Attenuated neurotransmitter release and spreading depression-like depolarizations after focal ischemia in mutant mice with disrupted type I nitric oxide synthase gene.

Nitric oxide (NO) plays a complex role in the pathophysiology of cerebral ischemia. In this study, mutant mice with disrupted type I (neuronal) NO synthase (nNOS) were compared with wild-type littermates after permanent focal ischemia. Cerebral blood flow in the central and peripheral zones of the ischemic distribution were measured with laser doppler flowmetry. Simultaneously, microdialysis electrodes were used to measure extracellular amino acid concentrations and DC potential in these same locations. Blood flow was reduced to <25 and 60% of baseline levels in the central and peripheral zones, respectively; there were no differences in nNOS mutants versus wild-type mice. Within the central ischemic zone, DC potentials rapidly shifted to -20 mV in all mice. In the ischemic periphery, spreading depression (SD)-like waves of depolarization were observed. SD-like events were significantly fewer in the nNOS mutant mice. Concurrent with these hemodynamic and electrophysiological perturbations, extracellular elevations in amino acids occurred after ischemia. There were no detectable differences between wild-type and mutant mice in the ischemic periphery. However, in the central zone of ischemia, elevations in glutamate and GABA were significantly lower in the nNOS mutants. Twenty-four hour infarct volumes in the nNOS mutant mice were significantly smaller than in their wild-type littermates. Overall, the number of SD-like depolarizations and the integrated efflux of glutamate were significantly correlated with infarct size. These results suggest that NO derived from the nNOS isoform contributes to tissue damage after focal ischemia by amplifying excitotoxic amino acid release in the core and deleterious waves of SD-like depolarizations in the periphery.