MPer1 and mper2 are essential for normal resetting of the circadian clock

Differential response of Period 1 expression within the suprachiasmatic nucleus

The suprachiasmatic nuclei (SCNs) of the hypothalamus contain a circadian clock that exerts profound control over rhythmic physiology and behavior. The clock consists of multiple autonomous cellular pacemakers distributed throughout the rat SCN. In response to a shift in the light schedule, the SCN rapidly changes phase to achieve the appropriate phase relationship with the shifted light schedule. Through use of a transgenic rat in which rhythmicity in transcription of the Period 1 gene was measured with a luciferase reporter (Per1-luc), we have been successful in tracking the time course of molecular rhythm phase readjustments in different regions of the SCN that occur in response to a shift in the light schedule. We find that different regions of the SCN phase adjust at different rates, leading to transient internal desynchrony in Per1-luc expression among SCN regions. This desynchrony among regions is most pronounced and prolonged when the light schedule is advanced compared with light schedule delays. A similar asymmetry in the speed of phase resetting is observed with locomotor behavior, suggesting that phase shifting kinetics within the SCN may underlay the differences observed in behavioral resetting to advances or delays in the light schedule.

Uncovering the design principles of circadian clocks: Mathematical analysis of flexibility and evolutionary goals

In this paper, we present the mathematical details underlying both an approach to the flexibility of regulatory networks and an analytical characterization of evolutionary goals of circadian clock networks. A fundamental problem in cellular regulation is to understand the relation between the form of regulatory networks and their function. Circadian clocks present a particularly interesting instance of this. Recent work has shown that they have complex structures involving multiple interconnected feedback loops with both positive and negative feedback. We address the question of why they have such a complex structure and argue that it is to provide the flexibility necessary to simultaneously attain multiple key properties of circadian clocks such as robust entrainment and temperature compensation. To do this we address two fundamental problems: (A) to understand the relationships between the key evolutionary aims of the clock and (B) to ascertain how flexible the clock's structure is. To address the first problem we use infinitesimal response curves (IRCs), a tool that we believe will be of general utility in the analysis of regulatory networks. To understand the second problem we introduce the flexibility dimension d, show how to calculate it and then use it to analyse a range of models. We believe our results will generalize to a broad range of regulatory networks.

Temporal and spatial distribution of immunoreactive PER1 and PER2 proteins in the suprachiasmatic nucleus and peri-suprachiasmatic region of the diurnal grass rat (Arvicanthis niloticus)

The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian pacemaker in both diurnal and nocturnal mammals. The lower subparaventricular zone (LSPV) immediately dorsal to the SCN may also play an important role in the regulation of circadian rhythms. The SCN contains a multitude of oscillator cells that generate circadian rhythms through transcriptional/translational feedback loops involving a set of clock genes including per1 and per2. Little is known about the temporal and spatial features of the proteins encoded by these genes in day-active mammals. The first objective of this study was to characterize the expression of PER1 and PER2 in the SCN of a diurnal rodent, the unstriped Nile grass rat (Arvicanthis niloticus). The second objective was to evaluate the hypothesis that a molecular clock could exist in the LSPV, where endogenous rhythms in Fos expression are seen in grass rats but not in laboratory rats. Animals were kept on a 12:12 light/dark cycle and perfused at 4-h intervals, and their brains were processed for immunohistochemical detection of PER1 and PER2. Both proteins were seen in the SCN where they peaked early in the dark phase, providing further evidence that the differences between diurnal and nocturnal patterns of behavior emerge from mechanisms lying downstream from the pacemaker within the SCN. Rhythmic expression of PER1 and PER2 was also seen in the LSPV providing support for the hypothesis that this region might participate in circadian time keeping in the diurnal grass rat. In addition, rhythms were seen lateral to the LSPV and the SCN. Results of this study are discussed in light of similarities and differences in the circadian time-keeping systems of day- and night-active animals.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

Ontogenetic development of the mammalian circadian system

This review summarizes the current knowledge about the ontogenetic development of the circadian system in mammals. The developmental changes of overt rhythms are discussed, although the main focus of the review is the underlying neuronal and molecular mechanisms. In addition, the review describes ontogenetic development, not only as a process of morpho-functional maturation. The need of repeated adaptations and readaptations due to changing developmental stage and environmental conditions is also considered. The review analyzes mainly rodent data, obtained from the literature and from the author's own studies. Results from other species, including humans, are presented to demonstrate common features and species-dependent differences. The review first describes the development of the suprachiasmatic nuclei as the central pacemaker system and shows that intrinsic circadian rhythms are already generated in the mammalian fetus. As in adult organisms, the period length is different from 24 h and needs continuous correction by environmental periodicities, or zeitgebers. The investigation of the ontogenetic development of the mechanisms of entrainment reveals that, at prenatal and early postnatal stages, non-photic cues deriving from the mother are effective. Light-dark entrainment develops later. At a certain age, both photic and non-photic zeitgebers may act in parallel, even though the respective time information is 12 h out of phase. That leads to a temporary internal desynchronization. Because rhythmic information needs to be transferred to effector organs, the corresponding neural and humoral signalling pathways are also briefly described. Finally, to be able to transform a rhythmic signal into an overt rhythm, the corresponding effector organs must be functionally mature. As many of these organs are able to generate their own intrinsic rhythms, another aspect of the review is dedicated to the development of peripheral oscillators and mechanisms of their entrainment. The latter includes control by the central pacemaker as well as by distinct environmental signals. Ecological aspects of the described developmental changes in the circadian system and some practical consequences are also briefly discussed.

AMPA/kainate receptor antagonist DNQX blocks the acute increase of Per2 mRNA levels in most but not all areas of the SCN

The daily light:dark cycle synchronizes the circadian timing system by resetting the phase of the circadian pacemaker on a daily basis. Light acutely increases mRNA levels of the clock genes Per1 and Per2 in the suprachiasmatic nucleus (SCN), the site of the primary circadian pacemaker in mammals. Light is conveyed to the SCN through the retinohypothalamic tract (RHT), an efferent projection from retinal ganglion cells that releases the excitatory amino acid (EAA) neurotransmitter glutamate in the SCN. EAA receptor activation in the SCN is critical for the ability of light to phase-shift the circadian pacemaker. In a previous study, we demonstrated that EAA receptor activation is necessary and sufficient for light to acutely increase Per1 mRNA levels in the SCN. In the current study, we determined whether EAA receptor activation in the SCN is necessary for the ability of light to increase Per2 mRNA levels in the SCN in Syrian hamsters. The NMDA receptor antagonist AP5 and the AMPA/kainate receptor antagonist DNQX inhibited the ability of light and NMDA to acutely increase Per2 mRNA levels in the SCN. In hamsters injected with DNQX, Per1 and Per2 mRNA levels remained slightly elevated in the ventrolateral SCN, suggesting that AMPA/kainate receptor activation in this region is not critical for the effects of light on the circadian pacemaker.

