Aδ Fibers Research Articles

Microiontophoretic Application of Dynorphin in Dental Pain: Excitatory or Inhibitory Effects.

The tooth exhibits increased sensitivity to noxious stimuli due to the dense innervation of thin myelinated Aδ fibers and unmyelinated C fibers within the dental pulp. While prior research has identified dynorphin expression in layers I-II of the dorsal horn across the spinal cord in various pain models, its functional role in trigeminal nociception, including tooth pain, remains underexplored. This study examines the potential role of dynorphin in the nociceptive processing of dental stimuli. Experiments were performed on adult male ferrets weighing 0.9-1.4 kg. The effects of dynorphin on electrically evoked responses of tooth pulp neurons were recorded extracellularly. The results demonstrated that the microiontophoretic application of dynorphin A induced excitatory and inhibitory effects on N-methyl-D-aspartate (NMDA)-evoked responses in electrically stimulated tooth pulp neurons. Specifically, dynorphin A attenuated NMDA-evoked responses in 16 out of 32 neurons by 61 ± 6%, facilitated NMDA-evoked responses in 10 out of 32 neurons by 69 ± 17%, and elicited mixed inhibitory and facilitatory responses in six out of 32 neurons. The inhibitory effects of dynorphin were blocked by nor-binaltorphimine, a kappa receptor antagonist, whereas the facilitatory effects were inhibited by D,L-2-amino-5-phosphonovaleric acid, an NMDA receptor antagonist. These findings suggest that dynorphin A-induced excitatory responses are mediated by NMDA receptors, whereas its inhibitory responses are mediated through kappa opioid receptors in dental pain. Thus, dynorphin exerts diverse effects, highlighting its role in the perception and modulation of dental pain.

Dermal macrophages control tactile perception under physiological conditions via NGF signaling

We demonstrated previously that sorting nexin 25 (SNX25) in nerve-associated macrophages plays critical roles in pain sensation by regulating tissue NGF content under both physiological and neuropathic conditions. In the present study, we apply the SNX25–NGF paradigm to tactile perception by showing that Snx25+/- mice or macrophage-specific Snx25 conditional knock-out (mcKO) mice had weaker responses to tactile stimuli in normal conditions. Snx25 mcKO mice responded poorly to transcutaneous electrical stimuli at a frequency of 5 Hz (C fiber responses), but normally to stimuli at a frequency of 250 Hz (Aδ fiber responses) or of 2000 Hz (Aβ fiber responses). CX3CR1-positive dermal macrophages were frequently found near calcitonin gene–related peptide (CGRP)- positive nerves and, less frequently, tyrosine hydroxylase (TH)-positive nerves. We confirmed that the tissue content of NGF was lower in Snx25 mcKO mice than in wild-type mice, and in turn, dermal NGF injection restored tactile sensitivity in Snx25+/- mice and Snx25 mcKO mice to normal levels. These results indicate that CGRP-positive C-nociceptors (possibly also TH-positive C-LTMRs) associated dermal macrophages control tactile perception by producing NGF and secreting it into the dermis.

Sensory dysfunction in SMA type 2 and 3 - adaptive mechanism or concomitant target of damage?

BackgroundThe motor neuron survival protein performs numerous cellular functions; hence, spinal muscular atrophy (SMA) is considered to be a multi-organ disease with possible sensory system damage. The controversy surrounding the presence of sensory disturbances, prompted us to conduct standard electrophysiological studies and assess the sensory thresholds for different modalities in adults with SMA types 2 and 3. The study group consisted of 44 adult SMA patients (types 2 and 3). All patients underwent neurological examination using the Hammersmith Functional Motor Scale – Expanded (HFMSE). Standard sensory electrophysiological studies in the ulnar nerve and the estimation of vibratory, temperature, and warm- and cold-induced pain thresholds with temperature dispersion assessment were performed using quantitative sensory testing (QST).ResultsThe most repeatable result was the high amplitude of the sensory nerve action potentials (SNAP) in SMA patients compared to controls. This was higher in type 2 patients compared to type 3a and 3b patients and patients with low HFSME scores. Patients with SMA, especially type 3b presented a longer sensory latency and slower conduction velocity than did controls. Cold pain threshold was higher and warm dispersion larger in SMA. The vibratory limit was higher in patients with high HFSME scores.ConclusionsA high SNAP amplitude suggests sensory fibre hyperactivity, which may be based on overactivation of metabolic pathways as an adaptive mechanism in response to SMN protein deficiency with additionally coexisting small C- and A-delta fibre damage. SMA patients seem to have a concomitant, mild demyelinating process present at the early SMA stage.

