Fluorometric assay of nitrite and nitrate in brain tissue after traumatic brain injury and cerebral ischemia

Nitric oxide synthase (NOS) is distributed within the brain, and nitric oxide (NO) is felt to be involved in the pathophysiology of deterioration after head injury and cerebral ischemia. This study determined the levels of the stable end products of NOS (NO x =nitrite+nitrate) after traumatic brain injury (TBI) and transient cerebral ischemia. A fluorometric assay using nitrate reductase and the NADPH regenerating system was used to quantitate NO x in ultrafiltered (10-kDa cutoff) cortical and hippocampal extracts after reduction of nitrate. In TBI rats, both the plasma and tissue showed a sharp increase in NO x levels 5 min after injury. Plasma NO x returned to control levels by 2 h after injury. Ipsilateral-cortex NO x levels returned to control levels ∼6 h after injury and remained constant from 6–24 h. Contralateral-cortex returned near to control levels after 1 h. Hippocampus also followed a similar trend. In gerbils, there was a significant elevation in tissue NO x levels immediately after 10 min transient cerebral ischemia, which gradually returned to control levels over 24 h reperfusion. This striking burst of NO synthesis immediately after injury is clearly evident whether the injury is head trauma or ischemia, or whether the measurements were performed on tissue or plasma. It is unknown whether endothelial NOS, neuronal NOS, or both caused the elevation of the NO end products seen after the CNS insults.