NMDA receptor-mediated excitotoxicity depends on the coactivation of synaptic and extrasynaptic receptors

N-methyl-D-aspartate receptors (NMDAR) overactivation is linked to neurodegeneration. The current prevailing theory suggests that synaptic and extrasynaptic NMDAR (syn-and ex-NMDAR) impose counteracting effects on cell fate, and neuronal cell death is mainly mediated by the activation of ex-NMDAR. However, several lines of evidence implicate the limitation of this theory. Here, we demonstrate that activation of NMDAR bi-directionally regulated cell fate through stimulating pro-survival or pro-death signaling. While low-dose NMDA preferentially activated syn-NMDAR and stimulated the extracellular signal-regulated kinase 1/2-cAMP responsive element-binding protein-brain-derived neurotrophic factor pro-survival signaling, higher doses progressively activated increasing amount of ex-NMDAR along with syn-NMDAR and triggered cell death program. Interestingly, the activation of syn- or ex-NMDAR alone did not cause measurable cell death. Consistently, activation of syn- or ex-NMDAR alone stimulated pro-survival but not pro-death signaling. Next, we found that memantine, which was previously identified as an ex-NMDAR blocker, inhibited intracellular signaling mediated by syn- or ex-NMDAR. Simultaneous blockade of syn-and ex-NMDAR by memantine dose-dependently attenuated NMDAR-mediated death. Moreover, long-but not short-term treatment with high-dose NMDA or oxygen-glucose deprivation triggered cell death and suppressed pro-survival signaling. These data implicate that activation of syn-or ex-NMDAR alone is not neurotoxic. The degree of excitotoxicity depends on the magnitude and duration of syn-and ex-NMDAR coactivation. Finally, genome-wide examination demonstrated that the activation of syn-and ex-NMDAR lead to significant overlapping rather than counteracting transcriptional responses. Cell Death and Disease (2013) 4, e560; doi:10.1038/cddis.2013.82; published online 28 March 2013