Age-related changes in electrophysiological properties of the mouse suprachiasmatic nucleus in vitro

Brain-muscle communication prevents muscle aging by maintaining daily physiology

A molecular clock network is crucial for daily physiology and maintaining organismal health. We examined the interactions and importance of intratissue clock networks in muscle tissue maintenance. In arrhythmic mice showing premature aging, we created a basic clock module involving a central and a peripheral (muscle) clock. Reconstituting the brain-muscle clock network is sufficient to preserve fundamental daily homeostatic functions and prevent premature muscle aging. However, achieving whole muscle physiology requires contributions from other peripheral clocks. Mechanistically, the muscle peripheral clock acts as a gatekeeper, selectively suppressing detrimental signals from the central clock while integrating important muscle homeostatic functions. Our research reveals the interplay between the central and peripheral clocks in daily muscle function and underscores the impact of eating patterns on these interactions.

ASIC1a affects hypothalamic signaling and regulates the daily rhythm of body temperature in mice

The body temperature of mice is higher at night than during the day. We show here that global deletion of acid-sensing ion channel 1a (ASIC1a) results in lower body temperature during a part of the night. ASICs are pH sensors that modulate neuronal activity. The deletion of ASIC1a decreased the voluntary activity at night of mice that had access to a running wheel but did not affect their spontaneous activity. Daily rhythms of thyrotropin-releasing hormone mRNA in the hypothalamus and of thyroid-stimulating hormone β mRNA in the pituitary, and of prolactin mRNA in the hypothalamus and pituitary were suppressed in ASIC1a-/- mice. The serum thyroid hormone levels were however not significantly changed by ASIC1a deletion. Our findings indicate that ASIC1a regulates activity and signaling in the hypothalamus and pituitary. This likely leads to the observed changes in body temperature by affecting the metabolism or energy expenditure.

Aging affects GABAergic function and calcium homeostasis in the mammalian central clock

Introduction

Aging impairs the function of the central circadian clock in mammals, the suprachiasmatic nucleus (SCN), leading to a reduction in the output signal. The weaker timing signal from the SCN results in a decline in rhythm strength in many physiological functions, including sleep-wake patterns. Accumulating evidence suggests that the reduced amplitude of the SCN signal is caused by a decreased synchrony among the SCN neurons. The present study was aimed to investigate the hypothesis that the excitation/inhibition (E/I) balance plays a role in synchronization within the network.

Methods

Using calcium (Ca²⁺) imaging, the polarity of Ca²⁺ transients in response to GABA stimulation in SCN slices of old mice (20-24 months) and young controls was studied.

Results

We found that the amount of GABAergic excitation was increased, and that concordantly the E/I balance was higher in SCN slices of old mice when compared to young controls. Moreover, we showed an effect of aging on the baseline intracellular Ca²⁺ concentration, with higher Ca²⁺ levels in SCN neurons of old mice, indicating an alteration in Ca²⁺ homeostasis in the aged SCN. We conclude that the change in GABAergic function, and possibly the Ca²⁺ homeostasis, in SCN neurons may contribute to the altered synchrony within the aged SCN network.

The shared molecular mechanisms underlying aging of the brain, major depressive disorder, and Alzheimer's disease: The role of circadian rhythm disturbances

An association with circadian clock function and pathophysiology of aging, major depressive disorder (MDD), and Alzheimer's disease (AD) is well established and has been proposed as a factor in the development of these diseases. Depression and changes in circadian rhythm have been increasingly suggested as the two primary overlapping and interpenetrating changes that occur with aging. The relationship between AD and depression in late life is not completely understood and probably is complex. Patients with major depression or AD suffer from disturbed sleep/wake cycles and altered rhythms in daily activities. Although classical monoaminergic hypotheses are traditionally proposed to explain the pathophysiology of MDD, several clinical and preclinical studies have reported a strong association between circadian rhythm and mood regulation. In addition, a large body of evidence supports an association between disruption of circadian rhythm and AD. Some clock genes are dysregulated in rodent models of depression. If aging, AD, and MDD share a common biological basis in pathophysiology, common therapeutic tools could be investigated for their prevention and treatment. Nitro-oxidative stress (NOS), for example, plays a fundamental role in aging, as well as in the pathogenesis of AD and MDD and is associated with circadian clock disturbances. Thus, development of therapeutic possibilities with these NOS-related conditions is advisable. This review describes recent findings that link disrupted circadian clocks to aging, MDD, and AD and summarizes the experimental evidence that supports connections between the circadian clock and molecular pathologic factors as shared common pathophysiological mechanisms underlying aging, AD, and MDD.

To sleep or not to sleep - Effects on memory in normal aging and disease

Sleep behavior undergoes significant changes across the lifespan, and aging is associated with marked alterations in sleep amounts and quality. The primary sleep changes in healthy older adults include a shift in sleep timing, reduced slow-wave sleep, and impaired sleep maintenance. However, neurodegenerative and psychiatric disorders are more common among the elderly, which further worsen their sleep health. Irrespective of the cause, insufficient sleep adversely affects various bodily functions including energy metabolism, mood, and cognition. In this review, we will focus on the cognitive changes associated with inadequate sleep during normal aging and the underlying neural mechanisms.

