Essential value and drug addiction

The purpose of the present work was fourfold: First, to compare the reinforcing strength of cocaine and food in rats (Chapter 2); second to compare the reinforcing strength of cocaine before and after a cocaine self-administration history and food before and after a food self-administration history (Chapter 3); third, to compare the reinforcing strength of food after a cocaine self-administration history (Chapter 4); and last, to compare the reinforcing strength of food and cocaine in two inbred rat strains (Chapter 5). To determine reinforcing strength, consumption (number of reinforcers earned) was plotted as a function of price (fixed-ratio requirement) resulting in a demand curve, which was then fit with the exponential equation (Hursh & Silberberg, 2008) to determine the essential value (their measure of reinforcer strength). It was observed that food was always more reinforcing than cocaine (Chapter 2). However, the reinforcing strength of cocaine increased with an increased cocaine self-administration history whereas food strength remained the same following increased food history (Chapter 3). The reinforcing strength of food decreased with an increased cocaine self-administration history (Chapter 4). These processes are proposed to model drug addiction. In addition, using Fischer and Lewis rat strains, it was shown that Fischer rats appear to be vulnerable to drug addiction because food was less reinforcing in Fischer relative to Lewis rats and cocaine was more reinforcing in Fischer relative to Lewis rats (Chapter 5).