Differences between Dorsal Root and Trigeminal Ganglion Nociceptors in Mice Revealed by Translational Profiling

ephrin-B2 promotes nociceptive plasticity and hyperalgesic priming through EphB2-MNK-eIF4E signaling in both mice and humans

Ephrin-B-EphB signaling can promote pain through ligand-receptor interactions between peripheral cells, like immune cells expressing ephrin-Bs, and EphB receptors expressed by DRG neurons. Previous studies have shown increased ephrin-B2 expression in peripheral tissues like synovium of rheumatoid and osteoarthritis patients, indicating the clinical significance of this signaling. The primary goal of this study was to understand how ephrin-B2 acts on mouse and human DRG neurons, which express EphB receptors, to promote pain and nociceptor plasticity. We hypothesized that ephrin-B2 would promote nociceptor plasticity and hyperalgesic priming through MNK-eIF4E signaling, a critical mechanism for nociceptive plasticity induced by growth factors, cytokines and nerve injury. Both male and female mice developed dose-dependent mechanical hypersensitivity in response to ephrin-B2, and both sexes showed hyperalgesic priming when challenged with PGE2 injection either to the paw or the cranial dura. Acute nociceptive behaviors and hyperalgesic priming were blocked in mice lacking MNK1 (Mknk1 knockout mice) and by eFT508, a specific MNK inhibitor. Sensory neuron-specific knockout of EphB2 using Pirt-Cre demonstrated that ephrin-B2 actions require this receptor. In Ca2+-imaging experiments on cultured DRG neurons, ephrin-B2 treatment enhanced Ca2+ transients in response to PGE2 and these effects were absent in DRG neurons from MNK1-/- and EphB2-PirtCre mice. In experiments on human DRG neurons, ephrin-B2 increased eIF4E phosphorylation and enhanced Ca2+ responses to PGE2 treatment, both blocked by eFT508. We conclude that ephrin-B2 acts directly on mouse and human sensory neurons to induce nociceptor plasticity via MNK-eIF4E signaling, offering new insight into how ephrin-B signaling promotes pain.

The protective barrier role of satellite glial cells in sensory ganglia

Neurons in sensory ganglia are wrapped completely by satellite glial cells (SGCs). One putative function of SGCs is to regulate the neuronal microenvironment, but this role has received only little attention. In this study we investigated whether the SGC envelope serves a barrier function and how SGCs may control the neuronal microenvironment. We studied this question on short-term (<24 h) cell cultures of dorsal root ganglia and trigeminal ganglia from adult mice, which contain neurons surrounded with SGCs, and neurons that are not. Using calcium imaging, we measured neuronal responses to molecules with established actions on sensory neurons. We found that neurons surrounded by SGCs had a smaller response to molecules such as adenosine triphosphate (ATP), glutamate, GABA, and bradykinin than neurons without glial cover. When we inhibited the activity of NTPDases, which hydrolyze the ATP, and also when we inhibited the glutamate and GABA transporters on SGCs, this difference in the neuronal response was no longer observed. We conclude that the SGC envelope does not hinder diffusional passage, but acts as a metabolic barrier that regulates the neuronal microenvironment, and can protect the neurons and modulate their activity.

Schisandrin B from Schisandra chinensis alleviated pain via glycine receptors, Nav1.7 channels and Cav2.2 channels

ETHNOPHARMACOLOGICAL RELEVANCE

Schisandra chinensis, the dried and ripe fruit of the magnolia family plant Schisandra chinensis (Turcz.) Baill, was commonly used in traditional analgesic prescription. Studies have shown that the extract of Schisandra chinensis (SC) displayed analgesic activity. However, the analgesic active component and the exact mechanisms have yet to be revealed.

AIM OF THE STUDY

The present study was to investigate the anti-nociceptive constituent of Schisandra chinensis, assess its analgesic effect, and explore the potential molecular mechanisms.

MATERIALS AND METHODS

The effects of a series of well-recognized compounds from SC on glycine receptors were investigated. The analgesic effect of the identified compound was evaluated in three pain models. Mechanistic studies were performed using patch clamp technique on various targets expressed in recombinant cells. These targets included glycine receptors, Nav1.7 sodium channels, Cav2.2 calcium channels et al. Meanwhile, primary cultured spinal dorsal horn (SDH) neurons and dorsal root ganglion (DRG) neurons were also utilized.

RESULTS

Schisandrin B (SchB) was a positive allosteric modulator of glycine receptors in spinal dorsal horn neurons. The EC50 of SchB on glycine receptors in spinal dorsal horn neurons was 2.94 ± 0.28 μM. In three pain models, the analgesic effect of SchB was comparable to that of indomethacin at the same dose. Besides, SchB rescued PGE2-induced suppression of α3 GlyR activity and alleviated persistent pain. Notably, SchB could also potently decrease the frequency of action potentials and inhibit sodium and calcium channels in DRG neurons. Consistent with the data from DRG neurons, SchB was also found to significantly block Nav1.7 sodium channels and Cav2.2 channels in recombinant cells.

CONCLUSION

Our results demonstrated that, Schisandrin B, the primary lignan component of Schisandra chinensis, may exert its analgesic effect by acting on multiple ion channels, including glycine receptors, Nav1.7 channels, and Cav2.2 channels.

Neuronal Panx1 drives peripheral sensitization in experimental plantar inflammatory pain

BACKGROUND

The channel-forming protein Pannexin1 (Panx1) has been implicated in both human studies and animal models of chronic pain, but the underlying mechanisms remain incompletely understood.

METHODS

Wild-type (WT, n = 24), global Panx1 KO (n = 24), neuron-specific Panx1 KO (n = 20), and glia-specific Panx1 KO (n = 20) mice were used in this study at Albert Einstein College of Medicine. The von Frey test was used to quantify pain sensitivity in these mice following complete Freund's adjuvant (CFA) injection (7, 14, and 21 d). The qRT-PCR was employed to measure mRNA levels of Panx1, Panx2, Panx3, Cx43, Calhm1, and β-catenin. Laser scanning confocal microscopy imaging, Sholl analysis, and electrophysiology were utilized to evaluate the impact of Panx1 on neuronal excitability and morphology in Neuro2a and dorsal root ganglion neurons (DRGNs) in which Panx1 expression or function was manipulated. Ethidium bromide (EtBr) dye uptake assay and calcium imaging were employed to investigate the role of Panx1 in adenosine triphosphate (ATP) sensitivity. β-galactosidase (β-gal) staining was applied to determine the relative cellular expression levels of Panx1 in trigeminal ganglia (TG) and DRG of transgenic mice.

RESULTS

Global or neuron-specific Panx1 deletion markedly decreased pain thresholds after CFA stimuli (7, 14, and 21 d; P < 0.01 vs. WT group), indicating that Panx1 was positively correlated with pain sensitivity. In Neuro2a, global Panx1 deletion dramatically reduced neurite extension and inward currents compared to the WT group (P < 0.05), revealing that Panx1 enhanced neurogenesis and excitability. Similarly, global Panx1 deletion significantly suppressed Wnt/β-catenin dependent DRG neurogenesis following 5 d of nerve growth factor (NGF) treatment (P < 0.01 vs. WT group). Moreover, Panx1 channels enhanced DRG neuron response to ATP after CFA injection (P < 0.01 vs. Panx1 KO group). Furthermore, ATP release increased Ca2+ responses in DRGNs and satellite glial cells surrounding them following 7 d of CFA treatment (P < 0.01 vs. Panx1 KO group), suggesting that Panx1 in glia also impacts exaggerated neuronal excitability. Interestingly, neuron-specific Panx1 deletion was found to markedly reduce differentiation in cultured DRGNs, as evidenced by stunted neurite outgrowth (P < 0.05 vs. Panx1 KO group; P < 0.01 vs. WT group or GFAP-Cre group), blunted activation of Wnt/β-catenin signaling (P < 0.01 vs. WT, Panx1 KO and GFAP-Cre groups), and diminished cell excitability (P < 0.01 vs. GFAP-Cre group) and response to ATP stimulation (P < 0.01 vs. WT group). Analysis of β-gal staining showed that cellular expression levels of Panx1 in neurons are significantly higher (2.5-fold increase) in the DRG than in the TG.

CONCLUSIONS

The present study revealed that neuronal Panx1 is a prominent driver of peripheral sensitivity in the setting of inflammatory pain through cell-autonomous effects on neuronal excitability. This hyperexcitability dependence on neuronal Panx1 contrasts with inflammatory orofacial pain, where similar studies revealed a prominent role for glial Panx1. The apparent differences in Panx1 expression in neuronal and non-neuronal TG and DRG cells are likely responsible for the distinct impact of these cell types in the two pain models.

