Trigeminal Spinal Subnucleus Caudalis Research Articles

The role of voltage-gated calcium channel α2δ-1 in the occurrence and development in myofascial orofacial pain

Patients who suffer from myofascial orofacial pain could affect their quality of life deeply. The pathogenesis of pain is still unclear. Our objective was to assess Whether Voltage-gated calcium channel α2δ-1(Cavα2δ-1) is related to myofascial orofacial pain. Rats were divided into the masseter tendon ligation group and the sham group. Compared with the sham group, the mechanical pain threshold of the masseter tendon ligation group was reduced on the 4th, 7th, 10th and 14th day after operation(P < 0.05). On the 14th day after operation, Cavα2δ-1 mRNA expression levels in trigeminal ganglion (TG) and the trigeminal spinal subnucleus caudalis and C1-C2 spinal cervical dorsal horn (Vc/C2) of the masseter tendon ligation group were increased (PTG=0.021, PVc/C2=0.012). Rats were divided into three groups. On the 4th day after ligating the superficial tendon of the left masseter muscle of the rats, 10 ul Cavα2δ-1 antisense oligonucleotide, 10 ul Cavα2δ-1 mismatched oligonucleotides and 10 ul normal saline was separately injected into the left masseter muscle of rats in Cavα2δ-1 antisense oligonucleotide group, Cavα2δ-1 mismatched oligonucleotides group and normal saline control group twice a day for 4 days. The mechanical pain threshold of the Cavα2δ-1 antisense oligonucleotides group was higher than Cavα2δ-1 mismatched oligonucleotides group on the 7th and 10th day after operation (P < 0.01). After PC12 cells were treated with lipopolysaccharide, Cavα2δ-1 mRNA expression level increased (P < 0.001). Cavα2δ-1 may be involved in the occurrence and development in myofascial orofacial pain.

Oxidative stress plays an important role in the central regulatory mechanism of orofacial hyperalgesia under low estrogen conditions

Hyperalgesia occurs in the orofacial region of rats when estrogen levels are low, although the specific mechanism needs to be investigated further. Furthermore, oxidative stress plays an important role in the transmission of pain signals. This study aimed to explore the role of oxidative stress in orofacial hyperalgesia under low estrogen conditions. We firstly found an imbalance between oxidative and antioxidant capacity within the spinal trigeminal subnucleus caudalis (SP5C) of rats after ovariectomy (OVX), resulting in oxidative stress and then a decrease in the orofacial pain threshold. To investigate the mechanism by which oxidative stress occurs, we used virus as a tool to inhibit or overexpress the excitatory amino acid transporter 3 (EAAT3) gene. Further investigation revealed that the regulation of glutathione (GSH) and reactive oxygen species (ROS) can be achieved by regulating EAAT3, which in turn impacts the occurrence of oxidative stress. In summary, our findings suggest that reduced expression of EAAT3 within the SP5C of rats in the low estrogen state may decrease GSH content and increase ROS levels, resulting in oxidative stress and ultimately lead to orofacial hyperalgesia. This suggests that antioxidants could be a potential therapeutic direction for orofacial hyperalgesia under low estrogen conditions, though more research is needed to understand its mechanism.

Effect of chronic intermittent hypoxia on ocular and intraoral mechanical allodynia mediated via the calcitonin gene-related peptide in a rat.

Obstructive sleep apnea, a significant hypoxic condition, may exacerbate several orofacial pain conditions. The study aims to define the involvement of calcitonin gene-related peptide (CGRP) in peripheral and central sensitization and in evoking orofacial mechanical allodynia under chronic intermittent hypoxia (CIH). Male rats were exposed to CIH. Orofacial mechanical allodynia was assessed using the eyeblink test and the two-bottle preference drinking test. The CGRP-immunoreactive neurons in the trigeminal ganglion (TG), CGRP-positive primary afferents projecting to laminae I-II of the trigeminal spinal subnucleus caudalis (Vc), and neural responses in the second-order neurons of the Vc were determined by immunohistochemistry. CGRP receptor antagonist was administrated in the TG. CIH-induced ocular and intraoral mechanical allodynia. CGRP-immunoreactive neurons and activated satellite glial cells (SGCs) were significantly increased in the TG and the number of cFos-immunoreactive cells in laminae I-II of the Vc were significantly higher in CIH rats compared to normoxic rats. Local administration of the CGRP receptor antagonist in the TG of CIH rats attenuated orofacial mechanical allodynia; the number of CGRP-immunoreactive neurons and activated SGCs in the TG, and the density of CGRP-positive primary afferent terminals and the number of cFos-immunoreactive cells in laminae I-II of the Vc were significantly lower compared to vehicle-administrated CIH rats. An increase in CGRP in the TG induced by CIH, as well as orofacial mechanical allodynia and central sensitization of second-order neurons in the Vc, supported the notion that CGRP plays a critical role in CIH-induced orofacial mechanical allodynia.

