Does neurogenic inflammation alter the sensitivity of unmyelinated nociceptors in the rat?

Cortical spreading depression and meningeal nociception

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CSD evoked persistent activation and mechanical sensitization of dural nociceptors is likely to drive the headache phase in migraine with aura.

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The development of neurogenic-mediated dural vasodilatation and increased plasma protein extravasation in the wake of CSD may not contribute to meningeal nociception.

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Cortical vasoconstriction and reduced oxygen availability following CSD do not contribute to meningeal nociception.

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Cortical neuroinflammation, involving neuronal pannexin1 and calcium-independent astrocytic signaling drive meningeal nociception following CSD.

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CSD-related closing of K(ATP) channels and release of COX-driven prostanoids mediate the activation and sensitization of dural nociceptors respectively.

Migraine results in an enormous burden on individuals and societies due to its high prevalence, significant disability, and considerable economic costs. Current treatment options for migraine remain inadequate, and the development of novel therapies is severely hindered by the incomplete understanding of the mechanisms responsible for the pain. The sensory innervation of the cranial meninges is now considered a key player in migraine headache genesis. Recent studies have significantly advanced our understanding of some of the processes that drive meningeal nociceptive neurons, which may be targeted therapeutically to abort or prevent migraine pain. In this review we will summarize our current understanding of the mechanisms that contribute to the genesis of the headache in one migraine subtype – migraine with aura. We will focus on animal studies that address the notion that cortical spreading depression is a critical process that drives meningeal nociception in migraine with aura, and discuss recent insights into some of the proposed underlying mechanisms.

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Dry eye sensitizes cool cells to capsaicin-induced changes in activity via TRPV1

Corneal cool cells are sensitive to the ocular fluid status of the corneal surface and may be responsible for the regulation of basal tear production. Previously, we have shown that dry eye, induced by lacrimal gland excision (LGE) in rats, sensitized corneal cool cells to the transient receptor potential melastatin 8 (TRPM8) agonist menthol and to cool stimulation. In the present study, we examined the effect of dry eye on the sensitivity of cool cells to the transient receptor potential vanilloid 1 (TRPV1) agonist capsaicin. Single-unit recordings in the trigeminal ganglion were performed 7-10 days after LGE. At a concentration of 0.3 μM, capsaicin did not affect ongoing or cool-evoked activity in control animals yet facilitated ongoing activity and suppressed cool-evoked activity in LGE animals. At higher concentrations (3 μM), capsaicin continued to facilitate ongoing activity in LGE animals but suppressed ongoing activity in control animals. Higher concentrations of capsaicin also suppressed cool-evoked activity in both groups of animals, with an overall greater effect in LGE animals. In addition to altering cool-evoked activity, capsaicin enhanced the sensitivity of cool cells to heat in LGE animals. Capsaicin-induced changes were prevented by the application of the TRPV1 antagonist capsazepine. With the use of fluorescent in situ hybridization, TRPV1 and TRPM8 expression was examined in retrograde tracer-identified corneal neurons. The coexpression of TRPV1 and TRPM8 in corneal neurons was significantly greater in LGE-treated animals when compared with sham controls. These results indicate that LGE-induced dry eye increases TRPV1-mediated responses in corneal cool cells at least in part through the increased expression of TRPV1. NEW & NOTEWORTHY Corneal cool cells are known to detect drying of the ocular surface. Our study is the first to report that dry eye induced alterations in cool cell response properties, including the increased responsiveness to noxious heat and activation by capsaicin. Along with the changes in cell response properties, it is possible these neurons also function differently in dry eye, relaying information related to the perception of ocular irritation in addition to regulating tearing and blinking.

Antinuclear antibody and rheumatoid factor positivity in temporomandibular disorders

Background

To investigate the differences in clinical characteristics and long-term treatment outcomes according to antinuclear antibody(ANA) and rheumatoid factor(RF) positivity and the correlation between pain-related and hematological indices in temporomandibular disorders(TMD) patients.

Methods

Clinical examinations were done following the Research Diagnostic Criteria for TMD in 257 patients. Comprehensive screening along with psychological and hematological evaluations (ANA, RF, complete blood cell count, C-reactive protein[CRP] and erythrocyte sedimentation rate[ESR]) were conducted. Clinical characteristics and treatment outcomes were statistically compared between ANA/RF positive and negative groups.

Results

Thirty-nine patients showed ANA/RF positivity. Male patients had smaller comfortable mouth opening(CMO)(p = 0.033) and maximum mouth opening(MMO)(p = 0.016) ranges with more painful neck muscles on palpation when RF/ANA positive. Pain duration, intensity, disability days and psychological distress levels were also higher in RF/ANA positive male patients. Significant correlation was shown in ESR with pain duration(p < 0.05) and numeric rating scale(NRS) before treatment(p < 0.05), CRP with NRS before treatment(p < 0.01), and red blood cell (RBC) with pain intensity(p < 0.05), NRS before treatment(p < 0.01), CMO(p < 0.01), pain on palpation of cervical muscles(p < 0.01), CMO(p < 0.05), and MMO(p < 0.05) 6 months after treatment.

Conclusions

These results may point towards a nonspecific autoimmune disposition in a subgroup of TMD patients. RF and ANA could be considered as a screening test for the detection of autoimmune phenomena in TMD.

Burst-Like Subcutaneous Electrical Stimulation Induces BDNF-Mediated, Cyclotraxin B-Sensitive Central Sensitization in Rat Spinal Cord

Intrathecal administration of brain derived neurotrophic factor (BDNF) induces long-term potentiation (LTP) and generates long-lasting central sensitization in spinal cord thus mimicking chronic pain, but the relevance of these observations to chronic pain mechanisms is uncertain. Since C-fiber activation by a high-frequency subcutaneous electrical stimulation (SES) protocol causes spinal release of BDNF and induces spinal cord LTP, we propose that application of such protocol would be a sufficient condition for generating long-lasting BDNF-mediated central sensitization. Results showed that application of burst-like SES to rat toes produced (i) rapid induction of hyperalgesia that lasted for more than 3 weeks, (ii) early increase of C-reflex activity followed by increased wind-up scores lasting for more than 1 week, and (iii) early increase followed by late decrease in BDNF protein levels and phosphorylated TrkB that lasted for more than 1 week. These changes were prevented by the TrkB antagonist cyclotraxin-B administered shortly before SES, while hyperalgesia was reversed by cyclotraxin-B administered 3 days after SES. Results suggest that mechanisms underlying central sensitization first involve BDNF release of probably neuronal origin, followed by brief increased expression of likely glial BDNF and pTrkB that could switch early phase sensitization into late one.

Current understanding of meningeal and cerebral vascular function underlying migraine headache

BACKGROUND

The exact mechanisms underlying the onset of a migraine attack are not completely understood. It is, however, now well accepted that the onset of the excruciating throbbing headache of migraine is mediated by the activation and increased mechanosensitivity (i.e. sensitization) of trigeminal nociceptive afferents that innervate the cranial meninges and their related large blood vessels.

OBJECTIVES

To provide a critical summary of current understanding of the role that the cranial meninges, their associated vasculature, and immune cells play in meningeal nociception and the ensuing migraine headache.

METHODS

We discuss the anatomy of the cranial meninges, their associated vasculature, innervation and immune cell population. We then debate the meningeal neurogenic inflammation hypothesis of migraine and its putative contribution to migraine pain. Finally, we provide insights into potential sources of meningeal inflammation and nociception beyond neurogenic inflammation, and their potential contribution to migraine headache.

