A study of corticospinal remodelling using retrograde fluorescent tracers in rats.

The retrogradely transported fluorescent tracers Fast Blue (FB) and Diamidino Yellow (DY) were injected into the spinal cord of adult rats that sustained unilateral frontoparietal cortical lesions at birth. Analysis of the resulting cortical labeling pattern in comparison to comparably injected control animals demonstrated an increase of retrogradely-labeled neurons within the unablated cerebral hemisphere ipsilateral to spinal cord injections. These ipsilateral labeled cells corresponded to previous descriptions, based on anterograde tracing techniques, of anomalous uncrossed corticospinal tract (CST) fibers. Additional findings indicated that the ipsilateral CST fibers are not axonal collaterals of normal, crossed CST fibers. Fluorescent tracer injections into cervical and lumbar spinal cord levels demonstrated a distribution pattern of labeled cells within the ipsilateral cortex that was similar to the topographic pattern found contralateral to spinal cord injections in normal animals. These findings support previous anatomical and electrophysiological data indicating that the neonatal cortical lesion-induced increase of ipsilateral CST fibers may be functional.