Compounds exhibiting selective efficacy for different β subunits of human recombinant γ-aminobutyric acidA receptors

Inhibitory GABA(A) receptor modulators are widely used therapeutic agents for a variety of central nervous system disorders. Ltk(-) cells stably expressing human recombinant GABA(A) subunits (alpha1beta1 - 3gamma2s) were seeded into 96-well plates, loaded with chlorocoumarin-2-dimyristoyl phosphatidylethanolamine and bis(1,3-diethyl-2-thiobarbiturate) trimethineoxonol, and rapid fluorescence resonance energy transfer technique (FRET) measurements were made of GABA-evoked depolarizations in low-Cl- buffer using a voltage/ion probe reader. The influence of different beta subunits on the ability of agents to modulate and directly activate the ion channel was examined. GABA evoked concentration-dependent decreases in FRET, increasing fluorescence emission ratio (460/580 nm) at alpha1beta1gamma2, alpha1beta2gamma2, and alpha1beta3gamma2 receptors with similar maximal amplitude (P > 0.05, n = 17) and EC50 values of 2.4 +/- 0.2, 2.5 +/- 0.2, and 1.3 +/- 0.1 muM, respectively. Piperidine-4-sulfonic acid and 4,5,6,7-tetrahydroisoxazolo[ 5,4-c] pyridin-3-ol were less potent, with EC50 values of 8.7 +/- 0.9, 9.2 +/- 0.5, and 11.7 +/- 1.2, and 43.7 +/- 6.4, 24.8 +/- 1.6, and 26.1 +/- 2.4 muM, respectively. Potency and maximal efficacy of propofol, methyl 6,7-dimethoxy-4-ethyl-beta-carboline- 3-carboxylate, pentobarbital, and steroids, 5alpha-pregnan-3alpha-ol-20-one and 5beta-pregnan-3alpha-ol-20-one, were unaffected by the beta isoform present in the receptor complex. However, several compounds displayed beta2/3 subunit selectivity, notably loreclezole, R(-)-etomidate, and a group of antiinflammatory agents including mefenamic acid, flufenamic acid, meclofenamic acid, tolfenamic acid, niflumic acid, and diflunisal. The anti-inflammatories exhibited varying levels of efficacy at beta2/3 subunits, with micromolar potency, while having antagonist or weak inverse agonist profiles at alpha1beta1gamma2. Diflunisal was the most efficacious compound, eliciting greater potentiation than loreclezole ( 90 +/- 14% and 109 +/- 14% at beta3 and beta2, respectively, compared with 62 +/- 6% and 56 +/- 3%), whereas niflumic acid exhibited the lowest efficacy. An additional agent, olsalazine, weakly potentiated responses at all three receptors without any selectivity. This study identifies and characterizes a variety of allosteric modulators for which beta subunits are an important determinant of efficacy and potency.