Mechanically insensitive afferents (MIAs) in cutaneous nerves of monkey

Piezo2 voltage-block regulates mechanical pain sensitivity

PIEZO2 is a trimeric mechanically-gated ion channel expressed by most sensory neurons in the dorsal root ganglia. Mechanosensitive PIEZO2 channels are also genetically required for normal touch sensation in both mice and humans. We previously showed that PIEZO2 channels are also strongly modulated by membrane voltage. Specifically, it is only at very positive voltages that all channels are available for opening by mechanical force. Conversely, most PIEZO2 channels are blocked at normal negative resting membrane potentials. The physiological function of this unusual biophysical property of PIEZO2 channels, however, remained unknown. We characterized the biophysical properties of three PIEZO2 ion channel mutations at an evolutionarily conserved arginine (R2756). Using genome engineering in mice we generated Piezo2R2756H/R2756H and Piezo2R2756K/R2756K knock-in mice to characterize the physiological consequences of altering PIEZO2 voltage sensitivity in vivo. We measured endogenous mechanosensitive currents in sensory neurons isolated from the dorsal root ganglia and characterized mechanoreceptor and nociceptor function using electrophysiology. Mice were also assessed behaviourally and morphologically. Mutations at the conserved Arginine (R2756) dramatically changed the biophysical properties of the channel relieving voltage block and lowering mechanical thresholds for channel activation. Piezo2R2756H/R2756H and Piezo2R2756K/R2756K knock-in mice that were homozygous for gain-of-function mutations were viable and were tested for sensory changes. Surprisingly, mechanosensitive currents in nociceptors, neurons that detect noxious mechanical stimuli, were substantially sensitized in Piezo2 knock-in mice, but mechanosensitive currents in most mechanoreceptors that underlie touch sensation were only mildly affected by the same mutations. Single-unit electrophysiological recordings from sensory neurons innervating the glabrous skin revealed that rapidly-adapting mechanoreceptors that innervate Meissner's corpuscles exhibited slightly decreased mechanical thresholds in Piezo2 knock-in mice. Consistent with measurements of mechanically activated currents in isolated sensory neurons essentially all cutaneous nociceptors, both fast conducting Aδ-mechanonociceptors and unmyelinated C-fibre nociceptors were substantially more sensitive to mechanical stimuli and indeed acquired receptor properties similar to ultrasensitive touch receptors in Piezo2 knock-in mice. Mechanical stimuli also induced enhanced ongoing activity in cutaneous nociceptors in Piezo2 knock-in mice and hyper-sensitive PIEZO2 channels were sufficient alone to drive ongoing activity, even in isolated nociceptive neurons. Consistently, Piezo2 knock-in mice showed substantial behavioural hypersensitivity to noxious mechanical stimuli. Our data indicate that ongoing activity and sensitization of nociceptors, phenomena commonly found in human chronic pain syndromes, can be driven by relieving the voltage-block of PIEZO2 ion channels. Indeed, membrane depolarization caused by multiple noxious stimuli may sensitize nociceptors by relieving voltage-block of PIEZO2 channels.

Luteolin Alleviates Oxidative Stress in Chronic Obstructive Pulmonary Disease Induced by Cigarette Smoke via Modulation of the TRPV1 and CYP2A13/NRF2 Signaling Pathways

The current study aims to investigate the therapeutic potential of luteolin (Lut), a naturally occurring flavonoid found in various medicinal plants, for treating chronic obstructive pulmonary disease (COPD) through both in vitro and in vivo studies. The results demonstrated that Lut increased body weight, reduced lung tissue swelling and lung damage indices, mitigated systemic oxidative stress levels, and decreased alveolar fusion in cigarette smoke (CS)- and lipopolysaccharide (LPS)-induced COPD mice. Additionally, Lut was observed to downregulate the expression of the TRPV1 and CYP2A13 proteins while upregulating SIRT6 and NRF2 protein expression in CS + LPS-induced COPD mice and cigarette smoke extract (CSE)-treated A549 cells. The concentrations of total reactive oxygen species (ROS) and mitochondrial ROS in A549 cells induced by CSE significantly increased. Moreover, CSE caused a notable elevation of intracellular Ca2+ levels in A549 cells. Importantly, Lut exhibited inhibitory effects on the inward flow of Ca2+ and attenuated the overproduction of mitochondrial and intracellular ROS in A549 cells treated with CSE. In conclusion, Lut demonstrated a protective role in alleviating oxidative stress and inflammation in CS + LPS-induced COPD mice and CSE-treated A549 cells by regulating TRPV1/SIRT6 and CYP2A13/NRF2 signaling pathways.

Advancing the Understanding of Acupoint Sensitization and Plasticity Through Cutaneous C-Nociceptors

Acupoint is the key area for needling treatment, but its physiology is not yet understood. Nociceptors, one of the responders in acupoints, are responsible for acupuncture manipulation and delivering acupuncture signals to the spinal or supraspinal level. Recent evidence has shown that various diseases led to sensory hypersensitivity and functional plasticity in sensitized acupoints, namely, acupoint sensitization. Neurogenic inflammation is the predominant pathological characteristic for sensitized acupoints; however, the underlying mechanism in acupoint sensitization remains unclear. Recent studies have reported that silent C-nociceptors (SNs), a subtype of C nociceptors, can be “awakened” by inflammatory substances released by sensory terminals and immune cells under tissue injury or visceral dysfunction. SNs can transform from mechano-insensitive nociceptors in a healthy state to mechanosensitive nociceptors. Activated SNs play a vital role in sensory and pain modulation and can amplify sensory inputs from the injured tissue and then mediate sensory hyperalgesia. Whether activated SNs is involved in the mechanism of acupoint sensitization and contributes to the delivery of mechanical signals from needling manipulation remains unclear? In this review, we discuss the known functions of cutaneous C nociceptors and SNs and focus on recent studies highlighting the role of activated SNs in acupoint functional plasticity.

