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The Effect and Possible Mode of Action of -Melanocyte-Stimulating Hormone on Gonadotropin Release in the Ovariectomized Rat: An in Vivo and in Vitro Analysis^*

Department of Physiology, University of Texas Health Science Center at Dallas Dallas, Texas 75235

Address requests for reprints to: Dr. S. M. McCann, Department of Physiology, University of Texas Health Science Center at Dallas, 5323 Harry Hines Boulevard, Dallas, Texas 75235.

Abstract

This study was designed to examine the possible effects of cvMSH on gonadotropin release. Injection of MSH into the third ventricle of the brain and sampling at 5, 10, 15, 30, and 60 min postinjection produced a significant lowering of plasma LH, but not of FSH, in conscious ovariectomized (OVX) rats. The minimum effective dose was 1 µg. This procedure did not affect plasma levels of LH or FSH in estradiol benzoateprimed, OVX rats. Multiple blood sampling every 10 min before and after intraventricular injection of MSH (2 µg) produced a significant reduction in the area under the secretion curve of LH. Intravenous injection of MSH had little effect; however, a slight lowering of plasma LH occurred in OVX rats after a 50-µg dose. The effect of MSH on LH release was blocked by pretreatment of the animals with -methyl-paratyrosine, an inhibitor of catecholamine synthesis, as well as by pretreatment with an iv injection of spiroperidol, a dopamine receptor blocker. The peptide failed to alter basal or K⁺-stimulated LHRH release from median eminence fragments of OVX rats incubated in vitro. MSH had no effect on LHRH-induced LH release in vivo and failed to alter the release of FSH and LH from hemipituitaries of OVX rats or dispersed cells from OVX, estradiol benzoate-primed rats in vitro. It is concluded that MSH exerts an inhibitory effect on LH release by an action on the hypothalamus, probably via activation of the tuberoinfundibular dopaminergic system. (Endocrinology 114: 227, 1984)

Footnotes

^* This work was supported by NIH Grants HD-09988 and AM-10073.

Received May 3, 1982.

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