Effects of the endogenously occurring kynurenate are biochemically linked to 5-HT, as both are synthesized from tryptophan, and both pathways interact [
40]–[
43]. Kynurenate is an antagonist of NMDA, AMPA and kainate receptors [
44], and therefore frequently used in experimental models to investigate ionotropic glutamate receptors effects. The concentration of 2 mM kynurenate is frequently used; a sensitization of capsaicin-induced release observed at 20 mM occurs probably through a different target. Glutamate receptor inhibition has been suggested as potential therapeutic mechanism for headaches. Activity induced by electrical stimulation of the trigeminal ganglion, resulting in increased c-fos immunoreactivity in the caudal trigeminal nucleus, was almost abolished by kynurenate infusion in rats [
45]. Kynurenate was shown to inhibit the delayed trigeminal activation induced by glyceroltrinitrate, using CamKII or cFos as a marker [
46,
47]. Combined inhibition of AMPA and kainate receptors was effective as treatment for migraine [
48]. All ionotropic glutamate receptors were found in the superficial laminae of the trigeminal nucleus [
49]. Subunits of the NMDA receptor were found on about 20% of rat trigeminal sensory neurons [
50,
51], also AMPA, kainate and metabotropic glutamate mGluR5 receptors were found in the rat trigeminal ganglion [
52,
53]. The somata of rat trigeminal sensory neurons were shown to respond to stimulation with AMPA, NMDA, kainate and mGluR agonists [
54]. For NMDA receptors, sensitization of TRPV1 via phosphorylation was evidenced as potential downstream mechanism in trigeminal sensory neurons [
55]. Since the central terminals of primary afferents are the most important if not the only source for CGRP in the medulla, it is likely that presynaptic glutamate receptors are responsible for the glutamate effect. In the presented experiments, capsaicin-released glutamate might contribute to the calcium-dependent CGRP release via presynaptic glutamatergic autoreceptors [
56]. The activation of calcium-permeable NMDA receptors could directly admit calcium to central afferent terminals, providing a secondary fraction of calcium in addition to the calcium entering via TRPV1 upon capsaicin stimulation. At the central terminals this contribution seems significant, given that kynurenate blocked about half of the capsaicin-stimulated release in brainstem slices. It should be noted that kynurenate could exert its effects through other targets than glutamate receptors, e.g. the G-protein-coupled receptor GPR35 or alpha7 nicotinic acetylcholine [
57]–[
60]. The production of glutamate receptors in the trigeminal ganglion suggests a transport of glutamate receptors peripherally and centrally. Since kynurenate did not inhibit CGRP release in the dura mater, it might by hypothesized that the required cascades are only fully assembled in presynaptic terminals, but not complete at other parts of the trigeminal afferents.
Administration of CGRP has facilitated glutamatergic transmission [
16]. The basis for this cooperative effect of CGRP and glutamate is not clear. We speculate that CGRP itself might facilitate glutamate release, which would fit to the immunohistochemical observation that CGRP receptor components are localized presynaptically on primary afferent terminals, but not on second order neuron cell bodies in the Sp5C [
61].