Relation of the [3H] gamma-hydroxybutyric acid (GHB) binding site to the gamma-aminobutyric acidB (GABAB) receptor in rat brain

GABAB receptor-positive modulators: brain region-dependent effects

This study examined the positive modulatory properties of 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) at γ-aminobutyric acid B (GABA(B)) receptors in different brain regions. Using quantitative autoradiography, we measured GABA(B) receptor-stimulated binding of guanosine 5'-O-(3-[³⁵S]thiotriphosphate) ([³⁵S]GTPγS) to G proteins in medial prefrontal cortex (mPFC), hippocampus, and cerebellum. CGP7930 and rac-BHFF enhanced baclofen-stimulated [³⁵S]GTPγS binding similarly in mPFC and hippocampus, but were more effective in cerebellum. CGP7930 (100 μM) increased [³⁵S]GTPγS binding stimulated by baclofen (30 μM) from 29 to 241% above basal in mPFC and from 13 to 1530% above basal in cerebellum. Likewise, rac-BHFF (10 μM) increased baclofen-stimulated [³⁵S]GTPγS binding more in cerebellum (from 13 to 1778% above basal) than in mPFC (from 29 to 514% above basal). rac-BHFF (10 μM) in combination with γ-hydroxybutyrate (20 mM) increased [³⁵S]GTPγS binding in cerebellum but not in mPFC. rac-BHFF also enhanced the effects of 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348). Consistent with its partial agonist properties, CGP35348 stimulated [³⁵S]GTPγS binding in mPFC when given alone (to 18% above basal), but less extensively than baclofen (140% above basal), and antagonized baclofen when given together. CGP35348 (1 mM) in combination with rac-BHFF (100 μM) produced an increase in [³⁵S]GTPγS binding that was larger in cerebellum (from 61 to 1260% above basal) than in mPFC (from 18 to 118% above basal). Taken together, the results show that GABA(B) receptor-positive modulators enhance [³⁵S]GTPγS binding stimulated by GABA(B) receptor agonists in a brain region-dependent manner. This regionally selective enhancement is further evidence of pharmacologically distinct GABA(B) receptor populations, possibly allowing for more selective therapeutic targeting of the GABA(B) system.

Acute toxicity and withdrawal syndromes related to γ-hydroxybutyrate (GHB) and its analogues γ-butyrolactone (GBL) and 1,4-butanediol (1,4-BD)

Gamma-hydroxybutyrate (GHB) has been used as a recreational drug since the 1990s and over the last few years there has been increasing use of its analogues gamma-butyrolactone (GBL) and to a lesser extent 1,4-butanediol (1,4BD). This review will summarize the literature on the pharmacology of these compounds; the patterns and management of acute toxicity associated with their use; and the clinical patterns of presentation and management of chronic dependency associated with GHB and its analogues.

Novel radioiodinated {gamma}-hydroxybutyric acid analogues for radiolabeling and Photolinking of high-affinity {gamma}-hydroxybutyric acid binding sites

γ-Hydroxybutyric acid (GHB) is a therapeutic drug, a drug of abuse, and an endogenous substance that binds to low- and high-affinity sites in the mammalian brain. To target the specific GHB binding sites, we have developed a (125)I-labeled GHB analog and characterized its binding in rat brain homogenate and slices. Our data show that [(125)I]4-hydroxy-4-[4-(2-iodobenzyloxy)phenyl]butanoate ([(125)I]BnOPh-GHB) binds to one site in rat brain cortical membranes with low nanomolar affinity (K(d), 7 nM; B(max), 61 pmol/mg protein). The binding is inhibited by GHB and selected analogs, but not by γ-aminobutyric acid. Autoradiography using horizontal slices from rat brain demonstrates the highest density of binding in hippocampus and cortical regions and the lowest density in the cerebellum. Altogether, the findings correlate with the labeling and brain regional distribution of high-affinity GHB sites or [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid ([(3)H]NCS-382) binding sites. Using a (125)I-labeled photoaffinity derivative of the new GHB ligand, we have performed denaturing protein electrophoresis and detected one major protein band with an apparent mass of 50 kDa from cortical and hippocampal membranes. [(125)I]BnOPh-GHB is the first reported (125)I-labeled GHB radioligand and is a useful tool for in vitro studies of the specific high-affinity GHB binding sites. The related photoaffinity linker [(125)I]4-hydroxy-4-[4-(2-azido-5-iodobenzyloxy)phenyl]butanoate can be used as a probe for isolation of the elusive GHB binding protein.

Characterization and pharmacology of the GHB receptor

Radioligand binding using [(3)H]NCS-382, an antagonist of the GHB receptor, revealed specific binding sites in the rat cerebrocortical and hippocampal membranes. Scatchard analysis of saturation isotherms revealed two different populations of binding sites. NCS-382 was about 10 times more potent than GHB in inhibiting [(3)H]NCS-382 binding. A variety of ligands for other receptors did not affect [(3)H]NCS-382 binding. Quantitative autoradiographic analysis of [(3)H]NCS-382 binding revealed similar characteristics. Thus [(3)H]NCS-382, being more potent and selective, offers advantage over [(3)H]GHB as a radioligand. Unlike GHB, several analogues of GHB such as UMB68 (a tertiary alcohol analogue of GHB), UMB86 (4-hydroxy-4-napthylbutanoic acid, sodium salt), UMB72 [4-(3-phenylpropyloxy)butyric acid, sodium salt], UMB73 (4-benzyloxybutyric acid, sodium salt), UMB66 (3-chloropropanoic acid), gamma-hydroxyvaleric acid (that is, GHV, a 4-methyl-substituted analogue of GHB), 3-HPA (3-hydroxyphenylacetic acid), and ethers of 3-hydroxyphenylacetic acid (UMB108, UMB109, and UMB119) displaced [(3)H]NCS-382 without affecting [(3)H]GABA binding to GABA(B) receptor. Thus these compounds offer an advantage over GHB as an experimental tool. Our study, aimed at exploring the potential involvement of the GHB receptor in the pharmacology of ethanol, indicated that ethanol does not affect [(3)H]NCS-382 binding in the rat brain, thereby suggesting that ethanol does not interact directly with the GHB receptor. Our study, aimed at exploring the involvement of the GHB receptor in the pathology of succinate semialdehyde dehydrogenase deficiency, which is known to cause elevation of GHB levels, revealed no change in the affinity, receptor density or displacement potency as determined by using [(3)H]NCS-382 as a radioligand in Aldh5a1(-/-) vs. Aldh5a1(+/+) mouse brain.

