Photoperiod regulates clock gene rhythms in the ovine liver

Developmental Programming: Impact of Prenatal Exposure to Bisphenol A on Senescence and Circadian Mediators in the Liver of Sheep

Prenatal exposure to endocrine disruptors such as bisphenol A (BPA) plays a critical role in the developmental programming of liver dysfunction that is characteristic of nonalcoholic fatty liver disease (NAFLD). Circadian and aging processes have been implicated in the pathogenesis of NAFLD. We hypothesized that the prenatal BPA-induced fatty-liver phenotype of female sheep is associated with premature hepatic senescence and disruption in circadian clock genes. The expression of circadian rhythm and aging-associated genes, along with other markers of senescence such as telomere length, mitochondrial DNA copy number, and lipofuscin accumulation, were evaluated in the liver tissue of control and prenatal BPA groups. Prenatal BPA exposure significantly elevated the expression of aging-associated genes GLB1 and CISD2 and induced large magnitude differences in the expression of other aging genes-APOE, HGF, KLOTHO, and the clock genes PER2 and CLOCK-in the liver; the other senescence markers remained unaffected. Prenatal BPA-programmed aging-related transcriptional changes in the liver may contribute to pathological changes in liver function, elucidating the involvement of aging genes in the pathogenesis of liver steatosis.

Transcriptome analysis reveals disruption of circadian rhythms in late gestation dairy cows may increase risk for fatty liver and reduced mammary remodeling

Circadian disruption increased insulin resistance and decreased mammary development in late gestation, nonlactating (dry) cows. The objective was to measure the effect of circadian disruption on transcriptomes of the liver and mammary gland. At 35 days before expected calving (BEC), multiparous dry cows were assigned to either control (CON) or phase-shifted treatments (PS). CON was exposed to 16-h light and 8-h dark. PS was exposed to 16-h light to 8-h dark, but phase of the light-dark cycle was shifted 6 h every 3 days. On day 21 BEC, liver and mammary were biopsied. RNA was isolated (n = 6 CON, n = 6 PS per tissue), and libraries were prepared and sequenced using paired-end reads. Reads mapping to bovine genome averaged 27 ± 2 million and aligned to 14,222 protein-coding genes in liver and 15,480 in mammary analysis. In the liver, 834 genes, and in the mammary gland, 862 genes were different (nominal P < 0.05) between PS and CON. In the liver, genes upregulated in PS functioned in cholesterol biosynthesis, endoplasmic reticulum stress, wound healing, and inflammation. Genes downregulated in liver function in cholesterol efflux. In the mammary gland, genes upregulated functioned in mRNA processing and transcription and downregulated genes encoded extracellular matrix proteins and proteases, cathepsins and lysosomal proteases, lipid transporters, and regulated oxidative phosphorylation. Increased cholesterol synthesis and decreased efflux suggest that circadian disruption potentially increases the risk of fatty liver in cows. Decreased remodeling and lipid transport in mammary may decrease milk production capacity during lactation.

The Circadian Physiology: Implications in Livestock Health

Circadian rhythms exist in almost all types of cells in mammals. Thousands of genes exhibit approximately 24 h oscillations in their expression levels, making the circadian clock a crucial regulator of their normal functioning. In this regard, environmental factors to which internal physiological processes are synchronized (e.g., nutrition, feeding/eating patterns, timing and light exposure), become critical to optimize animal physiology, both by managing energy use and by realigning the incompatible processes. Once the circadian clock is disrupted, animals will face the increased risks of diseases, especially metabolic phenotypes. However, little is known about the molecular components of these clocks in domestic species and by which they respond to external stimuli. Here we review evidence for rhythmic control of livestock production and summarize the associated physiological functions, and the molecular mechanisms of the circadian regulation in pig, sheep and cattle. Identification of environmental and physiological inputs that affect circadian gene expressions will help development of novel targets and the corresponding approaches to optimize production efficiency in farm animals.

