Secondary hyperalgesia and perceptual wind-up following intradermal injection of capsaicin in humans

Positive and negative affect dimensions in chronic knee osteoarthritis: effects on clinical and laboratory pain

OBJECTIVE

This study investigated whether daily and laboratory assessed pain differs as a function of the temporal stability and valence of affect in individuals with chronic knee osteoarthritis (KOA).

METHODS

One hundred fifty-one men and women with KOA completed 14 days of electronic diaries assessing positive affect (PA), negative affect (NA), and clinical pain. A subset of participants (n =79) engaged in quantitative sensory testing (QST). State PA and NA were assessed prior to administration of stimuli that induced suprathreshold pain and temporal summation. Multilevel modeling and multiple regression evaluated associations of affect and pain as a function of valence (i.e., positive versus negative) and stability (i.e., stable versus state).

RESULTS

In the diary, stable NA (B = -.63, standard error [SE] = .13, p < .001) was a stronger predictor of clinical KOA pain than stable PA (B = -.18, SE = .11, p = .091), and state PA (B = -.09, p < .001) was a stronger predictor of concurrent daily clinical pain than state NA (B = .04, SE = .02, p = .068). In the laboratory, state PA (B = -.05, SE = .02, p = .042), but not state NA (p = .46), predicted diminished temporal summation of mechanical pain.

CONCLUSIONS

Stable NA is more predictive of clinical pain than stable PA, whereas state PA is more predictive of both clinical and laboratory pain than state NA. The findings suggest that dynamic affect-pain processes in the field may reflect individual differences in central pain facilitation.

Individual modulation of pain sensitivity under stress

OBJECTIVES

Stress has a strong influence on pain sensitivity. However, the direction of this influence is unclear. Recent studies reported both decreased and increased pain sensitivities under stress, and one hypothesis is that interindividual differences account for these differences. The aim of our study was to investigate the effect of stress on individual pain sensitivity in a relatively large female sample.

METHODS

Eighty female participants were included. Pain thresholds and temporal summation of pain were tested before and after stress, which was induced by the Mannheim Multicomponent Stress Test. In an independent sample of 20 women, correlation coefficients between 0.45 and 0.89 indicated relatively high test-retest reliability for pain measurements.

RESULTS

On average, there were significant differences between pain thresholds under non-stress and stress conditions, indicating an increased sensitivity to pain under stress. No significant differences between non-stress and stress conditions were found for temporal summation of pain. On an individual basis, both decreased and increased pain sensitivities under stress conditions based on Jacobson's criteria for reliable change were observed. Furthermore, we found significant negative associations between pain sensitivity under non-stress conditions and individual change of pain sensitivity under stress. Participants with relatively high pain sensitivity under non-stress conditions became less sensitive under stress and vice versa.

DISCUSSION

These findings support the view that pain sensitivity under stress shows large interindividual variability, and point to a possible dichotomy of altered pain sensitivity under stress.

Discordance between pain and radiographic severity in knee osteoarthritis: findings from quantitative sensory testing of central sensitization

OBJECTIVE

Radiographic measures of the pathologic changes of knee osteoarthritis (OA) have shown modest associations with clinical pain. We sought to evaluate possible differences in quantitative sensory testing (QST) results and psychosocial distress profiles between knee OA patients with discordant versus congruent clinical pain reports relative to radiographic severity measures.

METHODS

A total of 113 participants (66.7% women; mean ± SD age 61.05 ± 8.93 years) with knee OA participated in the study. Radiographic evidence of joint pathology was graded according to the Kellgren/Lawrence scale. Central sensitization was indexed through quantitative sensory testing, including heat and pressure-pain thresholds, tonic suprathreshold pain (cold pressor test), and repeated phasic suprathreshold mechanical and thermal pain. Subgroups were constructed by dichotomizing clinical knee pain scores (median split) and knee OA grade scores (grades 1-2 versus 3-4), resulting in 4 groups: low pain/low knee OA grade (n = 24), high pain/high knee OA grade (n = 32), low pain/high knee OA grade (n = 27), and high pain/low knee OA grade (n = 30).

RESULTS

Multivariate analyses revealed significantly heightened pain sensitivity in the high pain/low knee OA grade group, while the low pain/high knee OA grade group was less pain-sensitive. Group differences remained significant after adjusting for differences on psychosocial measures, as well as age, sex, and race.

CONCLUSION

The results suggest that central sensitization in knee OA is especially apparent among patients with reports of high levels of clinical pain in the absence of moderate-to-severe radiographic evidence of pathologic changes of knee OA.

Modulation of central sensitisation by detoxification in MOH: results of a 12-month detoxification study

BACKGROUND

Human and animal models suggest that central sensitisation plays a role in medication-overuse headache (MOH). We aimed to study pain perception in MOH patients before and a year after withdrawal.

