Enantioselective ecotoxicity of psychoactive substances in Daphnia magna

Abstract

Psychoactive substances (PAS) are emergent contaminants frequently detected in aquatic ecosystems that may pose environmental risks in aquatic organisms even at low concentrations (ng to µg/L). Many PAS are chiral substances commercialized as racemate. Amphetamine (AMP) is a central nervous system stimulant used in the treatment of attention deficit hyperactivity disorder narcolepsy, and obesity. AMP is a chiral substance that exhibits enantioselective in its pharmacological activity being the (S)-AMP more potent and clinically effective than (R)-AMP. On the other hand, AMP is frequently used as a recreative drug. Due to its high consume and low biodegradability AMP has been detected in wastewaters and surface waters and can occur as enantiomeric mixtures or pure enantiomers. In this context, the evaluation of its enantioselectivity in eco-toxicity is crucial for a better understanding of AMP environmental risk on non-target organisms in freshwater ecosystems. Therefore, the present study aimed to investigate the enantioselectivity of AMP in toxicity on the aquatic invertebrate daphnia (Daphnia magna), used as a model organism to assess different biomarkers of toxicity. For that, neonates (less than 24h old) were exposed to 0.1; 1, and 10 µg/L of the racemate (rac-AMP) and to 0.1 and 1 µg/L of pure enantiomers, (R)-AMP and (S)-AMP, for 8 days. At selected 3, 5, and 8-days of exposure, different parameters were determined as morphophysiology (on days 3 and 8, as body size and heart rate, area, and length); swimming behaviour (on day 5, as swimming speed, active time, and total distance); reproduction (on day 8, number of eggs per daphnia, number of daphnia with eggs and number of neonates) and biochemical parameters (on day 8, like oxidative stress, catalase (CAT) and acetylcholinesterase (AChE) enzymatic activities). Data showed a significant decrease in body size found for organisms exposed to (S)-AMP comparatively to (R)-enantiomer and to the racemate. Effects on the development and functioning of heart were observed with a significant decrease in heart rate for both racemate and enantiomers though a decrease was observed in the organisms exposed to the racemate at days 3 and 8, while for the enantiomers a decrease was observed at day 3 with an enantioselective effect at 0.1 ug/L (lower decrease for (R)-AMP) but on day 8 no differences were found. A significant decrease in the heart area was observed for both enantiomers. Regarding swimming behaviour, different results were observed between racemate and enantiomers. (rac)-AMP caused a reduction of the total travelled distance while for both enantiomers an increase in total travelled distance was noted though no enantioselective effects were observed. No changes were observed in active time in the organism exposed to (rac)-AMP while a reduction of the active time was observed for both enantiomers. No changes in the number of eggs per daphnia or the number of daphnia with eggs were observed for the racemate, however, a tendency to increase of the number of neonates at 0.1 and 1 μg/L was observed while a significant increase was found at 10 μg/L. Regarding enantiomers, a significant difference was found between enantiomers with a decrease in the organisms exposed to (S)-AMP in contrast to the increase in the organisms exposed to (R)-AMP at 1 μg/L. Though no significant differences were observed for the number of daphnia with eggs, a tendency to increase was observed in the organisms exposed to (R)-AMP. These results show that AMP affects reproductive performance of daphnia, and these effects are enantiomer dependent. Changes in biochemical parameters were also observed with a significant decrease in reactive oxygen species (ROS) and CAT activity for enantiomers. These results showed that AMP can interfere with different biomarkers of toxicity and these effects can be enantioselective demonstrating the relevance and providing evidence for the need for this kind of study for an accurate environmental risk assessment. Additionally, some of the effects were observed at environmental reported concentrations (0.1 and 1 μg/L) AMP both racemate and enantiomers can cause adverse effects on D. magna reinforcing the concern of invertebrate medium- and long-term exposure to AMP.