Neurotoxicity of low-dose repeatedly intranasal instillation of nano- and submicron-sized ferric oxide particles in mice

Olfactory tract has been demonstrated to be an important portal for inhaled solid nanoparticle transportation into the central nervous system (CNS). We have previously demonstrated that intranasally instilled Fe₂O₃ nanoparticles could transport into the CNS via olfactory pathway. In this study, we investigated the neurotoxicity and size effect of repeatedly low-dose (130 μg) intranasal exposure of nano- and submicron-sized Fe₂O₃ particles (21 nm and 280 nm) to mice. The biomarkers of oxidative stress, activity of nitric oxide synthases and release of monoamine neurotransmitter in the brain were studied. Our results showed that significant oxidative stress was induced by the two sizes of Fe₂O₃ particles. The activities of GSH-Px, Cu,Zn-SOD, and cNOS significantly elevated and the total GSH and GSH/GSSG ratio significantly decreased in the olfactory bulb and hippocampus after the nano- and submicron-sized Fe₂O₃ particle treatment (p < 0.05). The nano-sized Fe₂O₃ generally induced greater alteration and more significant dose–effect response than the submicron-sized particle did. Some slight perturbation of monoamine neurotransmitters were found in the hippocampus after exposure to the two sizes of Fe₂O₃ particle. The TEM image showed that some ultrastructural alterations in nerve cells, including neurodendron degeneration, membranous structure disruption and lysosome increase in the olfactory bulb, slight dilation in the rough endoplasmic reticulum and lysosome increase in the hippocampus were induced by the nano-sized Fe₂O₃ treatment. In contrast, in the submicron-sized Fe₂O₃ treated mice, slightly swollen mitochondria and some vacuoles were observed in the olfactory bulb and hippocampus, respectively. These results indicate that intranasal exposure of Fe₂O₃ nanoparticles could induce more severe oxidative stress and nerve cell damage in the brain than the larger particle did. This is the first study to compare the neurotoxicity of nano- and submicron-sized Fe₂O₃ particles in the central nervous system after long-term and low-dose intranasal exposure.