A likely placental barrier against methylmercury in pregnant rats exposed to fish-containing diets.

Methylmercury (MeHg) taken up through fish consumption can be transferred from the mother to the fetus during pregnancy. In the present study, pregnant rat mothers were contaminated with environmentally relevant doses of 36 and 76 ng MeHg/g of food using diets containing naturally mercury-containing fish. Young female rats fed with fish-containing food after weaning showed decreased locomotion in Y maze for accumulated concentrations in brain as low as 75 ng Hg/g dry weight (15 ng Hg/g wet weight). Young female rats fed the control diet after weaning yet borne by mothers fed the diet containing 76 ng MeHg/g, presented a 58% reduced activity in the open-field labyrinth, meaning that the maternal exposure to fish-containing food exerted an effect in utero that lasted several weeks after birth. Newborns were protected against Hg exposure by the placental barrier since in newborns from mothers fed the diet containing 76 ng MeHg/g of food, the concentrations of Hg in brain, kidney, liver and skeletal muscles represented 12, 3, 21 and 18% of those of their mother's tissues, respectively. These results suggest the existence, at least in rats, of a threshold level in terms of MeHg exposure above which the placental barrier collapses.