The circadian clock as a molecular calendar

Reliable reference gene selection for quantitative real-time PCR (qRT-PCR) in floral developmental phases of dioecious species Coccinia grandis

The flowering process is intricate and regulated by a combination of external and internal factors. Delving into gene expression research has the potential to enhance our comprehension of the molecular foundations underlying floral development. Because of its accuracy, specificity, reproducibility, and efficiency, qRT-PCR is now a biological research tool for studying expression pattern of desired genes. The gene expression investigations using qRT-PCR required a reference gene with relatively uniform expression levels in multiple biological samples, including different developmental stages, tissues, and experimental conditions. In this study, experimental sets offloral and floral organ development in the male and female plants of C. grandis, a dioecious Cucurbitaceae species, qRT-PCR profiling was performed using six reference genes as internal control with B-class floral identity gene, PISTILLATA (PI). To analyse the data, algorithms such as geNorm, NormFinder, RefFinder, and BestKeeper were used to pick out the best internal controls from a group of candidates. The optimal reference gene for qRT-PCR studies with floral samples has been recommended as β-actin combined with β-tubulin. This is the first report on the validation of candidate reference genes across flower developmental stages in the dioecious species C. grandis, which will provide basic data for research on the molecular mechanism underlying flower development in this species and lay the groundwork for similar studies in other related species.

Preferences of Dairy Cattle for Supplemental Light-Emitting Diode Lighting in the Resting Area

Light from the environment is important for vision and regulating various biological processes. Providing supplemental lighting in the stall area could allow for individually targeted or group-level control of light. This study aimed to determine whether dairy cattle had preferences for short-term exposure to white (full-spectrum) light-emitting diode (LED) light or no LED light, yellow-green or white LED light, and blue or white LED light in the stall area. In total, 14 lactating cows were housed in a free-stall pen with unrestricted access to 28 stalls. LED light was controlled separately for each side of the stall platform. Two combinations of light were tested per week, and each week consisted of three adaptation days and four treatment days. Lying behaviour and video data were recorded continuously using leg-mounted pedometers and cameras, respectively. Preference was assessed by the amount of time spent lying and the number of bouts under each light treatment. No differences occurred between treatments within each week for daily lying time and number of bouts. Similarly, no differences occurred between treatments within each time period. Further controlled studies of long-term exposure to different LED wavelengths and intensities are required to determine potential benefits on metabolic processes.

[Melatonin: A short clarification for the over-enthusiasts]

Melatonin is a naturally occurring molecule derived from tryptophan. Melatonin is a key player in relaying the circadian rhythm between our environment and our body. It has also a key role in rhythming the seasons (more production during long nights and less during short ones) as well as in the reproduction cycles of the mammals. Melatonin is often and surprisingly presented as a molecule with multiple therapeutic properties that can fix (or help to fix) many health issues, such as diseases (cancer, ageing, virus-induced affections including COVID-19, etc…) or toxicological situations (metals, venoms, chemical such as adriamycin [doxorubicin], methotrexate or paclitaxel). The mechanistics behind those wonders is still missing and this is puzzling. In the present commentary, the main well-established biological properties are presented and briefly discussed with the aim of delineating the borders between facts and wishful thinking.

Variations in Rainbow Trout Immune Responses against A. salmonicida: Evidence of an Internal Seasonal Clock in Oncorhynchus mykiss

In poikilothermic vertebrates, seasonality influences different immunological parameters such as leukocyte numbers, phagocytic activity, and antibody titers. This phenomenon has been described in different teleost species, with immunological parameters peaking during warmer months and decreased levels during winter. In this study, the cellular immune responses of rainbow trout (Oncorhynchus mykiss) kept under constant photoperiod and water temperature against intraperitoneally injected Aeromonas salmonicida during the summer and winter were investigated. The kinetics of different leukocyte subpopulations from peritoneal cavity, spleen, and head kidney in response to the bacteria was measured by flow cytometry. Furthermore, the kinetics of induced A. salmonicida-specific antibodies was evaluated by ELISA. Despite maintaining the photoperiod and water temperature as constant, different cell baselines were detected in all organs analyzed. During the winter months, B- and T-cell responses were decreased, contrary to what was observed during summer months. However, the specific antibody titers were similar between the two seasons. Natural antibodies, however, were greatly increased 12 h post-injection only during the wintertime. Altogether, our results suggest a bias toward innate immune responses and potential lymphoid immunosuppression in the wintertime in trout. These seasonal differences, despite photoperiod and water temperature being kept constant, suggest an internal inter-seasonal or circannual clock controlling the immune system and physiology of this teleost fish.

