COMPARATIVE-STUDY BETWEEN PONY MARES AND EWES EVALUATING GONADOTROPICRESPONSE TO ADMINISTRATION OF GONADOTROPIN-RELEASING-HORMONE

This study compared equine and ovine LH secretory responses to GnRH tr eatment. Dioestrous mares and ewes were challenged with continuous GnR H for 15 h. Mares that received constant GnRH (110 mu g h(-1)) had sus tained LH secretion (P < 0.01), whereas LH concentrations in ewes trea ted with continuous GnRH (25 mu g h(-1)) initially increased, then dec lined and remained low, suggesting GnRH receptor desensitization or do wnregulation. In addition, progesterone-primed, ovariectomized mares a nd ewes were challenged with pulsatile or continuous GnRH for 5 days. Plasma LH concentrations were increased by day 5 in mares treated with pulsatile (25 mu g pulse(-1) h(-1)) and continuous (110 mu g h(-1)) G nRH (P<0.01). Furthermore, mean LH concentrations and time-response cu rves were not different. In contrast, ewes treated with continuous GnR H (2.5 mu g h(-1)) demonstrated LH secretory patterns indicative of Gn RH receptor downregulation on day I of treatment. LH concentrations in ewes treated with pulsatile GnRH (250 ng pulse(-1) h(-1)) did not dif fer from controls. In conclusion, pony mares responded continuously to GnRH treatment (pulsatile and continuous), whereas ewes treated with continuous GnRH experienced reduced LH secretion. These findings sugge st a unique hypothalamic-pituitary axis in pony mares.