Opioid regulation of parental behavior in juvenile rats.

When exposed to young rats for a period of days, juvenile rats will respond with full parental behavior (FPB: retrieval and grouping of, and crouching over, pups). Because the parental behavior of juveniles is so robust, and because opiates have been shown to be involved in the regulation of parental behavior in adult animals, we examined morphine's ability to disrupt the display of parental behavior in the juvenile animal. In Experiment 1, 25-day-old males and females were administered one of three injection regimens of morphine (MOR), saline (SAL), or naloxone (NAL)--[MOR (5.0 mg/kg) + SAL; MOR + NAL (0.5 mg/kg); or SAL + SAL]--and 1 h later were exposed to three 1- to 6-day-old neonates. Behavior was scored over a 60-min period and animals were considered parental if they responded with FPB for 2 consecutive testing days. Whereas the SAL + SAL and MOR + NAL were not different, MOR + SAL virtually failed to respond to young over a 10-day period. Experiment 2 exposed juveniles to young for a period of days until they displayed 2 consecutive days of FPB. Next, separate groups of juveniles were treated with the same regimen as above. Again, SAL + SAL and MOR + NAL responded rapidly to young, whereas MOR + SAL did not. These data suggest that the display of parental behavior, both its onset (Experiment 1) and maintenance (Experiment 2), appears to be regulated by opiates.