Strong resetting of the mammalian clock by constant light followed by constant darkness

Sex-specific attenuation of constant light-induced memory impairment and Clock gene expression in brain in hepatic Npas2 knockout mice

NPAS2 (Neuronal PAS Domain Protein 2) is a component of the core circadian clock and the coordinated activity between central brain and peripheral liver clock proteins postulated to be instrumental for linking behaviour and metabolism. We investigated a conditional liver-specific knockout mouse model (Npas2-/- or cKO) to explore its function in activity, circadian rhythms and cognition (novel object recognition-NOR). Circadian rhythms showed no genotype differences. Constant-light reduced NOR in floxxed controls but remarkably not in Npas2-/- mice, particularly females. Consistent with entrainment of systemic and central circadian biology, Npas2-/- mice showed altered expression of circadian gene Clock in frontal cortex. Sex differences independent of genotype were found in expression of circadian genes Clock, Bmal1 and Reverb-b in brain. Sex differences in Clock were absent in Npas2-/- mice. Females showed greater period length and phase response to constant light independently of genotype. The data suggest that a role for peripheral NPAS2 in constant light-induced memory impairment in females, and potential mediation by altered cortical circadian Clock gene expression, merit further investigation. These findings have implications for the interaction between peripheral and central circadian clocks, circadian sex differences and the deleterious effects of constant light on cognition.

Light-Modulated Circadian Synaptic Plasticity in the Somatosensory Cortex: Link to Locomotor Activity

The circadian clock controls various physiological processes, including synaptic function and neuronal activity, affecting the functioning of the entire organism. Light is an important external factor regulating the day-night cycle. This study examined the effects of the circadian clock and light on synaptic plasticity, and explored how locomotor activity contributes to these processes. We analyzed synaptic protein expression and excitatory synapse density in the somatosensory cortex of mice from four groups exposed to different lighting conditions (LD 12:12, DD, LD 16:8, and LL). Locomotor activity was assessed through individual wheel-running monitoring. To explore daily and circadian changes in synaptic proteins, we performed double-immunofluorescence labeling and laser scanning confocal microscopy imaging, targeting three pairs of presynaptic and postsynaptic proteins (Synaptophysin 1/PSD95, Piccolo/Homer 1, Neurexins/PICK1). Excitatory synapse density was evaluated by co-labeling presynaptic and postsynaptic markers. Our results demonstrated that all the analyzed synaptic proteins exhibited circadian regulation modulated by light. Under constant light conditions, only Piccolo and Homer 1 showed rhythmicity. Locomotor activity was also associated with the circadian clock's effects on synaptic proteins, showing a stronger connection to changes in postsynaptic protein levels. Excitatory synapse density peaked during the day/subjective day and exhibited an inverse relationship with locomotor activity. Continued light exposure disrupted cyclic changes in synapse density but kept it consistently elevated. These findings underscore the crucial roles of light and locomotor activity in regulating synaptic plasticity.

Persistence of clock gene expression in peripheral blood in dogs maintained under different photoperiod schedules

Dogs are the common pets adopted by humans, and their circadian behavior and physiology are influenced by human habits. In many families, there is a change of lifestyle with respect to the natural daylight (NDL) cycle. Exposure to constant light disrupts some central and peripheral circadian rhythms. The aim of the present study was to improve the knowledge about the circadian changes of clock components in the peripheral blood in dogs housed under NDL and constant light (LL) conditions. Blood samples were collected on five female Beagle dogs (2 years old, 14 ± 0.5 kg) every 4 hours for a 24-hour period during an NDL (Sunrise 05:05 h - Sunset 20:55  h) and 24-hour period of constant light (LL). Blood samples were stored in a PAX gene Blood RNA Tube, real-time RT-quantitative polymerase chain reaction was performed to determine Clock, Per1-3, and Cry1-2 gene expression. During the NDL, all genes investigated showed robust diurnal daily rhythmicity. During the constant light, only Clock maintained its daily rhythmicity. Clock acrophase was observed close to sunrise (ZT 0) and was statistically different from the other clock genes except for Per3. Per3 daily oscillations were not statistically significant. No differences were observed among the clock genes tested in the amplitude and robustness values. Our results can be considered preliminary data to provide new insights into the adaptation mechanism of the canine peripheral circadian clock. The persistence of Clock gene expression during the LL indicated the presence of an endogenously generated signal in blood. Because peripheral blood is an easily accessible sample in dogs, the analysis of clock gene expression in this tissue could be useful to investigate the adaptive capacity of this species housed in different environmental conditions linked to the owner's lifestyle.

