Dissociation between circadian Per1 and neuronal and behavioral rhythms following a shifted environmental cycle

Modeling of Jet Lag and Searching for an Optimal Light Treatment

The role of the hierarchical organization of the suprachiasmatic nucleus (SCN) in its functioning, jet lag, and the light treatment of jet lag remains poorly understood. Using the core-shell model, we mimic collective behavior of the core and shell populations of the SCN oscillators in transient states after rapid traveling east and west. The existence of a special region of slow dynamical states of the SCN oscillators can explain phenomena such as the east-west asymmetry of jet lag, instances when entrainment to an advance is via delay shifts, and the dynamics of jet lag recovery time. If jet lag brings the SCN state into this region, it will take a long time to leave it and restore synchronization among oscillators. We show that the population of oscillators in the core responds quickly to a rapid phase shift of the light-dark cycle, in contrast to the shell, which responds slowly. A slow recovery of the synchronization among the shell oscillators in transient states may strongly affect reentrainment in peripheral tissues and behavioral rhythms. We discuss the relationship between molecular, electrical, and behavioral rhythms. We also describe how light pulses affect the SCN and analyze the efficiency of the light treatment in facilitating the adaptation of the SCN to a new time zone. Light pulses of a moderate duration and intensity reduce the recovery time after traveling east, but not west. However, long duration and high intensity of light pulses are more detrimental than beneficial for speeding up reentrainment. The results of the core-shell model are compared with experimental data and other biologically motivated models of the SCN.

Cell Signaling in the Circadian Pacemaker: New Insights from in vivo Imaging

BACKGROUND

"One for all, and all for one," the famous rallying cry of the Three Musketeers, in Alexandre Dumas's popular novel, certainly applies to the 20,000 cells composing the suprachiasmatic nuclei (SCN). These cells work together to form the central clock that coordinates body rhythms in tune with the day-night cycle. Like virtually every body cell, individual SCN cells exhibit autonomous circadian oscillations, but this rhythmicity only reaches a high level of precision and robustness when the cells are coupled with their neighbors. Therefore, understanding the functional network organization of SCN cells beyond their core rhythmicity is an important issue in circadian biology.

SUMMARY

The present review summarizes the main results from our recent study demonstrating the feasibility of recording SCN cells in freely moving mice and the significance of variations in intracellular calcium over several timescales.

KEY MESSAGE

We discuss how in vivo imaging at the cell level will be pivotal to interrogate the mammalian master clock, in an integrated context that preserves the SCN network organization, with intact inputs and outputs.

A Journey in the Brain's Clock: In Vivo Veritas?

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the circadian pacemaker that coordinates mammalian rhythms in tune with the day-night cycle. Understanding the determinants of the intrinsic rhythmicity of this biological clock, its outputs, and resetting by environmental cues, has been a longstanding goal of the field. Integrated techniques of neurophysiology, including lesion studies and in vivo multi-unit electrophysiology, have been key to characterizing the rhythmic nature and outputs of the SCN in animal models. In parallel, reduced ex vivo and in vitro approaches have permitted us to unravel molecular, cellular, and multicellular mechanisms underlying the pacemaker properties of the SCN. New questions have emerged in recent years that will require combining investigation at a cell resolution within the physiological context of the living animal: What is the role of specific cell subpopulations in the SCN neural network? How do they integrate various external and internal inputs? What are the circuits involved in controlling other body rhythms? Here, we review what we have already learned about the SCN from in vivo studies, and how the recent development of new genetically encoded tools and cutting-edge imaging technology in neuroscience offers chronobiologists the opportunity to meet these challenges.

Circadian and kisspeptin regulation of the preovulatory surge

Fertility in mammals is ultimately controlled by a small population of neurons - the gonadotropin-releasing hormone (GnRH) neurons - located in the ventral forebrain. GnRH neurons control gonadal function through the release of GnRH, which in turn stimulates the secretion of the anterior pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In spontaneous ovulators, ovarian follicle maturation eventually stimulates, via sex steroid feedback, the mid-cycle surge in GnRH and LH secretion that causes ovulation. The GnRH/LH surge is initiated in many species just before the onset of activity through processes controlled by the central circadian clock, ensuring that the neuroendocrine control of ovulation and sex behavior are coordinated. This review aims to give an overview of anatomical and functional studies that collectively reveal some of the mechanisms through which the central circadian clock regulates GnRH neurons and their afferent circuits to drive the preovulatory surge.

Network-driven intracellular cAMP coordinates circadian rhythm in the suprachiasmatic nucleus

The mammalian central circadian clock, located in the suprachiasmatic nucleus (SCN), coordinates the timing of physiology and behavior to local time cues. In the SCN, second messengers, such as cAMP and Ca2+, are suggested to be involved in the input and/or output of the molecular circadian clock. However, the functional roles of second messengers and their dynamics in the SCN remain largely unclear. In the present study, we visualized the spatiotemporal patterns of circadian rhythms of second messengers and neurotransmitter release in the SCN. Here, we show that neuronal activity regulates the rhythmic release of vasoactive intestinal peptides from the SCN, which drives the circadian rhythms of intracellular cAMP in the SCN. Furthermore, optical manipulation of intracellular cAMP levels in the SCN shifts molecular and behavioral circadian rhythms. Together, our study demonstrates that intracellular cAMP is a key molecule in the organization of the SCN circadian neuronal network.

