Subunit dependent modulation of GABA A receptor function by neuroactive steroids

Abstract

Neurosteroids are potent, endogenous modulators of GABA A receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABA A receptor-activated membrane current ( I GABA). The role of different α and γ subunit subtypes in modulation of I GABA by ALP and AFX was investigated using recombinant GABA A receptor isoforms expressed in Xenopus oocytes. Changing or removal of the α subunit subtype altered the efficacy of both ALP and AFX (α2β1γ2L>α1β1γ2L≫β1γ2L) to potentiate I GABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance I GABA was also dependent on the γ subunit subtype (α1β1γ3>α1β1γ2L=α1β1γ1). AFX also had higher efficacy in the α1β1γ3 receptor isoform compared to α1β1γ1. In contrast to ALP, the potency of AFX was greater in the α1β1γ3 and α1β1γ1 receptor isoforms compared to α1β1γ2L. This study provides evidence that the α subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate I GABA. The γ3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other γ subunit subtypes. These results suggest that the heteromeric assembly of different GABA A receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABA A receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABA A receptor.