A guideline for analyzing circadian wheel-running behavior in rodents under different lighting conditions

Most behavioral experiments within circadian research are based on the analysis of locomotor activity. This paper introduces scientists to chronobiology by explaining the basic terminology used within the field. Furthermore, it aims to assist in designing, carrying out, and evaluating wheel-running experiments with rodents, particularly mice. Since light is an easily applicable stimulus that provokes strong effects on clock phase, the paper focuses on the application of different lighting conditions.

Measuring seasonal time within the circadian system: regulation of the suprachiasmatic nuclei by photoperiod

Day-length (photoperiod) is the primary environmental signal used to synchronise endogenous rhythms of physiology and behaviour. In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus house the master circadian clock. The SCN incorporate photoperiodic information and therefore measure both daily and seasonal time. Over the past decade, there have been significant advances in the understanding of the molecular basis of circadian clocks. It is now becoming apparent that the core molecular clock mechanism is itself regulated by photoperiod, although there is currently debate as to how this occurs. One recent model proposes that distinct groups of core 'clock genes' are associated with either morning or evening phases of the daily light/dark cycle. However, the validity of associating particular genes to morning and evening has been questioned. This article reviews the evidence for photoperiodic regulation of circadian clock function and then discusses alternative models that may explain the available data.

Isolation and phenogenetics of a novel circadian rhythm mutant in zebrafish

Widespread use of zebrafish (Danio rerio) in genetic analysis of embryonic development has led to rapid advances in the technology required to generate, map and clone mutated genes. To identify genes involved in the generation and regulation of vertebrate circadian rhythmicity, we screened for dominant mutations that affect the circadian periodicity of larval zebrafish locomotor behavior. In a screen of 6,500 genomes, we recovered 8 homozygous viable, semi-dominant mutants, and describe one of them here. The circadian period of the lager and lime (lag(dg2)) mutant is shortened by 0.7 h in heterozygotes,and 1.3 h in homozygotes. This mutation also shortens the period of the melatonin production rhythm measured from cultured pineal glands, indicating that the mutant gene product affects circadian rhythmicity at the tissue level, as well as at the behavioral level. This mutation also alters the sensitivity of pineal circadian period to temperature, but does not affect phase shifting responses to light. Linkage mapping with microsatellite markers indicates that the lag mutation is on chromosome 7. A zebrafish homolog of period1(per1) is the only known clock gene homolog that maps near the lag locus. However, all sequence variants found in per1 cDNA from lag(dg2) mutants are also present in wild type lines, and we were unable to detect any defect in per1 mRNA splicing, so this mutation may identify a novel clock gene.

Tetrodotoxin resets the clock

In mammals, circadian rhythms are driven by a pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. We measured the rhythm of arginine vasopressin release in rat organotypic SCN slices following application of tetrodotoxin (TTX) or N-methyl-D-aspartate (NMDA) at various times throughout the circadian cycle. TTX resets the clock in a manner similar to dark pulses. A 4-h application of TTX starting in mid subjective day, at around circadian time (CT) 7.0, induced phase advances, while TTX treatment started in early subjective morning, at about CT 2.0, induced phase delays. On the other hand, NMDA resets the clock in a manner similar to a light pulse; that is, NMDA treatment in the early evening induced phase delays while treatment in the late night induced phase advances. The data indicate that deprivation of neuronal firing changes the circadian rhythm.

The opioid fentanyl affects light input, electrical activity andPergene expression in the hamster suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain a major circadian pacemaker, which is regulated by photic and nonphotic stimuli. Although enkephalins are present in the SCN, their role in phase regulation of the pacemaker is largely unknown. The opioid agonist fentanyl, a homologue of morphine, is an addictive drug that induces phase shifts of circadian rhythms in hamsters. We observed that these phase shifts are blocked by naloxone, which is a critical test for true opioid receptor involvement, and conclude that opioid receptors are the sole mediators of the actions of fentanyl on the circadian timing system. A strong interaction between opioids and light input was shown by the ability of fentanyl and light to completely block each other's phase shifts of behavioural activity rhythms. Neuronal ensemble recordings in vitro provide first evidence that SCN cells show direct responses to fentanyl and react with a suppression of firing rate. Moreover, we show that fentanyl induces a strong attenuation of light-induced Syrian hamster Period 1 (shPer1) gene expression during the night. During the subjective day, we found no evidence for a role of shPer1 in mediation of fentanyl-induced phase shifts. Based on the present results, however, we cannot exclude the involvement of shPer2. Our data indicate that opioids can strongly modify the photic responsiveness of the circadian pacemaker and may do so via direct effects on SCN electrical activity and regulation of Per genes. This suggests that the pathways regulating addictive behaviour and the circadian clock intersect.

Functional development of the zebrafish pineal gland: light-induced expression of period2 is required for onset of the circadian clock

In zebrafish, the pineal gland is a photoreceptive organ that contains an intrinsic circadian oscillator and exhibits rhythmic arylalkylamine-N-acetyltransferase (zfaanat2) mRNA expression. In the present study, we investigated the role of light and of a clock gene, zperiod2 (zper2), in the development of this rhythm. Analysis of zfaanat2 mRNA expression in the pineal gland of 3-day-old zebrafish embryos after exposure to different photoperiodic regimes indicated that light is required for proper development of the circadian clock-controlled rhythmic expression of zfaanat2, and that a 1-h light pulse is sufficient to initiate this rhythm. Analysis of zper2 mRNA expression in zebrafish embryos exposed to different photoperiodic regimes indicated that zper2 expression is transiently up-regulated by light but is not regulated by the circadian oscillator. To establish the association between light-induced zper2 expression and light-induced clock-controlled zfaanat2 rhythm, zPer2 knock-down experiments were performed. The zfaanat2 mRNA rhythm, induced by a 1-h light pulse, was abolished in zPer2 knock-down embryos. These experiments indicated that light-induced zper2 expression is crucial for establishment of the clock-controlled zfaanat2 rhythm in the zebrafish pineal gland.

Regulation of prokineticin 2 expression by light and the circadian clock

Background

The suprachiasmatic nucleus (SCN) contains the master circadian clock that regulates daily rhythms of many physiological and behavioural processes in mammals. Previously we have shown that prokineticin 2 (PK2) is a clock-controlled gene that may function as a critical SCN output molecule responsible for circadian locomotor rhythms. As light is the principal zeitgeber that entrains the circadian oscillator, and PK2 expression is responsive to nocturnal light pulses, we further investigated the effects of light on the molecular rhythm of PK2 in the SCN. In particular, we examined how PK2 responds to shifts of light/dark cycles and changes in photoperiod. We also investigated which photoreceptors are responsible for the light-induced PK2 expression in the SCN. To determine whether light requires an intact functional circadian pacemaker to regulate PK2, we examined PK2 expression in cryptochrome1,2-deficient (Cry1-/-Cry2-/-) mice that lack functional circadian clock under normal light/dark cycles and constant darkness.