Kisspeptin neuron electrophysiology: Intrinsic properties, hormonal modulation, and regulation of homeostatic circuits

The obligatory role of kisspeptin (KISS1) and its receptor (KISS1R) in regulating the hypothalamic-pituitary-gonadal axis, puberty and fertility was uncovered in 2003. In the few years that followed, an impressive body of work undertaken in many species established that neurons producing kisspeptin orchestrate gonadotropin-releasing hormone (GnRH) neuron activity and subsequent GnRH and gonadotropin hormone secretory patterns, through kisspeptin-KISS1R signaling, and mediate many aspects of gonadal steroid hormone feedback regulation of GnRH neurons. Here, we review knowledge accrued over the past decade, mainly in genetically modified mouse models, of the electrophysiological properties of kisspeptin neurons and their regulation by hormonal feedback. We also discuss recent progress in our understanding of the role of these cells within neuronal circuits that control GnRH neuron activity and GnRH secretion, energy balance and, potentially, other homeostatic and reproductive functions.

Conceptual Models of Entrainment, Jet Lag, and Seasonality

Understanding entrainment of circadian rhythms is a central goal of chronobiology. Many factors, such as period, amplitude, Zeitgeber strength, and daylength, govern entrainment ranges and phases of entrainment. We have tested whether simple amplitude-phase models can provide insight into the control of entrainment phases. Using global optimization, we derived conceptual models with just three free parameters (period, amplitude, and relaxation rate) that reproduce known phenotypic features of vertebrate clocks: phase response curves (PRCs) with relatively small phase shifts, fast re-entrainment after jet lag, and seasonal variability to track light onset or offset. Since optimization found multiple sets of model parameters, we could study this model ensemble to gain insight into the underlying design principles. We found complex associations between model parameters and entrainment features. Arnold onions of representative models visualize strong dependencies of entrainment on periods, relative Zeitgeber strength, and photoperiods. Our results support the use of oscillator theory as a framework for understanding the entrainment of circadian clocks.

Amplitude Effects Allow Short Jet Lags and Large Seasonal Phase Shifts in Minimal Clock Models

Mathematical models of varying complexity have helped shed light on different aspects of circadian clock function. In this work, we question whether minimal clock models (Goodwin models) are sufficient to reproduce essential phenotypes of the clock: a small phase response curve (PRC), fast jet lag, and seasonal phase shifts. Instead of building a single best model, we take an approach where we study the properties of a set of models satisfying certain constraints; here, a 1h-pulse PRC with a range of 3h and clock periods between 22h and 26h is designed. Surprisingly, almost all these randomly parameterized models showed a 4h change in phase of entrainment between long and short days and jet lag durations of three to seven days in advance and delay. Moreover, intrinsic clock period influenced jet lag duration and entrainment amplitude and phase. Fast jet lag was realized in this model by means of an interesting amplitude effect: the association between clock amplitude and clock period termed "twist." This twist allows amplitude changes to speed up and slow down clocks enabling faster shifts. These findings were robust to the addition of positive feedback to the model. In summary, the known design principles of rhythm generation - negative feedback, long delay, and switch-like inhibition (we review these in detail) - are sufficient to reproduce the essential clock phenotypes. Furthermore, amplitudes play a role in determining clock properties and must be always considered, although they are difficult to measure.

Molecular and Cellular Networks in The Suprachiasmatic Nuclei

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

Recurring circadian disruption alters circadian clock sensitivity to resetting

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

Role of GABA in the regulation of the central circadian clock of the suprachiasmatic nucleus

In mammals, circadian rhythms, such as sleep/wake cycles, are regulated by the central circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN consists of thousands of individual neurons, which exhibit circadian rhythms. They synchronize with each other and produce robust and stable oscillations. Although several neurotransmitters are expressed in the SCN, almost all SCN neurons are γ-amino butyric acid (GABA)-ergic. Several studies have attempted to understand the roles of GABA in the SCN; however, precise mechanisms of the action of GABA in the SCN are still unclear. GABA exhibits excitatory and/or inhibitory characteristics depending on the circadian phase or region in the SCN. It can both synchronize and destabilize cellular circadian rhythms in individual SCN cells. Differing environmental light conditions, such as a long photoperiod, result in the decoupling of circadian oscillators of the dorsal and ventral SCN. This is due to high intracellular chloride concentrations in the dorsal SCN. Because mice with functional GABA deficiency, such as vesicular GABA transporter- and glutamate decarboxylase-deficient mice, are neonatal lethal, research has been limited to pharmacological approaches. Furthermore, different recording methods have been used to understand the roles of GABA in the SCN. The excitability of GABAergic neurons also changes during the postnatal period. Although there are technical difficulties in understanding the functions of GABA in the SCN, technical developments may help uncover new roles of GABA in circadian physiology and behavior.

