Studies on Regulatory Mechanisms for Gonadotropins Secretion in Lactating Rats

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  • TAYA Kazuyoshi
    Laboratory of Veterinary Physiology, Tokyo University of Agriculture and Technology

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Other Title
  • 泌乳ラットにおける性腺刺激ホルモン分泌調節機構に関する研究
  • 泌乳ラットにおける性腺刺激ホルモン分泌調節機構に関する研究(1992年度島村賞受賞講演論文)
  • ヒニュウ ラット ニ オケル セイセン シゲキ ホルモン ブンピ チョウセツ

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Abstract

The present study was designed to clarify mechanisms responsible for regulation of FSH and LH secretion and ovarian follicular maturation during lactation in rats. Throughout lactation in rats, plasma levels of follicle-stimulating hormone (FSH) remain within the range normally found during the diestrous phase of the estrous cycle, regardless of litter size, although levels of luteinizing hormone (LH) in plasma are consistently lower in dams nursing 8 pups than in dams nursing 2 pups. All antral follicles larger than 401μm in diameter degenerate during the first half of lactation and do not reappear until the second half of lactation in rats nursing 8 pups. Healthy follicles of this size, however, are always present in mothers nursing 2 pups. These changes in follicular degeneration corresponded with a decrease in concentrations of plasma LH in the first half of lactation, and follicular development was noted with an increase in basal levels of plasma LH in the second half of lactation. Corpora lutea formed after postpartum ovulation secrete a large amount of progesterone under the influence of prolactin released as a result of suckling. Despite continued suckling, plasma concentrations of prolactin and progesterone decline gradually and a slight increase in plasma concentrations of estradiol-17β occurs during the second half of lactation. Concentrations of bioactive and immunoreactive inhibin in plasma vary with the number of healthy antral follicles and no positive correlation has been observed with luteal function throughout lactation in the rat.<BR>On the basis of the present results it is indicated that 1) tonic secretion of LH is an important factor in normal follicular maturation and maintenance in lactating rats, 2) the suckling stimulus, rather than ovarian factors, is mainly responsible for the suppression of FSH as well as LH secretion during the first half of lactation in rats nursing 8 pups. On the other hand, during the second half of lactation in rats nursing 8 pups, ovarian inhibin plays a primary role in the suppression of FSH secretion, whereas ovarian steroids act to suppress LH secretion. The present findings also indicate that 3) the suckling stimulus alone suppressed the secretion of both LH and FSH and this effect may be mediated by the inhibition of LH releasing hormone (LH-RH) secretion from the hypothalamus, 4) the suckling-induced inhibition of LH-RH may be primarily mediated by endogenous corticotropin-releasing hormone and opioid peptides and 5) the pituitary-adrenal system is capable of influencing the maintenance of a normal secretion of gonadotropins and prolactin as well as the maintenance of ovarian function during the lactation in the rat.

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