Projections from the subparaventricular zone define four channels of output from the circadian timing system

Efferent projections of Nps-expressing neurons in the parabrachial region

In the brain, connectivity determines function. Neurons in the parabrachial nucleus (PB) relay diverse information to widespread brain regions, but the connections and functions of PB neurons that express Nps (neuropeptide S) remain mysterious. Here, we use Cre-dependent anterograde tracing and whole-brain analysis to map their output connections. While many other PB neurons project ascending axons through the central tegmental tract, NPS axons reach the forebrain via distinct periventricular and ventral pathways. Along the periventricular pathway, NPS axons target the tectal longitudinal column and periaqueductal gray then continue rostrally to target the paraventricular nucleus of the thalamus. Along the ventral pathway, NPS axons blanket much of the hypothalamus but avoid the ventromedial and mammillary nuclei. They also project prominently to the ventral bed nucleus of the stria terminalis, A13 cell group, and magnocellular subparafasciular nucleus. In the hindbrain, NPS axons have fewer descending projections, targeting primarily the superior salivatory nucleus, nucleus of the lateral lemniscus, and periolivary region. Combined with what is known about NPS and its receptor, the output pattern of Nps-expressing neurons in the PB region predicts a role in threat response and circadian behavior.

Rhythms in barriers and fluids: Circadian clock regulation in the aging neurovascular unit

The neurovascular unit is where two very distinct physiological systems meet: The central nervous system (CNS) and the blood. The permeability of the barriers separating these systems is regulated by time, including both the 24 h circadian clock and the longer processes of aging. An endogenous circadian rhythm regulates the transport of molecules across the blood-brain barrier and the circulation of the cerebrospinal fluid and the glymphatic system. These fluid dynamics change with time of day, and with age, and especially in the context of neurodegeneration. Factors may differ depending on brain region, as can be highlighted by consideration of circadian regulation of the neurovascular niche in white matter. As an example of a potential target for clinical applications, we highlight chaperone-mediated autophagy as one mechanism at the intersection of circadian dysregulation, aging and neurodegenerative disease. In this review we emphasize key areas for future research.

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.

Circadian Rhythms Disrupted by Light at Night and Mistimed Food Intake Alter Hormonal Rhythms and Metabolism

Availability of artificial light and light-emitting devices have altered human temporal life, allowing 24-hour healthcare, commerce and production, and expanding social life around the clock. However, physiology and behavior that evolved in the context of 24 h solar days are frequently perturbed by exposure to artificial light at night. This is particularly salient in the context of circadian rhythms, the result of endogenous biological clocks with a rhythm of ~24 h. Circadian rhythms govern the temporal features of physiology and behavior, and are set to precisely 24 h primarily by exposure to light during the solar day, though other factors, such as the timing of meals, can also affect circadian rhythms. Circadian rhythms are significantly affected by night shift work because of exposure to nocturnal light, electronic devices, and shifts in the timing of meals. Night shift workers are at increased risk for metabolic disorder, as well as several types of cancer. Others who are exposed to artificial light at night or late mealtimes also show disrupted circadian rhythms and increased metabolic and cardiac disorders. It is imperative to understand how disrupted circadian rhythms alter metabolic function to develop strategies to mitigate their negative effects. In this review, we provide an introduction to circadian rhythms, physiological regulation of homeostasis by the suprachiasmatic nucleus (SCN), and SCN-mediated hormones that display circadian rhythms, including melatonin and glucocorticoids. Next, we discuss circadian-gated physiological processes including sleep and food intake, followed by types of disrupted circadian rhythms and how modern lighting disrupts molecular clock rhythms. Lastly, we identify how disruptions to hormones and metabolism can increase susceptibility to metabolic syndrome and risk for cardiovascular diseases, and discuss various strategies to mitigate the harmful consequences associated with disrupted circadian rhythms on human health.

Retinorecipient areas in the common marmoset (Callithrix jacchus): An image-forming and non-image forming circuitry

The mammalian retina captures a multitude of diverse features from the external environment and conveys them via the optic nerve to a myriad of retinorecipient nuclei. Understanding how retinal signals act in distinct brain functions is one of the most central and established goals of neuroscience. Using the common marmoset (Callithrix jacchus), a monkey from Northeastern Brazil, as an animal model for parsing how retinal innervation works in the brain, started decades ago due to their marmoset's small bodies, rapid reproduction rate, and brain features. In the course of that research, a large amount of new and sophisticated neuroanatomical techniques was developed and employed to explain retinal connectivity. As a consequence, image and non-image-forming regions, functions, and pathways, as well as retinal cell types were described. Image-forming circuits give rise directly to vision, while the non-image-forming territories support circadian physiological processes, although part of their functional significance is uncertain. Here, we reviewed the current state of knowledge concerning retinal circuitry in marmosets from neuroanatomical investigations. We have also highlighted the aspects of marmoset retinal circuitry that remain obscure, in addition, to identify what further research is needed to better understand the connections and functions of retinorecipient structures.

The anatomy of head pain

Pain-sensitive structures in the head and neck, including the scalp, periosteum, meninges, and blood vessels, are innervated predominantly by the trigeminal and upper cervical nerves. The trigeminal nerve supplies most of the sensation to the head and face, with the ophthalmic division (V1) providing innervation to much of the supratentorial dura mater and vessels. This creates referral patterns for pain that may be misleading to clinicians and patients, as described by studies involving awake craniotomies and stimulation with electrical and mechanical stimuli. Most brain parenchyma and supratentorial vessels refer pain to the ipsilateral V1 territory, and less commonly the V2 or V3 region. The upper cervical nerves provide innervation to the posterior scalp, while the periauricular region and posterior fossa are territories with shared innervation. Afferent fibers that innervate the head and neck send nociceptive input to the trigeminocervical complex, which then projects to additional pain processing areas in the brainstem, thalamus, hypothalamus, and cortex. This chapter discusses the pain-sensitive structures in the head and neck, including pain referral patterns for many of these structures. It also provides an overview of peripheral and central nervous system structures responsible for transmitting and interpreting these nociceptive signals.

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.

Neuropeptide S (NPS) neurons: Parabrachial identity and novel distributions

Neuropeptide S (NPS) increases wakefulness. A small number of neurons in the brainstem express Nps. These neurons are located in or near the parabrachial nucleus (PB), but we know very little about their ontogeny, connectivity, and function. To identify Nps-expressing neurons within the molecular framework of the PB region, we used in situ hybridization, immunofluorescence, and Cre-reporter labeling in mice. The primary concentration of Nps-expressing neurons borders the lateral lemniscus at far-rostral levels of the lateral PB. Caudal to this main cluster, Nps-expressing neurons scatter through the PB and form a secondary concentration medial to the locus coeruleus (LC). Most Nps-expressing neurons in the PB region are Atoh1-derived, Foxp2-expressing, and mutually exclusive with neurons expressing Calca or Lmx1b. Among Foxp2-expressing PB neurons, those expressing Nps are distinct from intermingled subsets expressing Cck or Pdyn. Examining Nps Cre-reporter expression throughout the brain identified novel populations of neurons in the nucleus incertus, anterior hypothalamus, and lateral habenula. This information will help focus experimental questions about the connectivity and function of NPS neurons.