(195) QUANTITATIVE SENSORY TESTING OF THE CLITORIS, VULVA AND VESTIBULE IN MENOPAUSAL PATIENTS WITH SEXUAL DYSFUNCTION

Abstract Introduction Individuals in menopause often complain of sexual dysfunction (low libido, reduced sensation, muted orgasm). In sexual-health related menopause concerns, hormonal pathophysiologies have been well studied. However, the effects of menopause on the integrity of the female genital sensory pathways have not been well researched. There is one report of reduced vulvar sensitivity with menopause in the literature. Somatic afferent pathways of the clitoris, vulva and vestibule include the dorsal and perineal nerve branches of the pudendal nerve (S2-S4). To assess the integrity of these somatic afferent pathways, genital quantitative sensory testing (QST) may be performed. QST consists of vibration perception threshold testing for large myelinated A-beta fibers and temperature perception threshold testing for small myelinated A-delta fibers (cold) and for unmyelinated c-fibers (warm). Objective We wished to assess genital QST in 2 cohorts of menopausal patients with sexual dysfunction. Methods This is a chart review of menopausal patients presenting to a sexual medicine clinic for sexual dysfunction between January 2022 and May 2024. They all presented with limited or no previous menopause management by systemic and/or local sex steroid hormone treatment. The patient cohort was divided into those with genitourinary syndrome of menopause (GSM) alone where neurologic disease is not obvious, and those with persistent genital arousal disorder/genito-pelvic dysesthesia (PGAD/GPD), a disease state in which neurologic disease is well documented. Quantitative sensory testing of vibratory, cold and warm sensation was performed at their initial visit, prior to any treatment from our clinic, using a biothesiometer (Bio-Medical Instruments, Newbury, OH) to test for vibration perception threshold (VPT) measured in volts, and a thermal sensitivity tester (Sensortek, Inc., Clifton, NJ) to test for cold perception threshold (CPT) and warmth perception threshold (WPT) measured in degrees Centigrade, comparing the genital readings to a non-genital location as control. Abnormal QST was defined when the test value was ≥2 standard deviations from the control value. Results A total of 30 charts of patients in menopause were reviewed. Sixteen individuals, mean age 59 (range 50 to 81 years), presented for GSM alone. Abnormal VPT testing was found in 63%, mean VPT value 15 V compared to 6 V for control. Abnormal CPT testing was found in 44%, mean CPT value 19°C compared to 22°C for control. Abnormal WPT testing was found in 68%, mean WPT value 35°C compared to 27°C for control. Fourteen individuals, mean age 64 (range 50 to 78 years), presented with concomitant PGAD/GPD. Abnormal VPT testing was found in 71%, mean VPT value 16 V compared to 5 V for control. Abnormal CPT testing was found in 43%, mean CPT value 20°C compared to 22°C for control. Abnormal WPT testing was found in 71%, mean WPT value 35°C compared to 28°C for control. Conclusions A similar prevalence was found of abnormal QST testing in 44%-68% and 43% - 71%, respectively, of menopausal individuals without and with PGAD/GPD. We conclude that genital sensory impairment is common in menopause, may contribute to menopause-related sexual dysfunction, and should be documented objectively by baseline neurogenital testing. Disclosure No.

Quantitative sensory testing in notalgia paresthetica reveals small fiber-type-specific differences in non-pruritic sensitivity: a pilot study.

