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Sukhanova KY, Koirala A, Elmslie KS. Na V1.9 current in muscle afferent neurons is enhanced by substances released during muscle activity. J Neurophysiol 2022; 128:739-750. [PMID: 36043704 PMCID: PMC9512110 DOI: 10.1152/jn.00116.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Revised: 08/26/2022] [Accepted: 08/29/2022] [Indexed: 11/22/2022] Open
Abstract
Skeletal muscle contraction triggers the exercise pressor reflex (EPR) to regulate the cardiovascular system response to exercise. During muscle contraction, substances are released that generate action potential activity in group III and IV afferents that mediate the EPR. Some of these substances increase afferent activity via G-protein-coupled receptor (GPCR) activation, but the mechanisms are incompletely understood. We were interested in determining if tetrodotoxin-resistant (TTX-R) voltage-dependent sodium channels (NaV) were involved and investigated the effect of a mixture of such compounds (bradykinin, prostaglandin, norepinephrine, and ATP, called muscle metabolites). Using whole cell patch-clamp electrophysiology, we show that the muscle metabolites significantly increased TTX-R NaV currents. The rise time of this enhancement averaged ∼2 min, which suggests the involvement of a diffusible second messenger pathway. The effect of muscle metabolites on the current-voltage relationship, channel activation and inactivation kinetics support NaV1.9 channels as the target for this enhancement. When applied individually at the concentration used in the mixture, only prostaglandin and bradykinin significantly enhanced NaV current, but the sum of these enhancements was <1/3 that observed when the muscle metabolites were applied together. This suggests synergism between the activated GPCRs to enhance NaV1.9 current. When applied at a higher concentration, all four substances could enhance the current, which demonstrates that the GPCRs activated by each metabolite can enhance channel activity. The enhancement of NaV1.9 channel activity is a likely mechanism by which GPCR activation increases action potential activity in afferents generating the EPR.NEW & NOTEWORTHY G-protein-coupled receptor (GPCR) activation increases action potential activity in muscle afferents to produce the exercise pressor reflex (EPR), but the mechanisms are incompletely understood. We provide evidence that NaV1.9 current is synergistically enhanced by application of a mixture of metabolites potentially released during muscle contraction. The enhancement of NaV1.9 current is likely one mechanism by which GPCR activation generates the EPR and the inappropriate activation of the EPR in patients with cardiovascular disease.
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Affiliation(s)
- Khrystyna Yu Sukhanova
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
| | - Ankeeta Koirala
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
| | - Keith S Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
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2
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McCollum MM, Larmore M, Ishihara S, Ng LCT, Kimura LF, Guadarrama E, Ta MC, Vien TN, Frost GB, Scheidt KA, Miller RE, DeCaen PG. Targeting the tamoxifen receptor within sodium channels to block osteoarthritic pain. Cell Rep 2022; 40:111248. [PMID: 36001977 PMCID: PMC9523973 DOI: 10.1016/j.celrep.2022.111248] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2021] [Revised: 06/29/2022] [Accepted: 08/01/2022] [Indexed: 01/11/2023] Open
Abstract
Voltage-gated sodium channels (NaV) in nociceptive neurons initiate action potentials required for transmission of aberrant painful stimuli observed in osteoarthritis (OA). Targeting NaV subtypes with drugs to produce analgesic effects for OA pain management is a developing therapeutic area. Previously, we determined the receptor site for the tamoxifen analog N-desmethyltamoxifen (ND-Tam) within a prokaryotic NaV. Here, we report the pharmacology of ND-Tam against eukaryotic NaVs natively expressed in nociceptive neurons. ND-Tam and analogs occupy two conserved intracellular receptor sites in domains II and IV of NaV1.7 to block ion entry using a "bind and plug" mechanism. We find that ND-Tam inhibition of the sodium current is state dependent, conferring a potent frequency- and voltage-dependent block of hyperexcitable nociceptive neuron action potentials implicated in OA pain. When evaluated using a mouse OA pain model, ND-Tam has long-lasting efficacy, which supports the potential of repurposing ND-Tam analogs as NaV antagonists for OA pain management.
