Quantitative study of the coexpression of Fos and N-methyl-D aspartate (NMDA) receptor subunits in otolith-related vestibular nuclear neurons of rats

The expression of NMDA receptor subunits (NR1 and NR2A/B) was demonstrated immunocytochemically in otolith-related neurons within the vestibular nuclear complex and its subnuclei of conscious Sprague-Dawley adult rats. All experimental animals were subjected to constant velocity off-vertical axis rotation (OVAR). The rotating gravity vector during OVAR sequentially activates hair cells on all sectors of the utricular maculae; neurons so activated within the vestibular nuclei were denoted by the expression of Fos protein. Control animals, i.e., labyrinthectomized rats subjected to OVAR and normal rats that remained stationary, showed only a few sporadically scattered labeled neurons. In the brainstem of normal rats subjected to OVAR, a high density of Fos-immunoreactive (Fos-ir) neurons was found in the vestibular nuclear complex (namely, spinal vestibular nucleus, SpVe; medial vestibular nucleus, Mve; superior vestibular nucleus, SuVe) and subnuclei (namely, group x and group y), whereas a lower density was found in the lateral vestibular nucleus (LVe). A double-immunofluorescence study indicated that both NR1 and NR2A/B subunits were highly expressed in Fos-ir neurons within the vestibular nuclei. Fos/NR1 or Fos/NR2A/B double-labeled neurons constitute over three-quarters of the total number of Fos-ir neurons in SpVe, MVe, LVe, SuVe, and groups x and y. Our findings suggest that NMDA-type ionotropic glutamate receptors play a key role in the OVAR-induced neuronal activation of the vestibular nuclei, thus providing a morphological basis for further study of glutamatergic central otolith neurons and their involvement in sensorimotor regulation and autonomic functions of rats.