Submitted to: Reproduction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2002
Publication Date: 2/1/2003
Citation: JIMENEZ-SEVERIANO, H., D'OCCHIO, M.J., LUNSTRA, D.D., MUSSARD, M.L., KOCH, J., EHNIS, L., ENRIGHT, W.J., KINDER, J.E. 2003. EFFECT OF CHRONIC TREATMENT WITH THE GONADOTROPHIN-RELEASING HORMONE AGONIST AZAGLY-NAFARELIN ON BASAL CONCENTRATIONS OF LH IN PREPUBERTAL BULLS. REPRODUCTION. v. 125(2). p. 225-232. Interpretive Summary: The use of herd sire registries and artificial insemination in the cattle industry have placed greater importance on larger testicular size and increased sperm production in stud bulls. Pituitary hormones play an important role in both pubertal development and mature testicular function because they contribute to an environment that promotes the production of germ cells and assists their development into viable sperm in the testis. Increased basal levels of the pituitary hormones LH and FSH in bulls during development results in higher levels of testosterone, larger testes and increased sperm production. The present study evaluated the effects of 28-day treatment with continuous infusion of a gonadotropin-releasing hormone (GnRH) analog, azagly-nafarelin (a GnRH agonist), on blood levels of LH, FSH and testosterone in neopubertal bulls (i.e., at 5 mo of age). Basal concentrations of serum LH and FSH were both increased above levels seen in control bulls from Day 2 through Day 28, and basal levels of serum testosterone also were increased significantly during the same period in animals treated with the GnRH agonist. While testis size was not increased by this relatively short duration of treatment, the increased serum testosterone levels imply that testis function was increased by treatment, probably induced by increased secretion of LH and FSH and increased testicular stimulation by these gonadotropins in animals treated with the GnRH agonist. These results indicate that pituitary hormone secretion and testicular steroidogenesis can be enhanced by use of a GnRH agonist in young bulls during development. Thus, this approach provides a unique animal model for further studies of factors that program and control testis function. In addition, manipulation of these factors may ultimately allow producers to enhance mature testis size, sperm production and reproductive efficiency in bulls used as sires in the cattle industry.
Technical Abstract: Infusion of GnRH agonists for extended periods inhibits pulsatile LH release, but enhances testicular function of bulls. The reason long-term infusion of GnRH agonist enhances testosterone (T) concentration in blood of cattle treated for extended periods with GnRH agonists has not been delineated. Therefore, a primary objective was to demonstrate unequivocally whether chronic treatment with the GnRH agonist, azagly-nafarelin, increases blood concentrations of LH and FSH in prepubertal bulls. Treatments were administered via Alzet mini-osmotic pumps for 28 d. Blood samples were collected at 20-min intervals for 24 h at 2, 13 and 25 d of treatment. Both agonist-treated groups had reduced T pulse frequency, and increased mean and basal concentrations of T, compared with the control group. Basal LH concentrations were greater in agonist-treated bulls during all three periods and overall (1 ng/ml greater, as compared with control bulls). Agonist treatment decreased LH pulse frequency to less than one pulse in 24 h. Control bulls had the greatest overall LH pulse amplitude compared with the two agonist-treated groups. Agonist-treated bulls had a slight but significant increase in FSH concentration. In conclusion, the greater T concentration in bulls with prolonged treatment with GnRH agonist may result, at least in part, from functional changes in the testes, induced by enhanced basal secretion of LH.