Reorganisation of circadian activity and the pacemaker circuit under novel light regimes
Authors: Sharma Pragya , Sheeba Vasu 
Published: May 2024

Many environmental features are cyclic, with predictable daily and yearly changes which vary across latitudes. Organisms cope with such change using internal timekeepers or circadian clocks which have evolved remarkable flexibility. This flexibility is evident in the waveforms of behavioural and underlying molecular rhythms. In today’s world, many ecosystems experience artificial light at night, leading to unusual photoperiodic conditions. Additionally, occupational demands expose many humans to unconventional light cycles. Yet, practical means of manipulating activity waveforms for beneficial purposes are lacking. This requires an understanding of principles and factors governing waveform plasticity of activity rhythms. Even though waveform plasticity remains underexplored, few recent studies have used novel light regimes, inspired by shift work schedules, with alternating bright light and dim light (LDimLDim) to manipulate the activity waveform of nocturnal rodents. We undertook this study to understand what aspects of light regimes contribute to waveform flexibility and how the underlying neuronal circuitry regulates the behaviour by subjectingDrosophila melanogasterto novel light regimes. Using a range of LDimLDimregimes, we found that dim scotopic illumination of specific durations induces activity bifurcation in fruit flies, similar to mammals. Thus, we suggest evolutionarily conserved effects of features of the light regime on waveform plasticity. Further, we demonstrate that the circadian photoreceptor CRYPTOCHROME is necessary for activity bifurcation. We also find evidence for circadian reorganisation of the pacemaker circuit wherein the ‘evening’ neurons regulate the timing of both bouts of activity under novel light regimes. Thus, such light regimes can be explored further to understand the dynamics and coupling within the circadian circuit. The conserved effects of specific features of the light regime open up the possibility of designing other regimes to test their physiological impact and leverage them for waveform manipulation to minimise the ill effects of unusual light regimes.

Author Summary

It is thought that the appropriate timing of physiological and behavioural rhythms of organisms with respect to the environmental cycle confers an adaptive advantage. Endogenous timekeepers or circadian clocks regulate such rhythms. To optimally time biological rhythms, its waveform must be plastic and respond to changes in external cycles. Changes in external cycles may be natural, as seen across latitudes or seasons, or anthropogenic, such as artificial light induced changes in photoperiod or shiftwork driven novel light/dark cycles. Previous studies using a nocturnal rodent model showed that novel light regimes (LDimLDim) caused locomotor activity to bifurcate such that mice showed two bouts of activity restricted to the dimly lit phases.

Here, we first demonstrate that conserved features of the light regime - dim scotopic illumination of specific light durations induce activity bifurcation in the fly model. We leverage the genetic toolkit of the Drosophila model to also show evidence for the reorganisation of the circadian pacemaker neuronal network upon exposure to novel light regimes. Our findings indicate that conserved effects of specific features of the environmental regimes can be exploited to design light regimes that ease the waveform into synchronising with challenging conditions such as during shift work, jetlag, and photoperiodic changes.