Modulation of blood glucose by melatonin: a direct action on melatonin receptors in mouse hepatocytes

Therapeutic applications of melatonin in disorders related to the gastrointestinal tract and control of appetite

Most animals have large amounts of the special substance melatonin, which is controlled by the light/dark cycle in the suprachiasmatic nucleus. According to what is now understood, the gastrointestinal tract (GIT) and other areas of the body are sites of melatonin production. According to recent studies, the GIT and adjacent organs depend critically on a massive amount of melatonin. Not unexpectedly, melatonin's many biological properties, such as its antioxidant, anti-inflammatory, pro-apoptotic, anti-proliferative, anti-metastasis, and antiangiogenic properties, have drawn the attention of researchers more and more. Because melatonin is an antioxidant, it produces a lot of secretions in the GIT's mucus and saliva, which shields cells from damage and promotes the development of certain GIT-related disorders. Melatonin's ability to alter cellular behavior in the GIT and other associated organs, such as the liver and pancreas, is another way that it functions. This behavior alters the secretory and metabolic activities of these cells. In this review, we attempted to shed fresh light on the many roles that melatonin plays in the various regions of the gastrointestinal tract by focusing on its activities for the first time.

Circadian desynchrony and glucose metabolism

The circadian timing system controls glucose metabolism in a time-of-day dependent manner. In mammals, the circadian timing system consists of the main central clock in the bilateral suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks in peripheral tissues. The oscillations produced by these different clocks with a period of approximately 24-h are generated by the transcriptional-translational feedback loops of a set of core clock genes. Glucose homeostasis is one of the daily rhythms controlled by this circadian timing system. The central pacemaker in the SCN controls glucose homeostasis through its neural projections to hypothalamic hubs that are in control of feeding behavior and energy metabolism. Using hormones such as adrenal glucocorticoids and melatonin and the autonomic nervous system, the SCN modulates critical processes such as glucose production and insulin sensitivity. Peripheral clocks in tissues, such as the liver, muscle, and adipose tissue serve to enhance and sustain these SCN signals. In the optimal situation all these clocks are synchronized and aligned with behavior and the environmental light/dark cycle. A negative impact on glucose metabolism becomes apparent when the internal timing system becomes disturbed, also known as circadian desynchrony or circadian misalignment. Circadian desynchrony may occur at several levels, as the mistiming of light exposure or sleep will especially affect the central clock, whereas mistiming of food intake or physical activity will especially involve the peripheral clocks. In this review, we will summarize the literature investigating the impact of circadian desynchrony on glucose metabolism and how it may result in the development of insulin resistance. In addition, we will discuss potential strategies aimed at reinstating circadian synchrony to improve insulin sensitivity and contribute to the prevention of type 2 diabetes.

Maternal nutrient metabolism in the liver during pregnancy

The liver plays pivotal roles in nutrient metabolism, and correct hepatic adaptations are required in maternal nutrient metabolism during pregnancy. In this review, hepatic nutrient metabolism, including glucose metabolism, lipid and cholesterol metabolism, and protein and amino acid metabolism, is first addressed. In addition, recent progress on maternal hepatic adaptations in nutrient metabolism during pregnancy is discussed. Finally, the factors that regulate hepatic nutrient metabolism during pregnancy are highlighted, and the factors include follicle-stimulating hormone, estrogen, progesterone, insulin-like growth factor 1, prostaglandins fibroblast growth factor 21, serotonin, growth hormone, adrenocorticotropic hormone, prolactin, thyroid stimulating hormone, melatonin, adrenal hormone, leptin, glucagon-like peptide-1, insulin glucagon and thyroid hormone. Our vision is that more attention should be paid to liver nutrient metabolism during pregnancy, which will be helpful for utilizing nutrient appropriately and efficiently, and avoiding liver diseases during pregnancy.

Comparative metabolomics analysis investigating the impact of melatonin-enriched diet on energy metabolism in the crayfish, Cherax destructor

Melatonin is a multifunctional bioactive molecule present in almost all organisms and has been gradually used in the aquaculture industry in recent years. Energy metabolism is an essential process for individuals to maintain their life activities; however, the process through which melatonin regulates energy metabolism in aquatic animals remains unclear. The present study aimed to conduct a comprehensive analysis of the regulatory mechanism of melatonin for energy metabolism in Cherax destructor by combining metabolomics analysis with the detection of the key substance content, enzymatic activity, and gene expression levels in the energy metabolism process after culturing with dietary melatonin supplementation for 8 weeks. Our results showed that dietary melatonin increased the content of glycogen, triglycerides, and free fatty acids; decreased lactate levels; and promoted the enzymatic activity of pyruvate kinase (PK), malate dehydrogenase (MDH), and acetyl-CoA carboxylase. The results of gene expression analysis showed that dietary melatonin also increased the expression levels of hexokinase, PK, MDH, lactate dehydrogenase, lipase, and fatty acid synthase genes. The results of metabolomics analysis showed that differentially expressed metabolites were significantly enriched in lysine degradation and glycerophospholipid metabolism. In conclusion, our study demonstrates that dietary melatonin increased oxidative phosphorylation, improved glucose utilization, and promoted storage of glycogen and lipids in C. destructor. These lipids are used not only for energy storage but also to maintain the structure and function of cell membranes. Our results further add to the understanding of the mechanisms of energy regulation by melatonin in crustaceans.

Maternal hepatic immunology during pregnancy

The liver plays pivotal roles in immunologic responses, and correct hepatic adaptations in maternal immunology are required during pregnancy. In this review, we focus on anatomical and immunological maternal hepatic adaptations during pregnancy, including our recent reports in this area. Moreover, we summarize maternal pregnancy-associated liver diseases, including hyperemesis gravidarum; intrahepatic cholestasis of pregnancy; preeclampsia, specifically hemolysis, elevated liver enzymes, and low platelet count syndrome; and acute fatty liver of pregnancy. In addition, the latest information about the factors that regulate hepatic immunology during pregnancy are reviewed for the first time, including human chorionic gonadotropin, estrogen, progesterone, growth hormone, insulin like growth factor 1, oxytocin, adrenocorticotropic hormone, adrenal hormone, prolactin, melatonin and prostaglandins. In summary, the latest progress on maternal hepatic anatomy and immunological adaptations, maternal pregnancy-associated diseases and the factors that regulate hepatic immunology during pregnancy are discussed, which may be used to prevent embryo loss and abortion, as well as pregnancy-associated liver diseases.

Experimental Data on the Role of Melatonin in the Pathogenesis of Nonalcoholic Fatty Liver Disease

Despite the increasing prevalence of nonalcoholic fatty liver disease (NAFLD) worldwide, its complex pathogenesis remains incompletely understood. The currently stated hypotheses cannot fully clarify the interrelationships between individual pathogenetic mechanisms of the disease. No appropriate health strategies have been developed for treating NAFLD. NAFLD is characterized by an accumulation of triglycerides in hepatic cells (steatosis), with the advanced form known as nonalcoholic steatohepatitis. In the latter, superimposed inflammation can lead to fibrosis. There are scientific data on NAFLD's association with components of metabolic syndrome. Hormonal factors are thought to play a role in the development of metabolic syndrome. Endogenous melatonin, an indoleamine hormone synthesized by the pineal gland mainly at night, is a powerful chronobiotic that probably regulates metabolic processes and has antioxidant, anti-inflammatory, and genomic effects. Extrapineal melatonin has been found in various tissues and organs, including the liver, pancreas, and gastrointestinal tract, where it likely maintains cellular homeostasis. Melatonin exerts its effects on NAFLD at the cellular, subcellular, and molecular levels, affecting numerous signaling pathways. In this review article, we discuss the experimental scientific data accumulated on the involvement of melatonin in the intimate processes of the pathogenesis of NAFLD.

