Effect of Cdk5 cKO on instrumental responding for reward. A, Performance for control (Con) versus cKO mice for food reinforcement under a PR schedule. The graphs depict active (left) and inactive (right) nose poke responses ± SEM for 3 consecutive days of PR testing (*p < 0.05, **p < 0.01, ***p < 0.005 compared with control, post hoc Tukey, n = 16–20 per group). B, Breakpoints achieved for control versus cKO mice during 3 d of PR responding. The graphs show mean final ratios achieved ± SEM (*p < 0.05, **p < 0.01, ***p < 0.001 compared with control, post hoc Tukey, n = 16–20 per group). C, Performance in progressive ratio instrumental responding after prefeeding.