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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.
Highlights
The concept of central pattern generator (CPG) networks that produce rhythmic physiologic motor activities emerged from studies concerned with invertebrate flight, swimming, feeding, and locomotion (Marder and Calabrese, 1996)
To test the hypothesis that the pre-BötC/BötC core circuit is embedded within an anatomically distributed respiratory motor pattern generating network, we locally disinhibited the nucleus of the solitary tract (NTS), the pre-Bötzinger complex/Bötzinger complex area, the Kölliker-Fuse nuclei (KFn) or the periaqueductal gray (PAG)—brainstem and midbrain areas known to be involved in the generation or modulation of the respiratory rhythm and pattern—by microinjecting the GABA(A)R antagonist bicuculline (50 nl, 10 mM). (A–D) Representative traces are shown at baseline and after local disinhibition of the NTS (A), pre-BötC/BötC (B), KFn (C), or PAG (D)
To investigate the distributed organization of the brainstem network regulating the respiratory motor pattern, we locally disinhibited the nucleus of the solitary tract (NTS), pre-Bötzinger complex/Bötzinger complex area, the KöllikerFuse nuclei (KFn), or the periaqueductal gray (PAG)
Summary
The concept of central pattern generator (CPG) networks that produce rhythmic physiologic motor activities emerged from studies concerned with invertebrate flight, swimming, feeding, and locomotion (Marder and Calabrese, 1996). The rhythmogenic pre-BötC core contains inspiratory GABAergic and glycinergic neurons that are thought to provide inhibition to expiratory neuron populations in the neighboring BötC and thereby inter-nuclear connectivity is thought to form an inhibitory ring of mutual inhibition between inspiratory, post-inspiratory and late-expiratory neurons (Richter, 1982; Rybak et al, 2004; Smith et al, 2007, 2009; Ausborn et al, 2018) This rhythm- and pattern-generating core is embedded into a larger anatomically functionally compartmentalized lateral respiratory column spanning from the caudal medulla to dorsal pons (Alheid et al, 2004; Rybak et al, 2004; Smith et al, 2007; Alheid and McCrimmon, 2008; Dutschmann and Dick, 2012). The respiratory areas outside of the core circuit, such as the pontine respiratory group (see Dutschmann and Dick, 2012) or the caudal ventral respiratory group of the medulla oblongata are thought to provide tonic modulatory inputs to the core or to relay the phasic respiratory activity to spinal (e.g., phrenic, intercostal, or abdominal motor neurons) or cranial (e.g., vagal-laryngeal, hypoglossal motor neurons) respiratory motor pools
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