Anatomical and functional evidence for lesion‐specific sprouting of corticostriatal input in the adult rat

Previous studies in our laboratory have shown that cortical lesions induced by thermocoagulation of pial blood vessels, but not by acute aspiration, result in 1) the preservation of control levels of the growth‐associated protein (GAP)‐43 and 2) a prolonged increase in neurotransmitter gene expression in the denervated dorsolateral striatum. We have examined whether corticostriatal projections from the spared homotypic contralateral cortex contribute to these effects. Adult rats received either a thermocoagulatory or aspiration lesion of the cerebral cortex and, after 30 days, received an injection of the anterograde tracer, Fluoro‐Ruby, in the contralateral homotypic cortex. Rats were killed 7 days later, and labeled fibers were examined with fluorescence microscopy in the ipsilateral and contralateral striata. Ipsilateral corticostriatal projections were detected in lesioned and unlesioned rats. Numerous labeled fibers were detected in the contralateral striatum of thermocoagulatory‐lesioned but not aspiration‐lesioned or control animals, suggesting that contralateral cortical neurons may undergo axonal sprouting in the denervated striatum following a thermocoagulatory lesion of the cortex. To determine whether contralateral corticostriatal fibers play a role in the changes in striatal gene expression induced by the thermocoagulatory lesions, the effects of aspiration lesions, as well as unilateral and bilateral thermocoagulatory lesions of the cortex were compared. Confirming previous results, striatal enkephalin mRNA levels were increased after a unilateral thermocoagulatory lesion. However, they were unchanged after aspiration or bilateral thermocoagulatory lesions, suggesting that sprouting or overactivity of contralateral corticostriatal input contributes to the increase seen after unilateral thermocoagulatory lesions. © 1996 Wiley‐Liss, Inc.