Postnatal lead exposure and the cholinergic system

Previous reports have suggested that the behavioral effects of early lead (Pb) exposure may be due to an underlying deficiency in cholinergic function. To further examine this possibility, Long-Evans hooded rat pups were exposed to Pb for the first 25 postnatal days via the maternal milk. Dams were fed either 4.0% PbCO3 (High Pb), 0.4% PbCO3 (low Pb) or a Na2CO3 control diet throughout this period. Beginning at 65 days of age, animals were tested on behavioral tasks sensitive to both Pb exposure and cholinergic deficiency. Exposure to both levels of Pb impaired passive avoidance acquisition and produced lower rates of spontaneous alternation. Pb, however, had no clear effects on open field activity. The cholinergic agonist physostigmine (0.05 and 0.075 mg/kg) did not affect the behavior of control animals on any task, but in both Pb exposed groups physostigmine improved passive avoidance acquisition, increased the rate of spontaneous alternation and lowered open field activity scores. The cholinergic antagonist scopolamine (0.4 mg/kg) impaired passive avoidance acquisition, lowered the rate of spontaneous alternation and increased open field activity scores in control animals. Consistent with behavior characteristic of an inverted U shaped response curve, scopolamine (0.2 and 0.4 mg/kg) improved passive avoidance acquisition in both Pb exposed groups and decreased open field activity scores in the High Pb group. In all cases, the behavioral response of the Pb exposed animals may be interpreted as responses characteristic of cholinergically deficient animals. These results thus provide further evidence for cholinergic system involvement in the behavioral changes observed following early exposure to Pb.