Subunit dependent modulation of GABAA receptor function by neuroactive steroids

Neurosteroids are potent, endogenous modulators of GABA, receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound aIphaxalone (AFX) have been shown to both directly activate and potentiate GABA, receptor-activated membrane current (I-GABA). The role of different alpha and gamma subunit subtypes in modulation of I-GABA by ALP and AFX was investigated using recombinant GABA, receptor isoforms expressed in Xenopus oocytes. Changing or removal of the alpha subunit subtype altered the efficacy of both ALP and AFX (alpha 2 beta 1 gamma 2L > alpha 1 beta 1 gamma 2L much greater than beta 1 gamma 2L) 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 gamma subunit subtype (alpha 1 beta 1 gamma 3 > alpha 1 beta 1 gamma 2L = alpha 1 beta 1 gamma 1). AFX also had higher efficacy in the alpha 1 beta 1 gamma 3 receptor isoform compared to alpha 1 beta 1 gamma 1. In contrast to ALP, the potency of AFX was greater in the alpha 1 beta 1 gamma 3 and alpha 1 beta 1 gamma 1 receptor isoforms compared to alpha 1 beta 1 gamma 2L. This study provides evidence that the alpha subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate I-GABA. The gamma 3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other gamma 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. (C) 1999 Elsevier Science B.V. All rights reserved.