Connexin 43 contributes to ectopic orofacial pain following inferior alveolar nerve injury

Satellite glial cells in sensory ganglia play a wider role in chronic pain via multiple mechanisms

Satellite glial cells are unique glial cells that surround the cell body of primary sensory neurons. An increasing body of evidence suggests that in the presence of inflammation and nerve damage, a significant number of satellite glial cells become activated, thus triggering a series of functional changes. This suggests that satellite glial cells are closely related to the occurrence of chronic pain. In this review, we first summarize the morphological structure, molecular markers, and physiological functions of satellite glial cells. Then, we clarify the multiple key roles of satellite glial cells in chronic pain, including gap junction hemichannel Cx43, membrane channel Pannexin1, K channel subunit 4.1, ATP, purinergic P2 receptors, and a series of additional factors and their receptors, including tumor necrosis factor, glutamate, endothelin, and bradykinin. Finally, we propose that future research should focus on the specific sorting of satellite glial cells, and identify genomic differences between physiological and pathological conditions. This review provides an important perspective for clarifying mechanisms underlying the peripheral regulation of chronic pain and will facilitate the formulation of new treatment plans for chronic pain.

Targeting connexins: possible game changer in managing neuropathic pain?

Neuropathic pain is a chronic debilitating condition caused by nerve injury or a variety of diseases. At the core of neuropathic pain lies the aberrant neuronal excitability in the peripheral and/or central nervous system (PNS and CNS). Enhanced connexin expression and abnormal activation of connexin-assembled gap junctional channels are prominent in neuropathic pain along with reactive gliosis, contributing to neuronal hypersensitivity and hyperexcitability. In this review, we delve into the current understanding of how connexin expression and function contribute to the pathogenesis and pathophysiology of neuropathic pain and argue for connexins as potential therapeutic targets for neuropathic pain management.

Three-dimensional topography of rat trigeminal ganglion neurons using a combination of retrograde labeling and tissue-clearing techniques

The trigeminal nerve is the sensory afferent of the orofacial regions and divided into three major branches. Cell bodies of the trigeminal nerve lie in the trigeminal ganglion and are surrounded by satellite cells. There is a close interaction between ganglion cells via satellite cells, but the function is not fully understood. In the present study, we clarified the ganglion cells' three-dimensional (3D) localization, which is essential to understand the functions of cell-cell interactions in the trigeminal ganglion. Fast blue was injected into 12 sites of the rat orofacial regions, and ganglion cells were retrogradely labeled. The labeled trigeminal ganglia were cleared by modified 3DISCO, imaged with confocal laser-scanning microscopy, and reconstructed in 3D. Histograms of the major axes of the fast blue-positive somata revealed that the peak major axes of the cells innervating the skin/mucosa were smaller than those of cells innervating the deep structures. Ganglion cells innervating the ophthalmic, maxillary, and mandibular divisions were distributed in the anterodorsal, central, and posterolateral portions of the trigeminal ganglion, respectively, with considerable overlap in the border region. The intermingling in the distribution of ganglion cells within each division was also high, in particular, within the mandibular division. Specifically, intermingling was observed in combinations of tongue and masseter/temporal muscles, maxillary/mandibular molars and masseter/temporal muscles, and tongue and mandibular molars. Double retrograde labeling confirmed that some ganglion cells innervating these combinations were closely apposed. Our data provide essential information for understanding the function of ganglion cell-cell interactions via satellite cells.

Blockage of the fractalkine pathway reduces hyperalgesia and prevents morphological glial alterations-Comparison between inflammatory and neuropathic orofacial pain in male rats

This study aimed to evaluate the effects of inhibitors of the fractalkine pathway in hyperalgesia in inflammatory and neuropathic orofacial pain in male rats and the morphological changes in microglia and satellite glial cells (SGCs). Rats were submitted to zymosan-induced arthritis of the temporomandibular joint or infraorbital nerve constriction, and treated intrathecally with a P2 X7 antagonist, a cathepsin S inhibitor or a p-38 mitogen-activated protein kinase (MAPK) inhibitor. Mechanical hyperalgesia was evaluated 4 and 6 h following arthritis induction or 7 and 14 days following nerve ligation. The expression of the receptor CX3 CR1 , phospho-p-38 MAPK, ionized calcium-binding adapter molecule-1 (Iba-1), and glutamine synthetase and the morphological changes in microglia and SGCs were evaluated by confocal microscopy. In both inflammatory and neuropathic models, untreated animals presented a higher expression of CX3 CR1 and developed hyperalgesia and up-regulation of phospho-p-38 MAPK, which was prevented by all drugs (p < .05). The number of microglial processes endpoints and the total branch length were lower in the untreated animals, but the overall immunolabeling of Iba-1 was altered only in neuropathic rats (p < .05). The mean area of SGCs per neuron was significantly altered only in the inflammatory model (p < .05). All morphological alterations were reverted by modulating the fractalkine pathway (p < .05). In conclusion, the blockage of the fractalkine pathway seemed to be a possible therapeutic strategy for inflammatory and neuropathic orofacial pain, reducing mechanical hyperalgesia by impairing the phosphorylation of p-38 MAPK and reverting morphological alterations in microglia and SGCs.

Analgesic effect of linalool odor on oral ulcerative mucositis-induced pain in rats

Oral ulcerative mucositis (OUM) induces severe pain, leading to a low quality of life. Linalool odor exposure has recently been reported to suppress inflammatory pain in the hind paws. However, the analgesic effect of linalool odor on orofacial pain remains unclear. In this study, we examined the mechanism underlying the analgesic effect of linalool odor on oral pain caused by OUM using nocifensive behavioral and immunohistochemical analyses in rats. OUM was developed by treating the labial fornix region of the inferior incisors with acetic acid. Linalool at 1% was exposed for 5 min at 30 min before nocifensive behavioral measurements. OUM induced spontaneous pain and mechanical allodynia, which were suppressed by the linalool odor. Mechanical allodynia in the hind paw following the injection of complete Freund's adjuvant was also suppressed by linalool odor. Application of lidocaine to the olfactory bulb attenuated the inhibition of spontaneous pain and hyperactivation of trigeminal spinal nucleus caudalis neurons in OUM model rats. Linalool odor exposure-induced neuronal activation in the locus coeruleus (LC) of OUM model rats was decreased by lidocaine application to the olfactory bulb. The decrease in neuronal activation in the LC was attenuated by the administration of orexin 1 receptor (OX-1) antagonist to the LC. These results suggest that linalool odor stimulation through the olfactory pathway activates LC neurons via OX-1 signaling, leading to the suppression of OUM-induced oral pain.

Cross-species gene expression patterns of purinergic signaling in the human and mouse trigeminal ganglion

Purinergic signaling system plays a pivotal role in the trigeminal ganglion (TG) which is a primary sensory tissue in vertebrate nervous systems involving orofacial nociception and peripheral sensitization. Despite previous efforts to reveal the expression patterns of purinergic components in the mouse TG, it is still unknown the interspecies differences between human and mouse. In this study, we provide a comprehensive transcriptome profile of the purinergic signaling system across diverse cell types and neuronal subpopulations within the human TG, systematically comparing it with mouse TG. In addition, the evolutionary conservation and species-specific expression patterns of the purinergic components are also discussed. We propose that the data can improve our understanding of purinergic signaling in the peripheral nervous system and facilitate the identification of novel therapeutic targets.

