Is Dietary Mercury of Neurotoxicological Concern to Wild Polar Bears (Ursus maritimus)?
Polar bears are exposed to high concentrations of mercury because they are apex predators in the Arctic ecosystem. While mercury is a potent neurotoxic heavy metal, it is not known whether current exposures are of neurotoxicological concern to polar bears. We tested the hypotheses that: 1) polar bears accumulate levels of mercury in their brains that exceed the estimated lowest observable adverse effect level (20 microg/g dry weight, d.w.) for mammalian wildlife; and 2) such exposures are associated with subtle neurological damage, as determined by measuring neurochemical biomarkers previously shown to be disrupted by mercury in other high-trophic wildlife. Brain stem (medulla oblongata) tissues from 82 polar bears subsistence hunted in East Greenland were studied. Despite surprisingly low levels of mercury in the brain stem region (total mercury = 0.36 +/- 0.12 microg/g d.w.), a significant negative correlation was measured between N-methyl-D-aspartate (NMDA) receptor levels and both total mercury (r = -0.34, p < 0.01) and methylmercury (r = -0.89, p < 0.05). There were no relationships among mercury, selenium, and several other neurochemical biomarkers (dopamine-2, GABA-A, muscarinic cholinergic, nicotinic cholinergic receptors; cholinesterase, monoamine oxidase enzymes). These data show that East Greenland polar bears do not accumulate high levels of mercury in their brain stems. However, decreased levels of NMDA receptors may be one of the most sensitive indicators of mercury's sub-clinical and early effects.
PMID: 18717617 [PubMed - as supplied by publisher]