Investigation of the aging clock's intermittent-light responses uncovers selective deficits to green millisecond flashes

The central pacemaker of flies, rodents, and humans generates less robust circadian output signals across normative aging. It is not well understood how changes in light sensitivity might contribute to this phenomenon. In the present study, we summarize results from an extended data series (n = 5681) showing that the locomotor activity rhythm of aged Drosophila can phase-shift normally to intermittently spaced episodes of bright polychromatic light exposure (600 lx) but that deficits emerge in response to 8, 16, and 120-millisecond flashes of narrowband blue (λ_m, 452 nm) and green (λ_m, 525 nm) LED light. For blue, phase-resetting of the activity rhythm of older flies is not as energy efficient as it is in younger flies at the fastest flash-exposures tested (8 milliseconds), suggesting there might be different floors of light duration necessary to incur photohabituation in each age group. For green, the responses of older flies are universally crippled relative to those of younger flies across the slate of protocols we tested. The difference in green flash photosensitivity is one of the most salient age-related phenotypes that has been documented in the circadian phase-shifting literature thus far. These data provide further impetus for investigations on pacemaker aging and how it might relate to changes in the circadian system's responses to particular sequences of light exposure tuned for wavelength, intensity, duration, and tempo.

The aging brain: sleep, the circadian clock and exercise

Aging is a multifactorial process likely stemming from damage accumulation and/or a decline in maintenance and repair mechanisms in the organisms that eventually determine their lifespan. In our review, we focus on the morphological and functional alterations that the aging brain undergoes affecting sleep and the circadian clock in both human and rodent models. Although both species share mammalian features, differences have been identified on several experimental levels, which we outline in this review. Additionally, we delineate some challenges on the preferred analysis and we suggest that a uniform route is followed so that findings can be smoothly compared. We conclude by discussing potential interventions and highlight the influence of physical exercise as a beneficial lifestyle intervention, and its effect on healthy aging and longevity. We emphasize that even moderate age-matched exercise is able to ameliorate several aging characteristics as far as sleep and circadian rhythms are concerned, independent of the species studied.

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.

Seasonality and light phase-resetting in the mammalian circadian rhythm

We study the impact of light on the mammalian circadian system using the theory of phase response curves. Using a recently developed ansatz we derive a low-dimensional macroscopic model for the core circadian clock in mammals. Significantly, the variables and parameters in our model have physiological interpretations and may be compared with experimental results. We focus on the effect of four key factors which help shape the mammalian phase response to light: heterogeneity in the population of oscillators, the structure of the typical light phase response curve, the fraction of oscillators which receive direct light input and changes in the coupling strengths associated with seasonal day-lengths. We find these factors can explain several experimental results and provide insight into the processing of light information in the mammalian circadian system. In particular, we find that the sensitivity of the circadian system to light may be modulated by changes in the relative coupling forces between the light sensing and non-sensing populations. Finally, we show how seasonal day-length, after-effects to light entrainment and seasonal variations in light sensitivity in the mammalian circadian clock are interrelated.

Circadian Rhythms, Sleep, and Aging

Abstract. Circadian mechanisms and the sleep-wakefulness rhythms guarantee survival, adaptation, efficient action in everyday life or in emergencies and well-being. Disordered circadian processes at central and/or cellular levels, sleep disorders, and unhealthy wakefulness/sleep rhythms can impair the physiological circadian organization and result in subjective, professional, or behavioral changes ranging from functional inadequacy to higher risks at work or on the road to medical relevance. Circadian rhythms and the sleep organization change ontogenetically; major changes result from normal aging and from the multiple diseases that are often associated. There are circular functional interactions involving sleep/sleep disorders, the autonomic and immune systems, and the functional changes in the circadian system due to aging that deserve attention but have been overlooked thus far.

Advances in the Involvement of Gut Microbiota in Pathophysiology of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis and progresses to non-steatohepatitis (NASH) when the liver displays overt inflammatory damage. Increasing evidence has implicated critical roles for dysbiosis and microbiota-host interactions in NAFLD pathophysiology. In particular, microbiota alter intestine absorption of nutrients and intestine permeability, whose dysregulation enhances the delivery of nutrients, endotoxin, and microbiota metabolites to the liver and exacerbates hepatic fat deposition and inflammation. While how altered composition of gut microbiota attributes to NAFLD remains to be elucidated, microbiota metabolites are shown to be involved in the regulation of hepatocyte fat metabolism and liver inflammatory responses. In addition, intestinal microbes and circadian coordinately adjust metabolic regulation in different stages of life. During aging, altered composition of gut microbiota, along with circadian clock dysregulation, appears to contribute to increased incidence and/or severity of NAFLD.

Resetting the Aging Clock: Implications for Managing Age-Related Diseases

Worldwide, individuals are living longer due to medical and scientific advances, increased availability of medical care and changes in public health policies. Consequently, increasing attention has been focused on managing chronic conditions and age-related diseases to ensure healthy aging. The endogenous circadian system regulates molecular, physiological and behavioral rhythms orchestrating functional coordination and processes across tissues and organs. Circadian disruption or desynchronization of circadian oscillators increases disease risk and appears to accelerate aging. Reciprocally, aging weakens circadian function aggravating age-related diseases and pathologies. In this review, we summarize the molecular composition and structural organization of the circadian system in mammals and humans, and evaluate the technological and societal factors contributing to the increasing incidence of circadian disorders. Furthermore, we discuss the adverse effects of circadian dysfunction on aging and longevity and the bidirectional interactions through which aging affects circadian function using examples from mammalian research models and humans. Additionally, we review promising methods for managing healthy aging through behavioral and pharmacological reinforcement of the circadian system. Understanding age-related changes in the circadian clock and minimizing circadian dysfunction may be crucial components to promote healthy aging.

Mood-related central and peripheral clocks

Mood disorders, including major depression, bipolar disorder, and seasonal affective disorder, are debilitating disorders that affect a significant portion of the global population. Individuals suffering from mood disorders often show significant disturbances in circadian rhythms and sleep. Moreover, environmental disruptions to circadian rhythms can precipitate or exacerbate mood symptoms in vulnerable individuals. Circadian clocks exist throughout the central nervous system and periphery, where they regulate a wide variety of physiological processes implicated in mood regulation. These processes include monoaminergic and glutamatergic transmission, hypothalamic-pituitary-adrenal axis function, metabolism, and immune function. While there seems to be a clear link between circadian rhythm disruption and mood regulation, the mechanisms that underlie this association remain unclear. This review will touch on the interactions between the circadian system and each of these processes and discuss their potential role in the development of mood disorders. While clinical studies are presented, much of the review will focus on studies in animal models, which are attempting to elucidate the molecular and cellular mechanisms in which circadian genes regulate mood.