METHODS

We examined pain perception in 35 MOH patients before and two, six and 12 months after detoxification. For baseline comparison, we tested 40 healthy controls. We measured cephalic and extra-cephalic pressure-pain thresholds and supra-threshold pressure-pain scores and extra-cephalic pain thresholds, supra-threshold pain scores and temporal summation for electrical stimulation.

RESULTS

Of the 35 patients, 21 patients completed the entire study and remained cured of MOH. Statistically significant differences between patients and healthy controls were found in cephalic pressure-pain thresholds (137.3 kPa vs. 170 kPa, P < 0.05), extra-cephalic pressure pain thresholds (213.3 vs. 274.3 kPa, P < 0.05), in cephalic supra-threshold pressure-pain scores measured on a 100 mm visual analogue scale (61 vs. 27 mm, P < 0.05) and extra-cephalic supra-threshold pain scores for electrical stimulation (19.0 vs. 10.0 mm, P < 0.05). Cephalic supra-threshold pain scores decreased statistically significantly from 50.3 mm at baseline to 28.0 mm at the 12-month follow-up. In contrast to controls, temporal summation was not found in MOH patients before withdrawal, but after detoxification temporal summation normalised.

CONCLUSION

The central nervous system is sensitised in patients with MOH. For the first time we demonstrate that the pain perception continues to normalise up to a year after detoxification. This emphasises the importance of detoxification and follow-up to prevent relapse.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Sedative and analgesic effects of buprenorphine, combined with either acepromazine or dexmedetomidine, for premedication prior to elective surgery in cats and dogs

OBJECTIVE

To evaluate the sedative and analgesic effects of intramuscular buprenorphine with either dexmedetomidine or acepromazine, administered as premedication to cats and dogs undergoing elective surgery.

STUDY DESIGN

Prospective, randomized, blinded clinical study.

ANIMALS

Forty dogs and 48 cats.

METHODS

Animals were assigned to one of four groups, according to anaesthetic premedication and induction agent: buprenorphine 20 μg kg(-1) with either dexmedetomidine (dex) 250 μg m(-2) or acepromazine (acp) 0.03 mg kg(-1), followed by alfaxalone (ALF) or propofol (PRO). Meloxicam was administered preoperatively to all animals and anaesthesia was always maintained using isoflurane. Physiological measures and assessments of pain, sedation and mechanical nociceptive threshold (MNT) were made before and after premedication, intraoperatively, and for up to 24 hours after premedication. Data were analyzed with one-way, two-way and mixed between-within subjects anova, Kruskall-Wallis analyses and Chi squared tests. Results were deemed significant if p ≤ 0.05, except where multiple comparisons were performed (p ≤ 0.005).

RESULTS

Cats premedicated with dex were more sedated than cats premedicated with acp (p < 0.001) and ALF doses were lower in dex cats (1.2 ± 1.0 mg kg(-1) ) than acp cats (2.5 ± 1.9 mg kg(-1)) (p = 0.041). There were no differences in sedation in dogs however PRO doses were lower in dex dogs (1.5 ± 0.8 mg kg(-1) ) compared to acp dogs (3.3 ± 1.1 mg kg(-1) ) (p < 0.001). There were no differences between groups with respect to pain scores or MNT for cats or dogs.

CONCLUSION

Choice of dex or acp, when given with buprenorphine, caused minor, clinically detectable, differences in various characteristics of anaesthesia, but not in the level of analgesia.

CLINICAL RELEVANCE

A combination of buprenorphine with either acp or dex, followed by either PRO or ALF, and then isoflurane, accompanied by an NSAID, was suitable for anaesthesia in dogs and cats undergoing elective surgery. Choice of sedative agent may influence dose of anaesthetic induction agent.

No effect of a single session of transcranial direct current stimulation on experimentally induced pain in patients with chronic low back pain--an exploratory study

Transcranial direct current stimulation (tDCS) has been shown to modulate cortical excitability. A small number of studies suggested that tDCS modulates the response to experimental pain paradigms. No trials have been conducted to evaluate the response of patients already suffering from pain, to an additional experimental pain before and after tDCS. The present study investigated the effect of a single session of anodal, cathodal and sham stimulation (15 mins/1 mA) over the primary motor cortex on the perceived intensity of repeated noxious thermal and electrical stimuli and on elements of quantitative sensory testing (thermal pain and perception thresholds) applied to the right hand in 15 patients with chronic low back pain. The study was conducted in a double-blind sham-controlled and cross-over design. No significant alterations of pain ratings were found. Modalities of quantitative sensory testing remained equally unchanged. It is therefore hypothesized that a single 15 mins session of tDCS at 1 mA may not be sufficient to alter the perception of experimental pain and in patients with chronic pain. Further studies applying repetitive tDCS to patients with chronic pain are required to fully answer the question whether experimental pain perception may be influenced by tDCS over the motor cortex.

