September 29, 2008

Bisphenol A Blocks Growth of Brain Connections in Monkeys

Photo of a woman drinking from a hard plastic water bottle

Bisphenol A (BPA), a chemical widely used to make plastic food containers, can prevent connections from forming between nerve cells in the brains of monkeys, a new study suggests. Although similar findings have been seen in studies of rats and mice, this is the first to show that BPA may impair brain function in nonhuman primates. The results add to growing concerns about how widespread exposure to BPA may affect human health.

BPA has been used since the 1950s to manufacture a hard, often clear plastic called polycarbonate. Today a wide range of items—including water cooler bottles, baby bottles, plastic coatings inside food cans, dental sealants and medical devices—are made with BPA.

Many studies have shown that trace amounts of BPA can leach out of containers into foods. But it's unclear how this exposure affects the human body. Dozens of animal studies, mostly in rodents, have shown that BPA can influence behavior and harm the development of the brain and reproductive organs. But rodents differ from humans and other primates in how they process BPA and how hormones affect brain development.

In the new study, published in the September 16, 2008, issue of the Proceedings of the National Academy of Sciences, Dr. Csaba Leranth of Yale University School of Medicine and his colleagues looked at how BPA might affect estrogen activity and the formation of nerve cell connections, or synapses, in the brains of nonhuman primates. Their research was funded in part by NIH's National Institute of Environmental Health Sciences (NIEHS).

The researchers studied female African green monkeys that lacked ovaries and so produced little estrogen. Some of the animals received infusions of estradiol, a major form of the hormonal estrogen that promotes synapse growth in the brain. To mimic the effects of constant human exposure to BPA, some of the animals also received continuous infusions of BPA via an under-the-skin pump for nearly a month. The daily dose of BPA, adjusted for weight (50 μg/kg), was at the level considered safe for human consumption by the U.S. Environmental Protection Agency.

Compared to a control group that received no active compounds, animals given estradiol alone showed significant synapse formation in the brain's hippocampus and prefrontal cortex. But these estradiol-induced neuron connections did not form in monkeys that also received BPA infusions. Essentially, BPA appeared to block the synapse-forming effects of estradiol in those 2 brain regions, which both contribute to mood, learning and memory.

One limitation of the study is that BPA was infused beneath the skin and not given orally, which is the most common route of human exposure. The researchers note that more studies are needed to clarify the effects of BPA, especially considering the widespread use of the compound.

Earlier this month, the U.S. National Toxicology Program, headquartered at NIEHS, reported the results of its 2-year review of available data on BPA. The report cited “some concern” for the effects of BPA on development of the brain and prostate gland and for behavioral effects following fetal or childhood exposure to BPA. The review authors also called for additional studies help clarify how BPA exposure affects human health.

—by Vicki Contie

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