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.

The distribution of gamma-hydroxybutyrate-induced Fos expression in rat brain: comparison with baclofen

gamma-Hydroxybutyrate (GHB) is a euphoric, prosocial and sleep inducing drug that binds with high affinity to its own GHB receptor site and also more weakly to GABA(B) receptors. GHB is efficacious in the treatment of narcolepsy and alcoholism, but heavy use can lead to dependence and withdrawal. Many effects of GHB (sedation, hypothermia, catalepsy) are mimicked by GABA(B) receptor agonists (e.g. baclofen). However other effects (euphoric and prosocial effects and a therapeutic effect in narcolepsy) are not. The present study used Fos immunohistochemistry to assess the neural activation produced in rat brain by medium to high doses of GHB (250, 500 and 1000 mg/kg) and a high dose of baclofen (10 mg/kg) that produced similar sedation to 500 mg/kg GHB. Results showed many common regions of activation with these two drugs including the supraoptic, paraventricular, median preoptic and ventral premammillary nuclei of the hypothalamus, the central nucleus of the amygdala, Edinger-Westphal nucleus, lateral parabrachial nucleus, locus coeruleus, and nucleus of the solitary tract. GHB (500 mg/kg), but not baclofen (10 mg/kg), induced significant Fos expression in the median raphe nucleus and lateral habenula, while a higher dose of GHB (1000 mg/kg) induced additional Fos expression in the islands of Calleja, dentate gyrus (polymorphic layer) and arcuate nucleus, and in various regions implicated in rapid and non-rapid eye movement sleep (laterodorsal tegmental nucleus, tuberomammillary nucleus and the ventrolateral and anterodorsal preoptic nuclei). Surprisingly, Fos immunoreactivity was not observed with either GHB or baclofen in reward-relevant regions such as the nucleus accumbens, striatum and ventral tegmental area. Overall these results indicate a distinctive signature of brain activation with GHB that may be only partly due to GABA(B) receptor effects. This confirms a unique neuropharmacological profile for GHB and indicates key neural substrates that may underlie its characteristic influence on sleep, body temperature, sociability and endocrine function.

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.

Metabolic GHB precursor succinate binds to gamma-hydroxybutyrate receptors: characterization of human basal ganglia areas nucleus accumbens and globus pallidus

Binding of the metabolic gamma-hydroxybutyrate (GHB) precursor succinate to NCS-382-sensitive [3H]GHB-labeled sites in crude synaptosomal or purified synaptic membrane fractions prepared from the human nucleus accumbens (NA), globus pallidus (GP) and rat forebrain has been shown. This site can be characterized by binding of ethyl hemisuccinate and gap-junction blockers, including carbenoxolone hemisuccinate and beta-GRA. There was no significant binding interaction between GABAB receptor ligands (CGP 55845, (R)-baclofen) and these [3H]GHB-labeled sites. GHB, NCS-382 and succinate binding profile of [3H]GHB-labeled sites in rat forebrain, human NA or GP synaptic membranes were similar. The synaptic fraction isolated from the rat forebrain was characterized by GHB binding inhibition constants: Ki,NCS-382 = 1.2 +/- 0.2 microM, Ki,GHB = 1.6 +/- 0.3 microM and Ki,SUCCINATE = 212 +/- 66 microM. In crude membranes containing mainly extrasynaptic membranes, distinct GHB and GABAB receptor sites were found in the NA. By contrast, extrasynaptic GABAB receptor sites of rat forebrain and GP were GHB- and succinate-sensitive, respectively. The heterogeneity of GABAB sites found in native membranes indicates GABAB receptor-dependent differences in GHB action. Based on these findings, we suggest that succinate (and possibly drugs available as succinate salt derivatives) can mimic some of the actions of GHB.

Effects of drinking and smoking on endogenous levels of urinary γ-hydroxybutyric acid, a preliminary study

The aim of this study was to determine if the endogenous levels of gamma-hydroxybutyric acid (GHB) in urine were affected by drinking and smoking. Urine samples were obtained from 20 healthy volunteers (15 males, 21-45 years; 5 females, 22-24 years). This population included four average drinkers (males), 4 average smokers (males), and 12 nonsmokers/nondrinkers (seven males and five females). Urinary levels of GHB were measured by gas chromatography. No gender differences were observed in the urinary levels of endogenous GHB. The urinary levels of GHB in males were 0.52+/-0.37 microg/ml in smokers, 0.28+/-0.21 microg/ml in nonsmokers/nondrinkers, and 0.23+/-0.04 microg/ml in drinkers. Urinary GHB levels were measured three times a day for 5 consecutive days in a male from each group. Large intra-individual differences were observed over the 5-day period in a smoker and a nonsmoker/nondrinker. No significant changes in daily endogenous GHB levels were observed in a drinker during the period. Our preliminary results suggest that stimulatory effects of nicotine on the central nervous system (CNS) may result in an increase in nocturnal formation of GHB and the depressive effects of ethanol on the CNS may not affect, even may inhibit, nocturnal production of GHB.

Immunohistochemical localization of a GHB receptor-like protein isolated from rat brain

Gamma-hydroxybutyrate (GHB) is a substance derived from the metabolism of GABA and is heterogeneously distributed in various regions of the brain. This compound possesses a neuromodulatory role on several types of synapses, particularly those using GABA as a neurotransmitter. At physiological concentrations, this effect of GHB is mediated via specific receptors that induce neuronal hyperpolarization and bind radioactive GHB with a specific distribution, ontogenesis, kinetics, and pharmacology. A membrane protein that possesses six to seven transmembrane domains and which binds and is activated by micromolar amounts of GHB was recently cloned from rat brain hippocampus. In order to study the regional and cellular distribution of this receptor in rat brain, we selected several specific peptides belonging to the extracellular domains of the receptor to be used as specific immunogens to raise polyclonal antibodies in the rabbit. Among the antisera obtained, one of them gave particularly good results in terms of specificity and reactivity at high dilution. Immunohistochemical analyses, both at the confocal and electron microscopic level, showed receptor protein distribution closely resembling the distribution of GHB high-affinity binding sites, except for cerebellum, where GHB receptor(s) of lower affinity exist(s). In all regions studied the GHB receptor-like protein labeling appears to be distributed specifically in neurons and not in glial cells. At the cellular level the antibody specifically labels dendrites, and no immunoreactivity was detected in presynaptic endings or in axons. Accordingly, electron microscopy reveals strong labeling of postsynaptic densities and of neuronal cytosol.

