gms | German Medical Science

Objective: NO has been related to both protective and detrimental processes in CNS. Our aim is to elucidate the role of NO in TBI pathophysiology, to reveal protective and detrimental levels.

Method: TBI was induced by FPI on Sprague Dawley rats (mTBI<2.5atm<sTBI). DETC-Fe spin trapping was used for detection of NO at 4h, 1d, 2d, 3d post-TBI. NO was quantified by EPR. Hippocampal and cortical nNOS and iNOS were analyzed by RT-PCR. G6PDH was used as internal control. Nitrotyrosine levels were assessed by immunofluorescence. Mitochondrial activity was measured in cortical and hippocampal homogenates by high resolution respirometry using either glutamate or pyruvate (Complex I substrates) or succinate (Complex II substrate).

Results: There was a strong difference in basal NO levels of brain sub-regions (cerebellum, ~6 times higher than hippocampus). At 4h post-TBI there was 2-fold increase in NO in ipsilateral side with a concomitant impairment in mitochondrial activity and increase in iNOS expression, but all returned to basal levels at day one. Mitochondrial dysfunction was rather pronounced on glutamate dependent respiration indicative of an inhibition of 2-oxoglutarate dehydrogenase complex (OGDHC). Exposure of sham cortex homogenates to NO solution (20µM) resulted similarly in the inhibition of respiration with glutamate, but not with pyruvate and succinate. In the contralateral cortex and hippocampus, NO was doubled upon severe TBI without a concomitant iNOS expression. Cortical nitrotyrosine levels increased at day one, despite NO peak at 4h post-TBI. Nitrotyrosination was not solely dependent on NO levels as it increased in ipsi- but not in contralateral cortex although NO levels of both sides were comparable at 4h. Cerebellar NO was only affected by severe TBI. NO levels in liver and heart showed a similar profile as seen in cortex.

Conclusions: TBI-induced NO changes in brain are site specific, severity dependent, differ in source and consequences. Our data suggest that TBI-induced NO leads to reversible impairment in mitochondrial function through inhibition of OGDHC resulting in glutamate accumulation in injured brain. Our results indicate a link between NO elevation and glutamate excitotoxicity in TBI. TBI-induced NO increase in liver and heart suggests a central role for NO in brain-body axis and has implications for organ transplantation from brain-dead donors.