Hippocampal regulation of contextual cue-induced reinstatement of cocaine-seeking behavior

Chemogenetic inhibition of the ventral hippocampus but not its direct projection to the prelimbic cortex attenuates context-specific operant responding

Previous work has demonstrated the importance of the prelimbic cortex (PL) in contextual control of operant behavior. However, the associated neural circuitry responsible for providing contextual information to the PL is not well understood. In Pavlovian fear conditioning the ventral hippocampus (vH) and its projection to the PL have been shown to be important in supporting the effects of context on learning. The present experiments used chemogenetic inhibition of the direct vH-PL projection or the vH to determine involvement in expression of context-specific operant behavior. Rats were injected with an inhibitory DREADD (hM4Di) or mCherry-only into the vH, and subsequently trained to perform a lever press response for a food pellet in a distinct context. The DREADD ligand clozapine-n-oxide (CNO) was then delivered directly into the PL (experiment 1) and then systemically (experiment 2) prior to tests of the response in the training context as well as an equally familiar but untrained context. vH (systemic CNO) but not vH-PL (intra-PL CNO) inhibition was found to attenuate operant responding in its acquisition context. A third experiment, using the same rats, showed that chemogenetic inhibition of vH also reduced Pavlovian contextual fear. The present results suggest that multisynapatic connections between the vH and PL may be responsible for integration of contextual information with operant behavior.

Glucagon-like peptide-1 receptors in nucleus accumbens, ventral hippocampus, and lateral septum reduce alcohol reinforcement in mice

Glucagon-like peptide 1 (GLP-1) receptor agonists can decrease alcohol intake by central mechanisms that are still poorly understood. The lateral septum (LS) and the ventral/caudal part of the hippocampus are enriched in GLP-1 receptors, and activity in these regions was shown to modulate reward-related behaviors. Using microinfusions of the GLP-1 receptor agonist exendin-4 in mice trained to self-administer oral alcohol in an operant assay, we tested whether pharmacological stimulation of GLP-1 receptors in hippocampus and LS decrease alcohol self-administration. We report that infusion of exendin-4 in the ventral hippocampus or LS was sufficient to reduce alcohol self-administration with as large effect sizes as we previously reported with systemic exendin-4 administration. Infusion of exendin-4 into the nucleus accumbens also reduced alcohol self-administration, as anticipated based on earlier reports, while infusion of exendin-4 into the caudate-putamen (dorsal striatum) had little effect, consistent with lack of GLP-1 receptor expression in this region. The distribution of exendin-4 after infusion into the LS or caudate putamen was visualized using a fluorescently labeled ligand. These findings add to our understanding of the circuit-level mechanisms underlying the ability of GLP-1 receptor agonists to reduce alcohol self-administration. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Cdc42 signaling regulated by dopamine D2 receptor correlatively links specific brain regions of hippocampus to cocaine addiction

BACKGROUND

Hippocampus plays critical roles in drug addiction. Cocaine-induced modifications in dopamine receptor function and the downstream signaling are important regulation mechanisms in cocaine addiction. Rac regulates actin filament accumulation while Cdc42 stimulates the formation of filopodia and neurite outgrowth. Based on the region specific roles of small GTPases in brain, we focused on the hippocampal subregions to detect the regulation of Cdc42 signaling in long-term morphological and behavioral adaptations to cocaine.

METHODS

Genetically modified mouse models of Cdc42, dopamine receptor D1 (D1R) and D2 (D2R) and expressed Cdc42 point mutants that are defective in binding to and activation of its downstream effector molecules PAK and N-WASP were generated, respectively, in CA1 or dentate gyrus (DG) subregion.

RESULTS

Cocaine induced upregulation of Cdc42 signaling activity. Cdc42 knockout or mutants blocked cocaine-induced increase in spine plasticity in hippocampal CA1 pyramidal neurons, leading to a decreased conditional place preference (CPP)-associated memories and spatial learning and memory in water maze. Cdc42 knockout or mutants promoted cocaine-induced loss of neurogenesis in DG, leading to a decreased CPP-associated memories and spatial learning and memory in water maze. Furthermore, by using D1R knockout, D2R knockout, and D2R/Cdc42 double knockout mice, we found that D2R, but not D1R, regulated Cdc42 signaling in cocaine-induced neural plasticity and behavioral changes.

CONCLUSIONS

Cdc42 acts downstream of D2R in the hippocampus and plays an important role in cocaine-induced neural plasticity through N-WASP and PAK-LIMK-Cofilin, and Cdc42 signaling pathway correlatively links specific brain regions (CA1, dentate gyrus) to cocaine-induced CPP behavior.

