Effect of GABA agonists and GABA-A receptor modulators on cocaine- and food-maintained responding and cocaine discrimination in rats

Ethanol blocks a novel form of iLTD, but not iLTP of inhibitory inputs to VTA GABA neurons

The ventral tegmental area (VTA) is an essential component of the mesocorticolimbic dopamine (DA) circuit that processes reward and motivated behaviors. The VTA contains DA neurons essential in this process, as well as GABAergic inhibitory cells that regulate DA cell activity. In response to drug exposure, synaptic connections of the VTA circuit can be rewired via synaptic plasticity-a phenomenon thought to be responsible for the pathology of drug dependence. While synaptic plasticity to VTA DA neurons as well as prefrontal cortex to nucleus accumbens GABA neurons are well studied, VTA GABA cell plasticity, specifically inhibitory inputs to VTA GABA neurons, is less understood. Therefore, we investigated the plasticity of these inhibitory inputs. Using whole cell electrophysiology in GAD67-GFP mice to identify GABA cells, we observed that these VTA GABA cells experience either inhibitory GABAergic long-term potentiation (iLTP) or inhibitory long-term depression (iLTD) in response to a 5 Hz stimulus. Paired pulse ratios, coefficient of variance, and failure rates suggest a presynaptic mechanism for both plasticity types, where iLTP is NMDA receptor-dependent and iLTD is GABA_B receptor-dependent-this being the first report of iLTD onto VTA GABA cells. As illicit drug exposure can alter VTA plasticity, we employed chronic intermittent exposure (CIE) to ethanol (EtOH) vapor in male and female mice to examine its potential impact on VTA GABA input plasticity. Chronic EtOH vapor exposure produced measurable behavioral changes illustrating dependence and concomitantly prevented previously observed iLTD, which continued in air-exposed controls, illustrating the impact of EtOH on VTA neurocircuitry and suggesting physiologic mechanisms at play in alcohol use disorder and withdrawal states. Taken together, these novel findings of unique GABAergic synapses exhibiting either iLTP or iLTD within the mesolimbic circuit, and EtOH blockade specifically of iLTD, characterize inhibitory VTA plasticity as a malleable, experience-dependent system modified by EtOH.

Diazepam attenuates the effects of cocaine on locomotion, 50-kHz ultrasonic vocalizations and phasic dopamine in the nucleus accumbens of rats

BACKGROUND AND PURPOSE

Currently, there is no effective drug to treat cocaine-use disorder, which affects millions of people worldwide. Benzodiazepines are potential therapeutic candidates, as microdialysis and voltammetry studies have shown that they can decrease dopamine concentrations in the nucleus accumbens of rodents and block the increase in dopamine levels and appetitive 50-kHz ultrasonic vocalizations (USVs) induced by amphetamine in rats.

EXPERIMENTAL APPROACH

Here, we tested whether administration of 2.5-mg·kg^-1 diazepam (i.p.) in adult male rats could block the effects of 20-mg·kg^-1 cocaine (i.p.) on electrically evoked phasic dopamine signals in the nucleus accumbens measured by fast-scan cyclic voltammetry, as well as 50-kHz USV and locomotor activity.

KEY RESULTS

Cocaine injection increased evoked dopamine signals up to threefold within 5 min, and the increase was significantly higher than baseline for at least 75 min. The injection of diazepam, 5 min after cocaine, attenuated the cocaine effect by nearly 50%, and this attenuation was maintained for at least 40 min. Behaviourally, cocaine increased the number of appetitive 50-kHz calls by about 12-fold. Diazepam significantly blocked this effect for the entire duration of the session. Also, cocaine-treated rats were more active than controls and diazepam significantly attenuated cocaine-induced locomotion, by up to 50%.

CONCLUSION AND IMPLICATIONS

These results suggest that the neurochemical and psychostimulant effects of cocaine can be mitigated by diazepam.

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.

Reinforcing effectiveness of midazolam, ethanol, and sucrose: behavioral economic comparison of a mixture relative to its component solutions

Benzodiazepines (BZs) are relatively safe when administered alone. However, these drugs can produce severe side effects when coadministered with ethanol. Despite these adverse consequences, rates of concurrent BZ and ethanol misuse are increasing, and it is unclear whether this behavior is maintained by an enhanced reinforcing effect of the mixture. To address this issue, the current study compared the reinforcing effectiveness of sucrose solutions mixed with midazolam, ethanol, or both. Eight male rats were trained to orally self-administer solutions of either sucrose (S), sucrose+midazolam (SM), sucrose+ethanol (SE), or sucrose+midazolam+ethanol (SME). The response requirement was increased between sessions until the number of reinforcers earned was zero and the relationship between response requirement and reinforcers earned was analyzed using the exponential model of demand. Although baseline intake was similar across drug conditions, consumption of SM was least affected by increases in cost, indicating that it possessed the highest reinforcing effectiveness (i.e. least elastic). The reinforcing effectiveness of S, SE, and SME did not differ significantly. The finding that the reinforcing effectiveness of the SME was less than that of SM does not support the supposition that BZ and ethanol coadministration is maintained by a higher reinforcing effectiveness of the mixture.

