Background and Objective: Zinc oxide (ZnO) nanoparticles are widely used in the medical, industrial, pharmaceutical and nutritional fields and contradictory results in neurological studies of these compounds in comparison with the conventional pattern demand further investigation. The purpose of this study was to evaluate the histology of hippocampus (an area involved in memory) following chronic administration of ZnO nanoparticles in comparison with the conventional pattern.

Materials and Methods: NMRI adult male mice weighing 25 ± 5 g were assigned to five groups: control (saline), recipient of nano ZnO and conventional ZnO in doses of 1 and 5 mg/kg. After one month of treatment with these, animals were euthanized and their brains were removed from the hippocampus for histological study.

Results: While ZnO nanoparticles led to necrosis of cells in some areas of the hippocampus, conventional ZnO did not exhibit such considerable damage. These changes were more pronounced at a dose of 1 mg/kg of ZnO nanoparticles.

Conclusion: ZnO nanoparticles may pass through the blood-brain barrier and induce harmful effects on cells in hippocampus. It is recommended to take necessary precautions while using combinations of nano drugs.