Reversible suppression of circadian-driven locomotor rhythms in mice using a gradual fragmentation of the day-night cycle

Circadian rhythms are regulated by molecular clockwork and drive 24-h behaviors such as locomotor activity, which can be rendered non-functional through genetic knockouts of clock genes. Circadian rhythms are robust in constant darkness (DD) but are modulated to become exactly 24 h by the external day-night cycle. Whether ill-timed light and dark exposure can render circadian behaviors non-functional to the extent of genetic knockouts is less clear. In this study, we discovered an environmental approach that led to a reduction or lack in rhythmic 24-h-circadian wheel-running locomotor behavior in mice (referred to as arrhythmicity). We first observed behavioral circadian arrhythmicity when mice were gradually exposed to a previously published disruptive environment called the fragmented day-night cycle (FDN-G), while maintaining activity alignment with the four dispersed fragments of darkness. Remarkably, upon exposure to constant darkness (DD) or constant light (LL), FDN-G mice lost any resemblance to the FDN-G-only phenotype and instead, exhibited sporadic activity bursts. Circadian rhythms are maintained in control mice with sudden FDN exposure (FDN-S) and fully restored in FDN-G mice either spontaneously in DD or after 12 h:12 h light-dark exposure. This is the first study to generate a light-dark environment that induces reversible suppression of circadian locomotor rhythms in mice.

Prolonged Light Exposure Induces Circadian Impairment in Aquaporin-4-Knockout Mice

Astrocytes are densely present in the suprachiasmatic nucleus (SCN), which is the master circadian oscillator in mammals, and are presumed to play a key role in circadian oscillation. However, specific astrocytic molecules that regulate the circadian clock are not yet well understood. In our study, we found that the water channel aquaporin-4 (AQP4) was abundantly expressed in SCN astrocytes, and we further examined its circadian role using AQP4-knockout mice. There was no prominent difference in circadian behavioral rhythms between Aqp4^-/- and Aqp4^+/+ mice subjected to light-dark cycles and constant dark conditions. However, exposure to constant light induced a greater decrease in the Aqp4^-/- mice rhythmicity. Although the damped rhythm in long-term constant light recovered after transfer to constant dark conditions in both genotypes, the period until the reappearance of original rhythmicity was severely prolonged in Aqp4^-/- mice. In conclusion, AQP4 absence exacerbates the prolonged light-induced impairment of circadian oscillations and delays their recovery to normal rhythmicity.

Relationship of Circadian Rhythm in Behavioral Characteristics and Lipid Peroxidation of Brain Tissues in Mice

OBJECTIVE

This study aimed to explore the relationship among several indices of circadian rhythms and lipid peroxidation of brain tissue in mice.

METHODS

After entrainment of 4-week-old mice, one group was disrupted their circadian rhythms for three days and the other group for seven days (n = 10, respectively). After a recovery period, the Y-maze test, the elevated plus maze test, the tail suspension test, and the forced swimming test were conducted. To assess lipid peroxidation in brain tissue, thiobarbituric acid reactive substances were measured in the cortex, hippocampus, and cerebellum.

RESULTS

When circadian rhythms were disrupted and adapted back to their original rhythm, the recovery time of the 7-day disruption group (median 3.35 days) was significiantly faster than one of the 3-day disruption group (median 4.87 days). In the group with a 7-day disruption, mice that had recovered their rhythms early had higher malondialdehyde levels in their hippocampus compared to those with delayed recovery. The entrainment of circadian rhythms was negatively correlated with the malondialdehyde level of brain tissue. The behavioral test results showed no differences depending on the disruption durations or recovery patterns of circadian rhythms.

CONCLUSION

These results suggest that disruption types, recovery patterns, and the entrainment of circadian rhythms are likely to affect oxidative stress in adolescents or young adult mice. Future study is needed to confirm and specify these results on the effects of circadian rhythms on oxidative stress and age-dependent effects.

Protocol for setup and circadian analysis of inverted feeding in mice

Inverted feeding is a paradigm to study synchronization of circadian clocks by feeding rhythm in tissues more directly. Here, we provide a protocol for performing inverted feeding in mice and analyzing circadian rhythmicity in mouse tissues. We describe setting up inverted feeding and performing tissue dissection, followed by RNA extraction and gene expression analysis, and lastly R software-based analysis of circadian rhythmicity. This protocol can be combined with the use of CircaMetDB database for mechanistic studies of inverted feeding. For complete details on the use and execution of this protocol, please refer to Xin et al. (2021).

CRYPTOCHROMES confer robustness, not rhythmicity, to circadian timekeeping

Circadian rhythms are a pervasive property of mammalian cells, tissues and behaviour, ensuring physiological adaptation to solar time. Models of cellular timekeeping revolve around transcriptional feedback repression, whereby CLOCK and BMAL1 activate the expression of PERIOD (PER) and CRYPTOCHROME (CRY), which in turn repress CLOCK/BMAL1 activity. CRY proteins are therefore considered essential components of the cellular clock mechanism, supported by behavioural arrhythmicity of CRY-deficient (CKO) mice under constant conditions. Challenging this interpretation, we find locomotor rhythms in adult CKO mice under specific environmental conditions and circadian rhythms in cellular PER2 levels when CRY is absent. CRY-less oscillations are variable in their expression and have shorter periods than wild-type controls. Importantly, we find classic circadian hallmarks such as temperature compensation and period determination by CK1δ/ε activity to be maintained. In the absence of CRY-mediated feedback repression and rhythmic Per2 transcription, PER2 protein rhythms are sustained for several cycles, accompanied by circadian variation in protein stability. We suggest that, whereas circadian transcriptional feedback imparts robustness and functionality onto biological clocks, the core timekeeping mechanism is post-translational.

