Restricted access to food, but not sucrose, saccharine, or salt, synchronizes the expression of Period2 protein in the limbic forebrain

Food Restriction in Mice Induces Food-Anticipatory Activity and Circadian-Rhythm-Related Activity Changes

Anorexia nervosa (AN) is characterized by emaciation, hyperactivity, and amenorrhea. To what extent AN-related symptoms are due to food restriction or neuronal dysfunction is currently unknown. Thus, we investigated the relevance of food restriction on AN-related symptoms. Disrupted circadian rhythms are hypothesized to contribute to the pathophysiology of AN. Starvation was induced by restricting food access in early adolescent or adolescent mice to 40% of their baseline food intake until a 20% weight reduction was reached (acute starvation). To mimic chronic starvation, the reduced weight was maintained for a further 2 weeks. Locomotor activity was analyzed using running wheel sensors. The circadian-rhythm-related activity was measured using the tracking system Goblotrop. Amenorrhea was determined by histological examination of vaginal smears. All cohorts showed an increase in locomotor activity up to 4 h before food presentation (food-anticipatory activity, FAA). While amenorrhea was present in all groups except in early adolescent acutely starved mice, hyperactivity was exclusively found in chronically starved groups. Adolescent chronically starved mice showed a decrease in circadian-rhythm-related activity at night. Chronic starvation most closely mimics AN-related behavioral changes. It appears that the FAA is a direct consequence of starvation. The circadian activity changes might underlie the pathophysiology of AN.

Aberrant Lighting Causes Anxiety-like Behavior in Mice but Curcumin Ameliorates the Symptoms

Simple Summary

In the present study, we exposed mice to aberrant lighting system and noticed anxiety-like behavior. These symptoms were ameliorated by oral administration of curcumin. The study was carried out on the animals for three weeks in dim light at night (dLAN) and complete darkness (DD), monitoring the body weight, daily food intake, anxiety-like behavior, and expression of the period (PER1) gene. The exposure to dim light at night was found to significantly enhance the anxiety-like behavior and increased the body weight possibly through altered metabolism in mice. In contrast, exposure to DD caused increased anxiety but no significant difference in the body weight. Moreover, the expression of the PER1 gene involved in sleep was also found to be decreased in the aberrant light conditions (dLAN and DD). Although the treatment of curcumin had no effect on the body weight, it had ameliorated the anxiety-like behavior possibly by modulating the expression of the PER1 gene. Thus, the alteration in the light/dark cycle has negative influences on body weight, affecting even the emotional quotient. This study identifies the risk factors associated with aberrant lighting conditions in laboratory animal and ameliorative effects of curcumin.

Abstract

In the modern research field, laboratory animals are constantly kept under artificial lighting conditions. However, recent studies have shown the effect of artificial light on animal behavior and metabolism. In the present study on mice, following three weeks of housing in dim light at night (dLAN; 5lux) and complete darkness (DD; 0lux), we monitored the effect on body weight, daily food intake, anxiety-like behavior by employing the open field test, and expression of the period (PER1) gene. We also studied the effect of oral administration of different concentrations of curcumin (50, 100, and 150 mg/kg) for three weeks in the same mice and monitored these parameters. The exposure to dLAN had significantly increased the anxiety-like behavior and body weight possibly through the altered metabolism in mice, whereas exposure to DD caused increased anxiety but no significant difference in weight gain. Moreover, the expression of the PER1 gene involved in sleep was also found to be decreased in the aberrant light conditions (dLAN and DD). Although the treatment of curcumin had no effect on body weight, it ameliorated the anxiety-like behavior possibly by modulating the expression of the PER1 gene. Thus, alteration in the light/dark cycle had a negative effect on laboratory animals on the body weight and emotions of animals. The present study identifies the risk factors associated with artificial lighting systems on the behavior of laboratory animals and the ameliorative effects of curcumin, with a focus on anxiety-like behavior.

Dorsal striatum dopamine oscillations: Setting the pace of food anticipatory activity

Predicting the uncertainties of the ever-changing environment provides a competitive advantage for animals. The need to anticipate food sources has provided a strong evolutionary drive for synchronizing behavioural and internal processes with daily circadian cycles. When food is restricted to a few hours per day, rodents exhibit increased wakefulness and foraging behaviour preceding the arrival of food. Interestingly, while the master clock located in the suprachiasmatic nucleus entrains daily rhythms to the light cycle, it is not necessary for this food anticipatory activity. This suggests the existence of a food-entrained oscillator located elsewhere. Based on the role of nigrostriatal dopamine in reward processing, motor function, working memory and internal timekeeping, we propose a working model by which food-entrained dopamine oscillations in the dorsal striatum can enable animals maintained on a restricted feeding schedule to anticipate food arrival. Finally, we summarize how metabolic signals in the gut are conveyed to the nigrostriatal pathway to suggest possible insight into potential input mechanisms for food anticipatory activity.

Circadian-based Treatment Strategy Effective in the BACHD Mouse Model of Huntington's Disease

Huntington's disease (HD) patients suffer from progressive neurodegeneration that results in cognitive, psychiatric, cardiovascular, and motor dysfunction. Disturbances in sleep-wake cycles are common among HD patients with reports of delayed sleep onset, frequent bedtime awakenings, and excessive fatigue. The BACHD mouse model exhibits many HD core symptoms including circadian dysfunction. Because circadian dysfunction manifests early in the disease in both patients and mouse models, we sought to determine if early interventions that improve circadian rhythmicity could benefit HD symptoms and delay disease progression. We evaluated the effects of time-restricted feeding (TRF) on the BACHD mouse model. At 3 months of age, the animals were divided into 2 groups: ad lib and TRF. The TRF-treated BACHD mice were exposed to a 6-h feeding/18-h fasting regimen that was designed to be aligned with the middle (ZT 15-21) of the period when mice are normally active (ZT 12-24). Following 3 months of treatment (when mice reached the early disease stage), the TRF-treated BACHD mice showed improvements in their locomotor activity and sleep behavioral rhythms. Furthermore, we found improved heart rate variability, suggesting that their autonomic nervous system dysfunction was improved. On a molecular level, TRF altered the phase but not the amplitude of the PER2::LUC rhythms measured in vivo and in vitro. Importantly, treated BACHD mice exhibited improved motor performance compared with untreated BACHD controls, and the motor improvements were correlated with improved circadian output. It is worth emphasizing that HD is a genetically caused disease with no known cure. Lifestyle changes that not only improve the quality of life but also delay disease progression for HD patients are greatly needed. Our study demonstrates the therapeutic potential of circadian-based treatment strategies in a preclinical model of HD.

