Firing properties of ventral medullary respiratory neurons in sino-aortic denervated rats

Abstract

New findings: What is the central question of this study? After sino-aortic denervation (SAD), rats present normal levels of mean arterial pressure (MAP), high MAP variability and changes in breathing. However, mechanisms involved in SAD-induced respiratory changes and their impact on the modulation of sympathetic activity remain unclear. Herein, we characterized the firing frequency of medullary respiratory neurons after SAD. What is the main finding and its importance? Sino-aortic denervation-induced prolonged inspiration was associated with a reduced interburst frequency of pre-inspiratory/inspiratory neurons and an increased long-term variability of late inspiratory neurons, but no changes were observed in the ramp-inspiratory and post-inspiratory neurons. This imbalance in the respiratory network might contribute to the modulation of sympathetic activity after SAD.

Abstract: In previous studies, we documented that after sino-aortic denervation (SAD) in rats there are significant changes in the breathing pattern, but no significant changes in sympathetic activity and mean arterial pressure compared with sham-operated rats. However, the neural mechanisms involved in the respiratory changes after SAD and the extent to which they might contribute to the observed normal sympathetic activity and mean arterial pressure remain unclear. Here, we hypothesized that after SAD, rats present with changes in the firing frequency of the ventral medullary inspiratory and post-inspiratory neurons. To test this hypothesis, male Wistar rats underwent SAD or sham surgery and 3 days later were surgically prepared for an in situ experiment. The duration of inspiration significantly increased in SAD rats. During inspiration, the total firing frequency of ramp-inspiratory, pre-inspiratory/inspiratory and late-inspiratory neurons was not different between groups. During post-inspiration, the total firing frequency of post-inspiratory neurons was also not different between groups. Furthermore, the data demonstrate a reduced interburst frequency of pre-inspiratory/inspiratory neurons and an increased long-term variability of late-inspiratory neurons in SAD compared with sham-operated rats. These findings indicate that the SAD-induced prolongation of inspiration was not accompanied by alterations in the total firing frequency of the ventral medullary respiratory neurons, but it was associated with changes in the long-term variability of late-inspiratory neurons. We suggest that the timing imbalance in the respiratory network in SAD rats might contribute to the modulation of presympathetic neurons after removal of baroreceptor afferents.

Keywords: autonomic nervous system; baroreflex; breathing; respiration.