Circadian rhythms are defined as approximately 24-hour oscillations in physiology and behavior. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is known as the central circadian ...clock. Based on current understanding, circadian rhythms are believed to be generated by transcription-translation feedback loops (TTFL) involving several clock genes and their protein products. However, several studies have shown that circadian oscillation in single SCN cells is still detectable in several clock gene deficient mice. These results suggest that there might be some oscillatory mechanisms without TTFL in mammalian cells. Other important aspects of circadian rhythms include neuronal circuits in the brain that regulate timing of physiological functions. Especially, functional output pathways from the SCN that regulate sleep and wakefulness have not been identified. In this review, I describe recent findings on circadian rhythm in the SCN, and of neuronal mechanisms that control circadian clock regulated sleep and wakefulness in mice.
•Clock gene, cryptochrome, is not essential for circadian rhythms in mammals.•Mammalian cryptochrome is necessary for the development of oscillatory neuronal networks in the SCN.•SCNGABA-PVNCRF-LHorexin pathway plays a role in circadian clock regulated wakefulness.
In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Individual SCN cells exhibit intrinsic oscillations, and their circadian period and ...robustness are different cell by cell in the absence of cellular coupling, indicating that cellular coupling is important for coherent circadian rhythms in the SCN. Several neuropeptides such as arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are expressed in the SCN, where these neuropeptides function as synchronizers and are important for entrainment to environmental light and for determining the circadian period. These neuropeptides are also related to developmental changes of the circadian system of the SCN. Transcription factors are required for the formation of neuropeptide-related neuronal networks. Although VIP is critical for synchrony of circadian rhythms in the neonatal SCN, it is not required for synchrony in the embryonic SCN. During postnatal development, the clock genes
and
are involved in the maturation of cellular networks, and AVP is involved in SCN networks. This mini-review focuses on the functional roles of neuropeptides in the SCN based on recent findings in the literature.
The suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals, is a network structure composed of multiple types of neurons. Here, we report that mice with a Bmal1 deletion specific ...to arginine vasopressin (AVP)-producing neurons showed marked lengthening in the free-running period and activity time of behavior rhythms. When exposed to an abrupt 8-hr advance of the light/dark cycle, these mice reentrained faster than control mice did. In these mice, the circadian expression of genes involved in intercellular communications, including Avp, Prokineticin 2, and Rgs16, was drastically reduced in the dorsal SCN, where AVP neurons predominate. In slices, dorsal SCN cells showed attenuated PER2::LUC oscillation with highly variable and lengthened periods. Thus, Bmal1-dependent oscillators of AVP neurons may modulate the coupling of the SCN network, eventually coupling morning and evening behavioral rhythms, by regulating expression of multiple factors important for the network property of these neurons.
•Bmal1 deficiency in AVP neurons lengthens the free-running period and activity time•Bmal1 deficiency in AVP neurons weakens coupling among SCN neurons•Bmal1 coordinates expression of multiple key factors in AVP neurons•Cellular oscillators in AVP neurons stabilize SCN network circadian oscillations
The primary circadian pacemaker SCN is a network structure composed of multiple types of neurons. Mieda et al. find that Bmal1-dependent oscillators of AVP neurons modulate the coupling of SCN neurons to determine free-running period and activity time.
Abstract
The circadian clock controls daily rhythms of physiological processes. The presence of the clock mechanism throughout the body is hampering its local regulation by small molecules. A ...photoresponsive clock modulator would enable precise and reversible regulation of circadian rhythms using light as a bio-orthogonal external stimulus. Here we show, through judicious molecular design and state-of-the-art photopharmacological tools, the development of a visible light-responsive inhibitor of casein kinase I (CKI) that controls the period and phase of cellular and tissue circadian rhythms in a reversible manner. The dark isomer of photoswitchable inhibitor
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exhibits almost identical affinity towards the CKIα and CKIδ isoforms, while upon irradiation it becomes more selective towards CKIδ, revealing the higher importance of CKIδ in the period regulation. Our studies enable long-term regulation of CKI activity in cells for multiple days and show the reversible modulation of circadian rhythms with a several hour period and phase change through chronophotopharmacology.
Methicillin-susceptible S. aureus changes to methicillin-resistant S. aureus (MRSA) upon the acquisition of staphylococcal cassette chromosome mec (SCCmec), a genomic island that encodes methicillin ...resistance. SCCmec elements in S. aureus are classified into different types based on the combination of mec gene complexes and ccr gene complexes, which share variations, five classes in mec and eight in ccr. To date, at least 13 types of SCCmec elements have been identified and each SCCmec type has individual characteristics. It is known that hospital-associated MRSA strains carry SCCmec elements of types, I, II, and III, and the majority of community-acquired MRSA strains carry characteristic SCCmec elements, type IV SCCmec or type V SCCmec. We herein describe multiplex PCR methods to type SCCmec elements by identifying the mec gene complex class and ccr gene complex type.
Sleep/wakefulness cycle is regulated by coordinated interactions between sleep- and wakefulness-regulating neural circuitry. However, the detailed mechanism is far from understood. Here, we found ...that glutamic acid decarboxylase 67-positive GABAergic neurons in the ventral tegmental area (VTA
) are a key regulator of non-rapid eye movement (NREM) sleep in mice. VTA
project to multiple brain areas implicated in sleep/wakefulness regulation such as the lateral hypothalamus (LH). Chemogenetic activation of VTA
promoted NREM sleep with higher delta power whereas optogenetic inhibition of these induced prompt arousal from NREM sleep, even under highly somnolescent conditions, but not from REM sleep. VTA
showed the highest activity in NREM sleep and the lowest activity in REM sleep. Moreover, VTA
directly innervated and inhibited wake-promoting orexin/hypocretin neurons by releasing GABA. As such, optogenetic activation of VTA
terminals in the LH promoted NREM sleep. Taken together, we revealed that VTA
play an important role in the regulation of NREM sleep.
