Zinc toxicity on cultured cortical neurons: Involvement of N-methyl- d-aspartate receptors

Abstract

Neuronal injury induced by the excessive release of endogenous Zn 2+ at central glutamatergic synapses may contribute to the pathogenesis of epileptic brain damage. We explored the possibility that N-methyl- d-aspartate receptors might be involved in Zn 2+ neurotoxicity. Exposure of murine cortical cell cultures to 300–1000 μM concentrations of Zn 2+ for 15 min resulted in widespread neuronal degeneration, accompanied by the release of lactate dehydrogenase to the bathing medium. Both non-competitive and competitive N-methyl- d-aspartate antagonists attenuated this degeneration. However, the participation of N-methyl- d-aspartate receptors in Zn 2+ neurotoxicity was atypical. Removal of extracellular Ca 2+ attenuated N-methyl- d-aspartate neurotoxicity but potentiated Zn 2+ neurotoxicity, whereas increasing extracellular Ca 2+ potentiated N-methyl- d-aspartate neurotoxicity but attenuated Zn 2+ neurotoxicity. Furthermore, the nature of the antagonism of Zn 2+ neurotoxicity induced by N-methyl- d-aspartate antagonists was qualitatively different from that seen with other N-methyl- d-aspartate receptor-mediated events. The block of Zn 2+ neurotoxicity induced by the non-competitive N-methyl- d-aspartate antagonist MK-801 was better overcome by increasing Zn 2+ concentration than the block induced by the competitive antagonists d-aminophosphonovalerate and CGS-19755. We hypothesize that N-methyl- d-aspartate receptor-gated channels contribute to Zn 2+ toxicity by provicling a route of Zn 2+ influx into neurons. Consistent with this idea, intracellular Zn 2+ visualized by the fluorescent Zn 2+ chelator, N-(6-methoxy-8-quinolyl-p-toluenesulfonamide, rose during Zn 2+ exposure; this rise was increased by N-methyl- d-aspartate and reduced by either N-methyl- d-aspartate antagonists or high Ca 2+. Since Zn 2+ affects activity of many neuronal proteins, intracellular Zn 2+ overload may produce lethal disturbances in neuronal cell homeostasis.