Testosterone immunomodulation in free-living and captive Rhinella icterica male toads.

Testosterone (T) regulates immune function, with both immunostimulatory and immunosuppressive effects on several vertebrates. We investigated the covariation between plasma T and corticosterone (CORT) levels and immunity (plasma bacterial killing ability (BKA), and neutrophil to lymphocyte ratio (NLR)) in free-living Rhinella icterica male toads inside and outside the reproductive season. We found an overall positive correlation between steroids and immune traits, with toads during the reproductive season displaying increased T, CORT and BKA. We also investigated the T transdermal application effects on T, CORT, phagocytosis of blood cells, BKA and NLR in captive toads. Toads were treated with T (1, 10 or 100 µg) or vehicle (sesame oil) for eight consecutive days. Animals were bled on the first and eighth days of treatment. Increased plasma T was observed on the first and last day of T-treatment, while increased BKA was observed following all T doses on the last day, with a positive correlation between T and BKA. Plasma CORT, NLR and phagocytosis increased on the last day for all T-treated and vehicle groups. Overall, we demonstrated a positive covariation between T and immune traits in the field and T-induced augmented BKA in captive toads, indicating a T immunoenhancing effect in R. icterica males. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Plasma steroids and immune measures vary with restraint duration in a toad (Rhinella icterica).

Immunoenhancing effects have been widely described following acute stressors in several vertebrates, and valuable contributions have been made from studies on acute stress to understand hormonal-immune interactions. However, most studies focus on hormonal and immune responses after standardized time lapses, neglecting potential influence of duration of exposition to stressor. Herein, we investigate fluctuations of plasma hormone concentrations (corticosterone and testosterone) and immunity (neutrophil to lymphocyte ratio, phagocytosis of blood cells, and plasma bacterial killing ability) in a toad species (Rhinella icterica) in response to six different periods of exposure to restraint stress. We observed increased plasma corticosterone concentrations following restraint in all sampled times (0.5 to 48 h), with the highest values being observed during the first hour (0.5 to 1 h). Restraint-induced increases in the neutrophil to lymphocyte ratio and phagocytosis percentage were observed from the first 0.5 h, gradually increasing after that with the time of restraint. We also observed decreased testosterone plasma concentrations in response to a more prolonged restraint (24 and 48 h). No changes were observed in plasma bacterial killing ability following restraint. Together, our results demonstrate dynamic time-related hormonal and immune changes. These results point to the fact that for some species measuring hormonal and immune variables at single time points following a stressor might work better when preceded by a study of the temporal changes of the response variables to the stimuli applied. Also, time of response needs to be considered when different variables are used as proxies of stress.