Phase Responses to Light Pulses in Mice Lacking Functional per or cry Genes

Melatonin Does Not Affect the Stress-Induced Phase Shifts of Peripheral Clocks in Male Mice

Repeated or chronic stress can change the phase of peripheral circadian rhythms. Melatonin (Mel) is thought to be a circadian clock-controlled signal that might play a role in synchronizing peripheral rhythms, in addition to its direct suppressing effects on the stress axis. In this study we test whether Mel can reduce the social-defeat stress-induced phase shifts in peripheral rhythms, either by modulating circadian phase or by modulating the stress axis. Two experiments were performed with male Mel-deficient C57BL/6J mice carrying the circadian reporter gene construct (PER2::LUC). In the first experiment, mice received night-restricted (ZT11-21) Mel in their drinking water, resulting in physiological levels of plasma Mel peaking in the early dark phase. This treatment facilitated re-entrainment of the activity rhythm to a shifted light-dark cycle, but did not prevent the stress-induced (ZT21-22) reduction of activity during stress days. Also, this treatment did not attenuate the phase-delaying effects of stress in peripheral clocks in the pituitary, lung, and kidney. In a second experiment, pituitary, lung, and kidney collected from naive mice (ZT22-23), were treated with Mel, dexamethasone (Dex), or a combination of the two. Dex application affected PER2 rhythms in the pituitary, kidney, and lung by changing period, phase, or both. Administering Mel did not influence PER2 rhythms nor did it alleviate Dex-induced delays in PER2 rhythms in those tissues. We conclude that exogenous Mel is insufficient to affect peripheral PER2 rhythms and reduce stress effects on locomotor activity and phase changes in peripheral tissues.

Pharmacokinetic Model-Based Control across the Blood-Brain Barrier for Circadian Entrainment

The ability to shift circadian phase in vivo has the potential to offer substantial health benefits. However, the blood-brain barrier prevents the absorption of the majority of large and many small molecules, posing a challenge to neurological pharmaceutical development. Motivated by the presence of the circadian molecule KL001, which is capable of causing phase shifts in a circadian oscillator, we investigated the pharmacokinetics of different neurological pharmaceuticals on the dynamics of circadian phase. Specifically, we developed and validated five different transport models that describe drug concentration profiles of a circadian pharmaceutical at the brain level under oral administration and designed a nonlinear model predictive control (MPC)-based framework for phase resetting. Performance of the novel control algorithm based on the identified pharmacokinetic models was demonstrated through simulations of real-world misalignment scenarios due to jet lag. The time to achieve a complete phase reset for 11-h phase delay ranged between 48 and 72 h, while a 5-h phase advance was compensated in 30 to 60 h. This approach provides mechanistic insight into the underlying structure of the circadian oscillatory system and thus leads to a better understanding of the feasibility of therapeutic manipulations of the system.

The duper mutation reveals previously unsuspected functions of Cryptochrome 1 in circadian entrainment and heart disease

The Cryptochrome 1 (Cry1)-deficient duper mutant hamster has a short free-running period in constant darkness (τ_DD) and shows large phase shifts in response to brief light pulses. We tested whether this measure of the lability of the circadian phase is a general characteristic of Cry1-null animals and whether it indicates resistance to jet lag. Upon advance of the light:dark (LD) cycle, both duper hamsters and Cry1^-/- mice re-entrained locomotor rhythms three times as fast as wild types. However, accelerated re-entrainment was dissociated from the amplified phase-response curve (PRC): unlike duper hamsters, Cry1^-/- mice show no amplification of the phase response to 15' light pulses. Neither the amplified acute shifts nor the increased rate of re-entrainment in duper mutants is due to acceleration of the circadian clock: when mutants drank heavy water to lengthen the period, these aspects of the phenotype persisted. In light of the health consequences of circadian misalignment, we examined effects of duper and phase shifts on a hamster model of heart disease previously shown to be aggravated by repeated phase shifts. The mutation shortened the lifespan of cardiomyopathic hamsters relative to wild types, but this effect was eliminated when mutants experienced 8-h phase shifts every second week, to which they rapidly re-entrained. Our results reveal previously unsuspected roles of Cry1 in phase shifting and longevity in the face of heart disease. The duper mutant offers new opportunities to understand the basis of circadian disruption and jet lag.

duper is a null mutation of Cryptochrome 1 in Syrian hamsters

The duper mutation is a recessive mutation that shortens the period length of the circadian rhythm in Syrian hamsters. These animals show a large phase shift when responding to light pulses. Limited genetic resources for the Syrian hamster (Mesocricetus auratus) presented a major obstacle to cloning duper. This caused the duper mutation to remain unknown for over a decade. In this study, we did a de novo genome assembly of Syrian hamsters with long-read sequencing data from two different platforms, Pacific Biosciences and Oxford Nanopore Technologies. Using two distinct ecotypes and a fast homozygosity mapping strategy, we identified duper as an early nonsense allele of Cryptochrome 1 (Cry1) leading to a short, unstable protein. CRY1 is known as a highly conserved component of the repressive limb of the core circadian clock. The genome assembly and other genomic datasets generated in this study will facilitate the use of the Syrian hamster in biomedical research.

Clocks in the Wild: Entrainment to Natural Light

Entrainment denotes a process of coordinating the internal circadian clock to external rhythmic time-cues (Zeitgeber), mainly light. It is facilitated by stronger Zeitgeber signals and smaller period differences between the internal clock and the external Zeitgeber. The phase of entrainment ψ is a result of this process on the side of the circadian clock. On Earth, the period of the day-night cycle is fixed to 24 h, while the periods of circadian clocks distribute widely due to natural variation within and between species. The strength and duration of light depend locally on season and geographic latitude. Therefore, entrainment characteristics of a circadian clock vary under a local light environment and distribute along geoecological settings. Using conceptual models of circadian clocks, we investigate how local conditions of natural light shape global patterning of entrainment through seasons. This clock-side entrainment paradigm enables us to predict systematic changes in the global distribution of chronotypes.

