Food-entrainable circadian oscillators in the brain

Leptin-sensitive neurons in the arcuate nucleus integrate activity and temperature circadian rhythms and anticipatory responses to food restriction

Previously, we investigated the role of neuropeptide Y and leptin-sensitive networks in the mediobasal hypothalamus in sleep and feeding and found profound homeostatic and circadian deficits with an intact suprachiasmatic nucleus. We propose that the arcuate nuclei (Arc) are required for the integration of homeostatic circadian systems, including temperature and activity. We tested this hypothesis using saporin toxin conjugated to leptin (Lep-SAP) injected into Arc in rats. Lep-SAP rats became obese and hyperphagic and progressed through a dynamic phase to a static phase of growth. Circadian rhythms were examined over 49 days during the static phase. Rats were maintained on a 12:12-h light-dark (LD) schedule for 13 days and, thereafter, maintained in continuous dark (DD). After the first 13 days of DD, food was restricted to 4 h/day for 10 days. We found that the activity of Lep-SAP rats was arrhythmic in DD, but that food anticipatory activity was, nevertheless, entrainable to the restricted feeding schedule, and the entrained rhythm persisted during the subsequent 3-day fast in DD. Thus, for activity, the circuitry for the light-entrainable oscillator, but not for the food-entrainable oscillator, was disabled by the Arc lesion. In contrast, temperature remained rhythmic in DD in the Lep-SAP rats and did not entrain to restricted feeding. We conclude that the leptin-sensitive network that includes the Arc is required for entrainment of activity by photic cues and entrainment of temperature by food, but is not required for entrainment of activity by food or temperature by photic cues.

Recording and analysis of circadian rhythms in running-wheel activity in rodents

When rodents have free access to a running wheel in their home cage, voluntary use of this wheel will depend on the time of day. Nocturnal rodents, including rats, hamsters, and mice, are active during the night and relatively inactive during the day. Many other behavioral and physiological measures also exhibit daily rhythms, but in rodents, running-wheel activity serves as a particularly reliable and convenient measure of the output of the master circadian clock, the suprachiasmatic nucleus (SCN) of the hypothalamus. In general, through a process called entrainment, the daily pattern of running-wheel activity will naturally align with the environmental light-dark cycle (LD cycle; e.g. 12 hr-light:12 hr-dark). However circadian rhythms are endogenously generated patterns in behavior that exhibit a ~24 hr period, and persist in constant darkness. Thus, in the absence of an LD cycle, the recording and analysis of running-wheel activity can be used to determine the subjective time-of-day. Because these rhythms are directed by the circadian clock the subjective time-of-day is referred to as the circadian time (CT). In contrast, when an LD cycle is present, the time-of-day that is determined by the environmental LD cycle is called the zeitgeber time (ZT). Although circadian rhythms in running-wheel activity are typically linked to the SCN clock, circadian oscillators in many other regions of the brain and body could also be involved in the regulation of daily activity rhythms. For instance, daily rhythms in food-anticipatory activity do not require the SCN and instead, are correlated with changes in the activity of extra-SCN oscillators. Thus, running-wheel activity recordings can provide important behavioral information not only about the output of the master SCN clock, but also on the activity of extra-SCN oscillators. Below we describe the equipment and methods used to record, analyze and display circadian locomotor activity rhythms in laboratory rodents.

PKCγ participates in food entrainment by regulating BMAL1

Temporally restricted feeding (RF) can phase reset the circadian clocks in numerous tissues in mammals, contributing to altered timing of behavioral and physiological rhythms. However, little is known regarding the underlying molecular mechanism. Here we demonstrate a role for the gamma isotype of protein kinase C (PKCγ) in food-mediated entrainment of behavior and the molecular clock. We found that daytime RF reduced late-night activity in wild-type mice but not mice homozygous for a null mutation of PKCγ (PKCγ(-/-)). Molecular analysis revealed that PKCγ exhibited RF-induced changes in activation patterns in the cerebral cortex and that RF failed to substantially phase shift the oscillation of clock gene transcripts in the absence of PKCγ. PKCγ exerts effects on the clock, at least in part, by stabilizing the core clock component brain and muscle aryl hydrocarbon receptor nuclear translocator like 1 (BMAL1) and reducing its ubiquitylation in a deubiquitination-dependent manner. Taken together, these results suggest that PKCγ plays a role in food entrainment by regulating BMAL1 stability.

