Inhibitory effects of naltrexone on the induction of parental behavior in juvenile rats.

Juvenile rats are rapidly responsive to pups soon after weaning, displaying maternal-like behaviors such as licking, retrieving, grouping, and crouching over pups. As juveniles reach 30 days of age, they become less responsive to pups and show increased latencies to display the same parental behaviors. In light of previous data implicating opiates in the display of ongoing maternal behavior, we administered naltrexone, a long-acting opiate antagonist, beginning 5 and 9 days prior to and continuing throughout the period of behavioral testing, which started at 26 or 30 days of age. Male and female juveniles treated with 10 mg/kg of naltrexone SC for 9 days (days 21 to 29 of age) prior to and during behavioral testing (days 30 to 37) showed longer latencies to retrieve, group, and crouch over pups than did the vehicle-injected controls. These results suggest that opioids may have a stimulatory role in parental behavior during this prepubertal period.