Relevance of interstitial glutamate to selective vulnerability in focal cerebral ischemia

Glutamate concentrations in striatum and cortex were measured by means of in vivo cerebral microdialysis before and for 4 h after middle cerebral and ipsilateral common carotid artery occlusion in rats. The peak glutamate concentration reached 7.28 +/- 3.60 microM in dialysate from striatum and 5.64 +/- 2.24 microM in that from cortex. An index of exposure of each region to glutamate was calculated by integrating glutamate concentrations after occlusion. During ischemia the striatum was exposed to statistically higher cumulative concentrations of glutamate than the cortex (p < 0.01). The difference in vulnerability between striatum and cortex may arise from the additional time needed for the cortex to be exposed to cumulative threshold levels of glutamate.