CONICET | Buscador de Institutos y Recursos Humanos

The role of opioid peptides in the secretion of oxytocin and prolactin induced by sucking was studied in goats. Seven goats were isolated with their kids (four singletons and three twins) in individual corrals 3-4 weeks after parturition. On day 1 of the experiment, the kids were separated from the does for 7 h and were weighed before and 15 min after being reunited with their mothers to assess the amount of milk obtained by sucking. The does were blood-sampled 10 min before and at the end of the sucking period. On the second day, a similar protocol was followed, but naloxone was given immediately after the first blood sample. On day 3, the protocol was repeated but saline vehicle was injected instead of naloxone. On day 5, the naloxone experiment was repeated as on day 2. Milk ejection was evaluated as the difference in the weight of the kids before and after sucking for 15 min, and the maternal serum levels of oxytocin and prolactin were measured by radioimmunoassay. A significant decrease in the weight gain of the kids was obtained when the mothers were treated with naloxone on day 2. Consistently, serum levels of oxytocin and prolactin induced by sucking were significantly reduced; indicating that sucking induced oxytocin secretion for milk ejection in lactating goats is facilitated by opioid peptides. In a second experiment performed in the same animals 10 days later, the administration of oxytocin, immediately after naloxone administration, prevented the decrease in the weight gain induced by naloxone, suggesting that the effect of the opioid antagonist on milk ejection in goats is a result of a reduced oxytocin secretion. The present results confirm the importance of sucking-induced oxytocin secretion for milk ejection in lactating goats, and indicate that oxytocin and prolactin secretion are regulated by opioid peptides in this species.