Inhibitory control of active expiration by the Bötzinger complex in rats

ABSTRACTThe expiratory neurons of the Bötzinger complex (BötC) provide inhibitory inputs to the respiratory network, which, during eupnea, are critically important for respiratory phase transition and duration control. Herein, we investigated how the BötC neurons interact with the expiratory oscillator located in the parafacial respiratory group (pFRG) and control the abdominal activity during active expiration. Using the decerebrated, arterially perfused in situ rat preparations, we recorded the neuronal activity and performed pharmacological manipulations of the BötC and pFRG during hypercapnia or after the exposure to short-term sustained hypoxia – conditions that generate active expiration. The experimental data were integrated in a mathematical model to gain new insights in the inhibitory connectome within the respiratory central pattern generator. Our results reveal a complex inhibitory circuitry within the BötC that provides inhibitory inputs to the pFRG thus restraining abdominal activity under resting conditions and contributing to abdominal expiratory pattern formation during active expiration.

Chemogenetic modulation of the parafacial respiratory group influences the recruitment of abdominal activity during REM sleep

Current theories on respiratory control postulate that the respiratory rhythm is generated by oscillatory networks in the medulla: preBötzinger complex (preBötC) is the master oscillator responsible for generating inspiration, while parafacial respiratory group (pFRG) drives active expiration through recruitment of expiratory abdominal (ABD) muscle activity. Research addressing the role of pFRG in ventilation and rhythm generation across sleep states is limited. We recently reported the occurrence of ABD recruitment occurring despite the induction of muscle paralysis during REM sleep. This ABD recruitment was associated with increased tidal volume and regularization of the respiratory period in rats. As pFRG generates active expiration through the engagement of ABD muscles, we hypothesized that the expiratory oscillator is also responsible for the ABD recruitment observed during REM sleep. To test this hypothesis, we inhibited and activated pFRG using chemogenetics (i.e. designer receptors exclusively activated by designer drugs) while recording EEG and respiratory muscle EMG activities across sleep–wake cycles in male Sprague–Dawley rats. Our results suggest that inhibition of pFRG reduced the number of REM events expressing ABD recruitment, in addition to the intensity and prevalence of these events. Conversely, activation of pFRG resulted in an increase in the number of REM events in which ABD recruitment was observed, as well as the intensity and prevalence of ABD recruitment. Interestingly, modulation of pFRG activity did not affect ABD recruitment during NREM sleep or wakefulness. These results suggest that the occurrence of ABD recruitment during sleep is dependent on pFRG activity and is state dependent.

Distinct pathways to the parafacial respiratory group to trigger active expiration in adult rats

Active expiration (AE) is part of the breathing phase; it is conditional and occurs when we increase our metabolic demand, such as during hypercapnia, hypoxia, or exercise. The parafacial respiratory group (pFRG) is involved in AE. Data from the literature suggest that excitatory and the absence of inhibitory inputs to the pFRG are necessary to determine AE. However, the source of the inputs to the pFRG that trigger AE remains unclear. We show in adult urethane-anesthetized Wistar rats that the pharmacological inhibition of the medial aspect of the nucleus of the solitary tract (mNTS) or the rostral aspect of the pedunculopontine tegmental nucleus (rPPTg) is able to generate AE. In addition, direct inhibitory projection from the mNTS or indirect cholinergic projection from the rPPTg is able to contact pFRG to trigger AE. The inhibition of the mNTS or the rPPTg under conditions of high metabolic demand, such as hypercapnia (9–10% CO2), did not affect the AE. The present results suggest for the first time that inhibitory sources from the mNTS and a cholinergic pathway from the rPPTg, involving M2/M4 muscarinic receptors, could be important sources to modulate and sustain AE.

RESPIRATORY RESPONSE ON THE ACTIVATION OF GABAА- AND GABAВ-RECEPTORS OF A PARAFACIAL RESPIRATORY GROUP BY SPECIFIC AGONISTS

The purpose of the paper is to conduct comparative analysis of GABAА and GABAВ receptors of the parafacial respiratory group (pfRG) in central mechanisms of respiratory regulation in adult rats. Materials and Methods. The authors studied the reactions of external respiration and diaphragmatic muscle on microinjections of muscimol and baclofen solutions (10-6 M, 200 nl) in the region of pFHR on rats, anesthetized with urethane. Injections were administered through a glass microcannula according to stereotactic coordinates. A spirogram and electromyogram (EMG) of the diaphragm were recorded in the initial state and within 60 minutes after microinjection of the abovementioned agonists of GABAА and GABAВ receptors. Results. Microinjections of muscimol, the agonist of GABAA receptors, in pfGR had an inhibitory effect on respiration. The inhibitory effect was characterized by a short latent period. Minute lung ventilation decreased in rats due to changes in tidal volume and respiratory rate, while the inspiratory and expiratory time increased. On the EMG of the diaphragm, the oscillation amplitude decreased, while the duration of inspiratory bursts and inter-burst intervals increased. Microinjections of baclofen solution, GABAВ agonist receptor, in pfGR also caused respiratory depression, but with a longer latency period. In that case, a decrease in pulmonary ventilation was provided by a decrease in tidal volume in combination with inspiration lengthening. Changes in the breathing pattern corresponded to a decrease in the oscillation amplitude and an increase in the duration of inspiratory bursts on the EMG of the diaphragm. Conclusions. The role of pfGR in the respiratory regulation is mediated by ionotropic GABAА- and metabotropic GABAB-receptors. GABAA-receptors in the pfHR area contribute to expiration-inspiration regulation, while GABAB-receptors are predominantly involved in inspiratory activity modulation. Keywords: parafacial respiratory group, GABAA-receptors, GABAB-receptors, muscimol, baclofen, external respiration, electromyogram of the diaphragm.