Age‐Related Effects on the Biological Clock and its Behavioral Output in a Primate

Advancements in research on the association between the biological CLOCK and type 2 diabetes

Due to the Earth's rotation, the natural environment exhibits a light-dark diurnal cycle close to 24 hours. To adapt to this energy intake pattern, organisms have developed a 24-hour rhythmic diurnal cycle over long periods, known as the circadian rhythm, or biological clock. With the gradual advancement of research on the biological clock, it has become increasingly evident that disruptions in the circadian rhythm are closely associated with the occurrence of type 2 diabetes (T2D). To further understand the progress of research on T2D and the biological clock, this paper reviews the correlation between the biological clock and glucose metabolism and analyzes its potential mechanisms. Based on this, we discuss the potential factors contributing to circadian rhythm disruption and their impact on the risk of developing T2D, aiming to explore new possible intervention measures for the prevention and treatment of T2D in the future. Under the light-dark circadian rhythm, in order to adapt to this change, the human body forms an internal biological clock involving a variety of genes, proteins and other molecules. The main mechanism is the transcription-translation feedback loop centered on the CLOCK/BMAL1 heterodimer. The expression of important circadian clock genes that constitute this loop can regulate T2DM-related blood glucose traits such as glucose uptake, fat metabolism, insulin secretion/glucagon secretion and sensitivity in various peripheral tissues and organs. In addition, sleep, light, and dietary factors under circadian rhythms also affect the occurrence of T2DM.

Melatonin suppression by light involves different retinal photoreceptors in young and older adults

Age-related sleep and circadian rhythm disturbances may be due to altered nonvisual photoreception. Here, we investigated the temporal dynamics of light-induced melatonin suppression in young and older individuals. In a within-subject design study, young and older participants were exposed for 60 min (0030-0130 at night) to nine narrow-band lights (range: 420-620 nm). Plasma melatonin suppression was calculated at 15, 30, 45, and 60 min time intervals. Individual spectral sensitivity of melatonin suppression and photoreceptor contribution were predicted for each interval and age group. In young participants, melanopsin solely drove melatonin suppression at all time intervals, with a peak sensitivity at 485.3 nm established only after 15 min of light exposure. Conversely, in older participants, spectral light-driven melatonin suppression was best explained by a more complex model combining melanopsin, S-cone, and M-cone functions, with a stable peak (~500 nm) at 30, 45, and 60 min of light exposure. Aging is associated with a distinct photoreceptor contribution to melatonin suppression by light. While in young adults melanopsin-only photoreception is a reliable predictor of melatonin suppression, in older individuals this process is jointly driven by melanopsin, S-cone, and M-cone functions. These findings offer new prospects for customizing light therapy for older individuals.

Age-related and individual features of the HPA axis stress responsiveness under constant light in nonhuman primates

The hypothalamic-pituitary-adrenal (HPA) axis is a key adaptive neuroendocrine system, dysfunction of which plays an important role in the increasing incidence of stress-dependent age-related pathology. Among the environmental factors effecting increase age-related diseases, great importance is given to disturbances of the light-dark schedule, particularly with increased illumination at night. While disruption of the light-dark schedule has long been recognized as a powerful behavioral stressor, little is known regarding stress reactivity of the HPA under constant light (CL) conditions, especially with aging and depending on the features of stress behavior. The purpose of this investigation was to study the age-related and individual features of the HPA axis response to acute stress exposure (ASE) under chronic CL in nonhuman primates that are known to differ in behavioral responsiveness to stress. Young and old female rhesus monkeys (with control standard behavior or anxiety and depression-like behavior) were exposed to CL (24 h light/day, 330-400 lux for 4 to 8 weeks). Control young and old monkeys were exposed to standard lighting (SL) with natural light during the day and darkness at night. All animals were subjected to ASE (restriction of mobility for 2 hours), functional tests with corticotrophin-releasing hormone and arginine-vasopressin, and study of circadian rhythms of cortisol and pineal melatonin secretion. For the first time an inhibitory effect of CL on the reaction of the adrenal cortex to ASE was revealed in all individuals, regardless of age and preexisting behavior stress reactivity, the mechanisms of which were age-dependent: due to inhibition of the pituitary ACTH secretion in young animals and mainly not affecting the ACTH secretion in old individuals. There were no significant changes in melatonin secretion both in young and old animals. The observed CL inhibition of adrenal cortical reactivity to ASE may be useful to correct increased vulnerability to ASE observed in individuals with preexisting anxiety and depression-like stress behaviors. On the other hand, the CL induced decrease in adrenal stress reactivity of behaviorally normal animals suggests a potential risk of reducing the adaptive capacity of the organism under conditions of continuous light exposure.

Developmental features of sleep electrophysiology in family dogs

Age-related differences in dog sleep and the age at which dogs reach adulthood as indexed by sleep electrophysiology are unknown. We assessed, in (1) a Juvenile sample (n = 60) of 2-14-month-old dogs (weight range: 4-68 kg), associations between age, sleep macrostructure, and non-rapid eye movement (NREM) EEG power spectrum, whether weight moderates associations, and (2) an extended sample (n = 91) of 2-30-months-old dogs, when sleep parameters stabilise. In Juvenile dogs, age was positively associated with time in drowsiness between 2 and 8 months, and negatively with time in rapid eye movement (REM) sleep between 2 and 6 months. Age was negatively associated with delta and positively with theta and alpha power activity, between 8 and 14 months. Older dogs exhibited greater sigma and beta power activity. Larger, > 8-month-old dogs had less delta and more alpha and beta activity. In extended sample, descriptive data suggest age-related power spectrum differences do not stabilise by 14 months. Drowsiness, REM, and delta power findings are consistent with prior results. Sleep electrophysiology is a promising index of dog neurodevelopment; some parameters stabilise in adolescence and some later than one year. Determination of the effect of weight and timing of power spectrum stabilisation needs further inquiry. The dog central nervous system is not fully mature by 12 months of age.

