Therapeutic treatments potentially mediated by melatonin receptors: potential clinical uses in the prevention of osteoporosis, cancer and as an adjuvant therapy

Update on the role of melatonin in the prevention of cancer tumorigenesis and in the management of cancer correlates, such as sleep-wake and mood disturbances: review and remarks

The aim of this article was to perform a systematic review on the role of melatonin in the prevention of cancer tumorigenesis--in vivo and in vitro--as well as in the management of cancer correlates, such as sleep-wake and mood disturbances. The International Agency for Research on Cancer recently classified "shift-work that involves circadian disruption" as "probably carcinogenic to humans" (Group 2A) based on "limited evidence in humans for the carcinogenicity of shift-work that involves night-work", and "sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)". The clinical implications and the potential uses of melatonin in terms of biologic clock influence (e.g. sleep and mood), immune function, cancer initiation and growth, as well as the correlation between melatonin levels and cancer risk, are hereinafter recorded and summarized. Additionally, this paper includes a description of the newly discovered effects that melatonin has on the management of sleep-wake and mood disturbances as well as with regard to cancer patients' life quality. In cancer patients depression and insomnia are frequent and serious comorbid conditions which definitely require a special attention. The data presented in this review encourage the performance of new clinical trials to investigate the possible use of melatonin in cancer patients suffering from sleep-wake and mood disturbances, also considering that melatonin registered a low toxicity in cancer patients.

Urinary 6-sulphatoxymelatonin levels in patients with senile cataracts

Background

The antioxidant melatonin effectively scavenges highly toxic hydroxyl radicals. Decreases in circulating melatonin levels have been reported in patients with diseases that become more serious with advancing age. The purpose of the present study was to explore the relationship between circulatory melatonin level and the extent of senile cataracts. To this end, we assessed the urinary excretion levels of 6-sulphatoxymelatonin (aMTS6), a major metabolite of melatonin.

Methods

A total of 22 patients (aged 64 ± 7 years; 12 males and 10 females) with senile cataracts and 22 healthy controls (aged 61 ± 8 years, 12 males and 10 females) were studied. aMTS6 urine levels were measured using commercial ELISA kits. Each aMTS6 level was expressed as [aMTS6] (in ng)/[mg] creatinine. As the data were not normally distributed, the Mann–Whitney U-test was employed to assess the statistical validity of the difference observed.

Results

The aMT6 level in nocturnal urine was 17.87 ± 14.43 ng aMTS6/mg creatinine (mean ± SD) in senile cataract patients; this was 76% of the level measured in age- and gender-matched controls (23.28 ± 16.27 ng aMTS6/mg creatinine). This difference in nocturnal urine aMTS6 level between senile cataract patients and controls was not statistically significant (p = 0.358).

Conclusion

The urinary aMTS6 level did not differ between subjects with and without senile cataracts.

Toxicity induced by chemical warfare agents: insights on the protective role of melatonin

Chemical Warfare Agents (CWAs) are substances that can be used to kill, injure or incapacitate an enemy in warfare, but also against civilian population in terrorist attacks. Many chemical agents are able to generate free radicals and derived reactants, excitotoxicity process, or inflammation, and as consequence they can cause neurological symptoms and damage in different organs. Nowadays, taking into account that total immediate decontamination after exposure is difficult to achieve and there are not completely effective antidotes and treatments against all CWAs, we advance and propose that medical countermeasures against CWAs poisoning would benefit from a broad-spectrum multipotent molecule. Melatonin, a versatile and ubiquitous antioxidant molecule, originally discovered as a hormone synthesized mainly in the pineal gland, has low toxicity and high efficacy in reducing oxidative damage, anti-inflammatory effects by regulation of multiple cellular pathways and properties to prevent excitotoxicity, among others. The purpose of this review is to show the multiple and diverse properties of melatonin, as a pleiotropic indole derivative, and its marked potential for improving human health against the most widely used chemical weapons.

Melatonin effects on hard tissues: bone and tooth

Melatonin is an endogenous hormone rhythmically produced in the pineal gland under the control of the suprachiasmatic nucleus (SCN) and the light/dark cycle. This indole plays an important role in many physiological processes including circadian entrainment, blood pressure regulation, seasonal reproduction, ovarian physiology, immune function, etc. Recently, the investigation and applications of melatonin in the hard tissues bone and tooth have received great attention. Melatonin has been investigated relative to bone remolding, osteoporosis, osseointegration of dental implants and dentine formation. In the present review, we discuss the large body of published evidence and review data of melatonin effects on hard tissues, specifically, bone and tooth.

A novel therapeutic strategy for adolescent idiopathic scoliosis based on osteoporotic concept

Adolescent idiopathic scoliosis (AIS) is a complex three dimensional spinal deformity which occurs mostly in prepubertal and pubertal girls. Although bracing and surgery have been the mainstays of treatment for AIS, because of the complications and poor compliance, many patients with this disorder continue to experience significant residual symptoms. The etiology and pathogenesis of AIS is unclear, but recent studies show the association between osteopenia and AIS and imply that osteopenia play a causative role in the development of AIS. Anti-osteoporosis treatment can improve bone strength, prevent osteoporosis and rebalance the OPG-RANK-RANKL system, which may help to prevent curve progression in AIS. This report proposes that anti-osteoporosis treatment may be an effective treatment for AIS.

Peripheral reproductive organ health and melatonin: ready for prime time

Melatonin has a wide variety of beneficial actions at the level of the gonads and their adnexa. Some actions are mediated via its classic membrane melatonin receptors while others seem to be receptor-independent. This review summarizes many of the published reports which confirm that melatonin, which is produced in the ovary, aids in advancing follicular maturation and preserving the integrity of the ovum prior to and at the time of ovulation. Likewise, when ova are collected for in vitro fertilization-embryo transfer, treating them with melatonin improves implantation and pregnancy rates. Melatonin synthesis as well as its receptors have also been identified in the placenta. In this organ, melatonin seems to be of particular importance for the maintenance of the optimal turnover of cells in the villous trophoblast via its ability to regulate apoptosis. For male gametes, melatonin has also proven useful in protecting them from oxidative damage and preserving their viability. Incubation of ejaculated animal sperm improves their motility and prolongs their viability. For human sperm as well, melatonin is also a valuable agent for protecting them from free radical damage. In general, the direct actions of melatonin on the gonads and adnexa of mammals indicate it is an important agent for maintaining optimal reproductive physiology.