Notalgia paresthetica (NP) is a chronic condition characterized by pruritus and other unpleasant dysesthetic sensations unilaterally on the subscapular back. Its specific underlying mechanisms are largely unknown, though hypothesized to be neuropathic. Determination of possible somatosensory contributors to the condition could pave the way for novel treatments. Given the potential involvement of non-pruritic mechanisms in NP, our objective was to broadly characterize the somatosensory function in NP-affected and unaffected skin using methods that have been standardized in pain-free controls and painful neuropathic disorders. We hypothesized that if NP is caused by neuropathic mechanisms not targeted directly to pruritoceptors in the skin, somatosensory abnormalities would not be itchspecific. Second, given the lack of symptoms on the contralateral side of the back, we hypothesized that this region would be normally sensitive. In this study, quantitative sensory testing (QST) was used to comprehensively assess the somatosensory function in 15 adult patients with NP. Standardized QST metrics were performed in the NP-affected region and compared with the contralateral asymptomatic skin and itch-free individuals using an age, gender, and site-matched reference data set. There were no significant differences in sensitivity between symptomatic and asymptomatic skin, except for increased mechanical-evoked itch on the itchy side. However, reference data set comparisons revealed bilateral hyposensitivity to innocuous cold and noxious pinprick and higher temporal summation of pain in patients with NP. In addition, compared with reference data, patients with NP demonstrated decreased sensitivity to cold and pinprick, presence of paradoxical heat sensations, and increased wind-up of pain. These results suggest a role for Aδ fiber pathways and central sensitization in NP-associated itch. More research is needed to determine whether sensory differences extend beyond the NP-affected dermatomal level and what might cause neuropathy specifically targeting Aδ fibers.

Role of Piezo2 in Schwann Cell Volume Regulation and Its Impact on Neurotrophic Release Regulation.

Tactile perception relies on mechanoreceptors and nerve fibers, including c-fibers, Aβ-fibers and Aδ-fibers. Schwann cells (SCs) play a crucial role in supporting nerve fibers, with non-myelinating SCs enwrapping c-fibers and myelinating SCs ensheathing Aβ and Aδ fibers. Recent research has unveiled new functions for cutaneous sensory SCs, highlighting the involvement of nociceptive SCs in pain perception and Meissner corpuscle SCs in tactile sensation. Furthermore, Piezo2, previously associated with Merkel cell tactile sensitivity, has been identified in SCs. The goal of this study was to investigate the channels implicated in SC mechanosensitivity and the release process of neurotrophic factor secretion. Immortalized IFRS1 SCs and human primary SCs generated two distinct subtypes of SCs: undifferentiated and differentiated SCs. Quantitative PCR was employed to evaluate the expression of differentiation markers and mechanosensitive channels, including TRP channels (TRPV4, TRPM7 and TRPA1) and Piezo channels (Piezo1 and Piezo2). To validate the functionality of specific mechanosensitive channels, Ca2+ imaging and electronic cell sizing experiments were conducted under hypotonic conditions, and inhibitors and siRNAs were used. Protein expression was assessed by Western blotting and immunostaining. Additionally, secretome analysis was performed to evaluate the release of neurotrophic factors in response to hypotonic stimulation, with BDNF, a representative trophic factor, quantified using ELISA. Induction of differentiation increased Piezo2 mRNA expression levels both in IFRS1 and in human primary SCs. Both cell types were responsive to hypotonic solutions, with differentiated SCs displaying a more pronounced response. Gd3+ and FM1-43 effectively inhibited hypotonicity-induced Ca2+ transients in differentiated SCs, implicating Piezo2 channels. Conversely, inhibitors of Piezo1 and TRPM7 (Dooku1 and NS8593, respectively) had no discernible impact. Moreover, Piezo2 in differentiated SCs appeared to participate in regulatory volume decreases (RVD) after cell swelling induced by hypotonic stimulation. A Piezo2 deficiency correlated with reduced RVD and prolonged cell swelling, leading to heightened release of the neurotrophic factor BDNF by upregulating the function of endogenously expressed Ca2+-permeable TRPV4. Our study unveils the mechanosensitivity of SCs and implicates Piezo2 channels in the release of neurotrophic factors from SCs. These results suggest that Piezo2 may contribute to RVD, thereby maintaining cellular homeostasis, and may also serve as a negative regulator of neurotrophic factor release. These findings underscore the need for further investigation into the role of Piezo2 in SC function and neurotrophic regulation.

