Demand for food and cocaine in Fischer and Lewis rats.

Fischer and Lewis rat strains often serve as animal vulnerability models for drug abuse and addiction. When these strains respond for drugs of abuse, several measures, including total drug intake, response rate and progressive-ratio breakpoints, have been reported to be strain-dependent, a result suggesting genetic differences in drug reactivity and vulnerability. The present study extends these strain comparisons to a previously untested measure--demand analysis. In Experiment 1, four Fischer and four Lewis rats earned their daily food ration by lever pressing under a fixed-ratio schedule, the size of which was increased every three sessions from 3 to 1,000 in logarithmic steps. Consumption was plotted as a function of ratio size, and modeled by the exponential-demand equation (Hursh & Silberberg, 2008). Experiment 2 replicated Experiment 1 except that different rats were used, and cocaine reinforced lever pressing. A between-experiment comparison showed a commodity-by-strain interaction: Fischer rats defended consumption with greater vigor when cocaine served as the reinforcer than did Lewis rats; for food, this relation was reversed. However, for both strains, defense of consumption of food exceeded that of cocaine.