Different mechanisms of phase delays and phase advances of the circadian rhythm in rat pineal N-acetyltransferase activity

The circadian rhythm in rat pineal N-acetyltransferase (NAT) activity, which drives the rhythm in melatonin production, is controlled by a pacemaker located in the suprachiasmatic nucleus of the hypothalamus. As the NAT rhythm has two well-defined phase markers--namely, the time of the evening activity rise and of the morning decline--it is suitable for studies of the entrainment of the pacemaker by environmental light. Phase delays of the NAT rhythm proceed more rapidly than phase advances. One day after a brief light pulse applied before midnight, or after a delay in evening lights-off, or a delay of a light-dark (LD) cycle, phase delays of the evening NAT rise result in almost corresponding delays of the morning NAT decline. Consequently, the NAT rhythm is phase-shifted, but its pattern does not change. One day after a brief light pulse applied past midnight, or after bringing forward morning lights-on, or after an advance of an LD cycle, the morning NAT decline is phase-advanced, but the evening rise is not phase-advanced at all or may even by phase-delayed. Consequently, the phase relationship between the evening NAT activity onset and the morning offset may be compressed considerably, and it may take several transient cycles before phase advances of the morning NAT decline are followed by corresponding advances of the evening NAT rise. Due to the phase-delaying effect of evening light on the NAT rise and to the phase-advancing effect of morning light on the NAT decline, the phase relationship between the NAT rise and the decline is compressed on long days and decompressed on short days. Different phase shifts of the evening NAT rise and of the morning decline, even in opposite directions, are consistent with the hypothesis of a complex, two-component (evening-morning, or E-M) pacemaker controlling the NAT rhythm. As the E-M phase relationship determines duration of the high night melatonin production, and the duration of the nocturnal melatonin pulse may convey information on daylength, the data are consistent with the internal coincidence model for photoperiodic time measurement.     

Dynamics of discrete entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity during transient cycles

To elucidate entrainment of a pacemaker controlling the N-acetyltransferase (NAT) rhythm in the rat pineal gland, we studied the phase response curves (PRCs) of this rhythm. We exposed 50- to 60-day-old male Wistar rats maintained in a light-dark cycle (LD 12:12) to a 1-min light pulse at different times before midnight or at various times throughout the whole night. We then released them into constant darkness and studied the morning NAT decline during the night when rats were pulsed before midnight, as well as the evening NAT rise and the morning decline after 4 days following the pulses. The PRC for the first NAT decline and the PRCs for the NAT rise and decline after 4 days were compared with published transient PRCs (Illnerová and Van?cek, 1982b), in order to obtain a complete picture of the dynamics of the NAT rhythm entrainment during the transient cycles. Phase delays in the NAT rise due to a pulse before midnight were complete (i.e., identical to those of day 4) on day 1. Phase delays in the NAT decline were almost complete on day 1, while incomplete phase delays were observed on day 0. Phase advances in the NAT rise and decline due to a pulse past midnight had different dynamics: Advances in the decline were complete on day 1, while advances in the rise were absent on day 1 and much smaller than in the decline on day 4. The results are discussed in terms of a two-component (E-M) pacemaker controlling the NAT rhythm. The NAT rise may reflect the phase of the E-component, while the decline reflects the M-component. Phase delays of the E-component are accomplished within one cycle, and so are phase advances of the M-component. However, although delays of E already result in delays of M one cycle after the pulse, it takes several transient cycles before advances of M begin to induce advances of E.     

Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by prolonged periods of light

Summary Entrainment of the circadian rhythm in the pineal N-acetyltranferase activity by prolonged periods of light was studied in rats synchronized with a light:dark regime of 1212 h by observing phase-shifts in rhythm after delays in switching off the light in the evening or after bringing forward of the morning onset of light. When rats were subjected to delays in switching off the light of up to 10 h and then were released into darkness, phase-delays of the evening N-acetyltransferase rise during the same night corresponded roughly to delays in the light switch off. However, phasedelays of the morning decline were much smaller. After a delay in the evening switch off of 11 h, no N-acetyltransferase rhythm was found in the subsequent darkness. The evening N-acetyltransferase rise was phase-delayed by 6.2 h at most 1 day after delays. Phase-delays of the morning Nacetyltransferase decline were shorter than phasedelays of the N-acetyltransferase rise by only 0.7 h to 0.9 h at most. Hence, 1 day after delays in the evening switch off, the period of the high night N-acetyltransferase activity may be shortened only slightly. The N-acetyltransferase rhythm was abolished only after a 12 h delay in switching off the light.Rats were subjected to a bringing forward of the morning light onset and then were released into darkness 4 h before the usual switch off of light. In the following night, the morning N-acetyltransferase decline, but not the evening rise, was phase advanced considerably. Moreover, when the onset of light was brought forward to before midnight, the N-acetyltransferase rise was even phase-delayed. Hence, 1 day after bringing forward the morning onset of light, the period of the high night N-acetyltransferase activity may be drastically reduced. When rats were subjected to a 4 h light pulse around midnight and then released into darkness, the N-acetyltransferase rhythm in the next night was abolished.The data are discussed in terms of a two-component pacemaker controlling the N-acetyltransferase rhythm. It is suggested that delays in the evening switch off of light may disturb the N-acetyltransferase rhythm the next day only a little, as the morning component may adjust to phasedelays of the evening component almost within one cycle. On the other hand, bringing forward the morning onset of light may disturb the N-acetyltransferase rhythm heavily the next day, as the evening component not only does not adjust to phase-advances of the morning component, but it may even be phase-delayed when the light onset occurs before midnight.Abbreviations NAT N-acetyltransferase - PRC phase response curve - E evening component of the N-acetyltransferase rhythm or of its pacemaker - M morning component of the N-acetyltransferase rhythm or of its pacemaker - LD xy light dark cycle comprising x h of light and y h of darkness     

Melatonin accelerates reentrainment of the circadian rhythm of its own production after an 8-h advance of the light-dark cycle

Summary The rhythm in melatonin production in the rat is driven by a circadian rhythm in the pineal N-acetyltransferase (NAT) activity. Rats adapted to an artificial lighting regime of 12 h of light and 12 h of darkness per day were exposed to an 8-h advance of the light-dark regime accomplished by the shortening of one dark period; the effect of melatonin, triazolam and fluoxetine, together with 5-hydroxytryptophan, on the reentrainment of the NAT rhythm was studied.In control rats, the NAT rhythm was abolished during the first 3 cycles following the advance shift. It reappeared during the 4th cycle; however, the phase relationship between the evening rise in activity and the morning decline was still compressed.Melatonin accelerated the NAT rhythm reentrainment. In rats treated chronically with melatonin at the new dark onset, the rhythm had already reappeared during the 3rd cycle, in the middle of the advanced night, and during the 4th cycle, the phase relationship between the evening onset and the morning decline of the NAT activity was the same as before the advance shift. In rats treated chronically with melatonin at the old dark onset or in those treated with melatonin 8 h, 5 h and 2 h after the new dark onset during the 1st, 2nd and 3rd cycle, respectively, following the advance shift, the NAT rhythm reappeared during the 3rd cycle as well but in the last third of the advanced night only.Neither triazolam nor fluoxetine together with 5-hydroxytryptophan administered around the new dark onset facilitated NAT rhythm reentrainment after the 8-h advance of the light-dark cycle.Abbreviations NAT N-acetyltransferase - LD cycle light-dark cycle - CT circadian time - LD xy light dark cycle comprising x h of light and y h of darkness     

Dynamics of the adjustment of clock gene expression in the rat suprachiasmatic nucleus to an asymmetrical change from a long to a short photoperiod

