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date: 21 February 2019

Noxious Mechanosensation

Abstract and Keywords

This article introduces a number of critical features of mechanical transduction and neuronal sensory coding, with particular reference to mechanical pain. High threshold mechanoreceptors (HTMRs) in skin are the first relay point for the transduction of noxious mechanical force into the nervous system, and their properties determine how and when nociceptive signal is relayed up to the central nervous system. The article describes the structure and physiology of the mechano-nociceptors and explains the peripheral molecular mechanisms of transduction leading to mechanical pain. In addition, this article also describes a set of computer models of HTMRs and relevant ion channels representing structural and biophysical features of the biological prototype. A better understanding of HTMRs’ function may lead to the development of novel therapeutic pathways in the treatment of chronic mechanical pain.

Keywords: mechanonociception, inflammation, neuropathic pain, sensory neurons, mechanosensitive channels, piezo, Nav1.9, Kv1.1, ectopic discharges, modeling pain

Introduction

Nociceptors are specialized peripheral sensory neurons that alert us to potentially damaging stimuli, such as extreme temperature or pressure, and that also respond to injury-related chemicals, including inflammatory mediators and toxic chemicals. Like other sensations, nociception is detected, encoded, and transmitted to the central nervous system (CNS). However, while most of the sensory and somatosensory modalities are primarily informative, pain serves a protective role (Basbaum et al., 2009).

Like other sensory neurons, nociceptive neurons are pseudo-unipolar, with one axonal branch that extends to the periphery and another branch that forms synapses with second-order neurons in the spinal cord. The cell soma of nociceptive neurons innervating the skin, muscle, joints, bone, and viscera of the body resides para-spinally in the dorsal root ganglion (DRG), whereas sensory neurons responsible for cranial sensation have their cell bodies in the trigeminal ganglion (TG) located at the base of the cranium.

Nociceptors are classified according to the sensory modalities that activate them; i.e., thermal, chemical, and mechanical. Thus, different subgroups of nociceptors have been identified, including mechano-heat, heat, mechano-heat-cold, and mechano-insensitive-heat-insensitive (dubbed “silent”) subgroups (Kumazawa, 1996; Woolf & Ma, 2007). An additional distinction between nociceptors is made based on the diameter and conduction velocity of their axons (Kumazawa, 1996; Djouhri & Lawson, 2004). Three categories can be loosely defined based on axon characteristics: large diameter, heavily myelinated Aβ fibers (cross-sectional area ranging from 6–20 μm) with fast conduction velocity (range 30–75 m/s); medium diameter, thinly myelinated Aδ fibers (cross-sectional area ranging from 2–5 μm) with conduction velocities of 5–30 m/s; and small diameter, unmyelinated C fibers (cross-sectional area ranging from 0.3–3.0 μm) with slow conduction velocity (0.5–2.0 m/s). Myelinated Aδ/β fibers carry the nociceptive input responsible for sharp, pricking pain, whereas the small, unmyelinated C fibers carry the nociceptive input responsible for dull, aching pain. Silent nociceptors become sensitive to noxious mechanical or extreme temperature stimuli only after being sensitized by inflammatory mediators (Meyer et al., 1991). These nociceptors have small, unmyelinated C fibers that conduct impulses at a velocity of less than 3 m/s. Units responding to thermal, mechanical, and chemical stimuli (i.e., polymodal) are the most common C-fiber type observed in situ.

By definition, mechanonociceptors (e.g., HTMRs) are activated by intense (i.e., noxious) mechanical stimuli that impose the risk of tissue injury. They remain silent without stimulation or under conditions of soft touch, becoming activated only after stimulation reaches a relatively high threshold. This is in contrast with the so-called low-threshold mechanoreceptors (LTMRs) that encode a continuous range of stimulus intensities below the noxious threshold (e.g., mechanoreceptors for soft touch) (Delmas et al., 2011; Abraira & Ginty, 2013; Fleming & Luo, 2013). The majority of thinly myelinated Aδ and C fibers are thought to be nociceptors, based on their responses to noxious stimuli. However, large subsets of Aδ (D-hair afferents) and a particular subset of C-fibers (C-LTMRs) display thresholds well below the nociceptive range (Brown & Iggo, 1967; Abraira & Ginty, 2013) and respond to slow-moving gentle touch such as that produced by lightly stroking the skin.

Another important property of mechanoreceptors is their capacity to adapt upon static mechanical stimulation (Delmas et al., 2011; Abraira & Ginty, 2013) (Table 1). Mechanoreceptors with adaptation exhibit high-frequency firing activity at the onset of the mechanical stimulus but return to silence during the static phase of the stimulus (e.g., rapidly adapting, RA). This phenomenon accounts for the perceptual adaptation in which the stimulus vanishes from consciousness. RA mechanoreceptors typically signal the velocity and acceleration of the stimulation in the non-noxious range. A number of mechanoreceptors also activate (or reactivate) at the end of the stimulus, conveying information about the changing sensory environment to the brain. LTMRs classified as RA include the hair follicle and the Meissner and Pacini corpuscles, while those classified as slowly adapting (SA) include the Merkel cell-neurite complex and the Ruffini corpuscle (Lumpkin & Caterina, 2007; Zimmerman et al., 2014). HTMRs that transmit pain and some proprioceptive sensory neurons, which sustain postural states, exhibit little or no adaptation (Lewin & Moshourab, 2004; Delmas et al., 2011). HTMRs are able to signal stimulus magnitude for several minutes or hours. The stimulus duration is signaled by persistent generation of action potentials (APs) throughout the period of stimulation, which contributes to the persistence of pain (Table 1).

Table 1 The Different Types of Cutaneous Mechanoreceptors

Receptor Subtype

Associated Nerve Fiber

Skin

Ending Structure

Location

Mechanical Stimulus

Response Properties

SAI-LTMR

Glabrous

Merkel cell

Epidermis

Indentation

Noxious Mechanosensation

Hairy

Merkel touch dome

Guard hair follicles

SAII-LTMR

Glabrous

Ruffini

Dermis

Stretch

Noxious Mechanosensation

RAI-LTMR

Glabrous

Meissner corpuscle

Dermal papillae

Skin movement

Noxious Mechanosensation

Hairy

Longitudinal lanceolate ending

Guard/Awl-Auchene hair follicles

Hair follicle deflection

RAII-LTMR

Glabrous

Pacinian corpuscle

Dermis

Vibration

Noxious Mechanosensation

Aδ-LTMR

Hairy

Lanceolate ending

Awl-Auchene, Zigzag hair follicles

Hair follicle deflection

Noxious Mechanosensation

C-LTMR

C

Hairy

Lanceolate ending

Awl-Auchene, Zigzag hair follicles

Hair follicle deflection

Noxious Mechanosensation

HTMR

Aβ/Aδ/C

Glabrous

Hairy

Free nerve ending

Epidermis

Dermis

Tissue injury

Noxious Mechanosensation

Abbreviations: SAI-LTMR, Slow adapting type-I low threshold mechanoreceptor; SAII-LTMR, Slow adapting type-II low threshold mechanoreceptor; RAI-LTMR, Rapidly adapting type-I low threshold mechanoreceptor; RAII-LTMR, Rapidly adapting type-II low threshold mechanoreceptor; Aδ-LTMR, A delta fibres- low threshold mechanoreceptor; C-LTMR, C fibres-low threshold mechanoreceptor; HTMR, high threshold mechanoreceptor.

