Primary Afferent Nociceptors Research Articles

The dual role of TRPV1 in peripheral neuropathic pain: pain switches caused by its sensitization or desensitization.

The transient receptor potential vanilloid 1 (TRPV1) channel plays a dual role in peripheral neuropathic pain (NeuP) by acting as a "pain switch" through its sensitization and desensitization. Hyperalgesia, commonly resulting from tissue injury or inflammation, involves the sensitization of TRPV1 channels, which modulates sensory transmission from primary afferent nociceptors to spinal dorsal horn neurons. In chemotherapy-induced peripheral neuropathy (CIPN), TRPV1 is implicated in neuropathic pain mechanisms due to its interaction with ion channels, neurotransmitter signaling, and oxidative stress. Sensitization of TRPV1 in dorsal root ganglion neurons contributes to CIPN development, and inhibition of TRPV1 channels can reduce chemotherapy-induced mechanical hypersensitivity. In diabetic peripheral neuropathy (DPN), TRPV1 is involved in pain modulation through pathways including reactive oxygen species and cytokine production. TRPV1's interaction with TRPA1 channels further influences chronic pain onset and progression. Therapeutically, capsaicin, a TRPV1 agonist, can induce analgesia through receptor desensitization, while TRPV1 antagonists and siRNA targeting TRPV1 show promise in preclinical studies. Cannabinoid modulation of TRPV1 provides another potential pathway for alleviating neuropathic pain. This review summarizes recent preclinical research on TRPV1 in association with peripheral NeuP.

A mouse model of chronic primary pain that integrates clinically relevant genetic vulnerability, stress, and minor injury.

Chronic primary pain conditions (CPPCs) affect over 100 million Americans, predominantly women. They remain ineffectively treated, in large part because of a lack of valid animal models with translational relevance. Here, we characterized a CPPC mouse model that integrated clinically relevant genetic (catechol-O-methyltransferase; COMT knockdown) and environmental (stress and injury) factors. Compared with wild-type mice, Comt+/- mice undergoing repeated swim stress and molar extraction surgery intervention exhibited pronounced multisite body pain and depressive-like behavior lasting >3 months. Comt+/- mice undergoing the intervention also exhibited enhanced activity of primary afferent nociceptors innervating hindpaw and low back sites and increased plasma concentrations of norepinephrine and pro-inflammatory cytokines interleukin-6 (IL-6) and IL-17A. The pain and depressive-like behavior were of greater magnitude and longer duration (≥12 months) in females versus males. Furthermore, increases in anxiety-like behavior and IL-6 were female-specific. The effect of COMT genotype × stress interactions on pain, IL-6, and IL-17A was validated in a cohort of 549 patients with CPPCs, demonstrating clinical relevance. Last, we assessed the predictive validity of the model for analgesic screening and found that it successfully predicted the lack of efficacy of minocycline and the CB2 agonist GW842166X, which were effective in spared nerve injury and complete Freund's adjuvant models, respectively, but failed in clinical trials. Yet, pain in the CPPC model was alleviated by the beta-3 adrenergic antagonist SR59230A. Thus, the CPPC mouse model reliably recapitulates clinically and biologically relevant features of CPPCs and may be implemented to test underlying mechanisms and find new therapeutics.

Contact-Heat Evoked Potentials: Insights into Pain Processing in CRPS Type I.

The pathophysiological mechanisms underlying the development of chronic pain in complex regional pain syndrome (CRPS) are diverse and involve both peripheral and central changes in pain processing, such as sensitization of the nociceptive system. The aim of this study was to objectively distinguish the specific changes occurring at both peripheral and central levels in nociceptive processing in individuals with chronic CRPS type I. Nineteen individuals with chronic CRPS type I and 16 age- and sex-matched healthy controls (HC) were recruited. All individuals underwent a clinical examination and pain assessment in the most painful limb, the contralateral limb, and a pain-free control area to distinguish between peripheral and central mechanisms. Contact-heat evoked potentials (CHEPs) were recorded after heat stimulation of the three different areas and amplitudes and latencies were analyzed. Additionally, quantitative sensory testing (QST) was performed in all three areas. Compared to HC, CHEP amplitudes in CRPS were only increased after stimulation of the painful area (p=0.025), while no increases were observed for the pain-free control area (p=0.14). None of the CHEP latencies were different between the two cohorts (all p>0.23). Furthermore, individuals with CRPS showed higher pain ratings after stimulation of the painful limb compared to their contralateral limb (p=0.013). Lastly, compared to HC, mechanical (p=0.012) and thermal (p=0.046) sensitivity was higher in the painful area of the CRPS cohort. This study provides neurophysiological evidence supporting an intact thermo-nociceptive pathway with signs of peripheral sensitization, such as hyperexcitable primary afferent nociceptors, in individuals with CRPS type I. This is further supported by the observation of mechanical and thermal gain of sensation only in the painful limb. Additionally, the increased CHEP amplitudes might be related to fear-induced alterations of nociceptive processing.

