Distinct Influx Pathways, Not Calcium Load, Determine Neuronal Vulnerability to Calcium Neurotoxicity

Abstract: Many forms of neurodegeneration are ascribed to excessive cellular Ca 2+ loading (Ca 2+ hypothesis). We examined quantitatively whether factors other than Ca 2+ loading were determinants of excitotoxic neurodegeneration. Cell survival, morphology, free intracellular Ca 2+ concentration ([Ca 2+ ] i ), and 45 Ca 2+ accumulation were measured in cultured cortical neurons loaded with known quantities of Ca 2+ through distinct transmembrane pathways triggered by excitatory amino acids, cell membrane depolarization, or Ca 2+ ionophores. Contrary to the Ca 2+ hypothesis, the relationships between Ca 2+ load and cell survival, free [Ca 2+ ] i , and Ca 2+ ‐induced morphological alterations depended primarily on the route of Ca 2+ influx, not the Ca 2+ load. Notably, Ca 2+ loading via NMDA receptor channels was toxic, whereas identical Ca 2+ loads incurred through voltage‐sensitive Ca 2+ channels were completely innocuous. Furthermore, accounting quantitatively for Ca 2+ loading via NMDA receptors uncovered a previously unreported component of l ‐glutamate neurotoxicity apparently not mediated by ionotropic or metabotropic glutamate receptors. It was synergistic with toxicity attributable to glutamate‐evoked Ca 2+ loading, and correlated with enhanced cellular ATP depletion. This previously unrecognized toxic action of glutamate constituted a chief excitotoxic mechanism under conditions producing submaximal Ca 2+ loading. We conclude that (a) Ca 2+ neurotoxicity is a function of the Ca 2+ influx pathway, not Ca 2+ load, and (b) glutamate toxicity may not be restricted to its actions on glutamate receptors.