Nitric oxide as a potential pathological mechanism in demyelination: its differential effects on primary glial cells in vitro.

Because we believe that macrophage-derived nitric oxide contributes to pathology of demyelinating diseases, we have determined the differential effects of nitric oxide on primary rat glial cells in vitro. Enriched cultures of microglia, astrocytes and oligodendrocytes were treated with S-nitroso,N-acetyl-DL-penicillamine, a nitric oxide-releasing chemical. There was a significantly decreased function of one of the ferrosulfur-containing mitochondrial enzymes after S-nitroso,N-acetyl-DL-penicillamine/nitric oxide treatment in oligodendrocytes and astrocytes compared to microglia, which were much less sensitive to S-nitroso,N-acetyl-DL-penicillamine/nitric oxide at all concentrations. At 0.5 mM S-nitroso,N-acetyl-DL-penicillamine/nitric oxide, astrocytes and oligodendrocytes suffered a 40% loss in succinate dehydrogenase activity, while microglia were unaffected. A control non-ferrosulfur-containing mitochondrial enzyme, isocitrate dehydrogenase, was not affected in any glial cell type. Although the per cent of mitochondrial damage in oligodendrocytes and astrocytes was the same for all concentrations of S-nitroso,N-acetyl-DL-penicillamine/nitric oxide, significant cell death occurred in oligodendrocytes at 1.0 mM; at this concentration there was no significant killing of microglia or astrocytes. Furthermore, at a 0.5 mM concentration of S-nitroso,N-acetyl-DL-penicillamine/nitric oxide, which inhibited mitochondrial respiration but did not kill oligodendrocytes, significant changes in oligodendrocyte morphology (e.g. retraction of processes) occurred. Morphological changes were not seen in microglia and astrocytes at any concentration of S-nitroso,N-acetyl-DL-penicillamine/nitric oxide. In addition, oligodendrocytes were more sensitive to S-nitroso,N-acetyl-DL-penicillamine/nitric oxide-induced single stranded DNA breaks than microglia or astrocytes. The mitochondrial damage was attributable to nitric oxide since N-acetyl-DL-penicillamine had no effect. Oxyhemoglobin, which competitively inhibits toxic effects of nitric oxide, protected these glial cells from mitochondrial damage, single stranded breaks in DNA and cell death in a time- and dose-dependent manner. Once again, oligodendrocytes were less easily rescued from nitric oxide effects by oxyhemoglobin than were astrocytes, suggesting greater vulnerability of the myelin-producing cell to nitric oxide. These findings suggest that there is differential sensitivity of glial cells to nitric oxide. Although oligodendrocytes and astrocytes are equally susceptible to nitric oxide-induced mitochondrial damage, oligodendrocytes are more sensitive to nitric oxide-induced single stranded DNA breaks, morphological changes and cell death. Compared to both oligodendrocytes and astrocytes, microglia, nitric oxide-producing cells, are resistant to nitric oxide-induced damage.