The molecular clockwork of the rat suprachiasmatic nucleus, the site of the circadian clock, is affected by the photoperiod (Sumová et al., 2003). The aim of the present study was to partly elucidate the dynamics of the adjustment of the clockwork to a change from a long to a short photoperiod accomplished by an asymmetrical prolongation of the dark period into the morning hours. Rats maintained under a regime with 16 h of light and 8 h of darkness per day (LD 16:8) were transferred to LD 8:16, and after 2, 3, and 13 days, daily profiles of Per1, Per2, Bmal1, and Cry1 mRNA were assessed by in situ hybridization. The rhythms of Per1, Per2, and Bmal1 expression adjusted to the change from a long to a short photoperiod with larger phase delays of the morning Per mRNA rise and Bmal1 mRNA decline than of the evening and nighttime Per mRNA decline and Bmal1 mRNA rise. The rhythm of Cry1 expression adjusted to the change by parallel delays of the Cry1 mRNA rise and decline. Adjustment of the Cry1 mRNA rhythm to short days was almost accomplished within 13 days, whereas adjustment of the Per1 and Bmal1 mRNA rhythms took longer. Different dynamics of the adjustment of rhythms in clock gene expression to a change from a long to a short photoperiod suggests complex resetting effects of the photoperiod change.     

Human cone light sensitivity and melatonin rhythms following 24-hour continuous illumination

This study investigates the possibility of an endogenous circadian rhythm in retinal cone function in humans. A full-field cone electroretinogram (ERG) was performed every 2 h for 24 h under continuous rod-saturating ambient white light (53 ± 30 lux; pupils dilated) in nine healthy subjects. Distinct circadian variations were superimposed upon a gradual decrease in cone responsiveness to light, demonstrated most reliably in the implicit times of b-wave and oscillatory potentials, and to a lesser extent in amplitude and a-wave implicit times. After mathematical correction of the linear trend, the cone response was found to be greatest around 20:00 h and least around 06:00 h. The phase of the ERG circadian rhythm was not synchronized with the phase of the salivary melatonin rhythm measured the previous evening. Melatonin levels measured under constant light on the day of ERG assessments were suppressed by 53% on average compared to melatonin profiles obtained previously under near-total darkness in seven participants. The progressive decline in cone responsiveness to light over the 24 h may reflect an adaptation of the cone-driven retinal system to constant light, although the mechanism is unclear. The endogenous rhythm of cone responsiveness to light may be used as an additional index of central or retinal circadian clock time.     

Clock gene daily profiles and their phase relationship in the rat suprachiasmatic nucleus are affected by photoperiod

Rhythmicity of the rat suprachiasmatic nucleus (SCN), a site of the circadian pacemaker, is affected by daylength; that is, by the photoperiod. Whereas various markers of rhythmicity have been followed, so far there have been no studies on the effect of the photoperiod on the expression of the clock genes in the rat SCN. To fill the gap and to better understand the photoperiodic modulation of the SCN state, rats were maintained either under a long photoperiod with 16 h of light and 8 h of darkness per day (LD16:8) or under a short LD8:16 photoperiod, and daily profiles of Per1, Cry1, Bmal1 and Clock mRNA in darkness were assessed by in situ hybridization method. The photoperiod affected phase, waveform, and amplitude of the rhythmic gene expression as well as phase relationship between their profiles. Under the long period, the interval of elevated Per1 mRNA lasted for a longer and that of elevated Bmal1 mRNA for a shorter time than under the short photoperiod. Under both photoperiods, the morning and the daytime Per1 and Cry1 mRNA rise as well as the morning Bmal1 mRNA decline were closely linked to the morning light onset. Amplitude of Per1, Cry1, and Bmal1 mRNA rhythms was larger under the short than under the long photoperiod. Also, under the short photoperiod, the daily Clock mRNA profile exhibited a significant rhythm. Altogether, the data indicate that the whole complex molecular clockwork in the rat SCN is photoperiod dependent and hence may differ according to the season of the year.     

