Continued desensitization of the pituitary gland in young bulls after treatment with the luteinizing hormone-releasing hormone agonist deslorelin

Abstract

The objective in this study was to evaluate pituitary and testicular function in young bulls during and after treatment with the LHRH agonist deslorelin. The primary focus was to ascertain the period required for a return to typical LH and testosterone secretion after cessation of treatment with deslorelin, in order to establish whether desensitization was a phenomenon restricted to the pituitary or also occurred at the testes. Brahman bulls, 13.0 +/- 0.6 mo of age and 224 +/- 5 kg, were allocated to four groups and received treatment as follows: group C (n = 5), control, received no treatment; group C + LHRH (n = 5), control, received LHRH tests (50 micrograms LHRH i.m.) at the same times as group D + LHRH below; group D (n = 5), received deslorelin (approximately 200 micrograms/day) for 28 days; group D + LHRH (n = 5), received deslorelin for 28 days and were given LHRH test (50 micrograms LHRH i.m.) on Day 28 of treatment and on Days 2, 4, 8, 12, 16, and 20 after treatment. Deslorelin induced acute increases (p < 0.01) in plasma concentrations of LH and testosterone within 2 h, after which concentrations of both hormones declined by 24 h. From Day 6 to Day 28 of the treatment period, plasma LH concentrations were similar for treated and control bulls. During the same period, concentrations of plasma testosterone were greater (p < 0.01) in treated than in control bulls. Mean concentrations of both LH and testosterone were lower in treated than in control bulls for approximately one week after cessation of deslorelin treatment. Bulls treated with deslorelin did not exhibit a release of LH in response to exogenous LHRH on Day 28 of treatment. A relatively small but significant (p < 0.05) release of LH occurred on Day 4 after cessation of treatment with deslorelin (group D + LHRH, 0.49 +/- 0.11 ng/ml; group C + LHRH, 7.17 +/- 0.90 ng/ml). LH release in response to LHRH in bulls previously treated with deslorelin increased to Day 12 and then remained constant to Day 20 after cessation of treatment. However, LH release in these bulls remained significantly lower compared with that of control bulls during the 20 days after cessation of treatment with deslorelin. Bulls treated with deslorelin had a typical release of testosterone after administration of LHRH on Days 2-20 after discontinuation of treatment, even though there were relatively small releases of LH. Therefore, the pituitary in bulls remained desensitized to LHRH for at least 20 days after cessation of treatment with an LHRH agonist. In contrast, desensitization did not occur at the testes, which retained the capacity for a typical testosterone response to the LH that was released after administration of LHRH.