Attachment site of grafted SCN influences precision of restored circadian rhythm

Blood flows from the SCN toward the OVLT within a new brain vascular portal pathway

The suprachiasmatic nucleus (SCN) sets the phase of oscillation throughout the brain and body. Anatomical evidence reveals a portal system linking the SCN and the organum vasculosum of the lamina terminalis (OVLT), begging the question of the direction of blood flow and the nature of diffusible signals that flow in this specialized vasculature. Using a combination of anatomical and in vivo two-photon imaging approaches, we unequivocally show that blood flows unidirectionally from the SCN to the OVLT, that blood flow rate displays daily oscillations with a higher rate at night than in the day, and that circulating vasopressin can access portal vessels. These findings highlight a previously unknown central nervous system communication pathway, which, like that of the pituitary portal system, could allow neurosecretions to reach nearby target sites in OVLT, avoiding dilution in the systemic blood. In both of these brain portal pathways, the target sites relay signals broadly to both the brain and the rest of the body.

Vasculature of the Suprachiasmatic Nucleus: Pathways for Diffusible Output Signals

Transplant studies demonstrate unequivocally that the suprachiasmatic nucleus (SCN) produces diffusible signals that can sustain circadian locomotor rhythms. There is a vascular portal pathway between the SCN and the organum vasculosum of the lamina terminalis in mouse brain. Portal pathways enable low concentrations of neurosecretions to reach specialized local targets without dilution in the systemic circulation. To explore the SCN vasculature and the capillary vessels whereby SCN neurosecretions might reach portal vessels, we investigated the blood vessels (BVs) of the core and shell SCN. The arterial supply of the SCN differs among animals, and in some animals, there are differences between the 2 sides. The rostral SCN is supplied by branches from either the superior hypophyseal artery (SHpA) or the anterior cerebral artery or the anterior communicating artery. The caudal SCN is consistently supplied by the SHpA. The rostral SCN is drained by the preoptic vein, while the caudal is drained by the basal vein, with variations in laterality of draining vessels. In addition, several key features of the core and shell SCN regions differ: Median BV diameter is significantly smaller in the shell than the core based on confocal image measurements, and a similar trend occurs in iDISCO-cleared tissue. In the cleared tissue, whole BV length density and surface area density are significantly greater in the shell than the core. Finally, capillary length density is also greater in the shell than the core. The results suggest three hypotheses: First, the distinct arterial and venous systems of the rostral and caudal SCN may contribute to the in vivo variations of metabolic and neural activities observed in SCN networks. Second, the dense capillaries of the SCN shell are well positioned to transport blood-borne signals. Finally, variations in SCN vascular supply and drainage may contribute to inter-animal differences.

Identification of the suprachiasmatic nucleus venous portal system in the mammalian brain

There is only one known portal system in the mammalian brain - that of the pituitary gland, first identified in 1933 by Popa and Fielding. Here we describe a second portal pathway in the mouse linking the capillary vessels of the brain's clock suprachiasmatic nucleus (SCN) to those of the organum vasculosum of the lamina terminalis (OVLT), a circumventricular organ. The localized blood vessels of portal pathways enable small amounts of important secretions to reach their specialized targets in high concentrations without dilution in the general circulatory system. These brain clock portal vessels point to an entirely new route and targets for secreted SCN signals, and potentially restructures our understanding of brain communication pathways.

Resilience in the suprachiasmatic nucleus: Implications for aging and Alzheimer's disease

Many believe that the circadian impairments associated with aging and Alzheimer's disease are, simply enough, a byproduct of tissue degeneration within the central pacemaker, the suprachiasmatic nucleus (SCN). However, the findings that have accumulated to date examining the SCNs obtained postmortem from the brains of older individuals, or those diagnosed with Alzheimer's disease upon autopsy, suggest only limited atrophy. We review this literature as well as a complementary one concerning fetal-donor SCN transplant, which established that many circadian timekeeping functions can be maintained with rudimentary (structurally limited) representations of the SCN. Together, these corpora of data suggest that the SCN is a resilient brain region that cannot be directly (or solely) implicated in the behavioral manifestations of circadian disorganization often witnessed during aging as well as early and late progression of Alzheimer's disease. We complete our review by suggesting future directions of research that may bridge this conceptual divide and briefly discuss the implications of it for improving health outcomes in later adulthood.

