SSA: Investigating a role for the gut microbiota in stress axis dysfunction and anxiety behaviour in prenatally stressed rats

Importance of the early life environment: The development and neuronal organisation of the fetal/neonatal brain is significantly influenced by the physiological status of the mother and the early environment1. Maternal stress exposure during pregnancy detrimentally 'programmes' the offspring's brain resulting in profound alterations in physiology and behaviour in later life. Using a social stress paradigm in rats we have shown that the principal neuroendocrine stress system, the hypothalamo-pituitary-adrenal (HPA) axis, is particularly sensitive to prenatal stress exposure: the adult offspring of stressed dams display markedly greater stress responses1. HPA axis dysfunction is involved in the neurobiology of a range of mood disorders, including anxiety and depression. Indeed, adult prenatally stressed offspring also exhibit heightened anxiety-like behaviour1. These changes in stress responsivity and anxiety-like behaviour have been linked with changes in the central expression of stress-related genes, however what mediates these changes in the offspring's brain is not known.
The gut microbiota influences stress responses and stress influences the gut microbiota: Bidirectional communication between the brain and the gut is well established; however recent studies have highlighted the importance of the gut microbiota in influencing stress responses and related behaviours2. For example, germ free mice (with no commensal microbiota) display reduced anxiety-like behaviour compared with conventionally-reared mice and this behaviour can be reversed by colonization of the gut with microbiota from control mice2. Similarly mice treated with probiotics display reduced anxiety-like behaviour and lower corticosterone responses to stress2. In contrast treatment of mice with subclinical doses of pathogenic bacteria results in increased anxiety-like behaviour and activation of the HPA axis2. Changes in anxiety behaviour and stress responsiveness induced by manipulating the gut microbiota are associated with altered expression of GABA receptors in the brain3. This is important given dysfunctional GABA signalling has been linked to anxiety and depression and the GABAA receptor is a target for some anxiolytic drugs. Importantly, stress also influences the composition of the gut microbiota e.g. repeated social stress exposure disrupts commensal microbial populations in adult mice. Moreover early life stress (via post-natal maternal separation) in rats results in increased stress reactivity and anxiety-like behaviour and is associated with long-term changes in the composition and diversity of the gut microbiota2.
Commensal microbiota colonise the mammalian gut in early postnatal life, shortly after birth. The predominant source of microbes for the initial colonisation of the gut are the maternal microbiota. Studies using germ free mice have clearly demonstrated that the gut microbiota in early life is critical for the normal development of the HPA axis and appropriate stress responses in later life. Given that stress affects the gut microbiota, it is reasonable to hypothesise that social stress exposure during late pregnancy may alter the maternal microbiota and subsequently influence the composition/diversity of the gut microbiota of her offspring. Whether this may influence stress responsiveness and behaviour in the offspring is not known.
Project Aims: The overarching aim of this project is to establish whether alterations in the gut microbiota contributes to HPA axis dysfunction and anxiety-like behaviour in prenatally stressed (PNS) rats, whether there are any sex differences in the contribution of the gut microbiota and whether the adverse phenotypes in PNS rats can be reversed by manipulating the gut microbiota.