Melatonin and human skin aging

Like the whole organism, skin follows the process of aging during life-time. Additional to internal factors, several environmental factors, such as solar radiation, considerably contribute to this process. While fundamental mechanisms regarding skin aging are known, new aspects of anti-aging agents such as melatonin are introduced. Melatonin is a hormone produced in the glandula pinealis that follows a circadian light-dependent rhythm of secretion. It has been experimentally implicated in skin functions such as hair cycling and fur pigmentation, and melatonin receptors are expressed in many skin cell types including normal and malignant keratinocytes, melanocytes and fibroblasts. It possesses a wide range of endocrine properties as well as strong antioxidative activity. Regarding UV-induced solar damage, melatonin distinctly counteracts massive generation of reactive oxygen species, mitochondrial and DNA damage. Thus, there is considerable evidence for melatonin to be an effective anti-skin aging compound, and its various properties in this context are described in this review.

Sustained release of melatonin from poly (lactic-co-glycolic acid) (PLGA) microspheres to induce osteogenesis of human mesenchymal stem cells in vitro

Melatonin promotes bone formation and prevents bone degradation via receptor-dependent or receptor-independent actions. The aim of this study is to encapsulate melatonin into poly (lactic-co-glycolic acid) (PLGA) microspheres (PLGA-MEL-MS) and create a melatonin sustained release system, then to evaluate its effect on the osteogenesis of human mesenchymal stem cells (hMSCs) in vitro. PLGA-MEL-MS were prepared by single emulsion solvent evaporation technique. Scanning electron microscopy demonstrated the incorporation of melatonin did not disturb the conventional generation of PLGA microspheres in size and morphology. In vitro drug release assay showed that PLGA-MEL-MS exhibited a biphasic drug release pattern: a low initial burst release effect with approximately 40% drug release at the first 3 days and a relatively retarded and continuous release with about 85% drug release over the 25 days. Cell proliferation assay demonstrated that PLGA-MEL-MS had no apparent effect on proliferation of human MSCs. In an osteogenesis assay, PLGA-MEL-MS obviously enhanced alkaline phosphatase (ALP) mRNA expression and increased ALP activity compared to that in the control group. Meanwhile, several markers of osteoblast differentiation were also significantly upregulated, including runx2, osteopontin, and osteocalcin. Furthermore, quantificational alizarin red-based assay demonstrated that PLGA-MEL-MS significantly enhanced calcium deposit of hMSCs compared to the controls. Therefore, this simple melatonin sustained release system can control released melatonin to generate a microenvironment with a relatively stable concentration of melatonin for a period of time to support osteogenic differentiation of hMSCs in vitro. This suggests that this system may be used as bone growth stimulator in bone healing in vivo.

Effects on bone by the light/dark cycle and chronic treatment with melatonin and/or hormone replacement therapy in intact female mice

In this study, the effects of the light/dark cycle, hormone replacement therapy (HRT), and nocturnal melatonin supplementation on osteogenic markers and serum melatonin levels were examined in a blind mouse model (MMTV-Neu transgenic mice). Melatonin levels in this mouse strain (FVB/N) with retinal degeneration (rd-/-) fluctuate in a diurnal manner, suggesting that these mice, although blind, still perceive light. Real-time RT-PCR analyses demonstrated that Runx2, Bmp2, Bmp6, Bglap, and Per2 mRNA levels coincide with melatonin levels. The effect of chronic HRT (0.5 mg 17β-estradiol + 50 mg progesterone in 1800 kcal of diet) alone and in combination with melatonin (15 mg/L drinking water) on bone quality and density was also assessed by histomorphometry and microcomputed tomography, respectively. Bone density was significantly increased (P < 0.05) after 1 yr of treatment with the individual therapies, HRT (22% increase) and nocturnal melatonin (20% increase) compared to control. Hormone replacement therapy alone also increased surface bone, decreased trabecular space, and decreased the number of osteoclasts without affecting osteoblast numbers compared to the control group (P < 0.05). Chronic HRT + melatonin therapy did not significantly increase bone density, even though this combination significantly increased Bglap mRNA levels. These data suggest that the endogenous melatonin rhythm modulates markers important to bone physiology. Hormone replacement therapy with or without nocturnal melatonin in cycling mice produces unique effects on bone markers and bone density. The effects of these therapies alone and combined may improve bone health in women in perimenopause and with low nocturnal melatonin levels from too little sleep, too much light, or age.

A novel calcium aluminate-melatonin scaffold enhances bone regeneration within a calvarial defect

Over 500,000 bone graft or bio-implant procedures are performed annually in the United States. It has been reported that osseous autograft procurement may result in donor site complications and bio-implant allografts have been associated with disease transmission. Ceramic scaffolds are only osteoconductive, limiting their clinical use. The objective of this study was to create a bone filler substitute with regenerating properties similar to natural bone. Therefore, melatonin and platelet-rich plasma (PRP) were utilized for their known osteoinductive properties. It was hypothesized that melatonin and/or PRP would enhance the osteoinductive and osteoconductive properties of calcium aluminate (CA) scaffolds to promote bone regeneration in a model of calvarial defects. The biocompatibility of CA and CA-Mel scaffolds was tested in vitro and in vivo. Data show that CA-Mel scaffolds, in comparison with CA scaffolds, enhanced the adhesion, viability, and proliferation of normal human osteoblasts cells but not that of NIH3T3 fibroblasts. Data also showed that human adult mesenchymal stem cells grown on CA or CA-Mel scaffolds showed a time-dependent induction into osteoblasts over 14days revealed through scanning electron microscopy and by alkaline phosphatase analyses. Implantation of CA-Mel scaffolds into critical size calvarial defects in female, ovariectomized rats showed that the CA-Mel scaffolds were biocompatible, allowed for tissue infiltration, and showed evidence of scaffold biodegradation by 3 and 6months. Bone regeneration, assessed using fluorochrome labeling at 3 and 6months, was greatest in animals implanted with the CA-Mel scaffold. Overall, results from this study show that CA-Mel scaffolds were osteoconductive and osteoinductive.

Melatonin: bone metabolism in oral cavity

Throughout life, bone tissue undergoes a continuous process of resorption and formation. Melatonin, with its antioxidant properties and its ability to detoxify free radicals, as suggested by Conconi et al. (2000) may interfere in the osteoclast function and thereby inhibit bone resorption, as suggested by Schroeder et al. (1981). Inhibition of bone resorption may be enhanced by a reaction of indoleamine in osteoclastogenesis. That it has been observed melatonin, at pharmacological doses, decrease bone mass resorption by suppressing through down regulation of the RANK-L, as suggested by Penarrocha Diago et al. (2005) and Steflik et al. (1994). These data point an osteogenic effect towards that may be of melatonin of clinical importance, as it could be used as a therapeutic agent in situations in which would be advantageous bone formation, such as in the treatment of fractures or osteoporosis or their use as, a bioactive surface on implant as suggested by Lissoni et al. (1991).

