Kynurenine diminishes the ischemia-induced histological and electrophysiological deficits in the rat hippocampus

Causal relationship between human blood metabolites and risk of ischemic stroke: a Mendelian randomization study

Background: Ischemic stroke (IS) is a major cause of death and disability worldwide. Previous studies have reported associations between metabolic disorders and IS. However, evidence regarding the causal relationship between blood metabolites and IS lacking. Methods: A two-sample Mendelian randomization analysis (MR) was used to assess the causal relationship between 1,400 serum metabolites and IS. The inverse variance-weighted (IVW) method was employed to estimate the causal effect between exposure and outcome. Additionally, MR-Egger regression, weighted median, simple mode, and weighted mode approaches were employed as supplementary comprehensive evaluations of the causal effects between blood metabolites and IS. Tests for pleiotropy and heterogeneity were conducted. Results: After rigorous selection, 23 known and 5 unknown metabolites were identified to be associated with IS. Among the 23 known metabolites, 13 showed significant causal effects with IS based on 2 MR methods, including 5-acetylamino-6-formylamino-3-methyluracil, 1-ribosyl-imidazoleacetate, Behenoylcarnitine (C22), N-acetyltyrosine, and N-acetylputrescine to (N (1) + N (8))-acetate,these five metabolites were positively associated with increased IS risk. Xanthurenate, Glycosyl-N-tricosanoyl-sphingadienine, Orotate, Bilirubin (E,E), Bilirubin degradation product, C17H18N2O, Bilirubin (Z,Z) to androsterone glucuronide, Bilirubin (Z,Z) to etiocholanolone glucuronide, Biliverdin, and Uridine to pseudouridine ratio were associated with decreased IS risk. Conclusion: Among 1,400 blood metabolites, this study identified 23 known metabolites that are significantly associated with IS risk, with 13 being more prominent. The integration of genomics and metabolomics provides important insights for the screening and prevention of IS.

Ion Channel Disturbances in Migraine Headache: Exploring the Potential Role of the Kynurenine System in the Context of the Trigeminovascular System

Migraine is a primary headache disorder, which is an enormous burden to the healthcare system. While some aspects of the pathomechanism of migraines remain unknown, the most accepted theory is that activation and sensitization of the trigeminovascular system are essential during migraine attacks. In recent decades, it has been suggested that ion channels may be important participants in the pathogenesis of migraine. Numerous ion channels are expressed in the peripheral and central nervous systems, including the trigeminovascular system, affecting neuron excitability, synaptic energy homeostasis, inflammatory signaling, and pain sensation. Dysfunction of ion channels could result in neuronal excitability and peripheral or central sensitization. This narrative review covers the current understanding of the biological mechanisms leading to activation and sensitization of the trigeminovascular pain pathway, with a focus on recent findings on ion channel activation and modulation. Furthermore, we focus on the kynurenine pathway since this system contains kynurenic acid, which is an endogenous glutamate receptor antagonist substance, and it has a role in migraine pathophysiology.

The role of the kynurenine pathway in cognitive functioning after stroke: A prospective clinical study

BACKGROUND

The kynurenine pathway is the main metabolic pathway of tryptophan degradation and has been associated with stroke and impaired cognitive functioning, but studies on its role in post-stroke cognitive impairment (PSCI) are scarce. We aimed to investigate associations between metabolites of the kynurenine pathway at baseline and post-stroke cognitive functioning over time.

METHODS

Baseline plasma kynurenines were quantified in 198 stroke patients aged 65.4 ± 10.8 years, 138 (69.7%) men, who were followed up over a period of three years after stroke. Baseline and longitudinal associations of kynurenines with PSCI and cognitive domain scores were investigated using linear mixed models, adjusted for several confounders.

RESULTS

No evidence of associations between kynurenines and odds of PSCI were found. However, considering individual cognitive domains, higher plasma levels of anthranilic acid (AA) were associated with better episodic memory at baseline (β per SD 0.16 [0.05, 0.28]). Additionally, a linear-quadratic association was found for the kynurenic acid/ quinolinic acid ratio (KA/QA), a neuroprotective index, with episodic memory (Wald χ2 = 8.27, p = .016). Higher levels of KA were associated with better processing speed in women only (pinteraction = .008; β per SD 0.15 [95% CI 0.02, 0.27]). These associations did not change over time.

CONCLUSIONS

Higher levels of KA, AA and KA/QA were associated with better scores on some cognitive domains at baseline. These associations did not change over time. Given the exploratory nature and heterogeneity of findings, these results should be interpreted with caution, and verified in other prospective studies.

Inflammation in the pathogenesis of depression: a disorder of neuroimmune origin

There are several hypotheses concerning the underlying pathophysiological mechanisms of major depression, which centre largely around adaptive changes in neuronal transmission and plasticity, neurogenesis, and circuit and regional connectivity. The immune and endocrine systems are commonly implicated in driving these changes. An intricate interaction of stress hormones, innate immune cells and the actions of soluble mediators of immunity within the nervous system is described as being associated with the symptoms of depression. Bridging endocrine and immune processes to neurotransmission and signalling within key cortical and limbic brain circuits are critical to understanding depression as a disorder of neuroimmune origins. Emergent areas of research include a growing recognition of the adaptive immune system, advances in neuroimaging techniques and mechanistic insights gained from transgenic animals. Elucidation of glial-neuronal interactions is providing additional avenues into promising areas of research, the development of clinically relevant disease models and the discovery of novel therapies. This narrative review focuses on molecular and cellular mechanisms that are influenced by inflammation and stress. The aim of this review is to provide an overview of our current understanding of depression as a disorder of neuroimmune origin, focusing on neuroendocrine and neuroimmune dysregulation in depression pathophysiology. Advances in current understanding lie in pursuit of relevant biomarkers, as the potential of biomarker signatures to improve clinical outcomes is yet to be fully realised. Further investigations to expand biomarker panels including integration with neuroimaging, utilising individual symptoms to stratify patients into more homogenous subpopulations and targeting the immune system for new treatment approaches will help to address current unmet clinical need.

