Peptidergic transmitters of the suprachiasmatic nuclei and the control of circadian rhythmicity

Sleep Disorders and Circadian Disruption in Huntington's Disease

Sleep and circadian alterations are common in patients with Huntington's disease (HD). Understanding the pathophysiology of these alterations and their association with disease progression and morbidity can guide HD management. We provide a narrative review of the clinical and basic-science studies centered on sleep and circadian function on HD. Sleep/wake disturbances among HD patients share many similarities with other neurodegenerative diseases. Overall, HD patients and animal models of the disease present with sleep changes early in the clinical course of the disease, including difficulties with sleep initiation and maintenance leading to decreased sleep efficiency, and progressive deterioration of normal sleep architecture. Despite this, sleep alterations remain frequently under-reported by patients and under-recognized by health professionals. The degree of sleep and circadian alterations has not consistently shown to be CAG dose-dependent. Evidence based treatment recommendations are insufficient due to lack of well-designed intervention trials. Approaches aimed at improving circadian entrainment, such as including light therapy, and time-restricted feeding have demonstrated a potential to delay symptom progression in some basic HD investigations. Larger study cohorts, comprehensive assessment of sleep and circadian function, and reproducibility of findings are needed in future in order to better understand sleep and circadian function in HD and to develop effective treatments.

Circadian rhythms: a possible new player in non-alcoholic fatty liver disease pathophysiology

Over the last decades, a better knowledge of the molecular machinery supervising the regulation of circadian clocks has been achieved, and numerous findings have helped in unravelling the outstanding significance of the molecular clock for the proper regulation of our physiologic and metabolic homeostasis. Non-alcoholic fatty liver disease (NAFLD) is currently considered as one of the emerging liver pathologies in the Western countries due to the modification of eating habits and lifestyle. Although NAFLD is considered a pretty benign condition, it can progress towards non-alcoholic steatohepatitis (NASH) and eventually hepatocellular carcinoma (HCC). The pathogenic mechanisms involved in NAFLD development are complex, since this disease is a multifactorial condition. Major metabolic deregulations along with a genetic background are believed to take part in this process. In this light, the aim of this review is to give a comprehensive description of how our circadian machinery is regulated and to describe to what extent our internal clock is involved in the regulation of hormonal and metabolic homeostasis, and by extension in the development and progression of NAFLD/NASH and eventually in the onset of HCC.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

The Suprachiasmatic Nucleus of the Dromedary Camel (Camelus dromedarius): Cytoarchitecture and Neurochemical Anatomy

In mammals, biological rhythms are driven by a master circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Recently, we have demonstrated that in the camel, the daily cycle of environmental temperature is able to entrain the master clock. This raises several questions about the structure and function of the SCN in this species. The current work is the first neuroanatomical investigation of the camel SCN. We carried out a cartography and cytoarchitectural study of the nucleus and then studied its cell types and chemical neuroanatomy. Relevant neuropeptides involved in the circadian system were investigated, including arginine-vasopressin (AVP), vasoactive intestinal polypeptide (VIP), met-enkephalin (Met-Enk), neuropeptide Y (NPY), as well as oxytocin (OT). The neurotransmitter serotonin (5-HT) and the enzymes tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) were also studied. The camel SCN is a large and elongated nucleus, extending rostrocaudally for 9.55 ± 0.10 mm. Based on histological and immunofluorescence findings, we subdivided the camel SCN into rostral/preoptic (rSCN), middle/main body (mSCN) and caudal/retrochiasmatic (cSCN) divisions. Among mammals, the rSCN is unusual and appears as an assembly of neurons that protrudes from the main mass of the hypothalamus. The mSCN exhibits the triangular shape described in rodents, while the cSCN is located in the retrochiasmatic area. As expected, VIP-immunoreactive (ir) neurons were observed in the ventral part of mSCN. AVP-ir neurons were located in the rSCN and mSCN. Results also showed the presence of OT-ir and TH-ir neurons which seem to be a peculiarity of the camel SCN. OT-ir neurons were either scattered or gathered in one isolated cluster, while TH-ir neurons constituted two defined populations, dorsal parvicellular and ventral magnocellular neurons, respectively. TH colocalized with VIP in some rSCN neurons. Moreover, a high density of Met-Enk-ir, 5-HT-ir and NPY-ir fibers were observed within the SCN. Both the cytoarchitecture and the distribution of neuropeptides are unusual in the camel SCN as compared to other mammals. The presence of OT and TH in the camel SCN suggests their role in the modulation of circadian rhythms and the adaptation to photic and non-photic cues under desert conditions.

