CHRONIC EXPOSURE TO METHYLPHENIDATE-CONTAMINATED WATER ELICITS SOCIAL IMPAIRMENT TO ZEBRAFISH
Graphical abstract
Introduction
Methylphenidate (MPH) is a widely prescribed psychostimulant for the treatment of Attention Deficit Hyperactivity Disorder (CHENG et al., 2014; KIMKO et al., 1999)
The presence of main metabolic of MPH, ritalinic acid, has already been described in residual waters (BURGARD et al., 2013; LETZEL et al., 2010). MPH is rapidly absorbed and about 80% of the dose is metabolized to ritalinic acid, which shows non or very low pharmacological activity (SHIMIZU et al., 2019). Traces of unchanged MPH (<1% of the original dose) have already been found in urine and feces (MORTON and STOCKTON, 2000).
Behavior is the connection between the internal physiological processes of an animal and its interaction with the environment (CLIFT et al., 2014; ORGER and POLAVIEJA, 2017). Based on this definition, a complete and intact behavioral repertoire is crucial for fish survival, at both the individual and population level (STEWART et al., 2013). One of the most important behaviors is the social organization. Zebrafish is a highly social species exhibiting a high preference for being close to conspecifics (SAVERINO and GERLAI, 2008).
Reports of pharmaceutical residues in the environment and their effects on the behavior of exposed species are common. We recently showed that acute exposure to low levels of MPH increases cortisol levels and promotes an anxiolytic-like behavior in zebrafish. When animals were submitted to previous stress, acute exposure to MPH residues attenuates the stress response (ENDRES et al., 2017). Considering a long-term exposure is closer to the reality in the aquatic environment and the relevance of the social and exploratory behaviors to zebrafish, here we evaluated the effects of chronic exposure to MPH-contaminated water.
Section snippets
Study strategy
Aiming to evaluate the effects of chronic exposure, we distributed zebrafish into four groups of twelve animals (both sexes): three groups, which we treated with increasing doses of MPH, and a control group, with no treatment added. MPH was added on day 1 and the exposure lasted 15 days, without any additional spike. We performed behavioral tests: the novel tank test (NTT) and social preference test (SPT). After euthanasia, since MPH oxidative stress induction was already described in rats (
MPH induces antisocial behavior
In the SPT, zebrafish exposed to MPH (3 μg/L) spent less time in the conspecific segment (Fig. 2A) and more time in the empty segment (Fig. 2C) when compared to control group. In addition, despite no changes were seen in the latency to enter conspecific segment (Fig. 2B), fish exposed to 3 μg/L of MPH presented a smaller latency to enter the empty segment (Fig. 2D).
Changes in the anxiety pattern induced by MPH
Zebrafish exposed to the lowest dose of MPH spent more time at the top (Fig. 3A), while no changes in locomotor patterns were
Discussion
Here we show that chronic exposure of zebrafish to MPH impairs their social behavior. In fact, fish exposed to 3 μg/L spent less time in the segment nearest to conspecifics and spent more time and low latency to enter in the empty segment (Fig. 2 A, C and D). This behavior can be interpreted as an antisocial behavior, since the zebrafish is a highly social fish species with a strong natural tendency to live in shoals (FILBY et al., 2010; SCHROEDER et al., 2014; CHERESIZ et al., 2020), a clear
Credit author statement
Izadora Borgmann Frizzo: conception and design, acquisition of data, analysis and interpretation of data.
Gessi Koakoski: acquisition of data, analysis and interpretation of data.
Natália Freddo: acquisition of data, analysis and interpretation of data.
Victoria Costa Maffi: acquisition of data, analysis and interpretation of data.
Charise Dallazem Bertol: acquisition of data, analysis and interpretation of data, revising the article critically for important intellectual content.
Leonardo José Gil
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work received financial support from FAPERGS (Fundação de Amparo a Pesquisa do Rio Grande do Sul), Project ARD 17/2551-000804-9.
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