ROLE OF 5-HT3 RECEPTORS IN PERISTALTIC REFLEX ELICITED BY STROKING THE MUCOSA IN THE CANINE JEJUNUM

1. The role played by the 5-HT3 receptor, a serotonin subtype receptor , in peristaltic reflexes was studied in dogs first given ketamine, th en anaesthetized with urethane (1.0 g kg-1, i.v.) and alpha-chloralose (100 mg kg-1, i.v.). The jejunal loop was partitioned into two segmen ts with respect to blood supply. Drugs were infused intra-arterially i nto each segment. 2. Stroking of the mucosa of the aboral and oral seg ments elicited an ascending contraction and a descending relaxation, r espectively. 3. The ascending contraction was concentration-dependentl y inhibited by treatment of the aboral segment with the 5-HT3 receptor antagonists ICS 205-930 and ondansetron (1.4 pmol min-1 to 14 nmol mi n-1 for both). The maximal inhibition was 49.5 and 69.3 %, respectivel y. The response was not affected by treatment of the oral segment with these drugs. The descending relaxation was inhibited by 51.4 and 60.8 %, respectively, by treatment of the oral segment with ICS 205-930 an d ondansetron (1.4 nmol min-1 for both). 4. The ascending contraction was markedly inhibited by treatment of either segment with hexamethoni um (140 nmol min-1). The response was abolished by treating both segme nts with hexamethonium and by treating the oral segment with atropine (14 nmol min-1). 5. These results suggest firstly that, in the canine jejunum, enteric neurons with 5-HT3 receptors play a role as sensory n eurons or interneurons in the ascending excitatory and the descending inhibitory pathways of the peristaltic reflex elicited by stroking the mucosa, and secondly, that the ascending limb is composed of choliner gic interneurons and motoneurons.