Neural and Molecular Mechanisms Involved in Controlling the Quality of Feeding Behavior: Diet Selection and Feeding Patterns

We are what we eat. There are three aspects of feeding: what, when, and how much. These aspects represent the quantity (how much) and quality (what and when) of feeding. The quantitative aspect of feeding has been studied extensively, because weight is primarily determined by the balance between caloric intake and expenditure. In contrast, less is known about the mechanisms that regulate the qualitative aspects of feeding, although they also significantly impact the control of weight and health. However, two aspects of feeding quality relevant to weight loss and weight regain are discussed in this review: macronutrient-based diet selection (what) and feeding pattern (when). This review covers the importance of these two factors in controlling weight and health, and the central mechanisms that regulate them. The relatively limited and fragmented knowledge on these topics indicates that we lack an integrated understanding of the qualitative aspects of feeding behavior. To promote better understanding of weight control, research efforts must focus more on the mechanisms that control the quality and quantity of feeding behavior. This understanding will contribute to improving dietary interventions for achieving weight control and for preventing weight regain following weight loss.

Circadian Rhythms and Hormonal Homeostasis: Pathophysiological Implications

Over recent years, a deeper comprehension of the molecular mechanisms that control biological clocks and circadian rhythms has been achieved. In fact, many studies have contributed to unravelling the importance of the molecular clock for the regulation of our physiology, including hormonal and metabolic homeostasis. Here we will review the structure, organisation and molecular machinery that make our circadian clock work, and its relevance for the proper functioning of physiological processes. We will also describe the interconnections between circadian rhythms and endocrine homeostasis, as well as the underlying consequences that circadian dysregulations might have in the development of several pathologic affections. Finally, we will discuss how a better knowledge of such relationships might prove helpful in designing new therapeutic approaches for endocrine and metabolic diseases.

Lipids around the Clock: Focus on Circadian Rhythms and Lipid Metabolism

Disorders of lipid and lipoprotein metabolism and transport are responsible for the development of a large spectrum of pathologies, ranging from cardiovascular diseases, to metabolic syndrome, even to tumour development. Recently, a deeper knowledge of the molecular mechanisms that control our biological clock and circadian rhythms has been achieved. From these studies it has clearly emerged how the molecular clock tightly regulates every aspect of our lives, including our metabolism. This review analyses the organisation and functioning of the circadian clock and its relevance in the regulation of physiological processes. We also describe metabolism and transport of lipids and lipoproteins as an essential aspect for our health, and we will focus on how the circadian clock and lipid metabolism are greatly interconnected. Finally, we discuss how a deeper knowledge of this relationship might be useful to improve the recent spread of metabolic diseases.

The suprachiasmatic nucleus changes the daily activity of the arcuate nucleus α-MSH neurons in male rats

Timing of metabolic processes is crucial for balanced physiology; many studies have shown the deleterious effects of untimely food intake. The basis for this might be an interaction between the arcuate nucleus (ARC) as the main integration site for metabolic information and the suprachiasmatic nucleus (SCN) as the master clock. Here we show in male rats that the SCN influences ARC daily neuronal activity by imposing a daily rhythm on the α-MSH neurons with a peak in neuronal activity at the end of the dark phase. Bilateral SCN lesions showed a complete disappearance of ARC neuronal rhythms and unilateral SCN lesions showed a decreased activation in the ARC at the lesioned side. Moreover light exposure during the dark phase inhibited ARC and α-MSH neuronal activity. The daily inhibition of ARC neuronal activity occurred in light-dark conditions as well as in dark-dark conditions, demonstrating the inhibitory effect to be mediated by increased SCN (subjective) day neuronal activity. Injections into the SCN with the neuronal tracer cholera toxin B showed that α-MSH neurons receive direct projections from the SCN. The present study demonstrates that the SCN activates and possibly also inhibits depending on the moment of the circadian cycle ARC α-MSH neurons via direct neuronal input. The persistence of these activity patterns in fasted animals demonstrates that this SCN-ARC interaction is not necessarily satiety associated but may support physiological functions associated with changes in the sleep-wake cycle.

