Contrary to other non-photic cues, acute melatonin injection does not induce immediate changes of clock gene mRNA expression in the rat suprachiasmatic nuclei

Melatonin-related dysfunction in chronic restraint stress triggers sleep disorders in mice

Stress may trigger sleep disorders and are also risk factors for depression. The study explored the melatonin-related mechanisms of stress-associated sleep disorders on a mouse model of chronic stress by exploring the alteration in sleep architecture, melatonin, and related small molecule levels, transcription and expression of melatonin-related genes as well as proteins. Mice undergoing chronic restraint stress modeling for 28 days showed body weight loss and reduced locomotor activity. Sleep fragmentation, circadian rhythm disorders, and insomnia exhibited in CRS-treated mice formed sleep disorders. Tryptophan and 5-hydroxytryptamine levels were increased in the hypothalamus, while melatonin level was decreased. The transcription and expression of melatonin receptors were reduced, and circadian rhythm related genes were altered. Expression of downstream effectors to melatonin receptors was also affected. These results identified sleep disorders in a mice model of chronic stress. The alteration of melatonin-related pathways was shown to trigger sleep disorders.

Dopamine D1 Receptor-Mediated Regulation of Per1, Per2, CLOCK, and BMAL1 Expression in the Suprachiasmatic Nucleus in Adult Male Rats

Photic and non-photic inputs are reported to affect clock gene expressions and behavioral activities in the SCN. However, it is not known whether dopaminergic input mediates these regulatory effects on clock genes. The present study examined the molecular effects of dopamine D1 agonist on Per1, Per2, CLOCK, and Bmal1 expressions in the SCN and its effect on behavioral activities to determine the role of dopamine D1 receptor in regulation of these gene expressions and behavioral activities in adult male Wistar rats. To examine the molecular effects of dopamine D1 agonist day and night, we injected 20 mg/kg SKF38393 to the first group of rats at 6 a.m. and the second group at 6 p.m. We also injected saline to the third and fourth groups of rats at 6 a.m. and 6 p.m. as control groups. All rats were sacrificed 2 h following the injections. The real-time PCR technique was used to evaluate the clock gene expression. In addition, to examine the effects of dopamine D1 agonists on behavioral activities, we injected 20 mg/kg SKF38393 to SKF receiving group and saline to control group. The behavioral activities of the rats were monitored on the running wheel for 21 days, 1 week following the injections. SKF injections increased the Per2 and CLOCK expressions in the daytime and significantly decreased the Per1 and Bmal1 expressions. However, at night, SKF injections increased only Per2 expressions significantly and decreased the Per1, CLOCK, and Bmal1 genes expressions. Both saline receiving groups showed that all gene expressions were significantly higher except Per2 during nighttime. SKF injection increased the running wheel activity during nighttime significantly. Based on the obtained result, clock gene expression and behavioral activities in adult male Wistar rats may be altered or monitored by administration of exogenous dopamine.

Pinealectomy and gonadectomy modulate amplitude, but not photoperiodic modulation of Clock gene expression in the Syrian hamster suprachiasmatic nuclei

The duration of daytime light phase (photoperiod) controls reproduction in seasonal mammals. Syrian hamsters are sexually active when exposed to long photoperiod, while gonadal atrophy is observed after exposure to short photoperiod. The photorefractory period, or photorefractoriness, is a particular state of spontaneous recrudescence of sexual activity that occurs after a long-term exposure to short photoperiod. Expression of core clock genes in the master circadian clock contained in the suprachiasmatic nuclei depends on photoperiodic conditions. Interestingly, the expression of the Clock gene is also modified in photorefractory Syrian hamsters. Since melatonin and testosterone levels in seasonal species are dependent on photoperiod, photoperiodic variations of Clock mRNA levels in the suprachiasmatic clock could be a consequence of these hormonal changes. To test this hypothesis, we analysed the effects of pinealectomy on Clock mRNA changes due to long to short photoperiod transition and of gonadectomy on Clock mRNA levels in photorefractory period. Our data show that the suprachiasmatic integration of the short photoperiod (assessed by a rhythmic expression profile of Clock) is independent of the presence of melatonin. Furthermore, constitutively low expression of Clock observed during the photorefractory period does not require the presence of either melatonin or testosterone. However, we show that both hormones provide positive feedback on average levels of Clock expression. Thus, our data support the hypothesis that daily variations of Clock levels in the suprachiasmatic nuclei are influenced by photoperiodic changes and the time spent in short photoperiod, independently of seasonal modifications of melatonin or testosterone levels.

Circadian rhythm as a therapeutic target

The circadian clock evolved in diverse organisms to integrate external environmental changes and internal physiology. The clock endows the host with temporal precision and robust adaptation to the surrounding environment. When circadian rhythms are perturbed or misaligned, as a result of jet lag, shiftwork or other lifestyle factors, adverse health consequences arise, and the risks of diseases such as cancer, cardiovascular diseases or metabolic disorders increase. Although the negative impact of circadian rhythm disruption is now well established, it remains underappreciated how to take advantage of biological timing, or correct it, for health benefits. In this Review, we provide an updated account of the circadian system and highlight several key disease areas with altered circadian signalling. We discuss environmental and lifestyle modifications of circadian rhythm and clock-based therapeutic strategies, including chronotherapy, in which dosing time is deliberately optimized for maximum therapeutic index, and pharmacological agents that target core clock components and proximal regulators. Promising progress in research, disease models and clinical applications should encourage a concerted effort towards a new era of circadian medicine.

Melatonin and the circadian system: Keys for health with a focus on sleep

Melatonin (MLT), secreted during the night by the pineal gland, is an efferent hormonal signal of the master circadian clock located in the suprachiasmatic nucleus (SCN). Consequently, it is a reliable phase marker of the SCN clock. If one defines as "chronobiotic," a drug able to influence the phase and/or the period of the circadian clock, MLT is a very potent one. The most convincing data obtained so far come from studies on totally blind individuals. Exogenous MLT administered daily entrains the sleep-wake cycle of these individuals to a 24-h cycle. MLT, however, is not essential to sleep. In nocturnally, active mammals, MLT is released during the night concomitantly with the daily period of wakefulness. Therefore, MLT cannot be simply considered as a sleep hormone, but rather as a signal of darkness. Its role in the circadian system is to reinforce nighttime physiology, including timing of the sleep-wake cycle and other circadian rhythms. MLT exerts its effects on the sleep cycle especially by a direct action on the master circadian clock. The sleep-wake cycle is depending not only on the circadian clock but also on an orchestrated network of different centers in the brain. Thus, the control of sleep-wake rhythm might be explained by a parallel and concomitant action of MLT on the master clock (chronobiotic effect) and on sleep-related structures within the brain. MLT acts through two high-affinity membrane receptors (MT1 and MT2) with striking differences in their distribution pattern. MLT is a powerful synchronizer of human circadian rhythms, thus justifying the use of MLT and MLT agonists in clinical medicine as pharmacological tools to manipulate the sleep-wake cycle, and to treat sleep disorders and other circadian disorders. Available MLT analogs/drugs are all nonspecific MT1/MT2 agonists. The development of new ligands which are highly selectivity for each subtype is clearly a new challenge for the field and will be at the root of new therapeutic agents for curing specific pathologies, including sleep disorders.

