Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

Circadian phase inversion causes insulin resistance in a rat model of night work and jet lag

Shift workers have an increased risk to develop type 2 diabetes. We aimed to investigate the underlying mechanisms and the role of the timing of food intake by subjecting rats to an acute phase inversion of the light/dark (L/D) cycle. In the first experiment, with food available ad libitum, male Wistar rats were implanted with jugular vein catheters and intravenous glucose tolerance tests were performed at either ZT2 or ZT14. Three days after the 12 h phase shift, these glucose tolerance tests were repeated. In the second experiment, rats were housed in metabolic cages for the continuous measurement of multiple behavioral and metabolic parameters after the 12 h phase shift, food was available ad libitum or restricted to the light or dark period. The daily rhythm of glucose tolerance, and the peak and trough corticosterone levels, adapted within three days after exposure to the inverted L/D cycle. However, phase inversion caused insulin resistance at the onset of the active phase. Under ad libitum feeding conditions, the daily rhythms of locomotor activity and energy expenditure adapted faster to the inverted L/D cycle compared to the other behavioral rhythms measured. Food restriction to the dark period facilitated behavioral adaptation to the new circadian phase.

Chronic circadian advance shifts abolish melatonin secretion for days in rats

Melatonin deficiency has been proposed to underlie higher risks for cardiovascular and several other diseases in humans experiencing prolonged shiftwork. However, melatonin secretion has not been monitored longitudinally during consecutive shifts of the light:dark (LD) cycles in the same individuals (animals or humans) and the extent of melatonin deficiency is unknown in individuals experiencing consecutive LD shifts. We investigated the effect of consecutive LD shifts on melatonin secretion in adult F344 rats using continuous online pineal-microdialysis. The rats were entrained to the 12 h:12 h LD cycle before the shifts. The LD cycle was then advanced (n=5) or delayed (n=4) for six hours every four days for four consecutive times. The rats exhibited marked asymmetry in response to delay or advance LD shifts. While rats exposed to the repeated LD delay shifts always exhibited melatonin secretion throughout the entire periods, repeated LD advance shifts suppressed nocturnal melatonin secretion for several consecutive days in the middle of the 3-week period. Moreover, melatonin offset after LD delay and melatonin onset after LD advance determined the rate of circadian pacemaker reentrainment. Additionally, melatonin offset was phase locked at the new dark/light junctions for days following LD advance. These data demonstrate that chronic LD shifts are deleterious to melatonin rhythms, and that this effect is much more pronounced during advance shifts. These data may enhance our understanding of impact of LD shifts on our circadian timing system and benefit better design of shiftwork schedules to avoid melatonin disruption.

The search for preoperative biomarkers for thyroid carcinoma: application of the thyroid circadian clock properties

Accumulating evidence suggests that alterations in the molecular clocks underlying the circadian time-keeping system might be connected to changes in cell cycle, resulting in oncogenic transformation. The hypothalamic–pituitary–thyroid axis is driven by a circadian clock at several levels, with an endocrine feedback loop regulating thyroid-stimulating hormone. Changes in the expression levels of circadian and cell cycle markers may correlate with clinic-pathological characteristics in differentiated follicular thyroid carcinomas. Here we summarize recent advances in exploring complex regulation of the thyroid gland transcriptome and function by the circadian oscillator. We particularly focus on clinical implications of the parallel assessment of the circadian clock, cell-cycle and cell functionality markers in human thyroid tissue, which might help improving preoperative diagnostics of thyroid malignancies.

Neural activity in the suprachiasmatic circadian clock of nocturnal mice anticipating a daytime meal

