Differential expression of the mRNA for the vanilloid receptor subtype 1 in cells of the adult rat dorsal root and nodose ganglia and its downregulation by axotomy

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.

Chronic chemogenetic inhibition of TRPV1 bladder afferent promotes micturition recovery post SCI

Spinal cord injury often results in chronic loss of micturition control, which is featured by bladder hyperreflexia and detrusor sphincter dyssynergia. Previous studies showed that treatment of capsaicin reduces non-voiding bladder contractions in multiple animal injury models and human patients. However, its underlying neural mechanisms remain largely unknown. Here, by injecting a RetroAAV into the bladder wall, we specifically targeted TRPV1+, a capsaicin receptor, bladder afferent neurons. Morphometric analysis revealed borderline increase of the soma size and significant spinal axon sprouting of TRPV1+ bladder afferent neurons post a complete T8 spinal cord crush. We further demonstrated that chronic chemogenetic inhibition of these DRG neurons improved micturition recovery after SCI by increasing voiding efficiency and alleviating bladder hyperreflexia, along with reduced morphological changes caused by injury. Our study provided novel insights into the structural and functional changes of TRPV1+ bladder afferent post SCI and further supports the clinical use of capsaicin as an effective treatment to improve bladder functions in patients with SCI.

Peripheral Neuroinflammation and Pain: How Acute Pain Becomes Chronic

The number of individuals suffering from severe chronic pain and its social and financial impact is staggering. Without significant advances in our understanding of how acute pain becomes chronic, effective treatments will remain out of reach. This mini review will briefly summarize how critical signaling pathways initiated during the early phases of peripheral nervous system inflammation/ neuroinflammation establish long-term modifications of sensory neuronal function. Together with the recruitment of non-neuronal cellular elements, nociceptive transduction is transformed into a pathophysiologic state sustaining chronic peripheral sensitization and pain. Inflammatory mediators, such as nerve growth factor (NGF), can lower activation thresholds of sensory neurons through posttranslational modification of the pain-transducing ion channels transient-receptor potential TRPV1 and TRPA1. Performing a dual role, NGF also drives increased expression of TRPV1 in sensory neurons through the recruitment of transcription factor Sp4. More broadly, Sp4 appears to modulate a nociceptive transcriptome including TRPA1 and other genes encoding components of pain transduction. Together, these findings suggest a model where acute pain evoked by peripheral injury-induced inflammation becomes persistent through repeated cycles of TRP channel modification, Sp4-dependent overexpression of TRP channels and ongoing production of inflammatory mediators.

Infectious and Inflammatory Pathways to Cough

Coughing is a dynamic physiological process resulting from input of vagal sensory neurons innervating the airways and perceived airway irritation. Although cough serves to protect and clear the airways, it can also be exploited by respiratory pathogens to facilitate disease transmission. Microbial components or infection-induced inflammatory mediators can directly interact with sensory nerve receptors to induce a cough response. Analysis of cough-generated aerosols and transmission studies have further demonstrated how infectious disease is spread through coughing. This review summarizes the neurophysiology of cough, cough induction by respiratory pathogens and inflammation, and cough-mediated disease transmission.

Phenotypic screen identifies the natural product silymarin as a novel anti-inflammatory analgesic

Sensory neuron hyperexcitability is a critical driver of pathological pain and can result from axon damage, inflammation, or neuronal stress. G-protein coupled receptor signaling can induce pain amplification by modulating the activation of Trp-family ionotropic receptors and voltage-gated ion channels. Here, we sought to use calcium imaging to identify novel inhibitors of the intracellular pathways that mediate sensory neuron sensitization and lead to hyperexcitability. We identified a novel stimulus cocktail, consisting of the SSTR2 agonist L-054,264 and the S1PR3 agonist CYM5541, that elicits calcium responses in mouse primary sensory neurons in vitro as well as pain and thermal hypersensitivity in mice in vivo. We screened a library of 906 bioactive compounds and identified 24 hits that reduced calcium flux elicited by L-054,264/CYM5541. Among these hits, silymarin, a natural product derived from milk thistle, strongly reduced activation by the stimulation cocktail, as well as by a distinct inflammatory cocktail containing bradykinin and prostaglandin E2. Silymarin had no effect on sensory neuron excitability at baseline, but reduced calcium flux via Orai channels and downstream mediators of phospholipase C signaling. In vivo, silymarin pretreatment blocked development of adjuvant-mediated thermal hypersensitivity, indicating potential use as an anti-inflammatory analgesic.

Cutaneous Injection of Resiniferatoxin Completely Alleviates and Prevents Nerve-Injury-Induced Neuropathic Pain

Fifth lumbar (L5) nerve injury in rodent produces neuropathic manifestations in the corresponding hind paw. The aim of this study was to investigate the effect of cutaneous injection of resiniferatoxin (RTX), a TRPV1 receptor agonist, in the rat's hind paw on the neuropathic pain induced by L5 nerve injury. The results showed that intraplantar injection of RTX (0.002%, 100 µL) (1) completely reversed the development of chronic thermal and mechanical hypersensitivity; (2) completely prevented the development of nerve-injury-induced thermal and mechanical hypersensitivity when applied one week earlier; (3) caused downregulation of nociceptive pain markers, including TRPV1, IB4 and CGRP, and upregulation of VIP in the ipsilateral dorsal horn of spinal cord and dorsal root ganglion (DRG) immunohistochemically and a significant reduction in the expression of TRPV1 mRNA and protein in the ipsilateral DRG using Western blot and qRT-PCR techniques; (4) caused downregulation of PGP 9.5- and CGRP-immunoreactivity in the injected skin; (5) produced significant suppression of c-fos expression, as a neuronal activity marker, in the spinal neurons in response to a second intraplantar RTX injection two weeks later. This work identifies the ability of cutaneous injection of RTX to completely alleviate and prevent the development of different types of neuropathic pain in animals and humans.

Transient receptor potential melastatin-7 in the rat dorsal root ganglion

AIMS

Transient receptor potential melastatin-7 (TRPM7) is a selective cation permeable channel which plays important roles in cellular and developmental biology such as cell proliferation, survival, differentiation and migration. This channel is also known to be necessary for transmitter release in the peripheral nervous system. In this study, immunohistochemistry for TRPM7 was conducted in the rat lumbar dorsal root ganglion (DRG).

METHODS

Triple immunofluorescence methods were used to demonstrate distribution of TRPM7 and its relationship to other TRP channels in the DRG. Retrograde tracing and double immunofluorescence methods were also performed to know peripheral targets of DRG neurons containing TRPM7 and TRP vanilloid 1 (TRPV1). In addition, transection of the sciatic nerve was conducted to demonstrate an effect of the nerve injury on TRPM7expression in the DRG.

RESULTS

TRPM7-immunoreactivity was expressed by 53.9% of sensory neurons in the 4th lumbar DRG. TRPM7-immunoreactive (-IR) DRG neurons mostly had small (<600 µm²) and medium-sized (600-1200 µm²) cell bodies. By triple and double immunofluorescence methods, approximately 70% of TRPM7-IR DRG neurons contained TRPV1-immunoreactivity. Although the number of DRG neurons co-expressing TRPM7 and TRPM8 was small in the DRG, almost all of TRPM8-IR DRG neurons co-expressed TRPM7-immunoreactivity. By combination of retrograde tracing method and immunohistochemistry, TRPM7 was expressed by half of DRG neurons innervating the plantar skin (61.9%) and gastrocnemius muscle (51.2%), and 79.6% of DRG neurons innervating the periosteum. Co-expression of TRPM7 and TRPV1 among periosteum DRG neurons (75.7%) was more abundant than among cutaneous (53.2%) and muscular (40.4%) DRG neurons. DRG neurons which co-expressed these ion channels in the periosteum had smaller cell bodies compared to the skin and muscle. In addition, the sciatic nerve transection decreased the number of TRPM7-IR neurons in the DRG (approximately 60% reduction). The RT-qPCR analysis also demonstrated reduction of TRPM7 mRNA in the injured DRG.

CONCLUSION

The present study suggests that TRPM7 is mainly located in small nociceptors in the DRG. The content of TRPM7 in DRG neurons is probably different among their peripheral targets. TRPM7 in DRG neurons may be able to respond to noxious stimulation from their peripheral tissues. The nerve injury can decrease the level of TRPM7 mRNA and protein in DRG neurons.

