Melatonin-mediated regulation of human MT1 melatonin receptors expressed in mammalian cells

Beyond the Nucleus: Plastic Chemicals Activate G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are central mediators of cell signaling and physiological function. Despite their biological significance, GPCRs have not been widely studied in the field of toxicology. Herein, we investigated these receptors as novel targets of plastic chemicals using a high-throughput drug screening assay with 126 human non-olfactory GPCRs. In a first-pass screen, we tested the activity of triphenol phosphate, bisphenol A, and diethyl phthalate, as well as three real-world mixtures of chemicals extracted from plastic food packaging covering all major polymer types. We found 11 GPCR-chemical interactions, of which the chemical mixtures exhibited the most robust activity at adenosine receptor 1 (ADORA1) and melatonin receptor 1 (MTNR1A). We further confirm that polyvinyl chloride and polyurethane products contain ADORA1 or MTNRA1 agonists using a confirmatory secondary screen and pharmacological knockdown experiments. Finally, an analysis of the associated gene ontology terms suggests that ADORA1 and MTNR1A activation may be linked to downstream effects on circadian and metabolic processes. This work highlights that signaling disruption caused by plastic chemicals is broader than that previously believed and demonstrates the relevance of nongenomic pathways, which have, thus far, remained unexplored.

The Relationship Between Melatonin 1-2 Receptor Expression in Patients With Epithelial Ovarian Cancer and Survival

Melatonin has antiproliferative, antiangiogenic, apoptotic, and immunomodulatory properties in ovarian cancer. Considering those, we evaluated the relationship between melatonin 1 (MT1) and melatonin 2 receptor (MT2) expression in tumor tissues of patients with epithelial ovarian cancer, disease-free survival (DFS), and overall survival (OS). Patients who received primary surgical treatment for epithelial ovarian cancer in our clinic between 2000 and 2019 were retrospectively scanned through patient files, electronic databases, and telephone calls. One hundred forty-two eligible patients were included in the study, their tumoral tissues were examined to determine MT1 and MT2 expression by immunohistochemical methods. The percentage of receptor-positive cells and intensity of staining were determined. MT1 receptor expression ( P = 0.002 for DFS and P = 0.002 for OS) showed a significant effect on DFS and OS. MT2 expression had no effect on survival ( P = 0.593 for DFS and P = 0.209 for OS). The results showed that the higher the MT1 receptor expression, the longer the DFS and OS. It is suggested that melatonin should be considered as adjuvant therapy for ovarian cancer patients in addition to standard treatment, and clinical progress should be observed.

Expression and role of melatonin membrane receptors in the hypothalamic-pituitary-testicular axis of Tibetan sheep in a plateau pastoral area

MTNR1A and MTNR1B, two high-affinity MT membrane receptors found in mammals, mediate the activity of MT on the HPGA to regulate animal reproduction. Nevertheless, the expression patterns and function of the MTNR1A and MTNR1B genes in the HPTA of seasonal estrus sheep and perennial estrus sheep have not been elucidated. We studied the expression of MTNR1A and MTNR1B in the hypothalamic-pituitary-testicular axis (HPTA) of Tibetan sheep at different reproductive stages using histochemistry, enzyme linked immunosorbent assay (ELSIA), scanning electron microscopy, transmission electron microscopy, quantitative Real-time PCR (qRT-PCR), and Western blot (WB), and analyzed the relationship between their expression and reproductive hormone receptors. We also compared relevant characteristics between seasonal Tibetan sheep and non-seasonal Small Tail Han sheep in the same pastoral area. The results showed that MTNR1A and MTNR1B were expressed in all tissues of the Tibetan sheep HPTA, and both were co-expressed in the cytoplasm of epididymis basal and halo cells located at common sites of the epididymis basement membrane, forming an immune barrier. The qRT-PCR analysis showed that not only MTNR1A but also N-acetyltransferase (AANAT), hydroxyindole-oxygen- methyltransferase (HIOMT), androgen receptor (AR), and estrogen receptor α (ERα) mRNA expression was significantly upregulated in the testis and epididymis of Tibetan sheep during the breeding season, whereas no clear upregulation of these genes was observed in the tissues of Small Tail Han sheep. MTNR1A and MTNR1B are important regulators of the HPTA in sheep. MTNR1A mediates seasonal estrus regulation in Tibetan sheep. Both MTNR1A and MTNR1B may play important roles in formation of the blood-epididymal barrier. The results of this study should help advance research on the mechanism of reproductive regulation of the HPTA in male animals and provide reference data for improving the reproductive rate of seasonal breeding animals.

Therapeutic Potential of Melatonin Counteracting Chemotherapy-Induced Toxicity in Breast Cancer Patients: A Systematic Review

The purpose of this systematic review is to provide an overview of the existing knowledge on the therapeutic potential of melatonin to counteract the undesirable effects of chemotherapy in breast cancer patients. To this aim, we summarized and critically reviewed preclinical- and clinical-related evidence according to the PRISMA guidelines. Additionally, we developed an extrapolation of melatonin doses in animal studies to the human equivalent doses (HEDs) for randomized clinical trials (RCTs) with breast cancer patients. For the revision, 341 primary records were screened, which were reduced to 8 selected RCTs that met the inclusion criteria. We assembled the evidence drawn from these studies by analyzing the remaining gaps and treatment efficacy and suggested future translational research and clinical trials. Overall, the selected RCTs allow us to conclude that melatonin combined with standard chemotherapy lines would derive, at least, a better quality of life for breast cancer patients. Moreover, regular doses of 20 mg/day seemed to increase partial response and 1-year survival rates. Accordingly, this systematic review leads us to draw attention to the need for more RCTs to provide a comprehensive view of the promising actions of melatonin in breast cancer and, given the safety profile of this molecule, adequate translational doses should be established in further RCTs.

Melatonin Prescription in Children and Adolescents in Relation to Body Weight and Age

The prescription of melatonin to children and adolescents has increased dramatically in Sweden and internationally during the last ten years. In the present study we aimed to evaluate the prescribed melatonin dose in relation to body weight and age in children. The population-based BMI Epidemiology Study Gothenburg cohort has weight available from school health care records, and information on melatonin prescription through linkage with high-quality national registers. We included prescriptions of melatonin to individuals below 18 years of age where a weight measurement not earlier than three months before, or later than six months after the dispensing date, was available (n = 1554). Similar maximum doses were prescribed to individuals with overweight orobesity as to individuals with normal weight, and to individuals below and above 9 years of age. Age and weight only explained a marginal part of the variance in maximum dose, but were inversely associated and explained a substantial part of the variance in maximum dose per kg. As a result, individuals overweight or with obesity, or age above 9 years, received lower maximum dose per kg of body weight, compared with individuals with normal weight or below 9 years of age. Thus, the prescribed melatonin dose to individuals under 18 years of age is not primarily informed by body weight or age, resulting in substantial differences in prescribed dose per kg of body weight across BMI and age distribution.

