Circadian clock resetting by sleep deprivation without exercise in Syrian hamsters: dark pulses revisited

Chronic sleep loss disrupts rhythmic gene expression in Drosophila

Genome-wide profiling of rhythmic gene expression has offered new avenues for studying the contribution of circadian clock to diverse biological processes. Sleep has been considered one of the most important physiological processes that are regulated by the circadian clock, however, the effects of chronic sleep loss on rhythmic gene expression remain poorly understood. In the present study, we exploited Drosophila sleep mutants insomniac 1 (inc 1 ) and wide awake D2 (wake D2 ) as models for chronic sleep loss. We profiled the transcriptomes of head tissues collected from 4-week-old wild type flies, inc 1 and wake D2 at timepoints around the clock. Analysis of gene oscillation revealed a substantial loss of rhythmicity in inc 1 and wake D2 compared to wild type flies, with most of the affected genes common to both mutants. The disruption of gene oscillation was not due to changes in average gene expression levels. We also identified a subset of genes whose loss of rhythmicity was shared among animals with chronic sleep loss and old flies, suggesting a contribution of aging to chronic, sleep-loss-induced disruption of gene oscillation.

Keep Your Mask On: The Benefits of Masking for Behavior and the Contributions of Aging and Disease on Dysfunctional Masking Pathways

Environmental cues (e.g., light-dark cycle) have an immediate and direct effect on behavior, but these cues are also capable of “masking” the expression of the circadian pacemaker, depending on the type of cue presented, the time-of-day when they are presented, and the temporal niche of the organism. Masking is capable of complementing entrainment, the process by which an organism is synchronized to environmental cues, if the cues are presented at an expected or predictable time-of-day, but masking can also disrupt entrainment if the cues are presented at an inappropriate time-of-day. Therefore, masking is independent of but complementary to the biological circadian pacemaker that resides within the brain (i.e., suprachiasmatic nucleus) when exogenous stimuli are presented at predictable times of day. Importantly, environmental cues are capable of either inducing sleep or wakefulness depending on the organism’s temporal niche; therefore, the same presentation of a stimulus can affect behavior quite differently in diurnal vs. nocturnal organisms. There is a growing literature examining the neural mechanisms underlying masking behavior based on the temporal niche of the organism. However, the importance of these mechanisms in governing the daily behaviors of mammals and the possible implications on human health have been gravely overlooked even as modern society enables the manipulation of these environmental cues. Recent publications have demonstrated that the effects of masking weakens significantly with old age resulting in deleterious effects on many behaviors, including sleep and wakefulness. This review will clearly outline the history, definition, and importance of masking, the environmental cues that induce the behavior, the neural mechanisms that drive them, and the possible implications for human health and medicine. New insights about how masking is affected by intrinsically photosensitive retinal ganglion cells, temporal niche, and age will be discussed as each relates to human health. The overarching goals of this review include highlighting the importance of masking in the expression of daily rhythms, elucidating the impact of aging, discussing the relationship between dysfunctional masking behavior and the development of sleep-related disorders, and considering the use of masking as a non-invasive treatment to help treat humans suffering from sleep-related disorders.

Sleep drive reconfigures wake-promoting clock circuitry to regulate adaptive behavior

Circadian rhythms help animals synchronize motivated behaviors to match environmental demands. Recent evidence indicates that clock neurons influence the timing of behavior by differentially altering the activity of a distributed network of downstream neurons. Downstream circuits can be remodeled by Hebbian plasticity, synaptic scaling, and, under some circumstances, activity-dependent addition of cell surface receptors; the role of this receptor respecification phenomena is not well studied. We demonstrate that high sleep pressure quickly reprograms the wake-promoting large ventrolateral clock neurons to express the pigment dispersing factor receptor (PDFR). The addition of this signaling input into the circuit is associated with increased waking and early mating success. The respecification of PDFR in both young and adult large ventrolateral neurons requires 2 dopamine (DA) receptors and activation of the transcriptional regulator nejire (cAMP response element-binding protein [CREBBP]). These data identify receptor respecification as an important mechanism to sculpt circuit function to match sleep levels with demand.

New Insights Into the Circadian Rhythm and Its Related Diseases

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

Entrainment of the mouse circadian clock: Effects of stress, exercise, and nutrition

The circadian clock system in mammals plays a fundamental role in maintaining homeostasis. Entrainment is an important characteristic of the internal clock, by which appropriate timing is maintained according to external daily stimuli, such as light, stress, exercise, and/or food. Disorganized entrainment or a misaligned clock time, such as jet lag, increases health disturbances. The central clock in the suprachiasmatic nuclei, located in the hypothalamus, receives information about arousal stimuli, such as physical stress or exercise, and changes the clock time by modifying neural activity or the expression of circadian clock genes. Although feeding stimuli cannot entrain the central clock in a normal light-dark cycle, the central clock can partially detect the metabolic status. Local clocks in the peripheral tissues, including liver and kidney, have a strong direct response to the external stimuli of stress, exercise, and/or food that is independent of the central clock. The mechanism underlying entrainment by stress/exercise is mediated by glucocorticoids, sympathetic nerves, oxidative stress, hypoxia, pH, cytokines, and temperature. Food/nutrition-induced entrainment is mediated by fasting-induced hormonal or metabolic changes and re-feeding-induced insulin or oxyntomodulin secretion. Chrono-nutrition is a clinical application based on chronobiology research. Future studies are required to elucidate the effects of eating and nutrient composition on the human circadian clock. Here, we focus on the central and peripheral clocks mostly in rodents' studies and review the findings of recent investigations of the effects of stress, exercise, and food on the entrainment system.

