DISTRIBUTION OF CHOLINE-ACETYLTRANSFERASE IMMUNOREACTIVITY IN THE VESTIBULAR NUCLEI OF NORMAL AND UNILATERAL VESTIBULAR NEURECTOMIZED CATS

Post-lesion recovery of vestibular functions is a suitable model for s tudying adult central nervous system plasticity. The vestibular nuclei complex (VN) plays a major role in the recovery process and neurochem ical reorganizations have been described at this brainstem level. The cholinergic system should be involved because administration of cholin ergic agonists and antagonists modify the recovery time course. This s tudy was aimed at analysing the postlesion changes in choline acetyltr ansferase immunoreactivity (ChAT-Ir) in the VN of cats killed 1 week, 3 weeks or 1 year following unilateral vestibular neurectomy. ChAT-pos itive neurons and varicosities were immunohistochemically labelled and quantified (cell count and surface measurement, respectively) by mean s of an image analysing system. The spatial distribution of ChAT-Ir wi thin the VN of control cats showed darkly stained neurons and varicosi ties mainly located in the caudal parts of the medial (MVN) and inferi or (IVN) VN, the nucleus prepositus hypoglossi (PH) and, to a lesser e xtent, in the medial part of the superior vestibular nucleus (SVN). Le sion-induced changes consisted in a significant increase in both the n umber of ChAT-positive neurons (IVN, SVN) and the surface of ChAT-posi tive varicosities (IVN, SVN, PH). They were observed bilaterally in th e acute (1 year and 3 weeks) and compensated (1 year) cats for the SVN and PH, while they persisted only in the IVN on the lesioned side in the compensated cats. These findings demonstrate vestibular lesion-ind uced reorganization of the cholinergic system in the IVN, SVN and PH w hich could contribute to postural and oculomotor function recovery.