Central Nervous Effects of Perinatal Exposure to Lead or Methylmercury in the Monkey

Abstract

The behavioral consequences of chronic exposure to lead or methylmercury have been studied in monkeys (Macaca fascicularis). Monkeys exposed to lead from birth at a dose resulting in a blood lead level of 55 μg/dl preweaning and a steady state level of 33 μg/dl after weaning at 200 days of age produced impairment on a discrimination reversal task, as well as clear differences from control on an intermittent schedule of reinforcement, the fixed interval. Lead exposures to doses that produce blood levels in the range of 12–22 μg/dl results in performance outside of control range on these tasks for certain individuals. Monkeys exposed to methylmercury from birth at a dose resulting in a peak blood level of 1.2 ppm preweaning and .6–.9 ppm postweaning produced dificits in spatial visual function under conditions of both high and low luminance, while temporal visual function was superior in treated monkeys. Exposure to this same dose in utero plus postnatally resulted in two monkeys out of five exhibiting clinical signs, with monkeys exposed to half that dose apparently normal. Blood mercury levels of the infants at birth were considerably higher than those of the mother, and decreased to levels well below those of the mother over the course of several months. This was true despite the fact that infants were dosed from birth with the same dose the mothers had received. These monkeys were not severely impaired relative to controls on a series of discrimination reversal tasks, although some individuals failed to acquire the lever press response, so an easier operant had to be employed.