VESTIBULAR NUCLEUS PROJECTIONS TO THE PARABRACHIAL NUCLEUS IN RABBITS- IMPLICATIONS FOR VESTIBULAR INFLUENCES ON THE AUTONOMIC NERVOUS-SYSTEM

Acute vestibular dysfunction and motion sickness are characterized by autonomic effects such as pallor, nausea, and vomiting. Previous anato mic and physiologic studies suggest that one potential mediator of the se effects may be Light, direct vestibular nuclear projections to the nucleus tractus solitarius and the dorsal motor nucleus of the vagus n erve. This study presents evidence for relatively dense, direct projec tions from the vestibular nuclei to the parabrachial nucleus. Male alb ino rabbits received injections of Phaseolus vulgaris leucoag-glutinin into the vestibular nuclei. The tracer was visualized immunocytochemi cally with standard techniques. Anterogradely labeled axons were trace d bilaterally from the vestibular nuclei to the parabrachial nuclear c omplex, where they terminated around somata in the Kolliker-Fuse nucle us, external medial parabrachial nucleus, medial parabrachial nucleus, and lateral parabrachial nucleus. Less dense terminations were observ ed in the ventrolateral aspect of the medullary reticular formation, t he subtrigeminal nucleus, lateral tegmental field, and nucleus ambiguu s. These findings have several important implications. First, they sug gest that vestibular input converges directly at brain stem levels wit h visceral sensory input: in both nucleus of the solitary tract and th e para-brachial complex. Second, they suggest that vestibular input: i nfluences brain stem autonomic outflow via two parallel pathways: (1) direct, light projections to the nucleus of the solitary tract, dorsal motor nucleus of the vagus nerve, and ventrolateral medullary reticul ar formation; and (2) denser projection to regions of the parabrachial nucleus that project to these brain stem regions. Finally, since the parabrachial nucleus regions that receive vestibular input also projec t to the hypothalamus and the insular and infralimbic prefrontal corte x, the parabrachial nucleus may serve as an important relay and integr ative structure for the cognitive impairment and vegetative symptoms a ssociated with motion sickness, vestibular dysfunction, and responses to altered gravitational environments.