Priming of central- and peripheral mechanisms with heat and cutaneous capsaicin facilitates secondary hyperalgesia to high frequency electrical stimulation

Heat/capsaicin sensitization and electrical high frequency stimulation (HFS) are well known model of secondary hyperalgesia, a phenomenon related to chronic pain conditions. This study investigated whether priming with heat/capsaicin would facilitate hyperalgesia to HFS in healthy subjects. Heat/capsaicin priming consisted of a 45 °C heat stimulation for 5 min followed by a topical capsaicin patch (4x4 cm) for 30 minutes on the volar forearm of 20 subjects. HFS (100 Hz, 5 times 1s, minimum 1.5 mA) was subsequently delivered through a transcutaneous pin electrode approximately 1.5 cm proximal to the heat/capsaicin application. Two sessions were applied in a crossover design; traditional HFS (HFS) and heat/capsaicin sensitization followed by HFS (HFS+HEAT/CAPS). Heat pain threshold (HPT), mechanical pain sensitivity (MPS) and superficial blood perfusion were assessed at baseline, after capsaicin removal, and up to 40 min after HFS. MPS was assessed with pinprick stimulation (128 mN and 256 mN) in the area adjacent to both HFS and heat/capsaicin, distal but adjacent to heat/capsaicin and in a distal control area. HPT was assessed in the area of heat/capsaicin. Higher sensitivity to 128 mN pinprick stimulation (difference from baseline and control area) was observed in the HFS+HEAT/CAPS session than in the HFS session 20 and 30 minutes after HFS. Furthermore, sensitivity was increased after HFS+HEAT/CAPS compared to after heat/capsaicin in the area adjacent to both paradigms, but not in the area distal to heat/capsaicin. Results indicate that heat/capsaicin causes priming of the central- and peripheral nervous system, which facilitates secondary mechanical hyperalgesia to HFS.

Transient Receptor Potential Channels and Botulinum Neurotoxins in Chronic Pain

Pain afflicts more than 1.5 billion people worldwide, with hundreds of millions suffering from unrelieved chronic pain. Despite widespread recognition of the importance of developing better interventions for the relief of chronic pain, little is known about the mechanisms underlying this condition. However, transient receptor potential (TRP) ion channels in nociceptors have been shown to be essential players in the generation and progression of pain and have attracted the attention of several pharmaceutical companies as therapeutic targets. Unfortunately, TRP channel inhibitors have failed in clinical trials, at least in part due to their thermoregulatory function. Botulinum neurotoxins (BoNTs) have emerged as novel and safe pain therapeutics because of their regulation of exocytosis and pro-nociceptive neurotransmitters. However, it is becoming evident that BoNTs also regulate the expression and function of TRP channels, which may explain their analgesic effects. Here, we summarize the roles of TRP channels in pain, with a particular focus on TRPV1 and TRPA1, their regulation by BoNTs, and briefly discuss the use of BoNTs for the treatment of chronic pain.

2D <em>vs</em> 3D morphological analysis of dorsal root ganglia in health and painful neuropathy

Dorsal root ganglia (DRGs) are clusters of sensory neurons that transmit the sensory information from the periphery to the central nervous system, and satellite glial cells (SGCs), their supporting trophic cells. Sensory neurons are pseudounipolar neurons with a heterogeneous neurochemistry reflecting their functional features. DRGs, not protected by the blood brain barrier, are vulnerable to stress and damage of different origin (i.e., toxic, mechanical, metabolic, genetic) that can involve sensory neurons, SGCs or, considering their intimate intercommunication, both cell populations. DRG damage, primary or secondary to nerve damage, produces a sensory peripheral neuropathy, characterized by neurophysiological abnormalities, numbness, paraesthesia and dysesthesia, tingling and burning sensations and neuropathic pain. DRG stress can be morphologically detected by light and electron microscope analysis with alterations in cell size (swelling/atrophy) and in different subcellular compartments (i.e., mitochondria, endoplasmic reticulum, and nucleus) of neurons and/or SGCs. In addition, neurochemical changes can be used to portray abnormalities of neurons and SGC. Conventional immunostaining, i.e., immunohistochemical detection of specific molecules in tissue slices, can be employed to detect, localize and quantify particular markers of damage in neurons (i.e., nuclear expression of ATF3) or SGCs (i.e., increased expression of GFAP), markers of apoptosis (i.e., caspases), markers of mitochondrial suffering and oxidative stress (i.e., 8-OHdG), markers of tissue inflammation (i.e., CD68 for macrophage infiltration) etc. However classical (2D) methods of immunostaining disrupt the overall organization of the DRG, thus resulting in the loss of some crucial information. Whole-mount (3D) methods have been recently developed to investigate DRG morphology and neurochemistry without tissue slicing, giving the opportunity to study the intimate relationship between SGCs and sensory neurons in health and disease. Here, we aim to compare classical (2D) vs whole-mount (3D) approaches to highlight “pros” and “cons” of the two methodologies when analysing neuropathy-induced alterations in DRGs.

Ca2+ Signalling Induced by NGF Identifies a Subset of Capsaicin-Excitable Neurons Displaying Enhanced Chemo-Nociception in Dorsal Root Ganglion Explants from Adult pirt-GCaMP3 Mouse

Nociceptors sense hazards via plasmalemmal cation channels, including transient receptor potential vanilloid 1 (TRPV1). Nerve growth factor (NGF) sensitises TRPV1 to capsaicin (CAPS), modulates nociceptor excitability and induces thermal hyperalgesia, but cellular mechanisms remain unclear. Confocal microscopy was used to image changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) across neuronal populations in dorsal root ganglia (DRG) explants from pirt-GCaMP3 adult mice, which express a fluorescent reporter in their sensory neurons. Raised [Ca²⁺]_i was detected in 84 neurons of three DRG explants exposed to NGF (100 ng/mL) and most (96%) of these were also excited by 1 μM CAPS. NGF elevated [Ca²⁺]_i in about one-third of the neurons stimulated by 1 μM CAPS, whether applied before or after the latter. In neurons excitable by NGF, CAPS-evoked [Ca²⁺]_i signals appeared significantly sooner (e.g., respective lags of 1.0 ± 0.1 and 1.9 ± 0.1 min), were much (>30%) brighter and lasted longer (6.6 ± 0.4 vs. 3.9 ± 0.2 min) relative to those non-responsive to the neurotrophin. CAPS tachyphylaxis lowered signal intensity by ~60% but was largely prevented by NGF. Increasing CAPS from 1 to 10 μM nearly doubled the number of cells activated but only modestly increased the amount co-activated by NGF. In conclusion, a sub-population of the CAPS-sensitive neurons in adult mouse DRG that can be excited by NGF is more sensitive to CAPS, responds with stronger signals and is further sensitised by transient exposure to the neurotrophin.

Resolvins: Potent Pain Inhibiting Lipid Mediators via Transient Receptor Potential Regulation

Chronic pain is a serious condition that occurs in the peripheral nervous system (PNS) and the central nervous system (CNS). It is caused by inflammation or nerve damage that induces the release of inflammatory mediators from immune cells and/or protein kinase activation in neuronal cells. Both nervous systems are closely linked; therefore, inflammation or nerve damage in the PNS can affect the CNS (central sensitization). In this process, nociceptive transient receptor potential (TRP) channel activation and expression are increased. As a result, nociceptive neurons are activated, and pain signals to the brain are amplified and prolonged. In other words, suppressing the onset of pain signals in the PNS can suppress pain signals to the CNS. Resolvins, endogenous lipid mediators generated during the resolution phase of acute inflammation, inhibit nociceptive TRP ion channels and alleviate chronic pain. This paper summarizes the effect of resolvins in chronic pain control and discusses future scientific perspectives. Further study on the effect of resolvins on neuropathic pain will expand the scope of pain research.

Molecular identity, anatomy, gene expression and function of neural crest vs. placode-derived nociceptors in the lower airways

The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.

Single-cell q-PCR derived expression profiles of identified sensory neurons

Sensory neurons are chemically and functionally heterogeneous, and this heterogeneity has been examined extensively over the last several decades. These studies have employed a variety of different methodologies, including anatomical, electrophysiological, and molecular approaches. Recent studies using next-generation sequencing techniques have examined the transcriptome of single sensory neurons. Although these reports have provided a wealth of exciting new information on the heterogeneity of sensory neurons, correlation with functional types is lacking. Here, we employed retrograde tracing of cutaneous and muscle afferents to examine the variety of mRNA expression profiles of individual, target-specific sensory neurons. In addition, we used an ex vivo skin/nerve/dorsal root ganglion/spinal cord preparation to record and characterize the functional response properties of individual cutaneous sensory neurons that were then intracellularly labeled with fluorescent dyes, recovered from dissociated cultures, and analyzed for gene expression. We found that by using single-cell quantitative polymerase chain reaction techniques and a set of 28 genes, we can identify transcriptionally distinct groups. We have also used calcium imaging and single-cell quantitative polymerase chain reaction to determine the correlation between levels of mRNA expression and functional protein expression and how functional properties correlated with the different transcriptional groups. These studies show that although transcriptomics does map to functional types, within any one functional subgroup, there are highly variable patterns of gene expression. Thus, studies that rely on the expression pattern of one or a few genes as a stand in for physiological experiments, runs a high risk of data misinterpretation with respect to function.