A novel combination of sitagliptin and melatonin ameliorates T2D manifestations: studies on experimental diabetic models

INTRODUCTION

Type 2 diabetes (T2D) is an endocrine disorder characterized by hyperglycemia, insulin resistance, dysregulated glucose and lipid metabolism, reduced pancreatic β-cell function and mass, and a reduced incretin effect. Circadian rhythm disruption is associated with increased T2D risk. We have investigated the therapeutic potential of a combination of melatonin (M) and sitagliptin (S), a dipeptidyl peptidase IV (DPP-IV) inhibitor, in the amelioration of T2D manifestations in high-fat diet (HFD) induced T2D mouse model and also on β-cell proliferation under gluco-lipotoxicity stress in vitro.

METHODS

For in vivo study, mice were fed with HFD for 25 weeks to induce T2D and were treated with monotherapies and S + M for four weeks. For the in vitro study, primary mouse islets were exposed to normal glucose and high glucose + palmitate to induce gluco-lipotoxic stress.

RESULTS

Our results suggest that monotherapies and S + M improve metabolic parameters and glyco-lipid metabolism in the liver and adipose tissue, respectively, and improve mitochondrial function in the skeletal muscle. Moreover, it increases peripheral insulin sensitivity. Our in vitro and in vivo studies suggest that β-cell mass was preserved in all the drug-treated groups.

CONCLUSION

The combination treatment is superior to monotherapies in the management of T2D.

The role of melatonin implantation and the associated effect of insulin-like growth factor-1 on milk composition of Barki ewes

ContextManipulation of milk composition in lactating ewes represents an important mechanism by which sheep breeders can improve profits through increased milk prices for cheese making and higher lamb growth rates. To this end, melatonin implantation may affect milk composition by alleviating reproductive seasonality and augmenting insulin like growth factor-1 (IGF-1).AimsThis research aimed to evaluate the effects of melatonin administration on hormone levels and milk composition, and to determine its potential to manipulate milk composition of lactating Barki ewes.MethodsFifty lactating Barki ewes were divided into two groups of 25. One group received 18mg melatonin in a single implant at 40 days after delivery, and the other group served as an untreated control. Over the following 8 weeks, milk components (fat, protein, lactose and total solids percentages), milk energy value, and blood serum hormones including IGF-1 were measured.Key resultsMelatonin implantation increased (P<0.05) IGF-1 concentration, which showed continued elevation (P<0.05) throughout the treatment period (from Day14 to Day56) relative to the control. Analysis of milk composition of implanted ewes revealed higher (P<0.05) milk fat, protein, lactose and total solids percentages as well as milk energy value than in milk of control ewes.ConclusionAn integrated effect of melatonin and IGF-1 successfully improved milk composition of lactating Barki ewes.ImplicationsMelatonin implantation can be used to improve milk composition of lactating Barki ewes, potentially increasing profits from cheese making and lamb production.

Acute metabolic effects of melatonin-A randomized crossover study in healthy young men

Melatonin regulates circadian rhythm, but may also have effects on glucose homeostasis. A common G-allele in the MTNR1B locus has been associated with an increased risk of type 2 diabetes (T2DM). We aimed to examine acute effects of high doses of melatonin on glucose metabolism with attention to MTNR1B genotype. Twenty men were examined in a double-blinded, randomized crossover study on two nonconsecutive days with four doses of 10 mg oral melatonin or placebo. Insulin sensitivity and insulin secretion were assessed by an intravenous glucose tolerance test (IVGTT) and a hyperinsulinaemic-euglycaemic clamp (HEC). Blood samples were drawn to determine the metabolic profile and MTNR1B rs10830963 genotype. Indirect calorimetry and blood pressure measurements were also performed. Insulin sensitivity index was significantly reduced on the melatonin day (P = .028) in the whole group and in homozygous carriers of the rs10830963 C-allele (P = .041). Glucose during the IVGTT was unaffected, but there was a tendency towards lower insulin and C-peptide levels in the first minutes after glucose administration in G-allele carriers. Systolic blood pressure decreased and lipid oxidation increased significantly on the melatonin day in rs10830963 G-allele carriers. Overall, our study reports that acute administration of melatonin in supra-physiological doses may have a negative impact on insulin sensitivity. Clinical trial registration number (clinicaltrial.gov): NCT03204877.

Melatonin and circadian rhythms in liver diseases: Functional roles and potential therapies

Circadian rhythms and clock gene expressions are regulated by the suprachiasmatic nucleus in the hypothalamus, and melatonin is produced in the pineal gland. Although the brain detects the light through retinas and regulates rhythms and melatonin secretion throughout the body, the liver has independent circadian rhythms and expressions as well as melatonin production. Previous studies indicate the association between circadian rhythms with various liver diseases, and disruption of rhythms or clock gene expression may promote liver steatosis, inflammation, or cancer development. It is well known that melatonin has strong antioxidant effects. Alcohol drinking or excess fatty acid accumulation produces reactive oxygen species and oxidative stress in the liver leading to liver injuries. Melatonin administration protects these oxidative stress-induced liver damage and improves liver conditions. Recent studies have demonstrated that melatonin administration is not limited to antioxidant effects and it has various other effects contributing to the management of liver conditions. Accumulating evidence suggests that restoring circadian rhythms or expressions as well as melatonin supplementation may be promising therapeutic strategies for liver diseases. This review summarizes recent findings for the functional roles and therapeutic potentials of circadian rhythms and melatonin in liver diseases.

Melatonin Orchestrates Lipid Homeostasis through the Hepatointestinal Circadian Clock and Microbiota during Constant Light Exposure

Misalignment between natural light rhythm and modern life activities induces disruption of the circadian rhythm. It is mainly evident that light at night (LAN) interferes with the human endocrine system and contributes to the increasing rates of obesity and lipid metabolic disease. Maintaining hepatointestinal circadian homeostasis is vital for improving lipid homeostasis. Melatonin is a chronobiotic substance that plays a main role in stabilizing bodily rhythm and has shown beneficial effects in protecting against obesity. Based on the dual effect of circadian rhythm regulation and antiobesity, we tested the effect of melatonin in mice under constant light exposure. Exposure to 24-h constant light (LL) increased weight and insulin resistance compared with those of the control group (12-h light–12-h dark cycle, LD), and simultaneous supplementation in the melatonin group (LLM) ameliorated this phenotype. Constant light exposure disturbed the expression pattern of a series of transcripts, including lipid metabolism, circadian regulation and nuclear receptors in the liver. Melatonin also showed beneficial effects in improving lipid metabolism and circadian rhythm homeostasis. Furthermore, the LL group had increased absorption and digestion of lipids in the intestine as evidenced by the elevated influx of lipids in the duodenum and decrease in the efflux of lipids in the jejunum. More interestingly, melatonin ameliorated the gut microbiota dysbiosis and improved lipid efflux from the intestine. Thus, these findings offer a novel clue regarding the obesity-promoting effect attributed to LAN and suggest a possibility for obesity therapy by melatonin in which melatonin could ameliorate rhythm disorder and intestinal dysbiosis.