Neonatal injury modulates incisional pain sensitivity in adulthood: An animal study

Neonatal pain experiences including traumatic injury influences negatively on development of nociceptive circuit developments, resulting in persistent pain hypersensitivity in adults. However, the detailed mechanism is not yet well understood. In the present study, to clarify the pathogenesis of orofacial pain hypersensitivity associated with neonatal injury, the involvement of the voltage-gated sodium channel (Na_v) 1.8 and the C-C chemokine ligand 2 (CCL2)/C-C chemokine receptor 2 (CCR2) signaling in the trigeminal ganglion (TG) in facial skin incisional pain hypersensitivity was examined in 190 neonatal facial-injured and sham male rats. The whisker pad skin was incised on postnatal day 4 and week 7 (Incision-Incision group). Compared to the group without neonatal incision (Sham-Incision group), mechanical hypersensitivity in the whisker pad skin was enhanced in Incision-Incision group. The number of Na_v1.8-immunoreactive TG neurons and the amount of CCL2 expressed in the macrophages and satellite glial cells in the TG were increased on day 14 after re-incision in the Incision-Incision group, compared with Sham-Incision group. Blockages of Na_v1.8 in the incised region and CCR2 in the TG suppressed the enhancement of mechanical hypersensitivity in the Incision-Incision group. Administration of CCL2 into the TG enhanced mechanical hypersensitivity in the Sham-Sham, Incision-Sham and Sham-Incision group. Our results suggest that neonatal facial injury accelerates the TG neuronal hyperexcitability following orofacial skin injury in adult in association with Na_v1.8 overexpression via CCL2 signaling, resulting in the enhancement of orofacial incisional pain hypersensitivity in the adulthood.

Metabotropic glutamate receptor 5-mediated inhibition of inward-rectifying K+ channel 4.1 contributes to orofacial ectopic mechanical allodynia following inferior alveolar nerve transection in male mice

Inward-rectifying K⁺ channel 4.1 (Kir4.1), which regulates the electrophysiological properties of neurons and glia by affecting K⁺ homeostasis, plays a critical role in neuropathic pain. Metabotropic glutamate receptor 5 (mGluR5) regulates the expression of Kir4.1 in retinal Müller cells. However, the role of Kir4.1 and its expressional regulatory mechanisms underlying orofacial ectopic allodynia remain unclear. This study aimed to investigate the biological roles of Kir4.1 and mGluR5 in the trigeminal ganglion (TG) in orofacial ectopic mechanical allodynia and the role of mGluR5 in Kir4.1 regulation. An animal model of nerve injury was established via inferior alveolar nerve transection (IANX) in male C57BL/6J mice. Behavioral tests indicated that mechanical allodynia in the ipsilateral whisker pad lasted at least 14 days after IANX surgery and was alleviated by the overexpression of Kir4.1 in the TG, as well as intraganglionic injection of an mGluR5 antagonist (MPEP hydrochloride) or a protein kinase C (PKC) inhibitor (chelerythrine chloride); Conditional knockdown of the Kir4.1 gene downregulated mechanical thresholds in the whisker pad. Double immunostaining revealed that Kir4.1 and mGluR5 were co-expressed in satellite glial cells in the TG. IANX downregulated Kir4.1 and upregulated mGluR5 and phosphorylated PKC (p-PKC) in the TG; Inhibition of mGluR5 reversed the changes in Kir4.1 and p-PKC that were induced by IANX; Inhibition of PKC activation reversed the downregulation of Kir4.1 expression caused by IANX (p < .05). In conclusion, activation of mGluR5 in the TG after IANX contributed to orofacial ectopic mechanical allodynia by suppressing Kir4.1 via the PKC signaling pathway.

Age-Related Changes in Neurons and Satellite Glial Cells in Mouse Dorsal Root Ganglia

The effects of aging on the nervous system are well documented. However, most previous studies on this topic were performed on the central nervous system. The present study was carried out on the dorsal root ganglia (DRGs) of mice, and focused on age-related changes in DRG neurons and satellite glial cells (SGCs). Intracellular electrodes were used for dye injection to examine the gap junction-mediated coupling between neurons and SGCs, and for intracellular electrical recordings from the neurons. Tactile sensitivity was assessed with von Frey hairs. We found that 3-23% of DRG neurons were dye-coupled to SGCs surrounding neighboring neurons in 8-24-month (Mo)-old mice, whereas in young adult (3 Mo) mice, the figure was 0%. The threshold current for firing an action potential in sensory neurons was significantly lower in DRGs from 12 Mo mice compared with those from 3 Mo mice. The percentage of neurons with spontaneous subthreshold membrane potential oscillation was greater by two-fold in 12 Mo mice. The withdrawal threshold was lower by 22% in 12 Mo mice compared with 3 Mo ones. These results show that in the aged mice, a proportion of DRG neurons is coupled to SGCs, and that the membrane excitability of the DRG neurons increases with age. We propose that augmented neuron-SGC communications via gap junctions are caused by low-grade inflammation associated with aging, and this may contribute to pain behavior.

Age-related Changes in Trigeminal Ganglion Macrophages Enhance Orofacial Ectopic Pain After Inferior Alveolar Nerve Injury

BACKGROUND/AIM

The ectopic pain associated with inferior alveolar nerve (IAN) injury has been reported to involve macrophage expression in the trigeminal ganglion (TG). However, the effect of age-related changes on this abnormal pain conditions are still unknown. This study sought to clarify the involvement of age-related changes in macrophage expression and phenotypic conversion in the TG and how these changes enhance ectopic mechanical allodynia after IAN transection (IANX).

MATERIALS AND METHODS

We used senescence-accelerated mouse (SAM)-prone 8 (SAMP8) and SAM-resistance 1 (SAMR1) mice, which are commonly used to study ageing-related changes. Mechanical stimulation was applied to the whisker pad skin under light anaesthesia; the mechanical head withdrawal threshold (MHWT) was measured for 21 d post-IANX. We subsequently counted the numbers of Iba1 (macrophage marker)-immunoreactive (IR) cells, Iba1/CD11c (M1-like inflammatory macrophage marker)-co-IR cells, and Iba1/CD206 (M2-like anti-inflammatory macrophage marker)-co-IR cells in the TG innervating the whisker pad skin. After continuous intra-TG administration of liposomal clodronate Clophosome^®-A (LCCA) to IANX-treated SAMP8-mice, the MHWT values of the whisker pad skin were examined.

RESULTS

Five days post-IANX, the MHWT had significantly decreased in SAMP8 mice compared to SAMR1-mice. Iba1-IR and Iba1/CD11c-co-IR cell counts were significantly increased in SAMP8 mice compared to SAMR1 mice 5 d post-IANX. LCCA administration significantly restored MHWT compared to control-LCCA administration.

CONCLUSION

Ectopic mechanical allodynia of whisker pad skin after IANX is exacerbated by ageing, which involves increases in M1-like inflammatory macrophages in the TG.

Differential roles of NMDAR subunits 2A and 2B in mediating peripheral and central sensitization contributing to orofacial neuropathic pain

The spinal N-methyl-d-aspartate receptor (NMDAR), particularly their subtypes NR2A and NR2B, plays pivotal roles in neuropathic and inflammatory pain. However, the roles of NR2A and NR2B in orofacial pain and the exact molecular and cellular mechanisms mediating nervous system sensitization are still poorly understood. Here, we exhaustively assessed the regulatory effect of NMDAR in mediating peripheral and central sensitization in orofacial neuropathic pain. Von-Frey filament tests showed that the inferior alveolar nerve transection (IANX) induced ectopic allodynia behavior in the whisker pad of mice. Interestingly, mechanical allodynia was reversed in mice lacking NR2A and NR2B. IANX also promoted the production of peripheral sensitization-related molecules, such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, brain-derived neurotrophic factor (BDNF), and chemokine upregulation (CC motif) ligand 2 (CCL2), and decreased the inward potassium channel (Kir) 4.1 on glial cells in the trigeminal ganglion, but NR2A conditional knockout (CKO) mice prevented these alterations. In contrast, NR2B CKO only blocked the changes of Kir4.1, IL-1β, and TNF-α and further promoted the production of CCL2. Central sensitization-related c-fos, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adaptor molecule 1 (Iba-1) were promoted and Kir4.1 was reduced in the spinal trigeminal caudate nucleus by IANX. Differential actions of NR2A and NR2B in mediating central sensitization were also observed. Silencing of NR2B was effective in reducing c-fos, GFAP, and Iba-1 but did not affect Kir4.1. In contrast, NR2A CKO only altered Iba-1 and Kir4.1 and further increased c-fos and GFAP. Gain-of-function and loss-of-function approaches provided insight into the differential roles of NR2A and NR2B in mediating peripheral and central nociceptive sensitization induced by IANX, which may be a fundamental basis for advancing knowledge of the neural mechanisms' reaction to nerve injury.