The choroid plexus is an important circadian clock component

Mammalian circadian clocks have a hierarchical organization, governed by the suprachiasmatic nucleus (SCN) in the hypothalamus. The brain itself contains multiple loci that maintain autonomous circadian rhythmicity, but the contribution of the non-SCN clocks to this hierarchy remains unclear. We examine circadian oscillations of clock gene expression in various brain loci and discovered that in mouse, robust, higher amplitude, relatively faster oscillations occur in the choroid plexus (CP) compared to the SCN. Our computational analysis and modeling show that the CP achieves these properties by synchronization of “twist” circadian oscillators via gap-junctional connections. Using an in vitro tissue coculture model and in vivo targeted deletion of the Bmal1 gene to silence the CP circadian clock, we demonstrate that the CP clock adjusts the SCN clock likely via circulation of cerebrospinal fluid, thus finely tuning behavioral circadian rhythms.

The suprachiasmatic nucleus (SCN) has been thought of as the master circadian clock, but peripheral circadian clocks do exist. Here, the authors show that the choroid plexus displays oscillations more robust than the SCN and that can be described as a Poincaré oscillator with negative twist.

Clock-dependent and system-driven oscillators interact in the suprachiasmatic nuclei to pace mammalian circadian rhythms

Circadian clocks drive biological rhythms with a period of approximately 24 hours and keep in time with the outside world through daily resetting by environmental cues. While this external entrainment has been extensively investigated in the suprachiasmatic nuclei (SCN), the role of internal systemic rhythms, including daily fluctuations in core temperature or circulating hormones remains debated. Here, we show that lactating mice, which exhibit dampened systemic rhythms, possess normal molecular clockwork but impaired rhythms in both heat shock response gene expression and electrophysiological output in their SCN. This suggests that body rhythms regulate SCN activity downstream of the clock. Mathematical modeling predicts that systemic feedback upon the SCN functions as an internal oscillator that accounts for in vivo and ex vivo observations. Thus we are able to propose a new bottom-up hierarchical organization of circadian timekeeping in mammals, based on the interaction in the SCN between clock-dependent and system-driven oscillators.

Encoding seasonal information in a two-oscillator model of the multi-oscillator circadian clock

The suprachiasmatic nucleus (SCN) is a collection of about 10 000 neurons, each of which functions as a circadian clock with slightly different periods and phases, that work in concert with form and maintain the master circadian clock for the organism. The diversity among neurons confers on the SCN the ability to robustly encode both the 24-h light pattern as well as the seasonal time. Cluster synchronization brings the different neurons into line and reduces the large population to essentially two oscillators, coordinated by a macroscopic network motif of asymmetric repulsive-attractive coupling. We recount the steps leading to this simplification and rigorously examine the two-oscillator case by seeking an analytical solution. Through these steps, we identify physiologically relevant parameters that shape the behaviour of the SCN network and delineate its ability to store past details of seasonal variation in photoperiod.

Dissociation of Per1 and Bmal1 circadian rhythms in the suprachiasmatic nucleus in parallel with behavioral outputs

The temporal order of physiology and behavior in mammals is primarily regulated by the circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Taking advantage of bioluminescence reporters, we monitored the circadian rhythms of the expression of clock genes Per1 and Bmal1 in the SCN of freely moving mice and found that the rate of phase shifts induced by a single light pulse was different in the two rhythms. The Per1-luc rhythm was phase-delayed instantaneously by the light presented at the subjective evening in parallel with the activity onset of behavioral rhythm, whereas the Bmal1-ELuc rhythm was phase-delayed gradually, similar to the activity offset. The dissociation was confirmed in cultured SCN slices of mice carrying both Per1-luc and Bmal1-ELuc reporters. The two rhythms in a single SCN slice showed significantly different periods in a long-term (3 wk) culture and were internally desynchronized. Regional specificity in the SCN was not detected for the period of Per1-luc and Bmal1-ELuc rhythms. Furthermore, neither is synchronized with circadian intracellular Ca2+ rhythms monitored by a calcium indicator, GCaMP6s, or with firing rhythms monitored on a multielectrode array dish, although the coupling between the circadian firing and Ca2+ rhythms persisted during culture. These findings indicate that the expressions of two key clock genes, Per1 and Bmal1, in the SCN are regulated in such a way that they may adopt different phases and free-running periods relative to each other and are respectively associated with the expression of activity onset and offset.

Suprachiasmatic Nucleus Interaction with the Arcuate Nucleus; Essential for Organizing Physiological Rhythms

The suprachiasmatic nucleus (SCN) is generally considered the master clock, independently driving all circadian rhythms. We recently demonstrated the SCN receives metabolic and cardiovascular feedback adeptly altering its neuronal activity. In the present study, we show that microcuts effectively removing SCN-arcuate nucleus (ARC) interconnectivity in Wistar rats result in a loss of rhythmicity in locomotor activity, corticosterone levels, and body temperature in constant dark (DD) conditions. Elimination of these reciprocal connections did not affect SCN clock gene rhythmicity but did cause the ARC to desynchronize. Moreover, unilateral SCN lesions with contralateral retrochiasmatic microcuts resulted in identical arrhythmicity, proving that for the expression of physiological rhythms this reciprocal SCN-ARC interaction is essential. The unaltered SCN c-Fos expression following glucose administration in disconnected animals as compared to a significant decrease in controls demonstrates the importance of the ARC as metabolic modulator of SCN neuronal activity. Together, these results indicate that the SCN is more than an autonomous clock, and forms an essential component of a larger network controlling homeostasis. The present novel findings illustrate how an imbalance between SCN and ARC communication through circadian disruption could be involved in the etiology of metabolic disorders.