Results

Upon abrupt shifts of the light/dark cycle, PK2 expression exhibits transients in response to phase advances but rapidly entrains to phase delays. Photoperiod studies indicate that PK2 responds differentially to changes in light period. Although the phase of PK2 expression expands as the light period increases, decreasing light period does not further condense the phase of PK2 expression. Genetic knockout studies revealed that functional melanopsin and rod-cone photoreceptive systems are required for the light-inducibility of PK2. In Cry1-/-Cry2-/- mice that lack a functional circadian clock, a low amplitude PK2 rhythm is detected under light/dark conditions, but not in constant darkness. This suggests that light can directly regulate PK2 expression in the SCN.

Conclusion

These data demonstrate that the molecular rhythm of PK2 in the SCN is regulated by both the circadian clock and light. PK2 is predominantly controlled by the endogenous circadian clock, while light plays a modulatory role. The Cry1-/-Cry2-/- mice studies reveal a light-driven PK2 rhythm, indicating that light can induce PK2 expression independent of the circadian oscillator. The light inducibility of PK2 suggests that in addition to its role in clock-driven rhythms of locomotor behaviour, PK2 may also participate in the photic entrainment of circadian locomotor rhythms.

Orchestrating time: arrangements of the brain circadian clock

Daily oscillations in physiology and behavior are regulated by a brain clock located in the suprachiasmatic nucleus (SCN). Individual cells within this nucleus contain an autonomous molecular clock. Recent discoveries that make use of new molecular and genetic data and tools highlight the conclusion that the SCN is a heterogeneous network of functionally and phenotypically differentiated cells. Neurons within SCN subregions serve distinctly separate functions in regulating the overall activity of the circadian clock: some cells within the SCN rhythmically express "clock" genes, whereas others exhibit induced expression of these genes after the organism has been exposed to a light pulse. The coordinated interaction of these functionally distinct cells is integral to the coherent functioning of the brain clock.

Feeding cues alter clock gene oscillations and photic responses in the suprachiasmatic nuclei of mice exposed to a light/dark cycle

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the master mammalian circadian clock, which is mainly reset by light. Temporal restricted feeding, a potent synchronizer of peripheral oscillators, has only weak influence on light-entrained rhythms via the SCN, unless restricted feeding is coupled with calorie restriction, thereby altering phase angle of photic synchronization. Effects of daytime restricted feeding were investigated on the mouse circadian system. Normocaloric feeding at midday led to a predominantly diurnal (60%) food intake and decreased blood glucose in the afternoon, but it did not affect the phase of locomotor activity rhythm or vasopressin expression in the SCN. In contrast, hypocaloric feeding at midday led to 2-4 h phase advances of three circadian outputs, locomotor activity rhythm, pineal melatonin, and vasopressin mRNA cycle in the SCN, and it decreased daily levels of blood glucose. Furthermore, Per1 and Cry2 oscillations in the SCN were phase advanced by 1 and 3 h, respectively, in hypocalorie- but not in normocalorie-fed mice. The phase of Per2 and Bmal1 expression remained unchanged regardless of feeding condition. Moreover, the shape of behavioral phase-response curve to light and light-induced expression of Per1 in the SCN were markedly modified in hypocalorie-fed mice compared with animals fed ad libitum. The present study shows that diurnal hypocaloric feeding affects not only the temporal organization of the SCN clockwork and circadian outputs in mice under light/dark cycle but also photic responses of the circadian system, thus indicating that energy metabolism modulates circadian rhythmicity and gating of photic inputs in mammals.

Developmental alcohol exposure alters light-induced phase shifts of the circadian activity rhythm in rats

BACKGROUND

Developmental alcohol (EtOH) exposure produces long-term changes in the photic regulation of rat circadian behavior. Because entrainment of circadian rhythms to 24-hr light/dark cycles is mediated by phase shifting or resetting the clock mechanism, we examined whether developmental EtOH exposure also alters the phase-shifting effects of light pulses on the rat activity rhythm.

METHODS

Artificially reared Sprague-Dawley rat pups were exposed to EtOH (4.5 g/kg/day) or an isocaloric milk formula (gastrostomy control; GC) on postnatal days 4 to 9. At 2 months of age, rats from the EtOH, GC, and suckle control groups were housed individually, and wheel-running behavior was continuously recorded first in a 12-hr light/12-hr dark photoperiod for 10 to 14 days and thereafter in constant darkness (DD). Once the activity rhythm was observed to stably free-run in DD for at least 14 days, animals were exposed to a 15-min light pulse at either 2 or 10 hr after the onset of activity [i.e., circadian time (CT) 14 or 22, respectively], because light exposure at these times induces maximal phase delays or advances of the rat activity rhythm.

RESULTS

EtOH-treated rats were distinguished by robust increases in their phase-shifting responses to light. In the suckle control and GC groups, light pulses shifted the activity rhythm as expected, inducing phase delays of approximately 2 hr at CT 14 and advances of similar amplitude at CT 22. In contrast, the same light stimulus produced phase delays at CT 14 and advances at CT 22 of longer than 3 hr in EtOH-treated rats. The mean phase delay at CT 14 and advance at CT 22 in EtOH rats were significantly greater (p < 0.05) than the light-induced shifts observed in control animals.

CONCLUSIONS

The data indicate that developmental EtOH exposure alters the phase-shifting responses of the rat activity rhythm to light. This finding, coupled with changes in the circadian period and light/dark entrainment observed in EtOH-treated rats, suggests that developmental EtOH exposure may permanently alter the clock mechanism in the suprachiasmatic nucleus and its regulation of circadian behavior.

The clock gene Per2 influences the glutamatergic system and modulates alcohol consumption

Period (Per) genes are involved in regulation of the circadian clock and are thought to modulate several brain functions. We demonstrate that Per2(Brdm1) mutant mice, which have a deletion in the PAS domain of the Per2 protein, show alterations in the glutamatergic system. Lowered expression of the glutamate transporter Eaat1 is observed in these animals, leading to reduced uptake of glutamate by astrocytes. As a consequence, glutamate levels increase in the extracellular space of Per2(Brdm1) mutant mouse brains. This is accompanied by increased alcohol intake in these animals. In humans, variations of the PER2 gene are associated with regulation of alcohol consumption. Acamprosate, a drug used to prevent craving and relapse in alcoholic patients is thought to act by dampening a hyper-glutamatergic state. This drug reduced augmented glutamate levels and normalized increased alcohol consumption in Per2(Brdm1) mutant mice. Collectively, these data establish glutamate as a link between dysfunction of the circadian clock gene Per2 and enhanced alcohol intake.

