Treating cocaine addiction: From preclinical to clinical trial experience with ?-vinyl GABA

Astrocytes: Role in pathogenesis and effect of commonly misused drugs in the HIV infected brain

The roles of astrocytes as reservoirs and producers of a subset of viral proteins in the HIV infected brain have been studied extensively as a key to understanding HIV-associated neurocognitive disorders (HAND). However, their comprehensive role in the context of intersecting substance use and neurocircuitry of the reward pathway and HAND has yet to be fully explained. Use of methamphetamines, cocaine, or opioids in the context of HIV infection have been shown to lead to a faster progression of HAND. Glutamatergic, dopaminergic, and GABAergic systems are implicated in the development of HAND-induced cognitive impairments. A thorough review of scientific literature exploring the variety of mechanisms in which these drugs exert their effects on the HIV brain and astrocytes has revealed marked areas of convergence in overexcitation leading to increased drug-seeking behavior, inflammation, apoptosis, and irreversible neurotoxicity. The present review investigates astrocytes, the neural pathways, and mechanisms of drug disruption that ultimately play a larger holistic role in terms of HIV progression and drug use. There are opportunities for future research, therapeutic intervention, and preventive strategies to diminish HAND in the subset population of patients with HIV and substance use disorder.

Drug addiction: from bench to bedside

Drug addiction is responsible for millions of deaths per year around the world. Still, its management as a chronic disease is shadowed by misconceptions from the general public. Indeed, drug consumers are often labelled as "weak", "immoral" or "depraved". Consequently, drug addiction is often perceived as an individual problem and not societal. In technical terms, drug addiction is defined as a chronic, relapsing disease resulting from sustained effects of drugs on the brain. Through a better characterisation of the cerebral circuits involved, and the long-term modifications of the brain induced by addictive drugs administrations, first, we might be able to change the way the general public see the patient who is suffering from drug addiction, and second, we might be able to find new treatments to normalise the altered brain homeostasis. In this review, we synthetise the contribution of fundamental research to the understanding drug addiction and its contribution to potential novel therapeutics. Mostly based on drug-induced modifications of synaptic plasticity and epigenetic mechanisms (and their behavioural correlates) and after demonstration of their reversibility, we tried to highlight promising therapeutics. We also underline the specific temporal dynamics and psychosocial aspects of this complex psychiatric disease adding parameters to be considered in clinical trials and paving the way to test new therapeutic venues.

Comparison of Treatments for Cocaine Use Disorder Among Adults: A Systematic Review and Meta-analysis

IMPORTANCE

In the US and the United Kingdom, cocaine use is the second leading cause of illicit drug overdose death. Psychosocial treatments for cocaine use disorder are limited, and no pharmacotherapy is approved for use in the US or Europe.

OBJECTIVE

To compare treatments for active cocaine use among adults.

DATA SOURCES

PubMed and the Cochrane Database of Systematic Reviews were searched for clinical trials published between December 31, 1995, and December 31, 2017.

STUDY SELECTION

This meta-analysis was registered on Covidence.org (study 8731) on December 31, 2015. Clinical trials were included if they (1) had the term cocaine in the article title; (2) were published between December 31, 1995, and December 31, 2017; (3) were written in English; (4) enrolled outpatients 18 years or older with active cocaine use at baseline; and (5) reported treatment group size, treatment duration, retention rates, and urinalysis results for the presence of cocaine metabolites. A study was excluded if (1) more than 25% of participants were not active cocaine users or more than 80% of participants had negative test results for the presence of cocaine metabolites at baseline and (2) it reported only pooled urinalysis results indicating the presence of multiple substances and did not report the specific proportion of positive test results for cocaine metabolites. Multiple reviewers reached criteria consensus. Of 831 records screened, 157 studies (18.9%) met selection criteria and were included in the analysis.

DATA EXTRACTION AND SYNTHESIS

This study followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guideline. Search results were imported from PubMed XML into Covidence.org then Microsoft Excel. Data extraction was completed in 2 iterations to ensure fidelity. Analyses included a multilevel random-effects model, a multilevel mixed-effects meta-regression model, and sensitivity analyses. Treatments were clustered into 11 categories (psychotherapy, contingency management programs, placebo, opioids, psychostimulants, anticonvulsants, dopamine agonists, antidepressants, antipsychotics, miscellaneous medications, and other therapies). Missing data were imputed using multiple imputation by chained equations. The significance threshold for all analyses was P = .05. Data were analyzed using the metafor and mice packages in R software, version 3.3.2 (R Foundation for Statistical Computing). Data were analyzed from January 1, 2018, to February 28, 2021.

MAIN OUTCOMES AND MEASURES

The primary outcome was the intention-to-treat logarithm of the odds ratio (OR) of having a negative urinalysis result for the presence of cocaine metabolites at the end of each treatment period compared with baseline. The hypothesis, which was formulated after data collection, was that no treatment category would have a significant association with objective reductions in cocaine use.

RESULTS

A total of 157 studies comprising 402 treatment groups and 15 842 participants were included. Excluding other therapies, the largest treatment groups across all studies were psychotherapy (mean [SD] number of participants, 40.04 [36.88]) and contingency management programs (mean [SD] number of participants, 37.51 [25.51]). Only contingency management programs were significantly associated with an increased likelihood of having a negative test result for the presence of cocaine (OR, 2.13; 95% CI, 1.62-2.80), and this association remained significant in all sensitivity analyses.

CONCLUSIONS AND RELEVANCE

In this meta-analysis, contingency management programs were associated with reductions in cocaine use among adults. Research efforts and policies that align with this treatment modality may benefit those who actively use cocaine and attenuate societal burdens.

Effects of Isoniazid on Tolerance and Sensitization to the Rewarding Properties of Morphine: A Conditioned Place Preference Procedure Investigation in Mice

Introduction:

The GABAergic system of the brain plays a key role in morphine tolerance and sensitization. As isoniazid is a modulator of the GABAergic system, the present study aims to understand whether isoniazid can influence the induction of tolerance and sensitization to the rewarding effects of morphine.

Methods:

The rewarding effects of morphine and isoniazid were assessed using a Conditioned Place Preference (CPP) procedure in female mice. Tolerance to the rewarding effects of morphine was induced with high-dose morphine (25 mg/kg, SC), twice a day, for four days. Also, the sensitization was induced with an effective dose of morphine (5 mg/kg, SC), once a day, for three days. During the induction of tolerance or sensitization, the different groups of mice received saline or isoniazid (25, 50, and 75 mg/kg, IP) one hour before each morphine injection.

Results:

Morphine (0.5–10 mg/kg, SC) produced a significant CPP, but isoniazid (25, 50, and 75 mg/kg, IP) did not induce place preference or place aversion in mice. Although an effective dose of morphine (5 mg/kg, SC) did not induce CPP in morphine tolerated mice, an ineffective dose (0.5 mg/kg, SC) could produce a significant CPP in morphine-sensitized animals. The administration of isoniazid before morphine (on the days of tolerance or sensitization induction) inhibited the development of tolerance or sensitization to the rewarding effect of morphine in the CPP paradigm.

