Pharmacokinetics and pharmacodynamics of gamma-hydroxybutyric acid during tolerance in rats: effects on extracellular dopamine

Does sodium oxybate inhibit brain dopamine release in humans? An exploratory neuroimaging study

OBJECTIVE

To establish in an exploratory neuroimaging study whether γ-hydroxybutyrate (sodium oxybate [SO]), a sedative, anti-narcoleptic drug with abuse potential, transiently inhibits striatal dopamine release in the human.

METHODS

Ten healthy participants (30 years; 6M, 4F) and one participant with narcolepsy received a baseline positron emission tomography scan of [C-11]raclopride, a D2/3 dopamine receptor radioligand sensitive to dopamine occupancy, followed approximately one week later by an oral sedative 3g dose of SO and two [C-11]raclopride scans (1 h, 7 h post SO). Plasma SO levels and drowsiness duration were assessed.

RESULTS

No significant changes were detected in [C-11]raclopride binding in striatum overall 1 or 7 h after SO, but a small non-significant increase in [C-11]raclopride binding, implying decreased dopamine occupancy, was noted in limbic striatal subdivision at one hour (+6.5%; p uncorrected = 0.045; +13.2%, narcolepsy participant), returning to baseline at 7 h. A positive correlation was observed between drowsiness duration and percent change in [C-11]raclopride binding in limbic striatum (r = 0.73; p = 0.017).

CONCLUSIONS

We did not find evidence in this sample of human subjects of a robust striatal dopamine change, as was reported in non-human primates. Our preliminary data, requiring extension, suggest that a 3g sedative SO dose might cause slight transient inhibition of dopamine release in limbic striatum.

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.

Effect of chronic γ-hydroxybutyrate (GHB) administration on GHB toxicokinetics and GHB-induced respiratory depression

BACKGROUND

γ-hydroxybutyrate (GHB) has a high potential for illicit use; overdose of this compound results in sedation, respiratory depression and death. Tolerance to the hypnotic/sedative and electroencephalogram effects of GHB occurs with chronic GHB administration; however, tolerance to respiratory depression has not been evaluated. GHB toxicodynamic effects are mediated predominantly by GABA_B receptors. Chronic treatment may affect monocarboxylate transporters (MCTs) and alter the absorption, renal clearance and brain uptake of GHB.

OBJECTIVES

To determine effects of chronic GHB dosing on GHB toxicokinetics, GHB-induced respiratory depression, and MCT expression.

METHODS

Rats were administered GHB 600 mg/kg intravenously daily for 5 days. Plasma, urine and tissue samples and respiratory measurements were obtained on days 1 and 5. Plasma and urine were analyzed for GHB by LC/MS/MS and tissue samples for expression of MCT1, 2 and 4 and their accessory proteins by QRT-PCR.

RESULTS

No differences in GHB pharmacokinetics or respiratory depression were observed between days 1 and 5. Opposing changes in MCT1 and MCT4 mRNA expression were observed in kidney samples on day 5 compared to GHB-naïve animals, and MCT4 expression was increased in the intestine.

CONCLUSIONS

The lack of tolerance observed with GHB-induced respiratory depression, in contrast to the tolerance reported for the sedative/hypnotic and electroencephalogram effects, suggests that different GABA_B receptor subtypes may be involved in different GABA_B-mediated toxicodynamic effects of GHB. Chronic or binge users of GHB may be at no less risk for fatality from respiratory arrest with a GHB overdose than with a single dose of GHB.

γ-Hydroxybutyric Acid (GHB) Pharmacokinetics and Pharmacodynamics: Semi-Mechanistic and Physiologically Relevant PK/PD Model

An overdose of γ-hydroxybutyric acid (GHB), a drug of abuse, results in fatality caused by severe respiratory depression. In this study, a semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model was developed to characterize monocarboxylate transporter 1 (MCT1)-mediated transport of GHB, as well as effects of GHB on respiration frequency, for IV doses of 200, 600, and 1500 mg/kg in rats. The proposed PK/PD model for GHB consists of nonlinear metabolism of GHB in the liver, MCT1-mediated renal reabsorption with physiologically relevant concurrent fluid reabsorption, MCT1-mediated uptake into the brain, and direct effects of binding of GHB to GABA_B receptors on the PD parameter, respiration frequency. Michaelis-Menten affinity constants for metabolism, renal reabsorption, and uptake into and efflux from the brain were fixed to the observed in vitro values. The IC ₅₀ value for the effect of GHB on respiration frequency was fixed to a reported value for binding of GHB to GABA_B receptors. All physiological parameters were fixed to the reported values for a 300-g rat. The model successfully captured the GHB PK/PD data and was further validated using the data for a 600-mg/kg dose of GHB after IV bolus administration. Unbound GHB brain ECF/blood partition coefficient (Kp _u,u ) values obtained from the model agreed well with values calculated using experimental ECF concentrations obtained with brain microdialysis, demonstrating the physiological relevance of this model. Sensitivity analysis indicated that the PK/PD model was stable. In conclusion, we developed a semi-mechanistic and physiologically relevant PK/PD model of GHB using in vitro drug-transporter kinetics and in vivo PK/PD data in rats.