Melatonin and oral cavity

While initially the oral cavity was considered to be mainly a source of various bacteria, their toxins and antigens, recent studies showed that it may also be a location of oxidative stress and periodontal inflammation. Accordingly, this paper focuses on the involvement of melatonin in oxidative stress diseases of oral cavity as well as on potential therapeutic implications of melatonin in dental disorders. Melatonin has immunomodulatory and antioxidant activities, stimulates the proliferation of collagen and osseous tissue, and acts as a protector against cellular degeneration associated with aging and toxin exposure. Arising out of its antioxidant actions, melatonin protects against inflammatory processes and cellular damage caused by the toxic derivates of oxygen. As a result of these actions, melatonin may be useful as a coadjuvant in the treatment of certain conditions of the oral cavity. However, the most important effect of melatonin seems to result from its potent antioxidant, immunomodulatory, protective, and anticancer properties. Thus, melatonin could be used therapeutically for instance, locally, in the oral cavity damage of mechanical, bacterial, fungal, or viral origin, in postsurgical wounds caused by tooth extractions and other oral surgeries. Additionally, it can help bone formation in various autoimmunological disorders such as Sjorgen syndrome, in periodontal diseases, in toxic effects of dental materials, in dental implants, and in oral cancers.

New spectrofluorimetric methods for determination of melatonin in the presence of N-{2-[1-({3-[2-(acetylamino)ethyl]-5-methoxy-1H-indol-2-yl}methyl)-5-methoxy-1H-indol-3-yl]- ethyl}acetamide: a contaminant in commercial melatonin preparations

Background

Melatonin (MLT) has many health implications, therefore it is of valuable importance to develop specific analytical methods for determination of MLT in the presence of its main contaminant, N-{2-[1-({3-[2-(acetylamino)ethyl]-5-methoxy-1H-indol-2-yl}methyl)-5-methoxy-1H-indol-3-yl]ethyl}acetamide (10). For development of these analytical methods, compound 10 had to be prepared in an adequate amount.

Results

Compound 10 was synthesized in six steps starting from 5-methoxyindole-2-carboxylic acid (1). Analytical performance of the proposed spectrofluorimetric methods was statistically validated with respect to linearity, accuracy, precision and specificity. The proposed methods were successfully applied for the assay of MLT in laboratory prepared mixtures containing up to 60 % of compound 10 and in commercial MLT tablets with recoveries not less than 99.00 %. No interference was observed from common pharmaceutical additives and the results were favorably compared with those obtained by a reference method.

Conclusions

This work describes simple, sensitive, and reliable second derivative spectrofluorimetric method in addition to two multivariate calibration methods, principal component regression (PCR) and partial least square (PLS), for the determination of MLT in the presence of compound 10.

Melatonin osteoporosis prevention study (MOPS): a randomized, double-blind, placebo-controlled study examining the effects of melatonin on bone health and quality of life in perimenopausal women

The purpose of this double-blind study was to assess the effects of nightly melatonin supplementation on bone health and quality of life in perimenopausal women. A total of 18 women (ages 45-54) were randomized to receive melatonin (3mg, p.o., n=13) or placebo (n=5) nightly for 6months. Bone density was measured by calcaneal ultrasound. Bone turnover marker (osteocalcin, OC for bone formation and NTX for bone resorption) levels were measured bimonthly in serum. Participants completed Menopause-Specific Quality of Life-Intervention (MENQOL) and Pittsburgh Sleep Quality Index (PSQI) questionnaires before and after treatment. Subjects also kept daily diaries recording menstrual cycling, well-being, and sleep patterns. The results from this study showed no significant change (6-month-baseline) in bone density, NTX, or OC between groups; however, the ratio of NTX:OC trended downward over time toward a ratio of 1:1 in the melatonin group. Melatonin had no effect on vasomotor, psychosocial, or sexual MENQOL domain scores; however, it did improve physical domain scores compared to placebo (mean change melatonin: -0.6 versus placebo: 0.1, P<0.05). Menstrual cycling was reduced in women taking melatonin (mean cycles melatonin: 4.3 versus placebo: 6.5, P<0.05), and days between cycles were longer (mean days melatonin: 51.2 versus placebo: 24.1, P<0.05). No differences in duration of menses occurred between groups. The overall PSQI score and average number of hours slept were similar between groups. These findings show that melatonin supplementation was well tolerated, improved physical symptoms associated with perimenopause, and may restore imbalances in bone remodeling to prevent bone loss. Further investigation is warranted.

Melatonin uses in oncology: breast cancer prevention and reduction of the side effects of chemotherapy and radiation

INTRODUCTION

The possible oncostatic properties of melatonin on different types of neoplasias have been studied especially in hormone-dependent adenocarcinomas. Despite the promising results of these experimental investigations, the use of melatonin in breast cancer treatment in humans is still uncommon.

AREAS COVERED

This article reviews the usefulness of this indoleamine for specific aspects of breast cancer management, particularly in reference to melatonin's antiestrogenic and antioxidant properties: i) treatments oriented to breast cancer prevention, especially when the risk factors are obesity, steroid hormone treatment or chronodisruption by exposure to light at night (LAN); ii) treatment of the side effects associated with chemo- or radiotherapy.

EXPERT OPINION

The clinical utility of melatonin depends on the appropriate identification of its actions. Because of its SERM (selective estrogen receptor modulators) and SEEM (selective estrogen enzyme modulators) properties, and its virtual absence of contraindications, melatonin could be an excellent adjuvant with the drugs currently used for breast cancer prevention (antiestrogens and antiaromatases). The antioxidant actions also make melatonin a suitable treatment to reduce oxidative stress associated with chemotherapy, especially with anthracyclines, and radiotherapy.

5-Meth-oxy-1-[(5-meth-oxy-1H-indol-2-yl)meth-yl]-1H-indole

In the title compound, C(19)H(18)N(2)O(2), the two indole ring systems are essentially planar [maximum deviation = 0.015 (2) Å in both indole ring systems] and make a dihedral angle of 72.17 (7)° with each other. In the crystal, the mol-ecules are linked into a zigzag chain along the a axis via N-H⋯O hydrogen bonds.

Melatonin, the circadian multioscillator system and health: the need for detailed analyses of peripheral melatonin signaling

Evidence is accumulating regarding the importance of circadian core oscillators, several associated factors, and melatonin signaling in the maintenance of health. Dysfunction of endogenous clocks, melatonin receptor polymorphisms, age- and disease-associated declines of melatonin likely contribute to numerous diseases including cancer, metabolic syndrome, diabetes type 2, hypertension, and several mood and cognitive disorders. Consequences of gene silencing, overexpression, gene polymorphisms, and deviant expression levels in diseases are summarized. The circadian system is a complex network of central and peripheral oscillators, some of them being relatively independent of the pacemaker, the suprachiasmatic nucleus. Actions of melatonin on peripheral oscillators are poorly understood. Various lines of evidence indicate that these clocks are also influenced or phase-reset by melatonin. This includes phase differences of core oscillator gene expression under impaired melatonin signaling, effects of melatonin and melatonin receptor knockouts on oscillator mRNAs or proteins. Cross-connections between melatonin signaling pathways and oscillator proteins, including associated factors, are discussed in this review. The high complexity of the multioscillator system comprises alternate or parallel oscillators based on orthologs and paralogs of the core components and a high number of associated factors with varying tissue-specific importance, which offers numerous possibilities for interactions with melatonin. It is an aim of this review to stimulate research on melatonin signaling in peripheral tissues. This should not be restricted to primary signal molecules but rather include various secondarily connected pathways and discriminate between direct effects of the pineal indoleamine at the target organ and others mediated by modulation of oscillators.

