Melatonin is an endogenous hormone secreted by the pineal gland. The suprachiasmatic nuclei of the hypothalamus controls the numerous physiologic and endocrine circadian rhythms of the body, including that of rest and activity. The circadian clock is set via a process called entrainment, which is a response of the suprachiasmatic nuclei to photic input. Synthesis and secretion of endogenous melatonin is controlled by enzymes secreted by the hypothalamus which are activated by darkness and depressed by environmental light. Exactly how melatonin induces sleep is not clear, but it is probably not through a direct hypnotic effect. In patients with jet lag or circadian rhythm disorders, endogenous melatonin secretion does not correspond to the social or solar sleep-wake cycles imposed by their surroundings, and they experience sleep disruption. Administration of exogenous melatonin appears to re-set the body to the environmental clock and allow patients to normalize physiologic and behavioral sleep patterns. Exogenous melatonin maximally advances delayed rhythms when administered before endogenous melatonin levels begin to increase in the evening hours. In addition to circadian phase-shifting effects, melatonin has been shown to decrease nocturnal core body temperature, which helps to facilitate sleep. To date, pharmacological tolerance to melatonin has not been described.
Melatonin is involved in other physiologic processes besides the sleep-wake cycle. Secretion of melatonin from the pineal gland is highest during the pediatric years and tends to decrease with age. This age-related secretion performs important endocrine functions. It is thought that higher pre-pubertal melatonin levels are responsible for keeping the hypothalamic-pituitary-gonadal axis in quiescence, and that decreasing melatonin levels with age play a role in the onset of adolescence and sexual maturation. Melatonin receptors have been found in all male and female sexually responsive tissues, indicating that melatonin has a significant role in normal reproductive capacity. Exogenous melatonin can suppress the release of gonadotropin releasing hormone and lutenizing hormone, leading to anovulation and changes in steroid responsive tissues, especially in higher doses. Contraceptive activity has been noted when women are given melatonin in combination with norethindrone.
Melatonin also exhibits immunostimulatory and antioxidant actions. In neurodegenerative disease models, melatonin appears to neutralize oxidizing free radicals, specifically by preventing the reduction of antioxidant enzyme activity, and reducing beta-amyloid mediated lipid peroxidation of cell membranes. These actions appear to decrease apoptosis of neuronal cells. Further research is needed to determine if melatonin may preserve function in neurologic diseases where free radicals have been implicated as partially causative of the conditions. In epilepsy, the rise and fall of endogenous melatonin levels may influence seizure activity. Melatonin may play a role in certain cancers, and in some cases, may have antiproliferative effects on some tumors. The actions and role of melatonin in other body processes, such as regulation of the gastrointestinal system, continues to be investigated. Melatonin may also stimulate the activity of natural killer (NK) cells, lymphocytes, and various cytokines. Further study in well-controlled trials should answer further questions regarding melatonin’s neurologic, immunologic, and oncostatic activities.
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