Effects of dehydration on glucocorticoid secretion and immune function in toads

Author(s)

Adriana Maria Giorgi Barsotti

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Biociências (IBIOC/SB)
• Defense date: 2021-06-11

Examining board members

Fernando Ribeiro Gomes; José Eduardo de Carvalho; Pedro Augusto Carlos Magno Fernandes

• Advisor: Fernando Ribeiro Gomes

Abstract

Among tetrapods, amphibians represent the group with the most permeable skin and the highest risk of water stress. Currently, it is known that dehydration can trigger a stress response in vertebrates, activating the hypothalamus-pituitary-adrenal/interrenal axis (HPA/I), increasing the levels of glucocorticoids (GC) and, consequently, modulating the immune response. Although they have a clear implication for the fitness, these effects of dehydration on immunocompetence remain little explored in the literature, particularly for anuran amphibians. The objective of this thesis was to understand the impact of dehydration as a stressor on aspects of innate immune function in anuran amphibians. Particularly, it was investigated whether individuals of Rhinella ornate, a species of toad associated with the mesic and forest regions of Brazil, is able to activate a stress response when dehydrated, the consequences of this water stress on immunocompetence and the ability to respond to a secondary stressor. The impact of dehydration as a stressor on immune function was also investigated in native and invasive populations of the toad Sclerophrys gutturalis in South Africa, considering the different climatic conditions in which these populations live. Briefly, individuals were subjected to dehydration of 10% and 20% of standard body mass and had a blood sample collected to assess variables such as plasma corticosterone concentration (CORT), hematocrit (HCT), neutrophil: lymphocyte ratio (N:L), bacterial killing ability (BKA) and phagocytic activity of leukocytes (PP). Then, the animals were submitted to movement restriction under non-dehydrating conditions (secondary stressor). Our results showed that dehydration increased CORT, HCT and N:L in toads. Secondary restriction stress resulted in the maintenance of a high plasma CORT concentration and an increase in N:L and PP. Individuals of S. gutturalis from the invasive population showed lower body index, higher BKA and N:L in the field than individuals from the native population. After experimental submission to stressors, invasive and native toads showed an increase in CORT, and the invaders maintained BKA comparatively higher than the native ones. These results indicate that dehydration is a stressor for toads, being able to activate the HHI axis, increasing the secretion of CORT and stimulating immune function. In addition, invasive toads have a constitutively high innate immune function compared to that of natives, which could increase their fitness in the new environment and favor the success of dispersion. (AU)

• FAPESP's process: 16/05024-3 - Hidric stress and immune response in frogs (Rhinella ornata) inhabiting Atlantic Forest fragments
• Grantee: Adriana Maria Giorgi Barsotti
• Support Opportunities: Scholarships in Brazil - Doctorate