Symmetrical generalization between the discriminative stimulus effects of gamma-hydroxybutyric acid and ethanol: occurrence within narrow dose ranges

Discriminative stimulus properties of α-ethyltryptamine (α-ET) in rats: α-ET-like effects of MDMA, MDA and aryl-monomethoxy substituted derivatives of α-ET

Rationale α-ET (α-ethyltryptamine), a homolog of the classical hallucinogen α-methyltryptamine, was once prescribed clinically as an antidepressant. Classical psychedelic drugs are currently of interest as potential pharmacotherapy for psychiatric disorders. Objectives Drug discrimination was used to (a) determine if α-ET-like stimulus effects could be engendered by the prototypical phenylalkylamines MDMA ("Ecstasy") or MDA ("Love Drug") and (b) evaluate the α-ET-like stimulus effects of four synthesized aryl-substituted monomethoxy analogs of α-ET (4-OMe-, 5-OMe-, 6-OMe- and 7-OMe-α-ET). Methods Rats were trained to discriminate α-ET (2.5 mg/kg) from saline using a two-lever operant task. Results The α-ET (ED50 = 1.04 mg/kg) stimulus generalized to MDMA (ED50 = 0.72 mg/kg) and MDA (ED50 = 0.48 mg/kg). The four α-ET derivatives produced various results; 4-OMe α-ET yielded negligible (20% maximum) α-ET-like responding; 5-OMe α-ET occasioned a modest level (40% maximum) of α-ET-like substitution; 6-OMe α-ET (ED50 = 6.26 mg/kg) generalized completely, but in a narrow dose range and in an inverted U-shaped manner; 7-OMe α-ET (ED50 = 2.78 mg/kg) generalized completely. Conclusions α-ET stimulus effects are similar to those of MDMA, but appear more closely aligned to those of MDA and are produced by its stereoisomers which, when combined, exert MDA/MDMA-, hallucinogen- and some stimulant-like stimulus actions. Thus, α-ET exerts a complex (compound) stimulus and appears to be a tryptamine counterpart of these prototypic phenylalkylamines. The monomethoxy analogs of α-ET produced an assortment of α-ET-like outcomes such that future investigations of these agents will likely need to be performed on an individual basis; extrapolations of α-ET-like effects to these analogs should be done judiciously.

GABAergic mechanisms in alcohol dependence

The target of alcohol's effect on the central nervous system has been sought for more than 50 years in the brain's GABA system. The behavioral and emotional effects of alcohol in humans and rodents are very similar to those of barbiturates and benzodiazepines, and GABAA receptors have been shown to be one of the sites of alcohol action. The mechanisms of GABAergic inhibition have been a hotspot of research but have turned out to be complex and controversial. Genetics support the involvement of some GABAA receptor subunits in the development of alcohol dependence and in alcohol use disorders (AUD). Since the effect of alcohol on the GABAA system resembles that of a GABAergic positive modulator, it may be possible to develop GABAergic drug treatments that could substitute for alcohol. The adaptation mechanisms of the GABA system and the plasticity of the brain are a big challenge for drug development: the drugs that act on GABAA receptors developed so far also may cause adaptation and development of additional addiction. Human polymorphisms should be studied further to get insight about how they affect receptor function, expression or other factors to make reasonable predictions/hypotheses about what non-addictive interventions would help in alcohol dependence and AUD.

Substitution therapy for patients with alcohol dependence: Mechanisms of action and efficacy

New approaches for the treatment of alcohol dependence (AD) may improve patient outcomes. Substitution maintenance therapy is one of the most effective treatment options for opioid and nicotine use disorders. So far, there has been little attention to substitution therapy for the treatment of AD. Here, we explain the mechanistic foundations of alcohol substitution maintenance therapy. Alcohol has many primary targets in the brain (and other organs) and the physical interaction of ethanol molecules with these specific ethanol-sensitive sites on a variety of ionotropic receptors (e.g. GABA-A, NMDA, and nicotinic acetylcholine (nACh) receptors) and ion channels provides the rationale for substitution. As such, a variety of compounds can interact with those ethanol-sensitive sites and can thus substitute for some of the effects of alcohol. For some of these compounds, alcohol discrimination studies have shown their substitution potential. Accordingly, potential substitution treatments include agonists acting at GABA receptors such as sodium oxybate, baclofen and benzodiazepines, NMDA receptor antagonists such as ketamine and memantine, or nAChRs agonists such as varenicline. All these compounds are already approved for other indications and we present clinical evidence for these drugs in the treatment of alcohol withdrawal syndrome (AWS) and in the long-term treatment of AD, and outline future steps for their acceptance as substitution treatment in AD. Finally, we discuss the substitution approach of managed alcohol programs for the most severely affected homeless populations. Results showed that sodium oxybate is probably the closest to a substitution therapy for AD and is already approved for the treatment of AWS and in the long-term treatment of AD in some countries. In conclusion, we argue that better AD treatment can be provided if substitution maintenance treatments for alcohol are implemented at a similar scale as for opioid and nicotine use disorder.

Treating alcohol dependence with an abuse and misuse deterrent formulation of sodium oxybate: Results of a randomised, double-blind, placebo-controlled study

Sodium oxybate (SMO) has been approved in Italy and Austria for the maintenance of abstinence in alcohol dependent (AD) patients. Although SMO is well tolerated in AD patients, cases of abuse and misuse have been reported outside the therapeutic setting. Here we report on a phase IIb double-blind, randomized, placebo-controlled trial for the maintenance of abstinence in AD patients with a new abuse and misuse deterrent formulation of SMO. A total of 509 AD patients were randomized to 12 weeks of placebo or one of four SMO doses (0.75, 1.25, 1.75 or 2.25 g t.i.d.) followed by a one-week medication-free period. The primary endpoint was the percentage of days abstinent (PDA) at end of treatment. An unexpectedly high placebo response (mean 73%, median 92%) was observed. This probably compromised the demonstration of efficacy in the PDA, but several secondary endpoints showed statistically significant improvements. A post-hoc subgroup analysis based on baseline severity showed no improvements in the mild group, but statistically significant improvements in the severe group: PDA: mean difference +15%, Cohen's d = 0.42; abstinence: risk difference +18%, risk ratio = 2.22. No safety concerns were reported. Although the primary endpoint was not significant in the overall population, several secondary endpoints were significant in the intent-to-treat population and post-hoc results showed that treatment with SMO was associated with a significant improvement in severe AD patients which is consistent with previous findings. New trials are warranted that take baseline severity into consideration.

Modulation of sensitivity to alcohol by cortical and thalamic brain regions

The nucleus accumbens core (AcbC) is a key brain region known to regulate the discriminative stimulus/interoceptive effects of alcohol. As such, the goal of the present work was to identify AcbC projection regions that may also modulate sensitivity to alcohol. Accordingly, AcbC afferent projections were identified in behaviorally naïve rats using a retrograde tracer which led to the focus on the medial prefrontal cortex (mPFC), insular cortex (IC) and rhomboid thalamic nucleus (Rh). Next, to examine the possible role of these brain regions in modulating sensitivity to alcohol, neuronal response to alcohol in rats trained to discriminate alcohol (1 g/kg, intragastric [IG]) vs. water was examined using a two-lever drug discrimination task. As such, rats were administered water or alcohol (1 g/kg, IG) and brain tissue was processed for c-Fos immunoreactivity (IR), a marker of neuronal activity. Alcohol decreased c-Fos IR in the mPFC, IC, Rh and AcbC. Lastly, site-specific pharmacological inactivation with muscimol + baclofen (GABA_A agonist + GABA_B agonist) was used to determine the functional role of the mPFC, IC and Rh in modulating the interoceptive effects of alcohol in rats trained to discriminate alcohol (1 g/kg, IG) vs. water. mPFC inactivation resulted in full substitution for the alcohol training dose, and IC and Rh inactivation produced partial alcohol-like effects, demonstrating the importance of these regions, with known projections to the AcbC, in modulating sensitivity to alcohol. Together, these data demonstrate a site of action of alcohol and the recruitment of cortical/thalamic regions in modulating sensitivity to the interoceptive effects of alcohol.

