New issues about melatonin and its effects on the digestive system.

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.