Increased collateralization of the cerebellothalamic pathway following neonatal hemicerebellectomy

The postlesional reorganization of the cerebellothalamic cells was here studied using a multiple retrograde tracing technique. To this purpose, the cerebellothalamic cell population was investigated in 3 groups of adult rats: (a) cases hemicerebellectomized at birth; (b) cases hemicerebellectomized in adulthood; and (c) control unlesioned ones. In all of the groups the fluorescent tracers Diamidino Yellow and Fast Blue were injected bilaterally in the thalamus and the retrograde labeling obtained in the cerebellar nuclei was analyzed quantitatively. In the adult unlesioned rats the cerebellar cells projecting to the ipsilateral thalamus represented 3-4% of the cell population projecting to the contralateral thalamus. Furthermore, in agreement with previous results, it was demonstrated that the ipsilateral component was almost totally formed by axon collaterals of the main contralateral cerebellothalamic pathway. The organization of the bilateral cerebellothalamic pathway was unaffected by a hemicerebellectomy performed in adulthood. However, when the hemicerebellectomy had been performed at birth, the number of cerebellar cells that project to the ipsilateral thalamus was 3 times higher than in the controls. In these lesioned cases, as in the other ones, the ipsilateral projecting cell population was mainly represented by branched cerebellar cells which project bilaterally upon the thalamus. These results indicate that the bilaterality of the cerebellothalamic pathway is enhanced after an early hemicerebellectomy, and that this phenomenon is responsible for the reinnervation of the deafferented thalamus. Further, the present study shows that the increase in the bilaterality of the system is sustained by cerebellar cells which bifurcate bilaterally upon the thalamus, and therefore that the reinnervation of the deafferented thalamus is performed by axon collaterals of the contralateral spared cerebellothalamic pathway.