Dehydration followed by restraint sustains high circulating corticosterone and improves immunity in toads

Amphibians are suffering population declines due to a variety of factors such as increased ultraviolet radiation, climate change, habitat loss, pathogens, and pollution, or a combination of these. Such changes are associated with a reduction in the availability of water, exposing these animals to a greater risk of desiccation. In this context, understanding how dehydration can modulate the hypothalamic-pituitary-interrenal axis (HPI) and the immune response is an imperative question to predict how stressors can affect amphibian species. We investigated whether dehydration promotes long-lasting effects on toads' ability to respond to a consecutive stressor (restraint) even if the toads are allowed to rehydrate, as well as its effects on the immune function. We also tested the hypothesis that the toads showing more severe dehydration would exhibit lower responsiveness to restraint challenge, even if the animals were allowed to rehydrate. Individuals of R. ornata were dehydrated mildly and severely. Thereafter, they were submitted to a restraint stress challenge for 1 and 24 h. Our results show that dehydration increased hematocrit and CORT in R. ornata toads. The restraint induced an acute stress response in fully hydrated toads (increased CORT and neutrophil: lymphocyte ratio). Otherwise, restraint in moistened cloth bags allowed full rehydration in previously dehydrated toads and did not induce an additional increase in CORT, but those toads sustained elevated CORT up to 24 h of restraint. Also, these animals showed increased neutrophil: lymphocyte ratio and the phagocytic activity of blood cells, even when they rehydrated during restraint. These results point to a continuous activation of the HPA during dehydration and subsequent restraint, even when they recovered from the dehydration state. Also, acute stressors seem to promote immune cell redistribution and augmentation of immune cellular function in R. ornata toads.