Excitation-inhibition balance regulates the patterning of spinal and cranial inspiratory motor outputs in rats in situ
Introduction
Respiratory motor activities are expressed in a variety of functionally diverse muscles and muscle groups in order to modulate the depth and duration of inspiratory and particularly expiratory airflow (Bartlett, 1989; Dutschmann and Dick, 2012; Dutschmann and Paton, 2002; Sant’Ambrogio et al., 1995; Smith et al., 2013; Widdicombe, 1982). Briefly, respiration is governed by two motor systems: (i) the respiratory motor pools in the spinal cord govern respiratory pump muscles in the abdomen and thorax that generate pressure gradients to move air into or out of the lungs, and (ii) brainstem motoneurons supply muscles in the upper airways via specific branches of cranial nerves V, VII, IX X and XII that adjust and modulate airway resistance to determine the duration and/or force of pulmonary airflow. The complexity of respiratory motor activity is reflected in its motor pattern of inspiration, post-inspiration (controlled expiration, see (Dutschmann et al., 2014) and expiration (passive- or active-expiration, see Iscoe, 1998; Molkov et al., 2010; Pisanski and Pagliardini, 2018).
The pre-motor networks that generate the three-phase respiratory rhythm are thought to be localized within the ventrolateral medulla (Del Negro et al., 2018; Ramirez and Baertsch, 2018; Richter and Smith, 2014; Smith et al., 2009). However, recent evidence exists that the generation of the three-phase respiratory motor pattern depends on the rhythm-generating pool’s embedding within a larger ponto-medullary network (Dhingra et al., 2019). This most recent study showed that local disinhibition of the nucleus tractus solitarius (NTS), the pre-Bötzinger/Bötzinger complexes or the Kölliker-Fuse nuclei, but not the mibrain periaqueductal gray, was sufficient to perturb global respiratory network dynamics thereby evoking a disruption of the alternation between inspiration and post-inspiration as observed in the primary motor pattern (Dhingra et al., 2019). We concluded (1) that the inspiratory off-switch mechanism is distributed across the ponto-medullary brainstem and is an emergent property of the rCPG network; and (2) that excitation-inhibition balance within and across nodes of the distributed rCPG network is a mechanism that underlies the generation of the eupneic respiratory motor pattern.
In the present study, we use similar experimental and analytical approaches to investigate whether the functional structure of the network regulating spinal and cranial inspiratory motor outputs in the phrenic and hypoglossal nerves, respectively, is equally as distributed as the network underlying inspiratory and post-inspiratory respiratory pattern formation. We show that while spinal and cranial inspiratory motor outputs are perturbed by local modulation of circuit excitability in the nucleus tractus solitarius (NTS), pre-Bötzinger complex/Bötzinger complexes (pre-BötC/BötC) or KFn, local disinhibition of medullary targets evokes a stronger disruption of the eupneic pattern of hypoglossal nerve activity and of the phase synchronization between phrenic and hypoglossal nerve activity.
Section snippets
Materials and methods
Experimental protocols were approved by and conducted with strict adherence to the guidelines established by the Animal Ethics Committee of The Florey Institute of Neuroscience & Mental Health, Melbourne, Australia. Phrenic nerve recordings presented in this study comprised a subset of those published in a previous study (Dhingra et al., 2019).
Results
To investigate the influence of excitation-inhibition balance within the NTS, pre-BötC/BötC or KFn on phrenic and hypoglossal motor patterns, we recorded these motor patterns before and after local disinhibition of each target site. Representative traces are shown before and after local disinhibition in Fig. 1. At baseline (Fig. 1A, C & E), we observed the typical interaction between PNA and HNA such that HNA begins before PNA during pre-inspiration (pre-I), and both show activity throughout
Discussion
In the present study, we have shown that increasing local circuit excitability via disinhibition of either the NTS, pre-BötC/BötC or KFn was sufficient to perturb and desynchronize the spinal and cranial inspiratory motor pattern. Qualitatively, the nature of the disruption evoked by local disinhibition differed between medullary and pontine target sites such that stronger disruptions in HNA patterning were evoked by local disinhibition of medullary target sites. Quantitatively, local
Acknowledgement
This work was supported by a Discovery project funded by the Australian Research Council [DP170104861].
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