Concentration-Effect Relationships for the Drug of Abuse γ-Hydroxybutyric Acid

Abstract

␥-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 ␥-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 dosedependent manner (20-fold increase from 200 to 1000 mg/kg). GHB concentrations in plasma (300 -400 g/ml), whole brain (70 g/g), discrete brain regions (80 -100 g/g), and brain microdialysate (29 -39 g/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.

Brain Uptake of the Drug of Abuse γ-Hydroxybutyric Acid in Rats

“…HPLC conditions, mass spectrometer parameters, and linear calibration ranges are detailed in Felmlee et al, 2010a.…”

“…Plasma proteins were precipitated with acetonitrile (0.4 ml) followed by centrifugation. Supernatant (0.2 ml) was diluted with 0.8 ml of double-distilled water and extracted using Bond Elut SAX cartridges (Varian, Palo Alto, CA) that were prepared as described previously (Felmlee et al, 2010a). After evaporation, samples were reconstituted in 1.25 ml of 0.1% formic acid in double-distilled water and 5% acetonitrile.…”

“…Rats were anesthetized with ketamine/xylazine (90/9 mg/kg), implanted with a jugular vein cannula, and subsequently mounted in a stereotaxic frame (Harvard Apparatus). As described previously (Felmlee et al, 2010a), microdialysis guide cannulae (for CMA 11 probes; CMA Microdialysis, North Chelmsford, MA) containing dummy probes were implanted in the frontal cortex (AP ϩ 3.2 mm and ML Ϯ 2.5 mm from Bregma, and DV Ϫ0.5 mm from dura (Paxinos and Watson, 1986) with the incisor bar set 0.5 mm below the interaural line). The side of the brain in which the cannula was placed was alternated across animals.…”

“…We recently reported that GHB concentrations in plasma, whole brain, discrete brain regions, and brain extracellular fluid (ECF) correlate with GHB's sedative/hypnotic effects (Felmlee et al, 2010a). In these studies, we seek to further examine unbound brain concentration-effect relationships by determining the brain ECF concentrations of GHB by microdialysis and correlating ECF concentrations with its sedative/hypnotic effects.…”

γ-Hydroxybutyrate Blood/Plasma Partitioning: Effect of Physiologic pH on Transport by Monocarboxylate Transporters

“…The limit of quantification for blood and plasma was 1 and 4 g/ml for urine, respectively. Mass spectrometer conditions and calibration ranges have been described previously (Felmlee et al, 2010a). L-Lactate plasma concentrations were determined by a YSI 1500 Sport Lactate Analyzer (YSI, Inc., Yellow Springs, OH).…”