Patterns of activity-dependent conduction velocity changes differentiate classes of unmyelinated mechano-insensitive afferents including cold nociceptors, in pig and in human

Human pain ratings to electrical sinusoids increase with cooling through a cold-induced increase in C-fibre excitability

ABSTRACT

Low-frequency sinusoidal current applied to human skin evokes local axon reflex flare and burning pain, indicative of C-fibre activation. Because topical cooling works well as a local analgesic, we examined the effect of cooling on human pain ratings to sinusoidal and rectangular profiles of constant current stimulation. Unexpectedly, pain ratings increased upon cooling the skin from 32 to 18°C. To explore this paradoxical observation, the effects of cooling on C-fibre responses to stimulation with sinusoidal and rectangular current profiles were determined in ex vivo segments of mouse sural and pig saphenous nerve. As expected by thermodynamics, the absolute value of electrical charge required to activate C-fibre axons increased with cooling from 32°C to 20°C, irrespective of the stimulus profile used. However, for sinusoidal stimulus profiles, cooling enabled a more effective integration of low-intensity currents over tens of milliseconds resulting in a delayed initiation of action potentials. Our findings indicate that the paradoxical cooling-induced enhancement of electrically evoked pain in people can be explained by an enhancement of C-fibre responsiveness to slow depolarization at lower temperatures. This property may contribute to symptoms of enhanced cold sensitivity, especially cold allodynia, associated with many forms of neuropathic pain.

Optimized Electrical Stimulation of C-Nociceptors in Humans Based on the Chronaxie of Porcine C-Fibers

Classically, to electrically excite C-nociceptors, rectangular pulses are used with a duration close to the estimated chronaxie of C-fibres (about 2 ms). Recent results using slow depolarizing stimuli suggest longer chronaxies. We therefore set out to optimize C-fiber stimulation based on recordings of single C-nociceptors in-vivo and C-fiber compound-action-potentials (C-CAP) ex-vivo using half-sine shaped stimuli of durations between 1 and 250ms. Single fiber (n = 45) recording in pigs revealed high chronaxie values for C-touch fibers (15.8 ms), polymodal- (14.2 ms) and silent-nociceptors (16.8 ms). Activation thresholds decreased 2 to 3-fold in all fibre classes when increasing the duration of half-sine pulses from 1 to 25 ms (P < .05). C-CAPs strength-duration curves of the pig saphenous nerve (n = 7) showed the highest sensitivity for half-sine durations between 10 and 25 ms. Half-maximum currents for C-CAPS were reduced 3-fold compared to rectangular pulses (P < .01) whereas the opposite was found for A-fiber compound action potentials. Psychophysics in humans (n = 23) revealed that half-sine stimulus durations >10 ms reduced detection thresholds, pain thresholds, and stimulus current amplitudes required to generate a pain rating of 3 on an 11-point Numeric Rating Scale (NRS) as compared to 1 ms rectangular pulses (P < 0.05). Increasing the duration from 1 to 25 ms led to a 4-fold amplitude reduction for pain-thresholds and stimuli caused an axon-reflex flare. Excitability of single polymodal nociceptors in animals paralleled human psychophysics and we conclude optimized half-sine pulses facilitate C-nociceptor activation. PERSPECTIVE: Electrical stimulation with longer lasting half-sine wave pulses preferentially activates C-nociceptors and changes in the strength duration curve may identify nociceptor hyperexcitability in patients with neuropathic pain.

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

The cutaneous mechanisms that trigger spontaneous neuropathic pain in diabetic peripheral neuropathy (PDPN) are far from clear. Two types of nociceptors are found within the epidermal and dermal skin layers. Small-diameter lightly myelinated Aδ and unmyelinated C cutaneous mechano and heat-sensitive (AMH and CMH) and C mechanoinsensitive (CMi) nociceptors transmit pain from the periphery to central nervous system. AMH and CMH fibers are mainly located in the epidermis, and CMi fibers are distributed in the dermis. In DPN, dying back intra-epidermal AMH and CMH fibers leads to reduced pain sensitivity, and the patients exhibit significantly increased pain thresholds to acute pain when tested using traditional methods. The role of CMi fibers in painful neuropathies has not been fully explored. Microneurography has been the only tool to access CMi fibers and differentiate AMH, CMH, and CMi fiber types. Due to the complexity, its use is impractical in clinical settings. In contrast, a newly developed diode laser fiber selective stimulation (DLss) technique allows to safely and selectively stimulate Aδ and C fibers in the superficial and deep skin layers. DLss data demonstrate that patients with painful DPN have increased Aδ fiber pain thresholds, while C-fiber thresholds are intact because, in these patients, CMi fibers are abnormally spontaneously active. It is also possible to determine the involvement of CMi fibers by measuring the area of DLss-induced neurogenic axon reflex flare. The differences in AMH, CMH, and CMi fibers identify patients with painful and painless neuropathy. In this review, we will discuss the role of CMi fibers in PDPN.

Multisite silicon probes enable simultaneous recording of spontaneous and evoked activity in multiple isolated C-fibres in rat saphenous nerve

BACKGROUND

Recordings of electrical activity in nerves have provided valuable insights into normal function and pathological behaviours of the nervous system. Current high-resolution techniques (e.g. teased fibre recordings) typically utilise electrodes with a single recording site, capturing the activity of a single isolated neuron per recording.

NEW METHOD

We conducted proof-of-principle C-fibre recordings in the saphenous nerve of urethane-anaesthetised adult Wistar rats using 32-channel multisite silicon electrodes. Data was acquired using the OpenEphys recording system and clustered offline with Kilosort 2.5.

RESULTS

In single recordings in 5 rats, 32 units with conduction velocities in the C-fibre range (< 1 m/s) were identified via constant latency responses and classified using activity dependent slowing. In two animals, 6 C-fibres (5 classified as nociceptors) were well isolated after clustering. Their activity could be tracked throughout the recording - including during periods of spontaneous activity. Axonal conduction velocities were calculated from spontaneous activity and/or low frequency electrical stimulation using only the differences in action potential latency as it propagated past multiple probe sites.

COMPARISON WITH EXISTING METHODS

Single electrode approaches have a low data yield and generating group data for specific fibre types is challenging as it requires multiple experimental subjects and recording sessions. This is particularly true when the experimental targets are the small, unmyelinated C-fibres carrying nociceptive information.

CONCLUSIONS

We demonstrate that multisite recordings can greatly increase experimental yields and enhance fibre identification. The approach is of particular utility when coupled with clustering analysis. Multisite probes and analysis approaches constitute a valuable new toolbox for researchers studying the peripheral nervous system.