Novel cyclic gamma-hydroxybutyrate (GHB) analogs with high affinity and stereoselectivity of binding to GHB sites in rat brain

Gamma-hydroxybutyrate (GHB) is a psychotropic compound endogenous to the brain. Despite its potentially great physiological significance, its exact molecular mechanism of action is unknown. GHB is a weak agonist at GABA(B) receptors, but there is also evidence of specific GHB receptor sites, the molecular cloning of which remains a challenge. Ligands with high affinity and specificity for the reported GHB binding site are needed for pharmacological dissection of the GHB and GABA(B) effects and for mapping the structural requirements of the GHB receptor-ligand interactions. For this purpose, we have synthesized and assayed three conformationally restricted GHB analogs for binding against the GHB-specific ligand [3H]NCS-382 [(E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene-)acetic acid] in rat brain homogenate. The cyclohexene and cyclopentene analogs, 3-hydroxycyclohex-1-enecarboxylic acid [(RS)-HOCHCA] and 3-hydroxycyclopent-1-enecarboxylic acid [(RS)-HOCPCA], were found to be high-affinity GHB ligands, with IC50 values in the nanomolar range, and had 9 and 27 times, respectively, higher affinity than GHB. The stereo-selectively synthesized R,R-isomer of the trans-cyclopropyl GHB analog, HOCPrCA, proved to have 10-fold higher affinity than its enantiomer. Likewise, the R-enantiomers of HOCHCA and HOCPCA selectively inhibited [3H]NCS-382 binding. The best inhibitor of these, (R)-HOCPCA, has an affinity 39 times higher than GHB and is thus among the best GHB ligands reported to date. Neither of the cycloalkenes showed any affinity (IC50 > 1 mM) for GABA(A) or GABA(B) receptors. These compounds show excellent potential as lead structures and novel tools for studying specific GHB receptor-mediated pharmacology.

Endogenous gamma-aminobutyric acid (GABA)(A) receptor active neurosteroids and the sedative/hypnotic action of gamma-hydroxybutyric acid (GHB): a study in GHB-S (sensitive) and GHB-R (resistant) rat lines

In the rat brain, gamma-hydroxybutyric-acid (GHB) increases the concentrations of 3alpha-hydroxy,5alpha-pregnan-20-one (allopregnanolone, 3alpha,5alpha-THP) and 3alpha,21-dihydroxy,5alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone/3alpha,5alphaTHDOC), two neurosteroids acting as positive allosteric modulators of gamma-aminobutyric acid (GABA)(A) receptors. This study was aimed at assessing whether neurosteroids play a role in GHB-induced loss of righting reflex (LORR). Basal and GHB-stimulated brain concentrations of endogenous 3alpha,5alpha-THP and 3alpha,5alpha-THDOC were analyzed in two rat lines, GHB-sensitive (GHB-S) and GHB-resistant (GHB-R), selectively bred for opposite sensitivity to GHB-induced sedation/hypnosis. Basal neurosteroid concentrations were similar in brain cortex of the two rat lines. However, in male GHB-S rats, administration of GHB (1000 mg/kg, i.p., 30 min) increased brain cortical concentrations of 3alpha,5alpha-THP and 3alpha,5alpha-THDOC 7- and 2.5-fold, respectively, whilst male GHB-R animals displayed only a 4- and 2-fold increase, respectively. In GHB-S rats this increase lasted up to 90 min and declined 180 min following GHB administration, a time course that matches LORR onset and duration. In contrast, in GHB-R rats, which failed to show GHB-induced LORR, brain cortical 3alpha,5alpha-THP and 3alpha,5alpha-THDOC had returned to control values within 90 min. At onset of LORR, a similar increase in brain cortical levels of 3alpha,5alpha-THP and 3alpha,5alpha-THDOC (2-3-fold) was observed in GHB-S female rats and in the few female GHB-R rats that lost the righting reflex after GHB administration, but not in female GHB-R rats failing to show LORR. Sub-hypnotic doses (7.5 and 12.5 mg/kg, i.p.) of pregnanolone, administered 10 min before GHB, dose-dependently facilitated the expression of GHB-induced LORR in GHB-R male rats. These results suggest that the GHB-induced increases of brain 3alpha,5alpha-THP and 3alpha,5alpha-THDOC concentrations are implicated in the eliciting of the sedative/hypnotic action of GHB.

Gamma-hydroxybutyric acid (GHB) and gamma-aminobutyric acidB receptor (GABABR) binding sites are distinctive from one another: molecular evidence

gamma-Hydroxybutyric Acid (GHB) is thought to be a weak partial agonist at the gamma-aminobutyric acid(B) Receptor (GABA(B)R), but the precise relationship of the GHB receptor (GHBR) to the GABA(B)R remains unclear. In order to test the hypothesis that the GHBR is not identical to the GABA(B)R, we conducted two groups of experiments. First, GABA(B)R subtype 1 (R1) and/or subtype 2 (R2) were over expressed in HEK 293 cells and membrane binding studies on the transfected cells done using [(3)H]GHB and [(3)H] (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid ([(3)H]NCS-382). The latter is a specific antagonist at the GHB binding site. Second, [(3)H]GHB and [(3)H]NCS-382 autoradiographic binding studies were done on the brains of mice in which the gene for GABA(B)R1a was deleted. Such mice do not have a functioning GABA(B)R. There was no detectable specific [(3)H]GHB or [(3)H]NCS-382 binding in HEK 293 cells transfected with GABA(B)R1, R2, or R1/R2. Binding to [(3)H]CGP54626A, a high affinity GABA(B)R antagonist, was absent in GABA(B)R1a(-/-) mice. There was no difference in [(3)H]NCS-382 binding observed in the brains of GABA(B)R1a(-/-), GABA(B)R1a(+/-) or GABA(B)R1a(+/+) mice. Specific [(3)H]GHB binding was observed in the brain of GABA(B)R1a(-/-) mice but was significantly lower than in wild type mice. These data support the hypothesis that the GHB binding site is separate and distinct from the GABA(B)R.

