Changes in cis(Z)-flupentixol-induced dopamine blockade produce contrast effects in rats

Two groups of rats were trained under a 45-sec variable-interval schedule. The magnitude of reinforcement used in each test session alternated daily between one and four 45-mg food pellets, the magnitude of reinforcement available at any time being signalled by lights in the operant chamber. Testing took place over two consecutive 11-day phases. During the first phase one group of rats was injected with 0.06 mg/kg of cis(Z)-flupentixol prior to testing; the other group received vehicle injections. In the second phase drug conditions were reversed. The change in drug conditions between phases produced both positive and negative successive contrast effects, consistent with the hypothesis that dopamine blockade attenuated the hedonic impact of reinforcement. Embedded within each session were two short signalled probe periods during which the reinforcement magnitude was switched to that used on the alternate days. No contrast effects were found during these brief daily probe periods.