Mercury exposure and neurochemical biomarkers in multiple brain regions of Wisconsin river otters (Lontra canadensis)

Mercury and neurochemical biomarkers in multiple brain regions of five Arctic marine mammals

Mercury is a neurotoxic chemical that represents one of the greatest pollution threats to Arctic ecosystem health. Evaluating the direct neurotoxic effects of mercury in free ranging wildlife is challenging, necessitating the use of neurochemical biomarkers to assess potential sub-clinical neurological changes. The objective of this study was to characterize the distribution and speciation of mercury, as well as exposure-associated changes in neurochemistry, across multiple brain regions (n = 10) and marine mammal species (n = 5) that each occupy a trophic niche in the Arctic ecosystem. We found consistent species differences in mean brain and brain region-specific concentrations of total mercury (THg) and methyl mercury (MeHg), with higher concentrations in toothed whales (narwhal, pilot whales and harbour porpoise) compared to fur-bearing mammals (polar bear and ringed seal). Mean THg (μg/g dw) in decreasing rank order was: pilot whale (11.9) > narwhal (7.7) > harbour porpoise (3.6) > polar bear (0.6) > ringed seal (0.2). The higher THg concentrations in toothed whales was associated with a marked reduction in the percentage of MeHg (<40 %) compared to polar bears (>70 %) that had lower brain THg concentrations. This pattern in mercury concentration and speciation corresponded broadly to an overall higher number of mercury-associated neurochemical biomarker correlations in toothed whales. Of the 226 correlations between mercury and neurochemical biomarkers across brain regions, we found 60 (27 %) meaningful relationships (r>0.60 or p < 0.10). We add to the growing weight of evidence that wildlife accumulate mercury in their brains and demonstrate that there is variance in accumulation across species as well as across distinct brain regions, and that some of these exposures may be associated with sub-clinical changes in neurochemistry.

Spatial patterns of the exposure-response relationship between mercury and cortisol in the fur of river otter (Lontra canadensis)

Fur has been validated as a useful biomarker medium for chemical exposures and biological responses in wildlife. Mercury (Hg) is known to act as an endocrine disruptor by altering brain neurochemistry. In this study, we investigated the spatial patterns of relationships between total Hg (THg) and cortisol in the fur of river otter (Lontra canadensis). Geotagged fur samples were obtained from a wildlife biomonitoring program (n = 72) and the North American Fur Auction (n = 37) between 2014 and 2017. Fur THg was measured using direct thermal decomposition and fur cortisol was measured using an enzyme-linked immunosorbent assay (ELISA). The average fur THg concentration was 11.50 ± 12.40 μg/g fur weight (f.w.), and the fur cortisol concentration was 5.71 ± 8.24 pg/mg. Results from the global ordinary least squares regression show no relationship between THg and fur cortisol concentrations. However, both Hg and cortisol were heterogeneously distributed across the landscape. When a localized geographically weighted regression (GWR) was used, a geographically distinct bi-phasic relationship was observed. We suggest this bi-phasic relationship is associated with a threshold THg concentration, beyond which, there was a negative association with measured fur cortisol. Results of a break-point analysis, with one break, indicate that the threshold is 16 ± 1.27 μg/g f. w of THg in fur. This research highlights the need to use appropriate spatial methods when assessing exposure-response relationships in wildlife across large geographical areas. The identified threshold can be used for regulatory purposes.

Metal brain bioaccumulation and neurobehavioral effects on the wild rodent Liomys irroratus inhabiting mine tailing areas

