Tonically active GABAA receptors in hippocampal pyramidal neurons exhibit constitutive GABA-independent gating.
Human alpha(1)beta(3) epsilon GABA(A) receptors were expressed in Xenopus oocytes and examined using the conventional two-electrode voltage-clamp technique and compared to alpha(1)beta(3)gamma(2) receptors. The effects of several GABA(A) agonists were studied, and the allosteric modulation of the channel by a number of GABAergic modulators investigated. The presence of the epsilon subunit increased the potency and efficacy of direct activation by partial GABA(A) agonists (piperidine-4-sulphonic acid and thio-4-PIOL), pentobarbital and neuro-steroids. Direct activation by 3-hydroxylated neurosteroids was restricted to 3alpha epimers, while chirality at C5 was indifferent. The 3beta-sulfate esters of pregnenolone and dehydroepiandrosterone inhibited the spontaneous currents with efficacies higher, while bicuculline methiodide and SR 95531 did so lower than picrotoxin and TBPS. Furosemide, fipronil, triphenylcyanoborate and Zn(2+) blocked the spontaneous currents of alpha(1)beta(3) epsilon receptors with different efficacies. Flunitrazepam and 4'-chlorodiazepam inhibited the spontaneous currents with micromolar potencies. In conclusion, spontaneously active alpha(1)beta(3) epsilon GABA(A) receptors can be potentiated and blocked by GABAergic agents within a broad range of efficacy.