Effects of Photoperiod on Rat Motor Activity Rhythm at the Lower Limit of Entrainment

Impaired Morris water task retention following T21 light dark cycle exposure is not due to reduced hippocampal c-FOS expression

Circadian rhythms influence virtually all aspects of physiology and behavior. This is problematic when circadian rhythms no longer reliably predict time. Circadian rhythm disruption can impair memory, yet we don’t know how this fully works at the systems and molecular level. When trying to determine the root of a memory impairment, assessing neuronal activation with c-FOS is useful. This has yet to be assessed in the hippocampi of circadian rhythm disrupted rats in a hippocampal gold standard task. Rats were trained on the Morris water task (MWT), then received 6 days of a 21-h day (T21), 13 days of a normal light dark cycle, probe trial, and tissue extraction an hour later. Despite having impaired memory in the probe trial, compared to controls there were no differences in c-FOS expression in hippocampal sub regions: CA1; CA3; Dentate gyrus. These data confirm others in hamsters demonstrating that arrhythmicity which produces an impairment in spontaneous alternation does not affect c-FOS in the dentate gyrus. The current study indicates that the memory impairment induced by a lighting manipulation is likely not due to attenuated neuronal activation. Determining how the master clock in the brain communicates with the hippocampus is needed to untangle the relationship between circadian rhythms and memory.

A local circadian clock for memory?

The master clock, suprachiasmatic nucleus, is believed to control peripheral circadian oscillators throughout the brain and body. However, recent data suggest there is a circadian clock involved in learning and memory, potentially housed in the hippocampus, which is capable of acting independently of the master clock. Curiously, the hippocampal clock appears to be influenced by the master clock and by hippocampal dependent learning, while under certain conditions it may also revert to its endogenous circadian rhythm. Here we propose a mechanism by which the hippocampal clock could locally determine the nature of its entrainment. We introduce a novel theoretical framework, inspired by but extending beyond the hippocampal memory clock, which provides a new perspective on how circadian clocks throughout the brain coordinate their rhythms. Importantly, a local clock for memory would suggest that hippocampal-dependent learning at the same time every day should improve memory, opening up a range of possibilities for non-invasive therapies to alleviate the detrimental effects of circadian rhythm disruption on human health.

Assessment of Sleep, K-Complexes, and Sleep Spindles in a T21 Light-Dark Cycle

Circadian rhythm misalignment has a deleterious impact on the brain and the body. In rats, exposure to a 21-hour day length impairs hippocampal dependent memory. Sleep, and particularly K-complexes and sleep spindles in the cortex, have been hypothesized to be involved in memory consolidation. Altered K-complexes, sleep spindles, or interaction between the cortex and hippocampus could be a mechanism for the memory consolidation failure but has yet to be assessed in any circadian misalignment paradigm. In the current study, continuous local field potential recordings from five rats were used to assess the changes in aspects of behavior and sleep, including wheel running activity, quiet wakefulness, motionless sleep, slow wave sleep, REM sleep, K-complexes and sleep spindles, in rats exposed to six consecutive days of a T21 light-dark cycle (L9:D12). Except for a temporal redistribution of sleep and activity during the T21, there were no changes in period, or total amount for any aspect of sleep or activity. These data suggest that the memory impairment elicited from 6 days of T21 exposure is likely not due to changes in sleep architecture. It remains possible that hippocampal plasticity is affected by experiencing light when subjective circadian phase is calling for dark. However, if there is a reduction in hippocampal plasticity, changes in sleep appear not to be driving this effect.

