PRENATAL COEXPOSURE TO METALLIC MERCURY-VAPOR AND METHYLMERCURY PRODUCE INTERACTIVE BEHAVIORAL-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 me tallic mercury (Hg degrees) 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 degrees vapour by inhalation (MeHg + Hg degrees), or 4) were given co mbined vehicle administration for each of the two treatments (control) . The inhalation regimen corresponded to an approximate dose of 0.1 mg Hg degrees/kg/day. Clinical observations and developmental markers up to weaning showed no differences between any of the groups. Testing o f behavioural function was performed between 4 and 5 months of age and included spontaneous motor activity, spatial learning in a circular b ath, and instrumental maze learning for food reward. Offspring of dams exposed to Hg degrees showed hyperactivity in the motor activity test chambers over all three parameters: locomotion, rearing and total act ivity; this effect was potentiated in the animals of the MeHg + Hg deg rees group. In the swim maze test, the MeHg + Hg degrees and Hg degree s groups evidenced longer latencies to reach a submerged platform, whi ch they had learned to mount the day before, compared to either the co ntrol or MeHg groups. In the modified, enclosed radial arm maze, both the MeHg + Hg degrees and Hg degrees groups showed more ambulations an d rearings in the activity test prior to the learning test. During the learning trial, the same groups (i.e., MeHg + Hg degrees and Hg degre es) showed longer latencies and made more errors in acquiring all eigh t pellets. Generally, the results indicate that prenatal exposure to H g degrees causes alterations to both spontaneous and learned behaviour s, suggesting some deficit in adaptive functions. Coexposure to MeHg, which by itself did not alter these functions at the dose given in thi s study, served to significantly aggravate the changes.