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

Abstract

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Past questions, present results, future perspectives: Interacting chemosensitive areas and the hypercapnic ventilatory response

“…In this issue of The Journal of Physiology , Shi et al . (2022) revisit a previous hypothesis that raphe‐derived serotonin (5‐HT) drives the chemosensitivity of RTN neurons through the activation of 5‐HT 7 receptors (5‐HT 7 R). This possibility was proposed by Wu et al .…”

“…Finally, it was demonstrated that the bilateral 5-HT 7 R antagonism in the parafacial region did not modify the hypercapnic ventilatory response in freely behaving mice, although microinjections of a 5-HT 7 R agonist in the same area stimulated breathing. The data provided by Shi et al (2022) showing that the RTN chemosensitivity and the hypercapnic ventilatory response do not require the activation of 5-HT 7 R in the parafacial region are compelling and in contrast to the observations reported by Wu et al (2019). Both studies show scientific quality and relevance; however, the work by Shi et al (2022) highlights the importance of revisiting and confirming critical questions further using refined and more accurate methodologies.…”

“…Combining experiments in brain slices and cell culture containing RTN and raphe neurons, Wu et al (2019) reported that the response to acidosis was substantially reduced in a subset of Phox2b-expressing RTN neurons after blocking the 5-HT synthesis or antagonizing 5-HT 7 R. It was also noted that the systemic administration of ketanserin (a 5-HT 2/7 receptor antagonist) reduced the hypercapnic ventilatory response in vivo, speculating that raphe serotoninergic inputs would be necessary to express the RTN chemosensitivity fully. Shi et al (2022) explored this possibility further. First, three methods were used to evaluate the 5-HT 7 R expression in the parafacial region thoroughly: single-cell RNA sequencing, single-cell qPCR and fluorescence in situ hybridization.…”

“…The main findings were two-fold: (1) 5-HT 7 R transcripts were scarcely detected in Nmb-positive RTN chemosensitive cells; when present, the 5-HT 7 R mRNA expression was either significantly lower than the GPR4 and TASK-2 transcripts or found dorsomedial to the cluster of RTN neurons -a region similar to where the 5-HT 7 R-sensitive cells were recorded by Wu et al (2019); (2) the neurons of the parafacial region containing detectable levels of 5-HT 7 R were either Phox2b-negative or Phox2b-positive and Nmb-negative and, hence, were not RTN neurons. In another set of experiments, using brain slices of wild-type or reporter mice expressing a fluorescent protein in Phox2b neurons, Shi et al (2022) identified RTN chemosensitive neurons in which the excitatory responses to either hypercapnia or exogenous 5-HT were unaltered by the 5-HT 7 R antagonism, showing a parallel between the absence of mRNA expression and lack of receptor function. Finally, it was demonstrated that the bilateral 5-HT 7 R antagonism in the parafacial region did not modify the hypercapnic ventilatory response in freely behaving mice, although microinjections of a 5-HT 7 R agonist in the same area stimulated breathing.…”

“…Shi et al . (2022) explored this possibility further. First, three methods were used to evaluate the 5‐HT 7 R expression in the parafacial region thoroughly: single‐cell RNA sequencing, single‐cell qPCR and fluorescence in situ hybridization.…”