37 These latter observations have led to the investigation of NMDA antagonists as putative novel antidepressants.29,37 NMDA receptors play a critical role in regulating synaptic plasticity.38 The best-studied forms of synaptic plasticity in the CNS are long-term potentiation (LTP) and longterm depression (LTD) of excitatory synaptic transmission. The molecular mechanisms of LTP and LTD have been extensively Inhibitors,research,lifescience,medical characterized and have been proposed to represent cellular models of learning and memory.38
Induction of LTP and LTD in the CA1 region of the hippocampus and in many regions of the brain has now clearly been demonstrated to be dependent on NMDA receptor activation. COX phosphorylation During NMDA-receptor-dependent synaptic plasticity, Ca2+ influx through NMDA receptors can activate a wide variety of kinases and/or phosphatases that, in turn, modulate synaptic strength. An important recent development is the finding that two of the primary molecules involved – Ca2+/ calmodulin-dependcnt Inhibitors,research,lifescience,medical protein kinase II (CAMKII) and the NMDA subtype of glutamate receptor – form a tight complex with each other at the synapse.39 Interestingly, this binding appears to enhance both the autophosphorylation Inhibitors,research,lifescience,medical of the kinase and the ability of the entire holoenzyme, which has 12 subunits, to become hyperphosphorylated.39 This hyperphosphorylated state has been postulated to represent a “memory switch,” which can lead to long-term strengthening of
the synapse by multiple mechanisms. One important mechanism involves direct phosphorylation
Inhibitors,research,lifescience,medical of the glutamate-activated AMPA receptors, which increases their conductance. Furthermore, once CAMKII is bound to the NM.DA receptor, it may organize additional anchoring sites for AMPA receptors at the synapse. It is intriguing that activation of synaptic NMDA receptor versus nonsynaptic receptor has an opposite effect on cell survival via differential regulation of CREB (cyclic adenosine monophosphate [cAMP]-respouse element binding protein) function. Calcium entry through synaptic NMDA receptors induced CREB activity and brain-derived Inhibitors,research,lifescience,medical neurotrophic factor (BDNF) gene expression as strongly as did stimulation of L-type calcium Ketanserin channels. In contrast, calcium entry through nonsynaptic NMDA receptors, triggered by glutamate exposure or hypoxic/ischemic conditions, activated a general and dominant CREB shut-off pathway that blocked induction of BDNF expression. Synaptic NMDA receptors have antiapoptotic activity, whereas stimulation of extrasynaptic NMDA receptors caused loss of mitochondrial membrane potential (an early marker for glutamate-induced neuronal damage) and cell death.40 AMPA receptor trafficking plays critical roles in the regulation of various forms of neural plasticity The AMPA receptor is stimulated by the presence of glutamate and characteristically produces a fast excitatory synaptic signal that is responsible for the initial reaction to glutamate in the synapse.