Lack of evidence for ectopic sprouting of genetically labeled Aβ touch afferents in inflammatory and neuropathic trigeminal pain

TET1 Participates in Complete Freund's Adjuvant-induced Trigeminal Inflammatory Pain by Regulating Kv7.2 in a Mouse Model

Trigeminal inflammatory pain is one of the most severe pain-related disorders in humans; however, the underlying mechanisms remain largely unknown. In this study, we investigated the possible contribution of interaction between ten-eleven translocation methylcytosine dioxygenase 1 (TET1) and the voltage-gated K+ channel Kv7.2 (encoded by Kcnq2) to orofacial inflammatory pain in mice. We found that complete Freund's adjuvant (CFA) injection reduced the expression of Kcnq2/Kv7.2 in the trigeminal ganglion (TG) and induced orofacial inflammatory pain. The involvement of Kv7.2 in CFA-induced orofacial pain was further confirmed by Kv7.2 knockdown or overexpression. Moreover, TET1 knockdown in Tet1flox/flox mice significantly reduced the expression of Kv7.2 and M currents in the TG and led to pain-like behaviors. Conversely, TET1 overexpression by lentivirus rescued the CFA-induced decreases of Kcnq2 and M currents and alleviated mechanical allodynia. Our data suggest that TET1 is implicated in CFA-induced trigeminal inflammatory pain by positively regulating Kv7.2 in TG neurons.

Chemokine CCL7 mediates trigeminal neuropathic pain via CCR2/CCR3-ERK pathway in the trigeminal ganglion of mice

BACKGROUND

Chemokine-mediated neuroinflammation plays an important role in the pathogenesis of neuropathic pain. The chemokine CC motif ligand 7 (CCL7) and its receptor CCR2 have been reported to contribute to neuropathic pain via astrocyte-microglial interaction in the spinal cord. Whether CCL7 in the trigeminal ganglion (TG) involves in trigeminal neuropathic pain and the involved mechanism remain largely unknown.

METHODS

The partial infraorbital nerve transection (pIONT) was used to induce trigeminal neuropathic pain in mice. The expression of Ccl7, Ccr1, Ccr2, and Ccr3 was examined by real-time quantitative polymerase chain reaction. The distribution of CCL7, CCR2, and CCR3 was detected by immunofluorescence double-staining. The activation of extracellular signal-regulated kinase (ERK) was examined by Western blot and immunofluorescence. The effect of CCL7 on neuronal excitability was tested by whole-cell patch clamp recording. The effect of selective antagonists for CCR1, CCR2, and CCR3 on pain hypersensitivity was checked by behavioral testing.

RESULTS

Ccl7 was persistently increased in neurons of TG after pIONT, and specific inhibition of CCL7 in the TG effectively relieved pIONT-induced orofacial mechanical allodynia. Intra-TG injection of recombinant CCL7 induced mechanical allodynia and increased the phosphorylation of ERK in the TG. Incubation of CCL7 with TG neurons also dose-dependently enhanced the neuronal excitability. Furthermore, pIONT increased the expression of CCL7 receptors Ccr1, Ccr2, and Ccr3. The intra-TG injection of the specific antagonist of CCR2 or CCR3 but not of CCR1 alleviated pIONT-induced orofacial mechanical allodynia and reduced ERK activation. Immunostaining showed that CCR2 and CCR3 are expressed in TG neurons, and CCL7-induced hyperexcitability of TG neurons was decreased by antagonists of CCR2 or CCR3.

CONCLUSION

CCL7 activates ERK in TG neurons via CCR2 and CCR3 to enhance neuronal excitability, which contributes to the maintenance of trigeminal neuropathic pain. CCL7-CCR2/CCR3-ERK pathway may be potential targets for treating trigeminal neuropathic pain.

MiR-223-3p alleviates trigeminal neuropathic pain in the male mouse by targeting MKNK2 and MAPK/ERK signaling

BACKGROUND

Trigeminal neuralgia (TN) is a neuropathic pain that occurs in branches of the trigeminal nerve. MicroRNAs (miRNAs) have been considered key mediators of neuropathic pain. This study was aimed to elucidate the pathophysiological function and mechanisms of miR-223-3p in mouse models of TN.

