Effects of continuous administration of gonadotropin-releasing hormone (GnRH) or a potent GnRH analogue on blood luteinizing hormone and testosterone concentrations in prepubertal bulls.

The objective was to determine if continuous administration of gonadotropin-releasing hormone (GnRH) or a potent analogue (GnRH-A) for 28 or 56 d would decrease blood concentrations of luteinizing hormone (LH) and testosterone (T) in 5-month old bull calves. Treatments (5 calves/treatment), using a completely randomized design, were: control (vehicle), 3.3, 10 and 30 micrograms GnRH, and 3.3 and 10 micrograms GnRH-A (Leuprolide) per kg bodyweight/d for 28 d, administered via subcutaneously implanted mini-osmotic pumps. A second pump was implanted on day 28 in controls and bulls receiving 10 micrograms GnRH-A until day 56. Blood samples were taken every second day for plasma LH and T concentrations, and every 15 min for 6 hr on days 1, 14 and 27 (and days 40 and 55 where applicable) for plasma LH concentrations. There was an increase (P < 0.05) in basal plasma LH in response to GnRH and GnRH-A on day 1 (1.6, 2.1, 2.1, 2.5, 2.1 and 1.9 ng/ml for control, 3.3, 10 and 30 micrograms GnRH, and 3.3 and 10 micrograms GnRH-A, respectively; pooled s.e.d. = 0.2), but not on days 14 or 27. The number of LH pulses/6 hr was similar on day 1 for GnRH-treated and control calves, but there was a linear decrease (P < 0.05) in pulse frequency in response to GnRH doses on days 14 (1.8, 1.2, 0.6 and 0.0 for control, 3.3, 10 and 30 micrograms GnRH; s.e.d. = 0.5) and 27 (1.8, 1.0, 0.0 and 0.0; s.e.d. = 0.3). On days 1, 14 and 27, both GnRH-A doses suppressed (P < 0.05) LH pulsatility. GnRH (days 1-14) and GnRH-A (days 1-27) increased (P < 0.05) plasma T in a dose-dependent manner. Mean T was greater (P < 0.05) in 10 micrograms GnRH-A-treated than in control calves during days 29-41 (4.6 and 1.8 ng/ml; s.e.d. = 0.6) and days 43-55 (4.1 and 1.8 ng/ml; s.e.d. = 0.4). In conclusion, continuous administration of GnRH or GnRH-A to 5-month old bulls for 28 or 56 d chronically decreased LH pulse frequency, had no effect on basal LH, but increased testosterone concentrations.