Lifetime low-level lead exposure produces deficits in delayed alternation in adult monkeys

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Cynomolgus monkeys ( Macaca fascicularis) were dosed continuously from birth onward with 100, 50, or 0 μg/kg/day of lead. This resulted in blood lead concentrations of 25, 15, or 3 μg/dl respectively before withdrawal of infant formula at 200 days of age. Blood lead concentrations declined thereafter over the next 100–150 days to steady-state concentrations of 13, 11, or 3 μg/dl. At seven to eight years of age, monkeys were tested on a delayed alternation task. The task required the monkey to alternate responses between two pushbuttons; each alternation was rewarded with a small amount of apple juice. After each monkey learned the task, a delay was instituted between trials. The initial delay was 100 msec, and was increased in steps to 15 sec by the end of the experiment. Treated monkeys were impaired in their ability to learn the alternation task, but were not different from controls at short delay values (1 and 3 sec). At longer delay values (5 and 15 sec), treated monkeys again exhibited impairment. At the 15 sec delay value, some individuals in both treated groups exhibited marked perseveration, responding on the same button in some instances for hours at a time. Treated monkeys were also more variable in their performance across sessions than were controls. The data are interpreted as indicative of spatial learning and short-term memory deficits in the lead-exposed monkeys.