Mechanical sensitization, increased axonal excitability, and spontaneous activity in C-nociceptors after ultraviolet B irradiation in pig skin

ABSTRACT

Ultraviolet B (UVB) irradiation induces hyperalgesia in human and animal pain models. We investigated mechanical sensitization, increase in axonal excitability, and spontaneous activity in different C-nociceptor classes after UVB in pig skin. We focused on units with receptive fields covering both irradiated and nonirradiated skin allowing intraindividual comparisons. Thirty-five pigs were irradiated in a chessboard pattern, and extracellular single-fibre recordings were obtained 10 to 28 hours later (152 fibers). Units from the contralateral hind limb served as a control (n = 112). Irradiated and nonirradiated parts of the same innervation territory were compared in 36 neurons; low threshold C-touch fibers (n = 10) and sympathetic efferents (n = 2) were unchanged, but lower mechanical thresholds and higher discharge frequency at threshold were found in mechanosensitive nociceptors (n = 12). Half of them could be activated with nonnoxious brush stimuli in the sunburn. Four of 12 mechanoinsensitive nociceptors were found sensitized to mechanical stimulation in the irradiated part of the receptive field. Activity-dependent slowing of conduction was reduced in the irradiated and in the nonirradiated skin as compared with the control leg, whereas increased ability to follow high stimulation frequencies was restricted to the sunburn (108.5 ± 37 Hz UVB vs 6.3 ± 1 Hz control). Spontaneous activity was more frequent in the sunburn (72/152 vs 31/112). Mechanical sensitization of primary nociceptors and higher maximum after frequency are suggested to contribute to primary hyperalgesia, whereas the spontaneous activity of silent nociceptors might offer a mechanistic link contributing to ongoing pain and facilitated induction of spinal sensitization.

Lysophosphatidic acid activates nociceptors and causes pain or itch depending on the application mode in human skin

ABSTRACT

Lysophosphatidic acid (LPA) is involved in the pathophysiology of cholestatic pruritus and neuropathic pain. Slowly conducting peripheral afferent C-nerve fibers are crucial in the sensations of itch and pain. In animal studies, specialized neurons ("pruriceptors") have been described, expressing specific receptors e.g. from the Mrgpr family. Human nerve fibers involved in pain signaling ("nociceptors") can elicit itch if activated by focalized stimuli such as cowhage spicules.In this study, we scrutinized the effects of LPA in humans by two different application modes on the level of psychophysics and single nerve fiber recordings (microneurography). In healthy human subjects, intracutaneous LPA microinjections elicited burning pain, whereas LPA application via inactivated cowhage spicules evoked a moderate itch sensation. LPA microinjections induced heat hyperalgesia and hypersensitivity to higher electrical stimulus frequencies. Pharmacological blockade of TRPA1 or TRPV1 reduced heat hyperalgesia but not acute chemical pain. Microneurography revealed an application mode-dependent differential activation of mechano-sensitive (CM) and mechano-insensitive (CMi) C-fibers. LPA microinjections activated a greater proportion of CMi and more strongly than CM fibers; spicule-application of LPA activated CM and CMi fibers to a similar extent but excited CM more and CMi fibers less intensely than microinjections.In conclusion, we show for the first time in humans that LPA can cause pain as well as itch dependent on the mode of application and activates afferent human C-fibers. Itch may arise from focal activation of few nerve fibers with distinct spatial contrast to unexcited surrounding afferents, and a specific combination of activated fiber subclasses might contribute.

Studying human nociceptors: from fundamentals to clinic

Chronic pain affects one in five of the general population and is the third most important cause of disability-adjusted life-years globally. Unfortunately, treatment remains inadequate due to poor efficacy and tolerability. There has been a failure in translating promising preclinical drug targets into clinic use. This reflects challenges across the whole drug development pathway, from preclinical models to trial design. Nociceptors remain an attractive therapeutic target: their sensitization makes an important contribution to many chronic pain states, they are located outside the blood-brain barrier, and they are relatively specific. The past decade has seen significant advances in the techniques available to study human nociceptors, including: the use of corneal confocal microscopy and biopsy samples to observe nociceptor morphology, the culture of human nociceptors (either from surgical or post-mortem tissue or using human induced pluripotent stem cell derived nociceptors), the application of high throughput technologies such as transcriptomics, the in vitro and in vivo electrophysiological characterization through microneurography, and the correlation with pain percepts provided by quantitative sensory testing. Genome editing in human induced pluripotent stem cell-derived nociceptors enables the interrogation of the causal role of genes in the regulation of nociceptor function. Both human and rodent nociceptors are more heterogeneous at a molecular level than previously appreciated, and while we find that there are broad similarities between human and rodent nociceptors there are also important differences involving ion channel function, expression, and cellular excitability. These technological advances have emphasized the maladaptive plastic changes occurring in human nociceptors following injury that contribute to chronic pain. Studying human nociceptors has revealed new therapeutic targets for the suppression of chronic pain and enhanced repair. Cellular models of human nociceptors have enabled the screening of small molecule and gene therapy approaches on nociceptor function, and in some cases have enabled correlation with clinical outcomes. Undoubtedly, challenges remain. Many of these techniques are difficult to implement at scale, current induced pluripotent stem cell differentiation protocols do not generate the full diversity of nociceptor populations, and we still have a relatively poor understanding of inter-individual variation in nociceptors due to factors such as age, sex, or ethnicity. We hope our ability to directly investigate human nociceptors will not only aid our understanding of the fundamental neurobiology underlying acute and chronic pain but also help bridge the translational gap.

Methylglyoxal causes pain and hyperalgesia in human through C-fiber activation

The endogenous metabolite methylglyoxal (MG) accumulates in diabetic patients with neuropathic pain. Methylglyoxal could be a mediator of diabetes-induced neuropathic pain through TRPA1 activation and sensitization of the voltage-gated sodium channel subtype 1.8. In this study, we tested the algogenic and sensitizing effect of MG in healthy human subjects using intracutaneous microinjections. The involvement of C fibers was assessed through selective A-fiber nerve block, axon-reflex-erythema, and through single nerve fiber recordings in humans (microneurography). Involvement of the transduction channels TRPA1 and TRPV1 in MG-induced pain sensation was investigated with specific ion channel blockers. We showed for the first time in healthy humans that MG induces pain, axon-reflex-erythema, and long-lasting hyperalgesia through the activation of C nociceptors. Predominantly, the subclass of mechano-insensitive C fibers is activated by MG. A fibers contribute only negligibly to the burning pain sensation. Selective pharmacological blockade of TRPA1 or TRPV1 showed that TRPA1 is crucially involved in MG-induced chemical pain sensation and heat hyperalgesia. In conclusion, the actions of MG through TRPA1 activation on predominantly mechano-insensitive C fibers might be involved in spontaneously perceived pain in diabetic neuropathy and hyperalgesia as well as allodynia.

