Evidence for Transient Receptor Potential (TRP) Channel Contribution to Arthritis Pain and Pathogenesis.

Based on clinical and preclinical evidence, Transient Receptor Potential (TRP) channels have emerged as potential drug targets for the treatment of osteoarthritis, rheumatoid arthritis, and gout. This review summarizes the relevant data supporting a role for various TRP channels in arthritis pain and pathogenesis, as well as the current state of pharmacological efforts to ameliorate arthritis symptoms in patient populations.

Chemokine (c-c motif) receptor 2 mediates mechanical and cold hypersensitivity in sickle cell disease mice.

Approximately one-third of individuals with sickle cell disease (SCD) develop chronic pain. This debilitating pain is inadequately treated because the underlying mechanisms driving the pain are poorly understood. In addition to persistent pain, patients with SCD are also in a tonically proinflammatory state. Previous studies have revealed that there are elevated plasma levels of many inflammatory mediators including chemokine (c-c motif) ligand 2 (CCL2) in individuals with SCD. Using a transgenic mouse model of SCD, we investigated the contributions of CCL2 signaling to SCD-related pain. Inhibition of chemokine receptor 2 (CCR2), but not CCR4, alleviated the behavioral mechanical and cold hypersensitivity in SCD. Furthermore, acute CCR2 blockade reversed both the behavioral and the in vitro responsiveness of sensory neurons to an agonist of TRPV1, a neuronal ion channel previously implicated in SCD pain. These results provide insight into the immune-mediated regulation of hypersensitivity in SCD and could inform future development of analgesics or therapeutic measures to prevent chronic pain.

Keratinocytes mediate innocuous and noxious touch via ATP-P2X4 signaling.

The first point of our body's contact with tactile stimuli (innocuous and noxious) is the epidermis, the outermost layer of skin that is largely composed of keratinocytes. Here, we sought to define the role that keratinocytes play in touch sensation in vivo and ex vivo. We show that optogenetic inhibition of keratinocytes decreases behavioral and cellular mechanosensitivity. These processes are inherently mediated by ATP signaling, as demonstrated by complementary cutaneous ATP release and degradation experiments. Specific deletion of P2X4 receptors in sensory neurons markedly decreases behavioral and primary afferent mechanical sensitivity, thus positioning keratinocyte-released ATP to sensory neuron P2X4 signaling as a critical component of baseline mammalian tactile sensation. These experiments lay a vital foundation for subsequent studies into the dysfunctional signaling that occurs in cutaneous pain and itch disorders, and ultimately, the development of novel topical therapeutics for these conditions.

Development of TRPM8 Antagonists to Treat Chronic Pain and Migraine.

A review. Development of pharmaceutical antagonists of transient receptor potential melastatin 8 (TRPM8) have been pursued for the treatment of chronic pain and migraine. This review focuses on the current state of this progress.

Selective antagonism of TRPA1 produces limited efficacy in models of inflammatory- and neuropathic-induced mechanical hypersensitivity in rats.

The transient receptor potential ankyrin 1 (TRPA1) channel has been implicated in pathophysiological processes that include asthma, cough, 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 rodents, and TRPA1 antagonists have been reported to be effective in rodent models of pain. In our pursuit of TRPA1 antagonists as potential therapeutics, we generated AMG0902, a potent (IC90 of 300 nM against rat TRPA1), selective, brain penetrant (brain to plasma ratio of 0.2), and orally bioavailable small molecule TRPA1 antagonist. AMG0902 reduced mechanically evoked C-fiber action potential firing in a skin-nerve preparation from mice previously injected with complete Freund's adjuvant, supporting the role of TRPA1 in inflammatory mechanosensation. In vivo target coverage of TRPA1 by AMG0902 was demonstrated by the prevention of AITC-induced flinching/licking in rats. However, oral administration of AMG0902 to rats resulted in little to no efficacy in models of inflammatory, mechanically evoked hypersensitivity; and no efficacy was observed in a neuropathic pain model. Unbound plasma concentrations achieved in pain models were about 4-fold higher than the IC90 concentration in the AITC target coverage model, suggesting that either greater target coverage is required for efficacy in the pain models studied or TRPA1 may not contribute significantly to the underlying mechanisms.

