The acute glucocorticoid stress response, regulated by the hypothalamic-pituitary-adrenal (HPA) axis, is critical to enabling an individual to cope with stressors through reallocation of energy needs. There is now evidence in a number of species that individuals are repeatable in certain components of this stress response. Individual differences in the behavioral response to stress are often called coping styles and with these differences in behavior should come a constellation of physiological and neural traits—the nature of that trait constellation is the subject of this thesis. We tested the hypothesis that there is phenotypic integration across behavioral, hormonal and neuroendocrine traits and focus here on the hormonal and neuroendocrine components. We tested the idea that individual differences in patterns of circulating glucocorticoids (CORT) in response to a standardized and multi-part stress assessment would be correlated with the expression of two hormone receptors critical in glucocorticoid secretion patterns (mineralocorticoid, MR and glucocorticoid, GR). We predicted that there would be variation in receptor expression correlated with individual differences in hormonal stress reactivity. In support of this prediction, we found strong negative associations between levels of MR mRNA in the hippocampus and initial and stress-induced CORT as well as a negative relationship between GR expression in the hippocampus and stress-induced CORT. These results suggest these receptors and regions may be critical in regulating certain components of the stress response and supports the phenotypic integration hypothesis. We also explored the expression of GR and MR throughout the brain of a great tit as the species is popular for work on animal personality but the distribution of receptors has not yet been evaluated throughout the brain