Pharmacodynamic interactions of a solid formulation of sodium oxybate and ethanol in healthy volunteers

AIM

The pharmacologic effects of sodium oxybate (SO) have a number of similarities with those of alcohol. This study evaluated the pharmacodynamic interaction of SMO.IR (a solid immediate release formulation of SO) and alcohol (0.7 (males) or 0.57 (females) g kg(-1) alcohol using 40% vodka).

METHODS

In a randomized, double-blind, double-dummy, crossover trial, 24 healthy volunteers received randomly a) 2.25 g SMO.IR and placebo alcohol preparation, b) 2.25 g f SMO.IR and alcohol, c) 2.25 g SMO.IR matching placebo and alcohol and d) 2.25 g of SMO.IR matching placebo and placebo alcohol preparation. Objective and subjective cognitive parameters, adverse events and vital signs were assessed before, 15 and 165 min after treatment administration.

RESULTS

Alcohol produced the expected cognitive impairment and the expected subjective sedation rapidly after intake (from 15 min). The objective effects of SMO.IR were much less pronounced than those of alcohol. The reverse was observed for subjective complaints, which were related to lesser stimulation and greater sedation. Nevertheless, 165 min after administration this sedation feeling was less with SMO.IR than with alcohol. There was a significant interaction between SMO.IR and alcohol at 15 min (i.e. increase in alertness and stimulation and decrease in sedation). In addition, an isolated mild decrease in digit vigilance accuracy occurred at 165 min post-dose after the combination. The co-administration of SMO.IR and alcohol was safe and well-tolerated.

CONCLUSION

SMO.IR and alcohol have distinct adverse effect profiles. The objective effects of SMO.IR are much less marked than those of alcohol. No deleterious interaction was observed.

Preference for gamma-hydroxybutyrate (GHB) in current users

Gamma-hydroxybutyrate (GHB) is a drug with significant abuse potential. The present study aimed to assess the relative value of escalating doses of GHB to current GHB users via the Multiple Choice Procedure (MCP), and to validate that the dose rated highest with the MCP would be self-administered at a greater rate than placebo. Participants were 5 current GHB users who were not currently trying to stop using GHB. To examine the value of escalating doses of GHB, the following doses of GHB were used: 0 (placebo), 12.5, 25, 37.5, and 50 mg/kg. Participants typically assigned higher doses of GHB had higher crossover points on the MCP. During choice sessions, participants made repeated choices between administering GHB, placebo or nothing. All participants selected GHB exclusively (5 out of 5 instances) except for one participant who selected GHB on 4 out of 5 instances, thus 96% (i.e., 24/25) of choices were for active GHB. Based on these data, GHB appears likely to function as a dose-dependent reinforcer for humans based on our sample.

Efficacy and safety of pharmacological interventions for the treatment of the Alcohol Withdrawal Syndrome

BACKGROUND

Alcohol abuse and dependence represents a very serious health problem worldwide with major social, interpersonal and legal interpolations. Pharmacological treatments presently used are of uncertain effectiveness and there is even more doubt on the comparative effects and value for money.

OBJECTIVES

To summarize Cochrane reviews that assess the effectiveness and safety of pharmacological interventions in the treatment of alcohol withdrawal.

METHODS

We searched the Cochrane Database of Systematic Reviews (30 November 2010). Two authors independently screened, extracted data, summarised key characteristics of the included reviews and assessed their quality using AMSTAR; the quality of the evidence was summarised according to the GRADE methodology.

MAIN RESULTS

Five reviews, 114 studies, 7333 participants, satisfied criteria for inclusions. The outcomes considered were alcohol withdrawal seizures, adverse events and dropouts. Comparing the five treatments with placebo, benzodiazepines performed better for seizures, three studies, 324 participants, RR 0.16 (95% CI 0.04 to 0.69), moderate quality of evidence. Comparing each of the five treatments versus specific class of drugs, benzodiazepines performed better than antipsychotics for seizures, 4 studies, 633 participants, RR 0.24 (95% CI 0.07 to 0.88) high quality of the evidence. Comparing different benzodiazepines and anticonvulsants among themselves, 28 comparisons, results never reached statistical significance but chlordiazepoxide performed better. The quality of evidence was high for 3% of the results, moderate for 28%, low for 48% and very low for 20%.

AUTHORS' CONCLUSIONS

Among the treatments considered, benzodiazepines showed a protective benefit against seizures, when compared to placebo and a potentially protective benefit for many outcomes when compared with antipsychotics. Nevertheless, no definite conclusions about the effectiveness and safety of benzodiazepines were possible, because of the heterogeneity of the trials both in interventions and in the assessment of outcomes. Data on potential harms are sparse and fragmented. Results do not provide sufficient evidence in favour of anticonvulsants for the treatment of AWS, but anticonvulsants seem to have limited side effects. There is also not enough evidence of effectiveness and safety of baclofen, because only one study consider this treatment and of GHB for which no strong differences were observed in the comparisons with placebo, benzodiazepines and anticonvulsants.

Gamma-hydroxybutyrate (GHB) for treatment of alcohol withdrawal and prevention of relapses

BACKGROUND

Chronic excessive alcohol consumption may lead to dependence, and to alcohol withdrawal syndrome (AWS) in case of abrupt drinking cessation. Gamma-hydroxybutyric acid (GHB) can prevent and suppress withdrawal symptoms, and improve the medium-term abstinence rate. A clear balance between effectiveness and harmfulness has not been yet established.

OBJECTIVES

To evaluate the efficacy and safety of GHB for treatment of AWS and prevention of relapse

SEARCH STRATEGY

We searched Cochrane Drugs and Alcohol Group' Register of Trials (October 2008), PubMed, EMBASE, CINAHL (January 2005 - October 2008), EconLIT (1969 to February 2008), reference list of retrieved articles

SELECTION CRITERIA

Randomized controlled trials (RCTs) and Controlled Prospective Studies (CPS) evaluating the efficacy and the safety of GHB vs placebo or other pharmacological treatments.

DATA COLLECTION AND ANALYSIS

Three authors independently extracted data and assessed the methodological quality of studies.

MAIN RESULTS

Thirteen RCTs were included. Eleven studies were conducted in Italy.For withdrawal syndrome, comparing GHB 50mg with placebo, results from 1 study, 23 participants favour GHB for withdrawal symptoms: WMD -12.1 (95% CI, -15.9 to -8.29) and side effects were more frequent in the placebo group: RR 16.2 (95% CI, 1.04 to 254.9).In the comparison with Chlormetiazole, for GHB 50mg, results from 1 study, 21 participants favour GHB for withdrawal symptoms: MD -3.40 (95% CI -5.09 to -1.71), for GHB 100mg, results from 1 study, 98 participants favour anticonvulsants for side effects: RR 1.84 (95% CI 1.19 to 2.85).At mid-term, comparing GHB with placebo, results favour GHB for abstinence rate (RR 5.35; 1.28-22.4), controlled drinking (RR 2.13; 1.07-5.54), relapses (RR 0.36; 0.21-0.63), and number of daily drinks (WMD -4.60; -6.18 to -3.02). GHB performed better than NTX and Disulfiram on abstinence (RR 2.59; 1.35-4.98, RR 1.66; 0.99-2.80 respectively). The association of GHB and NTX was better than NTX on abstinence (RR 12.2; 1.79-83.9), as well was the association of NTX, GHB and Escitalopram versus Escitalopram alone (RR 4.58; 1.28-16.5). For Alcohol Craving Scale results favour GHB versus placebo (WMD -1.90; -2.45 to 1.35) and Disulfiram (WMD -1.40; -1.86 to-0.94).

