Melatonin generates an outward potassium current in rat suprachiasmatic nucleus neurones in vitro independent of their circadian rhythm

Circadian desynchrony and glucose metabolism

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

Melatonin receptor structure and signaling

Melatonin (5-methoxy-N-acetyltryptamine) binds with high affinity and specificity to membrane receptors. Several receptor subtypes exist in different species, of which the mammalian MT1 and MT2 receptors are the best-characterized. They are members of the G protein-coupled receptor superfamily, preferentially coupling to Gi/o proteins but also to other G proteins in a cell-context-depending manner. In this review, experts on melatonin receptors will summarize the current state of the field. We briefly report on the discovery and classification of melatonin receptors, then focus on the molecular structure of human MT1 and MT2 receptors and highlight the importance of molecular simulations to identify new ligands and to understand the structural dynamics of these receptors. We then describe the state-of-the-art of the intracellular signaling pathways activated by melatonin receptors and their complexes. Brief statements on the molecular toolbox available for melatonin receptor studies and future perspectives will round-up this review.

Melatonin as a Chronobiotic with Sleep-promoting Properties

The use of exogenous melatonin (exo-MEL) as a sleep-promoting drug has been under extensive debate due to the lack of consistency of its described effects. In this study, we conduct a systematic and comprehensive review of the literature on the chronobiotic, sleep-inducing, and overall sleep-promoting properties of exo-MEL. To this aim, we first describe the possible pharmacological mechanisms involved in the sleep-promoting properties and then report the corresponding effects of exo-MEL administration on clinical outcomes in: a) healthy subjects, b) circadian rhythm sleep disorders, c) primary insomnia. Timing of administration and doses of exo-MEL received particular attention in this work. The exo-MEL pharmacological effects are hereby interpreted in view of changes in the physiological properties and rhythmicity of endogenous melatonin. Finally, we discuss some translational implications for the personalized use of exo-MEL in the clinical practice.

Design and Validation of the First Family of Photo-Activatable Ligands for Melatonin Receptors

Melatonin is a neurohormone released in a circadian manner with peak levels at night. Melatonin mediates its effects mainly through G protein-coupled MT₁ and MT₂ receptors. Drugs acting on melatonin receptors are indicated for circadian rhythm- and sleep-related disorders. Tools to study the activation of these receptors with high temporal resolution are lacking. Here, we synthesized a family of light-activatable caged compounds by attaching o-nitrobenzyl (o-NB) or coumarin photocleavable groups to melatonin indolic nitrogen. All caged compounds showed the expected decrease in binding affinity for MT₁ and MT₂. The o-NB derivative MCS-0382 showed the best uncaging and biological properties, with 250-fold increase in affinity and potency upon illumination. Generation of melatonin from MCS-0382 was further demonstrated by its ability to modulate the excitation of SCN neurons in rat brain slices. MCS-0382 is available to study melatonin effects in a temporally controlled manner in cellular and physiological settings.

The use of melatonin to mitigate the adverse metabolic side effects of antipsychotics

Antipsychotic drugs are efficacious first-line treatments for many individuals diagnosed with a psychiatric illness. However, their adverse metabolic side-effect profile, which resembles the metabolic syndrome, represents a significant clinical problem that increases morbidity and limits treatment adherence. Moreover, the mechanisms involved in antipsychotic-induced adverse metabolic effects (AMEs) are unknown and mitigating strategies and interventions are limited. However, recent clinical trials show that nightly administration of exogenous melatonin may mitigate or even prevent antipsychotic-induced AMEs. This clinical evidence in combination with recent preclinical data implicate the circadian system in antipsychotic-induced AMEs and their mitigation. In this chapter, we provide an overview on the circadian system and its involvement in antipsychotic-induced AMEs, as well as the potential beneficial effect of nightly melatonin administration to mitigate them.

Melatonin and morphine: potential beneficial effects of co-use

Morphine is a potent analgesic agent used to control acute or chronic pain. Chronic administration of morphine results in analgesic tolerance, hyperalgesia, and other side effects including dependence, addiction, respiratory depression, and constipation, which limit its clinical usage. Therefore, identifying the new analgesics with fewer side effects which could increase the effect of morphine and reduce its side effects is crucial. Melatonin, a multifunctional molecule produced in the body, is known to play an important role in pain regulation. The strong anti-inflammatory effect of melatonin is suggested to be involved in the attenuation of the pain associated with inflammation. Melatonin also increases the anti-nociceptive actions of opioids, such as morphine, and reverses their tolerance through regulating several cellular signaling pathways. In this review, published articles evaluating the effect of the co-consumption of melatonin and morphine in different conditions were investigated. Our results show that melatonin has pain-killing properties when administered alone or in combination with other anti-nociceptive drugs. Melatonin decreases morphine consumption in different pathologies. Furthermore, attenuation of morphine intake can be accompanied by reduction of morphine-associated side-effects, including physical dependence, morphine tolerance, and morphine-related hyperalgesia. Therefore, it is reasonable to believe that the combination of melatonin with morphine could reduce morphine-induced tolerance and hyperalgesia, which may result from anti-inflammatory and antioxidant properties of melatonin. Overall, we underscore that, to further ameliorate patients' life quality and control their pain in various pathological conditions, melatonin deserves to be used with morphine by anesthesiologists in clinical practice.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Cellular localization of melatonin receptor Mel1b in pigeon retina

Melatonin, an important neuromodulator involved in circadian rhythms, modulates a series of physiological processes via activating its specific receptors, namely Mel1a (MT₁), Mel1b (MT₂) and Mel1c receptors. In this work, the localization of Mel1b receptor was studied in pigeon retina using double immunohistochemistry staining and confocal scanning microscopy. Our results showed that Mel1b receptor widely existed in the outer segment of photoreceptors and in the somata of dopaminergic amacrine cells, cholinergic amacrine cells, glycinergic AII amacrine cells, conventional ganglion cells and intrinsically photosensitive retinal ganglion cells, while horizontal cells, bipolar cells and Müller glial cells seemed to lack immunoreactivity of Mel1b receptor. That multiple types of retinal cells expressing Mel1b receptor suggests melatonin may directly modulate the activities of retina via activating Mel1b receptor.

