Melatonin receptor antagonists that differentiate between the human Mel1a and Mel1b recombinant subtypes are used to assess the pharmacological profile of the rabbit retina ML1 presynaptic heteroreceptor

Ramelteon protects against social defeat stress-associated abnormal behaviors

Psychological stress affects the neuroendocrine regulation, which modulates mental status and behaviors. Melatonin, a hormone synthesized primarily by the pineal gland, regulates many brain functions, including circadian rhythms, pain, sleep, and mood. Selective pharmacological melatonin agonist ramelteon has been clinically used to treat mood and sleep disorders. Posttraumatic stress disorder (PTSD) is a psychiatric condition associated with severe trauma; it is generally triggered by traumatic events, which lead to severe anxiety and uncontrollable trauma recall. We recently reported that repeated social defeat stress (RSDS) may induce robust anxiety-like behaviors and social avoidance in mice. In the present study, we investigated whether melatonin receptor activation by melatonin and ramelteon regulates RSDS-induced behavioral changes. Melatonin treatment improved social avoidance and anxiety-like behaviors in RSDS mice. Moreover, treatment of the non-selective MT1/MT2 receptor agonist, ramelteon, markedly ameliorated RSDS-induced social avoidance and anxiety-like behaviors. Moreover, activating melatonin receptors also balanced the expression of monoamine oxidases, glucocorticoid receptors, and endogenous antioxidants in the hippocampus. Taken together, our findings indicate that the activation of both melatonin and ramelteon regulates RSDS-induced anxiety-like behaviors and PTSD symptoms. The current study also showed that the regulatory effects of neuroendocrine mechanisms and cognitive behaviors on melatonin receptor activation in repeated social defeat stress.

Industrial and academic approaches to the search for alternative melatonin receptor ligands: An historical survey

The search for melatonin receptor agonists formed the main part of melatonin medicinal chemistry programs for the last three decades. In this short review, we summarize the two main aspects of these programs: the development of all the necessary tools to characterize the newly synthesized ligands at the two melatonin receptors MT1 and MT2, and the medicinal chemist's approaches to find chemically diverse ligands at these receptors. Both strategies are described. It turns out that the main source of tools were industrial laboratories, while the medicinal chemistry was mainly carried out in academia. Such complete accounts are interesting, as they delineate the spirits in which the teams were working demonstrating their strength and innovative character. Most of the programs were focused on nonselective agonists and few of them reached the market. In contrast, discovery of MT1-selective agonists and melatonergic antagonists with proven in vivo activity and MT1 or MT2-selectivity is still in its infancy, despite the considerable interest that subtype selective compounds may bring in the domain, as the physiological respective roles of the two subtypes of melatonin receptors, is still poorly understood. Poly-pharmacology applications and multitarget ligands have also been considered.

Binding and unbinding of potent melatonin receptor ligands: Mechanistic simulations and experimental evidence

The labeled ligand commonly employed in competition binding studies for melatonin receptor ligands, 2-[125I]iodomelatonin, showed slow dissociation with different half-lives at the two receptor subtypes. This may affect the operational measures of affinity constants, which at short incubation times could not be obtained in equilibrium conditions, and structure-activity relationships, as the Ki values of tested ligands could depend on either interaction at the binding site or the dissociation path. To address these issues, the kinetic and saturation binding parameters of 2-[125I]iodomelatonin as well as the competition constants for a series of representative ligands were measured at a short (2 h) and a long (20 h) incubation time. Concurrently, we simulated by molecular modeling the dissociation path of 2-iodomelatonin from MT1 and MT2 receptors and investigated the role of interactions at the binding site on the stereoselectivity observed for the enantiomers of the subtype-selective ligand UCM1014. We found that equilibrium conditions for 2-[125I]iodomelatonin binding can be reached only with long incubation times, particularly for the MT2 receptor subtype, for which a time of 20 h approximates this condition. On the other hand, measured Ki values for a set of ligands including agonists, antagonists, nonselective, and subtype-selective compounds were not significantly affected by the length of incubation, suggesting that structure-activity relationships based on data collected at shorter time reflect different interactions at the binding site. Molecular modeling simulations evidenced that the slower dissociation of 2-iodomelatonin from the MT2 receptor can be related to the restricted mobility of a gatekeeper tyrosine along a lipophilic path from the binding site to the membrane bilayer. The enantiomers of the potent, MT2-selective agonist UCM1014 were separately synthesized and tested. Molecular dynamics simulations of the receptor-ligand complexes provided an explanation for their stereoselectivity as due to the preference shown by the eutomer at the binding site for the most abundant axial conformation adopted by the ligand in solution. These results suggest that, despite the slow-binding kinetics occurring for the labeled ligand, affinity measures at shorter incubation times give robust results consistent with known structure-activity relationships and with interactions taken at the receptor binding site.

High-Throughput GPCRome Screen of Pollutants Reveals the Activity of Polychlorinated Biphenyls at Melatonin and Sphingosine-1-phosphate Receptors

Exposure to environmental pollutants is linked to numerous toxic outcomes, warranting concern about the effect of pollutants on human health. To assess the threat of pollutant exposure, it is essential to understand their biological activity. Unfortunately, gaps remain for many pollutants' specific biological activity and molecular targets. A superfamily of signaling proteins, G-protein-coupled receptors (GPCRs), has been shown as potential targets for pollutant activity. However, research investigating the pollutant activity at the GPCRome is scarce. This work explores pollutant activity across a library of human GPCRs by leveraging modern high-throughput screening techniques devised for drug discovery and pharmacology. We designed and implemented a pilot screen of eight pollutants at 314 human GPCRs and discovered specific polychlorinated biphenyl (PCB) activity at sphingosine-1-phosphate and melatonin receptors. The method utilizes open-source resources available to academic and governmental institutions to enable future campaigns that screen large numbers of pollutants. Thus, we present a novel high-throughput approach to assess the biological activity and specific targets of pollutants.

The location, physiology, pathology of hippocampus Melatonin MT2 receptor and MT2-selective modulators

Melatonin, a neurohormone secreted by the pineal gland and regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus, is synthesized and directly released into the cerebrospinal fluid (CSF) of the third ventricle (3rdv), where it undergoes rapid absorption by surrounding tissues to exert its physiological function. The hippocampus, a vital structure in the limbic system adjacent to the ventricles, plays a pivotal role in emotional response and memory formation. Melatonin MT1 and MT2 receptors are G protein-coupled receptors (GPCRs) that primarily mediate melatonin's receptor-dependent effects. In comparison to the MT1 receptor, the widely expressed MT2 receptor is crucial for mediating melatonin's biological functions within the hippocampus. Specifically, MT2 receptor is implicated in hippocampal synaptic plasticity and memory processes, as well as neurogenesis and axogenesis. Numerous studies have demonstrated the involvement of MT2 receptors in the pathophysiology and pharmacology of Alzheimer's disease, depression, and epilepsy. This review focuses on the anatomical localization of MT2 receptor in the hippocampus, their physiological function in this region, and their signal transduction and pharmacological roles in neurological disorders. Additionally, we conducted a comprehensive review of MT2 receptor ligands used in psychopharmacology and other MT2-selective ligands over recent years. Ultimately, we provide an outlook on future research for selective MT2 receptor drug candidates.

