MT1 and MT2 Melatonin Receptors: A Therapeutic Perspective

Melatonin receptors, brain functions, and therapies

In mammals, including humans, the neurohormone melatonin is mainly secreted from the pineal gland at night and acts on two high-affinity G protein-coupled receptors, the melatonin MT₁ and MT₂ receptors. Major functions of melatonin receptors in the brain are the regulation of circadian rhythms and sleep. Correspondingly, the main indications of the currently available drugs for these receptors indicate this as targets. Yet these drugs may not only improve circadian rhythm- and sleep-related disorders but may also be beneficial for complex diseases like major depression, Alzheimer's disease, autism, and attention-deficit/hyperactivity disorders. Here, we will focus on the hypothalamic functions of melatonin receptors by updating our knowledge on their hypothalamic expression pattern at normal, aged, and disease states, by discussing their capacity to regulate circadian rhythms and sleep and by presenting the clinical applications of the melatonin receptor-targeting drugs ramelteon, tasimelteon, and agomelatine or of prolonged-release melatonin formulations. Finally, we speculate about future trends in the field of melatonin receptor drugs.

Effects of Melatonin on Neurobehavior and Cognition in a Cerebral Palsy Model of plppr5-/- Mice

Cerebral palsy (CP), a group of clinical syndromes caused by non-progressive brain damage in the developing fetus or infant, is one of the most common causes of lifelong physical disability in children in most countries. At present, many researchers believe that perinatal cerebral hypoxic ischemic injury or inflammatory injury are the main causes of cerebral palsy. Previous studies including our works confirmed that melatonin has a protective effect against convulsive brain damage during development and that it affects the expression of various molecules involved in processes such as metabolism, plasticity and signaling in the brain. Integral membrane protein plppr5 is a new member of the plasticity-related protein family, which is specifically expressed in brain and spinal cord, and induces filopodia formation as well as neurite growth. It is highly expressed in the brain, especially in areas of high plasticity, such as the hippocampus. The signals are slightly lower in the cortex, the cerebellum, and in striatum. Noteworthy, during development plppr5 mRNA is expressed in the spinal cord, i.e., in neuron rich regions such as in medial motor nuclei, suggesting that plppr5 plays an important role in the regulation of neurons. However, the existing literature only states that plppr5 is involved in the occurrence and stability of dendritic spines, and research on its possible involvement in neonatal ischemic hypoxic encephalopathy has not been previously reported. We used plppr5 knockout (plppr5-/-) mice and their wild-type littermates to establish a model of hypoxicischemic brain injury (HI) to further explore the effects of melatonin on brain injury and the role of plppr5 in this treatment in an HI model, which mainly focuses on cognition, exercise, learning, and memory. All the tests were performed at 3-4 weeks after HI. As for melatonin treatment, which was performed 5 min after HI injury and followed by every 24h. In these experiments, we found that there was a significant interaction between genotype and treatment in novel object recognition tests, surface righting reflex tests and forelimb suspension reflex tests, which represent learning and memory, motor function and coordination, and the forelimb grip of the mice, respectively. However, a significant main effect of genotype and treatment on performance in all behavioral tests were observed. Specifically, wild-type mice with HI injury performed better than plppr5-/- mice, regardless of treatment with melatonin or vehicle. Moreover, treatment with melatonin could improve behavior in the tests for wild-type mice with HI injury, but not for plppr5-/- mice. This study showed that plppr5 knockout aggravated HI damage and partially weakened the neuroprotection of melatonin in some aspects (such as novel object recognition test and partial nerve reflexes), which deserves further study.

Aberrant functional connectivity between the suprachiasmatic nucleus and the superior temporal gyrus: Bridging RORA gene polymorphism with diurnal mood variation in major depressive disorder

Diurnal mood variation (DMV), a common symptom of major depressive disorder (MDD), is associated with circadian related genes and dysregulation of the suprachiasmatic nucleus (SCN). Previous research confirmed that the RORA gene is involved in the regulation of circadian rhythms. In this study, we hypothesized that polymorphisms of RORA may affect DMV symptoms of MDD through functional changes in the SCN. A total of 208 patients diagnosed with depression and 120 control subjects were enrolled and underwent a resting-state functional magnetic resonance imaging (rs-fMRI). Blood samples were collected and genotyping of 9 RORA gene SNPs were performed using next-generation sequencing technology. Patients were categorized as an AA genotype or C allele carriers based on RORA rs72752802 polymorphism. SCN-seed functional connectivity (FC) was compared between the two groups and correlation with severity of DMV was analyzed. Finally, a mediation analysis was performed to further determine FC intermediary effects. We observed that rs72752802 was significantly associated with patients' DMV symptoms. C allele carriers of rs72752802 showed significantly decreased FC between the right SCN and right superior temporal gyrus (rSTG). This was also correlated with DMV symptoms. In addition, the rs72752802 SNP influenced DMV symptoms through intermediary effects of SCN-rSTG connectivity. The study presented here provides a neurological and genetic basis for understanding depressed patients experiencing DMV.

Ramelteon modulates gamma oscillations in the rat primary motor cortex during non-REM sleep

Sleep disorders adversely affect daily activities and cause physiological and psychiatric problems. The shortcomings of benzodiazepine hypnotics have led to the development of ramelteon, a melatonin MT1 and MT2 agonist. Although the sleep-promoting effects of ramelteon have been documented, few studies have precisely investigated the structure of sleep and neural oscillatory activities. In this study, we recorded electrocorticograms in the primary motor cortex, the primary somatosensory cortex and the olfactory bulb as well as electromyograms in unrestrained rats treated with either ramelteon or vehicle. A neural-oscillation-based algorithm was used to classify the behavior of the rats into three vigilance states (e.g., awake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep). Moreover, we investigated the region-, frequency- and state-specific modulation of extracellular oscillations in the ramelteon-treated rats. We demonstrated that in contrast to benzodiazepine treatment, ramelteon treatment promoted NREM sleep and enhanced fast gamma power in the primary motor cortex during NREM sleep, while REM sleep was unaffected. Gamma oscillations locally coordinate neuronal firing, and thus, ramelteon modulates neural oscillations in sleep states in a unique manner and may contribute to off-line information processing during sleep.

Acute Ramelteon Treatment Maintains the Cardiac Rhythms of Rats during Non-REM Sleep

Sleep curtailment negatively affects cardiac activities and thus should be ameliorated by pharmacological methods. One of the therapeutic targets is melatonin receptors, which tune circadian rhythms. Ramelteon, a melatonin MT1/MT2 receptor agonist, has recently been developed to modulate sleep-wake rhythms. To date, the sleep-promoting effect of ramelteon has been widely delineated, but whether ramelteon treatment physiologically influences cardiac function is not well understood. To address this question, we recorded electrocardiograms, electromyograms, and electrocorticograms in the frontal cortex and the olfactory bulb of unrestrained rats treated with either ramelteon or vehicle. We detected vigilance states based on physiological measurements and analyzed cardiac and muscular activities. We found that during non-rapid eye movement (non-REM) sleep, heartrate variability was maintained by ramelteon treatment. Analysis of the electromyograms confirmed that neither microarousal during non-REM sleep nor the occupancy of phasic periods during REM sleep was altered by ramelteon. Our results indicate that ramelteon has a remedial effect on cardiac activity by keeping the heartrate variability and may reduce cardiac dysfunction during sleep.

Melatonin decreases cocaine-induced locomotor sensitization and cocaine-conditioned place preference in rats

Melatonin is a hormone that produces behavioral, pharmacological, and physiological effects through the activation of MT1 and MT2 melatonin receptors. Melatonin receptors participate in the modulation of the reinforcing effects of cocaine. Some studies report that dosing of melatonin decreases cocaine-induced locomotor activity and cocaine self-administration and that luzindole, an MT1, and MT2 melatonin receptor antagonist, blocks the melatonin-dependent decrease in cocaine-induced locomotor activity. The objective of this study was to evaluate the effect of acute or chronic dosing of melatonin on the induction and expression of cocaine-induced locomotor sensitization and cocaine-CPP in rats. Male Wistar rats received cocaine during the induction and expression of locomotor sensitization. Melatonin was administered 30 min before cocaine. After each treatment, locomotor activity was recorded for 30 min. Additionally, dopamine levels were determined in the ventral striatum, the prefrontal cortex (PFc), and the ventral tegmental area (VTA) by HPLC in animals treated with melatonin and cocaine. Melatonin decreased cocaine-induced locomotor sensitization and intracellular dopamine levels, as well as cocaine-CPP. Luzindole blocked the melatonin-induced decrease in the expression of locomotor sensitization in rats. These data suggest that melatonin may be a useful therapeutic agent to reduce cocaine abuse; additionally, they suggest that MT1 and MT2 receptors could be therapeutic targets, useful for the treatment of drug abuse disorder.