Circadian Clock and Sirtuins in Diabetic Lung: A Mechanistic Perspective

Diabetes-induced tissue injuries in target organs such as the kidney, heart, eye, liver, skin, and nervous system contribute significantly to the morbidity and mortality of diabetes. However, whether the lung should be considered a diabetic target organ has been discussed for decades. Accumulating evidence shows that both pulmonary histological changes and functional abnormalities have been observed in diabetic patients, suggesting that the lung is a diabetic target organ. Mechanisms underlying diabetic lung are unclear, however, oxidative stress, systemic inflammation, and premature aging convincingly contribute to them. Circadian system and Sirtuins have been well-documented to play important roles in above mechanisms. Circadian rhythms are intrinsic mammalian biological oscillations with a period of near 24 h driven by the circadian clock system. This system plays an important role in the regulation of energy metabolism, oxidative stress, inflammation, cellular proliferation and senescence, thus impacting metabolism-related diseases, chronic airway diseases and cancers. Sirtuins, a family of adenine dinucleotide (NAD⁺)-dependent histone deacetylases, have been demonstrated to regulate a series of physiological processes and affect diseases such as obesity, insulin resistance, type 2 diabetes (T2DM), heart disease, cancer, and aging. In this review, we summarize recent advances in the understanding of the roles of the circadian clock and Sirtuins in regulating cellular processes and highlight the potential interactions of the circadian clock and Sirtuins in the context of diabetic lung.

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

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

Melatonin in type 2 diabetes mellitus and obesity

Despite considerable advances in the past few years, obesity and type 2 diabetes mellitus (T2DM) remain two major challenges for public health systems globally. In the past 9 years, genome-wide association studies (GWAS) have established a major role for genetic variation within the MTNR1B locus in regulating fasting plasma levels of glucose and in affecting the risk of T2DM. This discovery generated a major interest in the melatonergic system, in particular the melatonin MT₂ receptor (which is encoded by MTNR1B). In this Review, we discuss the effect of melatonin and its receptors on glucose homeostasis, obesity and T2DM. Preclinical and clinical post-GWAS evidence of frequent and rare variants of the MTNR1B locus confirmed its importance in regulating glucose homeostasis and T2DM risk with minor effects on obesity. However, these studies did not solve the question of whether melatonin is beneficial or detrimental, an issue that will be discussed in the context of the peculiarities of the melatonergic system. Melatonin receptors might have therapeutic potential as they belong to the highly druggable G protein-coupled receptor superfamily. Clarifying the precise role of melatonin and its receptors on glucose homeostasis is urgent, as melatonin is widely used for other indications, either as a prescribed medication or as a supplement without medical prescription, in many countries in Europe and in the USA.

Adverse events associated with oral administration of melatonin: A critical systematic review of clinical evidence

While melatonin was once thought of simply as a sleep-inducing hormone, recent research has resulted in development of a deeper understanding of the complex physiological activity of melatonin in the human body. Along with this understanding has come widespread, increasing use of melatonin supplementation, extending beyond its traditional use as a sleep aid into novel fields of application. This increased use often involves off-label and self-prescription, escalating the importance of safety data. In order to examine the current knowledge relating to safety of the exogenous neurohormone, we conducted a comprehensive, critical systematic review of clinical evidence. We examined controlled studies of oral melatonin supplementation in humans when they presented any statistical analysis of adverse events. Of the fifty articles identified, twenty-six found no statistically significant adverse events, while twenty-four articles reported on at least one statistically significant adverse event. Adverse events were generally minor, short-lived and easily managed, with the most commonly reported adverse events relating to fatigue, mood, or psychomotor and neurocognitive performance. A few studies noted adverse events relating to endocrine (e.g. reproductive parameters, glucose metabolism) and cardiovascular (e.g. blood pressure, heart rate) function, which appear to be influenced by dosage, dose timing and potential interactions with antihypertensive drugs. Oral melatonin supplementation in humans has a generally favourable safety profile with some exceptions. Most adverse effects can likely be easily avoided or managed by dosing in accordance with natural circadian rhythms. Further research is required to explore the potential for melatonin to interact with endogenous hormones and pharmaceuticals.

Potential therapeutic role of melatonin on STZ-induced diabetic central neuropathy: A biochemical, histopathological, immunohistochemical and ultrastructural study

The aim of the present study was to assess the therapeutic potential of melatonin (Mel) in diabetic central neuropathy in a rat model of streptozotocin (STZ)-induced diabetes. The rats were injected with 60 mg/kg STZ and diabetes was confirmed by blood glucose levels (BGL) ≥ 250 mg/dL. Mel treatment (50 mg/kg) was started 72 h before the STZ injection and continued for 45 days. In addition, normal control, vehicle (5% ethanol) control, and Mel-treated non-diabetic control were also included. STZ induced a diabetic phenotype with persistent hyperglycemia and elevated oxidative stress in the brain, liver, and kidneys compared to the control groups. In addition, the diabetic rats showed severe β-cell necrosis with reduced insulin levels, cerebral neuronopathy, myelinopathy, axonopathy, microglial and astroglial activation, and vascular damage. While Mel treatment did not prevent the development of STZ-induced diabetes mellitus and had no significant effect on the BGLs of the diabetic rats, it significantly ameliorated the diabetes-induced oxidative stress and neurodegeneration. Taken together, Mel showed potent therapeutic effects against the neurological complications of hyperglycemia and therefore can be used to treat diabetic neuropathy.

Circadian dysrhythmia-linked diabetes mellitus: Examining melatonin’s roles in prophylaxis and management

Diabetes mellitus is a chronic, life-threatening metabolic disorder that occurs worldwide. Despite an increase in the knowledge of the risk factors that are associated with diabetes mellitus, its worldwide prevalence has continued to rise; thus, necessitating more research into its aetiology. Recent researches are beginning to link a dysregulation of the circadian rhythm to impairment of intermediary metabolism; with evidences that circadian rhythm dysfunction might play an important role in the aetiology, course or prognosis of some cases of diabetes mellitus. These evidences thereby suggest possible relationships between the circadian rhythm regulator melatonin, and diabetes mellitus. In this review, we discuss the roles of the circadian rhythm in the regulation of the metabolism of carbohydrates and other macronutrients; with emphasis on the importance of melatonin and the impacts of its deficiency on carbohydrate homeostasis. Also, the possibility of using melatonin and its analogs for the “prophylaxis” or management of diabetes mellitus is also considered.

Melatonin levels in human diabetic dental pulp tissue and its effects on dental pulp cells under hyperglycaemic conditions

AIM

To investigate melatonin (MEL) levels in human dental pulp tissue (hDP) in type 2 diabetic (T2D) participants and the underlying molecular mechanisms of its effects in human dental pulp cells (hDPCs) under hyperglycaemia.

