Analysis of modulation of the ρ1 GABAA receptor by combinations of inhibitory and potentiating neurosteroids reveals shared and distinct binding sites.

The ρ1 GABAA receptor is prominently expressed in the retina and is present at lower levels in several brain regions and other tissues. Although the ρ1 receptor is insensitive to many anesthetic drugs that modulate the heteromeric GABAA receptor, it maintains a rich and multifaceted steroid pharmacology. The receptor is negatively modulated by 5β-reduced steroids, sulfated or carboxylated steroids, and β-estradiol, while many 5α-reduced steroids potentiate the receptor. In this study, we analyzed modulation of the human ρ1 receptor by several neurosteroids, individually and in combination, in the framework of the co-agonist concerted transition model. Experiments involving coapplication of two or more steroids revealed that the receptor contains at least three classes of distinct, non-overlapping sites for steroids, one each for the inhibitory steroids 3αβP, 3α5βPS, and β-estradiol. The site for 3α5βP can accommodate the potentiating steroid 5αTHDOC. The findings are discussed with respect to receptor modulation by combinations of endogenous neurosteroids. SIGNIFICANCE STATEMENT: The study describes modulation of the ρ1 GABAA receptor by neurosteroids. The co-agonist concerted transition model was used to determine overlap of binding sites for several inhibitory and potentiating steroids.