The role of endogenous opioids in the ovulatory LH surge in mares.

The research study explores the role of endogenous opioids (natural opioids created within body) in controlling the ovulation process in mares. The study identifies that the use of an opioid antagonist, naloxone, affects the release of Gonadotropin-releasing hormone (GnRH), Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH), implying that endogenous opioids have a part to play in ovulation.

Research Methodology

• Eight mares were subjected to a process of blood sample collection from pituitary venous every 0.5-1.0 minute for an hour before and after the administration of naloxone (0.2 mg per kg of body weight).
• Jugular blood samples were taken at 10-15 minutes interval.
• The mares were studied either 0, 1 or 2 days before ovulation.

Research Findings

• The administration of naloxone increased the average rate of secretions of GnRH, LH and FSH hormones significantly.
• The effect of naloxone on hormone secretion did not vary with proximity to ovulation period.
• There was increased frequency and amplitude of both GnRH and LH pulses after the administration of naloxone.
• While there was an apparent increase in LH and FSH concentrations in the jugular blood after naloxone administration, these changes were not statistically significant.

Conclusion and Implication

• The study concludes that endogenous opioids hamper GnRH secretion during ovulatory surge, thus slowing down the increase of LH concentration.
• The research suggests that using opioid antagonists could naturally speed up and enhance the ovulatory surge during the breeding season.
• The study alludes to the possibility of using a long-acting, orally-active analogue such as naltrexone, in place of a single injection of naloxone, in order to maintain elevated levels of GnRH and LH secretion, with the potential of raising peripheral LH concentrations and speeding up ovulation.