Characterization of the muscarinic and serotoninergic receptors of the intestine of the rainbow trout (Salmo gairdneri)

The ability of carbachol and 5-hydroxytryptamine (5-HT) to contract isolated segments of rainbow trout intestine in a concentration-dependent manner indicates the presence of muscarinic and serotoninergic receptors in this tissue. The activity of these agonists appears to be directly on the smooth muscle, since ganglionic blockers and inhibitors of neurotransmission did not inhibit contractions. The carbachol-induced contractions were selectively inhibited by atropine and (±)-3-quinuclidinyl xanthene-9-carboxylate hemioxalate hydrate, an M-2 muscarinic receptor antagonist. However, the inhibition was not competitive. McN-A-343, an M-1 muscarinic agonist had no effect on intrinsic tone. The 5-HT-induced contractions were selectively inhibited by methysergide and the 5-HT2 receptor blockers, ketanserin and 1-(1-naphthyl)piperazine. Again, the inhibition by these agents was not competitive. 5-HT1 and 5-HT3 receptor antagonists did not inhibit contractions. The results thus suggest that the smooth muscle of the rainbow trout intestine contains M-2 muscarinic and 5-HT2 receptors.Key words: smooth muscle, gastrointestinal tract, trout, muscarinic receptors, serotonin.