哺乳动物昼夜节律组构中的下丘脑视交叉上核和松果腺

哺乳动物下丘脑视交叉上核（SCN）是昼夜节律最主要的起搏器,控制着机体的生理和行为的节律。它具有自身内在的节律性,同时也受光照周期信号和一些内源性化学物质的调节。检查腺分泌裉黑素（MEL）受SCN的调控,MEL通过作用于SCN上高亲和性MEL受体,启动第二、第三信使系统,调整SCN的昼夜节律活动。这种调整具有时间敏感性。     

Paraventricular-subparaventricular hypothalamic lesions selectively affect circadian function

The circadian timing system has three principal components: (i) entrainment pathways, (ii) pacemakers, and (iii) efferent pathways from the pacemakers that convey the circadian signal to effector systems. The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal mammalian circadian pacemaker and, although we understand the organization of entrainment pathways to the SCN and the pacemaker itself, we know much less about the functional organization of SCN projections mediating control of effector systems. It is unclear, for example, whether specific subsets of SCN projections control specific effector systems. In this study, we analyzed the effects of lesions ablating the paraventricular hypothalamic nucleus (PVH), with variable extension into the subparaventricular zone (SPVZ) and adjacent structures, on nocturnal pineal melatonin production and rhythms in core body temperature (T_b) and rest-activity (R-A). In accordance with prior work, ablation of the PVH abolishes the nocturnal rise in pineal melatonin. Lesions restricted to the PVH do not affect rhythms in T_b and R-A but lesions extending caudally and ventrally into the SPVZ disrupt the R-A rhythm proportionate to the interruption of caudal SCN projections without affecting the rhythm in T_b. We conclude that pacemaker regulation of the circadian rhythms analyzed in this study is mediated by discrete sets of SCN projections: (i) dorsal projections to the PVH control pineal melatonin production; (ii) rostral projections to the anterior hypothalamic/preoptic areas mediate the T_b rhythm; and (iii) caudal projections to the SPVZ and hypothalamic arousal systems located in the posterior and lateral hypothalamic areas control the rhythm in R-A.     

松果体昼夜节律生物钟分子机制的研究进展

在各种非哺乳类脊椎动物中 ,松果体起着中枢昼夜节律振荡器的作用。近来 ,在鸟类松果体中相继发现了几种钟基因 ,如Per、Cry、Clock和Bmal等 ,其表达的时间变化规律与哺乳类视交叉上核 (SCN)的非常相似。钟的振荡由其自身调控反馈环路的转录和翻译组成 ,鸟类松果体和哺乳类SCN似乎具有共同的钟振荡基本分子构架 ;若干钟基因产物作为正向或负向调节子影响钟的振荡 ;昼夜性的控时机制同时也需要翻译后事件的参与。这些过程对钟振荡器的稳定性和 /或钟导引的光输入通路有着重要的调控作用     

Hypothalamic regulation of circadian noradrenergic input to the chick pineal gland

Summary While the avian pineal gland contains circadian oscillators and photoreceptors capable of producing circadian rhythms of the hormone melatonin, it is extensively innervated by post-ganglionic fibers of the superior cervical ganglia which release norepinephrine (NE) rhythmically. Norepinephrine turnover is higher during subjective day than during subjective night. In mammals, this rhythmic input, which is higher in subjective night than subjective day, derives from the hypothalamic suprachiasmatic nuclei (SCN) and is essential for rhythmic melatonin production. The present study was designed to determine whether one of two candidates for the avian homologue of the mammalian SCN is necessary for rhythmic NE turnover in the chick pineal gland. Either electrolytic lesions or sham lesions were delivered to the periventricular preoptic nuclei (PPN) or to the visual suprachiasmatic nucleus (vSCN). After recovery, the rates of decline in [NE] were determined following pretreatment with -methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, at mid-subjective day or at mid-subjective night. Birds receiving sham surgeries in either PPN or vSCN and birds receiving lesions of the PPN exhibited rhythmicity in NE turnover. No rhythm of NE turnover could be determined in birds with ablated vSCN.Abbreviations AMPT -methyl-p-tyrosine - DS supraoptic decussation - EBZ ear bar zero (see Methods) - GLv ventral lateral geniculate body - NE norepinephrine - PPN periventricular preoptic nuclei - RH retinohypothalamic projection - SCN suprachiasmatic nuclei - vSCN visual suprachiasmatic nucleus     

