Effects of tritiated water on locomotion of zebrafish larvae: a new insight in tritium toxic effects on a vertebrate model species.

Tritium (H), a radioactive isotope of hydrogen, is ubiquitously present in the environment. In a previous study, we highlighted a mis-regulation of genes involved in muscle contraction, eye transparency and response to DNA damages after exposure of zebrafish embryo-larvae from 3 hpf to 96 hpf at 0.4 and 4 mGy/h of tritiated water (HTO). The present study aimed to link this gene mis-regulation to responses observed at higher biological levels. Analyses on spontaneous tail movement, locomotor activity and heart rate were performed. Histological sections of eyes were made to evaluate the impact of HTO on eye transparency and whole embryo immunostainings were realized to assess DNA double strand breaks repair using gamma-H2AX foci. We found a decrease of basal velocity as well as a decrease of response in 96 hpf larvae exposed at 0.4 mGy/h after a tactile stimulus as compared to controls. Histological sections of larvae eyes performed after the exposure to 4 mGy/h did not show obvious differences in lens transparency or retinal development between contaminated and control organisms. Gamma-H2AX foci detection revealed no differences in the number of foci between contaminated organisms and controls, for both dose rates. Overall, results highlighted more detrimental effects of HTO exposure on locomotor behavior in 96 hpf larvae exposed at the lowest dose rate. Those results could be linked to mis-regulation of genes involved in muscle contraction found in a previous study at the same dose rate. It appears that not all effects found at the molecular scale were confirmed using higher biological scales. These results could be due to a delay between gene expression modulation and the onset of physiological disruption or homeostatic mechanisms to deal with tritium effects. However, crossing data from different scales highlighted new pathways to explore, i.e. neurotoxic pathways, for better understanding HTO effects on organisms.