Neuroprotective Effect of White Nelumbo nucifera Gaertn. Petal Tea in Rats Poisoned with Mancozeb

Nelumbo nucifera Gaertn. (N. nucifera) tea is used as food and folk medicine to reduce toxicity in Southeast Asia. Mancozeb (Mz) is used for controlling fungi in agriculture and contains heavy metals. This study aimed to examine the effect of white N. nucifera petal tea on cognitive behavior, hippocampus histology, oxidative stress, and amino acid metabolism in rats poisoned with mancozeb. Seventy-two male Wistar rats were divided into nine groups (n = 8 in each). Y-maze spontaneous alternation test was used to assess cognitive behavior, and amino acid metabolism was investigated by nuclear magnetic resonance spectroscopy (¹H-NMR) from blood. There was a significant increase in relative brain weight in the Mz co-administered with the highest dose (2.20 mg/kg bw) of white N. nucifera group. The levels of tryptophan, kynurenine, picolinic acid, and serotonin in blood showed a significant decrease in the Mz group and a significant increase in the Mz co-administered with low dose (0.55 mg/kg bw) of white N. nucifera group. However, there was no significant difference in cognitive behavior, hippocampus histology, oxidative stress, and corticosterone. This study demonstrated that a low dose of white N. nucifera petal tea has a neuroprotective effect against mancozeb.

Oxygen–Glucose Deprivation Promoted Fibroblast Senescence and Collagen Expression via IL11

Cell senescence is one of the most important forms of injury induced by cardiovascular and other ischemic diseases. Fibroblasts are important participants in tissue repair after ischemic injury and the main source of IL11 secretion. However, the roles of oxygen–glucose deprivation (OGD) and IL11 in promoting fibroblast senescence and their regulatory mechanisms remain unclear. This study selected the NIH3T3 and L929 fibroblast cell lines as research objects. We found that OGD could induce the expression of p53, P16, p21, and collagen in fibroblasts. In the condition of OGD, when IL11 intervened, fibroblasts’ senescence and collagen expression were changed. Some studies have found that changes in kynurenine (KYN) metabolism are related to aging diseases, and indoleamine 2,3-dioxygenase 1 (IDO1) is a key rate-limiting enzyme in the KYN metabolic pathway. We found that KYN secretion decreased after OGD increased fibroblast senescence, and inhibition of IL11 promoted IDO1 and increased KYN secretion. These results suggest that OGD may promote fibroblast senescence and collagen expression via IL11 inhibition of the IDO1/KYN metabolic pathway. Therefore, the revealed mechanism of OGD-promoted fibroblast senescence could provide an effective theoretical basis for the clinical treatment of aging-related ischemic diseases.

Memory Enhancement with Kynurenic Acid and Its Mechanisms in Neurotransmission

Kynurenic acid (KYNA) is an endogenous tryptophan (Trp) metabolite known to possess neuroprotective property. KYNA plays critical roles in nociception, neurodegeneration, and neuroinflammation. A lower level of KYNA is observed in patients with neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases or psychiatric disorders such as depression and autism spectrum disorders, whereas a higher level of KYNA is associated with the pathogenesis of schizophrenia. Little is known about the optimal concentration for neuroprotection and the threshold for neurotoxicity. In this study the effects of KYNA on memory functions were investigated by passive avoidance test in mice. Six different doses of KYNA were administered intracerebroventricularly to previously trained CFLP mice and they were observed for 24 h. High doses of KYNA (i.e., 20–40 μg/2 μL) significantly decreased the avoidance latency, whereas a low dose of KYNA (0.5 μg/2 μL) significantly elevated it compared with controls, suggesting that the low dose of KYNA enhanced memory function. Furthermore, six different receptor blockers were applied to reveal the mechanisms underlying the memory enhancement induced by KYNA. The series of tests revealed the possible involvement of the serotonergic, dopaminergic, α and β adrenergic, and opiate systems in the nootropic effect. This study confirmed that a low dose of KYNA improved a memory component of cognitive domain, which was mediated by, at least in part, four systems of neurotransmission in an animal model of learning and memory.

On the Antioxidant Properties of L-Kynurenine: An Efficient ROS Scavenger and Enhancer of Rat Brain Antioxidant Defense

L-kynurenine (L-KYN) is an endogenous metabolite, that has been used as a neuroprotective strategy in experimental models. The protective effects of L-KYN have been attributed mainly to kynurenic acid (KYNA). However, considering that L-KYN is prone to oxidation, this redox property may play a substantial role in its protective effects. The aim of this work was to characterize the potential impact of the redox properties of L-KYN, in both synthetic and biological systems. First, we determined whether L-KYN scavenges reactive oxygen species (ROS) and prevents DNA and protein oxidative degradation in synthetic systems. The effect of L-KYN and KYNA (0.1–100 µM) on redox markers (ROS production, lipoperoxidation and cellular function) was compared in rat brain homogenates when exposed to FeSO₄ (10 µM). Then, the effect of L-KYN administration (75 mg/kg/day for 5 days) on the GSH content and the enzymatic activity of glutathione reductase (GR) and glutathione peroxidase (GPx) was determined in rat brain tissue. Finally, brain homogenates from rats pretreated with L-KYN were exposed to pro-oxidants and oxidative markers were evaluated. The results show that L-KYN is an efficient scavenger of ^●OH and ONOO⁻, but not O₂^●– or H₂O₂ and that it prevents DNA and protein oxidative degradation in synthetic systems. L-KYN diminishes the oxidative effect induced by FeSO₄ on brain homogenates at lower concentrations (1 µM) when compared to KYNA (100 µM). Furthermore, the sub-chronic administration of L-KYN increased the GSH content and the activity of both GR and GPx, and also prevented the oxidative damage induced by the ex vivo exposure to pro-oxidants. Altogether, these findings strongly suggest that L-KYN can be considered as a potential endogenous antioxidant.

Increased kynurenine concentration attenuates serotonergic neurotoxicity induced by 3,4-methylenedioxymethamphetamine (MDMA) in rats through activation of aryl hydrocarbon receptor

3,4-Methylenedioxymethamphetamine (MDMA) is an amphetamine derivative that has been shown to produce serotonergic damage in the brains of primates, including humans, and of rats. Tryptophan, the precursor of serotonin, is primarily degraded through the kynurenine (KYN) pathway, producing among others KYN, the main metabolite of this route. KYN has been reported as an endogenous agonist of the aryl hydrocarbon receptor (AhR), a transcription factor involved in several neurological functions. This study aims to determine the effect of MDMA on the KYN pathway and on AhR activity and to establish their role in the long-term serotonergic neurotoxicity induced by the drug in rats. Our results show that MDMA induces the activation of the KYN pathway, mediated by hepatic tryptophan 2,3-dioxygenase (TDO). MDMA also activated AhR as evidenced by increased AhR nuclear translocation and CYP1B1 mRNA expression. Autoradiographic quantification of serotonin transporters showed that both the TDO inhibitor 680C91 and the AhR antagonist CH-223191 potentiated the neurotoxicity induced by MDMA, while administration of exogenous l-kynurenine or of the AhR positive modulator 3,3'-diindolylmethane (DIM) partially prevented the serotonergic damage induced by the drug. The results demonstrate for the first time that MDMA increases KYN levels and AhR activity, and these changes appear to play a role in limiting the neurotoxicity induced by the drug. This work provides a better understanding of the physiological mechanisms that attenuate the brain damage induced by MDMA and identify modulation of the KYN pathway and of AhR as potential therapeutic strategies to limit the negative effects of MDMA.

