Food intake and addictive-like eating behaviors: Time to think about the circadian clock(s)

Sex differences in sleep, circadian rhythms, and metabolism: Implications for precision medicine

The number of individuals experiencing sleep loss has exponentially risen over the past decades. Extrapolation of laboratory findings to the real world suggests that females are more affected by extended wakefulness and circadian misalignment than males are. Therefore, long-term effects such as sleep and metabolic disorders are likely to be more prevalent in females than in males. Despite emerging evidence for sex differences in key aspects of sleep-wake and circadian regulation, much remains unknown, as females are often underrepresented in sleep and circadian research. This narrative review aims at highlighting 1) how sex differences systematically impinge on the sleep-wake and circadian regulation in humans, 2) how sex differences in sleep and circadian factors modulate metabolic control, and 3) the relevance of these differences for precision medicine. Ultimately, the findings justify factoring in sex differences when optimizing individually targeted sleep and circadian interventions in humans.

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.

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.

Variability in Sleep Timing and Dietary Intake: A Scoping Review of the Literature

The objective of this scoping review was to summarize previous studies which examined the effect of day-to-day variability in sleep timing and social jetlag (SJL) on dietary intake. A systematic literature search was conducted in PubMed, Embase, and Clarivate Analytics Web of Science and we identified 22 records. No difference in caloric and macronutrient intake between SJL groups was observed in studies that enrolled healthy young adults. However, studies that enrolled participants with obesity and obesity-related chronic conditions reported a higher caloric intake and a higher intake of carbohydrates, total fat, saturated fats, and cholesterol in participants with SJL than in those without. Most studies reported a lower quality of diet, a delayed mealtime, and eating jetlag in participants with SJL vs. those without SJL. No correlation of day-to-day variability in sleep timing with average caloric intake was observed, but bed-time variability was negatively associated with diet quality. Methodological issues have been identified in sources assessed including study design, power calculation, population enrolled, and tools/metrics used for sleep timing variability assessment. Future well powered longitudinal studies, with clear protocols, standardized metrics, including all age groups from general population are needed to clarify the dietary intake consequences of variability in sleep timing.

Knockout of the Circadian Clock Protein PER1 (Period1) Exacerbates Hypertension and Increases Kidney Injury in Dahl Salt-Sensitive Rats

BACKGROUND

Circadian rhythms play an essential role in physiological function. The molecular clock that underlies circadian physiological function consists of a core group of transcription factors, including the protein PER1 (Period1). Studies in mice show that PER1 plays a role in the regulation of blood pressure and renal sodium handling; however, the results are dependent on the strain being studied. Using male Dahl salt-sensitive (SS) rats with global knockout of PER1 (SSPer1-/-), we aim to test the hypothesis that PER1 plays a key role in the regulation of salt-sensitive blood pressure.

METHODS

The model was generated using CRISPR/Cas9 and was characterized using radiotelemetry and measures of renal function and circadian rhythm.

RESULTS

SSPer1-/- rats had similar mean arterial pressure when fed a normal 0.4% NaCl diet but developed augmented hypertension after three weeks on a high-salt (4% NaCl) diet. Despite being maintained on a normal 12:12 light:dark cycle, SSPer1-/- rats exhibited desynchrony mean arterial pressure rhythms on a high-salt diet, as evidenced by increased variability in the time of peak mean arterial pressure. SSPer1-/- rats excrete less sodium after three weeks on the high-salt diet. Furthermore, SSPer1-/- rats exhibited decreased creatinine clearance, a measurement of renal function, as well as increased signs of kidney tissue damage. SSPer1-/- rats also exhibited higher plasma aldosterone levels.

CONCLUSIONS

Altogether, our findings demonstrate that loss of PER1 in Dahl SS rats causes an array of deleterious effects, including exacerbation of the development of salt-sensitive hypertension and renal damage.

Suprachiasmatic to paraventricular nuclei interaction generates normal food searching rhythms in mice

Searching for food follows a well-organized decision process in mammals to take up food only if necessary. Moreover, scavenging is preferred during their activity phase. Various time-dependent regulatory processes have been identified originating from the suprachiasmatic nuclei (SCN), which convert external light information into synchronizing output signals. However, a direct impact of the SCN on the timing of normal food searching has not yet been found. Here, we revisited the function of the SCN to affect when mice look for food. We found that this process was independent of light but modified by the palatability of the food source. Surprisingly, reducing the output from the SCN, in particular from the vasopressin releasing neurons, reduced the amount of scavenging during the early activity phase. The SCN appeared to transmit a signal to the paraventricular nuclei (PVN) via GABA receptor A1. Finally, the interaction of SCN and PVN was verified by retrograde transport-mediated complementation. None of the genetic manipulations affected the uptake of more palatable food. The data indicate that the PVN are sufficient to produce blunted food searching rhythms and are responsive to hedonistic feeding. Nevertheless, the search for normal food during the early activity phase is significantly enhanced by the SCN.