Melatonin Reduces Excitability in Dorsal Root Ganglia Neurons with Inflection on the Repolarization Phase of the Action Potential

Melatonin is a neurohormone produced and secreted at night by pineal gland. Many effects of melatonin have already been described, for example: Activation of potassium channels in the suprachiasmatic nucleus and inhibition of excitability of a sub-population of neurons of the dorsal root ganglia (DRG). The DRG is described as a structure with several neuronal populations. One classification, based on the repolarizing phase of the action potential (AP), divides DRG neurons into two types: Without (N₀) and with (N_inf) inflection on the repolarization phase of the action potential. We have previously demonstrated that melatonin inhibits excitability in N₀ neurons, and in the present work, we aimed to investigate the melatonin effects on the other neurons (N_inf) of the DRG neuronal population. This investigation was done using sharp microelectrode technique in the current clamp mode. Melatonin (0.01–1000.0 nM) showed inhibitory activity on neuronal excitability, which can be observed by the blockade of the AP and by the increase in rheobase. However, we observed that, while some neurons were sensitive to melatonin effect on excitability (excitability melatonin sensitive—EMS), other neurons were not sensitive to melatonin effect on excitability (excitability melatonin not sensitive—EMNS). Concerning the passive electrophysiological properties of the neurons, melatonin caused a hyperpolarization of the resting membrane potential in both cell types. Regarding the input resistance (R_in), melatonin did not change this parameter in the EMS cells, but increased its values in the EMNS cells. Melatonin also altered several AP parameters in EMS cells, the most conspicuously changed was the (dV/dt)_max of AP depolarization, which is in coherence with melatonin effects on excitability. Otherwise, in EMNS cells, melatonin (0.1–1000.0 nM) induced no alteration of (dV/dt)_max of AP depolarization. Thus, taking these data together, and the data of previous publication on melatonin effect on N₀ neurons shows that this substance has a greater pharmacological potency on N_inf neurons. We suggest that melatonin has important physiological function related to N_inf neurons and this is likely to bear a potential relevant therapeutic use, since N_inf neurons are related to nociception.

Melatonin receptors: molecular pharmacology and signalling in the context of system bias

Melatonin, N-acetyl-5-methoxytryptamine, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Photoperiod-Induced Neuroplasticity in the Circadian System

Seasonal changes in light exposure have profound effects on behavioral and physiological functions in many species, including effects on mood and cognitive function in humans. The mammalian brain's master circadian clock, the suprachiasmatic nucleus (SCN), transmits information about external light conditions to other brain regions, including some implicated in mood and cognition. Although the detailed mechanisms are not yet known, the SCN undergoes highly plastic changes at the cellular and network levels under different light conditions. We therefore propose that the SCN may be an essential mediator of the effects of seasonal changes of day length on mental health. In this review, we explore various forms of neuroplasticity that occur in the SCN and other brain regions to facilitate seasonal adaptation, particularly altered phase distribution of cellular circadian oscillators in the SCN and changes in hypothalamic neurotransmitter expression.

Melatonin receptor activation increases glutamatergic synaptic transmission in the rat medial lateral habenula

Melatonin (MLT) is secreted from the pineal gland and mediates its physiological effects through activation of two G protein-coupled receptors, MT1 and MT2 . These receptors are expressed in several brain areas, including the habenular complex, a pair of nuclei that relay information from forebrain to midbrain and modulate a plethora of behaviors, including sleep, mood, and pain. However, so far, the precise mechanisms by which MLT control the function of habenula neurons remain unknown. Using whole cell recordings from male rat brain slices, we examined the effects of MLT on the excitability of medial lateral habenula (MLHb) neurons. We found that MLT had no significant effects on the intrinsic excitability of MLHb neurons, but profoundly increased the amplitude of glutamate-mediated evoked excitatory post-synaptic currents (EPSC). The increase in strength of glutamate synapses onto MLHb neurons was mediated by an increase in glutamate release. The MLT-induced increase in glutamatergic synaptic transmission was blocked by the competitive MT1 /MT2 receptor antagonist luzindole (LUZ). These results unravel a potential cellular mechanism by which MLT receptor activation enhances the excitability of MLHb neurons. The MLT-mediated control of glutamatergic inputs to the MLHb may play a key role in the modulation of various behaviors controlled by the habenular complex.

Oncostatic-Cytoprotective Effect of Melatonin and Other Bioactive Molecules: A Common Target in Mitochondrial Respiration

For several years, oncostatic and antiproliferative properties, as well as thoses of cell death induction through 5-methoxy-N-acetiltryptamine or melatonin treatment, have been known. Paradoxically, its remarkable scavenger, cytoprotective and anti-apoptotic characteristics in neurodegeneration models, such as Alzheimer’s disease and Parkinson’s disease are known too. Analogous results have been confirmed by a large literature to be associated to the use of many other bioactive molecules such as resveratrol, tocopherol derivatives or vitamin E and others. It is interesting to note that the two opposite situations, namely the neoplastic pathology and the neurodegeneration, are characterized by deep alterations of the metabolome, of mitochondrial function and of oxygen consumption, so that the oncostatic and cytoprotective action can find a potential rationalization because of the different metabolic and mitochondrial situations, and in the effect that these molecules exercise on the mitochondrial function. In this review we discuss historical and general aspects of melatonin, relations between cancers and the metabolome and between neurodegeneration and the metabolome, and the possible effects of melatonin and of other bioactive molecules on metabolic and mitochondrial dynamics. Finally, we suggest a common general mechanism as responsible for the oncostatic/cytoprotective effect of melatonin and of other molecules examined.

GIRK Channels Mediate the Nonphotic Effects of Exogenous Melatonin

Melatonin supplementation has been used as a therapeutic agent for several diseases, yet little is known about the underlying mechanisms by which melatonin synchronizes circadian rhythms. G-protein signaling plays a large role in melatonin-induced phase shifts of locomotor behavior and melatonin receptors activate G-protein-coupled inwardly rectifying potassium (GIRK) channels in Xenopus oocytes. The present study tested the hypothesis that melatonin influences circadian phase and electrical activity within the central clock in the suprachiasmatic nucleus (SCN) through GIRK channel activation. Unlike wild-type littermates, GIRK2 knock-out (KO) mice failed to phase advance wheel-running behavior in response to 3 d subcutaneous injections of melatonin in the late day. Moreover, in vitro phase resetting of the SCN circadian clock by melatonin was blocked by coadministration of a GIRK channel antagonist tertiapin-q (TPQ). Loose-patch electrophysiological recordings of SCN neurons revealed a significant reduction in the average action potential rate in response to melatonin. This effect was lost in SCN slices treated with TPQ and SCN slices from GIRK2 KO mice. The melatonin-induced suppression of firing rate corresponded with an increased inward current that was blocked by TPQ. Finally, application of ramelteon, a potent melatonin receptor agonist, significantly decreased firing rate and increased inward current within SCN neurons in a GIRK-dependent manner. These results are the first to show that GIRK channels are necessary for the effects of melatonin and ramelteon within the SCN. This study suggests that GIRK channels may be an alternative therapeutic target for diseases with evidence of circadian disruption, including aberrant melatonin signaling.

