Acute administration of the GABA reuptake inhibitor tiagabine does not alter the effects of oral cocaine in humans

Evaluation of the "Pipeline" for Development of Medications for Cocaine Use Disorder: A Review of Translational Preclinical, Human Laboratory, and Clinical Trial Research

Cocaine use disorder is a persistent public health problem for which no widely effective medications exist. Self-administration procedures, which have shown good predictive validity in estimating the abuse potential of drugs, have been used in rodent, nonhuman primate, and human laboratory studies to screen putative medications. This review assessed the effectiveness of the medications development process regarding pharmacotherapies for cocaine use disorder. The primary objective was to determine whether data from animal and human laboratory self-administration studies predicted the results of clinical trials. In addition, the concordance between laboratory studies in animals and humans was assessed. More than 100 blinded, randomized, fully placebo-controlled studies of putative medications for cocaine use disorder were identified. Of the 64 drugs tested in these trials, only 10 had been examined in both human and well-controlled animal laboratory studies. Within all three stages, few studies had been conducted for each drug and when multiple studies had been conducted conclusions were sometimes contradictory. Overall, however, there was good concordance between animal and human laboratory results when the former assessed chronic drug treatment. Although only seven of the ten reviewed drugs showed fully concordant results across all three types of studies reviewed, the analysis revealed several subject-related, procedural, and environmental factors that differ between the laboratory and clinical trial settings that help explain the disagreement for other drugs. The review closes with several recommendations to enhance translation and communication across stages of the medications development process that will ultimately speed the progress toward effective pharmacotherapeutic strategies for cocaine use disorder.

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.

Separate and combined effects of the GABA reuptake inhibitor tiagabine and Δ9-THC in humans discriminating Δ9-THC

BACKGROUND

The involvement of non-cannabinoid neurotransmitter systems in the abuse-related behavioral effects of cannabis has not been well characterized in humans. GABAergic drugs have overlapping effects with cannabis and Δ(9)-tetrahydrocannabinol (Δ(9)-THC) on certain behavioral measures, but those measures lack the specificity to draw conclusions regarding the involvement of GABA in cannabinoid effects. The aim of this study was to assess the separate and combined effects of the GABA reuptake inhibitor tiagabine and Δ(9)-THC using more pharmacologically specific drug-discrimination procedures.

METHODS

Eight cannabis users learned to discriminate 30 mg oral Δ(9)-THC from placebo and then received tiagabine (6 and 12 mg), Δ(9)-THC (5, 15 and 30 mg) and placebo, alone and in combination. Self-report, task performance and physiological measures were also collected.

RESULTS

Δ(9)-THC produced subjective effects typically associated with cannabinoids (e.g., High, Stoned, Like Drug), elevated heart rate and impaired rate and accuracy on psychomotor performance tasks. The higher tiagabine dose substituted for the Δ(9)-THC discriminative stimulus and engendered subjective and performance-impairing effects that overlapped with those of Δ(9)-THC when administered alone. In combination, tiagabine shifted the discriminative-stimulus effects of Δ(9)-THC leftward/upward and enhanced Δ(9)-THC effects on other outcomes.

CONCLUSIONS

These results indicate that GABA is involved in the clinical effects of Δ(9)-THC, and by extension, cannabis. Future studies should test selective GABAergic compounds to determine which receptor subtype(s) are responsible for the effects observed when combined with cannabinoids.

