Melatonin and circadian control in mammals

Melatonin: a ferroptosis inhibitor with potential therapeutic efficacy for the post-COVID-19 trajectory of accelerated brain aging and neurodegeneration

The unprecedented pandemic of COVID-19 swept millions of lives in a short period, yet its menace continues among its survivors in the form of post-COVID syndrome. An exponentially growing number of COVID-19 survivors suffer from cognitive impairment, with compelling evidence of a trajectory of accelerated aging and neurodegeneration. The novel and enigmatic nature of this yet-to-unfold pathology demands extensive research seeking answers for both the molecular underpinnings and potential therapeutic targets. Ferroptosis, an iron-dependent cell death, is a strongly proposed underlying mechanism in post-COVID-19 aging and neurodegeneration discourse. COVID-19 incites neuroinflammation, iron dysregulation, reactive oxygen species (ROS) accumulation, antioxidant system repression, renin-angiotensin system (RAS) disruption, and clock gene alteration. These events pave the way for ferroptosis, which shows its signature in COVID-19, premature aging, and neurodegenerative disorders. In the search for a treatment, melatonin shines as a promising ferroptosis inhibitor with its repeatedly reported safety and tolerability. According to various studies, melatonin has proven efficacy in attenuating the severity of certain COVID-19 manifestations, validating its reputation as an anti-viral compound. Melatonin has well-documented anti-aging properties and combating neurodegenerative-related pathologies. Melatonin can block the leading events of ferroptosis since it is an efficient anti-inflammatory, iron chelator, antioxidant, angiotensin II antagonist, and clock gene regulator. Therefore, we propose ferroptosis as the culprit behind the post-COVID-19 trajectory of aging and neurodegeneration and melatonin, a well-fitting ferroptosis inhibitor, as a potential treatment.

Melatonin and Kidney Health: From Fetal Stage to Later Life

Melatonin, an endogenous hormone mainly released at night by the pineal gland, has multifaceted biofunctions. Emerging evidence points to melatonin having a crucial role in kidney health and disease. As the prevalence of chronic kidney disease (CKD) is still rising, a superior strategy to advance global kidney health is needed to not just treat CKD, but prevent it early on. Adult kidney disease can have its origins in early life. This review aims to evaluate the recent literature regarding melatonin's effect on kidney development, its clinical uses in the early stage of life, animal models documenting preventive applications of melatonin on offspring's kidney-related disease, and a thorough summary of therapeutic considerations concerning melatonin supplementation.

Morphofunctional State and Circadian Rhythms of the Liver of Female Rats under the Influence of Chronic Alcohol Intoxication and Constant Lighting

A separate and combined effect of constant illumination and chronic alcohol intoxication (CAI) on diurnal dynamics of micromorphometric parameters of hepatocytes in female Wistar rats and p53, Ki-67, PER2, BMAL1, and ADH5 expression in these cells were studied. The increase in apoptotic activity and proliferation in all animals under the action of chronodestructors is shown. All experimental animals showed a decrease in BMAL1 expression and increase in PER2 expression; ADH5 is overexpressed under the influence of ethanol. Circadian rhythms (CRs) of BMAL1, PER2, p53, and Ki-67 expression persist in all groups, except combined action of chronodestructors, and ADH5 CRs persist in all groups—thus, these rhythms in females are quite stable. CRs of the hepatocyte nuclei area are preserved in all the studied groups, although they undergo a significant shift. At the same time, the CRs of the hepatocyte area are destroyed under the action of light, both independently and in combination with CAI, and the CR of the nuclear-cytoplasmic ratio (NCR) is destroyed by exposure to CAI. It can be assumed that CRs of the hepatocyte area are significantly affected by dark deprivation and NCR rhythm is sensitive to ethanol consumption, while the stability of studied genes’ expression rhythms at separate influences of studied chronodestructors is maintained by yet unknown adaptation mechanisms. It is necessary to note that, according to our previous studies of male rats, rat females show significantly greater stability of the studied CRs.

Circadian rhythms-related disorders in diurnal fat sand rats under modern lifestyle conditions: A review

Modern lifestyle reduces environmental rhythmicity and may lead to circadian desynchrony. We are exposed to poor day-time lighting indoors and excessive night-time artificial light. We use air-conditioning to reduce ambient temperature cycle, and food is regularly available at all times. These disruptions of daily rhythms may lead to type 2 diabetes mellitus (T2DM), obesity, cardiometabolic diseases (CMD), depression and anxiety, all of which impose major public health and economic burden on societies. Therefore, we need appropriate animal models to gain a better understanding of their etiologic mechanisms, prevention, and management.We argue that the fat sand rat (Psammomys obesus), a diurnal animal model, is most suitable for studying the effects of modern-life conditions. Numerous attributes make it an excellent model to study human health disorders including T2DM, CMD, depression and anxiety. Here we review a comprehensive series of studies we and others conducted, utilizing the fat sand rat to study the underlying interactions between biological rhythms and health. Understanding these interactions will help deciphering the biological basis of these diseases, which often occur concurrently. We found that when kept in the laboratory (compared with natural and semi-wild outdoors conditions where they are diurnal), fat sand rats show low amplitude, nocturnal or arrhythmic activity patterns, dampened daily glucose rhythm, glucose intolerance, obesity and decreased survival rates. Short photoperiod acclimation exacerbates these pathologies and further dampens behavioral and molecular daily rhythms, resulting in CMD, T2DM, obesity, adipocyte dysfunction, cataracts, depression and anxiety. Increasing environmental rhythmicity by morning bright light exposure or by access to running wheels strengthens daily rhythms, and results in higher peak-to-trough difference in activity, better rhythmicity in clock genes expression, lower blood glucose and insulin levels, improved glucose tolerance, lower body and heart weight, and lower anxiety and depression. In summary, we have demonstrated that fat sand rats living under the correspondent of “human modern lifestyle” conditions exhibit dampened behavioral and biological rhythms and develop circadian desynchrony, which leads to what we have named “The Circadian Syndrome”. Environmental manipulations that increase rhythmicity result in improvement or prevention of these pathologies. Similar interventions in human subjects could have the same positive results and further research on this should be undertaken.

Effects of photoperiod and diet on BDNF daily rhythms in diurnal sand rats

Brain-derived neurotrophic factor (BDNF), its receptors and epigenetic modulators, are implicated in the pathophysiology of affective disorders, T2DM and the circadian system function. We used diurnal sand rats, which develop type 2 diabetes (T2DM), anxiety and depressive-like behavior under laboratory conditions. The development of these disorders is accelerated when animals are maintained under short photoperiod (5:19L:D, SP) compared to neutral photoperiod (12:12L:D, NP). We compared rhythms in plasma BDNF as well as BDNF and PER2 expression in the frontal cortex and suprachiasmatic nucleus (SCN) of sand rats acclimated to SP and NP. Acclimation to SP resulted in higher insulin levels, significantly higher glucose levels in the glucose tolerance test, and significantly higher anxiety- and depression-like behaviors compared with animals acclimated to NP. NP Animals exhibited a significant daily rhythm in plasma BDNF levels with higher levels during the night, and in BDNF expression levels in the frontal cortex and SCN. No significant BDNF rhythm was found in the plasma, frontal cortex or SCN of SP acclimated animals. We propose that in sand rats, BDNF may, at least in part, mediate the effects of circadian disruption on the development of anxiety and depressive-like behavior and T2DM.

