The HPA axis
The HPA axis is the cornerstone of biological stress responses, together with the autonomic nervous system, in concert with behavioural adaptive processes. The main output elements of the axis are the glucocorticoid (GR) hormones, cortisol (mammals, fish) or corticosterone (birds) synthesized in and released by the adrenal cortex in response to the adrenocorticotropin hormone (ACTH) released by the anterior pituitary gland under the control of hypothalamic neurohormones corticotrophin-releasing hormone (CRH) and vasopressin. GR hormones act on a wide range of cells and tissues via the GR and mineralocorticoid nuclear receptors. They influence numerous metabolic pathways, the immune system, inflammatory processes and brain functions, to mention the most important. They also exert a strong feedback on the HPA axis (Chrousos, 1998).