Output from VIP cells of the mammalian central clock regulates daily physiological rhythms

Defining the brain control of physiological stability

The last few decades have seen major advances in neurobiology and uncovered novel genetic and cellular substrates involved in the control of physiological set points. In this Review, I discuss the limitations in the definition of homeostatic set points established by Walter B Canon and highlight evidence that two other physiological systems, namely rheostasis and allostasis provide distinct inputs to independently modify set-point levels. Using data collected over the past decade, the hypothalamic and genetic basis of regulated changes in set-point values by rheostatic mechanisms are described. Then, the role of higher-order brain regions, such as hippocampal circuits, for experience-dependent, allostatic induced changes in set-points are outlined. I propose that these systems provide a hierarchical organization of physiological stability that exists to maintain set-point values. The hierarchical organization of physiology has direct implications for basic and medical research, and clinical practice.

Turn off that night light! Light-at-night as a stressor for adolescents

Light-at-night is known to produce a wide variety of behavioral outcomes including promoting anxiety, depression, hyperactivity, abnormal sociability, and learning and memory deficits. Unfortunately, we all live in a 24-h society where people are exposed to light-at-night or light pollution through night-shift work - the need for all-hours emergency services - as well as building and street-lights, making light-at-night exposure practically unavoidable. Additionally, the increase in screentime (tvs and smart devices) during the night also contributes to poorer sleep and behavioral impairments. Compounding these factors is the fact that adolescents tend to be "night owls" and prefer an evening chronotype compared to younger children and adults, so these teenagers will have a higher likelihood of being exposed to light-at-night. Making matters worse is the prevalence of high-school start times of 8 am or earlier - a combination of too early school start times, light exposure during the night, and preference for evening chronotypes is a recipe for reduced and poorer sleep, which can contribute to increased susceptibility for behavioral issues for this population. As such, this mini-review will show, using both human and rodent model studies, how light-at-night affects behavioral outcomes and stress responses, connecting photic signaling and the circadian timing system to the hypothalamic-pituitary adrenal axis. Additionally, this review will also demonstrate that adolescents are more likely to exhibit abnormal behavior in response to light-at-night due to changes in development and hormone regulation during this time period, as well as discuss potential interventions that can help mitigate these negative effects.

Diverse genetic alteration dysregulates neuropeptide and intracellular signalling in the suprachiasmatic nuclei

In mammals, intrinsic 24 h or circadian rhythms are primarily generated by the suprachiasmatic nuclei (SCN). Rhythmic daily changes in the transcriptome and proteome of SCN cells are controlled by interlocking transcription-translation feedback loops (TTFLs) of core clock genes and their proteins. SCN cells function as autonomous circadian oscillators, which synchronize through intercellular neuropeptide signalling. Physiological and behavioural rhythms can be severely disrupted by genetic modification of a diverse range of genes and proteins in the SCN. With the advent of next generation sequencing, there is unprecedented information on the molecular profile of the SCN and how it is affected by genetically targeted alteration. However, whether the expression of some genes is more readily affected by genetic alteration of the SCN is unclear. Here, using publicly available datasets from recent RNA-seq assessments of the SCN from genetically altered and control mice, we evaluated whether there are commonalities in transcriptome dysregulation. This was completed for four different phases across the 24 h cycle and was augmented by Gene Ontology Molecular Function (GO:MF) and promoter analysis. Common differentially expressed genes (DEGs) and/or enriched GO:MF terms included signalling molecules, their receptors, and core clock components. Finally, examination of the JASPAR database indicated that E-box and CRE elements in the promoter regions of several commonly dysregulated genes. From this analysis, we identify differential expression of genes coding for molecules involved in SCN intra- and intercellular signalling as a potential cause of abnormal circadian rhythms.

System-level time computation and representation in the suprachiasmatic nucleus revealed by large-scale calcium imaging and machine learning

The suprachiasmatic nucleus (SCN) is the mammalian central circadian pacemaker with heterogeneous neurons acting in concert while each neuron harbors a self-sustained molecular clockwork. Nevertheless, how system-level SCN signals encode time of the day remains enigmatic. Here we show that population-level Ca2+ signals predict hourly time, via a group decision-making mechanism coupled with a spatially modular time feature representation in the SCN. Specifically, we developed a high-speed dual-view two-photon microscope for volumetric Ca2+ imaging of up to 9000 GABAergic neurons in adult SCN slices, and leveraged machine learning methods to capture emergent properties from multiscale Ca2+ signals as a whole. We achieved hourly time prediction by polling random cohorts of SCN neurons, reaching 99.0% accuracy at a cohort size of 900. Further, we revealed that functional neuron subtypes identified by contrastive learning tend to aggregate separately in the SCN space, giving rise to bilaterally symmetrical ripple-like modular patterns. Individual modules represent distinctive time features, such that a module-specifically learned time predictor can also accurately decode hourly time from random polling of the same module. These findings open a new paradigm in deciphering the design principle of the biological clock at the system level.

Estrogen and testosterone secretion from the mouse brain

Estrogen and testosterone are typically thought of as gonadal or adrenal derived steroids that cross the blood brain barrier to signal via both rapid nongenomic and slower genomic signalling pathways. Estrogen and testosterone signalling has been shown to drive interlinked behaviours such as social behaviours and cognition by binding to their cognate receptors in hypothalamic and forebrain nuclei. So far, acute brain slices have been used to study short-term actions of 17β-estradiol, typically using electrophysiological measures. For example, these techniques have been used to investigate, nongenomic signalling by estrogen such as the estrogen modulation of long-term potentiation (LTP) in the hippocampus. Using a modified method that preserves the slice architecture, we show, for the first time, that acute coronal slices from the prefrontal cortex and from the hypothalamus maintained in aCSF over longer periods i.e. 24 h can be steroidogenic, increasing their secretion of testosterone and estrogen. We also show that the hypothalamic nuclei produce more estrogen and testosterone than the prefrontal cortex. Therefore, this extended acute slice system can be used to study the regulation of steroid production and secretion by discrete nuclei in the brain.

