Glutamate release in the nucleus accumbens core is necessary for heroin seeking

Systemic administration of N-acetylcysteine during the extinction period and on the reinstatement day decreased the maintenance of morphine rewarding properties in the rats

Many animal studies and early clinical trials suggested that N-acetylcysteine (NAC) may benefit addiction treatment. The present study tried to evaluate whether chronic administration of systemic NAC during the extinction period and acute administration of systemic NAC on the reinstatement day could reduce the maintenance of the morphine rewarding properties in the conditioned place preference (CPP) paradigm in the rats. Ninety-six adult male Wistar rats (190-220 g) were examined with morphine (7 mg/kg; sc) and saline (1 mL/kg; sc) during the 3-day conditioning phase in the CPP paradigm. After the acquisition of morphine CPP, different doses of NAC were daily administered during the extinction period (5, 10, 25, and 50 mg/kg; ip), or 30 min before the CPP test on the reinstatement day (2, 5, 10, 25, and 50 mg/kg; ip). Conditioning score and locomotor activity were recorded by the video tracking system and Ethovision software after acquisition on the post-conditioning day, the extinction period, and reinstatement day. Daily NAC administration in high doses (25 and 50 mg/kg; ip) reduced extinction-responding compared with the vehicle-control group during the extinction period. Although a single injection of NAC in doses 10, 25, 50 mg/kg decreased the reinstatement of morphine-induced CPP, two lower doses (2 and 5 mg/kg) could not significantly reduce the CPP scores. These are the first data suggesting that NAC's application during the extinction period could attenuate the morphine reward-associated behaviors in the rats. Moreover, NAC could inhibit the reinstatement of morphine CPP, which adds to the growing appreciation that the NAC may have potential therapeutic use in combating morphine dependence. It can be consistent with the hypothesis of the involvement of the glutamatergic system in the pathophysiology of addiction.

Behavioural and biochemical responses to methamphetamine are differentially regulated by mGlu2 and mGlu3 metabotropic glutamate receptors in male mice

Group II metabotropic glutamate receptors (mGlu2 and mGlu3 receptors) shape mechanisms of methamphetamine addiction, but the individual role played by the two subtypes is unclear. We measured methamphetamine-induced conditioned place preference (CPP) and motor responses to single or repeated injections of methamphetamine in wild-type, mGlu2^-/-, and mGlu3^-/-mice. Only mGlu3^-/-mice showed methamphetamine preference in the CPP test. Motor response to the first methamphetamine injection was dramatically reduced in mGlu2^-/-mice, unless these mice were treated with the mGlu5 receptor antagonist, MTEP. In contrast, methamphetamine-induced sensitization was increased in mGlu3^-/-mice compared to wild-type mice. Only mGlu3^-/-mice sensitized to methamphetamine showed increases in phospho-ERK1/2 levels in the nucleus accumbens (NAc) and free radical formation in the NAc and medial prefrontal cortex. These changes were not detected in mGlu2^-/-mice. We also measured a series of biochemical parameters related to the mechanism of action of methamphetamine in naïve mice to disclose the nature of the differential behavioural responses of the three genotypes. We found a reduced expression and activity of dopamine transporter (DAT) and vesicular monoamine transporter-2 in the NAc and striatum of mGlu2^-/-and mGlu3^-/-mice, whereas expression of the DAT adaptor, syntaxin 1A, was selectively increased in the striatum of mGlu3^-/-mice. Methamphetamine-stimulated dopamine release in striatal slices was largely reduced in mGlu2^-/-, but not in mGlu3^-/-, mice. These findings suggest that drugs that selectively enhance mGlu3 receptor activity or negatively modulate mGlu2 receptors might be beneficial in the treatment of methamphetamine addiction and associated brain damage.

CB1R activation in nucleus accumbens core promotes stress-induced reinstatement of cocaine seeking by elevating extracellular glutamate in a drug-paired context

Preclinical models of stress-induced relapse to drug use have shown that the dysregulation of glutamatergic transmission within the nucleus accumbens (NA) contributes notably to the reinstatement of cocaine-seeking behavior in rodents. In this sense, there has been increasing interest in the cannabinoid type-1 receptor (CB1R), due to its crucial role in modulating glutamatergic neurotransmission within brain areas involved in drug-related behaviors. This study explored the involvement of CB1R within the NA subregions in the restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP), as well as in the regulation of glutamatergic transmission, by using a pharmacological approach and the in vivo microdialysis sampling technique in freely moving rats. CB1R blockade by the antagonist/inverse agonist AM251 (5 nmol/0.5 μl/side) or CB1R activation by the agonist ACEA (0.01 fmol/0.5 μl/side), prevented or potentiated restraint stress-induced reinstatement of cocaine-CPP, respectively, after local administration into NAcore, but not NAshell. In addition, microdialysis experiments demonstrated that restraint stress elicited a significant increase in extracellular glutamate in NAcore under reinstatement conditions, with the local administration of AM251 or ACEA inhibiting or potentiating this, respectively. Interestingly, this rise specifically corresponded to the cocaine-associated CPP compartment. We also showed that this context-dependent change in glutamate paralleled the expression of cocaine-CPP, and disappeared after the extinction of this response. Taken together, these findings demonstrated the key role played by CB1R in mediating reinstatement of cocaine-CPP after restraint stress, through modulation of the context-specific glutamate release within NAcore. Additionally, CB1R regulation of basal extracellular glutamate was demonstrated and proposed as the underlying mechanism.

Neurochemical substrates linked to impulsive and compulsive phenotypes in addiction: A preclinical perspective

Drug compulsion manifests in some but not all individuals and implicates multifaceted processes including failures in top-down cognitive control as drivers for the hazardous pursuit of drug use in some individuals. As a closely related construct, impulsivity encompasses rash or risky behaviour without foresight and underlies most forms of drug taking behaviour, including drug use during adverse emotional states (i.e., negative urgency). While impulsive behavioural dimensions emerge from drug-induced brain plasticity, burgeoning evidence suggests that impulsivity also predates the emergence of compulsive drug use. Although the neural substrates underlying the apparently causal relationship between trait impulsivity and drug compulsion are poorly understood, significant advances have come from the interrogation of defined limbic cortico-striatal circuits involved in motivated behaviour and response inhibition, together with chemical neuromodulatory influences from the ascending neurotransmitter systems. We review what is presently known about the neurochemical mediation of impulsivity, in its various forms, and ask whether commonalities exist in the neurochemistry of compulsive drug-motivated behaviours that might explain individual risk for addiction.

NMDA Receptors in Accumbal D1 Neurons Influence Chronic Sugar Consumption and Relapse

Glutamatergic input via NMDA and AMPA receptors within the mesolimbic dopamine (DA) pathway plays a critical role in the development of addictive behavior and relapse toward drugs of abuse. Although well-established for drugs of abuse, it is not clear whether glutamate receptors within the mesolimbic system are involved in mediating chronic consumption and relapse following abstinence from a non-drug reward. Here, we evaluated the contribution of mesolimbic glutamate receptors in mediating chronic sugar consumption and the sugar-deprivation effect (SDE), which is used as a measure of relapse-like behavior following abstinence. We studied four inducible mutant mouse lines lacking the GluA1 or GluN1 subunit in either DA transporter (DAT) or D1R-expressing neurons in an automated monitoring system for free-choice sugar drinking in the home cage. Mice lacking either GluA1 or GluN1 in D1R-expressing neurons (GluA1^D1CreERT2 or GluN1^D1CreERT2mice) have altered sugar consumption in both sexes, whereas GluA1^DATCreERT2 and GluN1^DATCreERT2do not differ from their respective littermate controls. In terms of relapse-like behavior, female GluN1^D1CreERT2mice show a more pronounced SDE. Given that glutamate receptors within the mesolimbic system play a critical role in mediating relapse behavior of alcohol and other drugs of abuse, it is surprising that these receptors do not mediate the SDE, or in the case of female GluN1^D1CreERT2 mice, show an opposing effect. We conclude that a relapse-like phenotype of sugar consumption differs from that of drugs of abuse on the molecular level, at least with respect to the contribution of mesolimbic glutamate receptors.

