Cycle of period gene expression in a diurnal mammal (Spermophilus tridecemlineatus): implications for nonphotic phase shifting

Energy balance drives diurnal and nocturnal brain transcriptome rhythms

Plasticity in daily timing of activity has been observed in many species, yet the underlying mechanisms driving nocturnality and diurnality are unknown. By regulating how much wheel-running activity will be rewarded with a food pellet, we can manipulate energy balance and switch mice to be nocturnal or diurnal. Here, we present the rhythmic transcriptome of 21 tissues, including 17 brain regions, sampled every 4 h over a 24-h period from nocturnal and diurnal male CBA/CaJ mice. Rhythmic gene expression across tissues comprised different sets of genes with minimal overlap between nocturnal and diurnal mice. We show that non-clock genes in the suprachiasmatic nucleus (SCN) change, and the habenula was most affected. Our results indicate that adaptive flexibility in daily timing of behavior is supported by gene expression dynamics in many tissues and brain regions, especially in the habenula, which suggests a crucial role for the observed nocturnal-diurnal switch.

The genomic basis of temporal niche evolution in a diurnal rodent

Patterns of diel activity-how animals allocate their activity throughout the 24-h daily cycle-play key roles in shaping the internal physiology of an animal and its relationship with the external environment.1,2,3,4,5 Although shifts in diel activity patterns have occurred numerous times over the course of vertebrate evolution,6 the genomic correlates of such transitions remain unknown. Here, we use the African striped mouse (Rhabdomys pumilio), a species that transitioned from the ancestrally nocturnal diel niche of its close relatives to a diurnal one,7,8,9,10,11 to define patterns of naturally occurring molecular variation in diel niche traits. First, to facilitate genomic analyses, we generate a chromosome-level genome assembly of the striped mouse. Next, using transcriptomics, we show that the switch to daytime activity in this species is associated with a realignment of daily rhythms in peripheral tissues with respect to the light:dark cycle and the central circadian clock. To uncover selection pressures associated with this temporal niche shift, we perform comparative genomic analyses with closely related rodent species and find evidence of relaxation of purifying selection on striped mouse genes in the rod phototransduction pathway. In agreement with this, electroretinogram measurements demonstrate that striped mice have functional differences in dim-light visual responses compared with nocturnal rodents. Taken together, our results show that striped mice have undergone a drastic change in circadian organization and provide evidence that the visual system has been a major target of selection as this species transitioned to a novel temporal niche.

Circadian Rhythms and Melatonin Metabolism in Patients With Disorders of Gut-Brain Interactions

Circadian rhythms are cyclic patterns of physiological, behavioural and molecular events that occur over a 24-h period. They are controlled by the suprachiasmatic nucleus (SCN), the brain’s master pacemaker which governs peripheral clocks and melatonin release. While circadian systems are endogenous, there are external factors that synchronise the SCN to the ambient environment including light/dark cycles, fasting/fed state, temperature and physical activity. Circadian rhythms also provide internal temporal organisation which ensures that any internal changes that take place are centrally coordinated. Melatonin synchronises peripheral clocks to the external time and circadian rhythms are regulated by gene expression to control physiological function. Synchronisation of the circadian system with the external environment is vital for the health and survival of an organism and as circadian rhythms play a pivotal role in regulating GI physiology, disruption may lead to gastrointestinal (GI) dysfunction. Disorders of gut-brain interactions (DGBIs), also known as functional gastrointestinal disorders (FGIDs), are a group of diseases where patients experience reoccurring gastrointestinal symptoms which cannot be explained by obvious structural abnormalities and include functional dyspepsia (FD) and irritable bowel syndrome (IBS). Food timing impacts on the production of melatonin and given the correlation between food intake and symptom onset reported by patients with DGBIs, chronodisruption may be a feature of these conditions. Recent advances in immunology implicate circadian rhythms in the regulation of immune responses, and DGBI patients report fatigue and disordered sleep, suggesting circadian disruption. Further, melatonin treatment has been demonstrated to improve symptom burden in IBS patients, however, the mechanisms underlying this efficacy are unclear. Given the influence of circadian rhythms on gastrointestinal physiology and the immune system, modulation of these rhythms may be a potential therapeutic option for reducing symptom burden in these patients.

Nighttime Light Hurts Mammalian Physiology: What Diurnal Rodent Models Are Telling Us

Natural sunlight permits organisms to synchronize their physiology to the external world. However, in current times, natural sunlight has been replaced by artificial light in both day and nighttime. While in the daytime, indoor artificial light is of lower intensity than natural sunlight, leading to a weak entrainment signal for our internal biological clock, at night the exposure to artificial light perturbs the body clock and sleep. Although electric light at night allows us "to live in darkness", our current lifestyle facilitates nighttime exposure to light by the use, or abuse, of electronic devices (e.g., smartphones). The chronic exposure to light at nighttime has been correlated to mood alterations, metabolic dysfunctions, and poor cognition. To decipher the brain mechanisms underlying these alterations, fundamental research has been conducted using animal models, principally of nocturnal nature (e.g., mice). Nevertheless, because of the diurnal nature of human physiology, it is also important to find and propose diurnal animal models for the study of the light effects in circadian biology. The present review provides an overview of the effects of light at nighttime on physiology and behavior in diurnal mammals, including humans. Knowing how the brain reacts to artificial light exposure, using diurnal rodent models, is fundamental for the development of new strategies in human health based in circadian biology.