Complexities in cardiovascular rhythmicity: perspectives on circadian normality, ageing and disease

Biological rhythms exist in organisms at all levels of complexity, in most organs and at myriad time scales. Our own biological rhythms are driven by energy emitted by the sun, interacting via our retinas with brain stem centres, which then send out complex messages designed to synchronize the behaviour of peripheral non-light sensing organs, to ensure optimal physiological responsiveness and performance of the organism based on the time of day. Peripheral organs themselves have autonomous rhythmic behaviours that can act independently from central nervous system control but is entrainable. Dysregulation of biological rhythms either through environment or disease has far-reaching consequences on health that we are only now beginning to appreciate. In this review, we focus on cardiovascular rhythms in health, with ageing and under disease conditions.

Effects of Melatonin on Anterior Pituitary Plasticity: A Comparison Between Mammals and Teleosts

Melatonin is a key hormone involved in the photoperiodic signaling pathway. In both teleosts and mammals, melatonin produced in the pineal gland at night is released into the blood and cerebrospinal fluid, providing rhythmic information to the whole organism. Melatonin acts via specific receptors, allowing the synchronization of daily and annual physiological rhythms to environmental conditions. The pituitary gland, which produces several hormones involved in a variety of physiological processes such as growth, metabolism, stress and reproduction, is an important target of melatonin. Melatonin modulates pituitary cellular activities, adjusting the synthesis and release of the different pituitary hormones to the functional demands, which changes during the day, seasons and life stages. It is, however, not always clear whether melatonin acts directly or indirectly on the pituitary. Indeed, melatonin also acts both upstream, on brain centers that control the pituitary hormone production and release, as well as downstream, on the tissues targeted by the pituitary hormones, which provide positive and negative feedback to the pituitary gland. In this review, we describe the known pathways through which melatonin modulates anterior pituitary hormonal production, distinguishing indirect effects mediated by brain centers from direct effects on the anterior pituitary. We also highlight similarities and differences between teleosts and mammals, drawing attention to knowledge gaps, and suggesting aims for future research.

Molecular and Cellular Networks in The Suprachiasmatic Nuclei

Why do we experience the ailments of jetlag when we travel across time zones? Why is working night-shifts so detrimental to our health? In other words, why can’t we readily choose and stick to non-24 h rhythms? Actually, our daily behavior and physiology do not simply result from the passive reaction of our organism to the external cycle of days and nights. Instead, an internal clock drives the variations in our bodily functions with a period close to 24 h, which is supposed to enhance fitness to regular and predictable changes of our natural environment. This so-called circadian clock relies on a molecular mechanism that generates rhythmicity in virtually all of our cells. However, the robustness of the circadian clock and its resilience to phase shifts emerge from the interaction between cell-autonomous oscillators within the suprachiasmatic nuclei (SCN) of the hypothalamus. Thus, managing jetlag and other circadian disorders will undoubtedly require extensive knowledge of the functional organization of SCN cell networks. Here, we review the molecular and cellular principles of circadian timekeeping, and their integration in the multi-cellular complexity of the SCN. We propose that new, in vivo imaging techniques now enable to address these questions directly in freely moving animals.

Synchronizing effects of melatonin on diurnal and circadian rhythms

In mammals, the rhythmic secretion of melatonin from the pineal gland is driven by the circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. The robust nightly peak of melatonin secretion is an output signal of the circadian clock and is supposed to deliver the circadian message to the whole of the organism. Since the circadian system regulates many behavioral and physiological processes, its disruption by external (shift-work, jet-lag) or internal desynchronization (blindness, aging) causes many different health problems. Externally applied melatonin is used in humans as a chronobiotic drug to treat desynchronization and circadian disorders, and the success of these treatments does, at first glance, underline the supposed pivotal role of melatonin in the synchronization of the circadian system. On the other hand, pinealectomy in experimental animals and humans does not abolish their rhythms of rest and activity. Furthermore, mice with deficient melatoninergic systems neither display overt defects in their rhythmic behavior nor do they show obvious signs of disease susceptibility, let alone premature mortality. During the last years, our laboratory has investigated several mouse stains with intact or compromised internal melatonin signaling systems in order to better understand the physiological role of the melatoninergic system. These and other investigations which will be reviewed in the present contribution confirm the synchronizing effect of endogenous melatonin and the melatoninergic system. However, these effects are subtle. Thus melatonin does not appear as the master of internal synchronization, but as one component in a cocktail of synchronizing agents.

Altered Circadian Timing System-Mediated Non-Dipping Pattern of Blood Pressure and Associated Cardiovascular Disorders in Metabolic and Kidney Diseases

The morning surge in blood pressure (BP) coincides with increased cardiovascular (CV) events. This strongly suggests that an altered circadian rhythm of BP plays a crucial role in the development of CV disease (CVD). A disrupted circadian rhythm of BP, such as the non-dipping type of hypertension (i.e., absence of nocturnal BP decline), is frequently observed in metabolic disorders and chronic kidney disease (CKD). The circadian timing system, controlled by the central clock in the suprachiasmatic nucleus of the hypothalamus and/or by peripheral clocks in the heart, vasculature, and kidneys, modulates the 24 h oscillation of BP. However, little information is available regarding the molecular and cellular mechanisms of an altered circadian timing system-mediated disrupted dipping pattern of BP in metabolic disorders and CKD that can lead to the development of CV events. A more thorough understanding of this pathogenesis could provide novel therapeutic strategies for the management of CVD. This short review will address our and others' recent findings on the molecular mechanisms that may affect the dipping pattern of BP in metabolic dysfunction and kidney disease and its association with CV disorders.

The hormone melatonin: Animal studies

The Melatonin (MLT), secreted rhythmically by the pineal, is an efferent hormonal signal of the circadian clock. MLT presents overall pleitropic effects but it is the role of MLT as a hormonal circadian signal which is the best documented. MLT-receptors are present in numerous structures/organs and the MLT is now considered as an endogenous synchronizer within the circadian system. The presence of MLT-receptors within the circadian clock, explains that exogenous MLT is a chronobiotic drug. Trials in humans, have confirmed the efficacy of MLT in circadian rhythm disorders. Subtypes of MLT-receptors have been characterized (MT1 and MT2). Striking differences are observed in the distribution pattern of these 2 subtypes. Up to now, MTL-analogues commercialized as drugs, are all non-specific MT₁/MT₂ agonists acting on the SCN. The development of new specific agonists/antagonists for both subtypes, the identification of the link between MLT target sites within different parts of the brain or the body and the association of specific MLT receptor subtypes and particular physiological effects open great therapeutic potential.

