NEURONAL SURVIVAL IS ASSOCIATED WITH 72-KDA HEAT-SHOCK PROTEIN EXPRESSION AFTER TRANSIENT MIDDLE CEREBRAL-ARTERY OCCLUSION IN THE RAT

Induction of the 72-kDa heat shock protein expression is thought to protect neurons against the subsequent effects of ischemia. However, it is not clear whether the induction of 72-kDa heat shock protein expression by an ischemic event improves neuronal survival. To address this question, we outlined the temporal profile of neuronal induction and expression of the 72-kDa heat shock protein in a model of transient focal ischemia in the rat. Fifty two adult Wistar rats were subjected to middle cerebral artery occlusion of 2 h duration. At 0.5, 3, 6, 9, 12, 24, 48, 96 and 168 h after reopening the artery, coronal brain sections were analyzed using both immunohistochemical methods and hematoxylin and eosin staining to determine the topographic and cellular distribution of the 72-kDa heat shock protein, as well as the extent of neuronal damage. Immunoreactivity to the 72-kDa heat shock protein was not detected in neurons that were destined to become necrotic, and were located in the ischemic core of the brain lesions. However, 72-kDa heat shock protein expression was evident in morphologically intact neurons located in the peripheral zone. The earliest neuronal expression of 72-kDa heat shock protein was detected in animals in which the 2 h occlusion of the middle cerebral artery was followed by 6 h recirculation; the intensity of the 72-kDa heat shock protein immunoreactivity peaked at 48 h, and progressively disappeared 7 days after the ischemic reperfusion event. These studies suggest that (1) 72-kDa heat shock protein is not expressed in morphologically intact neurons destined to become necrotic after 2 h of focal ischemia; (2) the 72-kDa heat shock protein is expressed only in morphologically intact neurons located at the periphery of the ischemic territory where they may be subjected to only sublethal stress; these neurons preserved their integrity 7 days after the ischemic episode. These data support the hypothesis that the expression of 72-kDa heat shock protein in ischemic brain may confer ''protection'' to the neurons.