Effects of gonadal steroids on the opioid regulation of LH and prolactin release in ovariectomized pony mares.

This research study explored how ovarian hormones influence the way opioids regulate the release of two other hormones, luteinizing hormone (LH) and prolactin, in female horses that have undergone ovary removal. The results suggest that extended exposure to the hormone estrogen triggers opioid systems to limit the release of LH and prolactin, while the hormone progesterone alone does not appear to activate this process, but adjusts the effect of estrogen.

Goal of the Research

• The research set out to explore the impact of ovarian hormones, particularly progesterone and oestradiol benzoate (a form of estrogen), on the opioid management of LH and prolactin hormones in mares, or adult female horses.
• The aim was to gain a more thorough understanding of the hormonal changes that occur over a mare’s estrous cycle, and the effects of these hormones on LH and prolactin release.

Methodology

• The study used naloxone, an opioid antagonist, to measure LH and prolactin secretion in ovariectomized (ovaries removed) pony mares.
• These mares were first pretreated with either progesterone or oestradiol benzoate for a period of 8 days. Following this, they received a combination of both hormones for another 8 days.
• The researchers then administered naloxone to the mares and studied the resultant impact on the release of LH and prolactin hormones.

Findings

• The study observed a significant release of LH and prolactin in mares pretreated with either oestradiol benzoate or a combination of progesterone and oestradiol benzoate.
• No notable changes were observed in LH and prolactin secretion in mares treated with only progesterone or those that received no steroid treatment.

Interpretation and Conclusion

• The study concluded that extended estrogen exposure activates opioid suppression of LH and prolactin release in mares.
• It was also observed that unlike other species, progesterone on its own does not activate a tonic opioid suppression of LH and prolactin, but it does alter the effect of estrogens.
• The researchers note that the opioid systems appear to be influenced by an array of different steroid environments, following the pattern noted during the estrous cycle.
• The simultaneous increases in prolactin and LH release could suggest that these two hormones share a common regulatory pathway.