Different levels of circadian (de)synchrony -- where does it hurt?

A network of cellular timers ensures the maintenance of homeostasis by temporal modulation of physiological processes across the day. These so-called circadian clocks are synchronized to geophysical time by external time cues (or zeitgebers). In modern societies, natural environmental cycles are disrupted by artificial lighting, around-the-clock availability of food or shiftwork. Such contradictory zeitgeber input promotes chronodisruption, i.e., the perturbation of internal circadian rhythms, resulting in adverse health outcomes. While this phenomenon is well described, it is still poorly understood at which level of organization perturbed rhythms impact on health and wellbeing. In this review, we discuss different levels of chronodisruption and what is known about their health effects. We summarize the results of disrupted phase coherence between external and internal time vs. misalignment of tissue clocks amongst each other, i.e., internal desynchrony. Last, phase incoherence can also occur at the tissue level itself. Here, alterations in phase coordination can emerge between cellular clocks of the same tissue or between different clock genes within the single cell. A better understanding of the mechanisms of circadian misalignment and its effects on physiology will help to find effective tools to prevent or treat disorders arising from modern-day chronodisruptive environments.

Different levels of circadian (de)synchrony -- where does it hurt?

A network of cellular timers ensures the maintenance of homeostasis by temporal modulation of physiological processes across the day. These so-called circadian clocks are synchronized to geophysical time by external time cues (or zeitgebers). In modern societies, natural environmental cycles are disrupted by artificial lighting, around-the-clock availability of food or shift work. Such contradictory zeitgeber input promotes chronodisruption, i.e., the perturbation of internal circadian rhythms, resulting in adverse health outcomes. While this phenomenon is well described, it is still poorly understood at which level of organization perturbed rhythms impact on health and wellbeing. In this review, we discuss different levels of chronodisruption and what is known about their health effects. We summarize the results of disrupted phase coherence between external and internal time vs. misalignment of tissue clocks amongst each other, i.e., internal desynchrony. Last, phase incoherence can also occur at the tissue level itself. Here, alterations in phase coordination can emerge between cellular clocks of the same tissue or between different clock genes within the single cell. A better understanding of the mechanisms of circadian misalignment and its effects on physiology will help to find effective tools to prevent or treat disorders arising from modern-day chronodisruptive environments.

Circadian Oscillations in the Murine Preoptic Area Are Reset by Temperature, but Not Light

Mammals maintain their internal body temperature within a physiologically optimal range. This involves the regulation of core body temperature in response to changing environmental temperatures and a natural circadian oscillation of internal temperatures. The preoptic area (POA) of the hypothalamus coordinates body temperature by responding to both external temperature cues and internal brain temperature. Here we describe an autonomous circadian clock system in the murine ventromedial POA (VMPO) in close proximity to cells which express the atypical violet-light sensitive opsin, Opn5. We analyzed the light-sensitivity and thermal-sensitivity of the VMPO circadian clocks ex vivo. The phase of the VMPO circadian oscillations was not influenced by light. However, the VMPO clocks were reset by temperature changes within the physiological internal temperature range. This thermal-sensitivity of the VMPO circadian clock did not require functional Opn5 expression or a functional circadian clock within the Opn5-expressing cells. The presence of temperature-sensitive circadian clocks in the VMPO provides an advancement in the understanding of mechanisms involved in the dynamic regulation of core body temperature.

Optimizing Sleep and Circadian Health in the NeuroICU

Purpose of Review

This article introduces fundamental concepts in circadian biology and the neuroscience of sleep, reviews recent studies characterizing circadian rhythm and sleep disruption among critically ill patients and potentially links to functional outcomes, and draws upon existing literature to propose therapeutic strategies to mitigate those harms. Particular attention is given to patients with critical neurologic conditions and the unique environment of the neuro-intensive care unit.

Recent Findings

Circadian rhythm disruption is widespread among critically ill patients and sleep time is reduced and abnormally fragmented. There is a strong association between the degree of arousal suppression observed at the bedside and the extent of circadian disruption at the system (e.g., melatonin concentration rhythms) and cellular levels (e.g., core clock gene transcription rhythms). There is a paucity of electrographically normal sleep, and rest-activity rhythms are severely disturbed. Common care interventions such as neurochecks introduce unique disruptions in neurologic patients. There are no pharmacologic interventions proven to normalize circadian rhythms or restore physiologically normal sleep. Instead, interventions are focused on reducing pharmacologic and environmental factors that perpetuate disruption.

Summary

The intensive care environment introduces numerous potent disruptors to sleep and circadian rhythms. Direct neurologic injury and neuro-monitoring practices likely compound those factors to further derange circadian and sleep functions. In the absence of direct interventions to induce normalized rhythms and sleep, current therapy depends upon normalizing external stimuli.

Neurobiology of dream activity and effects of stimulants on dreams

The sleep-wake cycle is the result of the activity of a multiple neurobiological network interaction. Dreaming feature is one interesting sleep phenomena that represents sensorial components, mostly visual perceptions, accompanied with intense emotions. Further complexity has been added to the topic of the neurobiological mechanism of dreams generation by the current data that suggests the influence of drugs on dream generation. Here, we discuss the review on some of the neurobiological mechanism of the regulation of dream activity, with special emphasis on the effects of stimulants on dreaming.

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.

Reduced numbers of corticotropin‐releasing hormone neurons in narcolepsy type 1

Narcolepsy type 1 (NT1) is a chronic sleep disorder correlated with loss of hypocretin(orexin). In NT1 post‐mortem brains, we observed 88% reduction in corticotropin‐releasing hormone (CRH)‐positive neurons in the paraventricular nucleus (PVN) and significantly less CRH‐positive fibers in the median eminence, whereas CRH‐neurons in the locus coeruleus and thalamus, and other PVN neuronal populations were spared: that is, vasopressin, oxytocin, tyrosine hydroxylase, and thyrotropin releasing hormone‐expressing neurons. Other hypothalamic cell groups, that is, the suprachiasmatic, ventrolateral preoptic, infundibular, and supraoptic nuclei and nucleus basalis of Meynert, were unaffected. The surprising selective decrease in CRH‐neurons provide novel targets for diagnostics and therapeutic interventions. ANN NEUROL 2022;91:282–288

Food reward induction of rhythmic clock gene expression in the prefrontal cortex of rats is accompanied by changes in miR-34a-5p expression

