Circadian rhythmicity after neural transplant to hamster third ventricle: specificity of suprachiasmatic nuclei

The Suprachiasmatic Nucleus at 50: Looking Back, Then Looking Forward

It has been 50 years since the suprachiasmatic nucleus (SCN) was first identified as the central circadian clock and 25 years since the last overview of developments in the field was published in the Journal of Biological Rhythms. Here, we explore new mechanisms and concepts that have emerged in the subsequent 25 years. Since 1997, methodological developments, such as luminescent and fluorescent reporter techniques, have revealed intricate relationships between cellular and network-level mechanisms. In particular, specific neuropeptides such as arginine vasopressin, vasoactive intestinal peptide, and gastrin-releasing peptide have been identified as key players in the synchronization of cellular circadian rhythms within the SCN. The discovery of multiple oscillators governing behavioral and physiological rhythms has significantly advanced our understanding of the circadian clock. The interaction between neurons and glial cells has been found to play a crucial role in regulating these circadian rhythms within the SCN. Furthermore, the properties of the SCN network vary across ontogenetic stages. The application of cell type-specific genetic manipulations has revealed components of the functional input-output system of the SCN and their correlation with physiological functions. This review concludes with the high-risk effort of identifying open questions and challenges that lie ahead.

Assessing Sex-Specific Circadian, Metabolic, and Cognitive Phenotypes in the AβPP/PS1 and APPNL-F/NL-F Models of Alzheimer's Disease

BACKGROUND

Circadian disruption has long been recognized as a symptom of Alzheimer's disease (AD); however, emerging data suggests that circadian dysfunction occurs early on in disease development, potentially preceding any noticeable cognitive deficits.

OBJECTIVE

This study compares the onset of AD in male and female wild type (C57BL6/J), transgenic (AβPP/PS1), and knock-in (APPNL-F/NL-F) AD mouse models from the period of plaque initiation (6 months) through 12 months.

METHODS

Rhythmic daily activity patterns, glucose sensitivity, cognitive function (Morris water maze, MWM), and AD pathology (plaques formation) were assessed. A comparison was made across sexes.

RESULTS

Sex-dependent hyperactivity in AβPP/PS1 mice was observed. In comparison to C57BL/6J animals, 6-month-old male AβPP/PS1 demonstrated nighttime hyperactivity, as did 12-month-old females. Female AβPP/PS1 animals performed significantly worse on a MWM task than AβPP/PS1 males at 12 months and trended toward increased plaque pathology. APPNL-F/NL-F 12-month-old males performed significantly worse on the MWM task compared to 12-month-old females. Significantly greater plaque pathology occurred in AβPP/PS1 animals as compared to APPNL-F/NL-F animals. Female AβPP/PS1 animals performed significantly worse than APPNL-F/NL-F animals in spatial learning and memory tasks, though this was reversed in males.

CONCLUSION

Taken together, this study provides novel insights into baseline sex differences, as well as characterizes baseline diurnal activity variations, in the AβPP/PS1 and APPNL-F/NL-F AD mouse models.

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.

Sleep and Alzheimer's disease: A pivotal role for the suprachiasmatic nucleus

Alzheimer's disease (AD), which accounts for most of the dementia cases, is, aside from cognitive deterioration, often characterized by the presence of non-cognitive symptoms. Society is desperately in need for interventions that alleviate the economic and social burden related to AD. Circadian dysrhythmia, one of these symptoms in particular, immensely decreases the self-care ability of AD patients and is one of the main reasons of caregiver exhaustion. Studies suggest that these circadian disturbances form the root of sleep-wake problems, diagnosed in more than half of AD patients. Sleep abnormalities have generally been considered merely a consequence of AD pathology. Recent evidence suggests that a bidirectional relationship exists between sleep and AD, and that poor sleep might negatively impact amyloid burden, as well as cognition. The suprachiasmatic nucleus (SCN), the main circadian pacemaker, is subjected to several alterations during the course of the disease. Its functional deterioration might fulfill a crucial role in the relation between AD pathophysiology and the development of sleep abnormalities. This review aims to give a concise overview of the anatomy and physiology of the SCN, address how AD pathology precisely impacts the SCN and to what degree these alterations can contribute to the progression of the disease.

How do Fetal Grafts of the Suprachiasmatic Nucleus Communicate with the Host Brain?

Fetal grafts containing the hypothalamic suprachiasmatic nucleus (SCN), the site of an endogenous circadian pacemaker, can reinstate behavioral rhythms in lesioned recipients but the precise routes of communication between the graft and the host brain remain unknown. Grafts containing the SCN may convey temporal information to the host brain via neural efferents, diffusible factors, or a combination of both. We examined graft-host connections in anterior hypothalamic homografts (hamster-to hamster) and heterografts (rat-to hamster) implanted in the third ventricle by: (a) applying the carbocyanine dye, dil, directly onto homo- and heterografts in fixed tissue sections; and (b) using a donor-specific neurofilament (NF) antibody to immuno-cytochemically visualize heterograft efferents. Dil applied onto either homografts or heterografts labeled relatively few graft efferents which could be followed only short distances into the host brain. In contrast, NF-labeled heterograft efferents were both more numerous and extended for longer distances into the host brain than anticipated on the basis of dil tract tracing. The results suggest that anterior hypothalamic grafts implanted in the third ventricle provide substantial input to the adjacent host hypothalamus although it is not known whether these projections arise from SCN cells or from other extra-SCN hypothalamic tissue within these grafts. Nor is it known whether these projections are functional. To determine if neural efferents are required for the restoration of rhythmicity after grafting, we have encapsulated fetal anterior hypothalamus in a permselective polymer which prevents neurite outgrowth but allows diffusible signals to reach the host brain. Polymer-encapsulated grafts of fetal anterior hypothalamus from wild-type hamster fetuses have been implanted into the third ventricle of heterozygote tau mutant, SCN-lesioned hamsters. Because the free-running period of tau mutant hamsters is significantly shorter than that of wild-type hamsters, restored rhythms when they occur can be unambiguously attributed to the presence of donor tissue. Encapsulated grafts that survive contain neuropeptide cell markers characteristic of the intact SCN, but the survival rate of encapsulated neural tissue is low. Nevertheless, if we find that even a few encapsulated grafts restore donor-specific rhythms, this would suggest that diffusible signals emitted from SCN grafts may be sufficient to support circadian function. It may be that the SCN in the intact animal communicates with the rest of the brain by redundant signals, either efferent fibers or diffusible signals. Alternatively, different circadian rhythms may be mediated by distinct output signals from the SCN.

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.

