ESTRADIOL IMPLANTS IN THE ARCUATE NUCLEUS INDUCE LACTOGENESIS IN VIRGIN RATS - ROLE OF PROGESTERONE

The aim of this study was to determine the effect of the centrally adm inistered estradiol, and the effects of the consequent hypersecretion of prolactin (PRL) and progesterone, on lactogenesis as evaluated by m ammary accumulation of casein and lactose. Bilateral cannulae containi ng 17 beta-estradiol or cholesterol were implanted in the arcuate nucl eus of virgin rats on the day of estrus (Day 0). In the first experime nt different groups of rats were killed on Days 6, 9, 15, 17, or 19. T runk blood was collected and abdominal mammary glands were taken. In t he second experiment, estradiol-implanted rats received the progestero ne antagonist mifepristone or vehicle at 14.00 h on Day 8 or 16 post-i mplant, and were killed 28 or 48 h later. Serum PRL and progesterone a nd mammary casein were measured by RIA and lactose was determined by a n enzymatic assay. Estradiol-implanted rats showed a significant incre ase in both milk components at ail time points after implant compared to controls. On Day 9 after estradiol implant, mifepristone had no eff ect on mammary content of casein or lactose. By contrast, on Day 16, m ifepristone markedly increased both casein and lactose contents withou t modifying serum PRL and progesterone concentrations.