Non-Photic Modulation of Phase Shifts to Long Light Pulses

Circadian rhythms can be reset by both photic and non-photic stimuli. Recent studies have used long light exposure to produce photic phase shifts or to enhance non-photic phase shifts. The presence or absence of light can also influence the expression of locomotor rhythms through masking; light during the night attenuates locomotor activity, while darkness during the day induces locomotor activity in nocturnal animals. Given this dual role of light, the current study was designed to examine the relative contributions of photic and non-photic components present in a long light pulse paradigm. Mice entrained to a light/dark cycle were exposed to light pulses of various durations (0, 3, 6, 9, or 12 h) starting at the time of lights-off. After the light exposure, animals were placed in DD and were either left undisturbed in their home cages or had their wheels locked for the remainder of the subjective night and subsequent subjective day. Light treatments of 6, 9, and 12 h produced large phase delays. These treatments were associated with decreased activity during the nocturnal light and increased activity during the initial hours of darkness following light exposure. When the wheels were locked to prevent high-amplitude activity, the resulting phase delays to the light were significantly attenuated, suggesting that the activity following the light exposure may have contributed to the overall phase shift. In a second experiment, telemetry probes were used to assess what effect permanently locking the wheels had on the phase shift to the long light pulses. These animals had phase shifts fully as large as animals without any form of wheel lock, suggesting that while non-photic events can modulate photic phase shifts, they do not play a role in the full phase-shift response observed in animals exposed to long light pulses. This paradigm will facilitate investigations into non-photic responses of the mouse circadian system.

Phase resetting in duper hamsters: specificity to photic zeitgebers and circadian phase

The duper mutation in Syrian hamsters shortens the free-running period of locomotor activity (τDD) to about 23 h and results in a type 0 phase-response curve (PRC) to 15-min light pulses. To determine whether exaggerated phase shifts are specific to photic cues and/or restricted to subjective night, we subjected hamsters to novel wheel confinements and dark pulses during subjective day. Small phase shifts elicited by the nonphotic cue were comparable in mutant and wild-type (WT) hamsters, but dark pulses triggered larger shifts in dupers. To assess further the effects of the duper mutation on light-dark transitions, we transferred hamsters between constant light (LL) and constant dark (DD) or between DD and LL at various circadian phases. Duper hamsters displayed significantly larger phase shifts than WT hamsters when transferred from LL to DD during subjective day and from DD to LL during subjective night. The variability of phase shifts in response to all light/dark transitions was significantly greater in duper hamsters at all time points. In addition, most duper hamsters, but none of the WTs, displayed transient ultradian wheel-running patterns for 5 to 12 days when transferred from light to dark at CT 18. The χ(2) periodogram and autocorrelation analyses indicate that these ultradian patterns differ from the disruption of rhythmicity by SCN lesions or exposure to constant bright light. We conclude that the duper mutation specifically amplifies phase shifts to photic cues and may destabilize coupling of circadian organization upon photic challenge due to weakened coupling among components of the circadian pacemaker. Mathematical modeling of the circadian pacemaker supports this hypothesis.

Cell injury and repair resulting from sleep loss and sleep recovery in laboratory rats

STUDY OBJECTIVES

Increased cell injury would provide the type of change in constitution that would underlie sleep disruption as a risk factor for multiple diseases. The current study was undertaken to investigate cell injury and altered cell fate as consequences of sleep deprivation, which were predicted from systemic clues.

DESIGN

Partial (35% sleep reduction) and total sleep deprivation were produced in rats for 10 days, which was tolerated and without overtly deteriorated health. Recovery rats were similarly sleep deprived for 10 days, then allowed undisturbed sleep for 2 days. The plasma, liver, lung, intestine, heart, and spleen were analyzed and compared to control values for damage to DNA, proteins, and lipids; apoptotic cell signaling and death; cell proliferation; and concentrations of glutathione peroxidase and catalase.

MEASUREMENTS AND RESULTS

Oxidative DNA damage in totally sleep deprived rats was 139% of control values, with organ-specific effects in the liver (247%), lung (166%), and small intestine (145%). Overall and organ-specific DNA damage was also increased in partially sleep deprived rats. In the intestinal epithelium, total sleep deprivation resulted in 5.3-fold increases in dying cells and 1.5-fold increases in proliferating cells, compared with control. Recovery sleep restored the balance between DNA damage and repair, and resulted in normal or below-normal metabolic burdens and oxidative damage.

CONCLUSIONS

These findings provide physical evidence that sleep loss causes cell damage, and in a manner expected to predispose to replication errors and metabolic abnormalities; thereby providing linkage between sleep loss and disease risk observed in epidemiological findings. Properties of recovery sleep include biochemical and molecular events that restore balance and decrease cell injury.

