Glutamine uptake contributes to central sensitization in the medullary dorsal horn

Post-Traumatic Trigeminal Neuropathy: Neurobiology and Pathophysiology

Painful traumatic trigeminal neuropathy (PTTN) is a chronic neuropathic pain that may develop following injury to the trigeminal nerve. Etiologies include cranio-orofacial trauma that may result from dental, surgical, or anesthetic procedures or physical trauma, such as a motor vehicle accident. Following nerve injury, there are various mechanisms, including peripheral and central, as well as phenotypic changes and genetic predispositions that may contribute to the development of neuropathic pain. In this article, we review current literature pertaining to the cellular processes that occur following traumatic damage to the trigeminal nerve, also called cranial nerve V, that results in chronic neuropathic pain. We examine the neurobiology and pathophysiology based mostly on pre-clinical animal models of neuropathic/trigeminal pain.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

Role of spinal astrocytes through the perisynaptic astrocytic process in pathological pain

Pathological pain is caused by abnormal activity in the neural circuit that transmits nociceptive stimuli. Beyond homeostatic functions, astrocytes actively participate in regulating synaptic transmission as members of tripartite synapses. The perisynaptic astrocytic process (PAP) is the key structure that allows astrocytes to play these roles and not only physically supports synapse formation through cell adhesion molecules (CAMs) but also regulates the efficiency of chemical signaling. Accumulating evidence has revealed that spinal astrocytes are involved in pathological pain by modulating the efficacy of neurotransmitters such as glutamate and GABA through transporters located in the PAP and by directly regulating synaptic transmission through various gliotransmitters. Although various CAMs contribute to pathological pain, insufficient evidence is available as to whether astrocytic CAMs also have this role. Therefore, more in-depth research is needed on how pathological pain is induced and maintained by astrocytes, especially in the PAP surrounding the synapse, and this will subsequently increase our understanding and treatment of pathological pain.

Satellite Glial Cells: Morphology, functional heterogeneity, and role in pain

Neurons in the somatic, sympathetic, and parasympathetic ganglia are surrounded by envelopes consisting of satellite glial cells (SGCs). Recently, it has become clear that SGCs are highly altered after nerve injury, which influences neuronal excitability and, consequently, the development and maintenance of pain in different animal models of chronic pain. However, the exact mechanism underlying chronic pain is not fully understood yet because it is assumed that SGCs in different ganglia share many common peculiarities, making the process complex. Here, we review recent data on morphological and functional heterogeneity and changes in SGCs in various pain conditions and their role in response to injury. More research is required to decipher the role of SGCs in diseases, such as chronic pain, neuropathology, and neurodegenerative diseases.

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Molecular mechanisms of painful traumatic trigeminal neuropathy-Evidence from animal research and clinical correlates

Painful traumatic trigeminal neuropathy (PTTN) may occur following major craniofacial or oral trauma, or may be subsequent to relatively minor dental interventions. Following injury, pain may originate from a peripheral nerve, a ganglion, or from the central nervous system. In this review, we focus on molecular mechanisms of pain resulting from injury to the peripheral branch of the trigeminal nerve. This syndrome has been termed painful traumatic trigeminal neuropathy (PTTN) by the International Headache Society and replaces previous terms including atypical odontalgia, deafferentation pain, traumatic neuropathy and phantom toothache. We emphasize the scientific evidence supporting the events purported to lead to PTTN by reviewing the pathophysiology of PTTN based on relevant animal models. Additionally, we briefly overview clinical correlates and pathophysiological manifestations of PTTN.

(-)-α-Bisabolol reduces nociception and trigeminal central sensitisation in acute orofacial neuropathic pain induced by infraorbital nerve injury

Neuropathic orofacial pain conditions represent a challenge to diagnose and treat. Natural substances are promising therapeutic options for the control of pain.

AIMS

This study aimed to examine whether (-)-α-bisabolol (BISA), a natural terpene, can attenuate nociceptive behaviour and central sensitisation in a rodent model of trigeminal neuropathic pain.

