Increase in proliferation and gliogenesis but decrease of early neurogenesis in the rat forebrain shortly after transient global ischemia

Abstract

Regarding regenerative strategies early post-ischemic therapeutic interventions might have a great impact on further pathophysiological cascades. To understand the early post-ischemic events we analyzed proliferation and neurogenesis as early as on day 3 after transient global ischemia in rats. Evaluations were performed not only in the dorsal hippocampus, where post-ischemic cell death develops selectively in the cornu ammonis, subfield 1 area, but also in distant areas like the ventricle wall and the striatum. Ischemia was induced by a transient two-vessel occlusion combined with hypotension. Animals received daily i.p. injections of 5-bromo-2-deoxyuridine until decapitation 1 or 3 days after ischemia. Immunohistochemistry was performed to detect 5-bromo-2-deoxyuridine and co-labeling with cell-specific markers. Three days after ischemia, proliferation significantly increased throughout the forebrain. Early neurogenesis, detected by doublecortin labeling, on the other hand, was restricted to the neurogenic zones of the dentate gyrus and the lateral ventricle. Global ischemia reduced the overall number of doublecortin-positive cells in the dentate gyrus, particularly in the upper blade of the dentate gyrus. However, the number of newly generated doublecortin- and 5-bromo-2-deoxyuridine double-labeled cells was unchanged. The vast majority of newly generated cells were microglia/macrophages, which invaded morphologically damaged as well as undamaged regions. Astroglial cells were activated all over the forebrain by the ischemic insult. They were co-localized almost completely with nestin in many areas, yet, sparsely proliferated after the insult. Interestingly, in locally defined zones we found nestin- and glial fibrillary acidic protein-signals clearly separated. In sham-operated animals, nestin could be detected in both neurogenic zones only without co-labeling with glial markers. In conclusion, during the first days after global ischemia, cell death of cornu ammonis, subfield 1-neurons was accompanied by a massive overall proliferation and activation of microglia/macrophages, a reduction of pre-ischemia existing doublecortin-positive precursors in the dentate gyrus and a re-expression of nestin in glial fibrillary acidic protein-positive astrocytes.