GABAA receptor positive allosteric modulators modify the abuse-related behavioral and neurochemical effects of methamphetamine in rhesus monkeys

GABA_A receptor positive allosteric modulators (GABA_A receptor modulators) are commonly used for the treatment of insomnia. Nevertheless, the effects of these compounds on psychostimulant-induced sleep impairment are poorly understood. Because GABA_A receptor modulators have been shown to decrease the abuse-related effects of psychostimulants, the aim of the present study was to evaluate the effects of temazepam (0.3, 1.0 or 3.0 mg/kg) and eszopiclone (0.3, 1.0 or 3.0 mg/kg), two GABA_A receptor modulators, on the behavioral neuropharmacology of methamphetamine in adult rhesus macaques (n = 5). Sleep-like measures and general daytime activity were evaluated with Actiwatch monitors. Methamphetamine self-administration (0.03 mg/kg/inf) was evaluated during morning sessions. Methamphetamine-induced dopamine overflow was assessed through in vivo microdialysis targeting the nucleus accumbens. Nighttime treatment with either temazepam or eszopiclone was ineffective in improving sleep-like measures disrupted by methamphetamine self-administration. Acute pretreatment with a low dose of temazepam before self-administration sessions increased methamphetamine self-administration without affecting normal daytime home-cage activity. At a high dose, acute temazepam pretreatment decreased methamphetamine self-administration and attenuated methamphetamine-induced increases in dopamine in the nucleus accumbens, without decreasing general daytime activity. Acute eszopiclone treatment exerted no effects on methamphetamine intake or drug-induced increases in dopamine. Our study suggests that treatments based on GABA_A receptor modulators are not effective for the treatment of sleep disruption in the context of psychostimulant use. In addition, distinct GABA_A receptor modulators differentially modulated the abuse-related effects of methamphetamine, with acute treatment with the high efficacy GABA_A receptor modulator temazepam decreasing the behavioral and neurochemical effects of methamphetamine.

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.

Separate and Combined Effects of Naltrexone and Extended-Release Alprazolam on the Reinforcing, Subject-Rated, and Cardiovascular Effects of Methamphetamine

Opioid antagonists (eg, naltrexone) and positive modulators of γ-aminobutyric acid type A receptors (eg, alprazolam) each modestly attenuate the abuse-related effects of stimulants. A previous study demonstrated that acute pretreatment with the combination of naltrexone and alprazolam attenuated a greater number of the subject-rated effects of D-amphetamine than the constituent drugs alone. This study tested the hypothesis that maintenance on the combination of naltrexone and alprazolam XR would attenuate the reinforcing and "positive" subject-rated effects of methamphetamine to a greater extent than the constituent drugs alone.Eight non-treatment-seeking, stimulant-using individuals completed a placebo-controlled, crossover, double-blind inpatient protocol. Participants were maintained on naltrexone (0 and 50 mg), alprazolam XR (0 and 1 mg), and the combination of naltrexone and alprazolam XR (50 mg and 1 mg, respectively) for 6 to 7 days. Under each maintenance condition, participants sampled intranasal doses of methamphetamine (0, 10, and 30 mg), and were then offered the opportunity to work for the sampled dose on a modified progressive-ratio procedure. Subject-rated drug effect questionnaires, psychomotor, and physiology assessments were collected.Intranasal methamphetamine functioned as a reinforcer and produced prototypical stimulant-like "positive" subject-rated and physiological effects. Maintenance on naltrexone significantly decreased the reinforcing, but not subject-rated drug effects of 10-mg methamphetamine. Alprazolam XR and the combination of naltrexone and alprazolam XR did not impact methamphetamine self-administration or subject-rated drug effects. The results support the continued evaluation of naltrexone for methamphetamine dependence, as well as the identification of other drugs that enhance its ability to reduce drug-taking behavior.

Effects of gaboxadol on the expression of cocaine sensitization in rats

Behavioral sensitization to psychostimulants is associated with changes in dopamine (DA), glutamate, and GABA within the mesocorticolimbic and nigrostriatal DA systems. Because GABAA receptors are highly expressed within these systems, we examined the role of these receptors containing a δ subunit in cocaine behavioral sensitization. Experiment 1 examined the effects of Gaboxadol (GBX, also known as THIP [4,5,6,7-tetrahydro-isoxazolo[5,4-c]pyridin-3-ol]), a selective δ-GABAA receptor agonist, on the locomotor responses to acute cocaine. GBX at 1.25 mg/kg produced locomotor depression in female rats alone. We then examined the effects of GBX on the expression of cocaine-induced locomotion and stereotypy in female and male rats treated with 5 days of cocaine (15 mg/kg) followed by cocaine challenge 7 days later. We administered systemic (Experiment 2) or intranucleus accumbens (intra-NAC; Experiment 3) injections of GBX (0, 1.25, 2.5, 5, or 10 mg/kg subcutaneously, or 1 μmol/L or 1 mM intra-NAC, respectively) prior to cocaine challenge (10 mg/kg). In our experiments females were robustly sensitized to cocaine at low dose whereas males did not show such sensitization-limiting comparisons between the 2 sexes. Sensitized females showed a biphasic response to low (1.25 mg/kg and 1 μmol/L) and high (10 mg/kg and 1 mM) dose GBX whereas nonsensitized males showed this pattern only following intra-NAC injection. Immunohistochemical analysis of the NAC revealed that females have more δ-containing GABAA receptors than do males and that following chronic cocaine injections this difference persisted (Experiment 4). Together, our results support the notion of the key role of extrasynaptic GABAA δ-subunit containing receptors in cocaine sensitization.

Topiramate impairs cognitive function in methadone-maintained individuals with concurrent cocaine dependence

Topiramate is being investigated as a potential pharmacotherapy for the treatment of addictive disorders. However, its cognitive side effects raise concerns about its use, especially in populations with cognitive impairment, such as persons with chronic substance use disorders. This study investigated topiramate's cognitive effects in individuals dually dependent on cocaine and opioids as part of a double-blind, randomized, controlled trial of topiramate for cocaine dependence treatment. After 5 weeks of stabilization on daily oral methadone (M = 96 mg), participants were randomized to topiramate (n = 18) or placebo (n = 22). Cognitive testing took place at 2 time points: study weeks 4 through 5 to assess baseline performance and 10 to 13 weeks later to assess performance during stable dosing (300 mg topiramate or placebo). All participants were maintained on methadone at both testing times, and testing occurred 2 hours after the daily methadone plus topiramate/placebo administration. The topiramate and placebo groups did not differ on sex, level of education, premorbid intelligence, methadone dose, or illicit drug use. Topiramate slowed psychomotor and information processing speed, worsened divided attention, reduced n-back working memory accuracy, and increased the false alarm rate in recognition memory. Topiramate had no effects on visual processing, other measures of psychomotor function, risk-taking, self-control, Sternberg working memory, free recall, and metamemory. These findings indicate that topiramate may cause cognitive impairment in this population. This effect may limit its acceptability and use as a treatment in individuals with chronic opioid and cocaine use disorders, among whom preexisting cognitive impairments are common. (PsycINFO Database Record