Multiple oscillators in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of the pacemaker that controls circadian rhythms of a variety of physiological functions. Data strongly indicate the majority of the SCN neurons express self-sustaining oscillations that can be detected as rhythms in the spontaneous firing of individual neurons. The period of single SCN neurons in a dissociated cell culture is dispersed in a wide range (from 20h to 28h in rats), but that of the locomotor rhythm is close to 24h, suggesting individual oscillators are coupled to generate an averaged circadian period in the nucleus. Electrical coupling via gap junctions, glial regulation, calcium spikes, ephaptic interactions, extracellular ion flux, and diffusible substances have been discussed as possible mechanisms that mediate the interneuronal rhythm synchrony. Recently, GABA (γ-aminobutyric acid), a major neurotransmitter in the SCN, was reported to regulate cellular communication and to synchronize rhythms through GABA_A receptors. At present, subsequent intracellular processes that are able to reset the genetic loop of oscillations are unknown. There may be diverse mechanisms for integrating the multiple circadian oscillators in the SCN. This article reviews the knowledge about the various circadian oscillations intrinsic to the SCN, with particular focus on the intercellular signaling of coupled oscillators. (Chronobiology International, 18(3), 371-387, 2001)     

Melatonin's Role in Vertebrate Orcadian Rhythms

The circadian secretion of melatonin by the pineal gland and retinae is a direct output of circadian oscillators and of the circadian system in many species of vertebrates. This signal affects a broad array of physiological and behavioral processes, making a generalized hypothesis for melatonin function an elusive objective. Still, there are some common features of melatonin function. First, melatonin biosynthesis is always associated with photoreceptors and/or cells that are embryonically derived from photoreceptors. Second, melatonin frequently affects the perception of the photic environment and has as its site of action structures involved in vision. Finally, melatonin affects overt circadian function at least partially via regulation of the hypothalamic suprachiasmatic nucleus (SCN) or its hofnologues. The mechanisms by which melatonin affects circadian rhythms and other downstream processes are unknown, but they include interaction with a class of membrane-bound receptors that affect intracellular processes through guanosine triphosphate (GTP)-binding protein second messenger systems. Investigation of mechanisms by which melatonin affects its target tissues may unveil basic concepts of neuromodulation, visual system function, and the circadian clock.     

The circadian regulation of extracellular ATP

Extracellular ATP is a potent signaling molecule released from various cells throughout the body and is intimately involved in the pathophysiological functions of the nervous system and immune system by activating P2 purinergic receptors. Recent increasingly studies showed that extracellular ATP exhibits circadian oscillation with an approximately 24-h periodicity, which participates in regulatory pathways of central oscillator suprachiasmatic nucleus and peripheral oscillator bladder, respectively. Oscillators modulate the protein expression of ATP release channels and ectonucleotidase activity through clock genes; indeed, real-time alterations of ATP release and degradation determine outcomes of temporal character on extracellular ATP rhythm. The regulatory pathways on extracellular ATP rhythm are different in central and peripheral systems. In this review, we summarize the circadian rhythm of extracellular ATP and discuss several circadian regulatory pathways in different organs via ATP release and degradation, to provide a new understanding for purinergic signaling in the regulatory mechanism of circadian rhythm and a potential target to research the circadian regulation of extracellular ATP in other circadian oscillators.     

Different patterns of circadian oscillation in the suprachiasmatic nucleus of hamster,mouse, and rat

Although spontaneous neural firing in the mammalian suprachiasmatic nucleus is accepted to peak once during mid-subjective day, dual activity peaks have been reported in horizontal brain slices taken from hamsters. These two peaks were interpreted as new evidence for the theory of dual circadian oscillators and raised the expectation that such activity would be found in other circadian model systems. We examined hamster, mouse, and rat slices in both coronal and horizontal planes and found a second peak of activity only in hamster horizontal preparations. This raises interesting questions about the relative circadian physiology of these important experimental animals.Abbreviations CT circadian time - SCN suprachiasmatic nucleus P.W. Burgoon and P.T. Lindberg contributed equally to this work.     