Model based conjectures on mammalian clock controversies

We explore some predictions of a previously developed detailed model of molecular timekeeping in mammals (Forger and Peskin, PNAS, 100:14806) in areas where our understanding of clock mechanisms are incomplete. It is conjectured that: (1) the clock's 24-h period depends on mRNA stability. (2) REV-ERBalpha suppresses and/or entrains rhythms in peripheral tissues by regulating CRY1 transcription. (3) CLK:BMAL1 oscillations are suppressed in the suprachiasmatic nuclei to enhance oscillations in other proteins. (4) PER2 is ineffective at causing phase advances because it is not induced by light during the late night. (5) The clock is a limit cycle oscillator that shows characteristics of the evening and morning oscillator model.

Circadian and light-induced transcription of clock gene Per1 depends on histone acetylation and deacetylation

Circadian clock genes are regulated through a transcriptional-translational feedback loop. Alterations of the chromatin structure by histone acetyltransferases and histone deacetylases (HDACs) are commonly implicated in the regulation of gene transcription. However, little is known about the transcriptional regulation of mammalian clock genes by chromatin modification. Here, we show that the state of acetylated histones fluctuated in parallel with the rhythm of mouse Per1 (mPer1) or mPer2 expression in fibroblast cells and liver. Mouse CRY1 (mCRY1) repressed transcription with HDACs and mSin3B, which was relieved by the HDAC inhibitor trichostatin A (TSA). In turn, TSA induced endogenous mPer1 expression as well as the acetylation of histones H3 and H4, which interacted with the mPer1 promoter region in fibroblast cells. Moreover, a light pulse stimulated rapid histone acetylation associated with the promoters of mPer1 or mPer2 in the suprachiasmatic nucleus (SCN) and the binding of phospho-CREB in the CRE of mPer1. We also showed that TSA administration into the lateral ventricle induced mPer1 and mPer2 expression in the SCN. Taken together, these data indicate that the rhythmic transcription and light induction of clock genes are regulated by histone acetylation and deacetylation.

Let there be “more” light: enhancement of light actions on the circadian system through non-photic pathways

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.

N-ethyl-N-nitrosourea mouse mutants in the dissection of behavioural and psychiatric disorders

Twin and adoption studies have consistently implicated genetics in the aetiology of psychiatric and behavioural disorders. The identification of the genes and molecular pathways that are associated with these traits using linkage studies has been difficult because psychiatric disorders are almost always non-mendelian, heterogeneous, involve multiple genetic loci and are influenced significantly by environmental factors. Mouse models that are based on intermediate signatures of psychiatric disease and pharmacological responsiveness hold promise as a complementary approach to dissecting the molecular basis of neurobehavioural disorders. This has been made possible by the development and refinement of gene targeting technologies and the use of super-efficient chemical mutagens. N-ethyl-N-nitrosourea (ENU) mutagenesis in the mouse, when coupled to a battery of sensitive behavioural screens, is an effective way of creating and identifying novel mouse behavioural mutants. Here, the concept of screening for ENU mutants is introduced while progress with two behavioural screens, an "anxiety" screen and a circadian screen, are presented. It is hoped that the study of mouse mutants that have arisen from these screens will provide new insights into the genetic basis of abnormal behaviour and that they might lead to the development of novel therapeutic compounds for human psychiatric disease.

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.

Resetting Mechanism of Central and Peripheral Circadian Clocks in Mammals

Almost all organisms on earth exhibit diurnal rhythms in physiology and behavior under the control of autonomous time-measuring system called circadian clock. The circadian clock is generally reset by environmental time cues, such as light, in order to synchronize with the external 24-h cycles. In mammals, the core oscillator of the circadian clock is composed of transcription/translation-based negative feedback loops regulating the cyclic expression of a limited number of clock genes (such as Per, Cry, Bmal1, etc.) and hundreds of output genes in a well-concerted manner. The central clock controlling the behavioral rhythm is localized in the hypothalamic suprachiasmatic nucleus (SCN), and peripheral clocks are present in other various tissues. The phase of the central clock is amenable to ambient light signal captured by the visual rod-cone photoreceptors and non-visual melanopsin in the retina. These light signals are transmitted to the SCN through the retinohypothalamic tract, and transduced therein by mitogen-activated protein kinase and other signaling molecules to induce Per gene expression, which eventually elicits phase-dependent phase shifts of the clock. The central clock controls peripheral clocks directly and indirectly by virtue of neural, humoral, and other signals in a coordinated manner. The change in feeding time resets the peripheral clocks in a SCN-independent manner, possibly by food metabolites and body temperature rhythms. In this article, we will provide an overview of recent molecular and genetic studies on the resetting mechanism of the central and peripheral circadian clocks in mammals.

E-box function in a period gene repressed by light

In most organisms, light plays a key role in the synchronization of the circadian timing system with the environmental day-night cycle. Light pulses that phase-shift the circadian clock also induce the expression of period (per) genes in vertebrates. Here, we report the cloning of a zebrafish per gene, zfper4, which is remarkable in being repressed by light. We have developed an in vivo luciferase reporter assay for this gene in cells that contain a light-entrainable clock. High-definition bioluminescence traces have enabled us to accurately measure phase-shifting of the clock by light. We have also exploited this model to study how four E-box elements in the zfper4 promoter regulate expression. Mutagenesis reveals that the integrity of these four E-boxes is crucial for maintaining low basal expression together with robust rhythmicity and repression by light. Importantly, in the context of a minimal heterologous promoter, the E-box elements also direct a robust circadian rhythm of expression that is significantly phase-advanced compared with the original zfper4 promoter and lacks the light-repression property. Thus, these results reveal flexibility in the phase and light responsiveness of E-box-directed rhythmic expression, depending on the promoter context.

Resetting the brain clock: time course and localization of mPER1 and mPER2 protein expression in suprachiasmatic nuclei during phase shifts

The mechanism whereby brief light pulses reset the mammalian circadian clock involves acute Per gene induction. In a previous study we investigated light-induced expression of mPer1 and mPer2 mRNA in the suprachiasmatic nuclei (SCN), with the aim of understanding the relationship between gene expression and behavioural phase shifts. In the present study, we examine the protein products of mPer1 and mPer2 genes in the core and shell region of SCN for 34 h following a phase-shifting light pulse, in order to further explore the molecular mechanism of photic entrainment. The results indicate that, during the delay zone of the phase response curve, while endogenous levels of mPER1 and mPER2 protein are falling, a light pulse produces an increase in the expression of both proteins. In contrast, during the advance zone of the phase response curve, while levels of endogenous mPER1 and mPER2 proteins are rising, a light pulse results in a further increase in mPER1 but not mPER2 protein. The regional distribution of mPER1 and mPER2 protein in the SCN follows the same pattern as their respective mRNAs, with mPER1 expression in the shell region of SCN correlated with phase advances and mPER2 in the shell region correlated with phase delays.

