Sensitivity changes after morphine treatment in the mouse uterus.

The addition of morphine to a bath containing the vas deferens from chronically morphinized mice induces a facilitatory effect on noradrenaline contractile responses. This facilitatory effect of morphine has been thought to be a dependence-like effect. In the present work the possibility that a pretreatment with morphine might induce a similar effect on the contractile responses of the mouse uterus to acetylcholine and serotonin was examined. The acute effect of morphine on the uteri of untreated mice consisted in an attenuation of the responses to both substances, whereas a long term pretreatment with morphine induced a supersensitivity state. Tolerance to the depressant action of morphine on the contractile responses induced by the stimulating compounds was also observed. Acute morphine in the uteri from morphinized mice did not induce a facilitatory effect on acetylcholine or serotonin responses. Naloxone did not modify the effects of morphine in the naive or chronically treated mice. The supersensitivity state and the intensity of tolerance were unaffected by changes in the concentration of calcium in the bath medium. Caffeine or theophylline decreased the tolerance observed in the uterus from chronically morphinized mice. The attenuation of tolerance suggests that methylxanthines induce effects opposed to those of chronic morphine in the calcium distribution within the cell.