The Molecular Fingerprint of Dorsal Root and Trigeminal Ganglion Neurons

Trigeminal innervation and tactile responses in mouse tongue

The mammalian tongue is richly innervated with somatosensory, gustatory and motor fibers. These form the basis of many ethologically important functions such as eating, speaking and social grooming. Despite its high tactile acuity and sensitivity, the neural basis of tongue mechanosensation remains largely mysterious. Here we explored the organization of mechanosensory afferents in the tongue and found that each lingual papilla is innervated by Piezo2 + trigeminal neurons. Notably, each fungiform papilla contained highly specialized ring-like sensory neuron terminations that asymmetrically circumscribe the taste buds. Myelinated lingual afferents in the mouse lingual papillae did not form corpuscular sensory end organs but rather had only free nerve endings. In vivo single-unit recordings from the trigeminal ganglion revealed lingual low-threshold mechanoreceptors (LTMRs) with conduction velocities in the Aδ range or above and distinct adaptation properties ranging from intermediately adapting (IA) to rapidly adapting (RA). IA units were sensitive to both static indentation and stroking, while RA units had a preference for tangential forces applied by stroking. Lingual LTMRs were not directly responsive to rapid cooling or chemicals that can induce astringent or numbing sensations. Sparse labeling of lingual afferents in the tongue revealed distinct terminal morphologies and innervation patterns in fungiform and filiform papillae. Together, our results indicate that fungiform papillae are mechanosensory structures, while suggesting a simple model that links the functional and anatomical properties of tactile sensory neurons in the tongue.

Models of Herpes Simplex Virus Latency

Our current understanding of HSV latency is based on a variety of clinical observations, and in vivo, ex vivo, and in vitro model systems, each with unique advantages and drawbacks. The criteria for authentically modeling HSV latency include the ability to easily manipulate host genetics and biological pathways, as well as mimicking the immune response and viral pathogenesis in human infections. Although realistically modeling HSV latency is necessary when choosing a model, the cost, time requirement, ethical constraints, and reagent availability are also equally important. Presently, there remains a pressing need for in vivo models that more closely recapitulate human HSV infection. While the current in vivo, ex vivo, and in vitro models used to study HSV latency have limitations, they provide further insights that add to our understanding of latency. In vivo models have shed light on natural infection routes and the interplay between the host immune response and the virus during latency, while in vitro models have been invaluable in elucidating molecular pathways involved in latency. Below, we review the relative advantages and disadvantages of current HSV models and highlight insights gained through each.

Expression of MHC II in DRG neurons attenuates paclitaxel-induced cold hypersensitivity in male and female mice

Chemotherapy is often a life-saving treatment, but the development of intractable pain caused by chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting toxicity that restricts cancer survival rates. Recent reports demonstrate that paclitaxel (PTX) robustly increases anti-inflammatory CD4+ T cells in the dorsal root ganglion (DRG), and that T cells and anti-inflammatory cytokines are protective against CIPN. However, the mechanism by which CD4+ T cells are activated, and the extent cytokines released by CD4+ T cells target DRG neurons are unknown. Here, we are the first to detect major histocompatibility complex II (MHCII) protein in mouse DRG neurons and to find CD4+ T cells breaching the satellite glial cell barrier to be in close proximity to neurons, together suggesting CD4+ T cell activation and targeted cytokine release. MHCII protein is primarily expressed in small nociceptive neurons in male and female mouse DRG but increased after PTX in small nociceptive neurons in only female DRG. Reducing one copy of MHCII in small nociceptive neurons decreased anti-inflammatory IL-10 and IL-4 producing CD4+ T cells in naïve male DRG and increased their hypersensitivity to cold. Administration of PTX to male and female mice that lacked one copy of MHCII in nociceptive neurons decreased anti-inflammatory CD4+ T cells in the DRG and increased the severity of PTX-induced cold hypersensitivity. Collectively, our results demonstrate expression of MHCII protein in mouse DRG neurons, which modulates cytokine producing CD4+ T cells in the DRG and attenuates cold hypersensitivity during homeostasis and after PTX treatment.

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.

Bipolar radiofrequency ablation for cancer pain in the trigeminal distribution

Tumours in patients with head and neck cancer (HNC) are associated with a more significant decrease in quality of life compared with the rest of patients with cancer. We present a patient with pain due to HNC successfully treated with bipolar radiofrequency ablation. A man in his 70s presented with a tumour in the left V2 and V3 region, with disabling pain, Visual Analogue Scale (VAS) score of 10/10, pain on swallowing, chewing and speaking, 3 months of evolution. The patient was evaluated in the pain management department, and the interventional treatment proposed consisted of bipolar pulsed radiofrequency, followed by bipolar thermal radiofrequency of the left V2 and V3 branches with fluoroscopic guidance to achieve better control and coverage of the affected trigeminal branches. Immediately after the procedure, the patient reported a significant improvement in pain with a 0-10 VAS; hypoesthesia in the affected V2 and V3 territory was identified, but no motor weakness. The improvement in pain was maintained for 6 months with a significant improvement in quality of life and pain, which allowed him to speak, chew and swallow without pain. Later, the patient died from complications associated with the disease. The treatment approach in these patients is both pain treatment and achieving independence by allowing better speech ability and improving eating, the above as a pillar of treatment focused on improving the patient's quality of life. This approach is a potential tool in the early stage of the disease in patients with pain due to HNC.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Novel expression of major histocompatibility complex II in dorsal root ganglion neurons attenuates paclitaxel-induced cold hypersensitivity in male and female mice

Chemotherapy is often a life-saving treatment, but the development of intractable pain caused by chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting toxicity that restricts survival rates. Recent reports demonstrate that paclitaxel (PTX) robustly increases anti-inflammatory CD4+ T cells in the dorsal root ganglion (DRG), and that T cells and anti-inflammatory cytokines are protective against CIPN. However, the mechanism by which CD4+ T cells are activated, and the extent cytokines released by CD4+ T cells target DRG neurons are unknown. Here, we found novel expression of functional major histocompatibility complex II (MHCII) protein in DRG neurons, and CD4+ T cells in close proximity to DRG neurons, together suggesting CD4+ T cell activation and targeted cytokine release. MHCII protein is primarily expressed in small nociceptive neurons in male mouse DRG regardless of PTX, while MHCII is induced in small nociceptive neurons in female DRG after PTX. Accordingly, reducing MHCII in small nociceptive neurons increased hypersensitivity to cold only in naive male mice, but increased severity of PTX-induced cold hypersensitivity in both sexes. Collectively, our results demonstrate expression of MHCII on DRG neurons and a functional role during homeostasis and inflammation.

