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Corticosterone transdermal application in toads (Rhinella icterica): Effects on cellular and humoral immunity and steroid plasma levels

Previous studies in a variety of vertebrates show stressed animals to exhibit acute increases in glucocorticoid levels, with consequences for immune modulation. Chronically elevated glucocorticoid levels are mostly associated with immunosuppression. However, there is still a paucity of knowledge regarding the amphibian physiological reaction to short- and long-term stress conditions, including glucocorticoid effects on the immune response. In order to better clarify the relationship between glucocorticoids and immune response, newly captured Brazilian toads (Rhinella icterica) (1 week in captivity) were subjected to a daily transdermal application (TA), of corticosterone or vehicle, for 30 consecutive days. Measures were made on the first day (acute stressor) and last day (chronic stressor), at timepoints 1, 6, and 12 hr post TA. A number of variables were analyzed: corticosterone plasma levels (CORT); neutrophil/lymphocyte ratio (N:L); testosterone plasma levels (T); innate immune response, as indicated by bacterial killing ability (BKA); and whole blood phagocytosis. The corticosterone TA only simulated hormonal changes associated with the acute stress response, even after 30 consecutive days of the treatment, with the increased CORT in response after corticosterone TA being evident only 1 hr postapplication and not thereafter. The general responses to corticosterone TA included increased CORT and N:L at first day of the treatment, and increased CORT and phagocytosis on the last day of the treatment. A decrease in T and BKA associated with the time in captivity was also evident, suggesting that captivity may be a chronic stressor for these toads.