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 systemic administration of nitroglycerine induces attacks in migraineurs and is able to activate and sensitize the trigeminal system in animals involving glutamate and α7-nicotinic acetylcholine receptors, among others. Kynurenic acid is one of the endogenous glutamate receptor antagonists, and exerts inhibitory action on the α7-nicotinic acetylcholine receptors. Since kynurenic acid penetrates the blood-brain barrier poorly, therefore a newly synthesized kynurenic acid amide, N-(2-N-pyrrolidinylethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride (KYNAa) was used with such a side-chain substitution to facilitate brain penetration in our study. We evaluated its modulatory effect on kynurenic acid concentration in the cervical part of trigemino-cervical complex (C1-C2) and in the model of nitroglycerine-induced trigeminal activation using male Sprague-Dawley rats. One hour after 1 mmol/kg bodyweight KYNAa administration, the kynurenic acid level increased significantly in C1-C2, which returned to the basal level at 300 min measured by high-performance liquid chromatography. KYNAa pre-treatment had dose-dependent, mitigating action on nitroglycerine-induced decrease in calcitonin gene-related peptide and increase in c-Fos, neuronal nitric oxide synthase and calmodulin-dependent protein kinase II alpha expression in the C1-C2. KYNAa also mitigated the behavioural changes after nitroglycerine. Thus, in this model KYNAa is able to modulate in a dose-dependent manner the changes in neurochemical markers of activation and sensitization of the trigeminal system directly and indirectly--via forming kynurenic acid, possibly acting on peripheral and central glutamate or α7-nicotinic acetylcholine receptors. These results suggest that application of kynurenic acid derivatives could be a useful therapeutic strategy in migraine headache in the future with a different mechanism of action.

Background The topical inflammatory soup can model the inflammation of the dura mater causing hypersensitivity and activation of the trigeminal system, a phenomenon present in migraineurs. Calcitonin gene-related peptide, transient receptor potential vanilloid-1 receptor, and neuronal nitric oxide synthase are important in the sensitization process there. 5-HT1B/1D receptor agonists, triptans are used as a treatment of migraine. Kynurenic acid an NMDA antagonist can act on structures involved in trigeminal activation. Aim We investigated the effect of inflammatory soup induced dural inflammation on the calcitonin gene-related peptide, transient receptor potential vanilloid-1 receptor, and neuronal nitric oxide synthase levels in the caudal trigeminal nucleus. We also tested whether pretreatment with a well-known antimigraine drug, such as sumatriptan and kynurenic acid, a compound with a different mechanism of action, can affect these changes and if their modulatory effects are comparable. Material and methods After subcutaneous sumatriptan or intraperitoneal kynurenic acid the dura mater of adult male Sprague-Dawley rats (n = 72) was treated with inflammatory soup or its vehicle (synthetic interstitial fluid). Two and a half or four hours later perfusion was performed and the caudal trigeminal nucleus was removed for immunohistochemistry. Results and conclusion Inflammatory soup increased calcitonin gene-related peptide, transient receptor potential vanilloid-1 receptor, and neuronal nitric oxide synthase in the caudal trigeminal nucleus compared to placebo, which was attenuated by sumatriptan and kynurenic acid. This suggests the involvement of 5-HT1B/1D and NMDA receptors in neurogenic inflammation development of the dura and thus in migraine attacks.

Abstract:Kynurenic acid (KYNA) has well-established protective properties against glutamatergic neurotransmission, which plays an essential role in the activation and sensitization process during headache disorders. The goal of this study was to Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporationcompare 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 a 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 10-fold 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. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems CorporationReviewer #2: The Authors have tried to address the comments of the referees and partially improved the manuscript. Some issues still need improvement.The sentence 'glutamatergic neurotransmission, which plays an essential role in the activation and sensitization process during headache disorders' is incorrect. This statement may be true for some primary headaches (migraine and chronic migraine), but not for 'headaches' in general.The sentence was modified accordingly.The way the data are presented is still quite confusing:Description of the time boundaries for Phase I and II is missing in the Methods section. The Methods section was supplemented with the requested information.A Figure ( Table 1 reports the levels of significance for data presented in Figure 2: the table should be inglobated in said figure otherwise the reader is forced to go back and forth.The requested modification was done in Figure 2 and Table 1 was removed. Table 2 should be associated to a figure that illustrates mean+sd of time spent in rubbing during the 2 phases of formalin in the di...