Mu opioid receptors on vGluT2-expressing glutamatergic neurons modulate opioid reward

The role of Mu opioid receptor (MOR)‐mediated regulation of GABA transmission in opioid reward is well established. Much less is known about MOR‐mediated regulation of glutamate transmission in the brain and how this relates to drug reward. We previously found that MORs inhibit glutamate transmission at synapses that express the Type 2 vesicular glutamate transporter (vGluT2). We created a transgenic mouse that lacks MORs in vGluT2‐expressing neurons (MORflox‐vGluT2cre) to demonstrate that MORs on the vGluT2 neurons themselves mediate this synaptic inhibition. We then explored the role of MORs in vGluT2‐expressing neurons in opioid‐related behaviors. In tests of conditioned place preference, MORflox‐vGluT2cre mice did not acquire place preference for a low dose of the opioid, oxycodone, but displayed conditioned place aversion at a higher dose, whereas control mice displayed preference for both doses. In an oral consumption assessment, these mice consumed less oxycodone and had reduced preference for oxycodone compared with controls. MORflox‐vGluT2cre mice also failed to show oxycodone‐induced locomotor stimulation. These mice displayed baseline withdrawal‐like responses following the development of oxycodone dependence that were not seen in littermate controls. In addition, withdrawal‐like responses in these mice did not increase following treatment with the opioid antagonist, naloxone. However, other MOR‐mediated behaviors were unaffected, including oxycodone‐induced analgesia. These data reveal that MOR‐mediated regulation of glutamate transmission is a critical component of opioid reward.

The 5α-reductase inhibitor finasteride reduces opioid self-administration in animal models of opioid use disorder

Opioid use disorder (OUD) has become a leading cause of death in the United States, yet current therapeutic strategies remain highly inadequate. To identify potential treatments for OUD, we screened a targeted selection of over 100 drugs using a recently developed opioid self-administration assay in zebrafish. This paradigm showed that finasteride, a steroidogenesis inhibitor approved for the treatment of benign prostatic hyperplasia and androgenetic alopecia, reduced self-administration of multiple opioids without affecting locomotion or feeding behavior. These findings were confirmed in rats; furthermore, finasteride reduced the physical signs associated with opioid withdrawal. In rat models of neuropathic pain, finasteride did not alter the antinociceptive effect of opioids and reduced withdrawal-induced hyperalgesia. Steroidomic analyses of the brains of fish treated with finasteride revealed a significant increase in dehydroepiandrosterone sulfate (DHEAS). Treatment with precursors of DHEAS reduced opioid self-administration in zebrafish in a fashion akin to the effects of finasteride. These results highlight the importance of steroidogenic pathways as a rich source of therapeutic targets for OUD and point to the potential of finasteride as a new treatment option for this disorder.

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.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Interactions of neuroimmune signaling and glutamate plasticity in addiction

Chronic use of drugs of abuse affects neuroimmune signaling; however, there are still many open questions regarding the interactions between neuroimmune mechanisms and substance use disorders (SUDs). Further, chronic use of drugs of abuse can induce glutamatergic changes in the brain, but the relationship between the glutamate system and neuroimmune signaling in addiction is not well understood. Therefore, the purpose of this review is to bring into focus the role of neuroimmune signaling and its interactions with the glutamate system following chronic drug use, and how this may guide pharmacotherapeutic treatment strategies for SUDs. In this review, we first describe neuroimmune mechanisms that may be linked to aberrant glutamate signaling in addiction. We focus specifically on the nuclear factor-kappa B (NF-κB) pathway, a potentially important neuroimmune mechanism that may be a key player in driving drug-seeking behavior. We highlight the importance of astroglial-microglial crosstalk, and how this interacts with known glutamatergic dysregulations in addiction. Then, we describe the importance of studying non-neuronal cells with unprecedented precision because understanding structure-function relationships in these cells is critical in understanding their role in addiction neurobiology. Here we propose a working model of neuroimmune-glutamate interactions that underlie drug use motivation, which we argue may aid strategies for small molecule drug development to treat substance use disorders. Together, the synthesis of this review shows that interactions between glutamate and neuroimmune signaling may play an important and understudied role in addiction processes and may be critical in developing more efficacious pharmacotherapies to treat SUDs.

Administration of N-acetylcysteine Plus Acetylsalicylic Acid Markedly Inhibits Nicotine Reinstatement Following Chronic Oral Nicotine Intake in Female Rats

Background

Nicotine is the major addictive component of cigarette smoke and the prime culprit of the failure to quit smoking. Common elements perpetuating the use of addictive drugs are (i) cues associated with the setting in which drug was used and (ii) relapse/reinstatement mediated by an increased glutamatergic tone (iii) associated with drug-induced neuroinflammation and oxidative stress.

Aims

The present study assessed the effect of the coadministration of the antioxidant N-acetylcysteine (NAC) plus the anti-inflammatory acetylsalicylic acid (ASA) on oral nicotine reinstatement intake following a post-deprivation re-access in female rats that had chronically and voluntarily consumed a nicotine solution orally. The nicotine-induced oxidative stress and neuroinflammation in the hippocampus and its effects on the glutamate transporters GLT-1 and XCT mRNA levels in prefrontal cortex were also analyzed.

Results

The oral coadministration of NAC (40 mg/kg/day) and ASA (15 mg/kg/day) inhibited by 85% of the oral nicotine reinstatement intake compared to control (vehicle), showing an additive effect of both drugs. Acetylsalicylic acid and N-acetylcysteine normalized hippocampal oxidative stress and blunted the hippocampal neuroinflammation observed upon oral nicotine reinstatement. Nicotine downregulated GLT-1 and xCT gene expression in the prefrontal cortex, an effect reversed by N-acetylcysteine, while acetylsalicylic acid reversed the nicotine-induced downregulation of GLT-1 gene expression. The inhibitory effect of N-acetylcysteine on chronic nicotine intake was blocked by the administration of sulfasalazine, an inhibitor of the xCT transporter.

Conclusion

Nicotine reinstatement, following post-deprivation of chronic oral nicotine intake, downregulates the mRNA levels of GLT-1 and xCT transporters, an effect reversed by the coadministration of N-acetylcysteine and acetylsalicylic acid, leading to a marked inhibition of nicotine intake. The combination of these drugs may constitute a valuable adjunct in the treatment of nicotine-dependent behaviors.

Opioid-induced structural and functional plasticity of medium-spiny neurons in the nucleus accumbens

Opioid Use Disorder (OUD) is a chronic relapsing clinical condition with tremendous morbidity and mortality that frequently persists, despite treatment, due to an individual's underlying psychological, neurobiological, and genetic vulnerabilities. Evidence suggests that these vulnerabilities may have neurochemical, cellular, and molecular bases. Key neuroplastic events within the mesocorticolimbic system that emerge through chronic exposure to opioids may have a determinative influence on behavioral symptoms associated with OUD. In particular, structural and functional alterations in the dendritic spines of medium spiny neurons (MSNs) within the nucleus accumbens (NAc) and its dopaminergic projections from the ventral tegmental area (VTA) are believed to facilitate these behavioral sequelae. Additionally, glutamatergic neurons from the prefrontal cortex, the basolateral amygdala, the hippocampus, and the thalamus project to these same MSNs, providing an enriched target for synaptic plasticity. Here, we review literature related to neuroadaptations in NAc MSNs from dopaminergic and glutamatergic pathways in OUD. We also describe new findings related to transcriptional, epigenetic, and molecular mechanisms in MSN plasticity in the different stages of OUD.

