H. O. Handwerker

Selective excitation by capsaicin of mechano-heat sensitive nociceptors in rat skin

The effect of close-by arterial injections of capsaicin on single afferent fibers of the saphenous nerve was studied on 82 units from control rats and on 44 units from rats pretreated with capsaicin (total dose 200 mg/kg applied subcutaneously under anesthesia 3 days before the experiment). In control rats low doses of capsaicin selectively excited mechano-heat sensitive cutaneous nociceptors (polymodal C fiber nociceptors and MH-A delta nociceptors). The median threshold dose for polymodal nociceptors was 0.1 micrograms. Repeated injections of capsaicin in near-threshold doses evoked reproducible effects without obvious signs of desensitization. In contrast A delta high-threshold mechanoreceptors, hair follicle receptors, cold receptors and C fiber mechanoreceptors were not excited by capsaicin even at doses of 5 micrograms. These high doses activated, however, some SA-II mechanoreceptors after a time lag, probably due to increased tissue turgor induced by plasma extravasation. Systemic capsaicin pretreatment of adult rats resulted in a selective decrease in the proportion of polymodal nociceptors among the afferent C-units, and an increment in the threshold dose of capsaicin of the responding polymodal nociceptors. It is concluded that in the adult rat capsaicin exerts a selective stimulatory and blocking effect on cutaneous mechano-heat sensitive nociceptors conducting both in the C fiber and A delta fiber range.

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.

The effect of capsaicin application to a peripheral nerve on impulse conduction in functionally identified afferent nerve fibres

Capsaicin applied locally to a coccygeal or saphenous nerve of rats was shown to block impulse conduction in unmyelinated afferent nerve fibres. Shortly after application of capsaicin (1% dissolved in 10% Tween 80 in paraffin oil) conduction of C-fibre--but not A-fibre--compound action potentials across the application site was markedly diminished. No recovery occurred during the period of observation, i.e. up to 2 h afterwards. C-Fibre compound action potentials stimulated and recorded proximal to the application site seemed to be unaffected. Solvent application had no effect. Recording from single units revealed that nociceptive C-fibres responding to strong mechanical and heat stimulation (MH units, polymodal nociceptors) were blocked. In contrast, unmyelinated cold fibres were not affected. The selective block of MH units indicates different membrane properties of unmyelinated MH and cold units not only at the receptive nerve endings, but also at the axons, to which capsaicin was applied.

Excitation of cutaneous afferent nerve endings in vitro by a combination of inflammatory mediators and conditioning effect of substance P.

SummaryA broad mixture of inflammatory mediators (“inflammatory soup”) was used to investigate the responsiveness of primary afferents from rat hairy skin in an in vitro skin-saphenous nerve preparation. In addition, a conditioning effect of the tachykinin substance P on chemosensitivity of nociceptors was examined. Inflammatory soup (IS) was made up in synthetic interstitial fluid from bradykinin, serotonin, histamin and prostaglandin E2 (all 10-5 M). In addition, the potassium and the hydrogen ion concentration (7 mM, pH 7.0) and the temperature (39.5°C) were elevated. The latter agents, in a control solution, did not excite nociceptors (n = 5). IS was repeatedly superfused over the receptive fields for 5 min at 10 min intervals; substance P (SP 10-6 and 10-5 M) was applied during the last 5 min of the interval and during the subsequent IS stimulation. IS excited more than 80% of the mechano-heat sensitive (“polymodal”) afferents with slowly conducting nerve fibres (n = 72), but none of the low-threshold mechanoreceptive slow and fast conducting units (n = 17). Slow conducting afferents with high mechanical threshold (n = 35) were weakly, and less frequently (<20%), driven by IS. A majority, but not all, of the responsive units showed tachyphylaxis upon repeated IS application. None, however, lost its responsiveness completely. Conditioning heat stimulation (32–46.5°C in 20 s) did not enhance the subsequent IS response, which may indicate that sensitizing substances normally released by a noxious heat stimulus were already contained in IS. No sensitization to mechanical (von Frey) or heat stimulation could be established in the period after the IS response had subsided and after the washout was completed, respectively. A short-lived sensitization may have been overlooked under these temporal restrictions. Conditioning SP in 10-5 M but not in 10-6 M concentration significantly increased the IS response of polymodal C fibres, by 58% on average (n = 14). SP did not excite the units. Comparing with previous data, we conclude that there is a significant synergism between inflammatory mediators, acting to induce more intense and more sustained discharge via many nociceptors than single mediators alone could achieve. Conditioning substance P can further enhance this algogenic action. Mechanisms of interaction and relative contributions of single substances remain to be elucidated.

