497 Environmental Toxicology and Chemistry, Vol. 26, No. 3, pp. 497–505, 2007  2007 SETAC Printed in the USA 0730-7268/07 $12.00 + .00 BIOACCUMULATION, OXIDATIVE STRESS, AND NEUROTOXICITY IN DANIO RERIO EXPOSED TO DIFFERENT ISOTOPIC COMPOSITIONS OF URANIUM SABRINA BARILLET,*† CHRISTELLE ADAM,† OLIVIER PALLUEL,‡ and ALAIN DEVAUX§ †Laboratory of Radioecology and Ecotoxicology, IRSN (Institute for Radiological Protection and Nuclear Safety), Cadarache, Building 186, BP 3, 13115 St-Paul-Lez-Durance cedex, France ‡Ecotoxicological Risk Assessment Unit, INERIS (National Institute for Industrial Environment and Risks), Parc technologique ALATA, 60 550 Verneuil-en-Halatte, France §Environmental Science Laboratory, ENTPE (National School of Civil Engineering), 69518, Vaulx-en-Velin cedex, France INRA (National Institute for Agronomic Research), EFPA (Department for Forest, Grassland, and Freshwater Ecology), 54280, Champenoux, France ( Received 17 May 2006; Accepted 28 September 2006) Abstract—Experiments were carried out on adult male zebrafish (Danio rerio) to assess early changes induced by waterborne exposure to different isotopic compositions of uranium (depleted uranium associated or not with 233 U). Oxidative stress and neurotoxicity were selected as effect endpoints to characterize uranium chemo- and radiotoxicity. Catalase, glutathione peroxidase, and superoxide dismutase activities and total glutathione content of hepatic extracts, as well as brain acetylcholinesterase activity and uranium bioaccumulation, were measured. Oxidative stress induced by uranium exposure led to decreases in superoxide dismutase and catalase activity levels as well as total glutathione content in liver extracts. These perturbations were significantly more marked in 233 U-exposed fish. Furthermore, significant increase in acetylcholinesterase activity was observed in brain extracts at the same level, whatever the isotopic composition of uranium. Keywords—Danio rerio Uranium Bioaccumulation Oxidative stress Neurotoxicity INTRODUCTION Uranium is the heaviest of all the naturally occurring ele- ments. It is ubiquitous throughout the natural environment, being found in varying but small amounts in rocks (e.g., 3– 10 mg/kg in the earth’s crust), soil, water, air, plants, animals, and human beings [1]. This metal essentially exists in the U(+IV) or U(+VI) oxidation state in water systems depending on their reduction–oxidation potential, U(+VI) being domi- nant in oxic surface waters. The U(+VI) state is highly soluble, unlike U(+IV), which is soluble to a much lesser extent [2]. Uranium is found in surface water as well as groundwater at an extremely wide range of concentrations from below 0.01 g/L to more than 1,500 g/L [3]. Various anthropogenic activities involving the processing or use of materials rich in uranium have the potential to re- mobilize radionuclides and heavy metals and make them avail- able in the environment, hence altering the natural abundance of uranium in environmental compartments. These activities include the use of phosphate fertilizers, various mining activ- ities, and the industrial processing of uranium for the manu- facture of nuclear fuel and other products, including the use of depleted uranium (DU) as a reinforcing material [4]. Natural uranium consists of a mixture of three isotopes ( 238 U, 235 U, and 234 U) that all emit alpha particles. Because of their relatively large size and charge, alpha particles rapidly lose their kinetic energy and have a low penetrating power (4 cm in air and only 50 m in soft tissue). Therefore, they are unable to penetrate even the superficial layer of organisms. As a result, from a radiologic standpoint, uranium is mainly an internal radiation hazard [1]. * To whom correspondence may be addressed (sabrina.barillet@irsn.fr). Uranium toxicity has not been studied extensively for non- human biota, particularly for aquatic vertebrates such as fish. Current knowledge of uranium toxicology in fish is generally limited to acute lethality data from waterborne exposure [5– 8]. Information regarding early and sublethal effects of ura- nium exposure in freshwater fish is scarce [9–12]. Better knowledge of interactions between U and living organisms at environmentally relevant concentrations is therefore needed to predict the possible consequences of uranium exposure. Among the indicators of primary subcellular damage po- tentially induced by the chemical and radiological properties of uranium, measurements of oxidative stress seem to be a relevant endpoint. Indeed, like any heavy metal, this radio- element is able to chemically activate oxygen species in the course of redox reactions via the redox chemistry of transition metals [13,14]. Furthermore, uranium can enhance the pro- duction of free radicals via the ionization phenomenon induced by alpha particle emissions [15]. The damage, in this case, would not be the direct result of radiation, but rather an indirect consequence as a result of reactive oxygen species stemming from the radiation [16]. When the quantity of free oxygen species generated exceeds the level that the cell’s protective system can control, cell proteins, nucleic acids, and lipids can be damaged. Several studies were carried out considering ox- idative stress markers in various biological models exposed to uranium, but none of them attempted to understand the re- spective role of chemotoxicity and radiotoxicity [9–12,16–18]. In addition, recent data have linked DU exposure to neu- rotoxic effects [9,17,19–21]. However, uranium exposure sce- narios reported in the literature demonstrated either inhibiting [9] or activating [17] effects of uranium on acetylcholinester- ase (AChE) activity. Because of the conflicting conclusions