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Affiliation(s)
- Megan M McCollum
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Megan Larmore
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Shingo Ishihara
- Division of Rheumatology, Rush University Medical Center, Chicago, IL 60612, USA
| | - Leo C T Ng
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Louise F Kimura
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Eduardo Guadarrama
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - My C Ta
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Thuy N Vien
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA
| | - Grant B Frost
- Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
| | - Karl A Scheidt
- Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
| | - Rachel E Miller
- Division of Rheumatology, Rush University Medical Center, Chicago, IL 60612, USA
| | - Paul G DeCaen
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA.
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3
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Kitano Y, Shinozuka T. Inhibition of Na V1.7: the possibility of ideal analgesics. RSC Med Chem 2022; 13:895-920. [PMID: 36092147 PMCID: PMC9384491 DOI: 10.1039/d2md00081d] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2022] [Accepted: 07/25/2022] [Indexed: 08/03/2023] Open
Abstract
The selective inhibition of NaV1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of NaV1.7 has been recently achieved, multiple NaV1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and NaV1.7 coverage among three types of voltage-gated sodium channel (VGSC) inhibitors, namely conventional VGSC inhibitors, sulphonamides and acyl sulphonamides, is discussed. By demonstrating the PK/PD discrepancy of preclinical studies versus in vivo models and clinical results, the potential reasons behind the disconnect between preclinical results and clinical outcomes are discussed together with strategies for developing ideal analgesic agents.
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Affiliation(s)
- Yutaka Kitano
- R&D Division, Daiichi Sankyo Co., Ltd. 1-2-58 Hiromachi Shinagawa-ku Tokyo 140-8710 Japan
| | - Tsuyoshi Shinozuka
- R&D Division, Daiichi Sankyo Co., Ltd. 1-2-58 Hiromachi Shinagawa-ku Tokyo 140-8710 Japan
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4
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Alvarez P, Bogen O, Green PG, Levine JD. Nociceptor Overexpression of Na V1.7 Contributes to Chronic Muscle Pain Induced by Early-Life Stress. THE JOURNAL OF PAIN 2021; 22:806-816. [PMID: 33636374 PMCID: PMC8406703 DOI: 10.1016/j.jpain.2021.02.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/18/2020] [Revised: 01/27/2021] [Accepted: 02/07/2021] [Indexed: 01/06/2023]
Abstract
Adult rats previously submitted to neonatal limited bedding (NLB), a model of early-life stress, display muscle mechanical hyperalgesia and nociceptor hyperexcitability, the underlying mechanism for which is unknown. Since voltage-gated sodium channel subtype 7 (NaV1.7) contributes to mechanical hyperalgesia in several preclinical pain models and is critical for nociceptor excitability, we explored its role in the muscle hyperalgesia exhibited by adult NLB rats. Western blot analyses demonstrated increased NaV1.7 protein expression in L4-L5 dorsal root ganglia (DRG) from adult NLB rats, and antisense oligodeoxynucleotide (AS ODN) targeting NaV1.7 alpha subunit mRNA attenuated the expression of NaV1.7 in DRG extracts. While this AS ODN did not affect nociceptive threshold in normal rats it significantly attenuated hyperalgesia in NLB rats. The selective NaV1.7 activator OD1 produced dose-dependent mechanical hyperalgesia that was enhanced in NLB rats, whereas the NaV1.7 blocker ProTx-II prevented OD1-induced hyperalgesia in control rats and ongoing hyperalgesia in NLB rats. AS ODN knockdown of extracellular signal-regulated kinase 1/2, which enhances NaV1.7 function, also inhibited mechanical hyperalgesia in NLB rats. Our results support the hypothesis that overexpression of NaV1.7 in muscle nociceptors play a role in chronic muscle pain induced by early-life stress, suggesting that NaV1.7 is a target for the treatment of chronic muscle pain. PERSPECTIVE: We demonstrate that early-life adversity, induced by exposure to inconsistent maternal care, produces chronic muscle hyperalgesia, which depends, at least in part, on increased expression of NaV1.7 in nociceptors.
