Acamprosate attenuates cocaine- and cue-induced reinstatement of cocaine-seeking behavior in rats

Oxycodone withdrawal is associated with increased cocaine self-administration and aberrant accumbens glutamate plasticity in rats

Individuals with opioid use disorder (OUD) frequently use other substances, including cocaine. Opioid withdrawal is associated with increased likelihood of cocaine use, which may represent an attempt to ameliorate opioid withdrawal effects. Clinically, 30% of co-using individuals take opioids and cocaine exclusively in a sequential manner. Preclinical studies evaluating mechanisms of drug use typically study drugs in isolation. However, polysubstance use is a highly prevalent clinical issue and thus, we established a novel preclinical model of sequential oxycodone and cocaine self-administration (SA) whereby rats acquired oxycodone and cocaine SA in an A-B-A-B design. Somatic signs of withdrawal were evaluated at 0, 22, and 24h following oxycodone SA, with the 24h timepoint representing somatic signs immediately following cocaine SA. Preclinically, aberrant glutamate signaling within the nucleus accumbens core (NAcore) occurs following use of cocaine or opioids, whereby medium spiny neurons (MSNs) rest in a potentiated or depotentiated state, respectively. Further, NAcore glial glutamate transport via GLT-1 is downregulated following SA of either drug alone. However, it is not clear if cocaine can exacerbate opioid-induced changes in glutamate signaling. In this study, NAcore GLT-1 protein and glutamate plasticity were measured (via AMPA/NMDA ratio) following SA. Rats acquired SA of both oxycodone and cocaine regardless of sex, and the acute oxycodone-induced increase in somatic signs at 22h was positively correlated with cocaine consumption during the cocaine testing phase. Cocaine use following oxycodone SA downregulated GLT-1 and reduced AMPA/NMDA ratios compared to cocaine use following food SA. Further, oxycodone SA alone was associated with reduced AMPA/NMDA ratio. Together, behavioral signs of oxycodone withdrawal may drive cocaine use and further dysregulate NAcore glutamate signaling.

GSK3β Activity in Reward Circuit Functioning and Addiction

Glycogen synthase kinase-3β (GSK3β), primarily described as a regulator of glycogen metabolism, is a molecular hub linking numerous signaling pathways and regulates many cellular processes like cytoskeletal rearrangement, cell migration, apoptosis, and proliferation. In neurons, the kinase is engaged in molecular events related to the strengthening and weakening of synapses, which is a subcellular manifestation of neuroplasticity. Dysregulation of GSK3β activity has been reported in many neuropsychiatric conditions, like schizophrenia, major depressive disorder, bipolar disorder, and Alzheimer’s disease. In this review, we describe the kinase action in reward circuit-related structures in health and disease. The effect of pharmaceuticals used in the treatment of addiction in the context of GSK3β activity is also discussed.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

The clinical pharmacology of acamprosate

Acamprosate is one of the few medications licensed for prevention of relapse in alcohol dependence, and over time it has proved to be significantly, if moderately, effective, safe and tolerable. Its use is now being extended into other addictions and neurodevelopmental disorders. The mechanism of action of acamprosate has been less clear, but in the decade or more that has elapsed since its licensing, a body of translational evidence has accumulated, in which preclinical findings are replicated in clinical populations. Acamprosate modulates N-methyl-d-aspartic acid receptor transmission and may have indirect effects on γ-aminobutyric acid type A receptor transmission. It is known to decrease brain glutamate and increase β-endorphins in rodents and man. Acamprosate diminishes reinstatement in ethanolized rodents and promotes abstinence in humans. Although acamprosate has been called an anticraving drug, its subjective effects are subtle and relate to diminished arousal, anxiety and insomnia, which parallel preclinical findings of decreased withdrawal symptoms in animals treated with acamprosate. Further understanding of the pharmacology of acamprosate will allow appropriate targeting of therapy in individuals with alcohol dependence and extension of its use to other addictions.

Reactivation of cocaine reward memory engages the Akt/GSK3/mTOR signaling pathway and can be disrupted by GSK3 inhibition

RATIONAL

Memories return to a labile state following their retrieval and must undergo a process of reconsolidation to be maintained. Thus, disruption of cocaine reward memories by interference with reconsolidation may be therapeutically beneficial in the treatment of cocaine addiction.

