Raffaella Carpenedo

Presynaptic kynurenate-sensitive receptors inhibit glutamate release

Kynurenic acid is a tryptophan metabolite provided with antagonist activity on ionotropic glutamate and α7 nicotinic acetylcholine receptors. We noticed that in rats with a dialysis probe placed in the head of their caudate nuclei, local administration of kynurenic acid (30–100 nm) significantly reduced glutamate output. Qualitatively and quantitatively similar effects were observed after systemic administration of kynurenine hydroxylase inhibitors, a procedure able to increase brain kynurenate concentrations. Interestingly, in microdialysis studies, methyllycaconitine (0.3–10 nm), a selective α7 nicotinic receptor antagonist, also reduced glutamate output. In isolated superfused striatal synaptosomes, kynurenic acid (100 nm), but not methyllycaconitine, inhibited the depolarization (KCl 12.5 mm)‐induced release of transmitter or previously taken‐up [3H]‐D‐aspartate. This inhibition was not modified by glycine, N‐methyl‐d‐aspartate or subtype‐selective kainate receptor agents, while CNQX or DNQX (10 µm), two AMPA and kainate receptor antagonists, reduced kynurenic acid effects. Low concentrations of kynurenic acid, however, did not modify [3H]‐kainate (high and low affinity) or [3H]‐AMPA binding to rat brain membranes. Finally, because metabotropic glutamate (mGlu) receptors modulate transmitter release in striatal preparations, we evaluated, with negative results, kynurenic acid (1–100 nm) effects in cells transfected with mGlu1, mGlu2, mGlu4 or mGlu5 receptors. In conclusion, our data show that kynurenate‐induced inhibition of glutamate release is not mediated by glutamate receptors. Nicotinic acetylcholine receptors, however, may contribute to the inhibitory effects of kynurenate found in microdialysis studies, but not in those found in isolated synaptosomes.

Inhibitors of kynurenine hydroxylase and kynureninase increase cerebral formation of kynurenate and have sedative and anticonvulsant activities

Kynurenate is an endogenous antagonist of the ionotropic glutamate receptors. It is synthesized from kynurenine, a tryptophan metabolite, and a significant increase in its brain concentration could be useful in pathological situations. We attempted to increase its neosynthesis by modifying kynurenine catabolism. Several kynurenine analogues were synthesized and tested as inhibitors of kynurenine hydroxylase (E.C.1.14.13.9) and of kynureninase (E.C.3.7.1.3), the two enzymes which catalyse the conversion of kynurenine to excitotoxin quinolinate. Among these analogues we observed that nicotinylalanine, a compound whose pharmacological properties have previously been reported, had an IC50 of 900 +/- 180 microM as inhibitor of kynurenine hydroxylase and of 800 +/- 120 microM as inhibitor of kynureninase. In the search for more potent molecules we noticed that meta-nitrobenzoylalanine had an IC50 of 0.9 +/- 0.1 microM as inhibitor of kynurenine hydroxylase and of 100 +/- 12 microM as inhibitor of kynureninase. When administered to rats meta-nitrobenzoylalanine (400 mg/kg) significantly increased the concentration of kynurenine (up to 10 times) and kynurenate (up to five times) in the brain. Similar results were obtained in the blood and in the liver. Furthermore meta-nitrobenzoylalanine increased in a dose dependent, long lasting (up to 13 times and up to 4 h) manner the concentration of kynurenate in the hippocampal extracellular fluid, as evaluated with a microdialysis technique. This increase was associated with a decrease in the locomotor activity and with protection from maximal electroshock-induced seizures in rats or from audiogenic seizures in DBA/2 mice. The conclusions drawn from the present study are: (i) meta-nitrobenzoylalanine is a potent inhibitor of kynurenine hydroxylase also affecting kynureninase; (ii) the inhibition of these enzymes causes a significant increase in the brain extracellular concentration of kynurenate; (iii) this increase is associated with sedative and anticonvulsant actions, suggesting a functional antagonism of the excitatory amino acid receptors.

