Sensitive periods for lead-induced behavioral impairment (nonspatial discrimination reversal) in monkeys

A total of 52 nursery-reared monkeys (Macaca fascicularis) were dosed orally with 1.5 mg/kg/day of lead on one of four dosing regimens (13 monkeys/group): Group 1, vehicle only; Group 2, dosed with lead continuously from birth; Group 3, dosed with lead from birth to 400 days of age and vehicle thereafter; and Group 4, dosed with vehicle from birth to 300 days of age and lead thereafter. This dosing regimen allowed evaluation of differential infant vulnerability as well as reversibility of the behavioral toxicity of lead. Blood lead concentrations averaged 3-6 micrograms/dl when monkeys were not being exposed to lead, 32-36 micrograms/dl when being dosed with lead and having access to infant formula, and 19-26 micrograms/dl during lead exposure after weaning from infant formula. When monkeys were 5-6 years old, they were tested on a series of nonspatial discrimination reversal tasks: form, form with irrelevant color cues, color with irrelevant form cues, and alternating form and color. Group 2 exhibited the greatest degree of impairment compared to controls. Group 4 also exhibited impaired performance, although less marked than that of Group 2. Group 3 was not impaired on this series of tasks. These results confirm findings observed in other monkeys exposed continuously to lead and suggest that while exposure beginning after infancy produces impairment, exposure during infancy as well exacerbates the effect.