Lingual Denervation Improves the Efficacy of Anti-PD-1 Immunotherapy in Oral Squamous Cell Carcinomas by Downregulating TGFβ Signaling

PURPOSE

Intratumoral nerve infiltration relates to tumor progression and poor survival in oral squamous cell carcinoma (OSCC). How neural involvement regulates antitumor immunity has not been well characterized. This study aims to investigate molecular mechanisms of regulating tumor aggressiveness and impairing antitumor immunity by nerve-derived factors.

EXPERIMENTAL DESIGN

We performed the surgical lingual denervation in an immunocompetent mouse OSCC model to investigate its effect on tumor growth and the efficacy of anti-PD-1 immunotherapy. A trigeminal ganglion neuron and OSCC cell coculture system was established to investigate the proliferation, migration, and invasion of tumor cells and the PD-L1 expression. Both the neuron-tumor cell coculture in vitro model and the OSCC animal model were explored.

RESULTS

Lingual denervation slowed down tumor growth and improved the efficacy of anti-PD-1 treatment in the OSCC model. Coculturing with neurons not only enhanced the proliferation, migration, and invasion but also upregulated TGFβ-SMAD2 signaling and PD-L1 expression of tumor cells. Treatment with the TGFβ signaling inhibitor galunisertib reversed nerve-derived tumor aggressiveness and downregulated PD-L1 on tumor cells. Similarly, lingual denervation in vivo decreased TGFβ and PD-L1 expression and increased CD8+ T-cell infiltration and the expression of IFNγ and TNFα within tumor.

CONCLUSIONS

Neural involvement enhanced tumor aggressiveness through upregulating TGFβ signaling and PD-L1 expression in OSCC, while denervation of OSCC inhibited tumor growth, downregulated TGFβ signaling, enhanced activities of CD8+ T cells, and improved the efficacy of anti-PD-1 immunotherapy. This study will encourage further research focusing on denervation as a potential adjuvant therapeutic approach in OSCC.

SIGNIFICANCE

This study revealed the specific mechanisms for nerve-derived cancer progression and impaired antitumor immunity in OSCC, providing a novel insight into the cancer-neuron-immune network as well as pointing the way for new strategies targeting nerve-cancer cross-talk as a potential adjuvant therapeutic approach for OSCC.

In-Vivo Calcium Imaging of Sensory Neurons in the Rat Trigeminal Ganglion

Genetically encoded calcium indicators (GECIs) enable imaging techniques to monitor changes in intracellular calcium in targeted cell populations. Their large signal-to-noise ratio makes GECIs a powerful tool for detecting stimulus-evoked activity in sensory neurons. GECIs facilitate population-level analysis of stimulus encoding with the number of neurons that can be studied simultaneously. This population encoding is most appropriately done in vivo. Dorsal root ganglia (DRG), which house the soma of sensory neurons innervating somatic and visceral structures below the neck, are used most extensively for in vivo imaging because these structures are accessed relatively easily. More recently, this technique was used in mice to study sensory neurons in the trigeminal ganglion (TG) that innervate oral and craniofacial structures. There are many reasons to study TG in addition to DRG, including the long list of pain syndromes specific to oral and craniofacial structures that appear to reflect changes in sensory neuron activity, such as trigeminal neuralgia. Mice are used most extensively in the study of DRG and TG neurons because of the availability of genetic tools. However, with differences in size, ease of handling, and potentially important species differences, there are reasons to study rat rather than mouse TG neurons. Thus, we developed an approach for imaging rat TG neurons in vivo. We injected neonatal pups (p2) intraperitoneally with an AAV encoding GCaMP6s, resulting in >90% infection of both TG and DRG neurons. TG was visualized in the adult following craniotomy and decortication, and changes in GCaMP6s fluorescence were monitored in TG neurons following stimulation of mandibular and maxillary regions of the face. We confirmed that increases in fluorescence were stimulus-evoked with peripheral nerve block. While this approach has many potential uses, we are using it to characterize the subpopulation(s) of TG neurons changed following peripheral nerve injury.

FOXD3-mediated transactivation of ALKBH5 promotes neuropathic pain via m6A-dependent stabilization of 5-HT3A mRNA in sensory neurons

The N6-methyladenosine (m6A) modification of RNA is an emerging epigenetic regulatory mechanism that has been shown to participate in various pathophysiological processes. However, its involvement in modulating neuropathic pain is still poorly understood. In this study, we elucidate a functional role of the m6A demethylase alkylation repair homolog 5 (ALKBH5) in modulating trigeminal-mediated neuropathic pain. Peripheral nerve injury selectively upregulated the expression level of ALKBH5 in the injured trigeminal ganglion (TG) of rats. Blocking this upregulation in injured TGs alleviated trigeminal neuropathic pain, while mimicking the upregulation of ALKBH5 in intact TG neurons sufficiently induced pain-related behaviors. Mechanistically, histone deacetylase 11 downregulation induced by nerve injury increases histone H3 lysine 27 acetylation (H3K27ac), facilitating the binding of the transcription factor forkhead box protein D3 (FOXD3) to the Alkbh5 promoter and promoting Alkbh5 transcription. The increased ALKBH5 erases m6A sites in Htr3a messenger RNA (mRNA), resulting in an inability of YT521-B homology domain 2 (YTHDF2) to bind to Htr3a mRNA, thus causing an increase in 5-HT3A protein expression and 5-HT3 channel currents. Conversely, blocking the increased expression of ALKBH5 in the injured TG destabilizes nerve injury-induced 5-HT3A upregulation and reverses mechanical allodynia, and the effect can be blocked by 5-HT3A knockdown. Together, FOXD3-mediated transactivation of ALKBH5 promotes neuropathic pain through m6A-dependent stabilization of Htr3a mRNA in TG neurons. This mechanistic understanding may advance the discovery of new therapeutic targets for neuropathic pain management.

Regulation of headache response and transcriptomic network by the trigeminal ganglion clock

OBJECTIVE

To characterize the circadian features of the trigeminal ganglion in a mouse model of headache.

BACKGROUND

Several headache disorders, such as migraine and cluster headache, are known to exhibit distinct circadian rhythms of attacks. The circadian basis for these rhythmic pain responses, however, remains poorly understood.

METHODS

We examined trigeminal ganglion ex vivo and single-cell cultures from Per2::LucSV reporter mice and performed immunohistochemistry. Circadian behavior and transcriptomics were investigated using a novel combination of trigeminovascular and circadian models: a nitroglycerin mouse headache model with mechanical thresholds measured every 6 h, and trigeminal ganglion RNA sequencing measured every 4 h for 24 h. Finally, we performed pharmacogenomic analysis of gene targets for migraine, cluster headache, and trigeminal neuralgia treatments as well as trigeminal ganglion neuropeptides; this information was cross-referenced with our cycling genes from RNA sequencing data to identify potential targets for chronotherapy.

RESULTS

The trigeminal ganglion demonstrates strong circadian rhythms in both ex vivo and single-cell cultures, with core circadian proteins found in both neuronal and non-neuronal cells. Using our novel behavioral model, we showed that nitroglycerin-treated mice display circadian rhythms of pain sensitivity which were abolished in arrhythmic Per1/2 double knockout mice. Furthermore, RNA-sequencing analysis of the trigeminal ganglion revealed 466 genes that displayed circadian oscillations in the control group, including core clock genes and clock-regulated pain neurotransmitters. In the nitroglycerin group, we observed a profound circadian reprogramming of gene expression, as 331 of circadian genes in the control group lost rhythm and another 584 genes gained rhythm. Finally, pharmacogenetics analysis identified 10 genes in our trigeminal ganglion circadian transcriptome that encode target proteins of current medications used to treat migraine, cluster headache, or trigeminal neuralgia.

CONCLUSION

Our study unveiled robust circadian rhythms in the trigeminal ganglion at the behavioral, transcriptomic, and pharmacogenetic levels. These results support a fundamental role of the clock in pain pathophysiology.

PLAIN LANGUAGE SUMMARY

Several headache diseases, such as migraine and cluster headache, have headaches that occur at the same time each day. We learned that the trigeminal ganglion, an important pain structure in several headache diseases, has a 24-hour cycle that might be related to this daily cycle of headaches. Our genetic analysis suggests that some medications may be more effective in treating migraine and cluster headache when taken at specific times of the day.

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

[Specificities of orofacial neuropathic pain]

Head pain and notably orofacial pain differs from spinal pain on pathophysiological, clinical, therapeutic and prognostic levels. Its high prevalence, important impact on quality of life and significant socio-economical burden justify specific study of such type of pain. Among them, neuropathic orofacial pain resulting from disease or trauma of the trigeminal nervous system is among the most difficult types of pain to diagnose and to treat. Deciphering of underlying peripheral and central mechanisms has allowed numerous conceptual, clinical and therapeutic advances, notably the role of neural and non neural cell types, such as glia, immunocytes, vascular endothelial cells or the role of trigeminal sensory complex neural circuitry reconfiguration in the development of post-traumatic trigeminal neuropathic pain. Cellular interactions within the trigeminal ganglion, allowing a better understanding of several painful dental, ocular or cephalalgic comorbidities, are also described.