The Suprachiasmatic Nucleus at 50: Looking Back, Then Looking Forward

It has been 50 years since the suprachiasmatic nucleus (SCN) was first identified as the central circadian clock and 25 years since the last overview of developments in the field was published in the Journal of Biological Rhythms. Here, we explore new mechanisms and concepts that have emerged in the subsequent 25 years. Since 1997, methodological developments, such as luminescent and fluorescent reporter techniques, have revealed intricate relationships between cellular and network-level mechanisms. In particular, specific neuropeptides such as arginine vasopressin, vasoactive intestinal peptide, and gastrin-releasing peptide have been identified as key players in the synchronization of cellular circadian rhythms within the SCN. The discovery of multiple oscillators governing behavioral and physiological rhythms has significantly advanced our understanding of the circadian clock. The interaction between neurons and glial cells has been found to play a crucial role in regulating these circadian rhythms within the SCN. Furthermore, the properties of the SCN network vary across ontogenetic stages. The application of cell type-specific genetic manipulations has revealed components of the functional input-output system of the SCN and their correlation with physiological functions. This review concludes with the high-risk effort of identifying open questions and challenges that lie ahead.

Integrated neural tracing and in-situ barcoded sequencing reveals the logic of SCN efferent circuits in regulating circadian behaviors

The circadian clock coordinates rhythms in numerous physiological processes to maintain organismal homeostasis. Since the suprachiasmatic nucleus (SCN) is widely accepted as the circadian pacemaker, it is critical to understand the neural mechanisms by which rhythmic information is transferred from the SCN to peripheral clocks. Here, we present the first comprehensive map of SCN efferent connections and suggest a molecular logic underlying these projections. The SCN projects broadly to most major regions of the brain, rather than solely to the hypothalamus and thalamus. The efferent projections from different subtypes of SCN neurons vary in distance and intensity, and blocking synaptic transmission of these circuits affects circadian rhythms in locomotion and feeding to different extents. We also developed a barcoding system to integrate retrograde tracing with in-situ sequencing, allowing us to link circuit anatomy and spatial patterns of gene expression. Analyses using this system revealed that brain regions functioning downstream of the SCN receive input from multiple neuropeptidergic cell types within the SCN, and that individual SCN neurons generally project to a single downstream brain region. This map of SCN efferent connections provides a critical foundation for future investigations into the neural circuits underlying SCN-mediated rhythms in physiology. Further, our new barcoded tracing method provides a tool for revealing the molecular logic of neuronal circuits within heterogeneous brain regions.

Phenotyping of light-activated neurons in the mouse SCN based on the expression of FOS and EGR1

Light-sensitive neurons are located in the ventral and central core of the suprachiasmatic nucleus (SCN), whereas stably oscillating clock neurons are found mainly in the dorsal shell. Signals between the SCN core and shell are believed to play an important role in light entrainment. Core neurons express vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), and Neuroglobin (Ngb), whereas the shell neurons express vasopressin (AVP), prokineticin 2, and the VIP type 2 (VPAC2) receptor. In rodents, light has a phase-shifting capacity at night, which induces rapid and transient expression of the EGR1 and FOS in the SCN. Methods: The present study used immunohistochemical staining of FOS, EGR1, and phenotypical markers of SCN neurons (VIP, AVP, Ngb) to identify subtypes/populations of light-responsive neurons at early night. Results: Double immunohistochemistry and cell counting were used to evaluate the number of SCN neurons expressing FOS and EGR1 in the SCN. The number of neurons expressing either EGR1 or FOS was higher than the total number of neurons co-storing EGR1 and FOS. Of the total number of light-responsive cells, 42% expressed only EGR1, 43% expressed only FOS, and 15% expressed both EGR1 and FOS. Light-responsive VIP neurons represented only 31% of all VIP neurons, and EGR1 represents the largest group of light-responsive VIP neurons (18%). VIP neurons expressing only FOS represented 1% of the total light-responsive VIP neurons. 81% of the Ngb neurons in the mouse SCN were light-responsive, and of these neurons expressing only EGR1 after light stimulation represented 44%, whereas 24% expressed FOS. Although most light-responsive neurons are found in the core of the SCN, 29% of the AVP neurons in the shell were light-responsive, of which 8% expressed EGR1, 10% expressed FOS, and 11% co-expressed both EGR1 and FOS after light stimulation. Discussion: Our analysis revealed cell-specific differences in light responsiveness between different peptidergic and Ngb-expressing neurons in different compartments of the mouse SCN, indicating that light activates diverse neuronal networks in the SCN, some of which participate in photoentrainment.

The Physiological and Pharmacological Significance of the Circadian Timing of the HPA Axis: A Mathematical Modeling Approach

As a potent endogenous regulator of homeostasis, the circadian time-keeping system synchronizes internal physiology to periodic changes in the external environment to enhance survival. Adapting endogenous rhythms to the external time is accomplished hierarchically with the central pacemaker located in the suprachiasmatic nucleus (SCN) signaling the hypothalamus-pituitary-adrenal (HPA) axis to release hormones, notably cortisol, which help maintain the body's circadian rhythm. Given the essential role of HPA-releasing hormones in regulating physiological functions, including immune response, cell cycle, and energy metabolism, their daily variation is critical for the proper function of the circadian timing system. In this review, we focus on cortisol and key fundamental properties of the HPA axis and highlight their importance in controlling circadian dynamics. We demonstrate how systems-driven, mathematical modeling of the HPA axis complements experimental findings, enhances our understanding of complex physiological systems, helps predict potential mechanisms of action, and elucidates the consequences of circadian disruption. Finally, we outline the implications of circadian regulation in the context of personalized chronotherapy. Focusing on the chrono-pharmacology of synthetic glucocorticoids, we review the challenges and opportunities associated with moving toward personalized therapies that capitalize on circadian rhythms.

