Microglial and astrocytic cell responses in the rat hippocampus after an intracerebroventricular kainic acid injection.

After central nervous system injury activated microglial cells and reactive astrocytes secrete neurotrophic factors which may provide an environment conducive to axonal sprouting. The present study has used a unilateral intracerebroventricular (ICV) injection of kainic acid (KA) to produce a lesion of the CA3 pyramidal neurons in the rat hippocampus. The time course of the microglial and astrocytic response was studied throughout a 3-month period using an antibody to proliferating cell nuclear antigen to identify proliferating cells as well as OX-42 and GFAP antibodies to identify the activated microglia and reactive astrocytes, respectively. There was no proliferation of reactive astrocytes whereas activated microglial cells continued to proliferate throughout the duration of the study. During the first month there were some activated microglial cells in the CA1 field and in the fascia dentata but this was short-lived in comparison to the persistence of activated microglia and reactive astrocytes in the CA3 field which were still present 3 months after the initial injection. The discussion attempts to correlate this ipsilateral microglial and astrocytic response with the bilateral mossy fiber axonal sprouting, which occurs in the dentate gyrus after a unilateral ICV injection of KA. The discussion concludes that the two events, the microglial and astrocytic response and the mossy fiber sprouting, are not directly related since the contralateral sprouting occurs in the absence of any astrocytic or microglial response.