Neurons and networks in daily rhythms

Spatiotemporal distribution of vasoactive intestinal polypeptide receptor 2 in mouse suprachiasmatic nucleus

Vasoactive intestinal polypeptide (VIP) signaling is critical for circadian rhythms. For example, the expression of VIP and its main receptor, VPAC2R, is necessary for maintaining synchronous daily rhythms among neurons in the suprachiasmatic nucleus (SCN), a master circadian pacemaker in animals. Where and when VPAC2R protein is expressed in the SCN and other brain areas has not been examined. Using immunohistochemistry, we characterized a new antibody and found that VPAC2R was highly enriched in the SCN and detectable at low levels in many brain areas. Within the SCN, VPAC2R was circadian, peaking in the subjective morning, and abundantly expressed from the rostral to caudal margins with more in the dorsomedial than ventrolateral area. VPAC2R was found in nearly all SCN cells including neurons expressing either VIP or vasopressin (AVP). SCN neurons mainly expressed VPAC2R in their somata and dendrites, not axons. Finally, constant light increased VIP and AVP expression, but not VPAC2R. We conclude that the circadian clock, not the ambient light level, regulates VPAC2R protein localization. These results are consistent with VPAC2R playing a role in VIP signaling at all times of day, broadly throughout the brain and in all SCN cells.

The output signal of Purkinje cells of the cerebellum and circadian rhythmicity

Measurement of clock gene expression has recently provided evidence that the cerebellum, like the master clock in the SCN, contains a circadian oscillator. The cerebellar oscillator is involved in anticipation of mealtime and possibly resides in Purkinje cells. However, the rhythmic gene expression is likely transduced into a circadian cerebellar output signal to exert an effective control of neuronal brain circuits that are responsible for feeding behavior. Using electrophysiological recordings from acute and organotypic cerebellar slices, we tested the hypothesis whether Purkinje cells transmit a circadian modulated signal to their targets in the brain. Extracellular recordings from brain slices revealed the typical discharge pattern previously described in vivo in single cell recordings showing basically a tonic or a trimodal-like firing pattern. However, in acute sagittal cerebellar slices the average spike rate of randomly selected Purkinje cells did not exhibit significant circadian variations, irrespective of their specific firing pattern. Also, frequency and amplitude of spontaneous inhibitory postsynaptic currents and the amplitude of GABA- and glutamate-evoked currents did not vary with circadian time. Long-term recordings using multielectrode arrays (MEA) allowed to monitor neuronal activity at multiple sites in organotypic cerebellar slices for several days to weeks. With this recording technique we observed oscillations of the firing rate of cerebellar neurons, presumably of Purkinje cells, with a period of about 24 hours which were stable for periods up to three days. The daily renewal of culture medium could induce circadian oscillations of the firing rate of Purkinje cells, a feature that is compatible with the behavior of slave oscillators. However, from the present results it appears that the circadian expression of cerebellar clock genes exerts only a weak influence on the electrical output of cerebellar neurons.

Molecular components of the Mammalian circadian clock

Mammals synchronize their circadian activity primarily to the cycles of light and darkness in the environment. This is achieved by ocular photoreception relaying signals to the suprachiasmatic nucleus (SCN) in the hypothalamus. Signals from the SCN cause the synchronization of independent circadian clocks throughout the body to appropriate phases. Signals that can entrain these peripheral clocks include humoral signals, metabolic factors, and body temperature. At the level of individual tissues, thousands of genes are brought to unique phases through the actions of a local transcription/translation-based feedback oscillator and systemic cues. In this molecular clock, the proteins CLOCK and BMAL1 cause the transcription of genes which ultimately feedback and inhibit CLOCK and BMAL1 transcriptional activity. Finally, there are also other molecular circadian oscillators which can act independently of the transcription-based clock in all species which have been tested.

Differential patterns in the periodicity and dynamics of clock gene expression in mouse liver and stomach

The rhythmic recurrence of biological processes is driven by the functioning of cellular circadian clocks, operated by a set of genes and proteins that generate self-sustaining transcriptional-translational feedback loops with a free-running period of about 24 h. In the gastrointestinal apparatus, the functioning of the biological clocks shows distinct patterns in the different organs. The aim of this study was to evaluate the time-related variation of clock gene expression in mouse liver and stomach, two components of the digestive system sharing vascular and autonomic supply, but performing completely different functions. The authors analyzed the periodicity by cosinor analysis and the dynamics of variation by computing the fractional variation to assess the rate of change in gene expression. Five-week-old male Balb/c mice were exposed to 2 wks of 12-h light/12-h dark cycles, then kept in complete darkness for 3 d as a continuation of the dark span of the last light-dark cycle. The authors evaluated the expression of Bmal1, Clock, Cry1, Cry2, Per1, Per2, Per3, Rev-erbα, Rev-erbβ, Npas2, Timeless, Dbp, Csnk1d, and Csnk1e by using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in mouse liver and stomach. A significant 24-h rhythmic component was found for 10 genes in the liver (Bmal1, Clock, Cry1, Per1, Per2, Per3, Rev-erbα, Rev-erbβ, Npas2, and Dbp), and for 9 genes in the stomach (Bmal1, Cry1, Per1, Per2, Per3, Rev-erbα, Rev-erbβ, Npas2, and Dbp). In particular, Clock showed marked rhythm differences between liver and stomach, putatively due to some compensation by Npas2. The acrophase of the original values of Bmal1, Per2, Per3, Rev-erbα, Rev-erbβ, Npas2, and Dbp expression was delayed in the stomach, and the average delay expressed as mean ± SD was 14.30 ± 7.94 degrees (57.20 ± 31.78 minutes). A statistically significant difference was found in the acrophases of Bmal1 (p = .015) and Npas2 (p = .011). Fractional variations provided significant circadian rhythms for nine genes in the liver (Bmal1, Per1, Per2, Per3, Rev-erbα, Rev-erbβ, Npas2, Timeless, and Dbp), and for seven genes in the stomach (Bmal1, Clock, Per2, Rev-erbα, Npas2, Dbp, and Csnk1e). The acrophase of the fractional variations of Bmal1, Per2, Per3, Rev-erbα, Rev-erbβ, and Dbp expression was delayed in the stomach, and the average delay expressed as mean ± SD was 19.10 ± 9.39 degrees (76.40 ± 37.59 minutes). A significantly greater fractional variation was found in the liver for Clock at 06:00 h (p = .034), Per1 at 02:00 h (p = .037), and Per3 at 02:00 h (p = .029), whereas the fractional variation was greater in the stomach for Clock at 10:00 h (p = .016), and for Npas2 at 02:00 h (p = .029) and at 06:00 h (p = .044). In conclusion, liver and stomach show different phasing and dynamics of clock gene expression, which are probably related to prevailing control by different driving cues, and allow them to keep going the various metabolic pathways and diverse functional processes that they manage.