Clock-dependent and system-driven oscillators interact in the suprachiasmatic nuclei to pace mammalian circadian rhythms

Circadian clocks drive biological rhythms with a period of approximately 24 hours and keep in time with the outside world through daily resetting by environmental cues. While this external entrainment has been extensively investigated in the suprachiasmatic nuclei (SCN), the role of internal systemic rhythms, including daily fluctuations in core temperature or circulating hormones remains debated. Here, we show that lactating mice, which exhibit dampened systemic rhythms, possess normal molecular clockwork but impaired rhythms in both heat shock response gene expression and electrophysiological output in their SCN. This suggests that body rhythms regulate SCN activity downstream of the clock. Mathematical modeling predicts that systemic feedback upon the SCN functions as an internal oscillator that accounts for in vivo and ex vivo observations. Thus we are able to propose a new bottom-up hierarchical organization of circadian timekeeping in mammals, based on the interaction in the SCN between clock-dependent and system-driven oscillators.

Melatonin Signal Transduction Pathways Require E-Box-Mediated Transcription of Per1 and Per2 to Reset the SCN Clock at Dusk

Melatonin is released from the pineal gland into the circulatory system at night in the absence of light, acting as “hormone of darkness” to the brain and body. Melatonin also can regulate circadian phasing of the suprachiasmatic nucleus (SCN). During the day-to-night transition, melatonin exposure advances intrinsic SCN neural activity rhythms via the melatonin type-2 (MT₂) receptor and downstream activation of protein kinase C (PKC). The effects of melatonin on SCN phasing have not been linked to daily changes in the expression of core genes that constitute the molecular framework of the circadian clock. Using real-time RT-PCR, we found that melatonin induces an increase in the expression of two clock genes, Period 1 (Per1) and Period 2 (Per2). This effect occurs at CT 10, when melatonin advances SCN phase, but not at CT 6, when it does not. Using anti-sense oligodeoxynucleotides (α ODNs) to Per 1 and Per 2, as well as to E-box enhancer sequences in the promoters of these genes, we show that their specific induction is necessary for the phase-altering effects of melatonin on SCN neural activity rhythms in the rat. These effects of melatonin on Per1 and Per2 were mediated by PKC. This is unlike day-active non-photic signals that reset the SCN clock by non-PCK signal transduction mechanisms and by decreasing Per1 expression. Rather, this finding extends roles for Per1 and Per2, which are critical to photic phase-resetting, to a nonphotic zeitgeber, melatonin, and suggest that the regulation of these clock gene transcripts is required for clock resetting by diverse regulatory cues.

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.

Melatonin feedback on clock genes: a theory involving the proteasome

The expression of 'clock' genes occurs in all tissues, but especially in the suprachiasmatic nuclei (SCN) of the hypothalamus, groups of neurons in the brain that regulate circadian rhythms. Melatonin is secreted by the pineal gland in a circadian manner as influenced by the SCN. There is also considerable evidence that melatonin, in turn, acts on the SCN directly influencing the circadian 'clock' mechanisms. The most direct route by which melatonin could reach the SCN would be via the cerebrospinal fluid of the third ventricle. Melatonin could also reach the pars tuberalis (PT) of the pituitary, another melatonin-sensitive tissue, via this route. The major 'clock' genes include the period genes, Per1 and Per2, the cryptochrome genes, Cry1 and Cry2, the clock (circadian locomotor output cycles kaput) gene, and the Bmal1 (aryl hydrocarbon receptor nuclear translocator-like) gene. Clock and Bmal1 heterodimers act on E-box components of the promoters of the Per and Cry genes to stimulate transcription. A negative feedback loop between the cryptochrome proteins and the nucleus allows the Cry and Per proteins to regulate their own transcription. A cycle of ubiquitination and deubiquitination controls the levels of CRY protein degraded by the proteasome and, hence, the amount of protein available for feedback. Thus, it provides a post-translational component to the circadian clock mechanism. BMAL1 also stimulates transcription of REV-ERBα and, in turn, is also partially regulated by negative feedback by REV-ERBα. In the 'black widow' model of transcription, proteasomes destroy transcription factors that are needed only for a particular period of time. In the model proposed herein, the interaction of melatonin and the proteasome is required to adjust the SCN clock to changes in the environmental photoperiod. In particular, we predict that melatonin inhibition of the proteasome interferes with negative feedback loops (CRY/PER and REV-ERBα) on Bmal1 transcription genes in both the SCN and PT. Melatonin inhibition of the proteasome would also tend to stabilize BMAL1 protein itself in the SCN, particularly at night when melatonin is naturally elevated. Melatonin inhibition of the proteasome could account for the effects of melatonin on circadian rhythms associated with molecular timing genes. The interaction of melatonin with the proteasome in the hypothalamus also provides a model for explaining the dramatic 'time of day' effect of melatonin injections on reproductive status of seasonal breeders. Finally, the model predicts that a proteasome inhibitor such as bortezomib would modify circadian rhythms in a manner similar to melatonin.

The internal time-giver role of melatonin. A key for our health

Daily rhythms in physiological and behavioural processes are controlled by a network of circadian clocks. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock. 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 tissues expressing melatonin receptors. In some target tissues, 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. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock. 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. 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.

Acute melatonin treatment alters dendritic morphology and circadian clock gene expression in the hippocampus of Siberian hamsters

In the hippocampus of Siberian hamsters, dendritic length and dendritic complexity increase in the CA1 region whereas dendritic spine density decreases in the dentate gyrus region at night. However, the underlying mechanism of the diurnal rhythmicity in hippocampal neuronal remodeling is unknown. In mammals, most daily rhythms in physiology and behaviors are regulated by a network of circadian clocks. The central clock, located in the hypothalamus, controls melatonin secretion at night and melatonin modifies peripheral clocks by altering expression of circadian clock genes. In this study, we examined the effects of acute melatonin treatment on the circadian clock system as well as on morphological changes of hippocampal neurons. Male Siberian hamsters were injected with melatonin in the afternoon; 4 h later, mRNA levels of hypothalamic and hippocampal circadian clock genes and hippocampal neuron dendritic morphology were assessed. In the hypothalamus, melatonin treatment did not alter Period1 and Bmal1 expression. However, melatonin treatment increased both Period1 and Bmal1 expression in the hippocampus, suggesting that melatonin affected molecular oscillations in the hippocampus. Melatonin treatment also induced rapid remodeling of hippocampal neurons; melatonin increased apical dendritic length and dendritic complexity in the CA1 region and reduced the dendritic spine density in the dentate gyrus region. These data suggest that structural changes in hippocampal neurons are regulated by a circadian clock and that melatonin functions as a nighttime signal to coordinate the diurnal rhythm in neuronal remodeling.

