Human Suprachiasmatic Nucleus Research Articles

Diurnal Variation of Brain Activity in the Human Suprachiasmatic Nucleus.

The suprachiasmatic nucleus (SCN) is the central clock for circadian rhythms. Animal studies have revealed daily rhythms in the neuronal activity in the SCN. However, the circadian activity of the human SCN has remained elusive. In this study, to reveal the diurnal variation of the SCN activity in humans, we localized the SCN by employing an areal boundary mapping technique to resting-state functional images and investigated the SCN activity using perfusion imaging. In the first experiment (n = 27, including both sexes), we scanned each participant four times a day, every 6 h. Higher activity was observed at noon, while lower activity was recorded in the early morning. In the second experiment (n = 20, including both sexes), the SCN activity was measured every 30 min for 6 h from midnight to dawn. The results showed that the SCN activity gradually decreased and was not associated with the electroencephalography. Furthermore, the SCN activity was compatible with the rodent SCN activity after switching off the lights. These results suggest that the diurnal variation of the human SCN follows the zeitgeber cycles of nocturnal and diurnal mammals and is modulated by physical lights rather than the local time.

Aging of the Suprachiasmatic Nucleus, CIRCLONSA Syndrome, Implications for Regenerative Medicine and Restoration of the Master Body Clock

The suprachiasmatic nucleus (SCN) in the brain is the master regulator of the circadian clocks throughout the human body. With increasing age the circadian clock in humans and other mammals becomes increasingly disorganized leading to a large number of more or less well categorized problems. While a lot of aging research has focused on the peripheral clocks in tissues across organisms, it remains a paramount task to quantify aging of the most important master clock, the human SCN. Furthermore, a pipeline needs to be developed with therapies to mitigate the systemic cellular circadian dysfunction in the elderly and ultimately repair and reverse aging of the SCN itself. A disease classification for the aging SCN, Circadian Clock Neuronal Senile Atrophy, (CIRCLONSA syndrome), would improve research funding and goal-oriented biotechnological entrepreneurship.

The suprachiasmatic nuclei as a seasonal clock.

In mammals, the suprachiasmatic nucleus (SCN) contains a central clock that synchronizes daily (i.e., 24-h) rhythms in physiology and behavior. SCN neurons are cell-autonomous oscillators that act synchronously to produce a coherent circadian rhythm. In addition, the SCN helps regulate seasonal rhythmicity. Photic information is perceived by the SCN and transmitted to the pineal gland, where it regulates melatonin production. Within the SCN, adaptations to changing photoperiod are reflected in changes in neurotransmitters and clock gene expression, resulting in waveform changes in rhythmic electrical activity, a major output of the SCN. Efferent pathways regulate the seasonal timing of breeding and hibernation. In humans, seasonal physiology and behavioral rhythms are also present, and the human SCN has seasonally rhythmic neurotransmitter levels and morphology. In summary, the SCN perceives and encodes changes in day length and drives seasonal changes in downstream pathways and structures in order to adapt to the changing seasons.

Activation of suprachiasmatic nuclei and primary visual cortex depends upon time of day

The human suprachiasmatic nucleus (SCN), the master biological clock, is a small (approximately 2 mm(3)) and deep structure located in the anterior hypothalamus. Previous methods do not allow in vivo study of the human SCN in a non-invasive manner. Therefore, we explored blood oxygen level-dependent (BOLD)-functional magnetic resonance imaging (fMRI) with OFF-ON-OFF block-designed visual stimuli to record the activities in the 'SCN and peri SCN in the anterior hypothalamus' (SCN+) and the primary visual area V1 using a 3T Siemens scanner and six normal subjects. We found that: (i) the BOLD-fMRI response to light and the mean of percentage activation in the SCN+ at midday was significantly less than that at night; and (ii) the number of activated voxels in most of the visual area V1 at midday was significantly higher than that at night. We conclude that BOLD-fMRI responses to light in the SCN+ and the V1 areas vary with time of day. This conclusion is consistent with: (i) the previously measured phase-response curve to light [J. Physiol., 549.3 (2003) 945] for the SCN activity at critical intensity threshold; and (ii) the interaction of the melanopsin-based signals with the rod-cone signals at the 'giant' retinal ganglion cells [Nature, 433 (2005) 749] for the V1 activity.

