Microbiota, Tryptophan and Aryl Hydrocarbon Receptors as the Target Triad in Parkinson's Disease-A Narrative Review

In the era of a steadily increasing lifespan, neurodegenerative diseases among the elderly present a significant therapeutic and socio-economic challenge. A properly balanced diet and microbiome diversity have been receiving increasing attention as targets for therapeutic interventions in neurodegeneration. Microbiota may affect cognitive function, neuronal survival and death, and gut dysbiosis was identified in Parkinson's disease (PD). Tryptophan (Trp), an essential amino acid, is degraded by microbiota and hosts numerous compounds with immune- and neuromodulating properties. This broad narrative review presents data supporting the concept that microbiota, the Trp-kynurenine (KYN) pathway and aryl hydrocarbon receptors (AhRs) form a triad involved in PD. A disturbed gut-brain axis allows the bidirectional spread of pro-inflammatory molecules and α-synuclein, which may contribute to the development/progression of the disease. We suggest that the peripheral levels of kynurenines and AhR ligands are strongly linked to the Trp metabolism in the gut and should be studied together with the composition of the microbiota. Such an approach can clearly delineate the sub-populations of PD patients manifesting with a disturbed microbiota-Trp-KYN-brain triad, who would benefit from modifications in the Trp metabolism. Analyses of the microbiome, Trp-KYN pathway metabolites and AhR signaling may shed light on the mechanisms of intestinal distress and identify new targets for the diagnosis and treatment in early-stage PD. Therapeutic interventions based on the combination of a well-defined food regimen, Trp and probiotics seem of potential benefit and require further experimental and clinical research.

Novel Activity of Oral Hypoglycemic Agents Linked with Decreased Formation of Tryptophan Metabolite, Kynurenic Acid

Kynurenic acid is a tryptophan (Trp) metabolite formed along the kynurenine (KYN) pathway in the brain and in peripheral tissues. The disturbed formation of kynurenic acid, which targets glutamate-mediated neurotransmission, GPR35, and aryl hydrocarbon receptors of immune or redox status, was implicated in the development of neuropsychiatric and metabolic disorders among others. Kynurenic acid exerts neuroprotective and immunomodulatory effects, yet its high brain levels may negatively impact cognition. Changes in the Trp-KYN pathway are also linked with the pathogenesis of diabetes mellitus, which is an established risk factor for cardiovascular and neurological diseases or cognitive deficits. Here, the effects of metformin and glibenclamide on the brain synthesis of kynurenic acid were evaluated. Acute exposure of rat cortical slices in vitro to either of the drugs reduced kynurenic acid production de novo. Glibenclamide, but not metformin, inhibited the activity of kynurenic acid biosynthetic enzymes, kynurenine aminotransferases (KATs) I and II, in semi-purified cortical homogenates. The reduced availability of kynurenic acid may be regarded as an unwanted effect, possibly alleviating the neuroprotective action of oral hypoglycemic agents. On the other hand, considering that both compounds ameliorate the cognitive deficits in animal and human studies and that high brain kynurenic acid may hamper learning and memory, its diminished synthesis may improve cognition.

Fractalkine, sICAM-1 and Kynurenine Pathway in Restrictive Anorexia Nervosa-Exploratory Study

The link between the kynurenine pathway and immunomodulatory molecules—fractalkine and soluble intercellular adhesion molecule-1 (sICAM-1)—in anorexia nervosa (AN) remains unknown. Fractalkine, sICAM-1, tryptophan (TRP), kynurenine (KYN), neuroprotective kynurenic acid (KYNA), neurotoxic 3-OH-kynurenine (3-OH-KYN), and the expression of mRNA for kynurenine aminotransferases (KAT1-3) were studied in 20 female patients with restrictive AN (mostly drug-free, all during first episode of the disease) and in 24 controls. In AN, serum fractalkine, but not sICAM-1, KYNA, KYN, TRP or 3-OH-KYN, was higher; ratios TRP/KYN, KYN/KYNA, KYN/3-OH-KYN and KYNA/3-OH-KYN were unaltered. The expression of the gene encoding KAT3, but not of genes encoding KAT1 and KAT2 (measured in blood mononuclear cells), was higher in patients with AN. In AN, fractalkine positively correlated with TRP, while sICAM-1 was negatively associated with 3-OH-KYN and positively linked with the ratio KYN/3-OH-KYN. Furthermore, TRP and fractalkine were negatively associated with the body mass index (BMI) in AN. Expression of KAT1, KAT2 and KAT3 did not correlate with fractalkine, sICAM-1 or BMI, either in AN or control. Increased fractalkine may be an independent factor associated with the restrictive type of AN. Excessive physical activity probably underlies increased expression of KAT3 observed among enrolled patients. Further, longitudinal studies on a larger cohort of patients should be aimed to clarify the contribution of fractalkine and KAT3 to the pathogenesis of AN.