In their function as transducers of noxious stimuli, HTMRs convert mechanical force into electrical signals. At the transduction site, mechanical energy is transduced into a change in membrane potential that is called the receptor/generator potential. The receptor potential is then transformed into a neural pulse code, in which the frequency of APs reflects to some extent the intensity of the stimulus. The ion channels expressed in peripheral nociceptors are directly activated by mechanical noxious stimuli, leading to nociceptor activation (Delmas et al., 2011; Delmas & Coste, 2013; Ranade et al., 2015) (Figure 1A). Surprisingly, transduction mechanisms underlying mechanical nociception are poorly understood, despite their clinical relevance in pain conditions such as diabetes, osteoarthritis, and inflammatory diseases (Wood & Eijkelkamp, 2012; Lolignier et al., 2015). With the recent cloning and functional characterization of the mechanically activated piezo proteins (Coste et al., 2010; Coste et al., 2012; Parpaite & Coste, 2017; Wu et al., 2017), the prototype of the mechanotransducer ion channels involved in soft touch and proprioception has been identified (Ranade et al., 2014; Woo et al., 2015; Florez-Paz et al., 2016). A major issue, then, is to understand how noxious information is transduced into electrical signals in nociceptors and encoded, via voltage-gated ion channels, into a neural pulse code.

Here we will review the nociceptive mechanisms of mechanical pain perception, focusing mainly on nociceptors innervating the skin. We provide an overview of how mechanical noxious stimuli are detected and encoded. Since reviews have described and discussed the biological components of both physiological and pathological pain processing (Dubin & Patapoutian, 2010; Gebhart & Bielefeldt, 2016; Eitner et al., 2017; Woller et al., 2017), we will focus here on the molecules potentially involved in transducing mechanical noxious stimuli, and will highlight, using computational models of nociceptors, how specific ion channels may contribute to the generation of mechanical pain in healthy and sensitized conditions. Current work in this field is providing researchers with a more thorough understanding of mechanonociceptor cell biology at molecular level. A better understanding of mechanical nociception is required to develop new therapeutic strategies.

Noxious MechanosensationClick to view larger

Figure 1 Mechanotransduction in sensory neurons.

A: Schematic representation of a C-fiber free nerve ending. Afferent mechanical signal generation involves different “specialized” domains responsible for transducing mechanical stimuli into APs propagated to central synapses. These specialized structures are differentially populated with ion channels, involved in transduction, system gain, and spike-generation.

B: Schematic drawing of a sensory neuron during recording of mechanically activated currents. The soma of a cultured neuron is stimulated using an electrically driven mechanical probe while patch-clamp recorded in the whole-cell configuration.

C: Three types of MS currents, differing in their rate of adaptation, have been identified in cell bodies of sensory neurons. They are found to be expressed differentially in DRG neurons, classified from their cell somata diameter and mechanical threshold for activation. Piezo2 is the molecular correlate of rapidly adapting MS currents, but ion channel(s) responsible for intermediately and slowly adapting MS currents, present in some nociceptors, is (are) still unknown. Displayed MS currents are representative for mouse DRG neurons at a holding potential of –60 mV.

Transduction of Noxious Mechanical Stimuli

Properties of Excitatory Mechanosensitive Currents in Native DRG Neurons

The challenges encountered in identifying molecular transducers of noxious mechanical stimuli include the relevance of stimulation protocols applied in cellular assays, and their correlation/extrapolation with nociception and pain models. Several assays of cellular responses to mechanostimulation have been developed in recent years and have begun to uncover the molecular basis of mechanotransduction (see, for review, Delmas et al., 2011; Roudaut et al., 2012). The most widely used technique (e.g., poking, whole-cell mechano-clamp) consists of mechanically challenging the cell soma or neurites of isolated DRG neurons using a glass probe while recording the evoked mechanotransducer currents (McCarter et al., 1999; Drew et al., 2002; Hu & Lewin, 2006; Coste et al., 2007; Hao & Delmas, 2011) (Figure 1B). Recording of isolated DRG neurons is a standard approach in sensory studies because cultured DRG neurons retain many aspects of their native properties, including sensitivity to a range of thermal and mechanical stimuli (Tominaga et al., 1998; Peier et al., 2002; Story et al., 2003; Coste et al., 2010). These experiments have helped to characterize the basic properties of mechanosensitive (MS) currents in DRG neurons. Based on kinetics analysis of inactivation, three main types of MS currents have been described in DRG neurons: rapidly adapting (τ = 2–6 ms), intermediately adapting (τ = 15–30 ms), and slowly adapting/ultraslow current (τ ≥ 100 ms) (McCarter et al., 1999; Drew et al., 2002; Coste et al., 2007; Hao & Delmas, 2010; Rugiero et al., 2010) (Figure 1C). Relative anion-cation permeability deduced from reversal potential measurements suggest a cationic non-selective permeability of these MS inward currents (McCarter et al., 1999; Drew et al., 2002; McCarter & Levine, 2006; Hao & Delmas, 2010). Slowly adapting/ultra-slow currents are preferentially, but not exclusively, encountered in small-to-medium–diameter DRG neurons with properties consistent with HTMRs or polymodal nociceptors, based on the expression of the nociceptor-specific Na+ channels, Nav1.8 and Nav1.9, and the noxious heat, transient receptor potential vanilloid type 1 ion channel (TRPV1) (Drew et al., 2002; Hu & Lewin, 2006; Coste et al., 2007; Drew et al., 2007; Hao & Delmas, 2010; Li et al., 2011). However, a subset of nociceptors also displays MS inward currents with a rapid component (Drew et al., 2002). Although relationship between in vitro and in vivo data remains speculative, these findings suggest that slowly adapting/ultraslow MS currents play a critical role in generating sustained activities of nociceptors in vivo (Hao & Delmas, 2010).

There is now compelling evidence that indicate that rapidly adapting MS currents recorded in LTMRs from DRGs and mesencephalic trigeminal nucleus neurons are mediated by Piezo2 subunits (Coste et al., 2010; Eijkelkamp et al., 2013; Lou et al., 2013; Florez-Paz et al., 2016) (Figure 1C). The deletion of Piezo2 abrogates rapidly adapting currents in light-touch mechanoreceptors and proprioceptors, without affecting mechanosensitivity in neurons with intermediately and slowly adapting MS currents (Ranade et al., 2014; Woo et al., 2015). TMEM150C/Tentonin3 was recently proposed to act as an ion channel mediating slowly adapting MS currents in proprioceptive neurons and in expression systems (Hong et al., 2016). However, it was recently shown that heterologous expression of TMEM150C fails to generate MS currents in cells with genomic ablation of the piezo1 gene (Dubin et al., 2017), suggesting that TMEM150C may be a regulator of piezo channel function rather than a pore-forming channel subunit contributing to slowly adapting MS currents.

Molecular Candidates of Excitatory Mechanotransducers in Nociceptors

Transient receptor potential (TRP) channels are candidates based on expression and functional similarities to their evolutionary counterparts (Christensen & Corey, 2007; Delmas & Coste, 2013), but whether these contribute to mechanical transduction in nociceptors is still uncertain. In mammalian sensory neurons, TRP channels are best known for sensing thermal information and mediating neurogenic inflammation, and only two TRP channels, TRPA1 and TRPV4, have been directly or indirectly linked with mechanosensation.

