GnRH antagonist inhibition of gonadotropin and steroid secretion in boars in vivo and steroid production in vitro.

The hormone GnRH has a stimulatory effect on gonadotropin synthesis and secretion. The objective of the first study was to evaluate concentrations of FSH and LH in plasma of boars after successive treatment with SB75, a GnRH antagonist. Thirteen boars greater than 1 yr of age (eight White Composite [WC] and five Meishan [MS]) were injected once daily with SB75 (10 microg/kg of body weight) for 4 d. Plasma concentrations of LH and testosterone (T) decreased after 1 h from the first dose of SB75. After 12 h of treatment, LH gradually returned to pretreatment concentrations, but T remained suppressed (< 2 ng/mL) until after the last injection of SB75. There was a modest, but significant, reduction in FSH during treatment with SB75. The prolonged inhibitory effect of SB75 on suppression of plasma T concentrations, in the presence of pretreatment concentrations of LH, implied direct effects of SB75 at the testis. In the second experiment, testicular tissue from adult boars was incubated in the presence of three doses of human chorionic gonadotropin (hCG; 0, .5, and 5 IU) with SB75 (250 ng/mL) or with Deslorelin, a GnRH agonist (500 ng/mL). Samples of media were collected every hour for 3 h, and concentrations of T and estrone (E1) were determined by RIA. Concentrations of T and E1 increased with time in response to treatment with hCG. Co-treatment with SB75 decreased media concentrations of T (P < .01) and E1 (P < .03) compared to controls (77.9 vs 85.7 +/- 2.0 and 4.7 vs 5.3 +/- .2 ng/g). In contrast, treatment with Deslorelin had no effect on the amount of T (P > .50) or E1 (P > .26) released with all dosages of hCG. These results indicate that a GnRH antagonist has a direct effect on the testis, decreasing amounts of T and E1 released from the Leydig cells; however, treatment with a GnRH agonist had no direct effect on release of these gonadal steroids. Thus, it remains unresolved whether the site of action of GnRH antagonist on testicular steroidogenesis is through a testicular GnRH receptor or through some other mechanism.