The modality-specific contribution of peptidergic and non-peptidergic nociceptors is manifest at the level of dorsal horn nociresponsive neurons

The nociceptive activity of peripheral sensory neurons is modulated by the neuronal membrane proteasome

Proteasomes are critical for peripheral nervous system (PNS) function. Here, we investigate mammalian PNS proteasomes and reveal the presence of the neuronal membrane proteasome (NMP). We show that specific inhibition of the NMP on distal nerve fibers innervating the mouse hind paw leads to reduction in mechanical and pain sensitivity. Through investigating PNS NMPs, we demonstrate their presence on the somata and proximal and distal axons of a subset of dorsal root ganglion (DRG) neurons. Single-cell RNA sequencing experiments reveal that the NMP-expressing DRGs are primarily MrgprA3+ and Cysltr2+. NMP inhibition in DRG cultures leads to cell-autonomous and non-cell-autonomous changes in Ca2+ signaling induced by KCl depolarization, αβ-meATP, or the pruritogen histamine. Taken together, these data support a model whereby NMPs are expressed on a subset of somatosensory DRGs to modulate signaling between neurons of distinct sensory modalities and indicate the NMP as a potential target for controlling pain.

4E-BP1-dependent translation in nociceptors controls mechanical hypersensitivity via TRIM32/type I interferon signaling

Activation of the mechanistic target of rapamycin complex 1 (mTORC1) contributes to the development of chronic pain. However, the specific mechanisms by which mTORC1 causes hypersensitivity remain elusive. The eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is a key mTORC1 downstream effector that represses translation initiation. Here, we show that nociceptor-specific deletion of 4E-BP1, mimicking activation of mTORC1-dependent translation, is sufficient to cause mechanical hypersensitivity. Using translating ribosome affinity purification in nociceptors lacking 4E-BP1, we identified a pronounced translational up-regulation of tripartite motif-containing protein 32 (TRIM32), an E3 ubiquitin ligase that promotes interferon signaling. Down-regulation of TRIM32 in nociceptors or blocking type I interferon signaling reversed the mechanical hypersensitivity in mice lacking 4E-BP1. Furthermore, nociceptor-specific ablation of TRIM32 alleviated mechanical hypersensitivity caused by tissue inflammation. These results show that mTORC1 in nociceptors promotes hypersensitivity via 4E-BP1-dependent up-regulation of TRIM32/interferon signaling and identify TRIM32 as a therapeutic target in inflammatory pain.

Paradoxical increases in anterior cingulate cortex activity during nitrous oxide-induced analgesia reveal a signature of pain affect

The general consensus is that increases in neuronal activity in the anterior cingulate cortex (ACC) contribute to pain's negative affect. Here, using in vivo imaging of neuronal calcium dynamics in mice, we report that nitrous oxide, a general anesthetic that reduces pain affect, paradoxically, increases ACC spontaneous activity. As expected, a noxious stimulus also increased ACC activity. However, as nitrous oxide increases baseline activity, the relative change in activity from pre-stimulus baseline was significantly less than the change in the absence of the general anesthetic. We suggest that this relative change in activity represents a neural signature of the affective pain experience. Furthermore, this signature of pain persists under general anesthesia induced by isoflurane, at concentrations in which the mouse is unresponsive. We suggest that this signature underlies the phenomenon of connected consciousness, in which use of the isolated forelimb technique revealed that pain percepts can persist in anesthetized patients.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Hesperidin Methyl Chalcone Reduces the Arthritis Caused by TiO2 in Mice: Targeting Inflammation, Oxidative Stress, Cytokine Production, and Nociceptor Sensory Neuron Activation

Arthroplasty is an orthopedic surgical procedure that replaces a dysfunctional joint by an orthopedic prosthesis, thereby restoring joint function. Upon the use of the joint prosthesis, a wearing process begins, which releases components such as titanium dioxide (TiO₂) that trigger an immune response in the periprosthetic tissue, leading to arthritis, arthroplasty failure, and the need for revision. Flavonoids belong to a class of natural polyphenolic compounds that possess antioxidant and anti-inflammatory activities. Hesperidin methyl chalcone's (HMC) analgesic, anti-inflammatory, and antioxidant effects have been investigated in some models, but its activity against the arthritis caused by prosthesis-wearing molecules, such as TiO₂, has not been investigated. Mice were treated with HMC (100 mg/kg, intraperitoneally (i.p.)) 24 h after intra-articular injection of 3 mg/joint of TiO₂, which was used to induce chronic arthritis. HMC inhibited mechanical hyperalgesia, thermal hyperalgesia, joint edema, leukocyte recruitment, and oxidative stress in the knee joint (alterations in gp91^phox, GSH, superoxide anion, and lipid peroxidation) and in recruited leukocytes (total reactive oxygen species and GSH); reduced patellar proteoglycan degradation; and decreased pro-inflammatory cytokine production. HMC also reduced the activation of nociceptor-sensory TRPV1⁺ and TRPA1⁺ neurons. These effects occurred without renal, hepatic, or gastric damage. Thus, HMC reduces arthritis triggered by TiO₂, a component released upon wearing of prosthesis.

Increased Heat Pain Tolerance but Hyperalgesia to Tonic Inflammatory Pain in the CRND8 Mouse Model of Alzheimer's Disease

BACKGROUND

The effects of Alzheimer's disease (AD) pathology on the experience of pain are poorly understood.

OBJECTIVE

To understand the pathophysiological mechanisms underlying pain sensory transmission in the transgenic mouse model of AD, CRND8.

METHODS

We explored AD-related pathology in the spinal cord and dorsal root ganglia of 18-week-old female CRND8 mice. We assessed nociceptive responses to both acute heat stimuli and persistent inflammatory pain in CRND8 mice and non-transgenic (non-Tg) littermates. In addition, we searched for differences in biochemical correlates of inflammatory pain between CRND8 and non-Tg mice. Finally, we investigated the excitability of dorsal horn noc iceptive neurons in spinal cord slices from CRND8 and non-Tg mice.

RESULTS

We demonstrated the presence of intracellular AD-like pathology in the spinal cord and in the dorsal root ganglia nociceptive sensory neurons of CRND8 mice. We found that CRND8 mice had a reduced susceptibility to acute noxious heat stimuli and an increased sensitivity to tonic inflammatory pain. Tonic inflammatory pain correlated with a lack of induction of pro-opiomelanocortin in the spinal cord of CRND8 mice as compared to non-Tg mice. Electrophysiological recording in acute spinal cord slice preparations indicated an increased probability of glutamate release at the membrane of dorsal horn nociceptive neurons in CRND8 mice.

