Coordination Deficits Induced in Young Adult Mice Treated with Methylmercury

Male and female C57BL/6J mice starting at postnatal (P) day 34 were exposed orally to five divided doses totaling 1.0 or 5.0 mg/kg of methylmercury (MeHg; given as methylmercuric chloride) or sterile deionized water in moistened rodent chow. After a 5-day waiting period, control and MeHg-treated mice were subjected to a standard battery of behavior tests for balance and motor coordination. Latency to falling on the accelerating rota-rod was significantly decreased in 5.0 mg/kg MeHg-exposed mice when compared to control mice. In the open field, horizontal exploration with respect to total distance traveled during the first 5 min on the first test day was significantly reduced in 1.0 mg/kg MeHg-exposed mice when compared to control mice. Rearing activity was not affected by MeHg treatment. In the footprint analysis, angle of foot placement measured in 1.0 mg/kg MeHg-treated mice was significantly greater compared to control mice. Base stance and stride length were unaffected by MeHg treatment. On the vertical pole test, 10 mice from each treatment group fell off the pole during the time the pole was shifted from a horizontal position to a vertical position, whereas none of the control mice fell. These results indicate that short-term, low to moderate doses of MeHg in young adult mice can be detrimental to motor coordination and balance.