Daily rhythms of serum leptin in ewes: effects of feeding, pregnancy and lactation

The aim of the present study was to test whether serum concentrations of leptin in ewes vary with a daily rhythm. For this purpose, we examined 24 h serum leptin profiles of ewes exposed to natural photoperiodic conditions and subjected to two different feeding schedules (regular feeding and fasting). The results show for the first time the existence of daily rhythm of plasma leptin in regularly fed ewes, with a minimum during the light phase and a peak during the dark phase. Daily rhythms of serum leptin persisted after 50 h of fasting, although fasting shifted the peak of the rhythm to the beginning of the light phase and significantly reduced daily leptin production. To gain a better understanding of the role of leptin in the temporal organization of physiological events related to pregnancy and lactation, we measured serum leptin profiles throughout 24 h in ewes either during pregnancy or lactation. Daily leptin rhythms were found to persist during pregnancy and lactation, but both physiological conditions altered leptin concentrations. Maternal serum leptin concentration rose between early and mid pregnancy, then decreased in the late pregnancy and during lactation. Daily serum leptin concentration was significantly lower in nonpregnant, nonlactating ewes, compared either to lactating or to early pregnant ewes.     

8-hour rhythm of ovarian progesterone secretion in ewe

Pulse character of hormones secretion in the hypothalamus-pituitary-gonads system is a necessary condition of physiological regulation of reproduction. At the same time, the rhythms of ovarian hormones secretion have not been adequately explored. The researches study mainly three sexually mature ewes. The stages of oestrus cycle were determined on behavioral reactions of females in the presence of ram. Blood samples from jugular vein were collected hourly over 24-hour period during follicular (15-16 days), early (3-4 days) and middle (7-9 days) luteal phase of oestrus cycle, pregnancy (40-105 days) and lactation (30-45 days). 27 experiments were performed. Plasma progesterone was determined by enzyme-immunoassay method. There was no diurnal rhythm of ovarian progesterone secretion in ewes. During early and middle luteal phase of oestrus cycle and lactation, an 8-hour rhythm of progesterone secretion was detected. Follicular phase of oestrus cycle and pregnancy were characterized by irregular rises of fluctuations of progesterone level. It seems that the 8-hour rhythm of progesterone secretion during luteal phase and lactation is controlled by action of intraovarian generator of ultradian rhythms.     

Influence of fasting and exercise on the daily rhythm of serum leptin in the horse

The hormone leptin is secreted by white adipocytes and regulates food intake and energy expenditure in rodents and humans. The goal of the present study was to investigate the existence of a daily rhythm of serum leptin in horses and its dependence on fasting and physical exercise. A robust daily rhythm of leptin was found in both athletic and sedentary horses, with a daytime trough and a peak in the dark phase. While physical exercise never induced changes in circulating leptin, fasting reliably affected serum leptin levels. Food deprivation did not abolish the daily rhythm of serum leptin, but daily mean leptin levels in fasted horses were significantly lower than in regularly fed horses. This result indicates that leptin production is not a mere consequence of feeding behavior. The fact that in a large animal such as the horse a short fast decreases leptin without significantly changing the body weight demonstrates that changes in levels of circulating leptin associated with food restriction do not solely reflect changes in amount of body fat.     

Rhythms of ghrelin, leptin, and sleep in rats: effects of the normal diurnal cycle, restricted feeding, and sleep deprivation

