Opioidergic inhibition of luteinising hormone and prolactin release changes during pregnancy in pony mares.

This study investigates how hormonal changes during pregnancy in pony mares, particularly the release of luteinising hormone and prolactin, are influenced by opioids. Both hormones were proven to drop during pregnancy, but experience a surge post-childbirth, suggesting a potential role of endogenous opioids in this regulation pattern.

Research Methodology and Procedures

• The study involved a group of six pregnant Shetland mares at different stages of gestation (26.4, 75.4, 171.8, 226.2, 282.7 and 319.8 days). The experiment was designed to simulate a consistent environment which led to mares serving as their own controls.
• Mares were given injections of naloxone, an opioid antagonist known for blocking the opiate receptors, and saline. The doses of naloxone were kept constant at 0.5 mg per kg of body weight.
• To stimulate hormonal response, two hours post naloxone and saline administration, mares received a dose of buserelin, a Gonadotropin-releasing hormone (GnRH) analog. This experiment was duplicated two days post-foaling.
• The team monitored the levels of LH and PRL in the mares’ bloodstream, through blood samples taken at 15-minute intervals over a course of four hours, with injections of either naloxone or saline given after the first 60 minutes.

Key Findings

• Naloxone induced a significant release of LH on days 172, 226, and 283 of pregnancy. However, it didn’t cause any significant changes on days 26, 76, and 320 or two days after the mares gave birth.
• Buserelin, when introduced into the system, caused a surge in plasma LH concentrations on days 172, 226, 282, and 320 of pregnancy.
• In terms of PRL levels, the study observed a substantial increase post naloxone administration on days 226, 282 and 320 of gestation.
• The surge in LH and PRL release post naloxone administration suggests that naturally produced opioids could be suppressing these hormones during the latter half of pregnancy in ponies.
• This deactivation of opioids’ effects on LH function could be a necessary step for initiating postpartum ovarian activity.

Implications

• The opioid-mediated control of hormonal release could help in understanding the complex hormonal mechanics during equine pregnancy and parturition.
• The insights could be pivotal for developing methods to manage potential complications related to horse breeding or equine reproductive disorders.