Enhanced sensitivity to punctate painful stimuli in female patients with chronic low back pain

Background

Chronic low back pain (CLBP) has been shown to be associated with various pathophysiological changes at several level of the sensorimotor system, pointing to a general hypersensitivity in CLBP patients. The aim of the present study was to investigate signs of generalized mechanical pain hypersensitivity in CLBP patients on the hand and on the painful site of the back.

Methods

Pinprick stimulation according to a validated standardized quantitative sensory testing protocol was used in 14 female CLBP patients and 14 healthy controls (HC) matched for sex and age. Stimulus response functions to pinprick stimulation on the skin were examined at the affected back and reference sites (hand palmar and hand dorsum). Data from CLBP patients were compared with HC and with reference data from the German Research Network on Neuropathic Pain.

Results

We found significant differences in the stimulus response functions between CLBP patients and HC. Pain ratings to the pinpricks were increased for low and moderate pinprick stimuli in CLBP patients. Importantly, this kind of specific pinprick hyperalgesia was found not only for the affected body site (back), but also for the remote reference sites (hand dorsum and hand palmar).

Conclusions

We interpret our results as pointing to changes in the nociceptive processing in CLBP at higher levels of the neuraxis, possibly thalamus and/or attentional control, rather than changes of spinal processing. Alternatively, there might be a higher vulnerability to noxious stimulation in CLBP patients.

Neuropathic pain: is quantitative sensory testing helpful?

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory system and is characterised by a combination of positive and negative sensory symptoms. Quantitative sensory testing (QST) examines the sensory perception after application of different mechanical and thermal stimuli of controlled intensity and the function of both large (A-beta) and small (A-delta and C) nerve fibres, including the corresponding central pathways. QST can be used to determine detection, pain thresholds and stimulus-response curves and can thus detect both negative and positive sensory signs, the second ones not being assessed by other methods. Similarly to all other psychophysical tests QST requires standardised examination, instructions and data evaluation to receive valid and reliable results. Since normative data are available, QST can contribute also to the individual diagnosis of neuropathy, especially in the case of isolated small-fibre neuropathy, in contrast to the conventional electrophysiology which assesses only large myelinated fibres. For example, detection of early stages of subclinical neuropathy in symptomatic or asymptomatic patients with diabetes mellitus can be helpful to optimise treatment and identify diabetic foot at risk of ulceration. QST assessed the individual's sensory profile and thus can be valuable to evaluate the underlying pain mechanisms which occur in different frequencies even in the same neuropathic pain syndromes. Furthermore, assessing the exact sensory phenotype by QST might be useful in the future to identify responders to certain treatments in accordance to the underlying pain mechanisms.

The science of migraine

The cardinal symptom of migraine is headache pain. In this paper we review the neurobiology of this pain as it is currently understood. In recent years, we discovered that the network of neurons that sense pain signals from the dura changes rapidly during the course of a single migraine attack and that the treatment of an attack is a moving target. We found that if the pain is not stopped within 10-20 minutes after it starts, the first set of neurons in the network, those located in the trigeminal ganglion, undergo molecular changes that make them hypersensitive to the changing pressure inside the head, which explains why migraine headache throbs and is worsened by bending over and sneezing. We found that if the pain is not stopped within 60-120 minutes, the second group of neurons in the network, those located in the spinal trigeminal nucleus, undergoes molecular changes that convert them from being dependent on sensory signals they receive from the dura by the first set of neurons, into an independent state in which they themselves become the pain generator of the headache. When this happens, patients notice that brushing their hair, taking a shower, touching their periorbital skin, shaving, wearing earrings, etc become painful, a condition called cutaneous allodynia. Based on this scenario, we showed recently that the success rate of rendering migraine patients pain-free increased dramatically if medication was given before the establishment of cutaneous allodynia and central sensitization. The molecular shift from activity-dependent to activity-independent central sensitization together with our recent conclusion that triptans have the ability to disrupt communications between peripheral and central trigeminovascular neurons (rather than inhibiting directly peripheral or central neurons) explain their clinical effects. Both our clinical and pre-clinical findings of the last five years point to possible short- and long-term advantages in using an early-treatment approach in the treatment of acute migraine attacks.

Preoperative scar hyperalgesia is associated with post-operative pain in women undergoing a repeat Caesarean delivery

BACKGROUND

Over 1.4 million Caesarean deliveries are performed annually in the United States, out of which 30% are elective repeat procedures. Post-operative hyperalgesia is associated with an increased risk for persistent post-surgical pain; however, there are no data on whether residual scar hyperalgesia (SHA) from a previous Caesarean delivery (CD) persists until the next delivery. We hypothesized that residual SHA may be present in a substantial proportion of women and is associated with increased post-operative pain.

