Glutamate Excitotoxicity Induced by Nitric Oxide Mediated Dysfunction of the Mitochondrial 2-Oxoglutarate Dehydrogenase Complex

摘要

Increased levels of nitric oxide (NO) and peroxynitrite (ONOO ? ), mitochondrial dysfunction at complex I, and excessive accumulation of glutamate have been suggested as the key pathophysiological events leading to brain damage upon traumatic brain injury (TBI). The aim of this study was to delineate the relationship between these three events and to begin to evaluate their potential pathophysiological relevance. In experiments with rat cortex homogenates we have shown that NO, at the concentrations previously shown to be present in the cortex upon TBI, substantially reduced 2-oxoglutarate dehydrogenase complex (OGDHC) activity, but did not affect mitochondrial respiration. Mitochondrial respiration was impaired by peroxynitrite (ONOO ? ) and tert -butylperoxide (TBP) in a manner similar to that observed in the in vivo model of TBI. Incubation of neuronal cell line B35 with an NO donor did not induce cell necrosis (estimated by measurement of lactate dehydrogenase (LDH) release to the supernatant). Cell viability was also largely unaffected by either thiamine (TH) or by succinyl phosphonate (SP), an activator and an inhibitor of OGDHC, respectively. In contrast, incubation of the cells with glutamate increased extracellular LDH concentrations, which was inhibited by TH, with the effect reversed by SP. A similar protective effect of TH was also observed in primary murine mesencephalic cultures exposed to glutamate. Glutamate drastically increased the rate of cell death and this effect was ameliorated by TH. Taken together, these data suggest that NO generated from NOS upon neuro-inflammation inhibits OGDHC, causing reduced glutamate uptake by the tricarboxylic acid cycle (TCA) cycle, followed by the intracellular accumulation of glutamate, culminating in glutamate toxicity and neuronal death. Cell injury can be prevented by TH. Thus, the impairment of OGDHC plays a key role in the glutamate mediated neurotoxicity in neurons during TBI; pharmacological activation of OGDHC may thus be of neuroprotective potential.