Environmental caffeine triggers oxidative stress and behavioral changes in Cyprinus carpio: Insights into neurotoxicity

Caffeine (CAF) is categorized as a central nervous system (CNS) stimulant, with effects varying based on factors like species, exposure duration, and dosage. The CNS regulates vital physiological processes and is notably sensitive to environmental pollutants, particularly oxidative stress due to its high lipid content. This study investigates the impact of CAF a known CNS stimulant, on juvenile Cyprinus carpio at various exposure levels (0, 500, 1250, 1750 and 2500 ng/L). Employing a holistic approach, we assessed the oxidative stress markers such as lipid peroxidation (LPX), hydroperoxides content (HPC), protein carbonyl content (PCC) and antioxidant responses superoxide dismutase (SOD) and catalase (CAT), alongside AChsterase (AChE) activity, behavioral patterns (Novel Tank Test, Dark & Light Test), histological changes (H&E staining), and gene expression (adenosine receptor A1, adenosine receptor A2A, serca, ryr, i3pr). Our findings indicate that increasing CAF concentrations correlate with heightened oxidative and antioxidant biomarker levels in the carp brain. Specifically, AChE activity diminished, reflecting neurotoxicity. Behaviorally, CAF displayed a biphasic influence: at lower concentrations (500 ng/L), it promoted locomotion and reduced anxiety, whereas at concentrations above 2500 ng/L, it suppressed locomotor activity and heightened anxiety-like behaviors. Notably, the highest concentration (2500 ng/L) resulted in significant histopathological changes, with a 45.9 % incidence of pathological alterations, predominantly vacuolization in the telencephalon and optic tectum. Gene expression analysis revealed an upregulation of adenosine receptor A1 and adenosine receptor A2A, indicating variations in the CAF mechanism based on receptor interaction. Additionally, there was a positive regulation of genes involved in Ca^2 + signaling (serca, ryr, i3pr), crucial for cellular homeostasis and implicated in cellular neoplasia’s. The study conclusively demonstrates that while low CAF doses can have beneficial effects on mobility, higher doses lead to pronounced neurotoxicity, indicated by oxidative stress, enzymatic inhibition, anxiety-like behavior, cerebral histopathology, and gene expression dysregulation. These findings provide critical insights into the dose-dependent neurotoxic effects of environmental CAF exposure in aquatic vertebrates.