Increasing Local Excitability of Brainstem Respiratory Nuclei Reveals a Distributed Network Underlying Respiratory Motor Pattern Formation

Abstract

The core circuit of the respiratory central pattern generator (rCPG) is located in the ventrolateral medulla, especially in the pre-Bötzinger complex (pre-BötC) and the neighboring Bötzinger complex (BötC). To test the hypothesis that this core circuit is embedded within an anatomically distributed pattern-generating network, we investigated whether local disinhibition of the nucleus tractus solitarius (NTS), the Kölliker-Fuse nuclei (KFn), or the midbrain periaqueductal gray area (PAG) can similarly affect the respiratory pattern compared to disinhibition of the pre-BötC/BötC core. In arterially-perfused brainstem preparations of rats, we recorded the three-phase respiratory pattern (inspiration, post-inspiration and late-expiration) from phrenic and vagal nerves before and after bilateral microinjections of the GABA(A)R antagonist bicuculline (50 nl, 10 mM). Local disinhibition of either NTS, pre-BötC/BötC, or KFn, but not PAG, triggered qualitatively similar disruptions of the respiratory pattern resulting in a highly significant increase in the variability of the respiratory cycle length, including inspiratory and expiratory phase durations. To quantitatively analyze these motor pattern perturbations, we measured the strength of phase synchronization between phrenic and vagal motor outputs. This analysis showed that local disinhibition of all brainstem target nuclei, but not the midbrain PAG, significantly decreased the strength of phase synchronization. The convergent perturbations of the respiratory pattern suggest that the rCPG expands rostrally and dorsally from the designated core but does not include higher mid-brain structures. Our data also suggest that excitation-inhibition balance of respiratory network synaptic interactions critically determines the network dynamics that underlie vital respiratory rhythm and pattern formation.

Volumetric mapping of the functional neuroanatomy of the respiratory network in the perfused brainstem preparation of rats

“…In line with recent publications from our laboratory, which showed that local disinhibition of the VRC, DRG or PRG caused similar disruptions of the I–PI phase transition (Dhingra et al . 2019 a , b ), the spatial distribution of I–PI LFPs support the hypothesis that distributed neuronal populations compete to determine the precise timing of the inspiratory off‐switch. Consistent with this hypothesis, it remains well established, though often overlooked, that without the pons, medullary connectivity is insufficient to generate the alternating patterns of activity that result from inspiratory off‐switch mechanisms (Marckwald, 1887; Lumsden, 1923; Jones & Dutschmann, 2016).…”

“…Previous research from our laboratory which used local (dis‐) inhibition of individual respiratory network compartments to functionally assess their role in the generation of the post‐inspiratory motor pattern suggested that neuronal activity within the PRG, DRG and VRC are necessary to generate a eupneic post‐inspiratory motor pattern (Dutschmann & Herbert, 2006; Dutschmann & Dick, 2012; Jones & Dutschmann, 2016; Dhingra et al . 2017, 2019 a , b ). Anatomically, these brainstem areas occupy a volume of ca .…”

Volumetric mapping of the functional neuroanatomy of the brainstem respiratory network in the perfused brainstem preparation of rats

“…Consistent with these anatomical frameworks, recent work from our laboratory has shown that expression of a eupnoeic three-phase respiratory motor pattern (inspiration, post-inspiration and late expiration) depends on the integrity of synaptic interactions across the pons and medulla, particularly between the medullary ventrolateral respiratory column (VRC), the medullary dorsal respiratory group (DRG) and the pontine respiratory group (PRG) (Dutschmann & Herbert, 2006;Smith et al 2007;Dutschmann & Dick, 2012;Dhingra et al 2017Dhingra et al , 2019a.…”

“…However, electrophysiological studies did confirm a potential excitation of DRG neurons arising from the VRC (Tian & Duffin, 1998;Duffin, 2004). Recently, we and others have provided functional evidence suggesting that the DRG is necessary for the formation of the post-inspiratory motor pattern (Wasserman et al 2002;Dhingra et al 2019a).…”

“…However, in the present study, because we had an a priori rationale as to the expected populations involved in respiratory pattern generation, we took advantage of the volume conduction of LFPs to effectively sample from a larger volume than the sampled electrode sites. Previous research from our laboratory which used local (dis-) inhibition of individual respiratory network compartments to functionally assess their role in the generation of the post-inspiratory motor pattern suggested that neuronal activity within the PRG, DRG and VRC are necessary to generate a eupneic post-inspiratory motor pattern (Dutschmann & Herbert, 2006;Dutschmann & Dick, 2012;Dhingra et al 2017Dhingra et al , 2019a. Anatomically, these brainstem areas occupy a volume of ca.…”