Direct channel-gating and modulatory effects of triiodothyronine on recombinant GABA A receptors

Abstract

We have previously shown that triiodothyronine (T3) inhibits γ-aminobutyric acid type A (GABA A) receptors in synaptoneurosomes and transfected cells. To further characterize this phenomenon, the effect of T3 on recombinant GABA A receptors expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp method. T3 inhibited GABA-gated chloride currents in a non-competitive manner and yielded an IC 50 of 7.3±0.8 μM in oocytes coexpressing α 1 β 2 γ 2L receptor subunits. T3 had no inhibitory effect on α 6 β 2 γ 2L or β 2 γ 2L receptor constructs, indicating that the α 1 subunit imparts T3 sensitivity to the receptor. In addition to the inhibitory effect of T3 on GABA responses, T3 alone induced a current in oocytes expressing α 1 β 2 γ 2L, α 6 β 2 γ 2L and β 2 γ 2L constructs. This current displayed a reversal potential identical to that of GABA-gated chloride currents, and was blocked by picrotoxin (10 μM), but not by bicuculline (50 μM), indicating that T3 gates the chloride channel by binding to a site other than the GABA-binding site. The direct channel-gating action of T3 was concentration-dependent, with an EC 50 of 23±5 μM. The actions of T3 are unique in that T3 acts as a noncompetitive antagonist in the presence of GABA but can directly gate the chloride channel in the absence of GABA.