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Photoperiodic Requirements for Induction and Maintenance of Rhythm Bifurcation and Extraordinary Entrainment in Male Mice

Department of Psychology and Center for Circadian Biology, University of California, San Diego, CA 92093, USA
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Author to whom correspondence should be addressed.
Current address: Department of Biomedical Sciences, Marquette University, Milwaukee, WI 53233, USA.
Current address: Department of Psychology, University of Georgia, Athens, GA 30602, USA.
Clocks & Sleep 2019, 1(3), 290-305; https://doi.org/10.3390/clockssleep1030025
Received: 23 May 2019 / Revised: 21 June 2019 / Accepted: 27 June 2019 / Published: 4 July 2019
(This article belongs to the Section Clocks & Sleep and Zeitgebers (Light))
Exposure of mice to a 24 h light:dark:light:dark (LDLD) cycle with dimly illuminated nights induces the circadian timing system to program two intervals of activity and two intervals of rest per 24 h cycle and subsequently allows entrainment to a variety of extraordinary light regimens including 30 h LDLD cycles. Little is known about critical lighting requirements to induce and maintain this non-standard entrainment pattern, termed “bifurcation,” and to enhance the range of apparent entrainment. The current study determined the necessary duration of the photophase for animals to bifurcate and assessed whether requirements for maintenance differed from those for induction. An objective index of bifurcated entrainment varied with length of the photophase over 4–10 h durations, with highest values at 8 h. To assess photic requirements for the maintenance of bifurcation, mice from each group were subsequently exposed to the LDLD cycle with 4 h photophases. While insufficient to induce bifurcation, this photoperiod maintained bifurcation in mice transferred from inductive LDLD cycles. Entrainment to 30 h LDLD cycles also varied with photoperiod duration. These studies characterize non-invasive tools that reveal latent flexibility in the circadian control of rest/activity cycles with important translational potential for addressing needs of human shift-workers. View Full-Text
Keywords: circadian entrainment; T cycles; bifurcation; phase-shifting; shift-work; SCN; oscillator coupling circadian entrainment; T cycles; bifurcation; phase-shifting; shift-work; SCN; oscillator coupling
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Sun, J.; Joye, D.A.M.; Farkas, A.H.; Gorman, M.R. Photoperiodic Requirements for Induction and Maintenance of Rhythm Bifurcation and Extraordinary Entrainment in Male Mice. Clocks & Sleep 2019, 1, 290-305.

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