The effects of morphine on responding under variable-interval schedules: rate-related effects, behavioral mechanisms and Herrnstein's hyperbola.

The study was designed to integrate rate-related principles and Herrnstein's hyperbolic matching equation (1970) as they apply to drug effects. It was also designed to assess the validity of an interpretation of Herrnstein's equation used to account for the behavioral mechanisms of drug action. Eight rats were trained to press a lever under a range of variable-interval (VI) schedules of reinforcement using sucrose solution (0.1 M) as a reinforcer. After 90 baseline sessions the effect of morphine sulfate, 3, 6, 12 and 17 mg/kg, on responding was tested. Response rates were suppressed in a dose-dependent manner after morphine administration. Herrnstein's equation, which states that responding is a function of relative reinforcement rate, provided a good description of the relationship between responding and reinforcement under most conditions. Therefore, the traditional variable used to predict the drug effect in rate-dependency functions, baseline response rate, was compared to a new variable, baseline relative reinforcement rate. Relative reinforcement rate performed as well as response rate in predicting the drug effect. In order to test if Herrnstein's hyperbola can identify the behavioral mechanisms of the action of morphine, the concentration of the sucrose solution was increased (0.32 M). The y-asymptote of Herrnstein's equation, k, increased as sucrose concentration increased. This calls into question one common interpretation of Herrnstein's equation used to identify the behavioral mechanisms of drug action.