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 μg/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 feedbac for the LH surge antecedes overt ovarian hyperstimulation.