Résumé
It has been known that granule neurons of the dentate gyrus (DG) are born in adulthood as well as during development. Apoptotic cell death also occurs normally throughout the life of the rat brain. The present study was designed to determine the effect of transient global ischemia on the apoptosis and/or neurogenesis of granule cells in the dentate gyrus. TUNEL study revealed that the ischemia produced an significant increase in apoptosis mainly in the granular zone (GZ) of the DG. The percentage of TUNEL-positive cells in the DG was maximal (37.3+/-2.5%) 8 hr after ischemia and declined thereafter. However, immunocytochemical studies showed that there was an increase in neurogenesis mainly in the subgranular zone (SGZ) although the induction of neurogenesis took longer than the apoptosis. As a neurogenesis marker, proliferating cell nuclear antigen (PCNA)-positive cells, possibly progenitor cells, were significantly increased by 34.1+/-2.2%(n=3, p<0.05) mainly in the dentate SGZ 4 days after ischemia. In addition, the gradual increase in Bcl-2 expression was only paralleled with the neurogenesis in the SGZ, but not with the apoptosis in the GZ of the DG. The expression level of Bcl-2 in the SGZ was increased significantly (optical density 43.7+/-3.4; n = 3, p<0.05) 4 days after the ischemic insult. Furthermore, the ischemia-induced neurogenesis in the SGZ was also indirectly supported by the observation that the expression of synapsin-alpha was significantly increased (176%; n=3 p<0.05) in the CA3 region 4 days after the ischemia. Taken together, these results strongly suggest that the transient global ischemia induces the apoptosis in the GZ, whereas the cell proliferation in the SCZ of the DG. In situ hybridization using the antisense probes to the NR2A and NR2B subunits of NMDA receptors revealed that the ischemia produced a more profound effect on the mRNA expression of NR2A (61.9% reduction) than NR2B (20.5% reduction). Thus, we also suggest a possibility that ischemia could induce the neurogenesis in the SGZ of the DG through downregulation of the number of functional NMDA receptors.