Coding of gravity-related 3D spatial orientations in brainstem nuclei of adult rats

Abstract

To chart the distribution of neurons encoding gravity-related 3-D spatial orientations within the vestibular and prepositus hypoglossal nuclei, conscious adult Sprague-Dawley rats were subjected to sinusoidal linear acceleration along the vertical, horizontal transverse or horizontal antero-posterior plane. Neuronal activation was identified by immuno-/hybridization histochemistry for Fos or c-fos mRNA expression. Labyrinthectomized and/or stationary controls showed only sporadic Fos-labeled neurons in individual brainstem nuclei, confirming the otolithic origin of c-fos expression in test animals. In response to linear acceleration in 3-D space, Fos-labeled neurons were observed throughout the rostro-caudal and medio-lateral lengths of the medial vestibular nucleus (MV), spinal vestibular nucleus (SpV), the ventral part of the lateral vestibular nucleus, group x, and group y. Dense subpopulations were found in the middle/caudal half of SpV, centro-lateral part of MV, and middle/caudal half of group y. Some of these areas were innervated by otolith afferent terminals. The superior vestibular nucleus was the only region within the vestibular nuclear complex where neurons were sensitive only to horizontal linear accelerations, i.e. 2-D spatial orientations. Similar to the vestibular nuclear complex, neurons within the prepositus hypoglossal nucleus were sensitive to linear acceleration in 3-D space and these were located mostly in the caudal half. These results suggest that there is regional segregation in the recognition of gravity-related 2-D and 3-D space within the vestibular nuclear complex. Supported by HKRGC