Expression of basic helix-loop-helix/PAS genes in the mouse suprachiasmatic nucleus

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Clock genes expression and locomotor activity are altered along the light-dark cycle in transgenic zebrafish overexpressing growth hormone

In the present work it was demonstrated that transgenic Danio rerio overexpressing growth hormone (GH-transgenic) present either altered gene expression at a determined time point, or different expression pattern along the LD cycle, when compared with non-transgenic (NT) animals, in the positive and negative loops of the circadian system. Gene expression of clock paralogs was reduced in GH fish at the beginning of the dark phase, leading to diminished expression amplitude along the LD cycle. Furthermore, although no differences were observed between NT and GH animals for bmal1a and cry2b expression at each time point, only GH fish presented amplitude along the LD cycle. Also, the locomotor activity behavior was evaluated for both groups. GH-transgenic animals presented higher locomotor activity along the whole LD cycle when compared with NT animals. These data suggest that alterations in the gene expression patterns along the LD cycle of the positive and negative loops of the circadian system, could lead to altered locomotor activity behavior in GH-transgenic fish, and GH overexpression could be responsible for these alterations, either affecting the pathways involved in the expression of genes from the circadian system or altering the metabolism.

Dec1 and Dec2 Expression is Disrupted in the Suprachiasmatic Nuclei of Clock Mutant Mice

DEC1 and DEC2 are basic helix-loop-helix transcription factors that functionally resemble negative feedback components of the mammalian circadian clock. The genes Dec1 and Dec2 are expressed rhythmically in the rat suprachiasmatic nuclei, and Dec1 expression is stimulated by light in a timedependent manner with the kinetics of an immediate early gene. DEC1 and DEC2 can inhibit CLOCK:BMAL1 transactivation of the clock gene Per1, suggesting that these transcription factors may help regulate circadian timing. The authors present data on the expression pattern of Dec1 and Dec2 in wild-type and homozygous Clock mutant mice. In the suprachiasmatic nuclei, the Clock mutation significantly reduces the expression of Dec1 and Dec2. Dec1 becomes arrhythmic; Dec2 remains weakly rhythmic in a 12L:12D light-dark cycle but is arrhythmic in constant darkness. A robust attenuation of the Dec1 and Dec2 signals in Clock mutant mice was detected in all brain areas examined. These data point to up-regulation of Dec1 and Dec2 by Clock in vivo.

Circadian rhythms of clock gene expression in the cerebellum of serotonin-deficient Pet-1 knockout mice

Serotonin plays an important role in the central regulation of circadian clock function. Serotonin levels are generally higher in the brain during periods of high activity, and these periods are in turn heavily regulated by the circadian clock located in the suprachiasmatic nucleus. However, the role of serotonin as a regulator of circadian rhythms elsewhere in the brain has not been extensively examined. In this study, we examined circadian rhythms of clock gene expression in the cerebellum in mice lacking the Pet-1 transcription factor, which results in a developed brain that is deficient in serotonin neurons. If serotonin helps to synchronize rhythms in brain regions other than the suprachiasmatic nucleus, we would expect to see differences in clock gene expression in these serotonin deficient mice. We found minor differences in the expression of Per1 and Per2 in the knockout mice as compared to wild type, but these differences were small and of questionable functional importance. We also measured the response of cerebellar clocks to injections of the serotonin agonist 8-OH-DPAT during the early part of the night. No effect on clock genes was observed, though the immediate-early gene Fos showed increased expression in wild type mice but not the knockouts. These results suggest that serotonin is not an important mediator of circadian rhythms in the cerebellum in a way that parallels its regulation of the circadian clock in the suprachiasmatic nucleus.

Circadian oscillators in the mouse brain: molecular clock components in the neocortex and cerebellar cortex

The circadian timekeeper of the mammalian brain resides in the suprachiasmatic nucleus of the hypothalamus (SCN), and is characterized by rhythmic expression of a set of clock genes with specific 24-h daily profiles. An increasing amount of data suggests that additional circadian oscillators residing outside the SCN have the capacity to generate peripheral circadian rhythms. We have recently shown the presence of SCN-controlled oscillators in the neocortex and cerebellum of the rat. The function of these peripheral brain clocks is unknown, and elucidating this could involve mice with conditional cell-specific clock gene deletions. This prompted us to analyze the molecular clockwork of the mouse neocortex and cerebellum in detail. Here, by use of in situ hybridization and quantitative RT-PCR, we show that clock genes are expressed in all six layers of the neocortex and the Purkinje and granular cell layers of the cerebellar cortex of the mouse brain. Among these, Per1, Per2, Cry1, Arntl, and Nr1d1 exhibit circadian rhythms suggesting that local running circadian oscillators reside within neurons of the mouse neocortex and cerebellar cortex. The temporal expression profiles of clock genes are similar in the neocortex and cerebellum, but they are delayed by 5 h as compared to the SCN, suggestively reflecting a master-slave relationship between the SCN and extra-hypothalamic oscillators. Furthermore, ARNTL protein products are detectable in neurons of the mouse neocortex and cerebellum, as revealed by immunohistochemistry. These findings give reason to further pursue the physiological significance of circadian oscillators in the mouse neocortex and cerebellum.

Circadian rhythms and addiction: mechanistic insights and future directions

Circadian rhythms are prominent in many physiological and behavioral functions. Circadian disruptions either by environmental or molecular perturbation can have profound health consequences, including the development and progression of addiction. Both animal and humans studies indicate extensive bidirectional relationships between the circadian system and drugs of abuse. Addicted individuals display disrupted rhythms, and chronic disruption or particular chronotypes may increase the risk for substance abuse and relapse. Moreover, polymorphisms in circadian genes and an evening chronotype have been linked to mood and addiction disorders, and recent efforts suggest an association with the function of reward neurocircuitry. Animal studies are beginning to determine how altered circadian gene function results in drug-induced neuroplasticity and behaviors. Many studies suggest a critical role for circadian rhythms in reward-related pathways in the brain and indicate that drugs of abuse directly affect the central circadian pacemaker. In this review, we highlight key findings demonstrating the importance of circadian rhythms in addiction and how future studies will reveal important mechanistic insights into the involvement of circadian rhythms in drug addiction.

