Twilight and photoperiod affect behavioral entrainment in the house mouse (Mus musculus)

Seasonal light hours modulate peripheral clocks and energy metabolism in mice

Except for latitudes close to the equator, seasonal variation in light hours can change dramatically between summer and winter. Yet investigations into the interplay between energy metabolism and circadian rhythms typically use a 12 h light:12 h dark photoperiod corresponding to the light duration at the equator. We hypothesized that altering the seasonal photoperiod affects both the rhythmicity of peripheral tissue clocks and energy homeostasis. Mice were housed at photoperiods representing either light hours in summer, winter, or the equinox. Mice housed at a winter photoperiod exhibited an increase in the amplitude of rhythmic lipid metabolism and a modest reduction in fat mass and liver triglyceride content. Comparing melatonin-proficient and -deficient mice, the effect of seasonal light on energy metabolism was largely driven by differences in the rhythmicity of food intake and not melatonin. Together, these data indicate that seasonal light impacts energy metabolism by modulating the timing of eating.

The Drosophila circadian phase response curve to light: Conservation across seasonally relevant photoperiods and anchorage to sunset

Photic history, including the relative duration of day versus night in a 24-hour cycle, is known to influence subsequent circadian responses to light in mammals. Whether such modulation is present in Drosophila is currently unknown. To date, all photic phase-response curves (PRCs) generated from Drosophila have done so with animals housed under seasonally agnostic equatorial photoperiods with alternating 12-hour segments of light and darkness. However, the genus contains thousands of species, some of which populate high and low-latitude habitats (20-50° north or south of the Equator) where seasonal variations in the light-dark schedule are pronounced. Here, we address this disconnect by constructing the first high-resolution Drosophila seasonal atlas for light-induced circadian phase-resetting. Testing the light responses of over 4,000 Drosophila at 120 timepoints across 5 seasonally-relevant rectangular photoperiods (i.e., LD 8:16, 10:14, 12:12, 14:10, and 16:8; 24 hourly intervals surveyed in each), we determined that many aspects of the fly circadian PRC waveform are conserved with increasing daylength. Surprisingly though, irrespective of LD schedule, the start of the PRCs always remained anchored to the timing of subjective sunset, creating a differential overlap of the advance zone with the morning hours after subjective sunrise that was maximized under summer photoperiods and minimized under winter photoperiods. These data suggest that there may be differences in flies versus mammals as to how the photoperiod modulates the waveform and amplitude of the circadian PRC to light. On the other hand, they support the possibility that the lights-off transition determines the phase-positioning of photic PRCs across seasons and across species. More work is necessary to test this claim and whether it might factor into the timing of seasonal light responses in humans.

Inhibitor of DNA binding 2 (Id2) Regulates Photic Entrainment Responses in Mice: Differential Responses of the Id2-/- Mouse Circadian System Are Dependent on Circadian Phase and on Duration and Intensity of Light

ID2 is a rhythmically expressed helix-loop-helix transcriptional repressor, and its deletion results in abnormal properties of photoentrainment. By examining parametric and nonparametric models of entrainment, we have started to explore the mechanism underlying this circadian phenotype. Id2-/- mice were exposed to differing photoperiods, and the phase angle of entrainment under short days was delayed 2 h as compared with controls. When exposed to long durations of continuous light, enhanced entrainment responses were observed after a delay of the clock but not with phase advances. However, the magnitude of phase shifts was not different in Id2-/- mice tested in constant darkness using a discrete pulse of saturating light. No differences were observed in the speed of clock resetting when challenged by a series of discrete pulses interspaced by varying time intervals. A photic phase-response curve was constructed, although no genotypic differences were observed. Although phase shifts produced by discrete saturating light pulses at CT16 were similar, treatment with a subsaturating pulse revealed a ~2-fold increase in the magnitude of the Id2-/- shift. A corresponding elevation of light-induced per1 expression was observed in the Id2-/- suprachiasmatic nucleus (SCN). To test whether the phenotype is based on a sensitivity change at the level of the retina, pupil constriction responses were measured. No differences were observed in responses or in retinal histology, suggesting that the phenotype occurs downstream of the retina and retinal hypothalamic tract. To test whether the phenotype is due to a reduced amplitude of state variables of the clock, the expression of clock genes per1 and per2 was assessed in vivo and in SCN tissue explants. Amplitude, phase, and period length were normal in Id2-/- mice. These findings suggest that ID2 contributes to a photoregulatory mechanism at the level of the SCN central pacemaker through control of the photic induction of negative elements of the clock.

