Thoracic sympathetic chain stimulation modulates and entrains the respiratory pattern

Stimulation of thoracic sympathetic chain (TSC) afferents has been shown to slow the respiratory rhythm in dogs, monkeys and humans. However, sparse information exists about the physiological role of TSC afferents in modulating respiration or the central pathways of these afferents. Here, we sought to investigate whether the perfused preparation of juvenile rats is a suitable experimental model to study the role of TSC-afferents in the modulation of respiration. We show that tonic (30s) TSC stimulation initially triggered either prolonged post-inspiratory vagal nerve discharge, or when the stimulus onset occurred in the second half of expiration, TSC stimulation also modulated late-expiratory abdominal nerve activity. Independent of the timing of the TSC-stimulation the net effect was lengthening of the expiratory interval and subtle shortening of inspiration. TSC-evoked respiratory modulation showed progressive habituation during the stimulus period. Importantly, high thoracic spinal cord transections abolished the TSC-evoked respiratory modulation, indicating that TSC afferents are likely to be relayed within the thoracic spinal cord. Next, we repeatedly applied 400 ms trains of stimuli at an inter-burst interval near that of the intrinsic respiratory rate and show that rhythmic TSC stimulation has a strong potential to entrain the central respiratory rhythm. Importantly, under the imposed rhythm, TSC stimuli became aligned with the late expiratory phase. The entrainment pattern supports the hypothesis that the TSC pathway may convey extra-pulmonary visceral mechano-sensory feedback that might be sensitive to visceral mass movements during locomotion. The latter was previously discussed to significantly contribute to the locomotor-respiratory coupling in various mammalian species.