Spatial coding property of dorsomedial cell column neurons in the vestibule-olivary pathway

Abstract

The inferior olivary complex is an important relay of vestibular information to the cerebellum. Experiments were conducted in adult Sprague-Dawley rats to characterize the spatial coding features of neurons in dorsomedial cell column (DMCC), an inferior olive subnucleus that is implicated to process otolithic signals. Connection between vestibular nucleus and DMCC was identified by combined retrograde tracing (Fluorogold, FG) and c-fos expression, which has been used as a means to document neurons functionally activated by natural otolithic stimulation. FG-labeled neurons were mainly found in the spinal vestibular nucleus (VN) while a smaller number was found in the medial VN and prepositus hypoglossal nucleus. Fos-positive DMCC neurons were observed with horizontal linear acceleration along the transverse direction but not the antero-posterior direction. Control animals, namely labyrinthectomized rats subjected to linear acceleration and normal rats that remained stationary, showed only a few sporadically scattered Fos-positive neurons. That FG-labeled VN neurons showed c-fos expression indicates direct projection of spatial signals from VN to DMCC. The directional responsiveness of otolith-related DMCC neurons was confirmed with in vivo extracellular recoding. The best response orientations of these neurons pointed only in the left-side-down quadrant or right-side-down quadrant, but not in the head-down quadrant and head-up-quadrant. Taken together, our results indicate that DMCC neurons receive otolithic inputs from the VN and encode translational head movements along the transverse direction on the horizontal plane.