Circadian rhythms, the molecular clock, and skeletal muscle

Almost all organisms ranging from single cell bacteria to humans exhibit a variety of behavioral, physiological, and biochemical rhythms. In mammals, circadian rhythms control the timing of many physiological processes over a 24-h period, including sleep-wake cycles, body temperature, feeding, and hormone production. This body of research has led to defined characteristics of circadian rhythms based on period length, phase, and amplitude. Underlying circadian behaviors is a molecular clock mechanism found in most, if not all, cell types including skeletal muscle. The mammalian molecular clock is a complex of multiple oscillating networks that are regulated through transcriptional mechanisms, timed protein turnover, and input from small molecules. At this time, very little is known about circadian aspects of skeletal muscle function/metabolism but some progress has been made on understanding the molecular clock in skeletal muscle. The goal of this chapter is to provide the basic terminology and concepts of circadian rhythms with a more detailed review of the current state of knowledge of the molecular clock, with reference to what is known in skeletal muscle. Research has demonstrated that the molecular clock is active in skeletal muscles and that the muscle-specific transcription factor, MyoD, is a direct target of the molecular clock. Skeletal muscle of clock-compromised mice, Bmal1(-/-) and Clock(Δ19) mice, are weak and exhibit significant disruptions in expression of many genes required for adult muscle structure and metabolism. We suggest that the interaction between the molecular clock, MyoD, and metabolic factors, such as PGC-1, provide a potential system of feedback loops that may be critical for both maintenance and adaptation of skeletal muscle.

Changes in vasoactive intestinal peptide and arginine vasopressin expression in the suprachiasmatic nucleus of the rat brain following footshock stress

The neuropeptides, arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are synthesized by neurons of the suprachiasmatic nucleus (SCN) of the hypothalamus and are important regulators of SCN function. Previous studies have demonstrated that acute exposure to stressors can disrupt circadian activity rhythms, suggesting the possibility of stress-related alterations in the expression of these neuropeptides within SCN neurons. In this study, we examined the effect of intermittent footshock stress on AVP mRNA and heterogeneous nuclear RNA (hnRNA) and VIP mRNA expression in neurons of the SCN. Young adult male Sprague/Dawley rats were subjected to 15 s of scrambled intermittent footshock (0.50 mA pulses, 1 pulse/s, 300 ms duration) every 5 min for 30 min. Animals were sacrificed 75 or 135 min after the onset of stress and brains examined for AVP mRNA and hnRNA, and VIP mRNA using in situ hybridization. Footshock stress increased AVP hnRNA levels at the 75 min time point whereas AVP mRNA was elevated at both the 75 and 135 min time points. In contrast, footshock stress decreased the number of cells expressing VIP mRNA in the SCN without changing hybridization level per cell. These data indicate that the disruptive effect of stress on activity rhythms correlate with alterations in the expression of regulatory peptides within the SCN.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Biotinylated dextran amine as a marker for fetal hypothalamic homografts and their efferents

We have explored the use of biotinylated dextran amine (BDA) as a marker for labeling fetal brain grafts and their connections with the host. As a model system we used transplantation of the hamster suprachiasmatic nucleus, the site of an endogenous biological clock governing circadian rhythms. Similar transplants into arrhythmic hosts have been shown to restore behavioral function with a period specific to the donor. For locomotor rhythms, efferent connections are not necessary. For other responses, including endocrine rhythms, efferent connections may be necessary. In order to visualize homografts and their efferents, injections of BDA, an anterograde tracer, were made into the anterior hypothalamic (AH) region containing the SCN or into the dorsal cortex (CTX) of fetal hamster brains. The fetal AH or CTX was microdissected out and stereotaxically implanted into the third ventricle of intact, adult hamsters. After 2, 4, 8, or 12 weeks, hosts were sacrificed and their brains were processed for detection of BDA by either histochemistry or immunofluorescence. BDA intensely labeled graft neurons, their dendrites, and axons with minimal or no spread to the adjacent host brain. Labeled graft axons could be followed for long distances (>1 mm) into the host brain and graft-derived varicosities formed close contacts with host neurons. BDA-labeled graft neurons, located at the perimeter of the graft, also extended dendrite-like processes into the host parenchyma. We conclude that BDA is a useful marker for fetal homografts and their efferents for survival times of less than 2 months.

NPAS2: an analog of clock operative in the mammalian forebrain

Neuronal PAS domain protein 2 (NPAS2) is a transcription factor expressed primarily in the mammalian forebrain. NPAS2 is highly related in primary amino acid sequence to Clock, a transcription factor expressed in the suprachiasmatic nucleus that heterodimerizes with BMAL1 and regulates circadian rhythm. To investigate the biological role of NPAS2, we prepared a neuroblastoma cell line capable of conditional induction of the NPAS2:BMAL1 heterodimer and identified putative target genes by representational difference analysis, DNA microarrays, and Northern blotting. Coinduction of NPAS2 and BMAL1 activated transcription of the endogenous Per1, Per2, and Cry1 genes, which encode negatively activating components of the circadian regulatory apparatus, and repressed transcription of the endogenous BMAL1 gene. Analysis of the frontal cortex of wild-type mice kept in a 24-hour light-dark cycle revealed that Per1, Per2, and Cry1 mRNA levels were elevated during darkness and reduced during light, whereas BMAL1 mRNA displayed the opposite pattern. In situ hybridization assays of mice kept in constant darkness revealed that Per2 mRNA abundance did not oscillate as a function of the circadian cycle in NPAS2-deficient mice. Thus, NPAS2 likely functions as part of a molecular clock operative in the mammalian forebrain.

Adenoviral Vector-Mediated Expression of Neurotrophin-3 Increases Neuronal Survival in Suprachiasmatic Nucleus Grafts

To improve transplantation results of fetal suprachiasmatic nucleus (SCN) in SCN-lesioned (SCNX) rats, grafts were ex vivo transduced with an adenoviral vector encoding for neurotrophin-3 (AdNT-3) before implantation. Mock- and AdLacZ-transduced grafts were used as controls. First, transplants were evaluated microscopically and by image analysis for the presence of vasopressinergic (VPergic) and vasoactive intestinal polypeptidergic (VIPergic) SCN neurons at 10 weeks or later postgrafting. Ex vivo AdNT-3-transduced transplants displayed increased volume areas of VPergic and VIPergic SCN cells in comparison with those in mock- and AdLacZ-transduced transplants, but significantly improved graft-to-host VPergic and VIPergic SCN fiber growth was not reached (though AdNT-3-transduced transplants tended to grow more VPergic fibers into the brain of VP-deficient SCNX Brattleboro rat recipients, which were chosen as recipients to circumvent the presence of non-SCN VP fiber staining). Second, a small group of arrhythmic Wistar rats received AdNT-3- or control-treated SCN grafts while continuously on-line for the monitoring of overt circadian activities in the pre- and postgrafting periods. The results indicated that ex vivo transduced SCN grafts can still restore arrhythmia, but that the NT-3-mediated anatomical improvements of the grafting results were not sufficient to enhance efficacy of reinstatement of circadian rhythm in SCN-lesioned rats. However, in this group VIP staining volume area, not VP staining volume area, correlated significantly with reinstatement of circadian rhythm.