The current study is focused on mechanisms by which the peripheral circadian oscillator in the prefrontal cortex (PFC) participates in food reward-induced activity. The experimental group of male Wistar rats was trained to receive a food reward with a low hedonic and caloric value. Afterwards, animals were exposed to a 5 h phase advance. Experimental animals could access a small food reward as they had been accustomed to, while control rats were exposed to the same phase shift without access to a food reward. When synchronisation to a new light:dark cycle was accompanied by intake of food reward, animals exerted more exact phase shift compared to the controls. In rats with access to a food reward, a rhythm in dopamine receptors types 1 and 2 in the PFC was detected. Rhythmic clock gene expression was induced in the PFC of rats when a food reward was provided together with a phase shift. The per2 and clock genes are predicted targets of miR-34a-5p. The precursor form of miR-34a-5p (pre-miR-34a-5p) showed a daily rhythm in expression in the PFC of the control and experimental groups. On the other hand, the mature form of miR-34a-5p exerted an inverted rhythm compared to pre-miR-34a-5p and negative correlation with per and clock genes expression only in the PFC of rewarded rats. A difference in the pattern of mature and pre-miR-34a-5p values was not related to expression of enzymes drosha, dicer and dgcr8. A role of the clock genes and miR-34a-5p in reward-facilitated synchronisation has been hypothesised.

Dim light in the evening causes coordinated realignment of circadian rhythms, sleep, and short-term memory

In modern societies, people are regularly exposed to artificial light (e.g., light-emitting electronic devices). Dim light in the evening (DLE) imposes an artificial extension of the solar day, increasing our alertness before bedtime, delaying melatonin timing and sleep onset, and increasing sleepiness in the next morning. Using laboratory mice as a model organism, we show that 2 wk of 4-h, 20-lux DLE postpones rest–activity rhythms, delays molecular rhythms in the brain and body, and reverses the diurnal pattern of short-term memory performance. These results highlight the biological impact of DLE and emphasize the need to optimize our evening light exposure if we are to avoid shifting our biological clocks.

Light provides the primary signal for entraining circadian rhythms to the day/night cycle. In addition to rods and cones, the retina contains a small population of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). Concerns have been raised that exposure to dim artificial lighting in the evening (DLE) may perturb circadian rhythms and sleep patterns, and OPN4 is presumed to mediate these effects. Here, we examine the effects of 4-h, 20-lux DLE on circadian physiology and behavior in mice and the role of OPN4 in these responses. We show that 2 wk of DLE induces a phase delay of ∼2 to 3 h in mice, comparable to that reported in humans. DLE-induced phase shifts are unaffected in Opn4^−/− mice, indicating that rods and cones are capable of driving these responses in the absence of melanopsin. DLE delays molecular clock rhythms in the heart, liver, adrenal gland, and dorsal hippocampus. It also reverses short-term recognition memory performance, which is associated with changes in preceding sleep history. In addition, DLE modifies patterns of hypothalamic and cortical cFos signals, a molecular correlate of recent neuronal activity. Together, our data show that DLE causes coordinated realignment of circadian rhythms, sleep patterns, and short-term memory process in mice. These effects are particularly relevant as DLE conditions―due to artificial light exposure―are experienced by the majority of the populace on a daily basis.

Drosophila clock cells use multiple mechanisms to transmit time-of-day signals in the brain

Regulation of circadian behavior and physiology by the Drosophila brain clock requires communication from central clock neurons to downstream output regions, but the mechanism by which clock cells regulate downstream targets is not known. We show here that the pars intercerebralis (PI), previously identified as a target of the morning cells in the clock network, also receives input from evening cells. We determined that morning and evening clock neurons have time-of-day-dependent connectivity to the PI, which is regulated by specific peptides as well as by fast neurotransmitters. Interestingly, PI cells that secrete the peptide DH44, and control rest:activity rhythms, are inhibited by clock inputs while insulin-producing cells (IPCs) are activated, indicating that the same clock cells can use different mechanisms to drive cycling in output neurons. Inputs of morning cells to IPCs are relevant for the circadian rhythm of feeding, reinforcing the role of the PI as a circadian relay that controls multiple behavioral outputs. Our findings provide mechanisms by which clock neurons signal to nonclock cells to drive rhythms of behavior.

Systems and Circuits Linking Chronic Pain and Circadian Rhythms

Research over the last 20 years regarding the link between circadian rhythms and chronic pain pathology has suggested interconnected mechanisms that are not fully understood. Strong evidence for a bidirectional relationship between circadian function and pain has been revealed through inflammatory and immune studies as well as neuropathic ones. However, one limitation of many of these studies is a focus on only a few molecules or cell types, often within only one region of the brain or spinal cord, rather than systems-level interactions. To address this, our review will examine the circadian system as a whole, from the intracellular genetic machinery that controls its timing mechanism to its input and output circuits, and how chronic pain, whether inflammatory or neuropathic, may mediate or be driven by changes in these processes. We will investigate how rhythms of circadian clock gene expression and behavior, immune cells, cytokines, chemokines, intracellular signaling, and glial cells affect and are affected by chronic pain in animal models and human pathologies. We will also discuss key areas in both circadian rhythms and chronic pain that are sexually dimorphic. Understanding the overlapping mechanisms and complex interplay between pain and circadian mediators, the various nuclei they affect, and how they differ between sexes, will be crucial to move forward in developing treatments for chronic pain and for determining how and when they will achieve their maximum efficacy.

Chronic lead exposure alters photic entrainment of locomotor activity rhythm and neuronal photoactivation in the suprachiasmatic nucleus of the adult rat

Chronic lead (Pb) exposure affects the circadian physiological processes regulated by suprachiasmatic nucleus (SCN), which is synchronized (entrainment) by light. Disorders in the entrainment capacity of an organism alter its performance to interact with the environment, thus affecting its health status. The objectives of the present study were to evaluate whether chronic early Pb exposure affects the entrainment of the circadian rhythm of locomotor activity by light and to explore the possible mechanisms involved. Adult male Wistar rats, control and chronically exposed to Pb (320 ppm) in drinking water from gestation to adult age, were used. Assessment of the metal level showed a significant increase of Pb in the blood, hypothalamus and prefrontal cortex of the experimental rats. Continuous registrations of locomotor activity (12 h:12 h light-dark cycle) depicted that Pb induces important delay of this activity when the light was turned off. The Pb exposed animals entrained faster with a photoperiod delay of 6 h, (lights on at 13:00 h), and maintained the significant delay in the onset of activity at lights out. In continuous darkness, the animals were exposed to a light pulse at circadian time 23. This resulted in a significant decrease of photo-stimulated neurons (immunoreactivity to c-Fos) in the SCN of the metal-exposed animals. These results show that chronic early Pb exposure alters the photic entrainment of the rhythm of locomotor activity, which is evidenced by a significant decrease in both the number of photo-stimulated neurons and neuronal population (Nissl stain) of the SCN.