CONCLUSION

This study suggests that an increased thermal tolerance and a facilitation of nociception by peripheral inflammation can coexist in AD.

Ehrlich Tumor Induces TRPV1-Dependent Evoked and Non-Evoked Pain-like Behavior in Mice

We standardized a model by injecting Ehrlich tumor cells into the paw to evaluate cancer pain mechanisms and pharmacological treatments. Opioid treatment, but not cyclooxygenase inhibitor or tricyclic antidepressant treatments reduces Ehrlich tumor pain. To best use this model for drug screening it is essential to understand its pathophysiological mechanisms. Herein, we investigated the contribution of the transient receptor potential cation channel subfamily V member 1 (TRPV1) in the Ehrlich tumor-induced pain model. Dorsal root ganglia (DRG) neurons from the Ehrlich tumor mice presented higher activity (calcium levels using fluo-4 fluorescent probe) and an increased response to capsaicin (TRPV1 agonist) than the saline-injected animals (p < 0.05). We also observed diminished mechanical (electronic von Frey) and thermal (hot plate) hyperalgesia, paw flinching, and normalization of weight distribution imbalance in TRPV1 deficient mice (p < 0.05). On the other hand, TRPV1 deficiency did not alter paw volume or weight, indicating no significant alteration in tumor growth. Intrathecal injection of AMG9810 (TRPV1 antagonist) reduced ongoing Ehrlich tumor-triggered mechanical and thermal hyperalgesia (p < 0.05). Therefore, the contribution of TRPV1 to Ehrlich tumor pain behavior was revealed by genetic and pharmacological approaches, thus, supporting the use of this model to investigate TRPV1-targeting therapies for the treatment of cancer pain.

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.

mTORC2 mediates structural plasticity in distal nociceptive endings that contributes to pain hypersensitivity following inflammation

The encoding of noxious stimuli into action potential firing is largely mediated by nociceptive free nerve endings. Tissue inflammation, by changing the intrinsic properties of the nociceptive endings, leads to nociceptive hyperexcitability and thus to the development of inflammatory pain. Here, we showed that tissue inflammation–induced activation of the mammalian target of rapamycin complex 2 (mTORC2) triggers changes in the architecture of nociceptive terminals and leads to inflammatory pain. Pharmacological activation of mTORC2 induced elongation and branching of nociceptor peripheral endings and caused long-lasting pain hypersensitivity. Conversely, nociceptor-specific deletion of the mTORC2 regulatory protein rapamycin-insensitive companion of mTOR (Rictor) prevented inflammation-induced elongation and branching of cutaneous nociceptive fibers and attenuated inflammatory pain hypersensitivity. Computational modeling demonstrated that mTORC2-mediated structural changes in the nociceptive terminal tree are sufficient to increase the excitability of nociceptors. Targeting mTORC2 using a single injection of antisense oligonucleotide against Rictor provided long-lasting alleviation of inflammatory pain hypersensitivity. Collectively, we showed that tissue inflammation–induced activation of mTORC2 causes structural plasticity of nociceptive free nerve endings in the epidermis and inflammatory hyperalgesia, representing a therapeutic target for inflammatory pain.

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

The cutaneous mechanisms that trigger spontaneous neuropathic pain in diabetic peripheral neuropathy (PDPN) are far from clear. Two types of nociceptors are found within the epidermal and dermal skin layers. Small-diameter lightly myelinated Aδ and unmyelinated C cutaneous mechano and heat-sensitive (AMH and CMH) and C mechanoinsensitive (CMi) nociceptors transmit pain from the periphery to central nervous system. AMH and CMH fibers are mainly located in the epidermis, and CMi fibers are distributed in the dermis. In DPN, dying back intra-epidermal AMH and CMH fibers leads to reduced pain sensitivity, and the patients exhibit significantly increased pain thresholds to acute pain when tested using traditional methods. The role of CMi fibers in painful neuropathies has not been fully explored. Microneurography has been the only tool to access CMi fibers and differentiate AMH, CMH, and CMi fiber types. Due to the complexity, its use is impractical in clinical settings. In contrast, a newly developed diode laser fiber selective stimulation (DLss) technique allows to safely and selectively stimulate Aδ and C fibers in the superficial and deep skin layers. DLss data demonstrate that patients with painful DPN have increased Aδ fiber pain thresholds, while C-fiber thresholds are intact because, in these patients, CMi fibers are abnormally spontaneously active. It is also possible to determine the involvement of CMi fibers by measuring the area of DLss-induced neurogenic axon reflex flare. The differences in AMH, CMH, and CMi fibers identify patients with painful and painless neuropathy. In this review, we will discuss the role of CMi fibers in PDPN.

Sensory defunctionalization induced by 8% topical capsaicin treatment in a model of ultraviolet-B-induced cutaneous hyperalgesia

Subpopulations of primary nociceptors (C- and Aδ-fibers), express the TRPV1 receptor for heat and capsaicin. During cutaneous inflammation, these afferents may become sensitized, leading to primary hyperalgesia. It is known that TRPV1⁺ nociceptors are involved in heat hyperalgesia; however, their involvement in mechanical hyperalgesia is unclear. This study explored the contribution of capsaicin-sensitive nociceptors in the development of mechanical and heat hyperalgesia in humans following ultraviolet-B (UVB) irradiation. Skin areas in 18 healthy volunteers were randomized to treatment with 8% capsaicin/vehicle patches for 24 h. After patches removal, one capsaicin-treated area and one vehicle area were irradiated with 2xMED (minimal erythema dose) of UVB. 1, 3 and 7 days post-UVB exposure, tests were performed to evaluate the development of UVB-induced cutaneous hyperalgesia: thermal detection and pain thresholds, pain sensitivity to supra-threshold heat stimuli, mechanical pain threshold and sensitivity, touch pleasantness, trans-epidermal water loss (TEWL), inflammatory response, pigmentation and micro-vascular reactivity. Capsaicin pre-treatment, in the UVB-irradiated area (Capsaicin + UVB area), increased heat pain thresholds (P < 0.05), and decreased supra-threshold heat pain sensitivity (P < 0.05) 1, 3 and 7 days post-UVB irradiation, while mechanical hyperalgesia resulted unchanged (P > 0.2). No effects of capsaicin were reported on touch pleasantness (P = 1), TEWL (P = 0.31), inflammatory response and pigmentation (P > 0.3) or micro-vascular reactivity (P > 0.8) in response to the UVB irradiation. 8% capsaicin ablation predominantly defunctionalizes TRPV1⁺-expressing cutaneous nociceptors responsible for heat pain transduction, suggesting that sensitization of these fibers is required for development of heat hyperalgesia following cutaneous UVB-induced inflammation but they are likely only partially necessary for the establishment of robust primary mechanical hyperalgesia.

