Finding a Balance: When Mercury is Part of the Meal
August 22, 2012
by Robin Tricoles
Signs of exposure to methylmercury can be subtle. Or sometimes shockingly obvious. Much depends on where and when one encounters the substance; that is, in utero, in childhood, or in adulthood.
But subtle or not, methylmercury can permanently disturb neural development, motor circuitry, organ function—and even the aging process.
Methylmercury is a form of mercury which, in its elemental state, is the silvery liquid metal found in old thermometers and fluorescent lights. When mercury enters the food web by way of microorganisms in lake- and seabeds, mercury morphs into methylmercury, a chemical compound with a strong affinity for living tissue, including that of fish.
Methylmercury burst onto the world scene in the 1950’s and 60’s after it was dumped into waters of Minamata, a fishing village in Japan. Symptoms of exposure included muscle weakness, lack of coordination, and damage to hearing, speech, and vision. Initially, scientists didn’t know what was behind the onset of neurological and physiological problems in Minamata. Finally, investigators traced the symptoms to the ingestion of methylmercury lodged in the tissue of the area’s fish and shellfish, the region’s dietary staple.
Since then, regulations prohibiting release of mercury into waterways have been implemented. But combustion of coal and waste products containing mercury emits the heavy metal into the atmosphere where it falls back to Earth and into the waterways.
Chris Newland, an experimental psychologist who studies methylmercury’s cognitive effects on rats, says researchers don’t yet know with certainty how much methylmercury can be tolerated without adverse effects. However, the EPA has determined an acceptable level of intake that corresponds roughly to one can of tuna a week. What researchers do know: exposure to excess methylmercury during fetal development can be especially harmful to the developing brain.
Using animal models, Newland exposed groups of rats to different levels of methylmercury in utero, while a control group was not exposed. When the rats were a few months old, the researchers asked both groups to press one of two levers in order to receive a reward. And they did. But after the lever stopped dispensing a reward, most of the rats that were exposed to methylmercury couldn’t or wouldn’t press the second lever to get a reward. Yet, the rats that were not exposed to methylmercury, quickly stopped pressing the first lever and started pressing the alternative lever, and so received a reward for their effort.
“What’s important about this finding is that we couldn’t see any overt signs of exposure in the rats,” says Newland. “They looked healthy. You could see the exposure only in their behavior when they were asked to stop pressing one lever and start pressing another, called a 'spatial discrimination reversal.' So, what I think we’re seeing in these prenatally exposed animals is the result of damage of the prefrontal cortex during development.
This kind of damage to development can affect voluntary behavior and decision-making for the remainder of one’s life. That’s because this region of the brain is in charge of our executive function, a host of complex behaviors that includes long-term planning, working memory, and the impact of current behavior on future outcomes."
Newland and his colleagues are also finding that gestational exposure to methylmercury accelerates aging. Researchers asked rats exposed to methylmercury during gestation to press a lever nine times in four-second intervals for a span of 30 minutes while being rewarded for their efforts. “The exposed animals’ responding deteriorated,” says Newland. “They looked okay, they looked healthy. But their times dropped, then the reinforcement rate dropped, then the response rate dropped. They wound up in a downward spiral.” The mice’s deterioration looked much like that seen in elderly animals.
All of which again brings up the question of how much methylmercury is too much. Or put another way, from a public-health viewpoint, how much fish is safe to consume especially for women who are pregnant. After all, fish is a source of healthy protein, something Newland says most Americans don’t get enough of.
Meanwhile, Newland and his colleagues are trying to come up with answers to these questions as well as whether substances such as selenium and calcium-channel-blockers, drugs often used in cardiac care, can offer neuroprotection to those who want to include fish in their diet.
M. Christopher Newland, PhD, is a professor of psychology at Auburn University. Trained in behavior analysis, neurobiology and mathematics, Newland studies ways in which experimental psychology, especially the experimental analysis of behavior, can contribute to an understanding of how drugs and environmental contaminants influence development and aging. He serves on the FABBS Council of Representatives for the Association for Behavior Analysis International, one of FABBS Foundation’s 22 member scientific societies.