The role of urea in neuronal degeneration and sensitization: An in vitro model of uremic neuropathy

Background

Uremic neuropathy commonly affects patients with chronic kidney disease, with painful sensations in the feet, followed by numbness and weakness in the legs and hands. The symptoms usually resolve following kidney transplantation, but the mechanisms of uremic neuropathy and associated pain symptoms remain unknown. As blood urea levels are elevated in patients with chronic kidney disease, we examined the morphological and functional effects of clinically observed levels of urea on sensory neurons.

Methods

Rat dorsal root ganglion neurons were treated with 10 or 50 mmol/L urea for 48 h, fixed and immunostained for PGP9.5 and βIII tubulin immunofluorescence. Neurons were also immunostained for TRPV1, TRPM8 and Gap43 expression, and the capsaicin sensitivity of urea- or vehicle-treated neurons was determined.

Results

Urea-treated neurons had degenerating neurites with diminished PGP9.5 immunofluorescence, and swollen, retracted growth cones. βIII tubulin appeared clumped after urea treatment. After 48 hours urea treatment, neurite lengths were significantly reduced to 60 ± 2.6% (10 mmol/L, **P < 0.01), and to 56.2 ± 3.3% (50 mmol/L, **P < 0.01), compared with control neurons. Fewer neurons survived urea treatment, with 70.08 ± 13.3% remaining after 10 mmol/L (*P < 0.05) and 61.49 ± 7.4% after 50 mmol/L urea treatment (**P < 0.01), compared with controls. The proportion of neurons expressing TRPV1 was reduced after urea treatment, but not TRPM8 expressing neurons. In functional studies, treatment with urea resulted in dose-dependent neuronal sensitization. Capsaicin responses were significantly increased to 115.29 ± 3.4% (10 mmol/L, **P < 0.01) and 125.3 ± 4.2% (50 mmol/L, **P < 0.01), compared with controls. Sensitization due to urea was eliminated in the presence of the TRPV1 inhibitor SB705498, the mitogen-activated protein kinase kinase inhibitor PD98059, the PI3 kinase inhibitor LY294002 and the TRPM8 inhibitor N-(3-Aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)benzamide (AMTB hydrochloride).

Conclusion

Neurite degeneration and sensitization are consistent with uremic neuropathy and provide a disease-relevant model to test new treatments.

Ablation of TRPV1 Elevates Nocturnal Blood Pressure in Western Diet-fed Mice

Background: This study tested the hypothesis that genetically ablation of transient receptor potential vanilloid type 1 (TRPV1) exacerbates impairment of baroreflex in mice fed a western diet (WD) and leads to distinct diurnal and nocturnal blood pressure patterns.

Methods: TRPV1 gene knockout (TRPV1^-/-) and wild-type (WT) mice were given a WD or normal diet (CON) for 4 months.

Results: Capsaicin, a selective TRPV1 agonist, increased ipsilateral afferent renal nerve activity in WT but not TRPV1^-/- mice. The sensitivity of renal sympathetic nerve activity and heart rate responses to baroreflex were reduced in TRPV1^-/--CON and WT-WD and further decreased in TRPV1^-/--WD compared to the WT-CON group. Urinary norepinephrine and serum insulin and leptin at day and night were increased in WT-WD and TRPV1^-/--WD, with further elevation at night in TRPV1^-/--WD. WD intake increased leptin, IL-6, and TNF-α in adipose tissue, and TNF-α antagonist III, R-7050, decreased leptin in TRPV1^-/--WD. The urinary albumin level was higher in TRPV1^-/--WD than WT-WD. Blood pressure was not dif-ferent during daytime among all groups, but increased at night in the TRPV1^-/--WD group compared with other groups.

Conclusions: TRPV1 ablation leads to elevated nocturnal but not diurnal blood pressure, which is probably attributed to fur-ther enhancement of sympathetic drives at night.

Neurochemical and Ultrastructural Characterization of Unmyelinated Non-peptidergic C-Nociceptors and C-Low Threshold Mechanoreceptors Projecting to Lamina II of the Mouse Spinal Cord

C-nociceptors (C-Ncs) and non-nociceptive C-low threshold mechanoreceptors (C-LTMRs) are two subpopulations of small unmyelinated non-peptidergic C-type neurons of the dorsal root ganglia (DRGs) with central projections displaying a specific pattern of termination in the spinal cord dorsal horn. Although these two subpopulations exist in several animals, remarkable neurochemical differences occur between mammals, particularly rat/humans from one side and mouse from the other. Mouse is widely investigated by transcriptomics. Therefore, we here studied the immunocytochemistry of murine C-type DRG neurons and their central terminals in spinal lamina II at light and electron microscopic levels. We used a panel of markers for peptidergic (CGRP), non-peptidergic (IB4), nociceptive (TRPV1), non-nociceptive (VGLUT3) C-type neurons and two strains of transgenic mice: the TAFA4^Venus knock-in mouse to localize the TAFA4⁺ C-LTMRs, and a genetically engineered ginip mouse that allows an inducible and tissue-specific ablation of the DRG neurons expressing GINIP, a key modulator of GABA_BR-mediated analgesia. We confirmed that IB4 and TAFA4 did not coexist in small non-peptidergic C-type DRG neurons and separately tagged the C-Ncs and the C-LTMRs. We then showed that TRPV1 was expressed in only about 7% of the IB4⁺ non-peptidergic C-Ncs and their type Ia glomerular terminals within lamina II. Notably, the selective ablation of GINIP did not affect these neurons, whereas it reduced IB4 labeling in the medial part of lamina II and the density of C-LTMRs glomerular terminals to about one half throughout the entire lamina. We discuss the significance of these findings for interspecies differences and functional relevance.

Lipid Nanoparticle Inclusion Prevents Capsaicin-Induced TRPV1 Defunctionalization

BACKGROUND

Capsaicin (CPS) is a highly selective agonist of the transient receptor potential vanilloid type 1 (TRPV1) with a nanomolar affinity. High doses or prolonged exposure to CPS induces TRPV1 defunctionalization and, although this effect is currently used for the treatment of thermal hyperalgesia in chronic pain conditions, it is responsible of detrimental effects, such as denervation of sensory fibers. The aim of the present study was to formulate CPS loaded lipid nanocarriers (CPS-LN) in order to optimize CPS release, thus preventing TRPV1 internalization and degradation.

METHODS

CPS-LNs were formulated and characterized by in vitro studies. The activation of TRPV1 receptors after CPS-LN administration was evaluated by measuring spontaneous pain that was induced by local injection into the plantar surface of the mouse hind-paw. Moreover, the expression of TRPV1 in the skin was evaluated by western blot analysis in CPS-LN injected mice and then compared to a standard CPS solution (CPS-STD).

RESULTS

CPS inclusion in LN induced a lower pain response when compared to CPS-STD; further, it prevented TRPV1 down-regulation in the skin, while CPS-STD induced a significant reduction of TRPV1 expression.

CONCLUSIONS

Drug encapsulation in lipid nanoparticles produced an optimization of CPS release, thus reducing mice pain behavior and avoiding the effects that are caused by TRPV1 defunctionalization related to a prolonged activation of this receptor.

Modulation of Sensory Nerve Function by Insulin: Possible Relevance to Pain, Inflammation and Axon Growth

Insulin, besides its pivotal role in energy metabolism, may also modulate neuronal processes through acting on insulin receptors (InsRs) expressed by neurons of both the central and the peripheral nervous system. Recently, the distribution and functional significance of InsRs localized on a subset of multifunctional primary sensory neurons (PSNs) have been revealed. Systematic investigations into the cellular electrophysiology, neurochemistry and morphological traits of InsR-expressing PSNs indicated complex functional interactions among specific ion channels, proteins and neuropeptides localized in these neurons. Quantitative immunohistochemical studies have revealed disparate localization of the InsRs in somatic and visceral PSNs with a dominance of InsR-positive neurons innervating visceral organs. These findings suggested that visceral spinal PSNs involved in nociceptive and inflammatory processes are more prone to the modulatory effects of insulin than somatic PSNs. Co-localization of the InsR and transient receptor potential vanilloid 1 (TRPV1) receptor with vasoactive neuropeptides calcitonin gene-related peptide and substance P bears of crucial importance in the pathogenesis of inflammatory pathologies affecting visceral organs, such as the pancreas and the urinary bladder. Recent studies have also revealed significant novel aspects of the neurotrophic propensities of insulin with respect to axonal growth, development and regeneration.

A novel approach for detection of functional expression of TRPV1 channels on regenerated neurons following nerve injury

Transient receptor potential vanilloid 1 (TRPV1) is a polymodal receptor channel, which plays an important role in pain transduction. It is important to understand the functional expression of this channel under neuropathic pain (NP) conditions. A novel method was used to investigate the dynamics of functional expression of this channel on regenerated neurons under NP conditions following trigeminal nerve injury using a combination of a permanently charged sodium channel blocker (QX-314) and a TRPV1 agonist (capsaicin; QX-CAP). The combination was originally introduced as a local anesthetic. Synchronization between the local anesthetic effect of QX-CAP and TRPV1 expression on regenerated neurons was observed following the nerve injury. QX-CAP had no local anesthetic effect under NP conditions 2 weeks after the injury when TRPV1 expression on regenerated neurons was low. However, this combination was effective under NP conditions 3 and 4 weeks following injury when TRPV1 expression in regenerated neurons was moderate to high. The current review, discusses the potential of QX-314 as a local anesthetic and a novel approach of using QX-CAP to reveal the dynamics of functional expression of TRPV1 on regenerated neurons following trigeminal nerve injury.

