Melatonin is a hormone secreted by the pineal gland in the brain that helps regulate other hormones and maintains the body's circadian rhythm.
The circadian rhythm is an internal 24-hour time-keeping system that plays a critical role in determining when we fall asleep and when we wake up. Darkness stimulates the production of melatonin while light suppresses its activity. Exposure to excessive light in the evening or too little light during the day can disrupt the body’s normal melatonin cycles. For example, jet lag, shift work, and poor vision can disrupt melatonin cycles. In addition, some experts claim that exposure to low-frequency electromagnetic fields (common in household appliances) may disrupt normal cycles and production of melatonin. Melatonin also helps control the timing and release of female reproductive hormones. It helps determine when menstruation begins, the frequency and duration of menstrual cycles, and when menstruation ends (menopause). Many researchers also believe that melatonin levels are related to the aging process. For example, young children have the highest levels of night time melatonin. Researchers believe these levels diminish as we age. In fact, the decline in melatonin may explain why many older adults have disrupted sleep patterns and tend to go to bed and wake up earlier than when they were younger. However, emerging research calls this theory into question. In addition to its hormonal actions, melatonin has strong antioxidant effects. Melatonin is not only a hormone but is also synthesized in numerous extrapineal sites, in which it sometimes attains much higher quantities than in the pineal and the circulation. It is also present in many taxonomically distant groups of organisms, including bacteria, fungi, and plants. Moreover, melatonin is a source of bioactive metabolites, such as 5-methoxytryptamine, N (1)-acetyl-N (2)-formyl-5-methoxykynuramine and N (1)-acetyl-5-methoxykynuramine.
SYNTHESIS AND METABOLISM
Melatonin is synthesized within the pinealocytes- from tryptophan. Most synthetic activity occurs during the dark phase, with a major increase (7-150 fold) in the activity of serotonin-N-acetyltransferase (arylalkylamine N-acetyl transferase, AA-NAT). AA-NAT is usually rate limiting in melatonin production, but serotonin availability may also play a role. The rhythm of production is endogenous, being generated by interacting networks of clock genes in the suprachiasmatic nuclei (SCN), the major central rhythm-generating system or "clock" in mammals (the pineal itself is a self sustaining "clock" in some if not all lower vertebrates). The main feedback loop involving transcription of a number of genes (Per1, Per2, Per3, Cry1, Cry2, and Reverbα) is activated by heteromeric complexes of CLOCK and BMAL1. This transcription continues into the night until nuclear levels of PER and CRY proteins become sufficiently high to repress CLOCK/BMAL1 activation. Declining levels of PER/CRY in the early morning then allows transcription of the genes again and the cycle continues (most, if not all peripheral tissues also express this sequence. The SCN rhythm is synchronized to 24 hours primarily by the light-dark cycle acting via the retina and the retinohypothalamic projection to the SCN. The cDNAs encoding both AA-NAT and the O-methylating enzyme HIOMT have both been cloned. There are substantial species differences in regulation of AA-NAT. It is likely that in humans and ovines the enzyme is regulated primarily at a post transcriptional level, whereas in rodents the key event appears to be cyclic AMP-dependent phosporylation of a transcription factor that binds to the AA-NAT promoter. Rapid decline in activity with light treatment at night appears to depend on proteasomal proteolysis. According to distribution studies of AA-NAT mRNA this enzyme is expressed in the pineal, retina and, to a much lesser extent, some other brain areas, the pituitary and the testis, but apart from the pineal these structures contribute little to circulating concentrations in mammals. There is also evidence that melatonin may be synthesised in the gastro-intestinal tract and may contribute to gut function. Within the rodent retina a self sustaining 'clock' maintains rhythmic production of melatonin in vitro as it does in many lower vertebrates. Whether or not this is true of humans remains to be seen. In humans and rodents melatonin is metabolized to 6-sulphatoxymelatonin (aMT6s), primarily within the liver, by 6-hydroxylation, followed by sulfate conjugation. A number of minor metabolites are also formed, including the glucuronide conjugate. Exogenous oral or intravenous melatonin has a short metabolic half-life (20 to 60 minutes, depending on author and species), with a large hepatic first-pass effect and a biphasic elimination pattern. In ruminants longer half-lives are seen after oral administration. Most effects of pinealectomy can be reversed by melatonin, administered appropriately in physiological concentrations
In its role as a pineal hormone, melatonin is a pleiotropic, nocturnally peaking and systemically acting chronobiotic. These effects are largely explained by actions via G protein-coupled membrane receptors found in the suprachiasmatic nucleus, but also in numerous other sites. Nuclear (ROR/RZR), cytoplasmic (quinone reductase-2, calmodulin, calreticulin) and mitochondrial binding sites and radical-scavenging properties contribute to the actions of melatonin.
