Characterization of opioid dependence of isolated ileum of morphine dependent and of chronically stressed Wistar-Kyoto (WKY)-rats.

The opioid antagonist naloxone induced tonic contraction ("gut dependence") of isolated ilia of both morphine dependent WKY-rats and WKY-rats chronically stressed by immobilization. Morphine and met-enkephalin but neither dynorphine nor leu-enkephalin antagonized the naloxone induced "gut dependence" of isolated ilia obtained from morphine dependent WKY-rats. Met-enkephalin as well as dynorphine inhibited the "gut dependence" of chronically stressed rats; morphine and leu-enkephalin had no inhibitory effect. These differences between influences of different opioid peptides on "gut dependence" of morphine dependent rats versus "gut dependence" of rats chronically stressed by immobilization suggest that chronic stress induced or facilitated production of a different population of receptors.