Perineural application of resiniferatoxin on uninjured L3 and L4 nerves completely alleviates thermal and mechanical hypersensitivity following L5 nerve injury in rats

Fifth lumbar (L5) nerve injury in rats causes neuropathic pain manifested with thermal and mechanical hypersensitivity in the ipsilateral hind paw. This study aimed to determine whether the elimination of unmyelinated primary afferents of the adjacent uninjured nerves (L3 and L4) would alleviate peripheral neuropathic pain. Different concentrations of capsaicin or its analog, resiniferatoxin (RTX), were applied perineurally on either the left L4 or L3 and L4 nerves in Wistar rats whose left L5 nerves were ligated and cut. The application of both capsaicin and RTX on the L4 nerve significantly reduced both thermal and mechanical hypersensitivity. However, only the application of RTX on both L3 and L4 nerves completely alleviated all neuropathic manifestations. Interestingly, responses to thermal and mechanical stimuli were preserved, despite RTX application on uninjured L3, L4, and L5 nerves, which supply the plantar skin in rats. Perineural application of RTX caused downregulation of TRPV1, CGRP, and IB4 binding and upregulation of VIP in the corresponding dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. In comparison, VGLUT1 and NPY immunoreactivities were not altered. RTX application did not cause degenerative or ultrastructural changes in the treated nerves and corresponding DRGs. The results demonstrate that RTX induces neuroplasticity, rather than structural changes in primary afferents, that are responsible for alleviating hypersensitivity and chronic pain. Furthermore, this study suggests that treating uninjured adjacent spinal nerves may be used to manage chronic neuropathic pain following peripheral nerve injury.

Sensory neurons in the human jugular ganglion

The jugular ganglion (JG) contains sensory neurons of the vagus nerve which innervate somatic and visceral structures in cranial and cervical regions. In this study, the number of sensory neurons in the human JG was investigated. And, the morphology of sensory neurons in the human JG and nodose ganglion (NG) was compared. The estimated number of JG neurons was 2721.8-9301.1 (average number of sensory neurons ± S.D. = 7975.1 ± 3312.8). There was no significant difference in sizes of the neuronal cell body and nucleus within the JG (cell body, 1128.8 ± 99.7 μ m²; nucleus, 127.7 ± 20.8 μ m²) and NG (cell body, 963.8 ± 225.7 μ m²; nucleus, 123.2 ± 32.3 μ m²). These findings indicate that most of sensory neurons show the similar morphology in the JG and NG. Our immunohistochemical method also demonstrated the distribution of ion channels, neurotransmitter agents and calcium-binding proteins in the human JG. Numerous JG neurons were immunoreactive for transient receptor potential cation channel subfamily V member 1 (TRPV1, mean ± SD = 19.9 ± 11.5 %) and calcitonin gene-related peptide (CGRP, 28.4 ± 6.7 %). A moderate number of JG neurons contained TRPV2 (12.0 ± 4.7 %), substance P (SP, 15.7 ± 6.9 %) and secreted protein, acidic and rich in cysteine-like 1 (SPARCL1, 14.6 ± 7.4 %). A few JG neurons had vesicular glutamate transporter 2 (VGLUT2, 5.6 ± 2.9 %) and parvalbumin (PV, 2.3 ± 1.4 %). SP- and TRPV2-containing JG neurons had mainly small and medium-sized cell bodies, respectively. TRPV1- and VGLUT2- containing JG neurons were small to medium-sized. CGRP- and SPARCL1-containing JG neurons were of various cell body sizes. Sensory neurons in the human JG were mostly free of vasoactive intestinal polypeptide (VIP), tyrosine hydroxylase (TH) and neuropeptide Y (NPY). In the external auditory canal skin, subepithelial nerve fibers contained TRPV1, TRPV2, SP, CGRP and VGLUT2. Perivascular nerve fibers also had TRPV1, TRPV2, SP, CGRP, VIP, NPY and TH. However, PV- and SPARCL1-containing nerve endings could not be seen in the external auditory canal. It is likely that sensory neurons in the human JG can transduce nociceptive and mechanoreceptive information from the external auditory canal. Theses neurons may be also associated with neurogenic inflammation in the external auditory canal and ear-cough reflex through the vagus nerve.

Role of Gangliosides in Peripheral Pain Mechanisms

Gangliosides are abundantly occurring sialylated glycosphingolipids serving diverse functions in the nervous system. Membrane-localized gangliosides are important components of lipid microdomains (rafts) which determine the distribution of and the interaction among specific membrane proteins. Different classes of gangliosides are expressed in nociceptive primary sensory neurons involved in the transmission of nerve impulses evoked by noxious mechanical, thermal, and chemical stimuli. Gangliosides, in particular GM1, have been shown to participate in the regulation of the function of ion channels, such as transient receptor potential vanilloid type 1 (TRPV1), a molecular integrator of noxious stimuli of distinct nature. Gangliosides may influence nociceptive functions through their association with lipid rafts participating in the organization of functional assemblies of specific nociceptive ion channels with neurotrophins, membrane receptors, and intracellular signaling pathways. Genetic and experimentally induced alterations in the expression and/or metabolism of distinct ganglioside species are involved in pathologies associated with nerve injuries, neuropathic, and inflammatory pain in both men and animals. Genetic and/or pharmacological manipulation of neuronal ganglioside expression, metabolism, and action may offer a novel approach to understanding and management of pain.

Capsaicin: Physicochemical properties, cutaneous reactions and potential applications in painful and inflammatory conditions

Capsaicin is a natural protoalkaloid recognized as the main pungent component in hot peppers (Capsicum annuum L.). The capsaicin receptor is highly expressed in the unmyelinated type C nerve fibers originating from small diameter sensory neurons in dorsal root ganglia and cranial nerve ganglia correspondents. Capsaicin and related vanilloids have a variety of effects on primary sensory neurons function, from sensory neuron excitation characterized by local burning sensation and neurogenic inflammation, followed by conduction blockage accompanied by reversible ultrastructural changes of peripheral nociceptive endings (desensitization), going as far as irreversible degenerative changes (neurotoxicity). The main role in capsaicin-induced neurogenic inflammation relies on the capsaicin sensitive, small diameter primary sensory neurons, therefore its evaluation could be used as a diagnostic instrument in functional alterations of cutaneous sensory nerve fibers. Moreover, capsaicin-induced desensitization and neurotoxicity explain the analgesic/anti-nociceptive and anti-inflammatory effects of topical capsaicin and its potential use in the management of painful and inflammatory conditions. In this study, we describe the effects of capsaicin on neurogenic inflammation and nociception, as well as its potential diagnostic value and therapeutic impact in various conditions involving impairment of sensory nerve fibers.

Enhanced histamine-induced itch in diacylglycerol kinase iota knockout mice

Subsets of small-diameter dorsal root ganglia (DRG) neurons detect pruritogenic (itch-causing) and algogenic (pain-causing) stimuli and can be activated or sensitized by chemical mediators. Many of these chemical mediators activate receptors that are coupled to lipid hydrolysis and diacylglycerol (DAG) production. Diacylglycerol kinase iota (DGKI) can phosphorylate DAG and is expressed at high levels in small-diameter mouse DRG neurons. Given the importance of these neurons in sensing pruritogenic and algogenic chemicals, we sought to determine if loss of DGKI impaired responses to itch- or pain-producing stimuli. Using male and female Dgki-knockout mice, we found that in vivo sensitivity to histamine—but not other pruritogens—was enhanced. In contrast, baseline pain sensitivity and pain sensitization following inflammatory or neuropathic injury were equivalent between wild type and Dgki^-/- mice. In vitro calcium responses in DRG neurons to histamine was enhanced, while responses to algogenic ligands were unaffected by Dgki deletion. These data suggest Dgki regulates sensory neuron and behavioral responses to histamine, without affecting responses to other pruritogenic or algogenic agents.

