Circadian rhythm bifurcation induces flexible phase resetting by reducing circadian amplitude

Abstract

Shift-work and jet-lag-related disorders are caused by the limited flexibility of the suprachiasmatic nucleus (SCN), a master circadian clock in the hypothalamus, to adjust to new light-dark (LD) cycles. Recent findings confirmed here establish that behavioral jet lag after simulated time-zone travel is virtually eliminated following bifurcated circadian entrainment under a novel and atypical 24-h light:dark:light:dark (LDLD) cycle. To investigate the mechanisms of this fast resetting, we examined the oscillatory stability of the SCN and peripheral tissues in LDLD-bifurcated mice employing the dissection procedure as a perturbing resetting stimulus. SCN, lung, liver, and adrenal tissue were extracted at times throughout the day from female and male PER2::Luciferase knock-in mice entrained to either LDLD or a normal LD cycle. Except for adrenals, the phase of the cultured explants was more strongly set by dissection under LDLD than under normal LD. Acute bioluminescence levels of SCN explants indicate that the rhythm amplitude of PER2 is reduced and phase is altered in LDLD. Real-time quantitative PCR suggests that amplitude and rhythmicity of canonical clock genes in the lung, liver, and kidney are also significantly reduced in LDLD in vivo. Furthermore, spatiotemporal patterns of PER2 peak time in cultured SCN were altered in LDLD. These results suggest that altered gene expression patterns in the SCN caused by bifurcation likely result in fast resetting of behavior and cultured explants, consistent with previously reported mathematical models. Thus, non-invasive, simple light manipulations can make circadian rhythms more adaptable to abrupt shifts in the environmental LD cycle.

The Suprachiasmatic Nucleus Regulates Anxiety-Like Behavior in Mice

“…In control mice, having slightly less robust, more flexible rhythms under times of acute stress may be beneficial. There is evidence to support that having a smaller amplitude of SCN rhythms, intercellular desynchrony of the SCN, leads to a more flexible SCN that can adapt to environmental changes 66 . Another possibility is that an SCN-independent mechanism underlies the relationship between the change in amplitude in homecage activity rhythms and anxiety-like behavior in control animals.…”

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

“…Moreover, upon release in DD, animals rapidly reverted to typical free-running periods with a phase determined by the time of release, rather than at random phases as would be expected from a free-running circadian oscillator or at a specific time which would be predicted by an entrained 24 h-oscillator [14,17,20]. Although our neurobiological investigations have not yet included T-cycles, in bifurcation, clock gene expression in the SCN shows dampened rhythmicity and strong resetting [22]. Recently, another group reported similar enhanced resetting of SCN PER2::LUC rhythms in mice exposed to T-cycles without successful entrainment [54].…”

“…Combining the observations from all studies, activity rhythms during or following bifurcation and extreme T-cycles appear to be more directly controlled by light than driven by a strong underlying circadian oscillator. At the same time, an explanation of only positive and negative masking of a strong oscillator driving behavior in these conditions has also been rejected by prior evidence [13,14,16,17,[20][21][22][23][24][25][48][49][50] as well as by the lack of 24 h rhythmicity in Study 3/Continental ( Figure 6). Therefore, we propose an alternative explanation that remains to be empirically tested.…”

“…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]. This suggests that the enhanced circadian plasticity described above may differ mechanistically from classical entrainment.…”

“…By what mechanism are mice able to adapt their behavior to these highly artificial schedules? A variety of studies with different light cycles and species show converging evidence that an explanation of simple masking can be rejected in favor of a true reorganization of the circadian system [13,14,16,17,[20][21][22][23][24][25][48][49][50]. Whereas Study 2/DuPont and 3/Continental took a phenomenological approach to explore overall entrainment under complex photoperiod regimes, Study 1/Jitter was designed to test specific entrainment hypotheses by evaluating oscillator characteristics in bifurcation and T-cycle entrainment in two ways.…”