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.

Nuclear Receptors (PPARs, REV-ERBs, RORs) and Clock Gene Rhythms in Goldfish (Carassius auratus) Are Differently Regulated in Hypothalamus and Liver

The circadian system is formed by a network of oscillators located in central and peripheral tissues that are tightly linked to generate rhythms in vertebrates to adapt the organism to the cyclic environmental changes. The nuclear receptors PPARs, REV-ERBs and RORs are transcription factors controlled by the circadian system that regulate, among others, a large number of genes that control metabolic processes for which they have been proposed as key genes that link metabolism and temporal homeostasis. To date it is unclear whether these nuclear receptors show circadian expression and which zeitgebers are important for their synchronization in fish. Therefore, the objective of this study was to investigate whether the two main zeitgebers (light-dark cycle and feeding time) could affect the synchronization of central (hypothalamus) and peripheral (liver) core clocks and nuclear receptors in goldfish. To this aim, three experimental groups were established: fish under a 12 h light-12 h darkness and fed at Zeitgeber Time 2; fish with the same photoperiod but randomly fed; and fish under constant darkness and fed at Circadian Time 2. After one month, clock genes and nuclear receptors expression in hypothalamus and liver and circulating glucose were studied. Clock genes displayed daily rhythms in both tissues of goldfish if the light-dark cycle was present, with shifted-acrophases of negative and positive elements, as expected for proper functioning clocks. In darkness-maintained fish hypothalamic clock genes were fully arrhythmic while the hepatic ones were still rhythmic. Among studied nuclear receptors, in the hypothalamus only nr1d1 was rhythmic and only when the light-dark cycle was present. In the liver all nuclear receptors were rhythmic when both zeitgebers were present, but only nr1d1 when one of them was removed. Plasma glucose levels showed significant rhythms in fish maintained under random fed regimen or constant darkness, with the highest levels at 1-h postprandially in all groups. Altogether these results support that hypothalamus is mainly a light-entrained-oscillator, while the liver is a food-entrained-oscillator. Moreover, nuclear receptors are revealed as clear outputs of the circadian system acting as key elements in the timekeeping of temporal homeostasis, particularly in the liver.

The Ventral Tegmental Area and Nucleus Accumbens as Circadian Oscillators: Implications for Drug Abuse and Substance Use Disorders

Circadian rhythms convergently evolved to allow for optimal synchronization of individuals’ physiological and behavioral processes with the Earth’s 24-h periodic cycling of environmental light and temperature. Whereas the suprachiasmatic nucleus (SCN) is considered the primary pacemaker of the mammalian circadian system, many extra-SCN oscillatory brain regions have been identified to not only exhibit sustainable rhythms in circadian molecular clock function, but also rhythms in overall region activity/function and mediated behaviors. In this review, we present the most recent evidence for the ventral tegmental area (VTA) and nucleus accumbens (NAc) to serve as extra-SCN oscillators and highlight studies that illustrate the functional significance of the VTA’s and NAc’s inherent circadian properties as they relate to reward-processing, drug abuse, and vulnerability to develop substance use disorders (SUDs).

The Mind After Midnight: Nocturnal Wakefulness, Behavioral Dysregulation, and Psychopathology

Sufficient sleep with minimal interruption during the circadian/biological night supports daytime cognition and emotional regulation. Conversely, disrupted sleep involving significant nocturnal wakefulness leads to cognitive and behavioral dysregulation. Most studies to-date have examined how fragmented or insufficient sleep affects next-day functioning, but recent work highlights changes in cognition and behavior that occur when someone is awake during the night. This review summarizes the evidence for day-night alterations in maladaptive behaviors, including suicide, violent crime, and substance use, and examines how mood, reward processing, and executive function differ during nocturnal wakefulness. Based on this evidence, we propose the Mind after Midnight hypothesis in which attentional biases, negative affect, altered reward processing, and prefrontal disinhibition interact to promote behavioral dysregulation and psychiatric disorders.

Genome-Wide Association Study on Three Behaviors Tested in an Open Field in Heterogeneous Stock Rats Identifies Multiple Loci Implicated in Psychiatric Disorders

Many personality traits are influenced by genetic factors. Rodents models provide an efficient system for analyzing genetic contribution to these traits. Using 1,246 adolescent heterogeneous stock (HS) male and female rats, we conducted a genome-wide association study (GWAS) of behaviors measured in an open field, including locomotion, novel object interaction, and social interaction. We identified 30 genome-wide significant quantitative trait loci (QTL). Using multiple criteria, including the presence of high impact genomic variants and co-localization of cis-eQTL, we identified 17 candidate genes (Adarb2, Ankrd26, Cacna1c, Cacng4, Clock, Ctu2, Cyp26b1, Dnah9, Gda, Grxcr1, Eva1a, Fam114a1, Kcnj9, Mlf2, Rab27b, Sec11a, and Ube2h) for these traits. Many of these genes have been implicated by human GWAS of various psychiatric or drug abuse related traits. In addition, there are other candidate genes that likely represent novel findings that can be the catalyst for future molecular and genetic insights into human psychiatric diseases. Together, these findings provide strong support for the use of the HS population to study psychiatric disorders.