The role of the circadian clock system in nutrition and metabolism

Mammals have an endogenous timing system in the suprachiasmatic nuclei (SCN) of the hypothalamic region of the brain. This internal clock system is composed of an intracellular feedback loop that drives the expression of molecular components and their constitutive protein products to oscillate over a period of about 24 h (hence the term 'circadian'). These circadian oscillations bring about rhythmic changes in downstream molecular pathways and physiological processes such as those involved in nutrition and metabolism. It is now emerging that the molecular components of the clock system are also found within the cells of peripheral tissues, including the gastrointestinal tract, liver and pancreas. The present review examines their role in regulating nutritional and metabolic processes. In turn, metabolic status and feeding cycles are able to feed back onto the circadian clock in the SCN and in peripheral tissues. This feedback mechanism maintains the integrity and temporal coordination between various components of the circadian clock system. Thus, alterations in environmental cues could disrupt normal clock function, which may have profound effects on the health and well-being of an individual.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

The Biological Clock: The Bodyguard of Temporal Homeostasis

In order for any organism to function properly, it is crucial that it be table to control the timing of its biological functions. An internal biological clock, located, in mammals, in the suprachiasmatic nucleus of the hypothalamus (SCN), therefore carefully guards this temporal homeostasis by delivering its message of time throughout the body. In view of the large variety of body functions (behavioral, physiological, and endocrine) as well as the large variety in their preferred time of main activity along the light:dark cycle, it seems logical to envision different means of time distribution by the SCN. In the present review, we propose that even though it presents a unimodal circadian rhythm of general electrical and metabolic activity, the SCN seems to use several sorts of output connections that are active at different times along the light:dark cycle to control the rhythmic expression of different body functions. Although the SCN is suggested to use diffusion of synchronizing factors in the rhythmic control of behavioral functions, it also needs neuronal connections for the control of endocrine functions. The distribution of the time-of-day message to neuroendocrine systems is either directly onto endocrine neurons or via intermediate neurons located in specific SCN targets. In addition, the SCN uses its connections with the autonomic nervous system for spreading its time-of-day message, either by setting the sensitivity of endocrine glands (i.e., thyroid, adrenal, ovary) or by directly controlling an endocrine output (i.e., melatonin synthesis). Moreover, the SCN seems to use different neurotransmitters released at different times along the light:dark cycle for each of the different connection types presented. Clearly, the temporal homeostasis of endocrine functions results from a diverse set of biological clock outputs.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.

Vasoactive intestinal peptide modulates luteinizing hormone subunit gene expression in the anterior pituitary in female rat

The direct monosynaptic pathway which exists between vasoactive intestinal peptide (VIP) and GnRH neurons in the hypothalamic preoptic area provides a neuroanatomical background for the modulatory effects of VIP exerted on GnRH neurons activity. Though central microinjection of VIP revealed its involvement in the modulation of LH release pattern, there is a lack of data concerning a possible VIP influence on the alpha and LHbeta subunit gene expression in the pituitary gland. Using a model based on intracerebroventricular pulsatile peptide(s) microinjections (1 pulse/h [10 microl/5 min] over 5 h) the effect of exogenous VIP (5 nM dose) microinjection on subunits mRNA content in ovariectomized/oestrogen-pretreated rats was studied. Subsequently, to obtain data concerning the involvement of GnRH and VIP receptor(s) in the regulation of alpha and LHbeta subunit mRNA expression, OVX/estrogen-primed rats received a pulsatile microinjections of 5 nM VIP with 3 nM antide (GnRH receptor antagonist) or 5 nM VIP with 15 nM VIP 6-28 (VIP receptor antagonist). In this case, substances were given separately with a 30 min lag according to which each antagonist pulse preceded a VIP pulse. Northern-blot analysis revealed that VIP microinjection resulted in a decreased alpha and LHbeta mRNA content in pituitary gland and this effect was dependent on GnRH receptor activity. Moreover, obtained results indicated that centrally administered VIP might operate through its own receptor(s) because a receptor antagonist, VIP 6-28, blocked the inhibitory effect of VIP exerted on both LH subunit mRNA content and LH release.

TGFα and AVP in the mouse suprachiasmatic nucleus: Anatomical relationship and daily profiles

Daily rhythms in behavior and physiology are under control of the suprachiasmatic nucleus (SCN), the main mammalian circadian pacemaker located in the hypothalamus. The SCN communicates with the rest of the brain via various output systems. The aim of the present study was to determine the neuroanatomical and temporal relationship between two output systems, arginine-vasopressin (AVP) and transforming growth factor alpha (TGFalpha), in the mouse SCN. TGFalpha-positive cells were found throughout the SCN, but more abundantly in the core than the shell area, while AVP was predominantly found in the shell. Fluorescent double labeling revealed a total lack of co-expression for the two proteins in SCN cells. The circadian profile, studied by way of optical density in immunostaining at 3 h intervals, showed peak values for AVP shortly after the LD transitions. Immunoreactivity for TGFalpha was highly variable, especially at time points before the LD transitions. In addition, strong lateralization in TGFalpha immunostaining in the SCN was found in some individuals. Daily fluctuations in the paraventricular nucleus were absent for TGFalpha, and only weakly present for AVP. The main conclusion derived from this study is that these two output systems of the biological clock are anatomically separated with different daily profiles in expression.