MATERIALS AND METHODS

Infraorbital nerve transection (IONX) or sham operation was performed in adult male rats. Head withdrawal thresholds as a measure of facial mechanical sensitivity were tested with von Frey monofilaments applied bilaterally to the facial vibrissal pad pre-operatively (baseline) and then post-operatively before and at 60, 120, 240 and 360 min after administration of vehicle control per oris (p.o.) or BISA (200 mg/kg p.o.) (n = 8/group). Effects of BISA or vehicle on the activity of nociceptive neurons recorded in the medullary dorsal horn (MDH) were tested on post - operative day 8-10. ANOVA followed by post-hoc Bonferroni tested for statistically significant differences (p < 0.05) across study groups and time points.

KEY FINDINGS

IONX animals (but not sham or naïve animals) showed post-operative facial mechanical hypersensitivity that was unaffected by vehicle. However, administration of BISA at post-operative day 7 significantly reversed the mechanical hypersensitivity in IONX rats; this effect lasted for at least 6 h. BISA also attenuated IONX-induced central sensitisation of MDH nociceptive neurons, as reflected in reversal of their reduced activation thresholds, increased responses to graded mechanical stimuli and enhanced spontaneous activity.

SIGNIFICANCE

BISA may attenuate nociceptive behaviour and central sensitisation in a rat model of acute trigeminal neuropathic pain.

Acupuncture-Analgesia-Mediated Alleviation of Central Sensitization

Pain can trigger central amplification called central sensitization, which ultimately results in hyperalgesia and/or allodynia. Many reports have showed acupuncture has an analgesic effect. We searched the related article on PubMed database and Cochrane database to discover central sensitization pathway in acupuncture analgesia. We summarized that acupuncture enhances the descending inhibitory effect and modulates the feeling of pain, thus modifying central sensitization. The possible mechanisms underlying the analgesic effects of acupuncture include segmental inhibition and the activation of the endogenous opioid, adrenergic, 5-hydroxytryptamine, and N-methyl-D-aspartic acid, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate pathways. Moreover, acupuncture can locally reduce the levels of inflammatory mediators. In clinical settings, acupuncture can be used to treat headache, neuropathic pain, low back pain, osteoarthritis, and irritable bowel syndrome. These mechanisms of acupuncture analgesia may be involved in the alleviation of central sensitization.

Electrophysiological Evidence for Functional Astrocytic P2X3 Receptors in the Mouse Trigeminal Caudal Nucleus

Recently, we reported that astrocytes in the trigeminal caudal nucleus (Vc) of the brain stem express a purinergic receptor P2X₃, which is involved in the craniofacial pathologic pain. Although we observed protein expression of P2X₃ receptors (P2X₃ Rs) in the astrocyte of the Vc, it is still unclear that astrocyte has functional P2X₃Rs in Vc. To address this issue, we recorded asrtocytic P2X₃Rs by using whole cell voltage-clamp recording in the Vc of the GFAP-GFP mice, which was used as a guide to astrocytes with green fluorescence. While measuring voltage ramp-induced astrocytic membrane current, we found the amplitude of the current was increased when we applied P2-purinoreceptor agonist, α,β-meATP. This increase was blocked by co-application of A317491, P2X₃R antagonist. These results demonstrate that astrocytes in the Vc express functional P2X₃Rs, which might be critical in craniofacial pathologic pain.

A role for the purinergic receptor P2X3 in astrocytes in the mechanism of craniofacial neuropathic pain

The purinergic receptor P2X₃, expressed in the central terminals of primary nociceptive neurons in the brainstem, plays an important role in pathological pain. However, little is known about expression of P2X₃ in the brainstem astrocytes and its involvement in craniofacial pathologic pain. To address this issue, we investigated the expression of P2X₃ in astrocytes in the trigeminal caudal nucleus (Vc) in a rat model of craniofacial neuropathic pain, chronic constriction injury of infraorbital nerve (CCI-ION). We found that 1) P2X₃-immunoreactivity is observed in the brainstem astrocytes, preferentially in their fine processes, 2) the number of P2X₃-positive fine astrocytic processes and the density of P2X₃ in these processes were increased significantly in CCI-ION rats, compared to control rats, and 3) administration of MPEP, a specific mGluR5 antagonist, alleviated the mechanical allodynia and abolished the increase in density of P2X₃ in fine astrocytic processes caused by CCI-ION. These findings reveal preferential expression of P2X₃ in the fine astrocytic processes in the brainstem, propose a novel role of P2X₃ in the fine astrocytic process in the mechanism of craniofacial neuropathic pain, and suggest that the expression of astrocytic P2X₃ may be regulated by astrocytic mGluR5.