Circadian and photic modulation of daily rhythms in diurnal mammals

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

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

Evidence of a molecular clock in the ovine ovary and the influence of photoperiod

The influence of the central circadian clock on reproductive timing is well established. Much less is known about the role of peripheral oscillators such as those in the ovary. We investigated the influence of photoperiod and timing of the LH surge on expression of circadian clock genes and genes involved in steroidogenesis in ovine ovarian stroma. Seventy-two Suffolk cross ewes were divided into two groups, and their estrous cycles were synchronized. Progestagen sponge removal was staggered by 12 hours between the groups such that expected LH peak would occur midway through either the light or dark phase of the photoperiodic cycle. Four animals from each group were killed, and their ovaries were harvested beginning 36 hours after sponge removal, at 6-hour intervals for 48 hours. Blood was sampled every 3 hours for the period 24 to 48 hours after sponge removal to detect the LH surge. The interval to peak LH did not differ between the groups (36.2 ± 1.2 and 35.6 ± 1.1 hours, respectively). There was an interaction between group and the time of sponge removal on the expression of the core clock genes ARNTL, PER1, CRY1, CLOCK, and DBP (P < 0.01, P < 0.05, P < 0.01, P < 0.01, and P < 0.01, respectively). As no significant interaction between group and time of day was detected, the datasets were combined. Statistically significant rhythmic oscillation was observed for ARNTL, CLOCK, CRY1 (P < 0.01, respectively), PTGS2, DBP, PTGER2, and CYP17A1 (P < 0.05, respectively), confirming the existence of a time-sensitive functionality within the ovary, which may influence steroidogenesis and is independent of the ovulatory cycle.

REV-ERBα inhibits the PTGS2 expression in bovine uterus endometrium stromal and epithelial cells exposed to ovarian steroids

The nuclear receptor REV-ERBα (encoded by NR1D1) has a critical role in metabolism and physiology as well as circadian rhythm. Here, we investigated the possible contribution of clock genes including NR1D1 to the secretion of prostaglandin F₂α (PGF₂α) from bovine uterine stromal (USCs) and epithelial cells (UECs) by modulating the expression of PTGS2. The circadian oscillation of clock genes in the cells was weak compared with that reported in rodents, but the expression of BMAL1, PER1, and NR1D1 was changed temporally by treatment with ovarian steroids. Significant expression of clock genes including NR1D1 was detected in USCs exposed to progesterone. NR1D1 was also significantly expressed in UECs exposed to estradiol. The expression of PTGS2 was suppressed in USCs exposed to progesterone, while the expression was initially suppressed in UECs exposed to estradiol and then increased after long-term exposure to estradiol. BMAL1 knockdown with specific siRNA caused a significant decrease in the transcript levels of NR1D1 and PTGS2 in USCs, but not in UECs. The production of PGF₂α also decreased in USCs after BMAL1 knockdown, while its level did not significantly change in UECs. The transcript level of PTGS2 was increased by treatment with the antagonist of REV-ERBα in both cell types, but the agonist was ineffective. In these two cell types treated with the agonist or antagonist, the PGF₂α production coincided well with the PTGS2 expression. Collectively, these results indicate that REV-ERBα plays an inhibitory role in the expression of PTGS2 in both bovine USCs and UECs treated with ovarian steroids.

Tissue-specific changes in molecular clocks during the transition from pregnancy to lactation in mice

Circadian clocks regulate homeostasis and mediate responses to stressors. Lactation is one of the most energetically demanding periods of an adult female's life. Peripartum changes occur in almost every organ so the dam can support neonatal growth through milk production while homeostasis is maintained. How circadian clocks are involved in adaptation to lactation is currently unknown. The abundance and temporal pattern of core clock genes' expression were measured in suprachiasmatic nucleus, liver, and mammary from late pregnant and early lactation mice. Tissue-specific changes in molecular clocks occurred between physiological states. Amplitude and robustness of rhythms increased in suprachiasmatic nucleus and liver. Mammary rhythms of core molecular clock genes were suppressed. Attenuated rhythms appeared to be a physiological adaptation of mammary to lactation, because manipulation of timing of suckling resulting in significant differences in plasma prolactin and corticosterone had no effect on amplitude. Analysis of core clock proteins revealed that the stoichiometric relationship between positive (CLOCK) and negative (PER2) components remained 1:1 in liver but was increased to 4:1 in mammary during physiological transition. Induction of differentiation of mammary epithelial cell line HC11 with dexamethasone, insulin, and prolactin resulted in similar stoichiometric changes among positive and negative clock regulators, and prolactin induced phase shifts in HC11 Arntl expression rhythm. Data support that distinct mechanisms drive periparturient changes in mammary clock. Stoichiometric change in clock regulators occurs with gland differentiation. Suppression of mammary clock gene expression rhythms represents a physiological adaptation to suckling cues. Adaptations in mammary clock are likely needed in part to support suckling demands of neonates.

