Low-threshold Mechanoreceptors Research Articles

Mechanically-gated currents in mouse sensory neurons lacking PIEZO2.

Peripheral somatosensory neurons in the dorsal root ganglia (DRG) transduce mechanical force in the skin and other organs into electrical signals using specialized mechanically activated (MA) ion channels that initiate neuronal activation in response to force. Increasing evidence highlights PIEZO2 as the primary transducer of low-threshold mechanical force in DRG neurons. However, in the absence of Piezo2, mice and humans still respond to noxious painful stimuli like pinch, suggesting that additional MA channel(s) likely exist in DRG neurons. Strategies to identify Cre lines and DRG subpopulations that select for non-PIEZO2 expressing neurons is therefore an ongoing effort in the field to discover unknown mechanosensors. Here, we investigated a Vglut3 labeled mouse line as a candidate to identify non-PIEZO2 MA channels in a subtype of DRG neurons called C-fiber low threshold mechanoreceptors (C-LTMRs). Our study carefully demonstrates that the Vglut3-IRES-Cre mouse line specifically and efficiently labels C-LTMR neurons of the DRG. Electrophysiological recordings using two different in vitro mechanical stimulation assays show that the genetically labelled Vglut3 neurons have robust indentation- and stretch-activated MA currents that are exclusively slowly or ultra-slowly adapting. To determine whether the Vglut3-IRES-Cre mouse line can be used to delete genes of interest and identify the underlying MA ion channels in C-LTMRs we attempted to generate a Tmem63b conditional knockout using this Cre line but detected incomplete loss of Tmem63b transcript and lack of TMEM63B-dependent effect on C-LTMR MA currents. Together, our results emphasize that although the Vglut3-IRES-Cre line is robust in driving expression of a conditional reporter gene, it is inefficient in deleting genes like Tmem63b as well as Piezo2.

Fibromyalgia syndrome (FM) is characterized by widespread pain and fatigue. People living with FM also experience tactile allodynia, cold-evoked pain, paraesthesia and dysaesthesia. There is evidence of small fibre neuropathy and hyperexcitability of nociceptors in FM; however, the presence of other sensory abnormalities suggests involvement of large diameter sensory fibres. The passive transfer of FM IgG to mice causes cold and mechanical hyperalgesia associated with changes in A- and C-nociceptor function. However, whether FM IgG also confers sensitivity to light touch and whether large diameter sensory fibres contribute to symptoms evoked by cold is unknown.Here we demonstrate that the presence of sensory abnormalities such as tingling, correlate with the impact of FM, and that people with FM describe the sensation of cutaneous cooling with neuropathic descriptors such as tingling/pins and needles. We find a causal link between circulating FM IgG and the sensitization of large diameter, Aβ low threshold mechanoreceptors (Aβ-LTMRs) to mechanical and cold stimuli in mice ex vivo and in vivo. In keeping with our experimental observations, a larger proportion of Aβ-LTMRs respond to cold stimulation in people with FM, but in contrast to our results ex vivo, the same fibres display reduced responses to mechanical stimuli.These results expand the pathophysiological role of IgG in FM and will inform future studies of sensory symptoms and pain in people with FM.

Neural basis of transcutaneous electrical nerve stimulation for neuropathic pain relief.

Alloknesis refers to itch caused by normally non itch-inducing stimuli, particularly light mechanical stimuli, such as contacts with clothes or other human bodies. This symptom occurs in patients suffering from chronic itch. While it has been mainly described in patients with atopic dermatitis, it is probably present in numerous other conditions and it could induce a severe burden. Until now, it is mainly diagnosed using Von Frey filaments and validated questionnaires are lacking. Alloknesis differs from mechanical pruritus in that it is linked to sensitization to pruritus and therefore occurs in pathological conditions, whereas mechanical pruritus (triggered by the presence of insects on the skin, for example) is a physiological phenomenon. While the role of central sensitization to pruritus in alloknesis is still poorly understood, the role of peripheral sensitization is becoming clearer. Interactions between low-threshold mechanoreceptors (LTMRs) and spinal interneurons are especially involved. Both the mechanical labelled pathway and the polymodal pathway have been shown to contribute to mechanical alloknesis. The mechanical labelled pathway comprises dedicated primary sensory neurons, spinal interneurons, and projection neurons that are functionally distinct from those involved in chemical itch. The polymodal pathway relies on a subset of primary sensory neurons traditionally associated with chemical itch, which can also transduce light mechanical stimuli through the activation of the mechanosensitive ion channel PIEZO1. Both converge onto the gastrin-releasing peptide (GRP) - GRP receptor (GRPR) chemical itch pathway in the spinal cord. Alloknesis is largely unknown to healthcare professionals and even more so to patients, and is not actively investigated. The objective of reducing alloknesis should be considered a therapeutic goal. To date, it has not been investigated in clinical trials. A novel research domain is emerging concerning this symptom, which exerts a substantial impact on the daily lives of numerous patients.

