Melatonin, the time keeper: biosynthesis and effects in fish
Melatonin is the time-keeping molecule of vertebrates. The daily and annual variations of its rhythmic production allow synchronizing metabolism, physiological functions and behaviour to the environment changes. In fish, melatonin is produced by the photoreceptors of the pineal organ and retina as well as by other retinal cell types in the inner nuclear and ganglion cell layers. In most species, the melatonin rhythm displays a high-at-night profile, resulting from the circadian control of the arylalkylamine N-acetyltranferase (AANAT) activity. AANAT is the penultimate enzyme in the melatonin biosynthesis pathway. This review summarizes our current knowledge on the molecular and cellular mechanisms controlling the rhythmic production of melatonin, as well as on the targets and modes of action of the hormone in fish, with special emphasis on neuroendocrine functions.