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

Area of Science:

• Endocrinology
• Animal Physiology
• Conservation Biology

Background:

• The hypothalamic-pituitary-adrenal (HPA) axis regulates stress response via glucocorticoids (GCs).
• Chronic stress leads to sustained GC secretion, potentially disrupting homeostasis.
• Koalas, like other mammals, use cortisol as their primary GC, excreted as fecal cortisol metabolites (FCMs).

Purpose of the Study:

• To investigate the negative feedback mechanism of the HPA axis in koalas.
• To assess plasma cortisol and FCM responses to a dexamethasone suppression test (DST).

Main Methods:

• Administered dexamethasone (an exogenous GC) to four koalas.
• Measured plasma cortisol concentrations.
• Quantified fecal cortisol metabolites (FCMs) using the 50c EIA.

Main Results:

• No suppression of plasma cortisol was observed after dexamethasone administration.
• Plasma cortisol levels increased in all koalas.
• Fecal cortisol metabolite (FCM) levels also increased, mirroring plasma cortisol trends.

Conclusions:

• The HPA axis feedback loop appears unresponsive in the studied koalas.
• Sustained high cortisol levels may increase disease susceptibility and impair natural coping mechanisms.
• This suggests potential physiological challenges for koala populations facing chronic stress.