Agomelatine Attenuates Isoflurane-Induced Inflammation and Damage in Brain Endothelial Cells

Background and Purpose

Neurotoxicity of anesthetics has been widely observed by clinicians. It is reported that inflammation and oxidative stress are involved in the pathological process. In the present study, we aimed to assess the therapeutic effects of agomelatine against isoflurane-induced inflammation and damage to brain endothelial cells.

Materials and Methods

MTT assay was used to detect cell viability in order to determine the optimized concentration of agomelatine. The bEnd.3 brain endothelial cells were treated with 2% isoflurane in the presence or absence of agomelatine (5, 10 μM) for 24 h. LDH release was evaluated and the ROS levels were checked using DHE staining assay. The expressions of IL-6, IL-8, TNF-α, VEGF, TF, VCAM-1, and ICAM-1 were evaluated using real-time PCR and ELISA. Real-time PCR and Western blot analysis were used to determine the expression level of Egr-1.

Results

The decreased cell viability promoted LDH release and elevated ROS levels induced by isoflurane were significantly reversed by the introduction of agomelatine in a dose-dependent manner. The expression levels of IL-6, IL-8, TNF-α, VEGF, TF, VCAM-1, and ICAM-1 were elevated by stimulation with isoflurane, which were significantly suppressed by the administration of agomelatine. The up-regulation of transcriptional factor Egr-1 induced by isoflurane was down-regulated by agomelatine.

Conclusion

Agomelatine might attenuate isoflurane-induced inflammation and damage via down-regulating Egr-1 in brain endothelial cells.

Development of an immune-related prognostic index associated with osteosarcoma

Tumor immunity is closely associated with the prognosis of tumors, including osteosarcoma (OS). The aim of the present study was to construct an immune-related prognostic index (PI) to predict the prognosis of OS. Herein, OS expression data were sourced from the Therapeutically Applicable Research to Generate Effective Treatments (TARGET) database. We divided the OS patients into nonmetastatic and metastatic groups, allowing differentially immune-related genes (DIRGs) to be selected. After DIRGs were further investigated by enrichment analysis, four keys prognostic IRGs (CD79A, CSF3R, MTNR1B and NPPC) were identified using a Cox proportional hazards model. Then, an immune-related prognostic index was constructed. Finally, gene set enrichment analysis (GSEA) was employed to further explore the underlying mechanisms. The difference in tumor-infiltrating immune cell (TIIC) abundance was also discussed. In our study, eight upregulated genes and 30 downregulated genes were identified. Several Gene Ontology (GO) terms and the most significantly enriched KEGG pathways were immune-associated functions and pathways. Four genes, including CD79A, CSF3R, MTNR1B and NPPC, were used to establish a risk assessment model for evaluating OS prognosis. GSEA revealed that the risk score was related to cytokine receptor interaction and to the chemokine and B cell receptor signaling pathways. Furthermore, high risk markedly related to the infiltration of several immune cell types, including M2 macrophages, naïve CD4 T cells, and CD8 T cells. In sum, we developed a survival model for OS. The underlying molecular mechanisms of the high-risk group may affect immune-related biological processes and TIICs.Abbreviations TARGET: Therapeutically Applicable Research To Generate Effective Treatments; PI: Prognostic index; OS: Osteosarcoma; DIRGs: Differentially immune-related genes; GSEA: Gene set enrichment analysis; TIIC: Tumor-infiltrating immune cell.

Melatonin and regulation of miRNAs: novel targeted therapy for cancerous and noncancerous disease

miRNAs, small noncoding RNAs with crucial diagnostic and prognostic capabilities, play essential therapeutic roles in different human diseases. These biomarkers are involved in several biological mechanisms and are responsible for the regulation of multiple genes expressions in cells. miRNA-based therapy has shown a very bright future in the case of clinical interventions. Melatonin, the main product of the pineal gland, is a multifunctional neurohormone with numerous therapeutic potentials in human diseases. Melatonin is able to regulate miRNAs in different pathologies such as malignant and nonmalignant diseases, which can be considered as a novel kind of targeted therapy. Herein, this review discusses possible therapeutic utility of melatonin for the regulation of miRNAs in various pathological conditions.

Melatonin Reduces NLRP3 Inflammasome Activation by Increasing α7 nAChR-Mediated Autophagic Flux

Microglia controls the immune system response in the brain. Specifically, the activation and dysregulation of the NLRP3 inflammasome is responsible for the initiation of the inflammatory process through IL-1β and IL-18 release. In this work, we have focused on studying the effect of melatonin on the regulation of the NLRP3 inflammasome through α7 nicotinic receptor (nAChR) and its relationship with autophagy. For this purpose, we have used pharmacological and genetic approaches in lipopolysaccharide (LPS)-induced inflammation models in both in vitro and in vivo models. In the BV2 cell line, LPS inhibited autophagy, which increased NLRP3 protein levels. However, melatonin promoted an increase in the autophagic flux. Treatment of glial cultures from wild-type (WT) mice with LPS followed by extracellular adenosine triphosphate (ATP) produced the release of IL-1β, which was reversed by melatonin pretreatment. In cultures from α7 nAChR knock-out (KO) mice, melatonin did not reduce IL-1β release. Furthermore, melatonin decreased the expression of inflammasome components and reactive oxygen species (ROS) induced by LPS; co-incubation of melatonin with α-bungarotoxin (α-bgt) or luzindole abolished the anti-inflammatory and antioxidant effects. In vivo, melatonin reverted LPS-induced cognitive decline, reduced NLRP3 levels and promoted autophagic flux in the hippocampi of WT mice, whereas in α7 nAChR KO mice melatonin effect was not observed. These results suggest that melatonin may modulate the complex interplay between α7 nAChR and autophagy signaling.

A Topical Formulation of Melatoninergic Compounds Exerts Strong Hypotensive and Neuroprotective Effects in a Rat Model of Hypertensive Glaucoma

Melatonin is of great importance for regulating several eye processes, including pressure homeostasis. Melatonin in combination with agomelatine has been recently reported to reduce intraocular pressure (IOP) with higher efficacy than each compound alone. Here, we used the methylcellulose (MCE) rat model of hypertensive glaucoma, an optic neuropathy characterized by the apoptotic death of retinal ganglion cells (RGCs), to evaluate the hypotensive and neuroprotective efficacy of an eye drop nanomicellar formulation containing melatonin/agomelatine. Eye tissue distribution of melatonin/agomelatine in healthy rats was evaluated by HPLC/MS/MS. In the MCE model, we assessed by tonometry the hypotensive efficacy of melatonin/agomelatine. Neuroprotection was revealed by electroretinography; by levels of inflammatory and apoptotic markers; and by RGC density. The effects of melatonin/agomelatine were compared with those of timolol (a beta blocker with prevalent hypotensive activity) or brimonidine (an alpha 2 adrenergic agonist with potential neuroprotective efficacy), two drugs commonly used to treat glaucoma. Both melatonin and agomelatine penetrate the posterior segment of the eye. In the MCE model, IOP elevation was drastically reduced by melatonin/agomelatine with higher efficacy than that of timolol or brimonidine. Concomitantly, gliosis-related inflammation and the Bax-associated apoptosis were partially prevented, thus leading to RGC survival and recovered retinal dysfunction. We suggest that topical melatoninergic compounds might be beneficial for ocular health.