Pushing the Frontiers: Optogenetics for Illuminating the Neural Pathophysiology of Bipolar Disorder

Bipolar disorder (BD), a disabling mental disorder, is featured by the oscillation between episodes of depression and mania, along with disturbance in the biological rhythms. It is on an urgent demand to identify the intricate mechanisms of BD pathophysiology. Based on the continuous progression of neural science techniques, the dysfunction of circuits in the central nervous system was currently thought to be tightly associated with BD development. Yet, challenge exists since it depends on techniques that can manipulate spatiotemporal dynamics of neuron activity. Notably, the emergence of optogenetics has empowered researchers with precise timing and local manipulation, providing a possible approach for deciphering the pathological underpinnings of mental disorders. Although the application of optogenetics in BD research remains preliminary due to the scarcity of valid animal models, this technique will advance the psychiatric research at neural circuit level. In this review, we summarized the crucial aberrant brain activity and function pertaining to emotion and rhythm abnormities, thereby elucidating the underlying neural substrates of BD, and highlighted the importance of optogenetics in the pursuit of BD research.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

Circadian Rhythms and Astrocytes: The Good, the Bad, and the Ugly

This review explores the interface between circadian timekeeping and the regulation of brain function by astrocytes. Although astrocytes regulate neuronal activity across many time domains, their cell-autonomous circadian clocks exert a particular role in controlling longer-term oscillations of brain function: the maintenance of sleep states and the circadian ordering of sleep and wakefulness. This is most evident in the central circadian pacemaker, the suprachiasmatic nucleus, where the molecular clock of astrocytes suffices to drive daily cycles of neuronal activity and behavior. In Alzheimer's disease, sleep impairments accompany cognitive decline. In mouse models of the disease, circadian disturbances accelerate astroglial activation and other brain pathologies, suggesting that daily functions in astrocytes protect neuronal homeostasis. In brain cancer, treatment in the morning has been associated with prolonged survival, and gliomas have daily rhythms in gene expression and drug sensitivity. Thus, circadian time is fast becoming critical to elucidating reciprocal astrocytic-neuronal interactions in health and disease.

The DNA methylation status of the serotonin metabolic pathway associated with reproductive inactivation induced by long-light exposure in Magang geese

BACKGROUND

Domestic geese are seasonal breeders and have the lowest reproductive capacity among all poultry species. Magang geese is a topical short-day breeder, short photoperiod exposure stimulates its reproductive activity while long photoperiod inhibits. To explore epigenetic change that could influence reproductive activity, we performed whole genome bisulfite sequencing and transcriptome sequencing in the hypothalamus at three reproductive stages during long-light exposure in male Magang geese.

RESULTS

A total number of 10,602 differentially methylated regions (DMRs) were identified among three comparison groups. We observed that the vast majority of DMRs were enriched in intron regions. By integrating the BS-sequencing and RNA-seq data, the correlation between methylation changes of CG DMRs and expression changes of their associated genes was significant only for genes containing CG DMRs in their intron. A total of 278 DMR-associated DEGs were obtained among the three stages. KEGG analysis revealed that the DMR-associated DEGs were mainly involved in 11 pathways. Among them, the neuroactive ligand-receptor interaction pathway was significantly enriched in both two comparisons (RA vs.RD and RD vs.RI); the Wnt signaling pathway, apelin signaling pathway, melanogenesis, calcium signaling pathway, focal adhesion, and adherens junction were significantly enriched in the RA vs. RI comparison. In addition, the expression level of two serotonin-metabolic genes was significantly altered during reproductive axis inactivation by the methylation status of their promoter region (TPH2) and intron region (SLC18A2), respectively. These results were confirmed by Bisulfite sequencing PCR (BSP), pyrosequencing, and real-time qPCR, indicating that serotonin metabolic signaling may play a key role in decreasing the reproductive activity of Magang geese induced by long-light exposure. Furthermore, we performed a metabolomics approach to investigate the concentration of neurotransmitters among the three stages, and found that 5-HIAA, the last product of the serotonin metabolic pathway, was significantly decreased in the hypothalamus during RI.

CONCLUSIONS

Our study reveals that the methylation status of the serotonin metabolic pathway in the hypothalamus is associated with reproductive inactivation, and provided new insight into the effect of DNA methylation on the reproductive regulation of the hypothalamus in Magang geese.

Circadian rhythm in cardiovascular diseases: a bibliometric analysis of the past, present, and future

BACKGROUND

One of the most prominent features of living organisms is their circadian rhythm, which governs a wide range of physiological processes and plays a critical role in maintaining optimal health and function in response to daily environmental changes. This work applied bibliometric analysis to explore quantitative and qualitative trends in circadian rhythm in cardiovascular diseases (CVD). It also aims to identify research hotspots and provide fresh suggestions for future research.

METHODS

The Web of Science Core Collection was used to search the data on circadian rhythm in CVD. HistCite, CiteSpace, and VOSviewer were used for bibliometric analysis and visualization. The analysis included the overall distribution of yearly outputs, top nations, active institutions and authors, core journals, co-cited references, and keywords. To assess the quality and efficacy of publications, the total global citation score (TGCS) and total local citation score (TLCS) were calculated.

