Christian Weidner

Which nerve fibers mediate the axon reflex flare in human skin

Axon reflex vasodilatation due to transcutaneous electrical stimulation in human skin was measured by laser Doppler imaging. Constant current pulses of 10 mA, 0.2 ms, delivered at 1 or 10 Hz for 2 min through a probe of 30 mm2 surface area did not induce a significant flare response, though this stimulus previously has been found supra-maximal for cutaneous poly-modal (mechano-heat responsive) C-nociceptors in microneurography experiments. Pulses of the same strength from a pointed probe yielding a higher current density induced an extended and persistent flare. This type of stimulus previously has been proven to recruit mechano-insensitive C-units in microneurography experiments, in contrast to stimuli from the 30 mm2 probe. It is concluded that mechano-insensitive C-nociceptors and not polymodal C-units mediate the axon reflex flare in human skin.

Low-dose lidocaine reduces secondary hyperalgesia by a central mode of action

Abstract Sodium channel blockers are approved for intravenous administration in the treatment of neuropathic pain states. Preclinical studies have suggested antihyperalgesic effects on the peripheral as well as the central nervous system. The objective of this study was to determine mechanisms of action of low‐dose lidocaine in experimental induced, secondary hyperalgesia. In a first experimental trial, participants (n=12) received lidocaine systemically (a bolus injection of 2 mg/kg in 10 min followed by an intravenous infusion of 2 mg kg−1 h−1 for another 50 min). In a second trial, a modified intravenous regional anesthesia (IVRA) was administered to exclude possible central analgesic effects. In one arm, patients received an infusion of 40 ml lidocaine, 0.05%; in the other arm 40 ml NaCl, 0.9%, served as a control. In both trials capsaicin, 20 &mgr;g, was injected intradermally and time course of capsaicin‐induced pain, allodynia and hyperalgesia as well as axon reflex flare was determined. The capsaicin‐induced pain was slightly reduced after systemic and regional application of the anesthetic. The area of pin‐prick hyperalgesia was significantly reduced by systemic lidocaine, whereas the inhibition of hyperalgesia was absent during regional administration of lidocaine. In contrast, capsaicin‐induced flare was significantly decreased after both treatments. We conclude that systemic lidocaine reduces pin‐prick hyperalgesia by a central mode of action, which could involve blockade of terminal branches of nociceptors. A possible role for tetrodotoxin resistant sodium channels in the antihyperalgesic effect of low‐dose lidocaine is discussed.

Abnormal Function of C-Fibers in Patients with Diabetic Neuropathy

The mechanisms underlying the development of painful and nonpainful neuropathy associated with diabetes mellitus are unclear. We have obtained microneurographic recordings from unmyelinated fibers in eight patients with diabetes mellitus, five with painful neuropathy, and three with neuropathy without pain. All eight patients had large-fiber neuropathy, and seven patients had pathological thermal thresholds in their feet, indicating the involvement of small-caliber nerve fibers. A total of 163 C-fibers were recorded at knee level from the common peroneal nerve in the patients (36–67 years old), and these were compared with 77 C-fibers from healthy controls (41–64 years old). The ratio of mechano-responsive to mechano-insensitive nociceptors was ∼2:1 in the healthy controls, whereas in the patients, it was 1:2. In patients, a fairly large percentage of characterized fibers (12.5% in nonpainful and 18.9% in painful neuropathy) resembled mechano-responsive nociceptors that had lost their mechanical and heat responsiveness. Such fibers were rarely encountered in age-matched controls (3.2%). Afferent fibers with spontaneous activity or mechanical sensitization were found in both patient groups. We conclude that small-fiber neuropathy in diabetes affects receptive properties of nociceptors that leads to an impairment of mechano-responsive nociceptors.

Active "itch fibers" in chronic pruritus.

An itch-specific neuronal pathway was recently discovered in healthy humans and animals. Here the authors report that activity in this specific pathway coincides with itch under pathophysiologic conditions in a patient with chronic pruritus. Microneurographic recordings from the symptomatic area revealed spontaneous activity in six single C-fiber afferents that had the characteristic features of “itch fibers.” Itch may be caused by activity in a specific subpopulation of C-fiber afferents.

