dian or circadian variations), it is necessary to take into consideration and control for their normal patterns of secretion in order to accurately interpret their concentration levels. Moreover, their usefulness is subjected to the same limitations as discussed above, although chronic indwelling vascular catheters and automated blood collection systems may circumvent this limitation to some degree (Abelson et al. 2005; for more references see Additional References).
Stressors activate the autonomic nervous system, specific brain areas, and various neurotransmitters, yet a cause and effect relationship has not yet been firmly established. Candidates for the role of a master integrator include the region of the amydgala and the neuropeptide corticotrophin-releasing factor.
Many different types of stressors cause the rapid activation of the sympathetic division of the autonomic nervous system (ANS) (Blanc et al. 1991; for more references see Additional References). This activation leads to increased cardiac output via increased heart rate and stroke volume; redistribution of blood flow from splanchnic, renal, and cutaneous vascular beds to active muscle; increased mobilization of nutrients; and increased heat production. Some stressors may also increase the activity of the parasympathetic division, affecting both core body temperature and the gastrointestinal system (e.g., disturbed intestinal absorption, gastric ulceration, colitis; Johnson et al. 2002; for more references see Additional References).
Direct monitoring of autonomic activity to assess the presence of distress in conscious animals is technically challenging, while indirect measures are somewhat easier to acquire (Li et al. 1997; Randall et al. 1994; Zhang and Thoren 1998). For example, telemetry in conscious, unrestrained animals is an effective method for the continuous monitoring of physiologic alterations in heart rate, respiration, blood pressure, ECG, and body temperature (Akutsu et al. 2002; for more references see Additional References). Once again, however, changes in these parameters do not necessarily indicate stress as they may result from nonstressful stimuli (e.g., circadian variations).
Considerable effort has been directed at exploring the neurotransmitter systems and brain areas activated in response to stress as different insults