Zsuzsanna Bohár

L-kynurenine combined with probenecid and the novel synthetic kynurenic acid derivative attenuate nitroglycerin-induced nNOS in the rat caudal trigeminal nucleus

Systemic administration of the nitric oxide (NO) donor nitroglycerin (NTG) triggers a delayed attack without aura in many migraineurs, but not in healthy volunteers. In rats, 4 h after the systemic administration of NTG (10 mg/kg bw, s.c.), the neurons of the caudal trigeminal nucleus (TNC) are activated and the expression of neuronal NO synthase (nNOS) in the same area is increased suggesting a self-amplifying process in the trigeminal system, which seems to be crucial in migraine pathogenesis. Kynurenic acid (KYNA) and its analogues may exert modulatory effects in many neuropathological conditions, probably via N-methyl-D-aspartate (NMDA) antagonism. Since NMDA receptors play a crucial role in trigeminal pain processing, the aim of our experiments was to compare the effects of L-kynurenine (L-KYN) combined with probenecid (PROB) or with 2-(2-N,N-dimethylaminoethylamine-1-carbonyl)-1H-quinolin-4-one hydrochloride alone, a newly synthetized KYNA derivative, on the NTG-induced nNOS expression in the rat TNC. Pretreatment with L-KYN (300 mg/kg bw, i.p.) together with PROB (200 mg/kg bw, i.p.) and KYNA derivative (300 mg/kg bw, i.p.) attenuated the NTG-induced nNOS expression in the rat TNC. Our data suggest that the stimulating effect of NTG, and thus of NO, on the expression of nNOS might be modulated by increasing the KYNA level in the brain, probably through the NMDA receptors. These data could help promote a better understanding of the pathogenesis of headaches and the action of antimigraine drugs.

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.

Kynurenines and headache

In parallel to serotonin synthesis, the major route of tryptophan catabolism is the kynurenine pathway, which produces neuroactive metabolites. Among these substances, kynurenic acid has potential neuroprotective action blocking glutamate release and glutamatergic neurotransmission. Glutamate is a key player in migraine pathogenesis; it is crucial in the communication of first and second-order neurons, and it has an important role in the genesis of cortical spreading depression, which is the electrophysiological correlate for migraine aura and may be involved in the activation of the trigeminal system. Thus, kynurenines may affect the pathogenesis directly, by acting on glutamate receptors and exerting other neuromodulatory effects, and indirectly via an altered serotonin metabolism. This work summarizes our current results regarding the role of the kynurenine system in trigeminal activation and other events occurring during migraine headache.

Selective inhibition of cyclooxygenase-2 attenuates nitroglycerin-induced calmodulin-dependent protein kinase II alpha in rat trigeminal nucleus caudalis

The nitric oxide donor, nitroglycerin (NTG) can trigger a migraine attack, after a delay of several hours in migraineurs, but not in healthy people. This long delay does not favor a pure vasodilatatory action. In rats, subcutaneous administration of NTG (10mg/kg) significantly and selectively increases the number of calmodulin-dependent protein kinase II alpha (CamKIIalpha)-immunoreactive neurons in the trigeminal caudal nucleus (TNC) after 4h. The aim of our study was to determine if any isoforms of the cyclooxygenase (COX) enzyme might have a role in the NTG-induced increase of CamKIIalpha expression. In our experiments, we demonstrated that pretreatment with NS398, the selective COX-2 inhibitor attenuated the NTG-induced CamKIIalpha expression in the TNC at doses of 3 and 5mg/kg. In contrast, SC560, a selective COX-1 inhibitor failed to modulate this phenomenon in any of the dosages used (1, 5 and 10mg/kg). These findings suggest that COX-2, but not COX-1 derived metabolites are important factors in the NTG-induced CamKIIalpha expression. Thus this isoform may play a significant role in the induction of migraine. These data could help in the better understanding of the pathogenesis of headaches and the action of antimigraine drugs.

The modulatory effect of anandamide on nitroglycerin-induced sensitization in the trigeminal system of the rat.