OBJECTIVE

The objectives were to elucidate the signaling pathway involved in reconsolidation of cocaine reward memory and to test whether targeting this pathway could disrupt cocaine-associated contextual memory.

METHODS

Using a mouse model of conditioned place preference, regulation of the activity of glycogen synthase kinase-3 (GSK3), mammalian target of Rapamycin complex 1 (mTORC1), P70S6K, β-catenin, and the upstream signaling molecule Akt, was studied in cortico-limbic-striatal circuitry after re-exposure to an environment previously paired with cocaine.

RESULT

Levels of phosporylated Akt-Thr308, GSK3α-Ser21, GSK3β-Ser9, mTORC1, and P70S6K were reduced in the nucleus accumbens and hippocampus 10 min after the reactivation of cocaine cue memories. Levels of pAkt and pGSK3 were also reduced in the prefrontal cortex. Since reduced phosphorylation of GSK3 indicates heightened enzyme activity, the effect of a selective GSK3 inhibitor, SB216763, on reconsolidation was tested. Administration of SB216763 immediately after exposure to an environment previously paired with cocaine abrogated a previously established place preference, suggesting that GSK3 inhibition interfered with reconsolidation of cocaine-associated reward memories.

CONCLUSIONS

These findings suggest that the Akt/GSK3/mTORC1 signaling pathway in the nucleus accumbens, hippocampus, and/or prefrontal cortex is critically involved in the reconsolidation of cocaine contextual reward memory. Inhibition of GSK3 activity during memory retrieval can erase an established cocaine place preference.

The influence of ionotropic and metabotropic glutamate receptor ligands on anxiety-like effect of amphetamine withdrawal in rats

Chronic amphetamine use results in anxiety-like states after drug cessation. The aim of the study was to determine a role of ionotropic and metabotropic glutamate receptor ligands in amphetamine-evoked withdrawal anxiety in the elevated plus-maze test in rats. In our study memantine (8 and 12 mg/kg), a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist did not reduce amphetamine withdrawal anxiety. Acamprosate (NMDA and metabotropic glutamate 5 receptor (mGluR5) antagonist) at the dose 200 and 400mg/kg showed anxiolytic-like effect, thus increasing the percent of time spent in open arms and a number of open arm entries. mGluR5 selective antagonist, MTEP (3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine hydrochloride) and mGluR2/3 agonist, LY354740 (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid), caused effects similar to acamprosate at doses 1.25-5mg/kg and 2.5-5mg/kg, respectively. None of the glutamate ligands influenced locomotor activity of rats when given to the saline-treated group. Taking into account the positive correlation between amphetamine withdrawal-induced anxiety and relapse to amphetamine taking, our results suggest that modulation of mGluRs may prevent relapse to amphetamine and might pose a new direction in amphetamine abuse therapy.

NMDA Receptor Modulators in the Treatment of Drug Addiction

Glutamate plays a pivotal role in drug addiction, and the N-methyl-d-aspartate (NMDA) glutamate receptor subtype serves as a molecular target for several drugs of abuse. In this review, we will provide an overview of NMDA receptor structure and function, followed by a review of the mechanism of action, clinical efficacy, and side effect profile of NMDA receptor ligands that are currently in use or being explored for the treatment of drug addiction. These ligands include the NMDA receptor modulators memantine and acamprosate, as well as the partial NMDA agonist d-Cycloserine. Data collected to date suggest that direct NMDA receptor modulators have relatively limited efficacy in the treatment of drug addiction, and that partial agonism of NMDA receptors may have some efficacy with regards to extinction learning during cue exposure therapy. However, the lack of consistency in results to date clearly indicates that additional studies are needed, as are studies examining novel ligands with indirect mechanisms for altering NMDA receptor function.

Acamprosate modulates experimental autoimmune encephalomyelitis

OBJECTIVE

This pilot study aimed to determine the efficacy of acamprosate (N-acetyl homotaurine) in reducing the pathological features of experimental autoimmune encephalomyelitis (EAE) which is an animal model for multiple sclerosis (MS).

