Inhibition of the Postovariectomy Depletion of Hypothalamic Luteinizing Hormone Releasing Hormone (LHRH) by Suckling1

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Abstract
A study was undertaken to further the understanding of the mechanism by which suckling inhibits the release of pituitary LH [lutropin] and depresses the postovariectomy rise of plasma LH in lactating mammals. To that end, the effect of suckling (10 pups/animal) for 1 or 3 wk on the LHRH content of the hypothalamus and preoptic area (POA) in ovariectomized and intact rats was examined. Controls consisted of intact and 1 or 3 wk ovariectomized, nonlactating animals. Following decapitation, the brains were rapidly removed and blocks containing the POA and the hypothalamus (with median eminenece) were isolated. Tissue was extracted with acetic acid and LHRH quantitated via validated radioimmunoassays utilizing 2 antisera specific for different portions of the LHRH molecule. Ovariectomy of nonlactating, diestrous animals resulted in a significant decline in hypothalamic LHRH, reaching 50% of control levels by 3 wk. During the same intervals, plasma LH increased dramatically to 20- and 60-fold over intact controls by 1 and 3 wk, respectively. In contrast, LHRH levels were not decreased at 1 or 3 wk in ovariectomized rats which were suckled, at which time plasma LH was greatly depressed. When intact animals were evaluated, the suckling stimulus failed to induce a detectable change in LHRH content of the hypothalamus, altough plasma LH was suppressed. No significant differences were observed in the concentration of LHRH in the POA between any of the treatment or control groups. These data from ovariectomized rats suggest that suckling inhibits LHRH release from the hypothalamus and hence provides an explanation for the depression of plasma LH observed in suckled ovariectomized and intact animals.

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