Scheduled feeding restores memory and modulates c-Fos expression in the suprachiasmatic nucleus and septohippocampal complex

Disruptions in circadian timing impair spatial memory in humans and rodents. Circadian-arrhythmic Siberian hamsters (Phodopus sungorus) exhibit substantial deficits in spatial working memory as assessed by a spontaneous alternation (SA) task. The present study found that daily scheduled feeding rescued spatial memory deficits in these arrhythmic animals. Improvements in memory persisted for at least 3 weeks after the arrhythmic hamsters were switched back to ad libitum feeding. During ad libitum feeding, locomotor activity resumed its arrhythmic state, but performance on the SA task varied across the day with a peak in daily performance that corresponded to the previous daily window of food anticipation. At the end of scheduled feeding, c-Fos brain mapping revealed differential gene expression in entrained versus arrhythmic hamsters in the suprachiasmatic nucleus (SCN) that paralleled changes in the medial septum and hippocampus, but not in other neural structures. These data show that scheduled feeding can improve cognitive performance when SCN timing has been compromised, possibly by coordinating activity in the SCN and septohippocampal pathway.

Metabolic activation of amygdala, lateral septum and accumbens circuits during food anticipatory behavior

When food is restricted to a brief fixed period every day, animals show an increase in temperature, corticosterone concentration and locomotor activity for 2-3h before feeding time, termed food anticipatory activity. Mechanisms and neuroanatomical circuits responsible for food anticipatory activity remain unclear, and may involve both oscillators and networks related to temporal conditioning. Rabbit pups are nursed once-a-day so they represent a natural model of circadian food anticipatory activity. Food anticipatory behavior in pups may be associated with neural circuits that temporally anticipate feeding, while the nursing event may produce consummatory effects. Therefore, we used New Zealand white rabbit pups entrained to circadian feeding to investigate the hypothesis that structures related to reward expectation and conditioned emotional responses would show a metabolic rhythm anticipatory of the nursing event, different from that shown by structures related to reward delivery. Quantitative cytochrome oxidase histochemistry was used to measure regional brain metabolic activity at eight different times during the day. We found that neural metabolism peaked before nursing, during food anticipatory behavior, in nuclei of the extended amygdala (basolateral, medial and central nuclei, bed nucleus of the stria terminalis), lateral septum and accumbens core. After pups were fed, however, maximal metabolic activity was expressed in the accumbens shell, caudate, putamen and cortical amygdala. Neural and behavioral activation persisted when animals were fasted by two cycles, at the time of expected nursing. These findings suggest that metabolic activation of amygdala-septal-accumbens circuits involved in temporal conditioning may contribute to food anticipatory activity.

Chronobesity: role of the circadian system in the obesity epidemic

Although obesity is considered to result from an imbalance between energy uptake and energy expenditure, the strategy of dietary changes and physical exercise has failed to tackle the global obesity epidemic. In search of alternative and more adequate treatment options, research has aimed at further unravelling the mechanisms underlying this excessive weight gain. While numerous studies are focusing on the neuroendocrine alterations that occur after bariatric Roux-en-Y gastric bypass surgery, an increasing amount of chronobiological studies have started to raise awareness concerning the pivotal role of the circadian system in the development and exacerbation of obesity. This internal timekeeping mechanism rhythmically regulates metabolic and physiological processes in order to meet the fluctuating demands in energy use and supply throughout the 24-h day. This review elaborates on the extensive bidirectional interaction between the circadian system and metabolism and explains how disruption of body clocks by means of shift work, frequent time zone travelling or non-stop consumption of calorie-dense foods can evoke detrimental metabolic alterations that contribute to obesity. Altering the body's circadian rhythms by means of time-related dietary approaches (chrononutrition) or pharmacological substances (chronobiotics) may therefore represent a novel and interesting way to prevent or treat obesity and associated comorbidities.

Food reward without a timing component does not alter the timing of activity under positive energy balance

Circadian clocks drive daily rhythms in physiology and behavior which allow organisms to anticipate predictable daily changes in the environment. In most mammals, circadian rhythms result in nocturnal activity patterns although plasticity of the circadian system allows activity patterns to shift to different times of day. Such plasticity is seen when food access is restricted to a few hours during the resting (light) phase resulting in food anticipatory activity (FAA) in the hours preceding food availability. The mechanisms underlying FAA are unknown but data suggest the involvement of the reward system and homeostatic regulation of metabolism. We previously demonstrated the isolated effect of metabolism by inducing diurnality in response to energetic challenges. Here the importance of reward timing in inducing daytime activity is assessed. The daily activity distribution of mice earning palatable chocolate at their preferred time by working in a running wheel was compared with that of mice receiving a timed palatable meal at noon. Mice working for chocolate (WFC) without being energetically challenged increased their total daily activity but this did not result in a shift to diurnality. Providing a chocolate meal at noon each day increased daytime activity, identifying food timing as a factor capable of altering the daily distribution of activity and rest. These results show that timing of food reward and energetic challenges are both independently sufficient to induce diurnality in nocturnal mammals. FAA observed following timed food restriction is likely the result of an additive effect of distinct regulatory pathways activated by energetic challenges and food reward.

Diurnal and circadian regulation of reward-related neurophysiology and behavior

Here, we review work over the past two decades that has indicated drug reward is modulated by the circadian system that generates daily (i.e., 24h) rhythms in physiology and behavior. Specifically, drug-self administration, psychomotor stimulant-induced conditioned place preference, and locomotor sensitization vary widely across the day in various species. These drug-related behavioral rhythms are associated with rhythmic neural activity and dopaminergic signaling in the mesocorticolimbic pathways, with a tendency toward increased activity during the species typical wake period. While the mechanisms responsible for such cellular rhythmicity remain to be fully identified, circadian clock genes are expressed in these brain areas and can function locally to modulate both dopaminergic neurotransmission and drug-associated behavior. In addition, neural and endocrine inputs to these brain areas contribute to cellular and reward-related behavioral rhythms, with the medial prefrontal cortex playing a pivotal role. Acute or chronic administration of drugs of abuse can also alter clock gene expression in reward-related brain regions. Emerging evidence suggests that drug craving in humans is under a diurnal regulation and that drug reward may be influenced by clock gene polymorphisms. These latter findings, in particular, indicate that the development of therapeutic strategies to modulate the circadian influence on drug reward may prove beneficial in the treatment of substance abuse disorders.

