Effect of haloperidol (0.05–0.4 mg/kg, i.v.) on A) firing rate and B) percent burst firing of VTA DA neurons in WT (n = 6) and KMO K/O (n = 2–6) mice. In A, the percent change was compared to the corresponding pre-drug value (*p < 0.05; Mann–Whitney U test) and between WT and KMO K/O mice at corresponding doses (no significance, Mann-Whiney U test). In B, percent burst firing was compared to the pre-drug value within subjects (no significance, Wilcoxon matched pairs sign rank test). Each value is the mean ± SEM.