Early- and late-emerging Drosophila melanogaster fruit flies differ in their sensitivity to light during morning and evening

The authors derived early and late populations of fruit flies showing increased incidence of emergence during morning or evening hours by imposing selection for timing of emergence under 12:12 h light/dark (LD) cycles. From previous studies, it was clear that the increased incidence of adult emergence during morning and evening hours in early and late populations was a result of evolution of divergent and characteristic emergence waveforms in these populations. Such characteristic waveforms are henceforth referred to as "evolved emergence waveforms" (EEWs). The early and late populations also evolved different circadian clocks, which is evident from the divergence in their clock period (τ) and photic phase response curve (PRC). Although correlation between emergence waveforms and clock properties suggests functional significance of circadian clocks, τ and PRCs do not satisfactorily explain the early and late emergence phenotypes. In order to understand the functional significance of the PRC for early and late emergence phenotypes, the authors investigated whether circadian clocks of these flies exhibit any difference in photosensitivity under entrained conditions. Such differences would suggest that the light requirement for circadian entrainment of the emergence rhythm in early and late populations is different. To test this, they examined if early and late flies differ in their light utilization behavior, first by assaying their emergence rhythm under complete photoperiod and then in three different skeleton photoperiods. The results showed that early and late populations require different durations of light during the morning and evening to achieve their EEWs, suggesting that for the circadian entrainment of the emergence rhythm, early and late flies utilize light from different parts of the day.     

Exercise elicits phase shifts and acute alterations of melatonin that vary with circadian phase

To examine the immediate phase-shifting effects of high-intensity exercise of a practical duration (1 h) on human circadian phase, five groups of healthy men 20-30 yr of age participated in studies involving no exercise or exposure to morning, afternoon, evening, or nocturnal exercise. Except during scheduled sleep/dark and exercise periods, subjects remained under modified constant routine conditions allowing a sleep period and including constant posture, knowledge of clock time, and exposure to dim light intensities averaging (+/-SD) 42 +/- 19 lx. The nocturnal onset of plasma melatonin secretion was used as a marker of circadian phase. A phase response curve was used to summarize the phase-shifting effects of exercise as a function of the timing of exercise. A significant effect of time of day on circadian phase shifts was observed (P < 0.004). Over the interval from the melatonin onset before exercise to the first onset after exercise, circadian phase was significantly advanced in the evening exercise group by 30 +/- 15 min (SE) compared with the phase delays observed in the no-exercise group (-25 +/- 14 min, P < 0.05). Phase shifts in response to evening exercise exposure were attenuated on the second day after exercise exposure and no longer significantly different from phase shifts observed in the absence of exercise. Unanticipated transient elevations of melatonin levels were observed in response to nocturnal exercise and in some evening exercise subjects. Taken together with the results from previous studies in humans and diurnal rodents, the current results suggest that 1) a longer duration of exercise exposure and/or repeated daily exposure to exercise may be necessary for reliable phase-shifting of the human circadian system and that 2) early evening exercise of high intensity may induce phase advances relevant for nonphotic entrainment of the human circadian system.     

Data-driven modelling captures dynamics of the circadian clock of Neurospora crassa

Eukaryotic circadian clocks are based on self-sustaining, cell-autonomous oscillatory feedback loops that can synchronize with the environment via recurrent stimuli (zeitgebers) such as light. The components of biological clocks and their network interactions are becoming increasingly known, calling for a quantitative understanding of their role for clock function. However, the development of data-driven mathematical clock models has remained limited by the lack of sufficiently accurate data. Here we present a comprehensive model of the circadian clock of Neurospora crassa that describe free-running oscillations in constant darkness and entrainment in light-dark cycles. To parameterize the model, we measured high-resolution time courses of luciferase reporters of morning and evening specific clock genes in WT and a mutant strain. Fitting the model to such comprehensive data allowed estimating parameters governing circadian phase, period length and amplitude, and the response of genes to light cues. Our model suggests that functional maturation of the core clock protein Frequency causes a delay in negative feedback that is critical for generating circadian rhythms.     