Differential role of melatonin in restoration of age-induced alterations in daily rhythms of expression of various clock genes in suprachiasmatic nucleus of male Wistar rats

Aging is associated with changes in several basic parameters of circadian rhythms in mammals leading to circadian dysfunction. The hypothalamic Suprachiasmatic nucleus (SCN) regulates neuronal, endocrine and behavioral rhythms through the expression of various clock genes and release of melatonin from pineal gland. In the present study, we investigated the effect of aging on daily rhythms of various clock genes such as rPer1, rPer2, rCry1, rCry2 and rBmal1 in the SCN of male Wistar rats. The m-RNA expression levels of these genes were studied by using quantitative Polymerase Chain Reaction (qPCR) in 3 age groups [3 (adult), 12 and 24 month (m)] at variable time points (Zeitgeber time (ZT)-0, 6, 12 and 18). The m-RNA expression for all genes studied was rhythmic in SCN of adult rats with maximum for rPer1 at ZT-6, rPer2, rCry1 and rCry2 at ZT-12 and rBmal1 at ZT-18. However in 12 and 24 m, the phases of expression of these genes were significantly altered with abolition of daily rhythms of rCry1, rCry2 and rBmal1 in 24 m. Melatonin, messenger of darkness, an endogenous synchronizer of rhythm, an antioxidant and an antiaging drug, declines with aging. We therefore studied the effects of melatonin administered subcutaneously at 1 h before the onset of darkness (ZT-11) for 11 days on age induced desynchronization in expression of these genes. We report here differential restoration of daily rhythm, phase, levels and stoichiometric interaction of m-RNA expression of these genes in various age groups in rat SCN with melatonin treatment.

Pineal oscillator functioning in the chicken--effect of photoperiod and melatonin

The avian pineal gland, apart from the hypothalamic master clock (suprachiasmatic nuclei, SCN) and retina, functions as an independent circadian oscillator, receiving external photic cues that it translates into the rhythmical synthesis of melatonin, a biochemical signal of darkness. Functional similarity to the mammalian SCN makes the avian pineal gland a convenient model for studies on biological clock mechanisms in general. Pineal melatonin is produced not only in a light-dependent manner but also remains under the control of the endogenous oscillator, while the possible involvement of melatonin in maintaining cyclic expression of the avian clock genes remains to be elucidated. The aim of the present study was to characterize the diurnal profiles of main clock genes transcription in the pineal glands of chickens exposed to continuous light (LL) and supplemented with exogenous melatonin. We hypothesized that rearing chickens from the day of hatch under LL conditions would evoke a functional pinealectomy, influencing, in turn, pineal clock function. To verify this hypothesis, we examined the diurnal transcriptional profiles of selected clock genes as well as the essential parameters of pineal gland function: transcription of the genes encoding arylalkylamine N-acetyltransferase (Aanat), a key enzyme in melatonin biosynthesis, and the melatonin receptor (Mel1c), along with the blood melatonin level. Chickens hatched in summer or winter were maintained under LD 16:8 and 8:16, corresponding to the respective photoperiods, as the seasonal control groups. Another set of chickens was kept in parallel under LL conditions and some were supplemented with melatonin to check the ability of exogenous hormone to antagonize the effects evoked by continuous light. Twelve-day-old chickens were sacrificed every 3 h over a 24-h period and the mRNAs of selected clock genes, Bmal1, Cry1, Per3, E4bp4, together with those of Aanat and Mel1c, were quantified in the isolated pineal glands. Our results indicate that the profiles of clock gene transcription are not dependent on the duration of the light phase, while LL conditions decrease the amplitude of diurnal changes, but do not abolish them entirely. Melatonin supplied in drinking water to the birds kept in LL seems to desynchronize transcription of the majority of clock genes in the summer, while in the winter, it restores the pattern, but not the diurnal rhythmicity. Rhythmic expression of Bmal1 appears to provide a direct link between the circadian clock and the melatonin output pathway, while the availability of cyclic melatonin is clearly involved in the canonical transcription pattern of Per3 in the chicken pineal gland. Regardless of the experimental conditions, a negative correlation was identified between the transcription of genes involved in melatonin biosynthesis (Aanat) and melatonin signal perception (Mel1c receptor).

Astakine 2--the dark knight linking melatonin to circadian regulation in crustaceans

Daily, circadian rhythms influence essentially all living organisms and affect many physiological processes from sleep and nutrition to immunity. This ability to respond to environmental daily rhythms has been conserved along evolution, and it is found among species from bacteria to mammals. The hematopoietic process of the crayfish Pacifastacus leniusculus is under circadian control and is tightly regulated by astakines, a new family of cytokines sharing a prokineticin (PROK) domain. The expression of AST1 and AST2 are light-dependent, and this suggests an evolutionarily conserved function for PROK domain proteins in mediating circadian rhythms. Vertebrate PROKs are transmitters of circadian rhythms of the suprachiasmatic nucleus (SCN) in the brain of mammals, but the mechanism by which they function is unknown. Here we demonstrate that high AST2 expression is induced by melatonin in the brain. We identify RACK1 as a binding protein of AST2 and further provide evidence that a complex between AST2 and RACK1 functions as a negative-feedback regulator of the circadian clock. By DNA mobility shift assay, we showed that the AST2-RACK1 complex will interfere with the binding between BMAL1 and CLK and inhibit the E-box binding activity of the complex BMAL1-CLK. Finally, we demonstrate by gene knockdown that AST2 is necessary for melatonin-induced inhibition of the complex formation between BMAL1 and CLK during the dark period. In summary, we provide evidence that melatonin regulates AST2 expression and thereby affects the core clock of the crustacean brain. This process may be very important in all animals that have AST2 molecules, i.e. spiders, ticks, crustaceans, scorpions, several insect groups such as Hymenoptera, Hemiptera, and Blattodea, but not Diptera and Coleoptera. Our findings further reveal an ancient evolutionary role for the prokineticin superfamily protein that links melatonin to direct regulation of the core clock gene feedback loops.

