Simple Lighting Manipulations Facilitate Behavioral Entrainment of Mice to 18-h Days

Abstract

In an invariantly rhythmic world, a robust and stable mammalian circadian clock is presumed to confer fitness advantages. In shift-work or after rapid transmeridian travel, however, a stable clock might be maladaptive and a more flexibly resettable clock may have advantages. The rate at which rodents can adjust to simulated time zone travel and the range of entrainment can be markedly increased through simple light manipulations, namely, by exposing animals to extremely dim light (<0.01 lux) at night or by bifurcating rhythms under 24-h light-dark-light-dark (LDLD) cycles. Here we investigated the separate effects of dim light and bifurcation on the ability of mice to entrain to 18-h days (LD 13:5; T18). Incorporating dim light at night, mice in Experiment 1 were exposed either to LD cycles with photophases that were progressively shortened from LD 19:5 to LD 13:5 or to bifurcating LDLD cycles with photophases that were lengthened from LDLD 7:5:7:5 to LDLD 13:5:13:5. In both cases, wheel-running rhythms were robustly synchronized to T18 and the phase of the free-running circadian rhythm was controlled by the timing of release into constant conditions. In Experiment 2, either dimly illuminated nights or a history of bifurcation without continuing dim light was sufficient to allow behavioral entrainment to T18 whereas previously unbifurcated mice under dark nights failed to entrain to T18. Additionally, concurrent measurement of body temperature rhythms in T24 LDLD revealed them to be bimodal. These studies suggest that the circadian system is markedly more flexible than conventionally thought and that this flexibility can be achieved in a noninvasive and nonpharmacological way. Facilitation of behavioral entrainment to extreme light-dark cycles may have translational potential for human shift-workers.

Enhanced Circadian Entrainment in Mice and Its Utility under Human Shiftwork Schedules

“…13 h of light, 5 h dark; LD13:5 or T18LD) and LDLD (e.g. LDLD5:4:5:4 or T18LDLD) cycles (see methods for explanation of nomenclature) [14,17,18]. Under those cycles that have been carefully characterized, behavioral adaptation reflects bona fide entrainment.…”

“…Notably, an explanation of simple masking for entrained behavior has been rejected. For example, following both LDLD and extreme T-cycles there is phase control in complete darkness (DD) [14,17,20] and reorganized rhythms of core body temperature [14,21]. In LDLD conditions alone, further evidence includes persistence of biphasic behavior in skeleton cycles [13], enhanced phase-resetting [16,22], entrainment after-effects [20], biphasic melatonin secretion [23], and altered c-Fos and clock gene expression in the Suprachiasmatic nucleus (SCN) [22,24,25] On the other hand, entrainment to extreme T-cycles lacks the characteristic large phase angle modulation [18] and period after-effects predicted by classical non-parametric entrainment theory [26,27].…”

“…We have only just begun to evaluate parameters of the lighting environment on entrainment flexibility primarily in T24 LDLD, but see [20]. Factors that predict entrainment to LDLD or T-cycle paradigms include sex, age of the animal, and light parameters like duration and intensity [14,17,20,40,41]. The superior entrainment flexibility in females observed here (in Studies 1 and 3) corroborates previous findings from our lab [14,40], but is seemingly opposite to that reported in humans where males may have higher tolerance than females as reflected in self-reported level of complaints related to sleep or health [42], but see [43].…”

“…Accumulating rodent studies have shown that non-invasive environmental manipulations can markedly increase circadian flexibility. First, compared to complete darkness, the mere addition of dim light at night (<0.1 lux) (a) cut in half the time required to re-entrain to acute phase-shifts in the light-dark cycle (LD) [12]; (b) facilitated induction of stable bimodal activity patterns in 24 h light:dark:light:dark cycles (LDLD) (i.e., rhythm bifurcation) [13]; and (c) expanded the range of behavioral entrainment far beyond conventionally understood limits, including 18 and 30 h cycles [14,15]. Second, following rhythm bifurcation under dimly lit nights, hamsters almost immediately adjusted to light:dark (LD) shifts to any phase [16].…”

“…Second, following rhythm bifurcation under dimly lit nights, hamsters almost immediately adjusted to light:dark (LD) shifts to any phase [16]. In hamsters and mice, the range of entrainment to LDLD cycles was greatly expanded to include 18-30 h zeitgeber periods [14,17,18]. In at least some cases, this enhanced entrainability after bifurcation persisted without the continuation of dim night-time lighting [17].…”