Simple Lighting Manipulations Facilitate Behavioral Entrainment of Mice to 18-h Days

In an invariantly rhythmic world, a robust and stable mammalian circadian clock is presumed to confer fitness advantages. In shift-work or after rapid transmeridian travel, however, a stable clock might be maladaptive and a more flexibly resettable clock may have advantages. The rate at which rodents can adjust to simulated time zone travel and the range of entrainment can be markedly increased through simple light manipulations, namely, by exposing animals to extremely dim light (<0.01 lux) at night or by bifurcating rhythms under 24-h light-dark-light-dark (LDLD) cycles. Here we investigated the separate effects of dim light and bifurcation on the ability of mice to entrain to 18-h days (LD 13:5; T18). Incorporating dim light at night, mice in Experiment 1 were exposed either to LD cycles with photophases that were progressively shortened from LD 19:5 to LD 13:5 or to bifurcating LDLD cycles with photophases that were lengthened from LDLD 7:5:7:5 to LDLD 13:5:13:5. In both cases, wheel-running rhythms were robustly synchronized to T18 and the phase of the free-running circadian rhythm was controlled by the timing of release into constant conditions. In Experiment 2, either dimly illuminated nights or a history of bifurcation without continuing dim light was sufficient to allow behavioral entrainment to T18 whereas previously unbifurcated mice under dark nights failed to entrain to T18. Additionally, concurrent measurement of body temperature rhythms in T24 LDLD revealed them to be bimodal. These studies suggest that the circadian system is markedly more flexible than conventionally thought and that this flexibility can be achieved in a noninvasive and nonpharmacological way. Facilitation of behavioral entrainment to extreme light-dark cycles may have translational potential for human shift-workers.

Beyond Emotional and Spatial Processes: Cognitive Dysfunction in a Depressive Phenotype Produced by Long Photoperiod Exposure

Cognitive dysfunction in depression has recently been given more attention and legitimacy as a core symptom of the disorder. However, animal investigations of depression-related cognitive deficits have generally focused on emotional or spatial memory processing. Additionally, the relationship between the cognitive and affective disturbances that are present in depression remains obscure. Interestingly, sleep disruption is one aspect of depression that can be related both to cognition and affect, and may serve as a link between the two. Previous studies have correlated sleep disruption with negative mood and impaired cognition. The present study investigated whether a long photoperiod-induced depressive phenotype showed cognitive deficits, as measured by novel object recognition, and displayed a cognitive vulnerability to an acute period of total sleep deprivation. Adult male Wistar rats were subjected to a long photoperiod (21L:3D) or a normal photoperiod (12L:12D) condition. Our results indicate that our long photoperiod exposed animals showed behaviors in the forced swim test consistent with a depressive phenotype, and showed significant deficits in novel object recognition. Three hours of total sleep deprivation, however, did not significantly change novel object recognition in either group, but the trends suggest that the long photoperiod and normal photoperiod groups had different cognitive responses to total sleep deprivation. Collectively, these results underline the extent of cognitive dysfunction present in depression, and suggest that altered sleep plays a role in generating both the affective and cognitive symptoms of depression.

Scotopic Illumination Enhances Entrainment of Circadian Rhythms to Lengthening Light:Dark Cycles

Endogenously generated circadian rhythms are synchronized with the environment through phase-resetting actions of light. Starlight and dim moonlight are of insufficient intensity to reset the phase of the clock directly, but recent studies have indicated that dim nocturnal illumination may otherwise substantially alter entrainment to bright lighting regimes. In this article, the authors demonstrate that, compared to total darkness, dim illumination at night (< 0.010 lux) alters the entrainment of male Syrian hamsters to bright-light T cycles, gradually increasing in cycle length (T) from 24 h to 30 h. Only 1 of 18 hamsters exposed to complete darkness at night entrained to cycles of T > 26 h. In the presence of dim nocturnal illumination, however, a majority of hamsters entrained to Ts of 28 h or longer. The presence or absence of a running wheel had only minor effects on entrainment to lengthening light cycles. The results further establish the potent effects of scotopic illumination on circadian entrainment and suggest that naturalistic ambient lighting at night may enhance the plasticity of the circadian pacemaker.

In synch but not in step: Circadian clock circuits regulating plasticity in daily rhythms

The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms.