Mechanisms for the Specific Properties of γ-Hydroxybutyrate in Brain

γ-Hydroxybutyrate (GHB) is both a natural brain compound with neuromodulatory properties at central GABAergic synapses (micromolar concentration range) and also a drug (Xyrem(R) ) clinically used for the treatment of various neurological symptoms (millimolar dose range). However, this drug has abuse potential and can be addictive for some patients. Here, we review the basic mechanistic role of endogenous GHB in brain as well as the properties and mechanisms of action for therapeutic clinical doses of exogenous GHB. Several hypotheses are discussed with a preference for a molecular mechanism that conciliates most of the findings available. This conciliatory model may help for the design of GHB-like drugs active at lower doses and devoid of major side effects.

Mechanistic modeling of monocarboxylate transporter-mediated toxicokinetic/toxicodynamic interactions between γ-hydroxybutyrate and L-lactate

Overdose of γ-hydroxybutyrate (GHB) can result in severe respiratory depression. Monocarboxylate transporter (MCT) inhibitors, including L-lactate, increase GHB clearance and represent a potential treatment for GHB intoxication. GHB can also affect L-lactate clearance, and L-lactate has been reported to affect respiration. In this research, we characterize these toxicokinetic/toxicodynamic interactions between GHB and L-lactate using mechanistic modeling. Plasma, urine, and respiration data were taken from our previous study in which GHB and sodium L-lactate were administered alone and concomitantly in rats. A model incorporating active renal reabsorption for both agents fit GHB and L-lactate toxicokinetic data. The Km for renal reabsorption of GHB (650 μg/mL) was close to its Km for the proton-dependent MCT1 and that for L-lactate (13.5 μg/mL) close to its Km for the sodium-dependent SMCT1. Inhibition of reabsorption by both agents was necessary to model concomitant drug administration. The metabolic Km for L-lactate closely resembled that for MCT-mediated hepatic uptake in vitro, and GHB inhibited this process. L-lactate significantly inhibited respiration at a high dose, and an indirect response model was used to fit these data. GHB toxicodynamics was modeled as a direct effect delayed by nonlinear transport into the brain extracellular fluid, with a Km value of 1,865 μg/mL for brain uptake which is similar to the in vitro Km value determined in rat brain endothelial cells. This model was useful for characterizing multiple MCT-mediated interactions. Incorporation of many parameters that can be determined in vitro may allow for clinical translation of these interactions.

Efficacy comparison of the novel water-soluble propofol prodrug HX0969w and fospropofol in mice and rats

BACKGROUND

HX0969w is a novel water-soluble prodrug designed to release propofol and gamma-hydroxybutyrate (GHB) and has a sedative-hypnotic effect. This study was performed to compare the efficacy of HX0969w with fospropofol in mice and rats.

METHODS

We performed hydrolysis studies in the plasma from mice and rats. The half-maximal effective doses (ED50) and half-maximal lethal doses (LD50) of fospropofol and HX0969w were determined. A pharmacodynamics comparison of these two compounds was also performed. Time to loss of righting reflex, time to return of righting reflex, recovery time, and adverse effects were recorded.

RESULTS

The hydrolysis studies demonstrated that HX0969w released propofol as expected. HX0969w ED50 values in mice and rats were 133.03 and 53.79 mg kg(-1), respectively, and LD50 values were 607.11 and 283.79 mg kg(-1), respectively. The calculated therapeutic index (TI), safety index (SI), and certain safety factor (CSF) of HX0969w were 4.56, 3.33, and 2.92 for mice, and 5.28, 3.94, and 3.49 for rats, respectively. The pharmacodynamic comparison studies suggest that HX0969w has a longer onset time and shorter duration than fospropofol.

