Melatonin potentiates the effects of metformin on glucose metabolism and food intake in high-fat-fed rats.

BACKGROUND: Melatonin is a hormone synthesized mainly by the pineal gland, and secreted only at night. Melatonin has been proposed as a modulator of glucose metabolism. METHODS: Here we studied the metabolic effects of melatonin administration alone (s.c. 10 mg/kg) or in combination with metformin (p.o. 300 mg/kg), a widely used anti-diabetic drug. These treatments were tested on glucose tolerance, insulin sensitivity and food intake in Zucker fatty rats (i.e., bearing a missense mutation in the leptin receptor gene) and high-fat fed Sprague-Dawley rats. RESULTS: Melatonin alone or in combination did not significantly modify glucose tolerance in either model. Melatonin alone in high-fat fed Sprague-Dawley improved insulin sensitivity to the level of metformin. In addition, combined treatment further ameliorated insulin sensitivity (+13%), especially during the late phase of rising glycemia. The lack of similar effects in Zucker rats suggests an involvement of leptin signaling in mediating the positive effects of melatonin. Body mass gain in Sprague-Dawley rats was decreased by both metformin, and combined metformin and melatonin. While melatonin alone did not markedly affect food intake, its combination with metformin led to a more pronounced anorexia (-17% food intake during the last week), as compared to metformin alone. CONCLUSIONS: Melatonin improves the beneficial effects of metformin on insulin sensitivity and body mass gain in high-fat fed Sprague-Dawley rats. Therefore, the combination of melatonin and metformin could be beneficial to develop dual therapies to treat or delay type 2 diabetes associated with obesity.

Circadian phenotyping of obese and diabetic db/db mice.

Growing evidence links metabolic disorders to circadian alterations. Genetically obese db/db mice, lacking the long isoform of leptin receptor, are a recognized model of type 2 diabetes. In this study, we aimed at characterizing the potential circadian alterations of db/db mice in comparison to db/+ control mice. By using telemetry devices, we first reported arrhythmicity in general activity of most db/db mice under both light-dark cycle and constant darkness, while their rhythm of body temperature is less dramatically disrupted. Water access restricted to nighttime restores significant rhythmicity in behaviorally arrhythmic db/db mice, indicating a masking effect of polydipsia when water is available ad libitum. Endogenous period of temperature rhythm under constant dark conditions is significantly increased (+30 min) in db/db compared with db/+ mice. Next, we studied the oscillations of clock proteins (PER1, PER2 and BMAL1) in the suprachiasmatic nuclei (SCN), the site of the master clock, and detected no difference according to the genotype. Furthermore, c-FOS and P-ERK1/2 expression in response to a light pulse in late night was significantly increased (+80 and +55%, respectively) in the SCN of these diabetic mice. We previously showed that, in addition to altered activity rhythms, db/db mice exhibit altered feeding rhythm. Therefore, we investigated daily patterns of clock protein expression in medial hypothalamic oscillators involved in feeding behavior (arcuate nucleus, ventro- and dorso-medial hypothalamic nuclei). Compared with db/+ mice, very subtle or no difference in oscillations of PER1 and BMAL1 is found in the medial hypothalamus. Although we did not find a clear link between altered hypothalamic clockwork and behavioral rhythms in db/db mice, our results highlight a lengthened endogenous period and altered photic integration in these genetically obese and diabetic mice.

Leptin modulates the daily rhythmicity of blood glucose.

Leptin may affect central and/or peripheral timing, in addition to its well-known regulatory effects on metabolism. Here, we investigated whether leptin can impact rhythmicity of blood glucose and lipids. For that purpose, daily variations of blood glucose and lipids were compared between mice lacking functional leptin receptor (db/db) or deficient for leptin (ob/ob) and controls (db/+ and ob/+, respectively). Next, we investigated whether timed treatment with exogenous leptin in ob/ob mice could modulate blood glucose rhythm. Mice with defective leptin signaling (db/db and ob/ob) have the same phase-opposed timing in glycemia (11 and 9 h shift, respectively) compared to respective controls. By contrast, the phase of plasma lipids rhythms (e.g. triglycerides, non-esterified fatty acid - NEFA, high density lipoprotein - HDL, low density lipoprotein - LDL) remained essentially unchanged, whatever the genotype. Daily injections of leptin (1 mg/kg) in ob/ob mice during nighttime or daytime led to 1-2 h phase-advances of blood glucose rhythm and glucose arrhythmicity, respectively. These injections induced additional phase-dependent shifts of feeding rhythm (ranging from 2.6 h phase-delays to 2.6 h advances). The present study reveals a chronomodulatory role of leptin, and highlights that rhythmic leptin can be a determinant of daily variations of blood glucose and food intake, though not for lipids.

In vitro and in vivo antitumor activity of melatonin receptor agonists.

