Differential responses in anterior pituitary luteinizing hormone (LH) content and LH beta- and alpha-subunit mRNA, and plasma concentrations of LH and testosterone, in bulls treated with the LH-releasing hormone agonist deslorelin.

Anterior pituitary gland contents of LH and LH beta- and alpha-subunit mRNAs, and circulating concentrations of LH and testosterone, were determined in bulls treated with the LH-releasing hormone (LHRH) agonist deslorelin. Brahman (Bos indicus) bulls (14-month-old) were allocated to two groups and received the following: Control (n = 5), no treatment; Deslorelin (n = 4), four deslorelin implants (approximately 200 micrograms total deslorelin/day) for 36 d. Plasma concentrations of LH were higher in bulls treated with deslorelin on Day 1, had returned to typical levels by Day 8, and did not differ for control bulls and bulls treated with deslorelin from Day 8 to Day 29. Pituitary content of LH on Day 36 was reduced (P < 0.001) in bulls treated with deslorelin (33 +/- 4 ng/mg) compared with control bulls (553 +/- 142 ng/mg). Relative pituitary content of LH beta-subunit mRNA was also reduced on Day 36 in bulls treated with deslorelin (Control, 0.65 +/- 0.10; Deslorelin, 0.22 +/- 0.04; P = 0.003). However, alpha-subunit mRNA relative content did not differ (Control, 0.73 +/- 0.15; Deslorelin, 1.06 +/- 0.12; P > 0.05). Plasma concentrations of testosterone were increased over the period of the experiment in the bulls treated with deslorelin compared with control bulls. This is the first demonstration of reduced pituitary content of LH beta-subunit mRNA and LH, and unaltered content of alpha-subunit mRNA, in bulls treated with LHRH agonist. This was associated with apparently typical plasma concentrations of LH and elevated plasma testosterone. The anterior pituitary in bulls treated with LHRH agonist, therefore, undergoes classical desensitization and downregulation, but plasma LH and testosterone are not suppressed.