Skeletal muscle PGC-1α1 modulates kynurenine metabolism and mediates resilience to stress-induced depression.

Depression is a debilitating condition with a profound impact on quality of life for millions of people worldwide. Physical exercise is used as a treatment strategy for many patients, but the mechanisms that underlie its beneficial effects remain unknown. Here, we describe a mechanism by which skeletal muscle PGC-1α1 induced by exercise training changes kynurenine metabolism and protects from stress-induced depression. Activation of the PGC-1α1-PPARα/δ pathway increases skeletal muscle expression of kynurenine aminotransferases, thus enhancing the conversion of kynurenine into kynurenic acid, a metabolite unable to cross the blood-brain barrier. Reducing plasma kynurenine protects the brain from stress-induced changes associated with depression and renders skeletal muscle-specific PGC-1α1 transgenic mice resistant to depression induced by chronic mild stress or direct kynurenine administration. This study opens therapeutic avenues for the treatment of depression by targeting the PGC-1α1-PPAR axis in skeletal muscle, without the need to cross the blood-brain barrier.

GRK3 deficiency elicits brain immune activation and psychosis.

The G protein-coupled receptor kinase (GRK) family member protein GRK3 has been linked to the pathophysiology of schizophrenia and bipolar disorder. Expression, as well as protein levels, of GRK3 are reduced in post-mortem prefrontal cortex of schizophrenia subjects. Here, we investigate functional behavior and neurotransmission related to immune activation and psychosis using mice lacking functional Grk3 and utilizing a variety of methods, including behavioral, biochemical, electrophysiological, molecular, and imaging methods. Compared to wildtype controls, the Grk3-/- mice show a number of aberrations linked to psychosis, including elevated brain levels of IL-1ß, increased turnover of kynurenic acid (KYNA), hyper-responsiveness to D-amphetamine, elevated spontaneous firing of midbrain dopamine neurons, and disruption in prepulse inhibition. Analyzing human genetic data, we observe a link between psychotic features in bipolar disorder, decreased GRK expression, and increased concentration of CSF KYNA. Taken together, our data suggest that Grk3-/- mice show face and construct validity relating to the psychosis phenotype with glial activation and would be suitable for translational studies of novel immunomodulatory agents in psychotic disorders.

Effects of IDO1 and TDO2 inhibition on cognitive deficits and anxiety following LPS-induced neuroinflammation.

OBJECTIVE: Sustained immune activation leads to cognitive dysfunctions, depression-, and anxiety-like behaviours in humans and rodents. It is modelled by administration of lipopolysaccharides (LPS) to induce expression of pro-inflammatory cytokines that then activate indoleamine 2,3 dioxygenase (IDO1), the rate-limiting enzyme in the kynurenine pathway of tryptophan metabolism. Here, we ask whether chronic IDO1 inhibition by 1-methyl-tryptophan (1-MT, added at 2 g/l in the drinking water) or chronic inhibition of tryptophan 2,3 dioxygenase (TDO2), another enzyme capable of converting tryptophan to kynurenine, by 680C91 (15 mg/kg per os), can rescue LPS-induced (0.83-mg/kg intraperitoneally) anxiety and cognitive deficits. We also investigate the acute effects of 680C91 on serotonergic, dopaminergic, and kynurenine pathway metabolites. METHODS: We examined LPS-induced deficits in trace fear conditioning and anxiety in the light-dark box and elevated plus maze (EPM) in group-housed C57Bl6/N mice. Kynurenine pathway metabolites and monoamine levels were measured via high-performance liquid chromatography. RESULTS: Chronic blockade of IDO1 with 1-MT did not rescue cognitive deficits or abrogate the anxiogenic behaviour caused by LPS despite a decrease in the brain kynurenine:tryptophan ratio. However, 1-MT by itself demonstrated anxiolytic properties in the EPM. Acute and chronic inhibition of TDO2 elevated brain levels of tryptophan, while chronic inhibition of TDO2 was unsuccessful in rescuing cognitive deficits and abrogating the anxiety caused by LPS. CONCLUSIONS: In line with previous studies, we show that LPS administration induces anxiety and cognitive dysfunctions in mice that however were not reversed by chronic blockade of IDO1 or TDO2 at the doses used.

Decreased levels of kynurenic acid in prefrontal cortex in a genetic animal model of depression.