Reconstructing mammalian sleep dynamics with data assimilation

Data assimilation is a valuable tool in the study of any complex system, where measurements are incomplete, uncertain, or both. It enables the user to take advantage of all available information including experimental measurements and short-term model forecasts of a system. Although data assimilation has been used to study other biological systems, the study of the sleep-wake regulatory network has yet to benefit from this toolset. We present a data assimilation framework based on the unscented Kalman filter (UKF) for combining sparse measurements together with a relatively high-dimensional nonlinear computational model to estimate the state of a model of the sleep-wake regulatory system. We demonstrate with simulation studies that a few noisy variables can be used to accurately reconstruct the remaining hidden variables. We introduce a metric for ranking relative partial observability of computational models, within the UKF framework, that allows us to choose the optimal variables for measurement and also provides a methodology for optimizing framework parameters such as UKF covariance inflation. In addition, we demonstrate a parameter estimation method that allows us to track non-stationary model parameters and accommodate slow dynamics not included in the UKF filter model. Finally, we show that we can even use observed discretized sleep-state, which is not one of the model variables, to reconstruct model state and estimate unknown parameters. Sleep is implicated in many neurological disorders from epilepsy to schizophrenia, but simultaneous observation of the many brain components that regulate this behavior is difficult. We anticipate that this data assimilation framework will enable better understanding of the detailed interactions governing sleep and wake behavior and provide for better, more targeted, therapies.

Mathematical models are developed to better understand interactions between components of a system that together govern the overall behavior. Mathematical models of sleep have helped to elucidate the neuronal cell groups that are involved in promoting sleep and wake behavior and the transitions between them. However, to be able to take full advantage of these models one must be able to estimate the value of all included variables accurately. Data assimilation refers to methods that allow the user to combine noisy measurements of just a few system variables with the mathematical model of that system to estimate all variables, including those originally inaccessible for measurement. Using these techniques we show that we can reconstruct the unmeasured variables and parameters of a mathematical model of the sleep-wake network. These reconstructed estimates can then be used to better understand the underlying neuronal behavior that results in sleep and wake activity. Because sleep is implicated in a wide array of neurological disorders from epilepsy to schizophrenia, we anticipate that this framework will enable better understanding of the link between sleep and the rest of the brain and provide for better, more targeted, therapies.

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

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

Circadian and sleep disorder in Huntington's disease

Huntington's disease is a progressive neurological disorder that starts insidiously with motor, cognitive or psychiatric disturbance, and progresses through a distressing range of symptoms to end with a devastating loss of function, both motor and executive. There is a growing awareness that, in addition to cognitive and psychiatric symptoms, there are other important non-motor symptoms in HD, including sleep and circadian abnormalities. It is not clear if sleep-wake changes are caused directly by HD gene-related pathology, or if they are simply a consequence of having a neurodegenerative disease. From a patient point of view, the answer is irrelevant, since sleep and circadian disturbances are deleterious to good daily living, even in neurologically normal people. The assumption should be that, at the very least, sleep and/or circadian disturbance in HD patients will contribute to their symptoms. At worst, they may contribute to the progressive decline in HD. Here I review the state of our understanding of sleep and circadian abnormalities in HD. I also outline a set of simple rules that can be followed to improve the chances of a good night's sleep, since preventing any 'preventable' symptoms is the a logical first step in treating disease. The long-term impact of sleep disruption in HD is unknown. There have been no large-scale systematic studies of in sleep in HD. Furthermore, there has never been a study of the efficacy of pharmaceuticals that are typically used to treat sleep deficits in HD patients. Thus treatment of sleep disturbance in HD is necessarily empirical. A better understanding of the relationship between sleep/circadian abnormalities and HD pathology is needed, if treatment of this aspect of HD is to be optimized.

Scheduled daily mating induces circadian anticipatory activity rhythms in the male rat

Daily schedules of limited access to food, palatable high calorie snacks, water and salt can induce circadian rhythms of anticipatory locomotor activity in rats and mice. All of these stimuli are rewarding, but whether anticipation can be induced by neural correlates of reward independent of metabolic perturbations associated with manipulations of food and hydration is unclear. Three experiments were conducted to determine whether mating, a non-ingestive behavior that is potently rewarding, can induce circadian anticipatory activity rhythms in male rats provided scheduled daily access to steroid-primed estrous female rats. In Experiment 1, rats anticipated access to estrous females in the mid-light period, but also exhibited post-coital eating and running. In Experiment 2, post-coital eating and running were prevented and only a minority of rats exhibited anticipation. Rats allowed to see and smell estrous females showed no anticipation. In both experiments, all rats exhibited sustained behavioral arousal and multiple mounts and intromissions during every session, but ejaculated only every 2–3 days. In Experiment 3, the rats were given more time with individual females, late at night for 28 days, and then in the midday for 28 days. Ejaculation rates increased and anticipation was robust to night sessions and significant although weaker to day sessions. The anticipation rhythm persisted during 3 days of constant dark without mating. During anticipation of nocturnal mating, the rats exhibited a significant preference for a tube to the mating cage over a tube to a locked cage with mating cage litter. This apparent place preference was absent during anticipation of midday mating, which may reflect a daily rhythm of sexual reward. The results establish mating as a reward stimulus capable of inducing circadian rhythms of anticipatory behavior in the male rat, and reveal a critical role for ejaculation, a modulatory role for time of day, and a potential confound role for uncontrolled food intake.

