## 3 Neurotoxicity of Micro and Nano-Plastics in Fish  
Nanoplastics are small hydrophobic particles that can reach the brain by crossing the blood-brain barrier [35; 43; 51]. Nanoplastic particles have been identified in the brain of fish after waterborne or food-mediated exposure, indicating that they are capable of crossing the blood-brain barrier [44]. Then, in aquatic models, the brain is essential for evaluating the toxic effect of nanoparticles (NPs) [52; 53]. In fish at different stages of development, NPs accumulate in the tissues, resulting in multiple negative effects including the nervous system [54]. [PERSON] et al. [45] suggest that the effects of NPs and MPs on the nervous system depend not only on the size and time of exposure but also on indirect mechanisms such as oxidative stress. In this regard, [PERSON] et al. [51] observed higher levels of LPO and greater AChE activity in the brains of _Dicentrachus labrax_, _Trachurus_, and _Scomber colias_ exposed to plastics than in unexposed specimens of the same species, probably because \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimens** & \begin{tabular}{c} **Micro and** \\ **Nano-Plastics Type** \\ \end{tabular} & \begin{tabular}{c} **Micro and** \\ **Nanoplastics Size** \\ \end{tabular} & **References** \\ \hline \begin{tabular}{c} Data not referred to a \\ particular species \\ \end{tabular} & \begin{tabular}{c} Data not referred to a \\ particular micro and \\ nano-plastics type \\ \end{tabular} & \textless{}10 \(\upmu\)m & [46; 47] \\ \hline \begin{tabular}{c} Red tilapia \\ (_Oreochromis niloticus_) \\ \end{tabular} & Polystyrene & 0.1 \(\upmu\)m & [40] \\ \hline \begin{tabular}{c} Red tilapia \\ (_Oreochromis niloticus_) \\ \end{tabular} & Polystyrene & 0.3; 5 \(\upmu\)m & [45] \\ \hline \
\begin{tabular}{c} See-through medaka \\ (_Oryzias latips_) \\ \end{tabular} & Polystyrene & 39.4 nm & [43] \\ \hline
\begin{tabular}{c} Crucian carp \\ (_Carassius carcassius_) \\ \end{tabular} & Polystyrene & 53 and 180 nm & [35] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Micro and nanoplastics crossing the blood brain barrier in fish. higher concentrations of LPO induce the break of acetylcholine-containing vesicles resulting in increased neurotransmitter released in synaptic clefts and increased production of acetylcholine as a compensation mechanism [51]. [PERSON] et al. [55] showed that neurotoxicity following polystyrene microplastics (PS-MPs) exposure in zebrafish larvae could be associated with some metabolic changes. Instead, [PERSON] et al. [45] show, in adult specimens of tilapia, a downregulation of the amino acids phenylalanine and tyrosine, associated with neurological functions and consequently the polystyrene nanoplastics (PS-NPs) PS-MPs exposure. According to [PERSON] et al. [45], amino acids associated with neurological functions could interact with nanoplastics and microplastics resulting in alteration of the metabolic pathways of these amino acids and alterations in the formation of various neurotransmitters. Furthermore, it has been observed in tilapia that PS between one and 100 micrometers in size can induce more severe stress than 0.3 micrometers particles and this stress seems to be mainly due to oxidative stress and damage caused by LPO, more severe in MPs than NPs exposure. This could be explained by the fact that medium-sized MPs can move freely between cells and induce mechanical injury and alterations in biochemical pathways such as oxidative damage and inflammatory response [45]. Furthermore, NPPs can favor the accumulation of reactive oxygen species (ROS) and cause negative effects at the cellular level [43; 56; 57]. In adult specimens of medaka fish (_Oryza lastipes_) polystyrene nanoplastics in both organs and blood have been found. Furthermore, after exposure for seven days at 10 mg/L, polystyrene nanoplastics crossed the blood brain barrier in this species and get to the brain [43].