Comparison of neurobehavioral changes in three inbred strains of mice prenatally exposed to methylmercury

Pregnant mice of three inbred strains (BALB/c, C57BL/6J, C57BL/6Cr) were orally given methylmercury (MMC; 3 x 3 mg/kg body weight) or the equivalent volume of phosphate-buffered saline during days 12-14 of gestation and allowed to deliver. The behaviors of their male offspring were evaluated in an open field and their home cage and in a Morris water maze. In the open field test, the BALB/c and C57BL/6Cr MMC groups exhibited less total locomotor activity than did their respective control groups. However, there was no significant difference observed between the MMC and control C57BL/6J strain. In the BALB/c strain, the MMC group exhibited significantly more central locomotion and significantly less peripheral locomotion than did the control group. These results indicated that the prenatal exposure to MMC caused decreases in open-field activity in the C57BL/6Cr and BALB/c strains, concomitantly with a change in emotional status in BALB/c strain. For spontaneous activity in their home cage, all groups moved more actively in the dark phase than in the light phase except BALB/c MMC group. The BALB/c MMC group moved in the light phase as much as in the dark phase, indicating a disturbance of nocturnal rhythm of spontaneous activity. In the Morris water maze, the C57BL/6Cr and C57BL/6J control groups perform very well over the 5 consecutive days. The prenatal exposure to MMC caused significantly prolonged latency in the C57BL/6Cr and C57BL/6J, but not in BALB/c strain. This result indicated that the prenatal exposure to MMC impaired the performance in the Morris water maze differently among the strains. This study provides a basis for evaluating strain-specific neurobehavioral changes when the widely used three inbred strains of mice are chronically exposed to MMC.