Prenatal methyl mercury exposure: I. Alterations in neonatal activity.

Pregnant Long-Evans rats were intubated with 0, 5, or 8 mg/kg of methyl mercury on day 8 or 15 of gestation. Maternal weight gain during gestation was reduced significantly only in those animals that had received 8 mg/kg on day 8 of gestation and had resorbed their litters. In litters that were delivered, the mercury treatment did not affect litter size or weight gain of the pups in the preweaning period. Methyl mercury content of the 1 day old rat brains was directly related to both the dose and time of treatment. Neonatal activity was significantly elevated on postnatal day 4 in rats treated with 5 mg/kg on day 8 of gestation, on postnatal day 8 in the group that received 8 mg/kg on day 8 of gestation, and on postnatal day 8 and 15 in the group that received 5 mg/kg on day 15 of gestation. The results indicate that neonatal activity measures can be used as sensitive indicators of low prenatal neurotoxic exposures.