Mercury's Scary Migration
Environment News Service
3:00 a.m. 10.Sep.99.PDT
Researchers say they have found the first evidence that mercury can circumvent
the blood-brain barrier that usually prevents toxins from entering the brain. Though the
studies involved fish, the findings have implications for humans, particularly children,
and for other species as well.
Scientists at Canada's Maurice Lamontagne Institute and the Swedish University
of Agricultural Sciences 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 brains of brown and rainbow trout.
The mercury can go directly to the brain, circumventing the blood-brain barrier
-- a nearly impermeable membrane that prevents most toxins from reaching the brain. The
researchers 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.
"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," says Claude Rouleau, Ph.D.,
a research scientist at Environment Canada's National Water Research Institute and the
study's primary investigator. Rouleau performed the research 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.
Mercury pollution in waterways comes from unsafe manufacturing processes, and
the combustion of fossil fuels that contain mercury. Natural sources such as the degassing
of the earth's crust, forest fires, the evaporation of seawater, and volcanoes also add
mercury to the environment. But an estimated two-thirds of environmental mercury is the
result of human activities.
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.
Exposure to mercury can damage the brain and nervous system, affecting language,
attention, and memory, particularly in children. The environmental group Clean Water
Action has calculated that the average mercury level in tuna is high enough that eating as
little as two ounces of tuna a week would be unsafe for a child weighing 35 pounds. In
May, the Environmental Working Group and Health Care Without Harm issued a report warning
pregnant mothers to avoid canned tuna due to mercury contamination risks.
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.
"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," says Rouleau. "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 says that while chemicals in the brains of such fish may not have direct
human implications, as most people do not eat fish brains, the survival of these species
does affect humans. More importantly, mercury may reach the brains of humans along similar
pathways.
Earlier research has shown that manganese, cadmium, and mercury can be taken
through the nose and mouth linings of rodents and transported to the brain through the
olfactory nerves.
"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."
Rouleau's research is published in the 1 October issue of Environmental Science
and Technology, a peer reviewed publication of the American Chemical Society.
Copyright 1999 Environment News Service (ENS).