Neural Signal Processing: The Underestimated Contribution of Peripheral Human C-Fibers

The microneurography technique was used to analyze use-dependent frequency modulation of action potential (AP) trains in human nociceptive peripheral nerves. Fifty-one single C-afferent units (31 mechano-responsive, 20 mechano-insensitive) were recorded from cutaneous fascicles of the peroneal nerve in awake human subjects. Trains of two and four suprathreshold electrical stimuli at interstimulus intervals of 20 and 50 msec were applied to the receptive fields of single identified nociceptive units at varying repetition rates. The output frequency (interspike interval) recorded at knee level was compared with the input frequency (interstimulus interval) at different levels of accumulated neural accommodation. At low levels of use-dependent accommodation (measured as conduction velocity slowing of the first action potential in a train), intervals between spikes increased during conduction along the nerve. At increasing levels of neural accommodation, intervals decreased because of a relative supernormal period (SNP) and asymptotically approached the minimum “entrainment” interval of the nerve fiber (11 ± 1.4 msec) corresponding to a maximum instantaneous discharge frequency (up to 190 Hz). For neural coding, this pattern of frequency decrease at low activity levels and frequency increase at high levels serves as a mechanism of peripheral contrast enhancement. The entrainment interval is a good minimum estimate for the duration of the refractory period of human C-fibers. At a given degree of neural accommodation, all afferent C-units exhibit a uniform pattern of aftereffects, independent of fiber class. The receptive class of a fiber only determines its susceptibility to accommodate. Thus, the time course of aftereffects and existence or absence of an SNP is fully explained by the amount of preexisting accommodation.

ATP responses in human C nociceptors.

&NA; Microelectrode recordings of impulse activity in nociceptive C fibres were performed in cutaneous fascicles of the peroneal nerve at the knee level in healthy human subjects. Mechano‐heat responsive C units (CMH), mechano‐insensitive but heat‐responsive (CH) as well as mechano‐insensitive and heat‐insensitive C units (CMiHi) were identified. A subgroup of the mechano‐insensitive units was readily activated by histamine. We studied the responsiveness of these nociceptor classes to injection of 20 &mgr;l 5 mM adenosintriphosphate (ATP) using saline injections as control. Because of mechanical distension during injection, which typically activates mechano‐responsive C fibres, interest was focused on responsiveness to ATP after withdrawal of the injection needle. Post‐injection responses were observed in 17/27 (63%) mechano‐responsive units and in 14/22 (64%) mechano‐insensitive units. Excitation by ATP occurred in 9/11 CH units and in 5/11 CMiHi units. ATP responsive units were found both within the histamine‐responsive and the histamine‐insensitive group of mechano‐insensitive fibres. ATP responses appeared with a delay of 0–180 s after completion of injection; responses were most pronounced during the first 1–3 min of activation, and irregular ongoing activity was observed for up to 10 or even 20 min. ATP responses were dose‐dependent, concentrations lower than 5 mM gave weaker responses. No heat or mechanical sensitisation was observed in any of the major fibre classes. In conclusion, we have shown that ATP injections at high concentrations activate C‐nociceptors in healthy human skin, without preference for mechano‐responsive or mechano‐insensitive units. ATP did not sensitise human C fibres for mechanical or heat stimuli. We discuss how various mechanisms might contribute to the observed responses to ATP.

Time course of post-excitatory effects separates afferent human C fibre classes

1 To study post‐excitatory changes of conduction velocity, action potentials were recorded from 132 unmyelinated nerve fibres (C fibres) in cutaneous fascicles of the peroneal nerve using microneurography in healthy human subjects. The ‘marking’ technique was used to assess responsiveness to mechanical and heat stimuli or sympathetic reflex provocation. 2 C fibres were classified into three major classes: mechano‐responsive afferent (n= 76), mechano‐insensitive afferent (n= 48) and sympathetic efferent C fibres (n= 8). 3 During regular stimulation at 0.25 Hz, conditioning pulses were intermittently interposed. Changes of conduction velocity were assessed for different numbers of conditioning impulses and varying interstimulus intervals (ISIs). For all three fibre classes the latency shift following conditioning pulses at an ISI of 1000 ms increased linearly with their number (n= 1, 2 and 4). However, the absolute degree of conduction velocity slowing was much higher in the 32 mechano‐insensitive fibres as compared with 56 mechano‐responsive or 8 sympathetic fibres. 4 Single additional pulses were interposed at different ISIs from 20 to 2000 ms. For 20 mechano‐responsive fibres conduction velocity slowing increased with decreasing ISI (subnormal phase). In contrast, for 16 mechano‐insensitive C fibres the conduction velocity slowing decreased with shorter ISIs, and at values lower than 417 ± 49 ms (mean ±s.e.m.) the conduction velocity of the conditioned action potential was faster than before (conduction velocity speeding). This supernormal phase had its maximum at 69 ± 10 ms. 5 In this study we provide, for the first time, direct evidence of relative supernormal conduction in human mechano‐insensitive C fibres. The implications for temporal coding in different afferent C fibre classes are discussed.

