Effects of luteolysis during late pregnancy on pituitary responsiveness to gonadotrophin-releasing hormone in the rat.

We investigated whether the increase in the gonadotrophin response to gonadotrophin-releasing hormone (GnRH) during the last days of pregnancy and the occurrence of parturition on day 22 of pregnancy in rats are due to the increase in the plasma concentrations of oestradiol-17 beta after luteolysis, which occurs around day 20. In a first series of experiments we studied the effects of s.c. implantation of two capsules containing oestradiol on basal and GnRH-stimulated secretion of LH and FSH before and after luteolysis. Before luteolysis, ovariectomy increased basal LH and FSH; oestradiol treatment prevented this increase partly (FSH) or completely (LH). Ovariectomy also lowered the LH response to the infusion of GnRH (100 ng/h). Oestradiol treatment on the other hand, increased the LH and FSH responses of both intact and ovariectomized rats above the level in intact non-treated control rats. After luteolysis, ovariectomy increased basal FSH only. Treatment with oestradiol did not prevent the increase in basal FSH and ovariectomy diminished the LH response to GnRH infusion. Oestradiol treatment maintained the LH response in ovariectomized rats at the control level and increased the FSH responses of both intact and ovariectomized rats to a higher level than in control rats. Furthermore, the LH and FSH responses of the oestradiol-treated groups of intact and ovariectomized rats were higher after luteolysis than before. In a second series of experiments two capsules containing progesterone were s.c. implanted before or after luteolysis. Progesterone treatment suppressed the plasma concentration of oestradiol and the gonadotrophin responses to infusion of GnRH on the expected day of parturition in both groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)