The Impacts of Transdermal Application of Corticosterone On Toad (Rhinella Icterica) Immunity.

Recent studies show that acute physiological increases in endogenous glucocorticoid levels have immunostimulatory effects. Although post-acute stress immunosuppressive effects have also been described, the difference between enhancing or suppressing the immune response seems mediated by the stressor's duration, intensity, and the immune component under analysis. To elicit physiologically relevant corticosterone levels that can be found in Rhinella icterica toads after stressful events (e.g., restraint or captivity) and understand how acute increased glucocorticoid levels of different intensities affect corticosterone and testosterone plasma levels and immune parameters (in vitro plasma bacterial killing ability, neutrophil to lymphocyte ratio, and in vivo phagocytosis of peritoneal leukocytes), we submitted toads to the transdermal application of two corticosterone doses (1 and 10ug). Corticosterone transdermal application increased corticosterone plasma levels with different intensities: three times for 1ug and fourteen times for 10ug, compared to the vehicle, and the neutrophil to lymphocyte ratio increased regardless of the corticosterone dose. However, there was no effect on testosterone levels and bacterial killing ability. Interestingly, both corticosterone doses promoted immunosuppression, decreasing peritoneal leukocytes' phagocytosis activity by 60% for toads receiving the dose of 1ug and 40% for those receiving 10ug. Our results show the complexity of the relationship between increased corticosterone levels and immunomodulation. The different corticosterone doses promoted increases of distinct magnitudes in corticosterone plasma levels, with the less intense increase in corticosterone levels generating greater cell-mediated immunosuppression. Future studies using different corticosterone doses to achieve and compare physiological vs. pharmacological hormone levels are imperative to understanding these interrelationships between corticosterone and immune response.