Circadian rhythms in mammals are regulated by a system of circadian oscillators that includes a light-entrainable pacemaker in the suprachiasmatic nucleus (SCN) and food-entrainable oscillators (FEOs) elsewhere in the brain and body. In nocturnal rodents, the SCN promotes sleep in the day and wake at night, while FEOs promote an active state in anticipation of a predictable daily meal. For nocturnal animals to anticipate a daytime meal, wake-promoting signals from FEOs must compete with sleep-promoting signals from the SCN pacemaker. One hypothesis is that FEOs impose a daily rhythm of inhibition on SCN output that is timed to permit the expression of activity prior to a daytime meal. This hypothesis predicts that SCN activity should decrease prior to the onset of anticipatory activity and remain suppressed through the scheduled mealtime. To assess the hypothesis, neural activity in the SCN of mice anticipating a 4-5-h daily meal in the light period was measured using FOS immunohistochemistry and in vivo multiple unit electrophysiology. SCN FOS, quantified by optical density, was significantly reduced at the expected mealtime in food-anticipating mice with access to a running disk, compared to ad libitum-fed and acutely fasted controls. Group differences were not significant when FOS was quantified by other methods, or in mice without running disks. SCN electrical activity was markedly decreased during locomotion in some mice but increased in others. Changes in either direction were concurrent with locomotion, were not specific to food anticipation, and were not sustained during longer pauses. Reduced FOS indicates a net suppression of SCN activity that may depend on the intensity or duration of locomotion. The timing of changes in SCN activity relative to locomotion suggests that any effect of FEOs on SCN output is mediated indirectly, by feedback from neural or systemic correlates of locomotion.

Thyroid circadian timing: roles in physiology and thyroid malignancies

The circadian clock represents an anticipatory mechanism, well preserved in evolution. It has a critical impact on most aspects of the physiology of light-sensitive organisms. These rhythmic processes are governed by environmental cues (fluctuations in light intensity and temperature), an internal circadian timing system, and interactions between this timekeeping system and environmental signals. Endocrine body rhythms, including hypothalamic-pituitary-thyroid (HPT) axis rhythms, are tightly regulated by the circadian system. Although the circadian profiles of thyroid-releasing hormone (TRH), thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) in blood have been well described, relatively few studies have analyzed molecular mechanisms governing the circadian regulation of HPT axis function. In this review, we will discuss the latest findings in the area of complex regulation of thyroid gland function by the circadian oscillator. We will also highlight the molecular makeup of the human thyroid oscillator as well as the potential link between thyroid malignant transformation and alterations in the clockwork.

Nighttime snacking reduces whole body fat oxidation and increases LDL cholesterol in healthy young women

The increase in obesity and lipid disorders in industrialized countries may be due to irregular eating patterns. Few studies have investigated the effects of nighttime snacking on energy metabolism. We examined the effects of nighttime snacking for 13 days on energy metabolism. Eleven healthy women (means ± SD; age: 23 ± 1 yr; body mass index: 20.6 ± 2.6 kg/m2) participated in this randomized crossover trial for a 13-day intervention period. Subjects consumed a specified snack (192.4 ± 18.3 kcal) either during the daytime (10:00) or the night time (23:00) for 13 days. On day 14, energy metabolism was measured in a respiratory chamber without snack consumption. An oral glucose tolerance test was performed on day 15. Relative to daytime snacking, nighttime snacking significantly decreased fat oxidation (daytime snacking: 52.0 ± 13.6 g/day; nighttime snacking: 45.8 ± 14.0 g/day; P = 0.02) and tended to increase the respiratory quotient (daytime snacking: 0.878 ± 0.022; nighttime snacking: 0.888 ± 0.021; P = 0.09). The frequency of snack intake and energy intake, body weight, and energy expenditure were not affected. Total and low-density lipoprotein (LDL) cholesterol significantly increased after nighttime snacking (152 ± 26 mg/dl and 161 ± 29 mg/dl; P = 0.03 and 76 ± 20 mg/dl and 83 ± 24 mg/dl; P = 0.01, respectively), but glucose and insulin levels after the glucose load were not affected. Nighttime snacking increased total and LDL cholesterol and reduced fat oxidation, suggesting that eating at night changes fat metabolism and increases the risk of obesity.