Effects of inhibitory GABA-active neurosteroids on cocaine seeking and cocaine taking in rats

RATIONALE

Several compounds that potentiate GABA-induced inhibitory currents also decrease stress, anxiety and addiction-related behaviors. Because of the well-established connection between stress and addiction, compounds that reduce stress-induced responses might be efficacious in treating addiction. Since endogenous neurosteroids such as allopregnanolone may function in a manner similar to benzodiazepines to reduce HPA axis activation and anxiety following stressful stimuli, we hypothesized that exogenously applied neurosteroids would reduce cocaine reinforcement in two animal models.

METHODS

Male Wistar rats were trained to self-administer cocaine and food under a concurrent alternating operant schedule of reinforcement. Two separate groups of rats were trained to self-administer cocaine or food pellets and were then exposed to similar cue-induced reinstatement paradigms. Both groups of rats were pretreated with various doses of neurosteroids.

RESULTS

Allopregnanolone and 3α-hydroxy-3β-methyl-17β-nitro-5α-androstane (R6305-7, a synthetic neurosteroid) were ineffective in selectively decreasing cocaine relative to food self-administration. On the other hand, both allopregnanolone and R6305-7 significantly decreased the cue-induced reinstatement of extinguished cocaine seeking, confirmed by one-way ANOVA.

CONCLUSIONS

These results suggest that neurosteroids may be effective in reducing the relapse to cocaine use without affecting ongoing cocaine self-administration.

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

RATIONALE

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

OBJECTIVES

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

METHODS

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A1 receptor agonist, N(6)-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D2 receptor agonist, quinpirole.

RESULTS

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

CONCLUSIONS

These findings demonstrate that adenosine A1 or A2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A1 receptor stimulation or presynaptic adenosine A2A receptor blockade during extinction produces lasting changes in relapse susceptibility.

Topiramate for cocaine dependence during methadone maintenance treatment: a randomized controlled trial

BACKGROUND

Dual dependence on opiate and cocaine occurs in about 60% of patients admitted to methadone maintenance and negatively impacts prognosis (Kosten et al. 2003. Drug Alcohol Depend. 70, 315). Topiramate (TOP) is an antiepileptic drug that may have utility in the treatment of cocaine dependence because it enhances the GABAergic system, antagonizes the glutamatergic system, and has been identified by NIDA as one of only a few medications providing a "positive signal" warranting further clinical investigation. (Vocci and Ling, 2005. Pharmacol. Ther. 108, 94).

METHOD

In this double-blind controlled clinical trial, cocaine dependent methadone maintenance patients (N=171) were randomly assigned to one of four groups. Under a factorial design, participants received either TOP or placebo, and monetary voucher incentives that were either contingent (CM) or non-contingent (Non-CM) on drug abstinence. TOP participants were inducted onto TOP over 7 weeks, stabilized for 8 weeks at 300 mg daily then tapered over 3 weeks. Voucher incentives were supplied for 12 weeks, starting during the fourth week of TOP induction. Primary outcome measures were cocaine abstinence (Y/N) as measured by thrice weekly urinalysis and analyzed using Generalized Estimating Equations (GEE) and treatment retention. All analyses were intent to treat and included the 12-week evaluation phase of combined TOP/P treatment and voucher intervention period.

RESULTS

There was no significant difference in cocaine abstinence between the TOP vs. P conditions nor between the CM vs. Non-CM conditions. There was no significant TOP/CM interaction. Retention was not significantly different between the groups.

CONCLUSION

Topiramate is not efficacious for increasing cocaine abstinence in methadone patients.

Separate and combined impact of acute naltrexone and alprazolam on subjective and physiological effects of oral d-amphetamine in stimulant users

RATIONALE

Opioid antagonists (e.g., naltrexone) and positive modulators of γ-aminobutyric-acidA (GABAA) receptors (e.g., alprazolam) modestly attenuate the abuse-related effects of stimulants like amphetamine. The use of higher doses to achieve greater efficacy is precluded by side effects. Combining naltrexone and alprazolam might safely maximize efficacy while avoiding the untoward effects of the constituent compounds.

OBJECTIVES

The present pilot study tested the hypothesis that acute pretreatment with the combination of naltrexone and alprazolam would not produce clinically problematic physiological effects or negative subjective effects and would reduce the positive subjective effects of d-amphetamine to a greater extent than the constituent drugs alone.

METHODS

Eight nontreatment-seeking, stimulant-using individuals completed an outpatient experiment in which oral d-amphetamine (0, 15, and 30 mg) was administered following acute pretreatment with naltrexone (0 and 50 mg) and alprazolam (0 and 0.5 mg). Subjective effects, psychomotor task performance, and physiological measures were collected.

RESULTS

Oral d-amphetamine produced prototypical physiological and stimulant-like positive subjective effects (e.g., VAS ratings of Active/Alert/Energetic, Good Effect, and High). Pretreatment with naltrexone, alprazolam, and their combination did not produce clinically problematic acute physiological effects or negative subjective effects. Naltrexone and alprazolam each significantly attenuated some of the subjective effects of d-amphetamine. The combination attenuated a greater number of subjective effects than the constituent drugs alone.

CONCLUSIONS

The present results support the continued evaluation of an opioid receptor antagonist combined with a GABAA-positive modulator using more clinically relevant experimental conditions like examining the effect of chronic dosing with these drugs on methamphetamine self-administration.