RESULTS

There were 2102 papers found to be associated with the circadian rhythm in CVD, with the overall number of publications increasing year after year. The United States had the most research citations and was the most prolific country. Hermida RC, Young ME, and Ayala DE were the top three writers. The three most notable journals on the subject were Chronobiology International, Hypertension Research, and Hypertension. In the early years, the major emphasis of circadian rhythm in CVD was hormones. Inflammation, atherosclerosis, and myocardial infarction were the top developing research hotspots.

CONCLUSION

Circadian rhythm in CVD has recently received a lot of interest from the medical field. These topics, namely inflammation, atherosclerosis, and myocardial infarction, are critical areas of investigation for understanding the role of circadian rhythm in CVD. Although they may not be future research priorities, they remain of significant importance. In addition, how to implement these chronotherapy theories in clinical practice will depend on additional clinical trials to get sufficient trustworthy clinical evidence.

Cholecystokinin neurons in mouse suprachiasmatic nucleus regulate the robustness of circadian clock

The suprachiasmatic nucleus (SCN) can generate robust circadian behaviors in mammals under different environments, but the underlying neural mechanisms remained unclear. Here, we showed that the activities of cholecystokinin (CCK) neurons in the mouse SCN preceded the onset of behavioral activities under different photoperiods. CCK-neuron-deficient mice displayed shortened free-running periods, failed to compress their activities under a long photoperiod, and developed rapid splitting or became arrhythmic under constant light. Furthermore, unlike vasoactive intestinal polypeptide (VIP) neurons, CCK neurons are not directly light sensitive, but their activation can elicit phase advance and counter light-induced phase delay mediated by VIP neurons. Under long photoperiods, the impact of CCK neurons on SCN dominates over that of VIP neurons. Finally, we found that the slow-responding CCK neurons control the rate of recovery during jet lag. Together, our results demonstrated that SCN CCK neurons are crucial for the robustness and plasticity of the mammalian circadian clock.

Somatostatin regulates central clock function and circadian responses to light

Daily and annual changes in light are processed by central clock circuits that control the timing of behavior and physiology. The suprachiasmatic nucleus (SCN) in the anterior hypothalamus processes daily photic inputs and encodes changes in day length (i.e., photoperiod), but the SCN circuits that regulate circadian and photoperiodic responses to light remain unclear. Somatostatin (SST) expression in the hypothalamus is modulated by photoperiod, but the role of SST in SCN responses to light has not been examined. Our results indicate that SST signaling regulates daily rhythms in behavior and SCN function in a manner influenced by sex. First, we use cell-fate mapping to provide evidence that SST in the SCN is regulated by light via de novo Sst activation. Next, we demonstrate that Sst  -/- mice display enhanced circadian responses to light, with increased behavioral plasticity to photoperiod, jetlag, and constant light conditions. Notably, lack of Sst  -/- eliminated sex differences in photic responses due to increased plasticity in males, suggesting that SST interacts with clock circuits that process light differently in each sex. Sst  -/- mice also displayed an increase in the number of retinorecipient neurons in the SCN core, which express a type of SST receptor capable of resetting the molecular clock. Last, we show that lack of SST signaling modulates central clock function by influencing SCN photoperiodic encoding, network after-effects, and intercellular synchrony in a sex-specific manner. Collectively, these results provide insight into peptide signaling mechanisms that regulate central clock function and its response to light.

Inputs and Outputs of the Mammalian Circadian Clock

Circadian rhythms in mammals are coordinated by the central circadian pacemaker, the suprachiasmatic nucleus (SCN). Light and other environmental inputs change the timing of the SCN neural network oscillator, which, in turn, sends output signals that entrain daily behavioral and physiological rhythms. While much is known about the molecular, neuronal, and network properties of the SCN itself, the circuits linking the outside world to the SCN and the SCN to rhythmic outputs are understudied. In this article, we review our current understanding of the synaptic and non-synaptic inputs onto and outputs from the SCN. We propose that a more complete description of SCN connectivity is needed to better explain how rhythms in nearly all behaviors and physiological processes are generated and to determine how, mechanistically, these rhythms are disrupted by disease or lifestyle.

Evolutionary conservation and diversification of auditory neural circuits that process courtship songs in Drosophila

Acoustic communication signals diversify even on short evolutionary time scales. To understand how the auditory system underlying acoustic communication could evolve, we conducted a systematic comparison of the early stages of the auditory neural circuit involved in song information processing between closely-related fruit-fly species. Male Drosophila melanogaster and D. simulans produce different sound signals during mating rituals, known as courtship songs. Female flies from these species selectively increase their receptivity when they hear songs with conspecific temporal patterns. Here, we firstly confirmed interspecific differences in temporal pattern preferences; D. simulans preferred pulse songs with longer intervals than D. melanogaster. Primary and secondary song-relay neurons, JO neurons and AMMC-B1 neurons, shared similar morphology and neurotransmitters between species. The temporal pattern preferences of AMMC-B1 neurons were also relatively similar between species, with slight but significant differences in their band-pass properties. Although the shift direction of the response property matched that of the behavior, these differences are not large enough to explain behavioral differences in song preferences. This study enhances our understanding of the conservation and diversification of the architecture of the early-stage neural circuit which processes acoustic communication signals.

