Anticipatory contrast effect in rats: a new view with lick response analysis and the effect of dopamine blocking

A negative anticipatory contrast effect (NACE) is manifested as a suppression of the effect of a reward stimulus to elicit consummatory or instrumental responses when a second more potent reward is presented right afterward. No well-supported theory explains the effect. We analyzed NACE with rats' licking responses to 3 solutions with varying reward value: 8% sucrose mixed with 0.008% quinine (A), 8% sucrose with 0.002% quinine (B), and 32% sucrose with 0.008% quinine (C). 3 groups (n = 4) of rats were given 2 solutions in succession, each 5 min with 15 s in between, for a total of 11 days: Group AA (control), A then A; Group AB, A then B; Group AC, A then C. On days 13 to 16, and 12 and 17, pimozide (0.4 mg/kg, i.p.), and the vehicle solution were injected, respectively. Results show that the licking of A was a function of what solution to follow: In AC, the total number of licks, the short interlick interval (SILI), and the number of licks per s during the initial contact were less than AA. In AB, only SILI was less. Licking of A was initially low but increased over days, except for AC. The failure of AC to increase the licking was attributed to a "retroactive overshadowing" effect of a more potent stimulus C blocking stimulus A to form a reward learning, thus preventing its rewarding property to be fully manifested. It may also be due to stimulus A becoming a "signal" for incoming reward, thus degrading its own rewarding function. The shorter SILI in AB and AC suggested a reward devaluation; however, devaluation alone did not predict the consummatory response. Pimozide reduced licking number overall without altering the relative reward property or SILI suggesting that NACE was not a dopamine-mediated motivated choosing behavior.