Inhalation of Silver Silicate Nanoparticles Leads to Transient and Differential Microglial Activation in the Rodent Olfactory Bulb.

Engineered silver nanoparticles (AgNPs), including silver silicate nanoparticles (Ag-SiO2 NPs), are used in a wide variety of medical and consumer applications. Inhaled AgNPs have been found to translocate to the olfactory bulb (OB) after inhalation and intranasal instillation. However, the biological effects of Ag-SiO2 NPs and their potential nose-to-brain transport have not been evaluated. The present study assessed whether inhaled Ag-SiO2 NPs can elicit microglial activation in the OB. Adult Sprague-Dawley rats inhaled aerosolized Ag-SiO2 NPs at a concentration of 1 mg/ml for 6 hours. On day 0, 1, 7, and 21 post-exposure, rats were necropsied and OB were harvested. Immunohistochemistry on OB tissues were performed with anti-ionized calcium-binding adapter molecule 1 and heme oxygenase-1 as markers of microglial activation and oxidative stress, respectively. Aerosol characterization indicated Ag-SiO2 NPs were sufficiently aerosolized with moderate agglomeration and high-efficiency deposition in the nasal cavity and olfactory epithelium. Findings suggested that acute inhalation of Ag-SiO2 NPs elicited transient and differential microglial activation in the OB without significant microglial recruitment or oxidative stress. The delayed and differential pattern of microglial activation in the OB implied that inhaled Ag-SiO2 may have translocated to the central nervous system via intra-neuronal pathways.