Excitotoxicity induced by enhanced excitatory neurotransmission in cultured hippocampal pyramidal neurons

Cultures of rat hippocampal pyramidal neurons were used to examine the roles of excitatory synaptic transmission, NMDA receptors, and elevated [Ca²⁺]_i in the production of excitotoxicity. In ∼70% of the cells observed, perfusion with Mg²⁺-free, glycine-supplemented medium induced large spontaneous fluctuations or maintained plateaus of [Ca²⁺]_i. [Ca²⁺]_i fluctuations could be blocked by tetrodotoxin, NMDA receptor antagonists, dihydropyridines, or compounds that inhibit synaptic transmission in the hippocampus, but not by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. When cells were treated with Mg²⁺-free, glycine-supplemented medium and examined 24 hr later, ∼30% of the neurons were found to have died. Cell death could be inhibited by the same agents that reduced [Ca²⁺]_i fluctuations. These results support a role for direct excitatory synaptic transmission, as opposed to the general release of glutamate, in excitotoxicity. A major role for synaptically activated NMDA receptors, rather than kainate/quisqualate receptors, is also indicated Neuronal death may be produced by abnormal changes in neuronal [Ca²⁺]_i.