Food as circadian time cue for appetitive behavior

Circadian influences on feeding behavior

Feeding, like many other biological functions, displays a daily rhythm. This daily rhythmicity is controlled by the circadian timing system of which the central master clock is located in the hypothalamic suprachiasmatic nucleus (SCN). Other brain areas and tissues throughout the body also display rhythmic functions and contain the molecular clock mechanism known as peripheral oscillators. To generate the daily feeding rhythm, the SCN signals to different hypothalamic areas with the lateral hypothalamus, paraventricular nucleus and arcuate nucleus being the most prominent. With respect to the rewarding aspects of feeding behavior, the dopaminergic system is also under circadian influence. However the SCN projects only indirectly to the different reward regions, such as the ventral tegmental area where dopamine neurons are located. In addition, high palatable, high caloric diets have the potential to disturb the normal daily rhythms of physiology and have been shown to alter for example meal patterns. Around a meal several hormones and peptides are released that are also under circadian influence. For example, the release of postprandial insulin and glucagon-like peptide following a meal depend on the time of the day. Finally, we review the effect of deletion of different clock genes on feeding behavior. The most prominent effect on feeding behavior has been observed in Clock mutants, whereas deletion of Bmal1 and Per1/2 only disrupts the day-night rhythm, but not overall intake. Data presented here focus on the rodent literature as only limited data are available on the mechanisms underlying daily rhythms in human eating behavior.

The circadian molecular clock in the suprachiasmatic nucleus is necessary but not sufficient for fear entrainment

We show that nocturnal aversive stimuli presented to mice while they are eating and drinking outside of their safe nest can entrain circadian behaviors, leading to a shift toward daytime activity. We also show that the canonical molecular circadian clock is necessary for fear entrainment and that an intact molecular clockwork in the suprachiasmatic nucleus, the site of the central circadian pacemaker, is necessary but not sufficient to sustain fear entrainment of circadian rhythms. Our results demonstrate that entrainment of a circadian clock by cyclic fearful stimuli can lead to severely mistimed circadian behavior that persists even after the aversive stimulus is removed. Together, our findings support the interpretation that circadian and sleep symptoms associated with fear and anxiety disorders are, in part, the output of a fear-entrained clock, and provide a mechanistic insight into this clock.

Development of activity-based anorexia requires PKC-δ neurons in two central extended amygdala nuclei

Anorexia nervosa (AN) is a serious psychiatric disease, but the neural mechanisms underlying its development are unclear. A subpopulation of amygdala neurons, marked by expression of protein kinase C-delta (PKC-δ), has previously been shown to regulate diverse anorexigenic signals. Here, we demonstrate that these neurons regulate development of activity-based anorexia (ABA), a common animal model for AN. PKC-δ neurons are located in two nuclei of the central extended amygdala (EAc): the central nucleus (CeA) and oval region of the bed nucleus of the stria terminalis (ovBNST). Simultaneous ablation of CeAPKC-δ and ovBNSTPKC-δ neurons prevents ABA, but ablating PKC-δ neurons in the CeA or ovBNST alone is not sufficient. Correspondingly, PKC-δ neurons in both nuclei show increased activity with ABA development. Our study shows how neurons in the amygdala regulate ABA by impacting both feeding and wheel activity behaviors and support a complex heterogeneous etiology of AN.

AgRP neurons encode circadian feeding time

Food intake follows a predictable daily pattern and synchronizes metabolic rhythms. Neurons expressing agouti-related protein (AgRP) read out physiological energetic state and elicit feeding, but the regulation of these neurons across daily timescales is poorly understood. Using a combination of neuron dynamics measurements and timed optogenetic activation in mice, we show that daily AgRP-neuron activity was not fully consistent with existing models of homeostatic regulation. Instead of operating as a 'deprivation counter', AgRP-neuron activity primarily followed the circadian rest-activity cycle through a process that required an intact suprachiasmatic nucleus and synchronization by light. Imposing novel feeding patterns through time-restricted food access or periodic AgRP-neuron stimulation was sufficient to resynchronize the daily AgRP-neuron activity rhythm and drive anticipatory-like behavior through a process that required DMHPDYN neurons. These results indicate that AgRP neurons integrate time-of-day information of past feeding experience with current metabolic needs to predict circadian feeding time.

Astrocytic insulin receptor controls circadian behavior via dopamine signaling in a sexually dimorphic manner

Mammalian circadian clocks respond to feeding and light cues, adjusting internal rhythms with day/night cycles. Astrocytes serve as circadian timekeepers, driving daily physiological rhythms; however, it's unknown how they ensure precise cycle-to-cycle rhythmicity. This is critical for understanding why mistimed or erratic feeding, as in shift work, disrupts circadian physiology- a condition linked to type 2 diabetes and obesity. Here, we show that astrocytic insulin signaling sets the free-running period of locomotor activity in female mice and food entrainment in male mice. Additionally, ablating the insulin receptor in hypothalamic astrocytes alters cyclic energy homeostasis differently in male and female mice. Remarkably, the mutants exhibit altered dopamine metabolism, and the pharmacological modulation of dopaminergic signaling partially restores distinct circadian traits in both male and female mutant mice. Our findings highlight the role of astrocytic insulin-dopaminergic signaling in conveying time-of-feeding or lighting cues to the astrocyte clock, thus governing circadian behavior in a sex-specific manner.

