Extrapineal melatonin: A hormonal microcircuit for local adaptations

Abstract

The indoleamine melatonin can no more be considered as an exclusive product of the pineal gland. In the last few decades the presence of melatonin has been reported in a wide variety of organs/cells that includes carotid body, bone marrow, Harderian gland, skin, liver, gastrointestinal tract, endothelial cells, endocrine glands, cells of immune system, reproductive organs and many more. These melatonin secreting organs/cells are supposed to be a part of the 'diffuse neuroendocrine circuit' which is independent of scotoperiod for activation of its melatonin bio-synthesis machinery. The absence of circadian melatonin rhythm in these tissues suggests local melatonin to be "out of step" with its chronobiological functions. However, the remarkably high concentration might meritoriously support the protective role played by the melatonin in these tissues. The concentration of melatonin is further known to vary in a tissue specific manner thus presenting an adaptive strategy to exercise homeostatic control depending upon the physiological status of the cell/tissue. Melatonin derived from these extra-pineal sources can act in an autocrine, intracrine or paracrine manner to exert its pleiotropic actions on cellular protection and survival. The present review will focus on the physiological relevance of the emerging roles of locally produced melatonin as a modulator of intracellular relationships and cellular homeostasis. © 2021 Nova Science Publishers, Inc.