Sphingosine-1-phosphate selectively activates vagal afferent C-fiber subtype in guinea pig esophagus

Capsaicin-Sensitive Vagal Afferent Nerve-Mediated Interoceptive Signals in the Esophagus

Heartburn and non-cardiac chest pain are the predominant symptoms in many esophageal disorders, such as gastroesophageal reflux disease (GERD), non-erosive reflux disease (NERD), functional heartburn and chest pain, and eosinophilic esophagitis (EoE). At present, neuronal mechanisms underlying the process of interoceptive signals in the esophagus are still less clear. Noxious stimuli can activate a subpopulation of primary afferent neurons at their nerve terminals in the esophagus. The evoked action potentials are transmitted through both the spinal and vagal pathways to their central terminals, which synapse with the neurons in the central nervous system to induce esophageal nociception. Over the last few decades, progress has been made in our understanding on the peripheral and central neuronal mechanisms of esophageal nociception. In this review, we focus on the roles of capsaicin-sensitive vagal primary afferent nodose and jugular C-fiber neurons in processing nociceptive signals in the esophagus. We briefly compare their distinctive phenotypic features and functional responses to mechanical and chemical stimulations in the esophagus. Then, we summarize activation and/or sensitization effects of acid, inflammatory cells (eosinophils and mast cells), and mediators (ATP, 5-HT, bradykinin, adenosine, S1P) on these two nociceptive C-fiber subtypes. Lastly, we discuss the potential roles of capsaicin-sensitive esophageal afferent nerves in processing esophageal sensation and nociception. A better knowledge of the mechanism of nociceptive signal processes in primary afferent nerves in the esophagus will help to develop novel treatment approaches to relieve esophageal nociceptive symptoms, especially those that are refractory to proton pump inhibitors.

Sphingosine-1-phosphate activates mouse vagal airway afferent C-fibres via S1PR3 receptors

KEY POINTS

Sphingosine-1-phosphate (S1P) strongly activates mouse vagal C-fibres in the airways. Airway-specific nodose and jugular C-fibre neurons express mRNA coding for the S1P receptor S1PR3. S1P activation of nodose C-fibres is inhibited by a S1PR3 antagonist. S1P activation of nodose C-fibres does not occur in S1PR3 knockout mice.

ABSTRACT

We evaluated the effect of sphingosine-1-phosphate (S1P), a lipid that is elevated during airway inflammatory conditions like asthma, for its ability to stimulate vagal afferent C-fibres in mouse lungs. Single cell RT-PCR on lung-specific vagal afferent neurons revealed that both TRPV1-expressing and TRPV1-non-expressing nodose neurons express mRNA coding for the S1P receptor S1PR3. TRPV1-expressing airway-specific jugular ganglion neurons also express S1PR3 mRNA. S1PR1 and S1PR2 mRNAs were also found to be expressed but only in a limited subset (32% and 22%, respectively) of airway-specific vagal sensory neurons; whereas S1PR4 and S1PR5 were rarely expressed. We used large scale two-photon imaging of the nodose ganglia from our ex vivo preparation isolated from Pirt-Cre;R26-GCaMP6s transgenic mice, which allows for simultaneous monitoring of calcium transients in ∼1000 neuronal cell bodies in the ganglia during tracheal perfusion with S1P (10 μM). We found that S1P in the lungs strongly activated 81.5% of nodose fibres, 70% of which were also activated by capsaicin. Single fibre electrophysiological recordings confirmed that S1P evoked action potential (AP) generation in a concentration-dependent manner (0.1-10 μM). Action potential generation by S1P in nodose C-fibres was effectively inhibited by the S1PR3 antagonist TY 52156 (10 μM). Finally, in S1PR3 knockout mice, S1P was not able to activate any of the airway nodose C-fibres analysed. These results support the hypothesis that S1P may play a role in evoking C-fibre-mediated airway sensations and reflexes that are associated with airway inflammatory diseases.