Melatonin improves oxidative state and lactate metabolism in rodent Sertoli cells

Antioxidant actions of melatonin and its impact on testicular function and fertility have already been described. Considering that Sertoli cells contribute to provide structural support and nutrition to germ cells, we evaluated the effect of melatonin on oxidative state and lactate metabolism in the immature murine TM4 cell line and in immature hamster Sertoli cells. A prooxidant stimulus applied to rodent Sertoli cells expressing MT1/MT2 receptors, increased lipid peroxidation whereas decreased antioxidant enzymes (superoxide dismutase 1, catalase, peroxiredoxin 1) expression and catalase activity. These changes were prevented by melatonin. Furthermore, melatonin stimulated lactate dehydrogenase (LDH) expression/activity via melatonin receptors, and increased intracellular lactate production in rodent Sertoli cells. Interestingly, oral melatonin supplementation in infertile men positively regulated LDHA testicular mRNA expression. Overall, our work provides insights into the potential benefits of melatonin on Sertoli cells contributing to testicular development and the future establishment of a sustainable spermatogenesis.

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Structural Basis for Agonistic Activity and Selectivity toward Melatonin Receptors hMT1 and hMT2

Glaucoma, a major ocular neuropathy originating from a progressive degeneration of retinal ganglion cells, is often associated with increased intraocular pressure (IOP). Daily IOP fluctuations are physiologically influenced by the antioxidant and signaling activities of melatonin. This endogenous modulator has limited employment in treating altered IOP disorders due to its low stability and bioavailability. The search for low-toxic compounds as potential melatonin agonists with higher stability and bioavailability than melatonin itself could start only from knowing the molecular basis of melatonergic activity. Thus, using a computational approach, we studied the melatonin binding toward its natural macromolecular targets, namely melatonin receptors 1 (MT1) and 2 (MT2), both involved in IOP signaling regulation. Besides, agomelatine, a melatonin-derivative agonist and, at the same time, an atypical antidepressant, was also included in the study due to its powerful IOP-lowering effects. For both ligands, we evaluated both stability and ligand positioning inside the orthosteric site of MTs, mapping the main molecular interactions responsible for receptor activation. Affinity values in terms of free binding energy (ΔG_bind) were calculated for the selected poses of the chosen compounds after stabilization through a dynamic molecular docking protocol. The results were compared with experimental in vivo effects, showing a higher potency and more durable effect for agomelatine with respect to melatonin, which could be ascribed both to its higher affinity for hMT2 and to its additional activity as an antagonist for the serotonin receptor 5-HT2c, in agreement with the in silico results.

High and low dose of luzindole or 4-phenyl-2-propionamidotetralin (4-P-PDOT) reverse bovine granulosa cell response to melatonin

Background

Communication between oocytes and granulosa cells ultimately dictate follicle development or atresia. Melatonin is also involved in follicle development. This study aimed to investigate the effects of melatonin and its receptor antagonists on hormone secretion, as well as gene expression related to hormone synthesis, TGF-β superfamily, and follicle development in bovine granulosa cells, and assess the effects of melatonin in the presence of 4-P-PDOT and luzindole.

Methods

Bovine ovaries were collected from a local abattoir and follicular fluid (follicle diameter 5-8 mm) was collected for granulosa cell isolation and culture. Granulosa cells and culture medium were collected 48 h after treatment with melatonin at high dose concentrations (10^-5 M) and low dose concentrations (10^-9 M) in the absence/presence of 4-P-PDOT and luzindole (10^-5 M or 10^-9 M). Furthermore, the expression level of genes related to hormonal synthesis (CYP11A1, CYP19A1, StAR, and RUNX2), TGF-β superfamily (BMP6, INHA, INHBA, INHBB, and TGFBR3), and development (EGFR, DNMT1A, and FSHR) were detected in each experimental group by real-time quantitative PCR. In addition, the level of hormones in culture medium were detected using ELISA.

Results

Both 10^-5 M and 10^-9 M melatonin doses promoted the secretion of inhibin A and progesterone without affecting the production of inhibin B and estradiol. In addition, both promoted the gene expression of INHA, StAR, RUNX2, TGFBR3, EGFR, and DNMT1A, and inhibited the expression of BMP6, INHBB, CYP11A1, CYP19A1, and FSHR. When combined with different doses of 4-P-PDOT and luzindole, they exhibited different effects on the secretion of inhibin B, estradiol, inhibin A, and progesterone, and the expression of CYP19A1, RUNX2, BMP6, INHBB, EGFR, and DNMT1A induced by melatonin.

Conclusion

High and low dose melatonin receptor antagonists exhibited different effects in regulating hormone secretion and the expression of various genes in response to melatonin. Therefore, concentration effects must be considered when using luzindole or 4-P-PDOT.

Characteristic of Ultrastructure of Mice B16 Melanoma Cells under the Influence of Different Lighting Regimes

Circadian rhythms of physiological processes, constantly being in a state of dynamic equilibrium and plastically associated with changes in environmental conditions, are the basis of homeostasis of an organism of human and other mammals. Violation of circadian rhythms due to significant disturbances in parameters of main environmental effectors (desynchronosis) leads to the development of pathological conditions and a more severe course of preexisting pathologies. We conducted the study of the ultrastructure of cells of mice transplantable malignant melanoma B16 under the condition of normal (fixed) lighting regime and under the influence of constant lighting. Results of the study show that melanoma B16 under fixed light regime represents a characteristic picture of this tumor-predominantly intact tissue with safe junctions of large, functionally active cells with highly irregular nuclei, developed organelles and a relatively low content of melanin. The picture of the B16 melanoma tissue structure and the ultrastructure of its cells under the action of constant lighting stand in marked contrast to the group with fixed light: under these conditions the tumor exhibits accelerated growth, a significant number of cells in the state of apoptosis and necrosis, ultrastructural signs of degradation of the structure and functions, and signs of embryonization of cells with the background of adaptation to oxygen deficiency.

A novel mechanism for the protection against acute lung injury by melatonin: mitochondrial quality control of lung epithelial cells is preserved through SIRT3-dependent deacetylation of SOD2

The mitochondrial quality control of lung epithelial cells is disturbed during sepsis, which contributes to abnormal mitochondrial function and acute lung injury. Melatonin is one of the primary hormones secreted by the pineal gland, displaying favorable antioxidative actions in sepsis and cardiopulmonary disease. However, the potential roles and molecular basis of melatonin in lipopolysaccharide (LPS)-treated lung epithelial cells have not been explored and reported. Herein, we investigated whether melatonin could protect against sepsis-induced acute lung injury (ALI) and LPS-treated lung epithelial cells through the mitochondrial quality control as well as its possible molecular targets. Wild type and Sirt3 knockout mice were intratracheally instilled with LPS for 12 h to construct an in vivo acute lung injury model. Both A549 lung epithelial cells and primary alveolar type II (AT-II) cells were used to explore the possible roles of melatonin in vitro by incubating with small interfering RNA against Sirt3. To determine the involvement of the melatonin receptor, cells and mice were treated with si Mtnr1b and luzindole. Melatonin pretreatment significantly inhibited pathological injury, inflammatory response, oxidative stress, and apoptosis in LPS-treated lung tissues and LPS-treated lung epithelial cells. Furthermore, melatonin also shifted the dynamic course of mitochondria from fission to fusion, inhibited mitophagy and fatty acid oxidation in LPS-treated lung epithelial cells in vitro and in vivo. However, SIRT3 inhibition abolished the protective roles of melatonin in acute lung injury. Mechanistically, we found that melatonin increased the activity and expression of SIRT3, which further promoted the deacetylation of SOD2 at K122 and K68. More importantly, melatonin exerted pulmonary protection by activating MTNR1B but not MTNR1A during ALI. Collectively, melatonin could preserve the mitochondrial quality control of lung epithelial cells through the deacetylation of SOD2 in a SIRT3-dependent manner, which eventually alleviated sepsis-induced injury, inflammation, oxidative stress, and apoptosis. Thus, melatonin may serve as a promising candidate against ALI in the future.