Daily regulation of hormone profiles

The highly coordinated output of the hypothalamic biological clock does not only govern the daily rhythm in sleep/wake (or feeding/fasting) behaviour but also has direct control over many aspects of hormone release. In fact, a significant proportion of our current understanding of the circadian clock has its roots in the study of the intimate connections between the hypothalamic clock and multiple endocrine axes. This chapter will focus on the anatomical connections used by the mammalian biological clock to enforce its endogenous rhythmicity on the rest of the body, using a number of different hormone systems as a representative example. Experimental studies have revealed a highly specialised organisation of the connections between the mammalian circadian clock neurons and neuroendocrine as well as pre-autonomic neurons in the hypothalamus. These complex connections ensure a logical coordination between behavioural, endocrine and metabolic functions that will help the organism adjust to the time of day most efficiently. For example, activation of the orexin system by the hypothalamic biological clock at the start of the active phase not only ensures that we wake up on time but also that our glucose metabolism and cardiovascular system are prepared for this increased activity. Nevertheless, it is very likely that the circadian clock present within the endocrine glands plays a significant role as well, for instance, by altering these glands' sensitivity to specific stimuli throughout the day. In this way the net result of the activity of the hypothalamic and peripheral clocks ensures an optimal endocrine adaptation of the metabolism of the organism to its time-structured environment.

Behavioral responses to combinations of timed light, food availability, and ultradian rhythms in the common vole (Microtus arvalis)

Light is the main entraining signal of the central circadian clock, which drives circadian organization of activity. When food is made available during only certain parts of the day, it can entrain the clock in the liver without changing the phase of the central circadian clock. Although a hallmark of food entrainment is a behavioral anticipation of food availability, the extent of behavioral alterations in response to food availability has not been fully characterized. The authors have investigated interactions between light and temporal food availability in the timing of activity in the common vole. Temporally restricted food availability enhanced or attenuated re-entrainment to a phase advance in light entrainment when it was shifted together with the light or remained at the same time of day, respectively. When light-entrained behavior was challenged with temporal food availability cycles with a different period, two distinct activity components were observed. More so, the present data indicate that in the presence of cycles of different period length of food and light, an activity component emerged that appeared to be driven by a free-running (light-entrainable) clock. Because the authors have previously shown that in the common vole altering activity through running-wheel availability can alter the effectiveness of food availability to entrain the clock in the liver, the authors included running-wheel availability as a parameter that alters the circadian/ultradian balance in activity. In the current protocols, running-wheel availability enhanced the entraining potential of both light and food availability in a differential way. The data presented here show that in the vole activity is a complex of individually driven components and that this activity is, itself, an important modulator of the effectiveness of entraining signals such as light and food.

Circadian disruption and SCN control of energy metabolism

In this review we first present the anatomical pathways used by the suprachiasmatic nuclei to enforce its rhythmicity onto the body, especially its energy homeostatic system. The experimental data show that by activating the orexin system at the start of the active phase, the biological clock not only ensures that we wake up on time, but also that our glucose metabolism and cardiovascular system are prepared for increased activity. The drawback of such a highly integrated system, however, becomes visible when our daily lives are not fully synchronized with the environment. Thus, in addition to increased physical activity and decreased intake of high-energy food, also a well-lighted and fully resonating biological clock may help to withstand the increasing "diabetogenic" pressure of today's 24/7 society.

Interactions between light, mealtime and calorie restriction to control daily timing in mammals

Daily variations in behaviour and physiology are controlled by a circadian timing system consisting of a network of oscillatory structures. In mammals, a master clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, adjusts timing of other self-sustained oscillators in the brain and peripheral organs. Synchronisation to external cues is mainly achieved by ambient light, which resets the SCN clock. Other environmental factors, in particular food availability and time of feeding, also influence internal timing. Timed feeding can reset the phase of the peripheral oscillators whilst having almost no effect in shifting the phase of the SCN clockwork when animals are exposed (synchronised) to a light-dark cycle. Food deprivation and calorie restriction lead not only to loss of body mass (>15%) and increased motor activity, but also affect the timing of daily activity, nocturnal animals becoming partially diurnal (i.e. they are active during their usual sleep period). This change in behavioural timing is due in part to the fact that metabolic cues associated with calorie restriction affect the SCN clock and its synchronisation to light.