Experimental hypothyroidism raises brain kynurenic acid - Novel aspect of thyroid dysfunction

Hypothyroidism frequently manifests with altered mood and disturbed cognition. Kynurenic acid may influence cognition through antagonism of N-methyl-d-aspartate receptors (NMDA) and α7 nicotinic receptors. In here, thyroid hormones effects on kynurenic acid synthesis in rat cortical slices and on kynurenine aminotransferases (KATs) activity in semi-purified cortical homogenates were studied. Furthermore, brain kynurenic acid levels and KATs activities were evaluated in experimental model of hypothyroidism, induced by chronic administration of 0.05% propylthiouracil in drinking water. In vitro, L-thyroxine (T₄) and 3,3,5-triiodothyronine (T₃), reduced kynurenic acid synthesis and KATs activities (IC₅₀ ~ 50-150 μM). In vivo, propylthiouracil increased cortical, hippocampal and striatal, but not cerebellar kynurenic acid content (192%, 142% and 124% of control, respectively), despite uniformly decreased KAT II activity and lower cortical and striatal KAT I activity. T₄ application to hypothyroid animals restored kynurenic acid levels to control values and reversed enzymatic changes. T₄ alone did not change brain kynurenic acid levels, despite increased activities of brain KATs. Hence, thyroid hormones modulate kynurenic acid levels by two opposing mechanisms, stimulation of KATs activity, most probably transcriptional, and direct, post-translational inhibition of KATs. Lack of correlation between KATs activity and kynurenic acid level may reflect the influence of T₄ on organic anion transporter and result from impaired removal of kynurenic acid from the brain during hypothyroidism. Our data reveal novel mechanism linked with thyroid hormones deficiency and imply the potential involvement of increased brain kynurenic acid in the hypothyroidism-related cognitive disturbance.

Losartan Reverses Hippocampal Increase of Kynurenic Acid in Type 1 Diabetic Rats: A Novel Procognitive Aspect of Sartan Action

Patients with diabetes mellitus (DM) type 1 and 2 are at a higher risk of cognitive decline and dementia; however, the underlying pathology is poorly understood. Kynurenic acid (KYNA), endogenous kynurenine metabolite, displays pleiotropic effects, including a blockade of glutamatergic and cholinergic receptors. Apart from well-known glial origin, kynurenic acid is robustly synthesized in the endothelium and its serum levels correlate with homocysteine, a risk factor for cognitive decline. Studies in an experimental DM model suggest that a selective, hippocampal increase of the kynurenic acid level may be an important factor contributing to diabetes-related cognitive impairment. The aim of this study was to assess the effects of chronic, four-week administration of losartan, angiotensin receptor blocker (ARB), on the brain KYNA in diabetic rats. Chromatographic and rt-PCR techniques were used to measure the level of KYNA and the expression of genes encoding kynurenine aminotransferases, KYNA biosynthetic enzymes, in the hippocampi of rats with streptozotocin-induced DM, treated with losartan. The effect of losartan on KYNA synthesis de novo was also evaluated in vitro, in brain cortical slices. The hippocampal increase of KYNA content occurred in diabetic rats treated and nontreated with insulin. Losartan did not affect KYNA levels when administered per se to naïve or diabetic animals but normalized KYNA content in diabetic rats receiving concomitantly insulin. The expression of CCBL1 (kat 1), AADAT (kat 2), and KAT3 (kat 3) genes did not differ between analyzed groups. Low concentrations of losartan did not affect KYNA production in vitro. The neuroprotective effect of ARBs in diabetic individuals may be, at least partially, linked to modulation of KYNA metabolism. The ability of ARB to modulate synthesis of KYNA in diabetic brain does not seem to result from changed expression of genes encoding KATs. We propose possible involvement of angiotensin AT₄ receptors in the observed action of losartan.

