Effect of clozapine (0.625–10.0 mg/kg, i.v.) on A) firing rate and B) burst firing of VTA DA neurons in WT (n = 4–8) and KMO K/O (n = 8–9) mice. In A, the percent change was compared to corresponding pre-drug values (***p < 0.001, **p < 0.01, *p < 0.05, Mann–Whitney U test) and between WT and KMO K/O mice at corresponding doses (++p < 0.01, +p < 0.05, Mann–Whitney U test). In B, burst firing was compared to the pre-drug value within subjects (**p < 0.01, *p < 0.05, Wilcoxon matched pairs sign rank test). Each value is the mean ± SEM.