Relationships between endogenous circadian period, physiological and cognitive parameters and sex in aged gray mouse lemurs (Microcebus murinus)

The biological clock generates circadian rhythms, with an endogenous period tau close to 24 h. The circadian resonance theory proposes that lifespan is reduced when endogenous period goes far from 24 h. It has been suggested that daily resetting of the circadian clock to the 24 h external photoperiod might induce marginal costs that would accumulate over time and forward accelerate aging and affect fitness. In this study, we aimed to evaluate the link between the endogenous period and biomarkers of aging in order to investigate the mechanisms of the circadian resonance theory. We studied 39 middle-aged and aged Microcebus murinus, a nocturnal non-human primate whose endogenous period is about 23.1 h, measuring the endogenous period of locomotor activity, as well as several physiological and behavioral parameters (rhythm fragmentation and amplitude, energetic expenditure, oxidative stress, insulin-like growth factor-1 (IGF-1) concentrations and cognitive performances) in both males and females. We found that aged males with tau far from 24 h displayed increased oxidative stress. We also demonstrated a positive correlation between tau and IGF-1 concentrations, as well as learning performances, in males and females. Together these results suggest that a great deviation of tau from 24 h leads to increased biomarkers of age-related impairments.

Sleep in Normal Aging, Homeostatic and Circadian Regulation and Vulnerability to Sleep Deprivation

In the context of geriatric research, a growing body of evidence links normal age-related changes in sleep with many adverse health outcomes, especially a decline in cognition in older adults. The most important sleep alterations that continue to worsen after 60 years involve sleep timing, (especially early wake time, phase advance), sleep maintenance (continuity of sleep interrupted by numerous awakenings) and reduced amount of sigma activity (during non-rapid eye movement (NREM) sleep) associated with modifications of sleep spindle characteristics (density, amplitude, frequency) and spindle-Slow Wave coupling. After 60 years, there is a very clear gender-dependent deterioration in sleep. Even if there are degradations of sleep after 60 years, daytime wake level and especially daytime sleepiness is not modified with age. On the other hand, under sleep deprivation condition, older adults show smaller cognitive impairments than younger adults, suggesting an age-related lower vulnerability to extended wakefulness. These sleep and cognitive age-related modifications would be due to a reduced homeostatic drive and consequently a reduced sleep need, an attenuation of circadian drive (reduction of sleep forbidden zone in late afternoon and wake forbidden zone in early morning), a modification of the interaction of the circadian and homeostatic processes and/or an alteration of subcortical structures involved in generation of circadian and homeostatic drive, or connections to the cerebral cortex with age. The modifications and interactions of these two processes with age are still uncertain, and still require further investigation. The understanding of the respective contribution of circadian and homeostatic processes in the regulation of neurobehavioral function with aging present a challenge for improving health, management of cognitive decline and potential early chronobiological or sleep-wake interventions.

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.

Survival is reduced when endogenous period deviates from 24 h in a non-human primate, supporting the circadian resonance theory

Circadian rhythms are ubiquitous attributes across living organisms and allow the coordination of internal biological functions with optimal phases of the environment, suggesting a significant adaptive advantage. The endogenous period called tau lies close to 24 h and is thought to be implicated in individuals' fitness: according to the circadian resonance theory, fitness is reduced when tau gets far from 24 h. In this study, we measured the endogenous period of 142 mouse lemurs (Microcebus murinus), and analyzed how it is related to their survival. We found different effects according to sex and season. No impact of tau on mortality was found in females. However, in males, the deviation of tau from 24 h substantially correlates with an increase in mortality, particularly during the inactive season (winter). These results, comparable to other observations in mice or drosophila, show that captive gray mouse lemurs enjoy better fitness when their circadian period closely matches the environmental periodicity. In addition to their deep implications in health and aging research, these results raise further ecological and evolutionary issues regarding the relationships between fitness and circadian clock.

Aging-related changes on social synchronization of circadian activity rhythm in a diurnal primate (Callithrix jacchus)

The input of environmental time cues and expression of circadian activity rhythms may change with aging. Among nonphotic zeitgebers, social cues from conspecific vocalizations may contribute to the stability and survival of individuals of social species, such as nonhuman primates. We evaluated aging-related changes on social synchronization of the circadian activity rhythm (CAR) in a social diurnal primate, the common marmoset. The activity of 18 male marmosets was recorded by actiwatches in two conditions. (1) Experimental - 4 young adult (5 ± 2 yrs of age) and 4 older (10 ± 2 yrs of age) animals maintained under LD 12/12 h and LL in a room with full insulation for light but only partial insulation for sound from vocalizations of conspecifics maintained outdoors in the colony; and (2) Control - 10 young adult animals maintained outdoors in the colony (5 animals as a control per age group). In LL, the CAR of young adults showed more stable synchronization with controls. Among the aged marmosets, two free-ran with τ > 24 h, whereas the other two showed relative coordination during the first 30 days in LL, but free-ran thereafter. These differences were reflected in the "social" phase angles (ψ_on and ψ_off ) between rhythms of experimental and control animal groups. Moreover, the activity patterns of aged animals showed lower social synchrony with controls compared to young adults, with the time lags of the time series between each experimental group and control group being negative in aged and positive in young adult animals (t-test, p < 0.05). The index of stability of the CAR showed no differences according to age, while the intradaily variability of the CAR was higher in the aged animals during LD-resynchronization, who took additional days to resynchronize. Thus, the social modulation on CAR may vary with age in marmosets. In the aged group, there was a lower effect of social synchronization, which may be associated with aging-related changes in the synchronization and generation of the CAR as well as in system outputs.

Linking cognition to age and amyloid-β burden in the brain of a nonhuman primate (Microcebus murinus)

The gray mouse lemur (Microcebus murinus) is a valuable model in research on age-related proteopathies. This nonhuman primate, comparable to humans, naturally develops tau and amyloid-β proteopathies during aging. Whether these are linked to cognitive alterations is unknown. Here, standardized cognitive testing in pairwise discrimination and reversal learning in a sample of 37 aged (>5 years) subjects was combined with tau and amyloid-β histochemistry in individuals that died naturally. Correlation analyses in successfully tested subjects (n = 22) revealed a significant relation between object discrimination learning and age, strongly influenced by outliers, suggesting pathological cases. Where neuroimmunohistochemistry was possible, as subjects deceased, the naturally developed cortical amyloid-β burden was significantly linked to pretraining success (intraneuronal accumulations) and discrimination learning (extracellular deposits), showing that cognitive (pairwise discrimination) performance in old age predicts the natural accumulation of amyloid-β at death. This is the first description of a direct relation between the cortical amyloid-β burden and cognition in a nonhuman primate.

