LACK OF TOLERANCE TO CONTRAST-REDUCING ACTIONS OF CHLORDIAZEPOXIDE WITH REPEATED REWARD REDUCTIONS

In each of eight cycles of repeated reward reduction, the performance of rats given brief access to 32% sucrose for 3 days, and then 4% sucr ose for 2 days, was compared to rats that received 4% on all 5 days. S hifted rats consumed less than unshifted rats following each shift, wi th little evidence of diminution of negative contrast across the eight shifts. Acute administration of chlordiazepoxide (CDP, 8 mg/kg) on th e second postshift day reduced contrast on each shift with no evidence of tolerance development to these anticontrast actions (Experiment 1a ). Acute administration of CDP on the first postshift day had no effec t on contrast through the first four shifts, but reliably reduced cont rast on the following four shifts (Experiment 2a). There was tolerance to the sedative effects of CDP, as measured in an open field (Experim ents 1b and 2b). The data thus show: a) that contrast is not lost with repeated shifts; b) no tolerance develops to the anticontrast actions of CDP and, instead, CDP gains anticontrast actions, in regard to ini tial contrast occurrence; c) but, concurrently, tolerance does develop to the sedative effects of CDP in an open field.