[The kynurenic acid hypothesis - a new look at etiopathogenesis and treatment of schizophrenia]

Kynurenic acid (KYNA) is a neuroactive metabolite of tryptophan formed in the brain and in the periphery, known to block ionotropic glutamate receptors and α7 nicotinic receptors, and to act as a ligand of G protein-coupled GPR35 receptors and human aryl hydrocarbon (AHR) receptors. KYNA seems to modulate a number of mechanisms involved in the pathogenesis of schizophrenia including dopaminergic transmission in mesolimbic and mesocortical areas or glutamatemediated neurotransmission. The kynurenine hypothesis of schizophrenia links the occurrence of positive and negative symptoms of schizophrenia and cognitive impairments characteristic for the disease with the disturbances of kynurenine pathway function. Available data suggest that antipsychotic drugs may restore balance among kynurenine pathway metabolites, and that co-administration of glycine with antipsychotics may reduce extrapyramidal symptoms in patients suffering from schizophrenia. Central level of KYNA may increase in the course of inflammation, which is consistent with the inflammatory hypothesis of schizophrenia. Alterations of immune response and disturbed functioning of kynurenine pathway may lead to disproportion between neuroprotective and neurotoxic mechanisms in the brain. Currently, intense research efforts are focused on the role of kynurenine pathway metabolites in pathogenesis of schizophrenia, their association with the response to antipsychotic treatment, and search for novel medications modulating the function of kynurenine pathway.

Experimental diabetes mellitus type 1 increases hippocampal content of kynurenic acid in rats

BACKGROUND

Diabetes mellitus (DM) is frequently associated with peripheral and central complications and has recently emerged as a risk factor for cognitive impairment and dementia. Kynurenic acid (KYNA), a unique tryptophan derivative, displays pleiotropic effects including blockade of ionotropic glutamate and α7 nicotinic receptors. Here, the influence of experimental diabetes on KYNA synthesis was studied in rat brain.

METHODS

DM was induced by i.p. administration of streptozotocin (STZ). Five weeks later, KYNA content and the activity of semi-purified kynurenine aminotransferases (KATs) were measured in frontal cortex, hippocampus and striatum of diabetic and insulin-treated rats, using HPLC-based methods.

RESULTS

Hippocampal but not cortical or striatal KYNA concentration was considerably increased during DM, either untreated or treated with insulin (220% and 170% of CTR, respectively). The activity of kynurenine aminotransferase I (KAT I) was not affected by DM in all of the studied structures. KAT II activity was moderately increased in cortex (145% of CTR) and hippocampus (126% of CTR), but not in striatum of diabetic animals. Insulin treatment normalized cortical but not hippocampal KAT II activity.

CONCLUSIONS

A novel factor potentially implicated in diabetic hippocampal dysfunction has been identified. Observed increase of KYNA level may stem from the activation of endogenous neuroprotection, however, it may also have negative impact on cognition.

Clenbuterol enhances the production of kynurenic acid in brain cortical slices and glial cultures

The effect of a beta(2)-adrenergic agonist, clenbuterol on the production of a glutamate receptor antagonist, kynurenic acid was studied in vitro. Clenbuterol enhanced the production of kynurenic acid in brain cortical slices (0.1-1.0 mM) and in glial cultures (1-50 muM). Timolol, a non-selective beta-adrenergic antagonist prevented this effect. The presented data indicate a novel mechanism of action of beta(2)-adrenoceptor agonists and suggest that an increased formation of the endogenous glutamate receptor antagonist, kynurenic acid could partially contribute to their neuroprotective activity.