Single-cell RNA sequencing reveals distinct transcriptional features of the purinergic signaling in mouse trigeminal ganglion

Trigeminal ganglion (TG) is the first station of sensory pathways in the orofacial region. The TG neurons communicate with satellite glial cells (SGCs), macrophages and other cells forming a functional unit that is responsible for processing of orofacial sensory information. Purinergic signaling, one of the most widespread autocrine and paracrine pathways, plays a crucial role in intercellular communication. The multidirectional action of purinergic signaling in different cell types contributes to the neuromodulation and orofacial sensation. To fully understand the purinergic signaling in these processes, it is essential to determine the shared and unique expression patterns of genes associated with purinergic signaling in different cell types. Here, we performed single-cell RNA sequencing of 22,969 cells isolated from normal mouse TGs. We identified 18 distinct cell populations, including 6 neuron subpopulations, 3 glial subpopulations, 7 immune cell subpopulations, fibroblasts, and endothelial cells. We also revealed the transcriptional features of genes associated with purinergic signaling, including purinergic receptors, extracellular adenosine triphosphate (eATP) release channels, eATP metabolism-associated enzymes, and eATP transporters in each cell type. Our results have important implications for understanding and predicting the cell type-specific roles of the purinergic signaling in orofacial signal processing in the trigeminal primary sensory system.

Satellite Glial Cells and Neurons in Trigeminal Ganglia Are Altered in an Itch Model in Mice

Itch (pruritus) is a common chronic condition with a lifetime prevalence of over 20%. The mechanisms underlying itch are poorly understood, and its therapy is difficult. There is recent evidence that following nerve injury or inflammation, intercellular communications in sensory ganglia are augmented, which may lead to abnormal neuronal activity, and hence to pain, but there is no information whether such changes take place in an itch model. We studied changes in neurons and satellite glial cells (SGCs) in trigeminal ganglia in an itch model in mice using repeated applications of 2,4,6-trinitro-1-chlorobenzene (TNCB) to the external ear over a period of 11 days. Treated mice showed augmented scratching behavior as compared with controls during the application period and for several days afterwards. Immunostaining for the activation marker glial fibrillary acidic protein in SGCs was greater by about 35% after TNCB application, and gap junction-mediated coupling between neurons increased from about 2% to 13%. The injection of gap junction blockers reduced scratching behavior, suggesting that gap junctions contribute to itch. Calcium imaging studies showed increased responses of SGCs to the pain (and presumed itch) mediator ATP. We conclude that changes in both neurons and SGCs in sensory ganglia may play a role in itch.

Pannexin 1 role in the trigeminal ganglion in infraorbital nerve injury-induced mechanical allodynia

OBJECTIVES

The detailed pathological mechanism of orofacial neuropathic pain remains unknown. We aimed to examine the pannexin 1 (Panx1) signaling in the trigeminal ganglion (TG) involvement in infraorbital nerve injury (IONI)-induced orofacial neuropathic pain.

MATERIALS AND METHODS

Mechanical head-withdrawal threshold (MHWT) was measured in IONI-treated rats receiving intra-TG Panx1 inhibitor or metabotropic glutamate receptor 5 (mGluR5) antagonist administration and MHWTs in naive rats receiving intra-TG mGluR5 agonist administration post-IONI. Glutamate and Panx1 in the TG were measured post-IONI. Panx1, mGluR5, and glutamine synthetase expression in TG were immunohistochemically identified, and changes in the number of mGluR5-P2X₃ -expressed TG neurons were examined.

RESULTS

MHWT was significantly decreased post-IONI, and this decrease was reversed by Panx1 inhibition or mGluR5 antagonism. mGluR5 agonism induced a decrease in the MHWT. IONI increased extracellular glutamate in TG. Panx1 was expressed in satellite glial cells and TG neurons, and intra-TG mGluR5 antagonism decreased the number of mGluR5 and P2X₃ positive TG neurons post-IONI.

CONCLUSIONS

IONI facilitates glutamate release via Panx1 that activates mGluR5 which was expressed in the nociceptive TG neurons innervating the orofacial region. In turn, P2X₃ receptor-expressed TG neurons is enhanced via mGluR5 signaling, resulting in orofacial neuropathic pain.

Role of Connexin 43 in an Inflammatory Model for TMJ Hyperalgesia

Temporomandibular joint disorders (TMD) consist of a heterogeneous group of conditions that present with pain in the temporomandibular joint (TMJ) region and muscles of mastication. This project assessed the role of connexin 43 (Cx43), a gap junction protein, in the trigeminal ganglion (TG) in an animal model for persistent inflammatory TMJ hyperalgesia. Experiments were performed in male and female rats to determine if sex differences influence the expression and/or function of Cx43 in persistent TMJ hyperalgesia. Intra-TMJ injection of Complete Freund's Adjuvant (CFA) caused a significant increase in Cx43 expression in the TG at 4 days and 10 days post-injection in ovariectomized (OvX) female rats and OvX females treated with estradiol (OvXE), while TG samples in males revealed only marginal increases. Intra-TG injection of interference RNA for Cx43 (siRNA Cx43) 3 days prior to recording, markedly reduced TMJ-evoked masseter muscle electromyographic (MMemg) activity in all CFA-inflamed rats, while activity in sham animals was not affected. Western blot analysis revealed that at 3 days after intra-TG injection of siRNA Cx43 protein levels for Cx43 were significantly reduced in TG samples of all CFA-inflamed rats. Intra-TG injection of the mimetic peptide GAP19, which inhibits Cx43 hemichannel formation, greatly reduced TMJ-evoked MMemg activity in all CFA-inflamed groups, while activity in sham groups was not affected. These results revealed that TMJ inflammation caused a persistent increase in Cx43 protein in the TG in a sex-dependent manner. However, intra-TG blockade of Cx43 by siRNA or by GAP19 significantly reduced TMJ-evoked MMemg activity in both males and females following TMJ inflammation. These results indicated that Cx43 was necessary for enhanced jaw muscle activity after TMJ inflammation in males and females, a result that could not be predicted on the basis of TG expression of Cx43 alone.

Orofacial Neuropathic Pain-Basic Research and Their Clinical Relevancies

Trigeminal nerve injury is known to cause severe persistent pain in the orofacial region. This pain is difficult to diagnose and treat. Recently, many animal studies have reported that rewiring of the peripheral and central nervous systems, non-neuronal cell activation, and up- and down-regulation of various molecules in non-neuronal cells are involved in the development of this pain following trigeminal nerve injury. However, there are many unknown mechanisms underlying the persistent orofacial pain associated with trigeminal nerve injury. In this review, we address recent animal data regarding the involvement of various molecules in the communication of neuronal and non-neuronal cells and examine the possible involvement of ascending pathways in processing pathological orofacial pain. We also address the clinical observations of persistent orofacial pain associated with trigeminal nerve injury and clinical approaches to their diagnosis and treatment.

Rapamycin Accelerates Axon Regeneration Through Schwann Cell-mediated Autophagy Following Inferior Alveolar Nerve Transection in Rats

Sensory disturbance in the orofacial region owing to trigeminal nerve injury is caused by dental treatment or accident. Commercially available therapeutics are ineffective for the treatment of sensory disturbance. Additionally, the therapeutic effects of rapamycin, an allosteric inhibitor of mammalian target of rapamycin (mTOR), which negatively regulates autophagy, on the sensory disturbance are not fully investigated. Thus, we investigated the therapeutic effects of rapamycin on the sensory disturbance in the mandibular region caused by inferior alveolar nerve (IAN) transection (IANX) in rats. The expression levels of the phosphorylated p70S6K, a downstream molecule of mTOR, in the proximal and distal stumps of the transected IAN were significantly reduced by rapamycin administration to the injured site. Conversely, the increments of both Beclin 1 and microtubule-associated protein-1 light chain 3-II protein levels in the proximal and distal stumps of the transected IAN was induced by rapamycin administration. Immunohistochemical analyses revealed that Beclin 1 was located in Schwann cells in the proximal stump of the IAN. Accumulation of myelin protein zero and myelin basic protein in the proximal and distal stumps of the IAN was significantly reduced by rapamycin administration. Rapamycin administration facilitated axon regeneration after IANX and increased the number of brain-derived neurotrophic factor positive neurons in the trigeminal ganglion. Thus, recovery from sensory disturbance in the lower lip caused by IANX was markedly facilitated by rapamycin. These findings suggest that rapamycin administration is a promising treatment for the sensory disturbance caused by IANX.

