Medullary GABAergic neurons constitute a neural brake coordinating respiratory rhythm in vivo

Motor behaviors, such as walking and breathing, depend on neural circuits combining excitatory and inhibitory neurons to produce rhythmic motor activities. Breathing is a critical autonomic motor function that requires excitatory and inhibitory circuits to orchestrate inhalation and exhalation. Inspiration is produced by excitatory medullary neurons of the preBötzinger Complex (preBötC), a small region of the ventral medulla dedicated to breathing. Expiration is characterized by passive recoil of the lungs and thorax and is a critical phase of the respiratory cycle allowing air to be exhaled until the next inspiration happens. The brain circuits regulating the timing and pattern of expiration and determining when the next inspiration is produced have not been identified. Here, we propose that inhibitory GABAergic preBötC neurons act as a neural brake to prevent the next inspiration to be produced until expiration is fully completed. Using optogenetic tools to modulate GABAergic neurons in male and female mice in vivo, we showed that photostimulation of GABAergic preBötzinger Complex neurons prevented the next inspiration to occur and considerably prolonged expiration. On the other hand, photoinhibition of GABAergic neurons quickly produced inspiration therefore aborting expiration. Collectively, GABAergic preBötC neurons are pivotal in orchestrating the transition from expiration to inspiration to properly complete exhalation. Importantly, understanding inhibitory circuits in the context of rhythmic breathing may be relevant to comprehend the universal properties of other rhythmic motor behaviors, such as walking, that also require alternating motor activity.Significance Statement Neural circuits producing rhythmic behaviors, such as walking and breathing, often involve excitatory and inhibitory neurons. Breathing is a fundamental rhythmic function alternating inspiration and expiration. Inspiration is mediated by excitatory neurons in the preBötzinger Complex (preBötC), a region of the ventral medulla dedicated to respiratory rhythm. Expiration is a critical phase of the respiratory cycle, allowing air to be exhaled until the next inspiration occurs. Here, we show that inhibitory GABAergic preBötC neurons act as a neural brake to prevent the next inspiration from being produced until expiration is fully completed. Importantly, understanding inhibitory circuits regulating rhythmic breathing may be relevant to understand the universal properties of rhythmic circuits mediating behaviors.