Effect of light on the development of the circadian rhythm of motor activity in the mouse

Impact of Prenatal LPS and Early-life Constant Light Exposure on Circadian Gene Expression Profiles in Various Rat Tissues

Exposure to lipopolysaccharide (LPS) during prenatal development leads to various changes in neurobiological and behavioural patterns. Similarly, continuous exposure to constant light (LL) during the critical developmental period of the circadian system affects gene expression in various tissues in adulthood. Given the reciprocal nature of the interaction between the circadian and the immune systems, our study primarily investigated the individual effects of both interventions and, more importantly, their combined effect. We aimed to explore whether there might be a potential synergistic effect on circadian rhythms and their parameters, focussing on the expression of clock genes, immune-related genes, and specific genes in the hippocampus, pineal gland, spleen and adrenal gland of rats at postnatal day 30. Our results show a significant influence of prenatal LPS and postnatal LL on the expression profiles of all genes assessed. However, the combination of prenatal LPS and postnatal LL only revealed an enhanced negative effect in a minority of the comparisons. In most cases, it appeared to attenuate the changes induced by the individual interventions, restoring the measured parameters to values closer to those of the control group. In particular, genes such as Nr1d1, Aanat and Tph1 showed increased amplitude in the pineal gland and spleen, while the kynurenine enzymes Kynu and KatII developed circadian rhythmicity in the adrenal glands only after the combined interventions. Our data suggest that a mild immunological challenge during prenatal development may play a critical role in triggering an adaptive response of the circadian clock later in life.

The circadian clock of the bacterium B. subtilis evokes properties of complex, multicellular circadian systems

Circadian clocks are pervasive throughout nature, yet only recently has this adaptive regulatory program been described in nonphotosynthetic bacteria. Here, we describe an inherent complexity in the Bacillus subtilis circadian clock. We find that B. subtilis entrains to blue and red light and that circadian entrainment is separable from masking through fluence titration and frequency demultiplication protocols. We identify circadian rhythmicity in constant light, consistent with the Aschoff's rule, and entrainment aftereffects, both of which are properties described for eukaryotic circadian clocks. We report that circadian rhythms occur in wild isolates of this prokaryote, thus establishing them as a general property of this species, and that its circadian system responds to the environment in a complex fashion that is consistent with multicellular eukaryotic circadian systems.

Constant Light in Critical Postnatal Days Affects Circadian Rhythms in Locomotion and Gene Expression in the Suprachiasmatic Nucleus, Retina, and Pineal Gland Later in Life

The circadian clock regulates bodily rhythms by time cues that result from the integration of genetically encoded endogenous rhythms with external cycles, most potently with the light/dark cycle. Chronic exposure to constant light in adulthood disrupts circadian system function and can induce behavioral and physiological arrhythmicity with potential clinical consequences. Since the developing nervous system is particularly vulnerable to experiences during the critical period, we hypothesized that early-life circadian disruption would negatively impact the development of the circadian clock and its adult function. Newborn rats were subjected to a constant light of 16 lux from the day of birth through until postnatal day 20, and then they were housed in conditions of L12 h (16 lux): D12 h (darkness). The circadian period was measured by locomotor activity rhythm at postnatal day 60, and the rhythmic expressions of clock genes and tissue-specific genes were detected in the suprachiasmatic nuclei, retinas, and pineal glands at postnatal days 30 and 90. Our data show that early postnatal exposure to constant light leads to a prolonged endogenous period of locomotor activity rhythm and affects the rhythmic gene expression in all studied brain structures later in life.