Simultaneous Assessment of Circadian Rhythms and Sleep in Mice Using Passive Infrared Sensors: A User's Guide

The 24-hr cycle of activity and sleep provides perhaps the most familiar example of circadian rhythms. In mammals, circadian activity rhythms are generated by a master biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). This clock is synchronized (entrained) to the external light environment via light input from retinal photoreceptors. However, sleep is not a simple circadian output and also is regulated by a homeostatic process whereby extended wakefulness increases the need for subsequent sleep. As such, the amount and distribution of sleep depends upon the interaction between both circadian and homeostatic processes. Moreover, the study of circadian activity and sleep is not confined only to these specialized fields. Sleep and circadian rhythm disruption is common in many conditions, ranging from neurological and metabolic disorders to aging. Such disruption is associated with a range of negative consequences including cognitive impairment and mood disorders, as well as immune and metabolic dysfunction. As circadian activity and sleep are hallmarks of normal healthy physiology, they also provide valuable welfare indicators. However, traditional methods for the monitoring of circadian rhythms and sleep in mice can require separate specialized resources as well as significant expertise. Here, we outline a low-cost, non-invasive, and open-source method for the simultaneous assessment of circadian activity and sleep in mice. This protocol describes both the assembly of the hardware used and the capture and analysis of data without the need for expertise in electronics or data processing. © 2020 Wiley Periodicals LLC. Basic Protocol: Assembly of a PIR system for basic activity and sleep recordings Alternate Protocol: Data collection using Raspberry Pi Support Protocol: Circadian analysis using PIR sensors.

New Insights Into the Circadian Rhythm and Its Related Diseases

Circadian rhythms (CR) are a series of endogenous autonomous oscillators generated by the molecular circadian clock which acting on coordinating internal time with the external environment in a 24-h daily cycle. The circadian clock system is a major regulatory factor for nearly all physiological activities and its disorder has severe consequences on human health. CR disruption is a common issue in modern society, and researches about people with jet lag or shift works have revealed that CR disruption can cause cognitive impairment, psychiatric illness, metabolic syndrome, dysplasia, and cancer. In this review, we summarized the synchronizers and the synchronization methods used in experimental research, and introduced CR monitoring and detection methods. Moreover, we evaluated conventional CR databases, and analyzed experiments that characterized the underlying causes of CR disorder. Finally, we further discussed the latest developments in understanding of CR disruption, and how it may be relevant to health and disease. Briefly, this review aimed to synthesize previous studies to aid in future studies of CR and CR-related diseases.

Twilight Dominates Over Moonlight in Adjusting Drosophila’s Activity Pattern

Light is the most important zeitgeber for the synchronization of the Drosophila melanogaster circadian clock. In nature, there is twilight, and the nights are rarely completely dark, a fact that is usually disregarded in lab experiments. Recent studies showed contrary effects of simulated twilight and moonlight on fly locomotor activity, with twilight shifting morning and evening activity into the day and moonlight shifting it into the night. A currently unanswered question is, what may happen to locomotor activity when flies are exposed to more natural conditions in which both moonlight and twilight are simulated? Our data demonstrate that flies are able to integrate twilight and moonlight. However, twilight seems to dominate over moonlight as both, morning and evening activity peaks, take place at dawn or at dusk, respectively, and not during the night. Furthermore, nocturnal activity decreases in the presence of twilight. The compound eyes are essential for this behavior, and by investigating different photoreceptor mutants, we unraveled the importance of photoreceptor cells 7 and 8 for wild-type phases of the activity peaks. To adjust nocturnal activity levels to a wild-type manner, all photoreceptor cells work together in a complex way, with rhodopsin 6 having a prominent role.