All-trans-retinoic acid ameliorated high fat diet-induced atherosclerosis in rabbits by inhibiting platelet activation and inflammation

Background. All-trans-retinoic acid (atRA) is effective for many proliferative diseases. We investigated the protective effects of atRA against atherosclerosis. Methods. Rabbits were randomly allocated to receive basal diet or an HFD for 4 weeks. HFD group then received rosuvastatin (3 mg/day), atRA (5 mg/kg/day), or the same volume of vehicle, respectively, for next 8 weeks. Results. HFD group showed increases in plasma lipids and aortic plaque formation. P-selectin expression and fibrinogen binding on platelets or deposition on the intima of the aorta also increased significantly as did the levels of TNF-α, IL-6, and fibrinogen in plasma. After 8 weeks of treatment with atRA, there was a significant decrease in plasma lipids and improvement in aortic lesions. AtRA also inhibited the expression of P-selectin and fibrinogen binding on platelets and deposition on the intima of the aorta. Conclusion. AtRA can ameliorate HFD-induced AS in rabbits by inhibiting platelet activation and inflammation.

Circadian regulation of molecular, dietary, and metabolic signaling mechanisms of human breast cancer growth by the nocturnal melatonin signal and the consequences of its disruption by light at night

This review article discusses recent work on the melatonin-mediated circadian regulation and integration of molecular, dietary, and metabolic signaling mechanisms involved in human breast cancer growth and the consequences of circadian disruption by exposure to light at night (LAN). The antiproliferative effects of the circadian melatonin signal are mediated through a major mechanism involving the activation of MT(1) melatonin receptors expressed in human breast cancer cell lines and xenografts. In estrogen receptor (ERα+) human breast cancer cells, melatonin suppresses both ERα mRNA expression and estrogen-induced transcriptional activity of the ERα via MT(1) -induced activation of G(αi2) signaling and reduction of 3',5'-cyclic adenosine monophosphate (cAMP) levels. Melatonin also regulates the transactivation of additional members of the steroid hormone/nuclear receptor super-family, enzymes involved in estrogen metabolism, expression/activation of telomerase, and the expression of core clock and clock-related genes. The anti-invasive/anti-metastatic actions of melatonin involve the blockade of p38 phosphorylation and the expression of matrix metalloproteinases. Melatonin also inhibits the growth of human breast cancer xenografts via another critical pathway involving MT(1) -mediated suppression of cAMP leading to blockade of linoleic acid uptake and its metabolism to the mitogenic signaling molecule 13-hydroxyoctadecadienoic acid (13-HODE). Down-regulation of 13-HODE reduces the activation of growth factor pathways supporting cell proliferation and survival. Experimental evidence in rats and humans indicating that LAN-induced circadian disruption of the nocturnal melatonin signal activates human breast cancer growth, metabolism, and signaling provides the strongest mechanistic support, thus far, for population and ecological studies demonstrating elevated breast cancer risk in night shift workers and other individuals increasingly exposed to LAN.

Melatonin combats molecular terrorism at the mitochondrial level

The intracellular environmental is a hostile one. Free radicals and related oxygen and nitrogen-based oxidizing agents persistently pulverize and damage molecules in the vicinity of where they are formed. The mitochondria especially are subjected to frequent and abundant oxidative abuse. The carnage that is left in the wake of these oxygen and nitrogen-related reactants is referred to as oxidative damage or oxidative stress. When mitochondrial electron transport complex inhibitors are used, e.g., rotenone, 1-methyl-1-phenyl-1,2,3,6-tetrahydropyridine, 3-nitropropionic acid or cyanide, pandemonium breaks loose within mitochondria as electron leakage leads to the generation of massive amounts of free radicals and related toxicants. The resulting oxidative stress initiates a series of events that leads to cellular apoptosis. To alleviate mitochondrial destruction and the associated cellular implosion, the cell has at its disposal a variety of free radical scavengers and antioxidants. Among these are melatonin and its metabolites. While melatonin stimulates several antioxidative enzymes it, as well as its metabolites (cyclic 3-hydroxymelatonin, N¹-acetyl-N²-formyl-5-methoxykynuramine and N¹-acetyl-5-methoxykynuramine), likewise effectively neutralize free radicals. The resulting cascade of reactions greatly magnifies melatonin's efficacy in reducing oxidative stress and apoptosis even in the presence of mitochondrial electron transport inhibitors. The actions of melatonin at the mitochondrial level are a consequence of melatonin and/or any of its metabolites. Thus, the molecular terrorism meted out by reactive oxygen and nitrogen species is held in check by melatonin and its derivatives.

Melatonin has membrane receptor-independent hypnotic action on neurons: an hypothesis

Melatonin, which is known to have sleep-promoting properties, has no morpho-physiological barriers and readily enters neurons and their subcellular compartments from both the blood and cerebrospinal fluid. It has multiple receptor-dependent and receptor-independent functions. Sleep is a neuronal function, and it can no longer be postulated that one or more anatomical structures fully control sleep. Neurons require sleep for metabolically driven restorative purposes, and as a result, the process of sleep is modulated by peripheral and central mechanisms. This is an important finding because it suggests that melatonin should have intracellular sleep-inducing properties. Based on recent evidence, it is proposed that melatonin induces sleep at the neuronal level independently of its membrane receptors. Thus, the hypnotic action of melatonin and the mechanisms involving the circadian rhythms are separate neurological functions. This is contrary to the presently accepted view.

Hepatoprotective actions of melatonin: Possible mediation by melatonin receptors

Melatonin, the hormone of darkness and messenger of the photoperiod, is also well known to exhibit strong direct and indirect antioxidant properties. Melatonin has previously been demonstrated to be a powerful organ protective substance in numerous models of injury; these beneficial effects have been attributed to the hormone’s intense radical scavenging capacity. The present report reviews the hepatoprotective potential of the pineal hormone in various models of oxidative stress in vivo, and summarizes the extensive literature showing that melatonin may be a suitable experimental substance to reduce liver damage after sepsis, hemorrhagic shock, ischemia/reperfusion, and in numerous models of toxic liver injury. Melatonin’s influence on hepatic antioxidant enzymes and other potentially relevant pathways, such as nitric oxide signaling, hepatic cytokine and heat shock protein expression, are evaluated. Based on recent literature demonstrating the functional relevance of melatonin receptor activation for hepatic organ protection, this article finally suggests that melatonin receptors could mediate the hepatoprotective actions of melatonin therapy.

