Desensitization of the peristaltic reflex induced by mucosal stimulation with the selective 5-HT4agonist tegaserod

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Abstract
The intestinal peristaltic reflex induced by mucosal stimulation is mediated by mucosal release of serotonin (5-HT), which acts on 5-HT4receptors located on CGRP-containing afferent nerve terminals. Exposure of the colonic mucosa to the 5-HT4receptor agonist tegaserod in the range of 1 nM to 10 μM elicits a peristaltic reflex and stimulates colonic propulsion. The present study was designed to identify the 5-HT4receptor subtype mediating the reflex and determine whether functionally effective concentrations of tegaserod desensitize the reflex induced by mucosal stimulation. Exposure of rat colonic mucosa to tegaserod in the range of 5 nM to 5 μM for 5 or 10 min caused rapid time- and concentration-dependent desensitization of the peristaltic reflex induced by mucosal stroking, consistent with the operation of a rapidly desensitizing 5-HT4breceptor subtype. Desensitization was accompanied by a decrease in CGRP release. The rate of recovery of peristaltic response depended on the desensitizing concentration of tegaserod: ascending contraction and descending relaxation recovered within 15 min after 5–50 nM tegaserod, 30 min after 0.5 μM, and 60 min after 5 μM. Neither CGRP release nor the peristaltic reflex induced by muscle stretch was affected by 5-HT4receptor desensitization, providing further evidence that 5-HT does not mediate the reflex induced by muscle stretch. These results suggest in cases of increased 5-HT availability or prolonged exposure, such as colitis, that it is likely the peristaltic reflex will be blunted.