Impact of nutrients on circadian rhythmicity

The suprachiasmatic nucleus (SCN) in the mammalian hypothalamus functions as an endogenous pacemaker that generates and maintains circadian rhythms throughout the body. Next to this central clock, peripheral oscillators exist in almost all mammalian tissues. Whereas the SCN is mainly entrained to the environment by light, peripheral clocks are entrained by various factors, of which feeding/fasting is the most important. Desynchronization between the central and peripheral clocks by, for instance, altered timing of food intake can lead to uncoupling of peripheral clocks from the central pacemaker and is, in humans, related to the development of metabolic disorders, including obesity and Type 2 diabetes. Diets high in fat or sugar have been shown to alter circadian clock function. This review discusses the recent findings concerning the influence of nutrients, in particular fatty acids and glucose, on behavioral and molecular circadian rhythms and will summarize critical studies describing putative mechanisms by which these nutrients are able to alter normal circadian rhythmicity, in the SCN, in non-SCN brain areas, as well as in peripheral organs. As the effects of fat and sugar on the clock could be through alterations in energy status, the role of specific nutrient sensors will be outlined, as well as the molecular studies linking these components to metabolism. Understanding the impact of specific macronutrients on the circadian clock will allow for guidance toward the composition and timing of meals optimal for physiological health, as well as putative therapeutic targets to regulate the molecular clock.

Comprehensive mapping of regional expression of the clock protein PERIOD2 in rat forebrain across the 24-h day

In mammals, a light-entrainable clock located in the suprachiasmatic nucleus (SCN) regulates circadian rhythms by synchronizing oscillators throughout the brain and body. Notably, the nature of the relation between the SCN clock and subordinate oscillators in the rest of the brain is not well defined. We performed a high temporal resolution analysis of the expression of the circadian clock protein PERIOD2 (PER2) in the rat forebrain to characterize the distribution, amplitude and phase of PER2 rhythms across different regions. Eighty-four LEW/Crl male rats were entrained to a 12-h: 12-h light/dark cycle, and subsequently perfused every 30 min across the 24-h day for a total of 48 time-points. PER2 expression was assessed with immunohistochemistry and analyzed using automated cell counts. We report the presence of PER2 expression in 20 forebrain areas important for a wide range of motivated and appetitive behaviors including the SCN, bed nucleus, and several regions of the amygdala, hippocampus, striatum, and cortex. Eighteen areas displayed significant PER2 rhythms, which peaked at different times of day. Our data demonstrate a previously uncharacterized regional distribution of rhythms of a clock protein expression in the brain that provides a sound basis for future studies of circadian clock function in animal models of disease.

Time of day influences the voluntary intake and behavioral response to methamphetamine and food reward

The circadian timing system influences a vast array of behavioral responses. Substantial evidence indicates a role for the circadian system in regulating reward processing. Here we explore time of day effects on drug anticipation, locomotor activity, and voluntary methamphetamine (MA) and food intake in animals with ad libitum food access. We compared responses to drug versus a palatable treat during their normal sleep times in early day (zeitgeber time (ZT) 0400) or late day (ZT 1000). In the first study, using a between-subjects design, mice were given daily 1-h access to either peanut butter (PB-Alone) or to a low or high concentration of MA mixed in PB (MA+PB). In study 2, we repeated the experiment using a within-subjects design in which mice could choose between PB-Alone and MA+PB at either ZT 0400 or 1000. In study 3, the effects of MA-alone were investigated by evaluating anticipatory activity preceding exposure to nebulized MA at ZT 0400 vs. ZT 1000. Time of day effects were observed for both drug and palatable treat, such that in the between groups design, animals showed greater intake, anticipatory activity, and post-ingestional activity in the early day. Furthermore, there were differences among mice in the amount of MA ingested but individuals were self-consistent in their daily intake. The results for the within-subjects experiment also revealed robust individual differences in preference for MA+PB or PB-Alone. Interestingly, time of day effects on intake were observed only for the preferred substance. Anticipatory activity preceding administration of MA by nebulization was also greater at ZT 0400 than ZT 1000. Finally, pharmacokinetic response to MA administered intraperitoneally did not vary as a function of time of administration. The results indicate that time of day is an important variable mediating the voluntary intake and behavioral effects of reinforcers.

Differential responses of circadian Per2 expression rhythms in discrete brain areas to daily injection of methamphetamine and restricted feeding in rats

Behavioral rhythms induced by methamphetamine (MAP) and daily restricted feeding (RF) in rats are independent of the circadian pacemaker in the suprachiasmatic nucleus (SCN), and have been regarded to share a common oscillatory mechanism. In the present study, in order to examine the responses of brain oscillatory systems to MAP and RF, circadian rhythms in clock gene, Period2, expression were measured in several brain areas in rats. Transgenic rats carrying a bioluminescence reporter of Period2-dLuciferase were subjected to either daily injection of MAP or RF of 2 h at a fixed time of day for 14 days. As a result, spontaneous movement and wheel-running activity were greatly enhanced following MAP injection and prior to daily meal under RF. Circadian Per2 rhythms were measured in the cultured brain tissues containing one of the following structures: the olfactory bulb; caudate-putamen; parietal cortex; substantia nigra; and SCN. Except for the SCN, the circadian Per2 rhythms in the brain tissues were significantly phase-delayed by 1.9 h on average in MAP-injected rats as compared with the saline-controls. On the other hand, the circadian rhythms outside the SCN were significantly phase-advanced by 6.3 h on average in rats under RF as compared with those under ad libitum feeding. These findings indicate that the circadian rhythms in specific brain areas of the central dopaminergic system respond differentially to MAP injection and RF, suggesting that different oscillatory mechanisms in the brain underlie the MAP-induced behavior and pre-feeding activity under RF.