Culture of Primary Neurons from Dissociated and Cryopreserved Mouse Trigeminal Ganglion

Corneal nerves originate from the ophthalmic branch of the trigeminal nerve, which enters the cornea at the limbus radially from all directions toward the central cornea. The cell bodies of the sensory neurons of trigeminal nerve are located in the trigeminal ganglion (TG), while the axons are extended into the three divisions, including ophthalmic branch that supplies corneal nerves. Study of primary neuronal cultures established from the TG fibers can therefore provide a knowledge basis for corneal nerve biology and potentially be developed as an in vitro platform for drug testing. However, setting up primary neuron cultures from animal TG has been dubious with inconsistency among laboratories due to a lack of efficient isolation protocol, resulting in low yield and heterogenous cultures. In this study, we used a combined enzymatic digestion with collagenase and TrypLE to dissociate mouse TG while preserving nerve cell viability. A subsequent discontinuous Percoll density gradient followed by mitotic inhibitor treatment effectively diminished the contamination of non-neuronal cells. Using this method, we reproducibly generated high yield and homogenous primary TG neuron cultures. Similar efficiency of nerve cell isolation and culture was further obtained for TG tissue cryopreserved for short (1 week) and long duration (3 months), compared to freshly isolated tissues. In conclusion, this optimized protocol shows a promising potential to standardize TG nerve culture and generate a high-quality corneal nerve model for drug testing and neurotoxicity studies.

Molecular and Cellular Neurobiology of Circadian and Circannual Rhythms in Migraine: A Narrative Review

Migraine-a primary headache-has circadian and circannual rhythms in the onset of attacks. The circadian and circannual rhythms involve the hypothalamus, which is strongly associated with pain processing in migraines. Moreover, the role of melatonin in circadian rhythms has been implied in the pathophysiology of migraines. However, the prophylactic effect of melatonin in migraines is controversial. Calcitonin gene-related peptide (CGRP) has recently attracted attention in the pathophysiology and treatment of migraines. Pituitary adenylate cyclase-activating peptide (PACAP)-a neuropeptide identical to CGRP-is a potential therapeutic target after CGRP. PACAP is involved in the regulation of circadian entrainment to light. This review provides an overview of circadian and circannual rhythms in the hypothalamus and describes the relationship between migraines and the molecular and cellular neurobiology of circadian and circannual rhythms. Furthermore, the potential clinical applications of PACAP are presented.

EGR4 is critical for cell-fate determination and phenotypic maintenance of geniculate ganglion neurons underlying sweet and umami taste

The sense of taste starts with activation of receptor cells in taste buds by chemical stimuli which then communicate this signal via innervating oral sensory neurons to the CNS. The cell bodies of oral sensory neurons reside in the geniculate ganglion (GG) and nodose/petrosal/jugular ganglion. The geniculate ganglion contains two main neuronal populations: BRN3A+ somatosensory neurons that innervate the pinna and PHOX2B+ sensory neurons that innervate the oral cavity. While much is known about the different taste bud cell subtypes, considerably less is known about the molecular identities of PHOX2B+ sensory subpopulations. In the GG, as many as 12 different subpopulations have been predicted from electrophysiological studies, while transcriptional identities exist for only 3 to 6. Importantly, the cell fate pathways that diversify PHOX2B+ oral sensory neurons into these subpopulations are unknown. The transcription factor EGR4 was identified as being highly expressed in GG neurons. EGR4 deletion causes GG oral sensory neurons to lose their expression of PHOX2B and other oral sensory genes and up-regulate BRN3A. This is followed by a loss of chemosensory innervation of taste buds, a loss of type II taste cells responsive to bitter, sweet, and umami stimuli, and a concomitant increase in type I glial-like taste bud cells. These deficits culminate in a loss of nerve responses to sweet and umami taste qualities. Taken together, we identify a critical role of EGR4 in cell fate specification and maintenance of subpopulations of GG neurons, which in turn maintain the appropriate sweet and umami taste receptor cells.

Sensory Neuron-TRPV4 Modulates Temporomandibular Disorder Pain Via CGRP in Mice

Temporomandibular disorder (TMD) pain that involves inflammation and injury in the temporomandibular joint (TMJ) and/or masticatory muscle is the most common form of orofacial pain. We recently found that transient receptor potential vanilloid-4 (TRPV4) in trigeminal ganglion (TG) neurons is upregulated after TMJ inflammation, and TRPV4 coexpresses with calcitonin gene-related peptide (CGRP) in TMJ-innervating TG neurons. Here, we extended these findings to determine the specific contribution of TRPV4 in TG neurons to TMD pain, and examine whether sensory neuron-TRPV4 modulates TMD pain via CGRP. In mouse models of TMJ inflammation or masseter muscle injury, sensory neuron-Trpv4 conditional knockout (cKO) mice displayed reduced pain. Coexpression of TRPV4 and CGRP in TMJ- or masseter muscle-innervating TG neurons was increased after TMJ inflammation and masseter muscle injury, respectively. Activation of TRPV4-expressing TG neurons triggered secretion of CGRP, which was associated with increased levels of CGRP in peri-TMJ tissues, masseter muscle, spinal trigeminal nucleus, and plasma in both models. Local injection of CGRP into the TMJ or masseter muscle evoked acute pain in naïve mice, while blockade of CGRP receptor attenuated pain in mouse models of TMD. These results suggest that TRPV4 in TG neurons contributes to TMD pain by potentiating CGRP secretion. PERSPECTIVE: This study demonstrates that activation of TRPV4 in TG sensory neurons drives pain by potentiating the release of pain mediator CGRP in mouse models of TMJ inflammation and masseter muscle injury. Targeting TRPV4 and CGRP may be of clinical potential in alleviating TMD pain.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Endogenous Glycoprotein GPM6a Is Involved in Neurite Outgrowth in Rat Dorsal Root Ganglion Neurons