A multi-tissue multi-omics analysis reveals distinct kineztics in entrainment of diurnal transcriptomes by inverted feeding

Time of eating synchronizes circadian rhythms of metabolism and physiology. Inverted feeding can uncouple peripheral circadian clocks from the central clock located in the suprachiasmatic nucleus. However, system-wide changes of circadian metabolism and physiology entrained to inverted feeding in peripheral tissues remain largely unexplored. Here, we performed a 24-h global profiling of transcripts and metabolites in mouse peripheral tissues to study the transition kinetics during inverted feeding, and revealed distinct kinetics in phase entrainment of diurnal transcriptomes by inverted feeding, which graded from fat tissue (near-completely entrained), liver, kidney, to heart. Phase kinetics of tissue clocks tracked with those of transcriptomes and were gated by light-related cues. Integrated analysis of transcripts and metabolites demonstrated that fatty acid oxidation entrained completely to inverted feeding in heart despite the slow kinetics/resistance of the heart clock to entrainment by feeding. This multi-omics resource defines circadian signatures of inverted feeding in peripheral tissues (www.CircaMetDB.org.cn).

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A multi-omics analysis of food entrainment in mouse peripheral tissues

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Inverted feeding rhythm entrains diurnal transcriptomes with distinct kinetics

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Phase kinetics of tissue clocks is conditioned by constant light

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Cardiac metabolism entrains to feeding fast with slow kinetics of the heart clock

Does a Red House Affect Rhythms in Mice with a Corrupted Circadian System?

The circadian rhythms of body functions in mammals are controlled by the circadian system. The suprachiasmatic nucleus (SCN) in the hypothalamus orchestrates subordinate oscillators. Time information is conveyed from the retina to the SCN to coordinate an organism's physiology and behavior with the light/dark cycle. At the cellular level, molecular clockwork composed of interlocked transcriptional/translational feedback loops of clock genes drives rhythmic gene expression. Mice with targeted deletion of the essential clock gene Bmal1 (Bmal1-/-) have an impaired light input pathway into the circadian system and show a loss of circadian rhythms. The red house (RH) is an animal welfare measure widely used for rodents as a hiding place. Red plastic provides light at a low irradiance and long wavelength-conditions which affect the circadian system. It is not known yet whether the RH affects rhythmic behavior in mice with a corrupted circadian system. Here, we analyzed whether the RH affects spontaneous locomotor activity in Bmal1-/- mice under standard laboratory light conditions. In addition, mPER1- and p-ERK-immunoreactions, as markers for rhythmic SCN neuronal activity, and day/night plasma corticosterone levels were evaluated. Our findings indicate that application of the RH to Bmal1-/- abolishes rhythmic locomotor behavior and dampens rhythmic SCN neuronal activity. However, RH had no effect on the day/night difference in corticosterone levels.

Constant Light Dysregulates Cochlear Circadian Clock and Exacerbates Noise-Induced Hearing Loss

Noise-induced hearing loss is one of the major causes of acquired sensorineural hearing loss in modern society. While people with excessive exposure to noise are frequently the population with a lifestyle of irregular circadian rhythms, the effects of circadian dysregulation on the auditory system are still little known. Here, we disturbed the circadian clock in the cochlea of male CBA/CaJ mice by constant light (LL) or constant dark. LL significantly repressed circadian rhythmicity of circadian clock genes Per1, Per2, Rev-erbα, Bmal1, and Clock in the cochlea, whereas the auditory brainstem response thresholds were unaffected. After exposure to low-intensity (92 dB) noise, mice under LL condition initially showed similar temporary threshold shifts to mice under normal light-dark cycle, and mice under both conditions returned to normal thresholds after 3 weeks. However, LL augmented high-intensity (106 dB) noise-induced permanent threshold shifts, particularly at 32 kHz. The loss of outer hair cells (OHCs) and the reduction of synaptic ribbons were also higher in mice under LL after noise exposure. Additionally, LL enhanced high-intensity noise-induced 4-hydroxynonenal in the OHCs. Our findings convey new insight into the deleterious effect of an irregular biological clock on the auditory system.