Isolating neural correlates of the pacemaker for food anticipation

Mice fed a single daily meal at intervals within the circadian range exhibit food anticipatory activity. Previous investigations strongly suggest that this behaviour is regulated by a circadian pacemaker entrained to the timing of fasting/refeeding. The neural correlate(s) of this pacemaker, the food entrainable oscillator (FEO), whether found in a neural network or a single locus, remain unknown. This study used a canonical property of circadian pacemakers, the ability to continue oscillating after removal of the entraining stimulus, to isolate activation within the neural correlates of food entrainable oscillator from all other mechanisms driving food anticipatory activity. It was hypothesized that continued anticipatory activation of central nuclei, after restricted feeding and a return to ad libitum feeding, would elucidate a neural representation of the signaling circuits responsible for the timekeeping component of the food entrainable oscillator. Animals were entrained to a temporally constrained meal then placed back on ad libitum feeding for several days until food anticipatory activity was abolished. Activation of nuclei throughout the brain was quantified using stereological analysis of c-FOS expressing cells and compared against both ad libitum fed and food entrained controls. Several hypothalamic and brainstem nuclei remained activated at the previous time of food anticipation, implicating them in the timekeeping mechanism necessary to track previous meal presentation. This study also provides a proof of concept for an experimental paradigm useful to further investigate the anatomical and molecular substrates of the FEO.

Circadian clocks for all meal-times: anticipation of 2 daily meals in rats

Anticipation of a daily meal in rats has been conceptualized as a rest-activity rhythm driven by a food-entrained circadian oscillator separate from the pacemaker generating light-dark (LD) entrained rhythms. Rats can also anticipate two daily mealtimes, but whether this involves independently entrained oscillators, one ‘continuously consulted’ clock, cue-dependent non-circadian interval timing or a combination of processes, is unclear. Rats received two daily meals, beginning 3-h (meal 1) and 13-h (meal 2) after lights-on (LD 14∶10). Anticipatory wheel running began 68±8 min prior to meal 1 and 101±9 min prior to meal 2 but neither the duration nor the variability of anticipation bout lengths exhibited the scalar property, a hallmark of interval timing. Meal omission tests in LD and constant dark (DD) did not alter the timing of either bout of anticipation, and anticipation of meal 2 was not altered by a 3-h advance of meal 1. Food anticipatory running in this 2-meal protocol thus does not exhibit properties of interval timing despite the availability of external time cues in LD. Across all days, the two bouts of anticipation were uncorrelated, a result more consistent with two independently entrained oscillators than a single consulted clock. Similar results were obtained for meals scheduled 3-h and 10-h after lights-on, and for a food-bin measure of anticipation. Most rats that showed weak or no anticipation to one or both meals exhibited elevated activity at mealtime during 1 or 2 day food deprivation tests in DD, suggesting covert operation of circadian timing in the absence of anticipatory behavior. A control experiment confirmed that daytime feeding did not shift LD-entrained rhythms, ruling out displaced nocturnal activity as an explanation for daytime activity. The results favor a multiple oscillator basis for 2-meal anticipatory rhythms and provide no evidence for involvement of cue-dependent interval timing.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Artificial feeding synchronizes behavioral, hormonal, metabolic and neural parameters in mother-deprived neonatal rabbit pups

Nursing in the rabbit is under circadian control, and pups have a daily anticipatory behavioral arousal synchronized to this unique event, but it is not known which signal is the main entraining cue. In the present study, we hypothesized that food is the main entraining signal. Using mother-deprived pups, we tested the effects of artificial feeding on the synchronization of locomotor behavior, plasma glucose, corticosterone, c-Fos (FOS) and PERIOD1 (PER1) rhythms in suprachiasmatic, supraoptic, paraventricular and tuberomammillary nuclei. At postnatal day 1, an intragastric tube was placed by gastrostomy. The next day and for the rest of the experiment, pups were fed with a milk formula through the cannula at either 02:00 h or 10:00 h [feeding time = zeitgeber time (ZT)0]. At postnatal days 5-7, pups exhibited behavioral arousal, with a significant increase in locomotor behavior 60 min before feeding. Glucose levels increased after feeding, peaking at ZT4-ZT12 and then declining. Corticosterone levels were highest around the time of feeding, and then decreased to trough concentrations at ZT12-ZT16, increasing again in anticipation of the next feeding bout. In the brain, the suprachiasmatic nucleus had a rhythm of FOS and PER1 that was not significantly affected by the feeding schedule. Conversely, the supraoptic, paraventricular and tuberomammillary nuclei had rhythms of both FOS and PER1 induced by the time of scheduled feeding. We conclude that the nursing rabbit pup is a natural model of food entrainment, as food, in this case milk formula, is a strong synchronizing signal for behavioral, hormonal, metabolic and neural parameters.

