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¹ Department of Physiology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan
² Department of Oral Functional Science, Hokkaido University Graduate School of Dentistry, Sapporo 060-8586, Japan
³ Research Institute for Cell Engineering, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan
⁴ Molecular Clock Project, Research Center for Genomic Medicine, Saitama Medical School, Hidaka, Saitama 350-1241, Japan
⁵ Department of Physiology, Saitama Medical School, Moroyama, Saitama 350-0495, Japan

A new reporter system for monitoring expressions of two clock genes, Per1 and Bmal1, from a single tissue in culture was developed in mice. Reporters are Vargula hilgendorfii luciferase (VL) and firefly luciferase (FL), whose activities are increased in parallel with Per1 and Bmal1 expressions, respectively. Formal properties of the circadian system in transgenic mice are indistinguishable from those in wild-type animals. Circadian rhythms in Per1-VL and Bmal1-FL in the suprachiasmatic nucleus (SCN) were robust and anti-phasic, although they were phase delayed by 4–8 h as compared with circadian rhythms in respective transcript levels in vivo. In peripheral tissues such as liver, circadian rhythms in Bmal1-FL persisted for more than 3 weeks. In the course of prolonged culture, circadian rhythms apparently damped out, but were restored immediately by refreshment of the culture medium. Restoration of the circadian rhythm is unlikely to be due to resetting of desynchronized population oscillation, because peripheral circadian rhythms did not show a type 0 phase response curve (PRC) for medium refreshment, a requirement for instantaneous resetting of circadian oscillation. Long-term persistence of circadian oscillation in spite of external perturbations supports an idea that circadian oscillations in peripheral tissues are self-sustained.

^* Correspondence: E-mail: sathonma{at}med.hokudai.ac.jp

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