Melatonin: A discussion of its evolution and actions in vertebrates

SYNOPSIS. Melatonin has many actions in vertebrates, with some considered hormonal. But are some melatonin actions more ancient than others? A survey of the tissues which synthesizemelatonin demonstrates that some are more recent vertebrate characters compared to others, indicating that melatonin action in these tissues also is more recent. The lateral eyes and pineal organs appear to be very ancient sources of melatonin and any action this molecule has within these tissues should be considered primordial. We hypothesize that melatonin's first actions (functions) were paracrine, that is, operating within these photoreceptive structures to facilitate the process of photoreception. Such actions have been documented. It is hypothesized that melatonin synthesis occurred at nightwithin the pineal organs and retinas of ancient vertebrates, as is the case among extant vertebrates. Accompanying the nightly synthesis of melatonin for paracrine function, secretion of melatonin either incidental or for detoxification by the liver occurred, providing a faithful template of the onset and/or duration of the scotophase. This nightly pulse of melatonin could provide important timing information to distant tissues capable of receiving the signal. The number of physiological systems within vertebrates "using" thenightly circulating melatonin pulse, and the apparent increased importance of circulating melatonin in timing physiological events in mammals, like reproduction, is the result of recent cooptation.