Effects of methylmercury on brain biogenic amines in the developing rat pup

Neonatal rat pups were dosed ip with methylmercury chloride, 5 mg/kg/day, on Days 5, 6, and 7 after birth. Peak brain concentrations of methylmercury were observed 1–3 days after the last injection. The t 1 2 of methylmercury in the brain was 6.6 days. This nonlethal dose produced an initial decrease (on Day 8) in brain 5-HT, 5-HIAA, NE, and DA in the rat pups. Subsequently, the concentrations of these brain constituents increased toward, and then exceeded, control values by Day 15. The initial decreases in the monoamine concentrations were associated with high amounts of methylmercury in the brain, while increased concentrations were seen when the amount of the poison was low. After Day 15 no consistent trends were observed in brain catecholamine concentrations after methylmercury treatment; large deviations from control were noted. Serotonin, on the other hand, continued to rise above control and remained elevated until the end of the experiment (Day 60). This monoamine was further studied with respect to early methylmercury exposure. Methylmercury-treated rat pups displayed decreases in brain tryptophan content, tryptophan hydroxylase activity, and serotonin turnover rate on Day 8 of the experiment. The rat pups matured normally, and no overt signs of the early poisoning were observed.