Background One of the human and animal models of migraine is the systemic administration of the nitric oxide donor (NO) nitroglycerin (NTG). NO can provoke migraine-like attacks in migraineurs and initiates a self-amplifying process in the trigeminal system, probably leading to central sensitization. Recent studies suggest that the endocannabinoid system is involved in nociceptive signal processing and cannabinoid receptor (CB) agonists are able to attenuate nociception in animal models of pain. Aim The purpose of the present study was to investigate the modulatory effects of a CB agonist anandamide (AEA) on the NTG-induced expression of transient receptor potential vanilloid type 1 (TRPV1), neuronal nitric oxide synthase (nNOS), nuclear factor kappa B (NF-κB), cyclooxygenase-2 (COX-2) and kynurenine aminotransferase-II (KAT-II) in the upper cervical spinal cord (C1–C2) of the rat, where most of the trigeminal nociceptive afferents convey. Methods A half hour before and one hour after NTG (10 mg/kg) or placebo injection, adult male Sprague-Dawley rats (n = 44) were treated with AEA (2 × 5 mg/kg). Four hours after placebo/NTG injection, the animals were perfused and the cervical spinal cords were removed for immunohistochemistry and Western blotting. Results and conclusion Our results show that NTG is able to increase TRPV1, nNOS, NF-κB and COX-2 and decrease KAT-II expression in the C1–C2 segments. On the other hand, we have found that AEA modulates the NTG-induced changes, thus it influences the activation and central sensitization process in the trigeminal system, probably via CBs.

The inimitable kynurenic acid: The roles of different ionotropic receptors in the action of kynurenic acid at a spinal level

Kynurenic acid (KYNA) is a neuroactive metabolite that interacts with NMDA, AMPA/kainate and alpha 7 nicotinic receptors. The goal of this study was to clarify the roles of these receptors in the action of KYNA at a spinal level by using highly specific receptor antagonists alone or in triple combinations. Chronic osteoarthritis-like joint pain was induced with monosodium-iodoacetate in male Wistar rats. Mechanical allodynia and motor function were quantified. In the first series we determined the dose-response and time course effects of intrathecally administered KYNA (10-100 μg), D-(-)-2-amino-5-phosphonopentanoic acid (AP5; an NMDA receptor antagonist; 10-200 μg), methyllycaconitine (MLA; an alpha 7 nicotinic receptor antagonist; 100-200 μg) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulfonamide (NBQX; an AMPA/kainate receptor antagonist; 1-20 μg). In the second series, four different triple combinations of MLA, AP5 and NBQX were investigated. Intrathecal administration of KYNA caused a dose-dependent motor impairment and antinociception. The highly specific NMDA receptor antagonist AP5 caused a motor impairment and antinociception with lower potency. High doses of NBQX resulted in significant antinociception with a slight motor impairment, while only the highest dose of MLA gave rise to significant antinociception with a slight motor impairment. After the coadministration of these ligands as combinations, no potentiation was observed. It may be supposed that the effects of KYNA are primarily due to the inhibition of NMDA receptors at both glycine and phencyclidine (PCP) binding sites, and not to the interactions at the different ionotropic receptors, but the mechanisms behind its high bio-efficiency are still unknown.

Effect of Probenecid on the Pain-Related Behaviour and Morphological Markers in Orofacial Formalin Test of the Rat