The role of intrinsically photosensitive retinal ganglion cells in nonimage-forming responses to light

Light exerts many effects on behavior and physiology. These effects can be characterized as either image-forming or nonimage-forming (NIF) visual processes. Image-forming vision refers to the process of detecting objects and organisms in the environment and distinguishing their physical characteristics, such as size, shape, and direction of motion. NIF vision, in contrast, refers to effects of light that are independent of fine spatiotemporal vision. NIF effects are many and varied, ranging from modulation of basal physiology, such as heart rate and body temperature, to changes in higher functions, such as mood and cognitive performance. In mammals, many NIF effects of light are dependent upon the inner retinal photopigment melanopsin and the cells in which melanopsin is expressed, the intrinsically photosensitive retinal ganglion cells (ipRGCs). The ipRGCs project broadly throughout the brain. Many of these projections terminate in areas known to mediate NIF effects, while others terminate in regions whose link to photoreception remains to be established. Additionally, the presence of ipRGC projections to areas of the brain with no known link to photoreception suggests the existence of additional ipRGC-mediated NIF effects. This review summarizes the known NIF effects of light and the role of melanopsin and ipRGCs in driving these effects, with an eye toward stimulating further investigation of the many and varied effects of light on physiology and behavior.

Exponential synchronization rate of Kuramoto oscillators in the presence of a pacemaker

The exponential synchronization rate is addressed for Kuramoto oscillators in the presence of a pacemaker. When natural frequencies are identical, we prove that synchronization can be ensured even when the phases are not constrained in an open half-circle, which improves the existing results in the literature. We derive a lower bound on the exponential synchronization rate, which is proven to be an increasing function of pacemaker strength, but may be an increasing or decreasing function of local coupling strength. A similar conclusion is obtained for phase locking when the natural frequencies are non-identical. An approach to trapping phase differences in an arbitrary interval is also given, which ensures synchronization in the sense that synchronization error can be reduced to an arbitrary level.

Regulation of alternative splicing by the circadian clock and food related cues

Background

The circadian clock orchestrates daily rhythms in metabolism, physiology and behaviour that allow organisms to anticipate regular changes in their environment, increasing their adaptation. Such circadian phenotypes are underpinned by daily rhythms in gene expression. Little is known, however, about the contribution of post-transcriptional processes, particularly alternative splicing.

Results

Using Affymetrix mouse exon-arrays, we identified exons with circadian alternative splicing in the liver. Validated circadian exons were regulated in a tissue-dependent manner and were present in genes with circadian transcript abundance. Furthermore, an analysis of circadian mutant Vipr2^-/- mice revealed the existence of distinct physiological pathways controlling circadian alternative splicing and RNA binding protein expression, with contrasting dependence on Vipr2-mediated physiological signals. This view was corroborated by the analysis of the effect of fasting on circadian alternative splicing. Feeding is an important circadian stimulus, and we found that fasting both modulates hepatic circadian alternative splicing in an exon-dependent manner and changes the temporal relationship with transcript-level expression.

Conclusions

The circadian clock regulates alternative splicing in a manner that is both tissue-dependent and concurrent with circadian transcript abundance. This adds a novel temporal dimension to the regulation of mammalian alternative splicing. Moreover, our results demonstrate that circadian alternative splicing is regulated by the interaction between distinct physiological cues, and illustrates the capability of single genes to integrate circadian signals at different levels of regulation.

Transcription-based oscillator model for light-induced splitting as antiphase circadian gene expression in the suprachiasmatic nuclei

Daily locomotor patterns of a variety of organisms have been interpreted as driven by dual circadian oscillators. Yet, in mammals, cellular data have revealed many circadian oscillators in the bilateral suprachiasmatic nucleus (SCN). To test how large numbers of oscillators could respond to environmental cues as a pair of oscillators, the authors developed a computational model composed of 2 groups of oscillators with strong local interactions and with weaker coupling between the 2 groups. Unlike previous models that assumed that light affects the timing or polarity of coupling between a pair of oscillators, this simulation assumed that light increased the transcription rate of a clock gene and consequently altered circadian properties of individual cells. In constant dark, weak local (within each of the 2 groups) and distant (between group) coupling established in-phase oscillations and a typical single bout of daily activity. In constant light, local synchrony developed only if coupling was strong and resulted in antiphase synchrony between the 2 groups and bimodal daily activity reminiscent of split behavior. These numerical simulations thus showed that splitting behavior can develop with increased light intensity without structural changes in the coupling topology or sign. Instead, the authors propose that light changes intrinsic oscillator properties through the increase of maximal transcription rate of a clock gene, so that as light intensity increases, the output of the coupled network transitions from a single bout of activity through irregular beating to 2 bouts and, in bright constant light, arrhythmicity.

Central and peripheral circadian clocks in mammals

The circadian system of mammals is composed of a hierarchy of oscillators that function at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator throughout the body, yet this clock system is adapted to different functional contexts. In the central suprachiasmatic nucleus (SCN) of the hypothalamus, a coupled population of neuronal circadian oscillators acts as a master pacemaker for the organism to drive rhythms in activity and rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which in turn provides stability to the overall temporal architecture of the organism. Throughout the majority of the cells in the body, cell-autonomous circadian clocks are intimately enmeshed within metabolic pathways. Thus, an emerging view for the adaptive significance of circadian clocks is their fundamental role in orchestrating metabolism.