Perinatal light imprinting of circadian clocks and systems (PLICCS): A signature of photoperiod around birth on circadian system stability and association with cancer

Recent findings from animal models suggest that plasticity of human circadian clocks and systems may be differentially affected by different paradigms of perinatal photoperiod exposure to the detriment of health in later life, including cancer development. Focusing on the example of cancer, we carry out a series of systematic literature reviews concerning perinatal light imprinting of circadian clocks and systems (PLICCS) in animal models, and concerning the risk of cancer development with the primary determinants of the perinatal photoperiod, namely season of birth or latitude of birth. The results from these systematic reviews provide supporting evidence of the PLICCS and cancer rationale and highlight that investigations of PLICCS in humans are warranted. Overall, we discuss findings from experimental research and insights from epidemiological studies. Considerations as to how to "test" PLICCS in epidemiological studies and as to the potential for non-invasive preventative measures during perinatal periods close our synthesis. If the PLICCS rationale holds true, it opens the exciting prospect for amenable, early-life, preventative measures against cancer development (and other disorders) in later life. Indeed, non-invasive anthropogenic light exposure may have enormous potential to alleviate the public health and economic burden of circadian-related diseases.

Perinatal Light Imprinting of Circadian Clocks and Systems (PLICCS): The PLICCS and Cancer Hypothesis

Circadian disruption is associated with sleep, mood, and metabolic disorders, and—according to the International Agency for Research on Cancer—even with cancer. Mechanistically, the source of disease may be circadian system instability which likely arises during development. In animal experiments, both low perinatal light:dark ratios and chronic perinatal photoperiod phase shifting yield enduring, detrimental effects on neuroendocrine physiology via circadian system instability. Certainly, accumulating disturbances to neuroendocrine physiology and detrimental downstream effects could predispose to internal cancers. Epidemiologically, either season of birth or latitude of birth, both of which co-determine perinatal photoperiod-zeitgeber strengths, have been utilized independently as proxies for other environmental co-etiologies of cancer. Both have been independently associated with cancer; however, the evidence is inconclusive. We hypothesize that time of birth and location of birth, together determining perinatal photoperiod, contribute to cancer development through Perinatal Light Imprinting of Circadian Clocks and Systems.

Postnatal Light Effects on Pup Stress Axis Development Are Independent of Maternal Behavior

Postnatal environment shapes brain development during key critical periods. We have recently found that postnatal light environment has long-term effects on the stress and circadian systems, which can lead to altered stress responses, circadian behavior and a depressive phenotype in adulthood. However, it is still unclear how light experience affects the postnatal development of specific stress markers in the pup brain and the role played by maternal behavior and stress. To test this, we raised mice under either light-dark cycles (LD), constant light (LL) or constant darkness (DD) during the suckling stage. After weaning, all mice were exposed to LD until adulthood. Results show that postnatal light environment does not have any significant effects on dam stress levels (plasma corticosterone concentration, Arginine-vasopressin and Glucocorticoid receptor (GR) protein expression in the brain) or maternal behavior, including licking and grooming. Light environment does not have a major effect on litter characteristics or pup growth either. Interestingly, light environment during the suckling stage significantly impacted Corticotrophin-releasing hormone (CRH) and Gr mRNA expression in pup brain during development. Furthermore, a difference in Crh mRNA expression between LL- and DD-raised mice was still observed in adulthood, long after the exposure to abnormal light environments had stopped. Taken together, these data suggest that the long-term effects of postnatal light environment on the pups' stress system cannot be attributed to alterations in either maternal behavior and/or stress axis function. Instead, postnatal light experience may act directly on the pup stress axis and/or indirectly via circadian system alterations.

Postnatal light alters hypothalamic-pituitary-adrenal axis function and induces a depressive-like phenotype in adult mice

The postnatal light environment that a mouse experiences during the critical first three postnatal weeks has long-term effects on both its circadian rhythm output and clock gene expression. Furthermore, data from our lab suggest that postnatal light may also impact the hypothalamic-pituitary-adrenal (HPA) axis, which is a key regulator of stress. To test the effect of postnatal light exposure on adult stress responses and circadian rhythmicity, we raised mice under either 24-h light-dark cycles (LD), constant light (LL) or constant dark (DD) during the first three postnatal weeks. After weaning we then exposed all animals to LD cycles (basal conditions), followed by LL (stressed conditions) environments. We examined brain neuropeptide and glucocorticoid receptor (GR) expression, plasma corticosterone concentration rhythm and body temperature rhythm, together with depression- and anxiety-related behaviour. Results showed that LL- and DD-raised mice exhibited decreased GR expression in the hippocampus, increased plasma corticosterone concentration at the onset of the dark phase and a depressive phenotype when exposed to LD cycles later in life. Furthermore, LL-raised mice showed increased corticotrophin-releasing hormone mRNA expression in the paraventricular nucleus of the hypothalamus. When exposed to LL as adults, LL-raised mice showed a significant circadian rhythm of plasma corticosterone concentration, together with a shorter period and stronger circadian rhythm of body temperature compared to DD-raised mice. Taken together, these data suggest that altered postnatal light environments have long-term effects on the HPA axis and the circadian system, which can lead to altered stress responses and a depressive phenotype in adulthood.

