Importance of kynurenine 3-monooxygenase for spontaneous firing and pharmacological responses of midbrain dopamine neurons: Relevance for schizophrenia

Abstract

Kynurenine 3-monooxygenase (KMO) is an essential enzyme of the kynurenine pathway, converting kynurenine into 3-hydroxykynurenine. Inhibition of KMO increases kynurenine, resulting in elevated levels of kynurenic acid (KYNA), an endogenous N-methyl-d-aspartate and α*7-nicotinic receptor antagonist. The concentration of KYNA is elevated in the brain of patients with schizophrenia, possibly as a result of a reduced KMO activity. In the present study, using in vivo single cell recording techniques, we investigated the electrophysiological characteristics of ventral tegmental area dopamine (VTA DA) neurons and their response to antipsychotic drugs in a KMO knock-out (K/O) mouse model. KMO K/O mice exhibited a marked increase in spontaneous VTA DA neuron activity as compared to wild-type (WT) mice. Furthermore, VTA DA neurons showed clear-cut, yet qualitatively opposite, responses to the antipsychotic drugs haloperidol and clozapine in the two genotypes. The anti-inflammatory drug parecoxib successfully lowered the firing activity of VTA DA neurons in KMO K/O, but not in WT mice. Minocycline, an antibiotic and anti-inflammatory drug, produced no effect in this regard. Taken together, the present data further support the usefulness of KMO K/O mice for studying distinct aspects of the pathophysiology and pharmacological treatment of psychiatric disorders such as schizophrenia.

Keywords: Dopamine; Electrophysiology; Kynurenic acid; NMDA receptor; Schizophrenia.

Fig. 1.

Whole brain levels of A) kynurenine and B) KYNA in untreated WT (n = 9) and KMO K/O (n = 7) mice. Bars represent the mean ± SEM, ***p < 0.001 vs. WT, Mann–Whitney U test.

Fig. 2.

Basal VTA DA cell activity in WT mice (n = 7 mice, 37 cells) compared to KMO K/O mice (n = 6, 44 cells). A) Numbers of spontaneously active cells found per electrode track, B) firing rate (Hz) and C) percent of action potentials fired in bursts. Bars represent the mean ± SEM. ***p < 0.001, **p < 0.01, *p < 0.05 vs. WT, Mann–Whitney U test.

Fig. 3.

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.

Fig. 4.

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.

Fig. 5.

Effect of parecoxib (25 mg/kg, i.v.), administered over 45 min, on A) firing rate and B) burst firing of VTA DA neurons in WT (n = 6–7) and KMO K/O (n = 6–8) mice. In A, the percent change in firing rate at different timepoints was compared to the corresponding value before the administration of parecoxib (***p < 0.001, *p < 0.05, Mann–Whitney U test). The change in firing rate between WT and KMO K/O mice was compared at corresponding timepoints (⁺⁺p < 0.01, ⁺p < 0.05, Mann–Whitney U test). In B, burst firing was compared within subjects to the basal value before parecoxib administration (*p < 0.05, Wilcoxon matched pairs sign rank test). Each value is the mean ± SEM.

Fig. 6.

Pharmacological treatments do not affect whole brain levels of kynurenine and KYNA in WT (A, B) and KMO K/O (C, D) mice. Animals were euthanized 5 min and 50 min following completed administration of haloperidol/clozapine and parecoxib/minocycline respectively. Individual and mean values are indicated. No significant within-group differences were seen (Dunn’s multiple comparison test).