SOC-IV-05 From molecules to phenotype: mechanisms involved in stress, sociability and hypolocomotion after chronic gamma radiation in adult zebrafish
Résumé
High levels of ionizing radiation (IR) are known to induce neurogenesis defects with harmful consequences on brain morphogenesis and cognitive functions. For instance, reductions of mitotically active cells and of mature neurons were observed in the brain of young rats exposed to 1 Gy1. Similar data showed that these neurogenesis defects were linked to the impairment in learning and memory2. However, the impact of chronic exposure to low and moderate dose rate of IR on adult brain functions remains largely unknown, as well as how they can affect, behavior, reproduction and survival. In this study, we aim at evaluating this impact, by integrating the cellular and molecular mechanisms underlying central nervous system damages with individual behavioral parameters.
Adult zebrafish were exposed at dose rates of 50 µGy/h, 0.5 mGy/h and 5 mGy/h during 36 days. Transcriptomics were carried out and showed a dose effect relation with high number of deregulated genes (DEG) at 5 mGy/h (258) and only 8 DEG at 50 µGy/h. Behavioral tests demonstrated a stress response that can be related to a decrease of acetylcholinesterase activity in fish brains and an up-regulation of oxytocine (oxt) after transcriptomic analysis. This neuropeptide is involved in anxiety and sociability behaviors3–7. Notably, we also detected a reduced contact among irradiated fish when testing in social preference test. Biological pathways analysis highlighted that several processes were impacted at both 0.5 mGy/h and 5 mGy/h, such as visual perception, circadian rhythm and photoreceptor cell differentiation. These results suggest alterations in a photoreceptive organ of zebrafish, the pineal gland. This gland is responsible for the synthesis of melatonin, known as the sleep hormone. It has been demonstrated that melatonin regulates appetite, cell proliferation and locomotor activity in zebrafish8. Using Novel Tank test, social preference and shoaling test, we confirmed a hypolocomotion state in irradiated fish that might be related to an increase of melatonin synthesis. Finally, RNA sequencing results were confirmed by qualitative analysis after in situ hybridization. We could detect ectopic expression of oxt with cells lining the ventricular zone in the preoptic area. Additionally, we could identify by RNA in situ hybridization a higher proportion of cells expressing aanat2, an enzyme involved in melatonin synthesis8, that were located in the lateral zone of the dorsal telencephalon. After chronic gamma radiation at moderate dose rates, we were able to identify molecular pathways with high probability of causing phenotype effect, stress, hypolocomotion and reduced sociability.
This project seeks the enhancement of a better knowledge concerning chronic and low or moderate dose rates of IR and their effects on the adult central nervous system. It will contribute to the development of radiosensitivity indicators and the improvement of risk assessment of IR in ecotoxicology and for humans.