The influence of time, dose and gonadal steroids on LH-RH-induced gonadotrophin release in rats.

Hypophyseal responses to LH-releasing hormone (LH-RH) were studied in ovariectomized rats, pre-treated once with 50 mug oestradiol-3-benzoate alone or in combination with 25 mg progesterone either 16, 24, 48 or 72 h before the administration of LH-RH. The highest serum gonadotrophin increase was detected in ovariectomized rats pre-treated with oestrogen 48 h before the LH-RH injection. The serum LH concentration change in intact male rats was markedly lower in terms of order of magnitude than in ovariectomized, oestradiol-3-benzoate, progesterone-treated rats ("O.Oe. P."-rats). These latter test animals showed the maximal increase of serum LH concentration, depending on the dose of LH-RH, after 10 to 25 min and that of FSH after 40 min. The dose-response curves for LH release in "O.Oe.P."-rats were linear for short (10 and 15 min) and sigmoid for longer time intervals (20, 30 and 40 min) between LH-RH injection and sacrifice. The possible reasons for this difference in both magnitude and time-lapse for maximal increase of serum LH and FSH concentration, the influence of steroid pre-treatment and the variation of the time course of serum LH concentration, depending on the dose of LH-RH, are discussed.