Conclusion:

Isoniazid can be a useful drug for the prevention of tolerance and sensitization to the rewarding effects of morphine.

Clinical Trials for Stimulant Use Disorders: Addressing Heterogeneities That May Undermine Treatment Outcomes

In recent years, use of cocaine and amphetamines and deaths associated with stimulants have been on the rise, and there are still no FDA-approved medications for stimulant use disorders. One contributing factor may involve heterogeneity. At the neurobiological level, dual dopamine dysfunction may be undermining medication efficacy, suggesting a need for combination pharmacotherapies. At the population level, individual variability is expressed in a number of ways and, if left unaddressed, may interfere with medication efficacy. This chapter reviews studies investigating medications to address dopamine dysfunction, and it also identifies several prominent heterogeneities associated with stimulant (and other substance) use disorders. The chapter has implications for improving interventions to treat stimulant use disorders, and the theme of individual heterogeneity may have broader application across substance use disorders.

Design and Mechanism of GABA Aminotransferase Inactivators. Treatments for Epilepsies and Addictions

When the brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) diminishes below a threshold level, the excess neuronal excitation can lead to convulsions. This imbalance in neurotransmission can be corrected by inhibition of the enzyme γ-aminobutyric acid aminotransferase (GABA-AT), which catalyzes the conversion of GABA to the excitatory neurotransmitter l-glutamic acid. It also has been found that raising GABA levels can antagonize the rapid elevation and release of dopamine in the nucleus accumbens, which is responsible for the reward response in addiction. Therefore, the design of new inhibitors of GABA-AT, which increases brain GABA levels, is an important approach to new treatments for epilepsy and addiction. This review summarizes findings over the last 40 or so years of mechanism-based inactivators (unreactive compounds that require the target enzyme to catalyze their conversion to the inactivating species, which inactivate the enzyme prior to their release) of GABA-AT with emphasis on their catalytic mechanisms of inactivation, presented according to organic chemical mechanism, with minimal pharmacology, except where important for activity in epilepsy and addiction. Patents, abstracts, and conference proceedings are not covered in this review. The inactivation mechanisms described here can be applied to the inactivations of a wide variety of unrelated enzymes.

Design and Mechanism of (S)-3-Amino-4-(difluoromethylenyl)cyclopent-1-ene-1-carboxylic Acid, a Highly Potent γ-Aminobutyric Acid Aminotransferase Inactivator for the Treatment of Addiction

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. Inhibition of GABA aminotransferase (GABA-AT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme that degrades GABA, has been established as a possible strategy for the treatment of substance abuse. The raised GABA levels that occur as a consequence of this inhibition have been found to antagonize the rapid release of dopamine in the ventral striatum (nucleus accumbens) that follows an acute challenge by an addictive substance. In addition, increased GABA levels are also known to elicit an anticonvulsant effect in patients with epilepsy. We previously designed the mechanism-based inactivator (1S,3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (2), now called CPP-115, that is 186 times more efficient in inactivating GABA-AT than vigabatrin, the only FDA-approved drug that is an inactivator of GABA-AT. CPP-115 was found to have high therapeutic potential for the treatment of cocaine addiction and for a variety of epilepsies, has successfully completed a Phase I safety clinical trial, and was found to be effective in the treatment of infantile spasms (West syndrome). Herein we report the design, using molecular dynamics simulations, synthesis, and biological evaluation of a new mechanism-based inactivator, (S)-3-amino-4-(difluoromethylenyl)cyclopent-1-ene-1-carboxylic acid (5), which was found to be almost 10 times more efficient as an inactivator of GABA-AT than CPP-115. We also present the unexpected crystal structure of 5 bound to GABA-AT, as well as computational analyses used to assist the structure elucidation process. Furthermore, 5 was found to have favorable pharmacokinetic properties and low off-target activities. In vivo studies in freely moving rats showed that 5 was dramatically superior to CPP-115 in suppressing the release of dopamine in the corpus striatum, which occurs subsequent to either an acute cocaine or nicotine challenge. Compound 5 also attenuated increased metabolic demands (neuronal glucose metabolism) in the hippocampus, a brain region that encodes spatial information concerning the environment in which an animal receives a reinforcing or aversive drug. This multidisciplinary computational design to preclinical efficacy approach should be applicable to the design and improvement of mechanism-based inhibitors of other enzymes whose crystal structures and inactivation mechanisms are known.

Acupuncture reduces relapse to cocaine-seeking behavior via activation of GABA neurons in the ventral tegmental area

There is growing public interest in alternative approaches to addiction treatment and scientific interest in elucidating the neurobiological underpinnings of acupuncture. Our previous studies showed that acupuncture at a specific Shenmen (HT7) points reduced dopamine (DA) release in the nucleus accumbens (NAc) induced by drugs of abuse. The present study was carried out to evaluate the effects of HT7 acupuncture on γ-aminobutyric acid (GABA) neuronal activity in the ventral tegmental area (VTA) and the reinstatement of cocaine-seeking behavior. Using microdialysis and in vivo single-unit electrophysiology, we evaluated the effects of HT7 acupuncture on VTA GABA and NAc DA release and VTA GABA neuronal activity in rats. Using a within-session reinstatement paradigm in rats self-administering cocaine, we evaluated the effects of HT7 stimulation on cocaine-primed reinstatement. Acupuncture at HT7 significantly reduced cocaine suppression of GABA release and GABA neuron firing rates in the VTA. HT7 acupuncture attenuated cocaine-primed reinstatement, which was blocked by VTA infusions of the selective GABA_B receptor antagonist 2-hydroxysaclofen. HT7 stimulation significantly decreased acute cocaine-induced DA release in the NAc, which was also blocked by 2-hydroxysaclofen. HT7 acupuncture also attenuated cocaine-induced sensitization of extracellular DA levels in the NAc. Moreover, HT7 acupuncture reduced both locomotor activity and neuronal activation in the NAc induced by acute cocaine in a needle-penetration depth-dependent fashion. These results suggest that acupuncture may suppress cocaine-induced DA release in the NAc and cocaine-seeking behavior through activation of VTA GABA neurons. Acupuncture may be an effective therapy to reduce cocaine relapse by enhancing GABAergic inhibition in the VTA.