The peripheral nervous system (PNS) has a unique ability for self-repair. Dorsal root ganglion (DRG) neurons regulate the expression of different molecules, such as neurotrophins and their receptors, to promote axon regeneration after injury. However, the molecular players driving axonal regrowth need to be better defined. The membrane glycoprotein GPM6a has been described to contribute to neuronal development and structural plasticity in central-nervous-system neurons. Recent evidence indicates that GPM6a interacts with molecules from the PNS, although its role in DRG neurons remains unknown. Here, we characterized the expression of GPM6a in embryonic and adult DRGs by combining analysis of public RNA-seq datasets with immunochemical approaches utilizing cultures of rat DRG explants and dissociated neuronal cells. M6a was detected on the cell surfaces of DRG neurons throughout development. Moreover, GPM6a was required for DRG neurite elongation in vitro. In summary, we provide evidence on GPM6a being present in DRG neurons for the first time. Data from our functional experiments support the idea that GPM6a could contribute to axon regeneration in the PNS.

Cellular complexity of the peripheral nervous system: Insights from single-cell resolution

Single-cell RNA sequencing allows the division of cell populations, offers precise transcriptional profiling of individual cells, and fundamentally advances the comprehension of cellular diversity. In the peripheral nervous system (PNS), the application of single-cell RNA sequencing identifies multiple types of cells, including neurons, glial cells, ependymal cells, immune cells, and vascular cells. Sub-types of neurons and glial cells have further been recognized in nerve tissues, especially tissues in different physiological and pathological states. In the current article, we compile the heterogeneities of cells that have been reported in the PNS and describe cellular variability during development and regeneration. The discovery of the architecture of peripheral nerves benefits the understanding of the cellular complexity of the PNS and provides a considerable cellular basis for future genetic manipulation.

Fluorescence visualization of the neuropathic pain triad in trigeminal neuralgia

Trigeminal neuralgia (TN), an exemplary condition of neuropathic facial pain, seriously affects the physical and mental health of patients, becoming a major medical and social problem. So far, the mechanism of TN and its relation to neuronal activity remain unclear, largely limited by the spatial resolution of visualization methods. In the meanwhile, current therapeutic strategies targeting neurons have not achieved satisfactory outcome. Here, we investigate the neuropathic pain triad in TN by establishing an animal model of TN by chronic constriction injury of the unilateral infraorbital nerve (ION-CCI) and leveraging the single-cell resolution of confocal microscopy, including neuronal hyperexcitability, glial activation, and macrophage polarization. These results can broaden the understanding of TN pathogenesis from neurons to the neuropathic pain triad, and suggest that optical microscopy can provide new opportunities for understanding the complex pathogenesis of TN at single-cell resolution, potentially contributing to the identification of more precise therapeutic targets and the development of more effective treatment modalities.

Up-regulation of oxytocin receptors on peripheral sensory neurons mediates analgesia in chemotherapy-induced neuropathic pain

BACKGROUND AND PURPOSE

Chemotherapy-induced neuropathic pain (CINP) currently has limited effective treatment. Although the roles of oxytocin (OXT) and the oxytocin receptor (OXTR) in central analgesia have been well documented, the expression and function of OXTR in the peripheral nervous system remain unclear. Here, we evaluated the peripheral antinociceptive profiles of OXTR in CINP.

EXPERIMENTAL APPROACH

Paclitaxel (PTX) was used to establish CINP. Quantitative real-time polymerase chain reaction (qRT-PCR), in situ hybridization, and immunohistochemistry were used to observe OXTR expression in dorsal root ganglia (DRG). The antinociceptive effects of OXT were assessed by hot-plate and von Frey tests. Whole-cell patch clamp was performed to record sodium currents, excitability of DRG neurons, and excitatory synapse transmission.

KEY RESULTS

Expression of OXTR in DRG neurons was enhanced significantly after PTX treatment. Activation of OXTR exhibited antinociceptive effects, by decreasing the hyperexcitability of DRG neurons in PTX-treated mice. Additionally, OXTR activation up-regulated the phosphorylation of protein kinase C (pPKC) and, in turn, impaired voltage-gated sodium currents, particularly the voltage-gated sodium channel 1.7 (Na_V 1.7) current, that plays an indispensable role in PTX-induced neuropathic pain. OXT suppressed excitatory transmission in the spinal dorsal horn as well as excitatory inputs from primary afferents in PTX-treated mice.

CONCLUSION AND IMPLICATIONS

The OXTR in small-sized DRG neurons is up-regulated in CINP and its activation relieved CINP by inhibiting the neural excitability by impairment of Na_V 1.7 currents via pPKC. Our results suggest that OXTR on peripheral sensory neurons is a potential therapeutic target to relieve CINP.

Peripheral sensory neurons and non-neuronal cells express functional Piezo1 channels

Here, we present evidence showing Piezo1 protein expression in the primary sensory neurons (PSNs) and non-neuronal cells of rat peripheral nervous system. Using a knockdown/knockout validated antibody, we detected Piezo1 immunoreactivity (IR) in ∼60% of PSNs of rat dorsal root ganglia (DRG) with higher IR density in the small- and medium-sized neurons. Piezo1-IR was clearly identified in DRG perineuronal glia, including satellite glial cells (SGCs) and Schwann cells; in sciatic nerve Schwann cells surrounding the axons and cutaneous afferent endings; and in skin epidermal Merkel cells and melanocytes. Neuronal and non-neuronal Piezo1 channels were functional since various cells (dissociated PSNs and SGCs from DRGs, isolated Schwann cells, and primary human melanocytes) exhibited a robust response to Piezo1 agonist Yoda1 by an increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i). These responses were abolished by non-specific Piezo1 antagonist GsMTx4. Immunoblots showed elevated Piezo1 protein in DRG proximal to peripheral nerve injury-induced painful neuropathy, while PSNs and SGCs from rats with neuropathic pain showed greater Yoda1-evoked elevation of [Ca²⁺]_i and an increased frequency of cells responding to Yoda1, compared to controls. Sciatic nerve application of GsMTx4 alleviated mechanical hypersensitivity induced by Yoda1. Overall, our data show that Piezo1 is widely expressed by the neuronal and non-neuronal cells in the peripheral sensory pathways and that painful nerve injury appeared associated with activation of Piezo1 in PSNs and peripheral glial cells.