METHODS

Infraorbital nerve chronic constriction injury (CCI-ION) was applied in male C57BL/6J mice to establish mouse models of TN. Pain responses were assessed utilizing Von Frey method. The expression of miR-223-3p, MKNK2, and MAPK/ERK pathway protein in trigeminal ganglions (TGs) of CCI-ION mice was measured using RT-qPCR and Western blotting. The concentrations of inflammatory cytokines were evaluated using Western blotting. The relationship between miR-223-3p and MKNK2 was tested by a luciferase reporter assay.

RESULTS

We found that miR-223-3p was downregulated, while MKNK2 was upregulated in TGs of CCI-ION mice. MiR-223-3p overexpression by an intracerebroventricular injection of Lv-miR-223-3p attenuated trigeminal neuropathic pain in CCI-ION mice, as well as reduced the protein levels of pro-inflammatory cytokines in TGs of CCI-ION mice. MKNK2 was verified to be targeted by miR-223-3p. Additionally, miR-223-3p overexpression decreased the phosphorylation levels of ERK1/2, JNK, and p38 protein in TGs of CCI-ION mice to inhibit MAPK/ERK signaling.

CONCLUSIONS

Overall, miR-223-3p attenuates the development of TN by targeting MKNK2 to suppress MAPK/ERK signaling.

The Up-regulation of TNF-α Maintains Trigeminal Neuralgia by Modulating MAPKs Phosphorylation and BKCa Channels in Trigeminal Nucleus Caudalis

Trigeminal neuralgia (TN) is a severe chronic neuropathic pain. Despite numerous available medical interventions, the therapeutic effects are not ideal. To control the pain attacks, the need for more contemporary drugs continues to be a real challenge. Our previous study reported that Ca²⁺-activated K⁺ channels (BK_Ca) channels modulated by mitogen-activated protein kinases (MAPKs) in the trigeminal ganglia (TG) neurons play crucial roles in regulating TN, and some research studies demonstrated that inflammatory cytokine tumor necrosis factor alpha (TNF-α) could promote neuropathic pain. Meanwhile, the trigeminal nucleus caudalis (TNC), the first central site of the trigeminal nociceptive pathway, is responsible for processing sensory and pain signals from the peripheral orofacial area. Thus, this study is aimed to further investigate whether TNF-α and MAPKs phosphorylation in the TNC could mediate the pathogenesis of TN by modulating BK_Ca channels. The results showed that TNF-α of the TNC region is upregulated significantly in the chronic constriction injury of infraorbital nerve (ION-CCI) rats model, which displayed persistent facial mechanical allodynia. The normal rats with target injection of exogenous TNF-α to the fourth brain ventricle behaved just like the ION-CCI model rats, the orofacial mechanical pain threshold decreased clearly. Meanwhile, the exogenous TNF-α increased the action potential frequency and reduced the BK_Ca currents of TNC neurons significantly, which could be reversed by U0126 and SB203580, the inhibitors of MAPK. In addition, U0126, SB203580, and another MAPK inhibitor SP600125 could relieve the facial mechanical allodynia by being injected into the fourth brain ventricle of ION-CCI model rats, respectively. Taken together, our work suggests that the upregulation of TNF-α in the TNC region would cause the increase of MAPKs phosphorylation and then the negative regulation of BK_Ca channels, resulting in the TN.

Postnatal development of inner lamina II interneurons of the rat medullary dorsal horn

ABSTRACT

Pain processing in young mammals is immature. Despite the central role of the medullary dorsal horn (MDH) in processing orofacial sensory information, the maturation of the neurons within the MDH has been largely overlooked. Combining in vitro electrophysiological recordings and 3D morphological analysis over the first postnatal month in rats, we investigated the age-dependent development of the neurons within the inner lamina II (IIi) of the MDH. We show the lamina IIi neuronal population transition into a more hyperpolarized state, with modification of the action potential waveform, and a shift from single spiking, at early postnatal ages, to tonic firing and initial bursting at later stages. These physiological changes are associated with a strong structural remodelling of the neuronal morphology with most of the modifications occurring after the third postnatal week. Among the lamina IIi neuronal population, the subpopulation of interneurons expressing the γ isoform of the protein kinase C (PKCγ+) are key elements for the circuits underlying facial mechanical allodynia. How do they develop from the rest of the lamina IIi constitute an important question that remained to be addressed. Here, we show that PKCγ+ interneurons display electrophysiological changes over time comparable with the PKCγ- population. However, they show a distinctive increase of the soma volume and primary branches length, as opposed to the PKCγ- population. Together, our data demonstrate a novel pattern of late postnatal maturation of lamina IIi interneurons, with a spotlight on PKCγ+ interneurons, that may be relevant for the development of orofacial sensitivity.

