Since the pineal complex only exists in the nervous system of vertebrates [11–14], and melatonin’s
described actions were endocrine in nature [15,16], melatonin was referred to initially as a
neurohormone and it was assumed to be only produced in the pineal gland. This image began to
change, however, when this indoleamine was identified in other organs as well, for example, in the
lateral eyes of vertebrates [17,18]. Subsequently, melatonin has been uncovered in numerous organs [19]
and, theoretically at least, it may be synthesized in every vertebrate cell [20]. Moreover, research
within the last two decades has definitively established a diversity of functions of this ubiquitously-distributed
indoleamine [21–25], many of which are not related to its actions as a hormone and some of which
seem not to require melatonin to interact with a receptor [26].