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 anatomic and physiologic studies suggest that one potential mediator of these effects may be light, direct vestibular nuclear projections to the nucleus tractus solitarius and the dorsal motor nucleus of the vagus nerve. This study presents evidence for relatively dense, direct projections from the vestibular nuclei to the parabrachial nucleus. Male albino rabbits received injections of Phaseolus vulgaris leucoagglutinin into the vestibular nuclei. The tracer was visualized immunocytochemically with standard techniques. Anterogradely labeled axons were traced bilaterally from the vestibular nuclei to the parabrachial nuclear complex, where they terminated around somata in the Kölliker-Fuse nucleus, external medial parabrachial nucleus, medial parabrachial nucleus, and lateral parabrachial nucleus. Less dense terminations were observed in the ventrolateral aspect of the medullary reticular formation, the subtrigeminal nucleus, lateral tegmental field, and nucleus ambiguus. These findings have several important implications. First, they suggest that vestibular input converges directly at brain stem levels with visceral sensory input in both nucleus of the solitary tract and the parabrachial complex. Second, they suggest that vestibular input influences 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 reticular 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 project to the hypothalamus and the insular and infralimbic prefrontal cortex, the parabrachial nucleus may serve as an important relay and integrative structure for the cognitive impairment and vegetative symptoms associated with motion sickness, vestibular dysfunction, and responses to altered gravitational environments.