Transcription factor Sp4 is required for hyperalgesic state persistence

Understanding how painful hypersensitive states develop and persist beyond the initial hours to days is critically important in the effort to devise strategies to prevent and/or reverse chronic painful states. Changes in nociceptor transcription can alter the abundance of nociceptive signaling elements, resulting in longer-term change in nociceptor phenotype. As a result, sensitized nociceptive signaling can be further amplified and nocifensive behaviors sustained for weeks to months. Building on our previous finding that transcription factor Sp4 positively regulates the expression of the pain transducing channel TRPV1 in Dorsal Root Ganglion (DRG) neurons, we sought to determine if Sp4 serves a broader role in the development and persistence of hypersensitive states in mice. We observed that more than 90% of Sp4 staining DRG neurons were small to medium sized, primarily unmyelinated (NF200 neg) and the majority co-expressed nociceptor markers TRPV1 and/or isolectin B4 (IB4). Genetically modified mice (Sp4+/-) with a 50% reduction of Sp4 showed a reduction in DRG TRPV1 mRNA and neuronal responses to the TRPV1 agonist-capsaicin. Importantly, Sp4+/- mice failed to develop persistent inflammatory thermal hyperalgesia, showing a reversal to control values after 6 hours. Despite a reversal of inflammatory thermal hyperalgesia, there was no difference in CFA-induced hindpaw swelling between CFA Sp4+/- and CFA wild type mice. Similarly, Sp4+/- mice failed to develop persistent mechanical hypersensitivity to hind-paw injection of NGF. Although Sp4+/- mice developed hypersensitivity to traumatic nerve injury, Sp4+/- mice failed to develop persistent cold or mechanical hypersensitivity to the platinum-based chemotherapeutic agent oxaliplatin, a non-traumatic model of neuropathic pain. Overall, Sp4+/- mice displayed a remarkable ability to reverse the development of multiple models of persistent inflammatory and neuropathic hypersensitivity. This suggests that Sp4 functions as a critical control point for a network of genes that conspire in the persistence of painful hypersensitive states.

TRPV1-Like Immunoreactivity in the Human Locus K, a Distinct Subregion of the Cuneate Nucleus

The presence of transient receptor potential vanilloid type-1 receptor (TRPV1)-like immunoreactivity (LI), in the form of nerve fibres and terminals, is shown in a set of discrete gray matter subregions placed in the territory of the human cuneate nucleus. We showed previously that those subregions share neurochemical and structural features with the protopathic nuclei and, after the ancient name of our town, collectively call them Locus Karalis, and briefly Locus K. TRPV1-LI in the Locus K is codistributed, though not perfectly overlapped, with that of the neuropeptides calcitonin gene-related peptide and substance P, the topography of the elements immunoreactive to the three markers, in relation to each other, reflecting that previously described in the caudal spinal trigeminal nucleus. Myelin stainings show that myelinated fibres, abundant in the cuneate, gracile and trigeminal magnocellular nuclei, are scarce in the Locus K as in the trigeminal substantia gelatinosa. Morphometric analysis shows that cell size and density of Locus K neurons are consistent with those of the trigeminal substantia gelatinosa and significantly different from those of the magnocellular trigeminal, solitary and dorsal column nuclei. We propose that Locus K is a special component of the human dorsal column nuclei. Its functional role remains to be determined, but TRPV1 appears to play a part in it.

The insulin receptor is differentially expressed in somatic and visceral primary sensory neurons

Recent studies demonstrated the expression of the insulin receptor (InsR) and its functional interaction with the transient receptor potential vanilloid type 1 receptor (TRPV1) in primary sensory neurons (PSNs). The present study was undertaken to reveal the target-specific expression of the InsR and its co-localization with the TRPV1 in rat PSNs. We assessed the localization of the InsR and its co-localization with the TRPV1 in PSNs retrogradely labelled with biotin-conjugated wheat germ agglutinin injected into the dorsal hind paw skin, the gastrocnemius muscle, the pancreas and the urinary bladder wall. The largest proportions of retrogradely labelled InsR-immunoreactive neurons were identified among PSNs serving the pancreas (~ 54%) and the urinary bladder (~ 53%). The proportions of retrogradely labelled InsR-immunoreactive neurons innervating the dorsal hind paw skin and the gastrocnemius muscle amounted to ~ 22 and ~ 21%. TRPV1-immunoreactive neurons amounted to ~ 63, ~ 62, ~ 67 and ~ 65% of retrogradely labelled cutaneous, muscle, pancreatic and urinary bladder PSNs, respectively. Co-localization of the TRPV1 with the InsR was observed in ~ 16, ~ 15, ~ 29 and ~ 30% of retrogradely labelled cutaneous, muscle, pancreatic and urinary bladder PSNs. These quantitative immunohistochemical data demonstrate a preponderance of InsR-immunoreactivity among PSNs, which innervate visceral targets. The present findings suggest that visceral spinal PSNs are more likely to be exposed to the modulatory effects of insulin on sensory functions, including neurotrophic, nociceptive and inflammatory processes.

Involvement of transient receptor potential vanilloid 2 in intra-oral incisional pain

OBJECTIVE

To examine whether transient receptor potential vanilloid 2 (TRPV2) contributes to the changes in intra-oral thermal and mechanical sensitivity following the incision of buccal mucosa.

MATERIALS AND METHODS

Buccal mucosal pain threshold was measured after the incision. Changes in the number of TRPV2-immunoreactive (IR) trigeminal ganglion (TG) neurons which innervate the whisker pad skin and buccal mucosa, changes in the number of isolectin B4-negative/isolectin B4-positive TRPV2-IR TG neurons which innervate the whisker pad skin and the buccal mucosa, and the effect of peripheral TRPV2 antagonism on the pain threshold of incisional whisker pad skin and buccal mucosa were examined after these injuries.

RESULTS

Buccal mucosal pain hypersensitivities were induced on day 3 following the incision. The total number of TRPV2-IR TG neurons and the number of isolectin B4-negative TRPV2-IR TG neurons which innervate the whisker pad skin and buccal mucosa were increased. Buccal mucosal TRPV2 antagonism completely suppressed the heat and mechanical hypersensitivities, but not cold hypersensitivity. TRPV2 antagonist administration to the incisional whisker pad skin only partially suppressed pain hypersensitivities.

CONCLUSION

The increased expression of TRPV2 in peptidergic TG neurons innervating the incisional buccal mucosa is predominantly involved in buccal mucosal heat hyperalgesia and mechanical allodynia following buccal mucosal incision.

Attenuation of TRPV1 by AMG-517 after nerve injury promotes peripheral axonal regeneration in rats

Aims

The main objective was to investigate the effects of the transient receptor potential cation channel subfamily V member 1 (TRPV1) on nerve regeneration following sciatic transection injury by functional blockage of TRPV1 using AMG-517, a specific blocker of TRPV1.

Methods

AMG-517 was injected into the area surrounding ipsilateral lumbar dorsal root ganglia 30 min after unilateral sciatic nerve transection. The number of sciatic axons and the expression of growth-associated protein-43 (GAP-43) and glial fibrillary acidic protein was examined using semithin sections, Western blot, and immunofluorescence analyses.

Results

Blockage of TRPV1 with AMG-517 markedly promoted axonal regeneration, especially at two weeks after sciatic injury; the number of axons was similar to the uninjured control group. After sciatic nerve transection, expression of glial fibrillary acidic protein was decreased and GAP-43 was increased at the proximal stump. However, the expression of both glial fibrillary acidic protein and GAP-43 increased significantly in AMG-517-treated groups.

Conclusions

TRPV1 may be an important therapeutic target to promote peripheral nerve regeneration after injury.

Mitochondrial modulation-induced activation of vagal sensory neuronal subsets by antimycin A, but not CCCP or rotenone, correlates with mitochondrial superoxide production

Inflammation causes nociceptive sensory neuron activation, evoking debilitating symptoms and reflexes. Inflammatory signaling pathways are capable of modulating mitochondrial function, resulting in reactive oxygen species (ROS) production, mitochondrial depolarization and calcium release. Previously we showed that mitochondrial modulation with antimycin A, a complex III inhibitor, selectively stimulated nociceptive bronchopulmonary C-fibers via the activation of transient receptor potential (TRP) ankyrin 1 (A1) and vanilloid 1 (V1) cation channels. TRPA1 is ROS-sensitive, but there is little evidence that TRPV1 is activated by ROS. Here, we used dual imaging of dissociated vagal neurons to investigate the correlation of mitochondrial superoxide production (mitoSOX) or mitochondrial depolarization (JC-1) with cytosolic calcium (Fura-2AM), following mitochondrial modulation by antimycin A, rotenone (complex I inhibitor) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP, mitochondrial uncoupling agent). Mitochondrial modulation by all agents selectively increased cytosolic calcium in a subset of TRPA1/TRPV1-expressing (A1/V1+) neurons. There was a significant correlation between antimycin A-induced calcium responses and mitochondrial superoxide in wild-type 'responding' A1/V1+ neurons, which was eliminated in TRPA1-/- neurons, but not TRPV1-/- neurons. Nevertheless, antimycin A-induced superoxide production did not always increase calcium in A1/V1+ neurons, suggesting a critical role of an unknown factor. CCCP caused both superoxide production and mitochondrial depolarization but neither correlated with calcium fluxes in A1/V1+ neurons. Rotenone-induced calcium responses in 'responding' A1/V1+ neurons correlated with mitochondrial depolarization but not superoxide production. Our data are consistent with the hypothesis that mitochondrial dysfunction causes calcium fluxes in a subset of A1/V1+ neurons via ROS-dependent and ROS-independent mechanisms.

