Suppression of circulating concentrations of FSH and LH by inhibin and estradiol during the initiation of follicle deviation in mares.

The study investigates the role of hormones estradiol and inhibin in suppressing follicle-stimulating hormone (FSH) and luteinizing hormone (LH) during the initial stages of follicle deviation in mares.

Study Design

• The research design involved two main experiments. Experiment 1 examined changes in hormone levels after manipulating the sizes and presence of follicles (F1, F2), while Experiment 2 examined the hormone alterations after administration of estradiol.
• In Experiment 1, two main follicles were retained during a wave, and all other follicles of 10 mm or greater were removed. Depending on the subgroup of the experiment, the largest follicle (F1) was either left untouched or removed when it reached a size of 20 mm or more (Day 0). Different patterns of growth and regression were observed in the second largest follicle (F2) after potential alteration of F1.
• Experiment 2 focused on the effect of estradiol administration at the projected beginning of deviation (Hour 0), in situations where 0 or 1 follicle was preserved.

Findings from Experiment 1

• The levels of circulating estradiol and total inhibin decreased after Day 0 in the F2-regressing group. However, in the control and F2-growing groups, estradiol levels increased after Day 0.5 and inhibin levels remained stable.
• In terms of FSH, its levels decreased in the control group, while it increased in the F2-regressing group post Day 0. For the F2-growing group, there was a surge in FSH during Days 0 and 0.5, which later declined.
• The levels of LH increased between Days 0 and 2 in the F2-regressing group, and between Days 0 and 0.5 in the F2-growing group.

Findings from Experiment 2

• Total inhibin levels were higher and FSH levels were lower at Hour 0 in those groups which retained a single follicle, compared to those which did not retain any.
• Following the administration of estradiol, FSH decreased in those groups that received 1 mg of estradiol but didn’t change in those receiving no estradiol. In the group that had no follicle but received 1 mg of estradiol, the FSH decrease did not match levels seen in the 1-follicle/1-mg group.
• Luteinizing hormone levels were not affected by the number of follicles present but were reduced following estradiol administration.

Conclusions

• The results of the study support the hypothesis that the largest follicle (F1) near the beginning of deviation produces inhibin and estradiol. Moreover, the increase in circulating estradiol at the beginning of deviation causes FSH suppression in conjunction with other substances produced by the follicle, presumed to be inhibin.
• The findings also suggest that the rise in estradiol also leads to a suppression of LH.