Role of the flocculus in the development of vestibular compensation: immunohistochemical studies with retrograde tracing and flocculectomy using Fos expression as a marker in the rat brainstem.

After unilateral labyrinthectomy in rats, Fos-like immunoreactive neurons appeared in the ipsilateral medial vestibular nucleus, contralateral prepositus hypoglossal nucleus and contralateral inferior olive beta subnucleus. and thereafter gradually disappeared in accordance with the development of vestibular compensation. This finding indicated that the activation of these nuclei is the initial event of vestibular compensation. In the present study, retrograde tracing experiments revealed that these Fos-like immunoreactive neurons project a proportion of their axons to the vestibulocerebellum (uvula-nodulus, flocculus). Before vestibular compensation was accomplished, right, left or bilateral flocculectomy was performed in right-labyrinthectomized rats. All these treatments caused reappearance of unilateral labyrinthectomy-induced behavioral deficits and Fos expression in the left medial vestibular nucleus and right prepositus hypoglossal nucleus. Since floccular efferents are GABAergic, these results indicate that the neurons in which Fos expression was detected by flocculectomy had been inhibited after unilateral labyrinthectomy by floccular Purkinje neurons and that disinhibition of these neurons induced by flocculectomy caused decompensation. Based on our present findings, we propose a hypothesis that the bilateral flocculus serves the restoration of balance between intervestibular nuclear activities to induce vestibular compensation after unilateral labyrinthectomy.