Circadian Gene Expression Rhythms During Critical Illness

OBJECTIVES

Core clock genes regulate tissue-specific transcriptome oscillations that synchronize physiologic processes throughout the body, held in phase by the central circadian rhythm. The central circadian rhythm rapidly dampens with onset of critical illness, but the effect of critical illness on gene expression oscillations is unknown. The objective of this study was to characterize the rhythmicity and phase coherence of core clock genes and the broader transcriptome after onset of critical illness.

DESIGN

Cross-sectional study.

SETTING

ICUs and hospital clinical research unit.

PATIENTS

Critically ill patients within the first day of presenting from the community and healthy volunteers.

INTERVENTIONS

Usual care (critically ill patients) and modified constant routine (healthy volunteers).

MEASUREMENTS AND MAIN RESULTS

We studied 15 critically ill patients, including 10 with sepsis and five with intracerebral hemorrhage, and 11 healthy controls. The central circadian rhythm and rest-activity rhythms were profiled by continuous wrist actigraphy, and serum melatonin sampled every 2 hours along with whole blood for RNA isolation over 24 hours. The gene expression transcriptome was obtained by RNA sequencing. Core clock genes were analyzed for rhythmicity by cosinor fit. Significant circadian rhythmicity was identified in five of six core clock genes in healthy controls, but none in critically ill patients. TimeSignature, a validated algorithm based on 41 genes, was applied to assess overall transcriptome phase coherence. Median absolute error of TimeSignature was higher in individual critically ill patients than healthy patients (4.90 vs 1.48 hr) and was correlated with encephalopathy severity by Glasgow Coma Scale in critically ill patients (rho, -0.54; p = 0.036).

CONCLUSIONS

Gene expression rhythms rapidly become abnormal during critical illness. The association between disrupted transcriptome rhythms and encephalopathy suggests a path for future work to elucidate the underlying pathophysiology.

Factors Disrupting Melatonin Secretion Rhythms During Critical Illness

OBJECTIVES

The circadian system modulates many important physiologic processes, synchronizing tissue-specific functions throughout the body. We sought to characterize acute alterations of circadian rhythms in critically ill patients and to evaluate associations between brain dysfunction, systemic multiple organ dysfunction, environmental stimuli that entrain the circadian rhythm (zeitgebers), rest-activity rhythms, and the central circadian rhythm-controlled melatonin secretion profile.

DESIGN

Prospective study observing a cohort for 24-48 hours beginning within the first day of ICU admission.

SETTING

Multiple specialized ICUs within an academic medical center.

PATIENTS

Patients presenting from the community with acute onset of either intracerebral hemorrhage as a representative neurologic critical illness or sepsis as a representative systemic critical illness. Healthy control patients were studied in using modified constant routine in a clinical research unit.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Light, feeding, activity, medications, and other treatment exposures were evaluated along with validated measures of encephalopathy (Glasgow Coma Scale), multiple organ system function (Sequential Organ Failure Assessment score), and circadian rhythms (profiles of serum melatonin and its urinary metabolite 6-sulphatoxymelatonin). We studied 112 critically ill patients, including 53 with sepsis and 59 with intracerebral hemorrhage. Environmental exposures were abnormal, including light (dim), nutritional intake (reduced or absent and mistimed), and arousal stimuli (increased and mistimed). Melatonin amplitude and acrophase timing were generally preserved in awake patients but dampened and delayed with increasing encephalopathy severity. Melatonin hypersecretion was observed in patients exposed to catecholamine vasopressor infusions, but unaffected by sedatives. Change in vasopressor exposure was the only factor associated with changes in melatonin rhythms between days 1 and 2.

CONCLUSIONS

Encephalopathy severity and adrenergic agonist medication exposure were the primary factors contributing to abnormal melatonin rhythms. Improvements in encephalopathy and medical stabilization did not rapidly normalize rhythms. Urinary 6-sulphatoxymelatonin is not a reliable measure of the central circadian rhythm in critically ill patients.

Sleep-Wake Neurobiology

Sleep and wakefulness are complex, tightly regulated behaviors that occur in virtually all animals. With recent exciting developments in neuroscience methodologies such as optogenetics, chemogenetics, and cell-specific calcium imaging technology, researchers can advance our understanding of how discrete neuronal groups precisely modulate states of sleep and wakefulness. In this chapter, we provide an overview of key neurotransmitter systems, neurons, and circuits that regulate states of sleep and wakefulness. We also describe long-standing models for the regulation of sleep/wake and non-rapid eye movement/rapid eye movement cycling. We contrast previous knowledge derived from classic approaches such as brain stimulation, lesions, cFos expression, and single-unit recordings, with emerging data using the newest technologies. Our understanding of neural circuits underlying the regulation of sleep and wakefulness is rapidly evolving, and this knowledge is critical for our field to elucidate the enigmatic function(s) of sleep.

Resilience in the suprachiasmatic nucleus: Implications for aging and Alzheimer's disease

Many believe that the circadian impairments associated with aging and Alzheimer's disease are, simply enough, a byproduct of tissue degeneration within the central pacemaker, the suprachiasmatic nucleus (SCN). However, the findings that have accumulated to date examining the SCNs obtained postmortem from the brains of older individuals, or those diagnosed with Alzheimer's disease upon autopsy, suggest only limited atrophy. We review this literature as well as a complementary one concerning fetal-donor SCN transplant, which established that many circadian timekeeping functions can be maintained with rudimentary (structurally limited) representations of the SCN. Together, these corpora of data suggest that the SCN is a resilient brain region that cannot be directly (or solely) implicated in the behavioral manifestations of circadian disorganization often witnessed during aging as well as early and late progression of Alzheimer's disease. We complete our review by suggesting future directions of research that may bridge this conceptual divide and briefly discuss the implications of it for improving health outcomes in later adulthood.

Circadian Gene Expression Rhythms During Critical Illness

OBJECTIVES

Core clock genes regulate tissue-specific transcriptome oscillations that synchronize physiologic processes throughout the body, held in phase by the central circadian rhythm. The central circadian rhythm rapidly dampens with onset of critical illness, but the effect of critical illness on gene expression oscillations is unknown. The objective of this study was to characterize the rhythmicity and phase coherence of core clock genes and the broader transcriptome after onset of critical illness.

DESIGN

Cross-sectional study.

SETTING

ICUs and hospital clinical research unit.

PATIENTS

Critically ill patients within the first day of presenting from the community and healthy volunteers.

INTERVENTIONS

Usual care (critically ill patients) and modified constant routine (healthy volunteers).

