Objectives: In this study, we focused on the qualitative and quantitative differences of the lung lesions induced by single or multiple intratracheal administration of nickel oxide nanoparticles (NiO). Methods: Male rats were randomized into groups receiving intratracheal administrations in a single dose or two to four divided doses of 2 mg/kg/bw. Bronchoalveolar lavage fluid (BALF) analyses were performed at 3 and 28 d post-dose. Histopathological analyses were performed at 28 and 91 d post-dose. Results: BALF analyses revealed pulmonary injury, inflammation, and increases in the parameters indicating processing the foreign material in all the NiO-treated groups. Histopathological analyses showed the phagocytosis of NiO by alveolar macrophages, degeneration and necrosis of alveolar macrophages, and inflammatory responses. In the comparison between single and multiple administrations, the trend for stronger toxicity effects was observed after multiple application at 3 d post-dose, while the obvious toxicity effects were also seen in case of single administration. No particular differences of lung lesions depending on the frequency of administration at 28 and 91 d post-dose were evident. Conclusion: Intratracheal NiO administration induced strong toxic response thoroughly even by single administration. Therefore, single administration was concluded to be applicable to assess the inhalation toxicity of nanomaterials and can be used in the screening test.

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