Ecotoxicological studies are necessary in order to evaluate the effects of environmental exposure of chemicals on wild animals and their ecological consequences. Particularly, neurobehavioral effects of heavy metal elements on wild rodents have been scarcely investigated. In the present study, we analyzed the effect of metal bioaccumulation (Pb, As, Mg, Ni, and Zn) in the brain and in the liver on exploratory activity, learning, memory, and on some dopaminergic markers in the wild rodent Liomys irroratus living inside mine tailings, at Huautla, Morelos, Mexico. We found higher Pb concentration but lower Zn in striatum, nucleus accumbens, midbrain, and hippocampus in exposed animals in comparison to rodents from the reference site. Exposed rodents exhibited anxious behavior evaluated in the open field, while no alterations in learning were found. However, they displayed slight changes in the memory test in comparison to reference group. The neurochemical evaluation showed higher levels of dopamine and 5-hydroxyindolacetic acid in midbrain, while lower levels of metabolites dihydroxyphenyl acetic acid and homovanillic acid in striatum of exposed rodents. In addition, mRNA expression levels of dopaminergic D2 receptors in nucleus accumbens were lower in animals from the mining zone than in animals from the reference zone. This is the first study that shows that chronic environmental exposure to metals results in behavioral and neurochemical alterations in the wild rodent L. irroratus, a fact that may comprise the survival of the individuals resulting in long-term effects at the population level. Finally, we suggest the use of L. irroratus as a sentinel species for environmental biomonitoring of mining sites.

Relationships between mercury concentrations in fur and stomach contents of river otter (Lontra canadensis) and mink (Neovison vison) in Northern Alberta Canada and their applications as proxies for environmental factors determining mercury bioavailability

The fur of piscivorous animals such as river otter (Lontra canadensis) and mink (Neovison vison) has been proposed to be used as a biomarker medium to assess mercury (Hg) exposure, but the relationship with dietary and environmental Hg exposure has not been fully characterized. The objective of this study was to investigate the relationship between fur total mercury (THg) and stomach content THg in river otter and mink, and their relationships with environmental factors. THg concentrations were measured in fur and stomach contents of river otter (n = 35) and mink (n = 30) collected from northern Alberta, Canada between 2014 and 2017. The fur THg concentration (mean ± standard deviation) was 6.36 ± 4.12 μg/g fur weight and 5.25 ± 3.50 μg/g fur weight and the average stomach content THg was 0.95 ± 0.56 μg/g dry weight and 0.71 ± 0.54 μg/g dry weight in river otter and mink respectively. There was a positive relationship between the log fur THg and log stomach contents THg for both species (p < 0.05). There was a positive relationship between the log THg of stomach contents and the percent of deciduous forest and a negative relationship with soil pH. There was a positive relationship between the log THg of fur and the total area burned by forest fire and a negative relationship with the percentage of wetlands. These results provide field evidence that fur can be used to reflect dietary Hg exposure and to identify sources and environmental factors that affect the bioavailable Hg in the habitats of these wildlife species.

Distribution of organic and inorganic mercury across the pelts of Canadian river otter (Lontra canadensis)

Fur is a common biomarker of environmental mercury (Hg) exposure. Further, there are well-established relationships between total mercury (THg) in fur and organs. However, these models assumed that THg is uniformly distributed across the fur in a pelt. In this study, we assess the distribution of THg and methylmercury (MeHg) across the pelts of four river otters (Lontra canadensis). THg concentrations were measured in the topcoat (n = 95) and undercoat fur (n = 95). MeHg was measured in a subset of these samples (n = 10). Patterns of THg and MeHg were explored using cluster analyses and ANOVAs. Significant differences existed between THg in topcoat and undercoat and between anatomical region (head/body/tail/legs) and fur regions (dorsal/ventral/furline). The cluster analysis showed significant THg clusters in undercoat fur and to a lesser extent topcoat fur. Further, the error rate for predicting internal THg is lowest in the forebody region of the topcoat, thus, making this the optimal region to sample for biomonitoring. Fur samples taken outside of this region could result in prediction error as high as 140% when estimating internal organ THg. The ratio of MeHg in THg in topcoat fur was measured at 95.7 ± 3.4% indicating THg concentrations can be used to assess MeHg exposure.