Impact of Melatonin on RAW264.7 Macrophages during Mechanical Strain

The concentration of melatonin is elevated during the night when patients mainly wear removable orthodontic appliances. Next to periodontal ligament fibroblasts and osteoblasts, macrophages react to mechanical strain with an increased expression of inflammatory mediators. Here, we investigated the impact of melatonin on RAW264.7 macrophages exposed to tensile or compressive strain occurring during orthodontic tooth movement in the periodontal ligament. Before exposure to mechanical strain for 4 h, macrophages were pre-incubated with different melatonin concentrations for 24 h, to determine the dependence of melatonin concentration. Afterwards, we performed experiments with and without mechanical strain, the most effective melatonin concentration (25 µM), and the melatonin receptor 2 (MT2) specific antagonist 4P-PDOT. The expression of inflammatory genes and proteins was investigated by RT-qPCR, ELISAs, and immunoblot. Both tensile and compressive strain increased the expression of the investigated inflammatory factors interleukin-1-beta, interleukin-6, tumor necrosis factor alpha, and prostaglandin endoperoxide synthase-2. This effect was inhibited by the addition of melatonin. Incubation with 4P-PDOT blocked this anti-inflammatory effect of melatonin. Melatonin had an anti-inflammatory effect on macrophages exposed to mechanical strain, independent of the type of mechanical strain. As inhibition was possible with 4P-PDOT, the MT2 receptor might be involved in the regulation of the observed effects.

2-Arylmelatonin analogues: Probing the 2-phenyl binding pocket of melatonin MT1 and MT2 receptors

In crystal structures of melatonin MT₁ and MT₂ receptors, a lipophilic subpocket has been characterized which accommodates the phenyl ring of the potent agonist 2-phenylmelatonin. This subpocket appears a key structural element to achieve high binding affinity and selectivity for the MT₂ receptor. A series of 2-arylindole ligands was synthesized to probe the requirements for the optimal occupation and interaction with the 2-phenyl binding pocket. Thermodynamic integration simulations applied to MT₁ and MT₂ receptors in complex with the α-naphthyl derivative provided a rationale for the MT₂-selectivity and investigation on the binding mode of a couple of atropisomers allowed to define the available space and arrangement of substituents inside the subpocket. Interestingly, more hydrophilic 2-aza-substituted compounds displayed high binding affinity and molecular dynamics simulations highlighted polar interaction with residues from the subpocket that could be responsible for their potency.

Altered Retinal Dopamine Levels in a Melatonin-proficient Mouse Model of Form-deprivation Myopia

Reduced levels of retinal dopamine, a key regulator of eye development, are associated with experimental myopia in various species, but are not seen in the myopic eyes of C57BL/6 mice, which are deficient in melatonin, a neurohormone having extensive interactions with dopamine. Here, we examined the relationship between form-deprivation myopia (FDM) and retinal dopamine levels in melatonin-proficient CBA/CaJ mice. We found that these mice exhibited a myopic refractive shift in form-deprived eyes, which was accompanied by altered retinal dopamine levels. When melatonin receptors were pharmacologically blocked, FDM could still be induced, but its magnitude was reduced, and retinal dopamine levels were no longer altered in FDM animals, indicating that melatonin-related changes in retinal dopamine levels contribute to FDM. Thus, FDM is mediated by both dopamine level-independent and melatonin-related dopamine level-dependent mechanisms in CBA/CaJ mice. The previously reported unaltered retinal dopamine levels in myopic C57BL/6 mice may be attributed to melatonin deficiency.

Prolonged Administration of Melatonin Ameliorates Liver Phenotypes in Cholestatic Murine Model

BACKGROUND & AIMS

Primary sclerosing cholangitis (PSC) is characterized by biliary senescence and hepatic fibrosis. Melatonin exerts its effects by interacting with Melatonin receptor 1 and 2 (MT1/MT2) melatonin receptors. Short-term (1 wk) melatonin treatment reduces a ductular reaction and liver fibrosis in bile duct-ligated rats by down-regulation of MT1 and clock genes, and in multidrug resistance gene 2 knockout (Mdr2-/-) mice by decreased miR200b-dependent angiogenesis. We aimed to evaluate the long-term effects of melatonin on liver phenotype that may be mediated by changes in MT1/clock genes/miR200b/maspin/glutathione-S transferase (GST) signaling.

METHODS

Male wild-type and Mdr2-/- mice had access to drinking water with/without melatonin for 3 months. Liver damage, biliary proliferation/senescence, liver fibrosis, peribiliary inflammation, and angiogenesis were measured by staining in liver sections, and by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay in liver samples. We confirmed a link between MT1/clock genes/miR200b/maspin/GST/angiogenesis signaling by Ingenuity Pathway Analysis software and measured liver phenotypes and the aforementioned signaling pathway in liver samples from the mouse groups, healthy controls, and PSC patients and immortalized human PSC cholangiocytes.

RESULTS

Chronic administration of melatonin to Mdr2-/- mice ameliorates liver phenotypes, which were associated with decreased MT1 and clock gene expression.

CONCLUSIONS

Melatonin improves liver histology and restores the circadian rhythm by interaction with MT1 through decreased angiogenesis and increased maspin/GST activity.

Mammalian Melatonin Agonist Pharmaceuticals Stimulate Rhomboid Proteins in Plants

Melatonin is a human neurotransmitter and plant signalling metabolite that perceives and directs plant metabolism. The mechanisms of melatonin action in plants remain undefined. We hypothesized that roots have a melatonin-specific receptor and/or transporter that can respond to melatonin-mediating pharmaceuticals. To test this hypothesis Arabidopsis seedlings were grown with melatonin pharmaceutical receptor agonists: ramelteon and tasimelteon, and/or antagonists: luzindole and 4-P-PDOT. Ramelteon was found both to mimic and competitively inhibit melatonin metabolism in plants. Due to the higher selectivity of ramelteon for the MT1 receptor type in humans, a sequence homology search for MT1 in Arabidopsis identified the rhomboid-like protein 7 (RBL7). In physiological studies, Arabidopsis rbl7 mutants were less responsive to ramelteon and melatonin. Quantum dot visualizations of the effects of ramelteon on melatonin binding to root cell membranes revealed a potential mechanism. We propose that RBL7 is a melatonin-interacting protein that directs root architecture and growth in a mechanism that is responsive to environmental factors.

