Nociceptive Nerve Fibers Research Articles

Cytokine-mediated inflammation mediates painful neuropathy from metabolic syndrome.

Painful neuropathy (PN) is a prevalent condition in patients with metabolic syndrome (MetS). However, the pathogenic mechanisms of metabolic syndrome-associated painful neuropathy (MetSPN) remain unclear. In the current study, high-fat-fed mice (HF mice) were used to study MetSPN. HF mice developed MetS phenotypes, including increased body weight, elevated plasma cholesterol levels, and insulin resistance in comparison with control-fat-fed (CF) mice. Subsequently, HF mice developed mechanical allodynia and thermal hyperalgesia in hind paws after 8 wk of diet treatment. These pain behaviors coincided with increased densities of nociceptive epidermal nerve fibers and inflammatory cells such as Langerhans cells and macrophages in hind paw skin. To study the effect of MetS on profiles of cytokine expression in HF mice, we used a multiplex cytokine assay to study the protein expression of 12 pro-inflammatory and anti-inflammatory cytokines in dorsal root ganglion and serum samples. This method detected the elevated levels of proinflammatory cytokines, including tumor necrosis factor (TNF)-α, and interleukin (IL)-6, IL-1β as well as reduced anti-inflammatory IL-10 in lumbar dorsal root ganglia (LDRG) of HF mice. Intraperitoneal administration of IL-10 reduced the upregulation of pro-inflammatory cytokines and alleviated pain behaviors in HF mice without affecting MetS phenotypes. Our findings suggested targeting HF-induced cytokine dysregulation could be an effective strategy for treating MetSPN.

PPARγ Agonists Attenuate Trigeminal Neuropathic Pain.

The aim of this study is to investigate the role of peroxisome proliferator-activated receptor-gamma isoform (PPARγ), in trigeminal neuropathic pain utilizing a novel mouse trigeminal inflammatory compression (TIC) injury model. The study determined that the PPARγ nuclear receptor plays a significant role in trigeminal nociception transmission, evidenced by: 1) Intense PPARγ immunoreactivity is expressed 3 weeks after TIC nerve injury in the spinal trigeminal caudalis, the termination site of trigeminal nociceptive nerve fibers. 2) Systemic administration of a PPARγ agonist, pioglitazone (PIO), attenuates whisker pad mechanical allodynia at doses of 300 mg/kg i.p. and 600 mg/kg p.o. 3) Administration of a PPARγ antagonist, GW9662 (30 mg/kg i.p.), prior to providing the optimal dose of PIO (300 mg/kg i.p.) blocked the analgesic effect of PIO. This is the first study localizing PPARγ immunoreactivity throughout the brainstem trigeminal sensory spinal nucleus (spV) and its increase three weeks after TIC nerve injury. This is also the first study to demonstrate that activation of PPARγ attenuates trigeminal hypersensitivity in the mouse TIC nerve injury model. The findings presented here suggest the possibility of utilizing the FDA approved diabetic treatment drug, PIO, as a new therapeutic that targets PPARγ for treatment of patients suffering from orofacial neuropathic pain.

In vivo optogenetic activation of Nav1.8+ cutaneous nociceptors and their responses to natural stimuli.

Optogenetic methods that utilize expression of the light-sensitive protein channelrhodopsin-2 (ChR2) in neurons have enabled selective activation of specific subtypes or groups of neurons to determine their functions. Using a transgenic mouse model in which neurons natively expressing Nav1.8 (a tetrodotoxin-resistant voltage-gated sodium channel) also express the light-gated channel ChR2, we have been able to determine the functional properties of Nav1.8-expressing cutaneous nociceptors of the glabrous skin in vivo. Most (44 of 53) of the C-fiber nociceptors isolated from Nav1.8-ChR2+ mice were found to be responsive to blue (470 nm) light. Response characteristics, including conduction velocity and responses to mechanical stimuli, were comparable between nociceptors isolated from Nav1.8-ChR2+ and control mice. Interestingly, while none of the non-light-responsive C-fibers were sensitive to heat or cold, nearly all (77%) light-sensitive fibers were excited by mechanical and thermal stimuli, suggesting that Nav1.8 is predominantly expressed by C-fiber nociceptors that are responsive to multiple stimulus modalities. The ability to activate peripheral nociceptors with light provides a method of stimulation that is noninvasive, does not require mechanical interruption of the skin, and accesses receptive fields that might be difficult or impossible to stimulate with standard stimuli while allowing repeated stimulation without injuring the skin.NEW & NOTEWORTHY Transgenic mice that express the blue light-sensitive protein channelrhodopsin2 (ChR2) in nociceptive nerve fibers that contain voltage-gated sodium channel Nav1.8 were used to determine functional properties of these afferent fibers. Electrophysiological recordings in vivo revealed that most nociceptive fibers that possess Nav1.8 are C-fiber nociceptors that respond to multiple stimulus modalities. Furthermore, responses evoked by blue light stimulation were comparable to those elicited by noxious mechanical, heat, and cold stimuli.

