30 June 2011

BPA exposure makes male animals "less desirable"

by Kate Melville

New experiments with the ubiquitous plastic chemical bisphenol A (BPA) show that it causes male deer mice to become demasculinized and behave more like females, leading the researchers to posit that daily exposure in the developed world could be reducing men's reproductive fitness.

BPA is an estrogen-like chemical that is a common component of plastics used to contain food. It is also used in baby bottles, refillable water bottles and the linings of metal food cans.

Findings from the experiments, carried out by researchers from the University of Missouri, are set to be published in the Proceedings of the National Academy of Sciences.

"The BPA-exposed deer mice in our study look normal; there is nothing obviously wrong with them. Yet, they are clearly different," said researcher Cheryl Rosenfeld. "Females do not want to mate with BPA-exposed male deer mice, and BPA-exposed males perform worse on spatial navigation tasks that assess their ability to find female partners in the wild. This study sets the stage for BPA researchers to examine how BPA might differentially impact the behavioral and cognitive patterns of boys versus girls. Investigators looking for obvious BPA-induced differences, such as chromosome deletions or DNA mutations, could be missing subtle behavioral differences that eventually lead to long-term adverse outcomes, including demasculinization of male behaviors with ensuing decreased reproductive fitness."

For the study, female deer mice were fed BPA-supplemented diets two weeks prior to breeding and throughout lactation. The mothers were given a dosage equivalent to what the FDA considers a non-toxic dose and safe for mothers to ingest. At weaning (25 days), the deer mice offspring were placed on a non-supplemented BPA diet and their behavior tested when they matured into adults.

The researchers tested each mouse's ability to navigate a maze - an ability that allows them to find mates that are dispersed throughout the environment. Rosenfeld said that many of the male mice that had been exposed to BPA early in their development never found the correct route. By comparison, male mice that had not been exposed to BPA consistently found the exit leading to their home cage. In addition, the untreated mice quickly learned the most direct approach to finding the correct route, while the exposed males appeared to employ a random, inefficient trial and error strategy.

Interestingly, the male deer mice exposed to BPA were less desirable to female deer mice. Females primed to breed were tested in a so-called mate choice experiment. The females' level of interest in a stranger male was measured by observing specific preferential behaviors, such as nose-to-nose sniffing and the amount of time the female spent evaluating her potential partner. These behaviors assess a potential mate's genetic fitness. Rosenfeld said that both non-exposed and BPA-exposed females favored control males over BPA-exposed males on a two-to-one basis.

"These findings presumably have broad implications to other species, including humans, where there are also innate differences between males and females in cognitive and behavioral patterns," Rosenfeld said. "In the wide scheme of things, these behavioral deficits could, in the long term, undermine the ability of a species such as the deer mouse to reproduce in the wild. Whether there are comparable health threats to humans remains unclear, but there clearly must be a concern."

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Source: University of Missouri-Columbia