Novel crossed corticothalamic projections after neonatal cerebral hemispherectomy. A quantitative autoradiography study in cats.

This is a quantitative study of remodeling of the corticothalamic projections in cats with a left cerebral hemispherectomy performed neonatally or in adulthood. Kittens were lesioned at a mean postnatal age of 10 days and compared, as adults, to adult-lesioned cats of prolonged survival time. All cats received injections of [3H]leucine-proline in the right pericruciate cortex and were sacrificed 5 days later. Injection site and terminal field areas were reconstructed from autoradiography-processed tissue. For the 3 animal groups the label filled a similar extent of cortical areas 4 gamma and 3a. Computerized procedures were used to count labeled particles from multiple bilateral sites of ventral thalamic nuclei (VTN), intralaminar nuclei and midline/paramedial regions at 2 thalamic coronal planes. In control cats the only labeling in the hemithalamus contralateral to the injection was in the intralaminar nuclei and medial-most region of VTN but the counts were low. In contrast, the contralateral hemithalamus of neonatal-lesioned cats showed: a significant increase in terminal field density relative to control cats throughout the VTN, at both coronal planes, and with a distribution of labeling different to the intact side (suggesting heterotypic growth); an increase in counts of labeled particles in intralaminar nuclei which was significant for one sampling site; and an increase in counts for midline/paramedial regions which was significant for all sites at the caudal plane. Adult hemispherectomized cats were similar to intacts except for a tendency to increased VTN counts. These novel terminals were interpreted as collateral sprouting of corticofugal fibers from the intact motor cortex which crossed the midline to reinnervate the decorticate hemithalamus. Findings are discussed in the context of our reported behavioral and anatomical results.