Extrasynaptic αβ subunit GABAA receptors on rat hippocampal pyramidal neurons

Abstract

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 α5βγ and/or α4βδ subunits. Our present studies reveal that a component of the tonic current in the hippocampus is highly sensitive to inhibition by Zn 2+. This component is probably not mediated by either α5βγ or α4βδ receptors, but might be explained by the presence of αβ 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 Zn 2+ inhibition (IC 50 values, 1.89 and 223 μ 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 γ2 subunits. To distinguish whether the benzodiazepine‐insensitive receptors were αβ or αβδ isoforms, we used single‐channel recording. Expressing recombinant α1β3γ2, α5β3γ2, α4β3δ and α1β3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (∼25–28 pS) for γ2 or δ subunit‐containing receptors whereas αβ receptors were characterized by a lower main conductance state (∼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 (∼11 pS) was the most sensitive to Zn 2+ inhibition in accord with the presence of αβ receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABA A receptors on hippocampal pyramidal neurons.