The Mammalian Circadian Time-Keeping System

Our physiology and behavior follow precise daily programs that adapt us to the alternating opportunities and challenges of day and night. Under experimental isolation, these rhythms persist with a period of approximately one day (circadian), demonstrating their control by an internal autonomous clock. Circadian time is created at the cellular level by a transcriptional/translational feedback loop (TTFL) in which the protein products of the Period and Cryptochrome genes inhibit their own transcription. Because the accumulation of protein is slow and delayed, the system oscillates spontaneously with a period of ∼24 hours. This cell-autonomous TTFL controls cycles of gene expression in all major tissues and these cycles underpin our daily metabolic programs. In turn, our innumerable cellular clocks are coordinated by a central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. When isolated in slice culture, the SCN TTFL and its dependent cycles of neural activity persist indefinitely, operating as "a clock in a dish". In vivo, SCN time is synchronized to solar time by direct innervation from specialized retinal photoreceptors. In turn, the precise circadian cycle of action potential firing signals SCN-generated time to hypothalamic and brain stem targets, which co-ordinate downstream autonomic, endocrine, and behavioral (feeding) cues to synchronize and sustain the distributed cellular clock network. Circadian time therefore pervades every level of biological organization, from molecules to society. Understanding its mechanisms offers important opportunities to mitigate the consequences of circadian disruption, so prevalent in modern societies, that arise from shiftwork, aging, and neurodegenerative diseases, not least Huntington's disease.

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.

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Cell-Based Phenotypic Screens to Discover Circadian Clock-Modulating Compounds

There is increasing demand to control circadian clock functions in a conditional manner for deeper understanding of the circadian system as well as for potential treatment of clock-related diseases. Small-molecule compounds provide powerful tools to reveal novel functions of target proteins in the circadian clock mechanism, and can be great therapeutic candidates. Here we describe the detailed methods of measuring cellular circadian rhythms in a high-throughput manner for chemical screening to identify compounds that affect circadian rhythms by targeting clock-related proteins.

Neural circuits in the central circadian clock and their regulation of sleep and wakefulness in mammals☆

Circadian rhythms are defined as approximately 24-hour oscillations in physiology and behavior. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is known as the central circadian clock. Based on current understanding, circadian rhythms are believed to be generated by transcription-translation feedback loops (TTFL) involving several clock genes and their protein products. However, several studies have shown that circadian oscillation in single SCN cells is still detectable in several clock gene deficient mice. These results suggest that there might be some oscillatory mechanisms without TTFL in mammalian cells. Other important aspects of circadian rhythms include neuronal circuits in the brain that regulate timing of physiological functions. Especially, functional output pathways from the SCN that regulate sleep and wakefulness have not been identified. In this review, I describe recent findings on circadian rhythm in the SCN, and of neuronal mechanisms that control circadian clock regulated sleep and wakefulness in mice.

Conditional Knockout of Bmal1 in Corticotropin-Releasing Factor Neurons Does Not Alter Sleep–Wake Rhythm in Mice

Sleep and wakefulness are regulated by both the homeostatic mechanism and circadian clock. In mammals, the central circadian clock, the suprachiasmatic nucleus, in the hypothalamus plays a crucial role in the timing of physiology and behavior. Recently, we found that the circadian regulation of wakefulness was transmitted via corticotropin-releasing factor (CRF) neurons in the paraventricular nucleus of the hypothalamus to orexin neurons in the lateral hypothalamus. However, it is still unclear how the molecular clock in the CRF neurons contributes to the regulation of sleep and wakefulness. In the present study, we established CRF neuron-specific Bmal1-deficient mice and measured locomotor activity or electroencephalography and electromyography. We found that these mice showed normal circadian locomotor activity rhythms in both light–dark cycle and constant darkness. Furthermore, they showed normal daily patterns of sleep and wakefulness. These results suggest that Bmal1 in CRF neurons has no effect on either circadian locomotor activity or sleep and wakefulness.

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

Circadian Rhythms, Disease and Chronotherapy

Circadian clocks are biological timing mechanisms that generate 24-h rhythms of physiology and behavior, exemplified by cycles of sleep/wake, hormone release, and metabolism. The adaptive value of clocks is evident when internal body clocks and daily environmental cycles are mismatched, such as in the case of shift work and jet lag or even mistimed eating, all of which are associated with physiological disruption and disease. Studies with animal and human models have also unraveled an important role of functional circadian clocks in modulating cellular and organismal responses to physiological cues (ex., food intake, exercise), pathological insults (e.g. virus and parasite infections), and medical interventions (e.g. medication). With growing knowledge of the molecular and cellular mechanisms underlying circadian physiology and pathophysiology, it is becoming possible to target circadian rhythms for disease prevention and treatment. In this review, we discuss recent advances in circadian research and the potential for therapeutic applications that take patient circadian rhythms into account in treating disease.

Optogenetic stimulation of VIPergic SCN neurons induces photoperiodic-like changes in the mammalian circadian clock

Circadian clocks play key roles in how organisms respond to and even anticipate seasonal change in day length, or photoperiod. In mammals, photoperiod is encoded by the central circadian pacemaker in the brain, the suprachiasmatic nucleus (SCN). The subpopulation of SCN neurons that secrete the neuropeptide VIP mediates the transmission of light information within the SCN neural network, suggesting a role for these neurons in circadian plasticity in response to light information that has yet to be directly tested. Here, we used in vivo optogenetic stimulation of VIPergic SCN neurons followed by ex vivo PERIOD 2::LUCIFERASE (PER2::LUC) bioluminescent imaging to test whether activation of this SCN neuron subpopulation can induce SCN network changes that are hallmarks of photoperiodic encoding. We found that optogenetic stimulation designed to mimic a long photoperiod indeed altered subsequent SCN entrained phase, increased the phase dispersal of PER2 rhythms within the SCN network, and shortened SCN free-running period-similar to the effects of a true extension of photoperiod. Optogenetic stimulation also induced analogous changes on related aspects of locomotor behaviour in vivo. Thus, selective activation of VIPergic SCN neurons induces photoperiodic network plasticity in the SCN that underpins photoperiodic entrainment of behaviour.