Dorsomedial SCN neuronal subpopulations subserve different functions in human dementia

The suprachiasmatic nuclei (SCN) are necessary and sufficient for the maintenance of circadian rhythms in primate and other mammalian species. The human dorsomedial SCN contains populations of non-species-specific vasopressin and species-specific neurotensin neurons. We made time-series recordings of core body temperature and locomotor activity in 19 elderly, male, end-stage dementia patients and 8 normal elderly controls. Following the death of the dementia patients, neuropathological diagnostic information and tissue samples from the hypothalamus were obtained. Hypothalamic tissue was also obtained from eight normal control cases that had not had activity or core temperature recordings previously. Core temperature was analysed for parametric, circadian features, and activity was analysed for non-parametric and parametric circadian features. These indices were then correlated with the degree of degeneration seen in the SCN (glia/neuron ratio) and neuronal counts from the dorsomedial SCN (vasopressin, neurotensin). Specific loss of SCN neurotensin neurons was associated with loss of activity and temperature amplitude without increase in activity fragmentation. Loss of SCN vasopressin neurons was associated with increased activity fragmentation but not loss of amplitude. Evidence for a circadian rhythm of vasopressinergic activity was seen in the dementia cases but no evidence was seen for a circadian rhythm in neurotensinergic activity. These results provide evidence that the SCN is necessary for the maintenance of the circadian rhythm in humans, information on the role of neuronal subpopulations in subserving this function and the utility of dementia in elaborating brain-behaviour relationships in the human.

Glucocorticoids suppress vasopressin gene expression in human suprachiasmatic nucleus

Sleep impairment is one of the major side effects of glucocorticoid therapy. The mechanism responsible for this circadian disorder is unknown, but alterations in the suprachiasmatic nucleus (SCN), the biological clock of the human brain, are presumed to play a major role. In the present study, the amount of vasopressin mRNA (AVP mRNA) expression in the SCN was investigated in 10 glucocorticoid-exposed patients and 10 glucocorticoid free, age- and clock time of death-matched controls. The total amount of AVP mRNA, expressed as masked silver grains in the SCN, was two times lower in glucocorticoid-exposed patients ( n = 10; 5115 ± 1314 μm 2) than that in controls ( n = 10; 11,021 ± 1408 μm 2) ( P = 0.006). There was also a 53% decrease in the total number of profiles in the SCN that expressed AVP mRNA in glucocorticoid-exposed patients (16,759 ± 3110) compared with those in controls (31,490 ± 3816) ( P = 0.01). In conclusion, glucocorticoids have an inhibitory effect on AVP mRNA expression in the human SCN, which may be the biological basis of the circadian rhythm disturbances during glucocorticoid therapy.

Circadian Rhythms in Infants

After completing this article, readers should be able to: 1. Describe how circadian rhythms are generated. 2. Describe how lighting influences the circadian system. 3. Delineate the period during which the origin of circadian rhythms develops. 4. Describe the relationship between maternal circadian rhythms and the development of circadian rhythms in infants. 5. Characterize the potential sequelae of impaired fetal and early neonatal circadian rhythms related to sleep patterns. 6. Describe beneficial lighting patterns for preterm infants in the neonatal intensive care nursery. Circadian rhythms are endogenously generated rhythms that have a period length of about 24 hours. Evidence gathered over the past decade indicates that the circadian timing system develops prenatally, with the suprachiasmatic nuclei (SCN) in the anterior hypothalamus, the site of a circadian clock, present by mid-gestation in primates. Recent evidence also shows that the circadian system of primate infants is responsive to light at very early stages (as early as 25 to 28 weeks’ gestation in humans) and that low-intensity lighting can regulate the developing clock. After birth, circadian system outputs mature progressively, with rhythms in sleep-wake cycles, body temperature, and hormone production generally developing between 1 and 3 months of age. The importance of light in regulation of circadian rhythm in infants is highlighted by the early establishment of rest-activity patterns that are in phase with the 24-hour light-dark cycle in preterm infants exposed to low-intensity cycled lighting. With the continued elucidation of circadian system development and influences on human physiology and illness, it is anticipated that consideration of circadian biology will become an increasingly important component of neonatal care. ### The Circadian Timing System Notable examples of circadian rhythms include the sleep-wake cycle and daily rhythms in body temperature and hormone production. Circadian rhythms are also involved in the pathogenesis of illnesses, such as reactive airway disease (eg, asthma) and myocardial infarction. The system responsible …

Circadian oscillations of neuropeptide expression in the human biological clock.

