IMMUNIZATION AGAINST GROWTH-HORMONE RELEASING-FACTOR OR CHRONIC FEED RESTRICTION INITIATED AT 3.5 MONTHS OF AGE REDUCES OVARIAN RESPONSE TOPULSATILE ADMINISTRATION OF GONADOTROPIN-RELEASING-HORMONE AT 6 MONTHS OF AGE AND DELAYS ONSET OF PUBERTY IN HEIFERS

A severe or moderate suppression of serum insulin-like growth factor I (IGF-I) was induced in heifers, beginning at 104 days of age, by acti ve immunization against growth hormone-releasing factor (GRFi) or by c hronic feed restriction (RES), respectively. We hypothesized that redu ced serum IGE-I results in decreased serum estradiol-17 beta (E(2)), w hich in turn delays onset of puberty. The objectives of this experimen t were to determine 1) whether GRFi and RES would alter follicular dev elopment and delay onset of puberty through similar mechanisms, and 2) whether GnRH would enhance follicular growth in control, GRFi, and RE S heifers at 6 mo of age. Changes in IGF-I, somatotropin, LH, FSH, and E(2) were evaluated. Serum IGF-I was greater in control than in RES h eifers, and was greater in both these groups than in GRFi heifers by 1 69 days of age. Basal LH decreased in control and RES but not in GRFi heifers from 136 to 157 days of age. During the same period, a decreas e in mean FSH was detected in control but not in GRFi and RES heifers. RES decreased mean serum E(2) from 148 to 183 days of age. At 6 mo of age, pulsatile administration of GnRH (5 mu g every 2 h for 42-46 h) increased serum LH and FSH similarly across treatments but had no effe ct on the number of follicles less than or equal to 8 mm in GRFi and R ES heifers relative to saline treatment. Serum E(2) and IGF-I in folli cular fluid from follicles greater than or equal to 8 mm were increase d in all GnRH-treated heifers; however, concentrations of both hormone s were lower in GRFi than in control or RES heifers. The main effect o f treatments on serum IGF-I was reflected in follicles less than or eq ual to 7 mm; follicular fluid IGF-I was greater in control than in RES heifers and was greater in both these groups than in GRFi heifers. Se rum E(2) was lower in RES than in control and GRFi heifers from 253 to 281 days of age. Because of an interaction, E(2) was lower in GRFi-Gn RH than in control-GnRH heifers but similar in GRFi-saline and control -saline heifers. By 393 days of age, 0% of RES and 32% of GRFi heifers had reached puberty compared to 71% of control heifers. These data su pport our hypothesis that decreased serum IGF-I results in decreased s erum E(2). GRFi appears to delay puberty in heifers because decreased serum IGF-I impairs the ovary's ability to synthesize preovulatory con centrations of E(2), thereby delaying stimulation of an LH surge. In c ontrast, RES may delay puberty by delaying follicular development at t wo stages: a) decreased IGF-I in follicles less than or equal to 7 mm may delay predominant follicular growth, and b) decreased LH may delay maturation of the preovulatory follicle.