Gamma-hydroxybutyric acid: neurobiology and toxicology of a recreational drug

gamma-Hydroxybutyric acid (GHB) is a short-chain fatty acid that occurs naturally in mammalian brain where it is derived metabolically from gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain. GHB was synthesised over 40 years ago and its presence in the brain and a number of aspects of its biological, pharmacological and toxicological properties have been elucidated over the last 20-30 years. However, widespread interest in this compound has arisen only in the past 5-10 years, primarily as a result of the emergence of GHB as a major recreational drug and public health problem in the US. There is considerable evidence that GHB may be a neuromodulator in the brain. GHB has multiple neuronal mechanisms including activation of both the gamma-aminobutyric acid type B (GABA(B)) receptor, and a separate GHB-specific receptor. This complex GHB-GABA(B) receptor interaction is probably responsible for the protean pharmacological, electroencephalographic, behavioural and toxicological effects of GHB, as well as the perturbations of learning and memory associated with supra-physiological concentrations of GHB in the brain that result from the exogenous administration of this drug in the clinical context of GHB abuse, addiction and withdrawal. Investigation of the inborn error of metabolism succinic semialdehyde deficiency (SSADH) and the murine model of this disorder (SSADH knockout mice), in which GHB plays a major role, may help dissect out GHB- and GABA(B) receptor-mediated mechanisms. In particular, the mechanisms that are operative in the molecular pathogenesis of GHB addiction and withdrawal as well as the absence seizures observed in the GHB-treated animals.

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.

Protection by the GABAB receptor antagonist, SCH 50911, of γ-hydroxybutyric acid-induced mortality in mice

Different effects of moderate to high doses of gamma-hydroxybutyric acid, including sedation/hypnosis, have been found to be blocked by gamma-aminobutyric acidB (GABAB) receptor antagonists. The present study investigated whether the protective effect of GABAB receptor antagonists extends also to gamma-hydroxybutyric acid-induced mortality. To this aim, the present study investigated the effect of the GABAB receptor antagonist, (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911; 100 mg/kg, ip), on mortality induced by gamma-hydroxybutyric acid (1-6 g/kg, ip) in DBA mice. Pretreatment with SCH 50911 resulted in a significant shift to the right of the dose-response curve of gamma-hydroxybutyric acid-induced mortality. Accordingly, the LD50 in SCH 50911-pretreated mice was significantly higher than that obtained in water-pretreated mice. The results of the present study support the hypothesis that (a) the GABAB receptor is a relevant site of action of gamma-hydroxybutyric acid, and (b) GABAB receptor antagonists may constitute potentially effective therapeutic interventions for gamma-hydroxybutyric acid intoxication.

From the street to the brain: neurobiology of the recreational drug gamma-hydroxybutyric acid

gamma-Hydroxybutyric acid (GHB) is a short-chain fatty acid that occurs naturally in the mammalian brain and is formed primarily from the precursor gamma-aminobutyric acid (GABA). The properties of GHB suggest that it has a neuromodulatory role in the brain and has the ability to induce several pharmacological and behavioral effects. GHB has been used clinically as an anesthetic and to treat alcoholism and narcolepsy. Furthermore, GHB has emerged recently as a major recreational drug of abuse. GHB appears to have dual mechanisms of action in the brain. Biochemical data suggest that the intrinsic neurobiological activity of GHB might be mediated through the GHB receptor, which is separate and distinct from the GABA(B) receptor. However, many of the pharmacological and clinical effects of exogenously administered GHB, including the properties of addiction, tolerance, withdrawal and intoxication, are probably mediated via the GABA(B) receptor, where GHB might act both directly as a partial agonist and indirectly through GHB-derived GABA.

Specific gamma-hydroxybutyrate-binding sites but loss of pharmacological effects of gamma-hydroxybutyrate in GABA(B)(1)-deficient mice

gamma-Hydroxybutyrate (GHB), a metabolite of gamma-aminobutyric acid (GABA), is proposed to function as a neurotransmitter or neuromodulator. gamma-Hydroxybutyrate and its prodrug, gamma-butyrolactone (GBL), recently received increased public attention as they emerged as popular drugs of abuse. The actions of GHB/GBL are believed to be mediated by GABAB and/or specific GHB receptors, the latter corresponding to high-affinity [3H]GHB-binding sites coupled to G-proteins. To investigate the contribution of GABAB receptors to GHB actions we studied the effects of GHB in GABAB(1)-/- mice, which lack functional GABAB receptors. Autoradiography reveals a similar spatial distribution of [3H]GHB-binding sites in brains of GABAB(1)-/- and wild-type mice. The maximal number of binding sites and the KD values for the putative GHB antagonist [3H]6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene acetic acid (NCS-382) appear unchanged in GABAB(1)-/- compared with wild-type mice, demonstrating that GHB- are distinct from GABAB-binding sites. In the presence of the GABAB receptor positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol GHB induced functional GTPgamma[35S] responses in brain membrane preparations from wild-type but not GABAB(1)-/- mice. The GTPgamma[35S] responses in wild-type mice were blocked by the GABAB antagonist [3-[[1-(S)-(3,4dichlorophenyl)ethyl]amino]-2-(S)-hydroxy-propyl]-cyclohexylmethyl phosphinic acid hydrochloride (CGP54626) but not by NCS-382. Altogether, these findings suggest that the GHB-induced GTPgamma[35S] responses are mediated by GABAB receptors. Following GHB or GBL application, GABAB(1)-/- mice showed neither the hypolocomotion, hypothermia, increase in striatal dopamine synthesis nor electroencephalogram delta-wave induction seen in wild-type mice. It, therefore, appears that all studied GHB effects are GABAB receptor dependent. The molecular nature and the signalling properties of the specific [3H]GHB-binding sites remain elusive.

GABA, gamma-hydroxybutyric acid, and neurological disease

gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. GABA is converted from glutamic acid by the action of glutamic acid decarboxylase (GAD) of which two isoforms exist GAD65 and GAD67. GABA then is broken down, both within the cell and in the synaptic cleft by GABA transaminase to form succinic semialdehyde. In turn, succinic semialdehyde is converted either to succinic acid by succinic semialdehyde dehydrogenase or into gamma-hydroxybutyric acid (GHB) by succinic semialdehyde reductase. Because GABA modulates the majority of inhibition that is ongoing in the brain, perturbations in GABAergic inhibition have the potential to result in seizures. Therefore, the most common disorder in which GABA is targeted as a treatment is epilepsy. However, other disorders such as psychiatric disease, spasticity, and stiff-person syndrome all have been related to disorders of GABAergic function in the brain. This review covers the roles of GABAergic neurotransmission in epilepsy, anxiety disorders, schizophrenia, stiff-person syndrome, and premenstrual dysphoric disorder. In the final section of this review, the GABA metabolite GHB is discussed in terms of its physiological significance and its role in epilepsy, sleep disorders, drug and alcohol addiction, and an inborn error of GABA metabolism, succinic semialdehyde dehydrogenase deficiency.