Differential regulation of the period genes in striatal regions following cocaine exposure

Several studies have suggested that disruptions in circadian rhythms contribute to the pathophysiology of multiple psychiatric diseases, including drug addiction. In fact, a number of the genes involved in the regulation of circadian rhythms are also involved in modulating the reward value for drugs of abuse, like cocaine. Thus, we wanted to determine the effects of chronic cocaine on the expression of several circadian genes in the Nucleus Accumbens (NAc) and Caudate Putamen (CP), regions of the brain known to be involved in the behavioral responses to drugs of abuse. Moreover, we wanted to explore the mechanism by which these genes are regulated following cocaine exposure. Here we find that after repeated cocaine exposure, expression of the Period (Per) genes and Neuronal PAS Domain Protein 2 (Npas2) are elevated, in a somewhat regionally selective fashion. Moreover, NPAS2 (but not CLOCK (Circadian Locomotor Output Cycles Kaput)) protein binding at Per gene promoters was enhanced following cocaine treatment. Mice lacking a functional Npas2 gene failed to exhibit any induction of Per gene expression after cocaine, suggesting that NPAS2 is necessary for this cocaine-induced regulation. Examination of Per gene and Npas2 expression over twenty-four hours identified changes in diurnal rhythmicity of these genes following chronic cocaine, which were regionally specific. Taken together, these studies point to selective disruptions in Per gene rhythmicity in striatial regions following chronic cocaine treatment, which are mediated primarily by NPAS2.

Circadian oscillations of molecular clock components in the cerebellar cortex of the rat

The central circadian clock of the mammalian brain resides in the suprachiasmatic nucleus (SCN) of the hypothalamus. At the molecular level, the circadian clockwork of the SCN constitutes a self-sustained autoregulatory feedback mechanism reflected by the rhythmic expression of clock genes. However, recent studies have shown the presence of extrahypothalamic oscillators in other areas of the brain including the cerebellum. In the present study, the authors unravel the cerebellar molecular clock by analyzing clock gene expression in the cerebellum of the rat by use of radiochemical in situ hybridization and quantitative real-time polymerase chain reaction. The authors here show that all core clock genes, i.e., Per1, Per2, Per3, Cry1, Cry2, Clock, Arntl, and Nr1d1, as well as the clock-controlled gene Dbp, are expressed in the granular and Purkinje cell layers of the cerebellar cortex. Among these genes, Per1, Per2, Per3, Cry1, Arntl, Nr1d1, and Dbp were found to exhibit circadian rhythms in a sequential temporal manner similar to that of the SCN, but with several hours of delay. The results of lesion studies indicate that the molecular oscillatory profiles of Per1, Per2, and Cry1 in the cerebellum are controlled, though possibly indirectly, by the central clock of the SCN. These data support the presence of a circadian oscillator in the cortex of the rat cerebellum.

Convergence of multiple mechanisms of steroid hormone action

Steroid hormones modulate a wide array of physiological processes including development, metabolism, and reproduction in various species. It is generally believed that these biological effects are predominantly mediated by their binding to specific intracellular receptors resulting in conformational change, dimerization, and recruitment of coregulators for transcription-dependent genomic actions (classical mechanism). In addition, to their cognate ligands, intracellular steroid receptors can also be activated in a "ligand-independent" manner by other factors including neurotransmitters. Recent studies indicate that rapid, nonclassical steroid effects involve extranuclear steroid receptors located at the membrane, which interact with cytoplasmic kinase signaling molecules and G-proteins. The current review deals with various mechanisms that function together in an integrated manner to promote hormone-dependent actions on the central and sympathetic nervous systems.

Expression pattern of clock under acute phase-delay of the light/dark cycle in the chicken pineal model

Shift workers have a higher risk of metabolic syndrome, a condition that also develops in mice carrying mutation in their circadian clock gene clock. To collect more data on the transcriptional changes of clock under phase-shifted light/dark LD conditions, we examined the 24h patterns of clock mRNA expression in vivo and in vitro in chickens exposed acutely to a reversed LD (DL) cycle. Under controlled LD conditions (lights on at 6:00, lights off at 20:00), clock mRNA expression peaked in vivo at 2:00 (Zeitgeber Time 20, ZT20) and in vitro at 22:00 (ZT16). Even higher mRNA contents were measured in the first cycle of in vivo DL conditions between 22:00 and 6:00 (lights at night), but in the second cycle by 2:00, lower mRNA contents were detected than the control peak values seen at this time point. Furthermore, no alterations were found in vitro in clock mRNA content during the first 12h of DL conditions (lights at night). The differences seen between the first and the second DL cycles in vivo and between the in vivo and in vitro data for the first DL cycle support the idea that neurohumoral signals perturbed by a phase-delayed light-dark cycle may also play a role in the in vivo rapid transcriptional resetting of the circadian clock in the chicken pineal model.

It's time to swim! Zebrafish and the circadian clock

The zebrafish represents a fascinating model for studying key aspects of the vertebrate circadian timing system. Easy access to early embryonic development has made this species ideal for investigating how the clock is first established during embryogenesis. In particular, the molecular basis for the functional development of the zebrafish pineal gland has received much attention. In addition to this dedicated clock and photoreceptor organ, and unlike the situation in mammals, the clocks in zebrafish peripheral tissues and even cell lines are entrainable by direct exposure to light thus providing unique insight into the function and evolution of the light input pathway. Finally, the small size, low maintenance costs and high fecundity of this fish together with the availability of genetic tools make this an attractive model for forward genetic analysis of the circadian clock. Here, we review the work that has established the zebrafish as a valuable clock model organism and highlight the key questions that will shape the future direction of research.

Nuclear receptor coactivators are coexpressed with steroid receptors and regulated by estradiol in mouse brain

BACKGROUND/AIMS

The steroid hormones, including estradiol (E) and progesterone, act in the brain to regulate female reproductive behavior and physiology. These hormones mediate many of their biological effects by binding to their respective intracellular receptors. The receptors for estrogens (ER) and progestins (PR) interact with nuclear receptor coactivators to initiate transcription of steroid-responsive genes. Work from our laboratory and others reveals that nuclear receptor coactivators, including steroid receptor coactivator-1 (SRC-1) and SRC-2, function in brain to modulate ER-mediated induction of the PR gene and hormone-dependent behaviors. In order for steroid receptors and coactivators to function together, both must be expressed in the same cells.

