Separation of opioid from nonopioid mediation of affect in neonatal rats: nonopioid mechanisms mediate maternal contact influences.

A causal distinction is established in infant Norway rats between opioid- and nonopioid-mediated determinants of behavior. Contact influences are shown to be mediated by nonopioid pathways, whereas gustatory influences are shown to be opioid mediated. Specifically, naltrexone (0.5 and 1.0 mg/kg) did not at all diminish quieting exerted by contact with an anesthetized dam but completely reversed the quieting effects of morphine in isolated rats. Naloxone (5 mg/kg) did not affect the latencies with which nondeprived or 8-hr deprived rats 9, 12, 15, and 18 days of age attached to the nipples of anesthetized dams, nor did naloxone (5 and 10 mg/kg) cause any systematic change in nipple attachment in 10- and 18-day-old rats that had been deprived of their dam for either 0, 8, or 24 hr. In a 3rd experiment, naloxone (5 mg/kg) did not significantly reduce milk intake by 9-, 12-, 15-, or 18-day-old rats from the nipple when milk letdown was induced by oxytocin. Moreover, naloxone (5 and 10 mg/kg) did not reduce milk intake in Day-10 rats that, while suckling, received milk via a cannula placed in the posterior portion of the tongue at the level of the intermolar eminence or in rats that obtained milk directly from their awake mother. In contrast, milk intake was significantly reduced by naltrexone (0.25-1.0 mg/kg) in Day-10 rats that obtained milk (a) by licking it off a saturated substrate or (b) through an indwelling cannula located in the anterior portion of the lower jaw. (Milk delivered at this placement is thought to engage feeding systems by its taste and texture.) In a final set of experiments in Day-10 rats, intake of milk delivered via anterior jaw cannulae was reduced by naloxone (5 and 10 mg/kg) in rats that were either isolated, in contact with an anesthetized dam, or attached to her nipples. On the basis of resistance to naloxone and naltrexone administration, these experiments demonstrate that behavioral influences of the tactile (and possibly olfactory) qualities of the mother are not mediated by opioid systems. Implications for understanding the means through which mothers can influence their young and the infantile mediators of these maternal influences are discussed.