The roles of TRPV1, TRPA1 and TRPM8 channels in chemical and thermal sensitivity of the mouse oral mucosa

Spices in food and beverages and compounds in tobacco smoke interact with sensory irritant receptors of the transient receptor potential (TRP) cation channel family. TRPV1 (vanilloid type 1), TRPA1 (ankyrin 1) and TRPM8 (melastatin 8) not only elicit action potential signaling through trigeminal nerves, eventually evoking pungent or cooling sensations, but by their calcium conductance they also stimulate the release of calcitonin gene-related peptide (CGRP). This is measured as an index of neuronal activation to elucidate the chemo- and thermosensory transduction in the isolated mouse buccal mucosa of wild types and pertinent knockouts. We found that the lipophilic capsaicin, mustard oil and menthol effectively get access to the nerve endings below the multilayered squamous epithelium, while cigarette smoke and its gaseous phase were weakly effective releasing CGRP. The hydrophilic nicotine was ineffective unless applied unprotonated in alkaline (pH9) solution, activating TRPA1 and TRPV1. Also, mustard oil activated both these irritant receptors in millimolar but only TRPA1 in micromolar concentrations; in combination (1 mm) with heat (45 °C), it showed supraadditive, that is heat sensitizing, effects in TRPV1 and TRPA1 knockouts, suggesting action on an unknown heat-activated channel and mustard oil receptor. Menthol caused little CGRP release by itself, but in subliminal concentration (2 mm), it enabled a robust cold response that was absent in TRPM8^-/- but retained in TRPA1^-/- and strongly reduced by TRPM8 inhibitors. In conclusion, all three relevant irritant receptors are functionally expressed in the oral mucosa and play their specific roles in inducing neurogenic inflammation and sensitization to heat and cold.

The CGRP receptor antagonist BIBN4096 inhibits prolonged meningeal afferent activation evoked by brief local K+ stimulation but not cortical spreading depression-induced afferent sensitization

Calcitonin gene-related peptide mediates K⁺-evoked delayed and prolonged activation of cranial meningeal afferents but does not contribute to their enhanced responsiveness following cortical spreading depression.

Introduction:

Cortical spreading depression (CSD) is believed to promote migraine headache by enhancing the activity and mechanosensitivity of trigeminal intracranial meningeal afferents. One putative mechanism underlying this afferent response involves an acute excitation of meningeal afferents by cortical efflux of K⁺ and the ensuing antidromic release of proinflammatory sensory neuropeptides, such as calcitonin gene-related peptide (CGRP).

Objectives:

We sought to investigate whether (1) a brief meningeal K⁺ stimulus leads to CGRP-dependent enhancement of meningeal afferent responses and (2) CSD-induced meningeal afferent activation and sensitization involve CGRP receptor signaling.

Methods:

Extracellular single-unit recording were used to record the activity of meningeal afferents in anesthetized male rats. Stimulations included a brief meningeal application of K⁺ or induction of CSD in the frontal cortex using pinprick. Cortical spreading depression was documented by recording changes in cerebral blood flow using laser Doppler flowmetery. Calcitonin gene-related peptide receptor activity was inhibited with BIBN4096 (333 μM, i.v.).

Results:

Meningeal K⁺ stimulation acutely activated 86% of the afferents tested and also promoted in ∼65% of the afferents a 3-fold increase in ongoing activity, which was delayed by 23.3 ± 4.1 minutes and lasted for 22.2 ± 5.6 minutes. K⁺ stimulation did not promote mechanical sensitization. Pretreatment with BIBN4096 suppressed the K⁺-induced delayed afferent activation, reduced CSD-evoked cortical hyperemia, but had no effect on the enhanced activation or mechanical sensitization of meningeal afferents following CSD.

Conclusion:

While CGRP-mediated activation of meningeal afferents evoked by cortical efflux of K⁺ could promote headache, acute activation of CGRP receptors may not play a key role in mediating CSD-evoked headache.

TRPA1 and TRPV1 are required for lidocaine-evoked calcium influx and neuropeptide release but not cytotoxicity in mouse sensory neurons

Background

Local anaesthetics (LA) reduce neuronal excitability by inhibiting voltage-gated Na⁺ channels. When applied at high concentrations in the direct vicinity of nerves, LAs can also induce relevant irritation and neurotoxicity via mechanisms involving an increase of intracellular Ca²⁺. In the present study we explored the role of the Ca²⁺-permeable ion channels TRPA1 and TRPV1 for lidocaine-induced Ca²⁺-influx, neuropeptide release and neurotoxicity in mouse sensory neurons.

Methods

Cultured dorsal root ganglion (DRG) neurons from wildtype and mutant mice lacking TRPV1, TRPA1 or both channels were explored by means of calcium imaging, whole-cell patch clamp recordings and trypan blue staining for cell death. Release of calcitonin gene-related peptide (CGRP) from isolated mouse peripheral nerves was determined with ELISA.

Results

Lidocaine up to 10 mM induced a concentration-dependent reversible increase in intracellular Ca²⁺ in DRG neurons from wildtype and mutant mice lacking one of the two receptors, but not in neurons lacking both TRPA1 and TRPV1. 30 mM lidocaine also released Ca²⁺ from intracellular stores, presumably from the endoplasmic reticulum. While 10 mM lidocaine evoked an axonal CGRP release requiring expression of either TRPA1 or TRPV1, CGRP release induced by 30 mM lidocaine again mobilized internal Ca²⁺ stores. Lidocaine-evoked cell death required neither TRPV1 nor TRPA1.

Summary

Depending on the concentration, lidocaine employs TRPV1, TRPA1 and intracellular Ca²⁺ stores to induce a Ca²⁺-dependent release of the neuropeptide CGRP. Lidocaine-evoked cell death does not seem to require Ca²⁺ influx through TRPV1 or TRPV1.

Modulation of C-nociceptive Activities by Inputs from Myelinated Fibers

To understand the mechanisms of neuropathic pain caused by demyelination, a rapid-onset, completed but reversible demyelination of peripheral A-fibers and neuropathic pain behaviors in adult rats by single injection of cobra venom into the sciatic nerve, was created. Microfilament recording revealed that cobra venom selectively blocked A-fibers, but not C-fibers. Selective blockade of A-fibers may result from A-fiber demyelination at the site of venom injection as demonstrated by microscope examination. Neuropathic pain behaviors including inflammatory response appeared almost immediately after venom injection and lasted about 3 weeks. Electrophysiological studies indicated that venom injection induced loss of conduction in A-fibers, increased sensitivity of C-polymodal nociceptors to innocuous stimuli, and triggered spontaneous activity from peripheral and central terminals of C-fiber nociceptors. Neurogenic inflammatory responses were also observed in the affected skin via Evans blue extravasation experiments. Both antidromic C-fiber spontaneous activity and neurogenic inflammation were substantially decreased by continuous A-fiber threshold electric stimuli applied proximally to the venom injection site. The data suggest that normal activity of peripheral A-fibers may produce inhibitory modulation of C-polymodal nociceptors. Removal of inhibition to C-fiber polymodal nociceptors following demyelination of A-fibers may result in pain and neurogenic inflammation in the affected receptive field.

Peripheral NMDA Receptors Mediate Antidromic Nerve Stimulation-Induced Tactile Hypersensitivity in the Rat

We investigated the role of peripheral NMDA receptors (NMDARs) in antidromic nerve stimulation-induced tactile hypersensitivity outside the skin area innervated by stimulated nerve. Tetanic electrical stimulation (ES) of the decentralized L5 spinal nerve, which induced enlargement of plasma extravasation, resulted in tactile hypersensitivity in the L4 plantar dermatome of the hind-paw. When intraplantar (i.pl.) injection was administered into the L4 dermatome before ES, NMDAR and group-I metabotropic Glu receptor (mGluR) antagonists and group-II mGluR agonist but not AMPA/kainate receptor antagonist prevented ES-induced hypersensitivity. I.pl. injection of PKA or PKC inhibitors also prevented ES-induced hypersensitivity. When the same injections were administered after establishment of ES-induced hypersensitivity, hypersensitivity was partially reduced by NMDAR antagonist only. In naïve animals, i.pl. Glu injection into the L4 dermatome induced tactile hypersensitivity, which was blocked by NMDAR antagonist and PKA and PKC inhibitors. These results suggest that the peripheral release of Glu, induced by antidromic nerve stimulation, leads to the expansion of tactile hypersensitive skin probably via nociceptor sensitization spread due to the diffusion of Glu into the skin near the release site. In addition, intracellular PKA- and PKC-dependent mechanisms mediated mainly by NMDAR activation are involved in Glu-induced nociceptor sensitization and subsequent hypersensitivity.