METHODS

One hundred and sixty-three women scheduled for a repeat CD under spinal anaesthesia were enrolled into the study. Mechanical temporal summation (mTS) and SHA index were measured preoperatively. SHA was considered present when the index was >0. Post-operative pain scores at 12, 24 and 48 h and wound hyperalgesia (WHA) at 48 h were recorded.

RESULTS

SHA was present in 67 women 41% with a median SHA index of 0.42 (Q (25)  = 0.25; Q (75)  = 1.1, range 0.03-4.25). Women with SHA had overall higher post-operative pain scores and SHA was correlated with preoperative mTS (r = 0.164, p < 0.05), post-operative pain severity (r = 0.25, p < 0.002) and WHA at 48 h (r = 0.608, p < 0.001). Severe pain (visual analogue pain scale-S48 ≥ 7, n = 20) was predicted with a sensitivity and specificity of 60% and 62%, respectively. Positive predictive value was 18% and negative predictive value was 92%.

CONCLUSIONS

Preoperative SHA is present in 41% of women scheduled for repeat CD and is associated with increased mTS and post-operative pain. Screening for preoperative SHA may predict women at risk for increased post-operative pain, and guide post-operative analgesia to include anti-hyperalgesic drugs.

Multiple chemical sensitivity: on the scent of central sensitization

BACKGROUND

Multiple Chemical Sensitivity (MCS) is a chronic condition characterized by recurrent, non-specific symptoms in response to chemically unrelated exposures in non-toxic concentrations. Although the pathophysiology of MCS remains unknown, central sensitization may be an important factor contributing to the clinical manifestations.

PURPOSE

To use quantitative sensory testing (QST) to study central hyperexcitability and multiple aspects of central sensory processing in MCS patients without comorbid overlapping disorders and to compare the results with those among matched controls.

METHODS

15 MCS patients and 15 healthy matched controls underwent QST to assess the following aspects of pain: capsaicin-induced secondary punctate hyperalgesia, stimulus response function (SRF) to punctate mechanical stimuli before and after capsaicin injection, temporal summation to punctate stimuli post capsaicin injection, pressure pain thresholds, heat pain thresholds, tonic heat stimulation and conditioning pain modulation (CPM: formerly known as diffuse noxious inhibitory control or DNIC).

RESULTS

The mean area of capsaicin-induced secondary punctate hyperalgesia was significantly larger in MCS patients than in controls at 5, 30 and 60 min post capsaicin injection (p=0.01). In addition MCS patients reported higher ratings in response to punctate mechanical stimuli assessed by SRF compared with controls (p<0.001). The CPM test induced significantly higher pain ratings in patients than in controls (p=0.002). We found no group differences in pressure pain and heat pain thresholds, temporal summation to punctate stimuli post capsaicin injection, capsaicin and tonic heat pain ratings or CPM effect.

CONCLUSION

Increased capsaicin-induced secondary punctate hyperalgesia was demonstrated in MCS patients without comorbid, overlapping disorders, suggesting facilitated central sensitization in MCS.

Pain-associated mild sensory deficits without hyperalgesia in chronic non-neuropathic pain

OBJECTIVES

A mixture of sensory loss and gain is a hallmark of neuropathic pain. But hypesthesia and hyperalgesia also occur with experimentally induced acute pain. Here, we assessed sensory profiles in chronic non-neuropathic pain (osteoarthritis, OA) using the quantitative sensory testing (QST) protocol of the German Research Network on Neuropathic Pain (DFNS).

METHODS

Twenty individuals with OA [mean pain intensity on the numerical rating scale (NRS, 0-10): 5.6±1.5] were tested on the painful and contralateral hand and compared with 20 healthy volunteers matched for age, sex, and handedness.

RESULTS

In the OA group, analysis of variance revealed increased detection thresholds to tactile stimuli bilaterally and to thermal stimuli restricted to the more painful hand (all P<0.05). Pin-prick hypoalgesia was present restricted to the patients' more affected hand. Neither hyperalgesia nor allodynia was found. QST parameters were correlated with average pain intensity (r between 0.48 and 0.51).

CONCLUSIONS

These results suggest that chronic non-neuropathic pain may induce slight sensory impairment for large fiber function (bilateral) and small fiber function (ipsilateral). However, all changes are within the normal range, in contrast to patients with neuropathy. Inhibition of central pathways by nociceptive input and altered sensory processing due to disuse of the hand are possible mechanisms. These functional sensory alterations do not interfere with the diagnosis of neuropathy.