Peripheral circadian rhythms in the liver and white adipose tissue of mice are attenuated by constant light and restored by time-restricted feeding

Disturbance of circadian rhythms underlies various metabolic diseases. Constant light exposure (LL) is known to disrupt both central and peripheral circadian rhythms. Here, we attempted to determine whether the effects of LL are different between various peripheral tissues and whether time-restricted feeding restores the circadian rhythms especially in white adipose tissue (WAT). Six-week-old mice were subjected to three feeding regimes: ad libitum feeding under light/dark phase (LD), ad libitum feeding under LL cycle, and restricted feeding at night-time under LL cycle with a normal chow. After 3 weeks, we compared body weight, food intake, plasma levels of lipids and glucose, and the expression patterns of the clock genes and the genes involved in lipid metabolism in the liver and WAT. The mice kept under LL with or without time-restricted feeding were 5.2% heavier (p<0.001, n = 16) than the mice kept under LD even though the food intakes of the two groups were the same. Food intake occurred mostly in the dark phase. LL disrupted this pattern, causing disruptions in circadian rhythms of plasma levels of triglycerides (TG) and glucose. Time-restricted feeding partially restored the rhythms. LL eliminated the circadian rhythms of the expression of the clock genes as well as most of the genes involved in lipid metabolism in both liver and WAT. More notably, LL markedly decreased not only the amplitude but also the average levels of the expression of the genes in the liver, but not in the WAT, suggesting that transcription in the liver is sensitive to constant light exposure. Time-restricted feeding restored the circadian rhythms of most of the genes to various degrees in both liver and WAT. In conclusion, LL disrupted the peripheral circadian rhythms more severely in liver than in WAT. Time-restricted feeding restored the circadian rhythms in both tissues.

Circadian rhythms in skin and other elastic tissues

Circadian rhythms are daily oscillations that, in mammals, are driven by both a master clock, located in the brain, and peripheral clocks in cells and tissues. Approximately 10% of the transcriptome, including extracellular matrix components, is estimated to be under circadian control. Whilst it has been established that certain collagens and extracellular matrix proteases are diurnally regulated (for example in tendon, cartilage and intervertebral disc) the role played by circadian rhythms in mediating elastic fiber homeostasis is poorly understood. Skin, arteries and lungs are dynamic, resilient, elastic fiber-rich organs and tissues. In skin, circadian rhythms influence cell migration and proliferation, wound healing and susceptibility of the tissues to damage (from protease activity, oxidative stress and ultraviolet radiation). In the cardiovascular system, blood pressure and heart rate also follow age-dependent circadian rhythms whilst the lungs exhibit diurnal variations in immune response. In order to better understand these processes it will be necessary to characterise diurnal changes in extracellular matrix biology. In particular, given the sensitivity of peripheral clocks to external factors, the timed delivery of interventions (chronotherapy) has the potential to significantly improve the efficacy of treatments designed to repair and regenerate damaged cutaneous, vascular and pulmonary tissues.

Insulin/IGF-1 Drives PERIOD Synthesis to Entrain Circadian Rhythms with Feeding Time

In mammals, endogenous circadian clocks sense and respond to daily feeding and lighting cues, adjusting internal ∼24 h rhythms to resonate with, and anticipate, external cycles of day and night. The mechanism underlying circadian entrainment to feeding time is critical for understanding why mistimed feeding, as occurs during shift work, disrupts circadian physiology, a state that is associated with increased incidence of chronic diseases such as type 2 (T2) diabetes. We show that feeding-regulated hormones insulin and insulin-like growth factor 1 (IGF-1) reset circadian clocks in vivo and in vitro by induction of PERIOD proteins, and mistimed insulin signaling disrupts circadian organization of mouse behavior and clock gene expression. Insulin and IGF-1 receptor signaling is sufficient to determine essential circadian parameters, principally via increased PERIOD protein synthesis. This requires coincident mechanistic target of rapamycin (mTOR) activation, increased phosphoinositide signaling, and microRNA downregulation. Besides its well-known homeostatic functions, we propose insulin and IGF-1 are primary signals of feeding time to cellular clocks throughout the body.

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Insulin and IGF-1 are a systemic synchronizing cue for circadian rhythms in mammals

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Insulin and IGF-1 signaling rapidly upregulates translation of PERIOD clock proteins

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Coincident signaling facilitates selective induction of PERIOD synthesis

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Circadian disruption is recapitulated by mistimed insulin in cell and animal models

Light and Cognition: Roles for Circadian Rhythms, Sleep, and Arousal

Light exerts a wide range of effects on mammalian physiology and behavior. As well as synchronizing circadian rhythms to the external environment, light has been shown to modulate autonomic and neuroendocrine responses as well as regulating sleep and influencing cognitive processes such as attention, arousal, and performance. The last two decades have seen major advances in our understanding of the retinal photoreceptors that mediate these non-image forming responses to light, as well as the neural pathways and molecular mechanisms by which circadian rhythms are generated and entrained to the external light/dark (LD) cycle. By contrast, our understanding of the mechanisms by which lighting influences cognitive processes is more equivocal. The effects of light on different cognitive processes are complex. As well as the direct effects of light on alertness, indirect effects may also occur due to disrupted circadian entrainment. Despite the widespread use of disrupted LD cycles to study the role circadian rhythms on cognition, the different experimental protocols used have subtly different effects on circadian function which are not always comparable. Moreover, these protocols will also disrupt sleep and alter physiological arousal, both of which are known to modulate cognition. Studies have used different assays that are dependent on different cognitive and sensory processes, which may also contribute to their variable findings. Here, we propose that studies addressing the effects of different lighting conditions on cognitive processes must also account for their effects on circadian rhythms, sleep, and arousal if we are to fully understand the physiological basis of these responses.