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

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

Studying human nociceptors: from fundamentals to clinic

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

Pruriception and neuronal coding in nociceptor subtypes in human and nonhuman primates

In humans, intradermal administration of β-alanine (ALA) and bovine adrenal medulla peptide 8–22 (BAM8-22) evokes the sensation of itch. Currently, it is unknown which human dorsal root ganglion (DRG) neurons express the receptors of these pruritogens, MRGPRD and MRGPRX1, respectively, and which cutaneous afferents these pruritogens activate in primate. In situ hybridization studies revealed that MRGPRD and MRGPRX1 are co-expressed in a subpopulation of TRPV1+ human DRG neurons. In electrophysiological recordings in nonhuman primates (Macaca nemestrina), subtypes of polymodal C-fiber nociceptors are preferentially activated by ALA and BAM8-22, with significant overlap. When pruritogens ALA, BAM8-22, and histamine, which activate different subclasses of C-fiber afferents, are administered in combination, human volunteers report itch and nociceptive sensations similar to those induced by a single pruritogen. Our results provide evidence for differences in pruriceptive processing between primates and rodents, and do not support the spatial contrast theory of coding of itch and pain.

Pain in Axial Spondyloarthritis: Insights from Immunology and Brain Imaging

Inflammatory back pain is characteristic of spondyloarthritis (SpA); however, this pain may not respond to treatment with NSAIDs or biologics. Pain is multifactorial and a combination of mechanical and inflammatory factors. A growing body of literature examines the impact of emotions on pain in SpA; many patients with this condition suffer from depression and fibromyalgia. Advanced imaging techniques can investigate the interplay of various brain networks in pain perception. Animal models have helped understand the interplay between the immune and nervous systems in pain generation and have highlighted differences in pain perception between the sexes.

Peripheral Deltorphin II Inhibits Nociceptors Following Nerve Injury

Clinical and preclinical studies have revealed that local administration of opioid agonists into peripheral tissue attenuates inflammatory pain. However, few studies have examined whether peripherally restricted opioids are effective in reducing mechanical allodynia and hyperalgesia that usually follows nerve injury. The aim of the present study was to determine whether the mechanical responsiveness of C-fiber mechanical nociceptors innervating skin under neuropathic pain conditions is depressed by direct activation of delta opioid receptors (DORs) on their peripheral terminals. A murine model of peripheral neuropathic pain was induced with a spared nerve (tibial) injury, in which mice survived 7 or 28 days after surgery before electrophysiological testing began. Control groups comprised naïve and sham-operated animals. An ex vivo preparation of mouse plantar skin with attached tibial nerve was used to examine electrophysiologically the effects of the selective DOR agonist, deltorphin II, on the response properties of individual cutaneous C-fiber nociceptors. In contrast to naïve and sham-operated animals, deltorphin II induced an inhibition of the mechanical responsiveness of C-fiber mechanical nociceptors innervating skin under neuropathic conditions. The effects of deltorphin II were concentration-dependent and prevented by pretreatment with naltrindole indicating DOR-mediated inhibitory effects of deltorphin II. Our results provide the first direct evidence for expression of functional DORs on mechanical nociceptors innervating skin in an animal model of neuropathic pain.

Nociceptor subtypes and their incidence in rat lumbar dorsal root ganglia (DRGs): focussing on C-polymodal nociceptors, Aβ-nociceptors, moderate pressure receptors and their receptive field depths

A recent study with Ca++-sensitive-dyes in neurons in whole DRGs (Table 5) found that much lower percentages of nociceptors were polymodal-nociceptors (PMNs) (Emery et al., 2016), than the 50-80% values in many electrophysiological fiber studies. This conflict highlighted the lack of knowledge about percentages of nociceptor-subtypes in the DRG. This was analysed from intracellularly-recorded neurons in rat lumbar DRGs stimulated from outside the skin. Polymodal nociceptors (PMNs) were 11% of all neurons and 19% of all nociceptors. Most PMNs had C-fibers (CPMNs). Percentages of C-nociceptors that were CPMNs varied with receptive field (RF) depths, whether superficial (∼80%), dermal (25%), deep (0%) or cutaneous (superficial + dermal) (40%). This explains CPMN percentages 40-90%, being highest, in electrophysiological studies using cutaneous nerves, and lowest in studies that also include deep RFs, including ours, and the recent Ca++-imaging studies in whole DRGs. Despite having been originally described in 1967 (Burgess and Perl), both Aβ-nociceptors and Aβ-moderate pressure receptors (MPRs) remain overlooked. Most A-fiber nociceptors in rodents have Aβ-fibers. Of rat lumbar Aβ-nociceptors with superficial RFs, 50% were MPRs with variable medium-low trkA-expression. Despite having conduction velocities at the two extremes for nociceptors, both CPMNs and MPRs have relatively low thresholds, superficial/epidermal RFs and low trkA-expression. For abbreviations used see Table 5.