CONCLUSIONS

Similar to fospropofol, HX0969w is an effective, water-soluble prodrug that is capable of inducing a sedative-hypnotic effect in mice and rats. Unlike fospropofol, HX0969w releases GHB instead of formaldehyde. Further studies regarding the efficacy and safety of HX0969w are necessary.

Pharmacokinetics and dopamine/acetylcholine releasing effects of ginsenoside Re in hippocampus and mPFC of freely moving rats

AIM

To investigate the pharmacokinetics and dopamine/acetylcholine-releasing effects of ginsenoside Re (Re) in brain regions related to learning and memory, and to clarify the neurochemical mechanisms underlying its anti-dementia activity.

METHODS

Microdialysis was conducted on awake, freely moving adult male SD rats with dialysis probes implanted into the hippocampus, medial prefrontal cortex (mPFC) or the third ventricle. The concentrations of Re, dopamine (DA) and acetylcholine (ACh) in dialysates were determined using LC-MS/MS.

RESULTS

Subcutaneous administration of a single dose of Re (12.5, 25 or 50 mg/kg) rapidly distributed to the cerebrospinal fluid and exhibited linear pharmacokinetics. The peak concentration (C(max)) occurred at 60 min for all doses. Re was not detectable after 240 min in the dialysates for the low dose of 12.5 mg/kg. At the same time, Re dose-dependently increased extracellular levels of DA and ACh in the hippocampus and mPFC, and more prominent effects were observed in the hippocampus.

CONCLUSION

The combined study of the pharmacokinetics and pharmacodynamics of Re demonstrate that increase of extracellular levels of DA and ACh, particularly in the hippocampus, may contribute, at least in part, to the anti-dementia activity of Re.

Comparative study of equimolar doses of gamma-hydroxybutyrate (GHB), 1,4-butanediol (1,4-BD) and gamma-butyrolactone (GBL) on catalepsy after acute and chronic administration

Gamma-hydroxybutyrate (GHB), and its precursors 1,4-butanediol (1,4-BD) and gamma-butyrolactone (GBL) are known drugs of abuse. The ability of acute and chronic administration of equimolar doses of GHB (200mg/kg), 1,4-BD (174mg/kg) and GBL (166mg/kg) to produce catalepsy in male Swiss Webster mice was examined. GHB, 1,4-BD, GBL produced catalepsy when injected acutely. Drug treatment was then continued for 14days. Tolerance development was determined on days 6, 14, and challenged with a higher dose on day 15 in those chronically pretreated mice, and compared with naïve mice. Chronic GHB produced tolerance to catalepsy, as evidenced from area under the curve (AUC) of catalepsy versus time (min-sec) on days 6 (678±254), 14 (272±247), which were less than those on day 1 (1923±269). However, less tolerance was seen from GBL or 1,4-BD, as AUCs on days 6 and 14 were not significantly lower than that of day 1. In conclusion, although equimolar doses were used, expecting similar levels of GHB in the body, 1,4-BD and GBL shared only some of the in vivo effects of GHB. The rate of metabolic conversion of 1,4-BD and GBL into GHB might be responsible for the differences in the tolerance development to these drugs.

Analysis of electrical brain waves in neurotoxicology: γ-hydroxybutyrate

Advances in computer technology have allowed quantification of the electroencephalogram (EEG) and expansion of quantitative EEG (qEEG) analysis in neurophysiology, as well as clinical neurology, with great success. Among the variety of techniques in this field, frequency (spectral) analysis using Fast Fourier Transforms (FFT) provides a sensitive tool for time-course studies of different compounds acting on particular neurotransmitter systems. Studies presented here include Electrocorticogram (ECoG) analysis following exposure to a glutamic acid analogue - domoic acid (DOM), psychoactive indole alkaloid - ibogaine, as well as cocaine and gamma-hydroxybutyrate (GHB). The ECoG was recorded in conscious rats via a tether and swivel system. The EEG signal frequency analysis revealed an association between slow-wave EEG activity delta and theta and the type of behavioral seizures following DOM administration. Analyses of power spectra obtained in rats exposed to cocaine alone or after pretreatment with ibogaine indicated the contribution of the serotonergic system in ibogaine mediated response to cocaine (increased power in alpha(1) band). Ibogaine also lowered the threshold for cocaine-induced electrographic seizures (increased power in the low-frequency bands, delta and theta). Daily intraperitoneal administration of cocaine for two weeks was associated with a reduction in slow-wave ECoG activity 24 hrs following the last injection when compared with controls. Similar decreased cortical activity in low-frequency bands observed in chronic cocaine users has been associated with reduced metabolic activity in the frontal cortex. The FFT analyses of power spectra relative to baseline indicated a significant energy increase over all except beta(2) frequency bands following exposure to 400 and 800 mg/kg GHB. The EEG alterations detected in rats following exposure to GHB resemble absence seizures observed in human petit mal epilepsy. Spectral analysis of the EEG signals combined with behavioral observations may prove to be a useful approach in studying chronic exposure to drugs of abuse and treatment of drug dependence.

