MELATONIN DESENSITIZING EFFECTS ON THE IN-VITRO RESPONSES TO MCH, ALPHA-MSH, ISOPROTERENOL AND MELATONIN IN PIGMENT-CELLS OF A FISH (S-MARMORATUS), A TOAD (B-ICTERICUS), A FROG (R-PIPIENS), AND A LIZARD (A-CAROLINENSIS), EXPOSED TO VARYING PHOTOPERIODIC REGIMENS

Melatonin is a weak dose-independent lightening agonist;in fish skin, a moderate dose-dependent lightening agonist in toad skin and a potent lightening agent in frog and lizard skins (reversing in a dose-depend ent manner the darkening caused by alpha-melanocyte-stimulating hormon e). In frog skins, previous exposure to melatonin reduced further ligh tening actions of the indoleamine, and in toad skins, increasing conce ntrations of melatonin elicited decreasing lightening responses, sugge sting an autodesensitizing action of the hormone. Various concentratio ns of melatonin diminished the responses to the lightening agonist mel anin-concentrating hormone (MCH) in fish skins and to the darkening ag onists alpha-MSH in toad, frog and lizard skins and isoproterenol in f rog skins. In vitro inhibitory actions of melatonin are mimicked in th e absence of the hormone in skin preparations from toads kept in conti nuous darkness for 48 hr. The lipophylic nature of the indoleamine ass ociated with the results herein described suggests intracellular actio ns of melatonin on vertebrate pigment cells.