In vitro approach points to a chemotactic effect of melatonin on ram spermatozoa

Sperm orientation mechanisms, such as chemotaxis, are essential for the sperm to reach the oocyte and fertilize it. Melatonin is secreted by the cumulus cells and is also present in the follicular fluid in mammals. The presence of membrane receptors for melatonin in ram spermatozoa, and its proven involvement in the sperm functionality, may suggest a possible role in the guided movement towards the oocyte. Hence, the objective of the present work is to study the in vitro potential chemotactic action of melatonin on ram spermatozoa, analysing the influence of the season (breeding and non-breeding) and the sperm capacitation state. The first experimental approach consisted in the inclusion of melatonin in the upper layer of a swim-up selection method. During the non-breeding season, the presence of melatonin at 100 pM and 1 μM concentrations significantly increased the cell recovery rate, and induced changes in the sperm location of the MT₂ melatonin receptor, compared with the standard swim-up. Moreover, the selected sperm population with 100 pM melatonin presented a higher percentage of capacitated spermatozoa. The greater recovery rate obtained with melatonin could be due to the stimulation of sperm movement in random directions, i.e., a chemokinetic effect, or due to a guided movement (chemotaxis) towards the gradient of the melatonin. To elucidate this issue, together with the study of the influence of the sperm capacitation status, we performed a second experimental approach which consisted in the use of chemotaxis chambers and an open-source software (Open-CASA) that analyses the sperm trajectories towards the hormone gradient and calculates a chemotaxis index (SL index). There was a significant difference between the SL index in the presence of 1 μM melatonin and the control without hormone. This effect was only observed in capacitated spermatozoa with cAMP-elevating agents (Cap-CK samples) obtained during the non-breeding season. These results would point to an in vitro chemotactic effect of melatonin on ram spermatozoa, although chemokinesis cannot be ruled out. Nonetheless, the inclusion of this hormone in the swim-up procedure could enhance the sperm recovery rate.

Changes on proteomic and metabolomic profiling of cryopreserved sperm effected by melatonin

Cryopreservation may reduce sperm fertility due to cryodamage including physical-chemical and oxidative stress damages. As a powerful antioxidant, melatonin has been reported to improve cryoprotective effect of sperm. However, the molecular mechanism of melatonin on cryopreserved ram sperm hasn't been fully understand. Give this, this study aimed to investigate the postthaw motility parameters, antioxidative enzyme activities and lipid peroxidation, as well as proteomic, metabolomic changes of Huang-huai ram spermatozoa with freezing medium supplemented with melatonin. Melatonin was firstly replenished to the medium to yield five different final concentrations: 0.1, 0.5, 1.0, 1.5, and 2.0 mM. A control (NC) group without melatonin replenishment was included. Protective effects of melatonin as evidenced by postthaw motility, activities of T-AOC, T-SOD, GSH-Px, CAT, contents of MDA, 4-HNE, as well as acrosome integrity, plasma membrane integrity, with 0.5 mM being the most effective concentration (MC group). Furthermore, 29 differentially abundant proteins involving in sperm functions were screened among Fresh, NC and MC groups of samples (n = 5) based on the 4D-LFQ, with 7 of them upregulated in Fresh and MC groups. 26 differentially abundant metabolites were obtained involving in sperm metabolism among the three groups of samples (n = 8) based on the UHPLC-QE-MS, with 18 of them upregulated in Fresh and MC groups. According to the bioinformatic analysis, melatonin may have positive effects on frozen ram spermatozoa by regulating the abundance changes of vital proteins and metabolites related to sperm function. Particularly, several proteins such as PRCP, NDUFB8, NDUFB9, SDHC, DCTN1, TUBB6, TUBA3E, SSNA1, as well as metabolites like L-histidine, L-targinine, ursolic acid, xanthine may be potential novel biomarkers for evaluating the postthaw quality of ram spermatozoa. In conclusion, a dose-dependent replenishment of melatonin to freezing medium protected ram spermatozoa during cryopreservation, which can improve motility, antioxidant enzyme activities, reduce levels of lipid peroxidation products, modify the proteomic and metabolomic profiling of cryopreserved ram spermatozoa through reduction of oxidative stress, maintenance of OXPHOS and microtubule structure. SIGNIFICANCE: Melatonin, a powerful antioxidant protects ram spermatozoa from cryopreservation injuries in a dose-dependent manner, with 0.5 mM being the most effective concentration. Furthermore, sequencing results based on the 4D-LFQ combined with the UHPLC-QE-MS indicated that melatonin modifies proteomic and metabolomic profiling of ram sperm during cryopreservation. According to the bioinformatic analysis, melatonin may have positive effects on frozen ram spermatozoa by regulating the expression changes of vital proteins and metabolites related to sperm metabolism and function. Particularly, several potential novel biomarkers for evaluating the postthaw quality of ram spermatozoa were acquired, proteins such as PRCP, NDUFB8, NDUFB9, SDHC, DCTN1, TUBB6, TUBA3E, SSNA1, as well as metabolites like L-histidine, L-targinine, ursolic acid, xanthine.

Melatonin as a Chronobiotic with Sleep-promoting Properties

The use of exogenous melatonin (exo-MEL) as a sleep-promoting drug has been under extensive debate due to the lack of consistency of its described effects. In this study, we conduct a systematic and comprehensive review of the literature on the chronobiotic, sleep-inducing, and overall sleep-promoting properties of exo-MEL. To this aim, we first describe the possible pharmacological mechanisms involved in the sleep-promoting properties and then report the corresponding effects of exo-MEL administration on clinical outcomes in: a) healthy subjects, b) circadian rhythm sleep disorders, c) primary insomnia. Timing of administration and doses of exo-MEL received particular attention in this work. The exo-MEL pharmacological effects are hereby interpreted in view of changes in the physiological properties and rhythmicity of endogenous melatonin. Finally, we discuss some translational implications for the personalized use of exo-MEL in the clinical practice.

Intestinal microbiota and melatonin in the treatment of secondary injury and complications after spinal cord injury

Spinal cord injury (SCI) is a central nervous system (CNS) disease that can cause sensory and motor impairment below the level of injury. Currently, the treatment scheme for SCI mainly focuses on secondary injury and complications. Recent studies have shown that SCI leads to an imbalance of intestinal microbiota and the imbalance is also associated with complications after SCI, possibly through the microbial-brain-gut axis. Melatonin is secreted in many parts of the body including pineal gland and gut, effectively protecting the spinal cord from secondary damage. The secretion of melatonin is affected by circadian rhythms, known as the dark light cycle, and SCI would also cause dysregulation of melatonin secretion. In addition, melatonin is closely related to the intestinal microbiota, which protects the barrier function of the gut through its antioxidant and anti-inflammatory effects, and increases the abundance of intestinal microbiota by influencing the metabolism of the intestinal microbiota. Furthermore, the intestinal microbiota can influence melatonin formation by regulating tryptophan and serotonin metabolism. This paper summarizes and reviews the knowledge on the relationship among intestinal microbiota, melatonin, and SCI in recent years, to provide new theories and ideas for clinical research related to SCI treatment.