Mechanisms of Action of the Peptide Toxins Targeting Human and Rodent Acid-Sensing Ion Channels and Relevance to Their In Vivo Analgesic Effects

Acid-sensing ion channels (ASICs) are voltage-independent H⁺-gated cation channels largely expressed in the nervous system of rodents and humans. At least six isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) associate into homotrimers or heterotrimers to form functional channels with highly pH-dependent gating properties. This review provides an update on the pharmacological profiles of animal peptide toxins targeting ASICs, including PcTx1 from tarantula and related spider toxins, APETx2 and APETx-like peptides from sea anemone, and mambalgin from snake, as well as the dimeric protein snake toxin MitTx that have all been instrumental to understanding the structure and the pH-dependent gating of rodent and human cloned ASICs and to study the physiological and pathological roles of native ASICs in vitro and in vivo. ASICs are expressed all along the pain pathways and the pharmacological data clearly support a role for these channels in pain. ASIC-targeting peptide toxins interfere with ASIC gating by complex and pH-dependent mechanisms sometimes leading to opposite effects. However, these dual pH-dependent effects of ASIC-inhibiting toxins (PcTx1, mambalgin and APETx2) are fully compatible with, and even support, their analgesic effects in vivo, both in the central and the peripheral nervous system, as well as potential effects in humans.

Unveil the pain of endometriosis: from the perspective of the nervous system

Endometriosis is a chronic inflammatory disease with pelvic pain and uncharacteristic accompanying symptoms. Endometriosis-associated pain often persists despite treatment of the disease, thus it brings a deleterious impact on their personal lives as well as imposing a substantial economic burden on them. At present, mechanisms underlie endometriosis-associated pain including inflammatory reaction, injury, aberrant blood vessels and the morphological and functional anomaly of the peripheral and central nervous systems. The nerve endings are influenced by the physical and chemical factors surrounding the lesion, via afferent nerve to the posterior root of the spinal nerve, then to the specific cerebral cortex involved in nociception. However, our understanding of the aetiology and mechanism of this complex pain process caused by endometriosis remains incomplete. Identifying the pathogenesis of endometriosis is crucial to disease management, offering proper treatment, and helping patients to seek novel targets for the maintenance and contributors of chronic pain. The main aim of this review is to focus on every possible mechanism of pain related to endometriosis in both peripheral and central nervous systems, and to present related mechanisms of action from the interaction between peripheral lesions and nerves to the changes in transmission of pain, resulting in hyperalgesia and the corresponding alterations in cerebral cortex and brain metabolism.

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.

Towards bridging the translational gap by improved modeling of human nociception in health and disease

Despite numerous studies which have explored the pathogenesis of pain disorders in preclinical models, there is a pronounced translational gap, which is at least partially caused by differences between the human and rodent nociceptive system. An elegant way to bridge this divide is the exploitation of human-induced pluripotent stem cell (iPSC) reprogramming into human iPSC-derived nociceptors (iDNs). Several protocols were developed and optimized to model nociceptive processes in health and disease. Here we provide an overview of the different approaches and summarize the knowledge obtained from such models on pain pathologies associated with monogenetic sensory disorders so far. In addition, novel perspectives offered by increasing the complexity of the model systems further to better reflect the natural environment of nociceptive neurons by involving other cell types in 3D model systems are described.

Transcriptional Alterations of Mouse Trigeminal Ganglion Neurons Following Orofacial Inflammation Revealed by Single-Cell Analysis

Orofacial inflammation leads to transcriptional alterations in trigeminal ganglion (TG) neurons. However, diverse alterations and regulatory mechanisms following orofacial inflammatory pain in different types of TG neurons remain unclear. Here, orofacial inflammation was induced by injection of complete Freund’s adjuvant (CFA) in mice. After 7 days, we performed single-cell RNA-sequencing on TG cells of mice from control and treatment groups. We identified primary sensory neurons, Schwann cells, satellite glial cells, oligodendrocyte-like cells, immune cells, fibroblasts, and endothelial cells in TG tissue. After principal component analysis and hierarchical clustering, we identified six TG neuronal subpopulations: peptidergic nociceptors (PEP1 and PEP2), non-peptidergic nociceptors (NP1 and NP2), C-fiber low-threshold mechanoreceptors (cLTMR) and myelinated neurons (Nefh-positive neurons, NF) based on annotated marker gene expression. We also performed differential gene expression analysis among TG neuronal subtypes, identifying several differential genes involved in the inflammatory response, neuronal excitability, neuroprotection, and metabolic processes. Notably, we identified several potential novel targets associated with pain modulation, including Arl6ip1, Gsk3b, Scn7a, and Zbtb20 in PEP1, Rgs7bp in PEP2, and Bhlha9 in cLTMR. The established protein–protein interaction network identified some hub genes, implying their critical involvement in regulating orofacial inflammatory pain. Our study revealed the heterogeneity of TG neurons and their diverse neuronal transcriptomic responses to orofacial inflammation, providing a basis for the development of therapeutic strategies for orofacial inflammatory pain.