Antihyperalgesic and analgesic properties of the N-methyl-d-aspartate (NMDA) receptor antagonist neramexane in a human surrogate model of neurogenic hyperalgesia

NMDA-receptors are a major target in the prevention and treatment of hyperalgesic pain states in neuropathic pain. However, previous studies revealed equivocal results depending on study design and efficacy parameters. We tested the analgesic (generalized reduction of generation and processing of nociceptive signalling) and anti-hyperalgesic (prevention of central sensitization) properties of the NMDA-receptor antagonist neramexane and the potassium channel opener flupirtine in the intradermal capsaicin injection model. Furthermore, we tested the effect on pain summation (wind up). Eighteen healthy subjects received either a single dose of neramexane (40 mg p.o.), flupirtine (100 mg) or placebo in a double-blind, randomized, cross-over study. Pain evoked by intradermal capsaicin injection as well as pain evoked by pinpricks was significantly reduced by neramexane (-22% to -30% vs. placebo) in the non-sensitized skin indicating a marked analgesic effect. Moreover, dynamic mechanical allodynia (pain to light touch) was also significantly attenuated by neramexane (-28% vs. placebo). However, static secondary hyperalgesia to pinprick stimuli after capsaicin injection was not significantly reduced (-9% vs. placebo). Flupirtine showed no analgesic or anti-hyperalgesic effect. Mechanically-evoked wind up of pain sensation was not affected by any treatment. The results suggests that in a human surrogate model of neurogenic hyperalgesia a single low-dose of neramexane had a marked analgesic effect in the sensitized and in the non-sensitized state and thus may be a useful drug to treat the enhanced pain sensitivity in neuropathic pain patients. Its efficacy may be based on analgesia rather than anti-hyperalgesia or anti-windup. In contrast, flupirtine showed neither an analgesic nor an anti-hyperalgesic effect at a dose used for the treatment of postoperative pain.

Differential effects of experimental central sensitization on the time-course and magnitude of offset analgesia

Pain perception is temporally altered during states of chronic pain and acute central sensitization; however, the mechanisms contributing to temporal processing of nociceptive information remain poorly understood. Offset analgesia is a phenomenon that reflects the presence of temporal contrast mechanisms for nociceptive information and can provide an end point to study temporal aspects of pain processing. In order to investigate whether offset analgesia is disrupted during sensitized states, 23 healthy volunteers provided real-time continuous visual analogue scale responses to noxious heat stimuli that evoke offset analgesia. Responses to these stimuli were evaluated during capsaicin-heat sensitization (45°C stimulus, capsaicin cream 0.1%) and heat-only sensitization (40°C stimulus, placebo cream). Capsaicin-heat sensitization produced significantly larger regions of secondary mechanical allodynia compared to heat-only sensitization. Although areas of mechanical allodynia were positively related to individual differences in heat pain sensitivity, this relationship was altered at later time points after capsaicin-heat sensitization. Heat hyperalgesia was observed in the secondary region following both capsaicin-heat and heat-only sensitization. Increased latencies to maximal offset analgesia and prolonged aftersensations were observed only in the primary regions directly treated by capsaicin-heat or heat alone. However, contrary to the hypothesis that offset analgesia would be reduced following capsaicin-heat sensitization, the magnitude of offset analgesia remained remarkably intact after both capsaicin-heat and heat-only sensitization in zones of both primary and secondary mechanical allodynia. These data indicate that offset analgesia is a robust phenomenon and engages mechanisms that interact minimally with those supporting acute central sensitization.

Dynamic mechanical allodynia in the secondary hyperalgesic area in the capsaicin model—Perceptually similar phenomena as in painful neuropathy?

Introduction

In order to develop valid experimental human pain models, i.e., models potentially reflecting mechanisms underlying certain expressions of clinical pain conditions, similarities and discrepancies of symptoms/signs must first and foremost be evaluated comparing the two. In a situation where symptoms/signs appear to be similar, a potential pitfall with surrogate models would be that pathophysiological mechanisms in clinical conditions and experimental models might differ, i.e., one symptom/sign may be due to several different mechanisms. Symptoms and signs caused by intradermally injected capsaicin have been suggested to reflect aspects of the clinical phenomenology of neuropathic pain, e.g., dynamic mechanical allodynia. Psychophysical characteristics of brush-evoked pain in the pain area in patients with painful peripheral neuropathy were compared with brush-evoked pain in the secondary hyperalgesic area in capsaicin-treated skin in patients and in healthy subjects using different temporo-spatial stimulus parameters.

