Different α subunits of human γ-aminobutyric acid type A (GABAA) receptors were transiently expressed together with β3 and γ2 subunits in Xenopus oocytes to examine the interactions of various GABAA 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 α2β3γ2 GABAA 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-β-carboline-3-carboxylic acid methyl ester (DMCM), with no effect on the maximal currents (Imax).
In contrast, maximal responses for different partial GABAA agonists on all benzodiazepine-sensitive αxβ3γ2 GABAA receptors were enhanced by chlordiazepoxide. Imax values for piperidine-4-sulphonic acid (P4S) on α1β3γ2, THIP on α3β3γ2, and 5-(4-piperidyl)isothiazol-3-ol (thio-4-PIOL) on α2β3γ2 and α5β3γ2 GABAA receptors were increased by chlordiazepoxide, while that for P4S on α1β3γ2 receptors was decreased by DMCM.
The Imax 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.
British Journal of Pharmacology (2000) 129, 1794–1800; doi:10.1038/sj.bjp.0703259