Inducible nitric oxide synthase gene expression in vascular cells after transient focal cerebral ischemia

Background and Purpose: We investigated whether inducible nitric oxide synthase (iNOS) is expressed after transient cerebral ischemia and, if so, we sought to define the temporal profile and cellular localization of the expression and the role of iNOS in the mechanism of ischemic brain injury. Methods: The middle cerebral artery in rats was occluded for 2 hours by an intraluminal filament. The occurrence of transient ischemia and reperfusion was confirmed by laser-Doppler flowmetry (n=5). iNOS message in the ischemic neocortex was determined by reverse-transcription polymerase chain reaction. iNOS enzymatic activity was assessed by citrulline assay. The cellular localization of iNOS expression was determined by immunohistochemistry. Results: iNOS mRNA was maximally expressed in postischemic brain at 12 hours and was not present at 4 days (n=3 per time point). iNOS mRNA was not observed in the contralateral cerebral cortex. iNOS enzymatic activity developed in the postischemic brain between 12 and 24 hours (P<.05) and subsided at 4 days (n=4 to 8 per time point). iNOS immunoreactivity in the ischemic region was restricted to the wall of capillaries and of larger blood vessels at 12 to 24 hours. In regions of early necrosis, inflammatory cells were iNOS positive. Treatment with the iNOS inhibitor aminoguanidine (n=5; 100 mg/kg IP, BID for 4 days), starting 6 hours after ischemia, reduced infarct size in neocortex by 36±7% in comparison with vehicle-treated controls (n=5) (P<.05). Conclusions: Transient focal ischemia leads to iNOS expression in postischemic brain. However, the spatial and temporal patterns of expression differ from those occurring in permanent ischemia: iNOS is induced earlier and predominantly in vascular cells rather than in neutrophils. Thus, the temporal profile and localization of postischemic iNOS expression depend on the nature of the ischemic insult. The finding that aminoguanidine reduces infarct size adds further support to the hypothesis that postischemic iNOS expression contributes to ischemic brain damage.