In silico drug discovery of melatonin receptor ligands with therapeutic potential

INTRODUCTION

The neurohormone melatonin (N-acetyl-5-methoxytryptamine) regulates circadian rhythms exerting a variety of effects in the central nervous system and in periphery. These activities are mainly mediated by activation of MT₁ and MT₂ GPCRs. MT₁/MT₂ agonist compounds are used clinically for insomnia, depression, and circadian rhythm disturbances.

AREA COVERED

The following review describes the design strategies that have led to the identification of melatonin receptor ligands, guided by in silico approaches and molecular modeling. Initial ligand-based design, mainly relying on pharmacophore modeling and 3D-QSAR, has been flanked by structure-based virtual screening, given the recent availability of MT₁ and MT₂ crystal structures. Receptor ligands with different activity profiles, agonist/antagonist and subtype-selective compounds, are available.

EXPERT OPINION

An insight on the pharmacological characterization and therapeutic perspectives for relevant ligands is provided. In silico drug discovery has been instrumental in the design of novel ligands targeting melatonin receptors. Ligand-based approaches has led to the construction of a solid framework defining structure-activity relationships to obtain compounds with a tailored pharmacological profile. Structure-based techniques could integrate previous knowledge and provide compounds with novel chemotypes and pharmacological activity as drug candidates for disease conditions in which melatonin receptor ligands are currently being investigated, including cancer and pain.

Non-24-Hour Sleep–Wake Disorder and Tasimelteon: A Review for Practitioners Who Work with Blind People

Introduction

This narrative review summarizes the biology of human circadian rhythms; details the epidemiology, clinical manifestation, and diagnosis of non-24-hour sleep–wake disorder (N24SWD); and reviews the efficacy of possible treatments.

Methods

Searches of targeted phrases, such as “non-24-hour sleep–wake disorder” and “tasimelteon,” were conducted on PubMed between December 2016 and March 2020.

Results

As the world’s population ages, health practitioners frequently work with people who are blind. Damage to the retinal ganglion cells that signal environmental irradiance levels to the suprachiasmatic nucleus prevents many of these individuals from synchronizing their internal clocks to the 24-hour day. As a result, they experience a condition called N24SWD, where the body’s circadian rhythms fall in and out of phase with the solar cycle. The ability to fall asleep and remain asleep is a complex process that depends on many variables, including the release of the neurohormone melatonin. Melatonin is produced at night and is a key regulator of regular sleep cycles. Periods of interrupted sleep, increased sleep latency, and reduced total sleep time occur when melatonin production peaks during daytime. Thus, many persons with N24SWD have difficulty maintaining normal schedules due in part to the mistimed release of melatonin. Randomized clinical trials have shown that melatonin receptor agonist tasimelteon is an effective therapy for individuals with N24SWD. Other treatments have varying efficacy profiles.

Conclusions

Although rare, N24SWD is a serious condition that can impair quality of life for blind persons. Tasimelteon appears to be a safe and efficacious treatment option.

Implications for practitioners

Practitioners can use this information to better understand why blind persons often report difficulties sleeping and to realize that therapeutic options are available to these individuals.

Alternative Ligands at Melatonin Receptors

Melatonin (N-acetyl-5-methoxytryptamine) is a neurohormone that possesses a wide range of biological effects. Most of the main recognized effects of this hormone in mammals are due to its interaction with two G protein-coupled receptors, MT1 and MT2. Ligand-binding studies have been based on the use of its radioligand analog, 2[125I]-iodomelatonin, a super agonist discovered in the early 1990s. This compound has been used in most of the binding studies reported in the literature. Nevertheless, more recently other possibilities arose. This chapter is a brief summary of those alternative radioligands and of their benefits one can find in using them.

Why Are We Still Cloning Melatonin Receptors? A Commentary

Cloning may seem to be a view from the past. The time before software, computers and AI were invented. It seems to us worth discussing these points in view of our favorite target: the melatoninergic system. In a few stances, it might be important to point out that even in the new era of dry science, there is still a need to experiment and to prove at the bench that our in silico assertions are right. Most of the living animals express to some extend the melatonin receptors. Some of these animal genomes were completely or partially sequenced, and it is tempting to extract from this huge information the sequence(s) of our favorite genes (MLT receptors). Then, why bother cloning, as opposed to simply built the gene and express it in a host cell? Because the genetic boundaries of the expressed sequence(s) are not 100% sure. Because the melatonin receptor gene(s) comprise a first exon 25,000 base pair far from the second one and the limits between this Ex1 and In1-as between In1 and Ex2-are subject to changes that might have a huge impact on the biochemical properties of the receptor, once expressed. Because a receptor is a biochemical entity with characteristics that are important for the functioning of this particular pathway, and more generally, for the functioning of life.

Membrane Melatonin Receptors Activated Cell Signaling in Physiology and Disease

The pineal hormone melatonin has attracted great scientific interest since its discovery in 1958. Despite the enormous number of basic and clinical studies the exact role of melatonin in respect to human physiology remains elusive. In humans, two high-affinity receptors for melatonin, MT1 and MT2, belonging to the family of G protein-coupled receptors (GPCRs) have been cloned and identified. The two receptor types activate Gi proteins and MT2 couples additionally to Gq proteins to modulate intracellular events. The individual effects of MT1 and MT2 receptor activation in a variety of cells are complemented by their ability to form homo- and heterodimers, the functional relevance of which is yet to be confirmed. Recently, several melatonin receptor genetic polymorphisms were discovered and implicated in pathology-for instance in type 2 diabetes, autoimmune disease, and cancer. The circadian patterns of melatonin secretion, its pleiotropic effects depending on cell type and condition, and the already demonstrated cross-talks of melatonin receptors with other signal transduction pathways further contribute to the perplexity of research on the role of the pineal hormone in humans. In this review we try to summarize the current knowledge on the membrane melatonin receptor activated cell signaling in physiology and pathology and their relevance to certain disease conditions including cancer.

Negative regulation of melatonin secretion by melatonin receptors in ovine pinealocytes

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms and reproduction. The photoperiodic information is detected by retinal photoreceptors and transmitted through nerve transmissions to the pineal gland, where MLT is synthesized and secreted at night into the blood. MLT interacts with two G protein-coupled receptors, MT1 and MT2. The aim of our work was to provide evidence for the presence of MLT receptors in the ovine pineal gland and define their involvement on melatonin secretion. For the first time, we identified the expression of MLT receptors with the specific 2-[125I]-MLT agonistic radioligand in ovin pinealocytes. The values of Kd and Bmax are 2.24 ± 1.1 nM and 20 ± 6.8 fmol/mg. MLT receptors are functional and inhibit cAMP production and activate ERK1/2 through pertussis toxin-sensitive Gi/o proteins. The MLT receptor antagonist/ inverse agonist luzindole increased cAMP production (189 ± 30%) and MLT secretion (866 ± 13%). The effect of luzindole on MLT secretion was additive with the effect of well-described activators of this pathway such as the β-adrenergic agonist isoproterenol and the α-adrenergic agonist phenylephrine. Co-incubation of all three compounds increased MLT secretion by 1236 ± 199%. These results suggest that MLT receptors are involved in the negative regulation of the synthesis of its own ligand in pinealocytes. While adrenergic receptors promote MLT secretion, MLT receptors mitigate this effect to limit the quantity of MLT secreted by the pineal gland.