In vivo quantification of excitation and kilohertz frequency block of the rat vagus nerve

OBJECTIVE

There is growing interest in treating diseases by electrical stimulation and block of peripheral autonomic nerves, but a paucity of studies on the excitation and block of small-diameter autonomic axons. We conducted in vivo quantification of the strength-duration properties, activity-dependent slowing (ADS), and responses to kilohertz frequency (KHF) signals for the rat vagus nerve (VN).

APPROACH

We conducted acute in vivo experiments in urethane-anaesthetized rats. We placed two cuff electrodes on the left cervical VN and one cuff electrode on the anterior subdiaphragmatic VN. The rostral cervical cuff was used to deliver pulses to quantify recruitment and ADS. The caudal cervical cuff was used to deliver KHF signals. The subdiaphragmatic cuff was used to record compound action potentials (CAPs).

MAIN RESULTS

We quantified the input-output recruitment and strength-duration curves. Fits to the data using standard strength-duration equations were qualitatively similar, but the resulting chronaxie and rheobase estimates varied substantially. We measured larger thresholds for the slowest fibres (0.5-1 m s^-1), especially at shorter pulse widths. Using a novel cross-correlation CAP-based analysis, we measured ADS of ~2.3% after 3 min of 2 Hz stimulation, which is comparable to the ADS reported for sympathetic efferents in somatic nerves, but much smaller than the ADS in cutaneous nociceptors. We found greater ADS with higher stimulation frequency and non-monotonic changes in CV in select cases. We found monotonically increasing block thresholds across frequencies from 10 to 80 kHz for both fast and slow fibres. Further, following 25 s of KHF signal, neural conduction could require tens of seconds to recover.

SIGNIFICANCE

The quantification of mammalian autonomic nerve responses to conventional and KHF signals provides essential information for the development of peripheral nerve stimulation therapies and for understanding their mechanisms of action.

TTX-Resistant Sodium Channels Functionally Separate Silent From Polymodal C-nociceptors

Pronounced activity-dependent slowing of conduction has been used to characterize mechano-insensitive, “silent” nociceptors and might be due to high expression of Na_V1.8 and could, therefore, be characterized by their tetrodotoxin-resistance (TTX-r). Nociceptor-class specific differences in action potential characteristics were studied by: (i) in vitro calcium imaging in single porcine nerve growth factor (NGF)-responsive neurites; (ii) in vivo extracellular recordings in functionally identified porcine silent nociceptors; and (iii) in vitro patch-clamp recordings from murine silent nociceptors, genetically defined by nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) expression. Porcine TTX-r neurites (n = 26) in vitro had more than twice as high calcium transients per action potential as compared to TTX-s neurites (n = 18). In pig skin, silent nociceptors (n = 14) characterized by pronounced activity-dependent slowing of conduction were found to be TTX-r, whereas polymodal nociceptors were TTX-s (n = 12) and had only moderate slowing. Mechano-insensitive cold nociceptors were also TTX-r but showed less activity-dependent slowing than polymodal nociceptors. Action potentials in murine silent nociceptors differed from putative polymodal nociceptors by longer duration and higher peak amplitudes. Longer duration AP in silent murine nociceptors linked to increased sodium load would be compatible with a pronounced activity-dependent slowing in pig silent nociceptors and longer AP durations could be in line with increased calcium transients per action potential observed in vitro in TTX-resistant NGF responsive porcine neurites. Even though there is no direct link between slowing and TTX-resistant channels, the results indicate that axons of silent nociceptors not only differ in their receptive but also in their axonal properties.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

Local NGF and GDNF levels modulate morphology and function of porcine DRG neurites, In Vitro

Nerve terminals of primary sensory neurons are influenced by their environment through target derived trophic factors, like nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF). In mice, subpopulations of DRG neurons express receptors either for NGF or GDNF and therefore differentially respond to these neurotrophic factors. We probed neurite endings from porcine DRG neurons cultured in either NGF or GDNF and examined their shape, elongation and stimulus-evoked CGRP release. A compartmentalized culture system was employed allowing spatial separation of outgrown neurites from their somata and use of different growth factors in the compartments. We show that neurites of GDNF cultured somata extend into lateral compartments without added growth factor, unlike neurites of NGF cultured ones. Neurites of NGF cultured somata extend not only into NGF- but also into GDNF-containing compartments. GDNF at the site of terminals of NGF responsive somata led to a strong neurite arborization and formation of large growth cones, compared to neurites in medium with NGF. Functionally, we could detect evoked CGRP release from as few as 7 outgrown neurites per compartment and calculated release per mm neurite length. CGRP release was detected both in neurites from NGF and GDNF cultured somata, suggesting that also the latter ones are peptidergic in pig. When neurites of NGF cultured somata were grown in GDNF, capsaicin evoked a lower CGRP release than high potassium, compared to those grown in NGF. Our experiments demonstrate that the compartmented culture chamber can be a suitable model to assess neurite properties from trophic factor specific primary sensory neurons. With this model, insights into mechanisms of gain or loss of function of specific nociceptive neurites may be achieved.

Microneurography as a tool to study the function of individual C-fiber afferents in humans: responses from nociceptors, thermoreceptors, and mechanoreceptors

The technique of microneurography-recording neural traffic from nerves in awake humans-has provided us with unrivaled insights into afferent and efferent processes in the peripheral nervous system for over 50 years. We review the use of microneurography to study single C-fiber afferents and provide an overview of the knowledge gained, with views to future investigations. C-fibers have slowly conducting, thin-diameter, unmyelinated axons and make up the majority of the fibers in peripheral nerves (~80%). With the use of microneurography in humans, C-fiber afferents have been differentiated into discrete subclasses that encode specific qualities of stimuli on the skin, and their functional roles have been investigated. Afferent somatosensory information provided by C-fibers underpins various positive and negative affective sensations from the periphery, including mechanical, thermal, and chemical pain (C-nociceptors), temperature (C-thermoreceptors), and positive affective aspects of touch (C-tactile afferents). Insights from microneurographic investigations have revealed the complexity of the C-fiber system, methods for delineating fundamental C-fiber populations in a translational manner, how C-fiber firing can be used to identify nerve deficits in pathological states, and how the responses from C-fibers may be modified to change sensory percepts, including decreasing pain. Understanding these processes may lead to future medical interventions to diagnose and treat C-fiber dysfunction. NEW & NOTEWORTHY The technique of microneurography allows us to directly investigate the functional roles of single C-fiber afferents in awake human beings. Here we outline and discuss the current field of C-fiber research on this heterogeneous population of afferents in healthy subjects, in pathological states, and from a translational perspective. We cover C-fibers encoding touch, temperature, and pain and provide perspectives on the future of C-fiber microneurography investigations in humans.