Circadian neurons in the paraventricular nucleus entrain and sustain daily rhythms in glucocorticoids

Signals from the central circadian pacemaker, the suprachiasmatic nucleus (SCN), must be decoded to generate daily rhythms in hormone release. Here, we hypothesized that the SCN entrains rhythms in the paraventricular nucleus (PVN) to time the daily release of corticosterone. In vivo recording revealed a critical circuit from SCN vasoactive intestinal peptide (SCNVIP)-producing neurons to PVN corticotropin-releasing hormone (PVNCRH)-producing neurons. PVNCRH neurons peak in clock gene expression around midday and in calcium activity about three hours later. Loss of the clock gene Bmal1 in CRH neurons results in arrhythmic PVNCRH calcium activity and dramatically reduces the amplitude and precision of daily corticosterone release. SCNVIP activation reduces (and inactivation increases) corticosterone release and PVNCRH calcium activity, and daily SCNVIP activation entrains PVN clock gene rhythms by inhibiting PVNCRH neurons. We conclude that daily corticosterone release depends on coordinated clock gene and neuronal activity rhythms in both SCNVIP and PVNCRH neurons.

Neurohumoral Cardiac Regulation: Optogenetics Gets Into the Groove

The cardiac autonomic nervous system (ANS) is the main modulator of heart function, adapting contraction force, and rate to the continuous variations of intrinsic and extrinsic environmental conditions. While the parasympathetic branch dominates during rest-and-digest sympathetic neuron (SN) activation ensures the rapid, efficient, and repeatable increase of heart performance, e.g., during the "fight-or-flight response." Although the key role of the nervous system in cardiac homeostasis was evident to the eyes of physiologists and cardiologists, the degree of cardiac innervation, and the complexity of its circuits has remained underestimated for too long. In addition, the mechanisms allowing elevated efficiency and precision of neurogenic control of heart function have somehow lingered in the dark. This can be ascribed to the absence of methods adequate to study complex cardiac electric circuits in the unceasingly moving heart. An increasing number of studies adds to the scenario the evidence of an intracardiac neuron system, which, together with the autonomic components, define a little brain inside the heart, in fervent dialogue with the central nervous system (CNS). The advent of optogenetics, allowing control the activity of excitable cells with cell specificity, spatial selectivity, and temporal resolution, has allowed to shed light on basic neuro-cardiology. This review describes how optogenetics, which has extensively been used to interrogate the circuits of the CNS, has been applied to untangle the knots of heart innervation, unveiling the cellular mechanisms of neurogenic control of heart function, in physiology and pathology, as well as those participating to brain-heart communication, back and forth. We discuss existing literature, providing a comprehensive view of the advancement in the understanding of the mechanisms of neurogenic heart control. In addition, we weigh the limits and potential of optogenetics in basic and applied research in neuro-cardiology.

Timed daily exercise remodels circadian rhythms in mice

Regular exercise is important for physical and mental health. An underexplored and intriguing property of exercise is its actions on the body’s 24 h or circadian rhythms. Molecular clock cells in the brain’s suprachiasmatic nuclei (SCN) use electrical and chemical signals to orchestrate their activity and convey time of day information to the rest of the brain and body. To date, the long-lasting effects of regular physical exercise on SCN clock cell coordination and communication remain unresolved. Utilizing mouse models in which SCN intercellular neuropeptide signaling is impaired as well as those with intact SCN neurochemical signaling, we examined how daily scheduled voluntary exercise (SVE) influenced behavioral rhythms and SCN molecular and neuronal activities. We show that in mice with disrupted neuropeptide signaling, SVE promotes SCN clock cell synchrony and robust 24 h rhythms in behavior. Interestingly, in both intact and neuropeptide signaling deficient animals, SVE reduces SCN neural activity and alters GABAergic signaling. These findings illustrate the potential utility of regular exercise as a long-lasting and effective non-invasive intervention in the elderly or mentally ill where circadian rhythms can be blunted and poorly aligned to the external world.

Using mice with disrupted neuropeptide signaling, Hughes et al. show that daily scheduled voluntary exercise (SVE) promotes suprachiasmatic nuclei (SCN) clock cell synchrony and robust 24 h rhythms in behavior. This study suggests the potential utility of regular exercise as a non-invasive intervention for the elderly or mentally ill, where circadian rhythms can be poorly aligned to the external world.

Bright daytime light enhances circadian amplitude in a diurnal mammal

Mammalian circadian rhythms are orchestrated by a master pacemaker in the hypothalamic suprachiasmatic nuclei (SCN), which receives information about the 24 h light-dark cycle from the retina. The accepted function of this light signal is to reset circadian phase in order to ensure appropriate synchronization with the celestial day. Here, we ask whether light also impacts another key property of the circadian oscillation, its amplitude. To this end, we measured circadian rhythms in behavioral activity, body temperature, and SCN electrophysiological activity in the diurnal murid rodent Rhabdomys pumilio following stable entrainment to 12:12 light-dark cycles at four different daytime intensities (ranging from 18 to 1,900 lx melanopic equivalent daylight illuminance). R. pumilio showed strongly diurnal activity and body temperature rhythms in all conditions, but measures of rhythm robustness were positively correlated with daytime irradiance under both entrainment and subsequent free run. Whole-cell and extracellular recordings of electrophysiological activity in ex vivo SCN revealed substantial differences in electrophysiological activity between dim and bright light conditions. At lower daytime irradiance, daytime peaks in SCN spontaneous firing rate and membrane depolarization were substantially depressed, leading to an overall marked reduction in the amplitude of circadian rhythms in spontaneous activity. Our data reveal a previously unappreciated impact of daytime light intensity on SCN physiology and the amplitude of circadian rhythms and highlight the potential importance of daytime light exposure for circadian health.