Ghrelin-deficient mice have fewer orexin cells and reduced cFOS expression in the mesolimbic dopamine pathway under a restricted feeding paradigm

Ghrelin is an orexigenic stomach peptide previously found to be important for the full display of anticipatory locomotor activity and hypothalamic neuronal activation that precedes a daily scheduled meal in mice. Ghrelin is also important for food-related motivation and seems to have direct effects in the mesocorticolimbic dopamine reward system. Here we hypothesized that neuronal activation in reward-related areas in anticipation of a scheduled meal could be mediated by elevated ghrelin induced by scheduled feeding, and therefore this would be attenuated in ghrelin receptor knock-out (GHSR KO) animals. We found that this was indeed the case for the ventral tegmental area and the shell, but not the core, of the nucleus accumbens. In addition, our results show a reduction in the proportion of activated orexin-immunoreactive (IR) neurons in GHSR KO animals in anticipation of the scheduled meal in comparison to the proportion of activated orexin neurons in wild type (WT) mice. Interestingly we observed that both GHSR and ghrelin KO mice had fewer orexin-IR cells than their WT littermates suggesting that lack of ghrelin or sensitivity to ghrelin may play a role in the development of the orexin system. Our data also suggest that ghrelin may mediate food anticipation, in part, by stimulating both the orexin system and the mesolimbic reward system.

Daily scheduled high fat meals moderately entrain behavioral anticipatory activity, body temperature, and hypothalamic c-Fos activation

When fed in restricted amounts, rodents show robust activity in the hours preceding expected meal delivery. This process, termed food anticipatory activity (FAA), is independent of the light-entrained clock, the suprachiasmatic nucleus, yet beyond this basic observation there is little agreement on the neuronal underpinnings of FAA. One complication in studying FAA using a calorie restriction model is that much of the brain is activated in response to this strong hunger signal. Thus, daily timed access to palatable meals in the presence of continuous access to standard chow has been employed as a model to study FAA in rats. In order to exploit the extensive genetic resources available in the murine system we extended this model to mice, which will anticipate rodent high fat diet but not chocolate or other sweet daily meals (Hsu, Patton, Mistlberger, and Steele; 2010, PLoS ONE e12903). In this study we test additional fatty meals, including peanut butter and cheese, both of which induced modest FAA. Measurement of core body temperature revealed a moderate preprandial increase in temperature in mice fed high fat diet but entrainment due to handling complicated interpretation of these results. Finally, we examined activation patterns of neurons by immunostaining for the immediate early gene c-Fos and observed a modest amount of entrainment of gene expression in the hypothalamus of mice fed a daily fatty palatable meal.

Ghrelin mediates anticipation to a palatable meal in rats

Food anticipatory activity (FAA) is displayed in rats when access to food is restricted to a specific time frame of their circadian phase, a behavior thought to reflect both hunger and the motivation to eat. Rats also display FAA in a feeding schedule with ad libitum access to normal chow, but limited availability of a palatable meal, which is thought to involve mainly motivational aspects. The orexigenic hormone ghrelin has been implicated in FAA in rodents with restricted access to chow. Because ghrelin plays an important role not only in the control of food intake, but also in reward, we sought to determine the role of ghrelin in anticipation to a palatable meal. Plasma ghrelin levels of non-restricted rats that anticipated chocolate correlated positively with FAA and were increased compared with chow-fed control rats. Furthermore, centrally injected ghrelin increased, whereas an antagonist of the ghrelin receptor decreased, the anticipation to chocolate. Therefore, we hypothesize that central ghrelin signaling is able to mediate the motivational drive to eat.

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.

A meta-analysis of caloric restriction gene expression profiles to infer common signatures and regulatory mechanisms

Caloric restriction, a reduction in calorie intake without malnutrition, retards age-related degeneration and extends lifespan in several organisms. CR induces multiple changes, yet its underlying mechanisms remain poorly understood. In this work, we first performed a meta-analysis of microarray CR studies in mammals and identified genes and processes robustly altered due to CR. Our results reveal a complex array of CR-induced changes and we re-identified several genes and processes previously associated with CR, such as growth hormone signalling, lipid metabolism and immune response. Moreover, our results highlight novel associations with CR, such as retinol metabolism and copper ion detoxification, as well as hint of a strong effect of CR on circadian rhythms that in turn may contribute to metabolic changes. Analyses of our signatures by integrating co-expression data, information on genetic mutants, and transcription factor binding site analysis revealed candidate regulators of transcriptional modules in CR. Our results hint at a transcriptional module involved in sterol metabolism regulated by Srebf1. A putative regulatory role of Ppara was also identified. Overall, our conserved molecular signatures of CR provide a comprehensive picture of CR-induced changes and help understand its regulatory mechanisms.

Sucrose modifies c-fos mRNA expression in the brain of rats maintained on feeding schedules

Food intake is regulated according to circadian activity, metabolic needs and the hedonic value of food. Rodents placed on a fixed feeding schedule show behavioral and physiological anticipation of mealtime referred to as food-anticipatory activity (FAA). FAA is driven by the food-entrainable oscillator (FEO), whose anatomical substrate is not yet known. Recent data have shown that restricted feeding schedules for regular chow and daily limited access to palatable food in free-feeding rats activate distinct brain regions during FAA. The combination of a deprivation regimen and scheduled access to palatable food may give rise to a more global anticipatory mechanism because the temporal cycles of energy balance would be strongly modulated by the incentive properties of palatable food; however, the neuronal response to this combined treatment is not yet known. The present study investigated how adding palatable sucrose to feeding schedules affects the pattern of brain c-fos mRNA expression during FAA (0-3 h) and 1 h following feeding. The rats maintained on scheduled chow access increased their daily chow intake, while the rats maintained on scheduled sucrose and chow mainly increased their daily sucrose intake. Adding sucrose to scheduled feeding displaced c-fos mRNA expression from the dorsomedial and paraventricular hypothalamic nuclei and posterior lateral hypothalamus (LH) to the prefrontal cortex, lateral septum, nucleus accumbens and anterior LH. During refeeding, the rats on scheduled sucrose demonstrated higher activation of the nucleus of the solitary tract. The present results suggest that palatable sucrose combined with restricted feeding schedules activate a distinct neuronal network compared to neuronal activation produced by scheduled access to regular chow. These data provide evidence that the brain may contain different food-oscillatory systems and that food palatability may shift the neuronal activity from the medial hypothalamus to the limbic and reward-related areas even at the negative metabolic state.