DNA Methylation Changes in Blood Cells of Fibromyalgia and Chronic Fatigue Syndrome Patients

Purpose

Fibromyalgia (FM) and Chronic Fatigue Syndrome (CFS) affect 0.4% and 1% of society, respectively, and the prevalence of these pain syndromes is increasing. To date, no strong association between these syndromes and the genetic background of affected individuals has been shown. Therefore, it is plausible that epigenetic changes might play a role in the development of these syndromes.

Patients and Methods

Three previous studies have attempted to elaborate the involvement of genome-wide methylation changes in blood cells in the development of fibromyalgia and chronic fatigue syndrome. These studies included 22 patients with fibromyalgia and 127 patients with CFS, and the results of the studies were largely discrepant. Contradicting results of those studies may be attributed to differences in the omics data analysis approaches used in each study. We reanalyzed the data collected in these studies using an updated and coherent data-analysis framework.

Results

Overall, the methylation changes that we observed overlapped with previous results only to some extent. However, the gene set enrichment analyses based on genes annotated to methylation changes identified in each of the analyzed datasets were surprisingly coherent and uniformly associated with the physiological processes that, when affected, may result in symptoms characteristic of fibromyalgia and chronic fatigue syndrome.

Conclusion

Methylomes of the blood cells of patients with FM and CFS in three independent studies have shown methylation changes that appear to be implicated in the pathogenesis of these syndromes.

The Ca2+ channel ORAI1 is a regulator of oral cancer growth and nociceptive pain

Oral cancer causes pain associated with cancer progression. We report here that the function of the Ca2+ channel ORAI1 is an important regulator of oral cancer pain. ORAI1 was highly expressed in tumor samples from patients with oral cancer, and ORAI1 activation caused sustained Ca2+ influx in human oral cancer cells. RNA-seq analysis showed that ORAI1 regulated many genes encoding oral cancer markers such as metalloproteases (MMPs) and pain modulators. Compared with control cells, oral cancer cells lacking ORAI1 formed smaller tumors that elicited decreased allodynia when inoculated into mouse paws. Exposure of trigeminal ganglia neurons to MMP1 evoked an increase in action potentials. These data demonstrate an important role of ORAI1 in oral cancer progression and pain, potentially by controlling MMP1 abundance.

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

Scientific and Regulatory Policy Committee Points to Consider: Sampling, Processing, Evaluation, Interpretation, and Reporting of Test Article-Related Ganglion Pathology for Nonclinical Toxicity Studies

Certain biopharmaceutical products consistently affect dorsal root ganglia, trigeminal ganglia, and/or autonomic ganglia. Product classes targeting ganglia include antineoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, and anti-nerve growth factor agents. This article outlines "points to consider" for sample collection, processing, evaluation, interpretation, and reporting of ganglion findings; these points are consistent with published best practices for peripheral nervous system evaluation in nonclinical toxicity studies. Ganglion findings often occur as a combination of neuronal injury (e.g., degeneration, necrosis, and/or loss) and/or glial effects (e.g., increased satellite glial cell cellularity) with leukocyte accumulation (e.g., mononuclear cell infiltration or inflammation). Nerve fiber degeneration and/or glial reactions may be seen in nerves, dorsal spinal nerve roots, spinal cord, and occasionally brainstem. Interpretation of test article (TA)-associated effects may be confounded by incidental background changes or experimental procedure-related changes and limited historical control data. Reports should describe findings at these sites, any TA relationship, and the criteria used for assigning severity grades. Contextualizing adversity of ganglia findings can require a weight-of-evidence approach because morphologic changes of variable severity occur in ganglia but often are not accompanied by observable overt in-life functional alterations detectable by conventional behavioral and neurological testing techniques.

Transient Receptor Potential Channel Vanilloid 1 Contributes to Facial Mechanical Hypersensitivity in a Mouse Model of Atopic Asthma

Sensitive skin, a common pathophysiological feature of allergic diseases, is defined as an unpleasant sensation in response to stimuli that normally should not provoke such sensations. However, the relationship between allergic inflammation and hypersensitive skin in the trigeminal system remains to be elucidated. To explore whether bronchial allergic inflammation affects facial skin and primary sensory neurons, we used an ovalbumin (OVA)-induced asthma mouse model. Significant mechanical hypersensitivity was observed in the facial skin of mice with pulmonary inflammation induced by OVA sensitization compared to mice treated with adjuvant or vehicle as controls. The skin of OVA-treated mice showed an increased number of nerve fibers, especially rich intraepithelial nerves, compared to controls. Transient receptor potential channel vanilloid 1 (TRPV1)-immunoreactive nerves were enriched in the skin of OVA-treated mice. Moreover, epithelial TRPV1 expression was higher in OVA-treated mice than in controls. Trigeminal ganglia of OVA-treated mice displayed larger numbers of activated microglia/macrophages and satellite glia. In addition, more TRPV1 immunoreactive neurons were found in the trigeminal ganglia of OVA-treated mice than in controls. Mechanical hypersensitivity was suppressed in OVA-treated Trpv1-deficient mice, while topical skin application of a TRPV1 antagonist before behavioral testing reduced the reaction induced by mechanical stimulation. Our findings reveal that mice with allergic inflammation of the bronchi had mechanical hypersensitivity in the facial skin that may have resulted from TRPV1-mediated neuronal plasticity and glial activation in the trigeminal ganglion.

Major Differences in Transcriptional Alterations in Dorsal Root Ganglia Between Spinal Cord Injury and Peripheral Neuropathic Pain Models

Chronic, often intractable, pain is caused by neuropathic conditions such as traumatic peripheral nerve injury (PNI) and spinal cord injury (SCI). These conditions are associated with alterations in gene and protein expression correlated with functional changes in somatosensory neurons having cell bodies in dorsal root ganglia (DRGs). Most studies of DRG transcriptional alterations have utilized PNI models where axotomy-induced changes important for neural regeneration may overshadow changes that drive neuropathic pain. Both PNI and SCI produce DRG neuron hyperexcitability linked to pain, but contusive SCI produces little peripheral axotomy or peripheral nerve inflammation. Thus, comparison of transcriptional signatures of DRGs across PNI and SCI models may highlight pain-associated transcriptional alterations in sensory ganglia that do not depend on peripheral axotomy or associated effects such as peripheral Wallerian degeneration. Data from our rat thoracic SCI experiments were combined with meta-analysis of published whole-DRG RNA-seq datasets from prominent rat PNI models. Striking differences were found between transcriptional responses to PNI and SCI, especially in regeneration-associated genes (RAGs) and long noncoding RNAs (lncRNAs). Many transcriptomic changes after SCI also were found after corresponding sham surgery, indicating they were caused by injury to surrounding tissue, including bone and muscle, rather than to the spinal cord itself. Another unexpected finding was of few transcriptomic similarities between rat neuropathic pain models and the only reported transcriptional analysis of human DRGs linked to neuropathic pain. These findings show that DRGs exhibit complex transcriptional responses to central and peripheral neural injury and associated tissue damage. Although only a few genes in DRG cells exhibited similar changes in expression across all the painful conditions examined here, these genes may represent a core set whose transcription in various DRG cell types is sensitive to significant bodily injury, and which may play a fundamental role in promoting neuropathic pain.

Phytol from Faeces Bombycis alleviated migraine pain by inhibiting Nav1.7 sodium channels

ETHNOPHARMACOLOGICAL RELEVANCE

Faeces Bombycis (silkworm excrement, called Cansha in Chinese), is the dried faeces of the larvae of silkworm. According to the theories of traditional Chinese medicine recorded in "Compendium of Materia Medica", Faeces Bombycis has often been prescribed in traditional Chinese medicine for the treatment of recurrent headache, rheumatalgia, rubella and itching et al. However, the bioactive components and their exact mechanisms underlying the pain-relieving effects remain to be revealed.

AIM OF THE STUDY

The present study aimed to evaluate the analgesic effect of Faeces Bombycis extract (FBE) on migraine, explore the main active constituents and investigate the pharmacological mechanisms for its pain relief.

MATERIALS AND METHODS

The bioactivity of different extracts from Faeces Bombycis was tracked by the nitroglycerin (NTG)-induced migraine model on rats and identified by NMR spectroscopic data. Whole-cell patch clamp technique, an electrophysiological method, was used to screen the potential targets and study the mechanism of action for the bioactive compound. The following targets have been screened and studied, including Nav1.7 sodium channels, Nav1.8 sodium channels, TRPV1 channels and TRPA1 channels. The trigeminal ganglion neurons were further used to study the effects of the identified compound on neuronal excitability.

