ORGANIZATION OF VESTIBULAR NUCLEUS PROJECTIONS TO THE CAUDAL DORSAL CAP OF KOOY IN RABBITS

This study used retrograde and anterograde tracing methods to characterize the origin and terminal distribution of vestibular nuclear projections to the caudal dorsal cap of the inferior olive in albino rabbits. Comparisons of the retrograde labeling patterns from Cholera toxin B fragment-horseradish peroxidase and Fluoro-Gold injection sites centred in either the caudal dorsal cap or the rostral dorsal cap plus ventrolateral outgrowth revealed that they receive projections from different vestibular nuclear regions. Tracer injections in the rostral dorsal cap and ventrolateral outgrowth produced a sparse bilateral distribution of labeled neurons in the superior vestibular nucleus and an almost exclusively ipsilateral pattern of labeled neurons in caudal pars alpha of the lateral vestibular nucleus. Injections in the caudal dorsal cap, though, labeled neurons bilaterally in the rostral medial vestibular nucleus, predominantly ipsilaterally in pars beta of the lateral vestibular nucleus and almost exclusively ipsilaterally in caudal pars alpha of the lateral vestibular nucleus and the rostral aspect of the inferior vestibular nucleus. Vestibular nucleus injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin indicated (1) that a predominantly ipsilateral projection to the caudal dorsal cap originates bilaterally from the pars beta of the lateral vestibular nucleus and the rostroventral aspect of the rostral medial vestibular nucleus, (2) that the medial half of the caudal medial vestibular nucleus is the source of a predominantly contralateral projection to dorsal cap, (3) that the caudal aspect of nucleus prepositus hypoglossi contributes a predominantly ipsilateral projection to the medial accessory olive and (4) that the rostral aspect of inferior vestibular nucleus and the dorsal and lateral aspects of the caudal medial vestibular nucleus project to nucleus beta of the medial accessory olive. In addition, axons containing anterogradely transported PHA-L were observed bilaterally in the oculomotor and abducens nuclei from injection sites involving pars beta of the lateral vestibular nucleus. It is hypothesized that bilateral vestibulo-caudal dorsal cap pathways coordinate activity in the left and right flocculus and nodulus during horizontal head movements to facilitate the performance of conjugate vestibule-ocular and optokinetic reflexes.