Sympathetic denervation of the pineal in mammals abolishes the rhythmic synthesis of melatonin and the light-dark control of its production. Norepinephrine is clearly the major transmitter, acting via beta-1-adrenoceptors with potentiation by alpha-1 stimulation, but the role of neural serotonin is probably not negligible. There is a day-night variation in pineal norepinephrine, with highest values at night, approximately 180 degrees out of phase with the pineal serotonin rhythm. cAMP acts as a second messenger and stimulates AA-NAT activity. Beta-adrenergic receptor binding sites in the rat pineal vary over a 24-hour period, the lowest number being found toward the end of the dark phase, increasing shortly after lights on. There is evidence for modulation of melatonin synthesis in vitro by other factors, notably neuropeptides, but their physiological importance remains obscure.Regulation of pineal melatonin biosynthesis is largely explained by control mechanisms acting on arylalkylamine N-acetyltransferase, at the levels of gene expression and/or enzyme stability influenced by phosphorylation and interaction with 14-3-3 proteins.
Although results are still controversial, studies suggest that melatonin supplements help induce sleep in people with disrupted circadian rhythms (such as those suffering from jet lag or poor vision or those who work the night shift) and those with low melatonin levels (such as some elderly and individuals with schizophrenia). In fact, a recent review of scientific studies found that melatonin supplements help prevent jet lag, particularly in people who cross five or more time zones. A few studies suggest that when taken for short periods of time (days to weeks) melatonin is significantly more effective than a placebo, or “dummy pill,” in decreasing the amount of time required to fall asleep, increasing the number of sleeping hours, and boosting daytime alertness. In addition, at least one study suggests that melatonin may improve the quality of life in people who suffer from insomnia and some experts suggest that melatonin may be helpful for children with learning disabilities who suffer from insomnia. Although research suggests that melatonin may be modestly effective for treating certain types of insomnia, few studies have investigated whether melatonin supplements are safe and effective for long term use.
Melatonin has been shown to stimulate cells called osteoblasts that promote bone growth. Since melatonin levels may be lower in some older individuals such as postmenopausal women, current studies are investigating whether decreased melatonin levels contribute to the development of osteoporosis, and whether treatment with melatonin can help prevent this condition.
Melatonin supplements may benefit menopausal women by promoting and sustaining sleep. Peri- or postmenopausal women who use melatonin supplements to regulate sleep patterns should do so only for a short period of time since long term effects are not known.
A recent study of postmenopausal women found that melatonin alleviated both depression and anxiety. Other studies show that people who suffer from major depression or panic disorder have low levels of melatonin. Healthy individuals with mild episodic depression and patients who have Seasonal Affective Disorder, (SAD -- a mild depression that correlates with fall and winter -- periods of light-phase shortening) also have lower than normal melatonin levels. Experimental studies show that melatonin causes a surge in the chemical serotonin, which helps alleviate symptoms of depressive illness, including major and mild depression and SAD. Melatonin should be used with caution in people with depression and should be appropriately timed with light therapy and sleep-phase changes. Disruption of normal circadian rhythm by poorly timed melatonin administration may worsen depression.
Melatonin levels may play a role in the symptoms of anorexia. For example, abnormally low melatonin levels may cause depressed mood in people with this condition. However, researchers do not know whether supplementation will change the course of the disease. Some researchers speculate that low melatonin levels in people with anorexia may indicate who is likely to benefit from antidepressant medications (a treatment often used for eating disorders).