RESULTS

By testing the bioactivity of the different extracts proceedingly, fraction petroleum ether showed higher anti-migraine activity. Through further step-by-step isolations, 7 compounds were isolated. Among them, phytol was identified with the highest yield and displayed a potent anti-migraine effect. By screening the potential ion channel targets for migraine, phytol was found to preferentially block the inactivated state of Nav1.7 sodium channels with half-inhibition concentration 0.32 ± 0.05 μM. Thus, the effects of phytol on the biophysical properties of Nav1.7 sodium channels were further characterized. Phytol induced a hyperpolarizing shift of voltage-dependent inactivation and slowed the recovery from inactivation. The affinity of phytol became weaker in the inactivation-deficient Nav1.7 channels (Nav1.7-WCW). And such an effect was independent on the local anesthetic site (Nav1.7 F1737A). Consistent with the data from recombinant channels, the compound also displayed state-dependent inhibition on neuronal sodium channels and further decreased the neuronal excitability in trigeminal ganglion neurons. Moreover, besides Nav1.7 channel, phytol also antagonized the activation of TRPV1 and TRPA1 channels at micromolar concentrations with a weaker affinity.

CONCLUSION

Our results demonstrated that phytol is the major anti-migraine ingredient of Faeces Bombycis and alleviates migraine behaviors by acting on Nav1.7 sodium channels in the trigeminal ganglion neurons. This study provided evidences for the therapeutic application of Faeces Bombycis and phytol on migraine disease.

Systemic sclerosis gastrointestinal dysmotility: risk factors, pathophysiology, diagnosis and management

Nearly all patients with systemic sclerosis (SSc) are negatively affected by dysfunction in the gastrointestinal tract, and the severity of gastrointestinal disease in SSc correlates with high mortality. The clinical complications of this dysfunction are heterogeneous and include gastro-oesophageal reflux disease, gastroparesis, small intestinal bacterial overgrowth, intestinal pseudo-obstruction, malabsorption and the requirement for total parenteral nutrition. The abnormal gastrointestinal physiology that promotes the clinical manifestations of SSc gastrointestinal disease throughout the gastrointestinal tract are diverse and present a range of therapeutic targets. Furthermore, the armamentarium of medications and non-pharmacological interventions that can benefit affected patients has substantially expanded in the past 10 years, and research is increasingly focused in this area. Here, we review the details of the gastrointestinal complications in SSc, tie physiological abnormalities to clinical manifestations, detail the roles of standard and novel therapies and lay a foundation for future investigative work.

CGRP-dependent sensitization of PKC-δ positive neurons in central amygdala mediates chronic migraine

BACKGROUND

To investigate specific brain regions and neural circuits that are responsible for migraine chronification.

METHODS

We established a mouse model of chronic migraine with intermittent injections of clinically-relevant dose of nitroglycerin (0.1 mg/kg for 9 days) and validated the model with cephalic and extracephalic mechanical sensitivity, calcitonin gene-related peptide (CGRP) expression in trigeminal ganglion, and responsiveness to sumatriptan or central CGRP blockade. We explored the neurons that were sensitized along with migraine chronification and investigated their roles on migraine phenotypes with chemogenetics.

RESULTS

After repetitive nitroglycerin injections, mice displayed sustained supraorbital and hind paw mechanical hyperalgesia, which lasted beyond discontinuation of nitroglycerin infusion and could be transiently reversed by sumatriptan. The CGRP expression in trigeminal ganglion was also upregulated. We found the pERK positive cells were significantly increased in the central nucleus of the amygdala (CeA), and these sensitized cells in the CeA were predominantly protein kinase C-delta (PKC-δ) positive neurons co-expressing CGRP receptors. Remarkably, blockade of the parabrachial nucleus (PBN)-CeA CGRP neurotransmission by CGRP_8-37 microinjection to the CeA attenuated the sustained cephalic and extracephalic mechanical hyperalgesia. Furthermore, chemogenetic silencing of the sensitized CeA PKC-δ positive neurons reversed the mechanical hyperalgesia and CGRP expression in the trigeminal ganglion. In contrast, repetitive chemogenetic activation of the CeA PKC-δ positive neurons recapitulated chronic migraine-like phenotypes in naïve mice.

CONCLUSIONS

Our data suggest that CeA PKC-δ positive neurons innervated by PBN CGRP positive neurons might contribute to the chronification of migraine, which may serve as future therapeutic targets for chronic migraine.

Identification of the NRF2 transcriptional network as a therapeutic target for trigeminal neuropathic pain

Trigeminal neuralgia, historically dubbed the "suicide disease," is an exceedingly painful neurologic condition characterized by sudden episodes of intense facial pain. Unfortunately, the only U.S. Food and Drug Administration (FDA)-approved medication for trigeminal neuralgia carries substantial side effects, with many patients requiring surgery. Here, we identify the NRF2 transcriptional network as a potential therapeutic target. We report that cerebrospinal fluid from patients with trigeminal neuralgia accumulates reactive oxygen species, several of which directly activate the pain-transducing channel TRPA1. Similar to our patient cohort, a mouse model of trigeminal neuropathic pain also exhibits notable oxidative stress. We discover that stimulating the NRF2 antioxidant transcriptional network is as analgesic as inhibiting TRPA1, in part by reversing the underlying oxidative stress. Using a transcriptome-guided drug discovery strategy, we identify two NRF2 network modulators as potential treatments. One of these candidates, exemestane, is already FDA-approved and may thus be a promising alternative treatment for trigeminal neuropathic pain.

Galcanezumab modulates Capsaicin-induced C-fiber reactivity

Background

The vasodilatory calcitonin-gene related peptide (CGRP) is understood as pivotal mediator in migraine pathophysiology. Blocking CGRP with small molecules or monoclonal antibodies (CGRP-mAb) reduces migraine frequency. However, prescription of CGRP-mAbs is still regulated and possible predictive measures of therapeutic success would be useful.

Methods

Using standardized capsaicin-induced dermal blood flow model, 29 migraine patients underwent a laser speckle imaging measurement before and after administration of galcanezumab. At both sessions dermal blood flow before and after capsaicin stimulation as well as flare size were analyzed over all three trigeminal branches and the volar forearm for extracranial control. Long-term measures were repeated in 14 patients after continuous treatment ranging from 6 to 12 months.

Results

Resting dermal blood flow remained unchanged after administration of galcanezumab. Capsaicin-induced dermal blood flow decreased significantly after CGRP-mAb in all tested areas compared to baseline and this was consistent even after 12 months of treatment. However, following galcanezumab administration, the flare size decreased only in the three trigeminal dermatomes, not the arm and was therefore specific for the trigemino-vascular system. None of these two markers distinguished between responders and non-responders.

Conclusion

CGRP-mAb changed blood flow response to capsaicin stimulation profoundly and this effect did not change over a 12-month application. Neither capsaicin-induced flare nor dermal blood flow can be used as a predictor for treatment efficacy. These data suggest that the mechanism of headache development in migraine is not entirely CGRP-mediated.

Decoding gene expression signatures in mice trigeminal ganglion across trigeminal neuropathic pain stages via high-throughput sequencing

Trigeminal neuropathic pain (TNP) arises due to peripheral nerve injury, the mechanisms underlying which are little known. The altered gene expression profile in sensory ganglia is critical for neuropathic pain generation and maintenance. We, therefore, assessed the transcriptome of the trigeminal ganglion (TG) from mice at different periods of pain progression. Trigeminal neuropathic pain was established by partial infraorbital nerve transection (pIONT). High-throughput RNA sequencing was applied to detect the mRNA profiles of TG collected at 3 and 10 days after modeling. Injured TG displayed dramatically altered mRNA expression profiles compared to Sham. Different gene expression profiles were obtained at 3 and 10 days after pIONT. Moreover, 314 genes were significantly upregulated, and 81 were significantly downregulated at both 3 and 10 days post-pIONT. Meanwhile, enrichment analysis of these persistent differentially expressed genes (DEGs) showed that the MAPK pathway was the most significantly enriched pathway for upregulated DEGs, validated by immunostaining. In addition, TG cell populations defined by single-nuclei RNA sequencing displayed cellular localization of DEGs at a single-cell resolution. Protein-protein interaction (PPI) and sub-PPI network analyses constructed networks and identified the top 10 hub genes for DEGs at different time points. The present data provide novel information on the gene expression signatures of TG during the development and maintenance phases of TNP, and the identified hub genes and pathways may serve as potential targets for treatment.

cytoNet: Spatiotemporal network analysis of cell communities

We introduce cytoNet, a cloud-based tool to characterize cell populations from microscopy images. cytoNet quantifies spatial topology and functional relationships in cell communities using principles of network science. Capturing multicellular dynamics through graph features, cytoNet also evaluates the effect of cell-cell interactions on individual cell phenotypes. We demonstrate cytoNet’s capabilities in four case studies: 1) characterizing the temporal dynamics of neural progenitor cell communities during neural differentiation, 2) identifying communities of pain-sensing neurons in vivo, 3) capturing the effect of cell community on endothelial cell morphology, and 4) investigating the effect of laminin α4 on perivascular niches in adipose tissue. The analytical framework introduced here can be used to study the dynamics of complex cell communities in a quantitative manner, leading to a deeper understanding of environmental effects on cellular behavior. The versatile, cloud-based format of cytoNet makes the image analysis framework accessible to researchers across domains.

