Ji-Guang Jin

Possible involvement of calcitonin gene-related peptide (CGRP) in non-cholinergic non-adrenergic relaxation induced by mesenteric nerve stimulation in guinea pig ileum

Mesenteric nerve (MN) stimulation produced a biphasic response, i.e., a contraction followed by a prolonged relaxation in the isolated guinea pig ileum after complete adrenergic neuron blockade with guanethidine. This biphasic response mimicked the effect of capsaicin by itself. The latter relaxation response to MN stimulation was unaffected by hexamethonium, but was abolished by capsaicin in an irreversible fashion. Calcitonin gene-related peptide desensitization (CGRP-D) reduced the relaxation response to about 20% of the control. After pretreatment with atropine and guanethidine, MN stimulation provoked a pure relaxation, which was significantly reduced to about 20% of the control by CGRP-D. Therefore, it seems likely that the non-adrenergic non-cholinergic relaxation response to MN stimulation is partly mediated via a release of CGRP from sensory nerve endings.

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.

Ruthenium red antagonism of the effect of capsaicin on the motility of the isolated guinea-pig ileum

Ruthenium red (1 microM), an inorganic dye which blocks transmembrane calcium (Ca) fluxes in neural tissues, selectively reduced the capsaicin (1 microM)-induced contraction of the guinea-pig ileum and protected the sensory fibers from capsaicin-induced desensitization. The ruthenium red (0.5-1 microM) antagonism of capsaicin-induced inhibition of responses to mesenteric nerve stimulation or field stimulation in the isolated guinea-pig ileum was an example of a similar antagonism of the effect of capsaicin. In view of the known action of ruthenium red on the depolarization-coupled entry of Ca into synaptosomes and the release of transmitter, our results support the proposal that ruthenium red could antagonize the action of capsaicin on the peripheral terminals of sensory nerves by a similar mechanism, thereby suppressing transmitter release and preventing the establishment of desensitization.

Ruthenium red prevents capsaicin-induced neurotoxic action on sensory fibers of the guinea pig ileum

Capsaicin, a neurotoxin which impairs some primary afferent nerve fibers, was locally applied to the mesenteric nerve trunks of adult guinea pigs. The effect of capsaicin was studied on contractile responses of the ileum to mesenteric nerve stimulation (20 Hz). Exposure of the mesenteric nerve to capsaicin (1 microM) irreversibly abolished the contractile response to mesenteric nerve stimulation, without influencing the resting tension and spontaneous motility of the ileum. However, if the mesenteric nerve had been incubated in a Ca-free medium containing EGTA (0.1 mM) or pretreated with Ruthenium red (10 microM), capsaicin only partially inhibited (about 30%), but did not abolish, the contractile response to the mesenteric nerve stimulation, thus indicating protection from desensitization.

Myenteric 5-HT-containing neurones activate the descending cholinergic excitatory pathway to the circular muscle of guinea-pig ileum.

1 Participation of myenteric 5‐hydroxytryptamine (5‐HT)‐containing neurones in the ascending and descending pathways of the guinea‐pig isolated ileum was investigated in a new preparation. Transmural electrical stimulation of the longitudinal muscle‐myenteric plexus (LM‐MP) portion of the preparation caused ascending and descending contractions of circular or longitudinal muscle in the attached, intact segments situated orally or anally to the point of stimulation 2 All contractions to LM‐MP stimulation were abolished by tetrodotoxin (0.2 μm). The ascending and descending contractions of circular muscles were also abolished by atropine and inhibited to about 50% by hexamethonium. They were not affected by desensitization to substance P (SP) or by the SP antagonist, (D‐Pro2,D‐Trp7,9)‐substance P. The contractions of longitudinal muscles were inhibited by about 45% by hexamethonium and abolished by a combination of atropine with SP desensitization or the SP antagonist, (D‐Pro2,D‐Trp7,9)‐substance P. 3 Desensitization to 5‐HT, ICS 205–930 (1 μm) or cocaine (1 μm) reduced the descending contraction of circular muscle by 80–90%, without significantly affecting the ascending contraction. Methysergide (0.2 μm) failed to alter either contraction. 4 5‐HT desensitization, ICS 205–930 and cocaine only partially reduced the descending contraction of longitudinal muscle. A similar reduction of the ascending contraction (20–30%) was also observed. Methysergide had no effects on either contraction. 5 Contractions of either circular or longitudinal muscle produced by field stimulation of the intact segment were not significantly affected by any of the 5‐HT receptor antagonists tested. 6 The results imply that 5‐HT‐containing neurones, as interneurones, are involved mainly in the descending cholinergic excitatory pathway to the circular muscles.

Bile salt potentiates the action of capsaicin on sensory neurones of guinea-pig ileum

In the isolated guinea-pig ileum, exposure to the sensory stimulant drug capsaicin (1 microM) produced a contraction thought to involve substance P(SP) release from sensory nerves. Bile salt, sodium deoxycholate, potentiated the capsaicin-induced contraction in a concentration-dependent (0.03-10 microM) manner, without influencing contractions produced by exogenous SP or by electrical stimulation of efferent nerves. The bile salt-induced potentiation of the capsaicin response was not modified by hexamethonium or indomethacin. It was, however, abolished by concomitant incubation with Ruthenium Red, which was reported to block transmembrane calcium fluxes and then suppress the SP release from the capsaicin-sensitive sensory nerve terminals. We propose that bile salt, as a calcium ionophore, could activate or sensitize the action of capsaicin on the peripheral terminals of sensory nerves.

Contractions of the guinea-pig ileum evoked by stimulation of the submucous plexus

Neural pathways from the submucous plexus to the longitudinal muscle of an adjacent segment of isolated guinea-pig ileum were studied. It was found that electrical field stimulation of a strip of submucosa-submucous plexus produced frequency-dependent longitudinal contractions of an intact segment of intestine lying oral to the point of stimulation. The responses were reduced to less than 10% of control by tetrodotoxin, atropine, morphine and chymotrypsin and by desensitization to substance P (SP). The responses were only inhibited by one-third by hexamethonium and were not affected by desensitization to 5-hydroxytryptamine. The effect of desensitization to SP was reversible, but the effect of chymotrypsin was irreversible. SP-induced desensitization and chymotrypsin did not inhibit the twitch response produced by field stimulation of the whole ileal segment. The same results were observed with preparations made from ileal segments that had been extrinsically denervated. The results suggest that intrinsic neurons with processes in the submucous plexus can excite cholinergic and SP-containing neurons in the myenteric plexus, thereby causing the longitudinal muscle to contract.