University of Tehran Iranian Journal of Veterinary Medicine 2251-8894 20 2 2026 03 01 Thymol and Brain Tissue Oxidative Stress Responses Caused by Mercury Metal Poisoning in Common Carp (Cyprinus carpio) 395 402 102960 10.32598/ijvm.20.2.1005859 EN Seyedeh Mohadeseh PourMortazavi Bahambari Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. HosseinAli Ebrahimzadeh Mousavi Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. Akram Vatannejad Department of Comparative Biosciences, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. Aghil Sharifzadeh Department of Microbiology and Immunology, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. Ali Taheri Mirghaed Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. Journal Article 2025 07 29 <strong>Background</strong>: One of the most significant heavy metals that is not biodegradable is mercury. Fish exposed to mercury may experience adverse effects, including reduced brain tissue. Using herbal compounds or their active constituents is one of the best ways to lower the negative effects of heavy metals. The essential oils of plants like thyme contain thymol, a naturally occurring monoterpene.<br /><strong>Objectives</strong>: This study aimed to evaluate the protective effects of thymol on oxidative stress responses in the brain tissue of juvenile carp (Cyprinus carpio) following mercury exposure.<br />Methods: The 120 common carp (C. carpio) used in this study were randomly divided into four groups: Control, HgCl2, thymol, and thymol+HgCl2. Each group had 10 fish and was reproduced three times. The fish spent 56 days in captivity. For the control group, the fish were kept in water devoid of mercury(II) chloride and fed a simple diet. Thymol and thymol+HgCl2 fish were fed food containing 100 mg/kg of thymol for 56 days. Fish in the thymol+HgCl2 and HgCl2 groups had 0.44 mg/L of mercury chloride in their tank water. A necropsy and removal of the fish’s brain tissue followed their euthanasia after 56 days. Brain tissues were rinsed with phosphate-buffered saline (PBS) and homogenized in 0.9 M PBS (pH 7.4). The supernatant obtained after centrifugation was used to assess oxidative stress markers. Malondialdehyde (MDA), total antioxidant capacity (TAC), and catalase (CAT) activity in brain tissue were quantified using commercial colorimetric assays<br /><strong>Results</strong>: TAC and CAT activity significantly increased in the thymol group (P<0.0001), while both were markedly reduced in the mercury group (P<0.0001). Thymol treatment restored TAC and CAT levels in mercury-exposed fish to near-control values (P<0.0001). MDA levels decreased in the thymol and mercury+thymol groups, but the changes were not statistically significant. These results highlight thymol’s antioxidant potential against mercury-induced stress.<br /><strong>Conclusion</strong>: Thymol significantly enhanced antioxidant defenses in juvenile carp exposed to mercury by increasing TAC and CAT levels. It partially restored oxidative balance and reduced lipid peroxidation, though MDA levels did not differ significantly. These findings support thymol’s potential as a natural protective agent against heavy metal-induced neurotoxicity. https://ijvm.ut.ac.ir/article_102960_37b012d9c7e758d72c908f79cd8a661b.pdf