The mammalian suprachiasmatic nucleus is the principal component of a neural timing system implicated in the temporal organization of circadian and seasonal processes. The present study was performed to analyze the circadian profiles of two major neuropeptidergic cell groups in the human suprachiasmatic nucleus. To that end the brains of 40 human subjects collected at autopsy were investigated. The populations of arginine vasopressin- and vasoactive intestinal polypeptide-expressing neurons, located in the shell and core of the suprachiasmatic nucleus, respectively, showed marked circadian rhythms with an asymmetrical, bimodal waveform. Time series analysis revealed that these circadian cycles in neuronal activity could be described by a composite model consisting of a nonlinear periodic function, with mono- and diphasic cycles. The findings suggest that the 24-h biosynthesis of neuropeptides in the human suprachiasmatic nucleus, being part of the neural output pathway of the clock, is driven by a complex pacemaker system consisting of coupled nonlinear oscillators, in accordance with a multioscillator model of circadian timekeeping.

Circasemiannual chronomics: half-yearly biospheric changes in their own right and as a circannual waveform

Geomagnetic activity has a strong half-yearly but no precise yearly component in its spectrum, as Armin Grafe suggested nearly half a century ago. We have postulated elsewhere that non-photic cycles such as those in geomagnetics may have signatures in the biosphere and vice versa that biological rhythms have likely counterparts in the physical environment. Accordingly, we document phenomena characterized by a prominent about half-yearly variation, re-analyzed to constitute the start of a transdisciplinary chronomic (time structural) map, aligning these conditions with a half-yearly cycle in the geomagnetic index K p. At least some biospheric phenomena fitted concomitantly with 1– and 0.5–year cosine curves exhibit an amplitude ( A) ratio of A 0.5–year/ A 1–year larger than unity. Methodologically, it is pertinent that even if data were read off published graphs, the resulting analyses were practically the same as those in the original data received subsequently. The main point is a circasemiannual pattern in status epilepticus, in several morbid oral conditions, in the cell density of vasopressin-containing neurons in the human suprachiasmatic nuclei (SCN), in circulating melatonin at middle latitudes at night during years of minimal solar activity or around noon at high latitudes, and in an unusual circasemiannual aspect of a birth-month-dependence of human longevity. Others have asked whether annual rhythms in human reproduction are biological, sociological or both. We show some other possibilities herein, involving the physical environment, hardly to be neglected in the case of open systems. As to almost certainly multifactorial circasemiannual rhythms, geomagnetics may also be a signal, a proxy or a putative, at least partial mechanism. Geomagnetic activity is related in its turn to solar and galactic activity, and may be a marker for other cyclic events that affect the biosphere. The similarity of cycle lengths in itself can only be a hint prompting the search for causal relations.

The biological clock tunes the organs of the body: timing by hormones and the autonomic nervous system

The biological clock, the suprachiasmatic nucleus (SCN), is essential for our daily well-being. It prepares us for the upcoming period of activity by an anticipatory rise in heart rate, glucose and cortisol. At the same time the 'hormone of the darkness', melatonin, decreases. Thus, the time-of-day message penetrates into all tissues, interestingly not only by means of hormones but also by a direct neuronal influence of the SCN on the organs of the body. The axis between the SCN and the paraventricular nucleus of the hypothalamus (PVN) is crucial for the organization/synchronization of the neuroendocrine and autonomic nervous system with the time of day. This SCN-neuroendocrine PVN axis takes care of a timely hormonal secretion. At the same time, the SCN-autonomic PVN axis fine-tunes the organs by means of the autonomic nervous system for the reception of these hormones. Finally, the similar organization of the projections of the human SCN as compared with that in the rodent brain suggests that these basic principles of neuroendocrine autonomic interaction may also be true in the human. The physiological data collected in humans thus far seem to support this hypothesis, while pathological changes in the SCN of humans suffering from depression or hypertension indicate a role for the SCN in the etiology of these diseases.

Neurotensin expressing neurons developed earlier than vasoactive intestinal polypeptide and vasopressin expressing neurons in the human suprachiasmatic nucleus

Development of neurotensin (NT), vasoactive intestinal polypeptide (VIP), vasopressin (AVP) and neuropeptide-Y (NPY) expressing neurons was investigated in the human fetal suprachiasmatic nucleus of nine subjects ranging from 20–40 weeks of gestation using immunocytochemistry and morphometry. Results obtained showed that NT expressing neurons developed earlier than VIP, AVP and NPY expressing neurons. Consistent with results obtained from animal studies, we also found VIP expressing neurons were born earlier than AVP expressing neurons. Whether the NT expressing neurons play a role in generating circadian rhythms in the early life of humans needs to be further investigated.