Generation of a TRPV1 knockout human pluripotent stem cell line (WAe009-A-U) using CRISPR/Cas9

The transient receptor potential vanilloid subfamily 1 (TRPV1) is a polymodal nociceptor that is highly expressed in sensory nerves. Activation of TRPV1 receptors excites primary afferent nociceptors by opening cation channels, allowing the influx of Na+ and Ca2+ ions into the cytoplasm. Here, a TRPV1 knockout human embryonic stem cell line was generated using the CRISPR/Cas9 genome-editing technology to further study the function of TRPV1. The cell line confirmed with normal pluripotency and karyotype.

Protein kinase C epsilon activation regulates proliferation, migration, and epithelial to mesenchymal-like transition in rat Schwann cells.

Protein kinase type C-ε (PKCε) plays an important role in the sensitization of primary afferent nociceptors, promoting mechanical hyperalgesia. In accordance, we showed that PKCε is present in sensory neurons of the peripheral nervous system (PNS), participating in the control of pain onset and chronification. Recently, it was found that PKCε is also implicated in the control of cell proliferation, promoting mitogenesis and metastatic invasion in some types of cancer. However, its role in the main glial cell of the PNS, the Schwann cells (SCs), was still not investigated. Rat primary SCs culture were treated with different pharmacologic approaches, including the PKCε agonist dicyclopropyl-linoleic acid (DCP-LA) 500 nM, the human recombinant brain derived neurotrophic factor (BDNF) 1 nM and the TrkB receptor antagonist cyclotraxin B 10 nM. The proliferation (by cell count), the migration (by scratch test and Boyden assay) as well as some markers of SCs differentiation and epithelial-mesenchymal transition (EMT) process (by qRT-PCR and western blot) were analyzed. Overall, we found that PKCε is constitutively expressed in SCs, where it is likely involved in the switch from the proliferative toward the differentiated state. Indeed, we demonstrated that PKCε activation regulates SCs proliferation, increases their migration, and the expression of some markers (e.g., glycoprotein P0 and the transcription factor Krox20) of SCs differentiation. Through an autocrine mechanism, BDNF activates TrkB receptor, and controls SCs proliferation via PKCε. Importantly, PKCε activation likely promoted a partial EMT process in SCs. PKCε mediates relevant actions in the neuronal and glial compartment of the PNS. In particular, we posit a novel function for PKCε in the transformation of SCs, assuming a role in the mechanisms controlling SCs' fate and plasticity.

Preclinical Studies of Posttraumatic Headache and the Potential Therapeutics

Posttraumatic headache (PTH) attributed to traumatic brain injury (TBI) is a secondary headache developed within 7 days after head injury, and in a substantial number of patients PTH becomes chronic and lasts for more than 3 months. Current medications are almost entirely relied on the treatment of primary headache such as migraine, due to its migraine-like phenotype and the limited understanding on the PTH pathogenic mechanisms. To this end, increasing preclinical studies have been conducted in the last decade. We focus in this review on the trigeminovascular system from the animal studies since it provides the primary nociceptive sensory afferents innervating the head and face region, and the pathological changes in the trigeminal pathway are thought to play a key role in the development of PTH. In addition to the pathologies, PTH-like behaviors induced by TBI and further exacerbated by nitroglycerin, a general headache inducer through vasodilation are reviewed. We will overview the current pharmacotherapies including calcitonin gene-related peptide (CGRP) monoclonal antibody and sumatriptan in the PTH animal models. Given that modulation of the endocannabinoid (eCB) system has been well-documented in the treatment of migraine and TBI, the therapeutic potential of eCB in PTH will also be discussed.

