Consequence of the presence of two different β subunit isoforms in a GABAA receptor

The major isoforms of GABA(A) receptors are thought to be composed of two alpha, two beta and one gamma subunit(s). GABA(A) receptors containing two beta(1) subunits respond differently to the anticonvulsive compound loreclezole and the general anaesthetic etomidate than receptors containing two beta(2) subunits. Receptors containing beta(2) subunits show a much larger allosteric stimulation by these agents than those containing beta(1) subunits. We were interested to know how receptors containing both beta(1) and beta(2) subunits, in different positions respond to loreclezole and etomidate. To answer this question, subunits were fused at the DNA level to form dimeric and trimeric subunits. Concatenated receptors (alpha(1)-beta(1)-alpha(1)/gamma(2)-beta(1), alpha(1)-beta(2)-alpha(1)/gamma(2)-beta(1), alpha(1)-beta(1)-alpha(1)/gamma(2)-beta(2) and alpha(1)-beta(2)-alpha(1)/gamma(2)-beta(2)) were expressed in Xenopus ooctyes and functionally compared in their response to the agonist GABA and to the positive allosteric modulators, loreclezole and etomidate. We have shown that (I) in the presence of both beta(1) and beta(2) subunits in the same pentamer (mixed receptors) direct gating by etomidate is similar to exclusively beta(1) containing receptors; (II) In mixed receptors, stimulation by etomidate assumed characteristics intermediate to exclusively beta(1) or beta(2) containing receptors, but the values for the concentrations < 10 mu M were always much closer to those observed in alpha(1)-beta(1)-alpha(1)/gamma(2)-beta(1) receptors; and (III) mixed receptors show no positional effects.