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.

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.

Involvement of astroglial glutamate-glutamine shuttle in modulation of the jaw-opening reflex following infraorbital nerve injury

To evaluate the mechanisms underlying orofacial motor dysfunction associated with trigeminal nerve injury, we studied the astroglial cell activation following chronic constriction injury (CCI) of the infraorbital nerve (ION) immunohistochemically, nocifensive behavior in ION-CCI rats, and the effect of the glutamine synthase (GS) blocker methionine sulfoximine (MSO) on the jaw-opening reflex (JOR), and also studied whether glutamate-glutamine shuttle mechanism is involved in orofacial motor dysfunction. GFAP-immunoreactive (IR) cells were observed in the trigeminal motor nucleus (motV) 3 and 14 days after ION-CCI, and the nocifensive behavior and JOR amplitude were also strongly enhanced at these times. The number of GS- and GFAP-IR cells was also significantly higher in ION-CCI rats on day 7. The amplitude and duration of the JOR were strongly suppressed after MSO microinjection (m.i.) into the motV compared with that before MSO administration in ION-CCI rats. After MSO administration, the JOR amplitude was strongly suppressed, and the duration of the JOR was shortened. Forty minutes after m.i. of glutamine, the JOR amplitude was gradually returned to the control level and the strongest attenuation of the suppressive effect of MSO was observed at 180 min after glutamine m.i. In addition, glutamine also attenuated the MSO effect on the JOR duration, and the JOR duration was extended and returned to the control level thereafter. The present findings suggest that astroglial glutamate-glutamine shuttle in the motV is involved in the modulation of excitability of the trigeminal motoneurons affecting the enhancement of various jaw reflexes associated with trigeminal nerve injury.

The gap junction blocker carbenoxolone attenuates nociceptive behavior and medullary dorsal horn central sensitization induced by partial infraorbital nerve transection in rats

Glial cells are being increasingly implicated in mechanisms underlying pathological pain, and recent studies suggest glial gap junctions involving astrocytes may contribute. The aim of this study was to examine the effect of a gap junction blocker, carbenoxolone (CBX), on medullary dorsal horn (MDH) nociceptive neuronal properties and facial mechanical nociceptive behavior in a rat trigeminal neuropathic pain model involving partial transection of the infraorbital nerve (p-IONX). p-IONX produced facial mechanical hypersensitivity reflected in significantly reduced head withdrawal thresholds that lasted for more than 3weeks. p-IONX also produced central sensitization in MDH nociceptive neurons that was reflected in significantly increased receptive field size, reduction of mechanical activation threshold, and increases in noxious stimulation-evoked responses. Intrathecal CBX treatment significantly attenuated the p-IONX-induced mechanical hypersensitivity and the MDH central sensitization parameters, compared to intrathecal vehicle treatment. These results provide the first documentation that gap junctions may be critically involved in orofacial neuropathic pain mechanisms.

Trigeminal nerve injury-induced thrombospondin-4 up-regulation contributes to orofacial neuropathic pain states in a rat model

BACKGROUND

Injury to the trigeminal nerve often results in the development of chronic pain states including tactile allodynia, or hypersensitivity to light touch, in orofacial area, but its underlying mechanisms are poorly understood. Peripheral nerve injury has been shown to cause up-regulation of thrombospondin-4 (TSP4) in dorsal spinal cord that correlates with neuropathic pain development. In this study, we examined whether injury-induced TSP4 is critical in mediating orofacial pain development in a rat model of chronic constriction injury to the infraorbital nerve.