Effects of caffeine on circadian phase, amplitude and period evaluated in cells in vitro and peripheral organs in vivo in PER2::LUCIFERASE mice

BACKGROUND AND PURPOSE

Caffeine is one of the most commonly used psychoactive substances. Circadian rhythms consist of the main suprachiasmatic nucleus (SCN) clocks and peripheral clocks. Although caffeine lengthens circadian rhythms and modifies phase changes in SCN-operated rhythms, the effects on caffeine on the phase, period and amplitude of peripheral organ clocks are not known. In addition, the role of cAMP/Ca(2+) signalling in effects of caffeine on rhythm has not been fully elucidated.

EXPERIMENTAL APPROACH

We examined whether chronic or transient application of caffeine affects circadian period/amplitude and phase by evaluating bioluminescence rhythm in PER2::LUCIFERASE knock-in mice. Circadian rhythms were monitored in vitro using fibroblasts and ex vivo and in vivo for monitoring of peripheral clocks.

KEY RESULTS

Chronic application of caffeine (0.1-10 mM) increased period and amplitude in vitro. Transient application of caffeine (10 mM) near the bottom of the decreasing phase of bioluminescence rhythm caused phase advance in vitro. Caffeine (0.1%) intake caused a phase delay under light-dark or constant dark conditions, suggesting a period-lengthening effect in vivo. Caffeine (20 mg·kg(-1) ) at daytime or at late night-time caused phase advance or delay in bioluminescence rhythm in the liver and kidney respectively. The complicated roles of cAMP/Ca(2+) signalling may be involved in the caffeine-induced increase of period and amplitude in vitro.

CONCLUSIONS AND IMPLICATIONS

Caffeine affects circadian rhythm in mice by lengthening the period and causing a phase shift of peripheral clocks. These results suggest that caffeine intake with food/drink may help with food-induced resetting of peripheral circadian clocks.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

Chronically inadequate sleep results in abnormal bone formation and abnormal bone marrow in rats

Insufficient sleep over long durations of the lifespan is believed to adversely affect proper development and healthful aging, although how this might become manifested is unknown. In the present study, rats were repeatedly sleep-restricted during 72 days to permit maladaptations to evolve, thereby permitting study. Densitometric and histomorphometric analyses were performed on harvested bone. In sleep-restricted rats, bone lined by osteoid was reduced 45-fold and osteoid thickness was decreased, compared with controls. This corresponded to a decrease in osteoblast number and activity. The percentage of bone lined by osteoclasts did not differ from that of controls. Plasma concentrations of an osteoclast marker (TRACP 5b) were increased in sleep-restricted rats, indicating increased bone resorption. The low amount of new bone formation without a reduction in bone resorption is diagnostic of osteopenia. Bone mineral density was decreased in femurs from sleep-restricted rats compared with controls, indicating osteoporosis. Red marrow in sleep-restricted rats contained only 37% of the fat and more than twice the number of megakaryocytes compared with that of the control rats. These findings in marrow suggest changed plasticity and increased hematopoiesis. Plasma concentrations of insulin-like growth factor-1, a known, major mediator of osteoblast differentiation and the proliferation of progenitor cells, was decreased by 30% in sleep-restricted rats. Taken together, these findings suggest that chronically inadequate sleep affects bone metabolism and bone marrow composition in ways that have implications for development, aging, bone healing and repair, and blood cell differentiation.

Circadian rhythmicity in serotonin transporter knockout mice

AIMS

Serotonin transporter knockout (5-HTT KO) mice exhibit elevated basal extracellular serotonin, increased depressive-like behaviors and increased rapid eye movement sleep. Because abnormalities of circadian rhythms are associated with mood disorders, we tested the hypothesis that 5-HTT KO mice would have altered circadian rhythmicity.

MAIN METHODS

Homecage locomotor activity was recorded in wild-type (WT) and KO mice under a standard 12:12 light-dark cycle. After 4weeks of recording, mice received a one-hour light pulse at circadian time (CT) 14 and then were kept under constant darkness for 3weeks.

KEY FINDINGS

There were no significant differences in amplitude, period, acrophase or total home cage locomotor activity between WT and KO mice during the 12:12 light-dark cycle or during constant darkness. The mean phase delay to a CT 14 light pulse was significantly attenuated in KO compared to WT mice.

SIGNIFICANCE

Acute increases in serotonin have been reported to attenuate photic phase shifts. The current study demonstrates that this effect is maintained in the face of a lifelong absence of 5-HTT.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Repeated exposure to severely limited sleep results in distinctive and persistent physiological imbalances in rats