SIGNIFICANCE STATEMENT

Despite the widespread use of melatonin supplementation for the treatment of sleep disruption and other neurological diseases such as epilepsy and depression, no studies have elucidated the molecular mechanisms linking melatonin-induced changes in neuronal activity to its therapeutic effects. Here, we used behavioral and electrophysiological techniques to address this scientific gap. Our results show that melatonin and ramelteon, a potent and clinically relevant melatonin receptor agonist, significantly affect the neurophysiological function of suprachiasmatic nucleus neurons through activation of G-protein-coupled inwardly rectifying potassium (GIRK) channels. Given the importance of GIRK channels for neuronal excitability (with >600 publications on these channels to date), our study should generate broad interest from neuroscientists in fields such as epilepsy, addiction, and cognition.

Protein interactome mining defines melatonin MT1 receptors as integral component of presynaptic protein complexes of neurons

In mammals, the hormone melatonin is mainly produced by the pineal gland with nocturnal peak levels. Its peripheral and central actions rely either on its intrinsic antioxidant properties or on binding to melatonin MT1 and MT2 receptors, belonging to the G protein-coupled receptor (GPCR) super-family. Melatonin has been reported to be involved in many functions of the central nervous system such as circadian rhythm regulation, neurotransmission, synaptic plasticity, memory, sleep, and also in Alzheimer's disease and depression. However, little is known about the subcellular localization of melatonin receptors and the molecular aspects involved in neuronal functions of melatonin. Identification of protein complexes associated with GPCRs has been shown to be a valid approach to improve our understanding of their function. By combining proteomic and genomic approaches we built an interactome of MT1 and MT2 receptors, which comprises 378 individual proteins. Among the proteins interacting with MT1 , but not with MT2 , we identified several presynaptic proteins, suggesting a potential role of MT1 in neurotransmission. Presynaptic localization of MT1 receptors in the hypothalamus, striatum, and cortex was confirmed by subcellular fractionation experiments and immunofluorescence microscopy. MT1 physically interacts with the voltage-gated calcium channel Cav 2.2 and inhibits Cav 2.2-promoted Ca(2+) entry in an agonist-independent manner. In conclusion, we show that MT1 is part of the presynaptic protein network and negatively regulates Cav 2.2 activity, providing a first hint for potential synaptic functions of MT1.

[A role of melatonin in the treatment of low back pain]

OBJECTIVE

To study an analgesic role of melatonin in the treatment of low back pain.

MATERIAL AND METHODS

A study included 178 patients, aged from 40 to 65 years, with low back pain during at least 12 weeks and the VAS score > 3. Patients were stratified into 6 groups (3 pairs of comparison). In the first pair, patients of the main group (n = 31) received APTPA (a combination of 500 mg of glucosamine hydrochloride and 500 mg of chondroitin sulfate) in dosage 1 tablet twice a day during 1 month and then 1 tablet during 2 months plus melaxen (3 mg of melatonin 30-40 min before sleep), patients of the control group (n = 29) received only APTPA. In the second pair, patients of the comparison group (n = 30) received APTPA in dosage 1 tablet twice a day and diclofenac in dosage 25 mg 2-3 times a day, patients of the main group (n = 30) received additionally melaxen (3 mg of melatonin 30-40 min before sleep). In the third pair, patients of the main group (n = 29) received APTPA in dosage 1 tablet twice a day, diclofenac in dosage 25 mg 2-3 times a day and melaxen (3 mg of melatonin 30-40 min before sleep), patients of the comparison group (n = 29) did not receive melaxen. Treatment results were assessed after 3 months for the first pair and after 1 month for the second and third pairs.

RESULTS

A significant reduction in pain intensity at movement and resting state was noted in the main groups compared to controls.

CONCLUSION

Possible mechanisms of analgesic properties of melatonin and world experience in chronic low back pain treatment are discussed.

Co-expression of two subtypes of melatonin receptor on rat M1-type intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in circadian and other non-image forming visual responses. An open question is whether the activity of these neurons may also be under the regulation mediated by the neurohormone melatonin. In the present work, by double-staining immunohistochemical technique, we studied the expression of MT₁ and MT₂, two known subtypes of mammalian melatonin receptors, in rat ipRGCs. A single subset of retinal ganglion cells labeled by the specific antibody against melanopsin exhibited the morphology typical of M1-type ipRGCs. Immunoreactivity for both MT₁ and MT₂ receptors was clearly seen in the cytoplasm of all labeled ipRGCs, indicating that these two receptors were co-expressed in each of these neurons. Furthermore, labeling for both the receptors were found in neonatal M1 cells as early as the day of birth. It is therefore highly plausible that retinal melatonin may directly modulate the activity of ipRGCs, thus regulating non-image forming visual functions.

Melatonin reduces tachycardia in postural tachycardia syndrome: a randomized, crossover trial

BACKGROUND

Postural tachycardia syndrome (POTS) induces disabling chronic orthostatic intolerance with an excessive increase in heart rate (HR) upon standing, and many POTS patients have a hyperadrenergic state. Medications that restrain HR are a promising approach to this problem.

OBJECTIVE

We tested the hypothesis that melatonin will attenuate the tachycardia and improve symptom burden in patients with POTS.

METHODS

Patients with POTS (n = 78) underwent acute drug trials with melatonin 3 mg orally and placebo, on separate mornings, in a randomized crossover design. Blood pressure, HR, and symptoms were assessed while seated and after standing for up to 10 min prior to, and hourly for 4 h following study drug administration.

RESULTS

The reduction in standing HR was significantly greater 2 h after melatonin compared with placebo (P = 0.017). There was no significant difference in the reduction of systolic blood pressure between melatonin and placebo, either with standing or while seated. The symptom burden was not improved with melatonin compared with placebo.

