Diesel exhaust inhalation exposure induced toxicity on olfactory bulb in mice through inflammatory response mediated by activating glial cells

Background Air pollution is related to the occurrence and development of mental diseases. Olfactory bulb damage might be the potential prodromal symptom and sign of these diseases. The toxicity of diesel exhaust (DE), one of the main sources of air pollution, on olfactory bulb and the underlying mechanisms remain to be elucidated.

Objective To explore the toxicity of DE on mouse olfactory bulb and underlying mechanisms.

Methods A total of 40 C57BL/6 mice were randomly divided into four groups for exposure to DE by systemic inhalation: control group (filtered air), low exposure group (750 μg·m⁻³ DE), medium exposure group (1500 μg·m⁻³ DE), and high exposure group (3000 μg·m⁻³ DE). The mouse inhalation exposure to DE was performed 1 h per day for 28 d. HE staining was performed to observe pathological changes in mouse olfactory bulb tissue. TUNEL assay was used to observe apop-tosis in olfactory bulb. Kyoto Encyclopedia of Genes and Genomes (KEGG) was exhibited to explore potential mechanisms of olfactory bulb damage associated with DE. Quantitative real-time PCR (qPCR) was used to determine mRNA expression levels of inflammatory factors including tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6). Immunofluorescence staining was conducted to observed the microglia and astrocyte activation in olfactory bulb.

Results The HE staining results showed that the number of periglomerular cells in the glomerular layer of olfactory bulb decreased in a dose-dependent manner, and the cells in the granule cell layer of olfactory bulb became disordered after DE exposure. The TUNEL staining showed that TUNEL positive cells in olfactory bulb tissue and neuronal apoptosis increased in the exposed groups compared with the control group (P＜0.05). The KEGG pathway analysis showed that DE associated with significant enrichment of TNF signaling pathway in olfactory bulb tissue. The qPCR results showed that the TNF-α relative expression level significantly increased by 67% and the IL-6 relative expression level by 340% in the DE high exposure dose group compared with the control group (P＜0.05). According to the immunofluorescence staining results, the numbers of activated microglia and astrocytes in olfactory bulb tissue significantly increased in the DE high exposure group, the relative fluorescence intensity of ionized calcium binding adaptor molecule 1 (IBA-1) increased by 120%, the granule cell layer relative fluorescence intensity of glial fibrillary acidic protein (GFAP) increased by 400%, and the glomerular layer relative fluorescence intensity of GFAP increased by 240% than those in the control group (P＜0.05).

Conclusion Inhalation exposure to DE can lead to glial cell activation including microglia and astrocytes in olfactory bulb tissue by activating inflammatory pathways and releasing inflammatory factors TNF-α and IL-6, leading to neuronal apoptosis in olfactory bulb tissue.