Selective spider toxins reveal a role for the Nav1.1 channel in mechanical pain.

Voltage-gated sodium (Nav) channels initiate action potentials in most neurons, including primary afferent nerve fibres of the pain pathway. Local anaesthetics block pain through non-specific actions at all Nav channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes of these channels and their contributions to chemical, mechanical, or thermal pain. Here we identify and characterize spider (Heteroscodra maculata) toxins that selectively activate the Nav1.1 subtype, the role of which in nociception and pain has not been elucidated. We use these probes to show that Nav1.1-expressing fibres are modality-specific nociceptors: their activation elicits robust pain behaviours without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibres also express Nav1.1 and show enhanced toxin sensitivity in a mouse model of irritable bowel syndrome. Together, these findings establish an unexpected role for Nav1.1 channels in regulating the excitability of sensory nerve fibres that mediate mechanical pain.

Mechanosensory and ATP Release Deficits following Keratin14-Cre-Mediated TRPA1 Deletion Despite Absence of TRPA1 in Murine Keratinocytes.

Keratinocytes are the first cells that come into direct contact with external tactile stimuli; however, their role in touch transduction in vivo is not clear. The ion channel Transient Receptor Potential Ankyrin 1 (TRPA1) is essential for some mechanically-gated currents in sensory neurons, amplifies mechanical responses after inflammation, and has been reported to be expressed in human and mouse skin. Other reports have not detected Trpa1 mRNA transcripts in human or mouse epidermis. Therefore, we set out to determine whether selective deletion of Trpa1 from keratinocytes would impact mechanosensation. We generated K14Cre-Trpa1fl/fl mice lacking TRPA1 in K14-expressing cells, including keratinocytes. Surprisingly, Trpa1 transcripts were very poorly detected in epidermis of these mice or in controls, and detection was minimal enough to preclude observation of Trpa1 mRNA knockdown in the K14Cre-Trpa1fl/fl mice. Unexpectedly, these K14Cre-Trpa1fl/fl mice nonetheless exhibited a pronounced deficit in mechanosensitivity at the behavioral and primary afferent levels, and decreased mechanically-evoked ATP release from skin. Overall, while these data suggest that the intended targeted deletion of Trpa1 from keratin 14-expressing cells of the epidermis induces functional deficits in mechanotransduction and ATP release, these deficits are in fact likely due to factors other than reduction of Trpa1 expression in adult mouse keratinocytes because they express very little, if any, Trpa1.

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.

CaMKII Controls Whether Touch Is Painful.

The sensation of touch is initiated when fast conducting low-threshold mechanoreceptors (Aß-LTMRs) generate impulses at their terminals in the skin. Plasticity in this system is evident in the process of adaption, in which a period of diminished sensitivity follows prior stimulation. CaMKII is an ideal candidate for mediating activity-dependent plasticity in touch because it shifts into an enhanced activation state after neuronal depolarizations and can thereby reflect past firing history. Here we show that sensory neuron CaMKII autophosphorylation encodes the level of Aß-LTMR activity in rat models of sensory deprivation (whisker clipping, tail suspension, casting). Blockade of CaMKII signaling limits normal adaptation of action potential generation in Aß-LTMRs in excised skin. CaMKII activity is also required for natural filtering of impulse trains as they travel through the sensory neuron T-junction in the DRG. Blockade of CaMKII selectively in presynaptic Aß-LTMRs removes dorsal horn inhibition that otherwise prevents Aß-LTMR input from activating nociceptive lamina I neurons. Together, these consequences of reduced CaMKII function in Aß-LTMRs cause low-intensity mechanical stimulation to produce pain behavior. We conclude that, without normal sensory activity to maintain adequate levels of CaMKII function, the touch pathway shifts into a pain system. In the clinical setting, sensory disuse may be a critical factor that enhances and prolongs chronic pain initiated by other conditions. SIGNIFICANCE STATEMENT: The sensation of touch is served by specialized sensory neurons termed low-threshold mechanoreceptors (LTMRs). We examined the role of CaMKII in regulating the function of these neurons. Loss of CaMKII function, such as occurred in rats during sensory deprivation, elevated the generation and propagation of impulses by LTMRs, and altered the spinal cord circuitry in such a way that low-threshold mechanical stimuli produced pain behavior. Because limbs are protected from use during a painful condition, this sensitization of LTMRs may perpetuate pain and prevent functional rehabilitation.