METHODOLOGY

The study included 16 healthy and 16 T2D participants who underwent vital pulp extirpation for hDP and four healthy participants undergoing third molar extraction for hDPCs analyses. MTT and NRU were used as tests for cytotoxicity. The pulp tissue levels of MEL, inducible NO synthase (iNOS) and superoxide dismutase (SOD) activity, as well as iNOS, histone acetyltransferase p300 (p300) and SOD activity levels in hDPCs incubated with MEL (0.1 and 1.0 mmol L^-1 ) under normoglycaemia and hyperglycaemia were measured by enzyme-linked immunosorbent assay. Comparisons between the two groups were made by unpaired t-tests or Mann-Whitney test whilst the chi-square test was used for dichotomous variables. To compare more groups, the Kruskal-Wallis test with Dunn's multiple comparison was used, whilst Spearman correlation was used to assess association between two variables.

RESULTS

Melatonin was decreased (124.30 ± 21.6 vs. 240.0 ± 19.1 pg mL^-1 , P < 0.01), whilst iNOS levels increased (0.92 ± 0.08 vs. 0.32 ± 0.09 ng mL^-1 , P < 0.01) in hDP from T2D compared to nondiabetic participants. In hDPCs, MEL (0.1 and 1.0 mmol L^-1 ) had no cytotoxicity. Incubation with 1.0 mmol L^-1 of MEL (24 h) decreased hyperglycaemia-induced increases of iNOS (0.34 ± 0.01 ng mL^-1 vs. 0.40 ± 0.01 ng mL^-1 , P < 0.01) and p300 (11.59 ± 0.58 ng mL^-1 vs. 16.12 ± 0.39 ng mL^-1 , P < 0.01), and also, increased SOD activity (87.11 ± 3.10% vs. 68.56 ± 3.77%, P < 0.01) to the levels comparable to the normoglycaemic; iNOS and p300 protein expression levels showed strong positive correlation under hyperglycaemia (Spearman r = 0.8242, P < 0.001).

CONCLUSION

Type 2 diabetic participants had decreased MEL in hDP. At pharmacological concentrations, MEL is not cytotoxic for hDPCs and normalizes iNOS and SOD activity levels in hyperglyceamic hDPCs suggesting its antioxidant and protective effects in human dental pulp tissue under hyperglycaemia.

Chronomedicine and type 2 diabetes: shining some light on melatonin

In mammals, the circadian timing system drives rhythms of physiology and behaviour, including the daily rhythms of feeding and activity. The timing system coordinates temporal variation in the biochemical landscape with changes in nutrient intake in order to optimise energy balance and maintain metabolic homeostasis. Circadian disruption (e.g. as a result of shift work or jet lag) can disturb this continuity and increase the risk of cardiometabolic disease. Obesity and metabolic disease can also disturb the timing and amplitude of the clock in multiple organ systems, further exacerbating disease progression. As our understanding of the synergy between the timing system and metabolism has grown, an interest has emerged in the development of novel clock-targeting pharmaceuticals or nutraceuticals for the treatment of metabolic dysfunction. Recently, the pineal hormone melatonin has received some attention as a potential chronotherapeutic drug for metabolic disease. Melatonin is well known for its sleep-promoting effects and putative activity as a chronobiotic drug, stimulating coordination of biochemical oscillations through targeting the internal timing system. Melatonin affects the insulin secretory activity of the pancreatic beta cell, hepatic glucose metabolism and insulin sensitivity. Individuals with type 2 diabetes mellitus have lower night-time serum melatonin levels and increased risk of comorbid sleep disturbances compared with healthy individuals. Further, reduced melatonin levels, and mutations and/or genetic polymorphisms of the melatonin receptors are associated with an increased risk of developing type 2 diabetes. Herein we review our understanding of molecular clock control of glucose homeostasis, detail the influence of circadian disruption on glucose metabolism in critical peripheral tissues, explore the contribution of melatonin signalling to the aetiology of type 2 diabetes, and discuss the pros and cons of melatonin chronopharmacotherapy in disease management.

Decreased concentration of serum melatonin in nighttime compared with daytime female medical technologists in South Korea

Working during the night can disrupt the normal circadian rhythm by altering the melatonin level. A low level of melatonin is associated with an increased risk of cancer, possibly by decreasing the expression of tumor-suppressor genes, such as p53. To determine whether nighttime work is associated with melatonin level in serum as well as the expression of related genetic markers, we enrolled 100 female nighttime medical technologists employed at a hospital in South Korea. Melatonin concentration and melatonin receptor 1 (MT1) expression were significantly lower in nighttime than in daytime workers (1.84 pg/mL versus 4.04 pg/mL; 1.16 versus 1.61, respectively). However, p53 expression showed no difference between the groups. In summary, nighttime work could be an important risk factor for circadian disruption, but not a direct risk factor for cancer in medical technologists in South Korea.

Beneficial effects of melatonin and alpha lipoic acid on lopinavir/ ritonavir-induced alterations in serum lipid and glucose levels of male albino rats

The use of lopinavir/ritonavir (LPV/r) has been associated with alterations in serum lipid and glucose levels. This study was designed to investigate the effects of melatonin (MT) and alpha lipoic acid (ALA) on LPV/r-induced changes in serum levels of triglycerides (TG), total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), very low density lipoprotein cholesterol (VLDL-C) and glucose (GL) levels in male albino rats. Rats in group A1 (placebo control) and group A2 (solvent control) were orally treated with normal saline and 1% ethanol respectively. Groups B-F contained 15 rats each which were divided into 3 groups of 5 rats each. Rats in group B were orally treated with MT (10 mg kg-1/day), ALA (10 mg kg-1/day) and a combination of MT and ALA, accordingly. Rats in group C were orally treated with 22.9/5.71, 45.6/11.4 and 91.4/22.9 mg kg-1/day of LPV/r, accordingly. Rats in groups D-F were orally treated with MT (10 mg kg-1/ day), ALA (10 mg kg-1/day) and combined doses of ALA and MT before oral treatment with 22.9/5.71, 45.6/11.4 and 91.4/22.9 mg kg-1/day of LPV/r, accordingly. All rats were treated for 30 days and at the end of the drug treatment, the serum levels of lipid fractions and glucose were evaluated. Treatment with MT and ALA significantly (p<0.05) decreased baseline serum levels of TG, TC, VLDL-C, LDL-C and GL, but these parameters were significantly (p<0.05) increased in a dose-dependent manner in LPV/r-treated rats when compared to placebo control. Administration of MT and ALA prior to treatment with LPV/r significantly (p<0.05) decreased serum levels of TG, TC, VLDL-C, LDL-C and GL when compared to LPV/r-treated rats. However, decreases obtained in rats pretreated with combined doses of MT and ALA were significantly (p<0.05) different when compared to their individual doses. This study showed that MT and ALA can serve as remedies for LPV/r-induced alterations in serum lipid and glucose levels.

The role of melatonin in diabetes: therapeutic implications

Melatonin referred as the hormone of darkness is mainly secreted by pineal gland, its levels being elevated during night and low during the day. The effects of melatonin on insulin secretion are mediated through the melatonin receptors (MT1 and MT2). It decreases insulin secretion by inhibiting cAMP and cGMP pathways but activates the phospholipaseC/IP3 pathway, which mobilizes Ca2+from organelles and, consequently increases insulin secretion. Both in vivo and in vitro, insulin secretion by the pancreatic islets in a circadian manner, is due to the melatonin action on the melatonin receptors inducing a phase shift in the cells. Melatonin may be involved in the genesis of diabetes as a reduction in melatonin levels and a functional interrelationship between melatonin and insulin was observed in diabetic patients. Evidences from experimental studies proved that melatonin induces production of insulin growth factor and promotes insulin receptor tyrosine phosphorylation. The disturbance of internal circadian system induces glucose intolerance and insulin resistance, which could be restored by melatonin supplementation. Therefore, the presence of melatonin receptors on human pancreatic islets may have an impact on pharmacotherapy of type 2 diabetes.