CONCLUSION

Oral melatonin produced a modest decrease in standing tachycardia in POTS. Further research is needed to determine the effects of regular night-time use of this medication in POTS.

Melatonin attenuates antipsychotic metabolic effects: an eight-week randomized, double-blind, parallel-group, placebo-controlled clinical trial

OBJECTIVE

Second-generation antipsychotics (SGAs) are among the first-line treatments for bipolar disorder and schizophrenia, but have a tendency to generate metabolic disturbances. These features resemble a metabolic syndrome for which a central autonomic imbalance has been proposed that may originate from the hypothalamic suprachiasmatic nuclei. In a clinical trial, we hypothesized that melatonin, a hormone that regulates the suprachiasmatic nucleus, could attenuate SGA-induced adverse metabolic effects.

METHODS

In an eight-week, double-blind, randomized, placebo-controlled, parallel-group clinical trial, we evaluated the metabolic effect of melatonin in SGA-treated patients in terms of weight, blood pressure, lipid, glucose, body composition, and anthropometric measures. A total of 44 patients treated with SGAs, 20 with bipolar disorder and 24 with schizophrenia, randomly received placebo (n = 24) or melatonin 5 mg (n = 20).

RESULTS

The melatonin group showed a decrease in diastolic blood pressure (5.1 versus 1.1 mmHg for placebo, p = 0.003) and attenuated weight gain (1.5 versus 2.2 kg for placebo, F = 4.512, p = 0.040) compared to the placebo group. The strong beneficial metabolic effects of melatonin in comparison to placebo on fat mass (0.2 versus 2.7 kg, respectively, p = 0.032) and diastolic blood pressure (5.7 versus 5.5 mmHg, respectively, p = 0.001) were observed in the bipolar disorder and not in the schizophrenia group. No adverse events were reported.

CONCLUSIONS

Our results show that melatonin is effective in attenuating SGAs' adverse metabolic effects, particularly in bipolar disorder. The clinical findings allow us to propose that SGAs may disturb a centrally mediated metabolic balance that causes adverse metabolic effects and that nightly administration of melatonin helps to restore. Melatonin could become a safe and cost-effective therapeutic option to attenuate or prevent SGA metabolic effects.

Melatonin

Melatonin (MEL) is a hormone synthesized and secreted by the pineal gland deep within the brain in response to photoperiodic cues relayed from the retina via an endogenous circadian oscillator within the suprachiasmatic nucleus in the hypothalamus. The circadian rhythm of melatonin production and release, characterized by nocturnal activity and daytime quiescence, is an important temporal signal to the body structures that can read it. Melatonin acts through high-affinity receptors located centrally and in numerous peripheral organs. Different receptor subtypes have been cloned and characterized: MT(1) and MT(2) (transmembrane G-protein-coupled receptors), and MT(3). However, their physiological role remains unelucidated, although livestock management applications already include the control of seasonal breeding and milk production. As for potential therapeutic applications, exogenous melatonin or a melatonin agonist and selective 5-hydroxytrypiamine receptor (5-HT(2c)) antagonist, eg, S 20098, can be used to manipulate circadian processes such as the sleep-vake cycle, which are frequently disrupted in many conditions, most notably seasonal affective disorder.

Melatonin in animal models

Melatonin is a hormone synthesized and secreted during the night by the pineal gland. Its production is mainly driven by the Orcadian clock, which, in mammals, is situated in the suprachiasmatic nucleus of the hypothalamus. The melatonin production and release displays characteristic daily (nocturnal) and seasonal patterns (changes in duration proportional to the length of the night) of secretion. These rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes. In mammals, the role of melatonin in the control of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be identified. The exact role of the hormone in the diurnal (Orcadian) timing system remains to be determined. However, exogenous melatonin has been shown to affect the circadian clock. The molecular and cellular mechanisms involved in this well-characterized “chronobiotic” effect have also begun to be characterized. The circadian clock itself appears to be an important site for the entrapment effect of melatonin and the presence of melatonin receptors appears to be a prerequisite. A better understanding of such “chronobiotic” effects of melatonin will allow clarification of the role of endogenous melatonin in circadian organization.

Melatonin: both master clock output and internal time-giver in the circadian clocks network

Daily rhythms in physiological and behavioral processes are controlled by a network of circadian clocks, reset by inputs and delivering circadian signals to the brain and peripheral organs. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus, mainly reset by ambient light. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock and inhibited by light exposure. Several roles of melatonin in the circadian system have been identified. As a major hormonal output, melatonin distributes temporal cues generated by the SCN to the multitude of tissue targets expressing melatonin receptors. In some target structures, like the Pars tuberalis of the adenohypophysis, these melatonin signals can drive daily rhythmicity that would otherwise be lacking. In other target structures, melatonin signals are used for the synchronization (i.e., adjustment of the timing of existing oscillations) of peripheral oscillators, such as the fetal adrenal gland. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock, although its physiological significance needs further characterization. Of note, pharmacological treatment with exogenous melatonin can synchronize the SCN clock. From a clinical point of view, provided that the subject is not exposed to light at night, the daily profile of circulating melatonin provides a reliable estimate of the timing of the human SCN. During the past decade, a number of melatonin agonists have been developed for treating circadian, psychiatric and sleep disorders. These drugs may target the SCN for improving circadian timing or act indirectly at some downstream level of the circadian network to restore proper internal synchronization.

Circadian disruption and SCN control of energy metabolism

In this review we first present the anatomical pathways used by the suprachiasmatic nuclei to enforce its rhythmicity onto the body, especially its energy homeostatic system. The experimental data show that by activating the orexin system at the start of the active phase, the biological clock not only ensures that we wake up on time, but also that our glucose metabolism and cardiovascular system are prepared for increased activity. The drawback of such a highly integrated system, however, becomes visible when our daily lives are not fully synchronized with the environment. Thus, in addition to increased physical activity and decreased intake of high-energy food, also a well-lighted and fully resonating biological clock may help to withstand the increasing "diabetogenic" pressure of today's 24/7 society.

