[Differences in the action of enkephalins, beta-endorphin, and morphine on spontaneous rhythmic movements of the isolated rabbit ileum].

Methionine-, leucine-enkephalin and beta-endorphin produced dose-related depression followed, not regularly, by stimulation of the spontaneous rhythmic activity of rabbit isolated ileum. The stimulant effect was obtained in one third to one half of the experiments. Naloxone antagonized the depressant, but not the stimulant effects of enkephalins and beta-endorphin. Morphine depressed or stimulated the spontaneous rhythmic activity of rabbit isolated ileum, but only when this narcotic analgesic was used in extremely large amounts. Naloxone had no effect or potentiated the depressant effect, while it potentiated the stimulant effect of morphine. It is apparent, therefore, that only enkephalins and beta-endorphin depressed the spontaneous rhythmic activity of rabbit isolated ileum by acting on enkephalinergic receptors. Further, these results suggest that these enkephalinergic receptors may be involved in the modulation or transmission of spontaneous rhythmic activity of rabbit isolated ileum.