ADVANCEMENT OF POSTNATAL PULSATILE LUTEINIZING-HORMONE SECRETION IN THE BULL CALF BY PULSATILE ADMINISTRATION OF GONADOTROPIN-RELEASING-HORMONE DURING INFANTILE DEVELOPMENT

The transition from infantile to prepubertal development in bull calves coincides with the onset of postnatal LH secretion following differentiation of pituitary gonadotropes. Increased pulsatile secretion of hypothalamic GnRH may contribute to maturational development of the hypothalamic-pituitary axis. To evaluate the role of GnRH pulse frequency in mediating changes in pituitary response that may lead to altered LH release, serum LH concentrations in Holstein bull calves treated with exogenous GnRH were examined. Nine calves received 200 ng GnRH for 5 min every hour from 1-6 wk of age (GnRH-T), while 5 animals served as controls. Weekly through 6 wk of age, jugular venous blood was collected every 10 min for 8 h. Blood samples from GnRH-T animals were taken during 2 exogenous GnRH pulses followed by 6 h of sampling without exogenous GnRH. Mean serum LH concentrations were greater (p less-than-or-equal-to 0.01) in GnRH-T calves than in controls at 2 through 6 wk of age. LH pulse height did not change in either group during the study period. However, LH pulse frequency in GnRH-T calves increased with age (p less-than-or-equal-to 0.01), and at 4-6 wk of age, the rate of pulsatile LH secretion was greater (p less-than-or-equal-to 0.01) in GnRH-T calves than in controls. Pituitary LH content increased 2.4-fold after GnRH infusion (p less-than-or-equal-to 0.01); this increase was associated with a doubling of the pituitary LH-beta mRNA concentration found in controls. Infusion of GnRH elevated pituitary concentrations of estradiol receptor 3.5-fold and of GnRH receptor 2-fold as compared with concentrations in controls. These results indicate that an increase in frequency of GnRH release provides the stimulus for increased pituitary response to GnRH and initiates pulsatile LH release, leading to pubertal development.