Ipsilateral interpositorubral projection in the kitten and its relation to post-hemicerebellectomy plasticity.

Hemicerebellectomy has been reported to induce aberrant ipsilateral projection from the nucleus interpositus (IN) of the remaining hemisphere to the red nucleus (RN) in the kitten but not in adult cats. The cellular mechanisms for this age-at-lesion effect were investigated. The experiments were designed to compare interpositorubral (IN-RN) projections in normal kittens and in kittens hemicerebellectomized at corresponding ages. A sparse ipsilateral IN-RN projection was found in normal kittens aged 7-16 postnatal days (PND) with the PHA-L method, but was not found in those aged 26-43 PND. Among the hemicerebellectomized kittens, ipsilateral IN-RN projections were observed only in those which received the lesion on PND 17 or earlier. Such temporal coincidence between the two events strongly suggests that the exuberant ipsilateral IN-RN projection in normal kittens is related to the establishment of the aberrant ipsilateral IN-RN projection observed after hemicerebellectomy. This notion is strengthened by other lines of evidence; the fibers projecting to the ipsilateral RNs appeared to enter the nucleus from the contralateral one by recrossing the midline between them in both the normal and the lesioned animals. In kittens in which hemicerebellectomy induced the aberrant ipsilateral projection, labeling of fibers within the ipsilateral RNs was much more extensive than that in the normal kittens, while labeling on the pathway of the projection was comparable to that in the normal animals. These observations support the interpretation for aberrant projections observed after a neonatal lesion as being terminal proliferations of pre-existing exuberant projections.