Axon reflexes evoked by transient receptor potential vanilloid 1 activation are mediated by tetrodotoxin-resistant voltage-gated Na+ channels in intestinal afferent nerves

Capsaicin-sensitive nerves mediate axon vasodilator reflexes in the intestine, but the ion channels underlying action potential (AP) propagation are poorly understood. To examine the role of voltage-gated Na(+) channels underlying these reflexes, we measured vasomotor and electrophysiological responses elicited by capsaicin in guinea pig and mouse dorsal root ganglia (DRG) neurons, submucosal arterioles, and mesenteric arteries in vitro. Transient receptor potential vanilloid 1 (TRPV1) agonists dilated guinea pig ileal submucosal arterioles and were blocked by capsazepine and ruthenium red. In double-chamber baths, capsaicin-evoked activation of TRPV1 on proximal perivascular nerves in the left chamber evoked dilations of the distal segment of the submucosal arteriole in the right chamber. Dilations were tetrodotoxin (TTX) (1 microM)-resistant, but reducing extracellular Na(+) (10% solution) or applying the Na(v) 1.8 antagonist A-803467 [5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide] (1 microM) in the proximal chamber blocked capsaicin-evoked dilations in the distal chamber (88%; P = 0.01 and 75% and P < 0.02, respectively). In mouse mesenteric arteries, electrical field stimulation and capsaicin (2 microM) evoked dilations that were also TTX-resistant. In perforated patch-clamp recordings, APs in mouse and guinea pig capsaicin-sensitive DRG neurons were TTX-resistant but blocked by 10% extracellular Na(+). When capsaicin-evoked AP conduction was studied in in vitro ileal multiunit afferent nerve preparations, capsaicin responses were elicited in the presence of TTX, whereas distention-evoked responses were almost completely blocked by TTX. Together, these data provide evidence for TTX-resistant AP conduction in extrinsic sensory neurons that innervate guinea pig and mouse intestine and suggest this neural propagation is sufficient to mediate axon reflexes in the intestine.