Stability of Wake-Sleep Cycles Requires Robust Degradation of the PERIOD Protein

Robustness in biology is the stability of phenotype under diverse genetic and/or environmental perturbations. The circadian clock has remarkable stability of period and phase that-unlike other biological oscillators-is maintained over a wide range of conditions. Here, we show that the high fidelity of the circadian system stems from robust degradation of the clock protein PERIOD. We show that PERIOD degradation is regulated by a balance between ubiquitination and deubiquitination, and that disruption of this balance can destabilize the clock. In mice with a loss-of-function mutation of the E3 ligase gene β-Trcp2, the balance of PERIOD degradation is perturbed and the clock becomes dramatically unstable, presenting a unique behavioral phenotype unlike other circadian mutant animal models. We believe that our data provide a molecular explanation for how circadian phases, such as wake-sleep onset times, can become unstable in humans, and we present a unique mouse model to study human circadian disorders with unstable circadian rhythm phases.

Environmental 24-hr Cycles Are Essential for Health

Circadian rhythms are deeply rooted in the biology of virtually all organisms. The pervasive use of artificial lighting in modern society disrupts circadian rhythms and can be detrimental to our health. To investigate the relationship between disrupting circadian rhythmicity and disease, we exposed mice to continuous light (LL) for 24 weeks and measured several major health parameters. Long-term neuronal recordings revealed that 24 weeks of LL reduced rhythmicity in the central circadian pacemaker of the suprachiasmatic nucleus (SCN) by 70%. Strikingly, LL exposure also reduced skeletal muscle function (forelimb grip strength, wire hanging duration, and grid hanging duration), caused trabecular bone deterioration, and induced a transient pro-inflammatory state. After the mice were returned to a standard light-dark cycle, the SCN neurons rapidly recovered their normal high-amplitude rhythm, and the aforementioned health parameters returned to normal. These findings strongly suggest that a disrupted circadian rhythm reversibly induces detrimental effects on multiple biological processes.

A tunable artificial circadian clock in clock-defective mice

Self-sustaining oscillations are essential for diverse physiological functions such as the cell cycle, insulin secretion and circadian rhythms. Synthetic oscillators using biochemical feedback circuits have been generated in cell culture. These synthetic systems provide important insight into design principles for biological oscillators, but have limited similarity to physiological pathways. Here we report the generation of an artificial, mammalian circadian clock in vivo, capable of generating robust, tunable circadian rhythms. In mice deficient in Per1 and Per2 genes (thus lacking circadian rhythms), we artificially generate PER2 rhythms and restore circadian sleep/wake cycles with an inducible Per2 transgene. Our artificial clock is tunable as the period and phase of the rhythms can be modulated predictably. This feature, and other design principles of our work, might enhance the study and treatment of circadian dysfunction and broader aspects of physiology involving biological oscillators.

Circadian rhythms are central to health and disease and there is renewed interest in chronotherapy. Here, the authors present a mouse with an artificial circadian clock that can be pharmacologically tuned, providing a tool for future studies of circadian biology and therapy.

Parkinson's disease, lights and melanocytes: looking beyond the retina

Critical analysis of recent research suggesting that light pollution causes Parkinson's disease (PD) reveals that such a hypothesis is unsustainable in the context of therapeutic use of light in treating various neuropsychiatric conditions. Reinterpretation of their findings suggests that retinal damage caused by prolonged light exposure may have contributed to the observed enhancement of experimental PD. To test this hypothesis further, forty-two Sprague Dawley rats received microinjections of 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-2, 4, 6-tetrahydropyridine (MPTP), paraquat or rotenone into the vitreal mass in doses so minute that the effects could not be attributed to diffusion into brain. Significant changes in five motor parameters consistent with symptoms of experimental PD were observed. These findings support the interpretation that the retina is involved in the control of motor function and in the aetiology of PD.

miRNAs are required for generating a time delay critical for the circadian oscillator

BACKGROUND

Circadian clocks coordinate an organism's activities and regulate metabolic homeostasis in relation to daily environmental changes, most notably light/dark cycles. As in other organisms, the timekeeping mechanism in mammals depends on a self-sustaining transcriptional negative feedback loop with a built-in time delay in feedback inhibition. Although the time delay is essential for generating a slow, self-sustaining negative feedback loop with a period close to 24 hr, the exact mechanisms underlying the time delay are not known.

RESULTS

Here, we show that RNAi mediated by microRNAs (miRNAs) is an essential mechanism in generating the time delay. In Dicer-deficient (and thus miRNA-deficient) cells and mice, circadian rhythms were dramatically shortened (by ∼2 hr), although the rhythms remained robust. The period shortening was caused by faster PER1 and PER2 translation in the Dicer-deficient cells. We also identified three specific miRNAs that regulate Per expression and showed that knockdown of these miRNAs in wild-type cells also shortened the circadian period.

