Assessment of Pb neurotoxicity in European brown bears: Evidence from Pb–Ca associations and oxidative stress biomarkers

Throughout the last decades, our research group has monitored non-essential trace element levels in brown bears (Ursus arctos) from the Dinara-Pindos population to assess exposure to these environmental pollutants, their impact on health, and the risk of chronic toxicity. Lead (Pb) is a high-priority neurotoxicant whose mechanisms of toxicity include molecular mimicry, oxidative stress, mitochondrial dysfunction and neuroinflammation. Given the brain’s limited capacity to compensate for toxic damage, this study aimed to investigate the most probable mechanisms by examining the associations between Pb and calcium (Ca) levels as well as biomarkers of oxidative stress. Brain stem samples were collected from 69 free-living brown bears hunted in Croatia between 2022 and 2025 in accordance with the Brown Bear Management Plan. Elements were quantified by inductively coupled plasma-mass spectrometry (ICP-MS). Levels of oxidative stress biomarkers malondialdehyde (MDA), reactive oxygen species (ROS), catalase (CAT) activity and total antioxidant capacity (TAC) were measured in tissue homogenates. Brain Pb levels (mean ± SD, median, range: 26.8 ± 23.9, 19.9, 2.11–175 μg/kg dry mass) did not exceed values considered normal for cattle (used as a reference due to the lack of species-specific reference values). A moderate positive association was observed between Pb and the essential Ca (Spearman’s r = 0.45, p = 0.000172, n = 64). This correlation may reflect divalent cation mimicry, whereby Pb²⁺ mimics Ca²⁺ by binding to similar sites and entering cells via Ca²⁺ channels. No significant relationships were observed between Pb levels and the investigated oxidative stress biomarkers thereby implying successful maintenance of redox balance in conditions of environmental exposure. This study suggests that Pb levels detected in the bear brain are unlikely to induce oxidative stress, while confirming an association between Pb and Ca, a key essential element involved in brain function.