Astrocytes in Addictive Disorders

Astroglia are key regulators of synaptic function, playing central roles in homeostatic ion buffering, energy dynamics, transmitter uptake, maintenance of neurotransmitter pools, and regulation of synaptic plasticity through release of neuroactive chemicals. Given the myriad of crucial homeostatic and signaling functions attributed to astrocytes and the variety of neurotransmitter receptors expressed by astroglia, they serve as prime cellular candidates for establishing maladaptive synaptic plasticity following drug exposure. Initial studies on astroglia and addiction have placed drug-mediated disruptions in the homeostatic regulation of glutamate as a central aspect of relapse vulnerability. However, the generation of sophisticated tools to study and manipulate astroglia have proven that the interaction between addictive substances, astroglia, and relapse-relevant synaptic plasticity extends far beyond the homeostatic regulation of glutamate. Here we present astroglial systems impacted by drug exposure and discuss how changes in astroglial biology contribute to addiction biology.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Facilitative effects of environmental enrichment for cocaine relapse prevention are dependent on extinction training context and involve increased TrkB signaling in dorsal hippocampus and ventromedial prefrontal cortex

Cocaine-cue extinction training combined with brief interventions of environmental enrichment (EE) was shown previously to facilitate extinction and attenuate reacquisition of cocaine self-administration in rats. It is unknown whether or not the usefulness of this approach would be undermined if extinction training took place in a novel rather than familiar context. Drawing on previous studies involving pharmacological interventions, we hypothesized that the facilitative effects of EE for cocaine relapse prevention would be independent of the context used for extinction training. Rats trained to self-administer cocaine underwent cocaine-cue extinction training in either the familiar self-administration context or a novel context, with or without EE. Rats then were tested for reacquisition of cocaine self-administration in the familiar context. Target brain regions were lysed and probed for memory-related changes in receptors for glutamate and BDNF by western blotting. Contrary to our hypothesis, the facilitative effects of EE for cocaine relapse prevention were dependent on the context used for extinction training. While EE facilitated extinction regardless of context used, it inhibited cocaine relapse only after extinction training in the familiar context. EE was associated with increased GluA2 in nucleus accumbens, TrkB in dorsal hippocampus and activated TrkB in ventromedial prefrontal cortex. Of these, the changes in dorsal hippocampus and ventromedial prefrontal cortex mirrored outcomes of the cocaine relapse tests in that these changes were specific to rats receiving EE plus extinction training in the familiar context. These findings support a role for hippocampal-prefrontal BDNF-TrkB signaling in extinction-based relapse prevention strategies involving EE.

Glycogen Synthase Kinase 3β in the Ventral Hippocampus is Important for Cocaine Reward and Object Location Memory

The ventral hippocampus is a component of the neural circuitry involved with context-associated memory for reward and generation of appropriate behavioral responses to context. Glycogen synthase kinase 3 beta (GSK3β) has been linked to the maintenance of synaptic plasticity, contextual memory retrieval, and is involved in the reconsolidation of cocaine-associated contextual memory. In this study, the effects of targeted downregulation of GSK3β in the ventral hippocampus were examined on a series of behavioral tests for assessing drug reward-context association and non-reward related memory. The Cre/loxP site-specific recombination system was used to knockdown GSK3β through bilateral stereotaxic delivery of an adeno-associated virus expressing Cre-recombinase (AAV-Cre) into the ventral hippocampus of adult mice homozygous for a floxed GSK3β allele. GSK3β floxed mice injected with AAV-Cre had a loss of 56-75% of GSK3β in the ventral hippocampus and displayed diminished development of cocaine conditioned place preference, but not morphine place preference as compared with wild-type mice injected with AAV-Cre or GSK3β floxed mice injected with a control virus, AAV-GFP. Impaired object location memory was observed in mice with GSK3β downregulation in the ventral hippocampus, but novel object recognition remained intact. These results indicate that GSK3β signaling in the ventral hippocampus is differentially involved in the formation of place-drug reward association dependent upon drug class. Additionally, ventral hippocampal GSK3β signaling is important in detection of discrete spatial cues, but not recognition memory for objects.

Regulation of Adult Neurogenesis by Non-coding RNAs: Implications for Substance Use Disorders

The discovery of non-coding RNAs (ncRNAs)has been one of the central findings from early genomic sequencing studies. Not only was the presence of these genes unknown previously, it was the staggering disproportionate share of the genome that was predicted to be encoded by ncRNAs that was truly significant in genomic research. Over the years the function of various classes of these ncRNAs has been revealed. One of the first and enduring regulatory programs associated with these factors was development. In the neurosciences, the discovery of adult derived populations of dividing cells within the brain was equally substantial. The brain was hypothesized to be plastic only in its neuronal connectivity, but the discovery of the generation of new neurons was a novel mechanism of neuronal and behavioral plasticity. The process of adult neurogenesis resembles early neuronal development and has been found to share many parallels in the proper stages of specified genetic programs. Adult neurogenesis has also been found to play a role in learning and memory involved in particular hippocampal-dependent behaviors. Substance use disorders (SUDs) are an example of a behavioral condition that is associated with and possibly driven by hippocampal alterations. Our laboratory has determined that hippocampal adult neurogenesis is necessary for a rodent model of methamphetamine relapse. Due to the previous research on ncRNAs in development and in other brain regions involved in SUDs, we posit that ncRNAs may play a role in adult neurogenesis associated with this disorder. This review will cover the regulatory mechanisms of various classes of ncRNAs on the coordinated genetic program associated with adult neurogenesis with a special focus on how these programs could be dysregulated in SUDs.