Circadian and photic regulation of clock and clock-controlled proteins in the suprachiasmatic nuclei of calorie-restricted mice

In mammals, behavioural and physiological rhythms as well as clock gene expression in the central suprachiasmatic clock (SCN) are phase-shifted by a timed calorie restriction (T-CR; animals receiving at midday 66% of their daily food intake). The molecular mechanism of SCN depends on feedback loops involving clock genes and their protein products. To understand how T-CR mediates its synchronizing effects, we examined the rhythmic expression of three clock proteins, PERIOD (PER) 1, 2 and CLOCK, and one clock-controlled protein (i.e. vasopressin; AVP) in the SCN of mice either fed ad libitum (AL) or with T-CR. Moreover, we evaluated expression of these proteins in the SCN of AL and T-CR mice following a 1-h light pulse. The results indicate that, while PER1 and AVP rhythms were phase-advanced in T-CR mice, the PER2 rhythm showed an increased amplitude. CLOCK was expressed constitutively in AL mice while in T-CR it was significantly reduced, especially after feeding time. A light pulse produced a delayed increase in PER1 and a larger increase in PER2 expression in the SCN of T-CR mice than in AL animals. In addition, light exposure triggered an increase in AVP-ir cells in both AL and T-CR mice, and also of CLOCK expression but in T-CR mice only. The circadian changes in clock and clock-controlled proteins and their acute responses to light in the SCN of T-CR mice demonstrate that metabolic cues induced by a calorie restriction modulate the translational regulation of the SCN clock.

Combined effects of high-fat feeding and circadian desynchronization

OBJECTIVE

To assess whether circadian desynchronization leads to metabolic alterations capable of promoting dietary obesity and/or impairing glucose tolerance.

DESIGN

Rats fed either with chow pellets (i.e., low-fat diet with 4% mass of fat) or high-fat diet (34% mass of fat). Half of each diet group was exposed to a fixed light-dark cycle or to a 10-h weekly shift in the light-dark cycle from Thursday to Sunday (20 shifts). To enforce the shifted animals to be active at unusual times of the day, food was available only during the daily dark period for all groups.

RESULTS

Shifting the light-dark cycle on a weekly basis was efficient to induce circadian desynchronization, as evidenced by strong disturbances in the daily expression of locomotor activity. Shifted rats fed with a nocturnal low-fat diet had lower plasma insulin and similar blood glucose compared to rats fed with the same diet under a fixed light-dark cycle. Nocturnal high-fat feeding led to an abdominal fat overload associated with increased plasma leptin and basal glucose. These metabolic changes were not significantly modified by circadian desynchronization.

CONCLUSION

Chronic desynchronization with low-fat diet impaired insulin regulation. Metabolic changes induced by the high-fat diet were not aggravated by chronic desynchronization.

Odor-specific effects on reentrainment following phase advances in the diurnal rodent, Octodon degus

Reentrainment following phase shifts of the light-dark (LD) cycle is accelerated in Octodon degus in the presence of olfactory social cues (i.e., odors) produced by conspecifics. However, not all odors from conspecifics were effective in facilitating reentrainment after a phase advance. In the current experiments, we examined whether nonanimal odors, odors from another species, or conspecific odors, including those manipulated by steroid hormones, can cause the same increased reentrainment of wheel-running activity as odors from an intact, adult female degu. A variety of odors, each selected to probe a particular aspect of the reentrainment acceleration phenomenon, were presented to a group of phase-shifting female degus. The shifting females (test animals) responded to odors of intact, female degu donors with decreased reentrainment time, but odors of ovariectomized (OVX), OVX with a single hormone replacement capsule (estradiol or progesterone) or phase-shifting females had no effect. Multiple males were effective odor donors, whereas a single male was ineffective in earlier studies. Rats and cloves were not effective in accelerating reentrainment. Furthermore, odors from rats delayed reentrainment. We conclude that the odors that effectively accelerate degu reentrainment after a phase advance of the LD cycle are species specific. We also report that repeated phase shifts, followed by complete recovery of phase relationships, do not alter the rate of recovery from a phase shift over time. These data suggest that in O. degus, a social species, odors may reinforce and strengthen the salience of the photic zeitgeber and/or facilitate synchronization of rhythms between animals.

