NEW YORK, Sep 21 (Reuters Health) -- Despite its inability to pass through the brain's protective barrier by way of the blood circulation, studies in fish suggest that mercury, which is toxic to brain cells, can travel directly to the brain through nerves.
According to the research team, this is the first study to establish that mercury can enter the brain via nerves. The finding "can be extrapolated to humans, as nerve transport also occurs in mammals, including humans," said Dr. Claude Rouleau of the National Water Research Institute in Burlington, Ontario, Canada in a statement issued by the American Chemical Society. Rouleau headed a multinational team whose research results are published in the October 1st issue of Environmental Science and Technology.
The investigators exposed brown trout and rainbow trout to mercury by adding it to the water surrounding them. The researchers also injected fish with solutions containing mercury, then looked for mercury in their bodies using a technique called whole-body autoradiography.
Among fish exposed to mercury in the water, mercury accumulated in specific areas of their brains associated with nerves that make direct contact with the water, the results indicate. Such nerves include those associated with the sensation of smell, taste, balance, and vibration.
Fish injected with mercury solutions showed no accumulation of mercury in their brains, the investigators report, indicating that mercury cannot traverse the blood-brain barrier, a protective wall between the blood vessels and the brain.
Having previously reported similar findings with other chemicals, such as cadmium and manganese, the scientists surmise that waterborne mercury entered the fish's nerve endings and traveled along the nerves to the brain. According to the report, this is the first time mercury has been demonstrated to enter fish brains directly through their sensory receptors and connecting nerves.
The investigators speculate that this form of transport could also lead to the accumulation of other toxins, such as insecticides, that would not otherwise breach the blood-brain barrier.
"Thus, mercury and other toxins could possibly accumulate in human brains via nerve transport," said Rouleau in the news release, citing similarities between fish and mammals.
The team concludes that further study is needed to determine the role this route plays in mercury's effects on the brain.