Selective gamma-hydroxybutyric acid receptor ligands increase extracellular glutamate in the hippocampus, but fail to activate G protein and to produce the sedative/hypnotic effect of gamma-hydroxybutyric acid

Two gamma-hydroxybutyric acid (GHB) analogues, trans-gamma-hydroxycrotonic acid (t-HCA) and gamma-(p-methoxybenzyl)-gamma-hydroxybutyric acid (NCS-435) displaced [3H]GHB from GHB receptors with the same affinity as GHB but, unlike GHB, failed to displace [3H]baclofen from GABAB receptors. The effect of the GHB analogues, GHB and baclofen, on G protein activity and hippocampal extracellular glutamate levels was compared. While GHB and baclofen stimulated 5'-O-(3-[35S]thiotriphospate) [35S]GTPgammaS binding both in cortex homogenate and cortical slices, t-HCA and NCS-435 were ineffective up to 1 mm concentration. GHB and baclofen effect was suppressed by the GABAB antagonist CGP 35348 but not by the GHB receptor antagonist NCS-382. Perfused into rat hippocampus, 500 nm and 1 mm GHB increased and decreased extracellular glutamate levels, respectively. GHB stimulation was suppressed by NCS-382, while GHB inhibition by CGP 35348. t-HCA and NCS-435 (0.1-1000 microm) locally perfused into hippocampus increased extracellular glutamate; this effect was inhibited by NCS-382 (10 microm) but not by CGP 35348 (500 microm). The results indicate that GHB-induced G protein activation and reduction of glutamate levels are GABAB-mediated effects, while the increase of glutamate levels is a GHB-mediated effect. Neither t-HCA nor NCS-435 reproduced GHB sedative/hypnotic effect in mice, confirming that this effect is GABAB-mediated. The GHB analogues constitute important tools for understanding the physiological role of endogenous GHB and its receptor.

Effects of γ-Hydroxybutyrate (GHB) on Schedule-Controlled Responding in Rats: Role of GHB and GABAB Receptors

Gamma-hydroxybutyrate (GHB), a metabolite of gamma-aminobutyric acid (GABA), is an increasingly popular drug of abuse and was recently approved for the treatment of narcolepsy (Xyrem). GHB and GABA receptors have been implicated in mediating effects of GHB; however, the relative importance of each of these receptors is unclear. This study evaluated the effects of selective antagonists in combination with GHB and related compounds on schedule-controlled responding. Eight male Sprague-Dawley rats responded under a fixed-ratio schedule of food presentation. Cumulative dose-effect curves were generated and ED50 values calculated to evaluate the relative potency at decreasing responding. The rank-order potency was as follows: diazepam = baclofen > gamma-butyrolactone (GBL) > 1,4-butanediol (1,4-BDL) = GHB. All compounds decreased responding 20 min after administration. The duration of action of diazepam, GHB, and GBL was shorter than that of 1,4-BDL and baclofen. p-3-Aminopropyl-p-diethoxymethyl phosphinic acid (CGP 35348) antagonized the rate-decreasing effects of baclofen and not GHB; flumazenil antagonized the effects of diazepam and not GHB. The GHB receptor antagonist (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid (NCS-382) did not attenuate the rate-decreasing effects of GHB, baclofen, or diazepam; larger doses of NCS-382 further decreased rate of responding when given in combination with each of these compounds. These studies show that GBL, 1,4-BDL, and GHB differ significantly in potency and duration of action. The ability of CGP 35348 to antagonize the rate-decreasing effects of baclofen may be limited by the involvement of multiple GABAB receptor subtypes and the lack of antagonism of GHB by NCS-382 may be due to its own GHB-like effects.

Quantitative autoradiographic analysis of the new radioligand [(3)H](2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid ([(3)H]NCS-382) at gamma-hydroxybutyric acid (GHB) binding sites in rat brain

(2E)-(5-Hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid (NCS-382) is an antagonist for gamma-hydroxybutyric acid (GHB) at GHB receptor sites. Advantages of using [(3)H]NCS-382 over [(3)H]GHB in radioligand binding studies are that unlike GHB, NCS-382 does not appear to bind to, activate, or interfere with the functioning of GABA(B) or GABA(A) receptors, either directly or indirectly. Herein we establish a protocol for use of [(3)H]NCS-382 by quantitative autoradiography. GHB was used to define non-specific binding, since it displaced [(3)H]NCS-382 to an extent equivalent to NCS-382. Among many areas of brain examined, two regions in which high specific binding of [(3)H]NCS-382 occurred were the hippocampus and cerebral cortex. Areas such as the striatum and nucleus accumbens exhibited intermediate levels of specific binding. No or very low binding was observed in other areas such as the cerebellum and dorsal raphe nucleus. The distribution of GHB binding sites as defined by [(3)H]NCS-382 suggests that GHB may play a role in neuromodulation or neurotransmission in frontal brain areas.

A tertiary alcohol analog of gamma-hydroxybutyric acid as a specific gamma-hydroxybutyric acid receptor ligand

gamma-Hydroxybutyric acid (GHB) shows great promise as a treatment for sleeping disorders but is also increasingly abused. The exact mechanism of action of GHB is yet to be delineated, but it is known to interact with specific GHB binding sites or receptors, to act as a weak agonist at GABA(B) receptors, and that GHB undergoes metabolism to GABA. In drug discrimination studies, GABA(B) agonists, and to a lesser extent GABA(A)-positive modulators, substitute for GHB. To delineate the relative contributions of each receptor system to the profile of GHB, tertiary alcohol analogs of GHB and its homolog, 5-hydroxypentanoic acid (UMB58), were prepared (UMB68 and UMB75, respectively), which cannot be metabolized to GABA-active compounds. Binding studies against [(3)H]NCS-382 [(2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid] showed that the tertiary alcohol analog of GHB (UMB68) has similar affinity to GHB, with the longer chain analogs possessing lower affinity. Against [(3)H]GABA, UMB68 showed no affinity (IC(50) >100 microM) at GABA(A) or GABA(B) receptors. In vivo studies showed that, at behaviorally active doses, rats trained to discriminate GHB did not recognize the novel ligands as GHB. Thus, UMB68 is a selective GHB receptor ligand in binding assays, will not undergo metabolism to GABA-active compounds, and does not show the same effects as GHB in vivo. These data suggest that, although UMB68 binds to the GHB receptor, it does not have the observed GABA receptor-mediated effects of GHB in vivo and could provide a novel tool for studying the pharmacology of the GHB receptor in the absence of complicating GABAergic effects.