Chimera analysis of the Clock mutation in mice shows that complex cellular integration determines circadian behavior

The Clock mutation lengthens periodicity and reduces amplitude of circadian rhythms in mice. The effects of Clock are cell intrinsic and can be observed at the level of single neurons in the suprachiasmatic nucleus. To address how cells of contrasting genotype functionally interact in vivo to control circadian behavior, we have analyzed a series of Clock mutant mouse aggregation chimeras. Circadian behavior in Clock/Clock <--> wild-type chimeric individuals was determined by the proportion of mutant versus normal cells. Significantly, a number of intermediate phenotypes, including Clock/+ phenocopies and novel combinations of the parental behavioral characteristics, were seen in balanced chimeras. Multivariate statistical techniques were used to quantitatively analyze relationships among circadian period, amplitude, and suprachiasmatic nucleus composition. Together, our results demonstrate that complex integration of cellular phenotypes determines the generation and expression of coherent circadian rhythms at the organismal level.

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.

Circadian variation of cell proliferation and cell cycle protein expression in man: clinical implications

Most physiological, biochemical and behavioural processes have been shown to vary in a regular and predictable periodic manner with respect to time. This review focuses on the circadian rhythm in cell proliferation in bone marrow and gut and how this is associated with a circadian expression of cell cycle proteins in human oral mucosa. The control of circadian rhythms by the suprachiasmatic nuclei and the evolving understanding of the genetic and molecular biology of the circadian clock is outlined. Finally, the potential clinical impact of chronobiology in cancer medicine is discussed.

Vasopressin-deficient suprachiasmatic nucleus grafts re-instate circadian rhythmicity in suprachiasmatic nucleus-lesioned arrhythmic rats

It was investigated whether grafts of the suprachiasmatic nucleus could re-instate circadian rhythmicity in the absence of its endogenous vasopressin production and whether the restored rhythm would have the long period length of the donor. Grafts of 17-days-old vasopressin-deficient homozygous Brattleboro rat fetuses, homotopically placed in arrhythmic suprachiasmatic nucleus-lesioned Wistar rats, re-instated circadian drinking rhythm within 20-50 days similar as seen for grafts of heterozygous control fetuses. Period length of the recovered rhythm revealed a similar difference (average 24.3 vs. 23.8 h) as reported for the rhythm between the adult Brattleboro genotypes. In all transplants, also those of the two-third non-recovery rats, a surviving suprachiasmatic nucleus was visible as a vasoactive intestinal polypeptide-positive neuronal cell cluster, whereas heterozygous transplants also revealed the complementary vasopressinergic cell part. Explanation of the absence of recovery failed since no undisputable correlation emerged between recovery of rhythm and vasoactive intestinal polypeptide, vasopressin and/or somatostatin immunocytochemical characteristics of the suprachiasmatic nucleus of the transplant. Special focus on the somatostatinergic neurons revealed their presence only occasionally near or in between the vasoactive intestinal polypeptidergic and (in the case of heterozygous grafts) vasopressinergic cell cluster. However their aberrant cytoarchitectural position appeared not to have affected the possibility to restore drinking rhythm of the suprachiasmatic nucleus-lesioned arrhythmic rat. It was concluded that grafted Brattleboro fetal suprachiasmatic nucleus develop their intrinsic rhythm conform their genotype and that vasopressin is not a crucial component in the maintenance nor in the transfer of circadian activity of the biological clock for drinking activity. Vasopressin of the suprachiasmatic nucleus may instead serve modulation within the circadian system.

Circadian rhythmicity of vasopressin levels in the cerebrospinal fluid of suprachiasmatic nucleus-lesioned and -grafted rats

Transplantation of the fetal suprachiasmatic nucleus (SCN) in arrhythmic SCN-lesioned rats can reinstate circadian drinking rhythms in 40% to 50% of the cases. In the current article, it was investigated whether the failure in the other rats could be due to the absence of a circadian rhythm in the grafted SCN, using a circadian vasopressin (VP) rhythm in the cerebrospinal fluid (CSF) as the indicator for a rhythmic SCN. CSF was sampled in continuous darkness from-intact control rats and SCN-lesioned and -grafted rats. VP could be detected in all samples, with concentrations of 15 to 30 pg/ml in the control rats and 5 to 15 pg/ml in the grafted rats. A circadian VP rhythm with a two- to threefold difference between peak and nadir values was found in all 7 control rats but in only 4 of 13 experimental rats, despite the presence of a VP-positive SCN in all grafts. A circadian VP rhythm was present in 2 drinking rhythm-recovered rats (6 of 13) and in 2 nonrecovery rats. Apparently, in these latter rats, the failure of the grafted SCN to restore a circadian drinking rhythm cannot be attributed to a lack of rhythmicity in the SCN itself. Thus, the presence of a rhythmic grafted SCN, as is deduced from a circadian CSF VP rhythm, appears not to be sufficient for restoration of a circadian drinking rhythm in SCN-lesioned arrhythmic rats.

Effects of suprachiasmatic transplants on circadian rhythms of neuroendocrine function in golden hamsters

Grafts of fetal tissue including the suprachiasmatic nucleus (SCN) of the hypothalamus restore locomotor rhythmicity to behaviorally arrhythmic, SCN-lesioned Syrian hamsters. We sought to determine whether such transplants also reinstate endocrine rhythms in SCN-lesioned hamsters. In Exp 1, SCN lesions interrupted estrous cycles in a 14 h light, 10 h dark photoperiod and locomotor rhythms in constant dim red light (DD). SCN grafts that reinstated behavioral circadian rhythms consistently failed to reestablish estrous cycles. After ovariectomy, estradiol implants triggered LH surges at approximately circadian time 8 in 10 of 12 brain-intact control females and 0 of 9 SCN-lesioned, grafted females. Daily rhythms of the principal urinary melatonin metabolite, 6alpha-sulfatoxymelatonin, were not reestablished by behaviorally functional grafts. In Exp 2, SCN lesions eliminated locomotor rhythmicity in adult male hamsters maintained in DD. Seven to 12 weeks after restoration of locomotor activity rhythms by fetal grafts, hosts and sham-lesioned controls were decapitated at circadian times 4, 8, 12, 16, 20, or 24. Clear circadian rhythms of both serum corticosterone and cortisol were seen in sham-lesioned males, with peaks in late subjective day. No circadian rhythms in either adrenal hormone were evident in serum from lesioned-grafted males. Testicular regression, observed in intact and sham-lesioned males maintained in DD, was absent not only in arrhythmic SCN-lesioned hamsters given grafts of cerebral cortex, but also in animals in which hypothalamic grafts had reinstated locomotor rhythmicity. The pineal melatonin concentration rose sharply during the late subjective night in control hamsters, but not in SCN-lesioned animals bearing behaviorally effective transplants. Even though circadian rhythms of locomotor activity are restored by SCN transplants, circadian endocrine rhythms are not reestablished. Endocrine rhythms may require qualitatively different or more extensive SCN outputs than those established by fetal grafts.