The metabolism of phospholipids oscillates rhythmically in cultures of fibroblasts and is regulated by the clock protein PERIOD 1

The mammalian circadian timing system is composed of countless cell oscillators distributed throughout the body and central pacemakers regulating temporal physiology and behavior. Peripheral clocks display circadian rhythms in gene expression both in vivo and in culture. We examined the biosynthesis of phospholipids as well as the expression of the clock gene period 1 (Per1) and its potential involvement in the regulation of the phospholipid metabolism in cultured quiescent NIH 3T3 cells synchronized by a 2 h serum shock. A 30 min pulse of radiolabeled precursor was given at phases ranging from 0.5 to 62 h after serum treatment. We observed a daily rhythm in the phospholipid labeling that persisted at least for two cycles, with levels significantly decreasing 29 and 58 h after treatment. Per1 expression exhibited a rapid and transient induction and a daily rhythmicity in antiphase to the lipid labeling. After Per1 expression knockdown, the rhythm of phospholipid labeling was lost. Furthermore, in cultures of CLOCK mutant fibroblasts--cells with a clock mechanism impairment--PER1 was equally expressed at all times examined and the phospholipid labeling did not oscillate. The results demonstrate that the biosynthesis of phospholipids oscillates daily in cultured fibroblasts by an endogenous clock mechanism involving Per1 expression.

Dissociation between circadian Per1 and neuronal and behavioral rhythms following a shifted environmental cycle

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a major circadian pacemaker that imposes or entrains rhythmicity on other structures by generating a circadian pattern in electrical activity. The identification of "clock genes" within the SCN and the ability to dynamically measure their rhythmicity by using transgenic animals open up new opportunities to study the relationship between molecular rhythmicity and other well-documented rhythms within the SCN. We investigated SCN circadian rhythms in Per1-luc bioluminescence, electrical activity in vitro and in vivo, as well as the behavioral activity of rats exposed to a 6-hr advance in the light-dark cycle followed by constant darkness. The data indicate large and persisting phase advances in Per1-luc bioluminescence rhythmicity, transient phase advances in SCN electrical activity in vitro, and an absence of phase advances in SCN behavioral or electrical activity measured in vivo. Surprisingly, the in vitro phase-advanced electrical rhythm returns to the phase measured in vivo when the SCN remains in situ. Our study indicates that hierarchical levels of organization within the circadian timing system influence SCN output and suggests a strong and unforeseen role of extra-SCN areas in regulating pacemaker function.

An abrupt shift in the day/night cycle causes desynchrony in the mammalian circadian center

The suprachiasmatic nucleus (SCN) is the neuroanatomical locus of the mammalian circadian pacemaker. Here we demonstrate that an abrupt shift in the light/dark (LD) cycle disrupts the synchronous oscillation of circadian components in the rat SCN. The phases of the RNA cycles of the period genes Per1 and Per2 and the cryptochrome gene Cry1 shifted rapidly in the ventrolateral, photoreceptive region of the SCN, but were relatively slow to shift in the dorsomedial region. During the period of desynchrony, the animals displayed increased nighttime rest, the timing of which was inversely correlated with the expression of Per1 mRNA in the dorsomedial SCN. Molecular resynchrony required approximately 6 d after a 10 hr delay and 9 approximately 13 d after a 6 hr advance of the LD cycle and was accompanied by the reemergence of normal rest-activity patterns. This dissociation and slow resynchronization of endogenous oscillators within the SCN after an LD cycle shift suggests a mechanism for the physiological symptoms that constitute jet lag.

Differential responses of circadian activity onset and offset following GABA-ergic and opioid receptor activation

The circadian pacemaker in the mammalian suprachiasmatic nuclei is responsive to photic and nonphotic stimuli. In the present study, the authors have investigated the response of activity onset and offset to application of nonphotic stimuli: the benzodiazepine midazolam and the opioid receptor agonist fentanyl. In correspondence with previous studies, both stimuli induced phase advances of the activity onset when given in the mid- to late subjective day. In contrast, activity offset did not phase advance following these injections. Injections during the early subjective day induced small phase delays of the activity onset, while large phase delays occurred in activity offset. Phase shifts, induced at both circadian time zones, were paralleled by an increase in the length of daily activity (alpha). The increase in a remained present during several days after the injection. The different kinetics in phase shifting of the activity onset and offset indicate complexity in phase-shifting behavior of the circadian pacemaker in response to nonphotic stimuli. Moreover, the data show responsiveness of the circadian system to GABA-ergic and opioid receptor activation, not only during the mid- to late subjective day but also during the early subjective day. The data implicate that the early subjective day is an interesting phase for analysis of molecular and biochemical processes involved in nonphotic phase shifting.

Attenuation of circadian light induced phase advances and delays by neuropeptide Y and a neuropeptide Y Y1/Y5 receptor agonist

Circadian rhythms can be synchronised to photic and non-photic stimuli. The circadian clock, anatomically defined as the suprachiasmatic nucleus in mammals, can be phase shifted by light during the night. Non-photic stimuli reset the circadian rhythm during the day. Photic and non-photic stimuli have been shown to interact during the day and night. Precise mechanisms for these complex interactions are unknown. A possible pathway for non-photic resetting of the clock is thought to generate from the intergeniculate leaflet, which conveys information to the suprachiasmatic nucleus (SCN) through the geniculohypothalamic tract and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Interactions between light and NPY were investigated during the early (2 h after activity onset) and late (6 h after activity onset) night in male Syrian hamsters. NPY microinjections into the region of the SCN significantly attenuated light-induced phase delay, during the early subjective night. Phase advances to light were completely inhibited by the administration of NPY during the late night. The precise mechanism by which NPY attenuates or blocks photic phase shifts is unclear, but the NPY Y5 receptor has been implicated in the mediation of this inhibitory effect. The NPY Y1/Y5 receptor agonist, [Leu(31),Pro(34)]NPY, was administered via cannula microinjections following light exposure during the early and late night. [Leu(31),Pro(34)]NPY significantly attenuated phase delays to light during the early night and blocked phase advances during the late night, in a manner similar to NPY. These results show the ability of NPY to attenuate phase shifts to light during the early night and block light-induced phase advances during the late night. Furthermore, this is the first in vivo study implicating the involvement of the NPY Y1/Y5 receptors in the complex interaction of photic and non-photic stimuli during the night. The alteration of photic phase shifts by NPY may influence photic entrainment within the circadian system.