Drug-Induced Glucocorticoids and Memory for Substance Use

The biological stress response of the body forms one of the foundations of adaptive behavior, including promoting (and impairing) different forms of memory. This response transcends stressful experiences and underlies reactions to challenges and even reinforcers such as addictive substances. Nevertheless, drug-induced stress responses are rarely incorporated into models of addiction. We propose here that drug-induced stress responses (particularly glucocorticoids) play a crucial role in addictive behavior by modulating the formation of memories for substance-use experiences. We review the contributions of amygdala-, striatum-, and hippocampus-based memory systems to addiction, and reveal common effects of addictive drugs and acute stress on these different memories. We suggest that the contributions of drug-induced stress responses to memory may provide insights into the mechanisms driving addictive behavior.

Activation of Astrocytic μ-opioid Receptor Elicits Fast Glutamate Release Through TREK-1-Containing K2P Channel in Hippocampal Astrocytes

Recently, μ-opioid receptor (MOR), one of the well-known Gi-protein coupled receptors (Gi-GPCR), was reported to be highly expressed in the hippocampal astrocytes. However, the role of astrocytic MOR has not been investigated. Here we report that activation of astrocytic MOR by [D-Ala²,N-MePhe⁴,Gly-ol]-enkephalin (DAMGO), a selective MOR agonist, causes a fast glutamate release using sniffer patch technique. We also found that the DAMGO-induced glutamate release was not observed in the astrocytes from MOR-deficient mice and MOR-short hairpin RNA (shRNA)-expressed astrocytes. In addition, the glutamate release was significantly reduced by gene silencing of the TREK-1-containing two-pore potassium (K2P) channel, which mediates passive conductance in astrocytes. Our findings were consistent with the previous study demonstrating that activation of Gi-GPCR such as cannabinoid receptor CB1 and adenosine receptor A1 causes a glutamate release through TREK-1-containing K2P channel from hippocampal astrocytes. We also demonstrated that MOR and TREK-1 are significantly co-localized in the hippocampal astrocytes. Furthermore, we found that both MOR and TREK-1-containing K2P channels are localized in the same subcellular compartments, soma and processes, of astrocytes. Our study raises a novel possibility that astrocytic MOR may participate in several physiological and pathological actions of opioids, including analgesia and addiction, through astrocytically released glutamate and its signaling pathway.

Synaptic Plasticity in the Bed Nucleus of the Stria Terminalis: Underlying Mechanisms and Potential Ramifications for Reinstatement of Drug- and Alcohol-Seeking Behaviors

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that shows significant changes in activity and plasticity through chronic exposure to drugs and stress. The region is critical for stress- and cue-induced reinstatement of drug-seeking behaviors and is thus a candidate region for the plastic changes that occur in abstinence that prime addicted patients for reinstatement behaviors. Here, we discuss the various forms of long-term potentiation (LTP) and long-term depression (LTD) in the rodent BNST and highlight the way that these changes in excitatory transmission interact with exposure to alcohol and other drugs of abuse, as well as other stressors. In addition, we highlight potential areas for future research in this area, including investigating input- and cell-specific bidirectional changes in activity. As we continue to accrue foundational knowledge in the mechanisms and effects of plasticity in the BNST, molecular targets and treatment strategies that are relevant to reinstatement behaviors will also begin to emerge. Here, we briefly discuss the effects of catecholamine receptor modulators on synaptic plasticity in the BNST due to the role of norepinephrine in LTD and dopamine on the short-term component of LTP as well as the role that signaling at these receptors plays in reinstatement of drug- and alcohol-seeking behaviors. We hope that insights gained on the specific changes in plasticity that occur within the BNST during abstinence from alcohol and other drugs of abuse will provide insight into the biological underpinnings of relapse behavior in human addicts and inform future treatment modalities for addiction that tackle this complex biological problem.

Lewis and Fischer 344 rats as a model for genetic differences in spatial learning and memory: Cocaine effects

Lewis (LEW) and Fischer 344 (F344) rats are considered a model of genetic vulnerability to drug addiction. We previously showed important differences in spatial learning and memory between them, but in contrast with previous experiments demonstrating cocaine-induced enhanced learning in Morris water maze (MWM) highly demanding tasks, the eight-arm radial maze (RAM) performance was not modified either in LEW or F344 rats after chronic cocaine treatment. In the present work, chronically cocaine-treated LEW and F344 adult rats have been evaluated in learning and memory performance using the Y-maze, two RAM protocols that differ in difficulty, and a reversal protocol that tests cognitive flexibility. After one of the RAM protocols, we quantified dendritic spine density in hippocampal CA1 neurons and compared it to animals treated with cocaine but not submitted to RAM. LEW cocaine treated rats showed a better performance in the Y maze than their saline counterparts, an effect that was not evident in the F344 strain. F344 rats significantly took more time to learn the RAM task and made a greater number of errors than LEW animals in both protocols tested, whereas cocaine treatment induced deleterious effects in learning and memory in the highly difficult protocol. Moreover, hippocampal spine density was cocaine-modulated in LEW animals whereas no effects were found in F344 rats. We propose that differences in addictive-like behavior between LEW and F344 rats could be related to differences in hippocampal learning and memory processes that could be on the basis of individual vulnerability to cocaine addiction.

