Olivocerebellar projections to paramedian lobule in tree shrew (Tupaia glis): a horseradish peroxidase study.

Horseradish peroxidase (HRP) was used as a retrograde tracer to identify the distribution pattern of labeled cells in the inferior olivary nucleus (IO) of Tupaia. Crystallized HRP was implanted into dorsal (DPML) and ventral (VPML) divisions of the paramedian lobule (PML) and, following appropriate survival times, the tissues were processed using diaminobenzidine and tetramethylbenzidine as chromogens. Subsequent to implants into lateral DPML and VPML, HRP-labeling is seen in rostral subgroup a of the medial accessory olive (MAO) and in the medial part of the dorsal accessory olive (DAOm) and ventral lamellae of the principal olive (VLPO) close to their rostral poles. The lateral bend and adjacent dorsal lamella of the principal olive and rostral subgroup c of MAO also contain HRP-reactive somata following lateral DPML implants. Subsequent to implants in central DPML, labeling is seen in rostral DAOm and subgroup a of MAO. Central VPML implants result in additional clusters of labeled cells in VLPO, the lateral bend of the principal olive (PO), and subgroup c of MAO. Following implants of HRP into medial PML reactive somata are found in dorsomedial VLPO and DLPO, and clusters of labeled cells are present in caudal subgroup a of MAO and DAOm. In contrast to implants in central and lateral PML, rostral DAOm and PO are devoid of reactive neurons. These results show that olivocerebellar projections to PML of Tupaia are exclusively contralateral and topographically organized. Collectively these olivocerebellar data corroborate the existence of zones C1, C2, C3 and D in PML of Tupaia and show that their patterns are similar, in their essential features, to those seen in the corticonuclear pathway in this species.