Corticospinal tract transection reduces H-reflex circadian rhythm in rats

In freely moving rats and monkeys, H-reflex amplitude displays a marked circadian variation without change in background motoneuron tone. In rats, the H-reflex is largest around noon and smallest around midnight. The present study evaluated in rats the effects on this rhythm of calibrated contusions of mid-thoracic spinal cord and mid-thoracic transection of specific spinal cord pathways. In 33 control rats, rhythm amplitude averaged 29.0(+/-2.6 S.E.)% of H-reflex amplitude. Contusion injuries at T8-9 that destroyed 53-88% of the white matter significantly reduced the rhythm to 18.9(+/-2.4)% of H-reflex amplitude. Transection of the ipsilateral lateral column, which contains the rubrospinal, vestibulospinal, and reticulospinal tracts, or bilateral transection of the dorsal column ascending tract did not affect rhythm amplitude or phase. In contrast, bilateral transection of the main corticospinal tract significantly reduced the rhythm to 14.7(+/-6.6)%. These results indicate that the H-reflex circadian rhythm depends in part on descending influence from the brain and that this influence is conveyed by the main corticospinal tract.

How does the circadian clock send timing information to the brain?

This paper discusses circadian output in terms of the signaling mechanisms used by circadian pacemaker neurons. In mammals, the suprachiasmatic nucleus houses a clock controlling several rhythmic events. This nucleus contains one or more pacemaker circuits, and exhibits diversity in transmitter content and in axonal projections. In Drosophila, a comparable circadian clock is located among period -expressing neurons, a sub-set of which (called LN-vs) express the neuropeptide PDF. Genetic experiments indicate LN-vs are the primary pacemakers neurons controlling daily locomotion and that PDF is the principal circadian transmitter. Further definition of pacemaker properties in several model systems will provide a useful basis with which to describe circadian output mechanisms.

Electrophysiology and pharmacology of projections from the suprachiasmatic nucleus to the ventromedial preoptic area in rat

Extracellular and whole-cell patch-clamp recordings were made from neurons in the ventromedial preoptic area in rat horizontal brain slices. Responses to single-pulse electrical stimulation of the ipsilateral suprachiasmatic nucleus were characterized using peristimulus time histograms or postsynaptic current recordings, and bath application of neurotransmitter receptor antagonists. Extracellular recordings showed that suprachiasmatic nucleus stimulation (50-150 microA) elicited a short-latency suppression in 35 of 64 neurons (55%), with the majority (29/35, 83%) showing a biphasic response consisting of a short-latency suppression followed by a long-duration activation. In addition, 14 cells (22%) showed activation only, while 15 (23%) were unresponsive. The GABA(A) receptor antagonist bicuculline (5-10 microM) reversibly blocked suppressions evoked by suprachiasmatic nucleus stimulation (20/20 cells). In the majority of these neurons (13/20), bicuculline also unmasked an activation in response to stimulation. Activations elicited by suprachiasmatic nucleus stimulation, either in the presence or absence of bicuculline, were blocked by the non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10/10 cells). 6,7-Dinitroquinoxaline-2,3-dione (10 microM) selectively and reversibly blocked the initial, short-duration (<50 ms) activation, but had no effect on the longer-duration activation. In contrast, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 microM) appeared to inhibit the long-duration activation selectively without affecting the initial rapid activation. Combined applications of the two ionotropic glutamate receptor antagonists blocked stimulation-induced activations completely. All the pharmacological effects were concentration dependent. Whole-cell patch-clamp recordings showed that suprachiasmatic nucleus stimulation elicited inhibitory postsynaptic currents or a combination of inhibitory and excitatory postsynaptic currents in 25 of 33 neurons tested. The inhibitory postsynaptic currents had short onset latencies (4.9+/-0.3 ms) and a reversal potential of -56.0+/-3.8 mV (n=5), and were reversibly blocked by bicuculline (5-10 microM, 4/4 cells). In the presence of bicuculline (5-10 microM), excitatory postsynaptic currents had short onset latencies (4.7+/-0.5 ms), and had a fast and a slow component. (+/-) 3-(2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid blocked the slow, but not the fast, component, whereas 6,7-dinitroquinoxaline-2, 3-dione blocked the fast, but not the slow, component (n=7). These results suggest that the projection from the suprachiasmatic nucleus conveys both inhibitory (GABA) and excitatory (glutamate) inputs to the ventromedial preoptic area. GABA(A) receptor and both non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptors mediate these influences. These inputs may be responsible for conveying information related to circadian phase from the suprachiasmatic nucleus to regions of the preoptic area known to be involved in regulation of sleep/waking and other physiological functions.