Synchronous circadian voltage rhythms with asynchronous calcium rhythms in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), the master circadian clock, contains a network composed of multiple types of neurons which are thought to form a hierarchical and multioscillator system. The molecular clock machinery in SCN neurons drives membrane excitability and sends time cue signals to various brain regions and peripheral organs. However, how and at what time of the day these neurons transmit output signals remain largely unknown. Here, we successfully visualized circadian voltage rhythms optically for many days using a genetically encoded voltage sensor, ArcLightD. Unexpectedly, the voltage rhythms are synchronized across the entire SCN network of cultured slices, whereas simultaneously recorded Ca²⁺ rhythms are topologically specific to the dorsal and ventral regions. We further found that the temporal order of these two rhythms is cell-type specific: The Ca²⁺ rhythms phase-lead the voltage rhythms in AVP neurons but Ca²⁺ and voltage rhythms are nearly in phase in VIP neurons. We confirmed that circadian firing rhythms are also synchronous and are coupled with the voltage rhythms. These results indicate that SCN networks with asynchronous Ca²⁺ rhythms produce coherent voltage rhythms.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Circadian rhythms and metabolism: from the brain to the gut and back again

This paper focuses on the relationship between the circadian system and glucose metabolism. Research across the translational spectrum confirms the importance of the circadian system for glucose metabolism and offers promising clues as to when and why these systems go awry. In particular, basic research has started to clarify the molecular and genetic mechanisms through which the circadian system regulates metabolism. The study of human behavior, especially in the context of psychiatric disorders, such as bipolar disorder and major depression, forces us to see how inextricably linked mental health and metabolic health are. We also emphasize the remarkable opportunities for advancing circadian science through big data and advanced analytics. Advances in circadian research have translated into environmental and pharmacological interventions with tremendous therapeutic potential.

Clock Genes, Oscillators, and Cellular Networks in the Suprachiasmatic Nuclei

The mammalian SCN contains a biological clock that drives remarkably precise circadian rhythms in vivo and in vitro. Recent advances have revealed molecular and cellular mechanisms required for the generation of these daily rhythms and their synchronization between SCN neurons and to the environmental light cycle. This review of the evidence for a cell-autonomous circadian pacemaker within specialized neurons of the SCN focuses on 6 genes implicated within the pace making mechanism, an additional 4 genes implicated in pathways from the pacemaker, and the intercellular and intracellular mechanisms that synchronize SCN neurons to each other and to solar time.

Noise Induces Oscillation and Synchronization of the Circadian Neurons

The principle clock of mammals, named suprachiasmatic nucleus (SCN), coordinates the circadian rhythms of behavioral and physiological activity to the external 24 h light-dark cycle. In the absence of the daily cycle, the SCN acts as an endogenous clock that regulates the ~24 h rhythm of activity. Experimental and theoretical studies usually take the light-dark cycle as a main external influence, and often ignore light pollution as an external influence. However, in modern society, the light pollution such as induced by electrical lighting influences the circadian clock. In the present study, we examined the effect of external noise (light pollution) on the collective behavior of coupled circadian oscillators under constant darkness using a Goodwin model. We found that the external noise plays distinct roles in the network behavior of neurons for weak or strong coupling between the neurons. In the case of strong coupling, the noise reduces the synchronization and the period of the SCN network. Interestingly, in the case of weak coupling, the noise induces a circadian rhythm in the SCN network which is absent in noise-free condition. In addition, the noise increases the synchronization and decreases the period of the SCN network. Our findings may shed new light on the impact of the external noise on the collective behavior of SCN neurons.

Enhanced phase resetting in the synchronized suprachiasmatic nucleus network

The suprachiasmatic nucleus (SCN) adapts to both the external light-dark (LD) cycle and seasonal changes in day length. In short photoperiods, single-cell activity patterns are tightly synchronized (i.e., in phase); in long photoperiods, these patterns are relatively dispersed, causing lower amplitude rhythms. The limit cycle oscillator has been used to describe the SCN's circadian rhythmicity and predicts that following a given perturbation, high-amplitude SCN rhythms will shift less than low-amplitude rhythms. Some studies reported, however, that phase delays are larger when animals are entrained to a short photoperiod. Because phase advances and delays are mediated by partially distinct (i.e., nonoverlapping) biochemical pathways, we investigated the effect of a 4-h phase advance of the LD cycle in mice housed in either short (LD 8:16) or long (LD 16:8) photoperiods. In vitro recordings revealed a significantly larger phase advance in the SCN of mice entrained to short as compared to long photoperiods (4.2 ± 0.3 h v. 1.4 ± 0.9 h, respectively). Surprisingly, in mice with long photoperiods, the behavioral phase shift was larger than the phase shift of the SCN (3.7 ± 0.4 h v. 1.4 ± 0.9 h, respectively). To exclude a confounding influence of running-wheel activity on the magnitude of the shifts of the SCN, we repeated the experiments in the absence of running wheels and found similar shifts in the SCN in vitro in short and long days (3.0 ± 0.5 h v. 0.4 ± 0.9 h, respectively). Interestingly, removal of the running wheel reduced the phase-shifting capacity of mice in long days, leading to similar behavioral shifts in short and long photoperiods (1.0 ± 0.1 h v. 1.0 ± 0.4 h). As the behavioral shifts in the presence of wheels were larger than the shift of the SCN, it is suggested that additional, non-SCN neuronal networks in the brain are involved in regulating the timing of behavioral activity. On the basis of the phase shifts observed in vitro, we conclude that highly synchronized SCN networks with high-amplitude rhythms show a larger phase-shifting capacity than desynchronized networks of low amplitude.

