On the development of pituitary responsiveness to LRH and the characteristics of pre-ovulatory LH-surges in the rat. Effect of oestradiol benzoate.

Abstract

Administration of oestradiol benzoate (OeB) on the second day of dioestrus of 5-day cyclic rats may advance ovulation by 24 h. The threshold dose of OeB necessary to achieve this effect varies with the time of administration. At 09.00 h, the 50% effective dose (ED-50) of OeB for advancing ovulation was 3.1 microgram; at 17.00 h it was 31 microgram. In the present study OeB was injected at either 09.00 or 17.00 h at doses about 3 times the corresponding ED-50's: 10 and 100 microgram, respectively. Though both regimens of OeB administration resulted in advancement of ovulation, neither had more than a moderate effect on the development of pituitary responsiveness to LRH. In neither group could OeB-induced LH-surges be distinguished from "normal" 4-day rat-LH-surges and both differ from 5-day rat-LH-surges. It is concluded that (1) in the "Everett-model" OeB acts almost exclusively on the central nervous system; (2) the oestrogen-induced surge of LH is an all-or-none effect; and (3) there exists no relationship between the blood oestrogen concentrations and the characteristics of the induced LH-surges.