5-HT activates neural reflexes regulating secretion in the guinea-pig colon

The role of 5-hydroxytryptamine (5-HT) in neural reflexes regulating secretion was examined in muscle-stripped segments of guinea-pig colon set up in modified flux chambers. A 15-microL pulse of 5-HT (100 microM) to the mucosal bath (1.5 mL), which was continuously perfused, evoked an increase in short-circuit current (Isc). The 5-HT-induced increase in Isc was inhibited by tetrodotoxin, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP), GR82334 and atropine, but not by tropisetron. 5-HTP-DP reduced the response to a 5-HT pulse over the concentration range of 1 nM to 1 microM. The Isc response to a 5-HT pulse was unaffected by the cyclooxygenase inhibitor, piroxicam. This contrasted with a reduction in the Isc response to mucosal stroking with a brush by piroxicam. The results suggest that a 5-HT pulse, like mucosal stroking, activates a secretory reflex that includes tachykinin and cholinergic neurons but, unlike mucosal stroking, does not release prostaglandins.