Role of GABA-active neurosteroids in the efficacy of metyrapone against cocaine addiction

Previous research has demonstrated a complicated role for stress and HPA axis activation in potentiating various cocaine-related behaviors in preclinical models of drug dependence. However, the investigation of several antiglucocorticoid therapies has yielded equivocal results in reducing cocaine-related behaviors, possibly because of varying mechanisms of actions. Specifically, research suggests that metyrapone (a corticosterone synthesis inhibitor) may reduce cocaine self-administration in rats via a nongenomic, extra-adrenal mechanism without altering plasma corticosterone. In the current experiments, male rats were trained to self-administer cocaine infusions and food pellets in a multiple, alternating schedule of reinforcement. Metyrapone pretreatment dose-dependently decreased cocaine self-administration as demonstrated previously. Pharmacological inhibition of neurosteroid production by finasteride had significant effects on cocaine self-administration, regardless of metyrapone pretreatment. However, metyrapone's effects on cocaine self-administration were significantly attenuated with bicuculline pretreatment, suggesting a role for GABA-active neurosteroids in cocaine-reinforced behaviors. In vitro binding data also confirmed that metyrapone does not selectively bind to GABA-related proteins. The results of these experiments support the hypothesis that metyrapone may increase neurosteroidogenesis to produce effects on cocaine-related behaviors.

The extra-adrenal effects of metyrapone and oxazepam on ongoing cocaine self-administration

Investigation of the role of stress in cocaine addiction has yielded an efficacious combination of metyrapone and oxazepam, hypothesized to decrease relapse to cocaine use by reducing stress-induced craving. However, recent data suggest an extra-adrenal role for metyrapone in mediating stress- and addiction-related behaviors. The interactions between the physiological stress response and cocaine self-administration were characterized in rodents utilizing surgical adrenalectomy and pharmacological treatment. Male Wistar rats were trained to self-administer cocaine (0.25mg/kg/infusion) and food pellets under a concurrent alternating fixed-ratio schedule of reinforcement. Surgical removal of the adrenal glands resulted in a significant decrease in plasma corticosterone and a consequent increase in ACTH, as expected. However, adrenalectomy did not significantly affect ongoing cocaine self-administration. Pretreatment with metyrapone, oxazepam and their combinations in intact rats resulted in a significant decrease in cocaine-reinforced responses. These same pharmacological treatments were still effective in reducing cocaine- and food-reinforced responding in adrenalectomized rats. The results of these experiments demonstrate that adrenally-derived steroids are not necessary to maintain cocaine-reinforced responding in cocaine-experienced rats. These results also demonstrate that metyrapone may produce effects outside of the adrenal gland, presumably in the central nervous system, to affect cocaine-related behaviors.

The combination of metyrapone and oxazepam for the treatment of cocaine and other drug addictions

Although scientists have been investigating the neurobiology of psychomotor stimulant reward for many decades, there is still no FDA-approved treatment for cocaine or methamphetamine abuse. Research in our laboratory has focused on the relationship between stress, the subsequent activation of the hypothalamic-pituitary-adrenal (HPA) axis, and psychomotor stimulant reinforcement for almost 30 years. This research has led to the development of a combination of low doses of the cortisol synthesis inhibitor, metyrapone, and the benzodiazepine, oxazepam, as a potential pharmacological treatment for cocaine and other substance use disorders. In fact, we have conducted a pilot clinical trial that demonstrated that this combination can reduce cocaine craving and cocaine use. Our initial hypothesis underlying this effect was that the combination of metyrapone and oxazepam reduced cocaine seeking and taking by decreasing activity within the HPA axis. Even so, doses of the metyrapone and oxazepam combination that consistently reduced cocaine taking and seeking did not reliably alter plasma corticosterone (or cortisol in the pilot clinical trial). Furthermore, subsequent research has demonstrated that this drug combination is effective in adrenalectomized rats, suggesting that these effects must be mediated above the level of the adrenal gland. Our evolving hypothesis is that the combination of metyrapone and oxazepam produces its effects by increasing the levels of neuroactive steroids, most notably tetrahydrodeoxycorticosterone, in the medial prefrontal cortex and amygdala. Additional research will be necessary to confirm this hypothesis and may lead to the development of improved and specific pharmacotherapies for the treatment of psychomotor stimulant use.

Deletion of the GABAA α2-subunit does not alter self administration of cocaine or reinstatement of cocaine seeking

RATIONALE

GABAA receptors containing α2-subunits are highly represented in brain areas that are involved in motivation and reward, and have been associated with addiction to several drugs, including cocaine. We have shown previously that a deletion of the α2-subunit results in an absence of sensitisation to cocaine.

OBJECTIVE

We investigated the reinforcing properties of cocaine in GABAA α2-subunit knockout (KO) mice using an intravenous self-administration procedure.

METHODS

α2-subunit wildtype (WT), heterozygous (HT) and KO mice were trained to lever press for a 30 % condensed milk solution. After implantation with a jugular catheter, mice were trained to lever press for cocaine (0.5 mg/kg/infusion) during ten daily sessions. Responding was extinguished and the mice tested for cue- and cocaine-primed reinstatement. Separate groups of mice were trained to respond for decreasing doses of cocaine (0.25, 0.125, 0.06 and 0.03 mg/kg).

RESULTS

No differences were found in acquisition of lever pressing for milk. All genotypes acquired self-administration of cocaine and did not differ in rates of self-administration, dose dependency or reinstatement. However, whilst WT and HT mice showed a dose-dependent increase in lever pressing during the cue presentation, KO mice did not.

CONCLUSIONS

Despite a reported absence of sensitisation, motivation to obtain cocaine remains unchanged in KO and HT mice. Reinstatement of cocaine seeking by cocaine and cocaine-paired cues is also unaffected. We postulate that whilst not directly involved in reward perception, the α2-subunit may be involved in modulating the "energising" aspect of cocaine's effects on reward-seeking.