BACKGROUND

The amino acid taurine has multiple biological activities including immunomodulation and neuromodulation. The synthetic acetylated taurine derivative, acamprosate, which crosses the blood-brain barrier more readily compared to taurine, is currently being used for the prevention of alcohol withdrawal symptoms associated with enhanced glutamatergic receptor function and GABA receptor hypofunction.

METHODS

EAE was induced in C57BL/6 female mice with myelin oligodendrocyte glyocoprotein, amino acid 35-55. Mice were treated with 20, 100 and 500 mg/kg acamprosate for 21 days.

RESULTS

Neurological scores at disease peak were reduced by 21, 64 and 9% in the 20, 100 and 500 mg/kg groups, respectively. Neurological improvement in the 100 mg/kg group correlated with a reduction in numbers of inflammatory lesions and the extent of CNS demyelination. Blood TNF-α levels were significantly reduced in the 500 mg/kg group.

DISCUSSION

Acamprosate and other taurine analogs have a potential for future MS therapy.

New medications for drug addiction hiding in glutamatergic neuroplasticity

The repeated use of drugs that directly or indirectly stimulate dopamine transmission carry addiction liability and produce enduring pathological changes in the brain circuitry that normally regulates adaptive behavioral responding to a changing environment. This circuitry is rich in glutamatergic projections, and addiction-related behaviors in animal models have been linked to impairments in excitatory synaptic plasticity. Among the best-characterized glutamatergic projection in this circuit is the prefrontal efferent to the nucleus accumbens. A variety of molecular adaptations have been identified in the prefrontal glutamate synapses in the accumbens, many of which are induced by different classes of addictive drugs. Based largely on work with cocaine, we hypothesize that the drug-induced adaptations impair synaptic plasticity in the cortico-accumbens projection, and thereby dysregulate the ability of addicts to control their drug-taking habits. Accordingly, we go on to describe the literature implicating the drug-induced changes in protein content or function that impinge upon synaptic plasticity and have been targeted in preclinical models of relapse and, in some cases, in pilot clinical trials. Based upon modeling drug-induced impairments in neuroplasticity in the cortico-accumbens pathway, we argue for a concerted effort to clinically evaluate the hypothesis that targeting glial and neuronal proteins regulating excitatory synaptic plasticity may prove beneficial in treating addiction.

Glutamatergic medications for the treatment of drug and behavioral addictions

Historically, most pharmacological approaches to the treatment of addictive disorders have utilized either substitution-based methods (i.e., nicotine replacement or opioid maintenance) or have targeted monoaminergic or endogenous opioidergic neurotransmitter systems. However, substantial evidence has accumulated indicating that ligands acting on glutamatergic transmission are also of potential utility in the treatment of drug addiction, as well as various behavioral addictions such as pathological gambling. The purpose of this review is to summarize the pharmacological mechanisms of action and general clinical efficacy of glutamatergic medications that are currently approved or are being investigated for approval for the treatment of addictive disorders. Medications with effects on glutamatergic transmission that will be discussed include acamprosate, N-acetylcysteine, d-cycloserine, gabapentin, lamotrigine, memantine, modafinil, and topiramate. We conclude that manipulation of glutamatergic neurotransmission is a relatively young but promising avenue for the development of improved therapeutic agents for the treatment of drug and behavioral addictions.

A double-blind, placebo-controlled pilot trial of acamprosate for the treatment of cocaine dependence

BACKGROUND

Acamprosate is a medication shown to be effective for the treatment of alcohol dependence. Although the exact mechanism of action of acamprosate is unknown, evidence suggests that it decreases excitatory amino acid activity by post-synaptic inhibition of the NMDA subtype of glutamate receptors. It is possible that the activity of acamprosate via modulating glutamatergic activity could also reduce craving for cocaine and impact abstinence in cocaine dependence. Therefore, we conducted a double-blind placebo-controlled pilot trial of acamprosate for the treatment of cocaine dependence.

METHODS

Sixty male and female cocaine dependent patients were included in a nine week double-blind, placebo-controlled trial. After a one-week baseline, patients were randomized to receive acamprosate 666 mg three times daily or identical placebo tablets for eight weeks. The primary outcome measure was cocaine use as determined by twice weekly urine drug screens.