TRPA1

The contribution of TRPA1 to mechanosensory function and mechanically evoked pain remains unresolved due to a series of contrasting results. Consistent with its expression pattern in a subset of small DRG neurons, TRPA1 has been put forward as potential candidate to mediate slowly adapting MS currents in nociceptors. In vitro studies have shown that slowly adapting MS currents are absent in small-diameter DRG neurons from Trpa1–/– mice, raising the possibility that TRPA1 mediates slowly adapting MS currents (Drew & Wood, 2007; Kerstein et al., 2009; Vilceanu & Stucky, 2010). Chemical inhibition of TRPA1 evokes subtle behavioral deficits, suggesting that TRPA1 may play a modulatory role of noxious mechanical response (Petrus et al., 2007; Lennertz et al., 2012). At variance, Rugiero and Wood (2009) ruled out a role of TRPA1 in mediating slowly adapting MS currents, while another study (Brierley et al., 2011), using neurite probing, suggested that Trpa1–/– mice have reduced intermediately adapting MS currents in small-diameter DRG neurons but no alteration of mechanosensory currents in other DRG neurons.

In vivo studies have shown that mice lacking TRPA1 (constitutive knockout [KO]) display a higher mechanical threshold and reduced responses to supra-threshold mechanical stimuli, along with presenting reduced mechanical hyperalgesia during inflammation (Kwan et al., 2006). Recordings of skin–nerve preparations from Trpa1–/– mice also showed impaired firing rates of C-fiber nociceptors in response to noxious mechanical stimuli (Kwan et al., 2006). However, these conclusions have been challenged (Bautista et al., 2006). More recent experiments, using Advillin-Cre-Trpa1fl/fl mice with TRPA1 deletion in a majority of DRG neurons, showed mechanosensory behavior deficits, which manifested as altered responsiveness to light-touch stimuli and faint impairment to noxious stimuli (Zappia et al., 2017). Thus, although there is substantial evidence for a role of TRPA1 in the sensitization of nociceptors to mechanical, thermal, as well as chemical cues in vivo, its contribution as a mechanical transducer in baseline mechanosensation awaits definitive demonstration at the molecular level.

TRPV4

TRPV4 acts as an osmotransducer because, in addition to warm temperature and acidic pH, it is activated by osmotic forceinduced cell swelling and fluid shear stress. However, TRPV4 activation by swelling is indirect and requires fatty acid metabolites (Vriens et al., 2004; Liedtke, 2005), a requirement that disqualifies TRPV4 as a genuine mechanically activated channel. Moreover, disrupting TRPV4 expression in mice causes only modest deficits in response thresholds to a tail-pinch stimulus (Liedtke et al., 2000; Suzuki et al., 2003; Vriens et al., 2004), but strongly impairs responses of nociceptive neurons to osmotic stress and inflammation (Liedtke et al., 2000; Alessandri-Haber et al., 2006).

Acid-Sensitive Ion Channels (ASIC)

A role has been assigned for acid-sensitive ion channels (ASICs) in mechanosensation, but there is no definite evidence it is involved in mechanotransduction. ASICs belong to a proton-gated subgroup of the degenerin–epithelial Na+ channel family of cation channels (Waldmann & Lazdunski, 1998; Lingueglia, 2007). These channels were initially implicated in mechanotransduction because their phylogenetic homologues in C. elegans, the mechanosensory (MEC) channel subunits, are essential for the perception of touch. Three members of the ASIC family (ASIC1–3) are expressed in peripheral mechanoreceptors and nociceptors in mammals. Deletion of ASIC1a does not alter the function of cutaneous mechanoreceptors, but it increases mechanical sensitivity of sensory afferences innervating the gut (Page et al., 2004). Knocking out ASIC2 in mice led to contrasting results, showing either decreased sensitivity of rapidly adapting cutaneous LTMRs (Price et al., 2000; Price et al., 2001) or no alteration of cutaneous mechanosensation (Roza et al., 2004). More convincingly, it was shown that ASIC2 is expressed in aortic baroreceptor neuron somata and terminals, and contributes to the baroreceptor sensitivity (Lu et al., 2009). ASIC2-null mice develop hypertension and exhibit a decreased gain of the baroreflex, suggesting that mechanosensitivity is diminished in ASIC2-null mice. ASIC3 disruption also decreases mechanosensitivity of visceral afferents and reduces responses of cutaneous HTMRs to noxious stimuli (Price et al., 2001). Unpredictably, transgenic expression of a dominant-negative form of ASIC3 leads to an increased sensitivity to noxious mechanical stimuli (Mogil et al., 2005).

These data indicate that ASIC proteins are involved in mechanosensation in mammals; however, their role as prime contender for directly transducing mechanical force in native neurons is still unresolved. In addition, we lack evidence that the channels are mechanically sensitive when expressed in heterologous cells (Garcia-Anoveros et al., 2001; Roza et al., 2004). Moreover, the ion selectivity and permeability ratios of recombinant ASICs differ from those of mechanosensitive currents recorded in native sensory neurons. Consistent with this, no differences in amplitude, kinetics, or incidence of MS currents in DRG neuron somata were seen in transgenic mice lacking ASIC2 and ASIC3 (Drew et al., 2004; Lechner & Lewin, 2009). Altogether, these data indicate that ASIC channels alone are not sufficient to reconstruct mechanically activated channels, but they do support a modulatory role for these channels in mechanosensation in multiple systems (see Omerbasic et al., 2015, for review).

The stomatin-like protein 3, STOML3, evolutionarily related to MEC-2, has been shown to be involved in regulating innocuous touch (Wetzel et al., 2007). Loss of STOML3 increased the number of mechanically insensitive DRG nerve fibers and led to deficits in the ability of mice to sense a sandpaper-like textured surface. In addition, STOML3 can modulate the sensitivity of both Piezo1 and Piezo2 in vitro by decreasing the mechanical threshold needed for activation of these channels (Poole et al., 2014). However, genetic deletion of STOML3 in mouse does not compromise behavioral responses to noxious pressure (Wetzel et al., 2007), but STOML3 inhibitors can reverse mechanical hypersensitivity in nerve injury or diabetic neuropathy models (Wetzel et al., 2017).

PIEZO2

The discovery of Piezo channels (Coste et al., 2010), the first bona fide mechanically activated ion channels identified in mammals (Coste et al., 2012; Syeda et al., 2016), provided promising candidates regarding mechanical pain transducers. Piezo1 is not expressed in DRG neurons, but Piezo2 is detected in up to 45% of total DRG neurons (Lou et al., 2013; Woo et al., 2014). In situ hybridization experiments revealed that Piezo2-expressing neurons form a heterogeneous population including large-diameter, myelinated neurons encompassing LTMRs, as well as medium- and small-diameter putative nociceptors, based on peripherin- and TRPV1-stainings (Coste et al., 2010). Moreover, expression of Piezo2 has also been suggested in putative mechano-nociceptors innervating the cornea (Bron et al., 2014; Alamri et al., 2015) and in mechanically sensitive Aδ nociceptors innervating the bone marrow (Nencini & Ivanusic, 2017).