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 Nav1.7 and Nav1.9 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.

Interactions between the painful disorders and the autonomic nervous system.

he autonomic nervous system (ANS) controls the heart rate, blood pressure, digestion, respiration, pupillary reactivity, sweating, urination, sexual arousal, and regulates the functions of internal organs. This system provides the homeostasis of the cells, tissues, and organs throughout the body and protects against the disturbances imposed by the external and internal stressors. The ANS has three main divisions: The sympathetic nervous system (SNS), the parasympathetic nervous system (PNS), and the enteric nervous system. In general, the SNS and PNS have opposing effects. Each region belonging to the 'pain matrix' interacts with ANS. The descending system regulates pain and creates a regulatory effect by the contribution of aminergic neurotransmitters. Hypothalamus, amygdala, and periaqueductal gray are the main structures of this regulatory system. Dysfunction of the ANS is frequently observed in pain patients. The SNS induce, facilitate, or potentiate chronic pain. Increased responsiveness of injured sensory nerves to catecholamines, increased expression of α-1 adrenoreceptors on the primary afferent nociceptors and hyperalgesic skin, central sensitization rendering Aβ mechanoreceptors, enhanced discharge and sympathetic sprouting in dorsal root ganglia, central sensitization, and dysfunction of the pain modulation is proposed mechanisms. In this review, the anatomical, physiological and pathological aspects of ANS and pain, and laboratory tests to evaluate autonomic functions will be discussed. Pathophysiological role of ANS in migraine, trigeminal autonomic cephalgias, trigeminal neuralgia, peripheral nerve injuries, small fiber neuropathies, myofascial pain syndrome, fibromyalgia, painful joint diseases, visceral pain, phantom limb pain, complex regional pain syndrome, and spinal cord injury will be discussed.

Pathophysiological Role of Purinergic P2X Receptors in Digestive System Diseases.

P2X receptors (P2XRs) are trimeric, non-selective cation channels activated by extracellular ATP and widely distributed in the digestive system. P2XRs have an important role in the physiological function of the digestive system, such as neurotransmission, ion transports, proliferation and apoptosis, muscle contraction, and relaxation. P2XRs can be involved in pain mechanisms both centrally and in the periphery and confirmed the association of P2XRs with visceral pain. In the periphery, ATP can be released as a result of tissue injury, visceral distension, or sympathetic activation and can excite nociceptive primary afferents by acting at homomeric P2X(3)R or heteromeric P2X(2/3)R. Thus, peripheral P2XRs, and homomeric P2X(3) and/or heteromeric P2X(2/3)R in particular, constitute attractive targets for analgesic drugs. Recently studies have shown that P2XRs have made significant advances in inflammation and cancer. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. It is believed that with the further study of P2XRs and its subtypes, P2XRs and its specific antagonists will be expected to be widely used in the treatment of human digestive diseases in the future.

Expression of ectopic heat shock protein 90 in male and female primary afferent nociceptors regulates inflammatory pain.

Heat shock protein 90 (Hsp90) is a ubiquitously expressed integral cellular protein essential for regulating proteomic stress. Previous research has shown that Hsp90 regulates critical signaling pathways underlying chronic pain and inflammation. Recent discovery of membrane bound ectopic Hsp90 (eHsp90) on tumor cells has shown that Hsp90 induction to the plasma membrane can stabilize disease-relevant proteins. Here, we characterize eHsp90 expression in a mouse model of inflammation and demonstrate its role in nociception and pain. We found that intraplantar complete Freund adjuvant (CFA) induced robust expression of eHsp90 on the cell membranes of primary afferent nociceptors located in the L3-L5 dorsal root ganglia (DRG), bilaterally, with minimal to no expression in other tissues. Complete Freund adjuvant-induced increases in eHsp90 expression on lumbar DRG were significantly greater in females compared with males. Furthermore, exogenous Hsp90 applied to primary Pirt-GCaMP3 nociceptors induced increases in calcium responses. Responses were estrogen-dependent such that greater activity was observed in female or estrogen-primed male nociceptors compared with unprimed male nociceptors. Treatment of mice with the selective eHsp90 inhibitor HS-131 (10 nmol) significantly reversed CFA-induced mechanical pain, thermal heat pain, and hind paw edema. Notably, a higher dose (20 nmol) of HS-131 was required to achieve analgesic and anti-inflammatory effects in females. Here, we provide the first demonstration that inflammation leads to an upregulation of eHsp90 on DRG nociceptors in a sex-dependent manner and that inhibition of eHsp90 reduces nociceptor activity, pain, and inflammation. Thus, eHsp90 represents a novel therapeutic axis for the development of gender-tailored treatments for inflammatory pain.