Early menarche in visually impaired girls: evidence and hypothesis of light-dark cycle disruption and blindness effect on puberty onset

Puberty onset is tightly regulated by a broad spectrum of neuroendocrine signals and peripheral stimuli which coordinate the hypothalamic-pituitary-gonadal (HPG) axis activation. Numerous studies suggest that light stimulation influences HPG axis function; however, the effect of blindness on puberty timing remains controversial. Given that menarche is a suitable marker for sexual development initiation, the evaluation of the age at which blind girls attain it allows to indirectly assess the effect of light-dark cycle disruption on pubertal development. The present investigation aimed to review the evidence regarding menarcheal age drift in visually impaired girls, as well as to discuss the findings based on the existing hypotheses of the physiological mechanisms linking the light-dark cycle and photic sensitivity loss to the onset of puberty. Eleven studies were retrieved from a literature search conducted in PubMed, Scopus, ScienceDirect, SpringerLink, and Google Scholar databases. Eight studies concluded that light perception impairment is related to a moderately earlier age at menarche. Moreover, the evidence gathered in this review suggests a positive association between the degree of light perception loss and precocious menarcheal onset; yet, no conclusive outcomes were found regarding menarche advancement in acquired versus congenital blindness. We encourage further research aiming to elucidate the physiological mechanism underlying photosensitive regulation and blindness effect on the neuroendocrine pathways involved in human sexual maturation.

Biological Clocks and Rhythms of Anger and Aggression

The body’s internal timekeeping system is an under-recognized but highly influential force in behaviors and emotions including anger and reactive aggression. Predictable cycles or rhythms in behavior are expressed on several different time scales such as circadian (circa diem, or approximately 24-h rhythms) and infradian (exceeding 24 h, such as monthly or seasonal cycles). The circadian timekeeping system underlying rhythmic behaviors in mammals is constituted by a network of clocks distributed throughout the brain and body, the activity of which synchronizes to a central pacemaker, or master clock. Our daily experiences with the external environment including social activity strongly influence the exact timing of this network. In the present review, we examine evidence from a number of species and propose that anger and reactive aggression interact in multiple ways with circadian clocks. Specifically, we argue that: (i) there are predictable rhythms in the expression of aggression and anger; (ii) disruptions of the normal functioning of the circadian system increase the likelihood of aggressive behaviors; and (iii) conversely, chronic expression of anger can disrupt normal rhythmic cycles of physiological activities and create conditions for pathologies such as cardiovascular disease to develop. Taken together, these observations suggest that a comprehensive perspective on anger and reactive aggression must incorporate an understanding of the role of the circadian timing system in these intense affective states.

Hypothesis driven single cell dual oscillator mathematical model of circadian rhythms

