Noncompetitive inhibition of γ-aminobutyric acid_A channels by Zn

The action of zinc on chloride currents evoked by ã-aminobutyric acid (GABA) was examined on cultured hippocampal neurons using whole cell voltage clamp and outside-out patch recording. Zn (5-30 μM) noncompetitively blocked responses evoked by GABA (0.5-100 μM), but did not affect either the time-to-peak or desensitization of the macroscopic current. In outside-out patches, Zn had no effect on the mean conductance or lifetime of the 19 or 30 pS openings of the GABA channel; however, the frequency of channel opening was markedly decreased in a voltage-independent manner. Zn inhibition of GABA responses appeared to be independent of the benzodiazepine binding site as Zn was effective in the presence of either diazepam or Ro15-1788, a competitive antagonist of benzodiazepine agonists and inverse agonists. In contrast to prior reports, Zn also inhibited GABA currents in a similar manner on cultured superior cervical ganglion neurons. These results suggest that Zn acts at an extracellular site on the GABA_A receptor complex, which is distinct from either the GABA or benzodiazepine binding sites. The structural similarity of the Cys-Cys loop of the α and γ GABA_A receptor subunits to some Zn-binding proteins suggests one possible region for a Zn binding site.