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

Circadian and photic modulation of daily rhythms in diurnal mammals

The temporal niche that an animal occupies includes a coordinated suite of behavioral and physiological processes that set diurnal and nocturnal animals apart. The daily rhythms of the two chronotypes are regulated by both the circadian system and direct responses to light, a process called masking. Here we review the literature on circadian regulations and masking responses in diurnal mammals, focusing on our work using the diurnal Nile grass rat (Arvicanthis niloticus) and comparing our findings with those derived from other diurnal and nocturnal models. There are certainly similarities between the circadian systems of diurnal and nocturnal mammals, especially in the phase and functioning of the principal circadian oscillator within the hypothalamic suprachiasmatic nucleus (SCN). However, the downstream pathways, direct or indirect from the SCN, lead to drastic differences in the phase of extra-SCN oscillators, with most showing a complete reversal from the phase seen in nocturnal species. This reversal, however, is not universal and in some cases the phases of extra-SCN oscillators are only a few hours apart between diurnal and nocturnal species. The behavioral masking responses in general are opposite between diurnal and nocturnal species, and are matched by differential responses to light and dark in several retinorecipient sites in their brain. The available anatomical and functional data suggest that diurnal brains are not simply a phase-reversed version of nocturnal ones, and work with diurnal models contribute significantly to a better understanding of the circadian and photic modulation of daily rhythms in our own diurnal species.

Medicine in the Fourth Dimension

The importance of circadian biology has rarely been considered in pre-clinical studies, and even more when translating to the bedside. Circadian biology is becoming a critical factor for improving drug efficacy and diminishing drug toxicity. Indeed, there is emerging evidence showing that some drugs are more effective at nighttime than daytime, whereas for others it is the opposite. This suggests that the biology of the target cell will determine how an organ will respond to a drug at a specific time of the day, thus modulating pharmacodynamics. Thus, it is now time that circadian factors become an integral part of translational research.

Time-shifting effects of methylphenidate on daily rhythms in the diurnal rodent Arvicanthis ansorgei

People suffering of attention-deficit/hyperactivity disorder (ADHD) and treated with the psychostimulant methylphenidate (MPH) show sleep-wake cycle and daily rhythm alterations despite the beneficial effects of MPH on behavioral symptoms (i.e., hyperactivity, attention). In nocturnal rodents (i.e., mice), chronic exposure to MPH alters the neural activity of the circadian clock in the suprachiasmatic nucleus (SCN), behavioral rhythms, and the sleep-wake cycle. Here, we studied the effects of MPH on daily rhythms of behavior and body temperature of the diurnal rodent Arvicanthis ansorgei. Under a light-dark cycle, chronic exposure to MPH in drinking water delayed the onset of both activity and body temperature rhythms. Interestingly, delays were larger when MPH access was restricted to the first 6 h of the light phase (i.e., activity phase) of the 24-h cycle. Since MPH effects are dependent on animal's fluid intake, in a last experiment, we controlled the time and dose of MPH delivery in Arvicanthis using an intraperitoneal perfusion method. Similarly to the experiment with MPH in drinking water, Arvicanthis showed a delay in the onset of general activity and body temperature when MPH infusions, but not vehicle, were during the first 6 h of the light phase. This study indicates that MPH alters daily rhythms in a time-dependent manner and proposes the use of a diurnal rodent for the study of the effects of MPH on the circadian clock. Knowing the circadian modulation on the effects of MPH in behavior could give new insights in the treatment of ADHD.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

Sleep Deprivation and Caffeine Treatment Potentiate Photic Resetting of the Master Circadian Clock in a Diurnal Rodent

Circadian rhythms in nocturnal and diurnal mammals are primarily synchronized to local time by the light/dark cycle. However, nonphotic factors, such as behavioral arousal and metabolic cues, can also phase shift the master clock in the suprachiasmatic nuclei (SCNs) and/or reduce the synchronizing effects of light in nocturnal rodents. In diurnal rodents, the role of arousal or insufficient sleep in these functions is still poorly understood. In the present study, diurnal Sudanian grass rats, Arvicanthis ansorgei, were aroused at night by sleep deprivation (gentle handling) or caffeine treatment that both prevented sleep. Phase shifts of locomotor activity were analyzed in grass rats transferred from a light/dark cycle to constant darkness and aroused in early night or late night. Early night, but not late night, sleep deprivation induced a significant phase shift. Caffeine on its own induced no phase shifts. Both sleep deprivation and caffeine treatment potentiated light-induced phase delays and phase advances in response to a 30 min light pulse, respectively. Sleep deprivation in early night, but not late night, potentiated light-induced c-Fos expression in the ventral SCN. Caffeine treatment in midnight triggered c-Fos expression in dorsal SCN. Both sleep deprivation and caffeine treatment potentiated light-induced c-Fos expression in calbindin-containing cells of the ventral SCN in early and late night. These findings indicate that, in contrast to nocturnal rodents, behavioral arousal induced either by sleep deprivation or caffeine during the sleeping period potentiates light resetting of the master circadian clock in diurnal rodents, and activation of calbindin-containing suprachiasmatic cells may be involved in this effect.SIGNIFICANCE STATEMENT Arousing stimuli have the ability to regulate circadian rhythms in mammals. Behavioral arousal in the sleeping period phase shifts the master clock in the suprachiasmatic nuclei and/or slows down the photic entrainment in nocturnal animals. How these stimuli act in diurnal species remains to be established. Our study in a diurnal rodent, the Grass rat, indicates that sleep deprivation in the early rest period induces phase delays of circadian locomotor activity rhythm. Contrary to nocturnal rodents, both sleep deprivation and caffeine-induced arousal potentiate the photic entrainment in a diurnal rodent. Such enhanced light-induced circadian responses could be relevant for developing chronotherapeutic strategies.