BMP7 alleviates trigeminal neuralgia by reducing oligodendrocyte apoptosis and demyelination

BackgroundBMP7 has been shown to have neuroprotective effects and to alleviate demyelination. However, its role in trigeminal neuralgia (TN) has not been well investigated. The current study aims to determine whether BMP7 plays a role in demyelination, its effects on pain behaviors and mechanism of action in rats with TN.MethodsWe used an infraorbital-nerve chronic-constriction injury (ION-CCI) to establish a rat model of TN. Adeno-associated viruses (AAVs) were injected into the rats to upregulate or downregulate BMP7. The mechanical withdrawal thresholds (MWT) of the injured rats were detected using Von Frey filaments. The changes in expression levels of BMP7 and oligodendrocyte (OL) markers were examined by western blotting, quantitative real-time PCR, immunofluorescence, and transmission electron microscopy.ResultsThe ION-CCI induced mechanical allodynia, demyelination, and loss of OLs with a reduction of BMP7. Short-hairpin RNA (shRNA)-BMP7 that inhibited BMP7 expression also caused mechanical allodynia, demyelination, and loss of OLs, and its mechanism may be OL apoptosis. Overexpressing BMP7 in the trigeminal spinal subnucleus caudalis(VC) with AAV-BMP7 relieved all three phenotypes induced by the CCI, and its mechanism may be alleviating OLs apoptosis. Two signal pathways associated with apoptosis, STAT3 and p65, were significantly downregulated in the VC after CCI and rescued by BMP7 overexpression.ConclusionBMP7 can alleviate TN by reducing OLs apoptosis and subsequent demyelination. The mechanism behind this protection could be BMP7-mediated activation of the STAT3 and NF-κB/p65 signaling pathway and subsequent decrease in OL apoptosis. Importantly, our study presents clear evidence in support of BMP7 as a possible therapeutic target for the treatment of TN.

Pathophysiology, Clinical Implications and Management of Orofacial Neuropathic Pain- with special attention to Trigeminal neuralgia: A Narrative Review

Background: It is a widely held belief that if the trigeminal nerve is damaged, the victim would experience agonising and unrelenting external pain. A lesion to the trigeminal nerve may have a wide-reaching effect, such as on one side of the face in particular, or it might have a more localised effect, such as on some or all of your gums. The risk of damage increases the likelihood that it will be difficult to speak and swallow. This nerve provides sensation to a part of your face that may be constantly aching or tingling for some people. However, the trigeminal nerve injury-related persistent orofacial pain might be brought on by a wide variety of unknown triggers. Aim: In this study investigate the clinical manifestations of chronic orofacial pain brought on by a damage to the trigeminal nerve, as well as the diagnostic and therapeutic approaches available to treat this condition. Methodology Through the use of search phrases such as "Trigeminal nerve injury," "Trigeminal ganglion," "Trigeminal spinal subnucleus caudalis," "Craniofacial pain," "Oral prognosis," and "treatment," the computerised databases for the last twenty years have been investigated. There are now two hundred objects in total that have been accumulated. There have been around fifty of them that are pertinent to the discussion that is going on in this work. Majority of the patients fair enough with the pharmacology treatment/drugs like the carbamazepine &amp; oxcarbazepine which forms the first line treatment options followed by lamotrigine &amp; baclofen encompassing the second line of drugs along with adjuvant drug support of topiramate, levetiracetam, gabapentin, pregabalin. As the field of science has explored &amp;advanced for the latest treatment options include microvascular decompression, gamma knife radiosurgery, percutaneous rhizotomies variable based on the evidences &amp; guidelines 54 Conclusion: New diagnostic criteria and treatment alternatives have become available for people who suffer from trigeminal neuropathy and orofacial neuropathic pain as a result of recent developments in fundamental animal research that have led to their development. Despite the results, more research needs to investigate a greater variety of distinct non-neuronal cell feature approaches.