Several studies indicate that melatonin levels may be linked with breast cancer risk. For example, women with breast cancer tend to have lower levels of melatonin than those without the disease. In addition, laboratory experiments have found that low levels of melatonin stimulate the growth of certain types of breast cancer cells, while adding melatonin to these cells inhibits their growth. Preliminary laboratory and clinical evidence also suggests that melatonin may enhance the effects of some chemotherapy drugs used to treat breast cancer. In a study that included a small number of women with breast cancer, melatonin (administered 7 days before beginning chemotherapy) prevented the lowering of platelets in the blood. This is a common complication of chemotherapy, known as thrombocytopenia that can lead to bleeding. In another study of a small group of women whose breast cancer was not improving with tamoxifen (a commonly used chemotherapy medication), adding melatonin caused tumors to modestly shrink in over 28% of the women. People with breast cancer who are considering taking melatonin supplements should consult their doctors before beginning supplementation.
Like breast cancer, studies show that people with prostate cancer have lower melatonin levels than men without the disease. Melatonin blocks the growth of prostate cancer cells in test tube studies. In one small-scale study, melatonin (when used in combination with conventional medical treatment) improved survival rates in 9 out of 14 patients with metastatic prostate cancer. Interestingly, since meditation may cause melatonin levels to rise it appears to be a valuable addition to the treatment of prostate cancer. More research is needed before doctors can make recommendations in this area.
- Cancer-related Weight Loss
Weight loss and malnutrition are concerns for people with cancer. In one study of 100 people with advanced cancer, those who received melatonin supplements were less likely to lose weight than those who did not receive the supplements.
Some physicians use melatonin to help treat sarcoidosis (a condition where fibrous tissue develops in the lungs and other tissues). Two case reports suggest that melatonin may be helpful for those who do not improve from conventional steroid treatment.
Melatonin levels are lower in patients with rheumatoid arthritis than in healthy individuals without arthritis. However, when arthritis patients were treated with the anti-inflammatory medication indomethacin, melatonin levels returned to normal. The chemical structure of melatonin resembles indomethacin, so researchers suspect that melatonin supplements may work similarly to this medication for people with rheumatoid arthritis. However, this theory has not been tested.
Attention Deficit/Hyperactivity Disorder (ADHD)
Although melatonin supplementation does not appear to improve the key behavioral symptoms of ADHD, it may be effective in managing sleep disturbances in children with this condition.
Preliminary research suggests that melatonin reduces the number of seizures in certain animals and may reduce seizures in people with epilepsy. However, not all experts agree with these findings. In fact, some researchers are concerned that melatonin (1 to 5 mg per day) may actually induce seizures, particularly in children with neurologic disorders. Since this research is in the early stages, some experts suggest that doctors should administer melatonin only to a select group of people who suffer from seizures that cannot be controlled by any other type of therapy.
A few small studies suggest that gels, lotions, or ointments containing melatonin may protect against redness (erythema) and other skin damage when used alone or in combination with topical vitamin E prior to exposure to UV radiation from the sun.
Although melatonin has not been scientifically evaluated for use in treating human encephalitis (inflammation of the brain), some studies suggest that this supplement may protect animals from serious complications associated with the condition and even increase their survival rates. In one study of mice infected with Venezuelan equine virus (a type of organism that causes viral encephalitis), melatonin supplements significantly lowered the presence of virus in the blood and reduced death rates by more than 80%. More studies are needed to determine whether similar treatment may offer the same protection to people with viral encephalitis.
Low blood levels of melatonin are associated with heart disease, but it is not clear whether melatonin levels are low in response to having heart disease or if low levels of melatonin cause people to develop this condition. In addition, several animal studies suggest that melatonin may protect the heart from the damaging effects of ischemia (decreased blood flow and oxygen that often leads to a heart attack). However, researchers are unclear whether melatonin supplements may help prevent or treat heart disease in people. More studies are needed before scientists can draw any conclusions.