G protein-coupled receptor GPR151 is involved in trigeminal neuropathic pain through the induction of Gβγ/extracellular signal-regulated kinase-mediated neuroinflammation in the trigeminal ganglion

ABSTRACT

Trigeminal nerve injury-induced neuropathic pain is a debilitating chronic orofacial pain syndrome but lacks effective treatment. G protein-coupled receptors (GPCRs), especially orphan GPCRs (oGPCRs) are important therapeutic targets in pain medicine. Here, we screened upregulated oGPCRs in the trigeminal ganglion (TG) after partial infraorbital nerve transection (pIONT) and found that Gpr151 was the most significantly upregulated oGPCRs. Gpr151 mRNA was increased from pIONT day 3 and maintained for more than 21 days. Furthermore, GPR151 was expressed in the neurons of the TG after pIONT. Global mutation or knockdown of Gpr151 in the TG attenuated pIONT-induced mechanical allodynia. In addition, the excitability of TG neurons was increased after pIONT in wild-type (WT) mice, but not in Gpr151-/- mice. Notably, GPR151 bound to Gαi protein, but not Gαq, Gα12, or Gα13, and activated the extracellular signal-regulated kinase (ERK) through Gβγ. Extracellular signal-regulated kinase was also activated by pIONT in the TG of WT mice, but not in Gpr151-/- mice. Gene microarray showed that Gpr151 mutation reduced the expression of a large number of neuroinflammation-related genes that were upregulated in WT mice after pIONT, including chemokines CCL5, CCL7, CXCL9, and CXCL10. The mitogen-activated protein kinase inhibitor (PD98059) attenuated mechanical allodynia and reduced the upregulation of these chemokines after pIONT. Collectively, this study not only revealed the involvement of GPR151 in the maintenance of trigeminal neuropathic pain but also identified GPR151 as a Gαi-coupled receptor to induce ERK-dependent neuroinflammation. Thus, GPR151 may be a potential drug target for the treatment of trigeminal neuropathic pain.

CXCL10 and CXCR3 in the Trigeminal Ganglion Contribute to Trigeminal Neuropathic Pain in Mice

Purpose

Trigeminal neuropathic pain is very common clinically, but effective treatments are lacking. Chemokines and their receptors have been implicated in the pathogenesis of chronic pain. This study explored the role of the chemokine CXCL10 and its receptor, CXCR3, in trigeminal neuropathic pain in mice.

Materials and Methods

Trigeminal neuropathic pain was established by partial infraorbital nerve ligation (pIONL) in wild-type and Cxcr3^−/− mice. Facial mechanical allodynia was evaluated by behavioral testing. A lentivirus containing Cxcr3 shRNA (LV-Cxcr3 shRNA) was microinjected into the trigeminal ganglion (TG) to knock down Cxcr3 expression. Quantitative polymerase chain reaction assays and immunofluorescence staining were used to examine Cxcl10/Cxcr3 mRNA expression and protein distribution. Western blotting was performed to examine activation of extracellular signal-regulated kinase (ERK) and AKT in the TG. Intra-TG injection of an AKT inhibitor was performed to examine the role of AKT in trigeminal neuropathic pain.

Results

pIONL induced persistent trigeminal neuropathic pain, which was alleviated in Cxcr3^−/− mice. Intra-TG injection of LV-Cxcr3 shRNA attenuated pIONL-induced mechanical allodynia. Furthermore, pIONL increased the expression of CXCR3 and its major ligand, CXCL10, in TG neurons. Intra-TG injection of CXCL10 induced pain hypersensitivity in wild-type mice but not in Cxcr3^−/− mice. CXCL10 also induced activation of ERK and AKT in the TG of wild-type mice. Finally, pIONL-induced activation of ERK and AKT was reduced in Cxcr3^−/− mice. Intra-TG injection of the AKT inhibitor alleviated pIONL-induced mechanical allodynia in WT mice but not in Cxcr3^−/− mice.

