Seasonal variation and opioidergic regulation of growth hormone release in cyclic, ovariectomized, and pregnant pony mares.

This study explores the relationship between reproductive functions and growth hormone (GH) release in horses, with a special focus on the influence of seasonal changes, ovary removal, pregnancy, and opioidergic pathways.

Research Objectives and Methodology

• The main goal of the research was to understand how reproductive functions might influence the release of growth hormone (GH) in horses. The researchers examined GH concentrations in three types of mares: ovary-intact mares, mares with removed ovaries (ovariectomized), and pregnant mares.
• The study was conducted over two distinct periods: the non-breeding (anovulatory) season and the breeding season.
• The researchers also explored the possibility of opioidergic regulation of GH, that is, if GH release is connected with our body’s opioid system. To do this, they administered an opioid antagonist called naloxone.
• The GH concentration was measured using a Radioimmunoassay (RIA), a technique that measures the concentration of antigens in a sample.

Main Findings

• In both ovariectomized and ovary-intact, cyclic mares, the concentration of GH was found to be low during November and December, with no significant difference between the two groups. This suggests that the removal of ovaries does not influence GH’s seasonal fluctuation.
• During May and June, GH concentrations significantly increased in cyclic mares, while no change was observed in ovariectomized mares. This indicates a possible link between ovarian factors and GH release.
• In the case of pregnant mares, GH concentrations remained high throughout the pregnancy regardless of the season. Postpartum, the concentration of GH further increased significantly. This suggests that pregnancy can modulate GH release.
• Administering naloxone resulted in significant GH release in ovary-intact mares, with the effect being most pronounced during the breeding season. This illustrates a potential regulation of GH release by opioidergic pathways. However, such regulation was not observed in ovariectomized mares or during most of the pregnancy period.

Conclusions

• The research concludes that the release of growth hormone in horses is influenced by season.
• These seasonal fluctuations depend on ovarian factors, as shown by the absence of these changes in mares with their ovaries removed.
• Pregnancy was found to modulate GH release, which provides a different context for seasonal fluctuations in the hormone.
• Lastly, the release of GH appears to be regulated, in part, through opioidergic pathways. This was evident from the GH release in ovary-intact mares upon administering naloxone.