Induction of luteinizing hormone, follicle-stimulating hormone surge in the estrogen-primed castrated male rat by progesterone.

Male rats were castrated at 26 days of age and treated with 0.05, 0.1, 1.0 and 2.0 micrograms/kg of BW of estradiol for 4 days starting with the day of castration. On the morning of Day 30 of life they were sacrificed and serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels were determined. A significant reduction of LH and/or FSH occurred only with the 1.0 and 2.0 micrograms/kg of BW dose of estradiol. This was in sharp contrast to results in female rats at the same age in which 0.2 microgram/kg of BW of estradiol suppressed the post-castration rise of gonadotropins. This change in threshold sensitivity to the suppressive effects of estradiol may be one component of the changes accompanying the masculinization of the hypothalamus. The 1.0 and 2.0 micrograms/kg of BW doses of estradiol then were used in combination with 0.1, 0.5, 1.0, 2.0, 4.0 and 8.0 mg/kg of BW of progesterone. The addition of progesterone to the treatment regimen resulted in either no change or a suppression or stimulation of FSH and LH, depending upon the dose of progesterone used. With the 2 micrograms/kg of BW dose of estradiol, 1.0 mg/kg of BW of progesterone resulted in a surge of both FSH and LH, irrespective of whether it was given in combination with estradiol or a single injection following 4 days of estradiol treatment. This, to our knowledge, is the first demonstration of a gonadotropin surge induced by progesterone in the estrogen-primed castrated male rat.