Naltrexone delays the onset of maternal behavior in primiparous parturient ewes

Previous studies indicate that morphine facilitates the induction of maternal behavior in nonpregnant multiparous ewes but has no effect in nulliparous females. Naltrexone (NAL) has the opposite effect. The aim of the present experiment was to investigate whether this also applied at parturition in ewes lambing for the first time. We studied the behavior of parturient ewes that received either saline (n = 9) or 50 mg (n = 7) or 150 mg (n = 8) of naltrexone, intravenously, at the first signs of lambing. either dose of naltrexone was found to reduce significantly the duration of maternal licking of the neonate and the emission of low-pitched bleats during the first 30 min following parturition (p < 0.005). The proportion of mothers failing to display an immediate onset of maternal care (in < 5 min) was also significantly higher in NAL-treated ewes (six of 15 vs. none of nine; p = 0.05). On the other hand, there was no indication that NAL affected the establishment of selectivity or that it facilitated the manifestation of postpartum estrus. Our results therefore tend to confirm that opiates have a role in the facilitation of maternal behavior at parturition in the ewe.