Cocaine self-administration under conditions of restricted and unrestricted food access

Four rhesus monkeys (Macaca mulatta), maintained at reduced body weight and restricted food availability, had access to a 0.8-mg/ml cocaine solution and vehicle under a concurrent fixed-ratio (FR) 8 schedule. Over days, the cocaine concentration was reduced (0.57, 0.4, 0.2, 0.1, 0.05, 0.025) and then returned, gradually over days, to 0.8 mg/ml. The ratio value was then varied (to 16, 32, 64, 128, and 8). Food access was unrestricted, and the ratio and the concentration manipulations were then repeated. During food restriction cocaine served as a reinforcer for all monkeys, whereas during free feeding cocaine functioned as a reinforcer for 3 of 4 monkeys; with these monkeys, the dose-response curve obtained under free feeding was shifted to the right of that obtained under food restriction. There were no differences in FR response curves obtained during food restriction and unrestricted feeding. These data suggest that food restriction increases cocaine's reinforcing effects and that the higher the cocaine dose, the greater are the reinforcing effects. A demand curve analysis was completed, and data are discussed in terms of microeconomic principles.