Using circadian entrainment to find cryptic clocks

Three properties are most often attributed to the circadian clock: a ca. 24-h free-running rhythm, temperature compensation of the circadian rhythm, and its entrainment to zeitgeber cycles. Relatively few experiments, however, are performed under entrainment conditions. Rather, most chronobiology protocols concern constant conditions. We have turned this paradigm around and used entrainment to study the circadian clock in organisms where a free-running rhythm is weak or lacking. We describe two examples therein: Caenorhabditis elegans and Saccharomyces cerevisiae. By probing the system with zeitgeber cycles that have various structures and amplitudes, we can demonstrate the establishment of systematic entrained phase angles in these organisms. We conclude that entrainment can be utilized to discover hitherto unknown circadian clocks and we discuss the implications of using entrainment more broadly, even in model systems that show robust free-running rhythms.

Role of Temperature in Mediating Morning and Evening Emergence Chronotypes in Fruit Flies Drosophila melanogaster

Following decades of research under controlled laboratory conditions, there has been growing interest in the recent past to study circadian rhythms in nature. Recent studies conducted under natural conditions have been fruitful in exploring several characteristics of circadian rhythms that remained cryptic and previously masked under standard laboratory conditions, reemphasizing that the complexity of circadian rhythms in nature increases multifold under the influence of multiple zeitgebers. However, our understanding of the contributions of different zeitgebers in shaping various rhythm characteristics still remains elusive. Previously, Vaze et al. reported that chronotype differences between the morning emerging ( early) and evening emerging ( late) populations of Drosophila melanogaster are considerably enhanced under natural conditions compared to standard laboratory conditions. In the present study, we assess the role of 2 primary zeitgebers in nature—light and temperature—individually and in unison in driving chronotype differences. We report that when provided independently, temperature cycles enhance divergence between the early and late chronotypes more strongly than light, but when together, light and temperature appear to act antagonistically and that appropriate phase difference between light and temperature cycles is essential to promote chronotype divergence. Thus, our study highlights the importance of light and temperature, as well as their interaction with circadian clocks in mediating early and late chronotypes in fruit flies D. melanogaster.

Light pulse duration differentially regulates mouse locomotor suppression and phase shifts

Brief exposure of mice to nocturnal light causes circadian rhythm phase shifts, simultaneously inducing locomotor suppression, a drop in body temperature, and associated sleep. The exact nature of the relationship between these light-induced responses is uncertain, although locomotor suppression and phase shift magnitudes are related to stimulus irradiance. Whether stimulus duration has similar effects is less clear. Here, the relationship between stimulus duration and response magnitude was evaluated further using 100 µW/cm(2) white light-emitting diode pulses administered for 30, 300, 1200, or 3000 sec. The results show that, in general, shorter pulses yielded smaller responses and larger pulses yielded larger responses. However, the 300-sec pulse failed to augment locomotor suppression compared with the effect of a 30-sec pulse (44.7 ± 4.8 vs 40.6 ± 2.0 min) but simultaneously induced much larger phase shifts (1.28 ± 0.20 vs 0.52 ± 0.11 h). The larger phase shifts induced by the 300-sec stimulus did not differ from those induced by either the 1200- or 3000-sec pulses (1.43 ± 0.10 and 1.30 ± 0.17 h, respectively). The results demonstrate differential photic regulation of the two response types. Pulses ranging from 300 to 3000 sec produce equal phase shifts (present data); pulses ranging from 30 to 600 sec produce equal locomotor suppression levels. Greater suppression can occur additively in response to pulses of 1200 sec or more (present data), but this is not true for phase shifts. Nocturnal light appears to trigger a fixed duration event, locomotor suppression, or phase shift, with the latter followed by a light-refractory interval during which locomotor suppression can additively increase. The results also provide further support for the view that temporal integration of photic energy applies, at best, across a limited set of stimulus durations for both light-induced locomotor suppression/sleep and phase shift regulation.

