Potent Circadian Effects of Dim Illumination at Night in Hamsters

Conventional wisdom holds that the circadian pacemaker of rodents and humans is minimally responsive to light of the intensity provided by dim moonlight and starlight. However, dim illumination (<0.005 lux) provided during the daily dark periods markedly alters entrainment in hamsters. Under dimly lit scotophases, compared to completely dark ones phases, the upper range of entrainment is increased by ～4 h, and re‐entrainment is accelerated following transfer from long to short day lengths. Moreover, the incidence of bimodal entrainment to 24 h light:dark:light:dark cycles is increased fourfold. Notably, the nocturnal illumination inducing these pronounced effects is equivalent in photic energy to that of a 2 sec, 100 lux light pulse. These effects may be parsimoniously interpreted as an action of dim light on the phase relations between multiple oscillators comprising the circadian pacemaker. An action of dim light distinct from that underlying bright‐light phase‐resetting may promote more effective entrainment. Together, the present results refute the view that scotopic illumination is environmental “noise” and indicate that clock function is conspicuously altered by nighttime illumination like that experienced under dim moonlight and starlight. We interpret our results as evidence for a novel action of dim light on the coupling of multiple circadian oscillators.     

Potent circadian effects of dim illumination at night in hamsters

Conventional wisdom holds that the circadian pacemaker of rodents and humans is minimally responsive to light of the intensity provided by dim moonlight and starlight. However, dim illumination (<0.005 lux) provided during the daily dark periods markedly alters entrainment in hamsters. Under dimly lit scotophases, compared to completely dark ones phases, the upper range of entrainment is increased by approximately 4 h, and re-entrainment is accelerated following transfer from long to short day lengths. Moreover, the incidence of bimodal entrainment to 24 h light:dark:light:dark cycles is increased fourfold. Notably, the nocturnal illumination inducing these pronounced effects is equivalent in photic energy to that of a 2 sec, 100 lux light pulse. These effects may be parsimoniously interpreted as an action of dim light on the phase relations between multiple oscillators comprising the circadian pacemaker. An action of dim light distinct from that underlying bright-light phase-resetting may promote more effective entrainment. Together, the present results refute the view that scotopic illumination is environmental "noise" and indicate that clock function is conspicuously altered by nighttime illumination like that experienced under dim moonlight and starlight. We interpret our results as evidence for a novel action of dim light on the coupling of multiple circadian oscillators.     

Circadian organization and photoreception in an Australian dasyurid marsupial (Sminthopsis macroura)

Much is known about the formal properties of circadian rhythm regulation and the physiological substrates underlying rhythmicity in nocturnal rodents, but relatively few studies have addressed circadian rhythm regulation in other mammalian taxonomic groups. In this study, some formal and functional aspects of circadian organization in a nocturnal dasyurid marsupial, the stripe-faced dunnart (Sminthopsis macroura), were analyzed. To determine phasic responses to discrete pulses of light, dunnarts were placed in constant darkness (DD) and were periodically administered pulses of bright light at different times of the animals' circadian day. Analysis of phase shifts in response to light indicated a phase response curve that was similar to responses observed in nocturnal rodents. To determine the possibility of extraretinal photoreception mediating photic entrainment, dunnarts were anesthetized and orbitally enucleated while maintained in a light-dark regimen (LD 14:10). All blinded dunnarts free-ran with periods (tau) that were similar to those observed in DD, indicating that entrainment is mediated through ocular photoreception. However, the data also indicated a decrease in activity in blind dunnarts during the last 3-5 hr of the dark phase, raising the possibility of some retention of photoreceptive capacities.     

Phase response curve to 1 h light pulses for the European rabbit (Oryctolagus cuniculus)

While much is known about the circadian systems of rodents, chronobiological studies of other mammalian groups have been limited. One of the most extensively studied nonrodent species, both in the laboratory and in the wild, is the European rabbit. The aim of this study was to extend knowledge of the rabbit circadian system by examining its phasic response to light. Twelve Dutch-Himalayan cross rabbits of both sexes were allowed to free-run in constant darkness and then administered 1 h light pulses (1000 lux) at multiple predetermined circadian times. Changes in the phase of the rabbits’ circadian wheel-running rhythms were measured after each light pulse and used to construct a phase–response curve (PRC). The rabbits’ PRC and free-running period (τ) conformed to the empirical regularities reported for other predominantly nocturnal animals, including rodents and predatory marsupials. The results of the study are thus consistent with reports that the rabbit is essentially a nocturnal animal and show that it can entrain to light/dark (LD) cycles via discrete phase shifts. Knowledge about the rabbit’s circadian range of entrainment to LD cycles gained in this study will be useful for examining the putative circadian processes believed to underlie the unusual rhythm of very brief, once-daily nest visits by nursing rabbit mothers and other nursing lagomorphs.     