Pruritogenic mechanisms and gut sensation: putting the "irritant" into irritable bowel syndrome

Chronic abdominal pain is a common clinical condition experienced by patients with irritable bowel syndrome (IBS). A general lack of suitable treatment options for the management of visceral pain is the major contributing factor to the debilitating nature of the disease. Understanding the underlying causes of chronic visceral pain is pivotal to identifying new effective therapies for IBS. This review provides the current evidence, demonstrating that mediators and receptors that induce itch in the skin also act as "gut irritants" in the gastrointestinal tract. Activation of these receptors triggers specific changes in the neuronal excitability of sensory pathways responsible for the transmission of nociceptive information from the periphery to the central nervous system leading to visceral hypersensitivity and visceral pain. Accumulating evidence points to significant roles of irritant mediators and their receptors in visceral hypersensitivity and thus constitutes potential targets for the development of more effective therapeutic options for IBS.

A genome-wide screen reveals microRNAs in peripheral sensory neurons driving painful diabetic neuropathy

ABSTRACT

Diabetes is a leading cause of peripheral neuropathy (diabetic peripheral neuropathy, DPN), and uncontrolled long-lasting hyperglycemia leads to severe complications. A major proportion of diabetics develop excruciating pain with a variable course. Mechanisms leading to painful DPN are not completely understood and treatment options limited. We hypothesized that epigenetic modulation at the level of microRNA (miRNA) expression triggered by metabolic imbalance and nerve damage regulates the course of pain development. We used clinically relevant preclinical models, genome-wide screening, in silico analyses, cellular assays, miRNA fluorescent in situ hybridization, in vivo molecular manipulations, and behavioral analyses in the current study. We identified miRNAs and their targets that critically impact on nociceptive hypersensitivity in painful DPN. Our analyses identify miR-33 and miR-380 expressed in nociceptive neurons as critical denominators of diabetic pain and miR-124-1 as a mediator of physiological nociception. Our comprehensive analyses on the putative mRNA targets for miR-33 or miR-124-1 identified a set of mRNAs that are regulated after miR-33 or miR-124-1 overexpression in dorsal root ganglia in vivo. Our results shed light on the regulation of DPN pathophysiology and implicate specific miRNAs as novel therapeutic targets for treating painful DPN.

Spinal Inhibitory Interneurons: Gatekeepers of Sensorimotor Pathways

The ability to sense and move within an environment are complex functions necessary for the survival of nearly all species. The spinal cord is both the initial entry site for peripheral information and the final output site for motor response, placing spinal circuits as paramount in mediating sensory responses and coordinating movement. This is partly accomplished through the activation of complex spinal microcircuits that gate afferent signals to filter extraneous stimuli from various sensory modalities and determine which signals are transmitted to higher order structures in the CNS and to spinal motor pathways. A mechanistic understanding of how inhibitory interneurons are organized and employed within the spinal cord will provide potential access points for therapeutics targeting inhibitory deficits underlying various pathologies including sensory and movement disorders. Recent studies using transgenic manipulations, neurochemical profiling, and single-cell transcriptomics have identified distinct populations of inhibitory interneurons which express an array of genetic and/or neurochemical markers that constitute functional microcircuits. In this review, we provide an overview of identified neural components that make up inhibitory microcircuits within the dorsal and ventral spinal cord and highlight the importance of inhibitory control of sensorimotor pathways at the spinal level.

The role of Mrgprs in pain

Owing to their functional diversity, the Mas-related G-protein-coupled receptor (Mrgpr) family has a role in both itch and pain modulation. While primarily linked to pruritis, Mrgprs were originally characterized in small-diameter nociceptive neurons of dorsal root ganglia (DRG) and trigeminal ganglia. This review will focus on the role Mrgpr's have in pain physiology, discussing recent discoveries as well as how Mrgpr's may provide a new target for the treatment of pathological pain.

Upregulation of Nav1.7 by endogenous hydrogen sulfide contributes to maintenance of neuropathic pain

Na_v1.7 is closely associated with neuropathic pain. Hydrogen sulfide (H₂S) has recently been reported to be involved in numerous biological functions, and it has been shown that H₂S can enhance the sodium current density, and inhibiting the endogenous production of H₂S mediated by cystathionine β-synthetase (CBS) using O-(carboxymethyl) hydroxylamine hemihydrochloride (AOAA) can significantly reduce the expression of Na_v1.7 and thus the sodium current density in rat dorsal root ganglion (DRG) neurons. In the present study, it was shown that the fluorescence intensity of H₂S was increased in a spared nerve injury (SNI) model and AOAA inhibited this increase. Na_v1.7 is expressed in DRG neurons, and the expression of CBS and Na_v1.7 were increased in DRG neurons 7, 14 and 21 days post-operation. AOAA inhibited the increase in the expression of CBS, phosphorylated (p)-MEK1/2, p-ERK1/2 and Na_v1.7 induced by SNI, and U0126 (a MEK blocker) was able to inhibit the increase in p-MEK1/2, p-ERK1/2 and Na_v1.7 expression. However, PF-04856264 did not inhibit the increase in CBS, p-MEK1/2, p-ERK1/2 or Na_v1.7 expression induced by SNI surgery. The current density of Na_v1.7 was significantly increased in the SNI model and administration of AOAA and U0126 both significantly decreased the density. In addition, AOAA, U0126 and PF-04856264 inhibited the decrease in rheobase, and the increase in action potential induced by SNI in DRG neurons. There was no significant difference in thermal withdrawal latency among each group. However, the time the animals spent with their paw lifted increased significantly following SNI, and the time the animals spent with their paw lifted decreased significantly following the administration of AOAA, U0126 and PF-04856264. In conclusion, these data show that Na_v1.7 expression in DRG neurons is upregulated by CBS-derived endogenous H₂S in an SNI model, contributing to the maintenance of neuropathic pain.

