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

γ-Hydroxybutyric Acid: Pharmacokinetics, Pharmacodynamics, and Toxicology

Gamma-hydroxybutyrate (GHB) is a short-chain fatty acid present endogenously in the brain and used therapeutically for the treatment of narcolepsy, as sodium oxybate, and for alcohol abuse/withdrawal. GHB is better known however as a drug of abuse and is commonly referred to as the "date-rape drug"; current use in popular culture includes recreational "chemsex," due to its properties of euphoria, loss of inhibition, amnesia, and drowsiness. Due to the steep concentration-effect curve for GHB, overdoses occur commonly and symptoms include sedation, respiratory depression, coma, and death. GHB binds to both GHB and GABAB receptors in the brain, with pharmacological/toxicological effects mainly due to GABAB agonist effects. The pharmacokinetics of GHB are complex and include nonlinear absorption, metabolism, tissue uptake, and renal elimination processes. GHB is a substrate for monocarboxylate transporters, including both sodium-dependent transporters (SMCT1, 2; SLC5A8; SLC5A12) and proton-dependent transporters (MCT1-4; SLC16A1, 7, 8, and 3), which represent significant determinants of absorption, renal reabsorption, and brain and tissue uptake. This review will provide current information of the pharmacology, therapeutic effects, and pharmacokinetics/pharmacodynamics of GHB, as well as therapeutic strategies for the treatment of overdoses. Graphical abstract.

Physical dependence on gamma-hydroxybutrate (GHB) prodrug 1,4-butanediol (1,4-BD): time course and severity of withdrawal in baboons

BACKGROUND

1,4-Butanediol (1,4-BD) is a gamma-hydroxybutyrate (GHB) pro-drug, with multiple commercial uses, and a drug of abuse. Although there are case reports of a withdrawal syndrome following 1,4-BD use, no studies have evaluated the physical dependence potential of 1,4-BD and characterized the time course of withdrawal.

METHODS

Vehicle and then 1,4-BD were administered continuously 24 h/day via intragastric catheters in male baboons (Papio anubis, n=3). Dosing was initiated at 100 mg/kg and increased by 100mg/kg/day to 400mg/kg. After a stabilization period, doses of 500 and then 600 mg/kg/day were each maintained for 3-4 weeks. Plasma levels of 1,4-BD and GHB were determined for each dose condition. Physical dependence was assessed via administration of a GABA-B antagonist (precipitated withdrawal test) during administration of the 600 mg/kg dose and via abrupt termination of chronic 1,4-BD administration (spontaneous withdrawal test). Outcome measures included the number of food pellets earned, performance on a fine-motor task, observed behaviors, and plasma levels of GHB and 1,4-BD.

RESULTS

Following maintenance of 1,4-BD 600 mg/kg for 3 weeks, the number of food pellets earned was significantly decreased. At the end of chronic 1,4-BD dosing, the levels of GHB in plasma ranged from 1290 to 2300 μmol/L and levels of 1,4-BD in plasma ranged from 13.1 to 37.9 μmol/L. Signs of physical dependence were observed following precipitated and spontaneous withdrawal tests. Seizures were not observed.

CONCLUSIONS

These data indicate chronic 1,4-BD produced physical dependence in baboons and the withdrawal syndrome can be characterized as mild to intermediate.

Behavioral effects and pharmacokinetics of gamma-hydroxybutyrate (GHB) precursors gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD) in baboons

RATIONALE

Gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD) are prodrugs for gamma-hydroxybutyrate (GHB). Like GHB, GBL and 1,4-BD are drugs of abuse, but their behavioral effects may differ from GHB under some conditions.

OBJECTIVES

The first study compared the behavioral effects of GBL (32-240 mg/kg) and 1,4-BD (32-240 mg/kg) with each other and to effects previously reported for GHB (32-420 mg/kg). A second study determined GHB pharmacokinetics following intragastric administration of GHB, GBL, and 1,4-BD.

METHODS

Operant responding for food, observed behavioral effects, and a fine-motor task occurred at multiple time intervals after administration of drug or vehicle. In a separate pharmacokinetics study, blood samples were collected across multiple time points after administration of GHB, GBL, and 1,4-BD.

RESULTS

Like GHB, GBL, and 1,4-BD impaired performance on the fine-motor task, but the onset of motor impairment differed across drugs. GBL and 1,4-BD dose dependently decreased the number of food pellets earned, but at lower doses than previously observed for GHB. Similar to GHB, both GBL and 1,4-BD produced sedation, muscle relaxation, gastrointestinal symptoms, and tremors/jerks. Administration of GBL and 1,4-BD produced higher maximum concentrations of GHB with shorter times to maximum concentrations of GHB in plasma when compared to GHB administration.

