Methylmercury inhibits dopaminergic function in rat pup synaptosomes in an age-dependent manner

Methylmercury (MeHg) is an environmental neurotoxicant that is especially harmful during brain development. Previously, we found greater sensitivity to MeHg-induced oxidative stress and greater loss of mitochondrial membrane potential in synaptosomes from early postnatal rats than in synaptosomes from older rat pups and adults. Here, we determine whether MeHg exposure also leads to greater changes in dopamine (DA) levels and dopamine transporter (DAT) function in synaptosomes from early postnatal rats. We report that MeHg exposure leads to DAT inhibition, and increases the levels of released DA compared to control; further, the effects are much greater in synaptosomes prepared from postnatal day (PND) 7 rats than in synaptosomes from PND 14 or PND 21 animals. In addition to the effects of MeHg in young rats, we observed age-dependent differences in dopaminergic function in unexposed synaptosomes: synaptosomal DA levels increased with age, whereas medium (released) DA levels were high at PND 7 and were lower in PND 14 and PND 21 synaptosomes. DAT activity increased slightly from PND 7 to PND 14 and then increased more strongly to PND 21, suggesting that higher DA release, in addition to the lower DAT activity seen in PND 7 animals, was responsible for the age differences in levels of released DA. These results demonstrate that MeHg affects the dopaminergic system during early development; it thus may contribute to the neurobehavioral effects seen in MeHg-exposed children.