Molecular mechanisms responsible for 24 h circadian oscillations, entrainment to external cues, encoding of day length and the time-of-day effects have been well studied experimentally. However, it is still debated from the molecular network point of view whether each cell in suprachiasmatic nuclei harbors two molecular oscillators, where one tracks dawn and the other tracks dusk activities. A single cell dual morning and evening oscillator was proposed by Daan et al., based on the molecular network that has two sets of similar non-redundant per1/cry1 and per2/cry2 circadian genes and each can independently maintain their endogenous oscillations. Understanding of dual oscillator dynamics in a single cell at molecular level may provide insight about the circadian mechanisms that encodes day length variations and its response to external zeitgebers. We present here a realistic dual oscillator model of circadian rhythms based on the series of hypotheses proposed by Daan et al., in which they conjectured that the circadian genes per1/cry1 track dawn while per2/cry2 tracks dusk and they together constitute the morning and evening oscillators (dual oscillator). Their hypothesis also provides explanations about the encoding of day length in terms of molecular mechanisms of per/cry expression. We frame a minimal mathematical model with the assumption that per1 acts a morning oscillator and per2 acts as an evening oscillator and to support and interpret this assumption we fit the model to the experimental data of per1/per2 circadian temporal dynamics, phase response curves (PRC's), and entrainment phenomena under various light-dark conditions. We also capture different patterns of splitting phenomena by coupling two single cell dual oscillators with neuropeptides vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP) as the coupling agents and provide interpretation for the occurrence of splitting in terms of ME oscillators, though they are not required to explain the morning and evening oscillators. The proposed dual oscillator model based on Daan's hypothesis supports per1 and per2 playing the role of morning and evening oscillators respectively and this may be the first step towards the understanding of the core molecular mechanism responsible for encoding the day length.

Are Humans Seasonally Photoperiodic?

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here—namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.

Phase Sensitivity Analysis of Circadian Rhythm Entrainment

As a biological clock, circadian rhythms evolve to accomplish a stable (robust) entrainment to environmental cycles, of which light is the most obvious. The mechanism of photic entrainment is not known, but two models of entrainment have been proposed based on whether light has a continuous (parametric) or discrete (nonparametric) effect on the circadian pacemaker. A novel sensitivity analysis is developed to study the circadian entrainment in silico based on a limit cycle approach and applied to a model of Drosophila circadian rhythm. The comparative analyses of complete and skeleton photoperiods suggest a trade-off between the contribution of period modulation (parametric effect) and phase shift (nonparametric effect) in Drosophila circadian entrainment. The results also give suggestions for an experimental study to (in)validate the two models of entrainment.

Phylogeny and oscillating expression of period and cryptochrome in short and long photoperiods suggest a conserved function in Nasonia vitripennis

Photoperiodism, the ability to respond to seasonal varying day length with suitable life history changes, is a common trait in organisms that live in temperate regions. In most studied organisms, the circadian system appears to be the basis for photoperiodic time measurement. In insects this is still controversial: while some data indicate that the circadian system is causally involved in photoperiodism, others suggest that it may have a marginal or indirect role. Resonance experiments in the parasitic wasp Nasonia vitripennis have revealed a circadian component in photoperiodic time measurement compatible with a mechanism of internal coincidence where a two components oscillator system obtains information from dawn and dusk, respectively. The identity of this oscillator (or oscillators) is still unclear but possible candidates are the oscillating molecules of the auto-regulatory feedback loops in the heart of the circadian system. Here, we show for the first time the circadian oscillation of period and cryptochrome mRNAs in the heads of Nasonia females kept under short and long photoperiods. Period and cryptochrome mRNA levels display a synchronous oscillation in all conditions tested and persist, albeit with reduced amplitude, during the first day in constant light as well as constant darkness. More importantly, the signal for the period and cryptochrome oscillations is set by the light-on signal. These results, together with phylogenetic analyses, indicate that Nasonia’s period and cryptochrome display characteristics of homologous genes in other hymenopteran species.

Minutes, days and years: molecular interactions among different scales of biological timing

Biological clocks are genetically encoded oscillators that allow organisms to keep track of their environment. Among them, the circadian system is a highly conserved timing structure that regulates several physiological, metabolic and behavioural functions with periods close to 24 h. Time is also crucial for everyday activities that involve conscious time estimation. Timing behaviour in the second-to-minutes range, known as interval timing, involves the interaction of cortico-striatal circuits. In this review, we summarize current findings on the neurobiological basis of the circadian system, both at the genetic and behavioural level, and also focus on its interactions with interval timing and seasonal rhythms, in order to construct a multi-level biological clock.