Melatonin reverts methamphetamine-induced learning and memory impairments and hippocampal alterations in mice

AIMS

Methamphetamine (METH) has become a major public health problem because of its abuse and profound neurotoxic effects, causing alterations in brain structure and function, and impairing cognitive functions, including attention, decision making, emotional memory, and working memory. This study aimed to determine whether melatonin (MEL), the circadian-control hormone, which has roles beyond circadian rhythm regulation, could restore METH-induced cognitive and neuronal impairment.

MAIN METHODS

Mice were treated with either METH (1 mg/kg) or saline for 7 days, followed by MEL (10 mg/kg) or saline for another 14 days. The Morris water maze (MWM) test was performed one day after the last saline or MEL injection. The hippocampal neuronal density, synaptic density, and receptors involved in learning and memory, along with downstream signaling molecules (NMDA receptor subunits GluN2A, GluN2B, and CaMKII) were investigated by immunoblotting.

KEY FINDINGS

METH administration significantly extended escape latency in learning phase and reduced the number of target crossings in memory test-phase as well as decreased the expression of BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin, and synaptophysin. MEL treatment significantly ameliorated METH-induced increased escape latency, decreased the number of target crossings and decreased expression of BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin and synaptophysin.

SIGNIFICANCE

METH administration impairs learning and memory in mice, and MEL administration restores METH-induced neuronal impairments which is probably through the changes in BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin and synaptophysin. Therefore, MEL is potentially an innovative and promising treatment for learning and memory impairment of humans.

Sleep Deprivation and Neurological Disorders

Sleep plays an important role in maintaining neuronal circuitry, signalling and helps maintain overall health and wellbeing. Sleep deprivation (SD) disturbs the circadian physiology and exerts a negative impact on brain and behavioural functions. SD impairs the cellular clearance of misfolded neurotoxin proteins like α-synuclein, amyloid-β, and tau which are involved in major neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. In addition, SD is also shown to affect the glymphatic system, a glial-dependent metabolic waste clearance pathway, causing accumulation of misfolded faulty proteins in synaptic compartments resulting in cognitive decline. Also, SD affects the immunological and redox system resulting in neuroinflammation and oxidative stress. Hence, it is important to understand the molecular and biochemical alterations that are the causative factors leading to these pathophysiological effects on the neuronal system. This review is an attempt in this direction. It provides up-to-date information on the alterations in the key processes, pathways, and proteins that are negatively affected by SD and become reasons for neurological disorders over a prolonged period of time, if left unattended.

Insights into Potential Targets for Therapeutic Intervention in Epilepsy

Epilepsy is a chronic brain disease that affects approximately 65 million people worldwide. However, despite the continuous development of antiepileptic drugs, over 30% patients with epilepsy progress to drug-resistant epilepsy. For this reason, it is a high priority objective in preclinical research to find novel therapeutic targets and to develop effective drugs that prevent or reverse the molecular mechanisms underlying epilepsy progression. Among these potential therapeutic targets, we highlight currently available information involving signaling pathways (Wnt/β-catenin, Mammalian Target of Rapamycin (mTOR) signaling and zinc signaling), enzymes (carbonic anhydrase), proteins (erythropoietin, copine 6 and complement system), channels (Transient Receptor Potential Vanilloid Type 1 (TRPV1) channel) and receptors (galanin and melatonin receptors). All of them have demonstrated a certain degree of efficacy not only in controlling seizures but also in displaying neuroprotective activity and in modifying the progression of epilepsy. Although some research with these specific targets has been done in relation with epilepsy, they have not been fully explored as potential therapeutic targets that could help address the unsolved issue of drug-resistant epilepsy and develop new antiseizure therapies for the treatment of epilepsy.

Melatonin regulates Aβ production/clearance balance and Aβ neurotoxicity: A potential therapeutic molecule for Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease with multiple predisposing factors and complicated pathogenesis. Aβ peptide is one of the most important pathogenic factors in the etiology of AD. Accumulating evidence indicates that the imbalance of Aβ production and Aβ clearance in the brain of AD patients leads to Aβ deposition and neurotoxic Aβ oligomer formation. Melatonin shows a potent neuroprotective effect and can prevent or slow down the progression of AD, supporting the view that melatonin is a potential therapeutic molecule for AD. Melatonin modulates the regulatory network of secretase expression and affects the function of secretase, thereby inhibiting amyloidogenic APP processing and Aβ production. Additionally, melatonin ameliorates Aβ-induced neurotoxicity and probably promotes Aβ clearance through glymphatic-lymphatic drainage, BBB transportation and degradation pathways. In this review, we summarize and discuss the role of melatonin against Aβ-dependent AD pathogenesis. We explore the potential cellular and molecular mechanisms of melatonin on Aβ production and assembly, Aβ clearance, Aβ neurotoxicity and circadian cycle disruption. We summarize multiple clinical trials of melatonin treatment in AD patients, showing that melatonin has a promising effect on improving sleep quality and cognitive function. This review aims to stimulate further research on melatonin as a potential therapeutic agent for AD.

The genus Uncaria: A review on phytochemical metabolites and biological aspects

The genus Uncaira (Rubiaceae) comprises of 34 species, many of which are usually used as traditional Chinese medicines (TCMs) to treat hypertension, fever, headache, gastrointestinal illness, and fungal infection. Over the past twenty years, Uncaira species have been paid the considerable attentions in phytochemical and biological aspects, and about 100 new secondary metabolites, including alkaloids, triterpenes, and flavonoids, have been elucidated. This review aims to present a comprehensive and up-to date overview of the biological source, structures and their biosynthetic pathways, as well as the pharmacological of the compounds reported in the genus Uncaria for the past two decades. It would provide an insight into the emerging pharmacological applications of the genus Uncaria.

Prescription Drugs Used in Insomnia

The scope of this article is to review the effects on sleep of prescription drugs that are commonly prescribed for chronic insomnia in adults. The following groups are discussed: benzodiazepines and its receptor agonists, the dual orexin receptor antagonist suvorexant, melatonin and its receptor agonists, sedating antidepressants, and antipsychotics. Together with the neurobiologic and pharmacologic properties of these drugs, clinical effects are described, including subjective and objective effects on sleep duration, continuity, and architecture. Medical prescription information is given when available. Recently published American and European guidelines for the treatment of insomnia serve as reference frame.

Circadian rhythms in diabetic retinopathy: an overview of pathogenesis and investigational drugs

INTRODUCTION

Circadian rhythm is a natural endogenous process occurring roughly every 24 hours. Circadian rhythm dysfunction is involved in diabetic retinopathy (DR) pathogenesis. Interestingly, there are investigational drugs that exhibit potential in the treatment of DR by targeting circadian rhythm dysfunction.

AREAS COVERED

We performed a literature search in June 2020 using PubMed's Medical Subject Heading (MeSH) terms 'circadian clock,' 'circadian rhythms,' and 'diabetic retinopathy.' This article offers an overview of the physiology of the biological clock and clock regulatory genes and presents an examination of the retinal clock. It discusses the pathogenic mechanisms of DR and emphasizes how circadian rhythm dysfunction at structural, physiological, metabolic and cellular levels, plays a critical role in the development of DR. The latter part of the paper sheds light on those investigational drugs (such as melatonin, tasimelteon and metformin) which exhibit potential in the treatment of DR by the targeting of circadian rhythm dysfunction.

EXPERT OPINION

An enhanced understanding of circadian rhythm and its role in DR could offer therapeutic potential by targeting of circadian rhythm dysfunction.