Kynurenine Hydroxylase Inhibitors Reduce Ischemic Brain Damage: Studies with (m-Nitrobenzoyl)-Alanine (mNBA) and 3,4-Dimethoxy-[-N-4-(Nitrophenyl)Thiazol-2YL]-Benzenesulfonamide (Ro 61-8048) in Models of Focal or Global Brain Ischemia

Two kynurenine hydroxylase inhibitors, (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (Ro 61-8048), have been tested as neuroprotective agents on brain lesions induced by bilateral carotid occlusion in gerbils or by middle cerebral artery occlusion in rats. The percentage of lesioned pyramidal neurones found in the hippocampal CA1 region of gerbils subjected to bilateral carotid occlusion for 5 minutes decreased from 92 ± 10% in vehicle-treated animals to 7 ± 6% after mNBA (400 mg/kg intraperitoneally, three times at 1, 30, and 180 minutes after occlusion) or to 10±11% after Ro 61-8048 (40 mg/kg intraperitoneally, three times). A significant reduction in infarct volumes also was found when the kynurenine hydroxylase inhibitors were given to rats after permanent middle cerebral artery occlusion (from 207 ± 111 mm3 in vehicle-treated rats to 82 ± 18 and to 62 ± 57 mm3 in rats treated with mNBA, 400 mg/kg intraperitoneally, or with Ro 61-8048, 40 mg/kg intraperitoneally, respectively). The administration of mNBA (400 mg/kg intraperitoneally) or Ro 61-8048 (40 mg/kg intraperitoneally) to gerbils with a dialysis probe in their dorsal hippocampus or to rats with a dialysis probe in their parietal cortex significantly increased kynurenic acid concentration in the dialysates. The data suggest that inhibition of kynurenine hydroxylase could be a new avenue to reduce neuronal loss in brain ischemia.

Comparison of the Neurochemical and Behavioral Effects Resulting from the Inhibition of Kynurenine Hydroxylase and/or Kynureninase

Abstract: Several kynurenine analogues were synthesized and tested as inhibitors of the enzymes kynurenine hydroxylase and/or kynureninase with the aim of identifying new compounds able to inhibit the synthesis of quinolinic acid (an endogenous excitotoxin) and to increase that of kynurenic acid, an endogenous antagonist of ionotropic glutamate receptors. Among these analogues, we selected m‐nitrobenzoylalanine (mNBA) as an inhibitor of kynurenine hydroxylase and o‐methoxybenzoylalanine (oMBA) as an inhibitor of kynureninase. When administered to rats, mNBA was more potent than oMBA in increasing the content of kynurenine and of kynurenic acid in the brain, blood, liver, and kidney. This confirms that hydroxylation is the main pathway of kynurenine metabolism. Both mNBA and oMBA (50–400 mg/kg i.p.) increased the concentration of kynurenate in hippocampal extracellular spaces (as measured with a microdialysis technique) and, when simultaneously injected, their effects were additive. This biochemical effect was associated with a decrease in locomotor activity in rats and with a protection of audiogenic convulsions in DBA/2 mice. In conclusion, the results of the present experiments indicate the possibility of increasing the neosynthesis of kynurenic acid by inhibiting the enzymes that metabolize kynurenine to 3‐hydroxykynurenine or to anthranilic acid. The increased synthesis of kynurenate is associated with behavioral effects such as sedation and protection from seizures, which suggests a functional antagonism of the excitatory amino acid receptors.

Kynurenine Disposition in Blood and Brain of Mice: Effects of Selective Inhibitors of Kynurenine Hydroxylase and of Kynureninase

