Neonatal naltrexone treatment: effects on sexual and exploratory behavior in male and female rats

The effect of neonatal naltrexone treatment (100 micrograms SC from day 1 to day 10) on copulatory and exploratory behavior in male and female rats was studied. In the female, neonatal naltrexone treatment enhanced copulatory (lordosis response) and exploratory behavior. An altered response to morphine was obtained; the effect of morphine on copulatory behavior was diminished while morphine's effect on exploratory activity was potentiated. The neonatal naltrexone treatment did not cause analogous effects in copulatory or exploratory behavior in the male rat. These data suggest that opioid mechanisms involved in the female copulatory and exploratory activity are established perinatally and can be influenced by early exposure to an opioid antagonist. It is concluded that there exist sex differences in this respect, both as to the sex-typical copulatory behavior and as to exploratory activity.