Sleep health and asynchronization

Recent surveys in Japan reported that more than half of children interviewed complained of daytime sleepiness, approximately one quarter reported insomnia, and some complained of both nocturnal insomnia and daytime sleepiness. To explain the pathophysiology of this type of sleep disturbance, a novel clinical concept of asynchronization has been proposed. Asynchronization involves disturbances in various aspects of biological rhythms that normally exhibit circadian oscillations. The putative major triggers for asynchronization include a combination of nighttime light exposure, which can disturb the biological clock and decrease melatonin secretion, and a lack of morning light exposure, which can prohibit normal synchronization of the biological clock to a 24-h cycle and decrease activity in the serotonergic system. The early phase of asynchronization may be caused by inadequate sleep hygiene, is likely to be functional, and to be relatively easily resolved by establishing a regular sleep-wakefulness cycle. However, without adequate intervention, these disturbances may gradually worsen, resulting into the chronic phase. No single symptom appears to be specific for the clinical phases, and the chronic phase is defined in terms of the response to interventions. The factors causing the transition from the early to chronic phase of asynchronization and those producing the difficulties of recovering patients with the chronic phase of asynchronization are currently unclear.

Electrophysiological effects of melatonin on mouse Per1 and non-Per1 suprachiasmatic nuclei neurones in vitro

The master circadian pacemaker in the suprachiasmatic nuclei (SCN) regulates the nocturnal secretion of the pineal hormone melatonin. Melatonin, in turn, has feedback effects on SCN neuronal activity rhythms via high affinity G protein-coupled receptors (MT(1) and MT(2) ). However, the precise effects of melatonin on the electrical properties of individual SCN neurones are unclear. In the present study, we investigated the acute effects of exogenous melatonin on SCN neurones using whole-cell patch-clamp recordings in brain slices prepared from Per1::d2EGFP-expressing transgenic mice. In current-clamp mode, bath applied melatonin, at near-physiological concentrations (1 nM), hyperpolarised the majority (63.7%) of SCN neurones tested at all times of the projected light/dark cycle. In addition, melatonin depolarised a small proportion of cells (11.0%). No differences were observed for the effects of melatonin between Per1::GFP or non-Per1::GFP SCN neurones. Melatonin-induced effects were blocked by the MT(1)/MT(2) antagonist, luzindole (1 μM) and the proportion of SCN neurones responsive to melatonin was greatly reduced in the presence of either tetrodotoxin (200 or 500 nM) or gabazine (20 μM). In voltage-clamp recordings, 1 nM melatonin increased the frequency of GABA-mediated currents. These findings indicate, for the first time, that exogenous melatonin can alter neuronal excitability in the majority of SCN neurones, regardless of whether or not they overtly express the core clock gene Per1. The results also suggest that melatonin acts mainly by modulating inhibitory GABAergic transmission within the SCN. This may explain why exogenous application of melatonin has heterogenous effects on individual SCN neurones.

The hypothalamic clock and its control of glucose homeostasis

The everyday life of mammals, including humans, exhibits many behavioral, physiological and endocrine oscillations. The major timekeeping mechanism for these rhythms is contained in the central nervous system (CNS). The output of the CNS clock not only controls daily rhythms in sleep/wake (or feeding/fasting) behavior but also exerts a direct control over glucose metabolism. Here, we show how the biological clock plays an important role in determining early morning (fasting) plasma glucose concentrations by affecting hepatic glucose production and glucose uptake, as well as glucose tolerance, by determining feeding-induced insulin responses. Recently, large-scale genetic studies in humans provided the first evidence for the involvement of disrupted (clock gene) rhythms in the pathogenesis of type 2 diabetes.

Physiology and pharmacology of melatonin in relation to biological rhythms

Melatonin is an evolutionarily conserved molecule that serves a time-keeping function in various species. In vertebrates, melatonin is produced predominantly by the pineal gland with a marked circadian rhythm that is governed by the central circadian pacemaker (biological clock) in the suprachiasmatic nuclei of the hypothalamus. High levels of melatonin are normally found at night, and low levels are seen during daylight hours. As a consequence, melatonin has been called the "darkness hormone". This review surveys the current state of knowledge regarding the regulation of melatonin synthesis, receptor expression, and function. In particular, it addresses the physiological, pathological, and therapeutic aspects of melatonin in humans, with an emphasis on biological rhythms.

Leptin modulates spike coding in the rat suprachiasmatic nucleus

The mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN), contains receptors to the adipose tissue hormone leptin. In the present study, the effects of leptin on the electrophysiological activity of the SCN cells were characterised in vitro in rat brain slices. During extracellular recording, application of 20 nm leptin (n = 36) decreased mean spike frequency (Wilcoxon signed rank test, z = -3.390, P < 0.001) and increased the irregularity of firing measured by the entropy of the log interspike interval distribution (Student's paired t-test, t = 2.377, P = 0.023), but had no consistent effect on spike patterning as measured by the mutual information between adjacent log interspike intervals (z = 0.745, P = 0.456). Intracellular current-clamp recordings (n = 25) revealed a hyperpolarising effect of 20 nm leptin on SCN neurones (z = -2.290, P = 0.022). The hyperpolarisation largely resulted from the effect of leptin on the subgroup of cells (n = 13) that generated 'rebound' spikes upon termination of a hyperpolarising current pulse (z = -2.697, P = 0.007). Leptin application also increased the group mean duration of the afterhyperpolarisation (n = 25, t = 2.512, P = 0.023). The effects of leptin on extracellularly recorded spike activity were consistent with the changes in membrane potential and spike shape. They suggest that leptin can directly modulate the electrical properties of SCN neurones and, in this way, contribute to the mechanism by which metabolic processes influence the circadian clock.

Melatonin and melatonergic drugs on sleep: possible mechanisms of action

Pineal melatonin is synthesized and secreted in close association with the light/dark cycle. The temporal relationship between the nocturnal rise in melatonin secretion and the "opening of the sleep gate" (i.e., the increase in sleep propensity at the beginning of the night), coupled with the sleep-promoting effects of exogenous melatonin, suggest that melatonin is involved in the regulation of sleep. The sleep-promoting and sleep/wake rhythm regulating effects of melatonin are attributed to its action on MT(1) and MT(2) melatonin receptors present in the suprachiasmatic nucleus (SCN) of the hypothalamus. Animal experiments carried out in rats, cats, and monkeys have revealed that melatonin has the ability to reduce sleep onset time and increase sleep duration. However, clinical studies reveal inconsistent findings, with some of them reporting beneficial effects of melatonin on sleep, whereas in others only marginal effects are documented. Recently a prolonged-release 2-mg melatonin preparation (Circadin(TM)) was approved by the European Medicines Agency as a monotherapy for the short-term treatment of primary insomnia in patients who are aged 55 or above. Several melatonin derivatives have been shown to increase nonrapid eye movement (NREM) in rats and are of potential pharmacological importance. So far only one of these melatonin derivatives, ramelteon, has received approval from the U.S. Food and Drug Administration to be used as a sleep promoter. Ramelteon is a novel MT(1) and MT(2) melatonergic agonist that has specific effects on melatonin receptors in the SCN and is effective in promoting sleep in experimental animals such as cats and monkeys. In clinical trials, ramelteon reduced sleep onset latency and promoted sleep in patients with chronic insomnia, including an older adult population. Both melatonin and ramelteon promote sleep by regulating the sleep/wake rhythm through their actions on melatonin receptors in the SCN, a unique mechanism of action not shared by any other hypnotics. Moreover, unlike benzodiazepines, ramelteon causes neither withdrawal effects nor dependence. Agomelatine, another novel melatonergic antidepressant in its final phase of approval for clinical use, has been shown to improve sleep in depressed patients and to have an antidepressant efficacy that is partially attributed to its effects on sleep-regulating mechanisms.