Prolonged PGE2 treatment increased TTX-sensitive but not TTX-resistant sodium current in trigeminal ganglionic neurons

Prostaglandin E₂ (PGE₂) is an important inflammatory mediator for the initiation and maintenance of inflammatory and neuropathic pain. The acute effect of PGE₂ on sodium currents has been widely characterized in sensory neurons; however, the prolonged effect of PGE₂ remains to be determined. Here, we performed patch clamp recordings to evaluate the acute and prolonged effects of PGE₂ on sodium currents in trigeminal ganglionic (TG) neurons from male Sprague-Dawley rats. We found that 24-h treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 31% in a voltage-dependent manner and shifted the activation curve in a hyperpolarized direction but did not affect steady-state inactivation. Furthermore, treatment with PGE₂ for 24 h increased the current density of tetrodotoxin-sensitive (TTX-S) but not TTX-resistant (TTX-R) channels significantly. Interestingly, TTX-S current was increased mostly in medium-sized, but not in small-sized, neurons after 24 h of treatment with PGE₂. Moreover, the mRNA level of TTX-S Nav1.1 but not TTX-R Nav1.8 or Nav1.9 was significantly increased after 24 h of treatment with PGE_2. In contrast, 5-min treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 29% and increased TTX-R sodium currents, but not TTX-S currents, in both small- and medium-sized TG neurons. Our results presented a differential regulation of subtypes of sodium channels by acute and prolonged treatments of PGE₂, which may help to better understand the mechanism of PGE₂-mediated orofacial pain development.

Histone methylation-mediated microRNA-32-5p down-regulation in sensory neurons regulates pain behaviors via targeting Cav3.2 channels

In this study, we identify microRNA-32-5p (miR-32-5p) as a key functional noncoding RNA in trigeminal-mediated neuropathic pain. We report that injury-induced histone methylation attenuates the binding of glucocorticoid receptor to the promoter region of the miR-32-5p gene and decreases the expression of miR-32-5p, in turn promoting the development of neuropathic pain through regulation of Cav3.2 channels. miRNA-mediated gene regulation has been proposed as a therapeutic approach in neuropathic pain. Our findings identify miR-32-5p replenishment as a therapeutic strategy for treating chronic neuropathic pain.

microRNA (miRNA)–mediated gene regulation has been studied as a therapeutic approach, but its functional regulatory mechanism in neuropathic pain is not well understood. Here, we identify that miRNA-32-5p (miR-32-5p) is a functional RNA in regulating trigeminal-mediated neuropathic pain. High-throughput sequencing and qPCR analysis showed that miR-32-5p was the most down-regulated miRNA in the injured trigeminal ganglion (TG) of rats. Intra-TG injection of miR-32-5p agomir or overexpression of miR-32-5p by lentiviral delivery in neurons of the injured TG attenuated established trigeminal neuropathic pain. miR-32-5p overexpression did not affect acute physiological pain, while miR-32-5p down-regulation in intact rats was sufficient to cause pain-related behaviors. Nerve injury increased the methylated histone occupancy of binding sites for the transcription factor glucocorticoid receptor in the miR-32-5p promoter region. Inhibition of the enzymes that catalyze H3K9me2 and H3K27me3 restored the expression of miR-32-5p and markedly attenuated pain behaviors. Further, miR-32-5p–targeted Cav3.2 T-type Ca²⁺ channels and decreased miR-32-5p associated with neuropathic pain caused an increase in Cav3.2 protein expression and T-type channel currents. Conversely, miR-32-5p overexpression in injured TG suppressed the increased expression of Cav3.2 and reversed mechanical allodynia. Together, we conclude that histone methylation-mediated miR-32-5p down-regulation in TG neurons regulates trigeminal neuropathic pain by targeting Cav3.2 channels.

Epigenome-wide DNA methylation profiling of conditioned pain modulation in individuals with non-specific chronic low back pain

BACKGROUND

The pathoanatomic cause of chronic low back pain (cLBP) cannot be identified for up to 90% of individuals. However, dysfunctional processing of endogenous nociceptive input, measured as conditioned pain modulation (CPM), has been associated with cLBP and may involve changes in neuronal gene expression. Epigenetic-induced changes such as DNA methylation (DNAm) have been associated with cLBP.

METHODS

In the present study, the relationship between CPM and DNAm changes in a sample of community-dwelling adults with nonspecific cLBP (n = 48) and pain-free controls (PFC; n = 50) was examined using reduced representation bisulfite sequencing. Gene ontology (GO) term enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were applied to identify key pathways involved in efficient versus deficient CPM.

RESULTS

Based on CPM efficiency, we identified 6006 and 18,305 differentially methylated CpG sites (DMCs) with q values < 0.01 among individuals with cLBP and PFCs, respectively. Most of the DMCs were hypomethylated and annotated to genes of relevance to pain, including OPRM1, ADRB2, CACNA2D3, GNA12, LPL, NAXD, and ASPHD1. In both cLBP and PFC groups, the DMCs annotated genes enriched many GO terms relevant to pain processing, including transcription regulation by RNA polymerase II, nervous system development, generation of neurons, neuron differentiation, and neurogenesis. Both groups also enriched the pathways involved in Rap1-signaling, cancer, and dopaminergic neurogenesis. However, MAPK-Ras signaling pathways were enriched in the cLBP, not the PFC group.

CONCLUSIONS

This is the first study to investigate the genome-scale DNA methylation profiles of CPM phenotype in adults with cLBP and PFCs. Based on CPM efficiency, fewer DMC enrichment pathways were unique to the cLBP than the PFCs group. Our results suggest that epigenetically induced modification of neuronal development/differentiation pathways may affect CPM efficiency, suggesting novel potential therapeutic targets for central sensitization. However, CPM efficiency and the experience of nonspecific cLBP may be independent. Further mechanistic studies are required to confirm the relationship between CPM, central sensitization, and nonspecific cLBP.

Nectin-1 and Non-muscle Myosin Heavy Chain-IIB: Major Mediators of Herpes Simplex Virus-1 Entry Into Corneal Nerves

Herpes Simplex Virus 1 (HSV-1) invades corneal nerves upon its infection of the cornea and then establishes latency in the trigeminal ganglion (TG). The latent virus in TG is often reactivated and travels back to the cornea, causing recurrent herpes simplex keratitis (HSK). The entry of HSV-1 into the corneal nerve is considered the initial step of infection resulting in HSV-1 latency and HSK recurrence. Several gD and gB receptors have been identified, including nectin-1, herpes virus entry medium (HVEM) and 3-O-sulfated heparan sulfate (3-OS-HS) as gD receptors, and non-muscle myosin heavy chain IIA (NMHC-IIA), NMHC-IIB and myelin-associated glycoprotein (MAG) as gB receptors. However, which receptors contribute to the entry of HSV-1 into corneal nerves are yet to be determined. This study observed that receptors nectin-1, HVEM, 3-OS-HS, NMHC-IIA, and NMHC-IIB, not MAG, were expressed in healthy corneal nerves. Further, we cultured TG neurons extracted from mice in vitro to screen for functional gD/gB receptors. Both in vitro siRNA knockdown and in vivo antibody blocking of either nectin-1 or NMHC-IIB reduced the entry and the replication of HSV-1 as shown by qPCR analysis and immunofluorescence measure, respectively. Also, we observed that the re-localization and the upregulation expression of NMHC-IIB after HSV-1 exposure were inhibited when gD receptor nectin-1 was knocked down. These data suggest that nectin-1 was the main gD receptor and NMHC-IIB was the main gB receptor in mediating HSV-1 entry and hold promise as therapeutic targets for resolving HSV-1 latency and HSK recurrence.

A compendium of validated pain genes

The development of novel pain therapeutics hinges on the identification and rigorous validation of potential targets. Model organisms provide a means to test the involvement of specific genes and regulatory elements in pain. Here we provide a list of genes linked to pain-associated behaviors. We capitalize on results spanning over three decades to identify a set of 242 genes. They support a remarkable diversity of functions spanning action potential propagation, immune response, GPCR signaling, enzymatic catalysis, nucleic acid regulation, and intercellular signaling. Making use of existing tissue and single-cell high-throughput RNA sequencing datasets, we examine their patterns of expression. For each gene class, we discuss archetypal members, with an emphasis on opportunities for additional experimentation. Finally, we discuss how powerful and increasingly ubiquitous forward genetic screening approaches could be used to improve our ability to identify pain genes. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Molecular and Cellular Physiology.

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.