Migraine is a common neurological condition, causing high disability, but the pathomechanism of the disease is not yet fully understood. Activation of the trigeminovascular system could play a crucial role in the manifestation of the symptoms, but initial step of this activation remains unknown. Functional imaging studies have revealed that certain brainstem areas, referred to as migraine generators, are activated during a migraine attack, including the dorsal raphe, the periaqueductal gray, the locus coeruleus, and the nucleus raphe magnus. However, the studies performed to date have not demonstrated whether this activation is a trigger or a consequence of the migraine attack. With the aim of evaluating the functional relationship between activation of the trigeminal system and migraine generators, we examined the changes in c-Fos immunoreactivity in the above-mentioned nuclei after stimulation of the trigeminal ganglion, an animal model for trigeminovascular activation. The stimulation led to significant increases in the number of c-Fos immunoreactive cells in the nucleus raphe magnus and in the caudal part of the spinal trigeminal nucleus, 2 and 4 h after the stimulation. Activation of the trigeminal system failed to exhibit uniform activation of the brain stem nuclei related to migraine. Our results suggest that the activation of the trigeminal system in the rat by electrical stimulation of the trigeminal ganglion leads to the activation of the descending pain modulatory system, but not to the activation of "migraine generator" nuclei. Therefore, the activity pattern seen in functional studies may reflect a unique feature, exclusively present in migraine.

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.

Activation of the trigeminal system plays an important role in the pathomechanism of headaches. A better understanding of trigeminal pain processing is expected to provide information helping to unravel the background of these diseases. ATP, a key modulator of nociceptive processing, acts on ligand-gated P2X receptors. Antagonists of the P2X7 receptors, such as Brilliant Blue G (BBG), have proved effective in several models of pain. We have investigated the effects of BBG after electrical stimulation of the trigeminal ganglion and in the orofacial formalin test in the rat. The right trigeminal ganglion of male rats was stimulated either with 5 Hz, 0.5 mA pulses for 5 min (mild procedure) or with 10 Hz, 0.5 mA pulses for 30 min (robust procedure), preceded by 50 mg/kg i.v. BBG. The animals were processed for c-Fos and calcitonin gene-related peptide (CGRP) immunohistochemistry. In the orofacial formalin test, 50 μL of 1.5 % formalin was injected into the right whisker pad of awake rats, following the pre-treatment with BBG. Behaviour was monitored for 45 min, and c-Fos and CGRP immunohistochemistry was performed. BBG attenuated the increase in c-Fos-positive cells in the caudal trigeminal nucleus (TNC) after robust stimulation, but not after mild stimulation. No alterations in CGRP levels were found with either methodology. BBG did not mitigate either the behaviour or the increase in c-Fos-positive cells in the TNC during the orofacial formalin test. These results indicate that P2X7 receptors may have a role in the modulation of nociception in the trigeminal system.

Background: Although migraine is one of the most common primary headaches, its therapy is still limited in many cases. The use of animal models is crucial in the development of novel therapeutic strategies, but unfortunately, none of them show all aspects of the disease, therefore, there is a constant need for further improvement in this field. The application of inflammatory agents on the dura mater is a widely accepted method to mimic neurogenic inflammation in rodents, which plays a key role in the pathomechanism of migraine. Complete Freund's Adjuvant (CFA), and a mixture of inflammatory mediators, called inflammatory soup (IS) are often used for this purpose. Methods: To examine the activation pattern that is caused by chemical stimulation of dura mater, we applied CFA or IS over the right parietal lobe. After 2 h and 4 h (CFA groups), or 2.5 h and 4 h (IS groups), animals were perfused, and c-Fos immunoreactive cells were counted in the caudal trigeminal nucleus. To explore every pitfall, we examined whether our surgical procedure (anesthetic drug, stereotaxic apparatus, local lidocaine) can alter the results under the same experimental settings. c-Fos labeled cells were counted in the second-order neuron area based on the somatotopic organization of the trigeminal nerve branches. Results: We could not find any difference between the CFA and physiological saline group neither 2 h, nor 4 h after dural stimulation. IS caused significant difference after both time points between IS treated and control group, and between treated (right) and control (left) side. Stereotaxic frame usage had a substantial effect on the obtained results. Conclusions: Counting c-Fos immunoreactive cells based on somatotopic organization of the trigeminal nerve helped to examine the effect of chemical stimulation of dura in a more specific way. As a result, the use of IS over the parietal lobe caused activation in the area of the ophthalmic nerve. To see this effect, the use of lidocaine anesthesia is indispensable. In conclusion, application of IS on the dura mater induces short-term, more robust c-Fos activation than CFA, therefore it might offer a better approach to model acute migraine headache in rodents.