Difference between revisions of "Encrypted code: Hyperexcitability of the trigeminal system"

Glutamate release is obviously a much broader and more complex phenomenon. NMDA, kainate, and AMPA ionotrophic receptors, and the metabotropic glutamate receptors, have been found in the superficial lamina of the trigeminal nucleus caudalis in mice.<ref>Tallaksen-Greene SJ, Young AB, Penney JB, Beitz AJ. Excitatory amino acid binding sites in the trigeminal principal sensory and spinal trigeminal nuclei of the rat. Neurosci Let. 1992;141(1):79–83. doi: 10.1016/0304-3940(92)90339-9. [PubMed] [CrossRef] [Google Scholar]</ref> NMDA and AMPA receptor antagonists can block the transmission of the nociceptive trigeminal-vascular signals <ref>Storer RJ, Goadsby PJ. Trigeminovascular nociceptive transmission involves N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptors. Neuroscience. 1999;90(4):1371–1376. doi: 10.1016/S0306-4522(98)00536-3. [PubMed] [CrossRef] [Google Scholar]</ref><ref>Goadsby PJ, Classey JD. Glutamatergic transmission in the trigeminal nucleus assessed with local blood flow. Brain Res. 2000;875(1–2):119–124. [PubMed] [Google Scholar]</ref> and reduce the high level of c-fos observed in the trigeminal nucleus caudalis following cisternal injection of capsaicin.<ref>Waeber C, Moskowitz MA, Cutrer FM, Sanchez Del Rio M, Mitsikostas DD. The NMDA receptor antagonist MK-801 reduces capsaicin-induced c-fos expression within rat trigeminal nucleus caudalis. Pain. 1998;76(1–2):239–248. [PubMed] [Google Scholar]</ref> Furthermore, micro-injections of ω-agatoxin into the ventrolateral area of the periaqueductal gray cause a facilitatory response of nociceptive activity in the trigeminal nucleus caudalis (TNC) activated by tonic electrical stimulation of the supratentorial parietal dura, adjacent to the middle meningeal artery.<ref>Knight YE, Bartsch T, Kaube H, Goadsby PJ. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6758884/ P/Q-type calcium-channel blockade in the periaqueductal gray facilitates trigeminal nociception: a functional genetic link for migraine?] J Neurosci. 2002;22(5):RC213. [PMC free article] [PubMed] [Google Scholar]</ref>

Indeed, a blink reflex study on a 38-year-old patient with right hemicranial symptoms associated with a pontine cavernoma affecting the nucleus raphes magnus area revealed a reduction of the pain threshold and a persistent facilitation of the R2 response, with an onset latency difference of 4.4 ms less in the side displaying the symptoms [8]. This confirms a regulatory role for release of neurotransmitters by the nucleus raphes magnus, which exhibits a descending inhibitory control on the TNC <ref>Hentall ID. Interactions between brainstem and trigeminal neurons detected by cross-spectral analysis. Neuroscience. 2000;96(3):601–610. doi: 10.1016/S0306-4522(99)00593-X. [PubMed] [CrossRef] [Google Scholar]</ref>and on the entire antinociceptive mesencephalic complex.<ref>Jiang M, Behbehani MM. Physiological characteristics of the projection pathway from the medial preoptic to the nucleus raphe magnus of the rat and its modulation by the periaqueductal gray. Pain. 2001;94(2):139–147. doi: 10.1016/S0304-3959(01)00348-7. [PubMed] [CrossRef] [Google Scholar]</ref> Our results suggest a hyperexcitability of the trigeminal nervous system in our subject, as follows. First, we evoked a direct response of the trigeminal motor system (bR-MEPs) to provide a value for reference and for amplitude symmetry, as the direct response of the trigeminal motor branch was not affected by any conditioning. A comparison between the jaw jerk responses versus the ipsilateral responses of the R-MEPs showed a much higher amplitude ratio than in normal subjects <ref>Cruccu G, Berardelli A, Inghilleri M, Manfredi M. Functional organization of the trigeminal motor system in man. A neurophysiological study. Brain. 1989;112(5):1333–1350. doi: 10.1093/brain/112.5.1333. [PubMed] [CrossRef] [Google Scholar]</ref> (Table 1). Therefore, these data indicate the presence of hyperexcitability of the trigeminal system.

In an electrophysiological study that analyzed a population of neurons and interneurons in the NVmt,<ref>McDavid S, Verdier D, Lund JP, Kolta A. Electrical properties of interneurons found within the trigeminal motor nucleus. Eur J Neurosci. 2008;28(6):1136–1145. doi: 10.1111/j.1460-9568.2008.06413.x. [PubMed] [CrossRef] [Google Scholar]</ref> three types of AHP were seen: fast, slow, and biphasic. The majority of the motoneurons had a fast AHP (fAHP), whereas most of the interneurons had a slow AHP. The basic properties of these interneurons are similar to the previously described “last-order pre-motoneurons” in the PeriV,<ref>Kolta A, Westberg KG, Lund JP. Identification of brainstem interneurons projecting to the trigeminal motor nucleus and adjacent structures in the rabbit. J Chem Neuroanat. 2000;19(3):175–195. doi: 10.1016/S0891-0618(00)00061-2. [PubMed] [CrossRef] [Google Scholar]</ref> suggesting that the interneurons in the NVmt are part of an interneuronal matrix surrounding the NVmt in which the motoneurons are inserted. In this last study, the authors describe the possibility, although rare, of interneurons also having an fAHP.

It is also possible that bruxism may be a clinical form of dystonia. Our data indicate that bruxism may be a clinical manifestation linked to a CNS neurotransmitter imbalance, and therefore should be considered a subclinical condition of orofacial dystonia or dystonic syndrome. Nevertheless, this phenomenon also appears in a transitory form in children and is resolved with the eruption of mixed dentition.<ref>Watts MW, Tan EK, Jankovic J. Bruxism and cranial-cervical dystonia: is there a relationship? Cranio. 1999;17(3):196–201. [PubMed] [Google Scholar]</ref><ref>Monaco A, Ciammella NM, Marci MC, Pirro R, Giannoni M. The anxiety in bruxer child. A case–control study. Minerva Stomatol. 2002;51(6):247–250. [PubMed] [Google Scholar]</ref>

{{Q2|'Bruxism' is a form of instability of neural excitability of a functional and/or organic type, therefore not exclusively pertinent to dentistry.}}{{bib}}