Different sensitivity to the motor incoordinating effects of γ-hydroxybutyric acid (GHB) and baclofen in GHB-sensitive and GHB-resistant 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.

Differential development of tolerance to the functional and behavioral effects of repeated baclofen treatment in rats

Baclofen, a gamma-aminobutyric acid (GABA)B receptor agonist, has been used clinically to treat muscle spasticity, rigidity and pain. More recently, interest in the use of baclofen as an addiction medicine has grown, with promising preclinical cocaine and amphetamine data and demonstrated clinical benefit from alcohol and nicotine studies. Few preclinical investigations, however, have utilized chronic dosing of baclofen, which is important given that tolerance can occur to many of its effects. Thus the question of whether chronic treatment of baclofen maintains the efficacy of acute doses is imperative. The neural substrates that underlie the effects of baclofen, particularly those after chronic treatment, are also not known. In the present study, therefore, rats were treated with either a) vehicle, b) acute baclofen (5 mg/kg) or c) chronic baclofen (5 mg/kg, t.i.d. for 5 days). The effects of acute and chronic baclofen administration, compared to vehicle, were assessed using locomotor activity and changes in brain glucose metabolism (a measure of functional brain activity). Acute baclofen significantly reduced locomotor activity (horizontal and total distance traveled), while chronic baclofen failed to affect locomotor activity. Acute baclofen resulted in significantly lower rates of local cerebral glucose utilization throughout many areas of the brain, including the prefrontal cortex, caudate putamen, septum and hippocampus. The majority of these functional effects, with the exception of the caudate putamen and septum, were absent in animals chronically treated with baclofen. Despite the tolerance to the locomotor and functional effects of baclofen following repeated treatment, these persistent effects on functional activity in the caudate putamen and septum may provide insights into the way in which baclofen alters the reinforcing effects of abused substances such as cocaine, alcohol, and methamphetamine both in humans and animal models.

Enhanced expression of the neuronal K+/Cl- cotransporter, KCC2, in spontaneously depressed Flinders Sensitive Line rats

We used Flinder Sensitive Line (FSL) rats, a genetic model of unipolar depression, to examine whether changes in central GABAergic transmission are associated with a depressed phenotype. FSL rats showed an increased behavioral response to low doses of diazepam, as compared to either Sprague Dawley (SD) or Flinder Resistant Line (FRL) rats used as controls. Diazepam at a dose of 0.3 mg/kg, i.p., induced a robust impairment of motor coordination in FSL rats, but was virtually inactive in SD or FRL rats. The increased responsiveness of FSL rats was not due to changes in the brain levels of diazepam or its active metabolites, or to increases in the number or affinity of benzodiazepine recognition sites, as shown by the analysis of [(3)H]-flunitrazepam binding in the hippocampus, cerebral cortex or cerebellum. We therefore examined whether FSL rats differed from control rats for the expression levels of the K(+)/Cl(-) cotransporter, KCC2, which transports Cl(-) ions out of neurons, thus creating the concentration gradient that allows Cl(-) influx through the anion channel associated with GABA(A) receptors. Combined immunoblot and immunohistochemical data showed a widespread increase in KCC2 expression in FSL rats, as compared with control rats. The increase was more prominent in the cerebellum, where KCC2 was largely expressed in the granular layer. These data raise the interesting possibility that a spontaneous depressive state in animals is associated with an amplified GABAergic transmission in the CNS resulting from an enhanced expression of KCC2.

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.