(087) SMALL FIBER NEUROPATHY AND FEMALE SEXUAL FUNCTION

Abstract Introduction Small fiber neuropathy (SFN) can result in a variety of pain and autonomic symptoms due to its impact on small-diameter somatic and autonomic unmyelinated C-fibers and/or thinly myelinated A-delta (Aδ) fibers. It has been identified in 66% of patients with complex (refractory or multisystem) pelvic pain. Objective The objective of this study was to characterize the differences between sexual symptoms in patients with and without SFN. Methods This was a retrospective case control study involving patients seen at a subspecialty Urogynecology and Reconstructive Surgery clinic between January 2023 and October 2023. All patients presenting for evaluation were offered an electronic pre-visit intake form inclusive of the PISQ-IR (evaluation tool validated by the IUGA for sexual dysfunction symptoms). The primary outcome measured was self-reported quality of sexual function, represented by PISQ-IR score. Secondary outcomes included comparisons of survey scores for arousal, orgasm, and dyspareunia between SFN and non-SFN patients. Results A total of 89 patients were analyzed, 11 with definitive SFN and 78 without SFN. Results were compared between these two groups using chi-square tests and Mann-Whitney U tests. PISQ-IR scores were significantly different between SFN and non-SFN patients. Mean scores of 2.9 (SD 0.6) and 3.5 (SD 0.6) were calculated, respectively, indicating worse overall sexual function in SFN patients (p = 0.01). Secondary outcomes analyzed were significant for lower orgasm intensity scores in SFN patients (median of 2 vs 3 in non-SFN patients, p = 0.02 Mann-Whitney test). Arousal scores and pain scores were not significantly different between the two patient populations. Conclusions SFN is associated with worse sexual function and decreased orgasm intensity in the subspecialty Urogynecology and Reconstructive Surgery population. Disclosure No.

Mapping Somatosensory Afferent Circuitry to Bone Identifies Neurotrophic Signals Required for Fracture Healing.

The profound pain accompanying bone fracture is mediated by somatosensory neurons, which also appear to be required to initiate bone regeneration following fracture. Surprisingly, the precise neuroanatomical circuitry mediating skeletal nociception and regeneration remains incompletely understood. Here, we characterized somatosensory dorsal root ganglia (DRG) afferent neurons innervating murine long bones before and after experimental long bone fracture in mice. Retrograde labeling of DRG neurons by an adeno-associated virus with peripheral nerve tropism showed AAV-tdT signal. Single cell transcriptomic profiling of 6,648 DRG neurons showed highest labeling across CGRP+ neuron clusters (6.9-17.2%) belonging to unmyelinated C fibers, thinly myelinated Aδ fibers and Aβ-Field LTMR (9.2%). Gene expression profiles of retrograde labeled DRG neurons over multiple timepoints following experimental stress fracture revealed dynamic changes in gene expression corresponding to the acute inflammatory ( S100a8 , S100a9 ) and mechanical force ( Piezo2 ). Reparative phase after fracture included morphogens such as Tgfb1, Fgf9 and Fgf18 . Two methods to surgically or genetically denervate fractured bones were used in combination with scRNA-seq to implicate defective mesenchymal cell proliferation and osteodifferentiation as underlying the poor bone repair capacity in the presence of attenuated innervation. Finally, multi-tissue scRNA-seq and interactome analyses implicated neuron-derived FGF9 as a potent regulator of fracture repair, a finding compatible with in vitro assessments of neuron-to-skeletal mesenchyme interactions.

Eliciting the rubber hand illusion by the activation of nociceptive C and Aδ fibers.

The coherent perceptual experience of one's own body depends on the processing and integration of signals from multiple sensory modalities, including vision, touch, and proprioception. Although nociception provides critical information about damage to the tissues of one's body, little is known about how nociception contributes to own-body perception. A classic experimental approach to investigate the perceptual and neural mechanisms involved in the multisensory experience of one's own body is the rubber hand illusion (RHI). During the RHI, people experience a rubber hand as part of their own body (sense of body ownership) caused by synchronized stroking of the rubber hand in the participant's view and the hidden participant's real hand. We examined whether the RHI can be elicited by visual and "pure" nociceptive stimulation, ie, without tactile costimulation, and if so, whether it follows the basic perceptual rules of the illusion. In 6 separate experiments involving a total of 180 healthy participants, we used a Nd:YAP laser stimulator to specifically target C and Aδ fibers in the skin and compared the illusion condition (congruent visuonociceptive stimulation) to control conditions of incongruent visuonociceptive, incongruent visuoproprioceptive, and no nociceptive stimulation. The illusion was quantified through direct (questionnaire) and indirect (proprioceptive drift) behavioral measures. We found that a nociceptive rubber hand illusion (N-RHI) could be elicited and that depended on the spatiotemporal congruence of visuonociceptive signals, consistent with basic principles of multisensory integration. Our results suggest that nociceptive information shapes multisensory bodily awareness and contributes to the sense of body ownership.