Circadian Disruption: comparing humans with mice

Disruption of the 24-h light-dark cycle has been implicated as an endocrine disruptor and linked to increased morbidity and mortality in animal studies. Previously reported measurements of circadian disruption in day-shift and rotating-shift nurses were compared with new mouse data where the light-dark patterns simulated shiftwork. Phasor magnitudes, a measure of circadian entrainment, were shown to be similar for humans and for mice when exposed to similar patterns of light and dark. Phasor analyses may be a useful method for quantitatively bridging ecological measurements of circadian disruption in human with parametric studies of health outcomes in a mouse model.     

Photic and nonphotic inputs to the diurnal circadian clock

Diurnal animals occupy a different temporal niche from nocturnal animals and are consequently exposed to different amounts of light as well as different dangers. Accordingly, some variation exists in the way that diurnal animals synchronize their internal circadian clock to match the external 24-hour daily cycle. First, though the brain mechanisms underlying photic entrainment are very similar among species with different daily activity patterns, there is evidence that diurnal animals are less sensitive to photic stimuli compared to nocturnal animals. Second, stimuli other than light that synchronize rhythms (i.e. nonphotic stimuli) can also entrain and phase shift daily rhythms. Some of the rules that govern nonphotic entrainment in nocturnal animals as well as the brain mechanisms that control nonphotic influences on rhythms do not appear to apply to diurnal animals, however. Some evidence supports the idea that arousal or activity plays an important role in entraining rhythms in diurnal animals, either during the light (active) or dark (inactive) phases, though no consistent pattern is seen. GABAergic stimulation induces phase shifts during the subjective day in both diurnal and nocturnal animals. In diurnal Arvicanthis niloticus (Nile grass rats), SCN GABA_A receptor activation at this time results in phase delays while in nocturnal animals phase advances are induced. It appears that the effect of GABA at this circadian phase results from the inhibition of period gene expression in both diurnal and nocturnal animals. Nonetheless, the resulting phase shifts are in opposite directions. It is not known what stimuli or behaviours ultimately induce changes in GABA activity in the SCN that result in alterations of circadian phase in diurnal grass rats. Taken together, studies such as these suggest that it may be problematic to apply the principles governing nocturnal nonphotic entrainment and its underlying mechanisms to diurnal species including humans.     

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.     

Dim light at night increases immune function in Nile grass rats, a diurnal rodent

With the widespread adoption of electrical lighting during the 20th century, human and nonhuman animals became exposed to high levels of light at night for the first time in evolutionary history. This divergence from the natural environment may have significant implications for certain ecological niches because of the important influence light exerts on the circadian system. For example, circadian disruption and nighttime light exposure are linked to changes in immune function. The majority of studies investigating the effects of light exposure and circadian disruption on the immune system use nocturnal rodents. In diurnal species, many hormones and immune parameters vary with secretion patterns 180° out of phase to those of nocturnal rodents. Thus, the authors investigated the effects of nighttime light exposure on immunocompetence in diurnal Nile grass rats (Arvicanthis niloticus). Rats were housed in either standard 14-h light (L):10-h dark (D) cycles with L ～150 lux and D 0 lux or dim light at night (dLAN) cycles of LD 14:10 with L ～150 lux and D 5 lux for 3 wks, then tested for plasma bactericidal capacity, as well as humoral and cell-mediated immune responses. Rats exposed to dLAN showed increased delayed-type hypersensitivity pinna swelling, which is consistent with enhanced cell-mediated immune function. dLAN rats similarly showed increased antibody production following inoculation with keyhole lymphocyte hemocyanin (KLH) and increased bactericidal capacity. Daytime corticosterone concentrations were elevated in grass rats exposed to nighttime dim light, which may have influenced immunological measures. Overall, these results indicate nighttime light affects immune parameters in a diurnal rodent.     

Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

It is well established that in the absence of photic cues, the circadian rhythms of rodents can be readily phase-shifted and entrained by various nonphotic stimuli that induce increased levels of locomotor activity (i.e., benzodiazepines, a new running wheel, and limited food access). In the presence of an entraining light-dark (LD) cycle, however, the entraining effects of nonphotic stimuli on (parts of) the circadian oscillator are far less clear. Yet, an interesting finding is that appropriately timed exercise after a phase shift can accelerate the entrainment of circadian rhythms to the new LD cycle in both rodents and humans. The present study investigated whether restricted daytime feeding (RF) (1) induces a phase shift of the melatonin rhythm under entrained LD conditions and (2) accelerates resynchronization of circadian rhythms after an 8-h phase advance. Animals were adapted to RF with 2-h food access at the projected time of the new dark onset. Before and at several time points after the 8-h phase advance, nocturnal melatonin profiles were measured in RF animals and animals on ad libitum feeding (AL). In LD-entrained conditions, RF did not cause any significant changes in the nocturnal melatonin profile as compared to AL. Unexpectedly, after the 8-h phase advance, RF animals resynchronized more slowly to the new LD cycle than AL animals. These results indicate that prior entrainment to a nonphotic stimulus such as RF may "phase lock" the circadian oscillator and in that way hinder resynchronization after a phase shift.     

The effects of feedback lighting on the circadian drinking rhythm in the diurnal new world primate Saimiri sciureus

Feedback lighting provides illumination primarily during the subjective night (i.e., the photosensitive portion of the circadian cycle) in response to a given behavior. This technique has previously been used to test the nonparametric model of entrainment in nocturnal rodents. In three species (Rattus norvegicus, Mesocricetus auratus, and Mus musculus), the free-running period of the locomotor activity rhythm was similar whether the animals were exposed to continuous light or discrete light pulses occurring essentially only during the subjective night (i.e., feedback lighting). In the current experiments, feedback lighting was presented to squirrel monkeys so that light fell predominantly during the subjective night. Feedback lighting was linked to the drinking behavior in this diurnal primate so that when the animal drank, the lights went out. Despite the seemingly adverse predicament, the monkeys maintained regular circadian drinking rhythms. Furthermore, just as the period of the free-running activity rhythms of nocturnal rodents exposed to continuous light or feedback lighting were similar, the period of the drinking rhythms of the squirrel monkeys in continuous light and feedback lighting were comparable (25.6 +/- 0.1 and 25.9 +/- 0.1 hours, respectively), despite a substantial decrease in the total amount of light exposure associated with feedback lighting. The free-running period of monkeys exposed to continuous dark (24.5 +/- 0.1 hours) was significantly shorter than either of the two lighting conditions (P < 0.001). The results presented for the drinking rhythm were confirmed by examination of the temperature and activity rhythms. Therefore, discrete light pulses given predominately during the subjective night are capable of simulating the effects of continuous light on the free-running period of the circadian rhythms of a diurnal primate. The response of squirrel monkeys to feedback lighting thus lends further support for the model and suggests that the major entrainment mechanisms are similar in nocturnal rodents and diurnal primates.     

Is the PRC for Dark Pulses in LL a Mirror Image of the PRC for Light Pulses in DD in Mice?

The phase-response curves (PRC) for light pulses in continuous darkness (DD) have been described in many mammals, especially in nocturnal rodents. The PRC for dark pulses in continuous light (LL), however, has been described in a few mammals only, in nocturnal for bat and for hamster and in diurnal for Octodon degus, suggesting that this PRC is mirror imaging the PRC for light pulses. Therefore, the effect of 1-h and 3-h lasting dark pulses on the circadian wheel-running activity rhythm of mice in continuous light was investigated and then the PRC for dark pulses in LL was drawn up. For comparison, the effect of 1-h lasting light pulses on the circadian wheel-running activity rhythm of mice in DD was examined and the PRC for light pulses in DD was constructed. It appeared that the PRC for dark pulses, to a certain degree, represents a mirror image of the PRC for light pulses in mice. However, the advance region of this PRC is longer than that of delay. The mechanism of dark pulses action is discussed.     

