Tetrodotoxin-resistant voltage-dependent sodium channels in identified muscle afferent neurons

Muscle afferents are critical regulators of motor function (Group I and II) and cardiovascular responses to exercise (Group III and IV). However, little is known regarding the expressed voltage-dependent ion channels. We identified muscle afferent neurons in dorsal root ganglia (DRGs), using retrograde labeling to examine voltage-dependent sodium (Na V ) channels. In patch-clamp recordings, we found that the dominant Na V current in the majority of identified neurons was insensitive to tetrodotoxin (TTX-R), with Na V current in only a few (14%) neurons showing substantial (>50%) TTX sensitivity (TTX-S). The TTX-R current was sensitive to a Na V 1.8 channel blocker, A803467. Immunocytochemistry demonstrated labeling of muscle afferent neurons by a Na V 1.8 antibody, which further supported expression of these channels. A portion of the TTX-R Na V current appeared to be noninactivating during our 25-ms voltage steps, which suggested activity of Na V 1.9 channels. The majority of the noninactivating current was insensitive to A803467 but sensitive to extracellular sodium. Immunocytochemistry showed labeling of muscle afferent neurons by a Na V 1.9 channel antibody, which supports expression of these channels. Further examination of the muscle afferent neurons showed that functional TTX-S channels were expressed, but were largely inactivated at physiological membrane potentials. Immunocytochemistry showed expression of the TTX-S channels Na V 1.6 and Na V 1.7 but not Na V 1.1. Na V 1.8 and Na V 1.9 appear to be the dominant functional sodium channels in small- to medium-diameter muscle afferent neurons. The expression of these channels is consistent with the identification of these neurons as Group III and IV, which mediate the exercise pressor reflex.