Differential Phasing between Circadian Clocks in the Brain and Peripheral Organs in Humans

The daily timing of mammalian physiology is coordinated by circadian clocks throughout the body. Although measurements of clock gene expression indicate that these clocks in mice are normally in phase with each other, the situation in humans remains unclear. We used publicly available data from five studies, comprising over 1000 samples, to compare the phasing of circadian gene expression in human brain and human blood. Surprisingly, after controlling for age, clock gene expression in brain was phase-delayed by ~8.5 h relative to that of blood. We then examined clock gene expression in two additional human organs and in organs from nine other mammalian species, as well as in the suprachiasmatic nucleus (SCN). In most tissues outside the SCN, the expression of clock gene orthologs showed a phase difference of ~12 h between diurnal and nocturnal species. The exception to this pattern was human brain, whose phasing resembled that of the SCN. Our results highlight the value of a multi-tissue, multi-species meta-analysis, and have implications for our understanding of the human circadian system.

Clock Genes, Oscillators, and Cellular Networks in the Suprachiasmatic Nuclei

The mammalian SCN contains a biological clock that drives remarkably precise circadian rhythms in vivo and in vitro. Recent advances have revealed molecular and cellular mechanisms required for the generation of these daily rhythms and their synchronization between SCN neurons and to the environmental light cycle. This review of the evidence for a cell-autonomous circadian pacemaker within specialized neurons of the SCN focuses on 6 genes implicated within the pace making mechanism, an additional 4 genes implicated in pathways from the pacemaker, and the intercellular and intracellular mechanisms that synchronize SCN neurons to each other and to solar time.

Nocturnal Activity in a Diurnal Rodent (Arvicanthis Niloticus): The Importance of Masking

It is known that day-active Nile grass rats, Arvicanthis niloticus, increase the amount of activity in the night relative to that in the day when provided with running wheels. This was confirmed in the present study. Animals without a wheel displayed 69.0% of their general activity in the L phase of a 12:12 h light-dark cycle; animals provided with wheels had only 48.6% of their wheel revolutions in the light. The contribution of direct (masking) responses to light to the increased nocturnality of animals with wheels was examined in two experiments. In experiment 1, masking was tested by exposing the animals to repeated cycles of 30 min of entraining light and 30 min of a different, usually dimmer light, during the L phase of a 12:12 h light-dark cycle. For animals with wheels, there was more running during the 30-min pulses of dim light or darkness than during the 30-min periods of entraining light. In contrast, for animals without wheels, there was more general activity during the 30-min periods of entraining light than during the 30-min pulses of dim light or darkness. In experiment 2, the animals were first exposed to a 12:12 h light-dark cycle and then put on a 1:10:1:12 h LDLD skeleton photoperiod. Animals with wheels increased their running during the subjective day of the skeleton photoperiod compared to that in the actual day of the 12:12 h light-dark cycle. Animals without wheels showed similar levels of general activity during the subjective day of the skeleton photoperiod and the actual day of the 12:12 h cycle. These experiments demonstrate that when Nile rats have running wheels, their increased nocturnal activity is associated with an increased suppression of locomotion in direct response to light. It is possible that changes in masking responses to light may be an essential and integral component of switching between diurnal and nocturnal activity profiles.

Mammalian Diurnality: Some Facts and Gaps

A major factor contributing to the evolution of mammals was their ability to be active during the night, a niche previously underused by terrestrial vertebrates. Diurnality subsequently reemerged multiple times in a variety of independent lineages. This paper reviews some recent data on circadian mechanisms in diurnal mammals and considers general themes that appear to be emerging from this work. Careful examination of behavioral studies suggests that although subtle differences may exist, the fundamental functions of the circadian system are the same, as seems to be the case with respect to the molecular mechanisms of the clock. This suggests that responses to signals originating in the clock must be different, either within the SCN or at its targets or downstream from them. Some features of the SCN vary from species to species, but none of these has been clearly associated with diurnality. The region immediately dorsal to the SCN, which receives substantial input from it, exhibits dramatically different rhythms in nocturnal lab rats and diurnal grass rats. This raises the possibility that it functions as a relay that transforms the signal emitted by the SCN and transmits different patterns to downstream targets in nocturnal and diurnal animals. Other direct targets of the SCN include neurons containing orexin and those containing gonadotropin-releasing hormone, and both of these populations of cells exhibit patterns of rhythmicity that are inverted in at least one diurnal compared to one nocturnal species. The patterns that emerge from the data on diurnality are discussed in terms of the implications they have for the evolution and neural substrates of a day-active way of life.

Analysis of the Prokineticin 2 System in a Diurnal Rodent, the Unstriped Nile Grass Rat (Arvicanthis niloticus)

Prokineticin 2 (PK2) is a putative output molecule from the SCN. PK2 RNA levels are rhythmic in the mouse SCN, with high levels during the day, and PK2 administration suppresses nocturnal locomotor activity in rats. The authors examined the PK2 system in a diurnal rodent, Arvicanthis niloticus, to determine whether PK2 or PK2 receptors differ between diurnal and nocturnal species. The major transcript variant of A. niloticus PK2 ( AnPK2) encodes a 26-residue signal peptide followed by the presumed mature peptide of 81 residues. Within the grass rat signal sequence, polymorphic sequences and amino acid substitutions were observed relative to mouse and laboratory rats, but the hydrophobic core and cleavage site of the signal sequence were preserved. The mature PK2 peptide is identical among A. niloticus, rat, and mouse. AnPK2 mRNA is rhythmically expressed in the SCN, with peak RNAlevels occurring in the morning, preceding peaks of Per1 and Per2 as in mouse SCN. Analysis of prokineticin receptor 2 (PKR2) sequences revealed polymorphisms among the grass rats studied. PKR2 mRNAwas expressed in the SCN and paraventricular nuclei of the thalamus and hypothalamus. While further analysis is necessary, there is no clear evidence indicating that a difference in the PK2 ligand/receptor system accounts for diurnality in this rodent species. These data contribute to a growing body of evidence suggesting that the key to diurnality lies downstream of the SCN in A. niloticus.