Temporal organization of activity in the brown bear (Ursus arctos): roles of circadian rhythms, light, and food entrainment

Seasonal cycles of reproduction, migration, and hibernation are often synchronized to changes in daylength (photoperiod). Ecological and evolutionary pressures have resulted in physiological specializations enabling animals to occupy a particular temporal niche within the diel cycle leading to characteristic activity patterns. In this study, we characterized the annual locomotor activity of captive brown bears (Ursus arctos). Locomotor activity was observed in 18 bears of varying ages and sexes during the active (Mar-Oct) and hibernating (Nov-Feb) seasons. All bears exhibited either crepuscular or diurnal activity patterns. Estimates of activity duration (α) and synchronization to the daily light:dark cycle (phase angles) indirectly measured photoresponsiveness. α increased as daylength increased but diverged near the autumnal equinox. Phase angles varied widely between active and hibernating seasons and exhibited a clear annual rhythm. To directly test the role of photoperiod, bears were exposed to controlled photoperiod alterations. Bears failed to alter their daily activity patterns (entrain) to experimental photoperiods during the active season. In contrast, photic entrainment was evident during hibernation when the daily photocycle was shifted and when bears were exposed to a skeleton (11:1:11:1) photoperiod. To test whether entrainment to nonphotic cues superseded photic entrainment during the active season, bears were exposed to a reversed feeding regimen (dark-fed) under a natural photocycle. Activity shifted entirely to a nocturnal pattern. Thus daily activity in brown bears is highly modifiable by photoperiod and food availability in a stereotypic seasonal fashion.

Circadian rhythms, the molecular clock, and skeletal muscle

Almost all organisms ranging from single cell bacteria to humans exhibit a variety of behavioral, physiological, and biochemical rhythms. In mammals, circadian rhythms control the timing of many physiological processes over a 24-h period, including sleep-wake cycles, body temperature, feeding, and hormone production. This body of research has led to defined characteristics of circadian rhythms based on period length, phase, and amplitude. Underlying circadian behaviors is a molecular clock mechanism found in most, if not all, cell types including skeletal muscle. The mammalian molecular clock is a complex of multiple oscillating networks that are regulated through transcriptional mechanisms, timed protein turnover, and input from small molecules. At this time, very little is known about circadian aspects of skeletal muscle function/metabolism but some progress has been made on understanding the molecular clock in skeletal muscle. The goal of this chapter is to provide the basic terminology and concepts of circadian rhythms with a more detailed review of the current state of knowledge of the molecular clock, with reference to what is known in skeletal muscle. Research has demonstrated that the molecular clock is active in skeletal muscles and that the muscle-specific transcription factor, MyoD, is a direct target of the molecular clock. Skeletal muscle of clock-compromised mice, Bmal1(-/-) and Clock(Δ19) mice, are weak and exhibit significant disruptions in expression of many genes required for adult muscle structure and metabolism. We suggest that the interaction between the molecular clock, MyoD, and metabolic factors, such as PGC-1, provide a potential system of feedback loops that may be critical for both maintenance and adaptation of skeletal muscle.

Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod

Virtually all species have developed cellular oscillations and mechanisms that synchronize these cellular oscillations to environmental cycles. Such environmental cycles in biotic (e.g. food availability and predation risk) or abiotic (e.g. temperature and light) factors may occur on a daily, annual or tidal time scale. Internal timing mechanisms may facilitate behavioural or physiological adaptation to such changes in environmental conditions. These timing mechanisms commonly involve an internal molecular oscillator (a 'clock') that is synchronized ('entrained') to the environmental cycle by receptor mechanisms responding to relevant environmental signals ('Zeitgeber', i.e. German for time-giver). To understand the evolution of such timing mechanisms, we have to understand the mechanisms leading to selective advantage. Although major advances have been made in our understanding of the physiological and molecular mechanisms driving internal cycles (proximate questions), studies identifying mechanisms of natural selection on clock systems (ultimate questions) are rather limited. Here, we discuss the selective advantage of a circadian system and how its adaptation to day length variation may have a functional role in optimizing seasonal timing. We discuss various cases where selective advantages of circadian timing mechanisms have been shown and cases where temporarily loss of circadian timing may cause selective advantage. We suggest an explanation for why a circadian timing system has emerged in primitive life forms like cyanobacteria and we evaluate a possible molecular mechanism that enabled these bacteria to adapt to seasonal variation in day length. We further discuss how the role of the circadian system in photoperiodic time measurement may explain differential selection pressures on circadian period when species are exposed to changing climatic conditions (e.g. global warming) or when they expand their geographical range to different latitudes or altitudes.