Melatonin supplementation in the management of obesity and obesity-associated disorders: A review of physiological mechanisms and clinical applications

Despite the evolving advances in clinical approaches to obesity and its inherent comorbidities, the therapeutic challenge persists. Among several pharmacological tools already investigated, recent studies suggest that melatonin supplementation could be an efficient therapeutic approach in the context of obesity. In the present review, we have amalgamated the evidence so far available on physiological effects of melatonin supplementation in obesity therapies, addressing its effects upon neuroendocrine systems, cardiometabolic biomarkers and body composition. Most studies herein appraised employed melatonin supplementation at dosages ranging from 1 to 20 mg/day, and most studies followed up participants for periods from 3 weeks to 12 months. Overall, it was observed that melatonin plays an important role in glycaemic homeostasis, in addition to modulation of white adipose tissue activity and lipid metabolism, and mitochondrial activity. Additionally, melatonin increases brown adipose tissue volume and activity, and its antioxidant and anti-inflammatory properties have also been demonstrated. There appears to be a role for melatonin in adiposity reduction; however, several questions remain unanswered, for example melatonin baseline levels in obesity, and whether any seeming hypomelatonaemia or melatonin irresponsiveness could be clarifying factors. Supplementation dosage studies and more thorough clinical trials are needed to ascertain not only the relevance of such findings but also the efficacy of melatonin supplementation.

Melatonin and doxorubicin co-delivered via a functionalized graphene-dendrimeric system enhances apoptosis of osteosarcoma cells

A functionalized graphene-dendrimeric system was designed via Fe₃O₄ nanoparticle (NP) as a magnetic nanocarrier for co-delivery of doxorubicin (DOX) and melatonin (MLT). Accordingly, β-Cyclodextrin (β-CD) was modified by creating amine functional groups. The modified β-CD was grafted with Graphene oxide (GO), and the resulting platform gain many functional groups, including the hydroxyl (-OH), carboxylic acid (-COOH), and amine functional groups (-NH2). Finally, magnetic NPs were synthesized on the prepared platform to efficiently controlling and targeting drugs to tumor sites. The human osteosarcoma cell lines including Saos-2 and MG-63, as well as Human Bone Marrow Mesenchymal Stem Cells (hBM-MSC) line, were used to determine the in vitro biological effects of the functionalized graphene-dendrimeric system. The magnetic nanocarrier has encapsulation efficiency (EE) values of 99.92% for DOX and 21.5% for MLT. The biocompatibility tests of the nanocarrier revealed that the magnetic nanocarrier was appropriate as a drug carrier. Co-delivery of DOX and MLT with an efficiently anticancer performance was also was confirmed by cellular uptake, 4',6-diamidino-2-phenylindole (DAPI) staining, and apoptosis analysis in comparison with free DOX and MLT. Moreover, there was a synergy in the antitumor effect when MLT was combined with DOX, especially in the nano-formulation form, which may be due to the down-regulation of X-linked Inhibitor of Apoptosis (XIAP), survivin, and human telomerase catalytic subunit (hTERT) (p < 0.0001). Overall, the result of our study suggests that the designed carrier is a promising nanocarrier for targeted co-delivery of DOX and MLT with improved anticancer efficacy in cancer cells and thus reduced toxicity in normal cells.

Melatonin as a therapy in cardiac ischemia-reperfusion injury: Potential mechanisms by which MT2 activation mediates cardioprotection

Introduction

Previous studies reported the beneficial effects of pretreatment with melatonin on the heart during cardiac ischemia/reperfusion (I/R) injury. However, the effects of melatonin given after cardiac ischemia, as well as its comparative temporal effects are unknown. These include pretreatment, during ischemia, and at the onset of reperfusion. Also, the association between melatonin receptors and cardiac arrhythmias, mitochondrial function and dynamics, autophagy, and mitophagy during cardiac I/R have not been investigated.

Objectives

We tested two major hypotheses in this study. Firstly, the temporal effect of melatonin administration exerts different cardioprotective efficacy during cardiac I/R. Secondly, melatonin provides cardioprotective effects via MT2 activation, leading to improvement in cardiac mitochondrial function and dynamics, reduced excessive mitophagy and autophagy, and decreased cardiac arrhythmias, resulting in improved LV function.

Methods

Male rats were subjected to cardiac I/R, and divided into 4 intervention groups: vehicle, pretreatment with melatonin, melatonin given during ischemia, and melatonin given at the onset of reperfusion. In addition, either a non-specific melatonin receptor (MT) blocker or specific MT2 blocker was given to rats.

Results

Treatment with melatonin at all time points alleviated cardiac I/R injury to a similar extent, quantified by reduction in infarct size, arrhythmia score, LV dysfunction, cardiac mitochondrial dysfunction, imbalance of mitochondrial dynamics, excessive mitophagy, and a decreased Bax/Bcl2 ratio. In H9C2 cells, melatonin increased %cell viability by reducing mitochondrial dynamic imbalance and a decrease in Bax protein expression. The cardioprotective effects of melatonin were dependent on MT2 activation.

Conclusion

Melatonin given before or after ischemia exerted equal levels of cardioprotection on the heart with I/R injury, and its beneficial effects on cardiac arrhythmias, cardiac mitochondrial function and dynamics were dependent upon the activation of MT2.

Melatonin promotes sleep by activating the BK channel in C. elegans

Melatonin (Mel) promotes sleep through G protein-coupled receptors. However, the downstream molecular target(s) is unknown. We identified the Caenorhabditis elegans BK channel SLO-1 as a molecular target of the Mel receptor PCDR-1-. Knockout of pcdr-1, slo-1, or homt-1 (a gene required for Mel synthesis) causes substantially increased neurotransmitter release and shortened sleep duration, and these effects are nonadditive in double knockouts. Exogenous Mel inhibits neurotransmitter release and promotes sleep in wild-type (WT) but not pcdr-1 and slo-1 mutants. In a heterologous expression system, Mel activates the human BK channel (hSlo1) in a membrane-delimited manner in the presence of the Mel receptor MT₁ but not MT₂ A peptide acting to release free Gβγ also activates hSlo1 in a MT₁-dependent and membrane-delimited manner, whereas a Gβλ inhibitor abolishes the stimulating effect of Mel. Our results suggest that Mel promotes sleep by activating the BK channel through a specific Mel receptor and Gβλ.

Chiral Recognition of Flexible Melatonin Receptor Ligands Induced by Conformational Equilibria

N-anilinoethylamides are a class of melatoninergic agents with the aniline portion mimicking the indole ring of the natural ligand and the ethylamide chain reproducing that of melatonin. The simplest compound in this class, N-{2-[(3-methoxyphenyl)methylamino]ethyl}acetamide (UCM793), has nanomolar binding affinity for MT₁ and MT₂ membrane receptors. To explore the effect of chain conformation on receptor binding, a methyl group was inserted on the methylene alpha or beta to the amide nitrogen and conformational equilibria were investigated by NMR spectroscopy and molecular dynamics simulations. Receptor affinity was conserved only for the beta-methyl derivative, which also showed significant stereoselectivity, with the (S) enantiomer being the eutomer. Molecular dynamics simulations, validated by NMR spectroscopy, showed that the beta-methyl group affects the conformational preferences of the ethylamide chain. Docking into the receptor crystal structure provides a rationale for the observed chiral recognition, suggesting that the (S)-beta-methyl group favors the conformation that better fits the receptor binding site.

Melatonin improves uterine-conceptus interaction via regulation of SIRT1 during early pregnancy

Melatonin has been shown to improve in vitro fertilization and offspring survival after bacterial infection, but its role in regulating maternal-fetal communication during early pregnancy has not been investigated. Results of this study demonstrated expression of abundant melatonin receptors in conceptus and endometrium during early pregnancy. In gilts, expression of melatonin receptor 1A (MTNR1A or MT1) and melatonin receptor 1B (MTNR1B or MT2) increased in trophectoderm (Tr) and uterine luminal epithelium (LE) with advancing days during early pregnancy in a different manner. Melatonin increased proliferation and migration of porcine trophectoderm (pTr) cell, the percent pTr cells in the G2 phase of the cell cycle, and the expression of implantation-related genes by pTr cells and endometrial luminal epithelium (pLE). Melatonin also attenuated the production of LPS-induced pro-inflammatory cytokines and tunicamycin-induced endoplasmic reticulum (ER) stress-sensing proteins. The expression of sirtuin 1 (SIRT1) as a potential target of melatonin increased between Days 9 and 14 of gestation. Co-treatment with SIRT1 inhibitor EX527 and melatonin restored cell-cell interactions through PI3K and MAPK signaling. Knockdown of SIRT1 decreased the expression of implantation-related genes, as well as migration of pTr and pLE cells. The expression of microRNAs regulated by SIRT1 was suppressed in response to melatonin. Furthermore, melatonin significantly increased lipopolysaccharide (LPS)-reduced fertilization and embryogenesis in zebrafish model. These results suggest that melatonin may improve the uterine-conceptus interactions via the regulation of SIRT1 during early pregnancy.