Melatonin stimulates the growth of new bone around implants in the tibia of rabbits

This study evaluated the effect of the topical application of melatonin in accelerating bone formation associated with implants 2 months after their application to the tibiae of rabbits. Twenty New Zealand rabbits were used. Twenty implants treated with melatonin and 20 control implants without melatonin were placed in the proximal metaphyseal area of each tibia. Studies of new bone formation were subsequently made at 15, 30, 45 and 60 days. Cortical width and cortical length of new bone formation were measured. Following implantation, an anteroposterior and lateral radiologic study was carried out. Collected samples were sectioned at 5 μm and stained using hematoxylin-eosin, Masson's trichromic and Gordon-Switt reticulin stains. After a 60 day treatment period, melatonin increased the length of cortical bone (95.13±0.42%) versus that around control implants (62.91±1.45%). Related to the perimeter of cortical bone of the tibiae, melatonin induced new bone 88.35±1.56% versus 60.20±1.67% in the control implants. Melatonin regenerated the width and length of cortical bone around implants in tibiae of rabbits more quickly than around control implants without the addition of melatonin.

Melatonin inhibits adipogenesis and enhances osteogenesis of human mesenchymal stem cells by suppressing PPARγ expression and enhancing Runx2 expression

Adipogenesis and osteogenesis, a reciprocal relationship in bone marrow, are complex processes including proliferation of precursor cells, commitment to the specific lineage, and terminal differentiation. Accumulating evidence from in vitro and in vivo studies suggests that melatonin affects terminal differentiation of osteoblasts and adipocytes, but little is known about the effect of melatonin on the process of adipogenesis and osteogenesis, especially adipogenesis. This study was performed to determine the effect of melatonin on adipogenesis and osteogenesis in human mesenchymal stem cells (hMSCs). Cell proliferation assays demonstrated that melatonin had no apparent effect on the proliferation of hMSCs. When melatonin was added to the adipogenic/osteogenic medium, it directly inhibited adipogenesis and simultaneously promoted osteogenesis of hMSCs in a dose-dependent manner. Furthermore, quantitative RT-PCR demonstrated that melatonin significantly suppressed peroxisome proliferator-activated receptor gamma (PPARγ) expression (day 3, 25% decrease; day 6, 47% decrease), but promoted Runx2 expression (day 3, 87% increase; day 6, 56% increase) in the early stages of adipogenesis and osteogenesis of hMSCs. Moreover, melatonin down-regulated several markers of terminal adipocyte differentiation, including leptin (30%), lipoprotein lipase (LPL, 41%), adiponectin (51%), and adipocyte protein 2 (αP2, 45%). Meanwhile, melatonin up-regulated several markers of osteoblast differentiation, including alkaline phosphatase (110%), osteopontin (218%), and osteocalcin (310%). These results suggest that melatonin directly inhibits hMSCs adipogenic differentiation and significantly enhances hMSCs osteogenic differentiation by suppressing PPARγ expression and enhancing Runx2 expression; this provides further evidence for melatonin as an anti-osteoporosis drug.

Determination of the minimal melatonin exposure required to induce osteoblast differentiation from human mesenchymal stem cells and these effects on downstream signaling pathways

The purpose of this study was to determine the critical time periods of melatonin treatment required to induce human mesenchymal stem cells (hAMSCs) into osteoblasts and to determine which osteogenic genes are involved in the process. The study design consisted of adding melatonin for different times (2, 5, 10, 14 or 21 days) toward the end of a 21-day treatment containing osteogenic (OS+) medium or at the beginning of the 21-day treatment and then withdrawn. The results show that a 21-day continuous melatonin treatment was required to induce both alkaline phosphatase (ALP) activity and calcium deposition and these effects were mediated through MT₂Rs. Functional analysis revealed that peak ALP levels induced by melatonin were accompanied by attenuation of melatonin-mediated inhibition of forskolin-induced cAMP accumulation. Immunoprecipitation and western blot analyses, respectively, showed that MT₂R/β-arrestin scaffolds complexed to Gi, MEK1/2 and ERK1/2 formed in these differentiated hAMSCs (i.e., when ALP levels were highest) where ERK1/2 resided primarily in the cytosol. It is hypothesized that these complexes form to modulate the subcellular localization of ERK1/2 to affect osteogenic gene expression. Using real-time RT-PCR, chronic melatonin exposure induced the expression of osteogenic genes RUNX-2, osteocalcin and BMP-2, through MT₂Rs. No melatonin-mediated changes in the mRNA expression of ALP, BMP-6 or in the oxidative enzymes MtTFA, PGC-1α, Polγ, NRF-1, PDH, PDK and LDH occurred. These data show that a continuous 21-day melatonin exposure is required to induce osteoblast differentiation from hAMSCs through the formation of MT₂R/Gi/β-arrestin/MEK/ERK1/2 complexes to induce osteogenesis.

Melatonin Potentiates the Neuroprotective Properties of Resveratrol Against Beta-Amyloid-Induced Neurodegeneration by Modulating AMP-Activated Protein Kinase Pathways

BACKGROUND AND PURPOSE

Recent studies have demonstrated that resveratrol (RSV) reduces the incidence of age-related macular degeneration, Alzheimer's disease (AD), and stroke, while melatonin (MEL) supplementation reduces the progression of the cognitive impairment in AD patients. The purpose of this investigation was to assess whether the co-administration of MEL and RSV exerts synergistic effects on their neuroprotective properties against β-amyloid (Aβ)-induced neuronal death.

METHODS

The neuroprotective effects of co-treatment with MEL and RSV on Aβ1-42-induced cell death, was measured by MTT reduction assay. Aβ1-42 caused an increase in intracellular levels of reactive oxygen species (ROS), as assessed by H(2)-DCF-DA dye, and a reduction of total glutathione (GSH) levels and mitochondrial membrane potential, as assessed using monochlorobimane and rhodamine 123 fluorescence, respectively. Western blotting was used to investigate the intracellular signaling mechanism involved in these synergic effects.

RESULTS

We treated a murine HT22 hippocampal cell line with MEL or RSV alone or with both simultaneously. MEL and RSV alone significantly attenuated ROS production, mitochondrial membrane-potential disruption and the neurotoxicity induced by Aβ1-42. They also restored the Aβ1-42-induced depletion of GSH, back to within its normal range and prevented the Aβ1-42-induced activation of glycogen synthase kinase 3β (GSK3β). However, co-treatment with MEL and RSV did not exert any significant synergistic effects on either the recovery of the Aβ1-42-induced depletion of GSH or on the inhibition of Aβ1-42-induced GSK3β activation. Aβ1-42 treatment increased AMP-activated protein kinase (AMPK) activity, which is associated with subsequent neuronal death. We demonstrated that MEL and RSV treatment inhibited the phosphorylation of AMPK.