Method

Nine patients were examined in the area of painful neuropathy and subsequently in the corresponding contralateral secondary site, i.e., the secondary hyperalgesic area after an intradermal capsaicin injection. Nine healthy age- and sex-matched subjects were examined in a corresponding area after capsaicin injection. Brush-evoked allodynia was induced by lightly stroking 2 different distances of the skin 2 or 4 times with brushes of 2 different widths. Intensity and duration of brush-evoked allodynia was recorded using a computerized visual analogue scale. The total brush-evoked pain intensity, including aftersensation was calculated as the area under the curve. In addition, similarities and discrepancies in the selection of sensory-discriminative and affective descriptors of the painful experience have been surveyed in the area of neuropathy and in the area of secondary hyperalgesia.

Results

All patients reported brush-evoked pain in their area of painful neuropathy during all stimuli. Eight out of 9 patients reported brush-evoked pain in an area outside the flare in the capsaicin treated skin and only 3 out of 9 healthy subjects reported brush-evoked pain in an area outside the flare. Within patients there was no significant difference between sides regarding the influence of the various temporo-spatial stimulus parameters on the total brush-evoked pain intensity. Of all parameters tested, only increased number of strokes resulted in significantly higher brush-evoked pain intensity. The most commonly used sensory-discriminative descriptors during brush-evoked pain in the area of painful neuropathy and in the capsaicin-induced secondary hyperalgesic area in patients and controls were smarting and burning and for the affective descriptors troublesome and annoying.

Conclusions

Similarities were found regarding the influence of temporo-spatial stimulus parameters on brush-evoked allodynia in the capsaicin-induced secondary hyperalgesic area contralateral to the area of painful neuropathy and their influence when testing the area of neuropathic pain. Only 3/9 healthy subjects reported brush-evoked pain after capsaicin injection, a finding that may be related to this group reporting less spontaneous pain than the patients after injection. A hyperexcitable nervous system due to the contralateral clinical condition may also have a bearing on the frequent finding of capsaicin-induced allodynia in the patients (8/9).

Implications

The low prevalence of tactile allodynia in healthy volunteers makes the capsaicin model an unattractive strategy.

Analysis of hyperalgesia time courses in humans after painful electrical high-frequency stimulation identifies a possible transition from early to late LTP-like pain plasticity

Electrical high-frequency stimulation (HFS) of skin afferents elicits long-term potentiation (LTP)-like hyperalgesia in humans. Time courses were evaluated in the facilitating (homotopic) or facilitated (heterotopic) pathways to delineate the relative contributions of early or late LTP-like pain plasticity. HFS in healthy subjects (n=55) elicited highly significant pain increases to electrical stimuli via the conditioning electrode (to 145% of control, homotopic pain LTP) and to pinprick stimuli in adjacent skin (to 190% of control, secondary hyperalgesia). Individual time courses in subjects expressing a sufficient magnitude of hyperalgesia (>20% pain increase, n=28) revealed similar half-lives of homotopic pain LTP and secondary hyperalgesia of 6.9 h and 4.9 h (log(10) mean 0.839±0.395 and 0.687±0.306) and times to full recovery of 48 h and 24 h (log(10) mean 1.679±0.790 and 1.373±0.611). Time course and peak magnitudes were not correlated between (r=-0.19to+0.21, NS), nor within both readout (r=0.29 and 0.31, NS). In most subjects, time courses were consistent with early LTP1. Notably, in some subjects (10 of 28), estimated times to full recovery were much longer (>10 days), possibly indicating development of late LTP2-like pain plasticity. Dynamic mechanical allodynia (only present in 16 of 55 subjects) lasted for a shorter time than secondary hyperalgesia. Three different readouts of nociceptive central sensitization suggest that brief intense nociceptive input elicits early LTP1 of pain sensation (based on posttranslational modifications), but susceptible subjects may already develop longer-lasting late LTP2 (based on transcriptional modifications). These findings support the hypothesis that LTP may contribute to the development of persistent pain disorders.

A positron emission tomography study of wind-up pain in chronic postherniotomy pain

Many neuropathic pain conditions are characterized by abnormal responses to noxious or innocuous mechanical stimulation, including wind-up pain. Whereas previous brain imaging studies have explored the cerebral correlates of hyperalgesia and allodynia, no studies are available on mechanical-induced wind-up pain in neuropathic pain patients. We therefore used positron emission tomography (PET) to investigate the cerebral response pattern of mechanical wind-up pain in a homogenous group of 10 neuropathic pain patients with long-standing postherniotomy pain in the groin area. Patients were scanned in the following conditions: (1) rest; (2) wind-up pain, induced by 2 Hz von Frey stimulation in the painful area; (3) non-painful 2 Hz von Frey stimulation in the homologous contralateral area and (4) tonic pressure pain in the homologous contralateral area. A direct comparison between wind-up pain and non-painful von Frey stimulation revealed that the former more strongly activated contralateral secondary somatosensory cortex, insula, anterior cingulate cortex, right dorsolateral prefrontal cortex, thalamus and cerebellum. In addition, wind-up pain also activated the sublenticular extended amygdala (SLEA) and the brain stem. A direct comparison between wind-up pain and pressure pain revealed that both activated a largely overlapping network. Since no de novo brain areas were activated by wind-up pain, our data suggest that the processes specific to wind-up pain do not occur at the cerebral level.