Most living organisms are able to sense the time and in particular time of day by their internal clocks. So-called clock proteins control these internal clockworks. BMAL1 and CLK are two important clock proteins, which together form a complex that serves as a transcription factor and controls the production of diurnal proteins. These diurnal proteins, in turn, inhibit the formation of clock proteins so that the concentration of the different proteins in the cell oscillates back and forth throughout the day. External factors may affect the balance of clock proteins, and one such important factor is light. Melatonin is a darkness hormone produced in the brain of most animals during the night, and here we show that melatonin controls the formation of a protein named AST2 in crayfish. AST2 belongs to a group of proteins found in many arthropods, such as spiders, scorpions, crustaceans, and some insects, whose function has been unknown until now. Now we demonstrate that AST2 is induced by melatonin at night and then functions in the internal biological clock by preventing BMAL1 and CLK to form a complex. In this way, AST2 acts as a link between melatonin and the internal biological clock.

The endogenous melatonin (MT) signal facilitates reentrainment of the circadian system to light-induced phase advances by acting upon MT2 receptors

The indolamine melatonin is an important rhythmic endocrine signal in the circadian system. Exogenous melatonin can entrain circadian rhythms in physiology and behavior, but the role of endogenous melatonin and the two membrane-bound melatonin receptor types, MT1 and MT2, in reentrainment of daily rhythms to light-induced phase shifts is not understood. The present study analyzed locomotor activity rhythms and clock protein levels in the suprachiasmatic nuclei (SCN) of melatonin-deficient (C57BL/6J) and melatonin-proficient (C3H/HeN) mice, as well as in melatonin-proficient (C3H/HeN) mice with targeted deletion of the MT1, MT2, or both receptors, to determine effects associated with phase delays or phase advances of the light/dark (LD) cycle. In all mouse strains and genotypes, reentrainment of locomotor activity rhythms was significantly faster after a 6-h phase delay than a 6-h phase advance. Reentrainment after the phase advance was, however, significantly slower than in melatonin-deficient animals and in mice lacking functional MT2 receptors than melatonin-proficient animals with intact MT2 receptors. To investigate whether these behavioral differences coincide with differences in reentrainment of clock protein levels in the SCN, mPER1, mCRY1 immunoreactions were compared between control mice kept under the original LD cycle and killed at zeitgeber time 04 (ZT04) or at ZT10, respectively, and experimental mice subjected to a 6-h phase advance of the LD cycle and sacrificed at ZT10 on the third day after phase advance. This ZT corresponds to ZT04 of the original LD cycle. Under the original LD cycle, the numbers of mPER1- and mCRY1-immunoreactive cell nuclei were low at ZT04 and high at ZT10 in the SCN of all mouse strains and genotypes investigated. Notably, mouse strains with intact melatonin signaling and functional MT2 receptors showed a significant increase in the number of mPER1- and mCRY1-immunoreactive cell nuclei at the new ZT10 as compared to the former ZT04. These data suggest the endogenous melatonin signal facilitates reentrainment of the circadian system to phase advances on the level of the SCN molecular clockwork by acting upon MT2 receptors.

Circadian system from conception till adulthood

In mammals, the circadian system is composed of the central clock in the hypothalamic suprachiasmatic nuclei and of peripheral clocks that are located in other neural structures and in cells of the peripheral tissues and organs. In adults, the system is hierarchically organized so that the central clock provides the other clocks in the body with information about the time of day. This information is needed for the adaptation of their functions to cyclically changing external conditions. During ontogenesis, the system undergoes substantial development and its sensitivity to external signals changes. Perinatally, maternal cues are responsible for setting the phase of the developing clock, while later postnatally, the LD cycle is dominant. The central clock attains its functional properties during a gradual and programmed process. Peripheral clocks begin to exhibit rhythmicity independent of each other at various developmental stages. During the early developmental stages, the peripheral clocks are set or driven by maternal feeding, but later the central clock becomes fully functional and begins to entrain the periphery. During the perinatal period, the central and peripheral clocks seem to be vulnerable to disturbances in external conditions. Further studies are needed to understand the processes of how the circadian system develops and what degree of plasticity and resilience it possesses during ontogenesis. These data may lead to an assessment of the contribution of disturbances of the circadian system during early ontogenesis to the occurrence of circadian diseases in adulthood.

The circadian clock, reward, and memory

During our daily activities, we experience variations in our cognitive performance, which is often accompanied by cravings for small rewards, such as consuming coffee or chocolate. This indicates that the time of day, cognitive performance, and reward may be related to one another. This review will summarize data that describe the influence of the circadian clock on addiction and mood-related behavior and put the data into perspective in relation to memory processes.

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.

When brain clocks lose track of time: cause or consequence of neuropsychiatric disorders

Patients suffering from neuropsychiatric disorders often exhibit a loss of regulation of their biological rhythms which leads to altered sleep/wake cycle, body temperature rhythm and hormonal rhythms. Whereas these symptoms have long been considered to result from the pathology of the underlying disease, increasing evidence now indicates that the circadian system may be more directly involved in the etiology of psychiatric disorders. This emerging view originated with the discovery that the genes involved in the generation of biological rhythms are expressed in many brain structures where clocks function-and perhaps malfunction. It is also due to the interesting phenotypes of clock mutant mice. Here we summarize recent reports showing that alteration of circadian clocks within key brain regions associated with neuropsychiatric disorders may be an underlying cause of the development of mental illness. We discuss how these alterations take place at both systems and molecular levels.

Expression of circadian clock and melatonin receptors within cultured rat cardiomyocytes

Melatonin, the pineal gland hormone, provides entrainment of many circadian rhythms to the ambient light/dark cycle. Recently, cardiovascular studies have demonstrated melatonin interactions with many physiological processes and diseases, such as hypertension and cardiopathologies. Although membrane melatonin receptors (MT1, MT2) and the transcriptional factor RORα have been reported to be expressed in the heart, there is no evidence of the cell-type expressing receptors as well as the possible role of melatonin on the expression of the circadian clock of cardiomyocytes, which play an important role in cardiac metabolism and function. Therefore, the aim of this study was to evaluate the mRNA and protein expressions of MT1, MT2, and RORα and to determine whether melatonin directly influences expression of circadian clocks within cultured rat cardiomyocytes. Adult rat cardiomyocyte cultures were created, and the cells were stimulated with 1 nM melatonin or vehicle. Gene expressions were assayed by real-time polymerase chain reaction (PCR). The mRNA and protein expressions of membrane melatonin receptors and RORα were established within adult rat cardiomyocytes. Two hours of melatonin stimulation did not alter the expression pattern of the analyzed genes. However, given at the proper time, melatonin kept Rev-erbα expression chronically high, specifically 12 h after melatonin treatment, avoiding the rhythmic decline of Rev-erbα mRNA. The blockage of MT1 and MT2 by luzindole did not alter the observed melatonin-induced expression of Rev-erbα mRNA, suggesting the nonparticipation of MT1 and MT2 on the melatonin effect within cardiomyocytes. It is possible to speculate that melatonin, in adult rat cardiomyocytes, may play an important role in the light signal transduction to peripheral organs, such as the heart, modulating its intrinsic rhythmicity.