Using a bistable animal opsin for switchable and scalable optogenetic inhibition of neurons

There is no consensus on the best inhibitory optogenetic tool. Since Gi/o signalling is a native mechanism of neuronal inhibition, we asked whether Lamprey Parapinopsin ("Lamplight"), a Gi/o-coupled bistable animal opsin, could be used for optogenetic silencing. We show that short (405 nm) and long (525 nm) wavelength pulses repeatedly switch Lamplight between stable signalling active and inactive states, respectively, and that combining these wavelengths can be used to achieve intermediate levels of activity. These properties can be applied to produce switchable neuronal hyperpolarisation and suppression of spontaneous spike firing in the mouse hypothalamic suprachiasmatic nucleus. Expressing Lamplight in (predominantly) ON bipolar cells can photosensitise retinas following advanced photoreceptor degeneration, with 405 and 525 nm stimuli producing responses of opposite sign in the output neurons of the retina. We conclude that bistable animal opsins can co-opt endogenous signalling mechanisms to allow optogenetic inhibition that is scalable, sustained and reversible.

Distinct circadian mechanisms govern cardiac rhythms and susceptibility to arrhythmia

Electrical activity in the heart exhibits 24-hour rhythmicity, and potentially fatal arrhythmias are more likely to occur at specific times of day. Here, we demonstrate that circadian clocks within the brain and heart set daily rhythms in sinoatrial (SA) and atrioventricular (AV) node activity, and impose a time-of-day dependent susceptibility to ventricular arrhythmia. Critically, the balance of circadian inputs from the autonomic nervous system and cardiomyocyte clock to the SA and AV nodes differ, and this renders the cardiac conduction system sensitive to decoupling during abrupt shifts in behavioural routine and sleep-wake timing. Our findings reveal a functional segregation of circadian control across the heart's conduction system and inherent susceptibility to arrhythmia.

Roles of Neuropeptides, VIP and AVP, in the Mammalian Central Circadian Clock

In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Individual SCN cells exhibit intrinsic oscillations, and their circadian period and robustness are different cell by cell in the absence of cellular coupling, indicating that cellular coupling is important for coherent circadian rhythms in the SCN. Several neuropeptides such as arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are expressed in the SCN, where these neuropeptides function as synchronizers and are important for entrainment to environmental light and for determining the circadian period. These neuropeptides are also related to developmental changes of the circadian system of the SCN. Transcription factors are required for the formation of neuropeptide-related neuronal networks. Although VIP is critical for synchrony of circadian rhythms in the neonatal SCN, it is not required for synchrony in the embryonic SCN. During postnatal development, the clock genes cryptochrome (Cry)1 and Cry2 are involved in the maturation of cellular networks, and AVP is involved in SCN networks. This mini-review focuses on the functional roles of neuropeptides in the SCN based on recent findings in the literature.

A multi-level assessment of the bidirectional relationship between aging and the circadian clock

The daily temporal order of physiological processes and behavior contribute to the wellbeing of many organisms including humans. The central circadian clock, which coordinates the timing within our body, is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Like in other parts of the brain, aging impairs the SCN function, which in turn promotes the development and progression of aging-related diseases. We here review the impact of aging on the different levels of the circadian clock machinery-from molecules to organs-with a focus on the role of the SCN. We find that the molecular clock is less effected by aging compared to other cellular components of the clock. Proper rhythmic regulation of intracellular signaling, ion channels and neuronal excitability of SCN neurons are greatly disturbed in aging. This suggests a disconnection between the molecular clock and the electrophysiology of these cells. The neuronal network of the SCN is able to compensate for some of these cellular deficits. However, it still results in a clear reduction in the amplitude of the SCN electrical rhythm, suggesting a weakening of the output timing signal. Consequently, other brain areas and organs not only show aging-related deficits in their own local clocks, but also receive a weaker systemic timing signal. The negative spiral completes with the weakening of positive feedback from the periphery to the SCN. Consequently, chronotherapeutic interventions should aim at strengthening overall synchrony in the circadian system using life-style and/or pharmacological approaches.

The Circadian Clock Is Sustained in the Thyroid Gland of VIP Receptor 2 Deficient Mice