Role of Melatonin in Breast Carcinoma: Correlation of Expression Patterns of Melatonin-1 Receptor With Estrogen, Progesterone, and HER2 Receptors

Breast carcinoma is a multifaceted-etiology malignancy. The presence of estrogen (ER), progesterone (PR), and HER2 (human epidermal growth factor receptor 2) receptors in breast carcinoma tissue has therapeutic implications. Recent studies indicate that pineal hormone melatonin by its receptor melatonin 1 (MT1) also influences the development and growth of breast cancer cells. The aim of this cross-sectional study was to elucidate the expression pattern of MT1 receptor in relation to estrogen, progesterone, and HER2 receptors in breast carcinoma. Two groups (receptor positive and triple negative) of breast carcinoma were taken. For comparison, normal mammary tissue was used as control. Immunohistochemistry was carried out using anti-melatonin receptor 1A antibody. Membranous/cytoplasmic expression was seen more than the nuclear expression in the cancerous tissue. Positive correlation of the MT1 expression was seen with ER, PR, and HER 2 receptor. Higher MT1 receptor expression was seen in the receptor-positive cases in comparison with triple-negative cases, which might signify melatonin deficiency in the former, leading to reactive increase in cell receptors. No correlation of MT1 expression with Ki67 index or lymph node status in both receptor-positive and triple-negative cases was found. Normal mammary tissue mainly showed cytoplasmic MT1 immunoreactivity of epithelial cells (ducts and acini), myoepithelial cells, and lining epithelium of blood vessels. Receptor-positive cases would, therefore, benefit from the use of melatonin as supporting therapy. This indicates that melatonin receptor status can be used as an independent pathologic indicator to evaluate breast carcinoma tissue, and melatonin receptor status may help to determine treatment protocols.

Cascade intramolecular Prins/Friedel-Crafts cyclization for the synthesis of 4-aryltetralin-2-ols and 5-aryltetrahydro-5H-benzo[7]annulen-7-ols

The treatment of 2-(2-vinylphenyl)acetaldehydes or 3-(2-vinylphenyl)propanals with BF₃·Et₂O results in an intramolecular Prins reaction affording intermediary benzyl carbenium ions, which are then trapped by a variety of electron-rich aromatics via Friedel–Crafts alkylation. This cascade Prins/Friedel–Crafts cyclization protocol paves an expedient path to medicinally useful 4-aryltetralin-2-ol and 5-aryltetrahydro-5H-benzo[7]annulen-7-ol derivatives.

Journal of pineal research guideline for authors: Defining and characterizing melatonin targets

A multitude of effects has been attributed to melatonin at pmol/L to mmol/L concentrations. More than fifteen targets have been proposed for melatonin but only few of them are well characterized. The current guidelines intend to provide a framework to improve and rationalize the characterization of melatonin targets and effects. They should be considered as mandatory guidelines and minimum requirements for manuscripts submitted to the Journal of Pineal Research.

Retinal and extra-retinal photoreceptor responses and reproductive performance of Japanese quail (Coturnix coturnix japonica) following exposure to different photoperiodic regime

Japanese quail is a truly photoperiodic avian species. In general long days are gonado-stimulatory and short days are gonado-inhibitory for this poultry bird. To investigate the correlation of retinal and extra-retinal photoreceptors with different photoperiodic conditions quail were divided into 2 groups and kept under long day (16L: 8D) and short day (8L: 16D) condition separately to develop photosensitivity and scotosensitivity respectively. Transfer of long day quail to intermediate day-length (13.5L: 10.5D) developed photorefractoriness (relative) and prolonged exposure to short photoperiodic conditions led the birds to develop scotorefractoriness. Increased expression of mRNA and immunosignaling of photoreceptors rhodopsin, transducin in eye and hypothalamus while decreased mRNA expression of melatonin receptors (Mel1b, Mel1c) were noted in the eyes of photosensitive (PS) and scotorefractory (SR) quail compared to photorefractory (PR) and scotosensitive (SS) birds respectively. Decreased expression of hypothalamic GnIH and melatonin receptors mRNA was observed in PS and SR birds compared to PR and SS birds respectively. Modulation of retinal and extra retinal photoreceptors leads to increased spermatogenesis as well as mRNA expression of steroidogenic genes and androgen receptor in the testis of sexually active PS and SR quail. These results led us to conclude that gonadal stimulation in PS as well as SR quail is outcome of activated retinal and extra retinal photoreceptors which lowered melatonin receptors and GnIH expression. Contrarily testicular inhibition in PR and SS is the outcome of decreased photoperception. It is suggested that decreased photoperception in SS quail increases after prolong exposure of the short day (in SR) leading to increased activity of HPG axis.

MT2 melatonin receptors expressed in the olfactory bulb modulate depressive-like behavior and olfaction in the 6-OHDA model of Parkinson's disease

Melatonin MT1 and MT2 receptors are expressed in the glomerular layer of the olfactory bulb (OB); however, the role of these receptors has not been evaluated until now. Considering the association of the OB with olfactory and depressive disorders in Parkinson's disease (PD), we sought to investigate the involvement of melatonin receptors in these non-motor disturbances in an intranigral 6-hydroxydopamine (6-OHDA)-lesioned rat model of PD. We demonstrate the presence of functional melatonin receptors in dopaminergic neurons of the glomerular layer. Local administration of melatonin (MLT, 1 μg/μl), luzindole (LUZ, 5 μg/μl) or the MT2-selective receptor drug 4-P-PDOT (5 μg/μl) reversed the depressive-like behavior elicited by 6-OHDA. Sequential administration of 4-P-PDOT and MLT (5 μg/μl, 1 μg/μl) promoted additive antidepressant-like effects. In the evaluation of olfactory discrimination, LUZ induced an olfactory impairment when associated with the nigral lesion-induced impairment. Thus, our results suggest that melatonin MT2 receptors expressed in the glomerular layer are involved in depressive-like behaviors and in olfactory function associated with PD.

