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

Perinatal Photoperiod Has Long-Term Effects on the Rest-Activity Cycle and Sleep in Male and Female Mice

Environmental light conditions during development can have long-lasting effects on the physiology and behavior of an animal. Photoperiod, a clear example of environmental light conditions, is detected by and coded in the suprachiasmatic nucleus. It is therefore possible that differences observed in behavior in adulthood after exposure to different perinatal photoperiods are caused by lasting changes in the suprachiasmatic nucleus or alternatively, in other nuclei affected by perinatal photoperiod. It can then be expected that behavior with strong circadian aspects, like rest-activity and sleep, are affected by difference in photoperiod during development as well. To investigate this further, we exposed mice to different photoperiods during their development in the womb until weaning (long: 16 h of light, 8 h of darkness; short: 8 h of light, 16 h of darkness). After weaning, the animals were exposed to a 12 h:12 h light:dark cycle for at least 3 more weeks and some animals were subsequently exposed to constant darkness. We assessed their rest-activity patterns by recording voluntary locomotor activity and used EEG recordings to determine sleep architecture and electroencephalographic spectral density. Perinatal long photoperiod animals showed a shorter duration of locomotor activity than short photoperiod-developed mice in a 12:12 light-dark cycle. This difference disappeared in constant darkness. In the light phase, that is, during the day, perinatal long photoperiod mice spent less time awake and more time in NREM sleep than short photoperiod-developed mice. No effects of perinatal photoperiod were observed in the EEG spectral density or in response to sleep deprivation. We see lasting differences in behavioral locomotor activity and sleep in female and male mice after exposure to different perinatal photoperiods. We conclude that perinatal photoperiod programs a developing mammal for different external conditions and changes brain physiology, which in turn results in long-lasting, possibly even permanent, changes in the sleep and locomotor activity.

Constant light and high fat diet alter daily patterns of activity, feed intake and fecal corticosterone levels in pregnant and lactating female ICR mice

The prevalence of constant light exposure and high-fat diet in modern society raises concerns regarding their impact on maternal and offspring health outcomes. In rodents, exposure to maternal high-fat diet or continuous light negatively program metabolic and stress response outcomes of offspring. A 2x3 factorial study was conducted to investigate the impact of diet (control-CON, 10% fat, or high fat-HF, 60% fat) and exposure to different lighting conditions: regular 12-hour light-dark cycles (LD), continuous dim light (L5), or continuous bright light (L100) on female ICR mice daily patterns of time in and out of the nest, feed intake, and fecal corticosterone levels during gestation and lactation. Our previous analysis of these mice found HF diet decreased number of pups born, but increased litter growth rate to postnatal (PN) d12. Whereas continuous light increased gestation length and tended to increase PN litter growth. Here we report that patterns of grams of feed intake, an indicator of feeding activity, were affected by light, diet, period of the day (day versus night) and physiological state (gestation and lactation), with significant interactions among all these variables (P<0.05). HF diet and light treatment increased fecal corticosterone output (P<0.05) during lactation. Dams exhibited significant 12 h and 24 h rhythms of activity out of the nest in the first 48 h postnatal, with time outside of the nest greater in the second 24 h period. L100 treatment and HF diet attenuated rhythms and shifted phase of rhythms relative to LD and CON, respectively (P<0.05). Alterations in behavior affect maternal physiology, including level and timing of release of corticosteroids. Elevated fecal corticosterone levels due to high-fat diet and continuous light may have potential implications on maternal-offspring health, and potentially underlie some of the adverse effects of modern lifestyle factors on maternal and offspring health.

Chronic exposure to dim artificial light disrupts the daily rhythm in mitochondrial respiration in mouse suprachiasmatic nucleus

Circadian rhythms of physiology, behavior, and metabolism have an endogenous 24 h period that synchronizes with environmental cycles of light/dark and food availability. Alterations in light cycles are stressful and disrupt such diurnal oscillations. Recently, we witnessed a sudden rise in studies describing the mechanisms behind the interaction between the key characteristics of mitochondrial functions, peripheral clocks, and stress responses. To our knowledge, there is no study in the suprachiasmatic nuclei (SCN) describing the dysregulated mitochondrial bioenergetics under abnormal lighting conditions, which is common in today's modern world. Thus, we aimed to investigate the existence of daily changes in mitochondrial bioenergetics (respiratory control rate, RCR), mitochondrial abundance (mtDNA/nDNA), plasma corticosterone, and to test whether disturbances in the lighting conditions might influence such rhythms. To confirm this, mice were sacrificed, mitochondria were isolated from the suprachiasmatic nuclei in the brain and blood was collected, every 3 h at various time points zeitgeber time/circadian time, (0, 3, 6, 9, 12, 15, 18, 21, and 24 h) under 12:12 h light-dark (LD, 150 lux L: 0 lux D) cycle and chronic artificial dim lighting (LL, 5 lux: 5lux) conditions, of a 24 h period, respectively. Our results demonstrate the existence of robust daily rhythmicity in RCR, mtDNA/nDNA and plasma CORT under a normal LD cycle. However, these rhythms were significantly disrupted and clock genes expressions were dysregulated under chronic dim LL. Furthermore, mitochondrial abundance was significantly reduced during LL compared to their numbers under LD cycle. Our data demonstrate that the circadian clock regulates mitochondrial functions (RCR, number), essential for accomplishing daily energy demands and supply by the SCN neurons. Abnormal light exposure dysregulates mitochondrial functions in the SCN and may alter metabolism, resulting in obesity, diabetes, and other metabolic disorders. Therefore, properly designing lighting conditions in workplaces is essential to mitigate the adverse consequences of light on humans.