Lifespan is unaffected by size and direction of daily phase shifts in Nasonia, a hymenopteran insect with strong circadian light resetting

Most organisms have an endogenous circadian clock with a period length of approximately 24 h that enables adaptation, synchronization and anticipation to environmental cycles. The circadian system (circa = about or around, diem = a day) may provide evolutionary benefits when entrained to the 24-h light-dark cycle. The more the internal circadian period (τ) deviates from the external light-dark cycle, the larger the daily phase shifts need to be to synchronize to the environment. In some species, large daily phase shifts reduce survival rate. Here we tested this 'resonance fitness hypothesis' on the diurnal wasp Nasonia vitripennis, which exhibits a large latitudinal cline in free-running period with longer circadian period lengths in the north than in the south. Longevity was measured in northern and southern wasps placed into light-dark cycles (T-cycles) with periods ranging from 20 h to 28 h. Further, locomotor activity was recorded to estimate range and phase angle of entrainment under these various T-cycles. A light pulse induced phase response curve (PRC) was measured in both lines to understand entrainment results. We expected a concave survival curve with highest longevity at T = τ and a reduction in longevity the further τ deviates from T (τ/T<>1). Our results do not support this resonance fitness hypothesis. We did not observe a reduction in longevity when τ deviates from T. Our results may be understood by the strong circadian light resetting mechanism (type 0 PRC) to single light pulses that we measured in Nasonia, resulting in: (1) the broad range of entrainment, (2) the wide natural variation in circadian free-running period, and (3) the lack of reduced survival when τ/T ratio's deviates from 1. Together this indicates that circadian adaption to latitude may lead to changes in circadian period and light response, without negative influences on survival.

Artificial light at night shifts daily activity patterns but not the internal clock in the great tit (Parus major)

Artificial light at night has shown a dramatic increase over the last decades and continues to increase. Light at night can have strong effects on the behaviour and physiology of species, which includes changes in the daily timing of activity; a clear example is the advance in dawn song onset in songbirds by low levels of light at night. Although such effects are often referred to as changes in circadian timing, i.e. changes to the internal clock, two alternative mechanisms are possible. First, light at night can change the timing of clock controlled activity, without any change to the clock itself; e.g. by a change in the phase relation between the circadian clock and expression of activity. Second, changes in daily activity can be a direct response to light ('masking'), without any involvement of the circadian system. Here, we studied whether the advance in onset of activity by dim light at night in great tits (Parus major) is indeed attributable to a phase shift of the internal clock. We entrained birds to a normal light/dark (LD) cycle with bright light during daytime and darkness at night, and to a comparable (LDim) schedule with dim light at night. The dim light at night strongly advanced the onset of activity of the birds. After at least six days in LD or LDim, we kept birds in constant darkness (DD) by leaving off all lights so birds would revert to their endogenous, circadian system controlled timing of activity. We found that the timing of onset in DD was not dependent on whether the birds were kept at LD or LDim before the measurement. Thus, the advance of activity under light at night is caused by a direct effect of light rather than a phase shift of the internal clock. This demonstrates that birds are capable of changing their daily activity to low levels of light at night directly, without the need to alter their internal clock.

Modeling the influence of chronopharmacological administration of synthetic glucocorticoids on the hypothalamic-pituitary-adrenal axis

Natural glucocorticoids, a class of cholesterol-derived hormones, modulate an array of metabolic, anti-inflammatory, immunosuppressive and cognitive signaling. The synthesis of natural glucocorticoids, largely cortisol in humans, is regulated by the hypothalamic-pituitary-adrenal (HPA) axis and exhibits pronounced circadian variation. Considering the central regulatory function of endogenous glucocorticoids, maintenance of the circadian activity of the HPA axis is essential to host survival and chronic disruption of such activity leads to systemic complications. There is a great deal of interest in synthetic glucocorticoids due to the immunosuppressive and anti-inflammatory properties and the development of novel dosing regimens that can minimize the disruption of endogenous activity, while still maintaining the pharmacological benefits of long-term synthetic glucocorticoid therapy. Synthetic glucocorticoids are associated with an increased risk of developing the pathological disorders related to chronic suppression of cortisol rhythmicity as a result of the potent negative feedback by synthetic glucocorticoids on the HPA axis precursors. In this study, a mathematical model was developed to explore the influence of chronopharmacological dosing of exogenous glucocorticoids on the endogenous cortisol rhythm considering intra-venous and oral dosing. Chronic daily dosing resulted in modification of the circadian rhythmicity of endogenous cortisol with the amplitude and acrophase of the altered rhythm dependent on the administration time. Simulations revealed that the circadian features of the endogenous cortisol rhythm can be preserved by proper timing of administration. The response following a single dose was not indicative of the response following long-term, repeated chronopharmacological dosing of synthetic glucocorticoids. Furthermore, simulations revealed the inductive influence of long-term treatment was only associated with low to moderate doses, while high doses generally led to suppression of endogenous activity regardless of the chronopharmacological dose. Finally, chronic daily dosing was found to alter the responsiveness of the HPA axis, such that a decrease in the amplitude of the cortisol rhythm resulted in a partial loss in the time-of-day dependent response to CRH stimulation, while an increase in the amplitude was associated with a more pronounced time-of-day dependence of the response.

Responses to Intermittent Light Stimulation Late in the Night Phase Before Dawn

The circadian clock is comprised of two oscillators that independently track sunset (evening) and sunrise (morning), though little is known about how light responses differ in each. Here, we quantified the morning oscillator’s responses to 19 separate pulse trains, collecting observations from over 1300 Drosophila at ZT23. Our results show that the advances in activity onset produced by these protocols depended on the tempo of light administration even when total exposure was conserved across a 15-min window. Moreover, patterns of stimulation previously shown to optimize the evening oscillator’s delay resetting at ZT13 (an hour after dusk) were equally effective for the M oscillator at ZT23 (an hour before dawn), though the morning oscillator was by comparison more photosensitive and could benefit from a greater number of fractionation strategies that better converted light into phase-shifting drive. These data continue to build the case that the reading frames for the pacemaker’s time-of-day estimates at dusk and dawn are not uniform and suggest that the “photologic” for the evening versus morning oscillator’s resetting might be dissociable.

A novel photic entrainment mechanism for the circadian clock in an insect: involvement of c-fos and cryptochromes

BACKGROUND

Entrainment to the environmental light cycle is an essential property of the circadian clock. Although the compound eye is known to be the major photoreceptor necessary for entrainment in many insects, the molecular mechanisms of photic entrainment remain to be explored.

RESULTS

We found  that cryptochromes (crys) and c-fos mediate photic entrainment of the circadian clock in a hemimetabolous insect, the cricket Gryllus bimaculatus. We examined the effects of RNA interference (RNAi)-mediated knockdown of the cry genes, Gb'cry1 and Gb'cry2, on photic entrainment, and light-induced resetting of the circadian locomotor rhythm. Gb'cry2 RNAi accelerated entrainment for delay shifts, while Gb'cry1/ Gb'cry2 double RNAi resulted in significant lengthening of transient cycles in both advance and delay shifts, and even in entrainment failure in some crickets. Double RNAi also strongly suppressed light induced resetting. The Gb'cry-mediated phase shift or resetting of the rhythm was preceded by light-induced Gb'c-fosB expression. We also found that Gb'c-fosB, Gb'cry2 and Gb'period (Gb'per) were likely co-expressed in some optic lobe neurons.