Mouse monoclonal antibodies to transient receptor potential ankyrin 1 act as antagonists of multiple modes of channel activation

The transient receptor potential ankyrin 1 (TRPA1) channel has been implicated in different pathophysiologies that include asthma, cough, itch, and inflammatory pain. Agonists of TRPA1 such as mustard oil and its key component allyl isothiocyanate (AITC) cause pain and neurogenic inflammation in humans and pain behaviors in rodents. Hence, TRPA1 antagonists are being pursued as potential therapeutics. With the goal of generating monoclonal antibodies (mAbs) to human TRPA1 that could act as selective antagonists, we immunized mice with a variety of antigens expressing TRPA1 channels. After generation of hybridomas, the hybridoma conditioned media were screened to identify the mAbs that bind TRPA1 channels by a flow cytometry assay utilizing U2OS or Chinese hamster ovary (CHO) cells stably expressing TRPA1. The purified IgGs from the hybridomas that showed selective binding to TRPA1 were evaluated for antagonism in agonist-induced (45)Ca(2+) uptake assays using CHO-TRPA1 cells. Several of the mAbs showed concentration-dependent inhibition of AITC and cold (4°C) activation of TRPA1. The most potent mAb, 2B10, had IC50 values of approximately 260 and 90 nM in the two assays, respectively. These antagonist mAbs also blocked osmotically activated TRPA1 as well as activation by an endogenous agonist (4-oxo-2-nonenal). In summary, we generated mouse mAbs against TRPA1 that act as antagonists of multiple modes of TRPA1 activation.

TRP modulation by natural compounds

The use of medicinal plants or other naturally derived products to relieve illness can be traced back over several millennia, and these natural products are still extensively used nowadays. Studies on natural products have, over the years, enormously contributed to the development of therapeutic drugs used in modern medicine. By means of the use of these substances as selective agonists, antagonists, enzyme inhibitors or activators, it has been possible to understand the complex function of many relevant targets. For instance, in an attempt to understand how pepper species evoke hot and painful actions, the pungent and active constituent capsaicin (from Capsicum sp.) was isolated in 1846 and the receptor for the biological actions of capsaicin was cloned in 1997, which is now known as TRPV1 (transient receptor potential vanilloid 1). Thus, TRPV1 agonists and antagonists have currently been tested in order to find new drug classes to treat different disorders. Indeed, the transient receptor potential (TRP) proteins are targets for several natural compounds, and antagonists of TRPs have been synthesised based on the knowledge of naturally derived products. In this context, this chapter focuses on naturally derived compounds (from plants and animals) that are reported to be able to modulate TRP channels. To clarify and make the understanding of the modulatory effects of natural compounds on TRPs easier, this chapter is divided into groups according to TRP subfamilies: TRPV (TRP vanilloid), TRPA (TRP ankyrin), TRPM (TRP melastatin), TRPC (TRP canonical) and TRPP (TRP polycystin). A general overview on the naturally derived compounds that modulate TRPs is depicted in Table 1.

Changes in sensory activity of ocular surface sensory nerves during allergic keratoconjunctivitis

Peripheral neural mechanisms underlying the sensations of irritation, discomfort, and itch accompanying the eye allergic response have not been hitherto analyzed. We explored this question recording the changes in the electrical activity of corneoconjunctival sensory nerve fibers of the guinea pig after an ocular allergic challenge. Sensitization was produced by i.p. ovalbumin followed by repeated application in the eye of 10% ovalbumin on days 14 to 18. Blinking and tearing rate were measured. Spontaneous and stimulus-evoked (mechanical, thermal, chemical) impulse activity was recorded from mechanonociceptor, polymodal nociceptor and cold corneoscleral sensory afferent fibers. After a single (day 14) or repeated daily exposures to the allergen during the following 3 to 4days, tearing and blinking rate increased significantly. Also, sensitization was observed in mechanonociceptors (transient reduction of mechanical threshold only on day 14) and in polymodal nociceptors (sustained enhancement of the impulse response to acidic stimulation). In contrast, cold thermoreceptors showed a significant decrease in basal ongoing activity and in the response to cooling. Treatment with the TRPV1 and TRPA1 blockers capsazepine and HC-030031 reversed the augmented blinking. Only capsazepine attenuated tearing rate increase and sensitization of the polymodal nociceptors response to CO2. Capsazepine also prevented the decrease in cold thermoreceptor activity caused by the allergic challenge. We conclude that changes in nerve impulse activity accompanying the ocular allergic response, primarily mediated by activation of nociceptor's TRPV1 and to a lesser degree by activation of TRPA1 channels, explain the eye discomfort sensations accompanying allergic episodes.

Mechanisms of transient receptor potential vanilloid 1 activation and sensitization by allyl isothiocyanate

Allyl isothiocyanate (AITC; aka, mustard oil) is a powerful irritant produced by Brassica plants as a defensive trait against herbivores and confers pungency to mustard and wasabi. AITC is widely used experimentally as an inducer of acute pain and neurogenic inflammation, which are largely mediated by the activation of nociceptive cation channels transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1 (TRPV1). Although it is generally accepted that electrophilic agents activate these channels through covalent modification of cytosolic cysteine residues, the mechanism underlying TRPV1 activation by AITC remains unknown. Here we show that, surprisingly, AITC-induced activation of TRPV1 does not require interaction with cysteine residues, but is largely dependent on S513, a residue that is involved in capsaicin binding. Furthermore, AITC acts in a membrane-delimited manner and induces a shift of the voltage dependence of activation toward negative voltages, which is reminiscent of capsaicin effects. These data indicate that AITC acts through reversible interactions with the capsaicin binding site. In addition, we show that TRPV1 is a locus for cross-sensitization between AITC and acidosis in nociceptive neurons. Furthermore, we show that residue F660, which is known to determine the stimulation by low pH in human TRPV1, is also essential for the cross-sensitization of the effects of AITC and low pH. Taken together, these findings demonstrate that not all reactive electrophiles stimulate TRPV1 via cysteine modification and help understanding the molecular bases underlying the surprisingly large role of this channel as mediator of the algesic properties of AITC.

Allyl isothiocyanate sensitizes TRPV1 to heat stimulation

The powerful plant-derived irritant allyl isothiocyanate (AITC, aka mustard oil) induces hyperalgesia to heat in rodents and humans through mechanisms that are not yet fully understood. It is generally believed that AITC activates the broadly tuned chemosensory cation channel transient receptor potential cation channel subfamily A member 1 (TRPA1), triggering an inflammatory response that sensitizes the heat sensor transient receptor potential cation channel subfamily V member 1 (TRPV1). In the view of recent data demonstrating that AITC can directly activate TRPV1, we here explored the possibility that this compound sensitizes TRPV1 to heat stimulation in a TRPA1-independent manner. Patch-clamp recordings and intracellular Ca(2+) imaging experiments in HEK293T cells over-expressing mouse TRPV1 revealed that the increase in channel activation induced by heating is larger in the presence of AITC than in control conditions. The analysis of the effects of AITC and heat on the current-voltage relationship of TRPV1 indicates that the mechanism of sensitization is based on additive shifts of the voltage dependence of activation towards negative voltages. Finally, intracellular Ca(2+) imaging experiments in mouse sensory neurons isolated from Trpa1 KO mice yielded that AITC enhances the response to heat, specifically in the subpopulation expressing TRPV1. Furthermore, this effect was strongly reduced by the TRPV1 inhibitor capsazepine and virtually absent in neurons isolated from double Trpa1/Trpv1 KO mice. Taken together, these findings demonstrate that TRPV1 is a locus for cross sensitization between AITC and heat in sensory neurons and may help explaining, at least in part, the role of this channel in AITC-induced hyperalgesia to heat.