Melatonin alleviates hippocampal GR inhibition and depression-like behavior induced by constant light exposure in mice

Light pollution has become a potential health risk factor worldwide. Chronic exposure to constant light (CCL) leads to depressive-like behavior, yet the mechanism remains unclear. In this study, mice exposed to CCL for 3 weeks exhibited depression-like behaviors, with decreased melatonin in plasma and increased oxidative stress in hippocampus. Meanwhile, CCL-exposed mice showed elevated plasma corticosterone (CORT) levels and diminished glucocorticoid receptor (GR) phosphorylation in hippocampus. Concurrently, glycogen synthase kinase 3 beta (GSK3β) was inactivated with increased phosphorylation at Ser9. The interrelationship of GSK3β and GR was clarified in mouse hippocampal neuron (HT-22) cells. GSK3β inhibitor CHIR-99021 induced GR inhibition with diminished phosphorylation, while GR inhibitor RU486 did not affect GSK3β expression or phosphorylation. Furthermore, GSK3β-mediated GR inhibition was reproduced in vitro in HT-22 cells treated with melatonin receptor antagonist luzindole and H2O2 in combination. Finally, melatonin reversed GSK3β-mediated GR inhibition in hippocampus and improved CCL-induced depression-like behavior in mice. These results indicate that CCL induces melatonin deficiency and oxidative stress in hippocampus, which in turn leads to GSK3β-mediated GR inhibition and depression-like behavior in mice.

Loss of circadian protection against influenza infection in adult mice exposed to hyperoxia as neonates

Adverse early-life exposures have a lasting negative impact on health. Neonatal hyperoxia that is a risk factor for bronchopulmonary dysplasia confers susceptibility to influenza A virus (IAV) infection later in life. Given our previous findings that the circadian clock protects against IAV, we asked if the long-term impact of neonatal hyperoxia vis-à-vis IAV infection includes circadian disruption. Here, we show that neonatal hyperoxia abolishes the clock-mediated time of day protection from IAV in mice, independent of viral burden through host tolerance pathways. We discovered that the lung intrinsic clock (and not the central or immune clocks) mediated this dysregulation. Loss of circadian protein, Bmal1, in alveolar type 2 (AT2) cells recapitulates the increased mortality, loss of temporal gating, and other key features of hyperoxia-exposed animals. Our data suggest a novel role for the circadian clock in AT2 cells in mediating long-term effects of early-life exposures to the lungs.

Dim Light at Night and Constant Darkness: Two Frequently Used Lighting Conditions That Jeopardize the Health and Well-being of Laboratory Rodents

The influence of light on mammalian physiology and behavior is due to the entrainment of circadian rhythms complemented with a direct modulation of light that would be unlikely an outcome of circadian system. In mammals, physiological and behavioral circadian rhythms are regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus. This central control allows organisms to predict and anticipate environmental change, as well as to coordinate different rhythmic modalities within an individual. In adult mammals, direct retinal projections to the SCN are responsible for resetting and synchronizing physiological and behavioral rhythms to the light-dark (LD) cycle. Apart from its circadian effects, light also has direct effects on certain biological functions in such a way that the participation of the SCN would not be fundamental for this network. The objective of this review is to increase awareness, within the scientific community and commercial providers, of the fact that laboratory rodents can experience a number of adverse health and welfare outcomes attributed to commonly-used lighting conditions in animal facilities during routine husbandry and scientific procedures, widely considered as “environmentally friendly.” There is increasing evidence that exposure to dim light at night, as well as chronic constant darkness, challenges mammalian physiology and behavior resulting in disrupted circadian rhythms, neural death, a depressive-behavioral phenotype, cognitive impairment, and the deregulation of metabolic, physiological, and synaptic plasticity in both the short and long terms. The normal development and good health of laboratory rodents requires cyclical light entrainment, adapted to the solar cycle of day and night, with null light at night and safe illuminating qualities during the day. We therefore recommend increased awareness of the limited information available with regards to lighting conditions, and therefore that lighting protocols must be taken into consideration when designing experiments and duly highlighted in scientific papers. This practice will help to ensure the welfare of laboratory animals and increase the likelihood of producing reliable and reproducible results.

Perinatal Programming of Circadian Clock-Stress Crosstalk

An intact communication between circadian clocks and the stress system is important for maintaining physiological homeostasis under resting conditions and in response to external stimuli. There is accumulating evidence for a reciprocal interaction between both-from the systemic to the molecular level. Disruption of this interaction by external factors such as shiftwork, jetlag, or chronic stress increases the risk of developing metabolic, immune, or mood disorders. From experiments in rodents, we know that both systems maturate during the perinatal period. During that time, exogenous factors such as stress or alterations in the external photoperiod may critically affect-or program-physiological functions later in life. This developmental programming process has been attributed to maternal stress signals reaching the embryo, which lastingly change gene expression through the induction of epigenetic mechanisms. Despite the well-known function of the adult circadian system in temporal coordination of physiology and behavior, the role of maternal and embryonic circadian clocks during pregnancy and postnatal development is still poorly defined. A better understanding of the circadian-stress crosstalk at different periods of development may help to improve stress resistance and devise preventive and therapeutic strategies against chronic stress-associated disorders.