Sex Hormone Receptors Are Present in the Human Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is the clock of the brain that orchestrates circadian and circannual biological rhythms, such as the rhythms of hormones, body temperature, sleep and mood. These rhythms are frequently disturbed in menopause and even more so in dementia and can be restored in postmenopausal women by sex hormone replacement therapy (SHRT). Although it seems clear, both from clinical and experimental studies, that sex hormones influence circadian rhythms, it is not known whether this is by a direct or an indirect effect on the SCN. Therefore, using immunocytochemistry in the present study, we investigated whether the human SCN expresses sex hormone receptors in 5 premenopausal women and 5 young men. SCN neurons appeared to contain estrogen receptor-α (ERα), estrogen receptor-β (ERβ) and progesterone receptors. Median ratings of ER immunoreactivity per individual and per gender group revealed a statistically significantly stronger nuclear ERα expression pattern in female SCN neurons (p < 0.05). No significant sexual dimorphic tendency was observed for nuclear ERβ (p > 0.1) and progesterone receptors (p > 0.7). These data seem to support previously reported functional and structural SCN differences in relation to sex and sexual orientation and indicate for the first time that estrogen and progesterone may act directly on neurons of the human biological clock. In addition, the present findings provide a potential neuroendocrine mechanism by which SHRT can act to improve or restore SCN-related rhythm disturbances, such as body temperature, sleep and mood.

Postmortem stability of melatonin receptor binding and clock-relevant mRNAs in mouse suprachiasmatic nucleus.

The stability of receptor proteins and mRNAs in brain tissue is variable after death. As a prelude to quantitative studies of melatonin receptor density and clock gene expression in the human brain, the stability of these macromolecules was examined in the mouse brain under simulated postmortem conditions using the model of Spokes and Koch. In the mouse suprachiasmatic nucleus (SCN), melatonin receptor binding was significantly reduced after 18 to 24 h under postmortem conditions. Two mRNAs that are rhythmically expressed in the SCN, mPer1 and prepropressophysin (AVP), also decreased significantly over the interval studied, and mPer1 declined more rapidly than AVP. Both mPer1 and AVP mRNA levels in the SCN declined more rapidly in vivo than under postmortem conditions, suggesting that the degradation of these mRNAs is an active process. The results indicate that quantitative studies of melatonin receptor density on human postmortem material are feasible and that detection of rhythmic gene expression in the human SCN will likely require collection of specimens with a rather short (< 8 h) interval from death to tissue collection. The relative stability of melatonin receptor binding in the SCN also suggests that receptor binding may be a reliable marker for the location of the SCN in studies assessing clock gene expression in postmortem material.

Seasonal Rhythms of Neuronal Activity in the Human Biological Clock: A Mathematical Model

The mammalian suprachiasmatic nucleus (SCN) is implicated in the temporal organization of circadian and seasonal processes. Photic information may have a synchronizing effect on the endogenous clock of the SCN by inducing periodic changes in the functional activity of specific groups of neurons. The present study was performed to analyze the annual profiles of the arginine vasopressin (AVP)- and vasoactive intestinal polypeptide (VIP)-producing neurons in the human SCN. The populations of AVP- and VIP-expressing neurons in the SCN showed marked annual rhythms with an asymmetrical, bimodal waveform. Time series analysis indicated that these annual cycles in peptidergic activity could be described by a statistical model consisting of multiple-harmonic regression and ARMA components. The annual AVP cycle was adequately described by a two-harmonic model and a third-order autoregressive noise component, whereas the properties of the VIP cycle were best characterized by a two-harmonic model and a first-order autoregressive noise component. The models of both annual cycles reached a maximum in September–October and a minimum in May–June, and their estimated amplitudes, relative to the annual mean, were similar in size. These findings indicate that the biosynthesis of vasopressin and VIP in the human SCN exhibits an annual rhythmicity, and that the temporal organization of these processes is mainly controlled by environmental lighting conditions.

