Nanoparticles and neuropathy in man and animals - a review

Nanoparticles (NPs) are particles between 1 and 100 nanometres (nm) in size with a surrounding interfacial layer. Numerous medical products contain nanomaterials including sunscreens in which nanomaterials are used to provide UV protection while remaining transparent on the skin, and drugs in which nanomaterials are used as the means of drug delivery to the target organ. Because of their unique size and high surface area, after surface modifications, many NPs are capable of bypassing or crossing the blood-brain barrier. However, NPs can also be potentially hazardous to the central nervous system in terms of neurotoxicity via several possible mechanisms, including oxidative stress, autophagy, lysosome dysfunction and the activation of certain signalling pathways etc. Key factors influencing the neurotoxicity of NPs are size, shape, charge, surface modification and aggregation and dispersion. An increasing number of people are reported to suffer from neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease and primary brain tumours and it is suspected that these NPs play a role in aggravating these diseases. Some experiments performed in rats, mice, terrestrial and aquatic animals have shown that NPs enter the lungs, then reach to blood, and are able to cross the blood-brain barrier. They also can reach other organs such as bone marrow, lymph nodes, spleen or heart. Some NPs like SiO₂ and TiO₂ exert different cytotoxic effects on neural cell types of the peripheral nervous system. These NPs lower the survival of Schwann cells and dorsal root ganglion neurons in a dose and time related manner. The present review attempts to provide updated details regarding neuropathy in animals and human caused due to nanoparticles.