Sweet treats before sleep disrupt the clock system and increase metabolic risk markers in healthy rats

AIM

Biological rhythms are endogenously generated natural cycles that act as pacemakers of different physiological mechanisms and homeostasis in the organism, and whose disruption increases metabolic risk. The circadian rhythm is not only reset by light but it is also regulated by behavioral cues such as timing of food intake. This study investigates whether the chronic consumption of a sweet treat before sleeping can disrupt diurnal rhythmicity and metabolism in healthy rats.

METHODS

For this, 32 Fischer rats were administered daily a low dose of sugar (160 mg/kg, equivalent to 2.5 g in humans) as a sweet treat at 8:00 a.m. or 8:00 p.m. (ZT0 and ZT12, respectively) for 4 weeks. To elucidate diurnal rhythmicity of clock gene expression and metabolic parameters, animals were sacrificed at different times, including 1, 7, 13, and 19 h after the last sugar dose (ZT1, ZT7, ZT13, and ZT19).

RESULTS

Increased body weight gain and higher cardiometabolic risk were observed when sweet treat was administered at the beginning of the resting period. Moreover, central clock and food intake signaling genes varied depending on snack time. Specifically, the hypothalamic expression of Nampt, Bmal1, Rev-erbα, and Cart showed prominent changes in their diurnal expression pattern, highlighting that sweet treat before bedtime disrupts hypothalamic control of energy homeostasis.

CONCLUSIONS

These results show that central clock genes and metabolic effects following a low dose of sugar are strongly time-dependent, causing higher circadian metabolic disruption when it is consumed at the beginning of the resting period, that is, with the late-night snack.

Leptin receptor neurons in the dorsomedial hypothalamus input to the circadian feeding network

Salient cues, such as the rising sun or availability of food, entrain biological clocks for behavioral adaptation. The mechanisms underlying entrainment to food availability remain elusive. Using single-nucleus RNA sequencing during scheduled feeding, we identified a dorsomedial hypothalamus leptin receptor-expressing (DMHLepR) neuron population that up-regulates circadian entrainment genes and exhibits calcium activity before an anticipated meal. Exogenous leptin, silencing, or chemogenetic stimulation of DMHLepR neurons disrupts the development of molecular and behavioral food entrainment. Repetitive DMHLepR neuron activation leads to the partitioning of a secondary bout of circadian locomotor activity that is in phase with the stimulation and dependent on an intact suprachiasmatic nucleus (SCN). Last, we found a DMHLepR neuron subpopulation that projects to the SCN with the capacity to influence the phase of the circadian clock. This direct DMHLepR-SCN connection is well situated to integrate the metabolic and circadian systems, facilitating mealtime anticipation.

The circadian molecular clock in the suprachiasmatic nucleus is necessary but not sufficient for fear entrainment in the mouse

Nocturnal aversive stimuli presented to mice during eating and drinking outside of their safe nest can entrain circadian behaviors, leading to a shift toward daytime activity. We show that the canonical molecular circadian clock is necessary for fear entrainment and that an intact molecular clockwork in the suprachiasmatic nucleus (SCN), the site of the central circadian pacemaker, is necessary but not sufficient to sustain fear entrainment of circadian rhythms. Our results demonstrate that entrainment of a circadian clock by cyclic fearful stimuli can lead to severely mistimed circadian behavior that persists even after the aversive stimulus is removed. Together, our results support the interpretation that circadian and sleep symptoms associated with fear and anxiety disorders may represent the output of a fear-entrained clock.

One-Sentence Summary

Cyclic fearful stimuli can entrain circadian rhythms in mice, and the molecular clock within the central circadian pacemaker is necessary but not sufficient for fear-entrainment.

Effects of the timing of protein infusion on the daily rhythms of milk synthesis and plasma hormones and metabolites in dairy cows

Milk synthesis exhibits a daily rhythm that is modified by the timing of feed intake. However, it is unknown how specific nutrients entrain this daily rhythm. Amino acids have an important role in milk synthesis, and may have a role in entrainment of mammary circadian rhythms. The objective of this study was to determine the effects of intestinally absorbed protein on daily rhythms of milk and milk component synthesis and key plasma hormones and metabolites. Nine lactating Holstein cows were assigned to 1 of 3 treatment sequences in a 3 × 3 Latin square. Treatments included abomasal infusions of 500 g/d of sodium caseinate either continuously throughout the day (CON), for 8 h/d from 0900 to 1700 h (DAY), or for 8 h/d from 2100 to 0500 h (NGT). Cows were milked every 6 h during the final 8 d of each period. A 24-h rhythm was fit to data using cosine analysis and the amplitude and acrophase were determined. Night infusion of protein decreased the daily milk yield and milk protein yield by 8.2% and 9.2%, respectively. Milk fat yield was increased 5.5% by DAY and milk fat concentration was increased 8.8% by NGT. Milk yield exhibited a daily rhythm in all treatments, with NGT increasing the amplitude of the daily rhythm 33% compared with CON. Milk fat concentration fit a daily rhythm in CON and NGT, but not DAY, whereas milk protein concentration fit a daily rhythm in CON and DAY, but not NGT. Moreover, DAY abolished the daily rhythm of plasma glucose concentration, but induced rhythms of plasma insulin and nonesterified fatty acid concentrations. Results suggest that feeding increased protein levels during the early part of the day may increase milk fat yield and modify energy metabolism through increased daily variation in insulin-stimulated lipid release, but additional research focused on feeding multiple diets across the day is required.

