Induction of Follicular Development, Maturation and Ovulation by Gonadotropin Releasing Hormone Administration to Acyclic Mares

Abstract

Five acyclic mares with inactive ovaries were given three courses of gonadotropin releasing hormone (GnRH) beginning on Days 1, 11 and 21 of treatment to reproduce the gonadotropin surges responsible for causing follicular growth and ovulation during the normal estrous cycle. The first two courses consisted of 3–4 im injections of 1.0 mg GnRH, and the third course of 5 im injections of 0.4–1.4 mg GnRH, at approximately 12 h intervals. Exogenous progesterone was given on Days 5 or 6 through 16 to simulate luteal phase progesterone levels. Serum FSH, LH, progesterone and estradiol were measured daily throughout the experimental period and at frequent intervals following GnRH administration. The GnRH injections in all courses induced peak FSH concentrations of 2.5–4 times baseline by 0.1–5 h after injection, and the levels were almost back to pre-injection concentrations by 12 h. Each GnRH injection in the first two courses resulted in increases in serum LH to 1.5–3 times baseline by 0.5–1 h after injection. The third course caused a greater release of LH which was thought to be related to the increases in serum estradiol occurring at this time. Typical pre-estrous follicles were present in all mares by Day 19 of treatment. All mares were in estrus by Day 21, and each ovulated a single large follicle between Days 24 and 27 of treatment. In three mares high LH levels occurred for 5 days after the last GnRH course and these mares established corpora lutea; the FSH, LH and estradiol levels induced by GnRH were similar to those which lead to ovulation in the normal cycle. In the other two mares LH fell to pre-treatment levels by 12 h after the last GnRH injection; these mares failed to establish corpora lutes. Thus it appears that GnRH in combination with appropriate progesterone treatment in the acyclic mare treated towards the end of the nonbreeding season can consistently induce normal cyclic pituitary and ovarian activity culminating in ovulation.