P2X3 receptor upregulation in trigeminal ganglion neurons through TNFα production in macrophages contributes to trigeminal neuropathic pain in rats

BACKGROUND

Trigeminal neuralgia is a characteristic disease that manifests as orofacial phasic or continuous severe pain triggered by innocuous orofacial stimulation; its mechanisms are not fully understood. In this study, we established a new animal model of trigeminal neuralgia and investigated the role of P2X₃ receptor (P2X₃R) alteration in the trigeminal ganglion (TG) via tumor necrosis factor alpha (TNFα) signaling in persistent orofacial pain.

METHODS

Trigeminal nerve root compression (TNC) was performed in male Sprague-Dawley rats. Changes in the mechanical sensitivity of whisker pad skin, amount of TNFα in the TG, and number of P2X₃R and TNF receptor-2 (TNFR2)-positive TG neurons were assessed following TNC. The effects of TNFR2 antagonism in TG and subcutaneous P2X₃R antagonism on mechanical hypersensitivity following TNC were examined.

RESULTS

TNC induced unilateral continuous orofacial mechanical allodynia, which was depressed by carbamazepine. The accumulation of macrophages showing amoeboid-like morphological changes and expression of TNFα in the TG was remarkably increased following TNC treatment. The number of P2X₃R- and TNFR2-positive TG neurons innervating the orofacial skin was significantly increased following TNC. TNFα was released from activated macrophages that occurred in the TG following TNC, and TNFR2 antagonism in the TG significantly diminished the TNC-induced increase in P2X₃R-immunoreactive TG neurons. Moreover, subcutaneous P2X₃R antagonism in the whisker pad skin significantly depressed TNC-induced mechanical allodynia.

CONCLUSIONS

Therefore, it can be concluded that the signaling of TNFα released from activated macrophages in the TG induces the upregulation of P2X₃R expression in TG neurons innervating the orofacial region, resulting in orofacial mechanical allodynia following TNC.

The α2δ-1-NMDAR1 interaction in the trigeminal ganglion contributes to orofacial ectopic pain following inferior alveolar nerve injury

Orofacial ectopic pain can often arise following nerve injury. However, the exact mechanism responsible for orofacial ectopic pain induced by trigeminal nerve injury remains unknown. The α2δ-1 and glutamate N-methyl-d-aspartic acid receptor (NMDAR) interactions have been demonstrated to participate in neuropathic pain regulation in the spinal cord. In this study, a rat model of inferior alveolar nerve transection (IANX) was used to investigate the role of α2δ-1-NMDAR1 interaction in the trigeminal ganglion (TG) in regard to the regulation of orofacial ectopic pain. Western blot (WB) analysis indicated that α2δ-1 and NMDAR1 in the TG were substantially higher in IANX rats than they were in sham/naive rats. Additionally, immunofluorescence (IF) results revealed that α2δ-1 and NMDAR1 were co-expressed and distributed within neurons and activated satellite glial cells in the TG. Co-immunoprecipitation (Co-IP) results indicated that α2δ-1-NMDAR1 complex levels in the TG were higher in IANX rats than they were in sham rats. Furthermore, the results of behavioral tests demonstrated that intra-TG injection of gabapentin (α2δ-1 inhibitory ligand) or memantine hydrochloride (NMDAR antagonist) reversed the decrease in mechanical head-withdrawal threshold (HWT) in IANX rats. Moreover, inhibition of α2δ-1 by intra-TG administration of gabapentin suppressed the upregulation of the NMDAR1 protein, and the inhibition of NMDAR by intra-TG administration of memantine hydrochloride inhibited the increased expression of α2δ-1 protein induced by IANX. In conclusion, the physical and functional interaction between α2δ-1 and NMDAR1 is critical for the development of orofacial ectopic pain, indicating that α2δ-1, NMDAR1, and the α2δ-1-NMDAR1 complex may represent potential targets for the treatment of orofacial ectopic pain.

Satellite Glial Cells and Astrocytes, a Comparative Review

Astroglia are neural cells, heterogeneous in form and function, which act as supportive elements of the central nervous system; astrocytes contribute to all aspects of neural functions in health and disease. Through their highly ramified processes, astrocytes form close physical contacts with synapses and blood vessels, and are integrated into functional syncytia by gap junctions. Astrocytes interact among themselves and with other cells types (e.g., neurons, microglia, blood vessel cells) by an elaborate repertoire of chemical messengers and receptors; astrocytes also influence neural plasticity and synaptic transmission through maintaining homeostasis of neurotransmitters, K⁺ buffering, synaptic isolation and control over synaptogenesis and synaptic elimination. Satellite glial cells (SGCs) are the most abundant glial cells in sensory ganglia, and are believed to play major roles in sensory functions, but so far research into SGCs attracted relatively little attention. In this review we compare SGCs to astrocytes with the purpose of using the vast knowledge on astrocytes to explore new aspects of SGCs. We survey the main properties of these two cells types and highlight similarities and differences between them. We conclude that despite the much greater diversity in morphology and signaling mechanisms of astrocytes, there are some parallels between them and SGCs. Both types serve as boundary cells, separating different compartments in the nervous system, but much more needs to be learned on this aspect of SGCs. Astrocytes and SGCs employ chemical messengers and calcium waves for intercellular signaling, but their significance is still poorly understood for both cell types. Both types undergo major changes under pathological conditions, which have a protective function, but an also contribute to disease, and chronic pain in particular. The knowledge obtained on astrocytes is likely to benefit future research on SGCs.

Activation of oxytocin receptor in the trigeminal ganglion attenuates orofacial ectopic pain attributed to inferior alveolar nerve injury

This study explores the effects of oxytocin receptor (OXTR) in the trigeminal ganglion (TG) on orofacial neuropathic pain. We demonstrate that OXTR activation in the TG relieves the orofacial ectopic pain as well as inhibits the upregulated expression of calcitonin gene-related peptide (CGRP), IL-1β, and TNFα in the TG and spinal trigeminal nucleus caudalis (SpVc) of rats with inferior alveolar nerve transection. OXTR, a G protein-coupled receptor, has been demonstrated to play a significant role in analgesia after activation by its canonical agonist oxytocin (OXT) in the dorsal root ganglion. However, the role of OXTR in the trigeminal nervous system on the orofacial neuropathic pain is still little known. In the present study, we aimed to investigate the regulation effect and mechanism of OXTR in the TG) and SpVc) on orofacial ectopic pain induced by trigeminal nerve injury. The inferior alveolar nerve (IAN) was transected to establish a ectopic pain model. A behavioral test with electronic von Frey filament demonstrated IAN transection (IANX) evoked mechanical hypersensitivity in the whisker pad from day 1 to at least day 14 after surgery. In addition, administration of OXT (50 and 100 μM) into the TG attenuated the mechanical hypersensitivity induced by IANX, which was reversed by pretreatment with L-368,899 (a selective antagonist of OXTR) into the TG. In addition, immunofluorescence showed the expression of OXTR in neurons in the TG and SpVc. Furthermore, Western blot analysis indicated that the upregulated expression of OXTR, CGRP, IL-1β, and TNFα in the TG and SpVc after IANX was inhibited by the administration of OXT into the TG. And the inhibition effect of OXT on the expression of CGRP, IL-1β, and TNFα was abolished by preapplication of OXTR antagonist L-368,899 into the TG.NEW & NOTEWORTHY This study explores the effects of oxytocin receptor (OXTR) in the trigeminal ganglion (TG) on orofacial neuropathic pain. We demonstrate that OXTR activation in the TG relieves the orofacial ectopic pain as well as inhibits the upregulated expression of calcitonin gene-related peptide, IL-1β, and TNF-α in the TG and spinal trigeminal nucleus caudalis of rats with inferior alveolar nerve transection.