Food anticipation depends on oscillators and memories in both body and brain

Despite the importance of learning and circadian rhythms to feeding, there has been relatively little effort to integrate these separate lines of research. In this review, we focus on how light and food entrainable oscillators contribute to the anticipation of food. In particular, we examine the evidence for temporal conditioning of food entrainable oscillators throughout the body. The evidence suggests a shift away from previous notions of a single locus or neural network of food entrainable oscillators to a distributed system involving dynamic feedback among cells of the body and brain. Several recent advances, including documentation of peroxiredoxin metabolic circadian oscillation and anticipatory behavior in the absence of a central nervous system, support the possibility of conditioned signals from the periphery in determining anticipatory behavior. Individuals learn to detect changes in internal and external signals that occur as a consequence of the brain and body preparing for an impending meal. Cues temporally near and far from actual energy content can then be used to optimize responses to temporally predictable and unpredictable cues in the environment.

Evidence for time-of-day dependent effect of neurotoxic dorsomedial hypothalamic lesions on food anticipatory circadian rhythms in rats

The dorsomedial hypothalamus (DMH) is a site of circadian clock gene and immediate early gene expression inducible by daytime restricted feeding schedules that entrain food anticipatory circadian rhythms in rats and mice. The role of the DMH in the expression of anticipatory rhythms has been evaluated using different lesion methods. Partial lesions created with the neurotoxin ibotenic acid (IBO) have been reported to attenuate food anticipatory rhythms, while complete lesions made with radiofrequency current leave anticipatory rhythms largely intact. We tested a hypothesis that the DMH and fibers of passage spared by IBO lesions play a time-of-day dependent role in the expression of food anticipatory rhythms. Rats received intra-DMH microinjections of IBO and activity and body temperature (T_b) rhythms were recorded by telemetry during ad-lib food access, total food deprivation and scheduled feeding, with food provided for 4-h/day for 20 days in the middle of the light period and then for 20 days late in the dark period. During ad-lib food access, rats with DMH lesions exhibited a lower amplitude and mean level of light-dark entrained activity and T_b rhythms. During the daytime feeding schedule, all rats exhibited food anticipatory activity and T_b rhythms that persisted during 2 days without food in constant dark. In some rats with partial or total DMH ablation, the magnitude of the anticipatory rhythm was weak relative to most intact rats. When mealtime was shifted to the late night, the magnitude of the food anticipatory activity rhythms in these cases was restored to levels characteristic of intact rats. These results confirm that rats can anticipate scheduled daytime or nighttime meals without the DMH. Improved anticipation at night suggests a modulatory role for the DMH in the expression of food anticipatory activity rhythms during the daily light period, when nocturnal rodents normally sleep.

The methamphetamine-sensitive circadian oscillator is dysfunctional in a transgenic mouse model of Huntington's disease

A progressive disintegration of the rest-activity rhythm has been observed in the R6/2 mouse model of Huntington's disease (HD). Rest-activity rhythm is controlled by a circadian clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus, although SCN-independent oscillators such as the methamphetamine (MAP)-sensitive circadian oscillator (MASCO) can also control rhythmicity, even in SCN-lesioned animals. We aimed to test whether or not the administration of MAP could restore a normal rest-activity rhythm in R6/2 mice, via the activation of the MASCO. We administered chronic low doses of MAP to wild-type (WT) and presymptomatic (7-8 weeks) R6/2 mice, in constant darkness. As expected, ~40% of the WT mice expressed a rest-activity rhythm controlled by the MASCO, with a period of around 32 h. By contrast, the MASCO was missing from almost 95% of the R6/2 mice, even at early stages of disease. Interestingly, although the MASCO was deficient, initially MAP was able to stabilize the day/night activity ratio in R6/2 mice and delay the onset of disintegration of the rest-activity rhythm driven by the SCN. Furthermore, in presymptomatic R6/2 mice treated with L-DOPA, a MASCO-like component began to emerge, although this never became established. Our data show a major dysfunction of the MASCO in presymptomatic R6/2 mice that is likely to be due to an early abnormality of the catecholaminergic systems. We suggest that the dysfunction of the MASCO in humans could be partially responsible for circadian disturbances observed in HD patients, as well as patients with other neurological diseases in which both catecholaminergic and circadian abnormalities are present, such as Parkinson's disease and schizophrenia.

Melatonin: both master clock output and internal time-giver in the circadian clocks network

Daily rhythms in physiological and behavioral processes are controlled by a network of circadian clocks, reset by inputs and delivering circadian signals to the brain and peripheral organs. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus, mainly reset by ambient light. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock and inhibited by light exposure. Several roles of melatonin in the circadian system have been identified. As a major hormonal output, melatonin distributes temporal cues generated by the SCN to the multitude of tissue targets expressing melatonin receptors. In some target structures, like the Pars tuberalis of the adenohypophysis, these melatonin signals can drive daily rhythmicity that would otherwise be lacking. In other target structures, melatonin signals are used for the synchronization (i.e., adjustment of the timing of existing oscillations) of peripheral oscillators, such as the fetal adrenal gland. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock, although its physiological significance needs further characterization. Of note, pharmacological treatment with exogenous melatonin can synchronize the SCN clock. From a clinical point of view, provided that the subject is not exposed to light at night, the daily profile of circulating melatonin provides a reliable estimate of the timing of the human SCN. During the past decade, a number of melatonin agonists have been developed for treating circadian, psychiatric and sleep disorders. These drugs may target the SCN for improving circadian timing or act indirectly at some downstream level of the circadian network to restore proper internal synchronization.

