Edinburgh Napier University

  When vertebrate brain poly(A)+ RNA is expressed in Xenopus oocytes the response of the GABA receptors formed is found to be inhibited allosterically by a neurosteroid, pregnenolone sulphate (PS). This negative modulation was reproduced after expressing RNAs enconding bovine GABAA receptor subunits in the combinations α1 + ß1, or αi + ß1 + γ2 (where i = 1. 2 or 3). The characteristics of this inhibition vary significantly with the type of the α subunit (α1, α2, or α3) used. When the bovine γ2L alternate form of the γ2 subunit was replaced by the human γ2S subunit, the behaviour was unchanged: the human γ2S subunit used is a newly-cloned form, which encodes a polypeptide with two amino acid differences from the human γ2 subunit previously described. The results of co-application of PS and 3α-hydroxy-5α-pregnan-ol-20-one, a neurosteroid which is a positive modulator of the GABAA receptor, indicate that these act at different sites on the receptor. PS also increases the desensitisation of the receptor by GABA. This effect, also, is α-subunit-type dependent and occurs by an acceleration of the fast phase of desensitisation.