Protective effects of moderate hypothermia on behavioral deficits but not necrotic cavitation following cortical impact injury in the rat

A number of experimental studies have reported that moderate hypothermia can produce significant protection against behavioral deficits and/or morphopathological alterations following traumatic brain injury; a Phase 3 clinical trial is currently examining the therapeutic potential for moderate hypothermia (32 degrees C) to improve outcome following severe traumatic brain injury in humans. The current study examined whether hypothermia (32 degrees C) provided behavioral protection following experimental cortical impact injury. The extent of focal cortical contusion was also examined in the same rats. A total of 30 male Sprague-Dawley rats were trained on beam balance and beam walking tasks prior to injury. Under isoflurane anesthesia, cortical impact was produced on the right parietal cortex of 20 rats. Ten rats underwent all surgical procedures but were not impacted (sham-injured rats). Ten of the injured rats were cooled to 32 degrees C (measured in temporalis muscle) beginning 5 min postinjury, maintained for 2 h and rewarmed slowly for 1 h. In the other 10 injured rats, normothermic temperatures (37.5 degrees C) were maintained for the same duration. Beam balance and beam walking performance was assessed daily for 5 days following injury. At 11 days postinjury, rats were assessed for 5 days on acquisition of the Morris water maze task. Following behavioral assessments, rats were perfused and the brain removed. Coronal sections were cut through the site of cortical impact injury and stained with hematoxylin and eosin. Hypothermic treatment resulted in significantly less beam balance and beam walking deficits than observed in normothermic rats. Hypothermia also significantly attenuated spatial memory performance deficits. Quantitative morphometric analyses failed to detect any significant differences in volumes of necrotic tissue cavitation in cortices of hypothermic and normothermic rats. Hypothermic treatment also had no effect on volumes of dorsal hippocampal tissue or numbers of cells in CA1 or CA3 regions of the hippocampus. These data suggest that hypothermia, consistent with the reports of others, can produce significant behavioral protection following cortical impact injury that is not necessarily correlated with changes in focal cortical necrosis within the first 15 days following injury.