Melatonin in sleep disorders

Melatonin is the main hormone involved in the control of the sleep-wake cycle. It is easily synthesisable and can be administered orally, which has led to interest in its use as a treatment for insomnia. Moreover, as production of the hormone decreases with age, in inverse correlation with the frequency of poor sleep quality, it has been suggested that melatonin deficit is at least partly responsible for sleep disorders. Treating this age-related deficit would therefore appear to be a natural way of restoring sleep quality, which is lost as patients age. However, despite the undeniable theoretical appeal of this approach to insomnia, little scientific evidence is available that supports any benefit of this substitutive therapy. Furthermore, the most suitable dose ranges and pharmaceutical preparations for melatonin administration are yet to be clearly defined. This review addresses the physiology of melatonin, the different pharmaceutical preparations, and data on its clinical usefulness.

Transcutaneous auricular vagal nerve stimulation releases extrapineal melatonin and reduces thermal hypersensitivity in Zucker diabetic fatty rats

Type 2 diabetes (T2D) is the most common comorbidity of COVID-19, and both are related to the lack of circulating melatonin. In addition, chronic pain is a common sequela of both COVID-19 and T2D. Using a neuropathic pain model produced by sciatic nerve chronic constriction injury in Zucker diabetic fatty rats, a verified preclinical genetic T2D neuropathy animal model, this study aimed to show that transcutaneous auricular vagal nerve stimulation (taVNS) could elevate plasma melatonin concentration, upregulate the expression of melatonin receptors (MTRs) in the amygdala, and relieve peripheral neuropathic pain. Furthermore, taVNS would restore melatonin levels and relieve pain even in pinealectomized rats. On the contrary, intraperitoneally injected luzindole, a melatonin receptor antagonist, would attenuate the antinociceptive effects of taVNS. In conclusion, the mechanism of the therapeutic effect of taVNS on chronic pain involves the release of extrapineal melatonin and the positive regulation of the expression of central MTRs. This beneficial efficacy should be considered during COVID-19 rehabilitation in individuals with diabetes.

Melatonin receptor 1A, but not 1B, knockout decreases biliary damage and liver fibrosis during cholestatic liver injury

BACKGROUND AND AIMS

Melatonin reduces biliary damage and liver fibrosis in cholestatic models by interaction with melatonin receptors 1A (MT1) and 1B (MT2). MT1 and MT2 can form heterodimers and homodimers, but MT1 and MT2 can heterodimerize with the orphan receptor G protein-coupled receptor 50 (GPR50). MT1/GPR50 dimerization blocks melatonin binding, but MT2/GPR50 dimerization does not affect melatonin binding. GPR50 can dimerize with TGFβ receptor type I (TGFβRI) to activate this receptor. We aimed to determine the differential roles of MT1 and MT2 during cholestasis.

APPROACH AND RESULTS

Wild-type (WT), MT1 knockout (KO), MT2KO, and MT1/MT2 double KO (DKO) mice underwent sham or bile duct ligation (BDL); these mice were also treated with melatonin. BDL WT and multidrug resistance 2 KO (Mdr2^-/- ) mice received mismatch, MT1, or MT2 Vivo-Morpholino. Biliary expression of MT1 and GPR50 increases in cholestatic rodents and human primary sclerosing cholangitis (PSC) samples. Loss of MT1 in BDL and Mdr2^-/- mice ameliorated biliary and liver damage, whereas these parameters were enhanced following loss of MT2 and in DKO mice. Interestingly, melatonin treatment alleviated BDL-induced biliary and liver injury in BDL WT and BDL MT2KO mice but not in BDL MT1KO or BDL DKO mice, demonstrating melatonin's interaction with MT1. Loss of MT2 or DKO mice exhibited enhanced GPR50/TGFβR1 signaling, which was reduced by loss of MT1.

CONCLUSIONS

Melatonin ameliorates liver phenotypes through MT1, whereas down-regulation of MT2 promotes liver damage through GPR50/TGFβR1 activation. Blocking GPR50/TGFβR1 binding through modulation of melatonin signaling may be a therapeutic approach for PSC.

Adrenal MT1 melatonin receptor expression is linked with seasonal variation in social behavior in male Siberian hamsters

Many animals exhibit pronounced changes in physiology and behavior on a seasonal basis, and these adaptations have evolved to promote survival and reproductive success. While the neuroendocrine pathways mediating seasonal reproduction are well-studied, far less is known about the mechanisms underlying seasonal changes in social behavior, particularly outside of the context of the breeding season. Our previous work suggests that seasonal changes in melatonin secretion are important in regulating aggression in Siberian hamsters (Phodopus sungorus); it is unclear, however, how melatonin acts via its receptors to modulate seasonal variation in social behavior. In this study, we infused a MT₁ melatonin receptor-expressing (MT₁) or control (CON) lentivirus into the adrenal glands of male Siberian hamsters. We then housed hamsters in long-day (LD) or short-day (SD) photoperiods, administered timed melatonin or control injections, and quantified aggressive and non-aggressive social behaviors (e.g., investigation, self-grooming) following 10 weeks of treatment. LD hamsters infused with the MT₁ lentivirus had significantly higher adrenal mt1 expression than LD CON hamsters, as determined via quantitative PCR. While melatonin administration was necessary to induce SD-like reductions in body and relative reproductive mass, only LD hamsters infused with the MT₁ lentivirus displayed SD-like changes in social behavior, including increased aggression and decreased investigation and grooming. In addition, SD CON and LD hamsters infused with the MT₁ lentivirus exhibited similar relationships between adrenal mt1 expression and aggressive behavior. Together, our findings suggest a role for adrenal MT₁ receptor signaling in regulating behavior, but not energetics or reproduction in seasonally breeding species.

Empowering Melatonin Therapeutics with Drosophila Models

Melatonin functions as a central regulator of cell and organismal function as well as a neurohormone involved in several processes, e.g., the regulation of the circadian rhythm, sleep, aging, oxidative response, and more. As such, it holds immense pharmacological potential. Receptor-mediated melatonin function mainly occurs through MT1 and MT2, conserved amongst mammals. Other melatonin-binding proteins exist. Non-receptor-mediated activities involve regulating the mitochondrial function and antioxidant cascade, which are frequently affected by normal aging as well as disease. Several pathologies display diseased or dysfunctional mitochondria, suggesting melatonin may be used therapeutically. Drosophila models have extensively been employed to study disease pathogenesis and discover new drugs. Here, we review the multiple functions of melatonin through the lens of functional conservation and model organism research to empower potential melatonin therapeutics to treat neurodegenerative and renal diseases.

Effects of Duodenal 5-Hydroxytryptophan Perfusion on Melatonin Synthesis in GI Tract of Sheep

The purpose of this study is to investigate the potential effects of 5-hydroxytryptophan (5-HTP) duodenal perfusion on melatonin (MT) synthesis in the gastrointestinal (GI) tract of sheep. 5-hydroxytryptophan is a precursor in the melatonin synthetic pathway. The results showed that this method significantly increased melatonin production in the mucosa of all segments in GI tract including duodenum, jejunum, ileum, cecum and colon. The highest melatonin level was identified in the colon and this indicates that the microbiota located in the colon may also participate in the melatonin production. In addition, portion of the melatonin generated by the GI tract can pass the liver metabolism and enters the circulation via portal vein. The current study provides further evidence to support that GI tract is the major site for melatonin synthesis and the GI melatonin also contributes to the circulatory melatonin level since plasma melatonin concentrations in 5-HTP treated groups were significantly higher than those in the control group. In conclusion, the results show that 10–50 mg of 5-HTP flowing into the duodenum within 6 h effectively improve the production of melatonin in the GI tract and melatonin concentration in sheep blood circulation during the day.

