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Mercury Toxicity in Polar Bears--implications for humans

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Old 03-31-2010, 05:35 PM   #1
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Default Mercury Toxicity in Polar Bears--implications for humans

...Drawing on research indicating that no adverse clinical symptoms were detected below a blood mercury level of 58 parts per billion (ppb), the US Environmental Protection Agency factored in a 10-fold margin of safety and recommended a blood mercury maximum of 5.8 ppb. The European Food Safety Authority, however, was less conservative and set the bar at 10 ppb, while Health Canada and the World Health Organization agreed that 20 ppb was safe for nonpregnant adults.

Yet these guidelines were put into effect without studying any neurochemical reactions at the cellular level, which is where scientists see the early signs of toxicity. Now Chan’s group is taking biochemical measurements of brain receptors and enzymes to study the close links between neuronal cell death and mercury uptake, and his team is producing some surprising—and unsettling—findings. “We see subtle changes in the brain before the onset of clinical outcomes,” says Chan.

In short, he and others are seeing biochemical changes in the brains of polar bears, mink, wild river otters, and other species. These biochemical changes could translate into physiological changes—such as defects in memory, language, attention, motor function and visual-spatial abilities—that often go unnoticed until it’s too late and the animal has suffered significant damage from mercury. As Chan’s group struggles to replicate and understand those changes, they are beginning to wonder: Is there no safe level of mercury exposure?...


....A freezer full of sliced-up seal brains, beluga brains and Inuit food samples. “In the old days, we always thought there was a margin of safety that when we were exposed to a lower level of contaminants that our body could handle that and there wouldn’t be clinical observations,” says Chan. “That threshold may not be real.” If his findings prove correct, the tens of thousands of people, whales, bears, and innumerable other species living with mercury in their brains below the supposed maximum may, in fact, be experiencing toxic effects from this low-level exposure. Chan and his former graduate student Niladri Basu had already shown that mercury disrupted GABAergic signaling, impaired components of the cholinergic system, and lowered the levels of a glutamate receptor called the N-methyl-d-aspartate receptor (NMdAR) in mink and numerous other species. In 2008, Chan also discovered a partial reason for why mercury was wreaking havoc on the brain. He showed in human neuroblastoma cells that the heavy metal bound and activated NMDARs—a commonly studied benchmark of toxicology testing that contributes to excitatory synaptic transmission and memory function—thereby causing neurons to start firing too often, and ultimately leading to cell death.1

Specific to polar bears, last year a team led by Basu found that NMDAR levels dropped when mercury levels rose in the brain-stem region of polar bears from East Greenland.2...

Krey has turned to another indicator, the enzyme monoamine oxidase (MAO), which Chan previously showed was less active in the face of low-dose mercury in many regions of the brains of both wild river otters and lab rats. Krey found this inverse relationship between the enzyme and the metal in the bear’s occipital cortex, which processes visual signals, but she has not yet seen this association anywhere else in the brain....In contrast, mercury concentrations in the bear’s liver can exceed 100 ppm—one of the highest known mercury levels among all organisms. And Chan’s group isn’t yet looking at the cellular damage mercury can cause in the liver.

...In a 2006 study of blood platelets from 127 fish-eating humans living along the Saint Lawrence River in Quebec, Chan found that people with more mercury in their diets had lower MAO levels, even though they had no overt symptoms of mercury poisoning4—suggesting that this low-level exposure might be impacting their brains by causing a decrease in MAO activity, which in turn affects the neurotransmitters. This work builds on previous studies showing that Aboriginal children and adults with elevated blood levels of mercury had impaired motor ability.


...


...In December, Silbergeld, together with her former postdoc Jennifer Nyland, reported that low concentrations of mercury in human blood cells affect immune function by disrupting cytokine signaling pathways—elevating the levels of pro-inflammatory cytokines and decreasing the release of anti-inflammatory cytokines.5 Those findings dovetail with Silbergeld’s previous whole organism studies with mice and humans that linked mercury with hyper-autoimmunity.


Sandy Suppa eases the bear head into the band saw “Somehow mercury is changing the threshold of activation for these auto-reactive cells so that less of a stimulus will put them over the edge,” says Nyland, now at University of South Carolina School of Medicine in Columbia. “The toxic effects of mercury on the immune system have been neglected, pure and simple,” adds Silbergeld.

What’s more, mercury’s neurotoxic and immunotoxic effects might be inextricably linked. Silbergeld has found evidence that mercury inhibits the migration of neurons in mouse brain cells by disrupting the cytokine-mediated communication between neurons and specialized immune cells in the brain called microglia. Chan’s lab is also now starting to investigate mercury’s effects on the brain’s immune system.



Read more: Against the Element - The Scientist - Magazine of the Life Sciences http://www.the-scientist.com/article...#ixzz0jnLdmYfL
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Old 03-31-2010, 07:55 PM   #2
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do you think anyone at the CDC or FDA reads this stuff? thimerasol is still used as preservative in flu shots. madelyn
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