Medullary tachykinin precursor 1 neurons promote rhythmic breathing

Rhythmic breathing is generated by neural circuits located in the brainstem. At its core is the preBotzinger Complex (preBotC), a region of the medulla, necessary for the generation of rhythmic breathing in mammals. The preBotC is comprised of various neuronal populations expressing neurokinin-1 receptors, the cognate G-protein-coupled receptor of the neuropeptide substance P (encoded by the tachykinin precursor 1 or Tac1). Neurokinin-1 receptors are highly expressed in the preBotC and destruction or deletion of neurokinin-1 receptor-expressing preBotC neurons severely impair rhythmic breathing. Although, the application of substance P to the preBotC stimulates breathing in rodents, substance P is also involved in nociception and locomotion in various brain regions, suggesting that Tac1 neurons found in the preBotC may have diverse functional roles. Here, we characterized the role of Tac1-expressing preBotC neurons in the generation of rhythmic breathing in vivo, as well as motor behaviors. Using a cre-lox recombination approach, we injected adeno-associated virus containing the excitatory channelrhodopsin-2 ChETA in the preBotC region of Tac1-cre mice. Employing a combination of histological, optogenetics, respiratory, and behavioral assays, we showed that stimulation of glutamatergic or Tac1 preBotC neurons promoted rhythmic breathing in both anesthetized and freely moving animals, but also triggered locomotion and overcame respiratory depression by opioid drugs. Overall, our study identified a population of excitatory preBotC with major roles in rhythmic breathing and behaviors.