Expression of Melatonin Receptor 1 in Rat Mesenteric Artery and Perivascular Adipose Tissue and Vasoactive Action of Melatonin.

Melatonin is released by the pineal gland and can modulate cardiovascular system function via the G protein-coupled melatonin receptors MT1 and MT2. Most vessels are surrounded by perivascular adipose tissue (PVAT), which affects their contractility. The aim of our study was to evaluate mRNA and protein expression of MT1 and MT2 in the mesenteric artery (MA) and associated PVAT of male rats by RT-PCR and Western blot. Receptor localization was further studied by immunofluorescence microscopy. Effects of melatonin on neurogenic contractions were explored in isolated superior MA ex vivo by measurement of isometric contractile tension. MT1, but not MT2, was present in MA, and MT1 was localized mainly in vascular smooth muscle. Moreover, we proved the presence of MT1, but not MT2 receptors, in MA-associated PVAT. In isolated superior MA with intact PVAT, neuro-adrenergic contractile responses were significantly smaller when compared to arteries with removed PVAT. Pre-treatment with melatonin of PVAT-stripped arterial rings enhanced neurogenic contractions, while the potentiating effect of melatonin was not detected in preparations with preserved PVAT. We hypothesize that melatonin can stimulate the release of PVAT-derived relaxing factor(s) via MT1, which can override the direct pro-contractile effect of melatonin on vascular smooth muscle. Our results suggest that melatonin is involved in the control of vascular tone in a complex way, which is vessel specific and can reflect a sum of action on different layers of the vessel wall and surrounding PVAT.

Carbamate Insecticides Target Human Melatonin Receptors.

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

Effects and mechanisms of melatonin on neural differentiation of induced pluripotent stem cells.

Melatonin, a lipophilic molecule mainly synthesized in the pineal gland, has properties of antioxidation, anti-inflammation, and antiapoptosis to improve neuroprotective functions. Here, we investigate effects and mechanisms of melatonin on neural differentiation of induced pluripotent stem cells (iPSCs). iPSCs were induced into neural stem cells (NSCs), then further differentiated into neurons in medium with or without melatonin, melatonin receptor antagonist (Luzindole) or Phosphatidylinositide 3 kinase (PI3K) inhibitor (LY294002). Melatonin significantly promoted the number of neurospheres and cell viability. In addition, Melatonin markedly up-regulated gene and protein expression of Nestin and MAP2. However, Luzindole or LY294002 attenuated these increase. The expression of pAKT/AKT were increased by Melatonin, while Luzindole or LY294002 declined these melatonin-induced increase. These results suggest that melatonin significantly increased neural differentiation of iPSCs via activating PI3K/AKT signaling pathway through melatonin receptor.

Cytoprotective Effect of Afobazole and Its Main Metabolite M-11.

Cell damage depending on activity of quinone reductase 2 (MT3 receptor) was simulated in experiments on bone marrow cell suspension and assessed by menadione-induced DNA breaks measured by comet assay. We analyzed the protective effect of afobazole interacting with MT1, MT3, σ1 receptors, and monoamine oxidase A and its main metabolite M11 that specifi cally binds to MT3 receptors. Both compounds reduced the level of menadione-induced DNA damage (afobazole was effective in lower concentrations in comparison with M-11). Conclusion was made on the contribution of MT3 receptors to the protective effect of afobazole, but the observed concentration differences indicate possible contribution of other targets of anxiolytic drug to the protective mechanisms.

Melatonin suppresses doxorubicin-induced premature senescence of A549 lung cancer cells by ameliorating mitochondrial dysfunction.

Melatonin is an indolamine that is synthesized in the pineal gland and shows a wide range of physiological functions. Although the anti-aging properties of melatonin have been reported in a senescence-accelerated mouse model, whether melatonin modulates cellular senescence has not been determined. In this study, we examined the effect of melatonin on anticancer drug-induced cellular premature senescence. We found that the doxorubicin (DOX)-induced senescence of A549 human lung cancer cells and IMR90 normal lung cells was substantially inhibited by cotreatment with melatonin in a dose-dependent manner. Mechanistically, the DOX-induced G2/M phase cell cycle arrest and the decrease in cyclinB and cdc2 expression were not affected by melatonin. However, the DOX-induced increase in intracellular levels of ROS, which is necessary for premature senescence, was completely abolished upon melatonin cotreatment. In addition, the reduction in mitochondrial membrane potential that occurs upon DOX treatment was inhibited by melatonin. An aberrant increase in mitochondrial respiration was also significantly suppressed by melatonin, indicating that melatonin ameliorates the mitochondrial dysfunction induced by DOX treatment. The treatment of A549 cells with luzindole, a potent inhibitor of melatonin receptors, failed to prevent the effects of melatonin treatment on mitochondrial functions and premature senescence in cells also treated with DOX; this suggests that melatonin suppresses DOX-induced senescence in a melatonin receptor-independent manner. Together, these results reveal that melatonin has an inhibitory effect of melatonin on premature senescence at the cellular level and that melatonin protects A549 cells from DOX-induced senescence. Thus, melatonin might have the therapeutic potential to prevent the side effects of anticancer drug therapy.

