Effect of Chronic Administration of Oxytocin on Corpus Luteum Function in Cycling Mares.

The research study investigates the impact of long-term administration of oxytocin on the function of corpus luteum (CL) in female reproductive mares. It suggests that giving 60 units of oxytocin daily for 29 days, irrespective of estrous cycle timings, increases the CL function duration, which may not require monitoring estrous behavior or day of ovulation.

Objectives and Methodology of the Study

• The study aimed to examine if giving daily doses of 60 units of oxytocin for 29 days, independent of the stage of the estrous cycle, would result in prolonged CL function in reproductive mares.
• Two groups were formed for the study, one receiving saline solution (control group with 7 mares) and the other – oxytocin (treatment group with 9 mares). Both groups received their respective treatments intramuscularly for 29 successive days, starting from the same day, irrespective of the estrous cycle stage.
• Blood samples were collected three times a week for 21 days before initiation of the treatment to confirm that all mares had a normal duration luteal phase. Post-treatment initiation, blood samples were collected daily for eight days, followed by a thrice-weekly schedule until the 80th day.
• Increased CL function was determined by a serum progesterone level of over 1.0 ng/mL for at least 25 days after the treatment period ended.

Findings of the Study

• The study found a heightened number of mares with extended CL function in the oxytocin group as compared to the saline group (7 out of 9 mares vs. 1 out of 7, respectively).
• Three of seven mares that received oxytocin experienced luteolysis within 4-7 days from the treatment commencement, but subsequently developed prolonged CL function after ovulating during the treatment period.
• The remaining four mares in the oxytocin group that experienced prolonged CL function retained progesterone level over 1.0 ng/mL throughout the treatment and beyond.
• All mares with prolonged CL function maintained increased progesterone levels till at least the 55th day of the study.

Conclusion of the Study

• The study concluded that injecting 60 units of oxytocin for 29 consecutive days can effectively prolong the CL function in mares, irrespective of when the treatment is initiated during the estrous cycle.
• This research contributes to an alternative protocol for utilizing oxytocin treatment to extend CL function, which bypasses the need to detect estrous behavior or day of ovulation, thereby simplifying the treatment process.