MEASUREMENTS AND MAIN RESULTS

We studied 15 critically ill patients, including 10 with sepsis and five with intracerebral hemorrhage, and 11 healthy controls. The central circadian rhythm and rest-activity rhythms were profiled by continuous wrist actigraphy, and serum melatonin sampled every 2 hours along with whole blood for RNA isolation over 24 hours. The gene expression transcriptome was obtained by RNA sequencing. Core clock genes were analyzed for rhythmicity by cosinor fit. Significant circadian rhythmicity was identified in five of six core clock genes in healthy controls, but none in critically ill patients. TimeSignature, a validated algorithm based on 41 genes, was applied to assess overall transcriptome phase coherence. Median absolute error of TimeSignature was higher in individual critically ill patients than healthy patients (4.90 vs 1.48 hr) and was correlated with encephalopathy severity by Glasgow Coma Scale in critically ill patients (rho, -0.54; p = 0.036).

CONCLUSIONS

Gene expression rhythms rapidly become abnormal during critical illness. The association between disrupted transcriptome rhythms and encephalopathy suggests a path for future work to elucidate the underlying pathophysiology.

Adaptation and survival: hypotheses about the neural mechanisms of unihemispheric sleep

Sleep and wakefulness are opposite brain and body conditions that accomplish different but complementary functions. However, these opposing conditions have been combined in some animals by the adoption of a sleep/wake strategy that allows them to survive, while maintaining both an interaction with the environment at the same time as enabling brain and body recovery. They sleep with half of the brain while keeping the other half awake: a state known as unihemispheric sleep (US). Sleep of cetaceans is exclusively in the form of US; therefore, they experience neither bihemispheric sleep (BS) nor REM sleep. US episodes have also been recorded in eared seals and some species of birds. In those animals, US episodes are intermingled with episodes of BS and REM sleep. Studies have reported both a lateralized release of some neurotransmitters and a drop of brain temperature during US. The aims of this article are to formulate hypotheses about the neural mechanisms of unihemispheric sleep(US) based on findings regarding the neural mechanisms of the sleep/wake cycle of mammals. The neural mechanisms of the sleep/wake cycle are largely preserved across species, allowing to hypothesize about those triggering and regulating US.

Vasopressin Neurons: Master Integrators of Time and Homeostasis

A recent article by Gizowski and Bourque shows that vasopressinergic (VP) neurons within the suprachiasmatic nucleus (SCN) master circadian clock have the ability of encoding afferent input from osmosensors and generating appropriate homeostatic responses, suggesting that SCN neurons can integrate internal circadian time and acute changes in homeostatic markers.

Potential Pathways for Circadian Dysfunction and Sundowning-Related Behavioral Aggression in Alzheimer's Disease and Related Dementias

Patients with Alzheimer's disease (AD) and related dementias are commonly reported to exhibit aggressive behavior and other emotional behavioral disturbances, which create a tremendous caretaker burden. There has been an abundance of work highlighting the importance of circadian function on mood and emotional behavioral regulation, and recent evidence demonstrates that a specific hypothalamic pathway links the circadian system to neurons that modulate aggressive behavior, regulating the propensity for aggression across the day. Such shared circuitry may have important ramifications for clarifying the complex interactions underlying "sundowning syndrome," a poorly understood (and even controversial) clinical phenomenon in AD and dementia patients that is characterized by agitation, aggression, and delirium during the late afternoon and early evening hours. The goal of this review is to highlight the potential output and input pathways of the circadian system that may underlie circadian dysfunction and behavioral aggression associated with sundowning syndrome, and to discuss possible ways these pathways might inform specific interventions for treatment. Moreover, the apparent bidirectional relationship between chronic disruptions of circadian and sleep-wake regulation and the pathology and symptoms of AD suggest that understanding the role of these circuits in such neurobehavioral pathologies could lead to better diagnostic or even preventive measures.

Suprachiasmatic VIP neurons are required for normal circadian rhythmicity and comprised of molecularly distinct subpopulations

The hypothalamic suprachiasmatic (SCN) clock contains several neurochemically defined cell groups that contribute to the genesis of circadian rhythms. Using cell-specific and genetically targeted approaches we have confirmed an indispensable role for vasoactive intestinal polypeptide-expressing SCN (SCN^VIP) neurons, including their molecular clock, in generating the mammalian locomotor activity (LMA) circadian rhythm. Optogenetic-assisted circuit mapping revealed functional, di-synaptic connectivity between SCN^VIP neurons and dorsomedial hypothalamic neurons, providing a circuit substrate by which SCN^VIP neurons may regulate LMA rhythms. In vivo photometry revealed that while SCN^VIP neurons are acutely responsive to light, their activity is otherwise behavioral state invariant. Single-nuclei RNA-sequencing revealed that SCN^VIP neurons comprise two transcriptionally distinct subtypes, including putative pacemaker and non-pacemaker populations. Altogether, our work establishes necessity of SCN^VIP neurons for the LMA circadian rhythm, elucidates organization of circadian outflow from and modulatory input to SCN^VIP cells, and demonstrates a subpopulation-level molecular heterogeneity that suggests distinct functions for specific SCN^VIP subtypes.

Cell groups in the hypothalamic suprachiasmatic clock contribute to the genesis of circadian rhythms. The authors identified two populations of vasoactive intestinal polypeptide-expressing neurons in the suprachiasmatic nucleus which regulate locomotor circadian rhythm in mice.

Equation representing the dark-entrained transition from inaction to action in male and female mice

The activation of behaviour in a daily rhythm governed by the light cycle is a universal phenomenon among humans, laboratory mammals and other vertebrates. For mice, the active period is during the dark. We have quantified the increase in activity when the lights shut off (Light to Dark, L to D) using a generalized CNS arousal assay with 20 ms resolution, rather than traditional running wheels. Data analysis yielded the rare demonstration of an equation which precisely tracks this behavioural transition and, surprisingly, its reverse during D to L. This behavioural dynamic survives in constant darkness (experiment 2) and is hormone-sensitive (experiment 3). Finally (experiment 4), mice on a light schedule analogous to one which proved troublesome for U.S. Navy sailors, had dysregulated activity bursts which did not conform to the transitions between D and L. These experiments show the lawfulness of a behavioural phase transition and the consequence of deviating from that dynamic pattern. And, in a new way, they bring mathematics to the realm of behavioural neuroscience.

Factors Disrupting Melatonin Secretion Rhythms During Critical Illness

OBJECTIVES

The circadian system modulates many important physiologic processes, synchronizing tissue-specific functions throughout the body. We sought to characterize acute alterations of circadian rhythms in critically ill patients and to evaluate associations between brain dysfunction, systemic multiple organ dysfunction, environmental stimuli that entrain the circadian rhythm (zeitgebers), rest-activity rhythms, and the central circadian rhythm-controlled melatonin secretion profile.

DESIGN

Prospective study observing a cohort for 24-48 hours beginning within the first day of ICU admission.

SETTING

Multiple specialized ICUs within an academic medical center.

