The circadian rhythms in mammals are regulated by a pacemaker located in the suprachiasmatic nucleus of the hypothalamus. Four clock-gene families have been found to be involved in a transcription-translation feedback loop that generates the circadian rhythm at the intracellular level. The proteins Clock and Bmal1 form a heterodimer which activates the transcription of the Per gene from the E-box elements in its promoter region. Protein products of Per act together with Cry proteins to inhibit Per transcription, thus closing the autoregulatory feedback loop. We found that Dec1 and Dec2, basic helix-loop-helix transcription factors, repressed Clock/Bmal1-induced transactivation of the mouse Per1 promoter through direct protein-protein interactions with Bmal1 and/or competition for E-box elements. Dec1 and Dec2 are expressed in the suprachiasmic nucleus in a circadian fashion, with a peak in the subjective day. A brief light pulse induced Dec1 but not Dec2 expression in the suprachiasmic nucleus in a phase-dependent manner. Dec1 and Dec2 are regulators of the mammalian molecular clock, and form a fifth clock-gene family.

The pattern of circadian behavioral rhythms is photoperioddependent, highlighted by the conservation of a phase relation between the behavioral rhythm and photoperiod. A model of two separate, but mutually coupled, circadian oscillators has been proposed to explain photoperiodic responses of behavioral rhythm in nocturnal rodents: an evening oscillator, which drives the activity onset and entrains to dusk, and a morning oscillator, which drives the end of activity and entrains to dawn. Continuous measurement of circadian rhythms in clock gene Per1 expression by a bioluminescence reporter enabled us to identify the separate oscillating cell groups in the mouse suprachiasmatic nucleus (SCN), which composed circadian oscillations of different phases and responded to photoperiods differentially. The circadian oscillation in the posterior SCN was phase-locked to the end of activity under three photoperiods examined. On the other hand, the oscillation in the anterior SCN was phase-locked to the onset of activity but showed a bimodal pattern under a long photoperiod [light-dark cycle (LD)18:6]. The bimodality in the anterior SCN reflected two circadian oscillatory cell groups of early and late phases. The anterior oscillation was unimodal under intermediate (LD12:12) and short (LD6:18) photoperiods, which was always phase-lagged behind the posterior oscillation when the late phase in LD18:6 was taken. The phase difference was largest in LD18:6 and smallest in LD6:18. These findings indicate that three oscillating cell groups in the SCN constitute regionally specific circadian oscillations, and at least two of them are involved in photoperiodic response of behavioral rhythm.bioluminescence reporter ͉ circadian rhythm ͉ clock gene ͉ photoperiod ͉ behavioral rhythm A daptation to seasonal changes in environment is critical to the survival of many organisms. Photoperiodic time measurement by the circadian clock is one of the strategies by which they conserve the phase relation between behavioral events such as the activity onset and dawn or dusk (1). A dramatic change induced by photoperiod is in the length of an activity band, the duration of activity in behavioral rhythms. Nocturnal rodents such as rats and mice exhibit compressed activity bands in long photoperiods and decompressed bands in short photoperiods. A long-standing hypothesis for the photoperiodic time measurement assumes two separate, but mutually coupled, circadian oscillators that drive the activity onset and end of activity, respectively, and respond to dawn and dusk differentially. Therefore, their phase-relationship encodes day lengths and changes the length of an activity band (1).The circadian clock in mammals is located in the suprachiasmatic nucleus (SCN) of the hypothalamus; it entrains to a light-dark cycle (LD) and determines the phases of overt circadian rhythms in behavior and physiology (2). Over the last decade, our understanding of the circadian clock in the SCN has advanced tremendously (3-5). The SCN consists of a number of oscillating cel...

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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.

Cryptochrome (Cry) 1 and Cry2 are regarded as critical components for circadian rhythm generation in mammals. Nevertheless, cultured suprachiasmatic nucleus (SCN) of neonatal Cry double deficient (Cry1 À / À /Cry2 À / À ) mice exhibit circadian rhythms that damp out in several cycles. Here, by combining bioluminescence imaging of Per1-luc and PER2::LUC with multielectrode recording, we show developmental changes in SCN circadian rhythms in Cry1 À / À /Cry2 À / À mice. At the tissue level, circadian rhythms are found in neonatal but not in adult SCN, whereas at the cellular level, rhythms are detected in both SCN. Cellular circadian rhythms are synchronized in neonates, but not in adults, indicating a loss of rhythm synchrony in the course of development. Synchronized circadian rhythms in adult Cry1 À / À / Cry2 À / À SCN are restored by coculture of neonatal, but not of juvenile, SCN. These findings indicate that CRY1 and CRY2 are necessary for the development of intercellular networks that subserve coherent rhythm expression in adult SCN.