The comparison between circadian oscillators in mouse liver and pituitary gland reveals different integration of feeding and light schedules

The mammalian circadian system is composed of multiple peripheral clocks that are synchronized by a central pacemaker in the suprachiasmatic nuclei of the hypothalamus. This system keeps track of the external world rhythms through entrainment by various time cues, such as the light-dark cycle and the feeding schedule. Alterations of photoperiod and meal time modulate the phase coupling between central and peripheral oscillators. In this study, we used real-time quantitative PCR to assess circadian clock gene expression in the liver and pituitary gland from mice raised under various photoperiods, or under a temporal restricted feeding protocol. Our results revealed unexpected differences between both organs. Whereas the liver oscillator always tracked meal time, the pituitary circadian clockwork showed an intermediate response, in between entrainment by the light regimen and the feeding-fasting rhythm. The same composite response was also observed in the pituitary gland from adrenalectomized mice under daytime restricted feeding, suggesting that circulating glucocorticoids do not inhibit full entrainment of the pituitary clockwork by meal time. Altogether our results reveal further aspects in the complexity of phase entrainment in the circadian system, and suggest that the pituitary may host oscillators able to integrate multiple time cues.     

Plasticity of the intrinsic period of the human circadian timing system

Human expeditions to Mars will require adaptation to the 24.65-h Martian solar day-night cycle (sol), which is outside the range of entrainment of the human circadian pacemaker under lighting intensities to which astronauts are typically exposed. Failure to entrain the circadian time-keeping system to the desired rest-activity cycle disturbs sleep and impairs cognitive function. Furthermore, differences between the intrinsic circadian period and Earth's 24-h light-dark cycle underlie human circadian rhythm sleep disorders, such as advanced sleep phase disorder and non-24-hour sleep-wake disorders. Therefore, first, we tested whether exposure to a model-based lighting regimen would entrain the human circadian pacemaker at a normal phase angle to the 24.65-h Martian sol and to the 23.5-h day length often required of astronauts during short duration space exploration. Second, we tested here whether such prior entrainment to non-24-h light-dark cycles would lead to subsequent modification of the intrinsic period of the human circadian timing system. Here we show that exposure to moderately bright light ( approximately 450 lux; approximately 1.2 W/m(2)) for the second or first half of the scheduled wake episode is effective for entraining individuals to the 24.65-h Martian sol and a 23.5-h day length, respectively. Estimations of the circadian periods of plasma melatonin, plasma cortisol, and core body temperature rhythms collected under forced desynchrony protocols revealed that the intrinsic circadian period of the human circadian pacemaker was significantly longer following entrainment to the Martian sol as compared to following entrainment to the 23.5-h day. The latter finding of after-effects of entrainment reveals for the first time plasticity of the period of the human circadian timing system. Both findings have important implications for the treatment of circadian rhythm sleep disorders and human space exploration.     

History dependence of circadian pacemaker period in the cockroach

The freerunning period of circadian clocks in constant environmental conditions can be history-dependent, and one effect of entrainment of circadian clocks by light cycles is to cause long-lasting changes in the freerunning period that are termed after-effects. We have studied after-effects of entrainment to 22-h (LD 8:14) and 26-h (LD 8:18) light cycles in the cockroach Leucophaea maderae. We find that in cockroaches, the freerunning period of the locomotor activity rhythm, measured in constant darkness (DD), is 0.7h less after entrainment to T22 than after entrainment to T26. Induction of after-effects requires several days (>1 week) entrainment, and after induction, after-effects will persist in DD for over 40 days. Further after-effects are unaltered by phase-resetting of up to 12h caused by exposure to low-temperature pulses (7 degrees C) of 24 or 48h duration. After-effects also persist through re-entrainment for 2 weeks to 24-h light cycles. These results indicate that after-effects arise from stable changes in the circadian system that are likely to be independent of phase relationships among oscillators within the circadian system. We also show that entrainment to temperature cycles does not generate after-effects indicating that light may be unique in its ability to generate lasting changes in pacemaker period.     

