Role of nitric oxide in cerebral ischemia/reperfusion injury: A biomolecular overview.

Nitric oxide (NO) is a gaseous molecule produced by 3 different NO synthase (NOS) isoforms: Neural/brain NOS (nNOS/bNOS, type 1), endothelial NOS (eNOS, type 3) and inducible NOS (type 2). Type 1 and 3 NOS are constitutively expressed. NO can serve different purposes: As a vasoactive molecule, as a neurotransmitter or as an immunomodulator. It plays a key role in cerebral ischemia/reperfusion injury (CIRI). Hypoxic episodes simulate the production of oxygen free radicals, leading to mitochondrial and phospholipid damage. Upon reperfusion, increased levels of oxygen trigger oxide synthases; whose products are associated with neuronal damage by promoting lipid peroxidation, nitrosylation and excitotoxicity. Molecular pathways in CIRI can be altered by NOS. Neuroprotective effects are observed with eNOS activity. While nNOS interplay is prone to endothelial inflammation, oxidative stress and apoptosis. Therefore, nNOS appears to be detrimental. The interaction between NO and other free radicals develops peroxynitrite; which is a cytotoxic agent. It plays a main role in the likelihood of hemorrhagic events by tissue plasminogen activator (t-PA). Peroxynitrite scavengers are currently being studied as potential targets to prevent hemorrhagic transformation in CIRI.