Predictive meta-regressions relating mercury tissue concentrations of freshwater piscivorous mammals

Mercury (Hg) is a pollutant of global concern. Sentinel species such as river otter (Lontra canadensis) and mink (Neovison vison) are often used to monitor environmental concentrations in freshwater ecosystems. Tissue total Hg (THg) concentrations are frequently used as biomarkers of exposure. However, there is no comprehensive model relating Hg tissue concentrations in different tissues, making interstudy comparisons challenging. Our objective was to establish conversion factors relating fur, brain, liver, kidney, and muscle THg concentrations using mean concentrations and standard errors reported in the literature. We used data from more than 6000 samples, pooled across 16 studies and 96 sampling sites in North America and Europe. Sixteen regressions were derived for the river otter and mink models, which were statistically significant at a 95% confidence interval and yielded high explained variances. The models were validated using an external data set of individually measured THg tissue concentrations. The validated conversions were used to evaluate the current fur Hg screening guidelines of 20 µg/g and 30 µg/g. At both of these fur concentrations, brain concentrations are of concern for altering brain neurochemistry. We suggest a more conservative fur Hg screening guideline of 15 µg/g to protect sensitive furbearers. The conversion factors can be used to predict internal organ THg concentrations from fur measurements, eliminating the need for invasive tissue sampling. Environ Toxicol Chem 2017;36:2377-2384. © 2017 SETAC.

Methylmercury determination in seafood by photochemical vapor generation capacitively coupled plasma microtorch optical emission spectrometry

A non-chromatographic method based on double liquid-liquid extraction and measurements by UV photochemical vapor generation capacitively coupled plasma microtorch optical emission spectrometry was developed and characterized for methylmercury determination in seafood. Samples were prepared following the procedure recommended in JRC Technical Report of European Commission formerly proposed for the determination of methylmercury in seafood by thermal decomposition atomic absorption spectrometry, namely confinement of Hg species in 47% HBr solution, extraction of CH₃Hg⁺ in toluene and back-extraction in 1% l-cysteine aqueous solution. Mercury cold vapor was generated by flow injection UV photo-reduction from CH₃Hg⁺ in 0.6molL^-1 HCOOH, while quantification was performed against external Hg²⁺ aqueous standards and measuring Hg 253.652nm emission using a low power/Ar consumption plasma microtorch (15W, 100mLmin^-1) and a low resolution microspectrometer (Ocean Optics). The figures of merit and analytical capability were assessed by analyzing certified reference materials and test samples of fish fillet and discussed in relation with requirements for Hg determination in seafood in European legislation (Decisions 2007/333/EC and 2002/657/EC) as well as compared to performances achieved in thermal decomposition atomic absorption spectrometry. The limit of detection and quantification of 2µgkg^-1 and 6µgkg^-1 respectively, precision of 2.7-9.4% and accuracy of 99±8% of the proposed method for the determination of CH₃Hg⁺ fulfill the demands of European legislation for Hg quantification. The limit of detection and quantification were better than those in the used reference method or other non-/chromatographic methods taken for comparison. The analysis of certified reference materials and the Bland and Altman test performed on 12 test samples confirmed trueness of the proposed method and its reliability for the determination of traces of CH₃Hg⁺ with 95% confidence level. The proposed method fulfills several demands of the eco-scale concept, is sensitive, simple and safe related to sample preparation through elimination of classical, harmful reductants and attractive by using economical miniaturized instrumentation incorporating a low power and low Ar consumption plasma.

Brains of Native and Alien Mesocarnivores in Biomonitoring of Toxic Metals in Europe

Mercury (Hg), lead (Pb) and cadmium (Cd) are involved in mammalian brain damage. However, little is known about Pb and Cd brain levels in wildlife that reflect the geochemical background. The aims of the study include the estimation of Hg, Pb and Cd concentrations, and the determination of relationships between these elements in the brains of 94 mesocarnivores. Road-killed or hunted animals were obtained from north-western Poland near the Polish-German border. The investigation covered the native Eurasian otter Lutra lutra, badger Meles meles, pine marten Martes martes, beech marten M. foina, European polecat Mustela putorius, red fox Vulpes vulpes, and alien species: feral and ranch American mink Neovison vison, raccoon Procyon lotor and raccoon dog Nyctereutes procyonoides. Depending on the diet and environmental pollution, the carnivore brains accumulated toxic metals in varying amounts. The highest median Hg levels (in mg/kg dry weight, dw) were found in the piscivorous Eurasian otter and feral mink (2.44 and 3.96), Pb in the omnivorous raccoon (0.47), while Cd in minks (~0.06). We indicated that Pb-based ammunition is a significant source of the element in scavengers from hunting area, and we also found a significant correlation between Pb and Cd levels in the fox brain. Finally, this study is the first to suggest background levels for brain Pb and Cd in mesocarnivores (<0.50 and <0.04 mg/kg dw, respectively).

