Case Study: A Dexamethasone Suppression Test in Koalas (Phascolarctos cinereus)

The hypothalamic-pituitary-adrenal axis responds to stress by releasing the adrenocorticotropic hormone which, in turn, stimulates the release of glucocorticoids (GCs). During acute stress events, the GCs' function is to maintain homeostasis. Short-term stress events trigger psychophysiological responses which are fundamental for survival in the natural world. Chronic stress occurs when the effect of stressors persists, and GCs secretion continues. The increase of GCs interacts with receptors in the brain triggering a negative feedback loop which inhibits the secretion of ACTH, consequently down-regulating GC production. Cortisol is the main GC in most mammals, including koalas. Cortisol is metabolised by the liver and bacterial enzymes in the intestine, and its metabolites are excreted via the faeces. In a previous study, where we used the tetrahydrocorticosterone) enzyme immunoassay (aka 50c EIA) for measuring faecal cortisol metabolites (FCMs) to assess stress in koalas, we did not detect a decrease in FCM values (negative feedback) after administration of prednisolone, an exogenous GC. Using the dexamethasone suppression test (DST), this study aimed at determining the ability of the feedback loop to decrease the concentration of plasma cortisol. This was achieved by measuring the values of plasma cortisol as well as FCMs using the 50c EIA in four koalas. No cortisol suppression was observed, rather an increase in plasma cortisol concentration in all koalas. This was also reflected in the increase of FCMs. An unresponsive feedback loop, and consequent prolonged high levels of plasma cortisol, is likely to increase koalas' susceptibility to diseases and it may impact their coping mechanism in nature.