Conditional KO achieved in the majority of DRG neurons using Advil-creERT2 mice demonstrated the crucial implication of Piezo2 in innocuous light touch (Ranade et al., 2014). More specific conditional KO studies selectively targeting Piezo2 in proprioceptors innervating muscle spindles and Golgi tendon organs (Woo et al., 2015; Florez-Paz et al., 2016) and mesencephalic trigeminal nucleus proprioceptor neurons innervating head muscles (Florez-Paz et al., 2016) indicate that Piezo2 is the major mechanotransducer of mammalian proprioceptors. On the other hand, Piezo2 does not seem to be involved in acute mechanical pain under normal conditions, as Piezo2 conditional KO mice behave like wild-type animals when assayed for mechanical pain using von Frey, tail clip, or Randall Selitto assays (Ranade et al., 2014). Therefore, if Piezo2 has a function in nociceptive neurons, it is not in setting the noxious mechanical threshold under normal conditions.

Growing evidences suggest that Piezo2 is involved in mechanical allodynia/hyperalgesia occurring during neuropathic or inflammatory conditions. Indeed, Piezo2 activation has been shown to be enhanced by bradykinin through cAMP-dependent Protein Kinase A (PKA)/ protein kinase C (PKC ) pathways in a subset of capsaicin-sensitive DRG neurons tentatively classified as mechano-heat nociceptors (Dubin et al., 2012). Although Piezo2 appeared to be sensitized in response to inflammatory signals in vitro, no changes in mechanical allodynia were detected in Piezo2 conditional KO mice following bradykinin injection in the hind paw (Ranade et al., 2014), leading to questions about the target tissue(s) and the functional relevance of this neuronal subpopulation. Another study demonstrated the positive modulation of Piezo2 through the cyclic adenosine monophosphate (AMP) sensor Epac1 pathway in large-diameter DRG neurons (Eijkelkamp et al., 2013). Knock-down of Piezo2 attenuates Epac signaling-mediated mechanical allodynia, as well as allodynia induced by spinal nerve ligation or chronic constriction injury (Eijkelkamp et al., 2013). Importantly, Epac1-induced allodynia does not require Nav1.8-expressing neurons (Eijkelkamp et al., 2013), suggesting that Epac-induced mechanical allodynia involves a non-nociceptive neuronal population, which still awaits further characterization.

A recent study identified the nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) as a marker of a specific subpopulation of “silent” nociceptors (Prato et al., 2017). These neurons account for about 50% of all peptidergic nociceptive afferents innervating visceral organs and deep somatic tissues, but they do not project to the skin. Remarkably, these mechanically insensitive neurons under normal conditions display mechanically activated Piezo2 currents upon treatments with the neurotrophin nerve growth factor (NGF), notably involved in persistent pain states associated with inflammation (Prato et al., 2017). These data suggest a contribution of Piezo2 channels in the development of mechanical hyperalgesia in inflamed visceral organs produced by un-silencing CHRNA3-expressing nociceptors. Piezo2 contribution to visceral sensation, including acetic acid-induced hyperalgesia, has been also suggested by intrathecal Piezo2-shRNA experiments (Yang et al., 2016). Altogether, these observations suggest that Piezo2 contributes to various types of mechanical hyperalgesia/allodynia, but its specific role in nociceptive and/or non-nociceptive sensory neuron subpopulation remains to be determined.

Mechanosensitive Ion Channels That Dampen Excitation

DRG neurons express a variety of K+ channels linked to mechanosensation, including members of the potassium two pore domain channel (KCNK ) and Kv1 families (Alloui et al., 2006; Hao et al., 2013). The existence of these MS K+ channels adds a layer of complexity to the basic model of neuronal activation by mechanical force: whereas the opening of MS cation channels leads to membrane depolarization and excitation, activation of MS K+ channels hyperpolarizes the membrane potential and thus decreases the likelihood of reaching an AP threshold. By doing so, and assuming they open simultaneously, MS K+ channels can counteract, or balance, the activity of MS excitatory channels, thereby shaping mechanosensory responses.

KCNK Channels

The two-pore domain potassium (K2P, KCNK) channel family is composed of 15 genes in the human genome. Based on their primary structure, K2P channels are grouped into six distinct subfamilies denoted as TREK, TALK, TASK, TWIK, THIK, and TRESK (Goldstein et al., 2005). These channels mediate K+-selective leak currents, which regulate cell excitability through their influence on resting membrane potential and are known to be modulated by a variety of factors, such as endogenous ligands, anesthetics, temperature, and membrane tension (Patel et al., 1998; Maingret et al. 1999a; Maingret et al., 1999b; Honore, 2007). Three members of the KCNK family—KCNK2 (TREK1), KCNK4 (TRAAK), and KCNK10 (TREK2)—are intrinsically mechanosensitive in expression systems, and all three are expressed in DRGs (Medhurst et al., 2001; Alloui et al., 2006; Kang & Kim, 2006; Noel et al., 2009). KCNK2 is found exclusively in polymodal nociceptors responsive to high pressure and extreme heat (Alloui et al., 2006). Kcnk2–/– mice are more sensitive than wild-type mice to mechanical stimulation, suggesting that KCNK2 is important for tuning the mechanosensitivity of polymodal nociceptors (Alloui et al., 2006). KCNK4 is activated by both membrane stretch and membrane crenation (Maingret et al., 1999a). Kcnk4–/– mice are hypersensitive to mechanical stimulation, a phenotype that is exacerbated while additionally deleting the Kcnk2 gene (Noel et al., 2009).

One key question that remains is how mechanical forces are integrated in cells where KCNK channels are co-expressed with MS cation channels. Interestingly, Brohawn and coworkers showed that expression of KCNK4 in the neuroblastoma cell line N2A led to decreased depolarization of the membrane potential upon mechanical indentation (Brohawn et al., 2014). This effect is probably due to the simultaneous activation of KCNK4 channels with endogenous Piezo1, suggesting that a similar mechanism (the nature of the channel set aside) may be at play in sensory nerve endings. Despite remarkable phenotypes observed in transgenic mice, it is not yet clear whether KCNK channels act as mechanical transducers or as mere modulators of excitability in nociceptors.

Kv1

Although KCNK channels have attracted the most attention for their MS properties, the voltage-gated K+ channels of the Shaker family (Kv1) have also been linked to mechanotransduction both in vitro and in vivo. Members of the Kv1 channel family are well known to contribute to setting neuronal excitability in a variety of neurons (Coetzee et al., 1999). These channels can be divided into several subfamilies (Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, and Kv1.6) on the basis of sequence similarity and function. Recent work has shown that Kv channels made of Kv1.1 homomers or Kv1.1–Kv1.2 heteromers are mechanically activated in DRG neurons (Hao et al., 2013). Unlike KCNK channels, which are activated by negative pressure applied through the patch pipette, Kv1.1–Kv1.2 channels are activated by indentation of the cell membrane, in addition to negative pressure applied to the patch pipette. Mechanical activation occurs under physiological forces (–10 mmHg) at both macroscopic and single-channel levels, generating a slow activating/inactivating outward current. Kv1.1-containing channels retain mechanosensitivity in excised patches, indicating little contribution of cytoskeletal elements to mechanical sensitivity. Activation of Kv1.1 channels by mechanical challenge can be accounted for by a shift in the pore-opening equilibrium toward the open conformation, consistent with membrane tension acting on a late-opening transition through stabilization of a dilated pore. This effect causes a shift in the voltage range over which Kv channels open, as well as an increase in the maximum open probability (Schmidt et al., 2012; Hao et al., 2013). Mechanistically, opening of Kv1.1 does not depend on a mechanical sensor, as has been hypothesized to be the case for the mechanically gated piezo channels (Guo & MacKinnon, 2017; Saotome et al., 2017; Zhao et al., 2018), but instead makes the best use of the inherently stretch-sensitive properties of the voltage sensor (Mienville & Clay, 1996; Morris & Juranka, 2007; Morris, 2011). Paddle chimera of drosophila Kv1.1 and Kv2.1 reconstituted in Xenopus oocytes also exhibit low-threshold mechanosensitivity, indicating that tension-induced gating changes are a common property of Kv1.1 subunit-containing K+ channels (Schmidt et al., 2012).