Programming of mice circadian photic responses by postnatal light environment

Early life programming has important consequences for future health and wellbeing. A key new aspect is the impact of perinatal light on the circadian system. Postnatal light environment will program circadian behavior, together with cell morphology and clock gene function within the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. Nevertheless, it is still not clear whether the observed changes reflect a processing of an altered photic input from the retina, rather than an imprinting of the intrinsic molecular clock mechanisms. Here, we addressed the issue by systematically probing the mouse circadian system at various levels. Firstly, we used electroretinography, pupillometry and histology protocols to show that gross retinal function and morphology in the adult are largely independent of postnatal light experiences that modulate circadian photosensitivity. Secondly, we used circadian activity protocols to show that only the animal's behavioral responses to chronic light exposure, but not to constant darkness or the acute responses to a light stimulus depend on postnatal light experience. Thirdly, we used real-time PER2::LUC rhythm recording to show long-term changes in clock gene expression in the SCN, but also heart, lung and spleen. The data showed that perinatal light mainly targets the long-term adaptive responses of the circadian clock to environmental light, rather than the retina or intrinsic clock mechanisms. Finally, we found long-term effects on circadian peripheral clocks, suggesting far-reaching consequences for the animal's overall physiology.

Development of circadian rhythms: role of postnatal light environment

Mammals are born with an immature circadian system, which completes its development postnatally. Evidence suggests that the environment experienced by a newborn will impact and shape its development, which will have future consequences at the levels of circadian system function, circadian behaviour and physiology, and potentially, the animal's long-term health and welfare. Here we review the various stages in postnatal development of the circadian system, and discuss the data available on the long-term effects of early environment, in particular light environment, on the animal's brain, physiology and behaviour.

Resilience of Circadian Pacemaker Development in Hamsters

Disruptions of circadian rhythms have been linked to a wide range of pathologies from sleep disorders to cancer. The extent to which disruptions of circadian rhythms during development contribute to later conditions is not known. The present study tested the hypothesis that functional properties of the central circadian pacemaker, the suprachiasmatic nucleus (SCN), are affected by abnormal entrainment during development. The SCN is specialized for the generation of robust rhythms, for direct and indirect output to physiological and behavioral systems, and for entrainment to light/dark cycles via direct retinal input. It consists of thousands of neurons and glia with distinct phenotypes and has subdivisions delineated by both anatomical and functional criteria. In rodents, SCN rhythms develop within days after SCN cells are produced and before many other aspects of differentiation, such as synaptogenesis, are complete. We demonstrated that around the time of birth, the hamster SCN in vivo can undergo repeated phase shifts by a dopamine D1 receptor agonist (SKF-38393). For 2 days before and 2 days after birth, one group of hamsters received regular exposure to the drug at the same time of day, while another group was exposed at varying times to induce repeated phase shifts. Free-running and entrained activity rhythms were compared between the groups at different ages after weaning. Repeated phase shifts during SCN development had a significant effect on free-running period measured immediately after weaning. This effect was eliminated by subsequent entrainment to a light/dark cycle, indicating that the effect was not permanent. These and other results suggest that SCN development required for functional properties such as free-running period is resilient to perturbation.