Circadian organization in hemimetabolous insects

The circadian system of hemimetabolous insects is reviewed in respect to the locus of the circadian clock and multioscillatory organization. Because of relatively easy access to the nervous system, the neuronal organization of the clock system in hemimetabolous insects has been studied, yielding identification of the compound eye as the major photoreceptor for entrainment and the optic lobe for the circadian clock locus. The clock site within the optic lobe is inconsistent among reported species; in cockroaches the lobula was previously thought to be a most likely clock locus but accessory medulla is recently stressed to be a clock center, while more distal part of the optic lobe including the lamina and the outer medulla area for the cricket. Identification of the clock cells needs further critical studies. Although each optic lobe clock seems functionally identical, in respect to photic entrainment and generation of the rhythm, the bilaterally paired clocks form a functional unit. They interact to produce a stable time structure within individual insects by exchanging photic and temporal information through neural pathways, in which serotonin and pigment-dispersing factor (PDF) are involved as chemical messengers. The mutual interaction also plays an important role in seasonal adaptation of the rhythm.     

Period and phase adjustments of human circadian rhythms in the real world

Entrainment of the circadian rhythm has 2 aspects, period and phase adjustments, which are established simultaneously in most nonhuman circadian systems. The human circadian system is unique in its functional structure in which 2 different subsystems are involved; one is the circadian pacemaker analogous to that located in the suprachiasmatic nucleus, and the other is the oscillatory system of unknown nature that drives the rest-activity cycle. The human circadian system shows the endogenous period very close to 24 h under entrainment and less sensitive to photic stimuli than under free running, which may explain stable entrainment in the real word where natural sun lights are unpredictable in terms of the intensity and time of appearance. On the other hand, nonphotic entrainment seems to play a significant role in phase adjustment of the human circadian system. Nonphotic zeitgebers initially directed to the rest-activity cycle may affect the circadian pacemaker through feedback and/or associated LD cycles.     

High potassium treatment resets the circadian oscillator in Xenopus retinal photoreceptors

In vertebrate retina, light hyperpolarizes the photoreceptor membrane, and this is an essential cellular signal for vision. Cellular signals responsible for photic entrainment of some circadian oscillators appear to be distinct from those for vision, but it is not known whether changes in photoreceptor membrane potential play roles in photic entrainment of the photoreceptor circadian oscillator. The authors show that a depolarizing exposure to high potassium resets the circadian oscillator in cultured Xenopus retinal photoreceptor layers. A 4-h pulse of high [K(+)] (34 mM higher than in normal culture medium) caused phase shifts of the melatonin rhythm. This treatment caused phase delays during the early subjective day and phase advances during the late subjective day. In addition to the phase-shifting effect, high potassium pulses stimulated melatonin release acutely at all times. High [K(+)] therefore mimicked dark in its effects on oscillator phase and melatonin synthesis. These results suggest that membrane potential may play a role in photic entrainment of the photoreceptor circadian oscillator and in regulation of melatonin release.     

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.     

Entrainment of the human circadian system by light

The periodic light-dark cycle is the dominant environmental synchronizer used by humans to entrain to the geophysical 24-h day. Entrainment is a fundamental property of circadian systems by which the period of the internal clock (tau) is synchronized to the period of the entraining stimuli (T cycle). An important aspect of entrainment in humans is the maintenance of an appropriate phase relationship between the circadian system, the timing of sleep and wakefulness, and environmental time (a.k.a. the phase angle of entrainment) to maintain wakefulness throughout the day and consolidated sleep at night. In this article, we review these concepts and the methods for assessing circadian phase and period in humans, as well as discuss findings on the phase angle of entrainment in healthy adults. We review findings from studies that examine how the phase, intensity, duration, and spectral characteristics of light affect the response of the human biological clock and discuss studies on entrainment in humans, including recent studies of the minimum light intensity required for entrainment. We briefly review conditions and disorders in which failure of entrainment occurs. We provide an integrated perspective on circadian entrainment in humans with respect to recent advances in our knowledge of circadian period and of the effects of light on the biological clock in humans.     

