Prolonged Behavioral Effects of in Utero Exposure to Lead or Methyl Mercury: Reduced Sensitivity to Changes in Reinforcement Contingencies during Behavioral Transitions and in Steady State

Abstract

Postnatal exposure to lead or methyl mercury results in mental retardation, learning deficits, and other neurobehavioral effects in humans, and adverse consequences of prenatal exposure have been clearly documented with methyl mercury. To examine the developmental neurotoxicity of these metals, especially lead, concurrent schedules of food reinforcement were used to identify learning deficits in squirrel monkeys exposed during gestation to either methyl mercury or lead. Pregnant squirrel monkeys were administered methyl mercury (0.7 to 0.9 ppm in maternal blood) or lead (21 to 79 μg/dl in maternal blood) during the last half to two-thirds of gestation. At about 5-6 years of age, offspring were trained to lever press under concurrent schedules of reinforcement in which separate random interval reinforcement schedules operated independently on two levers. Reinforcement densities were varied such that 20 to 90% of the reinforcers were programmed to derive from the left lever (i.e., one lever was "richer" than the other). At steady state, the behavior of the controls was sensitive to reinforcement density and showed little lever bias, but the behavior of monkeys exposed to more than 40 μg/dl of lead and to methyl mercury was less sensitive to reinforcement rates and heavily biased. When relative reinforcement density on a lever changed, the unexposed animals′ response rates gradually shifted to the newly rich lever. The behavior of monkeys exposed to methyl mercury or more than 40 μg/dl of lead changed slowly, not at all, or in the wrong direction. Steady-state behavior of monkeys exposed to less than 40 μg/dl resembled controls, but acquisition progressed more slowly and required 2-4 times as many reinforcers to complete. These effects suggest a behavioral mechanism-insensitivity to changing reinforcement contingencies-by which learning deficits and behavioral changes associated with these metals might be related to toxicant exposure. Since maternal blood levels corresponded to those that could be experienced in occupational settings, the present data raise the possibility of fetal hazards associated with maternal lead exposures at levels tolerated in humans in occupational settings.