Cadmium-Induced Disruption in 24-h Expression of Clock and Redox Enzyme Genes in Rat Medial Basal Hypothalamus: Prevention by Melatonin

In a previous study we reported that a low daily p.o. dose of cadmium (Cd) disrupted the circadian expression of clock and redox enzyme genes in rat medial basal hypothalamus (MBH). To assess whether melatonin could counteract Cd activity, male Wistar rats (45 days of age) received CdCl₂ (5 ppm) and melatonin (3 μg/mL) or vehicle (0.015% ethanol) in drinking water. Groups of animals receiving melatonin or vehicle alone were also included. After 1 month, MBH mRNA levels were measured by real-time PCR analysis at six time intervals in a 24-h cycle. In control MBH Bmal1 expression peaked at early scotophase, Per1 expression at late afternoon, and Per2 and Cry2 expression at mid-scotophase, whereas neither Clock nor Cry1 expression showed significant 24-h variations. This pattern was significantly disrupted (Clock, Bmal1) or changed in phase (Per1, Per2, Cry2) by CdCl₂ while melatonin counteracted the changes brought about by Cd on Per1 expression only. In animals receiving melatonin alone the 24-h pattern of MBH Per2 and Cry2 expression was disrupted. CdCl₂ disrupted the 24-h rhythmicity of Cu/Zn- and Mn-superoxide dismutase (SOD), nitric oxide synthase (NOS)-1, NOS-2, heme oxygenase (HO)-1, and HO-2 gene expression, most of the effects being counteracted by melatonin. In particular, the co-administration of melatonin and CdCl₂ increased Cu/Zn-SOD gene expression and decreased that of glutathione peroxidase (GPx), glutathione reductase (GSR), and HO-2. In animals receiving melatonin alone, significant increases in mean Cu/Zn and Mn-SOD gene expression, and decreases in that of GPx, GSR, NOS-1, NOS-2, HO-1, and HO-2, were found. The results indicate that the interfering effect of melatonin on the activity of a low dose of CdCl₂ on MBH clock and redox enzyme genes is mainly exerted at the level of redox enzyme gene expression.

Circadian rhythms in gene expression: Relationship to physiology, disease, drug disposition and drug action

Circadian rhythms (24h cycles) are observed in virtually all aspects of mammalian function from expression of genes to complex physiological processes. The master clock is present in the suprachiasmatic nucleus (SCN) in the anterior part of the hypothalamus and controls peripheral clocks present in other parts of the body. Components of this core clock mechanism regulate the circadian rhythms in genome-wide mRNA expression, which in turn regulate various biological processes. Disruption of circadian rhythms can be either the cause or the effect of various disorders including metabolic syndrome, inflammatory diseases and cancer. Furthermore, circadian rhythms in gene expression regulate both the action and disposition of various drugs and affect therapeutic efficacy and toxicity based on dosing time. Understanding the regulation of circadian rhythms in gene expression plays an important role in both optimizing the dosing time for existing drugs and in the development of new therapeutics targeting the molecular clock.

Genetic suppression of the circadian Clock mutation by the melatonin biosynthesis pathway

Most laboratory mouse strains including C57BL/6J do not produce detectable levels of pineal melatonin owing to deficits in enzymatic activity of arylalkylamine N-acetyltransferase (AANAT) and N-acetylserotonin O-methyl transferase (ASMT), two enzymes necessary for melatonin biosynthesis. Here we report that alleles segregating at these two loci in C3H/HeJ mice, an inbred strain producing melatonin, suppress the circadian period-lengthening effect of the Clock mutation. Through a functional mapping approach, we localize mouse Asmt to chromosome X and show that it, and the Aanat locus on chromosome 11, are significantly associated with pineal melatonin levels. Treatment of suprachiasmatic nucleus (SCN) explant cultures from Period2(Luciferase) (Per2(Luc)) Clock/+ reporter mice with melatonin, or the melatonin agonist, ramelteon, phenocopies the genetic suppression of the Clock mutant phenotype observed in living animals. These results demonstrate that melatonin suppresses the Clock/+ mutant phenotype and interacts with Clock to affect the mammalian circadian system.

Effect of rhythmic melatonin administration on clock gene expression in the suprachiasmatic nucleus and the heart of hypertensive TGR(mRen2)27 rats

OBJECTIVES

Plasma melatonin concentrations in non-dipping patients show a blunted daily rhythm. Melatonin has a capacity to improve disturbances in biological rhythms. Hypertensive TGR(mRen2)27 (TGR) rats with an upregulated renin-angiotensin system and inverted blood pressure profile were used to elucidate whether melatonin is able to influence the control of blood pressure.

DESIGN

Melatonin was administered in drinking water to normotensive Sprague-Dawley (SD) and hypertensive TGR rats during the dark phase of the light: dark cycle 12: 12 for 4 weeks.

METHODS

The effect of melatonin on blood pressure was monitored, and the expression of clock genes per2 and bmal1 and melatonin receptor MT1 in the suprachiasmatic nucleus (SCN) and the heart was measured by real time polymerase chain reaction during a 24-h cycle.

RESULTS AND CONCLUSION

The administration of melatonin did not influence clock gene expression in the SCN but its effect on clock gene expression in the heart was phase dependent in both SD and TGR rats. Melatonin administration did not decrease the expression of melatonin receptors in the SCN and the heart. Melatonin did not decrease blood pressure in TGR rats but influenced the peripheral oscillator in the heart independently of the SCN. A modified function of molecular circadian oscillators in the heart can interfere with anticipation and disturb the adaptation of this organ to pressure overload.

Time's arrow flies like a bird: two paradoxes for avian circadian biology

Biological timekeeping in birds is a fundamental feature of avian physiology, behavior and ecology. The physiological basis for avian circadian rhythmicity has pointed to a multi-oscillator system of mutually coupled pacemakers in the pineal gland, eyes and hypothalamic suprachiasmatic nuclei (SCN). In passerines, the role of the pineal gland and its hormone melatonin is particularly important. More recent molecular biological studies have pointed to a highly conserved mechanism involving rhythmic transcription and translation of "clock genes". However, studies attempting to reconcile the physiological role of pineal melatonin with molecular studies have largely failed. Recent work in our laboratory has suggested that melatonin-sensitive physiological processes are only loosely coupled to transcriptional oscillations. Similarly, although the pineal gland has been shown to be critical for overt circadian behaviors, its role in annual cycles of reproductive function appears to be minimal. Recent work on the seasonal control of birdsong, however, suggests that, although the pineal gland does not directly affect gonadal cycles, it is important for seasonal changes in song. Experimental analyses that address these paradoxes will shed light on the roles the biological clock play in birds and in vertebrates in general.