Expression of prokineticin 2 and its receptor in the macaque monkey brain

Prokineticin 2 (PK2) has been indicated as an output signaling molecule for the suprachiasmatic nucleus (SCN) circadian clock. Most of these studies were performed with nocturnal animals, particularly mice and rats. In the current study, the PK2 and its receptor, PKR2, was cloned from a species of diurnal macaque monkey. The macaque monkey PK2 and PKR2 were found to be highly homologous to that of other mammalian species. The mRNA expression of PK2 and PKR2 in the macaque brain was examined by in situ hybridization. The expression patterns of PK2 and PKR2 in the macaque brain were found to be quite similar to that of the mouse brain. Particularly, PK2 mRNA was shown to oscillate in the SCN of the macaque brain in the same phase and with similar amplitude with that of nocturnal mouse brain. PKR2 expression was also detected in known primary SCN targets, including the midline thalamic and hypothalamic nuclei. In addition, we detected the expression of PKR2 mRNA in the dorsal raphe nucleus (DR) of both macaque and mouse brains. As a likely SCN to dorsal raphe projection has previously been indicated, the expression of PKR2 in the raphe nuclei of both macaque and mouse brain signifies a possible role of DR as a previously unrecognized primary SCN projection target.

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

Circadian modulation in the intestinal absorption of P-glycoprotein substrates in monkeys

Recent studies in laboratory rodents have revealed that circadian oscillation in the physiologic functions affecting drug disposition underlies the dosing time-dependent change in pharmacokinetics. However, it is difficult to predict the circadian change in the drug pharmacokinetics in a diurnal human by using the data collected from nocturnal rodents. In this study, we used cynomolgus monkeys, diurnal active animals, to evaluate the relevance of intestinal expression of P-glycoprotein (P-gp) to the dosing time dependency of the pharmacokinetics of its substrates. The rhythmic phases of circadian gene expression in the suprachiasmatic nuclei (the mammalian circadian pacemaker) of cynomolgus monkeys were similar to those reported in nocturnal rodents. On the other hand, the expression of circadian clock genes in the intestinal epithelial cells of monkeys oscillated at opposite phases in rodents. The intestinal expression of P-gp in the small intestine of monkeys was also oscillated in a circadian time-dependent manner. Furthermore, the intestinal absorption of P-gp substrates (quinidine and etoposide) was substantially suppressed by administering the drugs at the times of day when P-gp levels were abundant. By contrast, there was no significant dosing time-dependent difference in the absorption of the non-P-gp substrate (acetaminophen). The oscillation in the intestinal expression of P-gp appears to affect the pharmacokinetics of its substrates. Identification of circadian factors affecting the drug disposition in laboratory monkeys may improve the predictive accuracy of pharmacokinetics in humans.

The primate seahorse rhythm

The main Zeitgeber, the day-night cycle, synchronizes the central oscillator which determines behaviors rhythms as sleep-wake behavior, body temperature, the regulation of hormone secretion, and the acquisition and processing of memory. Thus, actions such as acquisition, consolidation, and retrieval performed in the hippocampus are modulated by the circadian system and show a varied dependence on light and dark. To investigate changes in the hippocampus' cellular mechanism invoked by the day and night in a diurnal primate, this study analyzed the expression of PER2 and the calcium binding proteins (CaBPs) calbindin, calretinin and parvalbumin in the hippocampus of Sapajus apella, a diurnal primate, at two different time points, one during the day and one during the dark phase. The PER2 protein expression peaked at night in the antiphase described for the suprachiasmatic nucleus (SCN) of the same primate, indicating that hippocampal cells can present independent rhythmicity. This hippocampal rhythm was similar to that presented by diurnal but not nocturnal rodents. The CaBPs immunoreactivity also showed day/night variations in the cell number and in the cell morphology. Our findings provide evidence for the claim that the circadian regulation in the hippocampus may involve rhythms of PER2 and CaBPs expression that may contribute to the adaptation of this species in events and activities relevant to the respective periods.

The circadian gene Clock oscillates in the suprachiasmatic nuclei of the diurnal rodent Barbary striped grass mouse, Lemniscomys barbarus: a general feature of diurnality?

