Abstract
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 heifers 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.
