Characterization of steroid interactions with gamma-aminobutyric acid receptor-gated chloride ion channels: evidence for multiple steroid recognition sites.

The potentiation of gamma-aminobutyric acid (GABA) receptor-mediated 36Cl- uptake by various steroids has been characterized in rat cerebral cortical synaptoneurosomes. Several of these steroids, including 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha-OH-DHP) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (THDOC), increase the potency of muscimol to stimulate 36Cl- uptake in a concentration-dependent and stereospecific manner. Concentration-response curves for 3 alpha-OH-DHP, THDOC, 3 alpha-hydroxy-pregn-4-en-20-one, and pentobarbital enhancement of muscimol-stimulated 36Cl- uptake are biphasic, with Hill coefficients significantly less than 1.0. Computer-modeling (ALLFIT analysis) of these curves suggests that these steroids and pentobarbital interact with multiple binding sites on GABAA receptor(s). In contrast, the concentration-response curve for THDOC 21-mesylate is monophasic, with a smaller maximal response, and yields a Hill coefficients of 1.0. In addition to modulating GABA receptor-mediated 36Cl- uptake, THDOC enhanced the ability of the benzodiazepine clonazepam to potentiate muscimol-stimulated 36Cl- uptake. The central benzodiazepine antagonist Ro15-1788 failed to inhibit THDOC-induced potentiation of muscimol-stimulated 36Cl- uptake, although it has been previously reported to inhibit some of the behavioral actions of THDOC. In contrast to the A ring-reduced metabolites and analogues of progesterone and deoxycorticosterone, glucocorticoids had no effect on muscimol-stimulated 36Cl- uptake in cerebral cortical synaptoneurosomes at concentrations between 20 nM and 5 microM.