Stress programming throughout life history : implications for physiological ageing

Abstract

Stress exposure throughout life can shape ageing phenotypes by causing changes within an individual’s physiology, such that growth, lifespan and many fitness-related traits are significantly and sometimes permanently affected. Developmental increases in glucocorticoids (GCs) and subsequent programming of the Hypothalamic Pituitary Adrenal (HPA) axis, are considered one of the primary mechanisms that program these age-related physiological changes. However, it is unclear how stress experienced throughout life interacts on physiological processes that may influence the trajectory of the ageing process within an individual. In this thesis, I used the Japanese quail (Coturnix japonica) to investigate both short and long-term effects of developmental and adult stress exposure on physiological processes that are known to be affected by the process of ageing: the neuroimmune system, neural cell death and survival and maintenance of oxidative balance. I aimed to establish at which point during life stress elicited the strongest programming effects; if these effects had physiological costs or benefits for the ageing individual and if there were cumulative or diminishing effects of repeated stress exposure on these physiological processes. Within the neuroimmune system, I found that post-natal stress induced neuroinflammation and caused a reduction in cell survival in later life within brain regions involved in HPA axis functioning. Repeated developmental stress also programmed a dampened apoptotic response in the ageing brain. I also found age and sex-specific effects on apoptosis, cell survival and oxidative balance measures that depended on the conditions birds experienced during development. My data show that programmed long-term changes in physiology are most likely driven by elevated GCs during the post-natal period. My research suggests that these programmed phenotypes may exhibit signs of accelerated physiological ageing or may be adaptive in promoting swifter and stronger recovery from subsequent stress exposure in later life.