Vasopressin neurotransmission and the control of circadian rhythms in the suprachiasmatic nucleus

Vasopressin (VP) is one of the principal transmitters in the suprachiasmatic nucleus (SCN). Approximately 20% of neurones in the dorsomedial division of the SCN synthesize the peptide and a high proportion of SCN neurones (> 40%) are excited by VP acting through the V1 receptor. This suggests that VP may act as a feedback regulator of electrical activity within the nucleus. Such an intrinsic excitatory signal can be demonstrated by perifusion with a V1 antagonist which reduces spontaneous neural activity. As the synthesis and release of VP occurs in a circadian manner, this leads to a variable feedback excitation which may contribute to the circadian pattern of activity of the neural clock. This role in amplifying rhythmicity is supported by observations that animals deficient in VP show a reduced circadian amplitude of behavioural rhythms (e.g. locomotor and cortical electroencephalographic rhythms). VP expression declines during ageing and although aged animals show no change in the proportion of SCN neurones excited by VP, the rhythm of spontaneous electrical activity shows a progressive decline, consistent with the reduced endogenous excitatory feedback. However, the homozygous Brattleboro rat which lacks any VP expression still maintains rhythms of electrical activity, indicating that VP is not the sole factor generating circadian activity. The generation of this rhythmicity may depend upon the interaction of VP with other transmitter systems, such as the inhibitory transmitters somatostatin and GABA which show a circadian variation in efficacy. In addition to its role in feedback amplification of the endogenous rhythm of electrical activity, VP also functions as part of the efferent signal to the rest of the CNS where it potentially regulates a number of behavioural and physiological rhythms, including the circadian activity of the hypothalamo-pituitary-adrenal axis. Thus, the combined amplification and signalling functions makes VP an important component of the neuronal clock function in mammals.

Fetal tissue containing the suprachiasmatic nucleus restores multiple circadian rhythms in old rats

The suprachiasmatic nucleus (SCN) is the major circadian pacemaker in mammals. When fetal tissue containing the SCN is transplanted into young rats whose circadian rhythms have been abolished by SCN lesions, the rhythms gradually reappear. Circadian rhythms in many rats deteriorate or disappear with age. The rationale of the present study was that old rats with poor circadian rhythms resemble young rats with SCN lesions. If there is a similar mechanism underlying this resemblance, then fetal tissue containing the SCN should restore rhythms in old rats. Therefore, we implanted such tissue into the third ventricle of intact aged rats with poor circadian rhythms. Body temperature, locomotor activity, and/or drinking were measured simultaneously within subjects. Grafts and hosts were stained immunocytochemically for vasoactive intestinal polypeptide (VIP), arginine vasopressin (AVP), and neuropeptide Y (NPY). Of 23 SCN grafts, 14 were viable (cells observable with Nissl or peptide staining). In 7 of the 14 aged hosts, up to three circadian rhythms were improved or restored. VIP cells were always observable, which was not the case for AVP cells or NPY fibers. In the other seven hosts, no circadian rhythm was improved. Compared with the successful grafts, these unsuccessful grafts had similar amounts of AVP and NPY staining but significantly less VIP cell and/or fiber staining. Fetal cerebellar grafts, which do not contain any of the three peptides, did not improve or restore any rhythms. Thus the degeneration of circadian rhythms in aged rats may be due, at least in part, to deterioration of the aged SCN and in particular, to a loss of function of VIP-containing neurons.

Postmortem anterograde tracing of intrahypothalamic projections of the human dorsomedial nucleus of the hypothalamus

Together with the paraventricular nucleus (PVN), the dorsomedial nucleus of the hypothalamus (DMH) acts as one of the hypothalamic centers that integrate autonomic and central information. The DMH in the rat brain has extensive intrahypothalamic connections and is implicated in a wide variety of functions. Up until now, no knowledge has been available to indicate that the human DMH might have functions similar to those of the rat DMH. In the present study, intrahypothalamic efferent projections of the human DMH were revealed by a recently developed in vitro postmortem tracing method. It was found that the most densely innervated areas are the PVN, the ventromedial nucleus of the hypothalamus, and the area below the PVN. Other significant terminal fields include the periventricular nucleus, the lateral hypothalamic area, and the medial part of the anteroventral hypothalamic area. Scarce fibers project to the suprachiasmatic nucleus, infundibular nucleus, posterior hypothalamic nucleus, and posterior part of the bed nucleus of the stria terminals. The projections of the ventral and dorsal part of the DMH show some differences. The dorsal part of the DMH has denser projections to the dorsal part of the PVN than to the ventral part of the PVN. In contrast, the ventral part of the DMH has denser projections to the ventral part of the PVN. Labeled fibers in the PVN from ventral and dorsal DMH appear to run near many vasopressin and oxytocin neurons of different sizes, and also near some corticotropin- releasing hormone neurons, suggesting that the DMH neurons may directly affect the functioning of these PVN neurons. In many aspects, the observed projections of the human DMH resemble those of the rat, indicating that the organization of DMH intrahypothalamic projections of human is similar to that of rat. The functional significance of DMH intrahypothalamic connections is discussed.