Human behavioral pharmacology, past, present, and future: symposium presented at the 50th annual meeting of the Behavioral Pharmacology Society

A symposium held at the 50th annual meeting of the Behavioral Pharmacology Society in May 2007 reviewed progress in the human behavioral pharmacology of drug abuse. Studies on drug self-administration in humans are reviewed that assessed reinforcing and subjective effects of drugs of abuse. The close parallels observed between studies in humans and laboratory animals using similar behavioral techniques have broadened our understanding of the complex nature of the pharmacological and behavioral factors controlling drug self-administration. The symposium also addressed the role that individual differences, such as sex, personality, and genotype play in determining the extent of self-administration of illicit drugs in human populations. Knowledge of how these factors influence human drug self-administration has helped validate similar differences observed in laboratory animals. In recognition that drug self-administration is but one of many choices available in the lives of humans, the symposium addressed the ways in which choice behavior can be studied in humans. These choice studies in human drug abusers have opened up new and exciting avenues of research in laboratory animals. Finally, the symposium reviewed behavioral pharmacology studies conducted in drug abuse treatment settings and the therapeutic benefits that have emerged from these studies.

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.

Principles of laboratory assessment of drug abuse liability and implications for clinical development

Abuse liability testing plays an important role in informing drug development, regulatory processes, and clinical practice. This paper describes the current "gold standard" methodologies that are used for laboratory assessments of abuse liability in non-human and human subjects. Particular emphasis is given to procedures such as non-human drug discrimination, self-administration, and physical dependence testing, and human dose-effect abuse liability studies that are commonly used in regulatory submissions to governmental agencies. The potential benefits and risks associated with the inclusion of measures of abuse liability in industry-sponsored clinical trials is discussed. Lastly, it is noted that many factors contribute to patterns of drug abuse and dependence outside of the laboratory setting and positive or negative signals in abuse liability studies do not always translate to high or low levels of actual abuse or dependence. Well-designed patient and physician education, pharmacovigilance, and postmarketing surveillance can reduce the diversion and misuse of drugs with abuse liability and can effectively foster the protection and promotion of public health.

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.

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.

Substitution profile of Delta9-tetrahydrocannabinol, triazolam, hydromorphone, and methylphenidate in humans discriminating Delta9-tetrahydrocannabinol

RATIONALE

Preclinical evidence suggests that non-cannabinoid neurotransmitter systems are involved in the behavioral and physiological effects of cannabinoids, but relatively little research has been conducted in humans.

OBJECTIVES

The aims of this study were to assess whether oral Delta(9)-tetrahydrocannabinol (Delta(9)-THC) would function as a discriminative stimulus in humans and to examine the substitution profile of drugs acting at opioid, GABA, and dopamine systems.

METHODS

Healthy subjects who reported moderate cannabis use were enrolled. Subjects learned to identify when they received oral 25 mg Delta(9)-THC or placebo under double-blind conditions. Once subjects acquired the discrimination (i.e., > or =80% drug-appropriate responding for four consecutive sessions), multiple doses of Delta(9)-THC, the GABA(A) positive modulator triazolam, the micro-opioid agonist hydromorphone and the dopamine reuptake inhibitor methylphenidate were tested to determine if they shared discriminative-stimulus effects with the training dose of Delta(9)-THC.

RESULTS

Eight subjects (N = 8) accurately discriminated Delta(9)-THC and completed the study. The training dose of Delta(9)-THC functioned as a discriminative stimulus and produced prototypical subject-rated drug effects. All of the drugs tested produced significant effects on the self-report questionnaires, but only Delta(9)-THC substituted for the training dose.

CONCLUSION

These results suggest that the discriminative-stimulus effects of Delta(9)-THC in humans are not directly mediated through central neurotransmitter systems acted upon by the drugs tested in this study.

The role of human drug self-administration procedures in the development of medications