CD73 Controls Extracellular Adenosine Generation in the Trigeminal Nociceptive Nerves

Purinergic signaling is involved in pain generation and modulation in the nociceptive sensory nervous system. Adenosine triphosphate (ATP) induces pain via activation of ionotropic P2X receptors while adenosine mediates analgesia via activation of metabotropic P1 receptors. These purinergic signaling are determined by ecto-nucleotidases that control ATP degradation and adenosine generation. Using enzymatic histochemistry, we detected ecto-AMPase activity in dental pulp, trigeminal ganglia (TG) neurons, and their nerve fibers. Using immunofluorescence staining, we confirmed the expression of ecto-5′-nucleotidase (CD73) in trigeminal nociceptive neurons and their axonal fibers, including the nociceptive nerve fibers projecting into the brainstem. In addition, we detected the existence of CD73 and ecto-AMPase activity in the nociceptive lamina of the trigeminal subnucleus caudalis (TSNC) in the brainstem. Furthermore, we demonstrated that incubation with specific anti-CD73 serum significantly reduced the ecto-AMPase activity in the nociceptive lamina in the brainstem. Our results indicate that CD73 might participate in nociceptive modulation by affecting extracellular adenosine generation in the trigeminal nociceptive pathway. Disruption of TG neuronal ecto-nucleotidase expression and axonal terminal localization under certain circumstances such as chronic inflammation, oxidant stress, local constriction, and injury in trigeminal nerves may contribute to the pathogenesis of orofacial neuropathic pain.

Phosphorylated Histone 3 at Serine 10 Identifies Activated Spinal Neurons and Contributes to the Development of Tissue Injury-Associated Pain

Transcriptional changes in superficial spinal dorsal horn neurons (SSDHN) are essential in the development and maintenance of prolonged pain. Epigenetic mechanisms including post-translational modifications in histones are pivotal in regulating transcription. Here, we report that phosphorylation of serine 10 (S10) in histone 3 (H3) specifically occurs in a group of rat SSDHN following the activation of nociceptive primary sensory neurons by burn injury, capsaicin application or sustained electrical activation of nociceptive primary sensory nerve fibres. In contrast, brief thermal or mechanical nociceptive stimuli, which fail to induce tissue injury or inflammation, do not produce the same effect. Blocking N-methyl-D-aspartate receptors or activation of extracellular signal-regulated kinases 1 and 2, or blocking or deleting the mitogen- and stress-activated kinases 1 and 2 (MSK1/2), which phosphorylate S10 in H3, inhibit up-regulation in phosphorylated S10 in H3 (p-S10H3) as well as fos transcription, a down-stream effect of p-S10H3. Deleting MSK1/2 also inhibits the development of carrageenan-induced inflammatory heat hyperalgesia in mice. We propose that p-S10H3 is a novel marker for nociceptive processing in SSDHN with high relevance to transcriptional changes and the development of prolonged pain.

Topical Review: Potential Use of Botulinum Toxin in the Management of Painful Posttraumatic Trigeminal Neuropathy.

