Interactive effects of nonylphenol ethoxylate (NPE) exposure and thermal stress on oxidative stress biomarkers and neurotoxicity in bullfrog tadpoles.

The global decline in amphibian populations has been well documented and is driven by multiple environmental stressors. Among these, the synergistic effects of chemical pollution and rising water temperatures can significantly contribute to ecological disturbances. Nonylphenol ethoxylate (NPE), a surfactant widely used in sanitary and agrochemical products, has been detected in aquatic ecosystems. Despite its ecological relevance, the sublethal effects of NPE on amphibians, particularly under conditions of thermal stress, remain poorly understood. This study evaluated the effects of acute exposure (48 h) to an environmentally relevant concentration of NPE (30 µg L⁻¹) on Aquarana catesbeiana tadpoles under two thermal regimes (25 °C and 30 °C), including the potential interaction between these conditions. We assessed antioxidant enzymatic activities (superoxide dismutase, catalase, glutathione S-transferase, and glutathione peroxidase), glutathione levels, and oxidative stress biomarkers (lipid peroxidation and protein carbonylation) in hepatic, branchial, and muscular tissues. Neurotoxicity was assessed via acetylcholinesterase activity measurements in brain and muscle tissues. NPE exposure caused hepatotoxicity, oxidative damage and redox imbalance in gills and muscle, as well as inhibition of muscular acetylcholinesterase activity. Elevated temperature further depleted branchial antioxidant defenses, intensified oxidative stress in hepatic and muscular tissues, and induced central neurotoxicity. These findings demonstrate that sublethal exposure to NPE compromises redox balance and cholinergic function in a tissue-specific and temperature-dependent manner. The absence of regulatory limits for NPE in several countries represents a critical ecological threat, highlighting the urgent need to update aquatic environmental guidelines in the context of global warming.