Formation of endogenous glutamatergic receptors antagonist kynurenic acid--differences between cortical and spinal cord slices

Binge-like mephedrone treatment induces memory impairment concomitant with brain kynurenic acid reduction in mice

While mephedrone (4-methylmethcathinone), a synthetic cathinone derivative, is widely abused by adolescents and young adults, the knowledge about its long-term effects on memory processes is limited. Kynurenic acid (KYNA) is a neuroactive metabolite of the kynurenine pathway of tryptophan degradation. KYNA is considered an important endogenous modulator influencing physiological and pathological processes, including learning and memory processes. The aim of this study was to determine whether (A) binge-like mephedrone administration (10.0 and 30.0 mg/kg, intraperitoneally, in 4 doses separated by 2 h) induces memory impairments, assessed 2, 8 and 15 days after mephedrone cessation in the passive avoidance test in mice, and whether (B) KYNA is involved in these memory processes. To clarify the role of KYNA in the mephedrone effects, its level in the murine brain in vivo, and in cortical slices in vitro, as well as the activities of kynurenine aminotransferases (KATs) I and II were assessed. Furthermore, cell line experiments were conducted to investigate the effects of mephedrone on normal human brain cells. Our results showed memory impairments 8 and 15 days after binge-like mephedrone administration. At the same time, reduction in the KYNA level in the murine brain was noted. In vitro studies showed no effect of mephedrone on the production of KYNA in cortical slices or on the activity of the KAT I and II enzymes. Finally, exposure of normal cells to mephedrone in vitro resulted in a modest reduction of cell viability and proliferation.

Kynurenic acid in synovial fluid and serum of patients with rheumatoid arthritis, spondyloarthropathy, and osteoarthritis

OBJECTIVE

Previously we demonstrated that kynurenic acid (KYNA), an endogenous metabolite of kynurenine, is present in the synovial fluid of patients with rheumatoid arthritis (RA). KYNA inhibits proliferation of synoviocytes in vitro. The goal of our study was to compare KYNA concentrations in synovial fluid and blood of patients with RA, inflammatory spondyloarthropathies (SpA), and osteoarthritis (OA).

METHODS

Serum and synovial fluid samples were obtained from 189 patients with RA, 56 patients with SpA, and 32 patients with OA. KYNA was separated using a high-performance liquid chromatography system and measured fluorometrically.

RESULTS

KYNA concentration in synovial fluid obtained from patients with RA and SpA was significantly lower than that in patients with OA (p < 0.05). The concentration of KYNA in serum of patients with RA, SpA, and OA did not differ among all groups studied. The positive correlation between KYNA content in synovial fluid and serum was found in patients with RA (p < 0.05). Univariate linear regression analysis demonstrated that fibrinogen was significantly associated with KYNA in synovial fluid (p < 0.05), and red blood cell counts, morning stiffness, and pain scores were significantly associated with KYNA level in serum (all p < 0.05). Multivariate regression analysis revealed correlation between the following independent variables: hemoglobin level, hematocrit, red blood cell count in conjunction with age and KYNA content in synovial fluid. A lack of correlation was observed between KYNA content in synovial fluid of patients with RA and other clinical and laboratory measures of disease activity.

CONCLUSION

Our data show a local deficit of KYNA in inflammatory states.

Kynurenic acid and kynurenine aminotransferases in retinal aging and neurodegeneration

The kynurenine aminotransferases (KATs) KAT I and KAT II are pivotal to the synthesis of kynurenic acid (KYNA), the only known endogenous glutamate receptor antagonist and neuroprotectant. KAT I and II have been found in avian, rodent, and human retina. Expression of KAT I in Müller cell endfeet and KAT II in retinal ganglion cells has been documented. Developmental changes in KAT expression and KYNA concentration in the avian and rodent retina have also been found. Studies of retinal neurodegeneration have shown alterations in KYNA synthesis in the retina in response to retinal ganglion cell loss. In DBA/2J mice, a model of ocular hypertension, an age-dependent decrease of retinal KYNA and KATs was found. In the corpora amylacea in the human retina intensive KAT I and II immunoreactivity was demonstrated. In summary, these findings point to the potential involvement of KYNA in the mechanisms of retinal aging and neurodegeneration.

Alterations of amino acids and glutamate transport in the DBA/2J mouse retina; possible clues to degeneration

BACKGROUND

The DBA/2J mouse spontaneously develops ocular hypertension and time-dependent progressive retinal ganglion cell (RGC) loss. This study examines changes in amino acid levels in the vitreous, and changes in the expression of retinal glutamate transporters and receptors that occur during the progression of this pathology.

METHODS

Retinas were obtained from DBA/2J mice at ages 3, 6 and 11 months. C57BL/6 mice were used as age-matched controls. Vitreal amino acid content was measured with HPLC. Western blotting and immunohistochemistry were performed using specific antibodies against the glutamate transporters (GLAST, GLT-1v, EAAC-1) and glutamate receptors, particularly NMDA (NR1, NR2A, NR2B) and AMPA (GluR1, GluR2/3, GluR4) receptors.

