The effects of opiate antagonists on milk intake of preweanling rats.

The observation that acute administration of opiate agonists to rodent adults increased food intake while the reverse was true with a single injection of an opiate antagonist has led to the suggestion that endogenous opioids may in part mediate feeding behaviour in adult rodents. Studies were performed in infant rats to explore further the generality and ontogeny of this function. In three models of food availability, the effects of naloxone and naltrexone were assessed in deprivation-induced feeding tests in 3-, 10-, 12, 14- and 19-day-old preweanling rats. Naloxone or naltrexone attenuated food intake only after 14 days. Functional immaturity of the opioid systems vis-a-vis ingestive behaviour may account for this developmental sequence.