Melatonin and the circadian system: Keys for health with a focus on sleep

Melatonin (MLT), secreted during the night by the pineal gland, is an efferent hormonal signal of the master circadian clock located in the suprachiasmatic nucleus (SCN). Consequently, it is a reliable phase marker of the SCN clock. If one defines as "chronobiotic," a drug able to influence the phase and/or the period of the circadian clock, MLT is a very potent one. The most convincing data obtained so far come from studies on totally blind individuals. Exogenous MLT administered daily entrains the sleep-wake cycle of these individuals to a 24-h cycle. MLT, however, is not essential to sleep. In nocturnally, active mammals, MLT is released during the night concomitantly with the daily period of wakefulness. Therefore, MLT cannot be simply considered as a sleep hormone, but rather as a signal of darkness. Its role in the circadian system is to reinforce nighttime physiology, including timing of the sleep-wake cycle and other circadian rhythms. MLT exerts its effects on the sleep cycle especially by a direct action on the master circadian clock. The sleep-wake cycle is depending not only on the circadian clock but also on an orchestrated network of different centers in the brain. Thus, the control of sleep-wake rhythm might be explained by a parallel and concomitant action of MLT on the master clock (chronobiotic effect) and on sleep-related structures within the brain. MLT acts through two high-affinity membrane receptors (MT1 and MT2) with striking differences in their distribution pattern. MLT is a powerful synchronizer of human circadian rhythms, thus justifying the use of MLT and MLT agonists in clinical medicine as pharmacological tools to manipulate the sleep-wake cycle, and to treat sleep disorders and other circadian disorders. Available MLT analogs/drugs are all nonspecific MT1/MT2 agonists. The development of new ligands which are highly selectivity for each subtype is clearly a new challenge for the field and will be at the root of new therapeutic agents for curing specific pathologies, including sleep disorders.

Role of Melatonin in Plant Tolerance to Soil Stressors: Salinity, pH and Heavy Metals

Melatonin (MT) is a pleiotropic molecule with diverse and numerous actions both in plants and animals. In plants, MT acts as an excellent promotor of tolerance against abiotic stress situations such as drought, cold, heat, salinity, and chemical pollutants. In all these situations, MT has a stimulating effect on plants, fomenting many changes in biochemical processes and stress-related gene expression. Melatonin plays vital roles as an antioxidant and can work as a free radical scavenger to protect plants from oxidative stress by stabilization cell redox status; however, MT can alleviate the toxic oxygen and nitrogen species. Beyond this, MT stimulates the antioxidant enzymes and augments antioxidants, as well as activates the ascorbate–glutathione (AsA–GSH) cycle to scavenge excess reactive oxygen species (ROS). In this review, we examine the recent data on the capacity of MT to alleviate the effects of common abiotic soil stressors, such as salinity, alkalinity, acidity, and the presence of heavy metals, reinforcing the general metabolism of plants and counteracting harmful agents. An exhaustive analysis of the latest advances in this regard is presented, and possible future applications of MT are discussed.

Two indoleamines are secreted from rat pineal gland at night and act on melatonin receptors but are not night hormones

INTRODUCTION

At night, the pineal gland produces the indoleamines, melatonin, N-acetylserotonin (NAS), and N-acetyltryptamine (NAT). Melatonin is accepted as a hormone of night. Could NAS and NAT serve that role too?

METHODS

Concentration-response measurements with overexpressed human melatonin receptors MT₁ and MT₂ ; mass spectrometry analysis of norepinephrine-stimulated secretions from isolated rat pineal glands; analysis of 24-hour periodic samples of rat blood.

RESULTS

We show that NAT and NAS do activate melatonin receptors MT₁ and MT₂ , although with lower potency than melatonin, and that in vitro, melatonin and NAS are secreted from stimulated, isolated pineal glands in roughly equimolar amounts, but secretion of NAT was much less. All three were found at roughly equal concentrations in blood during the night. However, during the day, serum melatonin fell to very low values creating a high-amplitude circadian rhythm that was absent after pinealectomy, whereas NAS and NAT showed only small or no circadian variation.

CONCLUSION

Blood levels of NAS and NAT were insufficient to activate peripheral melatonin receptors, and they were invariant, so they could not serve as circulating hormones of night. However, they could instead act in paracrine circadian fashion near the pineal gland or via other higher-affinity receptors.

Characterization of the various functional pathways elicited by synthetic agonists or antagonists at the melatonin MT1 and MT2 receptors

Melatonin is a neurohormone that translates the circadian rhythm to the peripheral organs through a series of binding sites identified as G protein-coupled receptors MT1 and MT2. Due to minute amounts of receptor proteins in target organs, the main tool of studies of the melatoninergic system is recombinant expression of the receptors in cellular hosts. Although a number of studies exist on these receptors, studies of several signaling pathways using a large number of melatoninergic compounds are rather limited. We chose to fill this gap to better describe a panel of compounds that have been only partially characterized in terms of functionality. First, we characterized HEK cells expressing MT1 or MT2, and several signaling routes with melatonin itself to validate the approach: GTPγS, cAMP production, internalization, β-arrestin recruitment, and cell morphology changes (CellKey ® ). Second, we chose 21 compounds from our large melatoninergic chemical library and characterized them using this panel of signaling pathways. Notably, antagonists were infrequent, and their functionality depended largely on the pathway studied. This will permit redefining the availability of molecular tools that can be used to better understand the in situ activity and roles of these receptors.

Melatonin receptor structures shed new light on melatonin research

The tryptophan derivative melatonin is an evolutionary old molecule that is involved in a pleiotropy of physiological functions. In humans, age-related decline of circulating melatonin levels and/or dysregulation of its circadian synthesis pattern have been associated with several disorders and disease states. Several molecular targets have been proposed for melatonin since its discovery, in 1959. Among them, melatonin MT₁ and MT₂ receptors are the best characterized melatonin targets, mediating melatonin effects in a variety of tissues. They belong to the superfamily of G protein-coupled receptors. Two back-to-back articles published in the "Nature" Journal earlier this year present the first crystal structures of the human MT₁ and MT₂ in its inactive states. Here, we will briefly outline the discovery path of melatonin receptors until their structural elucidation and discuss how these new findings will guide future research toward a better understanding of their function and rational drug design.

Activation of melatonin receptor 2 but not melatonin receptor 1 mediates melatonin-conferred cardioprotection against myocardial ischemia/reperfusion injury

Accumulated pieces of evidence have proved the beneficial effects of melatonin on myocardial ischemia/reperfusion (MI/R) injury, and these effects were largely dependent on melatonin membrane receptor activation. In humans and other mammals, there are two types of melatonin receptors, including the melatonin receptor 1 (MT1, melatonin receptor 1a or MTNR1A) and melatonin receptor 1 (MT2, melatonin receptor 1b or MTNR1B) receptor subtypes. However, which receptor mediates melatonin-conferred cardioprotection remains unclear. In this study, we employed both loss-of-function and gain-of-function approaches to reveal the answer. Mice (wild-type; MT1 or MT2 silencing by in vivo minicircle vector; and those overexpressing MT1 or MT2 by in vivo AAV9 vector) were exposed to MI/R injury. Both MT1 and MT2 were present in wild-type myocardium. MT2, but not MT1, was essentially upregulated after MI/R Melatonin administration significantly reduced myocardial injury and improved cardiac function after MI/R Mechanistically, melatonin treatment suppressed MI/R-initiated myocardial oxidative stress and nitrative stress, alleviated endoplasmic reticulum stress and mitochondrial injury, and inhibited myocardial apoptosis. These beneficial actions of melatonin were absent in MT2-silenced heart, but not the MT1 subtype. Furthermore, AAV9-mediated cardiomyocyte-specific overexpression of MT2, but not MT1, mitigated MI/R injury and improved cardiac dysfunction, which was accompanied by significant amelioration of oxidative stress, endoplasmic reticulum stress, and mitochondrial dysfunction. Mechanistically, MT2 protected primary cardiomyocytes against hypoxia/reoxygenation injury via MT2/Notch1/Hes1/RORα signaling. Our study presents the first direct evidence that the MT2 subtype, but not MT1, is a novel endogenous cardiac protective receptor against MI/R injury. Medications specifically targeting MT2 may hold promise in fighting ischemic heart disease.

