Prenatal exposure to morphine feminizes male sexual behavior in the adult rat.

The endogenous opiate system plays a role in fetal sexual differentiation during development. We examined long-term effects of prenatal morphine on adult sexual behavior in male rats. Pregnant Fischer 344 rats were given increasing doses of morphine (0.75-12.0 mg/day) in slow-release emulsion during gestational days 12-18. Control rats were injected with vehicle and were either pair-fed with morphine rats or ad lib fed. At birth, all litters were culled to eight pups and fostered to naive dams. Testing began when rats were 10-12 weeks old. Masculine behavior was assessed using receptive stimulus females and recording instances of mount, intromission, and ejaculation. Feminine receptivity of the male rats was assessed following castration and priming with ovarian hormones; lordosis quotient of the experimental males was recorded using stimulus male studs. Males prenatally exposed to morphine exhibited normal rates of male copulatory behavior but a significantly higher lordosis quotient, suggesting that prenatal morphine induced long-lasting feminizing effects.