In mammals, circadian rhythms, such as sleep/wake cycles, are regulated by the central circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN consists of thousands ...of individual neurons, which exhibit circadian rhythms. They synchronize with each other and produce robust and stable oscillations. Although several neurotransmitters are expressed in the SCN, almost all SCN neurons are γ-amino butyric acid (GABA)-ergic. Several studies have attempted to understand the roles of GABA in the SCN; however, precise mechanisms of the action of GABA in the SCN are still unclear. GABA exhibits excitatory and/or inhibitory characteristics depending on the circadian phase or region in the SCN. It can both synchronize and destabilize cellular circadian rhythms in individual SCN cells. Differing environmental light conditions, such as a long photoperiod, result in the decoupling of circadian oscillators of the dorsal and ventral SCN. This is due to high intracellular chloride concentrations in the dorsal SCN. Because mice with functional GABA deficiency, such as vesicular GABA transporter- and glutamate decarboxylase-deficient mice, are neonatal lethal, research has been limited to pharmacological approaches. Furthermore, different recording methods have been used to understand the roles of GABA in the SCN. The excitability of GABAergic neurons also changes during the postnatal period. Although there are technical difficulties in understanding the functions of GABA in the SCN, technical developments may help uncover new roles of GABA in circadian physiology and behavior.
In mammals, the temporal order of physiology and behavior is primarily regulated by the circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Rhythms are generated in cells ...by an auto-regulatory transcription/translation feedback loop, composed of several clock genes and their protein products. Taking advantage of bioluminescence reporters, we have succeeded in continuously monitoring the expression of clock gene reporters Per1-luc, PER2::LUC and Bmal1-ELuc in the SCN of freely moving mice for up to 3 weeks in constant darkness. Bioluminescence emitted from the SCN was collected with an implanted plastic optical fiber which was connected to a cooled photomultiplier tube. We found robust circadian rhythms in the clock gene expression, the phase-relation of which were the same as those observed ex vivo. The circadian rhythms were superimposed by episodic bursts which had ultradian periods of approximately 3.0 h. Episodic bursts often accompanied activity bouts, but stoichiometric as well as temporal analyses revealed no causality between them. Clock gene expression in the SCN in vivo is regulated by the circadian pacemaker and ultradian rhythms of unknown origin.
The food-entrainable oscillator, which underlies the prefeeding activity peak developed by restricted daily feeding (RF) in rodents, does not depend on the circadian pacemaker in the suprachiasmatic ...nucleus (SCN) or on the known clock genes. In the present study, to clarify the roles of SCN circadian pacemaker and nutrient conditions on the development of prefeeding activity peak, RF of 3-h daily feeding was imposed on four groups of adult male mice for 10 cycles at different circadian times, zeitgeber time (ZT)2, ZT8, ZT14, and ZT20, where ZT0 is the time of lights-on in LD12:12. Seven days after the termination of RF session with ad libitum feeding in between, total food deprivation (FD) for 72 h was imposed. Wheel-running activity and core body temperature were measured throughout the experiment. Immediately after the RF or FD session, the PER2::LUC rhythms were measured in the cultured SCN slices and peripheral tissues. Not only the buildup process and magnitude of the prefeeding activity peak, but also the percentages of nocturnal activity and hypothermia developed under RF were significantly different among the four groups, indicating the involvement of light entrained circadian pacemaker. The buildup of prefeeding activity peak was accomplished by either phase-advance or phase-delay shifts (or both) of activity bouts comprising a nocturnal band. Hypothermia under FD was less prominent in RF-exposed mice than in naïve counterparts, indicating that restricted feeding increases tolerance to caloric restriction as well as to the heat loss mechanism. RF phase-shifted the peripheral clocks but FD did not affect the clocks in any tissue examined. These findings are better understood by assuming multiple bout oscillators, which are located outside the SCN and directly drive activity bouts uncoupled from the circadian pacemaker by RF or hypothermia.
Orexin/hypocretin-producing and melanin-concentrating hormone-producing (MCH) neurons are co-extensive in the hypothalamus and project throughout the brain to regulate sleep/wakefulness. Ablation of ...orexin neurons decreases wakefulness and results in a narcolepsy-like phenotype, whereas ablation of MCH neurons increases wakefulness. Since it is unclear how orexin and MCH neurons interact to regulate sleep/wakefulness, we generated transgenic mice in which both orexin and MCH neurons could be ablated. Double-ablated mice exhibited increased wakefulness and decreased both rapid eye movement (REM) and non-REM (NREM) sleep. Double-ablated mice showed severe cataplexy compared with orexin neuron-ablated mice, suggesting that MCH neurons normally suppress cataplexy. Double-ablated mice also showed frequent sleep attacks with elevated spectral power in the delta and theta range, a unique state that we call 'delta-theta sleep'. Together, these results indicate a functional interaction between orexin and MCH neurons in vivo that suggests the synergistic involvement of these neuronal populations in the sleep/wakefulness cycle.
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