Effects of Chronic Cortisol Elevation on Thermal Tolerance, Stress Axis Activity and Habitat Choice in Rainbow Trout (Oncorhynchus mykiss)

Abstract

In a changing climate, salmonid fishes face increasing temperatures along with other stressors, and therefore it is important to understand how chronic stress influences their thermal biology. Rainbow trout (Oncorhynchus mykiss) treated with intraperitoneal implants to elevate cortisol showed reductions in thermal tolerance measured as the critical thermal maximum (CTmax). Co-treatment with the glucocorticoid receptor (GR) antagonist RU486 prevented the decrease in CTmax, suggesting that the effect is GR-dependent. Although acclimation to higher temperatures increased CTmax, the effects of chronic cortisol persisted (i.e. lower CTmax than control). To investigate the mechanisms through which cortisol alters thermal tolerance, I targeted TRPV1, a thermosensitive ion channel known to sense noxious warm temperatures. I confirmed that TRPV1 was involved in heat sensation in rainbow trout by altering TRPV1 activity and measuring CTmax. Trout exposed to the TRPV1 antagonist capsazepine showed an increase in CTmax, whereas CTmax decreased in trout exposed to the TRPV1 agonist capsaicin. In addition, cortisol-treated fish showed increased expression of trpv4α, which may be contributing to their heightened thermal sensitivity. Capsazepine treatment also blunted the cortisol response to acute elevation of temperature, and I therefore investigated the effects of cortisol treatment and thermal acclimation on the cortisol response to acute warming. Thermal acclimation increased the temperature at which the cortisol response to acute warming was detected, whereas cortisol-treated trout mounted a cortisol response at temperatures lower than those of their control counterparts, suggesting an increase in thermal sensitivity after chronic cortisol treatment. Measurement of the transcript abundances of key genes in the hypothalamic-pituitary-interrenal (HPI) axis supported activation of the HPI axis by acute warming but there was little evidence of a role for cortisol in altering this response. Lastly, I asked whether these lab-based results would translate into effects in the natural environment, by investigating whether cortisol treatment altered the thermal habitat selection of rainbow trout in a lake. Cortisol-treated trout appeared to prefer warmer waters. Collectively, these data implicate TRPV1 in thermal sensing in rainbow trout, provide insight into the effects of chronic cortisol elevation on thermal tolerance, and emphasize the complexity of the relationships among chronic stress, stress axis activity and thermal tolerance.