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.

Aging differentially affects the re-entrainment response of central and peripheral circadian oscillators

Aging produces a decline in the amplitude and precision of 24 h behavioral, endocrine, and metabolic rhythms, which are regulated in mammals by a central circadian pacemaker within the suprachiasmatic nucleus (SCN) and local oscillators in peripheral tissues. Disruption of the circadian system, as experienced during transmeridian travel, can lead to adverse health consequences, particularly in the elderly. To test the hypothesis that age-related changes in the response to simulated jet lag will reflect altered circadian function, we examined re-entrainment of central and peripheral oscillators from young and old PER2::luciferase mice. As in previous studies, locomotor activity rhythms in older mice required more days to re-entrain following a shift than younger mice. At the tissue level, effects of age on baseline entrainment were evident, with older mice displaying earlier phases for the majority of peripheral oscillators studied and later phases for cells within most SCN subregions. Following a 6 h advance of the light:dark cycle, old mice displayed slower rates of re-entrainment for peripheral tissues but a larger, more rapid SCN response compared to younger mice. Thus, aging alters the circadian timing system in a manner that differentially affects the re-entrainment responses of central and peripheral circadian clocks. This pattern of results suggests that a major consequence of aging is a decrease in pacemaker amplitude, which would slow re-entrainment of peripheral oscillators and reduce SCN resistance to external perturbation.

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.

Role of vasoactive intestinal peptide in seasonal encoding by the suprachiasmatic nucleus clock

The neuropeptide vasoactive intestinal peptide (VIP) is critical for the proper functioning of the neural circuit that generates circadian rhythms. Mice lacking VIP show profound deficits in the ability to generate many behavioral and physiological rhythms. To explore how the loss of VIP impacts on the intact circadian system, we carried out in vivo multiunit neural activity (MUA) recordings from the suprachiasmatic nucleus of freely moving VIP knockout (KO) mice. The MUA rhythms were largely unaltered in the VIP KO mice, with no significant differences being seen in the amplitude or phase of the rhythms in light-dark conditions. Robust differences between the genotypes were revealed when the mice were transferred from light-dark to constant darkness conditions. In addition, the ability of the VIP KO mice to encode changes in photoperiod was examined. Strikingly, the behavioral and physiological rhythms of VIP KO mice showed no adaptation to short or long photoperiods. The data indicate that the intact circadian system can compensate for some of the consequences of the loss of VIP, whereas this peptide is indispensable for endogenous encoding of seasonal information.

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

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

Phase resetting of the mammalian circadian clock relies on a rapid shift of a small population of pacemaker neurons

The circadian pacemaker of the suprachiasmatic nuclei (SCN) contains a major pacemaker for 24 h rhythms that is synchronized to the external light-dark cycle. In response to a shift in the external cycle, neurons of the SCN resynchronize with different pace. We performed electrical activity recordings of the SCN of rats in vitro following a 6 hour delay of the light-dark cycle and observed a bimodal electrical activity pattern with a shifted and an unshifted component. The shifted component was relatively narrow as compared to the unshifted component (2.2 h and 5.7 h, respectively). Curve fitting and simulations predicted that less than 30% of the neurons contribute to the shifted component and that their phase distribution is small. This prediction was confirmed by electrophysiological recordings of neuronal subpopulations. Only 25% of the neurons exhibited an immediate shift in the phase of the electrical activity rhythms, and the phases of the shifted subpopulations appeared significantly more synchronized as compared to the phases of the unshifted subpopulations (p<0.05). We also performed electrical activity recordings of the SCN following a 9 hour advance of the light-dark cycle. The phase advances induced a large desynchrony among the neurons, but consistent with the delays, only 19% of the neurons peaked at the mid of the new light phase. The data suggest that resetting of the central circadian pacemaker to both delays and advances is brought about by an initial shift of a relatively small group of neurons that becomes highly synchronized following a shift in the external cycle. The high degree of synchronization of the shifted neurons may add to the ability of this group to reset the pacemaker. The large desynchronization observed following advances may contribute to the relative difficulty of the circadian system to respond to advanced light cycles.

Age-related decline in circadian output

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

Linking neural activity and molecular oscillations in the SCN

Neurons in the suprachiasmatic nucleus (SCN) function as part of a central timing circuit that drives daily changes in our behaviour and underlying physiology. A hallmark feature of SCN neuronal populations is that they are mostly electrically silent during the night, start to fire action potentials near dawn and then continue to generate action potentials with a slow and steady pace all day long. Sets of currents are responsible for this daily rhythm, with the strongest evidence for persistent Na(+) currents, L-type Ca(2+) currents, hyperpolarization-activated currents (I(H)), large-conductance Ca(2+) activated K(+) (BK) currents and fast delayed rectifier (FDR) K(+) currents. These rhythms in electrical activity are crucial for the function of the circadian timing system, including the expression of clock genes, and decline with ageing and disease. This article reviews our current understanding of the ionic and molecular mechanisms that drive the rhythmic firing patterns in the SCN.