TRPC3 Antagonizes Pruritus in a Mouse Contact Dermatitis Model

Contact dermatitis (CD), including allergic and irritant CD, are common dermatological diseases and are characterized by an erythematous rash and severe itch. In this study, we investigated the function of TRPC3, a canonical transient receptor potential channel highly expressed in type 1 nonpeptidergic (NP1) nociceptive primary afferents and other cell types, in a mouse CD model. Although TrpC3 null mice had little deficits in acute somatosensation, they showed significantly increased scratching with CD. In addition, TrpC3 null mice displayed no differences in mechanical and thermal hypersensitivity in an inflammatory pain model, suggesting that this channel preferentially functions to antagonize CD-induced itch. Using dorsal root ganglia and panimmune-specific TrpC3 conditional knockout mice, we determined that TrpC3 in dorsal root ganglia neurons but not in immune cells is required for this phenotype. Furthermore, the number of MRGPRD+ NP1 afferents in CD-affected dorsal root ganglia is significantly reduced in TrpC3-mutant mice. Taken together, our results suggest that TrpC3 plays a critical role in NP1 afferents to cope with CD-induced excitotoxicity and that the degeneration of NP1 fibers may lead to an increased itch of CD. Our study identified a role of TrpC3 and NP1 afferents in CD pathology.

Genetic and functional evidence for gp130/IL6ST-induced transient receptor potential ankyrin 1 upregulation in uninjured but not injured neurons in a mouse model of neuropathic pain.

Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia, which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 (TRPA1) ion channel is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By using a transgenic mouse model with a conditional depletion of the interleukin-6 (IL-6) signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury (SNI) model. Spared nerve injury mice developed profound mechanical hypersensitivity as indicated by decreased withdrawal thresholds in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin-nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamaldehyde, and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to cinnamaldehyde. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model, and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation after nerve injury and stress its significance as an important target for neuropathic pain disorders.

P2X7-induced nociception in the temporomandibular joint of rats depends on inflammatory mechanisms and C-fibres sensitization.

P2X7 receptors are responsible for triggering inflammatory responses contributing to processes of pain in articular tissues. This study aimed to investigate whether the activation of the P2X7 receptor located in the temporomandibular joint (TMJ) tissues induces nociception through an inflammatory mechanisms and/or the activation of C-fibres (small-diameter primary afferents) of rats' TMJ. The TMJ hypernociception induced by the activation of P2X7 receptor was assessed by measuring the behavioural nociceptive responses. After behavioural experiments, the animals were terminally anaesthetized and periarticular tissues were removed and homogenate for enzyme-linked immunosorbent assay, leukocyte infiltration and western blotting analysis. The nonselective P2X7 receptor agonist BzATP induced a dose-dependent TMJ nociception, which was blocked by the selective P2X7 receptor antagonist A-438079. The co-administration of the selective β2-adrenoceptor antagonist (ICI-118,551) and the pre-treatment with cyclooxygenase inhibitor indomethacin or with the nonspecific selectin inhibitor Fucoidan significantly reduced BzATP-induced TMJ nociception. BzATP also induced an increase of pro-inflammatory cytokines TNFα, IL-1β and CINC-1 levels, as well as leukocyte recruitment in TMJ tissue, effects that were reduced by A-438079. Moreover BzATP-induced TMJ nociception was inhibited in rats neonatal-treated with Capsaicin (depleting C-fibers). Finally, BzATP-induced an increase in TRPV1 expression in TMJ tissue. These findings suggest that P2X7 receptor activation in TMJ of rats induces nociceptive responses mediated by sympathomimetic amines, prostaglandins, leukocyte migration and increased levels of pro-inflammatory cytokines. Furthermore, the P2X7 receptor activation induces nociceptive responses dependent on the activation of the primary afferent nociceptors of rats' TMJ. The activation of P2X7 receptors has an essential role in TMJ nociception and could be an interesting target to control the inflammatory pain in temporomandibular disorders.