Agomelatine in depressive disorders: its novel mechanisms of action.

Disruptions in sleep and sleep-wake cycle regulation have been identified as one of the main causes for the pathophysiology of depressive disorders. The search has been on for the identification of an ideal antidepressant that could improve both sleep disturbances and depressive symptomatology. Melatonin, the major hormone of the pineal gland, has been shown to improve sleep and is involved in the regulation of the sleep-wake cycle. Identification of high concentrations of MT1 and MT2 melatonergic receptors in the suprachiasmatic nucleus of the anterior hypothalamus, the structure concerned with regulation of circadian rhythms and sleep-wake cycles, has led to the development of melatonergic agonists with greater potency and longer durations of action. Agomelatine is one such melatonergic agonist that acts specifically on MT1/MT2 melatonergic receptors and at the same time exhibits 5-HT2C antagonism, a property that is utilized by current antidepressants that are in clinical use. Agomelatine has been shown to be effective in a number of animal models of depression. Clinical studies undertaken on patients with major depression, bipolar disorders, seasonal affective disorder, and generalized anxiety disorder have all shown that agomelatine is also very effective in ameliorating depressive symptoms and manifesting early onset of action with a good tolerability and safety profile. It improved sleep efficiency and also resynchronized the disrupted circadian rhythms. Hence, the melatonergic modulation by agomelatine is suggested as one of the mechanisms for its antidepressant effect. Agomelatine's action on dendritic neurogenesis in animal models of depression is also identified as yet another action.

Melatonin effects on Fundulus heteroclitus reproduction.

This study aimed to investigate the effects of two different doses (100nM (M1) and 1µM (M2)) of exogenous melatonin on the reproductive capacity of Fundulus heteroclitus. Eight days of melatonin exposure significantly increased the fecundity and embryo survival of F. heteroclitus only in the M2 group compared with the control; the hatching rate was unaffected. Moreover, increases in the local expression of the melatonin receptor (mtnr) gene during follicle maturation were found; however, there were no differences between the experimental groups. Furthermore, in vitro melatonin-treated follicles showed a significantly higher germinal vesicle break down percentage compared with the control, while SDS-PAGE showed no difference in the electrophoretic pattern of the major yolk proteins. Nevertheless, densitometry revealed a greater intensity of the 118-, 95- and 40-kDa components in groups treated with melatonin. Finally, Fourier transform infrared microspectroscopy was applied to classify the different stages of oocyte development (Stages I-II, III and IV) on the basis of their macromolecular composition. The effects induced by melatonin on oogenesis were investigated by comparing vibrational spectra of females exposed to melatonin with those of controls. Changes to the Amide I band, corresponding to an increase in ß-structure, were found in oocytes of females exposed to the highest melatonin dose. These results highlight the positive role of melatonin, which is able to enhance the reproductive capacity of F. heteroclitus. Further studies are in progress to better explain the molecular mechanisms by which melatonin treatment affects reproduction in this marine species.

Melatonin: neuritogenesis and neuroprotective effects in crustacean x-organ cells.

Melatonin has both neuritogenic and neuroprotective effects in mammalian cell lines such as neuroblastoma cells. The mechanisms of action include receptor-coupled processes, direct binding and modulation of calmodulin and protein kinase C, and direct scavenging of free radicals. While melatonin is produced in invertebrates and has influences on their physiology and behavior, little is known about its mechanisms of action. We studied the influence of melatonin on neuritogenesis in well-differentiated, extensively-arborized crustacean x-organ neurosecretory neurons. Melatonin significantly increased neurite area in the first 24h of culture. The more physiological concentrations, 1 nM and 1 pM, increased area at 48 h also, whereas the pharmacological 1 µM concentration appeared to have desensitizing effects by this time. Luzindole, a vertebrate melatonin receptor antagonist, had surprising and significant agonist-like effects in these invertebrate cells. Melatonin receptors have not yet been studied in invertebrates. However, the presence of membrane-bound receptors in this population of crustacean neurons is indicated by this study. Melatonin also has significant neuroprotective effects, reversing the inhibition of neuritogenesis by 200 and 500 µM hydrogen peroxide. Because this is at least in part a direct action not requiring a receptor, melatonin's protection from oxidative stress is not surprisingly phylogenetically-conserved.