AUTHORS' CONCLUSIONS

GHB 50mg is effective compared to placebo in the treatment of AWS, and in preventing relapses in previously detoxified alcoholics at 3 months follow-up, but the results of this review do not provide sufficient evidence in favour of GHB compared to benzodiazepines and Chlormethiazole for AWS prevention. GHB is better than NTX and Disulfiram in maintaining abstinence and it has a better effect on craving than placebo and Disulfiram. Side effects of GHB are not statistically different from those with BZD, NTX or Disulfiram. However, concern has been raised regarding the risk of developing addiction, misuse or abuse, especially in polydrug abusers.

Behavioral analyses of GHB: receptor mechanisms

GHB is used therapeutically and recreationally, although the precise mechanism of action responsible for its different behavioral effects is not entirely clear. The purpose of this review is to summarize how behavioral procedures, especially drug discrimination procedures, have been used to study the mechanism of action of GHB. More specifically, we will review several different drug discrimination procedures and discuss how they have been used to qualitatively and quantitatively study different components of the complex mechanism of action of GHB. A growing number of studies have provided evidence that the behavioral effects of GHB are mediated predominantly by GABAB receptors. However, there is also evidence that the mechanisms mediating the effects of GHB and the prototypical GABAB receptor agonist baclofen are not identical, and that other mechanisms such as GHB receptors and subtypes of GABAA and GABAB receptors might contribute to the effects of GHB. These findings are consistent with the different behavioral profile, abuse liability, and therapeutic indications of GHB and baclofen. A better understanding of the similarities and differences between GHB and baclofen, as well as the pharmacological mechanisms of action underlying the recreational and therapeutic effects of GHB, could lead to more effective medications with fewer adverse effects.

Gamma-hydroxybutyric acid in male and female cynomolgus monkeys trained to discriminate 1.0 or 2.0 g/kg ethanol

Gamma-hydroxybutyric acid has been proposed as a pharmacotherapy for alcoholism in part based on similar discriminative stimulus effects as ethanol. To date, drug discrimination studies with gamma-hydroxybutyric acid and ethanol have exclusively used rodents or pigeons as subjects. To evaluate possible differences between species, sex, and route of administration, this study investigated the substitution of gamma-hydroxybutyric acid (intragastrically or intramuscularly) for ethanol 30 or 60 min after administration in male (n=6) and female (n=7) cynomolgus monkeys trained to discriminate 1.0 and 2.0 g/kg ethanol. At least one dose of gamma-hydroxybutyric acid completely or partially substituted for ethanol in three of the 13 monkeys tested, with each case occurring in female monkeys. Ethanol-appropriate responding did not increase with gamma-hydroxybutyric acid dose. Monkeys were more sensitive to the response rate decreasing effects of gamma-hydroxybutyric acid administered intramuscularly compared with intragastrically. The lack of gamma-hydroxybutyric acid substitution for ethanol suggests that these drugs have different receptor bases for discrimination. Furthermore, the data do not strongly support shared discriminative stimulus effects as the rationale for gamma-hydroxybutyric acid pharmacotherapy for alcoholism.

Differentiating the discriminative stimulus effects of gamma-hydroxybutyrate and ethanol in a three-choice drug discrimination procedure in rats

Anecdotal reports indicate that GHB produces subjective effects similar to those of ethanol. However, recent investigations comparing the discriminative stimulus effects of GHB to those of ethanol suggest that the subjective effects of these substances may differ considerably. To explore further potential differences between GHB and ethanol, 16 male Sprague-Dawley rats were trained in a three-lever drug discrimination procedure to discriminate ethanol (1.0 g/kg, experiment 1; 1.5 g/kg, experiment 2) and GHB (300 mg/kg) from vehicle. Dose-response functions determined with both training compounds revealed a clear dissociation between the discriminative stimulus effects of these drugs. As expected, the GHB precursors gamma-butyrolactone and 1,4-butanediol produced full substitution for GHB. In addition, the GABA(B) receptor agonist baclofen substituted for GHB, whereas the benzodiazepine flunitrazepam and the NMDA receptor antagonist ketamine engendered greater responding on the ethanol-lever. GHB's discriminative stimulus effects were blocked by the GABA(B) receptor antagonist CGP-35348 but only partially blocked by the putative GHB receptor antagonist NCS 382. These findings are consistent with previous reports of GHB's discriminative stimulus effects in two-choice drug discrimination procedures and provide additional evidence that these effects are distinct from those of ethanol.

Anti-aggressive effects of GHB in OF.1 strain mice: involvement of dopamine D2 receptors

Numerous studies indicate that gamma-hydroxybutyric acid (GHB) influences the endogenous dopamine system. Both GHB and most dopaminergic D(2) receptor antagonists are effective anti-aggressive agents in animal models. The present study aimed to investigate the effects of GHB on agonistic behaviour and to implicate D(2) dopamine receptor on these behaviours. For this purpose, the effects of GHB (80, 120 and 160 mg/kg, IP) and tiapride (60 mg/kg) administered alone or in combination were examined on agonistic behaviour elicited by 'isolation' in male mice. Individually housed mice were exposed to anosmic "standard opponents" 30 min after drug administration, and the encounters were videotaped and evaluated using an ethologically based analysis. The administration of 80 and 120 mg/kg of GHB reduced threat without impairing motor activity, but the administration of 160 mg/kg of GHB or the co-administration of GHB+tiapride (a selective D(2) receptor antagonist) significantly reduced threat and attack but concomitantly increased immobility. The co-administration of GHB+tiapride had different effects to those observed by the administration of these drugs separately. It is concluded that the anti-aggressive effect of GHB appears to be mediated, at least in part, by D(2) dopamine receptors. This anti-dopaminergic activity is an indirect effect, probably induced by the activation of GHB receptors of low affinity, and in this way, this compound would reduce levels of dopamine without blockading of D(2) postsynaptic dopamine receptors.

Effects of combining ethanol (EtOH) with gamma-hydroxybutyrate (GHB) on the discriminative stimulus, locomotor, and motor-impairing functions of GHB in mice

RATIONALE

Gamma-hydroxybutyrate (GHB) is a drug of abuse that is often coabused with ethanol (EtOH) and has been implicated as a date rape agent in conjunction with EtOH. Much information is lacking regarding the manner in which GHB interacts with EtOH.

OBJECTIVE

This study was designed to further characterize the behavioral effects of GHB alone and in combination with EtOH in male Swiss-Webster mice.

METHODS

The effects of GHB (0.1-1.0 g/kg) and EtOH (2.0-5.0 g/kg) alone, as well as the effects of GHB in combination with EtOH, were examined using an automated locomotor activity procedure, a functional observational battery (FOB) and a GHB drug discrimination procedure.

RESULTS

GHB decreased, whereas EtOH had little effect on locomotor activity. In the FOB, EtOH dose-dependently decreased activity in combination with 0.3 g/kg GHB. Alone, each drug had little effect on the righting reflex, but combining ineffective doses of GHB and EtOH significantly impaired righting. GHB and EtOH decreased forelimb grip strength. Combinations of ineffective doses of GHB and EtOH decreased forelimb grip strength when given together. GHB and EtOH impaired inverted screen performance, and EtOH increased the impairing effects of low, but not high, doses of GHB. GHB and EtOH increased hind limb splay, and EtOH increased the effects of 0.1 and 0.3 g/kg GHB on splay. GHB and EtOH decreased body temperature, and EtOH augmented the temperature-decreasing effects of GHB. EtOH produced less than 50% GHB-like discriminative stimulus effects, and GHB failed to alter the GHB-like discriminative stimulus effects of EtOH.

CONCLUSIONS

Overall, EtOH increased the effects of GHB on several gross measures of behavior and only partially occasioned the discriminative stimulus properties of GHB.

