Excretion of Steroid Hormones in Rodents: An Overview on Species Differences for New Biomedical Animal Research Models

Living organisms have regular patterns and routines that involve obtaining food and carrying out life-history stages such as breeding, migrating, molting and hibernating. The acquisition, utilization, and storage of energy reserves (and other resources) are critical to lifetime reproductive success, and this reproductive process could be affected by predictable and unpredictable environmental changes (McEwen and Wingfield, 2003; Schneider, 2004). Allostasis is achieving physiological stability through change (see details in McEwen and Wingfield, 2003); the allostatic state refers to altered and sustained activity levels of the primary mediators, i.e., glucocorticosteroids that integrate physiology and associated behaviors in response to changing environments and challenges. Focused on these primary mediators, particularly in steroid hormones, it has been well accepted for a long time that variations (increases) of adrenal glucocorticoids are associated with stress responses. Moreover, measuring changes in glucocorticoid concentration (and also in levels of adrenaline and noradrenaline) has been the most frequently used strategy to monitor physiological responses to stress and distress challenges (Terlouw et al, 1997; Wielebnowski, 2003; Mormède et al, 2007; Sheriff et al., 2011). In terms of changes in steroid secretory patterns in response to a stressor, glucocorticoids are known to change over the course of minutes and those levels will subsequently (within hours to days) affect steroid reproductive hormones (such as testosterone, estradiol and progesterone) (Sapolsky et al., 2000). While experiencing severe stress, animals, as humans, can succumb to disease or fail to reproduce or develop properly (Moberg, 2000). Therefore, animals have evolved a suite of physiological and behavioral strategies to cope with environmental changes as well as to survive in a particular given time and space (Buchanan, 2000; Romero, 2004). Therefore, “environmental endocrinology” has developed in response to the need to understand how hormones modulate and control physiological processes in animals exposed to the exigencies of their particular natural environment. This has only been possible through