Functional and Molecular Characterization of Mechanoinsensitive "Silent" Nociceptors

Mechanical and thermal hyperalgesia (pain hypersensitivity) are cardinal signs of inflammation. Although the mechanism underlying thermal hyperalgesia is well understood, the cellular and molecular basis of mechanical hyperalgesia is poorly described. Here, we have identified a subset of peptidergic C-fiber nociceptors that are insensitive to noxious mechanical stimuli under normal conditions but become sensitized to such stimuli when exposed to the inflammatory mediator nerve growth factor (NGF). Strikingly, NGF did not affect mechanosensitivity of other nociceptors. We show that these mechanoinsensitive "silent" nociceptors are characterized by the expression of the nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) and that the mechanically gated ion channel PIEZO2 mediates NGF-induced mechanosensitivity in these neurons. Retrograde tracing revealed that CHRNA3⁺ nociceptors account for ∼50% of all peptidergic nociceptive afferents innervating visceral organs and deep somatic tissues. Hence, our data suggest that NGF-induced "un-silencing" of CHRNA3⁺ nociceptors significantly contributes to the development of mechanical hyperalgesia during inflammation.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

The human pain system exhibits higher-order plasticity (metaplasticity)

The human pain system can be bidirectionally modulated by high-frequency (HFS; 100 Hz) and low-frequency (LFS; 1 Hz) electrical stimulation of nociceptors leading to long-term potentiation or depression of pain perception (pain-LTP or pain-LTD). Here we show that priming a test site by very low-frequency stimulation (VLFS; 0.05 Hz) prevented pain-LTP probably by elevating the threshold (set point) for pain-LTP induction. Conversely, prior HFS-induced pain-LTP was substantially reversed by subsequent VLFS, suggesting that preceding HFS had primed the human nociceptive system for pain-LTD induction by VLFS. In contrast, the pain elicited by the pain-LTP-precipitating conditioning HFS stimulation remained unaffected. In aggregate these experiments demonstrate that the human pain system expresses two forms of higher-order plasticity (metaplasticity) acting in either direction along the pain-LTD to pain-LTP continuum with similar shifts in thresholds for LTD and LTP as in synaptic plasticity, indicating intriguing new mechanisms for the prevention of pain memory and the erasure of hyperalgesia related to an already established pain memory trace. There were no apparent gender differences in either pain-LTP or metaplasticity of pain-LTP. However, individual subjects appeared to present with an individual balance of pain-LTD to pain-LTP (a pain plasticity "fingerprint").

Molecular Mechanisms That Contribute to Bone Marrow Pain

Pain associated a bony pathology puts a significant burden on individuals, society, and the health-care systems worldwide. Pathology that involves the bone marrow activates sensory nerve terminal endings of peripheral bone marrow nociceptors, and is the likely trigger for pain. This review presents our current understanding of how bone marrow nociceptors are influenced by noxious stimuli presented in pathology associated with bone marrow. A number of ion channels and receptors are emerging as important modulators of the activity of peripheral bone marrow nociceptors. Nerve growth factor (NGF) sequestration has been trialed for the management of inflammatory bone pain (osteoarthritis), and there is significant evidence for interaction of NGF with bone marrow nociceptors. Activation of transient receptor potential cation channel subfamily V member 1 sensitizes bone marrow nociceptors and could contribute to increased sensitivity of patients to noxious stimuli in various bony pathologies. Acid-sensing ion channels sense changes to tissue pH in the bone marrow microenvironment and could be targeted to treat pathology that involves acidosis of the bone marrow. Piezo2 is a mechanically gated ion channel that has recently been reported to be expressed by most myelinated bone marrow nociceptors and might be a target for treatments directed against mechanically induced bone pain. These ion channels and receptors could be useful targets for the development of peripherally acting drugs to treat pain of bony origin.

Mechanically sensitive Aδ nociceptors that innervate bone marrow respond to changes in intra-osseous pressure

KEY POINTS

Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a novel in vivo bone-nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra-osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra-osseous pressure stimulus and express the Piezo2 mechano-transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and that the Piezo2 mechano-transducer may be involved.

ABSTRACT

Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Aδ nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic-tonic units that appear to code for different intensities of intra-osseous pressure, or phasic units that code for the rate of change in intra-osseous pressure. Three different subclasses of mechanically sensitive Aδ units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic-tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak-to-peak action potential amplitude. The majority of small-diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Aδ mechano-nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and imply that Piezo2 signalling may be involved in mechano-transduction in these receptors.

High-resolution functional MRI identified distinct global intrinsic functional networks of nociceptive posterior insula and S2 regions in squirrel monkey brain

Numerous functional imaging and electrophysiological studies in humans and animals indicate that the two contiguous areas of secondary somatosensory cortex (S2) and posterior insula (pIns) are core regions in nociceptive processing and pain perception. In this study, we tested the hypothesis that the S2-pIns connection serves as a hub for connecting distinct sensory and affective nociceptive processing networks in the squirrel monkey brain. At 9.4T, we first mapped the brain regions that respond to nociceptive heat stimuli with high-resolution fMRI, and then used seed-based resting-state fMRI (rsfMRI) analysis to delineate and refine the global intrinsic functional connectivity circuits of the proximal S2 and pIns regions. In each subject, nociceptive (47.5°C) heat-evoked fMRI activations were detected in many brain regions, including primary somatosensory (S1), S2, pIns, area 7b, anterior cingulate cortex (ACC), primary motor cortex, prefrontal cortex, supplementary motor area, thalamus, and caudate. Using the heat-evoked fMRI activation foci in S2 and pIns as the seeds, voxel-wise whole-brain resting-state functional connectivity (rsFC) analysis revealed strong functional connections between contralateral S2 and pIns, as well as their corresponding regions in the ipsilateral hemisphere. Spatial similarity and overlap analysis identified each region as part of two distinct intrinsic functional networks with 7% overlap: sensory S2-S1-area 7b and affective pIns-ACC-PCC networks. Moreover, a high degree of overlap was observed between the combined rsFC maps of nociceptive S2 and pIns regions and the nociceptive heat-evoked activation map. In summary, our study provides evidence for the existence of two distinct intrinsic functional networks for S2 and pIns nociceptive regions, and these two networks are joined via the S2-pIns connection. Brain regions that are involved in processing nociceptive inputs are also highly interconnected at rest. The presence of robust and distinct S1-S2-area 7b and pIns-ACC-PCC rsFC networks under anesthesia underscores their fundamental roles in processing nociceptive information.

