Allosteric modulators affect the efficacy of partial agonists for recombinant GABA(A) receptors

Different alpha subunits of human gamma-aminobutyric acid type A (GABA(A)) receptors were transiently expressed together with beta(3) and gamma(2) subunits in Xenopus oocytes to examine the interactions of various GABA(A) agonists and representative allosteric modulators. Chloride currents elicited by agonists were measured using two electrode voltage clamp electrophysiology. Where compounds behaved as full agonists, i.e. GABA on all subtypes and 4,5,6, 7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol (THIP) on alpha2beta(3)gamma(2) GABA(A) receptors, agonist concentration-response curves were shifted to the left by the benzodiazepine full agonist chlordiazepoxide and the anticonvulsant loreclezole, or to the right by the inverse agonist 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester (DMCM), with no effect on the maximal currents (I(max)). In contrast, maximal responses for different partial GABA(A) agonists on all benzodiazepine-sensitive alpha(x)beta(3)gamma(2) GABA(A) receptors were enhanced by chlordiazepoxide. I(max) values for piperidine-4-sulphonic acid (P4S) on alpha(1)beta(3)gamma(2), THIP on alpha(3)beta(3)gamma(2), and 5-(4-piperidyl)isothiazol-3-ol (thio-4-PIOL) on alpha(2)beta(3)gamma(2) and alpha(5)beta(3)gamma(2) GABA(A) receptors were increased by chlordiazepoxide, while that for P4S on alpha(1)beta(3)gamma(2) receptors was decreased by DMCM. The I(max) values for partial agonists were also enhanced by pentobarbitone, the neurosteroid allopregnanolone and loreclezole irrespective of receptor subtype or the nature of the partial agonist. In the light of models of ligand-gated ion channel receptor activation we suggest two possible mechanisms of action for the effects of allosteric modulators on partial agonist receptor activation: either selective modulation of agonist affinity for the open/closed state, or direct modulation of the gating process itself.