Prolonged lead exposure and fixed ratio performance.

Male rats were chronically treated via drinking water with 50 or 500 ppm sodium acetate or 50 or 500 ppm lead acetate from weaning. Behavioral testing on a fixed-ratio (FR) schedule of reinforcement began at 55 days of age. A series of increasing ratio values was studied. The lower exposure level was without effect at any ratio value. The 500 ppm concentration decreased response rates over the first 15-20 sessions of FR5 and FR25, after which rates reached control levels. Changes in response rate derived primarily from longer interresponse times (IRTs) and decreased running rates. Differences in performance were not evident at FR values of 50 and 100. The 50 ppm concentration produced blood lead (PbB) values averaging 30.3 micrograms/dl; the 500 ppm concentration produced PbBs of 58-94 micrograms/dl. Zinc protoporphyrin levels resulting from 500 ppm exposure differed from controls even after one month of exposure, and continued to increase over eight months of exposure, to 72 micrograms/dl. Comparison of these data to previous studies from this laboratory of FI performance in which the identical exposure protocol was employed, suggests that FR response rates are less sensitive than FI to disruption by lead.