PATIENTS

Patients presenting from the community with acute onset of either intracerebral hemorrhage as a representative neurologic critical illness or sepsis as a representative systemic critical illness. Healthy control patients were studied in using modified constant routine in a clinical research unit.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Light, feeding, activity, medications, and other treatment exposures were evaluated along with validated measures of encephalopathy (Glasgow Coma Scale), multiple organ system function (Sequential Organ Failure Assessment score), and circadian rhythms (profiles of serum melatonin and its urinary metabolite 6-sulphatoxymelatonin). We studied 112 critically ill patients, including 53 with sepsis and 59 with intracerebral hemorrhage. Environmental exposures were abnormal, including light (dim), nutritional intake (reduced or absent and mistimed), and arousal stimuli (increased and mistimed). Melatonin amplitude and acrophase timing were generally preserved in awake patients but dampened and delayed with increasing encephalopathy severity. Melatonin hypersecretion was observed in patients exposed to catecholamine vasopressor infusions, but unaffected by sedatives. Change in vasopressor exposure was the only factor associated with changes in melatonin rhythms between days 1 and 2.

CONCLUSIONS

Encephalopathy severity and adrenergic agonist medication exposure were the primary factors contributing to abnormal melatonin rhythms. Improvements in encephalopathy and medical stabilization did not rapidly normalize rhythms. Urinary 6-sulphatoxymelatonin is not a reliable measure of the central circadian rhythm in critically ill patients.

Newly identified sleep-wake and circadian circuits as potential therapeutic targets

Optogenetics and chemogenetics are powerful tools, allowing the specific activation or inhibition of targeted neuronal subpopulations. Application of these techniques to sleep and circadian research has resulted in the unveiling of several neuronal populations that are involved in sleep-wake control, and allowed a comprehensive interrogation of the circuitry through which these nodes are coordinated to orchestrate the sleep-wake cycle. In this review, we discuss six recently described sleep-wake and circadian circuits that show promise as therapeutic targets for sleep medicine. The parafacial zone (PZ) and the ventral tegmental area (VTA) are potential druggable targets for the treatment of insomnia. The brainstem circuit underlying rapid eye movement sleep behavior disorder (RBD) offers new possibilities for treating RBD and neurodegenerative synucleinopathies, whereas the parabrachial nucleus, as a nexus linking arousal state control and breathing, is a promising target for developing treatments for sleep apnea. Therapies that act upon the hypothalamic circuitry underlying the circadian regulation of aggression or the photic regulation of arousal and mood pathway carry enormous potential for helping to reduce the socioeconomic burden of neuropsychiatric and neurodegenerative disorders on society. Intriguingly, the development of chemogenetics as a therapeutic strategy is now well underway and such an approach has the capacity to lead to more focused and less invasive therapies for treating sleep-wake disorders and related comorbidities.

The day-night differences in ERK1/2, GSK3β activity and c-Fos levels in the brain, and the responsiveness of various brain structures to morphine

As with other drugs or pharmaceuticals, opioids differ in their rewarding or analgesic effects depending on when they are applied. In the previous study, we have demonstrated the day/night difference in the sensitivity of the major circadian clock in the suprachiasmatic nucleus to a low dose of morphine, and showed the bidirectional effect of morphine on pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus depending on the time of administration. The main aim of this study was to identify other brain structures that respond differently to morphine depending on the time of its administration. Using immunohistochemistry, we identified 44 structures that show time-of-day specific changes in c-Fos level and activity of ERK1/2 and GSK3β kinases in response to a single dose of 1 mg/kg morphine. Furthermore, comparison among control groups revealed the differences in the spontaneous levels of all markers with a generally higher level during the night, that is, in the active phase of the day. We thus provide further evidence for diurnal variations in the activity of brain regions outside the suprachiasmatic nucleus indicated by the temporal changes in the molecular substrate. We suggest that these changes are responsible for generating diurnal variation in the reward behavior or analgesic effect of opioid administration.

The Concept of Coupling in the Mammalian Circadian Clock Network

The circadian clock network regulates daily rhythms in mammalian physiology and behavior to optimally adapt the organism to the 24-h day/night cycle. A central pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), coordinates subordinate cellular oscillators in the brain, as well as in peripheral organs to align with each other and external time. Stability and coordination of this vast network of cellular oscillators is achieved through different levels of coupling. Although coupling at the molecular level and across the SCN is well established and believed to define its function as pacemaker structure, the notion of coupling in other tissues and across the whole system is less well understood. In this review, we describe the different levels of coupling in the mammalian circadian clock system - from molecules to the whole organism. We highlight recent advances in gaining knowledge of the complex organization and function of circadian network regulation and its significance for the generation of stable but plastic intrinsic 24-h rhythms.

Circadian Function in Multiple Cell Types Is Necessary for Proper Timing of the Preovulatory LH Surge

The timing of the preovulatory surge of luteinizing hormone (LH), which occurs on the evening of proestrus in female mice, is determined by the circadian system. The identity of cells that control the phase of the LH surge is unclear: evidence supports a role of arginine vasopressin (AVP) cells of the suprachiasmatic nucleus (SCN), but it is not known whether vasopressinergic neurons are necessary or sufficient to account for circadian control of ovulation. Among other cell types, evidence also suggests important roles of circadian function of kisspeptin cells of the anteroventral periventricular nucleus (AvPV) and gonadotropin-releasing hormone (GnRH) neurons of the preoptic area (POA), whose discharge is immediately responsible for the discharge of LH from the anterior pituitary. The present studies used an ovariectomized, estradiol-treated preparation to determine critical cell types whose clock function is critical to the timing of LH secretion. As expected, the LH surge occurred at or shortly after ZT12 in control mice. In further confirmation of circadian control, the surge was advanced by 2 h in tau mutant animals. The timing of the surge was altered to varying degrees by conditional deletion of Bmal1 in AVP^Cre, Kiss^CreBAC, and GnRH^CreBAC mice. Excision of the mutant Cnsk1e (tau) allele in AVP neurons resulted in a reversion of the surge to the ZT12. Conditional deletion of Bmal1 in Kiss1 or GnRH neurons had no noticeable effect on locomotor rhythms, but targeting of AVP neurons produced variable effects on circadian period that did not always correspond to changes in the phase of LH secretion. The results indicate that circadian function in multiple cell types is necessary for proper timing of the LH surge.

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).

Molecular and Cellular Networks in The Suprachiasmatic Nuclei

Why do we experience the ailments of jetlag when we travel across time zones? Why is working night-shifts so detrimental to our health? In other words, why can’t we readily choose and stick to non-24 h rhythms? Actually, our daily behavior and physiology do not simply result from the passive reaction of our organism to the external cycle of days and nights. Instead, an internal clock drives the variations in our bodily functions with a period close to 24 h, which is supposed to enhance fitness to regular and predictable changes of our natural environment. This so-called circadian clock relies on a molecular mechanism that generates rhythmicity in virtually all of our cells. However, the robustness of the circadian clock and its resilience to phase shifts emerge from the interaction between cell-autonomous oscillators within the suprachiasmatic nuclei (SCN) of the hypothalamus. Thus, managing jetlag and other circadian disorders will undoubtedly require extensive knowledge of the functional organization of SCN cell networks. Here, we review the molecular and cellular principles of circadian timekeeping, and their integration in the multi-cellular complexity of the SCN. We propose that new, in vivo imaging techniques now enable to address these questions directly in freely moving animals.