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

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

Circadian blueprint of metabolic pathways in the brain

The circadian clock is an endogenous, time-tracking system that directs multiple metabolic and physiological functions required for homeostasis. The master or central clock located within the suprachiasmatic nucleus in the hypothalamus governs peripheral clocks present in all systemic tissues, contributing to their alignment and ultimately to temporal coordination of physiology. Accumulating evidence reveals the presence of additional clocks in the brain and suggests the possibility that circadian circuits may feed back to these from the periphery. Here, we highlight recent advances in the communications between clocks and discuss how they relate to circadian physiology and metabolism.

Central Circadian Clock Regulates Energy Metabolism

Our body not only responds to environmental changes but also anticipates them. The light and dark cycle with the period of about 24 h is a recurring environmental change that determines the diurnal variation in food availability and safety from predators in nature. As a result, the circadian clock is evolved in most animals to align locomotor behaviors and energy metabolism with the light cue. The central circadian clock in mammals is located at the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. We here review the molecular and anatomic architecture of the central circadian clock in mammals, describe the experimental and observational evidence that suggests a critical role of the central circadian clock in shaping systemic energy metabolism, and discuss the involvement of endocrine factors, neuropeptides, and the autonomic nervous system in the metabolic functions of the central circadian clock.

Melatonin feedback on clock genes: a theory involving the proteasome

The expression of 'clock' genes occurs in all tissues, but especially in the suprachiasmatic nuclei (SCN) of the hypothalamus, groups of neurons in the brain that regulate circadian rhythms. Melatonin is secreted by the pineal gland in a circadian manner as influenced by the SCN. There is also considerable evidence that melatonin, in turn, acts on the SCN directly influencing the circadian 'clock' mechanisms. The most direct route by which melatonin could reach the SCN would be via the cerebrospinal fluid of the third ventricle. Melatonin could also reach the pars tuberalis (PT) of the pituitary, another melatonin-sensitive tissue, via this route. The major 'clock' genes include the period genes, Per1 and Per2, the cryptochrome genes, Cry1 and Cry2, the clock (circadian locomotor output cycles kaput) gene, and the Bmal1 (aryl hydrocarbon receptor nuclear translocator-like) gene. Clock and Bmal1 heterodimers act on E-box components of the promoters of the Per and Cry genes to stimulate transcription. A negative feedback loop between the cryptochrome proteins and the nucleus allows the Cry and Per proteins to regulate their own transcription. A cycle of ubiquitination and deubiquitination controls the levels of CRY protein degraded by the proteasome and, hence, the amount of protein available for feedback. Thus, it provides a post-translational component to the circadian clock mechanism. BMAL1 also stimulates transcription of REV-ERBα and, in turn, is also partially regulated by negative feedback by REV-ERBα. In the 'black widow' model of transcription, proteasomes destroy transcription factors that are needed only for a particular period of time. In the model proposed herein, the interaction of melatonin and the proteasome is required to adjust the SCN clock to changes in the environmental photoperiod. In particular, we predict that melatonin inhibition of the proteasome interferes with negative feedback loops (CRY/PER and REV-ERBα) on Bmal1 transcription genes in both the SCN and PT. Melatonin inhibition of the proteasome would also tend to stabilize BMAL1 protein itself in the SCN, particularly at night when melatonin is naturally elevated. Melatonin inhibition of the proteasome could account for the effects of melatonin on circadian rhythms associated with molecular timing genes. The interaction of melatonin with the proteasome in the hypothalamus also provides a model for explaining the dramatic 'time of day' effect of melatonin injections on reproductive status of seasonal breeders. Finally, the model predicts that a proteasome inhibitor such as bortezomib would modify circadian rhythms in a manner similar to melatonin.

Circadian clock control of endocrine factors

Organisms experience dramatic fluctuations in demands and stresses over the course of the day. In order to maintain biological processes within physiological boundaries, mechanisms have evolved for anticipation of, and adaptation to, these daily fluctuations. Endocrine factors have an integral role in homeostasis. Not only do circulating levels of various endocrine factors oscillate over the 24 h period, but so too does responsiveness of target tissues to these signals or stimuli. Emerging evidence suggests that these daily endocrine oscillations do not occur solely in response to behavioural fluctuations associated with sleep-wake and feeding-fasting cycles, but are orchestrated by an intrinsic timekeeping mechanism known as the circadian clock. Disruption of circadian clocks by genetic and/or environmental factors seems to precipitate numerous common disorders, including the metabolic syndrome and cancer. Collectively, these observations suggest that strategies designed to realign normal circadian rhythmicities hold potential for the treatment of various endocrine-related disorders.

