Glial reaction after seizure induced hippocampal lesion: immunohistochemical characterization of proliferating glial cells.

Kainic acid treatment (a model of temporal lobe epilepsy) induces Ammon's horn sclerosis, which is characterized by degeneration of CA3 pyramidal neurons and reactive gliosis. In the present study we have combined autoradiographic analysis of 3H-thymidine incorporation and immunocytochemistry to investigate this glial scarring phenomenon. The present results demonstrate that in the fields showing neuronal degeneration (i.e. CA3-CA4 fields of Ammon's horn and dentate hilus) the glial reaction consists of a proliferation and hypertrophy of astrocytes and microglia-macrophages. In the regions showing exclusively terminal axonal degeneration (i.e. the molecular layer of kainate-treated rats), glial cells do not proliferate but astrocytes show a transient hypertrophy. These results also demonstrate that oligodendrocytes do not proliferate in the hippocampus of kainate-treated rats. In agreement with our previous report we find that hippocampal astrocytes from kainate-treated rats express A2B5 immunoreactivity, a marker of type-2 astrocytes. A2B5 immunoreactivity was expressed by astrocytes not only in areas showing glial proliferation such as CA3-CA4 fields, but also in the molecular layer, where astrocytes do not proliferate. This suggests that in the CNS, normal resident astrocytes acquire the phenotypic properties of type-2 astrocytes.