In developing hippocampal neurons, NR2B-containing N-methyl- d-aspartate receptors (NMDARs) can mediate signaling to neuronal survival and synaptic potentiation, as well as neuronal death

Abstract

It has been suggested that NR2B-containing N-methyl- d-aspartate (NMDA) receptors have a selective tendency to promote pro-death signaling and synaptic depression, compared with the survival promoting, synapse potentiating properties of NR2A-containing NMDA receptors. A preferential localization of NR2A-containing NMDA receptors at the synapse in maturing neurons could thus explain differences in synaptic vs. extrasynaptic NMDA receptor signaling. We have investigated whether NMDA receptors can mediate signaling to survival, death, and synaptic potentiation, in dissociated rat neuronal cultures at a developmental stage prior to significant NR2A expression and subunit-specific differences between synaptic and extrasynaptic NMDA receptors. We show that in developing hippocampal neurons, the progressive reduction in sensitivity of NMDA receptor currents to the NR2B antagonist ifenprodil applies to both synaptic and extrasynaptic locations. However, the reduction is less acute in extrasynaptic currents, indicating that NR2A does partition preferentially, but not exclusively, into synaptic locations at DIV>12. We then studied NMDA receptor signaling at DIV10, when both synaptic and extrasynaptic NMDA receptors are both overwhelmingly and equally NR2B-dominated. To analyze pro-survival signaling we studied the influence of synaptic NMDA receptor activity on staurosporine-induced apoptosis. Blockade of spontaneous NMDAR activity with MK-801, or ifenprodil exacerbated the apoptotic insult. Furthermore, MK-801 and ifenprodil both antagonized neuroprotection promoted by enhancing synaptic activity. Pro-death signaling induced by a toxic dose of NMDA is also blocked by NR2B-specific antagonists. Using a cell culture model of synaptic NMDA receptor-dependent synaptic potentiation, we find that this is mediated exclusively by NR2B-containing N-methyl- d-aspartate receptors, as implicated by NR2B-specific antagonists and the use of selective vs. non-selective doses of the NR2A-preferring antagonist NVP-AAM077. Therefore, within a single neuron, NR2B-NMDA receptors are able to mediate both survival and death signaling, as well as model of NMDA receptor-dependent synaptic potentiation. In this instance, subunit differences cannot account for the dichotomous nature of NMDA receptor signaling.