This study was carried out to investigate changes in pituitary response to GnRH on gonadotrophin secretion in prepubertal heifers. A total of 50 prepubertal Holstein-Friesian heifers were treated with 1 µg/kg GnRH intravenously at 1, 2, 4, 6 and 8 months of age, and plasma samples were collected from the jugular vein at 0 to 360 min after GnRH treatment. Plasma concentrations of LH and FSH were measured by RIA. Increase in concentrations of LH and FSH, induced by GnRH, were observed in all

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... rs from 1 month of age, and the concentrations of LH and FSH at 30 min after GnRH treatment were significantly greater than those before GnRH treatment in heifers of all ages. The time from the GnRH administration to the appearance of the peak of LH and FSH rise was similar at each age and, was prolonged with age. The peak concentration and the amount of release from the pituitary of LH rise increased with age, while those of FSH had a tendency to decrease with age. These results indicate that the pituitary gland already has reactivity to GnRH in heifers by 1 month of age, that the capacity of LH release to GnRH in the pituitary gland develops with age, and that the regulatory mechanism for the secretion of LH and FSH develops differently in prepubertal heifers. This study suggests that the development of the capacity to secrete LH secretion in response to GnRH in the pituitary gland before puberty is one of the factors for deciding the time of the onset of puberty in heifers. (J. Reprod. Dev. 48: [545][546][547][548][549][550][551] 2002) t the puberty, LH surge release in response to GnRH surge release occurs in the pituitary gla nd, and then the rapid burst increase in peripheral LH concentration induces the first ovulation from fully-grown follicles in the ovary. B e fo r e t he fi r s t o v u la t i o n at p u b er t y , t h e hypothalamus-pituitary-gonad axis as main regulating system of reproduction has to govern. It is generally considered that the final component of the reproductive endocrine system to develop, that regulates the time of the onset of puberty in heifers and ew es, is th e hy po thal am us [1-3]. T he hypothalamic content of GnRH and the number of pituitary receptors for GnRH do not change during sexual ma tura tion in heifer s [4 ]. The fina l component for the onset of puberty is thought to be the development of a GnRH releasing pattern develop like in sexually mature cattle. The pattern o f G n R H r e l e a s e d e c i d e s s e c r e t i on o f gonadotrophin in the pituitary gland [5, 6]. Peripheral circulating hormones, estradiol-17β and inhibin, directly or indirectly regulate GnRH and