Time of day regulates subcellular trafficking, tripartite synaptic localization, and polyadenylation of the astrocytic Fabp7 mRNA

The astrocyte brain fatty acid binding protein (Fabp7) has previously been shown to have a coordinated diurnal regulation of mRNA and protein throughout mouse brain, and an age-dependent decline in protein expression within synaptoneurosomal fractions. Mechanisms that control time-of-day changes in expression and trafficking Fabp7 to the perisynaptic process are not known. In this study, we confirmed an enrichment of Fabp7 mRNA and protein in the astrocytic perisynaptic compartment, and observed a diurnal change in the intracellular distribution of Fabp7 mRNA in molecular layers of hippocampus. Northern blotting revealed a coordinated time-of-day-dependent oscillation for the Fabp7 mRNA poly(A) tail throughout murine brain. Cytoplasmic polyadenylation element-binding protein 1 (CPEB1) regulates subcellular trafficking and translation of synaptic plasticity-related mRNAs. Here we show that Fabp7 mRNA coimmunoprecipitated with CPEB1 from primary mouse astrocyte extracts, and its 3'UTR contains phylogenetically conserved cytoplasmic polyadenylation elements (CPEs) capable of regulating translation of reporter mRNAs during Xenopus oocyte maturation. Given that Fabp7 expression is confined to astrocytes and neural progenitors in adult mouse brain, the synchronized cycling pattern of Fabp7 mRNA is a novel discovery among known CPE-regulated transcripts. These results implicate circadian, sleep, and/or metabolic control of CPEB-mediated subcellular trafficking and localized translation of Fabp7 mRNA in the tripartite synapse of mammalian brain.

Effect of network architecture on synchronization and entrainment properties of the circadian oscillations in the suprachiasmatic nucleus

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus constitutes the central circadian pacemaker. The SCN receives light signals from the retina and controls peripheral circadian clocks (located in the cortex, the pineal gland, the liver, the kidney, the heart, etc.). This hierarchical organization of the circadian system ensures the proper timing of physiological processes. In each SCN neuron, interconnected transcriptional and translational feedback loops enable the circadian expression of the clock genes. Although all the neurons have the same genotype, the oscillations of individual cells are highly heterogeneous in dispersed cell culture: many cells present damped oscillations and the period of the oscillations varies from cell to cell. In addition, the neurotransmitters that ensure the intercellular coupling, and thereby the synchronization of the cellular rhythms, differ between the two main regions of the SCN. In this work, a mathematical model that accounts for this heterogeneous organization of the SCN is presented and used to study the implication of the SCN network topology on synchronization and entrainment properties. The results show that oscillations with larger amplitude can be obtained with scale-free networks, in contrast to random and local connections. Networks with the small-world property such as the scale-free networks used in this work can adapt faster to a delay or advance in the light/dark cycle (jet lag). Interestingly a certain level of cellular heterogeneity is not detrimental to synchronization performances, but on the contrary helps resynchronization after jet lag. When coupling two networks with different topologies that mimic the two regions of the SCN, efficient filtering of pulse-like perturbations in the entrainment pattern is observed. These results suggest that the complex and heterogeneous architecture of the SCN decreases the sensitivity of the network to short entrainment perturbations while, at the same time, improving its adaptation abilities to long term changes.

In order to adapt to their cycling environment, virtually all living organisms have developed an internal timer, the circadian clock. In mammals, the circadian pacemaker is composed of about 20,000 neurons, called the suprachiasmatic nucleus (SCN) located in the hypothalamus. The SCN receives light signals from the retina and controls peripheral circadian clocks to ensure the proper timing of physiological processes. In each SCN neuron, a genetic regulatory network enables the circadian expression of the clock genes, but individual dynamics are highly heterogeneous in dispersed cell culture: many cells present damped oscillations and the period of the oscillations varies from cell to cell. In addition, the neurotransmitters that ensure the intercellular coupling, and thereby the synchronization of the cellular rhythms, differ between the two main regions of the SCN. We present here a mathematical model that accounts for this heterogeneous organization of the SCN and study the implication of the network topology on synchronization and entrainment properties. Our results show that cellular heterogeneity may help the resynchronization after jet lag and suggest that the complex architecture of the SCN decreases the sensitivity of the network to short entrainment perturbations while, at the same time, improving its adaptation abilities to long term changes.

Comparison of β-adrenergic and glucocorticoid signaling on clock gene and osteoblast-related gene expressions in human osteoblast

Most living organisms exhibit circadian rhythms that are generated by endogenous circadian clocks, the master one being present in the suprachiasmatic nuclei (SCN). Output signals from the SCN are believed to transmit standard circadian time to peripheral tissue through sympathetic nervous system and humoral routes. Therefore, the authors examined the expression of clock genes following treatment with the β-adrenergic receptor agonist, isoprenaline, or the synthetic glucocorticoid, dexamethasone, in cultured human osteoblast SaM-1 cells. Cells were treated with 10(-6) M isoprenaline or 10(-7) M dexamethasone for 2 h and gene expressions were determined using real-time polymerase chain reaction (PCR) analysis. Treatment with isoprenaline or dexamethasone induced the circadian expression of clock genes human period 1 (hPer1), hPer2, hPer3, and human brain and muscle Arnt-like protein 1 (hBMAL1). Isoprenaline or dexamethasone treatment immediately increased hPer1 and hPer2 and caused circadian oscillation of hPer1 and hPer2 with three peaks within 48 h. hPer3 expression had one peak after isoprenaline or dexamethasone treatment. hBMAL expression had two peaks after isoprenaline or dexamethasone treatment, the temporal pattern being in antiphase to that of the other clock genes. Dexamethasone treatment delayed the oscillation of all clock genes for 2-6 h compared with isoprenaline treatment. The authors also examined the expression of osteoblast-related genes hα-1 type I collagen (hCol1a1), halkaline phosphatase (hALP), and hosteocalcin (hOC). Isoprenaline induced oscillation of hCol1a1, but not hALP and hOC. On the other hand, dexamethasone induced oscillation of hCol1a1 and hALP, but not hOC. Isoprenaline up-regulated hCol1a1 expression, but dexamethasone down-regulated hCol1a1 and hALP expression in the first phase.