Cellular requirements of suprachiasmatic nucleus transplants for restoration of circadian rhythm

Fetal neurografts containing the suprachiasmatic nucleus (SCN) can restore the circadian locomotor and drinking rhythm of SCN-lesioned (SCNX) rat and hamster. This functional outcome finally proves that the endogenous biological clock autonomously resides in the SCN. Observations on the cellular requirements of the "new" SCN for restoration of the arrhythmic SCNX animals have led to some new insights and confirmed findings from other studies. A critical mass of SCN neurons appeared necessary for functional effects, whereas the temporal profile of reinstatement of rhythm correlated with the delayed maturation of the grafted SCN. Cytoarchitectonically, the grafted SCN does not seem to develop normally for all anatomical aspects. Complementary clusters of vasoactive intestinal polypeptide(VIP)- and vasopressin(VP)ergic neurons are formed, but somatostatin(SOM)ergic neurons do not always "join" this group, as is normally seen in situ. Nevertheless, these new SCNs can restore the ablated functions. As the period length of restored rhythms tends to vary, it might be that the grafted SCN underwent an altered or impaired maturation that resulted in a different setting of its clock mechanism. A prominent role of VIPergic neurons seems indicated by their presence in all functional grafts, but, although they may be required, these cells do not appear to be a sufficient condition for restoration of rhythm. Many grafts exhibit the presence of VIPergic cells without counteracting the arrhythmia, whereas VP- and SOMergic SCN neurons are usually present as well. Findings with VP-deficient Brattleboro rat grafts indicated that VP is not the primary obligatory signal of circadian activity. It is argued that perhaps the role of SOMergic neurons in the clock function of the (grafted) SCN has been insufficiently considered. However, one should keep in mind that the peptides of the various types of SCN neurons may function only as cofactors, mutually modulating molecular or bioelectrical cellular activities within the nucleus or the message of the main transmitter gamma-aminobutyric acid.

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.

Restoration of circadian behavior by anterior hypothalamic grafts containing the suprachiasmatic nucleus: graft/host interconnections

Destruction of the hypothalamic suprachiasmatic nucleus (SCN) disrupts circadian behavior. Transplanting SCN tissue from fetal donors into SCN-lesioned recipients can restore circadian behavior to the arrhythmic hosts. In the transplantation model employing fetal hamster donors and SCN-lesioned hamsters as hosts, the period of the restored circadian behavior is hamster-typical. However, when fetal rat anterior hypothalamic tissue containing the SCN is implanted into SCN-lesioned rats, the period of the restored circadian rhythm is only rarely typical of that of the intact rat. The use of an anterior hypothalamic heterograft model provides new approaches to donor specificity of restored circadian behavior and with the aid of species-specific markers, provides a means for assessing connectivity between the graft and the host. Using an antibody that stains rat and mouse neuronal tissue but not hamster neurons, it has been demonstrated that rat and mouse anterior hypothalamic heterografts containing the SCN send numerous processes into the host (hamster) neuropil surrounding the graft, consistent with graft efferents reported in other hypothalamic transplantation models in which graft and host tissue can be differentiated (i.e., Brattleboro rat and hypogonadal mouse). Moreover, SCN neurons within anterior hypothalamic grafts send an appropriately restricted set of efferent projections to the host brain which may participate in the functional recovery of circadian locomotor activity.

Behavior of SCN-lesioned chipmunks in natural habitat: a pilot study

The necessity of a circadian pacemaker, the suprachiasmatic nuclei (SCN), for survival was evaluated in a population of approximately 65 wild eastern chipmunks, Tamias striatus. The research involved over 3000 h of field-work between May 1995 and October 1997 on a study site at Mountain Lake Biological Station, Virginia. The 28 chipmunks randomly designated as project animals included 10 SCN-lesioned chipmunks, 5 surgical controls (sham-lesioned), and 13 intact controls. Visual observation, live trapping, and radio telemetric tracking were used to assess 6 aspects of survival and reproduction. Upon release after surgery, every animal returned to its den site and was able to maintain its home territory. In warm months from May through October, all chipmunks were active above ground with a strictly day-active pattern. During the remaining cold months, they were normothermic but relatively torpid in their underground dens for extended periods of time. Short-term mortality for the initial 3 months included only a single intact control chipmunk; loss for the extended period from August 1995 to October 1996 was 40% for the SCN-lesioned animals, 0% for surgical controls, and 15.4% for the intact controls. Survival differences were not significant between surgical control and intact control groups but were significant (alpha = .10) between SCN-lesioned and pooled control groups. Annual body weight patterns were similar for both groups. Most individuals in both SCN-lesioned and control groups were reproductively active in the spring and fall breeding periods. Lack of major differences may be attributable to the exceptionally favorable conditions for survival such as a very abundant fall acorn crop, medium population size, and absence of heavy predation.

The suprachiasmatic nucleus: a 25-year retrospective

The suprachiasmatic nuclei (SCN) of the anterior hypothalamus contain the master circadian pacemaker in mammals. On the occasion of the 25th anniversary of the discovery of the SCN as the circadian clock, Charles A. Czeisler and Steven M. Reppert organized a meeting to review milestones and recent developments in the study of the SCN. The discovery that the SCN contain tissue necessary for generation of circadian rhythmicity was established by lesion studies published in 1972. The second phase of study demonstrated unequivocally that the SCN contain an autonomous circadian pacemaker. The principal studies in this period showed the presence of metabolic and electrical activity rhythms in the SCN in vivo and progressed to studies showing that the SCN maintain rhythmicity in vitro, demonstrating that the transplanted SCN can restore circadian function following destruction of the host SCN and ultimately showing that single SCN "clock cells" exhibit independent rhythms in firing rate. The third phase of study, aimed at identifying the biochemical and molecular mechanisms responsible for rhythmicity within the SCN, has begun with the identification of circadian mutants (tau mutant hamsters and Clock mutant mice) and the isolation of the Clock gene. This report traces the important steps forward in our understanding of the suprachiasmatic circadian clock by recounting the information presented at the SCN Silver Anniversary Celebration.