Daily morphine injection and withdrawal disrupt 24-h wheel running and PERIOD2 expression patterns in the rat limbic forebrain

Symptoms of opiate withdrawal include disturbances in circadian rhythms. We examined in male Wistar rats (n=48) the effects of a daily, mid-morning morphine injection (5-40 mg/kg, i.p.) and its withdrawal on 24-h wheel-running activity and on the expression of the clock protein, PERIOD2 (PER2), in the suprachiasmatic nucleus (SCN), oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central amygdala (CEA), and dorsal striatum. Rats were killed over 2 days at 10, 22, 46, and 58 h after the last daily morphine injection at zeitgeber times (ZT) 1 or ZT13. Daily morphine injections and their withdrawal suppressed nighttime wheel running, but did not entrain any increase in activity in advance of the injection. Neither morphine injection nor its withdrawal affected PER2 expression in the SCN, whereas the normal daily peaks of PER2 in the BNSTov, CEA, and dorsal striatum were blunted both during morphine administration and its withdrawal. Treatment with a dopaminergic agonist (the D2/3 agonist, quinpirole, 1.0 mg/kg) or a noradrenergic agonist (alpha2 agonist, clonidine, 0.1 mg/kg) in morphine withdrawal did not restore normal PER2 patterns in each affected region; however, both quinpirole and clonidine themselves altered normal daily PER2 expression patterns in morphine-naive rats. These findings confirm and extend previous observations that opiates disrupt daily patterns of clock gene expression in the limbic forebrain. Furthermore, catecholaminergic drugs, which have been previously found to alleviate symptoms of opiate withdrawal, do not alleviate the effects of morphine withdrawal on PER2, but do modulate daily patterns of PER2 expression in saline controls.

Global depletion of dopamine using intracerebroventricular 6-hydroxydopamine injection disrupts normal circadian wheel-running patterns and PERIOD2 expression in the rat forebrain

Normal circadian rhythms of behavior are disrupted in disorders involving the dopamine (DA) system, such as Parkinson's disease. We have reported previously using unilateral injections of the catecholamine toxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle that DA signaling regulates daily expression of the clock protein, PERIOD2 (PER2), in the dorsal striatum of the rat. In the present study, we made widespread lesions of DA fibers using large injections of 6-OHDA into the third ventricle to determine the involvement of DA in normal daily rhythms of wheel-running activity and PER2 patterns in the suprachiasmatic nucleus (SCN) and several regions of the limbic forebrain. Rats injected with 6-OHDA and housed in constant darkness were less active in the wheel and showed a disorganized pattern of activity in which wheel running was not confined to a specific phase over 24 h. The 6-OHDA injection had no effect on the daily PER2 pattern in the SCN, but blunted the normal rise in PER2 in the dorsal striatum. 6-OHDA also blunted PER2 expression in the periventricular nucleus of the hypothalamus, a region in which a daily PER2 pattern has not been previously reported in male rats, and in the oval nucleus of the bed nucleus of the stria terminalis, but not in the central nucleus of the amygdala. These results indicate that DA plays a prominent role in regulating circadian activity at both behavioral and molecular levels.

The adjustment and manipulation of biological rhythms by light, nutrition, and abused drugs

Daily restricted feeding entrains the circadian rhythm of mouse clock gene expression in the central nervous system, excluding the suprachiasmatic nucleus (SCN), as well as in the peripheral tissues such as the liver, lung, and heart. In addition to entrainment of the clock genes, daily restricted feeding induces a locomotor activity increase 2-3h before the restricted feeding time initiates. The increase in activity is called the food-anticipatory activity (FAA). In addition to FAA, daily restricted feeding can also entrain peripheral circadian clocks in other organs such as liver, lung, and heart. This type of oscillator is called the food-entrainable peripheral oscillator (FEPO). At present, the mechanisms for restricted feeding-induced entrainment of locomotor activity (FAA) and/or peripheral clock (FEPO) are still unknown. In this review, we describe the role of the central nervous system and peripheral tissues in FAA performance and also in the entrainment of clock gene expression. In addition, the mechanism for entrainment of circadian oscillators by the abuse of drugs, such as methamphetamine, is discussed.

Food for thoughts: feeding time and hormonal secretion

Many daily cycles are imposed on us by our environment, such as alternating days and nights, temperature fluctuations or rhythms in food availability. When food is accessible every day at the same time, animals will adapt their physiology and behaviour to match the daily meal. They will anticipate the access to food by waking up and being active in the hours prior to feeding, foraging for food. Adaptation of physiology to changing conditions of food availability is not only evident at the behavioural level, but also for hormonal systems. Thus, corticosteroids, melatonin, leptin/ghrelin, insulin/glucagon, orexins and thyroid hormones, which show rhythmic profiles of secretion in ad libitum feeding conditions, are sensitive to increase and/or depletion in energy supplies and will be influenced when food sources are limited or available at unusual times. The present review reports the influence of restricted feeding schedules on secretion profiles of diverse hormones compared to normal ad libitum feeding conditions in rodents. In the end, the interplay between these systems and their response to environmental challenges will allow the animal to maintain their fitness for survival.

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.

The food-entrainable oscillator: a network of interconnected brain structures entrained by humoral signals?

Food is critical for all animal species. Its temporal availability is a relevant signal for organizing behavioral and physiological parameters. When food is restricted to a few hours per day, rats, mice, and other mammals exhibit anticipatory activity before mealtime (food-anticipatory activity). There is considerable evidence suggesting that this anticipation is mediated by a food-entrainable oscillator (FEO) with circadian properties, but located outside the suprachiasmatic nucleus of the hypothalamus (the light-entrainable oscillator). However, the locus of the FEO as well as the mechanisms by which food entrainment occurs is unclear. In this review, we summarize data about the potential input pathways to the FEO and propose a model for understanding it as a network of interconnected brain structures entrained by fluctuation of different humoral signals.