Conclusion

CXCL10 acts on CXCR3 to induce ERK and AKT activation in TG neurons and contributes to the maintenance of trigeminal neuropathic pain.

TLR8 in the Trigeminal Ganglion Contributes to the Maintenance of Trigeminal Neuropathic Pain in Mice

Trigeminal neuropathic pain (TNP) is a significant health problem but the involved mechanism has not been completely elucidated. Toll-like receptors (TLRs) have recently been demonstrated to be expressed in the dorsal root ganglion and involved in chronic pain. Here, we show that TLR8 was persistently increased in the trigeminal ganglion (TG) neurons in model of TNP induced by partial infraorbital nerve ligation (pIONL). In addition, deletion or knockdown of Tlr8 in the TG attenuated pIONL-induced mechanical allodynia, reduced the activation of ERK and p38-MAPK, and decreased the expression of pro-inflammatory cytokines in the TG. Furthermore, intra-TG injection of the TLR8 agonist VTX-2337 induced pain hypersensitivity. VTX-2337 also increased the intracellular Ca²⁺ concentration, induced the activation of ERK and p38, and increased the expression of pro-inflammatory cytokines in the TG. These data indicate that TLR8 contributes to the maintenance of TNP through increasing MAPK-mediated neuroinflammation. Targeting TLR8 signaling may be effective for the treatment of TNP.

Microglia: sculptors of neuropathic pain?

Neuropathic pain presents a huge societal and individual burden. The limited efficacy of current analgesics, diagnostic markers and clinical trial outcome measures arises from an incomplete understanding of the underlying mechanisms. A large and growing body of evidence has established the important role of microglia in the onset and possible maintenance of neuropathic pain, and these cells may represent an important target for future therapy. Microglial research has further revealed their important role in structural remodelling of the nervous system. In this review, we aim to explore the evidence for microglia in sculpting nervous system structure and function, as well as their important role in neuropathic pain, and finally integrate these studies to synthesize a new model for microglia in somatosensory circuit remodelling, composed of six key and inter-related mechanisms. Summarizing the mechanisms through which microglia modulate nervous system structure and function helps to frame a better understanding of neuropathic pain, and provide a clear roadmap for future research.

General anesthetics activate a potent central pain-suppression circuit in the amygdala

General anesthesia (GA) can produce analgesia (loss of pain) independent of inducing loss of consciousness, but the underlying mechanisms remain unclear. We hypothesized that GA suppresses pain in part by activating supraspinal analgesic circuits. We discovered a distinct population of GABAergic neurons activated by GA in the mouse central amygdala (CeA_GA neurons). In vivo calcium imaging revealed that different GA drugs activate a shared ensemble of CeA_GA neurons. CeA_GA neurons also possess basal activity that mostly reflects animals' internal state rather than external stimuli. Optogenetic activation of CeA_GA potently suppressed both pain-elicited reflexive and self-recuperating behaviors across sensory modalities and abolished neuropathic pain-induced mechanical (hyper-)sensitivity. Conversely, inhibition of CeA_GA activity exacerbated pain, produced strong aversion and canceled the analgesic effect of low-dose ketamine. CeA_GA neurons have widespread inhibitory projections to many affective pain-processing centers. Our study points to CeA_GA as a potential powerful therapeutic target for alleviating chronic pain.

A craniofacial-specific monosynaptic circuit enables heightened affective pain

Humans often rank craniofacial pain as more severe than body pain. Evidence suggests that a stimulus of the same intensity induces stronger pain in the face than in the body. However, the underlying neural circuitry for the differential processing of facial versus bodily pain remains unknown. Interestingly, the lateral parabrachial nucleus (PBL), a critical node in the affective pain circuit, is activated more strongly by noxious stimulation of the face than of the hindpaw. Using a novel activity-dependent technology called CANE developed in our laboratory, we identified and selectively labeled noxious-stimulus-activated PBL neurons and performed comprehensive anatomical input-output mapping. Surprisingly, we uncovered a hitherto uncharacterized monosynaptic connection between cranial sensory neurons and the PBL-nociceptive neurons. Optogenetic activation of this monosynaptic craniofacial-to-PBL projection induced robust escape and avoidance behaviors and stress calls, whereas optogenetic silencing specifically reduced facial nociception. The monosynaptic circuit revealed here provides a neural substrate for heightened craniofacial affective pain.