Synthesis and Deployment of an Elusive Fluorovinyl Cation Equivalent: Access to Quaternary α-(1'-Fluoro)vinyl Amino Acids as Potential PLP Enzyme Inactivators

Developing specific chemical functionalities to deploy in biological environments for targeted enzyme inactivation lies at the heart of mechanism-based inhibitor development but also is central to other protein-tagging methods in modern chemical biology including activity-based protein profiling and proteolysis-targeting chimeras. We describe here a previously unknown class of potential PLP enzyme inactivators; namely, a family of quaternary, α-(1'-fluoro)vinyl amino acids, bearing the side chains of the cognate amino acids. These are obtained by the capture of suitably protected amino acid enolates with β,β-difluorovinyl phenyl sulfone, a new (1'-fluoro)vinyl cation equivalent, and an electrophile that previously eluded synthesis, capture and characterization. A significant variety of biologically relevant AA side chains are tolerated including those for alanine, valine, leucine, methionine, lysine, phenylalanine, tyrosine, and tryptophan. Following addition/elimination, the resulting transoid α-(1'-fluoro)-β-(phenylsulfonyl)vinyl AA-esters undergo smooth sulfone-stannane interchange to stereoselectively give the corresponding transoid α-(1'-fluoro)-β-(tributylstannyl)vinyl AA-esters. Protodestannylation and global deprotection then yield these sterically encumbered and densely functionalized quaternary amino acids. The α-(1'-fluoro)vinyl trigger, a potential allene-generating functionality originally proposed by Abeles, is now available in a quaternary AA context for the first time. In an initial test of this new inhibitor class, α-(1'-fluoro)vinyllysine is seen to act as a time-dependent, irreversible inactivator of lysine decarboxylase from Hafnia alvei. The enantiomers of the inhibitor could be resolved, and each is seen to give time-dependent inactivation with this enzyme. Kitz-Wilson analysis reveals similar inactivation parameters for the two antipodes, L-α-(1'-fluoro)vinyllysine (Ki = 630 ± 20 μM; t1/2 = 2.8 min) and D-α-(1'-fluoro)vinyllysine (Ki = 470 ± 30 μM; t1/2 = 3.6 min). The stage is now set for exploration of the efficacy of this trigger in other PLP-enzyme active sites.

Impulse Control Disorders and Related Complications of Parkinson's Disease Therapy

Impulsive and compulsive behaviors in Parkinson's disease (PD) patients are most often attributed to dopamine agonist therapy; dysregulation of the mesocorticolimbic system accounts for this behavioral phenotype. The clinical presentation is commonly termed impulse control disorder (ICD): Behaviors include hypersexuality, compulsive eating, shopping, pathological gambling, and compulsive hobby participation. However, not all PD individuals taking dopamine agonists develop these behavioral changes. In this review, the authors focus on the similarities between the phenotypic presentation of ICDs with that of other reward-based behavioral disorders, including binge eating disorder, pathological gambling, and substance use disorders. With this comparison, we emphasize that the transition from an impulsive to compulsive behavior likely follows a ventral to dorsal striatal pattern, where an altered dopaminergic reward system underlies the emergence of these problematic behaviors. The authors discuss the neurobiological similarities between these latter disorders and ICDs, emphasizing similar pathophysiological processes and discussing treatment options that have potential for translation to PD patients.

A Novel Strategy for Attenuating Opioid Withdrawal in Neonates

The rate of Neonatal Abstinence Syndrome (NAS) has drastically increased over the past decade. The average hospital expense per NAS patient has tripled, while the number of babies born to opioid-dependent mothers has increased to 5 in 1000 births. Current treatment options are limited to opioid replacement and tapering. Consequently, we examined the efficacy of prenatal, low-dose and short-term vigabatrin (γ-vinyl GABA, GVG) exposure for attenuating these symptoms as well as the metabolic changes observed in the brains of these animals upon reaching adolescence. Pregnant Sprague-Dawley rats were treated in one of four ways: 1) saline; 2) morphine alone; 3) morphine+GVG at 25 mg/kg; 4) morphine+GVG at 50 mg/kg. Morphine was administered throughout gestation, while GVG administration occurred only during the last 5 days of gestation. On post-natal day 1, naloxone-induced withdrawal behaviours were recorded in order to obtain a gross behaviour score. Approximately 28 days following birth, ¹⁸FDG microPET scans were obtained on these same animals (Groups 1, 2, and 4). Morphine-treated neonates demonstrated significantly higher withdrawal scores than saline controls. However, GVG at 50 but not 25 mg/kg/day significantly attenuated them. Upon reaching adolescence, morphine treated animals showed regionally specific changes in ¹⁸FDG uptake. Again, prenatal GVG exposure blocked them. These data demonstrate that low-dose, short-term prenatal GVG administration blocks naloxone-induced withdrawal in neonates. Taken together, these preliminary findings suggest that GVG may provide an alternative and long-lasting pharmacologic approach for the management of neonatal and adolescent symptoms associated with NAS.

Psychostimulants: Basic and Clinical Pharmacology

Substance use disorder, and particularly psychostimulant use disorder, has considerable socioeconomic burden globally. The psychostimulants include several chemical classes, being derivatives of benzoylecgonine, phenethylamine, phenylpropanolamine, or aminoaryloxazoline. Psychostimulant drugs activate the brain reward pathways of the mesoaccumbal system, and continued use leads to persistent neuroplastic and dysfunctional changes of a variety of structures involved in learning and memory, habit-forming learning, salience attribution, and inhibitory control. There are a variety of neurochemical and neurobehavioral changes in psychostimulant addiction, for example, dopaminergic, glutamatergic, serotonergic (5-HT-ergic), and γ-amino butyric acid (GABA) changes have all noted. In this chapter, we will review pharmacological changes associated with psychostimulant use and abuse in humans and animals, and on the basis of the best characterized and most widely abused psychostimulants (amphetamines, cocaine) discuss why use transitions into abuse and review basic science and clinical strategies that might assist in treating psychostimulant abuse.

Emerging drugs for the treatment of cocaine use disorder: a review of neurobiological targets and pharmacotherapy

INTRODUCTION

Cocaine use is a global public health concern of significant magnitude, negatively impacting both the individual as well as larger society. Despite numerous trials, the discovery of an effective medication for treatment of cocaine use disorder remains elusive.

AREAS COVERED

This article reviews the emerging pharmacotherapies for treatment of cocaine use disorder, focusing on those medications that are currently in Phase II or III human clinical trials. Articles reviewed were obtained through searches of PubMed, Ovid MEDLINE, Clinicaltrials.gov and the Pharmaprojects database.

EXPERT OPINION

Research into cocaine pharmacotherapy must continue to show innovation. Given that medications targeting single neurotransmitter systems have demonstrated little efficacy in treatment of cocaine use disorder, the recent focus on pharmacotherapeutic agents with multiple neurobiochemical targets represents an exciting shift in trial design and approach. Additionally, consideration of pharmacogenetics may be helpful in identification of subpopulations of cocaine-dependent individuals who may preferentially respond to medications.