To determine the relationships among plasma ghrelin and leptin concentrations and hypothalamic ghrelin contents, and sleep, cortical brain temperature (Tcrt), and feeding, we determined these parameters in rats in three experimental conditions: in free-feeding rats with normal diurnal rhythms, in rats with feeding restricted to the 12-h light period (RF), and in rats subjected to 5-h of sleep deprivation (SD) at the beginning of the light cycle. Plasma ghrelin and leptin displayed diurnal rhythms with the ghrelin peak preceding and the leptin peak following the major daily feeding peak in hour 1 after dark onset. RF reversed the diurnal rhythm of these hormones and the rhythm of rapid-eye-movement sleep (REMS) and significantly altered the rhythm of Tcrt. In contrast, the duration and intensity of non-REMS (NREMS) were hardly responsive to RF. SD failed to change leptin concentrations, but it promptly stimulated plasma ghrelin and induced eating. SD elicited biphasic variations in the hypothalamic ghrelin contents. SD increased plasma corticosterone, but corticosterone did not seem to influence either leptin or ghrelin. The results suggest a strong relationship between feeding and the diurnal rhythm of leptin and that feeding also fundamentally modulates the diurnal rhythm of ghrelin. The variations in hypothalamic ghrelin contents might be associated with sleep-wake activity in rats, but, unlike the previous observations in humans, obvious links could not be detected between sleep and the diurnal rhythms of plasma concentrations of either ghrelin or leptin in the rat.     

Influence of reproductive status on the daily rhythms of oxidative stress markers in <Emphasis Type="Italic">Ovis aries</Emphasis>

Changes in circadian rhythms of dROMs, Oxy-ads and SHp during reproductive stages were studied in Comisana ewes. Twelve ewes were divided in two equal groups. The experimental group consisted of ewes undergoing gestation and lactation following artificial insemination and the control group consisted of non-pregnant ewes. Blood samples were collected every 3 h over a 24 h period, 20 days before insemination, on days 100 and 140 of pregnancy, on days 10, 30 and 200 post-partum and during the dry period. In the control group, blood samples were collected on the same days and with the same procedures as those used for the experimental group. A significant effect of time on all parameters studied was observed in the experimental group. Daily rhythms of the parameters studied were observed in the control group in all experimental conditions, and in the experimental group during pre-pregnancy and dry periods. We conclude that the reproductive status of sheep affects oxidative stress markers in blood and their circadian rhythms.     

SYNCHRONIZATION TO LIGHT AND RESTRICTED-FEEDING SCHEDULES OF BEHAVIORAL AND HUMORAL DAILY RHYTHMS IN GILTHEAD SEA BREAM (SPARUS AURATA)

Food is not continuously available in the wild, and so most animals show a wide variety of circadian rhythms that can be entrained to feeding time. The aim of this research was to evaluate the effect of time-restricted feeding on the daily rhythms of gilthead sea bream, with food being provided during the day or night under a 12:12?h light-dark (LD) cycle or constant light (LL) conditions. Self-feeding and locomotor activity, as well as daily rhythms of cortisol, glucose, and melatonin, were evaluated. Fish synchronized their feeding behavior to the feeding phase, so that in LD they displayed 78% nocturnal feeding activity under night-feeding and 81% diurnal feeding activity under day-feeding, while under LL-feeding they displayed 72% of their daily activity during the 12?h feeding phase. In contrast, locomotor activity was mostly diurnal (66–71%), regardless of the feeding schedule, and it became arrhythmic under LL. Cortisol showed daily rhythms that peaked at different times, depending on the light and feeding schedule: one peak several hours before feeding under day-feeding and night-feeding, and two peaks under LL-feeding. Glucose displayed low-amplitude variations, with no daily rhythms being detected. Melatonin, however, showed a nocturnal rhythm, regardless of the feeding schedule, while the rhythm became attenuated under LL. Taken together, these results highlight the role of feeding on endocrine and metabolic rhythms, suggesting that feeding behavior should be considered when studying these variables. (Author correspondence: jflopez@um.es)     