Restricted daytime feeding modifies suprachiasmatic nucleus vasopressin release in rats

The authors have shown previously that vasopressin (VP) release from suprachiasmatic nucleus (SCN) efferents in rats is important for the timing of the circadian activity of the hypothalamo-pituitary-adrenal (HPA) axis, resulting in a circadian rise in corticosterone at dusk. When meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosterone peak at dusk. This feeding-associated rise in corticosterone is regarded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors investigated a putative involvement of SCN-derived VP in the control of the prefeeding corticosterone peak by measuring the intranuclear release of VP in the SCN and plasma corticosterone levels in rats in ad libitum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted daytime feeding caused clear changes in the daily release pattern of VP from SCN terminals. Both a delayed onset of the diurnal rise and a premature decline of the elevated daytime levels were observed, but the acrophase of the VP rhythm was not phase shifted. Concerning the circadian corticosterone peak, no phase shift of its acrophase was observed either. It is concluded that (1) restricted daytime feeding does affect SCN activity, (2) intranuclear release of VP within the SCN is an important mechanism to amplify and synchronize the circadian rhythms as dictated by the light/dark-entrained circadian pacemaker, and (3) VP release observed in animals on restricted feeding is completely compatible with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.

Evidence for a direct neuronal pathway from the suprachiasmatic nucleus to the gonadotropin-releasing hormone system: combined tracing and light and electron microscopic immunocytochemical studies

The timing and occurrence of the preovulatory luteinizing hormone (LH) surge in the female rodent are critically dependent on the integrity of the suprachiasmatic nucleus (SCN). Destruction of the SCN leads to a cessation of the ovarian cycle, whereas implantation of estrogen in ovariectomized rats results in daily LH surges. The anatomical substrate for these effects is not known. Previous studies involving lesions of the SCN have suggested the presence of a direct vasoactive intestinal polypeptide (VIP)-containing pathway to gonadotropin-releasing hormone (GnRH) neurons. To further investigate the direct connection between the SCN and the GnRH system, we have used tract-tracing with the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PhaL) in combination with an immunocytochemical staining for GnRH in light and electron microscopic studies. Small, unilateral PhaL deposits, especially when they were placed in the rostral ventrolateral portion of the SCN, revealed a bilateral projection to the preoptic area, where PhaL-immunoreactive fibers were regularly found in close apposition to GnRH neurons. Ultrastructural studies showed synaptic interaction of PhaL-containing fibers with GnRH-immunoreactive (IR) cell bodies, thus demonstrating a direct SCN-GnRH connection. Taken together, these data provide evidence for the existence of a monosynaptic pathway from the SCN to the GnRH system in the hypothalamus of the female rat. We suggest that this pathway may contain at least VIP as a putative transmitter and may play a role in the circadian regulation of the estrous cycle in the female rat.

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.

In situ hybridization analysis of vasopressin mRNA expression in the mouse hypothalamus: diurnal variation in the suprachiasmatic nucleus

The distribution, and diurnal variation of AVP mRNA-expressing neurons in the hypothalamus of the mouse has been investigated using in situ hybridization histochemistry. In general, cells hybridizing with an AVP mRNA-specific oligonucleotide probe in the mouse hypothalamus exhibit a similar distribution to the well-characterized distribution of AVP nuclei in the rat, but species-specific patterns of expression have been observed, a finding that confirms the results of earlier immunocytochemical studies. For example, prominent groups of AVP mRNA expressing cells are found in the region between the paraventricular (PVN) and suprachiasmatic (SCN) nuclei, forming the distinct mouse accessory nucleus, and a periventricular group that merges with the PVN neurons. Sampling of brains during both phases of the daily cycle (either 10.00 h (light) or 22.00 h (dark)) revealed a marked and significant variation in AVP mRNA abundance in the SCN whereas a similar variation was not consistently observed in the magnocellular neurons of the supraoptic nucleus (SON). This study has confirmed the distribution of AVP-synthesizing neurons in the mouse hypothalamus, and provided an anatomical substrate for molecular genetic studies in this species that are designed to investigate the basis of neuronal rhythmicity.

