GONADAL AND PITUITARY-RESPONSIVENESS OF STALLIONS IS NOT DOWN-REGULATED BY PROLONGED PULSATILE ADMINISTRATION OF GNRH

The objective of this study was to determine if prolonged pulsatile ad ministration of homologous gonadotropin-releasing hormone (GnRH) at th erapeutic or 5x therapeutic doses would cause down-regulation of the s tallion's hypothalamic-pituitary-testicular axis. Fifteen stallions we re randomly assigned to three treatment groups (n = 5(group) and recei ved a 0.5 ml subcutaneous dose of saline (group 1), 50 mu g GnRH (grou p 2), or 250 mu g GnRH (group 3) every 2 hours for 75 days. Weekly eva luations of follicle stimulating hormone, luteinizing hormone, and tes tosterone and monthly evaluations of daily sperm output and spermatozo al motility failed to demonstrate any decreased pituitary or gonadal r esponsiveness within or among treatment groups (P > 0.1) as a result o f treatment with GnRH. Results of th is study demon strate that the hy pothalamic-pituitary-testicular axis of the stallion, unlike that of o ther domestic species, is remarkably refractory to GnRH-induced down-r egulation.