IMMUNIZATION OF SHEEP AGAINST GNRH EARLY IN LIFE - EFFECTS ON GONADOTROPINS, FOLLICULAR-GROWTH AND RESPONSIVENESS OF GRANULOSA-CELLS TO FSHAND IGF-I IN 2 BREEDS OF SHEEP WITH DIFFERENT PROLIFICACY (ROMANOV AND ILE-DE-FRANCE)

The prolific Romanov (R, ovulation rate = 3) and non-prolific Ile-de-F rance (IF, ovulation rate = 1) breeds were compared for their ovarian sensitivity to gonadotropins and IGF-I before puberty. For this purpos e, the effects of in vivo immunization against GnRH on populations of ovarian follicles and in vitro sensitivity of granulosa cells to FSH a nd IGF-I were studied in prepuberal lambs from both breeds. Seventeen prepuberal lambs of each breed were actively immunized against GnRH be tween 3 wk and 6 mo of age. Relative to untreated lambs, FSH levels at 4, 5, and 6 mo of age were (respectively) 41%, 25%, and 29% for IF, a nd 43%, 24%, and 36% for R lambs. In a first experiment, histological analysis of ovaries was performed. Immunization treatment decreased th e number of small (100-390 mu m in diameter) and large size follicles (<1500 pm) in both breeds at 6 mo of age. In both breeds, gonadotropin (FSK - LH -hCG) treatment increased the number of large size follicle s (<1500 pm in diameter) and induced the formation of preovulatory fol licles in immunized as well as untreated lambs. The ovulation rate was less in immunized animals, but it was not different between breeds. I n a second experiment, the effects of FSH and IGF-I were studied on gr anulosa cells from follicles between 1000 and 2000 pm in diameter. In both breeds, IGF-I increased granulosa cell proliferation, but enhance d progesterone secretion was observed only in R lambs after FSH and IG F-I stimulation. Granulosa cell response to FSH treatment was lost by immunization, whereas response to IGF-I remained unchanged in both bre eds. These results indicate that long-term immunization of prepuberal lambs against GnRH reduced systemic concentrations of FSH, follicular development, and response to gonadotropins in vivo, similarly in the p rolific R and the non-prolific IF breed. However, granulosa cells from R lambs had higher steroidogenic capacities and were more responsive to FSH. In addition, these results suggest that IGF-I could play an im portant role in regulating growth of small follicles both in immunized and non-immunized lambs. (C) Elsevier Science Inc. 1998.