A leptin-responsive hypothalamic circuit inputs to the circadian feeding network

Salient cues, such as the rising sun or the availability of food, play a crucial role in entraining biological clocks, allowing for effective behavioral adaptation and ultimately, survival. While the light-dependent entrainment of the central circadian pacemaker (suprachiasmatic nucleus, SCN) is relatively well defined, the molecular and neural mechanisms underlying entrainment associated with food availability remains elusive. Using single nucleus RNA sequencing during scheduled feeding (SF), we identified a leptin receptor (LepR) expressing neuron population in the dorsomedial hypothalamus (DMH) that upregulates circadian entrainment genes and exhibits rhythmic calcium activity prior to an anticipated meal. We found that disrupting DMHLepR neuron activity had a profound impact on both molecular and behavioral food entrainment. Specifically, silencing DMHLepR neurons, mis-timed exogenous leptin administration, or mis-timed chemogenetic stimulation of these neurons all interfered with the development of food entrainment. In a state of energy abundance, repetitive activation of DMHLepR neurons led to the partitioning of a secondary bout of circadian locomotor activity that was in phase with the stimulation and dependent on an intact SCN. Lastly, we discovered that a subpopulation of DMHLepR neurons project to the SCN with the capacity to influence the phase of the circadian clock. This leptin regulated circuit serves as a point of integration between the metabolic and circadian systems, facilitating the anticipation of meal times.

Human glucose rhythms and subjective hunger anticipate meal timing

Circadian rhythms, metabolism, and nutrition are closely linked.¹ Timing of a three-meal daily feeding pattern synchronizes some human circadian rhythms.² Despite animal data showing anticipation of food availability, linked to a food-entrainable oscillator,³ it is unknown whether human physiology predicts mealtimes and restricted food availability. In a controlled laboratory protocol, we tested the hypothesis that the human circadian system anticipates large meals. Twenty-four male participants undertook an 8-day laboratory study, with strict sleep-wake schedules, light-dark schedules, and food intake. For 6 days, participants consumed either hourly small meals throughout the waking period or two large daily meals (7.5 and 14.5 h after wake-up). All participants then undertook a 37-h constant routine. Interstitial glucose was measured every 15 min throughout the protocol. Hunger was assessed hourly during waking periods. Saliva melatonin was measured in the constant routine. During the 6-day feeding pattern, both groups exhibited increasing glucose concentration early each morning. In the small meal group, glucose concentrations continued to increase across the day. However, in the large meal group, glucose concentrations decreased from 2 h after waking until the first meal. Average 24-h glucose concentration did not differ between groups. In the constant routine, there was no difference in melatonin onset between groups, but antiphasic glucose rhythms were observed, with low glucose at the time of previous meals in the large meal group. Moreover, in the large meal group, constant routine hunger scores increased before the predicted meal times. These data support the existence of human food anticipation.

Many faces of sleep regulation: beyond the time of day and prior wake time

The two-process model of sleep regulation posits two main processes regulating sleep: the circadian process controlled by the circadian clock and the homeostatic process that depends on the history of sleep and wakefulness. The model has provided a dominant conceptual framework for sleep research since its publication ~ 40 years ago. The time of day and prior wake time are the primary factors affecting the circadian and homeostatic processes, respectively. However, it is critical to consider other factors influencing sleep. Since sleep is incompatible with other behaviors, it is affected by the need for essential behaviors such as eating, foraging, mating, caring for offspring, and avoiding predators. Sleep is also affected by sensory inputs, sickness, increased need for memory consolidation after learning, and other factors. Here, we review multiple factors influencing sleep and discuss recent insights into the mechanisms balancing competing needs.

Mechanisms of gut microbiota-immune-host interaction on glucose regulation in type 2 diabetes

Intestinal absorption of food is one of the sources of glucose. Insulin resistance and impaired glucose tolerance caused by lifestyle and diet are the precursors of type 2 diabetes. Patients with type 2 diabetes have trouble controlling their blood sugar levels. For long-term health, strict glycemic management is necessary. Although it is thought to be well correlated with metabolic diseases like obesity, insulin resistance, and diabetes, its molecular mechanism is still not completely understood. Disturbed microbiota triggers the gut immune response to reshape the gut homeostasis. This interaction not only maintains the dynamic changes of intestinal flora, but also preserves the integrity of the intestinal barrier. Meanwhile, the microbiota establishes a systemic multiorgan dialog on the gut-brain and gut-liver axes, intestinal absorption of a high-fat diet affects the host's feeding preference and systemic metabolism. Intervention in the gut microbiota can combat the decreased glucose tolerance and insulin sensitivity linked to metabolic diseases both centrally and peripherally. Moreover, the pharmacokinetics of oral hypoglycemic medications are also influenced by gut microbiota. The accumulation of drugs in the gut microbiota not only affects the drug efficacy, but also changes the composition and function of them, thus may help to explain individual therapeutic variances in pharmacological efficacy. Regulating gut microbiota through healthy dietary patterns or supplementing pro/prebiotics can provide guidance for lifestyle interventions in people with poor glycemic control. Traditional Chinese medicine can also be used as complementary medicine to effectively regulate intestinal homeostasis. Intestinal microbiota is becoming a new target against metabolic diseases, so more evidence is needed to elucidate the intricate microbiota-immune-host relationship, and explore the therapeutic potential of targeting intestinal microbiota.