Brain network alterations in the inflammatory soup animal model of migraine

Advances in our understanding of the human pain experience have shifted much of the focus of pain research from the periphery to the brain. Current hypotheses suggest that the progression of migraine depends on abnormal functioning of neurons in multiple brain regions. Accordingly, we sought to capture functional brain changes induced by the application of an inflammatory cocktail known as inflammatory soup (IS), to the dura mater across multiple brain networks. Specifically, we aimed to determine whether IS alters additional neural networks indirectly related to the primary nociceptive pathways via the spinal cord to the thalamus and cortex. IS comprises an acidic combination of bradykinin, serotonin, histamine and prostaglandin PGE2 and was introduced to basic pain research as a tool to activate and sensitize peripheral nociceptors when studying pathological pain conditions associated with allodynia and hyperalgesia. Using this model of intracranial pain, we found that dural application of IS in awake, fully conscious, rats enhanced thalamic, hypothalamic, hippocampal and somatosensory cortex responses to mechanical stimulation of the face (compared to sham synthetic interstitial fluid administration). Furthermore, resting state MRI data revealed altered functional connectivity in a number of networks previously identified in clinical chronic pain populations. These included the default mode, sensorimotor, interoceptive (Salience) and autonomic networks. The findings suggest that activation and sensitization of meningeal nociceptors by IS can enhance the extent to which the brain processes nociceptive signaling, define new level of modulation of affective and cognitive responses to pain; set new tone for hypothalamic regulation of autonomic outflow to the cranium; and change cerebellar functions.

Profound alteration in cutaneous primary afferent activity produced by inflammatory mediators

Inflammatory pain is thought to arise from increased transmission from nociceptors and recruitment of 'silent' afferents. To evaluate inflammation-induced changes, mice expressing GCaMP3 in cutaneous sensory neurons were generated and neuronal responses to mechanical stimulation in vivo before and after subcutaneous infusion of an 'inflammatory soup' (IS) were imaged in an unanesthetized preparation. Infusion of IS rapidly altered mechanical responsiveness in the majority of neurons. Surprisingly, more cells lost, rather than gained, sensitivity and 'silent' afferents that were mechanically insensitive and gained mechanosensitivity after IS exposure were rare. However, the number of formerly 'silent' afferents that became mechanosensitive was increased five fold when the skin was heated briefly prior to infusion of IS. These findings suggest that pain arising from inflamed skin reflects a dramatic shift in the balance of sensory input, where gains and losses in neuronal populations results in novel output that is ultimately interpreted by the CNS as pain.

Alfaxalone Anaesthesia Facilitates Electrophysiological Recordings of Nociceptive Withdrawal Reflexes in Dogs (Canis familiaris)

Naturally occurring canine osteoarthritis represents a welfare issue for affected dogs (Canis familiaris), but is also considered very similar to human osteoarthritis and has therefore been proposed as a model of disease in humans. Central sensitisation is recognized in human osteoarthritis sufferers but identification in dogs is challenging. Electromyographic measurement of responses to nociceptive stimulation represents a potential means of investigating alterations in central nociceptive processing, and has been evaluated in conscious experimental dogs, but is likely to be aversive. Development of a suitable anaesthetic protocol in experimental dogs, which facilitated electrophysiological nociceptive withdrawal reflex assessment, may increase the acceptability of using the technique in owned dogs with naturally occurring osteoarthritis. Seven purpose bred male hound dogs underwent electromyographic recording sessions in each of three states: acepromazine sedation, alfaxalone sedation, and alfaxalone anaesthesia. Electromyographic responses to escalating mechanical and electrical, and repeated electrical, stimuli were recorded. Subsequently the integral of both early and late rectified responses was calculated. Natural logarithms of the integral values were analysed within and between the three states using multi level modeling. Alfaxalone increased nociceptive thresholds and decreased the magnitude of recorded responses, but characteristics of increasing responses with increasing stimulus magnitude were preserved. Behavioural signs of anxiety were noted in two out of seven dogs during recordings in the acepromazine sedated state. There were few significant differences in response magnitude or nociceptive threshold between the two alfaxalone states. Following acepromazine premedication, induction of anaesthesia with 1–2 mg kg^-1 alfaxalone, followed by a continuous rate infusion in the range 0.075–0.1 mg kg^-1 min^-1 produced suitable conditions to enable assessment of spinal nociceptive processing in dogs, without subjecting them to potentially aversive experiences. This methodology may be appropriate for obtaining electrophysiological nociceptive withdrawal reflex data in client-owned dogs with naturally occurring osteoarthritis.

