Cortisol release in response to UVB exposure in Xiphophorus fish

Xiphophorus fishes are comprised of 26 known species. Interspecies hybridization between select species has been utilized to produce experimental models to study melanoma development. Xiphophorus melanoma induction protocols utilize ultraviolet light (UVB) to induce DNA damage and associated downstream tumorigenesis. However, the impact of induced stress caused by the UVB treatment of the experimental animals undergoing tumor induction protocols has not been assessed. Stress is an adaptive physiological response to excessive or unpredictable environmental stimuli. The stress response in fishes may be measured by an assay of cortisol released into the water. Here, we present results from investigations of stress response during an experimental treatment and UVB exposure in Xiphophorus maculatus Jp 163 B, Xiphophorus couchianus, and F1 interspecies hybrids produced from the mating X. maculatus Jp 163 B×X. couchianus. Overall, cortisol release rates for males and females after UVB exposure showed no statistical differences. At lower UVB doses (8 and 16kJ/m2), X. couchianus exhibited 2 fold higher levels of DNA damage then either X. maculatus or the F1 hybrid. However, based on the cortisol release rates, none of the fish types tested induced a primary stress response at the UVB lower doses (8 and 16kJ/m2). In contrast, at a very high UVB dose (32kJ/m2) both X. maculatus and the F1 hybrid showed a 5 fold increase in the cortisol release rate. To determine the effect of pigmentation on UVB induced stress, wild type and albino Xiphophorus hellerii were exposed to UVB (32kJ/m2). Albino X. hellerii exhibited 3.7 fold increase in the cortisol release while wild type X. hellerii did not exhibit a significant cortisol response to UVB. Overall, the data suggest the rather low UVB doses often employed in tumor induction protocols do not induce a primary stress response in Xiphophorus fishes.