Unresponsive afferent nerve fibres in the sural nerve of the rat.

1. The proportion of primary afferent nerve fibres in a skin nerve of the rat that responded or failed to respond to mechanical or thermal stimulation of the skin in the noxious and non‐noxious range was analysed. 2. Activity of afferent nerve fibres was recorded from the dorsal roots. Units projecting into the sural nerve were selected using supramaximal electrical stimulation of the nerve stem. All other hindleg nerves were cut. 3. The receptive fields were searched by carefully examining the hindleg skin with noxious and innocuous mechanical, cooling and warming stimuli. Probing of the intrinsic foot muscles and manipulation of the ankle and toe joints was employed to recruit units projecting to deeper tissues. 4. In a first series of twenty‐two experiments, eighty‐nine rapidly conducting myelinated A beta units, thirty slowly conducting myelinated A delta units and 101 unmyelinated C units were investigated. Most units were identified as belonging to one of the established classes of cutaneous sensory units and this was also ascertained by a collision test. 5. Two A beta, eight A delta and forty‐six C fibres did not respond to any one of the stimuli. Electrical thresholds and conduction velocities of the unresponsive C fibres were not significantly different from those of the units responding to natural stimulation of their receptive fields. In the A delta group unresponsive and high threshold mechanoreceptive units were preferentially found among the units with the slowest conduction velocities. 6. In a second series of seven experiments, one single nerve filament containing responsive and unresponsive C fibres was tested repetitively at 30 min intervals. Twenty unresponsive units and seven units responding to noxious mechanical and/or heat stimuli were studied. Ten of the twenty initially unresponsive units became activated by mechanical and/or heat stimuli after observation times of 30‐150 min. Some of these units had mechanical thresholds as low as 64 mN (tested with calibrated von Frey hairs), or thermal thresholds down to 42 degrees C. 7. Two of the ten C units which became responsive in the course of an experiment later lost their responsiveness again. On the other hand, two of the C units which were initially responsive to noxious heat and/or noxious mechanical stimuli became completely unresponsive after repetitive stimulation, whereas one unit initially only responding to noxious heat became responsive to mechanical stimuli, suggesting that mechanical and heat responsiveness may be separately gained or lost by sensory C fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Painful stimuli evoke itch in patients with chronic pruritus Central sensitization for itch

Background: Central sensitization for pain is important for patients with chronic pain. The authors investigated a possible role of central sensitization for itch in patients with chronic pruritus. Methods: Noxious stimuli were applied in lesional and visually nonlesional skin areas of 25 patients with atopic dermatitis, in lesional skin areas of 9 patients with psoriasis vulgaris, and in 20 healthy subjects. The stimuli included mechanical pinpricks, electrical stimuli, contact heat, and injection of low-pH solution. Intensities of itch and pain were assessed separately on a numeric rating scale. Results: All the noxious stimuli primarily evoked pain in control subjects and patients with psoriasis vulgaris. In patients with atopic dermatitis, however, itch was evoked instead of burning pain. In their lesional skin, itch was the predominant sensation. Chemical stimuli evoked intense itch in lesional and visually healthy skin areas (the area under the curve of itch rating compared with the control, mean ± SEM, 668 ± 166 and 625 ± 192 vs 38 ± 23; p < 0.001; p < 0.01). Chemically induced itch also was observed in healthy subjects after a conditioning histamine stimulus of 15 minutes, but not after a conditioning histamine stimulus of 2 minutes. Conclusion: The chronic barrage of pruriceptive input may elicit central sensitization for itch so that nociceptive input no longer inhibits itch but on the contrary is perceived as itch. In contrast to the well-known A-fiber-mediated alloknesis and hyperknesis, this type of central sensitization appears to be elicited by C-nociceptors.

The effect of carrageenan-induced inflammation on the sensitivity of unmyelinated skin nociceptors in the rat

&NA; Carrageenan was subcutaneously injected in the area innervated by the saphenous nerve. Part of the mechano‐heat sensitive C‐fiber receptors (CHM) located inside or at the border of the inflamed area showed an enhanced responsiveness to heat stimulation (sensitization). Those CMH units exhibited spontaneous activity; their mechanical thresholds (von Frey) were higher than those of not spontaneously active fibers. None of the units located outside of the inflamed area displayed sensitization. The data reveal that only part of the CMH units in a uniformly inflamed skin area shows signs of sensitization. Our results are compared to those obtained in other inflammatory processes. The relation to inflammatory pain and to hyperalgesia and the contribution of endogenous substances to sensitization of CMH units are discussed.

Sensitization of insensitive branches of C nociceptors in human skin.