Does the circadian system regulate lactation?

Environmental variables such as photoperiod, heat, stress, nutrition and other external factors have profound effects on quality and quantity of a dairy cow's milk. The way in which the environment interacts with genotype to impact milk production is unknown; however, evidence from our laboratory suggests that circadian clocks play a role. Daily and seasonal endocrine rhythms are coordinated in mammals by the master circadian clock in the hypothalamus. Peripheral clocks are distributed in every organ and coordinated by signals from the master clock. We and others have shown that there is a circadian clock in the mammary gland. Approximately 7% of the genes expressed during lactation had circadian patterns including core clock and metabolic genes. Amplitude changes occurred in the core mammary clock genes during the transition from pregnancy to lactation and were coordinated with changes in molecular clocks among multiple tissues. In vitro studies using a bovine mammary cell line showed that external stimulation synchronized mammary clocks, and expression of the core clock gene, BMAL1, was induced by lactogens. Female clock/clock mutant mice, which have disrupted circadian rhythms, have impaired mammary development and their offspring failed to thrive suggesting that the dam's milk production was not adequate enough to nourish their young. We envision that, in mammals, during the transition from pregnancy to lactation the master clock is modified by environmental and physiological cues that it receives, including photoperiod length. In turn, the master clock coordinates changes in endocrine milieu that signals peripheral tissues. In dairy cows, it is clear that changes in photoperiod during the dry period and/or during lactation influences milk production. We believe that the photoperiod effect on milk production is mediated, in part by the 'setting' of the master clock with light, which modifies peripheral circadian clocks including the mammary core clock and subsequently impacts milk yield and may impact milk composition.

Lactation Biology Symposium: circadian clocks as mediators of the homeorhetic response to lactation

The transition from pregnancy to lactation is the most stressful period in the life of a cow. During this transition, homeorhetic adaptations are coordinated across almost every organ and are marked by changes in hormones and metabolism to accommodate the increased energetic demands of lactation. Recent data from our laboratory showed that changes in circadian clocks occur in multiple tissues during the transition period in rats and indicate that the circadian system coordinates changes in the physiology of the dam needed to support lactation. Circadian rhythms coordinate the timing of physiological processes and synchronize these processes with the environment of the animal. Circadian rhythms are generated by molecular circadian clocks located in the hypothalamus (the master clock) and peripherally in every organ of the body. The master clock receives environmental and physiological cues and, in turn, synchronizes internal physiology by coordinating endocrine rhythms and metabolism through peripheral clocks. The effect of the circadian clock on lactation may be inferred by the photoperiod effect on milk production, which is accompanied by coordinated changes in the endocrine system and metabolic capacity of the dam to respond to changes in day length. We have shown that bovine mammary epithelial cells possess a functional clock that can be synchronized by external stimuli, and the expression of the aryl hydrocarbon receptor nuclear translocator-like gene, a positive limb of the core clock, is responsive to prolactin in bovine mammary explants. Others showed that 7% of genes expressed in breasts of lactating women had circadian patterns of expression, and we report that the diurnal variation of composition of bovine milk is associated with changes in expression of mammary core clock genes. Together these studies indicate that the circadian system coordinates the metabolic and hormonal changes needed to initiate and sustain lactation, and we believe that the capacity of the dam to produce milk and cope with metabolic stresses in early lactation is related to her ability to set circadian rhythms during the transition period.

The effect of dexamethasone on clock gene mRNA levels in bovine neutrophils and lymphocytes

