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The effects of intra-arterially (i.a.) injected morphine and dextromoramide on 5-hydroxytryptamine (5-HT)-induced contrac tion of the jejunum of dog were tested. Dextromoramide increased the intestinal contractile action of 5-HT from 44.7 ± 8.8 cm H₂·to 154.6 ± 17.3 cm H₂O. This effect was reversible in about 15 min. The tryptamine antagonists LSD, BOL, dibenamine, and cyproheptadine failed in blocking 5-HT and the narcotic analgesics. Atropine (50 µg/kg i.a.), however, was active in this respect. We propose that 5-HT released by morphine acts on tryptamine receptors located on the ganglionar cells and is not blocked by LSD. Possibly morphine inhibits also the uptake of 5-HT by the myenteric plexus which would explain the potentiation of exogenous 5-HT.

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