c-fos Expression in the rat brain after unilateral labyrinthectomy and its relation to the uncompensated and compensated stages.

The expression of the immediate early gene c-fos has been studied in the entire brain of rats 3, 6 and 24 h after surgical unilateral labyrinthectomy. We combined in situ hybridization for c-fos messenger RNA with immunocytochemistry for Fos protein to document very early changes in c-fos expression and to identify with cellular resolution neuronal populations activated by unilateral labyrinthectomy. Three hours after unilateral labyrinthectomy a bilateral increase in both c-fos messenger RNA and protein levels was seen in the superior, medial and spinal vestibular nuclei, nucleus Y, and prepositus hypoglossal nucleus. These changes were asymmetric in the medial vestibular nucleus, being most prominent in the dorsal part of the contralateral nucleus (where second order vestibular neurons are located) and in the ventral part of the ipsilateral nucleus (where commissural neurons acting on the medial vestibular nucleus of the intact side are located). An increase in c-fos messenger RNA expression was seen bilaterally, but with an ipsilateral predominance, in the vermal and paravermal areas of the cerebellar cortex, flocculus and paraflocculus, as well as in the precerebellar lateral and paramedian reticular nuclei. c-fos messenger RNA and protein levels increased in a few regions of the contralateral inferior olive. A predominantly ipsilateral increase in c-fos expression also occurred in the caudate-putamen. A bilateral but not exactly symmetric increase in both c-fos messenger RNA and protein levels was present in several nuclei of the dorsal pontine tegmentum (parabrachial nucleus, locus coeruleus and laterodorsal tegmental nucleus), mesencephalic periaqueductal gray, and several hypothalamic, thalamic and cerebrocortical regions. No change was seen in the cerebellar nuclei, lateral vestibular nucleus and red nucleus. The increased expression of c-fos observed 3 h after unilateral labyrinthectomy, in conjunction with the sudden occurrence of postural and motor deficits, usually declined 6-24 h after the lesion, i.e. during the development of vestibular compensation. In the dorsal part of the medial vestibular nucleus, however, the pattern of c-fos expression observed 3 h after unilateral labyrinthectomy was reversed 6-24 h after the lesion: both c-fos messenger RNA and protein levels increased on the ipsilateral side, but greatly decreased on the contralateral side. In conclusion, asymmetric changes in c-fos expression occurred within 3 h after unilateral labyrinthectomy, but gradually declined or reversed 6 and 24 h after the lesion, thus being temporally related to the appearance and development of vestibular compensation.