Weak evidence of bright light effects on human LH and FSH

Background

Most mammals are seasonal breeders whose gonads grow to anticipate reproduction in the spring and summer. As day length increases, secretion increases for two gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). This response is largely controlled by light. Light effects on gonadotropins are mediated through effects on the suprachiasmatic nucleus and responses of the circadian system. There is some evidence that seasonal breeding in humans is regulated by similar mechanisms, and that light stimulates LH secretion, but primate responses seem complex.

Methods

To gain further information on effects of bright light on LH and FSH secretion in humans, we analyzed urine samples collected in three experiments conducted for other goals. First, volunteers ages 18-30 years and 60-75 commenced an ultra-short 90-min sleep-wake cycle, during which they were exposed to 3000 lux light for 3 hours at balanced times of day, repeated for 3 days. Urine samples were assayed to explore any LH phase response curve. Second, depressed participants 60-79 years of age were treated with bright light or dim placebo light for 28 days, with measurements of urinary LH and FSH before and after treatment. Third, women of ages 20-45 years with premenstrual dysphoric disorder (PMDD) were treated to one 3-hour exposure of morning light, measuring LH and FSH in urine before and after the treatments.

Results

Two of the three studies showed significant increases in LH after light treatment, and FSH also tended to increase, but there were no significant contrasts with parallel placebo treatments and no significant time-of-day treatment effects.

Conclusions

These results gave some support for the hypothesis that bright light may augment LH secretion. Longer-duration studies may be needed to clarify the effects of light on human LH and FSH.

A circadian clock is not required in an arctic mammal

Seasonally breeding mammals use the annual change in the photoperiod cycle to drive rhythmic nocturnal melatonin signals from the pineal gland, providing a critical cue to time seasonal reproduction. Paradoxically, species resident at high latitudes achieve tight regulation of the temporal pattern of growth and reproduction despite the absence of photoperiodic information for most of the year. In this study, we show that the melatonin rhythm of reindeer (Rangifer tarandus) is acutely responsive to the light/dark cycle but not to circadian phase, and also that two key clock genes monitored in reindeer fibroblast cells display little, if any, circadian rhythmicity. The molecular clockwork that normally drives cellular circadian rhythms is evidently weak or even absent in this species, and instead, melatonin-mediated seasonal timing may be driven directly by photic information received at a limited time of year specific to the equinoxes.

Environmental influences on the production of pre-implantation embryos

Generation and cryopreservation of transgenic mice depend on reliable and continuous production of pre-implantation embryos. To suppress circannual and circadian rhythms driving the physiological and sexual behaviour of free living animals, laboratory animals are housed under standardized conditions. It remains to be elucidated if the artificial climate can cover all environmental effects. Here, we report that the humidity in an animal facility affects the embryo yield. The weather at the location of the facility, especially the temperature, influences the climate within an animal facility; weather peaks are obviously covered in part only, even if the facility is equipped with a powerful air-conditioning supply. Subsequently, external weather changes interact with the environment within the facility, influencing the production of embryos. Furthermore, noise and/or vibrations as generated by construction works, negatively affect the embryo yield.

