α4β3δ GABAA receptors characterized by fluorescence resonance energy transfer-derived measurements of membrane potential

Selective modulators of gamma -aminobutyric acid, type A (GABA(A)) receptors containing alpha (4) subunits may provide new treatments for epilepsy and premenstrual syndrome. Using mouse L(-tk) cells, we stably expressed the native GABA(A) receptor subunit combinations alpha (3)beta (3)gamma (2), alpha (4)beta (3)gamma (2), and, for the first time, alpha (4)beta (3)delta and characterized their properties using a novel fluorescence resonance energy transfer assay of GABA-evoked depolarizations. GABA evoked concentration-dependent decreases in fluorescence resonance energy transfer that were blocked by GABA(A) receptor antagonists and, for alpha (3)beta (3)gamma (2) and alpha (4)beta (3)gamma (2) receptors, modulated by benzodiazepines with the expected subtype specificity. When combined with alpha (4) and beta (3), delta subunits, compared with gamma (2), conferred greater sensitivity to the agonists GABA, 4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol (THIP), and muscimol and greater maximal efficacy to TRIP. alpha (4)beta (3)delta responses were markedly modulated by steroids and anesthetics. Alphaxalone, pentobarbital, and pregnanolone were all 3-7-fold more efficacious at alpha (4)beta (3)delta compared with alpha (4)beta (3)gamma (2). The fluorescence technique used in this study has proven valuable for extensive characterization of a novel GABA(A) receptor. For GABA(A) receptors containing a4 subunits, our experiments reveal that inclusion of 6 instead of gamma (2) subunits can increase the affinity and in some cases the efficacy of agonists and can increase the efficacy of allosteric modulators. Pregnanolone was a particularly efficacious modulator of alpha (4)beta (3)delta receptors, consistent with a central role for this subunit combination in premenstrual syndrome.