Mercury exposure and neurochemical biomarkers in multiple brain regions of Wisconsin River Otters ( Lontra canadensis )
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Mercury exposure and neurochemical biomarkers in multiple brain regions of Wisconsin River Otters (Lontra canadensis) Peter Dornbos • Sean Strom • Niladri Basu
Accepted: 9 January 2013 / Published online: 23 January 2013 Ó Springer Science+Business Media New York 2013
Abstract River otters are fish-eating wildlife that bioaccumulate high levels of mercury (Hg). Mercury is a proven neurotoxicant to mammalian wildlife, but little is known about the underlying, sub-clinical effects. Here, the overall goal was to increase understanding of Hg’s neurological risk to otters. First, Hg values across several brain regions and tissues were characterized. Second, in three brain regions with known sensitivity to Hg (brainstem, cerebellum, and occipital cortex), potential associations among Hg levels and neurochemical biomarkers [N-methyl-D-aspartic acid (NMDA) and gamma-aminobutyric acid (GABAA) receptor] were explored. There were no significant differences in Hg levels across eight brain regions (rank order, highest to lowest: frontal cortex, cerebellum, temporal cortex, occipital cortex, parietal cortex, basal ganglia, brainstem, and thalamus), with mean values ranging from 0.7 to 1.3 ug/g dry weight. These brain levels were significantly lower than mean values in the muscle (2.1 ± 1.4 ug/g), liver (4.7 ± 4.3 ug/g), and fur (8.8 ± 4.8 ug/g). While a significant association was found between Hg and NMDA receptor levels in the brain stem (P = 0.028, rp = -0.293), no relationships were found in the cerebellum and occipital cortex. For the GABA receptor, no relationships were found. The lack of consistent Hg-associated neurochemical changes is likely due to low brain Hg levels in these river otters, which are amongst the lowest reported. P. Dornbos N. Basu (&) Department of Environmental Health Sciences, University of Michigan School of Public Health, Ann Arbor, MI 48109, USA e-mail: [email protected] S. Strom Wisconsin Department of Natural Resources, Bureau of Wildlife Management, Madison, WI 53707, USA
Keywords Neurochemistry Neurotoxicology Biomarker Wildlife Methylmercury
Introduction Mercury (Hg) is an element of toxicological concern (Clarkson and Magos 2006). While ubiquitous in the environment, increasing dispersal and bioavailability of Hg is primarily attributed to anthropogenic activities such as fossil fuel combustion and waste incineration (Mason et al. 2005). Following deposition in aquatic systems, inorganic mercury may be methylated by microbes to form methylmercury (MeHg). Methylmercury can biomagnify through aquatic food chains by a factor of 1–10 million times (Evers et al. 2011). Organisms most at risk of MeHg exposure are those that occupy high trophic levels and consume fish or other aquatic biota. Once ingested, MeHg is efficiently assimilated from the GI tract and may ultimately cross the blood brain barrier (Clarkson and Magos 2006). Once in the brain, MeHg causes hallmark structural and functional effects that are consistent across mammalian species, including humans and wildlife (Basu and
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