Glutamate release from satellite glial cells of the murine trigeminal ganglion

Increase of glutamate in satellite glial cells of the trigeminal ganglion in a rat model of craniofacial neuropathic pain

Introduction

Satellite glial cells (SGCs) that envelop the cell bodies of neurons in sensory ganglia have been shown to both release glutamate, and be activated by glutamate in the context of nociceptive signaling. However, little is known about the subpopulations of SGCs that are activated following nerve injury and whether glutamate mechanisms in the SGCs are involved in the pathologic pain.

Methods

To address this issue, we used light and electron microscopic immunohistochemistry to examine the change in the glutamate levels in the SGCs and the structural relationship between neighboring neurons in the trigeminal ganglion (TG) in a rat model of craniofacial neuropathic pain, CCI-ION.

Results

Administration of ionomycin, ATP and Bz-ATP induced an increase of extracellular glutamate concentration in cultured trigeminal SGCs, indicating a release of glutamate from SGCs. The level of glutamate immunostaining in the SGCs that envelop neurons of all sizes in the TG was significantly higher in rats with CCI-ION than in control rats, suggesting that SGCs enveloping nociceptive as well as non-nociceptive mechanosensitive neurons are activated following nerve injury, and that the glutamate release from SGCs increases in pathologic pain state. Close appositions between substance-P (SP)-immunopositive (+) or calcitonin gene-related peptide (CGRP)+, likely nociceptive neurons, between Piezo1+, likely non-nociceptive, mechanosensitive neurons and SP+ or CGRP+ neurons, and between SGCs of neighboring neurons were frequently observed.

Discussion

These findings suggest that glutamate in the trigeminal SGCs that envelop all types of neurons may play a role in the mechanisms of neuropathic pain, possibly via paracrine signaling.

Peripheral role of glutamate in orofacial pain

Glutamate is the principal excitatory neurotransmitter in the central nervous system. In the periphery, glutamate acts as a transmitter and involves in the signaling and processing of sensory input. Glutamate acts at several types of receptors and also interacts with other transmitters/mediators under various physiological and pathophysiological conditions including chronic pain. The increasing amount of evidence suggests that glutamate may play a role through multiple mechanisms in orofacial pain processing. In this study, we reviewed the current understanding of how peripheral glutamate mediates orofacial pain, how glutamate is regulated in the periphery, and how these findings are translated into therapies for pain conditions.

Glutamatergic systems in neuropathic pain and emerging non-opioid therapies

Neuropathic pain, a disease of the somatosensory nervous system, afflicts many individuals and adequate management with current pharmacotherapies remains elusive. The glutamatergic system of neurons, receptors and transporters are intimately involved in pain but, to date, there have been few drugs developed that therapeutically modulate this system. Glutamate transporters, or excitatory amino acid transporters (EAATs), remove excess glutamate around pain transmitting neurons to decrease nociception suggesting that the modulation of glutamate transporters may represent a novel approach to the treatment of pain. This review highlights and summarizes (1) the physiology of the glutamatergic system in neuropathic pain, (2) the preclinical evidence for dysregulation of glutamate transport in animal pain models, and (3) emerging novel therapies that modulate glutamate transporters. Successful drug discovery requires continuous focus on basic and translational methods to fully elucidate the etiologies of this disease to enable the development of targeted therapies. Increasing the efficacy of astrocytic EAATs may serve as a new way to successfully treat those suffering from this devastating disease.

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.

A novel hypothesis on metal dyshomeostasis and mitochondrial dysfunction in amyotrophic lateral sclerosis: Potential pathogenetic mechanism and therapeutic implications