Peripheral somatosensory neurons in the dorsal root ganglia (DRG) transduce mechanical force in the skin and other organs into electrical signals using specialized mechanically activated (MA) ion channels that initiate neuronal activation in response to force. Increasing evidence highlights PIEZO2 as the primary transducer of low-threshold mechanical force in DRG neurons. However, in the absence of Piezo2, mice and humans still respond to noxious painful stimuli like pinch, suggesting that additional MA channel(s) likely exist in DRG neurons. Strategies to identify Cre lines and DRG subpopulations that select for non-PIEZO2 expressing neurons is therefore an ongoing effort in the field to discover unknown mechanosensors. Here, we investigated a Vglut3 labeled mouse line as a candidate to identify non-PIEZO2 MA channels in a subtype of DRG neurons called C-fiber low threshold mechanoreceptors (C-LTMRs). Our study carefully demonstrates that the Vglut3-IRES-Cre mouse line specifically and efficiently labels C-LTMR neurons of the DRG. Electrophysiological recordings using two different in vitro mechanical stimulation assays show that the genetically labelled Vglut3 neurons have robust indentation- and stretch-activated MA currents that are exclusively slowly or ultra-slowly adapting. To determine whether the Vglut3-IRES-Cre mouse line can be used to delete genes of interest and identify the underlying MA ion channels in C-LTMRs we attempted to generate a Tmem63b conditional knockout using this Cre-line but detected incomplete loss of Tmem63b transcript and lack of TMEM63B-dependent effect on C-LTMR MA currents. Together, our results emphasize that although the Vglut3-IRES-Cre line is robust in driving expression of a conditional reporter gene, it is inefficient in deleting genes like Tmem63b as well as Piezo2.

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder marked by social deficits, repetitive behaviors and atypical sensory perception. The link between ASD and skin abnormalities, inducing itchiness, has never been investigated in depth. This study explores mechanical itch sensitivity in the Shank3ΔC/ΔC mouse model. Key observations include heightened scratching in response to skin deformation and hypersensitivity to mechanical itch (i.e. alloknesis) in Shank3ΔC/ΔC mice. In Shank3ΔC/ΔC mice, ex vivo electrophysiological experiments revealed that C-fiber low-threshold mechanoreceptors (C-LTMRs) were hyporesponsive and transcriptomic analysis showed a downregulation of TAFA4, a protein secreted by C-LMTRs. Interestingly, pharmacologically inhibiting Aβ-LTMR, important in mechanical itch initiation, abolished the itch hypersensitivity. Also, TAFA4 injections reduced the spontaneous scratching response to skin deformation but failed to restore itch sensitivity. Our data suggest that somatosensory deficits in Shank3ΔC/ΔC mice lead to a hypersensitivity to itchiness and indicate that two pathways might be regulating mechanical itchiness, dependent or not on TAFA4.

