New Issues About Melatonin and its Effects on the Digestive System

Abstract

In human beings, melatonin is secreted in a cyclic way by the pineal gland, although it has been detected in other tissues. Synthesis of melatonin takes place in the pinealocyte. It depends on adrenergic stimulation, and its secretion is related to the photoperiod in a circadian model of low activity during light phase and high activity at night time. Former studies aimed to establish the mechanisms by which melatonin carries out its biological function, proved the existance of high affinity binding sites. However, melatonin can pass through the plasmatic membrane; this property suggested a possible activity of the hormone inside the cell trough activation of nuclear receptors. Moreover, melatonin can act by itself as a potent oxygen-free-radical scavenger, which renders it a very strong antioxidant. It is currently accepted that melatonin plays an important role in numerous physiological processes. The gastrointestinal tract of numerous animal species contains melatonin, which is synthesized essentially by intestinal enterochrommaffin cells. Some investigations have revealed that its liberation follows also a circadian rhythm, although its secretion pattern might be influenced by nutritional factors. Receptors for melatonin have been identified in the digestive system, therefore the indolamine might play a leading role in different aspects of the vast digestive physiopathology. The hormone may interact with receptors and subsequently stimulate the synthesis of gastroprotective hormones and also exerts a direct defense on the epithelium, enhances submucosal blood flow and prevents the damage induced by ischemia followed by reperfusion. Moreover, studies have shown that treatment with melatonin reduces the severity of the lesions induced by NSAIDs on gastric mucosa suggesting a beneficial role of melatonin in preventing this gastropathy related to antiinflammatories.

Melatonin biosynthesis in the thymus of humans and rats

“…All these results lead us to suppose an endogenous melatonin synthesis in human thymus. In general, while pineal melatonin seems to act as a typical hormone in the organism, exerting an endocrine action reaching its target cells through the bloodstream, extrapineal melatonin may play intracrine, autocrine and paracrine key roles in those tissues where it is synthesized [23,37,53]. In this way, the resulting melatonin action on a tissue is the sum of the circulating melatonin action, most of which is of pineal origin, plus the endogenous melatonin action (Fig.…”

Amelioration of methotrexate‐induced enteritis by melatonin in rats

“…38 Apart from its radical scavenging activity, melatonin is believed to exert a direct defence on the epithelium and to enhance submucosal blood flow. 11 It was reported that melatonin enhances microcirculation by cyclo-oxygenasederived prostaglandins and nitric oxide. 38 Moreover, the hormone is reported to stimulate the synthesis of gastroprotective hormones.…”

Effect of Prolonged Melatonin Application on Metabolic Parameters and Organ Weights in Young Male and Female Sprague-Dawley rats

“…MEL administered in tap water is absorbed in gastrointestinal tract which itself is a source of synthesis and release of MEL in enterochrmaffin cells (Raikhlin and Kvetnoy 1976;Bubenik 1980). Gastrointestinal MEL regulates many physiological functions of digestion and nutrient absorption; its receptors in gastrointestinal tract have similar characteristics as MEL receptors in central nervous system (Motilva et al 2001). The fact remaining unclear is whether the MEL synthesis in gastrointestinal tract decreases with aging similarly to the MEL synthesis in epiphysis.…”