CONCLUSIONS

GBL and 1,4-BD produced behavioral effects similar to those previously reported with GHB and the time course of effects were related to blood levels of GHB. Given their higher potency and faster onset of effects, the abuse liability of GBL and 1,4-BD may be greater than GHB.

Gamma-butyrolactone (GBL) disruption of passive avoidance learning in the day-old chick appears to be due to its effect on GABAB not gamma-hydroxybutyric [corrected] acid (GHB) receptors

Gamma-butyrolactone (GBL) is a prodrug to gamma-hydroxybutyric acid (GHB) and metabolises to GHB when ingested. Discrimination stimulus studies report generalisation of effects of GHB to GBL. While amnesia is one of the most commonly reported symptoms of GHB's ingestion in human users, as yet few studies have examined this effect. Although an endogenous GHB specific receptor is present in the brain, several studies have indicated that the clinical effects of exogenous doses of GBL/GHB are due to its action on GABA(B) receptors rather than on the GHB receptor. In this series of studies, New Hampshire x White leghorn cockerels were trained using a modified version of the passive avoidance learning task. Subcutaneous injections of GBL induced a memory deficit by 10 min post-training, which persisted for at least 24 h. No effect on memory was seen with administration of the specific GHB agonist NCS-356 (gamma-p-chlorophenyl-trans-4-hydroxycrotonate). The GBL-induced memory deficit appeared similar to the deficit produced by baclofen, where the antagonist facilitated learning. Additionally, GBL-induced memory deficit was ameliorated by application of a GABA(B) antagonist. The results support the hypothesis that GBL exerts its influence on memory via the GABA(B) receptor rather than by the specific GHB receptor.

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.

Effects of gamma-hydroxybutyrate (GHB) and its metabolic precursors on delayed-matching-to-position performance in rats

The purpose of the present study was to provide further information about the effects of gamma-hydroxybutyrate (GHB) on memory. Initially, the acute effects of gamma-butyrolactone (GBL, 75-200 mg/kg IP), 1,4-butanediol (1,4-BD, 100-300 mg/kg IP), and ethanol (1.0-3.0 g/kg, oral), as well as GHB (100-300 mg/kg IP), were examined in rats responding under a delayed-matching-to-position (DMTP) procedure with delays from 0 to 32 s. Acute administration of all four drugs reduced the number of trials completed and also reduced accuracy during delay trials, but not during trials without a delay. Some tolerance developed to the disruptive effects of GHB following exposure to 300 mg/kg/day for 29 consecutive days. These data indicate that GHB can disrupt working memory and speed of responding, and that tolerance can develop to these effects. Moreover, the acute effects of GHB under the DMTP procedure resemble those of its metabolic precursors, GBL and 1,4-BD, and of the prototypical CNS depressant drug, ethanol.

Lack of effects of GHB precursors GBL and 1,4-BD following i.c.v. administration in rats

Gamma-hydroxybutyrate (GHB) is used therapeutically and recreationally worldwide. Since the scheduling of GHB by the USA and the United Nations in 2000-2001, the recreational use of GHB precursors has reportedly increased. The aim of this study was to examine if potency differences of GHB and GHB-like compounds are due to their blood-brain barrier permeability. The effects of peripheral and central administration of GHB, GHB precursors gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD), and the gamma-aminobutyric acid (GABA)(B) receptor agonist baclofen on schedule-controlled responding were examined in rats. GHB and baclofen were 276- and 253-fold more potent, respectively, after intracerebroventricular (i.c.v.) administration than after intraperitoneal (i.p.) administration, whereas GBL and 1,4-BD, up to a dose of 1780 microg were without effect after i.c.v. administration. These data suggest that GBL and 1,4-BD are not metabolically converted to GHB in the brain, that enhanced brain penetration cannot account for potency differences between compounds, and that baclofen, like GHB, can readily cross the blood-brain barrier.