The role of GABAB receptors in the discriminative stimulus effects of gamma-hydroxybutyrate in rats: time course and antagonism studies

gamma-Hydroxybutyrate (GHB) is a neurotransmitter in brain and an emerging drug of abuse, although its mechanism of action is poorly understood. This study characterized the role of GABA(A), GABA(B), and other receptors in the discriminative stimulus effects of GHB. Eight rats reliably discriminated 200 mg/kg GHB from saline after a median of 35 (range: 23-41) training sessions. GHB, a metabolic precursor 1,4-butanediol (1,4-BDL), and the GABA(B) agonist (+/-)baclofen all occasioned greater than 83% responding on the GHB lever. The onset of action was similar for GHB and 1,4-BDL; however, 1,4-BDL exhibited a longer duration of action than GHB. The GHB precursor gamma-butyrolactone, the benzodiazepine diazepam, the neuroactive steroid pregnanolone, the opioid agonist morphine, and the N-methyl-d-aspartate antagonist ketamine elicited substantial GHB-appropriate responding, although none occasioned greater than 66% drug-lever responding. The barbiturate pentobarbital and the GABA(A) receptor agonist muscimol did not occasion greater than 17% drug-lever responding at any dose tested. The benzodiazepine antagonist flumazenil attenuated GHB-lever responding occasioned by diazepam, but not GHB. The GABA(B) receptor antagonist CGP 35348 antagonized GHB-lever responding occasioned by baclofen or GHB. Small doses of the purported GHB receptor antagonist (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid (NCS-382) attenuated partially the effects of GHB, whereas larger doses of NCS-382 alone occasioned partial GHB-lever responding. These results implicate GABA(B) mechanisms in the discriminative stimulus effects of GHB and further suggest that the effects of 1,4-BDL under these conditions result from its conversion to GHB. That NCS-382 shares effects with GHB could explain the lack of antagonism reported for NCS-382 in some studies.

GABA(B)-receptor mediation of the inhibitory effect of gamma-hydroxybutyric acid on intestinal motility in mice

The effect of acutely administered gamma-hydroxybutyric acid (GHB) and GHB receptor antagonist, NCS-382, on the propulsive activity in the mouse small intestine was assessed by measuring the transit of an orally administered, non absorbable marker. Both GHB (0, 25, 50, 100, 200 and 300 mg/kg; i.p.) and NCS-382 (0, 25, 50 and 75 mg/kg; i.p.) induced a dose-dependent inhibition (up to 50-60%) of the marker transit. Pretreatment with the GABA(B) receptor antagonist, SCH 50911 (100 mg/kg; i.p.), resulted in the blockade of the inhibiting effect of both GHB and NCS-382. These results suggest that the constipating effect of GHB and NCS-382 are secondary to stimulation of the GABA(B) receptor.

GABA(B) receptor-mediated increase of neurosteroids by gamma-hydroxybutyric acid

Among the pharmacological actions of gamma-hydroxybutyric acid (GHB), some may involve GABA(A) receptor-mediated mechanisms. GHB, however, fails to directly interact with sites for agonists and modulators on the GABA(A) receptor complex. We hypothesized that, in vivo, GHB may interfere with GABA(A) receptor function by altering the brain concentrations of the neurosteroids 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone, AP) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone, THDOC), positive allosteric modulators of GABA-gated chloride currents. In male Wistar rats, GHB dose-dependently (75-1000 mg/kg, i.p.) increased AP, THDOC and their precursors pregnenolone and progesterone in brain cortex and hippocampus. The increases of AP (4-5 fold) and THDOC (3-4 fold) elicited by 300 mg/kg GHB peaked between 30 and 90 min and abated by 180 min. The selective GABA(B) receptor antagonist SCH 50911 (50 mg/kg, i.p.) prevented the action of GHB, while the GABA(B) receptor agonist baclofen (5-10 mg/kg) mimicked it. NCS-382 (50 mg/kg, i.p.), the purported selective antagonist of the GHB receptor, failed to antagonize GHB, but at 300 mg/kg increased brain cortical neurosteroids to the same extent as 300 mg/kg GHB; coadministration of GHB and NCS-382, however, failed to yield an additive effect. These results strongly suggest that GHB, via a GABA(B) receptor-mediated mechanism, increases the brain concentrations of neurosteroids, whose properties as amplifiers of the GABA-gated chloride conductances may play a role in the GABA(A) receptor-mediated pharmacological actions of GHB.

Central effects of 1,4-butanediol are mediated by GABA(B) receptors via its conversion into gamma-hydroxybutyric acid

The aliphatic alcohol 1,4-butanediol in converted into gamma-hydroxybutyric acid (GHB) via two enzymatic steps: first, it is oxidised by alcohol dehydrogenase in gamma-hydroxybutyraldehyde; second, the latter is transformed, likely by aldehyde dehydrogenase, into GHB. Initially, the present study compared the sedative/hypnotic effect of GHB and 1,4-butanediol, measured as loss of righting reflex. 1,4-Butanediol was more potent than GHB, presumably because of a more rapid penetration of the blood brain barrier. Further alcohol dehydrogenase inhibitors, 4-methylpyrazole and ethanol, totally prevented the sedative/hypnotic effect of 1,4-butanediol; the aldehyde dehydrogenase inhibitor disulfiram partially blocked the sedative/hypnotic effect of 1,4-butanediol. Finally, the sedative/hypnotic effect of 1,4-butanediol was antagonised by the GABA(B) receptor antagonists, SCH 50911 [(2S)(+)-5,5-dimethyl-2-morpholineacetic acid] and CGP 46381 [(3-aminopropyl)(cyclohexylmethyl)phosphinic acid], but not by the putative GHB receptor antagonist NCS-382 (6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylideneacetic acid), indicating that it is mediated by GABA(B) but not GHB receptors. Taken together, these results suggest that the sedative/hypnotic effect of 1,4-butanediol is mediated by its conversion in vivo into GHB which, in turn, binds to GABA(B) receptors. Accordingly 1,4-butanediol, unlike GHB, failed to displace [(3)H]GHB and [(3)H]baclofen in brain membranes.

