Voltage-gated sodium channels mediating conduction in vagal motor fibers innervating the esophageal striated muscle

The vagal motor fibers innervating the esophageal striated muscle are essential for esophageal motility including swallowing and vomiting. However, it is unknown which subtypes of voltage-gated sodium channels (NaV1s) regulate action potential conduction in these efferent nerve fibers. The information on the NaV1s subtypes is necessary for understanding their potential side effects on upper gut, as novel inhibitors of NaV1s are developed for treatment of pain. We used isolated superfused (35 °C) vagally-innervated mouse esophagus striated muscle preparation (mucosa removed) to measure isometric contractions of circular striated muscle evoked by electrical stimulation of the vagus nerve. NaV1 inhibitors were applied to the de-sheathed segment of the vagus nerve. Tetrodotoxin (TTX) applied to the vagus nerve completely abolished electrically evoked contractions. The selective NaV1.7 inhibitor PF-05089771 alone partially inhibited contractions and caused a >3-fold rightward shift in the TTX concentration-inhibition curve. The NaV1.1, NaV1.2 and NaV1.3 group inhibitor ICA-121431 failed to inhibit contractions, or to alter TTX concentration-inhibition curves in the absence or in the presence of PF-05089771. RT-PCR indicated lack of NaV1.4 expression in nucleus ambiguus and dorsal motor nucleus of the vagus nerve, which contain motor and preganglionic neurons projecting to the esophagus. We conclude that the action potential conduction in the vagal motor fibers to the esophageal striated muscle in the mouse is mediated by TTX-sensitive voltage gated sodium channels including NaV1.7 and most probably NaV1.6. The role of NaV1.6 is supported by ruling out other TTX-sensitive NaV1s (NaV1.1-1.4) in the NaV1.7-independent conduction.