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.

AMG2850, a potent and selective TRPM8 antagonist, is not effective in rat models of inflammatory mechanical hypersensitivity and neuropathic tactile allodynia.

TRPM8 has been implicated in pain and migraine based on dorsal root- and trigeminal ganglion-enriched expression, upregulation in preclinical models of pain, knockout mouse studies, and human genetics. Here, we evaluated the therapeutic potential in pain of AMG2850 ((R)-8-(4-(trifluoromethyl)phenyl)-N-((S)-1,1,1-trifluoropropan-2-yl)-5,6-dihydro-1,7-naphthyridine-7(8H)-carboxamide), a small molecule antagonist of TRPM8 by in vitro and in vivo characterization. AMG2850 is potent in vitro at rat TRPM8 (IC90 against icilin activation of 204 ± 28 nM), highly selective (>100-fold IC90 over TRPV1 and TRPA1 channels), and orally bioavailable (F po > 40 %). When tested in a skin-nerve preparation, AMG2850 blocked menthol-induced action potentials but not mechanical activation in C fibers. AMG2850 exhibited significant target coverage in vivo in a TRPM8-mediated icilin-induced wet-dog shake (WDS) model in rats (at 10 mg/kg p.o.). However, AMG2850 did not produce a significant therapeutic effect in rat models of inflammatory mechanical hypersensitivity or neuropathic tactile allodynia at doses up to 100 mg/kg. The lack of efficacy suggests that either TRPM8 does not play a role in mediating pain in these models or that a higher level of target coverage is required. The potential of TRPM8 antagonists as migraine therapeutics is yet to be determined.

Loosening pain's grip by tightening TRPV1-TRPA1 interactions.

TRPA1 and TRPV1 are ion channels crucial for pain sensation. In this issue of Neuron, Weng et al. (2015) demonstrate that the activity of TRPA1-TRPV1 heteromers is governed by Tmem100 and that disabling Tmem100 may be a novel pharmacologic strategy to combat pain.

Transcriptional profiling at whole population and single cell levels reveals somatosensory neuron molecular diversity.

The somatosensory nervous system is critical for the organism's ability to respond to mechanical, thermal, and nociceptive stimuli. Somatosensory neurons are functionally and anatomically diverse but their molecular profiles are not well-defined. Here, we used transcriptional profiling to analyze the detailed molecular signatures of dorsal root ganglion (DRG) sensory neurons. We used two mouse reporter lines and surface IB4 labeling to purify three major non-overlapping classes of neurons: 1) IB4(+)SNS-Cre/TdTomato(+), 2) IB4(-)SNS-Cre/TdTomato(+), and 3) Parv-Cre/TdTomato(+) cells, encompassing the majority of nociceptive, pruriceptive, and proprioceptive neurons. These neurons displayed distinct expression patterns of ion channels, transcription factors, and GPCRs. Highly parallel qRT-PCR analysis of 334 single neurons selected by membership of the three populations demonstrated further diversity, with unbiased clustering analysis identifying six distinct subgroups. These data significantly increase our knowledge of the molecular identities of known DRG populations and uncover potentially novel subsets, revealing the complexity and diversity of those neurons underlying somatosensation.

Piezo2 is required for Merkel-cell mechanotransduction.