VIP/VPAC2-receptor signaling is crucial for functioning of the circadian clock in the suprachiasmatic nucleus (SCN) since the lack results in disrupted synchrony between SCN cells and altered locomotor activity, body temperature, hormone secretion and heart rhythm. Endocrine glands, including the thyroid, show daily oscillations in clock gene expression and hormone secretion, and SCN projections target neurosecretory hypothalamic thyroid-stimulating hormone (TSH)-releasing hormone cells. The aim of the study was to gain knowledge of mechanisms important for regulation of the thyroid clock by evaluating the impact of VIP/VPAC2-receptor signaling. Quantifications of mRNAs of three clock genes (Per1, Per2 and Bmal1) in thyroids of wild type (WT) and VPAC2-receptor deficient mice were done by qPCR. Tissues were taken every 4^th h during 24-h 12:12 light-dark (LD) and constant darkness (DD) periods, both genders were used. PER1 immunoreactivity was visualized on sections of both WT and VPAC2 lacking mice during a LD cycle. Finally, TSH and the thyroid hormone T4 levels were measured in the sera by commercial ELISAs. During LD, rhythmic expression of all three mRNA was found in both the WT and knockout animals. In VPAC2-receptor knockout animals, the amplitudes were approximately halved compared to the ones in the WT mice. In the WT, Per1 mRNA peaked around "sunset", Per2 mRNA followed with approximately 2 h, while Bmal1 mRNA was in antiphase with Per1. In the VPAC2 knockout mice, the phases of the mRNAs were advanced approximately 5 h compared to the WT. During DD, the phases of all the mRNAs were identical to the ones found during LD in both groups of mice. PER1 immunoreactivity was delayed compared to its mRNA and peaked during the night in follicular cells of both the thyroid and parathyroid glands in the WT animals. In WT animals, TSH was high around the transition to darkness compared to light-on, while T4 did not change during the 24 h cycle. In conclusion, sustained and identical rhythms (phases and amplitudes) of three clock genes were found in VPAC2 deficient mice during LD and DD suggesting high degree of independence of the thyroid clock from the master SCN clock.

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.

Rheostatic Balance of Circadian Rhythm and Autophagy in Metabolism and Disease

Circadian rhythms are physical, behavioral and environmental cycles that respond primarily to light and dark, with a period of time of approximately 24 h. The most essential physiological functions of mammals are manifested in circadian rhythm patterns, including the sleep-wake cycle and nutrient and energy metabolism. Autophagy is a conserved biological process contributing to nutrient and cellular homeostasis. The factors affecting autophagy are numerous, such as diet, drugs, and aging. Recent studies have indicated that autophagy is activated rhythmically in a clock-dependent manner whether the organism is healthy or has certain diseases. In addition, autophagy can affect circadian rhythm by degrading circadian proteins. This review discusses the interaction and mechanisms between autophagy and circadian rhythm. Moreover, we introduce the molecules influencing both autophagy and circadian rhythm. We then discuss the drugs affecting the circadian rhythm of autophagy. Finally, we present the role of rhythmic autophagy in nutrient and energy metabolism and its significance in physiology and metabolic disease.

The mammalian circadian pacemaker regulates wakefulness via CRF neurons in the paraventricular nucleus of the hypothalamus

In mammals, the daily rhythms of physiological functions are timed by the central circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Although the importance of the SCN for the regulation of sleep/wakefulness has been suggested, little is known about the neuronal projections from the SCN, which regulate sleep/wakefulness. Here, we show that corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus mediate circadian rhythms in the SCN and regulate wakefulness. Optogenetic activation of CRF neurons promoted wakefulness through orexin/hypocretin neurons in the lateral hypothalamus. In vivo Ca2+ recording showed that CRF neurons were active at the initiation of wakefulness. Furthermore, chemogenetic suppression and ablation of CRF neurons decreased locomotor activity and time in wakefulness. Last, a combination of optical manipulation and Ca2+ imaging revealed that neuronal activity of CRF neurons was negatively regulated by GABAergic neurons in the SCN. Our findings provide notable insights into circadian regulation of sleep/wakefulness in mammals.

Suprachiasmatic nucleus-dependent and independent outputs driving rhythmic activity in hypothalamic and thalamic neurons

BACKGROUND

Daily variations in mammalian physiology are under control of a central clock in the suprachiasmatic nucleus (SCN). SCN timing signals are essential for coordinating cellular clocks and associated circadian variations in cell and tissue function across the body; however, direct SCN projections primarily target a restricted set of hypothalamic and thalamic nuclei involved in physiological and behavioural control. The role of the SCN in driving rhythmic activity in these targets remains largely unclear. Here, we address this issue via multielectrode recording and manipulations of SCN output in adult mouse brain slices.

RESULTS

Electrical stimulation identifies cells across the midline hypothalamus and ventral thalamus that receive inhibitory input from the SCN and/or excitatory input from the retina. Optogenetic manipulations confirm that SCN outputs arise from both VIP and, more frequently, non-VIP expressing cells and that both SCN and retinal projections almost exclusively target GABAergic downstream neurons. The majority of midline hypothalamic and ventral thalamic neurons exhibit circadian variation in firing and those receiving inhibitory SCN projections consistently exhibit peak activity during epochs when SCN output is low. Physical removal of the SCN confirms that neuronal rhythms in ~ 20% of the recorded neurons rely on central clock input but also reveals many neurons that can express circadian variation in firing independent of any SCN input.

CONCLUSIONS

We identify cell populations across the midline hypothalamus and ventral thalamus exhibiting SCN-dependent and independent rhythms in neural activity, providing new insight into the mechanisms by which the circadian system generates daily physiological rhythms.

Circadian VIPergic Neurons of the Suprachiasmatic Nuclei Sculpt the Sleep-Wake Cycle

Although the mammalian rest-activity cycle is controlled by a "master clock" in the suprachiasmatic nucleus (SCN) of the hypothalamus, it is unclear how firing of individual SCN neurons gates individual features of daily activity. Here, we demonstrate that a specific transcriptomically identified population of mouse VIP+ SCN neurons is active at the "wrong" time of day-nighttime-when most SCN neurons are silent. Using chemogenetic and optogenetic strategies, we show that these neurons and their cellular clocks are necessary and sufficient to gate and time nighttime sleep but have no effect upon daytime sleep. We propose that mouse nighttime sleep, analogous to the human siesta, is a "hard-wired" property gated by specific neurons of the master clock to favor subsequent alertness prior to dawn (a circadian "wake maintenance zone"). Thus, the SCN is not simply a 24-h metronome: specific populations sculpt critical features of the sleep-wake cycle.