Web edition: June 28, 2011
Print edition: July 30, 2011; Vol.180 #3 (p. 16)
An ingredient in many clear plastics also renders some gender-linked behaviors plastic, at least in mice. Two new studies link feminized behaviors in adult males with exposures during development to bisphenol A, a weak estrogen-mimicking chemical. In one study, some behaviors in BPA-exposed females morphed into features characteristic of males.
The findings come from laboratory studies conducted in different species. Each experiment also exposed animals at a different time during development — one from the womb through weaning, the other during the rodent equivalent of adolescence and early adulthood. The trials therefore identify different periods during which the brain appears vulnerable to pollutants that mimic or alter the activity of sex hormones.
Because early BPA exposures left no lasting changes in sex hormone levels, the authors of each study note, the behavioral changes they observed in adulthood probably trace to an earlier rewiring of brain circuitry — most likely in an area known as the hippocampus.
Cheryl Rosenfeld of the University of Missouri–Columbia and her colleagues added BPA to the chow they fed to pregnant deer mice. BPA concentrations in the moms peaked at around 9 nanograms per milliliter, Rosenfeld says, “which is in the range of what’s been measured in humans.”
Tests in offspring of the treated animals evaluated spatial navigational abilities — something at which males normally excel — and susceptibility to fear or anxiety in novel or unprotected spaces. (In contrast to males, females typically exhibit a hesitancy to explore.) Spatial learning abilities and exploratory behaviors were severely compromised in males that encountered early BPA doses, relative to unexposed males, Rosenfeld’s team reports online June 27 in the Proceedings of the National Academy of Sciences.
For instance, a male released into a pen with many portals will seek the exit to its own cage. Unexposed males quickly learned how to find their exit and made a beeline to it. BPA-exposed males, by contrast, tended to take random paths, in trial after trial, or serially visit all exits — often without committing to any.
In this study, BPA did not affect female behavior. However, all females found BPA-exposed males substantially less attractive. Rosenfeld is currently investigating what cues, such as scents or behaviors, the BPA-exposed males might be giving off.
“Males exposed to BPA in the womb are less able to learn and remember territory-based tasks,” observes Laura Vandenberg of Tufts University in Medford, Mass., who did not take part in the research. Because male mice need to explore large areas in search of females, she notes, BPA might jeopardize a guy’s ability to find a mate. And that gals prefer unexposed males over those exposed to BPA in the womb “is shocking,” she says, but provides further evidence that BPA “can interfere with the mating process.”
In a second study, researchers at Zhejiang Normal University in Jinhua, China, detail impaired spatial navigational abilities and exploratory behaviors in standard male lab mice that had been exposed to low levels of BPA beginning in adolescence. These later exposures also affected females, the researchers report online in Neuropharmacology. For instance, emboldened BPA-exposed females were willing to explore open areas as males typically would.
“These novel findings point to the fact that an early environmental exposure can manifest itself later in life,” says Dana Dolinoy of the University of Michigan in Ann Arbor. “A likely mechanism,” she says, “is epigenetics” — whereby some event leads genes to acquire a chemical appendage that alters their activity. Such changes can easily occur during development, she notes, yet remain stable across time.
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