Figure 2. Appetitive learning is attenuated, while preference for sweet rewards remains intact.

(A) Learning in a T-maze for a food reward was attenuated in D1R KO mice. D1R KO mice (n = 9) and WT mice (n = 10) were food restricted to 85% body weight, then trained (10 trials/day) for 10 days to turn right to obtain a food reward. The correct arm was denoted by horizontal stripes (Zweifel et al. 2009). Two-way repeated measures ANOVA shows a significant effect of genotype (F_1,153 = 6.32; p= 0.0223), time (F_9,153 = 4.50; p <0.001) and time × genotype interaction (F_9,153 = 2.62; p= 0.0076). One-way repeated-measures ANOVA with Bonferroni’s multiple comparison test shows that while WT mice learned the task by day 6 (p < 0.0001), D1R KO mice failed to acquire the task over 10 days (p=0.3341). (B) Saccharin preference was normal in D1R KO mice. D1R KO mice (n = 6) and WT mice (n = 7) were monitored for saccharin preference over 4 consecutive days. The first 2 days examined preference for 0.033% saccharin vs. H₂O with no difference between genotypes (Students t-test; p=0.152). The final 2 days examined preference for 0.066% saccharin with no difference between genotypes (Students t-test; p=0.059). The locations of the saccharin and H₂O bottles were alternated each day. (C) Instrumental conditioning is attenuated in D1R KO mice. D1R KO mice (n = 7) and WT mice (n = 8) were trained on a simple fixed-ratio schedule in which each lever press delivered a single food pellet up to a maximum of 50 (Zweifel et al., 2009). Two-way repeated- measures ANOVA shows an effect of both day (F_6,91 = 6.284; p < 0.0001) and genotype (F_1,91 = 498.1; p < 0.0001).