Postnatal development of the rat vestibulo-olivary pathway in the horizontal otolith system

Abstract

To examine the developmental profile of functionally activated otolith neurons in the vestibulo-olivary pathway, conscious Sprague-Dawley rats (P6 to adult) were subjected to sinusoidal linear acceleration along specific directions on the horizontal plane. Neuronal activation was defined by the expression of Fos protein and c-fos mRNA. Only neural areas that exhibited c-fos expression in experimental animals but not in the two control groups, viz. labyrinthectomized rats subjected to linear accelerations and normal rats that remained stationary, were analyzed. With interaural stimulation, neurons with c-fos expression were first observed in the dorsomedial cell column (DMCC), a subnucleus of the inferior olive, of P11 rats. The number of Fos-positive neurons in the DMCC increased significantly with age and reached adult level at P21. However, no labeling was evident in the DMCC of all age groups with stimulation along the antero-posterior axis. This finding indicates that DMCC neurons only receive otolith information about head movements along the interaural direction. Retrograde transport of Fluorogold identified vestibular-related areas that project to DMCC; these included medial and spinal vestibular nuclei, and prepositus hyopoglossal nucleus. Neurons in these areas showed c-fos expression with interaural stimulations at P7, i.e. 2 days earlier than those in DMCC. Our results suggest that during postnatal development, otolith neurons in the vestibulo-olivary pathway gradually achieve the capacity to code horizontal head movements. Supported by HK RGC grant