CONCLUSIONS

Consistent with the canonical function of miRNAs as translational modulators of target genes and their widespread roles in cell physiology, circadian rhythms are also modulated by miRNA-mediated RNAi acting on posttranscriptional regulation of key clock genes. Our present study definitively shows that RNAi is an important modulator of circadian rhythms by controlling the pace of PER synthesis and presents a novel layer of regulation for the clock.

The embryonic pineal gland of the chicken as a model for experimental jet lag

The circadian clock in the chicken pineal model develops before hatching, at around the 17th embryonic day (ED17). By this stage, it runs in synchrony with environmental cues. To address if phase resetting mechanisms are comparable to those of post-hatched chicken, we investigated ED19 stage chicken embryos under 12h light:12h dark (LD), under constant darkness (DD), or under acute 4h phase delay of the LD condition (LD+4). The 24h mRNA-expression patterns of clock gene clock and of clock controlled genes Aanat and hiomt were analyzed with qRT-PCR. Under DD the rhythm of Aanat did not change significantly, however the 24h pattern of hiomt was altered. Clock shows a delayed response to DD with a phase-shift in its rhythm. After the first cycle under LD+4 conditions, the 24h patterns of Aa-nat and hiomt mRNA-s were phase delayed. Clock showed both acute and delayed changes in response to LD+4. These results show that the embryonic chicken pineal gland has a fully functioning clock mechanism, and that it is a good model for phase-change experiments. In addition it demonstrates that only one cycle of altered light schedule is sufficient to trigger changes within the molecular clock mechanisms of the chicken embryonic pineal model.

Mechanisms of rapid antidepressant effects of sleep deprivation therapy: clock genes and circadian rhythms

A significant subset of both major depressive disorder and bipolar disorder patients rapidly (within 24 hours) and robustly improves with the chronotherapeutic intervention of sleep deprivation therapy (SDT). Major mood disorder patients are reported to have abnormal circadian rhythms including temperature, hormonal secretion, mood, and particularly sleep. These rhythms are modulated by the clock gene machinery and its products. It is hypothesized that SDT resets abnormal clock gene machinery, that relapse of depressive symptoms during recovery night sleep reactivates abnormal clock gene machinery, and that supplemental chronotherapies and medications can block relapse and help stabilize circadian-related improvement. The central circadian clock genes, BMAL1/CLOCK (NPAS2), bind to Enhancer Boxes to initiate the transcription of circadian genes, including the period genes (per1, per2, per3). It is suggested that a defect in BMAL1/CLOCK (NPAS2) or in the Enhancer Box binding contributes to altered circadian function associated, in part, with the period genes. The fact that chronotherapies, including SDT and sleep phase advance, are dramatically effective suggests that altered clock gene machinery may represent a core pathophysiological defect in a subset of mood disorder patients.

Detrimental effects of constant light exposure and high-fat diet on circadian energy metabolism and insulin sensitivity

Circadian rhythm disturbances are observed in, e.g., aging and neurodegenerative diseases and are associated with an increased incidence of obesity and diabetes. We subjected male C57Bl/6J mice to constant light [12-h light-light (LL) cycle] to examine the effects of a disturbed circadian rhythm on energy metabolism and insulin sensitivity. In vivo electrophysiological recordings in the central pacemaker of the suprachiasmatic nuclei (SCN) revealed an immediate reduction in rhythm amplitude, stabilizing at 44% of normal amplitude values after 4 d LL. Food intake was increased (+26%) and energy expenditure decreased (-13%), and we observed immediate body weight gain (d 4: +2.4%, d 14: +5.0%). Mixed model analysis revealed that weight gain developed more rapidly in response to LL as compared to high fat. After 4 wk in LL, the circadian pattern in feeding and energy expenditure was completely lost, despite continuing low-amplitude rhythms in the SCN and in behavior, whereas weight gain had stabilized. Hyperinsulinemic-euglycemic clamp analysis revealed complete abolishment of normal circadian variation in insulin sensitivity in LL. In conclusion, a reduction in amplitude of the SCN, to values previously observed in aged mice, is sufficient to induce a complete loss of circadian rhythms in energy metabolism and insulin sensitivity.

Phase-shifting the fruit fly clock without cryptochrome

The blue light photopigment cryptochrome (CRY) is thought to be the main circadian photoreceptor of Drosophila melanogaster. Nevertheless, entrainment to light-dark cycles is possible without functional CRY. Here, we monitored phase response curves of cry(01) mutants and control flies to 1-hour 1000-lux light pulses. We found that cry(01) mutants phase-shift their activity rhythm in the subjective early morning and late evening, although with reduced magnitude. This phase-shifting capability is sufficient for the slowed entrainment of the mutants, indicating that the eyes contribute to the clock's light sensitivity around dawn and dusk. With longer light pulses (3 hours and 6 hours), wild-type flies show greatly enhanced magnitude of phase shift, but CRY-less flies seem impaired in the ability to integrate duration of the light pulse in a wild-type manner: Only 6-hour light pulses at circadian time 21 significantly increased the magnitude of phase advances in cry(01) mutants. At circadian time 15, the mutants exhibited phase advances instead of the expected delays. These complex results are discussed.