Painful posttraumatic trigeminal neuropathy (PPTTN) is a chronic condition that is difficult to endure and has a poorly understood pathophysiology. Treatment options are limited and often unsatisfactory due to insufficient efficacy and significant adverse effects. Botulinum toxin type A (BTX-A), initially used in the management of pathologically sustained or twisting muscular contractions, has recently been advocated for treatment of neuropathic pain. Its action is not limited to the blockage of acetylcholine release at the neuromuscular junction, but also includes inhibition of exocytosis of other neurotransmitters by interfering with the SNARE complexes of synaptic membranes. When injected into the painful location, the toxin can be taken up by peripheral terminals of nociceptive afferent nerve fibers, and this action suppresses peripheral and central release of algogenic neurotransmitters such as glutamate or substance P, thus promoting analgesia. Several randomized controlled trials in humans have provided emerging evidence for the therapeutic use of BTX-A in neuropathic pain states, including trigeminal neuralgia. This evidence, in addition to its good safety profile and long-lasting effect, suggests that BTX-A could be a potential novel treatment for PPTTN.

Mechanical antihypersensitivity effect induced by repeated spinal administrations of a TRPA1 antagonist or a gap junction decoupler in peripheral neuropathy

Spinal transient receptor potential ankyrin 1 (TRPA1) channel is associated with various pain hypersensitivity conditions. Spinally, TRPA1 is expressed by central terminals of nociceptive nerve fibers and astrocytes. Among potential endogenous agonists of TRPA1 is H2O2 generated by d-amino acid oxidase (DAAO) in astrocytes. Here we studied whether prolonged block of the spinal TRPA1 or astrocytes starting at time of injury attenuates development and/or maintenance of neuropathic hypersensitivity. Additionally, TRPA1 and DAAO mRNA were determined in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH). Experiments were performed in rats with spared nerve injury (SNI) and chronic intrathecal catheter. Drugs were administered twice daily for the first seven injury days or only once seven days after injury. Mechanical hypersensitivity was assessed with monofilaments. Acute and prolonged treatment with Chembridge-5861528 (a TRPA1 antagonist), carbenoxolone (an inhibitor of activated astrocytes), or gabapentin (a comparison drug) attenuated tactile allodynia-like responses evoked by low (2g) stimulus. However, antihypersensitivity effect of these compounds was short of significance at a high (15g) stimulus intensity. No preemptive effects were observed. In healthy controls, carbenoxolone failed to prevent hypersensitivity induced by spinal cinnamaldehyde, a TRPA1 agonist. TRPA1 and DAAO mRNA in the DRG but not SDH were slightly increased in SNI, independent of drug treatment. The results indicate that prolonged peri-injury block of spinal TRPA1 or inhibition of spinal astrocyte activation attenuates maintenance but not development of mechanical (tactile allodynia-like) hypersensitivity after nerve injury.

Short lasting transient effects of a capsaicin 8% patch on nociceptor activation in humans.

At high concentration, the TRPV-1 agonist capsaicin de-sensitizes nociceptors and reduces the intra-epidermal nerve density. We investigated the effects of a 5×10cm capsaicin 8% patch on C- and A-delta-nociceptor activation in ten healthy subjects before and at days 1-3-7-21 after patch application. Thermal thresholds, infrared thulium-YAG laser-evoked potentials (LEP) and heat pain (numeric rating scale, NRS, 0-10), electrically induced pain (10 pulses, 1.5-fold pain threshold intensity, five randomized series of 5-10-20-50-100Hz), and axon-reflex flare (laser Doppler imaging) were recorded. Thermal hypoesthesia developed upon capsaicin 8% treatment. Warmth detection thresholds increased at day 1-3, heat pain thresholds were increased by about 2.6°C after day 3, and laser-evoked heat pain remained significantly reduced for 7days. Axon-reflex flare responses (days 1-3), but not supra-threshold electrically induced pain were significantly reduced by the capsaicin patch. Axonal nociceptor function assessed by electrical excitability tests supplements threshold tests of nociceptive endings. The differential analgesic effects of 8% capsaicin patches may be attributed to the kinetics of capsaicin and the different depth of nociceptive nerve fibres, yet, the time course does not match the long-lasting analgesia observed in neuropathic pain patients treated with the same patch. WHAT DOES THIS STUDY ADD?: Axonal nociceptor function assessed by supra-threshold electrical excitability tests did not coincide with capsaicin-induced transduction changes supplementing threshold measures of terminal nociceptor endings. Threshold measurements do not reflect the sustained effect of pain relief seen in neuropathic pain patients. Capsaicin-sensitive nociceptors responsible for spontaneous pain are either not specifically tested with currently available sensory stimulation protocols or have higher capsaicin sensitivity or slower recovery under neuropathic conditions.