Reciprocal cholinergic and GABAergic modulation of the small ventrolateral pacemaker neurons of Drosophila's circadian clock neuron network

The relatively simple clock neuron network of Drosophila is a valuable model system for the neuronal basis of circadian timekeeping. Unfortunately, many key neuronal classes of this network are inaccessible to electrophysiological analysis. We have therefore adopted the use of genetically encoded sensors to address the physiology of the fly's circadian clock network. Using genetically encoded Ca(2+) and cAMP sensors, we have investigated the physiological responses of two specific classes of clock neuron, the large and small ventrolateral neurons (l- and s-LN(v)s), to two neurotransmitters implicated in their modulation: acetylcholine (ACh) and γ-aminobutyric acid (GABA). Live imaging of l-LN(v) cAMP and Ca(2+) dynamics in response to cholinergic agonist and GABA application were well aligned with published electrophysiological data, indicating that our sensors were capable of faithfully reporting acute physiological responses to these transmitters within single adult clock neuron soma. We extended these live imaging methods to s-LN(v)s, critical neuronal pacemakers whose physiological properties in the adult brain are largely unknown. Our s-LN(v) experiments revealed the predicted excitatory responses to bath-applied cholinergic agonists and the predicted inhibitory effects of GABA and established that the antagonism of ACh and GABA extends to their effects on cAMP signaling. These data support recently published but physiologically untested models of s-LN(v) modulation and lead to the prediction that cholinergic and GABAergic inputs to s-LN(v)s will have opposing effects on the phase and/or period of the molecular clock within these critical pacemaker neurons.

Dynamic neuronal network organization of the circadian clock and possible deterioration in disease

In mammals, the suprachiasmatic nuclei (SCNs) function as a circadian pacemaker that drives 24-h rhythms in physiology and behavior. The SCN is a multicellular clock in which the constituent oscillators show dynamics in their functional organization and phase coherence. Evidence has emerged that plasticity in phase synchrony among SCN neurons determines (i) the amplitude of the rhythm, (ii) the response to continuous light, (iii) the capacity to respond to seasonal changes, and (iv) the phase-resetting capacity. A decrease in circadian amplitude and phase-resetting capacity is characteristic during aging and can be a result of disease processes. Whether the decrease in amplitude is caused by a loss of synchronization or by a loss of single-cell rhythmicity remains to be determined and is important for the development of strategies to ameliorate circadian disorders.

The adrenal peripheral clock: glucocorticoid and the circadian timing system

The mammalian circadian timing system is organized in a hierarchy, with the master clock residing in the suprachiasmatic nucleus (SCN) of the hypothalamus and subsidiary peripheral clocks in other brain regions as well as peripheral tissues. Since the local oscillators in most cells contain a similar molecular makeup to that in the central pacemaker, determining the role of the peripheral clocks in the regulation of rhythmic physiology and behavior is an important issue. Glucocorticoids (GCs) are a class of multi-functional adrenal steroid hormones, which exhibit a robust circadian rhythm, with a peak linked with the onset of the daily activity phase. It has long been believed that the production and secretion of GC is primarily governed through the hypothalamus-pituitary-adrenal (HPA) neuroendocrine axis in mammals. Growing evidence, however, strongly supports the notion that the periodicity of GC involves the integrated activity of multiple regulatory mechanisms related to circadian timing system along with the classical HPA neuroendocrine regulation. The adrenal-intrinsic oscillator as well as the central pacemaker plays a pivotal role in its rhythmicity. GC influences numerous biological processes, such as metabolic, cardiovascular, immune and even higher brain functions, and also acts as a resetting signal for the ubiquitous peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior. In this review, we will therefore focus on the recent advances in our understanding of the circadian regulation of adrenal GC and its functional relevance.

Linking neural activity and molecular oscillations in the SCN

Neurons in the suprachiasmatic nucleus (SCN) function as part of a central timing circuit that drives daily changes in our behaviour and underlying physiology. A hallmark feature of SCN neuronal populations is that they are mostly electrically silent during the night, start to fire action potentials near dawn and then continue to generate action potentials with a slow and steady pace all day long. Sets of currents are responsible for this daily rhythm, with the strongest evidence for persistent Na(+) currents, L-type Ca(2+) currents, hyperpolarization-activated currents (I(H)), large-conductance Ca(2+) activated K(+) (BK) currents and fast delayed rectifier (FDR) K(+) currents. These rhythms in electrical activity are crucial for the function of the circadian timing system, including the expression of clock genes, and decline with ageing and disease. This article reviews our current understanding of the ionic and molecular mechanisms that drive the rhythmic firing patterns in the SCN.

Uncovering the proteome response of the master circadian clock to light using an AutoProteome system

In mammals, the suprachiasmatic nucleus (SCN) is the central circadian pacemaker that governs rhythmic fluctuations in behavior and physiology in a 24-hr cycle and synchronizes them to the external environment by daily resetting in response to light. The bilateral SCN is comprised of a mere ~20,000 neurons serving as cellular oscillators, a fact that has, until now, hindered the systematic study of the SCN on a global proteome level. Here we developed a fully automated and integrated proteomics platform, termed AutoProteome system, for an in-depth analysis of the light-responsive proteome of the murine SCN. All requisite steps for a large-scale proteomic study, including preconcentration, buffer exchanging, reduction, alkylation, digestion and online two-dimensional liquid chromatography-tandem MS analysis, are performed automatically on a standard liquid chromatography-MS system. As low as 2 ng of model protein bovine serum albumin and up to 20 μg and 200 μg of SCN proteins can be readily processed and analyzed by this system. From the SCN tissue of a single mouse, we were able to confidently identify 2131 proteins, of which 387 were light-regulated based on a spectral counts quantification approach. Bioinformatics analysis of the light-inducible proteins reveals their diverse distribution in different canonical pathways and their heavy connection in 19 protein interaction networks. The AutoProteome system identified vasopressin-neurophysin 2-copeptin and casein kinase 1 delta, both of which had been previously implicated in clock timing processes, as light-inducible proteins in the SCN. Ras-specific guanine nucleotide-releasing factor 1, ubiquitin protein ligase E3A, and X-linked ubiquitin specific protease 9, none of which had previously been implicated in SCN clock timing processes, were also identified in this study as light-inducible proteins. The AutoProteome system opens a new avenue to systematically explore the proteome-wide events that occur in the SCN, either in response to light or other stimuli, or as a consequence of its intrinsic pacemaker capacity.