Circadian disruption, melatonin rhythm perturbations and their contributions to chaotic physiology

The aim of this report is to summarize the data documenting the vital nature of well-regulated cellular and organismal circadian rhythms, which are also reflected in a stable melatonin cycle, in supporting optimal health. Cellular fluctuations in physiology exist in most cells of multicellular organisms with their stability relying on the prevailing light:dark cycle, since it regulates, via specialized intrinsically-photoreceptive retinal ganglion cells (ipRGC) and the retinohypothalamic tract, the master circadian oscillator, i.e., the suprachiasmatic nuclei (SCN). The output message of the SCN, as determined by the light:dark cycle, is transferred to peripheral oscillators, so-called slave cellular oscillators, directly via the autonomic nervous system with its limited distribution. and indirectly via the pineal-derived circulating melatonin rhythm, which contacts every cell. Via its regulatory effects on the neuroendocrine system, particularly the hypothalamo-pituitary-adrenal axis, the SCN also has a major influence on the adrenal glucocorticoid rhythm which impacts neurological diseases and psychological behaviors. Moreover, the SCN regulates the circadian production and secretion of melatonin. When the central circadian oscillator is disturbed, such as by light at night, it passes misinformation to all organs in the body. When this occurs the physiology of cells becomes altered and normal cellular functions are compromised. This physiological upheaval is a precursor to pathologies. The deterioration of the SCN/pineal network is often a normal consequence of aging and its related diseases, but in today's societies where manufactured light is becoming progressively more common worldwide, the associated pathologies may also be occurring at an earlier age.

Melatonin's Impact on Antioxidative and Anti-Inflammatory Reprogramming in Homeostasis and Disease

There is a growing consensus that the antioxidant and anti-inflammatory properties of melatonin are of great importance in preserving the body functions and homeostasis, with great impact in the peripartum period and adult life. Melatonin promotes adaptation through allostasis and stands out as an endogenous, dietary, and therapeutic molecule with important health benefits. The anti-inflammatory and antioxidant effects of melatonin are intertwined and are exerted throughout pregnancy and later during development and aging. Melatonin supplementation during pregnancy can reduce ischemia-induced oxidative damage in the fetal brain, increase offspring survival in inflammatory states, and reduce blood pressure in the adult offspring. In adulthood, disturbances in melatonin production negatively impact the progression of cardiovascular risk factors and promote cardiovascular and neurodegenerative diseases. The most studied cardiovascular effects of melatonin are linked to hypertension and myocardial ischemia/reperfusion injury, while the most promising ones are linked to regaining control of metabolic syndrome components. In addition, there might be an emerging role for melatonin as an adjuvant in treating coronavirus disease 2019 (COVID 19). The present review summarizes and comments on important data regarding the roles exerted by melatonin in homeostasis and oxidative stress and inflammation related pathologies.

Melatonin 1A and 1B Receptors' Expression Decreases in the Placenta of Women with Fetal Growth Restriction

Melatonin and its metabolites prevent oxidative stress and apoptosis, and it is actively produced by the placenta during pregnancy. Melatonin 1A and 1B receptors are present in human villous trophoblastic cells. We aimed to investigate the expression of melatonin 1A and 1B receptors in human placental tissue in the case of placental insufficiency manifested as the intrauterine growth restriction syndrome of the fetus (IUGR). Thirty-two pregnant women aged 18-36 with placental insufficiency manifested at the term 36 weeks of gestation as the IUGR syndrome (the estimated fetal weight less than the 3rd percentile) were included in the experimental group; all their babies had the diagnosis confirmed at birth, which occurred after 37 weeks of gestation. The control group consisted of 30 women with uncomplicated pregnancy of the same term. Pieces of the placental tissue were obtained after deliveries, and melatonin 1A and 1B receptors were immunoassayed; the richness of melatonin receptors in the placental tissue was estimated on the basis of immunohistochemical (IHC) staining of receptors, calculated in the IHC image score. The optical density of melatonin 1A receptors in the placentas obtained from women whose pregnancies were complicated with IUGR was significantly lower than that in the placentas from uncomplicated pregnancies: generally in the trophoblast, it was 0.095 ± 0.0009 IHC image score (in the control group, 0.194 ± 0.0015, p < 0.0001); in the apical parts of the syncytiotrophoblast, 0.108 ± 0.0016 IHC image score (in the control group, 0.221 ± 0.0013, p < 0.0001); and in the stromal cells of placental villi, 0.112 ± 0.0013 IHC image score (in the control group, 0.156 ± 0.0011, p < 0.0001). The optical density of melatonin 1B receptors in placentas obtained from women whose pregnancies were complicated with IUGR was also lower than that in the placentas from uncomplicated pregnancies: generally in the trophoblast, it was 0.165 ± 0.0019 IHC image score (in the control group, 0.231 ± 0.0013, p < 0.0001), and in the apical parts of the syncytiotrophoblast, 0.188 ± 0.0028 IHC image score (in the control group, 0.252 ± 0.0009, p < 0.0001). There was no difference found in the optical density of melatonin 1B receptors in the stromal cells of placental villi between the two groups: in the experimental group, 0.109 ± 0.006 IHC image score, and in the control group, 0.114 ± 0.0011 (p = 0.65). Melatonin receptors 1A and 1B are significantly less expressed in the placental tissue in the case that pregnancy is complicated with placental insufficiency, manifested as the intrauterine growth restriction syndrome of the fetus.

Ionizing Radiation as a Source of Oxidative Stress-The Protective Role of Melatonin and Vitamin D

Ionizing radiation (IR) has found widespread application in modern medicine, including medical imaging and radiotherapy. As a result, both patients and healthcare professionals are exposed to various IR doses. To minimize the negative side effects of radiation associated with oxidative imbalance, antioxidant therapy has been considered. In this review, studies on the effects of melatonin and vitamin D on radiation-induced oxidative stress are discussed. According to the research data, both substances meet the conditions for use as agents that protect humans against IR-induced tissue damage. Numerous studies have confirmed that melatonin, a hydro- and lipophilic hormone with strong antioxidant properties, can potentially be used as a radioprotectant in humans. Less is known about the radioprotective effects of vitamin D, but the results to date have been promising. Deficiencies in melatonin and vitamin D are common in modern societies and may contribute to the severity of adverse side effects of medical IR exposure. Hence, supporting supplementation with both substances seems to be of first importance. Interestingly, both melatonin and vitamin D have been found to selectively radiosensitise cancer cells, which makes them promising adjuvants in radiotherapy. More research is needed in this area, especially in humans.

Melatonin alleviates intestinal injury, neuroinflammation and cognitive dysfunction caused by intestinal ischemia/reperfusion

Intestinal ischemia/reperfusion (I/R) can cause multiple organ damage with extremely high morbidity and mortality. Melatonin has anti-inflammatory, anti-oxidative and anti-apoptotic effects against various diseases. This study aimed to explore whether melatonin had a protective effect against intestinal I/R-induced neuroinflammation and cognitive dysfunction, and investigate its potential mechanisms. In this study, melatonin was administered to the rats with intestinal I/R, then histological changes in intestine and brain (frontal cortex and hippocampal CA1 area) tissues and cognitive function were detected, respectively. The encephaledema and blood-brain barrier (BBB) permeability were observed. Moreover, the alterations of proinflammatory factors (tumor necrosis factor-α, interleukin-6 and interleukin-1β), oxidative response (malondialdehyde, superoxide dismutase, and reactive oxygen species), apoptosis and proteins associated with inflammation,including Toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (Myd88) and phosphorylated nuclear factor kappa beta (NF-κB), and apoptosis (cleaved caspase-3) in brain tissues were examined. Furthermore, the expressions of TLR4, Myd88, and microglial activity were observed by multiple immunofluorescence staining. The results showed that intestinal I/R-induced abnormal neurobehavior and cerebral damage were ameliorated after melatonin treatment, which were demonstrated by improved cognitive dysfunction and aggravated histology. Furthermore, melatonin decreased the levels of proinflammatory factors and oxidative stress in plasma, intestine and brain tissues, attenuated apoptotic cell, and inhibited the expressions of related proteins and the immunoreactivity of TLR4 or Myd88 in microglia in brain tissues. These findings showed that melatonin might relieve neuroinflammation and cognitive dysfunction caused by intestinal I/R, which could be, at least partially, related to the inhibition of the TLR4/Myd88 signaling in microglia.