Discriminative stimulus effects of gamma-hydroxybutyrate: role of training dose

gamma-Hydroxybutyrate (GHB) is a drug of abuse with actions at GHB and GABA receptors. This study examined whether the relative importance of GABA(A), GABA(B), and GHB receptors in the discriminative stimulus effects of GHB depends on the training dose. In comparison with a previous 100 mg/kg GHB-saline discrimination, pigeons were trained to discriminate either 178 or 56 mg/kg GHB from saline. Increasing the training dose shifted the GHB gradient to the right, and decreasing it shifted the gradient to the left. Similar shifts occurred with the GHB precursor gamma-butyrolactone, which substituted for GHB, and with the GABA(B) agonists baclofen and 3-aminopropyl(methyl)phosphinic acid hydrochloride (SKF97541) and the benzodiazepine diazepam, each of which produced at most 54 to 68% GHB-appropriate responding. The benzodiazepine antagonist flumazenil, the benzodiazepine inverse agonist ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-alpha]-[1,4]-benzodiazepine-3-carboxylate (Ro 15-4513), and the GHB receptor antagonist (2E)-5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid (NCS-382) produced a maximum of 66 to 97% GHB-appropriate responding in animals discriminating 56 or 100 mg/kg GHB and a maximum of 1 to 49% in animals discriminating 178 mg/kg. NCS-382 did not attenuate the effects of GHB. The GABA(B) antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348) blocked GHB at all training doses. The results suggest that increasing the training dose of GHB increases the pharmacological selectivity of its discriminative stimulus effects. At a high training dose, diazepam-insensitive GABA(A) receptors, for which flumazenil and Ro 15-4513 have affinity, may no longer be involved. Diazepam-sensitive GABA(A) receptors and GABA(B) receptors appear to play a similar role at all training doses. There was no evidence for GHB receptor involvement.

Discriminative stimulus effects of gamma-hydroxybutyrate (GHB) and its metabolic precursor, gamma-butyrolactone (GBL) in rats

RATIONALE

Gamma-hydroxybutyrate (GHB) is becoming an increasingly popular drug of abuse. Metabolic precursors of GHB, gamma-butyrolactone (GBL) and 1,4-butanediol (BDL), are commercially available industrial solvents that may also present potential health risks. Relatively little is known about the neurobehavioral effects of GHB and its precursors.

OBJECTIVE

The aim of the present investigation was to characterize the discriminative stimulus effects of GHB and its precursor, GBL.

METHODS

Male Sprague-Dawley rats were trained to discriminate GHB [300 mg/kg, i.g.; n=16] or GBL (150 mg/kg, i.p.; n=8) from vehicle under a fixed ratio 20 (FR 20) schedule of food reinforcement. Stimulus generalization tests were then conducted with several compounds.

RESULTS

GHB and GBL produced cross-generalization and BDL was fully substituted for both GHB and GBL. Two benzodiazepines, alprazolam and diazepam, and the 5-HT1A agonist, buspirone, did not substitute for either training drug nor did ethanol or the NMDA antagonists, PCP and ketamine. The GHB antagonist, NCS-382, and the GABA(B) antagonist, CGP-35348, blocked the discriminative stimulus effects of GHB but not those of GBL.

CONCLUSIONS

These findings suggest that GHB and its metabolic precursors produce similar subjective effects that differ from those of other sedative-hypnotic drugs. Further investigations into the neurochemical actions underlying the subjective effects of these drugs are warranted.

Gamma-hydroxybutyric acid (GHB) and the mesoaccumbens reward circuit: evidence for GABA(B) receptor-mediated effects

Gamma-hydroxybutyric acid (GHB) is a short-chain fatty acid naturally occurring in the mammalian brain, which recently emerged as a major recreational drug of abuse. GHB has multiple neuronal mechanisms including activation of both the GABA(B) receptor, and a distinct GHB-specific receptor. This complex GHB-GABA(B) receptor interaction is probably responsible for the multifaceted pharmacological, behavioral and toxicological profile of GHB. Drugs of abuse exert remarkably similar effects upon reward-related circuits, in particular the mesolimbic dopaminergic system and the nucleus accumbens (NAc). We used single unit recordings in vivo from urethane-anesthetized rats to characterize the effects of GHB on evoked firing in NAc "shell" neurons and on spontaneous activity of antidromically identified dopamine (DA) cells located in the ventral tegmental area. GHB was studied in comparison with the GABA(B) receptor agonist baclofen and antagonist (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH50911). Additionally, we utilized a GHB analog, gamma-(p-methoxybenzil)-gamma-hydroxybutyric acid (NCS-435), devoid of GABA(B) binding properties, but with high affinity for specific GHB binding sites. In common with other drugs of abuse, GHB depressed firing in NAc neurons evoked by the stimulation of the basolateral amygdala. On DA neurons, GHB exerted heterogeneous effects, which were correlated to the baseline firing rate of the cells but led to a moderate stimulation of the DA system. All GHB actions were mediated by GABA(B) receptors, since they were blocked by SCH50911 and were not mimicked by NCS-435. Our study indicates that the electrophysiological profile of GHB is close to typical drugs of abuse: both inhibition of NAc neurons and moderate to strong stimulation of DA transmission are distinctive features of diverse classes of abused drugs. Moreover, it is concluded that addictive and rewarding properties of GHB do not necessarily involve a putative high affinity GHB receptor.

Discriminative stimulus effects of gamma-hydroxybutyrate (GHB) in rats discriminating GHB from baclofen and diazepam

Gamma-hydroxybutyrate (GHB) is a drug of abuse with actions at GHB and GABA receptors. This study tried to increase the selectivity of the discriminative stimulus effects of GHB by training animals to discriminate GHB from compounds that share pharmacological mechanisms with GHB. In comparison with a previous GHB versus saline discrimination (group 1), rats were trained to discriminate GHB (200 mg/kg) either from saline and the GABA(B) agonist baclofen (3.2 mg/kg) (group 2) or from saline, baclofen, and the positive GABA(A) modulator diazepam (1 mg/kg) (group 3). In all groups, GHB produced more than 80% GHB-appropriate responding. Baclofen produced 84% GHB-appropriate responding in group 1 but less than 30% in groups 2 and 3. Diazepam produced 68% GHB-appropriate responding in group 1, 30% in group 2, and only 5% in group 3. The GABA(B) receptor antagonists CGP35348 [3-[aminopropyl(diethoxymethyl)phosphinic acid] and CGP52432 [3-[[[((3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl)phosphinic acid] attenuated the discriminative stimulus effects of GHB; CGP35348 did so with similar potency in all groups, but CGP52432 was significantly less potent in groups 2 and 3 than in group 1. In all groups, the GHB antagonist NCS-382 [(2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid] partially attenuated the discriminative stimulus effects of GHB. The selective GHB receptor ligand UMB86 (4-hydroxy-4-napthylbutanoic acid sodium) tended to attenuate the discriminative stimulus effects of GHB more in group 3 than in the other groups. The finding that animals can discriminate GHB from baclofen is further evidence that the effects of GHB and baclofen are not identical. Effects that GHB does not share with baclofen may involve GHB receptors or differential interactions with GABA(B) receptors.

A review of pharmacology of NCS-382, a putative antagonist of gamma-hydroxybutyric acid (GHB) receptor

Gamma-hydroxybutyric acid (GHB), a naturally occurring metabolite of gamma-aminobutyric acid (GABA), has been postulated to act as a specific agonist of GHB receptors and as well as a weak GABA(B) receptor agonist. To date, 6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylideneacetic acid (NCS-382), a semirigid compound structurally related to GHB, is the only compound reported to be an antagonist of the GHB receptor sites. In this article we review the in vivo and in vitro pharmacological properties of NCS-382 and its interaction with GHB and GABA(B) receptors. Binding studies have demonstrated that NCS-382 is a stereoselective ligand for GHB-binding sites, with both, the high and the low component of population, showing the same distribution of GHB receptors. Indeed, this compound did not display affinity for GABA(A), GABA(B), or any other known receptors, while conflicting data have been reported as to its selective antagonist action at GHB receptor. Only a few studies have shown that NCS-382 antagonizes GHB-induced effect, but a re-evaluation of all data reported in the literature suggests that the antagonistic effect of this compound could be due to an indirect action at GABA(B) receptors. As revealed by several behavioral studies, NCS-382 fails to antagonize GHB discriminative stimuli, GHB-induced inhibition of locomotor activity and ataxia or suppression of operant responses. Moreover, it is capable of either eliciting qualitatively similar effects to those of GHB or enhancing some actions of GHB. In addition, the NCS-382-sensitive electrophysiological effects of endogenous and exogenous GHB observed in vivo have not been completely replicated in vitro. The only electrophysiological action of GHB antagonized in vitro by NCS-382 required a previous blockade of GABA(B) receptors. We concluded that NCS-382 is a good ligand but not a selective antagonist for GHB receptor.

