Synapses, NMDA receptor activity and neuronal Aβ production in Alzheimer's disease.

A direct relationship has been established between synaptic activity and amyloid-β secretion. Dysregulation of neuronal calcium homeostasis was shown to increase production of amyloid-β, contributing to the initiation of Alzheimer's disease. Among the different routes of Ca(2+) entry, N-methyl-d-aspartate (NMDA) receptors, a subtype of ionotropic glutamate receptors, are especially involved in this process because of their ability to gate high levels of Ca(2+) influx. These receptors have been extensively studied for their crucial roles in synaptic plasticity that underlies learning and memory but also in neurotoxicity occurring during acute brain injuries and neurodegenerative diseases. For one decade, several studies provided evidence that NMDA receptor activation could have distinct consequences on neuronal fate, depending on their location. Synaptic NMDA receptor activation is neuroprotective, whereas extrasynaptic NMDA receptors trigger neuronal death and/or neurodegenerative processes. Recent data suggest that chronic activation of extrasynaptic NMDA receptors leads to a sustained neuronal amyloid-β release and could be involved in the pathogenesis of Alzheimer's disease. Thus, as for other neurological diseases, therapeutic targeting of extrasynaptic NMDA receptors could be a promising strategy. Following this concept, memantine, unlike other NMDA receptor antagonists was shown, to preferentially target the extrasynaptic NMDA receptor signaling pathways, while relatively sparing normal synaptic activity. This molecular mechanism could therefore explain why memantine is, to date, the only clinically approved NMDA receptor antagonist for the treatment of dementia.