5-Hydroxytryptamine and narcotic-analgesics interactions in the intestine.

The effects of 5-hydroxytriptamine (5-HT), 5-HT blocking agents, morphine, narcotic-antagonists and ganglionic blocking agents were tested in the dog intestine by close intra-arterial injection. Morphine, as 5-HT, induced an immediate increase in the intestinal tonus followed by phasic contractions. When 5-HT was injected immediately after cessation of morphine induced phasic contractions, a significant potentiation of the 5-HT induced intestinal contraction could be observed. The potentiation could be demonstrated for other narcotic-analgesics like dextromoramide and it is specific for 5-HT. 5-HT blocking agents like LSD, BOL, and cyproheptadine did not block either 5-HT or morphine. However, bufotenidine, a neural tryptaminergic blocking agent, blocked the effects of both 5-HT and morphine. The effects of 5-HT and morphine upon intestinal motility were also diminished by ganglionic depolarizing agents such as nicotine and DMPP. This effect could, however, be prevented by the pretreatment with hexamethonium. These results seem to confirm the hypothesis of a 5-HT mediator role in the intestinal contractile effect induced by the narcotic-analgesics. On the other hand, narcotic-antagonists such as nalorphine and cyclazocine, not only lacked the 5-HT potentiation effect but also prevented the 5-HT potentiation induced by morphine. Cyclazocine also showed a long lasting 5-HT blocking effect. These results seem to show that the 5-HT potentiating effect of morphine in vivo is very specific and characteristic of the narcotics and thus could be implicated in some of their central effects.