Cisplatin induces TRPA1-mediated mechanical allodynia in the oral mucosa

OBJECTIVE

Cisplatin, a platinum-based anticancer drug, produces reactive oxygen species (ROS) in many cell types and induces mechanical allodynia in the hands and/or feet (chemotherapy-induced painful neuropathy: CIPN). In this study, we examined the possibility of inducing neuropathy in the oral region using oral keratinocytes and rats.

METHODS

Human oral keratinocytes (HOKs) were used to evaluate ROS generation after cisplatin application by a ROS-reactive fluorescent assay. In rats, after cisplatin administrations (two times), the trigeminal ganglion (TG) was investigated by electron microscopy and quantitative RT-PCR. Using our proprietary assay system, oral pain-related behaviors were observed in cisplatin-treated rats.

RESULTS

In rats, cisplatin administration reduced food intake and body weight. In electron microscopic analysis, glycogen granules in the TG were depleted following administration, although organelles were intact. In HOK cells, cisplatin significantly increased ROS generation with cell death, similar to glycolysis inhibitors. Cisplatin administration did not show any effects on Trpa1 mRNA levels in the TG. However, the same procedure induced hypersensitivity to mechanical stimulation and the TRPA1 agonist allyl isothiocyanate in the oral mucosa. Mechanical hypersensitivity was inhibited by the antioxidative drug α-lipoic acid and the TRPA1 antagonist HC-030031, similar to that of the hind paw.

CONCLUSION

The present findings suggest that cisplatin induces TRPA1-mediated CIPN due to ROS generation in the oral region. This study will provide a better understanding of persistent oral pain in cancer patients.

Transient Receptor Potential Channel 4 Small-Molecule Inhibition Alleviates Migraine-Like Behavior in Mice

Migraine is a common neurological disorder with few available treatment options. Recently, we have demonstrated the role of transient receptor potential cation channel subfamily C member 4 (TRPC4) in itch and the modulation of the calcitonin gene-related peptide (CGRP), a biomarker and emerging therapeutic target for migraine. In this study, we characterized the role of TRPC4 in pain and evaluated its inhibition as anti-migraine pain therapy in preclinical mouse models. First, we found that TRPC4 is highly expressed in trigeminal ganglia and its activation not only mediates itch but also pain. Second, we demonstrated that the small-molecule inhibitor ML204, a specific TRPC4 antagonist, significantly reduced episodic and chronic migraine-like behaviors in male and female mice after injection of nitroglycerin (NTG), a well-known migraine inducer in rodents and humans. Third, we found a significant decrease in CGRP protein levels in the plasma of both male and female mice treated with ML-204, which largely prevented the development of chronic migraine-like behavior. Using sensory neuron cultures, we confirmed that activation of TRPC4 elicited release of CGRP, which was significantly diminished by ML-204. Collectively, our findings identify TRPC4 in peripheral sensory neurons as a mediator of CGRP release and NTG-evoked migraine. Since a TRPC4 antagonist is already in clinical trials, we expect that this study will rapidly lead to novel and effective clinical treatments for migraineurs.

Patch-seq of mouse DRG neurons reveals candidate genes for specific mechanosensory functions

A variety of mechanosensory neurons are involved in touch, proprioception, and pain. Many molecular components of the mechanotransduction machinery subserving these sensory modalities remain to be discovered. Here, we combine recordings of mechanosensitive (MS) currents in mechanosensory neurons with single-cell RNA sequencing. Transcriptional profiles are mapped onto previously identified sensory neuron types to identify cell-type correlates between datasets. Correlation of current signatures with single-cell transcriptomes provides a one-to-one correspondence between mechanoelectric properties and transcriptomically defined neuronal populations. Moreover, a gene-expression differential comparison provides a set of candidate genes for mechanotransduction complexes. Piezo2 is expectedly found to be enriched in rapidly adapting MS current-expressing neurons, whereas Tmem120a and Tmem150c, thought to mediate slow-type MS currents, are uniformly expressed in all mechanosensory neuron subtypes. Further knockdown experiments disqualify them as mediating MS currents in sensory neurons. This dataset constitutes an open resource to explore further the cell-type-specific determinants of mechanosensory properties.

Pituitary hormones are specifically expressed in trigeminal sensory neurons and contribute to pain responses in the trigeminal system

Trigeminal (TG), dorsal root (DRG), and nodose/jugular (NG/JG) ganglia each possess specialized and distinct functions. We used RNA sequencing of two-cycle sorted Pirt-positive neurons to identify genes exclusively expressing in L3-L5 DRG, T10-L1 DRG, NG/JG, and TG mouse ganglion neurons. Transcription factor Phox2b and Efcab6 are specifically expressed in NG/JG while Hoxa7 is exclusively present in both T10-L1 and L3-L5 DRG neurons. Cyp2f2, Krt18, and Ptgds, along with pituitary hormone prolactin (Prl), growth hormone (Gh), and proopiomelanocortin (Pomc) encoding genes are almost exclusively in TG neurons. Immunohistochemistry confirmed selective expression of these hormones in TG neurons and dural nerves; and showed GH expression in subsets of TRPV1+ and CGRP+ TG neurons. We next examined GH roles in hypersensitivity in the spinal versus trigeminal systems. Exogenous GH produced mechanical hypersensitivity when injected intrathecally, but not intraplantarly. GH-induced thermal hypersensitivity was not detected in the spinal system. GH dose-dependently generated orofacial and headache-like periorbital mechanical hypersensitivity after administration into masseter muscle and dura, respectively. Periorbital mechanical hypersensitivity was reversed by a GH receptor antagonist, pegvisomant. Overall, pituitary hormone genes are selective for TG versus other ganglia somatotypes; and GH has distinctive functional significance in the trigeminal versus spinal systems.

Sensory-specific peripheral nerve pathology in a rat model of Fabry disease

Fabry disease (FD) causes life-long pain, the mechanisms of which are unclear. Patients with FD have chronic pain that mirrors symptoms of other painful peripheral neuropathies. However, it is unclear what underlying damage occurs in FD peripheral nerves that may contribute to chronic pain. Here, we characterized myelinated and unmyelinated fiber pathology in peripheral nerves of a rat model of FD. Decreased nerve fiber density and increased nerve fiber pathology were noted in unmyelinated and myelinated fibers from FD rats; both observations were dependent on sampled nerve fiber modality and anatomical location. FD myelinated axons exhibited lipid accumulations that were determined to be the FD-associated lipid globotriaosylceramide (Gb3), and to a lesser extent lysosomes. These findings suggest that axonal Gb3 accumulation may drive peripheral neuron dysfunction and subsequent pain in FD.

Chemokines and Pain in the Trigeminal System

Chemotactic cytokines or chemokines are a large family of secreted proteins able to induce chemotaxis. Chemokines are categorized according to their primary amino acid sequence, and in particular their cysteine residues that form disulphide bonds to maintain the structure: CC, CXC, CX3C, and XC, in which X represents variable amino acids. Among their many roles, chemokines are known to be key players in pain modulation in the peripheral and central nervous systems. Thus, they are promising candidates for novel therapeutics that could replace current, often ineffective treatments. The spinal and trigeminal systems are intrinsically different beyond their anatomical location, and it has been suggested that there are also differences in their sensory mechanisms. Hence, understanding the different mechanisms involved in pain modulation for each system could aid in developing appropriate pharmacological alternatives. Here, we aim to describe the current landscape of chemokines that have been studied specifically with regard to trigeminal pain. Searching PubMed and Google Scholar, we identified 30 reports describing chemokines in animal models of trigeminal pain, and 15 reports describing chemokines involved in human pain associated with the trigeminal system. This review highlights the chemokines studied to date at different levels of the trigeminal system, their cellular localization and, where available, their role in a variety of animal pain models.

A peptide encoded within a 5' untranslated region promotes pain sensitization in mice

ABSTRACT

Translational regulation permeates neuronal function. Nociceptors are sensory neurons responsible for the detection of harmful stimuli. Changes in their activity, termed plasticity, are intimately linked to the persistence of pain. Although inhibitors of protein synthesis robustly attenuate pain-associated behavior, the underlying targets that support plasticity are largely unknown. Here, we examine the contribution of protein synthesis in regions of RNA annotated as noncoding. Based on analyses of previously reported ribosome profiling data, we provide evidence for widespread translation in noncoding transcripts and regulatory regions of mRNAs. We identify an increase in ribosome occupancy in the 5' untranslated regions of the calcitonin gene-related peptide (CGRP/Calca). We validate the existence of an upstream open reading frame (uORF) using a series of reporter assays. Fusion of the uORF to a luciferase reporter revealed active translation in dorsal root ganglion neurons after nucleofection. Injection of the peptide corresponding to the calcitonin gene-related peptide-encoded uORF resulted in pain-associated behavioral responses in vivo and nociceptor sensitization in vitro. An inhibitor of heterotrimeric G protein signaling blocks both effects. Collectively, the data suggest pervasive translation in regions of the transcriptome annotated as noncoding in dorsal root ganglion neurons and identify a specific uORF-encoded peptide that promotes pain sensitization through GPCR signaling.

