Long-term effects of elevated stress hormone levels in early development

A young animal can experience stress in many ways whilst it is growing, including food shortages and high levels of disturbance from predators. The level of stress experienced at this time may itself play an important role in shaping the subsequent development of the animal such that it is either better able to maintain normal function when exposed to a stressful environment as an adult or more sensitive to stress. In this study we plan to examine the long-term effects of varying levels of stress, within the natural range. Studying the effects of stress in birds could help us answer the question of exactly how rearing conditions affect individuals when they are adults. It is difficult to measure the influence of stress in infancy on 'performance' as adults and the long-term 'performance of animals in future generations because many species are so long-lived, taking years to reach sexual maturity. In this study we plan to examine the long-term effects of varying levels of stress, within the natural range, in a relatively fast maturing bird. When an animal's environment changes unpredictably, e.g. bad weather or food shortages, they suffer stress. During this time specific chemical messengers, called stress hormones, are released into the bloodstream from the brain, pituitary and adrenal gland. In the short term these hormones can benefit the animal as they promote activities to help it survive, but if the stress continues for a long time, they can have harmful effects and can even make an animal more prone to infection. Stress can also disrupt other normal physiological systems like the reproductive system and may even stop animals breeding. Although we know a lot about how immediate stress changes an animal's behaviour, we know very little about the long-term effects of stress experienced during development on adult behaviour and an animal's ability to reproduce. Furthermore, the question of how such effects might be felt in future generations have received very little attention to date. We propose to expose young zebra finches to concentrations of the stress hormone corticosterone that would be experienced under natural conditions, in response for example to human disturbance, by feeding them very small amounts of corticosterone twice a day. It is during this time that the systems within the body that regulate a birds response to stress are developing and may be affected by exposure to high levels of stress hormones. We will then collect data on the behaviour and breeding success of these birds when they are adults. Since stress hormones also increase during breeding and breeding is known to be affected by stress we will examine whether exposure to elevated stress hormones during development will affect reproductive decision-making and have an impact on breeding success as an adult. The results of this work will provide us with information that will greatly enhance our understanding of how exposure to stress hormones early in life can program an individual response to it's environment in later life.

The potential influence that environmental conditions experienced during early development may have on adult phenotypes has recently developed as an important research focus. While much of this work has centred upon the effects of variation in nutrient supply during pre- and post-natal development, another important axis to consider is the role of stress hormones, which are released in response to a range of adverse environmental and social conditions. The hypothalamic-pituitary-adrenal (HPA) axis responsible for the initiation and regulation of the physiological stress response undergoes significant development following birth in altricial bird species. It is thought that exposure to stress hormones may also alter the development of this axis and in so doing re-program the hormonal phenotype of the individual in adulthood, shaping behavioural and physiological responses to environmental stress, including resource allocation decisions during breeding. Using the zebra finch as a model species, this project will investigate the long-term and transgenerational effects of early exposure to stress hormones, specifically corticosterone (CORT). In particular, this research will examine the role of postnatal corticosterone exposure on the physiological and behavioural responses to a range of stressors in adulthood, as well as reproductive success, maternal transfer of corticosteroids and long term effects on offspring quality. We plan to use direct administration of corticosterone to avoid any problems with habituation to extraneous stimuli. By using a direct manipulation of nestling stress hormones we can focus on the long-term effects of early activation of the HPA axis, removing the confounding effects of other potential stressors such as changes in nutrient supply, which will cause multiple effects in addition to an activation of the HPA axis. A counterbalanced breeding experiment using same sex sibling pairs as a unit of comparison (one developmentally CORT treated, one control) will provide a powerful investigation of these possible effects in both males and females. The project will further address these issues under different environmental conditions, e.g. under favourable or unfavourable food availability, to determine the potential role of developmental CORT exposure in mediating appropriate responses under adverse conditions later in life. This study takes a multi-disciplinary approach, drawing on endocrinology, behavioural ecology and reproductive physiology, to determine if postnatal rearing conditions might be just as important as current conditions to the reproductive individual in decision-making. Radioimmunoassay techniques will be used to determine plasma corticosterone levels in response to stressors, at key points throughout an individuals life: during growth, early independence and whilst breeding. This research will therefore provide detailed information on how early life CORT exposure could program the physiology of a bird throughout its life. The proposed research fits perfectly within the remit of the Animal Sciences committee's Theme of 'Integrative Animal Physiology' as it addresses several of the issues identified in the Priority area of 'Genes to Physiology'. It will integrate behavioural and physiological information on the whole organism to understand the effects of neonatal programming on growth, behaviour and reproductive potential in adulthood, and provide valuable data on the role of early physiological experience in shaping an individuals response to it's environment.