Mini-review: Aging of the neuroendocrine system: Insights from nonhuman primate models

The neuroendocrine system (NES) plays a crucial role in synchronizing the physiology and behavior of the whole organism in response to environmental constraints. The NES consists of a hypothalamic-pituitary-target organ axis that acts in coordination to regulate growth, reproduction, stress and basal metabolism. The growth (or somatotropic), hypothalamic-pituitary-gonadal (HPG), hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-thyroid (HPT) axes are therefore finely tuned by the hypothalamus through the successive release of hypothalamic and pituitary hormones to control the downstream physiological functions. These functions rely on a complex set of mechanisms requiring tight synchronization between peripheral organs and the hypothalamic-pituitary complex, whose functionality can be altered during aging. Here, we review the results of research on the effects of aging on the NES of nonhuman primate (NHP) species in wild and captive conditions. A focus on the age-related dysregulation of the master circadian pacemaker, which, in turn, alters the synchronization of the NES with the organism environment, is proposed. Finally, practical and ethical considerations of using NHP models to test the effects of nutrition-based or hormonal treatments to combat the deterioration of the NES are discussed.

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.

Aging and the clock: Perspective from flies to humans

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

Strengths and Weaknesses of the Gray Mouse Lemur (Microcebus murinus) as a Model for the Behavioral and Psychological Symptoms and Neuropsychiatric Symptoms of Dementia

To face the load of the prevalence of Alzheimer’s disease in the aging population, there is an urgent need to develop more translatable animal models with similarities to humans in both the symptomatology and physiopathology of dementia. Due to their close evolutionary similarity to humans, non-human primates (NHPs) are of primary interest. Of the NHPs, to date, the gray mouse lemur (Microcebus murinus) has shown promising evidence of its translatability to humans. The present review reports the known advantages and limitations of using this species at all levels of investigation in the context of neuropsychiatric conditions. In this easily bred Malagasy primate with a relatively short life span (approximately 12 years), age-related cognitive decline, amyloid angiopathy, and risk factors (i.e., glucoregulatory imbalance) are congruent with those observed in humans. More specifically, analogous behavioral and psychological symptoms and neuropsychiatric symptoms of dementia (BPSD/NPS) to those in humans can be found in the aging mouse lemur. Aged mouse lemurs show typical age-related alterations of locomotor activity daily rhythms such as decreased rhythm amplitude, increased fragmentation, and increased activity during the resting-sleeping phase of the day and desynchronization with the light-dark cycle. In addition, sleep deprivation successfully induces cognitive deficits in adult mouse lemurs, and the effectiveness of approved cognitive enhancers such as acetylcholinesterase inhibitors or N-methyl-D-aspartate antagonists is demonstrated in sleep–deprived animals. This result supports the translational potential of this animal model, especially for unraveling the mechanisms underlying dementia and for developing novel therapeutics to prevent age-associated cognitive decline. In conclusion, actual knowledge of BPSD/NPS-like symptoms of age-related cognitive deficits in the gray mouse lemur and the recent demonstration of the similarity of these symptoms with those seen in humans offer promising new ways of investigating both the prevention and treatment of pathological aging.

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.

Gene expression profiling of the SCN in young and old rhesus macaques

In mammals, the suprachiasmatic nucleus (SCN) is the location of a master circadian pacemaker. It receives photic signals from the environment via the retinal hypothalamic tract, which play a key role in synchronizing the body's endogenously generated circadian rhythms with the 24-h rhythm of the environment. Therefore, it is plausible that age-related changes within the SCN contribute to the etiology of perturbed activity-rest cycles that become prevalent in humans during aging. To test this hypothesis, we used gene arrays and quantitative RT-PCR to profile age-related gene expression changes within the SCN of male rhesus macaques - a pragmatic translational animal model of human aging, which similarly displays an age-related attenuation of daytime activity levels. As expected, the SCN showed high expression of arginine vasopressin, vasoactive intestinal polypeptide, calbindin and nuclear receptor subfamily 1, group D, member 1 (NR1D1) (also known as reverse strand of ERBA (REV-ERBα), both at the mRNA and protein level. However, no obvious difference was detected between the SCNs of young (7-12 years) and old animals (21-26 years), in terms of the expression of core clock genes or genes associated with SCN signaling and neurotransmission. These data demonstrate the resilience of the primate SCN to normal aging, at least at the transcriptional level and, at least in males, suggest that age-related disruption of activity-rest cycles in humans may instead stem from changes within other components of the circadian system, such as desynchronization of subordinate oscillators in other parts of the body.

The Mouse Lemur, a Genetic Model Organism for Primate Biology, Behavior, and Health

Systematic genetic studies of a handful of diverse organisms over the past 50 years have transformed our understanding of biology. However, many aspects of primate biology, behavior, and disease are absent or poorly modeled in any of the current genetic model organisms including mice. We surveyed the animal kingdom to find other animals with advantages similar to mice that might better exemplify primate biology, and identified mouse lemurs (Microcebus spp.) as the outstanding candidate. Mouse lemurs are prosimian primates, roughly half the genetic distance between mice and humans. They are the smallest, fastest developing, and among the most prolific and abundant primates in the world, distributed throughout the island of Madagascar, many in separate breeding populations due to habitat destruction. Their physiology, behavior, and phylogeny have been studied for decades in laboratory colonies in Europe and in field studies in Malagasy rainforests, and a high quality reference genome sequence has recently been completed. To initiate a classical genetic approach, we developed a deep phenotyping protocol and have screened hundreds of laboratory and wild mouse lemurs for interesting phenotypes and begun mapping the underlying mutations, in collaboration with leading mouse lemur biologists. We also seek to establish a mouse lemur gene "knockout" library by sequencing the genomes of thousands of mouse lemurs to identify null alleles in most genes from the large pool of natural genetic variants. As part of this effort, we have begun a citizen science project in which students across Madagascar explore the remarkable biology around their schools, including longitudinal studies of the local mouse lemurs. We hope this work spawns a new model organism and cultivates a deep genetic understanding of primate biology and health. We also hope it establishes a new and ethical method of genetics that bridges biological, behavioral, medical, and conservation disciplines, while providing an example of how hands-on science education can help transform developing countries.

