Acknowledgements
This study was supported by grants DA07262 (PJK), DA12062 (DKG), and MH067497 (DKG). We thank Katherine M. Suchland for her excellent technical assistance.

Figures and Tables
Figure 1  Mice lacking dopamine D2 receptors acquire a goal-directed behavior similar to wild-type mice. Both genotypes readily learned to retrieve food pellets from a small dish and significantly decreased their latencies to perform this task across trials (* p < 0.001).
Figure 2  Wild-type and D2R-deficient mice perform the digging task equally well. No differences in responding as a function of genotype were found, and similar latencies to retrieve the reinforcer were seen. These response patterns suggest that the training from the previous day influenced behavior, demonstrating that both genotypes are capable of learning and retaining goal-directed behaviors.
Figure 3  Dopamine D2 receptor-mediated signaling contributes to the acquisition of odor discrimination/associative learning. Wild-type (D2R+/+) mice outperformed (mean + standard error) the D2R-/- mice during acquisition of (A) odor discrimination (*p < 0.05), and (B) committed significantly fewer discrimination errors (*p < 0.01) during the discrimination task.
Figure 4  D2R-/- mice perseverate in unreinforced behavior during reversal learning trials. The number of necessary trials (mean + standard error) to (A) demonstrate reversal learning (*p < 0.01), and (B) reversal errors committed (*p < 0.01) by D2R-/- and D2R+/+ mice.
Figure 5  Reversal learning measures reveal D2R-/- mice emit significantly more errors of commission but not errors of omission than D2R+/+ mice. Response patterns (mean + standard error) by D2R-/- mice are more suggestive of stimulus bound perseveration and not extinction. D2R-/- mice committed significantly more errors of commission (A) than D2R+/+ mice (*p < 0.01), but no differences in errors of omission (B) were observed between genotypes (p > 0.6).
Figure 6  Mice lacking functional D2Rs display an inability to withhold inappropriate responses. Response patterns during first 10 reversal trials reveal deficits in mice lacking D2Rs. When the mice encountered the inverted reinforcement contingencies during the first reversal learning session, the D2R-/- mice demonstrated an almost complete inability to inhibit responding to previously reinforced stimuli compared to D2R+/+ mice (* p < 0.001).
Figure 7  D2R-/- mice commit significantly more perseverative errors across reversal sessions than wild-type mice. An analysis of the time course of responding for errors of commission across the first four reversal sessions revealed that the D2R-/- committed several more errors of commission than the D2R+/+ mice (*p < 0.001).