Independent ethanol- and sucrose-maintained responding on a multiple schedule of reinforcement.

Assessment of drug effects on two different reinforcers at the same time requires that each reinforcer be sampled at approximately the same time. One procedure that effectively produces this result is the use of a multiple schedule of reinforcement in which two different reinforcers are presented in alternating 2-min components. In our study, sucrose and ethanol solutions were made available after appropriate lever-press responding. Subjects were trained to self-administer 10% ethanol using a sucrose-substitution procedure and to discriminate light cues associated with the different reinforcers until stable reinforcer-directed responding was achieved on both the ethanol- and the sucrose-associated lever during the changing 2-min components of the schedule. Subsequently, the reinforcer solution presented on one lever was held constant while the concentration of the alternate reinforcer was manipulated, i.e., ethanol concentrations of 0 to 15% and sucrose concentrations of 0 to 10% were assessed. This multiple schedule procedure resulted in sustained and independent ethanol- and sucrose-reinforced behavior. Manipulation of the characteristics of one reinforcer had no effect on behavior maintained by the alternate reinforcer. Increases in sucrose concentration resulted in increases in sucrose-directed responding with no change in ethanol responding, and increases in ethanol concentration resulted in increases in ethanol intake with no change in sucrose responding. Our methods can be used to examine differential drug effects on ethanol- and sucrose-reinforced behavior in the same animal over a single time course without the confounds or interference from the concurrently available reinforcer.