Melatonin in the multi‐oscillatory mammalian circadian world

In mammals, the complex interaction of neural, hormonal, and behavioral outputs from the suprachiasmatic nucleus (SCN) drives circadian expression of events, either directly or through coordination of the timing of peripheral oscillators. Melatonin, one of the endocrine output signals of the clock, provides the organism with circadian information and can be considered as an endogenous synchronizer, able to stabilize and reinforce circadian rhythms and to maintain their mutual phase-relationship at the different levels of the circadian network. Moreover, exogenous melatonin, through an action on the circadian clock, affects all levels of the circadian network. The molecular mechanisms underlying this chronobiotic effect have also been investigated in rats. REV-ERB alpha seems to be the initial molecular target.

Compartmentalized expression of light-induced clock genes in the suprachiasmatic nucleus of the diurnal grass rat (Arvicanthis niloticus)

Photic responses of the circadian system are mediated through light-induced clock gene expression in the suprachiasmatic nucleus (SCN). In nocturnal rodents, depending on the timing of light exposure, Per1 and Per2 gene expression shows distinct compartmentalized patterns that correspond to the behavioral responses. Whether the gene- and region-specific induction patterns are unique to nocturnal animals, or are also present in diurnal species is unknown. We explored this question by examining the light-induced Per1 and Per2 gene expression in functionally distinct SCN subregions, using diurnal grass rats Arvicanthis niloticus. Light exposure during nighttime induced Per1 and Per2 expression in the SCN, showing unique spatiotemporal profiles depending on the phase of the light exposure. After a phase delaying light pulse (LP) in the early night, strong Per1 induction was observed in the retinorecipient core region of the SCN, while strong Per2 induction was observed throughout the entire SCN. After a phase advancing LP in the late night, Per1 was first induced in the core and then extended into the whole SCN, accompanied by a weak Per2 induction. This compartmentalized expression pattern is very similar to that observed in nocturnal rodents, suggesting that the same molecular and intercellular pathways underlying acute photic responses are present in both diurnal and nocturnal species. However, after an LP in early subjective day, which induces phase advances in diurnal grass rats, but not in nocturnal rodents, we did not observe any Per1 or Per2 induction in the SCN. This result suggests that in spite of remarkable similarities in the SCN of diurnal and nocturnal rodents, unique mechanisms are involved in mediating the phase shifts of diurnal animals during the subjective day.

Cry1 circadian phase in vitro: wrapped up with an E-box

The circadian timing of gene expression is determined by transcriptional regulation through upstream response elements present throughout the genome. Central to this regulation are the actions of a core group of transcriptional activators and repressors, which act through, and are themselves regulated by, a small set of canonical circadian response elements. Among these, the E-box (CACGTG) is crucial for daytime transcriptional activity. The mammalian Period (Per1-3) and Cryptochrome (Cry1-2) genes are E-box-regulated genes, but in peripheral tissues peak Cry1 mRNA expression is delayed by several hours relative to that of Per. It has been proposed that this delay originates from interactions between the proximal E-box and retinoic acid-related orphan receptor response elements (RORE) present in the Cry1 promoter. By using real-time luciferase reporter assays in NIH3T3 cells the authors show here that a proximal 47-bp E-box containing region of the Cry1 promoter is both necessary and sufficient to drive circadian Cry1 transcription with an appropriate phase delay (around 4 h) relative to Per2. The results therefore suggest that, at least in this in vitro model of the clock, RORE are not necessary for the appropriate circadian regulation of Cry1 expression and rather suggest that sequences surrounding the proximal E-boxes confer gene-specific circadian phasing.

Pineal melatonin acts as a circadian zeitgeber and growth factor in chick astrocytes

Melatonin is rhythmically synthesized and released by the avian pineal gland and retina during the night, targeting an array of tissues and affecting a variety of physiological and behavioral processes. Among these targets, astrocytes express two melatonin receptor subtypes in vitro, the Mel(1A) and Mel(1C) receptors, which play a role in regulating metabolic activity and calcium homeostasis in these cells. Molecular characterization of chick astrocytes has revealed the expression of orthologs of the mammalian clock genes including clock, cry1, cry2, per2, and per3. To test the hypothesis that pineal melatonin entrains molecular clockworks in downstream cells, we asked whether coculturing astrocytes with pinealocytes or administration of exogenous melatonin cycles would entrain metabolic rhythms of 2-deoxy [14C]-glucose (2DG] uptake and/or clock gene expression in cultured astrocytes. Rhythmic secretion of melatonin from light-entrained pinealocytes in coculture as well as cyclic administration of exogenous melatonin entrained rhythms of 2DG uptake and expression of Gallus per2 (gper2) and/or gper3, but not of gcry1 mRNA. Surprisingly, melatonin also caused a dose-dependent increase in mitotic activity of astrocytes, both in coculture and when administered exogenously. The observation that melatonin stimulates mitotic activity in diencephalic astrocytes suggests a trophic role of the hormone in brain development. The data suggest a dual role for melatonin in avian astrocytes: synchronization of rhythmic processes in these cells and regulation of growth and differentiation. These two processes may or may not be mutually exclusive.

Brain Clocks: From the Suprachiasmatic Nuclei to a Cerebral Network

Circadian timing affects almost all life’s processes. It not only dictates when we sleep, but also keeps every cell and tissue working under a tight temporal regimen. The daily variations of physiology and behavior are controlled by a highly complex system comprising of a master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus, extra-SCN cerebral clocks, and peripheral oscillators. Here are presented similarities and differences in the molecular mechanisms of the clock machinery between the primary SCN clock and extra-SCN brain clocks. Diversity of secondary clocks in the brain, their specific sensitivities to time-giving cues, as their differential coupling to the master SCN clock, may allow more plasticity in the ability of the circadian timing system to integrate a wide range of temporal information. Furthermore, it raises the possibility that pathophysiological alterations of internal timing that are deleterious for health may result from internal desynchronization within the network of cerebral clocks.