RESULTS

HPLC showed retinal concentrations of glutamate, glutamine, glycine, alanine, lysine, serine, and arginine to be significantly higher in DBA/2J mice at 11 months of age compared to age-matched controls. Western Blots revealed a moderate decrease of GLAST and GLT-1v expression in DBA/2J mice at 6 and 11 months as compared to age-matched controls while there was no change in EAAC1. Immunohistochemically, no changes in expression of NMDA and AMPA receptors were seen.

CONCLUSION

Alterations of amino acid content and enhanced glutamate neurotransmission might be involved in the pathogenesis of retinal neurodegeneration in the DBA/ 2J mouse model of ocular hypertension. Moreover, these mice provide an animal model for studying excitotoxic retinal damage.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

Kynurenic acid synthesis in bovine retinal slices--effect of glutamate agonists

The purpose of the present study was to investigate the effect of glutamate agonists upon kynurenic acid (KYNA) production in bovine retinal slices. Quantitative analysis of newly synthesized KYNA was carried out using an HPLC system and detected fluorimetrically. Glutamate at the concentration of 0.01, 0.1 and 1 mM reduced KYNA synthesis in the retinal slices to 70% (p < 0.05), 35% (p < 0.01) and 23% (p < 0.001), respectively. The concentration of glutamate reducing production of KYNA by 50% (IC(50)) was 0.035 mM (0.02-0.06). Aspartate at the concentration of 0.01, 0.1 and 1 mM lowered KYNA synthesis in the retinal slices to 80% (p < 0.01), 57% (p < 0.001) and 43% (p < 0.001), respectively. In contrast, kainic acid (up to 5 mM), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) (up to 1 mM) and 1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) (up to 3 mM) turned out to be ineffective in diminishing KYNA synthesis. These data demonstrate that glutamate, aspartate and N-methyl-D-aspartate (NMDA) inhibit KYNA synthesis in bovine retinal slices with different potency.

Demonstration of Kynurenine Aminotransferases I and II and Characterization of Kynurenic Acid Synthesis in Oligodendrocyte Cell Line (OLN-93)

In the present study we demonstrate for the first time that both kynurenine aminotransferase (KAT) isoforms I and II are present in the permanent immature rat oligodendrocytes cell line (OLN-93). Moreover, we provide evidence that OLN-93 cells are able to synthesize kynurenic acid (KYNA) from exogenously added L-kynurenine and we characterize its regulation by extrinsic factors. KYNA production in OLN-93 cells was depressed in the presence of aminotransferase inhibitor, aminooxyacetic acid and was not affected by depolarizing agents such as 50 mM K+ and 4-aminopyridine. Glutamate agonists, L-glutamate and D,L-homocysteine significantly decreased KYNA production. Selective agonist of ionotropic glutamate receptors Amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropionic acid (AMPA) lowered KYNA production in OLN-93 cell line, whereas N-methyl-D-aspartate (NMDA) had no influence on KYNA production. Furthermore, KYNA synthesis in OLN-93 cells was decreased in a concentration-dependent manner by amino acids transported by L-system, L-leucine, L-cysteine and L-tryptophan. The role of KYNA synthesis in oligodendrocytes needs further investigation.

Kynurenic acid synthesis in cerebral cortical slices of rats with progressing symptoms of thioacetamide-induced hepatic encephalopathy

Increased ammonia is a major pathogenic factor in hepatic encephalopathy (HE), a neurologic syndrome associated with glutamatergic dysfunction. Previous studies have shown that in rat cerebral cortical slices or a glia-derived cell line, acute treatment with ammonia in vitro and in vivo inhibits the production of a broad-spectrum antagonist of excitatory amino acid receptors, kynurenic acid (KYNA). The present study analyzed KYNA synthesis in cerebral cortical slices obtained from rats with progressing HE symptoms accompanying acute liver failure induced by one, two, or three intraperitoneal administrations of thioacetamide (TAA) at 24-hr intervals. KYNA synthesis was found decreased to 83% of control 24 hr after one administration of TAA and unaffected after two TAA injections, when moderate hyperammonemia was associated by metabolic and bioelectric activation of the central nervous system, but was not accompanied by typical HE symptoms. KYNA synthesis was elevated to 155% of control after three TAA administrations, a period in which the rats showed advanced HE symptoms including stupor or coma. KYNA synthesis at the advanced HE stage was inhibited by glutamate in a degree comparable to that observed in control slices. The elevation of KYNA synthesis was associated with increased activity of a kynurenine aminotransferase (KAT) isomer, KAT-II. KYNA synthesis did not differ from control 21 days after the third TAA administration when HE symptoms receded. The results suggest that alterations of KYNA synthesis may contribute to the imbalance between neural excitation and inhibition at different stages of HE.