CONCLUSION

Based on these results, we propose a novel model for photic entrainment of the insect circadian clock, which relies on the light information perceived by the compound eye.

Alterations in glutamatergic signaling contribute to the decline of circadian photoentrainment in aged mice

Robust physiological circadian rhythms form an integral part of well-being. The aging process has been found to negatively impact systems that drive circadian physiology, typically manifesting as symptoms associated with abnormal/disrupted sleeping patterns. Here, we investigated the age-related decline in light-driven circadian entrainment in male C57BL/6J mice. We compared light-driven resetting of circadian behavioral activity in young (1-2 months) and old (14-18 months) mice and explored alterations in the glutamatergic pathway at the level of the circadian pacemaker, the suprachiasmatic nucleus (SCN). Aged animals showed a significant reduction in sensitivity to behavioral phase resetting by light. We show that this change was through alterations in N-Methyl-D-aspartate (NMDA) signaling at the SCN, where NMDA, a glutamatergic agonist, was less potent in inducing clock resetting. Finally, we show that this shift in NMDA sensitivity was through the reduced SCN expression of this receptor's NR2B subunit. Only in young animals did an NR2B antagonist attenuate behavioral resetting. These results can help target treatments that aim to improve both physiological and behavioral circadian entrainment in aged populations.

Long-Term Constant Light Induces Constitutive Elevated Expression of mPER2 Protein in the Murine SCN: A Molecular Basis for Aschoff’s Rule?

Circadian rhythms in behavior, metabolism, and physiology are based upon transcriptional/translational feedback loops involving a core set of clock genes that interact to regulate their own expression. In mammals, the SCN is the site of a master biological clock regulating circadian locomotor rhythms. The products of the clock genes mPer1, mPer2, mCry1, and mCry2 form essential components of both negative and positive elements within the SCN oscillator. The primary aims of this study were to examine clock gene abundance under longterm LL in an attempt to provide molecular correlates of the lengthened tau and daily phase delays described by Aschoff’s rule. Wheel-running behavior was recorded frommice maintained in eitherDDor LLfor 50 days. The abundance of the clock genes mPer1, mPer2, mCry1, and mCry2 and their protein products was then examined (every ~4 h) within the SCN using in situ hybridization and immunocytochemistry. Under LL conditions, mPer1, mPer2, mCry1, and mCry2 messages remained rhythmic, although the waveform of mCry2 was altered compared to DD. In LL, mPER1, mCRY1, and mCRY2 protein levels were also rhythmic and comparable to the patterns observed in DD. However, mPER2 is elevated and constitutively expressed under LL. Thus, rhythmic expression of these clock genes is not dependent on the rhythmic production of mPER2, and the acute up-regulation of mPer1 and mPer2 described for short (nonparametric) light pulses is not sustained under LL conditions. These findings suggest that mPER2 is important for the generation of phase delays in the molecular clockwork, providing a possible molecular explanation for Aschoff’s rule: LL lengthens the circadian period by inhibiting the degeneration of mPER2, and constitutively elevated levels of mPER2 enhance the phase-delaying limb of the molecular oscillator.

Molecular Correlates of Circadian Clocks in Fruit Fly Drosophila melanogaster Populations Exhibiting early and late Emergence Chronotypes

Although association of circadian clock properties with the timing of rhythmic behaviors (chronotype) has been extensively documented over several decades, recent studies on mice and Drosophila have failed to observe such associations. In addition, studies on human populations that examined effects of clock gene mutations/polymorphisms on chronotypes have revealed disparate and often contradictory results, thereby highlighting the need for a suitable model organism to study circadian clocks' role in chronotype regulation, the lack of which has hindered exploration of the underlying molecular-genetic bases. We used a laboratory selection approach to raise populations of Drosophila melanogaster that emerge in the morning (early) or in the evening (late), and over 14 years of continued selection, we report clear divergence of their circadian phenotypes. We also assessed the molecular correlates of early and late emergence chronotypes and report significant divergence in transcriptional regulation, including the mean phase, amplitude and levels of period (per), timeless (tim), clock (clk) and vrille (vri) messenger RNA (mRNA) expression. Corroborating some of the previously reported light-sensitivity and oscillator network coupling differences between the early and the late populations, we also report differences in mRNA expression of the circadian photoreceptor cryptochrome (cry) and in the mean phase, amplitude and levels of the neuropeptide pigment-dispersing factor (PDF). These results provide the first-ever direct evidence for divergent evolution of molecular circadian clocks in response to selection imposed on an overt rhythmic behavior and highlight early and late populations as potential models for chronotype studies by providing a preliminary groundwork for further exploration of molecular-genetic correlates underlying circadian clock-chronotype association.

The Neurobiology of Circadian Rhythms

There is a growing recognition that the coordinated timing of behavioral, physiologic, and metabolic circadian rhythms is a requirement for a healthy body and mind. In mammals, the primary circadian oscillator is the hypothalamic suprachiasmatic nucleus (SCN), which is responsible for circadian coordination throughout the organism. Temporal homeostasis is recognized as a complex interplay between rhythmic clock gene expression in brain regions outside the SCN and in peripheral organs. Abnormalities in this intricate circadian orchestration may alter sleep patterns and contribute to the pathophysiology of affective disorders.

Mice lacking circadian clock components display different mood-related behaviors and do not respond uniformly to chronic lithium treatment

Genomic studies suggest an association of circadian clock genes with bipolar disorder (BD) and lithium response in humans. Therefore, we tested mice mutant in various clock genes before and after lithium treatment in the forced swim test (FST), a rodent behavioral test used for evaluation of depressive-like states. We find that expression of circadian clock components, including Per2, Cry1 and Rev-erbα, is affected by lithium treatment, and thus, these clock components may contribute to the beneficial effects of lithium therapy. In particular, we observed that Cry1 is important at specific times of the day to transmit lithium-mediated effects. Interestingly, the pathways involving Per2 and Cry1, which regulate the behavior in the FST and the response to lithium, are distinct as evidenced by the phosphorylation of GSK3β after lithium treatment and the modulation of dopamine levels in the striatum. Furthermore, we observed the co-existence of depressive and mania-like symptoms in Cry1 knock-out mice, which resembles the so-called mixed state seen in BD patients. Taken together our results strengthen the concept that a defective circadian timing system may impact directly or indirectly on mood-related behaviors.

