## 3 Neurotoxicity of Micro and Nano-Plastics in Fish
The central nervous system is particularly vulnerable to oxidative stress, in fact it determines developmental and motility alterations and neurotoxicity [58; 59]. The results of [PERSON] et al. [60; 61], who evaluated specific biomarkers of oxidative stress, the activity of catalase (CAT), glutathione, peroxidase activity (GPx), and the reduced form of glutathione (GSH), confirm the role of oxidative stress in neurotoxicity [60; 61]. Behavioral anomalies in fish exposed to pollution by micro and nano plastics have been observed [62]. It has been observed that zebrafish exposed to plastic particles show uncontrolled movements probably due to an improper activation of neurons, due to an altered expression of important neurotransmitters such as acetylcholine, glutamate, and \(\gamma\)-aminobutyric acid [63; 64; 65; 66; 67]. Such alterations can be considered indicators of neurotoxicity [68; 69]. In fish, exposure to microplastics triggers the activation of cellular oxidative stress processes with consequent peroxidation of cell membranes [46; 70]. The damage from lipid peroxidation in the brain can cause the break of the vesicles membranes containing neurotransmitters with consequent increase of the neurotransmitter concentration in the synaptic junctions [71; 72]. The neurotoxic effects of microplastics were confirmed by measuring the increase in activity of the enzyme acetylcholinesteraseanase [46; 73]. These results are severe because the activity of cholinesterase (ChE) enzymes is essential for cholinergic neurotransmission in neuromuscular junctions and in cholinergic brain synapses [51; 74]. Acetylcholinesteraseesrace (AChE) activity regulates brain function and is considered an important biomarker for neurotoxicity, also in zebrafish [75; 76]. A decrease in AchE activity [61; 77], as well as the inhibition of neurotransmitters dopamine, melatonin, aminobutyric acid, serotonin, vasopressin, kisspeptin, and oxytocin [77], in zebrafish exposed to NPs was found [78]. Inhibition of acetylcholinesterase cholinesterase (AChE) enzymatic activities also in Medaka, O. _latipes, Pomatoschistus microps_ exposed to micro and nano plastics has been reported [10; 54; 79]. It has been also observed that, in zebrafish larvae, the inhibition of AChE activity interferes with the functioning of the nervous system, causing growth retardation, paralysis, and death [75; 76]. Since EE2 (17\(\alpha\)-ethinylestradiol) modulates the development of the neuroendocrine system, it was considered a positive control in the evaluation of neurotoxicity [60; 80; 81]. In groups of larvae exposed to MPP or coexposed with MPP and EE2, the activity of AChE decreased [60]. Moreover, polyethylene microplastics (1-5 \(\upmu\)m) inhibited AChE activity in _P. microps_[60; 73]. The small particles can inhibit AChE activity causing neurotoxicity with adverse effects in the cholinergic system. Therefore, the suppression of locomotor capacity in zebrafish exposed to nanoparticles and nanoplastics can be explained by the inhibition of acetylcholinesteraseasive activity [60]. Downregulation of genes associated with neuronal functioning (glutamate receptors, potassium channels, synapsin), genes implicated in neuron differentiation and axon genesis (_neuro4_, neuronal differentiation 4, Gene ID: 266958; _Irrn2_, leucine rich repeat neuronal 2, Gene ID: 558051) support the neurotoxic effects of plastics in zebrafish [82] and plastics seem to influence the visual system by upregulating the gene expression of _zfrho_ (rhdopsin, Gene ID: 30295) [60,78]. In zebrafish, _gfap_ (glial fibrillary acidic protein, Gene ID: 30646) and _a1-tubulin_ (alpha-1 tubulin, Gene ID: 842782) genes are expressed in the central nervous system during the early stages of development and are considered important biomarkers of neurotoxicity [83]. _a1-tubulin_ is essential in the formation of microtubules, while _gfap_ intervenes in the development of astrocytes [84,85].