GABA levels in the dorsal anterior cingulate cortex associated with difficulty ignoring smoking-related cues in tobacco-dependent volunteers

Substance abusers have difficulty ignoring drug-related cues, which is associated with relapse vulnerability. This 'attentional bias' towards drug cues translates into an inability to ignore drug-related stimuli and may reflect deficits in the brain regions, such as the dorsal anterior cingulate cortex (dACC)-a key region in cognitive control and adaptive decision making. Quantifying relationships between attentional biases to drug cues and dACC neurochemistry could aid in identifying neurobiological mechanisms associated with increased relapse vulnerability precipitated by drug cues. As gamma-aminobutyric acid (GABA) deficits have been linked to impaired cognition and addictive disorders, we hypothesized that reduced GABA in the dACC would be associated with increased attentional biases towards smoking-related cues. We confirmed this hypothesis among nicotine-dependent tobacco smokers by combining an offline behavioral measure of attentional bias with magnetic resonance spectroscopy. Smokers with the greatest attentional bias also experienced more negative affect during early nicotine withdrawal. Findings revealed a relationship between heightened reactivity to drug cues, and both decreasing dACC GABA and early withdrawal symptoms. Because reduced GABA function in frontal brain regions disrupt cognitive function, our findings suggest that smokers with diminished dACC GABA may lack the cognitive resources to successfully ignore highly salient distractors such as tobacco-related stimuli and therefore might be more prone to cue-induced relapse. This newly discovered relationship between dACC GABA and attentional bias provides evidence for a neurochemical target, which may aid smoking cessation in highly cue-reactive individuals.

Bacterial cocaine esterase: a protein-based therapy for cocaine overdose and addiction

Cocaine is highly addictive and there are no pharmacotherapeutic drugs available to treat acute cocaine toxicity or chronic abuse. Antagonizing an inhibitor such as cocaine using a small molecule has proven difficult. The alternative approach is to modify cocaine's pharmacokinetic properties by sequestering or hydrolyzing it in serum and limiting access to its sites of action. We took advantage of a bacterial esterase (CocE) that has evolved to hydrolyze cocaine and have developed it as a therapeutic that rapidly and specifically clears cocaine from the subject. Native enzyme was unstable at 37°C, thus limiting CocE's potential. Innovative computational methods based on the protein's structure helped elucidate its mechanism of destabilization. Novel protein engineering methodologies were applied to substantially improve its stability in vitro and in vivo. These improvements rendered CocE as a powerful and efficacious therapeutic to treat cocaine intoxication and lead the way towards developing a therapy for addiction.

Positron emission tomography molecular imaging of dopaminergic system in drug addiction

Dopamine (DA) is involved in drug reinforcement, but its role in drug addiction remains unclear. Positron emission tomography (PET) is the first technology used for the direct measurement of components of the dopaminergic system in the living human brain. In this article, we reviewed the major findings of PET imaging studies on the involvement of DA in drug addiction, especially in heroin addiction. Furthermore, we summarized PET radiotracers that have been used to study the role of DA in drug addiction. To investigate presynaptic function in drug addiction, PET tracers have been developed to measure DA synthesis and transport. For the investigation of postsynaptic function, several radioligands targeting dopamine one (D1) receptor and dopamine two (D2) receptor are extensively used in PET imaging studies. Moreover, we also summarized the PET imaging findings of heroin addiction studies, including heroin-induced DA increases and the reinforcement, role of DA in the long-term effects of heroin abuse, DA and vulnerability to heroin abuse and the treatment implications. PET imaging studies have corroborated the role of DA in drug addiction and increase our understanding the mechanism of drug addiction.

(1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115), a potent γ-aminobutyric acid aminotransferase inactivator for the treatment of cocaine addiction

Vigabatrin, a GABA aminotransferase (GABA-AT) inactivator, is used to treat infantile spasms and refractory complex partial seizures and is in clinical trials to treat addiction. We evaluated a novel GABA-AT inactivator (1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115, compound 1) and observed that it does not exhibit other GABAergic or off-target activities and is rapidly and completely orally absorbed and eliminated. By use of in vivo microdialysis techniques in freely moving rats and microPET imaging techniques, 1 produced similar inhibition of cocaine-induced increases in extracellular dopamine and in synaptic dopamine in the nucleus accumbens at (1)/(300) to (1)/(600) the dose of vigabatrin. It also blocks expression of cocaine-induced conditioned place preference at a dose (1)/(300) that of vigabatrin. Electroretinographic (ERG) responses in rats treated with 1, at doses 20-40 times higher than those needed to treat addiction in rats, exhibited reductions in ERG responses, which were less than the reductions observed in rats treated with vigabatrin at the same dose needed to treat addiction in rats. In conclusion, 1 can be administered at significantly lower doses than vigabatrin, which suggests a potential new treatment for addiction with a significantly reduced risk of visual field defects.

Hidden Markov models for zero-inflated Poisson counts with an application to substance use

Paradigms for substance abuse cue-reactivity research involve pharmacological or stressful stimulation designed to elicit stress and craving responses in cocaine-dependent subjects. It is unclear as to whether stress induced from participation in such studies increases drug-seeking behavior. We propose a 2-state Hidden Markov model to model the number of cocaine abuses per week before and after participation in a stress-and cue-reactivity study. The hypothesized latent state corresponds to 'high' or 'low' use. To account for a preponderance of zeros, we assume a zero-inflated Poisson model for the count data. Transition probabilities depend on the prior week's state, fixed demographic variables, and time-varying covariates. We adopt a Bayesian approach to model fitting, and use the conditional predictive ordinate statistic to demonstrate that the zero-inflated Poisson hidden Markov model outperforms other models for longitudinal count data.

Antiepileptic drugs in the treatment of psychiatric disorders

The clinical interface between psychiatry and neurology is epilepsy; the pharmacological expression of this interface is antiepileptic drugs (AEDs), as they are used to treat both epilepsy and psychiatric disorders, especially bipolar disorders. The prevalence of psychiatric comorbidity and the risk of suicidal behavior/ideation/suicide are markedly increased in patients with epilepsy (PWE). Though AEDs receive initial indications for the treatment of epilepsy, currently the majority of AEDs are used to treat pain and psychiatric disorders. Thus in selecting the appropriate AEDs for treatment of PWE, consideration should be given to which AEDs best treat the epileptic disorder and the psychiatric comorbidity. This review is an overview of 21 AEDs in which negative psychotropic properties, approved indications in psychiatry, off-label studied uses in psychiatry, and principal uses in psychiatry are presented with literature review. A total of 40 psychiatric uses have been identified. Of the 21 AEDs reviewed, only 5 have U.S. Food and Drug Administration and/or European Medicines Agency psychiatric approval for limited uses; the majority of AEDs are used off-label. Many of these off-label uses are based on case reports, open-label studies, and poorly controlled or small-sample-size studies. In some instances, off-label use persists in the face of negative pivotal trials. Further placebo-controlled (augmentation and monotherapy) parallel-arm research with active comparators is required in the complex field of AED treatment of psychiatric disorders to minimize the treatment gap not only for PWE with psychiatric disorders, but also for psychiatric patients who would benefit from properly studied AEDs while minimizing adverse effects.

