Reversible suppression of circadian-driven locomotor rhythms in mice using a gradual fragmentation of the day-night cycle.

Circadian rhythms are regulated by molecular clockwork and drive 24-h behaviors such as locomotor activity, which can be rendered non-functional through genetic knockouts of clock genes. Circadian rhythms are robust in constant darkness (DD) but are modulated to become exactly 24 h by the external day-night cycle. Whether ill-timed light and dark exposure can render circadian behaviors non-functional to the extent of genetic knockouts is less clear. In this study, we discovered an environmental approach that led to a reduction or lack in rhythmic 24-h-circadian wheel-running locomotor behavior in mice (referred to as arrhythmicity). We first observed behavioral circadian arrhythmicity when mice were gradually exposed to a previously published disruptive environment called the fragmented day-night cycle (FDN-G), while maintaining activity alignment with the four dispersed fragments of darkness. Remarkably, upon exposure to constant darkness (DD) or constant light (LL), FDN-G mice lost any resemblance to the FDN-G-only phenotype and instead, exhibited sporadic activity bursts. Circadian rhythms are maintained in control mice with sudden FDN exposure (FDN-S) and fully restored in FDN-G mice either spontaneously in DD or after 12 h:12 h light-dark exposure. This is the first study to generate a light-dark environment that induces reversible suppression of circadian locomotor rhythms in mice.

Fragmented day-night cycle induces reduced light avoidance, excessive weight gain during early development, and binge-like eating during adulthood in mice.

Fragmented day-night (FDN) cycles are environments in which multiple periods of light and dark alternate across a 24 h period. Exposure to FDN cycles disrupts circadian rhythms, resulting in period lengthening and alterations to mood in mice. A constant light environment, which also induces period lengthening, is linked to mood and metabolic disturbances and disruption to the development of the circadian clock. This study aims to determine how exposure to the FDN cycle impacts development in mice, with the hypothesis that there would be similar and adverse effects as observed in constant light conditions. Our study used CD-1 mice reared under the FDN cycle compared to the commonly used 12 h light: 12 h dark consolidated day-night cycle. During the first week of development, mouse pups reared under the FDN cycle gained bodyweight at a faster rate and did not avoid aberrant light exposure in comparison to 12:12 LD reared mouse pups. Developmental exposure to the FDN cycle lasted two weeks, and then mice were transferred to the 12:12 LD cycle, where after 2 weeks, bodyweight was similar between FDN reared and 12:12 LD reared mice at 1-month and 2-months old. When re-exposed to the FDN cycle during adulthood, FDN reared pups exhibited binge-like eating behaviors and reduced light avoidance. This study shows that the unnatural distribution of light and dark across the 24 h day can cause disruptions during early development that can reappear during adulthood when placed in the same stressful light-dark environment as adults.

Fragmented day-night cycle induces period lengthening, lowered anxiety, and anhedonia in male mice.

Light exposure at night disrupts circadian-regulated biological functions, including mood. However, the consequence of fragmenting the night period and distributing it across the 24-hr period is less understood. Here we show that fragmenting an 8 -h and 6 -h night into equally distributed 2 -h periods throughout the 24-hr day results in period lengthening of the circadian rhythm in mice. Furthermore, mice exhibited less anxiety, which indicates increased risk-taking behavior, and a lack of pleasure-seeking, known as anhedonia. The successive alley and open field tests were used to assess anxiety, while the sucrose preference test was used to assess anhedonia. Analysis of depressive-like behaviors with the forced swim and tail suspension tests were not observed. After two weeks in 12 h light - 12 h dark, mice exposed to the fragmented night recovered and exhibited normal behaviors for both anxiety and anhedonia. Our results are congruent with published studies that describe the detrimental effects of constant light conditions on circadian rhythms and mood. These findings unveil the negative impact that fragmenting the day-night cycle has on circadian rhythms and mood.

Light Has Diverse Spatiotemporal Molecular Changes in the Mouse Suprachiasmatic Nucleus.

To be physiologically relevant, the period of the central circadian pacemaker, located in the suprachiasmatic nucleus (SCN), has to match the solar day in a process known as circadian photoentrainment. However, little is known about the spatiotemporal molecular changes that occur in the SCN in response to light. In this study, we sought to systematically characterize the circadian and light effects on activity-dependent markers of transcriptional (cFos), translational (pS6), and epigenetic (pH3) activities in the mouse SCN. To investigate circadian versus light influences on these molecular responses, we harvested brains from adult wild-type mice in darkness at different circadian times (CT) or from mice exposed to a 15-min light pulse at the middle of the subjective day (CT6, no phase shifts), early subjective night (CT14, large phase delays), or late subjective night (CT22, small phase advances). We found that cFos and pS6 exhibited rhythmic circadian expression in the SCN with distinct spatial rhythms, whereas pH3 expression was undetectable at all circadian phases. cFos rhythms were largely limited to the SCN shell, whereas pS6 rhythms encompassed the entire SCN. pH3, pS6, and cFos showed gating in response to light; however, we were surprised to find that the expression levels of these markers were not higher at phases when larger phase shifts are observed behaviorally (CT14 versus CT22). We then used animals lacking melanopsin (melanopsin knockout [MKO]), which show deficits in phase delays, to further investigate whether changes in these molecular markers correspond to behavioral phase shifts. Surprisingly, only pS6 showed deficits in MKOs at CT14. Therefore, our previous understanding of the molecular pathways that lead to circadian photoentrainment needs to be revised.

Jet Lag Recovery and Memory Functions Are Correlated with Direct Light Effects on Locomotion.

Jet lag is a circadian disruption that affects millions of people, resulting, among other things, in extreme sleepiness and memory loss. The hazardous implications of such effects are evident in situations in which focus and attention are required. Remarkably, there is a limited understanding of how jet lag recovery and associated memory loss vary year round under different photoperiods. Here we show, using different cycles representing winter, summer, and equinox in male mice, that jet lag recovery and memory vary significantly with photoperiod changes. We uncover a positive correlation of acute light effects on circadian-driven locomotion (known as negative masking) with photoentrainment speed and memory enhancement during jet lag. Specifically, we show that enhancing or reducing negative masking is correlated with better or worse memory performance, respectively. This study indicates that in addition to timed-light exposure for phase shifting, the negative masking response could also be biologically relevant when designing effective treatments of jet lag.