Daily and seasonal adaptation of the circadian clock requires plasticity of the SCN neuronal network

Circadian rhythms are an essential property of many living organisms, and arise from an internal pacemaker, or clock. In mammals, this clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus, and generates an intrinsic circadian rhythm that is transmitted to other parts of the CNS. We will review the evidence that basic adaptive functions of the circadian system rely on functional plasticity in the neuronal network organization, and involve a change in phase relation among oscillatory neurons. We will illustrate this for: (i) photic entrainment of the circadian clock to the light-dark cycle; and (ii) seasonal adaptation of the clock to changes in day length. Molecular studies have shown plasticity in the phase relation between the ventral and dorsal SCN during adjustment to a shifted environmental cycle. Seasonal adaptation relies predominantly on plasticity in the phase relation between the rostral and caudal SCN. Electrical activity is integrated in the SCN, and appears to reflect the sum of the differently phased molecular expression patterns. While both photic entrainment and seasonal adaptation arise from a redistribution of SCN oscillatory activity patterns, different neuronal coupling mechanisms are employed, which are reviewed in the present paper.

Rapid changes in the light/dark cycle disrupt memory of conditioned fear in mice

Background

Circadian rhythms govern many aspects of physiology and behavior including cognitive processes. Components of neural circuits involved in learning and memory, e.g., the amygdala and the hippocampus, exhibit circadian rhythms in gene expression and signaling pathways. The functional significance of these rhythms is still not understood. In the present study, we sought to determine the impact of transiently disrupting the circadian system by shifting the light/dark (LD) cycle. Such “jet lag” treatments alter daily rhythms of gene expression that underlie circadian oscillations as well as disrupt the synchrony between the multiple oscillators found within the body.

Methodology/Principal Findings

We subjected adult male C57Bl/6 mice to a contextual fear conditioning protocol either before or after acute phase shifts of the LD cycle. As part of this study, we examined the impact of phase advances and phase delays, and the effects of different magnitudes of phase shifts. Under all conditions tested, we found that recall of fear conditioned behavior was specifically affected by the jet lag. We found that phase shifts potentiated the stress-evoked corticosterone response without altering baseline levels of this hormone. The jet lag treatment did not result in overall sleep deprivation, but altered the temporal distribution of sleep. Finally, we found that prior experience of jet lag helps to compensate for the reduced recall due to acute phase shifts.

Conclusions/Significance

Acute changes to the LD cycle affect the recall of fear-conditioned behavior. This suggests that a synchronized circadian system may be broadly important for normal cognition and that the consolidation of memories may be particularly sensitive to disruptions of circadian timing.

Daily electrical silencing in the mammalian circadian clock

Neurons in the brain's suprachiasmatic nuclei (SCNs), which control the timing of daily rhythms, are thought to encode time of day by changing their firing frequency, with high rates during the day and lower rates at night. Some SCN neurons express a key clock gene, period 1 (per1). We found that during the day, neurons containing per1 sustain an electrically excited state and do not fire, whereas non-per1 neurons show the previously reported daily variation in firing activity. Using a combined experimental and theoretical approach, we explain how ionic currents lead to the unusual electrophysiological behaviors of per1 cells, which unlike other mammalian brain cells can survive and function at depolarized states.

Spatiotemporal Heterogeneity in the Electrical Activity of Suprachiasmatic Nuclei Neurons and their Response to Photoperiod

The coordinated activity of thousands of cellular oscillators in the suprachiasmatic nuclei (SCN) temporally regulates mammalian physiology to anticipate daily environmental changes across the seasons. The phasing of clock gene expression varies according to anatomical location in the SCN and is thought to encode photoperiodic information. However, it is unclear whether similar variations in phase occur in the electrical activity of SCN neurons, a measure of both intraSCN signaling and clock output. To address this, we recorded single-unit and multiunit activity (SUA/MUA) from dorsal and ventral subregions of the middle level of the rostrocaudal axis of the SCN in coronal brain slices prepared from mice housed under different photoperiods. We demonstrate that under a symmetrical (12 h light:12 h dark) photoperiod, cells in the dorsal SCN are less tightly synchronized than those in the ventral SCN. Comparison of recordings made from mice under short (8 h light:16 h dark) or long (16 h light:8 h dark) photoperiods shows that the phase distribution of ventral, but not dorsal, SCN neurons expands with increasing day length. Conversely, the duration that individual neurons are active increases in dorsal, but not ventral, SCN under long days. These data indicate that in the ventral SCN photoperiod is encoded at the network level, while this coding occurs at the level of individual cells in the dorsal SCN.

