BACKGROUND Kynurenine 3-monooxygenase (KMO) converts kynurenine to 3-hydroxykynurenine, and its inhibition shunts the kynurenine pathway - which is implicated as dysfunctional in various psychiatric disorders - towards enhanced synthesis of kynurenic acid (KYNA), an antagonist of both α7 nicotinic acetylcholine and NMDA receptors. Possibly as a result of reduced KMO activity, elevated central nervous system levels of KYNA have been found in patients with psychotic disorders, including schizophrenia (SZ). METHODS In the present study, we investigated adaptive – and possibly regulatory – changes in mice with a targeted deletion of Kmo (Kmo−/−) and characterized the KMO-deficient mice using six behavioral assays relevant for the study of SZ. RESULTS Genome-wide differential gene expression analyses in the cerebral cortex and cerebellum of these mice identified a network of SZ- and psychosis-related genes, with more pronounced alterations in cerebellar tissue. KYNA levels were also increased in these brain regions in Kmo−/− mice, with significantly higher levels in the cerebellum than in the cerebrum. Kmo−/− mice exhibited impairments in contextual memory and spent less time than controls interacting with an unfamiliar mouse in a social interaction paradigm. The mutant animals displayed increased anxiety-like behavior in the elevated plus maze and in a light-dark box. After a D-amphetamine challenge (5 mg/kg, i.p.), Kmo−/− mice showed potentiated horizontal activity in the open field paradigm. CONCLUSIONS Taken together, these results demonstrate that the elimination of Kmo in mice is associated with multiple gene and functional alterations that appear to duplicate aspects of the psychopathology of several neuropsychiatric disorders.

Exposure to infections in early life is considered a risk-factor for developing schizophrenia. Recently we reported that a neonatal CNS infection with influenza A virus in mice resulted in a transient induction of the brain kynurenine pathway, and subsequent behavioral disturbances in immune-deficient adult mice. The aim of the present study was to investigate a potential role in this regard of kynurenic acid (KYNA), an endogenous antagonist at the glycine site of the N-methyl-D-aspartic acid (NMDA) receptor and at the cholinergic α7 nicotinic receptor. C57BL/6 mice were injected i.p. with neurotropic influenza A/WSN/33 virus (2400 plaque-forming units) at postnatal day (P) 3 or with L-kynurenine (2×200 mg/kg/day) at P7-16. In mice neonatally treated with L-kynurenine prepulse inhibition of the acoustic startle, anxiety, and learning and memory were also assessed. Neonatally infected mice showed enhanced sensitivity to d-amphetamine-induced (5 mg/kg i.p.) increase in locomotor activity as adults. Neonatally L-kynurenine treated mice showed enhanced sensitivity to d-amphetamine-induced (5 mg/kg i.p.) increase in locomotor activity as well as mild impairments in prepulse inhibition and memory. Also, d-amphetamine tended to potentiate dopamine release in the striatum in kynurenine-treated mice. These long-lasting behavioral and neurochemical alterations suggest that the kynurenine pathway can link early-life infection with the development of neuropsychiatric disturbances in adulthood.

Our results suggest an imbalanced metabolism of the kynurenine pathway in the PFC of FSL rats.