Ultrasound-guided, open-source microneurography: Approaches to improve recordings from peripheral nerves in man

OBJECTIVE

Microneurography is the only method for recording from single neurons in intact human nerves. It is challenging - requiring technical expertise, investment in specialised equipment and has sparse data yields.

METHODS

We assessed whether ultrasound guidance in combination with an 'open access' amplifier and data capture system (Open-Ephys) would simplify and expand the scope of microneurographic recordings in humans.

RESULTS

In 32 healthy consenting volunteers, ultrasound-guidance improved success rates for obtaining cutaneous C-fibres and reduced "Skin to Nerve" times from 28.5 min to 4.5 min for recordings of the peroneal nerve (P < 0.0001). We illustrate the potential utility of ultrasound-guided microneurography for difficult to access nerves with phrenic nerve recording during a Valsalva manoeuvre. We show that Open Ephys is a viable alternative to commercially available recording systems and offers advantages in terms of cost and software customisability.

CONCLUSIONS

Ultrasound guidance for microneurography with Open Ephys facilitates cutaneous C nociceptor recordings and allows recordings to be made from nerves previously considered inaccessible.

SIGNIFICANCE

We anticipate that the adoption of these techniques will improve microneurography experimental efficiency, adds an important visual learning aid and increases the generalisability of the approach.

Tuning in C-nociceptors to reveal mechanisms in chronic neuropathic pain

OBJECTIVE

Develop and validate a low-intensity sinusoidal electrical stimulation paradigm to preferentially activate C-fibers in human skin.

METHODS

Sinusoidal transcutaneous stimulation (4Hz) was assessed psychophysically in healthy volunteers (n = 14) and neuropathic pain patients (n = 9). Pursuing laser Doppler imaging and single nociceptor recordings in vivo in humans (microneurography) and pigs confirmed the activation of "silent" C-nociceptors. Synchronized C-fiber compound action potentials were evoked in isolated human nerve fascicles in vitro. Live cell imaging of L4 dorsal root ganglia in anesthetized mice verified the recruitment of small-diameter neurons during transcutaneous 4-Hz stimulation of the hindpaw (0.4mA).

RESULTS

Transcutaneous sinusoidal current (0.05-0.4mA, 4Hz) activated "polymodal" C-fibers (50% at ∼0.03mA) and "silent" nociceptors (50% at ∼0.04mA), intensities substantially lower than that required with transcutaneous 1-ms rectangular pulses ("polymodal" ∼3mA, "silent" ∼50mA). The stimulation induced delayed burning (nonpulsating) pain and a pronounced axon-reflex erythema, both indicative of C-nociceptor activation. Pain ratings to repetitive stimulation (1 minute, 4Hz) adapted in healthy volunteers by Numeric Rating Scale (NRS) -3 and nonpainful skin sites of neuropathic pain patients by NRS -0.5, whereas pain even increased in painful neuropathic skin by approximately NRS +2.

INTERPRETATION

Sinusoidal electrical stimulation at 4Hz enables preferential activation of C-nociceptors in pig and human skin that accommodates during ongoing (1-minute) stimulation. Absence of such accommodation in neuropathic pain patients suggest axonal hyperexcitability that could be predictive of alterations in peripheral nociceptor encoding and offer a potential therapeutic entry point for topical analgesic treatment. Ann Neurol 2018;83:945-957.

Spike propagation in axons under stretch growth conditions in cultured neurons from dorsal root ganglion

Computational software NEURON was used to simulate the stretch growth neurons in order to investigate the ability of dorsal root ganglion neurons to generate and propagate action potentials after a period of rapid axon stretch growth in vitro, and under what stimulating parameters can evoke action potentials. In the simulation, we found the stretch growth neuron had higher spike amplitude than from the static culture neuron in the soma and all axonal branch. In addition, the conduction velocity was also faster in the stretch growth axon. When the stimulating frequency was less than 15 Hz or the stimulating voltage was lower than 15 mV, no spike was evoked. Increasing stimulating frequency from 15 Hz to 5000 Hz or stimulating voltage from 15 mV to 100 mV had almost no effect on the spike amplitude. Interestingly, the first spike time and absolute refractory period (ARP) in different axonal branches and somas decreased stepwise with incremental increase in the stimulating frequency. It is concluded that the stretch growth neuron had higher amplitude and faster conduction velocity than the static culture neuron. In addition, some stimulating parameters had been analyzed in this study, which provided guidelines for electrophysiological experiments in future.

Inflammatory Pain Reduces C Fiber Activity-Dependent Slowing in a Sex-Dependent Manner, Amplifying Nociceptive Input to the Spinal Cord

C fibers display activity-dependent slowing (ADS), whereby repetitive stimulation (≥1 Hz) results in a progressive slowing of action potential conduction velocity, which manifests as a progressive increase in response latency. However, the impact of ADS on spinal pain processing has not been explored, nor whether ADS is altered in inflammatory pain conditions. To investigate, compound action potentials were made, from dorsal roots isolated from rats with or without complete Freund's adjuvant (CFA) hindpaw inflammation, in response to electrical stimulus trains. CFA inflammation significantly reduced C fiber ADS at 1 and 2 Hz stimulation rates. Whole-cell patch-clamp recordings in the spinal cord slice preparation with attached dorsal roots also demonstrated that CFA inflammation reduced ADS in the monosynaptic C fiber input to lamina I neurokinin 1 receptor-expressing neurons (1–10 Hz stimulus trains) without altering the incidence of synaptic response failures. When analyzed by sex, it was revealed that females display a more pronounced ADS that is reduced by CFA inflammation to a level comparable with males. Cumulative ventral root potentials evoked by long and short dorsal root stimulation lengths, to maximize and minimize the impact of ADS, respectively, demonstrated that reducing ADS facilitates spinal summation, and this was also sex dependent. This finding correlated with the behavioral observation of increased noxious thermal thresholds and enhanced inflammatory thermal hypersensitivity in females. We propose that sex/inflammation-dependent regulation of C fiber ADS can, by controlling the temporal relay of nociceptive inputs, influence the spinal summation of nociceptive signals contributing to sex/inflammation-dependent differences in pain sensitivity.

SIGNIFICANCE STATEMENT The intensity of a noxious stimulus is encoded by the frequency of action potentials relayed by nociceptive C fibers to the spinal cord. C fibers conduct successive action potentials at progressively slower speeds, but the impact of this activity-dependent slowing (ADS) is unknown. Here we demonstrate that ADS is more prevalent in females than males and is reduced in an inflammatory pain model in females only. We also demonstrate a progressive delay of C fiber monosynaptic transmission to the spinal cord that is similarly sex and inflammation dependent. Experimentally manipulating ADS strongly influences spinal summation consistent with sex differences in behavioral pain thresholds. This suggests that ADS provides a peripheral mechanism that can regulate spinal nociceptive processing and pain sensation.