[Melatonin: A short clarification for the over-enthusiasts]

Melatonin is a naturally occurring molecule derived from tryptophan. Melatonin is a key player in relaying the circadian rhythm between our environment and our body. It has also a key role in rhythming the seasons (more production during long nights and less during short ones) as well as in the reproduction cycles of the mammals. Melatonin is often and surprisingly presented as a molecule with multiple therapeutic properties that can fix (or help to fix) many health issues, such as diseases (cancer, ageing, virus-induced affections including COVID-19, etc…) or toxicological situations (metals, venoms, chemical such as adriamycin [doxorubicin], methotrexate or paclitaxel). The mechanistics behind those wonders is still missing and this is puzzling. In the present commentary, the main well-established biological properties are presented and briefly discussed with the aim of delineating the borders between facts and wishful thinking.

Ovarian activation delays in peripubertal ewe lambs infected with Haemonchus contortus can be avoided by supplementing protein in their diets

BACKGROUND

The ewe lamb nutritional and physiological state interfere with the ovarian environment and fertility. The lack or excess of circulating nutrients reaching the ovary can change its gene expression. A protein deficiency in the blood caused by an Haemonchus contortus abomasal infection is detrimental to the organism's development during puberty. The peripubertal period is a time of intensive growth that requires a high level of nutrients. An essential feature controlling pubertal arousal and female reproductive potential is ovarian follicle growth activation. Protein supplementation improves the sheep's immune response to helminthic infections. We aimed to determine if supplementing protein in infected ewe lambs' diet would impact the ovarian environment leading to earlier ovarian follicle activation than in infected not supplemented animals.

METHODS

We fed 18 Santa Ines ewe lambs (Ovis aries) - bred by the same ram - with either 12% protein (Control groups) or 19% protein (Supplemented groups) in their diets. After 35 days of the diet, they were each artificially infected or not with 10,000 Haemonchus contortus L3 larvae. Following 77 days of the diet and 42 days of infection, we surgically collected their left ovaries and examined their genes expression through RNA sequencing.

RESULTS

We found that protein supplementation in infected animals led to an up-regulation of genes (FDR p-values < 0.05) and biological processes (p-value cut-off = 0.01) linked to meiotic activation in pre-ovulatory follicles and primordial follicle activation, among others. The supplemented not infected animals also up-regulated genes and processes linked to meiosis and others, such as circadian behaviour. The not supplemented animals had these same processes down-regulated while up-regulated processes related to tissue morphogenesis, inflammation and immune response.

CONCLUSION

Diet's protein supplementation of peripubertal infected animals allowed them to express genes related to a more mature ovarian follicle stage than their half-sisters that were not supplemented. These results could be modelling potential effects of the interaction between environmental factors, nutrition and infection on reproductive health. When ovarian activation is achieved in a timely fashion, the ewe may generate more lambs during its reproductive life, increasing sheep breeders' productivity.

Ruminal Microbes Exhibit a Robust Circadian Rhythm and Are Sensitive to Melatonin

Gut hormones are not only able to regulate digestive, absorptive, and immune mechanisms of the intestine through biological rhythms, but impact the host through their interactions with intestinal microorganisms. Whether hormones in ruminal fluid have an association with the ruminal ecology is unknown. Objectives of the study were to examine relationships between the diurnal change in ruminal hormones and microbiota in lactating cows, and their associations in vivo and in vitro. For the in vivo study, six cows of similar weight (566.8 ± 19.6 kg), parity (3.0 ± 0.0), and milk performance (8,398.7 ± 1,392.9 kg/y) were used. They were adapted to natural light for 2 weeks before sampling and fed twice daily at 07:00 a.m. and 14:00 p.m. Serum, saliva, and ruminal fluid samples were collected at 02:00, 10:00, and 18:00 on the first day and 06:00, 14:00, and 22:00 on the second day of the experimental period. The concentrations of melatonin (MLT), growth hormone (GH), and prolactin (PRL) were measured via radioimmunoassay, whereas amplicon sequencing data were used to analyze relative abundance of microbiota in ruminal fluid. JTK_CYCLE analysis was performed to analyze circadian rhythms of hormone concentrations as well as the relative abundance of microbiota. For the in vitro study, exogenous MLT (9 ng) was added into ruminal fluid incubations to investigate the impacts of MLT on ruminal microbiota. The results not only showed that rumen fluid contains MLT, but the diurnal variation of MLT and the relative abundance of 9% of total rumen bacterial operational taxonomic units (OTUs) follow a circadian rhythm. Although GH and PRL were also detected in ruminal fluid, there was no obvious circadian rhythm in their concentrations. Ruminal MLT was closely associated with Muribaculaceae, Succinivibrionaceae, Veillonellaceae, and Prevotellaceae families in vivo. In vitro, these families were significantly influenced by melatonin treatment, as melatonin treatment increased the relative abundance of families Prevotellaceae, Muribaculaceae while it reduced the relative abundance of Succinivibrionaceae, Veillonellaceae. Collectively, ruminal microbes appear to maintain a circadian rhythm that is associated with the profiles of melatonin. As such, data suggest that secretion of melatonin into the rumen could play a role in host-microbe interactions in ruminants.

Research Hot Spots and Trends on Melatonin From 2000 to 2019

Research on melatonin remains one of the major hot spots in the field of disease treatment, but relevant data are numerous. The purpose of this study was to quantitatively and qualitatively analyze the progress of melatonin research through the method of bibliometrics and to predict hot spots and trends in melatonin research. This study retrieved all the studies on melatonin from 2000 to 2019 in the Web of Science and PubMed and analysed the publishing trends in the literature on a bibliometric online analysis platform and CiteSpace software. The research results were also visually analysed to summarize melatonin research hot spots through gCLUTO and pubMR. The study retrieved a total of 20,351 publications, of which the number of US publications ranked first, accounting for 21.46%, with the greatest impact (centrality = 0.31). The University of Texas Health Science Center at San Antonio and Harvard University had the highest average number of citations at 43.19 and 33.96, respectively. Journal of Pineal Research had the highest average number of citations in 2,993 journals. Professor Reiter made the largest contribution to this area. We further analysed 100 highly cited articles for clinical applications and ongoing related clinical drug trials based on the first hot spot. We systematically analysed melatonin for nearly 20 years while predicting the main research trends in the future, which may provide new directions and ideas for melatonin research. The structure and normal physiological functions of melatonin have been intensively studied in the past few years. And clinical application research and target of melatonin treatment for different diseases and target-based drug design will certainly become the focus of melatonin research.

