluR5 antagonists may be a potential candidate for the therapy of heroin addiction.

Effects of cocaine combined with a social cue on conditioned place preference and nucleus accumbens monoamines after isolation rearing in rats

Social interaction during drug exposure can potentiate cocaine reward. Isolation rearing (ISO) during adolescence increases social interaction and may amplify this potentiation. The objectives of this study are to determine whether ISO alters conditioned place preference (CPP) for cocaine when combined with a social cue and to determine whether ISO alters the effects of cocaine when combined with social cue on nucleus accumbens shell (NAcS) dopamine (DA) and serotonin (5-HT). Male and female rats were either ISO or group (GRP) reared for 4weeks during adolescence. CPP was performed using a low dose of cocaine (2mg/kg or saline) with or without exposure to a novel same-sex conspecific during conditioning. In vivo microdialysis was performed using the same parameters. ISO rats engaged in more social and aggressive behaviors during conditioning relative to GRP. Cocaine reduced social and aggressive behaviors in all rats. CPP was not influenced by rearing condition. Cocaine produced significant CPP, and a social cue produced CPP only in males. In contrast, the interaction of cocaine and a social cue on NAcS DA and 5-HT differed depending upon rearing condition. In isolates, cocaine-induced DA was attenuated, while cocaine plus a social cue produced potentiated DA and 5-HT. Exposure to a low dose of cocaine in the presence of a social cue produced additive effects on CPP while producing synergistic effects on DA and 5-HT in the NAcS of ISO rats. The aversive effects of this compound stimulus may negate the rewarding effects in isolates.

Protein Kinase D1-Dependent Phosphorylation of Dopamine D1 Receptor Regulates Cocaine-Induced Behavioral Responses

The dopamine (DA) D1 receptor (D1R) is critically involved in reward and drug addiction. Phosphorylation-mediated desensitization or internalization of D1R has been extensively investigated. However, the potential for upregulation of D1R function through phosphorylation remains to be determined. Here we report that acute cocaine exposure induces protein kinase D1 (PKD1) activation in the rat striatum, and knockdown of PKD1 in the rat dorsal striatum attenuates cocaine-induced locomotor hyperactivity. Moreover, PKD1-mediated phosphorylation of serine 421 (S421) of D1R promotes surface localization of D1R and enhances downstream extracellular signal-regulated kinase signaling in D1R-transfected HEK 293 cells. Importantly, injection of the peptide Tat-S421, an engineered Tat fusion-peptide targeting S421 (Tat-S421), into the rat dorsal striatum inhibits cocaine-induced locomotor hyperactivity and injection of Tat-S421 into the rat hippocampus or the shell of the nucleus accumbens (NAc) also inhibits cocaine-induced conditioned place preference (CPP). However, injection of Tat-S421 into the rat NAc shell does not establish CPP by itself and injection of Tat-S421 into the hippocampus does not influence spatial learning and memory. Thus, targeting S421 of D1R represents a promising strategy for the development of pharmacotherapeutic treatments for drug addiction and other disorders that result from DA imbalances.

Reciprocal regulation of inhibitory synaptic transmission by nicotinic and muscarinic receptors in rat nucleus accumbens shell

Medium spiny neurones (MSNs) in the nucleus accumbens (NAc) are the principal neurones whose activities are regulated by GABAergic inputs from MSNs and fast-spiking interneurones (FSNs). Cholinergic interneurones play important roles in the regulation of activity in MSNs; however, how acetylcholine modulates inhibitory synaptic transmission from MSNs/FSNs to MSNs remains unknown. We performed paired whole-cell patch-clamp recordings from MSNs and FSNs in rat NAc shell slice preparations and examined cholinergic effects on unitary inhibitory postsynaptic currents (uIPSCs). Carbachol (1 μM) suppressed uIPSC amplitude by 58.3 ± 8.0% in MSN→MSN connections, accompanied by increases in paired-pulse ratio and failure rate, suggesting that acetylcholine reduces the probability of GABA release from the synaptic terminals of MSNs. Carbachol-induced uIPSC suppression was antagonised by 100 μM atropine, and was mimicked by pilocarpine (1 μM) and acetylcholine (1 μM) but not nicotine (1 μM). Application of AM251 slightly reduced carbachol-induced uIPSC suppression (30.8 ± 8.9%), suggesting an involvement of endocannabinoid signalling in muscarinic suppression of uIPSCs. In contrast, FSN→MSN connections showed that pilocarpine had little effect on the uIPSC amplitude, whereas both nicotine and acetylcholine facilitated uIPSC amplitude, with decreases in failure rate and paired-pulse ratio, suggesting that nicotine-induced uIPSC facilitation is mediated by presynaptic mechanisms. Miniature IPSC recordings support these hypotheses of presynaptic cholinergic mechanisms. These results suggest a differential role for muscarinic and nicotinic receptors in GABA release, which depends on presynaptic neuronal subtypes in the NAc shell.