Chronic sleep disruption in laboratory rats leads to increased energy expenditure, connective tissue abnormalities, and increased weights of major organs relative to body weight. Here we report on expanded findings and the extent to which abnormalities become long-lasting, potentially permanent changes to health status after apparent recuperation from chronic sleep disruption. Rats were exposed 6 times to long periods of disrupted sleep or control conditions during 10 weeks to produce adaptations and then were permitted nearly 4 months of undisturbed sleep. Measurements were made in tissues from these groups and in preserved tissue from the experimental and control groups of an antecedent study that lacked a lengthy recuperation period. Cycles of sleep restriction resulted in energy deficiency marked by a progressive course of hyperphagia and major (15%) weight loss. Analyses of tissue composition in chronically sleep-restricted rats indicated that protein and lipid amounts in internal organs were largely spared, while adipose tissue depots appeared depleted. This suggests high metabolic demands may have preserved the size of the vital organs relative to expectations of severe energy deficiency alone. Low plasma corticosterone and leptin concentrations appear to reflect low substrate availability and diminished adiposity. After nearly 4 months of recuperation, sleep-restricted rats were consuming 20% more food and 35% more water than did comparison control rats, despite normalized weight, normalized adipocytes, and elevated plasma leptin concentrations. Plasma cholesterol levels in recuperated sleep-restricted rats were diminished relative to those of controls. The chronically increased intake of nutriments and water, along with altered negative feedback regulation and substrate use, indicate that internal processes are modified long after a severe period of prolonged and insufficient sleep has ended.

Role of polysialylated neural cell adhesion molecule in rapid eye movement sleep regulation in rats

Recent evidence suggests that synaptic plasticity occurs during homeostatic processes, including sleep-wakefulness regulation, although the underlying mechanisms are not well understood. Polysialylated neural cell adhesion molecule (PSA NCAM) is a transmembrane protein that has been implicated in various forms of plasticity. To investigate whether PSA NCAM is involved in the neuronal plasticity associated with spontaneous sleep-wakefulness regulation and sleep homeostasis, four studies were conducted using rats. First, we showed that PSA NCAM immunoreactivity is present in close proximity to key neurons in several nuclei of the sleep-wakefulness system, including the tuberomammillary hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. Second, using western blot analysis and densitometric image analysis of immunoreactivity, we found that 6 h of sleep deprivation changed neither the levels nor the general location of PSA NCAM in the sleep-wakefulness system. Finally, we injected endoneuraminidase (Endo N) intracerebroventricularly to examine the effects of polysialic acid removal on sleep-wakefulness states and electroencephalogram (EEG) slow waves at both baseline and during recovery from 6 h of sleep deprivation. Endo N-treated rats showed a small but significant decrease in baseline rapid eye movement (REM) sleep selectively in the late light phase, and a facilitated REM sleep rebound after sleep deprivation, as compared with saline-injected controls. Non-REM sleep and wakefulness were unaffected by Endo N. These results suggest that PSA NCAM is not particularly involved in the regulation of wakefulness or non-REM sleep, but plays a role in the diurnal pattern of REM sleep as well as in some aspects of REM sleep homeostasis.

Recurrent restriction of sleep and inadequate recuperation induce both adaptive changes and pathological outcomes

Chronic restriction of a basic biological need induces adaptations to help meet requisites for survival. The adaptations to chronic restriction of sleep are unknown. A single episode of 10 days of partial sleep loss in rats previously was shown to be tolerated and to result in increased food intake and loss of body weight as principal signs. The purpose of the present experiment was to investigate the extent to which adaptation to chronic sleep restriction would ameliorate short-term effects and result in a changed internal phenotype. Rats were studied during 10 wk of multiple periods of restricted and unrestricted sleep to allow adaptive changes to develop. Control rats received the same ambulatory requirements only consolidated into periods that lessened interruptions of their sleep. The results indicate a latent period of relatively stable food and water intake without weight gain, followed by a dynamic phase marked by enormous increases in food and water intake and progressive loss of body weight, without malabsorption of calories. Severe consequences ensued, marked especially by changes to the connective tissues, and became fatal for two individuals. The most striking changes to internal organs in sleep-restricted rats included lengthening of the small intestine, decreased size of adipocytes, and increased incidence of multilocular adipocytes. Major organs accounted for an increased proportion of total body mass. These changes to internal tissues appear adaptive in response to high energy production, decomposition of lipids, and increased need to absorb nutrients, but ultimately insufficient to compensate for inadequate sleep.

Phenobarbital blockade of the preovulatory luteinizing hormone surge: association with phase-advanced circadian clock and altered suprachiasmatic nucleus Period1 gene expression

The suprachiasmatic nucleus (SCN) controls the timing of the preovulatory luteinizing hormone (LH) surge in laboratory rodents. Barbiturate administration during a critical period on proestrus delays the surge and prolongs the estrous cycle 1 day. Because a nonphotic timing signal (zeitgeber) during the critical period that phase advances activity rhythms can also induce the latter effect, we hypothesized that barbiturates delay the LH surge by phase-advancing its circadian timing signal beyond the critical period. In experiment 1, locomotor rhythms and estrous cycles were monitored in hamsters for 2-3 wk preinjection and postinjection of vehicle or phenobarbital and after transfer to darkness at zeitgeber time (ZT) 6 on proestrus. Phenobarbital delayed estrous cycles in five of seven hamsters, which exhibited phase shifts that averaged twofold greater than those exhibited by vehicle controls or phenobarbital-injected hamsters with normal cycles. Experiment 2 used a similar protocol, but injections were at ZT 5, and blood samples for LH determination were collected from 1200 to 1800 on proestrus and the next day via jugular cannulae inserted the day before proestrus. Phenobarbital delayed the LH surge 1 day in all six hamsters, but it occurred at an earlier circadian time, supporting the above hypothesis. Experiment 3 investigated whether phenobarbital, like other nonphotic zeitgebers, suppresses SCN Period1 and Period2 transcription. Two hours postinjection, phenobarbital decreased SCN expression of only Period1 mRNA, as determined by in situ hybridization. These results suggest that phenobarbital advances the SCN pacemaker, governing activity rhythms and hormone release in part by decreasing its Period1 gene expression.

