Changes in Fos Expression in the Rat Brainstem after Bilateral Labyrinthectomy

In order to elucidate the role of the vestibulocerebellar neural circuits during two-stage bilateral labyrinthectomy (BL) we examined Fos-like immunoreactive (-LIR) neurons as a marker of neural activation in the rat brainstem after BL and the projections of these neurons into the vestibulocerebellum using retrograde tracing and immunohistochemical techniques. Simultaneous BL did not show any Fos expression in the medial vestibular nucleus (MVe). However, with an interval of >6 h between the two stages of BL, Fos-LIR neurons were induced in the ipsilateral (ipsi-) MVe of the side operated on second, partially projecting into the ipsi-vestibulocerebellum. A previous study demonstrated that some of the unilateral labyrinthectomy-induced Fos-LIR neurons in the ipsi-MVe projected into the ipsi-vestibulocerebellum and inhibited the contra-MVe neurons, resulting in the restoration of the right-left balance. Taken together with our present data, it is suggested that some of the labyrinthectomy-induced Fos expression in the ipsi-MVe is representative of a switch-on signal for the vestibulo-cerebello-vestibular inhibitory neural circuits, which remain turned off after symmetrical lesions such as simultaneous BL but are turned on after severe asymmetrical lesions in order to restore the balance.