Melatonin: a hormone that modulates pain

AIMS

Melatonin is a hormone synthesized principally in the pineal gland that has been classically associated with endocrine actions. However, several lines of evidence suggest that melatonin plays a role in pain modulation. This paper reviews the available evidence on melatonin's analgesic effects in animals and human beings.

MAIN METHODS

A medline search was performed using the terms "melatonin", "inflammatory pain", "neuropathic pain", "functional pain", "rats", "mice", "human", "receptors", "opioid" and "free radicals" in combinations.

KEY FINDINGS

The antinociceptive effect of melatonin has been evaluated in diverse pain models, and several findings show that melatonin receptors modulate pain mechanisms as activation induces an antinociceptive effect at spinal and supraspinal levels under conditions of acute and inflammatory pain. More recently, melatonin induced-antinociception has been extended to neuropathic pain states. This effect agrees with the localization of melatonin receptors in thalamus, hypothalamus, dorsal horn of the spinal cord, spinal trigeminal tract, and trigeminal nucleus. The effects of melatonin result from activation of MT(1) and MT(2) melatonin receptors, which leads to reduced cyclic AMP formation and reduced nociception. In addition, melatonin is able to activate opioid receptors indirectly, to open several K(+) channels and to inhibit expression of 5-lipoxygenase and cyclooxygenase 2. This hormone also inhibits the production of pro-inflammatory cytokines, modulates GABA(A) receptor function and acts as a free radical scavenger.

SIGNIFICANCE

Melatonin receptors constitute attractive targets for developing analgesic drugs, and their activation may prove to be a useful strategy to generate analgesics with a novel mechanism of action.

Possible participation of the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway in the peripheral antinociception of melatonin

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG)-K(+) channel pathway on melatonin-induced local antinociception was assessed during the second phase of the formalin test. The local peripheral ipsilateral, but not contralateral, administration of melatonin (150-600 microg/paw) produced a dose-related antinociception during both phases of the formalin test in rats. Moreover, local pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, NO synthesis inhibitor, 10-100 microg/paw), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor, 5-50 microg/paw), (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo [1,2,3-fg:3',2',1'-kl]pyrrolo [3,4-i][1,6] benzodiazocine-10-carboxylic acid methyl ester (KT-5823, specific PKG inhibitor, 50-500 ng/paw), glibenclamide (ATP-sensitive K(+) channel blocker, 5-50 microg/paw), apamin (small-conductance Ca(2+)-activated K(+) channel blocker, 0.1-1 microg/paw) or charybdotoxin (large- and intermediate-conductance Ca(2+)-activated K(+) channel blocker, 0.03-0.3 microg/paw), but not N(G)-D-nitro-arginine methyl ester (D-NAME, inactive isomer of L-NAME, 100 microg/paw) or vehicle, significantly prevented melatonin (300 microg/paw)-induced antinociception. Data suggest that melatonin-induced local peripheral antinociception during the second phase of the test could be due to activation of the NO-cyclic GMP-PKG-ATP-sensitive and Ca(2+)-activated K(+) channels pathway.

Melatonin, consciousness, and traumatic stress

Descartes intuitively anticipated the so-called 'binding problem' of consciousness and thought that the pineal gland enables spatio-temporal integration in cognitive processing. Recent findings indicate that a major role in the process of temporal integration and binding involve neurons in suprachiasmatic nuclei, specifically targeting the pineal gland and other structures, and control the neuroendocrine rhythms. Melatonin is an endocrine output signal of the clock and provides circadian information as an endogenous synchronizer which stabilizes and reinforces circadian rhythms. This integrative process occurs at the different levels of the circadian network via gene expression in some brain regions and peripheral structures that enables integration of circadian, hormonal, and metabolic information and creating temporal order of bodily and mental experience. This specific temporal order is reflected in associative sequentiality that is necessary for cognition, behavior and all processes of memory consolidation that must preserve all information in the temporal causal order and synchrony. In this context, recent findings suggest that melatonin could be a potential regulator in the processes that contribute to memory formation, long-term potentiation, and synaptic plasticity in the hippocampus and other brain regions. There is evidence that stress disrupts normal activity and memory consolidation in the hippocampus and prefrontal cortex, and this process leads to memories that are stored without a contextual or spatiotemporal frame. These findings emphasize a specific role of melatonin in mechanisms of consciousness, memory and stress and are also consistent with reported studies that indicate melatonin alterations under stressful conditions and in mental disorders.

Circadian difference in firing rate of isolated rat suprachiasmatic nucleus neurons

The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian clock in mammals. Dissociated SCN neurons in long-term culture exhibit a circadian modulation of spontaneous electrical activity. To evaluate the presence of circadian differences in spontaneous activity of isolated SCN neurons without synaptic connections, dissociated rat SCN neurons were studied with on-cell recording 3-4 days after preparation, before the formation of dendrites, axons and synapses. A day-night difference in spontaneous electrical firing rate was found in acutely dissociated SCN neurons. During the first subjective day, the average firing rate (0.87+/-0.12 Hz) was significantly higher than during the first subjective night (0.24+/-0.06 Hz), while the firing rate on the next day (0.68+/-0.11 Hz) was significantly higher than during the preceding night. These data suggest that populations of isolated SCN neurons with no synaptic interactions contain a functioning circadian clock, and are particularly amenable to biophysical experiments.