OBJECTIVE: There is a growing interest in the role of kynurenine pathway and tryptophan metabolites in the pathophysiology of depression. In the present study, the metabolism of tryptophan along the kynurenine pathway was analysed in a rat model of depression. METHODS: Kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK) were measured by high-performance liquid chromatography (HPLC) in prefrontal cortex (PFC) and frontal cortex (FC) in a rat model of depression, the Flinders Sensitive Line (FSL) and their controls, the Flinders Resistant Line (FRL) rats. In addition, KYNA was also measured in hippocampus, striatum and cerebellum. RESULTS: KYNA levels were reduced in the PFC of FSL rats compared with FRL rats, but did not differ with regard to the FC, hippocampus, striatum or cerebellum. 3-HK levels in PFC and FC, representing the activity of the microglial branch of the kynurenine pathway, did not differ between the FSL and FRL strains. CONCLUSION: Our results suggest an imbalanced metabolism of the kynurenine pathway in the PFC of FSL rats.

Endurance exercise increases skeletal muscle kynurenine aminotransferases and plasma kynurenic acid in humans.

Physical exercise has emerged as an alternative treatment for patients with depressive disorder. Recent animal studies show that exercise protects from depression by increased skeletal muscle kynurenine aminotransferase (KAT) expression which shifts the kynurenine metabolism away from the neurotoxic kynurenine (KYN) to the production of kynurenic acid (KYNA). In the present study, we investigated the effect of exercise on kynurenine metabolism in humans. KAT gene and protein expression was increased in the muscles of endurance-trained subjects compared with untrained subjects. Endurance exercise caused an increase in plasma KYNA within the first hour after exercise. In contrast, a bout of high-intensity eccentric exercise did not lead to increased plasma KYNA concentration. Our results show that regular endurance exercise causes adaptations in kynurenine metabolism which can have implications for exercise recommendations for patients with depressive disorder.

Direct effects of exercise on kynurenine metabolism in people with normal glucose tolerance or type 2 diabetes.

BACKGROUND: Systemic kynurenine levels are associated with resistance to stress-induced depression and are modulated by exercise. Tryptophan is a precursor for serotonin and kynurenine synthesis. Kynurenine is transformed into the neuroprotective catabolite kynurenic acid by kynurenine aminotransferases (KATs). PGC-1α1 increases KAT mRNA and induces kynurenic acid synthesis. We tested the hypothesis that skeletal muscle PGC-1α1/KAT-kynurenine pathway is altered by exercise and type 2 diabetes. METHOD: Skeletal muscle and plasma from men with normal glucose tolerance (n = 12) or type 2 diabetes (n = 12) was studied at rest, after acute exercise and during recovery. Tryptophan, Kynurenine and kynurenic acid plasma concentration were measured as well as mRNA of genes related to exercise and kynurenine metabolism. RESULTS: mRNA expression of KAT1, KAT2 and PPARα was modestly reduced in type 2 diabetic patients. In response to exercise, mRNA expression of KAT4 decreased and PGC-1α1 increased in both groups. Exercise increased plasma kynurenic acid and reduced kynurenine in normal glucose tolerance and type 2 diabetic participants. Plasma tryptophan was reduced and the ratio of [kynurenic acid] * 1000/[kynurenine] increased in both groups at recovery, suggesting an improved balance between neurotoxic and neuroprotective influences. Tryptophan and kynurenine correlated with body mass index, suggesting a relationship with obesity. CONCLUSIONS: Acute exercise directly affects circulating levels of tryptophan, kynurenine and kynurenic acid, providing a potential mechanism for the anti-depressive effects of exercise. Furthermore, exercise-mediated changes in kynurenine metabolism are preserved in type 2 diabetic patients. Copyright © 2016 John Wiley & Sons, Ltd.

Behavioral disturbances in adult mice following neonatal virus infection or kynurenine treatment--role of brain kynurenic acid.

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.

High brain lactate is a hallmark of aging and caused by a shift in the lactate dehydrogenase A/B ratio.

At present, there are few means to track symptomatic stages of CNS aging. Thus, although metabolic changes are implicated in mtDNA mutation-driven aging, the manifestations remain unclear. Here, we used normally aging and prematurely aging mtDNA mutator mice to establish a molecular link between mitochondrial dysfunction and abnormal metabolism in the aging process. Using proton magnetic resonance spectroscopy and HPLC, we found that brain lactate levels were increased twofold in both normally and prematurely aging mice during aging. To correlate the striking increase in lactate with tissue pathology, we investigated the respiratory chain enzymes and detected mitochondrial failure in key brain areas from both normally and prematurely aging mice. We used in situ hybridization to show that increased brain lactate levels were caused by a shift in transcriptional activities of the lactate dehydrogenases to promote pyruvate to lactate conversion. Separation of the five tetrameric lactate dehydrogenase (LDH) isoenzymes revealed an increase of those dominated by the Ldh-A product and a decrease of those rich in the Ldh-B product, which, in turn, increases pyruvate to lactate conversion. Spectrophotometric assays measuring LDH activity from the pyruvate and lactate sides of the reaction showed a higher pyruvate → lactate activity in the brain. We argue for the use of lactate proton magnetic resonance spectroscopy as a noninvasive strategy for monitoring this hallmark of the aging process. The mtDNA mutator mouse allows us to conclude that the increased LDH-A/LDH-B ratio causes high brain lactate levels, which, in turn, are predictive of aging phenotypes.