Evaluation for the withdrawal syndrome from gamma-hydroxybutyric acid (GHB), gamma-butyrolactone (GBL), and 1,4-butanediol (1,4-BD) in different rat lines

A severe and life-threatening gamma-hydroxybutyric acid (GHB) withdrawal syndrome, clinically similar to the alcohol withdrawal syndrome, is increasingly being reported in GHB addicts. We investigated for the occurrence of withdrawal in Wistar and Sprague-Dawley rats, and in the selectively bred lines of GHB-sensitive (GHB-S) and Sardinian alcohol-preferring (sP) rats, following chronic administration of GHB, gamma-butyrolactone (GBL), and/or 1,4-butanediol (1,4-BD). Using validated rodent alcohol withdrawal scoring scales, little to no spontaneous or pharmacologically precipitated withdrawal effects were observed in Wistar, Sprague-Dawley, or GHB-S rats. Conversely, sP rats displayed both spontaneous and precipitated audiogenic seizures following abrupt cessation of chronic GHB or 1,4-BD administration and following pharmacological challenge with the GABA(B) receptor-selective antagonist, SCH 50911, respectively.

The GABAB receptor-positive modulator GS39783 and the GABAB receptor agonist baclofen attenuate the reward-facilitating effects of cocaine: intracranial self-stimulation studies in the rat

There is an increasing interest in the development of nondopaminergic pharmacotherapies for cocaine abuse. Emerging preclinical and clinical data with the metabotropic GABAB receptor agonist baclofen support a role for the modulation of GABAB receptors in the treatment of drug addiction. Nevertheless, the muscle relaxant, hypothermic, and sedative properties of baclofen somewhat limit its widespread potential therapeutic utility. Recently, positive modulators of the GABAB receptor such as GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine) have been identified. These positive modulators enhance the effects of GABA (gamma-aminobutyric acid) through actions at an allosteric site and are devoid of intrinsic agonistic efficacy. The aim of the present study was to assess the ability of the novel GABAB-positive modulator GS39873 or baclofen to modulate the behavioral effects of cocaine. Drugs of abuse such as cocaine lower brain reward thresholds obtained using intracranial self-stimulation (ICSS). We demonstrate here that GS39783 had no intrinsic effects on ICSS reward thresholds (10-100 mg/kg p.o.) in rats, whereas the full GABAB receptor agonist baclofen (2.5-5 mg/kg p.o.) dose dependently elevated thresholds. Moreover, both GS39783 and baclofen attenuated the threshold lowering effect of cocaine administration (10 mg/kg intraperitoneally) in a dose-related manner. These data strongly suggest that activation of GABAB receptors attenuates the rewarding effects of acute cocaine. Therefore, GABAB-positive modulation may represent a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse without the side effects of full GABAB receptor agonists.

GHB-C rats: The control line of GHB-sensitive (GHB-S) and GHB-resistant (GHB-R) rats

Gamma-hydroxybutyric acid (GHB)-sensitive (GHB-S) and GHB-resistant (GHB-R) rats have been selectively bred for their opposite sensitivity to the sedative/hypnotic effect of GHB. This opposite sensitivity has been found to generalize to the GABA(B) receptor agonist, baclofen. A control line [named GHB-control (GHB-C)] has been derived from the foundation stock of GHB-S and GHB-R rats. GHB-C rats have been bred without any evaluation of their sensitivity to GHB. The experiments described here were designed to evaluate the sensitivity of GHB-C rats, from the 13th generation, to the sedative/hypnotic effect of GHB (1 g/kg, i.p.) and baclofen (20 mg/kg, i.p.). All measures (onset, sleep time and r = sleep time/onset) of sensitivity to GHB- and baclofen-induced sedation/hypnosis in GHB-C rats were significantly different from and intermediate to those recorded in GHB-S and GHB-R rats. Furthermore, these values were similar to those recorded in the foundation stock. These results suggest that GHB-C rats may constitute a valid control line for GHB-S and GHB-R rats, representing the "general population" from which GHB-S and GHB-R rats were derived. Furthermore, the relative equidistance of sensitivity to GHB- and baclofen-induced sedation/hypnosis of GHB-C rats from those of GHB-S and GHB-R rats suggests that genetic factors contributes to the development of both sensitivity in GHB-S rats and resistance in GHB-R rats.

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.