Therapeutic potential of melatonin agonists

Melatonin, a hormone secreted by the pineal gland, has been successfully employed to improve sleep in both normal patients and insomniacs, and for the treatment of circadian rhythm sleep disorders. Melatonergic MT₁ and MT₂ receptors exist in high concentrations in the suprachiasmatic nucleus of the hypothalamus and have been shown to be instrumental for the sleep-promoting and circadian rhythm-regulating effects of melatonin. A lack of consistency among reports on the therapeutic efficacy of melatonin has been attributed to differences in melatonin's bioavailability and the short half-life of the hormone. In view of the need for longer acting melatonergic agonists that improve sleep efficiency without causing drug abuse or dependency, ramelteon (Rozerem™, Takeda) was developed. Ramelteon, which acts via MT₁/MT₂ melatonergic agonism, has been found clinically effective for improving total sleep time and sleep efficiency in insomniacs. Agomelatine (Valdoxan™, Servier) is another MT₁/MT₂ melatonergic agonist that also displays antagonist activity at 5-HT_2C serotonin receptors. Agomelatine has been found effective in treating depression and sleep disorders in patients with major depressive disorder. A slow-release preparation of melatonin (Circadin™, Neurim) has been shown to be effective in treating sleep disorders in the elderly population.

Melatonin modulates spike coding in the rat suprachiasmatic nucleus

The effects of the application of melatonin in vitro on the electrophysiological activity of suprachiasmatic neurones were characterised using novel measures of coding based on the analysis of interspike intervals. Perfusion of 1 nM melatonin in vitro (n = 53) had no consistent effect on mean spike frequency (Wilcoxon's sign rank, z = -0.01, P = 0.989), but increased the irregularity of firing (Student's paired t-test, t = -3.02, P = 0.004), as measured by the log interval entropy, and spike patterning (z = -3.43, P < 0.001), as measured by the mutual information between adjacent log intervals. Intracellular recordings in vitro in current clamp mode showed that 1 nM melatonin significantly hyperpolarised (n = 11, z = -2.35, P = 0.019) those cells that showed 'rebound' spikes upon termination of a hyperpolarising current pulse. Grouping all cells together (n = 27), melatonin application decreased the duration of the afterhyperpolarisation (z = -2.49, P = 0.013) and increased the amplitude of the depolarising afterpotential (z = -2.71, P = 0.007). The effects of melatonin seen in vitro from extracellular recordings on interspike interval coding were consistent with the changes in spike shape seen from intracellular recordings. A melatonin-induced increase in the size of the depolarising afterpotential of suprachiasmatic cells might underlie the increased irregularity of spike firing seen during the subjective night time. The method of analysis demonstrated a difference in spike firing that is not revealed by frequency alone and is consistent with the presence of a melatonin-induced depolarising current.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Oral and spinal melatonin reduces tactile allodynia in rats via activation of MT2 and opioid receptors

The antiallodynic effect of melatonin after intrathecal (it) and oral administration as well as the possible participation of MT(2) and opioid receptors in melatonin-induced antiallodynia in neuropathic rats were assessed. Ligation of the L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal (3-100 microg) and oral (37.5-300 mg/kg) administration of melatonin decreased tactile allodynia induced by spinal nerve ligation. Intrathecal administration of the preferential MT(2) receptor antagonist luzindole (1-100 microg), but not vehicle, significantly diminished in a dose-dependent manner the antiallodynic effect induced by melatonin (100 microg, it). Oral (0.01-1mg/kg) or intrathecal (0.1-10 microg) administration of the highly selective MT(2) receptor antagonist 4P-PDOT diminished the antiallodynic activity induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin, respectively. Subcutaneous (1mg/kg) or intrathecal (0.5-50 microg) treatment with naltrexone, but not vehicle, significantly diminished the antiallodynic effect induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin. Oral melatonin (150 mg/kg)-induced antiallodynia was partially reduced by the spinal administration of 4P-PDOT (10 microg). Moreover, the spinal effect of melatonin (100 microg) was significantly reduced by the combination 4P-PDOT (0.1 microg)-naltrexone (0.5 microg). At the greatest tested doses, the antagonist drugs did not modify tactile allodynia in neuropathic rats. Melatonin (100 microg or 300 mg/kg) did not affect motor co-ordination in the rotarod test. Results indicate that melatonin reduces tactile allodynia in neuropathic rats after intrathecal and oral administration. Moreover, data suggest the participation of spinal MT(2) and opioid receptors in the melatonin-induced antiallodynic effect in this model.

Melatonin and anesthesia: a clinical perspective

The hypnotic, antinociceptive, and anticonvulsant properties of melatonin endow this neurohormone with the profile of a novel hypnotic-anesthetic agent. Sublingually or orally administered melatonin is an effective premedicant in adults and children. Melatonin premedication like midazolam is associated with sedation and preoperative anxiolysis, however, unlike midazolam these effects are not associated with impaired psychomotor skills or the quality of recovery. Melatonin administration also is associated with a tendency toward faster recovery and a lower incidence of postoperative excitement than midazolam. Oral premedication with 0.2 mg/kg melatonin significantly reduces the propofol and thiopental doses required for loss of responses to verbal commands and eyelash stimulation. In rats, melatonin and the more potent melatonin analogs 2-bromomelatonin and phenylmelatonin have been found to have anesthetic properties similar to those of thiopental and propofol, with the added advantage of providing potent antinociceptive effects. The exact mechanism(s) by which structurally diverse intravenous and volatile anesthetics produce general anesthesia is still largely unknown, but positive modulation of gamma-aminobutyric acid type A (GABAA) receptor function has been recognized as an important and common pathway underlying the depressant effects of many of these agents. Accumulating evidence indicates that there is interplay between the melatonergic and GABAergic systems, and it has been demonstrated that melatonin administration produces significant, dose-dependent increases in GABA concentrations in the central nervous system. Additional in vitro data suggest that melatonin alters GABAergic transmission by modulating GABAA receptor function. Of greater importance, data from in vivo studies suggest that the central anesthetic effects of melatonin are mediated, at least in part, via GABAergic system activation, as they can be blocked or reversed by GABAA receptor antagonists. Further work is needed to better understand the general anesthetic properties of melatonin at the molecular, cellular, and systems levels.

