Otolithic influence on central cardiovascular control following natural vestibular stimulation in conscious rats

Abstract

To investigate the effect of prolonged otolith activation on the cardiovascular system, constant velocity off-vertical axis rotations (OVAR), which selectively activate the otolith organs, were delivered to conscious Sprague Dawley rats for continuous periods of 3 hours. Mean blood pressure (MBP), heart rate (HR) and phenylephrine-induced baroreflex response were evaluated before and after the natural stimulation. In response to sustained OVAR, the basal HR was significantly lowered. Such reduction in HR, though not observed in stationary control, was significantly lower than that in labyrinthectomized control. However, no significant difference in basal MBP and baroreflex response was observed in all 3 groups. With the use of Fos immunohistochemistry and anterograde tracing (PHA-L) methods, functional connection between the vestibular nucleus and cardiovascular-related brainstem area was also examined. The projections of PHA-L axonal terminals from the vestibular nucleus were found in loci within the lateral parabrachial nucleus, Koelliker-Fuse nucleus, gigantocellular reticular formation, and medial aspect of rostroventral lateral medulla where Fos-positive neurons were located. In the control groups, neurons within these brainstem regions, which are known to actively participate in cardiovascular homeostasis, did not express Fos in the cell nuclei. Our findings suggest the involvement of these brainstem areas in coordinating otolith-induced cardiovascular adjustments following prolonged postural changes.