Fiber outgrowth from anterior hypothalamic and cortical xenografts in the third ventricle

Fetal grafts of the anterior hypothalamus (SCN/AH) containing the suprachiasmatic nucleus (SCN) restore circadian rhythms to SCN-lesioned host hamsters and rats following implantation into the third ventricle. Previous studies suggest that intraventricular SCN/AH grafts are variable in their attachment sites, the extent of their outgrowth, and the precise targets innervated in the host brain. However, the use of different methods to analyze graft outgrowth in this model has previously led to inconsistent results. We have reevaluated the outgrowth of fetal rat SCN/AH grafts implanted in the third ventricle of hamsters by using two methods: the carbocyanine dye, 1,1'dioctadecyl-3,3'-tetramethylindocarbocyanine percholate (DiI), was placed directly onto grafted tissue; and a donor-specific neurofilament marker was used in conjunction with xenografts. We examined the specificity of outgrowth by comparing SCN/AH xenografts with that of control cortical (CTX) xenografts. To evaluate whether SCN/AH graft efferents arise from the donor SCN, we used micropunch grafts that contained minimal extra-SCN tissue. The results show that the use of a donor-specific neurofilament marker reveals more extensive SCN/AH graft outgrowth than DiI. SCN/AH graft efferents project into areas normally innervated by the intact SCN. However, this outgrowth is variable among graft recipients, is not specific to SCN/AH tissue, and does not necessarily derive from the donor SCN. The precise functional role of neural efferents arising from SCN/AH grafts in the restoration of circadian clock function and the extent of SCN-derived efferents remain to be determined.

Fetal grafts containing suprachiasmatic nuclei restore the diurnal rhythm of CRH and POMC mRNA in aging rats

We assessed whether fetal tissue containing the suprachiasmatic nuclei (SCN) can restore age-related changes in the diurnal rhythm of hypothalamic corticotropin-releasing hormone (CRH) and anterior pituitary proopiomelanocortin (POMC) mRNA. Young, middle-aged, and middle-aged SCN-transplanted rats were killed at seven times of day. In young rats, CRH mRNA exhibited a diurnal rhythm in the dorsomedial paraventricular nuclei but not in other subdivisions of the nuclei. No rhythm was detected in aging rats. SCN transplants restored a rhythm in CRH mRNA, but the timing was not precisely the same as in young animals. POMC mRNA exhibited a daily rhythm in young rats. Aging abolished the rhythm and decreased the average mRNA level; fetal transplants restored the rhythm, but the amplitude remained attenuated. These data are the first demonstration that fetal tissue can restore the diurnal rhythm of a neuroendocrine axis that is driven by the SCN. We conclude that the neuroendocrine substrate from the aging host remains capable of responding to diurnal cues to express diurnal rhythmicity in CRH/POMC mRNA when fetal SCN transplants confer the appropriate signals.

Attachment site of grafted SCN influences precision of restored circadian rhythm

Fetal hypothalamic grafts containing the suprachiasmatic nucleus (SCN) restore circadian locomotor rhythmicity when implanted into the third ventricle of SCN-lesioned hamsters. However, the quality of restored rhythms is variable, and the locomotor rhythms of grafted animals are generally less robust than those of intact animals. The present study explored whether anatomical features of the graft predict the quality of the recovered rhythm and whether such information might provide insight as to the target of the signal from the SCN that controls locomotor rhythmicity. The following graft parameters were assessed: distance between the attachment site of the graft and potential targets for the output signal from the SCN, number and overall size of SCN clusters, the size of the cluster closest to the SCN lesion site, and extent of vasoactive intestinal polypeptide (VIP) and vasopressin-associated neurophysin (NP) positive fiber outgrowth from the graft. The restored circadian activity rhythm was assessed by quantifying the precision of activity onset and the amount, period, and robustness of rhythmicity. The results indicate a significant positive correlation between the precision of activity onset and the proximity of the closest SCN cluster to the site of the lesioned host SCN. A more detailed analysis of the spatial location of the graft indicates that proximity of the graft in the dorsal and caudal directions, but not the rostral direction, is positively correlated with the precision of the recovered rhythm. This suggests two possibilities: the coupling signal may act on a site very near the SCN and travel preferentially in a rostro-caudal direction. Alternatively, the coupling signal may act on a site rostral to the SCN. That the site is not far rostral to the SCN was suggested by the lack of a correlation between the precision of the restored rhythm and the rostrally lying anterior medial preoptic nucleus. Finally, evaluation of NP- and VIP-ergic fibers in nuclei known to receive input from the SCN indicates that the extent of such innervation by graft efferents does not predict either the occurrence of recovery or the precision of the recovered rhythm. Overall, these results suggest that the target(s) of SCN pacemakers regulating locomotor rhythmicity lie in the hypothalamus, close to or rostral to the SCN.

The effect of transplanting one or two suprachiasmatic nuclei on the period of the restored rhythm

A fundamental property of circadian rhythms is the free-running period expressed by organisms when isolated from environmental periodicity. The physiological determinants of the free-running period, including variation among and within individuals and among species, are not known. The circadian rhythms of mammals are regulated by a circadian pacemaker within the suprachiasmatic nucleus (SCN) of the hypothalamus. To examine possible determinants of the free-running period, one or two SCNs were transplanted into hamsters that had their own SCNs ablated. Wheel-running behavior was measured to estimate the free-running period of restored rhythmicity. Hosts received grafts containing either the left or right SCN from a single fetus or both SCNs from a single fetus. In some cases, both the left and right SCNs from a single fetus restored rhythmicity in different hosts, demonstrating that each of the right and left SCN alone is a competent circadian pacemaker. The average free-running period of the restored rhythms was significantly longer in hamsters that received both of the SCNs from a single fetus. The sizes of grafts were estimated using immunoreactivity for vasoactive intestinal polypeptide as a marker of SCN tissue. Grafts never grew to be larger than an intact SCN, and a graft only 6.5% the size of the combined left and right intact SCNs restored rhythmicity. The average volume of grafted SCN in hamsters that received two SCNs was larger than that in hamsters that received a single SCN. The results demonstrate that SCN graft volume and/or the number of SCNs that comprise the graft influence the free-running period.

Circadian rhythms and autoregulatory transcription loops--going round in circles?

Recent advances in the molecular analysis of biological timing have appeared to bring us closer to an answer to the 'big question', namely, 'What is the timing mechanism that enables an organism to measure the circadian (around 24 h) period?'. In this minireview, we consider the validity of the fashionable concept that autoregulatory feedback loops, centered on transcription, form the basis of the clock, and we offer a fresh view of recent progress as it relates to mammalian systems.