How we sense touch remains fundamentally unknown. The Merkel cell-neurite complex is a gentle touch receptor in the skin that mediates slowly adapting responses of Aß sensory fibres to encode fine details of objects. This mechanoreceptor complex was recognized to have an essential role in sensing gentle touch nearly 50 years ago. However, whether Merkel cells or afferent fibres themselves sense mechanical force is still debated, and the molecular mechanism of mechanotransduction is unknown. Synapse-like junctions are observed between Merkel cells and associated afferents, and yet it is unclear whether Merkel cells are inherently mechanosensitive or whether they can rapidly transmit such information to the neighbouring nerve. Here we show that Merkel cells produce touch-sensitive currents in vitro. Piezo2, a mechanically activated cation channel, is expressed in Merkel cells. We engineered mice deficient in Piezo2 in the skin, but not in sensory neurons, and show that Merkel-cell mechanosensitivity completely depends on Piezo2. In these mice, slowly adapting responses in vivo mediated by the Merkel cell-neurite complex show reduced static firing rates, and moreover, the mice display moderately decreased behavioural responses to gentle touch. Our results indicate that Piezo2 is the Merkel-cell mechanotransduction channel and provide the first line of evidence that Piezo channels have a physiological role in mechanosensation in mammals. Furthermore, our data present evidence for a two-receptor-site model, in which both Merkel cells and innervating afferents act together as mechanosensors. The two-receptor system could provide this mechanoreceptor complex with a tuning mechanism to achieve highly sophisticated responses to a given mechanical stimulus.

Corticomotor excitability during a noxious stimulus before and after exercise in women with fibromyalgia.

The purposes of this study were to assess corticomotor excitability in people with fibromyalgia during a noxious stimulus before and after fatiguing exercise and examine associations with pain perception. Fifteen women with fibromyalgia completed three sessions: one familiarization and two experimental. The experimental sessions were randomized and involved measurement of pain perception and motor evoked potentials before and after (1) quiet rest and (2) isometric contraction of the elbow flexor muscles. Motor evoked potential amplitude of brachioradialis muscle was measured following transcranial magnetic stimulation delivered before, during, and after a noxious mechanical stimulus. After quiet rest, there was no change in pain perception. After the submaximal contraction, there was considerable variability in the pain response. Based on the changes in the experimental pain, subjects were divided into three groups (increase, decrease, and no change in pain). There was an interaction between pain response and the pain-induced change in motor evoked potentials. Those individuals who had an increase in motor evoked potentials during the pain test had an increase in pain after exercise. Thus, women with fibromyalgia were classified based on their pain response to exercise, and this response was associated with the change in corticomotor excitability during the application of a noxious stimulus.

A gain-of-function voltage-gated sodium channel 1.8 mutation drives intense hyperexcitability of A- and C-fiber neurons.

Therapeutic use of general sodium channel blockers, such as lidocaine, can substantially reduce the enhanced activity in sensory neurons that accompanies chronic pain after nerve or tissue injury. However, because these general blockers have significant side effects, there is great interest in developing inhibitors that specifically target subtypes of sodium channels. Moreover, some idiopathic small-fiber neuropathies are driven by gain-of-function mutations in specific sodium channel subtypes. In the current study, we focus on one subtype, the voltage-gated sodium channel 1.8 (Nav1.8). Nav1.8 is preferentially expressed in nociceptors, and gain-of-function mutations in Nav1.8 result in painful mechanical hypersensitivity in humans. Here, we used the recently developed gain-of-function Nav1.8 transgenic mouse strain, Possum, to investigate Nav1.8-mediated peripheral afferent hyperexcitability. This gain-of-function mutation resulted in markedly increased mechanically evoked action potential firing in subclasses of Aß, Aδ, and C fibers. Moreover, mechanical stimuli initiated bursts of action potential firing in specific subpopulations that continued for minutes after removal of the force and were not susceptible to conduction failure. Surprisingly, despite the intense afferent firing, the behavioral effects of the Nav1.8 mutation were quite modest, as only frankly noxious stimuli elicited enhanced pain behavior. These data demonstrate that a Nav1.8 gain-of-function point mutation contributes to intense hyperexcitability along the afferent axon within distinct sensory neuron subtypes.

