DNA FRAGMENTATION AND HSP70 PROTEIN INDUCTION IN HIPPOCAMPUS AND CORTEX OCCURS IN SEPARATE NEURONS FOLLOWING PERMANENT MIDDLE CEREBRAL-ARTERY OCCLUSIONS

DNA nick end-labeling (TUNEL) and heat shock protein (HSP)70 immunocyt ochemistry were performed on the same brain sections 1 (n = 6), 3 (n = 12), and 7 (n = 7) days following permanent middle cerebral artery (M CA) occlusions produced in adult rats using the endovascular carotid s uture method. In the cortex at 1 and 3 days following MCA occlusions, HSP70 immunoreactive neurons were located outside areas of infarction and showed little evidence of DNA fragmentation. HSP70-stained cortica l neurons were intermingled with TUNEL cells near the infarct, but ext ended for greater distances away from the infarct. DNA fragmentation o ccurred in CA1 hippocampal neurons in 39% of the animals at 1 and 3 da ys following ipsilateral MCA occlusion. Bilateral DNA fragmentation oc curred in CA1 neurons in one subject. HSP70 protein was expressed in C A1 hippocampal neurons in nine of 18 (50%) animals at 1 and 3 days fol lowing MCA occlusions, including all animals that exhibited hippocampa l DNA fragmentation. Three animals had bilateral expression of HSP70 i n CA1 neurons. Cells that stained for either HSP70 protein or DNA frag mentation existed in close proximity to one another. Approximately 5-7 % of HSP70-stained cells were TUNEL stained and 3% of TUNEL-positive c ells also stained for HSP70. There was no HSP70 staining or DNA fragme ntation in the brains of sham-operated controls (n = 4) or in the brai ns of animals 7 days following MCA occlusions. These data suggest that ischemic cells capable of translating HSP70 protein generally do not undergo DNA fragmentation. These data support the concept that most HS P70 protein-containing neurons in the cortical ''penumbra'' and hippoc ampus survive ischemic injury and are ''reversibly injured.'' It is sh own that CA1 hippocampal pyramidal neurons die or are reversibly injur ed in similar to 50% of animals following permanent MCA occlusions. Al though the mechanism of this hippocampal injury is unknown, it could r elate to transynaptic activation of N-methyl-D-aspartate (NMDA) recept ors that mediate induction of early genes in hippocampus.