The Biological Clock in Gray Mouse Lemur: Adaptive, Evolutionary and Aging Considerations in an Emerging Non-human Primate Model

Circadian rhythms, which measure time on a scale of 24 h, are genetically generated by the circadian clock, which plays a crucial role in the regulation of almost every physiological and metabolic process in most organisms. This review gathers all the available information about the circadian clock in a small Malagasy primate, the gray mouse lemur (Microcebus murinus), and reports 30 years data from the historical colony at Brunoy (France). Although the mouse lemur has long been seen as a "primitive" species, its clock displays high phenotypic plasticity, allowing perfect adaptation of its biological rhythms to environmental challenges (seasonality, food availability). The alterations of the circadian timing system in M. murinus during aging show many similarities with those in human aging. Comparisons are drawn with other mammalian species (more specifically, with rodents, other non-human primates and humans) to demonstrate that the gray mouse lemur is a good complementary and alternative model for studying the circadian clock and, more broadly, brain aging and pathologies.

The functional changes of the circadian system organization in aging

The circadian clock drives periodic oscillations at different levels of an organism from genes to behavior. This timing system is highly conserved across species from insects to mammals and human beings. The question of how the circadian clock is involved in the aging process continues to attract more attention. We aim to characterize the detrimental impact of aging on the circadian clock organization. We review studies on different components of the circadian clock at the central and periperal levels, and their changes in aged rodents and humans, and the fruit fly Drosophila. Intracellular signaling, cellular activity and intercellular coupling in the central pacemaker have been found to decline with advancing age. Evidence of degradation of the molecular clockwork reflected by clock gene expression in both central and peripheral oscillators due to aging is inadequate. The findings on age-associated molecular and functional changes of peripheral clocks are mixed. We conclude that aging can affect the circadian clock organization at various levels, and the impairment of the central network may be a fundamental mechanism of circadian disruption seen in aged species.

From clock to functional pacemaker

In mammals, the central pacemaker that coordinates 24‐hr rhythms is located in the suprachiasmatic nucleus (SCN). Individual neurons of the SCN have a molecular basis for rhythm generation and hence, they function as cell autonomous oscillators. Communication and synchronization among these neurons are crucial for obtaining a coherent rhythm at the population level, that can serve as a pace making signal for brain and body. Hence, the ability of single SCN neurons to produce circadian rhythms is equally important as the ability of these neurons to synchronize one another, to obtain a bona fide pacemaker at the SCN tissue level. In this chapter we will discuss the mechanisms underlying synchronization, and plasticity herein, which allows adaptation to changes in day length. Furthermore, we will discuss deterioration in synchronization among SCN neurons in aging, and gain in synchronization by voluntary physical activity or exercise.

Rhythms of life: circadian disruption and brain disorders across the lifespan

Many processes in the human body - including brain function - are regulated over the 24-hour cycle, and there are strong associations between disrupted circadian rhythms (for example, sleep-wake cycles) and disorders of the CNS. Brain disorders such as autism, depression and Parkinson disease typically develop at certain stages of life, and circadian rhythms are important during each stage of life for the regulation of processes that may influence the development of these disorders. Here, we describe circadian disruptions observed in various brain disorders throughout the human lifespan and highlight emerging evidence suggesting these disruptions affect the brain. Currently, much of the evidence linking brain disorders and circadian dysfunction is correlational, and so whether and what kind of causal relationships might exist are unclear. We therefore identify remaining questions that may direct future research towards a better understanding of the links between circadian disruption and CNS disorders.

Circadian rhythms, time-restricted feeding, and healthy aging

Circadian rhythms optimize physiology and health by temporally coordinating cellular function, tissue function, and behavior. These endogenous rhythms dampen with age and thus compromise temporal coordination. Feeding-fasting patterns are an external cue that profoundly influence the robustness of daily biological rhythms. Erratic eating patterns can disrupt the temporal coordination of metabolism and physiology leading to chronic diseases that are also characteristic of aging. However, sustaining a robust feeding-fasting cycle, even without altering nutrition quality or quantity, can prevent or reverse these chronic diseases in experimental models. In humans, epidemiological studies have shown erratic eating patterns increase the risk of disease, whereas sustained feeding-fasting cycles, or prolonged overnight fasting, is correlated with protection from breast cancer. Therefore, optimizing the timing of external cues with defined eating patterns can sustain a robust circadian clock, which may prevent disease and improve prognosis.

Circadian Clocks and Metabolism: Implications for Microbiome and Aging

The circadian clock directs many aspects of metabolism, to separate in time opposing metabolic pathways and optimize metabolic efficiency. The master circadian clock of the suprachiasmatic nucleus synchronizes to light, while environmental cues such as temperature and feeding, out of phase with the light schedule, may synchronize peripheral clocks. This misalignment of central and peripheral clocks may be involved in the development of disease and the acceleration of aging, possibly in a gender-specific manner. Here we discuss the interplay between the circadian clock and metabolism, the importance of the microbiome, and how they relate to aging.

Sex differences in age-related changes in the sleep-wake cycle

Age-related changes in sleep and circadian regulation occur as early as the middle years of life. Research also suggests that sleep and circadian rhythms are regulated differently between women and men. However, does sleep and circadian rhythms regulation age similarly in men and women? In this review, we present the mechanisms underlying age-related differences in sleep and the current state of knowledge on how they interact with sex. We also address how testosterone, estrogens, and progesterone fluctuations across adulthood interact with sleep and circadian regulation. Finally, we will propose research avenues to unravel the mechanisms underlying sex differences in age-related effects on sleep.

