The kynurenine pathway in schizophrenia and bipolar disorder

The kynurenine pathway of tryptophan degradation generates several neuroactive compounds. Of those, kynurenic acid is an N-methyl-d-aspartate (NMDA) and alpha7 nicotinic receptor antagonist. The kynurenic acid hypothesis of schizophrenia is built upon the fact that kynurenic acid blocks glutamate receptors and is elevated in schizophrenia. Kynurenic acid tightly controls glutamatergic and dopaminergic neurotransmission and elevated brain levels appear related to psychotic symptoms and cognitive impairments. Contributing to enhanced production of kynurenic acid, the expression and enzyme activity of kynurenine 3-monooxygenase (KMO) are reduced in schizophrenia and in bipolar patients with a history of psychosis. The kynurenine pathway is also critically regulated by cytokines, and, indeed, the pro-inflammatory cytokines interleukin (IL)-1β and IL-6 are elevated in schizophrenia and bipolar disorder and stimulate the production of kynurenic acid. One physiological mechanism controlling the activity of the kynurenine pathway originates from the protein sorting nexin 7 (SNX7). This glial signaling pathway initiates a caspase-8-driven activation of IL-1β that induces tryptophan-2,3-dioxygenase 2 (TDO2), an enzyme in the kynurenine pathway. A recent study shows that a genetic variation resulting in decreased expression of SNX7 is linked to increased central levels of kynurenic acid and ultimately to psychosis and cognitive dysfunction in bipolar disorder. Experimental studies highlight the detrimental effects of increased synthesis of kynurenic acid during sensitive periods of early brain development. Furthermore, experimental studies strongly support inhibition of kynurenine aminotransferase (KAT) II as a novel target and a valuable pharmacological strategy in the treatment of psychosis and for improving cognitive performance relevant for schizophrenia. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.