Chronic neuropathic pain involves complex molecular adaptations, and emerging evidence indicates that cerebellins (CBLNs) play a role in sensory processing. This study investigates the role of CBLN2 in trigeminal neuropathic pain (TNP) and examines its regulation through epigenetic mechanisms. Bioinformatics analyses of CBLNs were performed using publicly available expression data from the trigeminal ganglion (TG). A mouse model of TNP was established through partial infraorbital nerve transection (pIONT). Facial allodynia in mice was assessed using the von Frey test. Quantitative real-time PCR (qRT-PCR), Western blotting and enzyme-linked immunosorbent assay (ELISA) were performed to evaluate the expression of CBLN2. Immunofluorescence was used to determine CBLN2's cellular localization. DNA methylation of Cbln2's promoter region was examined using methylation-specific PCR (MSP) and bisulfite sequencing PCR (BSP). Neuronal excitability was assessed through whole-cell patch-clamp recordings. Integration of cross-species single-nucleus RNA sequencing (snRNA-seq) datasets identified dominant CBLN2 expression in Aδ, Aβ and c-fiber low-threshold mechanoreceptors (LTMRs), with significant upregulation observed in a murine model of inflammatory migraine. Consistently, CBLN2 expression was upregulated in the TG of pIONT-induced TNP mice and localized to neurons with myelinated axons, peptidergic and nonpeptidergic nociceptors. siRNA-mediated Cbln2 knockdown attenuated mechanical allodynia, confirming its role in pain initiation and maintenance. Notably, Cbln2 expression was partially dependent on promoter demethylation. MSP and BSP analyses revealed significantly reduced methylation of the Cbln2 promoter in pIONT mice compared to sham controls. Furthermore, pIONT induced persistent upregulation of ten-eleven translocation 3 (TET3) in the TG, while Tet3 knockdown alleviated neuropathic pain and downregulated both Tet3 and Cbln2 expression. Additionally, exogenous CBLN2 potentiated neuronal excitability and activated extracellular signal-regulated kinase (ERK) signaling. Inhibition of the mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) pathway abolished CBLN2-induced hypersensitivity and suppressed the expression of proinflammatory cytokines, including Cxcl1, Cxcr2, Cxcl9, Cxcl10, and Il-6. TET3-mediated demethylation of the Cbln2 promoter drives ERK-dependent neuronal hyperexcitability and neuroinflammation following pIONT. The dual regulatory effects on neuroinflammatory cascades establish CBLN2 as a novel therapeutic target for the treatment of TNP.

C-tactile afferents are low-threshold mechanoreceptors that innervate the hairy skin of mammals, essential for emotional interactions. Replication of such a mechanism could facilitate emotional interactions between humans and embodied intelligence robotic systems. Herein, we demonstrate a monolithic synaptic device that replicates and integrates tactile sensing and neuromorphic processing functions for in-sensor computing. The device is operable by both mechanical and electrical inputs, with the mechanoelectrical operation mechanism stemming from the synergistic effect of dynamic ionic migration and injection. As a proof of concept, the device effectively converts spatiotemporal tactile stimuli into distinct electrical signals, which are subsequently encoded to enable the microcomputer to classify multiple discrete emotional states, such as happiness, calmness, and excitement. This monolithic integrated device, which converges mild tactile perception with neuromorphic processing, with high tactile sensitivity and low-energy consumption, establishes an approach for emotional interaction between intelligent robots and human beings.

Neuropathic pain triggered by chemotherapy poses a significant clinical challenge. Investigating cell type-specific alterations through single-cell transcriptome analysis holds promise in understanding symptom development and pathogenesis. In this study, we performed single nuclei RNA (snRNA) sequencing of dorsal root ganglions (DRG) to explore the molecular mechanism underlying paclitaxel-induced neuropathic pain. Mouse exposed to repeated paclitaxel doses developed persistent pain hypersensitivity lasting at least 21 days. The snRNA sequencing unveiled seven major cell types within DRGs, with neurons further subdivided into 12 distinct subclusters using known markers. Notably, type C low-threshold mechanoreceptors (C_LTMR) exhibited the most pronounced transcriptomic changes post-paclitaxel administration. Differential gene expression and Gene Ontology (GO) analysis highlighted suppressed potassium-related currents, microtubule transport, and mitochondrial functions in C_LTMR following paclitaxel treatment. Pseudo-time analysis uncovered nine distinct states (state 1 to 9) of C_LTMR. State 1 exhibits higher prevalence in paclitaxel-treated mice and altered neurotransmission properties, likely contributing to paclitaxel-induced pain hypersensitivity. Additionally, Camk1d is involved in temperature hyperalgesia in CIPN, a key clinical symptom observed in human patients with CIPN. This comprehensive exploration sheds light on the molecular mechanisms driving paclitaxel-induced neuropathic pain, offering potential avenues for therapeutic intervention.

