Lesions of nucleus accumbens reduce instrumental but not consummatory negative contrast in rats.

In Experiment 1, rats provided with brief daily access to 4% sucrose which preceded brief access to 32% sucrose (4-32) suppressed licking the 4% solution relative to 4-4 controls. This anticipatory negative contrast (ANC) was diminished when the 32% solution was downshifted to 4%. Licking the second 4% solution in shifted rats (4-32-4) was lower than licking of the second 4% solution in 4-4 control rats - a successive negative contrast (SNC) effect. Neither SNC nor ANC or their recovery were influenced by electrolytic lesions of the nucleus accumbens (NAC). Latency to initiate licking followed a concentration function, with rats initiating licking faster for 32 than 4% sucrose, but was not affected by the lesion. In Experiment 2, rats showed clear SNC in consummatory behavior when shifted from 32 to 4% sucrose and also showed SNC in running speed when shifted from a 12- to 1-pellet reward in a straight runway. As in Experiment 1, consummatory SNC was not affected by the lesion. However, in the runway, lesioned animals showed contrast later (after more trials) than the sham-lesioned rats and did not show contrast in the goal section, when goal speed was averaged across the postshift period. Reward downshift also increased the animals' tendency to backtrack in the runway and backtracking was greater in the lesioned rats during both the preshift and postshift periods. These data suggest that the NAC is not a necessary structure for the generation of expectancies, the comparison of rewards or the modulation of ingestive behavior. However, the NAC may be involved in responding to unmet expectancies when the task involves approach or instrumental behavior.