Evidence for a hypothalamic site of action of clomiphene citrate in women

To examine the site of action of clomiphene citrate (CC), LH and FSH pulsatile amplitude, frequency, and responsiveness to GnRH (10 micrograms, iv) were studied in 11 women during the early follicular phase of the menstrual cycle. Six women received CC (150 mg/day) on cycle days 2, 3, and 4, while 5 women received placebo tablets. Blood samples were drawn at 10-min intervals for 8 h before and after the treatment regimen on cycle days 2 and 5, respectively. All women treated with CC had multiple follicular development, as determined by ultrasound. Peripheral levels of estradiol did not change after CC treatment, while progesterone levels decreased slightly. Mean levels of LH increased from 7.5 +/- 0.9 (+/- SEM) to 10.7 +/- 1.4 mIU/ml (P less than 0.05), and FSH increased from 6.7 +/- 0.9 to 10.1 +/- 0.9 mIU/ml (P less than 0.01). After exposure to CC, the pulse frequency of LH during an 8-h period increased significantly (3.3 +/- 0.7 on day 2 vs. 6.8 +/- 0.8 on day 5; P less than 0.01), while the pulse frequency of FSH increased from 3.8 +/- 0.6 to 5 +/- 1.4, as determined by computer pulse analyses. The pulse amplitude of LH and FSH was not significantly altered. In the placebo-treated group, neither pulse amplitude nor pulse frequency changed significantly between cycle days 2 and 5. Pituitary sensitivity to exogenous GnRH did not change after CC treatment. Since the pulsatile frequency of LH is governed by hypothalamic influences, these findings provide compelling evidence for a hypothalamic site of action for CC, probably by inducing an increase in the frequency of GnRH secretion.