Hormonal Correlates of Photoperiod-Induced Puberty in a Reflex Ovulator, the Female Ferret (Mustela furo),2

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Abstract
In an attempt to develop an animal model wherein the pubertal process could be initiated or accelerated by a readily controlled, noninvasive, external signal, this study examined sexual development of the female ferret, a photoperiodic, reflex ovulator. These animals exhibit signs of precocious sexual maturation within 6 wk of exposure to a stimulatory, long-day photoperiod at 15 wk of age. In the first study, females matured under a long-day stimulus were allowed to mate with a fertile male ferret. All six females ovulated after mating, and three of six became pregnant and successfully delivered and reared litters, demonstrating that vulvar edema induced by long days reflected the onset of true precocious sexual maturation. The second study attempted to determine the endocrine basis of this photoperiod-induced puberty. Immature ferrets were either left intact or were ovariectomized at 13 wk of age. These females either remained untreated or immediately received an estradiol implant. Controls in each group remained in short days, and others were transferred to stimulatory long days at 16 wk of age. Results indicated that immature ferrets exhibited a rise in plasma luteinizing hormone (LH) upon ovariectomy, and that the extradiol implant was an effective negative feedback signal for the duration of the study in animals in short days. In contrast, females in long days exhibited an "escape" from LH inhibition by the steroid implant at the same age at which the intact females in long days were beginning to mature. These data suggest the young ferret exhibits a high sensitivity to estradiol negative feedback which is decreased in response to a stimulatory photoperiod, and which decrease is coincident with sexual maturation in intact animals under the same conditions.

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