Use of hypothalamic releasing hormones to evaluate the reproductive endocrine status of rams

Abstract

Pituitary and testicular responses to exogenous gonadotrophic hormone releasing hormone (GnRH, 5 ng/kg body weight) and thyrotrophin releasing hormone (TRH, 50 ng/kg body weight) were assessed in four breeds of ram ( n = 5/breed) maintained under alternating short (8L:16D) and long (16L:8D) photoperiods each of 12 weeks duration. The amplitudes of LH responses (ΔLH) to a single injection of GnRH were higher ( P < 0.01) after 12 weeks of 16L:8D (9.8 ± 1.2 ng/ml, mean ± SEM for all rams) compared with responses after 12 weeks of 8L:16D (5.7 ± 0.8 ng/ml). Plasma testosterone concentrations were higher ( P < 0.01) after 12 weeks of 8L:16D (7.2 ± 1.0 ng/ml) than after 16L:8D (0.7 ± 0.1 ng/ml) and the amplitudes of testosterone responses after injection of GnRH were also greater ( P < 0.01) under 8L:16D (14.3 ± 1.3 ng/ml) than under 16L:8D (4.6 ± 0.3 ng/ml). Prolactin concentrations were elevated under 16L:8D (166 ± 31 ng/ml) and suppressed under 8L:16D (15 ± 2 ng/ml) and the areas under the prolactin response curves were likewise greater ( P < 0.01) under 16L:8D (83 ± 9, arbitrary units) than under 8L:16D (24 ± 5). The results indicate that hypothalamic releasing hormones can be used to obtain information on the endocrine status of rams. The acute testosterone response to GnRH injection is of particular interest since it not only reflects testosterone secretory capacity, but it also provides an index of prevailing LH secretion based on the absolute dependence of steroidogenesis on gonadotrophin support.