Cutaneous silent periods demonstrate involvement of A-delta fibres in Charcot-Marie-Tooth type 1 disease – A case report

Cutaneous silent periods demonstrate involvement of A-delta fibres in Charcot-Marie-Tooth type 1 disease – A case report

Hypersensitivity of myelinated A-fibers via toll-like receptor 5 promotes mechanical allodynia in tenascin-X-deficient mice associated with Ehlers–Danlos syndrome

Deficiency of an extracellular matrix glycoprotein tenascin-X (TNX) leads to a human heritable disorder Ehlers–Danlos syndrome, and TNX-deficient patients complain of chronic joint pain, myalgia, paresthesia, and axonal polyneuropathy. We previously reported that TNX-deficient (Tnxb−/−) mice exhibit mechanical allodynia and hypersensitivity to myelinated A-fibers. Here, we investigated the pain response of Tnxb−/− mice using pharmacological silencing of A-fibers with co-injection of N-(2,6-Dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX-314), a membrane-impermeable lidocaine analog, plus flagellin, a toll-like receptor 5 (TLR5) ligand. Intraplantar co-injection of QX-314 and flagellin significantly increased the paw withdrawal threshold to transcutaneous sine wave stimuli at frequencies of 250 Hz (Aδ fiber responses) and 2000 Hz (Aβ fiber responses), but not 5 Hz (C fiber responses) in wild-type mice. The QX-314 plus flagellin-induced silencing of Aδ- and Aβ-fibers was also observed in Tnxb−/− mice. Co-injection of QX-314 and flagellin significantly inhibited the mechanical allodynia and neuronal activation of the spinal dorsal horn in Tnxb−/− mice. Interestingly, QX-314 alone inhibited the mechanical allodynia in Tnxb−/− mice, and it increased the paw withdrawal threshold to stimuli at frequencies of 250 Hz and 2000 Hz in Tnxb−/− mice, but not in wild-type mice. The inhibition of mechanical allodynia induced by QX-314 alone was blocked by intraplantar injection of a TLR5 antagonist TH1020 in Tnxb−/− mice. These results suggest that mechanical allodynia due to TNX deficiency is caused by the hypersensitivity of Aδ- and Aβ-fibers, and it is induced by constitutive activation of TLR5.

The stability of perception threshold tracking for long session evaluation of Aβ- and Aδ-fiber function.

Research has proven that epidermal and transcutaneous stimulation can identify the function of Aβ and Aδ fibers (i.e., in diabetes) individually using different electrodes. In this study we aimed to determine the stability of perception thresholds when using such electrodes. Twenty healthy volunteers participated in this study. The perception threshold of Aβ fibers (patch electrode) and Aδ fibers (pin electrode) was estimated 30 times during a period of 60 minutes. A threshold was established every other minute, alternating between the two electrodes. The stimulus duration was 1 millisecond and the interstimulus interval was 1.5 to 2.5 seconds. Linear regressions of the perception threshold as a function of time were performed. The slopes were used as an estimate of habituation and were compared between the electrodes. The slope was significantly larger when assessed by the pin electrode (median: 0.020 [0.009 to 0.030] mA/trial) than when assessed by the patch electrode (median: 0.005 [0.001 to 0.018] mA/trial) (P = .017, paired t test). During the session, total increases in perception threshold of approximately 55% and 1% were seen for the pin and patch electrodes, respectively. The two fiber types assessed showed significant perception threshold increases. The higher slope of the pin electrode indicated that the Aδ fibers were more prone to habituation than the Aβ fibers, and that habituation should be considered during prolonged experiments. This assessment is valuable for future research on nerve fiber function using the technique for long session experiments.

Differences in the Ultrastructure of Neurons in the Spinal Ganglion and Dorsal Rootlet Between Rats Treated with Cisplatin Only versus Co-administration with a Sphingosine 1-Phosphate Receptor 2 Agonist in Attenuating Neuropathy and Allodynia.