Relationship between gamma-hydroxybutyrate plasma concentrations and its electroencephalographic effects in the rat

In view of the potential interest in an objective parameter for the depth of coma in intoxications with the recreational drug gamma-hydroxybutyrate (GHB), we have studied the relationship between the plasma concentrations and the electroencephalographic (EEG) changes induced by GHB in the rat. Fifteen rats randomly received either 150 (n = 3), 200 (n = 6) or 300 mg kg(-1) (n = 6) GHB over 5 min, followed by a supramaximal dose of 450 mg kg(-1) over 5 min at the end of the experiment. Plasma concentrations were determined with HPLC. The EEG was continuously recorded and the amplitude in the 15.5-30 Hz frequency band was quantified using aperiodic analysis. The plasma concentration-time profiles were fitted to a two-compartment model with Michaelis-Menten elimination. The pharmacokinetic parameters Vmax, Km and the apparent volume of distribution (Vd) proved to be independent of the dose and the mean pooled values were Vmax 2068 +/- 140 microg min(-1) kg(-1), Km 58 +/- 16 microg mL(-1) and Vd 476 +/- 12 mL kg(-1). The EEG amplitude in the 15.5-30 Hz frequency band displayed a monophasic inhibition and the effect-plasma concentration curve showed hysteresis. This hysteresis between EEG effect and plasma concentrations was minimized by simultaneous calculation of hypothetical effect-site concentrations and fitting the effect vs effect-site concentration curve to a sigmoid inhibitory Emax model. The descriptors of this Emax model (Emax, EC50, k(e,0), gamma and E0) were independent of the dose with an equilibration half-life t1/2k(e,0) of 5.6 +/- 0.3 min (mean value of the pooled results of the 5-min treatment groups). To investigate the origin of this hysteresis, a dose of 600 mg kg(-1) GHB was infused over either 45 or 60 min each in three animals. The hysteresis was much less pronounced with 45 min than with 5 min and was absent with 60-min infusions. This indicated that the hysteresis was due to a distribution delay between the central compartment and the effect site. This study showed that the concentration-effect relationship of GHB could be characterized in individual rats using aperiodic analysis in the 15.5-30 Hz frequency band.

Role of GABA(B) receptors in the sedative/hypnotic effect of gamma-hydroxybutyric acid

The present study was aimed at identifying the receptor systems involved in the mediation of the sedative/hypnotic effect of gamma-hydroxybutyric acid (GHB) in DBA mice. Administration of the putative antagonist of the GHB binding site, 6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylideneacetic acid (NCS-382; 50-500 mg/kg, i.p.), significantly increased the duration of loss of righting reflex induced by GHB (1000 mg/kg, i.p.). In contrast, the GABA(B) receptor antagonists, (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911; 25-100 mg/kg, i.p.) and (3-aminopropyl)(cyclohexylmethyl)phosphinic acid (CGP 46381; 12.5-150 mg/kg, i.p.), completely prevented the sedative/hypnotic effect of GHB. SCH 50911 (100 and 300 mg/kg, i.p.) was also capable to readily reverse the sedative/hypnotic effect of GHB (1000 mg/kg, i.p.) in mice that had lost the righting reflex. SCH 50911 (100 mg/kg, i.p.) also completely abolished the sedative/hypnotic effect of the GABA(B) receptor agonist, baclofen. These results indicate that the sedative/hypnotic effect of GHB is mediated by the stimulation of GABA(B) receptors and add further support to the hypothesis that the GABA(B) receptor constitutes a central site of action of GHB.

Evidence for a G protein-coupled gamma-hydroxybutyric acid receptor

gamma-Hydroxybutyric acid (GHB) is a naturally occurring metabolite of GABA that has been postulated to exert ubiquitous neuropharmacological effects through GABA(B) receptor (GABA(B)R)-mediated mechanisms. The alternative hypothesis that GHB acts via a GHB-specific, G protein-coupled presynaptic receptor that is different from the GABA(B)R was tested. The effect of GHB on regional and subcellular brain adenylyl cyclase in adult and developing rats was determined and compared with that of the GABA(B)R agonist (-)-baclofen. Also, using guanosine 5'-O:-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and low-K:(m) GTPase activity as markers the effects of GHB and (-)-baclofen on G protein activity in the brain were determined. Neither GHB nor baclofen had an effect on basal cyclic AMP (cAMP) levels. GHB significantly decreased forskolin-stimulated cAMP levels by 40-50% in cortex and hippocampus but not thalamus or cerebellum, whereas (-)-baclofen had an effect throughout the brain. The effect of GHB on adenylyl cyclase was observed in presynaptic and not postsynaptic subcellular tissue preparations, but the effect of baclofen was observed in both subcellular preparations. The GHB-induced alteration in forskolin-induced cAMP formation was blocked by a specific GHB antagonist but not a specific GABA(B)R antagonist. The (-)-baclofen-induced alteration in forskolin-induced cAMP formation was blocked by a specific GABA(B)R antagonist but not a specific GHB antagonist. The negative coupling of GHB to adenylyl cyclase appeared at postnatal day 21, a developmental time point that is concordant with the developmental appearance of [(3)H]GHB binding in cerebral cortex, but the effects of (-)-baclofen were present by postnatal day 14. GHB and baclofen both stimulated [(35)S]GTPgammaS binding and low-K:(m) GTPase activity by 40-50%. The GHB-induced effect was blocked by GHB antagonists but not by GABA(B)R antagonists and was seen only in cortex and hippocampus. The (-)-baclofen-induced effect was blocked by GABA(B)R antagonists but not by GHB antagonists and was observed throughout the brain. These data support the hypothesis that GHB induces a G protein-mediated decrease in adenylyl cyclase via a GHB-specific G protein-coupled presynaptic receptor that is different from the GABA(B)R.