Melatonin regulation of biliary functions

The intrahepatic biliary epithelium is a three-dimensional tubular system lined by cholangiocytes, epithelial cells that in addition to modify ductal bile are also the targets of vanishing bile duct syndromes (i.e., cholangiopathies) such as primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) that are characterized by the damage/proliferation of cholangiocytes. Cholangiocyte proliferation is critical for the maintenance of the biliary mass and secretory function during the pathogenesis of cholangiopathies. Proliferating cholangiocytes serve as a neuroendocrine compartment during the progression of cholangiopathies, and as such secrete and respond to hormones, neurotransmitters and neuropeptides contributing to the autocrine and paracrine pathways that regulate biliary homeostasis. The focus of this review is to summarize the recent findings related to the role of melatonin in the modulation of biliary functions and liver damage in response to a number of insults. We first provide a general background on the general function of cholangiocytes including their anatomic characteristics, their innervation and vascularization as well the role of these cells on secretory and proliferation events. After a background on the synthesis and regulation of melatonin and its role on the maintenance of circadian rhythm, we will describe the specific effects of melatonin on biliary functions and liver damage. After a summary of the topics discussed, we provide a paragraph on the future perspectives related to melatonin and liver functions.

Influence of melatonin receptor signalling on parameters involved in blood glucose regulation

The pineal hormone melatonin is known to influence insulin secretion via the G-protein-coupled receptor isoforms MT1 and MT2. The present study was aimed to further elucide the impact of melatonin on blood glucose regulation. To this end, mouse lines were used, in which one of the two or both melatonin receptors were deleted. In comparison with wild-type mice of the same age (8-12 months old), increased plasma insulin and melatonin levels and decreased blood glucose levels and body weights were detected in the MT1- and double-knockout lines. The elimination of melatonin receptor signalling also altered blood glucose concentrations, body weight and melatonin and insulin levels when comparing wild-type and receptor knockout mice of different ages (6 wk and 8-12 months old); such changes, however, were dependent on the type of receptor deleted. Furthermore, reverse transcription polymerase chain reaction results provided evidence that melatonin receptor deficiency has an impact on transcript levels of pancreatic islet hormones as well as on pancreatic and hepatic glucose transporters (Glut1 and 2). Under stimulated insulin secretion in the presence of melatonin in the rat insulinoma β-cells INS-1, the Glut1 transcript level was decreased. In conclusion, the present findings demonstrate that melatonin receptor knockout types affect blood glucose levels, body weight, plasma levels of melatonin and insulin, as well as pancreatic hormone and Glut1 expression in significantly different manners.

Melatonin and pancreatic islets: interrelationships between melatonin, insulin and glucagon

The pineal hormone melatonin exerts its influence in the periphery through activation of two specific trans-membrane receptors: MT1 and MT2. Both isoforms are expressed in the islet of Langerhans and are involved in the modulation of insulin secretion from β-cells and in glucagon secretion from α-cells. De-synchrony of receptor signaling may lead to the development of type 2 diabetes. This notion has recently been supported by genome-wide association studies identifying particularly the MT2 as a risk factor for this rapidly spreading metabolic disturbance. Since melatonin is secreted in a clearly diurnal fashion, it is safe to assume that it also has a diurnal impact on the blood-glucose-regulating function of the islet. This factor has hitherto been underestimated; the disruption of diurnal signaling within the islet may be one of the most important mechanisms leading to metabolic disturbances. The study of melatonin–insulin interactions in diabetic rat models has revealed an inverse relationship: an increase in melatonin levels leads to a down-regulation of insulin secretion and vice versa. Elucidation of the possible inverse interrelationship in man may open new avenues in the therapy of diabetes.

Tired of diabetes genetics? Circadian rhythms and diabetes: the MTNR1B story?

Circadian rhythms are ubiquitous in biological systems and regulate metabolic processes throughout the body. Misalliance of these circadian rhythms and the systems they regulate has a profound impact on hormone levels and increases risk of developing metabolic diseases. Melatonin, a hormone secreted by the pineal gland, is one of the major signaling molecules used by the master circadian oscillator to entrain downstream circadian rhythms. Several recent genetic studies have pointed out that a common variant in the gene that encodes the melatonin receptor 2 (MTNR1B) is associated with impaired glucose homeostasis, reduced insulin secretion, and an increased risk of developing type 2 diabetes. Here, we try to review the role of this receptor and its signaling pathways in respect to glucose homeostasis and development of the disease.

Evidence of the receptor-mediated influence of melatonin on pancreatic glucagon secretion via the Gαq protein-coupled and PI3K signaling pathways

Melatonin has been shown to modulate glucose metabolism by influencing insulin secretion. Recent investigations have also indicated a regulatory function of melatonin on the pancreatic α-cells. The present in vitro and in vivo studies evaluated whether melatonin mediates its effects via melatonin receptors and which signaling cascade is involved. Incubation experiments using the glucagon-producing mouse pancreatic α-cell line αTC1 clone 9 (αTC1.9) as well as isolated pancreatic islets of rats and mice revealed that melatonin increases glucagon secretion. Preincubation of αTC1.9 cells with the melatonin receptor antagonists luzindole and 4P-PDOT abolished the glucagon-stimulatory effect of melatonin. In addition, glucagon secretion was lower in the pancreatic islets of melatonin receptor knockout mice than in the islets of the wild-type (WT) control animals. Investigations of melatonin receptor knockout mice revealed decreased plasma glucagon concentrations and elevated mRNA expression levels of the hepatic glucagon receptor when compared to WT mice. Furthermore, studies using pertussis toxin, as well as measurements of cAMP concentrations, ruled out the involvement of Gαi- and Gαs-coupled signaling cascades in mediating the glucagon increase induced by melatonin. In contrast, inhibition of phospholipase C in αTC1.9 cells prevented the melatonin-induced effect, indicating the physiological relevance of the Gαq-coupled pathway. Our data point to the involvement of the phosphatidylinositol 3-kinase signaling cascade in mediating melatonin effects in pancreatic α-cells. In conclusion, these findings provide evidence that the glucagon-stimulatory effect of melatonin in pancreatic α-cells is melatonin receptor mediated, thus supporting the concept of melatonin-modulated and diurnal glucagon release.