Melatonin and the pathologies of weakened or dysregulated circadian oscillators

Dynamic aspects of melatonin's actions merit increasing future attention. This concerns particularly entirely different effects in senescent, weakened oscillators and in dysregulated oscillators of cancer cells that may be epigenetically blocked. This is especially obvious in the case of sirtuin 1, which is upregulated by melatonin in aged tissues, but strongly downregulated in several cancer cells. These findings are not at all controversial, but are explained on the basis of divergent changes in weakened and dysregulated oscillators. Similar findings can be expected to occur in other accessory oscillator components that are modulated by melatonin, among them several transcription factors and metabolic sensors. Another cause of opposite effects concerns differences between nocturnally active laboratory rodents and the diurnally active human. This should be more thoroughly considered in the field of metabolic syndrome and related pathologies, especially with regard to type 2 diabetes and other aspects of insulin resistance. Melatonin was reported to impair glucose tolerance in humans, especially in carriers of the risk allele of the MT₂ receptor gene, MTNR1B, that contains the SNP rs10830963. These findings contrast with numerous reports on improvements of glucose tolerance in preclinical studies. However, the relationship between melatonin and insulin may be more complex, as indicated by loss-of-function mutants of the MT₂ receptor that are also prodiabetic, by the age-dependent time course of risk allele overexpression, by progressive reduction in circadian amplitudes and melatonin secretion, which are aggravated in diabetes. By supporting high-amplitude rhythms, melatonin may be beneficial in preventing or delaying diabetes.

Light-sensitive brain pathways and aging

Notwithstanding its effects on the classical visual system allowing image formation, light acts upon several non-image-forming (NIF) functions including body temperature, hormonal secretions, sleep-wake cycle, alertness, and cognitive performance. Studies have shown that NIF functions are maximally sensitive to blue wavelengths (460–480 nm), in comparison to longer light wavelengths. Higher blue light sensitivity has been reported for melatonin suppression, pupillary constriction, vigilance, and performance improvement but also for modulation of cognitive brain functions. Studies investigating acute stimulating effects of light on brain activity during the execution of cognitive tasks have suggested that brain activations progress from subcortical regions involved in alertness, such as the thalamus, the hypothalamus, and the brainstem, before reaching cortical regions associated with the ongoing task. In the course of aging, lower blue light sensitivity of some NIF functions has been reported. Here, we first describe neural pathways underlying effects of light on NIF functions and we discuss eye and cerebral mechanisms associated with aging which may affect NIF light sensitivity. Thereafter, we report results of investigations on pupillary constriction and cognitive brain sensitivity to light in the course of aging. Whereas the impact of light on cognitive brain responses appears to decrease substantially, pupillary constriction seems to remain more intact over the lifespan. Altogether, these results demonstrate that aging research should take into account the diversity of the pathways underlying the effects of light on specific NIF functions which may explain their differences in light sensitivity.

Age-related changes in neurochemical components and retinal projections of rat intergeniculate leaflet

Aging leads to several anatomical and functional deficits in circadian timing system. In previous works, we observed morphological alterations with age in hypothalamic suprachiasmatic nuclei, one central component of this system. However, there are few data regarding aging effects on other central components of this system, such as thalamic intergeniculate leaflet (IGL). In this context, we studied possible age-related alterations in neurochemical components and retinal projections of rat IGL. For this goal, young (3 months), adult (13 months), and aged (23 months) Wistar rats were submitted to an intraocular injection of neural tracer, cholera toxin subunit b (CTb), 5 days before a tissue fixation process by paraformaldehyde perfusion. Optical density measurements and cell count were performed at digital pictures of brain tissue slices processed by immunostaining for glutamic acid decarboxylase (GAD), enkephalin (ENK), neuropeptide Y (NPY) and CTb, characteristic markers of IGL and its retinal terminals. We found a significant age-related loss in NPY immunoreactive neurons, but not in immunoreactivity to GAD and ENK. We also found a decline of retinal projections to IGL with age. We conclude aging impairs both a photic environmental clue afferent to IGL and a neurochemical expression which has an important modulatory circadian function, providing strong anatomical correlates to functional deficits of the aged biological clock.

Aging and Circadian Rhythms

Aging is associated with numerous changes, including changes in sleep timing, duration, and quality. The circadian timing system interacts with a sleep-wake homeostatic system to regulate human sleep, including sleep timing and structure. This article reviews key features of the human circadian timing system, age-related changes in the circadian timing system, and how those changes may contribute to the observed alterations in sleep.

[Is the biological clock central to the aging process? Studies in a non-human primate]

The ability of organisms to adapt to their environment during aging is altered. Age-related disorders in Human include disturbances of biological rhythms, especially sleep-wake rhythms alterations, and perturbations of body temperature and hormone secretion. The alteration of biological rhythms with age leads to major health consequences, particularly due to the alteration of sleep-wake rhythms that causes a strong alteration of the general condition. The study of these changes is therefore a major health issue and requires the use of appropriate animal models such as the grey mouse lemur (Microcebus murinus), a small Madagascar primate with very pronounced biological rhythms.

Aging of non-visual spectral sensitivity to light in humans: compensatory mechanisms?

The deterioration of sleep in the older population is a prevalent feature that contributes to a decrease in quality of life. Inappropriate entrainment of the circadian clock by light is considered to contribute to the alteration of sleep structure and circadian rhythms in the elderly. The present study investigates the effects of aging on non-visual spectral sensitivity to light and tests the hypothesis that circadian disturbances are related to a decreased light transmittance. In a within-subject design, eight aged and five young subjects were exposed at night to 60 minute monochromatic light stimulations at 9 different wavelengths (420-620 nm). Individual sensitivity spectra were derived from measures of melatonin suppression. Lens density was assessed using a validated psychophysical technique. Although lens transmittance was decreased for short wavelength light in the older participants, melatonin suppression was not reduced. Peak of non-visual sensitivity was, however, shifted to longer wavelengths in the aged participants (494 nm) compared to young (484 nm). Our results indicate that increased lens filtering does not necessarily lead to a decreased non-visual sensitivity to light. The lack of age-related decrease in non-visual sensitivity to light may involve as yet undefined adaptive mechanisms.

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 resveratrol on daily rhythms of locomotor activity and body temperature in young and aged grey mouse lemurs

In several species, resveratrol, a polyphenolic compound, activates sirtuin proteins implicated in the regulation of energy balance and biological clock processes. To demonstrate the effect of resveratrol on clock function in an aged primate, young and aged mouse lemurs (Microcebus murinus) were studied over a 4-week dietary supplementation with resveratrol. Spontaneous locomotor activity and daily variations in body temperature were continuously recorded. Reduction in locomotor activity onset and changes in body temperature rhythm in resveratrol-supplemented aged animals suggest an improved synchronisation on the light-dark cycle. Resveratrol could be a good candidate to restore the circadian rhythms in the elderly.

