Serotonin and N-acetylserotonin effects on pigment cells of the toad Bufo ictericus: Pharmacological characterization of melatonin receptors

Melatonin is a skin lightening agonist of the toad Bufo ictericus. The hormone also exhibits an autodesensitizing action as well as an inhibitory activity on the darkening response to alpha-MSH. In an attempt to pharmacologically characterize the melatonin receptor of the toad pigment cells, serotonin and N-acetylserotonin (intermediate products of melatonin biosynthesis) biological activities were compared to melatonin effects. Serotonin (6.4 x 10(-5) to 10(-9) M) exhibited no lightening or inhibitory activity on MSH-elicited darkening responses or melatonin-elicited lightening responses. N-acetylserotonin, although less potent than melatonin, elicited a significant lightening response on previously MSH-darkened skins. Previous exposure of the skins with N-acetylserotonin reduced both MSH-induced darkening and melatonin-induced lightening in MSH-darkened skins. These results suggest a competitive inhibition of melatonin by N-acetylserotonin and indicate that melatonin receptors in B. ictericus pigment cells may be similar to ML-1 type found in chicken retina. (C) 1996 Academic Press, Inc.