Melatonin exerts by an autocrine loop antiproliferative effects in cholangiocarcinoma: its synthesis is reduced favoring cholangiocarcinoma growth

Cholangiocarcinoma (CCA) is a devastating biliary cancer. Melatonin is synthesized in the pineal gland and peripheral organs from serotonin by two enzymes, serotonin N-acetyltransferase (AANAT) and acetylserotonin O-methyltransferase (ASMT). Cholangiocytes secrete neuroendocrine factors, including serotonin-regulating CCA growth by autocrine mechanisms. Melatonin exerts its effects by interaction with melatonin receptor type 1A/1B (MT1/MT2) receptors. We propose that 1) in CCA, there is decreased expression of AANAT and ASMT and secretion of melatonin, changes that stimulate CCA growth; and 2) in vitro overexpression of AANAT decreases CCA growth. We evaluated the 1) expression of AANAT, ASMT, melatonin, and MT1/MT2 in human nonmalignant and CCA lines and control and CCA biopsy samples; 2) melatonin levels in nonmalignant and CCA lines, and bile and serum from controls and patients with intrahepatic CCA; 3) effect of melatonin on the growth and expression of AANAT/ASMT and MT1/MT2 in CCA lines implanted into nude mice; and 4) effect of AANAT overexpression on the proliferation, apoptosis, and expression of MT1/MT2 in Mz-ChA-1 cells. The expression of AANAT, ASMT, and melatonin decreased, whereas MT1/MT2 expression increased in CCA lines and biopsy samples. Melatonin secretion decreased in the supernatant of CCA lines and bile of CCA patients. Melatonin decreased xenograft CCA tumor growth in nude mice by increased AANAT/ASMT and melatonin, along with reduced MT1/MT2 expression. Overexpression of AANAT in Mz-ChA-1 cells inhibited proliferation and MT1/MT2 expression and increased apoptosis. There is dysregulation of the AANAT/ASMT/melatonin → melatonin receptor axis in CCA, which inhibited melatonin secretion and subsequently enhanced CCA growth.

Melatonin inhibits cholangiocyte hyperplasia in cholestatic rats by interaction with MT1 but not MT2 melatonin receptors

In bile duct-ligated (BDL) rats, large cholangiocytes proliferate by activation of cAMP-dependent signaling. Melatonin, which is secreted from pineal gland as well as extrapineal tissues, regulates cell mitosis by interacting with melatonin receptors (MT1 and MT2) modulating cAMP and clock genes. In the liver, melatonin suppresses oxidative damage and ameliorates fibrosis. No information exists regarding the role of melatonin in the regulation of biliary hyperplasia. We evaluated the mechanisms of action by which melatonin regulates the growth of cholangiocytes. In normal and BDL rats, we determined the hepatic distribution of MT1, MT2, and the clock genes, CLOCK, BMAL1, CRY1, and PER1. Normal and BDL (immediately after BDL) rats were treated in vivo with melatonin before evaluating 1) serum levels of melatonin, bilirubin, and transaminases; 2) intrahepatic bile duct mass (IBDM) in liver sections; and 3) the expression of MT1 and MT2, clock genes, and PKA phosphorylation. In vitro, large cholangiocytes were stimulated with melatonin in the absence/presence of luzindole (MT1/MT2 antagonist) and 4-phenyl-2-propionamidotetralin (MT2 antagonist) before evaluating cell proliferation, cAMP levels, and PKA phosphorylation. Cholangiocytes express MT1 and MT2, CLOCK, BMAL1, CRY1, and PER1 that were all upregulated following BDL. Administration of melatonin to BDL rats decreased IBDM, serum bilirubin and transaminases levels, the expression of all clock genes, cAMP levels, and PKA phosphorylation in cholangiocytes. In vitro, melatonin decreased the proliferation, cAMP levels, and PKA phosphorylation, decreases that were blocked by luzindole. Melatonin may be important in the management of biliary hyperplasia in human cholangiopathies.

Melatonin stimulates glucagon secretion in vitro and in vivo

Recent investigations have demonstrated that melatonin influences carbohydrate metabolism mediated by insulin-inhibiting effects on pancreatic β-cells. This study evaluated whether melatonin has also an effect on pancreatic α-cells and glucagon expression as well as the glucagon secretion in vitro and in vivo. Glucagon-producing pancreatic α-cell line αTC1 clone 9 (αTC1.9) was used, which was characterized as an appropriate model with glucose responsiveness and expression of the melatonin receptors MT1 and MT2. The results demonstrate that melatonin incubation significantly enhanced the expression as well as the secretion of glucagon. These effects appeared to be more pronounced under hyperglycemic conditions compared to basal glucose concentrations. Notably, in vivo studies demonstrated that long-term oral melatonin administration led to significantly elevated plasma glucagon concentrations in Wistar rats. In contrast, plasma glucagon levels were found to be slightly decreased in type 2 diabetic Goto-Kakizaki rats. Moreover, investigations measuring the relative glucagon receptor mRNA expression showed marked differences in the liver of melatonin-substituted rats as well as in melatonin receptor knockout mice. In conclusion, these findings revealed evidence that melatonin influences pancreatic glucagon expression and secretion as well as the peripheral glucagon action.

Hepatoprotective actions of melatonin: Possible mediation by melatonin receptors

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

Effects of Melatonin on Adrenal Cortical Functions of Indian Goats under Thermal Stress

The study was conducted with the primary objective to establish the influence of melatonin on adrenocortical functions to ameliorate thermal stress in goats. Endocrine secretions and several other blood biochemical parameters reflecting the animals adrenocortical stress response were determined over a one-week period after goats had been exposed to 40 degrees C and 60%RH for 10 days. The study was conducted for a period of 17 days in psychrometric chamber. The animals served as self-controls prior to start of the experiment. Blood samples were drawn on day 10 to establish effect of thermal stress. Chemical adrenalectomy was achieved using metyrapone followed by exogenous melatonin treatment. 40 degrees C of thermal stress which is quite normal in tropical zone significantly (P Collapse

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Affiliation(s)

Veerasamy Sejian Neurophysiology Laboratory, Division of Physiology and Climatology, Indian Veterinary Research Institute, Izatnagar, Bareilly 243122, India Scientist, Adaptation Physiology Laboratory, Division of Physiology & Biochemistry, Central Sheep & Wool Research Institute, Avikanagar, Via-Jaipur, Rajasthan 304501, India
Rajendra Swaroop Srivastava Neurophysiology Laboratory, Division of Physiology and Climatology, Indian Veterinary Research Institute, Izatnagar, Bareilly 243122, India

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The daily rhythms of melatonin and free fatty acids in goats under varying photoperiods and constant darkness

The purpose of the study was to explore parallel and divergent features of the daily rhythms of melatonin and plasma free fatty acids (FFA) in goats exposed to different lighting conditions. From these features, we attempted to analyze whether the endogenous melatonin rhythm plays any role in the maintenance of the FFA rhythm. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (longest photoperiod, 18 h; shortest, 6 h). The ambient temperature and feeding regimen were kept constant. Blood samples were collected 6 times a year at 2 h intervals for 2 d, first in the prevailing light-dark (LD) conditions and then after 3 d in constant darkness (DD). In LD conditions, the melatonin levels always increased immediately after lights-off and declined around lights-on, except in winter (18 h darkness), when the low daytime levels were restored clearly before lights-on. The FFA levels also displayed a consistent rhythmicity, with low levels at night and a transient peak around lights-on. In DD conditions, the melatonin profiles were very similar to those found in the habitual LD conditions, but the rhythm tended to advance. The FFA rhythm persisted also in DD, and the morning peak tended to advance. There was an overall parallelism between the two rhythms, with one significant exception. In winter in LD conditions, the morning rise in FFA levels coincided with lights-on and not with the declining phase of melatonin, whereas in DD conditions, the FFA peak advanced several hours and coincided with the declining phase of melatonin. From this finding and comparisons of the calculated rhythm characteristics, i.e., phase-shifts, phase differences, and correlations, we conclude that the daily rhythm of FFA levels is most probably generated by an endogenous oscillator, primarily adjusted by dawn, whereas the melatonin rhythm in this species is regulated by an oscillator primarily adjusted by dusk. The results did not exclude a modulatory effect of melatonin on the daily FFA profiles, but melatonin secretion, alone, does not explain the patterns sufficiently.