Circadian activity rhythms and risk of incident dementia and mild cognitive impairment in older women

OBJECTIVE

Previous cross-sectional studies have observed alterations in activity rhythms in dementia patients but the direction of causation is unclear. We determined whether circadian activity rhythms measured in community-dwelling older women are prospectively associated with incident dementia or mild cognitive impairment (MCI).

METHODS

Activity rhythm data were collected from 1,282 healthy community-dwelling women from the Study of Osteoporotic Fractures (SOF) cohort (mean age 83 years) with wrist actigraphy for a minimum of three 24-hour periods. Each participant completed a neuropsychological test battery and had clinical cognitive status (dementia, MCI, normal) adjudicated by an expert panel approximately 5 years later. All analyses were adjusted for demographics, body mass index (BMI), functional status, depression, medications, alcohol, caffeine, smoking, health status, and comorbidities.

RESULTS

After 4.9 years of follow-up, 195 (15%) women had developed dementia and 302 (24%) had developed MCI. Older women with decreased activity rhythms had a higher likelihood of developing dementia or MCI when comparing those in the lowest quartiles of amplitude (odds ratio [OR] = 1.57; 95% CI, 1.09-2.25) or rhythm robustness (OR = 1.57; 95% CI, 1.10-2.26) to women in the highest quartiles. An increased risk of dementia or MCI (OR = 1.83; 95% CI, 1.29-2.61) was found for women whose timing of peak activity occurred later in the day (after 3:51 PM) when compared to those with average timing (1:34 PM-3:51 PM).

INTERPRETATION

Older, healthy women with decreased circadian activity rhythm amplitude and robustness, and delayed rhythms have increased odds of developing dementia and MCI. If confirmed, future studies should examine whether interventions (physical activity, bright light exposure) that influence activity rhythms will reduce the risk of cognitive deterioration in the elderly.

The noradrenaline reuptake inhibitor atomoxetine phase-shifts the circadian clock in mice

Circadian rhythms are recurring cycles in physiology and behaviour that repeat with periods of near 24 h and are driven by an endogenous circadian timekeeping system with a master circadian pacemaker located in the suprachiasmatic nucleus (SCN). Atomoxetine is a specific noradrenaline reuptake inhibitor that is used in the clinical management of attention-deficit/hyperactivity disorder (ADHD). In the current study we examined the effects of atomoxetine on circadian rhythms in mice. Atomoxetine (i.p.; 3 mg/kg) treatment of mice free-running in constant light (LL) at circadian time (CT) 6 induced large phase delays that were significantly different to saline controls. Treatment of animals with atomoxetine at CT13 or CT18 did not elicit any significant phase shifts. We also examined the effects of atomoxetine treatment of animals free-running in constant darkness (DD). Atomoxetine treatment at CT6 in these animals leads to more modest, but significant, phase advances, whereas treatment at CT18 did not elicit significant phase shifts. The effects of atomoxetine in LL were attenuated by pretreatment with the α-1 adrenoreceptor antagonist prazosin and were mimicked by another noradrenaline reuptake inhibitor, reboxetine. Further, atomoxetine treatment at CT6 induced a downregulation of c-Fos and CLOCK in the SCN, but did not alter the expression of PER2 and BMAL1. Atomoxetine during the night phase did not alter any of these factors. Atomoxetine treatment preceding a light pulse at CT15 enhanced the magnitude of the photic-phase shift, whereas it altered photic induction of the immediate early gene products c-Fos and ARC in the SCN. These data indicate that atomoxetine can reset the circadian clock and indicate that part of the therapeutic profile of atomoxetine may be through circadian rhythm modulation.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Mathematical model of the Drosophila circadian clock: loop regulation and transcriptional integration

Eukaryotic circadian clocks include interconnected positive and negative feedback loops. The clock-cycle dimer (CLK-CYC) and its homolog, CLK-BMAL1, are key transcriptional activators of central components of the Drosophila and mammalian circadian networks, respectively. In Drosophila, negative loops include period-timeless and vrille; positive loops include par domain protein 1. Clockwork orange (CWO) is a recently discovered negative transcription factor with unusual effects on period, timeless, vrille, and par domain protein 1. To understand the actions of this protein, we introduced a new system of ordinary differential equations to model regulatory networks. The model is faithful in the sense that it replicates biological observations. CWO loop actions elevate CLK-CYC; the transcription of direct targets responds by integrating opposing signals from CWO and CLK-CYC. Loop regulation and integration of opposite transcriptional signals appear to be central mechanisms as they also explain paradoxical effects of period gain-of-function and null mutations.

Rhythmic PER abundance defines a critical nodal point for negative feedback within the circadian clock mechanism

Circadian rhythms in mammals are generated by a transcriptional negative feedback loop that is driven primarily by oscillations of PER and CRY, which inhibit their own transcriptional activators, CLOCK and BMAL1. Current models posit that CRY is the dominant repressor, while PER may play an accessory role. In this study, however, constitutive expression of PER, and not CRY1, severely disrupted the clock in fibroblasts and liver. Furthermore, constitutive expression of PER2 in the brain and SCN of transgenic mice caused a complete loss of behavioral circadian rhythms in a conditional and reversible manner. These results demonstrate that rhythmic levels of PER2, rather than CRY1, are critical for circadian oscillations in cells and in the intact organism. Our biochemical evidence supports an elegant mechanism for the disparity: PER2 directly and rhythmically binds to CLOCK:BMAL1, while CRY only interacts indirectly; PER2 bridges CRY and CLOCK:BMAL1 to drive the circadian negative feedback loop.