Melatonin has been shown to inhibit the proliferation of estrogen receptor α (ERα)-positive human breast cancer cells in vitro and suppress the growth of carcinogen-induced mammary tumors in rats. Melatonin's antiproliferative effect is mediated, at least in part, through the MT1 melatonin receptor and mechanisms involving modulation of the estrogen-signaling pathway. To develop melatonin analogs with greater therapeutic effects, we have examined the in vitro and in vivo antimitotic activity of two MT1/MT2 melatonin receptor agonists, S23219-1 and S23478-1. In our studies, both agonists are quite effective at suppressing the growth of MCF-7 human breast cancer cells. At a concentration of 10⁻6 m, S23219-1 and S23478-1 inhibited the growth of MCF-7 cells by 60% and 73%, respectively. However, S23478-1 is more effective than melatonin and S23219-1 at repressing the expression and transactivation of the ERα, and modulating the expression of pancreatic spasmolytic polypeptide (pS2), an estrogen-regulated gene. The melatonin agonist S23478-1 exhibited enhanced antitumor potency in the subsequent studies in our animal model. At a dosage of 25 mg/kg/day, S23478-1 is more efficacious than melatonin at inducing regression of the established N-nitroso-N-methyl-urea-induced rat mammary tumors. This dose of S23478-1 (25 mg/kg/day) generated a significant (P < 0.05) overall regression response of 52%. Furthermore, at this dosage, S23478-1 is more effective than melatonin at suppressing the estrogen-signaling pathway and promoting tumor cell apoptosis, significantly increasing the expression of the pro-apoptotic protein Bax, while decreasing the expression of ERα and the anti-apoptotic protein Bcl-2.

Agomelatine, the first melatonergic antidepressant: discovery, characterization and development.

Current management of major depression, a common and debilitating disorder with a high social and personal cost, is far from satisfactory. All available antidepressants act through monoaminergic mechanisms, so there is considerable interest in novel non-monoaminergic approaches for potentially improved treatment. One such strategy involves targeting melatonergic receptors, as melatonin has a key role in synchronizing circadian rhythms, which are known to be perturbed in depressed states. This article describes the discovery and development of agomelatine, which possesses both melatonergic agonist and complementary 5-hydroxytryptamine 2C (5-HT2C) antagonist properties. Following comprehensive pharmacological evaluation and extensive clinical trials, agomelatine (Valdoxan/Thymanax; Servier) was granted marketing authorization in 2009 for the treatment of major depression in Europe, thereby becoming the first approved antidepressant to incorporate a non-monoaminergic mechanism of action.

Short day-length increases sucrose consumption and adiposity in rats fed a high-fat diet.

BACKGROUND: Photoperiod, i.e., the relative day-length per 24h, may modulate the metabolic responses to high-fat diet (HFD) and sucrose consumption. METHODS: To test this hypothesis, hormonal changes, fat accretion and sucrose intake were measured in rats exposed to short- or long-day for 4 weeks and fed with a standard high-carbohydrate low-fat pelleted diet (high-carbohydrate diet (HCD)) or a high-fat, medium-carbohydrate pelleted diet (HFD), with or without free access to 10% sucrose solution in addition to water available ad libitum. RESULTS: Plasma leptin and adiposity index, defined as epididymal white fat expressed as percentage of body mass, were markedly increased only in HFD-fed animals drinking sucrose under short, but not long, photoperiods. Voluntary ingestion of sucrose under short days was greater in HFD rats compared with HCD animals over the experiment, while a trend for the opposite effect was visible under long days. Total energy intake was not changed overall, as rats proportionally decreased chow intake when they drank sucrose. A noteworthy exception was the HFD group with sucrose access under short days that significantly increased their total calorie intake. Fasting blood glucose was generally unaltered, except for an increase in HFD-fed animals drinking sucrose under long days compared to control animals, suggesting a decrease in glucose tolerance. Insulin resistance was not yet affected by nutritional or photoperiodic conditions after 4 experimental weeks. CONCLUSIONS: Even if photoperiod cannot be considered as an obesogenic environmental factor per se, the metabolic effects resulting from the combination of high-fat feeding and voluntary intake of sucrose were dependent on day-length. Exposure to short days triggers a larger increase of sucrose ingestion and hyperleptinemia in rats fed with HFD compared to the control diet. Considering that the cardinal symptoms of winter depression include carbohydrate craving and increased adiposity, the present data provide an experimental basis for developing new animal models of seasonal affective disorder.

[Melatoninergic receptor agonists and antagonists: therapeutic perspectives]. / Agonistes et antagonistes des récepteurs mélatoninergiques: perspectives thérapeutiques.