Transcription factor mesenchyme homeobox protein 2 (MEOX2) modulates nociceptor function

Mesenchyme homeobox protein 2 (MEOX2) is a transcription factor involved in mesoderm differentiation, including development of bones, muscles, vasculature and dermatomes. We have previously identified dysregulation of MEOX2 in fibroblasts from Congenital Insensitivity to Pain patients, and confirmed that btn, the Drosophila homologue of MEOX2, plays a role in nocifensive responses to noxious heat stimuli. To determine the importance of MEOX2 in the mammalian peripheral nervous system, we used a Meox2 heterozygous (Meox2^+/−) mouse model to characterise its function in the sensory nervous system, and more specifically, in nociception. MEOX2 is expressed in the mouse dorsal root ganglia (DRG) and spinal cord, and localises in the nuclei of a subset of sensory neurons. Functional studies of the mouse model, including behavioural, cellular and electrophysiological analyses, showed altered nociception encompassing impaired action potential initiation upon depolarisation. Mechanistically, we noted decreased expression of Scn9a and Scn11a genes encoding Na_v1.7 and Na_v1.9 voltage‐gated sodium channels respectively, that are crucial in subthreshold amplification and action potential initiation in nociceptors. Further transcriptomic analyses of Meox2^+/− DRG revealed downregulation of a specific subset of genes including those previously associated with pain perception, such as PENK and NPY. Based on these observations, we propose a novel role of MEOX2 in primary afferent nociceptor neurons for the maintenance of a transcriptional programme required for proper perception of acute and inflammatory noxious stimuli.

Trigeminal neurons control immune-bone cell interaction and metabolism in apical periodontitis

Abstract

Apical periodontitis (AP) is an inflammatory disease occurring following tooth infection with distinct osteolytic activity. Despite increasing evidence that sensory neurons participate in regulation of non-neuronal cells, their role in the development of AP is largely unknown. We hypothesized that trigeminal ganglia (TG) Nav1.8⁺ nociceptors regulate bone metabolism changes in response to AP. A selective ablation of nociceptive neurons in Nav1.8^Cre/Diphtheria toxin A (DTA)^Lox mouse line was used to evaluate the development and progression of AP using murine model of infection-induced AP. Ablation of Nav1.8⁺ nociceptors had earlier progression of AP with larger osteolytic lesions. Immunohistochemical and RNAscope analyses demonstrated greater number of macrophages, T-cells, osteoclast and osteoblast precursors and an increased RANKL:OPG ratio at earlier time points among Nav1.8^Cre/ DTA^Lox mice. There was an increased expression of IL-1α and IL-6 within lesions of nociceptor-ablated mice. Further, co-culture experiments demonstrated that TG neurons promoted osteoblast mineralization and inhibited osteoclastic function. The findings suggest that TG Nav1.8⁺ neurons contribute to modulation of the AP development by delaying the influx of immune cells, promoting osteoblastic differentiation, and decreasing osteoclastic activities. This newly uncovered mechanism could become a therapeutic strategy for the treatment of AP and minimize the persistence of osteolytic lesions in refractory cases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00018-022-04335-w.

Gene and protein expression of dorsal root ganglion sensory receptors in normotensive and hypertensive male rats

The exercise pressor reflex (EPR), a neurocirculatory control mechanism, is exaggerated in hypertensive humans and rats. Disease-related abnormalities within the afferent arm of the reflex loop, including mechano- and metabosensitive receptors located at the terminal end of group III/IV muscle afferents, may contribute to the dysfunctional EPR in hypertension. Using control (WKY) and spontaneous hypertensive (SHR) rats, we examined dorsal root ganglion (DRG) gene and protein expression of molecular receptors recognized as significant determinants of the EPR. Twelve lumbar DRGs (6-left, 6-right) were harvested from each of 10 WKY (arterial blood pressure (MAP): 96±9mmHg) and 10 SHR (MAP: 144±9mmHg). DRGs from the left side were used for protein expression (Western blotting; normalized to GAPDH), whereas right-side DRGs (i.e., parallel structure) were used to determine mRNA levels (RNA-sequencing, normalized to TPM). Analyses focused on metabosensitive (ASIC3, Bradykinin-receptor B2, EP4, P2X3, TRPv1) and mechanosensitive (Piezo1/2) receptors. While Piezo1 was similar in both groups (P=0.75), protein expression for all other receptors was significantly higher in SHR compared to WKY. With the exception of a greater Bradykinin-receptor B2 in SHR (P<0.05), mRNA expression of all other receptors was not different between groups (P>0.18). The higher protein content of these sensory receptors in SHR indirectly supports the previously proposed hypothesis that the exaggerated EPR in hypertension is, in part, due to disease-related abnormalities within the afferent arm of the reflex loop. The upregulated receptor content, combined with normal mRNA levels, insinuates that posttranscriptional regulation of sensory receptor protein expression might be impaired in hypertension.

Extended intergenic DNA contributes to neuron-specific expression of neighboring genes in the mammalian nervous system

Mammalian genomes comprise largely intergenic noncoding DNA with numerous cis-regulatory elements. Whether and how the size of intergenic DNA affects gene expression in a tissue-specific manner remain unknown. Here we show that genes with extended intergenic regions are preferentially expressed in neural tissues but repressed in other tissues in mice and humans. Extended intergenic regions contain twice as many active enhancers in neural tissues compared to other tissues. Neural genes with extended intergenic regions are globally co-expressed with neighboring neural genes controlled by distinct enhancers in the shared intergenic regions. Moreover, generic neural genes expressed in multiple tissues have significantly longer intergenic regions than neural genes expressed in fewer tissues. The intergenic regions of the generic neural genes have many tissue-specific active enhancers containing distinct transcription factor binding sites specific to each neural tissue. We also show that genes with extended intergenic regions are enriched for neural genes only in vertebrates. The expansion of intergenic regions may reflect the regulatory complexity of tissue-type-specific gene expression in the nervous system.

Smooth muscle ATP-sensitive potassium channels mediate migraine-relevant hypersensitivity in mouse models

BACKGROUND

Opening of KATP channels by systemic levcromakalim treatment triggers attacks in migraine patients and hypersensitivity to von Frey stimulation in a mouse model. Blocking of these channels is effective in several preclinical migraine models. It is unknown in what tissue and cell type KATP-induced migraine attacks are initiated and which KATP channel subtype is targeted.

