Effects of exogenous steroids on serum FSH and LH, and on follicular development in cyclic mares.

This research explores the effects of externally administered hormones on hormone levels and follicular development in mares that have regular reproductive cycles. The study used several treatment groups, including mares treated with progesterone, progesterone plus estradiol, or estradiol alone, alongside a control group. The research suggests that progesterone inhibits the increase in luteinizing hormone, which leads to low luteinizing hormone concentrations and failed ovulation, while combined progesterone and estradiol treatment causes suppression of follicular development.

Study Overview

• In this study, research was conducted to investigate how externally administered hormones, specifically progesterone and estradiol, affect hormone levels and follicular development in mares with regular reproductive cycles.
• The mares were divided into four groups: one treated with progesterone (Group P), one treated with progesterone and estradiol (Group PE), one treated with estradiol alone (Group E), and a control group treated with a cottonseed oil vehicle (Group C).

Findings

• Progesterone appeared to inhibit an increase in luteinizing hormone during estrus in the control group, resulting in low luteinizing hormone concentrations. This subsequently led to failed ovulation of preovulatory-sized follicles.
• Mares from Group PE displayed lower luteinizing hormone concentrations than those from Group P. This was associated with suppressing the development of the largest follicle.
• Group E displayed little change in follicle-stimulating hormone and luteinizing hormone levels, although follicular development was variable, with ovulations noted on days 25, 33, and 36.
• None of the hormone treatments appeared to directly suppress follicle-stimulating hormone levels, but these levels showed a reciprocal relationship with luteinizing hormone-dependent follicular development.

Conclusions and Implications

• The findings suggest that progesterone may play a role in the suppression of luteinizing hormone during estrus, which could consequently affect ovulation in mares.
• The combination of progesterone and estradiol appears to have the effect of inhibiting follicular growth, potentially impacting fertility.
• These findings provide valuable insights into how hormone levels can be managed for reproduction in mares, with potential implications for both animal breeding and veterinary medicine.