The purpose of this review is to illustrate the utility and value of employing human self-administration procedures in medication development, including abuse liability assessments of novel medications and evaluation of potential pharmacotherapies for substance use disorders. Traditionally, human abuse liability testing has relied primarily on subjective reports describing drug action by use of questionnaires; similarly, drug interactions between putative treatment agents and the drugs of abuse have relied on these measures. Subjective reports are highly valued because they provide qualitative and quantitative information about the characteristics of central and peripheral pharmacodynamic effects as well as safety and tolerability. However, self-administration procedures directly examine the behavior of interest-that is, drug taking. The present paper (1) reviews the most commonly used human self-administration procedures, (2) discusses the concordance of subjective reports and self-administration within the context of medications development for substance use disorders, focusing primarily on illustrative examples from development efforts with opioid and cocaine dependence, and (3) explores the utility of applying self-administration procedures to assess the abuse liability of novel compounds, including "abuse-deterrent" formulations (ADFs). The review will focus on opioid and cocaine dependence because a rich database from both clinical laboratory and clinical trial research exists for these two drug classes. The data reviewed suggest that drug-induced changes in self-administration and subjective effects are not always concordant. Therefore, assessment of self-administration in combination with subjective effects provides a more comprehensive picture that may have improved predictive validity for translating to the clinical setting.

Effects of acute tiagabine administration on aggressive responses of adult male parolees

Experimental and clinical studies have supported a relationship between gamma-aminobutyric acid (GABA) and aggressive behavior in non-humans and humans. Tiagabine is a GABA uptake inhibitor that has been shown to produce acute behavioral effects in animals. In addition, tiagabine has been shown to decrease aggression in agitated patients when administered chronically. The present study was designed to investigate the effects of acute administration of tiagabine on aggressive responding on a laboratory task in adult humans. Ten adult males participated in experimental sessions on the Point Subtraction Aggression Paradigm (PSAP), which provided subjects with aggressive, escape, and monetary-reinforced response options. All subjects received four acute oral doses of Tiagabine (4, 8, 12 and 16 mg) separated by placebo sessions. Tiagabine decreased aggression at doses that either did not affect, or affected to a lesser extent, monetary-reinforced responding. The results are consistent with some prior research using the PSAP showing a possible unique role for GABA in the regulation of human aggression. A possible behavioral mechanism for the rate-decreasing effects on aggressive responding produced in the present study is that tiagabine may modify aggressive responding by suppressing reactions to aversive stimuli.

Attenuation of cocaine-seeking by GABA B receptor agonists baclofen and CGP44532 but not the GABA reuptake inhibitor tiagabine in baboons

The current study evaluated the effects of drugs that increase GABA levels by activation of GABA(B) receptors (baclofen and CGP44532) or by inhibition of GABA reuptake (tiagabine) on the reinstatement of extinguished lever responding produced by priming doses of cocaine in baboons (i.e., cocaine-seeking). Cocaine self-injection was established and maintained under a fixed ratio (FR10) schedule of reinforcement during daily 2h sessions. Lever responding was extinguished by substituting vehicle (saline) for cocaine until the number of self-injections decreased to 10 or less per session for two consecutive sessions (defined as extinction). Once extinction occurred, priming doses of cocaine (0.1-3.2mg/kg, i.v.) were administered during extinction conditions. Administration of priming doses of cocaine significantly increased cocaine-seeking in a dose-dependent manner. Cocaine-seeking produced by priming doses of cocaine were attenuated by pretreatment with baclofen (N=5) or CGP44532 (N=5) but not tiagabine (N=3). The doses of baclofen (0.32 mg/kg), and CGP445532 (0.32 mg/kg) that reduced cocaine-seeking produced by cocaine priming doses did not reinstate cocaine-seeking and did not produce overt effects when administered alone. These data indicate that GABA(B) agonists may reduce relapse to cocaine taking.

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.

Diverse effects of GABA-mimetic drugs on cocaine-evoked self-administration and discriminative stimulus effects in rats

RATIONALE

Recent data indicate that gamma-aminobutyric acid (GABA) is a modulator of behavioral responses to cocaine.

OBJECTIVE

The efficacy of gabapentin (a cyclic GABA analogue), tiagabine (a GABA reuptake inhibitor), or vigabatrin (an inhibitor of GABA transaminase and reuptake) to alter cocaine-seeking behavior and discriminative effects was examined in rats.