Scheduled Food Hastens Re-Entrainment More Than Melatonin Does after a 6-h Phase Advance of the Light-Dark Cycle in Rats

Circadian desynchrony occurs when individuals are exposed to abrupt phase shifts of the light-dark cycle, as in jet lag. For reducing symptoms and for speeding up resynchronization, several strategies have been suggested, including scheduled exercise, exposure to bright light, drugs, and especially exogenous melatonin administration. Restricted feeding schedules have shown to be powerful entraining signals for metabolic and hormonal daily cycles, as well as for clock genes in tissues and organs of the periphery. This study explored in a rat model of jet lag the contribution of exogenous melatonin or scheduled feeding on the re-entrainment speed of spontaneous general activity and core temperature after a 6-h phase advance of the light-dark cycle. In a first phase, the treatment was scheduled for 5 days prior to the phase shift, while in a second stage, the treatment was simultaneous with the phase advance of the light-dark cycle. Melatonin administration and especially scheduled feeding simultaneous with the phase shift improved significantly the re-entrainment speed. The evaluation of the free-running activity and temperature following the 5-day treatment proved that both exogenous melatonin and specially scheduled feeding accelerated re-entrainment of the SCN-driven general activity and core temperature, respectively, with 7, 5 days ( p < 0.01) and 3, 3 days ( p < 0.001). The present results show the relevance of feeding schedules as entraining signals for the circadian system and highlight the importance of using them as a strategy for preventing internal desynchrony.

Entrainment of circadian clocks in mammals by arousal and food

Circadian rhythms in mammals are regulated by a system of endogenous circadian oscillators (clock cells) in the brain and in most peripheral organs and tissues. One group of clock cells in the hypothalamic SCN (suprachiasmatic nuclei) functions as a pacemaker for co-ordinating the timing of oscillators elsewhere in the brain and body. This master clock can be reset and entrained by daily LD (light–dark) cycles and thereby also serves to interface internal with external time, ensuring an appropriate alignment of behavioural and physiological rhythms with the solar day. Two features of the mammalian circadian system provide flexibility in circadian programming to exploit temporal regularities of social stimuli or food availability. One feature is the sensitivity of the SCN pacemaker to behavioural arousal stimulated during the usual sleep period, which can reset its phase and modulate its response to LD stimuli. Neural pathways from the brainstem and thalamus mediate these effects by releasing neurochemicals that inhibit retinal inputs to the SCN clock or that alter clock-gene expression in SCN clock cells. A second feature is the sensitivity of circadian oscillators outside of the SCN to stimuli associated with food intake, which enables animals to uncouple rhythms of behaviour and physiology from LD cycles and align these with predictable daily mealtimes. The location of oscillators necessary for food-entrained behavioural rhythms is not yet certain. Persistence of these rhythms in mice with clock-gene mutations that disable the SCN pacemaker suggests diversity in the molecular basis of light- and food-entrainable clocks.

A fear-inducing odor alters PER2 and c-Fos expression in brain regions involved in fear memory

Evidence demonstrates that rodents learn to associate a foot shock with time of day, indicating the formation of a fear related time-stamp memory, even in the absence of a functioning SCN. In addition, mice acquire and retain fear memory better during the early day compared to the early night. This type of memory may be regulated by circadian pacemakers outside of the SCN. As a first step in testing the hypothesis that clock genes are involved in the formation of a time-stamp fear memory, we exposed one group of mice to fox feces derived odor (TMT) at ZT 0 and one group at ZT 12 for 4 successive days. A separate group with no exposure to TMT was also included as a control. Animals were sacrificed one day after the last exposure to TMT, and PER2 and c-Fos protein were quantified in the SCN, amygdala, hippocampus, and piriform cortex. Exposure to TMT had a strong effect at ZT 0, decreasing PER2 expression at this time point in most regions except the SCN, and reversing the normal rhythm of PER2 expression in the amygdala and piriform cortex. These changes were accompanied by increased c-Fos expression at ZT0. In contrast, exposure to TMT at ZT 12 abolished the rhythm of PER2 expression in the amygdala. In addition, increased c-Fos expression at ZT 12 was only detected in the central nucleus of the amygdala in the TMT12 group. TMT exposure at either time point did not affect PER2 or c-Fos in the SCN, indicating that under a light-dark cycle, the SCN rhythm is stable in the presence of repeated exposure to a fear-inducing stimulus. Taken together, these results indicate that entrainment to a fear-inducing stimulus leads to changes in PER2 and c-Fos expression that are detected 24 hours following the last exposure to TMT, indicating entrainment of endogenous oscillators in these regions. The observed effects on PER2 expression and c-Fos were stronger during the early day than during the early night, possibly to prepare appropriate systems at ZT 0 to respond to a fear-inducing stimulus.