Aging-like circadian disturbances in folate-deficient mice

The elderly population shows various circadian disturbances, including dampened amplitude of rhythmicity and decreased responsiveness to light. The common poor folate status in the elderly might account for these aging-related circadian disturbances. To test this hypothesis, we investigated whether folate deficiency in mice affects circadian oscillations of the master clock in the suprachiasmatic nuclei, and the shifting responses to light. Mice fed a diet without folate for 6 weeks displayed markedly reduced (4.5-fold) erythrocyte folate concentration and increased (2.3-fold) homocysteinemia compared with control mice. Folate deficiency decreased the circadian amplitude of vasopressin and the clock protein PERIOD 2 (PER2) in the master clock, slowed the rate of re-entrainment of behavioral rhythms after delayed light-dark cycle and reduced light-induced phase-delays, without detectable morphologic changes in the retina, such as the number of melanopsinergic ganglion cells, that might have impaired photodetection. In conclusion, folate deficiency and consecutive hyperhomocysteinemia led to dampened PER2 and vasopressin oscillations in the master clock and reduced responsiveness to photic resetting, which constitute hallmarks of aging effects on circadian rhythmicity.

Effects of dietary resveratrol on the sleep-wake cycle in the non-human primate gray mouse lemur (Microcebus murinus)

Converging evidence shows that the non-human primate gray mouse lemur (Microcebus murinus) is ideal for the study of the aging process and for testing the effects of new therapies and dietary interventions on age-associated pathologies. One such dietary supplement is resveratrol (RSV), a dietary polyphenolic compound with several positive effects on metabolic functions and longevity. However, little is known about the effect of RSV on the lemur sleep-wake cycle, which reflects mammalian brain function and health. In the present study, the authors investigated this effect by comparing sleep-wake cycles in adult lemurs based on electroencephalographic (EEG) rhythms. The effect of short-term RSV supplementation on the sleep-wake cycle of mouse lemurs was evaluated in entrained conditions (long-day photoperiods, light:dark 14:10). After 3 wks of RSV supplementation, the animals exhibited a significantly increased proportion of active-wake time, occurring mainly during the resting phase of the sleep-wake cycle (+163%). The increase in active-wake time with RSV supplementation was accompanied by a significant reduction of both paradoxical sleep (-95%) and slow-wave sleep (-38%). These changes mainly occurred during the resting phase of the sleep-wake cycle (RSV supplementation induced negligible changes in active-wake time during the active phase of the sleep-wake cycle). The present data suggest that RSV may be a potent regulator of sleep-wake rhythms and could be of major interest in the study of sleep perturbations associated with aging and neuropathology.

Old flies have a robust central oscillator but weaker behavioral rhythms that can be improved by genetic and environmental manipulations

Sleep-wake cycles break down with age, but the causes of this degeneration are not clear. Using a Drosophila model, we addressed the contribution of circadian mechanisms to this age-induced deterioration. We found that in old flies, free-running circadian rhythms (behavioral rhythms assayed in constant darkness) have a longer period and an unstable phase before they eventually degenerate. Surprisingly, rhythms are weaker in light-dark cycles and the circadian-regulated morning peak of activity is diminished under these conditions. On a molecular level, aging results in reduced amplitude of circadian clock gene expression in peripheral tissues. However, oscillations of the clock protein PERIOD (PER) are robust and synchronized among different clock neurons, even in very old, arrhythmic flies. To improve rhythms in old flies, we manipulated environmental conditions, which can have direct effects on behavior, and also tested a role for molecules that act downstream of the clock. Coupling temperature cycles with a light-dark schedule or reducing expression of protein kinase A (PKA) improved behavioral rhythms and consolidated sleep. Our data demonstrate that a robust molecular timekeeping mechanism persists in the central pacemaker of aged flies, and reducing PKA can strengthen behavioral rhythms.

Age Affects Photoentrainment in a Nocturnal Primate

The endogenous circadian clock is entrained by external cues, mainly the light-dark cycle received by photopigments located in the retina. The authors investigated (1) the effect of aging on the synchronization of the rest-activity rhythm and (2) the physiological basis of light photoreception in the gray mouse lemur, a nocturnal Malagasy primate. Old individuals were tested at different irradiance levels under 3 different light wavelengths previously shown to trigger maximal response in young adults. Investigators analyzed animal activity and temperature waveforms and used 2 reference treatments, strong white light and continuous darkness. The investigation revealed manifold effects of aging picturing a general loss of responsiveness to light and age-related changes in activity and temperature. In addition, the investigation shows that photoentrainment is a continuous process. Short wavelengths (430 nm) are more efficient than longer wavelengths (470-540 nm) at low light levels at dawn and across all light levels at dusk. This suggests an implication of S-cones, differential at dawn and dusk. This results, surprising for several reasons, calls for further investigation. The study brings an interesting contribution to the understanding of the physiological processes underlying synchronization to light.

Aging and circadian disruption: causes and effects

The relationship between aging and daily “circadian” behavior in humans is bidirectional: on the one hand, dysfunction of circadian clocks promotes age-related maladies; on the other, aging per se leads to changes and disruption in circadian behavior and physiology. For the latter case, recent research suggests that changes to both homeostatic and circadian sleep regulatory mechanisms may play a role. Could hormonal changes be in part responsible?

The grey mouse lemur: a non-human primate model for ageing studies

The use of non-human primate models is required to understand the ageing process and evaluate new therapies against age-associated pathologies. The present article summarizes all the contributions of the grey mouse lemur Microcebus murinus, a small nocturnal prosimian primate, to the understanding of the mechanisms of ageing. Results from studies of both healthy and pathological ageing research on the grey mouse lemur demonstrated that this animal is a unique model to study age-dependent changes in endocrine systems, biological rhythms, thermoregulation, sensorial, cerebral and cognitive functions.

[Cellular aspects of aging in the pineal gland of the shrew, Crocidura russula]

The Greater White-toothed shrew Crocidura russula is short-lived species and the phase of senescence is greatly elongated in captivity. The loss of rhythmicity of biological functions that accompanies its aging is also well documented. C. russula is thus an excellent model to test the effects of aging on biological clocks. Melatonin is a key hormone in the synchronization of behaviors, metabolisms and physiological regulations with environmental factors. In the present work we want to know if the loss of rhythmicity and the reduced melatonin levels registered by the second year of life in this species could be associated to modified ultrastructural features of the pineal parenchyma, site of melatonin synthesis. Transmission electron microscopy (TEM) analysis of young (1-4 months) and old (25-28 months) shrew's pineals show that in older individuals, the parenchyma undergoes alterations affecting mainly nucleus, mitochondria and endoplasmic reticulum cisternae, with increased numbers of dense bodies and the formation of many concretions as well as a depletion of secretory products. These changes suggest a process of slowing pinealocytes metabolism which could explain the gradual reduction of melatonin levels registered during aging in C. russula.

