Extrasynaptic alphabeta 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 alpha5betagamma and/or alpha4betadelta 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 alpha5betagamma or alpha4betadelta receptors, but might be explained by the presence of alphabeta 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 microm, 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 gamma2 subunits. To distinguish whether the benzodiazepine-insensitive receptors were alphabeta or alphabetadelta isoforms, we used single-channel recording. Expressing recombinant alpha1beta3gamma2, alpha5beta3gamma2, alpha4beta3delta and alpha1beta3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (approximately 25-28 pS) for gamma2 or delta subunit-containing receptors whereas alphabeta receptors were characterized by a lower main conductance state (approximately 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 (approximately 11 pS) was the most sensitive to Zn(2+) inhibition in accord with the presence of alphabeta receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABA(A) receptors on hippocampal pyramidal neurons.