Episodic Ultradian Events-Ultradian Rhythms

In the fast lane of chronobiology, ultradian events are short-term rhythms that have been observed since the beginning of modern biology and were quantified about a century ago. They are ubiquitous in all biological systems and found in all organisms, from unicellular organisms to mammals, and from single cells to complex biological functions in multicellular animals. Since these events are aperiodic and last for a few minutes to a few hours, they are better classified as episodic ultradian events (EUEs). Their origin is unclear. However, they could have a molecular basis and could be controlled by hormonal inputs-in vertebrates, they originate from the activity of the central nervous system. EUEs are receiving increasing attention but their aperiodic nature requires specific sampling and analytic tools. While longer scale rhythms are adaptations to predictable changes in the environment, in theory, EUEs could contribute to adaptation by preparing organisms and biological functions for unpredictability.

Kisspeptin Neurons in the Arcuate Nucleus of the Hypothalamus Orchestrate Circadian Rhythms and Metabolism

Successful reproduction in female mammals is precisely timed and must be able to withstand the metabolic demand of pregnancy and lactation. We show that kisspeptin-expressing neurons in the arcuate hypothalamus (Kiss1^ARH) of female mice control the daily timing of food intake, along with the circadian regulation of locomotor activity, sleep, and core body temperature. Toxin-induced silencing of Kiss1^ARH neurons shifts wakefulness and food consumption to the light phase and induces weight gain. Toxin-silenced mice are less physically active and have attenuated temperature rhythms. Because the rhythm of the master clock in the suprachiasmatic nucleus (SCN) appears to be intact, we hypothesize that Kiss1^ARH neurons signal to neurons downstream of the master clock to modulate the output of the SCN. We conclude that, in addition to their well-established role in regulating fertility, Kiss1^ARH neurons are a critical component of the hypothalamic circadian oscillator network that times overt rhythms of physiology and behavior.

A time to fight: Circadian control of aggression and associated autonomic support

The central circadian clock, located in the suprachiasmatic nucleus of the mammalian hypothalamus (SCN), regulates daily behavioral rhythms including the temporal propensity for aggressive behavior. Such aggression propensity rhythms are regulated by a functional circuit from the SCN to neurons that drive attack behavior in the ventromedial hypothalamus (VMH), via a relay in the subparaventricular zone (SPZ). In addition to this pathway, the SCN also regulates sleep-wake and locomotor activity rhythms, via the SPZ, in a circuit to the dorsomedial hypothalamus (DMH), a structure that is also known to play a key role in autonomic function and the sympathetic "fight-or-flight" response (which prepares the body for action in stressful situations such as an agonistic encounter). While the autonomic nervous system is known to be under pronounced circadian control, it is less apparent how such autonomic rhythms and their underlying circuitry may support the temporal propensity for aggressive behavior. Additionally, it is unclear how circadian and autonomic dysfunction may contribute to aberrant social and emotional behavior, such as agitation and aggression. Here we review the literature concerning interactions between the circadian and autonomic systems and aggression, and we discuss the implications of these relationships for human neural and behavioral pathologies.

Novel insights into the spatial and temporal complexity of hypothalamic organization through precision methods allowing nanoscale resolution

The mammalian hypothalamus contains an astounding heterogeneity of neurons to achieve its role in coordinating central responses to virtually any environmental stressor over the life-span of an individual. Therefore, while core features of intrahypothalamic neuronal modalities and wiring patterns are stable during vertebrate evolution, integration of the hypothalamus into hierarchical brain-wide networks evolved to coordinate its output with emotionality, cognition and conscious decision-making. The advent of single-cell technologies represents a recent milestone in the study of hypothalamic organization by allowing the dissection of cellular heterogeneity and establishing causality between opto- and chemogenetic activity modulation of molecularly-resolved neuronal contingents and specific behaviours. Thus, organizational rules to accumulate an unprecedented variety of hierarchical neuroendocrine command networks into a minimal brain volume are being unravelled. Here, we review recent understanding at nanoscale resolution on how neuronal heterogeneity in the mammalian hypothalamus underpins the diversification of hormonal and synaptic output and keeps those sufficiently labile for continuous adaptation to meet environmental demands. Particular emphasis is directed towards the dissection of neuronal circuitry for aggression and food intake. Mechanistic data encompass cell identities, synaptic connectivity within and outside the hypothalamus to link vegetative and conscious levels of innate behaviours, and context- and circadian rhythm-dependent rules of synaptic neurophysiology to distinguish hypothalamic foci that either tune the body's metabolic set-point or specify behaviours. Consequently, novel insights emerge to explain the evolutionary advantages of non-laminar organization for neuroendocrine circuits coincidently using fast neurotransmitters and neuropeptides. These are then accrued into novel therapeutic principles that meet therapeutic criteria for human metabolic diseases.

To eat or to sleep: That is a lateral hypothalamic question

The lateral hypothalamus (LH) is a functionally and anatomically complex brain region that is involved in the regulation of many behavioral and physiological processes including feeding, arousal, energy balance, stress, reward and motivated behaviors, pain perception, body temperature regulation, digestive functions and blood pressure. Despite noteworthy experimental efforts over the past decades, the circuit, cellular and synaptic bases by which these different processes are regulated by the LH remains incompletely understood. This knowledge gap links in large part to the high cellular heterogeneity of the LH. Fortunately, the rapid evolution of newer genetic and electrophysiological tools is now permitting the selective manipulation, typically genetically-driven, of discrete LH cell populations. This, in turn, permits not only assignment of function to discrete cell groups, but also reveals that considerable synergistic and antagonistic interactions exist between key LH cell populations that regulate feeding and arousal. For example, we now know that while LH melanin-concentrating hormone (MCH) and orexin/hypocretin neurons both function as sensors of the internal metabolic environment, their roles regulating sleep and arousal are actually opposing. Additional studies have uncovered similarly important roles for subpopulations of LH GABAergic cells in the regulation of both feeding and arousal. Herein we review the role of LH MCH, orexin/hypocretin and GABAergic cell populations in the regulation of energy homeostasis (including feeding) and sleep-wake and discuss how these three cell populations, and their subpopulations, may interact to optimize and coordinate metabolism, sleep and arousal. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Recurring circadian disruption alters circadian clock sensitivity to resetting

