Lowering extracellular Na+ concentration causes NMDA receptor-mediated neuronal death in cultured rat hippocampal...

Reduction of the transmembrane Na+ gradient is expected to induce neurotoxic glutamate release via reversed uptake. We describe neuronal death induced by lowering extracellular Na+ concentration in cultured rat hippocampal slices. When slices were exposed to 3.6 mM Na+ for 30 min, almost all the neurons in the CA1 region were degenerated within 20-24 h. The marginal concentration of external Na+ for induction of neurotoxicity was 6.6 mM. N-methyl-D-aspartate (NMDA) receptor antagonists, MK-801 at 1-3 microM and (+/-)-CPP at 30 microM, significantly decreased these neurotoxic effects. A non-NMDA receptor antagonist, DNQX at 30-100 microM, had no protective effect against neurotoxicity. Removal of external Ca2+ completely eliminated neuronal death, but replacing external Cl- with SO4(2-) had no protective effect against neurotoxicity. A drastic 40-fold increase in glutamate release was produced by 30 min exposure to 3.6 mM Na+, and this release was partially independent of external Ca2+. These findings suggest that the low-Na(+)-induced neurotoxicity we observed is mediated by excessive release of glutamate via reversed uptake and subsequent Ca(2+)-influx through NMDA receptors. The model reported here may be useful for investigation of the mechanism of neuronal injury mediated by endogenous glutamate.