Circadian and dark-pulse activation of orexin/hypocretin neurons

Temporal control of brain and behavioral states emerges as a consequence of the interaction between circadian and homeostatic neural circuits. This interaction permits the daily rhythm of sleep and wake, regulated in parallel by circadian cues originating from the suprachiasmatic nuclei (SCN) and arousal-promoting signals arising from the orexin-containing neurons in the tuberal hypothalamus (TH). Intriguingly, the SCN circadian clock can be reset by arousal-promoting stimuli while activation of orexin/hypocretin neurons is believed to be under circadian control, suggesting the existence of a reciprocal relationship. Unfortunately, since orexin neurons are themselves activated by locomotor promoting cues, it is unclear how these two systems interact to regulate behavioral rhythms. Here mice were placed in conditions of constant light, which suppressed locomotor activity, but also revealed a highly pronounced circadian pattern in orexin neuronal activation. Significantly, activation of orexin neurons in the medial and lateral TH occurred prior to the onset of sustained wheel-running activity. Moreover, exposure to a 6 h dark pulse during the subjective day, a stimulus that promotes arousal and phase advances behavioral rhythms, activated neurons in the medial and lateral TH including those containing orexin. Concurrently, this stimulus suppressed SCN activity while activating cells in the median raphe. In contrast, dark pulse exposure during the subjective night did not reset SCN-controlled behavioral rhythms and caused a transient suppression of neuronal activation in the TH. Collectively these results demonstrate, for the first time, pronounced circadian control of orexin neuron activation and implicate recruitment of orexin cells in dark pulse resetting of the SCN circadian clock.

Phagocyte migration and cellular stress induced in liver, lung, and intestine during sleep loss and sleep recovery

Sleep is understood to possess recuperative properties and, conversely, sleep loss is associated with disease and shortened life span. Despite these critical attributes, the mechanisms and functions by which sleep and sleep loss impact health still are speculative. One of the most consistent, if largely overlooked, signs of sleep loss in both humans and laboratory rats is a progressive increase in circulating phagocytic cells, mainly neutrophils. The destination, if any, of the increased circulating populations has been unknown and, therefore, its medical significance has been uncertain. The purpose of the present experiment was to determine the content and location of neutrophils in liver and lung tissue of sleep-deprived rats. These are two principal sites affected by neutrophil migration during systemic inflammatory illness. The content of neutrophils in the intestine also was determined. Sleep deprivation in rats was produced for 5 and 10 days by the Bergmann-Rechtschaffen disk method, which has been validated for its high selectivity under freely moving conditions and which was tolerated and accompanied by a deep negative energy balance. Comparison groups included basal conditions and 48 h of sleep recovery after 10 days of sleep loss. Myeloperoxidase (MPO), an enzyme constituent of neutrophils, was extracted from liver, lung, and intestinal tissues, and its activity was determined by spectrophotometry. Leukocytes were located in vasculature and interstitial spaces in the liver and the lung by immunohistochemistry. Heme oxygenase-1, also known as heat shock protein-32 and a marker of cellular stress, and corticosterone also were measured. The results indicate neutrophil migration into extravascular liver and lung tissue concurrent with cell stress and consistent with tissue injury or infection induced by sleep loss. Plasma corticosterone was unchanged. Recovery sleep was marked by increased lung heme oxygenase-1, increased intestinal MPO activity, and abnormally low corticosterone, suggesting ongoing reactive processes as a result of prior sleep deprivation.

Shedding light on circadian clock resetting by dark exposure: differential effects between diurnal and nocturnal rodents

The master circadian clock in mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, is entrained by light and behavioural stimulation. In addition, the SCN can be reset by dark pulses in nocturnal rodents under constant light conditions. Here, the shifting effects of a dark pulse on the SCN clock were detailed at both a behavioural and molecular level in a nocturnal rodent (Syrian hamster), and were compared to those of a diurnal rodent (Arvicanthis ansorgei). Four-hour dark pulses led to phase advances in the circadian rhythm of locomotor activity from subjective midday to dusk in hamsters, but from subjective dusk to midnight in Arvicanthis. Moreover, dark pulses had no resetting effect during the middle of the subjective night in hamsters, while such a dead shifting zone occurred during most of the subjective day in Arvicanthis. The behavioural phase advances in both hamsters and Arvicanthis were most often accompanied by marked downregulation of the clock genes Per1 and/or Per2 in the SCN, and also by changes in the transforming growth factor-alpha expression, a neuropeptide that suppresses daytime activity in nocturnal mammals. Despite that both hamsters and Arvicanthis showed dark-induced phase advances at circadian time-12, Per1 gene and its protein PER1 were downregulated in Arvicanthis but not in hamsters. Altogether these results show that dark resetting of the SCN is always associated with downregulation of Per1 and/or Per2 expression, and mostly occurs during resting. Thus, the circadian window of sensitivity to dark differs between nocturnal and diurnal rodents.