CONCLUSIONS

Together, our results suggest that co-administration of MEL and RSV acts as an effective treatment for AD by attenuating Aβ1-42-induced oxidative stress and the AMPK-dependent pathway.

Scientific basis for the potential use of melatonin in bone diseases: osteoporosis and adolescent idiopathic scoliosis

The objective of this paper was to analyze the data supporting the possible role of melatonin on bone metabolism and its repercussion in the etiology and treatment of bone pathologies such as the osteoporosis and the adolescent idiopathic scoliosis (AIS). Melatonin may prevent bone degradation and promote bone formation through mechanisms involving both melatonin receptor-mediated and receptor-independent actions. The three principal mechanisms of melatonin effects on bone function could be: (a) the promotion of the osteoblast differentiation and activity; (b) an increase in the osteoprotegerin expression by osteoblasts, thereby preventing the differentiation of osteoclasts; (c) scavenging of free radicals generated by osteoclast activity and responsible for bone resorption. A variety of in vitro and in vivo experimental studies, although with some controversial results, point toward a possible role of melatonin deficits in the etiology of osteoporosis and AIS and open a new field related to the possible therapeutic use of melatonin in these bone diseases.

Beneficial effects of melatonin in cardiovascular disease

The experimental data obtained from both human and rodent studies suggest that melatonin may have utility in the treatment of several cardiovascular conditions. In particular, melatonin's use in reducing the severity of essential hypertension should be more widely considered. In rodent studies melatonin has been shown to be highly effective in limiting abnormal cardiac physiology and the loss of critical heart tissue resulting from ischemia/reperfusion injury. Melatonin may also be useful in reducing cardiac hypertrophy in some situations and thereby limiting the frequency of heart failure. Finally, some conventional drugs currently in use have cardiotoxicity as a side-effect. Based on studies in rodents, melatonin, due to its multiple anti-oxidative actions, is highly effective in abrogating drug-mediated damage to the heart. Taken together, the findings from human and animal studies support the consideration of melatonin as a cardioprotective agent.

Methylselenocysteine resets the rhythmic expression of circadian and growth-regulatory genes disrupted by nitrosomethylurea in vivo

Epidemiologic and animal studies indicate that disruption of circadian rhythm increases breast cancer risk. Previously, we showed that methylselenocysteine reduced the incidence of N-nitroso-N-methylurea (NMU)-induced mammary carcinomas in Fischer 344 rats by 63%. Methylselenocysteine also increased the expression of Period 2 (Per2) and D-binding protein (DBP), providing evidence for a link between circadian rhythm and chemoprevention. Here, we report that NMU disrupted the expression of core circadian genes (Per1, Per2, Cry1, Cry2, and RevErbAalpha) and circadian-controlled genes, including melatonin receptor 1alpha (MTNR1A), estrogen receptors (ERalpha and ERbeta), and growth-regulatory genes (Trp53, p21, Gadd45alpha, and c-Myc) in mammary glands of Fischer 344 rats. By contrast, dietary methylselenocysteine (3 ppm selenium) given for 30 days, significantly enhanced the circadian expression of these genes (except for Cry1 and Cry2). The largest effect was on the levels of the Per2, MTNR1A, and ERbeta mRNAs, which showed 16.5-fold, 4.7-fold, and 9.5-fold increases in their rhythm-adjusted means, respectively, and 44.5-fold, 6.5-fold, and 9.7-fold increases in amplitude as compared with the control diet, respectively. Methylselenocysteine also shifted the peak expression times of these genes to Zeitgeber time 12 (ZT12; lights off). Methylselenocysteine also induced rhythmic expression of Trp53, p21, and Gadd45alpha mRNAs with peak levels at ZT12, when c-Myc expression was at its lowest level. However, methylselenocysteine had no significant effect on the circadian expression of these genes in liver. These results suggest that dietary methylselenocysteine counteracted the disruptive effect of NMU on circadian expression of genes essential to normal mammary cell growth and differentiation.

Actions of melatonin mixed with collagenized porcine bone versus porcine bone only on osteointegration of dental implants

This study evaluated the effect of the topical application of melatonin mixed with collagenized porcine bone on the osteointegration on the rough discrete calcium deposit (DCD) surface implants in Beagle dogs 3 months after their insertion. In preparation for subsequent insertion of dental implants, lower molars were extracted from 12 Beagle dogs. Each mandible received two parallel wall expanded platform implants with a DCD surface of 4 mm in diameter and 10 mm in length. The implants were randomly assigned to the distal sites on each mandible in the molar area and the gaps were filled with 5 mg lyophilized powdered melatonin and porcine bone and collagenized porcine bone alone. Ten histological sections per implant were obtained for histomorphometric studies. After a 4-wk treatment period, melatonin plus porcine bone significantly increased the perimeter of bone that was in direct contact with the treated implants (P < 0.0001), bone density (P < 0.0001), and new bone formation (P < 0.0001) in comparison with porcine bone alone around the implants. Melatonin plus collagenized porcine bone on DCD surface may act as a biomimetic agent in the placement of endo-osseous dental implants and enhance the osteointegration. Melatonin combined with porcine bone on DCD implants reveals more bone in implant contact at 12 wk (84.5 +/- 1.5%) compared with porcine bone alone treated area (67.17 +/- 1.2%).

The inhibition of apoptosis by melatonin in VSC4.1 motoneurons exposed to oxidative stress, glutamate excitotoxicity, or TNF-alpha toxicity involves membrane melatonin receptors

Loss of motoneurons may underlie some of the deficits in motor function associated with the central nervous system (CNS) injuries and diseases. We tested whether melatonin, a potent antioxidant and free radical scavenger, would prevent motoneuron apoptosis following exposure to toxins and whether this neuroprotection is mediated by melatonin receptors. Exposure of VSC4.1 motoneurons to either 50 microm H(2)O(2), 25 microm glutamate (LGA), or 50 ng/mL tumor necrosis factor-alpha (TNF-alpha) for 24 h caused significant increases in apoptosis, as determined by Wright staining and ApopTag assay. Analyses of mRNA and proteins showed increased expression and activities of stress kinases and cysteine proteases and loss of mitochondrial membrane potential during apoptosis. These insults also caused increases in intracellular free [Ca(2+)] and activities of calpain and caspases. Cells exposed to stress stimuli for 15 min were then treated with 200 nm melatonin. Post-treatment of cells with melatonin attenuated production of reactive oxygen species (ROS) and phosphorylation of p38, MAPK, and JNK1, prevented cell death, and maintained whole-cell membrane potential, indicating functional neuroprotection. Melatonin receptors (MT1 and MT2) were upregulated following treatment with melatonin. To confirm the involvement of MT1 and MT2 in providing neuroprotection, cells were post-treated (20 min) with 10 microm luzindole (melatonin receptor antagonist). Luzindole significantly attenuated melatonin-induced neuroprotection, suggesting that melatonin worked, at least in part, via its receptors to prevent VSC4.1 motoneuron apoptosis. Results suggest that neuroprotection rendered by melatonin to motoneurons is receptor mediated and melatonin may be an effective neuroprotective agent to attenuate motoneuron death in CNS injuries and diseases.