The Peridural Membrane of the Human Spine is Well Innervated

There is evidence that low back pain may originate from a peridural membrane (PDM) at the inferior and medial aspect of neural foramen of the lumbar spine. The objective of this investigation was to determine if this membrane contains neural elements suggestive of sensory innervation with nociceptive function. Spines of four embalmed and three non-embalmed human cadavers were dissected using a sagittal approach to the neural foramen. Seventeen samples of the peridural membrane overlying the neural foramen were collected for immunohistochemistry (IHC) examination by light microscopy and transmission electron microscopy (TEM). Chromagin tagged antibodies to protein gene product 9.5 (PGP9.5) and S-100, and fluorescent antibodies to substance P and calcitonin gene related peptide (CGRP) were used to label neural structures in tissue sections cut from paraffin embedded blocks. This approach allows good visualization of all neural elements, small sensory, and nociceptive nerve fibers in particular. Neural elements were found in all samples. Marked presence of small nerve fibers was observed in 12 of 15 samples. IHC and TEM evaluation revealed myelinated as well as unmyelinated fibers in the peridural membrane. CGRP and substance P immunoreactive fibers indicative of nociceptive function were abundant. These findings confirm and expand evidence that the peridural membrane in human is well innervated and contains sensory nociceptive nerve fibers suggestive of a nociceptive function of the membrane.

GABAB receptors inhibit low-voltage activated and high-voltage activated Ca2+ channels in sensory neurons via distinct mechanisms

Growing evidence suggests that mammalian peripheral somatosensory neurons express functional receptors for gamma-aminobutyric acid, GABAA and GABAB. Moreover, local release of GABA by pain-sensing (nociceptive) nerve fibres has also been suggested. Yet, the functional significance of GABA receptor triggering in nociceptive neurons is not fully understood. Here we used patch-clamp recordings from small-diameter cultured DRG neurons to investigate effects of GABAB receptor agonist baclofen on voltage-gated Ca2+ currents. We found that baclofen inhibited both low-voltage activated (LVA, T-type) and high-voltage activated (HVA) Ca2+ currents in a proportion of DRG neurons by 22% and 32% respectively; both effects were sensitive to Gi/o inhibitor pertussis toxin. Inhibitory effect of baclofen on both current types was about twice less efficacious as compared to that of the μ-opioid receptor agonist DAMGO. Surprisingly, only HVA but not LVA current modulation by baclofen was partially prevented by G protein inhibitor GDP-β-S. In contrast, only LVA but not HVA current modulation was reversed by the application of a reducing agent dithiothreitol (DTT). Inhibition of T-type Ca2+ current by baclofen and the recovery of such inhibition by DTT were successfully reconstituted in the expression system. Our data suggest that inhibition of LVA current in DRG neurons by baclofen is partially mediated by an unconventional signaling pathway that involves a redox mechanism. These findings reinforce the idea of targeting peripheral GABA receptors for pain relief.

Somatic modulation of spinal reflex bladder activity mediated by nociceptive bladder afferent nerve fibers in cats.

The goal of the present study was to determine if supraspinal pathways are necessary for inhibition of bladder reflex activity induced by activation of somatic afferents in the pudendal or tibial nerve. Cats anesthetized with α-chloralose were studied after acute spinal cord transection at the thoracic T9/T10 level. Dilute (0.25%) acetic acid was used to irritate the bladder, activate nociceptive afferent C-fibers, and trigger spinal reflex bladder contractions (amplitude: 19.3 ± 2.9 cmH2O). Hexamethonium (a ganglionic blocker, intravenously) significantly (P < 0.01) reduced the amplitude of the reflex bladder contractions to 8.5 ± 1.9 cmH2O. Injection of lidocaine (2%, 1-2 ml) into the sacral spinal cord or transection of the sacral spinal roots and spinal cord further reduced the contraction amplitude to 4.2 ± 1.3 cmH2O. Pudendal nerve stimulation (PNS) at frequencies of 0.5-5 Hz and 40 Hz but not at 10-20 Hz inhibited reflex bladder contractions, whereas tibial nerve stimulation (TNS) failed to inhibit bladder contractions at all tested frequencies (0.5-40 Hz). These results indicate that PNS inhibition of nociceptive afferent C-fiber-mediated spinal reflex bladder contractions can occur at the spinal level in the absence of supraspinal pathways, but TNS inhibition requires supraspinal pathways. In addition, this study shows, for the first time, that after acute spinal cord transection reflex bladder contractions can be triggered by activating nociceptive bladder afferent C-fibers using acetic acid irritation. Understanding the sites of action for PNS or TNS inhibition is important for the clinical application of pudendal or tibial neuromodulation to treat bladder dysfunctions.