Resveratrol dietary supplementation shortens the free-running circadian period and decreases body temperature in a prosimian primate

Resveratrol (RSV) is a dietary polyphenolic compound with several positive effects on metabolic functions and longevity. We tested the effect of RSV on the circadian clock in a nonhuman primate, the gray mouse lemur. The impact of a 2-week dietary supplementation of RSV on the rhythms of locomotor activity and body temperature in constant dark conditions (DD) was investigated in young (n = 7) and old (n = 6) animals. RSV supplementation followed 2 weeks in DD under normal diet (CTL). In both young and old animals receiving RSV, we observed a shortening of the free-running period compared to those under CTL (-15 minutes in young animals and -45 minutes in old animals), accompanied by a lower mean body temperature in both age groups and decreased locomotor activity in young animals. Thus, RSV is a food component capable of influencing a primate's circadian clock. This property may be of interest clinically in the context of the treatment of circadian disruption and in the context of the effects of RSV ingestion on health.

Aging of Intrinsic Circadian Rhythms and Sleep in a Diurnal Nonhuman Primate, Macaca mulatta

There is growing evidence that alterations in the intrinsic circadian clock and sleep might affect the aging process. The rhesus monkey ( Macaca mulatta) provides unique opportunities to explore the role of the clock in successful and unsuccessful physiological and cognitive aging in a diurnal primate with consolidated nighttime sleep, complex cognitive functions, long life span, and phylogenetic proximity to humans. A longitudinal study was conducted to characterize the effects of aging on the entrained and intrinsic circadian rhythms of activity, polysomnographic sleep patterns, and melatonin production in unrestrained male rhesus monkeys [6—9 ( n = 6) and 24—28 ( n = 4) years of age]. An age-dependent decline was found in the stability of circadian rhythms of activity and in peak melatonin levels. The range of individual intrinsic circadian periods (τ) is not age-dependent. Aged monkeys do not display clearly defined “morningness-eveningness” chronotypes and, unlike the young, show no correlation between the chronotype under entrained conditions and the length of intrinsic circadian period. The daily activity period (α) is reduced with age and this is associated with high day-to-day variability in sleep quantity and quality, fragmentation of nighttime sleep and daytime wakefulness, increased daytime sleep time, overall increase in stage 1 sleep, and reduced time spent in rapid-eye movement and slow-wave sleep. In the absence of environmental time cues, age-dependent changes in sleep and circadian rhythms are exacerbated and circadian patterns of sleep in young rhesus monkeys start resembling those in aged animals, together suggesting important role of circadian regulation in aging sleep phenotype. This first characterization of age-dependent changes in the intrinsic rhythms and sleep in rhesus monkeys, demonstrating major similarities to human aging phenotype, should assist in the search for the mechanisms involved and for effective prophylactic and therapeutic strategies.

The development of small primate models for aging research

Nonhuman primate (NHP) aging research has traditionally relied mainly on the rhesus macaque. But the long lifespan, low reproductive rate, and relatively large body size of macaques and related Old World monkeys make them less than ideal models for aging research. Manifold advantages would attend the use of smaller, more rapidly developing, shorter-lived NHP species in aging studies, not the least of which are lower cost and the ability to do shorter research projects. Arbitrarily defining "small" primates as those weighing less than 500 g, we assess small, relatively short-lived species among the prosimians and callitrichids for suitability as models for human aging research. Using the criteria of availability, knowledge about (and ease of) maintenance, the possibility of genetic manipulation (a hallmark of 21st century biology), and similarities to humans in the physiology of age-related changes, we suggest three species--two prosimians (Microcebus murinus and Galago senegalensis) and one New World monkey (Callithrix jacchus)--that deserve scrutiny for development as major NHP models for aging studies. We discuss one other New World monkey group, Cebus spp., that might also be an effective NHP model of aging as these species are longer-lived for their body size than any primate except humans.

Lifespan daily locomotor activity rhythms in a mouse model of amyloid-induced neuropathology

Using a rodent model for neuropathology induced by human amyloid precursor protein, the present study tested the hypothesis that 24 h rest/activity rhythms deteriorate with age. A lifespan of rest/activity patterns was studied in transgenic Tg2576 mice and wild-type controls. Classic indices of circadian timekeeping, including onsets, offsets, and the duration of nighttime activity, were stable throughout the 96-week study. Analyses of ultradian bout activity revealed significant genotype and age-related changes in the duration and intensity of activity bouts, as well as amplitude of the 24 h rhythm. Tg2576 mice had more total activity counts, fewer bouts/24 h, more counts/bout, and longer bout time than wild-type controls. Amyloid deposits and plaques were solely found in specific cortex regions in aged postmortem Tg2576 mice, but were not evident in the hypothalamus or suprachiasmatic nucleus; this neuropathology was absent from brains of wild-type controls. These findings suggest that amyloidosis of the Tg2576 mouse exerts little influence on timing of locomotor activity in the circadian domain but significantly alters the temporal structure of ultradian activity.

Systemic and cellular reflections on ageing and the circadian oscillator: a mini-review

From circulation to digestion to excretion, a circadian clock synchronizes most aspects of mammalian physiology with the solar day. During normal ageing, this daily coordination gradually erodes, and during pathological ageing such erosion is exacerbated. Recent experiments suggest that therapies aimed at sustaining circadian function increase quality of life in elderly patients. Hence, a better understanding of the interactions between the circadian clock and ageing - at both cellular and systemic levels - could lead to direct benefits for aged individuals.

