A systematic review of the potential neurotoxicity of micro-and nanoplastics: the known and unknown

The nematode Caenorhabditis elegans (C. elegans) is a well-established model for studying the adverse effects of environmental toxicants on neuronal functions due to its well-characterized genetics, short lifespan, ease of culture, and transparent body [142, 143]. Exposure to PS MNPs ranging in size from 25 nm to 1 μm, at concentrations from 0.1 to 100 µg/L, caused a range of neurotoxic outcomes, including reduced growth, impaired movement, dopaminergic neuronal damage, and altered neurotransmitter levels [144 –148]. Functionalization of particles with carboxyl (-COOH) or amino (-NH₂) groups impacted toxicity profiles, with long-term exposure to NH₂-functionalized PS NPs showing the most pronounced effects on development, lifespan, and neuronal integrity [149 –151].

MNPs were primarily administered through ingestion in C. elegans, capitalizing on their natural feeding behavior. Particles were detected in the intestinal tract and associated with systemic oxidative stress and neuronal dysfunction, highlighting their potential for internal distribution.

Co-exposure studies demonstrated synergistic neurotoxicity. For example, 100 nm PE NPs combined with 6-PPD quinone (6-PPDQ), a potential neurotoxic environmental contaminant present in urban runoff and tire wear particles, led to increased neurodegeneration via ion channel dysregulation [152]. Combined exposure to 50 nm PS NPs and the flame retardant tetrabromobisphenol A (TBBPA) further inhibited survival, body length, and locomotor ability, while promoting reactive oxygen species (ROS) production and dopaminergic neuronal loss via upregulation of neurodegenerative genes such as pink-1 and hop-1 [153, 154].

Evidence from transgenerational exposure studies shows that neurotoxicity persists beyond parental exposure. Parental treatment with 1 μm PS MNPs reduced motor activity and increased ROS in F1 progeny, suggesting indirect developmental neurotoxicity mechanisms [155].

Photoaged and thermally degraded MNPs (Box 1) generated environmentally persistent free radicals (EPFRs) and ROS, resulting in oxidative damage, reduced neurotransmitter levels, and altered expression of genes involved in neuronal function and development [146, 156 –161].

Behavioral deficits such as impaired locomotion, reduced heat thrashing, and altered chemotaxis responses have been consistently reported in response to MNP exposure [144 –148]. Notably, long-term exposures and specific particle surface properties had more pronounced behavioral impacts. Overall, these studies indicate that neurotoxicity in C. elegans is influenced not only by the size and concentration of MNPs but also by particle surface properties, the occurrence of photoaging, and the presence of surface pollutants. This highlights the complex interplay between these factors and the observed toxicological effects.

In flatworms (Dugesia japonica), a common freshwater species that has often been used as an indicator for the detection of aquatic environments due to their high sensitivity to environmental pollutants [162], exposure to 5 μm PS MNPs, particularly in combination with perfluorooctane sulfonate (PFOS), induced DNA damage, delayed neuronal development, and disrupted gene expression [163]. In earthworms (Eisenia fetida), PS and PE MNPs (200 nm–50 μm, 0.05 to 20 g/kg soil) caused neurotoxicity through inhibited acetylcholinesterase (AChE) activity, oxidative stress, and metabolic dysregulation [164 –169]. Ragworms (Hediste diversicolor) exposed to both commercial and environmentally sourced MNPs (10–100 mg/kg sediment) exhibited size-dependent accumulation, survival reduction, and impaired antioxidant responses.

Exposure routes in these species varied by habitat: dermal absorption and ingestion dominated in earthworms; sediment ingestion in ragworms. MNPs were shown to accumulate internally and interfere with critical detoxification and antioxidant systems [170 –173].

In planarians, co-exposure with PFOS enhanced MNP-induced neurotoxicity [163]. In ragworms, comparison of MNPs and their additive dibutyl phthalate (DBP) revealed that MNPs themselves, not just additives, triggered lipid peroxidation and neuroimmune dysregulation [174].