Effect of Link N on the Expression of Neurotrophins and Substance P Release by Human Intervertebral Disc Cells Stimulated with Proinflammatory Cytokines

IntroductionAlthough there are different causes of low back pain, intervertebral disc (IVD) degeneration is one of them. In healthy discs, nociceptive nerve fibers and mechanoreceptors penetrate up...

Sensory Neuron Development in Mouse Coccygeal Vertebrae and Its Relationship to Tail Biopsies for Genotyping

A common method of genotyping mice is via tissue obtained from tail biopsies. However, there is no available information on the temporal development of sensory neurons in the tail and how their presence or absence might affect the age for performing tail biopsies. The goals of this study were to determine if afferent sensory neurons, and in particular nociceptive neurons, are present in the coccygeal vertebrae at or near the time of birth and if not, when they first can be visualized on or in those vertebrae. Using toluidine blue neuronal staining, transmission electron microscopy, and calcitonin-related gene peptide immunostaining, we found proximal to distal maturation of coccygeal nerve growth in the C57BL/6J mouse. Single nerve bundles were first seen on postpartum day (PPD) 0. On PPD 3 presumptive nociceptive sensory nerve fibers were seen entering the vertebral perichondrium. Neural development continued through the last time point (PPD 7) but at no time were neural fibers seen entering the body of the vertebrae. The effect of age on the development of pain perception in the neonatal mouse is discussed.

Mechanisms of chronic pain

Chronic pain is classified as nociceptive or neuropathic, depending on whether the integrity of the somatosensory nervous system is compromised by the underlying disease. Nociceptive pain results from the activation of receptors (nociceptors) sensitive to noxious stimuli. Prolonged or intense exposure to these stimuli, for example, chemical mediators released during inflammation, enhances the responsiveness of nociceptive nerve fibers [1]. This process, termed peripheral sensitization, involves a shift in the activation threshold of nociceptors and upregulation of voltage-gated sodium channels. Peripheral sensitization leads to increased action potential firing and transmitter release in the dorsal horn of the spinal cord, where somatosensory information is processed. Dorsal horn neurons react to the rising input with heightened excitability, a process termed central sensitization. Enhanced depolarization leads to the recruitment of N-methyl-D-aspartate (NMDA)-type glutamate receptors. NMDA and neuropeptide receptor activation produces a sharp increase in intracellular calcium, triggering signaling pathways and gene expression changes that promote a long-term shift in the activity of nociceptive circuits [2]. In some aspects, central sensitization even resembles long-term potentiation of excitatory transmission in the hippocampus. Central sensitization generates an exaggerated response to painful stimuli (hyperalgesia) and contributes to pain elicited by normally nonpainful stimuli (allodynia). Clinical findings suggest that pain hypersensitivity produces structural changes in the brain over time. These changes are, however, reversible upon pain relief. The pathophysiology of neuropathic pain is fundamentally different. Peripheral nerve lesion evokes stimulus-independent (ectopic) activity in nerve fibers. Innate immune cells react at the lesion site, in the dorsal root ganglion, where the cell bodies of peripheral somatosensory neurons reside, and in the dorsal horn of the spinal cord [3]. Active microglia of the dorsal horn releases chemical mediators that modulate the activity of neurons in the vicinity. One of these mediators, brain-derived neurotrophic factor, reduces the inhibitory effect of γ-aminobutyric acid (GABA) and glycine. Disinhibition opens polysynaptic connections in the dorsal horn, further enhancing the abnormal input from the lesioned nerve. Similarly to nociceptive pain, central sensitization occurs. Worsened by a relative deficit in transmitter uptake, increased glutamatergic transmission causes excitotoxic cell death, reducing the number of inhibitory interneurons. Their loss and a shift in descending modulatory pathways from the brainstem produce a profound imbalance between inhibition and excitation. The complexity of chronic pain mechanisms poses a major therapeutic challenge. Without biomarkers, it will remain difficult to develop targeted strategies for chronic pain reduction or prevention in the individual patient.