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Affiliation(s)
- Pedro Alvarez
- Department of Oral and Maxillofacial Surgery, University of California, San Francisco, San Francisco, California
| | - Oliver Bogen
- Department of Oral and Maxillofacial Surgery, University of California, San Francisco, San Francisco, California; UCSF Pain and Addiction Research Center, University of California, San Francisco, San Francisco, California
| | - Paul G Green
- Department of Oral and Maxillofacial Surgery, University of California, San Francisco, San Francisco, California; UCSF Pain and Addiction Research Center, University of California, San Francisco, San Francisco, California; Department of Preventative and Restorative Dental Sciences, University of California, San Francisco, San Francisco, California
| | - Jon D Levine
- Department of Oral and Maxillofacial Surgery, University of California, San Francisco, San Francisco, California; UCSF Pain and Addiction Research Center, University of California, San Francisco, San Francisco, California; Department of Medicine, University of California San Francisco, San Francisco, California.
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5
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Li Q, Qin L, Li J. Enhancement by TNF-α of TTX-resistant Na V current in muscle sensory neurons after femoral artery occlusion. Am J Physiol Regul Integr Comp Physiol 2020; 318:R772-R780. [PMID: 32101460 DOI: 10.1152/ajpregu.00338.2019] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Femoral artery occlusion in rats has been used to study human peripheral artery disease (PAD). Using this animal model, a recent study suggests that increases in levels of tumor necrosis factor-α (TNF-α) and its receptor lead to exaggerated responses of sympathetic nervous activity and arterial blood pressure as metabolically sensitive muscle afferents are activated. Note that voltage-dependent Na+ subtype NaV1.8 channels (NaV1.8) are predominately present in chemically sensitive thin fiber sensory nerves. The purpose of this study was to examine the role played by TNF-α in regulating activity of NaV1.8 currents in muscle dorsal root ganglion (DRG) neurons of rats with PAD induced by femoral artery occlusion. DRG neurons from control and occluded limbs of rats were labeled by injecting the fluorescent tracer DiI into the hindlimb muscles 5 days before the experiments. A voltage patch-clamp mode was used to examine TTX-resistant (TTX-R) NaV currents. Results were as follows: 72 h of femoral artery occlusion increased peak amplitude of TTX-R [1,922 ± 139 pA in occlusion (n = 11 DRG neurons) vs. 1,178 ± 39 pA in control (n = 10), means ± SE; P < 0.001 between the 2 groups] and NaV1.8 currents [1,461 ± 116 pA in occlusion (n = 11) and 766 ± 48 pA in control (n = 10); P < 0.001 between groups] in muscle DRG neurons. TNF-α exposure amplified TTX-R and NaV1.8 currents in DRG neurons of occluded muscles in a dose-dependent manner. Notably, the amplification of TTX-R and NaV1.8 currents induced by TNF-α was attenuated in DRG neurons with preincubation with respective inhibitors of the intracellular signaling pathways p38-MAPK, JNK, and ERK. In conclusion, our data suggest that NaV1.8 is engaged in the role of TNF-α in amplifying muscle afferent inputs as the hindlimb muscles are ischemic; p38-MAPK, JNK, and ERK pathways are likely necessary to mediate the effects of TNF-α.