Osteoporotic Pain is Associated with Increased Transient Receptor Vanilloid 4 Expression in the Dorsal Root Ganglia of Ovariectomized Osteoporotic Rats: A Pilot Basic Study

INTRODUCTION

Osteoporosis can produce a persistent state of pain known as osteoporotic pain. One proposed mechanism of this pathology is increased calcitonin gene-related peptide (CGRP; a marker related to inflammatory pain) expression in the dorsal root ganglia (DRG) innervating osteoporotic vertebrae. Alternatively, a previous study revealed that axial loading caused osteoporotic pain in a rodent model of coccygeal vertebrae compression. Because this compression model is associated with trauma, additional mechanistic studies of osteoporotic pain in the absence of trauma are required. The current study aimedto evaluate the expression and relative distribution of transient receptor potential vanilloid 4 (TRPV4), a pain-related mechanoreceptor, in ovariectomized (OVX) osteoporotic rats.

METHODS

CGRP-immunoreactive (-ir) and TRPV4-ir DRG neurons innervating the L3 vertebrae of Sprague-Dawley rats were labeled with a neurotracer, FluoroGold. Intravertebral pH was also measured during the neurotracer procedure. TRPV4-ir/CGRP-ir FluoroGold-positive DRG neurons were quantified in sham control and OVX rats (n = 10, ea). The threshold for statistical significance was set at P < 0.05.

RESULTS

There was no statistical difference in the number of FluoroGold-positive DRG neurons between groups; however, there were significantly more CGRP-ir/TRPV4-ir FluoroGold-positive DRG neurons in the OVX group compared with the sham control group (P < 0.05) as well as the significantly increased molecular production of each peptide. Intravertebral pH was also lower in the OVX group compared with the sham control group (P < 0.05).

CONCLUSION

Sensory neurons innervating osteoporotic vertebrae exhibited increased expression of co-localized CGRP and TRPV4 in OVX osteoporotic rats. Additionally, intravertebral pH was low in the vicinity osteoporotic vertebrae. Considering that TRPV4 is a mechanosensitive nociceptor that is activated in acidic environments, its upregulation may be associated with the pathology of osteoporotic pain derived from microinflammation involved in osteoporosis.

Differential Regulation of Bladder Pain and Voiding Function by Sensory Afferent Populations Revealed by Selective Optogenetic Activation

Bladder-innervating primary sensory neurons mediate reflex-driven bladder function under normal conditions, and contribute to debilitating bladder pain and/or overactivity in pathological states. The goal of this study was to examine the respective roles of defined subtypes of afferent neurons in bladder sensation and function in vivo via direct optogenetic activation. To accomplish this goal, we generated transgenic lines that express a Channelrhodopsin-2-eYFP fusion protein (ChR2-eYFP) in two distinct populations of sensory neurons: TRPV1-lineage neurons (Trpv1^Cre;Ai32, the majority of nociceptors) and Na_v1.8⁺ neurons (Scn10a^Cre;Ai32, nociceptors and some mechanosensitive fibers). In spinal cord, eYFP+ fibers in Trpv1^Cre;Ai32 mice were observed predominantly in dorsal horn (DH) laminae I-II, while in Scn10a^Cre;Ai32 mice they extended throughout the DH, including a dense projection to lamina X. Fiber density correlated with number of retrogradely-labeled eYFP+ dorsal root ganglion neurons (82.2% Scn10a^Cre;Ai32 vs. 62% Trpv1^Cre;Ai32) and degree of DH excitatory synaptic transmission. Photostimulation of peripheral afferent terminals significantly increased visceromotor responses to noxious bladder distension (30–50 mmHg) in both transgenic lines, and to non-noxious distension (20 mmHg) in Scn10a^Cre;Ai32 mice. Depolarization of ChR2+ afferents in Scn10a^Cre;Ai32 mice produced low- and high-amplitude bladder contractions respectively in 53% and 27% of stimulation trials, and frequency of high-amplitude contractions increased to 60% after engagement of low threshold (LT) mechanoreceptors by bladder filling. In Trpv1^Cre;Ai32 mice, low-amplitude contractions occurred in 27% of trials before bladder filling, which was pre-requisite for light-evoked high-amplitude contractions (observed in 53.3% of trials). Potential explanations for these observations include physiological differences in the thresholds of stimulated fibers and their connectivity to spinal circuits.

Tiotropium Attenuates Refractory Cough and Capsaicin Cough Reflex Sensitivity in Patients with Asthma

BACKGROUND

Asthmatic cough is often refractory to standard treatments such as inhaled corticosteroids (ICS) and long-acting β₂ agonists (LABA). Tiotropium may modulate cough reflex sensitivity of acute viral cough, but its efficacy in asthmatic cough remains unknown.

OBJECTIVE

To evaluate whether tiotropium improves cough and cough reflex sensitivity in patients with asthma refractory to ICS/LABA.

METHODS

Seventeen consecutive patients with asthma with chronic cough despite the use of ICS/LABA (13 women; 43.4 ± 19.0 years; average ICS dose, 651 ± 189 μg/d; fluticasone equivalent) were additionally treated with tiotropium (5 μg/d) for 4 to 8 weeks to examine its effects on pulmonary function and capsaicin cough reflex sensitivity (cough thresholds C2 and C5). Cough severity, cough-specific quality of life, and asthma control were also evaluated using cough visual analog scales (VASs), the Japanese version of Leicester Cough Questionnaire (J-LCQ), and Asthma Control Test (ACT), respectively. Patients with an improved cough VAS score of 15 mm or more were considered responders to tiotropium.

RESULTS

Tiotropium significantly improved cough VAS, J-LCQ, and ACT scores, but not FEV₁. Changes in cough VAS score correlated with those in C2 (r = -0.58; P = .03), C5 (r = -0.58; P = .03), and ACT scores (r = -0.62; P = .02), but not in FEV₁ in the overall patients. When analyses were confined to the 11 responders, tiotropium significantly improved capsaicin cough reflex sensitivity within the subgroup (C2: P = .01 and C5: P = .02) and versus the nonresponders (C2: P = .004 and C5: P = .02).

CONCLUSION

Tiotropium may alleviate asthmatic cough refractory to ICS/LABA by modulating cough reflex sensitivity but not through bronchodilation.

Oxaliplatin-induced changes in expression of transient receptor potential channels in the dorsal root ganglion as a neuropathic mechanism for cold hypersensitivity

Transient receptor potential (TRP) receptors are involved in the development of chemotherapy-induced peripheral neuropathic pain, which is a common side effect of selected chemotherapeutic agents such as oxaliplatin. However, the precise contribution of TRPs to this condition remains unknown. Cold hypersensitivity is the hallmark of oxaliplatin-induced neuropathy, so we used a preclinical model of oxaliplatin-induced cold hypersensitivity in rats to determine the effects of oxaliplatin on TRP channels. To this end, immunohistochemistry was used to examine TRP vanilloid 1 (TRPV1), TRP ankyrin 1 (TRPA1), and TRP melastatin 8 (TRPM8) expression in the rat dorsal root ganglion (DRG) after 4days of oxaliplatin treatment. Behavioral assessment using the acetone spray test showed that oxaliplatin significantly increased acute cold hypersensitivity after 4days of treatment. Double-staining immunohistochemistry showed that 4days after oxaliplatin treatment, there was increased co-expression of TRPA1 and TRPV1 in isolectin B4-positive small-sized DRG neurons, as well as a significant increase in the co-localization of TRPM8 and neurofilament 200 in medium-sized DRG neurons. In addition, in situ hybridization revealed that TRPV1 protein was co-expressed with TRPA1 mRNA on day 4 after oxaliplatin administration. Thus, at an early stage following oxaliplatin treatment there is an increased expression of TRPA1 and TRPV1 in small-sized DRG neurons and of TRPM8 in medium-sized DRG neurons. Collectively, these changes may contribute to the development of oxaliplatin-induced peripheral neuropathic pain.

Distinct Expression of Phenotypic Markers in Placodes- and Neural Crest-Derived Afferent Neurons Innervating the Rat Stomach

BACKGROUND

Visceral pain is initiated by activation of primary afferent neurons among which the capsaicin-sensitive (TRPV1-positive) neurons play an important role. The stomach is a common source of visceral pain. Similar to other organs, the stomach receives dual spinal and vagal afferent innervation. Developmentally, spinal dorsal root ganglia (DRG) and vagal jugular neurons originate from embryonic neural crest and vagal nodose neurons originate from placodes. In thoracic organs the neural crest- and placodes-derived TRPV1-positive neurons have distinct phenotypes differing in activation profile, neurotrophic regulation and reflex responses. It is unknown to whether such distinction exists in the stomach.

AIMS

We hypothesized that gastric neural crest- and placodes-derived TRPV1-positive neurons express phenotypic markers indicative of placodes and neural crest phenotypes.

