1. Introduction
Plastic pollution has been growing problem in recent years, gaining great public interest [
1]. Both terrestrial and aquatic environments are affected by the increase in polymer pollution. Larger plastics gradually degrade into mesoplastics (5-20 mm diameter) and microplastics (<5 mm), which are difficult to degrade naturally [
2]. This is an alarming problem for fresh water and sea water as well. Polyethylene and polypropylene are some of the most common plastics found in rivers and oceans. The two types of polymers have applications in both the construction and packaging industries [
3].
An additional sensitive issue is pharmaceutical pollution, which may adversely affect the environment and people [
4]. Even in small amounts, these pharmaceutical wastes can have a detrimental effect on the environment [
5]. Therefore, recent research has focused on the elimination of these contaminants [
6]. Nowadays, pharmaceutical residues have started to be monitored due to their presence in wastewater treatment plant effluents and surface waters [
7]. A current concern is that some of the active pharmaceutical ingredients partially persist in wastewater treatment processes [
8]. The presence of these pharmaceutical substances is due to consumers through the use and disposal of medicines [
9]. The use of wastewater for irrigation results in increased pharmaceutical pollution [
10].
Valproic acid is the tolerated [
11], prescribed medicine for the treatment of epilepsy [
12,
13], as a mood stabilizer [
14,
15]. It is also known as a pediatric hepatotoxic agent [
16]. VPA is also known to be prescribed to treat affective disorders, spinal muscular atrophy, and headache [
17]. Exposure to various environmental chemicals, both in air and in water, has been shown to be a trigger or even a cause of certain neurological disorders, for example, autism spectrum disorder [
18]. Valproic acid is one of the pharmaceutical substances presenting an environmental risk in Iraq [
19], also reported as high environmental risk in Switzerland since 2010 [
20]. Some studies showed that 130 ng L
-1 [
21], respectively 140 ng L
-1 of valproic acid [
22] were found in samples from the wastewater influents, taken from the Back River, USA, and from the waste water treatment plant in Baltimore, MD.
The animal model, zebrafish has been gained ground in several fields of research and as a model for complex brain disorders [
23]. It has also been used to evaluate the pathological mechanisms of affective disorders [
24]. This can be achieved through the zebrafish’s response to different stimuli, which allows researchers to study biological and pharmaceutical processes [
25].
The main purpose of this study was to evaluate the individual effects of polyethylene, polypropylene, and valproic acid, at environmentally relevant concentrations [
21], and their combined effects on the adult stage of
Danio rerio, at behavioral and biochemical levels. We hypothesize that all three contaminants, either alone or in combination, could have a toxicological impact on the zebrafish.
4. Discussion
Danio rerio has been established as a suitable animal model for pharmaceutical studies [
29], especially due to the possibility of immersion in water of the investigated substances and the high potential of zebrafish to absorb these compounds [
30]. Zebrafish has proven to be an excellent tool due to its high genetic similarity to humans [
31], thus motivating the study of some human diseases using the
Danio rerio animal model. In this sense, the zebrafish is an appropriate model for biomedical research [
32]. Due to the behavioral characteristics, and homology observed with humans with autism spectrum disorder (ASD), the zebrafish is also a relevant model for the study this disease [
29]. Studies have shown that VPA can induce ASD- like symptom behavior in zebrafish juvenile at a concentration of 48 µM VPA [
33]. This applies to both larval and adult zebrafish [
34]. Other studies have shown that valproic acid can negatively affect embryonic vasculature from exposure concentrations starting at 2.5 μM [
35] but can also affect larval social behavior [
36]. Exposure between 0.33 and 4.5 days post-fertilization (dpf) to 10 μM VPA was identified as an effective concentration to induce an early and persistent ASD-like phenotype in zebrafish, as embryonic exposure to VPA also reduces survival, induces malformations and delays hatching in a dose- and time-dependent manner, also triggering hyperactivity, anxiety-like behavior and social deficits [
34].
Regarding plastic pollution, limiting the flow of plastic from rivers to marine ecosystems is an important part of reducing the amount of plastic in the environment [
37]. The spread of polymers in both rivers and oceans can be stopped by effective management that removes plastics directly from their source. Plastic pollution has become a major environmental concern due to its extensive use and fast spread [
38]). Microplastics in freshwater ecosystems and marine environments can cause both physical and histological damage [
39]. Even more, PE and PP, are the most common plastics found in the marine environment [
40].
In terms of behavior analysis, in the case of the Novel Tank test, considering the distance moved and velocity, in the case of the PE-VPA treatments, PE has an anxiolytic effect immediately after the first dose, and it increases with the dose. The treatment with VPA increased an anxiogenic effect from the first dose and by day five, especially by the increased inactivity time. The latency to reach the upper part of the aquarium is lower after the first dose of PE treatment and then increases with the number of doses, with fish preferring the upper part of the aquarium, especially after VPA administration. In the case of PP-VPA treatments, PP induces opposite behavior to PE. Thus, from the first dose of PP, the fish prefer the upper part of the aquarium. This trend is maintained until the 5th dose of PP. This also holds true when treated with VPA.
In the case of PE we noticed that starting from the first dose of VPA, the latency decreased and the fish explored the entire aquarium again. However, at the first dose of PE and at the 5th dose of VPA thigmotaxis was most pronounced. In the case of PP, the thigmotaxis is more evident only at the 5th dose and is maintained during VPA treatment. VPA treatment significantly increases the total exploratory capacity expressed by the distance moved of the fish.