Modulation of metabolic and clock gene mRNA rhythms by pineal and retinal circadian oscillators

Avian circadian organization involves interactions between three neural pacemakers: the suprachiasmatic nuclei (SCN), pineal, and retina. Each of these structures is linked within a neuroendocrine loop to influence downstream processes and peripheral oscillations. However, the contribution of each structure to drive or synchronize peripheral oscillators or circadian outputs in avian species is largely unknown. To explore these interactions in the chick, we measured 2-deoxy[(14)C]-glucose (2DG) uptake and mRNA expression of the chick clock genes bmal1, cry1, and per3 in three brain areas and in two peripheral organs in chicks that underwent pinealectomy, enucleation, or sham surgery. We found that 2DG uptake rhythms damp under constant darkness in intact animals, while clock gene mRNA levels continue to cycle, demonstrating that metabolic rhythms are not directly driven by clock gene transcription. Moreover, 2DG rhythms are not phase-locked to rhythms of clock gene mRNA. However, pinealectomy and enucleation had similar disruptive effects on both metabolic and clock gene rhythms, suggesting that both of these oscillators act similarly to reinforce molecular and physiological rhythms in the chicken. Finally, we show that the relative phasing of at least one clock gene, cry1, varies between central and peripheral oscillators in a tissue specific manner. These data point to a complex, differential orchestration of central and peripheral oscillators in the chick, and, importantly, indicate a disconnect between canonical clock gene regulation and circadian control of metabolism.

Decreased MT1 and MT2 melatonin receptor expression in extrapineal tissues of the rat during physiological aging

Aging is a complex process associated with a diminished ability to respond to stress, a progressive increase in free radical generation and a decline in immune function. Melatonin, a molecule with a great functional versatility exerts anti-oxidant, oncostatic, immunomodulatory, and anti-aging properties. Melatonin levels drop during aging and it has been speculated that the loss of melatonin may accelerate aging. This study was designed to elucidate whether aging involves responsiveness to reduced melatonin. Melatonin membrane receptor (MT1 and MT2) expression and MT1 protein expression were analyzed in extrapineal tissues (thymus, spleen, liver, kidney, and heart) of 3- and 12-month-old rats using real time polymerase chain reaction and western blotting analysis. Moreover, melatonin in tissues was measured by high performance liquid chromatography. We report for the first time, an age-related reduction in mRNA MT1 and MT2 expression levels as well as MT1 protein expression in all tissues tested except the thymus, where surprisingly, both melatonin receptor levels were significantly higher in 12-month-old rats and MT1 protein expression maintained unchanged with age. Diminished melatonin concentrations were measured in spleen, liver, and heart during aging. As a conclusion, physiological aging seems to exert responsiveness to melatonin and consequently, the loss of this potent anti-oxidant may contribute to onset of aging.

Melatonin pretreatment improves liver function and hepatic perfusion after hemorrhagic shock

Exogenous administration of pineal hormone melatonin (MEL) has been demonstrated to attenuate organ damage in models of I/R and inflammation by antioxidative effects. However, specific organ-protective effects of MEL with respect to hemorrhagic shock have not been investigated yet. In the present study, we evaluated the role of MEL pretreatment for hepatic perfusion, redox state, and function after hemorrhage and resuscitation, with emphasis on MEL receptor activation. In a model of hemorrhagic shock (MAP 35 +/- 5 mmHg for 90 min) and reperfusion (2 h), we measured nicotinamide adenine dinucleotide phosphate (reduced form; NADPH) autofluorescence, hepatic microcirculation, and hepatocellular injury by intravital microscopy, as well as plasma disappearance rate of indocyanine green (PDRICG) as a sensitive maker of liver function in rat. Pretreatment with 10 mg kg(-1) MEL (i.v.) 15 min before induction of hemorrhage resulted in a significantly improved PDR(ICG) compared with controls (MEL/shock, 15.02% min(-1) +/- 2.9 SD vs. vehicle/shock, 6.18 +/- 4.6 SD; P = 0.001). Intravital microscopy after reperfusion revealed an improved hepatic perfusion index, redox state, and reduced hepatocellular injury in pretreated animals compared with the vehicle group. Melatonin receptor antagonist luzindole (LZN; 2.5 mg kg(-1)) almost completely abolished the protective effects of MEL pretreatment with respect to liver function (MEL + LZN/shock PDR(ICG), 7.31% min(-1) +/- 3.4 SD). Beneficial effects regarding hepatic perfusion, redox state, and cellular injury were not influenced by LZN, indicating that they may depend on antioxidative effects of MEL. However, liver function after hemorrhage is effectively maintained by MEL pretreatment via receptor-dependent pathways.

Melatonin receptors mediate improvements of liver function but not of hepatic perfusion and integrity after hemorrhagic shock in rats

OBJECTIVE

Melatonin has been demonstrated to attenuate organ damage in models of ischemia and reperfusion. Melatonin treatment before hemorrhagic shock has been shown to improve liver function and hepatic perfusion. Proposed mechanisms of the pineal hormone involve direct inactivation of reactive oxygen species and induction of antioxidative enzymes. However, recent evidence suggests a strong influence of melatonin receptor activation for these effects. Specific protection of organ function by melatonin after hemorrhage has not been investigated yet. In this study, we evaluated whether melatonin therapy after hemorrhagic shock improves liver function and hepatic perfusion, with emphasis on melatonin receptor activation.

DESIGN

Prospective, randomized, controlled study.

SETTING

University research laboratory.

SUBJECTS

Male Sprague-Dawley rats, 200-300 g (n = 10 per group).

INTERVENTIONS

Animals underwent hemorrhagic shock (mean arterial pressure, 35 +/- 5 mm Hg for 90 mins) and were resuscitated with shed blood and Ringer's solution. At the end of shock, animals were treated with either melatonin (10 mg/kg, intravenously), melatonin receptor antagonist luzindole (2.5 mg/kg, intravenously) plus melatonin (10 mg/kg, intravenously), luzindole alone (2.5 mg/kg, intravenously), or vehicle.

MEASUREMENTS AND MAIN RESULTS

After 2 hrs of reperfusion, either liver function was assessed by plasma disappearance rate of indocyanine green or intravital microscopy of the liver was performed for evaluation of hepatic perfusion, hepatocellular redox state, and hepatic integrity. Compared with vehicle controls, melatonin therapy after hemorrhagic shock significantly improved plasma disappearance rate of indocyanine green, hepatic redox state, hepatocellular injury, and hepatic perfusion index. Coadministration of luzindole completely abolished the protective effect with respect to liver function only, and improvements regarding hepatic redox state, perfusion, and integrity were comparable with melatonin treatment alone.

CONCLUSIONS

Melatonin therapy after hemorrhagic shock improves liver function, hepatic perfusion, redox state, and hepatic integrity. With respect to liver function, beneficial effects of the pineal hormone seem to be dependent on melatonin receptor activation.