Pain-related changes in cutaneous innervation of patients suffering from bortezomib-induced, diabetic or chronic idiopathic axonal polyneuropathy

Consistent associations between the severity of neuropathic pain and cutaneous innervation have not been described. We collected demographic and clinical data, McGill Pain Questionnaires (MPQ) and skin biopsies processed for PGP9.5 and CGRP immunohistochemistry from patients with bortezomib-induced peripheral neuropathy (BiPN; n = 22), painful diabetic neuropathy (PDN; n = 16), chronic idiopathic axonal polyneuropathy (CIAP; n = 16) and 17 age-matched healthy volunteers. Duration of neuropathic symptoms was significantly shorter in patients with BiPN in comparison with PDN and CIAP patients. BiPN was characterized by a significant increase in epidermal axonal swellings and upper dermis nerve fiber densities (UDNFD) and a decrease in subepidermal nerve fiber densities (SENFD) of PGP9.5-positive fibers and of PGP9.5 containing structures that did not show CGRP labeling, presumably non-peptidergic fibers. In PDN and CIAP patients, intraepidermal nerve fiber densities (IENFD) and SENFD of PGP9.5-positive and of non-peptidergic fibers were decreased in comparison with healthy volunteers. Significant unadjusted associations between IENFD and SENFD of CGRP-positive, i.e. peptidergic, fibers and the MPQ sensory-discriminative, as well as between UDNFD of PGP9.5-positive fibers and the MPQ evaluative/affective component of neuropathic pain, were found in BiPN and CIAP patients. No significant associations were found in PDN patients. Cutaneous innervation changes in BiPN confirm characteristic features of early, whereas those in CIAP and PDN are in line with late forms of neuropathic pathology. Our results allude to a distinct role for non-peptidergic nociceptors in BiPN and CIAP patients. The lack of significant associations in PDN may be caused by mixed ischemic and purely neuropathic pain pathology.

Understanding diverse TRPV1 signaling - an update

The transient receptor potential vanilloid 1 (TRPV1) is densely expressed in spinal sensory neurons as well as in cranial sensory neurons, including their central terminal endings. Recent work in the less familiar cranial sensory neurons, despite their many similarities with spinal sensory neurons, suggest that TRPV1 acts as a calcium channel to release a discrete population of synaptic vesicles. The modular and independent regulation of release offers new questions about nanodomain organization of release and selective actions of G protein–coupled receptors.

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

TRESK K+ Channel Activity Regulates Trigeminal Nociception and Headache

Although TWIK-related spinal cord K⁺ (TRESK) channel is expressed in all primary afferent neurons in trigeminal ganglia (TG) and dorsal root ganglia (DRG), whether TRESK activity regulates trigeminal pain processing is still not established. Dominant-negative TRESK mutations are associated with migraine but not with other types of pain in humans, suggesting that genetic TRESK dysfunction preferentially affects the generation of trigeminal pain, especially headache. Using TRESK global knock-out mice as a model system, we found that loss of TRESK in all TG neurons selectively increased the intrinsic excitability of small-diameter nociceptors, especially those that do not bind to isolectin B4 (IB4^-). Similarly, loss of TRESK resulted in hyper-excitation of the small IB4^- dural afferent neurons but not those that bind to IB4 (IB4⁺). Compared with wild-type littermates, both male and female TRESK knock-out mice exhibited more robust trigeminal nociceptive behaviors, including headache-related behaviors, whereas their body and visceral pain responses were normal. Interestingly, neither the total persistent outward current nor the intrinsic excitability was altered in adult TRESK knock-out DRG neurons, which may explain why genetic TRESK dysfunction is not associated with body and/or visceral pain in humans. We reveal for the first time that, among all primary afferent neurons, TG nociceptors are the most vulnerable to the genetic loss of TRESK. Our findings indicate that endogenous TRESK activity regulates trigeminal nociception, likely through controlling the intrinsic excitability of TG nociceptors. Importantly, we provide evidence that genetic loss of TRESK significantly increases the likelihood of developing headache.

Somatosensation a la mode: plasticity and polymodality in sensory neurons

Our understanding of how peripheral damage-sensing neurons (nociceptors) respond to noxious stimuli is fundamental to the development of effective analgesics. To date, numerous studies have presented diverging hypotheses on how nociceptors encode modality-specific stimuli, including labelled-line, intensity dependence or pattern theory. In this short review, we appraise data from electrophysiological, behavioural, imaging and molecular expression studies from the last 60 years, in order to obtain a coherent view of modality-specific sensing in peripheral sensory neurons. We propose a mechanistic explanation for the broad range of values obtained for the incidence of polymodal nociceptors that reconciles apparently contradictory data.

5-oxoETE triggers nociception in constipation-predominant irritable bowel syndrome through MAS-related G protein-coupled receptor D

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder that is characterized by chronic abdominal pain concurrent with altered bowel habit. Polyunsaturated fatty acid (PUFA) metabolites are increased in abundance in IBS and are implicated in the alteration of sensation to mechanical stimuli, which is defined as visceral hypersensitivity. We sought to quantify PUFA metabolites in patients with IBS and evaluate their role in pain. Quantification of PUFA metabolites by mass spectrometry in colonic biopsies showed an increased abundance of 5-oxoeicosatetraenoic acid (5-oxoETE) only in biopsies taken from patients with IBS with predominant constipation (IBS-C). Local administration of 5-oxoETE to mice induced somatic and visceral hypersensitivity to mechanical stimuli without causing tissue inflammation. We found that 5-oxoETE directly acted on both human and mouse sensory neurons as shown by lumbar splanchnic nerve recordings and Ca²⁺ imaging of dorsal root ganglion (DRG) neurons. We showed that 5-oxoETE selectively stimulated nonpeptidergic, isolectin B4 (IB4)-positive DRG neurons through a phospholipase C (PLC)- and pertussis toxin-dependent mechanism, suggesting that the effect was mediated by a G protein-coupled receptor (GPCR). The MAS-related GPCR D (Mrgprd) was found in mouse colonic DRG afferents and was identified as being implicated in the noxious effects of 5-oxoETE. Together, these data suggest that 5-oxoETE, a potential biomarker of IBS-C, induces somatic and visceral hyperalgesia without inflammation in an Mrgprd-dependent manner. Thus, 5-oxoETE may play a pivotal role in the abdominal pain associated with IBS-C.

