Inhibitory effect of capsaicin on the ascending pathway of the guinea-pig ileum and antagonism of this effect by ruthenium red.

A segment of guinea-pig ileum, which was continuous with a strip of longitudinal muscle-myenteric plexus (LM-MP) at the anal end, was used to examine the effect of capsaicin on ascending excitatory pathways. Electrical field stimulation of the LM-MP caused an ascending contraction of the segment. After initially causing contraction capsaicin (3 microM) inhibited the ascending contraction. This inhibitory effect of capsaicin exhibited rapid desensitization and was abolished after extrinsic (mesenteric) denervation. Desensitization to calcitonin gene-related peptide (CGRP) prevented the capsaicin-induced inhibition without affecting the ascending contraction. Neither naloxone nor alpha- and beta-adrenoceptor antagonists affected the capsaicin-induced inhibition. CGRP (25 nM) also inhibited the ascending contraction, mimicking the inhibition induced by capsaicin. Ruthenium red (0.1-3 microM) antagonized the capsaicin-induced inhibition in a concentration-related manner, but did not affect the CGRP-induced inhibition. These findings suggest that the inhibitory effect of capsaicin on the ascending pathways might be mediated via the release of CGRP from extrinsic nerve terminals, and that the site of the antagonism of the action of capsaicin by ruthenium red is prejunctional.