has been discovered 50 years ago by Lerner in the pineal gland
, but its physiological significance is still the subject of numerous studies. Melatonin is secreted primarily by pineal gland in response to environmental light/dark cycles, activated by supra-chiasmatic nuclei (SNC), the major circadian oscillator, regulating circadian rhythm of numerous biological functions.
In this picture there is a schematic presentation of the stimulation and inhibition of melatonin secretion from pineal gland. Activating the anteiror hypothalamus via the axons of retinal ganglion cells running in the optic nerves and forming retino-hypotalamic tract to reach the SCN
that are connected with pineal gland throught paraventricular nuclei, preganglionic sympathetic neurons supplying pineal gland. Norepinephrine, released from the postganglionic sympathetic fibers at pinealocyte membrane, stimulates its alfa-1/beta adrenoceptors leading to activation of membrane bound adenylate cyclase-cAMP system, resulting in the increase of the intracellular concentrations of cAMP as well as [Ca++], phosphatidylinositol, diacetylglycerol and protein kinase C. These second messangers stimulate the expression and of arylalkylamine-N-acetyltreansferase, the first rate-limiting enzyme in melatonine production, converting serotonin to N-acetylserotonin and the second rate-limiting enzyme, hydroxindole_methyltransferase, transforming acetylserotonin to melatonin. The enzymatic pathway for the biosynthesis of melatonin from tryptophan in pinealocytes was first identified by Axelrod. The circadian rhythm with a low light-time melatonin level and its marked increase at darkness exists in all animals irrespestive of whether the organism are active during the day-time or during the night.
Melatonin act on the target cells either directly or via G-protein coupled membrane receptors, such as MTR-1,MTR-2 and MTR-3 receptors
, that modulate several intracellular messengers such as cAMP
, cGMP and Ca++. In mammals, melatonin signals throught activation of at least two high-affinity G protein-coupled receptors, the MT1
This are unique receptors as they show distinct molecular structures, pharmacological characteristics and chromosomal localization. The MT1
receptors are 350 and 362 amino acids long, respectively, with calculated molecular weights of 39-40 kDa. This melatonin receptors have potential glycosilation sites in their N-terminus, protein kynase, casein kynase 1 and 2, and protein kynase A phosphorylation sites which may partecipate in the regulation of receptor function as demonstrated for other G-protein-coupled receptors. Mt1 and MT2
receptors signal by coupling to heterotrimeric Gi proteins formed by alfa, beta and gamma subunits. Activation of these receptors promoted dissociation of G-proteins into alfa and betagamma dimers, which interact with various effector molecules involved in the trasmission of cell signaling. Effector systems involved in MT1
melatonin receptor signaling throught G-protein coupling include adenylyl cyclase, phospholipase C, phospholipase A2,
potassium channels and potentially guanilil cyclase and calcium channels. MT1
melatonin receptors are discretely distributed in areas of the central nervous system and pheripheral target tissues. Receptors autoradiography with due-iodomelatonin binding, western analyses and immunohistochemistry with specific MT1
melatonin receptor antybodies; RT-PCR and/or in situ hibridization have been used to localize MT1
melatoni receptor proteins and mRNA expression in brain and pheripheral organs. Tissues endowed with fully charachterized functional MT1
melatonin receptor include retina, superchiasmatic nucleus, pars tuberalis, cerebral and peripheral arteries, kidney, pancreas, adrenal cortex, testes and immune cells. (Vedi file n° 3)
The melatonin is synthetized from an amino acid precursor; L-Triptophan
in the process involving Serotonin. Two main enzymes, which controlled the synthesis are: N-acetil serotonin transferasi (NAT) and hydrossi indolo-o-metil tranferasi (HIOMT). Melatonin received considerable attention because of its unique antioxidative property. This substance is able to neutralize reactive (ROS
) and nitrogen species (RNS), and it is highly effective in the protection of many tissue against toxic effects of free radicals. In the other hand the melatonin is involved in the regulation of many physiological systems: gastrointestinal system, cardiovascular system, immunity system and reproduction system. This indoleamine is also used in different situations in spite of the safety and effectiveness have not always have been proven, for example in the treatment of breast cancer
Molecular pharmacology, regulation and function of mammalian melatonin receptors
ROLE OF MELATONIN IN GASTRO-INTESTINAL TRACT
Gastrointestinal tract represents the most important extra pineal source of melatonin. Presence of melatonin suggests that this hormone is evolved digestive pathophysiology. Here melatonin is released from entero-endocrin cells (EE) of GIT wall, where this indole may act via endocrine, paracrine and luminal pathway through G-protein cupled receptors. Melatonin in GIT is should to be generated in about 500 times larger amounts then it is produced in pineal gland. Different study as focalized attention on the role of melatonin in upper portion of GIT, including oral cavity, esophagus, stomach and duodenum, where this indole is generated and released into the GIT lumen and into the portal circulation to be uptaken, metabolized by liver and realized with bile into duodenum. In the upper portion of GIT, melatonin exhibits a wide spectrum of activities such as circadian entrainment, free radical scavenging lesions such as stomatities, esophagities, gasrtities and peptic ulcer. There is a relationship between melatonin and food intake: abundant melatonin production in GIT, occurs mainly after good intake and maintains the indole concentration in peripheral blood, expecially following the intake of high dietary protein rich in tryptophan, wich serves as this indoleamine precursor. There are other neurohormonal factors originating from GIT such as ghrelin en.wikipedia.org/wiki/Ghrelin and orexins , wich increase food intake and leptin, CCK, glucagons-like peptide-1 (GLP-1) or peptide YY (PYY) that have been envolved in the decrease of food intake.