METHODS

Triple-label immunofluorescence was used to determine if E-induced PR cells also express SRC-1 or SRC-2 in reproductively relevant brain regions of the female mouse.

RESULTS

The majority of E-induced PR cells in the medial preoptic area (61%), ventromedial nucleus of the hypothalamus (63%) and arcuate nucleus (76%) coexpressed both SRC-1 and SRC-2. A smaller proportion of PR cells expressed either SRC-1 or SRC-2, while a few PR cells expressed neither coactivator. In addition, compared to control animals, 17β-estradiol benzoate (EB) treatment increased SRC-1 levels in the arcuate nucleus, but not the medial preoptic area or the ventromedial nucleus of the hypothalamus. EB did not alter SRC-2 expression in any of the three brain regions analyzed.

CONCLUSIONS

Taken together, the present findings identify a population of cells in which steroid receptors and nuclear receptor coactivators may interact to modulate steroid sensitivity in brain and regulate hormone-dependent behaviors in female mice. Given that cell culture studies reveal that SRC-1 and SRC-2 can mediate distinct steroid-signaling pathways, the present findings suggest that steroids can produce a variety of complex responses in these specialized brain cells.

Molecular cloning and functional analysis of the clock genes, Clock and cycle, in the firebrat Thermobia domestica

Comparative molecular analysis reveals a wide variation of clock mechanisms among insects. In this study, the clock gene homologues of Clock (Td'Clk) and cycle (Td'cyc) were cloned from an apterygote insect, Thermobia domestica. Structural analysis showed that Td'CLK includes bHLH, PAS-A, PAS-B domains but lacks a polyglutamine repeat in the C terminal region that is implicated for transcriptional activity in Drosophila CLK. Td'CYC contains a BCTR domain in its C terminal in addition to the common domains found in Drosophila CYC, i.e. bHLH, PAS-A, PAS-B domains. Unlike in Drosophila, Td'Clk mRNA levels showed no significant daily fluctuation, while Td'cyc exhibited rhythmic expression. A single injection of double-stranded RNA (dsRNA) of Td'Clk or Td'cyc into the abdomen of adult firebrats effectively knocked down respective mRNA levels and abolished the rhythmic expression of Td'cyc. Most Td'Clk or Td'cyc dsRNA-injected firebrats lost their circadian locomotor rhythm in constant darkness up to 30 days after injection, whereas those injected with DsRed2 dsRNA as a negative control clearly maintained it. From these results, it is likely that Td'Clk and Td'cyc are involved in the circadian clock machinery in the firebrat. However, the structure and expression profile of Td'Clk and Td'cyc more closely resembles those of mammals than Drosophila.

Who's in charge? Nuclear receptor coactivator and corepressor function in brain and behavior

Steroid hormones act in brain and throughout the body to regulate a variety of functions, including development, reproduction, stress and behavior. Many of these effects of steroid hormones are mediated by their respective receptors, which are members of the steroid/nuclear receptor superfamily of transcriptional activators. A variety of studies in cell lines reveal that nuclear receptor coregulators are critical in modulating steroid receptor-dependent transcription. Thus, in addition to the availability of the hormone and the expression of its receptor, nuclear receptor coregulators are essential for efficient steroid-dependent transactivation of genes. This review will highlight the importance of nuclear receptor coregulators in modulating steroid-dependent gene expression in brain and the regulation of behavior.

Oscillating perceptions: the ups and downs of the CLOCK protein in the mouse circadian system

A functional mouse CLOCK protein has long been thought to be essential for mammalian circadian clockwork function, based mainly on studies of mice bearing a dominant negative, antimorphic mutation in the Clock gene. However, new discoveries using recently developed Clock-null mutant mice have shaken up this view. In this review, I discuss how this recent work impacts and alters the previous view of the role of CLOCK in the mouse circadian clockwork.

Nuclear receptor coactivators: essential players for steroid hormone action in the brain and in behaviour

Steroid hormones act both in the brain and throughout the body to influence behaviour and physiology. Many of these effects of steroid hormones are elicited by transcriptional events mediated by their respective receptors. A variety of cell culture studies reveal that nuclear receptor coactivators are critical for modulating steroid receptor-dependent transcription. Thus, in addition to the availability of the hormone and the expression of its receptor, nuclear receptor coactivators are essential for steroid-dependent transactivation of genes. This review discusses the mounting evidence indicating that nuclear receptor coactivators are critical for modulating steroid hormone action in the brain and in the regulation of behaviour.

Moléculas que marcam o tempo: implicações para os fenótipos circadianos

OBJETIVO: Revisar resumidamente a literatura dos últimos 36 anos de pesquisa em cronobiologia molecular a fim de informar aos profissionais de saúde os avanços obtidos nesta área e os potenciais para aplicação na clínica médica. MÉTODO: Buscas na literatura foram realizadas utilizando as bases de dados PubMed e Scopus usando como palavras-chave "clock genes, circadian rhythms, diurnal preference, delayed sleep phase syndrome, advanced sleep phase syndrome, photoperiod and mood disorder". DISCUSSÃO: Atualmente, o mecanismo molecular da regulação da ritmicidade circadiana é compreendido em grande detalhe. Muitos estudos publicados mostram associações de polimorfismos nos genes relógio com transtornos do ritmo circadiano e com transtornos do humor. CONCLUSÕES: De maneira geral, o progresso obtido na área de cronobiologia molecular traz um melhor entendimento da regulação do sistema de temporização biológico. O desenvolvimento de estudos nesta área tem o potencial de ser aplicável ao tratamento dos transtornos dos ritmos circadianos e certos transtornos do humor, além de prevenir riscos à saúde causados por viagens intercontinentais (Jet Lag) e por trabalhos noturnos e por turnos.