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Affiliation(s)
- Qin Li
- Heart and Vascular Institute, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
| | - Lu Qin
- Heart and Vascular Institute, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
| | - Jianhua Li
- Heart and Vascular Institute, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
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Spider Knottin Pharmacology at Voltage-Gated Sodium Channels and Their Potential to Modulate Pain Pathways. Toxins (Basel) 2019; 11:toxins11110626. [PMID: 31671792 PMCID: PMC6891507 DOI: 10.3390/toxins11110626] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2019] [Revised: 10/24/2019] [Accepted: 10/24/2019] [Indexed: 12/15/2022] Open
Abstract
Voltage-gated sodium channels (NaVs) are a key determinant of neuronal signalling. Neurotoxins from diverse taxa that selectively activate or inhibit NaV channels have helped unravel the role of NaV channels in diseases, including chronic pain. Spider venoms contain the most diverse array of inhibitor cystine knot (ICK) toxins (knottins). This review provides an overview on how spider knottins modulate NaV channels and describes the structural features and molecular determinants that influence their affinity and subtype selectivity. Genetic and functional evidence support a major involvement of NaV subtypes in various chronic pain conditions. The exquisite inhibitory properties of spider knottins over key NaV subtypes make them the best lead molecules for the development of novel analgesics to treat chronic pain.
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Grotle AK, Garcia EA, Harrison ML, Huo Y, Crawford CK, Ybarbo KM, Stone AJ. Exaggerated mechanoreflex in early-stage type 1 diabetic rats: role of Piezo channels. Am J Physiol Regul Integr Comp Physiol 2019; 316:R417-R426. [DOI: 10.1152/ajpregu.00294.2018] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Recent findings have shown that muscle contraction evokes an exaggerated pressor response in type 1 diabetes mellitus (T1DM) rats; however, it is not known whether the mechanoreflex, which is commonly stimulated by stretching the Achilles tendon, contributes to this abnormal response. Furthermore, the role of mechano-gated Piezo channels, found on thin-fiber afferent endings, in evoking the mechanoreflex in T1DM is also unknown. Therefore, in male and female streptozotocin (STZ, 50 mg/kg)-induced T1DM and healthy control (CTL) rats, we examined the pressor and cardioaccelerator responses to tendon stretch during the early stage of the disease. To determine the role of Piezo channels, GsMTx-4, a selective Piezo channel inhibitor, was injected into the arterial supply of the hindlimb. At 1 wk after STZ injection in unanesthetized, decerebrate rats, we stretched the Achilles tendon for 30 s and measured pressor and cardioaccelerator responses. We then compared pressor and cardioaccelerator responses to tendon stretch before and after GsMTx-4 injection (10 µg/100 ml). We found that the pressor (change in mean arterial pressure) response [41 ± 5 mmHg ( n = 15) for STZ and 18 ± 3 mmHg ( n = 11) for CTL ( P < 0.01)] and cardioaccelerator (change in heart rate) response [18 ± 4 beats/min for STZ ( n = 15) and 8 ± 2 beats/min ( n = 11) for CTL ( P < 0.05)] to tendon stretch were exaggerated in STZ rats. Local injection of GsMTx-4 attenuated the pressor [55 ± 7 mmHg ( n = 6) before and 27 ± 9 mmHg ( n = 6) after GsMTx-4 ( P < 0.01)], but not the cardioaccelerator, response to tendon stretch in STZ rats and had no effect on either response in CTL rats. These data suggest that T1DM exaggerates the mechanoreflex response to tendon stretch and that Piezo channels play a role in this exaggeration.
