Neurobehavioral effects of postnatal exposure to low-level mercury vapor and/or methylmercury in mice.

This study examined the effects on neurobehavioral function of exposure to low-level mercury vapor (Hg ⁰ ), methylmercury (MeHg) in female mice and the combination of Hg ⁰ and MeHg during postnatal development. Postnatal mice were exposed to Hg ⁰ at a mean concentration of 0.188 mg/m ³ Hg ⁰ and supplied with food containing 3.85 μg/g of MeHg from day 2 to day 28 after delivery. The combined exposure group was exposed to both Hg ⁰ and MeHg, using the same procedure. When their offspring reached the age of 11 weeks, behavioral analyses were performed. The behavioral effects in mice were evaluated based on locomotive activity and rate of center entries in the open field (OPF), learning activity in the passive avoidance response (PA) and spatial learning ability in the radial maze (RM). Total locomotive activity in the OPF significantly decreased in the Hg ⁰ , MeHg and combined exposure groups compared with the control group. The proportion of entries to central area in the OPF was significantly higher in the combined exposure group than in the control group, while those in the Hg ⁰ or MeHg exposure group did not differ from the control group. Other behavioral tests did not reveal significant differences among the groups. Behavioral anomalies were more distinctive after combined exposure compared to Hg ⁰ or MeHg exposure alone. The brain Hg concentration of offspring, immediately after exposure, was highest in the combined exposure group, exceeding 2 μg/g, followed by the MeHg and Hg ⁰ exposure groups. Thus, the enhancement of neurobehavioral effects in the combined exposure group was associated with higher brain mercury concentration.