Connexin 36 Mediates Orofacial Pain Hypersensitivity Through GluK2 and TRPA1

Trigeminal neuralgia is a debilitating condition, and the pain easily spreads to other parts of the face. Here, we established a mouse model of partial transection of the infraorbital nerve (pT-ION) and found that the Connexin 36 (Cx36) inhibitor mefloquine caused greater alleviation of pT-ION-induced cold allodynia compared to the reduction of mechanical allodynia. Mefloquine reversed the pT-ION-induced upregulation of Cx36, glutamate receptor ionotropic kainate 2 (GluK2), transient receptor potential ankyrin 1 (TRPA1), and phosphorylated extracellular signal regulated kinase (p-ERK) in the trigeminal ganglion. Cold allodynia but not mechanical allodynia induced by pT-ION or by virus-mediated overexpression of Cx36 in the trigeminal ganglion was reversed by the GluK2 antagonist NS102, and knocking down Cx36 expression in Nav1.8-expressing nociceptors by injecting virus into the orofacial skin area of Nav1.8-Cre mice attenuated cold allodynia but not mechanical allodynia. In conclusion, we show that Cx36 contributes greatly to the development of orofacial pain hypersensitivity through GluK2, TRPA1, and p-ERK signaling.

Oxytocin-Dependent Regulation of TRPs Expression in Trigeminal Ganglion Neurons Attenuates Orofacial Neuropathic Pain Following Infraorbital Nerve Injury in Rats

We evaluated the mechanisms underlying the oxytocin (OXT)-induced analgesic effect on orofacial neuropathic pain following infraorbital nerve injury (IONI). IONI was established through tight ligation of one-third of the infraorbital nerve thickness. Subsequently, the head withdrawal threshold for mechanical stimulation (MHWT) of the whisker pad skin was measured using a von Frey filament. Trigeminal ganglion (TG) neurons innervating the whisker pad skin were identified using a retrograde labeling technique. OXT receptor-immunoreactive (IR), transient receptor potential vanilloid 1 (TRPV1)-IR, and TRPV4-IR TG neurons innervating the whisker pad skin were examined on post-IONI day 5. The MHWT remarkably decreased from post-IONI day 1 onward. OXT application to the nerve-injured site attenuated the decrease in MHWT from day 5 onward. TRPV1 or TRPV4 antagonism significantly suppressed the decrement of MHWT following IONI. OXT receptors were expressed in the uninjured and Fluoro-Gold (FG)-labeled TG neurons. Furthermore, there was an increase in the number of FG-labeled TRPV1-IR and TRPV4-IR TG neurons, which was inhibited by administering OXT. This inhibition was suppressed by co-administration with an OXT receptor antagonist. These findings suggest that OXT application inhibits the increase in TRPV1-IR and TRPV4-IR TG neurons innervating the whisker pad skin, which attenuates post-IONI orofacial mechanical allodynia.

Intraneural Application of microRNA-1 Mimetic Nucleotides Does Not Resolve Neuropathic Pain After Chronic Constriction Injury in Rats

Background

Alterations of the expression of microRNAs (miRNAs) in chronic pain models seem to play a crucial role in the development of neuropathic pain, with microRNA-1 (miR-1) being of particular interest. Recently, we were able to show that decreased miR-1 levels were associated with increased expression of brain-derived neurotrophic factor (BDNF) and Connexin 43 (Cx43). We hypothesized that miR-1 mimetic nucleotides could alleviate neuropathic pain caused by chronic constriction injury in rats.

Methods

MiR-1 mimetic nucleotides were evaluated for effectiveness, functionality, and intracellular stability by transfecting human glioblastoma cells (U-87 MG). In vivo transfection with miR-1 mimics and corresponding scrambled miRNAs serving as control was performed by repetitive injection (days 0, 3, and 7) into the sciatic nerve following chronic constriction injury (CCI) in rats. Quantitative PCR was used to measure miR-1 content. Cx43 expression was determined by Western blot analysis. Effects on neuropathic pain were assessed by detecting paw withdrawal thresholds using an automated filament application.

Results

Transfection of miR-1 mimics was confirmed in U-87 MG cells, with miR-1 content being increased significantly after 48 h and after 96 h (p<0.05). Effective downregulation of Cx43 expression was observed 48 and 96 h after transfection (−44 ± 0.07% and −40 ± 0.11%; p<0.05). In vivo, repetitive transfection with miR-1 mimetic nucleotides led to a 17.9-fold (± 14.2) increase of miR-1 in the sciatic nerve. However, the protein expression of Cx43 in sciatic nerves as well as paw withdrawal thresholds for mechanical stimulation was not significantly increased 10 days after chronic constriction injury.

Conclusion

While transfection with miR-1 mimics effective reduces Cx43 expression in vitro and restores miR-1 after CCI, we did neither observe altered levels of Cx43 protein level in nerves nor a beneficial effect on mechanical allodynia in vivo, most likely caused by insufficient Cx43 suppression.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

Regulation of the KATP-JNK gap junction signaling pathway by immunomodulator astragaloside IV attenuates neuropathic pain

BACKGROUND AND OBJECTIVES

Gap junctions play a pivotal role in contributing to the formation of astroglial networks and in chronic pain. However, the mechanisms underlying the dysfunction of astroglial gap junctions in chronic pain have not been fully elucidated.

METHODS

Chronic constriction injury (CCI) of the sciatic nerve was used to establish rat neuropathic pain model. C6 cells were used to perform experiments in vitro. Von Frey hairs and Hargreave's method were used to determine the withdrawal threshold of rats. Protein expression was detected by immunofluorescence and western blotting.

RESULTS

Astragaloside IV (AST IV) significantly attenuated neuropathic pain and suppressed the excitation of spinal astrocytes in rats with CCI. The antinociceptive effect of AST IV was reversed by the gap junction decoupler carbenoxolone (CBX). AST IV inhibited the high expression of phosphorylated connexin 43 (p-Cx43) and p-c-Jun N-terminal kinase (p-JNK) in spinal cord of rats with CCI. JNK inhibitor alleviated neuropathic pain, which was reversed by CBX. JNK inhibitor decreased the high expression of p-Cx43 in both rats with CCI and tumor necrosis factor-alpha (TNF-α)-treated C6 cells. Additionally, the analgesic effect of AST IV was reversed by the adenosine triphosphate-sensitive potassium (K_ATP) channel blocker, glibenclamide (Glib). Glib abolished the inhibitory effects of AST IV on p-JNK and p-Cx43 both in vivo and in vitro. K_ATP channel opener (KCO) mimicked the inhibitory effects of AST IV on p-JNK and p-Cx43 in TNF-α-treated C6 cells.

CONCLUSION

Our results indicate that the sciatic nerve CCI induces the dysfunction of gap junctions in the spinal cord by activating K_ATP/JNK signaling to contribute to neuropathic pain. AST IV attenuates neuropathic pain via regulating the K_ATP-JNK gap junction axis.

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Emerging importance of satellite glia in nervous system function and dysfunction

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC-SGC and neuron-SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

Pathophysiological mechanisms of persistent orofacial pain

Nociceptive stimuli to the orofacial region are typically received by the peripheral terminal of trigeminal ganglion (TG) neurons, and noxious orofacial information is subsequently conveyed to the trigeminal spinal subnucleus caudalis and the upper cervical spinal cord (C1-C2). This information is further transmitted to the cortical somatosensory regions and limbic system via the thalamus, which then leads to the perception of pain. It is a well-established fact that the presence of abnormal pain in the orofacial region is etiologically associated with neuroplastic changes that may occur at any point in the pain transmission pathway from the peripheral to the central nervous system (CNS). Recently, several studies have reported that functional plastic changes in a large number of cells, including TG neurons, glial cells (satellite cells, microglia, and astrocytes), and immune cells (macrophages and neutrophils), contribute to the sensitization and disinhibition of neurons in the peripheral and CNS, which results in orofacial pain hypersensitivity.

