Abstract

Because melatonin, a hormone apparently synthesized only in the pineal gland, is the most potent known antagonist of melanocyte-stimulating hormone (MSH) in frog skin melanocytes, its ability to counteract the lipid-mobilizing action of MSH (and of adrenocorticotropic hormone, ACTH) in rabbit adipose tissue was investigated. Melatonin did not interfere with glycerol release when the latter was stimulated by α-or β-MSH, but was inhibitory in the presence of ACTH. In rat adipose tissue, the effects of melatonin and a group of closely related hydroxyindole compounds were studied, with and without epinephrine present. Serotonin and N-acetylserotonin stimulated glycerol release slightly when present alone, but had little (N-acetylserotonin) or no (serotonin) effect when epinephrine was added. Both melatonin and 6-hydroxymelatonin produced an increase in glycerol (and free fatty acid) release in the presence of epinephrine, but not in its absence. Inhibition of glycerol release by 5-methoxytryptamine was more evident when epinephrine was present. These hydroxyindoles probably do not have a role in the regulation of lipolysis in adipose tissue. The findings suggest, however, that it may be useful to investigate the direct effects of these compounds on other mammalian tissues, particularly in relationship to the action of ACTH and epinephrine.