Actions of redox-related congeners of nitric oxide at the NMDA receptor.
The local redox milieu of a biological system is of critical importance in understanding the actions of the nitrogen monoxide (NO) moiety, as disparate chemical pathways involving distinct redox-related congeners of NO may trigger neurotoxic or neuroprotective pathways. The reactions of nitric oxide (NO.) with superoxide can lead to neurotoxicity through formation of peroxynitrite, whereas NO. alone does not, at least under certain conditions. Reaction (or transfer) of NO+ equivalents to thiol(s) on the NMDA receptor can lead to neuroprotection by inhibiting Ca2+ influx. These findings suggest that cell function can be controlled by, or through, protein S-nitrosylation, and raise the possibility that the NO group may initiate signal transduction in or at the plasma membrane. Neuroprotective effects of NO- suggest that acceleration of disulfide bond formation at the NMDA receptor is of mechanistic importance in the attenuation of Ca2+ influx. Our findings suggest novel therapeutic strategies. For example, downregulation of NMDA receptor activity can be obtained via sulfhydryl oxidation by S-nitros(yl)ation with NO+ donors (to form an RSNO at a cysteine residue on the receptor), or with NO- donors (with intermediate formation of RSNHOH). Pharmacologic intervention with these forms of NO donors could be implemented in the treatment of focal ischemia, neuropathic pain, Huntington's disease, AIDS dementia, and other neurological disorders associated, at least in part, with excessive activation of NMDA receptors.
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