Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content.

The research examined whether administering steroid hormones to mares that had their ovaries removed would affect the secretion of two hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), especially during the transition to the breeding season. The study observed how these hormones responded to administered GnRH, a hormone that triggers the release of LH and FSH, and the results varied depending on the type of steroid hormone administered.

Overall Design of the Study

• The research was performed using 30 long-term ovariectomized (OVX) mares and was divided into two separate experiments.
• Throughout the research, the concentrations of LH and FSH in the mares’ blood were regularly assessed, and the hormone GnRH was administered to trigger LH and FSH release.
• Consequently, the mares’ pituitary gonadotropin content was monitored to check the response to the introduced GnRH.

Experiment 1

• Ten mares were divided into two groups: those treated with a vehicle control (n = 5), and those treated with estradiol-17 beta (E2), a form of estrogen (n = 5).
• E2 was administered twice daily for 16 days, with blood samples taken daily to measure LH and FSH levels.
• On the tenth day, all mares received an administration of GnRH to stimulate LH and FSH secretion.
• Results showed that LH levels significantly increased over the course of treatment in the estradiol-treated group, but these mares had a weaker pituitary response to the introduced GnRH than controls.
• FSH levels tended to decrease in the estradiol-treated group, and this group’s pituitary response to GnRH was significantly reduced, demonstrating that E2 has a likely suppressive effect on FSH secretion.
• However, pituitary LH levels, but not FSH, rose on the 16th day of treatment in the mares administered estradiol.

Experiment 2

• In this round, 20 OVX mares were divided into four groups to receive either a vehicle control, E2, progesterone, or a combination of progesterone and E2.
• All treatments were administered twice daily over 14 days, and GnRH challenges were introduced on the 6th and 13th days of treatment to provoke LH and FSH release.
• Results of Experiment 2 are, unfortunately, truncated from the abstract.

Conclusion

• From the information, it can be concluded that E2 affects the secretion of LH and FSH in ovariectomized mares, during the period transitioning to the breeding season.
• It is inferred that E2 administration stimulates an increase in LH, but may be associated with reduced FSH levels, indicating a differential effect on the two hormones.
• Further studies, including the completion of experiment 2, are likely needed to define the effects of other steroid hormones, like progesterone, and combinations of hormones, on gonadotropin secretion in mares.