The brain, sirtuins, and ageing

In mammals, recent studies have demonstrated that the brain, the hypothalamus in particular, is a key bidirectional integrator of humoral and neural information from peripheral tissues, thus influencing ageing both in the brain and at the 'systemic' level. CNS decline drives the progressive impairment of cognitive, social and physical abilities, and the mechanisms underlying CNS regulation of the ageing process, such as microglia-neuron networks and the activities of sirtuins, a class of NAD⁺-dependent deacylases, are beginning to be understood. Such mechanisms are potential targets for the prevention or treatment of age-associated dysfunction and for the extension of a healthy lifespan.

Sleep and Alzheimer's disease: A pivotal role for the suprachiasmatic nucleus

Alzheimer's disease (AD), which accounts for most of the dementia cases, is, aside from cognitive deterioration, often characterized by the presence of non-cognitive symptoms. Society is desperately in need for interventions that alleviate the economic and social burden related to AD. Circadian dysrhythmia, one of these symptoms in particular, immensely decreases the self-care ability of AD patients and is one of the main reasons of caregiver exhaustion. Studies suggest that these circadian disturbances form the root of sleep-wake problems, diagnosed in more than half of AD patients. Sleep abnormalities have generally been considered merely a consequence of AD pathology. Recent evidence suggests that a bidirectional relationship exists between sleep and AD, and that poor sleep might negatively impact amyloid burden, as well as cognition. The suprachiasmatic nucleus (SCN), the main circadian pacemaker, is subjected to several alterations during the course of the disease. Its functional deterioration might fulfill a crucial role in the relation between AD pathophysiology and the development of sleep abnormalities. This review aims to give a concise overview of the anatomy and physiology of the SCN, address how AD pathology precisely impacts the SCN and to what degree these alterations can contribute to the progression of the disease.

The clock is ticking. Ageing of the circadian system: From physiology to cell cycle

The circadian system is the responsible to organise the internal temporal order in relation to the environment of every process of the organisms producing the circadian rhythms. These rhythms have a fixed phase relationship among them and with the environment in order to optimise the available energy and resources. From a cellular level, circadian rhythms are controlled by genetic positive and negative auto-regulated transcriptional and translational feedback loops, which generate 24h rhythms in mRNA and protein levels of the clock components. It has been described about 10% of the genome is controlled by clock genes, with special relevance, due to its implications, to the cell cycle. Ageing is a deleterious process which affects all the organisms' structures including circadian system. The circadian system's ageing may produce a disorganisation among the circadian rhythms, arrhythmicity and, even, disconnection from the environment, resulting in a detrimental situation to the organism. In addition, some environmental conditions can produce circadian disruption, also called chronodisruption, which may produce many pathologies including accelerated ageing. Finally, some strategies to prevent, palliate or counteract chronodisruption effects have been proposed to enhance the circadian system, also called chronoenhancement. This review tries to gather recent advances in the chronobiology of the ageing process, including cell cycle, neurogenesis process and physiology.

Membrane Currents, Gene Expression, and Circadian Clocks

Neuronal circadian oscillators in the mammalian and Drosophila brain express a circadian clock comprised of interlocking gene transcription feedback loops. The genetic clock regulates the membrane electrical activity by poorly understood signaling pathways to generate a circadian pattern of action potential firing. During the day, Na⁺ channels contribute an excitatory drive for the spontaneous activity of circadian clock neurons. Multiple types of K⁺ channels regulate the action potential firing pattern and the nightly reduction in neuronal activity. The membrane electrical activity possibly signaling by changes in intracellular Ca²⁺ and cyclic adenosine monophosphate (cAMP) regulates the activity of the gene clock. A decline in the signaling pathways that link the gene clock and neural activity during aging and disease may weaken the circadian output and generate significant impacts on human health.

Circadian Rhythm Neuropeptides in Drosophila: Signals for Normal Circadian Function and Circadian Neurodegenerative Disease

Circadian rhythm is a ubiquitous phenomenon in many organisms ranging from prokaryotes to eukaryotes. During more than four decades, the intrinsic and exogenous regulations of circadian rhythm have been studied. This review summarizes the core endogenous oscillation in Drosophila and then focuses on the neuropeptides, neurotransmitters and hormones that mediate its outputs and integration in Drosophila and the links between several of these (pigment dispersing factor (PDF) and insulin-like peptides) and neurodegenerative disease. These signaling molecules convey important network connectivity and signaling information for normal circadian function, but PDF and insulin-like peptides can also convey signals that lead to apoptosis, enhanced neurodegeneration and cognitive decline in flies carrying circadian mutations or in a senescent state.

Geniculohypothalamic GABAergic projections gate suprachiasmatic nucleus responses to retinal input

KEY POINTS

Visual input to the suprachiasmatic nucleus circadian clock is critical for animals to adapt their physiology and behaviour in line with the solar day. In addition to direct retinal projections, the clock receives input from the visual thalamus, although the role of this geniculohypothalamic pathway in circadian photoreception is poorly understood. In the present study, we develop a novel brain slice preparation that preserves the geniculohypothalamic pathway to show that GABAergic thalamic neurons inhibit retinally-driven activity in the central clock in a circadian time-dependent manner. We also show that in vivo manipulation of thalamic signalling adjusts specific features of the hypothalamic light response, indicating that the geniculohypothalamic pathway is primarily activated by crossed retinal inputs. Our data provide a mechanism by which geniculohypothalamic signals can adjust the magnitude of circadian and more acute hypothalamic light responses according to time-of-day and establish an important new model for future investigations of the circadian visual system.

