The vestibular system contributes to regulating sympathetic nerve activity and blood pressure. Initial studies in decerebrate animals showed that neurons in the rostral ventrolateral medulla (RVLM) respond to small-amplitude (<10°) rotations of the body, as in other brain areas that process vestibular signals, although such movements do not affect blood distribution in the body. However, a subsequent experiment in conscious animals showed that few RVLM neurons respond to small-amplitude movements. This study tested the hypothesis that RVLM neurons in conscious animals respond to signals from the vestibular otolith organs elicited by large-amplitude static tilts. The activity of approximately one-third of RVLM neurons whose firing rate was related to the cardiac cycle, and thus likely received baroreceptor inputs, was modulated by vestibular inputs elicited by 40° head-up tilts in conscious cats, but not during 10° sinusoidal rotations in the pitch plane that affected the activity of neurons in brain regions providing inputs to the RVLM. These data suggest the existence of brain circuitry that suppresses vestibular influences on the activity of RVLM neurons and the sympathetic nervous system unless these inputs are physiologically warranted. We also determined that RVLM neurons failed to respond to a light cue signaling the movement, suggesting that feedforward cardiovascular responses do not occur before passive movements that require cardiovascular adjustments.

Considerable evidence demonstrates that the vestibular system contributes to regulating sympathetic nerve activity and blood pressure. Initial studies in decerebrate animals showed that presumed pre-sympathetic neurons in the rostral ventrolateral medulla (RVLM) respond to small-amplitude (<10º) rotations of the body, as in other brain areas that process vestibular signals, despite the fact that such movements do not appreciably affect blood distribution in the body. However, a subsequent experiment in conscious animals showed that few RVLM neurons respond to small-amplitude movements. This study tested the hypothesis that vestibular inputs to RVLM neurons are modulated in conscious animals, such that vestibulosympathetic responses are only elicited when changes in body position are large enough to require changes in sympathetic nerve activity. The activity of approximately a third of RVLM neurons whose firing rate was related to the cardiac cycle, and thus likely received baroreceptor inputs, responded to vestibular inputs elicited by 40º head-up tilts in conscious cats, but not during 10º sinusoidal rotations in the pitch plane that affected the activity of neurons in brain regions providing inputs to the RVLM. These data suggest the existence of brain circuitry that suppresses vestibular influences on the activity of RVLM neurons and the sympathetic nervous system unless these inputs are physiologically warranted. We also determined that RVLM activity is not altered prior to tilts when a light cue is provided signaling the movement. The simplest interpretation of this findings is that feedforward cardiovascular responses are associated with active movement such as occurs during exercise, but not passive movements that require cardiovascular adjustments.

It is well‐established that the rostral ventrolateral medulla (RVLM) plays a predominant role in regulating the activity of sympathetic outflow to the cardiovascular system. We and others have previously shown that in addition to baroreceptor inputs, RVLM neurons respond to signals from the vestibular system. However, most recordings of RVLM unit activity have occurred in anesthetized or decerebrate preparations, limiting the assessment of cognitive effects on neuronal activity. In this study, felines were instrumented for chronic recordings of the electrocardiogram and/or carotid artery blood flow as well as activity of RVLM neurons during 40° head‐up tilts. RVLM units were identified through stereotaxic coordinates and having activity correlated with changes in blood pressure during the cardiac cycle. Tilts were preceded by a light cue, so animals anticipated the movement. We tested the hypothesis that head‐up tilts elicit an increase in the activity of RVLM neurons that is initiated between the light cue and the onset of movement. We also determined if there is a relationship between RVLM unit activity and heart rate, as would be expected if the changes in firing rate are dictated by baroreceptor signals. Head‐up tilts elicited an increase in firing rate for a third of RVLM units. However, for the majority of these units, there was not a significant change in firing rate following the light cue and prior to head‐up rotations. In addition, linear regression analyses showed that there was no appreciable relationship between unit firing rate and heart rate (R2 values ranged from 0.001–0.29, median of 0.03). These studies suggest that cognitive inputs do not produce feedforward alterations in excitability of RVLM neurons prior to perturbations that can alter blood pressure. In addition, they show that inputs to RVLM neurons are complex such that the firing rate of the units is not simply related to baroreceptor inputs.Support or Funding InformationNIH grant R01‐DC013788This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.