High‐affinity copper block of GABAA receptor‐mediated currents in acutely isolated cerebellar Purkinje cells of the rat

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Abstract
The actions of Cu2+ ions on GABAA receptor-mediated currents in acutely isolated Purkinje cells from rat cerebellum were studied using the whole-cell patch-clamp technique and a rapid perfusion system. Bath application of Cu2+ reduced currents induced by 2 μmγ-aminobutyric acid (GABA) in a concentration-dependent manner with an IC50 of 35 n m. The Cu2+-induced block of GABA responses was not voltage-dependent. Increasing the GABA concentration (from 2 to 50 μm) decreased the blocking effect of Cu2+. Dose–response analysis for activation of GABAA receptors revealed a twofold decrease in apparent affinity for GABA in the presence of 0.1 μm Cu2+. Recovery from the block required several minutes after removal of Cu2+ from the medium. The block was removed by histidine, which preferentially forms complexes with Cu2+, or by other chelating substances. Application of 10 μm histidine immediately before application of 2 μm GABA completely relieved the block of GABA responses produced by 0.1 μm Cu2+. The effect of histidine was concentration-dependent with an EC50 of 0.75 μm. The results demonstrate that Cu2+ is a potent inhibitor of GABA-evoked responses in rat Purkinje cells. Copper may be an endogenous synaptic modulating factor. Cu2+ toxicity, notably in Wilson’s disease, could result to some extent from chronic GABAA receptor blockade.