Activity Rhythms and Masking Response in the Diurnal Fat Sand Rat Under Laboratory Conditions

Daily rhythms are heavily influenced by light in two major ways. One is through photic entrainment of a circadian clock, and the other is through a more direct process, referred to as masking. Whereas entraining effects of photic stimuli are quite similar in nocturnal and diurnal species, masking is very different. Laboratory conditions differ greatly from what is experienced by individuals in their natural habitat, and several studies have shown that activity patterns can greatly differ between laboratory environment and natural condition. This is especially prevalent in diurnal rodents. We studied the daily rhythms and masking response in the fat sand rat (Psammomys obesus), a diurnal desert rodent, and activity rhythms of Tristram’s jird (Meriones tristrami), a nocturnal member of the same subfamily (Gerbillinae). We found that most sand rats kept on a 12?h:12?h light-dark (LD) cycles at two light intensities (500 and 1000?lux) have a nocturnal phase preferences of general activity and higher body temperature during the dark phase. In most individuals, activity was not as stable that of the nocturnal Tritram’s jirds, which showed a clear and stable nocturnal activity pattern under the same conditions. Sand rats responded to a 6-h phase advance and 6-h phase delay as expected, and, under constant conditions, all tested animals free ran. In contrast with the nocturnal phase preference, fat sand rats did not show a masking response to light pulses during the dark phase or to a dark pulse during the light phase. They did, however, have a significant preference to the light phase under a 3.5?h:3.5?h LD schedule. Currently, we could not identify the underlying mechanisms responsible for the temporal niche switch in this species. However, our results provide us with a valuable tool for further studies of the circadian system of diurnal species, and will hopefully lead us to understanding diurnality, its mechanisms, causes, and consequences.     

Duration of activity and period of circadian activity–rest rhythm in a photoperiod-dependent primate,Microcebus murinus

Under free-running conditions, the grey lesser mouse lemur expresses a circadian activity–rest rhythm with a period particularly short (22.50 ± 0.6 h) for a mammal. Light exerts a strong suppressive effect upon activity. After transfer from nycthemeral to free-running conditions the duration of activity was systematically increased. This extension took place in the first cycle and was characterized by both a phase advance in activity onset and an even larger phase delay in activity offset. This phenomenon was more pronounced after long day entrainment. Any shift of the light–dark cycle was followed the next day by a corresponding shift in activity onset. The phase response curve pattern was similar to that already described for nocturnal mammals. Due to the strength of light as a zeitgeber and the plasticity of the response to photic conditions, the mouse lemur appears as a convenient species for chronobiological studies on non-human primates.     

Ticking for Metabolic Health: The Skeletal‐Muscle Clocks

To be prepared for alternating metabolic demands occurring over the 24‐hour day, the body preserves information on time in skeletal muscle, and in all cells, through a circadian‐clock mechanism. Skeletal muscle can be considered the largest collection of peripheral clocks in the body, with a major contribution to whole‐body energy metabolism. Comparison of circadian‐clock gene expression between skeletal muscle of nocturnal rodents and diurnal humans reveals very common patterns based on rest/active cycles rather than light/dark cycles. Rodent studies in which the circadian clock is disrupted in skeletal muscle demonstrate impaired glucose handling and insulin resistance. Experimental circadian misalignment in humans modifies the skeletal‐muscle clocks and leads to disturbed energy metabolism and insulin resistance. Preclinical studies have revealed that timing of exercise over the day can influence the beneficial effects of exercise on skeletal‐muscle metabolism, and studies suggest similar applicability in humans. Current strategies to improve metabolic health (e.g., exercise) should be reinvestigated in their capability to modify the skeletal‐muscle clocks by taking timing of the intervention into account.     

Circadian system responses to nocturnal and diurnal hosts in the kissing bug,Triatoma infestans

ABSTRACT

Insects express diverse behavioral rhythms synchronized to environmental cycles. While circadian entrainment to light–dark cycles is ubiquitous in living organisms, synchronization to non-photic cycles may be critical for hematophagous bugs that depend on rhythmic hosts. The purpose was to determine whether Triatoma infestans are capable of synchronizing to the circadian rhythms of potential hosts with temporally distinct activity patterns; and, if so, if this synchronization occurs through masking or entrainment. Precise synchronization with the food source may be critical for the insects’ survival due to the specific predatory or defensive nature of each host. Kissing bugs were housed in a compartment in constant dark, air-flow-connected to another compartment with a nocturnal or a diurnal host; both hosts were synchronized to a light–dark cycle. The activity rhythms of kissing bugs were modulated by the daily activity rhythms of the vertebrates. Effects were a decrease in the endogenous circadian period, independent of the host being nocturnal or diurnal; in some cases relative coordination occurred and in others synchronization was clearly achieved. Moreover, splitting and bimodality arose, phenomena that were also affected by the host presence. The results indicate that T. infestans were able to detect the non-photic cycle of their potential hosts, an ability that surely facilitates feeding and hinders predation risk. Understanding triatomines behavior is of fundamental importance to the design of population control methods.     

