Neuroscience Letters, 22 (1981) 23-29 23 © Elsevier/North-Holland Scientific Publishers Ltd. EVIDENCE OF A COLLATERALIZED CLIMBING FIBER PROJECTION FROM THE INFERIOR OLIVE TO THE FLOCCULUS AND VESTIBULAR NUCLEI IN RABBITS CAREY D. BALABAN, YASUO KAWAGUCHI and EIJU WATANABE Department of Physiology, Faculty of Medicine, University c~/" Tokyo, 7-3-1, Hongo, Bunkyo-ku, 7"okvo (Japan) (Received November 6th, 1980; Accepted November 17th, 1980) In albino rabbits, horseradish peroxidase injections confined to vestibular nuclei retrogradely labeled neurons in the dorsal cap of the contralateral inferior olive. Mapping with stimulating electrodes revealed that stimulation of the lateral aspect of the medial vestibular nucleus and the medial aspect of the inferior vestibular nucleus evoked field potentials representing antidromic activation of contralateral dorsal cap neurons. These responses interfered with the antidromic response evoked from the contralateral flocculus and orthodromic responses evoked from the contralateral retina, suggesting that dorsal cap neurons which both receive retinal input and project to the flocculus send collaterals to vestibular nuclei. The dorsal cap (DC), a division of the medial accessory olive [ 19], is the source of a visual climbing fiber projection to the contralateral cerebellar flocculus (FL) [2, 14, 20]. FL Purkinje cells project, in turn, to relay cells in the vestibular nuclei which mediate vestibulo-ocular reflexes (VORs) [16, 24], to a caudal part of the lateral cerebellar nucleus and to nucleus prepositus hypoglossi [23]. In particular, DC-FL-VOR relay pathways have been implicated as a substrate for plastic modification of the VOR gain by altering the responsiveness of Purkinje cells to vestibular input [9], and have been used as a model system for investigating climbing fiber interactions with Purkinje cells. This study examined collateralization of DC-FL climbing fiber afferents. Anatomical and physiological investigations indicate that climbing fibers usually contribute collateral projections to both Purkinje cells in the cerebellar cortex and to their subcortical target neurons in deep cerebellar nuclei and the lateral vestibular nucleus [1, 6, 11, 17, 18]. Both anterograde [11, 18] and retrograde [4] tracing methods, though, have not revealed a projection from DC to regions of the vestibular nuclei (VN) that receive a projection from FL Purkinje cells. This article presents anatomical and physiological evidence for such a pathway in albino rabbits. Albino rabbits (2.7-3.3 kg body weight) were anesthetized with urethane (400 mg/kg) and e~-chloralose (30 mg/kg). Supplemental doses of urethane were