Enhanced glutamatergic transmission reduces the anticonvulsant potential of lamotrigine but not of felbamate against tonic-clonic seizures

The efficacy of lamotrigine and felbamate against maximal electroshock (MES)-induced seizures was assessed under conditions mimicking the pharmacoresistance associated with an increased excitatory neurotransmission. N-methyl-D-aspartate (NMDA), but not kainate applied at subconvulsive dose, reduced the activity of lamotrigine against MES-induced seizures increasing its ED50 value from 4.3 (3.2-5.6) to 6.1 (5.2-7.2) mg/kg (p < 0.001). This effect was reversed by co-application of an NMDAreceptor antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) at 0.1 mg/kg [4.5 (3.7-5.6) vs. 6.1 (5.2-7.2) mg/kg; p < 0.001]. The anticonvulsive action of felbamate was altered by neither NMDAnor kainate. In conclusion, the data presented here indicate that felbamate, but not lamotrigine, effectively prevents generalized tonic-clonic seizures, also when NMDA-mediated neurotransmission is enhanced. The impaired antiepileptic potential of lamotrigine might be restored in such scenario by the co-administration of a very low dose of NMDA receptor antagonist.

Hyperglycemia enhances the inhibitory effect of mitochondrial toxins and D,L-homocysteine on the brain production of kynurenic acid

We have evaluated the effect of diabetes-mimicking conditions on the inhibition of kynurenic acid (KYNA) production exerted by mitochondrial toxins: 3-nitropropionic acid (3-NPA) and aminooxyacetic acid (AOAA), by endogenous agonists of glutamate receptors: L-glutamate and L-cysteine sulfinate, and by a risk factor of atherosclerosis, D,L-homocysteine. Hyperglycemia (30 mM; 2 h) itself did not influence KYNA synthesis in brain cortical slices. However, it significantly enhanced the inhibitory effects of 3-NPA, AOAA and D,L-homocysteine, but not of L-glutamate and L-cysteine sulfinate, on KYNA production. Their IC(50) values were lowered from 5.8 (4.5-7.4) to 3.7 (3.1-4.5) mM (p < 0.01), from 11.6 (8.6-15.5) to 7.1 (4.9-10.3) microM (p < 0.05), and from 4.5 (3.5-5.8) to 2.4 (1.8-3.2) mM (p < 0.01), respectively. The obtained data suggest that during hyperglycemia, the mitochondrial impairment and high levels of D,L-homocysteine evoke stronger inhibition of KYNAsynthesis what may further exacerbate brain dysfunction and play a role in central complications of diabetes.

Serum kynurenic acid positively correlates with cardiovascular disease risk factor, homocysteine: a study in stroke patients

KYNA, an antagonist of ionotropic glutamate receptors and alpha7 nicotinic receptors, has been found as well in the brain as in the periphery. The altered metabolism of KYNA, especially its deficiency, can lead to the enhanced glutamate-mediated excitotoxicity, and was suggested to be a factor contributing to the development of neurodegeneration and seizures. Elevated serum concentration of homocysteine is considered to be an independent risk factor of atherosclerosis and is an emerging risk factor of cognitive dysfunction and stroke. In the present study, serum level of KYNA, homocysteine and other biochemical parameters were assessed in patients at early (up to 24 h after infarct) stage of stroke. Serum KYNA and homocysteine levels were similar in control (N = 26) and stroke (N = 24) groups. KYNA level correlated positively with the level of homocysteine in control and in stroke group, with p = 0.018; r = 0.462 and p = 0.027; r = 0.451, respectively. In control group, KYNA correlated positively also with age (p = 0.007; r = 0.514) and with creatinine level (p = 0.002; r = 0.581). In stroke group, serum KYNA correlated positively with creatinine (p = 0.001; r = 0.644) and with urea level (p < 0.001; r = 0.716). Homocysteine level correlated inversely with folate level in control (p = 0.01; r = -0.499) but not in stroke group (p = 0.13; r = -0.317). Serum homocysteine in stroke group correlated positively also with age (p = 0.001; r = 0.6401), and with urea level (p = 0.017; r = 0.4813). Clinical significance of the association between serum KYNA and homocysteine levels requires further investigation.

Effect of pesticides on kynurenic acid production in rat brain slices

Kynurenic acid (KYNA) is a broad spectrum antagonist of ionotropic glutamate receptors, preferentially active at the strychnine-insensitive glycine allosteric site of the N-methyl-D-aspartate (NMDA) receptor, and a noncompetitive antagonist of alpha7 nicotinic receptor. Animal studies showed that it possesses anticonvulsant and neuroprotective properties. Its involvement in the pathophysiology of various brain disorders was suggested. In this study, the effect of pesticides on KYNA production in brain cortical slices was investigated. Pyrethroids, deltamethrin and fenpropathrin significantly lowered KYNA production. Methomyl, bensultap, fipronil, diquat and MCPA were ineffective in this regard. In view of this data, the inhibition of KYNA synthesis appear to merit further investigation as a potential factor contributing to the toxicology of pyrethroids.