■ REVIEW : The Pathology of Pain

It has recently become recognized that neuropathic forms of chronic pain represent true neurologic disease. Current investigations are largely molecular, yet knowledge of the anatomy and cell biology of pain is also important for the development of more effective medications. Although acute pain is beneficial, neuropathic pain is pathological and creates devastating disability. It occurs when an abnormal somatosensory system chronically transmits pain signals despite the absence of acute injury. Any type of lesion anywhere in the peripheral or central spinothalamic pathway can cause it. The most common scenario involves interruption of peripheral sensory axons with distal Wallerian degeneration. Regenerating peripheral sensory axons can develop ongoing spontaneous action potentials or ectopic mechano- and chemosensitivity that contribute to pain. Axotomy also induces morphological and functional alterations proximally that can contribute to pain. Central axon terminals can degenerate or sprout aberrantly within the dorsal horn. Higher order sensory neurons within the CNS can experience trans-synaptic damage. Lesions wholly within the CNS, such as stroke and multiple sclerosis, can also produce neuropathic pain. This review of a nascent field is presented in hopes of stimulating further investigation into this common, under-recognized medical problem. NEURO SCIENTIST 5:302-310, 1999

Optogenetic activation of mechanically insensitive afferents in mouse colorectum reveals chemosensitivity

The sensory innervation of the distal colorectum includes mechanically insensitive afferents (MIAs; ∼25%), which acquire mechanosensitivity in persistent visceral hypersensitivity and thus generate de novo input to the central nervous system. We utilized an optogenetic approach to bypass the process of transduction (generator potential) and focus on transformation (spike initiation) at colorectal MIA sensory terminals, which is otherwise not possible in typical functional studies. From channelrhodopsin2-expressing mice (driven by Advillin-Cre), the distal colorectum with attached pelvic nerve was harvested for ex vivo single-fiber recordings. Afferent receptive fields (RFs) were identified by electrical stimulation and tested for response to mechanical stimuli (probing, stroking, and stretch), and afferents were classified as either MIAs or mechanosensitive afferents (MSAs). All MIA and MSA RFs were subsequently stimulated optically and MIAs were also tested for activation/sensitization with inflammatory soup (IS), acidic hypertonic solution (AHS), and/or bile salts (BS). Responses to pulsed optical stimuli (1-10 Hz) were comparable between MSAs and MIAs whereas 43% of MIAs compared with 86% of MSAs responded tonically to stepped optical stimuli. Tonic-spiking MIAs responded preferentially to AHS (an osmotic stimulus) whereas non-tonic-spiking MIAs responded to IS (an inflammatory stimulus). A significant proportion of MIAs were also sensitized by BS. These results reveal transformation as a critical factor underlying the differences between MIAs (osmosensors vs. inflammatory sensors), revealing a previously unappreciated heterogeneity of MIA endings. The current study draws attention to the sensory encoding of MIA nerve endings that likely contribute to afferent sensitization and thus have important roles in visceral pain.

Role of oxidative stress & transient receptor potential in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) affect millions of people worldwide and is known to be one of the leading causes of death. The highly sensitive airways protect themselves from irritants by cough and sneeze which propel endogenous and exogenous substances to minimize airway noxious effects. One noxious effect of these substances is activation of peripheral sensory nerve endings of nociceptor neurons innervating these airways lining thus transmitting dangerous signals from the environment to the central nervous system (CNS). Nociceptor neurons include transient receptor potential (TRP) ion channels, especially the vanilloid and ankyrin subfamilies, TRPV1/A1 which can be activated by noxious chemical challenges in models of airways disease. As oxidative stress may activate airways sensory neurons and contribute to COPD exacerbations we sought to review the role that TRP channel activation by oxidative signals may have on airway responses. It would be prudent to target the TRP channels with antagonists and lower systemic oxidative stress with agents that can modulate TRP expression and boost the endogenous levels of antioxidants for treatment and management of COPD.

Nociceptor Sensitization Depends on Age and Pain Chronicity(1,2,3)

Peripheral inflammation causes mechanical pain behavior and increased action potential firing. However, most studies examine inflammatory pain at acute, rather than chronic time points, despite the greater burden of chronic pain on patient populations, especially aged individuals. Furthermore, there is disagreement in the field about whether primary afferents contribute to chronic pain. Therefore, we sought to evaluate the contribution of nociceptor activity to the generation of pain behaviors during the acute and chronic phases of inflammation in both young and aged mice. We found that both young (2 months old) and aged (>18 months old) mice exhibited prominent pain behaviors during both acute (2 day) and chronic (8 week) inflammation. However, young mice exhibited greater behavioral sensitization to mechanical stimuli than their aged counterparts. Teased fiber recordings in young animals revealed a twofold mechanical sensitization in C fibers during acute inflammation, but an unexpected twofold reduction in firing during chronic inflammation. Responsiveness to capsaicin and mechanical responsiveness of A-mechanonociceptor (AM) fibers were also reduced chronically. Importantly, this lack of sensitization in afferent firing during chronic inflammation occurred even as these inflamed mice exhibited continued behavioral sensitization. Interestingly, C fibers from inflamed aged animals showed no change in mechanical firing compared with controls during either the acute or chronic inflammatory phases, despite strong behavioral sensitization to mechanical stimuli at these time points. These results reveal the following two important findings: (1) nociceptor sensitization to mechanical stimulation depends on age and the chronicity of injury; and (2) maintenance of chronic inflammatory pain does not rely on enhanced peripheral drive.

Laser-evoked potentials mediated by mechano-insensitive nociceptors in human skin

OBJECTIVES

Laser-evoked potentials (LEP) were assessed after peripheral nerve block of the lateral femoral cutaneous nerve (LFCN) in healthy volunteers from partially anesthetized skin areas to differentially stimulate mechano-insensitive nociceptors.

METHODS

An ultrasound-guided nerve block of the LFCN was performed in 12 healthy male subjects with Ropivacain 1%. After 30 min, the nerve block induced significantly larger anesthetic areas to mechanical stimuli than to electrical stimuli revealing an area of differential sensitivity. LEPs, reaction times and pain ratings were recorded in response to the laser stimuli of (1) completely anesthetic skin, (2) mechano-insensitive, but electrically excitable skin ('differential sensitivity'), (3) normal skin.

