Background: Transient ischemia (TI) in whole brain brings selective neuronal death in vulnerable areas of the brain: the striatum, neocortex and hippocampus selectively loose specific neurons after TI. Just 5 min of TI causes pyramidal neuronal death in the hippocampal CA1 field (CA1) from 4 days after TI. In this study, we examined the impact of severity of TI on neuronal death and gliosis (astrocytosis and microgliosis) in subregions (CA1-3 fields and dentate gyrus) of the gerbil hippocampal formation after 5 min (mild), 15 min (severe) and 20 min (lethal) TI by bilateral carotid artery occlusion (BCCAO). 
Methods: We used cresyl violet staining for cellular distribution, neuronal nuclei immunohistochemistry for neuronal distribution and Fluoro-Jade B histofluorescence for neuronal death. In addition, we used glial fibrillary acidic protein (GFAP) and ionized calcium-binding adapter molecule 1 (Iba1) immunohistochemistry for astrocytosis and microgliosis, respectively. 
Results: First of all, we found that animals with 20 min BCCAO died in at least 2 days after 20 min BCCAO. This study showed that 15 min BCCAO evoked pyramidal cell death in the CA1-3 from 2 days after TI and granule cell death in the dentate gyrus at 5 days after TI: this was not found in animals with 5 min BCCAO. In addition, gliosis was much more rapidly and severely progressed in the animals with 15 min BCCAO than that in the animals with 5 min BCCAO. 
Conclusion: Our results indicate that neuronal loss in the hippocampal formation following TI was significantly different according to subfields and severity of TI.

5 - Time-course Pattern of Neuronal Death and Gliosis in Gerbil Hippocampi induced by Mild, Severe and Lethal Transient Global Cerebral Ischemia