Autoradiographic and electron microscopic degeneration evidence for axonal sprouting in the rat corticopontine system

Newborn (2 or 3 days postnatal) rats were subjected to unilateral cerebral cortical lesions involving much of the anterior (sensorimotor) cortical surface in the right hemisphere. Three months later, the survivors were divided into two groups, one group receiving a [3H]leucine injection in the left sensorimotor cortex and the other sustaining a lesion involving the left sensorimotor cortex. Routine autoradiographic studies in the first group revealed abnormally dense axonal and terminal labeling in the pontine gray contralateral to the leucine-injected hemisphere, suggesting that much of the label was due to sprouting from intact corticopontine axons into the neonatally deafferented pontine gray. In the group with a second (adult) cortical lesion contralateral to the neonatal ablation, degenerating axons and boutons were abundant in the pontine gray contralateral to the adult lesion and at least some of these were interpreted to represent sprouted corticopontine axons and their terminals.