Intrinsically photosensitive retinal ganglion cells: many subtypes, diverse functions

For decades, rods and cones were thought to be the only photoreceptors in the mammalian retina. However, a population of atypical photoreceptive retinal ganglion cells (RGCs) expresses the photopigment melanopsin and is intrinsically photosensitive (ipRGCs). These ipRGCs are crucial for relaying light information from the retina to the brain to control circadian photoentrainment, pupillary light reflex, and sleep. ipRGCs were initially described as a uniform population involved solely in signaling irradiance for non-image forming functions. Recent work, however, has uncovered that ipRGCs are unexpectedly diverse at the molecular, cellular and functional levels, and could even be involved in image formation. This review summarizes our current understanding of the diversity of ipRGCs and their various roles in modulating behavior.

MAGED1: molecular insights and clinical implications

The melanoma antigen (MAGE) protein family contains more than 25 members that share a conserved MAGE homology domain (MHD). Type I MAGE genes exhibit cancer/testis-specific expression patterns and antigenic properties which render them ideal candidates for cancer immunotherapies. Maged1, a type II MAGE gene, is ubiquitously expressed and has been previously shown to play an important role in neuronal apoptosis during development. Recent studies have expanded the functional tissues and processes in which Maged1 activity is important and uncovered interacting partners of MAGED1 protein, adding novel layers to Maged1 functions. Maged1 plays a role in anti-tumorigenesis in a variety of cell types, and the down-regulation of MAGED1 has been observed in tumor cells. Moreover, MAGED1 can interact with a specific group of nuclear members and regulate circadian clock functions. These newly identified functions will enrich the molecular and clinical studies of the MAGE family of proteins.

On influences of global and local cues on the rate of synchronization of oscillator networks

Synchronization of connected oscillator networks under global and local cues is ubiquitous in both science and engineering. Over the last few decades, enormous attention has been paid to study synchronization conditions of connected oscillators in chemistry, physics, mechanics, and particularly in biology. However, the influences of global and local cues on the rate of synchronization have not been fully studied. It is widespread that synchronization is achieved in the simultaneous presence of both global and local cues, such as intercellular coupling signals and external entrainment signals in terms of biological oscillators, and inter-neighbor coupling signals between follower nodes and central guiding signals in terms of groups of mobile autonomous agents. We prove in this paper that strength of the global cue is the only determinant of the rate of synchronization. More specifically, we prove that a stronger global cue means a faster rate of synchronization whereas a stronger local cue does not necessarily make the synchronization rate faster. Our results not only apply to the noise free case, but also apply to the case that the oscillator natural frequencies are subject to white noise. The analysis does not require the interplay to be symmetric or balanced. Simulation results are given to illustrate the proposed results.

Phase synchronization of three locally coupled chaotic electrochemical oscillators: enhanced phase diffusion and identification of indirect coupling

Experiments are carried out with three locally coupled phase coherent chaotic electrochemical oscillators (A-B-C) in nickel dissolution in sulfuric acid. As the interaction strength is increased among the electrodes, an onset of synchronization is observed where the frequencies become identical and the phase differences are bounded. The precision of the period of the oscillators is characterized by phase diffusion coefficients from phases and phase differences. The transition to synchronization with increase of coupling strength was found to be accompanied by enhanced phase fluctuations that cause the precision of the oscillations to deteriorate. A parallel synchrony analysis showed that the direct (between A and B and B and C) and the indirect (between A and C) couplings can be correctly identified with the use of a partial phase synchrony index; therefore, the network topology can be deduced from dynamical measurements. Numerical simulations with a locally coupled model for electrochemical chaos confirm the presence of enhanced phase fluctuations close to the transition to synchronization and the usefulness of the partial phase synchrony index for differentiation of direct from indirect interactions in a small network of oscillators.

Emergence of noise-induced oscillations in the central circadian pacemaker

Bmal1 is an essential transcriptional activator within the mammalian circadian clock. We report here that the suprachiasmatic nucleus (SCN) of Bmal1-null mutant mice, unexpectedly, generates stochastic oscillations with periods that overlap the circadian range. Dissociated SCN neurons expressed fluctuating levels of PER2 detected by bioluminescence imaging but could not generate circadian oscillations intrinsically. Inhibition of intercellular communication or cyclic-AMP signaling in SCN slices, which provide a positive feed-forward signal to drive the intracellular negative feedback loop, abolished the stochastic oscillations. Propagation of this feed-forward signal between SCN neurons then promotes quasi-circadian oscillations that arise as an emergent property of the SCN network. Experimental analysis and mathematical modeling argue that both intercellular coupling and molecular noise are required for the stochastic rhythms, providing a novel biological example of noise-induced oscillations. The emergence of stochastic circadian oscillations from the SCN network in the absence of cell-autonomous circadian oscillatory function highlights a previously unrecognized level of circadian organization.

Increased late night response to light controls the circadian pacemaker in a nocturnal primate

The mammalian endogenous circadian clock, the suprachiasmatic nuclei, receives environmental inputs, namely the light-dark cycle, through photopigments located in the eye and from melanopsin-expressing retinal ganglion cells. The authors investigated the influence of light wavelength and intensity on the synchronization of the rest-activity rhythm of the gray mouse lemur, a nocturnal Malagasy primate. Animals were tested at different irradiance levels (320, 45, 13, and 6 nmol x m(-2) x s(- 1)) under several light wavelengths (from 400 to 610 nm). Several parameters including circadian period, activity, and body temperature waveforms were used to assess synchronization to a 12:12 light-dark cycle in comparison to control treatments (12:12 white light or continuous darkness). Entrainment of the circadian rest-activity cycle increased with light intensity. It was more efficient for mid wavelengths relative to shorter or longer wavelengths but not coincident with melanopsin maximal sensitivity, suggesting other photoreceptors are likely involved in lemurs' photoentrainment. The authors obtained a novel synchronization pattern characterized by a clear synchronization to lights-on only without phasing to lights-off. Changes in photo-responsiveness at dusk and dawn highlight differential responses of evening and morning oscillators in the circadian clock.