Endogenous enkephalin is necessary for cocaine-induced alteration in glutamate transmission within the nucleus accumbens

Altered glutamate transmission within the nucleus accumbens (NAc) has been proposed as a central mechanism underlying behavioural sensitisation associated with repeated cocaine exposure. In addition to glutamate, enkephalin, an endogenous opioid peptide derived from proenkephalin, is necessary for the neuroadaptations associated with chronic cocaine. However, the influence of enkephalin on long-term changes in glutamate transmission within the NAc associated with cocaine-induced sensitisation has not been described. This study used knockout proenkephalin mice (KO) to study the influence of endogenous enkephalin on the adaptations in glutamate neurotransmission associated with repeated cocaine treatment. Wild-type (WT) and KO mice were treated with daily cocaine injections for 9 days to induce sensitisation. On days 15 and 21, the animals received a cocaine challenge and locomotor sensitisation was evaluated, and microdialysis was performed to determine accumbens glutamate content on day 21. No expression of behavioural sensitisation to cocaine was evidenced in the KO mice. Consistently, these showed no changes in glutamate transmission in the NAc associated with repeated cocaine. This study reveals the central role of enkephalin in regulating the glutamate mechanisms associated with cocaine sensitisation.

The infralimbic cortex and mGlu5 mediate the effects of chronic intermittent ethanol exposure on fear learning and memory

RATIONALE AND OBJECTIVES

Alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD) often occur comorbidly. While the incidence of these disorders is increasing, there is little investigation into the interacting neural mechanisms between these disorders. These studies aim to identify cognitive deficits that occur as a consequence of fear and ethanol exposure, implement a novel pharmaceutical intervention, and determine relevant underlying neurocircuitry. Additionally, due to clinical sex differences in PTSD prevalence and alcohol abuse, these studies examine the nature of this relationship in rodent models.

METHODS

Animals were exposed to a model of PTSD+AUD using auditory fear conditioning followed by chronic intermittent ethanol exposure (CIE). Then, rats received extinction training consisting of multiple conditioned stimulus presentations in absence of the shock. Extinction recall and context-induced freezing were measured in subsequent tests. CDPPB, a metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulator, was used to treat these deficits, and region-specific effects were determined using microinjections.

RESULTS

These studies determined that CIE exposure led to deficits in fear extinction learning and heightened context-induced freezing while sex differences emerged in fear conditioning and extinction cue recall tests. Furthermore, using CDPPB, these studies found that enhancement of infralimbic (IfL) mGlu5 activity was able to recover CIE-induced deficits in both males and females.

CONCLUSIONS

These studies show that CIE induces deficits in fear-related behaviors and that enhancement of IfL glutamatergic activity can facilitate learning during extinction. Additionally, we identify novel pharmacological targets for the treatment of individuals who suffer from PTSD and AUD.

Relapse-Associated Transient Synaptic Potentiation Requires Integrin-Mediated Activation of Focal Adhesion Kinase and Cofilin in D1-Expressing Neurons

Relapse to drug use can be initiated by drug-associated cues. The intensity of cue-induced drug seeking in rodent models correlates with the induction of transient synaptic potentiation (t-SP) at glutamatergic synapses in the nucleus accumbens core (NAcore). Matrix metalloproteinases (MMPs) are inducible endopeptidases that degrade extracellular matrix (ECM) proteins, and reveal tripeptide Arginine-Glycine-Aspartate (RGD) domains that bind and signal through integrins. Integrins are heterodimeric receptors composed of αβ subunits, and a primary signaling kinase is focal adhesion kinase (FAK). We previously showed that MMP activation is necessary for and potentiates cued reinstatement of cocaine seeking, and MMP-induced catalysis stimulates β3-integrins to induce t-SP. Here, we determined whether β3-integrin signaling through FAK and cofilin (actin depolymerization factor) is necessary to promote synaptic growth during t-SP. Using a small molecule inhibitor to prevent FAK activation, we blocked cued-induced cocaine reinstatement and increased spine head diameter (dh). Immunohistochemistry on NAcore labeled spines with ChR2-EYFP virus, showed increased immunoreactivity of phosphorylation of FAK (p-FAK) and p-cofilin in dendrites of reinstated animals compared with extinguished and yoked saline, and the p-FAK and cofilin depended on β3-integrin signaling. Next, male and female transgenic rats were used to selectively label D1 or D2 neurons with ChR2-mCherry. We found that p-FAK was increased during drug seeking in both D1 and D2-medium spiny neurons (MSNs), but increased p-cofilin was observed only in D1-MSNs. These data indicate that β3-integrin, FAK and cofilin constitute a signaling pathway downstream of MMP activation that is involved in promoting the transient synaptic enlargement in D1-MSNs induced during reinstated cocaine by drug-paired cues.SIGNIFICANCE STATEMENT Drug-associated cues precipitate relapse, which is correlated with transient synaptic enlargement in the accumbens core. We showed that cocaine cue-induced synaptic enlargement depends on matrix metalloprotease signaling in the extracellular matrix (ECM) through β3-integrin to activate focal adhesion kinase (FAK) and phosphorylate the actin binding protein cofilin. The nucleus accumbens core (NAcore) contains two predominate neuronal subtypes selectively expressing either D1-dopamine or D2-dopamine receptors. We used transgenic rats to study each cell type and found that cue-induced signaling through cofilin phosphorylation occurred only in D1-expressing neurons. Thus, cocaine-paired cues initiate cocaine reinstatement and synaptic enlargement through a signaling cascade selectively in D1-expressing neurons requiring ECM stimulation of β3-integrin-mediated phosphorylation of FAK (p-FAK) and cofilin.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Chromatin accessibility mapping of the striatum identifies tyrosine kinase FYN as a therapeutic target for heroin use disorder

The current opioid epidemic necessitates a better understanding of human addiction neurobiology to develop efficacious treatment approaches. Here, we perform genome-wide assessment of chromatin accessibility of the human striatum in heroin users and matched controls. Our study reveals distinct neuronal and non-neuronal epigenetic signatures, and identifies a locus in the proximity of the gene encoding tyrosine kinase FYN as the most affected region in neurons. FYN expression, kinase activity and the phosphorylation of its target Tau are increased by heroin use in the post-mortem human striatum, as well as in rats trained to self-administer heroin and primary striatal neurons treated with chronic morphine in vitro. Pharmacological or genetic manipulation of FYN activity significantly attenuates heroin self-administration and responding for drug-paired cues in rodents. Our findings suggest that striatal FYN is an important driver of heroin-related neurodegenerative-like pathology and drug-taking behavior, making FYN a promising therapeutic target for heroin use disorder.

Epigenetic mechanisms have emerged as contributors to the molecular impairments caused by exposure to environmental factors such as abused substances. Here the authors perform epigenetic profiling of the striatum and identify the tyrosine kinase FYN is an important driver of neurodegenerative-like pathology and drug-taking behaviour.

Accumbens neuroimmune signaling and dysregulation of astrocytic glutamate transport underlie conditioned nicotine-seeking behavior

Nicotine self-administration is associated with decreased expression of the glial glutamate transporter (GLT-1) and the cystine-glutamate exchange protein xCT within the nucleus accumbens core (NAcore). N-acetylcysteine (NAC) has been shown to restore these proteins in a rodent model of drug addiction and relapse. However, the specific molecular mechanisms driving its inhibitory effects on cue-induced nicotine reinstatement are unknown. Here, we confirm that extinction of nicotine-seeking behavior is associated with impaired NAcore GLT-1 function and expression and demonstrates that reinstatement of nicotine seeking rapidly enhances membrane fraction GLT-1 expression. Extinction and cue-induced reinstatement of nicotine seeking was also associated with increased tumor necrosis factor alpha (TNFα) and decreased glial fibrillary acidic protein (GFAP) expression in the NAcore. NAC treatment (100 mg/kg/day, i.p., for 5 d) inhibited cue-induced nicotine seeking and suppressed AMPA to NMDA current ratios, suggesting that NAC reduces NAcore postsynaptic excitability. In separate experiments, rats received NAC and an antisense vivo-morpholino to selectively suppress GLT-1 expression in the NAcore during extinction and were subsequently tested for cue-induced reinstatement of nicotine seeking. NAC treatment rescued NAcore GLT-1 expression and attenuated cue-induced nicotine seeking, which was blocked by GLT-1 antisense. NAC also reduced TNFα expression in the NAcore. Viral manipulation of the NF-κB pathway, which is downstream of TNFα, revealed that cue-induced nicotine seeking is regulated by NF-κB pathway signaling in the NAcore independent of GLT-1 expression. Ultimately, these results are the first to show that immunomodulatory mechanisms may regulate known nicotine-induced alterations in glutamatergic plasticity that mediate cue-induced nicotine-seeking behavior.