Optimal delineation of single C-tactile and C-nociceptive afferents in humans by latency slowing

Human skin encodes a plethora of touch interactions, and affective tactile information is primarily signaled by slowly conducting C-mechanoreceptive afferents. We show that electrical stimulation of low-threshold C-tactile afferents produces markedly different patterns of activity compared with high-threshold C-mechanoreceptive nociceptors, although the populations overlap in their responses to mechanical stimulation. This fundamental distinction demonstrates a divergence in affective touch signaling from the first stage of sensory processing, having implications for the processing of interpersonal touch.

C-mechanoreceptors in humans comprise a population of unmyelinated afferents exhibiting a wide range of mechanical sensitivities. C-mechanoreceptors are putatively divided into those signaling gentle touch (C-tactile afferents, CTs) and nociception (C-mechanosensitive nociceptors, CMs), giving rise to positive and negative affect, respectively. We sought to distinguish, compare, and contrast the properties of a population of human C-mechanoreceptors to see how fundamental the divisions between these putative subpopulations are. We used microneurography to record from individual afferents in humans and applied electrical and mechanical stimulation to their receptive fields. We show that C-mechanoreceptors can be distinguished unequivocally into two putative populations, comprising CTs and CMs, by electrically evoked spike latency changes (slowing). After both natural mechanical stimulation and repetitive electrical stimulation there was markedly less latency slowing in CTs compared with CMs. Electrical receptive field stimulation, which bypasses the receptor end organ, was most effective in classifying C-mechanoreceptors, as responses to mechanical receptive field stimulation overlapped somewhat, which may lead to misclassification. Furthermore, we report a subclass of low-threshold CM responding to gentle mechanical stimulation and a potential subclass of CT afferent displaying burst firing. We show that substantial differences exist in the mechanisms governing axonal conduction between CTs and CMs. We provide clear electrophysiological “signatures” (extent of latency slowing) that can be used in unequivocally identifying populations of C-mechanoreceptors in single-unit and multiunit microneurography studies and in translational animal research into affective touch. Additionally, these differential mechanisms may be pharmacologically targetable for separate modulation of positive and negative affective touch information.

NEW & NOTEWORTHY Human skin encodes a plethora of touch interactions, and affective tactile information is primarily signaled by slowly conducting C-mechanoreceptive afferents. We show that electrical stimulation of low-threshold C-tactile afferents produces markedly different patterns of activity compared with high-threshold C-mechanoreceptive nociceptors, although the populations overlap in their responses to mechanical stimulation. This fundamental distinction demonstrates a divergence in affective touch signaling from the first stage of sensory processing, having implications for the processing of interpersonal touch.

Use dependence of peripheral nociceptive conduction in the absence of tetrodotoxin-resistant sodium channel subtypes

KEY POINTS

This study examines conduction in peripheral nerves and its use dependence in tetrodotoxin-resistant (TTXr) sodium channel (Nav 1.8, Nav 1.9) knockout and wildtype animals. We observed use-dependent decreases of single fibre and compound action potential amplitude in peripheral mouse C-fibres (wildtype). This matches the previously published hypothesis that increased Na/K-pump activity is not the underlying mechanism for use-dependent changes of neural conduction. Knocking out TTXr sodium channels influences use-dependent changes of conductive properties (action potential amplitude, latency, conduction safety) in the order Nav 1.8 KO > Nav 1.9KO > wildtype. This is most likely explained by different subsets of presumably (relatively) Nav 1.7-rich conducting fibres in knockout animals as compared to wildtypes, in combination with reduced per-pulse sodium influx.

ABSTRACT

Use dependency of peripheral nerves, especially of nociceptors, correlates with receptive properties. Slow inactivation of voltage-gated sodium channels has been discussed to be the underlying mechanism - pointing to a receptive class-related difference of sodium channel equipment. Using electrophysiological recordings of single unmyelinated cutaneous fibres and their compound action potential (AP), we evaluated use-dependent changes in mouse peripheral nerves, and the contribution of the tetrodotoxin-resistant (TTXr) sodium channels Nav 1.8 and Nav 1.9 to these changes. Nerve fibres were electrically stimulated using single or double pulses at 2 Hz. Use-dependent changes of latency, AP amplitude, and duration as well as the fibres' ability to follow the stimulus were evaluated. AP amplitudes substantially diminished in used fibres from C57BL/6 but increased in Nav 1.8 knockout (KO) mice, with Nav 1.9 KO in between. Activity-induced latency slowing was in contrast the most pronounced in Nav 1.8 KOs and the least in wildtype mice. The genotype was also predictive of how long fibres could follow the double pulsed stimulus with wildtype fibres blocking first and Nav 1.8 KO fibres enduring the longest. In contrast, changes in spike duration were less pronounced and displayed no significant tendency. Thus, all major measures of peripheral nerve accommodation (amplitude, latency and durability) depended on genotype. All use-dependent changes appeared in the order NaV 1.8 KO > NaV 1.9 KO > wildtype, which is most likely explained by the relative contribution of Nav 1.7 varying in the same order and the amounts of per-pulse sodium influx expected in the opposite order.

Peripheral and spinal mechanisms of nociception in a rat reserpine-induced pain model

Chronic widespread pain is a serious medical problem, yet the mechanisms of nociception and pain are poorly understood. Using a reserpine-induced pain model originally reported as a putative animal model for fibromyalgia, this study was undertaken to examine the following: (1) expression of several ion channels responsible for pain, mechanotransduction, and generation/propagation of action potentials in the dorsal root ganglion (DRG), (2) activities of peripheral nociceptive afferents, and (3) alterations in spinal microglial cells. A significant increase in mRNA expression of the acid-sensing ion channel (ASIC)-3 was detected in the DRG, and the behavioral mechanical hyperalgesia was significantly reversed by subcutaneous injection of APETx2, a selective blocker of ASIC3. Single-fiber recordings in vitro revealed facilitated mechanical responses of mechanoresponsive C-fibers both in the skin and muscle although the proportion of mechanoresponsive C-nociceptors was paradoxically decreased. In the spinal dorsal horn, microglial cells labeled with Iba1 immunoreactivity was activated, especially in laminae I-II where the nociceptive input is mainly processed compared with the other laminae. The activated microglia and behavioral hyperalgesia were significantly tranquilized by intraperitoneal injection of minocycline. These results suggest that the increase in ASIC3 in the DRG facilitated mechanical response of the remaining C-nociceptors and that activated spinal microglia may direct to intensify pain in this model. Pain may be further amplified by reserpine-induced dysfunction of the descending pain inhibitory system and by the decrease in peripheral drive to this system resulting from a reduced proportion of mechanoresponsive C-nociceptors.

