Changes in the sensitivity of smooth muscle to neurotransmitter during acute morphine treatment and its association with analgesia

Acute in vivo administration of morphine to mice produced dose- and time-dependent subsensitivity to acetylcholine in ileum and supersensitivity to norepinephrine in vas deferens. These responses were specific to the agonist as potassium chloride-induced responses were not altered. A correlation was observed between the analgesia and the sensitivity changes produced by morphine. The sensitivity changes were mediated through opiate receptors as naloxone antagonised the effect of morphine.