Neurogenesis and apoptosis in hippocampus following global ischemia

Using in situ hybridization, Northern blotting and RT-PCR we found that bcl-2, bcl-x and ICE mRNAs are induced in gerbil hippocampus 24-72 h following 5 or 10 min of global ischemia. bcl-2 and bcl-x mRNAs were induced in CA1 hippocampal pyramidal neurons at 24 h following ischemia and disappeared by 72 h when CA1 pyramidal neurons were TUNEL positive. ICE-like mRNA was induced at 72 h following ischemia in microglial cells. Other studies have shown that caspase 3 is induced in CA1 pyramidal neurons. These results show that different caspases are induced in different cells following ischemic injury, suggesting different cellular and functional specificity of the caspases. Though classic apoptosis may not occur in ischemic CA1 neurons, these neurons do die slowly and express apoptosis-associated genes that may play a role in the demise of the cells. To determine whether ischemia caused CA1 neurons to re-enter the cell cycle, and to determine whether ischemia affects neurogenesis, ischemic gerbils were administered BrdU and immunocytochemistry performed on the brain. The results show no BrdU uptake into CA1 neurons. However, there was a 12-fold increase in cell birth in the dentate subgranular zone 12 weeks following 10 min bilateral common carotid artery occlusions. Confocal microscopy demonstrated that BrdU immunoreactive cells in the granule cell layer co-localized with neuron-specific markers for neuronal nuclear anti-gen NeuN, microtubule-associated protein-a and calbindin D-28k indicating that the newly divided cells migrated from the subgranular zone into the granule cell layer and matured into neurons. Newborn cells with a neuronal phenotype were first seen 26 days after ischemia, survived for at least 7 months, were located only in the granule cell layer, and comprised approximately 60% of BrdU-labeled cells in the granule cell layer six weeks after ischemia. Ischemic preconditioning for two minutes, which protects CA1 neurons against subsequent ischemic damage, was also associated with increased neurogenesis in the,granule cell layer. Thus, ischemia-induced dentate neurogenesis is not due to CAI neuronal loss. Enhanced neuro,neurogenesis in the dentate gyrus may be a compensatory, adaptive response to ischemia-associated injury and could promote functional recovery following ischemic hippocampal injury.