Melatonin: a multitasking molecule

Melatonin (N-acetyl-5-methoxytryptamine) has revealed itself as an ubiquitously distributed and functionally diverse molecule. The mechanisms that control its synthesis within the pineal gland have been well characterized and the retinal and biological clock processes that modulate the circadian production of melatonin in the pineal gland are rapidly being unravelled. A feature that characterizes melatonin is the variety of mechanisms it employs to modulate the physiology and molecular biology of cells. While many of these actions are mediated by well-characterized, G-protein coupled melatonin receptors in cellular membranes, other actions of the indole seem to involve its interaction with orphan nuclear receptors and with molecules, for example calmodulin, in the cytosol. Additionally, by virtue of its ability to detoxify free radicals and related oxygen derivatives, melatonin influences the molecular physiology of cells via receptor-independent means. These uncommonly complex processes often make it difficult to determine specifically how melatonin functions to exert its obvious actions. What is apparent, however, is that the actions of melatonin contribute to improved cellular and organismal physiology. In view of this and its virtual absence of toxicity, melatonin may well find applications in both human and veterinary medicine.

Topical application of melatonin and growth hormone accelerates bone healing around dental implants in dogs

BACKGROUND

Growth hormone (GH) and melatonin belong to the group of growth factors. These substances have been proposed to improve and accelerate osseous healing using topical applications.

PURPOSE

The aim of this study was to evaluate the effect of the topical administration of GH and melatonin on osseointegration of dental implants in Beagle dogs 2, 5, and 8 weeks after their insertion.

MATERIALS AND METHODS

Twelve adult Beagle dogs and 48 implants were used in the study. The maxillary and mandibular premolars and molars were extracted. Each mandible received cylindrical screw implants of 3.25 mm in diameter and 10 mm in length. Prior to implanting, 4 IU of recombinant human GH and 1.2 mg of lyophilized powdered melatonin was applied to one osteotomy at each side of the mandible. None was applied at the control sites. The implants were retrieved at 2, 5, and 8 weeks for light microscopic examination, energy-dispersive x-ray microanalysis, and histomorphometric measurements in ground sections.

RESULTS

At week 2, BIC was significantly higher in the melatonin-growth hormone group than in the implant control one (34.20 vs 25.05%; p = .010). The M-GH group also increased significantly the peri-implant bone area (64.72 vs 53.20%; p = .038) and interthread bone area (35.62 vs 25.08%; p = .02). At weeks 5 and 8, BIC and bone density around implants were similar to both groups. Significant differences were detected in bone neoformation at 8 weeks in ML-GH group (9.04 vs 7.53%; p = .05). Regarding the mineral composition, in ML-GH group increments in concentrations of phosphorus (10.70 vs 10.34; p = .013) were observed at 2 weeks and of magnesium (0.29 vs 0.25; p = .019) 5 weeks after implantation.

CONCLUSION

The present study confirms that GH and melatonin synergistically enhance new bone formation around titanium implants in early stages of healing.

Melatonin plus porcine bone on discrete calcium deposit implant surface stimulates osteointegration in dental implants

The aim of this study was to evaluate the effect of the topical application of melatonin mixed with collagenized porcine bone to accelerate the osteointegration on the rough discrete calcium deposit (DCD) surface implants in Beagle dogs 3 months after their insertion. In preparation for subsequent insertion of dental implants, lower premolars and molars were extracted from 12 Beagle dogs. Each mandible received three parallel wall implants with discrete calcium deposit (DCD) surface of 4 mm in diameter and 10 mm in length. The implants were randomly assigned to the distal sites on each side of the mandible in three groups: group I implants alone, group II implants with melatonin and group III implants with melatonin and porcine bone. Prior to implanting, 5 mg lyophylized powdered melatonin was applied to one bone hole at each side of the mandible. None was applied at the control sites. Ten histological sections per implant were obtained for histomorphometric studies. After a 4-wk treatment period, melatonin significantly increased the perimeter of bone that was in direct contact with the treated implants (P < 0.0001), bone density (P < 0.0001), new bone formation (P < 0.0001) in comparison with control implants. Topical application of melatonin on DCD surface may act as a biomimetic agent in the placement of endo-osseous dental implants and enhance the osteointegration. Melatonin combined with porcine bone on DCD implants reveals more bone to implant contact at 12 wk (84.5 +/- 1.5%) compared with melatonin treated (75.1 +/- 1.4%) and nonmelatonin treated surface implants (64 +/- 1.4%).

Coexpression of the melatonin receptor 1 and nestin in human breast cancer specimens

Activation of the G-protein-coupled receptor (GPCR) for melatonin (MT1) suppresses breast cancer cell growth in experimental models. To elucidate whether MT1 might play a role in cancer cells positive for the stem cell marker nestin, we assessed paired carcinomatous (Ca) and adjacent noncancerous (NCa) samples from 42 patients with primary breast cancer for MT1 and nestin by double immunofluorescence staining and quantitative image analysis with Tissue-Quest software. MT1 was located in luminal and myoepithelial cells in milk ducts and in tumor cells in 40/42 and 39/42 of NCa and Ca specimens, respectively, independent of hormone receptor and HER-2 status. Nestin was located together with MT1 in myoepithelial cells in 38 NCa specimens (total n = 42) and in 18 Ca specimens with intact milk ducts. Quantitative evaluation of selected 16 NCa and Ca samples revealed that MT1 levels were higher in invasive Ca sections than in NCa specimens in eight and lower in six cases. Specimens from higher tumor stages (TII/III) with a higher risk of relapse were associated with MT1/nestin co-staining in more than 10% of tumor cells, whereas a lack of co-staining correlated with lower tumor stages. Abundant expression of MT1 and, particularly, coexpression of MT1 with nestin in invading tumor cells in more advanced tumors suggest an important role for this GPCR in the pathogenesis of breast cancer.