[Lights off? Neurobiological and pharmacological aspects of the melatonergic-serotonergic synergism]. / Lights off? A melatonerg és szerotonerg szinergizmus neurobiológiai és farmakológiai aspektusai.

SSRI antiepressants have been widely used for treating depressive symptoms for more than two decades. Despite their frequent usage, meta-analyses proved that only 20-25% of the patients had achieved long term remission. The introduction and spreading of dual-acting agents increased remission rate, but many of the patients with depressive symptoms still suffer from the disorder due to partial pharmacotherapeutic efficacy. Chronobiological disturbances might play an important role both in the pathophysiology and in the ongoing symptoms of depression. Pathological alterations in the melatonergic system may act as the first, obscure signs of the onset of depression. Agomelatine, a new antidepressive agent may offer new possibilities in the pharmacotherapy of depression, due to its synergistic melatonergic-serotonergic activity.

Melatonin as a protective agent in cardiac ischemia-reperfusion injury: Vision/Illusion?

Melatonin, an emphatic endogenous molecule exerts protective effects either via activation of G-protein coupled receptors (Melatonin receptors, MTR 1-3), tumor necrosis factor receptor (TNFR), toll like receptors (TLRS), nuclear receptors (NRS) or by directly scavenging the free radicals. MTRs are extensively expressed in the heart as well as in the coronary vasculature. Accumulating evidences have indicated the existence of a strong correlation between reduction in the circulating level of melatonin and precipitation of heart attack. Apparently, melatonin exhibits cardioprotective effects via modulating inextricably interlinked pathways including modulation of mitochondrial metabolism, mitochondrial permeability transition pore formation, nitric oxide release, autophagy, generation of inflammatory cytokines, regulation of calcium transporters, reactive oxygen species, glycosaminoglycans, collagen accumulation, and regulation of apoptosis. Convincingly, this review shall describe the various signaling pathways involved in salvaging the heart against ischemia-reperfusion injury.

Pathophysiology of depression: role of sleep and the melatonergic system.

Profound disturbances in sleep architecture occur in major depressive disorders (MDD) and in bipolar affective disorders. Reduction in slow wave sleep, decreased latency of rapid eye movement (REM) sleep and abnormalities in the timing of REM/non-REM sleep cycles have all been documented in patients with MDD. It is thus evident that an understanding of the basic mechanisms of sleep regulation is essential for an analysis of the pathophysiology of depressive disorders. The suprachiasmatic nucleus (SCN), which functions as the body's master circadian clock, plays a major role in the regulation of the sleep/wakefulness rhythm and interacts actively with the homeostatic processes that regulate sleep. The control of melatonin secretion by the SCN, the occurrence of high concentrations of melatonin receptors in the SCN, and the suppression of electrical activity in the SCN by melatonin all underscore the major influence which this neurohormone has in regulating the sleep/wake cycle. The transition from wakefulness to high sleep propensity is associated with the nocturnal rise of endogenous melatonin secretion. Various lines of evidence show that depressed patients exhibit disturbances in both the amplitude and shape of the melatonin secretion rhythm and that melatonin can improve the quality of sleep in these patients. The choice of a suitable antidepressant that improves sleep quality is thus important while treating a depressive disorder. The novel antidepressant agomelatine, which combines the properties of a 5-HT(2C) antagonist and a melatonergic MT(1)/MT(2) receptor agonist, has been found very effective for resetting the disturbed sleep/wake cycle and in improving the clinical status of MDD. Agomelatine has also been found useful in treating sleep problems and improving the clinical status of patients suffering from seasonal affective disorder.

The effects of ramelteon on respiration during sleep in subjects with moderate to severe chronic obstructive pulmonary disease.