Latitudinal clines: an evolutionary view on biological rhythms

Properties of the circadian and annual timing systems are expected to vary systematically with latitude on the basis of different annual light and temperature patterns at higher latitudes, creating specific selection pressures. We review literature with respect to latitudinal clines in circadian phenotypes as well as in polymorphisms of circadian clock genes and their possible association with annual timing. The use of latitudinal (and altitudinal) clines in identifying selective forces acting on biological rhythms is discussed, and we evaluate how these studies can reveal novel molecular and physiological components of these rhythms.

The Ability to Entrain to Long Photoperiods Differs between 3 Drosophila melanogaster Wild-Type Strains and Is Modified by Twilight Simulation

The ability to adapt to different environmental conditions including seasonal changes is a key feature of the circadian clock. Here, we compared the ability of 3 Drosophila melanogaster wild-type strains to adapt rhythmic activity to long photoperiods simulated in the laboratory. Fruit flies are predominantly crepuscular with activity bouts in the morning (M) and evening (E). The M peak follows dawn and the E peak follows dusk when the photoperiod is extended. We show that this ability is restricted to a certain extension of the phase angle between M and E peaks, such that the E peak does not delay beyond a certain phase under long days. We demonstrate that this ability is significantly improved by simulated twilight and that it depends additionally on the genetic background and the ambient temperature. At 20 °C, the laboratory strain CantonS had the most flexible phase angle between M and E peaks, a Northern wild-type strain had an intermediate one, and a Southern wild-type strain had the lowest flexibility. Furthermore, we found that the 3 strains differed in clock light sensitivity, with the CantonS and the Northern strains more light sensitive than the Southern strain. These results are generally in accord with the recently discovered polymorphisms in the timeless gene ( tim) that affect clock light sensitivity.

Modeling a Circadian Surface

Experiments that systematically varied T, τ, and photoperiod in Neurospora crassa revealed that the traditional nonparametric and parametric approaches could not explain entrainment for all of the tested conditions. The authors have developed a new approach to understanding entrainment that incorporates several features of the old paradigms but allows exploration of the underlying mechanisms in synchronized clocks, making extrapolations from constant conditions to entrained state unnecessary. It is based on a circadian integrated response characteristic (CIRC) that makes no assumptions about how entrainment occurs (by phase shifts or velocity changes). All it presumes is that, during entrainment, the clock’s cycle length must match that of the zeitgeber. With the help of the CIRC, entrainment to all zeitgeber conditions can be modeled by changing 3 parameters: the CIRC’s shape and asymmetry and an assumed internal cycle length (τ under entrainment: τ E) that the clock adopts under stable entrainment to produce a specific phase relationship to the zeitgeber (τE is reflected in a period aftereffect when clocks are released to constant conditions). The few parameters of the CIRC make it highly amenable to modeling. Here, the authors describe the results of modeling Neurospora’s circadian surface and show that the new approach can explain and unify all results of the circadian surface. The qualities of the CIRC are highly systematic for the respective entrainment condition and show that τE is an important variable in the entrainment process. The results also show that the wild-type strain is excellently tuned for entrainment under the natural 24-h cycle despite its shorter period (~22 h) in constant darkness. Experiments measuring aftereffects support the prediction that τE plays an important role in entrainment.

A circadian clock is not required in an arctic mammal

Seasonally breeding mammals use the annual change in the photoperiod cycle to drive rhythmic nocturnal melatonin signals from the pineal gland, providing a critical cue to time seasonal reproduction. Paradoxically, species resident at high latitudes achieve tight regulation of the temporal pattern of growth and reproduction despite the absence of photoperiodic information for most of the year. In this study, we show that the melatonin rhythm of reindeer (Rangifer tarandus) is acutely responsive to the light/dark cycle but not to circadian phase, and also that two key clock genes monitored in reindeer fibroblast cells display little, if any, circadian rhythmicity. The molecular clockwork that normally drives cellular circadian rhythms is evidently weak or even absent in this species, and instead, melatonin-mediated seasonal timing may be driven directly by photic information received at a limited time of year specific to the equinoxes.