Antagonist of transient receptor potential melastatin 2 suppresses mechanical hypersensitivity and activation of microglia induced by infraorbital nerve ligation in male rats

Activation of microglia is known to be involved in neuropathic pain. However, the pathway that regulates the microglial activation is not completely understood. Transient receptor potential (TRP) melastatin 2 (TRPM2), which is part of the TRP superfamily, is reportedly expressed on microglia and is suggested to be involved in neuropathic pain. To explore the effect of a TRPM2 antagonist on orofacial neuropathic pain and the relationship between TRPM2 and the activation of microglia, experiments were conducted using male rats that underwent infraorbital nerve (ION) ligation as orofacial neuropathic pain models. TRPM2 expression was detected on microglia in the trigeminal spinal subnucleus caudalis (Vc). The immunoreactivity of TRPM2 in the Vc increased after ION ligation. Mechanical threshold for head-withdrawal response was measured using von Frey filament, and it decreased after ION ligation. When the TRPM2 antagonist was administered to the ION-ligated rats, the low mechanical threshold for head-withdrawal response increased, and the number of phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells in the Vc decreased. The number of CD68-immunoreactive cells in the Vc also decreased after the administration of the TRPM2 antagonist in the ION-ligated rats. These findings suggest that TRPM2 antagonist administration suppresses hypersensitivity to mechanical stimulation induced by ION ligation and microglial activation, and TRPM2 is also involved in microglial activation in orofacial neuropathic pain.

Microglia cause structural remodeling of noradrenergic axon in the trigeminal spinal subnucleus caudalis after infraorbital nerve injury in rats

The dysfunction of descending noradrenergic (NAergic) modulation in second-order neurons has long been observed in neuropathic pain. In clinical practice, antidepressants that increase noradrenaline levels in the synaptic cleft are used as first-line agents, although adequate analgesia has not been occasionally achieved. One of the hallmarks of neuropathic pain in the orofacial regions is microglial abnormalities in the trigeminal spinal subnucleus caudalis (Vc). However, until now, the direct interaction between descending NAergic system and Vc microglia in orofacial neuropathic pain has not been explored. We found that reactive microglia ingested the dopamine-β-hydroxylase (DβH)-positive fraction, NAergic fibers, in the Vc after infraorbital nerve injury (IONI). Major histocompatibility complex class I (MHC-I) was upregulated in Vc microglia after IONI. Interferon-γ (IFNγ) was de novo induced in trigeminal ganglion (TG) neurons following IONI, especially in C-fiber neurons, which conveyed to the central terminal of TG neurons. Gene silencing of IFNγ in the TG reduced MHC-I expression in the Vc after IONI. Intracisternal administration of exosomes from IFNγ-stimulated microglia elicited mechanical allodynia and a decrease in DβH in the Vc, which did not occur when exosomal MHC-I was knocked down. Similarly, in vivo MHC-I knockdown in Vc microglia attenuated the development of mechanical allodynia and a decrease in DβH in the Vc after IONI. These results show that microglia-derived MHC-I causes a decrease in NAergic fibers, culminating in orofacial neuropathic pain.

Insular cortical descending projections facilitate neuronal responses to noxious but not innoxious stimulation in rat trigeminal spinal subnucleus caudalis

The insular cortex (IC) receives orofacial nociceptive information. Pyramidal neurons in IC layer V send their axons to various brain regions, such as the trigeminal spinal subnucleus caudalis (Sp5C), parabrachial nucleus, and periaqueductal gray. However, little information has been available about the functions of these descending projections from the IC. This study aimed to elucidate the effect of IC → Sp5C on neuronal spike firings responding to noxious and innoxious stimuli to the face of the rat receiving an injection of adeno-associated virus encoding modified channelrhodopsin-2 (ChR2) fused to mCherry under the control of the human synapsin promotor. We classified Sp5C neurons responding to mechanical stimuli into three groups: low-threshold (LT), nociceptive specific (NS), and wide dynamic range (WDR) neurons, which respond to innoxious stimuli (brushing) only, noxious mechanical stimuli (pinching) only, and both noxious and innoxious stimuli, respectively. Neuronal activities of IC neurons were activated by photostimulation (repetitive pulses at 20 Hz for 5 Hz) to the IC that consistently induced action potentials in IC layer V pyramidal neurons. LT neurons showed comparable spike firing rates to brushing the facial skin before and during ChR2 activation induced by photostimulation. In contrast, NS neurons showed an increase in their firing frequency to pinching during ChR2 activation. On the other hand, WDR neurons increased their Sp5C neuronal firing to pinching during ChR2 activation without changing their firing rates to innoxious mechanical stimuli. These results suggest that the IC descending projections facilitate nociception by increasing Sp5C neuronal activities responding to noxious mechanical stimuli.

