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Melatonin — More than Just a Pineal Hormone

Volume 1 - Issue 4

Rüdiger Hardeland*

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    • University of Goettingen, Germany

    *Corresponding author: Rüdiger Hardeland, Johann Friedrich Blumenbach Institute of Zoology and Anthropology, University of Goettingen, Buergerstr 50, D-37073 Göttingen, Germany

Received: August 30, 2017;   Published: September 12, 2017

DOI: 10.26717/BJSTR.2017.01.000351

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Melatonin is not only synthesized in the pineal gland and the retina, but also in numerous other tissues. Extrapineal melatonin is formed in amounts exceeding by orders of magnitude that found in the pineal gland. This is particularly valid for the gastrointestinal tract, in which several physiological functions such as duodenal bicarbonate secretion have been elucidated. Melatonin is produced upon neuronal stimuli in enteroendocrine cells. Duodenal signaling involves the melatonin receptor MT2 present in enterocytes and in Paneth cells, in the latter case causing uroguanylin/guanylin release that leads to enterocytic activation of the receptor guanylyl cyclase GUCY2C. High amounts of melatonin are also found in the bile fluid, with contributions of melatonin synthesized in cholangiocytes and, presumably, enterohepatic circulation. Melatonin is also formed and metabolized in the skin, in which it contributes to photoprotection. In the cerebellum, melatonin formation is upregulated by bacterial lipopolysaccharide and conveys local anti-inflammatory protection. Increasing evidence indicates that melatonin does not easily cross membranes because of lipophilicity, but that transporters are required.

Keywords: Bile; Cerebellum; Gastrointestinal tract; Membranes; Skin

Abbreviations: CSF: Cerebrospinal Fluid; GIT: Gastrointestinal Tract; CFTR: Cystic Fibrosis Trans-membrane Conductance Regulator; PKC: Protein Kinase C; CaMK II: Calmodulin-Dependent Protein Kinase II; cGKII: Cyclic GMP-Dependent Protein Kinase II, AFMK: N1-Acetyl-N2- formyl-5-methoxykynuramine; AMK: N1-Acetyl-5-methoxykynuramine, AANAT: Aralkylamine N -Acetyltransferse

Abstract| Introduction| Extrapineal sites and the problem of poor melatonin release| A few examples of the roles of extrapineal melatonin| Conclusion| References|