Stress Increases Peripheral Axon Growth and Regeneration through Glucocorticoid Receptor-Dependent Transcriptional Programs

Stress and glucocorticoid (GC) release are common behavioral and hormonal responses to injury or disease. In the brain, stress/GCs can alter neuron structure and function leading to cognitive impairment. Stress and GCs also exacerbate pain, but whether a corresponding change occurs in structural plasticity of sensory neurons is unknown. Here, we show that in female mice (Mus musculus) basal GC receptor (Nr3c1, also known as GR) expression in dorsal root ganglion (DRG) sensory neurons is 15-fold higher than in neurons in canonical stress-responsive brain regions (M. musculus). In response to stress or GCs, adult DRG neurite growth increases through mechanisms involving GR-dependent gene transcription. In vivo, prior exposure to an acute systemic stress increases peripheral nerve regeneration. These data have broad clinical implications and highlight the importance of stress and GCs as novel behavioral and circulating modifiers of neuronal plasticity.

Mechanisms of craniofacial pain

Aim To provide an overview of mechanisms underlying craniofacial pain; to highlight peripheral and central adaptations that may promote chronification of pain in craniofacial pain states such as migraine and temporomandibular disorders (TMD). Background Pain is a common symptom associated with disorders involving craniofacial tissues including the teeth and their supporting structures, the temporomandibular joint and the muscles of the head. Most acute painful craniofacial conditions are easily recognized and well managed, but others, especially those that are chronic (e.g., migraine, TMD and trigeminal neuropathies), present clinical challenges. Preclinical studies have provided substantial information about the anatomical and physiological mechanisms related to the initiation and modulation of nociceptive signals in the trigeminal system. While knowledge of the mechanisms underlying chronic craniofacial pain remains limited, both clinical and preclinical investigations suggest that changes in afferent inputs to the brain as well as in brain structure and modulatory pathways occur in chronic pain. Collectively, these changes result in amplification of nociception that promotes and sustains craniofacial chronic pain states. Conclusions The increased understanding gained of the physiological and pathological processing of nociception in the trigeminal system has provided new perspectives for the mechanistic understanding of acute craniofacial pain conditions and the peripheral and central adaptations that are related to pain chronification. Such knowledge may contribute to improvements in currently available treatments as well as to the development of novel analgesic therapies.

Neural circuit remodeling and structural plasticity in the cortex during chronic pain

Damage in the periphery or spinal cord induces maladaptive plastic changes along the somatosensory nervous system from the periphery to the cortex, often leading to chronic pain. Although the role of neural circuit remodeling and structural synaptic plasticity in the 'pain matrix' cortices in chronic pain has been thought as a secondary epiphenomenon to altered nociceptive signaling in the spinal cord, progress in whole brain imaging studies on human patients and animal models has suggested a possibility that plastic changes in cortical neural circuits may actively contribute to chronic pain symptoms. Furthermore, recent development in two-photon microscopy and fluorescence labeling techniques have enabled us to longitudinally trace the structural and functional changes in local circuits, single neurons and even individual synapses in the brain of living animals. These technical advances has started to reveal that cortical structural remodeling following tissue or nerve damage could rapidly occur within days, which are temporally correlated with functional plasticity of cortical circuits as well as the development and maintenance of chronic pain behavior, thereby modifying the previous concept that it takes much longer periods (e.g. months or years). In this review, we discuss the relation of neural circuit plasticity in the 'pain matrix' cortices, such as the anterior cingulate cortex, prefrontal cortex and primary somatosensory cortex, with chronic pain. We also introduce how to apply long-term in vivo two-photon imaging approaches for the study of pathophysiological mechanisms of chronic pain.