Respiratory Network Enhances the Sympathoinhibitory Component of Baroreflex of Rats Submitted to Chronic Intermittent Hypoxia

Abstract

Chronic intermittent hypoxia (CIH) produces respiratory-related sympathetic overactivity and hypertension in rats. In this study, we tested the hypothesis that the enhanced central respiratory modulation of sympathetic activity after CIH also decreases the sympathoinhibitory component of baroreflex of rats, which may contribute to the development of hypertension. Wistar rats were exposed to CIH or normoxia (control group) for 10 days. Phrenic nerve, thoracic sympathetic nerve, and neurons in the rostral ventrolateral medulla and caudal ventrolateral medulla were recorded in in situ preparations of rats. Baroreflex regulation of thoracic sympathetic nerve, rostral ventrolateral medulla, and caudal ventrolateral medulla neurons activities were evaluated in different phases of respiration in response to either aortic depressor nerve stimulation or pressure stimuli. CIH rats presented higher respiratory-related thoracic sympathetic nerve and rostral ventrolateral medulla presympathetic neurons activities at the end of expiration in relation to control rats, which are indexes of respiratory-related sympathetic overactivity. Baroreflex-evoked thoracic sympathetic nerve inhibition during expiration, but not during inspiration, was enhanced in CIH when compared with control rats. In addition, CIH selectively enhanced the expiratory-related baroreceptor inputs, probably through caudal ventrolateral medulla neurons, to the respiratory-modulated bulbospinal rostral ventrolateral medulla presympathetic neurons. These findings support the concept that the onset of hypertension, mediated by sympathetic overactivity, after 10 days of CIH is not secondary to a reduction in sympathoinhibitory component of baroreflex. Instead, it was observed an increase in the gain of sympathoinhibitory component in in situ preparations of rats, suggesting that changes in the respiratory-related sympathetic network after CIH also play a key role in preventing greater increase in arterial pressure.