Effects of an agonist of gonadotropin-releasing hormone on ovarian follicles in cattle

Three experiments were conducted to examine effects of Buserelin, a potent agonist of gonadotropin-releasing hormone, on characteristics of ovarian follicles in cycling cows and heifers. In experiment 1, heifers were injected once with 10 micrograms Buserelin on Day 11, 12, or 13 of the estrous cycle (estrus = Day 0), or once with 20 micrograms of Buserelin on Day 12. Additionally, two groups were injected with a luteolytic dose of prostaglandin F(2 alpha) (PGF(2 alpha)) on Day 13 preceded with or without a Buserelin injection (10 micrograms) on Day 12. A control group did not receive a Buserelin injection. Ovaries were recovered and weighed after animals were slaughtered on Day 15. Follicle diameters were measured with calipers. Follicles for all experiments were classified as small (class 1: 3-5 mm diameter), medium (class 2: 6-9 mm), or large (class 3: > 9 mm). Heifers receiving only Buserelin had an increased number of medium-sized follicles compared to controls. Buserelin injection administered 24 h before PGF(2 alpha) reduced the decline in the average weight of the ovaries containing the corpus luteum (7.8 g for Buserelin before PGF(2 alpha) vs. 6.7 g for no Buserelin before PGF(2 alpha)). Buserelin pretreatment appeared to delay or prevent complete luteolysis by the injected PGF(2 alpha). In experiment 2, 0, or 10 micrograms Buserelin was injected on Day 12 and follicle development was monitored by ultrasonography in situ from Day 12 to estrus. Follicles also were classified as clear or cloudy; cloudy was associated with flocculent material in the follicular fluid or with an indistinct follicular wall. Treatment with Buserelin increased average number of medium-sized follicles, reduced the average number of large follicles, and increased frequency of cloudy follicles. In experiment 3, 0 or 10 micrograms Buserelin was injected on Day 12. Animals were slaughtered on Day 17, follicle development was examined in vitro by ultrasonography, and follicular fluid was aspirated from follicles of > 5 mm in diameter. The number of medium-sized follicles was increased, whereas the number of large follicles was reduced by Buserelin treatment. A linear relationship between aspirated volume and calculated volume of follicular fluid was detected in the control group versus a linear-quadratic relationship between these responses in follicles of the treatment group. An induced ovulation was produced in 48%, 0%, and 76% of the treated animals in experiments 1, 2, and 3, respectively. In summary, a single injection of Buserelin during diestrus in cyclic cattle alters ovarian follicle development as determined by changes in the distribution of follicles among size classes, increased incidence of cloudy follicles, an alteration in expected volume of fluid aspirated from a follicle, and induced ovulation in some cows.