Autonomous onset of the circadian clock in the zebrafish embryo

On the first day of development a circadian clock becomes functional in the zebrafish embryo. How this oscillator is set in motion remains unclear. We demonstrate that zygotic period1 transcription begins independent of light exposure. Pooled embryos maintained in darkness and under constant temperature show elevated non-oscillating levels of period1 expression. Consequently, there is no maternal effect or developmental event that sets the phase of the circadian clock. Analysis of period1 transcription, at the cellular level in the absence of environmental stimuli, reveals oscillations in cells that are asynchronous within the embryo. Demonstrating an autonomous onset to rhythmic period1 expression. Transcription of clock1 and bmal1 is rhythmic in the adult, but constant during development in light-entrained embryos. Transient expression of dominant-negative DeltaCLOCK blocks period1 transcription, thus showing that endogenous CLOCK is essential for the transcriptional regulation of period1 in the embryo. We demonstrate a default mechanism in the embryo that initiates the autonomous onset of the circadian clock. This embryonic clock is differentially regulated from that in the adult, the transition coinciding with the appearance of several clock output processes.

[Synchronization and genetic redundancy in circadian clocks]

A network of feedback loops constitutes the basis for circadian timing in mammals. Complex transcriptional, post-transcriptional and post-translational events are also involved in the ticking of circadian clocks, allowing them to run autonomously with their characteristic, near-24h period. Central to the molecular mechanism is the CLOCK/BMAL1 heterodimer of transcription factors. Recent data using Clock knock-out mice however suggest that CLOCK may not be as mandatory as initially suggested from data gathered in the Clock mutant mouse model. Indeed, it appears that the Clock homolog Npas2 is able to functionally compensate for Clock genetic ablation. Furthermore, real-time imaging techniques using different clock genes knock-out lines established on a PER2 ::Luc knock-in background now demonstrate that persistent rhythmicity in the suprachiasmatic nuclei likely arises as a consequence of combined genetic redundancy and strong intercellular coupling, the latter characteristic being likely weakened in peripheral tissues such as liver or lung. The present review aims at summarizing current knowledge of the molecular basis of circadian clocks and possible differences between central and peripheral clocks in light of recent findings in Clock knock-out mice.

Molecular circadian rhythms in central and peripheral clocks in mammals

The last decade has seen tremendous progress in our understanding of the organization and function of the circadian clock. A number of so-called clock genes were discovered, and these genes and their protein products were shown to organize into feedback loops to give a near 24 h rhythmicity. However, the mechanism is much more complicated. First, many new clock components have been identified, increasing both our understanding and the overall complexity of the mechanism. Second, there is now evidence that transcription may not play a central role in determining the functioning of the clock: the identification of post-translational modifications of the clock proteins has revealed new levels of control. Finally, chromatin remodeling seems to be crucial in the regulation of the expression of major clock components. This review describes the recent advances in our knowledge of the molecular clockwork in mammals; in particular, the contribution of new clock components and of post-transcriptional and post-translational events to circadian timekeeping are discussed.

CLOCK and NPAS2 have overlapping roles in the suprachiasmatic circadian clock

Heterodimers of CLOCK and BMAL1, bHLH-PAS transcription factors, are believed to be the major transcriptional regulators of the circadian clock mechanism in mammals. However, a recent study shows that CLOCK-deficient mice continue to exhibit robust behavioral and molecular rhythms. Here we report that the transcription factor NPAS2 (MOP4) is able to functionally substitute for CLOCK in the master brain clock in mice to regulate circadian rhythmicity.

Cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and ovarian steroid receptors

Oestradiol and progesterone act in the brain to elicit profound effects on behaviour and physiology. One physiological function of oestradiol is the induction of progesterone receptor (PR) expression in a variety of behaviourally relevant brain regions, including the ventromedial nucleus of the hypothalamus (VMN), the medial preoptic nucleus of the preoptic area (MPOA), the arcuate nucleus (ARC) and the medial central grey (MCG). Ligand-dependent transcriptional activity of steroid receptors, including oestrogen receptors (ER) and Pr, is dramatically influenced by nuclear receptor coactivators. In previous studies, we have found that two of these nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), are important in ER-mediated induction of PR in the VMN and in steroid-dependent behaviours. For nuclear receptor coactivators to function in hormone-dependent transcription in the brain and regulate behaviour, both receptor and coactivator must be expressed in the same cell. In the present study, we used a dual-label immunohistochemical technique to investigate if individual cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and steroid receptors. Confocal analysis revealed that in oestrogen-primed rats, most of the E-induced PR cells in the VMN (89.6%), MPOA (63%), ARC (82.6%), and many in the MCG (39%), also express SRC-1. In addition, the majority of the cells containing E-induced PR in the VMN (78.3%), MPOA (83.1%), ARC (83.6%), and MCG (60%) also express CBP. These results, taken together with the findings that virtually all oestradiol-induced PR containing cells in the brain express ER, suggest that these neurones represent sites of functional interaction of nuclear receptor coactivators with ovarian steroid receptors in the brain. The present findings provide neuroanatomical evidence that nuclear receptor coactivators are integral in mediating steroid hormone action in behaviourally relevant brain regions.

Rhythmic expression of the cycle gene in a hematophagous insect vector

Background

A large number of organisms have internal circadian clocks that enable them to adapt to the cyclic changes of the external environment. In the model organism Drosophila melanogaster, feedback loops of transcription and translation are believed to be crucial for the maintenance of the central pacemaker. In this mechanism the cycle (or bmal1) gene, which is constitutively expressed, plays a critical role activating the expression of genes that will later inhibit their own activity, thereby closing the loop. Unlike Drosophila, the molecular clock of insect vectors is poorly understood, despite the importance of circadian behavior in the dynamic of disease transmission.

Results

Here we describe the sequence, genomic organization and circadian expression of cycle in the crepuscular/nocturnal hematophagous sandfly Lutzomyia longipalpis, the main vector of visceral leishmaniasis in the Americas. Deduced amino acid sequence revealed that sandfly cycle has a C-terminal transactivation domain highly conserved among eukaryotes but absent in D. melanogaster. Moreover, an alternative form of the transcript was also identified. Interestingly, while cycle expression in Drosophila and other Diptera is constitutive, in sandflies it is rhythmic in males and female heads but constitutive in the female body. Blood-feeding, which causes down-regulation of period and timeless in this species, does not affect cycle expression.

Conclusion

Sequence and expression analysis of cycle in L. longipalpis show interesting differences compared to Drosophila suggesting that hematophagous vector species might present interesting new models to study the molecular control of insect circadian clocks.

