Analysis of Modulation of the ρ1 GABAA Receptor by Combinations of Inhibitory and Potentiating Neurosteroids Reveals Shared and Distinct Binding Sites

The rho(1) GABA(A) receptor is prominently expressed in the retina and is present at lower levels in several brain regions and other tissues. Although the rho(1) receptor is insensitive to many anesthetic drugs that modulate the heteromeric GABA(A) receptor, it maintains a rich and multifaceted steroid pharmacology. The receptor is negatively modulated by 5 beta-reduced steroids, sulfated or carboxylated steroids, and p-estradiol, whereas many 5 alpha-reduced steroids potentiate the receptor. In this study, we analyzed modulation of the human rho(1) GABA(A) receptor by several neurosteroids, individually and in combination, in the framework of the coagonist concerted transition model. Experiments involving coapplication of two or more steroids revealed that the receptor contains at least three classes of distinct, nonoverlapping sites for steroids, one each for the inhibitory steroids pregnanolone (3 alpha 5 beta P), 3 alpha 5 beta P sulfate, and beta-estradiol. The site for 3 alpha 5 beta P can accommodate the potentiating steroid 5 alpha TH DOC. The findings are discussed with respect to receptor modulation by combinations of endogenous neurosteroids. SIGNIFICANCE STATEMENT The study describes modulation of the rho 1 GABA(A) receptor by neurosteroids. The coagonist concerted transition model was used to determine overlap of binding sites for several inhibitory and potentiating steroids.