Decreased PPARgamma in the trigeminal spinal subnucleus caudalis due to neonatal injury contributes to incision-induced mechanical allodynia in female rats

Whisker pad skin incision in infancy causes the prolongation of mechanical allodynia after re-incision in adulthood. A recent study also proposed the importance of sex differences in pain signaling in the spinal cord. However, the sex difference in re-incision-induced mechanical allodynia in the orofacial region is not fully understood. In the rats that experienced neonatal injury in the whisker pad skin, the mechanical allodynia in the whisker pad was significantly prolonged after re-incision in adulthood compared to sham injury in infancy. No significant sex differences were observed in the duration of mechanical allodynia. The duration of mechanical allodynia in male rats was shortened by intracisternal administration of minocycline. However, minocycline had no effects on the duration of mechanical allodynia in female rats. In contrast, intracisternal administration of pioglitazone markedly suppressed mechanical allodynia in female rats after re-incision. Following re-incision, the number of peroxisome proliferator-activated receptor gamma (PPARgamma)-positive cells were reduced in the trigeminal spinal subnucleus caudalis (Vc) in female rats that experienced neonatal injury. Immunohistochemical analyses revealed that PPARgamma was predominantly expressed in Vc neurons. Pioglitazone increased the number of PPARgamma-positive Vc neurons in female rats whose whisker pad skin was incised in both infancy and adulthood stages. Pioglitazone also upregulated heme oxygenase 1 and downregulated NR1 subunit in the Vc in female rats after re-incision. Together, PPARgamma signaling in Vc neurons is a female-specific pathway for whisker pad skin incision-induced mechanical allodynia.

Astrocytic and microglial interleukin-1β mediates complement C1q-triggered orofacial mechanical allodynia

Glial cells, such as microglia and astrocytes, in the trigeminal spinal subnucleus caudalis (Vc) are activated after trigeminal nerve injury and interact with Vc neurons to contribute to orofacial neuropathic pain. Complement C1q released from microglia has been reported to activate astrocytes and causes orofacial mechanical allodynia. However, how C1q-induced phenotypic alterations in Vc astrocytes are involved in orofacial pain remains to be elucidated. Intracisternal administration of C1q caused mechanical allodynia in the whisker pad skin and concurrent significant upregulation of glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 in the Vc. Immunohistochemical analyses clarified that C1q induces a significant increase in the cytokine interleukin (IL)-1β, predominantly in Vc astrocytes and partially in Vc microglia. The number of c-Fos-positive neurons in the Vc increased significantly in response to C1q. IL-1 receptor antagonist (IL-1Ra) was used to analyze the involvement of IL-1β in C1q-induced mechanical allodynia. Intracisternal administration of IL-1Ra ameliorated C1q-induced orofacial mechanical allodynia. The present findings suggest that IL-1β released from activated astrocytes and microglia in the Vc mediates C1q-induced orofacial pain.

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats.

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Attenuation of allodynia and microglial reactivity by inhibiting the degradation of 2-arachidonoylglycerol following injury to the trigeminal nerve in mice

Endocannabinoids have an important role for the regulation of neuropathic pain. In our previous study, we observed that preventing the degradation of a endocannabinoid, 2-arachidonoylglycerol (2-AG), using an inhibitor of monoacylglycerol lipase (JZL184), attenuated neuropathic orofacial pain (NOP). The present study aimed to investigate mechanisms underlying JZL184-induced attenuation of NOP. We hypothesized that JZL184 may suppress microglial reactivity in the trigeminal spinal subnucleus caudalis (Vc) under NOP. The infraorbital nerve (ION) was hemisected to model NOP in mice, resulting in a significant reduction of mechanical head-withdrawal threshold (MHWT) on day 4 following the ION hemisection. Chronic systemic application of JZL184 at a concentration of 8 or 16 mg/kg/day for 4 days significantly attenuated the reduction of MHWT in mice exposed to NOP. Administering JZL184 at 4 mg/kg/day or its vehicle, however, did not attenuate the MHWT of mice with NOP. The reactivity of microglial cells in the Vc increased in mice with NOP compared to sham-operated controls. The application of JZL184 at 8 or 16 mg/kg/day for 4 days significantly reduced the increased microglial reactivity in the Vc. The changes of microglia under NOP were, by contrast, not reduced by application of the drug at 4 mg/kg/day or its vehicle. The results indicate that preventing 2-AG degradation may increase its accumulation in the Vc and normalize microglial reactivity under NOP, which may contribute to suppressing NOP.

