Changes in Hypothalamic and Pituitary Content of Immunoreactive -Melanocyte-Stimulating Hormone during the Gestational and Postpartum Periods in the Rat

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Abstract
.alpha.-MSH was measured in the mediobasal hypothalamus (MH), median eminence (ME), preoptic-suprachiasmatic area (POA-SCN), anterior (AL) and posterior lobes (PL) of the pituitary gland during the gestational and postpartum periods in the rat. The content of .alpha.-MSH in the MH and POA-SCN compared to estrous levels was lower during the later days of gestation and decreased further in the MH during lactation in association with the elevated plasma prolactin (Prl). Distinct increases in the ME content of .alpha.-MSH compared to estrous levels occurred on days 8 and 12 of the gestational period and day 14 of the postpartum period. A significant increase in PL content of .alpha.-MSH compared to days 5-11 and 17-20 occurred on day 4 of gestation, and no significant changes were detected in the AP concentration of .alpha.-MSH throughout the period studied. In vitro, PL and AL from females on day 4 of gestation secreted more .alpha.-MSH into the incubation medium than tissues from animals on day 20. .alpha.-MSH of both brain and pituitary origin may play a role in mediating some of the physiological changes which occur during pregnancy and lactation.