The orthostatic hypotension in response to the assumption of an upright posture is regulated by activation of sympathetic nerves. Role of the vestibular system and neural pathway on orthostatic hypotension were investigated.

Changes of arterial blood pressure produced by head-up tilting, rotatory stimulation of the vestibular system, or electrical stimulation to the vestibular nerve, vestibular nuclei, and rostral ventrolateral medulla (RVLM) were measured in Sprague-Dawley rats. Also, field potentials were recorded in the vestibular nuclei and RVLM and c-Fos expression was evaluated in the brain stem in order to investigate the vestibulosympathetic pathways.

The three phasic blood pressure responses were elicited by head-up tilting: initial fall, early recovery, and late sustained pressure at near control levels, the magnitude of the pressure fall was parallel with the degree of head-up tilting in normal rats. Return position from head-up tilting recovered control level of blood pressure after a brief rapid elevation. However, bilateral labyrinthectomy resulted in exaggerated initial falling and devoid of early recovery phase during postural change. Sinusoidal rotation about off-vertical axis of the vestibular system elicited more elevation of blood pressure than rotation about earth vertical axis. Electrical stimulation of the vestibular nerve, vestibular nucleus, and RVLM produced elevation of blood pressure, which was the most prominent by stimulation of RVLM. Field potentials composed of P, N1, N2 waves in the vestibular nuclei were recorded by stimulation of the vestibular nerve, while weak potentials in RVLM were recorded by stimulation of the vestibular nuclei. An electrical stimulation of the vestibular nuclei expressed c-Fos immunoreactive cells in RVLM.

These results suggest that the otolith organ of the vestibular system plays a major role in control of orthostatic hypotension, and the pathway of vestibulosympathetic reflex in control of blood pressure involves the vestibular nuclei, RVLM, intermed-iolateral nuclei of the thoracic spinal cord.