As mentioned in Chapter 2, stress is ubiquitous, it can occur in both pleasurable and aversive situations, and its physiological parameters are well established. Our knowledge of the behavioral correlates of stress, however, is considerably smaller. The behavioral changes observed in a stressed animal (as opposed to a distressed one) may be more subtle and variable, depending on the environmental conditions in which the behavior is being evaluated. In addition to recognizing an animal’s normal patterns of behavior, the observer must be well trained and knowledgeable about the normal species-specific behavior in the context of species, strain, gender, and physiological state. Types of behavior commonly explored to investigate the presence of stress include open-field activity, movements in an elevated plus maze, changes in innate behaviors (e.g., movement, grooming, feeding, sexual behavior), defensive behaviors (to external threats), and avoidance/escape (Beck and Luine 2002; for more references see Additional References).
One of the primary endocrinological systems involved in the stress response is the hypothalamic-pituitary-adrenal (HPA) axis, which reacts to stress by releasing glucocorticoids. Glucocorticoid levels can be used as indicators for the impact and strength of a stressor, with two caveats: (1) they cannot inform as to the type of stressor (positive or aversive) that stimulates the HPA and (2) most sampling procedures are themselves stressful to the animals, thereby confounding the measurements. Therefore, the assessment of distress based on glucocorticoid levels has limitations, especially under the unproven assumption that a certain glucocorticoid concentration indicates the presence of distress. Furthermore, stress or distress may exist without the concomitant activation of the HPA axis.
The hypothalamic-pituitary-adrenal (HPA) axis, often referred to as the “stress response system”, plays an important role in an organism’s reaction to stressors. In response to a stressful situation the hypothalamic paraventricular nucleus synthesizes corticotrophin-releasing hormone (CRH), which is released into the median eminence and travels to the anterior pituitary where it causes the release of adrenocorticotropic hormone (ACTH) into the circulatory system. ACTH then acts selectively on specific