Circadian rhythms are driven by oscillating expression of a family of transcription factors called clock genes. In rodents, clock genes drive circadian rhythms in white blood cell function, and glucocorticoids are believed to regulate these rhythms. Little is known about circadian rhythms of cattle white blood cells. The objectives of this study were: (1) to quantify mRNA levels of clock genes in neutrophils and lymphocytes over 24h in healthy steers; and (2) to quantify effects of dexamethasone on clock gene mRNA levels in bovine neutrophils and lymphocytes. We hypothesized that bovine neutrophils and lymphocytes would display 24h variations in clock gene mRNA levels and that those patterns would be disrupted by glucocorticoid treatment. Six Holstein steers were injected with 0 or 0.10mg/kg body weight dexamethasone according to a crossover design. Neutrophils and lymphocytes were collected from jugular blood at 0, 4, 8, 12, 16, 20, and 24h relative to treatment administration. Neutrophil and lymphocyte mRNA levels of the clock genes Clock, Bmal1, Per1, Per2, Cry1, Cry2, Rev-erbα, and CK1ɛ were quantified. For neutrophils, an interaction between treatment and time was found for Clock, Cry1, and CK1ɛ. Time affected Clock, Per1, Cry1, Rev-erbα, and CK1ɛ. For all of those genes except Per1, neutrophils from control steers displayed 24h changes of mRNA levels characteristic of circadian regulated cells. The dexamethasone treatment increased neutrophil mRNA levels of Per1, decreased Clock, Cry1, Cry2, and Rev-erbα, and tended to decrease Bmal1. These results suggest that circadian rhythms have the potential to impact bovine neutrophil function, and that glucocorticoid-induced disruption of neutrophil circadian rhythms may contribute to periparturient immunosuppression. For lymphocytes, an interaction between treatment and time was observed for Per1 and tended to occur for Per2 and Cry2. Although time affected Per1 and Rev-erbα, distinct 24h patterns of lymphocyte clock gene mRNA levels were not evident as they were in neutrophils. Treatment increased Per1 and decreased Cry2, but the magnitude of the treatment effect was small. In summary, 24h patterns in clock gene mRNA levels were observed in bovine neutrophils and to some degree in lymphocytes, and these patterns were disrupted by dexamethasone administration. Although further research is needed, individual variation in white blood cell circadian rhythms and glucocorticoid responsiveness may help to explain individual differences in periparturient disease susceptibility.

Different mechanisms of adjustment to a change of the photoperiod in the suprachiasmatic and liver circadian clocks

Changes in photoperiod modulate the circadian system, affecting the function of the central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The aim of the present study was to elucidate the dynamics of adjustment to a change of a long photoperiod with 18 h of light to a short photoperiod with 6 h of light of clock gene expression rhythms in the mouse SCN and in the peripheral clock in the liver, as well as of the locomotor activity rhythm. Three, five, and thirteen days after the photoperiod change, daily profiles of Per1, Per2, and Rev-erbalpha expression in the rostral, middle, and caudal parts of the SCN and of Per2 and Rev-erbalpha in the liver were determined by in situ hybridization and real-time RT-PCR, respectively. The clock gene expression rhythms in the different SCN regions, desynchronized under the long photoperiod, attained synchrony gradually following the transition from long to short days, mostly via advancing the expression decline. The photoperiodic modulation of the SCN was due not only to the degree of synchrony among the SCN regions but also to different waveforms of the rhythms in the individual SCN parts. The locomotor activity rhythm adjusted gradually to short days by advancing the activity onset, and the liver rhythms adjusted by advancing the Rev-erbalpha expression rise and Per2 decline. These data indicate different mechanisms of adjustment to a change of the photoperiod in the central SCN clock and the peripheral liver clock.

Chronobiology and the horse: recent revelations and future directions

The circadian system provides animals with a means to adapt their internal physiology to the constantly changing environmental stimuli that exist on a rotating planet. Light information is translated into molecular timing mechanisms within pacemaker cells of the mammalian hypothalamic suprachiasmatic nucleus (SCN) via transcriptional-translational feedback loops. Humoral and neural outputs from this 'master' clock result in circadian rhythms of physiology and behaviour. The larger circadian system involves SCN synchronisation of cellular clocks throughout the organism such that individual organs can adapt their specific function to the time of day. In the short history of this scientific field, the vast majority of mammalian chronobiological research has been conducted using small laboratory animals. This review examines what these studies have revealed, discusses how recent chronobiological findings in the horse compare to what is known and highlights how the principles of circadian biology are applicable to equine husbandry and veterinary care.

Circadian clock genes of goldfish, Carassius auratus: cDNA cloning and rhythmic expression of period and cryptochrome transcripts in retina, liver, and gut

Clock genes are known to be the molecular core of biological clocks of vertebrates. They are expressed not only in those tissues considered central pacemakers, but also in peripheral tissues. In the present study, partial cDNAs for 6 of the principal clock genes (Period 1-3 and Cryptochrome 1-3) were cloned from a teleost fish, the goldfish (Carassius auratus ). These genes showed high homology (approximately 90%) with the respective cDNAs of zebrafish (Danio rerio), the only other teleost from which clock genes have been cloned. The daily expression pattern of each gene in retina, gut, and liver of goldfish was investigated using quantitative RT-PCR and cosinor analysis. All clock genes analyzed in the retina showed circadian rhythmicity; however, only Per 2-3 and Cry 2-3 were rhythmic in goldfish liver and gut. The amplitude and phase of the expression in liver and gut were different from those found in goldfish retina. Such differences suggest that other cues, such as feeding time, may contribute to the entrainment of oscillators in goldfish liver and gut. Our results support the use of goldfish as a teleost model to investigate the location and functioning of the circadian oscillators.

