Effect of oestradiol on LH secretion and pituitary responsiveness to GnRH in ovariectomized mares

Long-term ovariectomized Pony mares were treated with oestradiol (0.2-5.0 mg; i.m.) at 12 h intervals for 10 days. Blood samples were collected by jugular venepuncture three times a day throughout the experiment and additional blood samples were collected at 15 min intervals for 12 h on days 0 and 10 (sampling periods 1 and 2, respectively). There were significant effects of oestradiol treatment (P < 0.05) and oestradiol treatment x day (P < 0.0001) on the mean LH concentrations each day. Regression analysis of LH time trends each day indicates that there is heterogeneity (P < 0.001) of regression due to the increasing LH concentrations in mares treated with oestradiol. These results indicate that oestradiol has a strong dose-related positive feedback on LH release in long term ovariectomized Pony mares. In Expt 2, pituitary sensitivity to GnRH in long term ovariectomized Pony mares after oestradiol administration was examined by administering a 100 mg bolus of GnRH (i.v.) twice a day on days 4 and 10 of a 10 day period of oestradiol administration. Blood samples were collected at 30 min intervals for 8 h on days 4 and 10 and once a day on the other days of treatment. The mean LH concentrations each day increased in oestradiol-treated mares over the 10 days of treatment (P < 0.002) and the pituitary responsiveness to GnRH also increased. These results indicate that oestradiol has a strong positive feedback effect on LH secretion by increasing the amplitude of the LH episode.