Astrocytes as cellular mediators of cue reactivity in addiction

Relapse to addictive drug use remains a major medical problem worldwide. In rodents, glutamate release in the nucleus accumbens core triggers reinstated drug seeking in response to stress, and drug-associated cues and contexts. Glutamatergic dysregulation in addiction results in part from long-lasting adaptations in accumbens astroglia, including downregulation of the glutamate transporter GLT-1 and retraction from synapses after withdrawal from psychostimulants and opioids. While their capacity to clear glutamate is disrupted by drug use and withdrawal, accumbens astrocytes undergo rapid, transient plasticity in response to drug-associated cues that reinstate seeking. Cued reinstatement of heroin seeking, for example, restores synaptic proximity of astrocyte processes through ezrin phosphorylation, and enhances GLT-1 surface expression. These adaptations limit drug seeking behavior and largely occur on non-overlapping populations of astroglia. Here we review the growing literature supporting a critical role for accumbens astrocytes in modulating glutamate transmission during drug seeking in rodent models of relapse.

Pharmacological Inactivation of Medial Prefrontal Cortex Does Not Support Dichotomous "Go/Stop" Roles for Dorsal and Ventral Subdivisions in Natural Reward Seeking in Rats

Highlighted Research Paper:Differential Effects of Dorsal and Ventral Medial Prefrontal Cortex Inactivation during Natural Reward Seeking, Extinction, and Cue-Induced Reinstatement. Jessica P. Caballero, Garrett B. Scarpa, Luke Remage-Healey, David E. Moorman.

Chemogenetic modulation of accumbens direct or indirect pathways bidirectionally alters reinstatement of heroin-seeking in high- but not low-risk rats

Opioid addiction has been declared a public health emergency, with fatal overdoses following relapse reaching epidemic proportions and disease-associated costs continuing to escalate. Relapse is often triggered by re-exposure to drug-associated cues, and though the neural substrates responsible for relapse in vulnerable individuals remains ambiguous, the nucleus accumbens (NAc) has been shown to play a central role. NAc direct and indirect pathway medium spiny neurons (dMSNs and iMSNs) can have oppositional control over reward-seeking and associative learning and are critically involved in reinstatement of psychostimulant-seeking. However, whether these pathways similarly regulate reinstatement of opioid-seeking remains unknown, as is their role in modulating motivation to take opioids. Here, we describe a method for classifying addiction severity in outbred rats following intermittent-access heroin self-administration that identifies subgroups as addiction-vulnerable (high-risk) or addiction-resistant (low-risk). Using dual viral-mediated gene transfer of DREADDs, we show that transient inactivation of dMSNs or activation of iMSNs is capable of suppressing cue-induced reinstatement of heroin-seeking in high- but not low-risk rats. Surprisingly, however, the motivation to self-administer heroin was unchanged, indicating a divergence in the encoding of heroin-taking and heroin-seeking in rats. We further show that transient activation of dMSNs or inactivation of iMSNs exacerbates cue-induced reinstatement of heroin-seeking in high- but not low-risk rats, again with no effect on motivation. These findings demonstrate a critical role for dMSNs and iMSNs in encoding vulnerability to reinstatement of heroin-seeking and provide insight into the specific neurobiological changes that occur in vulnerable groups following heroin self-administration.

The effect of microinjection of CART 55-102 into the nucleus accumbens shell on morphine-induced conditioned place preference in rats: Involvement of the NMDA receptor

The addictive properties of opioids may be mediated to some extent by cocaine-and amphetamine-regulated transcript (CART) in the reward pathway. Moreover, some claims CART interacts with the glutamate system. Here, we evaluated whether intra-nucleus accumbens (NAc) shell infusions of CART induces Conditioned Place Preference (CPP) or Conditioned Place Aversion (CPA) and affects morphine reward. We also measured NR1 subunit expressions of the N-methyl-d-aspartate (NMDA) receptor in various parts of the reward pathway (NAc, prefrontal cortex and hippocampus) after conditioning tests. Animals with bilateral intra-NAc shell cannulas were place-conditioned with several doses of subcutaneous morphine prior to intra-NAc shell infusions of artificial cerebrospinal fluid (aCSF). Immunohistochemistry (IHC) showed a dose-dependent increase in the NR1 expression in all examined parts. When rats were conditioned with intra-NAc shell infusions of CART, CPP and CPA induced with 2.5 and 5 μg/side respectively and IHC showed NR1elevation with 2.5 and reduction with 5 μg/side in all areas. Sub-rewarding dose of CART administration (1.25 μg/side) prior to sub-rewarding dose of morphine (2.5 mg/kg) induced CPP and NR1 increased in all examined tissues in IHC. However, infusion of an aversive dose of CART (5 μg/side) prior to the rewarding dose of morphine (5 mg/kg) produced neither CPP nor CPA and NR1 in the NAc and hippocampus decreased significantly. It seems that the rewarding or aversive effects of intra-NAc shell CART and its facilitating or inhibiting effects on morphine reward are dose-dependent. Additionally, NMDA may be closely involved in the affective properties of opioids and CART in the reward pathway.

Impaired frontostriatal functional connectivity among chronic opioid using pain patients is associated with dysregulated affect

Preclinical studies have shown effects of chronic exposure to addictive drugs on glutamatergic-mediated neuroplasticity in frontostriatal circuitry. These initial findings have been paralleled by human functional magnetic resonance imaging (fMRI) research demonstrating weaker frontostriatal resting-state functional connectivity (rsFC) among individuals with psychostimulant use disorders. However, there is a dearth of human imaging literature describing associations between long-term prescription opioid use, frontostriatal rsFC, and brain morphology among chronic pain patients. We hypothesized that prescription opioid users with chronic pain, as compared with healthy control subjects, would evidence weaker frontostriatal rsFC coupled with less frontostriatal gray matter volume (GMV). Further, those opioid use-related deficits in frontostriatal circuitry would be associated with negative affect and drug misuse. Prescription opioid users with chronic pain (n = 31) and drug-free healthy controls (n = 30) underwent a high-resolution anatomical and an eyes-closed resting-state functional scan. The opioid group, relative to controls, exhibited weaker frontostriatal rsFC, and less frontostriatal GMV in both L.NAc and L.vmPFC. Frontostriatal rsFC partially mediated group differences in negative affect. Within opioid users, L.NAc GMV predicted opioid misuse severity. The current study revealed that prescription opioid use in the context of chronic pain is associated with functional and structural abnormalities in frontostriatal circuitry. These results suggest that opioid use-related abnormalities in frontostriatal circuitry may undergird disturbances in affect that may contribute to the ongoing maintenance of opioid use and misuse. These findings warrant further examination of interventions to treat opioid pathophysiology in frontostriatal circuitry over the course of treatment.

The Opioid-Addicted Tetrapartite Synapse

Opioid administration in preclinical models induces long-lasting adaptations in reward and habit circuitry. The latest research demonstrates that in the nucleus accumbens, opioid-induced excitatory synaptic plasticity involves presynaptic and postsynaptic elements as well as adjacent astroglial processes and the perisynaptic extracellular matrix. We outline opioid-induced modifications within each component of the tetrapartite synapse and provide a neurobiological perspective on how these adaptations converge to produce addiction-related behaviors in rodent models. By incorporating changes observed at each of the excitatory synaptic compartments into a unified framework of opioid-induced glutamate dysregulation, we highlight new avenues for restoring synaptic homeostasis that might limit opioid craving and relapse vulnerability.