Sweet dreams: therapeutic insights, targeting imaging and physiologic evidence linking sleep, melatonin and diabetic nephropathy

Melatonin is the main biochronologic molecular mediator of circadian rhythm and sleep. It is also a powerful antioxidant and has roles in other physiologic pathways. Melatonin deficiency is associated with metabolic derangements including glucose and cholesterol dysregulation, hypertension, disordered sleep and even cancer, likely due to altered immunity. Diabetic nephropathy (DN) is a key microvascular complication of both type 1 and 2 diabetes. DN is the end result of a complex combination of metabolic, haemodynamic, oxidative and inflammatory factors. Interestingly, these same factors have been linked to melatonin deficiency. This report will collate in a clinician-oriented fashion the mechanistic link between melatonin deficiency and factors contributing to DN.

Melatonin attenuates choroidal neovascularization by regulating macrophage/microglia polarization via inhibition of RhoA/ROCK signaling pathway

Choroidal neovascularization (CNV) is an important characteristic of advanced wet age-related macular degeneration (AMD) and leads to severe visual impairment among elderly patients. Previous studies have demonstrated that melatonin induces several biological effects related to antioxidation, anti-inflammation, and anti-angiogenesis. However, the role of melatonin in CNV, and its underlying mechanisms, has not been investigated thus far. In this study, we found that melatonin administration significantly reduced the scale and volume of CNV lesions, suppressed vascular leakage, and inhibited the capacity of vascular proliferation in the laser-induced mouse CNV model. Additionally, the results also show that the melatonin-treated retinal microglia in the laser-induced mice exhibited enhanced expression of M1-type markers, such as iNOS, CCL-3, CCL-5, and TNF-α, as well as decreased production of M2-type markers, such as Arg-1, Fizz-1, IL-10, YM-1, and CD206, indicating that melatonin switched the macrophage/microglia polarization from pro-angiogenic M2 phenotype to anti-angiogenic M1 phenotype. Furthermore, the RhoA/ROCK signaling pathway was activated during CNV formation, yet was suppressed after an intraperitoneal injection of melatonin. In conclusion, melatonin attenuated CNV, reduced vascular leakage, and inhibited vascular proliferation by switching the macrophage/microglia polarization from M2 phenotype to M1 phenotype via inhibition of RhoA/ROCK signaling pathway in CNV. This suggests that melatonin could be a novel agent for the treatment of AMD.

Do hypnotics increase the risk of driving accidents or near miss accidents due to hypovigilance? The effects of sex, chronic sleepiness, sleep habits and sleep pathology

Driving accidents due to hypovigilance are common but the role of hypnotics is unclear in patients suffering from sleep disorders. Our study examined factors influencing accidents and near miss accidents attributed to sleepiness at the wheel (ANMAS). Using data from an online questionnaire aimed at patients with sleep disorders, we analysed the associations between ANMAS, sociodemographic data, symptoms of sleep disorders, severity of insomnia (Insomnia Severity Index (ISI)) symptoms of anxiety and depression (Hospital Anxiety and Depression scale with depression (HADD) and anxiety (HADA) subscales), chronic sleepiness (Epworth sleepiness scale ESS), hypnotic use and information about sleep habits. Hypnotics were hierarchically grouped into Z-drugs, sedative medication, melatonin and over the counter (OTC) alternative treatments. Of 10802 participants; 9.1% reported ANMAS (Men 11.1% women 8.3%) and 24.4% took hypnotics (Z-drugs 8.5%, sedative medication 8%, melatonin 5.6% and alternative treatments 2.5%). Logistic regression analysis identified the following risk factors for ANMAS: moderate (OR 2.4; CI: 2.10-2.79) and severe sleepiness (ESS OR 5.66; CI: 4.74-6.77), depression (HADD OR 1.2; CI: 1.03-1.47), anxiety (HADA OR 1.2;CI: 1.01-1.47), and insufficient sleep (OR1.4; CI: 1.2-1.7). Hypnotics were not associated with an increased risk of ANMAS in patients suffering from insomnia. Risk factors varied according to sex: in females, sex (OR 0.; CI: 0.55-0.74), mild insomnia (OR 0.5; CI: 0.3-0.8) and use of alternative treatments (OR 0.455, CI:0.23-0.89) were protective factors and risk was increased by sleepiness, sleep debt, social jetlag, caffeine use, anxiety and depression. In men no protective factors were identified: sleepiness, sleep debt, and severe insomnia were associated with an increased risk of ANMAS. In clinical practice, all patients with daytime sleepiness and men with severe insomnia should be counselled concerning driving risk and encouraged to avoid sleep debt.

Melatonin inhibits Warburg-dependent cancer by redirecting glucose oxidation to the mitochondria: a mechanistic hypothesis

Melatonin has the ability to intervene in the initiation, progression and metastasis of some experimental cancers. A large variety of potential mechanisms have been advanced to describe the metabolic and molecular events associated with melatonin's interactions with cancer cells. There is one metabolic perturbation that is common to a large number of solid tumors and accounts for the ability of cancer cells to actively proliferate, avoid apoptosis, and readily metastasize, i.e., they use cytosolic aerobic glycolysis (the Warburg effect) to rapidly generate the necessary ATP required for the high metabolic demands of the cancer cells. There are several drugs, referred to as glycolytic agents, that cause cancer cells to abandon aerobic glycolysis and shift to the more conventional mitochondrial oxidative phosphorylation for ATP synthesis as in normal cells. In doing so, glycolytic agents also inhibit cancer growth. Herein, we hypothesize that melatonin also functions as an inhibitor of cytosolic glycolysis in cancer cells using mechanisms, i.e., downregulation of the enzyme (pyruvate dehydrogenase kinase) that interferes with the conversion of pyruvate to acetyl CoA in the mitochondria, as do other glycolytic drugs. In doing so, melatonin halts the proliferative activity of cancer cells, reduces their metastatic potential and causes them to more readily undergo apoptosis. This hypothesis is discussed in relation to the previously published reports. Whereas melatonin is synthesized in the mitochondria of normal cells, we hypothesize that this synthetic capability is not present in cancer cell mitochondria because of the depressed acetyl CoA; acetyl CoA is necessary for the rate limiting enzyme in melatonin synthesis, arylalkylamine-N-acetyltransferase. Finally, the ability of melatonin to switch glucose oxidation from the cytosol to the mitochondria also explains how tumors that become resistant to conventional chemotherapies are re-sensitized to the same treatment when melatonin is applied.

Melatonin: an endogenous miraculous indolamine, fights against cancer progression

PURPOSE

Melatonin is an amphipathic indolamine molecule ubiquitously present in all organisms ranging from cyanobacteria to humans. The pineal gland is the site of melatonin synthesis and secretion under the influence of the retinohypothalamic tract. Some extrapineal tissues (skin, lens, gastrointestinal tract, testis, ovary, lymphocytes, and astrocytes) also enable to produce melatonin. Physiologically, melatonin regulates various functions like circadian rhythm, sleep-wake cycle, gonadal activity, redox homeostasis, neuroprotection, immune-modulation, and anticancer effects in the body. Inappropriate melatonin secretion advances the aging process, tumorigenesis, visceral adiposity, etc. METHODS: For the preparation of this review, I had reviewed the literature on the multidimensional activities of melatonin from the NCBI website database PubMed, Springer Nature, Science Direct (Elsevier), Wiley Online ResearchGate, and Google Scholar databases to search relevant articles. Specifically, I focused on the roles and mechanisms of action of melatonin in cancer prevention.

