Developmental expression of metabotropic glutamate receptor 1α in rat vestibular nuclear neurons responding to linear accelerations

Abstract

To investigate the developmental expression of metabotropic glutamate receptor 1α (mGluR1α) in functionally activated otolith-related neurons within the vestibular nucleus, double-immunohistochemistry experiments were performed in conscious Sprague-Dawley rats (P4 to adult) subjected to sinusoidal linear acceleration along the vertical, horizontal transverse or horizontal antero-posterior plane. Neuronal activation was defined by the expression of c-fos in the vestibular nucleus. Labyrinthectomized and/or stationary controls showed only a few sporadically scattered Fos-labeled neurons, confirming the otolithic origin of expression in test animals. In response to linear acceleration in 3-D space, Fos/mGluR1α double-labeled neurons in adult rats were observed throughout the medial vestibular nucleus, spinal vestibular nucleus, the ventral part of the lateral vestibular nucleus, group x, and group y. The superior vestibular nucleus, however, was the only region within the vestibular nuclear complex where neurons were sensitive only to horizontal linear accelerations, i.e. 2-D spatial orientations. In P4-6 rats, no Fos-labeled neurons were found in the vestibular nuclei where immunoreactivity to mGluR1α was virtually absent. Fos/mGluR1α double-labeled neurons responsive to transverse or vertical stimulation were first observed at P7 while those to antero-posterior stimulation were first identified at P9. From P9 onwards, the number of Fos/mGluR1α double-labeled neurons responsive to stimulation along the horizontal or vertical axis increased with age and reached adult levels at P21. Our results suggest that mGluR1α in central otolith neurons might contribute to the developmental recognition of gravity-related spatial orientations.