Are Kynurenines Accomplices or Principal Villains in Dementia? Maintenance of Kynurenine Metabolism

Worldwide, 50 million people suffer from dementia, a group of symptoms affecting cognitive and social functions, progressing severely enough to interfere with daily life. Alzheimer’s disease (AD) accounts for most of the dementia cases. Pathological and clinical findings have led to proposing several hypotheses of AD pathogenesis, finding a presence of positive feedback loops and additionally observing the disturbance of a branch of tryptophan metabolism, the kynurenine (KYN) pathway. Either causative or resultant of dementia, elevated levels of neurotoxic KYN metabolites are observed, potentially upregulating multiple feedback loops of AD pathogenesis. Memantine is an N-methyl-D-aspartate glutamatergic receptor (NMDAR) antagonist, which belongs to one of only two classes of medications approved for clinical use, but other NMDAR modulators have been explored so far in vain. An endogenous KYN pathway metabolite, kynurenic acid (KYNA), likewise inhibits the excitotoxic NMDAR. Besides its anti-excitotoxicity, KYNA is a multitarget compound that triggers anti-inflammatory and antioxidant activities. Modifying the KYNA level is a potential multitarget strategy to normalize the disturbed KYN pathway and thus to alleviate juxtaposing AD pathogeneses. In this review, the maintenance of KYN metabolism by modifying the level of KYNA is proposed and discussed in search for a novel lead compound against the progression of dementia.

Structural Evaluation and Electrophysiological Effects of Some Kynurenic Acid Analogs

Kynurenic acid (KYNA), a metabolite of tryptophan, as an excitatory amino acid receptor antagonist is an effective neuroprotective agent in case of excitotoxicity, which is the hallmark of brain ischemia and several neurodegenerative processes. Therefore, kynurenine pathway, KYNA itself, and its derivatives came into the focus of research. During the past fifteen years, our research group has developed several neuroactive KYNA derivatives, some of which proved to be neuroprotective in preclinical studies. In this study, the synthesis of these KYNA derivatives and their evaluation with divergent molecular characteristics are presented together with their most typical effects on the monosynaptic transmission in CA1 region of the hippocampus of the rat. Their effects on the basic neuronal activity (on the field excitatory postsynaptic potentials: fEPSP) were studied in in vitro hippocampal slices in 1 and 200 μM concentrations. KYNA and its derivative 4 in both 1 and 200 μM concentrations proved to be inhibitory, while derivative 8 only in 200 μM decreased the amplitudes of fEPSPs. Derivative 5 facilitated the fEPSPs in 200 μM concentration. This is the first comparative study which evaluates the structural and functional differences of formerly and newly developed KYNA analogs. Considerations on possible relations between molecular structures and their physiological effects are presented.

Systematic Review on the Involvement of the Kynurenine Pathway in Stroke: Pre-clinical and Clinical Evidence

Background: Stroke is the second leading cause of death after ischemic heart disease and the third leading cause of disability-adjusted life-years lost worldwide. There is a great need for developing more effective strategies to treat stroke and its resulting impairments. Among several neuroprotective strategies tested so far, the kynurenine pathway (KP) seems to be promising, but the evidence is still sparse.

Methods: Here, we performed a systematic review of preclinical and clinical studies evaluating the involvement of KP in stroke. We searched for the keywords: (“kynurenine” or “kynurenic acid” or “quinolinic acid”) AND (“ischemia” or “stroke” or “occlusion) in the electronic databases PubMed, Scopus, and Embase. A total of 1,130 papers was initially retrieved.

Results: After careful screening, forty-five studies were included in this systematic review, being 39 pre-clinical and six clinical studies. Despite different experimental models of cerebral ischemia, the results are concordant in implicating the KP in the pathophysiology of stroke. Preclinical evidence also suggests that treatment with kynurenine and KMO inhibitors decrease infarct size and improve behavioral and cognitive outcomes. Few studies have investigated the KP in human stroke, and results are consistent with the experimental findings that the KP is activated after stroke.

Conclusion: Well-designed preclinical studies addressing the expression of KP enzymes and metabolites in specific cell types and their potential effects at cellular levels alongside more clinical studies are warranted to confirm the translational potential of this pathway as a pharmacological target for stroke and related complications.

The role of the kynurenine pathway of tryptophan metabolism in cardiovascular disease

Coronary heart disease and stroke, the deadliest forms of cardiovascular disease (CVD), are mainly caused by atherosclerosis, a chronic inflammatory disease of the artery wall driven by maladaptive immune responses in the vessel wall. Various risk factors for CVD influence this pathogenic process, including diabetes mellitus, hypertension, dyslipidaemia, and obesity. Indoleamine 2,3-dioxygenase (IDO), an enzyme catalyzing the rate-limiting step in the kynurenine pathway of tryptophan degradation, is strongly induced by inflammation in several tissues, including the artery wall. An increasing body of evidence indicates that IDO promotes immune tolerance, decreases inflammation, and functions as a homeostatic mechanism against excessive immune reactions.

This review provides an overview of the emerging field of the kynurenine pathway of tryptophan degradation in CVD, emphasizing the role of IDO-mediated tryptophan metabolism and its metabolites in the modulation of ‘classical’ cardiovascular risk factors, such as hypertension, obesity, lipid metabolism, diabetes mellitus, and in the development of atherosclerotic CVD.

Interactions between the Kynurenine and the Endocannabinoid System with Special Emphasis on Migraine

Both the kynurenine and the endocannabinoid systems are involved in several neurological disorders, such as migraine and there are increasing number of reports demonstrating that there are interactions of two systems. Although their cooperation has not yet been implicated in migraine, there are reports suggesting this possibility. Additionally, the individual role of the endocannabinoid and kynurenine system in migraine is reviewed here first, focusing on endocannabinoids, kynurenine metabolites, in particular kynurenic acid. Finally, the function of NMDA and cannabinoid receptors in the trigeminal system-which has a crucial role in the pathomechanisms of migraine-will also be discussed. The interaction of the endocannabinoid and kynurenine system has been demonstrated to be therapeutically relevant in a number of pathological conditions, such as cannabis addiction, psychosis, schizophrenia and epilepsy. Accordingly, the cross-talk of these two systems may imply potential mechanisms related to migraine, and may offer new approaches to manage the treatment of this neurological disorder.