Transduction of light in the suprachiasmatic nucleus: evidence for two different neurochemical cascades regulating the levels of Per1 mRNA and pineal melatonin

The suprachiasmatic nucleus (SCN) contains a circadian clock and regulates melatonin synthesis in the pineal gland. Light exposure during the subjective night acutely increases the mRNA levels of the Period (Per)1 gene in the SCN and acutely suppresses melatonin levels in the pineal gland. Activation of N-methyl-D-aspartate (NMDA) receptors in the SCN has been demonstrated to phase-shift the circadian clock in a manner similar to light. We tested the hypothesis that activation of excitatory amino acid (EAA) receptors in the SCN mediates the acute effects of light on Per1 mRNA levels and pineal melatonin. NMDA, injected into the SCN of Syrian hamsters during the night, acutely suppressed melatonin levels in the pineal gland. Both the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5) and the alpha-amino-3-hydroxy-5-methylisoxazoleproprionic acid (AMPA)/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) inhibited the light-induced increase of Per1 mRNA levels in the SCN. In the same animals, however, these antagonists had no effect on the ability of light to suppress pineal melatonin. These results support the hypothesis that EAA receptor activation in the SCN is necessary for the acute effects of light on Per1 mRNA levels. They also indicate that NMDA receptor activation in the SCN is sufficient but may not be necessary for the acute effects of light on pineal melatonin. These data suggest that there may be at least two different neurochemical cascades that transduce the effects of light in the SCN

Effect of phosphodiesterase type 4 on circadian clock gene Per1 transcription

The induction of Per1 gene in the suprachiasmatic nucleus, the center of the circadian clock, is assumed to play significant roles in the adjustment of the internal clock. cAMP is one of the intracellular mediators which activates Per1 transcription. Here, we showed that the amount of the rat Per1 (rPer1) transcript induced by forskolin (FK) was significantly upregulated by the inhibition of phosphodiesterase type 4 (PDE4), a specific phosphodiesterase for cAMP, in rat-1 fibroblasts. Administration of rolipram, a specific inhibitor of PDE4, increased intracellular cAMP concentration, phosphorylation of cAMP response element binding protein (CREB) and enhanced rPer1 induction at their peaks. However, in the falling phase of rPer1 induction, the inhibition of PDE4 hardly affected the profile of rPer1 expression. These findings suggest the involvement of PDE4 for the regulation of rPer1 expression via cAMP metabolism at peak of the induction but little or no participation of PDE4 in the decreasing phase of the gene expression.

Light does not degrade the constitutively expressed BMAL1 protein in the mouse suprachiasmatic nucleus

Biological rhythms in mammals are driven by a central circadian clock located in the suprachiasmatic nucleus (SCN). At the molecular level the biological clock is based on the rhythmic expression of clock genes. Two basic helix-loop-helix (bHLH)/PAS-containing transcription factors, CLOCK and BMAL1 (MOP3), provide the basic drive to the system by activating transcription of negative regulators through E box enhancer elements. A critical feature of circadian timing is the ability of the clockwork to be entrained to the environmental light/dark cycle. The light-resetting mechanism of the mammalian circadian clock is poorly understood. Light-induced phase shifts are correlated with the induction of the clock genes mPer1 and mPer2 and a subsequent increase in mPER1 protein levels. It has previously been suggested that rapid degradation of BMAL1 protein in the rat SCN is part of the resetting mechanism of the central pacemaker. Our study shows that BMAL1 and CLOCK proteins are continuously expressed at high levels in the mouse SCN, supporting the hypothesis that rhythmic negative feedback plays the major role in rhythm generation in the mammalian pacemaker. Using both immunocytochemistry and immunoblot analysis, our studies demonstrate that BMAL1 protein in the mouse SCN is not affected by a phase-resetting light pulse. These results indicate that rapid degradation of BMAL1 protein is not a consistent feature of resetting mechanisms in rodents.

Hypothalamic circadian organization in birds. II. Clock gene expression

While the site of the major circadian pacemaker in mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus, is very well characterized, little is known about hypothalamic circadian organization in birds. This paper reviews recent findings on clock gene expression in the hypothalamus of several bird species focusing on circadian pPer2 expression in the house sparrow. In contrast to mammals, rhythmic Per2 gene expression in the house sparrow hypothalamus is not restricted to a single cell group but occurs in two distinct hypothalamic nuclei, the SCN and the lateral hypothalamic nucleus (LHN). The complex temporal and spatial distribution of pPer2 expression suggests a longitudinal compartmentalization of the SCN with period gene expression being initiated in the most rostral portion before lights on. In the lateral hypothalamus, phasing of pPer2-rhythmicity appeared delayed. In pinealectomized house sparrows, the overall circadian pPer2 expression pattern is maintained indicating that rhythmic pPer2 transcription in the SCN and LHN of the house sparrow are not driven by the pineal gland. Rather, they reflect the activity of autonomous hypothalamic circadian oscillators. Certain changes in peak expression levels and the expression phase, however, suggest that the pineal melatonin rhythm affects both the phase and the amplitude of rhythmic hypothalamic pPer2 expression.

Phase shifts and Per gene expression in mouse suprachiasmatic nucleus

In mammals, circadian rhythms controlled by the suprachiasmatic nuclei are entrained by photic stimuli. To investigate the molecular mechanism of photic entrainment, we examined light-induced behavioral phase delays and associated changes in mPer1 and mPer2 gene expression in the suprachiasmatic nuclei of two mouse lines artificially selected for nest-building behavior. Big nest-builders show larger phase delays than small nest-builders. Light-induced mPer1 and mPer2 expression was examined in individual mice previously tested for phase shifting at circadian time 16. Light-induced mPer2 expression was significantly higher in big compared to small nest-builders. No difference was found between lines in light-induced mPer1 expression. The results suggest a more important role for mPer2 than for mPer1 gene expression in behavioral phase delays.

