Allosteric modulation in spontaneously active mutant gamma-aminobutyric acid(A) receptors in frogs

Tryptophan substitutions were made in the second transmembrane domain of th e gamma -aminobutyric acid(A) (GABAA) receptor alpha and beta subunits and the resulting mutant receptors, containing alpha (2)(S270W) and/or beta (1) (S265W), were expressed in Xenopus oocytes. Mutation of either or both subu nits resulted in receptors that exhibited enhanced sensitivity to agonist a nd were spontaneously active in the absence of GABA. The spontaneous activi ty was blocked by picrotoxin or bicuculline. The enhancement of GABA-induce d currents by pentobarbital, by the neurosteroid 5 alpha -pregnan-3 alpha - ol-20-one, and by the benzodiazepine flunitrazepam was dramatically reduced in the mutant receptors. These results are consistent with the idea that a mutation that promotes gating behavior in a ligand-gated ion channel will also show reduced effects of all positive allosteric modulators in a genera lized manner, even when these modulators act at distinct sites on the recep tor. (C) 2000 Published by Elsevier Science Ireland Ltd.