In earthworms, MNP exposure impaired antioxidant enzyme activity, such as AChE, S-transferase (GST), and energy metabolism, contributing to neurotoxicity. Similarly, H. diversicolor, MNPs altered AChE, GST, and lactate dehydrogenase (LDH) levels, indicating disrupted neuronal and metabolic homeostasis [174].

Delayed neuronal development in planarians and growth and survival impairments in ragworms suggest that MNP neurotoxicity extends beyond molecular responses to whole-organism functioning, even in non-neural tissues, due to systemic oxidative stress.

The fruit fly Drosophila melanogaster has been used as a model organism to study human diseases, including Parkinson's and Alzheimer's diseases, cardiovascular diseases, and immunologic and intestinal infections, among others [175], and it is currently employed as a model in toxicology to conduct mechanistic studies on several environmental contaminants and toxicants [176]. When exposed to 2–100 μm PET MNPs at concentrations of 10–40 g/L showed significant neurotoxicity [177]. Particle size and exposure durations were key variables influencing the accumulation and severity of effects.

PET MNPs accumulated in the midgut and hindgut following oral ingestion. The systemic impact of this accumulation was evidenced by changes in behavior and reduced physical performance in larvae and adult flies [177].

Though molecular-level details remain sparse in Drosophila studies, observed behavioral impairments point to underlying disruptions in neuromuscular coordination and neurotransmission likely mediated by oxidative damage [177].

Behavioral assays revealed dose- and time-dependent declines in climbing, jumping, and crawling activity [177]. These outcomes parallel findings in other invertebrate models, suggesting conserved neurotoxic effects of MNPs across species.

Amphibians such as Rana nigromaculata and Xenopus tropicalis are especially vulnerable to environmental contaminants due to their bare and permeable skin and aquatic developmental stages. These physiological traits facilitate the rapid absorption of pollutants, including MNPs [178]. Studies exposing tadpoles to PS MPs ranging from 0.1 to 10 μm, either alone or in combination with toxicants such as the antibiotic levofloxacin (LVFX), reported significant disruptions in cellular pathways associated with oxidative stress and neurotransmission [179]. Similarly, photoaged PE MNPs (36–38 μm) were shown to intensify neurotoxic effects when co-administered with triclosan, a widely used, chlorine-based antimicrobial compound [180].

Exposure primarily occurred through immersion in contaminated water during larval development. While studies consistently reported behavioral and biochemical changes, direct detection of MNPs in brain tissue or internal organs of amphibians has remained elusive, possibly due to technical limitations in detection or rapid excretion/metabolism of the particles.

Co-exposure to MNPs and other environmental toxicants, such as LVX or triclosan, increased oxidative stress responses, which were evident from increased levels of superoxide dismutase (SOD), catalase, and altered AChE activity [180]. Interestingly, these biochemical markers did not always correlate with observed behavioral effects such as swirling movements or abnormal surface breathing, suggesting involvement of alternative mechanisms, potentially including gut microbiota-neurotransmitter interactions [181].

Reported effects included disruptions in cell adhesion pathways, intestinal inflammation, and alterations in gut microbiota composition. These changes are believed to influence neurotoxicity through microbiome-mediated modulation of neurotransmitter metabolism [181].

Exposed tadpoles exhibited lethargy, impaired feeding, and abnormal respiratory behaviors. Notably, these behavioral phenotypes were more severe with both very small and relatively large particles, indicating a non-linear, size-dependent toxicity relationship that may reflect differences in biodistribution and cellular uptake.

Several studies investigated the neurotoxic effects of MNP exposure in various fish species, such as crucian carp (Carassius carassius), goldfish (Carassius auratus), Nile tilapia (Oreochromis niloticus), red tilapia (Oreochromis niloticus), red drum (Sciaenops ocellatus), and several wild marine species, including Dicentrarchus labrax, Platichthys flesus, and Mugil cephalus [182 –190]. Fish exposed to various polymers (PS, PE, PA) across a broad size range (nanometers to >70 μm) showed a spectrum of neurotoxic effects. Larger particles (5–70 μm) induced stronger oxidative stress, while smaller NPs were more likely to cross biological barriers and accumulate in tissues [186 –199].