Effects Of Exogenous Melatonin On Sleep And Circadian Rhythms In Women With Premenstrual Dysphoric Disorder

We previously found normal polysomnographic (PSG) sleep efficiency, increased slow wave sleep (SWS) and a blunted melatonin secretion in women with premenstrual dysphoric disorder (PMDD) compared to controls. Here, we investigated the effects of exogenous melatonin in five patients previously studied. They took 2 mg of slow-release melatonin 1 hour before bedtime during their luteal phase (LP) for three menstrual cycles. At baseline, patients spent every third night throughout one menstrual cycle sleeping in the laboratory. Measures included morning urinary 6-sulfatoxymelatonin (aMt6), PSG sleep, nocturnal core body temperature (CBT), visual analogue scale for mood (VAS-Mood), Prospective Record of the Impact and Severity of Menstrual Symptoms (PRISM), and ovarian hormones. Participants also underwent two 24-hour intensive physiological monitoring (during the follicular phase and LP) in time-isolation/constant conditions to determine 24-hour plasma melatonin and CBT rhythms. The same measures were repeated during their third menstrual cycle of melatonin administration. In the intervention condition compared to baseline, we found increased urinary aMt6 (p<0.001), reduced objective SOL (p=0.01), reduced SWS (p<0.001) and increased Stage 2 sleep (p<0.001). Increased urinary aMt6 was associated with reduced SWS (r=-0.51, p<0.001). Circadian parameters derived from 24-hour plasma melatonin and CBT did not differ between conditions, except for an increased melatonin mesor in the intervention condition (p=0.01). Ovarian hormones were comparable between the conditions (p≥0.28). Symptoms improved in the intervention condition, as measured by the VAS-Mood (p=0.02) and the PRISM (p<0.001). These findings support a role for disturbed melatonergic system in PMDD that can be partially corrected by exogenous melatonin.

Endogenous and Exogenous Melatonin Exposure Attenuates Hepatic MT1 Melatonin Receptor Protein Expression in Rat

Melatonin receptors are highly relevant for the hepatoprotective effects of the pineal hormone melatonin after experimental hemorrhagic shock in rats. In this study, we sought to determine the spatial expression pattern and a putative regulation of two melatonin receptors, membrane bound type 1 and 2 (MT₁ and MT₂), in the liver of rats. In a male rat model (Sprague Dawley) of hemorrhage and resuscitation, we investigated the gene expression and protein of MT₁ and MT₂ in rat liver by utilizing real-time quantitative polymerase chain reaction, a western blot analysis, and immunohistochemistry. Plasma melatonin content was measured by an enzyme-linked immunosorbent assay. Male rats underwent hemorrhage and were resuscitated with shed blood and a Ringer’s solution (n = 8 per group). After 90 min of hemorrhage, animals were given vehicle, melatonin, or ramelteon (each 1.0 mg/kg intravenously). Sham-operated controls did not undergo hemorrhage but were treated likewise. Plasma melatonin was significantly increased in all groups treated with melatonin and also after hemorrhagic shock. Only MT₁, but not the MT₂ messenger ribonucleic acid (mRNA) and protein, was detected in the rat liver. The MT₁ protein was located in pericentral fields of liver lobules in sham-operated animals. After hemorrhagic shock and treatment with melatonin or ramelteon, the hepatic MT₁ protein amount was significantly attenuated in all groups compared to sham controls (50% reduction; p < 0.001). With respect to MT₁ mRNA, no significant changes were observed between groups (p = 0.264). Our results indicate that both endogenous melatonin exposure from hemorrhagic shock, as well as exogenous melatonin and ramelteon exposure, may attenuate melatonin receptors in rat hepatocytes, possibly by means of desensitization.

Melatonin Administration Methods for Research in Mammals and Birds

Endocrine research in animals often entails exogenous hormone administration. Special issues arise when developing administration protocols for hormones with circadian and seasonal periodicity. This article reviews various methods for the exogenous administration of hormones with such periodicities by focusing on melatonin. We discuss that methodological variations across studies can affect experimental results. Melatonin administration techniques used in vertebrates includes infusion pumps, beeswax pellets, oral administration, injections, SILASTIC capsules, osmotic pumps, transdermal delivery, beads, and sponges.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

Carbamate Insecticides Target Human Melatonin Receptors

Carbaryl (1-naphthyl methylcarbamate) and carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) are among the most toxic insecticides, implicated in a variety of diseases including diabetes and cancer among others. Using an integrated pharmacoinformatics based screening approach, we have identified these insecticides to be structural mimics of the neurohormone melatonin and were able to bind to the putative melatonin binding sites in MT₁ and MT₂ melatonin receptors in silico. Carbaryl and carbofuran then were tested for competition with 2-[¹²⁵I]-iodomelatonin (300 pM) binding to hMT₁ or hMT₂ receptors stably expressed in CHO cells. Carbaryl and carbofuran showed higher affinity for competition with 2-[¹²⁵I]-iodomelatonin binding to the hMT₂ compared to the hMT₁ melatonin receptor (33 and 35-fold difference, respectively) as predicted by the molecular modeling. In the presence of GTP (100 μM), which decouples the G-protein linked receptors to modulate signaling, the apparent efficacy of carbaryl and carbofuran for 2-[¹²⁵I]-iodomelatonin binding for the hMT₁ melatonin receptor was not affected but significantly decreased for the hMT₂ melatonin receptor compatible with receptor antagonist/inverse agonist and agonist efficacy, respectively. Altogether, our data points to a potentially new mechanism through which carbamate insecticides carbaryl and carbofuran could impact human health by altering the homeostatic balance of key regulatory processes by directly binding to melatonin receptors.

New evidence of melatonin receptor contribution to ram sperm functionality

The present study analysed the involvement of melatonin, acting via its receptors (MT1 and MT2), in ram sperm functionality. Indirect immunofluorescence assays revealed no changes in the distribution or intensity of MT1 receptors, whereas different subpopulations were established for MT2 receptors in control, in vitro capacitated and acrosome-reacted ram spermatozoa. Chlortetracycline staining revealed the following correlations between the pattern of staining for MT2 receptors in: (1) non-capacitated (NC) sperm rate and the proportion of spermatozoa with equal immunostaining intensity in the acrosome and post-acrosome (r = 0.59, P < 0.001); (2) in capacitated (C) sperm rate and the proportion of spermatozoa with stronger reactivity in the acrosome (r = 0.60, P < 0.001); and (3) in acrosome-reacted (AR) sperm rate and the proportion of spermatozoa with more intense staining on the post-acrosome (r = 0.67, P < 0.001). Incubation of swim-up-selected samples with either 1 μM melatonin or MT1 and MT2 receptor agonists (2-phenylmelatonin 1 µM and 8-Methoxy-2-propionamidotetralin (8M-PDOT) 1 µM and 10 nM) at 39°C and 5% CO2 for 3 h resulted in a higher proportion of the NC pattern compared with the control group (P < 0.05), whereas treatment with MT1 and MT2 receptor antagonists (luzindole 1 µM and 4-phenyl-2-propionamidotetralin (4P-PDOT) 1 µM and 10 nM) decreased the proportion of spermatozoa exhibiting the NC pattern (P < 0.001) concomitant with an increase in those exhibiting the C pattern (P < 0.01). In conclusion, melatonin exerts a modulating effect on ram sperm functionality, primarily via activation of the MT2 receptor.