Anxiety-like behavior was triggered in different studies, for example in rats exposed to 600 mg kg
-1 VPA [
41] or 70 dpf zebrafish exposed to 48 µM VPA [
32]. The most obvious results were observed when analyzing the sociability test. The first dose of PE increased the presence of fish close to conspecifics. This presence began to decrease over the course of treatment. In the case of the first VPA treatment, the presence of fish near conspecifics is much higher and maintains an upward trend until the 5
th day of treatment. During both treatments, distance moved and velocity were steadily decreasing. Regarding social preference, as in the case of PE, the plastic treatment decreases sociability until the 5
th day. However, VPA has a great influence, so that from the first day of treatment with VPA, the presence of fish is majority in the arm where the conspecifics are located. Sociability is more affected at the fifth dose of PP. In this case, the affected mechanisms are more related to swimming performance.
In our study, VPA does not induce social impairments or hyperactivity deficits [
34] or other ASD-like events. VPA increases presence in the left arm but also increases inactivity time, indicating a possible anxiety mechanism. When testing aggressiveness, velocity and distance moved show similar trends. In the case of the first dose of PE they continue to decrease until the fifth dose. The first dose of VPA produces a slight increase in these two parameters, but by the 5
th dose both decrease to almost half of the control value. On the other hand, the first dose of PE induces more aggressive behavior compared to the control with an increasing trend up to the 5
th dose. In the case of PP, total distance moved and velocity decreased first and started to increase during VPA treatments. The presence of fish near the mirror is higher from the first day of treatment with PP and is maintained until the 5
th day of treatment with VPA. The first dose of VPA increased this presence. However, it decreased in direct proportion to the increase in dose. For the PE group, the first dose of VPA significantly decreases the presence of fish near the mirror, but by day 5, this presence increases again. Also, counterclockwise rotations, which are a marker of aggressiveness [
27], were observed with a high frequency compared to the control group in the first 24 hours after PP treatment and at 24 hours after VPA treatment, reaching limits significantly lower until the 5
th day of treatment. Significant results were obtained in the PE-VPA treatments. In the PE group, highest increase was observed on day 5, while the other groups maintained the same frequency as the control group. This reinforces the idea that PE has an anxiogenic effect on fish behavior. The frequency of swim bursts decreases with the PP treatments and increases with the VPA treatment. In this case, PP produces a non-aggressive behavior, but the situation changes radically from the first dose of VPA. Moreover, the frequency of swim bursts was lower at the 5
th dose of PE compared to the first, but treatment with VPA decreases this frequency by the 5
th dose.
It is worth noting that in the case of VPA treatments, behavioral differences appear with increasing doses [
32,
34,
42]. In the case of our study, the administration of VPA after intoxication with polymeric materials indicates the same thing, especially since depending on the polymeric material in some cases the first dose is the one that alleviates the effects and then they worsen, or only the fifth dose is the one that helps. In this regard, if VPA can be used for the treatment of polymer intoxication, it is important to know what the purpose of VPA administration would be. However, in the case of exposure to these pollutants under the conditions simulated in this study, the effects are visible and have a negative impact.
Superoxide dismutase (SOD) is an antioxidant enzyme that plays a critical role in the neutralizing superoxide radicals, being the first antioxidant enzyme within cellular the enzymatic defense against oxidative stress. It is currently thought that increased levels of SOD are the results of an adaptive mechanism that counteracts the pro-oxidative status and thus protects cells from potential damage. This response is often seen in a variety of physiological or pathological conditions where increased production of reactive oxygen species occurs [
43]. In our study, we observed increased levels of SOD following PE and PP+VPA exposure. Other studies have shown significantly increased levels of SOD and decreased levels of GPx in the liver of adult zebrafish exposed to PP for 21 consecutive days [
43]. However, no significant changes were observed in the brain of exposed fish [
44]. Lower levels of GPx may indicate a reduced ability to neutralize free radicals, which may make cells more susceptible to oxidative damage. This may be associated with increased oxidative stress, which has been implicated in several health problems, including chronic inflammation, neurodegenerative disease, and cardiovascular disease [
45]. In our study, lower levels of GPx were found in the PE group. Similarly, a 96-hour exposure to PE beads of different sizes reduced GPx activity in the brain and liver of adult zebrafish [
46]. Elevated levels of malondialdehyde (MDA) are often considered a marker of lipid peroxidation. MDA indicates oxidative damage to cell membranes. Elevated MDA levels may be associated with increased oxidative stress. MDA has been implicated in several health conditions, including inflammation, cardiovascular disease, and neurodegenerative disorders [
47]. In our study, all groups except VPA showed a high level of MDA compared to the control group. PP significantly elevated malondialdehyde (MDA) levels in the stomach in a 28-day study on male zebrafish. Moreover, combination of triclosan and PP significantly aggravated oxidative stress and lipid peroxidation in the liver as well as has enhanced neurotoxicity in the brain [
48]. In another 21-day study, PP-MPs resulted in increased levels of MDA in gills and liver cells compared to controls, indicating a dose-dependent effect [
49]. In our study, only VPA was shown to lower MDA levels, similar to another study that also found lower MDA levels when adult zebrafish were exposed to 0.5 mg mL
-1 VPA. Furthermore, compared to either substance alone, the combination of rotenone and VPA showed an increased level of MDA [
27]. The same was found when we combined PP and VPA.