Figure 1: Plastics degrade into micro- and nano-particles, acting as vectors for delivery of toxicmaterials[10] with profound negative conse-quences.There has historically been a lack of knowledge on ma-jor additives of concern in the plastic industry, and whatoccurs once they are disposed of into the environment[1][1] which canhave negative effects on human health[1, 7, 8] as well assurrounding marine and animal life[2, 6].
Inhaled nano-particles can affect neurons and depositin the brain, eliciting changed behaviour in animals[6].Animals with MNPs in their environment such as fishand rodents are also affected, with changes in biochem-ical expression[2], gut microbiota cross-talk[4], and neu-ronal damage[9].Animal models used to study human diseases haveshown concering effects such as brain abnormalities forprogeny with maternal exposure[3], negative learningand memory effects due to oxidative stress[11], inductionof Parkinson’s disease-like neurodegeneration[5], and in-duction of microglia causing neurotoxicity[9].
Mode of Action and Pharmacological Effects
Ingestion is the primary route of human exposure tomicroplastics, with about 50,000 particles ingested perperson per year through contaminated food or mucousmembrane inhalation [7]. It is estimated that of all par-ticles under 150μm which can cross the gastrointenstinalepithelium, that 0.3% are absorbed[1]. These particles,which penetrate the body through intestinal M-cells orare absorbed due to their small size, can lead to an in-flammatory response and disruption of gut membranesand microbiota[7].
World wide web template and diagram | Oxidative stress Cell damage/death Systemic uptake via gills, gut, lung (?) nose (?), and subsequent passage to the brain Micro- and Nanoplastics Neuro- inflammation Increased vulnerability neuronal disorders? AChE inhibition Altered neuro- transmitter levels Behavioral changes
Figure 2: Overview of neurotoxic effects of micro- andnano-plastics[8]. Areas with dashed lines orquestion marks warrant further conclusive re-search.
Inhaled microplastics range from 100-200 per personper day, with deposition inside the respiratory systeminfluenced by particle size and density[7][7][7]. Particles smaller than 1μm can cross into our bodies through lung mem-branes, and fibers of 15-20μm cannot be removed frommacrophages in the lungs[1][1][1][1].
Inhaled nanoplastics from polystyrene (PS) of 80 nmand smaller were able to be deposited in the brainsof mice, and resulted in less activity compared to in-haled water droplets – likely due to inhibition of Acetyl-cholinesterase (AChE) activities[6, 11]. Interestingly, af-ter treatment with vitamin E as an antioxidant agent,the learning and memory abilities of the mice was re-stored and release of neurotransmitters rebounded[11].
Additionally, in-vitro studies on zebrafish with realis-tic parameters of freshwater pollution show an increasein AChE activity[2][2]. Despite the opposing effects on AChEbetween mice and zebrafish, both still exhibit negativeneuro- and cyto-toxic symptoms as a result of MNPsexposure. This highlights also the need for more conclu-sive studies in models which are of clinical relevance tohumans.Additionally, PS particles of 50 nm and smaller arepassed onto progeny during postnatal stages in micethrough breast milk, and can accumulate in a dose-dependent manner in the hippocampus and cerebellum- increasing the risk for cognitive defects, but not of lo-comotive and emotional defects[3].
50 nm PS nano-particles were also examined in micein regards to Parkinson’s disease. It was found throughRNA sequencing that there were cell responses specific tothese particles which induced mitochondrial dysfunctionin excitatory neurons, and inflammatory turbulence inastrocytes and microglia[9], dysfunction of proteostasisand synaptic-function regulation in astrocytes, oligoden-drocytes, and endotheliocytes[5][5].
PS-MNPs have also been found to penetrate bone mar-row via the circulatory system, causing hematologicaldefects, the effects of which depend on particle size, com-position, and individual response[4][4].