Functional diversities of two activity components of circadian rhythm in genetical splitting mice (CS strain)

CS mice, an inbred strain, showed two distinctive characteristics in the circadian rhythm of locomotor activity: (1) large variation in the freerunning period, and (2) spontaneous rhythm splitting under continuous darkness. In the splitting rhythm there was a positive correlation between the freerunning period of the evening component and the activity time of the morning component. The phase-shifting effect of a 15-min light pulse was examined on the two activity components of the splitting rhythm. There were significant differences in the amount of light-induced phase response between the two components. A light pulse during the late subjective night induced a phase advance shift only in the morning component, while a light pulse during the early subjective night induced a phase delay shift only in the evening component. These results indicate functional diversities of the two activity components in the circadian locomotor rhythm of CS mice, and suggest that the circadian system in CS mice consists of two mutually coupled oscillators which have different circadian periods and different responsiveness to light. The CS mouse is a useful model to explore a genetic background of oscillator coupling in the circadian system of nocturnal rodents. Accepted: 19 November 1998     

Phase Angle of Entrainment in Morning‐ and Evening‐Types under Naturalistic Conditions

Differences in morningness‐eveningness among humans are commonly ascribed to circadian parameters, such as circadian period and responsivity to environmental time cues, as well as homeostatic sleep drive. Light is the primary synchronizer of the human biological clock, and if circadian differences exist between morning and evening types, they should have different phase angles of entrainment to the light/dark cycle; that is, morning and evening types should have different patterns of light exposure relative to endogenous circadian phase (ECP). When phase angle of entrainment is strictly defined as the relationship between a marker of ECP and the timing of light exposure, such differences have been demonstrated in the laboratory under controlled light/dark cycles and have recently been shown under conditions of spring and summer light exposure outside the laboratory, taking into account the variable intensity of light. Here, we report similar results from a large (n=66), diverse cohort of morning and evening types across the age span studied at all different times of the year. Differences between morning and evening types in light exposure relative to ECP, indicative of a difference in the phase angle of entrainment to the external light/dark cycle, were found. Specifically, evening types, compared to morning types, had a higher ratio of phase advancing to phase delaying by light. We interpret this as indicating a longer circadian period (τ) in evening types.     

Heart rate circadian rhythm as a biological marker of desynchronization in major depression: a methodological and preliminary report

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP greater than 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP greater than 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP less than 25%, decrease in amplitude greater than 70%). In the group of patients with a persisting HR circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.     

Daytime naps in darkness phase shift the human circadian rhythms of melatonin and thyrotropin secretion

To systematically determine the effects of daytime exposure to sleep in darkness on human circadian phase, four groups of subjects participated in 4-day studies involving either no nap (control), a morning nap (0900-1500), an afternoon nap (1400-2000), or an evening nap (1900-0100) in darkness. Except during the scheduled sleep/dark periods, subjects remained awake under constant conditions, i.e., constant dim light exposure (36 lx), recumbence, and caloric intake. Blood samples were collected at 20-min intervals for 64 h to determine the onsets of nocturnal melatonin and thyrotropin secretion as markers of circadian phase before and after stimulus exposure. Sleep was polygraphically recorded. Exposure to sleep and darkness in the morning resulted in phase delays, whereas exposure in the evening resulted in phase advances relative to controls. Afternoon naps did not change circadian phase. These findings indicate that human circadian phase is dependent on the timing of darkness and/or sleep exposure and that strategies to treat circadian misalignment should consider not only the timing and intensity of light, but also the timing of darkness and/or sleep.     

Human Cone Light Sensitivity and Melatonin Rhythms Following 24-hour Continuous Illumination

This study investigates the possibility of an endogenous circadian rhythm in retinal cone function in humans. A full-field cone electroretinogram (ERG) was performed every 2?h for 24?h under continuous rod-saturating ambient white light (53 ±?30 lux; pupils dilated) in nine healthy subjects. Distinct circadian variations were superimposed upon a gradual decrease in cone responsiveness to light, demonstrated most reliably in the implicit times of b-wave and oscillatory potentials, and to a lesser extent in amplitude and a-wave implicit times. After mathematical correction of the linear trend, the cone response was found to be greatest around 20:00?h and least around 06:00?h. The phase of the ERG circadian rhythm was not synchronized with the phase of the salivary melatonin rhythm measured the previous evening. Melatonin levels measured under constant light on the day of ERG assessments were suppressed by 53% on average compared to melatonin profiles obtained previously under near-total darkness in seven participants. The progressive decline in cone responsiveness to light over the 24?h may reflect an adaptation of the cone-driven retinal system to constant light, although the mechanism is unclear. The endogenous rhythm of cone responsiveness to light may be used as an additional index of central or retinal circadian clock time. (Author correspondence: kvdani@mail.ru)     