Persistent gene expression changes in NAc, mPFC, and OFC associated with previous nicotine or amphetamine exposure

Highly addictive drugs like nicotine and amphetamine not only change an individual's behaviour in the short and long-term, they also induce persistent changes in neuronal excitability and morphology. Although research has started to examine the epigenetic changes that occur immediately after drug exposure, there has been little investigation into the persistent modifications to the epigenome that likely moderate the stable maintenance of the neurological changes. Male Long-Evans rats were administered amphetamine, nicotine, or saline for 14 consecutive days, given a 14 day withdrawal period, and then sacrificed. DNA from the mPFC, OFC, and nucleus accumbens (NAc) was used for global DNA methylation analysis and RNA from the same brain regions was used for gene expression analysis. Following the two-week withdrawal period, exposure to amphetamine or nicotine was associated with a decrease in global DNA methylation in each brain region examined. Previous exposure to nicotine was associated with changes in expression of 16 genes (NAc:6, mPFC:5, OFC:5) whereas exposure to amphetamine was associated with changes in expression of 25 genes (NAc:13, OFC:8, mPFC:4). The persistent epigenetic changes associated with exposure to amphetamine and nicotine were region and drug dependent, and differ from the latent epigenetic changes that occur immediately after drug exposure. The changes in DNA methylation are consistent with the gene expression results and provide further support to the notion that DNA methylation is the key regulatory mechanism for experience dependent changes.

Accumbal and pallidal dopamine, glutamate and GABA overflow during cocaine self-administration and its extinction in rats

We investigated the changes in dopamine (DA), glutamate and γ-aminobutyric acid (GABA) during cocaine self-administration in rats implanted with guide cannulae into the nucleus accumbens and ventral pallidum. After stabilized cocaine self-administration, separate groups of rats underwent extinction (10 days) procedure in which cocaine infusion was replaced by saline injections. With using a 'yoked' procedure, the effects of cocaine or its withdrawal on the level of neurotransmitters were evaluated by dual-probe microdialysis. Repeated cocaine administration reduced basal glutamate levels in the nucleus accumbens and ventral pallidum, whereas it did not affect basal accumbal DA levels. Only rats that self-administered cocaine had increased basal GABA overflow in both examined brain structures. Active or passive cocaine administration elevated extracellular accumbal DA, however, the extent of cocaine-evoked DA level was significantly higher in rats that self-administered cocaine while both groups of animals showed also an attenuation of GABA level in the nucleus accumbens. On day 10 of extinction training, rats previously given cocaine revealed decreases in the basal accumbal concentration of glutamate while the basal GABA levels were significantly enhanced as compared with baseline of saline-yoked controls. Potassium depolarization delayed the reduction of the accumbal and pallidal extracellular glutamate levels in the active and passive cocaine groups. The present data indicate that changes in DA and GABA neurotransmission during maintenance phase mirror the motivational aspects of cocaine intake. Depending on acute (24 hours) or late (10 days) cocaine withdrawal, different neurotransmitter systems (i.e. glutamate or GABA) seem to be involved.

Long-lasting effects of a PEGylated mutant cocaine esterase (CocE) on the reinforcing and discriminative stimulus effects of cocaine in rats

Recent mutagenesis studies have identified a mutant G4C/S10C/T172R/G173Q cocaine esterase (CCRQ CocE) with an in vitro duration of action of >40 days. Although the in vivo duration of CCRQ CocE's action was <24 h, modification of this enzyme with polyethylene glycol (PEG) polymers resulted in a CocE (PEG-CCRQ CocE) capable of preventing cocaine-induced lethality for up to 72 h. The current studies were aimed at providing a detailed characterization of the effectiveness, selectivity, and duration of PEG-CCRQ CocE's actions in cocaine self-administration and discrimination assays in rats. Pretreatment with PEG-CCRQ CocE produced dose-dependent rightward shifts in the dose-response curves for cocaine self-administration and discrimination, with the highest dose of PEG-CCRQ CocE capable of producing an initial shift of cocaine's reinforcing and interoceptive effects of >30-fold to the right, with significant inhibition of these effects observed for up to 72 h. Although PEG-CCRQ CocE also produced slight reductions in the rates of methylphenidate- and food-reinforced responding, these effects were short-lived, lasting <24 h. Finally, when taken together with the finding that PEG-CCRQ CocE failed to alter the cocaine-like interoceptive effects of either methylphenidate or d-amphetamine, these results suggest that PEG-CCRQ CocE possesses a high degree of pharmacologic specificity for cocaine and a prolonged in vivo duration of action. In conclusion, these studies provide strong evidence to support the further development of long-lasting, highly efficient CocEs, such as PEG-CCRQ CocE, as a potential therapeutic option for the treatment of cocaine abuse in humans.

Diazepam promotes choice of abstinence in cocaine self-administering rats

When facing a choice between cocaine and a potent, albeit inessential, non-drug alternative (i.e. water sweetened with saccharin), most cocaine self-administering rats abstain from cocaine in favor of the non-drug pursuit, regardless of the dose available and even after extended drug use. Only a minority continues to take the drug despite the opportunity of making a different choice and increasing stakes. This pattern of individual variation could suggest that the majority of rats are resilient to addiction, taking cocaine by default of other options. Only a minority would be vulnerable to addiction. This study tested the hypothesis that rats choose to refrain from cocaine self-administration because cocaine would be conflictual, having both rewarding and anxiogenic properties. Contrary to this hypothesis, however, we report here that diazepam-a broad-spectrum benzodiazepine anxiolytic-did not decrease, but instead, further increased cocaine abstinence. Interestingly, although diazepam decreased locomotion, rats adapted to this effect by spending more time near the lever associated with the preferred reward, a behavior that minimized the need for locomotion at the moment of choice. When responding for cocaine or saccharin was analyzed separately, we found that diazepam decreased responding for cocaine without affecting responding for saccharin. Finally, the abstinence-promoting effects of diazepam were also induced in cocaine-preferring rats treated chronically with diazepam. Overall, this study demonstrates that abstinence from cocaine cannot be explained away by the anxiogenic effects of cocaine, thereby reinforcing the notion of resilience to addiction. It also supports the use of benzodiazepines in the treatment of cocaine addiction.

