Choice between cocaine and food in a discrete-trials procedure in monkeys: a unit price analysis.

In behavioral economics, the unit price (UP) model of drug consumption defines UP as the ratio of the response requirement to the dose of drug. This model makes two predictions: increasing UP will decrease consumption, and consumption at a given UP will be constant regardless of the response requirement and dose that make up the UP. The present experiment was designed to test the UP model in rhesus monkeys allowed to choose between an IV injection of cocaine and food in a discrete-trials choice procedure. Both response requirement/injection and dose of cocaine were varied in such a way as to yield UPs from 40 to 10,000 responses per mg/kg. The response requirement for food was always 30 and there was a 30-min time-out between trials to allow the direct effects of cocaine on responding to dissipate. Consistent with the UP model, cocaine consumption decreased as UP increased. However, at a given UP, cocaine consumption was usually higher at the higher dose. Thus, under the conditions of the present experiment an important component of the UP model of drug consumption was not supported. It may be that UP is not a reliable predictor of consumption under conditions in which the direct effects of a drug on responding are minimized.