A major challenge in the field of circadian rhythms is to understand the neural mechanisms controlling the oppositely phased temporal organization of physiology and behaviour between night- and day-active animals. Most identified components of the master clock in the suprachiasmatic nuclei (SCN), called circadian genes, display similar oscillations according to the time of day, independent of the temporal niche. This has led to the predominant view that the switch between night- and day-active animals occurs downstream of the master clock, likely also involving differential feedback of behavioral cues onto the SCN. The Barbary striped grass mouse, Lemniscomys barbarus is known as a day-active Muridae. Here we show that this rodent, when housed in constant darkness, displays a temporal rhythmicity of metabolism matching its diurnal behaviour (i.e., high levels of plasma leptin and hepatic glycogen during subjective midday and dusk, respectively). Regarding clockwork in their SCN, these mice show peaks in the mRNA profiles of the circadian gene Period1 (Per1) and the clock-controlled gene Vasopressin (Avp), which occur during the middle and late subjective day, respectively, in accordance with many observations in both diurnal and nocturnal species. Strikingly, expression of the circadian gene Clock in the SCN of the Barbary striped grass mouse was not constitutive as in nocturnal rodents, but it was rhythmic. As this is also the case for the other diurnal species investigated in the literature (sheep, marmoset, and quail), a hypothesis is that the transcriptional control of Clock within the SCN participates in the mechanisms underlying diurnality and nocturnality.

Retino-hypothalamic regulation of light-induced murine sleep

The temporal organization of sleep is regulated by an interaction between the circadian clock and homeostatic processes. Light indirectly modulates sleep through its ability to phase shift and entrain the circadian clock. Light can also exert a direct, circadian-independent effect on sleep. For example, acute exposure to light promotes sleep in nocturnal animals and wake in diurnal animals. The mechanisms whereby light directly influences sleep and arousal are not well understood. In this review, we discuss the direct effect of light on sleep at the level of the retina and hypothalamus in rodents. We review murine data from recent publications showing the roles of rod-, cone- and melanopsin-based photoreception on the initiation and maintenance of light-induced sleep. We also present hypotheses about hypothalamic mechanisms that have been advanced to explain the acute control of sleep by light. Specifically, we review recent studies assessing the roles of the ventrolateral preoptic area (VLPO) and the suprachiasmatic nucleus (SCN). We also discuss how light might differentially promote sleep and arousal in nocturnal and diurnal animals respectively. Lastly, we suggest new avenues for research on this topic which is still in its early stages.

Period Gene Expression in the Brain of a Dual-Phasing Rodent, the Octodon degus

Clock gene expression is not only confined to the master circadian clock in the suprachiasmatic nucleus (SCN) but is also found in many other brain regions. The phase relationship between SCN and extra-SCN oscillators may contribute to known differences in chronotypes. The Octodon degus is a diurnal rodent that can shift its activity-phase preference from diurnal to nocturnal when running wheels become available. To understand better the relationship between brain clock gene activity and chronotype, we studied the day-night expression of the Period genes, Per1 and Per2, in the SCN and extra-SCN brain areas in diurnal and nocturnal degus. Since negative masking to light and entrainment to the dark phase are involved in the nocturnalism of this species, we also compare, for the first time, Per expression between entrained (EN) and masked nocturnal (MN) degus. The brains of diurnal, MN, and EN degus housed with wheels were collected during the light (ZT4) and dark (ZT16) phases. Per1 and Per2 mRNA levels were analyzed by in situ hybridization. Within the SCN, signals for Per1 and Per2 were higher at ZT4 irrespective of chronotype. However, outside of the SCN, Per1 expression in the hippocampus of EN degus was out of phase (higher values at ZT16) with SCN values. Although a similar trend was seen in MN animals, this day-night difference in Per1 expression was not significant. Interestingly, daily differences in Per1 expression were not seen in the hippocampus of diurnal degus. For other putative brain areas analyzed (cortices, striatum, arcuate, ventromedial hypothalamus), no differences in Per1 levels were found between chronotypes. Both in diurnal and nocturnal degus, Per2 levels in the hippocampus and in the cingulate and piriform cortices were in phase with their activity rhythms. Thus, diurnal degus showed higher Per2 levels at ZT4, whereas in both types of nocturnal degus, Per2 expression was reversed, peaking at ZT16. Together, the present study supports the hypothesis that the mechanisms underlying activity-phase preference in diurnal and nocturnal mammals reside downstream from the SCN, but our data also indicate that there are fundamental differences between nocturnal masked and entrained degus.

The circadian control of sleep

The sleep/wake cycle is arguably the most familiar output of the circadian system, however, sleep is a complex biological process that arises from multiple brain regions and neurotransmitters, which is regulated by numerous physiological and environmental factors. These include a circadian drive for wakefulness as well as an increase in the requirement for sleep with prolonged waking (the sleep homeostat). In this chapter, we describe the regulation of sleep, with a particular emphasis on the contribution of the circadian system. Since their identification, the role of clock genes in the regulation of sleep has attracted considerable interest, and here, we provide an overview of the interplay between specific elements of the molecular clock with the sleep regulatory system. Finally, we summarise the role of the light environment, melatonin and social cues in the modulation of sleep, with a focus on the role of melanopsin ganglion cells.

The neuroendocrine control of the circadian system: adolescent chronotype

Scientists, public health and school officials are paying growing attention to the mechanism underlying the delayed sleep patterns common in human adolescents. Data suggest that a propensity towards evening chronotype develops during puberty, and may be caused by developmental alterations in internal daily timekeeping. New support for this theory has emerged from recent studies which show that pubertal changes in chronotype occur in many laboratory species similar to human adolescents. Using these species as models, we find that pubertal changes in chronotype differ by sex, are internally generated, and driven by reproductive hormones. These chronotype changes are accompanied by alterations in the fundamental properties of the circadian timekeeping system, including endogenous rhythm period and sensitivity to environmental time cues. After comparing the developmental progression of chronotype in different species, we propose a theory regarding the ecological relevance of adolescent chronotype, and provide suggestions for improving the sleep of human adolescents.

