ASTROGLIAL AND MICROGLIAL REACTIONS IN THE GERBIL HIPPOCAMPUS WITH INDUCED ISCHEMIC TOLERANCE

Preconditioning of the brain with sublethal ischemia protects against neuronal damage following subsequent longer periods of ischemia (ischemic tolerance). In this study, we investigated astroglial and microglial reactions in the hippocampus following ischemia in a gerbil model of ischemic tolerance. Two minutes of forebrain ischemia (preconditioning ischemia) or sham operation was followed by 3 min of ischemia (second ischemia)3 days later. The brains were perfusion-fixed after 4 h, 1 day, 2 days, and 7 days. Paraffin sections were used for the visualization of astrocytes by immunostaining against, glial fibrillary acidic protein (GFAP) and for the visualization of microglia by histochemical staining with isolectin-B4 from Griffonia simplicifolia. The preconditioning ischemia induced a moderate increase in astroglial and microglial staining. Two days after the second ischemia GFAP staining further increased in astrocytes in the hippocampus with ischemic tolerance. In the CA1 region of the hippocampus without ischemic tolerance, in contrast, microglial activation with increased staining and morphological changes was pronounced. After 7 days, neuronal destruction resulted in the CA1 region without tolerance, where hypertrophic reactive astroglia and reactive microglia with phagocytic transformation accumulated intensely. However, the ischemic preconditioning prevented the CA1 neuronal damage and the activation of microglia was subsiding after 7 days. Thus, activation of glial cells occurred in a graded fashion in response to different degrees of neuronal injury. Astroglial but not microglial activation may have implications in neuronal survival in ischemic tolerance. The findings also suggest that neuron-glial and glia-glial interactions are under strict control and play a critical role in neuronal survival and death after ischemia.