The neuroactive steroid 3α-hydroxy-5β-pregnan-20-one is a two-component modulator of ligand binding to the GABA A receptor

Abstract

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