Lacasse Pierre, Zhao Xin, Ollier Séverinne
Sherbrooke Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec J1M 0C8, Canada.
Department of Animal Science, McGill University, Sainte-Anne-de-Bellevue, Quebec H9X 3V9, Canada.
Domest Anim Endocrinol. 2019 Jan;66:72-85. doi: 10.1016/j.domaniend.2018.10.003. Epub 2018 Nov 5.
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.
进行了三项实验，以更好地理解为何随着泌乳期推进，挤奶诱导的催乳素（PRL）释放会减少。实验1比较了挤奶、对乳腺进行2分钟手动刺激（不挤奶）或注射1国际单位催产素（不挤奶）对初产奶牛、经产非妊娠奶牛以及经产妊娠奶牛（每种生理状态6头）激素释放的影响。在处理开始前20分钟至处理后60分钟采集血样。在挤奶过程中，初产奶牛的PRL释放（基线以上曲线下面积）高于经产奶牛，但不受妊娠影响。泌乳阶段和妊娠对通过手动刺激引起的PRL释放没有影响。催产素未诱导PRL或皮质醇的显著释放。皮质醇释放不受生理状态影响，挤奶和乳腺刺激时相似。挤奶诱导的β-内啡肽释放不受生理状态影响。实验2比较了挤奶、2分钟手动刺激或10分钟手动刺激对初产（6头）和经产（6头）奶牛的影响。在所有三种处理中，初产奶牛的催乳素释放均高于经产奶牛。10分钟手动刺激比2分钟刺激诱导的PRL释放更多。初产奶牛的皮质醇释放更高，但三种处理之间相似。实验3比较了5分钟手动刺激和注射多潘立酮（一种多巴胺拮抗剂）对初产（6头）和经产（6头）奶牛的影响。手动诱导的PRL释放在初产奶牛中高于经产奶牛。注射多潘立酮比手动刺激诱导的催乳素释放更多，且不受泌乳阶段影响。因此，泌乳后期挤奶诱导的PRL释放减少并非乳腺对刺激敏感性降低、挤奶时间缩短、妊娠阶段或垂体分泌PRL能力降低的结果。