METHODS

Orofacial sensitivity to mechanical stimulation was examined in a unilateral infraorbital nerve ligation rat model. The levels of TSP4 in trigeminal ganglia and associated spinal subnucleus caudalis and C1/C2 spinal cord (Vc/C2) from injured rats were examined at time points correlating with the initiation and peak orofacial hypersensitivity. TSP4 antisense and mismatch oligodeoxynucleotides were intrathecally injected into injured rats to see if antisense oligodeoxynucleotide treatment could reverse injury-induced TSP4 up-regulation and orofacial behavioural hypersensitivity.

RESULTS

Our data indicated that trigeminal nerve injury induced TSP4 up-regulation in Vc/C2 at a time point correlated with orofacial tactile allodynia. In addition, intrathecal treatment with TSP4 antisense, but not mismatch, oligodeoxynucleotides blocked both injury-induced TSP4 up-regulation in Vc/C2 and behavioural hypersensitivity.

CONCLUSIONS

Our data support that infraorbital nerve injury leads to TSP4 up-regulation in trigeminal spinal complex that contributes to orofacial neuropathic pain states. Blocking this pathway may provide an alternative approach in management of orofacial neuropathic pain states.

Systemic pregabalin attenuates sensorimotor responses and medullary glutamate release in inflammatory tooth pain model

Our previous studies have demonstrated that application of inflammatory irritant mustard oil (MO) to the tooth pulp induces medullary glutamate release and central sensitization in the rat medullary dorsal horn (MDH), as well as nociceptive sensorimotor responses in craniofacial muscles in rats. There is recent evidence that anticonvulsant drugs such as pregabalin that influence glutamatergic neurotransmission are effective in several pain states. The aim of this study was to examine whether systemic administration of pregabalin attenuated glutamate release in the medulla as well as these nociceptive effects reflected in increased electromyographic (EMG) activity induced by MO application to the tooth pulp. Male adult rats were anesthetized with isofluorane (1.0-1.2%), and jaw and tongue muscle EMG activities were recorded by needle electrodes inserted bilaterally into masseter and anterior digastric muscles and into the genioglossus muscle, and also the medullary release of glutamate was assessed by in vivo microdialysis. Pregabalin or vehicle control (isotonic saline) was administered 30 min before the pulpal application of MO or vehicle control (mineral oil). Application of mineral oil to the maxillary first molar tooth pulp produced no change in baseline EMG activity and glutamate release. However, application of MO to the pulp significantly increased both the medullary release of glutamate and EMG activity in the jaw and tongue muscles for several minutes. In contrast, pre-medication with pregabalin, but not vehicle control, significantly and dose-dependently attenuated the medullary glutamate release and EMG activity in these muscles after MO application to the tooth pulp (analysis of variance (ANOVA), p<0.05). These results suggest that pregabalin may attenuate the medullary release of glutamate and associated nociceptive sensorimotor responses in this acute inflammatory pulpal pain model, and that it may prove useful for the treatment of orofacial inflammatory pain states.

Amino acid transport system A is involved in inflammatory nociception in rats

Previous studies have indicated that central sensitization is a state of increased excitability of nociceptive neurons in the spinal dorsal horn following peripheral tissue injury and/or inflammation and astrocytes play an important role in the central sensitization. The current study investigated the role of amino acid transport system A in central sensitization and hyperalgesia induced by intraplantar injection of formalin in rats. Formalin (5%, 50μl) injected subcutaneously into the unilateral hindpaw pad induced typical biphase nociceptive behaviors, including licking/biting and flinching of the injected paw and an increase of glial fibrillary acid protein (GFAP, an activated astrocyte marker) expression in spinal dorsal horn, and these effects could be attenuated by intrathecal injection of the competitive inhibitor of amino acid system A transporter, methylaminoisobutyric acid (MeAIB, 0.1, 0.3, 0.5, and 0.7mmol), in a dose-dependent manner. Intrathecal injection of vehicle (PBS) had no effect on the formalin-induced nociceptive behaviors and increase of the GFAP. These findings suggest that amino acid transport system A is involved in inflammation-induced nociception, and inhibition of this transporter system results in inhibition of the central sensitization and hyperalgesia.

Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors

Central sensitization is a crucial process underlying the increased neuronal excitability of nociceptive pathways following peripheral tissue injury and inflammation. Our previous findings have suggested that extracellular adenosine 5'-triphosphate (ATP) molecules acting at purinergic receptors located on presynaptic terminals (e.g., P2X2/3, P2X3 subunits) and glial cells are involved in the glutamatergic-dependent central sensitization induced in medullary dorsal horn (MDH) nociceptive neurons by application to the tooth pulp of the inflammatory irritant mustard oil (MO). Since growing evidence indicates that activation of P2X7 receptors located on glia is involved in chronic inflammatory and neuropathic pain, the aim of the present study was to test in vivo for P2X7 receptor involvement in this acute inflammatory pain model. Experiments were carried out in anesthetized Sprague-Dawley male rats. Single unit recordings were made in MDH functionally identified nociceptive neurons for which mechanoreceptive field, mechanical activation threshold and responses to noxious stimuli were tested. We found that continuous intrathecal (i.t.) superfusion over MDH of the potent P2X7 receptor antagonists brilliant blue G and periodated oxidized ATP could each significantly attenuate the MO-induced MDH central sensitization. MDH central sensitization could also be produced by i.t. superfusion of ATP and even more effectively by the P2X7 receptor agonist benzoylbenzoyl ATP. Superfusion of the microglial blocker minocycline abolished the MO-induced MDH central sensitization, consistent with reports that dorsal horn P2X7 receptors are mostly expressed on microglia. In control experiments, superfusion over MDH of vehicle did not produce any significant changes. These novel findings suggest that activation of P2X7 receptors in vivo may be involved in the development of central sensitization in an acute inflammatory pain model.

Modulation of astroglial glutamine synthetase activity affects nociceptive behaviour and central sensitization of medullary dorsal horn nociceptive neurons in a rat model of chronic pulpitis

Previous studies indicate that the astroglial glutamate-glutamine shuttle may be involved in acute pulpal inflammatory pain by influencing central sensitization induced in nociceptive neurons in the trigeminal subnucleus caudalis [the medullary dorsal horn (MDH)] by application of an inflammatory irritant to the rat tooth pulp. The aim of this study was to test if intrathecal application to the rat medulla of the astroglial glutamine synthetase inhibitor methionine sulfoximine (MSO) can influence the central sensitization of MDH nociceptive neurons and the animal's associated behaviour that are manifested in a model of chronic pulpitis pain induced by exposure of a mandibular molar pulp. This model was found to be associated with nocifensive behaviour and enhanced reflex activity evoked by mechanical stimulation of the rat's facial skin and with immunocytochemical evidence of astroglial activation in the MDH. These features were apparent for up to 28 days post-operatively. During this post-operative period, the nocifensive behaviour and enhanced reflex activity were significantly attenuated by intrathecal application of MSO (5 μL, 10 mM) but not by vehicle application. In electrophysiological recordings of nociceptive neuronal activity in the MDH, central sensitization was also evident in pulp-exposed rats but not in intact rats and could be significantly attenuated by MSO application but not by vehicle application. These behavioural and neuronal findings suggest that the astroglial glutamate-glutamine shuttle is responsible for the maintenance of inflammation-induced nocifensive behavioural changes and the accompanying central sensitization in MDH nociceptive neurons in this chronic pulpitis pain model.

Role of glia in orofacial pain

Several acute and chronic pain conditions in the face or mouth are very common, and some are unique to the orofacial region. However, the etiology and pathogenesis of most orofacial chronic pain conditions are unresolved, and they are difficult to diagnose and manage. This article provides a brief overview of the neural mechanisms underlying orofacial pain and then highlights recent findings indicating that nonneural cells, specifically satellite cells in the sensory ganglia and astroglia and microglia cells in the central nervous system, are important players in both acute and chronic inflammatory and neuropathic orofacial pain conditions and may offer new targets for management of these conditions.

