New insights in the importance of prolactin in dairy ruminants

In most mammals, prolactin (PRL) is essential for maintaining lactationand its suppression inhibits lactation. However, the involvement of PRLin the control of ruminant lactation is less clear because inconsistenteffects on milk yield have been observed with short-term suppression ofPRL by bromocriptine. Therefore, several experiments were conductedto assess the galactopoietic role of PRL. In an initial experiment, cows inearly lactation received daily injections of the dopamine agonist quinagolide(QUIN) for 9 weeks. QUIN reduced milking-induced PRL releaseand caused a faster decline in milk production. Milk production wascorrelated with the amount of PRL released at milking. QUIN reducedmammary epithelial cell activity, survival and proliferation. In goats,QUIN did not affect either basal or milking induced PRL release and milkproduction, whereas injection of cabergoline, another dopamine agonist,caused a decrease of 28% of milk yield the day after the injection. Inanother experiment, cows were injected for 5 d with QUIN; QUIN +injection of bPRL at milking time; or vehicles. Again, milk, protein andlactose yield were decreased by QUIN. Although PRL injections werenot sufficient to restore milk yield, they tended to increase milk proteinand lactose yields and increased the viability of milk purified mammaryepithelial cells. In late lactation cows, QUIN decreased milk productionwithin the first day of treatment and induced a more rapid changes inseveral markers of mammary gland involution after drying-off. Similarly,injection at drying-off of cabergoline hastened mammary involution andenhanced mammary gland remodeling. Recently, we stimulated PRLsecretion with daily injection of the dopamine antagonist domperidonefor 5 weeks. Milk production increased gradually and was greater indomperidone-treated cows during the last 4 weeks of the treatmentperiod. Milk production of both groups became similar again 5d afterthe last injection. In conclusion, these data, combined with those fromother studies, provide a good body of evidence that PRL is galactopoieticin dairy ruminants.