Endogenous melatonin provides an effective circadian message to both the suprachiasmatic nuclei and the pars tuberalis of the rat

The suprachiasmatic nuclei (SCN) distribute the circadian neural message to the pineal gland which transforms it into a humoral circadian message, the nocturnal melatonin synthesis, which in turn modulates tissues expressing melatonin receptors such as the SCN or the pars tuberalis (PT). Nuclear orphan receptors (NOR), including rorbeta and rev-erbalpha, have been presented as functional links between the positive and negative loops of the molecular clock. Recent findings suggest that these NOR could be the initial targets of melatonin's chronobiotic message within the SCN. We investigated the role of these NOR in the physiological effect of endogenous melatonin on these tissues. We monitored rorbeta and rev-erbalpha mRNA expression levels by quantitative in situ hybridization after pinealectomy. Pinealectomy had no effect on NOR circadian expression rhythms in the SCN in 8-day pinealectomized (PX) animals. However in animals PX for 3 months, significant desynchronization between per1 and per2 transcription patterns appeared. These results suggest that endogenous melatonin could sustain the circadian rhythmicity and the phase relationship between the molecular partners of the SCN circadian system on a long-term basis. On the other hand, pinealectomy decreased the level and abolished the rhythmicity of NOR mRNA expression in the PT. These effects were partially prevented by daily melatonin administration in the drinking water. These results show that NOR can be regulated by the melatonin circadian rhythm in the PT and could be the link between the physiological action of melatonin and the core of the molecular circadian clock in this tissue.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Period gene expression in the diurnal degu (Octodon degus) differs from the nocturnal laboratory rat (Rattus norvegicus)

Recent data suggest that both nocturnal and diurnal mammals generate circadian rhythms using similarly phased feedback loops involving Period genes in the suprachiasmatic nuclei (SCN) of the hypothalamus. These molecular oscillations also exist in the brain outside of the SCN, but the relationship between SCN and extra-SCN oscillations is unclear. We hypothesized that a comparison of "diurnal" and "nocturnal" central nervous system Per rhythms would uncover differences in the underlying circadian mechanisms between these two chronotypes. Therefore, this study compared the 24-h oscillatory patterns of Per1 and Per2 mRNA in the SCN and putative striatum and cortex of Octodon degus (degu), a diurnal hystricognath rodent, with those of the nocturnal laboratory rat, Rattus norvegicus. The brains of adult male degus and rats were collected at 2-h intervals across 24 h in entrained light-dark and constant darkness conditions, and sections were analyzed via in situ hybridization. In the SCN, degu Per1 and Per2 hybridization signal exhibited 24-h oscillatory patterns similar in phasing to those seen in other rodents, with peaks occurring during the light period and troughs during the dark period. However, Per1 remained elevated for five fewer hours in the degu than in the rat, and Per2 remained elevated for two fewer hours in the degu. In brain areas outside of the SCN, the phase of Per2 hybridization signal rhythms in the degu were 180 degrees out of phase with those found in the rat, and Per1 hybridization signal lacked significant rhythmicity. These results suggest that, while certain basic components of the transcriptional-translational feedback loop generating circadian rhythms are similar in diurnal and nocturnal mammals, there are variations that may reflect adaptations to circadian niche.

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

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

Interactions of GABA A receptor activation and light on period mRNA expression in the suprachiasmatic nucleus

Activation of gamma-aminobutyric acid (GABA) A receptors in the suprachiasmatic nucleus (SCN) resets the circadian clock during the day and inhibits the ability of light to reset the clock at night. Light in turn acts during the day to inhibit the phase-resetting effects of GABA. Some evidence suggests that Period mRNA changes in the SCN are responsible for these interactions between light and GABA. Here, the hypothesis that light and the GABA A receptor interact by altering the expression of Period 1 and/or Period 2 mRNA in the SCN is tested. The GABA A agonist muscimol was injected near the SCN just prior to a light pulse, during the mid-subjective day and the early and late subjective night. Changes in Period 1 and Period 2 mRNA were measured in the SCN by in situ hybridization. Light-induced Period 1 mRNA was inhibited by GABA A receptor activation in the early and late subjective night, while Period 2 mRNA was only inhibited during the late night. During the subjective day, light had no effect on the ability of muscimol to suppress Period 1 mRNA hybridization signal. Thus, light and GABA A receptor activation inhibit each other's ability to induce behavioral phase shifts throughout the subjective day and night. However, only in the late night are these behavioral effects correlated with changes in Period gene expression. Together, our data support the hypothesis that the interacting effects of light and GABA are the result of the opposing actions of these stimuli on Period mRNA, but only during the subjective night.

Minireview: Entrainment of the suprachiasmatic clockwork in diurnal and nocturnal mammals

Daily rhythmicity, including timing of wakefulness and hormone secretion, is mainly controlled by a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN clockwork involves various clock genes, with specific temporal patterns of expression that are similar in nocturnal and diurnal species (e.g. the clock gene Per1 in the SCN peaks at midday in both categories). Timing of sensitivity to light is roughly similar, during nighttime, in diurnal and nocturnal species. Molecular mechanisms of photic resetting are also comparable in both species categories. By contrast, in animals housed in constant light, exposure to darkness can reset the SCN clock, mostly during the resting period, i.e. at opposite circadian times between diurnal and nocturnal species. Nonphotic stimuli, such as scheduled voluntary exercise, food shortage, exogenous melatonin, or serotonergic receptor activation, are also capable of shifting the master clock and/or modulating photic synchronization. Comparison between day- and night-active species allows classifications of nonphotic cues in two, arousal-independent and arousal-dependent, families of factors. Arousal-independent factors, such as melatonin (always secreted during nighttime, independently of daily activity pattern) or gamma-aminobutyric acid (GABA), have shifting effects at the same circadian times in both nocturnal and diurnal rodents. By contrast, arousal-dependent factors, such as serotonin (its cerebral levels follow activity pattern), induce phase shifts only during resting and have opposite modulating effects on photic resetting between diurnal and nocturnal species. Contrary to light and arousal-independent nonphotic cues, arousal-dependent nonphotic stimuli provide synchronizing feedback signals to the SCN clock in circadian antiphase between nocturnal and diurnal animals.

Organization of cell and tissue circadian pacemakers: a comparison among species

In most animal species, a circadian timing system has evolved as a strategy to cope with 24-hour rhythms in the environment. Circadian pacemakers are essential elements of the timing system and have been identified in anatomically discrete locations in animals ranging from insects to mammals. Rhythm generation occurs in single pacemaker neurons and is based on the interacting negative and positive molecular feedback loops. Rhythmicity in behavior and physiology is regulated by neuronal networks in which synchronization or coupling is required to produce coherent output signals. Coupling occurs among individual clock cells within an oscillating tissue, among functionally distinct subregions within the pacemaker, and between central pacemakers and the periphery. Recent evidence indicates that peripheral tissues can influence central pacemakers and contain autonomous circadian oscillators that contribute to the regulation of overt rhythmicity. The data discussed in this review describe coupling and synchronization mechanisms at the cell and tissue levels. By comparing the pacemaker systems of several multicellular animal species (Drosophila, cockroaches, crickets, snails, zebrafish and mammals), we will explore general organizational principles by which the circadian system regulates a 24-hour rhythmicity.