Barosensory neurons in the ventrolateral medulla in rabbits and their responses to various afferent inputs from peripheral and central sources.

In 55 anesthetized and paralyzed adult rabbits, 161 spontaneously active neurons which responded to electrical stimulation of A-fibers of the aortic nerve were found within the ventrolateral medulla (VLM). They were termed barosensory VLM neurons, since the aortic nerve A-fibers were considered to consist exclusively of afferents from arterial baroreceptors. Forty percent of barosensory VLM neurons tested (49/123) were activated antidromically by stimulation of the dorsolateral funiculus indicating that they send descending bulbospinal projections. Spontaneous discharges of barosensory VLM neurons were invariably inhibited by stimulation of aortic nerve A-fibers. Ninety-three percent of 80 neurons tested also responded to stimulation of aortic nerve C-fibers, a mixture of barosensory and nonbarosensory afferents. Natural stimulation of carotid sinus baroreceptors by an intravenous injection of phenylephrine in 19 vagotomized rabbits with aortic nerves disrupted inhibited spontaneous activity of all the 50 barosensory VLM neurons tested. By contrast, pharmacological stimulation of right or left carotid body chemoreceptors by close arterial injection of NaCN into the carotid sinus augmented activity of 93% of barosensory VLM neurons tested (41/44). The neuronal response was always greater to stimulation of chemoreceptors in the contralateral carotid sinus. Seven out of 8 barosensory VLM neurons tested (88%) were orthodromically excited by stimulation of the posterior hypothalamic area. In 74% of the 97 neurons examined in 29 vagotomized animals, a distinct respiratory-related rhythm, locked to that of phrenic nerve activity, was discerned. Thus, spontaneous activity of barosensory VLM neurons is inhibited by afferent inputs from aortic and carotid sinus baroreceptors, but is excited by incoming signals from carotid body chemoreceptors and the posterior hypothalamic area. It is also subject to the influence of the central mechanism generating the respiratory rhythm.