Crosstalk between Tryptophan Metabolism and Cardiovascular Disease, Mechanisms, and Therapeutic Implications

The cardiovascular diseases (CVD) associated with the highest rates of morbidity are coronary heart disease and stroke, and the primary etiological factor leading to these conditions is atherosclerosis. This long-lasting inflammatory disease, characterized by how it affects the artery wall, results from maladaptive immune responses linked to the vessel wall. Tryptophan (Trp) is oxidized in a constitutive manner by tryptophan 2,3-dioxygenase in liver cells, and for alternative cell types, it is catalyzed in the presence of a differently inducible indoleamine 2,3-dioxygenase (IDO1) in the context of a specific pathophysiological environment. Resultantly, this leads to a rise in the production of kynurenine (Kyn) metabolites. Inflammation in the preliminary stages of atherosclerosis has a significant impact on IDO1, and IDO1 and the IDO1-associated pathway constitute critical mediating agents associated with the immunoinflammatory responses that characterize advanced atherosclerosis. The purpose of this review is to survey the recent literature addressing the kynurenine pathway of tryptophan degradation in CVD, and the author will direct attention to the function performed by IDO1-mediated tryptophan metabolism.

Excitotoxins, Mitochondrial and Redox Disturbances in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). There is increasing evidence that MS is not only characterized by immune mediated inflammatory reactions, but also by neurodegenerative processes. There is cumulating evidence that neurodegenerative processes, for example mitochondrial dysfunction, oxidative stress, and glutamate (Glu) excitotoxicity, seem to play an important role in the pathogenesis of MS. The alteration of mitochondrial homeostasis leads to the formation of excitotoxins and redox disturbances. Mitochondrial dysfunction (energy disposal failure, apoptosis, etc.), redox disturbances (oxidative stress and enhanced reactive oxygen and nitrogen species production), and excitotoxicity (Glu mediated toxicity) may play an important role in the progression of the disease, causing axonal and neuronal damage. This review focuses on the mechanisms of mitochondrial dysfunction (including mitochondrial DNA (mtDNA) defects and mitochondrial structural/functional changes), oxidative stress (including reactive oxygen and nitric species), and excitotoxicity that are involved in MS and also discusses the potential targets and tools for therapeutic approaches in the future.

The Kynurenine Pathway in the Acute and Chronic Phases of Cerebral Ischemia

Kynurenines are a wide range of catabolites which derive from tryptophan through the "Kynurenine Pathway" (KP). In addition to its peripheral role, increasing evidence shows a role of the KP in the central nervous system (CNS), mediating both physiological and pathological functions. Indeed, an imbalance in this route has been associated with several neurodegenerative disorders such as Alzheimer´s and Huntington´s diseases. Altered KP catabolism has also been described during both acute and chronic phases of stroke; however the contribution of the KP to the pathophysiology of acute ischemic damage and of post-stroke disorders during the chronic phase including depression and vascular dementia, and the exact mechanisms implicated in the regulation of the KP after stroke are not well established yet. A better understanding of the regulation and activity of the KP after stroke could provide new pharmacological tools in both acute and chronic phases of stroke. In this review, we will make an overview of CNS modulation by the KP. We will detail the KP contribution in the ischemic damage, how the unbalance of the KP might trigger an alteration of the cognitive function after stroke as well as potential targets for the development of new drugs.

Altered hippocampal plasticity by prenatal kynurenine administration, kynurenine-3-monoxygenase (KMO) deletion or galantamine

Glutamate receptors sensitive to N-methyl-D-aspartate (NMDA) are involved in embryonic brain development but their activity may be modulated by the kynurenine pathway of tryptophan metabolism which includes an agonist (quinolinic acid) and an antagonist (kynurenic acid) at these receptors. Our previous work has shown that prenatal inhibition of the pathway produces abnormalities of brain development. In the present study kynurenine and probenecid (both 100mg/kg, doses known to increase kynurenic acid levels in the brain) were administered to female Wistar rats on embryonic days E14, E16 and E18 of gestation and the litter was allowed to develop to post-natal day P60. Western blotting revealed no changes in hippocampal expression of several proteins previously found to be altered by inhibition of the kynurenine pathway including the NMDA receptor subunits GluN1, GluN2A and GluN2B, as well as doublecortin, Proliferating Cell Nuclear Antigen (PCNA), sonic hedgehog and unco-ordinated (unc)-5H1 and 5H3. Mice lacking the enzyme kynurenine-3-monoxygenase (KMO) also showed no changes in hippocampal expression of several of these proteins or the 70-kDa and 100-kDa variants of Disrupted in Schizophrenia-1 (DISC1). Electrical excitability of pyramidal neurons in the CA1 region of hippocampal slices was unchanged, as was paired-pulse facilitation and inhibition. Long-term potentiation was decreased in the kynurenine-treated rats and in the KMO(-/-) mice, but galantamine reversed this effect in the presence of nicotinic receptor antagonists, consistent with evidence that it can potentiate glutamate at NMDA receptors. It is concluded that interference with the kynurenine pathway in utero can have lasting effects on brain function of the offspring, implying that the kynurenine pathway is involved in the regulation of early brain development.

Probenecid protects against cerebral ischemia/reperfusion injury by inhibiting lysosomal and inflammatory damage in rats

Probenecid has been used for decades to treat gout, and recent studies have revealed it is also a specific inhibitor of the pannexin-1 channel. It has been reported that the pannexin-1 channel is involved in ischemic injury. Here, we investigated the neuroprotective effect and the possible mechanisms of action of probenecid in global cerebral ischemia/reperfusion (I/R) injury in rats. Twenty minutes of transient global cerebral I/R injury was induced using the four-vessel occlusion (4-VO) method in male Sprague-Dawley rats. Different doses of probenecid were administered intravenously, intraperitoneally, or by gavage before or after reperfusion. Probenecid via all three routes protected against CA1 neuronal death when given before reperfusion. This protective effect continued when probenecid was given at 2h after reperfusion, but not at 6h. Interestingly, the protective effect regained if probenecid was given continuously for 7days after reperfusion. The release of cathepsin B and overexpression of calpain-1 after reperfusion were inhibited, while the upregulation of Hsp70 was strengthened by probenecid pre-treatment. Furthermore, the activation and proliferation of microglia and astrocytes after I/R injury were suppressed by continuous given for 7days, but only partly by a single dose at 6h of reperfusion. Thus, our data indicate that probenecid protects against transient global cerebral I/R injury probably by inhibiting calpain-cathepsin pathway and the inflammatory reaction.