In these studies, exposure primarily occurred through waterborne contamination. While the gastrointestinal tract, gills, and liver were common sites of particle accumulation, brain uptake was rarely confirmed. However, in one study involving wild Dicentrarchus labrax, MNPs ranging from 8 to 96 μm were detected in brain tissue, providing critical evidence for translocation across the BBB [184]. Complementing these findings, PS NPs fed to Australian bass (Macquaria novemaculeata) via trophic transfer accumulated in a size-dependent manner, with smaller particles (50 nm) localizing to brain and muscle and larger particles (1 μm) to gills and intestines [200].

MNPs have been shown to act as carriers and amplifiers for co-occurring pollutants. For example, co-exposure to PS MNPs and triphenyltin (TPT) resulted in synergistic neurotoxic effects, including altered locomotion and elevated oxidative stress. Notably, particle size affected toxicity, as 51 nm PS NPs enhanced TPT toxicity to a greater extent than 4.8 μm MPs, underscoring the increased risk of NPs exposure [201].

Neurotoxic responses included increased malondialdehyde (MDA), inhibited AChE activity, and observable histological changes in brain tissues [186 –199]. Larger MNPs tended to induce more oxidative damage, while smaller particles primarily disrupted neurotransmitter function and synaptic activity [202].

Exposed fish displayed a range of behavioral alterations, such as impaired olfactory response and abnormal locomotion. In red tilapia, 5 μm PS MNPs specifically inhibited brain AChE activity, while exposure to smaller NPs produced more subtle but widespread neurophysiological disruptions [202]. These findings support the role of particle size and surface characteristics in determining the severity and nature of neurobehavioral toxicity.

Over the last few decades, the zebrafish, Danio rerio, has emerged as a prominent vertebrate model in biomedical and environmental research due to its high genetic homology with humans [203], transparent embryos, ease of genetic manipulation, rapid development, and low maintenance cost. These traits, along with their high productivity and suitability for high-throughput screening, have made zebrafish widely used for disease modeling [204 –207], drug development [206, 208, 209], and especially for toxicological studies [210 –212], including investigations into the neurotoxic effects of MNPs [213, 214].

Numerous studies have examined the size- and charge-dependent toxicity of MNPs in zebrafish, testing polymers such as PS, PE, and polylactic acid (PLA) in a wide size range from 20 nm to over 50 μm [203, 215 –221]. Results consistently showed that smaller particles (20–100 nm) caused more severe neurotoxicity due to their enhanced ability to cross biological barriers and accumulate in brain tissues [222 –225]. Particle surface charge also significantly influenced toxicity; positively charged MNPs (e.g., amine-functionalized PS MNPs) induced greater apoptosis, oxidative stress, and behavioral abnormalities than neutral or negatively charged ones [226 –228].

Zebrafish have been exposed to MNPs through various routes, including waterborne exposure, dietary intake, and direct microinjection. In one comparative study, zebrafish exposed to PE MNPs (40–47 μm at concentrations of 0.1–10 mg/L) through both waterborne and foodborne routes exhibited distinct patterns of neurotoxicity. Although waterborne exposure led to higher ingestion rates, foodborne exposure caused more severe sublethal effects, such as abnormal hyperactivity and disrupted swimming behavior [229]. Neurochemical analysis revealed that the foodborne route predominantly affected the dopaminergic system, while the cholinergic pathway was more disrupted in the waterborne group, suggesting route-specific mechanisms of neurotoxicity [229].

MNP accumulation has been observed in multiple tissues, including the brain, liver, gills, gut, and eyes. Notably, smaller particles were capable of crossing the BBB and the chorion in developing embryos [222 –225].