Worldwide melatonin research: a bibliometric analysis of the published literature between 2015 and 2019

This study presents a bibliometric analysis of the publications on melatonin research from the Scopus database during the period 2015-2019. Based on the keywords used, which are related to melatonin in the article title, the study retrieved 4411 documents for further analysis using various tools. We used Microsoft Excel to conduct the frequency analysis, VOSviewer for data visualization, and Harzing's Publish or Perish for citation metrics and analysis. This study reports the results using standard bibliometric indicators such as the growth of publications, authorship patterns, collaboration, and prolific authors, country contribution, most active institutions, preferred journals, and top-cited articles. Based on our findings, there is a continuous growth of publications on melatonin research for 5 years since 2015. China was the largest contributor to melatonin research, followed by the United States. The Journal of Pineal Research published the most number of publications related to melatonin research. Our findings suggest that the role of melatonin in plant and food sciences, as well as in cancer, may in later years take over the clusters that earlier dominated melatonin research.

Investigation on the influence of isolated environment on human psychological and physiological health

Crewmembers are working and living in isolated environment lacking natural light and perception. Although their health problems have been documented, the mechanism has not been thoroughly investigated. The aim of the present study is to investigate the psychological and physiological influences of isolated environment on crewmember's health. On account of complexity of the isolated environment, it is necessary to have a manually controllable system to simulate research platform-Bioregenerative Life Support System (BLSS). Symptom checklist 90 (SCL-90) was used to complete emotional status test. Urine samples were collected for metabonomics and hormone secretion analysis. Fecal samples were collected for intestinal microorganisms analysis. Crewmembers (n = 4) followed strict daily schedule during the experimental period. Five emotional factors were significantly (P < 0.05) increased, differential metabolites were enriched in tryptophan metabolism pathway, the relative abundance of Prevotella decreased significantly (P < 0.0001) when crewmembers in isolated environment without natural light. Hormone (melatonin, cortisol) secretion rhythm also changed. Significant positive correlation (r = 0.805, P < 0.05) between cortisol secretion and anxiety was observed. In conclusion, natural light simulation in an isolated environment may have a positive effect on the physiological and psychological health of the crewmember.

Diabetes and atherothrombosis: The circadian rhythm and role of melatonin in vascular protection

Obesity-related euglycaemic insulin resistance clusters with cardiometabolic risk factors, contributing to the development of both type 2 diabetes and cardiovascular disease. An increased thrombotic tendency in diabetes stems from platelet hyperactivity, enhanced activity of prothrombotic coagulation factors and impaired fibrinolysis. Furthermore, a low-grade inflammatory response and increased oxidative stress accelerate the atherosclerotic process and, together with an enhanced thrombotic environment, result in premature and more severe cardiovascular disease. The disruption of circadian cycles in man secondary to chronic obesity and loss of circadian cues is implicated in the increased risk of developing diabetes and cardiovascular disease. Levels of melatonin, the endogenous synchronizer of circadian rhythm, are reduced in individuals with vascular disease and those with deranged glucose metabolism. The anti-inflammatory, antihypertensive, antioxidative and antithrombotic activities of melatonin make it a potential therapeutic agent to reduce the risk of vascular occlusive disease in diabetes. The mechanisms behind melatonin-associated reduction in procoagulant response are not fully known. Current evidence suggests that melatonin inhibits platelet aggregation and might affect the coagulation cascade, altering fibrin clot structure and/or resistance to fibrinolysis. Large-scale clinical trials are warranted to investigate the effects of modulating the circadian clock on insulin resistance, glycaemia and cardiovascular outcome.

Circumventricular Organs and Parasite Neurotropism: Neglected Gates to the Brain?

Circumventricular organs (CVOs), neural structures located around the third and fourth ventricles, harbor, similarly to the choroid plexus, vessels devoid of a blood-brain barrier (BBB). This enables them to sense immune-stimulatory molecules in the blood circulation, but may also increase chances of exposure to microbes. In spite of this, attacks to CVOs by microbes are rarely described. It is here highlighted that CVOs and choroid plexus can be infected by pathogens circulating in the bloodstream, providing a route for brain penetration, as shown by infections with the parasites Trypanosoma brucei. Immune responses elicited by pathogens or systemic infections in the choroid plexus and CVOs are briefly outlined. From the choroid plexus trypanosomes can seed into the ventricles and initiate accelerated infiltration of T cells and parasites in periventricular areas. The highly motile trypanosomes may also enter the brain parenchyma from the median eminence, a CVO located at the base of the third ventricle, by crossing the border into the BBB-protected hypothalamic arcuate nuclei. A gate may, thus, be provided for trypanosomes to move into brain areas connected to networks of regulation of circadian rhythms and sleep-wakefulness, to which other CVOs are also connected. Functional imbalances in these networks characterize human African trypanosomiasis, also called sleeping sickness. They are distinct from the sickness response to bacterial infections, but can occur in common neuropsychiatric diseases. Altogether the findings lead to the question: does the neglect in reporting microbe attacks to CVOs reflect lack of awareness in investigations or of gate-opening capability by microbes?

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

Reflections on melatonin: focus on child mental health

Aims and Method

To examine the propriety of the use of melatonin in child and adolescent psychiatry based on findings retrieved from multiple electronic databases including the Cochrane Library resource, covering 1950–2007.

Results

There is evidence for the effectiveness of melatonin in the initiation of sleep in children with a variety of neurodevelopmental disorders but its value in improving quality of sleep is doubtful. Like hormones in general, melatonin has multifarious action sites and hence potential for many side-effects. Posological issues and long-term side-effects are yet to be validly determined.

Clinical Implications

Sleep hygiene should be a major component of the routine intervention programme for insomnia and melatonin judiciously prescribed until the safety of long-term use is known.

