Synergetic effects of immune challenge and stress depress cortisol, inflammatory response and antioxidant activity in fish-eating Myotis

One of the most common tools in conservation physiology is the assessment of environmental stress via glucocorticoid measurement. However, little is known of its relationship with other stress related biomarkers, and how the incidence of an immune challenge during a long-term stress could affect individuals overall stress response. We investigated here the relationship between basal and post-acute stress fecal cortisol metabolite (FC) with different antioxidant enzymes, oxidative damage, and immune parameters in the fish-eating Myotis (Myotis vivesi). We found that in both basal and post-stress conditions, FC was highly related to a number of antioxidant enzymes and immune parameters, but no to oxidative damage. We also assessed changes of FC through the seasons. Basal FC samples and stress reactivity after a short-duration stress, displayed similar levels during summer, autumn and early winter, albeit lower concentrations in late winter. Stress reactivity after long-duration stress was greater in summer and early winter. Finally, we tested the effect of a simultaneous exposition to a long-strong-stress stimulus with an immune response stimulation by administrating adrenocorticotropic hormone (ACTH) and phytohemaglutanin (PHA) after 42 h. Results showed that when both stimulus were administrated, FC concentrations, inflammation and some antioxidant activity were lowered in comparison to the control and individual administration of the challenges. Our findings support the idea that animals maintain constant basal GC levels when living in challenging environments, but response to acute stress differs seasonally and immune defense mechanisms and stress response might be compromised when confronted with multiple challenges.