Kynurenic acid (KYNA or KYN) is a product of the normal metabolism of amino acid L-tryptophan. It has been shown that kynurenic acid possesses neuroactive activity. It acts as an antiexcitotoxic and anticonvulsant, most likely through acting as an antagonist at excitatory amino acid receptors. Because of this activity, it may influence important neurophysiological and neuropathological processes. As a result, kynurenic acid has been considered for use in therapy in certain neurobiological disorders. Conversely, increased levels of kynurenic acid have also been linked to certain pathological conditions.
As a noncompetitive antagonist at the glycine site of the NMDA receptor.
As an antagonist of the α7 nicotinic acetylcholine receptor. However, recently (2011) direct recording of α7 nicotinic acetylcholine receptor currents in adult (noncultured) hippocampal interneurons by the Cooper laboratory validated a 2009 study  that failed to find any blocking effect of kynurenic acid across a wide range of concentrations, thus suggesting that in noncultured, intact preparations from adult animals there is no effect of kynurenic acid on α7 nicotinic acetylcholine receptor currents 
High levels of kynurenic acid have been identified in patients suffering from tick-borne encephalitis, schizophrenia and HIV-related illnesses. In all these situations increased levels were associated with confusion and psychotic symptoms. Kynurenic acid acts in the brain as a glycine-site NMDAr antagonist, key in glutamatergic neurotransmission system, which is thought to be involved in the pathophysiology and pathogenesis of schizophrenia.
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^Hilmas, C., Pereira, EFR., Alkondon,M., Rassoulpour,A. Schwarcz,R., Albuquerque E.X.,(2001) The Brain Metabolite Kynurenic Acid Inhibits α7 Nicotinic Receptor Activity and Increases Non-α7 Nicotinic Receptor Expression: Physiopathological Implications. J. Neurosci 21(19):7463–7473.
^ abDobelis P., Varnell A., and Donald C. Cooper. Nicotinic α7 acetylcholine receptor-mediated currents are not modulated by the tryptophan metabolite kynurenic acid in adult hippocampal interneurons. (2011) Nature Precedings doi=10.1038/npre.2011.6277.1, http://www.neuro-cloud.net/nature-precedings/dobelis/
^ abMok MH, Fricker AC, Weil A, Kew JN (2009) Electrophysiological characterisation of the actions of kynurenic acid at ligand-gated ion channels. Neuropharmacology 57: 242-249.
^Wang J, Simonavicius N, Wu X, Swaminath G, Reagan J, Tian H, Ling L (2006). "Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35". J. Biol. Chem.281 (31): 22021–8. doi:10.1074/jbc.M603503200. PMID16754668.
^Grilli M, Raiteri L, Patti L, Parodi M, Robino F, Raiteri M, Marchi M (2006). "Modulation of the function of presynaptic α7 and non-α7 nicotinic receptors by the tryptophan metabolites, 5-hydroxyindole and kynurenate in mouse brain". Br. J. Pharmacol.149 (6): 724–32. doi:10.1038/sj.bjp.0706914. PMC2014664. PMID17016503.