Interaction between copper and zinc at GABAA receptors in acutely isolated cerebellar Purkinje cells of the rat

1 Nanomolar concentrations of Cu2+ induce a slowly reversible block of GABA(A) receptor-mediated currents which can be removed by chelating substances. 2 The possible interaction of Cu2+ with the Zn2+ binding site on the GABA(A) receptor complex was studied in acutely isolated Purkinje cells using whole-cell recording and a fast drug application system. 3 When Zn2+ was applied together with 2 mu M GABA, the Zn2+-induced block of GABA-mediated currents was not additive to the Cu2+-induced block. In the presence of 0.1 mu M Cu2+ in the bath solution the degree of inhibition of GABA-mediated responses by Zn2+ was strongly attenuated. 4 Preapplication of 100 mu M Zn2+ during 10 s, terminated 1 s before exposure to 2 mu M GABA did not affect the GABA current in Cu2+-free solution, but relieved its block by 0.1 mu M Cu2+. This effect of Zn2+ was concentration-dependent with an EC50 of 72 mu M. 5 When the Cu2+-induced block was removed by histidine, preapplication of Zn2+ did not increase the GABA current, indicating that the relief of Cu2+ block by Zn2+ is the result of its ability to actively remove Cu2+ from the GABA receptor complex. 6 It is proposed that the inhibitory effects of Zn2+ and Cu2+ on GABA-induced currents result from an action of these metal ions at distinct, but conformationally linked sites on the GABA(A) receptor protein. Under physiological conditions Zn2+ would liberate Cu2+ from the GABA(A) receptor, thus facilitating Cu2+ turnover and its binding by other endogenous chelating molecules.