TRPA1 mediates mechanical sensitization in nociceptors during inflammation

Inflammation is a part of the body's natural response to tissue injury which initiates the healing process. Unfortunately, inflammation is frequently painful and leads to hypersensitivity to mechanical stimuli, which is difficult to treat clinically. While it is well established that altered sensory processing in the spinal cord contributes to mechanical hypersensitivity (central sensitization), it is still debated whether primary afferent neurons become sensitized to mechanical stimuli after tissue inflammation. We induced inflammation in C57BL/6 mice via intraplantar injection of Complete Freund's Adjuvant. Cutaneous C fibers exhibited increased action potential firing to suprathreshold mechanical stimuli. We found that abnormal responses to intense mechanical stimuli were completely suppressed by acute incubation of the receptive terminals with the TRPA1 inhibitor, HC-030031. Further, elevated responses were predominantly exhibited by a specific subgroup of C fibers, which we determined to be C-Mechano Cold sensitive fibers. Thus, in the presence of HC-030031, C fiber mechanical responses in inflamed mice were not different than responses in saline-injected controls. We also demonstrate that injection of the HC-030031 compound into the hind paw of inflamed mice alleviates behavioral mechanical hyperalgesia without affecting heat hyperalgesia. Further, we pharmacologically anesthetized the TRPA1-expressing fibers in vivo by co-injecting the membrane-impermeable sodium channel inhibitor QX-314 and the TRPA1 agonist cinnamaldehyde into the hind paw. This approach also alleviated behavioral mechanical hyperalgesia in inflamed mice but left heat hypersensitivity intact. Our findings indicate that C-Mechano Cold sensitive fibers exhibit enhanced firing to suprathreshold mechanical stimuli in a TRPA1-dependent manner during inflammation, and that input from these fibers drives mechanical hyperalgesia in inflamed mice.

Involvement of the glutamatergic system in the nociception induced intrathecally for a TRPA1 agonist in rats

The transient receptor potential ankyrin 1 (TRPA1) is expressed in peripheral and spinal terminals of sensory neurons, jointly to the vanilloid receptor (TRPV1). A relevant peripheral role of TRPA1 receptor has been implicated in a variety of processes, including the detection of noxious cold, and diverse painful stimulus, but the functional role of TRPA1 receptor in nociceptive transmission at spinal cord in vivo is poorly known. Therefore, the aim of this study was to evaluate whether the glutamatergic system is involved in the transmission of nociceptive stimulus induced for a TRPA1 agonist in the rat spinal cord. We observed that cinnamaldehyde, a TRPA1 agonist, on spinal cord synaptosomes leads to an increase in [Ca(2+)](i) and a rapid release of glutamate, but was not able to change the specific [(3)H]-glutamate binding. In addition, spinally administered cinnamaldehyde produced heat hyperalgesia and mechanical allodynia in rats. This behavior was reduced by the co-injection (i.t.) of camphor (TRPA1 antagonist) or MK-801 (N-methyl-D-aspartate (NMDA) receptor antagonist) to cinnamaldehyde. Besides, the pretreatment with resiniferatoxin (RTX), a potent TRPV1 agonist, abolished the cinnamaldehyde-induced heat hyperalgesia. Here, we showed that intrathecal RTX results in a decrease in TRPA1 and TRPV1 immunoreactivity in dorsal root ganglion. Collectively, our results demonstrate the pertinent participation of spinal TRPA1 in the possible enhancement of glutamatergic transmission of nociceptive signals leading to increase of the hypersensitivity, here observed as heat hyperalgesia. Then the modulation of spinal TRPA1 might be a valuable target in painful conditions associated with central pain hypersensitivity.

Early demyelination of primary A-fibers induces a rapid-onset of neuropathic pain in rat

Some types of peripheral neuropathic pain are associated with damage to myelin rather than to axons of primary sensory neurons. It is extremely important to develop selective demyelination animal models for understanding neuropathic pain caused by demyelination. We induced a rapid-onset and reversible demyelination of peripheral A-fibers and neuropathic pain behaviors in adult rats by a single injection of cobra venom into the sciatic nerve. The relation between A-fiber demyelination and the abnormal pain behaviors was investigated using this model. Microfilament recordings revealed that cobra venom selectively blocked A-fibers, but not C-fibers. Selective blockade of A-fibers may result from A-fiber demyelination at the site of venom injection as demonstrated by microscope examination. The axons of the demyelinated A-fibers appeared to be otherwise normal. Neuropathic pain behaviors appeared almost immediately after venom injection and lasted about 3 weeks. Electrophysiological studies indicated that venom injection induced loss of conduction in A-fibers, increased sensitivity of C-polymodal nociceptors to innocuous stimuli, and triggered spontaneous activity from both peripheral and central terminals of C-fiber nociceptors. Neurogenic inflammatory responses were also observed in the affected skin via Evan's Blue extravasation experiments. Both antidromic C-fiber spontaneous activity and neurogenic inflammation were substantially decreased by continuous A-fiber threshold electric stimuli applied proximally to the venom injection site. The data suggest that normal activity of peripheral A-fibers may produce inhibitory modulation of C-fiber polymodal nociceptors. Removal of inhibition to C-fiber polymodal nociceptors following demyelination of A-fibers may result in pain and neurogenic inflammation in the affected receptive field.

The capsaicin receptor TRPV1 is a crucial mediator of the noxious effects of mustard oil

Mustard oil (MO) is a plant-derived irritant that has been extensively used in experimental models to induce pain and inflammation. The noxious effects of MO are currently ascribed to specific activation of the cation channel TRPA1 in nociceptive neurons. In contrast to this view, we show here that the capsaicin receptor TRPV1 has a surprisingly large contribution to aversive and pain responses and visceral irritation induced by MO. Furthermore, we found that this can be explained by previously unknown properties of this compound. First, MO has a bimodal effect on TRPA1, producing current inhibition at millimolar concentrations. Second, it directly and stably activates mouse and human recombinant TRPV1, as well as TRPV1 channels in mouse sensory neurons. Finally, physiological temperatures enhance MO-induced TRPV1 stimulation. Our results refute the dogma that TRPA1 is the sole nocisensor for MO and motivate a revision of the putative roles of these channels in models of MO-induced pain and inflammation. We propose that TRPV1 has a generalized role in the detection of irritant botanical defensive traits and in the coevolution of multiple mammalian and plant species.

Inhibition of ERK phosphorylation by substance P N-terminal fragment decreases capsaicin-induced nociceptive response

Previous research has demonstrated that substance P N-terminal fragments produced by the action of several different enzymes in the spinal cord could reduce nociception when injected intrathecally (i.t.) into mice. The present study examined the possible involvement of spinal extracellular signal-regulated protein kinase (ERK), a mitogen-activated protein kinase (MAPK), in i.t. substance P (1-7)-induced antinociception as assayed by the capsaicin test. The i.t. injection of substance P (1-7) (20-80 nmol) into mice resulted in a dose-dependent attenuation of paw-licking/biting behavior induced by intraplantar injection of capsaicin, which was reversed by co-injection of [D-Pro(2), D-Phe(7)]substance P (1-7), a D-isomer and antagonist of substance P (1-7). In Western blot analysis, intraplantar injection of capsaicin (400 and 1600 ng/paw) produced an increase of ERK phosphorylation in the dorsal spinal cord, whereas expression of p38 and c-Jun N-terminal kinase (JNK) phosphorylation was unchanged by capsaicin treatment. In parallel to the behavioral results, i.t. substance P (1-7) inhibited capsaicin-induced ERK phosphorylation, which was reversed by [D-Pro(2), D-Phe(7)]substance P (1-7), a substance P (1-7) antagonist. Both nociceptive behavioral response and spinal ERK activation induced by intraplantar capsaicin were reduced by U0126, an upstream inhibitor of ERK phosphorylation. Taken together, these findings suggest that the activation of ERK, but not p38 and JNK MAPKs in the spinal cord, contributes to intraplantar capsaicin-induced nociception, and that blocking ERK activation via substance P (1-7) binding sites may provide significant antinociception at the spinal cord level.