Melatonin enhances cartilage matrix synthesis by porcine articular chondrocytes

To develop a minimally invasive preventive measure for early osteoarthritis, the effect of melatonin on cartilage matrix synthesis of articular chondrocytes was evaluated in vitro in a pellet culture system. The chondrogenic markers were assessed using histology, TaqMan polymerase chain reaction, and western blot. Our results show that melatonin treatment yielded chondrocyte-pellets with a higher expression of chondrogenic markers consisting of collagen II, Sox 9, and aggrecan at both the mRNA and protein levels. A hypertrophic marker, collagen X, remained low. Moreover, up-regulation of internal transforming growth factor beta1 (TGF-beta1) expression was observed in the melatonin-treated cells. Our data indicate, for the first time, that the administration of melatonin enhances cartilage matrix synthesis of articular chondrocytes in a serum-containing pellet culture system, likely through the TGF-beta signal pathway. Melatonin may prove to be a highly valuable addition to current therapeutic models for degenerative cartilage repair.

Melatonin and breast cancer: cellular mechanisms, clinical studies and future perspectives

Recent studies have suggested that the pineal hormone melatonin may protect against breast cancer, and the mechanisms underlying its actions are becoming clearer. Melatonin works through receptors and distinct second messenger pathways to reduce cellular proliferation and to induce cellular differentiation. In addition, independently of receptors melatonin can modulate oestrogen-dependent pathways and reduce free-radical formation, thus preventing mutation and cellular toxicity. The fact that melatonin works through a myriad of signalling cascades that are protective to cells makes this hormone a good candidate for use in the clinic for the prevention and/or treatment of cancer. This review summarises cellular mechanisms governing the action of melatonin and then considers the potential use of melatonin in breast cancer prevention and treatment, with an emphasis on improving clinical outcomes.

Melatonin-evoked in vivo secretion of protein and amylase from the parotid gland of the anaesthetised rat

The intravenous infusion of melatonin (5 and 25 mg/kg over 10 min) evoked a dose-dependent output of protein and amylase but no overt fluid secretion from the parotid gland of the pentobarbitone-anaesthetised rat, as revealed by increased concentrations of protein and amylase activity in a subsequent wash-out flow of saliva in response to an intravenous bolus injection of methacholine (5 microg/kg) 10 min later. The secretory responses to melatonin occurred in the presence of alpha- and beta-adrenoceptor antagonists. They were not affected by the cholecystokinin A-receptor antagonist, lorglumide, and they were reproduced in eviscerated animals acutely subjected to postganglionic sympathetic and parasympathetic denervation of the gland. The responses to melatonin were partially dependent on nitric oxide generation, through the activity of nitric oxide synthase of the neuronal type. Immunoblotting showed both melatonin receptors of type 1 and type 2 to be expressed in parotid gland tissue. The relative specific melatonin 2-receptor antagonist luzindole prevented the expected secretory effects of melatonin. The results favour a direct action by melatonin on melatonin receptors of parotid secretory cells and suggest a potential physiological role for melatonin in the regulation of salivary glandular activities.

Melatonin stimulates osteointegration of dental implants

The aim of this study was to evaluate the effect of the topical application of melatonin on osteointegration of dental implants in Beagle dogs 14 days after their insertion. In preparation for subsequent insertion of dental implants, upper and lower premolars and molars were extracted from 12 Beagle dogs. Each mandible received cylindrical screw implants of 3.25 mm in diameter and 10 mm in length. The implants were randomly assigned to the mesial and distal sites on each side of the mandible. Prior to implanting, 1.2 mg lyophylized powdered melatonin was applied to one bone hole at each side of the mandible. None was applied at the control sites. Eight histological sections per implant were obtained for histomorphometric studies. After a 2-wk treatment period, melatonin significantly increased the perimeter of bone that was in direct contact with the treated implants (P < 0.0001), bone density (P < 0.0001), new bone formation (P < 0.0001) and inter-thread bone (P < 0.05) in comparison with control implants. Topical application of melatonin may act as a biomimetic agent in the placement of endo-osseous dental implants.

C-terminal domains within human MT1 and MT2 melatonin receptors are involved in internalization processes

Melatonin, a molecule implicated in a variety of diseases, including cancer, often exerts its effects through G-protein-coupled melatonin receptors, MT(1) and MT(2). In this study, we sought to understand further the domains involved in the function and desensitization patterns of these receptors through site-directed mutagenesis. Two mutations were constructed in the cytoplasmic C-terminal tail of each receptor subtype: (i) a cysteine residue in the C-terminal tail was mutated to alanine, thus removing a putative palmitoylation site, and a site possibly required for normal receptor function (MT(1)C7.72A and MT(2)C7.77A) and (ii) the C-terminal tail in the MT(1) and MT(2) receptors was truncated, removing the putative phosphorylation and beta-arrestin binding sites (MT(1)Y7.64 and MT(2)Y7.64). These mutations did not alter the affinity of 2-[(125)I]-iodomelatonin binding to the MT(1) or MT(2) receptors. Using confocal microscopy, it was determined that the putative palmitoylation site (cysteine residue) did not play a role in receptor internalization; however, this residue was essential for receptor function, as determined by 3',5'-cyclic adenosine monophosphate (cAMP) accumulation assays. Truncation of the C-terminal tail of both receptors (MT(1)Y7.64 and MT(2)Y7.64) inhibited internalization as well as the cAMP response, suggesting the importance of the C-terminal tail in these receptor functions.

Consumption of vegetables alters morning urinary 6-sulfatoxymelatonin concentration

Melatonin, which is contained in certain vegetables, may have an influence on circulatory melatonin concentrations. This study examined the effects of the consumption of vegetables on 6-sulfatoxymelatonin concentrations in morning urine. Ninety-four healthy women aged 24-55 were recruited through a city public health center in Japan. The women randomly allocated to the intervention group were requested to consume high amounts of six selected vegetables, with a target of 350 g/day for 65 days, while those in the control group were asked to avoid the same six vegetables during the same period. First-void morning urine was collected before and at the end of the intervention period, and creatinine-adjusted 6-sulfatoxymelatonin concentrations were measured. At the end of the intervention period, daily mean intake of melatonin from the six vegetables was 1288.0 ng in the intervention group and 5.3 ng in the control group. In the intervention group, the mean concentration of 6-sulfatoxymelatonin changed from 48.1 [95% confidence interval (CI): 40.4-57.2] ng/mg creatinine to 49.6 (95% CI: 42.8-57.3) ng/mg creatinine across the intervention period. In the control group, the mean concentration of 6-sulfatoxymelatonin changed from 55.5 (95% CI: 48.7-63.2) ng/mg creatinine to 50.8 (95% CI: 44.0-58.7) ng/mg creatinine across the intervention period. A comparison of the two groups with regard to the changes in the 6-sulfatoxymelatonin concentrations across the intervention period showed a significant difference (P = 0.03). The results indicate that increased consumption of vegetables raises circulatory melatonin concentrations.