Effect of ACTH (tetracosactide) on steroid hormone levels in the mare. Part A: effect in intact normal mares and mares with possible estrous related behavioral abnormalities.

This research investigates the effect of the hormone ACTH on steroid levels in mares, with a primary focus on those that have abnormal estrous or reproductive behaviors. The study found that ACTH notably raised the levels of several steroids, but mares with abnormal behaviors responded differently to the hormone treatment, indicating potential differences in adrenal function.

About the Study

• The research focuses on the effect of ACTH (a hormone that regulates the release of steroid hormones) on plasma levels of cortisol, progesterone, androstenedione, and testosterone. This was studied in intact mares with normal reproductive behavior and mares showing abnormal reproductive behavior.
• This study was driven by the fact that some mares, despite being ovariectomized or having inactive ovaries, show signs of estrus (being in heat). This phenomenon suggests that adrenal-derived steroid hormones may be involved.
• The investigation also considered prior observations claiming that abnormal adrenal hormone production might cause changes in the behavior of some mares.

Methodology

• The study involved a controlled setup with two groups of mares: ‘controls’ (n=5), who had normal reproductive behavior, and ‘problem’ mares (n=7), whose owners reported abnormal reproductive behavior.
• Both groups had blood samples collected hourly over 26 hours on two separate occasions with either a saline or ACTH treatment administered.

Findings

• ACTH treatment caused a significant increase in the plasma levels of cortisol, progesterone, androstenedione, and testosterone in all mares, revealing ACTH’s potent effect on the release of these steroid hormones.
• ‘Problem’ mares had a significantly lower cortisol response to ACTH treatment in the 30 min to 3h post-treatment window compared to the control group. They also showed a significantly higher increase in progesterone levels during the same period.
• The results suggest a reduced adreno-cortical reactivity in ‘problem’ mares under the response to ACTH treatment, which implies a difference in adrenal gland function in these mares compared to the control group. However, the reason behind this discrepancy was not identified in the study.

Implications

• This study adds valuable information about the hormonal responses of mares and suggests that differences in these responses could be related to estrous behavior abnormalities.
• However, as the cause of differing responses was not defined, further research is needed to gain an in-depth understanding of the adrenal activity in ‘problem’ mares.