MATERIALS AND METHODS

Rats were trained to press a lever for cocaine (0.5 mg/kg per infusion) paired with a cue (light + tone) using a fixed ratio (FR) 5 schedule of reinforcement. After extinction, the cocaine-seeking behavior was reinstated by cocaine priming (10 mg/kg). Another group of rats was trained to discriminate cocaine (10 mg/kg) from saline in a two-lever FR 20 task.

RESULTS

Vigabatrin (150-250 mg/kg) decreased cocaine-maintained responding, whereas tiagabine (10 mg/kg) significantly reduced responses on the "active" lever. Vigabatrin (150-250 mg/kg) significantly decreased responding to the cocaine-priming dose and a nonsignificant attenuation of cocaine-induced reinstatement was seen after tiagabine (5-10 mg/kg). Gabapentin (10-30 mg/kg) failed to alter maintenance of cocaine self-administration or drug-induced reinstatement. Pretreatment with either gabapentin, tiagabine, or vigabatrin resulted in neither reinstatement of cocaine seeking nor alterations in cocaine discrimination.

CONCLUSIONS

Our study demonstrates that vigabatrin (only at the 150 mg/kg dose) exerted inhibitory actions on cocaine-maintained responding and attenuated the reinstatement of extinguishing responding more effectively than gabapentin or tiagabine and with less evidence of motor impairment than the latter drugs. Present findings do not support a role for gabapentin or tiagabine for the possible treatment of cocaine relapse, whereas albeit limited effects of vigabatrin may be seen.

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.

Gabapentin does not reduce smoked cocaine self-administration: employment of a novel self-administration procedure

Previously, we reported that gabapentin, a nonselective gamma-aminobutyric acid agonist, reduced 'positive' subjective effects of cocaine without reducing cocaine self-administration. We speculated that the self-administration procedure used in that study was not sensitive to subtle shifts in the reinforcing effects of cocaine. Thus, this study examined the effects of gabapentin maintenance on cocaine self-administration using a purchase-cocaine choice procedure. During this 48-day inpatient/outpatient study, nontreatment-seeking cocaine abusers (n = 12) were maintained on gabapentin (0, 600, 1200 mg/day); four doses of cocaine (0, 12, 25, 50 mg) were each tested twice under each gabapentin condition. All cocaine testing was conducted while participants were inpatients. Before the start of each session, participants were provided with 25 dollars (five 5 dollar bills, one for each choice opportunity) and smoked the 'sample' cocaine dose once. Subsequently, participants were given five opportunities to purchase the sampled dose of cocaine (at 5 dollars per dose) or to keep 5 dollars for that choice trial. Choice to self-administer cocaine increased significantly with escalating cocaine doses; gabapentin maintenance did not alter choice to self-administer cocaine. These results concur with findings from our previous investigations of gabapentin and with those from a clinical trial examining the effects of larger gabapentin doses on cocaine use by treatment-seeking cocaine-dependent individuals. Together, the data indicate that gabapentin does not show promise as a treatment medication for cocaine dependence.

Various GABA-mimetic drugs differently affect cocaine-evoked hyperlocomotion and sensitization

To substantiate the notion that cocaine behavioral effects may be influenced by gamma-aminobutyric acid (GABA) neurotransmission male Wistar rats were injected with gabapentin (a cyclic GABA analogue), tiagabine (a GABA reuptake inhibitor), or vigabatrin (a GABA transaminase inhibitor) before acute or repeated treatment with cocaine evoking either locomotor hyperactivation or sensitization. Gabapentin (1-30 mg/kg), tiagabine (2.5-10 mg/kg) or vigabatrin (75-250 mg/kg) attenuated the cocaine (10 mg/kg)-induced hyperactivation and in the highest doses they also decreased basal locomotor activation. Vigabatrin (75-250 mg/kg) dose-dependently reduced the development of cocaine sensitization in rats treated repeatedly (days 1-5) with cocaine (10 mg/kg) and then challenged with cocaine (10 mg/kg) following 5-day withdrawal; the remaining drugs were ineffective. When injected acutely with a cocaine challenge dose, gabapentin (3-10 mg/kg) or vigabatrin (150 mg/kg), but not tiagabine, significantly attenuated the expression of cocaine sensitization. The present results show that enhanced GABA-ergic neurotransmission exerted inhibitory actions on acute responses to cocaine, however, only in a case of vigabatrin the inhibition seems to be unrelated to the inhibitory effect of the drugs on basal locomotor activity. The finding that vigabatrin protected against the development and the expression of cocaine sensitization further supports its therapeutic potential in the treatment of cocaine dependence.

