Blood and brain mercury levels after chronic gestational exposure to methylmercury in rats

Female rats were exposed to 0, 0.5, or 6 ppm Hg (as methylmercuric chloride, 10 rats/group) in drinking water. For half the rats, exposure began 4 weeks before mating and for the others, exposure began 7 weeks before mating. All mating was done with an unexposed male. Maternal exposure continued to post-natal day (PN) 16. Blood and whole-brain mercury concentrations were determined in pups on PN 0 (birth) and PN 21 (weaning). Maternal water consumption was monitored daily during gestation and lactation. Maternal water consumption increased 2- to 3-fold through gestation for all groups. Mercury levels in blood and brain were unrelated to the duration of exposure before mating, although reproductive success appeared to be so related. Mercury levels in both media were closely related to consumption during gestation, but apparently maternal exposure during lactation did not result in exposure to the nursing pups. Brain mercury in offspring decreased between birth and weaning from 0.49 to 0.045 ppm in the low-dose rats and from 9.8 to 0.53 ppm in the high-dose rats. The brain increased in weight only about 5.5-fold during this time, indicating that there was minimal mercury exposure and some net loss from brain during this period. Brain:blood ratios averaged about 0.14 at birth and 0.24 at weaning, suggesting differential loss from neural and non-neural tissue. These ratios are higher than those reported in studies using less chronic exposure conditions or with adult rats. Brain concentrations of mercury in females in the low-dose group were about 10-15% higher than those seen in their male siblings. At the higher dose, the males had slightly higher levels of mercury in the brain than did their female siblings at birth. The relationship between brain concentration (in ppm) and cumulative mercury consumption, also expressed on a ppm basis (cumulative mercury consumed divided by maternal body weight at parturition), was not linear but was well described by a power-function relationship: Hg = A*(cum exposure)(b) where the exponent, b, was 1.12 and 1.17 for blood and brain, respectively, at birth. This exponent was indistinguishable from 1.0 for both media at weaning, indicating that the relationship between exposure and blood and brain levels became linear.