The normal distribution and projections of constitutive NADPH-d/NOS neurons in the brainstem vestibular complex of the rat.

The vestibular system is a highly conserved sensory system in vertebrates that is largely responsible for maintenance of one's orientation in space, posture, and balance and for visual fixation of objects during motion. In light of the considerable literature indicating an involvement of nitric oxide (NO) in sensory systems, it is important to determine whether NO is associated with vestibular pathways. To study the relationship of NO to vestibular pathways, we first examined the normal distribution of constitutive NADPH-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the vestibular complex (VC) and then examined its association with selected vestibular projection neurons. Survey of the four major vestibular nuclei revealed that only the medial vestibular nucleus contained significant numbers of perikarya stained for NADPH-d/NOS. By contrast, all the vestibular nuclei contained a network of fine processes that stained positive for NADPH-d, although the density of this network varied among the individual nuclei. To determine whether NADPH-d/NOS neurons project to vestibular efferent targets, injections of the retrograde tracer Fluoro-Gold were made into known targets of second-order vestibular neurons. Vestibular neurons containing constitutive NADPH-d/NOS were found to project predominantly to the oculomotor nucleus. A small number of neurons also participate in vestibulothalamic and intrinsic vestibular connections. These results indicate that NADPH-d/NOS neurons are prevalent in the MVN and that a subpopulation of these neurons project to the oculomotor complex. Nitric oxide is probably released locally from axons located throughout the vestibular complex but may play a particularly important role in vestibulo-ocular pathways.