CSF dopamine is elevated in first-episode psychosis and associates to symptom severity and cognitive performance.

BACKGROUND: The hypothesis of dopamine dysfunction in psychosis has evolved since the mid-twentieth century. However, clinical support from biochemical analysis of the transmitter in patients is still missing. The present study assessed dopamine and related metabolites in the cerebrospinal fluid (CSF) of first-episode psychosis (FEP) subjects. METHODS: Forty first-episode psychosis subjects and twenty healthy age-matched volunteers were recruited via the Karolinska Schizophrenia Project, a multidisciplinary research consortium that investigates the pathophysiology of schizophrenia. Psychopathology, disease severity, and cognitive performance were rated as well as cerebrospinal fluid concentrations of dopamine and related metabolites were measured using a sensitive high-pressure liquid chromatography assay. RESULTS: CSF dopamine was reliably detected in 50 % of healthy controls and in 65 % of first-episode psychosis subjects and significantly higher in first-episode psychosis subjects compared to age-matched healthy controls. No difference in CSF dopamine levels was observed between drug-naive subjects and subjects with short exposure to antipsychotics. The dopamine concentrations were positively associated with illness severity and deficits in executive functioning. CONCLUSIONS: Dopamine dysfunction has long been considered a cornerstone of the pathophysiology of schizophrenia, although biochemical support for elevated brain dopamine levels has been lacking. The results of the present study, showing that FEP subjects have increased CSF dopamine levels that correlate to disease symptoms, should fill the knowledge gap in this regard.

The Kynurenine Pathway is Differentially Activated in Children with Lyme Disease and Tick-Borne Encephalitis.

In children, tick-borne encephalitis and neuroborreliosis are common infections affecting the central nervous system. As inflammatory pathways including cytokine expression are activated in these children and appear to be of importance for outcome, we hypothesized that induction of the kynurenine pathway may be part of the pathophysiological mechanism. Inflammatory biomarkers were analyzed in cerebrospinal fluid from 22 children with tick-borne encephalitis (TBE), 34 children with neuroborreliosis (NB) and 6 children with no central nervous system infection. Cerebrospinal fluid levels of kynurenine and kynurenic acid were increased in children with neuroborreliosis compared to the comparison group. A correlation was seen between expression of several cerebrospinal fluid cytokines and levels of kynurenine and kynurenic acid in children with neuroborreliosis but not in children with tick-borne encephalitis. These findings demonstrate a strong induction of the kynurenine pathway in children with neuroborreliosis which differs from that seen in children with tick-borne encephalitis. The importance of brain kynurenic acid (KYNA) in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in neuroborreliosis might be of importance for the pathophysiology of the disease. Drugs targeting the enzymes of this pathway may open the venue for novel therapeutic interventions.

Organic electronics for precise delivery of neurotransmitters to modulate mammalian sensory function.

Significant advances have been made in the understanding of the pathophysiology, molecular targets and therapies for the treatment of a variety of nervous-system disorders. Particular therapies involve electrical sensing and stimulation of neural activity, and significant effort has therefore been devoted to the refinement of neural electrodes. However, direct electrical interfacing suffers from some inherent problems, such as the inability to discriminate amongst cell types. Thus, there is a need for novel devices to specifically interface nerve cells. Here, we demonstrate an organic electronic device capable of precisely delivering neurotransmitters in vitro and in vivo. In converting electronic addressing into delivery of neurotransmitters, the device mimics the nerve synapse. Using the peripheral auditory system, we show that out of a diverse population of cells, the device can selectively stimulate nerve cells responding to a specific neurotransmitter. This is achieved by precise electronic control of electrophoretic migration through a polymer film. This mechanism provides several sought-after features for regulation of cell signalling: exact dosage determination through electrochemical relationships, minimally disruptive delivery due to lack of fluid flow, and on-off switching. This technology has great potential as a therapeutic platform and could help accelerate the development of therapeutic strategies for nervous-system disorders.

System A transporter SAT2 mediates replenishment of dendritic glutamate pools controlling retrograde signaling by glutamate.