Entrainment by a palatable meal induces food-anticipatory activity and c-Fos expression in reward-related areas of the brain

Rats maintained under restricted feeding schedules (RFS) develop food-anticipatory activity and entrainment of physiological parameters. Food entrainment is independent of the suprachiasmatic nucleus and depends on food-entrainable oscillators (FEO). Restricted feeding schedules lead animals toward a catabolic state and to increase their food driven motivation, suggesting that in this process metabolic- and reward-related mechanisms are implicated. This study explored if motivation driven by a palatable meal is sufficient to produce food-entrainment. To address this question, we evaluated whether daily fixed access to a highly palatable meal entrained (PME) locomotor activity, serum glucose and free fatty acids concentrations in rats maintained without food deprivation. The entrained response of PME rats was compared with rats entrained to RFS. In a second experiment, we used c-Fos-IR to identify structures in the central nervous system involved with PME. Rats showed anticipatory activity to a daily palatable meal, with a lower intensity than rats entrained to RFS. Anticipatory activity persisted at least for four cycles after interrupting palatable meal, suggesting that this persistence depends on an endogenous oscillator. Glucose and free fatty acids were not entrained in PME rats. c-Fos expression in limbic system nuclei was in phase with PME time, but not in the hypothalamus. Results suggest 1) that food deprivation, i.e. a catabolic state is not necessary for the expression of anticipatory activity; 2) that an increase in the motivational state due to taste and/or nutritional contents of palatable meal is sufficient to entrain behavior; and 3) that structures in the limbic system are involved in this entrainment process. The present study indicates that metabolic and motivational mechanisms are involved in food entrainment, and suggests that the FEO may be a multi-oscillatory system distributed over different regulatory systems in the brain.

Timed hypocaloric food restriction alters the synthesis and expression of vasopressin and vasoactive intestinal peptide in the suprachiasmatic nucleus

In mammals, the main circadian pacemaker is located in the suprachiasmatic nucleus (SCN) and its most potent synchronizer is the daily variation of the intensity of light. However, other nonphotic cues, such as timed food restriction, can induce changes in the circadian rhythms, leading also to the appearance of a food-entrained oscillator. The present study was designed to establish if the alterations of the circadian rhythms induced by timed hypocaloric food restriction are accompanied by structural changes in the SCN. Two groups of adult rats, both maintained on 12-h light/12-h dark cycles, were used; in one group, animals had permanent free access to food, whereas in the other they were subjected to a restricted hypocaloric early morning feeding during 7 months. Using stereological techniques and in situ hybridization, we have examined the structure of the SCN and the synthesis and expression of vasopressin (AVP) and vasoactive intestinal peptide (VIP). The volume of the SCN and the total number of neurons did not vary between the two groups. However, the total number of AVP- and VIP-immunoreactive neurons and the AVP and VIP mRNA levels were significantly decreased in timed hypocaloric food-restricted animals. The results indicate that timed hypocaloric food restriction has led to changes of AVP and VIP content of the neurons. They furthermore suggest the existence of a coupling between the food-entrainable oscillator and the light-entrainable pacemaker.

Bimodal circadian secretion of melatonin from the pineal gland in a living CBA mouse

Circadian melatonin secretion is the best-known output signal from the circadian pacemaker in the suprachiasmatic nucleus that indicates internal conditions of the body. We have established a system that enables long-term monitoring of melatonin secretion by implanting a transverse microdialysis probe in or near the pineal gland in a freely moving mouse. This in vivo method enabled continuous measurement of melatonin secretion over a period of >20 days in individual CBA mice, with simultaneous recording of the locomotor activity. Pineal melatonin secretion was completely matched to the circadian change of locomotor activity, and for the light-induced phase shift, the shift of melatonin secretion was clearer than the shift of locomotor rhythm. This analysis allowed us to detect rhythm with a high sensitivity: two peaks of daily secretion were observed, with the first small peak at the day-night transition time and the second large peak at midnight. The large nighttime peak was suppressed by tetrodotoxin, a Na+ channel blocker, and enhanced by both phenylephrine and isoproterenol, alpha- and beta-adrenergic agonists, whereas daytime melatonin levels were not affected by tetrodotoxin infusion. This finding suggests that, in CBA mice, melatonin release at night is activated by adrenergic signaling from the superior cervical ganglion, but the enhancement of melatonin during daytime is not mediated by neuronal signaling.