Melatonin affects nuclear orphan receptors mRNA in the rat suprachiasmatic nuclei

The pineal hormone melatonin nocturnal synthesis feeds back on the suprachiasmatic nuclei (SCN), the central circadian clock. Indeed, daily melatonin injections in free-running rats resynchronize their locomotor activity to 24 h. However, the molecular mechanisms underlying this chronobiotic effect of the hormone are poorly understood. The endogenous circadian machinery involves positive and negative transcriptional feedback loops implicating different genes (particularly period (Per) 1-3, Clock, Bmal1, cryptochrome (Cry) 1-2). While CLOCK:BMAL1 heterodimer activates the rhythmic transcription of per and cry genes, the PER and CRY proteins inhibit the CLOCK:BMAL1 complex. In previous studies, we observed that the immediate resetting effect of a melatonin injection at the end of the subjective day on the SCN circadian activity did not directly involve the above-mentioned clock genes. Recently, nuclear orphan receptors (NORs) have been presented as functional links between the regulatory loops of the molecular clock. These NORs bind to a retinoic acid receptor-related orphan receptor response element (RORE) domain and activate (RORalpha) or repress (REV-ERBalpha) bmal1 expression. In this study, we investigated whether melatonin exerts its chronobiotic effects through transcriptional regulation of these transcription factors. We monitored roralpha, rorbeta and rev-erbalpha messenger RNA (mRNA) expression levels by quantitative in situ hybridization, up to 36 h following a melatonin injection at circadian time (CT) 11.5. Results clearly showed that, while roralpha was not affected by melatonin, the hormone partially prevented the decrease of the rorbeta mRNA expression observed in control animals during the first hours following the injection. The major result is that the rev-erbalpha mRNA expression rhythm was 1.3+/-0.8-h phase-advanced in melatonin-treated animals during the first subjective night following the melatonin administration. Moreover, the bmal1 mRNA expression was 1.9+/-0.9-h phase-shifted in the second subjective night following the melatonin injection. These results clearly suggest that the NOR genes could be the link between the chronobiotic action of melatonin and the core of the molecular circadian clock.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Melatonin inhibits hippocampal long-term potentiation

The goal of this study is to investigate the effect of the hormone melatonin on long-term potentiation and excitability measured by stimulating the Schaffer collaterals and recording the field excitatory postsynaptic potential from the CA1 dendritic layer in hippocampal brain slices from mice. Application of melatonin produced a concentration-dependent inhibition of the induction of long-term potentiation, with a concentration of 100 nm producing an approximately 50% inhibition of long-term potentiation magnitude. Long-duration melatonin treatments of 6 h were also effective at reducing the magnitude of long-term potentiation. Melatonin (100 nm) did not alter baseline evoked responses or paired-pulse facilitation recorded at this synapse. The inhibitory actions of melatonin were prevented by application of the melatonin (MT) receptor antagonist luzindole as well as the MT2 receptor subtype antagonist 4-phenyl-2-propionamidotetraline. These inhibitory actions of melatonin were lost in mice deficient in MT2 receptors but not those deficient in MT1 receptors. In addition, application of the protein kinase A inhibitor H-89 both mimicked the effects of melatonin and precluded further inhibition by melatonin. Finally, the application an activator of adenylyl cyclase, forskolin, overcame the inhibitory effects of melatonin on LTP without affecting the induction of long-term potentiation on its own. These results suggest that hippocampal synaptic plasticity may be constrained by melatonin through a mechanism involving MT2-receptor-mediated regulation of the adenylyl cyclase-protein kinase A pathway.

Organization of circadian functions: interaction with the body

The hypothalamus integrates information from the brain and the body; this activity is essential for survival of the individual (adaptation to the environment) and the species (reproduction). As a result, countless functions are regulated by neuroendocrine and autonomic hypothalamic processes in concert with the appropriate behaviour that is mediated by neuronal influences on other brain areas. In the current chapter attention will be focussed on fundamental hypothalamic systems that control metabolism, circulation and the immune system. Herein a system is defined as a physiological and anatomical functional unit, responsible for the organisation of one of these functions. Interestingly probably because these systems are essential for survival, their function is highly dependent on each other's performance and often shares same hypothalamic structures. The functioning of these systems is strongly influenced by (environmental) factors such as the time of the day, stress and sensory autonomic feedback and by circulating hormones. In order to get insight in the mechanisms of hypothalamic integration we have focussed on the influence of the biological clock; the suprachiasmatic nucleus (SCN) on processes that are organized by and in the hypothalamus. The SCN imposes its rhythm onto the body via three different routes of communication: 1.Via the secretion of hormones; 2. via the parasympathetic and 3.via the sympathetic autonomous nervous system. The SCN uses separate connections via either the sympathetic or via the parasympathetic system not only to prepare the body for the coming change in activity cycle but also to prepare the body and its organs for the hormones that are associated with such change. Up till now relatively little attention has been given to the question how peripheral information might be transmitted back to the SCN. Apart from light and melatonin little is known about other systems from the periphery that may provide information to the SCN. In this chapter attention will be paid to e.g. the role of the circumventricular organs in passing info to the SCN. Herein especially the role of the arcuate nucleus (ARC) will be highlighted. The ARC is crucial in the maintenance of energy homeostasis as an integrator of long- and short-term hunger and satiety signals. Receptors for metabolic hormones like insulin, leptin and ghrelin allow the ARC to sense information from the periphery and signal it to the central nervous system. Neuroanatomical tracing studies using injections of a retrograde and anterograde tracer into the ARC and SCN showed a reciprocal connection between the ARC and the SCN which is used to transmit feeding related signals to the SCN. The implications of multiple inputs and outputs of the SCN to the body will be discussed in relation with metabolic functions.

Persistent subthreshold voltage-dependent cation single channels in suprachiasmatic nucleus neurons

The hypothalamic suprachiasmatic nucleus (SCN) contains the primary circadian pacemaker in mammals, and transmits circadian signals by diurnal modulation of neuronal firing frequency. The ionic mechanisms underlying the circadian regulation of firing frequency are unknown, but may involve changes in membrane potential and voltage-gated ion channels. Here we describe novel tetrodotoxin- and nifedipine-resistant subthreshold, voltage-dependent cation (SVC) channels that are active at resting potential of SCN neurons and increase their open probability (P(o)) with membrane depolarization. The increased P(o) reflects changes in the kinetics of the slow component of the channel closed-time, but not the channel open-time or fast closed-time. This study provides a background for investigation of the possible role of SVC channels in regulation of circadian oscillations of membrane excitability in SCN neurons.