Abstract: To study the regulation of the synthesis of quinolinic and kynurenic acids in vivo, we evaluated (a) the metabolism of administered kynurenine by measuring the content of its main metabolites 3‐hydroxykynurenine, anthranilic acid, and 3‐hydroxyanthranilic acid in blood and brain of mice; (b) the effects of (m‐nitrobenzoyl)alanine, a selective inhibitor of kynurenine hydroxylase and of (o‐methoxybenzoyl)alanine, a selective inhibitor of kynureninase, on this metabolism; and (c) the effects of (o‐methoxybenzoyl)alanine on liver kynureninase and 3‐hydroxykynureninase activity. The conclusions drawn from these experiments are (a) the disposition of administered kynurenine preferentially occurs through hydroxylation in brain and through hydrolysis in peripheral tissues; (b) (m‐nitrobenzoyl)alanine, the inhibitor of kynurenine hydroxylase, causes the expected changes in brain kynurenine metabolism, such as a decrease of 3‐hydroxykynurenine, and an increase of kynurenic acid; and (c) (o‐methoxybenzoyl)alanine, the kynureninase inhibitor, increases brain concentration of the cytotoxic compound 3‐hydroxykynurenine, and unexpectedly does not reduce brain concentration of 3‐hydroxyanthranilic acid, the direct precursor of quinolinic acid. Taken together, the experiments suggest that the systemic administration of a kynurenine hydroxylase inhibitor is a rational approach to increase the brain content of kynurenate and to decrease that of cytotoxic kynurenine metabolites, such as 3‐hydroxykynurenine and quinolinic acid.

Kynurenine 3‐mono‐oxygenase inhibitors attenuate post‐ischemic neuronal death in organotypic hippocampal slice cultures

Kynurenine 3‐mono‐oxygenase (KMO) inhibitors reduce 3‐hydroxykynurenine (3‐HK) and quinolinic acid (QUIN) neosynthesis and facilitate kynurenine metabolism towards kynurenic acid (KYNA) formation. They also reduce tissue damage in models of focal or transient global cerebral ischemia in vivo. We used organotypic hippocampal slice cultures exposed to oxygen and glucose deprivation (OGD) to investigate KMO mechanism(s) of neuroprotective activity. Exposure of the slices to 30 min of OGD caused CA1 pyramidal cell death and significantly decreased the amount of KYNA released in the incubation medium. The KMO inhibitors (m‐nitrobenzoyl)‐alanine (30–100 µm) or 3,4‐dimethoxy‐[‐N‐4‐(nitrophenyl)thiazol‐2yl]‐benzenesulfonamide (1–10 µm) reduced post‐ischemic neuronal death and increased KYNA concentrations in slice incubation media. The maximal concentration of KYNA detected in the incubation media of slices treated with KMO inhibitors was approximately 50 nm and was too low to efficiently interact with α7 nicotinic acetylcholine receptors or with the glycineb site of N‐methyl‐d‐aspartate (NMDA) receptors. On the other hand, the addition of either 3‐HK or QUIN (1–10 µm) to OGD‐exposed hippocampal slices prevented the neuroprotective activity of KMO inhibitors. Our results suggest that KMO inhibitors reduce the neuronal death found in the CA1 region of organotypic hippocampal slices exposed to 30 min of OGD by decreasing the local synthesis of 3‐HK and QUIN.

HCV patients, psychopathology and tryptophan metabolism: analysis of the effects of pegylated interferon plus ribavirin treatment

AIMS Depression and other psychiatric disorders are frequent in HCV-infected patients, especially during interferon treatment. The molecular mechanism(s) underlying this finding is still unknown but it has been suggested that HCV and/or interferon administration may increase indoleamine 2,3-dioxygenase (IDO) activity, and reduce plasma tryptophan (TRP) levels and brain serotonin synthesis thus leading to psychopathological disorders. METHODS We studied 89 subjects: (a) 39 patients with chronic hepatitis C virus (HCV) infection and mild liver damage; (b) 39 healthy controls; and (c) 10 patients with chronic hepatitis B virus (HBV) infection. 15 of the patients with HCV infection were re-evaluated after antiviral treatment with pegylated interferon alpha-2a plus ribavirin leading to viral eradication. We measured serum TRP and kynurenine levels and IDO activity in macrophages. Furthermore, each patient had an accurate psychopathological evaluation. RESULTS HCV-infected patients had lower (-28%) serum TRP and kynurenine levels than healthy volunteers or HBV-infected patients with comparable liver damage. Depression and anxiety symptoms were particularly common in HCV patients. After viral clearance, macrophage IDO activity, plasma TRP and kynurenine levels returned toward normal values and psychopathology improved. CONCLUSION Our study shows that HCV patients have reduced serum TRP levels and confirms that they frequently suffer from anxiety and depression-related symptoms. The reduced IDO activity found in the macrophages of these patients suggests that HCV infection may hamper macrophage functions. After successful antiviral treatment, in spite of the expected increase of IDO activity in macrophages, we noticed that TRP and kynurenine plasma levels returned toward physiological levels and psychopathology decreased significantly.

