Neurobehavioral changes in metallothionein-null mice prenatal and postnatal exposed to mercury vapor.

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We studied the neurobehavioral effects of prenatal and postnatal exposure of MT-null and wild-type mice to elemental mercury vapor (Hg^0). Pregnant mice of both strains were repeatedly exposed to Hg^0 vapor at 0.50 and 0.56 mg/m^3 for 6 h per day until the 18th day of gestation. The behavioral effects were evaluated with locomotor activity in the open field (OFT), learning ability in the passive avoidance response (PA) and spatial learning ability in the Morris water maze at 12 weeks of age. Hg^0-exposed MT-null mice showed a significant decrease in total locomotor activity in males, and a learning disability in the passive avoidance response and a retarded acquisition in the Morris water maze in females as compared with the control. In contrast, Hg^0-exposed wild-type mice did not differ from controls in three behavioral measurements.Next, we studied the neurobehavioral effects of postnatal exposure of MT-null and wild-type mice to elemental mercury vapor (Hg^0). After parturition, the dam and neonates were exposed to Hg^0 vapor at 0.023 and 0.043 mg/m^3 for 6 h per day until weanling and the behavioral effects of evaluation was carried out at 12weeks of age. In OFT, Hg^0-exposed MT-null mice showed a significant decrease in total locomotor activity in females. However, the learning ability in the PA test and spatial learning ability was not affected the exposed mice of both strains. The results indicate that MT-null mice in females would be more susceptible than wild-type mice to the behavioral neurotoxicity of prenatal and postnatal Hg^0 exposure. Mercury concentrations in the brain of both strains in females were slightly higher in the exposed group than in the control group, indicating the retention of residual mercury even 12 weeks after the cessation of the exposure. From these results, we suggest that the increased susceptibility of MT-null females to behavioral changes caused by prenatal and postnatal Hg^0 exposure.