Intrinsic organization and monoaminergic innervation of the suprachiasmatic nucleus transplanted to adult rats. A light- and electron-microscopic study

Light- and electron-microscopic immunocytochemistry was used to investigate grafts of foetal hypothalamic tissue implanted close to the site of the suprachiasmatic nucleus in adult rats with bilateral surgical ablation of this nucleus. The transplants contained vasoactive intestinal peptide and vasopressin cell clusters, which have previously been shown to characterize functional suprachiasmatic nucleus grafts. Vasoactive intestinal peptide and vasopressin neurons presented synaptic features that have not been described in the native suprachiasmatic nucleus. More specifically, their terminals within the graft were involved in 'double' synapses with separate unlabelled dendrites. Moreover, in dually stained sections, an unexpected synaptic investment of vasoactive intestinal peptide neurons by vasopressin endings was detected, which revealed reversed vasoactive intestinal peptide/vasopressin interactions compared to those described in the native nucleus. These observations could reflect some immature features of the grafted neurons. Ultrastructural relationships of monoaminergic fibres arising from host and/or intragraft neurons were also examined. Within the engrafted suprachiasmatic nucleus, tyrosine hydroxylase-labelled fibres, which probably belonged to cografted dopaminergic neurons, showed normal patterns of distribution and synaptic connections, with no preferential relationships with vasoactive intestinal peptide or vasopressin neurons. Serotoninergic axons arborized within transplants but, in agreement with previous data showing an inhibitory influence of the suprachiasmatic nucleus on ingrowing serotoninergic fibres, they had no predilection for the area corresponding to that nucleus. In spite of their relative scarcity, serotoninergic fibres within the engrafted suprachiasmatic nucleus showed an almost normal synaptic incidence, but synapses were not predominantly shared with the vasoactive intestinal peptide neurons, known to be their major targets in the native nucleus. This may contribute not only to the failure of functional grafts to synchronize with environmental conditions, but also to the inability of transplants to restore hormonal rhythms such as estrous cyclicity.

A diffusible coupling signal from the transplanted suprachiasmatic nucleus controlling circadian locomotor rhythms

The mammalian suprachiasmatic nuclei (SCN) transmit signals to the rest of the brain, organizing circadian rhythms throughout the body. Transplants of the SCN restore circadian activity rhythms to animals whose own SCN have been ablated. The nature of the coupling signal from the grafted SCN to the host brain is not known, although it has been presumed that functional recovery requires re-establishment of appropriate synaptic connections. We have isolated SCN tissue from hamsters within a semipermeable polymeric capsule before transplantation, thereby preventing neural outgrowth but allowing diffusion of humoral signals. Here we show that the transplanted SCN, like neural pacemakers of Drosophila and silkmoths, can sustain circadian activity rhythms by means of a diffusible signal.

Regulation of the phase and period of circadian rhythms restored by suprachiasmatic transplants

The influence of exogenous signals on circadian rhythms restored by transplants of the suprachiasmatic nucleus (SCN) of the hypothalamus has received little study. The authors tested the responsiveness of hamsters bearing SCN transplants to photic and pharmacological treatments. Light intensities as high as 6,500 lux were insufficient to produce entrainment, although masking was observed frequently. Triazolam failed to produce statistically significant phase shifts when administered during the subjective day, but 2 animals bearing functional SCN grafts responded to this benzodiazapine during the subjective night. The authors next tested the hypothesis that the host can retain circadian aftereffects that influence the period of the circadian system reconstituted by the graft. Intact hamsters were entrained to light:dark cycles of short (23.25-h) and long (25-h) period (T) for at least 3 months. Control hamsters released into constant darkness exhibited profound and long-lasting aftereffects of entrainment to T cycles. Hamsters that received SCN lesions after exposure to these T cycles and SCN grafts 3 weeks later exhibited marginal but statistically significant aftereffects that disappeared within 3 months. On subsequent transfer to constant light, tau lengthened by 0.25 +/- 0.6 h in hamsters with intact SCN (p < .05). Animals bearing SCN grafts continued to free run in constant light but differed from intact animals in that circadian period did not lengthen. Functional SCN grafts contained vasoactive intestinal polypeptide (VIP), neurophysin (NP), and cholecystokinin (CCK) immunoreactive (ir) cells. Inputs of neuropeptide Y-and serotonin-ir fibers from the host brain to grafted SCN peptide cell clusters were variable. Limited observations using retrograde and anterograde tracers do not support the existence of extensive input to the graft. Retinal input overlapped only rarely with clusters of VIP-ir, CCK-ir, or NP-ir cells. The authors conclude that the circadian system reinstated by SCN transplants is relatively impervious to photic influences that exert parametric and nonparametric influences in intact hamsters. The transient expression of aftereffects induced in the host before transplantation indicates that extra-SCN systems of the host can influence the period of the reconstituted circadian system to at least a limited degree.

Restoration of circadian rhythmicity by transplants of SCN "micropunches"

Although it is widely accepted that the suprachiasmatic nuclei (SCN) of the hypothalamus serve as biological pacemakers regulating circadian rhythmicity, a number of studies suggest that some circadian rhythms may be controlled by extra-SCN structures. Transplantation of fetal anterior hypothalamic tissue containing the SCN restores circadian locomotor rhythms in SCN-lesioned hosts. Such transplants, however, contain substantial extra-SCN hypothalamic tissue. In the present study, the authors examined the recovery of circadian locomotor rhythms in animals implanted with small grafts harvested by taking "micropunches" from vibratome-sectioned brain slices. Micropunches were taken from three areas of the hypothalamus known to receive retinal input: the SCN, the subparaventricular zone, and the supraoptic nucleus. The results indicate that transplants restricted to the SCN region are necessary and sufficient for restoration of circadian locomotor activity rhythms and that micropunches of tissues from other sources are ineffective.

Is blood pressure really a trigger for the circadian rhythm of subarachnoid hemorrhage?

BACKGROUND AND PURPOSE

Circadian blood pressure changes are not infrequently cited as a trigger for the onset of subarachnoid hemorrhage (SAH). Our purpose was to determine the reliability of this chronorisk and study the variability and consequences of it as it occurs in hypertensive and normotensive individuals.

METHODS

Of 273 consecutive patients with proven SAH of aneurysmal origin seen between January 1990 and December 1993, we studied 120 (44%) for whom the exact time of hemorrhage could be reliably determined. Beyond the recognition of a circadian rhythm for this collective, the patients were then sorted by blood pressure, yielding one group each of 80 normotensive (group N, 66.7%) and hypertensive (group H, 33.3%) individuals. The differential chronorisk of these two groups was studied.

RESULTS

A circadian rhythm with a definitive characteristic acrophase was observed for the entire group, occurring between 9 AM and 10 AM (chi 2 test, P < .0005) with a possible secondary peak in the afternoon hours. The separation into two blood pressure groups somewhat surprisingly revealed a different curve for each group (chi 2 test, P = .01). Statistical analysis of each group's separate chronorisk revealed that this acrophase only holds true for hypertensive individuals, whereas normotensive patients not only lack a morning peak, but an apparent elevation in the afternoon is statistically irrelevant, leading to the impression that SAH in normotensive persons seems to be subject to no circadian rhythm at all.