Spike propagation through the dorsal root ganglia in an unmyelinated sensory neuron: a modeling study

Unmyelinated C-fibers are a major type of sensory neurons conveying pain information. Action potential conduction is regulated by the bifurcation (T-junction) of sensory neuron axons within the dorsal root ganglia (DRG). Understanding how C-fiber signaling is influenced by the morphology of the T-junction and the local expression of ion channels is important for understanding pain signaling. In this study we used biophysical computer modeling to investigate the influence of axon morphology within the DRG and various membrane conductances on the reliability of spike propagation. As expected, calculated input impedance and the amplitude of propagating action potentials were both lowest at the T-junction. Propagation reliability for single spikes was highly sensitive to the diameter of the stem axon and the density of voltage-gated Na⁺ channels. A model containing only fast voltage-gated Na⁺ and delayed-rectifier K⁺ channels conducted trains of spikes up to frequencies of 110 Hz. The addition of slowly activating KCNQ channels (i.e., K_V7 or M-channels) to the model reduced the following frequency to 30 Hz. Hyperpolarization produced by addition of a much slower conductance, such as a Ca²⁺-dependent K⁺ current, was needed to reduce the following frequency to 6 Hz. Attenuation of driving force due to ion accumulation or hyperpolarization produced by a Na⁺-K⁺ pump had no effect on following frequency but could influence the reliability of spike propagation mutually with the voltage shift generated by a Ca²⁺-dependent K⁺ current. These simulations suggest how specific ion channels within the DRG may contribute toward therapeutic treatments for chronic pain.

Saltatory conduction in unmyelinated axons: clustering of Na(+) channels on lipid rafts enables micro-saltatory conduction in C-fibers

THE ACTION POTENTIAL (AP), THE FUNDAMENTAL SIGNAL OF THE NERVOUS SYSTEM, IS CARRIED BY TWO TYPES OF AXONS: unmyelinated and myelinated fibers. In the former the action potential propagates continuously along the axon as established in large-diameter fibers. In the latter axons the AP jumps along the nodes of Ranvier-discrete, anatomically specialized regions which contain very high densities of sodium ion (Na(+)) channels. Therefore, saltatory conduction is thought as the hallmark of myelinated axons, which enables faster and more reliable propagation of signals than in unmyelinated axons of same outer diameter. Recent molecular anatomy showed that in C-fibers, the very thin (0.1 μm diameter) axons of the peripheral nervous system, Nav1.8 channels are clustered together on lipid rafts that float in the cell membrane. This localized concentration of Na(+) channels resembles in structure the ion channel organization at the nodes of Ranvier, yet it is currently unknown whether this translates into an equivalent phenomenon of saltatory conduction or related-functional benefits and efficiencies. Therefore, we modeled biophysically realistic unmyelinated axons with both conventional and lipid-raft based organization of Na(+) channels. We find that APs are reliably conducted in a micro-saltatory fashion along lipid rafts. Comparing APs in unmyelinated fibers with and without lipid rafts did not reveal any significant difference in either the metabolic cost or AP propagation velocity. By investigating the efficiency of AP propagation over Nav1.8 channels, we find however that the specific inactivation properties of these channels significantly increase the metabolic cost of signaling in C-fibers.

Differential axonal conduction patterns of mechano-sensitive and mechano-insensitive nociceptors--a combined experimental and modelling study

Cutaneous pain sensations are mediated largely by C-nociceptors consisting of both mechano-sensitive (CM) and mechano-insensitive (CMi) fibres that can be distinguished from one another according to their characteristic axonal properties. In healthy skin and relative to CMi fibres, CM fibres show a higher initial conduction velocity, less activity-dependent conduction velocity slowing, and less prominent post-spike supernormality. However, after sensitization with nerve growth factor, the electrical signature of CMi fibres changes towards a profile similar to that of CM fibres. Here we take a combined experimental and modelling approach to examine the molecular basis of such alterations to the excitation thresholds. Changes in electrical activation thresholds and activity-dependent slowing were examined in vivo using single-fibre recordings of CM and CMi fibres in domestic pigs following NGF application. Using computational modelling, we investigated which axonal mechanisms contribute most to the electrophysiological differences between the fibre classes. Simulations of axonal conduction suggest that the differences between CMi and CM fibres are strongly influenced by the densities of the delayed rectifier potassium channel (Kdr), the voltage-gated sodium channels NaV1.7 and NaV1.8, and the Na+/K+-ATPase. Specifically, the CM fibre profile required less Kdr and NaV1.8 in combination with more NaV1.7 and Na+/K+-ATPase. The difference between CM and CMi fibres is thus likely to reflect a relative rather than an absolute difference in protein expression. In support of this, it was possible to replicate the experimental reduction of the ADS pattern of CMi nociceptors towards a CM-like pattern following intradermal injection of nerve growth factor by decreasing the contribution of Kdr (by 50%), increasing the Na+/K+-ATPase (by 10%), and reducing the branch length from 2 cm to 1 cm. The findings highlight key molecules that potentially contribute to the NGF-induced switch in nociceptors phenotype, in particular NaV1.7 which has already been identified clinically as a principal contributor to chronic pain states such as inherited erythromelalgia.

Three functionally distinct classes of C-fibre nociceptors in primates

In primates, C-fibre polymodal nociceptors are broadly classified into two groups based on mechanosensitivity. Here we demonstrate that mechanically sensitive polymodal nociceptors that respond either quickly (QC) or slowly (SC) to a heat stimulus differ in responses to a mild burn, heat sensitization, conductive properties and chemosensitivity. Superficially applied capsaicin and intradermal injection of β-alanine, an MrgprD agonist, excite vigorously all QCs. Only 40% of SCs respond to β-alanine, and their response is only half that of QCs. Mechanically insensitive C-fibres (C-MIAs) are β-alanine insensitive but vigorously respond to capsaicin and histamine with distinct discharge patterns. Calcium imaging reveals that β-alanine and histamine activate distinct populations of capsaicin-responsive neurons in primate dorsal root ganglion. We suggest that histamine itch and capsaicin pain are peripherally encoded in C-MIAs, and that primate polymodal nociceptive afferents form three functionally distinct subpopulations with β-alanine responsive QC fibres likely corresponding to murine MrgprD-expressing, non-peptidergic nociceptive afferents.