Cisplatin is a chemotherapeutic agent for many types of cancer. The neurotoxicity of cisplatin includes neuropathy and allodynia. We aimed to study structural changes by using CYM54-78, attenuates cisplatin-induced neuropathy and blocks the pathogenesis in neurons and promotes axonal regeneration. TEM (Transmission electron microscopy) was used to distinguish ultrastructural changes in dorsal root ganglion (DRG) and dorsal rootlets (DR) between rats treated with cisplatin alone and rats co-treated with cisplatin and sphingosine -1-phosphate receptor2 (S1P2) agonist, CYM-5478. In DRG of rats treated with cisplatin alone, TEM micrographs showed necrosis and apoptotic cells. Neuronal cytoplasm showed numerous vacuole (stage C) and swelling (stage B➔C) mitochondrial degeneration. Neurons in DRG from cisplatin+CYM-5478 group showed a higher percentage of healthy mitochondria (from 5.3% to 75.6%) than those treated with cisplatin alone. DR of cisplatin only group showed abnormal axoplasm, axolemma and focal detached myelin sheaths, especially in Aδ (fast pain) and Aβ (touch) fibers and revealed collateral branches that sprouted from Aβ fibers, which is characteristic of allodynia. Moreover, vasoconstriction was observed in DRG and DR. Rats in cisplatin+CYM-5478 group showed not only fewer abnormal structures than those in cisplatin only group, but also showed Bands of Büngner and onion bulb-like structures, which are characteristic of nerve regeneration. Together with our previous study, showed that CYM-5478 attenuated neuropathy and allodynia in a rat model of cisplatin-induced neuropathy, these results suggest S1P2 agonists as a potential approach for treatment of cancer due to reduction of side-effects of cisplatin. This article is protected by copyright. All rights reserved.

Members of the CUGBP Elav-like family of RNA-binding proteins are expressed in distinct populations of primary sensory neurons.

Primary sensory dorsal root ganglia (DRG) neurons are diverse, with distinct populations that respond to specific stimuli. Previously, we observed that functionally distinct populations of DRG neurons express mRNA transcript variants with different 3' untranslated regions (3'UTRs). 3'UTRs harbor binding sites for interaction with RNA-binding proteins (RBPs) for transporting mRNAs to subcellular domains, modulating transcript stability, and regulating the rate of translation. In the current study, analysis of publicly available single-cell RNA-sequencing data generated from adult mice revealed that 17 3'UTR-binding RBPs were enriched in specific populations of DRG neurons. This included four members of the CUG triplet repeat (CUGBP) Elav-like family (CELF): CELF2 and CELF4 were enriched in peptidergic, CELF6 in both peptidergic and nonpeptidergic, and CELF3 in tyrosine hydroxylase-expressing neurons. Immunofluorescence studies confirmed that 60% of CELF4+ neurons are small-diameter C fibers and 33% medium-diameter myelinated (likely Aδ) fibers and showed that CELF4 is distributed to peripheral termini. Coexpression analyses using transcriptomic data and immunofluorescence revealed that CELF4 is enriched in nociceptive neurons that express GFRA3, CGRP, and the capsaicin receptor TRPV1. Reanalysis of published transcriptomic data from macaque DRG revealed a highly similar distribution of CELF members, and reanalysis of single-nucleus RNA-sequencing data derived from mouse and rat DRG after sciatic injury revealed differential expression of CELFs in specific populations of sensory neurons. We propose that CELF RBPs may regulate the fate of mRNAs in populations of nociceptors, and may play a role in pain and/or neuronal regeneration following nerve injury.

Nociceptive flexion reflex in small fibers neuropathy and pain assessments†.

