Melanocyte-Stimulating Hormone-Releasing and Inhibiting Factors in Two Hypothalamic Extracts1

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Abstract
Two acidic extracts were prepared from rat hypothalamus; one from the stalk median eminence (SME) and the other from the supraoptic nucleus area (SON). These extracts were injected into male recipient rats and the content of pituitary melanocyte-stlmulatlng hormone (MSH) was determined 20 min later. SME extracts induced a drop in pituitary MSH activity. SON extract did not change pituitary MSH activity but counteracted the effect of SME extracts. Plasma melanophoretic activity increased after injecting SME extracts into normal or stalk-median eminence-lesioned rats but not into hypophysectomized animals. When SON extract was injected together with SME extract the increase in plasma melanophoretic activity induced by SME extract was significantly reduced. It is concluded that SON extract blocks the release of MSH induced by SME extract and this effect is assigned to an MSH-release inhibitory factor.