Understanding Stress and Immunity in Amphibians: Corticosterone, Hormonal Interactions, and Conservation Implications

Amphibians, like other vertebrates, respond to stressors through the activation of the hypothalamic-pituitary-interrenal (HPI) axis, leading to elevated levels of glucocorticoids in the bloodstream. The amphibian HPI axis is functionally analogous to the mammalian hypothalamic-pituitary-adrenal (HPA) axis, coordinating the stress response via glucocorticoid release. Among these, corticosterone (CORT) acts as the principal downstream effector hormone, exerting widespread effects on various physiological systems. As seen in many other vertebrates, physiologically increased CORT levels are commonly associated with immune modulation, which might enhance or suppress the immune response. This immune outcome is influenced by several factors, including the duration and intensity of the stressors, the body condition of individuals, life history, and species-specific traits. Here, we provide a literature review on the role of stressors and CORT in amphibian immunity, including studies conducted in natural environments and controlled settings. These studies involve standardized stress protocols (i.e., restraint, captivity, and exogenous hormone treatment), along with "in vivo" and "in vitro" immune assays. Overall, CORT levels and their effects on immunity are highly variable, yet they do not act in isolation. There is significant interaction between CORT and other hormones, such as testosterone and melatonin, which further influences the immune response in amphibians. This interplay underscores the complexity of the stress-immune relationship and suggests that a holistic approach is essential to fully understand the impact of stressors on amphibian health and conservation.