Responses of neurons in the rostral ventrolateral medulla to whole body rotations: comparisons in decerebrate and conscious cats

Abstract

The responses to vestibular stimulation of brain stem neurons that regulate sympathetic outflow and blood flow have been studied extensively in decerebrate preparations, but not in conscious animals. In the present study, we compared the responses of neurons in the rostral ventrolateral medulla (RVLM), a principal region of the brain stem involved in the regulation of blood pressure, to whole body rotations of conscious and decerebrate cats. In both preparations, RVLM neurons exhibited similar levels of spontaneous activity (median of ∼17 spikes/s). The firing of about half of the RVLM neurons recorded in decerebrate cats was modulated by rotations; these cells were activated by vertical tilts in a variety of directions, with response characteristics suggesting that their labyrinthine inputs originated in otolith organs. The activity of over one-third of RVLM neurons in decerebrate animals was altered by stimulation of baroreceptors; RVLM units with and without baroreceptor signals had similar responses to rotations. In contrast, only 6% of RVLM neurons studied in conscious cats exhibited cardiac-related activity, and the firing of just 1% of the cells was modulated by rotations. These data suggest that the brain stem circuitry mediating vestibulosympathetic reflexes is highly sensitive to changes in body position in space but that the responses to vestibular stimuli of neurons in the pathway are suppressed by higher brain centers in conscious animals. The findings also raise the possibility that autonomic responses to a variety of inputs, including those from the inner ear, could be gated according to behavioral context and attenuated when they are not necessary.

Rhythmic activity of neurons in the rostral ventrolateral medulla of conscious cats: effect of removal of vestibular inputs

“…Data were collected from four conscious female purpose-bred adult cats (Liberty Research, Waverly, NY) that were instrumented using methods described in previous studies (18,33,34) for single-unit recordings. Animals were spayed by a veterinarian prior to being included in this study to prevent cyclic changes in hormonal levels that could have affected cardiovascular regulation.…”

“…Animals were spayed by a veterinarian prior to being included in this study to prevent cyclic changes in hormonal levels that could have affected cardiovascular regulation. In addition, surgical procedures were only conducted on animals that were highly tractable and that were gradually acclimated over a 1-to 2-mo period for 90 min of restraint during recording sessions (18,33,34).…”

“…Two surgical procedures were performed aseptically in a dedicated operating suite, as described in detail in prior publications (18,33,34). During each surgery, animals were initially anesthetized using an intramuscular injection of ketamine (20 mg/kg) and acepromazine (0.2 mg/kg), an endotracheal tube was inserted, and anesthesia was maintained using 1-2% isoflurane vaporized in O 2.…”

“…We recently provided the first examples of recordings of RVLM neuronal activity in conscious animals (18). In that study, we compared in decerebrate and awake cats the responses of RVLM neurons to whole-body rotations that activate vestibular receptors.…”

“…In that study, we compared in decerebrate and awake cats the responses of RVLM neurons to whole-body rotations that activate vestibular receptors. We demonstrated that RVLM neuronal responses to vestibular inputs are ordinarily suppressed in conscious animals and hypothesized that autonomic responses to a variety of inputs, including those from the inner ear, are gated by higher brain centers, according to behavioral context and attenuated when they are not necessary (18).…”