Pulsatile gonadotropin secretion determined by frequent sampling from the intercavernous sinus of the mare: possible modulatory role of progesterone during luteolysis.

This study investigated the hormonal changes that occur during the luteolysis stage of the estrous cycle in horse mares. Researchers looked at the frequency of gonadotropin pulses and observed a potential regulatory influence of the hormone progesterone during this process.

Methodology

• The research team used twelve horse mares in a repeated-measures design. The study was arranged into three cycles of four mares each.
• Luteolysis, the process of the corpus luteum degenerating, was initiated by a prostaglandin F2 alpha (PGF2 alpha) injection on day 6 after ovulation.
• Blood samples were collected periodically from the intercavernous sinus (ICS) and jugular vein at specific time points, either 12 or 36 hours before the PGF2 alpha injection, and then 36 or 60 hours after the injection.
• The researchers also tested the pituitary gland’s reaction to exogenous Gonadotropin-Releasing Hormone (GnRH), a hormone crucial to reproductive processes.

Findings

• Progesterone levels were at their lowest 48 hours after the PGF2 alpha injection, while estradiol experienced a significant increase 156 hours after PGF2 alpha.
• Before luteolysis, Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH) pulse frequencies were 0.6 and 1.8, respectively.
• During this period, there were three high-amplitude concurrent LH and FSH pulses that correlated with higher gonadotropin levels in the jugular samples.
• Lower amplitude pulses were mostly FSH releases, which were not detectable in the jugular vein.
• After luteolysis, the gonadotropin pulse frequencies increased (targets of this study) significantly, which were mostly concurrent low-amplitude releases of LH and FSH but were not measurable in the jugular samples.

Implications

• This research provides insights for understanding the hormonal changes during the luteolysis phase of the mare’s reproductive cycle, with emphasis on gonadotropin pulse frequencies.
• The outcomes suggest that there might be a regulatory role of progesterone on the pituitary response during this period in mares.
• These findings can further advance studies on different aspects of equine reproduction and contribute to improving breeding strategy and fertility management in horses.