Skeptical approaches concerning the effect of exposure to electromagnetic fields on brain hormones and enzyme activities

This review discusses the effects of various frequencies of electromagnetic fields (EMF) on brain hormones and enzyme activity. In this context, the mechanism underlying the effects of EMF exposure on tissues generally and cellular pathway specifically has been discussed. The cell membrane plays important roles in mediating enzymatic activities as to response and reacts with extracellular environment. Alterations in the calcium signaling pathways in the cell membrane are activated in response to the effects of EMF exposure. Experimental and epidemiological studies have demonstrated that no changes occur in serum prolactin levels in humans following short-term exposure to 900 Mega Hertz (MHz) EMF emitted by mobile phones. The effects of EMF on melatonin and its metabolite, 6-sulfatoxymelatonin, in humans have also been investigated in the clinical studies to show a disturbance in metabolic activity of melatonin. In addition, although 900 MHz EMF effects on NF-κB inflammation, its effects on NF-κB are not clear. Abbreviations: ELF-EMF, extremely low frequency electromagnetic fields; EMF, electromagnetic fields; RF, Radiofrequency; ROS, reactive oxygen species; VGCCs, voltage-gated calcium channels; MAPK, mitogen-activated phosphokinase; NF-κB, nuclear factor kappa B; ERK-1/2, extracellular signal-regulated kinase; GSH-Px, glutathione peroxidase; JNK, Jun N-terminal kinases; SOD, superoxide dismutase; MnSOD, manganese-dependent superoxide dismutase; GLUT1, glucose transporter 1; GSSG-Rd, glutathione reductase MDA malondialdehyde; NO, nitric oxide; LH, luteinizing hormone; FSH, follicle-stimulating hormone.

Melatonin and Hippo Pathway: Is There Existing Cross-Talk?

Melatonin is an indolic hormone that regulates a plethora of functions ranging from the regulation of circadian rhythms and antioxidant properties to the induction and maintenance of tumor suppressor pathways. It binds to specific receptors as well as to some cytosolic proteins, leading to several cellular signaling cascades. Recently, the involvement of melatonin in cancer insurgence and progression has clearly been demonstrated. In this review, we will first describe the structure and functions of melatonin and its receptors, and then discuss both molecular and epidemiological evidence on melatonin anticancer effects. Finally, we will shed light on potential cross-talk between melatonin signaling and the Hippo signaling pathway, along with the possible implications for cancer therapy.

Good night, sleep tight: The effects of sleep deprivation on spatial associative learning in zebrafish

Learning and memory are vital to an animal's survival, and numerous factors can disrupt cognitive performance. Sleep is an evolutionarily conserved physiological process known to be important for the consolidation of learning and memory. The zebrafish has emerged as a powerful model organism sharing organizational and functional characteristics with other vertebrates, providing great translational relevance. In our study, we used a simple spatial associative learning task to quantify the effects of sleep deprivation (partial vs. total) on learning performance in zebrafish, using an animated conspecific shoal image as a reward. Control animals maintained on a regular light:dark cycle were able to acquire the association between the unconditioned and conditioned stimulus, reinforcing zebrafish as a valid and reliable model for appetitive conditioning tasks. Notably, sleep deprivation did not alter the perception of and response to the conspecific image. In contrast, although partial sleep deprivation did not impair cognitive performance, total sleep deprivation significantly impaired performance on the associative learning task. Our results suggest that sleep is important for learning and memory, and that the effects of sleep deprivation on these processes can be investigated in zebrafish.

Suprachiasmatic Nucleus Interaction with the Arcuate Nucleus; Essential for Organizing Physiological Rhythms

The suprachiasmatic nucleus (SCN) is generally considered the master clock, independently driving all circadian rhythms. We recently demonstrated the SCN receives metabolic and cardiovascular feedback adeptly altering its neuronal activity. In the present study, we show that microcuts effectively removing SCN-arcuate nucleus (ARC) interconnectivity in Wistar rats result in a loss of rhythmicity in locomotor activity, corticosterone levels, and body temperature in constant dark (DD) conditions. Elimination of these reciprocal connections did not affect SCN clock gene rhythmicity but did cause the ARC to desynchronize. Moreover, unilateral SCN lesions with contralateral retrochiasmatic microcuts resulted in identical arrhythmicity, proving that for the expression of physiological rhythms this reciprocal SCN-ARC interaction is essential. The unaltered SCN c-Fos expression following glucose administration in disconnected animals as compared to a significant decrease in controls demonstrates the importance of the ARC as metabolic modulator of SCN neuronal activity. Together, these results indicate that the SCN is more than an autonomous clock, and forms an essential component of a larger network controlling homeostasis. The present novel findings illustrate how an imbalance between SCN and ARC communication through circadian disruption could be involved in the etiology of metabolic disorders.

"Singing" Fish Rely on Circadian Rhythm and Melatonin for the Timing of Nocturnal Courtship Vocalization

The patterning of social acoustic signaling at multiple timescales, from day-night rhythms to acoustic temporal properties, enhances sender-receiver coupling and reproductive success [1-8]. In diurnal birds, the nocturnal production of melatonin, considered the major vertebrate timekeeping hormone [9, 10], suppresses vocal activity but increases song syllable duration over circadian and millisecond timescales, respectively [11, 12]. Comparable studies are lacking for nocturnal vertebrates, including many teleost fish species that are also highly vocal during periods of reproduction [4, 13-20]. Utilizing continuous sound recordings, light cycle manipulations, hormone implants, and in situ hybridization, we demonstrate in a nocturnally breeding teleost fish that (1) courtship vocalization exhibits an endogenous circadian rhythm under constant dark conditions that is suppressed under constant light, (2) exogenous delivery of a melatonin analog under inhibitory constant light conditions rescues courtship vocal activity as well as the duration of single calls, and (3) melatonin receptor 1b is highly expressed in evolutionarily conserved neuroendocrine and vocal-acoustic networks crucial for patterning reproductive and vocal behaviors in fishes and tetrapods. Our findings, together with those in birds, show melatonin's remarkable versatility as a timing signal in distantly related lineages. It exerts opposing effects on vocalization in nocturnal versus diurnal species at the circadian timescale but comparable effects at the finer timescale of acoustic features. We propose that melatonin's separable effects at different timescales depends on its actions within distinct neural networks that control circadian rhythms, reproduction, and vocalization, which may be selected upon over evolutionary time as dissociable modules to pattern and coordinate social behaviors. VIDEO ABSTRACT.

Adaptations of the aging animal to exercise: role of daily supplementation with melatonin

The pineal gland, through melatonin, seems to be of fundamental importance in determining the metabolic adaptations of adipose and muscle tissues to physical training. Evidence shows that pinealectomized animals fail to develop adaptive metabolic changes in response to aerobic exercise and therefore do not exhibit the same performance as control-trained animals. The known prominent reduction in melatonin synthesis in aging animals led us to investigate the metabolic adaptations to physical training in aged animals with and without daily melatonin replacement. Male Wistar rats were assigned to four groups: sedentary control (SC), trained control (TC), sedentary treated with melatonin (SM), and trained treated with melatonin (TM). Melatonin supplementation lasted 16 wk, and the animals were subjected to exercise during the last 8 wk of the experiment. After euthanasia, samples of liver, muscle, and adipose tissues were collected for analysis. Trained animals treated with melatonin presented better results in the following parameters: glucose tolerance, physical capacity, citrate synthase activity, hepatic and muscular glycogen content, body weight, protein expression of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and protein kinase activated by adenosine monophosphate (AMPK) in the liver, as well as the protein expression of the glucose transporter type 4 (GLUT4) and AMPK in the muscle. In conclusion, these results demonstrate that melatonin supplementation in aging animals is of great importance for the required metabolic adaptations induced by aerobic exercise. Adequate levels of circulating melatonin are, therefore, necessary to improve energetic metabolism efficiency, reducing body weight and increasing insulin sensitivity.