Expression of kynurenine aminotransferases in the rat retina during development

The study investigates the cellular expression of kynurenine aminotransferases (KAT I and II) in the rat retina during development. At P1 (the day of birth) and P7 (the 7th day after birth), KAT I expression was observed in the inner plexiform layer (IPL), the fiber layer (FL), and in vertically running processes in the ganglion cell layer (GCL) (but not in the cell bodies). At P14 (the 14th day after birth) a strong KAT I immunoreactivity was observed in Müller cell endfeet. KAT II was expressed in the IPL, the FL, and in cells in the GCL at P1 and P7. From P14 on, KAT II expression in the IPL decreased. Double labeling revealed that KAT I was expressed in Müller cell endfeet, whilst KAT II both on retinal ganglion cells (RGC) and Müller cell endfeet. In conclusion, KAT I and II are present in the rat retina during development. The heterogeneity of the KAT developmental profiles possibly reflects a neuromodulatory role in the retinal differentiation.

Endogenous protectant kynurenic acid in amyotrophic lateral sclerosis

OBJECTIVES

Excitotoxicity may play a role in neurodegeneration in amyotrophic lateral sclerosis (ALS). Kynurenic acid (KYNA), an endogenous antagonist of excitatory amino acid receptors, may inhibit excitotoxic lesions. The aim of this study was to determine the concentration of KYNA in ALS patients.

MATERIAL AND METHODS

KYNA was measured by high-performance liquid chromatography in the serum and cerebrospinal fluid (CSF) from ALS and control patients.

RESULTS

Our study revealed that CSF KYNA concentration was significantly higher in patients with bulbar onset of ALS compared to controls, and compared to patients with limb onset of the disease. CSF KYNA was also higher in patients with severe clinical status compared to controls. Serum KYNA was significantly lower in ALS patients with severe clinical status compared to controls, and compared to patients with mild clinical status. There were no significant differences in CSF and serum KYNA concentration between the whole ALS group of patients and controls. There was no difference in CSF KYNA concentration between males and females, and there was no correlation between KYNA concentration and age of patients, and duration of ALS.

CONCLUSIONS

An increased CSF KYNA concentration in patients with bulbar onset of ALS and in patients with severe clinical status may indicate neuroprotective role of KYNA against excitotoxicity. The difference of KYNA concentration in CSF of patients with bulbar and limb onset of ALS suggests that these two variants of motor neuron disease may have different etiopathogenetic mechanisms.

Ontogenic changes of kynurenine aminotransferase I activity and its expression in the chicken retina

Kynurenine aminotransferases are key enzymes for the synthesis of kynurenic acid (KYNA), an endogenous glutamate receptor antagonist. The study described here examined ontogenic changes of kynurenine aminotransferase I (KAT I) activity and its expression in the chicken retina. KAT I activity measured on embryonic day 16 (E16) was significantly higher than at all other stages (E12, P0 and P7). Double labeling with antibodies against glutamine synthetase showed that on P7 KAT I was expressed in Müller cell endfeet and their processes in the inner retina. Since KAT I activity is high in the late embryonic stages, it is conceivable that it plays a neuromodulatory role in the retina during the late phase of embryogenesis.

Decreased level of kynurenic acid in cerebrospinal fluid of relapsing-onset multiple sclerosis patients

The present study was undertaken to measure cerebrospinal fluid (CSF) levels of kynurenic acid (KYNA) in patients with relapsing-onset multiple sclerosis (MS) during remission or not progressing for at least 2 months. In these patients the levels of CSF KYNA were found to be significantly lower compared with subjects with non-inflammatory neurological diseases, as well as those with inflammatory disease (median (interquartile range): 0.41 (0.3-0.5) pmol/ml, n=26 vs. 0.67 (0.5-1.1), n=23, P<0.01 and 1.7 (1.5-2.6), n=16, P<0.001, respectively). These results provide further evidence of the alterations in the kynurenine pathway during remitting-onset MS.

Regulation of kynurenic acid synthesis in C6 glioma cells

Studies with brain slices have provided evidence that synthesis of kynurenic acid (KYNA) from kynurenine (KYN), which occurs in astrocytes, is modulated by changes in the ionic composition of the medium and the presence of depolarizing agents or the excitatory amino acid glutamate (Glu). The present study analyzed the effects of changes in incubation medium on KYNA synthesis in cultured C6 glioma cells. The synthesis was not affected by omission of Na(+) and raising K(+) concentration to 50 mM, conditions that in brain slices stimulate or inhibit KYNA formation, respectively. KYNA synthesis in C6 cells was inhibited by the absence of Ca(2+), which contrasts with its Ca(2+) independence in brain slices. Also, lack of Mg(2+) and addition of a chloride channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonate (SITS), did not affect the synthesis. KYNA synthesis in C6 cells was dose dependently inhibited by Glu. The inhibitory effect of Glu was not affected by GDPbetaS, an antagonist of metabotropic Glu receptors, the receptor class prevailing in C6 cells, suggesting that Glu acted intracellularly. NH(4)Cl and veratridine decreased KYNA production, mirroring the effects noted in brain slices. KYNA synthesis was strongly reduced in the presence of leucine (Leu), and the uptake of [(14)C]Leu was inhibited by the KYNA precursor KYN, which points to Leu as a potential endogenous modulator of KYNA formation in CNS cells.