Effect of chlordiazepoxide on the response to repeated reductions in sucrose concentration in free-fed rats

Free-fed rats were shifted from brief access to 32% sucrose (5 min per day for 3 days) to 4% sucrose (5 min per day for 2 days) 8 separate times. During preshift, rats given access to 32% sucrose licked less than rats given access to 4% sucrose (the opposite of what is typically obtained with deprived rats). The shift from 32% sucrose to 4% sucrose resulted in a further decline in lick frequency, suggesting a successive negative contrast effect. Degree of negative contrast diminished with repeated downshifts. Administration of chlordiazepoxide (CDP), 8 mg/kg, on either the first or second postshift day completely offset the effect of reward reduction. The diminution of contrast and the pattern of effectiveness of CDP differed from results obtained with food-deprived rats. The reduction of contrast by CDP was interpreted as being separate from an appetite-stimulation effect of the drug. Finally, all groups showed tolerance to the sedative effects of CDP in an open-field test, a result consistent with that obtained with deprived rats.