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder characterized by motor dysfunctions resulting from the loss of upper (UMNs) and lower (LMNs) motor neurons. While ALS symptoms are coincidental with pathological changes in LMNs and UMNs, the causal relationship between the two is unclear. For example, research on the extra-motor symptoms associated with this condition suggests that an imbalance of metals, including copper, zinc, iron, and manganese, is initially induced in the sensory ganglia due to a malfunction of metal binding proteins and transporters. It is proposed that the resultant metal dyshomeostasis may promote mitochondrial dysfunction in the satellite glial cells of these sensory ganglia, causing sensory neuron disturbances and sensory symptoms. Sensory neuron hyperactivation can result in LMN impairments, while metal dyshomeostasis in spinal cord and brain stem parenchyma induces mitochondrial dysfunction in LMNs and UMNs. These events could prompt intracellular calcium dyshomeostasis, pathological TDP-43 formation, and reactive microglia with neuroinflammation, which in turn activate the apoptosis signaling pathways within the LMNs and UMNs. Our model suggests that the degeneration of LMNs and UMNs is incidental to the metal-induced changes in the spinal cord and brain stem. Over time psychiatric symptoms may appear as the metal dyshomeostasis and mitochondrial dysfunction affect other brain regions, including the reticular formation, hippocampus, and prefrontal cortex. It is proposed that metal dyshomeostasis in combination with mitochondrial dysfunction could be the underlying mechanism responsible for the initiation and progression of the pathological changes associated with both the motor and extra-motor symptoms of ALS.

The effects of sympathetic nerve damage on satellite glial cells in the mouse superior cervical ganglion

Neurons in sensory, sympathetic, and parasympathetic ganglia are surrounded by satellite glial cell (SGCs). There is little information on the effects of nerve damage on SGCs in autonomic ganglia. We studied the consequences of damage to sympathetic nerve terminals by 6-hydroxydopamine (6-OHDA) on SGCs in the mouse superior cervical ganglia (Sup-CG). Immunostaining revealed that at 1-30 d post-6-OHDA injection, SGCs in Sup-CG were activated, as assayed by upregulation of glial fibrillary acidic protein. Intracellular labeling showed that dye coupling between SGCs around different neurons increased 4-6-fold 1-14 d after 6-OHDA injection. Behavioral testing 1-7 d post-6-OHDA showed that withdrawal threshold to tactile stimulation of the hind paws was reduced by 65-85%, consistent with hypersensitivity. A single intraperitoneal injection of the gap junction blocker carbenoxolone restored normal tactile thresholds in 6-OHDA-treated mice, suggesting a contribution of SGC gap junctions to pain. Using calcium imaging we found that after 6-OHDA treatment responses of SGCs to ATP were increased by about 30% compared with controls, but responses to ACh were reduced by 48%. The same experiments for SGCs in trigeminal ganglia from 6-OHDA injected mice showed no difference from controls, confirming that 6-OHDA acted selectively on sympathetic nerves. However, systemic inflammation induced by lipopolysaccharide did not affect SGCs of Sup-CG, but did influence SGCs in trigeminal ganglia in the same manner as 6-OHDA did on SGCs in Sup-CG. We conclude that even though SGCs in sympathetic and sensory ganglia are morphologically similar, they are quite different functionally, particularly after damage.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

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.

Participation of satellite glial cells of the dorsal root ganglia in acute nociception

At dorsal root ganglia, neurons and satellite glial cells (SGC) can communicate through ATP release and P2X7 receptor activation. SGCs are also interconnected by gap junctions and have been previously implicated in modulating inflammatory and chronic pain.We now present evidence that SGCs are also involved in processing acute nociception in rat dorsal root ganglia. Using primary dorsal root ganglia cultures we observed that calcium transients induced in neurons by capsaicin administration were followed by satellite glial cells activation. Only satellite glial cells response was reduced by administration of the P2X7 receptor antagonist A740003. In vivo, acute nociception induced by intraplantar injection of capsaicin in rats was inhibited by A740003 or by the gap junction blocker carbenoxolone administered at the dorsal root ganglia (L5 level). Both drugs also reduced the second phase of the formalin test. These results suggest that communication between neurons and satellite glial cells is not only involved in inflammatory or pathological pain, but also in the transmission of the nociceptive signal, possibly in situations involving C-fiber activation.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Connexin43 Hemichannels in Satellite Glial Cells, Can They Influence Sensory Neuron Activity?

In this review article, we summarize the current insight on the role of Connexin- and Pannexin-based channels as modulators of sensory neurons. The somas of sensory neurons are located in sensory ganglia (i.e., trigeminal and nodose ganglia). It is well known that within sensory ganglia, sensory neurons do not form neither electrical nor chemical synapses. One of the reasons for this is that each soma is surrounded by glial cells, known as satellite glial cells (SGCs). Recent evidence shows that connexin43 (Cx43) hemichannels and probably pannexons located at SGCs have an important role in paracrine communication between glial cells and sensory neurons. This communication may be exerted via the release of bioactive molecules from SGCs and their subsequent action on receptors located at the soma of sensory neurons. The glio-neuronal communication seems to be relevant for the establishment of chronic pain, hyperalgesia and pathologies associated with tissue inflammation. Based on the current literature, it is possible to propose that Cx43 hemichannels expressed in SGCs could be a novel pharmacological target for treating chronic pain, which need to be directly evaluated in future studies.