Metabolism and circadian rhythms--implications for obesity

Obesity has become a serious public health problem and a major risk factor for the development of illnesses, such as insulin resistance and hypertension. Human homeostatic systems have adapted to daily changes in light and dark in a way that the body anticipates the sleep and activity periods. Mammals have developed an endogenous circadian clock located in the suprachiasmatic nuclei of the anterior hypothalamus that responds to the environmental light-dark cycle. Similar clocks have been found in peripheral tissues, such as the liver, intestine, and adipose tissue, regulating cellular and physiological functions. The circadian clock has been reported to regulate metabolism and energy homeostasis in the liver and other peripheral tissues. This is achieved by mediating the expression and/or activity of certain metabolic enzymes and transport systems. In return, key metabolic enzymes and transcription activators interact with and affect the core clock mechanism. In addition, the core clock mechanism has been shown to be linked with lipogenic and adipogenic pathways. Animals with mutations in clock genes that disrupt cellular rhythmicity have provided evidence for the relationship between the circadian clock and metabolic homeostasis. In addition, clinical studies in shift workers and obese patients accentuate the link between the circadian clock and metabolism. This review will focus on the interconnection between the circadian clock and metabolism, with implications for obesity and how the circadian clock is influenced by hormones, nutrients, and timed meals.

The suprachiasmatic nucleus participates in food entrainment: a lesion study

Daily feeding schedules entrain temporal patterns of behavior, metabolism, neuronal activity and clock gene expression in several brain areas and periphery while the suprachiasmatic nucleus (SCN), the biological clock, remains coupled to the light/dark cycle. Because bilateral lesions of the SCN do not abolish food entrained behavioral and hormonal rhythms it is suggested that food entrained and light entrained systems are independent of each other. Special circumstances indicate a possible interaction between the light and the food entrained systems and indicate modulation of SCN activity by restricted feeding. This study explores the influence of the SCN on food entrained rhythms. Food entrained temporal profiles of behavior, core temperature, corticosterone and glucose, as well as Fos and PER1 immunoreactivity in the hypothalamus and corticolimbic structures were explored in rats bearing bilateral SCN lesions (SCNX). In SCNX rats food anticipatory activity and the food entrained temperature and corticosterone increase were expressed with earlier onset and higher values than in intact controls. Glucose levels were lower in SCNX rats in all time points and SCNX rats anticipation to a meal induced higher c-Fos positive neurons in the hypothalamus, while a decreased c-Fos response was observed in corticolimbic structures. SCNX rats also exhibited an upregulation of the PER1 peak in hypothalamic structures, especially in the dorsomedial hypothalamic nucleus (DMH), while in some limbic structures PER1 rhythmicity was dampened. The present results indicate that the SCN participates actively during food entrainment modulating the response of hypothalamic and corticolimbic structures, resulting in an increased anticipatory response.

Behavioral and hormonal regulation of expression of the clock protein, PER2, in the central extended amygdala

PER2, a key molecular component of the mammalian circadian clock, is expressed rhythmically in many brain areas and peripheral tissues in mammals. Here we review findings from our work on the nature and regulation of rhythms of expression of PER2 in two anatomically and neurochemically defined subregions of the central extended amygdala, the oval nucleus of the bed nucleus of the stria terminalis (BNSTov) and the central nucleus of the amygdala (CEA). Daily rhythms in the expression of PER2 in these regions are coupled to those of the master circadian pacemaker, the suprachiasmatic nucleus (SCN) but, importantly, they are sensitive to homeostatic perturbations and to hormonal states that directly influence motivated behavior.

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.

Food-entrainable circadian oscillators in the brain

Circadian rhythms in mammalian behaviour and physiology rely on daily oscillations in the expression of canonical clock genes. Circadian rhythms in clock gene expression are observed in the master circadian clock, the suprachiasmatic nucleus but are also observed in many other brain regions that have diverse roles, including influences on motivational and emotional state, learning, hormone release and feeding. Increasingly, important links between circadian rhythms and metabolism are being uncovered. In particular, restricted feeding (RF) schedules which limit food availability to a single meal each day lead to the induction and entrainment of circadian rhythms in food-anticipatory activities in rodents. Food-anticipatory activities include increases in core body temperature, activity and hormone release in the hours leading up to the predictable mealtime. Crucially, RF schedules and the accompanying food-anticipatory activities are also associated with shifts in the daily oscillation of clock gene expression in diverse brain areas involved in feeding, energy balance, learning and memory, and motivation. Moreover, lesions of specific brain nuclei can affect the way rats will respond to RF, but have generally failed to eliminate all food-anticipatory activities. As a consequence, it is likely that a distributed neural system underlies the generation and regulation of food-anticipatory activities under RF. Thus, in the future, we would suggest that a more comprehensive approach should be taken, one that investigates the interactions between multiple circadian oscillators in the brain and body, and starts to report on potential neural systems rather than individual and discrete brain areas.

Neurogenetics of food anticipation

Circadian clocks enable the organisms to anticipate predictable cycling events in the environment. The mechanisms of the main circadian clock, localized in the suprachiasmatic nuclei of the hypothalamus, involve intracellular autoregulatory transcriptional loops of specific genes, called clock genes. In the suprachiasmatic clock, circadian oscillations of clock genes are primarily reset by light, thus allowing the organisms to be in phase with the light-dark cycle. Another circadian timing system is dedicated to preparing the organisms for the ongoing meal or food availability: the so-called food-entrainable system, characterized by food-anticipatory processes depending on a circadian clock whose location in the brain is not yet identified with certainty. Here we review the current knowledge on food anticipation in mice lacking clock genes or feeding-related genes. The food-entrainable clockwork in the brain is currently thought to be made of transcriptional loops partly divergent from those described in the light-entrainable suprachiasmatic nuclei. Possible confounding effects associated with behavioral screening of meal anticipation in mutant mice are also discussed.