Working around the clock: circadian rhythms and skeletal muscle

The study of the circadian molecular clock in skeletal muscle is in the very early stages. Initial research has demonstrated the presence of the molecular clock in skeletal muscle and that skeletal muscle of a clock-compromised mouse, Clock mutant, exhibits significant disruption in normal expression of many genes required for adult muscle structure and metabolism. In light of the growing association between the molecular clock, metabolism, and metabolic disease, it will also be important to understand the contribution of circadian factors to normal metabolism, metabolic responses to muscle training, and contribution of the molecular clock in muscle-to-muscle disease (e.g., insulin resistance). Consistent with the potential for the skeletal muscle molecular clock modulating skeletal muscle physiology, there are findings in the literature that there is significant time-of-day effects for strength and metabolism. Additionally, there is some recent evidence that temporal specificity is important for optimizing training for muscular performance. While these studies do not prove that the molecular clock in skeletal muscle is important, they are suggestive of a circadian contribution to skeletal muscle function. The application of well-established models of skeletal muscle research in function and metabolism with available genetic models of molecular clock disruption will allow for more mechanistic understanding of potential relationships.

Seasonal Variation of Falling Asleep while Driving: An Examination of Fatal Road Accidents

Road accidents related to sleep show a clear time-of-day pattern. Following the previous finding that such accidents in Finland happen more often (absolutely and relatively) during the summer months, especially during the afternoon, the main aim of this study was to reveal the factors behind such seasonality. The data included the computerized database of the Finnish fatal road accident investigation system restricted to nonprofessional, non-intoxicated car drivers (N=1464) and sample of the original folders (N=101). The results of the analysis on the sample of the original folders showed that investigation teams adequately documented and explained their decisions concerning falling-asleep accidents and that the observed seasonality is a real phenomenon, not a result of bias of the teams. With the exception of 26-35-year-olds, men of all of the other age groups had a notable absolute and relative increase of sleep-related accidents in the summer. Young male drivers (< or =25 yrs), followed by the oldest group (> or =66 yrs) had the highest increase in the absolute number of sleep-related accidents between the summer and winter months, while the relative change was the highest for ages 56-65. Women showed similar trends. A detailed analysis of 44 cases of summer afternoon falling-asleep accidents revealed that many drivers were engaged in unusual activities the day or night before the accident. The findings are discussed in relation to different driving and lifestyle habits between seasons, including sleep quality. Preliminary results of this study were presented at the 17th Congress of the European Sleep Research Society, Prague 2004.

Sexual symptoms in endocrine diseases: psychosomatic perspectives

BACKGROUND

Not only the most frequent causes of endocrine sexual dysfunction, such as hypogonadism and hyperprolactinemia, but almost all extragonadal endocrinopathies (hyper- and hypothyroidism, hyper- and hypocortisolism, steroidal secreting tumors, etc.) may have a greater or lesser effect on sexual function.

METHODS

We analyzed scientific literature on the correlations between hormones and sexual behavior, analyzing the most important issue from a practical point of view. The aim of this review article was thus to summarize the sexual symptoms that may be observed with endocrine diseases.

RESULTS

Hormones directly or indirectly regulate all human sexual functions (desire, erection/lubrication, ejaculation, orgasm). Some sexual symptoms may occur as a psychosomatic consequence of hormonal impairment. However, in other cases, endocrine failure may be generated by the psychosomatic involvement.

CONCLUSIONS

The endocrinologist, as an expert in body chemistry, is ideally positioned to identify and evaluate the full range of medical, physical, and psychiatric problems disrupting sexual function.

A functional link between rhythmic changes in chromatin structure and the Arabidopsis biological clock