What's new in the treatment of cocaine addiction?

Cocaine dependence continues to be a significant public health problem in the United States. The number of regular cocaine users has not declined significantly in the United States since 1992. Although counseling remains the treatment of choice for cocaine dependence, many cocaine-dependent patients do not respond completely to standard drug counseling. Therefore, the development of new and more effective treatments for cocaine dependence is a research priority. Progress in the understanding of the neurobiology of cocaine dependence has led to the discovery of several promising medications that have already shown encouraging results in controlled clinical trials. Other promising compounds are just becoming available for clinical trials. The use of novel psychosocial techniques such as contingency management seems to increase the efficacy of several medications used to treat cocaine dependence. New medications and new psychosocial techniques are leading to significant improvements in the treatment of cocaine dependence.

Pharmacotherapy of addictive disorders

Substance use disorders are highly prevalent in the United States and cause considerable damage to our society. They are underrecognized and undertreated despite a vast body of literature demonstrating the efficacy of treatment using both psychosocial and psychopharmacological modalities. For the last decade, research and progress into the biological basis of the addictive process has led to a rapidly growing number of pharmacological agents used to interrupt the addictive process at its various stages such as the initiation of substance abuse, the transition from abuse to dependence, and the prevention of drug reinstatement or relapse. Food and Drug Administration-approved medications exist for nicotine, alcohol, and opioid use disorders, and progress is being made to develop agents for stimulant use disorders. Regarding nicotine use disorders, nicotine replacement therapies,bupropion and varenicline, have Food and Drug Administration approval, and future options exist with endocannabinoid antagonists and immune therapy. Aversive agents, opiate antagonists, and glutamate based interventions are currently approved to treat alcohol use disorders with future promise with GABAergic, serotonergic, and endocannabinoid system agents. Opiate addiction is treated by approved agonist and antagonist mu-opioid medications with the future potential for agents that can modulate the stress systems and the iboga alkaloids. Although no pharmacotherapies are currently approved for cocaine addiction, promising lines of research include agents that affect dopaminergic, GABAergic, serotonergic,and glutamatergic systems as well as the promise for immune therapies.

GABAB receptors in addiction and its treatment

The GABA(B) receptor plays an important role in the control of neurotransmitter release, and experiments using preclinical models have shown that modulation of this receptor can have profound effects on the reward process. This ability to affect the reward process has led to clinical investigations into the possibility that this could be a viable target in the treatment of addiction. Presented here is an overview of a number of studies testing this hypothesis in different drug dependencies. The studies reviewed have used the GABA(B) receptor agonist baclofen, which is currently the only GABA(B) agonist for use in humans. In addition, studies using the non-specific GABA(B) receptor agonists vigabatrin and tiagabine have been included. In some of the studies these were found to have efficacy in the initiation and maintenance of abstinence, as an anti-craving treatment and alleviation of withdrawal syndromes, while in other studies showing limited effects. However, there is enough evidence to suggest that modulators of the GABA(B) receptor have potential as adjunct treatments to aid in the initiation of abstinence, maintenance of abstinence, and prevention of cue-related relapse in some addictions. This potential is at present poorly understood or studied and warrants further investigation.

Novel pharmacological approaches to drug abuse treatment

The field of pharmacologic addiction treatment is expanding rapidly. While there are currently several FDA-approved medications for nicotine, alcohol, and opiate dependence, research into novel pharmacological approaches for these and additional substances is legion. Each drug of abuse, while sharing a common final neural pathway of increasing dopaminergic tone, has unique and individual characteristics that are important in developing improved and varied treatments. In this chapter, we discuss such research and present the neurobiological underpinnings of these explorations. In general, addiction treatment is focused on four areas: (1) reducing withdrawal discomfort, (2) diminishing cravings, (3) blocking rewarding effects of the drug, and (4) treating comorbidities, such as depression or ADHD. We present current ideas in pharmacologic research for nicotine, alcohol, cannabis, stimulants, and opiates.

Non-treatment laboratory stress- and cue-reactivity studies are associated with decreased substance use among drug-dependent individuals

INTRODUCTION

Human laboratory paradigms for examining stress- or cue-reactivity in substance-dependent individuals often involve exposure to pharmacological, psychosocial or physical laboratory procedures or drug paraphernalia. This study examines whether participation in such studies alters drug-seeking behavior and which patient attributes contribute to increased use.

METHODS

In two separate studies, the relationship between participation and drug use post-study were examined. Cocaine-dependent participants received 1 microg/kg of corticotropin releasing hormone intravenously, underwent the Trier Social Stress Task, and were exposed to drug cues and various measures obtained. Cocaine use for 90 days prior and 28 days following the study was assessed. Methamphetamine-dependent participants were exposed to drug cues and various measures obtained. Methamphetamine use for 90 days prior and 14 days following the study was assessed. Weekly drug use was modeled using a 2-state hidden Markov model assuming two possible underlying states at each week. Bayesian estimation was used. Results are presented as posterior mean odds ratios (OR) and 95% credible intervals (CI).

RESULTS

Participation decreased the odds of remaining in or transitioning to the high use state (cocaine study OR=0.04 [CI=0.01, 0.11]; methamphetamine study OR=0.39 [CI=0.07, 1.70]). In the cocaine study, older age increased the odds of remaining in or transitioning into the high use state (1.66 [CI=0.99, 2.96]). In the methamphetamine study, male gender increased the odds (2.70 [CI=1.10, 6.17]).

CONCLUSION

Stress and cue exposure paradigms were associated with a decreased odds of drug use following participation.

The cardiovascular and subjective effects of methamphetamine combined with gamma-vinyl-gamma-aminobutyric acid (GVG) in non-treatment seeking methamphetamine-dependent volunteers

Gamma-vinyl-gamma-aminobutyric acid (GVG) elevates central nervous system gamma-aminobutyric acid (GABA) levels by irreversibly inhibiting GABA transaminase. An open-label clinical trial in humans suggested that GVG may reduce cocaine and methamphetamine use. To test safety and to obtain preliminary data on efficacy of GVG for treating methamphetamine dependence, we conducted a double-blind, placebo-controlled, parallel group study of GVG interaction with the cardiovascular and subjective effects produced by methamphetamine. Non-treatment seeking methamphetamine-dependent volunteers received either GVG (N=8) or placebo (N=9) by random assignment. GVG treatment was initiated at 1 g/day and increased to 5 g/day. After reaching the target dose of 5 g/day, participants received methamphetamine (15+30 mg, IV), and cardiovascular and subjective effects were assessed. No serious adverse events were noted, and the total number of adverse events was similar between the treatment groups. Considering the full time course and peak effects independently, no significant differences were detected between the groups for systolic or diastolic blood pressures, or heart rate, following methamphetamine exposure. Some methamphetamine-induced cardiovascular changes approached significance (p<0.10) and may warrant attention in future trials. Methamphetamine-induced subjective effects ("any drug effect", "high", "crave methamphetamine") were statistically similar between GVG and placebo treatment groups. Pharmacokinetic data indicate that GVG treatment did not alter methamphetamine or amphetamine plasma levels, and there was no association between methamphetamine or amphetamine plasma levels and peak cardiovascular effects. Taken together, the data indicate that GVG treatment is generally well tolerated but not efficacious in attenuating the positive subjective effects of methamphetamine in the laboratory.

