The pineal gland influences rat circadian activity rhythms in constant light.

Mammalian circadian organization is believed to derive primarily from circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN). The SCN drives circadian rhythms of a wide array of functions (e.g., locomotion, body temperature, and several endocrine processes, including the circadian secretion of the pineal hormone melatonin). In contrast to the situation in several species of reptiles and birds, there is an extensive literature reporting little or no effect of pinealectomy on mammalian circadian rhythms. However, recent research has indicated that the SCN and circadian systems of several mammalian species are highly sensitive to exogenous melatonin, raising the possibility that endogenous pineal hormone may provide feedback in the control of overt circadian rhythms. To determine the role of the pineal gland in rat circadian rhythms, the effects of pinealectomy on locomotor rhythms in constant light (LL) and constant darkness (DD) were studied. The results indicated that the circadian rhythms of pinealectomized rats but not sham-operated controls dissociated into multiple ultradian components in LL and recoupled into circadian patterns only after 12-21 days in DD. The data suggest that pineal feedback may modulate sensitivity to light and/or provide coupling among multiple circadian oscillators within the SCN.     

Hierarchically coupled ultradian oscillators generating robust circadian rhythms

Ensembles of mutually coupled ultradian cellular oscillators have been proposed by a number of authors to explain the generation of circadian rhythms in mammals. Most mathematical models using many coupled oscillators predict that the output period should vary as the square root of the number of participating units, thus being inconsistent with the well-established experimental result that ablation of substantial parts of the suprachiasmatic nuclei (SCN), the main circadian pacemaker in mammals, does not eliminate the overt circadian functions, which show no changes in the phases or periods of the rhythms. From these observations, we have developed a theoretical model that exhibits the robustness of the circadian clock to changes in the number of cells in the SCN, and that is readily adaptable to include the successful features of other known models of circadian regulation, such as the phase response curves and light resetting of the phase.     

Effects of suprachiasmatic nuclei lesions on circadian and ultradian rhythms in body temperature in ocular enucleated rats

Abstract

To test the hypothesis that an oscillator located outside the suprachiasmatic nuclei (SCN) controls the circadian rhythm of body temperature, we conducted a study with 14 blinded rats, 10 of which receiving a SCN lesion. Body temperature was automatically and continuously recorded for about one month by intraperitoneal radio transmitters. Food intake, drinking and locomotor activity were also recorded. Periodograms revealed that 3 rats with histologically verified total bilateral SCN lesions did not exhibit any circadian rhythmicity. The 7 other rats appeared to have partial lesions. They showed shortening of period and severe amplitude reduction in all functions. Thus, no support was found for the hypothesis of a separate circadian ‘temperature oscillator’ located outside the SCN. Nevertheless, after large partial lesions body temperature showed more persistency than some of the other behavioral rhythms.

Ultradian rhythms in temperature persisted after partial and total lesions. Other functions showed parallel ultradian rhythms. In intact rats the ultradian peaks were restricted predominantly to the subjective night. After total lesions these peaks became more or less homogeneously distributed in time but more heterogeneously after partial lesions. So the SCN plays a role in the temporal structure of ultradian rhythms but does not generate them. Non‐24‐hour actograms showed instabilities of period and phase of ultradian rhythms. Intact and lesioned rats were similar with respect to the mean (about 3.5 hrs) and standard deviation (about 1.5 hrs) of ultradian periods in temperature. These features indicate that a mechanism outside the SCN is underlying ultradian rhythmicity, capable of generating short‐term oscillations. Two approaches, homeostatic sleep‐wake relaxation oscillations and multiple circadian oscillators, are discussed.     