Glutamate mediates several modes of neurotransmission in the central nervous system including recently discovered retrograde signaling from neuronal dendrites. We have previously identified the system N transporter SN1 as being responsible for glutamine efflux from astroglia and proposed a system A transporter (SAT) in subsequent transport of glutamine into neurons for neurotransmitter regeneration. Here, we demonstrate that SAT2 expression is primarily confined to glutamatergic neurons in many brain regions with SAT2 being predominantly targeted to the somatodendritic compartments in these neurons. SAT2 containing dendrites accumulate high levels of glutamine. Upon electrical stimulation in vivo and depolarization in vitro, glutamine is readily converted to glutamate in activated dendritic subsegments, suggesting that glutamine sustains release of the excitatory neurotransmitter via exocytosis from dendrites. The system A inhibitor MeAIB (alpha-methylamino-iso-butyric acid) reduces neuronal uptake of glutamine with concomitant reduction in intracellular glutamate concentrations, indicating that SAT2-mediated glutamine uptake can be a prerequisite for the formation of glutamate. Furthermore, MeAIB inhibited retrograde signaling from pyramidal cells in layer 2/3 of the neocortex by suppressing inhibitory inputs from fast-spiking interneurons. In summary, we demonstrate that SAT2 maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate.

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

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.

Exploring neurocircuitries of the basal ganglia by intracerebral administration of selective neurotoxins.

The detailed anatomy of the monoamine pathways of the rat, first described by the students of Nils Ake Hillarp in Sweden, provided the basis for a neurocircuitry targeted pharmacology, leading to important therapeutic breakthroughs. Progress was achieved by the introduction of accurate lesion techniques based on selective neurotoxins. Systematic intracerebral injections of 6-hydroxydopamine let Urban Ungerstedt at the Karolinska Institutet, Stockholm, Sweden, to propose the first stereotaxic mapping of the monoamine pathways in the rat brain; and the 'Rotational Behaviour', as a classical model for screening drugs useful for alleviating Parkinson's disease and other neuropathologies. The direction of the rotational behaviour induced by drugs administrated to unilaterally 6-hydroxydopamine-lesioned rats reveals their mechanism of action at dopamine synapses, as demonstrated when rotational behaviour was combined with microdialysis. The model was useful for proposing a role for dopamine receptors in the gating of the flow of information integrated and/or modulated by the basal ganglia, through different efferent pathways; notably the striatopallidal system, via D(2) receptors, and the striatonigral system, via D(1) receptors. The role of other dopamine receptor subtypes on rotational behaviour has not yet been clarified.

PHGDH Defines a Metabolic Subtype in Lung Adenocarcinomas with Poor Prognosis.

Molecular signatures are emerging determinants of choice of therapy for lung adenocarcinomas. An evolving therapeutic approach includes targeting metabolic dependencies in cancers. Here, using an integrative approach, we have dissected the metabolic fingerprints of lung adenocarcinomas, and we show that Phosphoglycerate dehydrogenase (PHGDH), the rate-limiting enzyme in serine biosynthesis, is highly expressed in a adenocarcinoma subset with poor prognosis. This subset harbors a gene signature for DNA replication and proliferation. Accordingly, models with high levels of PHGDH display rapid proliferation, migration, and selective channeling of serine-derived carbons to glutathione and pyrimidines, while depletion of PHGDH shows potent and selective toxicity to this subset. Differential PHGDH protein levels were defined by its degradation, and the deubiquitinating enzyme JOSD2 is a regulator of its protein stability. Our study provides evidence that a unique metabolic program is activated in a lung adenocarcinoma subset, described by PHGDH, which confers growth and survival and may have therapeutic implications.

Screening of microdialysates taken before and after induced liver damage; on-line solid phase extraction-electrospray ionization-mass spectrometry.

A novel method is described to follow known and unknown compounds in biological processes using microdialysis sampling and mass spectrometric detection. By implementation of internal standard, desalting/enrichment for the sample work-up, and multivariate data analysis, this methodology is a basis for future applications in early diagnosis of diseases and organ damage, as a complement to the routinely used clinical methods for biological samples. The present study includes screening without specific target analytes, of samples collected by microdialysis from liver of anaesthetized rats before and after local damage to this organ. Sample series were classified by principal component analysis, and the stimulation was identified in the chemical patterns produced by the presented analytical tool.

Neurochemical and behavioural characterisation of alkoxyamphetamine derivatives in rats.