Setting the main circadian clock of a diurnal mammal by hypocaloric feeding

Caloric restriction attenuates the onset of a number of pathologies related to ageing. In mammals, circadian rhythms, controlled by the hypothalamic suprachiasmatic (SCN) clock, are altered with ageing. Although light is the main synchronizer for the clock, a daily hypocaloric feeding (HF) may also modulate the SCN activity in nocturnal rodents. Here we report that a HF also affects behavioural, physiological and molecular circadian rhythms of the diurnal rodent Arvicanthis ansorgei. Under constant darkness HF, but not normocaloric feeding (NF), entrains circadian behaviour. Under a light–dark cycle, HF at midnight led to phase delays of the rhythms of locomotor activity and plasma corticosterone. Furthermore, Per2 and vasopressin gene oscillations in the SCN were phase delayed in HF Arvicanthis compared with animals fed ad libitum. Moreover, light-induced expression of Per genes in the SCN was modified in HF Arvicanthis, despite a non-significant effect on light-induced behavioural phase delays. Together, our data show that HF affects the circadian system of the diurnal rodent Arvicanthis ansorgei differentially from nocturnal rodents. The Arvicanthis model has relevance for the potential use of HF to manipulate circadian rhythms in diurnal species including humans.

The neural circadian system of mammals

Humans and other mammals exhibit a remarkable array of cyclical changes in physiology and behaviour. These are often synchronized to the changing environmental light–dark cycle and persist in constant conditions. Such circadian rhythms are controlled by an endogenous clock, located in the suprachiasmatic nuclei of the hypothalamus. This structure and its cells have unique properties, and some of these are reviewed to highlight how this central clock controls and sculpts our daily activities.

Immunocytochemical evidence for different patterns in daily rhythms of VIP and AVP peptides in the suprachiasmatic nucleus of diurnal Funambulus palmarum

The suprachiasmatic nucleus (SCN) is the principal pacemaker that coordinates circadian rhythmicity in mammals. The studies on understanding the circadian system in diurnal rodents are limited. In this study, we have used the 3 striped South Indian Palm Squirrel (Funambulus palmarum). The locomotor activity showed a diurnal pattern of activity in LD 12:12, constant darkness (DD) and light (LL) conditions with circadian periods (τ) of 24.19 ± 0.1, 24.11 ± 0.03 and 24.92 ± 0.35 h respectively. Anatomical study of the brain revealed that this animal had short, thick and stout optic nerves with SCN elliptical in shape with a higher neuronal population as distinct from nocturnal rodents. Since the neuropeptides, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP) play important roles in photic entrainment and relay of information respectively in nocturnal rodents, we studied the distribution and daily rhythms of VIP-ir and AVP-ir in squirrel SCN. The VIP-ir and AVP-ir cells in the SCN showed a ventrolateral and dorsomedial distribution with daily rhythmicity in their levels. The peak time of VIP-ir rhythm was found ahead of AVP-ir. The VIP-ir levels were higher for longer duration than AVP-ir levels. The maximum and minimum VIP-ir levels were at ZT-6 and ZT-0 respectively and AVP-ir levels at ZT-12 and ZT-0 respectively. Thus, VIP and AVP maximum and minimum levels appeared 6 and 12h apart respectively in squirrel, though 12 and 8h apart in rat. These findings in the present report could be a step towards underpinning the mechanisms regulating diurnality.

Phase preference for the display of activity is associated with the phase of extra-suprachiasmatic nucleus oscillators within and between species

Many features of the suprachiasmatic nucleus (SCN) are the same in diurnal and nocturnal animals, suggesting that differences in phase preference are determined by mechanisms downstream from the SCN. Here, we examined this hypothesis by characterizing rhythmic expression of Period 1 (PER1) and Period 2 (PER2) in several extra-SCN areas in the brains of a diurnal murid rodent, Arvicanthis niloticus (grass rats). In the shell of the nucleus accumbens, dorsal striatum, piriform cortex, and CA1 of the hippocampus, both PER1 and PER2 were rhythmic, with peak expression occurring at ZT10. PER1 in the dentate gyrus also peaked at ZT10, but PER2 was arrhythmic in this region. In general, these patterns are 180 degrees out of phase with those reported for nocturnal species. In a second study, we examined inter-individual differences in the multioscillator system of grass rats. Here, we housed grass rats in cages with running wheels, under which conditions some individuals spontaneously adopt a day active (DA) and others a night active (NA) phase preference. In the majority of the extra-SCN regions sampled, the patterns of PER1 and PER2 expression of NA grass rats resembled those of nocturnal species, while those of DA grass rats were similar to the ones seen in grass without access to running wheels. In contrast, the rhythmic expression of both PER proteins was identical in the SCN and ventral subparaventricular zone (vSPZ) of DA and NA animals. Differences in the phase of oscillators downstream from the SCN, and perhaps the vSPZ, appear to determine the phase preference of particular species, as well as that of members of a diurnal species that show voluntary phase reversals. The latter observation has important implications for the understanding of health problems associated with human shift work.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

The response of Per1 to light in the suprachiasmatic nucleus of the diurnal degu (Octodon degus)

Several studies suggest that the circadian systems of diurnal mammals respond differently to daytime light than those of nocturnal mammals. We hypothesized that the photosensitive "clock" gene Per1 would respond to light exposure during subjective day in the suprachiasmatic nucleus of the diurnal rodent, Octodon degus. Tissue was collected 1.5-2 h after a 30 min light pulse presented at five timepoints across the 24 h day and compared to controls maintained under conditions of constant darkness. Per1 mRNA was quantified using in situ hybridization. Results showed that the rhythmicity and photic responsiveness of Per1 in the degu resembles that of nocturnal animals.

