Distinct components of photoperiodic light are differentially encoded by the mammalian circadian clock

Michael C. Tackenberg, Jacob J. Hughey, and Douglas G. McMahon



Seasonal light cycles influence multiple physiological functions and are mediated through photoperiodic encoding by the circadian system. Despite our knowledge of the strong connection between seasonal light input and downstream circadian changes, less is known about the specific components of seasonal light cycles that are encoded and induce persistent changes in the circadian system. Using combinations of three T cycles (23, 24, and 26 h) and two photoperiods per T cycle (Long and Short, with duty cycles scaled to each T cycle), we investigate after-effects of entrainment to these six light cycles on locomotor behavior duration (alpha), period, and phase in vivo, and SCN ex vivo phase distribution, period, and phase in order to refine our understanding of critical light components for influencing particular circadian properties. We find that photoperiod and T cycle period both drive determination of entrained phase angle, and differentially influence after-effects in alpha and period, with photoperiod driving changes in alpha and photoperiod length and T cycle length combining to influence period. Using skeleton photoperiods, we demonstrate that phase angle determination is driven by both parametric and non-parametric components, while changes in alpha are driven non-parametrically. Within the SCN, we find that phase angle of entrainment and phase distribution follow closely with their likely behavioral counterparts (phase and alpha), while also confirming previous reports of free-running period after-effects of gene expression rhythms showing negative correlations with behavioral period after-effects in response to T cycles. We demonstrate that phase distribution changes, thought to underly alpha changes in vivo, are induced primarily non-parametrically. Taken together, our results demonstrate distinct components of seasonal light input differentially influence phase, activity duration, and period, and suggest the possibility of separate mechanisms driving the persistent changes in circadian behaviors mediated by seasonal light.

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