Substitution profiles of N-methyl-D-aspartate antagonists in ethanol-discriminating inbred mice

C57BL/6J (B6) and DBA/2J (D2) inbred mice show pronounced differences in ethanol-induced behaviors, such as loss of righting reflex and locomotor activation, among others. They also differ in measures of conditioned place preference and oral self-administration of ethanol. In the current study, I examined whether B6 and D2 mice differed in their expression of the N-methyl-D-aspartate (NMDA) receptor-mediated component of the discriminative stimulus effects of ethanol. B6 and D2 mice were trained to discriminate ethanol (1.5 g/kg, i.p.) from saline in a two-choice, milk-reinforced operant procedure. After training was completed, substitution and response rate dose-effect curves were generated for ethanol; the uncompetitive NMDA antagonists phencyclidine and ketamine; and the competitive NMDA antagonist D-CPPene. Dose-effect curves were also generated for midazolam, cocaine, m-chlorophenylpiperazine (mCPP), morphine, and gamma-hydroxybutyric acid (GHB). B6 and D2 mice learned the ethanol-versus-saline discrimination. Phencyclidine produced near full substitution for ethanol in both strains, whereas ketamine fully substituted for ethanol only in B6 mice. D-CPPene partially substituted for ethanol in both strains. Moderate doses of phencyclidine produced greater response rate-increasing effects in B6 mice than in D2 mice, and high doses of phencyclidine were more potent for suppressing response rates in D2 mice. In contrast, D-CPPene had similar response rate-increasing effects in both strains, but high doses produced more potent response rate-decreasing effects in B6 mice. Among the other drugs tested, only midazolam produced substantial substitution for ethanol. Taken together, these findings seem to indicate that the behavioral effects of NMDA antagonists differ between strains, but that the NMDA-mediated component of the discriminative stimulus effects of ethanol is similar in B6 and D2 mice.

Influence of reinforcer type and route of administration on gamma-hydroxybutyrate discrimination in rats

RATIONALE

Possible effects of reinforcer type on the results of drug discrimination studies have not been examined systematically, but different deprivation operations and differentially effective reinforcers might well influence outcomes.

OBJECTIVE

Therefore, this study examined the influence of reinforcer (food or water) as well as route of administration (IP or IG) on gamma-hydroxybutyrate (GHB) discrimination.

METHODS

Four separate groups of six rats were trained under a resetting fixed-ratio schedule to discriminate between 300 mg/kg GHB and vehicle under these conditions, then generalization tests were conducted with gamma-butyrolactone (GBL), 1,4-butanediol (1,4-BD), ethanol, and ethanol plus 150 mg/kg GHB.

RESULTS

Food maintained significantly higher response rates than water, but there were no significant differences among the four training groups in response accuracy or sessions required to meet the discrimination criterion. Training conditions significantly affected the results of stimulus generalization tests. The IG-Water group was most sensitive to a lower dose of GHB, and only the IP-Water group failed to generalize to orally-administered GHB. Gamma-butyrolactone and 1,4-butanediol fully substituted in all except the IP-Food group. Ethanol did not fully substitute for GHB in any group, and the combination of GHB (150 mg/kg) and ethanol did not have additive effects.

CONCLUSIONS

These results demonstrate that methodological variables during drug discrimination training can certainly influence the results of stimulus generalization. Future investigations into the behavioral and/or physiological mechanisms that account for these effects are warranted.

Discriminative stimulus effects of gamma-hydroxybutyrate in pigeons: role of diazepam-sensitive and -insensitive GABA(A) and GABA(B) receptors

gamma-Hydroxybutyrate (GHB) is an emerging drug of abuse with multiple mechanisms of action. This study is part of an effort to examine the role of GHB, GABA(A), and GABA(B) receptors in the discriminative stimulus (DS) effects of GHB. In pigeons trained to discriminate 100 mg/kg GHB from saline, GHB and its precursors gamma-butyrolactone and 1,4-butanediol produced 80 to 100% GHB-appropriate responding, whereas other compounds such as morphine, naltrexone, cocaine, and haloperidol produced no more than 34%. Compounds interacting with GABA receptors produced different maximal levels of GHB-appropriate responding. For example, the GABA(A) agonist muscimol produced 3%; the GABA(A)-positive modulators diazepam, pentobarbital, and ethanol, and the GABA(B) agonist baclofen produced levels ranging from 54 to 73%; and the benzodiazepine antagonist flumazenil and inverse agonist Ro 15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-alpha]-[1,4]-benzodiazepine-3-carboxylate) both produced 96%. The putative GHB receptor antagonist (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid (NCS-382) produced 70% GHB-appropriate responding. The GABA(B) antagonist (3-aminopropyl)(diethoxymethyl)phosphinic acid (CGP 35348) completely blocked the GHB-like DS effects of NCS-382 and baclofen at a dose of 56 mg/kg. CGP 35348 also blocked the DS effects of GHB, but incompletely and only at a dose of 560 mg/kg. Together, these results are consistent with a role for diazepam-sensitive and -insensitive GABA(A) and GABA(B) receptors in the DS effects of GHB. Together with previous findings, the present results suggest that diazepam-insensitive GABA(A) receptors are more prominently involved in the DS effects of GHB in pigeons than in rats, whereas GABA(B) receptors are less prominently involved. Exploring the role of GHB receptors with NCS-382 is hampered by its GABA(B) receptor-mediated, GHB-like agonist activity.

Interactions of gamma-hydroxy butyrate with ethanol and NCS 382

We examined the effects of gamma-hydroxy butyrate (GHB) alone and in combination with either ethanol or NCS 382 [(2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene], a purported antagonist at the GHB receptor. These effects were examined on the responding of rats under a fixed-ratio (FR) 10 schedule of sugar solution (14%, w/v; 0.1 ml) presentation. GHB dose-relatedly decreased responding. When GHB was combined with ethanol, the effects of the two drugs were less than additive. NCS 382 did not antagonize the rate-decreasing effects of GHB. These observations are consistent with the notion that many of the behavioral actions of exogenously administered GHB result from GHB's actions at sites other than the GHB receptor, and are inconsistent with the popular notion that the effects of GHB and ethanol are synergistic.

GHB use among Australians: characteristics, use patterns and associated harm

The aim of this study was to examine the characteristics of gamma-hydroxybutyrate (GHB) users, their GHB and other drug use patterns, and the harms associated with GHB use. Seventy-six GHB users were recruited and administered a structured interview on GHB use and related harms. GHB users appeared to be a stable, highly educated and well-functioning group. They had had extensive experience with a range of drugs, and GHB was typically used in conjunction with other drugs. Despite the fact that most GHB users had not had a long or extensive experience with GHB use, the proportion reporting significant negative side effects when using GHB was high (99% reported at least one), and the mean number of side effects ever experienced was 6.5. Notably, half (52%) reported becoming unconscious, 53% reported vomiting, 58% reported profuse sweating, and 8% reported having a fit or seizure. The high rate of problems reported by a group with limited use of this drug suggests that in a context of polydrug use, GHB use is associated with significant risks to users.

