Topography of olivocerebellar projections to the uvula and paramedian lobule in the rabbit: comparison with other species.

The organization of projections from the inferior olive (IO) to the uvula of the vermis and the paramedian lobule (PML) of the same hemisphere in the rabbit cerebellar cortex was studied using retrograde tracers: Diamidino Yellow (DY) and Fast Blue (FB) dyes were injected unilaterally into these two targets in the same animals. The distribution patterns of DY- and FB-labeled neurons consistently detected in spatially separate regions of the contralateral IO subnuclei show that the IO-uvular and IO-PML projections display a distinct topography. The absence of double labeling indicates that no neurons project divergently to both the uvula and PML. The lateral and medial regions of the medial accessory olive and of the ventral lamina of the caudal principal olive project to the uvula and PML, respectively. Several neuronal groups at more rostral levels of the principal olive send fibers solely to the PML. The medial and lateral regions of the beta nucleus and the dorsomedial cell column supply the uvula and PML, respectively. Neurons in the dorsal cap and the ventrolateral outgrowth also tend to display this type of localization. Moreover, the caudal part of the dorsal accessory olive and the dorsal lamina of the principal olive project exclusively to the PML. The most caudal and rostral aspects of the IO do not participate in projection. Topographical relationships in the IO-uvular and IO-PML projections have not been previously demonstrated in the rabbit, and they are discussed in the context of inter-animal differences and cerebellar zonal organization. The possible functional significance of the connections is also discussed.