Functional central rhythmicity and light entrainment, but not liver and muscle rhythmicity, are Clock independent

The circadian rhythmicity of hormone secretion, body temperature, and sleep/wakefulness results from an endogenous rhythm of neural activity generated by clock genes in the suprachiasmatic nucleus (SCN). One of these genes, Clock, has been considered essential for the generation of cellular rhythmicity centrally and in the periphery; however, melatonin-proficient Clock(Delta19) + MEL mutant mice retain melatonin rhythmicity, suggesting that their central rhythmicity is intact. Here we show that melatonin production in these mutants was rhythmic in constant darkness and could be entrained by brief single daily light pulses. Under normal light-dark conditions, per2 and prokineticin2 (PK2) mRNA expression was rhythmic in the SCN of Clock(Delta19) + MEL mice. Expression of Bmal1 and npas2 was not altered, whereas per1 expression was arrhythmic. In contrast to the SCN, per1 and per2 expression, as well as Bmal1 expression in liver and skeletal muscle, together with plasma corticosterone, was arrhythmic in Clock(Delta19) + MEL mutant mice in normal light-dark conditions. npas2 mRNA was also arrhythmic in liver but rhythmic in muscle. The Clock(Delta19) mutation does not abolish central rhythmicity and light entrainment, suggesting that a functional Clock homolog, possibly npas2, exists in the SCN. Nevertheless, the SCN of Clock(Delta19) + MEL mutant mice cannot maintain liver and muscle rhythmicity through rhythmic outputs, including melatonin secretion, in the absence of functional Clock expression in the tissues. Therefore, liver and muscle, but not SCN, have an absolute requirement for CLOCK, with as yet unknown Clock-independent factors able to generate the latter.

A CLOCK-less clock

Mammalian physiology is governed by a complex circadian timing system that involves interacting positive and negative transcriptional feedback loops. A key role in this feedback loop was attributed to the PAS domain helix-loop-helix protein CLOCK, on the basis of a dominant-negative mutation in this transcription factor. However, recent experiments by Reppert and coworkers with Clock knockout mice suggest that CLOCK is dispensable for rhythmic gene expression and behavior, presumably because other proteins can substitute for CLOCK in these animals.

The aryl hydrocarbon receptor pathway and sexual differentiation of neuroendocrine functions

Historically, much of the research on health effects of environmental pollutants focused on ascertaining whether compounds were carcinogenic. More recent findings show that environmental contaminants also exert insidious effects by disrupting hormone action. Of particular concern are findings that developmental exposure to dioxins, chemicals that act through the aryl hydrocarbon receptor pathway, permanently alters sexually differentiated neural functions in animal models. In this review, we focus on mechanisms through which dioxins disrupt neuroendocrine development as exemplified by effects on a brain region critical for ovulation in rodents. We also provide evidence that dysregulation of GABAergic neural development may be a general mechanism underlying a broad spectrum of effects seen after perinatal dioxin exposure.

A clock shock: mouse CLOCK is not required for circadian oscillator function

The circadian clock mechanism in the mouse is composed of interlocking transcriptional feedback loops. Two transcription factors, CLOCK and BMAL1, are believed to be essential components of the circadian clock. We have used the Cre-LoxP system to generate whole-animal knockouts of CLOCK and evaluated the resultant circadian phenotypes. Surprisingly, CLOCK-deficient mice continue to express robust circadian rhythms in locomotor activity, although they do have altered responses to light. At the molecular and biochemical levels, clock gene mRNA and protein levels in both the master clock in the suprachiasmatic nuclei and a peripheral clock in the liver show alterations in the CLOCK-deficient animals, although the molecular feedback loops continue to function. Our data challenge a central feature of the current mammalian circadian clock model regarding the necessity of CLOCK:BMAL1 heterodimers for clock function.

Plasticity in the expression of the steroid receptor coactivator 1 in the Japanese quail brain: effect of sex, testosterone, stress and time of the day

Analysis of nuclear receptor action on the eukaryotic genome highlights the importance of coactivators on gene transcription. The steroid receptor coactivator-1 in particular is the focus of an intense research and physiological or behavioral studies have confirmed that it plays a major role in the modulation of steroid and thyroid receptors activity. However, little is known about the regulation of steroid receptor coactivator-1 expression the brain. The goal of this study was to determine the potential factors modulating steroid receptor coactivator-1 synthesis in Japanese quail by quantification of its mRNA with real time quantitative polymerase chain reaction and of the corresponding protein via Western blotting. Contrary to previously published results from our laboratory [Charlier TD, Lakaye B, Ball GF, Balthazart J (2002) The steroid receptor coactivator SRC-1 exhibits high expression in steroid-sensitive brain areas regulating reproductive behaviors in the quail brain. Neuroendocrinology 76:297-315], we found here that sexually mature females had a higher concentration of steroid receptor coactivator-1 in the preoptic area/hypothalamus compared with males. Steroid receptor coactivator-1 expression in the male preoptic area/hypothalamus was up-regulated by testosterone and tended to be decreased by stress. We also identified a significant correlation between the time of the day and the expression of the coactivator in the optic lobes, hippocampus, telencephalon and hindbrain but the pattern of changes in expression as a function of the time of the day varied from one brain area to another. Together, these data support the idea that steroid receptor coactivator-1 is not constitutively expressed but rather is finely regulated by steroids, stress and possibly other unidentified factors.

Control of daily transcript oscillations in Drosophila by light and the circadian clock

The transcriptional circuits of circadian clocks control physiological and behavioral rhythms. Light may affect such overt rhythms in two ways: (1) by entraining the clock circuits and (2) via clock-independent molecular pathways. In this study we examine the relationship between autonomous transcript oscillations and light-driven transcript responses. Transcript profiles of wild-type and arrhythmic mutant Drosophila were recorded both in the presence of an environmental photocycle and in constant darkness. Systematic autonomous oscillations in the 12- to 48-h period range were detectable only in wild-type flies and occurred preferentially at the circadian period length. However, an extensive program of light-driven expression was confirmed in arrhythmic mutant flies. Many light-responsive transcripts are preferentially expressed in the compound eyes and the phospholipase C component of phototransduction, NORPA (no receptor potential), is required for their light-dependent regulation. Although there is evidence for the existence of multiple molecular clock circuits in cyanobacteria, protists, plants, and fungi, Drosophila appears to possess only one such system. The sustained photic expression responses identified here are partially coupled to the circadian clock and may reflect a mechanism for flies to modulate functions such as visual sensitivity and synaptic transmission in response to seasonal changes in photoperiod.