The nociceptive flexion reflex (NFR) is a polysynaptic and multisegmental spinal reflex that develops in response to a noxious stimulus and is characterized by the withdrawal of the affected body part. The NFR possesses two excitatory components: early RII and late RIII. Late RIII is derived from high-threshold cutaneous afferent A-delta fibers, which are prone to injury early in the course of diabetes mellitus (DM) and may lead to neuropathic pain. We investigated NFR in patients with DM with different types of polyneuropathies to analyze the role of NFR in small fiber neuropathy (SFN). We included 37 patients with DM and 20 healthy participants of similar age and sex. We performed the Composite Autonomic Neuropathy Scale-31, modified Toronto Neuropathy Scale, and routine nerve conduction studies. We grouped the patients into large fiber neuropathy (LFN), SFN, and no overt neurological symptom/sign groups. In all participants, NFR was recorded on anterior tibial (AT) and biceps femoris (BF) muscles after train stimuli on the sole of the foot, and NFR-RIII findings were compared. We identified 11 patients with LFN, 15 with SFN, and 11 with no overt neurological symptoms or signs. The RIII response on the AT was absent in 22 (60%) patients with DM and 8 (40%) healthy participants. The RIII response on the BF was absent in 31 (73.8%) patients and 7 (35%) healthy participants (P = .001). In DM, the latency of RIII was prolonged, and the magnitude was reduced. Abnormal findings were seen in all subgroups; however, they were more prominent in patients with LFN compared to other groups. The NFR-RIII was abnormal in patients with DM even before the emergence of the neuropathic symptoms. The pattern of involvement before neuropathic symptoms was possibly related to an earlier loss of A-delta fibers.

Cold-evoked potentials in clinical practice: A head-to-head contrast with laser-evoked responses.

Innocuous cooling of the skin activates cold-specific Aδ fibres, and hence, the recording of cold-evoked potentials (CEPs) may improve the objective assessment of human thermo-nociceptive function. While the feasibility of CEP recordings in healthy humans has been reported, their reliability and diagnostic use in clinical conditions have not been documented. Here, we report the results of CEP recordings in 60 consecutive patients with suspected neuropathic pain, compared with laser-evoked potentials (LEPs) which are the gold standard for thermo-algesic instrumental assessment. CEP recording was a well-tolerated procedure, with only ~15 min of surplus in exam duration. The reproducibility and signal-to-noise ratio of CEPs were lower than those of LEPs, in particular for distal lower limbs (LLs). While laser responses were interpretable in all patients, CEPs interpretation was inconclusive in 5/60 because of artefacts or lack of response on the unaffected side. Both techniques yielded concordant results in 73% of the patients. In 12 patients, CEPs yielded abnormal values while LEPs remained within normal limits; 3 of these patients had clinical symptoms limited to cold sensations, including cold-heat transformation. CEPs appear as a useful technique for exploring pain/temperature systems. Advantages are low cost of equipment and innocuity. Disadvantages are low signal-to-noise ratio for LL stimulation, and sensitivity to fatigue/habituation. Joint recording of CEPs and LEPs can increase the sensitivity of neurophysiological techniques to thin fibre- spinothalamic lesions, in particular, when abnormalities of cold perception predominate. Recording of cold-evoked potentials is a well-tolerated, inexpensive and easy-to-use procedure that can be helpful in the diagnosis of abnormalities in the thin fibre- spinothalamic pathways. Supplementing LEPs with CEPs allows consolidating the diagnosis and, for some patients suffering from symptoms limited only to cold, CEPs but not LEPs may allow the diagnosis of thin fibre pathology. Optimal CEP recording conditions are important to overcome the low signal-to-noise ratio and habituation phenomena, which are less favourable than with LEPs.

Perceptual simultaneity between nociceptive and visual stimuli depends on their spatial congruence.

To protect our body against physical threats, it is important to integrate the somatic and extra-somatic inputs generated by these stimuli. Temporal synchrony is an important parameter determining multisensory interaction, and the time taken by a given sensory input to reach the brain depends on the length and conduction velocity of the specific pathways through which it is transmitted. Nociceptive inputs are transmitted through very slow conducting unmyelinated C and thinly myelinated Aδ nociceptive fibers. It was previously shown that to perceive a visual stimulus and a thermo-nociceptive stimulus applied on the hand as coinciding in time, the nociceptive stimulus must precede the visual one by 76ms for nociceptive inputs conveyed by Aδ fibers and 577ms for inputs conveyed by C fibers. Since spatial proximity is also hypothesized to contribute to multisensory interaction, the present study investigated the effect of spatial congruence between visual and nociceptive stimuli. Participants judged the temporal order of visual and nociceptive stimuli, with the visual stimuli flashed either next to the stimulated hand or next to the opposite unstimulated hand, and with nociceptive stimuli evoking responses mediated by either Aδ or C fibers. The amount of time by which the nociceptive stimulus had to precede the visual stimulus for them to be perceived as appearing concomitantly was smaller when the visual stimulus occurred near the hand receiving the nociceptive stimulus as compared to when it occurred near the contralateral hand. This illustrates the challenge for the brain to process the synchrony between nociceptive and non-nociceptive stimuli to enable their efficient interaction to optimize defensive reaction against physical dangers.