Circadian rhythms and memory formation

There has been considerable progress in elucidating the molecular mechanisms that contribute to memory formation and the generation of circadian rhythms. However, it is not well understood how these two processes interact to generate long-term memory. Recent studies in both vertebrate and invertebrate models have shown time-of-day effects on neurophysiology and memory formation, and have revealed a possible role for cycling molecules in memory persistence. Together, these studies suggest that common mechanisms underlie circadian rhythmicity and long-term memory formation.

Stoichiometry of N-methyl-D-aspartate receptors within the suprachiasmatic nucleus

The circadian pacemaker within the suprachiasmatic nucleus (SCN) confers daily rhythms to bodily functions. In nature, the circadian clock will adopt a 24-h period by synchronizing to the solar light/dark cycle. This light entrainment process is mediated, in part, at glutamatergic synapses formed between retinal ganglion afferents and SCN neurons. N-methyl-D-aspartate receptors (NMDARs) located on SCN neurons gate light-induced phase resetting. Despite their importance in circadian physiology, little is known about their functional stoichiometry. We investigated the NR2-subunit composition with whole cell recordings of SCN neurons within the murine hypothalamic brain slice using a combination of subtype-selective NMDAR antagonists and voltage-clamp protocols. We found that extracellular magnesium ([Mg](o)) strongly blocks SCN NMDARs exhibiting affinities and voltage sensitivities associated with NR2A and NR2B subunits. These NMDAR currents were inhibited strongly by NR2B-selective antagonists, Ro 25-6981 (3.5 microM, 55.0 +/- 9.0% block; mean +/- SE) and ifenprodil (10 microM, 55.8 +/- 3.0% block). The current remaining showed decreased [Mg](o) affinities reminiscent of NR2C and NR2D subunits but was highly sensitive to [Zn](o), a potent NR2A blocker, showing a approximately 44.2 +/- 1.1% maximal inhibition at saturating concentrations with an IC(50) of 7.8 +/- 1.1 nM. Considering the selectivity, efficacy, and potency of the drugs used in combination with [Mg](o)-block characteristics of the NMDAR, our data show that both diheteromeric NR2B NMDARs and triheteromeric NR2A NMDARs (paired with an NR2C or NR2D subunits) account for the vast majority of the NMDAR current within the SCN.

PER2 rhythms in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central pacemaker that controls circadian rhythms in mammals. In diurnal grass rats (Arvicanthis niloticus), many functional aspects of the SCN are similar to those of nocturnal rodents, making it likely that the difference in the circadian system of diurnal and nocturnal animals lies downstream from the SCN. Rhythms in clock genes expression occur in several brain regions outside the SCN that may function as extra-SCN oscillators. In male grass rats PER1 is expressed in the oval nucleus of the bed nucleus of the stria terminalis (BNST-ov) and in the central and basolateral amygdala (CEA and BLA, respectively); several features of PER1 expression in these regions of the grass rat brain differ substantially from those of nocturnal species. Here we describe PER2 rhythms in the same three brain regions of the grass rat. In the BNST-ov and CEA PER2 expression peaked early in the light period Zeitgeber time (ZT) 2 and was low during the early night, which is the reverse of the pattern of nocturnal rodents. In the BLA, PER2 expression was relatively low for most of the 24-h cycle, but showed an acute elevation late in the light period (ZT10). This pattern is also different from that of nocturnal rodents that show elevated PER2 expression in the mid to late night and into the early day. These results are consistent with the hypothesis that diurnal behavior is associated with a phase change between the SCN and extra-SCN oscillators.

Cell-type specific distribution of chloride transporters in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is a circadian oscillator and biological clock. Cell-to-cell communication is important for synchronization among SCN neuronal oscillators and the great majority of SCN neurons use GABA as a neurotransmitter, the principal inhibitory neurotransmitter in the adult CNS. Acting via the ionotropic GABA(A) receptor, a chloride ion channel, GABA typically evokes inhibitory responses in neurons via Cl(-) influx. Within the SCN GABA evokes both inhibitory and excitatory responses although the mechanism underlying GABA-evoked excitation in the SCN is unknown. GABA-evoked depolarization in immature neurons in several regions of the brain is a function of intracellular chloride concentration, regulated largely by the cation-chloride cotransporters NKCC1 (sodium/potassium/chloride cotransporter for chloride entry) and KCC1-4 (potassium/chloride cotransporters for chloride egress). It is well established that changes in the expression of the cation-chloride cotransporters through development determines the polarity of the response to GABA. To understand the mechanisms underlying GABA-evoked excitation in the SCN, we examined the SCN expression of cation-chloride cotransporters. Previously we reported that the K(+)/Cl(-) cotransporter KCC2, a neuron-specific chloride extruder conferring GABA's more typical inhibitory effects, is expressed exclusively in vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) neurons in the SCN. Here we report that the K(+)/Cl(-) cotransporter isoforms KCC4 and KCC3 are expressed solely in vasopressin (VP) neurons in the rat SCN whereas KCC1 is expressed in VIP neurons, similar to KCC2. NKCC1 is expressed in VIP, GRP and VP neurons in the SCN as is WNK3, a chloride-sensitive neuron-specific with no serine-threonine kinase which modulates intracellular chloride concentration via opposing actions on NKCC and KCC cotransporters. The heterogeneous distribution of cation-chloride cotransporters in the SCN suggests that Cl(-) levels are differentially regulated within VIP/GRP and VP neurons. We suggest that GABA's excitatory action is more likely to be evoked in VP neurons that express KCC4.

Suprachiasmatic nucleus: cell autonomy and network properties

The suprachiasmatic nucleus (SCN) is the primary circadian pacemaker in mammals. Individual SCN neurons in dispersed culture can generate independent circadian oscillations of clock gene expression and neuronal firing. However, SCN rhythmicity depends on sufficient membrane depolarization and levels of intracellular calcium and cAMP. In the intact SCN, cellular oscillations are synchronized and reinforced by rhythmic synaptic input from other cells, resulting in a reproducible topographic pattern of distinct phases and amplitudes specified by SCN circuit organization. The SCN network synchronizes its component cellular oscillators, reinforces their oscillations, responds to light input by altering their phase distribution, increases their robustness to genetic perturbations, and enhances their precision. Thus, even though individual SCN neurons can be cell-autonomous circadian oscillators, neuronal network properties are integral to normal function of the SCN.