RESULTS

Thirty-six patients (60%) completed the trial, with no significant between-group difference in treatment retention. Percent cocaine positive urine drug screens did not differ between the two groups. Acamprosate was no better than placebo in reducing cocaine craving, reducing cocaine withdrawal symptoms, or improving measures of drug use severity from the Addiction Severity Index. Adverse events in this trial were generally mild and were evenly distributed between the two groups.

DISCUSSION

Acamprosate was well tolerated but was no more efficacious than placebo in promoting abstinence from cocaine in cocaine dependent patients. Acamprosate does not appear to be a promising medication for the treatment of cocaine dependence.

AMPA receptor synaptic plasticity induced by psychostimulants: the past, present, and therapeutic future

Experience-dependent plasticity at excitatory synapses of the mesocorticolimbic system is a fundamental brain mechanism that enables adaptation to an ever-changing environment. These synaptic responses are critical for the planning and execution of adaptive behaviors that maximize survival. The mesocorticolimbic system mediates procurement of positive reinforcers such as food and sex; however, drugs of abuse resculpt this crucial circuitry to promote compulsive drug-seeking behavior. This review will discuss the long-term changes in glutamatergic neurotransmission that occur within the mesolimbic system following cocaine exposure. In addition, we will examine how these long-lasting neuroadaptations may drive the pathology of psychostimulant addiction. Finally, we review clinical trials that highlight antagonists at excitatory AMPA receptors as promising targets against cocaine abuse.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Acamprosate and naltrexone treatment effects on ethanol and sucrose seeking and intake in ethanol-dependent and nondependent rats

RATIONALE

Two pharmacotherapies are approved for treating alcohol craving (acamprosate and naltrexone), but both have shown mixed findings in animals and humans.

OBJECTIVES

The present experiments utilized a "reinforcer blocking" approach (i.e., rats were able to consume ethanol during treatment) to better understand the efficacy of these treatments for ethanol seeking and drinking using ethanol-dependent and nondependent rats.

MATERIALS AND METHODS

In "nondependent" experiments, drugs (acamprosate 50, 100, and 200 mg/kg; naltrexone 0.1, 0.3, and 1.0 mg/kg) were administered over 3-week periods prior to operant sessions with a low response requirement to gain access to reinforcers for 20 min. For "dependent" experiments, rats were made dependent in vapor/inhalation chambers.

RESULTS

Acamprosate and naltrexone had similar effects on intake in nondependent and dependent rats; neither drug was selective for ethanol over sucrose drinking. In nondependent animals, naltrexone was more efficacious at more doses than acamprosate, and acamprosate's effects were limited to a dose that also had adverse effects on body weight. Both pharmacotherapies showed more selectivity when examining reinforcer seeking. In nondependent rats, acamprosate and naltrexone had response-attenuating effects in ethanol, but not sucrose, groups. In dependent animals, acamprosate had selective effects limited to a decrease in sucrose seeking. Naltrexone, however, selectively decreased ethanol-seeking in nondependent rats.

CONCLUSIONS

The naltrexone-induced decreases in seeking suggested a change in incentive motivation which was selective for ethanol in nondependent rats. The "nondependent" paradigm may model early stages of "problem drinking" in humans, and the findings suggest that naltrexone could be a good intervention for this level of alcohol abuse and relapse prevention.

Neurobiological mechanisms of the reinstatement of drug-conditioned place preference

Drug addiction is a chronic disorder characterized by a high rate of relapse following detoxification. There are two main versions of the reinstatement model that are employed to study relapse to drug abuse; one based on the operant self-administration procedure, and the other on the classical conditioned place preference procedure. In the last seven years, the use of the latter version has become more widespread, and the results obtained complement those obtained in self-administration studies. It has been observed that the conditioned place preference induced by opioids, psychostimulants, nicotine, ethanol and other drugs of abuse can be extinguished and reinstated by drug priming or exposure to stressful events. Herein, the neuroanatomical and neurochemical basis of drug priming- and stress-induced reinstatement of morphine and cocaine, together with the molecular correlates of reinstatement behavior, are reviewed. Differences between the conditioned place preference and self-administration studies are also discussed. Evidence suggests that data of reinstatement with the CPP are to be viewed with caution until more extensive analysis of operant procedures has been performed, and that further research will undoubtedly improve our understanding of the neurobiological mechanisms of relapse to drug seeking.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.