Two modes of corticospinal reinnervation occur close to spinal targets following unilateral lesion of the motor cortex in neonatal hamsters

Although it has been shown that unilateral neonatal cortical ablation induc es bilateral corticospinal projections, the explanation for the pathways re sponsible for this bilateral innervation remains controversial. We hypothes ized that such reinnervation may be supplied from newly formed fibers sprou ting at the level rostral to, or at, or caudal to the pyramidal decussation . In order to test our hypothesis, we examined the brain and spinal cord of young hamsters which had a unilateral ablation of the right motor cortex a t six days postnatally, and then received an injection of an anterograde ne uronal lectin tracer, Phaseolus vulgaris-leucoagglutinin, into the hindlimb area of the left motor cortex at 21 days postnatally. For the identificati on of motoneurons in the lumbar spinal cord, some of these animals also rec eived an injection of cholera toxin subunit B, a retrograde tracer, into th e gastrocnemius muscle. A quantitative analysis in the left gray matter of the lumbar spinal cord indicated that the lectin labeling was two to eight times higher in cortically ablated animals than in intact animals. Immunohi stochemical detection of the lectin revealed that innervation of the left s pinal cord occurred close to targets at lower levels in the spinal cord. Tw o modes of reinnervation (types I and II) by the intact corticospinal tract were recognized. The type I fibers consisted of recrossing axon collateral s sprouted from the intact dorsal funiculus near their targets, while the t ype II fibers were recrossing parent axons which entered the intact, right gray matter several levels rostral to their targets, and then changed direc tion toward the targets. The recrossing at lower spinal levels yielded a la rge number of ipsilaterally labeled axons and their terminals in the gray m atter of the denervated lumbar cord, with a distribution pattern similar to that seen on the intact side. The present results indicate that such ipsilateral innervation may play an important role in the sparing and recovery of function following neonatal h emicortical injury. (C) 1999 IBRO. Published by Elsevier Science Ltd.