Zebrafish co-exposed to MNPs and environmental toxicants, such as arsenic, polybrominated diphenyl ethers (PBDE-47), nonylphenol (4-NP), fluoxetine, and copper, displayed increased oxidative stress, neuroinflammation, and disruption of neurotransmitter systems [229 –243]. MNPs facilitated the bioaccumulation of these toxicants, especially in neural tissue, and altered the microbiota-gut-brain axis, leading to serotonin pathway disruption and depressive-like behaviors [244 –246].

For instance, exposure to aged 1 μm PS MNPs in combination with the insecticide thiamethoxam reduced heart rate and locomotor activity in larvae, suppressed antioxidant enzyme function, and disturbed neurotransmitter homeostasis [247]. Similarly, MNPs have been shown to carry methylmercury (MeHg), a potent neurotoxicant [248 –250], contributing to elevated oxidative stress and motor deficits in zebrafish [251, 252].

Other studies demonstrated synergistic effects of cosmetic ingredients such as avobenzone (AVO) co-occurring with MNPs in aquatic environments. This mixture impaired AChE activity, reduced antioxidant defenses, and altered retinal and neural development in zebrafish larvae [253]. Moreover, experiments combining different PS NPs (e.g., COOH-, NH₂-functionalized, and unmodified PS NPs) with acrylamide, a well-known neurotoxicant [254], revealed that 100 nm PS NPs-COOH most significantly exacerbated neurodevelopmental toxicity, increased embryo mortality, and elevated heart rate [255].

Exposure to MNPs mixed with the pesticide abamectin also resulted in hepatic and renal damage, inhibited catalase and acid phosphatase activity, but did not affect glutathione S-transferase or AChE levels [256]. A field-based study further evaluated the real-world implications of MNP-mediated pollutant transfer. MNPs composed of PE, PP, PS, and PVC were aged on the Rhine River surface for 21 days. While zebrafish exposed to water samples exhibited notable behavioral changes and altered cyclophilin 40 (CYP40) and AChE activity, the effects of clean or pollutant-loaded MNPs were considerably milder [257]. These results suggest that under natural conditions, MNPs may have limited capacity for transferring pollutants to aquatic organisms compared to naturally suspended matter.

Brain accumulation of MNPs has been confirmed through fluorescence imaging and electron microscopy [222, 223]. Smaller 20 nm PS NPs demonstrated higher translocation efficiency across the BBB and induced more severe neurobehavioral outcomes compared to 100 nm–1 μm particles [222 –225]. This supports a particle size-dependent model of neuroinvasion and toxicity.

MNP exposure in zebrafish has been linked to a range of neurotoxic responses, including neuroinflammation, apoptosis, altered expression of neurotransmitter-related genes (e.g., GABAergic, cholinergic, serotonergic pathways), and microglia/astrocyte activation in CNS tissues [216, 258, 259]. These effects were more often severe following exposure to photoaged MNPs, which are more chemically reactive due to surface oxidation [260 –264]. Charge- and size-dependent cellular effects were repeatedly observed [226 –228, 265 –267], underscoring the complex interaction between particle characteristics and toxicodynamic profiles. Furthermore, in elderly zebrafish, PS MP exposure induced dose-dependent hepatic oxidative stress, immune activation, and metabolic disruptions, especially in pathways linked to energy, immunity, and neurological function, suggesting systemic mechanisms that may exacerbate MNP-associated neurotoxicity [268].

Morphological abnormalities were reported in specific brain regions such as the optic tectum and hindbrain, including disrupted neural differentiation and impaired synaptic development [269, 270]. Organ-specific disruption patterns varied depending on MNP size and polymer type [271 –274], suggesting selective vulnerability of certain CNS regions.

Zebrafish exhibited a wide range of behavioral deficits following MNP exposure, including reduced locomotion, anxiety-like behavior, circadian rhythm disruption, and learning and memory impairments [275]. Notably, sex-specific differences were observed in co-exposures with endocrine-disrupting chemicals like 4-methylbenzylidene camphor (4-MBC) or flame retardants. Females showed autism spectrum disorder (ASD)-like behavior and impaired oocyte development, while males displayed Parkinson's-like motor symptoms and impaired spermatogenesis [276, 277].