Reproductive and diurnal rhythms regulate vocal motor plasticity in a teleost fish

Seasonal and circadian rhythms control fundamental physiological processes including neural excitability and synaptic plasticity that can lead to the periodic modulation of motor behaviors like social vocalizations. Parental male midshipman fish produce three call types during the breeding season: long duration (min to >1 h) advertisement 'hums', frequency and amplitude modulated agonistic 'growls' (s), and very brief (ms) agonistic 'grunts' produced either singly or repetitively as ;grunt trains' for up to several minutes. Fictive grunts that establish the temporal properties of natural grunts are readily evoked and recorded in vivo from vocal occipital nerve roots at any time of day or year by electrical microstimulation in either the midbrain periaqueductal gray or a hindbrain vocal pre-pacemaker nucleus. Now, as shown here, the longer duration fictive growls and hums can also be elicited, but are restricted to the nocturnal reproductive season. A significant drop in call threshold accompanies the fictive growls and hums that are distinguished by their much longer duration and lower and more regular firing frequency. Lastly, the long duration fictive calls are dependent upon increased stimulation time and intensity and hence may result from activity-dependent changes in the vocal motor circuit that are themselves modulated by seasonal and circadian rhythms.

RNA-interference knockdown of Drosophila pigment dispersing factor in neuronal subsets: the anatomical basis of a neuropeptide's circadian functions

Background

In animals, neuropeptide signaling is an important component of circadian timekeeping. The neuropeptide pigment dispersing factor (PDF) is required for several aspects of circadian activity rhythms in Drosophila.

Methodology/Principal Findings

Here we investigate the anatomical basis for PDF's various circadian functions by targeted PDF RNA-interference in specific classes of Drosophila neuron. We demonstrate that PDF is required in the ventro-lateral neurons (vLNs) of the central brain and not in the abdominal ganglion for normal activity rhythms. Differential knockdown of PDF in the large or small vLNs indicates that PDF from the small vLNs is likely responsible for the maintenance of free-running activity rhythms and that PDF is not required in the large vLNs for normal behavior. PDF's role in setting the period of free-running activity rhythms and the proper timing of evening activity under light:dark cycles emanates from both subtypes of vLN, since PDF in either class of vLN was sufficient for these aspects of behavior.

Conclusions/Significance

These results reveal the neuroanatomical basis PDF's various circadian functions and refine our understanding of the clock neuron circuitry of Drosophila.

Integrating ELF4 into the circadian system through combined structural and functional studies

The circadian clock is a timekeeping mechanism that enables anticipation of daily environmental changes. In the plant Arabidopsis thaliana, the circadian system is a multiloop series of interlocked transcription-translation feedbacks. Several genes have been arranged in these oscillation loops, but the position of the core-clock gene ELF4 in this network was previously undetermined. ELF4 lacks sequence similarity to known domains, and functional homologs have not yet been identified. Here we show that ELF4 is functionally conserved within a subclade of related sequences, and forms an alpha-helical homodimer with a likely electrostatic interface that could be structurally modeled. We support this hypothesis by expression analysis of new elf4 hypomorphic alleles. These weak mutants were found to have expression level phenotypes of both morning and evening clock genes, implicating multiple entry points of ELF4 within the multiloop network. This could be mathematically modeled. Furthermore, morning-expression defects were particular to some elf4 alleles, suggesting predominant ELF4 action just preceding dawn. We provide a new hypothesis about ELF4 in the oscillator-it acts as a homodimer to integrate two arms of the circadian clock.

Effects of brinzolamide vs timolol as an adjunctive medication to latanoprost on circadian intraocular pressure control in primary open-angle glaucoma Japanese patients

Purpose

To study the effect of the concomitant use of brinzolamide and latanoprost on the 24-hour variation in intraocular pressure (IOP) in primary open-angle glaucoma (POAG) patients first treated with timolol and latanoprost.

Methods

We studied 30 eyes from 30 POAG patients previously treated with latanoprost monotherapy. After a washout of four weeks in both eyes, all patients were treated with 0.5% timolol and latanoprost. Three months after the initiation of treatment with timolol and latanoprost, the 24-hour IOP variation was measured. Thereafter, all patients were treated with concomitant 1% brinzolamide and latanoprost. Three months after changing the therapeutic regimen, the 24-hour IOP variation was measured a second time. Latanoprost was administered once a day, and timolol and brinzolamide were twice a day. IOP was measured with a noncontact tomometer.

Results

On treatment with brinzolamide and latanoprost, a significant decrease in IOP compared to timolol and latanoprost was observed at all time points except at 3 PM and 6 PM. As a group, the patients had a significantly lower diurnal mean IOP and nocturnal mean IOP during treatment with brinzolamide and latanoprost than with timolol and latanoprost.

Conclusions

Treatment of POAG with a combination of brinzolamide and latanoprost demonstrated improved hypotensive effects compared with timolol and latanoprost during a 24-hour period.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Time-of-day determines neuronal damage and mortality after cardiac arrest

Ischemic events in humans are not evenly distributed across the day. To discriminate between temporal differences in the incidence of ischemia and susceptibility to ischemic events, we examined the outcome of global ischemia in a murine model at three time points during the day. Global cerebral ischemia in mice during the light phase impairs survival and exacerbates outcome compared to ischemia at other times of the day. Specifically, mice that underwent cardiac arrest during the light phase had greater numbers of degenerating neurons, greater microglial activation, and increased proinflammatory cytokine production in the ischemia-vulnerable hippocampus, as well as increased locomotor activity. Time-of-day differences were not altered by the melatonin receptor antagonist luzindole. Our results document that brain tissue displays endogenous fluctuations in susceptibility to ischemic damage and demonstrate that small differences in time of onset can significantly influence ischemic outcomes.

