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

Abstract

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.

Keywords: SCN; T cycles; bifurcation; circadian entrainment; oscillator coupling; phase-shifting; shift-work.

Figure 1

Illustrative wheel-running actograms of mice from each condition in Experiment 1. Actograms (A–D) are double-plotted modulo 24 h and scaled from 0 to 100 wheel revolutions per minute compiled in 6 min bins. Mice were initially exposed to LD_im4:8 (A), LD_im6:6 (B), LD_im8:4 (C) or LD_im10:2 (D) for 4 weeks (upper third of actograms) followed by 6 weeks of exposure to LD_im4:8 (middle of actograms) and, finally, 2 weeks of D_imD_im (lower portion of actograms). Scotophases are indicated with dark shading. BSI values are shown for these individual animals and indicate the intervals analyzed for all animals to generate data averaged in Table 2.

Figure 2

Mean ± sem BSI values (A) in various Phase 1 photoperiods (open bars) and in Phase 2 LD_im4:8 (filled bars) of Experiment 1. Individual data points and group mean vectors (B) of projected activity onsets relative to the beginning of D_imD_im.

Figure 3

Illustrative wheel-running actograms of mice from each condition in Experiment 2. Actograms are double-plotted modulo 24 h (A–D) or modulo 30 h (E–H). Mice were initially exposed to LD_im4:8 LD_im5:7 LD_im6:6 or LD_im7:5 for 8 weeks except that nights were completely dark in weeks 5–6 (Phase 2). Conditions were changed to LD_im7:8, LD_im8:7, LD_im9:6 or LD_im10:5 for 2 weeks before all were exposed to LD_im7:8 for 4 additional weeks (Phase 3). For clarity, shading is omitted during T15 LD_im cycles in modulo 24 h plots. Other conventions as described in Figure 1. The red line indicates missing data as a result of a technical failure.

Figure 4

Mean ± sem EQ values in T15 LD_im conditions of Phase 2 of Experiment 2 that differed in photoperiod (open bars) and in Phase 3 when all animals were maintained in identical LD_im7:8 (filled bars).