Abstract
Nitric oxide (NO) is a free radical molecule with a short half-life (<5 s). Because its synthesis from L-arginine by constitutive NO synthase (NOS) is low in many cell types, including neurons and endothelial cells, direct detection of NO in biological systems is a difficult task. During pathological conditions in the CNS, the inducible form of NOS (iNOS or NOS2) is expressed in activated astrocytes and microglial cells and can result in higher levels of NO. However, it may still be difficult to detect NO in these cell types using typical spectrophotometric methods. Of particular note, NO is readily oxidized to nitrite and nitrate (relatively stable products) in cells and medium, which can be measured as a valid indicator of NO synthesis. The conversion of NO to peroxynitrite leads to the formation of stable protein adducts that can be detected by immunohistochemical or immunofluorescence methods. Additionally, intracellular levels of NO can be detected in real time using fluorescence imaging and NO-specific, cell permeable indicator dyes.