Effects of copper on the acute cortisol response and associated physiology in rainbow trout

Abstract

The aim of this study was to determine the effects of chronic waterborne copper (Cu) exposure on the acute stress-induced cortisol response and associated physiological consequences in rainbow trout (Oncorhynchus mykiss). Trout were exposed to 30μg Cu/L in moderately hard water (120mg/L as CaCO3) for 40days, following which time the acute cortisol response was examined with a series of stressors. At 40days, a 65% increase in Cu was observed in the gill, but no accumulation was observed in the liver, brain or head kidney. Stressors such as air exposure or confinement did not elicit an increase in circulating cortisol levels for Cu-exposed fish, in contrast to controls. However, this inhibitory effect on the acute cortisol response appeared to have few implications on the ability of Cu-exposed fish to maintain ion and carbohydrate homeostasis. For example, plasma Na+, Ca2+ and glucose levels as well as hepatic glycogen levels were the same post-stress in control and Cu-exposed fish. Trout were also challenged with exposure to 50% seawater for 48h, where Cu-exposed trout maintained plasma Na+, glucose and hepatic glycogen levels. However, Cu-exposed fish experienced decreased plasma K+ levels throughout the Cu exposure and stress tests. In conclusion, chronic Cu exposure resulted in the abolition of an acute cortisol response post-stress. There was no Cu accumulation in the hypothalamus-pituitary-interrenal axis (HPI axis) suggesting this was not a direct toxic effect of Cu on the cortisol regulatory pathway. However, the lack of an acute cortisol response in Cu-exposed fish did not impair the ability of the fish to maintain ion and carbohydrate homeostasis. This effect on cortisol may be a strategy to reduce costs during the chronic stress of Cu exposure, and not endocrine disruption as a result of toxic injury.