Glia and Orofacial Pain: Progress and Future Directions

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

Increase in trigeminal ganglion neurons that respond to both calcitonin gene-related peptide and pituitary adenylate cyclase-activating polypeptide in mouse models of chronic migraine and posttraumatic headache

A large body of animal and human studies indicates that blocking peripheral calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) signaling pathways may prevent migraine episodes and reduce headache frequency. To investigate whether recurring migraine episodes alter the strength of CGRP and PACAP signaling in trigeminal ganglion (TG) neurons, we compared the number of TG neurons that respond to CGRP and to PACAP (CGRP-R and PACAP-R, respectively) under normal and chronic migraine-like conditions. In a mouse model of chronic migraine, repeated nitroglycerin (NTG) administration significantly increased the number of CGRP-R and PACAP-R neurons in TG but not dorsal root ganglia. In TG neurons that express endogenous αCGRP, repeated NTG led to a 7-fold increase in the number of neurons that respond to both CGRP and PACAP (CGRP-R&PACAP-R). Most of these neurons were unmyelinated C-fiber nociceptors. This suggests that a larger fraction of CGRP signaling in TG nociceptors may be mediated through the autocrine mechanism, and the release of endogenous αCGRP can be enhanced by both CGRP and PACAP signaling pathways under chronic migraine condition. The number of CGRP-R&PACAP-R TG neurons was also increased in a mouse model of posttraumatic headache (PTH). Interestingly, low-dose interleukin-2 treatment, which completely reverses chronic migraine-related and PTH-related behaviors in mouse models, also blocked the increase in both CGRP-R and PACAP-R TG neurons. Together, these results suggest that inhibition of both CGRP and PACAP signaling in TG neurons may be more effective in treating chronic migraine and PTH than targeting individual signaling pathways.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Insights Into Translatomics in the Nervous System

Most neurological disorders are caused by abnormal gene translation. Generally, dysregulation of elements involved in the translational process disrupts homeostasis in neurons and neuroglia. Better understanding of how the gene translation process occurs requires detailed analysis of transcriptomic and proteomic profile data. However, a lack of strictly direct correlations between mRNA and protein levels limits translational investigation by combining transcriptomic and proteomic profiling. The much better correlation between proteins and translated mRNAs than total mRNAs in abundance and insufficiently sensitive proteomics approach promote the requirement of advances in translatomics technology. Translatomics which capture and sequence the mRNAs associated with ribosomes has been effective in identifying translational changes by genetics or projections, ribosome stalling, local translation, and transcript isoforms in the nervous system. Here, we place emphasis on the main three translatomics methods currently used to profile mRNAs attached to ribosome-nascent chain complex (RNC-mRNA). Their prominent applications in neurological diseases including glioma, neuropathic pain, depression, fragile X syndrome (FXS), neurodegenerative disorders are outlined. The content reviewed here expands our understanding on the contributions of aberrant translation to neurological disease development.

Pharmacological Manipulation of Translation as a Therapeutic Target for Chronic Pain

Dysfunction in regulation of mRNA translation is an increasingly recognized characteristic of many diseases and disorders, including cancer, diabetes, autoimmunity, neurodegeneration, and chronic pain. Approximately 50 million adults in the United States experience chronic pain. This economic burden is greater than annual costs associated with heart disease, cancer, and diabetes combined. Treatment options for chronic pain are inadequately efficacious and riddled with adverse side effects. There is thus an urgent unmet need for novel approaches to treating chronic pain. Sensitization of neurons along the nociceptive pathway causes chronic pain states driving symptoms that include spontaneous pain and mechanical and thermal hypersensitivity. More than a decade of preclinical research demonstrates that translational mechanisms regulate the changes in gene expression that are required for ongoing sensitization of nociceptive sensory neurons. This review will describe how key translation regulation signaling pathways, including the integrated stress response, mammalian target of rapamycin, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase-interacting kinases, impact the translation of different subsets of mRNAs. We then place these mechanisms of translation regulation in the context of chronic pain states, evaluate currently available therapies, and examine the potential for developing novel drugs. Considering the large body of evidence now published in this area, we propose that pharmacologically manipulating specific aspects of the translational machinery may reverse key neuronal phenotypic changes causing different chronic pain conditions. Therapeutics targeting these pathways could eventually be first-line drugs used to treat chronic pain disorders. SIGNIFICANCE STATEMENT: Translational mechanisms regulating protein synthesis underlie phenotypic changes in the sensory nervous system that drive chronic pain states. This review highlights regulatory mechanisms that control translation initiation and how to exploit them in treating persistent pain conditions. We explore the role of mammalian/mechanistic target of rapamycin and mitogen-activated protein kinase-interacting kinase inhibitors and AMPK activators in alleviating pain hypersensitivity. Modulation of eukaryotic initiation factor 2α phosphorylation is also discussed as a potential therapy. Targeting specific translation regulation mechanisms may reverse changes in neuronal hyperexcitability associated with painful conditions.

De novo protein synthesis is necessary for priming in preclinical models of migraine

BACKGROUND

Migraine attacks are often triggered by normally innocuous stimuli, suggesting that sensitization within the nervous system is present. One mechanism that may contribute to neuronal sensitization in this context is translation regulation of new protein synthesis. The goal of this study was to determine whether protein synthesis contributes to behavioral responses and priming in preclinical models of migraine.

METHODS

Mice received a dural injection of interleukin-6 in the absence or presence of the protein synthesis inhibitor anisomycin or the translation initiation inhibitor 4EGI-1 and were tested for facial hypersensitivity. Upon returning to baseline, mice were given a second, non-noxious dural injection of pH 7.0 to test for priming. Additionally, eIF4E^S209Amice lacking phosphorylation of mRNA cap-binding protein eIF4E received dural interleukin-6 or were subjected to repeated restraint stress and then tested for facial hypersensitivity. After returning to baseline, mice were given either dural pH 7.0 or a systemic sub-threshold dose of the nitric oxide donor sodium nitroprusside and tested for priming.

RESULTS

Dural injection of interleukin-6 in the presence of anisomycin or 4EGI-1 or in eIF4E^S209Amice resulted in the partial attenuation of acute facial hypersensitivity and complete block of hyperalgesic priming. Additionally, hyperalgesic priming following repeated restraint stress was blocked in eIF4E^S209Amice.

CONCLUSIONS

These studies show that de novo protein synthesis regulated by activity-dependent translation is critical to the development of priming in two preclinical models of migraine. This suggests that targeting the regulation of protein synthesis may be a novel approach for new migraine treatment strategies.

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

Sensory fibers of the peripheral nervous system carry sensation from specific sense structures or use different tissues and organs as receptive fields, and convey this information to the central nervous system. In the head of vertebrates, each cranial sensory ganglia and associated nerves perform specific functions. Sensory ganglia are composed of different types of specialized neurons in which two broad categories can be distinguished, somatosensory neurons relaying all sensations that are felt and visceral sensory neurons sensing the internal milieu and controlling body homeostasis. While in the trunk somatosensory neurons composing the dorsal root ganglia are derived exclusively from neural crest cells, somato- and visceral sensory neurons of cranial sensory ganglia have a dual origin, with contributions from both neural crest and placodes. As most studies on sensory neurogenesis have focused on dorsal root ganglia, our understanding of the molecular mechanisms underlying the embryonic development of the different cranial sensory ganglia remains today rudimentary. However, using single-cell RNA sequencing, recent studies have made significant advances in the characterization of the neuronal diversity of most sensory ganglia. Here we summarize the general anatomy, function and neuronal diversity of cranial sensory ganglia. We then provide an overview of our current knowledge of the transcriptional networks controlling neurogenesis and neuronal diversification in the developing sensory system, focusing on cranial sensory ganglia, highlighting specific aspects of their development and comparing it to that of trunk sensory ganglia.

Sex Differences in Nociceptor Translatomes Contribute to Divergent Prostaglandin Signaling in Male and Female Mice

BACKGROUND

There are clinically relevant sex differences in acute and chronic pain mechanisms, but we are only beginning to understand their mechanistic basis. Transcriptome analyses of rodent whole dorsal root ganglion (DRG) have revealed sex differences, mostly in immune cells. We examined the transcriptome and translatome of the mouse DRG with the goal of identifying sex differences.

METHODS

We used translating ribosome affinity purification sequencing and behavioral pharmacology to test the hypothesis that in Nav1.8-positive neurons, most of which are nociceptors, translatomes would differ by sex.

RESULTS

We found 80 genes with sex differential expression in the whole DRG transcriptome and 66 genes whose messenger RNAs were sex differentially actively translated (translatome). We also identified different motifs in the 3' untranslated region of messenger RNAs that were sex differentially translated. In further validation studies, we focused on Ptgds, which was increased in the translatome of female mice. The messenger RNA encodes the prostaglandin PGD₂ synthesizing enzyme. We observed increased PTGDS protein and PGD₂ in female mouse DRG. The PTGDS inhibitor AT-56 caused intense pain behaviors in male mice but was only effective at high doses in female mice. Conversely, female mice responded more robustly to another major prostaglandin, PGE₂, than did male mice. PTGDS protein expression was also higher in female cortical neurons, suggesting that DRG findings may be generalizable to other nervous system structures.

