THE CORPUS LUTEUM-HYPOPHYSIS RELATIONSHIP: THE EFFECT OF PROGESTERONE TREATMENT ON THE RELEASE OF GONADOTROPHINS IN THE RAT

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Abstract
The effect of progesterone on pituitary gonadotropin release has been studied in the rat. Castrated female rats bearing ovarian autotransplants, or unilaterally ovariectomized rats, were given 1 to 5 mg of progesterone daily for intervals from 7 to 28 days, and then killed. The ovaries, uteri, and vaginas were examined grossly and microscopically, and the pituitary glands were assayed for total gonadotropin content by the mouse uterine weight method. Although treatment with 5 mg of progesterone for 28 days inhibited ovulation, it did not prevent Graafian follicle development (determined histo-logically) or secretory function (evaluated by the presence of vaginal mucification) in either of the animal preparations. Progesterone treatment was associated with a significant elevation of pituitary gonadotropic potency in the castrated rats bearing ovarian autotransplants, but not in the unilaterally ovariectomized rats. These findings suggest that the tonic rate of gonadotropin secretion is unaffected by treatment with 5 mg of progesterone daily. The acute discharge of the ovulation inducing hormone complex [probably primarily LH (luteinizing hormone)] is, however, inhibited by such a dose of progesterone, and probably accounts for the inhibition of ovulation. The findings also suggest that the amount of progesterone secreted by the functioning corpora lutea during pseudopregnancy in the rat is equivalent in effect to between 2 and 5 mg of progesterone injected once daily.