Clinical Pharmacology of 1,4-Butanediol and Gamma-hydroxybutyrate After Oral 1,4-Butanediol Administration to Healthy Volunteers

1,4-Butanediol (BD) is converted to gamma-hydroxybutyrate (GHB) after ingestion, and is associated with cases of dependence, coma, and death. The pharmacology of BD after oral ingestion has not been described in humans. Eight healthy volunteers (five men) were administered 25 mg/kg BD in a single oral dose after an overnight fast in a double-blinded, placebo-controlled, crossover study. Vital signs were monitored, and serial blood samples collected over 24 h for gas chromatography-mass spectrometry analysis of BD and GHB levels. Subjective mood and symptoms responses were assessed by visual analog scale. All subjects completed the study without significant adverse effects. BD was quickly absorbed and cleared, with time to maximal plasma concentration of 24+/-12 min, and elimination half-life (T(1/2)) of 39.3+/-11 min. BD was extensively converted to GHB, with a mean maximum GHB concentration of 45.6+/-19.7 mg/l reached 39.4+/-11.2 min after BD ingestion. GHB T(1/2) averaged 32.3+/-6.6 min. Some subjects exhibited slow oral clearance of BD, which tended to correlate with a variant haplotype of the alcohol dehydrogenase gene ADH-IB G143A. Mean CL/F was 151.5+/-176.5 ml/min kg for four subjects with variant haplotype versus 598.8+/-446.6 ml/min kg for four wild-type subjects (P=0.061). Subjects reported feeling less awake and alert, less able to concentrate, and more lightheaded in the first 90 min after BD ingestion. Pulse oximetry readings were lower 45 min after BD dosing with a mean oxygen saturation of 98.5% with BD versus 99.6% with placebo (P=0.031). Transient increases in mean systolic and diastolic blood pressure were observed, but other vital signs remained unchanged. BD was extensively converted to GHB after oral administration, but significant inter-individual variability in the rate of metabolism, possibly related to variants in ADH-IB, was observed. At the modest dose studied, significant clinical effects were not seen.

Chronic intragastric administration of gamma-butyrolactone produces physical dependence in baboons

RATIONALE

Abuse of gamma-hydroxybutyrate (GHB) and its precursors is a public health concern. Gamma-butyrolactone (GBL) is found in commercially available products and, when ingested, is metabolized to GHB.

OBJECTIVE

The goal was to evaluate the physical dependence potential and behavioral effects of GBL.

METHODS

Vehicle and then GBL were administered continuously (24 h per da y) in baboons (Papio anubis, n=5) via intragastric catheters. GBL dosing was initiated at 100 mg/kg/day and then progressively increased stepwise by increments of 100 mg/kg to a final dose of 600 mg/kg. The number of food pellets earned, fine-motor task performance, and observed behaviors were used as dependent measures. Precipitated withdrawal was evaluated after administration of GABA-B and benzodiazepine receptor antagonists during chronic GBL dosing (400-600 mg/kg). Spontaneous withdrawal was evaluated after discontinuation of chronic GBL 600 mg/kg. Blood GHB levels were determined during chronic dosing of each GBL dose by isotope dilution assay.

RESULTS

Chronic GBL dose-dependently decreased food-maintained behavior, disrupted performance on the fine-motor task, and produced signs of sedation and muscle relaxation. The GABA-B antagonist SGS742 [56 mg/kg, intramuscular (IM)] precipitated a withdrawal syndrome, whereas the benzodiazepine antagonist flumazenil (5 mg/kg, IM) produced little or no effect. Signs of physical dependence were also demonstrated when chronic GBL dosing was discontinued. Analysis of plasma indicated GBL was metabolized to GHB; levels were 825 to 1,690 micromol l(-1) GHB and 2,430 to 3,785 micromol l(-1) GHB after week 1 of 400 and 600 mg/kg/day, respectively.

CONCLUSIONS

These data indicate that, like GHB, chronic GBL dosing produced physical dependence that likely involved the GABA-B receptor.

Low-carb diets, fasting and euphoria: Is there a link between ketosis and gamma-hydroxybutyrate (GHB)?

Anecdotal evidence links the initial phase of fasting or a low-carbohydrate diet with feelings of well-being and mild euphoria. These feelings have often been attributed to ketosis, the production of ketone bodies which can replace glucose as an energy source for the brain. One of these ketone bodies, beta-hydroxybutyrate (BHB), is an isomer of the notorious drug of abuse, GHB (gamma-hydroxybutyrate). GHB is also of interest in relation to its potential as a treatment for alcohol and opiate dependence and narcolepsy-associated cataplexy. Here I hypothesize that, the mild euphoria often noted with fasting or low-carbohydrate diets may be due to shared actions of BHB and GHB on the brain. Specifically, I propose that BHB, like GHB, induces mild euphoria by being a weak partial agonist for GABA(B) receptors. I outline several approaches that would test the hypothesis, including receptor binding studies in cultured cells, perception studies in trained rodents, and psychometric testing and functional magnetic resonance imaging in humans. These and other studies investigating whether BHB and GHB share common effects on brain chemistry and mood are timely and warranted, especially when considering their structural similarities and the popularity of ketogenic diets and GHB as a drug of abuse.