Modality-specific peripheral antinociceptive effects of μ-opioid agonists on heat and mechanical stimuli: Contribution of sigma-1 receptors

Morphine induces peripherally μ-opioid-mediated antinociception to heat but not to mechanical stimulation. Peripheral sigma-1 receptors tonically inhibit μ-opioid antinociception to mechanical stimuli, but it is unknown whether they modulate μ-opioid heat antinociception. We hypothesized that sigma-1 receptors might play a role in the modality-specific peripheral antinociceptive effects of morphine and other clinically relevant μ-opioid agonists. Mechanical nociception was assessed in mice with the paw pressure test (450 g), and heat nociception with the unilateral hot plate (55 °C) test. Local peripheral (intraplantar) administration of morphine, buprenorphine or oxycodone did not induce antinociception to mechanical stimulation but had dose-dependent antinociceptive effects on heat stimuli. Local sigma-1 antagonism unmasked peripheral antinociception by μ-opioid agonists to mechanical stimuli, but did not modify their effects on heat stimulation. TRPV1+ and IB4+ cells are segregated populations of small neurons in the dorsal root ganglia (DRG) and the density of sigma-1 receptors was higher in IB4+ cells than in the rest of small nociceptive neurons. The in vivo ablation of TRPV1-expressing neurons with resiniferatoxin did not alter IB4+ neurons in the DRG, mechanical nociception, or the effects of sigma-1 antagonism on local morphine antinociception in this type of stimulus. However, it impaired the responses to heat stimuli and the effect of local morphine on heat nociception. In conclusion, peripheral opioid antinociception to mechanical stimuli is limited by sigma-1 tonic inhibitory actions, whereas peripheral opioid antinociception to heat stimuli (produced in TRPV1-expressing neurons) is not. Therefore, sigma-1 receptors contribute to the modality-specific peripheral effects of opioid analgesics.

The neurobiology of acute pain

The mechanisms by which noxious stimuli produce the sensation of pain in animals are complex. Noxious stimuli are transduced at the periphery and transmitted to the CNS, where this information is subject to considerable modulation. Finally, the information is projected to the brain where it is perceived as pain. Additionally, plasticity can develop in the pain pathway and hyperalgesia and allodynia may develop through sensitisation both peripherally and centrally. A large number of different ion channels, receptors, and cell types are involved in pain perception, and it is hoped that through a better understanding of these, new and refined treatments for pain will result.

Role of neurturin in spontaneous itch and increased nonpeptidergic intraepidermal fiber density in a mouse model of psoriasis

Psoriasis is often accompanied by itch, but the mechanisms behind this symptom remain elusive. Dynamic changes in epidermal innervation have been observed under chronic itch conditions. Therefore, we investigated whether epidermal innervation is altered in the imiquimod-induced psoriasis mouse model, whether blockade of neurotrophic growth factor signaling can reduce these changes, and whether this system can impact psoriatic itch. Over the 7-day time course of imiquimod treatment, the density of epidermal nonpeptidergic nerves significantly increased, whereas the density of peptidergic nerves significantly decreased. The nonpeptidergic epidermal nerves expressed glial cell line-derived neurotrophic factor (GDNF) family receptor GFRα-1 and GFRα-2, the ligand-binding domains for GDNF and neurturin (NRTN). The NRTN mRNA expression was elevated in the skin of imiquimod-treated mice, whereas the GDNF mRNA expression was decreased. Treatment of imiquimod-challenged mice with an NRTN-neutralizing antibody significantly reduced nonpeptidergic nerve density as well as spontaneous scratching. These results indicate that NRTN contributes to an increase in the epidermal density of nonpeptidergic nerves in the imiquimod-induced psoriasis model, and this increase may be a factor in chronic itch for these mice. Therefore, inhibition of NRTN could be a potential treatment for chronic itch in psoriasis.

Will optogenetics be used to treat chronic pain patients?

Chronic pain affects a third of the population and current treatments produce limited relief and severe side effects. An alternative strategy to decrease pain would be to directly modulate somatosensory pathways using optogenetics. Optogenetics involves the use of genetically encoded and optically active proteins, namely opsins, to control neuronal circuits. In preclinical animal models, optical silencing of peripheral nociceptors has been shown to alleviate both inflammatory and neuropathic pain. An opsin-based gene therapy to treat chronic pain patients is not ready yet, but encouraging advances have been made in optical and viral technology. In view of the increasing burden of chronic pain in our aging society, innovative analgesic approaches based on optogenetics are definitely worth exploring.

Genetic ablation of GINIP-expressing primary sensory neurons strongly impairs Formalin-evoked pain

Primary sensory neurons are heterogeneous by myriad of molecular criteria. However, the functional significance of this remarkable heterogeneity is just emerging. We precedently described the GINIP⁺ neurons as a new subpopulation of non peptidergic C-fibers encompassing the free nerve ending cutaneous MRGPRD⁺ neurons and C-LTMRs. Using our recently generated ginip mouse model, we have been able to selectively ablate the GINIP⁺ neurons and assess their functional role in the somatosensation. We found that ablation of GINIP⁺ neurons affected neither the molecular contents nor the central projections of the spared neurons. GINIP-DTR mice exhibited impaired sensation to gentle mechanical stimuli applied to their hairy skin and had normal responses to noxious mechanical stimuli applied to their glabrous skin, under acute and injury-induced conditions. Importantly, loss of GINIP⁺ neurons significantly altered formalin-evoked first pain and drastically suppressed the second pain response. Given that MRGPRD⁺ neurons have been shown to be dispensable for formalin-evoked pain, our study suggest that C-LTMRs play a critical role in the modulation of formalin-evoked pain.

In vivo characterization of distinct modality-specific subsets of somatosensory neurons using GCaMP

Mechanistic insights into pain pathways are essential for a rational approach to treating this vast and increasing clinical problem. Sensory neurons that respond to tissue damage (nociceptors) may evoke pain sensations and are typically classified on the basis of action potential velocity. Electrophysiological studies have suggested that most of the C-fiber nociceptors are polymodal, responding to a variety of insults. In contrast, gene deletion studies in the sensory neurons of transgenic mice have frequently resulted in modality-specific deficits. We have used an in vivo imaging approach using the genetically encoded fluorescent calcium indicator GCaMP to study the activity of dorsal root ganglion sensory neurons in live animals challenged with painful stimuli. Using this approach, we can visualize spatially distinct neuronal responses and find that >85% of responsive dorsal root ganglion neurons are modality-specific, responding to either noxious mechanical, cold, or heat stimuli. These observations are mirrored in behavioral studies of transgenic mice. For example, deleting sodium channel Nav1.8 silences mechanical- but not heat-sensing sensory neurons, consistent with behavioral deficits. In contrast, primary cultures of axotomized sensory neurons show high levels of polymodality. After intraplantar treatment with prostaglandin E₂, neurons in vivo respond more intensely to noxious thermal and mechanical stimuli, and additional neurons (silent nociceptors) are unmasked. Together, these studies define polymodality as an infrequent feature of nociceptive neurons in normal animals.