Oxindole, a Sedative Tryptophan Metabolite, Accumulates in Blood and Brain of Rats with Acute Hepatic Failure

Abstract: Rats treated with oxindole (10–100 mg/kg i.p.), a putative tryptophan metabolite, showed decreased spontaneous locomotor activity, loss of the righting reflex, hypotension, and reversible coma. Brain oxindole levels were 0.05 ± 0.01 nmol/g in controls and increased to 8.1 ± 1.7 or 103 ± 15 nmol/g after its administration at doses of 10 or 100 mg/kg i.p., respectively. To study the role that oxindole plays in the neurological symptoms associated with acute liver failure, we measured the changes of its concentration in the brain after massive liver damage, and we investigated the possible metabolic pathways leading to its synthesis. Rats treated with either thioacetamide (0.2 and 0.4 g/kg i.p., twice) or galactosamine (1 and 2 g/kg i.p.) showed acute liver failure and a large increase in blood or brain oxindole concentrations (from 0.05 ± 0.01 nmol/g in brains of controls to 1.8 ± 0.3 nmol/g in brains of thioacetamide‐treated animals). Administration of tryptophan (300–1,000 mg/kg p.o.) caused a twofold increase, whereas administration of indole (10–100 mg/kg p.o.) caused a 200‐fold increase, of oxindole content in liver, blood, and brain, thus suggesting that indole formation from tryptophan is a limiting step in oxindole synthesis. Oral administration of neomycin, a broad‐spectrum, locally acting antibiotic agent able to reduce intestinal flora, significantly decreased brain oxindole content. Taken together, our data show that oxindole is a neurodepressant tryptophan metabolite and suggest that it may play a significant role in the neurological symptoms associated with acute liver impairment.

Studies on the Neuroprotective Action of Kynurenine Mono-Oxygenase Inhibitors in Post-Ischemic Brain Damage

Kynurenine 3-mono-oxygenase (KMO) inhibitors facilitate kynurenic acid (KYNA) neosynthesis and reduce the formation of 3OH-kynurenine (3-HK) and quinolinic acid (QUIN). They also attenuate post-ischemic brain damage and decrease glutamate (Glu) content in brain extracellular spaces. To investigate KMO mechanism(s) of neuroprotection, we performed experiments in gerbils subjected to bilateral carotid occlusion and in organotypic rat hippocampal slice cultures exposed to oxygen and glucose deprivation (OGD). In gerbils, direct application of KYNA (100 nM, through reverse microdialysis in the hippocampus) completely prevented the increase in Glu output induced by transient (5 min) occlusion of the carotids. In rat hippocampal slices exposed for 30 min to OGD, KMO inhibitors (m-nitrobenzoyl)-alanine (mNBA, 30-100 microM) or 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (Ro 61-8048, 1-10 microM) reduced post-ischemic neuronal death and increased KYNA concentrations in the incubation medium. KYNA may antagonize glycineb or alpha7 nicotinic acetylcholine receptors but the concentrations in the incubation medium never reached values that could efficiently antagonize receptor function. On the contrary, 3-HK (1-10 microM) added to slices exposed to OGD in the presence of KMO inhibitors completely prevented the neuroprotective effects of the inhibitors. Our findings suggest that KMO inhibitors reduce OGD-induced pyramidal cell death by decreasing 3-HK (and possibly QUIN) synthesis.

Oxindole in pathogenesis of hepatic encephalopathy

concentrations in their plasma than those without (0·249 [0·05] vs 0·96 [0·25] nmol/mL). Because plasma and brain concentrations of oxindole rapidly equilibrate, it is reasonable to assume that hepatic encephalopathy patients have elevated brain concentrations of this endogenous sedative agent. Studies involving a larger number of patients, correlating selected neurological signs with plasma oxindole levels are necessary in order to further evaluate the role this endogenous sedative plays in the neurological diseases associated with acute or chronic liver failure.