Addition of a non-photic component to a light-based mathematical model of the human circadian pacemaker

Mathematical models have become vital to the study of many biological processes in humans due to the complexity of the physiological mechanisms underlying these processes and systems. While our current mathematical representation of the human circadian pacemaker has proven useful in many experimental situations, it uses as input only a direct effect of light on the circadian pacemaker. Although light (a photic stimulus) has been shown to be the primary synchronizer of the circadian pacemaker across a number of species, studies in both animals and humans have confirmed the existence of non-photic effects that also contribute to phase shifting and entrainment. We modified our light-based circadian mathematical model to reflect evidence from these studies that the sleep-wake cycle and/or associated behaviors have a non-photic effect on the circadian pacemaker. In our representation, the sleep-wake cycle and its associated behaviors provides a non-photic drive on the circadian pacemaker that acts both independently and concomitantly with light stimuli. Further experiments are required to validate fully our model and to understand the exact effect of the sleep-wake cycle as a non-photic stimulus for the human circadian pacemaker.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

The enigma of behavioral inputs to the circadian clock: A test of function using restraint

Wheel running stimulated during the daily rest period can acutely shift circadian rhythms in Syrian hamsters. Spontaneous running, defining the active phase of the circadian rest-activity cycle, can shorten the circadian periodicity in constant light or dark in several nocturnal rodent species. The adaptive significance of these behavioral effects on pacemaker phase and period is unclear. Here we consider a hypothesis that behavioral inputs to the circadian pacemaker serve primarily to enhance the precision of light-dark entrainment and maintain daily activity onset close to lights-off (i.e., dusk) by stabilizing entrainment on a steeper portion of the delay zone of the phase-response curve to light. This hypothesis rests on the evidence that spontaneous activity early in the active period feeds back on the pacemaker to advance its motion. If so, then preventing activity at this time should induce a phase delay shift. Such delay shifts have been reported in Syrian hamsters physically restrained early in the active period. We show here that restraint can induce phase delays but that, using the Aschoff Type 2 procedure for measuring shifts, these delays are very small, are inversely related to behavioral sleep during restraint, and are positively correlated with 'rebound' increases in running following restraint, at a circadian time when stimulated running is known to induce phase delay shifts. Repeated bouts of restraint, to promote habituation, were associated with strong attenuation of 'rebound' running and no significant delay shifts. These results suggest that, in Syrian hamsters, spontaneous activity early at night has little effect on pacemaker motion, and argue against the stated hypothesis.

Resetting of the hamster circadian system by dark pulses

Circadian rhythms of animals are reset by exposure to light as well as dark; however, although the parameters of photic entrainment are well characterized, the phase-shifting actions of dark pulses are poorly understood. Here, we determined the tonic and phasic effects of short (0.25 h), moderate (3 h), and long (6-9 h) duration dark pulses on the wheel-running rhythms of hamsters in constant light. Moderate- and long-duration dark pulses phase dependently reset behavioral rhythms, and the magnitude of these phase shifts increased as a function of the duration of the dark pulse. In contrast, the 0.25-h dark pulses failed to evoke consistent effects at any circadian phase tested. Interestingly, moderate- and long-dark pulses elevated locomotor activity (wheel-running) on the day of treatment. This induced wheel-running was highly correlated with phase shift magnitude when the pulse was given during the subjective day. This, together with the finding that animals pulsed during the subjective day are behaviorally active throughout the pulse, suggests that both locomotor activity and behavioral activation play an important role in the phase-resetting actions of dark pulses. We also found that the robustness of the wheel-running rhythm was weakened, and the amount of wheel-running decreased on the days after exposure to dark pulses; these effects were dependent on pulse duration. In summary, similarly to light, the resetting actions of dark pulses are dependent on both circadian phase and stimulus duration. However, dark pulses appear more complex stimuli, with both photic and nonphotic resetting properties.

Clinical assessment of blood leukocytes, serum cytokines, and serum immunoglobulins as responses to sleep deprivation in laboratory rats

The specific systems and mechanisms affected by sleep deprivation that may perpetuate disease processes in humans still are speculative. In laboratory rats, prolonged sleep deprivation induces a state marked by abnormal control over indigenous bacteria that results in transient infections of internal tissues and eventual lethal septicemia. The present studies investigated changes in blood, serum, and bone marrow parameters that may provide diagnostic clues to immunopathology. Prolonged sleep deprivation was produced in rats by the disk-over-water method, a well-established and selective means that does not interfere with normal waking behaviors. Measurements included bone and blood differential white blood cell counts, multiple serum cytokines and chemokines, several major Ig classes and subclasses, and serum endotoxin concentrations. The results indicated mild, regenerative neutrophilia in sleep-deprived rats, initially accompanied by immature neutrophils and later by monocytosis. The corresponding serum cytokine profile revealed an evolving proinflammatory state, particularly by high incidence of interleukin-1β, implicating mononuclear phagocytes and resident tissue cells as main intermediary sources. In addition, multiple serum Ig classes were increased by sleep deprivation without experimental administration of an exogenous antigen. Despite this immune activation, there was failure to eradicate invading bacteria and toxins, suggesting competing anti-inflammatory processes or interference with immune effector functions during sleep deprivation. Nearly all of the immune-related events that emerged as responses to sleep deprivation have been implicated as etiological or provocative factors in other disease processes and may provide means by which sleep deprivation as a risk factor in disease may become understood.

Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

Circadian rhythms in Syrian hamsters can be phase shifted by behavioral arousal during the usual rest phase of the circadian rest-activity cycle. Phase shifts can be greatly potentiated by exposing the animals to constant light for 1 or 2 cycles. This could reflect a change in a specific nonphotic input pathway to the suprachiasmatic nucleus (SCN) circadian pacemaker, or it could be caused by a change in the amplitude of the pacemaker. If the latter, then phase shifts to any stimulus, including those activating the photic input pathway, should be potentiated. This hypothesis was tested by measuring phase shifts induced by microinjections of NMDA (500 nl, 10 mM) into the SCN area of hamsters exposed to constant light or dark for 2 days. NMDA induced significant phase delay shifts that mimicked those induced by light exposure early in the night. The magnitude of these shifts did not differ by prior lighting condition. Shifts induced by NMDA (200 nl, 10 mM) microinjections on day 3 and 13 of LL also did not differ. Phase shifts induced by a nonphotic stimulus (3 h of running stimulated by confinement to a novel wheel) were significantly potentiated by 2 days of exposure to constant light. These results indicate that exposure to constant light for 2 circadian cycles differentially affects phase resetting responses to photic and nonphotic inputs to the circadian pacemaker, suggesting that potentiation of shifts to nonphotic stimuli reflect changes in a nonphotic input pathway rather than in an amplitude dimension of the circadian pacemaker.

Short-term exposure to constant light promotes strong circadian phase-resetting responses to nonphotic stimuli in Syrian hamsters

Behavioral (nonphotic) stimuli can shift circadian rhythms by serotonin (5-HT) and/or neuropeptide Y (NPY) inputs to the suprachiasmatic nucleus (SCN) circadian clock. Based on the idea that behavioral phase resetting is modulated by endogenous changes in postsynaptic sensitivity to such transmitters, hamsters were exposed to constant light (LL; approximately 250 lx) for 1-3 days, which suppresses locomotor activity and eliminates the daily rhythm of SCN 5-HT release measured by microdialysis. Groups subjected to brief LL or maintained under a light/dark cycle (LD) received phase-resetting treatments with the 5-HT(1A,7) agonist (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) or sleep deprivation (SD). Animals were released to constant darkness at the start of the treatments. Phase advances to 8-OH-DPAT and SD during the day were 11 and 3 h for LL vs. 2 and 1 h for LD, respectively. Phase delays during the night were -12 and -5 h for LL vs. no responses for LD, respectively. Phase-transition curves for both LL treatments had slopes approximating 0, indicative of Type 0 phase resetting. For all treatments, the degree of locomotor suppression by LL was not correlated with the phase shift magnitude. Re-establishing locomotor activity by overnight food deprivation did not prevent potentiated shifting to SD. However, re-establishing peak extracellular 5-HT levels by intra-SCN 5-HT reverse microdialysis perfusion in LL did significantly reduce potentiated 8-OH-DPAT phase advances. Constant light also enhanced intra-SCN NPY-induced phase advances during the day (6 vs. 2 h for LD). These results suggest that LL promotes Type 0 phase resetting by supersensitizing 5-HT and/or NPY postsynaptic responses and possibly by attenuating the amplitude of the circadian pacemaker, thus enhancing circadian clock resetting nonspecifically.

Activity patterns as a correlate for sleep–wake behaviour in mice

Sleep-wake behaviour in mice is known to interact with various behavioural dimensions. Therefore, it is necessary to control for such dimensions when evaluating sleep in mice. The characterisation of sleep in rodents usually is based on EEG signals. Since this method demands the invasive implantation of electrodes, it cannot be integrated into general behavioural phenotyping procedures. Thus, non- or minimum-invasive methods are needed for the analysis of sleep-wake behaviour. Although physiological parameters, like for instance general locomotor activity, allow for the assessment of sleep-wake behaviour in mice, existing methods lack reliability especially in measuring stationary and three-dimensional activities. In this study, a small magnet was implanted subcutaneously near the neck muscles of mice and each movement of the magnet was registered via a sensor plate. For validation of the described method, the effects of sleep deprivation were evaluated by both the magnet and the EEG in parallel. Our results show that the data obtained via the subcutaneously implanted magnet represent a reliable and sensitive measurement of quantitative aspects of sleep-wake behaviour: spatial variation as well as stationary activities could be dissociated from sleep. Qualitative sleep characteristics were not detected. In summary, this minimum invasive method allows for the detection of quantitative alterations in sleep-wake behaviour in mice, thus, offering a useful, rapid pre-screen in animal sleep research.