The coding of cutaneous temperature in the spinal cord

The spinal cord is the initial stage that integrates temperature information from peripheral inputs. Here we used molecular genetics and in vivo calcium imaging to investigate the coding of cutaneous temperature in the spinal cord in mice. We found that heating or cooling the skin evoked robust calcium responses in spinal neurons, and their activation threshold temperatures distributed smoothly over the entire range of stimulation temperatures. Once activated, heat-responding neurons encoded the absolute skin temperature without adaptation and received major inputs from transient receptor potential (TRP) channel V1 (TRPV1)-positive dorsal root ganglion (DRG) neurons. By contrast, cold-responding neurons rapidly adapted to ambient temperature and selectively encoded temperature changes. These neurons received TRP channel M8 (TRPM8)-positive DRG inputs as well as novel TRPV1(+) DRG inputs that were selectively activated by intense cooling. Our results provide a comprehensive examination of the temperature representation in the spinal cord and reveal fundamental differences in the coding of heat and cold.

CT-guided injection of a TRPV1 agonist around dorsal root ganglia decreases pain transmission in swine

One approach to analgesia is to block pain at the site of origin or along the peripheral pathway by selectively ablating pain-transmitting neurons or nerve terminals directly. The heat/capsaicin receptor (TRPV1) expressed by nociceptive neurons is a compelling target for selective interventional analgesia because it leaves somatosensory and proprioceptive neurons intact. Resiniferatoxin (RTX), like capsaicin, is a TRPV1 agonist but has greater potency. We combine RTX-mediated inactivation with the precision of computed tomography (CT)-guided delivery to ablate peripheral pain fibers in swine. Under CT guidance, RTX was delivered unilaterally around the lumbar dorsal root ganglia (DRG), and vehicle only was administered to the contralateral side. During a 4-week observation period, animals demonstrated delayed or absent withdrawal responses to infrared laser heat stimuli delivered to sensory dermatomes corresponding to DRG receiving RTX treatment. Motor function was unimpaired as assessed by disability scoring and gait analysis. In treated DRG, TRPV1 mRNA expression was reduced, as were nociceptive neuronal perikarya in ganglia and their nerve terminals in the ipsilateral dorsal horn. CT guidance to precisely deliver RTX to sites of peripheral pain transmission in swine may be an approach that could be tailored to block an array of clinical pain conditions in patients.

Mas-Related G Protein-Coupled Receptors Offer Potential New Targets for Pain Therapy

The founding member of the Mas-related G-protein-coupled receptor (Mrgpr) family was discovered in 1986. Since then, many more members of this receptor family have been identified in multiple species, and their physiologic functions have been investigated widely. Because they are expressed exclusively in small-diameter primary sensory neurons, the roles of Mrgpr proteins in pain and itch have been best studied. This review will focus specifically on the current knowledge of their roles in pathological pain and the potential development of new pharmacotherapies targeted at some Mrgprs for the treatment of chronic pain. We will also discuss the limitations and future scope of this receptor family in pain treatment.

Upregulation of Ih expressed in IB4-negative Aδ nociceptive DRG neurons contributes to mechanical hypersensitivity associated with cervical radiculopathic pain

Cervical radiculopathy represents aberrant mechanical hypersensitivity. Primary sensory neuron’s ability to sense mechanical force forms mechanotransduction. However, whether this property undergoes activity-dependent plastic changes and underlies mechanical hypersensitivity associated with cervical radiculopathic pain (CRP) is not clear. Here we show a new CRP model producing stable mechanical compression of dorsal root ganglion (DRG), which induces dramatic behavioral mechanical hypersensitivity. Amongst nociceptive DRG neurons, a mechanically sensitive neuron, isolectin B4 negative Aδ-type (IB4⁻ Aδ) DRG neuron displays spontaneous activity with hyperexcitability after chronic compression of cervical DRGs. Focal mechanical stimulation on somata of IB4^- Aδ neuron induces abnormal hypersensitivity. Upregulated HCN1 and HCN3 channels and increased I_h current on this subset of primary nociceptors underlies the spontaneous activity together with neuronal mechanical hypersensitivity, which further contributes to the behavioral mechanical hypersensitivity associated with CRP. This study sheds new light on the functional plasticity of a specific subset of nociceptive DRG neurons to mechanical stimulation and reveals a novel mechanism that could underlie the mechanical hypersensitivity associated with cervical radiculopathy.

Gate control of mechanical itch by a subpopulation of spinal cord interneurons

Light mechanical stimulation of hairy skin can induce a form of itch known as mechanical itch. This itch sensation is normally suppressed by inputs from mechanoreceptors; however, in many forms of chronic itch, including alloknesis, this gating mechanism is lost. Here we demonstrate that a population of spinal inhibitory interneurons that are defined by the expression of neuropeptide Y::Cre (NPY::Cre) act to gate mechanical itch. Mice in which dorsal NPY::Cre-derived neurons are selectively ablated or silenced develop mechanical itch without an increase in sensitivity to chemical itch or pain. This chronic itch state is histamine-independent and is transmitted independently of neurons that express the gastrin-releasing peptide receptor. Thus, our studies reveal a dedicated spinal cord inhibitory pathway that gates the transmission of mechanical itch.

Liposomal topical capsaicin in post-herpetic neuralgia: a safety pilot study

Topical treatments have gained popularity for general use as an adjunct to systemic drugs in neuropathic pain, but their use produces variable clinical results and local adverse events.

OBJECTIVE

To evaluate the safety and analgesic effect of a formulation of liposomal capsaicin (LC) (0.025%) in patients with post herpetic neuralgia (PHN).

METHOD

Patients who remained symptomatic after first-and second-line treatment were randomized to receive LC for six weeks in a placebo-controlled, crossover design study. Clinical assessment was performed at baseline, in the second, fourth and sixth week of treatment.

RESULTS

Thirteen patients completed both treatment periods. Visual Analog Scale (VAS) was significantly decreased after the end of the study (p = 0.008), however the effect of treatment was not significant (p = 0.076). There was no difference on global impression of change and other pain characteristics. LC was safe and well tolerated. However, at the concentration used, its analgesic effects were marginal and not significant.