Time-restricted feeding entrains daily rhythms of energy metabolism in mice

Energy metabolism, oxygen consumption rate (VO2), and respiratory quotient (RQ) in mice were monitored continuously throughout 12:12-h light-dark cycles before, during, and after time-restricted feeding (RF). Mice fed ad libitum showed robust daily rhythms in both parameters: high during the dark phase and low during the light phase. The daily profile of energy metabolism in mice under daytime-only feeding was reversed at the beginning of the first fasting night. A few days after daytime-only feeding began, RF also reversed the circadian core body temperature rhythm. Moreover, RF for 6 consecutive days shifted the phases of circadian expression patterns of clock genes in liver significantly by 8-10 h. When mice were fed a high-fat (HF) diet ad libitum, the daily rhythm of RQ dampened day by day and disappeared on the sixth day of RF, whereas VO2 showed a robust daily rhythm. Mice fed HF only in the daytime had reversed VO2 and RQ rhythms. Similarly, mice fed HF only in the daytime significantly phase shifted the clock gene expression in liver, whereas ad libitum feeding with HF had no significant effect on the expression phases of liver clock genes. These results suggested that VO2 is a sensitive indicator of entrainment in the mouse liver. Moreover, physiologically, it can be determined without any surgery or constraint. On the basis of these results, we hypothesize that a change in the daily VO2 rhythm, independent of the energy source, might drive phase shifts of circadian oscillators in peripheral tissues, at least in the liver.     

Systemic oscillator-driven and nutrient-responsive hormonal regulation of daily expression rhythms for gluconeogenic enzyme genes in the mouse liver

Gluconeogenesis is de novo glucose synthesis from substrates such as amino acids and is vital when glucose is lacking in the diurnal nutritional fluctuation. Accordingly, genes for hepatic gluconeogenic enzymes exhibit daily expression rhythms, whose detailed regulations under nutritional variations remain elusive. As a first step, we performed general systematic characterization of daily expression profiles of gluconeogenic enzyme genes for phosphoenolpyruvate carboxykinase (PEPCK), cytosolic form (Pck1), glucose-6-phosphatase (G6Pase), catalytic subunit (G6pc), and tyrosine aminotransferase (TAT) (Tat) in the mouse liver. On a standard diet fed ad libitum, mRNA levels of these genes showed robust daily rhythms with a peak or an elevation phase during the late sleep-fasting period in the diurnal feeding/fasting (wake/sleep) cycle. The rhythmicity was preserved in constant darkness, modulated with prolonged fasting, attenuated by Clock mutation, and entrained to varied photoperiods and time-restricted feedings. These results are concordant with the notion that gluconeogenic enzyme genes are under the control of the intrinsic circadian oscillator, which is entrained by the light/dark cycle, and which in turn entrains the feeding/fasting cycle and also drives systemic signaling pathways such as the hypothalamic-pituitary-adrenal axis. On the other hand, time-restricted feedings also showed that the ingestion schedule, when separated from the light/dark cycle, can serve as an independent entrainer to daily expression rhythms of gluconeogenic enzyme genes. Moreover, nutritional changes dramatically modified expression profiles of the genes. In addition to prolonged fasting, a high-fat diet and a high-carbohydrate (no-protein) diet caused modification of daily expression rhythms of the genes, with characteristic changes in profiles of glucoregulatory hormones such as corticosterone, glucagon, and insulin, as well as their modulators including ghrelin, leptin, resistin, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1). Remarkably, high-protein (60% casein or soy-protein) diets activated the gluconeogenic enzyme genes atypically during the wake-feeding period, with paradoxical up-regulation of glucagon, which frequently formed correlation networks with other humoral factors. Based on these results, we propose that daily expression rhythms of gluconeogenic enzyme genes are under the control of systemic oscillator-driven and nutrient-responsive hormones.     

Complex interaction between circadian rhythm and diet on bile acid homeostasis in male rats