Recently, also melatonin as been suggested to decrease the hypothalamic content of ghrelin, the most potent appetite stimulating hormone. In fact neuro-hormonal mechanism controlling of food intake by hypothalamus and the influence of various neurohormonal factors originating from the GIT (ghrelin, CCK, PYY, OXM, GLP-1, GIP), pancreas (insulin) or adipose tissue (adiponectins) that exert stimulatory (orexigenic) or inhibitory (anorexigenic) influence on food intake and energy metabolism. Melatonin inhibits food intake through the sensory vagal nerves and through the inhibition of ghrelin release in the hypothalamus. According to results of a study:
1. Digestive system especially duodenal cluster unit, and small bowel, highly effective in the biosynthesis of melatonin.
2. Melatonin, originating from the pineal gland, is responsible of nocturnal rise in plasma level of this hormone, whereas that produce during the day-time originates mainly the GIT.
3. Deliver is capable of accumulating melatonin from the portal blood and after metabolizing it, its metabolites as well as the unchanged relation molecule are excreted in the bile.
Additionally, melatonin is proposed to regulate pancreatic secretion and maintain the integrity of pancreas. In spite of the presence of melatonin receptors in the pancreatic tissue little is known about the role of this indole in pancreas. Experimental studies have shown that exogenous melatonin as well as this produced endougenously from its precursor L-tryptophan, strongly stimulates amylase secretion when given entraperitoneally, or into gut lumen. This accompanied by significant increase of CCK plasma level. Above pancreatic stimulatory effects of luminal administration of melatonin where completely reversed by bilateral vagotomy, capsaicin deactivation of sensory nerves or pre-treatment of the rats with CCK1 receptor antagonist. The beneficial effects of melatonin or L-tryptophan on acute pancreatitits could be related to the ability of melatonin to scavenge the free radicals, to activate anti-oxidative enzymes and modulate the cytokine production. (schema in file n°4)
MELATONIN AND ORAL CAVITY
Melatonin was found to be realised with saliva into oral cavity and to be implicated in various dental and periodontal diseases. The concentration of salivary melatonin under basal conditions is negligible but following local oral applications of this indoleamine, its plasma level increases dose-dependently and this is accompanied by the increase of salivary melatonin reaching about 20% of that in plasma. This suggests that locally applied melatonin to the oral cavity lining maybe useful in the treatments of oral lesions. Melatonin has several specifically functions in oral cavity. It acts as a potent antioxidant and free radical scavenge as an immuno-modularity agent, strong promoter of bone formation and anti-inflammatory factor in periodontal diseases.
Role of melatonin in upper gastro-intestinal tract
Melatonin as modulator of pancreatic enzymes secretion and pancreatorprotector
Melatonin and oral cavity
Melatonin and its role in oxidative stress related diseases of oral cavity
Melatonin and serotonin gastrointestinal motility effects
MELATONIN AND CARDIOVASCULAR SYSTEM
The recent data indicated that impaired melatonin production is involved in several cardiovascular pathologies including ipertension and ischemic heart diseases. In fact melatonin influences blood pressure, myocardial contractility and increases the antioxidant reserve.