In vivo, HTMRs have been shown to be mechanically tuned by the relative expression of Kv1.1–1.2 and slowly adapting cation currents, whose respective inhibitory and excitatory influences modulate the net mechanical response (Hao et al., 2013). MS Kv1.1–1.2 current shifts mechanical threshold for firing towards higher values and concurrently reduces the duration of mechanically induced AP discharges. Thus, the function of MS Kv1.1–1.2 in HTMRs is to tune generator potential by counteracting the action of MS excitatory channels, shifting the threshold for noxious mechano-perception to higher values. By contrast, MS Kv1.1–1.2 current has little impact on mechanical threshold properties of Aβ-type LTMRs because activation of Piezo2 in these cells is faster than that of MS Kv1.1–1.2 current. Thus, in LTMRs, Kv1.1–1.2 current does not impede detection but regulates duration and frequency of firing. Therefore, functional connection between excitatory and inhibitory MS channels is important to setting the properties of the receptor potential. The modulation of either component in pathological conditions may disrupt this balance and makes animals hyper- or hyposensitive to mechanical stimuli.

Regulation of Mechanical Receptor Potential by Subthreshold Voltage-Gated Channels

The net ionic current flowing through excitatory MS channels brings membrane potential towards the threshold for triggering APs. Deviation from the resting membrane potential may therefore activate multiple intrinsic ionic conductances in the subthreshold range (Heidenreich et al., 2011; Wang & Lewin, 2011; Hockley et al., 2014).

Cav3.2 Calcium Channel

The Cav3.2 T-type calcium channel exhibits a low threshold of activation (~–60 mV) and produces Ca 2+ current with rapid activation and inactivation phases in response to depolarization. Cav3.2 channels are strongly expressed in D-hair mechanoreceptors (Dubreuil et al., 2004; Coste et al., 2007; Swayne & Bourinet, 2008; Wang & Lewin, 2011; Hilaire et al., 2012), Aδ- and C-LTMRs, innervating the skin hair follicles (Delfini et al., 2013; Francois et al., 2014) and polymodal nociceptors (Bourinet et al., 2005; Coste et al., 2007). Functional analyses indicate that Cav3.2 has a proexcitatory impact in small-diameter nociceptors expressing mechanoactivated channels (Coste et al., 2007) and favors chronic pain states. Deletion or knockdown of Cav3.2 induces a marked analgesia in vivo (Lee, 2013; Francois et al., 2014). Specific KO of Cav3.2 in C-LTMR cutaneous fibers shows that T-type Ca2+ channels determine their low threshold of mechanical activation and contribute to both mechanical and cold allodynia in peripheral neuropathy (Francois et al., 2015). These findings support a role for Cav3.2 in touch/pain pathophysiology, validating their pharmacological relevance to alleviating mechanical and cold allodynia.

Pharmacological inhibition as well as genetic deletion of the Cav3.2 gene in D-hair LTMRs has been shown to increase mechanical threshold and impair temporal firing (Wang & Lewin, 2011). Altered properties are related to deficiency in receptor potential integration, since neither abnormal mechanotransducer current nor damaged nerve terminal structure was observed in Cav3.2 KO mice. Thus, in D-hair mechanoreceptors, T-type calcium channels serve as subthreshold amplifier and contribute to the high sensitivity of D-hair mechanoreceptors to mechanical stimuli.

KCNQ Channels

M channels, an important regulator of neural excitability, are composed of five subunits of the Kv7 (Kv7.1–Kv7.5) channel family (Delmas & Brown, 2005). Kv7 channels are of particular interest because they are activated at subthreshold membrane potentials. Kv7.2 and Kv7.3 subunits were found to be expressed in DRG nociceptors, generating a slow K+ current activated by retigabine and inhibited by XE991 (Passmore et al., 2003). KCNQ openers have demonstrated their analgesic effect in preclinical and clinical studies, and KCNQ channels are thus considered to be potential therapeutic targets as analgesics (Du et al., 2017). However, their specific role in regulating mechanonociception has not been explored.

The Kv7.4 channel subunit has been found essential to tuning firing activity and modulating stimulus–excitation coupling of many LTMRs. It is expressed in Meissner corpuscles, peripheral lanceolate endings, circular nerve fibers of hair follicles, and Meissner bodies (Delmas & Brown, 2005; Heidenreich et al., 2011). Kv7.4 generates a low-threshold, slow-activating, and non-inactivating current that typically regulates membrane potential (Kubisch et al., 1999). The presence of Kv7.4 near the mechanotransduction site suggests a role in the integration process of receptor potential. Genetic deletion of the Kcnq4 gene in mice enhances mechanosensitivity and alters frequency response of rapidly adapting LTMRs (Heidenreich et al., 2011). Human subjects with loss of function mutation of KCNQ4 outperformed control subjects when tested for vibrotactile acuity (Heidenreich et al., 2011). Thus, KCNQ4 may act as a brake on excitation of rapidly adapting LTMRs.

Nav1.9 Channel

The voltage-gated Nav1.9 channel generates an atypical Na+ current with hyperpolarized activation/inactivation properties, giving rise to a prominent, persistent Na+ current component at subthreshold voltages (Cummins et al., 1999; Coste et al., 2004). Nav1.9 is expressed in all types of nociceptors, in which it co-localizes with slow-adapting/ultraslow MS cation currents (Coste et al., 2007; Padilla et al., 2007). Nav1.9-null mice exhibit impaired mechanical and thermal sensory capacities and reduced electrical excitability of nociceptors (Hoffmann et al., 2017). Deletion of Nav1.9 in C-fibers elevates the electrical threshold of activation in single-fiber recordings and increases the von Frey mechanical threshold compared to wild type C-fibers. Interestingly, heat threshold in C mechano-heat-sensitive fibers was also altered (Hoffmann et al., 2017). Thus, Nav1.9, under normal conditions, contributes to both acute thermal and mechanical nociception in mice through increasing the excitability of nociceptors and also by amplifying receptor potentials, irrespective of the stimulus modality (Lolignier et al., 2015; Hoffmann et al., 2017).