Uptake of selenium and mercury by captive mink: Results of a controlled feeding experiment

Captive, juvenile, ranch-bred, male mink (Neovison vison) were fed diets containing various concentrations of methyl-mercury (MeHg) and selenium (Se) for a period of 13 weeks and then sacrificed to determine total Hg levels in fur, blood, brain, liver and kidneys and total Se concentrations in brain tissue. As MeHg concentrations in the diet increased, concentrations of total Hg in the tissues also increased with the highest level occurring in the fur > liver = kidney > brain > blood. Concentrations of Hg in the fur were correlated (r(2) > 0.97) with liver, kidney, blood and brain concentrations. The addition of Se to the mink diet did not appear to affect most tissue concentrations of total Hg nor did it affect the partitioning of Hg between the liver:blood, kidney:blood and brain:blood; however, partitioning of Hg between fur and blood was apparently affected.

Assessment of neurotoxic effects of mercury in beluga whales (Delphinapterus leucas), ringed seals (Pusa hispida), and polar bears (Ursus maritimus) from the Canadian Arctic

Marine mammals are indicator species of the Arctic ecosystem and an integral component of the traditional Inuit diet. The potential neurotoxic effects of increased mercury (Hg) in beluga whales (Delphinapterus leucas), ringed seals (Pusa hispida), and polar bears (Ursus maritimus) are not clear. We assessed the risk of Hg-associated neurotoxicity to these species by comparing their brain Hg concentrations with threshold concentrations for toxic endpoints detected in laboratory animals and field observations: clinical symptoms (>6.75 mg/kg wet weight (ww)), neuropathological signs (>4 mg/kg ww), neurochemical changes (>0.4 mg/kg ww), and neurobehavioral changes (>0.1mg/kg ww). The total Hg (THg) concentrations in the cerebellum and frontal lobe of ringed seals and polar bears were <0.5mg/kg ww, whereas the average concentration in beluga whale brain was >3mg/kg ww. Our results suggest that brain THg levels in polar bears are below levels that induce neurobehavioral effects as reported in the literature, while THg concentrations in ringed seals are within the range that elicit neurobehavioral effects and individual ringed seals exceed the threshold for neurochemical changes. The relatively high THg concentration in beluga whales exceeds all of the neurotoxicity thresholds assessed. High brain selenium (Se):Hg molar ratios were observed in all three species, suggesting that Se could protect the animals from Hg-associated neurotoxicity. This assessment was limited by several factors that influence neurotoxic effects in animals, including: animal species; form of Hg in the brain; and interactions with modifiers of Hg-associated toxicity, such as Se. Comparing brain Hg concentrations in wildlife with concentrations of appropriate laboratory studies can be used as a tool for risk characterization of the neurotoxic effects of Hg in Arctic marine mammals.

Hepatic polybrominated diphenyl ether (PBDE) levels in Wisconsin river otters (Lontra canadensis) and Michigan bald eagles (Haliaeetus leucocephalus)