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.

Hypothesis-driven medication discovery for the treatment of psychostimulant addiction

Psychostimulant abuse is a serious social and health problem, for which no effective treatments currently exist. A number of review articles have described predominantly 'clinic'-based pharmacotherapies for the treatment of psychostimulant addiction, but none have yet been shown to be definitively effective for use in humans. In the present article, we review various 'hypothesis'- or 'mechanism'-based pharmacological agents that have been studied at the preclinical level and evaluate their potential use in the treatment of psychostimulant addiction in humans. These compounds target brain neurotransmitter or neuromodulator systems, including dopamine (DA), gamma-aminobutyric acid (GABA), endocannabinoid, glutamate, opioid and serotonin, which have been shown to be critically involved in drug reward and addiction. For drugs in each category, we first briefly review the role of each neurotransmitter system in psychostimulant actions, and then discuss the mechanistic rationale for each drug's potential anti-addiction efficacy, major findings with each drug in animal models of psychostimulant addiction, abuse liability and potential problems, and future research directions. We conclude that hypothesis-based medication development strategies could significantly promote medication discovery for the effective treatment of psychostimulant addiction.

Plasticity of addiction: a mesolimbic dopamine short-circuit?

The development of drug addiction progresses along a continuum from acute drug use to compulsive use and drug seeking behavior. Many researchers have focused on identifying the physiological mechanisms involved in drug addiction in order to develop effective pharmacotherapies. Neuroplasticity, the putative mechanism underlying learning and memory, is modified by drugs of abuse and may contribute to the development of the eventual addicted state. Innovative treatments directly targeting these drug-induced changes in brain reward components and circuits may be efficacious in reducing drug use and relapse.

Valproate treatment and cocaine cue reactivity in cocaine dependent individuals

Based on prior clinical trials indicating that gamma-aminobutyric acid (GABA)-based anticonvulsant medications reduce drug craving in cocaine dependent study participants, we tested the effects of valproate treatment on cue-induced cocaine craving. Crack cocaine dependent individuals (N=20) were tested in a randomized, placebo-controlled, within-subjects, crossover study design. Valproate treatment was titrated up to 1500 mg/day by Day 6 of treatment, cue testing was completed on Day 8 of treatment, and all study participants underwent a washout period of 5 days between active and placebo medication treatment periods. Testing included both cocaine and neutral cue exposure sessions, presented in a random and counterbalanced order. Main effects of cue exposure were found for subjective ratings of "desire to use cocaine now", the cocaine craving index, cocaine-like high, and cocaine withdrawal. Treatment interaction effects were found with "desire to use cocaine now", which underwent a greater increase following cocaine cue exposure in the valproate condition. Main effects of medication treatment were found, in which lower blood pressure and heart rate, and higher plasma cortisol levels, were associated with valproate treatment. Valproate treatment was also associated, at a trend level, with higher pre-test cocaine craving levels. The results demonstrate that cocaine cue reactivity is a robust phenomena across two assessment sessions, but fail to support the use of valproate as a means of reducing spontaneous and cue-induced cocaine craving. The use of valproate as a treatment for cocaine dependence is not supported.

Cue-induced dopamine release predicts cocaine preference: positron emission tomography studies in freely moving rodents

Positron emission tomography studies in drug-addicted patients have shown that exposure to drug-related cues increases striatal dopamine, which displaces binding of the D(2) ligand, [(11)C]-raclopride. However, it is not known if animals will also show cue-induced displacement of [(11)C]-raclopride binding. In this study, we use [(11)C]-raclopride imaging in awake rodents to capture cue-induced changes in dopamine release associated with the conditioned place preference model of drug craving. Ten animals were conditioned to receive cocaine in a contextually distinct environment from where they received saline. Following conditioning, each animal was tested for preference and then received two separate [(11)C]-raclopride scans. For each scan, animals were confined to the cocaine and/or the saline-paired environment for the first 25 min of uptake, after which they were anesthetized and scanned. [(11)C]-raclopride uptake in the saline-paired environment served as a within-animal control for uptake in the cocaine-paired environment. Cocaine produced a significant place preference (p = 0.004) and exposure to the cocaine-paired environment decreased [(11)C]-raclopride binding relative to the saline-paired environment in both the dorsal (20%; p < 0.002) and ventral striatum (22%; p < 0.05). The change in [(11)C]-raclopride binding correlated with preference in the ventral striatum (R(2) = -0.87; p = 0.003). In this region, animals who showed little or no preference exhibited little or no change in [(11)C]-raclopride binding in the cocaine-paired environment. This noninvasive procedure of monitoring neurochemical events in freely moving, behaving animals advances preclinical molecular imaging by interrogating the degree to which animal models reflect the human condition on multiple dimensions, both biological and behavioral.

Racemic gamma vinyl-GABA (R,S-GVG) blocks methamphetamine-triggered reinstatement of conditioned place preference

Preventing relapse poses a significant challenge to the successful management of methamphetamine (METH) dependence. Although no effective medication currently exists for its treatment, racemic gamma vinyl-GABA (R,S-GVG, vigabatrin) shows enormous potential as it blocks both the neurochemical and behavioral effects of a variety of drugs, including METH, heroin, morphine, ethanol, nicotine, and cocaine. Using the reinstatement of a conditioned place preference (CPP) as an animal model of relapse, the present study specifically investigated the ability of an acute dose of R,S-GVG to block METH-triggered reinstatement of a METH-induced CPP. Animals acquired a METH CPP following a 20-day-period of conditioning, in which they received 10 pairings of alternating METH and saline injections. During conditioning, rats were assigned to one of four METH dosage groups: 1.0, 2.5, 5.0, or 10.0 mg/kg (i.p., n = 8/group). Animals in all dosage groups demonstrated a robust and consistent CPP. This CPP was subsequently extinguished in each dosage group with repeated saline administration. Upon extinction, all groups reinstated following an acute METH challenge. On the following day, an acute dose of R,S-GVG (300 mg/kg, i.p.) was administered 2.5 h prior to an identical METH challenge. R,S-GVG blocked METH-triggered reinstatement in all four groups. Given that drug re-exposure may potentiate relapse to drug-seeking behavior, the ability of R,S-GVG to block METH-triggered reinstatement offers further support for its use in the successful management of METH dependence.

