Activity of<i>Tachykinin1</i>-Expressing<i>Pet1</i>Raphe Neurons Modulates the Respiratory Chemoreflex

Abstract

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 coexpress other neurotransmitters such as glutamate, GABA, thyrotropin releasing hormone, and substance P encoded by the <i>Tachykinin-1</i> (<i>Tac1</i>) gene. Here, we characterize in mice a 5-HT neuron subtype identified by expression of <i>Tac1</i> and the serotonergic transcription factor gene <i>Pet1</i>, referred to as the <i>Tac1-Pet1</i> neuron subtype. Transgenic cell labeling showed <i>Tac1-Pet1</i> soma resident largely in the caudal medulla. Chemogenetic [clozapine<i>-N-</i>oxide (CNO)-hM4Di] perturbation of <i>Tac1-Pet1</i> 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<i>Tac1-Pet1</i> 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 <i>Pet1</i> neuron subtype with the 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 <i>Tac1-Pet1</i> neurons may act downstream of <i>Egr2-Pet1</i> serotonergic neurons, which were previously established in respiratory chemoreception, but do not innervate respiratory motor nuclei.<b>SIGNIFICANCE STATEMENT</b> Serotonin (5-HT) neurons modulate physiological processes and behaviors as diverse as body temperature, respiration, aggression, and mood. Using genetic tools, we characterize a 5-HT neuron subtype defined by expression of <i>Tachykinin1</i> and <i>Pet1</i> (<i>Tac1-Pet1</i> 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 CO₂<i>Tac1-Pet1</i> neurons thus appear distinct from and contrast previously described <i>Egr2-Pet1</i> neurons, which project primarily to chemosensory integration centers and are themselves chemosensitive.