Basic sleep and circadian science as building blocks for behavioral interventions: a translational approach for mood disorders

Sleep and circadian functioning has been of particular interest to researchers focused on improving treatments for psychiatric illness. The goal of the present paper is to highlight the exciting research that utilizes basic sleep and circadian science as building blocks for intervention in the mood disorders. The reviewed evidence suggests that the sleep and circadian systems are a) disrupted in the mood disorders and linked to symptoms, b) open systems that can be modified, c) the focus of interventions which have been developed to effectively treat sleep disturbance within mood disorders, and d) intimately linked with mood, such that improvements in sleep are associated with improvements in mood. Although significant positive treatment effects are evident, more research is needed to fill the gap in our basic understanding of the relationship between sleep and mood.

Chronic phase advance alters circadian physiological rhythms and peripheral molecular clocks

Shifting the onset of light, acutely or chronically, can profoundly affect responses to infection, tumor progression, development of metabolic disease, and mortality in mammals. To date, the majority of phase-shifting studies have focused on acute exposure to a shift in the timing of the light cycle, whereas the consequences of chronic phase shifts alone on molecular rhythms in peripheral tissues such as skeletal muscle have not been studied. In this study, we tested the effect of chronic phase advance on the molecular clock mechanism in two phenotypically different skeletal muscles. The phase advance protocol (CPA) involved 6-h phase advances (earlier light onset) every 4 days for 8 wk. Analysis of the molecular clock, via bioluminescence recording, in the soleus and flexor digitorum brevis (FDB) muscles and lung demonstrated that CPA advanced the phase of the rhythm when studied immediately after CPA. However, if the mice were placed into free-running conditions (DD) for 2 wk after CPA, the molecular clock was not phase shifted in the two muscles but was still shifted in the lung. Wheel running behavior remained rhythmic in CPA mice; however, the endogenous period length of the free-running rhythm was significantly shorter than that of control mice. Core body temperature, cage activity, and heart rate remained rhythmic throughout the experiment, although the onset of the rhythms was significantly delayed with CPA. These results provide clues that lifestyles associated with chronic environmental desynchrony, such as shift work, can have disruptive effects on the molecular clock mechanism in peripheral tissues, including both types of skeletal muscle. Whether this can contribute, long term, to increased incidence of insulin resistance/metabolic disease requires further study.

Psicologia da saúde e cronobiologia: diálogo possível?

A Psicologia da saúde visa a avaliar o papel do comportamento na etiologia da doença, a elaborar prognósticos dos comportamentos prejudiciais à saúde e a promover comportamentos saudáveis. Seu pressuposto epistemológico, o modelo biopsicossocial, envolve a interação de fatores biológicos, psicológicos e sociais na análise do processo saúde e doença. A cronobiologia ocupa-se da investigação temporal recorrente de variáveis fisiológicas e comportamentais assim como do ajuste temporal dessas variáveis ao ambiente e às suas demandas sociais. A proposta deste artigo, de caráter teórico, foi expor a interseção entre essas duas áreas, evidenciando que ambas contribuem para a compreensão dos efeitos biopsicossociais na saúde, para a identificação precoce de pessoas em situação de risco e para a prevenção de comportamentos saudáveis a fim de enfrentar situações conflituosas, além do seu papel na elaboração de programas de promoção à saúde, de estudos e de intervenções no campo comunitário e na saúde pública. São fundamentalmente especialidades interdisciplinares, que envolvem conhecimentos fisiológicos, comportamentais, cognitivos, educativos e ambientais e que fazem com que essa interseção se consolide.