γ-Hydroxybutyrate blood/plasma partitioning: effect of physiologic pH on transport by monocarboxylate transporters

The drug of abuse γ-hydroxybutyrate (GHB) displays nonlinear renal clearance, which has been attributed to saturable renal reabsorption by monocarboxylate transporters (MCTs) present in the kidney. MCT1 is also present in red blood cells (RBCs); however, the significance of this transporter on the blood/plasma partitioning of GHB is unknown. The purpose of this research was to characterize the transport of GHB across the RBC membrane and assess GHB blood/plasma partitioning in vivo in the presence and absence of a competitive MCT inhibitor, l-lactate. In vitro experiments were performed using freshly isolated rat erythrocytes at pH values of 6.5 and 7.4. Inhibition with p-chloromercuribenzene sulfonate and 4,4'-diisothiocyanostilbene-2,2'-disulfonate were used to determine the contribution of MCT1 and band 3, respectively, on GHB uptake. For in vivo experiments, rats were administered GHB (400-1500 mg/kg) with and without l-lactate. In vitro experiments demonstrated that GHB is transported across the RBC membrane primarily by MCT1 at relevant in vivo concentrations. The K(m) for MCT1 was lower at pH 6.5 than that at pH 7.4, 2.2 versus 17.0 mM, respectively. The in vivo blood/plasma partitioning of GHB displayed linearity across all concentrations. l-Lactate coadministration increased GHB renal clearance but had no effect on the blood/plasma ratio. Unlike its MCT-mediated transport in the intestine and kidneys, GHB blood/plasma partitioning appears to be linear and is unaffected by l-lactate. These findings can be attributed, at least in part, to differences in physiologic pH at different sites of MCT-mediated transport.

Mechanistic toxicokinetic model for gamma-hydroxybutyric acid: inhibition of active renal reabsorption as a potential therapeutic strategy

gamma-Hydroxybutyric acid (GHB), a drug of abuse, exhibits saturable renal clearance and capacity-limited metabolism. The objectives of this study were to construct a mechanistic toxicokinetic (TK) model describing saturable renal reabsorption and capacity-limited metabolism of GHB and to predict the effects of inhibition of renal reabsorption on GHB TK in the plasma and urine. GHB was administered by iv bolus (200-1,000 mg/kg) to male Sprague-Dawley rats and plasma and urine samples were collected for up to 6 h post-dose. GHB concentrations were determined by LC/MS/MS. GHB plasma concentration and urinary excretion were well-described by a TK model incorporating plasma and kidney compartments, along with two tissue and two ultrafiltrate compartments. The estimate of the Michaelis-Menten constant for renal reabsorption (K (m,R)) was 0.46 mg/ml which is consistent with in vitro estimates of monocarboxylate transporter (MCT)-mediated uptake of GHB (0.48 mg/ml). Simulation studies assessing inhibition of renal reabsorption of GHB demonstrated increased time-averaged renal clearance and GHB plasma AUC, independent of the inhibition mechanism assessed. Co-administration of GHB (600 mg/kg iv) and L: -lactate (330 mg/kg iv bolus plus 121 mg/kg/h iv infusion), a known inhibitor of MCTs, resulted in a significant decrease in GHB plasma AUC and an increase in time-averaged renal clearance, consistent with the model simulations. These results suggest that inhibition of renal reabsorption of GHB is a viable therapeutic strategy for the treatment of GHB overdoses. Furthermore, the mechanistic TK model provides a useful in silico tool for the evaluation of potential therapeutic strategies.