CONCLUSIONS

Our results demonstrate sex differences in nociceptor-enriched translatomes and reveal unexpected sex differences in one of the oldest known nociceptive signaling molecule families, the prostaglandins.

Transcriptomic sex differences in sensory neuronal populations of mice

Many chronic pain conditions show sex differences in their epidemiology. This could be attributed to sex-dependent differential expression of genes (DEGs) involved in nociceptive pathways, including sensory neurons. This study aimed to identify sex-dependent DEGs in estrous female versus male sensory neurons, which were prepared by using different approaches and ganglion types. RNA-seq on non-purified sensory neuronal preparations, such as whole dorsal root ganglion (DRG) and hindpaw tissues, revealed only a few sex-dependent DEGs. Sensory neuron purification increased numbers of sex-dependent DEGs. These DEG sets were substantially influenced by preparation approaches and ganglion types [DRG vs trigeminal ganglia (TG)]. Percoll-gradient enriched DRG and TG neuronal fractions produced distinct sex-dependent DEG groups. We next isolated a subset of sensory neurons by sorting DRG neurons back-labeled from paw and thigh muscle. These neurons have a unique sex-dependent DEG set, yet there is similarity in biological processes linked to these different groups of sex-dependent DEGs. Female-predominant DEGs in sensory neurons relate to inflammatory, synaptic transmission and extracellular matrix reorganization processes that could exacerbate neuro-inflammation severity, especially in TG. Male-selective DEGs were linked to oxidative phosphorylation and protein/molecule metabolism and production. Our findings catalog preparation-dependent sex differences in neuronal gene expressions in sensory ganglia.

Exacerbated Headache-Related Pain in the Single Prolonged Stress Preclinical Model of Post-traumatic Stress Disorder

Chronic headache pain is one of the most commonly reported comorbid pain conditions with post-traumatic stress disorder (PTSD) patients and resistant to effective treatment, yet no combined preclinical model of the two disorders has been reported. Here, we used a modified chronic headache pain model to investigate the contribution of single prolonged stress (SPS) model of PTSD with sodium nitroprusside (SNP)-induced hyperalgesia. Injection of SNP (2 mg/kg, i.p.) occurred every other day from day 7 to day 15 after initiation of SPS in rats. Paw withdrawal threshold (PWT) to von Frey stimuli and tail flick latencies (TFL) dramatically decreased as early as 7 days after SPS and lasted until at least day 21. Basal PWT and TFL also significantly decreased during the SNP treatment period. The lower nociceptive thresholds recovered in 6 days following the final SNP injection in SNP group, but not in SPS + SNP group. Elevated nociceptin/OFQ (N/OFQ) levels observed in cerebrospinal fluid of SPS rats were even higher in SPS + SNP group. Glial fibrillary acidic protein (GFAP) and N/OFQ peptide (NOP) receptor mRNA expression increased in dorsal root ganglia (DRG) 21 days after SPS exposure; mRNA increases in the SPS/SNP group was more pronounced than SPS or SNP alone. GFAP protein expression was upregulated in trigeminal ganglia by SPS. Our results indicate that traumatic stress exaggerated chronic SNP-induced nociceptive hypersensitivity, and that N/OFQ and activated satellite glia cells may play an important role in the interaction between both conditions.

Sex-dependent pronociceptive role of spinal α5 -GABAA receptor and its epigenetic regulation in neuropathic rodents

Extrasynaptic α₅ -subunit containing GABA_A (α₅ -GABA_A ) receptors participate in chronic pain. Previously, we reported a sex difference in the action of α₅ -GABA_A receptors in dysfunctional pain. However, the underlying mechanisms remain unknown. The aim of this study was to examine this sexual dimorphism in neuropathic rodents and the mechanisms involved. Female and male Wistar rats or ICR mice were subjected to nerve injury followed by α₅ -GABA_A receptor inverse agonist intrathecal administration, L-655,708. The drug produced an antiallodynic effect in nerve-injured female rats and mice, and a lower effect in males. We hypothesized that changes in α₅ -GABA_A receptor, probably influenced by hormonal and epigenetic status, might underlie this sex difference. Thus, we performed qPCR and western blot. Nerve injury increased α₅ -GABA_A mRNA and protein in female dorsal root ganglia (DRG) and decreased them in DRG and spinal cord of males. To investigate the hormonal influence over α₅ -GABA_A receptor actions, we performed nerve injury to ovariectomized rats and reconstituted them with 17β-estradiol (E2). Ovariectomy abrogated L-655,708 antiallodynic effect and E2 restored it. Ovariectomy decreased α₅ -GABA_A receptor and estrogen receptor α protein in DRG of neuropathic female rats, while E2 enhanced them. Since DNA methylation might contribute to α₅ -GABA_A receptor down-regulation in males, we examined CpG island DNA methylation of α₅ -GABA_A receptor coding gene through pyrosequencing. Nerve injury increased methylation in male, but not female rats. Pharmacological inhibition of DNA methyltransferases increased α₅ -GABA_A receptor and enabled L-655,708 antinociceptive effect in male rats. These results suggest that α₅ -GABA_A receptor is a suitable target to treat chronic pain in females.

Bipolar Pulsed Radiofrequency for Trigeminal Neuralgia: A Report of Two Cases

BACKGROUND

Trigeminal neuralgia (TN) is the most common cause of facial pain, leading to significant disability and impacting a patient's quality of life. Percutaneous procedures like continuous radiofrequency, pulsed radiofrequency (PRF), and combined continuous and pulsed radiofrequency have been studied in past years comparing different voltages in order to find more effective therapies with fewer complications (eg, numbness and masseter muscle weakness). With regard to young patients, there is still insufficient evidence on the most appropriate procedure in this patient population. PRF does not cause thermal damage. The mechanism of action involves delivering an electrical field to targeted nerves or tissues, modulating pain. We propose that bipolar pulsed radiofrequency (2 parallel cannulas) in the trigeminal ganglion produce a denser and larger field resulting in more effective controlled pain.

CASE PRESENTATION

We present 2 cases of 40- and 48-year-old men with severe V2 to V3 TN who underwent bipolar PRF. We performed bipolar PRF on the trigeminal ganglion through the foramen ovale using two 22-gauge 100-mm cannulas with 10-mm active tips. Parameters used were voltage of 85 V, pulse width of 20 milliseconds, and total duration time of 6 minutes. Both patients reported complete relief of pain after the procedure, and at 2-year follow-up they were pain free and experienced a better quality of life. No complications were reported.

CONCLUSION

Bipolar PRF could be a non-neurodestructive option for young people with TN and deserves further investigation as a treatment option.

Neuroendocrine Mechanisms Governing Sex Differences in Hyperalgesic Priming Involve Prolactin Receptor Sensory Neuron Signaling

Many clinical and preclinical studies report higher prevalence and severity of chronic pain in females. We used hyperalgesic priming with interleukin 6 (IL-6) priming and PGE₂ as a second stimulus as a model for pain chronicity. Intraplantar IL-6 induced hypersensitivity was similar in magnitude and duration in both males and females, while both paw and intrathecal PGE₂ hypersensitivity was more persistent in females. This difference in PGE₂ response was dependent on both circulating estrogen and translation regulation signaling in the spinal cord. In males, the duration of hypersensitivity was regulated by testosterone. Since the prolactin receptor (Prlr) is regulated by reproductive hormones and is female-selectively activated in sensory neurons, we evaluated whether Prlr signaling contributes to hyperalgesic priming. Using ΔPRL, a competitive Prlr antagonist, and a mouse line with ablated Prlr in the Nav1.8 sensory neuronal population, we show that Prlr in sensory neurons is necessary for the development of hyperalgesic priming in female, but not male, mice. Overall, sex-specific mechanisms in the initiation and maintenance of chronic pain are regulated by the neuroendocrine system and, specifically, sensory neuronal Prlr signaling.SIGNIFICANCE STATEMENT Females are more likely to experience chronic pain than males, but the mechanisms that underlie this sex difference are not completely understood. Here, we demonstrate that the duration of mechanical hypersensitivity is dependent on circulating sex hormones in mice, where estrogen caused an extension of sensitivity and testosterone was responsible for a decrease in the duration of the hyperalgesic priming model of chronic pain. Additionally, we demonstrated that prolactin receptor expression in Nav1.8⁺ neurons was necessary for hyperalgesic priming in female, but not male, mice. Our work demonstrates a female-specific mechanism for the promotion of chronic pain involving the neuroendrocrine system and mediated by sensory neuronal prolactin receptor.