The distribution of vestibular efferent neurons receiving innervation of secondary vestibular afferent nerves in rats.

OBJECTIVES/HYPOTHESIS To explore the innervation areas of the medial vestibular nucleus (MVN) afferent neurons onto vestibular efferent neurons in the brain stem of rats. STUDY DESIGN A morphology study in the central vestibular system. METHODS Two neuronal tracers were used. Lectin PHA-L Conjugates (PHA-L, Invitrogen L - 11270,) was injected into the MVN as an anterograde tracer, and 5% FluoSpheres carboxylate-modified microspheres (MFS, Molecular Probe F-8793) was injected into the contralateral peripheral vestibule using as a retrograde tracer. All animals were allowed to recover for 12 days to facilitate sufficient transportation of the tracers. Then brain stems were sliced coronally on a freezing microtome and observed under a fluorescence microscope and laser confocal microscopy. RESULTS Neurons in the MVN labeled with PHA-L exhibited green fluorescence, and their axons were distributed near the genu of the facial nerve (g7) and in the reticulation structure, as well as in the cerebellum or oculomotor-related nuclei. Neurons labeled with red fluorescence of MFS were mainly located dorsomedial and dorsolateral to g7 and in the caudal pontine reticular nucleus (PnC) bilaterally and presented different morphologies at different locations. The synaptic junctions would display color overlap (fluoresced yellow). Under three-dimensional reconstruction of the confocal laser microscopy, the synaptic junctions were visualized dorsomedial and dorsolateral to g7 bilaterally, predominantly ipsilateral to the MVN injection site. CONCLUSIONS Morphologic evidence of the distribution of vestibular efferent neurons synapsed by afferent nerves from MVN was demonstrated. These efferent neurons constitute short closed-loop circuits with neurons in the MVN.