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

This study used retrograde and anterograde tracing methods to characte rize the origin and terminal distribution of vestibular nuclear projec tions 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 centr ed in either the caudal dorsal cap or the rostral dorsal cap plus vent rolateral outgrowth revealed that they receive projections from differ ent vestibular nuclear regions. Tracer injections in the rostral dorsa l cap and ventrolateral outgrowth produced a sparse bilateral distribu tion of labeled neurons in the superior vestibular nucleus and an almo st exclusively ipsilateral pattern of labeled neurons in caudal pars a lpha of the lateral vestibular nucleus. Injections in the caudal dorsa l cap, though, labeled neurons bilaterally in the rostral medial vesti bular nucleus, predominantly ipsilaterally in pars beta of the lateral vestibular nucleus and almost exclusively ipsilaterally in caudal par s alpha of the lateral vestibular nucleus and the rostral aspect of th e inferior vestibular nucleus. Vestibular nucleus injections of the an terograde tracer Phaseolus vulgaris leucoagglutinin indicated (1) that a predominantly ipsilateral projection to the caudal dorsal cap origi nates 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 contribute s a predominantly ipsilateral projection to the medial accessory olive and (4) that the rostral aspect of inferior vestibular nucleus and th e dorsal and lateral aspects of the caudal medial vestibular nucleus p roject to nucleus beta of the medial accessory olive. In addition, axo ns containing anterogradely transported PHA-L were observed bilaterall y 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 moveme nts to facilitate the performance of conjugate vestibule-ocular and op tokinetic reflexes.