Light Pulse‐Induced Heme and Iron‐Associated Transcripts in Mouse Brain: A Microarray Analysis

Synchronization of circadian oscillators with the outside world is achieved by the acute effects of light on the levels of one or more clock components. In mammals the PAS transcription factors Clock, NPAS2, and BMAL1 regulate gene expression as a function of the day-night cycle. Both PAS domains of NPAS2 were found to bind heme as a prosthetic group, form a gas-regulated sensor, and exert heme-status control of DNA binding in vitro. In a microarray analysis comparing overall changes in brain transcript levels between mice subjected to light pulses during the dark phase with animals maintained in darkness, we traced consistent changes in more than 200 different transcripts. Of these, 20 are associated with heme and iron biosynthesis and catabolism. A model for the pathway of induction of heme and iron homeostasis-related transcripts resulting from light pulses suggests that light signals (as stressors) induce transcription of heme oxygenase 2 (Hmox2) and cytochrome P450 oxidoreductase (Por), which may serve as a primary line of cellular defense. HMOX2 degrades heme from proteins such as hemoglobin. This degradation generates CO, a signal molecule, and may also change the redox state of the cell by reducing the NADPH/NADP ratio. This could lead to up-regulation of globin gene transcription, thereby releasing iron that in turn controls production of ferritins, and further up-regulating aminolevulinate synthase 2 (Alas2).

Compartmentalized expression of light-induced clock genes in the suprachiasmatic nucleus of the diurnal grass rat (Arvicanthis niloticus)

Photic responses of the circadian system are mediated through light-induced clock gene expression in the suprachiasmatic nucleus (SCN). In nocturnal rodents, depending on the timing of light exposure, Per1 and Per2 gene expression shows distinct compartmentalized patterns that correspond to the behavioral responses. Whether the gene- and region-specific induction patterns are unique to nocturnal animals, or are also present in diurnal species is unknown. We explored this question by examining the light-induced Per1 and Per2 gene expression in functionally distinct SCN subregions, using diurnal grass rats Arvicanthis niloticus. Light exposure during nighttime induced Per1 and Per2 expression in the SCN, showing unique spatiotemporal profiles depending on the phase of the light exposure. After a phase delaying light pulse (LP) in the early night, strong Per1 induction was observed in the retinorecipient core region of the SCN, while strong Per2 induction was observed throughout the entire SCN. After a phase advancing LP in the late night, Per1 was first induced in the core and then extended into the whole SCN, accompanied by a weak Per2 induction. This compartmentalized expression pattern is very similar to that observed in nocturnal rodents, suggesting that the same molecular and intercellular pathways underlying acute photic responses are present in both diurnal and nocturnal species. However, after an LP in early subjective day, which induces phase advances in diurnal grass rats, but not in nocturnal rodents, we did not observe any Per1 or Per2 induction in the SCN. This result suggests that in spite of remarkable similarities in the SCN of diurnal and nocturnal rodents, unique mechanisms are involved in mediating the phase shifts of diurnal animals during the subjective day.

Circadian phase resetting in response to light-dark and dark-light transitions

Phase shifting of circadian systems by light has been attributed both to parametric effects on angular velocity elicited by a tonic response to the luminance level and to nonparametric instantaneous shifts induced by a phasic response to the dark-light (D>L) and light-dark (L>D) transitions. Claims of nonparametric responses are partly based on "step-PRCs," that is, phase response curves derived from such transitions. Step-PRCs in nocturnal mammals show mostly delays after lights-on and advances after lights-off, and therefore appear incompatible with phase delays generated by light around dusk and advances by light around dawn. We have pursued this paradox with 2 experimental protocols in mice. We first use the classic step-PRC protocol on wheel running activity, using the center of gravity as a phase marker to minimize the masking effects of light. The experiment was done for 3 different light intensities (1, 10, and 100 lux). D>L transitions evoke mostly delays and L>D transitions show no clear tendency to either delay or advance. Overall there is little or no circadian modulation. A 2nd protocol aimed to avoid the problem of masking by assessing phase before and after the light stimuli, both in DD. Light stimuli consisted of either a slow light intensity increase over 48 h followed by abruptly switching off the light, or an abrupt switch on followed by a slow decrease toward total darkness during 48 h. If the abrupt transitions were responsible for phase shifting, we expected large differences between the 2 stimuli. Both light stimuli yielded similar PRCs characterized by delays only with circadian modulation. The results can be adequately explained by a model in which all PRCs evoked by steps result in fact from tonic responses to the light following a step-up or preceding a step-down. In this model only the response reduction of tonic velocity change after the 1st hour is taken into account. The data obtained in both experiments are thus compatible with tonic velocity responses. Contrary to standard interpretation of step-PRCs, nonparametric responses to the transitions are unlikely since they would predict delays in response to lights-off, advances in response to lights-on, while the opposite was found. Although such responses cannot be fully excluded, parsimony does not require invocation of a role for transitions, since all the data can readily be explained by tonic velocity (parametric) effects, which must exist because of the dependence of tau on light intensity.