Biomarkers for Success: Using Neuroimaging to Predict Relapse and Develop Brain Stimulation Treatments for Cocaine-Dependent Individuals

Cocaine dependence is one of the most difficult substance use disorders to treat. While the powerful effects of cocaine use on behavior were documented in the 19th century, it was not until the late 20th century that we realized cocaine use was affecting brain tissue and function. Following a brief introduction (Section 1), this chapter will summarize our current knowledge regarding alterations in neural circuit function typically observed in chronic cocaine users (Section 2) and highlight an emerging body of literature which suggests that pretreatment limbic circuit activity may be a reliable predictor of clinical outcomes among individuals seeking treatment for cocaine (Section 3). Finally, as the field of addiction research strives to translate this neuroimaging data into something clinically meaningful, we will highlight several new brain stimulation approaches which utilize functional brain imaging data to design noninvasive brain stimulation interventions for individuals seeking treatment for substance dependence disorders (Section 4).

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

AIM

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Activity-regulated gene expression in immature neurons in the dentate gyrus following re-exposure to a cocaine-paired environment

Intense craving for drug and relapse are observed in addicts who are exposed to environmental stimuli associated with drug-taking behavior even after long periods of abstinence. The hippocampus is a brain region known to be involved in contextual processing, taking place predominantly in the septal hippocampus, and emotional processing, taking place predominantly in the temporal hippocampus. Conditioned place preference is an animal model of context-conditioned reward. The dentate gyrus is a hippocampal sub-region particularly important for the acquisition of cocaine-induced place preference and is a site of continuous neurogenesis, which has been implicated in the vulnerability to drug-taking behavior. Therefore, these experiments explored the role of newly generated neurons in drug reward-context association by examining the activation, as determined by expression of the immediate early gene cfos, of young and mature granule cells in the septal and temporal dentate gyrus of adult rats that were re-exposed to a drug-paired environment following the development of cocaine place preference. The overall level of cfos expression was increased in both the septal and temporal dentate gyrus of animals that developed place preference and were re-exposed to the drug paired environment compared with re-exposure to a neutral environment. Overall level of neurogenesis, as detected by the S-phase marker 5'-bromo-2'-deoxyuridine (BrdU) and the immature neuron marker doublecortin (DCX), was unaltered by cocaine conditioning. However, the number of activated new neurons (DCX + cfos) was greater in the temporal dentate gyrus of cocaine-conditioned rats re-exposed to the drug-paired environment as compared to those re-exposed to a neutral environment. Further understanding of the role of dentate gyrus neurogenesis on the conditioned effects of drugs of abuse may provide new insights into the role of this process in the expression of addictive behaviors.

Hippocampal neurogenesis protects against cocaine-primed relapse

Accumulating evidence demonstrates a functional role for the hippocampus in mediating relapse to cocaine-seeking behavior and extinction-induced inhibition of cocaine seeking, and dentate gyrus neurogenesis in the hippocampus may have a role. Here, we tested the hypothesis that disruption of normal hippocampal activity during extinction alters relapse to cocaine-seeking behavior as a function of dentate gyrus neurogenesis. Adult rats were trained to self-administer cocaine on a fixed-ratio schedule, followed by extinction and cocaine-primed reinstatement testing. Some rats received low-frequency stimulation (LFS; 2 Hz for 25 minutes) after each extinction session in the dorsal or ventral hippocampal formation. All rats received an injection of the mitotic marker 5-bromo-2'-deoxyuridine (BrdU) to label developing dentate gyrus neurons during self-administration, as well as before or after extinction and LFS. We found that LFS during extinction did not alter extinction behavior but enhanced cocaine-primed reinstatement. Cocaine self-administration reduced levels of 24-day-old BrdU cells and dentate gyrus neurogenesis, which was normalized by extinction. LFS during extinction prevented extinction-induced normalization of dentate gyrus neurogenesis and potentiated cocaine-induced reinstatement of drug seeking. LFS inhibition of extinction-induced neurogenesis was not due to enhanced cell death, revealed by quantification of activated caspase3-labeled cells. These data suggest that LFS during extinction disrupts hippocampal networking by disrupting neurogenesis and also strengthens relapse-like behaviors. Thus, newly born dentate gyrus neurons during withdrawal and extinction learning facilitate hippocampal networking that mediates extinction-induced inhibition of cocaine seeking and may play a key role in preventing relapse.

Repeated cocaine enhances ventral hippocampal-stimulated dopamine efflux in the nucleus accumbens and alters ventral hippocampal NMDA receptor subunit expression