Integration of human sleep-wake regulation and circadian rhythmicity.

The human sleep-wake cycle is generated by a circadian process, originating from the suprachiasmatic nuclei, in interaction with a separate oscillatory process: the sleep homeostat. The sleep-wake cycle is normally timed to occur at a specific phase relative to the external cycle of light-dark exposure. It is also timed at a specific phase relative to internal circadian rhythms, such as the pineal melatonin rhythm, the circadian sleep-wake propensity rhythm, and the rhythm of responsiveness of the circadian pacemaker to light. Variations in these internal and external phase relationships, such as those that occur in blindness, aging, morning and evening, and advanced and delayed sleep-phase syndrome, lead to sleep disruptions and complaints. Changes in ocular circadian photoreception, interindividual variation in the near-24-h intrinsic period of the circadian pacemaker, and sleep homeostasis can contribute to variations in external and internal phase. Recent findings on the physiological and molecular-genetic correlates of circadian sleep disorders suggest that the timing of the sleep-wake cycle and circadian rhythms is closely integrated but is, in part, regulated differentially.     

Heart Rate Orcadian Rhythm as a Biological Marker of Desynchronization in Major Depression: A Methodological and Preliminary Report

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP > 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP > 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP < 25%, decrease in amplitude > 70%). In the group of patients with a persisting HK circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.     

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.     

Feeding melatonin enhances the phase shifting response to triazolam in both young and old golden hamsters

Aging alters many aspects of circadian rhythmicity, including responsivity to phase-shifting stimuli and the amplitude of the rhythm of melatonin secretion. As melatonin is both an output from and an input to the circadian clock, we hypothesized that the decreased melatonin levels exhibited by old hamsters may adversely impact the circadian system as a whole. We enhanced the diurnal rhythm of melatonin by feeding melatonin to young and old hamsters. Animals of both age groups on the melatonin diet showed larger phase shifts than control-fed animals in response to an injection with the benzodiazepine triazolam at a circadian time known to induce phase advances in the activity rhythm of young animals. Thus melatonin treatment can increase the sensitivity of the circadian timing system of young animals to a nonphotic stimulus, and the ability to increase this sensitivity persists into old age, indicating exogenous melatonin might be useful in reversing at least some age-related changes in circadian clock function.     

Thermoperiodic regulation of the circadian eclosion rhythm in the flesh fly, Sarcophaga crassipalpis

We recorded the eclosion time of the flesh fly, Sarcophaga crassipalpis, at different depths in the outdoor soil and under temperature cycles with various amplitudes in the laboratory, to examine the timing adjustment of eclosion in response to temperature cycles and their amplitudes in the pupal stage. In the soil, most eclosions occurred in the late morning, which was consistent with the eclosion time under pseudo-sinusoidal temperature cycles in the laboratory. The circadian clock controlling eclosion was reset by temperature cycles and free-ran with a period close to 24 h. This clock likely helps pupae eclose at an optimal time even when the soil temperature does not show clear daily fluctuations. The eclosion phase of the circadian clock progressively advanced as the amplitude of the pseudo-sinusoidal temperature cycle decreased. This response allows pupae located at any depth in the soil to eclose at the appropriate time despite the depth-dependent phase delay of the temperature change. In contrast, the abrupt temperature increase in square-wave temperature cycles reset the phase of the circadian clock to the increasing time, regardless of the temperature amplitude. The rapid temperature increase may act as the late-morning signal for the eclosion clock.