Immunohistochemical visualization of heat shock protein-70 in the gerbil hippocampus following repeated brief cerebral ischemia.

Recent experiments have shown that neuronal damage following repeated cerebral ischemic insults is more extensive than the damage after a single equivalent period of ischemia. To clarify the mechanism of this cumulative neuronal damage after repeated ischemia, we visualized the localization of heat shock protein-70 (HSP70), a marker of neuronal stress, with immunohistochemistry using a monoclonal antibody. Mongolian gerbils were subjected to three 2-min forebrain ischemic insults spaced at 1-h intervals and to a single 6-min period of ischemia. The animals were killed 24 and 48 h after ischemia. Hippocampal CA1 pyramidal neurons, which are destined to die, showed no HSP70 staining 24 and 48 h after three 2-min ischemic insults, but showed a mild to moderate staining after 6 min of ischemia, suggesting more severe damage after repeated ischemia. CA3 neurons, which are resistant to ischemia, were intensely stained with HSP70 antibody following 6 min of ischemia but was stained only slightly after three 2-min ischemic insults, showing less stress after repeated ischemia. Thus, thresholds for cell damage are obviously different among different cell populations within the hippocampus; the different neuronal populations respond differently to single and repetitive ischemia. The result suggests that cumulative neuronal damage after repeated sublethal ischemic insults is produced by increased susceptibility to subsequent insults when ischemic stress reaches certain thresholds that are different among different neuronal populations.