Responses of seasonally anovulatory mares to daily administration of thyrotropin-releasing hormone and(or) gonadotropin-releasing hormone analog.

The research investigates the effects of administering gonadotropin-releasing hormone analogs (GnRH-A) and thyrotropin-releasing hormone (TRH) to seasonally anovulatory mares, aiming to stimulate follicular growth and development. The study concluded that while both hormones had initial impacts on certain hormone concentrations in the mares, these effects subsided quickly, indicating no long-term benefits.

Study Methodology

• The study involved 17 seasonally anovulatory light horse mares and lasted 28 days, starting from January 5th.
• Researchers treated the mares using a 2 x 2 factorial arrangement with GnRH-A and/or TRH, hypothesizing that combined treatment would boost follicular growth and development.
• Ovaries were examined via ultrasonography and blood samples were taken every three days, while frequent blood samples were collected after administering the treatment on specific days.
• On day 29, all mares received a mixture of GnRH, TRH, sulpiride, and EP51389 (a growth hormone secretagogue) to evaluate pituitary responsiveness.

Observations and Findings

• There were no consistent effects observed in the plasma LH, FSH, prolactin, or thyroxine concentration in samples taken every 3 days.
• The only effect noted on ovarian follicle numbers was the reduction in number of 11 to 19 mm follicles due to TRH treatment.
• No mare ovulated during the treatment period.
• Mares receiving GnRH-A had higher mean LH and FSH concentrations and had a tendency to increase from day 1 through 7.
• Mares receiving TRH had high mean prolactin and thyroid-stimulating hormone levels on day 1, which rapidly decreased thereafter.
• The hormone response to the secretagogue mixture on day 29 varied based on the hormone previously received by the mare, with greater LH response in mares that had previously received GnRH-A, and greater prolactin respond in mares who received TRH.
• No immediate growth hormone response to EP51389 was detected in any of the mares.

Conclusions

• The study concluded that administering GnRH-A daily stimulates plasma LH and FSH concentrations immediately after injection; however, no long-term elevation in hormone levels was achieved.
• Daily TRH treatment increased plasma TSH and prolactin concentrations, but this response decreased rapidly, indicating a depletion of pituitary stores and minimal stimulation of production.
• The study found no beneficial effect of adding TRH treatment to the daily GnRH-A regimen.