Received for publication July 6, 2006.
Revised November 6, 2006.
Accepted for publication November 7, 2006.

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

Abstract

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 mIPSCs and responses to exogenous GABA, had no effect on tonic currents. Since 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 gabazine resistant. 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 132 receptors recombinantly expressed in HEK293 cells. Propofol activated gabazine-resistant, bicuculline-sensitive currents when applied to either preparation. While 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 11/32 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, due to either anesthetic or spontaneous gating. Moreover, spontaneously active GABA_A receptors mediate gabazine resistant tonic currents in pyramidal neurons.

Key words: GABAA, GABAC, Glycine, Barbiturates, Benzodiazepines, Gases/general anesthetics

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