A morphological study of the circadian cycle of the pineal gland of the rat

Summary To seek a morphological expression of circadian rhythm, we investigated cytologically pineal glands taken from rats every 2 to 4 h under a lighting regime of 12 h of illumination (6:00 to 18:00) and 12 h of darkness. The changes in the number of synaptic ribbons and ribbon fields was observed by electron microscopy. The number of these intracellular elements was greatest at 2:00 and the lowest at 14:00, the difference being statistically significant. There were no significant local differences in numbers with respect to the part of the pineal gland examined. The data are similar to those of Vollrath from the pineal gland of a guinea pig, and seem to confirm a circadian function in the pineal gland in mammals.Supported by grants-in-aid from the Ministry of Education, Science and Culture, and the Ministry of Health and Welfare     

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

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

Hypothalamic circadian organization in birds. I. Anatomy,functional morphology,and terminology of the suprachiasmatic region

In mammals, the “master clock” controlling circadian rhythmicity is located in the hypothalamic suprachiasmatic nuclei (SCN). Until now, no comparable structure has been unambiguously described in the brain of any nonmammalian vertebrate. In birds, early anatomical and lesioning studies described a SCN located in the anterior hypothalamus, but whether birds possess a nucleus equivalent to the mammalian SCN remained controversial. By reviewing the existing literature it became evident that confusion in delineation and nomenclature of hypothalamic cell groups may be one of the major reasons that no coherent picture of the avian hypothalamus exists. In this review, we attempt to clarify certain aspects of the organization of the avian hypothalamus by summarizing anatomical and functional studies and comparing them to immunocytochemical results from our laboratory. There is no single cell group in the avian hypothalamus that combines the morphological and neurochemical features of the mammalian SCN. Instead, certain aspects of anatomy and morphology suggest that at least two anatomically distinct cell groups, the SCN and the lateral hypothalamic nucleus (LHN), bear some of the characteristics of the mammalian SCN.     

Coupling-induced synchronization in multicellular circadian oscillators of mammals

In mammals, circadian rhythms are controlled by the neurons located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Each neuron in the SCN contains an autonomous molecular clock. The fundamental question is how the individual cellular oscillators, expressing a wide range of periods, interact and assemble to achieve phase synchronization. Most of the studies carried out so far emphasize the crucial role of the periodicity imposed by the light-dark cycle in neuronal synchronization. However, in natural conditions, the interaction between the SCN neurons is non-negligible and coupling between cells in the SCN is achieved partly by neurotransmitters. In this paper, we use a model of nonidentical, globally coupled cellular clocks considered as Goodwin oscillators. We mainly study the synchronization induced by coupling from an analytical way. Our results show that the role of the coupling is to enhance the synchronization to the external forcing. The conclusion of this paper can help us better understand the mechanism of circadian rhythm.     

Diurnal, circadian and photic regulation of calcium/calmodulin-dependent kinase II and neuronal nitric oxide synthase in the hamster suprachiasmatic nuclei

Mammalian circadian rhythms are entrained by light pulses that induce phosphorylation events in the suprachiasmatic nuclei (SCN). Ca²⁺-dependent enzymes are known to be involved in circadian phase shifting. In this paper, we show that calcium/calmodulin-dependent kinase II (CaMKII) is rhythmically phosphorylated in the SCN both under entrained and free-running (constant dark) conditions while neuronal nitric oxide synthase (nNOS) is rhythmically phosphorylated in the SCN only under entrained conditions. Both p-CaMKII and p-NOS (specifically phosphorylated by CaMKII) levels peak during the day or subjective day. Light pulses administered during the subjective night, but not during the day, induced rapid phosphorylation of both enzymes. Moreover, we found an inhibitory effect of KN-62 and KN-93, both CaMKII inhibitors, on light-induced nNOS activity and nNOS phosphorylation respectively, suggesting a direct pathway between both enzymes which is at least partially responsible of photic circadian entrainment.     

Time-related changes in size of nuclei of pinealocytes in rats

Summary In a study of 96 adult male Sprague-Dawley rats, da-and time-related changes of the mean nuclear volume of pinealocytes were determined with the aim of testing the reproducibility of the karyometric findings described by Quay and Renzoni (1966). It is shown (i) that statistically significant differences exist between the central and peripheral mean volumes of pinealocyte nuclei/group of animals and time point (p<0.001), (ii) that the day and time related differences are statistically different in both regions (p<0.001), (iii) that in the center and periphery of the pineal body different diurnal patterns exist, and (iv) that the diurnal patterns are not parallel in the two regions.The question discussed is (i) whether or not probable superimposed infradian rhythms may account for the different diurnal patterns, and (ii) whether these patterns in the two regions indicate functional differences between the cortex and the medulla of the rat pineal body.Supported by the Deutsche Forschungsgemeinschaft (Grant Vo 135/4) within the Schwerpunkt-programm NeuroendokrinologieThis paper is an abridged version of a thesis submitted for obtaining the degree of Dr. med., Fachbereich Medizin, University of Mainz     

