Abstract
Introduction: Environmental exposure to nanoparticles is inevitable as they became part of our daily life. Nanoparticles are widely distributed in air, cosmetics and medicines and even in food and as a result, nanotoxicity research is gaining attention. Gold nanoparticles (AuNPs) can cause lung damage through increased oxidative stress. Melatonin has a huge protective effect on all organs of the body through regulation of antioxidant enzymes.

Objective: in the present study we examined the possible histological alterations in the pulmonary alveoli after administration of 10 nm AuNPs in an attempt to understand the toxicity of AuNPs and the possible protective effect of concomitant administration of melatonin.

Material & methods: 30 healthy adult male albino rats were used. They were divided into 3 groups; control group (10 animals), AuNPs group: 10 animals treated with 4 mg/kg of 10 nm AuNPs once daily for 7 consecutive days and AuNPs+melatonin group: 10 rats treated with 4mg/kg of 10 nm AuNPs and 10 mg/kg melatonin at the same time once daily for 7 consecutive days.

Results: Exposure to AuNPs induced alterations in the pulmonary alveoli which were summarized mainly as destruction of type 1and type 2 pneumocytes, thick interalveolar septa due to cellular infiltration. There was mild affection of pulmonary alveoli in the AuNPs+melatonin group; few alveoli appeared slightly collapsed, separated by slightly thickened interalveolar septa due to mild cellular infiltration.

Conclusion: AuNPs induced evident damage in rat pulmonary alveoli. Melatonin significantly ameliorated the alveolar damage.

Key words: AuNPs, Melatonin, Pulmonary alveoli, Nanotoxicity, Oxidative stress.