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

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Abstract
Estradiol benzoate administration (EB) on the 2nd day of diestrus of 5-day cyclic rats may advance ovulation by 24 h. The threshold dose of EB necessary to achieve this effect varies with the time of administration. At 0900 h, the 50% effective dose (ED-50) of EB for advancing ovulation was 3.1 .mu.g; at 1700 h it was 31 .mu.g. In the present study, EB was injected at either 0900 or 1700 h at doses about 3 times the corresponding ED-50: 10 and 100 .mu.g, respectively. Though both regimens of EB administration resulted in advancement of ovulation, neither had more than a moderate effect on the development of pituitary responsiveness to [luteinizing hormone releasing hormone, luliberin]. In neither group could EB-induced LH[luteinizing hormone, lutropin]-surges be distinguished from normal 4-day rat LH surges and both differ from 5-day rat LH surges. Apparently in the Everett-model, EB acts almost exclusively on the CNS; the estrogen-induced LH surge is an all-or-none effect; and there exists no relationship between the blood estrogen concentrations and the characteristics of the induced LH-surges.