The Effect of Old Age on the Free-Running Period of Circadian Rhythms in Rat

The free-running period is regarded to be an exclusive feature of the endogenous circadian clock. Changes during aging in the free-running period may therefore reflect age-related changes in the internal organization of this clock. However, the literature on alterations in the free-running period in aging is not unequivocal. In the present study, with various confounding factors kept to a minimum, it was found that the free-running periods for active wakefulness, body temperature, and drinking behavior were significantly shorter (by 12-17 min) in old than in young rats. In addition, it was found that the day-to-day stability of the different sleep states was reduced in old rats, whereas that of the drinking rhythm was enhanced. Transient cycles were not observed, nor were there any age-related differences in daily totals of the various sleep-wake states. The amplitudes of the circadian rhythms of active wakefulness, quiet sleep, and temperature were reduced, whereas those of paradoxical sleep and quiet wakefulness remained unchanged.     

Light-induced tyrosine phosphorylation of BIT in the rat suprachiasmatic nucleus

Circadian changes of protein tyrosine phosphorylation in the hypothalamic suprachiasmatic nucleus have been studied using rats maintained under 12-h light/ 12-h dark cycles as well as constant dark conditions. We found that tyrosine phosphorylation of BIT (brain immunoglobulin-like molecule with tyrosine-based activation motifs), a transmembrane glycoprotein of 90-95 kDa, was higher in the light period than in the dark period and was increased after light exposure in the dark period. Similar changes in tyrosine phosphorylation were observed under constant dark conditions, but its amplitude was weaker than that in 12-h light/12-h dark cycles. As the tyrosine-phosphorylated form of BIT is able to bind to the Src homology 2 domain of a protein tyrosine phosphatase, SHP-2, we examined association of these proteins in suprachiasmatic nucleus extracts and found that SHP-2 was coprecipitated with BIT in parallel with its tyrosine phosphorylation. These results suggest that tyrosine phosphorylation of BIT might be involved in light-induced entrainment of the circadian clock.     

Phase organization of circadian oscillators in extended gate and oscillator models

The suprachiasmatic nuclei (SCN) control daily oscillations in physiology and behavior. The gate-oscillator model captures functional heterogeneity in SCN and has been successful in reproducing many features of SCN. This paper investigates the mechanism of phase organization in the gate-oscillator model and finds that only stable fixed points of the phase transition function are essential to phase organization. This obvious finding forms the basis for understanding the complex phase distribution in the gate-oscillator scheme. Extending the model with a dead zone of the phase transition function and the propagation delay of the gate signal which may represent the spatial structure of SCN, the author discusses how some features of experimentally reported phase distribution, such as the existence of anti-phase neurons and fixed phase difference between neurons, could be understood in the framework of the gate-oscillator model. The extended model shows clearly the way in which the interplay between the single-cell property and the property of the network organization influence the phase distribution of SCN neurons.     

Circadian modulation in photic induction of Fos-like immunoreactivity in the suprachiasmatic nucleus cells of diurnal chipmunk,Eutamias asiaticus

Photic induction of immediate early genes including c-fos in the suprachiasmatic nucleus (SCN) has been well demonstrated in the nocturnal rodents. On the other hand, in diurnal rodents, no data is available whether the light can induce c-fos or Fos in the SCN. We therefore examined whether 60 min light exposure induces Fos-like immunoreactivity (Fos-lir) in the SCN cells of diurnal chipmunks and whether the induction is phase dependent, comparing with the results in nocturnal hamsters. We also examined an effect of light on the locomotor activity rhythm under continuous darkness. Fos-lir was induced in the chipmunk SCN. The induction was clearly phase dependent. The light during the subjective night induced strong expression of Fos-lir. This phase dependency is similar to that in hamsters. However, unlike in hamsters, the Fos-lir was induced in some SCN cells of chipmunks exposed to light during the subjective day. In the locomotor rhythm, on the other hand, the light pulse failed to induce the phase shift at phases at which the Fos-lir was induced. These results suggest that the photic induction of Fos-lir in the diurnal chipmunks is gated by a circadian oscillator as well as in the nocturnal hamsters. However, the functional role of Fos protein may be different in the diurnal rodents from in the nocturnal rodents.