Brain temperature alters hydroxyl radical production during cerebral ischemia/reperfusion in rats.

Selective neuronal cell death in the CA1 pyramidal cells of the hippocampus and neurons of the dorsolateral striatum as a consequence of brain ischemia/reperfusion (IR) can be ameliorated with brain hypothermia. Since postischemic injury is mediated partially by chemical production of reactive oxygen species (ROS), decreased ROS production may be one of the mechanisms responsible for cerebral protection by hypothermia. To determine if ischemic brain temperature alters ROS production, reversible IR was produced in rats by occlusion of both carotid arteries with hemorrhagic hypotension. After 15 min of ischemia, circulation was restored for 60 min. Brain temperature was maintained during ischemia at either 30, 36, or 39 degrees C and kept at 36-37 degrees C after reperfusion. Using cerebral microdialysis, we measured nonenzymatic hydroxylation of salicylate by HPLC with electrochemical detection in the hippocampus. CBF was also compared among the groups during IR. The results were that normothermic animals during reperfusion had persistently increased levels of the salicylate hydroxylation product, 2,3-dihydroxybenzoic acid (2,3-DHBA), reaching 251% of control at 60 min. This increase in 2,3-DHBA production was potentiated after 60 min of reperfusion (406% of control) with ischemic hyperthermia. In hypothermic ischemia, 2,3-DHBA production at 60 min was attenuated to 160% of control. CBF decreased to approximately 5% of baseline value during ischemia, but increased three- to four-fold relative to control in all three groups. Therefore, the effects of ischemic brain temperature on 2,3-DHBA production did not correlate with changes in CBF during IR. We conclude that brain-temperature-related changes in OH.production are readily detected in the rat and decreased ROS generation may contribute to cerebral protection afforded by hypothermia during brain ischemia.