Noncompetitive inhibition of gamma-aminobutyric acidA channels by Zn

The action of zinc on chloride currents evoked by gamma-aminobutyric acid (GABA) was examined on cultured hippocampal neurons using whole cell voltage clamp and outside-out patch recording. Zn (5-30 microM) noncompetitively blocked responses evoked by GABA (0.5-100 microM), 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 GABAA receptor complex, which is distinct from either the GABA or benzodiazepine binding sites. The structural similarity of the Cys-Cys loop of the alpha and gamma GABAA receptor subunits to some Zn-binding proteins suggests one possible region for a Zn binding site.