Temperature Modulation of Cerebral Depolarization during Focal Cerebral Ischemia in Rats: Correlation with Ischemic Injury

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Abstract
The role of cerebral depolarizations in focal cerebral ischemia is unknown. We therefore measured the direct current (DC) electrical activity in the cortex of Wistar rats subjected to transient occlusion of the middle cerebral artery (MCA). Focal ischemia was induced for 90 min by insertion of an intraluminal filament to occlude the MCA. To modulate cell damage, we subjected the rats to hypothermic (30°C, n = 4), normothermic (37°C, n = 4), and hyperthermic (40°C, n = 6) ischemia. Controlled temperatures were also maintained during 1 h of reperfusion. Continuous cortical DC potential changes were measured using two active Ag–AgCl electrodes placed in the cortical lesion. Animals were killed 1 week after ischemia. The brains were sectioned and stained with hematoxylin and eosin, for evaluation of neuronal damage, and calculation of infarct volume. All animals exhibited an initial depolarization within 30 min of ischemia, followed by a single depolarization event in hypothermic animals, and multiple periodic depolarization events in both normothermic and hyperthermic animals. Hyperthermic animals exhibited significantly more (p < 0.05) DC potential deflections (n = 6.17 ± 0.67) than normothermic animals (n = 2.75 ± 0.96). The ischemic infarct volume (% of hemisphere) was significantly different for the various groups; hypothermic animals exhibited no measurable infarct volume, while the ischemic infarct volume was 10.2 ± 12.3% in normothermic animals and 36.5 ± 3.4% in hyperthermic animals (p < 0.05). A significant correlation was detected between the volume of infarct and number of depolarization events ( r = 0.90, p < 0.001). Our data indicate that body temperature has a profound effect on the number of ischemic depolarization events, and ischemic cell damage after transient MCA occlusion, and suggest a role for ischemic depolarizations in mediating ischemic cell damage.