ABSTRACT

Sensory input to the master mammalian circadian clock, the suprachiasmatic nucleus (SCN), is vital in allowing animals to optimize physiology and behaviour alongside daily changes in the environment. Retinal inputs encoding changes in external illumination provide the principle source of such information. The SCN also receives input from other retinorecipient brain regions, primarily via the geniculohypothalamic tract (GHT), although the contribution of these indirect projections to circadian photoreception is currently poorly understood. To address this deficit, in the present study, we established an in vitro mouse brain slice preparation that retains connectivity across the extended circadian system. Using multi-electrode recordings, we first confirm that this preparation retains intact optic projections to the SCN, thalamus and pretectum and a functional GHT. We next show that optogenetic activation of GHT neurons selectively suppresses SCN responses to retinal input, and also that this effect exhibits a pronounced day/night variation and involves a GABAergic mechanism. This inhibitory action was not associated with overt circadian rhythmicity in GHT output, indicating modulation at the SCN level. Finally, we use in vivo electrophysiological recordings alongside pharmacological inactivation or optogenetic excitation to show that GHT signalling actively modulates specific features of the SCN light response, indicating that GHT cells are primarily activated by crossed retinal projections. Taken together, our data establish a new model for studying network communication in the extended circadian system and provide novel insight into the roles of GHT-signalling, revealing a mechanism by which thalamic activity can help gate retinal input to the SCN according to time of day.

Evidence for Weakened Intercellular Coupling in the Mammalian Circadian Clock under Long Photoperiod

For animals living in temperate latitudes, seasonal changes in day length are an important cue for adaptations of their physiology and behavior to the altered environmental conditions. The suprachiasmatic nucleus (SCN) is known as the central circadian clock in mammals, but may also play an important role in adaptations to different photoperiods. The SCN receives direct light input from the retina and is able to encode day-length by approximating the waveform of the electrical activity rhythm to the duration of daylight. Changing the overall waveform requires a reorganization of the neuronal network within the SCN with a change in the degree of synchrony between the neurons; however, the underlying mechanisms are yet unknown. In the present study we used PER2::LUC bioluminescence imaging in cultured SCN slices to characterize network dynamics on the single-cell level and we aimed to provide evidence for a role of modulations in coupling strength in the photoperiodic-induced phase dispersal. Exposure to long photoperiod (LP) induced a larger distribution of peak times of the single-cell PER2::LUC rhythms in the anterior SCN, compared to short photoperiod. Interestingly, the cycle-to-cycle variability in single-cell period of PER2::LUC rhythms is also higher in the anterior SCN in LP, and is positively correlated with peak time dispersal. Applying a new, impartial community detection method on the time series data of the PER2::LUC rhythm revealed two clusters of cells with a specific spatial distribution, which we define as dorsolateral and ventromedial SCN. Post hoc analysis of rhythm characteristics of these clusters showed larger cycle-to-cycle single-cell period variability in the dorsolateral compared to the ventromedial cluster in the anterior SCN. We conclude that a change in coupling strength within the SCN network is a plausible explanation to the observed changes in single-cell period variability, which can contribute to the photoperiod-induced phase distribution.

Photoperiod Modulates Fast Delayed Rectifier Potassium Currents in the Mammalian Circadian Clock

One feature of the mammalian circadian clock, situated in the suprachiasmatic nucleus (SCN), is its ability to measure day length and thereby contribute to the seasonal adaptation of physiology and behavior. The timing signal from the SCN, namely the 24 hr pattern of electrical activity, is adjusted according to the photoperiod being broader in long days and narrower in short days. Vasoactive intestinal peptide and gamma-aminobutyric acid play a crucial role in intercellular communication within the SCN and contribute to the seasonal changes in phase distribution. However, little is known about the underlying ionic mechanisms of synchronization. The present study was aimed to identify cellular mechanisms involved in seasonal encoding by the SCN. Mice were adapted to long-day (light–dark 16:8) and short-day (light–dark 8:16) photoperiods and membrane properties as well as K⁺ currents activity of SCN neurons were measured using patch-clamp recordings in acute slices. Remarkably, we found evidence for a photoperiodic effect on the fast delayed rectifier K⁺ current, that is, the circadian modulation of this ion channel’s activation reversed in long days resulting in 50% higher peak values during the night compared with the unaltered day values. Consistent with fast delayed rectifier enhancement, duration of action potentials during the night was shortened and afterhyperpolarization potentials increased in amplitude and duration. The slow delayed rectifier, transient K⁺ currents, and membrane excitability were not affected by photoperiod. We conclude that photoperiod can change intrinsic ion channel properties of the SCN neurons, which may influence cellular communication and contribute to photoperiodic phase adjustment.

Reciprocal interactions between circadian clocks and aging

Virtually, all biological processes in the body are modulated by an internal circadian clock which optimizes physiological and behavioral performance according to the changing demands of the external 24-h world. This circadian clock undergoes a number of age-related changes, at both the physiological and molecular levels. While these changes have been considered to be part of the normal aging process, there is increasing evidence that disruptions to the circadian system can substantially impact upon aging and these impacts will have clear health implications. Here we review the current data of how both the physiological and core molecular clocks change with age and how feedback from external cues may modulate the aging of the circadian system.

Circadian Rhythms, Sleep, and Disorders of Aging

Sleep-wake cycles are known to be disrupted in people with neurodegenerative disorders. These findings are now supported by data from animal models for some of these disorders, raising the question of whether the disrupted sleep/circadian regulation contributes to the loss of neural function. As circadian rhythms and sleep consolidation also break down with normal aging, changes in these may be part of what makes aging a risk factor for disorders like Alzheimer's disease (AD). Mechanisms underlying the connection between circadian/sleep dysregulation and neurodegeneration remain unclear, but several recent studies provide interesting possibilities. While mechanistic analysis is under way, it is worth considering treatment of circadian/sleep disruption as a means to alleviate symptoms of neurodegenerative disorders.

