Opiate regulation of maternal behavior in the rat

The effects of the opiate agonist morphine, the opiate antagonist naloxone and the weak opiate nonanalgesic dextrorphan on the expression of maternal behavior were investigated in a series of three experiments. In the first experiment treatment of rats with morphine (5 mg/kg, subcutaneously) after ovariectomy and hysterectomy on day 17 of gestation resulted in a disruption in the onset and quality of maternal responsiveness in the homecage and in a T-maze test. The duration of morphine's acute disruptive action was 2-4 hours. In the second experiment concurrent treatment of morphine-injected rats with naloxone prevented the disruptive effects of morphine in both the homecage and T-maze tests. The effects of morphine did not appear to result from a severe alteration in activity levels as measured in an open-field test, although morphine did increase activity slightly by the fifth day of treatment. In the third experiment treatment of rats after ovariectomy plus hysterectomy on day 17 of gestation with dextrorphan failed to disrupt maternal behavior. These results indicate that morphine disrupts maternal behavior through an opiate receptor mechanism, and suggests to us that endogenous opiates may mediate the expression of maternal behavior under certain physiological conditions.