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

Abstract

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, 26 hr.) 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. We measure locomotor behavior duration (), period ({tau}), and entrained phase angle ({Psi}) in vivo, and SCN phase distribution ({sigma}{phi}), {tau}, and {Psi} ex vivo in order to refine our understanding of critical light components for influencing particular circadian properties. We find that photoperiod and T cycle length both drive determination of in vivo {Psi} but differentially influence after-effects in and {tau}, with photoperiod driving changes in and photoperiod length and T cycle length combining to influence {tau}. Using skeleton photoperiods, we demonstrate that in vivo {Psi} is determined by both parametric and non-parametric components, while changes in are driven non-parametrically. Within the ex vivo SCN, we find that {Psi} and {sigma}{phi} of the PER2::LUCIFERASE rhythm follow closely with their likely behavioral counterparts ({Psi} and of the locomotor activity rhythm), while also confirming previous reports of {tau} after-effects of gene expression rhythms showing negative correlations with behavioral {tau} after-effects in response to T cycles. We demonstrate that within-SCN {sigma}{phi} changes, thought to underly changes in vivo, are induced primarily non-parametrically. Taken together, our results demonstrate distinct components of seasonal light input differentially influence {Psi}, , and {tau}, and suggest the possibility of separate mechanisms driving the persistent changes in circadian behaviors mediated by seasonal light.