Researchers say they have found the first evidence that mercury can circumvent the blood-brain barrier that usually prevents such toxins from entering the brain. Their studies were with brown and rainbow trout - two of the most popular species for anglers and fish consumers - but may have implications for humans and other species as well, they say. The study was carried out by researchers at Canada's Maurice Lamontagne Institute and the Swedish University of Agricultural Sciences and is published in the October 1 issue of Environmental Science and Technology, a peer-reviewed publication of the American Chemical Society, the world's largest scientific society.
The researchers found that mercury dissolved in lake and river water can enter the nerves that connect water-exposed sensory receptors (for odor, taste, vibration and touch) to the fishes' brains. It can go directly to the brain, they say, circumventing the blood-brain barrier, a nearly impermeable membrane that prevents most toxins from reaching the brain. They also say this is the first study concerning mercury levels in fish brains (as opposed to levels accumulated in other body areas) and the first time it has been established that mercury can enter fish brains through sensory receptors and their connected nerves.
Mercury's toxic effects on fish and human brains are well established. Fish depend on their nervous systems to find food, communicate, migrate, orient themselves and recognize predators. Dissolved mercury usually is taken in by fish through their gills and dispersed by blood as it circulates through the body. In most cases, little mercury accumulates in the brain, which is protected by the blood-brain barrier. However, mercury that does accumulate, having passed through the bloodstream or through nerves, is concentrated in specific sites connected to primary sensory nerves critical to the function of the nervous system.
"Considering the importance of complex behavior in the life of fish, and the well-known deleterious effects of mercury upon the nervous system, the toxicological significance of this uptake route needs to be assessed," said Claude Rouleau, Ph.D., a research scientist at Environment Canada's National Water Research Institute and the study's primary investigator (Rouleau did the work at the Swedish University of Agricultural Sciences, Uppsala, and completed it for publication while at the Maurice Lamontagne Institute-Department of Fisheries and Oceans Canada, in Mont-Joli, Quebec). "The accumulation of mercury or other toxic chemicals in the brain via water-exposed nerve terminals may result in an alteration of these functions and jeopardize fish survival. We believe that uptake of metals such as mercury and the subsequent transport along sensory nerves is a process common to all fish species, and in this respect, it is possible that other toxins (such as pesticides) also could reach fish brains in this way and this is a subject worthy of further study."
Rouleau also said that while chemicals in the brains of such fish may not have direct human implications (people generally don't eat fish brains), the survival of these species does affect humans. "However, the fact that mercury is transported along fish nerves can be extrapolated to humans, as nerve transport also occurs in mammals, including humans," said Rouleau. "Thus, mercury and other toxins could possibly accumulate in human brains via nerve transport." Earlier research has shown that manganese, cadmium and mercury can be taken through the nasal mucosa of rodents and transported to the brain through the olfactory nerves.
The study's other main investigator was Professor Hans Tjalve of the Swedish University of Agricultural Sciences.
The accumulated mercury was located by whole-body autoradiography (used by the pharmaceutical industry to see how drugs are distributed throughout the body). Fish were exposed to radioactive mercury, frozen, then cut into very thin slices. The slices were exposed to X-ray film for varying amounts of time - a few weeks to a few months.
The film blackened only in areas where the radioactive metal was present. The method is particularly useful for obtaining information on fragile organs or tissues, such as fish brains.
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