BACKGROUND: Individuals with moderate to severe chronic obstructive pulmonary disease (COPD) have poor sleep quality. This study evaluated the effects of ramelteon, an MT(1)/MT(2) melatonin receptor agonist indicated for insomnia treatment on respiration in this population. MATERIALS AND METHODS: This double-blind, crossover study enrolled 25 subjects (>or=40 years) with moderate to severe COPD (FEV(1)/FVC <70% and FEV(1) between 50 and 80% of predicted value [moderate], or FEV(1)/FVC <70% and FEV(1) <50% of predicted value [severe]). Subjects received ramelteon 8 mg or placebo for one night 30 min before polysomnographic monitoring, including measurement of oxygen saturation (SaO(2)) and respiratory effort and flow. Subjects crossed to alternate treatment after a 5- to 10-day washout. The primary endpoint was mean SaO(2) for the entire night. RESULTS: No significant difference in SaO(2) for the entire night was observed with ramelteon vs placebo (92.2% vs 92.5%, P = 0.576). Mean SaO(2) was similar between ramelteon and placebo for each hour of the night, each sleep stage, the number of minutes that SaO(2) was <80% and <90%, and mean apnea-hypopnea index. There was a significant difference in total sleep time (389.0 vs 348.4 min, P = 0.019) and sleep efficiency (81.0 vs 72.6%, P = 0.019), and latency to persistent sleep was shorter (23.1 vs 56.9 min, P = 0.051), with ramelteon vs placebo. All adverse events were mild to moderate; none led to study discontinuation. CONCLUSION: Ramelteon did not produce respiratory depressant effects as measured by oxygenation or abnormal breathing events in subjects with moderate to severe COPD. Ramelteon was well tolerated in this population.

Agomelatine: new drug. Adverse effects and no proven efficacy.

(1) When an antidepressant is considered a necessary addition to psychological support in treating patients with depression, the first-line drug is a tricyclic such as clomipramine or a selective serotonin reuptake inhibitor (SSRI) such as paroxetine; (2) Agomelatine, a melatonin receptor agonist, is approved in the European Union for the treatment of depression; (3) Available evaluation does not include any clinical trials designed to compare the efficacy of agomelatine with that of a tricyclic or a selective serotonin reuptake inhibitor. Most data come from 7 placebo-controlled trials; (4) Agomelatine (25 mg/day) was statistically more effective (on a rating scale) than placebo in only 3 of these 7 trials. The clinical relevance of the score improvements is questionable. No data are available on the cure rate or on suicide prevention; (5) In one trial, increasing the daily dose from 25 mg to 50 mg provided no supplementary benefit; (6) A trial in 367 patients failed to show that agomelatine was any more effective than placebo in preventing new depressive episodes (29% after one year). In another trial including 339 patients, the relapse rate was statistically lower after 6 months on agomelatine (20.6%) than on placebo (41.4%); (7) Very high doeses of agomelatine are oncogenic in animals. The risk in humans is not known. Dizziness, gastrointestinal and cutaneous disorders have been observed. Agomelatine is probably hepatotoxic; (8) In summary, agomelatine has unproven efficacy and poorly documented adverse effects. It is better to continue to use older antidepressants such as tricyclics or serotonin reuptake inhibitors.

Selective activation of melatonin receptors with ramelteon improves liver function and hepatic perfusion after hemorrhagic shock in rat.

OBJECTIVE: Melatonin may attenuate organ damage via direct antioxidative properties, and was recently demonstrated to reduce cardiac and hepatic injury through receptor-dependent effects. However, the relevance of an isolated activation of melatonin receptors for organ protection, excluding direct antioxidant effects, has not been established yet. This study was designed to investigate whether therapy with melatonin receptor agonist and hypnotic substance ramelteon may improve liver function, hepatic perfusion, and hepatic integrity after hemorrhagic shock in rat. DESIGN: Prospective, randomized, controlled study. SETTING: University research laboratory. SUBJECTS: Male Sprague-Dawley rats (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 lactate (2 hrs). At the end of shock, animals were treated with ramelteon (1.0 mg/kg intravenously), melatonin receptor antagonist luzindole plus ramelteon (each 1.0 mg/kg intravenously), or vehicle. MEASUREMENTS AND MAIN RESULTS: In vitro, ramelteon displayed no relevant antioxidant capacity in an 2,2'-Azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) assay, compared with melatonin. In vivo, liver function was assessed by plasma disappearance rate of indocyanine green, and intravital microscopy was performed for evaluation of hepatic perfusion index, nicotinamide adenine dinucleotide phosphate autofluorescence, and hepatic integrity. Compared with vehicle controls, ramelteon therapy significantly improved plasma disappearance rate of indocyanine green (7.89 +/- 2.12% vs. 13.67 +/- 2.51%; p = 0.006), hepatic perfusion index (352.04 +/- 111.78 pl/sec/mm vs. 848.81 +/- 181.38 pl/sec/mm; p = 0.002), nicotinamide adenine dinucleotide phosphate autofluorescence and hepatocellular injury. Coadministration of luzindole abolished the protective effect of ramelteon with respect to liver function and nicotinamide adenine dinucleotide phosphate autofluorescence. CONCLUSIONS: Ramelteon therapy improves liver function, hepatic perfusion, and hepatocellular integrity after hemorrhagic shock in rat. This demonstrates that an isolated activation of melatonin receptors may be sufficient for organ protection, independent from direct antioxidant effects. The hypnotic ramelteon could thus play an interesting role in future sedation concepts for critical care patients.