Forced desynchronization of activity rhythms in a model of chronic jet lag in mice

We studied locomotor activity rhythms of C57/Bl6 mice under a chronic jet lag (CJL) protocol (ChrA(6/2) ), which consisted of 6-hour phase advances of the light-dark schedule (LD) every 2 days. Through periodogram analysis, we found 2 components of the activity rhythm: a short-period component (21.01 ± 0.04 h) that was entrained by the LD schedule and a long-period component (24.68 ± 0.26 h). We developed a mathematical model comprising 2 coupled circadian oscillators that was tested experimentally with different CJL schedules. Our simulations suggested that under CJL, the system behaves as if it were under a zeitgeber with a period determined by (24 - [phase shift size/days between shifts]). Desynchronization within the system arises according to whether this effective zeitgeber is inside or outside the range of entrainment of the oscillators. In this sense, ChrA(6/2) is interpreted as a (24 - 6/2 = 21 h) zeitgeber, and simulations predicted the behavior of mice under other CJL schedules with an effective 21-hour zeitgeber. Animals studied under an asymmetric T = 21 h zeitgeber (carried out by a 3-hour shortening of every dark phase) showed 2 activity components as observed under ChrA(6/2): an entrained short-period (21.01 ± 0.03 h) and a long-period component (23.93 ± 0.31 h). Internal desynchronization was lost when mice were subjected to 9-hour advances every 3 days, a possibility also contemplated by the simulations. Simulations also predicted that desynchronization should be less prevalent under delaying than under advancing CJL. Indeed, most mice subjected to 6-hour delay shifts every 2 days (an effective 27-hour zeitgeber) displayed a single entrained activity component (26.92 ± 0.11 h). Our results demonstrate that the disruption provoked by CJL schedules is not dependent on the phase-shift magnitude or the frequency of the shifts separately but on the combination of both, through its ratio and additionally on their absolute values. In this study, we present a novel model of forced desynchronization in mice under a specific CJL schedule; in addition, our model provides theoretical tools for the evaluation of circadian disruption under CJL conditions that are currently used in circadian research.

Dissociation of the circadian system of Octodon degus by T28 and T21 light-dark cycles

Octodon degus is a primarily diurnal rodent that presents great variation in its circadian chronotypes due to the interaction between two phase angles of entrainment, diurnal and nocturnal, and the graded masking effects of environmental light and temperature. The aim of this study was to test whether the circadian system of this diurnal rodent can be internally dissociated by imposing cycles shorter and longer than 24 h, and to determine the influence of degus chronotypes and wheel-running availability on such dissociation. To this end, wheel-running activity and body temperature rhythms were studied in degus subjected to symmetrical light-dark (LD) cycles of T28h and T21h. The results show that both T-cycles dissociate the degus circadian system in two different components: one light-dependent component (LDC) that is influenced by the presence of light, and a second non-light-dependent component (NLDC) that free-runs with a period different from the external lighting cycle. The LDC was more evident in the nocturnal than diurnal chronotype, and also when wheel running was available. Our results show that, in addition to rats and mice, degus must be added to the list of species that show an internal dissociation in their circadian rhythms when exposed to forced desynchronization protocols. The existence of a multioscillatory circadian system having two groups of oscillators with low coupling strength may explain the flexibility of degus chronotypes.

Circadian desynchronization

The suprachiasmatic nucleus (SCN) coordinates via multiple outputs physiological and behavioural circadian rhythms. The SCN is composed of a heterogeneous network of coupled oscillators that entrain to the daily light-dark cycles. Outside the physiological entrainment range, rich locomotor patterns of desynchronized rhythms are observed. Previous studies interpreted these results as the output of different SCN neural subpopulations. We find, however, that even a single periodically driven oscillator can induce such complex desynchronized locomotor patterns. Using signal analysis, we show how the observed patterns can be consistently clustered into two generic oscillatory interaction groups: modulation and superposition. In seven of 17 rats undergoing forced desynchronization, we find a theoretically predicted third spectral component. Combining signal analysis with the theory of coupled oscillators, we provide a framework for the study of circadian desynchronization.