Variable restricted feeding disrupts the daily oscillations of Period2 expression in the limbic forebrain and dorsal striatum in rats

Predictable restricted feeding schedules limit food availability to a single meal at the same time each day, lead to the induction and entrainment of circadian rhythms in food-anticipatory activity, and shift daily rhythms of clock gene expression in areas of the brain that are important in the regulation of motivational and emotional state. In contrast, when food is delivered under a variable restricted feeding (VRF) schedule, at a new and unpredictable mealtime each day, circadian rhythms in food-anticipatory activity fail to develop. Here, we study the effects of VRF on the daily rhythm of plasma corticosterone and of clock gene expression in the limbic forebrain and dorsal striatum, of rats provided a 2-h access to a complete meal replacement (Ensure Plus) at an unpredictable time each day. VRF schedules varied the mealtimes within the 12 h of light (daytime VRF), the 12 h of dark (nighttime VRF), or across the 24 h light-dark cycle (anytime VRF). Our results show that contrary to the synchronizing effects of predictable restricted feeding, VRF blunts the daily corticosterone rhythm and disrupts daily rhythms of PER2 expression in a region-specific and mealtime-dependent manner.

In a rat model of night work, activity during the normal resting phase produces desynchrony in the hypothalamus

Internal synchrony among external cycles and internal oscillators allows adaptation of physiology to cyclic demands for homeostasis. Night work and shift work lead to a disrupted phase relationship between external time cues and internal rhythms, also losing internal coherence among oscillations. This process results in internal desynchrony (ID) in which behavioral, hormonal, and metabolic variables cycle out of phase. It is still not clear whether ID originates at a peripheral or at a central level. In order to determine the possible role of hypothalamic oscillators in ID, we explored with a rat model of "night work" daily rhythms of activity and clock gene expression in the hypothalamus. This study provides evidence that wakefulness and activity during the normal resting phase lead to a shift in the diurnal rhythms of c-Fos and induce a rhythm of PER1 in the arcuate and dorsomedial nucleus of the hypothalamus, both associated with metabolism and regulation of the sleep/wake cycle. Moreover, the number of orexin (ORX)-positive neurons and c-Fos in the perifornical area increased during the working period, suggesting a relevant switch of activity in this brain region induced by the scheduled activity; however, the colocalization of c-Fos in ORX-positive cells was not increased. In contrast, the suprachiasmatic nucleus and the paraventricular nucleus remained locked to the light/dark cycle, resulting in ID in the hypothalamus. Present data suggest that ID occurs already at the level of the first output projections from the SCN, relaying nuclei that transmit temporal signals to other brain areas and to the periphery.

Neurobiology of food anticipatory circadian rhythms

Circadian rhythms in mammals can be entrained by daily schedules of light or food availability. A master light-entrainable circadian pacemaker located in the suprachiasmatic nucleus (SCN) is comprised of a population of cell autonomous, transcriptionally based circadian oscillators with defined retinal inputs, circadian clock genes and neural outputs. By contrast, the neurobiology of food-entrainable circadian rhythmicity remains poorly understood at the systems and cellular levels. Induction of food-anticipatory activity rhythms by daily feeding schedules does not require the SCN, but these rhythms do exhibit defining properties of circadian clock control. Clock gene rhythms expressed in other brain regions and in peripheral organs are preferentially reset by mealtime, but lesions of specific hypothalamic, corticolimbic and brainstem structures do not eliminate all food anticipatory rhythms, suggesting control by a distributed, decentralized system of oscillators, or the existence of a critical oscillator at an unknown location. The melanocortin system and dorsomedial hypothalamus may play modulatory roles setting the level of anticipatory activity. The metabolic hormones ghrelin and leptin are not required to induce behavioral food anticipatory rhythms, but may also participate in gain setting. Clock gene mutations that disrupt light-entrainable rhythms generally do not eliminate food anticipatory rhythms, suggesting a novel timing mechanism. Recent evidence for non-transcriptional and network based circadian rhythmicity provides precedence, but any such mechanisms are likely to interact closely with known circadian clock genes, and some important double and triple clock gene knockouts remain to be phenotyped for food entrainment. Given the dominant role of food as an entraining stimulus for metabolic rhythms, the timing of daily food intake and the fidelity of food entrainment mechanisms are likely to have clinical relevance.

Endogenous dopamine regulates the rhythm of expression of the clock protein PER2 in the rat dorsal striatum via daily activation of D2 dopamine receptors

A role for dopamine (DA) in the regulation of clock genes in the mammalian brain is suggested by evidence that manipulations of DA receptors can alter the expression of some clock genes outside the suprachiasmatic nucleus (SCN), the master circadian clock. The role of endogenous DA in the regulation of clock gene expression is unknown. Here, we demonstrate a direct relationship between extracellular DA levels and the rhythm of expression of the clock protein PERIOD2 (PER2) in the dorsal striatum of the male Wistar rat. Specifically, we show that the peak of the daily rhythm of extracellular DA in the dorsal striatum precedes the peak of PER2 by ∼6 h and that depletion of striatal DA by 6-hydroxydopamine or α-methyl-para-tyrosine or blockade of D(2) DA receptors by raclopride blunts the rhythm of striatal PER2. Furthermore, timed daily activation of D(2) DA receptors, but not D(1) DA receptors, restores and entrains the PER2 rhythm in the DA-depleted striatum. None of these manipulations had any effect on the PER2 rhythm in the SCN. Our findings are consistent with the idea that the rhythm of expression of PER2 in the dorsal striatum depends on daily dopaminergic activation of D(2) DA receptors. These observations may have implications for circadian abnormalities seen in Parkinson's disease.