Adenosine A1 receptors and the anticonvulsant potential of drugs effective in the model of 3-nitropropionic acid-induced seizures in mice

The role of adenosine A1 receptors in the activity of drugs and substances protecting against seizures evoked by mitochondrial toxin, 3-nitropropionic acid (3-NPA) was studied in mice. Non-selective A1/A2 adenosine receptor antagonist, aminophylline and selective A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) diminished the anticonvulsive effects of diazepam, phenobarbital, valproate and gabapentin. In contrast, A1/A2 adenosine receptor antagonist, 8-(p-sulfophenyl)theophylline (8pSPT) not penetrating via blood-brain barrier was ineffective. Aminophylline and DPCPX but not 8pSPT also reversed the protective action of A1/A2 adenosine receptor agonist, 2-chloroadenosine (2-CADO) and selective A1 adenosine receptor agonist, R-N6-phenylisopropyloadenosine (R-PIA), against 3-NPA-evoked convulsions. Obtained results suggest that the central adenosine A1 receptor stimulation may play a role in the anticonvulsive potential of diazepam, phenobarbital, valproate and gabapentin in a novel model of 3-NPA-evoked seizures. Moreover, concomitant application of aminophylline with these drugs may reduce their clinical antiepileptic efficacy, especially among patients suffering from seizures related to the disturbances of mitochondrial respiratory chain.

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.

Endogenous level of kynurenic acid and activities of kynurenine aminotransferases following transient global ischemia in the gerbil hippocampus

The accumulated data indicate that massively released excitatory amino acids play a major role in mediating the acute ischemic neuronal degeneration. Kynurenic acid (KYNA), the endogenous glutamate receptor antagonist, displaying a particularly high affinity for the glycine-site of N-methyl-D-aspartate (NMDA) receptor, was shown to ameliorate ischemic brain damage and its altered metabolism was implicated in the pathogenesis of neurodegeneration during ischemia/anoxia. Thus, we investigated the effect of transient global ischemia in gerbils on the endogenous levels of KYNA and the activity of its biosynthethic enzymes, kynurenine aminotransferases I (KAT I) and II (KAT II) in the hippocampus, 24 and 72 h after the ischemic episode. The level of KYNA in CA1 area was not altered 24 and 72 h following transient global ischemia (39.7 +/- 3.1 vs. 44.8 +/- 4.2, and 46.3 +/- 4.0 vs. 47.8 +/- 3.9 fmol/mg of tissue). Similarly, the activities of KATs in CA1 area were not changed and reached 1.91 +/- 0.11 vs. 1.8 +/- 0.19 and 1.86 +/- 0.1 vs. 1.7 +/- 0.15 (KAT I), and 0.56 +/- 0.2 vs. 0.43 +/- 0.16 and 0.54 +/- 0.08 vs. 0.55 +/- 0.17 (KAT II) pmol KYNA/mg of tissue/h, respectively. The presented data indicate that KYNA production is preserved in CA1 area of gerbil hippocampus during early stages after ischemic insult.

Endothelium-dependent production and liberation of kynurenic acid by rat aortic rings exposed to L-kynurenine

Rat aortic slices produced and liberated the endogenous antagonist of glutamate receptors, kynurenic acid, upon exposure to L-kynurenine. Endothelium-denuded slices did not synthesize any measurable amount of kynurenic acid, indicating its endothelial origin. Aortic kynurenic acid production was diminished by modification of the ionic milieu, hypoxia and hypoglycemia, as well as by L-glutamate and L-aspartate, endogenous glutamate receptor agonists, and aminooxyacetic acid, a non-selective inhibitor of aminotransferases and mitochondrial respiration. These data pave the way for future research aimed to clarify the role of kynurenic acid in the physiology and pathology of the endothelium and vasculature.

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.