Dissociation of circadian and light inhibition of melatonin release through forced desynchronization in the rat

Pineal melatonin release exhibits a circadian rhythm with a tight nocturnal pattern. Melatonin synthesis is regulated by the master circadian clock within the hypothalamic suprachiasmatic nucleus (SCN) and is also directly inhibited by light. The SCN is necessary for both circadian regulation and light inhibition of melatonin synthesis and thus it has been difficult to isolate these two regulatory limbs to define the output pathways by which the SCN conveys circadian and light phase information to the pineal. A 22-h light-dark (LD) cycle forced desynchrony protocol leads to the stable dissociation of rhythmic clock gene expression within the ventrolateral SCN (vlSCN) and the dorsomedial SCN (dmSCN). In the present study, we have used this protocol to assess the pattern of melatonin release under forced desynchronization of these SCN subregions. In light of our reported patterns of clock gene expression in the forced desynchronized rat, we propose that the vlSCN oscillator entrains to the 22-h LD cycle whereas the dmSCN shows relative coordination to the light-entrained vlSCN, and that this dual-oscillator configuration accounts for the pattern of melatonin release. We present a simple mathematical model in which the relative coordination of a single oscillator within the dmSCN to a single light-entrained oscillator within the vlSCN faithfully portrays the circadian phase, duration and amplitude of melatonin release under forced desynchronization. Our results underscore the importance of the SCN's subregional organization to both photic input processing and rhythmic output control.

How to achieve fast entrainment? The timescale to synchronization

Entrainment, where oscillators synchronize to an external signal, is ubiquitous in nature. The transient time leading to entrainment plays a major role in many biological processes. Our goal is to unveil the specific dynamics that leads to fast entrainment. By studying a generic model, we characterize the transient time to entrainment and show how it is governed by two basic properties of an oscillator: the radial relaxation time and the phase velocity distribution around the limit cycle. Those two basic properties are inherent in every oscillator. This concept can be applied to many biological systems to predict the average transient time to entrainment or to infer properties of the underlying oscillator from the observed transients. We found that both a sinusoidal oscillator with fast radial relaxation and a spike-like oscillator with slow radial relaxation give rise to fast entrainment. As an example, we discuss the jet-lag experiments in the mammalian circadian pacemaker.

Effects of Transient and Continuous Wheel Running Activity on the Upper and Lower Limits of Entrainment to Light‐Dark Cycles in Female Hamsters

The entrainment limits to light-dark cycles can be modified by the experimental conditions under which they are tested. Among the factors that may influence entrainment is the amount of wheel running exerted by the animal. In the present work, the effects of transitory and continuous wheel running on entrainment to light-dark cycles were tested using a range of T cycles at the entrainment limits. Four groups of female hamsters were submitted to 1 h stepwise changes in T cycles. Two groups were exposed to T cycles of which the period was shortened at the lower limit from T22 to T18, and the other two groups were exposed to cycles that lengthened at the upper limit from T27 to T32. One of the groups at the lower limit and one at the upper limit had continuous access to a running wheel, while the others had the wheel locked, except at certain T when a lack of period control by T cycle appeared. The study demonstrates that access to running wheel widens the limits of entrainment to LD cycles. Specifically, the following observations were made: the effects of wheel running for entrainment were more evident in the groups with continuous access to wheel, as they did entrain to T19 and T32; continuous access to a wheel produced aftereffects only after T19, but not under T32; and when animals without a wheel showed relative coordination, unlocking the wheel favored entrainment in all the animals at T31, but in only 1 out 6 at T19. All of these indicate a different effect of the wheel running on the upper and lower limits of entrainment.