Sensing cooling temperatures is achieved by primary afferent endings located in the skin and is essential for the survival of animals. TRPM8 channels, primarily expressed in cutaneous C-fibers, have been established as receptors for cooling temperatures, sensing innocuous cooling from the normal skin temperature near 30°C to 17°C, and noxious cooling below 17°C. A cooling sensation is also felt when skin temperatures are first elevated to higher temperatures, for example, noxious heat, and then cool down to the normal skin temperature near 30°C. It is currently not clear what types of cutaneous afferent fibers are involved in sensing the cooling from a high heat to the normal skin temperature. Cutaneous Aβ-fiber low-threshold mechanoreceptors (Aβ-LTMRs) are primarily involved in the sense of touch and are thought to play no role in cooling sensation. In the present study, we conducted the opto-electrophysiological recordings from the skin-nerve preparations made from the hindpaw glabrous skin of Nav1.8-ChR2 transgenic mice. In these transgenic mice, Nav1.8-ChR2-negative Aβ-fiber mechanoreceptors are primarily Aβ-LTMRs, and Nav1.8-ChR2-positive Aβ-fiber mechanoreceptors are mainly high-threshold mechanoreceptors (Aβ-HTMRs). Neither Aβ-LTMRs nor Aβ-HTMRs responded to temperature rising from 30°C to the noxious heat of 43°C. However, a subpopulation of Aβ-LTMRs, but not Aβ-HTMRs, robustly fires action potential impulses in response to the temperature drop from 43°C to 30°C. This finding reveals for the first time that a subpopulation of Aβ-LTMRs senses the cooling for a temperature drop from noxious heat to normal skin temperature.

Background/Objectives: The male prepuce that covers the glans penis is richly innervated by low-threshold mechanoreceptors, which form cutaneous end-organ complexes (Meissner, Pacinian and Ruffini corpuscles) and mucous end-organ complexes (especially Krause-like corpuscles). The mechanosensory inputs from these formations are the beginning for spinal reflexes that regulate movements of intercourse and erection and, therefore, are required for sexual function. The study was aimed at analyzing the age-dependent changes in prepuce innervation. Methods: Here we used immunohistochemistry to investigate whether the innervation of the male prepuce undergoes age-dependent changes, analyzing subjects aged 4 months to 61 years. Results: Abundant Meissner corpuscles and Krause-like corpuscles were regularly found whose morphology, size, and topography were variable and were not correlated with age; however, Ruffini's and Pacinian corpuscles were scarcely observed. The earliest evidence of Meissner corpuscles was observed at 4 months, and thereafter they undergo significant age-dependent variations in density. Until the age of 20 years increases progressively, remains stable until 40 years, and then the density decreases. Meissner's corpuscle index paralleled that of density. Regarding Kause-like corpuscles already resemble the skin of 4-month-old subjects and from the age of 3 years they can be identified at all ages. Its density significantly increased until 10 years and then remained stable. Conclusions: Present results state that the mechanosensory innervation of the human foreskin reaches its maximum value around the age of 20, remains stable during adulthood and decreases with maturity. These findings contribute to a more complete understanding of foreskin innervation and add to the scientific knowledge base surrounding the potential harm of removing a richly innervated structure.