Propranolol and metoprolol enhance the anticonvulsant action of valproate and diazepam against maximal electroshock

The anticonvulsive potential of classical antiepileptics co-administered with beta-adrenergic receptor antagonists against generalized tonic-clonic seizures was evaluated in the model of maximal electroshock (MES)-induced convulsions. Propranolol, acebutolol, metoprolol and atenolol were tested in the doses not affecting the electroconvulsive threshold. Propranolol and metoprolol lowered the ED(50) of valproate and diazepam. Acebutolol reduced valproate's but not diazepam's ED(50) value. In contrast, hydrophilic atenolol, not penetrating via blood-brain barrier, affected neither the action of valproate nor diazepam. None of the studied drugs changed the protective activity of carbamazepine and phenytoin against MES. beta-blockers per se did not alter the motor performance of mice. Moreover, propranolol and metoprolol did not influence diazepam-evoked impairment of locomotor activity. The free plasma and brain levels of antiepileptic drugs were not affected by beta-blockers. In conclusion, the use of certain beta-adrenoceptor antagonists, such as propranolol and metoprolol, might improve the antiepileptic potential of valproate and diazepam.

N(G)-nitro-L-arginine and its methyl ester inhibit brain synthesis of kynurenic acid possibly via nitric oxide-independent mechanism

The effect of nitric oxide synthase (NOS) inhibitors on the brain production of endogenous glutamate receptor antagonist, kynurenic acid, was estimated in vitro. Under standard incubation conditions N(G)-nitro-L-arginine, but not N(G)-nitro-L-arginine methyl ester, up to 5 mM, or 7-nitroindazole, up to 100 microM, inhibited de novo synthesis of kynurenic acid in cortical slices. However, during prolonged incubation, N(G)-nitro-L-arginine methyl ester also reduced the production of kynurenic acid. The substrate for NOS, L-arginine (up to 5 mM), did not influence kynurenic acid synthesis and did not reverse the N(G)-nitro-L-arginine-evoked changes, suggesting that the observed effects are not related to disturbed generation of NO. Enzymatic studies revealed that N(G)-nitro-L-arginine and its methyl ester blocked the activity of brain kynurenine aminotransferase (KAT) I. The activity of KAT II was diminished only by N(G)-nitro-L-arginine. Kinetic analyses have shown that N(G)-nitro-L-arginine and its methyl ester reduce Vmax and increase Km of KAT I, whereas N(G)-nitro-L-arginine diminishes Vmax of KAT II. In conclusion, we report that N(G)-nitro-L-arginine and its methyl ester impair brain synthesis of kynurenic acid, probably via NO-independent mechanism, what could contribute, at least partially, to the enhancement of neurotoxicity or seizures observed in some experimental designs based on their use.

Protective effect of adenosine receptor agonists in a new model of epilepsy – seizures evoked by mitochondrial toxin, 3-nitropropionic acid, in mice

The role of adenosine receptor agonists in the convulsant activity of mitochondrial toxin, 3-nitropropionic acid (3-NPA), was studied in mice. The occurrence of seizures evoked by peripheral application of 3-NPA was inhibited with the use of A1 adenosine receptor agonist, R-N6-phenylisopropyladenosine and A1/A2 agonist, 2-chloroadenosine. Moreover, both drugs prevented 3-NPA-induced mortality. Similarly, A1/A2 agonist, 5'-N-ethylcarboxamidoadenosine, protected against seizures evoked by the intracerebral administration of 3-NPA, and this effect was reversed by the co-application of adenosine receptor antagonist, 8-(p-sulfophenyl)theophylline. Obtained results suggest that A1 adenosine receptor activation may modulate the chain of events leading to the development of 3-NPA-induced seizures.

Previous prolonged clonic seizures diminish antiepileptic activity of valproate against pentetrazol-evoked convulsions

Prolonged seizures may alter the brain function in numerous ways. It is conceivable that they might lead to modifications of seizure susceptibility or anticonvulsive drug efficacy, however, only limited data address this issue. Therefore, we have decided to estimate the antiepileptic activity of drugs interfering with GABA-ergic neurotransmission in mice subjected to prolonged clonic seizures 2 weeks before, using pentetrazol test. The activity of valproate, but not diazepam or phenobarbital, was diminished in animals following repetitive clonic seizures. It might be hypothesized that in humans suffering from epilepsy, prolonged seizures in the past might contribute to the lowered efficacy of valproic acid later on.

