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 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, GHB (1000 mg/kg, i.p., 30 minutes) increased the brain cortical concentrations of 3alpha,5alpha-THP and 3alpha,5alpha-THDOC 7 and 2.5 fold, respectively, in male GHB-S and only 4 and 2 fold, respectively, in male GHB-R rats. In GHB-S rats this increase lasted up to 90 minutes and declined 180 minutes 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 were back to control values by 90 minutes. Brain cortical 3alpha,5alpha-THP and 3alpha,5alpha-THDOC at LORR onset were increased to the same extent (2-3 fold) as in GHB-S females in those few female GHB-R rats that lost the righting reflex after GHB administration, but not in female GHB-R rats that failed to show LORR. Sub-hypnotic doses (7.5 and 12.5 mg/kg, i.p.) of pregnanolone, administered 10 minutes 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 have a permissive action in GHB's sedative/hypnotic action.