5‐HT7 receptors expressed in the mouse parafacial region are not required for respiratory chemosensitivity

Abstract

A brainstem homeostatic system senses CO2/H+ to regulate ventilation, blood gases and acid‐base balance. Neurons of the retrotrapezoid nucleus (RTN) and medullary raphe are both implicated in this mechanism as respiratory chemosensors, but recent pharmacological work suggested that the CO2/H+‐sensitivity of RTN neurons is mediated indirectly – by raphe‐derived serotonin acting on 5‐HT7 receptors. To investigate this further, we characterized Htr7 transcript expression in phenotypically identified RTN neurons using multiplex single cell qRT‐PCR and RNAscope. Although present in multiple neurons in the parafacial region of the ventrolateral medulla, Htr7 expression was undetectable in most RTN neurons (Nmb+/Phox2b+) concentrated in the densely packed cell group ventrolateral to the facial nucleus. Where detected, Htr7 expression was modest and often associated with RTN neurons that extend dorsolaterally to partially encircle the facial nucleus. These more dorsolateral Nmb+/Htr7+ neurons tended to express high levels of Nmb and low levels of the intrinsic RTN proton detectors, Gpr4 and Kcnk5. In mouse brainstem slices, CO2‐stimulated firing in RTN neurons was mostly unaffected by a 5‐HT7 receptor antagonist, SB269970 (n=11/12). At the whole animal level, microinjection of SB269970 into the RTN of conscious mice blocked respiratory stimulation by co‐injected LP‐44, a 5‐HT7 receptor agonist, but had no effect on CO2‐stimulated breathing in those same mice. We conclude that Htr7 is expressed by a minor subset of RTN neurons with a molecular profile distinct from the established chemoreceptors and that 5‐HT7 receptors have negligible effects on CO2‐evoked firing activity in RTN neurons or on CO2‐stimulated breathing in mice.