Spreading depression induces tolerance of cortical neurons to ischemia in rat brain

Cortical spreading depression (CSD) was induced in male Wistar rats by applying 2 M KCl to the frontal cortex of one hemisphere for 2 h. Saline was applied to the contralateral cortex in the same manner. Following recovery for 24 h, bilateral forebrain ischemia was induced for 6 min, and the animals were permitted to survive for 6 days for assessment of histopathology. The number of necrotic neurons was counted in the cerebral cortex, striatum, and hippocampus of both hemispheres. In separate sets of animals, the effects of KCl application on cortical direct current (DC) potential and regional expression of c-fos mRNA and 72-kDa heat shock protein (hsp72) mRNA were determined. Forebrain ischemia induced selective neuronal necrosis in both hemispheres, but the number of necrotic neurons in the cerebral cortex ipsilateral to the application of KCl was significantly smaller than that in the contralateral cortex (p < 0.02, Wilcoxon signed rank test, n = 7). In the striatum and hippocampus, there were no significant differences in neuronal necrosis between hemispheres. Application of KCl for 2 h induced 11 +/- 2 (mean +/- SD, n = 5) negative deflections of DC potential in the ipsilateral cortex; none were detected in the contralateral cortex. Widespread expression of c-fos mRNA was evident in the ipsilateral cortex, while hsp72 mRNA expression was restricted to the KCl application site. The present results demonstrate that CSD induces tolerance of cortical neurons to ischemia by mechanisms unrelated to hsp72.