Comparison of oxytocin, prolactin and cortisol release during the first and second lactation

摘要

This experiment was conducted to investigate whether the stimulus caused by the milking machine was effective enough to maintain oxytocin (OT), prolactin (PRL) and cortisol (CORT) release throughout lactation and to compare the evolution of these hormones during two successive lactations. Eight Lacaune ewes during the first and second lactations were used in this experiment. Blood samples, milk yield and milk composition were measured on days 1, 30, 60, 90, 120 and 150 after parturition. Only on day 1, the OT release was minimal and of no significance. From day 30 to 150, the onset of the milking was followed by a significant OT release. The maximum OT concentration was observed at 0.5min. after the cup attachment. The highest OT release was measured on day 30. However, on the other days, OT release was lower but remained significant. During milking, PRL release increased slowly, with the maximum level being observed 6min. after the beginning of the milking and plateaued afterwards. The PRL release was higher early in lactation (from day to 60). From day 90 to 150, however, the baseline levels increased at the same tune that the PRL release induced by milking decreased; consequently, PRL release was not significant. CORT levels increased significantly after the end of the milking and the maximum cortisol level was recorded at 10min. CORT release was significant throughout lactation with the highest release of CORT being measured on day 1 and the lowest on day 30. The total amount of OT, PRL andCORT released by milking machine on a given day of lactation was not affected by the lactation order. The results also suggest that neither the udder sensitivity nor the activation of the ascending nervous pathways is involved in this mechanism of regulation.