Distribution of inferior olivary projections to the vestibular nuclei of albino rabbits

Abstract

This study analyses the course and topography of olivo-vestibular projections originating in the dorsal cap, ventrolateral outgrowth and beta nucleus of albino rabbits. Rabbits were given either single pressure-injections of [ 3H] l-leucine (20 μCi in 50 nl) or single or multiple injections of 3-acetylpyridine (0.2–0.25 μ1 of 27.5μg/μ1 in saline) into the medial aspect of the inferior olive. Brains from the former animals were processed for autoradiography after 2–3 days survival; brains from the latter animals were stained for degeneration with cupric-silver methods after a 16–24 h survival. In addition, four rabbits with kainic acid lesions of the flocculus were used to document flocculo-vestibular projections. Olivo-vestibular projections from the dorsal cap-ventrolateral outgrowth, beta nucleus and the medial accessory olive diverge from olivo-cerebellar projections at the caudal margin of the flocculus stalk, and course medially in a broad sheet. Fibers (1) ascend in the superior fascicle, with flocculo-vestibular projections, to the superior vestibular nucleus, (2) enter the medial fascicle, with flocculo-vestibular fibers, and course along the dorsolateral border of the 4th ventricle to innervate a distinct rostral subdivision of the medial vestibular nucleus, and (3) enter the lateral fascicle, with flocculo-vestibular fibers, to terminate in pars α and β of the lateral vestibular nucleus and the caudal subdivision of the medial vestibular nucleus. Comparison of different injection cases indicate that the caudal half to two-thirds of the dorsal cap contributes projections to the rostral medial vestibular nucleus, centrolateral and dorsomedial aspects of the superior vestibular nucleus, and a projection to both central and dorsal aspects of the caudal medial vestibular nucleus. By contrast, the rostral third to half of the dorsal cap-ventrolateral outgrowth projects sparsely to the rostral medial vestibular nucleus, contributing dense projections to the central aspect of the superior vestibular nucleus and dorsomedial and lateral regions in the caudal medial vestibular nucleus. Pars β of the lateral vestibular nucleus received densest projections from the intermediate third of the dorsal cap; the projection to pars α did not display a distinct topography. These data and results of previous retrograde tracing experiments [C.D. Balaban (1984) Neuroscience 12, 129–149] suggest that distinct groups of dorsal cap-ventrolateral outgrowth neurons project differentially to the vestibular nuclei in rabbits. Comparisons with previous electrophysiologic studies raise the testable hypothesis that they demarcate distinct pools of vestibular nuclear output neurons.