Modulation of absence seizures by the GABA(A) receptor: a critical rolefor metabotropic glutamate receptor 4 (mGluR4)

Experimental absence seizures are associated with perturbations in the presynaptic release of GABA and glutamate within thalamocortical circuitry. The release of both glutamate and GABA is regulated by group III metabotropic glutamate receptors (mGluRs). Therefore, we examined the susceptibility of mice lacking the mGluR4 subtype of mGluR (mGluR4(-/-)) versus their wild-type controls (mGluR4(+/+)) to absence seizures induced either by gamma-hydroxybutyrate (GHB) or the GABA(B) agonist (-) baclofen or by low doses of the GABA(A) receptor (GABA(A)R) antagonists pentylenetetrazole, bicuculline, or picrotoxin. There was no difference between mGluR4(-/-) and mGluR4(+/+) mice in threshold to absence seizures induced by either GHB or (-) baclofen. In contrast, the mGluR4(-/-) mice were markedly resistant to absence seizures induced by low doses of GABA(A)R antagonists. No differences were observed between mGluR4(-/-) and mGluR4(+/+) mice in threshold to clonic or tonic seizures induced by higher doses of GABA(A)R antagonists, strychnine, or electroshock, indicating that seizure resistance in the mGluR4(-/-) mice was restricted solely to absence seizures. The resistance of mGluR4(-/-) mice to absence seizures induced by GABA(A)R antagonists was mimicked by bilateral administration of a mGluR4 antagonist into the nucleus reticularis thalami (nRT) of mGluR4(+/+) mice. Conversely, intra-nRT administration of a mGluR4 agonist in mGluR4(+/+) mice exacerbated GABA(A)R-induced absence seizures. These data indicate that the presence of mGluR4 within nRT is critical to GABAergic modulation of thalamocortical synchronization in normal and pathological states, such as generalized absence epilepsy.

gamma-aminobutyric acid type B receptors are expressed and functional in mammalian cardiomyocytes

gamma-Hydroxybutyrate (GHB), an anesthetic adjuvant analog of gamma-aminobutyrate (GABA), depresses cell excitability in hippocampal neurons by inducing hyperpolarization through the activation of a prominent inwardly rectifying K(+) (Kir3) conductance. These GABA type B (GABA(B))-like effects are clearly shown at high concentrations of GHB corresponding to blood levels usually reached during anesthesia and are mimicked by the GABA(B) agonist baclofen. Recent studies of native GABA(B) receptors (GABA(B)Rs) have favored the concept that GHB is also a selective agonist. Furthermore, cloning has demonstrated that GABA(B)Rs assemble heteromeric complexes from the GABA(B)R1 and GABA(B)R2 subtypes and that these assemblies are activated by GHB. The surprisingly high tissue content, together with anti-ischemic and protective effects of GHB in the heart, raises the question of a possible influence of GABA(B) agonists on excitable cardiac cells. In the present study, we provide electrophysiological evidence that GHB activates an inwardly rectifying K(+) current in rat ventricular myocytes. This effect is mimicked by baclofen, reversibly inhibited by GABA(B) antagonists, and prevented by pertussis toxin pretreatment. Both GABA(B)R1 and GABA(B)R2 are detected in cardiomyocytes by Western blotting and are shown to coimmunoprecipitate. Laser scanning confocal microscopy discloses an even distribution of the two receptors in the sarcolemma and along the transverse tubular system. Hence, we conclude that GABA(B)Rs are distributed not only in neuronal tissues but also in the heart, where they can be activated and induce electrophysiological alterations through G-protein-coupled inward rectifier potassium channels.

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.

Abuse and therapeutic potential of gamma-hydroxybutyric acid

Gamma-hydroxbutyric acid is a compound found in mammalian brain that is structurally related to the neurotransmitters gamma-aminobutyric acid and glutamic acid. Gamma-hydroxybutyric acid effects dopaminergic systems in the brain and may be a neurotransmitter. Gamma-hydroxybutyric acid was first reported as a drug of abuse in 1990 and continues to be abused by bodybuilders, participants of "rave" dance parties, and polydrug abusers. Physical dependence can develop after prolonged, high-dose use, and overdoses have been widely reported. Its use in sexual assaults as a "date rape" drug and availability on the internet have recently emerged. Gamma-hydroxybutyric acid has established efficacy as an anesthetic agent, and preliminary evidence supports its utility in the treatment of alcohol dependence, opiate dependence, and narcolepsy.

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.

Regulation of gamma-aminobutyric acid (GABA) release in cerebral cortex in the gamma-hydroxybutyric acid (GHB) model of absence seizures in rat

Gamma-hydroxybutyric acid (GHB) has the ability to induce absence seizures. The precise way in which GHB causes seizures remains unclear, but GABA(B)- and/or GHB-mediated presynaptic mechanisms within thalamocortical circuitry may play a role. In the present study, we determined the basal and K+-evoked release of GABA and glutamate in the superficial laminae of frontal cortex during GHB-induced absence seizures. Our data indicate that both the basal and K+-evoked release of GABA were significantly decreased in laminae I-III of frontal cortex at the onset of GHB-induced absence seizures. The appearance and disappearance of the observed changes in basal and K+-evoked extracellular levels of GABA correlated with the onset and offset of absence seizures. In contrast, neither the basal nor the K+-evoked release of glutamate was altered in superficial laminae of cerebral cortex at any time during the absence seizures. Intracortical perfusion of the GABA(B) receptor antagonists, CGP 35348 and phaclofen as well as the GHB receptor antagonist, NCS 382 attenuated GHB-mediated changes in the basal and K+-evoked release of GABA. These data suggest that GHB induces a selective decrease in the basal and depolarization-induced release of GABA in cerebral cortex, and further, that this action of GHB may play a role in the mechanism by which GHB induces absence seizures.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

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.

Sites of action of gamma-hydroxybutyrate (GHB)--a neuroactive drug with abuse potential

OBJECTIVE

This review highlights the biochemistry, pharmacology, and toxicology of the naturally-occurring fatty acid derivative, gamma-hydroxybutyrate (GHB). GHB is derived from gamma-aminobutyric acid (GABA) and is proposed to function as an inhibitory chemical transmitter in the central nervous system.

CONTENT

When administered in pharmacological doses, its powerful central nervous system depressant effects are readily observed. Although some of the neurophysiological actions of GHB could involve alterations in dopaminergic transmission in the basal ganglia, both its physiological and pharmacological actions are probably mediated through specific brain receptors for GHB. In addition, GHB might mediate some of its effects through interaction with the GABA(B) receptor. Experimentally, GHB has been used as a model for petit mal epilepsy; clinically, it has been used as a general anesthetic and as a drug to treat certain sleep disorders and related conditions. Owing to the purported ability of GHB to induce a state of euphoria, recreational use of this substance is popular. Although no deaths or long-term problems have been associated with GHB abuse, symptoms of GHB intoxication can be severe. The continued potential for GHB abuse makes it imperative for clinical toxicologists to be aware of the effects of this agent. Future research on the mechanism of action of GHB is needed to elucidate both its central nervous system depressant properties and its ability to effect a state of well-being.