Intake of melatonin is associated with amelioration of physiological changes, both metabolic and morphological pathologies associated with obesity: an animal model

Obesity and its associated metabolic pathologies are the most common and detrimental diseases, affecting over 50% of the adult population. Our knowledge about the protective effects of melatonin against high-fat diet (HFD)-induced obesity is still marginal. In this investigation, we hypothesized that melatonin can minimize the metabolic pathologies and morphological changes associated with obesity in animals receiving an HFD. To examine these effects, and to test our hypothesis, an animal model formed of male Boscat white rabbits was established. The animals were divided into three groups: (i) a control group fed regular diet; (ii) an obesity group fed an HFD for 12 weeks; and (iii) a treated group fed HFD for 12 weeks and then treated with melatonin for 4 weeks. The animals were killed and their serum and tissues were evaluated for: (i) lipid profile (cholesterol, triglycerides and low-density lipoprotein) and glucose; (ii) antioxidant enzyme (serum glutathione peroxidase, GSH-PX); and (iii) fatty changes (liver, kidney and blood vessels). Compared with the control group, intake of HFD (obesity group) was associated with: (i) a statistically significant increase in blood pressure, heart rate, sympathetic nerve activity, body weight, food consumption, serum lipids, blood glucose levels and atherogenic index; (ii) decreased level of GSH-PX and high-density lipoprotein (HDL); and (iii) fatty changes in the liver and kidney as well as atheromatous changes in the blood vessels. Compared with the obesity group, intake of melatonin (treated group) was associated with: (i) a statistically significant decrease in blood pressure, heart rate, sympathetic nerve activity, body weight, food consumption, serum lipids, blood glucose levels and atherogenic index; (ii) increased level of GSH-PX and HDL; and (iii) disappearance of fatty changes in the liver and kidney as well as atheromatous changes in the blood vessels. The administration of melatonin reduced the metabolic pathologies associated with the intake of HFD, suggesting a protective role. Although the underlying mechanisms are unclear, they may include its antioxidant and receptor-mediated effects. The clinical ramifications of these effects await further investigations.

Diurnal and circadian regulation of a melatonin receptor, MT1, in the golden rabbitfish, Siganus guttatus

The golden rabbitfish Siganus guttatus is a reef fish with a restricted lunar-synchronized spawning rhythmicity and releases gametes simultaneously around the first quarter moon period during the spawning season. In order to understand the molecular aspects of the "circa" rhythms in this species, the full-length melatonin receptor (MT1) cDNA was cloned, and its diurnal/circadian regulation was examined. The full-length MT1 cDNA (1257 bp) contained an open reading frame that encodes a protein of 350 amino acids; this protein is highly homologous to MT1 of nonmammalian species. A high expression of MT1 mRNA with a day-night difference was observed in the whole brain, retina, liver, and kidney. When diurnal variations in MT1 mRNA expression in the retina and whole brain were examined using real-time quantitative RT-PCR, an increase in the mRNA expression was observed during nighttime in both tissues under conditions of light/dark, constant darkness, and constant light. This suggests that MT1 mRNA expression is under circadian regulation. The expression of MT1 mRNA in the cultured pineal gland also showed diurnal variations with high expression levels during nighttime; this suggests that the increased expression level observed in the whole brain is partially of pineal origin. Alternation of light conditions in the pineal gland cultures resulted in the changes in melatonin release into the culture medium as well as MT1 mRNA expression in the pineal gland. The present results suggest that melatonin and its receptors play an important role in the exertion of daily and circadian variations in the neural tissues.

Melatonin receptor of a reef fish with lunar-related rhythmicity: cloning and daily variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the regulation of circadian rhythms in various species. For overall understanding of 'circa' rhythms in the golden rabbitfish, Siganus guttatus, which exhibits restricted lunar-related rhythms and spawns synchronously around the first quarter moon, the aim of the present study was to clone a melatonin receptor (Mel(lb)) cDNA and examine daily variations of Mel(lb) mRNA expression in certain tissues of the rabbitfish. The full-length Mel(lb) cDNA (1808 bp) contained an open reading frame to encode a protein with a length of 354 amino acids, which was highly homologous to a protein of nonmammalian species. Northern blot analysis showed transcripts of Mel(lb) in the brain and retina. Real-time quantitative polymerase chain reaction analysis also revealed expression of Mel(lb) in all tissues tested. Significantly high expression of the gene during daytime was evident in the liver and kidney. When the expression of Mel(lb) was examined in the brain and retina under conditions of light/dark cycles or constant darkness, daily and circadian variations of gene expression with two increases during daytime and nighttime for the brain and a single increase during nighttime for the retina were recognized. Moreover, daily variations in the expression of Mel(lb) were observed in the cultured pineal gland. These results suggest that the melatonin receptor plays a role in integration of melatonin actions in various tissues and that daily variations of Mel(lb) in the neural tissues may be related to regulation of circadian clock.

Photoperiod regulates clock gene rhythms in the ovine liver

To investigate the photoperiodic entrainment of peripheral rhythms in ruminants, we studied the expression of clock genes in the liver in the highly seasonal Soay sheep. Animals were kept under long (LD 16:8) or short photoperiod (LD 8:16). Daily rhythms in locomotor activity were recorded, and blood concentrations of melatonin and cortisol were measured by RIA. Per2, Bmal1, and Cry1 gene expression was determined by Northern blot analyses using ovine RNA probes in liver collected every 4h for 24h. Liver Per2 and Bmal1, but not Cry1, expression was rhythmic in all treatments. Under long days, peak Per2 expression occurred at end of the night with a similar timing to Bmal1, whereas, under short days the Per2 maximum was in the early night with an inverse pattern to Bmal1. There was a photoperiodxtime interaction for only Per2 (P < 0.001). The 24-h pattern in plasma cortisol matched the observed phasing of Per2 expression, suggesting that it may act as an endocrine entraining factor. The clock gene rhythms in the peripheral tissues were different in timing compared with the ovine suprachiasmatic nucleus (SCN, central pacemaker) and pars tuberalis (melatonin target tissue), and the hepatic rhythms were of lower amplitude compared with photoperiodic rodents. Thus, there are likely to be important species differences in the way the central and peripheral clockwork encodes external photoperiod.

Melatonin-induced modulation of glucose metabolism in primary cultures of rabbit kidney-cortex tubules

The effect of melatonin on glucose metabolism in the presence and absence of insulin has been investigated in the primary cultures of renal tubules grown in a defined medium. In the absence of glucose in the medium containing 5 microg/mL of insulin and 2 mm alanine + 5 mm glycerol + 0.5 mm octanoate, 100 nm melatonin stimulated both glucose and lactate synthesis, while in the medium devoid of insulin melatonin action was negligible. Melatonin-induced increase in glucose and lactate synthesis was accompanied by an enhancement of alanine and glycerol consumption. In view of measurements of [U-14C]L-alanine and [U-14C]L-glycerol incorporation into glucose, it is likely that melatonin increased alanine utilization for glucose production, while accelerated lactate synthesis was because of an enhanced glycerol consumption. As (i) 10 nm luzindole attenuated the stimulatory action of melatonin on glucose formation and (ii) the indole induced a decrease in intracellular cAMP level, it seems likely that in renal tubules melatonin binds to ML1 membrane receptor subtype. In view of a decline of intracellular fructose-1,6-bisphosphate content accompanied by a significant rise in hexose-6-phosphate and glucose levels, melatonin might result in an acceleration of flux through fructose-1,6-bisphosphatase probably because of an increase in the active, dephosphorylated form of this enzyme. Thus, the administration of melatonin in combination with insulin might be beneficial for diabetic therapy because of protection against hypoglycemia.