Glial transcripts and immune-challenged glia in the suprachiasmatic nucleus of young and aged mice

Biological rhythms are frequently disturbed with advancing age, and aging-related changes of glia in the hypothalamic suprachiasmatic nucleus (SCN), the master circadian pacemaker, require special attention. In particular, astrocytes contribute to SCN function, and aging is associated with increased inflammatory activity in the brain, in which microglia could be especially implicated. On this basis, we investigated in the SCN of young and old mice glial transcripts and cell features, and the glial cell response to a central inflammatory challenge. Quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to analyze the expression of mRNAs encoding the astrocytic glial fibrillary acidic protein and the microglial antigen CD11b. Both these transcripts, here investigated in the SCN for the first time, were significantly increased in the old SCN. Glial cell phenotyping with immunohistochemistry revealed hypertrophic and intensely stained astrocytes and microglia in the aged SCN. In both age groups, microglia were scattered throughout the SCN and astrocytes were prominent in the ventral portion, where retinal fibers are densest; in the aged SCN, astrocytes were also numerous in the dorsal portion. After intracerebroventricular injections of a mixture of interferon-gamma and tumor necrosis factor-alpha, or phosphate-buffered saline as control, immunolabeling was evaluated with stereological cell counts and confocal microscopy. Phenotypic features of astrocyte and microglia activation in response to cytokine injections were markedly enhanced in the aged SCN. Subregional variations in glial cell density were also documented in the aged compared to the young SCN. Altogether, the findings show increases in the expression of glial transcripts and hypertrophy of astrocytes and microglia in the aged SCN, as well as age-dependent variation in the responses of immune-challenged SCN glia. The data thus point out an involvement of glia in aging-related changes of the biological clock.

Region-specific changes in the immunoreactivity of vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptors (VPAC2, and PAC1 receptor) in the aged rat brains

Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) have been implicated in a large array of physiological and patho-physiological processes through their receptors (VPAC(1), VPAC(2), and PAC(1) receptor) in the central nervous system. Previously, we demonstrated age-related decreases in VPAC(1) receptor expression in the rat brain providing a possible basis of several age-induced functional changes in the aged brain. In the current study, we also examined age-related changes in PAC(1) and VPAC(2) receptors in aged rat brains using an immunohistochemical approach. We found that PAC1 immunoreactivity was significantly increased in the hippocampal formation, hypothalamus, thalamus, midbrain septal nuclei, and white matter of aged rats compared with young control rats although its distribution pattern was not altered. In contrast, both distribution pattern and immunoreactivity of VPAC(2) receptor remained unchanged in aged rat brains. These results suggest that the PACAP/VIP receptors exhibit specific expressional changes in the aged brain and that these specific changes could underlie age-associated memory and cognitive functional declines as well as several other age-induced functional changes in the brain. However, the exact regulatory mechanism and its functional significance require further elucidation.

Plasma levels of interferon-gamma correlate with age-related disturbances of circadian rhythms and survival in a non-human primate

Aging can be associated with changes in circadian rhythms and reduction in adaptive immune responses accompanied by expansion of memory T cells and elevated levels of pro-inflammatory cytokines. Recent findings suggest the cytokine interferon-gamma (IFN-gamma) can affect the function of the hypothalamic suprachiasmatic nucleus (SCN), the master mammalian circadian pacemaker, both in vitro and in vivo. We studied the correlation of plasma levels of IFN-gamma and changes in circadian rhythms in a non-human primate species, the nocturnal mouse lemur (Microcebus murinus). Plasma IFN-gamma and dehydroepiandrosterone sulfate (DHEA-S), a known biomarker of aging, were determined in middle- to old-age animals by immunoenzymoassay. Daily rhythms of locomotor activity and body temperature as well as survival time of the lemurs were recorded. With aging, mean levels of DHEA-S decreased whereas IFN-gamma increased. Aged animals showed biological rhythm alterations characterized by a high percentage of diurnal activity, anticipation of the activity onset relative to lights-off, short free-running period, and delayed occurrence of minimal body temperature. The magnitude of these disturbances was correlated with the plasma level of IFN-gamma but not DHEA-S. Most remarkably, in contrast to DHEA-S, increased levels of IFN-gamma correlated with duration of the lifetime of the lemurs. These results show the degree of circadian rhythm alterations in an individual is correlated with plasma IFN-gamma level during aging, and that plasma IFN-gamma level may predict survival, at least in this non-human primate.

Circadian temperature variation and ageing

In the present paper, an attempt is made to summarize current knowledge concerning the daily body temperature rhythm and its age-dependent alterations. Homeostatic and circadian control mechanisms are considered. Special attention is paid to the circadian system, as the mechanisms of autonomic control are the topic of another contribution to this special issue. Also, the interactions of the core body temperature rhythm with other circadian functions are discussed in detail as they constitute an essential part of the internal temporal order of living systems and thus guarantee their optimal functioning. In the second part of the paper, age-dependent changes in the circadian body temperature rhythm and their putative causes, considering circadian and homeostatic components, are described. Consequences for health and fitness and some possibilities to prevent adverse effect are mentioned in the final section.

Circadian clock resetting in the mouse changes with age

The most widely recognised consequence of normal age-related changes in biological timing is the sleep disruption that appears in old age and diminishes the quality of life. These sleep disorders are part of the normal ageing process and consist primarily of increased amounts of wakefulness and reduced amounts of deep sleep. Changes in the amplitude and timing of the sleep-wake cycle appear to represent, at least in part, a loss of effective circadian regulation of sleep. Understanding alterations in the characteristics of stimuli that help to consolidate internal rhythms will lead to recommendations to improve synchronisation in old age. Converging evidence from both human and animal studies indicate that senescence is associated with alterations in the neural structure thought to be primarily responsible for the generation of the circadian oscillation, the suprachiasmatic nuclei (SCN). Work has shown that there are changes in the anatomy, physiology and ability of the clock to reset in response to stimuli with age. Therefore it is possible that at least some of the observed age-related changes in sleep and circadian timing could be mediated at the level of the SCN. The SCN contain a circadian clock whose activity can be recorded in vitro for several days. We have tested the response of the circadian clock to a number of neurochemicals that reset the clock in a manner similar to light, including glutamate, N-methyl-D-aspartate (NMDA), gastrin-releasing peptide (GRP) and histamine (HA). In addition, we have also tested agents which phase shift in a pattern similar to behavioural 'non-photic' signals, including neuropeptide Y (NPY), serotonin (5HT) and gamma-aminobutyric acid (GABA). These were tested on the circadian clock in young and older mice (approximately 4 and 15 months old). We found deficits in the response to specific neurochemicals but not to others in our older mice. These results indicate that some changes seen in the responsiveness of the circadian clock to light with age may be mediated at the level of the SCN. Further, the responsiveness of the circadian clock with age is attenuated to some, but not all stimuli. This suggests that not all clock stimuli lose their effectiveness with age, and that it may be possible to compensate for deficits in clock performance by enhancing the strength of those stimulus pathways which are intact.