Desynchronization between the master clock in the brain, which is entrained by (day) light, and peripheral organ clocks, which are mainly entrained by food intake, may have negative effects on energy metabolism. Bile acid metabolism follows a clear day/night rhythm. We investigated whether in rats on a normal chow diet the daily rhythm of plasma bile acids and hepatic expression of bile acid metabolic genes is controlled by the light/dark cycle or the feeding/fasting rhythm. In addition, we investigated the effects of high caloric diets and time-restricted feeding on daily rhythms of plasma bile acids and hepatic genes involved in bile acid synthesis. In experiment 1 male Wistar rats were fed according to three different feeding paradigms: food was available ad libitum for 24 h (ad lib) or time-restricted for 10 h during the dark period (dark fed) or 10 h during the light period (light fed). To allow further metabolic phenotyping, we manipulated dietary macronutrient intake by providing rats with a chow diet, a free choice high-fat-high-sugar diet or a free choice high-fat (HF) diet. In experiment 2 rats were fed a normal chow diet, but food was either available in a 6-meals-a-day (6M) scheme or ad lib. During both experiments, we measured plasma bile acid levels and hepatic mRNA expression of genes involved in bile acid metabolism at eight different time points during 24 h. Time-restricted feeding enhanced the daily rhythm in plasma bile acid concentrations. Plasma bile acid concentrations are highest during fasting and dropped during the period of food intake with all diets. An HF-containing diet changed bile acid pool composition, but not the daily rhythmicity of plasma bile acid levels. Daily rhythms of hepatic Cyp7a1 and Cyp8b1 mRNA expression followed the hepatic molecular clock, whereas for Shp expression food intake was leading. Combining an HF diet with feeding in the light/inactive period annulled CYp7a1 and Cyp8b1 gene expression rhythms, whilst keeping that of Shp intact. In conclusion, plasma bile acids and key genes in bile acid biosynthesis are entrained by food intake as well as the hepatic molecular clock. Eating during the inactivity period induced changes in the plasma bile acid pool composition similar to those induced by HF feeding.     

Photic entrainment of daily rhythm pattern of locomotor activity in sea bass (Dicentrarcus labrax)

The aim of this study was to evaluate the circadian rhythm of daily locomotor activity in sea bass exposed to three different artificial lighting regimes including 12/12, 24/0, and 0/24 L/D periods, and scheduled daytime feeding. Daily rhythm of locomotor activity during 12/12 L/D was observed, whereas locomotor activity recorded during 24/0 and 0/24 L/D resulted arrhythmic. Statistical differences in mesor values among the photoperiods and between light and dark phases of each photoperiod were found. During the 12/12 and 0/24 L/D, the fish were active mostly during the light phase. During 24/0 L/D, a phase inversion occurred. The results showed that photoperiod is a dominant synchronizer in sea bass, and that the appetite in this fish follows daily rhythms which does not match the imposed feeding schedule, suggesting the absence or the lability of internal process that couples feeding cycles and biological rhythms.     

Daily and annual variations of free fatty acid, glycerol and leptin plasma concentrations in goats (Capra hircus) under different photoperiods

The aim of the study was to differentiate the impact of lighting conditions and feeding times on the regulation of lipid metabolism of goats under different photoperiods throughout the year. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (the longest light period 18 h, the shortest 6 h). Ambient temperature and feeding regime were kept constant. Blood samples were collected six times a year at 2-h intervals for 2 days, first in light/dark (LD) conditions and then after 3 days in constant darkness (DD). Significant daily variations were detected in the concentrations of plasma free fatty acids (FFA) and glycerol throughout the year. The nocturnal decrease and morning rise of FFA levels were related to the photoperiod, while the trough levels of glycerol were associated with the concentrate meal times. In DD conditions, FFA and glycerol rhythms were unstable. A significant seasonal variation was detected in the overall FFA and glycerol levels suggesting decreased lipogenesis in winter, increased lipolysis in spring and high lipogenesis in summer and fall. There was no significant daily rhythm in serum leptin levels, nor did the profiles in LD and DD conditions differ. The leptin level was slightly lower in early fall than in the other seasons, paralleling a small decrease of body mass in the goats after the grazing season. The daily or annual variations of FFA and glycerol levels were not clearly related to leptin concentrations. The results suggest that lipid metabolism of goats is regulated by light even in constant temperature and feeding conditions; however, no significant contribution of leptin levels could be shown.     

Metabolic cycles are linked to the cardiovascular diurnal rhythm in rats with essential hypertension

Background

The loss of diurnal rhythm in blood pressure (BP) is an important predictor of end-organ damage in hypertensive and diabetic patients. Recent evidence has suggested that two major physiological circadian rhythms, the metabolic and cardiovascular rhythms, are subject to regulation by overlapping molecular pathways, indicating that dysregulation of metabolic cycles could desynchronize the normal diurnal rhythm of BP with the daily light/dark cycle. However, little is known about the impact of changes in metabolic cycles on BP diurnal rhythm.

