Tonically active GABAA receptors in hippocampal pyramidal neurons exhibit constitutive GABA-independent gating

Phasic and tonic inhibitory currents of hippocampal pyramidal neurons exhibit distinct pharmacological properties. Picrotoxin and bicuculline methiodide inhibited both components, consistent with a role for GABA(A) receptors; however, gabazine, at a concentration that abolished miniature GABAergic inhibitory postsynaptic currents and responses to exogenous GABA, had no effect on tonic currents. Because all GABA-activated GABA(A) receptors in pyramidal neurons are gabazine- sensitive, it follows that tonic currents are not GABA- activated. Furthermore, picrotoxin- sensitive spontaneous single- channel events recorded from outside- out patches had the same chord conductance as GABA- activated channels and were gabazineresistant. Therefore, we hypothesize that GABA(A) receptors, constitutively active in the absence of GABA, mediate tonic current; the failure of gabazine to block tonic current reflects a lack of negative intrinsic efficacy of the antagonist. We compared the negative efficacies of bicuculline and gabazine using the general anesthetic propofol to directly activate GABA(A) receptors native to pyramidal neurons or alpha 1 beta 3 gamma 2 receptors recombinantly expressed in human embryonic kidney 293 cells. Propofol activated gabazine- resistant, bicuculline- sensitive currents when applied to either preparation. Although gabazine had negligible efficacy as an inhibitor of propofol- activated currents, it prevented inhibition by bicuculline, which acts as an inverse agonist inhibiting GABA- independent gating. Recombinant alpha 1 beta 1/3 gamma 2 receptors also mediated agonist- independent tonic currents that were resistant to gabazine and inhibited by bicuculline. Thus, gabazine is a competitive antagonist with negligible negative efficacy and is therefore unable to inhibit GABA(A) receptors that are active in the absence of GABA because of either anesthetic or spontaneous gating. Moreover, spontaneously active GABA(A) receptors mediate gabazine- resistant tonic currents in pyramidal neurons.