Owls and larks in mice

Humans come in different chronotypes and, particularly, the late chronotype (the so-called owl) has been shown to be associated with several health risks. A number of studies show that laboratory mice also display various chronotypes. In mice as well as in humans, the chronotype shows correlations with the period length and rhythm stability. In addition, some mouse models for human diseases show alterations in their chronotypic behavior, which are comparable to those humans. Thus, analysis of the behavior of mice is a powerful tool to unravel the molecular and genetic background of the chronotype and the prevalence of risks and diseases that are associated with it. In this review, we summarize the correlation of chronotype with free-running period length and rhythm stability in inbred mouse strains, in mice with a compromised molecular clockwork, and in a mouse model for neurodegeneration.

A Path to Sleep Is through the Eye

Sleep is expressed as a circadian rhythm and the two phenomena exist in a poorly understood relationship. Light affects each, simultaneously influencing rhythm phase and rapidly inducing sleep.

Light has long been known to modulate sleep, but recent discoveries support its use as an effective nocturnal stimulus for eliciting sleep in certain rodents. “Photosomnolence” is mediated by classical and ganglion cell photoreceptors and occurs despite the ongoing high levels of locomotion at the time of stimulus onset. Brief photic stimuli trigger rapid locomotor suppression, sleep, and a large drop in core body temperature (Tc; Phase 1), followed by a relatively fixed duration interval of sleep (Phase 2) and recovery (Phase 3) to pre-sleep activity levels. Additional light can lengthen Phase 2. Potential retinal pathways through which the sleep system might be light-activated are described and the potential roles of orexin (hypocretin) and melanin-concentrating hormone are discussed. The visual input route is a practical avenue to follow in pursuit of the neural circuitry and mechanisms governing sleep and arousal in small nocturnal mammals and the organizational principles may be similar in diurnal humans. Photosomnolence studies are likely to be particularly advantageous because the timing of sleep is largely under experimenter control. Sleep can now be effectively studied using uncomplicated, nonintrusive methods with behavior evaluation software tools; surgery for EEG electrode placement is avoidable. The research protocol for light-induced sleep is easily implemented and useful for assessing the effects of experimental manipulations on the sleep induction pathway. Moreover, the experimental designs and associated results benefit from a substantial amount of existing neuroanatomical and pharmacological literature that provides a solid framework guiding the conduct and interpretation of future investigations.

Acute inhibition of casein kinase 1δ/ε rapidly delays peripheral clock gene rhythms

Circadian rhythms are generated through a transcription-translation feedback loop involving clock genes and the casein kinases CSNK1D and CSNK1E. In this study, we investigated the effects of the casein kinase inhibitor PF-670462 (50 mg/kg) on rhythmic expression of clock genes in the liver, pancreas and suprachiasmatic nucleus (SCN) as well as plasma corticosterone, melatonin and running behaviour in rats and compared them to the responses to a 4 h extension of the light phase. PF-670462 acutely phase delayed the rhythmic transcription of Bmal1, Per1, Per2 and Nr1d1 in both liver and pancreas by 4.5 ± 1.3 and 4.5 ± 1.2 h, respectively, 1 day after administration. In the SCN, the rhythm of Nr1d1 and Dbp mRNA expression was delayed by 4.2 and 4 h, respectively. Despite these changes, the time of peak plasma melatonin secretion was not delayed, although the plasma corticosterone rhythm and onset of wheel-running activity were delayed by 2.1 and 1.1 h, respectively. These changes are in contrast to the effects of the 4 h light extension, which resulted in delays in peak expression of the clock genes of less than 1 h and no change in the melatonin or corticosterone rhythms. The ability of the casein kinase inhibitor to bring about large phase shifts in the rhythms of major metabolic target tissues may lead to new drugs being developed to rapidly phase adjust circadian rhythms to alleviate the metabolic impact of shift work.

Effects of adenylates on the circadian interaction of KaiB with the KaiC complex in the reconstituted cyanobacterial Kai protein oscillator

Cyanobacteria are photosynthetic prokaryotes that possess circadian oscillators. Clock proteins, KaiA, KaiB, KaiC compose the central circadian oscillator, which can be reconstituted in vitro in the presence of ATP. KaiC has ATPase, autokinase, and autophosphatase enzymatic activities. These activities are modulated by protein-protein interactions among the Kai proteins. The interaction of KaiB with the KaiC complex shows a circadian rhythm in the reconstituted system. We previously developed a quantitative, real-time monitoring system for the dynamic behavior of the complex using fluorescence correlation spectroscopy. Here, we examined the effects of ATP and ADP on the rhythmic interaction of KaiB. We show that increased concentration of ATP or ADP shortened period length. Adding ADP to the Kai protein oscillation shifted its phase in a phase-dependent manner. These results provide insight into how circadian oscillation entrainment mechanism is linked to cellular metabolism.

Nucleolar localization and circadian regulation of Per2S, a novel splicing variant of the Period 2 gene

In this work, we show for the first time that a second splicing variant of the core clock gene Period 2 (Per2), Per2S, is expressed at both the mRNA and protein levels in human keratinocytes and that it localizes in the nucleoli. Moreover, we show that a reversible perturbation of the nucleolar structure acts as a resetting stimulus for the cellular clock. Per2S expression and periodic oscillation upon dexamethasone treatment were assessed by qRT-PCR using specific primers. Western blot (WB) analysis using an antibody against the recombinant human PER2 (abRc) displayed an intense band at a molecular weight of ~55 kDa, close to the predicted size of Per2S, and a weaker band at the expected size of Per2 (~140 kDa). The antibody raised against PER2 pS662 (abS662), an epitope absent in PER2S, detected only the higher band. Immunolocalization studies with abRc revealed a peculiar nucleolar signal colocalizing with the nucleolar marker nucleophosmin, whereas with abS662 the signal was predominantly diffuse all over the nucleus and partially colocalized with abRc in the nucleolus. The analysis of cell fractions by WB confirmed the enrichment of PER2S and the presence of PER2 in the nucleolar compartment. Finally, a pulse (1 h) of actinomycin D (0.01 μg/ml) induced reversible nucleolar disruption, PER2S de-localization and circadian synchronization of clock and Per2S genes. Our work represents the first evidence that the Per2S splicing isoform is a clock component expressed in human cells localizing in the nucleolus. These results suggest a critical role for the nucleolus in the process of circadian synchronization in human keratinocytes.