Twelve-hour days in the brain and behavior of split hamsters

Hamsters will spontaneously 'split' and exhibit two rest-activity cycles each day when housed in constant light (LL). The suprachiasmatic nucleus (SCN) is the locus of a brain clock organizing circadian rhythmicity. In split hamsters, the right and left SCN oscillate 12 h out of phase with each other, and the twice-daily locomotor bouts alternately correspond to one or the other. This unique configuration of the circadian system is useful for investigation of SCN communication to efferent targets. To track phase and period in the SCN and its targets, we measured wheel-running and FOS expression in the brains of split and unsplit hamsters housed in LL or light-dark cycles. The amount and duration of activity before splitting were correlated with latency to split, suggesting behavioral feedback to circadian organization. LL induced a robust rhythm in the SCN core, regardless of splitting. The split hamsters' SCN exhibited 24-h rhythms of FOS that cycled in antiphase between left and right sides and between core and shell subregions. In contrast, the medial preoptic area, paraventricular nucleus of the hypothalamus, dorsomedial hypothalamus and orexin-A neurons all exhibited 12-h rhythms of FOS expression, in-phase between hemispheres, with some detectable right-left differences in amplitude. Importantly, in all conditions studied, the onset of FOS expression in targets occurred at a common phase reference point of the SCN oscillation, suggesting that each SCN may signal these targets once daily. Finally, the transduction of 24-h SCN rhythms to 12-h extra-SCN rhythms indicates that each SCN signals both ipsilateral and contralateral targets.

The regulation of brain states by neuroactive substances distributed via the cerebrospinal fluid; a review

The cerebrospinal fluid (CSF) system provides nutrients to and removes waste products from the brain. Recent findings suggest, however, that in addition, the CSF contains message molecules in the form of actively released neuroactive substances. The concentrations of these vary between locations, suggesting they are important for the changes in brain activity that underlie different brain states, and induce different sensory input and behavioral output relationships.The cranial CSF displays a rapid caudally-directed ventricular flow followed by a slower rostrally-directed subarachnoid flow (mainly towards the cribriform plate and from there into the nasal lymphatics). Thus, many brain areas are exposed to and can be influenced by substances contained in the CSF. In this review we discuss the production and flow of the CSF, including the mechanisms involved in the regulation of its composition. In addition, the available evidence for the release of neuropeptides and other neuroactive substances into the CSF is reviewed, with particular attention to the selective effects of these on distant downstream receptive brain areas. As a conclusion we suggest that (1) the flowing CSF is involved in more than just nutrient and waste control, but is also used as a broadcasting system consisting of coordinated messages to a variety of nearby and distant brain areas; (2) this special form of volume transmission underlies changes in behavioral states.

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."

A role for cardiotrophin-like cytokine in the circadian control of mammalian locomotor activity

The suprachiasmatic nucleus (SCN) drives circadian rhythms of locomotor behavior by releasing factors that act on receptor sites near the third ventricle. Here we show that cardiotrophin-like cytokine (CLC) satisfies multiple criteria for a circadian regulator of locomotor activity. In the mouse, CLC is expressed in a subpopulation of SCN vasopressin neurons with a circadian rhythm that peaks during the daily period of locomotor quiescence. CLC receptors flank the third ventricle, and acute infusion of CLC into the third ventricle produced a transient blockade of locomotor activity without affecting the circadian clock. The hypothalamic infusion of neutralizing antibodies to the CLC receptor produced extra daily locomotor activity at the time when CLC is maximally expressed. These results suggest that CLC is probably an SCN output signal important for shaping daily rhythms of behavior; moreover, they indicate an unexpected role for a cytokine in adult brain function.

A Screen for Secreted Factors of the Suprachiasmatic Nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) drives daily locomotor activity rhythms presumably by secreting diffusible factors whose target sites are accessible from the third ventricle of the hypothalamus. This article describes the methodology of a systematic molecular and behavioral screen to identify "locomotor factors" of the SCN. To find SCN-secreted factors not previously documented, a hamster SCN cDNA library was screened in a yeast signal sequence trap. In a subsequent behavioral screen, newly identified and previously documented SCN factors were tested for an effect on locomotor activity rhythms by chronic infusion into the third ventricle of hamsters. Using this approach combined with further experiments, we identified transforming growth factor-alpha (TGF-alpha) as a likely SCN inhibitor of locomotion.

