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The strength—duration characteristics of the increments in circulating estrogen required to elicit LH surges in the rhesus monkey were investigated by implanting (sc) Silastic capsules containing estradiol—17β for varying periods of time during the early follicular phase of the menstrual cycle. The magnitude of the resultant “square—wave” increments in serum estrogen concentration was varied by the number of capsules implanted. It was found that serum estrogen concentrations below 100 pg/ml were ineffective in eliciting LH surges, even when applied for as long as 120 hr. Serum estrogen concentrations of 100– 200 pg/ml had! to be maintained for approximately 42 hr to induce LH discharges, whereas 200–400 pg/ml were effective when applied for 36 hr. Supraphysiological estrogen levels of 1200 to 2000 pg/ml elicited LH surges when sustained for only 24 hr. In all instances, LH surges were initiated before the capsules were removed, showing that this response is not a consequence of estrogen withdrawal. The results further indicate that a stimulus, to be effective, must be applied until the response actually begins and that the initiation of the LH surge in the monkey does not appear to be synchronized with the diurnal light—dark cycle.(Endocrinology92: 1740, 1973)
Karsch et al. (Fri,) studied this question.