Bisphenol A Binds to the Local Anesthetic Receptor Site to Block the Human Cardiac Sodium Channel

Bisphenol A (BPA) has attracted considerable public attention as it leaches from plastic used in food containers, is detectable in human fluids and recent epidemiologic studies link BPA exposure with diseases including cardiovascular disorders. As heart-toxicity may derive from modified cardiac electrophysiology, we investigated the interaction between BPA and hNav1.5, the predominant voltage-gated sodium channel subtype expressed in the human heart. Electrophysiology studies of heterologously-expressed hNav1.5 determined that BPA blocks the channel with a Kd of 25.4±1.3 µM. By comparing the effects of BPA and the local anesthetic mexiletine on wild type hNav1.5 and the F1760A mutant, we demonstrate that both compounds share an overlapping binding site. With a key binding determinant thus identified, an homology model of hNav1.5 was generated based on the recently-reported crystal structure of the bacterial voltage-gated sodium channel NavAb. Docking predictions position both ligands in a cavity delimited by F1760 and contiguous with the DIII–IV pore fenestration. Steered molecular dynamics simulations used to assess routes of ligand ingress indicate that the DIII–IV pore fenestration is a viable access pathway. Therefore BPA block of the human heart sodium channel involves the local anesthetic receptor and both BPA and mexiletine may enter the closed-state pore via membrane-located side fenestrations.

Mechanically induced axon reflex and hyperalgesia in human UV‐B burn are reduced by systemic lidocaine

The mechanisms for the induction of primary mechanical hyperalgesia are unclear. We analyzed the neurogenic axon reflex erythema (flare) following phasic mechanical stimulation in normal and in UV‐B irradiated skin. In a cross‐over double blind design (n=10), low dose of systemic lidocaine suppressed mechanical hyperalgesia in sunburned skin and in the mechanically induced flare. Phasic mechanical stimulation, even at painful intensities, did not evoke a flare reaction in normal skin. However, stimulation within the UV‐B burn dose‐dependently provoked an immediate flare reaction. Systemic lidocaine suppressed the mechanically induced flare as well as the mechanical hyperalgesia in sunburned skin, while leaving the impact‐induced ratings in normal skin unchanged. Systemic lidocaine reduced these effects of sensitization, but did not reduce ratings in normal skin. As mechanically insensitive (“sleeping”) nociceptors have been shown to mediate the axon‐reflex in human skin, sensitization of this class of nociceptors might contribute also to the UV‐B‐induced primary mechanical hyperalgesia.

Action potential conduction in the terminal arborisation of nociceptive C‐fibre afferents

Recordings of single human peroneal C‐fibres and rat saphenous C‐fibres confirm two different patterns of conduction at branching points. In general, an action potential (AP) arising from one terminal branch may be propagated not only centrally, but also antidromically into the other branches of the terminal arborisation. If a stimulus activates several converging branches of one unit, at each branching point only the AP arriving first from the simultaneously activated daughter branches will be propagated centrally, resetting the slower branches. However, occasionally a single electrical stimulus may evoke a double response in the parent axon. In this case, these two responses apparently originate from different terminal branches and require unidirectional conduction block to prevent the faster AP from invading and resetting the slower‐conducting terminal. This conclusion is supported by the notion that when such a double response occurs, both responses immediately show additional activity‐dependent slowing of the conduction velocity due to frequency increase in the parent axon (two spikes per stimulus, one from each of the two excited branches). A comparable discharge pattern in the stem axon can be induced by repetitive paired stimulation of one terminal branch. Then the slowing is induced by the doubled frequency along the whole nerve fibre including the terminal branch. Since in this case not only the stem axon, but also the terminal branches carry two spikes per pulse, activity‐dependent slowing is predictably more pronounced. Unidirectional block thus provides insight into the differential amount of activity‐dependent slowing (and hence postexcitatory hyperpolarisation) in the stem axon and terminal branches of cutaneous C‐fibres. This comparison reveals that more than two‐thirds of the slowing can be attributed to the terminal branches, since it is two‐ to fourfold that observed during double stimulation as compared with the unidirectional block condition. This indicates that the terminal branches are equipped with membrane proteins that are different from those of the parent axon.