Effects of reinforcer sweetness and the D2/D3 antagonist raclopride on progressive ratio operant performance.

Previous studies have reported that DA receptor antagonists suppress most behaviours; however, a paradoxical increase in performance may be seen in tests of operant or consummatory behaviours maintained by very sweet rewards, which lie on the descending limb of the inverted U-shaped concentration-performance function (i.e., under conditions where performance decreases as sweetness increases). Despite the low performance levels associated with very sweet reinforcers, preference studies indicate that they are nonetheless more rewarding. In the present study, the hypothesis that reinforcer efficacy is monotonically related to reinforcer sweetness was tested using a geometric progressive ratio reinforcement schedule, in which increasing numbers of responses were required to earn successive reinforcers (1, 2, 4, 8,.); the amount of work the animal emits in order to obtain an increasingly infrequent reinforcer is assumed to provide a measure of the magnitude of its rewarding effect. Three groups of rats were trained on this schedule, using as reinforcers food pellets containing 1%, 10% and 95% sucrose, respectively. Under conditions of continuous sucrose-pellet reinforcement, the highest response rates were maintained by the 10% sucrose pellets. However, under the progressive ratio schedule, performance was monotonically related to sucrose concentration. The dopamine D2/D3 receptor antagonist raclopride dose-dependently suppressed progressive ratio performance in all three groups.