Extrasynaptic αβ subunit GABAA receptors on rat hippocampal pyramidal neurons

Extrasynaptic GABA(A) receptors that are tonically activated by ambient GABA are important for controlling neuronal excitability. In hippocampal pyramidal neurons, the subunit composition of these extrasynaptic receptors may include alpha 5 beta gamma and/or alpha 4 beta delta subunits. Our present studies reveal that a component of the tonic current in the hippocampus is highly sensitive to inhibition by Zn2+. This component is probably not mediated by either alpha 5 beta gamma or alpha 4 beta delta receptors, but might be explained by the presence of alpha beta isoforms. Using patch-clamp recording from pyramidal neurons, a small tonic current measured in the absence of exogenous GABA exhibited both high and low sensitivity to Zn2+ inhibition (IC50 values, 1.89 and 223 mu M, respectively). Using low nanomolar and micromolar GABA concentrations to replicate tonic currents, we identified two components that are mediated by benzodiazepine-sensitive and -insensitive receptors. The latter indicated that extrasynaptic GABA(A) receptors exist that are devoid of gamma 2 subunits. To distinguish whether the benzodiazepine-insensitive receptors were alpha beta or alpha beta delta isoforms, we used single-channel recording. Expressing recombinant alpha 1 beta 3 gamma 2, alpha 5 beta 3 gamma 2, alpha 4 beta 3 delta and alpha 1 beta 3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (similar to 25-28 pS) for gamma 2 or delta subunit-containing receptors whereas alpha beta receptors were characterized by a lower main conductance state (similar to 11 pS). Recording from pyramidal cell somata revealed a similar range of channel conductances, indicative of a mixture of GABA(A) receptors in the extrasynaptic membrane. The lowest conductance state (similar to 11 pS) was the most sensitive to Zn2+ inhibition in accord with the presence of alpha beta receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABA(A) receptors on hippocampal pyramidal neurons.