Studying food entrainment: Models, methods, and musings

The ability to tell time relative to predictable feeding opportunities has a long history of research, going back more than 100 years with behavioral observations of honeybees and rats. Animals that have access to food at a particular time of day exhibit “food anticipatory activity” (FAA), which is a preprandial increase in activity and arousal thought to be driven by food entrained circadian oscillator(s). However, the mechanisms behind adaptation of behavior to timed feeding continue to elude our grasp. Methods used to study circadian entrainment by food vary depending on the model system and the laboratory conducting the experiments. Most studies have relied on rodent model systems due to neuroanatomical tools and genetic tractability, but even among studies of laboratory mice, methods vary considerably. A lack of consistency within the field in experimental design, reporting, and definition of food entrainment, or even FAA, makes it difficult to compare results across studies or even within the same mutant mouse strain, hindering interpretation of replication studies. Here we examine the conditions used to study food as a time cue and make recommendations for study design and reporting.

Atypical behavioral and thermoregulatory circadian rhythms in mice lacking a microbiome

Trillions of microbial oscillators reside throughout the mammalian body, yet their contributions toward fundamental features of host circadian rhythms (CRs) have not been characterized. Here, we demonstrate that the microbiome contributes to host CRs in activity and thermoregulation. Mice devoid of microbes (germ-free, GF) exhibited higher-amplitude CRs in a light-dark cycle and longer circadian periods in constant darkness. Circadian entrainment to food was greater in GF mice, but resetting responses to simulated jet-lag were unaffected. Microbial transplantation with cecal contents of conventionally-raised mice normalized CRs of GF mice, indicating that the concurrent activity of gut microbes modulates host circadian networks. Obesogenic effects of high-fat diet were absent in GF mice, but some circadian-disruptive effects persisted. Transkingdom (host-microbe) interactions affect circadian period and entrainment of CRs in diverse traits, and microbes alter interactions among light- and food-entrainable circadian processes in the face of environmental (light, diet) perturbations.

Mistimed sleep and waking activity in humans disrupts glucocorticoid signalling transcripts and SP1, but not plasma cortisol rhythms

Cortisol is a robust circadian signal that synchronises peripheral circadian clocks with the central clock in the suprachiasmatic nucleus via glucocorticoid receptors that regulate peripheral gene expression. Misalignment of the cortisol rhythm with the sleep–wake cycle, as occurs in shift work, is associated with negative health outcomes, but underlying molecular mechanisms remain largely unknown. We experimentally induced misalignment between the sleep–wake cycle and melatonin and cortisol rhythms in humans and measured time series blood transcriptomics while participants slept in-phase and out-of-phase with the central clock. The cortisol rhythm remained unchanged, but many glucocorticoid signalling transcripts were disrupted by mistimed sleep. To investigate which factors drive this dissociation between cortisol and its signalling pathways, we conducted bioinformatic and temporal coherence analyses. We found that glucocorticoid signalling transcripts affected by mistimed sleep were enriched for binding sites for the transcription factor SP1. Furthermore, changes in the timing of the rhythms of SP1 transcripts, a major regulator of transcription, and changes in the timing of rhythms in transcripts of the glucocorticoid signalling pathways were closely associated. Associations between the rhythmic changes in factors that affect SP1 expression and its activity, such as STAT3, EP300, HSP90AA1, and MAPK1, were also observed. We conclude that plasma cortisol rhythms incompletely reflect the impact of mistimed sleep on glucocorticoid signalling pathways and that sleep–wake driven changes in SP1 may mediate disruption of these pathways. These results aid understanding of mechanisms by which mistimed sleep affects health.

Dopamine systems and biological rhythms: Let's get a move on

How dopamine signaling regulates biological rhythms is an area of emerging interest. Here we review experiments focused on delineating dopamine signaling in the suprachiasmatic nucleus, nucleus accumbens, and dorsal striatum to mediate a range of biological rhythms including photoentrainment, activity cycles, rest phase eating of palatable food, diet-induced obesity, and food anticipatory activity. Enthusiasm for causal roles for dopamine in the regulation of circadian rhythms, particularly those associated with food and other rewarding events, is warranted. However, determining that there is rhythmic gene expression in dopamine neurons and target structures does not mean that they are bona fide circadian pacemakers. Given that dopamine has such a profound role in promoting voluntary movements, interpretation of circadian phenotypes associated with locomotor activity must be differentiated at the molecular and behavioral levels. Here we review our current understanding of dopamine signaling in relation to biological rhythms and suggest future experiments that are aimed at teasing apart the roles of dopamine subpopulations and dopamine receptor expressing neurons in causally mediating biological rhythms, particularly in relation to feeding, reward, and activity.

Physiological Role of Orexinergic System for Health

Orexins, or hypocretins, are excitatory neuropeptides involved in the regulation of feeding behavior and the sleep and wakefulness states. Since their discovery, several lines of evidence have highlighted that orexin neurons regulate a great range of physiological functions, giving it the definition of a multitasking system. In the present review, we firstly describe the mechanisms underlining the orexin system and their interactions with the central nervous system (CNS). Then, the system’s involvement in goal-directed behaviors, sleep/wakefulness state regulation, feeding behavior and energy homeostasis, reward system, and aging and neurodegenerative diseases are described. Advanced evidence suggests that the orexin system is crucial for regulating many physiological functions and could represent a promising target for therapeutical approaches to obesity, drug addiction, and emotional stress.