Sensory innervation of the lumbar 5/6 intervertebral disk in mice.

Over the years, most back pain-related biological studies focused on the pathogenesis of disk degeneration. It is known that nerve distributions at the outer layer of the annulus fibrosus (AF) may be an important contributor to back pain symptoms. However, the types and origins of sensory nerve terminals in the mouse lumbar disks have not been widely studied. Using disk microinjection and nerve retrograde tracing methods, the current study aimed to characterize the nerve types and neuropathway of the lumbar 5/6 (L5/6) disk in mice. Using an anterior peritoneal approach, the L5/6 disk of adult C57BL/6 mice (males, 8-12 weeks) disk microinjection was performed. Fluorogold (FG) was injected into the L5/6 disk using the Hamilton syringe with a homemade glass needle driven by a pressure microinjector. The lumbar spine and bilateral thoracic 13 (Th13) to L6 DRGs were harvested at 10 days after injection. The number of FG+ neurons among different levels was counted and analyzed. Different nerve markers, including anti-neurofilament 160/200 (NF160/200), anti-calcitonin gene-related peptide (CGRP), anti-parvalbumin (PV), and anti-tyrosine hydroxylase (TH), were used to identify different types of nerve terminals in AF and their origins in DRG neurons. There were at least three types of nerve terminals at the outer layer of L5/6 AF in mice, including NF160/200+ (indicating Aβ fibers), CGRP+ (Aδ and C fibers), and PV+ (proprioceptive fibers). No TH+ fibers (sympathetic nerve fibers and some C-low threshold mechanoreceptors) were noticed in either. Using retrograde tracing methods, we found that nerve terminals in the L5/6 disk were multi-segmentally from Th13-L6 DRGs, with L1 and L5 predominately. An immunofluorescence analysis revealed that FG+ neurons in DRGs were co-localized with NF160/200, CGRP, and PV, but not TH. Intervertebral disks were innervated by multiple types of nerve fibers in mice, including Aβ, Aδ, C, and proprioceptive fibers. No sympathetic nerve fibers were found in AF. The nerve network of the L5/6 disk in mice was multi-segmentally innervated by the Th13-L6 DRGs (mainly L1 and L5 DRGs). Our results may serve as a reference for preclinical studies of discogenic pain in mice.

Effects of remifentanil on the noxiously stimulated somatosensory evoked potentials recorded at the spinal cord in dogs and cats

This study assessed the somatosensory evoked potentials (SEPs) in dogs and cats to compare the effect of remifentanil on the action potentials evoked by peripheral noxious stimulation in the spinal cord. Five healthy dogs and five healthy cats underwent general anaesthesia induced with propofol and maintained with isoflurane. Each animals received all dosage of a constant-rate infusion of remifentanil at 0 (control), 0.25, 0.5, 1.0 or 2.0 μg/kg/min. The hair of the dorsal foot of a hind limb was clipped and an intraepidermal stimulation electrode that could selectively stimulate the nociceptive Aδ and C fibres was attached. An electrical stimulus was generated by a portable peripheral nerve testing device. The evoked potentials were recorded by two needle electrodes inserted subcutaneously in the dorsal midline between the lumbar vertebra: L3-L4 and L4-L5. Bimodal waveforms were obtained by electrical stimulation in control dogs and cats. The inhibitory effect of remifentanil was evaluated by comparing the changes in the N1P2 and P2N2 amplitudes. The N1P2 amplitude was depressed by remifentanil in a dose-dependent manner in dogs, but it showed no remifentanil-induced changes in cats. While the P2N2 amplitude was also depressed in a dose-dependent manner in dogs, it showed milder remifentanil-induced effects in cats. The N1P2 and P2N2 amplitudes observed herein are assumed to represent the evoked potentials derived from the Aδ and C fibres, respectively. Thus, the inhibitory effect of remifentanil on nociceptive transmission at the spinal cord was much weaker in cats, especially for transmissions possibly derived from Aδ fibres.

Molecular taxonomy of nociceptors and pruriceptors.

Molecular taxonomy of nociceptors and pruriceptors.