Calcitonin gene-related peptide facilitates serotonin release from guinea-pig colonic mucosa via myenteric neurons and tachykinin NK2/NK3 receptors

I The ability of calcitonin gene-related peptide (CGRP), to alter the outflow of 5-hydroxytryptamine (5-HT) from the guinea-pig proximal colon, was evaluated using three different isolated preparations: whole colon, mucosa-free muscle layer and submucosa/mucosa preparations. 2 In the presence of the monoamine oxidase A inhibitor, clorgyline, CGRP elicited a concentration-dependent increase in 5-HT outflow from the whole colon, but not from mucosa-free muscle layer preparations. The CGRP-evoked 5-HT outflow was sensitive to tetrodotoxin (TTX) or hexamethonium, but was not detectable in submucosa/mucosa preparations. HCGRP(8-37) (3 muM) inhibited the submaximal effect of CGRP on the 5-HT outflow. [Cys(ACM)(2,7)]hCGRP had a slight stimulant influence on the 5-HT outflow. 3 The selective NK2 and NK3 receptor antagonists, SR48968 or SR142801, respectively, prevented the enhancing effect of CGRP. By contrast, a selective NK1 receptor antagonist L703606, failed to block the effect of CGRP. 4 The enhancing effect of CGRP was mimicked by the NK2 receptor agonist [beta-Ala(8)]-neurokinin A (NKA)(4-10) and the NK3 receptor agonist senktide. The effect of [beta-Ala(8)]-NKA(4-10) on the 5-HT outflow was unaffected by TTX, while the effect of senktide was prevented by TTX, hexamethonium or SR48968. The present data also demonstrated a synergistic action of the NK2 and NK3 receptor agonists on the CGRP-evoked 5-HT outflow. 5 We concluded that CGRP facilitates 5-HT release from the guinea-pig colonic mucosa through an action on myenteric neurons and that this effect is mediated by endogenously released tachykinins, acting via tachykinin NK2/NK3 receptors in cascade.