A potential endogenous ligand for the aryl hydrocarbon receptor has potent agonist activity in vitro and in vivo

The aryl hydrocarbon receptor (AhR) is best known as a mediator of toxicity of a diverse family of xenobiotic chemicals such as dioxins and PCBs. However, many naturally occurring compounds also activate AhR. One such compound, 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), was isolated from tissue and found to be potent in preliminary tests [J. Song, M. Clagett-Dame, R.E. Peterson, M.E. Hahn, W.M. Westler, R.R. Sicinski, H.F. DeLuca, Proc. Natl. Acad. Sci. USA 99 (2002) 14694-14699]. We have synthesized ITE and [(3)H]ITE and further evaluated its AhR activity in several in vitro and in vivo assays in comparison with the toxic ligand, TCDD. AhR in Hepa1c1c7 cell cytosol bound [(3)H]ITE with high affinity and the AhR.ITE complex formed in vitro bound dioxin response element (DRE) oligonucleotide as potently as TCDD.AhR. In cells treated with ITE, nuclear translocation of AhR, and induction of CYP1A1 protein and of a DRE-dependent luciferase reporter gene were observed. ITE administered to pregnant DRE-LacZ transgenic mice activated fetal AhR, observed as X-gal staining in the same sites as in TCDD-treated mice. However, unlike TCDD, ITE did not induce cleft palate or hydronephrosis. TCDD but not ITE induced thymic atrophy in young adult mice, but both ITE and TCDD caused similar loss of cells and alterations of cell profiles in cultured fetal thymi. These data demonstrate that ITE is a potent AhR agonist in cell extracts, cultured cells, and intact animals, but does not cause the toxicity associated with the more stable xenobiotic ligand, TCDD.

Molecular cloning of Clock cDNA from the prawn, Macrobrachium rosenbergii

CLOCK, which belongs to the basic helix-loop-helix (bHLH)/PER-ARNT-SIM (PAS) superfamily of transcription factors, is one of the most essential proteins involved in circadian systems of animals. Clock genes have been cloned from several species, including mammals, insects, birds, fish, and amphibians. In the present study, we successfully isolated a Clock homolog (termed Mar-Clock) from the giant prawn, Macrobrachium rosenbergii. The 2949-bp cDNA contained a 2115 bp open reading frame that encoded a putative CLOCK protein of 704 amino acids (termed Mar-CLOCK) exhibiting high identities with CLOCK homologs in other species (30-35%). This is the first report of a circadian clock gene from crustaceans. Mar-CLOCK possessed an exceptionally long glutamine-rich domain (140 amino acids) in its C-terminus, which usually ranges from 14 to 57 amino acids in other known CLOCKs and is supposed to function in transcriptional activation. Using RT-PCR, we observed that Mar-Clock was expressed in all tested tissues. Semiquantitative RT-PCR was performed to investigate the gene expression profile during the light-dark cycle. The results indicated that the expression of the Mar-Clock gene had no significant rhythmicity in central nervous tissues (thoracic ganglia and eyestalk) or peripheral tissues (gill, ovary, hepatopancreas, and muscle). Furthermore, gene expression tended to increase in the central nervous system (brain, thoracic, and abdominal ganglia) of eyestalk-ablated or constant dark (DD) prawns, and in the eyestalk-ablated gill. No expression change was found under constant light (LL) or in heart and muscle.

The tau mutation in the Syrian hamster differentially reprograms the circadian clock in the SCN and peripheral tissues

The hypothalamic suprachiasmatic nuclei (SCN), the principal circadian oscillator in mammals, are synchronized to the solar day by the light-dark cycle, and in turn, they coordinate circadian oscillations in peripheral tissues. The tau mutation in the Syrian hamster is caused by a point mutation leading to a deficiency in the ability of Casein Kinase 1epsilon to phosphorylate its targets, including circadian PER proteins. How this accelerates circadian period in neural tissues is not known, nor is its impact on peripheral circadian oscillators established. We show that this mutation has no effect on per mRNA expression nor the nuclear accumulation of PER proteins in the SCN. It does, however, accelerate the clearance of PER proteins from the nucleus to an extent sufficient to explain the shortened circadian period of behavioral rhythms. The mutation also has novel, unanticipated consequences for circadian timing in the periphery, including tissue-specific phase advances and/or reduced amplitude of circadian gene expression. The results suggest that the tau mutation accelerates a specific phase, during mid-late subjective night of the SCN circadian feedback loop, rather than cause a global compression of the entire cycle. This reprogrammed output from the clock is associated with peripheral desynchrony, which in turn could account for impaired growth and metabolic efficiency of the mutant.

Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina

Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.

Pax6 is a direct, positively regulated target of the circadian gene Clock

Clock is a member of a highly conserved transcription control network that underlies the circadian cycle. During early embryogenesis, its expression is developmentally regulated and may be required for the normal development of the head. In this report, the transcription factor Pax6, a highly conserved regulator of anterior development, is shown to be a direct target of Clock regulation.

Photoperiodic regulation of androgen receptor and steroid receptor coactivator-1 in Siberian hamster brain

Seasonal changes in the neuroendocrine actions of gonadal steroid hormones are triggered by fluctuations in daylength. The mechanisms responsible for photoperiodic influences upon the feedback and behavioral effects of testosterone in Siberian hamsters are poorly understood. We hypothesized that daylength regulates the expression of androgen receptor (AR) and/or steroid receptor coactivator-1 (SRC-1) in specific forebrain regions. Hamsters were castrated and implanted with either oil-filled capsules or low doses of testosterone; half of the animals remained in 16L/8D and the rest were kept in 10L/14D for the ensuing 70 days. The number of AR-immunoreactive (AR-ir) cells was regulated by testosterone in medial amygdala and caudal arcuate, and by photoperiod in the medial preoptic nucleus and the posterodorsal medial amygdala. A significant interaction between photoperiod and androgen treatment was found in medial preoptic nucleus and posterodorsal medial amygdala. The molecular weight and distribution of SRC-1 were similar to reports in other rodent species, and short days reduced the number of SRC-1-ir cells in posteromedial bed nucleus of the stria terminalis (BNST) and posterodorsal medial amygdala. A significant interaction between androgen treatment and daylength in regulation of SRC-1-ir was found in anterior medial amygdala. The present results indicate that daylength-induced fluctuations in SRC-1 and AR expression may contribute to seasonally changing effects of testosterone.

