抄録
This study was carried out to investigate changes in gonadotrophin secretion in response to estradiol, and to clarify the change of the positive feedback action of estradiol in prepubertal heifers. Four prepubertal Holstein-Friesian heifers were treated with 2 μg/kg estradiol benzoate intramuscularly at 1, 3, 5, 7 and 9 months of age, and plasma samples were collected every 3 h for 36 h after estradiol treatment. Plasma concentrations of LH, FSH and estradiol were measured by RIA. The changes in peripheral estradiol concentration after estradiol treatment were similar in pattern with peaks at 3 h in all ages. LH and FSH rise induced by estradiol were observed in heifers after 3 months of age, and the time from the estradiol administration to the appearance of the peak of gonadotrophin rise gradually shortened with the age. The peak concentration of LH rise increased, while the concentration of FSH rise did not change with age. These results indicate that the hypothalamus and pituitary gland already have reactivity to estradiol in heifers by 3 months of age, and that estradiol positive feedback action on LH secretion develops with age in prepubertal heifers. We concluded that the development of estradiol positive feedback action on LH secretion before puberty may be one of the essential factors for deciding the time of the onset of puberty in heifers.
