Exposure to chronic sustained hypoxia (SH), as experienced in high altitudes, elicits an increase in ventilation, named ventilatory acclimatization to hypoxia (VAH). We previously showed that rats exposed to short-term (24 h) SH exhibit enhanced abdominal expiratory motor activity at rest, accompanied by augmented baseline sympathetic vasoconstrictor activity. In the present study, we investigated whether the respiratory and sympathetic changes elicited by short-term SH are accompanied by carotid body chemoreceptor sensitization. Juvenile male Holtzman rats (60–80 g) were exposed to SH (10% O2 for 24 h) or normoxia (control) to examine basal and hypoxic-induced ventilatory parameters in unanesthetized conditions, as well as the sensory response of carotid body chemoreceptors in artificially perfused in situ preparations. Under resting conditions (normoxia/normocapnia), SH rats (n = 12) exhibited higher baseline respiratory frequency, tidal volume, and minute ventilation compared to controls (n = 11, P < 0.05). SH group also showed greater hypoxia ventilatory response than control group (P < 0.05). The in situ preparations of SH rats (n = 8) exhibited augmented baseline expiratory and sympathetic activities under normocapnia, with additional bursts in abdominal and thoracic sympathetic nerves during late expiratory phase that were not seen in controls (n = 8, P < 0.05). Interestingly, basal and potassium cyanide-induced afferent activity of carotid sinus nerve (CSN) was similar between SH and control rats. Our findings indicate that the maintenance of elevated resting ventilation, baseline sympathetic overactivity, and enhanced ventilatory responses to hypoxia in rats exposed to 24 h of SH are not dependent on increased basal and sensorial activity of carotid body chemoreceptors.

26The expiratory neurons of the Bötzinger complex (BötC) provide inhibitory inputs to 27 the respiratory network, which, during eupnea, are critically important for respiratory phase 28 transition and duration control. Herein, we investigated how the BötC neurons interact with the 29 expiratory oscillator located in the parafacial respiratory group (pFRG) and control the 30 abdominal activity during active expiration. Using the decerebrated, arterially perfused in situ 31 rat preparations, we recorded the neuronal activity and performed pharmacological 32 manipulations of the BötC and pFRG during hypercapnia or after the exposure to short-term 33 sustained hypoxia -conditions that generate active expiration. The experimental data were 34 integrated in a mathematical model to gain new insights in the inhibitory connectome within 35 the respiratory central pattern generator. Our results reveal a complex inhibitory circuitry within 36 the BötC that provides inhibitory inputs to the pFRG thus restraining abdominal activity under 37 resting conditions and contributing to abdominal expiratory pattern formation during active 38 expiration. 39 40 41 Keywords: abdominal activity, hypercapnia, hypoxia, parafacial respiratory group, ventral 42 respiratory column. 48 et al., 2015, Harris-Warrick, 2010, Ramirez and Baertsch, 2018. In mammals, rhythmical 49 contraction and relaxation of respiratory muscles emerges from interacting excitatory and 50 inhibitory neurons with specific cellular properties, distributed within the pons and the medulla 51 oblongata (Richter and Smith, 2014, Del Negro et al., 2018, Lindsey et al., 2012. Coupled 52 oscillators embedded in this brainstem respiratory network are essential to generate and 53 distribute synaptic inputs for the initiation of respiratory rhythmicity and the control of pattern 54 formation (Anderson and Ramirez, 2017, Del Negro et al., 2018). Defining the arrangement 55 and connections of the respiratory oscillators and circuitries are essential to understand how 56 breathing is generated and adjusted to attend metabolic and behavior demands.57

Key points Contraction of abdominal muscles at the end of expiration during metabolic challenges (such as hypercapnia and hypoxia) improves pulmonary ventilation. The emergence of this active expiratory pattern requires the recruitment of the expiratory oscillator located on the ventral surface of the medulla oblongata. Here we show that an inhibitory circuitry located in the Bötzinger complex is an important source of inhibitory drive to the expiratory oscillator. This circuitry, mediated by GABAergic and glycinergic synapses, provides expiratory inhibition that restrains the expiratory oscillator under resting condition and regulates the formation of abdominal expiratory activity during active expiration. By combining experimental and modelling approaches, we propose the organization and connections within the respiratory network that control the changes in the breathing pattern associated with elevated metabolic demand. Abstract The expiratory neurons of the Bötzinger complex (BötC) provide inhibitory inputs to the respiratory network, which, during eupnoea, are critically important for respiratory phase transition and duration control. Here, 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 preparations of juvenile rats, we recorded the activity of expiratory neurons 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 into the inhibitory connectome within the respiratory central pattern generator. Our results indicate that the BötC neurons may establish mutual connections with the pFRG, providing expiratory inhibition during the first stage of expiration and receiving excitatory inputs during late expiration. Moreover, we found that application of GABAergic and glycinergic antagonists in the BötC caused opposing effects on abdominal expiratory activity, suggesting complex inhibitory circuitry within the BötC. Using mathematical modelling, we propose that the BötC network organization and its interactions with the pFRG restrain abdominal activity under resting conditions and contribute to abdominal expiratory pattern formation during active expiration observed during hypercapnia or after the exposure to short‐term sustained hypoxia.