Discrimination of gamma-hydroxybutyrate and ethanol administered separately and as a mixture in rats

The physiological effects of gamma-hydroxybutyrate (GHB) are complex and not yet clearly defined. GHB has been labeled as a recreational drug and is reported to be frequently coabused with ethanol (ETH). Other studies have yielded discrepant results as to the interaction between GHB and ETH. Thus, the present study investigated extensively the discriminative stimulus of GHB and ETH and a mixture of the two compounds. Thirty male Long-Evans rats were divided into three groups and trained to discriminate doses of either 300 mg/kg GHB, 1000 mg/kg ETH, or a mixture (MIX: 150 mg/kg GHB+500 mg/kg ETH) from vehicle on a two-lever fixed-ratio (FR) 10 schedule of food reinforcement. Dose-response curves were attained in each group with its respective training drugs. GHB and ETH did not cross-generalize in the ETH- and GHB-trained rats, respectively. However, when the effects of the MIX were tested in the GHB- and ETH-trained rats, a greater than additive response was observed. Testing also revealed that the MIX-trained rats did not perceive a novel stimulus but a near-equal contribution from GHB and ETH. This study provides evidence of a complex relationship between GHB and ETH and opposes previous work reporting cross-generalization between GHB and ETH.

GHB: a new and novel drug of abuse

There has been increasing attention in the United States to problems of abuse of gamma-hydroxybutyrate (GHB), with some evidence for problems in other parts of the world as well. In vitro and animal research show that, while GHB shares some properties with abused central nervous system depressant drugs, it has unique aspects of its pharmacology as well, including actions at a specific neural receptor which probably mediates many of its effects. Abuse potential assessment of GHB using standard animal models has not yielded a picture of a highly abusable substance, but little human testing has yet been done. Very little systematic data exist on tolerance and dependence with GHB, but both have been seen in human users. Quantitative data on the prevalence of GHB abuse is incomplete, but various qualitative measures indicate that a mini-epidemic of abuse began in the late 1980s and continues to the present. GHB is often included with the group of 'club drugs', and can be used as an intoxicant. It also has been used as a growth promoter and sleep aid and has been implicated in cases of 'date rape', usually in combination with alcohol. Undoubtedly the easy availability of GHB and some of its precursors has contributed to its popularity. Recent changes in the control status of GHB in the US may reduce its availability with as yet unknown consequences for the scope of the public health problem. Drug abuse experts need to familiarize themselves with GHB as possibly representing a new type of drug abuse problem with some unique properties.

Gamma-hydroxybutyric acid in alcohol preference, dependence and withdrawal

Gamma-hydroxybutyric acid (GHB) is an endogenous constituent of the mammalian brain, where it likely functions as a neurotransmitter or a neuromodulator. Its exogenous administration exerts a number of pharmacological effects, including reduction of intensity of alcohol withdrawal syndrome and alcohol consumption in both laboratory animals and human alcoholics.The clinical studies conducted to date, although often testing samples of limited size, feature GHB as an effective, well-tolerated and safe drug for the treatment of alcohol dependence. Behavioural data in rats suggest that GHB may produce alcohol-like effects. This similarity may explain why GHB produces positively reinforcing properties, being subsequently self-administered by rodents and sometimes abused by humans (although episodes of self-directed intake of GHB appear to be a limited phenomenon in alcoholics); in addition it provides support to the hypothesis that GHB constitutes for alcoholism a replacement therapy similar to methadone in heroin addiction.

Opiate delta-2-receptor antagonist naltriben does not alter discriminative stimulus effects of ethanol

The ability of a selective 2-opiate receptor antagonist, naltriben, to modulate ethanol discrimination was investigated in a rat model using a drug discrimination procedure. Rats were trained to discriminate ethanol (1.25 g/kg, IP) from saline on a fixed-ratio schedule, FR10. Once rats had acquired the ethanol-saline discrimination, ethanol dose-response tests were conducted with 15-min pretest injections. Following the characterization of the ethanol dose-response curve, the effect of naltriben on ethanol's discriminative stimulus was assessed by administering naltriben (0. 032-5.6 mg/kg, IP) 15 min before the ethanol administration. In the present study, naltriben did not have any modulatory effect on ethanol discrimination, suggesting that either Delta(2)-opiate receptors are not involved in the formation of ethanol's discriminative stimulus or the antagonism of Delta(2)-opiate receptors is not sufficient to alter ethanol's compound discriminative stimulus.

Quantitative autoradiographic distribution of gamma-hydroxybutyric acid binding sites in human and monkey brain

gamma-Hydroxybutyric acid (GHB), a naturally occurring metabolite of GABA, is present in micromolar concentrations in various areas of the mammalian brain. Specific GHB binding sites, uptake system, synthetic and metabolizing enzymes have been identified in CNS. The present study shows the anatomical distribution of GHB binding sites in sections of primate (squirrel monkey) and human brain by radioligand quantitative autoradiography. In both species the highest densities of binding sites were found in the hippocampus, high to moderate densities in cortical areas (frontal, temporal, insular, cingulate and entorhinal) and low densities in the striatum; no binding sites were detected in the cerebellum. High density of GHB binding was found in the monkey amygdala. In addition the binding characteristics of [(3)H]GHB to membrane preparations of human brain cortex were examined. Scatchard analysis and saturation curves revealed both a high (K(d1) 92+/-4.4 nM; B(max1) 1027+/-110 fmol/mg protein) and a low-affinity binding site (K(d2) 916+/-42 nM; B(max2) 8770+/-159 fmol/mg protein). The present study is the first report on the autoradiographic distribution of specific GHB binding sites in the primate and human brain: such distribution is in both species in good agreement with the distribution found in the rat brain.

Mechanism of the antialcohol effect of gamma-hydroxybutyric acid

Treatment with gamma-hydroxybutyric acid has been reported to effectively decrease alcohol craving and consumption as well as alcohol withdrawal symptoms in alcoholics. We describe the results of animal studies demonstrating the ability of gamma-hydroxybutyric acid to reduce (1) the severity of ethanol withdrawal signs in rats rendered physically dependent on ethanol and (2) voluntary ethanol intake in selectively bred Sardinian alcohol-preferring rats. Furthermore, we review experimental data suggesting that gamma-hydroxybutyric acid and ethanol have several pharmacological effects in common. Relevant similarities are: (1) stimulation of firing rate of dopaminergic neurons and dopamine release in specific rat brain areas; (2) development of cross-tolerance to the motor-impairing effects after repeated administration in rats; 3) abuse potential, as indicated by self-administration of pharmacologically relevant doses of gamma-hydroxybutyric acid in rats and mice; (4) induction of anxiolytic effects in rats; and (5) induction of similar discriminative stimulus effects, as evidenced by symmetrical generalization in a drug discrimination study in rats. These lines of evidence are discussed in relation to gamma-hydroxybutyric acid exerting its antialcohol effects by a substitution mechanism.

Gamma-hydroxybutyric acid efficacy, potential abuse, and dependence in the treatment of alcohol addiction

The main objective in alcoholism therapy is to achieve and maintain abstinence and to prevent relapse. Pharmacotherapy may be necessary in treating persons who are not helped by group or psychosocial support alone. Among the substances experimented with in the past few years, gamma-hydroxybutyric acid has been effective in preventing alcohol withdrawal syndrome and in inducing a reduction in craving and an increase in the abstinence rate in treated alcoholics, in view of the alcohol-mimicking effects of the drug on the central nervous system. However, a possible development of craving for the drug and the risk of abuse and physical dependence have been reported in subjects who used gamma-hydroxybutyric acid for different reasons, including alcoholism therapy. The present review updates the existing differences in drug abuse behavior, side effects, and poisoning in the use of gamma-hydroxybutyric acid in a treatment alcoholism program and in self nonclinical illicit use.

