The development of acute tolerance to analgesia and the sensitivity changes in mouse vas deferens and ileum produced by morphine.

Repeated administration of morphine at 4 hr interval resulted in the development of tolerance to morphine-induced subsensitivity to acetylcholine in ileum and supersensitivity to norepinephrine in vas deferens of mouse. This was in parallel with analgesic tolerance to morphine. Pretreatment with naloxone antagonized the tolerance development to both morphine-induced analgesia and sensitivity changes, indicating a role for opiate receptors in this phenomenon.