Differential elimination of circadian and ultradian rhythmicity by hypothalamic lesions in the common vole, Microtus arvalis

Effects of hypothalamic lesions on the ultradian and circadian organization of wheel running and feeding were studied in the common vole, Microtus arvalis. Circadian organization broke down within 30 days in continuous darkness in 24% of intact voles (n = 135). Ultradian rhythmicity of feeding (period 2-3 hr) persisted in constant conditions in all intact voles. Following lesions of the suprachiasmatic nuclei (SCN), circadian rhythmicity disappeared when lesions were complete (n = 8) or more extensive than 25% of the total SCN volume (n = 5). Absence of circadian rhythmicity was also found in animals with substantial lesions in the diencephalic paraventricular area (PVA) and in the retrochiasmatic area (RCA) and/or adjacent arcuate nucleus (Arc). Complete loss of ultradian and circadian organization occurred in eight voles with damage to the RCA and/or Arc. In three of these, the SCN was intact. The SCN is a likely candidate for a circadian pacemaker in voles (as in other rodents), while the loss of circadian rhythmicity following PVA and RCA/Arc lesions may be due to destruction of efferent pathways from the SCN. The RCA/Arc area is apparently necessary for the expression of ultradian rhythms. The intact SCN is neither necessary nor sufficient for the generation of ultradian rhythmicity.     

A neural theory of circadian rhythms: The gated pacemaker

This article describes a behaviorally, physiologically, and anatomically predictive model of how circadian rhythms are generated by each suprachiasmatic nucleus (SCN) of the mammalian hypothalamus. This gated pacemaker model is defined in terms of competing on-cell off-cell populations whose positive feedback signals are gated by slowly accumulating chemical transmitter substances. These components have also been used to model other hypothalamic circuits, notably the eating circuit. A parametric analysis of the types of oscillations supported by the model is presented. The complementary reactions to light of diurnal and nocturnal mammals as well as their similar phase response curves are obtained. The “dead zone” of the phase response curve during the subjective day of a noctural rodent is also explained. Oscillations are suppressed by high intensities of steady light. Operations that alter the parameters of the model transmitters can phase shift or otherwise change its circadian oscillation. Effects of ablation and hormones on model oscillations are summarized. Observed oscillations include regular periodic solutions, periodic plateau solutions, rippled plateau solutions, period doubling solutions, slow modulation of oscillations over a period of months, and repeating sequences of oscillation clusters. The model period increases inversely with the transmitter accumulation rate but is insensitive to other parameter choices except near the breakdown of oscillations. The model's clocklike nature is thus a mathematical property rather than a formal postulate. A singular perturbation approach to the model's analysis is described.     

Daily rhythms of Fos expression in hypothalamic targets of the suprachiasmatic nucleus in diurnal and nocturnal rodents.

Little is known about the differences in the neural substrates of circadian rhythms that are responsible for the maintenance of differences between diurnal and nocturnal patterns of activity in mammals. In both groups of animals, the suprachiasmatic nucleus (SCN) functions as the principal circadian pacemaker, and surprisingly, several correlates of neuronal activity in the SCN show similar daily patterns in diurnal and nocturnal species. In this study, immunocytochemistry was used to monitor daily fluctuations in the expression of the nuclear phosphoprotein Fos in the SCN and in hypothalamic targets of the SCN axonal outputs in the nocturnal laboratory rat and in the diurnal murid rodent, Arvicanthis niloticus. The daily patterns of Fos expression in the SCN were very similar across the two species. However, clear species differences were seen in regions of the hypothalamus that receive inputs from the SCN including the subparaventricular zone. These results indicate that differences in the circadian system found downstream from the SCN contribute to the emergence of a diurnal or nocturnal profile in mammals.     

