Schematic of the HPA axis.

Under basal conditions, the pituitary gland releases readily-available ACTH in response to CRH secreted from the hypothalamus. In response to ACTH, the adrenal cortex synthesizes and secretes CORT, which feeds back primarily at the pituitary gland, but also at higher centres in the brain (depicted by the dashed line), to inhibit ACTH secretion. This dynamic balance between the positive feed-forward action of CRH and the negative feedback action of CORT at the level of the pituitary gland has been suggested theoretically to regulate the oscillatory activity of the pituitary-adrenal system. In addition to basal regulation, acute stressors result in additional hypothalamic secretion (CRH and AVP) which act to further stimulate the pituitary-adrenal network. The interplay between basal oscillatory activity and acute stress perturbations is the focus of this study.