Brain microdialysis in exercise research

During the last 5 to 10 years, the microdialysis technique has been used to explore neurotransmitter release during exercise. Microdialysis can collect virtually any substance from the brains of freely moving animals with a limited amount of tissue trauma. It allows the measurement of local neurotransmitter release in combination with ongoing behavioural changes such as exercise. Several groups examined the effect of treadmill running on extracellular neurotransmitter levels. Microdialysis probes were implanted in different brain areas to monitor diverse aspects of locomotion (striatum, hippocampus, nucleus accumbens, frontal cortex, spinal cord), food reward (hypothalamus, hippocampus, cerebral cortex), thermoregulation (hypothalamus). Some studies combined microdialysis with running on a treadmill to evaluate motor deficit and improvement following dopaminergic grafts in 6-hydroxydopamine lesioned rats, or combined proton nuclear magnetic resonance spectroscopy and cortical microdialysis to observe intra- plus extracellular brain glucose variations. This method allows us to understand neurotransmitter systems underlying normal physiological function and behaviour. Because of the growing interest in exercise and brain functioning, it should be possible to investigate increasingly subtle behavioural and physiological changes within the central nervous system. There is now compelling evidence that regular physical activity is associated with significant physiological, psychological and social benefits in the general population. In contrast with our knowledge about the peripheral adaptations to exercise, studies relating exercise to brain neurotransmitter levels are scarce. It is of interest to examine the effect of short and long term exercise on neurotransmitter release, since movement initiation and control of locomotion have been shown to be related to striatal neurotransmitter function, and one of the possible therapeutic modalities in movement, and mental disorders is exercise therapy. Until very recently most experimental studies on brain chemistry were conducted with postmortem tissue. However, in part because of shortcomings with postmortem methods, and in part because of the desire to be able to directly relate neurochemistry to behaviour, there has been considerable interest in the development of 'in vivo' neurochemical methods. Because total tissue levels may easily mask small but important neurochemical changes related to activity, it is important to sample directly in the extracellular compartment of nervous tissue in living animals. Since the chemical interplay between cells occurs in the extracellular fluid, there was a need to access this compartment in the intact brain of living and freely moving animals. Estimation of the transmitter content in this compartment is believed to be directly related to the concentration at the site where these compounds are functionally released: in the synaptic cleft. As measurements in the synapse are not yet possible, in vivo measurements in the extracellular fluid appear to provide the most directly relevant information currently available. This article provides an overview of the in vivo microdialysis technique as a method for measuring in the extracellular space, and its application in exercise science. Although this technique has been used in different tissues such as brain, adipose tissue, spinal cord and muscle, in animals as well as humans, we will focus on the use of this in vivo method in brain tissue. Recently two excellent reviews on the application of microdialysis in human experiments especially in subcutaneous tissue have been published, and we refer the interested reader to these articles.

Melatonin sees the light: blocking GABA-ergic transmission in the paraventricular nucleus induces daytime secretion of melatonin

Despite a pronounced inhibitory effect of light on pineal melatonin synthesis, usually the daily melatonin rhythm is not a passive response to the surrounding world. In mammals, and almost every other vertebrate species studied so far, the melatonin rhythm is coupled to an endogenous pacemaker, i.e. a circadian clock. In mammals the principal circadian pacemaker is located in the suprachiasmatic nuclei (SCN), a bilateral cluster of neurons in the anterior hypothalamus. In the present paper we show in the rat that bilateral abolition of gamma-aminobutyric acid (GABA), but not vasopressin, neurotransmission in an SCN target area, i.e. the paraventricular nucleus of the hypothalamus, during (subjective) daytime results in increased pineal melatonin levels. The fact that complete removal of the SCN results in a pronounced increase of daytime pineal mRNA levels for arylalkylamine N-acetyltransferase (AA-NAT), i.e. the rate-limiting enzyme of melatonin synthesis, further substantiates the existence of a major inhibitory SCN output controlling the circadian melatonin rhythm.