Ionic mechanisms in peripheral pain

Chronic pain constitutes an important and growing problem in society with large unmet needs with respect to treatment and clear implications for quality of life. Computational modeling is used to complement experimental studies to elucidate mechanisms involved in pain states. Models representing the peripheral nerve ending often address questions related to sensitization or reduction in pain detection threshold. In models of the axon or the cell body of the unmyelinated C-fiber, a large body of work concerns the role of particular sodium channels and mutations of these. Furthermore, in central structures: spinal cord or higher structures, sensitization often refers not only to enhanced synaptic efficacy but also to elevated intrinsic neuronal excitability. One of the recent developments in computational neuroscience is the emergence of computational neuropharmacology. In this area, computational modeling is used to study mechanisms of pathology with the objective of finding the means of restoring healthy function. This research has received increased attention from the pharmaceutical industry as ion channels have gained increased interest as drug targets. Computational modeling has several advantages, notably the ability to provide mechanistic links between molecular and cellular levels on the one hand and functions at the systems level on the other hand. These characteristics make computational modeling an additional tool to be used in the process of selecting pharmaceutical targets. Furthermore, large-scale simulations can provide a framework to systematically study the effects of several interacting disease parameters or effects from combinations of drugs.

Modeling activity-dependent changes of axonal spike conduction in primary afferent C-nociceptors

Action potential initiation and conduction along peripheral axons is a dynamic process that displays pronounced activity dependence. In patients with neuropathic pain, differences in the modulation of axonal conduction velocity by activity suggest that this property may provide insight into some of the pathomechanisms. To date, direct recordings of axonal membrane potential have been hampered by the small diameter of the fibers. We have therefore adopted an alternative approach to examine the basis of activity-dependent changes in axonal conduction by constructing a comprehensive mathematical model of human cutaneous C-fibers. Our model reproduced axonal spike propagation at a velocity of 0.69 m/s commensurate with recordings from human C-nociceptors. Activity-dependent slowing (ADS) of axonal propagation velocity was adequately simulated by the model. Interestingly, the property most readily associated with ADS was an increase in the concentration of intra-axonal sodium. This affected the driving potential of sodium currents, thereby producing latency changes comparable to those observed for experimental ADS. The model also adequately reproduced post-action potential excitability changes (i.e., recovery cycles) observed in vivo. We performed a series of control experiments replicating blockade of particular ion channels as well as changing temperature and extracellular ion concentrations. In the absence of direct experimental approaches, the model allows specific hypotheses to be formulated regarding the mechanisms underlying activity-dependent changes in C-fiber conduction. Because ADS might functionally act as a negative feedback to limit trains of nociceptor activity, we envisage that identifying its mechanisms may also direct efforts aimed at alleviating neuronal hyperexcitability in pain patients.

Assessing the permeability of the rat sciatic nerve epineural sheath against compounds with local anesthetic activity: an ex vivo electrophysiological study

Abstract Studies have shown that the sciatic nerve epineural sheath acts as a barrier and has a delaying effect on the diffusion of local anesthetics into the nerve fibers and endoneurium. The purpose of this work is to assess and to quantify the permeability of the epineural sheath. For this purpose, we isolated the rat sciatic nerve in a three-chamber recording bath that allowed us to monitor the constant in amplitude evoked nerve compound action potential (nCAP) for over 24 h. For nerves exposed to the compounds under investigation, we estimated the IT50 the time required to inhibit the nCAP to 50% of its initial value. For desheathed nerves, the half-vitality time was denoted as IT50(-) and for the ensheath normal nerves as IT50(+). There was no significant difference between the IT50 of desheathed and ensheathed nerves exposed to normal saline. The IT50(-) for nerves exposed to 40 mM lidocaine was 12.1 ± 0.95 s (n=14) and the IT50(+) was 341.4 ± 2.49 s (n=6). The permeability (P) coefficient of the epineural sheath was defined as the ratio IT50(+)/IT50(-). The P coefficient for 40 mM lidocaine and linalool was 28.2 and 3.48, correspondingly, and for 30 mM 2-heptanone was 4.87. This is an indication that the epineural sheath provided a stronger barrier against lidocaine, compared to natural local anesthetics, linalool and 2-heptanone. The methodology presented here is a useful tool for studying epineural sheath permeability to compounds with local anesthetic properties.

Discriminative sensory characteristics of the lateral femoral cutaneous nerve after mepivacaine-induced block

Background and objectives

Unmyelinated C-fibres comprise the largest group of somatic afferents and have demonstrated a crucial role not only in the perception of high-threshold mechanically, thermally or chemically induced pain, but also in non-harmful low-threshold mechanical stimuli [1,2]. The objective of our study was to characterize differential sensitivity changes of C-fibre related subclasses of high-threshold and low-threshold polymodal nociceptors and low-threshold mechanoreceptors to the local anaesthetic (LA) mepivacaine during nerve block of the purely sensory lateral femoral cutaneous nerve (LFCN) in human. We assumed a diverse response of different classes of afferents to the two different concentrations of the LA mepivacaine (Scandicaine).

Methods

In a double-blind randomized experimental setting, an ultrasound-guided nerve block of the LFCN was performed in 10 healthy male subjects, each with two different concentrations of mepivacaine (0.5 and 1%). Responsiveness of afferent nerve fibres to different noxious and non-noxious stimuli was tested by Quantitative Sensory Testing (QST) 30, 180, and 300 min after nerve block. Both LA concentrations of mepivacaine were compared for time course of the areas of anaesthesia for the tested sensory modalities.

Results

Initial extension of anaesthetic areas at 30 min did not differ between both LA concentrations. At 180 min only the anaesthetic areas to nociceptive stimuli were reduced at the site of lower mepivacaine injection (260mN: 204mm2 (18; 244; median difference and 95% confidence interval; p < 0.05), heat: 276mm2 (3; 305)). In contrast, no significant differences were found between the two concentration when non-nociceptive stimuli were used (100mN: 187mm2 (4; 240), p >0.05, brush: 159mm2 (–59; 202)).

Conclusion

Equal initial sizes of anaesthesia areas for all sensory modalities can be explained by supramaximal perineural LA molecule concentration in both administered mepivacaine dosages. Upon washout of the LA nociceptive function is restored faster as compared to non-nociceptive sensation and higher concentration of the LA are required to maintain the analgesia. Quantitative sensory testing is able to detect different susceptibility of low threshold mechanosensors and subtypes of nociceptive C-fibres to mepivacaine. Using painful mechanical, heat and electrical stimulation different classes of nociceptors will be activated. The analgesic areas to electrical stimulation were particularly small; one might therefore hypothesize that the proposed protocol allows to also differentiate mechano-insensitive (“silent”) and mechanosensitive (“polymodal”) nociceptors.

Implications

QST is a non-invasive method to functionally examine sensory modalities and their pharmacological modulation in humans. The method is sufficiently sensitive to differentiate the analgesic properties of mepivacaine at 0.5 and 1% and might also be adequate to different classes of nociceptors. Further development of nociceptive stimuli including supra-threshold encoding characteristics will enable to investigate peripheral analgesic effects more specifically and thus might help to design new analgesics with preferential effect on high frequency discharge of nociceptors.