Physiological and subjective effects of acute intranasal methamphetamine during extended-release alprazolam maintenance

BACKGROUND

Medications development for methamphetamine dependence is ongoing, but no widely accepted, effective pharmacotherapy has been identified. Previous studies have demonstrated neurobiological perturbations to central GABA(A) activity following chronic stimulant use, and that positive modulation of GABA(A) receptors attenuates the neurochemical and behavioral response to stimulant drugs such as methamphetamine. Therefore, GABA(A) modulators could be useful as pharmacotherapies for stimulant-use disorders.

METHODS

This study tested the hypothesis that intranasal methamphetamine would be safe and well tolerated during maintenance on extended-release alprazolam (XR), and that the effects of methamphetamine would be attenuated. Eight non-treatment-seeking, stimulant-dependent individuals completed an inpatient experiment in which ascending doses of intranasal methamphetamine (0, 5, 10, 20 and 30 mg) were administered after four days of alprazolam XR maintenance (0 and 1mg/day).

RESULTS

Intranasal methamphetamine produced prototypical effects (e.g., increased positive subjective ratings and elevated cardiovascular signs). The combination of intranasal methamphetamine and alprazolam XR was safe and well tolerated. Alprazolam XR produced small, but orderly, reductions in some of the subjective effects of methamphetamine, and performance impairment.

CONCLUSIONS

The present results demonstrate that methamphetamine use during alprazolam XR treatment would not pose a significant safety risk. Given the potential of GABA(A) positive modulators to manage certain aspects of stimulant abuse and dependence (i.e., drug-induced seizures, anxiety and stress), but the relatively small impact on the acute abuse-related effects of methamphetamine observed here, additional research with GABA(A) positive modulators is warranted, but should consider their use as an adjunct component of combination behavioral and/or drug treatment.

Gastrodia Elata Bl Attenuates Cocaine-Induced Conditioned Place Preference and Convulsion, but not Behavioral Sensitization in Mice: Importance of GABA(A) Receptors

It has been suggested that GABAergic neurotransmission can modulate cocaine dependence and seizure activity. Since Gastrodia elata Bl (GE), an oriental herb agent, has been shown to enhance GABAergic transmission, we examined whether GE affects cocaine-induced seizures, conditioned place preference (CPP), and behavioral sensitization in mice. Treatment with GE (500 or 1000 mg/kg, p.o.) significantly delayed seizure onset time and significantly shortened seizure duration induced by cocaine (90 mg/kg, i.p.). In addition, cocaine (15 mg/kg, i.p.)-induced CPP was significantly attenuated by GE in a dose-dependent manner. However, GE did not significantly alter behavioral sensitization induced by cocaine (15 mg/kg, i.p.). In order to understand whether GABAergic receptors are implicated in GE-mediated pharmacological action in response to cocaine, GABA_A receptor antagonist bicuculline and GABA_B receptor antagonist SCH 50911 were employed in the present study. GE-mediated attenuations on the cocaine-induced seizures and CPP were significantly reversed by bicuculline (0.25 or 0.5 mg/kg, i.p.), but not by SCH 50911 (1.5 or 3.0 mg/kg, i.p.). Therefore, our results suggest that GE attenuates cocaine-induced seizures and CPP via, at least in part, GABA_A receptor activation.

The discriminative stimulus effects of midazolam are resistant to modulation by morphine, amphetamine, dizocilpine, and γ-butyrolactone in rhesus monkeys

RATIONALE

Although abuse of benzodiazepines alone is uncommon, it is high in polydrug abusers, including those who primarily use opioids or stimulants.

OBJECTIVES

This study investigated whether drugs that are abused (e.g., amphetamine) or drugs that have mechanisms of action similar to abused drugs (e.g., morphine) alter the discriminative stimulus effects of the benzodiazepine midazolam.

METHODS

Three rhesus monkeys discriminated 0.56 mg/kg of midazolam while responding under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for midazolam alone and in the presence of morphine (opioid receptor agonist), amphetamine (dopamine receptor indirect agonist), dizocilpine (N-methyl-D: -aspartic acid receptor antagonist), or γ-butyrolactone (prodrug of γ-hydroxybutyrate, which acts primarily at GABA(B) receptors).

RESULTS

Doses of midazolam larger than 0.32 mg/kg produced ≥80% midazolam-lever responding. When administered alone, morphine, amphetamine, dizocilpine, and γ-butyrolactone did not produce midazolam-lever responding, although large doses of each drug eliminated responding; when administered in combination with midazolam, they did not alter the discriminative stimulus effects of midazolam up to doses that markedly decreased response rates.

CONCLUSIONS

The current study demonstrates a lack of modulation of the discriminative stimulus effects of midazolam by morphine, amphetamine, dizocilpine, and γ-butyrolactone. Other effects of benzodiazepines, such as their reinforcing effects, might be altered by these other drugs, or benzodiazepines might modulate the discriminative stimulus or reinforcing effects of the other drugs, which might contribute to the relatively high incidence of benzodiazepine abuse among polydrug abusers.