Daily timed sexual interaction induces moderate anticipatory activity in mice

Anticipation of resource availability is a vital skill yet it is poorly understood in terms of neuronal circuitry. Rodents display robust anticipatory activity in the several hours preceding timed daily access to food when access is limited to a short temporal duration. We tested whether this anticipatory behavior could be generalized to timed daily social interaction by examining if singly housed male mice could anticipate either a daily novel female or a familiar female. We observed that anticipatory activity was moderate under both conditions, although both a novel female partner and sexual experience are moderate contributing factors to increasing anticipatory activity. In contrast, restricted access to running wheels did not produce any anticipatory activity, suggesting that an increase in activity during the scheduled access time was not sufficient to induce anticipation. To tease apart social versus sexual interaction, we tested the effect of exposing singly housed female mice to a familiar companion female mouse daily. The female mice did not show anticipatory activity for restricted female access, despite a large amount of social interaction, suggesting that daily timed social interaction between mice of the same gender is insufficient to induce anticipatory activity. Our study demonstrates that male mice will show anticipatory activity, albeit inconsistently, for a daily timed sexual encounter.

Phase preference for the display of activity is associated with the phase of extra-suprachiasmatic nucleus oscillators within and between species

Many features of the suprachiasmatic nucleus (SCN) are the same in diurnal and nocturnal animals, suggesting that differences in phase preference are determined by mechanisms downstream from the SCN. Here, we examined this hypothesis by characterizing rhythmic expression of Period 1 (PER1) and Period 2 (PER2) in several extra-SCN areas in the brains of a diurnal murid rodent, Arvicanthis niloticus (grass rats). In the shell of the nucleus accumbens, dorsal striatum, piriform cortex, and CA1 of the hippocampus, both PER1 and PER2 were rhythmic, with peak expression occurring at ZT10. PER1 in the dentate gyrus also peaked at ZT10, but PER2 was arrhythmic in this region. In general, these patterns are 180 degrees out of phase with those reported for nocturnal species. In a second study, we examined inter-individual differences in the multioscillator system of grass rats. Here, we housed grass rats in cages with running wheels, under which conditions some individuals spontaneously adopt a day active (DA) and others a night active (NA) phase preference. In the majority of the extra-SCN regions sampled, the patterns of PER1 and PER2 expression of NA grass rats resembled those of nocturnal species, while those of DA grass rats were similar to the ones seen in grass without access to running wheels. In contrast, the rhythmic expression of both PER proteins was identical in the SCN and ventral subparaventricular zone (vSPZ) of DA and NA animals. Differences in the phase of oscillators downstream from the SCN, and perhaps the vSPZ, appear to determine the phase preference of particular species, as well as that of members of a diurnal species that show voluntary phase reversals. The latter observation has important implications for the understanding of health problems associated with human shift work.

Palatable meal anticipation in mice

The ability to sense time and anticipate events is a critical skill in nature. Most efforts to understand the neural and molecular mechanisms of anticipatory behavior in rodents rely on daily restricted food access, which induces a robust increase of locomotor activity in anticipation of daily meal time. Interestingly, rats also show increased activity in anticipation of a daily palatable meal even when they have an ample food supply, suggesting a role for brain reward systems in anticipatory behavior, and providing an alternate model by which to study the neurobiology of anticipation in species, such as mice, that are less well adapted to “stuff and starve” feeding schedules. To extend this model to mice, and exploit molecular genetic resources available for that species, we tested the ability of wild-type mice to anticipate a daily palatable meal. We observed that mice with free access to regular chow and limited access to highly palatable snacks of chocolate or “Fruit Crunchies” avidly consumed the snack but did not show anticipatory locomotor activity as measured by running wheels or video-based behavioral analysis. However, male mice receiving a snack of high fat chow did show increased food bin entry prior to access time and a modest increase in activity in the two hours preceding the scheduled meal. Interestingly, female mice did not show anticipation of a daily high fat meal but did show increased activity at scheduled mealtime when that meal was withdrawn. These results indicate that anticipation of a scheduled food reward in mice is behavior, diet, and gender specific.

