Importance of endogenous kynurenic acid in brainm catecholaminergic processes and in the pathophysiology of schizophrenia

Kynurenic acid is a metabolite of tryptophan and the only known naturally occurring N-MethylD-Aspartic acid (NMDA)-receptor antagonist (at the co-agonist glycine site) in the human brain. The aim of the present work was to investigate the physiological and pharmacological significance of endogenous kynurenic acid as well as its putative pathophysiological implications. For this purpose in vivo extracellular single unit recording from rat brain catecholamine neurons were used. An inhibitor of kynurenine 3-hydroxylase (PNU 156561A) was administered i.v. (40 mg/kg; 3-9 hours before the electrophysiological experiment) to cause an elevation of endogenous kynurenic acid levels. This effect was mediated by blockade of an alternative pathway for the precursor of kynurenine, thereby promoting the synthesis of kynurenic acid. After each experiment, the brains were rapidly taken out and analysis of kynurenic acid were performed using an isocratic reversed-phase HPLC system. Pharmacologically induced elevation (3-5 fold) of endogenous kynurenic acid levels was associated with increased firing rate and burst activity and a decreased regularity of firing of midbrain dopamine (DA) neurons. These actions were reversed by treatment with D-cycloserine (128 mg/kg i.v., 5 min), an agonist at the glycine site of the NMDA receptor. The effects of PNU 156561A on the neuronal activity of midbrain DA neurons are in all probability induced by a blockade of NMDA receptors by kynurenic acid and mediated via inhibition of GABAergic afferents. The GABAA-agonist muscimol produces a paradoxical hyperactivity of substantia nigra (SN) DA neurons. Pretreatment with PNU 156561A, producing an increase in endogenous kynurenic acid, as well as systemically administered MK-801 or intracerebroventricular administration of kynurenic acid clearly antagonized this excitatory action of muscimol. However, in rats with elevated levels of endogenous kynurenic acid muscimol s excitatory action was reversed into a pure inhibitory response. Our results suggest an excitatory glutamatergic component in the actions of muscimol on SN DA neurons. Previous studies have shown that systemically administered nicotine is associated with an activation of rat midbrain DA neurons and noradrenergic (NA) neurons in the locus coeruleus (LC). Elevation of endogenous kynurenic acid by PNU 156561A was found to antagonize the nicotine-induced (25-200 μg/kg i.v.) increase in firing rate of LC NA neurons. Similarly, the increase in firing rate and burst firing activity of ventral tegmental area (VTA) DA neurons elicited by nicotine (1.5-400 μg/kg i.v.) was antagonized but also reversed into an inhibitory response by elevated levels of kynurenic acid. These antagonistic actions induced by PNU 156561A pretreatment were prevented by administration of D-cycloserine (128 mg/kg i.v., 5 min). A novel finding in this study is that the activation of VTA DA neurons by nicotine was preceded by a short-lasting inhibition in firing rate. This inhibitory action of nicotine was antagonized by pretreatment with the selective GABAB receptor antagonist CGP 35348 (200 mg/kg i.v., 3 min). In addition, CGP 35348 also facilitated the nicotine-induced increase in burst firing activity of VTA DA neurons. We propose that nicotine exerts both excitatory and inhibitory actions of VTA DA neurons and that these effects are related to release of glutamate and GABA, respectively. Present experimental data support a contribution of endogenous kynurenic acid in brain glutamatergic neurotransmission. Further, our findings demonstrate that a moderate elevation in brain kynurenic acid levels produces dysregulation of neuronal firing of midbrain DA neurons, similarly to the effects of systemic administration of the NMDA receptor antagonists phencyclidine (PCP, angel dust) or ketamine. Schizophrenia is thought to be mediated, at least in part, by dysregulation of the midbrain DA pathways and interestingly, both PCP and ketamine induce schizophrenia-like positive and negative symptoms as well as cognitive dysfunction in healthy volunteers, and furthermore, aggravate psychotic symptoms in schizophrenic patients. Based upon these findings we investigated the endogenous levels of kynurenic acid in cerebrospinal fluid (CSF) in patients with schizophrenia. CSF levels of kynurenic acid from 28 schizophrenic patients (25 of whom were drug-naive) and 17 healthy volunteers were analyzed. Kynurenic acid was detected in all controls, with little inter-individual variation (0.97 ±0.07 nM). In patients suffering from schizophrenia, CSF levels of kynurenic acid levels were higher than in volunteers (1.67 ±0.27 nM; p=0.038, Mann-Whitney U-test) and the variation between individuals was larger, with a maximum value of 6.8 nM. These results indicate a contribution of kynurenic acid in the pathophysiology of schizophrenia.