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.

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.

Melatonin and the immune system in aging.

Aging is associated with a decline in immune function (immunosenescence), a condition known to correlate with increased incidence of cancer as well as infectious and degenerative diseases. Innate, cellular and humoral immunity all exhibit increased deterioration with age. Circulating melatonin decreases with age, and in recent years much interest has been focused on its immunomodulatory effect. Melatonin stimulates the production of progenitor cells for granulocytes and macrophages. It also stimulates the production of natural killer cells and CD4+ cells and inhibits CD8+ cells. The production and release of various cytokines from natural killer cells and T helper lymphocytes are enhanced by melatonin. Melatonin has the potential therapeutic value to enhance immune function in aged individuals.

Design and synthesis of benzofuranic derivatives as new ligands at the melatonin-binding site MT3.

Benzofuranic analogues of MCA-NAT (5-methoxycarbonylamino-N-acetyltryptamine) have been synthesized and evaluated as melatonin receptor ligands. Introduction of a methoxycarbonylamino substituent in the C-5 position of the benzofurane nucleus obtains MT(3) selective ligands. This selectivity can be modulated with suitable variations of the C-5 position and the acyl group on the C-3 side chain.

Ramelteon: new drug. Insomnia: no role for risky placebos.

(1) Patients complaining of insomnia should first be treated with non-drug measures (information, advice). Short-course benzodiazepine therapy can be tried if non-drug measures and established herbal remedies fail; (2) Ramelteon, a drug that antagonises receptors for melatonin, a hormone involved in circadian rhythms, is being considered for European marketing authorization in the treatment of insomnia; (3) Ramelteon has only been compared with placebo in clinical trials. Only one of three trials in which the patients were studied in their normal environment showed that ramelteon reduced the time to sleep onset, only by about 10 minutes. A similar reduction was observed in the artificial conditions of a sleep laboratory. There was no effect on sleep duration or on the number of night-time awakenings; (4) Ramelteon does not appear to have the disadvantages of benzodiazepines, such as residual daytime drowsiness, rebound insomnia on drug withdrawal, and dependence. But ramelteon provokes hyperprolactinaemia and was carcinogenic in experimental animals; (5) In practice, when a drug is needed for a patient complaining of insomnia, the best options are phytotherapy or short-course benzodiazepine treatment.

The pattern of melatonin receptor expression in the brain may influence antidepressant treatment.

The pineal hormone melatonin produces most of its biological effects via G protein-coupled receptors MT1 and MT2. In mammals, these receptors are expressed in various tissues and organs including in the brain. Recent research points to a putative role of MT1/MT2 dimerization as a mechanism that could determine the receptor-mediated biological effects of melatonin. Brain content and the ratios between MT1 and MT2 receptors are affected by illness, e.g., Alzheimer's disease, and by prolonged drug treatment, e.g., antidepressants. New drugs with antidepressant properties that bind and activate melatonin receptors have been discovered. We hypothesize that endogenous, i.e., low, levels of melatonin could contribute to antidepressant effects depending on the expression pattern of melatonin receptors in the brain. Hence, we propose that a prolonged treatment with classical antidepressant drugs alters the brain ratio of MT1/MT2 receptors to enable the endogenous melatonin, which is secreted during the night, to further improve the antidepressant effects. A corollary of this hypothesis is that antidepressants would be less effective in conditions of pathologically altered brain melatonin receptors, e.g., in Alzheimer's patients or due to genetic polymorphisms. If our hypothesis is confirmed, supplementing classical antidepressant treatment with an appropriate dose of a melatonin receptor agonist might be used to improve antidepressant effects in subjects with a susceptible pattern of brain melatonin receptor expression.