Interactions between Arousal State and CO₂Determine the Activity of Central Chemoreceptor Neurons That Drive Breathing

The homeostatic regulation of pulmonary ventilation, and ultimately arterial PCO₂, depends on interactions between respiratory chemoreflexes and arousal state. The ventilatory response to CO₂ is triggered by neurons in the retrotrapezoid nucleus (RTN) that function as sensors of central pH, which can be identified in adulthood by the expression of Phox2b and neuromedin B. Here, we examine the dynamic response of genetically defined RTN neurons to hypercapnia and arousal state in freely behaving adult male and female mice using the calcium indicator jGCaMP7 and fiber photometry. We found that hypercapnia vigorously activates RTN neurons with a low CO₂ recruitment threshold and with response kinetics that match respiratory activity whereas hypoxia had little effect. RTN activity increased transiently during wakefulness and respiratory-related arousals and rose persistently during rapid eye movement sleep, and their CO₂ response persisted under anesthesia. Complementary studies using inhibitory optogenetics show that RTN activity supports eupneic breathing under anesthesia as well as during states of high arousal, but their activity is redundant for voluntary breathing patterns. Collectively, this study demonstrates that CO₂-activated RTN neurons are exquisitely sensitive to the arousal state, which determines their contribution to alveolar ventilation in relation to arterial PCO₂.