Cross-talk signaling in the trigeminal ganglion: role of neuropeptides and other mediators

The trigeminal ganglion with its three trigeminal nerve tracts consists mainly of clusters of sensory neurons with their peripheral and central processes. Most neurons are surrounded by satellite glial cells and the axons are wrapped by myelinating and non-myelinating Schwann cells. Trigeminal neurons express various neuropeptides, most notably, calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide (PACAP). Two types of CGRP receptors are expressed in neurons and satellite glia. A variety of other signal molecules like ATP, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion neurons and signal to neighboring neurons or satellite glial cells, which can signal back to neurons with same or other mediators. This potential cross-talk of signals involves intracellular mechanisms, including gene expression, that can modulate mediators of sensory information, such as neuropeptides, receptors, and neurotrophic factors. From the ganglia cell bodies, which are outside the blood–brain barrier, the mediators are further distributed to peripheral sites and/or to the spinal trigeminal nucleus in the brainstem, where they can affect neural transmission. A major question is how the sensory neurons in the trigeminal ganglion differ from those in the dorsal root ganglion. Despite their functional overlap, there are distinct differences in their ontogeny, gene expression, signaling pathways, and responses to anti-migraine drugs. Consequently, drugs that modulate cross-talk in the trigeminal ganglion can modulate both peripheral and central sensitization, which may potentially be distinct from sensitization mediated in the dorsal root ganglion.

Involvement of Satellite Cell Activation via Nitric Oxide Signaling in Ectopic Orofacial Hypersensitivity

The mechanical head-withdrawal threshold (MHWT) was significantly reduced following inferior alveolar nerve transection (IANX) in rats. Nitrate and nitrite synthesis was dramatically increased in the trigeminal ganglion (TG) at 6 h after the IANX. The relative number of neuronal nitric oxide synthase (nNOS)-immunoreactive (IR) cells was significantly higher in IANX rats compared to sham-operated and N-propyl-L-arginine (NPLA)-treated IANX rats. On day 3 after NPLA administration, the MHWT recovered considerably in IANX rats. Following L-arginine injection into the TG, the MHWT was significantly reduced within 15 min, and the mean number of TG cells encircled by glial fibrillary acidic protein (GFAP)-IR cells was substantially higher. The relative number of nNOS-IR cells encircled by GFAP-IR cells was significantly increased in IANX rats. In contrast, after NPLA injection into the TG, the relative number of GFAP-IR cells was considerably reduced in IANX rats. Fluorocitrate administration into the TG significantly reduced the number of GFAP-IR cells and prevented the MHWT reduction in IANX rats. The present findings suggest that following IANX, satellite glial cells are activated via nitric oxide (NO) signaling from TG neurons. The spreading satellite glial cell activation within the TG results in mechanical hypersensitivity of face regions not directly associated with the trigeminal nerve injury.

Involvement of TNFα in the enhancement of hypersensitivity in the adulthood-injured face associated with facial injury in infancy

To evaluate the mechanisms underlying acceleration of hypersensitivity in the adulthood-injured face following facial injury in infants, we developed the rats model with facial skin injury in infants and adulthoods (incision + incision), and facial skin suture in infants and facial skin injury in adulthoods (sham + incision), and analyzed the mechanical head-withdrawal threshold (MHWT) of the facial skin, immunohistochemical analysis of trigeminal ganglion (TG) and the effects of intra-ganglionic administration of neutralizing ant-TNFα antibody and recombinant TNFα on nocifensive behavior. The MHWT became considerably lower in incision + incision rats than in sham + incision rats at 10-14 days after the surgery. We observed many TG neurons encircled by glial fibrillary acidic protein-immunoreactive (GFAP-IR) cells and those exhibited TNFα immunoreactivity. TNFα was also expressed in GFAP-IR cells in incision + inicision TG. TNFα protein levels and the relative number of TNFα-IR cells were significantly higher in incision + incision rats than in sham + incision rats. The MHWT was significantly recovered during the intra-ganglionic administration of neutralizing anti-TNFα antibody 4-14 days after the incision. Furthermore, the MHWT was significantly decreased in sham + incision rats following the intra-ganglionic administration of recombinant TNFα. The present findings suggest that the neuron-satellite glial cell communication via TNFα is a critical mechanism in the enhancement of mechanical hypersensitivity in the adulthood-injured face following facial injury in infants.

Increase in IGF-1 Expression in the Injured Infraorbital Nerve and Possible Implications for Orofacial Neuropathic Pain

Insulin-like growth factor-1 (IGF-1) is upregulated in the injured peripheral nerve bundle and controls nociceptive neuronal excitability associated with peripheral nerve injury. Here, we examined the involvement of IGF-1 signaling in orofacial neuropathic pain following infraorbital nerve injury (IONI) in rats. IONI promoted macrophage accumulation in the injured ION, as well as in the ipsilateral trigeminal ganglion (TG), and induced mechanical allodynia of the whisker pad skin together with the enhancement of neuronal activities in the subnucleus caudalis of the spinal trigeminal nucleus and in the upper cervical spinal cord. The levels of IGF-1 released by infiltrating macrophages into the injured ION and the TG were significantly increased. The IONI-induced the number of transient receptor potential vanilloid (TRPV) subfamily type 4 (TRPV4) upregulation in TRPV subfamily type 2 (TRPV2)-positive small-sized, and medium-sized TG neurons were inhibited by peripheral TRPV2 antagonism. Furthermore, the IONI-induced mechanical allodynia was suppressed by TRPV4 antagonism in the whisker pad skin. These results suggest that IGF-1 released by macrophages accumulating in the injured ION binds to TRPV2, which increases TRPV4 expression in TG neurons innervating the whisker pad skin, ultimately resulting in mechanical allodynia of the whisker pad skin.

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

Chronic pain, including inflammatory, neuropathic pain, is a serious clinical issue. There are increasing numbers of patients with chronic pain due to the growing number of elderly and it is estimated that about 25% of the global population will develop chronic pain. Chronic pain patients are refractory to medications used to treat acute pain such as opioids and non-steroidal anti-inflammatory drugs. Furthermore, the complexity and diversity of chronic pain mechanisms hinder the development of new analgesics. Thus, a better understanding of the mechanism of chronic pain is needed, which would facilitate the development of novel analgesics based on novel mechanisms. With this goal, connexins (Cxs) could be targeted for the development of new analgesics. Connexins are proteins with 20 subtypes, and function as channels, gap junctions between cells, and hemichannels that sample the extracellular space and release molecules such as neurotransmitters. Furthermore, Cxs could have functions independent of channel activity. Recent studies have shown that Cxs could be crucial in the induction and maintenance of chronic pain, and modulation of the activity or the expression of Cxs ameliorates nociceptive hypersensitivity in multiple chronic pain models. This review will cite novel findings on the role of of Cxs in the nociceptive transduction pathway under the chronic pain state and antinociceptive effects of various molecules modulating activity or expression of Cxs. Also, the potential of Cx modulation as a therapeutic strategy for intractable chronic pain will be discussed.

Peripheral and Central Mechanisms of Persistent Orofacial Pain

Neuroplastic changes in the neuronal networks involving the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1/C2) are considered the mechanisms underlying the ectopic orofacial hypersensitivity associated with trigeminal nerve injury or orofacial inflammation. It has been reported that peripheral nerve injury causes injury discharges in the TG neurons, and a barrage of action potentials is generated in TG neurons and conveyed to the Vc and C1/C2 after trigeminal nerve injury. Long after trigeminal nerve injury, various molecules are produced in the TG neurons, and these molecules are released from the soma of TG neurons and are transported to the central and peripheral terminals of TG neurons. These changes within the TG cause neuroplastic changes in TG neurons and they become sensitized. The neuronal activity of TG neurons is further accelerated, and Vc and C1/C2 neurons are also sensitized. In addition to this cascade, non-neuronal glial cells are also involved in the enhancement of the neuronal activity of TG, Vc, and C1/C2 neurons. Satellite glial cells and macrophages are activated in the TG after trigeminal nerve injury and orofacial inflammation. Microglial cells and astrocytes are also activated in the Vc and C1/C2 regions. It is considered that functional interaction between non-neuronal cells and neurons in the TG, Vc, and C1/C2 regions is a key mechanism involved in the enhancement of neuronal excitability after nerve injury or inflammation. In this article, the detailed mechanisms underlying ectopic orofacial hyperalgesia associated with trigeminal nerve injury and orofacial inflammation are addressed.