Effects of the duper mutation on responses to light: parametric and nonparametric responses, range of entrainment, and masking

The duper mutation in Syrian hamsters shortens the free-running period (τDD) of locomotor activity by approximately 1 h when expressed on the wild-type background and by 2 h on the tau mutant background ("super duper"). In either case, duper markedly amplifies the phase response curve (PRC) of the light pulse. This work examined whether the duper mutation alters parametric as well as nonparametric properties, intensity thresholds, and noncircadian responses to light. Furthermore, it assessed the effects of duper on the range of entrainment and circadian aftereffects. In the first study, duper mutant and wild-type (wt) hamsters showed a similar intensity threshold for light-induced phase shifts. In the second, wt, tau mutant, and super duper hamsters were exposed to LD cycles whose period (T) progressively shortened. Regardless of whether the light phase was held at 50% of T or fixed at 3 h, super duper mutants entrained to a wider range of T cycles and showed aftereffects upon release into DD. In the third study, τLL was measured in mutant and wt hamsters that were maintained for 30-day intervals in constant light of progressively greater intensities. With increasing light intensity, the circadian period shortened in duper mutants. Circadian rhythms of super duper hamsters were disrupted at light intensities considerably below those that induced arrhythmicity in wt, tau heterozygote, or duper homozygote hamsters. In the fourth study, hamsters that were wt or homozygous for duper received two 15-min light pulses: the first at CT14 to CT16 or CT17 to CT19 and the second 2 h later. As expected, wt and duper mutants showed weak and strong resetting, respectively. Light pulses in early subjective night had an additive effect in mutant but not in wt hamsters, indicating that larger phase shifts of the pacemaker take longer to complete. Finally, super duper hamsters showed slightly but not significantly more negative masking than did wt or duper mutant hamsters. These results indicate that the duper mutation affects the properties of the central circadian pacemaker. The mutant allele affects not only the PRC but also parametric responses to light.

Altered phase-relationship between peripheral oscillators and environmental time in Cry1 or Cry2 deficient mouse models for early and late chronotypes

The mammalian circadian system is composed of a light-entrainable central clock in the suprachiasmatic nuclei (SCN) of the brain and peripheral clocks in virtually any other tissue. It allows the organism to optimally adjust metabolic, physiological and behavioral functions to the physiological needs it will have at specific time of the day. According to the resonance theory, such rhythms are only advantageous to an organism when in tune with the environment, which is illustrated by the adverse health effects originating from chronic circadian disruption by jetlag and shift work. Using short-period Cry1 and long-period Cry2 deficient mice as models for morningness and eveningness, respectively, we explored the effect of chronotype on the phase relationship between the central SCN clock and peripheral clocks in other organs. Whereas the behavioral activity patterns and circadian gene expression in the SCN of light-entrained Cry1(-/-) and Cry2(-/-) mice largely overlapped with that of wild type mice, expression of clock and clock controlled genes in liver, kidney, small intestine, and skin was shown to be markedly phase-advanced or phase-delayed, respectively. Likewise, circadian rhythms in urinary corticosterone were shown to display a significantly altered phase relationship similar to that of gene expression in peripheral tissues. We show that the daily dissonance between peripheral clocks and the environment did not affect the lifespan of Cry1(-/-) or Cry2(-/-) mice. Nonetheless, the phase-shifted peripheral clocks in light-entrained mice with morningness and eveningness-like phenotypes may have implications for personalized preventive and therapeutic (i.e. chronomodulation-based) health care for people with early and late chronotypes.

Circadian behavior of mice deficient in PER1/PML or PER2/PML

BACKGROUND

Our recent studies demonstrate that the murine homolog of the human tumor suppressor promyelocytic leukemia (PML) regulates circadian behavior of mice. To further gather insight into PML's contribution to circadian behavior, we generated two strains of mice deficient in one of the two period (Per) genes and the PML gene, with Per1-/-/Pml-/- and Per2-/-/Pml-/- genotypes.

RESULTS

Here we report the circadian behavior of these mice based on wheel-running behavioral analysis. In a free-running environment, the Per1-/-/Pml-/- mice maintained circadian rhythm but displayed a significantly shorter period of 22.2 h. In addition, these mice displayed significantly enhanced phase response to a light pulse given at zeitgeber time (ZT) 14 and 22. The Per2-/-/Pml-/- mice lose persistent rhythm when in a free-running environment, as also the case for Per2-/- mice. A transient post-light pulse rhythm seen in the arrhythmic Per2-/- mice was less apparent in Per2-/-/Pml-/- mice. Both the Per1-/-/Pml-/- and Per2-/-/Pml-/- mice displayed a more advanced phase angle of entrainment activity during light-dark cycles than the single gene deficient mice.

CONCLUSIONS

Beyond merely regulating PER1 and PER2, the current behavioral studies suggest PML has additional roles in mouse circadian behavior.

Individual differences in circadian waveform of Siberian hamsters under multiple lighting conditions

Because the circadian clock in the mammalian brain derives from a network of interacting cellular oscillators, characterizing the nature and bases of circadian coupling is fundamental to understanding how the pacemaker operates. Various phenomena involving plasticity in circadian waveform have been theorized to reflect changes in oscillator coupling; however, it remains unclear whether these different behavioral paradigms reference a unitary underlying process. To test whether disparate coupling assays index a common mechanism, we examined whether there is covariation among behavioral responses to various lighting conditions that produce changes in circadian waveform. Siberian hamsters, Phodopus sungorus, were transferred from long to short photoperiods to distinguish short photoperiod responders (SP-R) from nonresponders (SP-NR). Short photoperiod chronotyped hamsters were subsequently transferred, along with unselected controls, to 24-h light:dark:light: dark cycles (LDLD) with dim nighttime illumination, a procedure that induces bifurcated entrainment. Under LDLD, SP-R hamsters were more likely to bifurcate their rhythms than were SP-NR hamsters or unselected controls. After transfer from LDLD to constant dim light, SP-R hamsters were also more likely to become arrhythmic compared to SP-NR hamsters and unselected controls. In contrast, short photoperiod chronotype did not influence more transient changes in circadian waveform. The present data reveal a clear relationship in the plasticity of circadian waveform across 3 distinct lighting conditions, suggesting a common mechanism wherein individual differences reflect variation in circadian coupling.

p53 regulates Period2 expression and the circadian clock

The mechanistic interconnectivity between circadian regulation and the genotoxic stress response remains poorly understood. Here we show that the expression of Period 2 (Per2), a circadian regulator, is directly regulated by p53 binding to a response element in the Per2 promoter. This p53 response element is evolutionarily conserved and overlaps with the E-Box element critical for BMAL1/CLOCK binding and its transcriptional activation of Per2 expression. Our studies reveal that p53 blocks BMAL1/CLOCK binding to the Per2 promoter, leading to repression of Per2 expression. In the suprachiasmatic nucleus (SCN), p53 expression and its binding to the Per2 promoter are under circadian control. Per2 expression in the SCN is altered by p53 deficiency or stabilization of p53 by Nutlin-3. Behaviourally, p53⁻/⁻ mice have a shorter period length that lacks stability, and they exhibit impaired photo-entrainment to a light pulse under a free-running state. Our studies demonstrate that p53 modulates mouse circadian behaviour.

