Vertebrate circadian and photoperiodic systems: role of the pineal gland and melatonin.

TLDR: This review focuses on melatonin’s role in two systems-the circadian system and the photoperiodic system because of the interrelationship between the circadian and photoperperiodic systems.

Abstract: first of these compounds isolated, and it had the interesting property of causing blanching of frog skin. A more important reason, however, arose from the belief that the terminal enzyme in the synthesis of melatonin, hydroxyindole-O-methyltransferase (HIOMT), was uniquely localized to pineal tissue. Consequently, melatonin and related methoxyindoles were, at least potentially, unique pineal products. Finally, interest in pineal products was generated because the pineal appeared to be involved in controlling important physiological functions, such as reproduction. Of the several related methoxyindoles, only 5-methoxytryptamine and 5-methoxytryptophol exhibit actions that, like those of melatonin, mimic certain influences of photoperiod on mammalian reproduction. Melatonin, however, appears to be at least 10 times more potent than any of the other naturally occurring methoxyindoles with respect to these actions (Reiter et al., 1975; Rollag, 1982; Goldman et al., 1984). This review focuses on melatonin’s role in two systems-the circadian system and the photoperiodic system. These two systems are discussed together because of the interrelationship between the circadian and photoperiodic systems: Many vertebrates utilize a circadian mechanism to measure daylength. A brief description of the structural evolution of the pineal and related organs is given as background for a more detailed discussion of melatonin’s role within the circadian and photoperiodic systems.