A single phase advance of the light:dark (LD) cycle can temporarily disrupt synchrony of neural circadian rhythms within the suprachiasmatic nucleus (SCN) and between the SCN and peripheral tissues. Compounding this, modern life can involve repeated disruptive light conditions. To model chronic disruption to the circadian system, we exposed male mice to more than a month of a 20-hr light cycle (LD10:10), which mice typically cannot entrain to. Control animals were housed under LD12:12. We measured locomotor activity and body temperature rhythms in vivo, and rhythms of PER2::LUC bioluminescence in SCN and peripheral tissues ex vivo. Unexpectedly, we discovered strong effects of the time of dissection on circadian phase of PER2::LUC bioluminescent rhythms, which varied across tissues. White adipose tissue was strongly reset by dissection, while thymus phase appeared independent of dissection timing. Prior light exposure impacted the SCN, resulting in strong resetting of SCN phase by dissection for mice housed under LD10:10, and weak phase shifts by time of dissection in SCN from control LD12:12 mice. These findings suggest that exposure to circadian disruption may desynchronize SCN neurons, increasing network sensitivity to perturbations. We propose that tissues with a weakened circadian network, such as the SCN under disruptive light conditions, or with little to no coupling, for example, some peripheral tissues, will show increased resetting effects. In particular, exposure to light at inconsistent circadian times on a recurring weekly basis disrupts circadian rhythms and alters sensitivity of the SCN neural pacemaker to dissection time.

Dim Light at Night and Constant Darkness: Two Frequently Used Lighting Conditions That Jeopardize the Health and Well-being of Laboratory Rodents

The influence of light on mammalian physiology and behavior is due to the entrainment of circadian rhythms complemented with a direct modulation of light that would be unlikely an outcome of circadian system. In mammals, physiological and behavioral circadian rhythms are regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus. This central control allows organisms to predict and anticipate environmental change, as well as to coordinate different rhythmic modalities within an individual. In adult mammals, direct retinal projections to the SCN are responsible for resetting and synchronizing physiological and behavioral rhythms to the light-dark (LD) cycle. Apart from its circadian effects, light also has direct effects on certain biological functions in such a way that the participation of the SCN would not be fundamental for this network. The objective of this review is to increase awareness, within the scientific community and commercial providers, of the fact that laboratory rodents can experience a number of adverse health and welfare outcomes attributed to commonly-used lighting conditions in animal facilities during routine husbandry and scientific procedures, widely considered as “environmentally friendly.” There is increasing evidence that exposure to dim light at night, as well as chronic constant darkness, challenges mammalian physiology and behavior resulting in disrupted circadian rhythms, neural death, a depressive-behavioral phenotype, cognitive impairment, and the deregulation of metabolic, physiological, and synaptic plasticity in both the short and long terms. The normal development and good health of laboratory rodents requires cyclical light entrainment, adapted to the solar cycle of day and night, with null light at night and safe illuminating qualities during the day. We therefore recommend increased awareness of the limited information available with regards to lighting conditions, and therefore that lighting protocols must be taken into consideration when designing experiments and duly highlighted in scientific papers. This practice will help to ensure the welfare of laboratory animals and increase the likelihood of producing reliable and reproducible results.

Defective daily temperature regulation in a mouse model of amyotrophic lateral sclerosis

Current understanding of the pathogenesis of the familial form of amyotrophic lateral sclerosis has been aided by the study of transgenic mice that over-express mutated forms of the human CuZn-superoxide dismutase (SOD1) gene. While mutant SOD1 in motor neurons determines disease onset, other non-cell autonomous factors are critical for disease progression, and altered energy metabolism has been implicated as a contributing factor. Since most energy expended by laboratory mice is utilized to defend body temperature (T_b), we analyzed thermoregulation in transgenic mice carrying the G93A mutation of the human SOD1 gene, using implantable temperature data loggers to continuously record T_b for up to 85 days. At room (22 °C) ambient temperature, G93A mice exhibited a diminished amplitude of the daily T_b rhythm compared to C57BL/6J controls, secondary to decreased T_b values during the dark (behaviorally active) phase of the light-dark cycle. The defect arose at 85-99 days of age, around the age of symptom onset (as assessed by grip strength), well before observable weakness and weight loss, and could not be accounted for by decreased levels of locomotor activity or food consumption. Housing under thermoneutral (29 °C) ambient temperature partially rescued the defect, but age-dependently (only in animals >100 days of age), suggesting that the deficit in older mice was due in part to inadequate thermogenesis by "peripheral" thermogenic organs as the disease progressed. In younger mice, we found that cold-induced thermogenesis and energy expenditure were intact, hinting that an initial "central" defect might localize to the subparaventricular zone, involving neural output pathways from the circadian clock in the hypothalamic suprachiasmatic nucleus to forebrain thermoregulatory circuitry.

Flexible clock systems: adjusting the temporal programme

Under natural conditions, many aspects of the abiotic and biotic environment vary with time of day, season or even era, while these conditions are typically kept constant in laboratory settings. The timing information contained within the environment serves as critical timing cues for the internal biological timing system, but how this system drives daily rhythms in behaviour and physiology may also depend on the internal state of the animal. The disparity between timing of these cues in natural and laboratory conditions can result in substantial differences in the scheduling of behaviour and physiology under these conditions. In nature, temporal coordination of biological processes is critical to maximize fitness because they optimize the balance between reproduction, foraging and predation risk. Here we focus on the role of peripheral circadian clocks, and the rhythms that they drive, in enabling adaptive phenotypes. We discuss how reproduction, endocrine activity and metabolism interact with peripheral clocks, and outline the complex phenotypes arising from changes in this system. We conclude that peripheral timing is critical to adaptive plasticity of circadian organization in the field, and that we must abandon standard laboratory conditions to understand the mechanisms that underlie this plasticity which maximizes fitness under natural conditions.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

A hypothalamic circuit for the circadian control of aggression

“Sundowning” in dementia and Alzheimer’s disease is characterized by early evening agitation and aggression. While such periodicity suggests a circadian origin, whether the circadian clock directly regulates aggressive behavior is unknown. We demonstrate that a daily rhythm in aggression propensity in male mice is gated by GABAergic subparaventricular zone (SPZ^GABA) neurons, the major postsynaptic targets of the central circadian clock, the suprachiasmatic nucleus (SCN). Optogenetic mapping revealed that SPZ^GABA neurons receive input from vasoactive intestinal polypeptide SCN neurons and innervate neurons in the ventrolateral part of the ventromedial hypothalamus (VMHvl) known to regulate aggression. Additionally, VMH-projecting dorsal SPZ neurons are more active during early day than early night, and acute chemogenetic inhibition of SPZ^GABA transmission phase-dependently increases aggression. Finally, SPZ^GABA-recipient central VMH neurons directly innervate VMHvl neurons and activation of this intra-VMH circuit drove attack behavior. Altogether, we reveal a functional polysynaptic circuit by which the SCN clock regulates aggression.