Effect of adenosine receptor agonists on neurodegenerative and convulsive activity of mitochondrial toxin, 3-nitropropionic acid

3-Nitropropionic acid (3-NPA) is a mitochondrial toxin inhibiting the activity of succinate dehydrogenase. Its experimental application in rodents causes lesions of the striatum resembling the course of Huntington's disease in humans. Recently, we have shown that 3-NPA is also a potent convulsive and proconvulsive agent. This study investigated the effects of adenosine receptor agonists on neurodegeneration and convulsions induced by 3-NPA. Adenosinergic agonists prevented seizures but not striatal neuronal loss evoked by 3-NPA, what suggests that different mechanisms might contribute to these pathologies associated with application of mitochondrial toxin.

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

Rat spinal cord slices produced kynurenic acid (KYNA) upon exposure to L-kynurenine. Aminooxyacetic acid, non-selective aminotransferase inhibitor, and L-glutamate, but neither N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-metyloisoxazolo-4-propionate (AMPA), nor kainate, diminished synthesis of KYNA. L-Glutamate action was less potent in spinal than in cortical slices. Metabotropic agonists, L-(+)-2-amino-4-phosphonobutyrate (L-AP4) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), used in concentrations inhibiting cortical KYNA synthesis, were ineffective in spinal cord. Spinal KYNA production seems less susceptible to inhibitory modulation.

Anticonvulsant action of chlormethiazole is prevented by subconvulsive amounts of strychnine and aminophylline but not by bicuculline and picrotoxin

The anticonvulsant action of chlormethiazole was evaluated with the use of subthreshold doses of convulsants affecting the purinergic, glycinergic and gamma-aminobutyric acid (GABA)-mediated transmission, i.e. aminophylline, strychnine, bicuculline and picrotoxin in the model of generalized tonic-clonic convulsions. Chlormethiazole protected mice against maximal electroshock-induced seizures with an ED50 of 130.8 mg/kg. Aminophylline (100 mg/kg) and strychnine (0.4 mg/kg) reversed the protective action of chlormethiazole against electroconvulsions raising the ED50 values of this drug to 218.6 and 208.6 mg/kg, respectively. In contrast, GABA antagonists, bicuculline and picrotoxin, neither affected the protection provided by chlormethiazole nor did they alter the protective activity of valproate, phenobarbital, diphenylhydantoin and carbamazepine against electroconvulsions. Our results indicate that (a) the anticonvulsant activity of chlormethiazole might be related to its interaction with strychnine-sensitive glycinergic as well as purinergic neurotransmission, (b) purinergic and strychnine-sensitive glycinergic events contribute more prominently than GABAergic ones to the anticonvulsant activity of the drugs providing protection against maximal electroshock-induced convulsions.

Seizures evoked by mitochondrial toxin, 3-nitropropionic acid: new mechanism of epileptogenesis?

A number of data concerning the central action of mitochondrial toxins, substances impairing mitochondrial synthesis of ATP and thus compromising cellular energy status, has emerged within last years. 3-Nitropropionic acid (3-NPA) is an irreversible inhibitor of succinate dehydrogenase and mitochondrial complex II. The experimental administration of 3-NPA may lead to selective neuronal loss and chorea-like behavioral alterations but, as was recently shown, it also evokes clonic convulsions in rodents. The gathered data suggest that disturbed mitochondrial energy metabolism might initiate the chain of events culminating in seizure episode and that 3-NPA might become a useful tool in studying "mitochondrial" seizures. It has been hypothesized that the resistance to standard anticonvulsive therapy occurring among high proportion of epilepsy sufferers may result from the impairment of mitochondrial energy status due to either genetic predispositions or environmental influences.