Racing and Pacing in the Reward System: A Multi-Clock Circadian Control Over Dopaminergic Signalling

Level of motivation, responsiveness to rewards and punishment, invigoration of exploratory behaviours, and motor performance are subject to daily fluctuations that emerge from circadian rhythms in neuronal activity of the midbrain’s dopaminergic system. While endogenous circadian rhythms are weak in the ventral tegmental area and substantia nigra pars compacta, daily changes in expression of core clock genes, ion channels, neurotransmitter receptors, dopamine-synthesising enzymes, and dopamine transporters, accompanied by changes in electrical activity, are readily observed in these nuclei. These processes cause dopamine levels released in structures innervated by midbrain dopaminergic neurons (e.g., the striatum) to oscillate in a circadian fashion. Additionally, growing evidence show that the master circadian clock located in the suprachiasmatic nucleus of the hypothalamus (SCN) rhythmically influences the activity of the dopaminergic system through various intermediate targets. Thus, circadian changes in the activity of the dopaminergic system and concomitant dopamine release observed on a daily scale are likely to be generated both intrinsically and entrained by the master clock. Previous studies have shown that the information about the value and salience of stimuli perceived by the animal is encoded in the neuronal activity of brain structures innervating midbrain dopaminergic centres. Some of these structures themselves are relatively autonomous oscillators, while others exhibit a weak endogenous circadian rhythm synchronised by the SCN. Here, we place the dopaminergic system as a hub in the extensive network of extra-SCN circadian oscillators and discuss the possible consequences of its daily entrainment for animal physiology and behaviour.

The circadian system, sleep, and the health/disease balance: a conceptual review

The field of "circadian medicine" is a recent addition to chronobiology and sleep research efforts. It represents a logical step arising from the increasing insights into the circadian system and its interactions with life in urbanised societies; applying these insights to the health/disease balance at home and in the medical practice (outpatient) and clinic (inpatient). Despite its fast expansion and proliferating research efforts, circadian medicine lacks a formal framework to categorise the many observations describing interactions among the circadian system, sleep, and the health/disease balance. A good framework allows us to categorise observations and then assign them to one or more components with hypothesised interactions. Such assignments can lead to experiments that document causal (rather than correlational) relationships and move from describing observations to discovering mechanisms. This review details such a proposed formal framework for circadian medicine and will hopefully trigger discussion among our colleagues, so that the framework can be improved and expanded. As the basis of the framework for circadian medicine, we define "circadian health" and how it links to general health. We then define interactions among the circadian system, sleep, and the health/disease balance and put the framework into the context of the literature with examples from six domains of health/disease balance: fertility, cancer, immune system, mental health, cardiovascular, and metabolism.

Multiple entrained oscillator model of food anticipatory circadian rhythms

For many animal species, knowing when to look for food may be as important as knowing where to look. Rats and other species use a feeding-responsive circadian timing mechanism to anticipate, behaviorally and physiologically, a predictable daily feeding opportunity. How this mechanism for anticipating a daily meal accommodates more than one predictable mealtime is unclear. Rats were trained to press a lever for food, and then limited to one or more daily meals at fixed or systematically varying times of day. The rats were able to anticipate up to 4 of 4 daily meals at fixed times of day and two ‘daily’ meals recurring at 24 h and 26 h intervals. When deprived of food, in constant dark, lever pressing recurred for multiple cycles at expected mealtimes, consistent with the periodicity of the prior feeding schedule. Anticipation did not require the suprachiasmatic nucleus circadian pacemaker. The anticipation rhythms could be simulated using a Kuramoto model in which clusters of coupled oscillators entrain to specific mealtimes based on initial phase and intrinsic circadian periodicity. A flexibly coupled system of food-entrainable circadian oscillators endows rats with adaptive plasticity in daily programming of foraging activity.

Assessing temporal eating pattern in free living humans through the myCircadianClock app

The quality and quantity of nutrition impact health. However, chrononutrition, the timing, and variation of food intake in relation to the daily sleep-wake cycle are also important contributors to health. This has necessitated an urgent need to measure, analyze, and optimize eating patterns to improve health and manage disease. While written food journals, questionnaires, and 24-hour dietary recalls are acceptable methods to assess the quantity and quality of energy consumption, they are insufficient to capture the timing and day-to-day variation of energy intake. Smartphone applications are novel methods for information-dense real-time food and beverage tracking. Despite the availability of thousands of commercial nutrient apps, they almost always ignore eating patterns, and the raw real-time data is not available to researchers for monitoring and intervening in eating patterns. Our lab developed a smartphone app called myCircadianClock (mCC) and associated software to enable long-term real-time logging that captures temporal components of eating patterns. The mCC app runs on iOS and android operating systems and can be used to track multiple cohorts in parallel studies. The logging burden is decreased by using a timestamped photo and annotation of the food/beverage being logged. Capturing temporal data of consumption in free-living individuals over weeks/months has provided new insights into diverse eating patterns in the real world. This review discusses (1) chrononutrition and the importance of understanding eating patterns, (2) the myCircadianClock app, (3) validation of the mCC app, (4) clinical trials to assess the timing of energy intake, and (5) strengths and limitations of the mCC app.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Depriving Mice of Sleep also Deprives of Food