Reestablishment of Redox Homeostasis in the Nociceptive Primary Afferent as a Mechanism of Antinociception Promoted by Mesenchymal Stem/Stromal Cells in Oxaliplatin-Induced Chronic Peripheral Neuropathy.

Painful neuropathy is a common adverse effect of oxaliplatin (OXL), a platinum-derivative chemotherapeutic agent. Oxidative stress and mitochondrial dysfunction are key factors contributing to the development of OXL-induced peripheral neuropathy (OIPN). Based on the antioxidant and antinociceptive properties of mesenchymal stem/stromal cells (MSC), the present study tested the hypothesis that MSC induce antinociceptive effects during OIPN by promoting regulation of redox environment and mitochondrial homeostasis in the nociceptive primary afferents. C57Bl/6 mice submitted to the OXL-chronic neuropathy induction protocol by repeated intravenous administration of OXL (1 mg/kg) were evaluated to determine the paw mechanical and thermal nociceptive thresholds using the von Frey filaments and cold plate tests, respectively. Two weeks after the neuropathy induction, mice were treated with bone marrow-derived MSC (1 × 106), vehicle, or gabapentin (GBP, 70 mg/kg). Four weeks later, mitochondrial morphology, gene expression profile, and oxidative stress markers in the sciatic nerve and dorsal root ganglia (DRG) were evaluated by transmission electron microscopy, RT-qPCR, and biochemical assays, respectively. OXL-treated mice presented behavioral signs of sensory neuropathy, such as mechanical allodynia and thermal hyperalgesia. The behavioral painful neuropathy was completely reverted by a single administration of MSC, while the daily treatment with GBP induced only a short-lived antinociceptive effect. The ultrastructural analysis of the sciatic nerve and DRG of OIPN mice revealed a high proportion of atypical mitochondria in both myelinated and unmyelinated fibers. Importantly, this mitochondrial atypia was strongly reduced in MSC-treated neuropathic mice. Moreover, MSC-treated neuropathic mice showed upregulation of Sod and Nrf2 mRNA in the sciatic nerve and DRG. In line with this result, MSC reduced markers of nitrosative stress and lipid peroxidation in the sciatic nerve and DRG from OIPN mice. Our data suggest that the reestablishment of redox homeostasis in the nociceptive primary afferents is a mechanism by which MSC transplantation reverts the OXL-induced chronic painful neuropathy.

Nerve growth factor sensitizes nociceptors to C-fibre selective supra-threshold electrical stimuli in human skin.

Intradermal injection of 1µg nerve growth factor (NGF) causes sustained nociceptor sensitization. Slowly depolarizing electrical current preferentially activates C-nociceptors. We explored the differential contribution of A-delta and C-nociceptors in NGF-sensitized skin using slowly depolarizing transcutaneous electrical current stimuli, CO2 laser heat, mechanical impact, and A-fibre compression block. In 14 healthy volunteers, pain rating was recorded on a numeric scale at days 1-14 after NGF treatment. Ratings during A-fibre conduction block were investigated at days 3 and 7 post-NGF. Pain ratings to electrical, CO2 heat and mechanical impact stimuli were enhanced (>30%, p<.0005, ANOVA) at NGF-injection sites. Axon reflex erythema evoked by electrical stimulation was also larger at NGF-injection sites (p<.02, ANOVA). Diminution of pain during continuous (1min) sinusoidal current stimulation at 4Hz was less pronounced after NGF (p<.05, ANOVA). Pain ratings to electrical sinusoidal and mechanical impact stimuli during A-fibre conduction block were significantly elevated at the NGF sites compared to NaCl-treated skin (p<.05, ANOVA). NGF-induced sensitization of human skin to electrical and mechanical stimuli is primarily driven by C-nociceptors with little contribution from A-delta fibres. Less-pronounced accommodation during ongoing sinusoidal stimulation suggests that NGF could facilitate axonal spike generation and conduction in primary afferent nociceptors in humans. Further studies using this sinusoidal electrical stimulation profile to investigate patients with chronic inflammatory pain may allow localized assessment of skin C-nociceptors and their putative excitability changes under pathologic conditions. The application of novel slowly depolarizing electrical stimuli demonstrated a predominant C-nociceptor sensitization in NGF-treated skin. Increased pain ratings, larger axon reflex erythema and less accommodation of C-fibres to ongoing sinusoidal stimulation all indicated an enhanced nociceptor discharge after NGF. A-fibre conduction block had little effect on electrical and mechanical hyperalgesia skin in NGF-treated compared to NaCl-treated skin. This electrical stimulus profile may be applicable for patients with chronic inflammatory pain, allowing localized assessment of skin C-nociceptors and their putative excitability changes under pathologic conditions.