Many MNP-induced phenotypes observed in zebrafish, such as altered neurotransmission, oxidative stress, and neurodevelopmental disruption, mirror features of human neuropsychiatric conditions, including depression, Parkinson's disease, ASD, and schizophrenia [278]. Studies using schizophrenia zebrafish models showed that MNP exposure exacerbated neural lesions and increased oxidative damage, reinforcing the model's translational value for studying environmental contributors to human brain diseases.

Studies using mouse models have primarily focused on PS NPs (25–400 nm) and MPs (up to 10 μm), administered at doses ranging from 1 mg to 50 mg via oral gavage. Behavioral outcomes included anxiety-like behavior, depression, social deficits, and, in some cases, impaired spatial learning and memory, though not all studies found consistent effects on cognition [279 –285]. Cognitive decline and memory impairments have been documented following exposure to both nano- (50 nm) and micro-sized (5–10 μm) PS particles [286, 287], with toxicity severity depending on size, dose, and surface chemistry [288].

Specific studies found that 30–50 nm PS NPs administered at 10–20 mg/kg/day impaired spatial and fear memory without altering locomotion or social behavior [289]. In contrast, other polymers have been less frequently studied. PP MNPs (7–20 μm), with or without Di-(2-Ethylhexyl)Phthalate (DEHP), administered at 5 mg/kg, induced hippocampal CA3 damage and cognitive deficits [290]. PET NPs (83 nm, 200 mg/kg) triggered neuroinflammation and oxidative stress [291], and oxidized and irregularly shaped LDPE MNPs (2.67–12.61 μm) disrupted cholinergic signaling more than unoxidized forms [292]. Moreover, PLA oligomers were shown to accumulate in the midbrain and cause mitochondrial calcium overload, leading to Parkinson's-like neurotoxicity [293].

Oral gavage remains the most common exposure method, although inhalation and intranasal routes have also been employed. Intranasal administration of amine-functionalized PS-NH₂ NPs resulted in greater brain accumulation compared to carboxylated (PS-COOH) or unmodified particles and led to NF-κB activation and impaired glymphatic dysfunction [294]. Ingested PS MPs have similarly been detected in distant tissues, including the brain, liver, and kidney, with concentration- and particle type-dependent metabolic alterations in the colon, liver, and brain, further supporting systemic distribution and potential CNS involvement [295]. Inhalation studies have demonstrated translocation to multiple brain regions, including the olfactory bulb, cerebrum, cerebellum, and pons, especially for PS-NH₂ particles, though particle detection in tissues is not consistently reported across studies [279, 280, 296 –313].

MNPs have been shown to enhance the toxicity of environmental co-contaminants. For instance, co-exposure with DEHP, a widely present endocrine disruptor in soil, air, water, and food [314], resulted in synergistic effects, including cognitive dysfunction, oxidative stress, and gut-brain axis disruption [315 –317]. These findings align with epidemiological links between DEPH and increased risk for anxiety and autism in children [314, 318].

Similar results were observed when MNPs were co-exposed with iron [317], silver nanoparticles [319], or antibiotics such as doxycycline. Combined exposure to 500 nm PS MNPs and doxycycline altered gut microbiota homeostasis, increased brain inflammation, and impaired learning and memory [320]. Importantly, fecal microbiota transplantation reversed many of these deficits, underscoring the role of the microbiota-gut-brain axis. However, further studies are needed to understand the dynamics of co-exposure uptake, bioaccumulation, and metabolism.

Several studies confirmed MNP accumulation in the brain through imaging or tissue analysis [307, 308, 311, 321], while others observed CNS dysfunction in the absence of detectable particles, suggesting indirect mechanisms, such as systemic inflammation or gut-brain communication [310]. Maternal exposure to PS MNPs (50 and 500 nm) during gestation and lactation induced sex-specific neurodevelopmental abnormalities and long-term cognitive deficits in offspring [322].