Modulation of Hormonal Signaling in the Brain by Steroid Receptor Coactivators

Nuclear receptors, such as estrogen, glucocorticoid or thyroid hormone receptors, have been shown to play a critical role in brain development and physiology. The activity of these receptors is modulated by the interaction with several proteins and, in particular, coactivators are required to enhance their transcriptional activity. The steroid receptor coactivators (SRC-1, -2 and -3) are currently the best characterized coactivators and we review here the current knowledge on the distribution and function of these proteins in the brain. Knock-out models and antisense techniques have demonstrated the requirement for SRC-1 and -2 in the brain, focusing mainly on steroid and thyroid hormone-dependent development and behavior. The precise function of SRC-3 in the brain is currently unknown but its presence throughout the brain suggests an important function. Although the molecular biology of SRCs is relatively well known, the in vivo control of their expression, post-translational modifications and time- and cell-specific interactions with the different nuclear receptors remain elusive. A complete understanding of hormone action on brain and behavior will not be attained until a better knowledge of coactivator physiology is achieved.

Enhanced running wheel activity of both Mch1r- and Pmch-deficient mice

Mch1r-deficient (Mch1r(-/-)) mice are hyperphagic, hyperactive, lean, and resistant to diet-induced obesity. To examine whether the MCH1R is involved in regulating activity-based energy expenditure, we investigated voluntary wheel running (WR) activity of wild-type (WT) and Mch1r(-/-) mice basally, in response to diets with different caloric density and with different feeding schedules. We also evaluated WR activity of mice with ablation of the prepro-MCH gene (Pmch(-/-) mice). Dark cycle WR activity of Mch1r(-/-) mice fed low fat (LF) chow was increased significantly relative to WT mice. Transition to moderate high-fat (MHF) diet was associated with an increase in nocturnal WR activity in both genotypes. Both Mch1r(-/-) and WT mice exhibited food anticipatory activity (FAA) before the daily scheduled feeding time, indicating that MCH1R is not required for FAA. Naloxone (3 mg/kg, i.p.) suppressed WR activity of both genotypes, suggesting opioid regulation of locomotor activity. WR increased nocturnal dynorphin mRNA levels in Mch1r(-/-) brain. Importantly, Pmch-deficient mice had significantly enhanced WR activity relative to WT controls. These results suggest that endogenous MCH plays an inhibitory role in regulating locomotor activity. In summary, we demonstrated enhanced WR activities in mice lacking either MCH or its cognate receptor.

Reproductive performance in female ClockΔ19 mutant mice

The relationship between circadian rhythmicity and rodent reproductive cyclicity is well established, but the impact of disrupted clock gene function on reproduction has not been well established. The present study evaluated the reproductive performance of mice carrying the ClockΔ19 mutation that were either melatonin deficient (ClockΔ19/Δ19) or had the capacity to synthesise melatonin reinstated (ClockΔ19/Δ19+MEL). The ClockΔ19/Δ19 mice took 2–3 days longer to mate, and to subsequently deliver pups, than their control line. The melatonin-competent mutants had a smaller, but still significant (P < 0.05), delay. The ClockΔ19 mutation resulted in smaller median litter sizes compared with control lines (seven v. eight pups; P < 0.05), whereas melatonin proficiency reversed this difference. Survival to weaning was 84% and 80% for the ClockΔ19/Δ19 and ClockΔ19/Δ19+MEL lines, respectively, compared with 94–96% for the two control lines. The ClockΔ19/Δ19 mutants became behaviourally arrhythmic in constant darkness but, despite this, seven of seven became pregnant when paired with males after at least 14 days of constant darkness (five of seven within 4 days of pairing). In the ClockΔ19/Δ19+MEL mice, seven of 15 became arrhythmic in constant darkness but still became pregnant. The seven mice that free ran for at least 14 days in constant darkness with a period of 27.1 h also became pregnant. The present study has demonstrated that the ClockΔ19 mutation has significant, but subtle, effects on reproductive performance. The reintroduction of melatonin competency and/or other genes as a result of crosses with CBA mice reduced the impact of the mutation further. It would appear that redundancy in genes in the circadian system allows the reproductive cyclicity to persist in mice, albeit at a suboptimal level.

Chronic MCH-1 receptor modulation alters appetite, body weight and adiposity in rats

Central administration of the neuropeptide melanin-concentrating hormone (MCH) stimulates feeding in rodents. We studied the effects of intracerebroventricular (i.c.v.) administration of an MCH-1 receptor agonist (Compound A) and an MCH-1 receptor antagonist (Compound B) on feeding in satiated rats. Compound B (10 microg, i.c.v.) blocked the acute orexigenic effect of Compound A (5 microg, i.c.v.). In an experiment designed to either stimulate or inhibit MCH-1 receptor signaling over an extended period, rats received continuous i.c.v. infusions of vehicle (saline), Compound A (30 microg/day), Compound B (30 or 48 microg/day) or neuropeptide Y (24 microg/day, as positive control) via implantable infusion pumps. Continuous MCH-1 receptor activation recapitulated the obese phenotype of MCH-over-expressor mice, manifest as enhanced feeding (+23%, P<0.001), caloric efficiency and body weight gain (+38%, P<0.005) over the 14-day period relative to controls. Chronic MCH-1 receptor activation also elevated plasma insulin and leptin levels significantly. Conversely, continuous MCH-1 receptor antagonism led to sustained reductions in food intake (-16%, P<0.001), body weight gain (-35%, P<0.01), and body fat gain relative to controls, without an effect on lean mass. Antagonism of the MCH-1 receptor may be an effective approach for the treatment of obesity.

