Besides the assessment of toxicological effects immediately
Besides the assessment of toxicological effects immediately after exposure, we also studied the potential recovery of the exposed organisms, through the quantification of ChE activity at specific time intervals after placing organisms in clean test medium. Recovery from chemical challenge, in this case uranium, is relevant because: (i) most animals are able to avoid contaminated areas (Araújo et al., 2016a.), recovering from stress and preventing irreversible damage to biological structures; (ii) the reversibility of ChE inhibition may help explain the mode of action of the toxic agent, since most unspecific anticholinesterasic agents are reversible in their mode of toxic action (Nunes, 2011).
Material and methods
Discussion The general lack of effects here reported may raise the issue of bioavailability of uranium to test organisms, and the likelihood of its Topotecan HCl and distribution. No effects may be ultimately interpreted as a sign of poor or even null absorption from the external media. However, this present work was in line with previous data from our work team, which thoroughly assessed uranium contamination and toxic effects using the same fish species, but with distinct toxicological end-points (Lourenço et al., 2010). It was possible to observe that uranium was bioaccumulated in muscle of C. auratus, a tendency that was particularly relevant for the two highest concentrations (see Lourenço et al., 2010). This set of data confirms that uranium was readily bioavailable at least to fish, and was also in fact absorbed. Furthermore, the same study evidenced the occurrence of significant deleterious effects, including impairment of catalase activity and irreversible DNA damage, supporting not only the presence of waterborne forms of uranium, but also their potential to be uptaken and distributed along the fish organs (Lourenço et al., 2010). On the contrary, our data show no effects in AChE for the same concentrations that elicited cyto- and geno-toxicity. The use of biomarkers to diagnose exposure of biota to chemical stress has been frequently and successfully used in ecotoxicological and biomonitoring studies. AChE (or ChE) inhibition has been used as tool for the assessment of environmental exposure of non-target organisms mainly to organophosphate and carbamate pesticides (Guimarães et al., 2007), but also to detergents, specific metals, and complex mixtures of pollutants (Labrot et al., 1996, Payne et al., 1996, Frasco et al., 2005, Nunes et al., 2005, Ramos et al., 2012, de Lima et al., 2013). However, anticholinesterasic effects elicited by metals are not always straightforward and simple to devise, since several authors showed that AChE inhibition is somewhat contradictory and even questionable (Frasco et al., 2005, Nunes et al., 2005; 2011; Sáenz et al., 2010). Some metals (e.g. cadmium, copper, mercury, lead and uranium) are known for their anticholinesterasic activity in some specific, sensitive species, such as Aiolopus thalassinus (Schmidt and Ibrahim, 1994); Daphnia magna (Guilhermino et al., 1996); Corbicula sp., Eisenia fetida and Brachydanio rerio (Labrot et al., 1996). However, cholinesterasic inhibition by metals is not an absolute rule, since some metallic species are not capable of exerting such effect, even for in vitro conditions. The study conducted by Araújo et al. (2016b) showed the selective in vitro responsiveness of brain cholinesterases from the fish jaguar cichlid (Parachromis managuensis) towards the metals aluminium, barium, copper, mercury, manganese, lead, iron and zinc. Among this set, only the metals mercury, copper, cadmium and zinc were capable of significant inhibitory effects. A similar pattern of in vitro inhibition was also found by Silva et al. (2013), on brain cholinesterases of the fish species peacock bass (Cichla ocellaris). Authors reported that, from a large set of metals, only mercury, arsenic, copper, and zinc were capable of significantly inhibiting cholinesterases. The specificity of metal cholinesterase inhibition was again demonstrated by Brandão et al. (2013). This study demonstrated that, from a list of different metals (including copper, zinc, lead, and cadmium), only copper had a significant inhibitory effect on the cholinesterase of the fish Gambusia holbrooki.