Decreased Vasopressin Gene Expression in the Biological Clock of Alzheimer Disease Patients With and Without Depression

Circadian rhythm disturbances are frequently present in Alzheimer disease (AD). In the present study, we investigated the expression of vasopressin (AVP) mRNA in the human suprachiasmatic nucleus (SCN). The in situ hybridization procedure on formalin-fixed paraffin-embedded material was improved to such a degree that we could, for the first time, visualize AVP mRNA expressing neurons in the human SCN and carry out quantitative measurements. The total amount of AVP mRNA expressed as masked silver grains in the SCN was 3 times lower in AD patients (n = 14; 2,135 +/- 597 microm2) than in age- and time-of-death-matched controls (n = 11; 6,667 +/- 1466 microm2) (p = 0.003). No significant difference was found in the amount of AVP mRNA between AD patients with depression (n = 7) and without depression (n = 7) (2,985 +/-1103 microm2 and 1,285 +/- 298 microm2, respectively; p = 0.38). In addition, the human SCN AVP mRNA expressing neurons showed a marked day-night difference in controls under 80 years of age. The amount of AVP mRNA was more than 3 times higher during the daytime (9,028 +/- 1709 microm2, n = 7) than at night (2,536 +/- 740 microm2, n = 4; p = 0.02), whereas no clear diurnal rhythm of AVP mRNA in the SCN was observed in AD patients. There was no relationship between the amount of AVP mRNA in the SCN and age at onset of dementia, duration of AD and the neuropathological changes in the cerebral cortex. These findings suggest that the neurobiological basis of the circadian rhythm disturbances that are responsible for behavioral rhythm disorders is located in the SCN. It also explains the beneficial effects of light therapy on nightly restlessness in AD patients.

THE HUMAN CIRCADIAN CLOCK AND AGING

The suprachiasmatic nucleus (SCN) of the hypothalamus is implicatedin the timing of a wide variety of circadian processes. Since the environmentallight-dark cycle is the main zeitgeber for many of the rhythms, photic informationmay have a synchronizing effect on the endogenous clock of the SCN by inducingperiodic changes in the biological activity of certain groups of neurons.By studying the brains obtained at autopsy of human subjects, marked diurnaloscillations were observed in the neuropeptide content of the SCN. Vasopressin,for example, one of the most abundant peptides in the human SCN, exhibiteda diurnal rhythm, with low values at night and peak values during the earlymorning. However, with advancing age, these diurnal fluctuations deteriorated,leading to a disrupted cycle with a reduced amplitude in elderly people. Thesefindings suggest that the synthesis of some peptides in the human SCN exhibitsan endogenous circadian rhythmicity, and that the temporal organization ofthese rhythms becomes progressively disturbed in senescence. (ChronobiologyInternational, 17(3), 245–259, 2000)

Colocalization of VIP with AVP in neurons of the human paraventricular, supraoptic and suprachiasmatic nucleus

Aim of this study was to investigate, with the aid of a recently developed immunofluorescence technique, cellular colocalization of vasoactive intestinal peptide (VIP) with arginine–vasopressin (AVP) in the paraventricular nucleus (PVN), the supraoptic nucleus (SON) and the suprachiasmatic nucleus (SCN) of the human hypothalamus. To this end, six hypothalami resected from patients who had died suddenly served as material of research. After formaldehyde fixation and subsequent storage in 30% sucrose, 25-μm thick cryosections were cut of one half of each hypothalamus. These sections were double-immunolabeled with primary antibodies against AVP and VIP followed by fluorophore-conjugated secondary antibodies. Autofluorescence, mainly caused by lipofuscin granules in neurons and glial cells, was blocked by a specially developed procedure consisting of incubating the immunolabeled sections in a Sudan Black B solution. Quantitative analysis with a confocal laser scanning microscope showed that of all stained cellular profiles the percentages of profiles immunoreactive exclusively for AVP or VIP or for both neuropeptides (colocalization) were for the SCN approximately 76.5%, 19.6% and 3.9%, for the SON 97.7%, 0.2% and 2.1% and for the PVN 93.2%, 1.6% and 5.2%, respectively. These data illustrate that colocalization between AVP and VIP is not only present in neurons of the PVN and SON, but also in neurons of the SCN. This unexpected finding illustrates that the human SCN may also use a highly differentiated language to transmit its circadian signal to the rest of the brain.

Pathologic evaluation of the human suprachiasmatic nucleus in severe dementia.