Myrtenol Reduces Orofacial Nociception and Inflammation in Mice Through p38-MAPK and Cytokine Inhibition.

Orofacial pain is one of the commonest and most complex complaints in dentistry, greatly impairing life quality. Preclinical studies using monoterpenes have shown pharmacological potential to treat painful conditions, but the reports of the effects of myrtenol on orofacial pain and inflammation are scarce. The aim of this study was to evaluate the effect of myrtenol in experimental models of orofacial pain and inflammation. Orofacial nociceptive behavior and the immunoreactivity of the phosphorylated p38 (P-p38)-MAPK in trigeminal ganglia (TG) and spinal trigeminal subnucleus caudalis (STSC) were determined after the injection of formalin in the upper lip of male Swiss mice pretreated with myrtenol (12.5 and 25 mg/kg, i.p.) or vehicle. Orofacial inflammation was induced by the injection of carrageenan (CGN) in the masseter muscle of mice pretreated with myrtenol (25 and 50 mg/kg, i.p.) or its vehicle (0.02% Tween 80 in saline). Myeloperoxidase (MPO) activity and histopathological changes in the masseter muscle and interleukin (IL)-1β levels in the TG and STSC were measured. The increase in face-rubbing behavior time induced by formalin and P-p38-MAPK immunostaining in trigeminal ganglia were significantly reduced by myrtenol treatment (12.5 and 25 mg/kg). Likewise, increased MPO activity and inflammatory histological scores in masseter muscle, as well as augmented levels of IL-1β in the TG AND STSC, observed after CGN injection, were significantly decreased by myrtenol (25 and 50 mg/kg). Myrtenol has potential to treat orofacial inflammation and pain, which is partially related to IL-1β levels in the trigeminal pathway and p38-MAPK modulation in trigeminal ganglia.

Cerebroventricular administration of anti-calcitonin gene-related peptide antibody reduces the increase of dopamine D2 receptor observed in the trigeminal spinal subnucleus caudalis following infraorbital nerve ligation

A dopamine D2 receptor (D2R) agonist and an anti-calcitonin gene-related peptide (CGRP) antibody were separately reported to reduce neuropathic pain. To further attenuate neuropathic pain, co-administration of a D2R agonist and an anti-CGRP antibody was performed in a rat with the infraorbital nerve (ION) ligation. However, this co-administration showed no further attenuation of mechanical hypersensitivity compared to the administration of anti-CGRP antibody alone. Our results also revealed that D2R immunoreactivity in the trigeminal spinal subnucleus caudalis (Vc) increased following the nerve ligation and decreased following administration of an anti-CGRP antibody. The ratio of immunoreactive neurons of phosphorylated cyclic adenosine monophosphate-response-element-binding protein in the Vc also increased following nerve ligation and decreased with the anti-CGRP antibody. Our results suggest that a decrease in D2R immunoreactivity reduces the effect of a D2R agonist, and transcription of D2R is activated following the ION ligation and suppressed by treatment with an anti-CGRP antibody.

Ca V 3.2 calcium channels contribute to trigeminal neuralgia.

