Recent progress on our understanding of the biological effects of mercury in fish and wildlife in the Canadian Arctic

This review summarizes our current state of knowledge regarding the potential biological effects of mercury (Hg) exposure on fish and wildlife in the Canadian Arctic. Although Hg in most freshwater fish from northern Canada was not sufficiently elevated to be of concern, a few lakes in the Northwest Territories and Nunavut contained fish of certain species (e.g. northern pike, Arctic char) whose muscle Hg concentrations exceeded an estimated threshold range (0.5-1.0 μg g(-1) wet weight) within which adverse biological effects begin to occur. Marine fish species generally had substantially lower Hg concentrations than freshwater fish; but the Greenland shark, a long-lived predatory species, had mean muscle Hg concentrations exceeding the threshold range for possible effects on health or reproduction. An examination of recent egg Hg concentrations for marine birds from the Canadian Arctic indicated that mean Hg concentration in ivory gulls from Seymour Island fell within the threshold range associated with adverse effects on reproduction in birds. Mercury concentrations in brain tissue of beluga whales and polar bears were generally lower than levels associated with neurotoxicity in mammals, but were sometimes high enough to cause subtle neurochemical changes that can precede overt neurotoxicity. Harbour seals from western Hudson Bay had elevated mean liver Hg concentrations along with comparatively high muscle Hg concentrations indicating potential health effects from methylmercury (MeHg) exposure on this subpopulation. Because current information is generally insufficient to determine with confidence whether Hg exposure is impacting the health of specific fish or wildlife populations in the Canadian Arctic, biological effects studies should comprise a major focus of future Hg research in the Canadian Arctic. Additionally, studies on cellular interactions between Hg and selenium (Se) are required to better account for potential protective effects of Se on Hg toxicity, especially in large predatory Arctic fish, birds, and mammals.

Mercury, selenium and neurochemical biomarkers in different brain regions of migrating common loons from Lake Erie, Canada

Common loons (Gavia immer) can be exposed to relatively high levels of dietary methylmercury (MeHg) through fish consumption, and several studies have documented MeHg-associated health effects in this species. To further study the neurological risks of MeHg accumulation, migrating loons dying of Type E botulism were collected opportunistically from the Lake Erie shore at Long Point (Ontario, Canada) and relationships between total mercury (THg), selenium (Se), and selected neurochemical receptors and brain enzymes were investigated. THg concentrations were 1-78 μg/g in liver; and 0.3-4 μg/g in the brain (all concentrations reported on a dry weight basis). A significant (p < 0.05) positive correlation was found between THg in liver and THg in 3 subregions of the brain (cerebral cortex: r = 0.433; cerebellum: r = 0.293; brain stem: r = 0.405). THg varied significantly among different brain regions, with the cortex having the highest concentrations. Se levels in the cortex and cerebellum were 1-29 and 1-10 μg/g, respectively, with no significant differences between regions. Se was not measured in brain stem due to insufficient tissue mass. There were molar excesses of Se over mercury (Hg) in both cortex and cerebellum at all Hg concentrations, and a significant positive relationship between THg and the Hg:Se molar ratio (cortex: r = 0.63; cerebellum: r = 0.47). No significant associations were observed between brain THg and the N-methyl-D-aspartic acid (NMDA) receptor concentration, nor between THg and muscarinic cholinergic (mACh) receptor concentration; however, brain THg levels were lower than in previous studies that reported significant Hg-associated changes in neuroreceptor densities. Together with previous studies, the current findings add to our understanding of Hg distribution in the brain of common loons, and the associations between Hg and sub-lethal neurochemical changes in fish-eating wildlife.

Investigation of spatial trends and neurochemical impacts of mercury in herring gulls across the Laurentian Great Lakes

Herring gulls (Larus argentatus) bioaccumulate mercury (Hg) but it is unknown whether they are exposed at levels of neurological concern. Here we studied brain tissues from gulls at five Great Lakes colonies and one non-Great Lakes colony during spring of 2001 and 2003. Total brain Hg concentrations ranged from 0.14 to 2.0 microg/g (dry weight) with a mean of 0.54 microg/g. Gulls from Scotch Bonnet Island, on the easternmost edge of the Great Lakes, had significantly higher brain Hg than other colonies. No association was found between brain Hg concentration and [3H]-ligand binding to neurochemical receptors (N-methyl-D-aspartate, muscarinic cholinergic, nicotinic cholinergic) or nicotinic receptor alpha-7 relative mRNA expression as previously documented in other wildlife. In conclusion, spatial trends in Hg contamination exist in herring gulls across the Great Lakes basin, and herring gulls accumulate brain Hg but not at levels associated with sub-clinical neurochemical alterations.

Variation of cholinergic biomarkers in brain regions and blood components of captive mink

Studies are increasingly using cholinergic parameters as biomarkers of early neurotoxicity, but few have characterized this system in ecologically relevant model organisms. In the present study, key neurochemicals in the cholinergic pathway were measured and analyzed from discrete parts of brain and blood from captive mink (Mustela vison). Similar to other mammals, the regional distribution of cholinergic parameters in the brain could be ranked from highest to lowest as: basal ganglia > occipital cortex > brain stem > cerebellum (F (3,192) = 172.1, p < 0.001). Higher variation in cholinergic parameters was found in the cerebellum (coefficient of variation = 34.9%), and the least variation was measured in the brain stem (19.7%). Variation was also assessed by calculating the difference between the lowest and highest measures among individual animals: choline acetyltransferase (1.6x fold difference), cholinesterase (2.0x), muscarinic receptor levels (2.4x), acetylcholine (3.7x), nicotinic receptor levels (3.9x), and choline transporter (5.0x). In blood samples, activity and inter-individual variation of cholinesterase was highest in whole blood and lowest in plasma and serum. By using captive mink of a common genetic source, age, gender, and rearing conditions, these data help establish normal levels, ranges, and variations of cholinergic biomarkers among brain regions, blood components, and individual animals. Such information may better enable the utility of cholinergic biomarkers in environmental assessments.

Effects of mercury on neurochemical receptors in wild river otters (Lontra canadensis)

Fish-eating wildlife, such as river otters (Lontra canadensis), accumulate mercury (Hg) at concentrations known to impair animal behavior, but few studies have explored the underlying biochemical changes that precede clinical neurotoxicity. The objective of this study was to determine if Hg exposure can be related to concentrations of neurochemical receptors in river otters. River otter carcasses (n = 66) were collected in Ontario and Nova Scotia (Canada) by local trappers in 2002-2004. Concentrations of Hg (total and organic) were measured in the cerebral cortex and cerebellum. Saturation binding curves for the cholinergic muscarinic acetylcholine (mACh) receptor and dopamine-2 (D2) receptor were completed for each animal to calculate receptor density (Bmax) and ligand affinity (Kd). Negative correlations were found between concentrations of Hg and mACh receptor Bmax (r(total) Hg = -0.458, r(inorganic) Hg = -0.454, r(organic) Hg = -0.443) in the cerebral cortex. A negative correlation was also found between concentrations of total Hg and D2 receptor Bmax (r = -0.292) in the cerebral cortex. These results suggest that neurochemical receptors may prove useful as novel biomarkers of Hg exposure and neurotoxic effects in wildlife. Given the importance of cholinergic and dopaminergic systems in animal physiology, the ecological implications of these changes need to be investigated.