Effects of circadian phase and melatonin injection on anxiety-like behavior in nocturnal and diurnal rodents

Animals show daily rhythms in most bodily functions, resulting from the integration of information from an endogenous circadian clock and external stimuli. These rhythms are adaptive and are expected to be related to activity patterns, i.e., to be opposite in diurnal and nocturnal species. Melatonin is secreted during the night in all mammalian species, regardless of their activity patterns. Consequently, in diurnal species the nocturnal secretion of melatonin is concurrent with the resting phase, whereas in nocturnal species it is related to an increase in activity. In this research, we examined in three diurnal and three nocturnal rodent species whether a daily rhythm in anxiety-like behavior exists; whether it differs between nocturnal and diurnal species; and how melatonin affects anxiety-like behavior in species with different activity patterns. Anxiety-like behavior levels were analyzed using the elevated plus-maze. We found a daily rhythm in anxiety-like behavior and a significant response to daytime melatonin administration in all three nocturnal species, which showed significantly lower levels of anxiety during the dark phase, and after melatonin administration. The diurnal species showed either an inverse pattern to that of the nocturnal species in anxiety-like behavior rhythm and in response to daytime melatonin injection, or no rhythm and, accordingly, no response to melatonin.

Pineal melatonin level disruption in humans due to electromagnetic fields and ICNIRP limits

The International Agency for Research on Cancer (IARC) classifies electromagnetic fields (EMFs) as 'possibly carcinogenic' to humans that might transform normal cells into cancer cells. Owing to high utilisation of electricity in day-to-day life, exposure to power-frequency (50 or 60 Hz) EMFs is unavoidable. Melatonin is a natural hormone produced by pineal gland activity in the brain that regulates the body's sleep-wake cycle. How man-made EMFs may influence the pineal gland is still unsolved. The pineal gland is likely to sense EMFs as light but, as a consequence, may decrease the melatonin production. In this study, more than one hundred experimental data of human and animal studies of changes in melatonin levels due to power-frequency electric and magnetic fields exposure were analysed. Then, the results of this study were compared with the International Committee of Non-Ionizing Radiation Protection (ICNIRP) limit and also with the existing experimental results in the literature for the biological effect of magnetic fields, in order to quantify the effects. The results show that this comparison does not seem to be consistent despite the fact that it offers an advantage of drawing attention to the importance of the exposure limits to weak EMFs. In addition to those inconsistent results, the following were also observedfrom this work: (i) the ICNIRP recommendations are meant for the well-known acute effects, because effects of the exposure duration cannot be considered and (ii) the significance of not replicating the existing experimental studies is another limitation in the power-frequency EMFs. Regardless of these issues, the above observation agrees with our earlier study in which it was confirmed that it is not a reliable method to characterise biological effects by observing only the ratio of AC magnetic field strength to frequency. This is because exposure duration does not include the ICNIRP limit. Furthermore, the results show the significance of disruption of melatonin due to exposure to weak EMFs, which may possibly lead to long-term health effects in humans.

The melanocyte photosensory system in the human skin

The pigment cells form the largest population of neural crest cells to migrate into the epidermis and hair follicle along each dermatomic area from the neural folds. The melanopsin system responsible for photoentrainment, was isolated from the photosensitive dermal melanophores of frogs Xenopus laevis responding to light. Melanocytes form a photoresponsive network which reads the environmental seasonal variations in the light cycles in the same manner. The present work was undertaken to study the organization of this system by: I. Experimental assessment of photoresponse and II. Evidence of an organized system of photoreception in the skin. Melanocytes, in whole skin organ cultures and epidermal strips, from margin of vitiligo in G₂ phase show prominent dendricity, and express pigment, biogenic amines and hormones on UV exposure. The photoresponse depends on the photosensitive enzymes NAT/HIOMT and dopaoxidase. Melanocytes interact with adjacent keratinocytes, dermal capillaries, and nerve endings. The melanocyte network reads the diurnal and seasonal photophase by the melatonin/serotonin switch like the pineal. Sleep disorders and winter depression are corrected by phototherapy utilising this mechanism. Melanocytes showing photoactivity, aplasia, hypoplasia and hyperplasia, and interactive keratinocytes occupy the trigeminal, brachial and lumbosacral dermatomes, zones of high embryonic induction, forming an ectodermal placodal system. Melanin units and hair follicles serve as photoreceptors. Migration of active melanocytes to defined areas is evident in pigment patterns in guinea pigs. This study identifies defined photoreceptor melanocyte/epidermal domains which read the seasonal photophase and control the sleep waking cycle in response to the environmental light. I. Whole skin organ cultures, and epidermal strips from margin of vitiligo in G₂ phase are exposed to UV and IR to study sequential and dose response of marginal melanocytes, using histochemistry, immunohistochemistry to assess pigment, biogenic amines and hormones on UV exposure. II. Dermatomic Distributions: Detailed maps of melanocyte photoresponse in 356 biopsies, lesions in 297 vitiligo, 100 melanosis, 165 melanomas 142 leprosy and 442 basal cell/keratinocytes lesions were assessed for patterns of dermatomic distribution. Embryonal melanocyte migration along dermatomes was assessed in 285 guinea pigs from an inbred colony having black, brown and white patches.

Abnormal Skeletal Growth in Adolescent Idiopathic Scoliosis Is Associated with Abnormal Quantitative Expression of Melatonin Receptor, MT2

The defect of the melatonin signaling pathway has been proposed to be one of the key etiopathogenic factors in adolescent idiopathic scoliosis (AIS). A previous report showed that melatonin receptor, MT2, was undetectable in some AIS girls. The present study aimed to investigate whether the abnormal MT2 expression in AIS is quantitative or qualitative. Cultured osteoblasts were obtained from 41 AIS girls and nine normal controls. Semi-quantification of protein expression by Western blot and mRNA expression by TaqMan real-time PCR for both MT1 and MT2 were performed. Anthropometric parameters were also compared and correlated with the protein expression and mRNA expression of the receptors. The results showed significantly lower protein and mRNA expression of MT2 in AIS girls compared with that in normal controls (p = 0.02 and p = 0.019, respectively). No differences were found in the expression of MT1. When dichotomizing the AIS girls according to their MT2 expression, the group with low expression was found to have a significantly longer arm span (p = 0.036). The results of this study showed for the first time a quantitative change of MT2 in AIS that was also correlated with abnormal arm span as part of abnormal systemic skeletal growth.