Concentration-effect relationships for the drug of abuse gamma-hydroxybutyric acid

gamma-Hydroxybutyric acid (GHB) is an endogenous neurotransmitter that is abused because of its sedative/hypnotic and euphoric effects. The objectives of this study were to evaluate the concentration-effect relationships of GHB in plasma, cerebrospinal fluid (CSF), brain (whole and discrete brain regions), and brain frontal cortex extracellular fluid. This information is crucial for future studies to evaluate effects of therapeutic interventions on the toxicodynamics of GHB. GHB (200-1000 mg/kg) was administered intravenously to rats, and plasma and frontal cortex microdialysate samples were collected for up to 6 h after the dose, or plasma, CSF, and brain (whole, frontal cortex, striatum, and hippocampus) concentrations were determined at the offset of its sedative/hypnotic effect [return to righting reflex (RRR)]. GHB-induced changes in the brain neurotransmitters gamma-aminobutyric acid (GABA) and glutamate were also determined. GHB, GABA, and glutamate concentrations were measured by liquid chromatography/tandem mass spectrometry. GHB-induced sleep time significantly increased in a dose-dependent manner (20-fold increase from 200 to 1000 mg/kg). GHB concentrations in plasma (300-400 microg/ml), whole brain (70 microg/g), discrete brain regions (80-100 microg/g), and brain microdialysate (29-39 microg/ml) correlated with RRR. In contrast, CSF GHB and GABA and glutamate concentrations in discrete brain regions exhibited no relationship with RRR. Our results suggest that GHB-induced sedative/hypnotic effects are mediated directly by GHB and that at high GHB doses, GABA formation from GHB may not contribute to the observed sedative/hypnotic effect. These results support the use of a clinical GHB detoxification strategy aimed at decreasing plasma and brain GHB concentrations after GHB overdoses.

GABA(B) receptors, schizophrenia and sleep dysfunction: a review of the relationship and its potential clinical and therapeutic implications

Evidence for an intrinsic relationship between sleep, cognition and the symptomatic manifestations of schizophrenia is accumulating. This review presents evidence for the possible utility of GABA(B) receptor agonists for the treatment of subjective and objective sleep abnormalities related to schizophrenia. At the phenotypic level, sleep disturbance occurs in 16-30% of patients with schizophrenia and is related to reduced quality of life and poor coping skills. On the neurophysiological level, studies suggest that sleep deficits reflect a core component of schizophrenia. Specifically, slow-wave sleep deficits, which are inversely correlated with cognition scores, are seen. Moreover, sleep plays an increasingly well documented role in memory consolidation in schizophrenia. Correlations of slow-wave sleep deficits with impaired reaction time and declarative memory have also been reported. Thus, both behavioural insomnia and sleep architecture are critical therapeutic targets in patients with schizophrenia. However, long-term treatment with antipsychotics often results in residual sleep dysfunction and does not improve slow-wave sleep, and adjunctive GABA(A) receptor modulators, such as benzodiazepines and zolpidem, can impair sleep architecture and cognition in schizophrenia. GABA(B) receptor agonists have therapeutic potential in schizophrenia. These agents have minimal effect on rapid eye movement sleep while increasing slow-wave sleep. Preclinical associations with increased expression of genes related to slow-wave sleep production and circadian rhythm function have also been reported. GABA(B) receptor deficits result in a sustained hyperdopaminergic state and can be reversed by a GABA(B) receptor agonist. Genetic, postmortem and electrophysiological studies also associate GABA(B) receptors with schizophrenia. While studies thus far have not shown significant effects, prior focus on the use of GABA(B) receptor agonists has been on the positive symptoms of schizophrenia, with minimal investigation of GABA(B) receptor agonists such as baclofen or gamma-hydroxybutyric acid and their effects on sleep architecture, cognition and negative symptoms in patients with schizophrenia. Further study is needed.

Sedative and hypothermic effects of gamma-hydroxybutyrate (GHB) in rats alone and in combination with other drugs: assessment using biotelemetry