Neurotrophin selectivity in organizing topographic regeneration of nociceptive afferents

Neurotrophins represent some of the best candidates to enhance regeneration. In the current study, we investigated the effects of artemin, a member of the glial derived neurotrophic factor (GDNF) family, on sensory axon regeneration following a lumbar dorsal root injury and compared these effects with that observed after either NGF or GDNF expression in the rat spinal cord. Unlike previously published data, artemin failed to induce regeneration of large-diameter myelinated sensory afferents when expressed within either the spinal cord or DRG. However, artemin or NGF induced regeneration of calcitonin gene related peptide positive (CGRP(+)) axons only when expressed within the spinal cord. Accordingly, artemin or NGF enhanced recovery of only nociceptive behavior and showed a cFos distribution similar to the topography of regenerating axons. Artemin and GDNF signaling requires binding to different co-receptors (GFRα3 or GFRα1, respectively) prior to binding to the signaling receptor, cRet. Approximately 70% of DRG neurons express cRet, but only 35% express either co-receptor. To enhance artemin-induced regeneration, we co-expressed artemin with either GFRα3 or GDNF. Co-expression of artemin and GFRα3 only slightly enhanced regeneration of IB4(+) non-peptidergic nociceptive axons, but not myelinated axons. Interestingly, this co-expression also disrupted the ability of artemin to produce topographic targeting and lead to significant increases in cFos immunoreactivity within the deep dorsal laminae. This study failed to demonstrate artemin-induced regeneration of myelinated axons, even with co-expression of GFRα3, which only promoted mistargeted regeneration.

Transplant-mediated enhancement of spinal cord GABAergic inhibition reverses paclitaxel-induced mechanical and heat hypersensitivity

Decreased spinal cord GABAergic inhibition is a major contributor to the persistent neuropathic pain that can follow peripheral nerve injury. Recently, we reported that restoring spinal cord GABAergic signaling by intraspinal transplantation of cortical precursors of GABAergic interneurons from the embryonic medial ganglionic eminence (MGE) can reverse the mechanical hypersensitivity (allodynia) that characterizes a neuropathic pain model in the mouse. We show that MGE cell transplants are also effective against both the mechanical allodynia and the heat hyperalgesia produced in a paclitaxel-induced chemotherapy model of neuropathic pain. To test the necessity of GABA release by the transplants, we also studied the utility of transplanting MGE cells from mice with a deletion of VGAT, the vesicular GABA transporter. Transplants from these mice, in which GABA is synthesized but cannot be stored or released, had no effect on mechanical hypersensitivity or heat hyperalgesia in the paclitaxel model. Taken together, these results demonstrate the therapeutic potential of GABAergic precursor cell transplantation in diverse neuropathic pain models and support our contention that restoration of inhibitory controls through release of GABA from the transplants is their mode of action.

An early diagnostic tool for diabetic peripheral neuropathy in rats

The skin’s rewarming rate of diabetic patients is used as a diagnostic tool for early diagnosis of diabetic neuropathy. At present, the relationship between microvascular changes in the skin and diabetic neuropathy is unclear in streptozotocin (STZ) diabetic rats. The aim of this study was to investigate whether the skin rewarming rate in diabetic rats is related to microvascular changes and whether this is accompanied by changes observed in classical diagnostic methods for diabetic peripheral neuropathy. Computer-assisted infrared thermography was used to assess the rewarming rate after cold exposure on the plantar skin of STZ diabetic rats’ hind paws. Peripheral neuropathy was determined by the density of intra-epidermal nerve fibers (IENFs), mechanical sensitivity, and electrophysiological recordings. Data were obtained in diabetic rats at four, six, and eight weeks after the induction of diabetes and in controls. Four weeks after the induction of diabetes, a delayed rewarming rate, decreased skin blood flow and decreased density of IENFs were observed. However, the mechanical hyposensitivity and decreased motor nerve conduction velocity (MNCV) developed 6 and 8 weeks after the induction of diabetes. Our study shows that the skin rewarming rate is related to microvascular changes in diabetic rats. Moreover, the skin rewarming rate is a non-invasive method that provides more information for an earlier diagnosis of peripheral neuropathy than the classical monofilament test and MNCV in STZ induced diabetic rats.

Pharmacology and signaling of MAS-related G protein-coupled receptors

Signaling by heptahelical G protein-coupled receptors (GPCR) regulates many vital body functions. Consequently, dysfunction of GPCR signaling leads to pathologic states, and approximately 30% of all modern clinical drugs target GPCR. One decade ago, an entire new GPCR family was discovered, which was recently named MAS-related G protein-coupled receptors (MRGPR) by the HUGO Gene Nomenclature Committee. The MRGPR family consists of ∼40 members that are grouped into nine distinct subfamilies (MRGPRA to -H and -X) and are predominantly expressed in primary sensory neurons and mast cells. All members are formally still considered "orphan" by the Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology. However, several distinct peptides and amino acids are discussed as potential ligands, including β-alanine, angiotensin-(1-7), alamandine, GABA, cortistatin-14, and cleavage products of proenkephalin, pro-opiomelanocortin, prodynorphin, or proneuropeptide-FF-A. The full spectrum of biologic roles of all MRGPR is still ill-defined, but there is evidence pointing to a role of distinct MRGPR subtypes in nociception, pruritus, sleep, cell proliferation, circulation, and mast cell degranulation. This review article summarizes findings published in the last 10 years on the phylogenetic relationships, pharmacology, signaling, physiology, and agonist-promoted regulation of all MRGPR subfamilies. Furthermore, we highlight interactions between MRGPR and other hormonal systems, paying particular attention to receptor multimerization and morphine tolerance. Finally, we discuss the challenges the field faces presently and emphasize future directions of research.

Update on the role of spinal cord TRPV1 receptors in pain modulation

The structure, expression and function of the transient receptor potential vanilloid 1 (TRPV1) receptor were intensively studied since the cloning in 1997 and TRPV1 receptors are now considered to act as transducers and molecular integrators of nociceptive stimuli in the periphery. In contrast, spinal TRPV1 receptors were studied less extensively and their role in pain modulation is still not fully understood. This short review is a follow up on our previous summary in this area (Spicarova and Palecek 2008). The aim was to review preferentially the most recent findings concerning the role of the spinal TRPV1 receptors, published within the last five years. The update is given on the expression and function of the spinal TRPV1 receptors, their activation by endogenous agonists, interaction between the endocannabinoid and endovanillod system and possible role of the spinal TRPV1 receptors in pathological pain states. There is now mounting evidence that TRPV1 receptors may be an important element in modulation of nociceptive information at the spinal cord level and represent an interesting target for analgesic therapy.

