Lewis rats are more sensitive than Fischer rats to successive negative contrast, but less sensitive to the anxiolytic and appetite-stimulating effects of chlordiazepoxide.

Lewis rats show greater anticipatory contrast effects than Fischer 344 rats. Specifically, relative to Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when it predicts the future availability of a preferred sucrose reward [Grigson, P.S., Freet, C.S. The suppressive effects of sucrose and cocaine, but not lithium chloride, are greater in Lewis than in Fischer rats: evidence for the reward comparison hypothesis. Behav Neurosci 2000;114:353-363.]. Experiment 1 was designed to determine whether Lewis rats also would demonstrate greater contrast in another paradigm, successive negative contrast (SNC). The results demonstrated a tendency for greater SNC in Lewis rats and then slower recovery from the unexpected loss of reward relative to the Fischer rats. Pretreatment with the anxiolytic agent, chlordiazepoxide (CDP), effectively eliminated contrast in the Fischer rats, but served to prolong recovery from contrast in the Lewis rats. Finally, the results of Experiment 2 demonstrated that Fischer rats, but not Lewis rats, increase consumption of a 0.1 M sucrose solution following pretreatment with CDP. Together, the results show that, while both Lewis and Fischer rats demonstrate SNC, the effect is more sustained in the Lewis rats and these rats are insensitive to both the anxiolytic and the appetite-stimulating effects of CDP.