Intranasal exposure to ZnO nanoparticles induces alterations in cholinergic neurotransmission in rat brain

The neurotoxicity of inhaled ZnO nanoparticles (NPs) and the underlying mechanisms remain largely unknown. In this study, ZnO NPs (30 ± 6 nm) were intranasally instilled to rats via a single dose (13 mg Zn/kg BW), with ZnSO<inf>4</inf> as the ionic control, and analysis 7-days post exposure. The hippocampus was found to be the main target for Zn accumulation for both ZnO NPs and ZnSO<inf>4</inf>. Synchrotron radiation based X-ray absorption fine structure (XAFS) analysis showed that no particulate ZnO was found, suggesting the occurrence of dissolution and transformation of ZnO NPs. Multi-omics analysis, including transcriptomics, proteomics and metabolomics, demonstrated that cholinergic neurotransmission was the main biological process affected following both treatments. The release of the key neurotransmitter acetylcholine (ACh) was increased by enhanced ACh synthesis, upregulation of vesicular ACh transporter, and suppression of the activity of ACh hydrolysis enzyme (AChE), either by direct Zn-AChE interaction or a transcriptional down-regulation mechanism. In addition, ZnO NPs and ZnSO<inf>4</inf> induced similar molecular consequences and exhibited the same Zn chemical speciation (100 % of Zn complexes) in the hippocampal region evidenced by XAFS analysis, suggesting that the observed biological effects were mainly derived from Zn²⁺ released from the ZnO NPs. This study not only evidences a new pathway for the impact of ZnO NPs on the brain, but also identifies the origin of the impact as ionic Zn, which provides the basis for safe-by-design of ZnO NPs.