UNLABELLED Probenecid has been widely used in the treatment of gout, but evidence suggests that it may also have antinociceptive effects in different inflammatory and pain conditions. We examined the potential modulatory effects of probenecid on behavioural and morphological markers in the orofacial formalin test of the rat. One hour after pre-treatment with vehicle or probenecid (1 mmol/kg body weight) intraperitoneally, 50μl 1.5% formalin solution or physiological saline was injected subcutaneously into the right whisker pad of rats. The rubbing activity directed to the injected whisker pad was then measured for a period of 45 minutes. Four hours after formalin injection, the caudal part of spinal trigeminal nucleus was removed and subjected to c-Fos and neuronal nitric oxide synthase (nNOS) immunohistochemistry and to interleukin-1β and NAD(P)H quinone oxidoreductase 1 (NQO1) Western blot. There was a significant decrease in formalin-induced biphasic behavioural response and c-Fos and nNOS immunoreactivity in the rats that were pre-treated with probenecid. However there were no alterations in expression of interleukin-1β or NQO1 after formalin administration. Our results suggest that probenecid has an anti-nociceptive effect in the trigeminal inflammatory pain model. This effect may be through influencing the release of prostaglandin E2 or desensitizing the transient receptor potential channel subtype A member 1 or the transient receptor potential channel subtype V member 2 or the effect may be through modulating kynurenic acid levels in the central nervous system. Thus, probenecid might be a potential candidate for the treatment of trigeminal activation related pain conditions.

The Effect of Systemic Nitroglycerin Administration on the Kynurenine Pathway in the Rat

The primary headache disorders include migraine, which is one of the most frequent neurological disorders, which influences more than 14% of the whole population. Despite the research efforts, its exact pathomechanism is not fully revealed, but evidence points to the role of glutamate and its receptors. Kynurenic acid is an endogenous glutamate receptor antagonist produced by the kynurenine pathway (KP). Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) convert l-tryptophan to N-formyl-l-kynurenine, to be further transformed to l-kynurenine. Kynurenine aminotransferase-II (KAT-II), l-kynurenine hydrolase (KYNU), and l-kynurenine 3-monooxygenase (KMO) are key enzymes in the later steps of the KP. Nitroglycerin (NTG) administration serves as both human and animal model of migraine, causing the activation and sensitization in the trigeminal system. A previous study demonstrated a reduction of KAT-II expression following NTG administration in animals. The goal of current tests was to identify the potential modulatory effect of NTG on other metabolizing enzymes of the KP in the caudal trigeminal nucleus (TNC) of rats. Four hours following the intraperitoneal injection of NTG (10 mg/kg), the rats were perfused transcardially and the TNC was extracted for Western blotting. Western blot studies revealed that the expression of TDO2, IDO1, KYNU, and KMO decreased in the TNC. The results demonstrated that NTG is able to downregulate the KP, with a potential influence on the glutamatergic system as well, contributing to the development of trigeminal activation and sensitization in animals.

A comparative assessment of two kynurenic acid analogs in the formalin model of trigeminal activation: a behavioral, immunohistochemical and pharmacokinetic study

Kynurenic acid (KYNA) has well-established protective properties against glutamatergic neurotransmission, which plays an essential role in the activation and sensitization process during some primary headache disorders. The goal of this study was to compare the effects of two KYNA analogs, N-(2-N,N-dimethylaminoethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride (KA-1) and N-(2-N-pyrrolidinylethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride (KA-2), in the orofacial formalin test of trigeminal pain. Following pretreatment with KA-1 or KA-2, rats were injected with subcutaneous formalin solution in the right whisker pad. Thereafter, the rubbing activity and c-Fos immunoreactivity changes in the spinal trigeminal nucleus pars caudalis (TNC) were investigated. To obtain pharmacokinetic data, KA-1, KA-2 and KYNA concentrations were measured following KA-1 or KA-2 injection. Behavioral tests demonstrated that KA-2 induced larger amelioration of formalin-evoked alterations as compared with KA-1 and the assessment of c-Fos immunoreactivity in the TNC yielded similar results. Although KA-1 treatment resulted in approximately four times larger area under the curve values in the serum relative to KA-2, the latter resulted in a higher KYNA elevation than in the case of KA-1. With regard to TNC, the concentration of KA-1 was under the limit of detection, while that of KA-2 was quite small and there was no major difference in the approximately tenfold KYNA elevations. These findings indicate that the differences between the beneficial effects of KA-1 and KA-2 may be explained by the markedly higher peripheral KYNA levels following KA-2 pretreatment. Targeting the peripheral component of trigeminal pain processing would provide an option for drug design which might prove beneficial in headache conditions.

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.