1. Eighteen cutaneous mechanosensitive C nociceptors were recorded from the peroneal nerves of healthy human subjects. Their identity was continuously monitored by intracutaneous electrical stimulation, and their activation by mechanical or transcutaneous electrical stimulation was detected by slowing of conduction velocity during the relative refractory period. 2. Mechanoreceptive fields (mRFs) mapped with suprathreshold von Frey hair stimuli covered an area of 99 +/‐ 21 mm2 (mean +/‐ S.E.M.). Two of the units had separate mRFs, with borders about 0.5‐1.5 cm apart from each other and the largest of these units had a maximal diameter of 4.5 cm. 3. Successive topical application of mustard oil and capsaicin induced expansions of mRFs by 57 +/‐ 14 mm2 in eight of fifteen units. 4. In twelve units transcutaneous electrical stimulation delivered through a pointed electrode was used for mapping the electroreceptive fields (eRFs). The borders of the eRFs and the mRFs were identical for two of twelve units only. In the other ten units additional mechano‐insensitive areas (55 +/‐ 22 mm2) were detected from which transcutaneous electrical stimuli could activate the respective unit. 5. Application of mustard oil and capsaicin to these mechano‐insensitive areas sensitized five of eight units to mechanical stimuli. In these cases the mRF after sensitization exactly corresponded to the eRF. 6. It is concluded that there are insensitive branches in human mechanosensitive cutaneous C nociceptors that can be detected by transcutaneous electrical stimulation and sensitized by topical application of chemical irritants. Activation of those branches in the course of inflammatory processes may contribute to spatial summation at central synapses and hence to hyperalgesia.

Discharge patterns of afferent cutaneous nerve fibers from the rat's tail during prolonged noxious mechanical stimulation

SummaryFeedback controlled constant force stimuli of 4, 6 and 8 N intensities and of 120 s duration were applied to the receptive fields of cutaneous afferent fibers in the rats tail. Two types of nociceptive units showed sustained discharges during these stimuli: “polymodal” unmyelinated C-units (MH-C units, N = 18, c.v. 0.5–0.9 m/s) and high-threshold mechanoreceptive A-delta-units(HTM-units, N=10, c.v. 1.9–11.2 m/s). In addition two classes of sensitive low threshold mechanoreceptors, SA I (N=6) and SA II (N=5) units, responded to the prolonged mechanical stimuli. At the onset of a noxious pressure, 11 of the 18 polymodal nociceptors exhibited dynamic responses (lasting about 10 s) which were followed by slowly adapting tonic discharges that lasted for the duration of the stimuli. The remaining polymodal C-fiber units (8/18) did not show dynamic discharges at 4 and 6 N. Phasic and tonic discharges were positively correlated with stimulus strength. The HTM-units encoded stimulation intensity mainly by their dynamic discharges. The tonic discharges of these units displayed faster adaptation rates with stronger mechanical stimuli, i.e. encoding of stimulation intensity became progressively weaker during the tonic phase. The discharges of sensitive SA I and SA II units with A beta axons were not positively correlated with the strength of noxious pressure stimuli. Tonic discharge rates of SA I units were negatively correlated to stimulus strength, whereas SA II units usually stopped firing in the course of a stimulus and became reversibly irresponsive to mechanical stimulation. Possible afferent mechanisms underlying the induction of pain by sustained noxious mechanical stimulation are discussed.

Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation

SummaryThis single fiber study on rat tail nerve afferents attempts to establish a peripheral neural correlate for the hyperalgesia to mechanical stimulation which follows injury to the skin. Mechano-heat sensitive C fibers (MH-C or “polymodal” nociceptors) and high-threshold mechanoreceptive A delta fibers (HTM-A delta) were examined with a series of constant noxious pressure stimulations (4-6-8-4 N on 25 mm2, 120 s each, 5 min intervals). These injurious stimuli were either directed to the most sensitive spot of the receptive fields (central stimulation) or closely outside their borders (1–5 mm). With this protocol no clear sensitization was seen in MH-C fibers apart from a stronger dynamic response to central stimulation in some of them. In contrast, most HTM-A delta units, irrespective of the site of noxious stimulation, developed spontaneous activity, lowering of their von Frey thresholds and expansion of their receptive fields. All HTM-A delta units responded to outside stimulation: upon the first stimulus (4 N) there was a delayed discharge of continuously increasing frequency (“recruited response”), but the onset of the last stimulation (4 N repeated) evoked vigorous dynamic responses in many fibers. The recruitment of HTM-A delta nociceptor activity may contribute to post-injury hyperalgesia to mechanical stimulation and it may counteract adaptation of the single afferent fiber during prolonged noxious influence.