Pineal and gonadal influences on ultradian locomotor rhythms of male Siberian hamsters

The extent to which changes in ultradian and circadian rhythms (URs and CRs) reflect seasonal variations in pineal melatonin secretion was assessed in male Siberian hamsters transferred from long to short day lengths. The period of the locomotor activity UR increased from 2.5 h in long days to 4.5 h in short day lengths, but this and most other features of the short-day ultradian phenotype were unaffected by pinealectomy; only the short-day increase in UR amplitude was counteracted by pineal extirpation. Virtually all UR components were unaffected by gonadectomy or replacement testosterone or estradiol treatment; changes in testicular hormone secretion appear insufficient to account for seasonal fluctuation in URs. Pinealectomy did not affect activity onsets and offsets or phase angles of CR entrainment in short and long day lengths; the duration of nocturnal activity was equivalently longer in short than long days in both pinealectomized and pineal-intact hamsters. CR robustness of pinealectomized hamsters in short days was intermediate between values of long-day and short-day sham-pinealectomized males. Hourly nocturnal locomotor activity was markedly reduced in SD, and this effect was completely reversed by PINx. We conclude that seasonal transitions in UR and CR waveforms controlled by day length are mediated primarily by melatonin-independent mechanisms, with lesser contributions from melatonin-dependent processes. Most seasonal changes in ultradian and circadian rhythms in males of this species are not influenced by gonadal hormones. URs may allow animals to respond appropriately to changing environmental contingencies. In winter reduced activity combined with temporal restructuring of activity to include longer intervals of rest may be adaptive in maintaining body temperature at lower values and down-regulating energy expenditure when above ground temperatures are extremely low.

Maternal melatonin programs the daily pattern of energy metabolism in adult offspring

Background

Shift work was recently described as a factor that increases the risk of Type 2 diabetes mellitus. In addition, rats born to mothers subjected to a phase shift throughout pregnancy are glucose intolerant. However, the mechanism by which a phase shift transmits metabolic information to the offspring has not been determined. Among several endocrine secretions, phase shifts in the light/dark cycle were described as altering the circadian profile of melatonin production by the pineal gland. The present study addresses the importance of maternal melatonin for the metabolic programming of the offspring.

Methodology/Principal Findings

Female Wistar rats were submitted to SHAM surgery or pinealectomy (PINX). The PINX rats were divided into two groups and received either melatonin (PM) or vehicle. The SHAM, the PINX vehicle and the PM females were housed with male Wistar rats. Rats were allowed to mate and after weaning, the male and female offspring were subjected to a glucose tolerance test (GTT), a pyruvate tolerance test (PTT) and an insulin tolerance test (ITT). Pancreatic islets were isolated for insulin secretion, and insulin signaling was assessed in the liver and in the skeletal muscle by western blots. We found that male and female rats born to PINX mothers display glucose intolerance at the end of the light phase of the light/dark cycle, but not at the beginning. We further demonstrate that impaired glucose-stimulated insulin secretion and hepatic insulin resistance are mechanisms that may contribute to glucose intolerance in the offspring of PINX mothers. The metabolic programming described here occurs due to an absence of maternal melatonin because the offspring born to PINX mothers treated with melatonin were not glucose intolerant.

Conclusions/Significance

The present results support the novel concept that maternal melatonin is responsible for the programming of the daily pattern of energy metabolism in their offspring.

Melatonin modulates cytoskeletal organization in the rat brain hippocampus

Melatonin concentration in plasma reaches high levels during the night and synchronizes body rhythms with the photoperiod. Previous evidence obtained in cultured cells suggests that melatonin synchronizes cytoskeletal re-arrangements at nocturnal plasma concentration. In this study, we determined the amount of microtubules and microfilaments in the rat hippocampus as an index of cytoskeletal organization in rats submitted to a photoperiodic regime. Additionally, these parameters were determined in control rats, sham rats, pinealectomized rats, and rats that were pinealectomized and treated with melatonin for 1 week. The results showed an increase in both the amount of microfilaments in the hippocampus of rats sacrificed in the dark phase, and in melatonin levels. In addition, a decrease in both microfilament and microtubule amounts occurred in pinealectomized rats. In contrast, melatonin treatment partially reestablished actin and tubulin proportions organized in microfilaments and microtubules, respectively. The results indicate that actin organization in microfilaments was associated with both the photoperiod and with melatonin levels. Together, the data support that cytoskeletal organization is regulated rhythmically by melatonin in synchrony with the photoperiod.

Melatonin elicits protein kinase C-mediated calcium response in immortalized GT1-7 GnRH neurons

Melatonin is suggested to have effects on hypothalamic-pituitary-gonadal (HPG) axis. The pulsatile pattern of GnRH release, which results in the intermittent release of gonadotropic hormones from the pituitary, has a critical importance for reproductive function but the factors responsible from this release pattern are not known. Calcium is a second messenger involved in hormone release. Therefore, investigation of the effects of melatonin on intracellular free calcium levels ([Ca(2+)](i)) would provide critical information on hormone release in immortalized GnRH neurons. The pattern of melatonin-induced intracellular calcium signaling was investigated by fluorescence calcium imaging using the immortalized GnRH-secreting GT1-7 hypothalamic neurons. Melatonin caused a significant increase in [Ca(2+)](i,) which was greatly blocked by luzindole, a melatonin antagonist, or attenuated by pre-treatment with protein kinase C inhibitor. This study suggests that melatonin seems to have a direct effect on GnRH neurons.

Therapeutic potential of melatonin and its analogs in Parkinson's disease: focus on sleep and neuroprotection

Sleep disorders constitute major nonmotor features of Parkinson's disease (PD) that have a substantial effect on patients' quality of life and can be related to the progression of the neurodegenerative disease. They can also serve as preclinical markers for PD, as it is the case for rapid eye movement (REM)-associated sleep behavior disorder (RBD). Although the etiology of sleep disorders in PD remains undefined, the assessment of the components of the circadian system, including melatonin secretion, could give therapeutically valuable insight on their pathophysiopathology. Melatonin is a regulator of the sleep/wake cycle and also acts as an effective antioxidant and mitochondrial function protector. A reduction in the expression of melatonin MT(1) and MT(2) receptors has been documented in the substantia nigra of PD patients. The efficacy of melatonin for preventing neuronal cell death and for ameliorating PD symptoms has been demonstrated in animal models of PD employing neurotoxins. A small number of controlled trials indicate that melatonin is useful in treating disturbed sleep in PD, in particular RBD. Whether melatonin and the recently developed melatonergic agents (ramelteon, tasimelteon, agomelatine) have therapeutic potential in PD is also discussed.