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.     

DISSOCIATION OF THE CIRCADIAN SYSTEM OF OCTODON DEGUS BY T28 AND T21 LIGHT-DARK CYCLES

Octodon degus is a primarily diurnal rodent that presents great variation in its circadian chronotypes due to the interaction between two phase angles of entrainment, diurnal and nocturnal, and the graded masking effects of environmental light and temperature. The aim of this study was to test whether the circadian system of this diurnal rodent can be internally dissociated by imposing cycles shorter and longer than 24?h, and to determine the influence of degus chronotypes and wheel-running availability on such dissociation. To this end, wheel-running activity and body temperature rhythms were studied in degus subjected to symmetrical light-dark (LD) cycles of T28h and T21h. The results show that both T-cycles dissociate the degus circadian system in two different components: one light-dependent component (LDC) that is influenced by the presence of light, and a second non–light-dependent component (NLDC) that free-runs with a period different from the external lighting cycle. The LDC was more evident in the nocturnal than diurnal chronotype, and also when wheel running was available. Our results show that, in addition to rats and mice, degus must be added to the list of species that show an internal dissociation in their circadian rhythms when exposed to forced desynchronization protocols. The existence of a multioscillatory circadian system having two groups of oscillators with low coupling strength may explain the flexibility of degus chronotypes. (Author correspondence: angerol@um.es)     

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.     

Action spectrum of Drosophila cryptochrome

Cryptochromes are a highly conserved class of UV-A/blue light photoreceptors. In Drosophila, cryptochrome is required for the normal entrainment of circadian rhythms to light dark cycles. The photocycle and molecular mechanism of animal cryptochrome photoreception are presently unknown. Drosophila cryptochrome undergoes light-dependent degradation when heterologously expressed in Schneider-2 cells. We have generated Drosophila luciferase-cryptochrome fusion proteins to more precisely monitor light-dependent cryptochrome degradation. We found that the luciferase-cryptochrome fusion protein undergoes light-dependent degradation with luciferase activity declining approximately 50% within 5 min of light exposure and approximately 85% within 1 h of light exposure. Degradation is inhibited by MG-132, consistent with a proteasomal degradation mechanism. Irradiance-response curves yield an action spectrum similar to absorption spectra for prokaryotic and eukaryotic cryptochromes with highest sensitivity in the UV-A. A luciferase-cryptochrome fusion protein lacking the terminal 15 amino acids is stably expressed in the dark but demonstrates increased sensitivity to light-induced degradation. The conferral of light-dependent degradation on a heterologous protein by fusion to cryptochrome may be a useful tool for probing protein function in cell expression systems.     

Scotopic illumination enhances entrainment of circadian rhythms to lengthening light:dark cycles

Endogenously generated circadian rhythms are synchronized with the environment through phase-resetting actions of light. Starlight and dim moonlight are of insufficient intensity to reset the phase of the clock directly, but recent studies have indicated that dim nocturnal illumination may otherwise substantially alter entrainment to bright lighting regimes. In this article, the authors demonstrate that, compared to total darkness, dim illumination at night (< 0.010 lux) alters the entrainment of male Syrian hamsters to bright-light T cycles, gradually increasing in cycle length (T) from 24 h to 30 h. Only 1 of 18 hamsters exposed to complete darkness at night entrained to cycles of T > 26 h. In the presence of dim nocturnal illumination, however, a majority of hamsters entrained to Ts of 28 h or longer. The presence or absence of a running wheel had only minor effects on entrainment to lengthening light cycles. The results further establish the potent effects of scotopic illumination on circadian entrainment and suggest that naturalistic ambient lighting at night may enhance the plasticity of the circadian pacemaker.