Zonal organization of flocculo-vestibular connections in rats

Anatomical and electrophysiological evidence has contributed to the hypothe sis that microzones in the mammalian flocculus are organized to reflect con trol of eye movements in the planes of semicircular canals. Adult male Long -Evans rats received iontophoretic injections of FluoroGold and/or tetramet hylrhodamine dextran amine (10,000 molecular weight, "Fluoro-Ruby") into th e vestibular nuclei. The distribution of retrogradely labeled Purkinje cell s revealed that efferent projections from the dorsal surface of the floccul us and the ventral paraflocculus to the superior vestibular nucleus, rostra l medial vestibular nucleus, ventral lateral vestibular nucleus, and caudal aspect of the vestibular nuclear complex (caudal medial vestibular nucleus , inferior vestibular nucleus and nucleus prepositus hypoglossi) tended to correspond to previously identified climbing fiber zones [Ruigrok et al. (1 992) J. comp. Neurol. 316, 129-150] in a manner consistent with other mamma ls. However, vestibular nucleus projections from the ventral surface of the flocculus did not appear to respect climbing fiber zonal boundaries. Rathe r, climbing fiber zones each contained interdigitated groups of Purkinje ce lls that project to different vestibular nuclear regions. It is suggested that this pattern of flocculus efferent organization is a s pecialization for controlling the activity of primary and accessory extraoc ular muscle pairs to confine vestibule-ocular reflexes within semicircular canal planes when the "center of regard" is located at different eccentrici ties. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved .