Homeostatic control of breathing, heart rate, and body temperature relies on circuits within the brainstem modulated by the neurotransmitter serotonin (5-HT). Mounting evidence points to specialized neuronal subtypes within the serotonergic neuronal system, borne out in functional studies, for the modulation of distinct facets of homeostasis. Such functional differences, read out at the organismal level, are likely subserved by differences among 5-HT neuron subtypes at the cellular and molecular levels, including differences in the capacity to co-express other neurotransmitters such as glutamate, GABA, thyrotropin releasing hormone, and substance P encoded by the Tachykinin-1 (Tac1) gene. Here we characterize in mice a 5-HT neuron subtype identified by expression of Tac1 and the serotonergic transcription factor gene Pet1, referred to as the Tac1-Pet1 neuron subtype. Transgenic cell labeling showed Tac1-Pet1 soma resident largely in the caudal medulla. Chemogenetic (CNO-hM4Di) perturbation of Tac1-Pet1 neuron activity blunted the ventilatory response of the respiratory CO2 chemoreflex, which normally augments ventilation in response to hypercapnic acidosis to restore normal pH and PCO2. Tac1-Pet1 axonal boutons were found localized to brainstem areas implicated in respiratory modulation, with highest density in motor regions. These findings demonstrate that the activity of a Pet1 neuron subtype with potential to release both 5-HT and substance P is necessary for normal respiratory dynamics, perhaps via motor outputs that engage muscles of respiration and maintain airway patency. These Tac1-Pet1 neurons may act downstream of Egr2-Pet1 serotonergic neurons, previously established in respiratory chemoreception, but which do not innervate respiratory motor nuclei.
5-HT neurons modulate physiological processes and behaviors as diverse as body temperature, respiration, aggression, and mood. Employing genetic tools, we characterize a 5-HT neuron subtype defined by expression of Tachykinin1 and Pet1 (Tac1-Pet1 neurons), mapping soma localization to the caudal medulla primarily and axonal projections to brainstem motor nuclei most prominently, and when silenced, observed blunting of the ventilatory response to inhaled carbon dioxide. Tac1-Pet1 neurons thus appear distinct from and contrast previously described Egr2-Pet1 neurons which project primarily to chemosensory integration centers and are themselves chemosensitive.
The authors declare no competing financial interests.
Grants supporting this work include P01 HD036379 (S.M.D, E.E.N., A.C.) NIGMS F31NS073276 (R.D.B), NIGMS T32GM007753 (M.L.H.) and NINDS F31 NS083165-02 (M.L.H). The project described was supported by award number T32GM007753 from the National Institute of General Medical Sciences. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of General Medical Sciences or the National Institutes of Health. The authors thank B. Rood, B. Okaty, M. Freret, and R. Dosumu-Johnson for discussion and J.J. Mai for technical support. Appreciation extends to the Harvard Neurobiology Imaging Center for microscopy support, M. Rice for help with figure graphics, and R. Ouillette for experimental assistance.