Period gene expression in the diurnal degu (Octodon degus) differs from the nocturnal laboratory rat (Rattus norvegicus)

Recent data suggest that both nocturnal and diurnal mammals generate circadian rhythms using similarly phased feedback loops involving Period genes in the suprachiasmatic nuclei (SCN) of the hypothalamus. These molecular oscillations also exist in the brain outside of the SCN, but the relationship between SCN and extra-SCN oscillations is unclear. We hypothesized that a comparison of "diurnal" and "nocturnal" central nervous system Per rhythms would uncover differences in the underlying circadian mechanisms between these two chronotypes. Therefore, this study compared the 24-h oscillatory patterns of Per1 and Per2 mRNA in the SCN and putative striatum and cortex of Octodon degus (degu), a diurnal hystricognath rodent, with those of the nocturnal laboratory rat, Rattus norvegicus. The brains of adult male degus and rats were collected at 2-h intervals across 24 h in entrained light-dark and constant darkness conditions, and sections were analyzed via in situ hybridization. In the SCN, degu Per1 and Per2 hybridization signal exhibited 24-h oscillatory patterns similar in phasing to those seen in other rodents, with peaks occurring during the light period and troughs during the dark period. However, Per1 remained elevated for five fewer hours in the degu than in the rat, and Per2 remained elevated for two fewer hours in the degu. In brain areas outside of the SCN, the phase of Per2 hybridization signal rhythms in the degu were 180 degrees out of phase with those found in the rat, and Per1 hybridization signal lacked significant rhythmicity. These results suggest that, while certain basic components of the transcriptional-translational feedback loop generating circadian rhythms are similar in diurnal and nocturnal mammals, there are variations that may reflect adaptations to circadian niche.

Mobilized hematopoietic stem cell yield depends on species-specific circadian timing

Endogenous rhythmicity likely evolved as a mechanism allowing organisms to anticipate predictable daily changes in the environment (Rutter et al., 2002). Under homeostasis, murine hematopoietic stem cell (HSC) egress is orchestrated by rhythmic beta 3 adrenergic signals delivered by the sympathetic nervous system (SNS) that regulate Cxcl12 expression in stromal cells (Mendez-Ferrer et al., 2008). Here, we show that CXCR4 is also regulated under circadian control whose rhythm is synchronized with its ligand, CXCL12, to optimize HSC trafficking. These circadian oscillations are inverted in humans compared to the mouse and continue to influence the yield even when stem cell mobilization is enforced. Our results suggest that the human HSC yield for clinical transplantation might be significantly greater if patients were harvested during the evening compared to the morning.

Daily rhythms in PER1 within and beyond the suprachiasmatic nucleus of female grass rats (Arvicanthis niloticus)

Although circadian rhythms of males and females are different in a variety of ways in many species, their mechanisms have been primarily studied in males. Furthermore, rhythms are dramatically different in diurnal and nocturnal animals but have been studied predominantly in nocturnal ones. In the present study, we examined rhythms in one element of the circadian oscillator, the PER1 protein, in a variety of cell populations in brains of diurnal female grass rats. Every 4 h five adult female grass rats kept on a 12-h light/dark (LD) cycle were perfused and their brains were processed for immunohistochemical detection of PER1. Numbers of PER1-labeled cells were rhythmic not only within the suprachiasmatic nucleus (SCN), the locus of the primary circadian clock in mammals, but also in the peri-suprachiasmatic region, the oval nucleus of the bed nucleus of the stria terminalis, the central amygdala, and the nucleus accumbens. In addition, rhythms were detected within populations of neuroendocrine cells that contain tyrosine hydroxylase. The phase of the rhythm within the SCN was advanced compared with that seen previously in male grass rats. Rhythms beyond the SCN were varied and different from those seen in most nocturnal species, suggesting that signals originating in the SCN are modified by its direct and/or indirect targets in different ways in nocturnal and diurnal species.

Retinal pathways influence temporal niche

In mammals, light input from the retina entrains central circadian oscillators located in the suprachiasmatic nuclei (SCN). The phase of circadian activity rhythms with respect to the external light:dark cycle is reversed in diurnal and nocturnal species, although the phase of SCN rhythms relative to the light cycle remains unchanged. Neural mechanisms downstream from the SCN are therefore believed to determine diurnality or nocturnality. Here, we report a switch from nocturnal to diurnal entrainment of circadian activity rhythms in double-knockout mice lacking the inner-retinal photopigment melanopsin (OPN4) and RPE65, a key protein used in retinal chromophore recycling. These mice retained only a small amount of rod function. The change in entrainment phase of Rpe65(-/-);Opn4(-/-) mice was accompanied by a reversal of the rhythm of clock gene expression in the SCN and a reversal in acute masking effects of both light and darkness on activity, suggesting that the nocturnal to diurnal switch is due to a change in the neural response to light upstream from the SCN. A switch from nocturnal to diurnal activity rhythms was also found in wild-type mice transferred from standard intensity light:dark cycles to light:dark cycles in which the intensity of the light phase was reduced to scotopic levels. These results reveal a novel mechanism by which changes in retinal input can mediate acute temporal-niche switching.

Circadian and homeostatic factors in arousal

In the course of evolution, mechanisms have evolved to anticipate the timing of regularly occurring events. These mechanisms are encompassed in a circadian timing system that include a master clock localized to the suprachiasmatic nucleus of the hypothalamus and "slave" oscillators distributed throughout the body. This system serves multiple functions so as to ensure that various physiological processes occur at optimal and nonoverlapping times, to synchronize our activities to local environmental time, and to permit changes required to respond to new environmental circumstances. We suggest that a generalized concept of arousal (which includes alterations in responsiveness to homeostatic pressures, sensory stimuli and emotional reactivity, and to changes in motor activity) serves as a rubric in which to explore the multiple ways in which the circadian system modulates behavior.