Social behavior of juvenile rats after in utero exposure to morphine: dose-time-effect relationship

In the present study, the effects of morphine exposure in utero on social b ehavior in juvenile male rats was investigated. Pinning, a measure for play behavior, and social grooming of the offspring were measured at postnatal day 21. The subjects were offspring of Wistar rat dams given sc, injections of 1 or 10 mg/kg body weight morphine HCl daily from gestational days 8 (G D8)-GD 21 and control dams injected daily with saline. Pinning and social g rooming of the morphine-treated offspring were significantly elevated compa red to saline controls. The doses of morphine used neither affected the ges tation of pregnant mother rats nor sensorimotor development of the juvenile rats. Prenatal exposure to morphine of 10 mg/kg daily increased both pinni ng and social grooming, prenatal exposure to a lower dose of 1 mg/kg increa sed pinning behavior but not social grooming in the offspring. To study the importance of the gestational period, offspring of dams given 10 mg/kg bod y weight morphine HCl from GD8-GD15 and saline from GD16-parturition or mor phine from GD16-parturtion and saline from GD8-GD15 was tested. Pinning was only increased when morphine exposure occurred during the third week of ge station, social grooming was increased when morphine exposure had been in t he second week of gestation. Subcutaneous administration of 1 mg/kg naltrex one 1 h before the test significantly decreased play behavior in control ra ts, but not in animals prenatally exposed to morphine. From these experimen ts we conclude that the long term effect of in utero exposure to morphine o n play behavior is established by affecting the endogenous opioid system. ( C) 1999 Elsevier Science Ltd. All rights reserved.