RESULTS

The actions of melatonin are primarily mediated by G-protein coupled MT1 and MT2 receptors; however, several intracellular protein and nuclear receptors can modulate the activity. Normal levels of the melatonin protect the cells from adverse effects including carcinogenesis. Therapeutically, melatonin has chronomedicinal value; it also shows a remarkable anticancer property. The oncostatic action of melatonin is multidimensional, associated with the advancement of apoptosis, the arrest of the cell cycle, inhibition of metastasis, and antioxidant activity.

CONCLUSION

The present review has emphasized the mechanism of the anti-neoplastic activity of melatonin that increases the possibilities of the new approaches in cancer therapy.

Infusion of Melatonin Into the Paraventricular Nucleus Ameliorates Myocardial Ischemia-Reperfusion Injury by Regulating Oxidative Stress and Inflammatory Cytokines

Melatonin, the receptors for which are abundant in the hypothalamic paraventricular nucleus (PVN), can protect the heart from myocardial ischemia–reperfusion (MI/R) injury. The aim of this study was to determine whether the infusion of melatonin into the PVN protects the heart from MI/R injury by suppressing oxidative stress or regulating the balance between proinflammatory cytokines and anti-inflammatory cytokines in MI/R rats. Male Sprague–Dawley rats were treated with a bilateral PVN infusion of melatonin. MI/R operation was performed 1 week after infusion. At the end of the third week after the infusion, all the rats were euthanized. This was followed by immunohistochemistry and immunofluorescence studies of the rats. MI/R rats showed larger infarct size, increased left ventricular (LV) end-diastolic volume, and decreased LV ejection fraction and LV fractional shortening. Moreover, MI/R rats had a higher level of norepinephrine in the plasma, heart, and PVN; higher PVN levels of reactive oxygen species, NOX2, NOX4, IL-1β, and NF-κB activity; and lower PVN levels of copper/zinc superoxide dismutase (Cu/Zn-SOD) and IL-10 compared with the sham group. Melatonin infusion in PVN reduced LV end-diastolic volume, norepinephrine, reactive oxygen species, NOX2, NOX4, IL-1β, and NF-κB activity, and increased LV ejection fraction, LV fractional shortening, Cu/Zn-SOD, and IL-10. Overall, these results suggest that the infusion of melatonin ameliorates sympathetic nerve activity and MI/R injury by attenuating oxidative stress and inflammatory cytokines in the PVN of MI/R rats.

Role and Therapeutic Potential of Melatonin in the Central Nervous System and Cancers

Melatonin (MLT) is a powerful chronobiotic hormone that controls a multitude of circadian rhythms at several levels and, in recent times, has garnered considerable attention both from academia and industry. In several studies, MLT has been discussed as a potent neuroprotectant, anti-apoptotic, anti-inflammatory, and antioxidative agent with no serious undesired side effects. These characteristics raise hopes that it could be used in humans for central nervous system (CNS)-related disorders. MLT is mainly secreted in the mammalian pineal gland during the dark phase, and it is associated with circadian rhythms. However, the production of MLT is not only restricted to the pineal gland; it also occurs in the retina, Harderian glands, gut, ovary, testes, bone marrow, and lens. Although most studies are limited to investigating the role of MLT in the CNS and related disorders, we explored a considerable amount of the existing literature. The objectives of this comprehensive review were to evaluate the impact of MLT on the CNS from the published literature, specifically to address the biological functions and potential mechanism of action of MLT in the CNS. We document the effectiveness of MLT in various animal models of brain injury and its curative effects in humans. Furthermore, this review discusses the synthesis, biology, function, and role of MLT in brain damage, and as a neuroprotective, antioxidative, anti-inflammatory, and anticancer agent through a collection of experimental evidence. Finally, it focuses on the effect of MLT on several neurological diseases, particularly CNS-related injuries.

Cardioprotective effects of melatonin: Focusing on its roles against diabetic cardiomyopathy

Melatonin is a pineal-produced indole known for its anti-aging, antiapoptotic and antioxidant properties. In past decades, the protective potentials of melatonin for cardiovascular diseases, such as atherosclerosis and myocardial infarction, have been widely revealed, triggering more investigations focused on other cardioprotective effects of melatonin. Recently, the roles of melatonin in diabetic cardiomyopathy (DCM) have attracted increased attention. In this regard, researchers found that melatonin attenuated cardiac fibrosis and hypertrophy, thus interrupting the development of DCM. Retinoid-related orphan receptor α is a key melatonin receptor that contributed to the cardioprotective effect of melatonin in hearts with DCM. For the downstream mechanisms, the inhibition of mammalian STE20-like kinase 1 plays a pivotal role, which exerts antiapoptotic and proautophagic effects, thus enhancing cardiac tolerance in high-glucose conditions. In addition, other signalling mechanisms, such as sirtuin-1/peroxisome proliferator-activated receptor gamma-coactivator alpha and endoplasmic reticulum-related signalling, are also involved in the protective effects of melatonin on cardiomyocytes under diabetic conditions. This review will focus on the protective signalling mechanisms regulated by melatonin and provide a better understanding of the therapeutic applications of melatonin signalling in DCM.

Molecular basis defining the selectivity of substituted isoquinolinones for the melatonin MT2 receptor

Melatonin MT₁ and MT₂ receptors represent attractive drug targets for the treatment of various disorders. However, the high conservation of the melatonin binding pocket has hindered the development of subtype-selective compounds. By leveraging on the recently resolved crystal structures of MT₁ and MT₂ receptors, this study aims to elucidate the structural basis of MT₂-selectivity of a panel of isoquinolinone derivatives. Molecular modelling and ligand docking approaches were employed to predict residues involved in forming interactions with the MT₂-selective isoquinolinones. Seven conserved residues (Asn¹⁷⁵, His²⁰⁸, Trp²⁶⁴, Asn²⁶⁸, Gly²⁷¹, Tyr²⁹⁴ and Tyr²⁹⁸) were selected as targets for site-directed mutagenesis. Ca²⁺ mobilization, cAMP inhibition, phosphorylation of extracellular signal-regulated kinase, and ligand binding assays were performed to functionally characterize the receptor mutants in transfected CHO cells. Unlike melatonin, isoquinolinones bearing a 3-methoxybenzyloxyl substituent were unaffected by alanine substitution at His²⁰⁸ of MT₂. Although alanine substitutions at Tyr²⁹⁴ or Tyr²⁹⁸ reduced the potency of melatonin and some isoquinolinones on MT₂, similar mutations on MT₁ allowed five hitherto ineffective isoquinolinones to act as agonists. An isoquinolinone antagonist bearing a 4-methoxybenzyloxyl moiety turned into an agonist at MT₂ mutants with alanine substitutions at His²⁰⁸, Tyr²⁹⁴ or Tyr²⁹⁸. A subset of residues is apparently involved in forming a hydrophobic binding cavity to confer selectivity upon the aromatic substituent of isoquinolinone compounds. Two conserved tyrosine residues on transmembrane helix 7 may confer ligand selectivity at MT₁ and MT₂ receptors, while a conserved histidine on transmembrane helix 5 is apparently involved in receptor activation.