Both sleep-wake behavior and circadian rhythms are tightly coupled to energy metabolism and food intake. Altered feeding times in mice are known to entrain clock gene rhythms in the brain and liver, and sleep-deprived humans tend to eat more and gain weight. Previous observations in mice showing that sleep deprivation (SD) changes clock gene expression might thus relate to altered food intake, and not to the loss of sleep per se. Whether SD affects food intake in the mouse and how this might affect clock gene expression is, however, unknown. We therefore quantified (i) the cortical expression of the clock genes Per1, Per2, Dbp, and Cry1 in mice that had access to food or not during a 6 h SD, and (ii) food intake during baseline, SD, and recovery sleep. We found that food deprivation did not modify the SD-incurred clock gene changes in the cortex. Moreover, we discovered that although food intake during SD did not differ from the baseline, mice lost weight and increased food intake during subsequent recovery. We conclude that SD is associated with food deprivation and that the resulting energy deficit might contribute to the effects of SD that are commonly interpreted as a response to sleep loss.

Mice hypomorphic for Pitx3 show robust entrainment of circadian behavioral and metabolic rhythms to scheduled feeding

Pitx3ak mice lack a functioning retina and develop fewer than 10% of dopamine neurons in the substantia nigra. Del Río-Martín et al. (2019) reported that entrainment of circadian rhythms to daily light-dark (LD) cycles is absent in these mice, and that rhythms of locomotor activity, energy expenditure, and other metabolic variables are disrupted with food available ad libitum and fail to entrain to a daily feeding. The authors propose that retinal innervation of the suprachiasmatic nucleus is required for development of cyclic metabolic homeostasis, but methodological issues limit interpretation of the results. Using standardized feeding schedules and procedures for distinguishing free-running from entrained circadian rhythms, we confirm that behavioral and metabolic rhythms in Pitx3ak mice do not entrain to LD cycles, but we find no impairment in circadian organization of metabolism with food available ad libitum and no impairment in entrainment of metabolic or behavioral rhythms by daily feeding schedules. This Matters Arising paper is in response to Del Río-Martín et al. (2019), published in Cell Reports. See also the response by Fernandez-Perez et al. (2022), published in this issue.

Studying Circadian Clock Entrainment by Hormonal Signals

In mammals, molecular circadian clocks not only exist in the suprachiasmatic nucleus (SCN) but in almost all organ systems. Intriguingly, tissue clocks can operate in both isolated tissues and cell lines with endocrine signals mediating the circadian expression of local transcriptomes. This can be demonstrated by treating tissue explants with endocrine cues in a phase- and dose-dependent manner. In this chapter we provide an overview of methods to study the effects of candidate hormonal time cues on tissue clock resetting. We propose an experimental procedure based on an in vitro setup consisting of several consecutive steps in which organotypic tissue cultures or cells can be used. Our approach targets the potential resetting mechanism at three levels: the hormone, the direct clock gene target, and the tissue clock response.

Brain Clocks, Sleep, and Mood

The suprachiasmatic nucleus houses the master clock, but the genes which encode the circadian clock components are also expressed throughout the brain. Here, we review how circadian clock transcription factors regulate neuromodulator systems such as histamine, dopamine, and orexin that promote arousal. These circadian transcription factors all lead to repression of the histamine, dopamine, and orexin systems during the sleep period, so ensuring integration with the ecology of the animal. If these transcription factors are deleted or mutated, in addition to the global disturbances in circadian rhythms, this causes a chronic up-regulation of neuromodulators leading to hyperactivity, elevated mood, and reduced sleep, which have been suggested to be states resembling mania.

Targeting Histamine and Histamine Receptors for the Precise Regulation of Feeding

Histamine has long been accepted as an anorexigenic agent. However, lines of evidence have suggested that the roles of histamine in feeding behaviors are much more complex than previously thought, being involved in satiety, satiation, feeding motivation, feeding circadian rhythm, and taste perception and memory. The functional diversity of histamine makes it a viable target for clinical management of obesity and other feeding-related disorders. Here, we update the current knowledge about the functions of histamine in feeding and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on weight control and discuss insights into future research on the roles of histamine in feeding. Despite the recent progress in histamine research, the histaminergic feeding circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

THE ROLE OF CIRCADIAN REGULATION OF GHRELIN LEVELS IN PARKINSON’S DISEASE (LITERATURE REVIEW)

The paper is aimed at the analysis of the role of the circadian regulation of ghrelin levels in patients with Parkinson’s disease. Based on the literature data, patients with Parkinson’s disease have clinical fluctuations in the symptoms of the disease, manifested by the diurnal changes in motor activity, autonomic functions, sleep-wake cycle, visual function, and the efficacy of dopaminergic therapy. Biological rhythms are controlled by central and peripheral oscillators which links with dopaminergic neurotransmission – core of the pathogenesis of Parkinson`s disease. Circadian system is altered in Parkinson`s disease due to that ghrelin fluctuations may be changed. Ghrelin is potential food-entrainable oscillator because it is linked with clock genes expression. In Parkinson`s disease this hormone may induce eating behavior changing and as a result metabolic disorder. The “hunger hormone” ghrelin can be a biomarker of the Parkinson’s disease, and the study of its role in the pathogenesis, as well as its dependence on the period of the day, intake of levodopa medications to improve the effectiveness of treatment is promising.

