Differential Regulation of Gonadotropins in Response to Continuous Infusion of Native Gonadotropin-Releasing Hormone in the Winter Anovulatory Mare and Effects of Treatment With Estradiol-17β.

This research examines how chronic daily injections of estradiol-17β (E2) and subcutaneous E2 implants, in combination with continuous subcutaneous infusion of Gonadotropin-Releasing Hormone (GnRH), could change pituitary secretion of gonadotropins in anovulatory mares during winter. The study also compares the secretion patterns of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) and analyses their variation in mares that develop or fail to develop a preovulatory follicle.

Research Methods and Objectives

• The research was divided into two experiments. Experiment 1 involved 20 anovulatory mares while experiment 2 involved 24 anovulatory mares. The goal was to test how different treatments affected pituitary secretion of gonadotropins and ovarian response to GnRH.
• The mares in both experiments were given varying treatments including the administration of E2 and GnRH, both individually and in combination. The effect of these treatments was measured by observing changes in LH, FSH, plasma E2 and estrone sulfate levels.

Key Findings

• In Experiment 1, E2 increased the response to GnRH as evidenced by a higher mean LH in GnRH + E2 treated mares on Day 3 as compared to mares treated with GnRH only.
• In Experiment 2, plasma E2 and estrone sulfate were increased in association with the development of large (35mm) follicles. However, these levels did not increase in response to E2 implant despite visible increases in uterine edema after their implementation.
• A steady increase in plasma LH was observed in all GnRH-treated mares, regardless of the type of E2 implant used.
• By Day 28, varying numbers of mares in different treatment groups developed large follicles, with none developed in the control group. These changes were not explained by variations in gonadotropin secretion.
• Notably, all GnRH-treated mares exhibited a marked increase in plasma FSH within 24 hours, although this returned to baseline by Day 4. In contrast, LH showed a sustained increase with the continuous infusion of GnRH.

Conclusions and Implications

• The results showed that twice-daily injection of 5 mg E2 enhanced pituitary secretion of LH in response to continuous administration of GnRH, a finding that could have implications for the treatment of anovulatory mares.
• In contrast, commercial E2 cattle implants failed to generate the same effects, suggesting that the method of E2 delivery may impact its effectiveness.
• The differential patterns of FSH and LH secrection raise new inquiries for detailed studies. By understanding the nuanced hormonal responses, it would be possible to further refine the treatments for anovulatory mares in winter.