Daily variation of clock output gene activation in behaviorally arrhythmic mPer/mCry triple mutant mice

The mammalian central pacemaker, driving circadian rhythms in behavior, physiology, and metabolism, is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. At the molecular level circadian clocks are based on a system of transcriptional/translational feedback loops oscillating with a period of about 24h. In mammals the CLOCK/BMAL1 transcriptional activator complex regulates a set of central clock genes like mPer1, mPer2, mCry1, and mCry2. The corresponding gene products form protein complexes that translocate into the nucleus and inhibit CLOCK/BMAL1-driven transcription of their own genes and other E-box containing genes. To elucidate whether only one of these four genes of the negative feedback loop is sufficient to generate a 24h rhythm we generated mPer/mCry triple mutant mice. As could be expected on the basis of the arrhythmicity of mPer1/mPer2 and mCry1/mCry2 double mutant mice, we show that none of the triple mutants is able to maintain circadian rhythmicity in constant darkness. This indicates that a single mPer or mCry gene is not sufficient to drive circadian rhythms. Interestingly however, under light-dark conditions (LD) the oscillation of some output genes is persisting in these animals indicating that the LD cycle is able to partially drive rhythmic signalling to the body, through an hour-glass mechanism.

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.

Loss of circadian rhythmicity in aging mPer1-/-mCry2-/- mutant mice

The mPer1, mPer2, mCry1, and mCry2 genes play a central role in the molecular mechanism driving the central pacemaker of the mammalian circadian clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus. In vitro studies suggest a close interaction of all mPER and mCRY proteins. We investigated mPER and mCRY interactions in vivo by generating different combinations of mPer/mCry double-mutant mice. We previously showed that mCry2 acts as a nonallelic suppressor of mPer2 in the core clock mechanism. Here, we focus on the circadian phenotypes of mPer1/mCry double-mutant animals and find a decay of the clock with age in mPer1-/- mCry2-/- mice at the behavioral and the molecular levels. Our findings indicate that complexes consisting of different combinations of mPER and mCRY proteins are not redundant in vivo and have different potentials in transcriptional regulation in the system of autoregulatory feedback loops driving the circadian clock.

A molecular perspective of human circadian rhythm disorders

A large number of physiological variables display 24-h or circadian rhythms. Genes dedicated to the generation and regulation of physiological circadian rhythms have now been identified in several species, including humans. These clock genes are involved in transcriptional regulatory feedback loops. The mutation of these genes in animals leads to abnormal rhythms or even to arrhythmicity in constant conditions. In this view, and given the similarities between the circadian system of humans and rodents, it is expected that mutations of clock genes in humans may give rise to health problems, in particular sleep and mood disorders. Here we first review the present knowledge of molecular mechanisms underlying circadian rhythmicity, and we then revisit human circadian rhythm syndromes in light of the molecular data.

Light exposure during daytime modulates expression of Per1 and Per2 clock genes in the suprachiasmatic nuclei of mice

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the master circadian clock in mammals. Nocturnal light pulses that reset the circadian clock also lead to rapid increases in levels of Per1 and Per2 mRNA in the SCN, suggesting that these genes are involved in the synchronization to light. During the day, when light has no phase-shifting effects in nocturnal rodents, the consequences of light exposure for Per expression have been less thoroughly studied. Therefore, the effects of light exposure during the day were assessed on Per1 and Per2 mRNA in the SCN of mice. Expression of Per1 and Per2 was generally increased by 30-min light pulses during the subjective day, with more pronounced effects in the morning. One exception was noted for a transient decrease in Per2 expression after a short light pulse applied at midday. Prolonged light exposure (up to 3 hr) starting at midday markedly increased Per2 expression but not that of Per1. Moreover, the amplitude of the daily variations of both Per and the duration of Per1 peak was increased in mice exposed to a light-dark cycle compared with those transferred to constant darkness. Finally, the amplitude of the daily variations of both Per and the basal level of Per1 were increased in mice under a light-dark cycle compared with animals synchronized to a skeleton photoperiod (i.e., with daily dawn and dusk 1-hr exposures to light). Taken together, the results indicate that prolonged light exposure during daytime positively modulates daily levels of Per1 and Per2 mRNA in the SCN of mice.

Synchronization of the molecular clockwork by light- and food-related cues in mammals

The molecular clockwork in mammals involves various clock genes with specific temporal expression patterns. Synchronization of the master circadian clock located in the suprachiasmatic nucleus (SCN) is accomplished mainly via daily resetting of the phase of the clock by light stimuli. Phase shifting responses to light are correlated with induction of Per1, Per2 and Dec1 expression and a possible reduction of Cry2 expression within SCN cells. The timing of peripheral oscillators is controlled by the SCN when food is available ad libitum. Time of feeding, as modulated by temporal restricted feeding, is a potent 'Zeitgeber' (synchronizer) for peripheral oscillators with only weak synchronizing influence on the SCN clockwork. When restricted feeding is coupled with caloric restriction, however, timing of clock gene expression is altered within the SCN, indicating that the SCN function is sensitive to metabolic cues. The components of the circadian timing system can be differentially synchronized according to distinct, sometimes conflicting, temporal (time of light exposure and feeding) and homeostatic (metabolic) cues.

cGMP-dependent protein kinase II modulates mPer1 and mPer2 gene induction and influences phase shifts of the circadian clock

BACKGROUND

In mammals, the master circadian clock that drives many biochemical, physiological, and behavioral rhythms is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Generation and maintenance of circadian rhythmicity rely on complex interlocked transcriptional/translational feedback loops involving a set of clock genes. Among the molecular components driving the mammalian circadian clock are the Period 1 and 2 (mPer1 and mPer2) genes. Because the periodicity of the clock is not exactly 24 hr, it has to be adjusted periodically. The major stimulus for adjustment (resetting) of the clock is nocturnal light. It evokes activation of signaling pathways in the SCN that ultimately lead to expression of mPer1 and mPer2 genes conveying adjustment of the clock.

RESULTS

We show that mice deficient in cGMP-dependent protein kinase II (cGKII, also known as PKGII), despite regular retinal function, are defective in resetting the circadian clock, as assessed by changes in the onset of wheel running activity after a light pulse. At the molecular level, light induction of mPer2 in the SCN is strongly reduced in the early period of the night, whereas mPer1 induction is elevated in cGKII-deficient mice. Additionally, we show that light induction of cfos and light-dependent phosphorylation of CREB at serine 133 are not affected in these animals.

CONCLUSIONS

cGKII plays a role in the clock-resetting mechanism. In particular, the ability to delay clock phase is affected in cGKII-deficient mice. It seems that the signaling pathway involving cGKII influences in an opposite manner the light-induced induction of mPer1 and mPer2 genes and thereby influences the direction of a phase shift of the circadian clock.

