Chronic, low-dose prenatal exposure to methylmercury impairs motor and mnemonic function in adult C57/B6 mice

Methylmercury (MeHg) has cytotoxic effects on animals and humans, and a major target organ for MeHg is the central nervous system (CNS). It is well known that the developing CNS is extremely vulnerable to MeHg-induced changes in comparison to the mature brain. Most studies have concentrated on the direct effects of high levels of prenatal MeHg exposure. Surprisingly, behavioral outcomes found in adult offspring exposed developmentally to the neurotoxic effects of chronic, low-dose mercury more akin to ingestion in humans are not well characterized. The objective of this study was to determine whether such exposure produces deleterious effects on behavior in adult mice, including motor/coordination abilities, overall activity and mnemonic function. Developing mouse fetuses were exposed in utero during gestational days 8-18 by giving pregnant C57Bl/6J female mice food containing MeHg at a daily dose of 0.01 mg/kg body weight. Adult mice prenatally exposed to MeHg exhibited significant deficits in motor abilities, coordination, and overall activity, as measured by rotarod, footprint analysis and open field. In addition, MeHg-exposed mice were impaired with respect to reference memory but not in a visible, cued version of the Morris water maze task. These results indicate that prenatal exposure to the lowest dose of MeHg examined to date can have long-lasting motor and cognitive consequences on adult offspring. These findings have far reaching implications related to putative safe levels of MeHg ingestion, particularly during pregnancy, and increasing rates of cognitive and psychological disorders (e.g. attention hyperactivity deficit disorder, autism) in our society.