Differential effects of formamidine pesticides on fixed-interval behavior in rats.

Chlordimeform (CDM), amitraz (AMZ), and formetanate (FMT) are members of the formamidine class of pesticides. To date, effects on operant behavior have been determined only for CDM. This experiment compared the effects of CDM, AMZ, and FMT on schedule-controlled responding. Nine male Long-Evans rats were trained during 1-hr sessions to lever-press under a multiple fixed-interval (FI)-1-min FI-5-min schedule of milk reinforcement. Dose-effect determinations for each compound administered ip, 20 min presession, were carried out in each subject. The dose ranges were: CDM HCl, 0.3-20 mg/kg; FMT HCl, 0.03-0.75 mg/kg; and AMZ, 5-75 mg/kg. Under baseline conditions response rates were higher under the FI-1-min than under FI-5-min, and index of curvature (IOC) values (a measure of within-interval response patterning) were generally higher under FI-5-min. All compounds produced dose-dependent decreases in response rate. CDM significantly decreased only FI-1-min response rates; a similar effect of AMZ was seen only at an intermediate dose. FMT decreased responding to the same extent in both components. CDM produced pronounced changes in the pattern of responding in both components, with IOC decreased more under FI-5-min than under FI-1-min. AMZ produced significant decreases in IOC only under FI-5-min. FMT did not appreciably decrease IOC in either component. High doses of AMZ produced general signs of poor health that persisted for several days. In addition, a greater effect on response rates and IOC in both components was obtained when AMZ (75 mg/kg) was given more than 10 days following another dose compared to when it was given 7 days or less after another dose. Formamidine pesticides produce differential effects on FI schedule-controlled behavior that are in turn modulated by the parameter value of the FI schedule.