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Affiliation(s)
- Ann-Katrin Grotle
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Elizabeth A. Garcia
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Michelle L. Harrison
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Yu Huo
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Charles K. Crawford
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Kai M. Ybarbo
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
| | - Audrey J. Stone
- Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas
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Marler TL, Wright AB, Elmslie KL, Heier AK, Remily E, Kim-Han JS, Ramachandra R, Elmslie KS. Na V1.9 channels in muscle afferent neurons and axons. J Neurophysiol 2018; 120:1032-1044. [PMID: 29847236 DOI: 10.1152/jn.00573.2017] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The exercise pressor reflex (EPR) is activated by muscle contractions to increase heart rate and blood pressure during exercise. While this reflex is beneficial in healthy individuals, the reflex activity is exaggerated in patients with cardiovascular disease, which is associated with increased mortality. Group III and IV afferents mediate the EPR and have been shown to express both tetrodotoxin-sensitive (TTX-S, NaV1.6, and NaV1.7) and -resistant (TTX-R, NaV1.8, and NaV1.9) voltage-gated sodium (NaV) channels, but NaV1.9 current has not yet been demonstrated. Using a F--containing internal solution, we found a NaV current in muscle afferent neurons that activates at around -70 mV with slow activation and inactivation kinetics, as expected from NaV1.9 current. However, this current ran down with time, which resulted, at least in part, from increased steady-state inactivation since it was slowed by both holding potential hyperpolarization and a depolarized shift of the gating properties. We further show that, following NaV1.9 current rundown (internal F-), application of the NaV1.8 channel blocker A803467 inhibited significantly more TTX-R current than we had previously observed (internal Cl-), which suggests that NaV1.9 current did not rundown with that internal solution. Using immunohistochemistry, we found that the majority of group IV somata and axons were NaV1.9 positive. The majority of small diameter myelinated afferent somata (putative group III) were also NaV1.9 positive, but myelinated muscle afferent axons were rarely labeled. The presence of NaV1.9 channels in muscle afferents supports a role for these channels in activation and maintenance of the EPR. NEW & NOTEWORTHY Small diameter muscle afferents signal pain and muscle activity levels. The muscle activity signals drive the cardiovascular system to increase muscle blood flow, but these signals can become exaggerated in cardiovascular disease to exacerbate cardiac damage. The voltage-dependent sodium channel NaV1.9 plays a unique role in controlling afferent excitability. We show that NaV1.9 channels are expressed in muscle afferents, which supports these channels as a target for drug development to control hyperactivity of these neurons.
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Affiliation(s)
- Tyler L Marler
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Andrew B Wright
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Kristina L Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Ankeeta K Heier
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Ethan Remily
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Jeong Sook Kim-Han
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Renuka Ramachandra
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Keith S Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
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Nociceptor interleukin 10 receptor 1 is critical for muscle analgesia induced by repeated bouts of eccentric exercise in the rat. Pain 2018. [PMID: 28628078 DOI: 10.1097/j.pain.0000000000000936] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Delayed-onset muscle soreness is typically observed after strenuous or unaccustomed eccentric exercise. Soon after recovery, blunted muscle soreness is observed on repeated eccentric exercise, a phenomenon known as repeated bout effect (RBE). Although regular physical activity decreases muscle hyperalgesia, likely because of increased production of the anti-inflammatory cytokine interleukin-10 (IL-10) in the skeletal muscle, whether IL-10 also contributes to the antinociceptive effect of RBE is unknown. Furthermore, whether IL-10 attenuates muscle hyperalgesia by acting on muscle nociceptors remains to be established. Here, we explored the hypothesis that blunted muscle nociception observed in RBE depends on a local effect of IL-10, acting on IL-10 receptor 1 (IL-10R1) expressed by muscle nociceptors. Results show that after a second bout of eccentric exercise, rats exhibited decreased muscle hyperalgesia, indicative of RBE, and increased expression of IL-10 in the exercised gastrocnemius muscle. Although knockdown of IL-10R1 protein in nociceptors innervating the gastrocnemius muscle by intrathecal antisense oligodeoxynucleotide did not change nociceptive threshold in naive rats, it unveiled latent muscle hyperalgesia in rats submitted to eccentric exercise 12 days ago. Furthermore, antisense also prevented the reduction of muscle hyperalgesia observed after a second bout of eccentric exercise. These data indicate that recovery of nociceptive threshold after eccentric exercise and RBE-induced analgesia depend on a local effect of IL-10, acting on its canonical receptor in muscle nociceptors.
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