Oscillators entrained by food and the emergence of anticipatory timing behaviors

Circadian rhythms are adjusted to the external environment by the light-dark cycle via the suprachiasmatic nucleus, and to the internal environment of the body by multiple cues that derive from feeding/fasting. These cues determine the timing of sleep/wake cycles and all the activities associated with these states. We suggest that numerous sources of temporal information, including hormonal cues such as corticoids, insulin, and ghrelin, as well as conditioned learned responses determined by the temporal relationships between photic and feeding/fasting signals, can determine the timing of regularly recurring circadian responses. We further propose that these temporal signals can act additively to modulate the pattern of daily activity. Based on such reasoning, we describe the rationale and methodology for separating the influences of these diverse sources of temporal information. The evidence indicates that there are individual differences in sensitivity to internal and external signals that vary over circadian time, time since the previous meal, time until the next meal, or with duration of food deprivation. All of these cues are integrated in sites and circuits modulating physiology and behavior. Individuals detect changes in internal and external signals, interpret those changes as "hunger," and adjust their physiological responses and activity levels accordingly.

Altered thermoregulation via sensitization of A1 adenosine receptors in dietary-restricted rats

RATIONALE

Evidence links longevity to dietary restriction (DR). A decrease in body temperature (T(b)) is thought to contribute to enhanced longevity because lower T(b) reduces oxidative metabolism and oxidative stress. It is as yet unclear how DR decreases T(b).

OBJECTIVE

Here, we test the hypothesis that prolonged DR decreases T(b) by sensitizing adenosine A(1) receptors (A(1)AR) and adenosine-induced cooling.

METHODS AND RESULTS

Sprague-Dawley rats were dietary restricted using an every-other-day feeding protocol. Rats were fed every other day for 27 days and then administered the A(1)AR agonist, N(6)-cyclohexyladenosine (CHA; 0.5 mg/kg, i.p.). Respiratory rate (RR) and subcutaneous T(b) measured using IPTT-300 transponders were monitored every day and after drug administration. DR animals displayed lower RR on day 20 and lower T(b) on day 22 compared to animals fed ad libitum and displayed a larger response to CHA. In all cases, RR declined before T(b). Contrary to previous reports, a higher dose of CHA (5 mg/kg, i.p.) was lethal in both dietary groups. We next tested the hypothesis that sensitization to the effects of CHA was due to increased surface expression of A(1)AR within the hypothalamus. We report that the abundance of A(1)AR in the membrane fraction increases in hypothalamus, but not cortex of DR rats.

CONCLUSION

These results suggest that every-other-day feeding lowers T(b) via sensitization of thermoregulatory effects of endogenous adenosine by increasing surface expression of A(1)AR.

DISCUSSION

Evidence that diet can modulate purinergic signaling has implications for the treatment of stroke, brain injury, epilepsy, and aging.

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.

Bidirectional interactions between the circadian and reward systems: is restricted food access a unique zeitgeber?

Reward is mediated by a distributed series of midbrain and basal forebrain structures collectively referred to as the brain reward system. Recent evidence indicates that an additional regulatory system, the circadian system, can modulate reward-related learning. Diurnal or circadian changes in drug self-administration, responsiveness to drugs of abuse and reward to natural stimuli have been reported. These variations are associated with daily rhythms in mesolimbic electrical activity, dopamine synthesis and metabolism, and local clock gene oscillations. Conversely, the presentation of rewards appears capable of influencing circadian timing. Rodents can anticipate a daily mealtime by the entrainment of a series of oscillators that are anatomically distinct from the suprachiasmatic nucleus. Other work has indicated that restricted access to non-nutritive reinforcers (e.g. drugs of abuse, sex) or to palatable food in the absence of an energy deficit is capable of inducing relatively weak anticipatory activity, suggesting that reward alone is sufficient to induce anticipation. Recent attempts to elucidate the neural correlates of anticipation have revealed that both restricted feeding and restricted palatable food access can entrain clock gene expression in many reward-related corticolimbic structures. By contrast, restricted feeding alone can induce or entrain clock gene expression in hypothalamic nuclei involved in energy homeostasis. Thus, under ad libitum feeding conditions, the weak anticipatory activity induced by restricted reward presentation may result from the entrainment of reward-associated corticolimbic structures. The additional induction or entrainment of oscillators in hypothalamic regulatory areas may contribute to the more robust anticipatory activity associated with restricted feeding schedules.

Neural basis of timing and anticipatory behaviors

The ability to anticipate physiological needs and to predict the availability of desirable resources optimizes the likelihood of survival for an organism. The neural basis of the complex behaviors associated with anticipatory responses is now being delineated. Anticipation likely involves learning and memory, reward and punishment, memory and cognition, arousal and feedback associated with changes in internal and external state, homeostatic processes and timing mechanisms. While anticipation can occur on a variety of timescales (seconds to minutes to hours to days to a year), there have been great strides made towards understanding the neural basis timing of events in the circadian realm. Anticipation of daily events, such as scheduled access to food, may serve as a useful model for a more broadly based understanding the neurobiology of anticipation. In this review we examine the historical, conceptual and experimental approaches to understanding the neural basis of anticipation with a focus on anticipation of scheduled daily meals. We also introduce the key topics represented in the papers in this issue. These papers focused on food anticipation, to explore the state of the art in the studies of the neural basis of timing and anticipatory behaviors.