Nav1.9 current plays also a crucial role in inflammation (Rush & Waxman, 2004; Maingret et al., 2008). Nav1.9 channel activity is potentiated by the concerted action of inflammatory mediators and contributes to nociceptor hyperexcitability during inflammation (Rush & Waxman, 2004; Maingret et al., 2008; Vanoye et al., 2013). For that reason, Nav1.9 plays a permissive role in the generation of mechanical pain hypersensitivity, both in subacute and chronic inflammatory pain models (Ritter et al., 2009; Lolignier et al., 2011). It is also required for normal gut mechanosensation and for the development of hypersensitivity of colonic afferents to mechanical stimuli under inflammatory conditions (Copel et al., 2013; Hockley et al., 2014). Thus, Nav1.9 endows nociceptors with a subthreshold-activating inward current that can amplify depolarizing drive induced by MS excitatory channels, a possible molecular mechanism for mechanical hyperalgesia.

Modeling High-Threshold Mechanical Receptors

Computer models provide a useful complementary tool to capture the cellular and molecular basis of noxious stimuli processing. Advantages of computational models lie in the noninvasive nature of the method and in their capabilities to predict functional consequences of ion channel modulation. The “dark side of the Moon” is that any model is a simplified representation of a prototype often based on guesses rather than on precise data. For HTMRs, the morphological structure and biophysical properties of main ion channels are relatively well established (see previous discussion). However, the data about combination of channels and their relative quantities are available mostly for the somas, but not for the peripheral and central processes. Electrical activity patterns of in vitro DRG neurons are thought to represent the nociceptor properties well enough and thus can be used for validation of computer (and conceptual) models. It is the reason why most of computer models of nociceptive neurons are the so-called “single-compartment” models of the soma (Table 2). The somatic recordings are also the main source of experimental data for the rare “multi compartment” models of DRG neurons, which include peripheral and central processes (Table 2).

Table 2 Complement of Ion Channels Used in Computational Models of Nociceptive Neurons

Scriven (1981)

Herzog et al. (2001)

Sheets et al. (2007)

Kouranova et al. (2008)

Maingret et al. (2008)

Choi & Waxman (2011)

Tigerholm et al. (2014)

Petersson et al. (2014)

Sundt et al. (2015)

Nav1.9 (TTX-R)

+

+

+

+

+

Nav1.8 (TTX-R)

+

+

+

+

+

Nav (TTX-S)

+

+

+

Nav1.7 (TTX-S)

+

+

+

+

+

NaP

+

Cav (Ca-T)

+

Cav (Ca-L)

+

Kdr

+

+

+

+

+

+

+

+

+

Ih

+

+

+

KA

+

+

+

+

Kv7 (KM)

+

+

+

+

K(Ca) (K-SK)

+

+

K(Na)

+

+

Na-K pump

+

+

+

Ca pump

+

Abbreviations: TTX, tetrodotoxin; Nav1.9 (TTX-R), TTX-resistant voltage-gated sodium channel isoform 1.9; Nav1.8 (TTX-R), TTX-resistant voltage-gated sodium channel isoform 1.8; Nav (TTX-S), TTX-sensitive voltage-gated sodium channel; Nav1.7 (TTX-S), TTX-sensitive voltage-gated sodium channel isoform 1.8; NaP, voltage-gated channel conducting persistent sodium current; Cav (Ca-T), T-type voltage-gated calcium channel; Cav (Ca-L), L-type voltage-gated calcium channel; Kdr, delayed rectifier potassium channel; Ih, hyperpolarization-activated cation conducting channel; KA, A-type potassium channel; Kv7 (KM), muscarinic-type voltage-gated family 7 potassium channel; K(Ca) (K-SK), calcium-dependent small-conductance potassium channel; K(Na), sodium-dependent potassium channel.

Insights from Models of Non-Mechanical C-Type Primary Afferent Neurons

To aid in understanding the neural basis of noxious stimuli processing, computational models have focused on describing in detail the dynamics of APs and analyzing the contribution of multiple aspects (e.g., morphology, ion channels, pumps) in their generation and transmission.

Patterns of AP firing evoked by depolarizing current injections were studied in a single-compartment model of C-fibers containing a variety of currents, including TTX-S Na+ current, Kdr, T-type Ca2+ current (Cav3.1), and Ca2+-dependent K+ current (SK-type, KSK), together with Na+/K+ and Ca2+ pumps (Scriven, 1981). The modeled nerve fiber generated a continuous discharge or periodical bursts of APs, depending on activity of the pumps and accumulation of current-conducting ions in the peri-axonal space (e.g., K+) and cytoplasm (e.g., Na+). An insightful point here is that any site of C-fiber containing such a cocktail of channels and pumps can become a source of continuous spiking or bursting activity.

Unmyelinated C-fibers that convey pain information usually fire at rates <20 Hz. How C-fiber firing is influenced by the morphology of the T-junction and the local expression of ion channels is important for understanding normal pain signaling. Sundt et al. (2015) used computational modeling to investigate the interaction of axonal morphology, somatic and axonal excitability, and membrane conductances on spike conduction through the DRGs. Using a computer model in which the voltage-gated Nav and Kdr channels were present with the same density in all compartments, except at the soma, the model demonstrated that propagation reliability for single spikes was highly sensitive to the diameter of the stem axon and the density of voltage-gated Na+ channels. The short AP refractory period provided by Nav and Kdr channels allows the modeled fiber to conduct trains of spikes up to frequencies of 110 Hz. However, the presence of Kv7 and the small-conductance (SK)–like Ca2+-dependent K+ channels are required to reduce the firing frequency to near physiological levels found in vivo. Altogether, these simulations demonstrate that the complex interplay of ion channels and temporal gating dynamics determines the specific firing pattern of C-type nociceptors.

Other models disclose the specific roles of Nav channel subunits in C-nociceptor activity under normal and pathological conditions (Herzog et al., 2001). Recordings from small DRG neurons were used to fit parameters of simulated currents, including fast inactivating TTX-S Na+ current, Nav1.9-type TTX-R Na+ current, and Kdr current. By varying the relative magnitude of TTX-S and TTX-R Nav currents and recording the model responses to sub- and supra-threshold current injections for AP firing, it was shown that although Nav1.9-type Na+ current does not contribute substantially to the inward current during AP upstroke, it provides an important depolarizing influence to resting membrane potential (RMP) and amplifies subthreshold excitatory inputs.

The hyperpolarization-activated cation current (Ih, HCN) was added to the earlier model (Herzog et al., 2001), and its role in excitability and shaping APs of small DRG neurons was explored (Kouranova et al., 2008). The characteristics of simulated Ih current (kinetics, steady-state, and conductance parameters) were based on experimental recordings from small DRG neurons, in which Ih current has significantly smaller density and slower activation kinetics than in large-diameter neurons. In the absence of Ih, the RMP was close to the reversal potential of Kdr. Inclusion of Ih resulted in a +16 mV-shift of RMP and the occurrence of characteristic excitatory rebounds after membrane hyperpolarization. In addition, Ih was shown to facilitate subthreshold membrane potential oscillations that are known to trigger ectopic discharges in afferent axons and cell somata after nerve trauma (Liu et al., 2000; Amir et al., 2002; Liu et al., 2002; Amir et al., 2005).

