Relative Uptake of Methylmercury was Higher in the Brain of Fetal and Neonate than Weanling and Adult Rats

Abstract

Fetuses and neonates are known to be highly susceptible to methylmercury (MeHg) toxicity. However, little is known about the relative uptake of MeHg from blood to the developing brain compared to adults. We measured time-course changes in MeHg concentrations in the brain of fetal, neonate, weanling, and adult rats following a single MeHg administration. In the experiment at late gestation, MeHg (0.08 mg Hg/kg) was subcutaneously injected to dams on embryonic days 17, 18, 18.5, 19, 19.5, and 20, and the dams and fetuses were dissected on embryonic day 21 (1 day before parturition). Hg concentrations were measured 1, 1.5, 2, 2.5, 3, and 4 days after the injection. Brain Hg levels in fetuses peaked 2 days after the injection and were approximately 1.5 times higher than levels in dams during that period. In the experiment at postnatal periods, the same dose of MeHg was injected subcutaneously to male pups on postnatal day 1 (neonate), 35 (weanling), and 56 (adult). The rats were then sacrificed 1, 2, 3, 4, 5, and 6 days after injection. Brain Hg levels peaked most rapidly in neonates, and levels were approximately 1.5 times higher than levels in weanling or adult rats. Throughout the experiment of the pregnancy and postnatal periods, the Hg level in the blood and Hg brain/blood ratio 24 h after MeHg injection were highest in fetuses, followed by neonates, and decreased with life stage. These results indicate that relative higher brain uptake of MeHg is an important factor for increased vulnerability of fetuses and neonates to MeHg poisoning.