The chronobiotic neurohormone melatonin, synthetized in the pineal gland during darkness periods governs the circadian and seasonal biological rhythms. Physiologically, melatonin regulates the sleep/activity alternance, together with the circadian cycle of body temperature and cortisol secretion, and influences various immune, endocrine and metabolic functions. Dysfunction of the endogenous melatonin secretion is associated with mood and behavioral disorders including body weight. Patients with severe depression exhibit desynchronized and reduced melatonin secretion, in parallel with marked sleep disturbances whereas exogenous melatonin administration and antidepressive drugs restore melatonin secretion. A dysregulated melatonin secretion is also observed in obese subjects. Implication of melatonin in these disorders stimulated the search for melatonin analogues with enhanced antidepressive and body weight control effects. The melatoninergic agonist S 20098, or agomelatin, disclosed a potent antidepressive and anxiolytic activity in preclinical studies, which was confirmed in clinical trials in patients with major depression. The antagonist S 20928 was shown to limit seasonal weight gain in an hibernating rodent model. Thus, development of melatoninergic agonists and antagonists appear as an innovative approach in the treatment of depression and obesity, two major public health problems.

Increasing the fat-to-carbohydrate ratio in a high-fat diet prevents the development of obesity but not a prediabetic state in rats.

Metabolic disorders induced by high-fat feeding in rodents evoke some, if not all, of the features of human metabolic syndrome. The occurrence and severity of metabolic disorders, however, varies according to rodent species, and even strain, as well as the diet. Therefore, in the present study, we investigated the long-term obesogenic and diabetogenic effects of three high-fat diets differing by their fat/carbohydrate ratios. Sprague-Dawley rats were fed a control high-carbohydrate and low-fat diet [HCD; 3:16:6 ratio of fat/carbohydrate/protein; 15.48 kJ/g (3.7 kcal/g)], a high-fat and medium-carbohydrate diet [HFD1; 53:30:17 ratio of fat/carbohydrate/protein; 19.66 kJ/g (4.7 kcal/g)], a very-high-fat and low-carbohydrate diet [HFD2; 67:9:24 ratio of fat/carbohydrate/protein; 21.76 kJ/g (5.2 kcal/g)] or a very-high-fat and carbohydrate-free diet [HFD3; 75:0:25 ratio of fat/carbohydrate/protein; 24.69 kJ/g (5.9 kcal/g)] for 10 weeks. Compared with the control diet (HCD), rats fed with high-fat combined with more (HFD1) or less (HFD2) carbohydrate exhibited higher BMI (body mass index; +13 and +10% respectively; P<0.05) and abdominal fat (+70% in both HFD1 and HFD2; P<0.05), higher plasma leptin (+130 and +135% respectively; P<0.05), lower plasma adiponectin levels (-23 and -30% respectively; P<0.05) and impaired glucose tolerance. Only the HFD1 group had insulin resistance. By contrast, a very-high-fat diet devoid of carbohydrate (HFD3) led to impaired glucose tolerance, insulin resistance and hypoadiponectinaemia (-50%; P<0.05), whereas BMI, adiposity and plasma leptin did not differ from respective values in animals fed the control diet. We conclude that increasing the fat-to-carbohydrate ratio to the uppermost (i.e. carbohydrate-free) in a high-fat diet prevents the development of obesity, but not the prediabetic state (i.e. altered glucose tolerance and insulin sensitivity).

Melatonin MT(1/2) receptor stimulation reduces cortical overflow of cholecystokinin-like material in a model of anticipation of social defeat in the rat.

The involvement of cholecystokinin (CCK) in the potential anxiolytic-like effects of melatonin and of the antitumor MT(1/2) receptor agonist, S23478, was assessed by measuring the cortical outflow of CCK-like material (CCKLM) in a rat model of anticipation of social defeat. After repeated social defeats by a male Tryon Maze Dull (TMD) rat, Sprague-Dawley (SD) rats were implanted for microdialysis in the frontal cortex and placed in the same environment as for the defeated sessions, but no confrontation with the TMD rat was allowed. Anticipation of social defeat induced anxiety-like behaviors (immobility, ultrasonic vocalization, defensive postures) associated with a significant increase (approximately +90%) in cortical CCKLM outflow in SD rats. Acute pretreatment with melatonin (5 or 40 mg/kg i.p.) or S23478, at 5 mg/kg i.p., had no or only minor effects on anxiety-like behaviors and did not affect CCKLM overflow. In contrast, at 40 mg/kg i.p., S23478 significantly reduced the duration of immobility and vocalization as well as the cortical CCKLM overflow (-30%) in defeated SD rats, and both effects were prevented by the MT(1/2) receptor antagonist S22153 (40 mg/kg i.p.). These data indicated that MT(1/2) receptor stimulation can exert anxiolytic-like effects associated with inhibition of cortical CCKergic neurotransmission in rats anticipating social defeat.