Systemic L-Kynurenine sulfate administration disrupts object recognition memory, alters open field behavior and decreases c-Fos immunopositivity in C57Bl/6 mice

L-Kynurenine (L-KYN) is a central metabolite of tryptophan degradation through the kynurenine pathway (KP). The systemic administration of L-KYN sulfate (L-KYNs) leads to a rapid elevation of the neuroactive KP metabolite kynurenic acid (KYNA). An elevated level of KYNA may have multiple effects on the synaptic transmission, resulting in complex behavioral changes, such as hypoactivity or spatial working memory deficits. These results emerged from studies that focused on rats, after low-dose L-KYNs treatment. However, in several studies neuroprotection was achieved through the administration of high-dose L-KYNs. In the present study, our aim was to investigate whether the systemic administration of a high dose of L-KYNs (300 mg/bwkg; i.p.) would produce alterations in behavioral tasks (open field or object recognition) in C57Bl/6j mice. To evaluate the changes in neuronal activity after L-KYNs treatment, in a separate group of animals we estimated c-Fos expression levels in the corresponding subcortical brain areas. The L-KYNs treatment did not affect the general ambulatory activity of C57Bl/6j mice, whereas it altered their moving patterns, elevating the movement velocity and resting time. Additionally, it seemed to increase anxiety-like behavior, as peripheral zone preference of the open field arena emerged and the rearing activity was attenuated. The treatment also completely abolished the formation of object recognition memory and resulted in decreases in the number of c-Fos-immunopositive-cells in the dorsal part of the striatum and in the CA1 pyramidal cell layer of the hippocampus. We conclude that a single exposure to L-KYNs leads to behavioral disturbances, which might be related to the altered basal c-Fos protein expression in C57Bl/6j mice.

Changing the face of kynurenines and neurotoxicity: therapeutic considerations

Kynurenines are the products of tryptophan metabolism. Among them, kynurenine and kynurenic acid are generally thought to have neuroprotective properties, while 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid are considered neurotoxic. They participate in immunoregulation and inflammation and possess pro- or anti-excitotoxic properties, and their involvement in oxidative stress has also been suggested. Consequently, it is not surprising that kynurenines have been closely related to neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and multiple sclerosis. More information about the less-known metabolites, picolinic and cinnabarinic acid, evaluation of new receptorial targets, such as aryl-hydrocarbon receptors, and intensive research on the field of the immunomodulatory function of kynurenines delineated the high importance of this pathway in general homeostasis. Emerging knowledge about the kynurenine pathway provides new target points for the development of therapeutical solutions against neurodegenerative diseases.

Hippocampal cAMP/PKA/CREB is required for neuroprotective effect of acupuncture

Acupuncture has beneficial effects in vascular dementia (VaD) patients. The underlying mechanism, however, remains unknown. The present study was designed to investigate whether the cAMP/PKA/CREB cascade is involved in the mechanism of acupuncture in cerebral multi-infarction rats. In this study, cerebral multi-infarction was modeled in adult Wistar rats by homologous blood clot emboli. After a two-week acupuncture treatment at Zusanli (ST36), hippocampal-dependent memory was tested by employing a radial arm maze test. The hippocampus was isolated for analyses of cAMP concentration, phosphodiesterase (PDE) activity and CREB/pCREB and ERK/pERK expressions. The Morris water maze (MWM) task and CREB phosphorylation were evaluated in the presence of PKA-selective peptide inhibitor (H89). The radial arm maze test results demonstrated that acupuncture treatment at ST36 reversed hippocampal-dependent memory in impaired animals. Compared to those of the impaired group, cAMP concentration, PKA activity and pCREB and pERK expressions were increased following acupuncture therapy. Finally, the blockade of PKA reversed the increase in CREB phosphorylation and the improvement in recognitive function induced by acupuncture treatment. These results suggest that acupuncture could improve hippocampus function by modulating the cAMP/PKA/CREB signaling pathway, which represents a molecular mechanism of acupuncture for recognitive function in cerebral multi-infarction rats.

Plasma kynurenic acid concentration in patients undergoing cardiac surgery: effect of anaesthesia

Increases in plasma kynurenic acid (KYNA) concentration relate to the severity of inflammation. The aim of this study was to analyse changes in plasma KYNA concentration and neutrophil/lymphocyte ratio (NLR) in cardiac surgery patients. Additionally, the effect of anaesthesia was analysed. Adult cardiac surgery patients under intravenous general anaesthesia were studied. Additionally, some patients received sevoflurane (SEV) prior to cardiopulmonary bypass. Plasma KYNA concentration and NLR were measured before anaesthesia, just after surgery and on postoperative days 1, 2 and 3. Patients were assigned to two groups: patients who did not receive SEV (NonSEV group) and patients who received SEV (SEV group). Forty-three patients were studied. Twenty-four of them received SEV. KYNA increased immediately after surgery and remained elevated through postoperative day 3 in the NonSEV group, whereas it was similar to the preoperative concentration in the SEV group. NLR increased immediately after surgery in both groups, and higher values were noted in the NonSEV group than in the SEV group at postoperative days 2 and 3. Plasma KYNA concentration correlated with NLR in the NonSEV group. Cardiac surgery caused an increase in NLR. Plasma KYNA increased in the NonSEV group and correlated with NLR. Administration of SEV inhibited the increase in KYNA, most likely due to its anti-inflammatory properties.

Kynurenines and other novel therapeutic strategies in the treatment of dementia

Dementia is a common neuropsychological disorder with an increasing incidence. The most prevalent type of dementia is Alzheimer's disease. The underlying pathophysiological features of the cognitive decline are neurodegenerative processes, a cerebrovascular dysfunction and immunological alterations. The therapeutic approaches are still limited, although intensive research is being conducted with the aim of finding neuroprotective strategies. The widely accepted cholinesterase inhibitors and glutamate antagonists did not meet expectations of preventing disease progression, and research is therefore currently focusing on novel targets. Nonsteroidal anti-inflammatory drugs, secretase inhibitors and statins are promising drug candidates for the prevention and management of different forms of dementia. The kynurenine pathway has been associated with various neurodegenerative disorders and cerebrovascular diseases. This pathway is also closely related to neuroinflammatory processes and it has been implicated in the pathomechanisms of certain kinds of dementia. Targeting the kynurenine system may be of therapeutic value in the future.

Post-ischemic treatment with L-kynurenine sulfate exacerbates neuronal damage after transient middle cerebral artery occlusion

Since brain ischemia is one of the leading causes of adult disability and death, neuroprotection of the ischemic brain is of particular importance. Acute neuroprotective strategies usually have the aim of suppressing glutamate excitotoxicity and an excessive N-methyl-d-aspartate (NMDA) receptor function. Clinically tolerated antagonists should antagonize an excessive NMDA receptor function without compromising the normal synaptic function. Kynurenic acid (KYNA) an endogenous metabolite of the tryptophan metabolism, may be an attractive neuroprotectant in this regard. The manipulation of brain KYNA levels was earlier found to effectively enhance the histopathological outcome of experimental ischemic/hypoxic states. The present investigation of the neuroprotective capacity of L-kynurenine sulfate (L-KYNs) administered systemically after reperfusion in a novel distal middle cerebral artery occlusion (dMCAO) model of focal ischemia/reperfusion revealed that in contrast with earlier results, treatment with L-KYNs worsened the histopathological outcome of dMCAO. This contradictory result indicates that post-ischemic treatment with L-KYNs may be harmful.