Dopaminergic neurotransmission in the nucleus accumbens is important for various reward-related cognitive processes including reinforcement learning. Repeated cocaine enhances hippocampal synaptic plasticity, and phasic elevations of accumbal dopamine evoked by unconditioned stimuli are dependent on impulse flow from the ventral hippocampus. Therefore, sensitized hippocampal activity may be one mechanism by which drugs of abuse enhance limbic dopaminergic activity. In this study, in vivo microdialysis in freely moving adult male Sprague-Dawley rats was used to investigate the effect of repeated cocaine on ventral hippocampus-mediated dopaminergic transmission within the medial shell of the nucleus accumbens. Following seven daily injections of saline or cocaine (20 mg/kg, ip), unilateral infusion of N-methyl-d-aspartate (NMDA, 0.5 μg) into the ventral hippocampus transiently increased both motoric activity and ipsilateral dopamine efflux in the medial shell of the nucleus accumbens, and this effect was greater in rats that received repeated cocaine compared to controls that received repeated saline. In addition, repeated cocaine altered NMDA receptor subunit expression in the ventral hippocampus, reducing the NR2A : NR2B subunit ratio. Together, these results suggest that repeated exposure to cocaine produces maladaptive ventral hippocampal-nucleus accumbens communication, in part through changes in glutamate receptor composition. A behaviorally sensitizing regimen of cocaine (20 mg/kg, ip 7 days) also sensitized ventral hippocampus (hipp)-mediated dopaminergic transmission within the nucleus accumbens (Nac) to NMDA stimulation (bolts). This was associated with reduced ventral hippocampal NR2A:NR2B subunit ratio, suggesting that repeated exposure to cocaine produces changes in hippocampal NMDA receptor composition that lead to enhanced ventral hippocampus-nucleus accumbens communication.

Contribution of a mesocorticolimbic subcircuit to drug context-induced reinstatement of cocaine-seeking behavior in rats

Cocaine-seeking behavior triggered by drug-paired environmental context exposure is dependent on orbitofrontal cortex (OFC)-basolateral amygdala (BLA) interactions. Here, we present evidence supporting the hypothesis that dopaminergic input from the ventral tegmental area (VTA) to the OFC critically regulates these interactions. In experiment 1, we employed site-specific pharmacological manipulations to show that dopamine D1-like receptor stimulation in the OFC is required for drug context-induced reinstatement of cocaine-seeking behavior following extinction training in an alternate context. Intra-OFC pretreatment with the dopamine D1-like receptor antagonist, SCH23390, dose-dependently attenuated cocaine-seeking behavior in an anatomically selective manner, without altering motor performance. Furthermore, the effects of SCH23390 could be surmounted by co-administration of a sub-threshold dose of the D1-like receptor agonist, SKF81297. In experiment 2, we examined effects of D1-like receptor antagonism in the OFC on OFC-BLA interactions using a functional disconnection manipulation. Unilateral SCH23390 administration into the OFC plus GABA agonist-induced neural inactivation of the contralateral or ipsilateral BLA disrupted drug context-induced cocaine-seeking behavior relative to vehicle, while independent unilateral manipulations of these brain regions were without effect. Finally, in experiment 3, we used fluorescent retrograde tracers to demonstrate that the VTA, but not the substantia nigra, sends dense intra- and interhemispheric projections to the OFC, which in turn has reciprocal bi-hemispheric connections with the BLA. These findings support that dopaminergic input from the VTA, via dopamine D1-like receptor stimulation in the OFC, is required for OFC-BLA functional interactions. Thus, a VTA-OFC-BLA neural circuit promotes drug context-induced motivated behavior.

Possible contributions of a novel form of synaptic plasticity in Aplysia to reward, memory, and their dysfunctions in mammalian brain

Recent studies in Aplysia have identified a new variation of synaptic plasticity in which modulatory transmitters enhance spontaneous release of glutamate, which then acts on postsynaptic receptors to recruit mechanisms of intermediate- and long-term plasticity. In this review I suggest the hypothesis that similar plasticity occurs in mammals, where it may contribute to reward, memory, and their dysfunctions in several psychiatric disorders. In Aplysia, spontaneous release is enhanced by activation of presynaptic serotonin receptors, but presynaptic D1 dopamine receptors or nicotinic acetylcholine receptors could play a similar role in mammals. Those receptors enhance spontaneous release of glutamate in hippocampus, entorhinal cortex, prefrontal cortex, ventral tegmental area, and nucleus accumbens. In all of those brain areas, glutamate can activate postsynaptic receptors to elevate Ca²⁺ and engage mechanisms of early-phase long-term potentiation (LTP), including AMPA receptor insertion, and of late-phase LTP, including protein synthesis and growth. Thus, presynaptic receptors and spontaneous release may contribute to postsynaptic mechanisms of plasticity in brain regions involved in reward and memory, and could play roles in disorders that affect plasticity in those regions, including addiction, Alzheimer’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD).

Stress differentially alters mu opioid receptor density and trafficking in parvalbumin-containing interneurons in the female and male rat hippocampus

Stress differentially affects hippocampal-dependent learning relevant to addiction and morphology in male and female rats. Mu opioid receptors (MORs), which are located in parvalbumin (PARV)-containing GABAergic interneurons and are trafficked in response to changes in the hormonal environment, play a critical role in promoting principal cell excitability and long-term potentiation. Here, we compared the effects of acute and chronic immobilization stress (AIS and CIS) on MOR trafficking in PARV-containing neurons in the hilus of the dentate gyrus in female and male rats using dual label immunoelectron microscopy. Following AIS, the density of MOR silver-intensified gold particles (SIGs) in the cytoplasm of PARV-labeled dendrites was significantly reduced in females (estrus stage). Conversely, AIS significantly increased the proportion of cytoplasmic MOR SIGs in PARV-labeled dendrites in male rats. CIS significantly reduced the number of PARV-labeled neurons in the dentate hilus of males but not females. However, MOR/PARV-labeled dendrites and terminals were significantly smaller in CIS females, but not males, compared with controls. Following CIS, the density of cytoplasmic MOR SIGs increased in PARV-labeled dendrites and terminals in females. Moreover, the proportion of near-plasmalemmal MOR SIGs relative to total decreased in large PARV-labeled dendrites in females. After CIS, no changes in the density or trafficking of MOR SIGs were seen in PARV-labeled dendrites or terminals in males. These data show that AIS and CIS differentially affect available MOR pools in PARV-containing interneurons in female and male rats. Furthermore, they suggest that CIS could affect principal cell excitability in a manner that maintains learning processes in females but not males.

