Inflammatory-activated glia are seen in numerous central nervous system (CNS) pathologies and can kill nearby neurons through the release of cytotoxic mediators. Glia, when activated, can express the inducible
isoform of
nitric oxide synthase (iNOS) producing high levels of
nitric oxide (NO), which can kill neurons in certain conditions. We show, however, that inflammatory activation of glia in a mature culture of cerebellar granule neurons and glia causes little or no neuronal death under normal (21%)
oxygen conditions. Similarly,
hypoxia (2%
oxygen) or low levels of an NO donor (100 microM
DETA/NO) caused little or no neuronal death in nonactivated cultures. If inflammatory activation of glia or addition of NO donor was combined with
hypoxia, however, extensive neuronal death occurred. Death in both cases was prevented by the
N-methyl-D-aspartate (
NMDA) receptor blocker
MK-801, implying that death was mediated by the
glutamate receptor. Low levels of NO were found to increase the apparent K(M) of cellular oxygen consumption for
oxygen, probably due to NO-induced inhibition of mitochondrial respiration, in competition with
oxygen, at
cytochrome oxidase. Necrotic death, induced by
hypoxia plus
DETA/NO, was increased further by
deoxyglucose, an inhibitor of glycolysis, suggesting that
necrosis was mediated by energy depletion.
Hypoxia was found to be a potent stimulator of microglia proliferation, but this proliferation was not significant in inflammatory-activated cultures. These results suggest that low levels of NO can induce neuronal death under hypoxic conditions, mediated by
glutamate after NO inhibition of respiration in competition with
oxygen.
Brain inflammation can thus sensitize to
hypoxia-induced death, which may be important in pathologies such as
stroke, neurodegeneration, and brain aging.