Simulated body temperature rhythms reveal the phase-shifting behavior and plasticity of mammalian circadian oscillators

The circadian pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus maintains phase coherence in peripheral cells through metabolic, neuronal, and humoral signaling pathways. Here, we investigated the role of daily body temperature fluctuations as possible systemic cues in the resetting of peripheral oscillators. Using precise temperature devices in conjunction with real-time monitoring of the bioluminescence produced by circadian luciferase reporter genes, we showed that simulated body temperature cycles of mice and even humans, with daily temperature differences of only 3°C and 1°C, respectively, could gradually synchronize circadian gene expression in cultured fibroblasts. The time required for establishing the new steady-state phase depended on the reporter gene, but after a few days, the expression of each gene oscillated with a precise phase relative to that of the temperature cycles. Smooth temperature oscillations with a very small amplitude could synchronize fibroblast clocks over a wide temperature range, and such temperature rhythms were also capable of entraining gene expression cycles to periods significantly longer or shorter than 24 h. As revealed by genetic loss-of-function experiments, heat-shock factor 1 (HSF1), but not HSF2, was required for the efficient synchronization of fibroblast oscillators to simulated body temperature cycles.

PML regulates PER2 nuclear localization and circadian function

Studies have suggested that the clock regulator PER2 is a tumour suppressor. A cancer network involving PER2 raises the possibility that some tumour suppressors are directly involved in the mammalian clock. Here, we show that the tumour suppressor promyelocytic leukaemia (PML) protein is a circadian clock regulator and can physically interact with PER2. In the suprachiasmatic nucleus (SCN), PML expression and PML-PER2 interaction are under clock control. Loss of PML disrupts and dampens the expression of clock regulators Per2, Per1, Cry1, Bmal1 and Npas2. In the presence of PML and PER2, BMAL1/CLOCK-mediated transcription is enhanced. In Pml(-/-) SCN and mouse embryo fibroblast cells, the cellular distribution of PER2 is primarily perinuclear/cytoplasmic. PML is acetylated at K487 and its deacetylation by SIRT1 promotes PML control of PER2 nuclear localization. The circadian period of Pml(-/-) mice displays reduced precision and stability consistent with PML having a role in the mammalian clock mechanism.

Photic resetting and entrainment in CLOCK-deficient mice

Mice lacking the CLOCK protein have a relatively subtle circadian phenotype, including a slightly shorter period in constant darkness, differences in phase resetting after 4-hour light pulses in the early and late night, and a variably advanced phase angle of entrainment in a light-dark (LD) cycle. The present series of experiments was conducted to more fully characterize the circadian phenotype of Clock(-/-) mice under various lighting conditions. A phase-response curve (PRC) to 4-hour light pulses in free-running mice was conducted; the results confirm that Clock(-/-) mice exhibit very large phase advances after 4-hour light pulses in the late subjective night but have relatively normal responses to light at other phases. The abnormal shape of the PRC to light may explain the tendency of CLOCK-deficient mice to begin activity before lights-out when housed in a 12-hour light:12-hour dark lighting schedule. To assess this relationship further, Clock(-/-) and wild-type control mice were entrained to skeleton lighting cycles (1L:23D and 1L:10D:1L:12D). Comparing entrainment under the 2 types of skeleton photoperiods revealed that exposure to 1-hour light in the morning leads to a phase advance of activity onset (expressed the following afternoon) in Clock(-/-) mice but not in the controls. Constant light typically causes an intensity-dependent increase in circadian period in mice, but this did not occur in CLOCK-deficient mice. The failure of Clock(-/-) mice to respond to the period-lengthening effect of constant light likely results from the increased functional impact of light falling in the phase advance zone of the PRC. Collectively, these experiments reveal that alterations in the response of CLOCK-deficient mice to light in several paradigms are likely due to an imbalance in the shape of the PRC to light.

Tissue-specific function of Period3 in circadian rhythmicity

The mammalian circadian system is composed of multiple central and peripheral clocks that are temporally coordinated to synchronize physiology and behavior with environmental cycles. Mammals have three homologs of the circadian Period gene (Per1, 2, 3). While numerous studies have demonstrated that Per1 and Per2 are necessary for molecular timekeeping and light responsiveness in the master circadian clock in the suprachiasmatic nuclei (SCN), the function of Per3 has been elusive. In the current study, we investigated the role of Per3 in circadian timekeeping in central and peripheral oscillators by analyzing PER2::LUCIFERASE expression in tissues explanted from C57BL/6J wild-type and Per3⁻/⁻ mice. We observed shortening of the periods in some tissues from Per3⁻/⁻ mice compared to wild-types. Importantly, the periods were not altered in other tissues, including the SCN, in Per3⁻/⁻ mice. We also found that Per3-dependent shortening of endogenous periods resulted in advanced phases of those tissues, demonstrating that the in vitro phenotype is also present in vivo. Our data demonstrate that Per3 is important for endogenous timekeeping in specific tissues and those tissue-specific changes in endogenous periods result in internal misalignment of circadian clocks in Per3⁻/⁻ mice. Taken together, our studies demonstrate that Per3 is a key player in the mammalian circadian system.

Genetic interaction of Per1 and Dec1/2 in the regulation of circadian locomotor activity

In mammals, 24-h rhythms are controlled by a hierarchical system of endogenous clocks, with a circadian pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus that synchronizes peripheral oscillators throughout the body. The molecular clock machinery is regulated by interlocked transcriptional translational feedback loops (TTLs). The core TTL includes the transcriptional modulators PER (1-3) and CRY (1/2) that feed back on their own expression by interaction with CLOCK/BMAL1. An accessory loop involving the transcription factors DEC1 and DEC2 has been described that also impinges on CLOCK/BMAL1-mediated transactivation. In Drosophila, the DEC ortholog CWO shows synergistic activity to PER. This prompted the authors to analyze PER1-DEC interaction in the mammalian SCN. They generated Per1/Dec double and triple mutant mice to monitor activity rhythms under entrained and free-running conditions. Furthermore, they analyzed expression of the clock genes Per2, Rev-Erbα, and Bmal1 in wild-type and Per1/Dec mutant SCN by in situ hybridization. The experiments reveal a critical role for Per1-Dec interaction in regulating activity phase under entrained conditions. In constant darkness, a synergistic function for Per1 and Dec1/2 in period regulation was found, correlating with disrupted clock gene mRNA levels in the SCN. Luciferase reporter gene assays indicate an activatory function of DECs on Bmal1 expression. Together, the results suggest a partially redundant and bidirectional regulatory function for the 2 Dec genes in the TTL and a conservation of Per-Dec (Cwo) synergism between vertebrate and invertebrate clocks.