Experience-independent development of the hamster circadian visual system

Experience-dependent functional plasticity is a hallmark of the primary visual system, but it is not known if analogous mechanisms govern development of the circadian visual system. Here we investigated molecular, anatomical, and behavioral consequences of complete monocular light deprivation during extended intervals of postnatal development in Syrian hamsters. Hamsters were raised in constant darkness and opaque contact lenses were applied shortly after eye opening and prior to the introduction of a light-dark cycle. In adulthood, previously-occluded eyes were challenged with visual stimuli. Whereas image-formation and motion-detection were markedly impaired by monocular occlusion, neither entrainment to a light-dark cycle, nor phase-resetting responses to shifts in the light-dark cycle were affected by prior monocular deprivation. Cholera toxin-b subunit fluorescent tract-tracing revealed that in monocularly-deprived hamsters the density of fibers projecting from the retina to the suprachiasmatic nucleus (SCN) was comparable regardless of whether such fibers originated from occluded or exposed eyes. In addition, long-term monocular deprivation did not attenuate light-induced c-Fos expression in the SCN. Thus, in contrast to the thalamocortical projections of the primary visual system, retinohypothalamic projections terminating in the SCN develop into normal adult patterns and mediate circadian responses to light largely independent of light experience during development. The data identify a categorical difference in the requirement for light input during postnatal development between circadian and non-circadian visual systems.

Postnatal development and functional adaptations of the melanopsin photoreceptive system in the albino mouse retina

PURPOSE

To study the melanopsin system of the albino CD1 mouse retina during postnatal development.

METHODS

Pups were kept under different ambient conditions: light/dark (LD) cycles, constant light (LL), constant darkness (DD), LL followed by LD, and DD followed by LL. Using immunohistochemistry, melanopsin-expressing cells were classified as M1 or M2 according to the location of their somata and dendritic processes and were counted.

RESULTS

Under LD cycles an increase in the number of immunoreactive cells was observed within the first week of postnatal development. When mice were maintained in DD, the increase in the number of immunopositive cells detected was significantly higher than that in LD. On the contrary, when mice were exposed to LL within the same period, no increase was detected. To determine whether the effect of LL during the early postnatal period was reversible, the authors studied animals born in LL and subsequently maintained under LD cycles. After 3 days in LD, these animals showed a significant increase in melanopsin cell number. However, after 1 month in LD, the number was similar to that of the LD controls. Surprisingly, when mice born in DD were exposed to LL, no decrease was detected, though the immunostaining was of low intensity.

CONCLUSIONS

The amount of melanopsin protein per cell varies, depending on ambient light conditions. Periods of darkness or, more likely, the sequence of light and dark periods occurring under the daily cycles might be necessary for the normal development of the melanopsin system.

Expression of circadian neuropeptides in the hypothalamus of adult mice is affected by postnatal light experience

The suprachiasmatic nuclei (SCN) of the hypothalamus are the principal pacemaker in mammals, controlling daily, circadian rhythms in physiology and behaviour. Environmental light during development has long-term effects on circadian behaviour, but it is still unclear what the relevant adaptations within the brain are. In the present study, we examined the manifestation of the circadian rhythm of locomotor activity, and the expression of arginine-vasopressin (AVP) and vasointestinal polypeptide (VIP) in the SCN of adult mice reared under different light environments during the suckling period, and synchronised to light/dark cycles after weaning. We found that animals reared under constant light had higher amplitude and more stable activity rhythms, together with lower levels of VIP- and AVP-immunostaining in the SCN, compared to mice reared under light/dark cycles or constant darkness. Differences in AVP expression were also found in the paraventricular nucleus and the supraoptic nucleus, two brain areas which receive SCN projections. These results indicate that the postnatal light experience may affect clock function and clock output, and suggest implications for the control of hormonal homeostasis and circadian behaviour.

