Minor interference of low-dose pulses of GnRH with the positive effect of oestradiol on the pituitary gland.

Abstract

In the ovariectomized rat we investigated the effects of prolonged treatment with oestradiol and 'physiological' pulses of exogenous GnRH on the GnRH-responsiveness of the pituitary gland. Rats were for 20 h pre-treated with either GnRH (3-ng pulses of 3 min duration every 20 min) or saline and for 19 h with either oestradiolbenzoate (OB; 3 micrograms/sc injection) or oil. Then LH and FSH responses were evoked by continuous test infusions of GnRH at the rates of either 60, 100, 150 or 1000 ng/h, lasting 10 h. It appeared that, irrespective of pre-treatment with either GnRH or saline, OB caused an increase of the maximal LH and FSH responses (that is the responses to the maximal stimulus of 1000 ng GnRH/h), and an increase of the GnRH-sensitivity of the pituitary gland, as far as the FSH secretion is concerned. GnRH pulses caused, irrespective of pre-treatment with OB or oil, a decrease of the GnRH-sensitivity of the pituitary gland, as well as a decrease of the maximal FSH response and of the pituitary FSH content. It was concluded that low-dose pulses of exogenous GnRH desensitize the pituitary gland and deplete the pituitary FSH stores, but do not change the positive effect of oestradiol on the maximal gonadotropin response.