Impact of melatonin and molecular clockwork components on the expression of thyrotropin beta-chain (Tshb) and the Tsh receptor in the mouse pars tuberalis

Photoperiodic regulation of reproduction in birds and mammals involves thyrotropin beta-chain (TSHb), which is secreted from the pars tuberalis (PT) and controls the expression of deiodinase type 2 and 3 in the ependymal cell layer of the infundibular recess (EC) via TSH receptors (TSHRs). To analyze the impact of melatonin and the molecular clockwork on the expression of Tshb and Tshr, we investigated melatonin-proficient C3H wild-type (WT), melatonin receptor 1-deficient (MT1-/-) or clockprotein PERIOD1-deficient (mPER1-/-) mice. Expression of Tshb and TSHb immunoreactivity in PT were low during day and high during the night in WT, high during the day and low during the night in mPER1-deficient, and equally high during the day and night in MT1-deficient mice. Melatonin injections into WT acutely suppressed Tshb expression. Transcription assays showed that the 5' upstream region of the Tshb gene could be controlled by clockproteins. Tshr levels in PT were low during the day and high during the night in WT and mPER1-deficient mice and equally low in MT1-deficient mice. Tshr expression in the EC did not show a day/night variation. Melatonin injections into WT acutely induced Tshr expression in PT but not in EC. TSH stimulation of hypothalamic slice cultures of WT induced phosphorylated cAMP response element-binding protein in PT and EC and deiodinase type 2 in the EC. Our data suggest that Tshb expression in PT is controlled by melatonin and the molecular clockwork and that melatonin activates Tshr expression in PT but not in EC. They also confirm the functional importance of TSHR in the PT and EC.

Biological rhythms, higher brain function, and behavior: Gaps, opportunities, and challenges

Increasing evidence suggests that disrupted temporal organization impairs behavior, cognition, and affect; further, disruption of circadian clock genes impairs sleep-wake cycle and social rhythms which may be implicated in mental disorders. Despite this strong evidence, a gap in understanding the neural mechanisms of this interaction obscures whether biological rhythms disturbances are the underlying causes or merely symptoms of mental disorder. Here, we review current understanding, emerging concepts, gaps, and opportunities pertinent to (1) the neurobiology of the interactions between circadian oscillators and the neural circuits subserving higher brain function and behaviors of relevance to mental health, (2) the most promising approaches to determine how biological rhythms regulate brain function and behavior under normal and pathological conditions, (3) the gaps and challenges to advancing knowledge on the link between disrupted circadian rhythms/sleep and psychiatric disorders, and (4) the novel strategies for translation of basic science discoveries in circadian biology to clinical settings to define risk, prevent or delay onset of mental illnesses, design diagnostic tools, and propose new therapeutic strategies. The review is organized around five themes pertinent to (1) the impact of molecular clocks on physiology and behavior, (2) the interactions between circadian signals and cognitive functions, (3) the interface of circadian rhythms with sleep, (4) a clinical perspective on the relationship between circadian rhythm abnormalities and affective disorders, and (5) the pre-clinical models of circadian rhythm abnormalities and mood disorders.

Chronotype influences diurnal variations in the excitability of the human motor cortex and the ability to generate torque during a maximum voluntary contraction

The ability to generate torque during a maximum voluntary contraction (MVC) changes over the day. The present experiments were designed to determine the influence of an individual's chronotype on this diurnal rhythm and on cortical, spinal, and peripheral mechanisms that may be related to torque production. After completing a questionnaire to determine chronotype, 18 subjects (9 morning people, 9 evening people) participated in 4 data collection sessions (at 09:00, 13:00, 17:00, and 21:00) over 1 day. We used magnetic stimulation of the cortex, electrical stimulation of the tibial nerve, electromyographic (EMG) recordings of muscle activity, and isometric torque measurements to evaluate the excitability of the motor cortex, the spinal cord, and the torque-generating capacity of the triceps surae (TS) muscles. We found that for morning people, cortical excitability was highest at 09:00, spinal excitability was highest at 21:00, and there were no significant differences in TS EMG or torque produced during MVCs over the day. In contrast, evening people showed parallel increases in cortical and spinal excitability over the day, and these were associated with increased TS EMG and MVC torque. There were no differences at the level of the muscle over the day between morning and evening people. We propose that the simultaneous increases in cortical and spinal excitability increased central nervous system drive to the muscles of evening people, thus increasing torque production over the day. These differences in cortical excitability and performance of a motor task between morning and evening people have implications for maximizing human performance and highlight the influence of chronotype on an individual's diurnal rhythms.