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.     

A phase response curve for circannual rhythm in the varied carpet beetle <Emphasis Type="Italic">Anthrenus verbasci</Emphasis>

We know that entrainment, a stable phase relationship with an environmental cycle, must be established for a biological clock to function properly. Phase response curves (PRCs), which are plots of phase shifts that result as a function of the phase of a stimulus, have been created to examine the mode of entrainment. In circadian rhythms, single-light pulse PRCs have been obtained by giving a light pulse to various phases of a free-running rhythm under continuous darkness. This successfully explains the entrainment to light-dark cycles. Some organisms show circannual rhythms. In some of these, changes in photoperiod entrain the circannual rhythms. However, no single-pulse PRCs have been created. Here we show the PRC to a long-day pulse superimposed for 4 weeks over constant short days in the circannual pupation rhythm in the varied carpet beetle Anthrenus verbasci. Because the shape of that PRC closely resembles that of the Type 0 PRC with large phase shifts in circadian rhythms, we suggest that an oscillator having a common feature in the phase response with the circadian clock, produces a circannual rhythm.     

Behavioral inhibition of circadian responses to light.

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.     

Vitamin B12 accelerates re-entrainment of activity rhythms in rats.

The effects of methyl vitamin B12 (5-6 mg/kg, p.o.) on the entrainment of circadian running wheel activity rhythm to a new lighting schedule were measured in rats. After the light-dark (LD) cycle was abruptly reversed, rats given vitamin B12 took less time to entrain their circadian locomotor activity rhythm to the new cycle than did controls. This result indicates that vitamin B12 accelerates the reentrainment of the mammalian circadian activity rhythm following an abrupt change in the environmental LD cycle.     

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.     

Phase response and entrainment: Model computations on their relation using Wever's oscillator model

Abstract

Enright's theory, which explains entrainment as periodically repeated phase response, is applied to Wever's self‐sustained oscillation model of are circadian rhythm and tested by computer simulation. Ranges of phase response and entrainment are compared and the oscillatory behaviour is shown in the phase diagram for the cases of phase response and entrainment. It is shown that Enright's theory is not valid for self‐sustained oscillations in general, but it need not necessarily fail in case of the biological circadian rhythm.     

Neurophysiological mechanisms involved in photo-entrainment of the circadian rhythm from the Aplysia eye

An attempt was made to identify the neurophysiological processes involved in entrainment of the circadian rhythm of spontaneous optic nerve potentials from the Aplysia eye by determining whether pharmacological agents or ion substitutions could block phase shifts produced by single light pulses. Knowing which physiological processes are involved in entrainment should help define the morphological pathway traveled by entrainment information. A secretory step does not appear to be involved in the flow of entrainment information from the environment to the circadian oscillator. A treatment (HiMg LoCa) capable of inhibiting secretion did not interfere with phase shifting by light. Furthermore, treating eyes with putative transmitters or extracts of eyes did not phase shift the free running rhythm. Also, the phase shifting information is not translated into action potentials before reaching the oscillator since TTX–HiMg LoCa solutions did not block the light-induced phase shift. The photoreceptor potential does seem to be important for light-induced phase shifts. A correlation was found between the effects of treatments on the ERG and their effects on the light-induced phase shift. Solutions which decreased the ERG by 90% or more blocked phase shifting whereas solutions which decreased the ERG by less than 74% had no effect on phase shifting by light. The results from these studies are consistent with two pathways for the flow of phase shifting information to the circadian oscillator. The circadian oscillator may be associated with receptor cells and the entrainment pathway would include a step involving the photoreceptor potential. Alternatively, the circadian oscillator may be associated with secondary cells and receive entrainment information via the photoreceptor potential and passive spread of current through a gap junction. Higher order cells than second-order ones are probably not involved in the entrainment pathway.     

Astronomical cycles

Abstract

Talorchestia quoyana, a sand beach amphipod, shows a rhythm of locomotor activity controlled by a circadian clock and an inhibitory circatidal clock. This article reports on an investigation of the entrainment of the circadian dock to skeleton photoperiods. Four important mathematical models for circadian rhythms are examined with respect to the results of the entrainment experiments and to predictions from the phase response curve for Talorchestia. Significant differences between the models are described, and properties of circadian rhythms not accounted for by present models are outlined.     

A phase dynamics model of human circadian rhythms

Nonphotic entrainment of an overt sleep-wake rhythm and a circadian pacemaker-driving temperature/melatonin rhythm suggests existence of feedback mechanisms in the human circadian system. In this study, the authors constructed a phase dynamics model that consisted of two oscillators driving temperature/melatonin and sleep-wake rhythms, and an additional oscillator generating an overt sleep-wake rhythm. The feedback mechanism was implemented by modifying couplings between the constituent oscillators according to the history of correlations between them. The model successfully simulated the behavior of human circadian rhythms in response to forced rest-activity schedules under free-run situations: the sleep-wake rhythm is reentrained with the circadian pacemaker after release from the schedule, there is a critical period for the schedule to fully entrain the sleep-wake rhythm, and the forced rest-activity schedule can entrain the circadian pacemaker with the aid of exercise. The behavior of human circadian rhythms was reproduced with variations in only a few model parameters. Because conventional models are unable to reproduce the experimental results concerned here, it was suggested that the feedback mechanisms included in this model underlie nonphotic entrainment of human circadian rhythms.     

