Antagonism of the gut-contracting effects of bombesin and neurotensin by opioid peptides, morphine, atropine or tetrodotoxin.

Morphine, met-enkephalin, beta-endorphin, tetrodotoxin (TTX), and atropine antagonized the gut-contracting effects of the peptides neurotensin and bombesin. The opioids and TTX shifted the concentration-response curves to the right and mostly depressed the maximum response to the agonists; atropine caused only depression of the maximum. Morphine was more potent than the opioid peptides. Naloxone did not modify the effects of neurotensin and bombesin. However, it completely abolished the antagonistic effects of the opioids, but not that of atropine. In conclusion, neurotensin and bombesin stimulate the intramural neurons via a process that is inhibited by the activation of opioid receptors.