Morphologic study of neuronal death, glial activation, and progenitor cell division in the hippocampus of rat models of epilepsy.

Purpose: To clarify the relationship of neuronal death to cellular responses, we studied neuronal death as well as reactions of glia and progenitor cells in the hippocampus of two rat models of epilepsy.

Methods: Seizures were induced by either kainic acid (KA) administration or electrical kindling. Neuronal degeneration was assessed by in situ DNA fragmentation analysis. Reactions of glial cells were studied by immunohistochemistry. Progenitor cell division was evaluated using the bromodeoxyuridine (BrdU) labeling method.

Results: DNA fragmentation and reactive microglia were observed in the CA1, CA3, and hilus region for 24 h to 4 weeks after KA injection, but not detected in the kindling model. Reactive astrocytes and enhancement of progenitor cell division were seen in both animal models. The number of BrdU-positive cells began to increase on day 3 after KA injection, peaked on day 5, and returned to baseline on day 10. After kindling, the number of BrdU-positive cells began to increase after five consecutive experience of stage I seizures.

Conclusions: These observations show that neuronal degeneration is not necessary for triggering the upregulation. Microglial activation is closely related to the neuronal death process induced by KA.