Cutaneous C-polymodal fibers lacking TRPV1 are sensitized to heat following inflammation, but fail to drive heat hyperalgesia in the absence of TPV1 containing C-heat fibers

BACKGROUND

Previous studies have shown that the TRPV1 ion channel plays a critical role in the development of heat hyperalgesia after inflammation, as inflamed TRPV1-/- mice develop mechanical allodynia but fail to develop thermal hyperalgesia. In order to further investigate the role of TRPV1, we have used an ex vivo skin/nerve/DRG preparation to examine the effects of CFA-induced-inflammation on the response properties of TRPV1-positive and TRPV1-negative cutaneous nociceptors.

RESULTS

In wildtype mice we found that polymodal C-fibers (CPMs) lacking TRPV1 were sensitized to heat within a day after CFA injection. This sensitization included both a drop in average heat threshold and an increase in firing rate to a heat ramp applied to the skin. No changes were observed in the mechanical response properties of these cells. Conversely, TRPV1-positive mechanically insensitive, heat sensitive fibers (CHs) were not sensitized following inflammation. However, results suggested that some of these fibers may have gained mechanical sensitivity and that some previous silent fibers gained heat sensitivity. In mice lacking TRPV1, inflammation only decreased heat threshold of CPMs but did not sensitize their responses to the heat ramp. No CH-fibers could be identified in naïve nor inflamed TRPV1-/- mice.

CONCLUSIONS

Results obtained here suggest that increased heat sensitivity in TRPV1-negative CPM fibers alone following inflammation is insufficient for the induction of heat hyperalgesia. On the other hand, TRPV1-positive CH fibers appear to play an essential role in this process that may include both afferent and efferent functions.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Transient receptor potential channels on sensory nerves

The somatosensory effects of natural products such as capsaicin, mustard oil, and menthol have been long recognized. Over the last decade, the identification of transient receptor potential (TRP) channels in primary sensory neurons as the targets for these agents has led to an explosion of research into the roles of "thermoTRPs" TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8 in nociception. In concert, through the efforts of many industrial and academic teams, a number of agonists and antagonists of these channels have been discovered, paving the way for a better understanding of sensory biology and, potentially, for novel treatments for diseases.

Trpv1 mediates spontaneous firing and heat sensitization of cutaneous primary afferents after plantar incision

TrpV1, the receptor for capsaicin, contributes to nociception in animals but appears to be much more important for signaling increased behavioral sensitivity in the injured state. The current study examined the relationship between the marked reduction in heat hyperalgesia after incision in TrpV1 knockout (KO) mice and the activity of the nociceptors in these same mice. Also, the role of TrpV1 in spontaneous activity (SA) of afferents after incision was examined. Standard teased-fiber techniques were used to record from glabrous skin afferents from incised and control TrpV1 KO and C57Bl6 mice. The loss of TrpV1 had minimal effect on the responses of mechano-heat-sensitive C-fiber afferents in the normal and incised states. However, a different group of heat sensitive afferents, termed unclassified afferents, was sensitized to heat by incision and had markedly reduced sensitization in the TrpV1 KO mice. These unclassified afferents also developed SA after incision, and generally had a lower threshold temperature compared to unclassified afferents without SA. The rate of SA was inversely correlated to the threshold temperature for heat; afferents that exhibited a higher rate of SA had a lower heat threshold. The proportion of unclassified afferents with SA was also reduced in incised TrpV1 KO mice compared to incised C57Bl6 mice. We conclude that a distinct class of afferents outside the mechano-heat-sensitive afferent population likely contributes to heat hypersensitivity after plantar incision. KO of TrpV1 influences SA in these unclassified afferents in incised skin. SA in these afferents is perhaps a manifestation of heat sensitization.

Inflammation reduces mechanical thresholds in a population of transient receptor potential channel A1-expressing nociceptors in the rat

Inflammatory hypersensitivity is characterized by behavioural reductions in withdrawal thresholds to noxious stimuli. Although cutaneous primary afferent neurones are known to have lowered thermal thresholds in inflammation, whether their mechanical thresholds are altered remains controversial. The transient receptor potential channel A1 (TRPA1) is a receptor localized to putative nociceptive neurones and is implicated in mechanical and thermal nociception. Herein, we examined changes in the properties of single primary afferents in normal and acutely inflamed rats and determined whether specific nociceptive properties, particularly mechanical thresholds, are altered in the subpopulation of afferents that responded to the TRPA1 agonist cinnamaldehyde (TRPA1-positive afferents). TRPA1-positive afferents in normal animals belonged to the mechanonociceptive populations, many of which also responded to heat or capsaicin but only a few of which responded to cold. In acute inflammation, a greater proportion of afferents responded to cinnamaldehyde and an increased proportion of dorsal root ganglion neurones expressed TRPA1 protein. Functionally, in inflammation, TRPA1-positive afferents showed significantly reduced mechanical thresholds and enhanced activity to agonist stimulation. Inflammation altered thermal thresholds in both TRPA1-positive and TRPA1-negative afferents. Our data show that a subset of afferents is sensitized to mechanical stimulation by inflammation and that these afferents are defined by expression of TRPA1.

Pronociceptive response elicited by TRPA1 receptor activation in mice

Ankyrin-repeat transient receptor potential 1 (TRPA1) is a member of the transient receptor potential (TRP) channel family and it is found in sensory neurons. In the present study, we found that TRPA1 receptor activation with allyl isothiocyanate or cinnamaldehyde caused dose-dependent spontaneous nociception when injected into the mouse hind paw. Very similar results were obtained when stimulating transient receptor potential vanilloid 1 (TRPV1) receptors with capsaicin. Pretreatment with the TRP receptor antagonist Ruthenium Red (1 nmol/paw) inhibited capsaicin-(0.1 nmol/paw) and allyl isothiocyanate-(1 nmol/paw) induced nociceptive responses. However, the nonselective TRPV1 receptor antagonist capsazepine (1 nmol/paw) and the selective TRPV1 receptor antagonist SB 366791 (1 nmol/paw) only attenuated capsaicin-induced nociception. In contrast, the intrathecal treatment with TRPA1 antisense oligodeoxynucleotide (2.5 nmol/site) and the degeneration of the subset of primary afferent fibers sensitive to capsaicin significantly reduced allyl isothiocyanate-induced nociception. Consequently to TRPA1 antisense oligodeoxynucleotide treatment there was a marked decrease of the expression of TRPA1 receptor in both sciatic nervous and spinal cord segments. Moreover, capsaicin and allyl isothiocyanate-induced nociception were not significantly changed by chemical sympathectomy produced by guanethidine. The previous degranulation of mast cells by compound 48/80 and treatment with antagonist H(1) receptor antagonist pyrilamine (400 microg/paw) both significantly inhibited the capsaicin- and allyl isothiocyanate-induced nociception. The selective NK(1) receptor antagonist N(2)-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl) carbony-1-L-prolyl]-N-methyl-N-phenylmethyl-3-2-(2-naphtyl)-L-alaninamide (10 nmol/paw) reduced either capsaicin- or allyl isothiocyanate-induced nociception. Collectively, the present findings demonstrate that the TRPA1 agonist allyl isothiocyanate produces a consistent nociceptive response when injected into the mouse paw, an effect that seems to be mediated via activation of TRPA1 receptor and dependent on the capsaicin-sensitive fibers, release of histamine by mast cells and participation of tachykinins. Thus, the TRPA1 receptor has an apparently relevant role in nociceptive processes and the selective TRPA1 antagonist might possess a potential antinociceptive property.