Role of Nonneuronal TRPV4 Signaling in Inflammatory Processes

Transient receptor potential (TRP) ion channels are important signaling components in nociceptive and inflammatory pathways. This is attributed to their ability to function as polymodal sensors of environmental stimuli (chemical and mechanical) and as effector molecules in receptor signaling pathways. TRP vanilloid 4 (TRPV4) is a nonselective cation channel that is activated by multiple endogenous stimuli including shear stress, membrane stretch, and arachidonic acid metabolites. TRPV4 contributes to many important physiological processes and dysregulation of its activity is associated with chronic conditions of metabolism, inflammation, peripheral neuropathies, musculoskeletal development, and cardiovascular regulation. Mechanosensory and receptor- or lipid-mediated signaling functions of TRPV4 have historically been attributed to central and peripheral neurons. However, with the development of potent and selective pharmacological tools, transgenic mice and improved molecular and imaging techniques, many new roles for TRPV4 have been revealed in nonneuronal cells. In this chapter, we discuss these recent findings and highlight the need for greater characterization of TRPV4-mediated signaling in nonneuronal cell types that are either directly associated with neurons or indirectly control their excitability through release of sensitizing cellular factors. We address the integral role of these cells in sensory and inflammatory processes as well as their importance when considering undesirable on-target effects that may be caused by systemic delivery of TRPV4-selective pharmaceutical agents for treatment of chronic diseases. In future, this will drive a need for targeted drug delivery strategies to regulate such a diverse and promiscuous protein.

RNA Sequencing of Trigeminal Ganglia in Rattus Norvegicus after Glyceryl Trinitrate Infusion with Relevance to Migraine

INTRODUCTION

Infusion of glyceryl trinitrate (GTN), a donor of nitric oxide, induces immediate headache in humans that in migraineurs is followed by a delayed migraine attack. In order to achieve increased knowledge of mechanisms activated during GTN-infusion this present study aims to investigate transcriptional responses to GTN-infusion in the rat trigeminal ganglia.

METHODS

Rats were infused with GTN or vehicle and trigeminal ganglia were isolated either 30 or 90 minutes post infusion. RNA sequencing was used to investigate transcriptomic changes in response to the treatment. Furthermore, we developed a novel method for Gene Set Analysis Of Variance (GSANOVA) to identify gene sets associated with transcriptional changes across time.

RESULTS

15 genes displayed significant changes in transcription levels in response to GTN-infusion. Ten of these genes showed either sustained up- or down-regulation in the 90-minute period after infusion. The GSANOVA analysis demonstrate enrichment of pathways pointing towards an increase in immune response, signal transduction, and neuroplasticity in response to GTN-infusion. Future functional in-depth studies of these mechanisms are expected to increase our understanding of migraine pathogenesis.

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

Background

Clinically, it is well known that injury of mandibular nerve fiber induces persistent ectopic pain which can spread to a wide area of the orofacial region innervated by the uninjured trigeminal nerve branches. However, the exact mechanism of such persistent ectopic orofacial pain is not still known. The present study was undertaken to determine the role of connexin 43 in the trigeminal ganglion on mechanical hypersensitivity in rat whisker pad skin induced by inferior alveolar nerve injury. Here, we examined changes in orofacial mechanical sensitivity following inferior alveolar nerve injury. Furthermore, changes in connexin 43 expression in the trigeminal ganglion and its localization in the trigeminal ganglion were also examined. In addition, we investigated the functional significance of connexin 43 in relation to mechanical allodynia by using a selective gap junction blocker (Gap27).