Neural control of gut homeostasis.

The gut-brain axis is a coordinated communication system that not only maintains homeostasis, but significantly influences higher cognitive functions and emotions, as well as neurological and behavioral disorders. Among the large populations of sensory and motor neurons that innervate the gut, insights into the function of primary afferent nociceptors, whose cell bodies reside in the dorsal root ganglia and nodose ganglia, have revealed their multiple crosstalk with several cell types within the gut wall, including epithelial, vascular, and immune cells. These bidirectional communications have immunoregulatory functions, control host response to pathogens, and modulate sensations associated with gastrointestinal disorders, through activation of immune cells and glia in the peripheral and central nervous system, respectively. Here, we will review the cellular and neurochemical basis of these interactions at the periphery, in dorsal root ganglia, and in the spinal cord. We will discuss the research gaps that should be addressed to get a better understanding of the multifunctional role of sensory neurons in maintaining gut homeostasis and regulating visceral sensitivity.

Schwann Cell Autocrine and Paracrine Regulatory Mechanisms, Mediated by Allopregnanolone and BDNF, Modulate PKCε in Peripheral Sensory Neurons.

Protein kinase type C-ε (PKCε) plays important roles in the sensitization of primary afferent nociceptors, such as ion channel phosphorylation, that in turn promotes mechanical hyperalgesia and pain chronification. In these neurons, PKCε is modulated through the local release of mediators by the surrounding Schwann cells (SCs). The progesterone metabolite allopregnanolone (ALLO) is endogenously synthesized by SCs, whereas it has proven to be a crucial mediator of neuron-glia interaction in peripheral nerve fibers. Biomolecular and pharmacological studies on rat primary SCs and dorsal root ganglia (DRG) neuronal cultures were aimed at investigating the hypothesis that ALLO modulates neuronal PKCε, playing a role in peripheral nociception. We found that SCs tonically release ALLO, which, in turn, autocrinally upregulated the synthesis of the growth factor brain-derived neurotrophic factor (BDNF). Subsequently, glial BDNF paracrinally activates PKCε via trkB in DRG sensory neurons. Herein, we report a novel mechanism of SCs-neuron cross-talk in the peripheral nervous system, highlighting a key role of ALLO and BDNF in nociceptor sensitization. These findings emphasize promising targets for inhibiting the development and chronification of neuropathic pain.

Differential expression of Na+/K+/Cl\u2212 cotransporter 1 in neurons and glial cells within the superficial spinal dorsal horn of rodents

Although convincing experimental evidence indicates that Na+/K+/Cl− cotransporter 1 (NKCC1) is involved in spinal nociceptive information processing and in the generation of hyperalgesia and allodynia in chronic pain states, the cellular distribution of NKCC1 in the superficial spinal dorsal horn is still poorly understood. Because this important piece of knowledge is missing, the effect of NKCC1 on pain processing is still open to conflicting interpretations. In this study, to provide the missing experimental data, we investigated the cellular distribution of NKCC1 in the superficial spinal dorsal horn by immunohistochemical methods. We demonstrated for the first time that almost all spinal axon terminals of peptidergic nociceptive primary afferents express NKCC1. In contrast, virtually all spinal axon terminals of nonpeptidergic nociceptive primary afferents were negative for NKCC1. Data on the colocalization of NKCC1 with axonal and glial markers indicated that it is almost exclusively expressed by axon terminals and glial cells in laminae I–IIo. In lamina IIi, however, we observed a strong immunostaining for NKCC1 also in the dendrites and cell bodies of PV-containing inhibitory neurons and a weak staining in PKCγ-containing excitatory neurons. Our results facilitate further thinking about the role of NKCC1 in spinal pain processing.