Circadian clocks rhythmically coordinate biological processes in resonance with the environmental cycle. The clock function relies on negative feedback loops that generate 24-h rhythms in multiple outputs. In Arabidopsis thaliana, the clock component TIMING OF CAB EXPRESSION1 (TOC1) integrates the environmental information to coordinate circadian responses. Here, we use chromatin immunoprecipitation as well as physiological and luminescence assays to demonstrate that proper photoperiodic phase of TOC1 expression is important for clock synchronization of plant development with the environment. Our studies show that TOC1 circadian induction is accompanied by clock-controlled cycles of histone acetylation that favor transcriptionally permissive chromatin structures at the TOC1 locus. At dawn, TOC1 repression relies on the in vivo circadian binding of the clock component CIRCADIAN CLOCK ASSOCIATED1 (CCA1), while histone deacetylase activities facilitate the switch to repressive chromatin structures and contribute to the declining phase of TOC1 waveform around dusk. The use of cca1 late elongated hypocotyl double mutant and CCA1-overexpressing plants suggests a highly repressing function of CCA1, antagonizing H3 acetylation to regulate TOC1 mRNA abundance. The chromatin remodeling activities relevant at the TOC1 locus are distinctively modulated by photoperiod, suggesting a mechanism by which the clock sets the phase of physiological and developmental outputs.

In SYNC: the ins and outs of circadian oscillations in calcium

Many stimuli induce short-term increases in the cytosolic concentration of free calcium ions ([Ca(2+)](i)) that encode signaling information about diverse physiological and developmental events. Slow cytosolic Ca(2+) oscillations that span an entire day have also been discovered in both plants and animals; it is thought that these daily Ca(2+) oscillations may encode circadian clock signaling information. A recent study focusing on the characterization of the extracellular Ca(2+)-sensing receptor (CAS) has provided insight into the molecular mechanisms by which the daily Ca(2+) oscillation in plants is generated. We summarize the major findings regarding daily oscillations of cytosolic Ca(2+) concentrations in plants and animals, and discuss hypothetical biological roles for the circadian clock-regulated physiology in plants.

The genetic basis of circadian behavior

In most species, an endogenous timing system synchronizes physiology and behavior to the rhythmic succession of day and night. The mammalian circadian pacemaker residing in the suprachiasmatic nuclei (SCN) of the hypothalamus controls peripheral clocks throughout the brain and the body via humoral and neuronal transmission. On the cellular level, these clockworks consist of a set of interwoven transcriptional/translational feedback loops. Recent work emphasizes the tissue specificity of some components of these molecular clockworks and the differential regulation of their rhythmicity by the SCN.

Free-running rhythms of pineal circadian output

Circadian rhythms are self-sustaining oscillations that free-run in constant conditions with a period close to 24 h. Overt circadian rhythms have been studied mostly using onset phase as the marker for the underlying pacemaker. Using in vivo online pineal microdialysis, the authors have performed detailed analysis of free-running profiles of rat pineal secretory products, including N-acetylserotonin (NAS) and melatonin that have precisely defined onsets and offsets. When rats entrained in LD 12:12 were released into constant darkness (DD), both onset and offset phases of melatonin and NAS free-run. However, while onsets free-run with a period closer to a day (FRP(on) = 24-24.17 h) at the beginning, offset phases free-run with significantly larger FRPs (free-running periods) (FRP(off) = 24.24-24.42 h). This asymmetric free-running of onset and offset of NAS and melatonin in DD resulted in a 60- to 120-min increase of secretion duration of both NAS and melatonin. The rate of expansion of melatonin duration was 10 to 15 min per circadian cycle. The expansion of melatonin secretion duration ended for some within 4 days, while others were still expanding by the end of 10th day in DD. These results revealed that upon release into DD, the pacemaker's oscillation is initially driven by 2 forces, free running and decompression, before reaching a stable state of free running, and suggest that the circadian pacemaker may be an elastic structure that can decompress and compress under varying photic conditions. They also illustrate the importance of using both onset and offset of a given rhythm as phase markers, as compression/decompression, and transient disparity between FRP(on) and FRP(off) may be a common phenomenon of the circadian pacemaker.