Use of animal models to develop antiaddiction medications

Although addiction is a uniquely human phenomenon, some of its pathognomonic features can be modeled at the animal level. Such features include the euphoric "high" produced by acute administration of addictive drugs; the dysphoric "crash" produced by acute withdrawal; drug-seeking and drug-taking behaviors; and relapse to drug-seeking behavior after achieving successful abstinence. Animal models exist for each of these features. In this review, I focus on various animal models of addiction and how they can be used to search for clinically effective antiaddiction medications. I conclude by noting some of the new and novel medications that have been developed preclinically using such models and the hope for further developments along such lines.

Effects of gabapentin on cocaine self-administration, cocaine-triggered relapse and cocaine-enhanced nucleus accumbens dopamine in rats

Gabapentin is a gamma-aminobutyric acid (GABA) analogue, with GABAmimetic pharmacological properties. Gabapentin is used for the treatment of seizures, anxiety and neuropathic pain. It has been proposed that gabapentin may be useful in the treatment of cocaine dependence. However, clinical trials with gabapentin have shown conflicting results, while preclinical studies are sparse. In the present study, we investigated the effects of gabapentin on intravenous cocaine self-administration and cocaine-triggered reinstatement of drug-seeking behavior, as well as on cocaine-enhanced dopamine (DA) in the nucleus accumbens (NAc). We found that gabapentin (25-200 mg/kg, i.p., 30 min or 2 h prior to cocaine) failed to inhibit intravenous cocaine (0.5 mg/kg/infusion) self-administration under a fixed-ratio reinforcement schedule or cocaine-triggered reinstatement of cocaine-seeking behavior. In vivo microdialysis showed that the same doses of gabapentin produced a modest increase (approximately 50%, p<0.05) in extracellular NAc GABA levels, but failed to alter either basal or cocaine-enhanced NAc DA. These data suggest that gabapentin is a weak GABA-mimic drug. At the doses tested, it has no effect in the addiction-related animal behavioral models here tested. This is in striking contrast to positive findings in the same animal models shown by another GABAmimetic--gamma-vinyl GABA (see companion piece to present article).

The search for medications to treat stimulant dependence

Progress in understanding the neurobiology of stimulant dependence has enabled researchers to identify medications whose pharmacological effects suggest that they might help patients initiate abstinence or avoid relapse. Several of these medications and a vaccine have shown encouraging results in controlled clinical trials with cocaine-dependent patients. The search for a medical treatment for methamphetamine dependence started more recently, due to the later emergence of this epidemic, but at least one candidate medication has shown promise in early clinical testing. Treatment approaches that combine efficacious medications and empirically proven behavioral interventions, such as voucher-based reinforcement therapy, will almost certainly produce the best results.

New treatments for cocaine dependence: a focused review

Cocaine, already a significant drug problem in North and South America, has become a more prominent part of the European drug scene. Cocaine dependence has major somatic, psychological, psychiatric, socio-economic, and legal implications. No specific effective pharmacological treatment exists for cocaine dependence. Recent advances in neurobiology have identified various neuronal mechanisms implicated in cocaine addiction and suggested several promising pharmacological approaches. Data were obtained from Medline, EMBASE, and PsycINFO searches of English-language articles published between 1985 and June 2007 using the key words: cocaine, addiction, cocaine dependence, clinical trials, pharmacotherapy(ies) singly and in combination. Large well-controlled studies with appropriate statistical methods were preferred. Pharmacological agents such as GABA agents (topiramate, tiagabine, baclofen and vigabatrin) and agonist replacement agents (modafinil, disulfiram, methylphenidate) seem to be the most promising in treatment of cocaine dependence. The results from trials of first- and second-generation neuroleptics are largely negative. Aripiprazole, a partial dopaminergic agonist that may modulate the serotonergic system, shows some promise. Preliminary results of human studies with anti-cocaine vaccine, N-acetylcysteine, and ondansetron, are promising, as are several compounds in preclinical development. While no medication has received regulatory approval for the treatment of cocaine dependence, several medications marketed for other indications have shown efficacy in clinical trials. An anti-cocaine vaccine and several compounds in preclinical development have also shown promise. Findings from early clinical trials must be confirmed in larger, less selective patient populations.

Endocannabinoid signaling mediates cocaine-induced inhibitory synaptic plasticity in midbrain dopamine neurons

Drugs that increase GABA levels in the brain reduce cocaine seeking in rodents and humans, suggesting that GABAergic inhibition regulates cocaine-seeking behavior. We previously reported that repeated cocaine exposure in vivo facilitates long-term potentiation by reducing the strength of GABAergic inhibition in dopamine neurons of the ventral tegmental area (VTA). Selective blockade of cocaine-induced reduction of GABAergic inhibition in the VTA might diminish cocaine-induced aberrant synaptic plasticity and addictive behavior. Here, we investigated the mechanism for cocaine-induced reduction of GABAergic inhibition. We show that a pathophysiologically relevant concentration of cocaine enables a normally ineffective stimulus to induce long-term depression (LTD) of IPSCs (I-LTD) in VTA dopamine neurons of midbrain slices. Activation of D2 dopamine receptors and group I metabotropic glutamate receptors and subsequent recruitment of endocannabinoid signaling are required for I-LTD induction. We further demonstrate that in vivo pretreatment with antagonists to these receptors blocks cocaine-induced reduction of GABAergic inhibition and that repeated cocaine exposure in vivo occludes the subsequent induction of I-LTD ex vivo. Together, these results suggest that repeated cocaine exposure reduces the strength of GABAergic inhibition in dopamine neurons by inducing I-LTD-like modification in vivo.

Imaging the addicted human brain

Modern imaging techniques enable researchers to observe drug actions and consequences as they occur and persist in the brains of abusing and addicted individuals. This article presents the five most commonly used techniques, explains how each produces images, and describes how researchers interpret them. The authors give examples of key findings illustrating how each technique has extended and deepened our knowledge of the neurobiological bases of drug abuse and addiction, and they address potential clinical and therapeutic applications.

Biological treatments for amfetamine dependence : recent progress

Amfetamine abuse has grown into a worldwide epidemic. Methamfetamine, a derivative of amfetamine made from readily accessible chemicals, has plagued the US since the 1960s, with an alarming recent surge in the numbers of those meeting the criteria for amfetamine abuse and dependence. We review this problem using a computerised literature search (PubMed 1964-2007) to summarise knowledge from animal and human studies about treatments for amfetamine dependence, while exploring the potential of pharmacogenetics to help uncover new treatment targets. Several promising therapeutic targets have come from animal models of reward, drug-taking behaviour and withdrawal. Although preclinical and selected clinical results have been promising, clinical studies have yielded inconsistent results. To improve these outcomes, pharmacogenetic studies may be used to identify candidate alleles that predict therapeutic response. Exciting preclinical findings and a steady progression of clinical results offer hope for the development of a treatment for amfetamine dependence.