Daily pattern of pituitary glutamine, glutamate, and aspartate content disrupted by cadmium exposure

Cadmium is a neurotoxic heavy metal and is considered endocrine disruptor. In this work, we investigate the effects of cadmium on the 24 h changes of aspartate, glutamate, and glutamine content in the pituitary. Adult male Sprague-Dawley rats were treated with 25 or 50 mg/l of cadmium chloride (CdCl(2)) in the drinking water for 30 days. Metal exposure with the lowest dose induced the disappearance of the nocturnal peak of anterior pituitary amino acid content, and the appearance of a peak of glutamine concentration during the resting phase of the photoperiod. After exposure to 50 mg/l of CdCl(2), the peaks of anterior pituitary amino acid content at 12:00 and 00:00 h disappeared, and two minimal values at these same hours and a peak at 08:00 h appeared. In the posterior pituitary, cadmium treatment with the lowest dose induced the appearance of a peak of aspartate and glutamate concentration at 12:00 h, and the disappearance of the peak of glutamine content at 16:00 h. After exposure to 50 mg/l of CdCl(2) aspartate and glutamate daily pattern presented two maximal values between 00:00 and 04:00 h, and the metal abolished glutamine daily pattern. These results suggest that cadmium disrupted aspartate, glutamate, and glutamine daily pattern in the pituitary.

Activation of suprachiasmatic nuclei and primary visual cortex depends upon time of day

The human suprachiasmatic nucleus (SCN), the master biological clock, is a small (approximately 2 mm(3)) and deep structure located in the anterior hypothalamus. Previous methods do not allow in vivo study of the human SCN in a non-invasive manner. Therefore, we explored blood oxygen level-dependent (BOLD)-functional magnetic resonance imaging (fMRI) with OFF-ON-OFF block-designed visual stimuli to record the activities in the 'SCN and peri SCN in the anterior hypothalamus' (SCN+) and the primary visual area V1 using a 3T Siemens scanner and six normal subjects. We found that: (i) the BOLD-fMRI response to light and the mean of percentage activation in the SCN+ at midday was significantly less than that at night; and (ii) the number of activated voxels in most of the visual area V1 at midday was significantly higher than that at night. We conclude that BOLD-fMRI responses to light in the SCN+ and the V1 areas vary with time of day. This conclusion is consistent with: (i) the previously measured phase-response curve to light [J. Physiol., 549.3 (2003) 945] for the SCN activity at critical intensity threshold; and (ii) the interaction of the melanopsin-based signals with the rod-cone signals at the 'giant' retinal ganglion cells [Nature, 433 (2005) 749] for the V1 activity.

Diurnal expression of period 2 and urocortin 1 in neurones of the non-preganglionic Edinger-Westphal nucleus in the rat

Period 2 (Per2) is an important clock gene involved in the regulation of the major circadian clock in the mammalian central nervous system, the suprachiasmatic nucleus. In addition, Per2 is expressed in many other stress-sensitive brain structures. We have previously showed that the non-preganglionic Edinger-Westphal nucleus (npEW) is the main site of the corticotropin-releasing factor peptide family member urocortin 1 (Ucn1) and that this peptide undergoes conspicuous expression changes in response to various stressors. Here, we hypothesized that in the rat npEW both Per2 and Ucn1 would be produced in a diurnal, rhythmical fashion. This hypothesis was tested by following this expected rhythm on two days in rats killed at four time points each day (Zeitgeber times 0, 6, 12, and 18). We showed the co-existence of Per2 and Ucn1 in the npEW with double-label immunofluorescence and demonstrated with quantitative RT-PCR and semi-quantitative immunocytochemistry diurnal rhythms in Per2 mRNA expression and Per2 protein content, each on a single different day, with a minimum at lights-off and a maximum at lights-on. We furthermore revealed a diurnal rhythm in the number of Ucn1-immunopositive neurones and in their Ucn1 peptide content, with a minimum at night and at the beginning of the light period and a peak at lights-off, while the Ucn1 mRNA content paralleled the Per2 mRNA rhythm. The rhythms were accompanied by a diurnal rhythm in plasma corticosterone concentration. Our results are in line with the hypothesis that both Per2 and Ucn1 in the rat npEW are produced in a diurnal fashion, a phenomenon that may be relevant for the regulation of the diurnal rhythm in the stress response.

Brain Clocks: From the Suprachiasmatic Nuclei to a Cerebral Network

Circadian timing affects almost all life’s processes. It not only dictates when we sleep, but also keeps every cell and tissue working under a tight temporal regimen. The daily variations of physiology and behavior are controlled by a highly complex system comprising of a master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus, extra-SCN cerebral clocks, and peripheral oscillators. Here are presented similarities and differences in the molecular mechanisms of the clock machinery between the primary SCN clock and extra-SCN brain clocks. Diversity of secondary clocks in the brain, their specific sensitivities to time-giving cues, as their differential coupling to the master SCN clock, may allow more plasticity in the ability of the circadian timing system to integrate a wide range of temporal information. Furthermore, it raises the possibility that pathophysiological alterations of internal timing that are deleterious for health may result from internal desynchronization within the network of cerebral clocks.

Melatonin and its relevance to jet lag

Jet lag is a disorder in which body rhythms are out of phase with the environment because of rapid travel across time zones. Although it often produces minor symptoms it can cause serious problems in those who need to make rapid critical decisions including airline pilots and business travelers. In this article the authors review basic knowledge underlying the body clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, and the manner in which it regulates the sleep/wake cycle. The regulation of melatonin by the SCN is described together with the role of the melatonin receptors which are integral to its function as the major hormonal output of the body clock. Several factors are known that help prevent and treat jet lag, including ensuring adequate sleep, appropriate timing of exposure to bright light and treatment with melatonin. Because travel can cross a variable number of time zones and in two different directions, recommendations for treatment are given that correspond with these different types of travel. In addition to use of bright light and melatonin, other factors including timed exercise, timed and selective diets and social stimuli deserve study as potential treatments. Moreover, new melatonin agonists are currently under investigation for treatment of jet lag.

The genetics of mammalian circadian order and disorder: implications for physiology and disease

Circadian cycles affect a variety of physiological processes, and disruptions of normal circadian biology therefore have the potential to influence a range of disease-related pathways. The genetic basis of circadian rhythms is well studied in model organisms and, more recently, studies of the genetic basis of circadian disorders has confirmed the conservation of key players in circadian biology from invertebrates to humans. In addition, important advances have been made in understanding how these molecules influence physiological functions in tissues throughout the body. Together, these studies set the scene for applying our knowledge of circadian biology to the understanding and treatment of a range of human diseases, including cancer and metabolic and behavioural disorders.