MT1 and MT2 melatonin receptors are expressed in nonoverlapping neuronal populations

Melatonin (MLT) exerts its physiological effects principally through two high-affinity membrane receptors MT1 and MT2. Understanding the exact mechanism of MLT action necessitates the use of highly selective agonists/antagonists to stimulate/inhibit a given MLT receptor. The respective distribution of MT1 and MT2 within the CNS and elsewhere is controversial, and here we used a "knock-in" strategy replacing MT1 or MT2 coding sequences with a LacZ reporter. The data show striking differences in the distribution of MT1 and MT2 receptors in the mouse brain: whereas the MT1 subtype was expressed in very few structures (notably including the suprachiasmatic nucleus and pars tuberalis), MT2 subtype receptors were identified within numerous brain regions including the olfactory bulb, forebrain, hippocampus, amygdala and superior colliculus. Co-expression of the two subtypes was observed in very few structures, and even within these areas they were rarely present in the same individual cell. In conclusion, the expression and distribution of MT2 receptors are much more widespread than previously thought, and there is virtually no correspondence between MT1 and MT2 cellular expression. The precise phenotyping of cells/neurons containing MT1 or MT2 receptor subtypes opens new perspectives for the characterization of links between MLT brain targets, MLT actions and specific MLT receptor subtypes.

Is There Sufficient Evidence that the Melatonin Binding Site MT3 Is Quinone Reductase 2?

In the 1980s, researchers used binding studies to show that there is a melatonin binding site in addition to the receptors described previously. It was first termed ML2 and then, in 1999, MT3 Purification efforts led to its identification as quinone reductase 2. Several lines of evidence support the notion that MT3 is the same as quinone reductase 2, including the detection and characterization of MT3 whenever quinone reductase 2 was added to various systems under various conditions. This evidence is discussed in this review, which summarizes the results of relevant cellular and animal experiments. The recent discovery that the quinone reductase 2 enzyme can be partly membrane-associated may unite the current body of evidence and allow us to conclude definitively that the third melatonin binding site, MT3 , is indeed quinone reductase 2.

Melatonin and their analogs as a potential use in the management of Neuropathic pain

Melatonin (N-acetyl-5-methoxytryptamine), secreted by the pineal gland is known to perform multiple functions including, antioxidant, anti-hypertensive, anti-cancerous, immunomodulatory, sedative and tranquilizing functions. Melatonin is also known to be involved in the regulation of body mass index, control the gastrointestinal system and play an important role in cardioprotection, thermoregulation, and reproduction. Recently, several studies have reported the efficacy of Melatonin in treating various pain syndromes. The current paper reviews the studies on Melatonin and its analogs, particularly in Neuropathic pain. Here, we first briefly summarized research in preclinical studies showing the possible mechanisms through which Melatonin and its analogs induce analgesia in Neuropathic pain. Second, we reviewed research indicating the role of Melatonin in attenuating analgesic tolerance. Finally, we discussed the recent studies that reported novel Melatonin agonists, which were proven to be effective in treating Neuropathic pain.

Distribution and density of melatonin receptors in human main pancreatic islet cell types

Recent investigations of our group established that melatonin modulates hormone secretion of pancreatic islets via melatonin receptor types MT1 and MT2. Expression of MT1 and MT2 has been shown in mouse, rat, and human pancreatic islets as well as in the β-, α-, and δ-cell lines INS-1, αTC1.9, and QGP-1. In view of these earlier investigations, this study was performed to analyze in detail the distribution and density of melatonin receptors on the main islet cell types in human pancreatic tissue obtained from nondiabetic and type 2 diabetic patients. Immunohistochemical analysis established the presence of MT1 and MT2 in β-, α-, and δ-cells, but notably, with differences in receptor density. In general, the lowest MT1 and MT2 receptor density was measured in α-cells compared to the 2 other cell types. In type 2 diabetic islets, MT1 and MT2 receptor density was increased in δ-cells compared to normoglycemic controls. In human islets in batch culture of a nondiabetic donor, an increase of somatostatin secretion was observed under melatonin treatment while in islets of a type 2 diabetic donor, an inhibitory influence could be observed, especially in the presence of 5.5 mmol/L glucose. These data suggest the following: i) cell-type-specific density of MT1 and MT2 receptors in human pancreatic islets, which should be considered in context of the hormone secretion of islets, ii) the influence of diabetes on density of MT1 and MT2 as well as iii) the differential impact of melatonin on somatostatin secretion of nondiabetic and type 2 diabetic islets.

Aerobic exercise enhanced endothelium-dependent vasorelaxation in mesenteric arteries in spontaneously hypertensive rats: the role of melatonin

Melatonin, a neuroendocrine hormone synthesized primarily by the pineal gland, provides various cardiovascular benefits. Regular physical activity is an effective non-pharmacological therapy for the prevention and control of hypertension. In the present study, we hypothesized that melatonin plays an important role in the aerobic exercise-induced increase of endothelium-dependent vasorelaxation in the mesenteric arteries (MAs) of spontaneously hypertensive rats (SHRs) in a melatonergic receptor-dependent manner. To test this hypothesis, we evaluated the vascular mechanical and functional properties in normotensive Wistar Kyoto (WKY), SHRs, and SHRs that were trained on a treadmill (SHR-EX) for 8 weeks. Exercise training produced a significant reduction in blood pressure and heart rate in SHR, which was significantly attenuated by the intraperitoneal administration of luzindole, a non-selective melatonin receptor (MT1/MT2) antagonist. Serum melatonin levels in the SHR group were significantly lower than those in the WKY group at 8:00-9:00 and 21:00-22:00, while exercise training reduced this difference. Endothelium-dependent vessel relaxation induced by acetylcholine was significantly blunted in SHR compared with age-matched WKY. Both exercise training and luzindole ameliorated this endothelium-dependent impairment of relaxation in hypertension. Immunohistochemistry and Western blotting showed that the protein expression of the MT2 receptor and eNOS, as well as their colocalization in the endothelial cell layer in SHRs, was significantly decreased; as exercise training suppressed this reduction. These results provide evidence that regular exercise has a beneficial effect on improving endothelium-dependent vasorelaxation in MAs, in which melatonin plays a critical role by acting on MT2 receptors to increase NO production and/or NO bioavailability.

Assessments of cellular melatonin receptor signaling pathways: β-arrestin recruitment, receptor internalization, and impedance variations

Melatonin receptors belong to the family of G-protein coupled receptors. Agonist-induced receptor activation is terminated with the recruitment of β-arrestin, which leads to receptor internalization. Furthermore, agonist binding induces a shift in cellular shape that translates into a change in the electric impedance of the cell. In the present study, we employed engineered cells to study these internalization-related processes in the context of the two melatonin receptors, MT₁ and MT₂. To assess these three receptor internalization-related functions and validate the results, we employed four classical ligands of melatonin receptors: the natural agonist melatonin; the super-agonist 2-iodo-melatonin and the two antagonists luzindole and 4-phenyl-2-propionamidotetralin. The assessments confirmed the nature of the agonistic ligands but showed that 4-phenyl-2-propionamidotetralin, a described antagonist, is a biased partial agonist at MT₂ with poorer affinity for MT₁. The methods are now available to be applied to any receptor system for which multiple signaling pathways must be evaluated for new molecules.

