This family of glutamate receptors are ligand-gated ion channels highly permeable to Ca
2+. In physiological situations, NMDARs are central to neuronal transmission, synaptic plasticity and memory, as well as being central to survival
[10]. They are heterotetramers formed by two obligatory GluN1 subunits and two GluN2 (GluN2A–D) or GluN3 (GluN3A–B) subunits, although the most frequently expressed NMDARs contain GluN1 in combination with either GluN2B or GluN2A subunits. The synaptic NMDARs form large and dynamic signaling complexes in the postsynaptic membrane, mostly by interactions established by their intracellular C-terminal domains with signaling, cytoskeleton and scaffold proteins such as PSD-95
[11], which functions as a postsynaptic density (PSD) organizer. Pro-survival signaling initiated by synaptic NMDAR activation and a moderate Ca
2+ increase stimulates, among others, antioxidant defenses
[12], and activates extracellular signal-regulated kinase
[13] and phosphorylation of the cAMP response element-binding protein (CREB)
[14]. The activation of CREB results in the expression of pro-survival genes such as those encoding brain-derived neurotrophic factor (BDNF)
[15][16] or its receptor, tropomyosin-related kinase B (TrkB)
[17]. In contrast, activation of NMDARs in pathological conditions—for example, after massive glutamate release in the ischemic brain—opposes these neuroprotective effects and is coupled to cell death pathways
[18]. In stroke, deprivation of oxygen and glucose causes an important decrease in ATP levels, followed by membrane depolarization and the release of excitatory neurotransmitters that induce NMDAR overactivation, massive Ca
2+-influx and excitotoxicity. Different mechanisms participate in this form of neuronal death, but a major contribution is made by calpain, a Ca
2+-dependent protease central to different acute and chronic CNS disorders associated with excitotoxicity
[19]. In neurons, calpain processes many different substrates in discreate specific sequences altering their stability, activity or localization
[20][21]. Importantly, some of these calpain substrates are proteins critical to neuronal survival and function, such as the neurotrophin receptor TrkB
[22], the GluN2 subunits of the NMDAR
[23] or their interacting protein PSD-95
[23][24].
The dual roles played by NMDARs in neurons have been related to their cellular location, whereas synaptic NMDARs mediate survival signaling pathways, and extrasynaptic receptors activate death pathways. However, another theory states that GluN2A-containing NMDARs are implicated in neuronal survival, while GluN2B-containing NMDARs induce neuronal death
[25]. Actually, the “NMDAR location” and “NMDAR subtype” hypotheses are not mutually exclusive since, in the adult cortex, GluN2A and GluN2B subunits are preferentially localized at, respectively, synaptic and extrasynaptic sites. Nevertheless, recent studies have pointed out that the activation of death signaling pathways requires the synergistic activation of both synaptic and extrasynaptic NMDARs
[25]. Although we still need a more in depth characterization, our current knowledge about the dual roles of NMDARs in physiopathology has helped to understand previous failure of NMDAR antagonists in stroke clinical trials. This knowledge has also provided solid ground for the development of novel neuroprotective strategies for stroke that are able to inhibit death signaling and downstream NMDAR overactivation, without interfering with survival or, alternatively, strategies aimed at preventing aberrant downregulation of neuronal survival cascades taking place in excitotoxicity. Since PSD-95 plays a central role in neuronal survival and death downstream NMDARs, it has become a very promising target for both types of therapeutic strategies. Such therapies might protect and recover the ischemic penumbra, contributing to reduced stroke damage, but the therapy could be also used to treat other acute and chronic diseases, such as neurodegenerative diseases, which are similarly associated with excitotoxicity
[26].