Ghrelin signaling in the ventral hippocampus stimulates learned and motivational aspects of feeding via PI3K-Akt signaling

BACKGROUND

The stomach-derived hormone ghrelin drives higher-order feeding processes related to food reward and food seeking via central nervous system signaling at its receptor (GHSR1A). The specific nuclei mediating these effects are only partially understood. Here, we use a rat model to examine whether ghrelin signaling in the ventral subregion of the hippocampus (VHPC), a brain substrate of recent interest in energy balance control, affects learned and motivational aspects of feeding behavior.

METHODS

The effects of VHPC ghrelin administration were examined on feeding-relevant behavioral paradigms, including meal pattern analysis, operant lever pressing for sucrose, and conditioned stimulus-induced feeding. The intracellular signaling and downstream neuronal pathways stimulated by VHPC GHSR1A activation were assessed with immunoblot analysis and behavioral pharmacology.

RESULTS

Ghrelin delivery to the VHPC but not the dorsal hippocampus increased food intake primarily by increasing meal frequency. Intra-VHPC ghrelin delivery also increased willingness to work for sucrose and increased spontaneous meal initiation in nondeprived rats after the presentation of a conditioned stimulus that previously signaled meal access when the rats were food-restricted. The food intake enhancing effects of VHPC ghrelin were blocked by co-administration of a phosphoinositide 3-kinase (PI3K) inhibitor (LY294002). Immunoblot analyses provided complementary support for ghrelin activated PI3K-Akt signaling in the VHPC and revealed that this activation is blunted with high-fat diet consumption. Other immunoblot results show that VHPC GHSR1A signaling activates downstream dopaminergic activity in the nucleus accumbens.

CONCLUSIONS

These findings illuminate novel neuronal and behavioral mechanisms mediating ghrelinergic control of cognitive aspects of feeding control.

Systems level neuroplasticity in drug addiction

Drug addiction is a chronic relapsing disorder for which research has been dedicated to understand the various factors that contribute to development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide a broad overview of various theories of addiction, drugs of abuse, and the neurobiology involved across the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the involvement of these pathways and associated circuits in mediating conditioned responses, drug craving, and loss of behavioral control thought to underlie withdrawal and relapse. With a better understanding of the neurobiological factors that underlie drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

18-Methoxycoronaridine blocks context-induced reinstatement following cocaine self-administration in rats

Numerous studies utilizing drug self-administration have shown the importance of conditioned cues in maintaining and reinstating addictive behaviors. However, most used simple cues that fail to replicate the complexity of cues present in human craving and addiction. We have recently shown that music can induce behavioral and neurochemical changes in rats following classical conditioning with psychostimulants. However, such effects have yet to be characterized utilizing operant self-administration procedures, particularly with regard to craving and relapse. The goal of the present study was to validate the effectiveness of music as a contextual conditioned stimulus using cocaine in an operant reinstatement model of relapse. Rats were trained to lever press for cocaine with a musical cue, and were subsequently tested during reinstatement sessions to determine how musical conditioning affected drug seeking behavior. Additionally, in vivo microdialysis was used to determine basolateral amygdala involvement during reinstatement. Lastly, tests were conducted to determine whether the putative anti-addictive agent 18-methoxycoronaridine (18-MC) could attenuate cue-induced drug seeking behavior. Our results show that music-conditioned animals exhibited increased drug seeking behaviors when compared to controls during reinstatement test sessions. Furthermore, music-conditioned subjects exhibited increased extracellular dopamine in the basolateral amygdala during reinstatement sessions. Perhaps most importantly, 18-MC blocked musical cue-induced reinstatement. Thus,music can be a powerful contextual conditioned cue in rats, capable of inducing changes in both brain neurochemistry and drug seeking behavior during abstinence. The fact that 18-MC blocked cue-induced reinstatement suggests that α3β4 nicotinic receptors may be involved in the mechanism of craving, and that 18-MC may help prevent relapse to drug addiction in humans.

Emerging, reemerging, and forgotten brain areas of the reward circuit: Notes from the 2010 Motivational Neural Networks conference

On April 24-27, 2010, the Motivational Neuronal Networks meeting took place in Wrightsville Beach, North Carolina. The conference was devoted to "Emerging, re-emerging, and forgotten brain areas" of the reward circuit. A central feature of the conference was four scholarly discussions of cutting-edge topics related to the conference's theme. These discussions form the basis of the present review, which summarizes areas of consensus and controversy, and serves as a roadmap for the next several years of research.