Dark pulse resetting of the suprachiasmatic clock in Syrian hamsters: behavioral phase-shifts and clock gene expression

In mammals, the circadian clock in the suprachiasmatic nuclei (SCN) is mainly synchronized to photic cues provided by the daily light/dark cycle. Phase-shifts produced by light exposure during the night are correlated with rapid induction of two clock genes, Per1 and Per2, in the SCN. Nonphotic stimuli such as behavioral and pharmacological cues, when presented during the subjective day, induce behavioral phase-advances and a down-regulation of Per1 and Per2 expression in the SCN. When applied during the subjective day, dark pulses in continuous light also produce phase-advances. These phase-shifting effects have been interpreted as reflecting either a photic image mirror, nonphotic cues, or a combination of both. Here we evaluated in Syrian hamsters housed in constant light how dark pulses applied in late subjective day affect levels of Per1, Per2 and Cry1 mRNA. Four-hour dark pulses with no access to a wheel produced 1.2+/-0.4 h phase-advances of locomotor activity rhythm while control manipulation induced non-significant shifts (0.1+/-0.2 h). Dark pulses transiently down-regulated Per1 and Per2 mRNA levels in the SCN by 40 and 20% respectively, while the levels of Cry1 mRNA remained unaffected. In behaviorally split hamsters in which Per oscillations were asymmetric between the left and right sides of the SCN, dark pulses reduced Per expression in the half-SCN with high Per. This study shows that exposure during the late subjective day to dark pulses independent of wheel-running have nonphotic-like effects on the SCN clock at both behavioral and molecular levels.

Circadian clock resetting by arousal in Syrian hamsters: the role of stress and activity

Circadian rhythms in the Syrian hamster can be markedly phase shifted by 3 h of wheel running or arousal stimulation during their usual daily rest period ("subjective day"). Continuous wheel running is predictive but not necessary for phase shifts of this "nonphotic" type; hamsters aroused by gentle handling without running can also show maximal shifts. By contrast, physical restraint, a standard stress procedure and thus presumably arousing, is ineffective. To resolve this apparent paradox, phase-shifting effects of 3-h sessions of restraint or other stress procedures were assessed. In a preliminary study, phase shifts to arousal by gentle handling were significantly potentiated by the cortisol synthesis inhibitor metyrapone, suggesting that stress-related cortisol release may inhibit phase shifts to arousal. Next, it was confirmed that restraint in the subjective day does not induce phase shifts, but behavioral observations revealed that it also does not sustain arousal. Restraint combined with noxious compressed air blasts did sustain arousal and induced a significant cortisol response compared with arousal by gentle handling but did not induce shifts. Restraint combined with continuous horizontal rotation was also ineffective, as was EEG-validated arousal via confinement to a pedestal over water. However, 3 h of resident-intruder interactions (an intense psychosocial stress) or exposure to an open field (a mild stress) did induce large shifts that were positively correlated with indexes of forward locomotion. The results indicate that large phase shifts associated with arousal in the usual sleep period are neither induced nor prevented by stress per se, but are dependent on the expression of at least low levels of locomotor activity. Sustained arousal alone is not sufficient.

Invited review: Sleeping flies don't lie: the use of Drosophila melanogaster to study sleep and circadian rhythms

During the past century, flies thoroughly proved their value as an animal model for the study of the genetics of development and basic cell processes. During the past three decades, they have also been extensively used to study the genetics of behavior. For both circadian rhythms and for sleep, flies are helping us to understand the genetic mechanisms that underlie these complex behaviors. Since 1971, discoveries in the fly have led the way to a number of significant discoveries, establishing a mechanistic framework that is now known to be conserved in the mammalian clock. The highlights of this history are described. For sleep, the use of the fly as a model is relatively new, that is, only within the past 2 yr. Nonetheless, studies have already established that two transcription factors alter rest and rest homeostasis. The implications of these advances for the future of sleep research are summarized.

Phase shifting the hamster circadian clock by 15-minute dark pulses

The mammalian circadian pacemaker can be phase shifted by exposure to a period of darkness interrupting otherwise continuous light. Circadian phase shifting by dark pulses was interpreted originally as reflecting a photic mirror-image mechanism, but more recent observations suggest that dark pulse-induced phase shifting may be mediated by a nonphotic, behavioral state-dependent mechanism. The authors recently presented evidence indicating that the dark-pulse phase response curve (PRC) is in fact a complex function, reflecting both photic mirror image and nonphotic mechanisms at different phases of the circadian cycle. Previous studies of dark pulse-induced phase shifting have universally employed relatively long (2 to 6 h) pulses, which complicates PRC analysis due to the extended segment of the underlying PRC spanned by such a long pulse. The present study was therefore designed to examine the phase-shifting effects of brief 15-min dark pulses presented at both mid-subjective day and subjective dusk, and to explore the possible activity dependence of these effects by using physical restraint to prevent evoked locomotor activity. The results indicate that 15-min dark pulses are effective phase-shifting stimuli at both midday and dusk. Furthermore, as with longer dark pulses, phase shifting by 15-min dark pulses is completely blocked by physical restraint during subjective day but combines in a simple additive manner with the independent phase-shifting effect of restraint at subjective dusk.