Abstract

The intensive production and use of plastics, poor biodegradability and inadequate recycling have caused excessive and alarming environmental pollution. This has led to the inevitable intake by humans, through different routes, of small plastic particles, the micro and nano-plastics (MNPs) with sizes ranging from nanometers (<1000nm) to micrometers (from 5mm to 1µm). MNPs can cause harmful effects in human tissues and organs, contributing to the early onset of aging and various age-related diseases. A growing body of evidence supports this toxic role of MNPs. In this regard, it has been shown that their different chemical and physical properties, including different chemical composition with different additives, different size, shape, solubility and ability to interact with metals and microbial agents, as well as the duration of multiple exposures, modulate their toxic action. In the brain, as documented mainly by studies conducted on brain tissues of deceased individuals, nanosized nanoparticles (NPs) of mostly 50nm or smaller, made of polyethylene, bioaccumulate, causing damage. The mechanisms involved do not seem to be fully understood. However, studies on animal models and human cell cultures using plastic particles made of synthetic polystyrene, of slightly larger dimensions, partially clarify this aspect. They demonstrated that these particles have the unique ability to cross the blood-brain barrier and evoke neurotoxicity, through the activation of pathways that determine oxidative stress, inflammation, apoptosis, altered synthesis of neurotransmitters, endocrine molecules and key enzymes related to nerve conduction, and able to influence the gut-brain axis. Despite the paucity of studies conducted directly in humans, this review collects a growing body of evidence demonstrating that exposure to MNPs, and essentially NPs, can damage neurons. This could lead to alterations in learning, memory and behaviour, and could evoke additional potential negative impacts, contributing to amplifying neuroinflammation and the onset of neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Preventive approaches and measures to limit their use and human exposure, as well as potential therapeutic strategies, are also suggested. SUMMARY: of available knowledge on the possible role of micro- and nano-plastics (MNPs) in the central nervous system. Environmental and modeling studies report that air (both indoor and outdoor), water (bottled and tap), food products and cosmetics are possible sources of exposure to MNPs in humans. Such MNPs can be absorbed through inhalation, ingestion or even skin contact, as suggested by animal models. Once in the bloodstream, such MNPs could reach the blood-brain barrier and cross it, as also demonstrated by studies using ex vivo human samples. In this case, they could promote the activation of a series of deleterious pathways that elicit low-grade inflammation, oxidative stress, endothelial dysfunction, altered neutrophil synthesis and activity, as proposed by in vitro and animal experiments. This might contribute to the onset of neurodegenerative diseases.