Entrainment of circadian clocks in mammals by arousal and food

Circadian rhythms in mammals are regulated by a system of endogenous circadian oscillators (clock cells) in the brain and in most peripheral organs and tissues. One group of clock cells in the hypothalamic SCN (suprachiasmatic nuclei) functions as a pacemaker for co-ordinating the timing of oscillators elsewhere in the brain and body. This master clock can be reset and entrained by daily LD (light-dark) cycles and thereby also serves to interface internal with external time, ensuring an appropriate alignment of behavioural and physiological rhythms with the solar day. Two features of the mammalian circadian system provide flexibility in circadian programming to exploit temporal regularities of social stimuli or food availability. One feature is the sensitivity of the SCN pacemaker to behavioural arousal stimulated during the usual sleep period, which can reset its phase and modulate its response to LD stimuli. Neural pathways from the brainstem and thalamus mediate these effects by releasing neurochemicals that inhibit retinal inputs to the SCN clock or that alter clock-gene expression in SCN clock cells. A second feature is the sensitivity of circadian oscillators outside of the SCN to stimuli associated with food intake, which enables animals to uncouple rhythms of behaviour and physiology from LD cycles and align these with predictable daily mealtimes. The location of oscillators necessary for food-entrained behavioural rhythms is not yet certain. Persistence of these rhythms in mice with clock-gene mutations that disable the SCN pacemaker suggests diversity in the molecular basis of light- and food-entrainable clocks.     

Mammalian cultured cells as a model system of peripheral circadian clocks

The mammalian circadian system consists of multiple oscillators with basically hierarchical relationship, in which the hypothalamic suprachiasmatic nucleus (SCN) is the master pacemaker and the other oscillators in the periphery are subordinate. Although peripheral oscillators have been preceded by the SCN in circadian studies, accumulating data have revealed the importance and characteristics of peripheral oscillators. Cultured cell lines have also provided valuable information about intracellular mechanisms of circadian rhythms. This review outlines the properties of peripheral clocks in several perspectives such as the mechanisms of autonomous oscillations, the clock resetting, and the clock outputs, and describes the usefulness of immortalized cultured cells as a model system of mammalian circadian clocks by introducing some fruits of related works.     

哺乳动物季节性繁殖的内源年生物钟及光敏神经环路研究进展

生活在温带和寒带的哺乳动物在长期的进化过程中形成了季节性繁殖的生活史特征。哺乳动物的繁殖功能主要受到下丘脑-垂体-性腺轴(hypothalamic-pituitary-gonadal axis,HPGA)的调控。视交叉上核(suprachiasmatic nucleus,SCN)能够自发振荡并响应光周期信号的变化,引发褪黑素分泌的改变,并介导下游通路中下丘脑甲状腺激素、Kisspeptin和RF酰胺相关肽(RF amide-related peptide,RFRP)的节律性表达变化,从而调控哺乳动物的季节性繁殖。本文综述了哺乳动物季节性繁殖的内源年生物钟调控,并强调了光敏通路中包括甲状腺激素、Kisspeptin和RFRP在季节性繁殖调控中的重要作用。     

Immortalized suprachiasmatic nucleus cells express components of multiple circadian regulatory pathways

We undertook an extensive antigenic characterization of the SCN 2.2 cell line in order to further evaluate whether the line expresses components of circadian regulatory pathways common to the hypothalamic suprachiasmatic nucleus (SCN), the central circadian clock in mammals. We found that differentiated SCN 2.2 cultures expressed a broad range of putative clock genes, as well as components of daytime, nighttime, and crepuscular circadian regulatory pathways found within the SCN in vivo. The line also exhibits several antigens that are highly expressed in a circadian pattern and/or differentially localized in the SCN relative to other hypothalamic regions. Expression of a broad complement of circadian regulatory proteins and putative clock genes further support growing evidence in recent reports that the SCN 2.2 cell line is an appropriate model for investigating the regulation of central mammalian pacemaker.     

