The neuroactive steroid 3 alpha -hydroxy-5 beta -pregnan-20-one is a two-component modulator of ligand binding to the GABA sub(A) receptor

Neuroactive steroids allosterically inhibit [ super(35)S]t-butylbicyclophosphorothionate ([ super(35)S]TBPS) and enhance [ super(3)H]flunitrazepam binding to the GABA sub(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 [ super(35)S]TBPS binding with high- and low-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. 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 [ super(3)H]flunitrazepam binding with high- (EC sub(50) 44-58 nM) and low-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. 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 [ super(35)S]TBPS and [ super(3)H]flunitrazepam binding in all regions examined. The data provide pharmacological evidence consistent with receptor heterogeneity for neuroactive steroids.