Bioactivity-guided synthesis of gramine derivatives as new MT1 and 5-HT1A receptors agonists

Twenty-four gramine derivatives were synthesized and evaluated on MT₁ and 5-HT_1A receptors in vitro. Among them, seven derivatives (7, 8, 16, 19, 20, 21, and 24) exhibited higher agonisting activities on MT₁ or 5-HT_1A receptors. Compared with gramine, derivatives 7, 8, 16, 19, 20, 21, and 24 displayed 1.6-3.5-fold increase in agonistic rates on 5-HT_1A receptor. Particularly, derivatives 7, 19, and 21 exhibited significant agonistic activities on MT₁ and 5-HT_1A receptors with EC₅₀ values of 0.51, 0.39, 0.50 mΜ and 0.28, 0.46, 0.23 mΜ, respectively. The preliminary structure-activity relationships of gramine derivatives were summarized for further investigation on MT₁ and 5-HT_1A receptors as new potential agonists.

The P2Y1 receptor-mediated leukocyte adhesion to endothelial cells is inhibited by melatonin

Extracellular ATP (released by endothelial and immune cells) and its metabolite ADP are important pro-inflammatory mediators via the activation of purinergic P2 receptors (P2Y and P2X), which represent potential new targets for anti-inflammatory therapy. Endothelial P2Y₁ receptor (P2Y₁R) induces endothelial cell activation triggering leukocyte adhesion. A number of data have implicated melatonin as a modulator of immunity, inflammation, and endothelial cell function, but to date no studies have investigated whether melatonin modulates endothelial P2YR signaling. Here, we evaluated the putative effect of melatonin on P2Y₁R-mediated leukocyte adhesion to endothelial cells and TNF-α production, using mesenteric endothelial cells and fresh peripheral blood mononuclear cells isolated from rats. Endothelial cells were treated with the P2Y₁R agonist 2MeSATP, alone or in combination with melatonin, and then exposed to mononuclear cells. 2MeSATP increased leukocyte adhesion to endothelial cells and TNF-α production in vitro, and melatonin inhibited both effects without altering P2Y₁R protein expression. In addition, assays with the Ca²⁺ chelator BAPTA-AM indicate that the effect of melatonin on 2MeSATP-stimulated leukocyte adhesion depends on intracellular Ca²⁺ modulation. P2Y₁R is considered a potential target to control chronic inflammation. Therefore, our data unveiled a new endothelial cell modulator of purinergic P2Y₁ receptor signaling.

Melatonin improves reprogramming efficiency and proliferation of bovine-induced pluripotent stem cells

Melatonin can modulate neural stem cell (NSC) functions such as proliferation and differentiation into NSC-derived pluripotent stem cells (N-iPS) in brain tissue, but the effect and mechanism underlying this are unclear. Thus, we studied how primary cultured bovine NSCs isolated from the retinal neural layer could transform into N-iPS cell. NSCs were exposed to 0.01, 0.1, 1, 10, or 100 μm melatonin, and cell viability studies indicated that 10 μm melatonin can significantly increase cell viability and promote cell proliferation in NSCs in vitro. Thus, 10 μm melatonin was used to study miR-302/367-mediated cell reprogramming of NSCs. We noted that this concentration of melatonin increased reprogramming efficiency of N-iPS cell generation from primary cultured bovine NSCs and that this was mediated by downregulation of apoptosis-related genes p53 and p21. Then, N-iPS cells were treated with 1, 10, 100, or 500 μm melatonin, and N-iPS (M-N-iPS) cell proliferation was measured. We noted that 100 μm melatonin increased proliferation of N-iPS cells via increased phosphorylation of intracellular ERK1/2 via activation of its pathway in M-N-iPS via melatonin receptors 1 (MT1). Finally, we verified that N-iPS cells and M-N-iPS cells are similar to typical embryonic stem cells including the expression of pluripotency markers (Oct4 and Nanog), the ability to form teratomas in vivo, and the capacity to differentiate into all three embryonic germ layers.

Update on melatonin receptors: IUPHAR Review 20

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.

Alternative Radioligands for Investigating the Molecular Pharmacology of Melatonin Receptors

Melatonin exerts a variety of physiologic activities that are mainly relayed through the melatonin receptors MT1 and MT2 Low expressions of these receptors in tissues have led to widespread experimental use of the agonist 2-[(125)I]-iodomelatonin as a substitute for melatonin. We describe three iodinated ligands: 2-(2-[(2-iodo-4,5-dimethoxyphenyl)methyl]-4,5-dimethoxy phenyl) (DIV880) and (2-iodo-N-2-[5-methoxy-2-(naphthalen-1-yl)-1H-pyrrolo[3,2-b]pyridine-3-yl])acetamide (S70254), which are specific ligands at MT2 receptors, and N-[2-(5-methoxy-1H-indol-3-yl)ethyl]iodoacetamide (SD6), an analog of 2-[(125)I]-iodomelatonin with slightly different characteristics. Here, we further characterized these new ligands with regards to their molecular pharmacology. We performed binding experiments, saturation assays, association/dissociation rate measurements, and autoradiography using sheep and rat tissues and recombinant cell lines. Our results showed that [(125)I]-S70254 is receptor, and can be used with both cells and tissue. This radioligand can be used in autoradiography. Similarly, DIV880, a partial agonist [43% of melatonin on guanosine 5'-3-O-(thio)triphosphate binding assay], selective for MT2, can be used as a tool to selectively describe the pharmacology of this receptor in tissue samples. The molecular pharmacology of both human melatonin receptors MT1 and MT2, using a series of 24 ligands at these receptors and the new radioligands, did not lead to noticeable variations in the profiles. For the first time, we described radiolabeled tools that are specific for one of the melatonin receptors (MT2). These tools are amenable to binding experiments and to autoradiography using sheep or rat tissues. These specific tools will permit better understanding of the role and implication in physiopathologic processes of the melatonin receptors.

Effects of melatonin and its analogues on neural stem cells

Neural stem cells (NSCs) are multipotent cells which are capable of self-replication and differentiation into neurons, astrocytes or oligodendrocytes in the central nervous system (CNS). NSCs are found in two main regions in the adult brain: the subgranular zone (SGZ) in the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ). The recent discovery of NSCs in the adult mammalian brain has fostered a plethora of translational and preclinical studies to investigate novel approaches for the therapy of neurodegenerative diseases. Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. Recently, accumulated experimental evidence showed that melatonin plays an important role in NSCs, including its proliferation, differentiation and survival, which are modulated by many factors including MAPK/ERK signaling pathway, histone acetylation, neurotrophic factors, transcription factors, and apoptotic genes. The purpose of this review is to summarize the beneficial effects of melatonin on NSCs and further to discuss the potential usage of melatonin and its derivatives or analogues in the treatment of CNS neurodegenerative diseases.