Quantitative and qualitative determination of the effects of ovariectomy and suckling on hypotalamic luteinizing hormone releasing hormone in the rat

Typescript (photocopy).A study was undertaken to further the understanding of the mechanism by which suckling inhibits the release of pituitary luteinizing hormone (LH) 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 weeks on the LHRH content of the hypothalamus and preoptic area (POA) in ovariectomized and intact rats was examined. Controls consisted of both intact and 1 and 3 week ovariectomized, nonlactating animals. Following decapitation of half the animals from each group, the brains were rapidly removed and the POA and a block of hypothalamus containing the median eminence (ME) were isolated. Tissue was extracted with 2N acetic acid and LHRH quantitated by validated RIAs utilizing two antisera, each specific for different portions of the LHRH molecule. Brains from the remaining animals were removed following transcardial perfusion with phosphate buffered formalin and postfixed in Bouin's solution. The hypothalamus and POA were isolated, sectioned and LHRH localized by immunocytochemical staining. Total radioimmunoassayable LHRH per ME was found to be inversely correlated (r = -0.98; P < .0001) with plasma LH. Ovariectomy of nonlactating, diestrous animals resulted in a significant decline in hypothalamic LHRH, reaching 50% of control levels by 3 weeks. During the same intervals, plasma LH increased dramatically to 20 and 60 fold over intact controls by 1 and 3 weeks, respectively. In contrast, LHRH levels were not decreased at 1 or 3 weeks 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, although 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. Immunocytochemical assessment of the relative concentration of LHRH within each group concurred with the results obtained using RIA. In addition, the concentrations of LHRH were observed to be localized in the axons and termini abutting the hypophyseal portal vessels in the external layer of the ME and the vasculature of the organum vasculosum of the lamina terminalis within the POA. These data from ovariectomized rats provide evidence that suckling inhibits LHRH release from the hypothalamus (ME) and hence provides and explanation for the depression of plasma LH observed in suckled ovariectomized and intact animals