Epidermal nerve fibers modulate keratinocyte growth via neuropeptide signaling in an innervated skin model

Atopic eczema is a chronic inflammatory skin disease characterized by cutaneous nerve fiber sprouting and epidermal hyperplasia, pointing to an involvement of the peripheral nervous system in cutaneous homeostasis. However, the interaction of sensory neurons and skin cells is poorly understood. Using an innervated skin model, we investigated the influence of sensory neurons on epidermal morphogenesis. Neurons induced the proliferation of keratinocytes, resulting in an increase in the epidermal thickness. Inhibition of calcitonin gene-related peptide (CGRP), but not substance P (SP) signaling, reversed this effect. Human CGRP enhanced keratinocyte proliferation and epidermal thickness in skin models, demonstrating a key role of CGRP in modulating epidermal morphogenesis, whereas SP had only a moderate effect. Innervated skin models composed of atopic skin cells showed increased neurite outgrowth, accompanied by elevated CGRP release. As atopic keratinocytes were sensitized to CGRP owing to higher expression levels of the CGRP receptor components, receptor activity-modifying protein 1 (RAMP1) and receptor component protein (RCP), atopic innervated skin models displayed a thicker epidermis than did healthy controls. We conclude that neural CGRP controls local keratinocyte growth. Our results show that the crosstalk of the cutaneous peripheral nervous system and skin cells significantly influences epidermal morphogenesis and homeostasis in healthy and atopic skin.

NON-INVASIVE EVALUATION OF NERVE CONDUCTION IN SMALL DIAMETER FIBERS IN THE RAT

A novel non-invasive technique was applied to measure velocity within slow conducting axons in the distal extreme of the sciatic nerve (i.e., digital nerve) in a rat model. The technique is based on the extraction of rectified multiple unit activity (MUA) from in vivo whole nerve compound responses. This method reliably identifies compound action potentials in thinly myelinated fibers conducting at a range of 9-18 m/s (Aδ axons), as well as in a subgroup of unmylinated C fibers conducting at approximately 1-2 m/s. The sensitivity of the method to C-fiber conduction was confirmed by the progressive decrement of the responses in the 1-2 m/s range over a 20-day period following the topical application of capsaicin (ANOVA p<0.03). Increasing the frequency of applied repetitive stimulation over a range of 0.75 Hz to 6.0 Hz produced slowing of conduction and a significant decrease in the magnitude of the compound C-fiber response (ANOVA p<0.01). This technique offers a unique opportunity for the non-invasive, repeatable, and quantitative assessment of velocity in the subsets of Aδ and C fibers in parallel with evaluation of fast nerve conduction.

Nociceptive responses to thermal and mechanical stimulations in awake pigs

BACKGROUND

Porcine skin exhibits a high degree of homology to human skin, and the pig has recently been used as a cutaneous pain model. However, before the full potential of this novel in vivo cutaneous pain model can be achieved, several methodological aspects related to the management of awake animal studies in a large species require further examination. This manuscript describes the initial development of a porcine model of cutaneous nociception and focuses on interactions between the sensory modality, body size and the anatomical location of the stimulation site.

METHODS

Pigs of different body sizes (30 and 60 kg) were exposed to thermal (CO2 laser) and mechanical (pressure application measurement device) stimulations to the flank and the hind legs in a balanced order. The median response latency and the type of behavioural response were recorded.

RESULTS

Small pigs exhibited significantly lower pain thresholds (shorter latency to response) than large pigs to thermal and mechanical stimulations. Stimulations at the two anatomical locations elicited very distinct sets of behavioural responses, with different levels of sensitivity between the flank and the hind legs. Furthermore, small animals exhibited lower levels of individual variability between single stimulations.

CONCLUSIONS

Our data indicate that this experimental approach may be valuable for use in studies that focus on porcine cutaneous nociception.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Activity-dependent slowing properties of an unmyelinated low threshold mechanoreceptor in human hairy skin

It has been previously shown that unmyelinated afferent fibres in human skin are differentiated not only by their receptor characteristics, but also by their profiles of activity-dependent slowing. One type of profile, described originally as 'type 3', is different from that of nociceptors (type 1), cold afferents (type 2) and sympathetic efferents (type 4), in that these fibres display a minimal activity-dependent slowing (∼1% at 2 Hz). However, their function remains to be determined. Here we describe one unit with a typical 'type 3' activity-dependent slowing profile recorded from an undamaged fascicle of the superficial peroneal nerve of a patient. Its conduction velocity was 1.8 m s(-1) and it slowed by 1.3% during the 2 Hz tetanus. This unit had a mechanical receptive field in the hairy skin and responded readily to weak mechanical stimuli, and not to cold. This suggests that the low threshold unmyelinated mechanoreceptors recently described in human hairy skin are probably endowed with a 'type 3' activity-dependent profile.

Nociceptors: the sensors of the pain pathway

Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury-related chemicals, and transducing these stimuli into long-ranging electrical signals that are relayed to higher brain centers. The activation of functionally distinct cutaneous nociceptor populations and the processing of information they convey provide a rich diversity of pain qualities. Current work in this field is providing researchers with a more thorough understanding of nociceptor cell biology at molecular and systems levels and insight that will allow the targeted design of novel pain therapeutics.

Cutaneous C-fiber nociceptor responses and nociceptive behaviors in aged Sprague-Dawley rats

The change with age in pain perception in humans and the nociceptive behaviors in animals elicited by noxious stimuli to the skin are not well understood, and little is known about the peripheral neural mechanisms of cutaneous nociception in the aged. We systematically examined cutaneous nociceptor responses and nociceptive behaviors in young (9-14 w) and in aged (127-138 w) Sprague-Dawley rats. C-fiber nociceptors in the skin were identified by mechanical and electrical stimulation, and extracellularly recorded from hind paw skin-saphenous nerve preparations in vitro. In the aged rats, the proportions of mechano-responsive and/or heat-responsive C-nociceptors were significantly lower. The proportion of mechano- and thermo-insensitive units, on the other hand, was significantly increased. In addition, the response threshold to mechanical stimulus tended to be higher and the magnitude of the response tended to be smaller. There were no differences between the two age groups in the response magnitudes of mechano-responsive C-nociceptors to bradykinin, cold or heat. Repetitive electrical stimulation of afferent fibers revealed exaggerated slowing of conduction velocity in mechano-responsive C-fibers in the aged. This showed for the first time that not only receptive properties of afferent terminals but also membrane properties of conducting axons are changed in aged rats. Nociceptive behaviors in response to noxious levels of cold (cold plate test) and heat (Hargreaves' radiant heat test) were facilitated in aged animals, while mechanical sensitivity measured by von Frey hairs remained unchanged. These discrepancies between the changes in peripheral afferents and the behavioral outcomes might be explained by facilitatory changes in the central nervous system.