Action, Targets, and Receptors of Melatonin

Abstract

Melatonin is the hormone providing animals and humans with information on the length of the night and thereby also on the length of the day. In amphibians it elicits the primary chromatic response, i.e., blanching of the skin in darkness, by causing pigment aggregation in dermal melanophores (Rollag 1988; Fig. 5). This well-known action was used as a bioassay when Lerner and colleagues isolated melatonin in the late 1950s, and dermal melanophores of the frog Xenopus laevis served as the source for the first molecular biological identification of the melatonin receptor (Ebisawa et al. 1994). In birds melatonin is necessary for maintaining normal circadian function and imposes periodicity on structures that ultimately control overt circadian rhythms (Cassone 1990). In mammals melatonin can entrain circadian rhythms and acts in concert with light to keep the circadian rhythm in phase with prevailing environmental conditions (Arendt 1995; Lewy et al. 1992). Thus melatonin is useful to treat jet lag and some circadian sleeping disorders of humans (Arendt 1995). Moreover, melatonin is involved in regulating reproduction in seasonally breeding mammals (Reiter 1991).