Brain expression of apurinic/apyrimidinic endonuclease (APE/Ref-1) multifunctional DNA repair enzyme gene in the mouse with special reference to the suprachiasmatic nucleus

Multifunctional mammalian apurinic/apyrimidinic endonuclease (APE, also known as redox factor-1; Ref-1) repairs baseless sites of damaged DNA caused by oxidative stress and regulates the redox state of various DNA binding proteins. Here, we examined the expression of APE/Ref-1 m-RNA in the mouse brain by in situ hybridization. We detected APE/Ref-1 transcripts throughout the mouse brain particularly in the clock oscillating neurons of the suprachiasmatic nucleus (SCN), hippocampal pyramidal cells, granular cells, and in monoaminergic neurons. In the circadian center SCN, levels of APE/ref-1 mRNA transcripts were constantly high, and were not influenced by either circadian rhythms or by exposure to light.

Tipin, a novel timeless-interacting protein, is developmentally co-expressed with timeless and disrupts its self-association

The mouse Timeless gene (mTim) was identified originally on the basis of its similarity to a Drosophila circadian gene, but has no substantiated role in the circadian clock mechanism. The importance of mTim in cellular processes involved in development, however, is undeniable, since targeted mutagenesis of this gene arrests embryonic development. To connect mTim to known pathways controlling cellular processes important for early development, a yeast two-hybrid approach was used to identify embryonic mTIM-interacting proteins. One positive interactor, a previously uncharacterized protein that is here termed TIPIN (TIMELESS interacting protein), was shown to interact with mTIM in vitro and in cultured cells. mTim and Tipin transcripts are co-expressed in similar tissues during embryonic development and in the adult brain. In transiently transfected cultures, mTIM promotes the nuclear localization of TIPIN. Immunoprecipitation experiments suggest that TIPIN is capable of regulating mTIM activity by disrupting the ability of mTIM to form homo-multimeric complexes. Together, these results indicate that mTIM forms a functional complex with TIPIN, and provide a starting point from which to link mTim to biochemical pathways controlling vital cellular functions.

Non-cyclic and developmental stage-specific expression of circadian clock proteins during murine spermatogenesis

The central circadian clock in mammals is housed in the brain and is based on cyclic transcription and translation of clock proteins. How the central clock regulates peripheral organ function is unclear. However, cyclic expression of circadian genes in peripheral tissues is well established, suggesting that these tissues have their own endogenous oscillators. Reproduction is a process influenced by circadian rhythms in many organisms, thus making the testis an attractive model for studying clock function in peripheral organs. However, results addressing cyclic expression of clock genes in the mammalian testis are inconsistent. To resolve this issue, RNA was extracted from testes of mice at various times of day. Expression of the circadian clock genes mPer1, mPer2, Bmal1, Clock, mCry1, and Npas2 was constant at all times. Immunohistochemical localization of mPER1 and CLOCK proteins revealed restricted expression only in cells at specific developmental stages of spermatogenesis. For mPER1, these stages are the spermatogonia and the condensing spermatids. In contrast, CLOCK expression was restricted to round spermatids, specifically within the developing acrosome. Expression of mPER1 and CLOCK was constant at all times of day. These results suggest that clock proteins have noncircadian functions in spermatogenesis. Noncircadian expression of clock genes was also found in the thymus, which, like the testis, is composed primarily of differentiating cells. We propose that cyclic expression of clock genes is suspended during cellular differentiation.

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.

Melatonin and activity rhythm responses to light pulses in mice with the Clock mutation

Melatonin and wheel-running rhythmicity and the effects of acute and chronic light pulses on these rhythms were studied in Clock(Delta19) mutant mice selectively bred to synthesize melatonin. Homozygous melatonin-proficient Clock(Delta19) mutant mice (Clock(Delta19/Delta19)-MEL) produced melatonin rhythmically, with peak production 2 h later than the wild-type controls (i.e., just before lights on). By contrast, the time of onset of wheel-running activity occurred within a 20-min period around lights off, irrespective of the genotype. Melatonin production in the mutants spontaneously decreased within 1 h of the expected time of lights on. On placement of the mice in continuous darkness, the melatonin rhythm persisted, and the peak occurred 2 h later in each cycle over the first two cycles, consistent with the endogenous period of the mutant. This contrasted with the onset of wheel-running activity, which did not shift for several days in constant darkness. A light pulse around the time of expected lights on followed by constant darkness reduced the expected 2-h delay of the melatonin peak of the mutants to approximately 1 h and advanced the time of the melatonin peak in the wild-type mice. When the Clock(Delta19/Delta19)-MEL mice were maintained in a skeleton photoperiod of daily 15-min light pulses, a higher proportion entrained to the schedule (57%) than melatonin-deficient mutants (9%). These results provide compelling evidence that mice with the Clock(Delta19) mutation express essentially normal rhythmicity, albeit with an underlying endogenous period of 26-27 h, and they can be entrained by brief exposure to light. They also raise important questions about the role of Clock in rhythmicity and the usefulness of monitoring behavioral rhythms compared with hormonal rhythms.

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.

Temporal-spatial characterization of chicken clock genes: circadian expression in retina, pineal gland, and peripheral tissues

The molecular core of the vertebrate circadian clock is a set of clock genes, whose products interact to control circadian changes in physiology. These clock genes are expressed in all tissues known to possess an endogenous self-sustaining clock, and many are also found in peripheral tissues. In the present study, the expression patterns of two clock genes, cBmal1 and cMOP4, were examined in the chicken, a useful model for analysis of the avian circadian system. In two tissues which contain endogenous clocks--the pineal gland and retina--circadian fluctuations of both cBmal1 and cMOP4 mRNAs were observed to be synchronous; highest levels occurred at Zeitgeber time 12. Expression of these genes is also rhythmic in several peripheral tissues; however, the phases of these rhythms differ from those in the pineal gland and retina: in the liver the peaks of cMOP4 and cBmal1 mRNAs are delayed 4-8 h and in the heart they are advanced by 4 h, relative to those in the pineal gland and retina. These results provide the first temporal characterization of cBmal1 and cMOP4 mRNAs in avian tissues: their presence in avian peripheral tissues indicates they may influence temporal features of daily rhythms in biochemical, physiological, and behavioral functions at these sites.