Polybrominated diphenyl ethers (PBDEs) are persistent and toxic flame-retardant chemicals widespread in the Great Lakes ecosystem. These chemicals are now being regulated and phased-out of the region; therefore it remains important to understand the extent of contamination in order to track the efficacy of recent actions. Here, Σ₄PBDE congeners (PBDE-47, 99, 100, 153;wetweight basis unless indicated)were determined in liver tissues from Wisconsin river otters (Lontra canadensis; n = 35; 2009-2010) and Michigan bald eagles (Haliaeetus leucocephalus; n = 33; 2009-2011). In otters, Σ₄PBDE ranged from0.5 to 72.9 ng/g, with a mean (±SD) and median (25th-75th percentile inter-quartile range) of 16.3 ± 16.4 ng/g and 11.3 (5.6-18.9) ng/g, respectively. The mean lipid-adjusted Σ₄PBDE was 1377 ± 1485 ng/g. In eagles, Σ₄PBDE ranged from 0 to 1,538.8 ng/g, with a mean and median of 74.3 ± 266.7 ng/g and 21.2 (5.7-28.9) ng/g, respectively. The mean lipid-adjusted Σ₄PBDE was 5274.5 ± 19,896.1 ng/g. In both species, PBDE-47 accounted for >50% of the Σ₄PBDE, followed by PBDE-99 and PBDE-100 (each ~17-19% of the total). The PBDE levels reported here in otters are similar to mammalian wildlife elsewhere, though the levels in eagles are among the highest worldwide across studied birds. The findings indicate that apex Great Lakes wildlife remain exposed to appreciable levels of PBDEs and more work is needed to understand whether such exposures are associated with adverse health outcomes.

Methylmercury accumulation and elimination in mink (Neovison vison) hair and blood: results of a controlled feeding experiment using stable isotope tracers

Concentrations of metals in hair are used often to develop pharmacokinetic models for both animals and humans. Although data on uptake are available, elimination kinetics are less well understood; stable isotope tracers provide an excellent tool for measuring uptake and elimination kinetics. In the present study, methylmercury concentrations through time were measured in the hair and blood of mink (Neovison vison) during a controlled 60-d feeding experiment. Thirty-four mink were fed a standard fish-based diet for 14 d, at the end of which (day 0), 4 mink were sacrificed to determine baseline methylmercury (MeHg) concentrations. From day 0 to day 10, the remaining mink were fed a diet consisting of the base diet supplemented with 0.513 ± 0.013 µg Me(199) Hg/g and 0.163 ± 0.003 µg Me(201) Hg/g. From day 10 to day 60, mink were fed the base diet supplemented with 0.175 ± 0.024 µg Me(201) Hg/g. Animals were sacrificed periodically to determine accumulation of Me(201) Hg in blood and hair over the entire 60-d period and the elimination of Me(199) Hg over the last 50 d. Hair samples, collected from each mink and cut into 2.0-mm lengths, indicate that both isotopes of MeHg appeared in the hair closest to the skin at approximately day 10, with concentrations in the hair reaching steady state from day 39 onward. The elimination rate of Me(199) Hg from the blood was 0.05/d, and the ratio of MeHg in the hair to blood was 119. A large fraction of MeHg (22% to >100%) was stored in the hair, suggesting that in fur-bearing mammals the hair is a major route of elimination of MeHg from the body.

Ecogenetics of mercury: from genetic polymorphisms and epigenetics to risk assessment and decision-making

The risk assessment of mercury (Hg), in both humans and wildlife, is made challenging by great variability in exposure and health effects. Although disease risk arises following complex interactions between genetic ("nature") and environmental ("nurture") factors, most Hg studies thus far have focused solely on environmental factors. In recent years, ecogenetic-based studies have emerged and have started to document genetic and epigenetic factors that may indeed influence the toxicokinetics or toxicodynamics of Hg. The present study reviews these studies and discusses their utility in terms of Hg risk assessment, management, and policy and offers perspectives on fruitful areas for future research. In brief, epidemiological studies on populations exposed to inorganic Hg (e.g., dentists and miners) or methylmercury (e.g., fish consumers) are showing that polymorphisms in a number of environmentally responsive genes can explain variations in Hg biomarker values and health outcomes. Studies on mammals (wildlife, humans, rodents) are showing Hg exposures to be related to epigenetic marks such as DNA methylation. Such findings are beginning to increase understanding of the mechanisms of action of Hg, and in doing so they may help identify candidate biomarkers and pinpoint susceptible groups or life stages. Furthermore, they may help refine uncertainty factors and thus lead to more accurate risk assessments and improved decision-making.