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

Peripheral and central neurobiological effects of botulinum toxin A (BoNT/A) in neuropathic pain: a systematic review

ABSTRACT

Botulinum toxin (BoNT), a presynaptic inhibitor of acetylcholine (Ach) release at the neuromuscular junction (NMJ), is a successful and safe drug for the treatment of several neurological disorders. However, a wide and recent literature review has demonstrated that BoNT exerts its effects not only at the "periphery" but also within the central nervous system (CNS). Studies from animal models, in fact, have shown a retrograde transport to the CNS, thus modulating synaptic function. The increasing number of articles reporting efficacy of BoNT on chronic neuropathic pain (CNP), a complex disease of the CNS, demonstrates that the central mechanisms of BoNT are far from being completely elucidated. In this new light, BoNT might interfere with the activity of spinal, brain stem, and cortical circuitry, modulating excitability and the functional organization of CNS in healthy conditions. Botulinum toxins efficacy on CNP is the result of a wide and complex action on many and diverse mechanisms at the basis of the maladaptive plasticity, the core of the pathogenesis of CNP. This systematic review aims to discuss in detail the BoNT's mechanisms and effects on peripheral and central neuroplasticity, at the basis for the clinical efficacy in CNP syndromes.

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.

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.

New Insights on the Role of Satellite Glial Cells

Satellite glial cells (SGCs) that surround sensory neurons in the peripheral nervous system ganglia originate from neural crest cells. Although several studies have focused on SGCs, the origin and characteristics of SGCs are unknown, and their lineage remains unidentified. Traditionally, it has been considered that SGCs regulate the environment around neurons under pathological conditions, and perform functions of supporting, nourishing, and protecting neurons. However, recent studies demonstrated that SGCs may have the characteristics of stem cells. After nerve injury, SGCs up-regulate the expression of stem cell markers and can differentiate into functional sensory neurons. Moreover, SGCs express several markers of Schwann cell precursors and Schwann cells, such as CDH19, MPZ, PLP1, SOX10, ERBB3, and FABP7. Schwann cell precursors have also been proposed as a potential source of neurons in the peripheral nervous system. The similarity in function and markers suggests that SGCs may represent a subgroup of Schwann cell precursors. Herein, we discuss the roles and functions of SGCs, and the lineage relationship between SGCs and Schwann cell precursors. We also describe a new perspective on the roles and functions of SGCs. In the DRG located on the posterior root of spinal nerves, satellite glial cells wrap around each sensory neuron to form an anatomically and functionally distinct unit with the sensory neurons. Following nerve injury, satellite glial cells up-regulate the expression of progenitor markers, and can differentiate into neurons.

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.

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.

Carbenoxolone has the potential to ameliorate acute incision pain in rats

Carbenoxolone (CBX) is primarily used to relieve various types of neuropathic and inflammatory pain. However, little is known concerning the role of CBX in acute pain and its functional mechanisms therein and this was investigated in the present study. Rats underwent toe incision and behavioral tests were performed to assess mechanical hypersensitivity. The expression levels of pannexin 1 (Px1) and connexin 43 (Cx43) were detected using western blot analysis 2, 4, 6 or 24 h after toe incision, and the expression of TNF-α, IL-1β and P substance (SP) was determined by ELISA; Px1 and Cx43 expression was also examined by immunofluorescence staining. At 2, 6 and 12 h post-toe incision, the postoperative pain threshold was significantly reduced, which was subsequently recovered at 2 and 6 h post-surgery following pretreatment with CBX or pannexin 1 mimetic inhibitory peptide. CBX reduced Px1 levels at 4 and 24 h post-incision. However, Cx43 levels were reduced by CBX as little as 2 h post-surgery. Furthermore, CBX not only distinctly decreased the levels of Px1 and Cx43, but also reduced the co-localization of Px1 or Cx43 with glial fibrillary acidic protein, 2 h after incision. It was also observed that the protein levels of inflammatory makers (IL-1β, SP and TNF-α) showed a tendency to decline at 2, 4, 6 and 24 h after incision. Collectively, the expression of Px1 and Cx43 in astrocytes may be involved in pain behaviors diminished by CBX, and CBX potentially reduces acute pain by decreasing Px1 and Cx43 levels. Px1 and Cx43 from spinal astrocytes may serve important roles in the early stages and maintenance of acute pain, while preoperative injection of CBX has the potential to relieve hyperalgesia.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

