Melatonin and 5-methoxytryptamine in non-metazoans

Melatonin seems to be an almost ubiquitous substance, which has been detect ed not only in metazoans, but also in all major non-metazoan taxa investiga ted, including bacteria, dinoflagellates, euglenoids, trypanosomids, fungi, rhodophyceans, pheophyceans, chlorophyceans and angiosperms. Despite its v ast abundance, little is known to date about its functions. Its presence is not necessarily associated with circadian rhythmicity, which is evident in yeast. Circadian rhythms of melatonin have been reported in non metazoans only for several unicellular organisms and in one angiosperm. In dinoflagel lates, which have been studied in the most detail, the effects on enzyme ac tivities and on phase shifting are known, but the most spectacular actions concerning the stimulation of bioluminescence, changes in cytoplasmic pH an d induction of resting stages, can be related to a metabolite of melatonin, the 5-methoxytryptamine; therefore, melatonin should also be considered as a source of other agonists. (C) Inra/Elsevier, Paris.