Evidence for a crossed nigrostriatal pathway in rats

In order to characterize further the origin of striatal afferents, adult rats underwent a unilateral intrastriatal infusion of 30% Evans Blue solution (0.2 - 0.5 mul). Labeled neurons were observed in ipsilateral substantia nigra, thalamus, cortex, ventromedial mesencephalic tegmentum and dorsal raphe. Several labeled neuronal somata, albeit considerably fewer than seen ipsilaterally, were found also in the contralateral substantia nigra and ventral tegmental area, a result contrary to previously reported findings in the rat. Control injections restricted to cortex overlying striatal target site did not result in similar labeling, and corpus callosum transection prior to intrastriatal injection did not prevent labeling of contralateral substantia nigra. These findings indicate that rats, like cats, have a sparse crossed nigrostriatal pathway.