Emerging pharmacological strategies in the fight against cocaine addiction

Cocaine addiction continues to be an important public health problem worldwide. At present, there are no proven pharmacotherapies for cocaine addiction. The studies reviewed here revealed a number of emerging targets for cocaine pharmacotherapy. First, disulfiram, a medication with dopaminergic effects, reduced cocaine use in a number of clinical trials. Second, GABA medications, tiagabine and topiramate, were found promising in clinical trials. Third, a beta-adrenergic blocker, propranolol, may be effective especially among cocaine-addicted individuals with high withdrawal severity. Fourth, treatment with a stimulant medication, modafinil, has reduced cocaine use. Last, a cocaine vaccine that slows entry of cocaine into the brain holds promise. These promising findings need to be further tested in controlled clinical trials.

Effects of GABAergic modulators on food and cocaine self-administration in baboons

Drugs that indirectly alter dopaminergic systems may alter the reinforcing effects of cocaine. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) has extensive neural connections in mesolimbic regions that appear to modulate dopamine. The current study evaluated the effects of GABA(B) receptor agonists baclofen and CGP44532, the benzodiazepine agonist alprazolam, and the GABA reuptake inhibitor tiagabine on lever responding maintained by low dose cocaine injections (0.032 mg/kg) or by food pellet (1 g) delivery in baboons. The benzodiazepine antagonist flumazenil was tested as a negative control. Cocaine or food was available under a fixed ratio (FR 10) schedule of reinforcement during daily 2-h sessions. During baseline conditions, cocaine and pellets maintained similar numbers of reinforcers per session. Baclofen, CGP44532 and tiagabine dose-dependently reduced the number of cocaine injections, where as the benzodiazepine antagonist flumazenil did not. Baclofen, CGP44532 and tiagabine also produced dose-related decreases in food-maintained behavior. In contrast, the benzodiazepine agonist alprazolam, which positively modulates GABA(A) receptors via the benzodiazepine site, produced decreases in cocaine self-injection, but not food-maintained behavior. Thus, the effects of alprazolam were specific for cocaine-maintained behavior, where as the effects of baclofen and CGP44532 were not.

Aripiprazole attenuates the discriminative-stimulus and subject-rated effects of D-amphetamine in humans

The results of animal research suggest that the use of partial agonists at dopamine (DA) D2 receptors may be an effective strategy for the treatment of stimulant dependence. Aripiprazole is an atypical antipsychotic that has partial agonist activity at D2 receptors. In this experiment, seven human participants with a history of nontherapeutic stimulant use learned to discriminate 15 mg oral D-amphetamine. After acquiring the discrimination (ie > or =80% correct responding on four consecutive sessions), the effects of a range of doses of D-amphetamine (0, 2.5, 5, 10, and 15 mg), alone and in combination with aripiprazole (0 and 20 mg), were assessed. D-Amphetamine alone functioned as a discriminative stimulus, produced prototypical subject-rated drug effects (eg increased ratings of Active, Alert, Energetic) and elevated cardiovascular indices. These effects were generally a function of dose. Aripiprazole alone did not occasion D-amphetamine-appropriate responding or produce subject-rated effects, but modestly impaired performance. Administration of aripiprazole significantly attenuated the discriminative-stimulus and cardiovascular effects of D-amphetamine, as well as some of the subject-rated drug effects. These data are consistent with previous preclinical findings and suggest that DA partial agonists deserve further evaluation as potential pharmacotherapies in the management of stimulant dependence. Future studies should investigate the ability of aripiprazole or related compounds to attenuate the behavioral effects of stimulants associated with a greater degree of dependence, such as methamphetamine or cocaine, in dependent individuals.

