The physiological stress response results in release of glucocorticoid hormones such as corticosterone (CORT). Whereas short-term activation of this response helps animals cope with environmental stressors, chronic activation can result in negative effects including metabolic dysregulation and reproductive failure. However, there is no consensus hormonal profile of a chronically stressed animal, suggesting that researchers may need to look beyond hormone titers to interpret the impacts of chronic stress. In this study, we brought wild house sparrows (Passer domesticus) into captivity. We then compared glucocorticoid and mineralocorticoid receptor concentrations in sparrows exposed either to a standardized chronic stress protocol (n=26) or to standard husbandry conditions (controls; n=20). We used radioligand binding assays to quantify receptors in whole brain, liver, kidneys, spleen, gonads, gastrocnemius and pectoralis muscle, omental and subcutaneous fat, and bib and back skin. In most tissues, CORT receptors did not differ between controls and stressed animals, although we found marginal increases in receptor density in kidney and testes in stressed birds at some time points. Only in pectoralis muscle was there a robust effect of chronic stress, with both receptor types higher in stressed animals. Increased pectoralis sensitivity to CORT with chronic stress may be part of the underlying mechanism for muscle wasting in animals administered exogenous CORT. Furthermore, the change in pectoralis was not paralleled by gastrocnemius receptors. This difference may help explain previous reports of a greater effect of CORT on pectoralis than on other muscle types, and indicate that birds use this muscle as a protein reserve.
KEY WORDS: Hypothalamic-pituitary-adrenal axis, Glucocorticoids, Peripheral receptors, Bird
INTRODUCTIONActivation of the hypothalamus-pituitary-adrenal (HPA) axis results in the release of glucocorticoid hormones (cortisol and/or corticosterone depending on the species, hereafter CORT). At baseline concentrations, CORT helps regulate metabolism, activity levels and feeding behavior; at the higher concentrations caused by a stressor, CORT is an essential component of the vertebrate stress response (Landys et al., 2006;Sapolsky et al., 2000). During the stress response, CORT helps animals mobilize energy stores, enhances certain immune components and promotes escape and selfmaintenance behaviors (Spencer et al., 2001;Wingfield and Kitaysky, 2002).
RESEARCH ARTICLEDepartment of Biology, Tufts University, Medford, MA 02155, USA.*Author for correspondence (christine.lattin@tufts.edu)
Received 7 February 2014; Accepted 26 April 2014However, sustained high levels of CORT due to frequent or prolonged stressors can result in a number of stress-related pathologies, including suppression of reproduction and the immune system, metabolic dysregulation and cognitive impairment (Dallman et al., 2003; de Kloet et al., 2005;Martin, 2009;Sapolsky et al., 2000). Chronic stress is thought to result once COR...