Phase advances of circadian rhythms in somatostatin depleted rats: effects of cysteamine on rhythms of locomotor activity and electrical discharge of the suprachiasmatic nucleus

Somatostatin is synthesized in the suprachiasmatic nucleus (SCN), a circadian pacemaker in mammals. To explore the functional significance of somatostatin in the circadian system, we examined rhythms of rat locomotor activity and electrical firing rate of SCN neurons in the brain slice after temporal depletion of somatostatin levels in the SCN. Intraperitoneal administration of cysteamine (200 mg/kg), a somatostatin depletor, significantly reduced somatostatin level in the in vivo SCN 5 min after injection and kept low level as long as 3 to 4 days. This administration, on the other hand, induced significant phase advances of about 51 min in the subsequent free-running rhythm of locomotor activity of the rat. A marked phase advance in the circadian rhythm of firing rate in the SCN was also observed after administration of cysteamine in coronal hypothalamic slices. These persistent phase shifts after administration of a somatostatin depletor may suggest that the change of somatostatin level in the SCN have a feedback influence on the circadian pacemaker.Abbreviations SCN suprachiasmatic nucleus - AVP arginine-vasopressin - VIP vasoactive intestinal polypeptide - CT circadian time - ZT zeitgeber time - i.p. intraperitoneally - 12L:12D 12 h light and 12 h dark - ANOVA analysis of variance     

Organization and function of a central nervous system circadian oscillator: the suprachiasmatic hypothalamic nucleus

Circadian rhythms in mammals are generated by endogenous neural oscillating systems entrained to the light-dark cycle by specific visual pathways. We conclude from available data that the suprachiasmatic hypothalamic nuclei (SCN) are the principal circadian oscillators in the rodent brain and that a retinohypothalamic projection terminating in the SCN is the primary visual pathway subserving entrainment of circadian rhythms. Recent anatomical studies demonstrate that the SCN have distinct subdivisions in the rat. A dorsomedial component is comprised of a distinct neuronal population and contains a large population of interneurons, many of which produce peptides. It receives no direct or indirect visual input and has only very limited projections outside the SCN. A ventrolateral component is also made up of a distinctive neuronal population, receives both direct and indirect visual projections, and provides the major external projections of the SCN, which are to the hypothalamus, particularly the hypophysiotrophic area. The SCN are viewed in this review as containing multiple, mutually coupled oscillating systems that arise from a developmental process of interconnecting individual neuronal circadian oscillators into circuits that form the oscillating systems. A model for the organization of the systems is presented.     

Come together, right...now: synchronization of rhythms in a mammalian circadian clock

In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus act as a dominant circadian pacemaker, coordinating rhythms throughout the body and regulating daily and seasonal changes in physiology and behavior. This review focuses on the mechanisms that mediate synchronization of circadian rhythms between SCN neurons. Understanding how these neurons communicate as a network of circadian oscillators has begun to shed light on the adaptability and dysfunction of the brain's master clock.     

Electrical and pharmacological properties of the suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) of the mammalian hypothalamus are in important circadian pacemaker. The electrical activity of these nuclei exhibits an intrinsic circadian rhythm. The rhythmicity of the SCN is also reflected in cyclic glucose consumption and serotonin metabolism. These rhythms are entrained to the light-dark cycle via the retinohypothalamic projection. This pathway, possibly together with a visual projection via the ventral lateral geniculate nuclei, innervates light-responsive SCN cells, which exhibit the functional properties of luminance detectors. The SCN contain various peptides, acetylcholine, and serotonin either intrinsically or in terminals of afferent projections. For acetylcholine it has been demonstrated that the SCN mediate the process of photic entrainment and light suppression of pineal synthetic activity. In the case of serotonin and vasopressin it seems certain that the SCN do not depend on their presence for generating circadian rhythms or for entrainment. Both substances may modulate the intrinsic pacemaker frequency through mechanisms that remain to be established.     

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.     