Activation of peripheral nerve fibers by electrical stimulation in the sole of the foot

BackgroundHuman nociceptive withdrawal reflexes (NWR) can be evoked by electrical stimulation applied to the sole of the foot. However, elicitation of NWRs is highly site dependent, and NWRs are especially difficult to elicit at the heel. The aim of the present study was to investigate potential peripheral mechanisms for any site dependent differences in reflex thresholds.ResultsThe first part of the study investigated the neural innervation in different sites of the sole of the foot using two different staining techniques. 1) Staining for the Nav1.7 antigen (small nociceptive fibers) and 2) the Sihler whole nerve technique (myelinated part of the nerve). No differences in innervation densities were found across the sole of the foot using the two staining techniques: Nav1.7 immunochemistry (small nociceptive fibers (1-way ANOVA, NS)) and the Sihler’s method (myelinated nerve fibers (1-way ANOVA, NS)). However, the results indicate that there are no nociceptive intraepidermal nerve fibers (IENFs) innervating the heel.Secondly, mathematical modeling was used to investigate to what degree differences in skin thicknesses affect the activation thresholds of Aδ and Aβ fibers in the sole of the foot. The modeling comprised finite element analysis of the volume conduction combined with a passive model of the activation of branching cutaneous nerve fibers. The model included three different sites in the sole of the foot (forefoot, arch and heel) and three different electrode sizes (diameters: 9.1, 12.9, and 18.3 mm). For each of the 9 combinations of site and electrode size, a total of 3000 Aβ fibers and 300 Aδ fibers was modeled. The computer simulation of the effects of skin thicknesses and innervation densities on thresholds of modeled Aδ and Aβ fibers did not reveal differences in pain and perception thresholds across the foot sole as have been observed experimentally. Instead a lack of IENFs at the heel decreased the electrical activation thresholds compared to models including IENFs.ConclusionsThe nerve staining and modeling results do not explain differences in NWR thresholds across the sole of the foot which may suggest that central mechanisms contribute to variation in NWR excitability across the sole of the foot.

TRPA1: A Transducer and Amplifier of Pain and Inflammation

The transient receptor potential ankyrin 1 (TRPA1) ion channel on peripheral terminals of nociceptive primary afferent nerve fibres contributes to the transduction of noxious stimuli to electrical signals, while on central endings in the spinal dorsal horn, it amplifies transmission to spinal interneurons and projection neurons. The centrally propagating nociceptive signal that is induced and amplified by TRPA1 not only elicits pain sensation but also contributes to peripheral neurogenic inflammation through a peripheral axon reflex or a centrally mediated back propagating dorsal root reflex that releases vasoactive agents from sensory neurons in the periphery. Endogenous TRPA1 agonists that are generated under various pathophysiological conditions both in the periphery and in the spinal cord have TRPA1-mediated pro-nociceptive and pro-inflammatory effects. Among endogenous TRPA1 agonists that have been shown to play a role in the pathogenesis of pain and inflammatory conditions are, for example, methylglyoxal, 4-hydroxynonenal, 12-lipoxygenase-derived hepoxilin A3, 5,6-epoxyeicosatrienoic acid and reactive oxygen species, while mustard oil and cinnamaldehyde are most commonly used exogenous TRPA1 agonists in experimental studies. Among selective TRPA1 antagonists are HC-030031, A-967079, AP-14 and Chembridge-5861528. Recent evidence indicates that TRPA1 plays a role also in transition of acute to chronic pain. Due to its location on a subpopulation of pain-mediating primary afferent nerve fibres, blocking the TRPA1 channel is expected to have antinociceptive, antiallodynic and anti-inflammatory effects.

