Jens Christian Laursen

Glutamate dysregulation in the trigeminal ganglion: A novel mechanism for peripheral sensitization of the craniofacial region

In the trigeminal ganglion (TG), satellite glial cells (SGCs) form a functional unit with neurons. It has been proposed that SGCs participate in regulating extracellular glutamate levels and that dysfunction of this SGC capacity can impact nociceptive transmission in craniofacial pain conditions. This study investigated whether SGCs release glutamate and whether elevation of TG glutamate concentration alters response properties of trigeminal afferent fibers. Immunohistochemistry was used to assess glutamate content and the expression of excitatory amino acid transporter (EAAT)1 and EAAT2 in TG sections. SGCs contained glutamate and expressed EAAT1 and EAAT2. Potassium chloride (10 mM) was used to evoke glutamate release from cultured rat SGCs treated with the EAAT1/2 inhibitor (3S)-3-[[3-[[4-(trifluoromethyl)ben zoyl]amino]phenyl]methoxy]-L-aspartic acid (TFB-TBOA) or control. Treatment with TFB-TBOA (1 and 10 μM) significantly reduced the glutamate concentration from 10.6 ± 1.1 to 5.8 ± 1.4 μM and 3.0 ± 0.8 μM, respectively (p<0.05). Electrophysiology experiments were conducted in anaesthetized rats to determine the effect of intraganglionic injections of glutamate on the response properties of ganglion neurons that innervated either the temporalis or masseter muscle. Intraganglionic injection of glutamate (500 mM, 3 μl) evoked afferent discharge and significantly reduced muscle afferent mechanical threshold. Glutamate-evoked discharge was attenuated bythe N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate (APV) and increased by TFB-TBOA, whereas mechanical sensitization was only sensitive to APV. Antidromic invasion of muscle afferent fibers by electrical stimulation of the caudal brainstem (10 Hz) or local anesthesia of the brainstem with lidocaine did not alter glutamate-induced mechanical sensitization. These findings provide a novel mechanism whereby dysfunctional trigeminal SGCs could contribute to cranial muscle tenderness in craniofacial pain conditions such as migraine headache.

Investigating the expression of metabotropic glutamate receptors in trigeminal ganglion neurons and satellite glial cells: implications for craniofacial pain

Abstract Context/objective: Previous studies have demonstrated that various subtypes of the metabotropic glutamate receptors (mGluRs) are expressed in the dorsal root ganglion (DRG) of the peripheral nervous system (PNS), implicating that glutamate potentially contributes to sensory transmission through these receptors. While mGluR expression has been investigated largely in the DRG, the present study focused on mGluR expression on neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG). Materials and methods: To address the presence of mGluRs in rat TG neurons and their corresponding SGCs, the trigeminal ganglia from six adult male Wistar rats were isolated and immunohistochemistry and immunocytochemistry were performed. The expression of mGluR1α-, mGluR2/3- and mGluR8 on TG neurons and SGCs was investigated in tissue slices and isolated cells. Results: 35.1 ± 6.0% of the TG neurons were positive for mGluR1α, whereas 39.9 ± 7.7% and 55.5 ± 6.3% were positive for mGluR2/3 and mGluR8, respectively. Immunoreactive neurons expressing mGluRs were mainly medium- to large sized, with a smaller population of small-sized neurons showing immunoreactivity. The SGCs showed immunoreactivity toward mGluR1α and mGluR8, but not mGluR2/3, both in the tissue and in isolated cells. Conclusions: Findings from the present study showed that trigeminal neurons express mGluR1α, mGluR2/3 and mGluR8, while SGCs only express mGluR1α and mGluR8. This novel evidence may advance investigations on a possible role of mGluRs in relation to trigeminal pain transmission within the craniofacial region.

Intraganglionic injection of a nitric oxide donator induces afferent mechanical sensitization that is attenuated by palmitoylethanolamide

Aim The aim of this article is to investigate whether the nitric oxide (NO) donator diethylenetriamine/nitric oxide (DETA/NO) affects trigeminal sensory processing through the trigeminal ganglion in part by activating trigeminal satellite glial cells (SGCs) and whether this effect is attenuated by the anti-inflammatory compound palmitoylethanolamide (PEA). Methods DETA/NO was administered to isolated rat trigeminal SGCs in vitro, and injected into the rat trigeminal ganglion in vivo, in the presence or absence of PEA. Results Administration of DETA/NO (1000 µM) increased the release of prostaglandin E2 by SGCs. PEA (1 and 10 µM) significantly attenuated prostaglandin E2 release. Two intraganglionic injections of DETA/NO (10 mM, 3 µl) or prostaglandin E2 at a 30-minute interval did not evoke discharge in trigeminal ganglion neurons that innervate the rat jaw-closer muscles, but did reduce the mechanical activation threshold of their peripheral endings by 30%–50%. Intravenous administration of PEA (1 mg/kg) or ketorolac (0.5 mg/kg) prevented DETA/NO-induced afferent mechanical sensitization. Conclusions Elevation of NO in the trigeminal ganglion results in the sensitization of the peripheral endings of masticatory muscle nociceptors to mechanical stimulation through a mechanism that involves prostaglandin E2 release from SGCs. Attenuation of this sensitization by PEA suggests a possible option for acute management of craniofacial pain and headache.

Glutamate attenuates nitric oxide release from isolated trigeminal ganglion satellite glial cells

Abstract Background/aims Elevated concentrations of nitric oxide (NO) and glutamate (Glu) in the trigeminal ganglion (TG) may contribute to the development and maintenance of migraine headache. The role of satellite glial cells (SGC) and their pattern of substance release in relation to the pathophysiology of migraine are currently under investigation. In the present study, we investigated the release of NO from isolated trigeminal SGCs and its modulation by Glu. Methods SGCs from the TG of adult male Sprague-Dawley rats (n = 8) were isolated and maintained in culture until used. SGCs were treated with graded concentrations of Glu (0, 10, 100, 1000 μM) and samples were withdrawn after 48 h of incubation. In subsequent experiments, SGCs were treated with vehicle medium, 10 μM forskolin (FSK) alone, or 10 μM FSK in conjunction with 100 μM Glu and incubated for 48 h. The NO concentration was determined using the Griess Reagent System and data was subjected to a one-way repeated measures ANOVA analysis, where p <0.05 was considered statistically significant. All experimental procedures were performed at minimum in triplicate. Results The concentration of NO was 3.59 ± 0.04 M under baseline conditions. Application of 10 or 100 μM Glu resulted in a significant drop in NO concentration (2.92 ± 0.017 μM and 2.83 ± 0.012 M, respectively) compared to baseline, whereas treatment with 1000 μM Glu did not significantly alter NO release. Treatment of SGCs with 10 μM FSK significantly increased NO release (to 125.94 ± 3.90% of baseline) compared to baseline. Coapplication of 10 M FSK with 100 M Glu significantly decreased FSK-induced NO release (83.73 ± 2.29% of baseline), compared to both FSK-mediated NO release and baseline levels. Conclusion These findings suggest that one mechanism by which SGCs protect the TG from elevated Glu concentrations that may occur in response to prolonged noxious stimulation is to reduce the release of NO.