The clinical utility of amphetamine and amphetamine analogues has been jeopardized by a number of side effects and toxicity, partly due to complex mechanisms of action. While some of the analogues have been individually characterised, there is still need for comparative studies, in particularly on their efficacy to release dopamine and 5-hydroxytryptamine, further enlightening some of the synaptic mechanisms conveying their actions. Thus, we have compared four alkoxyamphetamine derivatives, i.e., p-methoxyamphetamine; p-methoxymethamphetamine; methylenedioxyamphetamine, methylenedioxymethamphetamine, using methamphetamine, and D-amphetamine, as reference substances, on rotational behaviour and releasing mechanisms studied with in vivo microdialysis in rats. All alkoxylated-derivatives produced a long-lasting rotational behaviour at 10 mg/kg s.c., but the reference substances produced a strong rotation already at 2 mg/kg s.c. in 6-hydroxydopamine-lesioned rats. At the concentration of 100 micromolar, the alkoxylated-derivatives were equipotent to evoke dopamine and 5-hydroxytryptamine release in rat neostriatum, while D-amphetamine and methamphetamine were more efficient on dopamine release. Pre-treatment with methamphetamine or the alkoxylated-derivatives produced a remarkable decrease of the effect of K+ -depolarisation on both dopamine and 5-hydroxytryptamine release. The insertion of a methoxy or a methylenedioxy group on the benzene ring of D-amphetamine or methamphetamine, or N-methylation of the D-amphetamine molecule alters the selectivity of the compounds. The efficacy of the alkoxylated-derivatives on dopamine and 5-hydroxytryptamine release was similar, but stimulated less dopamine release and produced less rotational behaviour than D-amphetamine and methamphetamine. The lower efficacy of K+ -depolarisation following pre-treatments with the derivatives suggests an impairment of releasable monoamine stores. The present observations can enlighten the mechanisms of action of drugs showing a high risk for abuse among young populations.

Microdialysis for myocardial metabolic surveillance: developing a clinical technique.

Metabolic surveillance of the myocardium is of great interest in cardiac surgery. Microdialysis allows sampling of chemical substances from the interstitial fluid for immediate analysis. The two objectives of this study were to develop a technique for simple and safe implantation of a commercially available microdialysis probe (CMA-70) into the myocardium and to obtain reference data for further use and metabolic control. Eighteen pigs were used in an experimental ischaemic heart model where the left anterior descending coronary artery was occluded for 20 min. Microdialysis was performed proximally as well as distally to the arterial occlusion site corresponding to a control and an ischaemic area in the heart. Two techniques were tried for probe implantation, using either a pacemaker wire attached to the probe tip or a needle introducer. Metabolic substrates (glucose, lactate, glycerol and pyruvate) were collected before, during and after ischaemia, for up to 6 h. Both techniques were highly effective in registering metabolic changes due to ischaemia with sharp time resolution, but the needle introducer was superior regarding probe durability. It is concluded that the CMA-70 microdialysis probe implanted with the needle introducer allows for an accurate monitoring of myocardial metabolism during a prolonged period of time. Future studies in the human heart are warranted to further validate the technique.

Inhibition of kynurenine aminotransferase II reduces activity of midbrain dopamine neurons.

Kynurenic acid (KYNA), a neuroactive metabolite of tryptophan, is elevated in the brain of patients with psychotic disorders. Therefore, lowering brain KYNA levels might be a novel approach in the treatment of psychotic disorders. The present in vivo electrophysiological study aimed to investigate the effect of an inhibitor of kynurenine aminotransferase (KAT) II, the primary enzyme for KYNA synthesis, on dopamine (DA) neurons in the ventral tegmental area (VTA). Acute administration of the KAT II inhibitor PF-04859989 (5 or 10 mg/kg) was associated with a short-onset, time-dependent decrease in firing rate and burst activity of DA neurons, both parameters reaching a 50% reduction within 45 min. Furthermore, PF-04859989 reduced the number of spontaneously active DA cells as measured 4-6 after administration. Pretreatment with d-cycloserine (30 mg/kg) or CGP-52432 (10 mg/kg) prevented the inhibitory action of PF-04859989 (5 mg/kg) on firing rate and burst firing activity. In contrast, pretreatment with methyllycaconitine (MLA, 4 mg/kg) did not change the response, whereas picrotoxin (4.5 mg/kg) partially prevented the inhibitory effects of PF-04859989 (5 mg/kg, i.v.). Our results show that a specific inhibition of KAT II is associated with a marked reduction in VTA DA firing activity. This effect appears to be specifically executed by NMDA-receptors and mediated indirectly via a GABA(B)-receptor-induced disinhibition of DA neurons. Our findings are in line with the view that endogenous KYNA, by modulation of the NMDA-receptor, exerts important physiological roles in the brain.