Systemic stimulation of TLR2 impairs neonatal mouse brain development

BACKGROUND

Inflammation is associated with perinatal brain injury but the underlying mechanisms are not completely characterized. Stimulation of Toll-like receptors (TLRs) through specific agonists induces inflammatory responses that trigger both innate and adaptive immune responses. The impact of engagement of TLR2 signaling pathways on the neonatal brain is still unclear. The aim of this study was to investigate the potential effect of a TLR2 agonist on neonatal brain development.

METHODOLOGY/PRINCIPAL FINDINGS

Mice were injected intraperitoneally (i.p.) once a day from postnatal day (PND) 3 to PND11 with endotoxin-free saline, a TLR2 agonist Pam(3)CSK(4) (5 mg/kg) or Lipopolysaccharide (LPS, 0.3 mg/kg). Pups were sacrificed at PND12 or PND53 and brain, spleen and liver were collected and weighed. Brain sections were stained for brain injury markers. Long-term effects on memory function were assessed using the Trace Fear Conditioning test at PND50. After 9 days of Pam(3)CSK(4) administration, we found a decreased volume of cerebral gray matter, white matter in the forebrain and cerebellar molecular layer that was accompanied by an increase in spleen and liver weight at PND12. Such effects were not observed in Pam3CSK4-treated TLR 2-deficient mice. Pam3CSK4-treated mice also displayed decreased hippocampus neuronal density, and increased cerebral microglia density, while there was no effect on caspase-3 or general cell proliferation at PND12. Significantly elevated levels of IL-1β, IL-6, KC, and MCP-1 were detected after the first Pam3CSK4 injection in brain homogenates of PND3 mice. Pam(3)CSK(4) administration did not affect long-term memory function nor the volume of gray or white matter.

CONCLUSIONS/SIGNIFICANCE

Repeated systemic exposure to the TLR2 agonist Pam(3)CSK(4) can have a short-term negative impact on the neonatal mouse brain.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Involvement of the dorsal subiculum and rostral basolateral amygdala in cocaine cue extinction learning in rats

Memory system circuitry may regulate how cues associated with cocaine are extinguished, and understanding neurosubstrates of extinction may lead to the development of improved treatment strategies for cocaine addiction. Sites within the hippocampus and amygdala were investigated for their role in regulating cocaine cue extinction learning. Initially, rats were trained to self-administer cocaine under a second-order reinforcement schedule (cocaine and cocaine cues present) followed by a 2-week abstinence period. Using lidocaine, rats next underwent bilateral inactivation of the dorsal subiculum (dSUB) or rostral basolateral amygdala (rBLA), asymmetric inactivation of the dSUB and rBLA, unilateral inactivation of the dSUB or rBLA, or ipsilateral inactivation of the dSUB and rBLA prior to cocaine cue extinction training sessions (only cocaine cues present) on two consecutive days. Relative to vehicle, bilateral and asymmetric lidocaine treatments in the dSUB and rBLA slowed cocaine cue extinction learning. Specifically, vehicle-treated rats exhibited a significantly larger difference in responding from Day 1 to Day 2 of extinction training than lidocaine-treated rats. In comparison, unilateral or ipsilateral lidocaine treatments in the dSUB and rBLA did not slow cocaine cue extinction learning. Rats treated with lidocaine and vehicle exhibited a similar difference in responding from Day 1 to Day 2 of extinction training. These results indicate that sites within the hippocampus and amygdala need to be functionally active simultaneously in at least one brain hemisphere for acquisition of cocaine cue extinction learning. These results further suggest that a serial circuit within each hemisphere mediates acquisition of cocaine cue extinction learning.

Sub-region specific contribution of the ventral hippocampus to drug context-induced reinstatement of cocaine-seeking behavior in rats

The ventral hippocampus (VH) plays critical roles in cue-induced and cocaine-primed reinstatement of cocaine seeking [Rogers JL, See RE (2007) Neurobiol Learn Mem 87:688-692]. Subregions of the VH make distinct projections to elements of the brain relapse circuitry that mediate drug context-induced reinstatement. Thus, the VH may also critically contribute to this form of cocaine seeking in a subregion-specific manner. Accordingly, this study evaluated the hypothesis that functional inactivation of the ventral hippocampus proper (VHp)-but not of the dentate gyrus (DG)-impairs cocaine seeking elicited by re-exposure to a drug-paired environmental context. Rats were trained to lever press for un-signaled i.v. cocaine infusions (0.15 mg/infusion) in a distinct environmental context (cocaine-paired context) followed by extinction training in a distinctly different context (extinction context). Subsequently, cocaine-seeking behavior (i.e., non-reinforced active lever responding) was assessed in either the previously cocaine-paired context or the extinction context. Rats received bilateral microinfusions of the GABA agonist cocktail, baclofen+muscimol (BM: 1.0/.01 mM), or vehicle into the VHp, DG, or the posterior dorsal hippocampus (pDH; extra-VH control) immediately before each test session. Exposure to the previously cocaine-paired context, but not the extinction context, reinstated extinguished cocaine-seeking behavior following vehicle pretreatment. BM pretreatment administered into the VHp, but not the DG or pDH, significantly attenuated drug context-induced cocaine seeking. These results indicate that the VH contributes to drug context-induced cocaine seeking in a subregion-specific manner, with the functional integrity of the VHp being necessary for memory or motivational aspects of drug-paired environmental stimuli that sustain stimulus control over goal-directed behavior.