Pain perception after isometric exercise in women with fibromyalgia.

OBJECTIVE: The purpose of this study was to identify exercise protocols incorporating isometric contractions that provide pain relief in women with fibromyalgia. DESIGN: A before-after trial. SETTING: A physical therapy department in an academic setting. PARTICIPANTS: Fifteen women (mean ± SD, 52±11y) with fibromyalgia. INTERVENTIONS: Subjects completed 4 sessions: 1 familiarization and 3 experimental. The following randomized experimental sessions involved the performance of isometric contractions with the elbow flexor muscles that varied in intensity and duration: (1) 3 maximal voluntary contractions (MVCs), (2) 25% MVC held to task failure, and (3) 25% MVC held for 2 minutes. MAIN OUTCOME MEASURES: Experimental pain (pain threshold and pain rating), Fibromyalgia Impact Questionnaire, and fibromyalgia pain intensity (visual analog scale). RESULTS: After all 3 isometric contractions, there was considerable variability between subjects in the pain response. Based on the changes in experimental pain, subjects were divided into 3 groups (increase, decrease, no change in pain). Multiple regression analysis revealed that age, baseline experimental pain, and change in fibromyalgia pain intensity were significant predictors of the experimental pain response after the isometric contractions. CONCLUSIONS: We identified subgroups of women with fibromyalgia based on how they perceived pain after isometric contractions. The greatest pain relief for women with fibromyalgia occurred at a younger age and in women with the greatest experimental pain before exercise. Additionally, we established a link between experimental and clinical pain relief after the performance of isometric contractions.

Involvement of noradrenergic neurotransmission in the stress- but not cocaine-induced reinstatement of extinguished cocaine-induced conditioned place preference in mice: role for ß-2 adrenergic receptors.

The responsiveness of central noradrenergic systems to stressors and cocaine poses norepinephrine as a potential common mechanism through which drug re-exposure and stressful stimuli promote relapse. This study investigated the role of noradrenergic systems in the reinstatement of extinguished cocaine-induced conditioned place preference by cocaine and stress in male C57BL/6 mice. Cocaine- (15 mg/kg, i.p.) induced conditioned place preference was extinguished by repeated exposure to the apparatus in the absence of drug and reestablished by a cocaine challenge (15 mg/kg), exposure to a stressor (6-min forced swim (FS); 20-25°C water), or administration of the α-2 adrenergic receptor (AR) antagonists yohimbine (2 mg/kg, i.p.) or BRL44408 (5, 10 mg/kg, i.p.). To investigate the role of ARs, mice were administered the nonselective ß-AR antagonist, propranolol (5, 10 mg/kg, i.p.), the α-1 AR antagonist, prazosin (1, 2 mg/kg, i.p.), or the α-2 AR agonist, clonidine (0.03, 0.3 mg/kg, i.p.) before reinstatement testing. Clonidine, prazosin, and propranolol failed to block cocaine-induced reinstatement. The low (0.03 mg/kg) but not high (0.3 mg/kg) clonidine dose fully blocked FS-induced reinstatement but not reinstatement by yohimbine. Propranolol, but not prazosin, blocked reinstatement by both yohimbine and FS, suggesting the involvement of ß-ARs. The ß-2 AR antagonist ICI-118551 (1 mg/kg, i.p.), but not the ß-1 AR antagonist betaxolol (10 mg/kg, i.p.), also blocked FS-induced reinstatement. These findings suggest that stress-induced reinstatement requires noradrenergic signaling through ß-2 ARs and that cocaine-induced reinstatement does not require AR activation, even though stimulation of central noradrenergic neurotransmission is sufficient to reinstate.