Motivational Modulation of Rhythms of the Expression of the Clock Protein PER2 in the Limbic Forebrain

Key molecular components of the mammalian circadian clock are expressed rhythmically in many brain areas and peripheral tissues in mammals. Here we review findings from our work on rhythms of expression of the clock protein Period2 (PER2) in four regions of the limbic forebrain known to be important in the regulation of motivational and emotional states. These regions include the oval nucleus of the bed nucleus of the stria terminalis (BNSTov), the central nucleus of the amygdala (CEA), the basolateral amygdala (BLA), and the dentate gyrus (DG). Daily rhythms in the expression of PER2 in these regions are controlled by the master circadian pacemaker, the suprachiasmatic nucleus (SCN), but, importantly, they are also sensitive to homeostatic perturbations and to hormonal states that directly influence motivated behavior. Thus, circadian information from the SCN and homeostatic signals are integrated in these regions of the limbic forebrain to affect the temporal organization of motivational and emotional processes.

The dorsomedial hypothalamic nucleus is not necessary for food-anticipatory circadian rhythms of behavior, temperature or clock gene expression in mice

Circadian rhythms in mammals are regulated by a light-entrainable circadian pacemaker in the hypothalamic suprachiasmatic nucleus and food-entrainable oscillators located elsewhere in the brain and body. The dorsomedial hypothalamic nucleus (DMH) has been proposed to be the site of oscillators driving food-anticipatory circadian rhythms, but this is controversial. To further evaluate this hypothesis, we measured clock gene, temperature and activity rhythms in intact and DMH-ablated mice. A single 4-h midday feeding after an overnight fast induced mPer1 and mPer2 mRNA expression in the DMH, arcuate nucleus, nucleus of the solitary tract and area postrema, and reset daily rhythms of mPer1, mPer2 and mBMAL1 in the DMH, arcuate and neocortex. These rhythms persisted during 2 days of food deprivation after 12 days of scheduled daytime feeding. Acute induction of DMH mPer1 and mPer2 was N-methyl-D-aspartate (NMDA) receptor-dependent, whereas rhythmic expression after 6 days of restricted feeding was not. Thermal DMH lesions did not affect acute induction or rhythmic expression of clock genes in other brain regions in response to scheduled daytime feeding. DMH lesions attenuated mean daily activity levels and nocturnality but did not affect food-anticipatory rhythms of activity and body temperature in either light-dark or constant darkness. These results confirm that the DMH and other brain regions express circadian clock gene rhythms sensitive to daytime feeding schedules, but do not support the hypothesis that DMH oscillations drive food-anticipatory behavioral or temperature rhythms.

Brain Clocks: From the Suprachiasmatic Nuclei to a Cerebral Network

Circadian timing affects almost all life’s processes. It not only dictates when we sleep, but also keeps every cell and tissue working under a tight temporal regimen. The daily variations of physiology and behavior are controlled by a highly complex system comprising of a master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus, extra-SCN cerebral clocks, and peripheral oscillators. Here are presented similarities and differences in the molecular mechanisms of the clock machinery between the primary SCN clock and extra-SCN brain clocks. Diversity of secondary clocks in the brain, their specific sensitivities to time-giving cues, as their differential coupling to the master SCN clock, may allow more plasticity in the ability of the circadian timing system to integrate a wide range of temporal information. Furthermore, it raises the possibility that pathophysiological alterations of internal timing that are deleterious for health may result from internal desynchronization within the network of cerebral clocks.

Timed restricted feeding restores the rhythms of expression of the clock protein, Period2, in the oval nucleus of the bed nucleus of the stria terminalis and central nucleus of the amygdala in adrenalectomized rats

Feeding schedules that limit food availability to a set time of day are powerful synchronizers of the rhythms of expression of the circadian clock protein Period 2 (PER2) in the limbic forebrain in rats. Little is known, however, about the mechanisms that mediate the effect of such timed restricted feeding (TRF) schedules on the expression of PER2. Adrenal glucocorticoids have been implicated in the circadian regulation of clock genes expression in peripheral tissues as well as in the control of the rhythms of expression of PER2 in certain limbic forebrain regions, such as the oval nucleus of the bed nucleus of the stria terminalis (BNSTov) and central nucleus of the amygdala (CEA) in rats. To study the possible involvement of glucocorticoids in the regulation of PER2 expression by TRF, we assessed the effect of adrenalectomy on TRF-entrained PER2 rhythms in the limbic forebrain in rats. Adrenalectomy selectively abolished the rhythms of PER2 in the BNSTov and CEA in normally fed rats, as previously shown, but had no effect on TRF-entrained PER2 rhythms in the same structures. These findings show that the effect of TRF on PER2 rhythms in the limbic forebrain is independent of adrenal glucocorticoids and demonstrate that the involvement of glucocorticoids in the regulation PER2 rhythms in the limbic forebrain is not only region specific, as previously shown, but also state dependent.

Expression of Fos during sham sucrose intake in rats with central gustatory lesions

For humans and rodents, ingesting sucrose is rewarding. This experiment tested the prediction that the neural activity produced by sapid sucrose reaches reward systems via projections from the pons through the limbic system. Gastric cannulas drained ingested fluid before absorption. For 10 days, the rats alternated an hour of this sham ingestion between sucrose and water. On the final test day, half of them sham drank water and the other half 0.6 M sucrose. Thirty minutes later, the rats were killed and their brains immunohistochemically stained for Fos. The groups consisted of controls and rats with excitotoxic lesions in the gustatory thalamus (TTA), the medial (gustatory) parabrachial nucleus (PBN), or the lateral (visceral afferent) parabrachial nucleus. In controls, compared with water, sham ingesting sucrose produced significantly more Fos-positive neurons in the nucleus of the solitary tract, PBN, TTA, and gustatory cortex (GC). In the ventral forebrain, sucrose sham licking increased Fos in the bed nucleus of the stria terminalis, central nucleus of amygdala, and the shell of nucleus accumbens. Thalamic lesions blocked the sucrose effect in GC but not in the ventral forebrain. After lateral PBN lesions, the Fos distributions produced by distilled H(2)O or sucrose intake did not differ from controls. Bilateral medial PBN damage, however, eliminated the sucrose-induced Fos increase not only in the TTA and GC but also in the ventral forebrain. Thus ventral forebrain areas associated with affective responses appear to be activated directly by PBN gustatory neurons rather than via the thalamocortical taste system.

