Relationship between serotonin and tryptamine receptors in the rat stomach fundus

Tryptamine and serotonin (5-HT) are relatively potent contractile agonists in the rat fundus, a tissue in which contraction to 5-HT is not mediated by interaction with 5-HT1 or 5-HT2 receptors. The identification of [3H]tryptamine binding sites in the brain and fundus that show high affinity for certain beta-carbolines raised the possibility that 5-HT and tryptamine may be interacting with a similar receptor that is best described as a tryptaminergic receptor in the fundus. The affinity of five 5-HT receptor antagonists, ketanserin, metergoline, 1-(1-naphthyl)piperazine, LY154930 and LY175041 was similar when 5-HT or tryptamine was the agonist, indicating that 5-HT and tryptamine are interacting with the same receptor in the fundus. Furthermore, maximum contractile response to both 5-HT and tryptamine was reduced to the same extent by the calcium channel blocker, diltiazem, and by the calmodulin inhibitor, trifluoperazine. Inasmuch as diltiazem and trifluoperazine did not similarly inhibit contraction to agents interacting with other receptors (i.e., carbamylcholine), these data are consistent with the contention that 5-HT and tryptamine are interacting with the same receptor in the fundus. Consistent with this conclusion is the observation that affinity of the beta-carbolines, harmaline and harmine was also similar when tryptamine or 5-HT was used as the agonist. However, affinity of the beta-carbolines for the tryptamine/5-HT receptor in the fundus was dramatically lower than reported for [3H]tryptamine binding sites in brain membranes.(ABSTRACT TRUNCATED AT 250 WORDS)