Reciprocal modulation of Ca v 2.3 voltage‐gated calcium channels by copper( II ) ions and kainic acid

Kainic acid ( KA ) is a potent agonist at non‐N‐methyl‐D‐aspartate (non‐ NMDA ) ionotropic glutamate receptors and commonly used to induce seizures and excitotoxicity in animal models of human temporal lobe epilepsy. Among other factors, Ca v 2.3 voltage‐gated calcium channels have been implicated in the pathogenesis of KA ‐induced seizures. At physiologically relevant concentrations, endogenous trace metal ions (Cu 2+ , Zn 2+ ) occupy an allosteric binding site on the domain I gating module of these channels and interfere with voltage‐dependent gating. Using whole‐cell patch‐clamp recordings in human embryonic kidney ( HEK ‐293) cells stably transfected with human Ca v 2.3d and β 3 ‐subunits, we identified a novel, glutamate receptor‐independent mechanism by which KA can potently sensitize these channels. Our findings demonstrate that KA releases these channels from the tonic inhibition exerted by low nanomolar concentrations of Cu 2+ and produces a hyperpolarizing shift in channel voltage‐dependence by about 10 mV , thereby reconciling the effects of Cu 2+ chelation with tricine. When tricine was used as a surrogate to study the receptor‐independent action of KA in electroretinographic recordings from the isolated bovine retina, it selectively suppressed a late b‐wave component, which we have previously shown to be enhanced by genetic or pharmacological ablation of Ca v 2.3 channels. Although the pathophysiological relevance remains to be firmly established, we speculate that reversal of Cu 2+ ‐induced allosteric suppression, presumably via formation of stable kainate‐Cu 2+ complexes, could contribute to the receptor‐mediated excitatory effects of KA . In addition, we discuss experimental implications for the use of KA in vitro , with particular emphasis on the seemingly high incidence of trace metal contamination in common physiological solutions.