The network of time: understanding the molecular circadian system

The circadian clock provides a temporal structure that modulates biological functions from the level of gene expression to performance and behaviour. Pioneering work on the fruitfly Drosophila has provided a basis for understanding how the temporal sequence of daily events is controlled in mammals. New insights have come from work on mammals, specifically from studying the daily activity profiles of clock mutant mice; from more detailed recordings of clock gene expression under different experimental conditions and in different tissues; and from the discovery and analysis of a growing number of additional clock genes. These new results are moving the model paradigm away from a simple negative feedback loop to a molecular network. Understanding the coupling and interactions of this network will help us to understand the evolution of the circadian system, advance medical diagnosis and treatment, improve the health of shift workers and frequent travellers, and will generally enable the treatment of clock-related pathologies.     

Phase sensitivity analysis of circadian rhythm entrainment

As a biological clock, circadian rhythms evolve to accomplish a stable (robust) entrainment to environmental cycles, of which light is the most obvious. The mechanism of photic entrainment is not known, but two models of entrainment have been proposed based on whether light has a continuous (parametric) or discrete (nonparametric) effect on the circadian pacemaker. A novel sensitivity analysis is developed to study the circadian entrainment in silico based on a limit cycle approach and applied to a model of Drosophila circadian rhythm. The comparative analyses of complete and skeleton photoperiods suggest a trade-off between the contribution of period modulation (parametric effect) and phase shift (nonparametric effect) in Drosophila circadian entrainment. The results also give suggestions for an experimental study to (in)validate the two models of entrainment.     

Dim nighttime illumination interacts with parametric effects of bright light to increase the stability of circadian rhythm bifurcation in hamsters

The endogenous circadian pacemaker of mammals is synchronized to the environmental day by the ambient cycle of relative light and dark. The present studies assessed the actions of light in a novel circadian entrainment paradigm where activity rhythms are bifurcated following exposure to a 24-h light:dark:light:dark (LDLD) cycle. Bifurcated entrainment under LDLD reflects the temporal dissociation of component oscillators that comprise the circadian system and is facilitated when daily scotophases are dimly lit rather than completely dark. Although bifurcation can be stably maintained in LDLD, it is quickly reversed under constant conditions. Here the authors examine whether dim scotophase illumination acts to maintain bifurcated entrainment under LDLD through potential interactions with the parametric actions of bright light during the two daily photophases. In three experiments, wheel-running rhythms of Syrian hamsters were bifurcated under LDLD with dimly lit scotophases, and after several weeks, dim scotophase illumination was either retained or extinguished. Additionally, "full" and "skeleton" photophases were employed under LDLD cycles with dimly lit or completely dark scotophases to distinguish parametric from nonparametric effects of bright light. Rhythm bifurcation was more stable in full versus skeleton LDLD cycles. Dim light facilitated the maintenance of bifurcated entrainment under full LDLD cycles but did not prevent the loss of rhythm bifurcation in skeleton LDLD cycles. These studies indicate that parametric actions of bright light maintain the bifurcated entrainment state; that dim scotophase illumination increases the stability of the bifurcated state; and that dim light interacts with the parametric effects of bright light to increase the stability of rhythm bifurcation under full LDLD cycles. A further understanding of the novel actions of dim light may lead to new strategies for understanding, preventing, and treating chronobiological disturbances.     

