Plasma and pituitary MSH activities were measured in albino rats after various treatments. Administration of trifluoperazine increased plasma MSH activity and decreased pituitary MSH activity. Ether also elevated plasma MSH levels and lowered pituitary MSH content 2 min after anesthesia; however, plasma MSH activity in these rats exposed to ether vapors subsequently declined despite a persistent decrease in pituitary MSH levels. Pretreatment with dexamethasone failed to block the MSH releasing effects of ether at 2 min. MSH also appeared to be released by synthetic lysine vasopressin. Darkness diminished and pinealectomy elevated pituitary MSH content but did not seem to affect plasma MSH levels. The tendency for Nembutal to cause a lowering of plasma MSH activity without much change inpituitary MSH content was not statistically significant. Morphine raised plasma MSH levels but did not alter pituitary MSH levels. An interaction of the effects of treatments on MSH release was observed with morphine and ether: Pretreatment with morphine blocked the effects of ether, and, conversely, pretreatment with ether blocked the effects of morphine. The combination of Nembutal and morphine significantly elevated plasma MSH activity and greatly lowered pituitary MSH levels. Injection of purified MSH-release inhibiting factor (MIF) into these Nembutal-morphine treated rats was found to decrease plasma MSH activity and increase pituitary MSH content, thus supporting the concept that MIF inhibits the release of MSH. (Endocrinology 84: 20,1969)

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