Behavioral manifestations of prolonged lead exposure initiated at different stages of the life cycle: II. Delayed spatial alternation.

Since both Pb exposure and aging have been associated with alterations in memory functions, this study compared the effects of Pb exposure initiated at early, middle and later stages of the life cycle on delayed spatial alternation performance. Young (21 day old), adult (8 mon old) and old (16 mon old) rats were exposed for a total duration of 8.5 mon to 0, 2 or 10 mg Pb acetate/kg/day (young rats) or 0, 1.9 or 9.3 mg Pb acetate/kg/day (adult and old rats) in drinking water. These doses were designed to produce equivalent brain Pb concentrations across the three age groups. Behavioral testing began after 4 mon of exposure utilizing a standard delayed alternation paradigm with delay values of 0, 3, 6, 9 and 12 sec presented randomly during each experimental session for a total of 40 sessions. Aging, as expected, was associated with impairments of accuracy, but the disruption of accuracy at the 0 sec delay suggested that these were performance rather than memory deficits. Pb exposure actually improved delayed spatial alternation performance in both young and old animals: accuracy values were increased, while error frequencies declined in the two age groups. The increases in accuracy occurred primarily at the longest delay value in young rats, but were more prominent at short delay values in old rats. This difference may have been the result of a ceiling effect, since accuracy for young rats at short delay values was already quite high. Delayed spatial alternation performance of adult rats was generally unaffected by Pb exposure. A possible explanation of the improved performance of young and old Pb-exposed rats is one based on response perseveration, i.e., a facilitation of rote alternation behavior engendered by the cued alternation training program. The results of this study suggest that both early and late stages of the life cycle must be considered periods of enhanced vulnerability to Pb-induced behavioral changes.