BEHAVIORAL ECONOMICS AND DRUG CHOICE - EFFECTS OF UNIT PRICE ON COCAINE SELF-ADMINISTRATION BY MONKEYS

The application of microeconomic theory to the experimental analysis o f behavior has been termed behavioral economics. There has been an inc reasing interest in applying the concepts of behavioral economics to t he study of drug self-administration. In a previously published experi ment (Nader and Woolverton, 1992), rhesus monkeys (N = 3) were trained in a discrete-trials choice procedure and allowed to-choose between i ntravenous injections of cocaine (0.03 - 1.0 mg/kg/injection) and food presentation (1 or 4 pellets; 1 g/pellet) during daily 7-h experiment al sessions. When cocaine or food was available under a fixed-ratio (F R) 30 schedule, cocaine intake increased in a dose-related manner for all monkeys. When the response requirement (FR) for cocaine was differ entially increased by doubling or quadrupling, the frequency of cocain e choice decreased, shifting the cocaine dose-response function to the right. The present paper is a reanalysis of data from that experiment . Several mathematical models, differentially incorporating the effect s of FR, dose and number of food pellets, were compared. When cocaine consumption was analyzed using a multiple linear regression analysis w ith FR, dose and number of pellets as separate main effects (model I), the R2 was 0.82. When FR and dose were combined into one factor, unit price (UP, responses/mg/kg), and cocaine consumption was analyzed as a linear function of UP (model IIA), the R2 was 0.54. When cocaine con sumption was analyzed as a curvilinear, negatively accelerated functio n of UP (model IIB), the R2 was 0.53. The difference between models I and IIA was statistically significant while models IIA and IIB were no t different. Since the choice was mutually exclusive, cocaine consumpt ion was analyzed as a function of the ratio of the UP of cocaine and t he UP of food, the reinforcer that was lost when drug was chosen. The R2 for this analysis was 0.58. Multiplying that ratio by the sum of th e UPs for cocaine and food (Bickel, personal communication) resulted i n an increased R2 of 0.64. Eliminating data from low dose conditions ( Bickel, personal communication) improved the fit of model IIB but not of I or IIA. Thus, a UP analysis of cocaine consumption under the pres ent conditions significantly decreased the proportion of the variance that was accounted for by the multiple linear regression model. These results suggest that changes in FR and dose do not necessarily have th e same functional effect on drug consumption and that the applicabilit y of the UP model to drug self-administration may be limited.