Pathological Mechanisms and Therapeutic Targets for Trigeminal Neuropathic Pain

Trigeminal neuropathic pain is a chronic pain condition caused by damage or inflammation of the trigeminal nerve or its branches, with both peripheral and central nervous system dysfunction contributing to the disorder. Trigeminal pain conditions present with diagnostic and therapeutic challenges to healthcare providers and often require multiple therapeutic approaches for pain reduction. This review will provide the overview of pathophysiology in peripheral and central nociceptive circuits that are involved in neuropathic pain conditions involving the trigeminal nerve and the current therapeutics that are used to treat these disorders. Recent advances in treatment of trigeminal pain, including novel therapeutics that target ion channels and receptors, gene therapy and monoclonal antibodies that have shown great promise in preclinical studies and clinical trials will also be described.

Cell junctions and oral health

The oral cavity and its appendices are exposed to considerable environmental and mechanical stress. Cell junctions play a pivotal role in this context. Among those, gap junctions permit the exchange of compounds between cells, thereby controlling processes such as cell growth and differentiation. Tight junctions restrict paracellular transportation and inhibit movement of integral membrane proteins between the different plasma membrane poles. Adherens junctions attach cells one to another and provide a solid backbone for resisting to mechanistical stress. The integrity of oral mucosa, normal tooth development and saliva secretion depend on the proper function of all these types of cell junctions. Furthermore, deregulation of junctional proteins and/or mutations in their genes can alter tissue functioning and may result in various human disorders, including dental and periodontal problems, salivary gland malfunction, hereditary and infectious diseases as well as tumorigenesis. The present manuscript reviews the role of cell junctions in the (patho)physiology of the oral cavity and its appendices, including salivary glands.

Connexin 43 contributes to temporomandibular joint inflammation induced-hypernociception via sodium channel 1.7 in trigeminal ganglion

We previously demonstrated that sodium channel 1.7 (Nav1.7) in trigeminal ganglion (TG) was a critical factor in temporomandibular joint (TMJ) inflammation-induced hypernociception, but the mechanism underlying inflammation-induced upregulation of Nav1.7 remained unclear. Glial-neuron interaction plays a critical role in pain process and connexin 43 (Cx43), a gap junction protein expressed in satellite glial cells (SGCs) has been shown to play an important role in several pain models. In the present study, we investigate the role of Cx43 in TMJ inflammation-induced hypernociception and its possible impact on neuronal Nav1.7. We induced TMJ inflammation in rats by injecting complete Freund's adjuvant (CFA) into TMJ and observed a decrease in head withdraw threshold after 24 h. Electron microscopy showed morphological alterations of SGCs in TMJ-inflamed rats. The expression of Cx43, glial fibrillary acidic protein (GFAP), and Nav1.7 increased greatly compared with controls. In addition, pretreatment with Cx43 blockers in TMJ-inflamed rats could alleviate mechanical hypernociception, inhibit SGCs activation and IL-1βrelease, and thus block the upregulation of Nav1.7. These findings indicate that the propagation of SGCs activation via Cx43 plays a critical role in Nav1.7-involved mechanical hypernociception induced by TMJ inflammation.

Protective Effects of Fucoxanthin on Ultraviolet B-Induced Corneal Denervation and Inflammatory Pain in a Rat Model

Fucoxanthin is a carotenoid with many pharmaceutical properties that is found in brown seaweed. However, the effects of fucoxanthin on corneal innervation and intense eye pain have not been extensively examined. To clarify the protective roles and underlying mechanisms of fucoxanthin on ocular lesions, we investigated the beneficial effects and mechanisms by which fucoxanthin ameliorates ultraviolet B (UVB)-induced corneal denervation and trigeminal pain. Treatment with fucoxanthin enhanced the expression of nuclear factor erythroid 2-related factor 2 in the cornea. Inhibition of typical denervation and epithelial exfoliation in the cornea were observed in rats treated with fucoxanthin following UVB-induced nerve disorders. Moreover, the active phosphorylated form of p38 MAP kinase (pp38) and the number of glial fibrillary acidic protein (GFAP)-positive neural cells were significantly reduced. Decreased expression of neuron-selective transient receptor potential vanilloid type 1 (TRPV1) in the trigeminal ganglia neurons was also demonstrated in rats treated with fucoxanthin after UVB-induced keratitis. Symptoms of inflammatory pain, including difficulty in opening the eyes and eye wipe behaviour, were also reduced in fucoxanthin-treated groups. Pre-treatment with fucoxanthin may protect the eyes from denervation and inhibit trigeminal pain in UVB-induced photokeratitis models.

Connexin Hemichannels in Astrocytes: Role in CNS Disorders

In the central nervous system (CNS), astrocytes form networks interconnected by gap junctions made from connexins of the subtypes Cx30 and Cx43. When unopposed by an adjoining hemichannel, astrocytic connexins can act as hemichannels to control the release of small molecules such as ATP and glutamate into the extracellular space. Accruing evidence indicates that astrocytic connexins are crucial for the coordination and maintenance of physiologic CNS activity. Here we provide an update on the role of astrocytic connexins in neurodegenerative disorders, glioma, and ischemia. In addition, we address the regulation of Cx43 in chronic pain.

Trigeminal Nerve Transection-Induced Neuroplastic Changes in the Somatosensory and Insular Cortices in a Rat Ectopic Pain Model

The primary sensory cortex processes competitive sensory inputs. Ablation of these competitive inputs induces neuroplastic changes in local cortical circuits. However, information concerning cortical plasticity induced by a disturbance of competitive nociceptive inputs is limited. Nociceptive information from the maxillary and mandibular molar pulps converges at the border between the ventral secondary somatosensory cortex (S2) and insular oral region (IOR); therefore, S2/IOR is a suitable target for examining the cortical changes induced by a disturbance of noxious inputs, which often causes neuropathic pain and allodynia. We focused on the plastic changes in S2/IOR excitation in a model of rats subjected to inferior alveolar nerve transection (IANX). Our optical imaging using a voltage-sensitive dye (VSD) revealed that the maxillary molar pulp stimulation-induced excitatory propagation was expanded one to two weeks after IANX at the macroscopic level. At the cellular level, based on Ca²⁺ imaging using two-photon microscopy, the amplitude of the Ca²⁺ responses and the number of responding neurons in S2/IOR increased in both excitatory and inhibitory neurons. The in vitro laser scanning photostimulation (LSPS) revealed that Layer II/III pyramidal and GABAergic fast-spiking neurons in S2/IOR received larger excitatory inputs from Layer IV in the IANX models, which supports the findings obtained by the macroscopic and microscopic optical imaging. Furthermore, the inhibitory postsynaptic inputs to the pyramidal neurons were decreased in the IANX models, suggesting suppression of inhibitory synaptic transmission onto excitatory neurons. These results suggest that IANX induces plastic changes in S2/IOR by changing the local excitatory and inhibitory circuits.

The role of satellite glial cells in orofacial pain

Some chronic pain conditions in the orofacial region are common, the mechanisms underlying which are unresolved. Satellite glial cells (SGCs) are the glial cells of the peripheral nervous system. In the sensory ganglia, each neuronal body is surrounded by SGCs forming distinct functional units. The unique structural organization enables SGCs to communicate with each other and with their enwrapped neurons via a variety of ways. There is a growing body of evidence that SGCs can influence the level of neuronal excitability and are involved in the development and/or maintenance of pain. The aim of this review was to summarize the latest advances made about the implication of SGCs in orofacial pain. It may offer new targets for the development of orofacial pain treatment.