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.

Suprachiasmatic nucleus of the human brain: an immunocytochemical and morphometric analysis

BACKGROUND

The present paper describes the immunocytochemical and morphometric characteristics of two major cell groups of the suprachiasmatic nucleus (SCN) in the human hypothalamus: the vasopressin (VP) and vasoactive intestinal polypeptide (VIP) neuronal subdivisions. The dimensions (volume and length) and the number of neurons expressing each peptide in the two subdivisions were obtained, as well as the mean diameter of the cell nuclei. All morphometric parameters were studied in relation to sex and age.

METHODS

Brains of 42 human subjects (22 males and 20 females) ranging in age from 10 to 92 years were obtained at autopsy. The hypothalamic area containing the SCN was dissected from each brain, dehydrated, and embedded in paraffin. Serial sections of 6 microns were cut in a coronal plane and stained with thionin for general orientation. To determine the architectonic boundaries of the VP- and VIP-expressing cell populations every 25th section was immunocytochemically stained by means of antibodies against arginine VP or VIP using the peroxidase-antiperoxidase method. The VP- and VIP-expressing cell numbers in the SCN of each subject were estimated by unilaterally counting the number of nuclear profiles with the aid of a Zeiss microscope under x 500 magnification, using a deconvolution procedure and a correction for section thickness.

RESULTS

The main portion of the VP positive neurons is located in the dorsomedial part of the SCN and is rostrocaudally longer in females than in males (1.76 +/- 0.12 mm and 1.40 +/- 0.10 mm, respectively). The volume of the VP subdivision is 0.244 +/- 0.017 mm3 and contains 6,890 +/- 520 VP-immunoreactive neurons, with a mean density of about 29,000 neurons/mm3. No significant sexual dimorphism or age-related alterations in the population of VP neurons is found. The VIP positive neurons are mainly located in the ventral and central part of the SCN and extend rostrocaudally in a similar way in females and males (1.07 +/- 0.08 mm and 1.02 +/- 0.11 mm, respectively). The volume of the VIP subdivision is 0.034 +/- 0.004 mm3 and contains 1,700 +/- 140 VIP-immunoreactive neurons, with a mean density of about 63,000 neurons/mm3. An age-dependent sexual dimorphism is observed in the number of VIP-expressing neurons in the SCN: young males have about twice as many VIP neurons as females of the same age, whereas in middle-aged subjects this sexual difference is reversed, and less robust, with females now having about 1.7 times as many VIP neurons as males. In old subjects the difference in VIP cell number between men and women disappears.

CONCLUSIONS

The present study clearly shows that the population of VP neurons in the human SCN is considerably larger than the population of VIP neurons. Furthermore, the age-related sexual differences in the VIP cell number reinforces the idea that the SCN is not only involved in the timing of circadian rhythms but also in the temporal organization of reproductive functions.

Influence of aging on the seasonal rhythm of the vasopressin-expressing neurons in the human suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN) is considered to be a major component of the biological clock implicated in the temporal organization of a variety of physiological, endocrine, and behavioral processes. There is now a great deal of evidence indicating that many of these rhythms are progressively disturbed during senescence. The present study was aimed at investigating the influence of aging on the seasonal rhythm of the vasopressin (VP)-expressing neurons in the human SCN. To that end, brains obtained at autopsy of 48 human subjects, ranging in age from 6 to 91 years, were studied. Subjects were divided into two age groups, viz. "young subjects" (up to 50 years) and "elderly subjects" (over 50 years). It is shown that the number of VP-immunoreactive neurons in the human SCN exhibits a marked annual oscillation in young but not in elderly people. Whereas in young subjects low VP-immunoreactive neuron numbers were found during the summer (May-July) and peak values in autumn (September-November), the SCN of elderly people showed a disrupted annual cycle with a reduced amplitude. These data suggest that the biosynthesis of vasopressin in the human SCN exhibits a seasonal rhythm that becomes disturbed later in life.

