FURTHER ANALYSIS OF CHOICE BETWEEN COCAINE AND FOOD USING THE UNIT PRICE MODEL OF BEHAVIORAL ECONOMICS

Behavioral economics defines unit price (UP) as the ratio of the respo nse requirement to magnitude of reinforcer. When applied to drug self- administration, the UP model defines UP as the ratio of the response r equirement to the unit dose of drug. This model makes two predictions about drug self-administration: increasing UP decreases consumption an d consumption at a given UP will be constant regardless of the respons e requirement and dose that make up the UP. In previous experiments co nducted in rhesus monkeys allowed to choose between an i.v. injection of cocaine and food, the UP model has failed to adequately predict dru g consumption in that consumption varied (increased with dose) at a gi ven UP. However, previous experiments have allowed a fixed number of c hoice trials/day, thereby imposing a procedural ceiling on consumption that may have influenced conformity to the UP model. In the present e xperiment, the number of choice trials available was varied in such a way that constant drug consumption was possible over the range of UPs tested. The response requirement for cocaine was varied between 15 and 1200 lever presses/injection and the dose of cocaine was varied betwe en 0.05 and 0.2 mg/kg/inj, yielding UPs from 300 to 5600 responses/mg/ kg. The response requirement for food was always 30. As predicted by t he UP model, cocaine consumption decreased as UP increased. Moreover, in contrast to previous experiments, consumption did not vary signific antly across the response requirement/dose combinations that made up a UP. A detailed analysis suggested that a decrease in magnitude of the alternative reinforcer (one rather than three food pellets), rather t han the increase in trials, was responsible for the improved conformit y to the UP model in the present experiment relative to previous exper iments. Taken together with previous experiments, the present experime nt suggests that conformity to the UP model of drug consumption in a c hoice situation is dependent upon the magnitude of alternative reinfor cers that are available. Consumption was best predicted by the UP mode l when the magnitude of the alternative reinforcer was small. (C) 1997 Elsevier Science Ireland Ltd.