Food Restriction Engages Prefrontal Corticostriatal Cells and Local Microcircuitry to Drive the Decision to Run versus Conserve Energy

Food restriction (FR) evokes running, which may promote adaptive foraging in times of food scarcity, but can become lethal if energy expenditure exceeds caloric availability. Here, we demonstrate that chemogenetic activation of either the general medial prefrontal cortex (mPFC) pyramidal cell population, or the subpopulation projecting to dorsal striatum (DS) drives running specifically during hours preceding limited food availability, and not during ad libitum food availability. Conversely, suppression of mPFC pyramidal cells generally, or targeting mPFC-to-DS cells, reduced wheel running specifically during FR and not during ad libitum food access. Post mortem c-Fos analysis and electron microscopy of mPFC layer 5 revealed distinguishing characteristics of mPFC-to-DS cells, when compared to neighboring non-DS-projecting pyramidal cells: 1) greater recruitment of GABAergic activity and 2) less axo-somatic GABAergic innervation. Together, these attributes position the mPFC-to-DS subset of pyramidal cells to dominate mPFC excitatory outflow, particularly during FR, revealing a specific and causal role for mPFC-to-DS control of the decision to run during food scarcity. Individual differences in GABAergic activity correlate with running response to further support this interpretation. FR enhancement of PFC-to-DS activity may influence neural circuits both in studies using FR to motivate animal behavior and in human conditions hallmarked by FR.

Food Entrainment, Arousal, and Motivation in the Neonatal Rabbit Pup

In the newborn rabbit, the light entrainable circadian system is immature and once a day nursing provides the primary timing cue for entrainment. In advance of the mother's arrival, pups display food anticipatory activity (FAA), and metabolic and physiological parameters are synchronized to this daily event. Central structures in the brain are also entrained as indicated by expression of Fos and Per1 proteins, GFAP, a glial marker, and cytochrome oxidase activity. Under fasting conditions, several of these rhythmic parameters persist in the periphery and brain, including rhythms in the olfactory bulb (OB). Here we provide an overview of these physiological and neurobiological changes and focus on three issues, just beginning to be examined in the rabbit. First, we review evidence supporting roles for the organum vasculosum of lamina terminalis (OVLT) and median preoptic nucleus (MnPO) in homeostasis of fluid ingestion and the neural basis of arousal, the latter which also includes the role of the orexigenic system. Second, since FAA in association with the daily visit of the mother is an example of conditioned learning, we review evidence for changes in the corticolimbic system and identified nuclei in the amygdala and extended amygdala as part of the neural substrate responsible for FAA. Third, we review recent evidence supporting the role of oxytocinergic cells of the paraventricular hypothalamic nucleus (PVN) as a link to the autonomic system that underlies physiological events, which occur in preparation for the upcoming next daily meal. We conclude that the rabbit model has contributed to an overall understanding of food entrainment.

Type 1 dopamine receptor (D1R)-independent circadian food anticipatory activity in mice

Circadian rhythms are entrained by light and influenced by non-photic stimuli, such as feeding. The activity preceding scheduled mealtimes, food anticipatory activity (FAA), is elicited in rodents fed a limited amount at scheduled times. FAA is thought to be the output of an unidentified food entrained oscillator. Previous studies, using gene deletion and receptor pharmacology, implicated dopamine type receptor 1 (D1R) signaling in the dorsal striatum as necessary for FAA in mice. To further understand the role of D1R in promoting FAA, we utilized the Cre-lox system to create cell type-specific deletions of D1R, conditionally deleting D1R in GABA neurons using Vgat-ires-Cre line. This conditional deletion mutant had attenuated FAA, but the amount was higher than expected based on prior results using a constitutive knockout of D1R, D1R KO^Drago. This result prompted us to re-test the original D1R KO^Drago line, which expressed less FAA than controls, but only moderately so. To determine if genetic drift had diminished the effect of D1R deletion on FAA, we re-established the D1R KO^Drago knockout line from cryopreserved samples. The reestablished D1R KO^Drago-cryo had a clear impairment of FAA compared to controls, but still developed increased activity preceding mealtime across the 4 weeks of timed feeding. Finally, we tested a different deletion allele of D1R created by the Knockout Mouse Project. This line of D1R KO^KOMP mice had a significant impairment in the acquisition of FAA, but eventually reached similar levels of premeal activity compared to controls after 4 weeks of timed feeding. Taken together, our results suggest that D1R signaling promotes FAA, but other dopamine receptors likely contribute to FAA given that mice lacking the D1 receptor still retain some FAA.