Constant light housing during nursing causes human DSPS (delayed sleep phase syndrome) behaviour in Clock-mutant mice

Delayed sleep phase syndrome (DSPS) is very often seen among patients with sleep-wake rhythm disorders. Humans with the 3111C allele of the human Clock gene tend to demonstrate a higher evening preference on the morningness-eveningness (ME) preference test. DSPS is thought to be an extreme form of this evening preference. Clock-mutant mice have been proposed as an animal model of evening preference. In this study, we looked at whether constant light (LL) housing of Clock-mutant mice during lactation would result in evening preference and/or DSPS. Housed under light-dark (LD) or constant dark (DD) conditions during the lactation period, both wild-type and Clock-mutant mice did not show a phase-delay in the locomotor activity measured under light-dark conditions, whereas constant light housing during lactation significantly caused a delayed onset. The magnitude of the delay during the light-dark cycle was positively associated with free-running period measured during constant darkness. Among wild, heterozygote, and homozygote pups born from heterozygous dams, only homozygote pups showed a delayed onset. Constant light-housed Clock-mutant mice exhibited a lower number and delayed peak of phospho-MAPK-immunoreactive cells in core regions of the suprachiasmatic nucleus (SCN) compared to light-dark housed wild-type or Clock-mutant mice. Activity onset returned to normal with daily melatonin injection at the lights-off time for 5 days. The present results demonstrate that Clock-mutant mice exposed to constant light during lactation can function as an animal model of DSPS and can be used to gain an understanding of the ethological aspects of DSPS as well as to find medication for its treatment.

Constant light disrupts the developing mouse biological clock

The central biological clock of the brain, contained within the suprachiasmatic nuclei (SCN) of mammals, orchestrates an orderly "internal day" of physiology and behavior. The developing biological clock begins to respond to light at an early stage and a particular concern in humans is whether light exposure has disruptive effects on the developing biological clock of infants exposed to constant lighting conditions in neonatal intensive care units (NICUs). Worldwide, eighteen million, or 14%, of newborns estimated to be of low birth weight, are exposed to artificial lighting environments in hospital nurseries annually. Here, we have tested whether constant light (LL) exposure disrupts the developing biological clock of mice, using a circadian reporter transgenic mouse model in which the organization of the central biological clock can be assayed by real-time gene expression imaging. We now find that LL has both acute and long-term disruptive effects on developing biological clocks and that cyclic lighting conditions are critical for developing circadian clocks to coordinate their molecular circadian mechanisms. This suggests that, from the perspective of developing circadian organization in humans, cyclic light conditions in NICUs are likely to be most appropriate for infants.

Effect of Robertsonian Translocations on the Motor Activity Rhythm in the House Mouse

Here we studied the circadian rhythm of motor activity in two groups of wild house mice from the chromosomal polymorphic zone of Barcelona, which differed in diploid number (2n): standard (2n = 40), with all acrocentric chromosomes, and Robertsonian (2n = 29-32), with several Robertsonian translocations. Motor activity under three lighting conditions, light-dark cycle, constant darkness, and constant light, was recorded for each mouse. The motor activity rhythm was examined by Fourier analysis and the daily power spectra were obtained. On the basis of the mean power spectrum of each animal and under each lighting condition, stepwise discriminant analyses were performed to classify the two chromosomal groups. This method allowed the correct classification of a large number of animals, the rhythms of about 2-2.6 hour periods being the most significant, with higher values in Robertsonian than in standard mice. Our results indicate that the daily motor activity pattern differs between the two chromosomal groups and its analysis may have a valuable interest for behavioral investigations on Robertsonian polymorphic zones of this species.

Sleep responses to light and dark are shaped by early experience

Light regulates sleep timing through circadian entrapment and by eliciting acute changes in behavior. These behaviors are mediated by the subcortical visual system, retinorecipient nuclei distinct from the geniculocortical system. To test the hypothesis that early visual experience shapes light regulation of behavior, the authors recorded sleep in albino rats reared in continuous dark, continuous light, or a 12-hr light-dark cycle. Dark rearing strengthened and light rearing weakened acute responses to light, including light modulation of REM sleep, a marker for pretectal function in albino rats. However, neither dark nor light rearing altered daily amounts of wakefulness, non-REM sleep, or REM sleep. Thus, light and dark rearing might differentially affect the balance between acute and circadian responses to light that, in concert, govern sleep timing.