DEVELOPMENT OF THE MECHANISMS INVOLVED IN THE FACILITATORY AND INHIBITORY EFFECTS OF OVARIAN STEROIDS ON THE RELEASE OF FOLLICLE-STIMULATING HORMONE IN THE IMMATURE RAT

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Abstract
SUMMARY Serum follicle-stimulating hormone (FSH) concentration was found to be higher in neonatal female rats than in adults. The values increased to a maximum on day 15 and decreased thereafter. Ovariectomy soon after birth or at 5 days of age induced a significant rise in serum FSH concentration 9 days later. Administration of 10 μg oestradiol benzoate (OB) lowered FSH concentration in both intact and spayed animals. Progesterone (1 mg) injected 3 days after priming female rats with a single dose of 10 μg OB induced, on the same day, a significant rise in serum FSH concentration in animals older than 22 days of age. In younger animals progesterone reversed the effect of OB. The facilitatory effect of progesterone occurred when the hormone was given in the afternoon but not when it was given in the morning. Male rats, although showing the negative feedback effect of OB injection, failed to show the positive feedback effect of progesterone. It is concluded that the central nervous system—hypophysial mechanism responsible for FSH secretion is ready to function before puberty. In female rats initiation of puberty probably depends on the activation of that mechanism by appropriate facilitatory ovarian steroid signals.

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