Across studies, MNP exposure was associated with neuroinflammation, microglial activation, dendritic spine loss, altered mitochondrial function, decreased AChE activity, and apoptosis [279, 280, 296 –310]. Exposure also reduced BBB integrity via suppression of tight junction proteins, such as zonula occludens-1 (ZO-1) and occludin. Short-term exposure to PS MPs further induced behavioral alterations alongside immune marker changes in brain tissue, with age-dependent differences suggesting heightened vulnerability during both early-life development and aging [323].

Neurodegeneration was frequently observed in hippocampal neurons and cerebellar Purkinje cells, along with inhibition of neurogenesis and activation of glial cells [279, 280, 309]. Particle accumulation showed regional specificity, particularly in the olfactory tubercle and cerebellum [308].

Behavioral phenotypes included anxiety-like behaviors, social impairments, and cognitive deficits. Several studies highlighted sex-specific effects, with female mice showing greater oxidative stress and behavioral changes [322, 324], potentially due to interference with estrogen signaling and brain sexual differentiation.

Neurotoxic pathways activated by MNPs, such as ferroptosis, inflammation, mitochondrial dysfunction, and BBB disruption, overlap with the pathological features of human neurological diseases, including Parkinson's disease, Alzheimer's disease, and ASD.

Studies in rats have primarily used PS MNPs (17 nm and up), though data on particle accumulation are often lacking, making mechanistic interpretation more challenging [325 –327]. Nevertheless, consistent findings suggest potential neurotoxicity, especially following developmental exposure.

In these studies, most exposures involved oral gavage or maternal administration during gestation and lactation. Brain accumulation of MNPs was rarely measured directly. However, offspring from exposed dams exhibited cognitive impairment and hippocampal injury, indicating developmental and potentially transgenerational CNS effects [327].

Although brain uptake was generally not confirmed, indirect evidence from maternal exposure models, such as hippocampal damage and cognitive impairment in offspring, points to neurodevelopmental vulnerability during gestation [327].

Mechanistic studies identified elevated ROS production, ferroptosis, and P53-mediated ferritinophagy in brain tissues [327]. Altered astrocyte proportions and reduced antioxidant enzyme activity (e.g., GST, peroxidases) were observed, with more pronounced effects in females [325], likely involving disrupted estrogen pathways [326]. In contrast, a recent study of early-life oral exposure to nylon-11 or PS NPs in rat pups found no changes in brain neurotransmitter levels or related metabolites. However, plasma metabolomic alterations in amino acid and lipid pathways suggest systemic metabolic disruption that may indirectly affect neurodevelopment [328].

Behavioral assays revealed significant sex-specific effects, including differences in grooming behavior, stress responses, and fecal boli counts. These outcomes suggest altered neuroendocrine regulation and stress reactivity [326].

Neurobiological pathways disrupted by MNPs in rats, particularly those involving oxidative stress, hippocampal plasticity, and ferroptosis, mirror mechanisms implicated in human developmental neurodegenerative diseases.

Studies in chickens employed PS MNPs ranging from 500 nm to 50 μm, typically administered via drinking water or oral gavage. MNP accumulation was confirmed in peripheral tissues (e.g., liver, intestine, muscle), but not in the brain [329 –331].

Despite the lack of detected brain accumulation, systemic MNP exposure produced significant neurotoxic effects, likely mediated by peripheral inflammation, oxidative stress, or metabolic disruptions.

Neurotoxicity markers included reduced AChE activity, oxidative stress, inflammatory cytokine release, altered lipid metabolism, and loss of BBB integrity. Histopathological signs such as cerebral hemorrhage and microthrombi formation were also reported, despite no direct particle detection in brain tissue [329 –331].

Although behavioral outcomes were not directly assessed, histological damage, such as Purkinje cell loss and vascular injury, strongly suggests CNS dysfunction.

These findings provide additional evidence that neurotoxicity may occur via indirect pathways without direct CNS accumulation of MNPs. They emphasize the potential roles of systemic inflammation, barrier disruption, and microbiota in mediating MNP-related neuropathology.