New developments in the pharmacotherapy of cocaine abuse

Despite huge advances in the neuroscience of substance abuse and dependence in the past 20 years, no approved pharmacological treatment exists for cocaine abuse. The available drugs for the treatment of cocaine abuse are poorly effective, hence the need for new compounds to be screened and tested for efficacy: targeting symptoms might improve the effectiveness of the treatment of cocaine abuse and dependence. On the basis of the known neurochemistry of cocaine, some target compounds have been studied: among others, BP-897, a D3 partial agonist; vanoxerine, a highly selective inhibitor of dopamine uptake; aripiprazole, a partial mixed-action agonist approved for the treatment of schizophrenia. Recently modafinil, approved for the treatment of narcolepsy, proved effective in favouring cocaine abstinence in cocaine-abusing people. Some placebo-controlled studies also reported the effectiveness of topiramate, a licensed antiepileptic drug, and of tiagabine, a gamma-aminobutyric acid (GABA) re-uptake inhibitor also approved as an anticonvulsant; both compounds increased cocaine abstinence with no serious adverse events. Promising results came from two more compounds acting on the GABA circuits, baclofen and valproic acid. Finally disulfiram, prescribed with active psychosocial therapy, was found to favour higher retention rates and longer abstinence periods from both alcohol and cocaine in polydrug-abusing patients. An alternative approach rests on the use of vaccines, to date in the experimental stage still. Psychosocial treatments are a useful companion in the pharmacotherapy of cocaine abuse, with group therapy and contingency management therapies improving motivation and social functioning, particularly in patients abusing alcohol as well.

Opioids block long-term potentiation of inhibitory synapses

Excitatory brain synapses are strengthened or weakened in response to specific patterns of synaptic activation, and these changes in synaptic strength are thought to underlie persistent pathologies such as drug addiction, as well as learning. In contrast, there are few examples of synaptic plasticity of inhibitory GABA (gamma-aminobutyric acid)-releasing synapses. Here we report long-term potentiation of GABA(A)-mediated synaptic transmission (LTP(GABA)) onto dopamine neurons of the rat brain ventral tegmental area, a region required for the development of drug addiction. This novel form of LTP is heterosynaptic, requiring postsynaptic NMDA (N-methyl-d-aspartate) receptor activation at glutamate synapses, but resulting from increased GABA release at neighbouring inhibitory nerve terminals. NMDA receptor activation produces nitric oxide, a retrograde signal released from the postsynaptic dopamine neuron. Nitric oxide initiates LTP(GABA) by activating guanylate cyclase in GABA-releasing nerve terminals. Exposure to morphine both in vitro and in vivo prevents LTP(GABA). Whereas brief treatment with morphine in vitro blocks LTP(GABA) by inhibiting presynaptic glutamate release, in vivo exposure to morphine persistently interrupts signalling from nitric oxide to guanylate cyclase. These neuroadaptations to opioid drugs might contribute to early stages of addiction, and may potentially be exploited therapeutically using drugs targeting GABA(A) receptors.

Tiagabine does not attenuate alcohol-induced activation of the human reward system

RATIONALE

The rewarding effects of ethanol and other drugs of abuse are mediated by activation of the mesolimbic dopamine system. Recent neuroimaging studies in primates and humans suggest that cocaine-induced dopamine stimulation might be diminished by drugs augmenting gamma-aminobutyric acid A (GABA-A) receptor function such as the GABA transaminase inhibitor vigabatrin.

OBJECTIVES

The objective of this study was to test the property of the selective GABA transporter 1 (GAT1) inhibitor tiagabine to block ethanol-induced activation of the mesolimbic reward system in an i.v. ethanol challenge.

MATERIALS AND METHODS

Twenty nonaddicted healthy volunteers underwent an i.v. ethanol challenge after 1 week of tiagabine (15 mg/day) administration. Neuronal activation was measured using [(18)F]-fluoro-deoxyglucose positron emission tomography (PET).

RESULTS

Tiagabine did not prevent ethanol-induced stimulation of the mesolimbic reward system but augmented ethanol-induced hypometabolism within areas of the visual system and the cerebellum. Tiagabine alone also decreased neuronal metabolism within parts of the right temporal cortex that are highly enriched with GABA-ergic neurons.

CONCLUSIONS

Our ethanol challenge imaging study does not provide supporting evidence that the GAT1 inhibitor tiagabine diminishes the rewarding effects of ethanol. Further PET imaging studies using established anticraving compounds, such as the mu-opioid receptor antagonist naltrexone and antiepileptic drugs affecting the GABA-ergic system more broadly, will provide additional important insights on the interaction between the GABA-ergic and the brain reward system in vivo and the suitability of GABA-ergic drugs as anticraving compounds.

Approaches to the development of medications for the treatment of methamphetamine dependence

BACKGROUND

Methamphetamine abuse has become an increasing problem in both the United States and globally with concomitant increases in adverse medical, social and environmental sequelae. Behavioral therapies have been used with some success to treat methamphetamine abusers and dependent individuals, but are not universally efficacious. Methamphetamine has a rich pharmacology that theoretically provides many opportunities for potential pharmacotherapeutic intervention. Nevertheless, there are no approved medications with an indication for treating methamphetamine abusers or addicts at this time.

AIM

To describe briefly how methamphetamine functions and affects function in brain and report how basic researchers and clinicians are attempting to exploit and exploiting this knowledge to discover and develop effective pharmacotherapies.

RESULTS

Scientifically based approaches to medications development by evaluating medications that limit brain exposure to methamphetamine; modulate methamphetamine effects at vesicular monoamine transporter-2 (VMAT-2); or affect dopaminergic, serotonergic, GABAergic, and/or glutamatergic brain pathways that participate in methamphetamine's reinforcing effects are presented.

CONCLUSION

The evidence supports the rationale that pharmacotherapies to decrease methamphetamine use, or reduce craving during abstinence may be developed from altering the pharmacokinetics and pharmacodynamics of methamphetamine or its effects on appetitive systems in the brain.

Cellular and molecular mechanisms of drug dependence: An overview and update

Drug dependence is a major cause of morbidity and loss of productivity. Various theories ranging from economic to psychological have been invoked in an attempt to explain this condition. With the advent of research at the cellular and subcellular levels, perspectives on the etiology of drug dependence have also changed. Perhaps the greatest advance has been in the identification of specific receptors for each of the drugs, their target neurotransmitter systems and the intracellular changes produced by them. These receptors also provide potential targets for treatment strategies of drug dependence. This overview attempts to present the mechanisms in the development of dependence and the newer treatment strategies for the major drugs of abuse like alcohol, opioids, cannabis, nicotine and cocaine.