Nitric Oxide as a Mediator of Cerebral Blood-Flow, Synaptic Plasticity, and Superoxide-Mediated Brain Injury

We propose that excessive production of nitric oxide initiated by activation of glutamate receptors after cerebral ischemia potentiates free radical injury to the brain. Nitric oxide, produced by neurons and possibly astrocytes, helps regulate local cerebral blood flow and plays an essential role in synaptic plasticity and normal development of the brain. Nitric oxide itself is a weak oxidizing agent, but after reaction with superoxide (O 2 - ), it forms the strong and relatively long-lived oxidant peroxynitrite anion (ONOO-) (Beckman et al., 1990). Peroxynitrite is sufficiently stable even in the presence of physiological concentrations of glutathione and other cellular antioxidants to diffuse for up to several cell diameters. Depending on what peroxynitrite reacts with, it can produce oxidants with the reactivity of hydroxyl radical (HO⋅), nitrogen dioxide (NO2), nitronium ion (NO 2 + ), and possibly singlet oxygen. Thus, the conversion of nitric oxide to peroxynitrite and related secondary oxidants may provide a common link between glutamate and free radical-mediated injury. Glutamate antagonists and free radical scavengers can reduce infarct volume by approximately the same extent in rat middle cerebral artery occlusion models of stroke (Oh and Betz, 1991). Recently, nitroarginine, a competitive inhibitor of nitric oxide synthesis, has also been shown to reduce infarct volume in a mouse middle cerebral ischemia model (Nowicki et al., 1991).