Nitric oxide reactivity and toxicity in brain tissue in vitro
The role of nitric oxide (NO) in brain physiology and pathology is governed by its concentration, a function of the balance between synthesis and breakdown. Following cerebral ischaemia NO may play a protective or destructive role, and the literature is plagued by contradictory findings. Contributing to the confusion is a lack of knowledge as to what constitutes a toxic concentration of NO, how NO is inactivated in vivo, and a large number of potential pitfalls. Measured using an NO-sensitive electrode, most of the NO delivered using a NONOate donor was removed by reaction with tissue culture medium. The main constituent responsible was Hepes buffer, which consumed NO in a superoxide dismutase-sensitive manner, indicating formation of peroxynitrite from superoxide and NO. Given the widespread use of Hepes, the reaction may contribute artifactually to multiple effects of NO observed in vitro. The hypothesis that NO mediates neurodegeneration arising from NMDA receptor activity was then re-examined using organotypic slice cultures of rat hippocampus. The NO-cGMP signaling pathway was well preserved in such cultures but no component of NMDA-induced cell death was attributable to NO. At the same time, the tissue was remarkably resistant to exogenous NO at up to 1000-fold higher concentrations. Together, these results seriously question the proposed role of NO in NMDA receptor-mediated excitotoxicity. An avid NO consumption mechanism in rat cerebellar cells and brain homogenate had been previously described. A combination of transition metals and ascorbate was shown to be responsible for a component of this consumption. When this mechanism of consumption was inhibited dispersed rat brain preparations continued to consume NO by another powerful, as yet undetermined, mechanism.