Age-Related Changes in the Circadian System Unmasked by Constant Conditions

Circadian timing systems, like most physiological processes, cannot escape the effects of aging. With age, humans experience decreased duration and quality of sleep. Aged mice exhibit decreased amplitude and increased fragmentation of the activity rhythm, and lengthened circadian free-running period in both light-dark (LD) and constant dark (DD) conditions. Several studies have shown that aging impacts neural activity rhythms in the central circadian clock in the suprachiasmatic nucleus (SCN). However, evidence for age-related disruption of circadian oscillations of clock genes in the SCN has been equivocal. We hypothesized that daily exposure to LD cycles masks the full impact of aging on molecular rhythms in the SCN. We performed ex vivo bioluminescent imaging of cultured SCN slices of young and aged PER2::luciferase knock-in (PER2::LUC) mice housed under LD or prolonged DD conditions. Under LD conditions, the amplitude of PER2::LUC rhythms differed only slightly between SCN explants from young and aged animals; under DD conditions, the PER2::LUC rhythms of aged animals showed markedly lower amplitudes and longer circadian periods than those of young animals. Recordings of PER2::LUC rhythms in individual SCN cells using an electron multiplying charge-coupled device camera revealed that aged SCN cells showed longer circadian periods and that the rhythms of individual cells rapidly became desynchronized. These data suggest that aging degrades the SCN circadian ensemble, but that recurrent LD cycles mask these effects. We propose that these changes reflect a decline in pacemaker robustness that could increase vulnerability to environmental challenges, and partly explain age-related sleep and circadian disturbances.

The hypothalamus in Alzheimer's disease: a Golgi and electron microscope study

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, characterized by irreversible decline of mental faculties, emotional and behavioral changes, loss of motor skills, and dysfunction of autonomic nervous system and disruption of circadian rhythms (CRs). We attempted to describe the morphological findings of the hypothalamus in early cases of AD, focusing our study mostly on the suprachiasmatic nucleus (SCN), the supraoptic nucleus (SON), and the paraventricular nucleus (PVN). Samples were processed for electron microscopy and silver impregnation techniques. The hypothalamic nuclei demonstrated a substantial decrease in the neuronal population, which was particularly prominent in the SCN. Marked abbreviation of dendritic arborization, in association with spinal pathology, was also seen. The SON and PVN demonstrated a substantial number of dystrophic axons and abnormal spines. Alzheimer's pathology, such as deposits of amyloid-β peptide and neurofibrillary degeneration, was minimal. Electron microscopy revealed mitochondrial alterations in the cell body and the dendritic branches. The morphological alterations of the hypothalamic nuclei in early cases of AD may be related to the gradual alteration of CRs and the instability of autonomic regulation.

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

The proportion of light-responsive neurons determines the limit cycle properties of the suprachiasmatic nucleus

In mammals, the central clock in the suprachiasmatic nucleus (SCN) controls physiological and behavioral circadian rhythms and is entrained to the external light-dark cycle. The ability of the SCN to entrain can be measured by exposing the animal to a light-dark cycle with a duration that deviates from 24 h (T-cycles); a wider entrainment range reflects a higher ability to entrain. The neurons of the SCN are either light responsive or light unresponsive and are mutually synchronized. The coupling and synchronization between individual SCN neurons and between groups of neurons within the SCN influence the SCN's ability to entrain. Some studies suggest that enhanced coupling decreases the entrainment range, whereas others suggest that enhanced coupling increases the entrainment range. The latter results are surprising, as they are not consistent with the prevalent assumption that the SCN is a limit cycle oscillator that has larger phase shifts when the amplitude is smaller. Here, we used the Poincaré and Goodwin models to test entrainment properties using various proportions of neurons that are responsive to an external stimulus. If all neurons receive external input, the SCN shows limit cycle behavior in all conditions. If all neurons do not receive light input, we found that the entrainment range of the SCN was positively related to coupling strength when coupling was weak. When coupling strength was stronger and above a critical value, the entrainment range was negatively correlated with coupling strength. The results obtained from our simulations were confirmed by analytical studies. Thus, the limit cycle behavior of the SCN appears to be critically dependent on the coupling strength among the neurons and the proportion of neurons that respond to the entraining stimulus.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Aging in mice reduces the ability to sustain sleep/wake states

One of the most significant problems facing older individuals is difficulty staying asleep at night and awake during the day. Understanding the mechanisms by which the regulation of sleep/wake goes awry with age is a critical step in identifying novel therapeutic strategies to improve quality of life for the elderly. We measured wake, non-rapid eye movement (NREM) and rapid-eye movement (REM) sleep in young (2–4 months-old) and aged (22–24 months-old) C57BL6/NIA mice. We used both conventional measures (i.e., bout number and bout duration) and an innovative spike-and-slab statistical approach to characterize age-related fragmentation of sleep/wake. The short (spike) and long (slab) components of the spike-and-slab mixture model capture the distribution of bouts for each behavioral state in mice. Using this novel analytical approach, we found that aged animals are less able to sustain long episodes of wakefulness or NREM sleep. Additionally, spectral analysis of EEG recordings revealed that aging slows theta peak frequency, a correlate of arousal. These combined analyses provide a window into the mechanisms underlying the destabilization of long periods of sleep and wake and reduced vigilance that develop with aging.

