Nitric oxide toxicity in CNS white matter: an in vitro study using rat optic nerve

Excessive nitric oxide formation may contribute to the pathology occurring in diseases affecting central white matter, such as multiple sclerosis. The rat isolated optic nerve preparation was used to investigate the potential toxicity of the molecule towards such tissue. The nerves were exposed to a range of concentrations of different classes of nitric oxide donor for up to 23 h, with or without a subsequent period of recovery, and the damage assessed by quantitative histological methods. Degeneration of axons and macroglia occurred in a time- and concentration-dependent manner, the order of susceptibility being: axons>oligodendrocytes>astrocytes. Use of NONOate donors differing in half-life indicated that nitric oxide delivered in an enduring manner at relatively low concentration was more toxic than the same amount supplied rapidly at high concentration. The mechanism by which nitric oxide affects axons was studied using a donor [3-(n-propylamino)propylamine/NO adduct, PAPA/NO] with an intermediate half-life that produced selective axonopathy after a 2-h exposure (plus 2 h recovery). Axon damage was abolished if, during the exposure, Na(+) or Ca(2+) was removed from the bathing medium or the sodium channel inhibitors tetrodotoxin or BW619C89 (sipatrigine) were added. In electrophysiological experiments, the donor elicited a biphasic depolarisation. The second, larger component (occurring after 7-10 min) was associated with a block of nerve conduction and could be inhibited by tetrodotoxin. Coincident with the secondary depolarisation was a reduction in ATP levels by about 50%, an effect that was also inhibited by tetrodotoxin. It is concluded that nitric oxide, in submicromolar concentrations, can kill axons and macroglia in white matter. The findings lend support to the hypothesis that nitric oxide may be of importance to white matter pathologies, particularly those in which inducible nitric oxide synthase is expressed. The axonopathy, at least when elicited over relatively short time intervals, is likely to be caused by metabolic inhibition. As in anoxia and anoxia/aglycaemia, nitric oxide-induced destruction of axons is likely to be caused by the Ca(2+) overload that follows a reduction in ATP levels in the face of continued influx of Na(+) through voltage-dependent channels.