Modulatory effect of synthetic kynurenic acid derivatives on the nitroglycerin-induced trigeminal activation and sensitization in rats

Abstract

Migraine is one of the most common neurological disorders. Its pathomechanism is only partially known and therapy is not always effective. In migraine, activation of trigeminal system plays an important role involving peptides/enzymes like calcitonin gene-related peptide (CGRP), neuronal nitric oxide synthase (nNOS) and calcium/calmodulin-dependent protein kinase II α (CaMKIIα) and glutamate and α7-nicotinic acetylcholine (α7-nACh) receptors. Kynurenic acid is an endogenous antagonist of the above mentioned receptors, but its poor ability to cross the blood-brain barrier limits its therapeutic potential, therefore in our experiments we investigated effect of its pro-drug, L-kynurenine (L-KYN) combined with probenecid (PROB) and two newly synthesized kynurenic acid amides (KYNAa1, KYNAa2) on kynurenic acid concentration of the central nervous system in rats and on morphological and behavioural changes induced by systemic administration of nitroglycerin (NTG), an animal model of migraine. Our results showed that kynurenic acid concentration in cervical part of spinal trigeminal nucleus pars caudalis (C1-C2) - where most of the trigeminal nociceptors convey - was significantly increased one hour after L-KYN-PROB. Moreover, KYNAa2 also enhanced kynurenic acid levels suggesting that this new derivative, at least partially, transforms to kynurenic acid resulting in a possible direct and indirect action in the central nervous system. The significant decrease of CGRP and increase of c-Fos, nNOS and CaMKIIα four hours after NTG treatment in C1-C2 measured by immunohistochemistry and Western blotting reflect the activation and sensitization of the trigeminal system. These changes were attenuated by the pre-treatment with L-KYN-PROB, KYNAa1 and in a dose-dependent manner with KYNAa2 probably due to their effects on the target receptors of kynurenic acid located in the peripheral and central part of the trigeminal system. The NTG-induced lower ambulation distance demonstrated by Open Field Test may indicate pain condition in rats, which can not be observed after pre-treatment with KYNAa2. Furthermore, the KYNAa2 administration lowers the basic ambulation distance which can reflect its central effect. Our results suggest that by modulating the activation and sensitization of the trigeminal system in animals, kynurenic acid or its derivatives can be potential drug candidates in the treatment of headaches.