Depression of FSH and LH secretion following pulsatile GnRH administration in ovariectomized women

To investigate the mechanism by which pulsatile administration of gonadotrophin-releasing hormone (GnRH) modifies secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), we studied three groups of five women who had been ovariectomized for non-malignant gynaecological conditions at least 6 months previously, none of whom had received substitutional hormone therapy. Before and after 15 day treatment with subcutaneous pulsatile GnRH (one 20 microg dose every 90 min in group A, one 10 microg dose every 90 min in group B and one 20 microg dose every 120 min in group C), pulsatile secretion of LH and FSH was characterized by determining these hormones in 4 ml blood samples taken every 10 min for 8 h (9.00 a.m. to 5.00 p.m.). For both LH and FSH, mean serum concentration and pulse amplitude were lower after GnRH treatment than before (and in the case of LH the decrease depended upon both the size and frequency of exogenous GnRH pulses) but in no group was there a significant change in LH or FSH pulse frequency. We conclude that exogenous pulsatile GnRH probably acts by partially desensitizing the pituitary rather than by depressing endogenous GnRH secretion. Such partial desensitization would explain reports that exogenous pulsatile GnRH improves ovulation by women with polycystic ovary syndrome.