Pituitary response to steroid replacement therapy in fertile, subfertile and infertile stallions after castration

Recent studies in our laboratory investigating plasma gonadotrophin concentrations before and after castration indicate that the hypothalamic-pituitary axis is not the original site of dysfunction in stallions with idiopathic infertility. In the present study, fertile, subfertile and infertile stallions were subjected to replacement therapy with oestradiol and testosterone 1 year after castration to investigate hypothalamic-pituitary axis function further. Fertile (n=5), subfertile (n=3) and infertile (n=2) castrated stallions of Light horse breeds, aged 11-23 years, were given either oestradiol cypionate or testosterone propionate for 28 days during the breeding season. Oestradiol cypionate in cottonseed oil (30 mg; 6 mg ml(-1)) was administered i.m. at 2 day intervals starting in May for 28 days. Eight weeks after the last oestradiol cypionate injection, testosterone propionate in vegetable oil (85 mg; 8.5 mg ml(-1)) was administered i.m. at 2 day intervals for 28 days. Heparinized blood samples were taken from the jugular vein three times a week from 3 weeks before the start of oestradiol cypionate treatment until 4 weeks after the last testosterone propionate injection. Saline or 25 microg GnRH in 1 ml saline were administered to the stallions on days 24 and 25, respectively, of the 28 day steroid treatments. Jugular blood samples were collected periodically through a catheter from 60 min before saline or GnRH injection until 420 min after injection. The plasma samples were stored at -20 degrees C until analysed for plasma LH, FSH, oestradiol and testosterone concentrations by validated radioimmunoassay. Exogenous steroid treatment increased plasma oestradiol and testosterone concentrations to concentrations close to normal circulating values (oestradiol: 49 pg ml(-1); testosterone: 1.3 ng ml(-1)) in all the groups. There were no significant differences in plasma LH or FSH concentrations among the groups during any of the treatment periods. Oestradiol treatment significantly increased (P < 0.05) LH concentrations, whereas testosterone treatment decreased LH concentrations, although the difference was not significant. Oestradiol treatment significantly decreased FSH concentrations (P < 0.05), whereas testosterone treatment significantly increased FSH concentrations (P < 0.05). There were no differences in LH and FSH responses to GnRH challenge among the three groups of stallions. In conclusion, the results of the present study indicate that the hypothalamic-pituitary axes in these subfertile and infertile stallions were not the original sites of dysfunction. Further studies at the testicular level are required.