Differential Effects of Dorsal and Ventral Medial Prefrontal Cortex Inactivation during Natural Reward Seeking, Extinction, and Cue-Induced Reinstatement

Rodent dorsal medial prefrontal cortex (mPFC), typically prelimbic cortex, is often described as promoting actions such as reward seeking, whereas ventral mPFC, typically infralimbic cortex, is thought to promote response inhibition. However, both dorsal and ventral mPFC are necessary for both expression and suppression of different behaviors, and each region may contribute to different functions depending on the specifics of the behavior tested. To better understand the roles of dorsal and ventral mPFC in motivated behavior we pharmacologically inactivated each area during operant fixed ratio 1 (FR1) seeking for a natural reward (sucrose), extinction, cue-induced reinstatement, and progressive ratio (PR) sucrose seeking in male Long–Evans rats. Bilateral inactivation of dorsal mPFC, but not ventral mPFC increased reward seeking during FR1. Inactivation of both dorsal and ventral mPFC decreased seeking during extinction. Bilateral inactivation of ventral mPFC, but not dorsal mPFC decreased reward seeking during cue-induced reinstatement. No effect of inactivation was found during PR. Our data contrast sharply with observations seen during drug seeking and fear conditioning, indicating that previously established roles of dorsal mPFC = going versus ventral mPFC = stopping are not applicable to all motivated behaviors and/or outcomes. Our results indicate that dichotomous functions of dorsal versus ventral mPFC, if they exist, may align better with other models, or may require the development of a new framework in which these multifaceted brain areas play different roles in action control depending on the behavioral context in which they are engaged.

A novel role for the actin-binding protein drebrin in regulating opiate addiction

Persistent transcriptional and morphological events in the nucleus accumbens (NAc) and other brain reward regions contribute to the long-lasting behavioral adaptations that characterize drug addiction. Opiate exposure reduces the density of dendritic spines on medium spiny neurons of the NAc; however, the underlying transcriptional and cellular events mediating this remain unknown. We show that heroin self-administration negatively regulates the actin-binding protein drebrin in the NAc. Using virus-mediated gene transfer, we show that drebrin overexpression in the NAc is sufficient to decrease drug seeking and increase dendritic spine density, whereas drebrin knockdown potentiates these effects. We demonstrate that drebrin is transcriptionally repressed by the histone modifier HDAC2, which is relieved by pharmacological inhibition of histone deacetylases. Importantly, we demonstrate that heroin-induced adaptations occur only in the D1⁺ subset of medium spiny neurons. These findings establish an essential role for drebrin, and upstream transcriptional regulator HDAC2, in opiate-induced plasticity in the NAc.

The underlying transcriptional and cellular events mediating the reduction of dendritic spines on medium spiny neurons of the nucleus accumbens (NAc) remains unknown. Here, authors demonstrate that heroin self-administration negatively regulates the actin-binding protein drebrin in the NAc, which is shown to be transcriptionally repressed by the histone modifier HDAC2, and that overexpression of drebrin is sufficient to decrease drug seeking and increase dendritic spine density

Pharmacological Treatment of Youth Substance Use Disorders

While the majority of youth who experiment with alcohol and drugs do not develop problematic levels of use, 5% of adolescents and 15% of young adults meet criteria for a substance use disorder (SUD). Pharmacotherapy, in combination with behavioral interventions, has the potential to increase the likelihood of successful treatment for youth struggling with SUD; however, the literature in this area is limited. To date, there are no Food and Drug Administration (FDA)-approved medications for adolescent SUD, other than buprenorphine, which has been approved down to 16 years of age for opioid use disorder. Despite alcohol and cannabis being the most commonly used substances during adolescence, only three medications have been tested among this demographic, and only two have warranted further study (i.e., naltrexone for alcohol and N-acetylcysteine for cannabis use disorder). Although less common in adolescents and young adults, the most promising pharmacological findings for this age group are for opioid (buprenorphine) and tobacco (bupropion and varenicline) use disorders. In addition, despite the recent marked increases in electronic nicotine delivery systems (i.e., vaping) among youth, treatment strategies are still in their infancy and no recommendation exists for how to promote cessation for youth vaping. Current findings are limited by: small, demographically homogeneous samples; few trials, including a substantial number of youth younger than 18; low retention; medication adherence rates; and minimal information on effective dosing levels and long-term outcomes. Overall, pharmacotherapy may be a potentially effective strategy to increase treatment effects; however, more rigorous research trials are warranted before FDA approval would be granted for any of the potential adjunctive medications in this age group.

Effects of Chronic Inhalation of Electronic Cigarette Vapor Containing Nicotine on Neurotransmitters in the Frontal Cortex and Striatum of C57BL/6 Mice

Electronic (E)-cigarettes are the latest form of nicotine delivery device and are highly popular in the general population. It is currently unknown whether vaping E-cigarettes (E-CIGs) leads to nicotine addiction. Alterations in the levels of the neurotransmitters in the mesocorticolimbic areas have been reported to mediate the initiation and development of nicotine addiction. Therefore, to determine whether E-CIGs activate the same addiction pathways as conventional cigarettes, we investigated for the effects of daily inhalation of nicotine (24 mg/ml)-containing E-CIG vapor for 6 months on the concentrations of these neurotransmitters in the frontal cortex (FC) and striatum (STR) of male C57BL/6 mice as compared to control group that was exposed to air only. We reported here that 6-month E-CIG vapor containing nicotine inhalation decreased dopamine concentration only in the STR. There were no changes in serotonin concentrations in the FC or STR. Chronic E-CIG exposure also increased glutamate concentration in the STR alone, while glutamine concentrations were increased in both the FC and STR. We found that E-CIG exposure also decreased GABA concentration only in the FC. These data suggest that chronic E-CIG use alters homeostasis of several neurotransmitters in the mesocorticolimbic areas, which may result in the development of nicotine dependence in E-CIG users.

Synapse-specific opioid modulation of thalamo-cortico-striatal circuits

The medial thalamus (MThal), anterior cingulate cortex (ACC) and striatum play important roles in affective-motivational pain processing and reward learning. Opioids affect both pain and reward through uncharacterized modulation of this circuitry. This study examined opioid actions on glutamate transmission between these brain regions in mouse. Mu-opioid receptor (MOR) agonists potently inhibited MThal inputs without affecting ACC inputs to individual striatal medium spiny neurons (MSNs). MOR activation also inhibited MThal inputs to the pyramidal neurons in the ACC. In contrast, delta-opioid receptor (DOR) agonists disinhibited ACC pyramidal neuron responses to MThal inputs by suppressing local feed-forward GABA signaling from parvalbumin-positive interneurons. As a result, DOR activation in the ACC facilitated poly-synaptic (thalamo-cortico-striatal) excitation of MSNs by MThal inputs. These results suggest that opioid effects on pain and reward may be shaped by the relative selectivity of opioid drugs to the specific circuit components.

Beyond drug-induced alteration of glutamate homeostasis, astrocytes may contribute to dopamine-dependent intrastriatal functional shifts that underlie the development of drug addiction: A working hypothesis

The transition from recreational drug use to compulsive drug‐seeking habits, the hallmark of addiction, has been shown to depend on a shift in the locus of control over behaviour from the ventral to the dorsolateral striatum. This process has hitherto been considered to depend on the aberrant engagement of dopamine‐dependent plasticity processes within neuronal networks. However, exposure to drugs of abuse also triggers cellular and molecular adaptations in astrocytes within the striatum which could potentially contribute to the intrastriatal transitions observed during the development of drug addiction. Pharmacological interventions aiming to restore the astrocytic mechanisms responsible for maintaining homeostatic glutamate concentrations in the nucleus accumbens, that are altered by chronic exposure to addictive drugs, abolish the propensity to relapse in both preclinical and, to a lesser extent, clinical studies. Exposure to drugs of abuse also alters the function of astrocytes in the dorsolateral striatum, wherein dopaminergic mechanisms control drug‐seeking habits, associated compulsivity and relapse. This suggests that drug‐induced alterations in the glutamatergic homeostasis maintained by astrocytes throughout the entire striatum may interact with dopaminergic mechanisms to promote aberrant plasticity processes that contribute to the maintenance of maladaptive drug‐seeking habits. Capitalising on growing evidence that astrocytes play a fundamental regulatory role in glutamate and dopamine transmission in the striatum, we present an innovative model of a quadripartite synaptic microenvironment within which astrocytes channel functional interactions between the dopaminergic and glutamatergic systems that may represent the primary striatal functional unit that undergoes drug‐induced adaptations eventually leading to addiction.

