Mechanism of induction of LH surge in the rhesus monkey: positive feedback of estrogen?

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Abstract
Experiments were designed to determine whether the luteinizing hormone (LH) surge induced by estrogen results from a positive feedback effect of the steroid or from some alternative mechanism. Ovariectomized rhesus monkeys were implanted s.c. with different numbers of Silastic capsules containing 17.beta.-estradiol, producing serum estrogen concentrations of 149 .+-. 10, 245 .+-. 13, or 452 .+-. 26 pg/ml (mean .+-. SE). All treatments caused an initial decrease in serum LH levels followed by an LH surge. When serum estrogen was maintained at these levels throughout the ensuing LH surge, peak LH levels varied, with significantly lower LH peaks occurring in response to the lowest estrogen levels, a result suggestive of either positive feedback or of 149 pg/ml being a marginally sufficient estrogen stimulus. When the estradiol implants were removed as the LH surge was starting, all animals showed LH surges of normal magnitude, apparently synchronized in time. An interaction between the systems may control the tonic and surge modes of LH secretion. Positive feedback may not be a critical mechanism controlling the LH surge. Estrogen is apparently necessary to initiate the LH surge, but its continued presence is not required to sustain that phenomenon.

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