Circadian aspects of energy metabolism and aging

Life span extension has been a goal of research for several decades. Resetting circadian rhythms leads to well being and increased life span, while clock disruption is associated with increased morbidity accelerated aging. Increased longevity and improved health can be achieved by different feeding regimens that reset circadian rhythms and may lead to better synchrony in metabolism and physiology. This review focuses on the circadian aspects of energy metabolism and their relationship with aging in mammals.

Aging of the suprachiasmatic clock

More than half of the elderly in today's society suffer from sleep disorders with detrimental effects on brain function, behavior, and social life. A major contribution to the regulation of sleep stems from the circadian system. The central circadian clock located in the suprachiasmatic nucleus of the hypothalamus is like other brain regions subject to age-associated changes. Age affects different levels of the clock machinery from molecular rhythms, intracellular messenger, and membrane properties to neuronal network synchronization. While some of the age-sensitive components of the circadian clock, like ion channels and neurotransmitters, have been described, little is known about the underlying mechanisms. In any case, the result is a reduction in the amplitude of the circadian timing signal produced by the suprachiasmatic nucleus, a weakening in the control of peripheral oscillators and a decrease in amplitude and precision of daily rhythms in physiology and behavior. The distortion in temporal organization is thought to be related to a number of serious health problems and promote neurodegeneration. Understanding the mechanisms underlying age-related deficits in circadian clock function will therefore not only benefit rhythm disorders but also alleviate age-associated diseases aggravated by clock dysfunction.

Effects of age-related dopaminergic neuron loss in the substantia nigra on the circadian rhythms of locomotor activity in mice

Elderly people often develop sleep and autonomic dysfunctions, which are regulated by circadian rhythm. Recently, we reported on the degradation of neural output from the central circadian clock in the suprachiasmatic nucleus (SCN) with aging. However, it is likely that many other factors contribute to the age-related decline in the functioning of the circadian system. In this study, we examined the effects of dopaminergic neuronal loss in the substantia nigra (SN) on circadian rhythms of mice to assess whether age-related degeneration of the dopamine system influences circadian rhythm. Young male C57BL/6J mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a compound that selectively destroys dopaminergic neurons in the SN, and their wheel-running activities were recorded. We observed that MPTP-treated mice lost 43% of their dopaminergic neurons in the SN (on average) and demonstrated longer period of wheel-running activity rhythm in constant darkness compared with control mice. However, all the remaining circadian parameters in the MPTP-treated mice remained constant. Our findings suggest that in addition to SCN output dysfunction, age-related degeneration in the dopamine system of the brain leads to circadian rhythm irregularities.

Evidence for neuronal desynchrony in the aged suprachiasmatic nucleus clock

Aging is associated with a deterioration of daily (circadian) rhythms in physiology and behavior. Deficits in the function of the central circadian pacemaker in the suprachiasmatic nucleus (SCN) have been implicated, but the responsible mechanisms have not been clearly delineated. In this report, we characterize the progression of rhythm deterioration in mice to 900 d of age. Longitudinal behavioral and sleep-wake recordings in up to 30-month-old mice showed strong fragmentation of rhythms, starting at the age of 700 d. Patch-clamp recordings in this age group revealed deficits in membrane properties and GABAergic postsynaptic current amplitude. A selective loss of circadian modulation of fast delayed-rectifier and A-type K+ currents was observed. At the tissue level, phase synchrony of SCN neurons was grossly disturbed, with some subpopulations peaking in anti-phase and a reduction in amplitude of the overall multiunit activity rhythm. We propose that aberrant SCN rhythmicity in old animals--with electrophysiological arrhythmia at the single-cell level and phase desynchronization at the network level--can account for defective circadian function with aging.

The circadian clock and pathology of the ageing brain

Ageing leads to a functional deterioration of many brain systems, including the circadian clock--an internal time-keeping system that generates ∼24-hour rhythms in physiology and behaviour. Numerous clinical studies have established a direct correlation between abnormal circadian clock functions and the severity of neurodegenerative and sleep disorders. Latest data from experiments in model organisms, gene expression studies and clinical trials imply that dysfunctions of the circadian clock contribute to ageing and age-associated pathologies, thereby suggesting a functional link between the circadian clock and age-associated decline of brain functions. Potential molecular mechanisms underlying this link include the circadian control of physiological processes such as brain metabolism, reactive oxygen species homeostasis, hormone secretion, autophagy and stem cell proliferation.

Circadian activity rhythms and risk of incident dementia and mild cognitive impairment in older women

OBJECTIVE

Previous cross-sectional studies have observed alterations in activity rhythms in dementia patients but the direction of causation is unclear. We determined whether circadian activity rhythms measured in community-dwelling older women are prospectively associated with incident dementia or mild cognitive impairment (MCI).

METHODS

Activity rhythm data were collected from 1,282 healthy community-dwelling women from the Study of Osteoporotic Fractures (SOF) cohort (mean age 83 years) with wrist actigraphy for a minimum of three 24-hour periods. Each participant completed a neuropsychological test battery and had clinical cognitive status (dementia, MCI, normal) adjudicated by an expert panel approximately 5 years later. All analyses were adjusted for demographics, body mass index (BMI), functional status, depression, medications, alcohol, caffeine, smoking, health status, and comorbidities.

RESULTS

After 4.9 years of follow-up, 195 (15%) women had developed dementia and 302 (24%) had developed MCI. Older women with decreased activity rhythms had a higher likelihood of developing dementia or MCI when comparing those in the lowest quartiles of amplitude (odds ratio [OR] = 1.57; 95% CI, 1.09-2.25) or rhythm robustness (OR = 1.57; 95% CI, 1.10-2.26) to women in the highest quartiles. An increased risk of dementia or MCI (OR = 1.83; 95% CI, 1.29-2.61) was found for women whose timing of peak activity occurred later in the day (after 3:51 PM) when compared to those with average timing (1:34 PM-3:51 PM).