Clock genes are implicated in the human metabolic syndrome

BACKGROUND

Clock genes play a role in adipose tissue (AT) in animal experimental models. However, it remains to be elucidated whether these genes are expressed in human AT.

OBJECTIVE

We investigated the expression of several clock genes, Bmal1, Per2 and Cry1, in human AT from visceral and subcutaneous abdominal depots. A second objective was to elucidate whether these clock genes expressions were related to the metabolic syndrome features.

METHODS

Visceral and subcutaneous AT samples were obtained from morbid obese men (n=8), age: 42+/-13 years and body mass index>/=40 kg/m(2), undergoing laparoscopic surgery due to obesity. Biopsies were taken as paired samples at the beginning of the surgical process (1100 hour). Metabolic syndrome features such as waist circumference, plasma glucose, triglycerides, total cholesterol, high-density lipoprotein cholesterol and low-density lipoprotein (LDL) cholesterol were also studied. Homeostasis model assessment index of insulin resistance was also calculated. The expression of the different clock genes, hBmal1, hPer2 and hCry1, was determined by quantitative real-time PCR.

RESULTS

Clock genes were expressed in both human AT depots. hBmal1 expression was significantly lower than hPer2 and hCry1 in both AT (P<0.001). All genes were highly correlated to one another in the subcutaneous fat, while no correlation was found between Bmal1 and Per2 in the visceral AT. Clock genes AT expression was associated with the metabolic syndrome parameters: hPer2 expression level from visceral depot was inversely correlated to waist circumference (P<0.01), while the three clock genes studied were significantly and negatively correlated to total cholesterol and LDL cholesterol (P<0.01).

CONCLUSION

We have demonstrated for the first time in humans that clock genes are expressed in both subcutaneous and visceral fat. Their association with abdominal fat content and cardiovascular risk factors may be an indicator of the potential role of these clock genes in the metabolic syndrome disturbances.

Transcription factor dynamics in pineal gland and liver of the Syrian hamster (Mesocricetus auratus) adapts to prevailing photoperiod

The anticipation of day length and duration of darkness is necessary and advantageous for animals to survive and requires a photoperiodic memory. In the Syrian hamster this adaptation to photoperiod is mirrored by seasonal changes in the animal's reproductive state and its liver metabolism. Both events are linked to season-dependent alterations of the nocturnally elevated synthesis of the pineal hormone melatonin. To decipher molecules that are involved in this temporal gating, hamsters were exposed to long photoperiod (16 hr light:8 hr darkness; LP), or short photoperiod (8 hr light:16 hr darkness; SP). Dynamics in gene expression was investigated in the pineal gland [inducible cAMP early repressor (ICER)], and in the liver (ICER; C/EBPdelta; clock genes) using immunochemistry and reverse transcriptase PCR. While in the pineal, ICER rhythms tightly follow the prior duration of light and dark with decreasing levels at the beginning of the dark period in both LP and SP, ICER is not rhythmic in liver. In the liver, clock genes and their protein products reflect differences in photoperiodic history, with enhanced rhythm amplitudes of PER, CRY, CLOCK, and BMAL1 under SP conditions. Thus, in the Syrian hamster transcription factor expression patterns lock onto the prevailing photoperiod in two peripheral oscillators, the pineal gland and the liver, to function as mediators of suprachiasmatic nucleus-derived information on environmental light and dark. This tissue-specific gating in gene transcription represents a strategy to ameliorate consequences of altering environmental lighting conditions on endocrine and metabolic parameters that endow a strong circadian bias.