MT1 and MT2 Melatonin Receptors: A Therapeutic Perspective

Melatonin, or 5-methoxy-N-acetyltryptamine, is synthesized and released by the pineal gland and locally in the retina following a circadian rhythm, with low levels during the day and elevated levels at night. Melatonin activates two high-affinity G protein-coupled receptors, termed MT1 and MT2, to exert beneficial actions in sleep and circadian abnormality, mood disorders, learning and memory, neuroprotection, drug abuse, and cancer. Progress in understanding the role of melatonin receptors in the modulation of sleep and circadian rhythms has led to the discovery of a novel class of melatonin agonists for treating insomnia, circadian rhythms, mood disorders, and cancer. This review describes the pharmacological properties of a slow-release melatonin preparation (i.e., Circadin®) and synthetic ligands (i.e., agomelatine, ramelteon, tasimelteon), with emphasis on identifying specific therapeutic effects mediated through MT1 and MT2 receptor activation. Discovery of selective ligands targeting the MT1 or the MT2 melatonin receptors may promote the development of novel and more efficacious therapeutic agents.

Melatonin in sperm biology: breaking paradigms

Melatonin is a ubiquitous molecule, present in a wide range of organisms, and involved in multiple functions. Melatonin relays the information about the photoperiod to the tissues that express melatonin-binding sites in both central and peripheral nervous systems. This hormone has a complex mechanism of action. It can cross the cell plasma membrane and exert its actions in all cells of the body. Certain melatonin actions are mediated by receptors that belong to the superfamily of G-protein-coupled receptors (GPCRs), the MT1 and MT2 membrane. Melatonin can also bind to calmodulin as well as to nuclear receptors of the retinoic acid receptor family, RORα1, RORα2 and RZRβ. The purpose of this review is to report on recent developments in the physiological role of melatonin and its receptors. Specific issues concerning the biological function of melatonin in mammalian seasonal reproduction and spermatozoa are considered. The significance of the continuous presence of melatonin in seminal plasma with a fairly constant concentration is also discussed.

Transcutaneous vagus nerve stimulation induces tidal melatonin secretion and has an antidiabetic effect in Zucker fatty rats

Melatonin plays a protective role in type 2 diabetes (T2D) through regulation of glucose metabolism. Whether transcutaneous vagus nerve stimulation (taVNS) is antidiabetic and whether a modulated melatonin production is involved in the antidiabetic mechanism of taVNS is unknown. In this study, once daily 30min noninvasive taVNS was administered in Zucker diabetic fatty (ZDF, fa/fa) and Zucker lean (ZL, +/fa) littermates under anesthesia for 5 consecutive weeks. The acute and chronic influences of taVNS on the secretion of melatonin were studied as well as the effects of taVNS on blood glucose metabolism. We found that naïve ZDF rats develop hyperglycemia naturally with age. Each taVNS session would trigger a tidal secretion of melatonin both during and after the taVNS procedure and induce an acute two-phase glycemic change, a steep increase followed by a gradual decrease. Once daily taVNS sessions eventually reduced the glucose concentration to a normal level in seven days and effectively maintained the normal glycemic and plasma glycosylated hemoglobin (HbA_lc) levels when applied for five consecutive weeks. These beneficial effects of taVNS also exist in pinealectomized rats, which otherwise would show overt and continuous hyperglycemia, hyperinsulinemia, and high HbA_lc levels. We concluded that multiple taVNS sessions are antidiabetic in T2D through triggering of tidal secretion of melatonin. This finding may have potential importance in developing new approaches to the treatment of T2D, which is highly prevalent, incurable with any current approaches, and very costly to the world.

In vitro comparison of duration of action of melatonin agonists on melatonin MT1 receptor: possible link between duration of action and dissociation rate from receptor

Melatonin MT1 and MT2 receptors are Gi protein-coupled receptors and promising therapeutic targets for a number of diseases. A proportion of G protein-coupled receptor agonists and antagonists have been classified according to their duration of action, which influences their pharmacological efficacy. However, the duration of action of melatonin agonists remains unclear. In this study, we investigated the duration of action of melatonin agonists (melatonin, 2-iodomelatonin, ramelteon, and the ramelteon metabolite M-II) at the melatonin MT1 receptor, which is more resistant to agonist-induced desensitization than the melatonin MT2 receptor. In Chinese hamster ovary cells stably expressing the human melatonin MT1 receptor, significant differences in the duration of action were observed after 2-h pretreatment with agonists followed by washout. In contrast to melatonin and M-II, the agonist activities of ramelteon and 2-iodomelatonin were persistent (i.e. inhibition of forskolin-stimulated cAMP formation and increase in ERK 1/2 phosphorylation) even after repeated washouts. Similar activities were observed for INS-1 cells endogenously expressing the rat MT1 receptor. Further, we examined potential factors linked to the duration of action. Residual activities of melatonin agonists after washout strongly correlated with their dissociation rates from the human melatonin MT1 receptor, but not their lipophilicity or extent of desensitization. These data suggest that the in vitro duration of action significantly differs between melatonin agonists and might dictate dissociation kinetics. Characterization of these in vitro properties may facilitate further in vivo study of the duration of action.

Genetic deletion of MT1 melatonin receptors alters spontaneous behavioral rhythms in male and female C57BL/6 mice

Behaviors vary over the 24h light/dark cycle and these temporal patterns reflect in part modulation by circadian neural circuits and hormones, such as melatonin. The goal of this study was to investigate the involvement of MT1 melatonin receptors in behavioral regulation by comparing male and female C57 wild type (WT) mice with C57 mice with genetic deletion of the MT1 receptor (MT1KO). A comprehensive array of fifteen distinct spontaneous behaviors was recorded continuously in the homecage over multiple days using the HomeCageScan system. Behaviors assessed were activity-like (i.e. come down, hang, jump, walk), exploration-like (i.e. dig, groom, rear up, sniff, stretch), resting-like (i.e. awake, remain low, rest, twitch) and ingestion-like (i.e. drink, eat). Phenotypic array and temporal distribution analysis revealed distinct behavioral rhythms that differed between WT and MT1KO mice. The rhythms were consistent from day to day in males and varied with the estrous cycle in females. We also studied the role of MT1 receptors on depressive and anxiety-like behaviors. Genetic deletion of MT1 receptors increased immobility time in the forced swim test and decreased the number of marbles buried in the marble burying test in both male and female C57 mice. We conclude that MT1 melatonin receptors are involved in neural pathways modulating diurnal rhythms of spontaneous behavior in the homecage as well as pathways regulating depressive and anxiolytic-like behaviors.