Nociception originating from the crural fascia in rats

Little is documented in the literature as to the function of muscle fascia in nociception and pain. The aim of this study was to examine the distribution of presumptive nociceptive nerve fibers, to characterize fascial thin-fiber sensory receptors, and to examine the spinal projection of nociceptive input from the rat crural fascia (CF). Nerve fibers labeled with specific antibodies to calcitonin gene-related peptide (CGRP) and peripherin were found to be densely distributed in the distal third of the CF. Thin-fiber receptors (Aδ- and C-fibers) responding to pinching stimuli to the CF with sharpened watchmaker’s forceps, identified in vivo with the teased fiber technique from the common peroneal nerve, exist in the CF. Forty-three percent of the mechano-responsive fascial C-fibers were polymodal receptors (nociceptors) responding to mechanical, chemical (bradykinin), and heat stimuli, whereas almost all Aδ-fibers were responsive only to mechanical stimuli. Repetitive pinching stimulus to the CF induced c-Fos protein expression in the middle to medial part of superficial layers ie, laminae I–II of the spinal dorsal horn at segments L2 to L4, peaking at L3. These results clearly demonstrate the following: 1) peptidergic and non-peptidergic axons of unmyelinated C-fibers with nerve terminals are distributed in the CF; 2) peripheral afferents responding to noxious stimuli exist in the fascia, and 3) nociceptive information from the CF is mainly processed in the spinal dorsal horn at the segments L2 to L4. These results together indicate that the “muscle fascia,” a tissue often overlooked in pain research, can be an important source of nociception under normal conditions.

The distribution of nociceptive innervation in the painful hip

Our understanding of the origin of hip pain in degenerative disorders of the hip, including primary osteoarthritis, avascular necrosis and femoroacetabular impingement (FAI), is limited. We undertook a histological investigation of the nociceptive innervation of the acetabular labrum, ligamentum teres and capsule of the hip, in order to prove pain- and proprioceptive-associated marker expression. These structures were isolated from 57 patients who had undergone elective hip surgery (44 labral samples, 33 ligamentum teres specimens, 34 capsular samples; in 19 patients all three structures were harvested). A total of 15,000 histological sections were prepared that were investigated immunohistochemically for the presence of protein S-100, 68 kDa neurofilament, neuropeptide Y, nociceptin and substance P. The tissues were evaluated in six representative areas. Within the labrum, pain-associated free nerve ending expression was located predominantly at its base, decreasing in the periphery. In contrast, the distribution within the ligamentum teres showed a high local concentration in the centre. The hip capsule had an almost homogeneous marker expression in all investigated areas. This study showed characteristic distribution profiles of nociceptive and pain-related nerve fibres, which may help in understanding the origin of hip pain.

Pearls and pitfalls in experimental in vivo models of migraine: Dural trigeminovascular nociception

Migraine is a disorder of the brain and is thought to involve activation of the trigeminovascular system, which includes the peripheral afferent projection to the nociceptive specific dura mater, as well as the central afferent projection to the trigeminal nucleus caudalis. Stimulation of the blood vessels of the dura mater produces pain in patients that is referred to the head similar to headache. HEADACHE MECHANISMS: The likely reason for the pain is because the vascular structures of the dura mater, including the superior sagittal sinus and middle meningeal artery, are richly innervated by a plexus of largely unmyelinated sensory nerve fibers from the ophthalmic division of the trigeminal ganglion. Stimulation of these nociceptive specific nerve fibers is painful and produces neuronal activation in the trigeminal nucleus caudalis. Preclinical models of headache have taken advantage of this primarily nociceptive pathway, and various animal models use dural trigeminovascular nociception to assay aspects of head pain. These assays measure responses at the level of the dural vasculature and the central trigeminal nucleus caudalis as a correlate of trigeminovascular activation thought to be involved in headache. This review will summarize the history of the development of models of dural trigeminovascular nociception, including intravital microscopy and laser Doppler flowmetry at the level of the vasculature, and electrophysiology and Fos techniques used to observe neuronal activation at the trigeminal nucleus caudalis. It will also describe some of pitfalls of these assays and developments for the future.

Three-dimensional Imaging of Nociceptive Intraepidermal Nerve Fibers in Human Skin Biopsies

Three-dimensional Imaging of Nociceptive Intraepidermal Nerve Fibers in Human Skin Biopsies