Small-world network models of intercellular coupling predict enhanced synchronization in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is a multioscillator system that drives daily rhythms in mammalian behavior and physiology. Based on recent data implicating vasoactive intestinal polypeptide (VIP) as the key intercellular synchronizing agent, we developed a multicellular SCN model to investigate the effects of cellular heterogeneity and intercellular connectivity on circadian behavior. A 2-dimensional grid was populated with 400 model cells that were heterogeneous with respect to their uncoupled rhythmic behavior (intrinsic and damped pacemakers with a range of oscillation periods) and VIP release characteristics (VIP producers and nonproducers). We constructed small-world network architectures in which local connections between VIP producing cells and their 4 nearest neighbors were augmented with random connections, resulting in long-range coupling across the grid. With only 10% of the total possible connections, the small-world network model was able to produce similar phase synchronization indices as a mean-field model with VIP producing cells connected to all other cells. Partial removal of random connections decreased the synchrony among neurons, the amplitude of VIP and cAMP response element binding protein oscillations, the mean period of intrinsic periods across the population, and the percentage of oscillating cells. These results indicate that small-world connectivity provides the optimal compromise between the number of connections and control of circadian amplitude and synchrony. This model predicts that small decreases in long-range VIP connections in the SCN could have dramatic effects on period and amplitude of daily rhythms, features commonly described with aging.

The timing of the shrew: continuous melatonin treatment maintains youthful rhythmic activity in aging Crocidura russula

BACKGROUND

Laboratory conditions nullify the extrinsic factors that determine the wild expected lifespan and release the intrinsic or potential lifespan. Thus, wild animals reared in a laboratory often show an increased lifespan, and consequently an increased senescence phase. Senescence is associated with a broad suite of physiological changes, including a decreased responsiveness of the circadian system. The time-keeping hormone melatonin, an important chemical player in this system, is suspected to have an anti-aging role. The Greater White-toothed shrew Crocidura russula is an ideal study model to address questions related to aging and associated changes in biological functions: its lifespan is short and is substantially increased in captivity; daily and seasonal rhythms, while very marked the first year of life, are dramatically altered during the senescence process which starts during the second year. Here we report on an investigation of the effects of melatonin administration on locomotor activity of aging shrews.

METHODOLOGY/PRINCIPAL FINDINGS

1) The diel fluctuations of melatonin levels in young, adult and aging shrews were quantified in the pineal gland and plasma. In both, a marked diel rhythm (low diurnal concentration; high nocturnal concentration) was present in young animals but then decreased in adults, and, as a result of a loss in the nocturnal production, was absent in old animals. 2) Daily locomotor activity rhythm was monitored in pre-senescent animals that had received either a subcutaneous melatonin implant, an empty implant or no implant at all. In non-implanted and sham-implanted shrews, the rhythm was well marked in adults. A marked degradation in both period and amplitude, however, started after the age of 14-16 months. This pattern was considerably delayed in melatonin-implanted shrews who maintained the daily rhythm for significantly longer.

CONCLUSIONS

This is the first long term study (>500 days observation of the same individuals) that investigates the effects of continuous melatonin delivery. As such, it sheds new light on the putative anti-aging role of melatonin by demonstrating that continuous melatonin administration delays the onset of senescence. In addition, the shrew appears to be a promising mammalian model for elucidating the precise relationships between melatonin and aging.

Decline of the presynaptic network, including GABAergic terminals, in the aging suprachiasmatic nucleus of the mouse

Biological rhythms, and especially the sleep/wake cycle, are frequently disrupted during senescence. This draws attention to the study of aging-related changes in the hypothalamic suprachiasmatic nucleus (SCN), the master circadian pacemaker. The authors here compared the SCN of young and old mice, analyzing presynaptic terminals, including the gamma-aminobutyric acid (GABA)ergic network, and molecules related to the regulation of GABA, the main neurotransmitter of SCN neurons. Transcripts of the alpha3 subunit of the GABAA receptor and the GABA-synthesizing enzyme glutamic acid decarboxylase isoform 67 (GAD67) were analyzed with real-time RT-PCR and GAD67 protein with Western blotting. These parameters did not show significant changes between the 2 age groups. Presynaptic terminals were identified in confocal microscopy with synaptophysin immunofluorescence, and the GABAergic subset of those terminals was revealed by the colocalization of GAD67 and synaptophysin. Quantitative analysis of labeled synaptic endings performed in 2 SCN subregions, where retinal afferents are known to be, respectively, very dense or very sparse, revealed marked aging-related changes. In both subregions, the evaluated parameters (the number of and the area covered by presynaptic terminals and by their GABAergic subset) were significantly decreased in old versus young mice. No significant differences were found between SCN tissue samples from animals sacrificed at different times of day, in either age group. Altogether, the data point out marked reduction in the synaptic network of the aging biological clock, which also affects GABAergic terminals. Such alterations could underlie aging-related SCN dysfunction, including low-amplitude output during senescence.

Calbindin D28K protein cells in a primate suprachiasmatic nucleus: localization, daily rhythm and age-related changes

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker. The SCN controls daily rhythms and synchronizes the organism to its environment and especially to photic signals. Photic signals via the retinohypothalamic tract reach the ventral part of the SCN, where the majority of calbindin-containing neurons are located. Calbindin cells seem important for the control of circadian rhythmicity. As ageing leads to marked changes in the expression of circadian rhythms, we investigated in the mouse lemur, a nocturnal primate, age-related changes in the oscillation of calbindin protein expression in SCN neurons. We used immunohistochemistry and quantitative analysis of calbindin expression in the SCN of adult and aged mouse lemurs. In this primate, a dense cluster of calbindin-positive neurons was found in the ventral part of the SCN. In adult animals, calbindin-positive SCN neurons did not exhibit daily rhythms in their number or intensity, but exhibited significant daily variations in the percentage of cells with a calbindin-positive nucleus, characterized by high values during the daytime and low values during the night. Immunoreactive intensity peaked in the middle of the daytime. Calbindin expression in the nuclei of calbindin cells in the SCN tends to be modified by ageing. The amplitude of daily variation in calbindin expression was damped, with a lower immunointensity during the daytime and a delayed decrease during the night. These changes may affect the ability of the SCN to transmit rhythmic information to other SCN cells and thereby modify the synchronization of the different cell populations in the SCN.