Species-specific pharmacology of Trichloro(sulfanyl)ethyl benzamides as transient receptor potential ankyrin 1 (TRPA1) antagonists

Agonists of TRPA1 such as mustard oil and its key component AITC cause pain and neurogenic inflammation in humans and pain behavior in rodents. TRPA1 is activated by numerous reactive compounds making it a sensor for reactive compounds in the body. Failure of AITC, formalin and other reactive compounds to trigger pain behavior in TRPA1 knockout mice, as well as the ability of TRPA1 antisense to alleviate cold hyperalgesia after spinal nerve ligation, suggest that TRPA1 is a potential target for novel analgesic agents. Here, we have characterized CHO cells expressing human and rat TRPA1 driven by an inducible promoter. As reported previously, both human and rat TRPA1 are activated by AITC and inhibited by ruthenium red. We have also characterized noxious cold response of these cell lines and show that noxious cold activates both human and rat TRPA1. Further, we have used CHO cells expressing human TRPA1 to screen a small molecule compound library and discovered that 'trichloro(sulfanyl)ethyl benzamides' (AMG2504, AMG5445, AMG7160 and AMG9090) act as potent antagonists of human TRPA1 activated by AITC and noxious cold. However, trichloro(sulfanyl)ethyl benzamides' (TCEB compounds) displayed differential pharmacology at rat TRPA1. AMG2504 and AMG7160 marginally inhibited rat TRPA1 activation by AITC, whereas AMG5445 and AMG9090 acted as partial agonists. In summary, we conclude that both human and rat TRPA1 channels show similar AITC and noxious cold activation profiles, but TCEB compounds display species-specific differential pharmacology at TRPA1.

Effects of TRPA1 agonists mustard oil and cinnamaldehyde on lumbar spinal wide-dynamic range neuronal responses to innocuous and noxious cutaneous stimuli in rats

Mustard oil [allyl isothiocyanate (AITC)] and cinnamaldehyde (CA), agonists of the ion channel TRPA1 expressed in sensory neurons, elicit a burning sensation and heat hyperalgesia. We tested whether these phenomena are reflected in the responses of lumbar spinal wide-dynamic range (WDR) neurons recorded in pentobarbital-anesthetized rats. Responses to electrical and graded mechanical and noxious thermal stimulation were tested before and after cutaneous application of AITC or CA. Repetitive application of AITC initially increased the firing rate of 52% of units followed by rapid desensitization that persisted when AITC was reapplied 30 min later. Responses to noxious thermal, but not mechanical, stimuli were significantly enhanced irrespective of whether the neuron was directly activated by AITC. Windup elicited by percutaneous or sciatic nerve electrical stimulation was significantly reduced post-AITC. These results indicate that AITC produced central inhibition and peripheral sensitization of heat nociceptors. CA did not directly excite WDR neurons, and significantly enhanced responses to noxious heat while not affecting windup or responses to skin cooling or mechanical stimulation, indicating a peripheral sensitization of heat nociceptors.

Paracrine-like excitation of low-threshold mechanoceptive C-fibers innervating rat hairy skin is mediated by substance P via NK-1 receptors

We reported previously that C-fibers innervating rat skin can be excited by short trains of electrical shocks ('tetanus') applied to neighboring nerves. Since these nerves were disconnected from the CNS, the cross-talk is located peripherally. Here we tested if low-threshold mechanoceptive (LTM) C-fibers can be excited by this cross-talk and if this process is mediated by substance P (SP) via neurokinin-1 (NK-1) receptors. In urethane anesthetized rats we found that 80% (56/71) of LTM C-fibers, recorded in the lateral cutaneous branch of the dorsal ramus (CBDR) of T10 spinal nerve, were excited by a 10s, 20 Hz tetanus of the T9 CBDR. Compared to the spontaneous pre-tetanic firing frequency of 1.62+/-0.40 impulses/30s, the frequency significantly increased to 3.74+/-0.99, 3.17+/-0.69 and 2.92+/-0.63 impulses/30s, at 30, 60 and 90 s after the tetanus, respectively, and declined to the baseline frequency thereafter. When injected into their receptive fields, SP mimicked the tetanically induced increase of firing rate, whereas the NK-1 receptor antagonist WIN 51708 blocked the excitation in most fibers. The excitation was significantly diminished in adult rats that were neonatally treated with capsaicin, a treatment that destroys most SP-expressing afferent fibers. Thus, we conclude that peptidergic primary afferents are functionally linked with adjacent LTM C-fibers in a non-synaptic, paracrine-like signaling pathway via SP and NK-1 receptors, and perhaps also other agents as well. We propose that this cross-talk has evolved as a mechanism regulating the mechanoceptive characteristics of LTM C-fibers, presumably contributing to pain sensation elicited by tactile stimuli ('allodynia').

Transient receptor potential TRPA1 channel desensitization in sensory neurons is agonist dependent and regulated by TRPV1-directed internalization

The pharmacological desensitization of receptors is a fundamental mechanism for regulating the activity of neuronal systems. The TRPA1 channel plays a key role in the processing of noxious information and can undergo functional desensitization by unknown mechanisms. Here we show that TRPA1 is desensitized by homologous (mustard oil; a TRPA1 agonist) and heterologous (capsaicin; a TRPV1 agonist) agonists via Ca2+-independent and Ca2+-dependent pathways, respectively, in sensory neurons. The pharmacological desensitization of TRPA1 by capsaicin and mustard oil is not influenced by activation of protein phosphatase 2B. However, it is regulated by phosphatidylinositol-4,5-bisphosphate depletion after capsaicin, but not mustard oil, application. Using a biosensor, we establish that capsaicin, unlike mustard oil, consistently activates phospholipase C in sensory neurons. We next demonstrate that TRPA1 desensitization is regulated by TRPV1, and it appears that mustard oil-induced TRPA1 internalization is prevented by coexpression with TRPV1 in a heterologous expression system and in sensory neurons. In conclusion, we propose novel mechanisms whereby TRPA1 activity undergoes pharmacological desensitization through multiple cellular pathways that are agonist dependent and modulated by TRPV1.

Characterization of spinal alpha-adrenergic modulation of nociceptive transmission and hyperalgesia throughout postnatal development in rats

BACKGROUND AND PURPOSE

The selective alpha(2)-adrenergic agonist dexmedetomidine is used clinically for analgesia and sedation, but effects in early life are not well characterized. Investigation of age-related effects of dexmedetomidine is important for evaluating responses to exogenously administered analgesics and provides insight into postnatal function of noradrenergic pathways.

EXPERIMENTAL APPROACH

We examined effects of epidural dexmedetomidine in anaesthetized rat pups (3, 10 and 21 postnatal days) using a quantitative model of nociception and C-fibre induced hyperalgesia. Electromyographic recordings of withdrawal responses to hindpaw mechanical stimuli measured effects of dexmedetomidine upon the baseline reflex and the response to mustard oil application on the hindpaw (primary hyperalgesia) or hindlimb (secondary hyperalgesia). In addition, we compared epidural with systemic administration, examined effects of spinal transection and evaluated heart rate changes following dexmedetomidine.

KEY RESULTS

Epidural dexmedetomidine dose-dependently prevented mustard oil-induced hyperalgesia at all ages but dose requirements were lower in the youngest pups. Higher doses also suppressed the baseline nociceptive reflex when given epidurally, but had no effect when given systemically. Analgesic efficacy was the same for primary and secondary hyperalgesia, and was not diminished by spinal cord transection.

CONCLUSIONS AND IMPLICATIONS

Our laboratory studies predict that spinally mediated alpha(2)-agonist analgesia would be effective throughout postnatal development, dose requirements would be lower in early life and selective anti-hyperalgesic effects could be achieved with epidural administration at doses lower than associated with antinociceptive or cardiovascular effects. Clinical trials of alpha(2) agonists in neonates and infants should consider developmentally regulated changes.