High-fat feeding alters the clock synchronization to light

High-fat feeding in rodents leads to metabolic abnormalities mimicking the human metabolic syndrome, including obesity and insulin resistance. These metabolic diseases are associated with altered temporal organization of many physiological functions. The master circadian clock located in the suprachiasmatic nuclei controls most physiological functions and metabolic processes. Furthermore, under certain conditions of feeding (hypocaloric diet), metabolic cues are capable of altering the suprachiasmatic clock's responses to light. To determine whether high-fat feeding (hypercaloric diet) can also affect resetting properties of the suprachiasmatic clock, we investigated photic synchronization in mice fed a high-fat or chow (low-fat) diet for 3 months, using wheel-running activity and body temperature rhythms as daily phase markers (i.e. suprachiasmatic clock's hands). Compared with the control diet, mice fed with the high-fat diet exhibited increased body mass index, hyperleptinaemia, higher blood glucose, and increased insulinaemia. Concomitantly, high-fat feeding led to impaired adjustment to local time by photic resetting. At the behavioural and physiological levels, these alterations include slower rate of re-entrainment of behavioural and body temperature rhythms after 'jet-lag' test (6 h advanced light-dark cycle) and reduced phase-advancing responses to light. At a molecular level, light-induced phase shifts have been correlated, within suprachiasmatic cells, with a high induction of c-FOS, the protein product of immediate early gene c-fos, and phosphorylation of the extracellular signal-regulated kinases I/II (P-ERK). In mice fed a high-fat diet, photic induction of both c-FOS and P-ERK in the suprachiasmatic nuclei was markedly reduced. Taken together, the present data demonstrate that high-fat feeding modifies circadian synchronization to light.

Circadian entrainment aftereffects in suprachiasmatic nuclei and peripheral tissues in vitro

Circadian rhythms are endogenous 24-h rhythms. The suprachiasmatic nuclei (SCN) of the mammalian hypothalamus serve as the master circadian pacemaker, entraining peripheral organs which also demonstrate circadian rhythms. Entrainment to LD cycles of non-24 h duration (T-cycles) induces aftereffects on period that act to bring the intrinsic period closer to the entraining cycle. Both parametric effects, such as changes in endogenous period, and non-parametric effects of light, such as instantaneous phase shifts, act synergistically to accomplish entrainment of the SCN. It is not yet known if entrainment of peripheral oscillators similarly involves both parametric and non-parametric effects. In this study, mPer2(Luc) knockin mice were entrained to either long or short T-cycles, placed into constant darkness (DD) for 3 days to measure behavioral free-running period (FRP), and then PER2::LUC bioluminescence from SCN, spleen, esophagus, lung and thymus was measured in vitro. The FRP of SCN samples was negatively correlated with the FRP of behavioral rhythms, replicating prior results in mPer1-Luc mice. The FRP of the four peripheral oscillators tested did not correlate with behavioral rhythm FRP. Evidence that the SCN may entrain peripheral tissues by shifting phase relationships was observed, in that the phase of PER2::LUC in the SCN relative to peripheral tissues and also to the onset of behavioral activity varied between groups. Our study suggests that aftereffects on FRP may be an emergent property of the system that cannot be explained by the period changes in the system components. Further, we demonstrate that the phase relationship between the rhythm in PER2 in the SCN and these peripheral tissues is altered following T-cycle entrainment.

Organization of cell and tissue circadian pacemakers: a comparison among species

In most animal species, a circadian timing system has evolved as a strategy to cope with 24-hour rhythms in the environment. Circadian pacemakers are essential elements of the timing system and have been identified in anatomically discrete locations in animals ranging from insects to mammals. Rhythm generation occurs in single pacemaker neurons and is based on the interacting negative and positive molecular feedback loops. Rhythmicity in behavior and physiology is regulated by neuronal networks in which synchronization or coupling is required to produce coherent output signals. Coupling occurs among individual clock cells within an oscillating tissue, among functionally distinct subregions within the pacemaker, and between central pacemakers and the periphery. Recent evidence indicates that peripheral tissues can influence central pacemakers and contain autonomous circadian oscillators that contribute to the regulation of overt rhythmicity. The data discussed in this review describe coupling and synchronization mechanisms at the cell and tissue levels. By comparing the pacemaker systems of several multicellular animal species (Drosophila, cockroaches, crickets, snails, zebrafish and mammals), we will explore general organizational principles by which the circadian system regulates a 24-hour rhythmicity.

Processing of daily and seasonal light information in the mammalian circadian clock

It is necessary for an organism's survival that many physiological functions and behaviours demonstrate daily and seasonal variations. A crucial component for the temporal control in mammals is the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Neurons in the SCN generate a rhythm in electrical activity with a period of about 24h. The SCN receives photic information from photoreceptive ganglion cells in the retina and processes the information, detecting dawn and dusk as well as encoding day-length. Information processing by the SCN is optimized to extract relevant irradiance information and reduce interferences. Neuronal coupling pathways, including GABAergic signalling, are employed to distribute information and synchronize SCN subregions to form a uniform timing signal. Encoding of day-length is manifested in SCN neuronal activity patterns and may be the product of network interactions rather than being based on the single cell.