The neurotoxic potential of MNPs in murine neural models varies significantly with particle size, surface chemistry, and charge. Several studies have confirmed that smaller particles and positively charged surface modifications enhance cellular uptake and cytotoxicity. For instance, 25–50 nm PS MNPs were taken up by primary rat microglia and astrocytes, leading to concentration-dependent cytotoxicity and microglial activation, as evidenced by morphological changes and increased immunostaining of activation markers [289, 345].

Similarly, 80 nm PS-NH₂ particles were preferentially internalized in rat primary neurons compared to neutral or negatively charged variants, suggesting that surface charge is a key determinant of neuronal MNP accumulation [312]. Studies in rat and mouse neural stem cell (NSC)-derived cultures showed that astrocytes and NSCs internalized smaller MNPs more readily, with positively charged PS MNPs reducing cell viability and proliferation [346, 347]. Notably, 2 μm PS particles were too large to enter the plasma membrane of neurons but still induced significant neurotoxicity, such as dendritic shortening and apoptosis, through indirect mechanisms [324].

Most in vitro models use static exposure conditions, which may not fully capture the dynamics of particle-tissue interactions. However, experiments using HT-22 hippocampal cells in microfluidic (flow-based) devices demonstrated that dynamic exposure to 100 nm and 1 μm PS MNPs (5–75 µg/mL) led to greater cytotoxicity, increased ROS production, and cell cycle arrest compared to static cultures [348]. This suggests that more realistic exposure models are critical to accurately assess MNP neurotoxicity.

Although this role was not directly tested in murine in vitro systems, the physicochemical traits that enhance MNP uptake, i.e., small size and surface reactivity, could also facilitate the adsorption and transport of co-contaminants. This implies that MNPs may act as vectors that amplify the neurotoxic effects of environmental toxicants.

Studies using cell lines like PC12 and SH-SY5Y, as well as mouse cortical neurons, consistently report cytoskeletal alterations, dendritic atrophy, cell viability loss, and apoptotic cell death following exposure to 18 nm sulfonated PS MNPs [348]. These effects were both time- and dose-dependent. Key molecular markers included caspase-3 activation and loss of structural integrity (e.g., soma and dendritic shortening) [349 –351]. Additionally, signs of reactive astrogliosis and reduced oligodendrocyte markers suggest broader dysregulation of CNS glial cell function.

Different neural cell types exhibited varying susceptibility to MNPs. Microglia and astrocytes, due to their phagocytic and immune-responsive nature, appear to internalize MNPs more efficiently and respond with elevated pro-inflammatory signaling [346]. NSCs are particularly sensitive to disruption of proliferation and lineage specification, highlighting developmental vulnerability. Neurons, while less efficient at MNP uptake, appear to be affected indirectly through glia-mediated inflammation and oxidative stress.

Although in vitro systems cannot model complex behaviors, the cellular and molecular disruptions observed, especially oxidative stress, neuroinflammation, and apoptosis, are mechanistically linked to cognitive dysfunction and neurodegenerative conditions in vivo. Sexual dimorphism remains largely unexplored in these models; future studies using sex-specific primary cultures or hormone-supplemented media could address this critical gap.

Molecular changes such as caspase activation, mitochondrial dysfunction, and chronic inflammation mirror key features of human neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. These findings strengthen the translational relevance of murine in vitro studies for understanding human pathophysiology linked to environmental MNP exposure.

The brain endothelial phenotype of the hCMEC/D3 human brain microvascular endothelial cell line has been extensively characterized and is a widely used model of BBB function. Exposure to 100 nm PS MNPs (30 µg/mL) for 24 h did not significantly decrease cell viability; however, significant decreases in the expression of tight junction proteins ZO-1 and occludin were observed, particularly in cells exposed to PS-COOH and PS-NH₂ MNPs, suggesting subtle barrier compromise even in the absence of overt cytotoxicity [279].

