On the use of dexamethasone to block the pituitary-interrenal axis in the brown trout, Salmo trutta L.

Oral administration of dexamethasone in a single meal at a dose of 3 micrograms g-1 body wt was sufficient to block the secretion of ACTH and cortisol in brown trout subjected to a handling stress followed by 30 min confinement. This dose, however, also had a marked effect on several of the circulating blood cell types. Lymphocyte and neutrophil counts were reduced in dexamethasone-treated fish whereas erythrocyte counts increased. The value of dexamethasone as a tool for investigating the role of interrenal tissue during stress responses in teleost fish is limited by its cortisol-like effects on other steroid-sensitive tissues.