Circadian modulation of human erythrocyte plasma membrane redox system by melatonin

BACKGROUND

The pineal hormone melatonin works for the stabilization of biological rhythms, however, it also modulates several other functions such as cardioprotection, thermoregulation and immunomodulation. Melatonin also shows antioxidant activity. The erythrocyte plasma membrane redox system (PMRS) alongwith ascorbate free radical (AFR) reductase is involved in providing protection against oxidative stress. The present work is an ex vivo study addressing RBC PMRS and AFR reductase activities at two different times of the day. The in vitro modulatory effect of melatonin on PMRS and AFR reductase activities is also reported.

MATERIALS AND METHODS

The study was carried out on 61 healthy donors of both sexes (aged 20-30). Blood samples were collected at two different timings viz., 10:00 a.m. and 10:00 p.m. PMRS and AFR reductase were determined by methods already reported. The concentration-dependent effect of the melatonin was evaluated by incubating the RBCs with the hormone at different doses.

RESULTS

We present results to show that erythrocyte PMRS and AFR reductase activity are modulated by melatonin, a higher activity (p<0.05) of PMRS and AFR reductase is observed during night when the level of melatonin is high. The effect of in vitro treatment of erythrocytes with melatonin (10(-7) M to 10(-11) M final concentration) shows significant changes during day at a melatonin concentration of 10(-9)M.

CONCLUSION

To the best of our knowledge this study shows for the first time the circadian rhythmicity of erythrocyte PMRS and AFR reductase activities. The modulatory effect of melatonin on PMRS and AFR reductase opens the possibility of melatonin being used in treatment of such physiological and metabolic dysfunctions that involve photic cues in association with oxidative stress.

Mood disorders, circadian rhythms, melatonin and melatonin agonists

Recent advances in the understanding of circadian rhythms have led to an interest in the treatment of major depressive disorder with chronobiotic agents. Many tissues have autonomous circadian rhythms, which are orchestrated by the master clock, situated in the suprachiasmatic nucleus (SNC). Melatonin (N-acetyl-5-hydroxytryptamine) is secreted from the pineal gland during darkness. Melatonin acts mainly on MT1 and MT2 receptors, which are present in the SNC, regulating physiological and neuroendocrine functions, including circadian entrainment, referred to as the chronobiotic effet. Circadian rhythms has been shown to be either misaligned or phase shifted or decreased in amplitude in both acute episodes and relapse of major depressive disorder (MDD) and bipolar disorder. Manipulation of circadian rhythms either using physical treatments (such as high intensity light) or behavioral therapy has shown promise in improving symptoms. Pharmacotherapy using melatonin and pure melatonin receptor agonists, while improving sleep, has not been shown to improve symptoms of depression. A novel antidepressant, agomelatine, combines 5HT2c antagonist and melatonin agonist action, and has shown promise in both acute treatment of MDD and in preventing relapse.

Functional characterization of polymorphic variants for ovine MT1 melatonin receptors: possible implication for seasonal reproduction in sheep

In seasonal breeding species, the gene encoding for the melatonin MT(1) receptor (oMT(1)) is highly polymorphic and numerous data have reported the existence of an association between an allele of the receptor and a marked expression of the seasonality of reproduction in ewes. This allele called "m" (previously named "-" allele) carries a mutation leading to the absence of a MnlI restriction site as opposed to the "M" allele (previously named "+" allele) carrying the MnlI restriction site (previously "+" allele). This allows the determination of the three genotypes "M/M" (+/+), "M/m" (+/-) and "m/m" (-/-). This mutation is conservative and could therefore not be causal. However, it is associated with another mutation introducing the change of a valine to an isoleucine in the fifth transmembrane domain of the receptor. Homozygous "M/M" and "m/m" animals consequently express structurally different receptors respectively named oMT(1) Val(220) and oMT(1) Ile(220). The objective of this study was to test whether these polymorphic variants are functionally different. To achieve this goal, we characterized the binding properties and the transduction pathways associated with both variants of the receptors. Using a pharmacological approach, no variation in binding parameters between the two receptors when transiently expressed in COS-7. In stably transfected HEK293 cells, significant differences were detected in the inhibition of cAMP production whereas receptors internalization processes were not different. In conclusion, the possibility that subtle alterations induced by the non conservative mutation in "m/m" animals might modify the perception of the melatoninergic signal is discussed in the context of melatonin action.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

Effect of melatonin in the antioxidant defense system in the locomotor muscles of the estuarine crab Neohelice granulata (Decapoda, Brachyura)

In vertebrates, many studies verified different effects of melatonin in the antioxidant defense system (ADS). In crustaceans, few studies have been conducted to verify this possibility. We verified the melatonin effects in the crab Neohelice granulata using low (0.002 and 0.02 pmol/crab) and high (2.0 and 20.0 pmol/crab) melatonin dosages in short-term (0.5h) and long-term (9.5h) experiments. We analyzed the antioxidant capacity against peroxyl radicals (ACAP), reactive oxygen species (ROS) concentration, levels of by products of lipid peroxidation (LPO), oxygen consumption (VO(2)), the activity of glutamate cysteine ligase (gamma-GCL) and catalase (CAT) and glutathione content (GSH). Finally, the effects of exogenous melatonin were verified in terms of melatonin and N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) content in the muscles of N. granulata. In short-term experiment and low dosages, melatonin increased the VO(2), gamma-GCL activity and GSH content (p<0.05) and decreased melatonin content (p<0.05) without effects in ROS, ACAP and LPO (p>0.05). Possibly, melatonin is acting in the ADS increasing its efficiency and/or acting in mitochondrial activity and/or through signaling muscles to increase its consumption. AFMK was only detected in the eyestalk and cerebroid ganglia. In high dosages melatonin effects decreased, possibly by the desensitization of their receptors. In long-term experiment, melatonin decreased ACAP (p<0.05), and CAT activity (p<0.05) in low dosages. In high dosages melatonin reduced VO(2) (p<0.05) and increased ACAP (p<0.05), possibly stimulating others components of the ADS. In conclusion, melatonin in the locomotor muscles of N. granulata affects the antioxidant/pro-oxidant balance in a time and dosage dependent manner.