RESULTS

LEP latencies in the area of differential sensitivity were increased compared to unaffected skin (228 ± 8.5 ms, vs. 181 ± 3.6 ms, p < 0.01) and LEP amplitudes were reduced (14.8 ± 1.2 μV vs. 24.6 ± 1.7 μV, p < 0.01). Correspondingly, psychophysically assessed response latencies in the differentially anesthetic skin were increased (649 ms vs. 427 ms, p < 0.01) and pain ratings reduced (1.5/10 vs. 5/10 NRS, p < 0.01).

CONCLUSION

The increase in LEP latency suggests that mechano-insensitive heat-sensitive Aδ nociceptors (MIA, type II) have a slower conduction velocity or higher utilization time than mechano-sensitive type II Aδ nociceptors. Moreover, widely branched, slowly conducting and mechano-insensitive branches of Aδ nociceptors can explain our finding. LEPs in the differentially anesthetized skin provide specific information about a mechanically insensitive but heat-sensitive subpopulation of Aδ nociceptors. These findings support the concept that A-fibre nociceptors exhibit a similar degree of modality specificity as C-fibre nociceptors.

Amplified Mechanically Gated Currents in Distinct Subsets of Myelinated Sensory Neurons following In Vivo Inflammation of Skin and Muscle

Primary afferents are sensitized to mechanical stimuli following in vivo inflammation, but whether sensitization of mechanically gated ion channels contributes to this phenomenon is unknown. Here we identified two populations of murine A fiber-type sensory neurons that display markedly different responses to focal mechanical stimuli of the membrane based on their expression of calcitonin gene-related peptide (CGRP). Following inflammation of the hindpaw, myelinated, CGRP-positive neurons projecting to the paw skin displayed elevated mechanical currents in response to mechanical stimuli. Conversely, muscle inflammation markedly amplified mechanical currents in myelinated, CGRP-negative neurons projecting to muscle. These data show, for the first time, that mechanically gated currents are amplified following in vivo tissue inflammation, and also suggest that mechanical sensitization can occur in myelinated neurons after inflammation.

In vitro functional characterization of mouse colorectal afferent endings

This video demonstrates in detail an in vitro single-fiber electrophysiological recording protocol using a mouse colorectum-nerve preparation. The approach allows unbiased identification and functional characterization of individual colorectal afferents. Extracellular recordings of propagated action potentials (APs) that originate from one or a few afferent (i.e., single-fiber) receptive fields (RFs) in the colorectum are made from teased nerve fiber fascicles. The colorectum is removed with either the pelvic (PN) or lumbar splanchnic (LSN) nerve attached and opened longitudinally. The tissue is placed in a recording chamber, pinned flat and perfused with oxygenated Krebs solution. Focal electrical stimulation is used to locate the colorectal afferent endings, which are further tested by three distinct mechanical stimuli (blunt probing, mucosal stroking and circumferential stretch) to functionally categorize the afferents into five mechanosensitive classes. Endings responding to none of these mechanical stimuli are categorized as mechanically-insensitive afferents (MIAs). Both mechanosensitive and MIAs can be assessed for sensitization (i.e., enhanced response, reduced threshold, and/or acquisition of mechanosensitivity) by localized exposure of RFs to chemicals (e.g., inflammatory soup (IS), capsaicin, adenosine triphosphate (ATP)). We describe the equipment and colorectum-nerve recording preparation, harvest of colorectum with attached PN or LSN, identification of RFs in the colorectum, single-fiber recording from nerve fascicles, and localized application of chemicals to the RF. In addition, challenges of the preparation and application of standardized mechanical stimulation are also discussed.

Neurophysiology and itch pathways

As we all can easily differentiate the sensations of itch and pain, the most straightforward neurophysiologic concept would consist of two specific pathways that independently encode itch and pain. Indeed, a neuronal pathway for histamine-induced itch in the peripheral and central nervous system has been described in animals and humans, and recently several non-histaminergic pathways for itch have been discovered in rodents that support a dichotomous concept differentiated into a pain and an itch pathway, with both pathways being composed of different "flavors." Numerous markers and mediators have been found that are linked to itch processing pathways. Thus, the delineation of neuronal pathways for itch from pain pathways seemingly proves that all sensory aspects of itch are based on an itch-specific neuronal pathway. However, such a concept is incomplete as itch can also be induced by the activation of the pain pathway in particular when the stimulus is applied in a highly localized spatial pattern. These opposite views reflect the old dispute between specificity and pattern theories of itch. Rather than only being of theoretic interest, this conceptual problem has key implication for the strategy to treat chronic itch as key therapeutic targets would be either itch-specific pathways or unspecific nociceptive pathways.

Skin-nerve preparation to assay the function of opioid receptors in peripheral endings of sensory neurons

This chapter describes the methodology of the in vitro skin-saphenous nerve preparation and its application to test for the modulatory effects of opioids on the function of cutaneous sensory neurons in experimental models of pain. We detail the skin-nerve setup requirements and the technique to record action potentials from single sensory fibers. We address how to test for inhibitory effects of opioid receptor activation on mechanical and thermal sensitivity of nociceptors and mechanoreceptors in the complete Freund's adjuvant-induced inflammation and the chronic constriction injury model of neuropathic pain.