Tiagabine affects the subjective responses to cocaine in humans

In preclinical studies, medications which increase the synaptic GABA levels have been shown to block cocaine reinforcement. In this study, we examined the interaction between a GABA enhancing medication, tiagabine, and cocaine in cocaine users. A total of 7 subjects, 5 male and 2 female cocaine users had 2 experimental sessions. Before each session, subjects received either two oral doses of 4 mg of tiagabine or placebo. Starting 2 h after the second dose of medication treatment, subjects received an injection of saline followed by 2 escalating cocaine doses (0.15 and 0.3 mg/kg) intravenously. Tiagabine treatment did not affect the cocaine-induced blood pressure and heart rate changes. Tiagabine treatment attenuated the subjective ratings of "stimulated" and "crave cocaine" in response to cocaine administration. These results suggest that tiagabine treatment attenuates some of the subjective effects of cocaine without affecting its cardiovascular effects. GABA medications, including tiagabine, are currently being evaluated in controlled clinical trials for the treatment of cocaine dependence.

Novel pharmacotherapeutic targets for the management of drug addiction

Despite individual variation in the liability to the abuse of psychoactive substances, there is substantial commonality shared by drugs of abuse. The knowledge of these common mechanisms together with the continued elucidation of the neurobiological underpinnings of withdrawal symptoms, drug intake, craving, relapse, and co-morbid psychiatric associations are critically important for the development of new therapeutic strategies. The present review will focus on recent advances in the development of innovative pharmacotherapeutic agents, which should promote higher efficacy (abstinence, prevention of relapse, long-term recovery) and patient compliance, as well as improved safety profiles.

GABAergic modulation of the discriminative stimulus effects of methamphetamine

To assess whether gamma-aminobutyric acid (GABA) modulation of dopamine is important in mediation of the discriminative stimulus effects of methamphetamine, the GABA compounds chlordiazepoxide (benzodiazepine site agonist), pentobarbital (barbiturate site agonist), bicuculline and pentylenetetrazol (GABA(A) receptor antagonists) were tested in Sprague-Dawley rats trained to discriminate methamphetamine (1 mg/kg, i.p.) from saline. Each of the compounds produced modest amounts of methamphetamine-appropriate responding (20-35%) when tested alone. When tested in combination with methamphetamine, the antagonists (bicuculline and pentylenetetrazol) failed to shift the methamphetamine dose-effect curve. In contrast, chlordiazepoxide (25 mg/kg, i.p.) reduced methamphetamine-appropriate responding at each dose of methamphetamine tested, and pentobarbital (10 mg/kg, i.p.) dose-dependently decreased the discriminative stimulus effects of 1 mg/kg methamphetamine. In conclusion, GABA(A) antagonists and positive modulators likely do not produce methamphetamine-like stimulus effects. However, activation of GABA(A) receptors can interfere with the discriminative stimulus effects of methamphetamine.