GABAB receptors in reward processes

gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the brain which acts through different receptor subtypes. Metabotropic GABA(B) receptors are widely distributed throughout the brain. Alterations in GABA signaling through pharmacological activation or deactivation of the GABA(B) receptor regulate behavior and brain reward processes. GABA(B) receptor agonists and, most recently, positive modulators have been found to inhibit the reinforcing effects of drugs of abuse, such as cocaine, amphetamine, nicotine, ethanol, and opiates. This converging evidence of the effects of GABA(B) compounds on the reinforcing properties of addictive drugs is based on behavioral studies that used a variety of procedures with relevance to reward processes and drug abuse liability, including intracranial self-stimulation, intravenous self-administration under both fixed- and progressive-ratio schedules of reinforcement, reinstatement, and conditioned place preference. GABA(B) receptor agonists and positive modulators block the reinforcing effects of drugs of abuse in these animal models. However, GABA(B) receptor agonists also have undesirable side-effects. GABA(B) receptor modulators have potential advantages as medications for drug addiction. These compounds have a better side-effect profile than GABA(B) agonists because they are devoid of intrinsic agonistic activity in the absence of GABA. They only exert their modulatory actions in concert with endogenous GABAergic activity. Thus, GABA(B) receptor positive modulators are promising therapeutics for the treatment of various aspects of dependence (e.g., initiation, maintenance, and relapse) on various drugs of abuse, such as cocaine, nicotine, heroin, and alcohol.

Design, synthesis, and subtype selectivity of 3,6-disubstituted β-carbolines at Bz/GABA(A)ergic receptors. SAR and studies directed toward agents for treatment of alcohol abuse

A series of 3,6-disubstituted β-carbolines was synthesized and evaluated for their in vitro affinities at α(x)β(3)γ(2) GABA(A)/benzodiazepine receptor subtypes by radioligand binding assays in search of α(1) subtype selective ligands to treat alcohol abuse. Analogues of β-carboline-3-carboxylate-t-butyl ester (βCCt, 1) were synthesized via a CDI-mediated process and the related 6-substituted β-carboline-3-carboxylates 6 including WYS8 (7) were synthesized via a Sonogashira or Stille coupling processes from 6-iodo-βCCt (5). The bivalent ligands of βCCt (32 and 33) were also designed and prepared via a palladium-catalyzed homocoupling process to expand the structure-activity relationships (SAR) to larger ligands. Based on the pharmacophore/receptor model, a preliminary SAR study on 34 analogues illustrated that large substituents at position-6 of the β-carbolines were well tolerated. As expected, these groups are proposed to project into the extracellular domain (L(Di) region) of GABA(A)/Bz receptors (see 32 and 33). Moreover, substituents located at position-3 of the β-carboline nucleus exhibited a conserved stereo interaction in lipophilic pocket L(1), while N(2) presumably underwent a hydrogen bonding interaction with H(1). Three novel β-carboline ligands (βCCt, 3PBC and WYS8), which preferentially bound to α1 BzR subtypes permitted a comparison of the pharmacological efficacies with a range of classical BzR antagonists (flumazenil, ZK93426) from several different structural groups and indicated these β-carbolines were 'near GABA neutral antagonists'. Based on the SAR, the most potent (in vitro) α(1) selective ligand was the 6-substituted acetylenyl βCCt (WYS8, 7). Earlier both βCCt and 3PBC had been shown to reduce alcohol self-administration in alcohol preferring (P) and high alcohol drinking (HAD) rats but had little or no effect on sucrose self-administration.(1-3) Moreover, these two β-carbolines were orally active, and in addition, were anxiolytic in P rats but were only weakly anxiolytic in rodents. These data prompted the synthesis of the β-carbolines presented here.

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.

Self-administration of cocaine-pentobarbital mixtures by rhesus monkeys

A number of experiments have evaluated self-administration of the combination of a stimulant and an opioid. Less is known about the combination of a stimulant and a CNS depressant. The present experiment was designed to examine self-administration of the mixture of cocaine and pentobarbital (PB). Rhesus monkeys (n=4) prepared with i.v. catheters were allowed to self-administer cocaine or saline under a progressive-ratio schedule. When responding was stable, doses of cocaine and PB, alone or in combination, were made available in test sessions. Cocaine functioned as a positive reinforcer in a dose-related manner in all monkeys. PB functioned as a relatively weaker reinforcer in one of four monkeys. Self-administration of intermediate doses of cocaine (0.025-0.1mg/kg per injection) was decreased when mixed with PB (0.05-0.2mg/kg per injection); full maximum responding was re-established when cocaine dose was increased. The magnitude of the shift to the right in the cocaine dose-response function was directly related to PB dose. When PB was given as an i.v. pretreatment there was no effect on cocaine self-administration up to a sedative dose of PB (5.6 mg/kg), suggesting that responding was not non-specifically suppressed by PB. Thus, simultaneous self-administration of PB diminished the potency but not the strength of cocaine as a reinforcer, potentially encouraging self-administration of larger doses of cocaine.

Effects of allopregnanolone on the reinstatement of cocaine-seeking behavior in male and female rats

RATIONALE

Previous research indicates that progesterone (PROG) decreased cocaine-seeking behavior in female rats. This effect of PROG may be in part due to its metabolite allopregnanolone (ALLO), which has been shown to decrease the sensitizing effects of cocaine and reduce lethality associated with cocaine overdose in mice.

OBJECTIVE

The purpose of the present study was to examine the effects of ALLO on the reinstatement of cocaine-seeking behavior in female and male rats.

METHODS

Rats were trained to lever press for i.v. infusions of cocaine (0.4 mg/kg per infusion) during 2-h sessions, and once acquisition criteria were met, cocaine self-administration continued for 14 days. Cocaine was then replaced with saline, and lever pressing was allowed to extinguish over 21 days. After the extinction phase, rats received s.c. ALLO (15 or 30 mg/kg), PROG (0.5 mg/kg), PROG (0.5 mg/kg) plus the 5-alpha reductase inhibitor finasteride (25 mg/kg), or vehicle pretreatment for 3 days. Rats were then tested during reinstatement with three doses of cocaine (5, 10, and 15 mg/kg, i.p. in mixed order).

RESULTS

PROG, and to a greater extent ALLO, decreased cocaine-primed reinstatement in females, while finasteride blocked the attenuating effects of PROG on reinstatement. ALLO had no effect on cocaine-primed reinstatement in males.