Seasonal changes in entrainment cues for the circadian rhythm of the supratidal amphipod Talorchestia longicornis

The supratidal amphipod Talorchestia longicornis Say has a circadian rhythm in activity, in which it is active on the substrate surface at night and inactive in burrows during the day. The present study determined: (1) the circadian rhythms in individual versus groups of amphipods; (2) the range of temperature cycles that entrain the circadian rhythm; (3) entrainment by high-temperature cycles versus light?:?dark cycles, and (4) seasonal substrate temperature cycles. The circadian rhythm was determined by monitoring temporal changes in surface activity using a video system. Individual and groups of amphipods have similar circadian rhythms. Entrainment occurred only to temperature cycles that included temperatures below 20°C (10–20, 15–20, 17–19, 15–25°C) but not to temperatures above 20°C (20–25, 20–30°C), and required only a 2°C temperature cycle (17–19°C). Diel substrate temperatures were above 20°C in the summer and below 20°C during the winter. Upon simultaneous exposure to a diel high-temperature cycle (20–30°C) and a light?:?dark cycle phased differently, amphipods entrained to the light?:?dark cycle. Past studies found that a temperature cycle below 20°C overrode the light?:?dark cycle for entrainment. The functional significance of this change in entrainment cues may be that while buried during the winter, the activity rhythm remains in phase with the day?:?night cycle by the substrate temperature cycles. During the summer, T. longicornis switches to the light?:?dark cycle for entrainment, perhaps as a mechanism to phase activity precisely to the short summer nights.     

The role of Clock in the plasticity of circadian entrainment

The mammalian circadian clock lying in suprachiasmatic nucleus (SCN) is synchronized to about 24 h by the environmental light-dark cycle (LD). The circadian clock exhibits limits of entrainment above and below 24 h, beyond which it will not entrain. Little is known about the mechanisms regulating the limits of entrainment. In this study, we show that wild-type mice entrain to only an LD 24 h cycle, whereas Clock mutant mice can entrain to an LD 24, 28, and 32 h except for LD 20 h and LD 36 h cycle. Under an LD 28 h cycle, Clock mutant mice showed a clear rhythm in Per2 mRNA expression in the SCN and behavior. Light response was also increased. This is the first report to show that the Clock mutation makes it possible to adapt the circadian oscillator to a long period cycle and indicates that the clock gene may have an important role for the limits of entrainment of the SCN to LD cycle.     

Light-dark entrainment of proestrous LH surges and circadian locomotor activity in female hamsters

The temporal relationships of the proestrous LH surge and the circadian locomotor activity rhythm were compared in hamsters entrained to four different 24-hr light-dark (LD) cycles. Animals were housed in cages equipped with running wheels to obtain continuous activity records. Stable entrainment of locomotor activity was complete within 3 weeks of exposure to each photoperiod at which time hamsters were randomly assigned to hourly sample groups. Serum was obtained by cardiac puncture under light ether anesthesia on the day of proestrous and was assayed by RIA for LH. A computer-based least-squares sine wave-fitting technique determined a single objective phase reference point for the time of the hormone maximum. In each photoperiod, precise temporal relationships were maintained between the LH surge and activity onset, whereas the phase relationship between the LH surge and the LD cycle was more variable. These data indicate that the environmental LD cycle entrains the circadian timing system which, in turn, provides temporal information to the rhythms of proestrous gonadotropin and locomotor activity.     

Neural mechanisms for entrainment and generation of mammalian circadian rhythms.

The identification of a direct retinohypothalamic tract (RHT) terminating in the supra-chiasmatic nuclei (SCN) has focused attention on the role of these structures in the entrainment and generation of circadian rhythms in mammals. Light effects on circadian rhythms are mediated by both the RHT and portions of the classical visual system. The complex interactions of these systems are reflected both in their direct anatomical connections and in the functional changes in entrainment produced by interruption of either set of projections. Destruction of the RHT/SCN eliminated both normal entrainment and normal free-running circadian rhythms. No circadian rhythms has survived SCN ablation in rodents, but a variety of non-circadian cycles can be generated by lesioned animals. The complex behavioral patterns produced by SCN-lesioned hamsters suggest that circadian oscillators continue to function in these animals, but that their activity is no longer integrated into a single circadian framework. The available evidence indicates that the mammalian pacemaking system comprises a set of independent oscillators normally regulated by the SCN and by light information that is transmitted via several retinofugal pathways.