Alteration in AMPA receptor subunit expression and receptor binding among patients with addictive disorders: A systematic review of human postmortem studies

Background and Objectives

Altered trafficking of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptors has been reported in postmortem studies and suggested the involvement of AMPA receptors in the pathophysiology underpinning addictive disorders. However, these findings seemed mixed.

Methods

A systematic literature search was conducted, using PubMed and Embase (last search, August 2018), to identify human postmortem studies that examined the expression of proteins and mRNA of AMPA receptor subunits in patients with addictive disorders in comparison with healthy controls.

Results

Twelve (18 studies) out of 954 articles were identified to be relevant. Eight studies included alcohol use disorders, and four studies included heroin/cocaine abusers. The most frequently investigated regions were the hippocampus (three studies), amygdala (three studies), and putamen (three studies). In summary, two out of the three studies showed an increase in the expression of AMPA receptors in the hippocampus, while the other study found no change. Two studies to examine the amygdala demonstrated either a decreased or no change in receptor expression or binding. Concerning putamen, two studies showed no significant change whereas an overexpression of receptors was observed in the other.

Conclusions and Scientific Significance

The hippocampus and amygdala may be pertinent to addictive disorders through their functions on learning and memory, whereas findings in other regions were inconsistent across the studies. Human postmortem studies are prone to degenerative changes after death. Moreover, only qualitative assessment was conducted because of the limited, heterogenous data. These limitations emphasize the need to investigate AMPA receptors in the living human brains.

Postmortem studies on AMPA receptors in patients with addiction show that the hippocampus and amygdala may be pertinent to addictive disorders through their functions on learning and memory, whereas findings in other regions were inconsistent across the studies. Human postmortem studies are prone to degenerative changes after death, which emphasizes the need to investigate AMPA receptors in the living human brains.

Molecular windows into the human brain for psychiatric disorders

Delineating the pathophysiology of psychiatric disorders has been extremely challenging but technological advances in recent decades have facilitated a deeper interrogation of molecular processes in the human brain. Initial candidate gene expression studies of the postmortem brain have evolved into genome wide profiling of the transcriptome and the epigenome, a critical regulator of gene expression. Here, we review the potential and challenges of direct molecular characterization of the postmortem human brain, and provide a brief overview of recent transcriptional and epigenetic studies with respect to neuropsychiatric disorders. Such information can now be leveraged and integrated with the growing number of genome-wide association databases to provide a functional context of trait-associated genetic variants linked to psychiatric illnesses and related phenotypes. While it is clear that the field is still developing and challenges remain to be surmounted, these recent advances nevertheless hold tremendous promise for delineating the neurobiological underpinnings of mental diseases and accelerating the development of novel medication strategies.

NMDA receptor subunits change in the prefrontal cortex of pure-opioid and multi-drug abusers: a post-mortem study

Addiction is a chronic relapsing disorder and is one of the most important issues in the world. Changing the level of neurotransmitters and the activities of their receptors, play a major role in the pathophysiology of substance abuse disorders. It is well-established that N-methyl-D-aspartate receptors (NMDARs) play a significant role in the molecular basis of addiction. NMDAR has two obligatory GluN1 and two regionally localized GluN2 subunits. This study investigated changes in the protein level of GluN1, GluN2A, and GluN2B in the prefrontal cortex of drug abusers. The medial prefrontal cortex (mPFC), lateral prefrontal cortex (lPFC), and orbitofrontal cortex (OFC) were dissected from the brain of 101 drug addicts brains and were compared with the brains of non-addicts (N = 13). Western blotting technique was used to show the alteration in NMDAR subunits level. Data obtained using Western blotting technique showed a significant increase in the level of GluN1 and GluN2B, but not in GluN2A subunits in all the three regions (mPFC, lPFC, and OFC) of men whom suffered from addiction as compared to the appropriate controls. These findings showed a novel role for GluN1, GluN2B subunits, rather than the GluN2A subunit of NMDARs, in the pathophysiology of addiction and suggested their role in the drug-induced plasticity of NMDARs.

Relapse to opioid seeking in rat models: behavior, pharmacology and circuits

Lifetime relapse rates remain a major obstacle in addressing the current opioid crisis. Relapse to opioid use can be modeled in rodent studies where drug self-administration is followed by a period of abstinence and a subsequent test for drug seeking. Abstinence can be achieved through extinction training, forced abstinence, or voluntary abstinence. Voluntary abstinence can be accomplished by introducing adverse consequences of continued drug self-administration (e.g., punishment or electric barrier) or by introducing an alternative nondrug reward in a discrete choice procedure (drug versus palatable food or social interaction). In this review, we first discuss pharmacological and circuit mechanisms of opioid seeking, as assessed in the classical extinction-reinstatement model, where reinstatement is induced by reexposure to the self-administered drug (drug priming), discrete cues, discriminative cues, drug-associated contexts, different forms of stress, or withdrawal states. Next, we discuss pharmacological and circuit mechanisms of relapse after forced or voluntary abstinence, including the phenomenon of "incubation of heroin craving" (the time-dependent increases in heroin seeking during abstinence). We conclude by discussing future directions of preclinical relapse-related studies using opioid drugs.

Lamotrigine attenuates the motivation to self-administer ketamine and prevents cue- and prime-induced reinstatement of ketamine-seeking behavior in rats

BACKGROUND

Lamotrigine is an anticonvulsant drug used in the treatment of epilepsy and bipolar disorder. A case report has demonstrated that a ketamine addict experienced a significant reduction in craving and ketamine use after taking lamotrigine. The present study determined whether lamotrigine can reduce the motivation for ketamine and prevent the relapse to ketamine seeking behavior in rats.

METHODS

Male Sprague-Dawley rats were trained to respond for intravenous ketamine (0.5 mg/kg/infusion) self-administration or food pellets. The effects of lamotrigine on the motivation for ketamine or food were assessed using breakpoint test under a progressive ratio (PR) paradigm. Furthermore, the effects of lamotrigine on reinstatement of ketamine-seeking and food-seeking behaviors were examined after extinction.

RESULTS

Lamotrigine significantly decreased the breakpoint for ketamine and prevented cue- and ketamine priming-induced reinstatement of ketamine seeking behavior. However, lamotrigine did not affect the breakpoint for food reinforcement, cue-induced reinstatement of food-seeking behavior, or spontaneous locomotor activity.

CONCLUSIONS

Our data reveal that lamotrigine is capable of attenuating the reinforcing efficacy of ketamine and reducing ketamine craving and relapse risk, which lays the foundation for conducting clinical trials in patients with ketamine use disorder.

