Hypothalamic-pituitary responsiveness during lactation was studied by determining the ability of estrogen to induce surges of LH. In intact females nursing eight pups, 50 μg estradiol benzoate (EB) on either day 4, 9, or 14 postpartum induced surges of LH the following day that were similar in magnitude, timing, and duration. The results from females nursing two pups were similar to those from females nursing eight pups. When compared to the proestrous surge of LH, the surges in intact lactating animals had an earlier onset and decreased peak concentration of LH. The effect of high levels of progesterone on EB-induced LH surges was determined during pregnancy, when progesterone concentrations were similar to those during lactation. On day 14 of pregnancy 50 μg EB evoked LH surges the following day that did not differ from proestrous surges. Therefore, during lactation, the decreased ability of the hypothalamus to respond to estrogen is most likely due to the suckling stimulus or prolactin. The effects of suckling were most clearly demonstrated in the ovariectomized females nursing eight pups. On either day 4 or 9, 50 μg EB induced LH surges the following day that were significantly smaller than those in intact lactating females. However, by day 15, LH surges with similar peak concentrations were observed in ovariectomized and intact females. The factors responsible for the larger LH surges in intact females were studied by treating intact females nursing eight pups with CB-154 to inhibit prolactin and, indirectly, progesterone secretion. After treatment with CB-154, EB-induced LH surges were much smaller than in control animals. These results suggest that suckling, in the absence of ovarian steroids, can greatly suppress hypothalamic- pituitary responsiveness to estrogen, since this inhibitory influence of suckling is much less effective in intact lactating animals.
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