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April 13, 2009 -- Exposure to environmental toxins known as PCBs have long been linked with behavioral and developmental problems in children, but scientists could never say exactly how PCBs (polychlorinated biphenyls) might adversely affect the brain and lead to the problems.
Now, scientists think they may know.
The chemicals adversely affect the development of brain cells and also make brain circuits "overexcited," which has been linked in previous research to developmental problems, according to researcher Isaac N. Pessah, PhD, a professor of molecular biosciences and director of the University of California Davis Center for Children's Environmental Health.
"We think we have identified the way in which a broad class of environmental contaminants influences the developing nervous system and may contribute to neuro-developmental impairments such as hyperactivity, seizure disorders, and autism," says Pessah, a co-author on a trio of new studies examining the issue. The latest of the three is published online today in PLoS-Biology.
The findings of the three studies are called a "turning point" by another expert in the field.
One surprise finding: low levels of PCB exposure sometimes have greater ill effects than high-dose exposures.?
PCBs were widely used for years in many products such as electronic components, pesticides, caulking, and flame retardants, but their production was banned in the U.S. in 1979.
Even so, the chemicals persist in the environment because they don't break down easily, explaining why high levels of PCBs can still be detected in people and in animals. PCBs are found in air, water, soil, and contaminated foods such as fish.
In one of the three new studies, exposures to low doses of PCBs in animals hampered their ability to learn to swim a maze, a common test of animal learning.
The low PCB doses also adversely affected the plasticity of the animals' dendrites -- small projections branching out from the neurons or nerve cells that get signals from other cells in the body.
"This plasticity is very important for learning and memory," says study researcher Pamela Lein, PhD, associate professor of neurotoxicology at the UC Davis School of Veterinary Medicine.
The study was published in March in Environmental and Health Perspectives.
Problems in dendrite plasticity and growth have already been implicated in disorders such as autism, schizophrenia, and mental retardation, Lein says.
Lein and colleagues compared the effects of low-dose PCB exposure, high-dose exposure, and no exposure in three groups of rats that had been trained to swim and find an escape platform in a maze, and three groups not trained to swim the maze.
"The PCB treatments did affect the learning and memory," she says. "The effects were seen in the low-dose group but not the high-dose." Those in the low-dose trained group took longer to learn to swim and escape the maze, she says.
In the high-dose group, she says, the exposure may have triggered a compensatory mechanism that protected the brain cells from harm.