Circadian clock genes and photoperiodism: Comprehensive analysis of clock gene expression in the mediobasal hypothalamus, the suprachiasmatic nucleus, and the pineal gland of Japanese Quail under various light schedules

In birds, the mediobasal hypothalamus (MBH) including the infundibular nucleus, inferior hypothalamic nucleus, and median eminence is considered to be an important center that controls the photoperiodic time measurement. Here we show expression patterns of circadian clock genes in the MBH, putative suprachiasmatic nucleus (SCN), and pineal gland, which constitute the circadian pacemaker under various light schedules. Although expression patterns of clock genes were different between long and short photoperiod in the SCN and pineal gland, the results were not consistent with those under night interruption schedule, which causes testicular growth. These results indicate that different expression patterns of the circadian clock genes in the SCN and pineal gland are not an absolute requirement for encoding and decoding of seasonal information. In contrast, expression patterns of clock genes in the MBH were stable under various light conditions, which enables animals to keep a steady-state photoinducible phase.

Encoding time of day and time of year by the avian circadian system

The most important zeitgeber for seasonal rhythmicity of physiology and behaviour in birds is the annual cycle of photoperiod. Regulatory mechanisms are less well understood in birds than in mammals since photic information can be perceived by photoreceptors in the retina and the pineal gland, as well as in the brain, and photoperiodic time measurement might be performed with reference to at least three autonomous circadian systems, the retina, the pineal gland and a hypothalamic oscillator. In many bird species, the pineal melatonin rhythm plays a central role in circadian organization. Durations of elevated melatonin in the blood reflect night length when animals are kept under natural photoperiodic conditions, as well as under different light/dark schedules in the laboratory. In the house sparrow, time of year is encoded in a particular melatonin signal, being short in duration and high in amplitude in long photoperiods and being long in duration and low in amplitude in short photoperiods, independent of whether the light zeitgeber is natural or artificial or varies in strength. Specific features of the melatonin signal are retained in vivo as well as in vitro when birds or isolated pineal glands are transferred to constant conditions. To regulate daily and seasonal changes of behaviour and physiology, melatonin may act at various target sites, including a complex hypothalamic oscillator that, unlike that in mammals, is not confined to a single cell group in the house sparrow. There is increasing evidence that interactions between two or more components of the songbird circadian pacemaking system are essential to encode and store biologically meaningful information about time, and thus provide the basis for photoperiodic time measurements and after effects in birds.

GIGANTEA and SPINDLY genes linked to the clock pathway that controls circadian characteristics of transpiration in Arabidopsis

Several "clock" genes that regulate the circadian system in Arabidopsis thaliana have been identified. The GIGANTEA (GI) gene has been shown to participate in the circadian system that is linked to overt rhythms in gene expression, leaf movements, hypocotyl elongation, and photoperiodic control of flowering in Arabidopsis. During continuous light (LL), circadian expression patterns in gi-2 mutants show reduced amplitudes and altered period lengths when compared with controls. Rhythms in stomatal function, such as transpiration, have been shown to be endogenous and persist in constant lighting conditions. In order to test for a physiologic variable that might be affected by the circadian clock via the GI gene, we compared circadian characteristics of transpiration between three Arabidopsis mutants (gi-2, spy-4, spy-4/gi-2) and wild-type (WT) controls in synchronized (LD for 2.5d) and free-running (LL for 3d) conditions. Each genotype showed a significant circadian rhythm in LD at p < 0.001, with acrophases located near the middle of the daily 14h L-span, with average amplitudes for WT: 18.9%, gi-2: 16.1%, spy-4: 7.7%, and spy-4/gi-2: 5.3%. On the first day in LL, the circadian amplitude was dramatically reduced to 3.1% for gi-2 compared with WT (11.9%), while amplitudes for spy-4 (6.9%) and spy-4/gi-2 (5.7%) were not significantly changed from LD. The amplitude for gi-2 remained low during days 2 (4.2%) and 3 (2.1%) in LL, while it slowly dampened for the WT (8.6 and 6.6%). The amplitudes for spy-4 (6.6%) and spy-4/gi-2 (5.6%) on day 2 in LL were indistinguishable from the LD span, but finally dampened on day 3 in LL (1.9 and 2.3%, respectively). These data suggest that transpiration is a physiologic variable controlled by a circadian system that involves both the GI and SPY proteins.