Role of prefrontal cortex in the extinction of drug memories

It has been recognized that drug addiction engages aberrant process of learning and memory, and substantial studies have focused on developing effective treatment to erase the enduring drug memories to reduce the propensity to relapse. Extinction, a behavioral intervention exposing the individuals to the drug-associated cues repeatedly, can weaken the craving and relapse induced by drug-associated cues, but its clinic efficacy is limited. A clear understanding of the neuronal circuitry and molecular mechanism underlying extinction of drug memory will facilitate the successful use of extinction therapy in clinic. As a key component of mesolimbic system, medial prefrontal cortex (mPFC) has received particular attention largely in that PFC stands at the core of neural circuits for memory extinction and manipulating mPFC influences extinction of drug memories and subsequent relapse. Here, we review the recent advances in both animal models of drug abuse and human addicted patients toward the understanding of the mechanistic link between mPFC and drug memory, with particular emphasis on how mPFC contributes to the extinction of drug memory at levels ranging from neuronal architecture, synaptic plasticity to molecular signaling and epigenetic regulation, and discuss the clinic relevance of manipulating the extinction process of drug memory to prevent craving and relapse through enhancing mPFC function.

Corticolimbic circuitry in the modulation of chronic pain and substance abuse

The transition from acute to chronic pain is accompanied by increased engagement of emotional and motivational circuits. Adaptations within this corticolimbic circuitry contribute to the cellular and behavioral maladaptations associated with chronic pain. Central regions within the corticolimbic brain include the mesolimbic dopamine system, the amygdala, and the medial prefrontal cortex. The evidence reviewed herein supports the notion that chronic pain induces significant changes within these corticolimbic regions that contribute to the chronicity and intractability of pain. In addition, pain-induced changes in corticolimbic circuitry are poised to impact motivated behavior and reward responsiveness to environmental stimuli, and may modulate the addiction liability of drugs of abuse, such as opioids.

Deletion of the type 2 metabotropic glutamate receptor increases heroin abuse vulnerability in transgenic rats

Opioid abuse is a rapidly growing public health crisis in the USA. Despite extensive research in the past decades, little is known about the etiology of opioid addiction or the neurobiological risk factors that increase vulnerability to opioid use and abuse. Recent studies suggest that the type 2 metabotropic glutamate receptor (mGluR2) is critically involved in substance abuse and addiction. In the present study, we evaluated whether low-mGluR2 expression may represent a risk factor for the development of opioid abuse and addiction using transgenic mGluR2-knockout (mGluR2-KO) rats. Compared to wild-type controls, mGluR2-KO rats exhibited higher nucleus accumbens (NAc) dopamine (DA) and locomotor responses to heroin, higher heroin self-administration and heroin intake, more potent morphine-induced analgesia and more severe naloxone-precipitated withdrawal symptoms. In contrast, mGluR2-KO rats displayed lower motivation for heroin self-administration under high price progressive-ratio (PR) reinforcement conditions. Taken together, these findings suggest that mGluR2 may play an inhibitory role in opioid action, such that deletion of this receptor results in an increase in brain DA responses to heroin and in acute opioid reward and analgesia. Low-mGluR2 expression in the brain may therefore be a risk factor for the initial development of opioid abuse and addiction.

Effects of Clavulanic Acid Treatment on Reinstatement to Methamphetamine, Glial Glutamate Transporters, and mGluR 2/3 Expression in P Rats Exposed to Ethanol

Evidence demonstrated that the glutamatergic system is implicated in mediating relapse to several drugs of abuse, including methamphetamine (METH). Glutamate homeostasis is maintained by a number of glutamate transporters, such as glutamate transporter type 1 (GLT-1), cystine/glutamate transporter (xCT), and glutamate aspartate transporter (GLAST). In addition, group II metabotropic glutamate receptors (mGluR2/3) were found to be implicated in relapse-seeking behavior. Ample evidence showed that β-lactam antibiotics are effective in upregulating GLT-1 and xCT expression, thus improving glutamate homeostasis and attenuating relapse to drugs of abuse. In this study, we investigated the reinstatement of METH using conditioned place preference (CPP) in male alcohol-preferring (P) rats exposed to home-cage free choice ethanol drinking. Here, we tested the effect of clavulanic acid (CA), a β-lactam, on the reinstatement of METH-seeking and ethanol drinking. In addition, we examined the expression of GLT-1, xCT, and GLAST as well as metabotropic glutamate receptor (mGluR2/3) in the nucleus accumbens (NAc) shell, NAc core, and dorsomedial prefrontal cortex (dmPFC). A priming i.p. injection of METH reinstated preference in METH-paired chamber following extinction. Chronic exposure to ethanol decreased the expression of GLT-1 and xCT in the NAc shell, but not in the NAc core or dmPFC. CA treatment blocked the reinstatement of METH-seeking, decreased ethanol intake, and restored the expression of GLT-1 and xCT in the NAc shell. In addition, the expression of mGluR2/3 was increased by CA treatment in the NAc shell and dmPFC. These findings suggest that these glutamate transporters and mGluR2/3 might be potential therapeutic targets for the attenuation of reinstatement to METH-seeking.

Normal extinction and reinstatement of morphine-induced conditioned place preference in the GluA1-KO mouse line

Extinction and reinstatement of morphine-induced conditioned place preference were studied in glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor GluA1 subunit-deficient mice (global GluA1-KO mice). In line with previous findings, both acquisition and expression of conditioned place preference to morphine (20 mg/kg, subcutaneously) were fully functional in GluA1 KO mice compared with wild-type littermate controls (GluA1-WT), thus enabling the study of extinction. With a 10-session extinction paradigm, the GluA1 KO mice showed complete extinction similar to that of the GluA1-WT mice. Morphine-induced reinstatement (10 mg/kg, subcutaneously) was detected in both mouse lines. GluA1 KO mice moved more during all the phases of the experiment, including the place conditioning trials, extinction sessions, and place preference tests. The results suggest that the GluA1 subunit may be dispensable or prone to compensation at the neural circuitries delineating extinction and reinstatement. The GluA1 KO mice show altered long-term between-session habituation, which extends longer than previously anticipated.

Role of glutamatergic system and mesocorticolimbic circuits in alcohol dependence

Emerging evidence demonstrates that alcohol dependence is associated with dysregulation of several neurotransmitters. Alterations in dopamine, glutamate and gamma-aminobutyric acid release are linked to chronic alcohol exposure. The effects of alcohol on the glutamatergic system in the mesocorticolimbic areas have been investigated extensively. Several studies have demonstrated dysregulation in the glutamatergic systems in animal models exposed to alcohol. Alcohol exposure can lead to an increase in extracellular glutamate concentrations in mesocorticolimbic brain regions. In addition, alcohol exposure affects the expression and functions of several glutamate receptors and glutamate transporters in these brain regions. In this review, we discussed the effects of alcohol exposure on glutamate receptors, glutamate transporters and glutamate homeostasis in each area of the mesocorticolimbic system. In addition, we discussed the genetic aspect of alcohol associated with glutamate and reward circuitry. We also discussed the potential therapeutic role of glutamate receptors and glutamate transporters in each brain region for the treatment of alcohol dependence. Finally, we provided some limitations on targeting the glutamatergic system for potential therapeutic options for the treatment alcohol use disorders.

Metaplasticity at the addicted tetrapartite synapse: A common denominator of drug induced adaptations and potential treatment target for addiction

In light of the current worldwide addiction epidemic, the need for successful therapies is more urgent than ever. Although we made substantial progress in our basic understanding of addiction, reliable therapies are lacking. Since 40-60% of patients treated for substance use disorder return to active substance use within a year following treatment discharge, alleviating the vulnerability to relapse is regarded as the most promising avenue for addiction therapy. Preclinical addiction research often focuses on maladaptive synaptic plasticity within the reward pathway. However, drug induced neuroadaptations do not only lead to a strengthening of distinct drug associated cues and drug conditioned behaviors, but also seem to increase plasticity thresholds for environmental stimuli that are not associated with the drug. This form of higher order plasticity, or synaptic metaplasticity, is not expressed as a change in the efficacy of synaptic transmission but as a change in the direction or degree of plasticity induced by a distinct stimulation pattern. Experimental addiction research has demonstrated metaplasticity after exposure to multiple classes of addictive drugs. In this review we will focus on the concept of synaptic metaplasticity in the context of preclinical addiction research. We will take a closer look at the tetrapartite glutamatergic synapse and outline forms of metaplasticity that have been described at the addicted synapse. Finally we will discuss the different potential avenues for pharmacotherapies that target glutamatergic synaptic plasticity and metaplasticity. Here we will argue that aberrant metaplasticity renders the reward seeking circuitry more rigid and hence less able to adapt to changing environmental contingencies. An understanding of the molecular mechanisms that underlie this metaplasticity is crucial for the development of new strategies for addiction therapy. The correction of drug-induced metaplasticity could be used to support behavioral and pharmacotherapies for the treatment of addiction.

