Topographic specificity of aberrant cerebellorubral projections following neonatal hemicerebellectomy in the rat

Anterograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) was used to examine the topographic specificity of ascending cerebellar efferent projections in adult rats which were hemicerebellectomized at birth. The results were compared to similar cerebellar projections in unlesioned adults. HRP-WGA placement in the nucleus interpositus of control rats resulted in a dense projection of labeled fibers which decussated in the midbrain, caudal to the red nucleus. In the red nucleus, dense terminal labeling was confined to the magnocellular region, while retrogradely labeled rubrocerebellar neurons were present throughout both parvo- and magnocellular areas. Similar HRP-WGA placements in the nucleus lateralis gave rise to fewer labeled fibers which terminated in the parvocellular red nucleus. In addition to the cerebellorubral projection, other areas of terminal labeling included the mid-brain reticular formation, nucleus parafascicularis prerubralis, zona incerta, fields of Forel and ventral thalamus. In neonatally lesioned adults, aberrant cerebellorubral and cerebellothalamic projections were observed deflecting ipsilaterally at the decussation of the normal contralateral projection. Topographic specificity of the aberrant ipsilateral cerebellorubral projection mirrored that of the normal contralateral fibers. In addition, an ipsilateral projection from the cerebellum could be followed rostral to the red nucleus, to terminate in the ipsilateral ventral thalamus. Lesioned animals also demonstrated marked cell loss in the red nucleus contralateral to the hemicerebellectomy.