Smoked cocaine discrimination in humans: effects of gabapentin

Gamma-aminobutyric acid (GABA) agonists, such as the GABA analogue, gabapentin, may provide new avenues for pharmacological treatment of cocaine dependence. The purpose of this study was to develop a smoked cocaine drug discrimination procedure in humans to test the effects of gabapentin maintenance on the discriminative stimulus, subjective, cognitive and cardiovascular effects of smoked cocaine. Eight male, nontreatment-seeking, cocaine-dependent volunteers, residing on an inpatient research unit for 47 days completed a within-subjects, counter-balanced design. Participants learned to discriminate between cocaine (25 mg) and placebo, and once the criterion for discrimination was met, smoked cocaine dose-effect functions (0, 6, 12, 25 and 50 mg) were determined under three gabapentin maintenance conditions (0, 600 and 1200 mg/day po). The highest dose of gabapentin tested (1200 mg/day) decreased the discriminative stimulus effects of cocaine (6 mg), decreased cocaine craving by 41-53% following cocaine administration (6 and 12 mg), and increased heart rate following either placebo or cocaine (12 mg) administration. Gabapentin did not significantly affect psychomotor task performance or the subjective effects of cocaine. Although the direction of gabapentin's effects was appropriate for a potential treatment medication, i.e., a decrease in cocaine-elicited craving and a decrease in cocaine's discriminative stimulus effects, these effects were limited to low doses of cocaine. The results suggest gabapentin may not produce effects sufficiently robust to be clinically useful, at least at this dose regimen.

Oxazepam does not modulate the behavioral effects of d-amphetamine in humans

Benzodiazepines, which are gamma-aminobutyric acid-A (GABA(A)) receptor positive modulators, can block the behavioral effects of psychomotor stimulants. In the present study, the ability of oxazepam, which may have less abuse potential compared to some other benzodiazepines, to attenuate the discriminative-stimulus, subject-rated and psychomotor performance effects of d-amphetamine in humans was determined. Six healthy participants (2 female, 4 male) learned to discriminate 15 mg oral d-amphetamine. After acquiring the discrimination (i.e., > or = 80% correct responding on 4 consecutive days), the effects of d-amphetamine (0, 2.5, 5, 10 and 15 mg), alone and in combination with acutely administered oxazepam (0 and 20 mg) were assessed. d-Amphetamine alone functioned as a discriminative stimulus, produced stimulant-like subject-rated effects (e.g., increased ratings of Stimulated on a Drug-Effect Questionnaire) and enhanced psychomotor performance. Oxazepam alone increased subject ratings of sedation (e.g., increased ratings of Sluggish, Fatigued and Lazy on a Drug-Effect Questionnaire) and impaired psychomotor performance. Oxazepam alone did not occasion d-amphetamine-like discriminative-stimulus effects, and had no effect on the discriminative-stimulus or subject-rated effects of d-amphetamine when given in combination. The results of this experiment are discordant with previous research and suggest that benzodiazepines differ in their ability to modulate the behavioral effects of d-amphetamine.

Effects of tiagabine in combination with intravenous nicotine in overnight abstinent smokers

RATIONALE

Preclinical studies suggest that medications enhancing the brain gamma amino butyric acid (GABA) system attenuate the rewarding effects of stimulants including nicotine. These preclinical studies have not been followed up in systematic human studies.

OBJECTIVES

This study was conducted to examine the effects of a GABAergic medication, tiagabine, on acute physiological and subjective effects of intravenous (i.v.) nicotine and on tobacco withdrawal symptoms in overnight abstinent smokers. The proposed mechanism of action for tiagabine is selective inhibition of GABA transporter type I, which leads to increases in synaptic GABA levels.

METHODS

Eight male and four female smokers participated in a double-blind, placebo-controlled, crossover study. In each of three experimental sessions, participants were treated orally with a single 4- or 8-mg dose of tiagabine or placebo. Two hours following the medication treatment, participants received i.v. saline, followed 30 min later by 1.5 mg/70 kg i.v. nicotine.

RESULTS

Tiagabine treatment did not affect the heart rate or blood pressure changes induced by nicotine. There was a significant treatment effect for the subjective responses to nicotine, such that tiagabine, compared to placebo, attenuated the ratings of "good effects" and "drug liking." Tiagabine treatment at 8 mg attenuated the craving for cigarettes and enhanced the cognitive performance in the Classical Stoop Tests, compared to placebo or 4 mg tiagabine condition.

CONCLUSIONS

These results suggest that GABA enhancing medication tiagabine may reduce the rewarding effects of nicotine and improve cognitive performance in abstinent smokers. The utility of GABA medications for smoking cessation needs to be examined further in controlled clinical trials.