Scheduled Food Hastens Re-Entrainment More Than Melatonin Does after a 6-h Phase Advance of the Light-Dark Cycle in Rats

Circadian desynchrony occurs when individuals are exposed to abrupt phase shifts of the light-dark cycle, as in jet lag. For reducing symptoms and for speeding up resynchronization, several strategies have been suggested, including scheduled exercise, exposure to bright light, drugs, and especially exogenous melatonin administration. Restricted feeding schedules have shown to be powerful entraining signals for metabolic and hormonal daily cycles, as well as for clock genes in tissues and organs of the periphery. This study explored in a rat model of jet lag the contribution of exogenous melatonin or scheduled feeding on the re-entrainment speed of spontaneous general activity and core temperature after a 6-h phase advance of the light-dark cycle. In a first phase, the treatment was scheduled for 5 days prior to the phase shift, while in a second stage, the treatment was simultaneous with the phase advance of the light-dark cycle. Melatonin administration and especially scheduled feeding simultaneous with the phase shift improved significantly the re-entrainment speed. The evaluation of the free-running activity and temperature following the 5-day treatment proved that both exogenous melatonin and specially scheduled feeding accelerated re-entrainment of the SCN-driven general activity and core temperature, respectively, with 7, 5 days ( p < 0.01) and 3, 3 days ( p < 0.001). The present results show the relevance of feeding schedules as entraining signals for the circadian system and highlight the importance of using them as a strategy for preventing internal desynchrony.

Melatonin in mitochondrial dysfunction and related disorders

Mitochondrial dysfunction is considered one of the major causative factors in the aging process, ischemia/reperfusion (I/R), septic shock, and neurodegenerative disorders like Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD). Increased free radical generation, enhanced mitochondrial inducible nitric oxide (NO) synthase activity, enhanced NO production, decreased respiratory complex activity, impaired electron transport system, and opening of mitochondrial permeability transition pore all have been suggested as factors responsible for impaired mitochondrial function. Melatonin, the major hormone of the pineal gland, also acts as an antioxidant and as a regulator of mitochondrial bioenergetic function. Both in vitro and in vivo, melatonin was effective for preventing oxidative stress/nitrosative stress-induced mitochondrial dysfunction seen in experimental models of PD, AD, and HD. In addition, melatonin is known to retard aging and to inhibit the lethal effects of septic shock or I/R lesions by maintaining respiratory complex activities, electron transport chain, and ATP production in mitochondria. Melatonin is selectively taken up by mitochondrial membranes, a function not shared by other antioxidants. Melatonin has thus emerged as a major potential therapeutic tool for treating neurodegenerative disorders such as PD or AD, and for preventing the lethal effects of septic shock or I/R.

Circadian periods of sensitivity for ramelteon on the onset of running-wheel activity and the peak of suprachiasmatic nucleus neuronal firing rhythms in C3H/HeN mice

Ramelteon, an MT(1)/MT(2) melatonin receptor agonist, is used for the treatment of sleep-onset insomnia and circadian sleep disorders. Ramelteon phase shifts circadian rhythms in rodents and humans when given at the end of the subjective day; however, its efficacy at other circadian times is not known. Here, the authors determined in C3H/HeN mice the maximal circadian sensitivity for ramelteon in vivo on the onset of circadian running-wheel activity rhythms, and in vitro on the peak of circadian rhythm of neuronal firing in suprachiasmatic nucleus (SCN) brain slices. The phase response curve (PRC) for ramelteon (90 µg/mouse, subcutaneous [sc]) on circadian wheel-activity rhythms shows maximal sensitivity during the late mid to end of the subjective day, between CT8 and CT12 (phase advance), and late subjective night and early subjective day, between CT20 and CT2 (phase delay), using a 3-day-pulse treatment regimen in C3H/HeN mice. The PRC for ramelteon resembles that for melatonin in C3H/HeN mice, showing the same magnitude of maximal shifts at CT10 and CT2, except that the range of sensitivity for ramelteon (CT8-CT12) during the subjective day is broader. Furthermore, in SCN brain slices in vitro, ramelteon (10 pM) administered at CT10 phase advances (5.6 ± 0.29 h, n = 3) and at CT2 phase delays (-3.2 ± 0.12 h, n = 6) the peak of circadian rhythm of neuronal firing, with the shifts being significantly larger than those induced by melatonin (10 pM) at the same circadian times (CT10: 2.7 ± 0.15 h, n = 4, p < .05; CT2: -1.13 ± 0.08 h, n = 6, p < .001, respectively). The phase shifts induced by both melatonin and ramelteon in the SCN brain slice at either CT10 or CT2 corresponded with the period of sensitivity observed in vivo. In conclusion, melatonin and ramelteon showed identical periods of circadian sensitivity at CT10 (advance) and CT2 (delay) to shift the onset of circadian activity rhythms in vivo and the peak of SCN neuronal firing rhythms in vitro.

Antioxidant enzyme and element status in heroin addiction or heroin withdrawal in rats: effect of melatonin and vitamin E plus Se

Heroin use, withdrawal syndrome, and heroin-related deaths are still the most serious public health problems. Antioxidants and bio-elements are essential for metabolism in living organisms. To our knowledge, there are no data about the effect of antioxidant therapy on the levels of bio-elements and antioxidant enzymes in the naloxone (NX)-induced heroin withdrawal syndrome. Therefore, in the present study for the first time, we have investigated the role of antioxidant therapy, melatonin, and vitamin E plus Se, on the trace and major elements and antioxidant enzymes in the heroin addiction or heroin withdrawal in rats. Glutathione peroxidase levels were increased and catalase levels were decreased in the all study groups when compared to the sham group. The level of superoxide dismutase (SOD) in the fixed dose of heroin (FDH) given group was lower; however, in the variable doses of heroin (VDH) given group SOD level was higher. Furthermore, in withdrawal syndrome, Fe, Mg, Mn, and Ti levels were diminished and Al, Ca, and Cu levels were increased in the FDH+NX group. Moreover, Mg, Mn, and Se levels were also diminished and Al level was increased in the VDH+NX group. In conclusion, our results obviously indicated that heroin effected both bio-element status and antioxidant enzyme activities and, exogenous melatonin or vE+Se therapy might relieve on the element and antioxidant enzyme the destructive activity caused by heroin.