Methodology/Principal Findings

To test the hypothesis that feeding-fasting cycles could affect the diurnal pattern of BP, we used spontaneously hypertensive rats (SHR) which develop essential hypertension with disrupted diurnal BP rhythms and examined whether abnormal BP rhythms in SHR were caused by alteration in the daily feeding rhythm. We found that SHR exhibit attenuated feeding rhythm which accompanies disrupted rhythms in metabolic gene expression not only in metabolic tissues but also in cardiovascular tissues. More importantly, the correction of abnormal feeding rhythms in SHR restored the daily BP rhythm and was accompanied by changes in the timing of expression of key circadian and metabolic genes in cardiovascular tissues.

Conclusions/Significance

These results indicate that the metabolic cycle is an important determinant of the cardiovascular diurnal rhythm and that disrupted BP rhythms in hypertensive patients can be normalized by manipulating feeding cycles.     

Effect of Lithium on the Rhythms of Melatonin in the Pineal Gland, Serum and Retina of Viscacha (Lagostomus maximus maximus)

The aim of this work was to describe the pineal, serum and retinal melatonin daily rhythms in viscacha (Lagostomus maximus maximus), a nocturnal and subterranean rodent. A daily rhythm with a nocturnal peak and an increase during the light phase was found in all cases, with the seric rhythm being delayed with respect to the pineal one. Chronic lithium administration to viscachas suppressed the diurnal increase of pineal melatonin, abolished the retinal rhythm and had no effect on seric melatonin.     

Circadian rhythms of melatonin in European starlings exposed to different lighting conditions: relationship with locomotor and feeding rhythms

In passerine birds, the periodic secretion of melatonin by the pineal organ represents an important component of the pacemaker that controls overt circadian functions. The daily phase of low melatonin secretion generally coincides with the phase of intense activity, but the precise relationship between the melatonin and the behavioral rhythms has not been studied. Therefore, we investigated in European starlings (Sturnus vulgaris) (1) the temporal relationship between the circadian plasma melatonin rhythm and the rhythms in locomotor activity and feeding; (2) the persistence of the melatonin rhythm in constant conditions; and (3) the effects of light intensity on synchronized and free-running melatonin and behavioral rhythms. There was a marked rhythm in plasma melatonin with high levels at night and/or the inactive phase of the behavioral cycles in almost all birds. Like the behavioral rhythms, the melatonin rhythm persisted for at least 50 days in constant dim light. In the synchronized state, higher daytime light intensity resulted in more tightly synchronized rhythms and a delayed melatonin peak. While all three rhythms usually assumed a rather constant phase relationship to each other, in one bird the two behavioral rhythms dissociated from each other. In this case, the melatonin rhythm retained the appropriate phase relationship with the feeding rhythm. Accepted: 10 December 1999     

Different circadian pacemakers control feeding and locomotor activity rhythms in European starlings

Summary In higher organisms, many physiological and behavioral functions exhibit daily variations, generated by endogenous circadian oscillators. It is not yet clear whether all the various rhythms that occur within an individual depend on one and the same pacemaker or whether different pacemakers are involved. To examine this question, the feeding and perch-hopping rhythms were measured in European starlings (Sturnus vulgaris) under light-dark cycles and continuous dim light. In dim light, the internal phase relationship between the feeding and perch-hopping rhythms changed systematically as a function of the circadian period, and the two rhythms could even dissociate and show different circadian periods in individuals with extremely long or extremely short circadian periods. Moreover, in some birds kept on lowamplitude light-dark cycles, the rhythm of feeding was synchronized 180° out of phase with the rhythm of locomotor activity. These results strongly suggest that in the European starling the feeding and locomotor activity rhythms are controlled by separate circadian pacemakers.     