METHODS

In mouse models, we administered levcromakalim intracerebroventricularly, intraperitoneally and intraplantarily and compared the nociceptive responses by von Frey and hotplate tests. Mice with a conditional loss-of-function mutation in the smooth muscle KATP channel subunit Kir6.1 were given levcromakalim and GTN and examined with von Frey filaments. Arteries were tested for their ability to dilate ex vivo. mRNA expression, western blotting and immunohistochemical stainings were made to identify relevant target tissue for migraine induced by KATP channel opening.

RESULTS

Systemic administration of levcromakalim induced hypersensitivity but central and local administration provided antinociception respectively no effect. The Kir6.1 smooth muscle knockout mouse was protected from both GTN and levcromakalim induced hypersensitivity, and their arteries had impaired dilatory response to the latter. mRNA and protein expression studies showed that trigeminal ganglia did not have significant KATP channel expression of any subtype, whereas brain arteries and dura mater primarily expressed the Kir6.1 + SUR2B subtype.

CONCLUSION

Hypersensitivity provoked by GTN and levcromakalim in mice is dependent on functional smooth muscle KATP channels of extracerebral origin. These results suggest a vascular contribution to hypersensitivity induced by migraine triggers.

Beyond CGRP: The calcitonin peptide family as targets for migraine and pain

The CGRP system has emerged as a key pharmacological target for the treatment of migraine. However, some individuals who suffer from migraine have low or no response to anti-CGRP or other treatments, suggesting the need for additional clinical targets. CGRP belongs to the calcitonin family of peptides, which includes calcitonin, amylin, adrenomedullin and adrenomedullin 2. These peptides display a range of pro-nociceptive and anti-nociceptive actions, in primary headache conditions such as migraine. Calcitonin family peptides also show expression at sites relevant to migraine and pain. This suggests that calcitonin family peptides and their receptors, beyond CGRP, may be therapeutically useful in the treatment of migraine and other pain disorders. This review considers the localisation of the calcitonin family in peripheral pain pathways and discusses how they may contribute to migraine and pain. LINKED ARTICLES: This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Functional Characterization of Mechanosensitive Piezo1 Channels in Trigeminal and Somatic Nerves in a Neuron-on-Chip Model

Mechanosensitive ion channels, Piezo1 and 2, are activated by pressure and involved in diverse physiological functions, including senses of touch and pain, proprioception and many more. Understanding their function is important for elucidating the mechanosensitive mechanisms of a range of human diseases. Recently, Piezo channels were suggested to be contributors to migraine pain generation. Migraine is typically characterized by allodynia and mechanical hyperalgesia associated with the activation and sensitization of trigeminal ganglion (TG) nerve fibers. Notably, migraine specific medicines are ineffective for other types of pain, suggesting a distinct underlying mechanism. To address, in a straightforward manner, the specificity of the mechanosensitivity of trigeminal vs. somatic nerves, we compared the activity of Piezo1 channels in mouse TG neurons vs. dorsal root ganglia (DRG) neurons. We assessed the functional expression of Piezo1 receptors using a conventional live calcium imaging setup equipped with a multibarrel application system and utilizing a microfluidic chip-based setup. Surprisingly, the TG neurons, despite higher expression of the Piezo1 gene, were less responsive to Piezo1 agonist Yoda1 than the DRG neurons. This difference was more prominent in the chip-based setup, suggesting that certain limitations of the conventional approach, such as turbulence, can be overcome by utilizing microfluidic devices with laminar solution flow.

Tmem160 contributes to the establishment of discrete nerve injury-induced pain behaviors in male mice

Chronic pain is a prevalent medical problem, and its molecular basis remains poorly understood. Here, we demonstrate the significance of the transmembrane protein (Tmem) 160 for nerve injury-induced neuropathic pain. An extensive behavioral assessment suggests a pain modality- and entity-specific phenotype in male Tmem160 global knockout (KO) mice: delayed establishment of tactile hypersensitivity and alterations in self-grooming after nerve injury. In contrast, Tmem160 seems to be dispensable for other nerve injury-induced pain modalities, such as non-evoked and movement-evoked pain, and for other pain entities. Mechanistically, we show that global KO males exhibit dampened neuroimmune signaling and diminished TRPA1-mediated activity in cultured dorsal root ganglia. Neither these changes nor altered pain-related behaviors are observed in global KO female and male peripheral sensory neuron-specific KO mice. Our findings reveal Tmem160 as a sexually dimorphic factor contributing to the establishment, but not maintenance, of discrete nerve injury-induced pain behaviors in male mice.

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.

A link between migraine and prolactin: the way forward

Migraine is an incapacitating neurological disorder that predominantly affects women. Sex and other hormones (e.g., oxytocin, and prolactin) may play a role in sexual dimorphic features of migraine. Initially, prolactin was recognized for its modulatory action in milk production and secretion; later, its roles in the regulation of the endocrine, immune and nervous systems were discovered. Higher prolactin levels in individuals with migraine were found in earlier studies, with a female sex-dominant trend. Studies that are more recent have identified that the expression of prolactin receptor in response to neuronal excitability and stress depends on sex with a dominant role in females. These findings have opened up potentials for explanation of sex-related pathophysiology of migraine, but have left some unanswered questions. This focused review examines the past and present of the link between prolactin and migraine, and presents open questions and directions for future experimental and clinical efforts.

Sex hormones (e.g., estrogen and progesterone) have been investigated to explain the sex-related manifestation of migraine, which is predominant in females. Prolactin is known for promoting lactation, but accumulating evidence supports that it can promote pain in females. An increasing number of studies have shown that the expression of a prolactin receptor in female nociceptors and their responses to external stimuli such as stress are different, which can help explain the female sex-dominant feature of migraine. In this focused review, the current knowledge is presented and the directions where prolactin research in migraine may evolve are proposed. The ultimate goal is to shape an overview toward considering sex-based treatments for migraine with highlighting the role of prolactin.

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.