Melatonin receptor were discovered in the heart and arteries. Moreover, decreased melatonin levels were reported in various pathological conditions including ipertension with non dipper pattern, impairment of heart failure, ischemic diseases or in patients after acute myocardial infarction. The interference of melatonin synthesis and action with other neurohumoral system plays an important role in the modulation of cardiovascular functions by melatonin. In the cardiovascular system there are melatonin receptors and it was hypothesized that wile MT1-receptor is localized primary on vascular smooth muscle cells, the MT2-receptor appears on endothelial as well as vascular smooth muscle cells. There are three hypothesis on mechanisms of the effect of melatonin on blood pressure.
1. the constrictive effect of melatonin can be explained by receptor mediated decrease in cAMP levels and phosphatidyl-inositol-4,5-bisphosphate hydrolysis which was reported by several in vitro studies. Despite the fact that the vasodilatation after melatonin is congruent with decreased blood pressure after melatonin administration, it is difficult to explain it the bases of melatonin receptors stimulation. The activation of melatonin receptors in majority of the experimental models was associated with cAMP decrease and phosphatidilinositol-4,5bisphosphate hydrolyse, which lead to inhibition of vasodilatation or to vasoconstriction. Nevertheless, the activation of MT2 receptors on endothelial cells could increase cytosolic Ca++ in endothelial cells, which was observed by Pogan. Activated endothelial cells are then stimulated to increase the production of NO, which is additionally protected by antioxidant properties of melatonin. This hypothesis is supported by the findings of Anwar et al who observed decreased oxidative blood and increased NO in blood serum associated with decreased cytosolic Ca++ and increased cGMP in vascular smooth muscle cells. This results of melatonin influence on blood pressure from vivo experiments are in contrast with data from in vitro experiments and suggest the involvement of central regulatory mechanisms in the mediation of melatonin effects on blood pressure in vivo. Improved baroreflex responses, decreased sympathetic output and association of decreased heart rate or cardiac output with blood pressure fall after melatonin administration.
2. Melatonin was reported to enhance Gaba-ergic signalization (Wang et al 2003), which may contribute to inhibition of paraventricular nucleus and rostral ventrolateral medulla in SNC and subsequent decrease in symphatetic tone.
3. NO formation was shown to potentiate Gaba-ergic inhibitory effects in paraventricular nucleus and rostral ventrolateral medulla. The potential of melatonin to increase NO availability may additionally augment inhibition in these areas. (vedi file fondo pagina)
Blood pressure modulation and cardiovascular protection by melatonin
Melatonin as a potential antihypertensive treatment
At the beginning of the 1900s, the first studies describing the anti-oxidative activity of melatonin started to appear. Among other finding, melatonin was found to protect not only proteins and structural lipids, but also cellular DNA from the damaging effects of reactive oxygen species.
During about the last 15 years, intense investigations have been conducted and published confirming the anti-oxidative effects of melatonin. The results demostred that it is one of the most effective endogenous scavengers of free oxygen radicals. This indoleamine donates electrons to the freeradicals becoming tiself indolyl cation radical. The later undergoes several other reaction to be inverted to N-acetil-N-formil-5-metoxykynuramine and N-acetil-5-metoxykynuramine which are excreted in the urine. Besides the direct scavenging of free radicals, melatonin influences the oxydative stresses status in an indirect way by stabilizing the inner mithocondrial membrane what improves the electron transport chain lacated there. Moreover, melatonin inhibits iNOS and inducible enzime which produces excessive amounts of NO, that are no any longer beneficial to the system, but increase oxydative stress due to its conversion together with ROS into the distructive RNS. Additionally, melatonin in pharmacological and in physiological doses increase gene expression and activity of andogenous anti-oxidant anzymes such as glutathione peroxidase, SOD, CAT, which are important in maintaining the integrity of vasculature and other tissues. These antioxydant properties of melatonin could turn turn out very beneficial for treatment of the local inflammatory lesions and for accelerating the healing process for example after tooth extraction and other surgical procedures in oral cavity.
melatonin adjuvant therapy of malignant tumours
melatonin and its role in oxidative stress related diseases of oral cavity
Actions of melatonin in the ruduction of oxydative stress
The detection of the numerous regulatory functions of melatonin pointed to the potential of the pineal gland to participate in the control of immune system functions.
