The use of antidepressants has been increasing considerably since 2000, and they were inevitably released into the environments and caused serious environmental pollution (Sehonova et al., 2018). As emerging contaminants, CTP and MTP are not target pollutants in the waste water treatment plant (WWTP). The average CTP removal efficiency in WWTP was 25% (Gros et al., 2020), therefore the effluent concentration of CTP was still considered high (Kuzmanovic et al., 2015; Petrie et al., 2015). CTP and MTP are typical antidepressant drugs, widely used for the depression treatment (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 and 6–20% of MTP was excreted into the aquatic environments (Bergheim et al., 2012; Gundlach et al., 2021).
Recently, the use of CTP and MTP continues to increase, both drugs can be found in most environment compartments, such as sediments, surface water, groundwater, etc. (Silva et al., 2015; Proctor et al., 2021). 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 (Styrishave et al., 2011). Antidepressants maybe highly dangerous for aquatic ecosystems (Minguez et al., 2016). The risks of disturbances in behavior and endocrine system may be expected in fish exposed to 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 received more attentions. 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 Acetyl cholinesterase (ACHE) activity was 73% 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, many pollutants usually exist simultaneously in the aquatic environments. When aquatic organisms were exposed to a mixture of pollutants simultaneously, 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 may cause significant decrease in swimming activity of zebrafish during dark conditions when compared with individuals CTP and tramadol (Bachour et al., 2020). In the depression treatment, CTP and MTP were often used in combination (Zhuang and Hospital, 2019), and appeared together in the environment. Therefore, the combined toxicity of CTP and MTP was worthy of more attentions.
Because of the characteristics of easy cultivation, short life cycle and high sensitivity to pollutants, D. magna is a good model organism for the evaluation of aquatic environment pollution (He et al., 2019; Tkaczyk et al., 2021). D. magna has been used to assess the acute toxic effects of MTP or CTP (Yang et al., 2017), while many adverse effects are not well characterized. Toxicology studies have suggested that the feeding behavior and heartbeat of D. magna were used to assess the sub-lethal effects of pollutants. Assessment of effects on feeding activity is a valuable tool for determination of early effects induced by bioactive substances. The daphnid heart responded to sublethal levels of various environmental stressors. Heart may be considered as a promising sensor of effects induced by stressful factors in the aquatic environment and as a model for testing cardioactive drugs (Bownik, 2017, 2020). The pollutant concentration of in the environment is constantly changing. However, few studies have analyzed the recovery effects after exposed to CTP/MTP of D. magna. In previous studies, Yan et al. (2018) revealed that after 7-day exposed-recovery to sulfamethazine (SMZ), the activities of SOD and MDA of zebrafish were reversed. However, after 1-week 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 deepen the understanding of potential toxicity of CTP and MTP, the individual and combined toxicity on the feeding behavior, heart rate, nutritional enzymes, and related gene transcription of D. magna were thoroughly studied in this study during exposure and recovery periods. There were three research objectives 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 feasible to evaluate the potential risks of CTP and MTP in aquatic ecosystems.