A Systematic Review of Rat Models With Temporomandibular Osteoarthritis Suitable for the Study of Emerging Prolonged Intra-Articular Drug Delivery Systems

PURPOSE

Development of minimally invasive therapies for temporomandibular joint osteoarthritis (TMJOA) has focused on drug intra-articular injections to avoid the systemic adverse effects experienced when these substances are administered orally. Therefore, we performed a systematic review to answer the question "Which method of induction of a TMJOA-related pain model in rats leads to prolonged painful symptoms, allowing the best assessment of a sustained drug delivery system?"

MATERIALS AND METHODS

Following the PRISMA guidelines, we searched MEDLINE for papers published from 1994 to July 2020 on a TMJ arthritis model using rats. We identified the means of pain induction and of nociception assessment. We assessed protocol bias using an adaptation of the QUADAS-2 tool. Animal selection, the reference standard method of pain assessment, applicability of a statistical assessment, and flow and timing were assessed.

RESULTS

Of the 59 full papers we reviewed, 41 performed no pain assessment after the first 7 days following induction of the TMJ-related pain model. We eventually identified 18 long-term TMJOA-related pain models. Pain was induced by injection of toxic substances, most commonly Freund's complete adjuvant (50 μg per 50 μl), formalin at various concentrations, or monosodium iodoacetate (0,5 mg per 50 μl), into the TMJ, or by physical methods. Few studies reported data on pain after 21 days of follow-up. Heterogeneity of induction methods, pain assessment methods, and flow and timing biases precluded a meta-analysis.

CONCLUSIONS

Given that pain is 1 of the main symptoms of TMJOA, experimental study protocols should include long-term pain assessment.

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.

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.

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.

Tumor necrosis factor alpha mediates orofacial discomfort in an occlusal dental interference model in rats: The role of trigeminal ganglion inflammation

BACKGROUND

Tumor necrosis factor alpha (TNF-α) is a proinflammatory cytokine that plays an important role in the early stages of inflammation. In this study, we investigated its role in orofacial discomfort in rats subjected to occlusal dental interference (ODI).

METHODS

Female Wistar rats (180-200 g) were divided in three groups (n = 30/group): sham group, without ODI, and two experimental groups with ODI pre-treated with 0.1 mL/kg saline (ODI + SAL) or 5 mg/kg infliximab (ODI + INF) and treated every 3 days. The animals were euthanized after 1, 3, and 7 days. The number of bites and scratches and grimace scale scores were determined daily, and the bilateral trigeminal ganglion was histomorphometrically (neuronal body area) analyzed and submitted for immunohistochemistry for TNF-α, nitric oxide synthesis (NOS) neuronal (nNOS) and inducible (iNOS), peroxisome proliferator-activated receptors (PPAR) y (PPARy) and δ/β (PPARδ/β), and glial fibrillary acidic protein (GFAP). One-way/two-way ANOVA/Bonferroni tests were used (P < .05, GraphPad Prism 5.0).

RESULTS

ODI + SAL showed a large number of bites (P = .002), scratches (P = .002), and grimace scores (P < .001) in the firsts days, and ODI + INF partially reduced these parameters. The contralateral and ipsilateral neuronal body area was significantly reduced on day 1 in ODI + SAL, but returned to the basal size on days 3 and 7, by increase in TNF-α, nNOS, PPARy, PPARδ/β, and GFAP immunostaining. The infliximab treatment attenuated these alterations (P < .05). There was no iNOS immunostaining.

CONCLUSION

Occlusal dental interference induced transitory orofacial discomfort by trigeminal inflammatory mediator overexpression, and TNF-α blockage attenuated these processes.