INTERPRETATION

Older, healthy women with decreased circadian activity rhythm amplitude and robustness, and delayed rhythms have increased odds of developing dementia and MCI. If confirmed, future studies should examine whether interventions (physical activity, bright light exposure) that influence activity rhythms will reduce the risk of cognitive deterioration in the elderly.

Circadian rhythms, aging, and life span in mammals

Resetting the circadian clock leads to well being and increased life span, whereas clock disruption is associated with aging and morbidity. Increased longevity and improved health can be achieved by different feeding regimens that reset circadian rhythms and may lead to better synchrony in metabolism and physiology. This review focuses on recent findings concerning the relationships between circadian rhythms, aging attenuation, and life-span extension in mammals.

Dynamic neuronal network organization of the circadian clock and possible deterioration in disease

In mammals, the suprachiasmatic nuclei (SCNs) function as a circadian pacemaker that drives 24-h rhythms in physiology and behavior. The SCN is a multicellular clock in which the constituent oscillators show dynamics in their functional organization and phase coherence. Evidence has emerged that plasticity in phase synchrony among SCN neurons determines (i) the amplitude of the rhythm, (ii) the response to continuous light, (iii) the capacity to respond to seasonal changes, and (iv) the phase-resetting capacity. A decrease in circadian amplitude and phase-resetting capacity is characteristic during aging and can be a result of disease processes. Whether the decrease in amplitude is caused by a loss of synchronization or by a loss of single-cell rhythmicity remains to be determined and is important for the development of strategies to ameliorate circadian disorders.

Age-related decline in circadian output

Disruptions in sleep/wake cycles, including decreased amplitude of rhythmic behaviors and fragmentation of the sleep episodes, are commonly associated with aging in humans and other mammals. While there are undoubtedly many factors contributing to these changes, a body of literature is emerging, suggesting that an age-related decline in the central circadian clock in the suprachiasmatic nucleus (SCN) may be a key element responsible. To explore age-related changes in the SCN, we have performed in vivo multiunit neural activity (MUA) recordings from the SCN of freely moving young (3-5 months) and middle-aged (13-18 months) mice. Importantly, the amplitude of day-night difference in MUA was significantly reduced in the older mice. We also found that the neural activity rhythms are clearly degraded in the subparaventricular zone, one of the main neural outputs of the SCN. Surprisingly, parallel studies indicate that the molecular clockwork in the SCN as measured by PER2 exhibited only minor deficits at this same age. Thus, the circadian output measured at the level of neural activity rhythms in the SCN is degraded by aging, and this decline occurs before the disruption of key components of the molecular clockwork.

Age-related changes in sleep in inbred mice are genotype dependent

Aging produces major changes in sleep structure and intensity which might be linked to cognitive impairment in the elderly. In this study, the genetic contribution to age-related changes in sleep was assessed in three inbred mouse strains of various ages. Baseline sleep and the response to 6 hours sleep deprivation (SD) achieved by gentle handling were quantified in young, middle-aged, and older male mice using electroencephalography. Total sleep time initially increased with age but then decreased in the oldest group mainly due to changes in sleep duration during the active phase. The effect of age on electroencephalographic (EEG) delta power depends on genotype and sleep pressure level with SD increasing the age-related differences. The strong effect of age upon the spectral profile of the different behavioral states was modulated by genetic background. Overall, our results suggest that sleep pressure can modulate the effect of age, that most sleep variables do not monotonically change with age in contrast to previous reports in humans and other species, and that genetic factors have a major impact on the aging processes affecting sleep.

Impact of aging on diurnal expression patterns of CLOCK and BMAL1 in the mouse brain

Mammalian circadian rhythms are generated by a network of transcriptional and translational loops in the expression of a panel of clock genes in various brain and peripheral sites. Many of the output rhythms controlled by this system are significantly affected by ageing, although the mechanisms of age-related circadian dysfunction remain opaque. The aim of this study was to investigate the effect of aging on the daily oscillation of two clock gene proteins (CLOCK, BMAL1) in the mouse brain. Clock gene protein expression in the brain was measured by means of immunohistochemistry in groups of young (4 months) and older (16 months) mice sampled every 4h over a 24-h cycle. CLOCK and BMAL1 were constitutively expressed in the suprachiasmatic nucleus (SCN; the master circadian pacemaker) in young adult animals. We report novel rhythmic expression of CLOCK and BMAL1 in a number of extra-SCN sites in the young mouse brain, including the hippocampus, amygdala and the paraventricular, arcuate and dorsomedial nuclei of the hypothalamus. Aging altered the amplitude and/or phase of expression in these regions. These results indicate hitherto unreported expression patterns of CLOCK and BMAL1 in non-SCN brain circadian oscillators, and suggest that alterations of these patterns may contribute to age-related circadian dysfunction.

Circadian clocks and antiaging: do non-aging microalgae like Euglena reveal anything?

Microalgae that divide symmetrically in all aspects do not age. While the evolutionary reason for this is obvious, little attention has been paid to the mechanistic explanations. A great deal of study involving many research fields would be needed to explain the mechanisms if we suppose that the immortality results from a lifelong sufficiency of defense from stress or from an essential part of counteracting age-accompanied damage accumulation. Additionally, little is known about the relationships between homeostasis and circadian clocks in antiaging, although each of these has been studied separately. Here, we present a conceptual generalization of those relationships, as suggested by evidence from non-aging microalgae, mainly Euglena. The circadian gating of mitosis and circadian temporal coordination may respectively reduce radiation- and disharmony-induced stress in which homeostasis cannot be involved, whereas circadian resistance rhythms may greatly help homeostatic defense from radiation- and metabolism-induced stress. We also briefly sketch mammalian aging research to compare the current status of knowledge with that of algal antiaging.