Follicle-stimulating hormone blocks estrogen-positive feedback during the early follicular phase in monkeys

Normally cycling rhesus monkeys were treated with 12.5 IU of follicle-stimulating hormone (FSH; n = 9) or saline (n = 9) on cycle days 1 to 4, 1 to 6, or 1 to 8 and 50 micrograms/kg estradiol benzoate (E2B, intramuscularly) on day 1, 3, or 5, respectively. Blood was collected daily, then every 8 hours for 96 hours after E2B treatment. Serum was assayed for FSH and luteinizing hormone (LH) by radioimmunoassay. Laparoscopy and serum E2 levels were used to assess ovarian follicular development. Eight of nine FSH-treated monkeys failed to demonstrate typical LH surges in response to E2B. One monkey treated with E2B on day 1 had an attenuated LH surge. All nine control monkeys demonstrated typical threefold LH increases within 72 hours after E2B treatment. Supraphysiologic ovarian stimulation was not visibly evident until 5 days after initiation of FSH treatment. These results suggest that the FSH-induced blockade of estrogen-positive feedback for the LH surge antecedes overt ovarian hyperstimulation.