Secretion of luteinizing hormone into pituitary venous effluent of the follicular and luteal phase mare: novel acceleration of episodic release during constant infusion of gonadotropin-releasing hormone.

This study investigates the effects of continuous infusion of gonadotropin-releasing hormone (GnRH) on the levels and release patterns of luteinizing hormone (LH) in mares during their estrous cycle. The findings indicate that GnRH infusion significantly increases LH concentrations and alters its release patterns, revealing potential new insights into the regulation of gonadotrope function in horses.

Objective and Methodology

• The research aimed to verify the hypothesis that continuously infusing mares with gonadotropin-releasing hormone (GnRH) during their estrous cycle would increase the circulating levels of luteinizing hormone (LH) without changing the endogenous episodic pattern of LH release.
• The experiment included ten cyclic mares that were divided into two groups: those who received a saline control (group 1) and those who were administered 100 µg/h of GnRH in saline (group 2).
• On the experiment’s start day (3-6 days after ovulation), osmotic pumps containing either saline or GnRH were subcutaneously placed and linked to a jugular infusion catheter in the mares.
• Blood samples were collected from the jugular catheters daily. On the fourth day of the experiment (during the luteal phase of the cycle), additional samples were taken every five minutes from catheters placed in the intercavernous sinus (ICS) for an eight-hour period.

Key Results

• Treatment with GnRH led to a 3-4-fold increase in LH concentrations in the peripheral circulation and a 4-5-fold increase in the ICS.
• The frequency of LH release accelerated, and the interval between LH release episodes decreased in both the luteal and follicular phases.
• The GnRH treatment during the luteal phase changed the pattern of episodic LH release from a low-frequency, long-duration pattern to a high-frequency, short-duration pattern, similar to the follicular phase’s pattern.
• The effects observed in this study seem to be unique to horses, suggesting this study’s model can be useful for future research into regulatory mechanisms of gonadotrope function in horses.