Physiological mechanisms of neuropathic pain: the orofacial region

Neuropathic pain in the orofacial region is the clinical manifestation of trigeminal nerve injury following oral surgeries such as tooth extraction, dental implantation or tooth pulp treatment. Normally non-noxious touching of the facial skin or oral mucosa elicits strong pain named allodynia, and normally noxious stimulation causes intolerable pain named hyperalgesia in the trigeminal neuropathic pain patients. Although the mechanisms underlying trigeminal neuropathic pain have been studied by many researchers, the detailed mechanisms are still unknown. In this chapter, we are focusing on trigeminal neuropathic pain, and describe our recent studies using animal models of trigeminal neuropathic pain. We also present the clinical assessment of trigeminal neuropathic pain patients to develop the appropriate treatment of trigeminal neuropathic pain.

Central sensitization in medullary dorsal horn involves gap junctions and hemichannels

Central sensitization is a fundamental mechanism contributing to acute and chronic pain conditions. Our previous studies have documented a glutamatergic, purinergic and glial-dependent central sensitization that can be induced in rat medullary dorsal horn nociceptive neurons by mustard oil application to the tooth pulp. This study showed that carbenoxolone, a potent gap junction and hemichannel blocker, completely blocked all parameters of mustard oil-induced central sensitization tested in functionally identified medullary dorsal horn nociceptive neurons. These results represent the first evidence suggesting that gap junctions and hemichannels may have a critical role in mediating central sensitization in dorsal horn nociceptive neurons and may account for the spread as well as development of central sensitization.

Emerging role of astroglia in pain hypersensitivity

Recent studies suggest that astroglia, a major non-neuronal cell type in the central nervous system, actively participate in synaptic activity and potentially contribute to neurological disorders including chronic pain. Astroglia exhibit a hyperactive phenotype, also referred to as reactive astrocytosis, in response to peripheral injury. This process is often referred to as astroglial activation. By immunostaining against glial fibrillary acidic proteins, an intermediate cytoskeleton filament protein selectively localized to matured astrocytes, hypertrophy of astrocytes are clearly visible in the spinal cord dorsal horn and spinal trigeminal nucleus following a variety of injuries. Injury-related astroglial activation correlates with behavioral hyperalgesia and conversely, astroglial inhibition attenuates pain hypersensitivity. Astrocytes have a close anatomical relationship with neurons. Interactions between astrocytes and neurons contribute to the mechanisms of chronic pain. Astroglial activation is accompanied by initiation of cellular signal transduction pathways that lead to transcriptional gene regulation and release of a variety of chemical mediators or gliotransmitters, down-regulation of glutamate transporter activity that directly affects synaptic transmission, changes in gap junction proteins by which calcium waves spread, and alterations of the blood brain barrier. These coordinated changes in astroglial functions in turn modulate neuronal activity and facilitate pain transmission.

Astroglia in medullary dorsal horn (trigeminal spinal subnucleus caudalis) are involved in trigeminal neuropathic pain mechanisms

The aim of this study was to investigate whether astroglia in the medullary dorsal horn (trigeminal spinal subnucleus caudalis; Vc) may be involved in orofacial neuropathic pain following trigeminal nerve injury. The effects of intrathecal administration of the astroglial aconitase inhibitor sodium fluoroacetate (FA) were tested on Vc astroglial hyperactivity [as revealed by glial fibrillary acid protein (GFAP) labeling], nocifensive behavior, Vc extracellular signal-regulated kinase phosphorylation (pERK), and Vc neuronal activity in inferior alveolar nerve-transected (IANX) rats. Compared with sham-control rats, a significant increase occurred in GFAP-positive cells in ipsilateral Vc at postoperative day 7 in IANX rats, which was prevented following FA administration. FA significantly increased the reduced head withdrawal latency to high-intensity heat stimulation of the maxillary whisker pad skin in IANX rats, although it did not significantly affect the reduced escape threshold to low-intensity mechanical stimulation of the whisker skin in IANX rats. FA also significantly reduced the increased number of pERK-like immunoreactive cells in Vc and the enhanced Vc nociceptive neuronal responses following high-intensity skin stimulation that were documented in IANX rats, and glutamine administration restored the enhanced responses. These various findings provide the first documentation that astroglia is involved in the enhanced nociceptive responses of functionally identified Vc nociceptive neurons and in the associated orofacial hyperalgesia following trigeminal nerve injury.