Vincristine is an important chemotherapy drug to treat various types of cancer, but it induces peripheral neuropathy, leading to numbness and mechanical allodynia in the hands and feet of patients. The peripheral neuropathy is a dose-limiting toxicity of vincristine chemotherapy. How vincristine treatment causes numbness and mechanical allodynia remains incompletely understood. In the present study, we utilized Nav1.8-ChR2 transgenic mice in which Nav1.8-ChR2-positive and Nav1.8-ChR2-negative mechanoreceptors could be characterized using the opto-electrophysiological method. Nav1.8-ChR2-negative Aβ- and Aδ-fiber mechanoreceptors are primarily low-threshold mechanoreceptors (LTMRs). On the other hand, Nav1.8-ChR2-positive Aβ- and Aδ-fiber mechanoreceptors are mainly high-threshold mechanoreceptors (HTMRs). We have shown that the mechanical threshold of Nav1.8-ChR2-negative Aβ-fiber mechanoreceptors, but not Nav1.8-ChR2-negative Aδ-fiber mechanoreceptors, were increased significantly in the animals treated with vincristine. In contrast, the mechanical threshold of Nav1.8-ChR2-positive Aβ-fiber mechanoreceptors were significantly reduced following vincristine treatment. Vincristine treatment did not significantly affect the mechanical sensitivity of Nav1.8-ChR2-positive Aδ- and C-fiber mechanoreceptors. Vincristine treatment also did not affect the opto-sensitivity of Nav1.8-ChR2-positive Aβ-, Aδ-, and C-fiber mechanoreceptors. Our findings suggest that mechanical sensitivity is decreased in Aβ-fiber LTMRs and increased in Aβ-HTMRs following vincristine treatment, providing insights into vincristine-induced numbness and mechanical allodynia.

Genetic targeting of select populations of cells in the mouse nervous system is often hampered by a lack of selectivity, as candidate genes for such targeting are commonly expressed by multiple cell populations, also in the same region. Intersectional targeting using two or more genes has been enabled by the development of reporter tools dependent on more than one recombinase or gene regulator. Still, widespread adoption of intersectional tools is complicated by a scarcity of driver mice expressing recombinases other than Cre. Here we report the generation and characterization of a new driver mouse that expresses the FlpO recombinase from the endogenous locus of the Scn10a gene encoding NaV1.8, a voltage-gated sodium channel that is almost exclusively expressed in the afferent limb of the peripheral nervous system. Moreover, among sensory neurons the channel is preferentially expressed in nociceptors and in low-threshold C-fiber mechanoreceptors (C-LTMRs). The mouse showed high recombination efficiency (97%) and selectivity (93%) in dorsal root ganglia. Reporter-expressing fibers were observed in a variety of peripheral tissues, including skin, skeletal muscle, genitalia, bladder and intestines. To validate the suitability of the FlpO mouse line for intersectional targeting, we crossed it with a mouse line expressing CreERT2 from the Th (tyrosine hydroxylase) locus. This approach resulted in strikingly selective and efficient targeting of C-LTMRs, showing robust visualization of nerve endings of these fibers in skin and spinal cord at the light and electron microscopic level. Thus, the NaV1.8Flpo mouse line presented here constitutes a selective and versatile tool for intersectional genetic targeting of NaV1.8 expressing primary afferent neurons.

Truncated TrkB: The predominant TrkB isoform in nociceptors.

Although the association between neural invasion and poor survival in oral cavity squamous cell carcinoma (OSCC) is known, innervating nerve types have not been definitively established; this has confounded mechanistic and translational studies. Therefore, we investigated innervation in human OSCC and further explored these findings in mice. Sensory, sympathetic, and parasympathetic nerves were identified by IHC and linked to neural phenotypes in 71 patients. Additionally, we investigated sensory innervation of OSCC using neuronal tracing with transcriptomic profiling in transgenic mice. In OSCC, most nerves are exclusively sensory or sensory mixed with other types. The presence of exclusively sensory nerves and mixed sensory and sympathetic nerves was significantly increased within the tumor bulk compared with the margin, whereas mixed sympathetic and parasympathetic nerves were decreased. The proportion of exclusively sensory and mixed sensory and sympathetic perineural invasion-positive nerves was significantly higher, whereas the proportion of mixed sympathetic and parasympathetic nerves was significantly lower than that of perineural invasion-negative nerves. Classification of tumor-innervating trigeminal sensory neurons in mice revealed an increase in Calca+ peptidergic nociceptors and reduction in low-threshold mechanoreceptors. Using transgenic reporter mice to verify innervation, we identified that mouse tongue SCC is innervated by Pirt+ and Calca+ nociceptors. This study is the first comprehensive characterization of nerve types in OSCC with classification of innervating trigeminal sensory neurons. Our findings emphasize the importance of sensory innervation in OSCC and are highly relevant for mechanistic and translational studies on treatment strategies.