Biological Rhythms Workshop IB: neurophysiology of SCN pacemaker function

Pacemakers are functional units capable of generating oscillations that synchronize downstream rhythms. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is a circadian pacemaker composed of individual neurons that intrinsically express a near 24-hour rhythm in gene expression. Rhythmic gene expression is tightly coupled to a rhythm in spontaneous firing rate via intrinsic daily regulation of potassium current. Recent progress in the field indicates that SCN pacemaking is a specialized property that emerges from intrinsic features of single cells, structural connectivity among cells, and activity dynamics within the SCN. The focus of this chapter is on how Nature built a functional pacemaker from many individual oscillators that is capable of coordinating the daily timing of essential brain and physiological processes.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Effects of preparation time on phase of cultured tissues reveal complexity of circadian organization

The phases of central (SCN) and peripheral circadian oscillators are held in specific relationships under LD cycles but, in the absence of external rhythmic input, may damp or drift out of phase with each other. Rats exposed to prolonged constant light become behaviorally arrhythmic, perhaps as a consequence of dissociation of phases among SCN cells. The authors asked whether individual central and peripheral circadian oscillators were rhythmic in LL-treated arrhythmic rats and, if rhythmic, what were the phase relationships between them. The authors prepared SCN, pineal gland, pituitary, and cornea cultures from transgenic Period1-luciferaserats whose body temperature and locomotor activity were arrhythmic and from several groups of rhythmic rats held in LD, DD, and short-term LL. The authors measured mPer1gene expression by recording light output with sensitive photomultipliers. Most of the cultures from all groups displayed circadian rhythms. This could reflect persistent rhythmicity in vivo prior to culture or, alternatively, rhythmicity that may have been initiated by the culture procedure. To test this, the authors cultured tissues at 2 different times 12 h apart and asked whether phase of the rhythm was related to culture time. The pineal, pituitary, and SCN cultures showed partial or complete dependence of phase on culture time, while peak phases of the cornea cultures were independent of culture time in rhythmic rats and were randomly distributed regardless of culture time in arrhythmic animals. These results suggest that in behaviorally arrhythmic rats, oscillators in the pineal, pituitary, and SCN had been arrhythmic or severely damped in vivo, while the cornea oscillator was free running. The peak phases of the SCN cultures were particularly sensitive to some aspect of the culture procedure since rhythmicity of SCN cultures from robustly rhythmic LD-entrained rats was strongly influenced when the procedure was carried out at any time except the 2nd half of the day.

Differential response of Period 1 expression within the suprachiasmatic nucleus

The suprachiasmatic nuclei (SCNs) of the hypothalamus contain a circadian clock that exerts profound control over rhythmic physiology and behavior. The clock consists of multiple autonomous cellular pacemakers distributed throughout the rat SCN. In response to a shift in the light schedule, the SCN rapidly changes phase to achieve the appropriate phase relationship with the shifted light schedule. Through use of a transgenic rat in which rhythmicity in transcription of the Period 1 gene was measured with a luciferase reporter (Per1-luc), we have been successful in tracking the time course of molecular rhythm phase readjustments in different regions of the SCN that occur in response to a shift in the light schedule. We find that different regions of the SCN phase adjust at different rates, leading to transient internal desynchrony in Per1-luc expression among SCN regions. This desynchrony among regions is most pronounced and prolonged when the light schedule is advanced compared with light schedule delays. A similar asymmetry in the speed of phase resetting is observed with locomotor behavior, suggesting that phase shifting kinetics within the SCN may underlay the differences observed in behavioral resetting to advances or delays in the light schedule.

The circadian cycle: daily rhythms from behaviour to genes

The daily recurrence of activity and rest are so common as to seem trivial. However, they reflect a ubiquitous temporal programme called the circadian clock. In the absence of either anatomical clock structures or clock genes, the timing of sleep and wakefulness is disrupted. The complex nature of circadian behaviour is evident in the fact that phasing of the cycle during the day varies widely for individuals, resulting in extremes colloquially called 'larks' and 'owls'. These behavioural oscillations are mirrored in the levels of physiology and gene expression. Deciphering the underlying mechanisms will provide important insights into how the circadian clock affects health and disease.

Reentrainment of the circadian pacemaker through three distinct stages

Circadian rhythms are endogenously generated by a central pacemaker and are synchronized to the environmental LD cycle. The rhythms can be resynchronized, or reentrained, after a shift of the LD cycle, as in traveling across time zones. The authors have performed high-resolution mapping of the pacemaker to analyze the reentrainment process using rat pineal melatonin onset (MT(on)) and melatonin offset (MT(off)) rhythms as markers. Following LD (12:12) delays of 3, 6, and 12 h, MT(on) was phase locked immediately, whereas MT(off) shifted rapidly during the initial 1 through 3 cycles. In all animals, the MT(off) shifted beyond their expected phase positions in the new LD cycle, which resulted in a transient expansion of melatonin secretion duration for several cycles. It took MT(off) only 1, 2, or 3 cycles to complete most of the required phase shifts after 3, 6, or 12 h of the LD cycle delays, respectively. However, the final stabilization of phase relationships of both MT(on) and MT(off) required at least 6 cycles for rats experiencing a 3-h LD delay and much longer for the rest. These results reaffirmed the notion that both onset and offset phases of melatonin rhythms are important markers for the pacemaker and demonstrated that the reentrainment of the central pacemaker to a delay shift of the LD cycle is a 3-step process: an immediate phase lock of onset and a rapid delay shift of offset rhythms, overshoot of the offset, and, finally, a slow adjustment of both onset and offset phases. This study represents the 1st detailed analysis of the pacemaker behavior during reentrainment using melatonin and supports the notion that the eventual adaptation of the circadian pacemaker to a new time zone is a time-consuming process.