Changes in corticotropin-releasing hormone (CRH) in the larval Euphlyctis cyanophlyctis exposed to predation risk

Abstract

Predation is a principal selection pressure acting on prey, often leading to changes in physiology, morphology, behaviour, and life-history traits of prey. Vertebrates respond to predation by activating their neuroendocrine stress axis (hypothalamus-pituitary-adrenal axis). Activation of the neuroendocrine stress axis results in the release of corticotropin-releasing factor (CRF) from the hypothalamus that acts on corticotropes of the anterior pituitary to release adrenocorticotrophic hormone (ACTH). ACTH reaches the adrenal cortex, through peripheral blood and stimulates the cortical cells to secrete glucocorticoids (GCs) which mediate the antipredator responses of prey animals by integrating many physiological processes. CRF being the key regulator of HPA axis, holds a key role in predator induced stress responses of animals. The present study intends to determine the changes in CRF distribution and expression after a chronic predation stress on larval Euphlyctis cyanophlyctis. After rearing the tadpoles under chronic predator stress from Gosner stages 25-44, changes in the distribution of CRF in the brain of tadpoles was checked using Immunohistochemistry and the CRF gene expression was assessed using qRT-PCR. The immunohistochemical results show that robust CRF expression was observed in the hypothalamus of tadpoles which were exposed to predator. These results were corroborated by RT-PCR studies which showed the increased levels of mRNA in the brain of tadpoles experiencing predation stress. The results of our study are discussed in the context of previous studies in other vertebrates.