Transient receptor potential ion channel of the vanilloid type 1 (TRPV1)-dependent pathway, consisting of capsaicin-sensitive tachykininergic primary afferent and myenteric nitrergic neurons, has been suggested to mediate the inhibitory effect of capsaicin on vagally mediated striated muscle contractions in the rat esophagus. In a recent study, similar but also different effects of capsaicin and piperine on TRPV1 were demonstrated. Therefore, this study aimed to compare the effects of these two drugs on vagally induced contractions in the mouse esophagus. Capsaicin and piperine inhibited vagally induced contractions of a thoracic esophageal segment in a concentration-dependent manner. Ruthenium red (10 μM; a non-selective blocker of transient receptor potential cation channels) and SB-366791 (10 μM; a novel selective antagonist of TRPV1) blocked the inhibitory effect of capsaicin but not that of piperine. Piperine inhibited the vagally mediated contractions in esophagi of adult mice neonatally injected with capsaicin, while capsaicin failed to do so. Desensitization of TRPV1 in the mouse esophagus by in vitro pretreatment with capsaicin failed to affect the inhibitory effect of piperine, whereas the piperine effect was cross-desensitized by capsaicin pretreatment in rat and hamster esophagi. Additionally, a tachykinin NK1 receptor antagonist, L-732,138 (1 μM), as well as a nitric oxide synthase inhibitor, NG-nitro-l-arginine methyl ester (l-NAME 200 μM), blocked the inhibitory effect of capsaicin but not that of piperine. Taken together, the results suggest that piperine inhibits the vagally mediated striated muscle contraction in the mouse esophagus through its action on a TRPV1-dependent pathway as well as a TRPV1-independent site.
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