Effect of the concentration of circulating prolactin on dairy cows' responsiveness to domperidone injection

Effect of prolactin concentration during the dry period on the subsequent milk production of dairy cows

Shortening the dry period has a negative impact on milk production of the following lactation. One possible explanation is that a period of low prolactin (PRL) concentration is necessary to restore mammary gland milk production capacity. Therefore, the objective of this study was to determine the effect of lowering blood PRL level on subsequent lactation milk production. In this experiment, quinagolide was used to inhibit PRL secretion during the dry period. Thirty Holstein cows were randomly assigned one of 3 dry period managements: a conventional (60 d) dry period (CD) and 2 short (35 d) dry period treatments (SD). Short dry period cows received either water (SDwater) or quinagolide (2 mg, SDquin) injections twice daily from dry-off until 14 d before calving. Cows were followed during the first 20 wks of the subsequent lactation. When CD cows were dry but SD cows were lactating, concentration of PRL was lower in the CD cows than in the SD cows. During the injection period, PRL of SDquin cows was lower than that of the other treatments and was greater in the blood of SDwater than in that of CD cows. After the injection period until calving, no difference in PRL concentration was observed between treatments. After calving, PRL concentration of the SDquin cows was greater than those of CD and SDwater cows. During the first 20 wks of lactation, energy corrected milk (ECM) was lower in SDwater cows than in CD and SDquin cows. The ECM of the latter groups were not different. During the same period, mammary expression of genes related to milk synthesis, pro-apoptotic genes, as well as the expression of the short and long isoforms of the PRLR genes were not affected; however, the expression of SOCS3 gene tended to be lower for the SDquin than the SDwater cows. Lowering the PRL level during short dry period restored milk production to the level normally observed after a conventional dry period, which suggests that higher PRL levels during short dry period are the cause of the lower milk production after a short dry period. Ultimately, strategies to lower blood PRL level may help the adoption of short dry period.

A Comparative Review of the Extrinsic and Intrinsic Factors Regulating Lactose Synthesis

Milk is critical for the survival of all mammalian offspring, where its production by a mammary gland is also positively associated with its lactose concentration. A clearer understanding of the factors that regulate lactose synthesis stands to direct strategies for improving neonatal health while also highlighting opportunities to manipulate and improve milk production and composition. In this review we draw a cross-species comparison of the extra- and intramammary factors that regulate lactose synthesis, with a special focus on humans, dairy animals, and rodents. We outline the various factors known to influence lactose synthesis including diet, hormones, and substrate supply, as well as the intracellular molecular and genetic mechanisms. We also discuss the strengths and limitations of various in vivo and in vitro systems for the study of lactose synthesis, which remains an important research gap.

Effect of stage of lactation and gestation on milking-induced hormone release in lactating dairy cows

Three experiments were conducted to better understand why milking-induced prolactin (PRL) release decreases as lactation advances. Experiment 1 compared the effects of milking, 2-min manual stimulation of the mammary gland (without milking), or injection of 1 IU of oxytocin (without milking) on hormonal release in early lactation cows, late-lactation and nongestating cows, and late-lactation and gestating cows (n = 6 per physiological status). Blood samples were collected from 20 min before the start of the treatments to 60 min after. During milking, PRL release (area under the curve above the baseline) was greater in the early lactation cows than in the late-lactation cows but was unaffected by gestation. Lactation stage and gestation did not affect PRL release by manual stimulation. Oxytocin did not induce a significant release of PRL or cortisol. Cortisol release was unaffected by physiological status and was similar for milking and mammary stimulation. Milking-induced β-endorphin release was not affected by physiological status. Experiment 2 compared the effects of milking, 2-min manual stimulation, or 10-min manual stimulation in cows in early (n = 6) and late (n = 6) lactation. Prolactin release was greater in the early lactation cows than in the late-lactation cows for all 3 treatments. A 10-min manual stimulation induced greater PRL release than a 2-min stimulation did. Cortisol release was greater in the early lactation cows but was similar among the 3 treatments. Experiment 3 compared the effects of a 5-min manual stimulation and the injection of domperidone (a dopamine antagonist) in cows in early (n = 6) and late (n = 6) lactation. Manually induced PRL release was greater in the early lactation cows than in the late-lactation cows. Prolactin release was greater with domperidone injection than with manual stimulation and was not affected by lactation stage. Thus, the reduction of milking-induced PRL release in late lactation is not a consequence of the lower sensitivity of the mammary gland to stimulation, a shorter milking time, the gestation stage, or the reduced capacity of the pituitary gland to secrete PRL.