Light-induced phase shifts in mice lacking mPER1 or mPER2

Three homologs of the Drosophila Period gene have been identified in mammals. In mice, these three genes (mPer1, mPer2, and mPer3) have distinct roles in the circadian clockwork. While products of mPer1 and mPer2 play important roles in the maintenance of circadian rhythmicity, mPer3 gene products are dispensable for rhythmicity. Several studies also implicate mPER1 and mPER2 in transduction of photic information to the core circadian clockwork. The phase-shifting effects of light were examined in mPER1-deficient and mPER2-deficient mice using T cycle paradigms, in which mice received 1 h of light per day at an interval of T hours. To assess phase delays, repeated exposure to 1 h of light per day at T = 24 was used. To assess phase advances, exposure to 1-h light pulses at T = 22-h intervals was used. The degeneration of rhythmicity in the mutant mice prevented assessment of a response in most cases. Nevertheless, clear examples of phase delays and phase advances were observed in both mPer1 and mPer2 mutant mice. These results are not consistent with the hypothesis that mPER1 and mPER2 play necessary and nonoverlapping roles in mediating the effects of light on the circadian dock.

The circadian clock: a manager of biochemical processes within the organism

The periodic succession of night and day has influenced life on earth for millions of years. Many organisms have "internalized" this periodic change in the form of the circadian clock. Its main function is to organize the time course of biochemical, physiological and behavioural processes thereby optimizing an organism's performance in anticipating changing environmental conditions. Therefore, it is important to understand the underlying mechanisms that connect the core pacemaker, which is located in the suprachiasmatic nuclei (SCN) of the hypothalamus, with peripheral organs. Several laboratories set out to identify genes that are under the influence of the circadian clock. It appears that the circadian clock coordinates transcription of key metabolic pathways thereby orchestrating the time course of physiological and behavioural processes. We review these investigations and put our experiments, the comparison of gene expression in SCN tissue of Per2 mutant and wild-type mice, in the context of these findings.

Phase resetting light pulses induce Per1 and persistent spike activity in a subpopulation of biological clock neurons

The endogenous circadian clock of the mammalian suprachiasmatic nucleus (SCN) can be reset by light to synchronize the biological clock of the brain with the external environment. This process involves induction of immediate-early genes such as the circadian clock gene Period1 (Per1) and results in a stable shift in the timing of behavioral and physiological rhythms on subsequent days. The mechanisms by which gene activation permanently alters the phase of clock neuron activity are unknown. To study the relationship between acute gene activation and persistent changes in the neurophysiology of SCN neurons, we recorded from SCN neurons marked with a dynamic green fluorescent protein (GFP) reporter of Per1 gene activity. Phase-resetting light pulses resulted in Per1 induction in a distinct subset of SCN neurons that also exhibited a persistent increase in action potential frequency 3-5 hr after a light pulse. By simultaneously quantifying Per1 gene activation and spike frequency in individual neurons, we found that the degree of Per1 induction was highly correlated with neuronal spike frequency on a cell-by-cell basis. Increased neuronal activity was mediated by membrane potential depolarization as a result of a reduction in outward potassium current. Double-label immunocytochemistry revealed that vasoactive intestinal peptide (VIP)-expressing cells, but not arginine vasopressin (AVP)-expressing cells, exhibited significant Per1 induction by light pulses. Rhythmic GFP expression occurred in both VIP and AVP neurons. Our results indicate that the steps that link acute molecular events to permanent changes in clock phase involve persistent suppression of potassium current, downstream of Per1 gene induction, in a specific subset of Per1-expressing neurons enriched for VIP.

Involvement of CLOCK:BMAL1 heterodimer in serum-responsive mPer1 induction

A rapid induction of mouse period1 (mPer1) gene expression is supposed to be critical in the clock gene regulation, especially in the phase resetting of the clock, but its molecular mechanism is poorly understood. Based on the previous finding that the process does not involve de novo synthesis of proteins, we postulated the involvement of CLOCK:BMAL1 heterodimer, a positive regulator of circadian oscillator, in the rapid induction of mPer1 transcription. To test this hypothesis, we utilized CLOCKdelta19, a dominant-negative mutant, to suppress the function of CLOCK:BMAL1 in vitro. Serum-evoked rapid increases of mPer1 mRNA expression and promoter activity were significantly blunted when CLOCK:BMAL1 function was interfered with. Furthermore, DNA binding activity of CLOCK:BMAL1 heterodimer to five E-boxes of mPer1 promoter markedly increased shortly after serum shock. Taken together, these results suggest that CLOCK:BMAL1 heterodimer is not only a core component of negative feedback loop driving circadian oscillator, but also involved in the rapid induction of mPer1during phase resetting of the clock.

Circadian profile and photic regulation of clock genes in the suprachiasmatic nucleus of a diurnal mammal Arvicanthis ansorgei

The molecular mechanisms of the mammalian circadian clock located in the suprachiasmatic nucleus have been essentially studied in nocturnal species. Currently, it is not clear if the clockwork and the synchronizing mechanisms are similar between diurnal and nocturnal species. Here we investigated in a day-active rodent Arvicanthis ansorgei, some of the molecular mechanisms that participate in the generation of circadian rhythmicity and processing of photic signals. In situ hybridization was used to characterize circadian profiles of expression of Per1, Per2, Cry2 and Bmal1 in the suprachiasmatic nucleus of A. ansorgei housed in constant dim red light. All the clock genes studied showed a circadian expression. Per1 and Per2 mRNA increased during the subjective day and decreased during the subjective night. Also, Bmal1 exhibited a circadian expression, but in anti-phase to that of Per1. The expression of Cry2 displayed a circadian pattern, increasing during the late subjective day and decreasing during the late subjective night. We also obtained the phase responses to light for wheel-running rhythm and clock gene expression. At a behavioral level, light was able to induce phase shifts only during the subjective night, like in other diurnal and nocturnal species. At a molecular level, light pulse exposure during the night led to an up-regulation of Per1 and Per2 concomitant with a down-regulation of Cry2 in the suprachiasmatic nucleus of A. ansorgei. In contrast, Bmal1 expression was not affected by light pulses at the circadian times investigated. This study demonstrates that light exposure during the subjective night has opposite effects on the expression of the clock genes Per1 and Per2 compared with that of Cry2. These differential effects can participate in photic resetting of the circadian clock. Our data also indicate that the molecular mechanisms underlying circadian rhythmicity and photic synchronization share clear similarities between diurnal and nocturnal mammals.

Genetic analysis of the circadian system in Drosophila melanogaster and mammals

The fruit fly, Drosophila melanogaster, has been a grateful object for circadian rhythm researchers over several decades. Behavioral, genetic, and molecular studies helped to reveal the genetic bases of circadian time keeping and rhythmic behaviors. Contrary, mammalian rhythm research until recently was mainly restricted to descriptive and physiologic approaches. As in many other areas of research, the surprising similarity of basic biologic principles between the little fly and our own species, boosted the progress of unraveling the genetic foundation of mammalian clock mechanisms. Once more, not only the basic mechanisms, but also the molecules involved in establishing our circadian system are taken or adapted from the fly. This review will try to give a comparative overview about the two systems, highlighting similarities as well as specifics of both insect and murine clocks.