Over the past several years, our laboratory has investigated the importance of brain temperature on ischemic outcome. In models of global and focal ischemia, we have shown that while mild intraischemic and postischemic hypothermia is neuroprotective, mild intraischemic hyperthermia worsens ischemic outcome. Periods of hyperthermia are commonly seen in patients after stroke and cardiac arrest. The general hypothesis to be tested in the present proposal is that brief hyperthermic periods, even when occurring days after the ischemic insult, will have detrimental effects on ischemic outcome. Pilot data summarized in this Project indicate that histopathological outcome is severely aggravated by a hyperthermic period induced 24 hr following global or focal ischemia. In proposed studies, we will first demonstrate by quantitative histopathological analysis the detrimental consequences of delayed hyperthermia. To obtain functional correlates, neurobehavioral assessment of animals undergoing postischemic hyperthermia will also be assessed. Potential mechanisms underlying the detrimental consequences of delayed hyperthermia will next be investigated. To determine whether delayed postischemic hyperthermia is associated with intravascular abnormalities, we will determine whether postischemic hyperthermia leads to blood-brain barrier breakdown, leukocyte or platelet accumulation, or increased vasogenic edema. In addition, evidence for metabolism/blood flow uncoupling after delayed hyperthermia will be assessed by autoradiographic techniques. Postischemic hyperthermia may also worsen outcome through excitotoxic and radical-mediated processes. Thus, by using microdialysis technology, we will determine whether postischemic hyperthermia is associated with increased extracellular levels of excitatory amino acids and/or the secondary generation of hydroxyl radicals. The breakdown of cytoskeletal proteins may be associated with irreversible neuronal injury. Thus, the effects of delayed hyperthermia on spectrin breakdown products will be assessed by immunocytochemistry. Finally, postischemic hyperthermia may also aggravate ischemic outcome by leading to the expression of novel genes; we will use mRNA differential PCR technology to test whether postischemic gene expression is altered by delayed postisdiemic hyperthermia. These studies should provide novel information regarding the significance of delayed hyperthermia in the pathophysiology of brain injury.
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