The contribution of Nav1.7 channels to the excitability of small DRG neurons was explored in a more complex model, which also included channels conducting Nav1.8 current, Kdr, and a transient A-type K+ (KA) current (Sheets et al., 2007). Such model composition was motivated by the abundant expression of Nav1.7 in nociceptive neurons (Djouhri et al., 2003) and the association of a severe phenotype of hereditary erythromelalgia with a particular (N395K) Nav1.7 mutation that lies within the local anesthetic binding site of the channel (Drenth et al., 2005). The Nav1.7-N395K mutant channel differs from wild type (WT) by a hyperpolarized voltage dependence of activation and impaired (right-shifted), steady-state, slow inactivation. The model revealed functional consequences of such difference, when the channels operate in the environment of other channel subtypes: the hyperpolarization shift in activation was found to be the major determinant of hyperexcitability induced by the erythromelalgia mutation, but changes in slow inactivation also contribute to enhanced excitability.

The respective roles of the excitability of small DRG neurons of Nav1.7 and Nav1.8, another Nav subunit strongly expressed in nociceptive neurons (Brock et al., 1998; Strassman & Raymond, 1999; Black & Waxman, 2002; Zimmermann et al., 2007; Persson et al., 2010) were further examined in a similar model (Choi & Waxman, 2011). To this end, the partial conductance of these currents was changed so that one of them (Nav1.7 or Nav1.8) was set on full level, and its counterpart varied or eliminated. In the presence of both Nav1.7 and Nav1.8 conductances, the simulated neuron responded to sustained depolarizations by a periodical sequence of spikes with subthreshold membrane potential oscillations of increasing amplitude within interspike intervals. The oscillations disappeared in the absence of Nav1.8, and were magnified by addition of Nav1.7 in a cell containing Nav1.8. Moreover, increasing levels of expression of Nav1.8 resulted in a decrease of Nav1.7 current activated during sustained depolarizations, due to the accumulation of channels into fast inactivated states. These simulations indicate that the respective levels of Nav1.7 and Nav1.8 provide a regulatory mechanism that tunes the excitability of small DRG neurons. The interplay of Nav1.7 and Nav1.8 currents is crucial for the repetitive firing of DRG neurons since Nav1.7 operates in the subthreshold range to amplify small inputs by producing graded depolarizing responses, and Nav1.8 provides the majority of the inward current responsible for the upstroke of APs. The relative magnitude of Nav1.7 and Nav1.8 may change during disease (Luo et al., 2008; Strickland et al., 2008), peripheral nerve injury, limb amputation (Coward et al., 2000; Coward et al., 2001; Kim et al., 2002; Kretschmer et al., 2002; Black et al., 2008), and the action of proinflammatory mediators (Black et al., 2004).

The contribution of Nav1.9 current to the hyperexcitability produced by inflammatory mediators was studied in a combined experimental and simulation study with the use of a model containing Nav1.8, Nav1.9, Kdr, and KCNQ (Kv7) channels (Maingret et al., 2008). Nav1.9 channels were shown to contribute to plateau potentials, oscillatory bursting activities, and conditional bistable behaviors. The model responded to the 5-ms depolarizing stimulus by generation of a subthreshold regenerative depolarization and AP, followed by a plateau potential triggering spike burst, as observed experimentally. The model allowed the observation of all component currents inaccessible in the in vitro experiments and thus revealed the mechanism underlying such response patterns. It was done with the use of a dynamic voltage-clamp protocol employing a special command voltage. The latter was a prerecorded plateau potential crowned with a burst of APs, which was evoked by a short depolarizing pulse. The partial current traces accompanying this voltage command confirmed that Nav1.9 current does not contribute substantially to the APs during burst, but is time-tuned with subthreshold depolarization triggering first AP and the subsequent depolarizing plateau phase. Conversely, Nav1.8 was responsible for the surge of Na+ current that occurs during the rising phase of APs, but it had only minor involvement in the later phase of the plateau depolarization. This mechanism may sustain nociceptor hyperexcitability during peripheral inflammation since inflammatory mediators were shown to upregulate Nav1.9 current, lowering the excitability threshold.

A Model of C-HTMR with Slow-Adapting MS Current as Mechanical Transducers

Mechanosensory transduction and evoked or spontaneous activity related to normal or pathological pain were out of the scope of earlier models of primary nociceptor neurons (see previous discussion). We developed a model of C-HTMR that provides a quantitative description of the neurobiological processes that precede or accompany the pain experience, including the transduction, transmission, and modulation of cutaneous noxious stimuli (Figure 2). The model included the peripheral and central non-myelinated thin (0.5 µm) branches formed by a trunk emerging from the soma, homogeneously populated with fast-inactivating TTX-S Na+ channels, Kdr, and passive leakage channels. The peripheral axon terminated with a transduction zone (TZ), which contains Nav1.8 and Nav1.9 channels, Kdr, and leak channels, as well as slowly adapting MS channels. Biophysical properties of Nav1.8, Nav1.9, and Kdr were derived from earlier models (Maingret et al., 2008). Numerical simulation for slowly adapting MS channels derived from a four-state gating mechanism similar to that described for Piezo1 channels (Lewis et al., 2017) but with slower kinetics (Figure 2B). Steady current-voltage relationships (IVs) characterizing the channels present in our C-HTMR model are shown in Figures 2B (Kdr), 2C (Nav1.8 and Nav1.9), and 2D (Na-TTXs). The effect of inflammation was simulated by a 6 or 20 mV-hyperpolarization shift of Nav1.9 activation/inactivation kinetics (Figure 2C, thick lines). The neuropathic condition associated with a post-lesional accumulation of slowly adapting MS channels was simulated by a 33% increase in MS channel maximal conductivity.

Noxious MechanosensationClick to view larger

Figure 2 Modeling of C-type high threshold mechanoreceptors.

A: Scheme of the model structure, showing the main channel types and their subcellular distribution (e.g., transduction zone, peripheral axon, trunk-soma, and central axon), together with the different recording sites. The model supports channels with Ohmic or Goldman-Hodgkin-Katz behavior and can simulate the time-course and steady-state conditions of ionic currents.

B: Channel kinetics scheme (top inset) used to simulate the slow-adapting MS current evoked by pressure clamp–like mechanical stimuli.

C–E: Steady-state IV relationships for Kdr (C), Nav1.8 and Nav1.9 (D), and TTX-S Nav (E) currents. Note the –6 mV (#1) and –20 mV (#2) shift of Nav1.9 IV relationship used to depict the properties of Nav1.9 in mild and strong inflammatory conditions, respectively (D).

Simulated C-HTMR Firing Patterns Related to Normal, Inflammatory, and Neuropathic Conditions

Under normal conditions (Figure 3, left column), application of a trapezoid supra-threshold mechanical stimulus to TZ (Figure 3A) activated slow-adapting MS channels conducting an inward current (Figure 3B). Notably, the MS current was non-zero before and after the stimulus because the open kinetics state was not empty—at basal plasma membrane tension—in the absence of the mechanical stimulation. This “basal” current contributed to the resting depolarization of TZ relative to the resting potential (RP) of the peripheral axon. However, it was insufficient for activation of spike-generating Na+ currents. When activated, the MS current (Figure 3B) produced a depolarization that was amplified by activation of Nav1.9 channels (Figure 3C). This resulted in a large receptor potential (Figure 3D), which spread into adjacent peripheral axon and triggered a burst of APs (Figure 3E) conducted to the central ending (Figure 3F). Such activity can be considered a normal pain signal, providing a reference for abnormal cases.

Noxious MechanosensationClick to view larger

Figure 3 Modeling mechanical hypersensitivity in nociceptive neurons.

