Luteinizing hormone release in intact and castrate rams is altered with immunoneutralization of endogenous estradiol

Abstract

Changes in the dynamics of luteinizing hormone (LH) release in the adult ram following immunoneutralization of endogenous estradiol were investigated. Castrate rams were actively immunized against estradiol-6 – bovine serum albumin for 7 months and then their patterns of episodic LH release and LH response to multiple injections of gonadotropin-releasing hormone (GnRH, two 5-μg doses given iv 2 h apart) were assessed (April). In comparison with control rams immunized against rabbit gamma globulin, estradiol-immunized rams (antibody titre ~1:5000) exhibited more frequent LH releases (11.7 ± 0.3 vs. 9.3 ± 0.8 pulses/8 h, P < 0.05) and a greater LH response to the first GnRH injection (peak Δ value 190 ± 8 vs. 130 ± 25 ng/mL, P < 0.01). Estradiol antiserum collected from the castrate rams was used in the passive immunization of intact rams (antibody titre ~1:200) for 1 month (beginning mid-July). Although episodic LH release was always similar for control and immunized rams, testosterone levels in the latter group increased approximately 150%. In contrast with the castrate ram response, GnRH treatment (two 5-μg doses given iv 80 min apart) produced a "self-priming" effect on LH release in the intact rams, an effect that was dampened with estradiol immunoneutralization. Consequently, peak 2: peak 1 ratios for Δ value and 80-min mean incremental increase were much smaller (P < 0.01) for the immunized rams (~2:1 vs. 4:1 for the control rams). Data indicate that estradiol is (i) biologically active at the level of the hypothalamic-pituitary axis in the castrate ram and (ii) involved in maintaining the self-priming effect of GnRH on LH release in the intact ram.