Design and structure-activity relationship analysis of ligands of gamma-hydroxybutyric acid receptors

With the use of [3H]gamma-hydroxybutyric acid, binding experiments allowed the screening of new compounds as ligands of gamma-hydroxybutyric acid receptors. Starting from the acid-alcohol gamma-hydroxybutyric acid structure, structure-activity relation analysis and lead optimization highlighted gamma-hydroxybutyric acid derivatives with significantly increased affinities, when compared with the affinity of gamma-hydroxybutyric acid. Further pharmacological studies with the use of gamma-hydroxybutyric acid derivatives allowed the characterization of the first competitive antagonist acting at gamma-hydroxybutyric acid receptors (NCS 382).

Characterization of the discriminative stimulus effects of gamma-hydroxybutyric acid as a means for unraveling the neurochemical basis of gamma-hydroxybutyric acid actions and its similarities to those of ethanol

The present paper reviews the drug discrimination studies, both from the literature and from this laboratory, conducted to investigate the pharmacological profile of the discriminative stimulus effects of gamma-hydroxybutyric acid. Collectively, the results of these studies suggest that: (1) the discriminative stimulus effects of gamma-hydroxybutyric acid are composed of different cues, each one being the effect of gamma-hydroxybutyric acid on a specific receptor system; (2) the proportion of each component cue varies as the training dose of gamma-hydroxybutyric acid is increased; (3) the gamma-aminobutyric acid B-mediated cue is a major ingredient of the mixed stimulus of gamma-hydroxybutyric acid, but it is more prominent at high training doses than at low training doses of gamma-hydroxybutyric acid; and (4) positive modulation of the gamma-aminobutyric acid A receptor is a relevant part of the discriminative stimulus effects of low gamma-hydroxybutyric acid doses. Finally, data indicating symmetrical generalization between the discriminative stimulus effects of a specific range of doses of gamma-hydroxybutyric acid and those of ethanol are discussed in regard to their further support of the hypothesis that gamma-hydroxybutyric acid may exert its antialcohol effects through a substitution mechanism.

Ability of baclofen in reducing alcohol intake and withdrawal severity: I--Preclinical evidence

BACKGROUND

The similarities between the pharmacological effects of the gamma-aminobutyric acid receptor agonist, baclofen, and the alcohol-substituting agent, gamma-hydroxybutyric acid, led us to investigate whether baclofen was capable of reducing (a) ethanol withdrawal syndrome in ethanol-dependent rats and (b) voluntary ethanol intake in ethanol-preferring rats.

METHODS

In experiment 1, Wistar rats were rendered physically dependent on ethanol by the repeated administration of intoxicating doses of ethanol for 6 consecutive days. Baclofen was acutely administered intraperitoneally at doses of 10, 20, and 40 mg/kg. In experiment 2, baclofen (0, 2.5, 5, and 10 mg/kg, intraperitoneally) was administered once a day for 14 consecutive days to ethanol-preferring sP rats that had continuous access to ethanol (10%, v/v) and water under the two-bottle free choice regimen.

RESULTS

In experiment 1, baclofen dose-dependently decreased the intensity of ethanol withdrawal signs; furthermore, 20 mg/kg of baclofen protected from audiogenic seizures in ethanol-withdrawn rats. In experiment 2, baclofen selectively and dose-dependently reduced voluntary ethanol intake; a compensatory increase in water intake left total fluid intake virtually unchanged.

CONCLUSIONS

These results are in close agreement with those of a preliminary clinical study and suggest that baclofen may constitute a novel therapeutic agent for alcoholism.

Gamma-Hydroxybutyrate inhibits excitatory postsynaptic potentials in rat hippocampal slices

Gamma-Hydroxybutyrate (GHB) has been shown to mimic different central actions of ethanol, to suppress alcohol withdrawal syndrome, and to reduce alcohol consumption both in rats and in humans. The aim of the present study was to determine if GHB shared with alcohol the ability to inhibit glutamate action at both NMDA and AMPA/kainate receptors. The NMDA or the AMPA/kainate receptors-mediated postsynaptic potentials were evoked in CA1 pyramidal neurons by stimulation of Schaffer-collateral commissural fibers in the presence of CGP 35348, bicuculline to block the GABA(B) and GABA(A) receptors, and 10 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX) or 30 microM DL-2-amino-5-phosphonovalerate (d-APV) to block AMPA/kainate or NMDA receptors, respectively. GHB (600 microM) produced a depression of both NMDA and AMPA/kainate receptors-mediated excitatory postsynaptic potentials with recovery on washout. The GHB receptors antagonist, NCS-382, at the concentration of 500 microM had no effect per se on these responses but prevented the depressant effect of GHB (600 microM) on the NMDA and AMPA/kainate-mediated responses. In the paired-pulse experiments, GHB (600 microM) depressed the amplitude of the first and the second evoked AMPA/kainate excitatory postsynaptic potentials, and significantly increased the paired-pulse facilitation (PPF). These results suggest that GHB inhibits excitatory synaptic transmission at Schaffer-collateral commissural-pyramidal neurons synapses by decreasing the probability of release of glutamate.

Evaluation of the reinforcing and discriminative stimulus effects of gamma-hydroxybutyrate in rhesus monkeys

Gamma-hydroxybutyrate (GHB) is a metabolite of GABA that is present in the CNS and fulfils at least some of the criteria for a neurotransmitter. Its effects are generally similar to those of CNS depressants and include ataxia, sleep and anesthesia. It has also been suggested that GHB is a drug of abuse. The present experiment was designed to evaluate GHB in procedures predictive of abuse and dependence potential in rhesus monkeys. Three monkeys were surgically prepared with indwelling silicone venous catheters and allowed to self-administer methohexital or saline in twice-daily experimental sessions. Other groups of monkeys were trained in drug discrimination paradigms to discriminate D-amphetamine (AMPH; n = 4), pentobarbital (PB; n = 3) or triazolam (n = 3) from saline. Another group was maintained on diazepam daily and trained to discriminate flumazenil from saline (n = 2). GHB (0.01-10 mg/kg per injection) maintained self-administration marginally above saline levels at one dose (3.2 or 10 mg/kg) in two of the three monkeys tested. GHB (1.0-178 mg/kg, subcutaneously (s.c.) or intragastrically (i.g.)) did not reliably substitute as a discriminative stimulus for any of the training conditions. Taken together with previous results, the present experiment suggests that GHB has, at most, low potential for abuse.

Reduction of blood ethanol levels by the gamma-hydroxybutyric acid receptor antagonist, NCS-382

The present study demonstrates that the gamma-hydroxybutyric acid receptor antagonist, NCS-382, markedly reduces blood ethanol levels (BELs) in rats when ethanol is administered via the intragastric route, whereas it is completely ineffective when ethanol is injected IP. The reducing effect of NCS-382 on BELs is likely due to a lessened absorption of ethanol from the gastrointestinal tract.

Discriminative stimulus effects of ethanol: neuropharmacological characterization

