Advancement of postnatal pulsatile luteinizing hormone secretion in the bull calf by pulsatile administration of gonadotropin-releasing hormone during infantile development. Biology of reproduction 44

The transition from infantile to prepubertal development in bull calves coincides with the onset of postnatal LII secretion following differentiation of pituitary gonadotropes. Increased pulsatile secretion of hypothalamic GnRII may conthbute to mat-urational 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 GnRJ3 were examined. Nine calves received 200 ng GnRH for 5 mm 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 mm 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 at 0.01) in GnRM-T calves than in controls at 2 through 6 wk of age. LII pulse height did not change in either group during the study period. However, LII pulse frequency in GnRH-T calves increased with age (p at 0.01), and at 4-6 wk of age, the rate of pulsatile LII secretion was greater (p at 0.01) in GnRH-T calves than in controls. Pituitary LII content increased 2.4-fold after GnRH infusion (p at 0.01); this increase was associated with a doubling of the pituitary LH- 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 GnR.H and initiates pulsatile LII release, leading to pubertal development