Test-retest and interobserver reliability of quantitative sensory testing according to the protocol of the German Research Network on Neuropathic Pain (DFNS): a multi-centre study

Quantitative sensory testing (QST) is an instrument to assess positive and negative sensory signs, helping to identify mechanisms underlying pathologic pain conditions. In this study, we evaluated the test-retest reliability (TR-R) and the interobserver reliability (IO-R) of QST in patients with sensory disturbances of different etiologies. In 4 centres, 60 patients (37 male and 23 female, 56.4±1.9years) with lesions or diseases of the somatosensory system were included. QST comprised 13 parameters including detection and pain thresholds for thermal and mechanical stimuli. QST was performed in the clinically most affected test area and a less or unaffected control area in a morning and an afternoon session on 2 consecutive days by examiner pairs (4 QSTs/patient). For both, TR-R and IO-R, there were high correlations (r=0.80-0.93) at the affected test area, except for wind-up ratio (TR-R: r=0.67; IO-R: r=0.56) and paradoxical heat sensations (TR-R: r=0.35; IO-R: r=0.44). Mean IO-R (r=0.83, 31% unexplained variance) was slightly lower than TR-R (r=0.86, 26% unexplained variance, P<.05); the difference in variance amounted to 5%. There were no differences between study centres. In a subgroup with an unaffected control area (n=43), reliabilities were significantly better in the test area (TR-R: r=0.86; IO-R: r=0.83) than in the control area (TR-R: r=0.79; IO-R: r=0.71, each P<.01), suggesting that disease-related systematic variance enhances reliability of QST. We conclude that standardized QST performed by trained examiners is a valuable diagnostic instrument with good test-retest and interobserver reliability within 2days. With standardized training, observer bias is much lower than random variance. Quantitative sensory testing performed by trained examiners is a valuable diagnostic instrument with good interobserver and test-retest reliability for use in patients with sensory disturbances of different etiologies to help identify mechanisms of neuropathic and non-neuropathic pain.

The perception threshold counterpart to dynamic and static mechanical allodynia assessed using von Frey filaments in peripheral neuropathic pain patients

Background and aim Pain due to a usually non-painful mechanical stimulus, mechanical allodynia, is an oppressive symptom in subgroups of patients with neuropathic pain. Dynamic mechanical allodynia (DMA) is evoked by a normally innocuous light moving mechanical stimulus on the skin and static mechanical allodynia (SMA) by a sustained, normally innocuous pressure against the skin. DMA is claimed to be mediated by myelinated fibres and SMA by C-fibres. Also A-delta fibres have been implicated in the static subtype. A low intensity vertically applied stimulus of 1 second (s) is expected to activate predominantly rapidly adapting A-beta mechanoreceptors thus recruiting the same peripheral substrate as a horizontally moving brush on top of the skin. In patients with SMA we assumed an activation of Cbut also A-delta fibres from a static 10 s von Frey filament stimulus. The aim was to investigate if DMA and SMA could be assessed at perception threshold level using short or longer lasting usually non-painful von Frey filament prodding of the neuropathic skin. Patients and methods Eighteen patients with painful unilateral partial peripheral traumatic nerve injury suffering from SMA (n = 9) and/or DMA (n = 18) in a limb were studied. A compression/ischemia-induced (differential) nerve block in conjunction with repeated quantitative sensory testing of A-delta and C-fibre function using cold and warm stimuli was used to assess which nerve fibre population that contributes to pain at perception threshold level using 1 s (vF1) and 10 s (vF10) von Frey filament stimulation of the skin. Results The main outcome was the finding that elevation of vF1 and vF10 occurred simultaneously and significantly prior to an increase in the perception level to cold or warmth during the continuous nerve block. Single patients demonstrated a slight decrease in cold perception levels at the time of elevation of vF1 or vF10 and a possible contribution to mechanical allodynia from A-delta-fibres can therefore not completely be ruled out although the recorded alterations were minor. None of the patients reported an elevation of the perception level to warmth at the time of elevation of vF1 or vF10 excluding contribution from C-fibres. Further, only patients with clinically established SMA (n = 9) reported continuous pain to a sustained 10 s von Frey filament stimulation (vF10). Patients with only DMA (n = 9) reported pain merely for the initial 1-3 s of the total stimulus duration of 10 s and for a few seconds after the filament was lifted from the skin. Conclusions These findings support the role of A-beta fibres as peripheral mediators of both vF1 and vF10 although different receptor organs may be involved, i.e., rapidly (RA) and slowly (SA-I) adapting mechanoreceptors. Implications Techniques to quantify the different allodynias at perception threshold level deserve further attention as possible adjuncts to suprathreshold stimuli in intervention studies aimed at modifying these stimulus-evoked phenomena.