Hippocampus and retrograde amnesia in the rat model: a modest proposal for the situation of systems consolidation

The properties of retrograde amnesia after damage to the hippocampus have been explicated with some success using a rat model of human medial temporal lobe amnesia. We review the results of this experimental work with rats focusing on several areas of consensus in this growing literature. We evaluate the theoretically significant hypothesis that hippocampal retrograde amnesia normally exhibits a temporal gradient, affecting recent, but sparing remote memories. Surprisingly, the evidence does not provide much support for the idea that there is a lengthy process of systems consolidation following a learning episode. Instead, recent and remote memories tend to be equally affected. The extent of damage to the hippocampus is a significant factor in this work since it is likely that spared hippocampal tissue can support at least partial memory retrieval. With extensive hippocampal damage gradients are flat or, in the case of memory tasks with flavour/odour retrieval cues, the retrograde amnesia covers a period of about 1-3 days. There is consistent evidence that at the time of learning the hippocampus interferes with or overshadows memory acquisition by other systems. This contributes to the breadth and severity of retrograde amnesia relative to anterograde amnesia in the rat. The fact that multiple, distributed learning episodes can overcome this overshadowing is consistent with a parallel dual-store theory or a Distributed Reinstatement Theory in which each learning episode triggers a short period of memory replay that provides a brief hippocampal-dependent systems consolidation.

Involvement of the lateral orbitofrontal cortex in drug context-induced reinstatement of cocaine-seeking behavior in rats

Orbitofrontal cortex (OFC) damage produces impaired decision-making, impulsivity and perseveration and potentially contributes to compulsive drug seeking in cocaine users. To further explore this phenomenon, we assessed the role of the lateral OFC (lOFC) in drug context-induced cocaine-seeking behavior in the reinstatement model of drug relapse. Rats were trained to lever press for intravenous cocaine infusions in a distinct environmental context (cocaine-paired context) followed by extinction training in a different context (extinction-paired context). Reinstatement of cocaine seeking (non-reinforced lever presses) was assessed in the cocaine context in the absence of response-contingent stimuli. In Experiment 1, we evaluated whether acute inhibition of lOFC output alters context-induced cocaine-seeking behavior by infusing the GABA(B + A) agonists (baclofen + muscimol) or vehicle into the lOFC immediately before exposure to the cocaine-paired context. In Experiments 2 and 3, we assessed how prolonged loss of lOFC output affects drug context-induced cocaine seeking by administering bilateral N-methyl-d-aspartic acid or sham lesions of the lOFC either before or after self-administration and extinction training. Remarkably, IOFC functional inactivation attenuated, post-training lesions failed to alter and pre-training lesions potentiated drug context-induced cocaine seeking without altering responding in the extinction context. These results suggest that neural activity in the lOFC promotes context-induced cocaine-seeking behavior. However, prolonged loss of lOFC output enhances the motivational salience of cocaine-paired contextual stimuli probably by eliciting compensatory neuroadaptations, with the effects of post-training lOFC lesions reflecting an intermediate state of compensatory neuroplasticity. Overall, these findings support the idea that OFC dysfunction may promote cue reactivity and enhance relapse propensity in cocaine users.

Ethanol seeking triggered by environmental context is attenuated by blocking dopamine D1 receptors in the nucleus accumbens core and shell in rats

RATIONALE

Conditioned behavioral responses to discrete drug-associated cues can be modulated by the environmental context in which those cues are experienced, a process that may facilitate relapse in humans. Rodent models of drug self-administration have been adapted to reveal the capacity of contexts to trigger drug seeking, thereby enabling neurobiological investigations of this effect.

OBJECTIVES

We tested the hypothesis that dopamine transmission in the nucleus accumbens, a neural structure that mediates reinforcement, is necessary for context-induced reinstatement of responding for ethanol-associated cues.

METHODS

Rats pressed one lever (active) for oral ethanol (0.1 ml; 10% v/v) in operant conditioning chambers distinguished by specific visual, olfactory, and tactile contextual stimuli. Ethanol delivery was paired with a discrete (4 s) light-noise stimulus. Responses on a second lever (inactive) were not reinforced. Behavior was then extinguished by withholding ethanol but not the discrete stimulus in a different context. Reinstatement, expressed as elevated responding for the discrete stimulus without ethanol delivery, was tested by placing rats into the prior self-administration context after administration of saline or the dopamine D1 receptor antagonist, SCH 23390 (0.006, 0.06, and 0.6 microg/side), into the nucleus accumbens core or shell.

RESULTS

Compared with extinction responding, active lever pressing in saline-pretreated rats was enhanced by placement into the prior ethanol self-administration context. SCH 23390 dose-dependently reduced reinstatement after infusion into the core or shell.

CONCLUSION

These findings suggest a critical role for dopamine acting via D1 receptors in the nucleus accumbens in the reinstatement of responding for ethanol cues triggered by placement into an ethanol-associated context.