CONCLUSION

These findings suggest that ALLO may explain part of PROG's inhibitory effect on cocaine-primed reinstatement, and it may serve as a novel approach for preventing relapse in female cocaine abusers.

Gamma-vinyl GABA inhibits cocaine-triggered reinstatement of drug-seeking behavior in rats by a non-dopaminergic mechanism

Relapse to drug use is a core feature of addiction. Previous studies demonstrate that gamma-vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, attenuates the acute rewarding effects of cocaine and other addictive drugs. We here report that systemic administration of GVG (25-300 mg/kg) dose-dependently inhibits cocaine- or sucrose-induced reinstatement of reward-seeking behavior in rats. In vivo microdialysis data indicated that the same doses of GVG dose-dependently elevate extracellular GABA levels in the nucleus accumbens (NAc). However, GVG, when administered systemically or locally into the NAc, failed to inhibit either basal or cocaine-priming enhanced NAc dopamine in either naïve rats or cocaine extinction rats. These data suggest that: (1) GVG significantly inhibits cocaine- or sucrose-triggered reinstatement of reward-seeking behavior; and (2) a GABAergic-, but not dopaminergic-, dependent mechanism may underlie the antagonism by GVG of cocaine-triggered reinstatement of drug-seeking behavior, at least with respect to GVG's action on the NAc.

Effects of the combination of metyrapone and oxazepam on cocaine and food self-administration in rats

For several years, our laboratory has investigated the role for the HPA axis in cocaine reinforcement. Two classes of drugs that we have studied include corticosterone synthesis inhibitors (e.g., metyrapone) and benzodiazepine receptor agonists (e.g., oxazepam). In the experiments described in this manuscript, we tested the effects of various doses of metyrapone and oxazepam against several doses of self-administered cocaine. Behavioral, endocrine and pharmacokinetic measures of the effects of the combination of metyrapone and oxazepam on cocaine reward are presented. Combinations of metyrapone and oxazepam at doses that produced no observable effects when administered separately significantly reduced cocaine self-administration without affecting food-maintained responding during the same sessions. Changes in pharmacokinetics or endocrine function do not appear to mediate these effects, suggesting a central mechanism of action. Therefore, although these drugs produce their effects through distinct mechanisms, an additive effect on cocaine self-administration is obtained when these drugs are administered together, suggesting that combinations of low doses of metyrapone and oxazepam may be useful in reducing cocaine seeking with a reduced incidence of unwanted side effects and a decreased potential for abuse.

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 GABAB receptor antagonist, agonists and allosteric positive modulator on the cocaine-induced self-administration and drug discrimination

Preclinical and clinical findings indicate that a GABA(B) receptor agonist baclofen decreases cocaine use. The present study investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phoshinic acid (SKF 97541) and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) in cocaine-and food-maintained responding under a fixed ratio 5 schedule of reinforcement in male Wistar rats. The effects of the GABA(B) receptor ligands on cocaine (10 mg/kg)-induced discriminative stimulus in a two-lever, water-reinforced fixed ratio 20 task and on basal locomotor activity were also assessed. Baclofen (2.5-5 mg/kg), SKF 97541 (0.1-0.3 mg/kg) and CGP 7930 (30-100 mg/kg) decreased the cocaine (0.5 mg/kg/injection)-maintained responding; SCH 50911 (3-10 mg/kg) was inactive in this respect. Baclofen (5 mg/kg) and SKF 97541 (0.3 mg/kg), but not CGP 7930 or SCH 50911 attenuated the food-maintained responding. The inhibitory effects of the GABA(B) receptor agonists and the modulator were blocked by SCH 50911. SKF 97541 (0.1 mg/kg) or CGP 9730 (30-100 mg/kg) did not produce a significant shift in the cocaine (1.25-10 mg/kg) dose-response curve in a drug discrimination procedure, while baclofen (1.5 mg/kg) or SCH 50911 (10 mg/kg) attenuated the effects of separate doses of cocaine. Baclofen (5 mg/kg) and CGP 7930 (100 mg/kg) significantly reduced basal horizontal activity. We found that pharmacological stimulation of GABA(B) receptors by direct agonists or allosteric positive modulation reduces cocaine reinforcement while this property of cocaine is not related to tonic activation of GABA(B) receptors. The GABA(B) receptor stimulation-induced reduction of cocaine reinforcement was separated from its discriminative stimulus effects. Moreover, a dissociation between effects of direct GABA(B) receptor agonists and a GABA(B) allosteric positive modulator on cocaine vs. food-maintained responding was demonstrated.

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.

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.

Effects of pregnanolone in rats discriminating cocaine

The discriminative stimulus effects of cocaine are typically attributed to its ability to increase dopaminergic transmission, although drugs that have different mechanisms of action can substitute for cocaine and modulation of the GABA(A) receptor system has been reported to alter its discriminative effects. Therefore, a discrimination procedure was used to extend the characterization of cocaine's discriminative effects and to examine the interaction between cocaine and pregnanolone, a drug that can modulate the GABA(A) receptor complex. Rats (n=15) were trained to discriminate saline from 5.6 or 10 mg/kg of cocaine under a fixed-ratio (FR) 20 schedule of food presentation. The dopamine releaser d-amphetamine and two monoamine uptake inhibitors bupropion and desipramine substituted for cocaine. In contrast, the positive GABA(A) modulators pregnanolone and lorazepam and the opioid agonist morphine did not substitute for cocaine. When administered prior to cocaine, the D(2) receptor antagonist haloperidol and pregnanolone, but not lorazepam, produced a small rightward shift of the cocaine dose-effect curve. The results of the present studies suggest that the discriminative stimulus effects of cocaine are not solely mediated by increases in dopaminergic transmission and that positive modulation of GABA(A) receptors by pregnanolone can alter these effects, albeit at doses that also decrease overall response rate.

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.