Zonal organization of the climbing fiber projection to the flocculus and nodulus of the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study.

The localization and termination of olivocerebellar fibers in the flocculus and nodulus of the rabbit were studied with anterograde axonal transport methods [wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) and tritiated leucine] and correlated with the compartments in the white matter of these lobules delineated with acetylcholinesterase histochemistry (Tan et al. J. Comp. Neurol., 1995, this issue). Olivocerebellar fibers originating from the caudal dorsal cap travel through floccular compartments FC2 and FC4 to terminate as climbing fibers in floccular zones FZII and FZIV. Fibers from the rostral dorsal cap and the ventrolateral outgrowth traverse compartments FC1 and FC3, which are interleaved with compartments FC2 and FC4, and terminate in zones FZI and FZIII. Fibers from the rostral pole of the medial accessory olive traverse the C2 compartment and terminate in the C2 zone. FZI-III extend into the adjoining folium (folium p) of the ventral paraflocculus. The C2 zone continues across folium p into other folia of the ventral paraflocculus and into the dorsal paraflocculus. Four compartments and five zones were distinguished in the nodulus. Medial compartment XC1 contains olivocerebellar fibers from the caudal dorsal cap and subnucleus beta that terminate in the XZI zone. Olivocerebellar fibers from the rostral dorsal cap and the ventrolateral outgrowth occupy XC2 and terminate in XZII. The XC4 compartment contains fibers from both the caudal dorsal cap and from the rostral dorsal cap and the ventrolateral outgrowth. The latter terminate in a central portion of the XZIV zone. The dorsomedial cell column projects to the XZIII zone, which is present only in the dorsal part of the nodulus. The rostral medial accessory olive projects to the XZV zone, which occupies the lateral border of the nodulus. These results confirm and extend the conclusions of Katayama and Nisimaru ([1988] Neurosci. Res. 5:424-438) on the zonal pattern in the olivo-nodular projection in the rabbit. Additional observations were made on the presence of a lateral A zone (Buisseret-Delmas [1988] Neurosci. Res. 5:475-493) in the hemisphere of lobules VI and VII. Retrograde labeling of the nucleo-olivary tract of Legendre and Courville ([1987] Neuroscience 21:877-891) was observed after WGA-HRP injections into the inferior olive including the rostral dorsal cap and the ventrolateral outgrowth. The anatomical and functional implications of these observations are discussed.