Modality-specific sensory changes in humans after the induction of long-term potentiation (LTP) in cutaneous nociceptive pathways

The impact of long-term potentiation (LTP) in nociceptive pathways on somatosensory perception was examined by means of quantitative sensory testing (QST) in the ventral forearm of 12 healthy human subjects. Electrical high-frequency stimulation of the forearm skin (HFS; 5 x 1 s at 100 Hz and 10 x detection threshold) led to an abrupt increase of pain to single electrical test stimuli, which were applied through the same electrode (perceptual LTP +72%, p<0.01). Perceptual LTP outlasted the 1-h observation period. The effects of HFS on somatosensory perception of natural test stimuli in the conditioned skin area were restricted to mechanical submodalities. Subjects exhibited a significant decrease of pain threshold and an increase of pain ratings to suprathreshold pinprick stimuli (p<0.01). In 5 out of 12 subjects (42%) light tactile stimuli led to painful sensations (dynamic mechanical allodynia). Furthermore, a small but significant decrease of threshold to blunt pressure stimuli (p<0.05) was found. In contrast, all thermal modalities comprising cold and warm detection thresholds, cold and heat pain thresholds as well as pain summation (perceptual wind up) remained unaltered. These data show that HFS of peptidergic cutaneous C-fiber afferents predominantly modulates Adelta- and Abeta-fiber mediated somatosensory functions, suggesting that LTP in nociceptive pathways enhances human pain sensitivity via interaction of two afferent pathways (extrinsic sensitization).

Contribution of substance P and neurokinin A to the differential injury-induced thermal and mechanical responsiveness of lamina I and V neurons

In a previous report, we compared the properties of lamina V neurons of the spinal cord dorsal horn in wild-type mice and in mice with a deletion of the preprotachykinin-A (PPT-A) gene, which encodes substance P (SP) and neurokinin A (NKA). The mutant mice had pronounced deficits in the response to thermal stimulation, both before and after mustard oil induced sensitization. Here, we extended our analysis to the properties of lamina I neurons and also examined responsiveness to mechanical stimulation. Consistent with the properties of lamina V neurons, in the PPT-A mutant mice we found significantly reduced responses of lamina I neurons to noxious thermal stimulation, and mustard oil sensitization of these neurons to heat was lost. In contrast, not only were the responses of lamina I neurons to noxious mechanical stimulation unchanged in the mutant mice, but in neither the wild-type nor the mutant mice could sensitization be induced. However, mustard oil profoundly sensitized lamina V neurons to mechanical stimulation in both wild-type and mutant mice. We conclude that SP and/or NKA are required for the transmission of noxious thermal stimulation by lamina I and V neurons, both before and after tissue injury. The persistence of mechanical sensitization of lamina V neurons in the mutant mice further shows that mustard oil induces mechanical and thermal sensitization through different mechanisms. Finally, we conclude that lamina I sensitization to mechanical stimulation is not required for this form of injury-increased responsiveness of lamina V neurons.

Involvement of peripheral ionotropic glutamate receptors in activation of cutaneous branches of spinal dorsal rami following antidromic electrical stimulation of adjacent afferent nerves in rats

The aim of the present study was to investigate the role of peripheral ionotropic glutamate receptors in the process of signal transmission between adjacent different peripheral sensory nerves. The T9 and T10 cutaneous branches of spinal dorsal rami were dissociated and cut proximally in pentobarbital anesthetized rats. Eighty-seven single afferents from T10 nerve filaments were recorded and characterized by assessing their spontaneous activities. Following 30 s antidromic electrical stimulation (intensity: 1 mA; duration: 0.5 ms; frequency: 20 Hz) of T9 cutaneous branches, the spontaneous activities of Abeta, Adelta and C fibers of T10 nerve were significantly enhanced from 2.00+/-0.34, 2.42+/-0.33, and 2.19+/-0.32 impulses/min to 4.31+/-0.58, 5.22+/-0.55, and 5.27+/-0.69 impulses/min, respectively (n=29 for each type, P<0.05). These enhanced spontaneous discharges of T10 nerve were significantly blocked by local treatment of its receptive field with either N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 or non-NMDA receptor antagonist DNQX (0.1 mM, 10 microl for each drug) (P<0.05). These results suggest that peripheral ionotropic glutamate receptors are involved in the activation of peripheral nerves following the antidromic stimulation of adjacent afferents from different spinal segments. We further provide the direct evidence that neurotransmitters released from adjacent peripheral nerves may also contribute to the occurrence of allodynia as well as secondary hyperalgesia during the pathological nociception.

Increased responsiveness of rat colonic splanchnic afferents to 5-HT after inflammation and recovery

5-Hydroxytryptamine (5-HT) activates colonic splanchnic afferents, a mechanism by which it has been implicated in generating symptoms in postinfectious and postinflammatory states in humans. Here we compared mechanisms of colonic afferent activation by 5-HT and mechanical stimuli in normal and inflamed rat colon, and after recovery from inflammation. Colonic inflammation was induced in rats by dextran sulphate sodium. Single-fibre recordings of colonic lumbar splanchnic afferents revealed that 58% of endings responded to 5-HT (10(-4) m) in controls, 88% in acute inflammation (P<0.05) and 75% after 21 days recovery (P < 0.05 versus control). Maximal responses to 5-HT were also larger, and the estimated EC50 was reduced from 3.2 x 10(-6) to 8 x 10(-7) m in acute inflammation and recovered to 2 x 10(-6) m after recovery. Responsiveness to mechanical stimulation was unaffected. 5-HT3 receptor antagonism with alosetron reduced responses to 5-HT in controls but not during inflammation. Responses to the mast cell degranulator 48/80 mimicked those to 5-HT in inflamed tissue but not in controls, and more 5-HT-containing mast cells were seen close to calcitonin gene-related peptide-containing fibres in inflamed serosa. We conclude that colonic serosal and mesenteric endings exhibit increased sensitivity to 5-HT in inflammation, with both an increase in proportion of responders and an increase in sensitivity, which is maintained after healing of inflammation. This is associated with alterations in the roles of 5-HT3 receptors and mast cells.

Peripheral mGluR5 antagonist attenuated craniofacial muscle pain and inflammation but not mGluR1 antagonist in lightly anesthetized rats

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300-400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10s. After 30 microl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans' blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration. Intramuscular pretreatment with 5 mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Windup in dorsal horn neurons is modulated by endogenous spinal mu-opioid mechanisms

The mu-opioid receptor (MOR) plays a critical role in morphine analgesia and nociceptive transmission. However, the physiological roles for endogenous MOR mechanisms in modulating spinal nociceptive transmission, and particularly in the enhanced excitability of spinal nociceptive neurons after repeated noxious inputs, are less well understood. Using a MOR gene knock-out (-/-) approach and an MOR-preferring antagonist, we investigated the roles of endogenous MOR mechanisms in processing of acute noxious input and in neuronal sensitization during windup-inducing stimuli in wide dynamic range (WDR) neurons. Extracellular single-unit activity of WDR neurons was recorded in isoflurane-anesthetized MOR(-/-) and wild-type C57BL/6 mice. There were no significant differences between the genotypes in the responses of deep WDR cells to acute mechanical stimuli, graded electrical stimuli, and noxious chemical stimuli applied to the receptive field. Intracutaneous electrical stimulation at 1.0 Hz produced similar levels of windup in both genotypes. In contrast, 0.2 Hz stimulation induced significantly higher levels of windup in MOR(-/-) mice compared with the wild-type group. In wild-type mice, spinal superfusion with naloxone hydrochloride (10 mM, 30 microl) significantly enhanced windup to 0.2 Hz stimulation in both deep and superficial WDR cells. A trend toward facilitation of windup was also observed during 1.0 Hz stimulation after naloxone treatment. These results suggest that endogenous MOR mechanisms are not essential in the processing of acute noxious mechanical and electrical stimuli by WDR neurons. However, MORs may play an important role in endogenous inhibitory mechanisms that regulate the development of spinal neuronal sensitization.