Trigeminal neuralgia (TN) is a rare but debilitating disorder characterized by excruciating facial pain, with a higher incidence in women. Recent studies demonstrated that TN patients present mutations in the gene encoding the Ca V 3.2 T-type calcium channel, an important player in peripheral pain pathways. We characterize the role of Ca V 3.2 channels in TN at 2 levels. First, we examined the biophysical properties of CACNA1H variants found in TN patients. Second, we investigated the role of Ca V 3.2 in an animal model of trigeminal neuropathic pain. Whole-cell patch-clamp recordings from 4 different mutants expressed in tsA-201 cells (E286K in the pore loop of domain I, H526Y, G563R, and P566T in the domain I-II linker) identified a loss of function in activation in the E286K mutation and gain of function in the G563R and P566T mutations. Moreover, a loss of function in inactivation was observed with the E286K and H526Y mutations. Cell surface biotinylation revealed no difference in channel trafficking among the variants. The G563R mutant also caused a gain of function in the firing properties of transfected trigeminal ganglion neurons. In female and male mice, constriction of the infraorbital nerve induced facial thermal heat hyperalgesia. Block of T-type channels with Z944 resulted in antihyperalgesia. The effect of Z944 was absent in Ca V 3.2 -/- mice, indicating that Ca V 3.2 is the molecular target of the antihyperalgesic Z944 effect. Finally, enzyme-linked immunosorbent assay analysis revealed increased Ca V 3.2 channel expression in the spinal trigeminal subnucleus caudalis. Altogether, the present study demonstrates an important role of Ca V 3.2 channels in trigeminal pain.

Enhanced Ocular Surface and Intraoral Nociception via a Transient Receptor Potential Vanilloid 1 Mechanism in a Rat Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA), characterized by low arterial oxygen saturation during sleep, is associated with an increased risk of orofacial pain. In this study, we simulated chronic intermittent hypoxia (CIH) during the sleep/rest phase (light phase) to determine the role of transient receptor potential vanilloid 1 (TRPV1) in mediating enhanced orofacial nocifensive behavior and trigeminal spinal subnucleus caudalis (Vc) neuronal responses to capsaicin (a TRPV1 agonist) stimulation in a rat model of OSA. Rats were subjected to CIH (nadir O2, 5%) during the light phase for 8 or 16 consecutive days. CIH yielded enhanced behavioral responses to capsaicin after application to the ocular surface and intraoral mucosa, which was reversed under normoxic conditions. The percentage of TRPV1-immunoreactive trigeminal ganglion neurons was greater in CIH rats than in normoxic rats and recovered under normoxic conditions after CIH. The ratio of large-sized TRPV1-immunoreactive trigeminal ganglion neurons increased in CIH rats. The density of TRPV1 positive primary afferent terminals in the superficial laminae of Vc was higher in CIH rats. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells intermingled with the central terminal of TRPV1-positive afferents in the Vc. The number of pERK-immunoreactive cells following low-dose capsaicin (0.33 µM) application to the tongue was significantly greater in the middle portion of the Vc of CIH rats than of normoxic rats and recovered under normoxic conditions after CIH. These data suggest that CIH during the sleep (light) phase is sufficient to transiently enhance pain on the ocular surface and intraoral mucosa via TRPV1-dependent mechanisms.

Modulation of excitatory synaptic transmissions by TRPV1 in the spinal trigeminal subnucleus caudalis neurons of neuropathic pain rats

The present study examined contribution of the transient receptor potential vanilloid 1 channel (TRPV1) to the chronic orofacial pain. Bilateral partial nerve ligation (PNL) of the mental nerve, a branch of trigeminal nerve, was performed to induce neuropathic pain. The withdrawal threshold in response to mechanical stimulation of the lower lip skin was substantially reduced after the surgery in the PNL rats while it remained unchanged in the sham rats. This reduction in the PNL rats was alleviated by pregabalin injected intraperitoneally (10 mg/kg) and intracisternally (10, 30, 100 μg). Furthermore, an intracisternal injection of AMG9810, an antagonist of TRPV1, (1.5, 5.0 μg) attenuated the reduction of withdrawal threshold. Spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) were recorded from the spinal trigeminal subnucleus caudalis (Vc) neurons in the brainstem slice, which receive the orofacial nociceptive signals. In the PNL rats, superfusion of capsaicin (0.03, 0.1 μM) enhanced their frequency without effect on the amplitude and the highest concentration (0.3 μM) increased both the frequency and amplitude. In the sham rats, only 0.3 μM capsaicin increased their frequency. Thus, capsaicin-induced facilitation of sEPSCs and mEPSCs in the PNL rats was significantly stronger than that in the sham rats. AMG9810 (0.1 μM) attenuated the capsaicin's effect. Capsaicin was ineffective on the trigeminal tract-evoked EPSCs in the PNL and sham rats. These results suggest that the chronic orofacial pain in the PNL model results from facilitation of the spontaneous excitatory synaptic transmission in the Vc region through TRPV1 at least partly.

Activation of parabrachial nucleus - ventral tegmental area pathway underlies the comorbid depression in chronic neuropathic pain in mice.

