Methylmercury Impairs Components of the Cholinergic System in Captive Mink (Mustela vison)

Abstract

The effects of methylmercury (MeHg) on components of the cholinergic system were evaluated in captive mink (Mustela vison). Cholinergic parameters were measured in brain regions (occipital cortex, cerebellum, brain stem, basal ganglia) and blood (whole blood, plasma, serum) following an 89-day exposure to MeHg at dietary concentrations of 0, 0.1, 0.5, 1, and 2 ppm (n = 12 animals per treatment). There were no effects of MeHg on brain choline acetyltransferase, acetylcholine, and choline transporter. However, significantly higher densities of muscarinic cholinergic receptors, as assessed by 3H-quinuclidinyl benzilate binding, were measured in the occipital cortex (30.2 and 39.0% higher in the 1 and 2 ppm groups, respectively), basal ganglia (67.5 and 69.1% higher in the 0.5 and 1 ppm groups, respectively), and brain stem (64.4% higher in the 0.5 ppm group), compared to nonexposed controls. The calculated positive relationship between MeHg exposure and muscarinic cholinergic receptor levels in this dosing study were consistent with observations in wild mink. There were no MeHg-related effects on blood cholinesterase (ChE) activity, but ChE activity was significantly higher in the occipital cortex (17.0% in the 1 ppm group) and basal ganglia (34.1% in the 0.5 ppm group), compared to nonexposed controls. The parallel increases in muscarinic cholinergic receptor levels and ChE activity following MeHg exposure highlight the autoregulatory nature of cholinergic neurotransmission. In conclusion, these laboratory data support findings from wild mink and demonstrate that ecologically relevant exposures to MeHg (i.e., 0.5 ppm in diet) have the potential to alter the cholinergic system in specific brain regions.

What are the toxicological effects of mercury in Arctic biota?

“…In these polar bear tissues, concentrations of several chlorinated and brominated organic chemicals were also measured; however, statistical analyses showed that these were not correlated with any of the neurochemical biomarkers (Basu et al, 2009). In a captive mink study, ingestion of food containing MeHg levels as low as 0.1 ppm (corresponding to brain Hg concentrations ranging from 1 to 2.2 μg/g ww) was linked to decreased on previous studies on fish-eating mammals (Basu et al, 2006b(Basu et al, , 2007b) and birds (Scheuhammer et al, 2008). For detailed data see Table S1.…”

“…For example, changes in the levels of muscarinic cholinergic receptors (increased) and N-methyl-D-aspartate (NMDA) glutamate receptors (decreased) were related to concentrations of brain Hg in wild mink (Basu et al, 2005(Basu et al, , 2007b, common eagles, and bald eagles (Scheuhammer et al, 2008). Several of these neurochemical effects have been substantiated in laboratory studies involving captive mink experimentally fed environmentally realistic MeHg doses (Basu et al, 2006b(Basu et al, , 2007b. These results suggest that Hg at ecologically relevant levels may be exerting subtle, sub-clinical neurological changes in the 3 to 5 μg/g dw range (dry weight concentration in brain tissue) in several fish-eating wildlife species.…”

Gold Mining and Mercury Bioaccumulation in a Floodplain Lake and Main Channel of the Tambopata River, Perú

“…There are important implications for the endangered giant otter; at least one of the otter's preferred prey species, huasaco, had a high MeHg body burden. Five of the eight species from the main channel (63%) had MeHg concentrations greater than those known to cause neurological changes in the brain of the American mink, a species from the same family as giant otter [17]. Giant otters often use oxbow lakes as core areas within their home ranges, so it is possible that the high sediment organic matter and lower Hg levels found in fish in protected lakes where gold mining does not occur could partially protect them from greater MeHg exposure.…”

“…While very little is known about the impacts of MeHg accumulation on giant otters of South America, studies indicate that mammalian wildlife and humans respond to MeHg in a comparable manner [14]. Furthermore, other members of the family Mustelidae including otter species in North America and Europe (Lutra canadensis, Lutra lutra) and American mink (Mustela vison) are highly sensitive to Hg, with neurochemical changes in the brain occurring at MeHg dietary concentrations of 0.5 mg/kg [15][16][17].…”

The role of metallothioneins, selenium and transfer to offspring in mercury detoxification in Franciscana dolphins ( Pontoporia blainvillei )

“…Sensitive fetuses and calves exposed to Hg could lead to neurodegenerative diseases or dysfunction of the central nervous system followed by premature death (Gaeta and Hider, 2005). Basu et al (2006) showed that subclinical neurological changes were observed from 0.78 to 1.3 mg·kg −1 (w.w.) Hg in mink (Mustela vison) brain.…”