Although no internalized MNPs were detected in hCMEC/D3 cells in this study, previous work using larger (0.2–2 μm) PS particles reported internalization and consequent inflammatory signaling, including ROS production and NF-κB activation [302, 352]. The discrepancy may reflect differences in particle size, surface chemistry, detection methods, or culture conditions.

PS MNP exposure altered the expression of junctional and matrix-modifying proteins, including upregulation of matrix metalloproteinase-9 (MMP-9), suggesting possible extracellular matrix remodeling and compromised BBB integrity. However, the lack of trans-endothelial electrical resistance (TEER) measurements limits definitive conclusions regarding barrier permeability.

BBB disruption is a key feature in many CNS disorders, including multiple sclerosis, epilepsy, and Alzheimer's disease. The observed protein expression changes suggest that chronic MNP exposure could contribute to neuroinflammation via BBB impairment.

Human neuroblastoma cell line SHSY-5Y, originating from the SK-N-SH cell subline, is a versatile in vitro model for neurotoxicity, ischemia, and neurological disorders, including Alzheimer's disease and amyotrophic lateral sclerosis [353]. Studies showed that they respond differentially to PS MNPs based on size and surface charge. For instance, exposure to 25–50 nm PS and PS-NH₂ particles induced reduced neuronal differentiation, nuclear swelling, and morphological changes [281, 354]. MNPs localized predominantly to the cytoplasm, supporting their ability to cross cellular membranes [145].

Neurodegenerative processes were recapitulated in vitro, with increased aggregation of α-synuclein and enhanced secretion of amyloid β (Aβ), biomarkers strongly linked to Parkinson's and Alzheimer's disease [145, 149]. Additional studies using 70 and 150 nm PS MNPs confirmed that even at low doses, these particles could accelerate Aβ aggregation and elevate intracellular calcium levels and ROS generation, leading to cell damage [355].

These findings suggest that MNP may not only contribute to cellular stress but also accelerate pathogenic processes associated with neurodegeneration. The ability of nano-sized plastic particles to enhance hallmark disease proteins like Aβ and α-synuclein warrants further exploration.

Human pluripotent stem cells (hPSCs) and their derivatives, including 2D neural cultures and 3D brain organoids, offer physiologically relevant platforms to model human neurodevelopment and disease. A recent study exposed cortical and sensory neurons to 20 nm, 100 nm, and 2 μm PS MNPs and polyester microfibers at environmentally relevant doses. Oxidative stress and neurodegeneration were observed in a size- and dose-dependent manner [356].

In cerebral organoids, 50 nm particles penetrated deeper into tissue than 100 nm particles, suggesting that small size enables parenchymal infiltration and damage. After 21 days of exposure to 10 mg/mL particles, gene expression was significantly altered in pathways associated with neural viability and toxicity [306].

Importantly, biofilms formed by Pseudomonas aeruginosa and Escherichia coli on 2 μm PS MNPs exacerbated neurotoxicity [356], supporting the concept that MNPs may serve as biological vectors for pathogens or co-pollutants in the CNS [321].

Exposure to MNPs disrupted key neurodevelopmental processes, including altered proliferation, increased apoptosis, and disrupted tissue patterning in organoids and retinal models [357]. Long-term studies (4–30 days) on 3D forebrain models showed decreased expression of neural markers (Nestin, PAX6), DNA damage, and inhibition of cadherins, indicating potential impairment of neural circuit formation and neurodevelopment [358, 359].

While behavior cannot be directly assessed in vitro, altered gene expression in critical neurodevelopmental pathways can serve as a surrogate for long-term neurological dysfunction. The role of sex in these models remains unexplored, but future studies incorporating sex-specific hPSC lines or hormone treatments could shed light on this variable.

These advanced human models offer unprecedented insight into how MNPs could impair human brain development and function. Findings suggest that environmentally relevant levels of MNPs may interfere with neurogenesis and brain architecture, raising important concerns for public health, particularly for vulnerable populations such as pregnant women and young children.