SummaryDepression symptoms are often found in patients suffering from chronic pain, a phenomenon that is yet to be understood mechanistically. Here, we systematically investigate the cellular mechanisms and circuits underlying the chronic-pain-induced depression behavior. We show that the development of chronic pain is accompanied by depressive-like behaviors in a mouse model of trigeminal neuralgia. In parallel, we observe increased activity of the dopaminergic (DA) neuron in the midbrain ventral tegmental area (VTA), and inhibition of this elevated VTA DA neuron activity reverses the behavioral manifestations of depression. Further studies establish a pathway of glutamatergic projections from the spinal trigeminal subnucleus caudalis (Sp5C) to the lateral parabrachial nucleus (LPBN) and then to the VTA. These glutamatergic projections form a direct circuit that controls the development of the depression-like behavior under the state of the chronic neuropathic pain.

IL-33 induces orofacial neuropathic pain through Fyn-dependent phosphorylation of GluN2B in the trigeminal spinal subnucleus caudalis

Orofacial neuropathic pain can cause considerable disruptions in patients' daily lives, especially because of a lack of effective medications as its underlying causative mechanisms are not fully understood. Here, we found neuron-specific expression of the interleukin (IL)-33 receptor in the trigeminal spinal subnucleus caudalis (Vc), distinct from the spinal dorsal horn. Reduction in head withdrawal threshold in response to von Frey filament stimulation of the whisker pad skin was inversely correlated with the upregulation of IL-33 in the Vc after infraorbital nerve injury (IONI). Neutralization of IL-33 in the Vc alleviated mechanical allodynia in the whisker pad skin after IONI; conversely, intracisternal administration of IL-33 elicited mechanical allodynia in the whisker pad skin, which was relieved by GluN2B antagonism. Moreover, IL-33 triggered the potentiation of GluN2B-containing N-methyl-D-aspartate receptor-mediated synaptic currents and phosphorylation of synaptosomal GluN2B in the Vc, whereas IONI-induced GluN2B phosphorylation was inhibited by neutralization of IL-33 in the Vc. IL-33-induced GluN2B phosphorylation was mediated by phosphorylation of Fyn kinase, and inhibition of the Fyn kinase pathway prevented the development of IL-33-induced mechanical allodynia. Our findings provide insights into a new mechanism by which IL-33 directly regulates synaptic transmission and suggest that IL-33 signaling could be a candidate target for therapeutic interventions for orofacial neuropathic pain.

Pannexin 1-Mediated ATP Signaling in the Trigeminal Spinal Subnucleus Caudalis Is Involved in Tongue Cancer Pain.

Pain is one of the most severe concerns in tongue cancer patients. However, the underlying mechanisms of tongue cancer pain are not fully understood. We investigated the molecular mechanisms of tongue cancer-induced mechanical allodynia in the tongue by squamous cell carcinoma (SCC) inoculation in rats. The head-withdrawal threshold of mechanical stimulation (MHWT) to the tongue was reduced following SCC inoculation, which was inhibited by intracisternal administration of 10Panx, an inhibitory peptide for pannexin 1 (PANX1) channels. Immunohistochemical analyses revealed that the expression of PANX1 was upregulated in the trigeminal spinal subnucleus caudalis (Vc) following SCC inoculation. The majority of PANX1 immunofluorescence was merged with ionized calcium-binding adapter molecule 1 (Iba1) fluorescence and a part of it was merged with glial fibrillary acidic protein (GFAP) fluorescence. Spike frequencies of Vc nociceptive neurons to noxious mechanical stimulation were significantly enhanced in SCC-inoculated rats, which was suppressed by intracisternal 10Panx administration. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) neurons increased significantly in the Vc after SCC inoculation, which was inhibited by intracisternal 10Panx administration. SCC inoculation-induced MHWT reduction and increased pERK-IR Vc neuron numbers were inhibited by P2X7 purinoceptor (P2X7R) antagonism. Conversely, these effects were observed in the presence of P2X7R agonist in SCC-inoculated rats with PANX1 inhibition. SCC inoculation-induced MHWT reduction was significantly recovered by intracisternal interleukin-1 receptor antagonist administration. These observations suggest that SCC inoculation causes PANX1 upregulation in Vc microglia and adenosine triphosphate released through PANX1 sensitizes nociceptive neurons in the Vc, resulting in tongue cancer pain.