Circadian timing of REM sleep is coupled to an oscillator within the dorsomedial suprachiasmatic nucleus

Sleep is consistently concentrated to a specific time of the day. Its timing and consolidation depend on the interplay between a homeostatic and a circadian process of sleep regulation [1-3]. Sleep propensity rises as a homeostatic response to increasing wake time, whereas a circadian clock determines the specific time when sleep will probably occur. This two-process regulation of sleep also determines which specific sleep stage will be manifested, and the circadian process governs tightly the manifestation of rapid eye movement sleep (REMS) [1, 4]. The role of the hypothalamic suprachiasmatic nucleus (SCN) in the circadian gating of sleep and wakefulness has been unequivocally established by lesion studies [5], but its role in the timing of specific sleep stages has remained unknown. Using a forced desynchrony paradigm that induces the stable dissociation of the ventrolateral (vl) and dorsomedial (dm) SCN, and a jetlag paradigm that induces desynchronization between these SCN subregions, we show that the SCN can time the occurrence of specific sleep stages. Specifically, the circadian regulation of REMS is associated with clock gene expression within the dmSCN. We provide the first neurophysiological model for the disruption of sleep architecture that may result from temporal challenges such as rotational-shift work and jetlag.

Circadian desynchronization of core body temperature and sleep stages in the rat

Proper functioning of the human circadian timing system is crucial to physical and mental health. Much of what we know about this system is based on experimental protocols that induce the desynchronization of behavioral and physiological rhythms within individual subjects, but the neural (or extraneural) substrates for such desynchronization are unknown. We have developed an animal model of human internal desynchrony in which rats are exposed to artificially short (22-h) light-dark cycles. Under these conditions, locomotor activity, sleep-wake, and slow-wave sleep (SWS) exhibit two rhythms within individual animals, one entrained to the 22-h light-dark cycle and the other free-running with a period >24 h (tau(>24 h)). Whereas core body temperature showed two rhythms as well, further analysis indicates this variable oscillates more according to the tau(>24 h) rhythm than to the 22-h rhythm, and that this oscillation is due to an activity-independent circadian regulation. Paradoxical sleep (PS), on the other hand, shows only one free-running rhythm. Our results show that, similarly to humans, (i) circadian rhythms can be internally dissociated in a controlled and predictable manner in the rat and (ii) the circadian rhythms of sleep-wake and SWS can be desynchronized from the rhythms of PS and core body temperature within individual animals. This model now allows for a deeper understanding of the human timekeeping mechanism, for testing potential therapies for circadian dysrhythmias, and for studying the biology of PS and SWS states in a neurologically intact model.

History-dependent changes in entrainment of the activity rhythm in the Syrian hamster (Mesocricetus auratus)

The authors have studied the activity rhythm of Syrian hamsters exposed to square LD cycles with a 22-h period (T22) with the aim of testing the effects of the previous history on the rhythmic pattern. To do so, sequential changes of different lighting environments were established, followed by the same LD condition. Also, the protocol included T22 cycles with varying lighting contrasts to test the extent to which a computational model predicts experimental outcomes. At the beginning of the experiment, exposure to T22 with 300 lux and dim red light occurring respectively at photophase and scotophase (LD300/dim red) mainly generated relative coordination. Subsequent transfer to cycles with approximately 0.1-lux dim light during the scotophase (LD300/0.1) promoted entrainment to T22. However, a further reduction in light intensity to 10 lux during the photophase (LD10/0.1) generated weak and unstable T22 rhythms. When, after that, animals were transferred again to the initial LD300/dim red cycles, the amplitude of the rhythm still remained very low, and the phases were very unstable. Exposure to constant darkness partially restored the activity rhythm, and when, afterwards, the animals were submitted again to LD300/dim red cycles, a robust T22 rhythm appeared. The results demonstrate history-dependent changes in the hamster circadian system because the locomotor activity pattern under the same T22 cycle can show relative coordination or unstable or robust entrainment depending on the prior lighting condition. This suggests that the circadian system responds to environmental stimuli depending on its previous history. Moreover, computer simulations allow the authors to predict entrainment under LD300/0.1 cycles and indicate that most of the patterns observed in the animals due to the light in the scotophase can be explained by different degrees of coupling among the oscillators of the circadian system.