In vivo studies show that melatonin exerts immunoenhancing properties. Thus, pinealectomy on newborn rats causes disorganization of thymic structure and suppression of pineal functions, whereas constant light diminished antibody responses to T-cell activity, whereas melatonin treatment enhances antibody dependent cellular citotoxicity and INF-gamma. Production by murine-splenocytes. Furthermore, in vitro studies show that melatonin acts on immune cells by regulation cytokine production. Melatonin activates T helper cells by increasing IL-2 production as well as activating monocyte by increasing the production of IL-1, IL-6, TNF, ROS and NO. Melatonin also enhances IL-12 production by monocytes driving T-cells differentiation toward the TH1 phenotype and causing an increase of INF-gamma production. In this sense, cytokine production could be considered as one of the main mechanism to modulate the immune system by melatonin.
Moreover, the existence of specific melatonin binding setes in lymphoid cells provides evidence for a direct effect on the regulation of the immune system. Thus, using the melatonin agonist 2-[125I]melatonin.
Melatonin, high affinity binding sites and a signal transduction pathway for melatonin have been characterized in human lymphocytes. In addition a recent study described a physiological role of the MT1 membrane melatonin receptor in the human system in which melatonin counteracts the inhibitory effect of prostaglandin E2 on IL-2 production in human lymphocytes via its MT1 membrane receptor. Theses melatonin-mediated effects were presumed to be driven by pineal-derived melatonin, not only because of the previously mentioned pinealectomy and constant light exposure experiments, but also because the reported circadian and seasonal variations in melatonin production ae associated with changes in immune cell activity. However, on believe that melatonin synthesized by lymphoid cells may also play a physiological role in lymphocyte regulation.
BIBLIOGRAFIA: evidence of melatonin synthesis by human lymphocytes and its physiological significance
Melatonin and its relation to the immune system and inflammation
Melatonin modulation of lymphocytic proliferation and th1/th2 cytokine
Melatonin-immune system relationship
EFFECT OF MELATONIN ON TUMOR
Melatonin plays a significant role in controlling the immune system mediated by its stimulatory effect on the lymphocytes the secretion of some cytokines (IL-1, IL-2, IL-6, INF-gamma) and its effect on the maturation and activity of NK cells. For this reason, melatonin may play a significant role in the promotion of immune system interactions with neoplastic cells.
Most of the studies on the effects of melatonin on tumor cells were conducted on the MCF-7 cell line of mammary carcinoma. The oncostatic effects of melatonin involved mainly inhibited proliferation of neoplastic cells, intensified apoptosis and decreased capacity to form metastases. The effect of melatonin’s action on MCF-7 cells depended on several factors, including the hormone concentration in the medium and the status of the estrogens receptors (ERS) on the studied cells. Later investigations confirmed that melatonin, added to the medium of MCF-7 (ERT) cells at a concentration of 1hM, inhibited their proliferation, augmented the expression of pro-apoptotic p53 and p21 WAF proteins, and reduced their metastatic potential due to increased expression of the adhesion-promoting protein E-cadherin flipper.diff.org/app/items/info/310 and B1-integrin. The oncostatic effect of melatonin on breast cancer (ERT) cell resulted from melatonin induced decreased expression of ER-alpha receptors, leading to a decreased number of active estradiol ER-alpha receptors, which activate DNA transcription factors, this stimulating cancer cell proliferation.
Results of studies on animals in which pinealectomy was performed supplied rich information on the role of the pineal gland in neoplastic process. The procedure was found to accelerate growth of earlier implanted tumor. Administration of melatonin was confirmed to show the growth rate of such tumors as breast cancer, prostate cancer, melanoma and leukaemia. The data presented on the effect of melatonin on tumor cells in vitro and in vivo suggest that it may play a certain role in anti-tumor therapy in humans.
Its oncostatic action may involve a few parallel mechanisms not necessarily linked to each other. The effects of the antioxidative action of the melatonin on tumor cells may prevent neoplastic transformation in the course of carcinogenesis. The anti-proliferative action of melatonin may involve a decreased expression of genes responsible for the synthesis of growth factors. An alternate mechanism through with melatonin may exert its oncostatic effect involves anti-angiogenic, anti-infiammatory and immune system-stimulating activities suche as breast cancer, uterine cancer, prostate cancer. There is currently great interest in the potential role of melatonin in the epigenetic regulation of tumor suppressor genes in cancer cells and thus in cancer inhibition.
Melatonin: adjuvant therapy of malignant tumors