Kynurenines in the CNS: recent advances and new questions

Various pathologies of the central nervous system (CNS) are accompanied by alterations in tryptophan metabolism. The main metabolic route of tryptophan degradation is the kynurenine pathway; its metabolites are responsible for a broad spectrum of effects, including the endogenous regulation of neuronal excitability and the initiation of immune tolerance. This Review highlights the involvement of the kynurenine system in the pathology of neurodegenerative disorders, pain syndromes and autoimmune diseases through a detailed discussion of its potential implications in Huntington's disease, migraine and multiple sclerosis. The most effective preclinical drug candidates are discussed and attention is paid to currently under-investigated roles of the kynurenine pathway in the CNS, where modulation of kynurenine metabolism might be of therapeutic value.

Fundamental interstrain differences in cortical activity between Wistar and Sprague-Dawley rats during global ischemia

Four-vessel occlusion (4VO), a frequently used model of global cerebral ischemia in rats, results in a dysfunction in wide brain areas, including the cerebral cortex and hippocampus. However, there are pronounced differences in response to global ischemia between the laboratory rat strains used in these studies. In the present work, the immediate acute effects of 4VO-induced global ischemia on the spontaneous electrocorticogram (ECoG) signals were analyzed in Wistar and Sprague-Dawley rats. The ECoG was isoelectric during the 10 min of global cerebral ischemia in Wistar rats and the first burst (FB) was seen 10-13 min after the start of reperfusion. In Sprague-Dawley rats, the FB was detected immediately after the start of 4VO or a few seconds later. The burst suppression ratio (BSR) in Wistar rats decreased to 45% in 5 min after FB, and after 25 min it was approximately 40%. In Sprague-Dawley rats, the BSR was 55% immediately after the FB and it decreased steeply to reach 0% by 10 min. There was also a significant difference between the two strains in the frequency composition of the ECoG pattern. The power spectral densities of the two strains differed virtually throughout the post-ischemic state. The histological results (Evans Blue, Cresyl Violet and Fluoro Jade C stainings) supplemented the electrophysiological data: the neuronal damage in the CA1 pyramids in Wistar rats was severe, whereas in the Sprague-Dawley animals it was only partial. These observations clearly demonstrate that the use of different rat strains (e.g. Wistar vs. Sprague-Dawley) can be a source of considerable variability in the results of acute experiments on global ischemia and it is important that the laboratory rats used in such experiments should be carefully chosen.

Mitochondrial disturbances, excitotoxicity, neuroinflammation and kynurenines: novel therapeutic strategies for neurodegenerative disorders

A mitochondrial dysfunction causes an abatement in ATP production, the induction of oxidative damage and the propagation of cell death pathways. It is additionally closely related to both glutamate excitotoxicity and neuroinflammation. All of these interconnected aspects of a cellular dysfunction are involved in the pathogenesis of numerous neurological disorders, including those with an acute (e.g. ischemic stroke) or a chronic (e.g. Huntington's disease) onset. Both acute and chronic neurodegenerative disorders have been demonstrated to involve multiple imbalances of the kynurenine pathway metabolism in the pathogenesis of the disease. As regards neuroactive compounds featuring in the pathway, quinolinic acid is a specific agonist of N-methyl-d-aspartate receptors, and a potent neurotoxin with additional and marked free radical-producing and lipid peroxidation-inducing properties. The toxic effects of 3-hydroxy-L-kynurenine are mediated by free radicals. Besides the possibility of increasing brain kynurenic acid concentrations, L-kynurenine may have vasoactive properties, too. Kynurenic acid has proven to be neuroprotective in several experimental settings, but in consequence of its pharmacokinetic properties it is not applicable as systemic administration in human cases. The aim of this short review is to emphasize the common features of cerebral ischemia and Huntington's disease and to highlight therapeutic strategies targeting the kynurenine pathway.

Protective function of nicotinamide against ketamine-induced apoptotic neurodegeneration in the infant rat brain

During development, anesthetics activate neuroapoptosis and produce damage in the central nervous system that leads to several types of neurological disorders. A single dose of ketamine (40 mg/kg) during synaptogenesis in a 7-day-old rat brain activated the apoptotic cascade and caused extensive neuronal cell death in the forebrain. In this study, we investigated the protective effect of nicotinamide against ketamine-induced apoptotic neurodegeneration. After 4 h, neuronal cell death induced by ketamine was associated with the induction of Bax, release of cytochrome c into the cytosol, and activation of caspase-3. One single dose of 1 mg/g nicotinamide was administered to a developing rat and was found to inhibit ketamine-induced neuroapoptosis by downregulating Bax, inhibiting cytochrome c release from mitochondria into cytosol, and inhibiting the expression of activated caspase-3. TUNEL and immunohistochemical analyses showed that ketamine-induced cell death occurred through apoptosis and that it was inhibited by nicotinamide. Fluoro-Jade-B staining demonstrated an increased number of dead cells in the cortex and thalamus after ketamine treatment; treatment with nicotinamide reduced the number of dead cells in these brain regions. Our findings suggest that nicotinamide attenuated ketamine-induced neuronal cell loss in the developing rat brain and is a promising therapeutic and neuroprotective agent for the treatment of neurodevelopmental disorders.

Synthesis and biological effects of some kynurenic acid analogs

The overactivation of excitatory amino acid receptors plays a key role in the pathomechanism of several neurodegenerative disorders and in ischemic and post-ischemic events. Kynurenic acid (KYNA) is an endogenous product of the tryptophan metabolism and, as a broad-spectrum antagonist of excitatory amino acid receptors, may serve as a protective agent in neurological disorders. The use of KYNA is excluded, however, because it hardly crosses the blood-brain barrier. Accordingly, new KYNA analogs which can readily cross this barrier and exert their complex anti-excitatory activity are generally needed. During the past 6 years, we have developed several KYNA derivatives, among others KYNA amides. These new analogs included one, N-(2-N,N-dimethylaminoethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride (KYNA-1), that has proved to be neuroprotective in several models. This paper reports on the synthesis of 10 new KYNA amides (KYNA-1-KYNA-10) and on the effectiveness of these molecules as inhibitors of excitatory synaptic transmission in the CA1 region of the hippocampus. The molecular structure and functional effects of KYNA-1 are compared with those of other KYNA amides. Behavioral studies with these KYNA amides demonstrated that they do not exert significant nonspecific general side-effects. KYNA-1 may therefore be considered a promising candidate for clinical studies.

