DOSE RELATED EFFECT OF A SINGLE INJECTION OF PROGESTERONE ON GONADOTROPIN-SECRETION AND PITUITARY SENSITIVITY TO LHRH IN ESTROGEN-PRIMED CASTRATE FEMALE RATS

This investigation studied the effect on gonadotropin secretion of varying doses of progesterone given as a single injection to estrogen-primed castrated rats. Female rats were ovariectomized at 26 days of age (Day 1) and administered 0.1 μg/kgBW estradiol-17β for 4 days starting on the day of castration. Progesterone, in various doses, was administered at 0930 h on the fourth postcastration day and groups of animals were sacrificed between 1000 h and 1600 h. Estrogen treatment reduced but did not suppress the postcastration rise of gonadotropins. In estrogen-primed castrated rats given 0.2 or 0.8 mg/kgBW progesterone, there was a significant increase in both serum follicle stimulating hormone (FSH) and luteinizing hormone (LH) by 1600 h. In estrogen-primed castrated rats given 0.4 or 3.2 mg/kgBW progesterone, there was a significant suppression of both FSH and LH by 1600 h. By 1800 h the serum LH had returned to baseline levels in all groups. Estrogen-primed castrated rats given 0.8 or 3.2 mg/kgBW progesterone were then administered 10 ng pulses of luteinizing hormone releasing hormone (LHRH) at 1800 h, 2000 h and 2200 h and bled at 0, 10, 60 and 120 min after each pulse. There was a dose related effect of progesterone on the pituitary's response to LHRH. The 0.8 mg dose was stimulatory and the 3.2 mg dose inhibitory as compared with estrogen treated controls for LH. For FSH, there was no difference in the increment after LHRH in the progesterone treated and nonprogesterone treated estrogen-primed castrated rats. Between the 2 progesterone treated groups, however, the increment in FSH in rats treated with the 0.8 mg/kgBW dose was significantly higher than in rats given the 3.2 mg/kgBW dose. Pituitary gonadotropin content was significantly increased by the 0.8 mg/kgBW progesterone dose. In addition to the demonstration of a direct pituitary effect, this study shows that the effect of progesterone is dose dependent under conditions of uniform estrogen stimulation and time of exposure to progesterone.