Active expiration represents an important mechanism to improve ventilation in conditions of augmented ventilatory demand, such as hypercapnia. While a rostral ventromedullary region, the parafacial respiratory group (pFRG), has been identified as a conditional expiratory oscillator, little is known about how central chemosensitive sites contribute to modulate active expiration under hypercapnia. In this study, we investigated the influence of the medullary raphe in the emergence of phasic expiratory abdominal activity during hypercapnia in unanesthetized adult male rats, in a state-dependent manner. To do so, reverse microdialysis of muscimol (GABA_A receptor agonist, 1 mM) or 8-OH-DPAT (5-HT_1A agonist, 1 mM) was applied in the MR during sleep and wakefulness periods, both in normocapnic (room air) and hypercapnic conditions (7% CO₂). Electromyography (EMG) of diaphragm and abdominal muscles was performed to measure inspiratory and expiratory motor outputs. We found that active expiration did not occur in room air exposure during wakefulness or sleep. However, hypercapnia did recruit active expiration, and differential effects were observed with the drug dialyses in the medullary raphe. Muscimol increased the diaphragm inspiratory motor output and also increased the amplitude and frequency of abdominal expiratory rhythmic activity during hypercapnia in wakefulness periods. On the other hand, the microdialysis of 8-OH-DPAT attenuated hypercapnia-induced active expiration in a state-dependent manner. Our data suggest that the medullary raphe can either inhibit or potentiate respiratory motor activity during hypercapnia, and the balance of these inhibitory or excitatory outputs may determine the expression of active expiration.

主动呼气是在通气需求增加的条件下（例如高碳酸血症）改善通气的重要机制。虽然头侧腹髓区，即面旁呼吸群 (pFRG)，已被确定为条件呼气振荡器，但对中央化学敏感位点如何在高碳酸血症下调节主动呼气知之甚少。在这项研究中，我们以状态依赖的方式研究了中缝延髓对高碳酸血症期间非麻醉成年雄性大鼠出现阶段性呼气腹部活动的影响。为此，对 muscimol（GABA _A受体激动剂，1 mM）或 8-OH-DPAT（5-HT _1A激动剂，1 mM）在睡眠和清醒期间应用于 MR，在正常二氧化碳（室内空气）和高二氧化碳条件（7% CO ₂）。进行膈肌和腹部肌肉的肌电图 (EMG) 以测量吸气和呼气运动输出。我们发现在清醒或睡眠期间暴露于室内空气中不会发生主动呼气。然而，高碳酸血症确实会引起主动呼气，并且在髓质中缝中观察到药物透析的不同效果。Muscimol 增加了膈肌吸气运动输出，还增加了清醒期高碳酸血症期间腹部呼气节律活动的幅度和频率。另一方面，8-OH-DPAT 的微透析以状态依赖的方式减弱了高碳酸血症引起的主动呼气。我们的数据表明，延髓中缝可以抑制或增强高碳酸血症期间的呼吸运动活动，