Central mechanisms of pain in orofacial pain patients: Implications for management.

Central sensitization (CS) is a form of neuroplasticity characterized by changes in the neural sensitivity, responsiveness, and/or output that are not contingent on peripheral input nor activity-dependent. CS is characterized by activation of unmyelinated C-fibers resulting in a cascade of events at molecular and cellular levels which eventuate into generation of synaptic currents at rest. CS, therefore, contributes to heightened generalized pain sensitivity, further complicates the process of reaching a diagnosis, and increases the possibility of treatment failure. BODY: Trigeminal nerve is the main sensory supplier of the anterior part of the head, including the intraoral structures. Primary afferent nociceptors of the trigeminal nerve and low threshold mechanoreceptors synapse with wide dynamic range (WDR) neurons in the pons. This multifaceted network of nerve interactions which is further complicated by the modulatory circuits that can suppress or heighten the activity of WDR neurons is one of the main contributors to CS. The importance of CS in orofacial pain disorders is emphasized in the context of chronic pain development. As for all chronic pain conditions, it is crucial to consider the biopsychosocial aspects of chronic orofacial pain in managing this diverse group of conditions. This review highlights current understanding of the biopsychosocial model and central mechanisms contributing to the pathogenesis of chronic orofacial pain.

ErbB1-dependent signalling and vesicular trafficking in primary afferent nociceptors associated with hypersensitivity in neuropathic pain

Effective analgesic treatment for neuropathic pain remains an unmet need, so previous evidence that epidermal growth factor receptor inhibitors (EGFRIs) provide unexpected rapid pain relief in a clinical setting points to a novel therapeutic opportunity. The present study utilises rodent models to address the cellular and molecular basis for the findings, focusing on primary sensory neurons because clinical pain relief is provided not only by small molecule EGFRIs, but also by the anti-EGFR antibodies cetuximab and panitumumab, which are unlikely to access the central nervous system in therapeutic concentrations. We report robust, rapid and dose-dependent analgesic effects of EGFRIs in two neuropathic pain models, matched by evidence with highly selective antibodies that expression of the EGFR (ErbB1 protein) is limited to small nociceptive afferent neurons. As other ErbB family members can heterodimerise with ErbB1, we investigated their distribution, showing consistent co-expression of ErbB2 but not ErbB3 or ErbB4, with ErbB1 in cell bodies of nociceptors, as well as providing evidence for direct molecular interaction of ErbB1 with ErbB2 in situ. Co-administration of selective ErbB1 and ErbB2 inhibitors produced clear evidence of greater-than-additive, synergistic analgesia; highlighting the prospect of a unique new combination therapy in which enhanced efficacy could be accompanied by minimisation of side-effects. Peripheral (intraplantar) administration of EGF elicited hypersensitivity only following nerve injury and this was reversed by local co-administration of selective inhibitors of either ErbB1 or ErbB2. Investigating how ErbB1 is activated in neuropathic pain, we found evidence for a role of Src tyrosine kinase, which can be activated by signals from inflammatory mediators, chemokines and cytokines during neuroinflammation. Considering downstream consequences of ErbB1 activation in neuropathic pain, we found direct recruitment to ErbB1 of an adapter for PI 3-kinase and Akt signalling together with clear Akt activation and robust analgesia from selective Akt inhibitors. The known Akt target and regulator of vesicular trafficking, AS160 was strongly phosphorylated at a perinuclear location during neuropathic pain in an ErbB1-, ErbB2- and Akt-dependent manner, corresponding to clustering and translocation of an AS160-partner, the vesicular chaperone, LRP1. Exploring whether neuronal ion channels that could contribute to hyperexcitability might be transported by this vesicular trafficking pathway we were able to identify Nav1.9, (Nav1.8) and Cav1.2 moving towards the plasma membrane or into proximal axonal locations – a process prevented by ErbB1 or Akt inhibitors. Overall these findings newly reveal both upstream and downstream signals to explain how ErbB1 can act as a signalling hub in neuropathic pain models and identify the trafficking of key ion channels to neuronal subcellular locations likely to contribute to hyperexcitability. The new concept of combined treatment with ErbB1 plus ErbB2 blockers is mechanistically validated as a promising strategy for the relief of neuropathic pain.