Molecular signatures of mouse TRPV1-lineage neurons revealed by RNA-Seq transcriptome analysis

Disorders of pain neural systems are frequently chronic and, when recalcitrant to treatment, can severely degrade the quality of life. The pain pathway begins with sensory neurons in dorsal root or trigeminal ganglia, and the neuronal subpopulations that express the transient receptor potential cation channel, subfamily V, member 1 (TRPV1) ion channel transduce sensations of painful heat and inflammation and play a fundamental role in clinical pain arising from cancer and arthritis. In the present study, we elucidate the complete transcriptomes of neurons from the TRPV1 lineage and a non-TRPV1 neuroglial population in sensory ganglia through the combined application of next-gen deep RNA-Seq, genetic neuronal labeling with fluorescence-activated cell sorting, or neuron-selective chemoablation. RNA-Seq accurately quantitates gene expression, a difficult parameter to determine with most other methods, especially for very low and very high expressed genes. Differentially expressed genes are present at every level of cellular function from the nucleus to the plasma membrane. We identified many ligand receptor pairs in the TRPV1 population, suggesting that autonomous presynaptic regulation may be a major regulatory mechanism in nociceptive neurons. The data define, in a quantitative, cell population-specific fashion, the molecular signature of a distinct and clinically important group of pain-sensing neurons and provide an overall framework for understanding the transcriptome of TRPV1 nociceptive neurons.

PERSPECTIVE

Next-gen RNA-Seq, combined with molecular genetics, provides a comprehensive and quantitative measurement of transcripts in TRPV1 lineage neurons and a contrasting transcriptome from non-TRPV1 neurons and cells. The transcriptome highlights previously unrecognized protein families, identifies multiple molecular circuits for excitatory or inhibitory autocrine and paracrine signaling, and suggests new combinatorial approaches to pain control.

Activation of transient receptor potential vanilloid 2-expressing primary afferents stimulates synaptic transmission in the deep dorsal horn of the rat spinal cord and elicits mechanical hyperalgesia

Probenecid, an agonist of transient receptor vanilloid (TRPV) type 2, was used to evaluate the effects of TRPV2 activation on excitatory and inhibitory synaptic transmission in the dorsal horn (DH) of the rat spinal cord and on nociceptive reflexes induced by thermal heat and mechanical stimuli. The effects of probenecid were compared with those of capsaicin, a TRPV1 agonist. Calcium imaging experiments on rat dorsal root ganglion (DRG) and DH cultures indicated that functional TRPV2 and TRPV1 were expressed by essentially non-overlapping subpopulations of DRG neurons, but were absent from DH neurons and DH and DRG glial cells. Pretreatment of DRG cultures with small interfering RNAs against TRPV2 suppressed the responses to probenecid. Patch-clamp recordings from spinal cord slices showed that probenecid and capsaicin increased the frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents in a subset of laminae III-V neurons. In contrast to capsaicin, probenecid failed to stimulate synaptic transmission in lamina II. Intrathecal or intraplantar injections of probenecid induced mechanical hyperalgesia/allodynia without affecting nociceptive heat responses. Capsaicin induced both mechanical hyperalgesia/allodynia and heat hyperalgesia. Activation of TRPV1 or TRPV2 in distinct sets of primary afferents increased the sEPSC frequencies in a largely common population of DH neurons in laminae III-V, and might underlie the development of mechanical hypersensitivity following probenecid or capsaicin treatment. However, only TRPV1-expressing afferents facilitated excitatory and/or inhibitory transmission in a subpopulation of lamina II neurons, and this phenomenon might be correlated with the induction of thermal heat hyperalgesia.

Functional distinction between NGF-mediated plasticity and regeneration of nociceptive axons within the spinal cord

Successful regeneration after injury requires either the direct reformation of the circuit or the formation of a bridge circuit to provide partial functional return through a more indirect route. Presently, little is known about the specificity of how regenerating axons reconnect or reconstruct functional circuits. We have established an in vivo Dorsal root entry zone (DREZ) model, which in the presence of Nerve Growth Factor (NGF), shows very robust regeneration of peptidergic nociceptive axons, but not other sensory axons. Expression of NGF in normal, non-injured animals leads to robust sprouting of only the peptidergic nociceptive axons. Interestingly, NGF-induced sprouting of these axons leads to severe chronic pain, whereas, regeneration leads to protective-like pain without chronic pain. Using this model we set out to compare differences in behavioral outcomes and circuit features between these two groups. In this study, we examined pre-synaptic and post-synaptic markers to evaluate the relationship between synaptic connections and behavioral responses. NGF-induced sprouting of calcitonin gene-related peptide (CGRP) axons resulted in a significant redistribution of synapses and cFos expression into the deeper dorsal horn. Regeneration of only the CGRP axons showed a general reduction in synapses and cFos expression within laminae I and II; however, inflammation of the hindpaw induced peripheral sensitization. These data show that although NGF-induced sprouting of peptidergic axons induces robust chronic pain and cFos expression throughout the entire dorsal horn, regeneration of the same axons resulted in normal protective pain with a synaptic and cFos distribution similar, albeit significantly less than that shown by the sprouting of CGRP axons.

Three functionally distinct classes of C-fibre nociceptors in primates

In primates, C-fibre polymodal nociceptors are broadly classified into two groups based on mechanosensitivity. Here we demonstrate that mechanically sensitive polymodal nociceptors that respond either quickly (QC) or slowly (SC) to a heat stimulus differ in responses to a mild burn, heat sensitization, conductive properties and chemosensitivity. Superficially applied capsaicin and intradermal injection of β-alanine, an MrgprD agonist, excite vigorously all QCs. Only 40% of SCs respond to β-alanine, and their response is only half that of QCs. Mechanically insensitive C-fibres (C-MIAs) are β-alanine insensitive but vigorously respond to capsaicin and histamine with distinct discharge patterns. Calcium imaging reveals that β-alanine and histamine activate distinct populations of capsaicin-responsive neurons in primate dorsal root ganglion. We suggest that histamine itch and capsaicin pain are peripherally encoded in C-MIAs, and that primate polymodal nociceptive afferents form three functionally distinct subpopulations with β-alanine responsive QC fibres likely corresponding to murine MrgprD-expressing, non-peptidergic nociceptive afferents.

Transmitting pain and itch messages: a contemporary view of the spinal cord circuits that generate gate control

The original formulation of Gate Control Theory (GCT) proposed that the perception of pain produced by spinal cord signaling to the brain depends on a balance of activity generated in large (nonnociceptive)- and small (nociceptive)-diameter primary afferent fibers. The theory proposed that activation of the large-diameter afferent "closes" the gate by engaging a superficial dorsal horn interneuron that inhibits the firing of projection neurons. Activation of the nociceptors "opens" the gate through concomitant excitation of projection neurons and inhibition of the inhibitory interneurons. Sixty years after publication of the GCT, we are faced with an ever-growing list of morphologically and neurochemically distinct spinal cord interneurons. The present Review highlights the complexity of superficial dorsal horn circuitry and addresses the question whether the premises outlined in GCT still have relevance today. By examining the dorsal horn circuits that underlie the transmission of "pain" and "itch" messages, we also address the extent to which labeled lines can be incorporated into a contemporary view of GCT.