EFFECTS OF CHRONIC TREATMENT WITH A GONADOTROPIN-RELEASING-HORMONE AGONIST ON PERIPHERAL CONCENTRATIONS OF FSH AND LH, AND OVARIAN-FUNCTIONIN HEIFERS

The effect of chronic treatment with a gonadotrophin-releasing hormone agonist (GnRHa) on ovarian function in cattle was investigated by inj ecting heifers i.m. twice a day with saline, 5 mu g GnRHa (Buserelin) or 10 mu g GnRHa (n = 7) for 21 days. Blood samples were taken twice a day during the treatment period, and then three times a day for 7 day s and once daily for a further 4 days. Frequent samples were also coll ected on day I, day 10 and day 21 of treatment. The ovaries of all hei fers were examined daily using real-time ultrasonography throughout th e experimental period. No significant differences in the response were observed between two doses of GnRHa. The first GnRHa injection produc ed a large LH and FSH surge and this acute response was still present by day 21 of treatment, but both the magnitude and duration of respons e were significantly attenuated (P < 0.01). After an initial increase, LH returned to the basal concentration, which was maintained until th e termination of treatment, when concentrations increased significantl y, with a preovulatory surge occurring approximately 6 days later. Per ipheral FSH concentrations during the oestrous cycle in control animal s displayed a pattern of three waves, each of which closely preceded a wave of follicular development. Concentrations of FSH in GnRHa-treate d heifers showed a normal pattern for the first wave after the start o f treatment. During the next wave, concentrations increased and remain ed at the peak values until about 4 days after the end of treatment. A n additional ovulation was induced in II of 14 GnRHa-treated heifers w ithin 2-3 days of the start of treatment, and a significant (P < 0.05) increase in serum progesterone concentrations was detected 2 days lat er. All GnRHa-treated heifers then showed a normal follicular wave, wi th the development and regression of a dominant follicle. The dominant follicles from the next wave grew to only 7-9 mm in diameter and rema ined at this size until the end of treatment, when they resumed growth , ovulated approximately 7 days later and formed corpora lutea. We con clude that chronic treatment of heifers with GnRHa for 3 weeks suppres ses pulsatile secretion of LH and blocks the development of dominant f ollicles beyond 9 mm in diameter, preventing the preovulatory LH surge and ovulation. However, GnRHa did not suppress the secretion of FSH w ithin the 3 week treatment period. The maintenance of the dominant fol licles for an extended period should provide an ideal model to study t he control of follicular atresia in cattle in vivo.