Generally, compounds discriminated by animals possess psychotropic effects in animals and humans. As with many other drugs of abuse, strength of the ethanol discriminative stimulus is dose related. The majority of studies show that doses close to 1.0 g/kg are close to the minimum at which the discrimination can be learned easily. Substitution studies suggest that anxiolytic, sedative, atactic, and myorelaxant effects of ethanol all play an important role in the formation of its intercoeptive stimulus. Low doses of ethanol produce more excitatory cues, similar to amphetamine-like subjective stimuli, whereas higher doses produce rather sedative/hypnotic stimuli similar to those elicited by barbiturates. Substitution studies have shown that the complete substitution for ethanol may be exerted by certain GABA-mimetic drugs acting through different sites within the GABA(A)-benzodiazepine receptor complex (e.g., diazepam, pentobarbital, certain neurosteroids), gamma-hydroxybutyrate, and antagonists of the glutamate NMDA receptor. Among the NMDA receptor antagonists both noncompetitive (e.g., dizocilpine) and competitive antagonists (e.g., CGP 40116) are capable of substituting for ethanol. Further, some antagonists of strychnine-insensitive glycine modulatory sites among the NMDA receptor complex (e.g., L-701,324) dose-dependently substitute for the ethanol discriminative stimulus. On the other hand, neither GABA-benzodiazepine antagonists nor NMDA receptor agonists produce contradictory effects (i.e., reduce the ethanol discriminative stimulus). There is influence of a particular training dose of ethanol on the substitution pattern of different compounds. For example, 5-HT(1B/2C) agonists substitute for intermediate (1.0 g/kg) but not higher (2.0 g/kg) ethanol training doses. Discrimination studies with ethanol and drugs acting on NMDA and GABA receptors consistently indicate asymmetrical generalization. For example, ethanol is able to generalize to barbiturates and benzodiazepines, but neither the benzodiazepine nor barbiturate response generalizes to ethanol. Only a few drugs are able to antagonize, at least to some extent, the discriminative stimulus of ethanol (e.g., partial inverse GABA-benzodiazepine receptor antagonist Ro 15-4513 and the opioid antagonist naloxone). The ethanol stimulus effect may be increased (i.e., stronger recognition) by N-cholinergic drugs (nicotine), dopaminergic drugs (apomorphine), and 5-HT3 receptor agonists (m-chlorophenylbiguanide). Thus, the ethanol stimulus is composed of the several components, with the NMDA receptor and GABA(A) receptor complex being of particular importance. This suggests that a drug mixture may be more capable of substituting for ethanol (or block its stimulus) than a single compound. The ability of drugs to substitute for the ethanol discriminative stimulus is frequently, although not preclusively, associated with the reduction of voluntary ethanol consumption. The examples of positive correlation are gamma-hydroxybutyrate, possibly memantine and certain serotonergic drugs such as fluoxetine. However, it remains uncertain to what extent the discriminative stimulus of ethanol can be seen as relevant in the understanding of the complex mechanisms of dependence.

Intravenous self-administration of gamma-hydroxybutyric acid in drug-naive mice

The reinforcing effects of gamma-hydroxybutyric acid (GHB) were studied by means of intravenous self-administration in drug-naive mice. GHB self-administration was concentration-dependent (0.01-0.5 mg/kg/inj.) according to a bell-shaped curve. Pretreatment with the specific GHB receptor antagonist NCS-382 at a dose of 12.5 mg/kg i.p. completely antagonized the reinforcing effects of GHB. These data suggest that GHB is able to induce reinforcing effects in mice and support the hypothesis of an abuse liability of this drug.

Involvement of GABA(A) and GABA(B) receptors in the mediation of discriminative stimulus effects of gamma-hydroxybutyric acid

The present study was designed to further investigate the pharmacological profile of the discriminative stimulus effects of gamma-hydroxybutyric acid (GHB). Drugs acting at the gamma-aminobutyric acid (GABA)B receptor (baclofen and CGP 35348), GABA(A)/benzodiazepine receptor complex (diazepam), N-methyl-D-aspartate (NMDA) receptor complex (dizocilpine), and cannabinoid receptor (WIN 55,212-2) were tested for substitution or blockade of the GHB interoceptive cue in rats trained to discriminate either 300 or 700 mg/kg of GHB i.g. from water in a T-maze, food-reinforced drug discrimination paradigm. Baclofen completely substituted for both training doses of GHB; however, its potency in substituting for GHB increased as the training dose of GHB was increased. CGP 35348 partially and completely blocked the cue elicited by 300 and 700 mg/kg of GHB, respectively. In contrast, diazepam partially substituted for 300 mg/kg of GHB, while failing to produce a GHB-appropriate response in the rat group trained to the higher GHB dose. Neither dizocilpine nor WIN 55,212-2 substituted for GHB. Collectively, these data suggest that: a) GHB produces a compound stimulus; and b) GABA(B)- and GABA(A)-mediated cues are prominent components of the mixed stimulus of GHB. However, the quality (i.e., the proportion of the component cues) of the stimulus varies as the training dose of GHB is increased; indeed, the contribution of the GABA(A)- and GABA(B)-mediated cues were smaller and greater, respectively, at 700 and 300 mg/kg of GHB training doses.

Gamma-hydroxybutyric acid intake in ethanol-preferring sP and -nonpreferring sNP rats

Gamma-hydroxybutyric acid (GHB) and ethanol share several pharmacological similarities, suggesting that GHB may exert ethanol-like effects in the central nervous system. The present study was designed to test whether selectively bred ethanol-preferring rats would, unlike ethanol-nonpreferring ones, self-administer GHB, consistent with their higher preference for ethanol. Male ethanol-naive Sardinian alcohol-preferring (sP) and Sardinian alcohol-nonpreferring (sNP) rats were used. In Experiment 1, GHB solution (1% (w/v) in water) was initially offered as the sole fluid available for 14 consecutive days and then presented under the two-bottle, free-choice regimen, one bottle containing water and the other the GHB solution, for an additional 14 consecutive days. During the free-choice phase, high preference for GHB and intake of pharmacologically relevant daily doses of GHB developed in both rat lines, presumably because the 14-day no-choice period would unmask the reinforcing properties of GHB and lead to acquisition of GHB preference also in the supposedly less susceptible sNP rats. In Experiment 2, the forced GHB drinking phase was reduced to 3 days. Under the subsequent free-choice regimen, daily GHB preference and intake were initially low in both sP and sNP rats; however, after approximately 10 days, GHB preference and intake in sP rats rose progressively and then stabilized to significantly higher levels than in sNP rats throughout the entire free-choice phase. It is likely that episodic binges of GHB intake occurring during the first 10 days resulted in experiencing the reinforcing properties of GHB by sP but not sNP rats. The results of the present study suggest that a) sP rats are genetically more sensitive to the reinforcing effects of both ethanol and GHB than sNP rats; and b) disclosure of the higher sensitivity of sP rats to the reinforcing effects of GHB is a function of the length of the induction procedure. The results are also discussed in terms of differences in GHB receptors contributing to the predisposition to ethanol preference and avoidance, respectively.

Gamma-hydroxybutyric acid decreases intravenous cocaine self-administration in rats

Gamma-hydroxybutyric acid (GHB) is an endogenous compound present in mammalian brain suggested as a putative neurotransmitter, which has been shown to affect several aspects of dependence from various classes of drugs of abuse. In the present study, two sets of experiments were performed to investigate the effects of acute pretreatment with GHB on intravenous cocaine self-administration in rats. In the first experiment GHB was administered intragastrically at the doses of 175, 350, and 700 mg/kg to Long-Evans rats trained to self-administer cocaine using nose-poke as operandum. In the second experiment, GHB was administered intraperitoneally at the doses of 100, 200, and 400 mg/kg to Wistar rats trained to self-administer cocaine intravenously using lever-pressing as operandum. In both experiments acute pretreatment with GHB significantly and dose dependently reduced cocaine self-administration. The effectiveness of GHB was similar in both experiments, indicating that the effect of GHB on cocaine self-administration is independent of animal strain. route of administration, and type of operant response required. These results indicate that GHB reduces cocaine-seeking behavior in rats, modulating the acute reinforcing effect of cocaine. The clinical effectiveness of GHB in dependence from various classes of abused drugs warrants further studies to evaluate the possibility that GHB might represent a useful therapeutic agent for cocaine addiction in humans.

Effects of gamma-hydroxybutyric acid (GHB) in opioid-dependent patients

Gamma-hydroxybutyric acid (GHB) is a GABA metabolite used clinically for sleep induction. The abuse liability of GHB is controversial. As part of a study of the effect of GHB pretreatment on naloxone-precipitated opiate withdrawal, eight opioid-stabilized subjects received a balanced, randomized, double-blind sequence of oral placebo, GHB 15 mg/kg and 30 mg/kg. GHB had no consistent physiological effects. After GHB and prior to naloxone, subjects rated "sluggish," "spaced," "carefree," and "good-mood" higher after GHB 30 mg/kg than after placebo. Subjects identified the 30 mg/kg dose as most similar to placebo (n = 3), benzodiazepine (n = 2), opiate (n = 2), and alcohol (n = 1).