Anterograde tracing of the rat olivocerebellar system with phaseolus vulgaris leucoagglutinin (PHA‐L). Demonstration of climbing fiber collateral innervation of the cerebellar nuclei

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Abstract
The olivocerebellar climbing fiber system was investigated in the rat with anterograde Phaseclus vulgaris leucoagglutinin (PHA‐L) tracing. The specific objective of the study was to find morphological evidence of climbing fiber collaterals innervating the cerebellar nuclei.Small iontophoretic injections of PHA‐L were placed in different parts of the inferior olivary complex, and labelled olivocerebellar fibers could be traced to their termination as climbing fibers in sagittal zones of the contralateral cerebellar cortex. Reaching the cerebellum via the restiform body, the labelled olivocerebellar axons entered the deep cerebellar white matter anterior to the cerebellar nuclei. Most of these thicker, nonterminal axons continued dorsally around the nuclei, but some ran through them. Bundles of fibers could be followed into the folial white matter toward their cortical zones of termination.Depending on which part of the olivary complex that was injected with PHA‐L, labelled axons were seen to converge on different regions of the cerebellar nuclei, where dense plexuses of thin varicose terminal fibers appeared. Quantitative estimates of the innervation ranged from 1.7 to 4.3 million boutons per mm3 in the fastigial (FN), interposed, and main parts of the lateral cerebellar (LCN) nuclei, whereas the parvicellular portion of LCN demonstrated 15–20 million varicosities per mm3. Frequently, thicker olivocerebellar axons, which seemed directed toward the cerebellar cortex, were seen to send a fine collateral branch toward these areas of nuclear innervation.As controls, PHA‐L was injected into the degenerated olivary complex of 3‐acetylpyridine‐treated rats. Neither cortical climbing fiber terminals nor nerve terminal plexuses in the nuclei appeared in these experiments. In cases with injection sites extending into the reticular formation, substantial mossy fiber labelling was present bilaterally in the cortex, but the cerebellar nuclei were devoid of labelled innervation or demonstrated only a few larger diameter fibers.The projection of the inferior olivary complex to the cerebellar nuclei was strictly topographically organized and agreed in principle with the organization described in the cat by Groenewegen et al. ('79). The caudal medial accessory olive (MAO) projected to FN, the rostral MAO to the posterior interposed nucleus (NIP), the rostral part of the dorsal accessory olive (DAO) to the anterior interposed nucleus (NIA), and the principal olive (PO) to LCN. Smaller injections into olivary subnuclei resulted in labelled innervation in restricted sectors of individual cerebellar nuclei, suggesting the existence of a more detailed topographical organization.

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