METHODS

Gastric DRG and vagal neurons were retrogradely traced by DiI injected into the rat stomach wall. Single-cell RT-PCR was performed on traced gastric neurons.

RESULTS

Retrograde tracing demonstrated that vagal gastric neurons locate exclusively into the nodose portion of the rat jugular/petrosal/nodose complex. Gastric DRG TRPV1-positive neurons preferentially expressed markers PPT-A, TrkA and GFRα₃ typical for neural crest-derived TRPV1-positive visceral neurons. In contrast, gastric nodose TRPV1-positive neurons preferentially expressed markers P2X₂ and TrkB typical for placodes-derived TRPV1-positive visceral neurons. Differential expression of neural crest and placodes markers was less pronounced in TRPV1-negative DRG and nodose populations.

CONCLUSIONS

There are phenotypic distinctions between the neural crest-derived DRG and placodes-derived vagal nodose TRPV1-positive neurons innervating the rat stomach that are similar to those described in thoracic organs.

Inflammation of peripheral tissues and injury to peripheral nerves induce differing effects in the expression of the calcium-sensitive N-arachydonoylethanolamine-synthesizing enzyme and related molecules in rat primary sensory neurons

Elevation of intracellular Ca²⁺ concentration induces the synthesis of N-arachydonoylethanolamine (anandamide) in a subpopulation of primary sensory neurons. N-acylphosphatidylethanolamine phospholipase D (NAPE-PLD) is the only known enzyme that synthesizes anandamide in a Ca²⁺ -dependent manner. NAPE-PLD mRNA as well as anandamide's main targets, the excitatory transient receptor potential vanilloid type 1 ion channel (TRPV1), the inhibitory cannabinoid type 1 (CB1) receptor, and the main anandamide-hydrolyzing enzyme fatty acid amide hydrolase (FAAH), are all expressed by subpopulations of nociceptive primary sensory neurons. Thus, NAPE-PLD, TRPV1, the CB1 receptor, and FAAH could form an autocrine signaling system that could shape the activity of a major subpopulation of nociceptive primary sensory neurons, contributing to the development of pain. Although the expression patterns of TRPV1, the CB1 receptor, and FAAH have been comprehensively elucidated, little is known about NAPE-PLD expression in primary sensory neurons under physiological and pathological conditions. This study shows that NAPE-PLD is expressed by about one-third of primary sensory neurons, the overwhelming majority of which also express nociceptive markers as well as the CB1 receptor, TRPV1, and FAAH. Inflammation of peripheral tissues and injury to peripheral nerves induce differing but concerted changes in the expression pattern of NAPE-PLD, the CB1 receptor, TRPV1, and FAAH. Together these data indicate the existence of the anatomical basis for an autocrine signaling system in a major proportion of nociceptive primary sensory neurons and that alterations in that autocrine signaling by peripheral pathologies could contribute to the development of both inflammatory and neuropathic pain.

Quantitative Thermal Testing Profiles as a Predictor of Treatment Response to Topical Capsaicin in Patients with Localized Neuropathic Pain

There are no reliable predictors of response to treatment with capsaicin. Given that capsaicin application causes heat sensation, differences in quantitative thermal testing (QTT) profiles may predict treatment response. The aim of this study was to determine whether different QTT profiles could predict treatment outcomes in patients with localized peripheral neuropathic pain (PeLNP). We obtained from medical records QTT results and treatment outcomes of 55 patients treated between 2010 and 2013. Warm sensation threshold (WST) and heat pain threshold (HPT) values were assessed at baseline at the treatment site and in the asymptomatic, contralateral area. Responders were defined as those who achieved a > 30% decrease in pain lasting > 30 days. Two distinct groups were identified based on differences in QTT profiles. Most patients (27/31; 87.1%) with a homogenous profile were nonresponders. By contrast, more than half of the patients (13/24, 54.2%) with a nonhomogenous profile were responders (p = 0.0028). A nonhomogenous QTT profile appears to be predictive of response to capsaicin. We hypothesize patients with a partial loss of cutaneous nerve fibers or receptors are more likely to respond. By contrast, when severe nerve damage or normal cutaneous sensations are present, the pain is likely due to central sensitization and thus not responsive to capsaicin. Prospective studies with larger patient samples are needed to confirm this hypothesis.

TRPV1 receptor in the human trigeminal ganglion and spinal nucleus: immunohistochemical localization and comparison with the neuropeptides CGRP and SP

This work presents new data concerning the immunohistochemical occurrence of the transient receptor potential vanilloid type-1 (TRPV1) receptor in the human trigeminal ganglion (TG) and spinal nucleus of subjects at different ontogenetic stages, from prenatal life to postnatal old age. Comparisons are made with the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). TRPV1-like immunoreactive (LI) material was detected by western blot in homogenates of TG and medulla oblongata of subjects at prenatal and adult stages of life. Immunohistochemistry showed that expression of the TRPV1 receptor is mostly restricted to the small- and medium-sized TG neurons and to the caudal subdivision of the spinal trigeminal nucleus (Sp5C). The extent of the TRPV1-LI TG neuronal subpopulation was greater in subjects at early perinatal age than at late perinatal age and in postnatal life. Centrally, the TRPV1 receptor localized to fibre tracts and punctate elements, which were mainly distributed in the spinal tract, lamina I and inner lamina II of the Sp5C, whereas stained cells were rare. The TRPV1 receptor colocalized partially with CGRP and SP in the TG, and was incompletely codistributed with both neuropeptides in the spinal tract and in the superficial laminae of the Sp5C. Substantial differences were noted with respect to the distribution of the TRPV1-LI structures described in the rat Sp5C and with respect to the temporal expression of the receptor during the development of the rat spinal dorsal horn. The distinctive localization of TRPV1-LI material supports the concept of the involvement of TRPV1 receptor in the functional activity of the protopathic compartment of the human trigeminal sensory system, i.e. the processing and neurotransmission of thermal and pain stimuli.

Cannabinoid and Vanilloid Effects of R(+)-Methanandamide in the Hemisected Meningeal Preparation

The endogenous cannabinoid R(+)-methanandamide (mAEA) exerts differential anti- and pronociceptive effects by activating both cannabinoid (CB1) and vanilloid (TRPV1) receptors of nociceptive primary afferents. The significance of these effects in meningeal nociception was evaluated by modulation of calcitonin gene-related peptide (CGRP) release from meningeal afferents measured in an in vitro preparation of the hemisected rat skull. Temperature steps to 39°C and 45°C caused heat-dependent increases in CGRP release. One micromolar mAEA inhibited CGRP release at 32°C but facilitated it at 45°C. This effect was abolished in the presence of the TRPV1 receptor antagonist capsazepine. Lower doses of mAEA had no effect on basal or heat-evoked release. In the presence of the CB1 receptor antagonist SR141716 (0.2 μM) heat-stimulated increase in CGRP release was facilitated. CGRP release in the presence of SR141716 (0.2 μM) was further increased by adding mAEA at a concentration which had no effect on its own. These results confirm an opposing functional role for anandamide at CB1 and TRPV1 receptors on meningeal afferents.

Separate functions for responses to oral temperature in thermo-gustatory and trigeminal neurons

Oral temperature is a component and modifier of taste perception. Both trigeminal (V) and taste-sensitive cells, including those in the nucleus of the solitary tract (NTS), can respond to oral temperature. However, functional associations in thermal sensitivity between V and gustatory neurons are poorly understood. To study this we recorded electrophysiological responses to oral stimulation with cool (9, 15, 25, 32, and 34 °C) and warm (40 and 45 °C) temperatures from medullary V (n = 45) and taste-sensitive NTS (n = 27) neurons in anesthetized mice. Results showed temperatures below 34 °C activated the majority of V neurons but only a minority of NTS units. V neurons displayed larger responses to cooling and responded to temperatures that poorly stimulated NTS cells. Multivariate analyses revealed different temperatures induced larger differences in responses across V compared with NTS neurons, indicating V pathways possess greater capacity to signal temperature. Conversely, responses to temperature in NTS units associated with gustatory tuning. Further analyses identified two types of cooling-sensitive V neurons oriented toward innocuous or noxious cooling. Multivariate analyses indicated the combined response of these cells afforded distinction among a broad range of cool temperatures, suggesting multiple types of V neurons work together to represent oral cooling.

PG110, A Humanized Anti-NGF Antibody, Reverses Established Pain Hypersensitivity in Persistent Inflammatory Pain, but not Peripheral Neuropathic Pain, Rat Models

BACKGROUND

Chronic inflammatory and peripheral neuropathic pain (PNP) is a major health problem for which effective drug treatment is lacking. The pathophysiology of these debilitating conditions is incompletely understood, but nerve growth factor (NGF) is believed to play a major role. NGF-antagonism has previously been shown to prevent pain hypersensitivity in rodent models of acute inflammatory pain and PNP, but most of those animal studies did not address the more clinically relevant issue of whether NGF-antagonism provides relief of established chronic pain behavior. Therefore, the aim of this study was to investigate whether blocking NGF actions with a humanized anti-NGF monoclonal antibody (PG110) would reverse/attenuate established pain hypersensitivity in rat models of chronic/persistent inflammatory pain and PNP.

