Thymol and p-Cymene Protect the Liver by Mitigating Oxidative Stress, Suppressing TNF-α/NF-κB, and Enhancing Nrf2/HO-1 Expression in Immobilized Rats

This study aimed to investigate the effects of the monoterpenes thymol and p-cymene on the liver of rats subjected to prolonged immobilization stress and to discover the possible mechanism behind this effect. For 14 consecutive days, the rats were placed in a restrainer for 2.5 h every day to expose them to stress. During the same period, thymol (10 mg/kg, gavage) and p-cymene (50 mg/kg, intraperitoneally) were also administered. Thymol and p-cymene prevented the increase in malondialdehyde levels and the decrease in glutathione content in the liver of rats exposed to chronic immobility. They also increased the activity of the glutathione peroxidase enzyme in the liver of stressed animals, but only thymol could increase the activity of superoxide dismutase. These monoterpenes reduced the expression of pro-inflammatory cytokines tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β, and IL-6 and nuclear factor kappa B (NF-κB) in the liver of stressed animals. They increased the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1). Thymol and p-cymene greatly prevented the infiltration of inflammatory cells in the liver parenchyma of stressed rats. In conclusion, the study found that thymol and p-cymene have a hepatoprotective effect on immobilized rats, likely exerted by suppressing oxidative stress and inflammation, stimulating Nrf2/HO-1 signaling, and inhibiting the TNF-α/NF-κB pathway.