Tissue trace and major element levels in organophosphate insecticide fenthion (Lebaycid) toxicity in rats: prophylactic and therapeutic effect of exogenous melatonin

Organophosphate compounds are very toxic chemicals and used in widespread applications. The present study was designed to examine the role of exogenous melatonin against organophosphate toxicity in tissues (brain, heart, jejunum, kidney, liver, lung, muscle and pancreas) trace and major element levels of rats. Trace and major element concentrations in the tissues were measured in the sham group, the control group, prophylaxis with the melatonin group and therapy with the melatonin group (TM) by inductively coupled plasma-optical emission spectroscopy. Statistically significant differences among the experimental groups were detected for some tissue trace and major element concentrations. In the brain tissue, the Al, Mn and Se concentrations in the sham group were significantly higher than those in the control group (p<0.05). In the heart tissue, the Cu, Mn and Se concentrations in the sham group were significantly increased than those in the control group (p<0.05). In the kidney tissue, trace and major element concentrations in the TM group were significantly lower than those in the sham group (Fe and Mn; p<0.05, Cu, Mo, Ni, Ti, V and Zn; p<0.01). In the liver, Mg, Al, Zn and Ca concentrations in the TM group were significantly higher than those in the fenthion-treated control group (p<0.01). In the muscle tissue, element concentrations in the TM group were significantly lower when compared with the sham groups (Ca and Si; p<0.01). The Al, Cr, Mo, Ni, Si and Zn element concentrations were markedly decreased in the control group as compared with the TM group in the pancreas tissue (p<0.01). In conclusion, according to the results of the present study the major findings are that the fenthion-treated rat's tissue element levels were effected and the melatonin may normalize the altered levels of some trace and major elements of the tissues in organophosphate toxicity.

Potential use of melatonergic drugs in analgesia: mechanisms of action

Melatonin is a remarkable molecule with diverse physiological functions. Some of its effects are mediated by receptors while other, like cytoprotection, seem to depend on direct and indirect scavenging of free radicals not involving receptors. Among melatonin's many effects, its antinociceptive actions have attracted attention. When given orally, intraperitoneally, locally, intrathecally or through intracerebroventricular routes, melatonin exerts antinociceptive and antiallodynic actions in a variety of animal models. These effects have been demonstrated in animal models of acute pain like the tail-flick test, formalin test or endotoxin-induced hyperalgesia as well as in models of neuropathic pain like nerve ligation. Glutamate, gamma-aminobutyric acid, and particularly, opioid neurotransmission have been demonstrated to be involved in melatonin's analgesia. Results using melatonin receptor antagonists support the participation of melatonin receptors in melatonin's analgesia. However, discrepancies between the affinity of the receptors and the very high doses of melatonin needed to cause effects in vivo raise doubts about the uniqueness of that physiopathological interpretation. Indeed, melatonin could play a role in pain through several alternative mechanisms including free radicals scavenging or nitric oxide synthase inhibition. The use of melatonin analogs like the MT(1)/MT(2) agonist ramelteon, which lacks free radical scavenging activity, could be useful to unravel the mechanism of action of melatonin in analgesia. Melatonin has a promising role as an analgesic drug that could be used for alleviating pain associated with cancer, headache or surgical procedures.

Ramelteon: a review of its therapeutic potential in sleep disorders

Ramelteon is a tricyclic synthetic analog of melatonin that acts specifically on MT(1) and MT(2) melatonin receptors. Ramelteon's half-life is longer than that of melatonin, being metabolized in the body to four main metabolites, M-I, M-II, M-III, and M-IV. M-II has an affinity to MT(1) and MT(2) of about one-tenth of the parent compound, but its concentration in the circulation exceeds that of ramelteon by more than an order of magnitude. Ramelteon is effective in decreasing latency to persistent sleep and increasing total sleep time in freely moving monkeys. A number of clinical studies have been undertaken to study the efficacy of ramelteon in subjects with chronic insomnia. In almost all of these studies, ramelteon, in various doses of 4, 8, or 16 mg most commonly, significantly reduced sleep latency and increased sleep duration. Its primary action in sleep promotion is not a generalized gamma-aminobutyric (GABA)-ergic central nervous system depression, but rather it acts as a melatonergic agonist in the suprachiasmatic nucleus (and at other central nervous system sites), from where downstream processes, including GABA-ergic effects, are controlled via the hypothalamic sleep switch. Unlike other commonly prescribed hypnotic drugs, ramelteon is not associated with next morning hangover effects or reductions in alertness, nor has it been shown to cause withdrawal symptoms. The adverse symptoms reported with ramelteon are mild. All long-term investigations that have been carried out support the conclusion that ramelteon is a well tolerated and effective drug for the treatment of insomnia.

Circadian rhythms of photorefractory siberian hamsters remain responsive to melatonin

Short day lengths increase the duration of nocturnal melatonin (Mel) secretion, which induces the winter phenotype in Siberian hamsters. After several months of continued exposure to short days, hamsters spontaneously revert to the spring-summer phenotype. This transition has been attributed to the development of refractoriness of Mel-binding tissues, including the suprachiasmatic nucleus (SCN), to long-duration Mel signals. The SCN of Siberian hamsters is required for the seasonal response to winter-like Mel signals, and becomes refractory to previously effective long-duration Mel signals restricted to this area. Acute Mel treatment phase shifts circadian locomotor rhythms of photosensitive Siberian hamsters, presumably by affecting circadian oscillators in the SCN. We tested whether seasonal refractoriness of the SCN to long-duration Mel signals also renders the circadian system of Siberian hamsters unresponsive to Mel. Males manifesting free-running circadian rhythms in constant dim red light were injected with Mel or vehicle for 5 days on a 23.5-h T-cycle beginning at circadian time 10. Mel injections caused significantly larger phase advances in activity onset than did the saline vehicle, but the magnitude of phase shifts to Mel did not differ between photorefractory and photosensitive hamsters. Similarly, when entrained to a 16-h light/8-h dark photocycle, photorefractory and photosensitive hamsters did not differ in their response to Mel injected 4 h before the onset of the dark phase. Activity onset in Mel-injected hamsters was masked by light but was revealed to be significantly earlier than in vehicle-injected hamsters upon transfer to constant dim red light. The acute effects of melatonin on circadian behavioral rhythms are preserved in photorefractory hamsters.

The effect of α- and β-adrenergic blockade on daily rhythms of body temperature, urine production, and urinary 6-sulfatoxymelatonin of social voles Microtus socialis

To examine the effect of adrenergic blockade on daily rhythms of rectal body temperature (T(b)), urine production rate, and melatonin (MEL; measured as urinary 6-sulfatoxymelatonin; 6-SMT), social voles Microtus socialis received a single intra-peritoneal injection of either prazosin (PRAZ, 1 mg/kg) or propranolol (PROP, 4.5 mg/kg); alpha- and beta-adrenergic blocking agents respectively, 1 h prior to scotophase onset (light/dark, 12L:12D; lights on 07:00 h). Both blockers caused significant decrease in T(b) values mainly during scotophase. Nocturnal urine production rates were higher for M. socialis treated with the drugs compared with controls. Overall, urine production rates were systematically higher in PROP-voles over the 24 h period when compared with PRAZ-voles; however these differences were not statistically significant. Interestingly, PROP caused significant elevation in urinary 6-SMT at the second half of the dark phase, whereas PRAZ had no effects. These data suggest that the mechanisms regulating MEL synthesis and thermoregulatory responses in M. socialis are different from those described in other rodents' species. Importantly, the data also suggest that the beta-blockade-induced elevation in MEL levels may be directly associated with increased urination in M. socialis.