The recreational drug gamma-hydroxybutyrate (GHB) has euphoric effects and can induce sedation and body temperature changes. GHB is frequently combined with other recreational drugs although these interactions are not well characterised. The present study used biotelemetry to provide a fine-grained analysis of the effects of GHB on body temperature and locomotor activity in freely moving rats, and investigated interactions between GHB and 3,4-methylenedioxymethamphetamine (MDMA), methamphetamine (METH) and various antagonist drugs. GHB (1000mg/kg) caused profound sedation for more than 2h and a complex triphasic effect on body temperature: an initial hypothermia (5-40min), followed by hyperthermia (40-140min), followed again by hypothermia (140-360min). A lower GHB dose (500mg/kg) also caused sedation but only a hypothermic effect that lasted up to 6h. The dopamine D(1) receptor antagonist SCH 23390 (1mg/kg), the opioid antagonist naltrexone (1mg/kg), the benzodiazepine antagonist flumazenil (10mg/kg), and the 5-HT(2A/2C) receptor antagonist ritanserin (1mg/kg) did not prevent the overall sedative or body temperature effects of GHB (1000mg/kg). However the GABA(B) antagonist SCH 50911 (50mg/kg) prevented the hyperthermia induced by GHB (1000mg/kg). Repeated daily administration of GHB (1000mg/kg) produced tolerance to the sedative and hyperthermic effects of the drug and cross-tolerance to the sedative effects of the GABA(B) receptor agonist baclofen (10mg/kg). A high ambient temperature of 28 degrees C prevented the hypothermia obtained with GHB (500mg/kg) at 20 degrees C, while GHB (500mg/kg) reduced the hyperthermia and hyperactivity produced by co-administered doses of MDMA (5mg/kg) or METH (1mg/kg) at 28 degrees C. These results further confirm a role for GABA(B) receptors in the hypothermic and sedative effects of GHB and show an interaction between GHB and MDMA, and GHB and METH, that may be relevant to the experience of recreational users who mix these drugs.

Pharmacological doses of gamma-hydroxybutyrate (GHB) potentiate histone acetylation in the rat brain by histone deacetylase inhibition

Several small chain fatty acids, including butyrate, valproate, phenylbutyrate and its derivatives, inhibit several HDAC activities in the brain at a several hundred micromolar concentration. Gamma-hydroxy-butyrate (GHB), a natural compound found in the brain originating from the metabolism of GABA, is structurally related to these fatty acids. The average physiological tissue concentration of GHB in the brain is below 50 microM, but when GHB is administered or absorbed for therapeutic or recreative purposes, its concentration reaches several hundred micromolars. In the present scenario, we demonstrate that pharmacological concentrations of GHB significantly induce brain histone H3 acetylation with a heterogeneous distribution in the brain and reduce in vitro HDAC activity. The degree of HDAC inhibition was also different according to the region of the brain considered. Taking into account the multiple physiological and functional roles attributed to the modification of histone acetylation and its consequences at the level of gene expression, we propose that part of the therapeutic or toxic effects of high concentrations of GHB in the brain after therapeutic administration of the drug could be partly due to GHB-induced epigenetic factors. In addition, we hypothesize that GHB, being naturally synthesized in the cytosolic compartment of certain neurons, could penetrate into the nuclei and may reach sufficient levels that could significantly modulate histone acetylation and may participate in the epigenetic modification of gene expression.

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.

Relative abuse liability of gamma-hydroxybutyric acid, flunitrazepam, and ethanol in club drug users

OBJECTIVES

Despite the increasing concern about gamma-hydroxybutyric acid (GHB) toxicity, there are few studies examining the clinical pharmacology of GHB and its abuse potential. To evaluate GHB-induced subjective and physiological effects, its relative abuse liability and its impact on psychomotor performance in club drug users.

MATERIALS AND METHODS

Twelve healthy male recreational users of GHB participated in 5 experimental sessions in the framework of a clinical trial. The study was randomized, double-blind, double-dummy, and crossover. Drug conditions were a single oral dose of GHB (40 or 60 mg/kg), ethanol (0.7 g/kg), flunitrazepam (1.25 mg), and placebo. Study variables included vital signs (blood pressure, heart rate, oral temperature, pupil diameter), psychomotor performance (digit symbol substitution test, balance, Maddox-Wing), subjective effects (a set of 13 visual analogue scales, Addiction Research Center Inventory-49 items, and Evaluation of the Subjective Effects of Substances with Potential of Abuse questionnaires), and pharmacokinetics.

RESULTS

All active conditions induced positive effects related to their abuse potential. The administration of GHB produced euphoria and pleasurable effects with slightly higher ratings than those observed for flunitrazepam and ethanol. Gamma-hydroxybutyric acid induced a biphasic time profile with an initial stimulant-like effect related to the simultaneous rise of plasma concentrations and a latter sedative effect not related to GHB kinetics. Gamma-hydroxybutyric acid increased blood pressure and pupil diameter. Ethanol induced its prototypical effects, and flunitrazepam produced marked sedation. Gamma-hydroxybutyric acid and flunitrazepam impaired psychomotor performance, digit symbol substitution test, and balance task, whereas ethanol, at the dose tested, induced only mild effects exclusively affecting the balance task.

CONCLUSIONS

Our results suggest a high abuse liability of GHB and flunitrazepam in club drug users.