THE NEUROACTIVE STEROID 3-ALPHA-HYDROXY-5-BETA-PREGNAN-20-ONE IS A 2-COMPONENT MODULATOR OF LIGAND-BINDING TO THE GABA(A) RECEPTOR

Neuroactive steroids allosterically inhibit [S-35]t-butylbicyclophosphorothionate ([S-35]TBPS) and enhance [H-3]flunitrazepam binding to the GABA(A) receptor complex. In the presence of 5 mu M GABA, 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha,5 beta-P) inhibits [S-35]TBPS binding with high- (IC50 21-32 nM) and low- (IC50 24-63 mu M) affinity components in bovine cortical, cerebellar, and hippocampal membranes. The percentage of high-affinity sites ranges from 53% in cortex to 65% in cerebellum and hippocampus. However, 3 alpha,5 beta-P is a single-site inhibitor in thalamus (IC50 43 nM). In the absence of GABA, similar affinities for the high- and low-affinity components were detected, although the percentages of high-affinity sites were reduced. Similarly, 3 alpha,5 beta-P enhances [H-3]flunitrazepam binding with high- (EC(50) 44-58 nM) and low- (EC(50) 2-13 mu M) affinity components which account for 71-77% and 23-29% of the sites, respectively, in cortex, cerebellum and hippocampus. 3 alpha,5 beta-P is a single-site enhancer in thalamus (EC(50) 80 nM). In contrast to 3 alpha,5 beta-P, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) is a single site modulator of [S-35]TBPS and [H-3]flunitrazepam binding in all regions examined. These data provide pharmacological evidence consistent with receptor heterogeneity for neuroactive steroids.