Colocalization of gamma-aminobutyric acid with vasopressin, vasoactive intestinal peptide, and somatostatin in the rat suprachiasmatic nucleus

The seemingly contradictory observations in previous publications that gamma-aminobutyric acid (GABA) is detected in all cell bodies of the suprachiasmatic nucleus (SCN) and that terminals originating from the SCN are only 20-30% GABA positive prompted us to investigate whether this might be explained by a preference of colocalization in terminals of certain peptidergic neurons in the SCN or by a day/night rhythm in GABA synthesis. At three different circadian times, animals were perfusion fixed, and their SCNs were stained for vasopressin (VP), somatostatin (SOM), or vasoactive intestinal polypeptide (VIP). Subsequently, the number of GABA peptide-positive terminals was determined using GABA postembedding staining in ultrathin sections. It appeared that the highest percentage of colocalization with GABA was detected in VIP terminals (38%) and the lowest in VP terminals (15%). No differences in colocalization percentages could be observed in any parameter at any circadian time. In the dorsomedial hypothalamus, one of the target areas of the VP and VIP fibers from the SCN, a colocalization of GABA within VP and VIP terminals was found similar to that in the SCN. In the region of the somatostatin-containing neurons in the SCN, a number of axoaxonal contacts could be observed that sometimes exhibited synaptic specializations. In nearly all cases, the axoaxonic terminals contained GABA and/or SOM. The conclusion is that the high level of intrinsic GABAergic connections in the SCN represents a putatively powerful mechanism to synchronize or shut down the activity of the SCN. We discuss the possibility that, depending on the firing frequency of the neurons, the colocalization of GABA with all peptides under investigation allows for the selection of which transmitter is released, the peptidergic one or the amino acid.

Ultrastructural evidence for intra- and extranuclear projections of GABAergic neurons of the suprachiasmatic nucleus

GABAergic projections of the suprachiasmatic nucleus (SCN) were demonstrated in a double-labelling ultrastructural study which visualised the efferents of the SCN by PHA-L tracing, diaminobenzidine (DAB) immunocytochemistry, and GABA with immunogold postembedding staining. The results show a strong contralateral projection of the SCN that is partly GABA-containing. In addition, ipsilateral SCN projections to the dorsomedial hypothalamus and periventricular part of the paraventricular nucleus and sub-paraventricular nucleus were shown to contain GABA. The present results indicate that the SCN may utilize this inhibitory neurotransmitter to regulate and organize its own circadian rhythm as well as using GABA to transmit its diurnal information to other regions of the brain.

Projections of the suprachiasmatic nucleus to stress-related areas in the rat hypothalamus: a light and electron microscopic study

The purpose of the present study was to investigate the sites in the hypothalamus where the suprachiasmatic nucleus (SCN) may influence corticosteroid secretion. In spite of the well established, SCN-mediated, daily rhythms in adrenocorticotrophic hormone (ACTH) and corticosteroid secretion, previous studies determining the projections of the suprachiasmatic nucleus failed to illustrate direct connections with corticotrophin-releasing hormone neurons (CRH). In order to identify where in the central nervous system the SCN may influence corticosteroid secretion, areas were selected that contained SCN efferents contacting neurons involved in the stress response. To achieve this in the present study, SCN efferents were visualized by Pha-L tract-tracing, together with the neurons involved in the stress response by immunocytochemical staining for c-fos protein. The sites where these efferents contacted c-fos-positive neurons were established by light microscopic double staining and electron microscopic immunocytochemical studies. It appeared that apart from the medial parvocellular area of the paraventricular nucleus (PVN) of the hypothalamus, many more regions showed fos-positive neurons. Sites where SCN efferents contacted such neurons are limited only to areas immediately adjacent to these putative CRH neurons but are not concentrated on these neurons themselves. These areas consist of the periventricular and rostral PVN together with the dorsomedial hypothalamus: all three regions are known to project into the PVN. Therefore, it is concluded that the SCN transmits its information related to corticosteroid secretion via interneurons in and around the PVN to the CRH-containing neurons, rather than by a direct interaction with these neurons themselves.

Vasopressin and vasoactive intestinal peptide infused in the paraventricular nucleus of the hypothalamus elevate plasma melatonin levels

The connection between the suprachiasmatic nucleus (SCN) and the paraventricular nucleus of the hypothalamus (PVN) forms an important component of the melatonin rhythm-generating system. However, the chemical identity of this projection is not known. To test the possible implication of the SCN peptides vasopressin (VP) and vasoactive intestinal peptide (VIP) in this projection, we performed microinfusions in the PVN during the first half of the dark period and subsequently monitored resulting plasma melatonin levels. Infusions for 7 hr of either VP or VIP, but not oxytocin, caused increased plasma melatonin levels in the middle of the dark period. These observations confirm the role of the PVN in the melatonin rhythm-generating pathway and indicate that both VP and VIP released at the level of the PVN, and probably derived from the SCN, are able to influence peripheral plasma melatonin levels.