Eating and Physical Activity Patterns in Day and Night Shift EMS Clinicians

Objective: EMS clinicians work in shifts to provide 24-hour care. Shift work is linked with metabolic disease and over 70% of EMS clinicians report having overweight or obesity. Inability to store food in their vehicles combined with limited overnight dining establishments, and unpredictable job demands leads to reliance on convenience and fast foods. The objective of this study was to describe the eating and physical activity patterns among EMS clinicians on days on and off shift.Methods: EMS clinicians throughout the United States participated in a study involving four 24-hour monitoring periods. Participants wore activity monitors to measure physical activity level and remote food photography was used to collect dietary data on two work days and two days off. Repeated measures analysis of variance was conducted to compare energy and macronutrient intake and activity levels in day and night workers on and off shift.Results: We analyzed data from 39 EMS clinicians (29.7 + 8.5yrs old). Controlling for sex, those working night shifts consumed more kilocalories (p=.037) and total fat (p=.043) compared to day shift workers. Night shift workers had fewer steps (p = 0.045), more sedentary time (p = 0.053), and less moderate activity (p = 0.037) during a shift compared to day workers.Conclusion: Among EMS clinicians, night shift is associated with greater energy intake, and decreased physical activity during shifts. This may contribute to positive energy balance and weight gain overtime, increasing risk for metabolic disease.

A single factor dominates the behavior of rhythmic genes in mouse organs

Background

Circadian rhythm, regulated by both internal and external environment of the body, is a multi-scale biological oscillator of great complexity. On the molecular level, thousands of genes exhibit rhythmic transcription, which is both organ- and species-specific, but it remains a mystery whether some common factors could potentially explain their rhythmicity in different organs. In this study we address this question by analyzing the transcriptome data in 12 mouse organs to determine such major impacting factors.

Results

We found a strong positive correlation between the transcriptional level and rhythmic amplitude of circadian rhythmic genes in mouse organs. Further, transcriptional level could explain over 70% of the variation in amplitude. In addition, the functionality and tissue specificity were not strong predictors of amplitude, and the expression level of rhythmic genes was linked to the energy consumption associated with transcription.

Conclusion

Expression level is a single major factor impacts the behavior of rhythmic genes in mouse organs. This single determinant implicates the importance of rhythmic expression itself on the design of the transcriptional system. So, rhythmic regulation of highly expressed genes can effectively reduce the energetic cost of transcription, facilitating the long-term adaptive evolution of the entire genetic system.

Neural circuits linking sleep and addiction: Animal models to understand why select individuals are more vulnerable to substance use disorders after sleep deprivation

Individuals differ widely in their drug-craving behaviors. One reason for these differences involves sleep. Sleep disturbances lead to an increased risk of substance use disorders and relapse in only some individuals. While animal studies have examined the impact of sleep on reward circuitry, few have addressed the role of individual differences in the effects of altered sleep. There does, however, exist a rodent model of individual differences in reward-seeking behavior: the sign/goal-tracker model of Pavlovian conditioned approach. In this model, only some rats show the key behavioral traits associated with addiction, including impulsivity and poor attentional control, making this an ideal model system to examine individually distinct sleep-reward interactions. Here, we describe how the limbic neural circuits responsible for individual differences in incentive motivation overlap with those involved in sleep-wake regulation, and how this model can elucidate the common underlying mechanisms. Consideration of individual differences in preclinical models would improve our understanding of how sleep interacts with motivational systems, and why sleep deprivation contributes to addiction in only select individuals.

Eating Rewards the Gears of the Clock

Eating behavior is regulated by metabolic and hedonic brain networks, which interact with each other to balance the physiological regulation of hunger and satiety. The daily balance of this regulation is controlled by the central circadian clock. Importantly, metabolic and reward properties of food impact the functioning of circadian clocks, altering the oscillatory activity of the molecular clockwork and circadian rhythms. However, when feeding (metabolic or reward) is timed, the whole circadian system is entrained. Furthermore, besides synchronizing the clock, the timing of both metabolic and reward eating might be crucial for health, to improve circadian physiology, as well as to treat metabolic (e.g., diabetes, obesity) and neurological diseases (e.g., mental, neurodegenerative).