Influences on cocaine tolerance assessed under a multiple conjunctive schedule of reinforcement

Abstract

Under multiple schedules of reinforcement, previous research has generally observed tolerance to the rate-decreasing effects of cocaine that has been dependent on schedule-parameter size in the context of fixed-ratio (FR) schedules, but not under the context of fixed-interval (FI) schedules of reinforcement. The current experiment examined the effects of cocaine on key-pecking responses of White Carneau pigeons maintained under a three-component multiple conjunctive FI (10 s, 30 s, & 120 s) FR (5 responses) schedule of food presentation. Dose-effect curves representing the effects of presession cocaine on responding were assessed in the context of (1) acute administration of cocaine (2) chronic administration of cocaine and (3) daily administration of saline. Chronic administration of cocaine generally resulted in tolerance to the response-rate decreasing effects of cocaine, and that tolerance was generally independent of relative FI value, as measured by changes in ED50 values. Daily administration of saline decreased ED50 values to those observed when cocaine was administered acutely. The results show that adding a FR requirement to FI schedules is not sufficient to produce schedule-parameter-specific tolerance. Tolerance to cocaine was generally independent of FI-parameter under the present conjunctive schedules, indicating that a ratio requirement, per se, is not sufficient for tolerance to be dependent on FI parameter.

Keywords: cocaine; conjunctive schedule; fixed-interval schedule; fixed-ratio schedule; key peck; multiple schedule; pigeons; tolerance.

Fig 1

Key-pecking rates as a function of cocaine dose during the Acute and Chronic Phases. Data for each subject are presented horizontally while components are presented vertically. Circles and squares represent mean rates at each dose during the Acute and Chronic Phases, respectively. Rates as a proportion of those observed during saline sessions are shown on the ordinate. Dose of cocaine is represented on the abscissa on a log scale. Error bars represent ±SEM. Errors bars that are not visible are subsumed by the data point. Note that ordinate extends to 1.5 for each plot except for Subjects 649 and 612 in the FI 120-s component, which extends to 1.75.

Fig 2

Key-pecking rates as a function of cocaine dose during the Acute and Saline Phases. Triangles represent mean rates at each dose during the Saline Phase. All other details are as in Figure 1.