TRPA1 Contributes to Cold, Mechanical, and Chemical Nociception but Is Not Essential for Hair-Cell Transduction

TRPA1, a member of the transient receptor potential (TRP) family of ion channels, is expressed by dorsal root ganglion neurons and by cells of the inner ear, where it has proposed roles in sensing sound, painful cold, and irritating chemicals. To test the in vivo roles of TRPA1, we generated a mouse in which the essential exons required for proper function of the Trpa1 gene were deleted. Knockout mice display behavioral deficits in response to mustard oil, to cold ( approximately 0 degrees C), and to punctate mechanical stimuli. These mice have a normal startle reflex to loud noise, a normal sense of balance, a normal auditory brainstem response, and normal transduction currents in vestibular hair cells. TRPA1 is apparently not essential for hair-cell transduction but contributes to the transduction of mechanical, cold, and chemical stimuli in nociceptor sensory neurons.

Role for Medullary Pain Facilitating Neurons in Secondary Thermal Hyperalgesia

The rostral ventromedial medulla (RVM) has recently received considerable attention in efforts to understand mechanisms of hyperalgesia and persistent pain states. Three classes of neurons can be identified in the RVM based on responses associated with nocifensive reflexes: on cells, off cells, and neutral cells. There is now direct evidence that on cells exert a net facilitating effect on spinal nociception and that off cells depress nociception. These experiments tested whether the secondary hyperalgesia produced by topical application of mustard oil involves an activation of on cells in RVM. Firing of a characterized RVM neuron and the latencies of withdrawal reflexes evoked by noxious heat were recorded in lightly anesthetized rats before and after application of mustard oil to the shaved skin of the leg above the knee. Mineral oil was applied as a control. Mustard oil produced a significant increase in ongoing and reflex-related discharge of on cells, as well as a decrease in the activity of off cells. neutral cell firing was uniformly unchanged after application of mustard oil. The alterations in on and off cell firing were associated with a significant decrease in the latency to withdraw the paw of the treated limb from the heat stimulus, and this hyperalgesia was blocked by microinjection of lidocaine within the RVM. Withdrawals evoked by heating the contralateral hindpaw, forepaw, and tail were unchanged after mustard oil application. These experiments support a pronociceptive role for on cells and suggest that these neurons contribute to secondary hyperalgesia in inflammation.

Kainate-induced excitation and sensitization of nociceptors in normal and inflamed rat glabrous skin

This study investigates contributions of peripheral kainate receptors to acute nociception and persistent inflammatory pain in rat. Immunohistochemical analysis of kainate receptor expression using antibodies recognizing glutamate receptor subunits 5, 6, and 7 demonstrates that 28% of unmyelinated axons in normal digital nerve are positively labeled. Following intraplantar injection of complete Freund's adjuvant, a significant increase in glutamate receptor subunits 5, 6, and 7-labeled axons occurs at 2 days (40%), but not 7 (31%) or 14 days (28%) post-complete Freund's adjuvant. In behavioral studies, we confirm an increased mechanical sensitivity in complete Freund's adjuvant-injected hind paws. Furthermore, activation of kainate receptors following intraplantar injection of 1.0 mM kainate in normal animals results in a mechanical sensitivity similar to that observed in inflamed animals. A 1.0 mM kainate injection into inflamed hind paws further enhances the mechanical sensitivity. Injection of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (0.1 mM) reverses complete Freund's adjuvant-induced mechanical sensitivity through a local effect. In single unit recordings from nociceptors in a glabrous skin-nerve preparation, mechanical sensitization is present in inflamed skin evidenced by a decrease in mechanical threshold and an increase in discharge rate during a suprathreshold, constant force stimulus. Thermal sensitization is also present evidenced by a decrease in heat threshold. There is a dose-dependent increase in kainate-induced nociceptor activity in both normal and inflamed skin but the kainate required to induce activation is reduced in inflamed skin. Although proportions of kainate-activated nociceptors are the same in normal and inflamed skin, the kainate-induced mean discharge rate is significantly enhanced in inflamed skin. Exposure of normal and inflamed nociceptors to 0.3 mM kainate sensitizes fibers to re-application of kainate and heat. This sensitization is blocked in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione or the glutamate receptor subunit 5 selective antagonist 3S,4aR,6S,8aR-6-[4-carboxy-phenyl] methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid. The data indicate that peripheral kainate receptors not only play an important role in normal nociception but also contribute to mechanical sensitivity and heat sensitization accompanying inflammatory pain.

Variable sensitivity to noxious heat is mediated by differential expression of the CGRP gene

Heat sensitivity shows considerable functional variability in humans and laboratory animals, and is fundamental to inflammatory and possibly neuropathic pain. In the mouse, at least, much of this variability is genetic because inbred strains differ robustly in their behavioral sensitivity to noxious heat. These strain differences are shown here to reflect differential responsiveness of primary afferent thermal nociceptors to heat stimuli. We further present convergent behavioral and electrophysiological evidence that the variable responses to noxious heat are due to strain-dependence of CGRP expression and sensitivity. Strain differences in behavioral response to noxious heat could be abolished by peripheral injection of CGRP, blockade of cutaneous and spinal CGRP receptors, or long-term inactivation of CGRP with a CGRP-binding Spiegelmer. Linkage mapping supports the contention that the genetic variant determining variable heat pain sensitivity across mouse strains affects the expression of the Calca gene that codes for CGRPalpha.

Mechanical and heat sensitization of cutaneous nociceptors in rats with experimental peripheral neuropathy

This study examined whether or not the properties of cutaneous nociceptive fibers are altered in the neuropathic state by comparing lumbars 5 and 6 spinal nerve ligation (SNL) rats with sham-operated controls. The rats with the unilateral SNL developed mechanical allodynia in the ipsilateral hind limb, whereas the sham group did not. Two to 5 weeks after the neuropathic or sham surgery, rats were subjected to single fiber-recording experiments to examine the properties of afferent fibers in the sural and plantar nerves. A total of 224 afferents in the C- and Adelta-ranges were characterized in the neuropathic and sham groups. Spontaneous activity was observed in 16 of 155 fibers in the neuropathic group and one of 69 fibers in the sham group. The response threshold of both the C- and Adelta-fibers to mechanical stimuli was lower in the neuropathic group than the sham group. The afferent fibers responsive to heat stimuli were all C-fibers, and none were Adelta-fibers. The response threshold of the C-fibers to the heat stimuli was lower in the neuropathic group than the sham group. The magnitude of the responses of both C- and Adelta-fibers to the suprathreshold intensity of the mechanical stimulus was greater in the neuropathic group than the sham group. However, the magnitude of the responses of C-fibers to the suprathreshold intensity of the heat stimulus in the neuropathic group was not different from that in the sham group. These results suggest that after a partial peripheral nerve injury, the nociceptors on the skin supplied by an uninjured nerve become sensitized to both mechanical and heat stimuli. This nociceptor sensitization can contribute to neuropathic pain.

Substance P does not play a critical role in neurogenic inflammation in the rat masseter muscle

In this study, we performed a series of experiments to investigate whether substance P (SP) contributes to neurogenic inflammation in the skeletal muscle tissue. Intramuscular injection of an inflammatory irritant, mustard oil (MO), induces significant edema formation in the rat masseter muscle. In order to study the contribution of endogenous SP in the MO-induced edema, groups of rats were pretreated with two different doses (100 nmol; 1 microl) of either peptidergic (Sendide) or non-peptidergic (L703, 606) neurokinin 1 (NK1) receptor antagonist in one masseter muscle 15 min prior to the MO injection in the same muscle. The extent of edema was assessed as the percent weight difference of the injected muscle compared to the non-injected muscle. Neither Sendide nor L703,606 pretreatment resulted in a significant inhibition of the MO-induced edema in the masseter muscle. Exogenous application of SP also produced a significant swelling of the muscle, which was blocked by L703,606 (1 microl) pretreatment, suggesting that evoked release of SP following MO injection is not sufficient to induce significant edema formation. Capsaicin (1% in 25 microl), which is known to cause neurogenic inflammation, failed to produce edema formation in the masseter muscle. The same concentration of capsaicin injected into the hindpaw produced significant swelling of the injected paw. Taken together, these results provide compelling evidence that, unlike cutaneous or joint tissue, SP does not play a critical role in inducing neurogenic inflammation in the skeletal muscle tissue.