Conserved site for neurosteroid modulation of GABA(A) receptors

This study addresses whether the potentiation site for neurosteroids on GABA(A) receptors is conserved amongst different GABA(A) receptor isoforms. The neurosteroid potentiation site was previously identified in the alpha 1 beta 2 gamma 2S receptor by mutation of Q241 to methionine or leucine, which reduced the potentiation of GABA currents by the naturally occurring neurosteroids, allopregnanolone or tetrahydrodeoxycorticosterone (THDOC). By using heterologous expression of GABA(A) receptors in HEK cells, in combination with whole-cell patch clamp recording methods, a relatively consistent potentiation by allopregnanolone of GABA-activated currents was evident for receptors composed of one alpha subunit isoform (alpha 2-5) assembled with beta 3 and gamma 2S subunits. Using mutant alpha beta gamma receptors, the neurosteroid potentiation was universally dependent on the conserved glutamine residue in M1 of the respective alpha subunit. Studying wild-type and mutant receptors composed of alpha 4 beta 3 delta subunits revealed that the delta subunit is unlikely to contribute to the neurosteroid potentiation binding site and probably affects the efficacy of potentiation. Thus, in keeping with the ability of neurosteroids to potentiate GABA currents via a broad variety of GABA(A) receptor isoforms in neurons, the potentiation site is structurally highly conserved on this important neurotransmitter receptor family. (C) 2008 Elsevier Ltd. All rights reserved.