Bayer School of Natural and Environmental Sciences
amphibian, ecology, HPA axis, immunity, physiology, stress
Many vertebrate stress responses are mediated by the hypothalamic-pituitary-adrenal/interrenal (HPA/I) axis, which involves hypothalamic secretion of corticotropin releasing factor (CRF) and increased plasma levels of glucocorticoid hormones (GCs) like corticosterone (CORT). Basal HPA/I activity is also imperative to maintaining homeostasis. Despite having crucial roles, HPA/I activity is variable and can be influenced by factors like environment, physiology, and life-history. HPA/I activity also influences reproduction and immunity. To better understand patterns of HPA/I variation (specifically CORT and CRF), we conducted several studies using plethodontid salamanders. Their low-energy lifestyle offers unique contrast to studies of stress physiology in high-energy animals like mammals and birds. Understanding amphibian stress physiology is also important because environmental stressors are linked to amphibian population declines. First, we sampled free-living Allegheny Mountain dusky salamanders (Desmognathus ochrophaeus) to identify physiological factors associated with CORT variation. White blood cells were the strongest predictors of HPA/I activity compared to reproductive investment, fat stores, or body condition. This indicated HPA/I activity is correlated with immunity and supported the facilitation hypothesis. Second, we investigated HPA/I variation in reproductive and nonreproductive female red-legged salamanders (Plethodon shermani) and found no evidence that reproductive condition influences HPA/I activity. Next, we tested the effects of CORT on immune function and wound healing in Allegheny Mountain dusky salamanders. Exogenous CORT elevation, but not a daily chasing stressor, delayed wound healing. This indicates that elevated CORT can delay healing, but endogenous elevation may be suppressed under natural conditions since a daily stressor did not suppress healing despite decreasing body mass and increasing mortality. Finally, we tested whether testosterone (T) suppressed healing as predicted by the immunocompetence handicap hypothesis and found no evidence that T affects healing in dusky salamanders, but a prior surgery enhanced healing of a subsequent induced wound. Together these studies highlight the relationship between HPA/I activity and immunity; whereas, we found no evidence that reproductive condition, fat stores, or body condition relate to HPA/I activity in plethodontid salamanders. This work has implications for understanding vertebrate stress physiology in general and for better understanding amphibian stress responses and how plasma GCs mediate these responses.
Thomas, J. (2016). Variation in the amphibian hypothalamic pituitary adrenal axis: Patterns, mechanisms, and implications (Doctoral dissertation, Duquesne University). Retrieved from http://ddc.duq.edu/etd/108