Characteristics of luteinizing hormone (LH) and testosterone secretion, pituitary responses to LH-releasing hormone (LHRH), and reproductive function in young bulls receiving the LHRH agonist deslorelin: effect of castration on LH responses to LHRH.

Young bulls were treated with graded dosages of the LHRH agonist deslorelin to ascertain 1) whether increased testosterone secretion persisted over a wide dose range of agonist; 2) whether elevated testosterone was maintained long-term and, if so, what effects there were on reproductive function; and 3) what pituitary responses to exogenous LHRH occurred in intact and castrated bulls receiving deslorelin. In three experiments, bulls received dosages of agonist ranging from approximately 0.15 to 29.0 micrograms deslorelin/kg live weight/day, by means of either bioimplants or injections. At all dosages, deslorelin induced an acute increase in plasma LH concentrations that declined after 24 h but remained at greater concentrations than in controls, although the differences were relatively small. Profiles of LH in bulls treated with deslorelin were characterized by tonic secretion with no clear evidence of LH pulses. Plasma testosterone concentrations were increased at all dosages of deslorelin and in one experiment remained greater than in controls for over 100 days of treatment. This increase was associated with an increase in the rate of testis growth; however, there were no apparent improvements in semen parameters. Bulls receiving deslorelin did not show a typical postcastration rise in plasma LH concentrations, and neither intact nor castrated bulls receiving deslorelin showed an increase in plasma LH after injection of natural sequence LHRH. The absence of endogenous LH pulses and lack of response to exogenous LHRH suggested that the anterior pituitary in bulls receiving LHRH agonist becomes desensitized. However, LH secretion persisted in a tonic manner and was associated with elevated plasma testosterone concentrations. The failure of both intact and castrated bulls receiving deslorelin to respond to exogenous LHRH suggested direct effects of deslorelin on the pituitary, rather than an interaction with steroid feedback.