Melatonin in fish: Circadian rhythm and functions

Melatonin (N-acetyl-5-methoxytryptamine) was isolated from bovine pineal tissue and chemically characterized by a group of dermatologists (Lerner et al., 1958, 1959) five decades ago who intended to use it in the treatment of pigmentary skin disorders. These studies were initiated because of a discovery 50 years earlier that something of pineal origin had a powerful blanching effect in the skin of tadpoles (McCord and Allen, 1917). At the time of melatonin’s discovery, there was widespread agreement among scientists that the pineal gland was a non-functional neural vestige. Since the characterization of melatonin, its synthesis in the pineal of all species has been documented and, because of its almost exclusive production at night, it is known as the chemical expression of darkness (Reiter, 1991). The light:dark dependency of the melatonin rhythm also has been characterized in a variety of fish species as in other classes of vertebrates (Bromage et al., 2001; Nikaido et al., 2009).