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.     

Maternal influence on activity rhythms and reproductive development in Djungarian hamster pups

Photoperiodism and entrainment of the circadian rhythm of locomotor activity were investigated in juvenile Djungarian hamsters. Animals were housed in simulated burrows. Activity was measured as the animal's emergence from a dark nest chamber into an outer box exposed to the room illumination. This burrow emergence activity exhibited marked circadian rhythmicity. Interactions between mother hamsters and their offspring were examined in the simulated burrow system. Male reproductive responses were determined by measuring testicular weights at the time of weaning. It was shown that photoperiodic information received between Days 1 and 15 of life failed to alter the rate of testicular development, but that after Day 15 testicular growth was photoperiod-dependent. The mother, when entrained to a long photoperiod, did not influence the photoperiodic responses of her pups when they were confined to a dark nest box. In contrast, the mother did influence the circadian entrainment patterns of her pups. Pups exhibited a well-developed circadian activity rhythm at weaning with a phase angle roughly similar to that of the mother's activity rhythm. When the maternal rhythms were discrepant with photoperiod information received by the pups directly from the environment, the pups' activity rhythms were synchronized with the light/dark cycle rather than with the rhythm of their mother. Thus, it appears that although pups may first become entrained by maternal cues, they rapidly adjust to the environmental light cycle after leaving the nest.     

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.     

Phase-shifting the fruit fly clock without cryptochrome

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

Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels

In golden-mantled ground squirrels, phase angles of entrainment of circadian locomotor activity to a fixed light-dark cycle differ markedly between subjective summer and winter. A change in ambient temperature affects entrainment only during subjective winter when it also produces pronounced effects on body temperature (Tb). It was previously proposed that variations in Tb are causally related to the circannual rhythm in circadian entrainment. To test this hypothesis, wheel-running activity and Tb were monitored for 12 to 14 months in castrated male ground squirrels housed in a 14:10 LD photocycle at 21 degrees C. Animals were treated with testosterone implants that eliminated hibernation and prevented the marked winter decline in Tb; these squirrels manifested circannual changes in circadian entrainment indistinguishable from those of untreated animals. Both groups exhibited pronounced changes in phase angle and alpha of circadian wheel-running and Tb rhythms. Seasonal variation in Tb is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.     

Endogenous rhythms of locomotor activity in the high-shore limpet, Helcion pectunculus (Patellogastropoda)

Helcion pectunculus, a high-shore, crevice-dwelling limpet, is active during nocturnal low tides and during daytime low tides whilst in the shade. We examined whether this activity is controlled by an internal clock or purely by exogenous stimuli, such as light levels and tidal phase. Maximum entropy spectral analysis (MESA) revealed that the limpets possess a free-running endogenous rhythm of locomotor activity with both circadian (period 28.1 h) and circatidal (period 13.8 h) components. We suggest that this rhythm plays a role in allowing individuals to avoid unfavourable environmental conditions. The exogenous entrainment factor of the endogenous circatidal rhythm in H. pectunculus is the time of exposure to air, whilst the zeitgeber for the circadian component is not yet known. Copyright 1999 The Association for the Study of Animal Behaviour.     

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.     

Temperature can entrain egg laying rhythm of Drosophila but may not be a stronger zeitgeber than light

In Drosophila multiple circadian oscillators and behavioral rhythms are known to exist, yet most previous studies that attempted to understand circadian entrainment have focused on the activity/rest rhythm and to some extent the adult emergence rhythm. Egg laying behavior of Drosophila females also follows circadian rhythmicity and has been seen to deviate substantially from the better characterized rhythms in a few aspects. Here we report the findings of our study aimed at evaluating how circadian egg laying rhythm in fruit flies Drosophila melanogaster entrains to time cues provided by light and temperature. Previous studies have shown that activity/rest rhythm of flies entrains readily to light/dark (LD) and temperature cycles (TC). Our present study revealed that egg laying rhythm of a greater percentage of females entrains to TC compared to LD cycles. Therefore, in the specific context of our study this result can be taken to suggest that egg laying clocks of D. melanogaster entrains to TC more readily than LD cycles. However, when TC were presented along with out-of-phase LD cycles, the rhythm displayed two peaks, one occurring close to lights-off and the other near the onset of low temperature phase, indicating that upon entrainment by TC, LD cycles may be able to exert a greater influence on the phase of the rhythm. These results suggest that temperature and light associatively entrain circadian egg laying clocks of Drosophila.