CONCLUSIONS

The incidence of SAH conforms to circadian blood pressure variation in hypertensive patients, similar to the diurnal rhythms observed with strokes and myocardial infarctions. This leads to the hypothesis that blood pressure elevation is a trigger for the onset of bleeding in this group. In clear contrast, normotensive individuals with cerebrovascular aneurysms seem to have a random 24-hour distribution of SAH onset times, thus leaving the nature of a possible trigger mechanism unresolved.

Suprachiasmatic nucleus grafts restore circadian function in aged hamsters

The regulation of circadian rhythms changes with age. In humans, changes in the timing of sleep and wakefulness are especially common. In Syrian hamsters Mesocricetus auratus the free running period of the activity/rest rhythm shortens with age. The present study tested the hypothesis that critical age-related changes occur within the hypothalamic suprachiasmatic nucleus (SCN), known to contain a circadian pacemaker. Fetal SCN were transplanted into the brains of younger (20 weeks) and older (81 weeks) hamsters which had had their own SCNs ablated. The restoration of rhythmicity and the free running period of the rhythmicity were determined from continuous records of wheel-running activity. Transplantation restored rhythmicity in hosts of both ages. In older hamsters, the mean free running period after transplantation was longer than that measured before SCN ablation, but a similar lengthening of period was not observed after transplantation to younger hamsters. In addition, the mean period after transplantation was the same for both younger and older hosts even when there was a difference between the groups before SCN ablation. When the grafts were allowed to age, the mean free running period of the restored rhythms became shorter, indicating that the grafts can also undergo age-related changes. The results indicate that age-related changes specifically in the SCN are responsible for an age-related change in free running period.

Host resets phase of grafted suprachiasmatic nucleus: a 2-DG study of time course of entrainment

The object of the present experiment was to examine whether in an intact animal implanted with a hypothalamic graft, the phase of the host and grafted suprachiasmatic nucleus (SCN) would become synchronized. To this end, we first established the time at which daily fluctuations in local cerebral glucose utilization were maximal in the SCN in our population of adult hamsters. Next, we verified that rhythms of (14C)2-deoxyglucose uptake could be measured on the day after birth in pups that were to provide donor tissue. Host and donor animals were housed in opposite light:dark cycles. We then transplanted fetal SCN tissue into the third ventricle of intact hamsters, placed the grafted animals in constant darkness with access to running wheels and examined the phase of metabolic activity in host and donor SCN. For several days after grafting, there was no circadian fluctuation in the metabolic activity of either the host SCN or of the grafted SCN. During this time, the circadian locomotor rhythms were not disrupted, suggesting that pacemaker activity was not interrupted. By day 14 after transplantation, metabolic activity in the host SCN was elevated during subjective day and host and donor SCN were in synchrony, invariably with the phase of the host animal. We conclude that a signal from the host SCN resets the grafted SCN and not vice versa and that pacemaker cells communicate with each other rather than exerting independent effects on target sites.

A possible glial role in the mammalian circadian clock

The primary mammalian circadian clock is located within the suprachiasmatic nuclei (SCN), but the cellular organization of the clock is not yet known. We investigated the potential role of glial cells in the clock mechanism by determining whether disrupting glial activity affects the in vitro circadian rhythm of neuronal activity and the in vivo circadian activity rhythm in rats. We used two agents (octanol and halothane) that block gap junctions, and one (fluorocitrate) that inhibits glial metabolism. All three agents disrupted the circadian pattern of neuronal activity. Octanol flattened the rhythm at the highest concentration (200 microM) and induced a small phase delay at a lower concentration (66 microM). Halothane and fluorocitrate induced ultradian rhythmicity. Fluorocitrate injected into the SCN of an intact rat induced arrhythmicity for about 1 week, after which the rhythm reappeared with a 1.6 h delay. These results suggest that glia play an important role in the SCN circadian clock.

The control of circadian rhythms and the levels of vasoactive intestinal peptide mRNA in the suprachiasmatic nucleus are altered in spontaneously hypertensive rats

Vasoactive intestinal peptide (VIP) has been localized within the suprachiasmatic nucleus of the hypothalamus (SCN) and appears to play an important role in the entrainment of circadian rhythms with the light-dark (LD) cycle. The spontaneously hypertensive rat (SHR), an inbred strain used extensively in research on primary hypertension, has significantly more VIP mRNA in its brain than normotensive Wistar-Kyoto control (WKY) rats. Because VIP levels are abnormally high in SHR rats the present study examined whether the mechanisms controlling circadian rhythms are also altered in SHR rats. When entrained to a 24 h LD cycle, SHR rats began their wheel-running rhythm approximately 1.5 h earlier than WKY controls. SHR rats re-entrained to a phase delay in the LD cycle more slowly than did WKY rats, but tended to re-entrain to a phase advance more rapidly. The free-running period of SHR rats in both constant light and constant dark was significantly shorter than that of WKY rats. In SHR rats, phase delays produced by 1-h pulses of light were less than one-half the magnitude of the delays seen in WKY rats; however, the phase advances were nearly twice that of WKY rats. Using in situ hybridization, the SCN levels of mRNA encoding VIP were found to be significantly greater in SHR rats, but the mRNA levels of another peptide important for entrainment, gastrin releasing peptide, did not differ between SHR and WKY rats. These data indicate that the mechanisms controlling circadian rhythms in SHR rats differ significantly from those controlling rhythms in WKY rats and that VIP mRNA is significantly elevated within the SCN of SHR rats. The role of VIP in the entrainment of circadian rhythms is discussed.

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

Heavy water lengthens the period of free-running rhythms in lesioned hamsters bearing SCN grafts

Heavy water (D2O) lengthens the period of free-running circadian rhythms in most organisms. We compared the effect of D2O on free-running locomotor activity rhythms in intact and SCN-lesioned (SCN-X) hamsters that had recovered circadian rhythmicity following implantation of SCN grafts. The animals were housed individually in cages equipped with running wheels, and locomotor activity was monitored using a computer-based data acquisition system. At the end of the behavioral tests, animals were anesthetized and perfused. Brain sections were immunostained for vasoactive intestinal peptide (VIP) and vasopressin (VP) to evaluate the extent of the lesion and the presence of a functional graft. The D2O similarly lengthened the period of free-running activity without affecting amount of activity in both intact and in SCN-X grafted animals. The results indicate that D2O acts directly on the SCN to lengthen the free-running period, and suggest that coupling between pacemakers within the grafted SCN is as efficient as in the intact SCN.

Lithium lengthens the period of circadian rhythms in lesioned hamsters bearing SCN grafts

Lithium lengthens the free-running period of circadian rhythms in a wide variety of organisms. The object of the present study was to examine the effects of lithium treatment on free-running activity rhythms in suprachiasmatic nuclei lesioned (SCN-X) hamsters that had recovered circadian rhythmicity following transplantation of fetal anterior hypothalamic grafts containing the suprachiasmatic nuclei (SCN). The animals were housed individually in cages equipped with running wheels, and locomotor activity was monitored using a computer-based data acquisition system. At the end of the behavioral tests, animals were anesthetized and perfused. Brain sections were immunostained for vasoactive intestinal peptide (VIP) and vasopressin-associated neurophysin (NP) to evaluate the extent of the lesion and the presence of a functional graft. In both intact and in SCN-X grafted animals, lithium lengthened the period of free running activity without affecting the amount of activity or the precision of the rhythm.