Peripheral circadian oscillators in mammals: time and food

Peripheral cells from mammalian tissues, while perfectly capable of circadian rhythm generation, are not light sensitive and thus have to be entrained by nonphotic cues. Feeding time is the dominant zeitgeber for peripheral mammalian clocks: Daytime feeding of nocturnal laboratory rodents completely inverts the phase of circadian gene expression in many tissues, including liver, heart, kidney, and pancreas, but it has no effect on the SCN pacemaker. It is thus plausible that in intact animals, the SCN synchronizes peripheral docks primarily through temporal feeding patterns that are imposed through behavioral rest-activity cycles. In addition, body temperature rhythms, which are themselves dependent on both feeding patterns and rest-activity cycles, can sustain circadian, clock gene activity in vivo and in vitro. The SCN may also influence the phase of rhythmic gene expression in peripheral tissues through direct chemical pathways. In fact, many chemical signals induce circadian gene expression in tissue culture cells. Some of these have been shown to elicit phase shifts when injected into intact animals and are thus candidates for physiologically relevant timing cues. While the response of the SCN to light is strictly gated to respond only during the night, peripheral oscillators can be chemically phase shifted throughout the day. For example, injection of dexamethasone, a glucocorticoid receptor agonist, resets the phase of circadian liver gene expression during the entire 24-h day. Given the bewildering array of agents capable of influencing peripheral clocks, the identification of physiologically relevant agents used by the SCN to synchronize peripheral clocks will clearly be an arduous undertaking. Nevertheless, we feel that experimental systems by which this enticing problem can be tackled are now at hand.

Ectopic transplantation of the accessory medulla restores circadian locomotor rhythms in arrhythmic cockroaches (Leucophaea maderae)

The presence of an endogenous circadian clock in the brain of an animal was first demonstrated in the cockroach Leucophaea maderae. However, the clock's cellular basis remained elusive until pigment-dispersing hormone-immunoreactive neurons, which express the clock genes period and timeless in Drosophila, were proposed as pacemaker candidates. In several insect species, pigment-dispersing hormone-immunoreactive neurons are closely associated with the accessory medulla, a small neuropil in the optic lobe, which was suggested to be a circadian clock neuropil. Here, we demonstrate that ectopic transplantation of adult accessory medulla into optic lobe-less cockroaches restores circadian locomotor activity rhythms in L. maderae. All histologically examined cockroaches that regained circadian activity regenerated pigment-dispersing hormone-immunoreactive fibres from the grafts to original targets in the protocerebrum. The data show that the accessory medulla is the circadian pacemaker controlling locomotor activity rhythms in the cockroach. Whether pigment-dispersing hormone-immunoreactive neurons are the only circadian pacemaker cells controlling locomotor activity rhythms remains to be examined.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Studies of cerebrospinal fluid flow and penetration into brain following lateral ventricle and cisterna magna injections of the tracer [14C]inulin in rat

Parasynaptic communication, also termed volume transmission, has been suggested as an important means to mediate information transfer within the central nervous system. The purpose of the present study was to visualize by autoradiography the available channels for fluid movement within the extracellular space following injection of the inert extracellular marker [14C]inulin into the lateral ventricle or cisterna magna. Bolus injections of 5 microl of 1 microCi of [14C]inulin were made in awake rats via chronically implanted cannulae. After survival times ranging from 5 min to 4 h, brains were processed for in vivo autoradiography. At 5 min the tracer distributed throughout the ventricles, subarachnoid spaces and cisterns "downstream" of the injection sites. Penetration into the brain from these sites was complex with preferential entry along the ventral side of the brain, especially into the hypothalamus and brainstem. By 4 h virtually the entire brain was labeled irrespective of the site of tracer application. Sustained tracer entry from subarachnoid spaces suggests that some areas act as depots to trap circulating material. This mechanism may contribute to the pattern of deep penetration at later time-points. The spatial and temporal characteristics of fluid movement throughout the brain are instructive in the interpretation of many experimental procedures involving injection of molecules into the cerebrospinal fluid.

Output signals of the SCN

The suprachiasmatic nucleus (SCN) of the hypothalamus controls circadian rhythmicity in mammals (for reviews, see Refs. 33 and 59). Responses modulated by the SCN are numerous and include rhythms in sleep/wake cycles, locomotor, gnawing and general activity, temperature, ingestive behavior, and rhythms of hormonal and peptide secretions. Though a great deal is known about the neuroanatomical organization of the SCN, many elements of the structure-function relationships remain to be discovered. For example, it is not known which cellular components of the SCN function as driving pacemakers or which output signal(s) of these pacemakers are important for each of its functions. While some signals from pacemaker cells reach target regions by neural efferents, there is also evidence that rhythmic responses can be controlled by diffusible signals. This article reviews output signals from the SCN. The data available suggest that neural efferents are not necessary for the control of locomotor activity rhythms. Evidence that a diffusible signal is sufficient to restore activity rhythms in SCN-lesioned animals is described. Finally, possible physiological mechanisms for diffusible signals are suggested.