Timing and consolidation of human sleep, wakefulness, and performance by a symphony of oscillators

Daily rhythms in sleep and waking performance are generated by the interplay of multiple external and internal oscillators. These include the light-dark and social cycles, a circadian hypothalamic oscillator oscillating virtually independently of behavior, and a homeostatic oscillator driven primarily by sleep-wake behavior. Both internal oscillators contribute to variation in many aspects of sleep and wakefulness (e.g., sleep timing and duration, REM sleep, non-REM sleep, REM density, sleep spindles, slow-wave sleep, electroencephalographic oscillations during wakefulness and sleep, and performance parameters, including attention and memory). The relative contribution of the oscillators varies greatly between these variables. Sleep and performance cannot be predicted by either oscillator independently but critically depend on their phase relationship and amplitude. The homeostatic oscillator feeds back onto the central pacemaker or its outputs. Thus, the amplitude of observed circadian variation in sleep and performance depends on how long we have been asleep or awake. During entrainment to external 24-h cycles, the opposing interplay between circadian and homeostatic changes in sleep propensity consolidates sleep and wakefulness. Some physiological correlates and mediators of both the circadian process (e.g., melatonin and hypocretin rhythms) and the homeostat (e.g., EEG, slow-wave activity, and adenosine release) have been established, offering targets for the development of countermeasures for circadian sleep and performance disorders. Interindividual differences in sleep timing, duration, and morning or evening preference are associated with changes of circadian or sleep homeostatic processes or both. Molecular genetic correlates, including polymorphisms in clock genes, of some of these interindividual differences are emerging.     

Acute exposure to 2G phase shifts the rat circadian timing system

The circadian timing system (CTS) provides internal and external temporal coordination of an animal's physiology and behavior. In mammals, the generation and coordination of these circadian rhythms is controlled by a neural pacemaker, the suprachiasmatic nucleus (SCN), located within the hypothalamus. The pacemaker is synchronized to the 24 hour day by time cues (zeitgebers) such as the light/dark cycle. When an animal is exposed to an environment without time cues, the circadian rhythms maintain internal temporal coordination but exhibit a "free-running" condition in which the period length is determined by the internal pacemaker. Maintenance of internal and external temporal coordination are critical for normal physiological and psychological function in human and non-human primates. Exposure to altered gravitational environments has been shown to affect the amplitude, mean, and timing of circadian rhythms in species ranging from unicellular organisms to man. However, it has not been determined whether altered gravitational fields have a direct effect on the neural pacemaker, or affect peripheral physiological systems that express these circadian parameters. In previous studies, the ability of a stimulus to phase shift circadian rhythms was used to determine whether a stimulus has a direct effect on the neural pacemaker. The present experiment was performed in order to determine whether acute exposure to a hyperdynamic field could phase shift circadian rhythms.     

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

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a major circadian pacemaker that imposes or entrains rhythmicity on other structures by generating a circadian pattern in electrical activity. The identification of "clock genes" within the SCN and the ability to dynamically measure their rhythmicity by using transgenic animals open up new opportunities to study the relationship between molecular rhythmicity and other well-documented rhythms within the SCN. We investigated SCN circadian rhythms in Per1-luc bioluminescence, electrical activity in vitro and in vivo, as well as the behavioral activity of rats exposed to a 6-hr advance in the light-dark cycle followed by constant darkness. The data indicate large and persisting phase advances in Per1-luc bioluminescence rhythmicity, transient phase advances in SCN electrical activity in vitro, and an absence of phase advances in SCN behavioral or electrical activity measured in vivo. Surprisingly, the in vitro phase-advanced electrical rhythm returns to the phase measured in vivo when the SCN remains in situ. Our study indicates that hierarchical levels of organization within the circadian timing system influence SCN output and suggests a strong and unforeseen role of extra-SCN areas in regulating pacemaker function.     

Encoding le quattro stagioni within the mammalian brain: photoperiodic orchestration through the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) is a pacemaker that not only drives circadian rhythmicity but also directs the circadian organization of photoperiodic (seasonal) timekeeping. Recent evidence using electrophysiological, molecular, and genetic tools now strongly supports this conclusion. Important questions remain regarding the SCN's precise role(s) in the brain's photoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesting that SCN "clock" genes may also function as "calendar" genes are a first step toward understanding how a photoperiodic clock is built from cycling molecules.