AMPA receptor-mediated excitotoxicity in human NT2-N neurons results from loss of intracellular Ca²⁺ homeostasis following marked elevation of intracellular Na⁺

Human NT2-N neurons express Ca²⁺-permeable α-amino-3-hydroxy-5- methylisoxazole-4-propionic acid glutamate receptors (AMPA-GluRs) and become vulnerable to excitotoxicity when AMPA-GluR desensitization is blocked with cyclothiazide. Although the initial increase in intracellular Ca²⁺ levels ([Ca²⁺](i)) was 1.9-fold greater in the presence than in the absence of cyclothiazide, Ca²⁺ entry via AMPA-GluRs in an early phase of the exposure was not necessary to elicit excitotoxicity in these neurons. Rather, subsequent necrosis was caused by a >40-fold rise in [Na⁺](i), which induced a delayed [Ca²⁺](i) rise. Transfer of the neurons to a 5 mM Na⁺ medium after AMPA-GluR activation accelerated the delayed [Ca²⁺](i) rise and intensified excitotoxicity. Low-Na⁺ medium-enhanced excitotoxicity was partially blocked by amiloride or dizocilpine (MK-801), and completely blocked by removal of extracellular Ca²⁺, suggesting that Ca²⁺ entry by reverse operation of Na⁺/Ca²⁺ exchangers and via NMDA glutamate receptors was responsible for the neuronal death after excessive Na⁺ loading. Our results serve to emphasize the central role of neuronal Na⁺ loading in AMPA- GluR-mediated excitotoxicity in human neurons.