Acute systemic inflammation transiently synchronizes clock gene expression in equine peripheral blood

Peripheral clocks receive timing signals from the master mammalian pacemaker in the suprachiasmatic nucleus (SCN) and function to adaptively anticipate daily changes that influence local physiology. Evidence suggests that peripheral immune activation may act as a resetting signal for circadian clocks in peripheral tissues. We wished to investigate whether acute systemic inflammation could synchronize clock gene expression in equine peripheral blood, a tissue that does not normally oscillate in this species. We report that in vivo administration of lipopolysaccharide (LPS) results in significant upregulation of the core clock genes Per2 and Bmal1 in equine blood, in association with an acute rise in tumor necrosis factor (TNF) alpha and core body temperature compared to vehicle-treated control animals. Furthermore, co-administration of LPS and phenylbutazone, a non-steroidal anti-inflammatory drug (NSAID) known to inhibit prostaglandin (PG) E(2) synthesis in the horse, prevents both the febrile response and the synchronized increase in clock gene expression. However, the rise in Per2 and Bmal1 expression cannot be replicated in equine peripheral blood mononuclear cells (PBMCs) ex vivo by treatment with PGE(2), LPS or a heat shock mimicking the in vivo febrile response. These results may suggest an indirect communication pathway between immune modulators and the molecular machinery of cell clocks in peripheral blood. This potential immune feedback regulation of an equine peripheral clock implies a role for the circadian system in contributing to innate immune reactions and maintaining homeostasis in a tissue that acts as the first line of defense during an infectious challenge.

Where clocks are redundant: weak circadian mechanisms in reindeer living under polar photic conditions

Biological rhythms are a result of interplay between endogenous clocks and the ambient light-dark (LD) cycle. Biological timing in resident polar organisms presents a conundrum because these experience distinct daily LD cycles for only a few weeks each year. We measured locomotor activity in reindeer, Rangifer tarandus platyrhynchus (SR, n = 5 and 6) and R. tarandus tarandus (NR, n = 6), ranging freely at 78 and 70 degrees N, respectively, continuously throughout 1 year using data loggers. NR, but not SR, are gregarious which enabled us to examine the integrated effects of differences in social organisation and the photic environment at two different latitudes on the organisation of activity. In both sub-species, ultradian bouts of activity and inactivity alternated across the 24-h day throughout the year. This pattern was modified by the LD cycle in NR but barely at all in SR. Periodogram analysis revealed significant ultradian rhythmicity in both sub-species; the frequency of daily cycles of activity increased from three per day in winter to nearly five in summer. We conclude that this increase, and a concomitant increase in the level of daily activity, reflected the seasonal increase in the animals' appetite and the quality of their forage. Secondly, the combination, most evident in SR, of a weak photic response, weak circadian mechanisms and a weak social synchronization reduces the constraints of biological timing in an environment which is effectively non-rhythmic most of the year and permits expression of the basic ultradian pattern of ruminant activity. Third, the weaker 24-h rhythmicity in SR compared to NR indicates a latitudinal decrease in circadian organization and photic responsiveness in Rangifer.

Seasonal photoperiodism in vertebrates: from coincidence to amplitude

In vertebrates living in regions that range from tropical to polar zones, the day length (photoperiod) is a powerful synchronizer of seasonal changes in endocrine and metabolic physiology. This seasonal photoperiodism depends on the responses of internal circadian clocks to changing patterns of light-dark exposure, which can be conceptualized in the form of "coincidence-timing" models. The structural basis for this timing function is formed by a specialized "photoperiodic axis" that links light reception to the neuroendocrine system. In this review we describe the essential elements of this axis in mammals and birds, and discuss recent progress in understanding the cellular and molecular mechanisms through which this axis transduces photoperiodic change into altered endocrine output.

Evidence of an oscillating peripheral clock in an equine fibroblast cell line and adipose tissue but not in peripheral blood

The master mammalian pacemaker in the brain controls numerous diverse physiological and behavioral processes throughout the organism. Timing information is continually transmitted from the master clock to peripheral organs to synchronize rhythmic daily oscillations of clock gene transcripts and control local physiology. To investigate the presence of peripheral clocks in the horse, quantitative real-time RT-PCR assays were designed to detect levels of equine clock genes. Expression profiles for Per2, Bmal1 and Cry1 were first determined in a synchronized equine cell line. Subsequently, expression in equine whole blood and adipose tissue was assessed. Robust circadian oscillations of Per2, Bmal1 and Cry1 were observed in vitro. A synchronized molecular clock was also demonstrated in equine adipose tissue although oscillation of Bmal1 was less robust than that of Per2 and Cry1. In contrast to previous studies in humans and rats however, there was no evidence of synchronized clock gene expression in equine peripheral blood. These studies suggest that synchronous control of clock gene oscillation in equine peripheral blood is not as tightly regulated as in other species and may reflect the influence of different evolutionary challenges modifying the function of a peripheral clock.