Pro-neurotrophins, sortilin, and nociception

Nerve growth factor (NGF) signaling is important in the development and functional maintenance of nociceptors, but it also plays a central role in initiating and sustaining heat and mechanical hyperalgesia following inflammation. NGF signaling in pain has traditionally been thought of as primarily engaging the classic high-affinity receptor tyrosine kinase receptor TrkA to initiate sensitization events. However, the discovery that secreted proforms of nerve NGF have biological functions distinct from the processed mature factors raised the possibility that these proneurotrophins (proNTs) may have distinct function in painful conditions. ProNTs engage a novel receptor system that is distinct from that of mature neurotrophins, consisting of sortilin, a type I membrane protein belonging to the VPS10p family, and its co-receptor, the classic low-affinity neurotrophin receptor p75NTR. Here, we review how this new receptor system may itself function with or independently of the classic TrkA system in regulating inflammatory or neuropathic pain.

Three functionally distinct classes of C-fibre nociceptors in primates

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

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

Sensory neurons and circuits mediating itch

Chemicals that are used experimentally to evoke itch elicit activity in diverse subpopulations of cutaneous pruriceptive neurons, all of which also respond to painful stimuli. However, itch is distinct from pain: it evokes different behaviours, such as scratching, and originates from the skin or certain mucosae but not from muscle, joints or viscera. New insights regarding the neurons that mediate the sensation of itch have been gained from experiments in which gene expression has been manipulated in different types of pruriceptive neurons as well as from comparisons between psychophysical measurements of itch and the neuronal discharges and other properties of peripheral and central pruriceptive neurons.

Nerve growth factor and nociception: from experimental embryology to new analgesic therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.

Offset analgesia is reduced in older adults

Recent studies indicate that aging is associated with dysfunctional changes in pain modulatory capacity, potentially contributing to increased incidence of pain in older adults. However, age-related changes in offset analgesia (offset), a form of temporal pain inhibition, remain poorly characterized. The purpose of this study was to investigate age differences in offset analgesia of heat pain in healthy younger and older adults. To explore the peripheral mechanisms underlying offset, an additional aim of the study was to test offset at 2 anatomical sites with known differences in nociceptor innervation. A total of 25 younger adults and 20 older adults completed 6 offset trials in which the experimental heat stimulus was presented to the volar forearm and glabrous skin of the palm. Each trial consisted of 3 continuous phases: an initial 15-second painful stimulus (T1), a slight increase in temperature from T1 for 5 seconds (T2), and a slight decrease back to the initial testing temperature for 10 seconds (T3). During each trial, subjects rated pain intensity continuously using an electronic visual analogue scale (0-100). Older adults demonstrated reduced offset compared to younger adults when tested on the volar forearm. Interestingly, offset analgesia was nonexistent on the palm for all subjects. The reduced offset found in older adults may reflect an age-related decline in endogenous inhibitory systems. However, although the exact mechanisms underlying offset remain unknown, the absence of offset at the palm suggests that peripheral mechanisms may be involved in initiating this phenomenon.

The fundamental unit of pain is the cell

The molecular/genetic era has seen the discovery of a staggering number of molecules implicated in pain mechanisms [18,35,61,69,96,133,150,202,224]. This has stimulated pharmaceutical and biotechnology companies to invest billions of dollars to develop drugs that enhance or inhibit the function of many these molecules. Unfortunately this effort has provided a remarkably small return on this investment. Inevitably, transformative progress in this field will require a better understanding of the functional links among the ever-growing ranks of "pain molecules," as well as their links with an even larger number of molecules with which they interact. Importantly, all of these molecules exist side-by-side, within a functional unit, the cell, and its adjacent matrix of extracellular molecules. To paraphrase a recent editorial in Science magazine [223], although we live in the Golden age of Genetics, the fundamental unit of biology is still arguably the cell, and the cell is the critical structural and functional setting in which the function of pain-related molecules must be understood. This review summarizes our current understanding of the nociceptor as a cell-biological unit that responds to a variety of extracellular inputs with a complex and highly organized interaction of signaling molecules. We also discuss the insights that this approach is providing into peripheral mechanisms of chronic pain and sex dependence in pain.

[Microneurography and research on peripheral neuropathic pain]

BACKGROUND

Microneurography is a neurophysiological technique which enables recording from single peripheral nerve fibres in persons who are awake. The method is only used in research. We discuss how microneurography has been used to map nerve-fibre functions under normal circumstances and in chronic pain conditions.

METHOD

The article is based on a literature search in PubMed and on the authors' own knowledge and experience of the method from their research.

RESULTS

Microneurography has contributed to the understanding of pain under physiological conditions and in chronic pain conditions, in particular peripheral neuropathic pain. For example, signs of hyperexcitability have been found in peripheral nerve fibres in connection with neuropathies and peripheral neuropathic pain conditions, and the proportion of hyperexcitable nerve fibres has been shown to be greater in neuropathy patients with chronic pain than in neuropathy patients without pain. Findings indicate that so-called CMi nociceptors play an important role in chronic neuropathic pain.

INTERPRETATION

In the longer term we hope that research using microneurography will help to reveal mechanisms of direct importance for the development of targeted treatment of neuropathic pain.

Fentanyl decreases discharges of C and A nociceptors to suprathreshold mechanical stimulation in chronic inflammation