Kainate postconditioning restores LTP in ischemic hippocampal CA1: onset-dependent second pathophysiological stress

Postconditioning can be induced by a broad range of stimuli within minutes to days after an ischemic cerebral insult. A special form is elicited by pharmacological intervention called second pathophysiological stress. The present study aimed to evaluate the effects of low-dose (5 mg/kg) kainate postconditioning with onsets 0, 24 and 48 h after the ischemic insult on the hippocampal synaptic plasticity in a 2-vessel occlusion model in rat. The hippocampal function was tested by LTP measurements of Schaffer collateral-CA1 pyramidal cell synapses in acute slices and the changes in density of Golgi-Cox-stained apical dendritic spines. Postconditioning 0 and 24 h after ischemia was not protective, whereas 48-h-onset postconditioning resulted in the reappearance of a normal spine density (>100,000 spines) 3 days after ischemia, in parallel with the long-term restoration of the damaged LTP function. Similar, but somewhat less effects were observed after 10 days. Our data clearly demonstrate the onset dependence of postconditioning elicited by a subconvulsant dose of kainate treatment in global ischemia, with restoration of the structural plasticity and hippocampal function.

On the antioxidant properties of kynurenic acid: free radical scavenging activity and inhibition of oxidative stress

Kynurenic acid (KYNA) is an endogenous metabolite of the kynurenine pathway for tryptophan degradation and an antagonist of both N-methyl-D-aspartate (NMDA) and alpha-7 nicotinic acetylcholine (α7nACh) receptors. KYNA has also been shown to scavenge hydroxyl radicals (OH) under controlled conditions of free radical production. In this work we evaluated the ability of KYNA to scavenge superoxide anion (O(2)(-)) and peroxynitrite (ONOO(-)). The scavenging ability of KYNA (expressed as IC(50) values) was as follows: OH=O(2)(-)>ONOO(-). In parallel, the antiperoxidative and scavenging capacities of KYNA (0-150 μM) were tested in cerebellum and forebrain homogenates exposed to 5 μM FeSO(4) and 2.5 mM 3-nitropropionic acid (3-NPA). Both FeSO(4) and 3-NPA increased lipid peroxidation (LP) and ROS formation in a significant manner in these preparations, whereas KYNA significantly reduced these markers. Reactive oxygen species (ROS) formation were determined in the presence of FeSO(4) and/or KYNA (0-100 μM), both at intra and extracellular levels. An increase in ROS formation was induced by FeSO(4) in forebrain and cerebellum in a time-dependent manner, and KYNA reduced this effect in a concentration-dependent manner. To further know whether the effect of KYNA on oxidative stress is independent of NMDA and nicotinic receptors, we also tested KYNA (0-100 μM) in a biological preparation free of these receptors - defolliculated Xenopus laevis oocytes - incubated with FeSO(4) for 1 h. A 3-fold increase in LP and a 2-fold increase in ROS formation were seen after exposure to FeSO(4), whereas KYNA attenuated these effects in a concentration-dependent manner. In addition, the in vivo formation of OH evoked by an acute infusion of FeSO(4) (100 μM) in the rat striatum was estimated by microdialysis and challenged by a topic infusion of KYNA (1 μM). FeSO(4) increased the striatal OH production, while KYNA mitigated this effect. Altogether, these data strongly suggest that KYNA, in addition to be a well-known antagonist acting on nicotinic and NMDA receptors, can be considered as a potential endogenous antioxidant.

Cell death/proliferation and alterations in glial morphology contribute to changes in diffusivity in the rat hippocampus after hypoxia-ischemia

To understand the structural alterations that underlie early and late changes in hippocampal diffusivity after hypoxia/ischemia (H/I), the changes in apparent diffusion coefficient of water (ADC(W)) were studied in 8-week-old rats after H/I using diffusion-weighted magnetic resonance imaging (DW-MRI). In the hippocampal CA1 region, ADC(W) analyses were performed during 6 months of reperfusion and compared with alterations in cell number/cell-type composition, glial morphology, and extracellular space (ECS) diffusion parameters obtained by the real-time iontophoretic method. In the early phases of reperfusion (1 to 3 days) neuronal cell death, glial proliferation, and developing gliosis were accompanied by an ADC(W) decrease and tortuosity increase. Interestingly, ECS volume fraction was decreased only first day after H/I. In the late phases of reperfusion (starting 1 month after H/I), when the CA1 region consisted mainly of microglia, astrocytes, and NG2-glia with markedly altered morphology, ADC(W), ECS volume fraction and tortuosity were increased. Three-dimensional confocal morphometry revealed enlarged astrocytes and shrunken NG2-glia, and in both the contribution of cell soma/processes to total cell volume was markedly increased/decreased. In summary, the ADC(W) increase in the CA1 region underlain by altered cellular composition and glial morphology suggests that considerable changes in extracellular signal transmission might occur in the late phases of reperfusion after H/I.

Vascular expression, activity and function of indoleamine 2,3-dioxygenase-1 following cerebral ischaemia-reperfusion in mice

Indoleamine 2,3-dioxygenases-1 (Ido1) and -2 initiate the kynurenine pathway of tryptophan metabolism. In addition to the established immune regulatory effects of Ido1 and the ability of nitric oxide to regulate Ido1 activity, it is now also known that Ido1-mediated metabolism of tryptophan to kynurenine can modulate vascular tone. Ido activity is reportedly elevated in stroke patients and correlates with increased risk of death. Thus, the present goals were to test whether, following cerebral ischaemia, Ido activity and cerebrovascular Ido1 expression are altered and whether expression of Ido1 contributes to stroke outcome. Transient cerebral ischaemia was induced in wild-type and Ido1 gene-deficient (Ido1 (-/-)) mice. Mice were pre-treated with vehicle, the Ido1 inhibitor, 1-methyl-D-tryptophan (1-MT; 50 mg/kg i.p.) or the inducible nitric oxide synthase (Nos2) inhibitor, aminoguanidine (AG, 100 mg/kg i.p.). At 24 h, neurological function, brain infarct size and swelling were assessed. In addition, Ido activity was estimated by plasma kynurenine and tryptophan, and Ido1 expression was examined in cerebral arterioles. Cerebral ischaemia-reperfusion in wild-type mice increased Ido activity and its expression in cerebral arterioles. Ido1 (-/-) and 1-MT-treated wild-type mice had lower Ido activity but similar post-stroke neurological function and similar total brain infarct volume and swelling, relative to control mice. Inhibition of Nos2 with AG also did not affect Ido activity or outcome following stroke. This study provides molecular and pharmacological evidence that the expression and the activity of Ido1 increase following stroke. However, such Ido1 expression does not appear to affect overall outcome following acute ischaemic stroke, and furthermore, a regulatory role of Nos2-derived nitric oxide on Ido activity following cerebral ischaemia-reperfusion appears unlikely.