Sleep disruption and other circadian rhythm disturbances are frequently seen in dementia patients. In this study, we examined the suprachiasmatic nucleus (SCN), the putative site of the hypothalamic circadian pacemaker, to determine the nature and degree of pathologic changes caused by severe dementia. Neuropathologic examination indicated that among 30 patients with a clinical history of severe dementia, 22 had Braak and Braak stage V-VI Alzheimer disease, 3 had combined Alzheimer and Parkinson disease, 3 had Pick disease and 2 had severe hippocampal sclerosis. Comparisons were made with a control group composed of 13 age-matched patients with no clinical or pathological evidence of dementia or other CNS disorders. To determine the pathologic involvement within the SCN, human hypothalami were stained with: Nissl, Bielchowsky silver, thioflavin S and specific antibodies directed against vasopressin (VP), neurotensin (NT), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), beta-amyloid (B/A4) and glial fibrillary acidic protein (GFAP). Pathologic damage was primarily limited to neuronal loss and neurofibrillary tangle formation. Only rare diffuse plaques were noted. The pathologic changes within the SCN were less severe than in the other brain regions. Morphometric analysis was accomplished using a stereological approach to sample the average total number of positively stained neurons and astrocytes in 10 different 0.1mm2 microscopic fields in the dorsal subdivision of the SCN. Patients with Alzheimer disease exhibited a significant decrease in vasopressin (9.75 vs 16.7, p < 0.001) and neurotensin (6.82 vs 9.63, p < 0.002) neurons, as well as a corresponding increase in the GFAP-stained astrocyte/Nissl-stained neuron ratio (0.54 vs 0.10, p < 0.009). These studies provide evidence that both vasopressin and neurotensin neurons are lost in Alzheimer disease, and that the astrocyte/neuron ratio is a reliable indicator of disease-related pathology within the SCN. Taken collectively, our data support the hypothesis that damage to the SCN may be an underlying anatomical substrate for the clinically observed changes in circadian rhythmicity that have been observed in Alzheimer patients.

Postmortem tracing reveals the organization of hypothalamic projections of the suprachiasmatic nucleus in the human brain.

The suprachiasmatic nucleus (SCN) is a small structure considered to be the site of the major circadian pacemaker of the mammalian brain. Disturbances in human biological clock function may occur in several diseases, such as Alzheimer's disease, sleep problems, and seasonal depression. Since basic knowledge of the anatomical connections of the human SCN is limited due to the lack of suitable neuroanatomical tracing methods, the understanding of physiological mechanisms of human SCN function has obviously been hampered. In the present study, the hypothalamic connections of the human SCN were revealed for the first time with a newly developed in vitro postmortem anterograde tracing method. The human SCN was found to be connected with nuclei in the hypothalamus that are involved in hormone secretion, cardiovascular regulation, and behavior activity. These human SCN projections appear to follow the same general patterns as those in the rodent brain. This homology may indicate an evolutionary conservation of the SCN projections from rodent to human. Through these connections, the human SCN may transmit its circadian information to regulate hormone secretion, body temperature, and behavioral functions as it does in animal species. In addition, the postmortem tracing technique may be a valuable tool that will contribute to our understanding of anatomical connections in the human brain, and may have other applications in the research on the physiology and pathology of the human brain.

Distribution of vasopressin and vasoactive intestinal polypeptide (VIP) fibers in the human hypothalamus with special emphasis on suprachiasmatic nucleus efferent projections.

The human suprachiasmatic nucleus (SCN) is located in the basal part of the anterior hypothalamus and is considered as the biological clock that generates circadian rhythms and synchronizes the daily activity pattern with the environmental light-dark cycle. However, the mechanisms and pathways by which the SCN transmits its information to the other brain areas are unknown. Therefore, in the present study, we investigated the efferent projections of the SCN by the immunocytochemical staining of two major peptidergic SCN neurotransmitters: vasopressin (VP) and vasoactive intestinal polypeptide (VIP). It confirmed that these peptides are present in different subdivisions of the SCN. The results of this investigation show that VP and VIP fibers arising from the SCN were detected to branch extensively and hence seem to innervate the SCN itself and the central and medial part of the anteroventral hypothalamic area (AVH), the area below the paraventricular nucleus (sub-PVN), the ventral part of the paraventricular nucleus (PVN), and the dorsomedial nucleus of the hypothalamus (DMH). There appeared to be substantial congruity between the presumptive human SCN projections and those as observed by tracing in rat or hamster. Regarding the anatomical organization of the human SCN projections, the main projection areas appeared to be the AVH, the sub-PVN, the ventral part of the PVN, and the DMH. The observation that VIP and in particular VP fibers pass between the SCN and the PVN suggests that the human SCN and the PVN may have a direct anatomical connection. In addition, VP and VIP fibers were detected in several other hypothalamic areas that are not known to have clear direct connections to the SCN. The possible origin of these VP and VIP fibers is discussed.