Prenatal coexposure to metallic mercury vapour and methylmercury produce interactive behavioural changes in adult rats

Pregnant rats were 1) administered methyl mercury (MeHg) by gavage, 2 mg/kg/day during days 6–9 of gestation, 2) exposed by inhalation to metallic mercury (Hg °) vapour (1.8 mg/m 3 air for 1.5 h per day) during gestation days 14–19, 3) exposed to both MeHg by gavage and Hg ° vapour by inhalation (MeHg + Hg °), or 4) were given combined vehicle administration for each of the two treatments (control). The inhalation regimen corresponded to an approximate dose of 0.1 mg Hg °/kg/day. Clinical observations and developmental markers up to weaning showed no differences between any of the groups. Testing of behavioural function was performed between 4 and 5 months of age and included spontaneous motor activity, spatial learning in a circular bath, and instrumental maze learning for food reward. Offspring of dams exposed to Hg ° showed hyperactivity in the motor activity test chambers over all three parameters: locomotion, rearing and total activity; this effect was potentiated in the animals of the MeHg + Hg ° group. In the swim maze test, the MeHg + Hg ° and Hg ° groups evidenced longer latencies to reach a submerged platform, which they had learned to mount the day before, compared to either the control or MeHg groups. In the modified, enclosed radial arm maze, both the MeHg + Hg ° and Hg ° groups showed more ambulations and rearings in the activity test prior to the learning test. During the learning trial, the same groups (i.e., MeHg + Hg ° and Hg °) showed longer latencies and made more errors in acquiring all eight pellets. Generally, the results indicate that prenatal exposure to Hg ° causes alterations to both spontaneous and learned behaviours, suggesting some deficit in adaptive functions. Coexposure to MeHg, which by itself did not alter these functions at the dose given in this study, served to significantly aggravate the changes.