MT1 melatonin receptor internalization underlies melatonin-induced morphologic changes in Chinese hamster ovary cells and these processes are dependent on Gi proteins, MEK 1/2 and microtubule modulation

Melatonin induces cellular differentiation in numerous cell types. Data show that multiple mechanisms are involved in these processes that are cell-type specific and may be receptor dependent or independent. The focus of this study was to specifically assess the role of human MT1 melatonin receptors in cellular differentiation using an MT1-Chinese hamster ovary (CHO) model; one that reproducibly produces measurable morphologic changes in response to melatonin. Using multiple approaches, we show that melatonin induces MT1-CHO cells to hyperelongate through a MEK 1/2, and ERK 1/2-dependent mechanism that is dependent upon MT1 receptor internalization, Gi protein activation, and clathrin-mediated endocytosis. Using immunoprecipitation analysis, we show that MT1 receptors form complexes with Gi(alpha) 2,3, Gq(alpha), beta-arrestin-2, MEK 1/2, and ERK 1/2 in the presence of melatonin. We also show that MEK and ERK activity that is induced by melatonin is dependent on Gi protein activation, clathrin-mediated endocytosis and is modulated by microtubules. We conclude from these studies that melatonin-induced internalization of human MT1 melatonin receptors in CHO cells is responsible for activating both MEK 1/2 and ERK 1/2 to drive these morphologic changes. These events, as mediated by melatonin, require Gi protein activation and endocytosis mediated through clathrin, to form MT1 receptor complexes with beta-arrestin-2/MEK 1/2 and ERK 1/2. The MT1-CHO model is invaluable to mapping out signaling cascades as mediated through MT1 receptors especially because it separates out MEK/ERK 1/2 activation by MT1 receptors from that of receptor tyrosine kinases.

Role of the melatonin system in the control of sleep: therapeutic implications

The circadian rhythm of pineal melatonin secretion, which is controlled by the suprachiasmatic nucleus (SCN), is reflective of mechanisms that are involved in the control of the sleep/wake cycle. Melatonin can influence sleep-promoting and sleep/wake rhythm-regulating actions through the specific activation of MT(1) (melatonin 1a) and MT(2) (melatonin 1b) receptors, the two major melatonin receptor subtypes found in mammals. Both receptors are highly concentrated in the SCN. In diurnal animals, exogenous melatonin induces sleep over a wide range of doses. In healthy humans, melatonin also induces sleep, although its maximum hypnotic effectiveness, as shown by studies of the timing of dose administration, is influenced by the circadian phase. In both young and elderly individuals with primary insomnia, nocturnal plasma melatonin levels tend to be lower than those in healthy controls. There are data indicating that, in affected individuals, melatonin therapy may be beneficial for ameliorating insomnia symptoms. Melatonin has been successfully used to treat insomnia in children with attention-deficit hyperactivity disorder or autism, as well as in other neurodevelopmental disorders in which sleep disturbance is commonly reported. In circadian rhythm sleep disorders, such as delayed sleep-phase syndrome, melatonin can significantly advance the phase of the sleep/wake rhythm. Similarly, among shift workers or individuals experiencing jet lag, melatonin is beneficial for promoting adjustment to work schedules and improving sleep quality. The hypnotic and rhythm-regulating properties of melatonin and its agonists (ramelteon, agomelatine) make them an important addition to the armamentarium of drugs for treating primary and secondary insomnia and circadian rhythm sleep disorders.

Diurnal differences in melatonin effect on intracellular Ca2+ concentration in chicken spleen leukocytes in vitro

Melatonin plays a pleiotropic role in the immune system of mammals and birds. Endogenous and exogenous melatonin modulates lymphocyte proliferation via specific MT(1), MT(2) and Mel(1c) membrane receptors, although the mechanisms behind this process are poorly understood. The diurnal changes in the expression and function of melatonin membrane receptors within the immune system have so far received little attention. We investigated the day/night differences in melatonin membrane receptor mRNA expression in chicken lymphoid organs and cultured splenocytes and examined the in vitro effect of melatonin and 2-iodomelatonin on the intracellular Ca(2+) concentration ([Ca(2+)](i)) in chicken splenocytes. In whole organs, expression of all subtypes of Mel membrane receptors was observed, and the level did not change significantly with the time of day. Interestingly, we observed a significant increase in the expression of the transcripts of all receptor subtypes in cultured splenocytes isolated at night compared with cells obtained during the day. In chicken spleen leukocytes isolated during the day, melatonin and 2-iodomelatonin increased [Ca(2+)](i), with only 2-iodomelatonin being effective in the 'night' cells. Luzindole modulated the [Ca(2+)](i) increase caused by melatonin receptor agonists: it potentiated the stimulatory effect of melatonin during the day, but counteracted that evoked by 2-iodomelatonin at night. The results of this study demonstrate that melatonin can induce changes in [Ca(2+)](i) in chicken spleen leukocytes that should modulate proliferation. The effect of melatonin on [Ca(2+)](i) is less pronounced at night, possibly caused by receptor desensitization.

Melatonin receptors: role on sleep and circadian rhythm regulation

The circadian release of the hormone melatonin is regulated by the suprachiasmatic nucleus (SCN), which feeds back into the nucleus to modulate sleep and circadian phase through activation of the MT(1) and/or MT(2) melatonin receptors. Considering the functions of the SCN as a sleep and circadian rhythm regulator, melatonin and melatonin receptor agonists have attracted interest as being possible treatments for sleep and circadian rhythm sleep disorders. Part of this interest has centered on elucidating which melatonin receptors are targets for the regulation of these functions within the SCN. Two G-protein coupled melatonin receptors, the MT(1) and MT(2), inhibit neuronal activity and phase shift circadian firing rhythms in the SCN, respectively. Recent reports have uncovered possible interactions between the two types of receptors in the mammalian SCN, as well as the role of physiological and supraphysiological levels of melatonin on the molecular pharmacology and cellular changes of human and rodent melatonin receptors via desensitization and internalization mechanisms. These data outline the complexity of the interplay between melatonin and its receptors in the SCN and their corresponding roles in sleep and circadian regulation. Although further studies are necessary, a great deal of progress has been made toward understanding how melatonin and its agonists contribute to sleep and circadian phase changes, and how best to develop compounds that can target the functions of the SCN specifically and effectively.

Inverse agonist exposure enhances ligand binding and G protein activation of the human MT1 melatonin receptor, but leads to receptor down-regulation

Melatonin binds and activates G protein-coupled melatonin receptors. The density and affinity of the endogenous melatonin receptors change throughout the 24-hr day, and the exposure of recombinant melatonin receptors to melatonin often results in desensitization of the receptors. Receptor density, G protein activation and expression level were analyzed in CHO cell lines stably expressing the human MT1 receptors after 1 or 72 hr of exposure to melatonin (agonist, 10 nm) and luzindole (antagonist/inverse agonist, 10 microm). The 72-hr exposure to luzindole significantly increased the apparent receptor density in cell lines with both high and low MT1 receptor expression levels (MT1(high) and MT1(low) cells, respectively). In the constitutively active MT1(high) cells, luzindole pretreatment also stimulated the functional response to melatonin in [(35)S]GTPgammaS binding assays, whereas melatonin pretreatment attenuated the functional response at both time points. Receptor ELISA was used to analyze the cell membrane and total expression level of the MT1 receptor in intact and permeabilized cells, respectively. Luzindole pretreatment decreased the total cellular level of MT1 receptor in the MT1(high) cells at both time points but increased the cell surface expression of MT1 receptor at 72 hr. Melatonin significantly decreased MT1 receptor cell surface expression only in MT1(high) cells after a 1-hr treatment. These results indicate that melatonin treatment desensitizes MT1 receptors, whereas luzindole increases ligand binding and G-protein activation. Luzindole also stimulates downregulation of the MT1 receptor protein, interfering with the synthesis and/or degradation of the receptor.

