ROLE OF THE FLOCCULUS IN THE DEVELOPMENT OF VESTIBULAR COMPENSATION -IMMUNOHISTOCHEMICAL STUDIES WITH RETROGRADE TRACING AND FLOCCULECTOMYUSING FOS EXPRESSION AS A MARKER IN THE RAT BRAIN-STEM

After unilateral labyrinthectomy in rats, Fos-like immunoreactive neur ons appeared in the ipsilateral medial vestibular nucleus, contralater al prepositus hypoglossal nucleus and contralateral inferior olive bet a subnucleus, and thereafter gradually disappeared in accordance with the development of vestibular compensation. This finding indicated tha t the activation of these nuclei is the initial event of vestibular co mpensation. Tn the present study, retrograde tracing experiments revea led that these Fos-like immunoreactive neurons project a proportion of their axons to the vestibulocerebellum (uvula-nodulus, flocculus). Be fore vestibular compensation was accomplished, right, left or bilatera l flocculectomy was performed in right-labyrinthectomized rats. All th ese treatments caused reappearance of unilateral labyrinthectomy-induc ed behavioral deficits and Fos expression in the left medial vestibula r nucleus and right prepositus hypoglossal nucleus. Since floccular ef ferents are GABAergic, these results indicate that the neurons in whic h Fos expression was detected by flocculectomy had been inhibited afte r unilateral labyrinthectomy by floccular Purkinje neurons and that di sinhibition of these neurons induced by flocculectomy caused decompens ation. Based on our present findings, we propose a hypothesis that the bilateral flocculus serves the restoration of balance between interve stibular nuclear activities to induce vestibular compensation after un ilateral labyrinthectomy. Copyright 1996 IBRO.