Results

Long-lasting mechanical allodynia in the whisker pad skin and the upper eyelid skin, and activation of satellite glial cells in the trigeminal ganglion, were induced after inferior alveolar nerve injury. Connexin 43 was expressed in the activated satellite glial cells encircling trigeminal ganglion neurons innervating the whisker pad skin, and the connexin 43 protein expression was significantly increased after inferior alveolar nerve injury. Administration of Gap27 in the trigeminal ganglion significantly reduced satellite glial cell activation and mechanical hypersensitivity in the whisker pad skin. Moreover, the marked activation of satellite glial cells encircling trigeminal ganglion neurons innervating the whisker pad skin following inferior alveolar nerve injury implies that the satellite glial cell activation exerts a major influence on the excitability of nociceptive trigeminal ganglion neurons.

Conclusions

These findings indicate that the propagation of satellite glial cell activation throughout the trigeminal ganglion via gap junctions, which are composed of connexin 43, plays a pivotal role in ectopic mechanical hypersensitivity in whisker pad skin following inferior alveolar nerve injury.

Involvement of aberrant calcium signalling in herpetic neuralgia

Alpha-herpesviruses, herpes simplex viruses (HSV) and varicella zoster virus (VZV), are pathogens of the peripheral nervous system. After primary infection, these viruses establish latency within sensory ganglia, while retaining the ability to reactivate. Reactivation of VZV results in herpes zoster, a condition characterized by skin lesions that leads to post-herpetic neuralgia. Recurrent reactivations of HSV, which cause mucocutaneous lesions, may also result in neuralgia. During reactivation of alpha-herpesviruses, satellite glial cells (SGCs), which surround neurons in sensory ganglia, become infected with the replicating virus. SGCs are known to contribute to neuropathic pain in a variety of animal pain models. Here we investigated how infection of short-term cultures of mouse trigeminal ganglia with HSV-1 affects communication between SGCs and neurons, and how this altered communication may increase neuronal excitability, thus contributing to herpetic neuralgia. Mechanical stimulation of single neurons or SGCs resulted in intercellular calcium waves, which were larger in cultures infected with HSV-1. Two differences were observed between control and HSV-1 infected cultures that could account for this augmentation. Firstly, HSV-1 infection induced cell fusion among SGCs and neurons, which would facilitate the spread of calcium signals over farther distances. Secondly, using calcium imaging and intracellular electrical recordings, we found that neurons in the HSV-1 infected cultures exhibited augmented influx of calcium upon depolarization. These virally induced changes may not only cause more neurons in the sensory ganglia to fire action potentials, but may also increase neurotransmitter release at the presynaptic terminals in the spinal cord. They are therefore likely to be contributing factors to herpetic neuralgia.

P2Y1 Receptor Activation of the TRPV4 Ion Channel Enhances Purinergic Signaling in Satellite Glial Cells

Transient receptor potential (TRP) ion channels of peripheral sensory pathways are important mediators of pain, itch, and neurogenic inflammation. They are expressed by primary sensory neurons and by glial cells in the central nervous system, but their expression and function in satellite glial cells (SGCs) of sensory ganglia have not been explored. SGCs tightly ensheath neurons of sensory ganglia and can regulate neuronal excitability in pain and inflammatory states. Using a modified dissociation protocol, we isolated neurons with attached SGCs from dorsal root ganglia of mice. SGCs, which were identified by expression of immunoreactive Kir4.1 and glutamine synthetase, were closely associated with neurons, identified using the pan-neuronal marker NeuN. A subpopulation of SGCs expressed immunoreactive TRP vanilloid 4 (TRPV4) and responded to the TRPV4-selective agonist GSK1016790A by an influx of Ca(2+) ions. SGCs did not express functional TRPV1, TRPV3, or TRP ankyrin 1 channels. Responses to GSK1016790A were abolished by the TRPV4 antagonist HC067047 and were absent in SGCs from Trpv4(-/-) mice. The P2Y1-selective agonist 2-methylthio-ADP increased [Ca(2+)]i in SGCs, and responses were prevented by the P2Y1-selective antagonist MRS2500. P2Y1 receptor-mediated responses were enhanced in TRPV4-expressing SGCs and HEK293 cells, suggesting that P2Y1 couples to and activates TRPV4. PKC inhibitors prevented P2Y1 receptor activation of TRPV4. Our results provide the first evidence for expression of TRPV4 in SGCs and demonstrate that TRPV4 is a purinergic receptor-operated channel in SGCs of sensory ganglia.