The inhibitory effect of progesterone on lactogenesis during pregnancy is already evident by mid- to late gestation in rodents.

Lactogenesis is a very complex process highly dependent on hormonal regulation. In the present study the time-course of the inhibitory actions of progesterone on prolactin secretion, mammary gland morphology and lactogenesis from mid- to late gestation in rodents was investigated. Groups of pregnant rats were luteectomised or administered with mifepristone on Day 10, 13, 15 or 17 of gestation and decapitated 28 or 48h later. Whole-blood samples and the inguinal mammary glands were taken for determinations of hormone levels and for measurement of mammary content of casein and lactose and for tissue morphology analyses, respectively. Luteectomy or mifepristone evoked prolactin increases only after Day 17 of gestation. Mammary content of casein was increased by both treatments regardless of timing or duration. Mifepristone was less effective than luteectomy in inducing lactose production and the effect was only observed after Day 15 of gestation. Analysis of mammary gland morphology confirmed the observed effect of progesterone on lactogenesis. Both treatments triggered remarkable secretory activity in the mammary gland, even without a parallel epithelial proliferation, demonstrating that the mammary epithelium is able to synthesise milk compounds long before its full lobulo-alveolar development is achieved, provided that progesterone action is abolished. Thus, the present study demonstrates that progesterone is a potent hormonal switch for the prolactin and prolactin-like effects on mammary gland development and its milk-synthesising capacity during pregnancy, and that its inhibitory action is already evident by mid-pregnancy in rodents.