METHODS

The complete Freund's adjuvant (CFA) rat model of persistent inflammatory pain, and the L5 spinal nerve axotomy (SNA) model of PNP, were used in the present study. The effect of a single intravenous injection (10, 30, and 300 µg/kg) of an anti-NGF antibody PG110 on heat and mechanical hypersensitivity was assessed 5 and 7 days after CFA and SNA, respectively.

RESULTS

Compared to vehicle treated group, PG110 dose dependently attenuated established heat and mechanical hypersensitivity induced by CFA, but not that induced by SNA. The anti-allodynic and anti-hyperalgesic effects of PG110 in the CFA model were similar to those of the positive control naproxen (30 mg/kg, i.v.).

CONCLUSION

These findings suggest that therapies that target NGF or its receptors may be effective for treatment of persistent/chronic inflammatory pain, but probably not PNP.

Emerging Role of Spinal Cord TRPV1 in Pain Exacerbation

TRPV1 is well known as a sensor ion channel that transduces a potentially harmful environment into electrical depolarization of the peripheral terminal of the nociceptive primary afferents. Although TRPV1 is also expressed in central regions of the nervous system, its roles in the area remain unclear. A series of recent reports on the spinal cord synapses have provided evidence that TRPV1 plays an important role in synaptic transmission in the pain pathway. Particularly, in pathologic pain states, TRPV1 in the central terminal of sensory neurons and interneurons is suggested to commonly contribute to pain exacerbation. These observations may lead to insights regarding novel synaptic mechanisms revealing veiled roles of spinal cord TRPV1 and may offer another opportunity to modulate pathological pain by controlling TRPV1. In this review, we introduce historical perspectives of this view and details of the recent promising results. We also focus on extended issues and unsolved problems to fully understand the role of TRPV1 in pathological pain. Together with recent findings, further efforts for fine analysis of TRPV1's plastic roles in pain synapses at different levels in the central nervous system will promote a better understanding of pathologic pain mechanisms and assist in developing novel analgesic strategies.

Changes in the Expressions of Iba1 and Calcitonin Gene-Related Peptide in Adjacent Lumbar Spinal Segments after Lumbar Disc Herniation in a Rat Model

Lumbar disc herniation is commonly encountered in clinical practice and can induce sciatica due to mechanical and/or chemical irritation and the release of proinflammatory cytokines. However, symptoms are not confined to the affected spinal cord segment. The purpose of this study was to determine whether multisegmental molecular changes exist between adjacent lumbar spinal segments using a rat model of lumbar disc herniation. Twenty-nine male Sprague-Dawley rats were randomly assigned to either a sham-operated group (n=10) or a nucleus pulposus (NP)-exposed group (n=19). Rats in the NP-exposed group were further subdivided into a significant pain subgroup (n=12) and a no significant pain subgroup (n=7) using mechanical pain thresholds determined von Frey filaments. Immunohistochemical stainings of microglia (ionized calcium-binding adapter molecule 1; Iba1), astrocytes (glial fibrillary acidic protein; GFAP), calcitonin gene-related peptide (CGRP), and transient receptor potential vanilloid 1 (TRPV1) was performed in spinal dorsal horns and dorsal root ganglions (DRGs) at 10 days after surgery. It was found immunoreactivity for Iba1-positive microglia was higher in the L5 (P=0.004) dorsal horn and in the ipsilateral L4 (P=0.009), L6 (P=0.002), and S1 (P=0.002) dorsal horns in the NP-exposed group than in the sham-operated group. The expression of CGRP was also significantly higher in ipsilateral L3, L4, L6, and S1 segments and in L5 DRGs at 10 days after surgery in the NP-exposed group than in the sham-operated group (P<0.001). Our results indicate that lumbar disc herniation upregulates microglial activity and CGRP expression in many adjacent and ipsilateral lumbar spinal segments.

Perineural capsaicin induces the uptake and transganglionic transport of choleratoxin B subunit by nociceptive C-fiber primary afferent neurons

The distribution of spinal primary afferent terminals labeled transganglionically with the choleratoxin B subunit (CTB) or its conjugates changes profoundly after perineural treatment with capsaicin. Injection of CTB conjugated with horseradish peroxidase (HRP) into an intact nerve labels somatotopically related areas in the ipsilateral dorsal horn with the exceptions of the marginal zone and the substantia gelatinosa, whereas injection of this tracer into a capsaicin-pretreated nerve also results in massive labeling of these most superficial layers of the dorsal horn. The present study was initiated to clarify the role of C-fiber primary afferent neurons in this phenomenon. In L5 dorsal root ganglia, analysis of the size frequency distribution of neurons labeled after injection of CTB-HRP into the ipsilateral sciatic nerve treated previously with capsaicin or resiniferatoxin revealed a significant increase in the proportion of small neurons. In the spinal dorsal horn, capsaicin or resiniferatoxin pretreatment resulted in intense CTB-HRP labeling of the marginal zone and the substantia gelatinosa. Electron microscopic histochemistry disclosed a dramatic, ∼10-fold increase in the proportion of CTB-HRP-labeled unmyelinated dorsal root axons following perineural capsaicin or resiniferatoxin. The present results indicate that CTB-HRP labeling of C-fiber dorsal root ganglion neurons and their central terminals after perineural treatment with vanilloid compounds may be explained by their phenotypic switch rather than a sprouting response of thick myelinated spinal afferents which, in an intact nerve, can be labeled selectively with CTB-HRP. The findings also suggest a role for GM1 ganglioside in the modulation of nociceptor function and pain.

Social stress in mice induces urinary bladder overactivity and increases TRPV1 channel-dependent afferent nerve activity

Social stress has been implicated as a cause of urinary bladder hypertrophy and dysfunction in humans. Using a murine model of social stress, we and others have shown that social stress leads to bladder overactivity. Here, we show that social stress leads to bladder overactivity, increased bladder compliance, and increased afferent nerve activity. In the social stress paradigm, 6-wk-old male C57BL/6 mice were exposed for a total of 2 wk, via barrier cage, to a C57BL/6 retired breeder aggressor mouse. We performed conscious cystometry with and without intravesical infusion of the TRPV1 inhibitor capsazepine, and measured pressure-volume relationships and afferent nerve activity during bladder filling using an ex vivo bladder model. Stress leads to a decrease in intermicturition interval and void volume in vivo, which was restored by capsazepine. Ex vivo studies demonstrated that at low pressures, bladder compliance and afferent activity were elevated in stressed bladders compared with unstressed bladders. Capsazepine did not significantly change afferent activity in unstressed mice, but significantly decreased afferent activity at all pressures in stressed bladders. Immunohistochemistry revealed that TRPV1 colocalizes with CGRP to stain nerve fibers in unstressed bladders. Colocalization significantly increased along the same nerve fibers in the stressed bladders. Our results support the concept that social stress induces TRPV1-dependent afferent nerve activity, ultimately leading to the development of overactive bladder symptoms.

Limited changes in spinal lamina I dorsal horn neurons following the cytotoxic ablation of non-peptidergic C-fibers

BACKGROUND

Non-peptidergic nociceptive neurons are a sub-population of small diameter primary sensory neurons that comprise approximately 50 % of the C fiber population. Together with the peptidergic sub-population, they transmit nociceptive information from the periphery to the superficial dorsal horn of the spinal cord. Despite the numerous studies investigating the role of the non-peptidergic primary afferents, their role in normal nociception and in pain remains poorly understood. Our lab has previously demonstrated that, in rat models of neuropathic and inflammatory pain, there is a de novo expression of substance P receptors (NK-1r) by lamina I pyramidal projection neurons, a neuronal population that normally does not express these receptors.

RESULTS

In this study, we used a ribosomal toxin, saporin, conjugated to the lectin IB4 to selectively ablate the non-peptidergic nociceptive C fibers, to investigate if the loss of these fibers was enough to induce a change in NK-1r expression by lamina I projection neurons. IB4-saporin treatment led to the permanent ablation of the IB4-positive afferents but also to a small non-significant reduction in CGRP-positive afferents. An overall increase in immunoreactivity for the NK-1r was observed in lamina I projection neurons, however, the lack of non-peptidergic afferents did not increase the number of lamina I pyramidal projection neurons immunoreactive for the receptor.

CONCLUSIONS

Our results demonstrate that the deletion of the non-peptidergic afferents, at the L4-L5 spinal levels, is not sufficient to trigger the de novo expression of NK-1r by projection pyramidal neurons but increases the expression of NK-1r in fusiform and multipolar projection neurons. Furthermore, our data suggest that a neuropathic component is essential to trigger the expression of NK-1r by pyramidal neurons.