Evidence for melatonin synthesis in the rat brain during development

Melatonin production is not restricted to the pineal gland. Several extrapineal sources of this indole such as retina, Harderian gland, and immune system are well documented. Melatonin of pineal origin is not present in the rat at early stages of development. To assess the potential capacity of local melatonin synthesis by the immature brain and to gain insight into the relationship between melatonin production by the brain (without the pineal gland) and pineal gland during rat development, the melatonin content as well as the expression and activity of the melatonin-synthesizing enzymes, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were studied at fetal and postnatal stages. Moreover, melatonin-membrane receptor (MT(1)) expression was also analyzed. Both, the expression and activity of NAT and HIOMT were found in the brain with significant day/night differences in enzymes activities. Additionally, melatonin content was detected in all stages showing day/night differences depending on the stage of development. The brain nocturnal melatonin content was higher than diurnal content on postnatal day 16 and in adult rats which is in accordance with the pineal melatonin synthesis. To investigate the origin of this brain melatonin, pinealectomized rats were used and we found that the developing brain produced its own melatonin. Also, MT(1) expression was detected in brain during development. These results demonstrate that, when the pineal is not yet producing melatonin, there is melatonin synthesis by the brain that could be used as protection from free radical damage and/or could exert some actions through MT(1) receptors.

Melatonin receptors in the eye: location, second messengers and role in ocular physiology

The pineal hormone melatonin, an important regulator of circadian and seasonal rhythms, has a role in ocular pathophysiology. In addition to the pineal gland, melatonin synthesis is carried out in several ocular structures. Moreover, specific melatonin receptors have been located in the retina, cornea, ciliary body, lens, choroid and sclera, which suggests that cells in these tissues may be targets for melatonin action. This review summarizes the current knowledge about melatonin receptor subtypes with the emphasis on those melatonin receptors, which have been identified in ocular tissues and their possible roles in biochemical and physiological processes in the eye.

Modulation of melatonin receptors and G-protein function by microtubules

Chronic melatonin exposure produces microtubule rearrangements in Chinese hamster ovary (CHO) cells expressing the human MT1 melatonin receptor while at the same time desensitizing MT1 receptors. Because microtubule rearrangements parallel MT1 receptor desensitization, we tested whether microtubules modulate receptor responsiveness. We determined whether depolymerization of microtubules by Colcemid, which prevents melatonin-induced outgrowths in MT1-expressing CHO cells, also prevents MT1 receptor desensitization by affecting G(alpha)-GTP exchange on G-proteins. In this study, we found that depolymerization of microtubules in MT1 receptor expressing cells, prevented melatonin-induced receptor desensitization reflected by an increase in the number of high potency sites when compared with melatonin-treated cells. Further examination of the mechanism(s) underlying this desensitization suggested that these effects occurred at the level of G-proteins. Depolymerization of microtubules during melatonin-induced desensitization, attenuated forskolin-induced cAMP accumulation, the opposite of which usually occurs following melatonin exposure alone. Concomitant to this attenuation in the forskolin response was a reduction in the amount of G(i alpha) protein coupled to MT1 receptors and an increase in [32P] azidoanilido GTP incorporation into G(i) proteins. These data are consistent with the findings that microtubule depolymerization did not affect MT1/G(q) coupling nor did it affect melatonin-induced phosphoinositide hydrolysis following melatonin exposure. However, interestingly, microtubule depolymerization enhanced melatonin-induced protein kinase C activation that was blocked in the presence of pertussis toxin. These data demonstrate that microtubule dynamics can modulate melatonin receptor function through their actions on G(i) proteins and impact on downstream signaling cascades.

Ramelteon: a novel treatment for the treatment of insomnia

Ramelteon, a potent agonist for the melatonin MT1 and MT2 brain receptors, has recently been granted approval by the US FDA for the treatment of insomnia associated with sleep onset. The drug has not exhibited potential for abuse or dependency in laboratory tests, nor does it interact with neurotransmitter receptors most associated with these phenomena, hence it has the great advantage of being a nonscheduled drug. Few data have been published in peer-reviewed journals describing its efficacy and side effects in patients with insomnia; however, side effects noted to date appear minor. No comparison study has been performed to determine whether the recommended dose of ramelteon 8 mg has any advantage over physiologic doses of melatonin (0.3 mg), particularly for long-term use.

Functional MT1 and MT2 melatonin receptors in mammals

Melatonin, dubbed the hormone of darkness, is known to regulate a wide variety of physiological processes in mammals. This review describes well-defined functional responses mediated through activation of high-affinity MT1 and MT2 G protein-coupled receptors viewed as potential targets for drug discovery. MT1 melatonin receptors modulate neuronal firing, arterial vasocon-striction, cell proliferation in cancer cells, and reproductive and metabolic functions. Activation of MT2 melatonin receptors phase shift circadian rhythms of neuronal firing in the suprachiasmatic nucleus, inhibit dopamine release in retina, induce vasodilation and inhibition of leukocyte rolling in arterial beds, and enhance immune responses. The melatonin-mediated responses elicited by activation of MT1 and MT2 native melatonin receptors are dependent on circadian time, duration and mode of exposure to endogenous or exogenous melatonin, and functional receptor sensitivity. Together, these studies underscore the importance of carefully linking each melatonin receptor type to specific functional responses in target tissues to facilitate the design and development of novel therapeutic agent.

Melatonin desensitizes endogenous MT2 melatonin receptors in the rat suprachiasmatic nucleus: relevance for defining the periods of sensitivity of the mammalian circadian clock to melatonin

The hormone melatonin phase shifts circadian rhythms generated by the mammalian biological clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, through activation of G protein-coupled MT2 melatonin receptors. This study demonstrated that pretreatment with physiological concentrations of melatonin (30-300 pM or 7-70 pg/mL) decreased the number of hMT2 melatonin receptors heterologously expressed in mammalian cells in a time and concentration-dependent manner. Furthermore, hMT2-GFP melatonin receptors heterologously expressed in immortalized SCN2.2 cells or in non-neuronal mammalian cells were internalized upon pretreatment with both physiological (300 pM or 70 pg/mL) and supraphysiological (10 nM or 2.3 ng/mL) concentrations of melatonin. The decrease in MT2 melatonin receptor number induced by melatonin (300 pM for 1 h) was reversible and reached almost full recovery after 8 h; however, after treatment with 10 nM melatonin full recovery was not attained even after 24 h. This recovery process was partially protein synthesis dependent. Furthermore, exposure to physiological concentrations of melatonin (300 pM) for a time mimicking the nocturnal surge (8 h) desensitized functional responses mediated through melatonin activation of endogenous MT2 receptors, i.e., stimulation of protein kinase C (PKC) in immortalized SCN2.2 cells and phase shifts of circadian rhythms of neuronal firing in the rat SCN brain slice. We conclude that in vivo the nightly secretion of melatonin desensitizes endogenous MT2 melatonin receptors in the mammalian SCN thereby providing a temporally integrated profile of sensitivity of the mammalian biological clock to a melatonin signal.