In the face of any threat or challenge, either real or perceived, an organism must mount a series of coordinated and specific hormonal, autonomic, immune, and behavioural responses that allow it to either escape or adapt (1–3). To be successful, the characteristics and intensity of the response must match that posed by the threat itself and should last no longer than is necessary. A response that is either inadequate or excessive in terms of its specificity, intensity or duration may result in one or more of a multitude of psychological or physical pathologies (2–5). This concept of threat and the organism’s response to it is frequently recognized and understood as ‘stress’ but is so diverse that it lacks a universally accepted definition (2) and thus is difficult to investigate or study (6).
In the early 1900s, Walter Cannon introduced the concept of homoeostasis (4)—an ideal steady state for all physiological processes. Stress has been defined as the state where this ideal is threatened. More easily appreciated, however, are those factors, both intrinsic and extrinsic, which represent a challenge to homoeostasis (termed stressors) and the complex physiological, hormonal, and behavioural responses that occur to restore the balance, the stress response (1). The importance of endocrine systems in this stress response was emphasized by Hans Selye (7), who described the need for multiple, integrated systems to respond in a coordinated fashion following exposure to a particular stressor. Nonspecific activation of the hypothalamic–pituitary–adrenal (HPA) and sympatho-adrenomedullary (SAM) axes occurred following initial exposure to a noxious stimulus. Continued exposure to the same agent has been shown to have lasting and damaging effects on various endocrine, immune, and other systems, although recovery from this state was possible provided the stress was terminated (7). In addition to various noxious agents, numerous potential stressors exist including exertion, physical extremes, trauma, injury, and psychological stress. Indeed, psychological stressors are some of the most potent stimuli of the endocrine stress response particularly when they involve elements of novelty, uncertainty, and unpredictability. This has been highlighted by the observation that anticipating an event can be as potent an activator of the stress response as the event itself (7).
Regulation of dopamine system responsivity and its adaptive and pathological response to stress