A: Mechanical stimuli applied to the transduction zone, and responses (B to F) of modeled HTMR representing normal, inflammatory and neuropathic conditions (as indicated).

BC: Shown are currents through slow-adapting MS channels (B) and Nav1.9 channels (C) of the transduction zone.

D–F: Membrane potentials recorded from the transduction zone (D), peripheral axon at the junction to the TZ (E), and central ending (F).

Inflammatory states were simulated by either –6 mV (inflammatory condition #1) or –20 mV (inflammatory condition #2) shift of Nav1.9 activation/inactivation kinetics (cf. IVs in Figure 2D). The neuropathic condition was simulated by a 33% increase in the maximal conductivity of slow-adapting MS channels mimicking post-lesional accumulation of channels near the TZ.

Inflammatory conditions #1 and #2 (see Figure 3, middle columns) each bring the neuron to a sustained pre-stimulus firing with a frequency of 97 Hz and 8 Hz, respectively. This happened because the depolarizing effects of both basal MS current and persistent Nav1.9 current components brought the membrane potential into a supra-threshold range for spiking. Compared to the normal case (left column), the pre-stimulus Nav1.9 current increased about 5.6 and 3.8 times, from –0.6 µA/cm2 in control to –2.3 and –3.4 µA/cm2, respectively (Figure 3C). During the stimulus, the firing frequency noticeably increased, reaching a peak value of 287 Hz and 281 Hz (Figure 3E) on the top of the depolarization receptor potential (Figure 3D), which reached –36.05 and –36.28 mV, respectively. At these potentials, Nav1.9 current with 6 mV negatively shifted IV was activated to 64% of its peak value (–2.66 µA/cm2 at –42.5 mV); whereas that with 20 mV negatively shifted IV was nearly inactivated (6% of peak –3.09 66 µA/cm2 at –56.5 mV) (Figure 2D). This explains the decrease in Nav1.9 current seen during the receptor potential compared to pre-stimulus basal value in the inflammatory condition #2 (Figure 3C). Such ongoing firings can be considered as coding the afferent message of sensitized HTMRs during acute inflammation.

The neuropathic condition (Figure 3, right column) differed from the normal one (left column) by a 33% increase in slow-adapting MS channel conductivity. Both peak and basal MS currents (Figure 3B) increased to –310.4 µA/cm2 (+30.5%) and –21.9 µA/cm2 (+23.2%), respectively. Depolarization induced by basal activity of MS currents in (Figure 3D) was amplified by activation of Nav1.9 channels (Figure 3C), triggering firing (Figure 3E, F) with a frequency of 33 Hz. During mechanical stimulation (Figure 3A), the firing rate increased to a peak value of 298 Hz (Figure 3E, F) that was nearly the same as in inflammatory conditions. The firing pattern observed in this condition can exemplify signaling of a neuropathic mechanical pain related to post-lesional accumulation of MS channels in peripheral terminals of C-HTMRs.

Computational experiments with our model of C-HTMRs draw attention to a point that earlier escaped from consideration and discussion. Conventionally, greater or smaller effects of sensory stimulus were thought of in terms of greater or smaller evoked depolarization receptor potentials. This concept still has no direct experimental evidence, because thin C-nociceptor fibers and their even thinner sensory endings remain inaccessible for electrophysiological recordings. The computer modeling allows virtual access to these sites. Simulated recordings (Figure 3) show that, not only the receptor potential, but also the basal level of the membrane potential in TZ and adjacent peripheral axon do matter. The cocktail of channels presumably present in the C-mechanonociceptor sensory ending (Figure 2A) does not provide by itself the generation of APs. It provides a source of the depolarizing current delivered to the electrically coupled pre-TZ peripheral axon containing channels specialized in the APs’ generation and propagation. Notably, existing estimates (Carr et al., 2002) suggest that, at rest, the sensory terminal ending can be more depolarized than the pre-terminal peripheral axon. The channel cocktail of TZ with those of other parts of our simulated C-HTMR form a system that robustly converts the mechanically evoked phasic receptor potential into a burst of APs. This system is fine-tuned by the basal level of membrane depolarization: sustained firing interpreted as a signal of unprovoked persisting pain occurs if basal depolarization is within a relatively narrow, few mV-width, range. The model demonstrates that Nav1.9 and MS channels with the given kinetic properties under pathological conditions can conduct abnormally increased basal inward currents, bringing the membrane depolarization into the range of sustained AP firing.

This set of simulations may help explain the paradoxical relationship between Nav1.9 channel dysfunction and clinical phenotypes. Mutations of Nav1.9 in humans have been associated with either the inability to sense pain (Leipold et al., 2013; Woods et al., 2015) or the opposite phenotype of painful peripheral neuropathy (Zhang et al., 2013; Huang et al., 2014; Leipold et al., 2015; Okuda et al., 2016). However, studies of mutant Nav1.9 channels all showed hyperpolarizing shifts in channel activation, consistent with a gain of function at the channel level. A recent study (Huang et al., 2017) has solved this apparent conundrum by demonstrating that the L1302F Nav1.9 mutation, which leads to substantially enhanced overlap between activation and steady-state inactivation relationships (e.g., persistent current), reduces the excitability of DRG neurons through inactivation of Nav channels, consistent with impaired pain sensation at the clinical level. This points out to the interplay between the RMP and AP threshold, where small depolarizations cause hyperexcitability (painful neuropathy), while larger depolarizations cause hypoexcitability (insensitivity to pain). Thus, input-output properties of nociceptors depend on the RMP, the effect of transducer depolarization on membrane potential, and on the dynamics/properties of the spike threshold.

Conclusion and Future Directions

Despite technical difficulties, enormous progress has been made in recent years in understanding many aspects of mechanosensation. The specialized cell types found in the skin and involved in sensing touch and vibration, as well as the evidence for subsets of sensory DRG neurons that respond to noxious mechanical stimuli, are now well established. The MS channel Piezo2 is intimately associated with soft touch and proprioception, but the molecular identity of the MS channels transducing noxious mechanical cues remains to be discovered. Although none of the recently tested candidates, including TRP, ASIC, and TMEM members unequivocally fulfill the criteria for genuine MS channels, this should not deter speculation regarding their contribution in mechanosensation.

Our knowledge is still far from complete with regard to the full complement and localization of voltage-gated channels and MS sensors in HTMR nerve endings, the coding of the quality of the stimulus, and the specific role of each ion channel in pathophysiological pain. Obviously, the clinically relevant question in the field is how mechanical allodynia and hyperalgesia occur, and how they can be blocked. Inflammatory mediators and neuropathic conditions are known to affect mechanical pain thresholds by altering both transduction mechanisms and the electrogenesis of nociceptors. Dissecting mechanisms of mechanical sensitization therefore requires an understanding of both general changes of excitability as well as specific effects on mechanotransducer channels. Studies using native somatosensory neurons and their computer avatars will thrust the direction of research toward a better understanding of mechanical pain hypersensitivity.

Acknowledgements

P.D. and B.C. are supported by the Centre national de la recherche scientifique (CNRS) and Aix-Marseille Université. B.C. Research is funded from the European Research Council/ERC Grant Agreement n. 678610; P.D research from grants from the Fondation pour la Recherche Médicale (FRM 2013 DEQ20130326482). S.M.K. was partly supported by a research visit scholarship from the Embassy of France in Ukraine.

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