FEEDING ENTRAINMENT OF DAILY RHYTHMS OF LOCOMOTOR ACTIVITY AND CLOCK GENE EXPRESSION IN ZEBRAFISH BRAIN

Light and feeding cycles strongly synchronize daily rhythms in animals, which may, as a consequence, develop food anticipatory activity (FAA). However, the light/food entraining mechanisms of the central circadian oscillator remain unknown. In this study, we investigate the existence of FAA in seven groups of zebrafish subjected to a light/dark (LD) cycle or constant light (LL) and different feeding regimes (random, fasting, and feeding in the middle of the light phase or dark phase). The aim was to ascertain whether the daily rhythm of behavior and clock gene (per1 and cry1) expression in the zebrafish brain was entrained by the light and feeding regime. The results revealed that FAA developed in zebrafish fed daily at a fixed time, under LD and under LL. Zebrafish displayed locomotor activity mostly during the daytime, although the percentage of activity during the light phase varied depending on feeding time (ranging from 93.2% to 63.1% in the mid-light and mid-dark fed groups, respectively). However, the different feeding regimes failed to modify the daily rhythm of per1 and cry1 expression in the zebrafish brain under LD (approximate acrophases [peak times] at ZT22 and ZT4, respectively; lights-on =?ZT0). Under LL, per1 and cry1 expression did not show significant daily rhythmicity, regardless of the feeding regime. These findings indicate that, although schedule-fed zebrafish developed FAA as regards locomotor activity, feeding had little effect on clock gene expression in whole brain homogenates, suggesting the feeding-entrainable oscillator may be located elsewhere or at specific brain sites. (Author correspondence: jasanchez@um.es)     

Impact of Chronodisruption during Primate Pregnancy on the Maternal and Newborn Temperature Rhythms

Disruption of the maternal environment during pregnancy is a key contributor to offspring diseases that develop in adult life. To explore the impact of chronodisruption during pregnancy in primates, we exposed pregnant capuchin monkeys to constant light (eliminating the maternal melatonin rhythm) from the last third of gestation to term. Maternal temperature and activity circadian rhythms were assessed as well as the newborn temperature rhythm. Additionally we studied the effect of daily maternal melatonin replacement during pregnancy on these rhythms. Ten pregnant capuchin monkeys were exposed to constant light from 60% of gestation to term. Five received a daily oral dose of melatonin (250 µg kg/body weight) at 1800 h (LL+Mel) and the other five a placebo (LL). Six additional pregnant females were maintained in a 14∶10 light:dark cycles and their newborns were used as controls (LD). Rhythms were recorded 96 h before delivery in the mother and at 4–6 days of age in the newborn. Exposure to constant light had no effect on the maternal body temperature rhythm however it delayed the acrophase of the activity rhythm. Neither rhythm was affected by melatonin replacement. In contrast, maternal exposure to constant light affected the newborn body temperature rhythm. This rhythm was entrained in control newborns whereas LL newborns showed a random distribution of the acrophases over 24-h. In addition, mean temperature was decreased (34.0±0.6 vs 36.1±0.2°C, in LL and control, respectively P<0.05). Maternal melatonin replacement during pregnancy re-synchronized the acrophases and restored mean temperature to the values in control newborns. Our findings demonstrate that prenatal melatonin is a Zeitgeber for the newborn temperature rhythm and supports normal body temperature maintenance. Altogether these prenatal melatonin effects highlight the physiological importance of the maternal melatonin rhythm during pregnancy for the newborn primate.     

Circadian Intraocular Pressure Rhythms in Athletic Horses under Different Lighting Regime

The present study was undertaken to investigate the existence of intraocular pressure (IOP) rhythms in athletic thoroughbred horses maintained under a 24 h cycle of light and darkness (LD) or under constant light (LL) or constant dark (DD) conditions. We identified an IOP circadian rhythm that is entrained to the 24 h LD cycle. IOP was low during the dark phase and high during the light phase, with a peak at the end of the light phase (ZT10). The circadian rhythm of IOP persisted in DD (with a peak at CT9.5), demonstrating an endogenous component in IOP rhythm. As previously shown in other mammalian species, horse IOP circadian rhythmicity was abolished in LL. Because tonometry is performed in horses for the diagnosis of ophthalmologic diseases, such as glaucoma or anterior uveitis, the daily variation in IOP must be taken into account in clinical practice to properly time tests and to interpret clinical findings.