The role of oxytocin, vasopressin, and their receptors at nociceptors in peripheral pain modulation

Oxytocin and vasopressin are neurohypophyseal hormones with sequence similarity and play a central role in bodily homeostatic regulation. Pain is currently understood to be an important phenotype that those two neurohormones strongly downregulate. Nociceptors, the first component of the ascending neural circuit for pain signals, have constantly been shown to be modulated by those peptides. The nociceptor modulation appears to be critical in pain attenuation, which has led to a gradual increase in scientific interest about their physiological processes and also drawn attention to their translational potentials. This review focused on what are recently understood and stay under investigation in the functional modulation of nociceptors by oxytocin and vasopressin. Effort to produce a nociceptor-specific view could help to construct a more systematic picture of the peripheral pain modulation by oxytocin and vasopressin.

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.

Identification of Trigeminal Sensory Neuronal Types Innervating Masseter Muscle

Understanding masseter muscle (MM) innervation is critical for the study of cell-specific mechanisms of pain induced by temporomandibular disorder (TMDs) or after facial surgery. Here, we identified trigeminal (TG) sensory neuronal subtypes (MM TG neurons) innervating MM fibers, masseteric fascia, tendons, and adjusted tissues. A combination of patch clamp electrophysiology and immunohistochemistry (IHC) on TG neurons back-traced from reporter mouse MM found nine distinct subtypes of MM TG neurons. Of these neurons, 24% belonged to non-peptidergic IB-4+/TRPA1- or IB-4+/TRPA1+ groups, while two TRPV1+ small-sized neuronal groups were classified as peptidergic/CGRP+ One small-sized CGRP+ neuronal group had a unique electrophysiological profile and were recorded from Nav1.8- or trkC+ neurons. The remaining CGRP+ neurons were medium-sized, could be divided into Nav1.8-/trkC- and Nav1.8low/trkC+ clusters, and showed large 5HT-induced current. The final two MM TG neuronal groups were trkC+ and had no Nav1.8 and CGRP. Among MM TG neurons, TRPV1+/CGRP- (somatostatin+), tyrosine hydroxylase (TH)+ (C-LTMR), TRPM8+, MrgprA3+, or trkB+ (Aδ-LTMR) subtypes have not been detected. Masseteric muscle fibers, tendons and masseteric fascia in mice and the common marmoset, a new world monkey, were exclusively innervated by either CGRP+/NFH+ or CGRP-/NFH+ medium-to-large neurons, which we found using a Nav1.8-YFP reporter, and labeling with CGRP, TRPV1, neurofilament heavy chain (NFH) and pgp9.5 antibodies. These nerves were mainly distributed in tendon and at junctions of deep-middle-superficial parts of MM. Overall, the data presented here demonstrates that MM is innervated by a distinct subset of TG neurons, which have unique characteristics and innervation patterns.

Colocalization of Oxtr with Prader-Willi syndrome transcripts in the trigeminal ganglion of neonatal mice

Prader-Willi syndrome (PWS) is caused by deficient expression of the paternal copy of several contiguous genes on chromosome 15q11-q13 and affects multiple organ systems in the body, including the nervous system. Feeding and suckling deficits in infants with PWS are replaced with excessive feeding and obesity in childhood through adulthood. Clinical trials using intranasal oxytocin (OXT) show promise to improve feeding deficits in infants with PWS. The mechanism and location of action of exogenous OXT are unknown. We have recently shown in neonatal mice that OXT receptors (OXTR) are present in several regions of the face with direct roles in feeding. Here we show that the trigeminal ganglion, which provides sensory innervation to the face, is a rich source of Oxtr and a site of cellular co-expression with PWS gene transcripts. We also quantified OXTR ligand binding in mice deficient in Magel2, a PWS gene, within the trigeminal ganglion and regions that are anatomically relevant to feeding behavior and innervated by the trigeminal ganglion including the lateral periodontium, rostral periodontium, tongue, olfactory epithelium, whisker pads and brainstem. We found that peripheral OXTR ligand binding in the head is mostly intact in Magel2-deficient mice, although it is reduced in the lateral periodontium (gums) of neonatal Magel2-deficient mice compared to wild-type controls. These data suggest that OXT via orofacial OXTR may play a peripheral role to modulate sensory-motor reflexes necessary for suckling and may be part of the mechanism by which intranasal OXT shows promise for therapeutic benefit in PWS.

Cellular models of pain: New technologies and their potential to progress preclinical research

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Human sensory neurons can reduce the translational gap in analgesic development.

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Access to dorsal root ganglion (hDRG) neurons is increasing.

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Diverse sensory neuron subtypes can now be generated via stem cell technology.

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Advances of these technologies will improve our understanding of human nociception.

In vitro models fill a vital niche in preclinical pain research, allowing detailed study of molecular pathways, and in the case of humanised systems, providing a translational bridge between in vivo animal models and human patients. Significant advances in cellular technology available to basic pain researchers have occurred in the last decade, including developing protocols to differentiate sensory neuron-like cells from stem cells and greater access to human dorsal root ganglion tissue. In this review, we discuss the use of both models in preclinical pain research: What can a human sensory neuron in a dish tell us that rodent in vivo models cannot? How similar are these models to their endogenous counterparts, and how should we judge them? What limitations do we need to consider? How can we leverage cell models to improve translational success? In vitro human sensory neuron models equip pain researchers with a valuable tool to investigate human nociception. With continual development, consideration for their advantages and limitations, and effective integration with other experimental strategies, they could become a driving force for the pain field's advancement.