Obesity alters circadian behavior and metabolism in sex dependent manner in the volcano mouse Neotomodon alstoni

The aim of the present study is to evaluate whether circadian locomotor activity, and the daily profile of plasma parameters related to metabolic syndrome (nutrients: glucose and triacylglycerides, and hormones: insulin and leptin), differ between male and female Neotomodon alstoni mice, both lean and obese. Young adult animals were captured in the field and kept at the laboratory animal facility. After 6 to 7 months feeding the animals ad libitum with a regular diet for laboratory rodents, 50-60% of mice became obese. Comparisons between sexes indicated that lean females were more active than males; however obese females reduced their nocturnal activity either in LD or DD, and advanced the phase of their activity-onset with respect to lights off. No differences in food intake between lean and obese mice, either during the day or night, were observed. Daily profiles of metabolic syndrome-related plasma parameters showed differences between sexes, and obesity was associated with increased values, especially leptin (500% in females and 273% in males) and insulin (150% in both females and males), as compared with lean mice. Our results indicate that lean mice display behavioral and endocrine differences between sexes, and obesity affects the parameters tested in a sex-dependent manner. The aforementioned leads us to propose N. alstoni, studied in captivity, could be an interesting model for the study of sex differences in the effects of obesity.

Distinct patterns of Period gene expression in the suprachiasmatic nucleus underlie circadian clock photoentrainment by advances or delays

The circadian clock in the mammalian hypothalamic suprachiasmatic nucleus (SCN) is entrained by the ambient light/dark cycle, which differentially acts to cause the clock to advance or delay. Light-induced changes in the rhythmic expression of SCN clock genes are believed to be a critical step in this process, but how the two entrainment modalities--advances vs. delays--engage the molecular clockwork remains incompletely understood. We investigated molecular substrates of photic entrainment of the clock in the SCN by stably entraining hamsters to T cycles (non-24-h light/dark cycles) consisting of a single 1-h light pulse repeated as either a short (23.33-h) or a long (24.67-h) cycle; under these conditions, the light pulse of the short cycle acts as "dawn," whereas that of the long cycle acts as "dusk." Analyses of the expression of the photoinducible and rhythmic clock genes Period 1 and 2 (Per1 and Per2) in the SCN revealed fundamental differences under these two entrainment modes. Light at dawn advanced the clock, advancing the onset of the Per1 mRNA rhythm and acutely increasing mRNA transcription, whereas light at dusk delayed the clock, delaying the offset of the Per2 mRNA rhythm and tonically increasing mRNA stability. The results suggest that the underlying molecular mechanisms of circadian entrainment differ with morning (advancing) or evening (delaying) light exposure, and such differences may reflect how entrainment takes place in nocturnal animals under natural conditions.

Masking responses to light in period mutant mice

Masking is an acute effect of an external signal on an overt rhythm and is distinct from the process of entrainment. In the current study, we investigated the phase dependence and molecular mechanisms regulating masking effects of light pulses on spontaneous locomotor activity in mice. The circadian genes, Period1 (Per1) and Per2, are necessary components of the timekeeping machinery and entrainment by light appears to involve the induction of the expression of Per1 and Per2 mRNAs in the suprachiasmatic nuclei (SCN). We assessed the roles of the Per genes in regulating masking by assessing the effects of light pulses on nocturnal locomotor activity in C57BL/6J Per mutant mice. We found that Per1(-/-) and Per2(-/-) mice had robust negative masking responses to light. In addition, the locomotor activity of Per1(-/-)/Per2(-/-) mice appeared to be rhythmic in the light-dark (LD) cycle, and the phase of activity onset was advanced (but varied among individual mice) relative to lights off. This rhythm persisted for 1 to 2 days in constant darkness in some Per1(-/-)/Per2(-/-) mice. Furthermore, Per1(-/-)/Per2(-/-) mice exhibited robust negative masking responses to light. Negative masking was phase dependent in wild-type mice such that maximal suppression was induced by light pulses at zeitgeber time 14 (ZT14) and gradually weaker suppression occurred during light pulses at ZT16 and ZT18. By measuring the phase shifts induced by the masking protocol (light pulses were administered to mice maintained in the LD cycle), we found that the phase responsiveness of Per mutant mice was altered compared to wild-types. Together, our data suggest that negative masking responses to light are robust in Per mutant mice and that the Per1(-/-)/Per2(-/-) SCN may be a light-driven, weak/damping oscillator.

Lab mice in the field: unorthodox daily activity and effects of a dysfunctional circadian clock allele

Daily patterns of animal behavior are potentially of vast functional importance. Fitness benefits have been identified in nature by the association between individual timing and survival or by the fate of individuals after experimental deletion of their circadian pacemaker. The recent advances in unraveling the molecular basis of circadian timing enable new approaches to natural selection on timing. The investigators report on the effect and fate of the mutant Per2(Brdm1) allele in 4 replicate populations of house mice in a seminatural outside environment over 2 years. This allele is known to compromise circadian organization and entrainment and to cause multiple physiological disturbances. Mice (N=250) bred from Per2(Brdm1) heterozygotes were implanted subcutaneously with transponders and released in approximately Mendelian ratios in four 400 m(2) pens. An electronic system stored the times of all visits to feeders of each individual. The study first demonstrates that mice are not explicitly nocturnal in this natural environment. Feeding activity was predominantly and sometimes exclusively diurnal and spread nearly equally over day and night under the protective snow cover in winter. The effect of Per2(Brdm1) on activity timing is negligible compared to seasonal changes in all genotypes. Second, the Per2(Brdm1) allele did not have persistent negative effects on fitness. In the first year, the allele gradually became less frequent by reducing survival. New cohorts captured had the same Per2(Brdm1) frequency as the survivors from previous cohorts, consistent with an absence of an effect on reproduction. In the second year, the allele recovered to about its initial frequency (0.54). These changes in selective advantage were primarily due to female mice, as females lived longer and the sex ratio dropped to about 25% males in the population. While it is unknown which selective advantage led to the recovery, the results caution against inferences from laboratory experiments on fitness consequences in the natural environment. It also demonstrates that the activity of mice, while strictly nocturnal in the laboratory, may be partially or completely diurnal in the field. The new method allows assessment of natural selection on specific alleles on a day-today basis.

