Abstract
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.
Highlights
The importance of early life programming on later health and wellbeing is becoming increasingly evident, in the fields of stress and obesity
Dams exposed to LD during the suckling stage weighed more than dams exposed to DD or LL environments and dams in LL showed no significant circadian rhythm of locomotor activity, light environment did not appear to have any significant effect on stress levels or maternal behavior, including licking and grooming
We found that dams exposed to LL or DD environments during the suckling period showed no differences in overall HPA axis function or maternal care behavior, including licking and grooming, compared to those exposed to control 24-h LD cycles
Summary
The importance of early life programming on later health and wellbeing is becoming increasingly evident, in the fields of stress and obesity. An important aspect of early programming is the long-term effects of postnatal light experience on brain development. Recent evidence suggests that, in addition to the effects on the circadian system, postnatal light experience will induce long-term changes on the stress system, the hypothalamic-pituitary-adrenal (HPA) axis, and mood regulation. Compared to mice raised under control 24-h light-dark (LD) cycles, mice raised under constant light (LL) or constant darkness (DD) during the first 3 postnatal weeks will show decreased glucocorticoid receptor (GR) expression in the hippocampus, increased plasma corticosterone concentration at the onset of the dark phase of a LD cycle and a depressive phenotype in adulthood (Coleman et al, 2016). The mechanisms underlying postnatal light experience effects on the developing circadian and stress systems are still unclear
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