Chlorpyrifos: lack of cognitive effects in adult Long-Evans rats

Female Long-Evans rats were gavaged 5 days a week for 4 weeks with chlorpyrifos in oil at dosages of 0, 1, 3, and 10 mg/kg/day. Clinical observations were conducted, and memory was tested with a delayed matching-to-position task (DMTP). Before exposure started, the rats were divided into four groups of ten of comparable overall performance. Then, they were tested during four weeks of dosing and for another four weeks thereafter. The observer was not provided information about each rat's dose group identification. Miosis was a prominent sign observed in the 3 and 10 mg/kg/day groups. Rectal temperature was reduced in the 10 mg/kg/day group. Noncognitive performance measures in the DMTP test (e.g., actual total delay, void trials) were affected and consistent with decreased motor activity. There was a statistically significant difference in the intercept at the zero delay (i.e., a measure of encoding/motivation/attention), which was attributed to deviations from controls in the high-dosage group during dosing weeks 2 and 3 (in opposite directions). This difference was not considered treatment related. The slope of the retention gradient (i.e., a measure of forgetting rate) did not show any statistically significant difference between groups at dosages that inhibited brain cholinesterase by up to approximately 85%. In conclusion, chlorpyrifos decreased motor activity but had no effects on short-term memory (i.e., information retention capability) and on encoding/motivation/attention.