The relationship between indoleamine 2,3-dioxygenase activity and post-stroke cognitive impairment

BACKGROUND

Activation of indoleamine 2,3-dioxygenase (IDO) and higher concentrations of several kynurenine metabolites have been observed post-stroke, where they have been associated with increased mortality. While lower tryptophan or a higher ratio of kynurenine/tryptophan (K/T) in peripheral blood have been associated with dementia and the severity of cognitive symptoms in Alzheimer's disease, the association between K/T ratios and post-stroke cognitive impairment (PSCI) has not been investigated.

METHODS

Patients were recruited from the acute stroke unit of a general hospital within 1 month post-stroke. Assessments included the Standardized Mini-Mental State Examination (sMMSE) for cognition, the National Institutes of Health Stroke Scale (NIHSS) for stroke severity, and the Center for Epidemiological Studies-Depression Scale (CES-D) for depressive symptoms. Tryptophan and kynurenine concentrations were determined by high-performance liquid chromatography.

RESULTS

A total of 41 patients with ischemic stroke ([mean ± SD] age 72.3 ± 12.2 years, 53.7% male, sMMSE 25.6 ± 4.1, NIHSS 7.27 ± 5.55) were recruited. Higher K/T ratios were associated with lower post-stroke global cognition (i.e. sMMSE scores; β = -.327, P = .037). A backward stepwise elimination linear regression (F1,40=6.15, P=.005, adjusted R2=.205) showed that the highest K/T ratio tertile (β = -.412, P = .006) predicted lower sMMSE scores, controlling for age (β = -.253, p = .081), with NIHSS (β = -.027, P = 0.859), and lesion volume (β = -.066, P = 0.659) removed from the model. In receiver operating characteristic analysis, a K/T ratio of 78.3 μmol/mmol (top tertile) predicted significant cognitive impairment (sMMSE score ≤ 24) with 67% sensitivity and 86% specificity (area under the curve = 0.730, p = .022).

CONCLUSIONS

These data suggest an inflammatory response characterized by IDO activation may be relevant to the development of PSCI. Since the neuroactivity of kynurenine metabolites may be amenable to pharmacotherapeutic intervention, the K/T ratio may be a clinically important biomarker.

Protective effect of L-kynurenine and probenecid on 6-hydroxydopamine-induced striatal toxicity in rats: implications of modulating kynurenate as a protective strategy

The neuroactive metabolite at the kynunerine pathway, kynurenic acid (KYNA), is a well-known competitive antagonist at the co-agonist glycine site of the N-methyl-D-aspartate receptor (NMDAr), and also decreases the extracellular levels of glutamate by blocking α7-nicotinic acetylcholine receptor (α7-nAchr) located on glutamatergic terminals. KYNA has been often reported to be neuroprotective in different neurotoxic models. The systemic administration of L-kynurenine (L-KYN)--the precursor of KYNA--together with probenecid (PROB)--an inhibitor of organic acids transport--to rodents increases KYNA levels in the brain in a dose-dependent manner. The striatal infusion of the toxin 6-hydroxydopamine (6-OHDA) to rodents is one of the common models used to simulate Parkinson's disease (PD). Different studies have linked PD alterations with excessive glutamatergic transmission in the striatum since NMDAr antagonists exert beneficial effects in PD models. In this work we investigated the effect that a systemic administration of L-KYN+PROB exerted on the toxic model induced by 6-OHDA in rats. PROB (50 mg/kg, i.p.) + L-KYN (75 mg/kg, i.p.) were given to rats for seven consecutive days. On day two of treatment, the animals were infused with a single injection of 6-OHDA (20 μg/2 μl) into the right striatum. Fourteen days post-lesion, rotation behavior was assessed as a marker of motor impairment. The total levels of dopamine (DA) were also estimated in striatal tissue samples of 6-OHDA-treated animals as a neurochemical marker of damage. In addition, twenty eight days post-lesion, the striatal damage was assessed by hematoxylin/eosin staining and immunohistochemistry against glial fibrillary acidic protein (GFAP) in the same animals. Neurodegeneration was also assessed by Fluoro Jade staining. 6-OHDA infusion increased rotation behavior, striatal reactive gliosis and neurodegeneration, while DA levels were decreased. For all markers evaluated, we observed protective effects of L-KYN+PROB on the dopaminergic damage induced by 6-OHDA. Our results suggest that this strategy was useful to mitigate dopaminergic toxicity in the hemiparkinsonian model. The combined use of L-KYN and PROB is a valuable tool to modulate glutamatergic and cholinergic activities, presumably by means of increased levels of endogenous KYNA.

Oxaloacetate restores the long-term potentiation impaired in rat hippocampus CA1 region by 2-vessel occlusion

Various acute brain pathological conditions are characterized by the presence of elevated glutamate concentrations in the brain interstitial fluids. It has been established that a decrease in the blood glutamate level enhances the brain-to-blood efflux of glutamate, removal of which from the brain may prevent glutamate excitotoxicity and its contribution to the long-lasting neurological deficits seen in stroke. A decrease in blood glutamate level can be achieved by exploiting the glutamate-scavenging properties of the blood-resident enzyme glutamate-oxaloacetate transaminase, which transforms glutamate into 2-ketoglutarate in the presence of the glutamate co-substrate oxaloacetate. The present study had the aim of an evaluation of the effects of the blood glutamate scavenger oxaloacetate on the impaired long-term potentiation (LTP) induced in the 2-vessel occlusion ischaemic model in rat. Transient (30-min) incomplete forebrain ischaemia was produced 72 h before LTP induction. Although the short transient brain hypoperfusion did not induce histologically identifiable injuries in the CA1 region (Fluoro-Jade B, S-100 and cresyl violet), it resulted in an impaired LTP function in the hippocampal CA1 region without damaging the basal synaptic transmission between the Schaffer collaterals and the pyramidal neurons. This impairment could be fended off in a dose-dependent manner by the intravenous administration of oxaloacetate in saline (at doses between 1.5 mmol and 0.1 mumol) immediately after the transient hypoperfusion. Our results suggest that oxaloacetate-mediated blood and brain glutamate scavenging contributes to the restoration of the LTP after its impairment by brain ischaemia.