Expectancy for food or expectancy for chocolate reveals timing systems for metabolism and reward

The clock gene protein Per 1 (PER1) is expressed in several brain structures and oscillates associated with the suprachiasmatic nucleus (SCN). Restricted feeding schedules (RFS) induce anticipatory activity and impose daily oscillations of c-Fos and clock proteins in brain structures. Daily access to a palatable treat (chocolate) also elicits anticipatory activity and induces c-Fos expression mainly in corticolimbic structures. Here the influence of daily access to food or chocolate was explored by the analysis of the oscillatory patterns of PER1 in hypothalamic and corticolimbic structures. Wistar rats were exposed to RFS or to daily access to chocolate for 3 weeks. Persistence of food or chocolate entrained rhythms was determined 8 days after cessation of the feeding protocols. RFS and chocolate induced a phase shift in PER1 rhythmicity in corticolimbic structures with peak values at zeitgeber time 12 and a higher amplitude in the chocolate group. Both RFS and chocolate groups showed an upregulation of PER1 in the SCN. Food and chocolate entrained rhythms persisted for 8 days in behavior and in PER1 expression in the dorsomedial hypothalamic nucleus, accumbens, prefrontal cortex and central amygdala. The present data demonstrate the existence of different oscillatory systems in the brain that can be activated by entrainment to metabolic stimuli or to reward and suggest the participation of PER1 in both entraining pathways. Persistence and amplification of PER1 oscillations in structures associated with reward suggest that this oscillatory process is fundamental to food addictive behavior.

Region-specific modulation of PER2 expression in the limbic forebrain and hypothalamus by nighttime restricted feeding in rats

Feeding schedules that restrict food access to a predictable daytime meal induce in rodents food-anticipatory behaviors, changes in physiological rhythms and shifts in the rhythm of clock gene expression in the brain and periphery. However, little is known about the effects of nighttime restricted feeding. Previously, we showed that daytime restricted access to a highly palatable complete meal replacement, Ensure Plus (Ensure), shifts the rhythm of expression of the clock protein PER2 in limbic forebrain areas including the oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central nucleus of the amygdala (CEA), basolateral amygdala (BLA) and dentate gyrus (DG), and induces a rhythm in the dorsomedial hypothalamic nucleus (DMH) in food deprived (restricted feeding), but not free-fed rats (restricted treat). In the present study we investigated the effects of nighttime restricted feeding (Ensure only, 2 h/night) and nighttime restricted treats (Ensure 2 h/night+free access to chow) in order to determine whether these effects were dependent on the time of day the meal was provided. We found that nighttime restricted feeding, like daytime restricted feeding, shifted the rhythm of PER2 expression in the BNSTov and CEA and peak expression was observed approximately 12 h after the mealtime. Also consistent with previous work, nighttime restricted feeding induced a rhythm of PER2 expression in the DMH and these effects occurred without affecting the rhythm in the suprachiasmatic nucleus (SCN). In contrast to previous work with daytime restricted feeding, nighttime restricted feeding had no effect on PER2 rhythms in the BLA and DG. Finally, nighttime restricted treats, as was the case for daytime restricted treats, had no effect on PER2 expression in any of the brain areas studied. The present results together with our previous findings show that the effect of restricted feeding on PER2 rhythms in the limbic forebrain and hypothalamus depend on a negative energy balance and vary as a function of time of day in a brain region-specific manner.

Unpredictable feeding schedules unmask a system for daily resetting of behavioural and metabolic food entrainment

Restricted feeding schedules (RFS) are a potent Zeitgeber that uncouples daily metabolic and clock gene oscillations in peripheral tissues from the suprachiasmatic nucleus (SCN), which remains entrained to the light-dark cycle. Under RFS, animals develop food anticipatory activity (FAA), characterized by arousal and increased locomotion. Food availability in nature is not precise, which suggests that animals need to adjust their food-associated activity on a daily basis. This study explored the capacity of rats to adjust to variable and unpredictable feeding schedules. Rats were exposed either to RFS with fixed daily meal (RF) or to a variable meal time (VAR) during the light phase. RF and VAR rats exhibited daily metabolic oscillations driven by the last meal event; however, VAR rats were not able to show a robust adjustment in the anticipating corticosterone peak. VAR rats were unable to exhibit FAA but exhibited a daily activation pattern in phase with the previous meal. In both groups the dorsomedial nucleus of the hypothalamus and arcuate nucleus, involved in energy balance, exhibited increased c-Fos expression 24 h after the last meal, while only RF rats exhibited low c-Fos expression in the SCN. Data show that metabolic and behavioural food-entrained rhythms can be reset on a daily basis; the two conditions elicit a similar hypothalamic response, while only the SCN is inhibited in rats exhibiting anticipatory activity. The variable feeding strategy uncovered a rapid (24-h basis) resetting mechanism for metabolism and general behaviour.

The dorsomedial hypothalamic nucleus is not necessary for the expression of circadian food-anticipatory activity in rats

Restricted daytime feeding generates food-anticipatory activity (FAA) by entrainment of a circadian pacemaker separate from the light-entrainable pacemaker located in the SCN. The dorsomedial hypothalamic nucleus (DMH) has been proposed as the site of food-entrainable oscillators critical for the expression of FAA, but another study found no effects of complete DMH ablation on FAA. To account for these different results, the authors examined methodological factors, including (1) cage configuration and feeding method and (2) use of social cues. Intact and DMH-ablated rats were maintained on one 4-h daily meal in the middle of the light period, using caging and feeding methods matching those of Gooley et al. (2006). Rats with partial or complete DMH ablation were less nocturnal during ad lib food access but exhibited normal FAA during restricted feeding, as quantified by FAA magnitude, ratios, latency to appearance, duration, and precision. To evaluate the use of social cues, intact rats naive to restricted-feeding schedules were food deprived for 72 h on 4 tests. Daytime activity increased during food deprivation, but the magnitude and waveform of this activity was not influenced by the presence of food-entrained rats exhibiting robust FAA in adjacent cages. Thus, hungry intact rats do not use social cues to anticipate a daily mealtime, suggesting that DMH-ablated rats do not anticipate meals by reacting to sounds from food-entrained intact rats in adjacent cabinets. These results confirm our previous finding that the DMH is not critical for normal expression of FAA in rats, and this observation is extended to food restriction methodologies used by other labs. The methodological differences that do underlie discrepant results remain unresolved, as does the location of food-entrainable oscillators, input pathways, and output pathways critical for FAA.