Effects of postnatal light environment on the development of the mouse stress system

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Abstract

The postnatal period is a critical time for development where external influences can help shape the long-term structure and function of the brain. Adverse experiences or stressors during the postnatal period, such as abuse or neglect, can have huge consequences on the long term function, health and susceptibility to disease. One environmental factor, whose importance is becoming increasingly more recognised for normal development, is light. Abnormal light during the first three weeks of life has been shown to have long term effects on the circadian system of rodents. On the other hand, the effects of abnormal light during the postnatal period on the stress system have yet been relatively unexplored. Therefore the aim of this thesis was to assess if altered postnatal light environment, such as that a preterm baby might be exposed to, has any long-term effects on the stress system.Mice were raised under constant light (LL), constant darkness (DD) or a normal 12:12hr light:dark cycle (LD) for the first three weeks of life from postnatal day (P)1 up until P21. From P21, all mice were then housed in LD conditions and the stress system was assessed by looking at several different levels of the HPA axis including neuropeptide expression in the brain, body and adrenal weight, and plasma corticosterone levels under both basal and stressed conditions. Learning and memory, anxiety-like behaviour and circadian output rhythms were also evaluated. Finally, mother-pup behaviour and maternal HPA axis were assessed to see if maternal care was changed by altered postnatal light.Both LL and DD rearing caused changes in the HPA axis of offspring with LL raised mice showing alterations in neuropeptide and glucocorticoid receptor expression in the brain. Postnatal DD resulted in a blunted corticosterone response to a stressor in females but had no effect in males. In terms of behaviour, LL raised mice had increased depressive-like behaviour. In contrast, postnatal light appears to have no effect on learning and memory or anxiety behaviour. When we looked at circadian output rhythms, we found that LL rearing appears to confer resilience to the rhythm disrupting effects of LL later on in life as seen by the maintenance of locomotor activity, body temperature and plasma corticosterone rhythms in LL. Maternal care and maternal stress systems appeared unaltered under the different postnatal light environments suggesting that the changes we see in the offspring are attributed to mechanisms other than alterations in maternal care.

Keyword(s)

circadian; development; hypothalamic pituitary adrenal axis; light; stress

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