An essential component of mechanical hyperalgesia resulting from tissue injury is an enhanced excitability of nociceptive neurons, termed mechanical sensitization. Local application of opioids to inflamed rat paws attenuates mechanical hyperalgesia and reduces electrical excitability of C-fiber nociceptors in acute injury. Here, we examined the effects of the opioid receptor agonist fentanyl on the mechanical coding properties of not only C- but also A-fiber nociceptors innervating the rat hind paw in a model of chronic pain, i.e., 4 days after Freund's complete adjuvant-induced inflammation. The peripheral mechanosensitive terminals of C-fibers (n = 143), A-fibers (n = 79), and low-threshold mechanoreceptors (n = 25) were characterized using the in vitro skin-nerve preparation from the saphenous nerve. Although mechanical activation thresholds were not changed, discharges to suprathreshold mechanical stimuli were elevated significantly in both A- and C-fiber nociceptors from inflamed tissue. In addition, the proportion of nociceptors as well as the frequency of spontaneous discharges in A (14% vs. 0%)- and C (28% vs. 8%)-fibers were increased in inflamed compared with normal tissue. Fentanyl inhibited responses to suprathreshold stimuli in a significantly higher proportion of not only C (36% vs. 7%)- but also A (41% vs. 8%)-fibers in inflamed tissue in a naloxone-reversible and concentration-dependent manner. Our results demonstrate that mechanical sensitization persists in chronic inflammation, in correlation with behavioral hyperalgesia. Opioid sensitivity of both A- and C-fibers is markedly augmented. This is consistent with an upregulation or enhanced functionality of opioid receptors located at the peripheral terminals of sensitized nociceptors.

Pain Management for Nerve Injury following Dental Implant Surgery at Tokyo Dental College Hospital

By allowing reconstruction of compromised occlusion, dental implants contribute to an improvement in quality of life (QOL) and diet. Injury to a nerve during such treatment, however, can result in a sudden decline in QOL. And once a nerve has been injured, the chances of a full recovery are slim unless the damage is only slight. If such damage causes neuropathic pain severe enough to prevent sleep, the patient's QOL will deteriorate dramatically. While damage to skin tissue or bone invariably heals over time, damage to nerves does not, indicating the need to avoid such injury while performing implant insertion, for example. This means not relying solely on X-ray images, which can be rather unclear, but also using computed tomography to allow preoperative planning and intraoperative execution to be performed as accurately as possible. Moreover, if sensory damage does occur it is essential to avoid breaking the bond of trust between dentist and patient by giving false assurances of recovery. In such cases, appropriate measures must be taken promptly. This paper describes pain management for nerve injury following dental implant surgery at the Orofacial Pain Center of Tokyo Dental College Suidoubashi Hospital.

TRPV1 antagonist, A-889425, inhibits mechanotransmission in a subclass of rat primary afferent neurons following peripheral inflammation

TRPV1 (transient receptor potential vanilloid family type 1) is a nonselective cation channel that is activated and/or sensitized by noxious heat, protons, and other endogenous molecules released following tissue injury. In addition, a role for TRPV1 in mechanotransmission is emerging. We have recently reported that a selective TRPV1 receptor antagonist, A-889425, reduces mechanical allodynia and spinal neuron responses to mechanical stimulation of complete Freund's adjuvant (CFA)-inflamed rat hind paws. The population of peripheral nerve fibers through which TRPV1 antagonists mediate their effect on mechanotransmission have not yet been described. The objective of this study was to characterize TRPV1-mediated modulation of mechanically evoked activity in sensory axons innervating rat hind paws. We used an in vitro skin-nerve preparation to record neural activity from single axons isolated from rat tibial nerve. Single fibers were classified by conduction velocity, mechanical threshold, and stimulus-response relationships. We used A-889425 to investigate uninjured and inflamed skin afferent neuron populations to evoked mechanical stimulation. Application of A-889425 had no effect on the mechanical responsiveness of Aδ and C-fiber units innervating uninjured skin. In contrast, A-889425 inhibited responses of slowly conducting Aδ fiber units to noxious mechanical stimulation in a population of axons innervating CFA-inflamed hind paws. These data support a role for TRPV1 in mechanotransmission following peripheral inflammation, and highlight the importance of a distinct subclass of primary afferent neurons in mediating this effect.

Characterization of silent afferents in the pelvic and splanchnic innervations of the mouse colorectum

Hypersensitivity in inflammatory/irritable bowel syndrome is contributed to in part by changes in the receptive properties of colorectal afferent endings, likely including mechanically insensitive afferents (MIAs; silent afferents) that have the ability to acquire mechanosensitivity. The proportion and attributes of colorectal MIAs, however, have not previously been characterized. The distal ∼3 cm of colorectum with either pelvic (PN) or lumbar splanchnic (LSN) nerve attached was removed, opened longitudinally, pinned flat in a recording chamber, and perfused with oxygenated Krebs solution. Colorectal receptive endings were located by electrical stimulation and characterized as mechanosensitive or not by blunt probing, mucosal stroking, and circumferential stretch. MIA endings were tested for response to and acquisition of mechanosensitivity by localized exposure to an inflammatory soup (IS). Colorectal afferents were also tested with twin-pulse and repetitive electrical stimulation paradigms. PN MIAs represented 23% of 211 afferents studied, 71% (30/42) of which acquired mechanosensitivity after application of IS to their receptive ending. LSN MIAs represented 33% of 156 afferents studied, only 23% (11/48) of which acquired mechanosensitivity after IS exposure. Mechanosensitive PN endings uniformly exhibited significant twin-pulse slowing whereas LSN endings showed no significant twin-pulse difference. PN MIAs displayed significantly greater activity-dependent slowing than LSN MIAs. In conclusion, significant proportions of MIAs are present in the colorectal innervation; significantly more in the PN than LSN acquire mechanosensitivity in an inflammatory environment. This knowledge contributes to our understanding of the possible roles of MIAs in colon-related disorders like inflammatory/irritable bowel syndrome.

Nociceptors: the sensors of the pain pathway

Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury-related chemicals, and transducing these stimuli into long-ranging electrical signals that are relayed to higher brain centers. The activation of functionally distinct cutaneous nociceptor populations and the processing of information they convey provide a rich diversity of pain qualities. Current work in this field is providing researchers with a more thorough understanding of nociceptor cell biology at molecular and systems levels and insight that will allow the targeted design of novel pain therapeutics.