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 t hat is largely responsible for maintenance of one's orientation in space, p osture, and balance and for visual fixation of objects during motion. In li ght of the considerable literature indicating an involvement of nitric oxid e (NO) in sensory systems, it is important to determine whether NO is assoc iated with vestibular pathways. To study the relationship of NO to vestibul ar pathways, we first examined the normal distribution of constitutive NADP H-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the ve stibular complex (VC) and then examined its association with selected vesti bular projection neurons. Survey of the four major vestibular nuclei reveal ed 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, al though the density of this network varied among the individual nuclei. To d etermine whether NADPH-d/NOS neurons project to vestibular efferent targets , injections of the retrograde tracer Fluoro-Gold were made into known targ ets of second-order vestibular newtons. Vestibular neurons containing const itutive NADPH-d/NOS were found to project predominantly to the oculomotor n ucleus. 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 locall y from axons located throughout the vestibular complex but may play a parti cularly important role in vestibuloocular pathways. J. Comp, Neurol. 425:97 -120, 2000. (C) 2000 Wiley-Liss, Inc.