Gut Melatonin in Vertebrates: Chronobiology and Physiology

Abstract

Melatonin, following discovery in the bovine pineal gland, has been detected in several extra-pineal sources including gastrointestinal tract or gut. Arylalkylamine N-acetyltransferase (AANAT) is the key regulator of its biosynthesis. Melatonin in pineal is rhythmically produced with a nocturnal peak in synchronization with environmental light–dark cycle. A recent study on carp reported first that melatonin levels and intensity of a ~23 kDa AANAT protein in each gut segment also exhibit significant daily variations but, unlike pineal, show a peak at midday in all seasons. Extensive experimental studies ruled out direct role of light–dark conditions in determining temporal pattern of gut melatoninergic system in carp, and opened up possible role of environmental non-photic cue(s) as its synchronizer. Based on mammalian findings, physiological significance of gut-derived melatonin also appears unique because its actions at local levels sharing paracrine and/or autocrine functions have been emphasized. The purpose of this mini review is to summarize the existing data on the chronobiology and physiology of gut melatonin and to emphasize their relation with the same hormone derived in the pineal in vertebrates including fish.

A Comparative Genomics Study on the Molecular Evolution of Serotonin/Melatonin Biosynthesizing Enzymes in Vertebrates

“…(B) The C-terminal of ASMTL is highly similar to ASMT1 and ASMT2. the preparatory phase (Velarde et al, 2009(Velarde et al, , 2010bMukherjee and Maitra, 2015).…”

Melatonin-Induced Cytoskeleton Reorganization Leads to Inhibition of Melanoma Cancer Cell Proliferation

“…The pineal indole melatonin is a tryptophan-derived hormone, synthesized and released by the pineal gland during the dark period in all vertebrates [1]. Even though this gland is the organ responsible for the serum concentrations at night, both precursors and synthetic enzymes as well as melatonin itself have been found in many other tissues including the Harderian gland, retina, and gut among others [2][3][4]. However, the contribution to the systemic circulation by these tissues is minimal-if any-as it is deduced from the fact that pinealectomy leads to a total drop in melatonin serum levels and, as a consequence, the lack of circadian rhythmicity [5].…”

Melatonin Bio-Synthesizing Machinery in Fish: A Current Knowledge with a Special Emphasis on Tropical Carp

“…Extra-pineal Sources of Melatonin: The pineal organ and retina are considered as the primary site for the melatonin biosynthesis, however, recently, the search for the melatonin bio-synthesising machinery in different organs has been expedited and the results were astonishing. Melatonin bio-synthesising machinery were reported in brain and peripheral tissues like ovary, gut, kidney, liver, muscle and heart which plays an important role in proper functioning of the body [2,4,[20][21][22][23][24][25]. This is paradigm shifting as it has increased the biological significance of melatonin and opened new vistas for melatonin research.…”

“…In the ovary, melatonin plays a crucial role in maintaining the ovarian function [10,20] and also induces oocytes maturation in Catla catla [27]. The expression pattern of melatonin bio-synthesizing enzyme genes in gut and brain were changed rhythmically and distinct from each other in natural photo-thermal conditions [4,22]. Remarkably, melatonin production in gut tissue is enormously higher than that of the pineal organ and plays a crucial role in many physiological functions [4,28].…”