COMPENSATORY CLIMBING FIBER INNERVATION AFTER UNILATERAL PEDUNCULOTOMY IN THE NEWBORN RAT - ORIGIN AND TOPOGRAPHIC ORGANIZATION

In neonatal rats the unilateral transection of the cerebellar peduncles causes a fast and complete degeneration of the contralateral inferior olive. Axons from the remaining olive recross the cerebellar midline and partially innervate the deprived hemicortex. The topographic organization of this compensatory projection was analyzed with the axonal tracing method. The bulk of compensatory afferents originates from neurons in the ipsilateral medial accessory olive, especially from its medial region, where afferents from the principal olive and the dorsal accessory olive contribute to a much lesser degree. In case of incomplete neonatal pedunculotomy, neurons with a similar location in the ipsilateral intact olive still contribute to the innervation of the partially deprived hemicortex, alone with atrophic contralateral olive. Important information about the organization of the compensation was revealed. Although its specificity was not totally maintained, the mediolateral distribution of sprouted afferents in the cerebellum matched the caudorostral disposition of parent neurons in the olive, as in the case in normal olivocerebellar projection. Compensatory fibers recrossing the cerebellar midline spread throughout the whole extent of the deprived cortex and terminate solely in the molecular layer as typical climbing fibers. The latter were not homogeneously distributed, their density being markedly reduced according to a mediolateral gradient. Compensatory projection followed a sagittal striped pattern, as does the normal climbing fiber projection. If the cortex is divided broadly into vermal, intermediate and hemispheral regions, an apparent reciprocity seems to exist concerning the relative involvement of the various cortical subdivision in both hemicerebella. The immature olivocerebellar system is capable of anatomical plasticity, although to a limited extent. A certain degree of specificity is maintained during the process of sprouting resulting in a topographical arrangement of the transcommissural climbing fiber projection. Cues which guide the growth of olivocerebellar fibers during normal development could also direct the compensatory innervation.