Neuron-glia interaction is a key mechanism underlying persistent orofacial pain

Excitability of neurons in the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1-C2) is greatly enhanced after orofacial inflammation and trigeminal nerve injury, and TG, Vc, and C1-C2 neurons remain sensitized long after such episodes. Sensitized neurons generate various molecules, which are released from nociceptive neurons in these areas and are involved in modulating the excitability of TG, Vc, and C1-C2 nociceptive neurons. Hyperexcitable nociceptive neurons also activate satellite glial cells in the TG and microglial cells and astrocytes in the Vc and C1-C2. Glial cell activation spreads throughout the TG, Vc, and C1-C2 and triggers the release of various molecules involved in modulating nociceptive neurons in TG, Vc, and C1-C2 neurons. These findings suggest that functional interaction between neurons and glial cells is critical in persistent orofacial pain associated with orofacial inflammation and trigeminal nerve injury.

Neurons and satellite glial cells in adult rat lumbar dorsal root ganglia express connexin 36

Previous studies have shown that following peripheral nerve injury there was a downregulation of the gap junction protein connexin 36 (Cx36) in the spinal cord; however, it is not known whether Cx36 protein is expressed in the dorsal root ganglia (DRGs), nor if its levels are altered following peripheral nerve injuries. Here we address these aspects in the adult rat lumbar DRG. Cx36 mRNA was detected using qRT-PCR, and Cx36 protein was identified in DRG sections using immunohistochemistry (IHC) and immunofluorescence (IF). Double staining revealed that Cx36 co-localizes with both anti-β-III tubulin, a neuronal marker, and anti-glutamine synthetase, a satellite glial cell (SGC) marker. In neurons, Cx36 staining was mostly uniform in somata and fibers of all sizes and its intensity increased at the cell membranes. This labeling pattern was in contrast with Cx36 IF dots mainly found at junctional membranes in islet beta cells used as a control tissue. Co-staining with anti-Cx43 and anti-Cx36 showed that whereas mostly uniform staining of Cx36 was found throughout neurons and SGCs, Cx43 IF puncta were localized to SGCs. Cx36 mRNA was expressed in normal lumbar DRG, and it was significantly down-regulated in L4 DRG of rats that underwent sciatic nerve injury resulting in persistent hypersensitivity. Collectively, these findings demonstrated that neurons and SGCs express Cx36 protein in normal DRG, and suggested that perturbation of Cx36 levels may contribute to chronic neuropathic pain resulting from a peripheral nerve injury.

Macrophages in trigeminal ganglion contribute to ectopic mechanical hypersensitivity following inferior alveolar nerve injury in rats

Background

Accidental mandibular nerve injury may occur during tooth extraction or implant procedures, causing ectopic orofacial pain. The exact mechanisms underlying ectopic orofacial pain following mandibular nerve injury is still unknown. Here, we investigated the role of macrophages and tumor necrosis factor alpha (TNFα) in the trigeminal ganglion (TG) in ectopic orofacial pain following inferior alveolar nerve transection (IANX).

Methods

IANX was performed and the mechanical head-withdrawal threshold (MHWT) in the whisker pad skin ipsilateral to IANX was measured for 15 days. Expression of Iba1 in the TG was examined on day 3 after IANX, and the MHWT in the whisker pad skin ipsilateral to IANX was measured following successive intra-ganglion administration of the macrophage depletion agent liposomal clodronate Clophosome-A (LCCA). TNFα expression in the TG and the MHWT in the whisker pad skin ipsilateral to IANX following successive intra-ganglion administration of the TNFα blocker etanercept were measured on day 3 after IANX, and tumor necrosis factor receptor-1 (TNFR1) immunoreactive (IR) cells in the TG were analyzed immunohistochemically on day 3.

Results

The MHWT in the whisker pad skin was significantly decreased for 15 days, and the number of Iba1-IR cells was significantly increased in the TG on day 3 after IANX. Successive intra-ganglion administration of the macrophage depletion agent LCCA significantly reduced the increased number of Iba1-IR cells in the TG and reversed the IANX-induced decrease in MHWT in the whisker pad skin. TNFα expression was increased in the TG on day 3 after IANX and was reduced following successive intra-ganglion administration of the TNFα inhibitor etanercept. The decreased MHWT was also recovered by etanercept administration, and TNFR1-IR cells in the TG were increased on day 3 following IANX.

Conclusions

These findings suggest that signaling cascades resulting from the production of TNFα by infiltrated macrophages in the TG contributes to the development of ectopic mechanical allodynia in whisker pad skin following IANX.

Role of Neuron-Glial Interaction Mediated by IL-1β in Ectopic Tooth Pain

Although many reports have demonstrated that ectopic pain develops in the orofacial region following tooth pulp inflammation, which often causes misdiagnosis and inappropriate treatment for patients with pulpitis, the precise mechanism remains unknown. In the present study, we hypothesized that the functional interaction between satellite glial cells and neurons mediated by interleukin 1β (IL-1β) in the trigeminal ganglion (TG) is involved in ectopic orofacial pain associated with tooth pulp inflammation. The digastric muscle electromyogram (D-EMG) activity elicited by capsaicin administration into the maxillary second molar tooth pulp was analyzed to evaluate the noxious reflex and was significantly increased in rats with inflammation of the maxillary first molar (M1) versus rats injected with saline. A significant increase in the expression of connexin43 (Cx43), a gap junction containing protein, was observed in activated satellite glial cells surrounding second molar-innervating neurons in the TG after M1 pulpitis. Daily administration of Gap26, a Cx43 mimetic peptide and inhibitor, in the TG significantly suppressed the enhancement of capsaicin-induced D-EMG activity and the percentage of Fluoro-Gold (FG)-labeled cells encircled by glial fibrillary acid protein-immunoreactive (IR) + Cx43-IR cells after M1 pulp inflammation ( P < 0.01). The percentage of FG-labeled cells encircled by glial fibrillary acid protein-IR + IL-1β-IR cells, IL-1 type I receptor-IR cells labeled with FG, and TRPV1-IR cells labeled with FG significantly increased after M1 pulp inflammation ( P < 0.01). Daily administration of IL-1ra, an IL-1 receptor antagonist, into the TG significantly reduced the enhancement of capsaicin-induced D-EMG activity and the percentage of TRPV1-IR neurons labeled with FG after M1 pulp inflammation ( P < 0.01). The present findings suggest that satellite glial cell is activated in the TG via activated gap junctions composed of Cx43 following tooth pulp inflammation, which leads to the hyperactivation of remote neurons via IL-1β mechanisms and results in ectopic tooth pulp pain in the adjacent tooth.

Neuron-Glia Crosstalk and Neuropathic Pain: Involvement in the Modulation of Motor Activity in the Orofacial Region

Neuropathic orofacial pain (NOP) is a debilitating condition. Although the pathophysiology remains unclear, accumulating evidence suggests the involvement of multiple mechanisms in the development of neuropathic pain. Recently, glial cells have been shown to play a key pathogenetic role. Nerve injury leads to an immune response near the site of injury. Satellite glial cells are activated in the peripheral ganglia. Various neural and immune mediators, released at the central terminals of primary afferents, lead to the sensitization of postsynaptic neurons and the activation of glia. The activated glia, in turn, release pro-inflammatory factors, further sensitizing the neurons, and resulting in central sensitization. Recently, we observed the involvement of glia in the alteration of orofacial motor activity in NOP. Microglia and astroglia were activated in the trigeminal sensory and motor nuclei, in parallel with altered motor functions and a decreased pain threshold. A microglial blocker attenuated the reduction in pain threshold, reduced the number of activated microglia, and restored motor activity. We also found an involvement of the astroglial glutamate–glutamine shuttle in the trigeminal motor nucleus in the alteration of the jaw reflex. Neuron–glia crosstalk thus plays an important role in the development of pain and altered motor activity in NOP.