Characterization of the suprachiasmatic nucleus in organotypic slice explant cultures

Suprachiasmatic nuclei (SCN) from hypothalami of postnatal rats were maintained for 18-39 days in vitro as organotypic slice explants. Neuronal subtypes containing vasopressin (VP), vasoactive intestinal polypeptide (VIP), gastrin releasing hormone (GRP), and GABA were immunocytochemically identifiable in these cultures. In situ hybridization histochemistry was compatible with these SCN slice explant cultures, and mRNA encoding for VP was detected bilaterally within these nuclei. After 18 days in vitro, both VP mRNA and VP immunoreactivity increased from levels present on postnatal days 4 (the earliest age from which the explanted tissue was derived) to levels typical of adult SCNs. In contrast, the GRP expression remained low, characteristic of early postnatal animals and far lower than adult levels. This suggests that the developmental cues or programs necessary for enhanced VP expression are maintained in these cultures, while those affecting GRP expression are absent or inhibited. VIP-containing neurons were numerous in the cultures. Culture slices appeared healthy, and similar numbers and distributions of identifiable neurons within the SCN were observed, whether or not the slices were grown in the presence of serum. EM analysis revealed that the SCN in vitro is composed of tightly packed neurons, processes, and abundant synapses containing both clear and dense core vesicles, closely resembling the SCN in vivo. Vasopressinergic neuronal somata contained extensive Golgi systems and labeled secretory granules, the latter organelle being present also within processes and synaptic terminals. GABA-immunopositive processes and synaptic profiles were abundant, with labeling occurring particularly over secretory vesicles and mitochondria. This slice culture system effectively maintained much of the intrinsic organization and cellular components of the SCN for long periods in vitro and should be an excellent model system for studying the intrinsic molecular mechanisms and extrinsic cues which regulate neuronal phenotype in this circadian pacemaker.

Age of donor influences ability of suprachiasmatic nucleus grafts to restore circadian rhythmicity

Previous studies have shown a high (80-90%) rate of restoration of circadian rhythmicity in suprachiasmatic nucleus (SCN)-lesioned adult hamsters given anterior hypothalamic tissue containing the SCN taken from fetal day 13-15 donors. In the present experiments we explored the influence of age of donor on morphological and functional characteristics of the SCN graft, using tissue taken from animals at postnatal day 1, 3, 5, 7 and 10. Grafts taken from older donors tend to reach a smaller overall final size than those from younger donors, and are more likely to contain isolated, medium sized NP-positive neurons. The rate of restoration of locomotor rhythmicity following transplantation of postnatal day (PN) 1 grafts is as high as that of embryonic grafts. By PN 3, the rate of restoration falls to about 50%, and grafts of PN 7 and 10 do not restore function. As in the case of fetal grafts, there is a strong correlation between the ability of a graft to restore locomotor rhythmicity, and the presence of a cluster of vasoactive intestinal peptide (VIP) and neurophysin (NP) cells characteristic of the intact SCN within the graft. Since the period of neurogenesis for the hamster SCN occurs between day 10.5 and day 13 postfertilization, the results indicate that the SCN can be transplanted successfully well beyond the period of neurogenesis.

Functional development of fetal suprachiasmatic nucleus grafts in suprachiasmatic nucleus-lesioned rats

Recovery of circadian drinking rhythms in suprachiasmatic nucleus (SCN)-lesioned rats after fetal SCN grafting was related to the immunocytochemical appearance and fiber outgrowth of vasopressin (VP)-, vasoactive intestinal polypeptide (VIP)-, and somatostatin (SOM)-containing neurons in the implants. At 4 weeks postgrafting, the first recovered animal was found. After longer survival times, 38% of the animals showed recovery. Immunocytochemical evaluation indicated that full maturation of the SCN grafts was not reached until 4 weeks postgrafting. Grafted VP and VIP cells were always located together, whereas SOM cells were clustered nearby but separate. Neuropeptide Y fibers were observed with an increasing fiber density between 2 and 5 weeks posttransplantation and were clustered particularly at the level of the SOM cells. In all rhythm-recovered animals transplants of VP and VIP fibers had grown laterally into the hypothalamus. A few nonrecovered animals also showed ingrowth of such fibers, though more caudally to the lesioned SCN. Many of the nonrecovered rats showed similar stainings but without these efferent outgrowth to the host. We conclude that neither a humoral factor nor the presence of VP and VIP efferents in the host brain alone are enough for the restoration of circadian drinking rhythms.

Perinatal development of human circadian rhythms: role of the foetal biological clock

The development of circadian rhythms and the neuronal mechanisms underlying their generation (particularly the suprachiasmatic nucleus of the hypothalamus) were reviewed. Based on perinatal animal studies and data from human foetuses and/or preterm infants it was concluded that human circadian rhythms are present as early as at 30 weeks of gestation. The significance of the mother and/or the environment regarding the entrainment of the "endogenous" foetal biological clock was emphasized.

Spontaneous and stimulated firing in cultured rat suprachiasmatic neurons

Neurons from the suprachiasmatic nucleus (SCN) of the hypothalamus, the site of a circadian pacemaker in mammals, were isolated from embryonic rat. After mechanical dissociation neurons were brought into culture for 1-2 weeks, using a chemically defined medium. Recordings were made from 74 bipolar neurons using two different configurations of the patch-clamp technique. During cell attached patch recordings, 45% of neurons fired spontaneously. The mean firing rate was 0.7 +/- 0.6 Hz and the firing pattern was irregular. In whole cell recordings 73% of the investigated neurons showed spontaneous activity with an irregular firing pattern. The mean spontaneous firing rate with an intracellular Cl- concentration of 145 mM was 1.0 +/- 0.6 Hz. The resting membrane potential of the bipolar neurons was estimated to be -62 +/- 24 mV. An intracellular Cl- concentration of 145 mM depolarised the membrane potential. It also increased the probability of spontaneous firing. A depolarising current stimulus produced an action potential with a threshold voltage of -46 +/- 9 mV. Suprathreshold stimuli resulted in repetitive firing with a mean frequency of 12 +/- 4 Hz. The minimum interspike interval was 52 +/- 14 ms. All action potentials either occurring spontaneously or elicited by current stimuli were abolished by the Na(+)-channel blocker TTX. These results indicate that our cultured neurons have some electrophysiological properties in common with SCN neurons in brain slices and in vivo.