Prefrontal circuit organization for executive control

The essential role of executive control is to select the most appropriate behavior among other candidates depending on the sensory information (exogenous information) and on the subject's internal state (endogenous information). Here I review series of the evidence implicating that the rodent prefrontal cortex (PFC) evaluates and compares the expected outcome for candidate actions that are automatically primed by exogenous and endogenous information, and selects the most appropriate action while inhibiting the others, with different PFC subregions contributing to distinct aspects of the computation via differential recruitments of the distributed networks. The recurrent nature of the PFC networks further facilitates the computation by integrating bottom-up signals over a long timescale. I also overview the local circuit organization in the PFC, where vasoactive intestinal peptide-positive (VIP) GABAergic interneurons are tightly linked with the cholinergic system and play significant roles in regulating executive control signals. The empirical evidence inspires the disinhibitory module hypothesis of the PFC organization that a group of pyramidal neurons and interneurons forms a disinhibitory module with similar task-variable selectivity in the PFC, and long-range inputs and neuromodulations in these modules exert a distributed gain modulation of the ongoing executive control signals by adjusting VIP neuron activity.

Blockade of Glutamate Receptors within the Prelimbic Cortex Attenuate Concentration of Excitatory Amino Acids in the Morphine Self-administration in Rats

Background

The attitude of research on addiction has been done on the key role of glutamate. As a regard, the prelimbic cortex (PrL) has an important role in addiction, learning, and memory. We tried to investigate the level of glutamate and aspartate concentration after glutamate receptors blockade in this region in the morphine-addicted rats.

Materials and Methods

In this study, we examined the effects of local infusion of the N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonists, 2-amino-5-phosphonovaleric acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), into the PrL cortex on the level of excitatory amino acids (EAAs) and glycine. After 11 days of self-administration, the prelimbic area of the brain was taken out, and the EAAs and glycine concentration was measured by high-performance liquid chromatography.

Results

Morphine resulted in the significant increase in the EAAs concentration within this area (P ≤ 0.001). Microinjection of AP5 into this region before using of morphine significantly decreased the morphine-induced glutamate and aspartate concentration (P ≤ 0.001). CNQX had the same effect and significantly reduced the EAAs concentration compared to the morphine group (P ≤ 0.001). In addition, microinjection of AP5 and CNQX simultaneously increased glycine concentration (P ≤ 0.001).

Conclusions

These results show that morphine stimulates the EAAs release in the prelimbic area. It seems that microinjection of AP5 or CNQX in this region is effective in reducing morphine-induced EAA. It is suggested that EAA transmission in the PrL cortex may be a possible target for treatment of morphine addiction.

bFGF expression is differentially regulated by cocaine seeking versus extinction in learning-related brain regions

In cocaine use disorder, relapse can be elicited by drug-associated cues despite long periods of abstinence. The persistence of drug-associated cues in eliciting drug seeking suggests enduring changes in structural and functional plasticity, which may be mediated by basic fibroblast growth factor (bFGF, FGF2). Stimulant drug use increases bFGF expression in reward- and learning-related brain regions, such as the infralimbic medial-prefrontal cortex (IL-mPFC), and we previously found that this increase was reversed by extinction. However, whether bFGF expression is similarly modified in other brain regions is unknown. Therefore, we used the conditioned place preference (CPP) paradigm to assess bFGF expression following cocaine-associated CPP or extinction of that CPP within the mPFC, nucleus accumbens (NAc), hippocampus (Hipp), and basolateral amygdala (BLA). bFGF expression was increased in IL-mPFC and NAc-Core and -Shell following a cocaine-associated CPP, an effect reversed by extinction. Conversely, bFGF expression was increased in BLA following extinction, but no significant changes were observed in PL-mPFC or either dorsal or ventral Hipp. These results demonstrate differential regulation of bFGF following cocaine-associated CPP or extinction of that CPP in discrete brain regions. Changes in bFGF expression may regulate long-lasting drug-induced plasticity that underlies persistent drug-associated memories, and therefore present potential prophylactic targets.

Effects of ceftriaxone on hydrocodone seeking behavior and glial glutamate transporters in P rats

Hydrocodone (HYD) is one of the most widely prescribed opioid analgesic drugs. Several neurotransmitters are involved in opioids relapse. Among these neurotransmitters, glutamate is suggested to be involved in opioid dependence and relapse. Glutamate is regulated by several glutamate transporters, including glutamate transporter 1 (GLT-1) and cystine/glutamate transporter (xCT). In this study, we investigated the effects of ceftriaxone (CEF) (200 mg/kg, i.p.), known to upregulate GLT-1 and xCT, on reinstatement to HYD (5 mg/kg, i.p.) using the conditioned place preference (CPP) paradigm in alcohol-preferring (P) rats. Animals were divided into three groups: 1) saline-saline group (SAL-SAL); 2) HYD-SAL group; and 3) HYD-CEF group. The CPP was conducted as follows: habituation phase, conditioning phase with HYD (i.p.) injections every other day for four sessions, extinction phase with CEF (i.p.) injections every other day for four sessions, and reinstatement phase with one priming dose of HYD. Time spent in the HYD-paired chamber after conditioning training was increased as compared to pre-conditioning. There was an increase in time spent in the HYD-paired chamber with one priming dose of HYD in the reinstatement test. HYD exposure downregulated xCT expression in the nucleus accumbens and hippocampus, but no effects were observed in the dorsomedial prefrontal cortex and amygdala. Importantly, CEF treatment attenuated the reinstatement effect of HYD and normalized xCT expression in the affected brain regions. These findings demonstrate that the attenuating effect of HYD reinstatement with CEF might be mediated through xCT.

Peri-adolescent drinking of ethanol and/or nicotine modulates astroglial glutamate transporters and metabotropic glutamate receptor-1 in female alcohol-preferring rats

Impairment in glutamate neurotransmission mediates the development of dependence upon nicotine (NIC) and ethanol (EtOH). Previous work indicates that continuous access to EtOH or phasic exposure to NIC reduces expression of the glutamate transporter-1 (GLT-1) and cystine/glutamate antiporter (xCT) but not the glutamate/aspartate transporter (GLAST). Additionally, metabotropic glutamate receptors (mGluRs) expression was affected following exposure to EtOH or NIC. However, little is known about the effects of EtOH and NIC co-consumption on GLT-1, xCT, GLAST, and mGluR1 expression. In this study, peri-adolescent female alcohol preferring (P) rats were given binge-like access to water, sucrose (SUC), SUC-NIC, EtOH, or EtOH-NIC for four weeks. The present study determined the effects of these reinforcers on GLT-1, xCT, GLAST, and mGluR1 expression in the nucleus accumbens (NAc), hippocampus (HIP) and prefrontal cortex (PFC). GLT-1 and xCT expression were decreased in the NAc following both SUC-NIC and EtOH-NIC. In addition, only xCT expression was downregulated in the HIP in both of these latter groups. Also, glutathione peroxidase (GPx) activity in the HIP was reduced following SUC, SUC-NIC, EtOH, and EtOH-NIC consumption. Similar to previous work, GLAST expression was not altered in any brain region by any of the reinforcers. However, mGluR1 expression was increased in the NAc in the SUC-NIC, EtOH, and EtOH-NIC groups. These results indicate that peri-adolescent binge-like drinking of EtOH or SUC with or without NIC may exert differential effects on astroglial glutamate transporters and receptors. Our data further parallel some of the previous findings observed in adult rats.