NG-nitro-L-arginine, a nitric oxide synthase inhibitor, and seizure susceptibility in four seizure models in mice

Nitric oxide may be involved in seizure phenomena even though data often seem to be contradictory. This prompted us to study the influence of nitric oxide upon electrically and chemically induced seizures. The effects of nitric oxide synthase inhibitor, NG-nitro-L-arginine (NNA), on pentylenetetrazol-, aminooxyacetic acid-, aminophylline-induced seizures or electroconvulsive shock were evaluated. NNA was applied at 1, 10 and 40 mg/ kg 0.5 and 2.0 h before chemical seizures and at 1 and 40 mg/kg 0.5 and 2.0 h prior to electroconvulsions. The nitric oxide synthase inhibitor (up to 40 mg/ kg) did not affect the susceptibility of mice to pentylenetetrazol, amino-oxyacetic acid or electroconvulsions. However, NNA significantly enhanced the convulsive properties of aminophylline when applied at 40 mg/kg, 0.5 h before the test. The CD50 value for aminophylline-induced clonus and tonus/ mortality was decreased from 233 to 191 and from 242 to 212 mg/kg, respectively. However, this pretreatment also led to a significant increase in the plasma levels of theophylline. Our results suggest that differential effects of NNA on chemically-induced convulsions might in some cases be associated with a pharmacokinetic interaction.

Dysfunction of brain kynurenic acid metabolism in Huntington's disease: focus on kynurenine aminotransferases

The levels of the neuroprotective excitatory amino acid receptor antagonist kynurenic acid (KYNA) have been previously shown to be reduced in several regions of the brain of Huntington's disease (HD) patients. Thus, KYNA has been speculatively linked to the pathogenesis of HD. We have examined KYNA levels and the activity of its two biosynthetic enzymes (kynurenine aminotransferases (KAT) I and II) in 12 regions of brains from late-stage HD patients and control donors (n = 17 each). KYNA levels were measured in the original tissue homogenate. Using [3H]kynurenine as the substrate, enzyme activities were determined in dialyzed tissue homogenates. KYNA levels in the caudate nucleus decreased from 733 +/- 95 in controls to 401 +/- 62 fmol/mg tissue in HD (p < 0.01). The activity of both enzymes was highest in cortical areas (e.g. control frontal cortex: KAT I: 148 +/- 18 fmol/mg tissue/h; KAT II: 25 +/- 2 fmol/mg tissue/h). The activities of both KAT I and KAT II, when expressed per mg original weight, showed significant decreases (48-55%) in the HD putamen (p < 0.01). Trends toward lower enzyme activities and KYNA concentrations were detected in other brain areas as well. Kinetic analyses, performed in putamen and cerebellum, showed an approximately 3-fold increase in Km values for both KAT I and KAT II in the putamen only. Vmax values remained unchanged in the HD brain. These findings indicate a selective impairment in KYNA biosynthesis in the neostriatum of HD patients, possibly due to the loss of (an) endogenous KAT activator(s).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of the astrocytic enzymes 3-hydroxyanthranilic acid oxygenase, kynurenine aminotransferase and glutamine synthetase in seizure-prone and non-epileptic mice

Previous investigations in seizure-prone mice have suggested that an abnormally elevated production of the astrocyte-derived neuroexcitant, quinolinic acid (QUIN), plays a role in seizure susceptibility. In order to evaluate further the role of QUIN metabolism in genetic murine seizure models, the activities of its biosynthetic enzyme 3-hydroxyanthranilic acid oxygenase (3HAO), and of two other astrocytic enzymes, kynurenine aminotransferase (KAT) and glutamine synthetase (GS), were measured in the brains of seizure-prone EL and DBA/2 mice and two non-epileptic strains (BALB/c and Swiss-Webster). 3HAO activity was found to be markedly higher in both EL and DBA/2 mice than in the non-epileptic strains in all brain regions examined. The activity of 3HAO was not modified by the tossing procedure employed to promote seizures in EL mice. While some strain differences were noted in the activities of KAT and GS, these enzymes did not distinguish seizure-prone from the non-epileptic mice. In order to delineate better the relationship between glial activation and 3HAO, KAT and GS, further studies were performed in the ibotenate-lesioned hippocampus. In mice (but not in rats), the activity of 3HAO was selectively increased in gliotic tissue. These data demonstrate substantial species and strain differences in astroglial enzymes and in their response to brain injury. The observation of widespread abnormally high 3HAO activity in two distinct seizure-prone mouse strains strengthens the hypothesis that enhanced production of QUIN contributes to seizure susceptibility in mice.