Afferent innervation of the kidney has been primarily associated with spinal afferents originating in dorsal root ganglia (DRG) between spinal levels T6-L2. These renal afferents were thought to preferentially innervate the renal pelvis, however, recent findings have identified renal afferents near glomeruli in the renal cortex. While renal afferents in the renal pelvis are known to either be chemosensitive or mechanosensitive, the physiological role of renal afferents innervating the renal cortex is unclear. Afferent tracing studies in the kidney have led to mixed abilities to selectively identify and isolate renal afferent cell bodies in DRG, and identify their functional roles. Here we used single cell RNA sequencing (scRNAseq) through Parse Biosciences to identify renal afferent neurons via the expression of neuronal injury markers after axotomy by unilateral nephrectomy. We hypothesized that sensory neurons innervating the kidney will express the neuronal injury marker Atf3 24 hours after nephrectomy of the right kidney, allowing for the identification of kidney-innervating afferent neurons based on comparing the left intact T6-L2 DRG and right axotomized DRG neurons. We additionally investigated vagal innervation of the kidney by including the left and right nodose ganglia (NG). scRNAseq data processing was completed in R using packages singleCellTK for quality control, Seurat for data visualization and analyses, and SingleR for cell type reference annotation. After quality control cell filtering steps, we annotated 7,411 cells into known DRG subtypes and Phox2b+ nodose neurons. Using Atf3 and Jun as neuronal injury markers, preliminary analyses identified clusters that highly expressed Atf3 in right axotomized DRG cell populations NF1/2, NP1, PEP2, and TH compared to left intact DRG neurons, and right nodose neurons (NG3-18). These clusters represent thinly myelinated Aδ low-threshold mechanoreceptors (LTMRs, NF1/2), Aδ peptidergic neurons (PEP2), unmyelinated C-LTMRs (TH), and non-peptidergic neurons (NP1). These cell types agree with prior renal afferent tracing and electrophysiology studies. Further gene expression analyses in these clusters will be completed after additional cell type annotation validation to identify more information about the potential modalities of these cells and how this information aligns with renal afferent physiological responses in the literature. This study aimed to preferentially identify kidney-innervating sensory neurons in DRG and NG with the goal of identifying genes that can be used to selectively label renal afferents. The gained ability to label these kidney-innervating neurons will provide the opportunity to investigate the physiological role and anatomy of these neurons in the kidney as well as transcriptional changes under pathophysiological conditions. Innovative Science Accelerator Program (ISAC) Award 23AU4374 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Exploring the impact of gentle stroking touch on psychophysiological regulation of inhibitory control.

Brain oscillatory dynamics during discriminative vs CT-optimal touch.

Preclinical studies addressing the peripheral effects of cancer perineural invasion report severe neuronal availability and excitability changes. Oral cell squamous cell carcinoma perineural invasion (MOC2-PNI) shows similar effects, modulating the afferent's sensibility (tactile desensitization with concurrent nociceptive sensitization) and demyelination without inducing spontaneous activity (see Part 1.). The current study addresses the electrical status (normal or abnormal) of both active (low threshold mechano receptors (LT) and high threshold mechano receptors (HT)) and inactive (F-type and S-type) afferents. Concurrently, we have also evaluated changes in the genetic landscape that may help to understand the physiological dynamics behind MOC2-PNI-induced functional disruption of the peripheral sensory system. We have observed that the altered cell distribution and mechanical sensibility of the animal's somatosensory system cannot be explained by cellular electrical dysfunction or MOC2-PNI-induced apoptosis. Although PNI does modify the expression of several genes related to cellular hypersensitivity, these changes are insufficient to explain the MOC2-PNI-induced aberrant neuronal excitability state. Our results indicate that genetic markers provide limited information about the functional hyperexcitable state of the peripheral system. Importantly, our results also highlight the emerging role of plasma membrane Ca2+-ATPase activity (PMCA) in explaining several aspects of the observed gender-specific neuronal plasticity and the reported cellular distribution switch generated by MOC2-PNI.