Stimulus intensity-dependent recruitment of Na1 subunits in action potential initiation in nerve terminals of vagal C-fibers innervating the esophagus.

We investigated voltage-gated sodium channel (Na1) subunits that regulate action potential initiation in the nerve terminals of vagal nodose C-fibers innervating the esophagus. Extracellular single fiber recordings were made from the nodose C-fibers, with mechanically sensitive nerve terminals in the isolated innervated guinea pig esophagus. Na1 inhibitors were selectively delivered to the tissue-containing nerve terminals. Graded esophageal distention was used for mechanical stimulation. The Na1.7 inhibitor PF-05089771 nearly abolished action potential initiation in response to low levels of esophageal distention but only partially inhibited the response to higher levels of esophageal distention. The PF-05089771-insensitive component of the response progressively increased (up to ≈50%) with increasing esophageal distention and was abolished by tetrodotoxin (TTX). In addition to Na1.7, nodose C-fiber [transient receptor potential channel-vanilloid subfamily member 1 (TRPV1)-positive] neurons retrogradely labeled from the esophagus expressed mRNA for multiple TTX-sensitive Na1s. The group Na1.1, Na1.2, and Na1.3 inhibitor ICA-121431 inhibited but did not abolish the PF-05089771-insensitive component of the response to high level of esophageal distention. However, combination of ICA-121431 with compound 801, which also inhibits Na1.7 and Na1.6, nearly abolished the response to the high level of esophageal distention. Our data indicate that the action potential initiation in esophageal nodose C-fibers evoked by low (innocuous) levels of esophageal distention is mediated by Na1.7. However, the response evoked by higher (noxious) levels of esophageal distention has a progressively increasing Na1.7-independent component that involves multiple TTX-sensitive Na1s. The stimulus intensity-dependent recruitment of Na1s may offer novel opportunities for strategic targeting of Na1 subunits for inhibition of nociceptive signaling in visceral C-fibers. We report that pharmacologically distinguishable voltage-gated sodium channels (Na1) mediate action potential initiation at low (innocuous) versus high (noxious) intensity of esophageal distention in nerve terminals of vagal nodose C-fibers. Action potential initiation at low intensity is entirely dependent on Na1.7; however, additional tetrodotoxin (TTX)-sensitive Na1s are recruited at higher intensity of distention. This is the first demonstration that Na1s underlying action potential initiation in visceral C-fibers depend on the intensity of the stimulus.