EVIDENCE FOR ACTIVATION OF ASTROCYTES VIA REACTIVE MICROGLIAL CELLS FOLLOWING HYPOGLOSSAL NERVE TRANSECTION

Following peripheral nerve injury, resident microglial cells prolifera te and astrocytes undergo hypertrophy, as evidenced, e.g., by an incre ase in the levels of glial fibrillary acidic protein (GFAP). In a prev ious study we have shown that infusion of cytosine arabinoside (ARA-C) into the rat brain blocks the axotomy-induced proliferation of microg lial cells. This experimental approach has been used in the present st udy in order to explore the issue of whether the reactive microglial c ells are mediators of the increased GFAP expression in the hypoglossal nucleus of the rat following axotomy. Quantitative analysis of sectio ns processed for immunocytochemistry or in situ hybridization demonstr ated a marked increase in GFAP-like immunoreactivity and GFAP-mRNA, re spectively, in the ipsilateral hypoglossal nucleus 4 and 7 days after axotomy in control experiments. These increases failed to occur in axo tomized animals treated with ARA-C. Therefore, our data are compatible with the hypothesis that activation of astrocytes following axotomy a s measured by increased expression of GFAP and its mRNA is induced sec ondarily to the microglial response.