Studying human nociceptors: from fundamentals to clinic

Chronic pain affects one in five of the general population and is the third most important cause of disability-adjusted life-years globally. Unfortunately, treatment remains inadequate due to poor efficacy and tolerability. There has been a failure in translating promising preclinical drug targets into clinic use. This reflects challenges across the whole drug development pathway, from preclinical models to trial design. Nociceptors remain an attractive therapeutic target: their sensitization makes an important contribution to many chronic pain states, they are located outside the blood-brain barrier, and they are relatively specific. The past decade has seen significant advances in the techniques available to study human nociceptors, including: the use of corneal confocal microscopy and biopsy samples to observe nociceptor morphology, the culture of human nociceptors (either from surgical or post-mortem tissue or using human induced pluripotent stem cell derived nociceptors), the application of high throughput technologies such as transcriptomics, the in vitro and in vivo electrophysiological characterization through microneurography, and the correlation with pain percepts provided by quantitative sensory testing. Genome editing in human induced pluripotent stem cell-derived nociceptors enables the interrogation of the causal role of genes in the regulation of nociceptor function. Both human and rodent nociceptors are more heterogeneous at a molecular level than previously appreciated, and while we find that there are broad similarities between human and rodent nociceptors there are also important differences involving ion channel function, expression, and cellular excitability. These technological advances have emphasized the maladaptive plastic changes occurring in human nociceptors following injury that contribute to chronic pain. Studying human nociceptors has revealed new therapeutic targets for the suppression of chronic pain and enhanced repair. Cellular models of human nociceptors have enabled the screening of small molecule and gene therapy approaches on nociceptor function, and in some cases have enabled correlation with clinical outcomes. Undoubtedly, challenges remain. Many of these techniques are difficult to implement at scale, current induced pluripotent stem cell differentiation protocols do not generate the full diversity of nociceptor populations, and we still have a relatively poor understanding of inter-individual variation in nociceptors due to factors such as age, sex, or ethnicity. We hope our ability to directly investigate human nociceptors will not only aid our understanding of the fundamental neurobiology underlying acute and chronic pain but also help bridge the translational gap.

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.

Transient Receptor Potential (TRP) Ion Channels in Orofacial Pain

Orofacial pain, including temporomandibular joint disorders pain, trigeminal neuralgia, dental pain, and debilitating headaches, affects millions of Americans each year with significant population health impact. Despite the existence of a large body of information on the subject, the molecular underpinnings of orofacial pain remain elusive. Two decades of research has identified that transient receptor potential (TRP) ion channels play a crucial role in pathological pain. A number of TRP ion channels are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. Although there are many similarities, the orofacial sensory system shows some distinct peripheral and central pain processing and different sensitivities from the spinal sensory system. Relative to the extensive review on TRPs in spinally-mediated pain, the summary of TRPs in trigeminally-mediated pain has not been well-documented. This review focuses on the current experimental evidence involving TRP ion channels, particularly TRPV1, TRPA1, TRPV4, and TRPM8 in orofacial pain, and discusses their possible cellular and molecular mechanisms.

Trigeminal Neuralgia TRPM8 Mutation: Enhanced Activation, Basal [Ca2+]i and Menthol Response

Objective

To assess the functional effects of a variant, c.89 G > A (p.Arg30Gln), in the transient receptor potential melastatin 8 (TRPM8) cold-sensing, nonselective cation channel, which we have previously identified in a patient with familial trigeminal neuralgia.

Methods

We carried out Ca²⁺ imaging and whole-cell patch-clamp recording.

Results

The TRPM8 mutation enhances channel activation, increases basal current amplitude and intracellular [Ca²⁺] in cells carrying the mutant channel, and enhances the response to menthol.

Conclusions

We propose that Arg30Gln confers gain-of-function attributes on TRPM8, which contribute to pathogenesis of trigeminal neuralgia in patients carrying this mutation.

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.

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

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.

The impact of bone cancer on the peripheral encoding of mechanical pressure stimuli

Skeletal metastases are frequently accompanied by chronic pain that is mechanoceptive in nature. Mechanistically, cancer-induced bone pain (CIBP) is mediated by peripheral sensory neurons innervating the cancerous site, the cell bodies of which are housed in the dorsal root ganglia (DRG). How these somatosensory neurons encode sensory information in CIBP remains only partly explained. Using a validated rat model, we first confirmed cortical bone destruction in CIBP but not sham-operated rats (day 14 after surgery, designated "late"-stage bone cancer). This occurred with behavioural mechanical hypersensitivity (Kruskal-Wallis H for independent samples; CIBP vs sham-operated, day 14; P < 0.0001). Next, hypothesising that the proportion and phenotype of primary afferents would be altered in the disease state, dorsal root ganglia in vivo imaging of genetically encoded calcium indicators and Markov Cluster Analysis were used to analyse 1748 late-stage CIBP (n = 10) and 757 sham-operated (n = 9), neurons. Distinct clusters of responses to peripheral stimuli were revealed. In CIBP rats, upon knee compression of the leg ipsilateral to the tumour, (1) 3 times as many sensory afferents responded (repeated-measures analysis of variance: P < 0.0001 [vs sham]); (2) there were significantly more small neurons responding (Kruskal-Wallis for independent samples (vs sham): P < 0.0001); and (3) approximately 13% of traced tibial cavity afferents responded (no difference observed between CIBP and sham-operated animals). We conclude that an increased sensory afferent response is present in CIBP rats, and this is likely to reflect afferent recruitment from outside of the bone rather than increased intraosseous afferent activity.

Cathepsin E in neutrophils contributes to the generation of neuropathic pain in experimental autoimmune encephalomyelitis

Pain is a frequent and disabling symptom in patients with multiple sclerosis (MS); however, the underlying mechanisms of MS-related pain are not fully understood. Here, we demonstrated that cathepsin E (CatE) in neutrophils contributes to the generation of mechanical allodynia in experimental autoimmune encephalomyelitis, an animal model of MS. We showed that CatE-deficient (CatE) mice were highly resistant to myelin oligodendrocyte glycoprotein (MOG35-55)-induced mechanical allodynia. After MOG35-55 immunization, neutrophils immediately accumulated in the dorsal root ganglion (DRG). Adoptive transfer of MOG35-55-stimulated wild-type neutrophils into the dorsal root ganglion induced mechanical allodynia in the recipient C57BL/6 mice. However, the pain threshold did not change when MOG35-55-stimulated CatE neutrophils were transferred into the recipient C57BL/6 mice. MOG35-55 stimulation caused CatE-dependent secretion of elastase in neutrophils. Behavioral analyses revealed that sivelestat, a selective neutrophil elastase inhibitor, suppressed mechanical allodynia induced by adoptively transferred MOG35-55-stimulated neutrophils. MOG35-55 directly bound to toll-like receptor 4, which led to increased production of CatE in neutrophils. Our findings suggest that inhibition of CatE-dependent elastase production in neutrophil might be a potential therapeutic target for pain in patients with MS.