The use of antidepressants has increased considerably since 2000, and they were inevitably released into the environments and caused serious environmental pollution (Sehonova et al., 2018). As emerging contaminants, their removal efficiency in wastewater treatment plants was low (Kuzmanovic et al., 2015; Petrie et al., 2015). CTP and MTP are typical antidepressant drugs, which are widely used for the treatment of depression (Cipriani et al., 2018). Previous studies have found that more than 10% of psychoactive drugs were excreted into the aquatic environments in their active form (Balakrishna et al., 2017), and about 12% of CTP was excreted into the aquatic environments (Bergheim et al., 2012).
Recently, the use of CTP and MTP continues to increase, both drugs can be found in most of the environment compartments, such as sediments, surface water, groundwater, etc (Silva et al., 2015; Faggio et al., 2016). Golovko et al. (2020) reported that the average concentrations of CTP and MTP in surface water of Lake Ekoln in Sweden were 0.59 and 1.1 ng/L, respectively. The detection frequency of MTP at a university hospital in Ioannina located in northwestern Greece was > 73% and the average concentration was 8.3 ng/L (Kosma et al., 2019). CTP was detected in untreated sewage in Denmark, with its concentration range of 0.19–10.3 µg/L (Kosma et al., 2019). Antidepressants have potentially highly dangerous for aquatic ecosystems (Minguez et al., 2016). The risks of disturbances in behavior, endocrine system may be expected in fish exposed to a psychotropic drugs-contaminated environment (Giang et al., 2018). However, the potential impacts of CTP/MTP pollution on aquatic organisms were limited.
The aquatic environmental risks of CTP and MTP have been concerned. For example, Bachour et al. (2020) observed a significant decrease in swimming activity of zebrafish exposed to CTP with a concentration of 373 µg/L. The inhibition rate of ACHE activity was73% of D. magna exposed to CTP with a concentration of 1 g/L (Yang et al., 2017). Assessment of individual chemical is a common tool for ecological risk assessment of pollutants. However, in the aquatic environments, many pollutants usually exist at the same time. When aquatic organisms were exposed to a mixture of pollutants at the same time, the toxicity of those pollutants to organisms may be superimposed or reduced. (Lari et al., 2017; Liu et al., 2018; Bachour et al., 2020; Hossain et al., 2021). It was revealed that the 1:1 binary mixture of CTP and tramadol caused significant decrease swimming activity of zebrafish during dark conditions in comparison with individuals CTP and tramadol (Backhaus, 2016). As a compound drug, the combined toxicity of CTP and MTP was worthy of attention.
Because of the characteristics of easy cultivation, short life cycle and high sensitivity to pollutants, D. magna is a good model organism for the aquatic environment pollution evaluation (Chai et al., 2021; Tkaczyk et al., 2021). D. magna have been used to assess the acute toxic effects of MTP or CTP (Yang et al., 2017), but studies about the damage of CTP and MTP on daphnia’s behavior were limited. Toxicology studies have suggested that the feeding behavior and heartbeat of D. magna were used to assess the sub-lethal effects of pollutants (Bownik, 2017, 2020). The concentration of pollutants in the environment is constantly changing. But in what concerns CTP/MTP, few studies analyzed post-exposure effects in D. magna. In previous studies, Yan et al. (2018) revealed that after 7days of exposed-recovery to sulfamethazine (SMZ), the activities of SOD and MDA of zebrafish were reversed. However, after 1-week of exposed-recovery to rifampicin, bacterial communities of Gambusia affinis were not able to recover in terms of diversity or composition (Carlson et al., 2017). It is worth studying whether the aquatic organisms could fully recover when the pollutants were removed. Thus, the studies of the abnormal behavior of D. magna caused by CTP and MTP were performed to understand the aquatic ecological risks of these two substances.
Thus, to improve the understanding of the potential toxicity of CTP and MTP, the individual and combined toxicity of them on the feeding behavior, heart rate, nutritional enzymes, and related gene expressions of D. magna was thoroughly studied in this study during exposure and recovery periods. There were three aims in this study: 1) the effects of CTP and MTP on the feeding behavior and heartbeat of D. magna were studied under single and mixed environmental stress; 2) the recovery of D. magna after exposure was study to evaluate the toxicity persistence of CTP and MTP.; 3) the potential toxic mechanism of CTP and MTP was studied by monitoring the digestive enzymes and related genes of D. magna. The findings were helpful to evaluate the potential risk of CTP and MTP in aquatic ecosystems.