Working for food shifts nocturnal mouse activity into the day

Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few hours in daytime. Timed food access also results in reduced food intake, but the role of food intake in circadian organization per se has not been described. By simulating natural food shortage in mice that work for food we show that reduced food intake alone shifts the activity phase from the night into the day and eventually causes nocturnal torpor (natural hypothermia). Release into continuous darkness with ad libitum food, elicits immediate reversal of activity to the previous nocturnal phase, indicating that the classical circadian pacemaker maintained its phase to the light-dark cycle. This flexibility in behavioral timing would allow mice to exploit the diurnal temporal niche while minimizing energy expenditure under poor feeding conditions in nature. This study reveals an intimate link between metabolism and mammalian circadian organization.

Behavioral and genetic dissection of a mouse model for advanced sleep phase syndrome

STUDY OBJECTIVE

The adaptive value of the endogenous circadian clock arises from its ability to synchronize (i.e., entrain) to external light-dark (LD) cycles at an appropriate phase. Studies have suggested that advanced circadian phase alignment might result from shortening of the period length of the clock. Here we explore mechanisms that contribute to an early activity phase in CAST/EiJ (CAST) mice.

METHODS

We investigated circadian rhythms of wheel-running activity in C57BL/6J (B6), CAST and 2 strains of B6.CAST congenic mice, which carry CAST segments introgressed in a B6 genome.

RESULTS

When entrained, all CAST mice initiate daily activity several hours earlier than normal mice. This difference could not be explained by alterations in the endogenous period, as activity onset did not correlate with period length. However, the photic phase-shifting responses in these mice were phase-lagged by 3 hours relative to their activity. Attenuated light masking responses were also found in CAST mice, which allow for activity normally inhibited by light. A previously identified quantitative trait locus (QTL), Era1, which contributes to the early activity trait, was confirmed and refined here using two B6.CAST congenic strains. Surprisingly, these B6.CAST mice exhibited longer rather than shorter endogenous periods, further demonstrating that the advanced phase in these mice is not due to alterations in period.

CONCLUSIONS

CAST mice have an advanced activity phase similar to human advanced sleep phase syndrome. This advanced phase is not due to its shorter period length or smaller light-induced phase shifts, but appears to be related to both light masking and altered coupling of the circadian pacemaker with various outputs. Lastly, a QTL influencing this trait was confirmed and narrowed using congenic mice as a first step toward gene identification.

Phase and period responses of the jerboa Jaculus orientalis to short light pulses

The phase and period responses to short light pulses were studied in the jerboa, a seasonal, hibernating, nocturnal rodent from the Atlas region in Morocco. The jerboa, which is a saltatory species, showed precise activity onsets and offsets under a light-dark (LD) cycle using infrared captors to record locomotor activity. When released into constant darkness (DD), the majority of animals showed a circadian period (tau) < 24 h (mean tau = 23.89 +/- 0.13 h) and a lengthening of the activity span, alpha. Animals were subsequently exposed to up to eight 15-min light pulses, each separated by at least 2 wks, for up to 160 days in DD. During this span, most individuals maintained robust circadian rhythmicity, with clearly defined activity onsets and offsets, similar levels of total activity, duration of alpha, and percent activity occurring during the subjective night. The phase response curve (PRC) is typical of other nocturnal rodents, with light eliciting delays during late subjective day and early subjective night (CT8-CT19) and advances during late subjective night to early subjective day (CT19-CT2). A dead zone, when light had no effect on phase, is observed during mid-subjective day (CT3-CT8). A few individuals showed large (> 9 h) Type 0 phase resetting near the singularity region (CT19) that resulted in a complete phase reversal, but otherwise displayed normal phase-shifting responses at other CT times. The tau response curve showed a decrease in period from early to late subjective night with increases at other times, but these changes were small (maximum < 9 min) and highly variable. There was a distinct tendency for animals that had an initial short tau in DD to conserve a short tau during the series of light pulses and, inversely, for animals with long tau to conserve a long tau. This suggests possible constraints on the plasticity of variation of tau in relation to the endogenous period of the animal.

Photic entrainment of period mutant mice is predicted from their phase response curves

A fundamental property of circadian clocks is that they entrain to environmental cues. The circadian genes, Period1 and Period2, are involved in entrainment of the mammalian circadian system. To investigate the roles of the Period genes in photic entrainment, we constructed phase response curves (PRC) to light pulses for C57BL/6J wild-type, Per1(-/-), Per2(-/-), and Per3(-/-) mice and tested whether the PRCs accurately predict entrainment to non-24 light-dark cycles (T-cycles) and constant light (LL). The PRCs of wild-type and Per3(-/-) mice are similar in shape and amplitude and have relatively large delay zones and small advance zones, resulting in successful entrainment to 26 h T-cycles (T26), but not T21, with similar phase angles. Per1(-/-) mice have a high-amplitude PRC, resulting in entrainment to a broad range of T-cycles. Per2(-/-) mice also entrain to a wide range of T-cycles because the advance portion of their PRC is larger than wild types. Period aftereffects following entrainment to T-cycles were similar among all genotypes. We found that the ratio of the advance portion to the delay portion of the PRC accurately predicts the lengthening of the period of the activity rhythm in LL. Wild-type, Per1(-/-), and Per3(-/-) mice had larger delay zones than advance zones and lengthened (>24 h) periods in LL, whereas Per2(-/-) mice had delay and advance zones that were equal in size and no period lengthening in LL. Together, these results demonstrate that PRCs are powerful tools for predicting and understanding photic entrainment of circadian mutant mice.

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.

Light-dependent behavioral phenotypes in PER3-deficient mice

A functional knockout of Period3 in mice (mPer3(-/-)) results in a mildly altered circadian phenotype, and mPer3 shows a redundant role within the circadian clock. In this study, the authors reevaluated the Per3(-/ -) behavioral phenotype on a C57Bl/6J background and report altered responses to light. In constant light, free-running activity period was shorter than that of wild-type, whereas in constant darkness, no difference was observed between genotypes. The effect of light was parametric, and the difference in free-running period between genotypes increased under constant light with increasing light intensity. An attenuated response to light in Per3(-/-) mice was also demonstrated through reduced negative masking in activity in an ultradian protocol and a slower reentrainment to a shifted light-dark cycle when activity falls in the light period of the new light-dark cycle. Behavioral phase-shifts in response to a single delaying or advancing light pulse in the Per3(-/-) mouse were not compromised. This demonstrates that the mPer3(- /-) phenotype is characterized predominantly by altered sensitivity to light and not by the ability of the circadian system to respond to light. In addition to its redundant role within the molecular clock, these data suggest a new role for Per3 outside of the circadian clock and contributing to light input pathways.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.