Regulation of diurnal energy balance by mitokines

The mammalian system of energy balance regulation is intrinsically rhythmic with diurnal oscillations of behavioral and metabolic traits according to the 24 h day/night cycle, driven by cellular circadian clocks and synchronized by environmental or internal cues such as metabolites and hormones associated with feeding rhythms. Mitochondria are crucial organelles for cellular energy generation and their biology is largely under the control of the circadian system. Whether mitochondrial status might also feed-back on the circadian system, possibly via mitokines that are induced by mitochondrial stress as endocrine-acting molecules, remains poorly understood. Here, we describe our current understanding of the diurnal regulation of systemic energy balance, with focus on fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15), two well-known endocrine-acting metabolic mediators. FGF21 shows a diurnal oscillation and directly affects the output of the brain master clock. Moreover, recent data demonstrated that mitochondrial stress-induced GDF15 promotes a day-time restricted anorexia and systemic metabolic remodeling as shown in UCP1-transgenic mice, where both FGF21 and GDF15 are induced as myomitokines. In this mouse model of slightly uncoupled skeletal muscle mitochondria GDF15 proved responsible for an increased metabolic flexibility and a number of beneficial metabolic adaptations. However, the molecular mechanisms underlying energy balance regulation by mitokines are just starting to emerge, and more data on diurnal patterns in mouse and man are required. This will open new perspectives into the diurnal nature of mitokines and action both in health and disease.

Chrono-nutrition: From molecular and neuronal mechanisms to human epidemiology and timed feeding patterns

The circadian timing system governs daily biological rhythms, synchronising physiology and behaviour to the temporal world. External time cues, including the light-dark cycle and timing of food intake, provide daily signals for entrainment of the central, master circadian clock in the hypothalamic suprachiasmatic nuclei (SCN), and of metabolic rhythms in peripheral tissues, respectively. Chrono-nutrition is an emerging field building on the relationship between temporal eating patterns, circadian rhythms, and metabolic health. Evidence from both animal and human research demonstrates adverse metabolic consequences of circadian disruption. Conversely, a growing body of evidence indicates that aligning food intake to periods of the day when circadian rhythms in metabolic processes are optimised for nutrition may be effective for improving metabolic health. Circadian rhythms in glucose and lipid homeostasis, insulin responsiveness and sensitivity, energy expenditure, and postprandial metabolism, may favour eating patterns characterised by earlier temporal distribution of energy. This review details the molecular basis for metabolic clocks, the regulation of feeding behaviour, and the evidence for meal timing as an entraining signal for the circadian system in animal models. The epidemiology of temporal eating patterns in humans is examined, together with evidence from human intervention studies investigating the metabolic effects of morning compared to evening energy intake, and emerging chrono-nutrition interventions such as time-restricted feeding. Chrono-nutrition may have therapeutic application for individuals with and at-risk of metabolic disease and convey health benefits within the general population.

Eat, sleep, repeat: the role of the circadian system in balancing sleep-wake control with metabolic need

Feeding and sleep are behaviours fundamental to survival, and as such are subject to powerful homeostatic control. Of course, these are mutually exclusive behaviours, and therefore require coordinated temporal organisation to ensure that both energy demands and sleep need are met. Under optimal conditions, foraging/feeding and sleep can be simply partitioned to appropriate phases of the circadian cycle so that they are in suitable alignment with the external environment. However, under conditions of negative energy balance, increased foraging activity must be balanced against sleep requirements and energy conservation. In mammals and many other species, neural circuits that regulate sleep and energy balance are intimately and reciprocally linked. Here, we examine this circuitry, discuss how homeostatic regulation and temporal patterning of sleep are modulated by altered food availability, and describe the role of circadian system in adaptation to metabolic stress.

DAILY-A Personalized Circadian Zeitgeber Therapy as an Adjunctive Treatment for Alcohol Use Disorder Patients: Study Protocol for a Randomized Controlled Trial

Background: Hallmarks of alcohol use disorder (AUD) are disturbances of circadian rhythms and everyday structures. While circadian rhythms dictate the timing of daily recurring activities such as sleep, activity, and meals, conversely, these activities represent time cues, so called Zeitgebers, that the circadian system uses to synchronize with the environment. Here we present a study protocol for our newly developed therapy approach for AUD patients, in which we take advantage of this mutual influence and stabilize and strengthen their circadian system by creating strict daily schedules for daily Zeitgeber activities. Since every person has a circadian system with its own characteristics and is subject to social obligations, the daily plans are personalized for each test person. Our hypothesis is that a regular exposure to Zeitgebers stabilizes behavioral and physiological circadian rhythms and thereby reduces the risk of alcohol relapses and depressive symptoms and facilitates physical recovery in AUD patients during the 1st weeks of their addiction therapy. Methods/design: The study is a 6-weeks single site trial with a controlled, randomized, single-blinded, parallel-group design including patients with a diagnosis of AUD. The study runs parallel to the standard addiction therapy of the clinic. Patients are randomly assigned to either an intervention group (DAILY) or a sham control group (placebo treatment). Questionnaires and physiological assessments of both groups are conducted before and immediately after the intervention or control treatment. According to our hypothesis, the primary outcomes of this study are improvements of regularity, alcohol consumption, and relapse rate in AUD patients compared to AUD patients receiving control treatment. Secondary outcomes are reduced depressive symptoms and increased physical recovery. Discussion: This study is a randomized controlled trial to investigate the efficacy of a personalized circadian Zeitgeber therapy as an adjunctive treatment for alcohol use disorder patients. The overall goal of this and more extended future studies is the development of an adjunctive therapy for AUD patients that is uncomplicated in its use and easy to implement in the clinical and everyday routine. Trial registration: This study is registered at the German Clinical Trial Register with the trial number DRKS00019093 on November 28, 2019.