Selective enhancement of domoic acid toxicity in primary cultures of cerebellar granule cells by lowering extracellular Na⁺ concentration

Domoic acid (DOM) is an excitatory amino acid analog of kainic acid (KA) that acts through glutamic acid (GLU) receptors, inducing a fast and potent neurotoxic response. Here, we present evidence for an enhancement of excitotoxicity following exposure of cultured cerebellar granule cells to DOM in the presence of lower than physiological Na⁺ concentrations. The concentration of DOM that reduced by 50% neuronal survival was approximately 3 mM in Na⁺-free conditions and 16 mM in presence of a physiological concentration of extracellular Na⁺. The enhanced neurotoxic effect of DOM was fully prevented by AMPA/KA receptor antagonist, while N-methyl-D-aspartate-receptor-mediated neurotoxicity did not seem to be involved, as the absence of extracellular Na⁺ failed to potentiate GLU excitotoxicity under the same experimental conditions. Lowering of extracellular Na⁺ concentration to 60mMeliminated extracellular recording of spontaneous electrophysiological activity from cultured neurons grown on a multi electrode array and prevented DOM stimulation of the electrical activity. Although changes in the extracellular Na⁺ concentration did not alter the magnitude of the rapid increase in intracellular Ca²⁺ levels associated to DOM exposure, they did change significantly the contribution of voltagesensitive calciumchannels (VScaCs) and the recovery time to baseline. The prevention of Ca²⁺ influx via VSCaCs by nifedipine failed to prevent DOM toxicity at any extracellular Na⁺ concentration, while the reduction of extracellular Ca²⁺ concentration ameliorated DOM toxicity only in the absence of extracellular Na⁺, enhancing it in physiological conditions. Our data suggest a crucial role for extracellular Na⁺ concentration in determining excitotoxicity by DOM.