Melatonin MT1 and MT2 receptor ERK signaling is differentially dependent on Gi/o and Gq/11 proteins

G protein-coupled receptors (GPCRs) transmit extracellular signals into cells by activating G protein- and β-arrestin-dependent pathways. Extracellular signal-regulated kinases (ERKs) play a central role in integrating these different linear inputs coming from a variety of GPCRs to regulate cellular functions. Here, we investigated human melatonin MT₁ and MT₂ receptors signaling through the ERK1/2 cascade by employing different biochemical techniques together with pharmacological inhibitors and siRNA molecules. We show that ERK1/2 activation by both receptors is exclusively G protein-dependent, without any participation of β-arrestin1/2 in HEK293 cells. ERK1/2 activation by MT₁ is only mediated though G_i/o proteins, while MT₂ is dependent on the cooperative activation of G_i/o and G_q/11 proteins. In the absence of G_q/11 proteins, however, MT₂ -induced ERK1/2 activation switches to a β-arrestin1/2-dependent mode. The signaling cascade downstream of G proteins is the same for both receptors and involves activation of the PI3K/PKCζ/c-Raf/MEK/ERK cascade. The differential G protein dependency of MT₁ - and MT₂ -mediated ERK activation was confirmed at the level of EGR1 and FOS gene expression, two ERK1/2 target genes. G_i/o /G_q/11 cooperativity was also observed in Neuroscreen-1 cells expressing endogenous MT₂ , whereas in the mouse retina, where MT₂ is engaged into MT₁ /MT₂ heterodimers, ERK1/2 signaling is exclusively G_i/o -dependent. Collectively, our data reveal differential signaling modes of MT₁ and MT₂ in terms of ERK1/2 activation, with an unexpected G_i/o /G_q/11 cooperativity exclusively for MT₂ . The plasticity of ERK activation by MT₂ is highlighted by the switch to a β-arrestin1/2-dependent mode in the absence of G_q/11 proteins and by the switch to a G_i/o mode when engaged into MT₁ /MT₂ heterodimers, revealing a new mechanism underlying tissue-specific responses to melatonin.

Cardioprotective Role of Melatonin in Acute Myocardial Infarction

Melatonin is a pleiotropic, indole secreted, and synthesized by the human pineal gland. Melatonin has biological effects including anti-apoptosis, protecting mitochondria, anti-oxidation, anti-inflammation, and stimulating target cells to secrete cytokines. Its protective effect on cardiomyocytes in acute myocardial infarction (AMI) has caused widespread interest in the actions of this molecule. The effects of melatonin against oxidative stress, promoting autophagic repair of cells, regulating immune and inflammatory responses, enhancing mitochondrial function, and relieving endoplasmic reticulum stress, play crucial roles in protecting cardiomyocytes from infarction. Mitochondrial apoptosis and dysfunction are common occurrence in cardiomyocyte injury after myocardial infarction. This review focuses on the targets of melatonin in protecting cardiomyocytes in AMI, the main molecular signaling pathways that melatonin influences in its endogenous protective role in myocardial infarction, and the developmental prospect of melatonin in myocardial infarction treatment.

Melatonin: An atypical hormone with major functions in the regulation of angiogenesis

Melatonin (N-acetyl-5-methoxytryptamine), a pleotropic molecule with a wide distribution, has received considerable attention in recent years, mostly because of its various major effects on tissues or cells since it has both receptor-dependent and receptor-independent actions over a wide range of concentrations. These biological and physiological functions of melatonin include regulation of circadian rhythms by modulating the expression of core oscillator genes, scavenging the reactive oxygen species and reactive nitrogen species, modulating the immune system and inflammatory response, and exerting cytoprotective and antiapoptotic effects. Given the multiple critical roles of melatonin, dysregulation of its production or any disruption in signaling through its receptors may have contributed in the development of a wide range of disorders including type 2 diabetes, aging, immune-mediated diseases, hypertension, and cancer. Herein, we focus on the modulatory effects of melatonin on angiogenesis and its implications as a therapeutic strategy in cancer and related diseases.

How Sure Can We Be about ML Methods-Based Evaluation of Compound Activity: Incorporation of Information about Prediction Uncertainty Using Deep Learning Techniques

A great variety of computational approaches support drug design processes, helping in selection of new potentially active compounds, and optimization of their physicochemical and ADMET properties. Machine learning is a group of methods that are able to evaluate in relatively short time enormous amounts of data. However, the quality of machine-learning-based prediction depends on the data supplied for model training. In this study, we used deep neural networks for the task of compound activity prediction and developed dropout-based approaches for estimating prediction uncertainty. Several types of analyses were performed: the relationships between the prediction error, similarity to the training set, prediction uncertainty, number and standard deviation of activity values were examined. It was tested whether incorporation of information about prediction uncertainty influences compounds ranking based on predicted activity and prediction uncertainty was used to search for the potential errors in the ChEMBL database. The obtained outcome indicates that incorporation of information about uncertainty of compound activity prediction can be of great help during virtual screening experiments.

Melatonin receptor agonist protects against acute lung injury induced by ventilator through up-regulation of IL-10 production

Background

It is well known that ventilation with high volume or pressure may damage healthy lungs or worsen injured lungs. Melatonin has been reported to be effective in animal models of acute lung injury. Melatonin exerts its beneficial effects by acting as a direct antioxidant and via melatonin receptor activation. However, it is not clear whether melatonin receptor agonist has a protective effect in ventilator-induced lung injury (VILI). Therefore, in this study, we determined whether ramelteon (a melatonin receptor agonist) can attenuate VILI and explore the possible mechanism for protection.

Methods

VILI was induced by high tidal volume ventilation in a rat model. The rats were randomly allotted into the following groups: control, control+melatonin, control+ramelteon, control+luzindole, VILI, VILI+luzindole, VILI + melatonin, VILI + melatonin + luzindole (melatonin receptor antagonist), VILI + ramelteon, and VILI + ramelteon + luzindole (n = 6 per group). The role of interleukin-10 (IL-10) in the melatonin- or ramelteon-mediated protection against VILI was also investigated.

Results

Ramelteon treatment markedly reduced lung edema, serum malondialdehyde levels, the concentration of inflammatory cytokines in bronchoalveolar lavage fluid (BALF), NF-κB activation, iNOS levels, and apoptosis in the lung tissue. Additionally, ramelteon treatment significantly increased heat shock protein 70 expression in the lung tissue and IL-10 levels in BALF. The protective effect of ramelteon was mitigated by the administration of luzindole or an anti-IL-10 antibody.

Conclusions

Our results suggest that a melatonin receptor agonist has a protective effect against VILI, and its protective mechanism is based on the upregulation of IL-10 production.

Structure-based discovery of potent and selective melatonin receptor agonists

Melatonin receptors MT1 and MT2 are involved in synchronizing circadian rhythms and are important targets for treating sleep and mood disorders, type-2 diabetes and cancer. Here, we performed large scale structure-based virtual screening for new ligand chemotypes using recently solved high-resolution 3D crystal structures of agonist-bound MT receptors. Experimental testing of 62 screening candidates yielded the discovery of 10 new agonist chemotypes with sub-micromolar potency at MT receptors, with compound 21 reaching EC50 of 0.36 nM. Six of these molecules displayed selectivity for MT2 over MT1. Moreover, two most potent agonists, including 21 and a close derivative of melatonin, 28, had dramatically reduced arrestin recruitment at MT2, while compound 37 was devoid of Gi signaling at MT1, implying biased signaling. This study validates the suitability of the agonist-bound orthosteric pocket in the MT receptor structures for the structure-based discovery of selective agonists.

Increased plasma melatonin in presymptomatic Huntington disease sheep (Ovis aries): Compensatory neuroprotection in a neurodegenerative disease?

Melatonin is a pleiotrophic hormone, synthesised primarily by the pineal gland under the control of the suprachiasmatic nuclei (SCN). It not only provides a hormonal signal of darkness but also has neuroprotective properties. Huntington's disease (HD) is a progressive neurodegenerative disorder characterised by abnormal motor, cognitive and psychiatric symptoms. There is growing evidence, particularly from animal models, that circadian rhythms may also be disturbed in HD. We measured two circadian-regulated hormones, melatonin and cortisol, in plasma samples collected around-the-clock from normal and presymptomatic transgenic HD sheep (Ovis aries) at 5 and 7 years of age, to assess SCN-driven rhythms and the effect of genotype, sex and age. Melatonin-related precursors and metabolites (tryptophan, serotonin, kynurenine) were also measured by liquid chromatography (LC)-mass spectrometry (MS). At 5 years of age in both rams and ewes, plasma melatonin levels were significantly elevated in HD sheep. In ewes measured 2 years later, there was still a significant elevation of nocturnal melatonin. Furthermore, the daytime baseline levels of melatonin were significantly higher in HD sheep. Since increased melatonin could have global beneficial effects on brain function, we suggest that the increased melatonin measured in presymptomatic HD sheep is part of an autoprotective response to mutant huntingtin toxicity that may account, at least in part, for the late onset of disease that characterises HD.