Experimental simulation of neuroendocrine dynamics at periparturient periods--a synthetic approach to pregnancy maintenance, parturition and lactation by programmed infusion of sex steroids in ovariectomized rats.

The blood levels of progesterone (P) and estradiol (E2) were artificially controlled in rats ovariectomized on days 19-20 of pregnancy (day 1: sperm in vaginal smears) by 48-168 h intravenous infusions according to 15 different temporally variable programs. The predominance of P infused in various curves for longer than 72 h, caused postmature deliveries on days 24-26 accompanying difficulties in labor and the lack of lactation, although changes in E2 levels antagonistically accelerated the timing of delivery. The most successful delivery and subsequent lactation were induced by an infusion of total amount of 1.5 mg P for 48 h (days 19-21) at a linearly decreasing rate in combination with an infusion of a total amount of 875 ng E2 for 72 h (days 19-22) at a slowly increasing rate, reaching a plateau in the morning of day 21 and rapidly decreasing to nil after that. Spontaneous delivery occurred on day 22 with a normal range of labor duration and no incidence of stillbirth. No further supply of sex steroids was required for the maintenance of lactation. Blood levels of prolactin shortly after delivery were much higher in these lactating ovariectomized rats than in non-lactating animals. The neuroendocrine dynamics at periparturient periods, which triggers the termination of pregnancy and then the induction of parturition and lactation, is discussed in relation to the results of these simulation experiments.