Wide-dynamic-range neurons are heterogeneous in windup responsiveness to changes in stimulus intensity and isoflurane anesthesia level in mice

The windup phenomenon in wide-dynamic-range (WDR) neurons represents a short-term neuronal sensitization to repetitive noxious inputs that may share similar mechanisms with those that trigger the development of persistent pain and hyperalgesia. Some WDR cells are readily sensitized and express prominent windup (windup(+)), whereas others do not (windup(-)). We recorded extracellular single-unit activity of deep laminae WDR neurons (350-700 microm) in C57BL/6 mice to determine how changes in stimulus intensity (1x and >2x C-component threshold, n = 53) and concentrations of isoflurane anesthesia (2.0% and 1.0%, n = 30) might differently modulate windup responsiveness in windup(+) and windup(-) cells. Two principally different analysis methods [absolute windup (the number of action potentials) and relative windup (the percentage of action potentials evoked by the first stimulus of the train)] were used to interpret windup data. We observed that increasing the stimulus intensity and decreasing the isoflurane concentration: 1) facilitated windup generation at 0.2-Hz stimulation and significantly enhanced absolute windup at both 0.2-Hz and 0.5-Hz stimulation predominantly in windup(+) cells but did not confer windup capability on windup(-) cells and 2) significantly increased relative windup at 0.2-Hz, but not 0.5-Hz, stimulation in windup(+) cells. Our findings advance our understanding of the neurobiology of deep WDR neurons in mice and demonstrate that two populations of cells differ in their windup responsiveness to changes in experimental conditions. We also elucidate the usefulness and potential limitations of two widely used methods for calculating and presenting windup data.

Central sensitization: implications for the diagnosis and treatment of pain

Nociceptor inputs can trigger a prolonged but reversible increase in the excitability and synaptic efficacy of neurons in central nociceptive pathways, the phenomenon of central sensitization. Central sensitization manifests as pain hypersensitivity, particularly dynamic tactile allodynia, secondary punctate or pressure hyperalgesia, aftersensations, and enhanced temporal summation. It can be readily and rapidly elicited in human volunteers by diverse experimental noxious conditioning stimuli to skin, muscles or viscera, and in addition to producing pain hypersensitivity, results in secondary changes in brain activity that can be detected by electrophysiological or imaging techniques. Studies in clinical cohorts reveal changes in pain sensitivity that have been interpreted as revealing an important contribution of central sensitization to the pain phenotype in patients with fibromyalgia, osteoarthritis, musculoskeletal disorders with generalized pain hypersensitivity, headache, temporomandibular joint disorders, dental pain, neuropathic pain, visceral pain hypersensitivity disorders and post-surgical pain. The comorbidity of those pain hypersensitivity syndromes that present in the absence of inflammation or a neural lesion, their similar pattern of clinical presentation and response to centrally acting analgesics, may reflect a commonality of central sensitization to their pathophysiology. An important question that still needs to be determined is whether there are individuals with a higher inherited propensity for developing central sensitization than others, and if so, whether this conveys an increased risk in both developing conditions with pain hypersensitivity, and their chronification. Diagnostic criteria to establish the presence of central sensitization in patients will greatly assist the phenotyping of patients for choosing treatments that produce analgesia by normalizing hyperexcitable central neural activity. We have certainly come a long way since the first discovery of activity-dependent synaptic plasticity in the spinal cord and the revelation that it occurs and produces pain hypersensitivity in patients. Nevertheless, discovering the genetic and environmental contributors to and objective biomarkers of central sensitization will be highly beneficial, as will additional treatment options to prevent or reduce this prevalent and promiscuous form of pain plasticity.

Quantitative sensory testing: a diagnostic tool for painful neuropathy

Quantitative sensory testing (QST) analyzes sensory perceptions of external stimuli of controlled intensity. Both large and small fiber function can be evaluated by applying thermal and mechanical stimuli, thus closing the diagnostic gap for the conventional electrophysiology in the examination of thin and unmyelinated nerve fibers. Detection, pain thresholds and stimulus–response curves can be determined; therefore, QST is a valuable diagnostic tool for identifying both sensory loss (i.e., hypoesthesia and hypoalgesia) and gain (i.e., hyperalgesia and allodynia) in patients with painful or painless neuropathy. Every psychophysical approach QST requires standardized stimuli administration, instructions and data evaluation to achieve valid results. QST can be used to evaluate distinct somatosensory profiles and thus give hints to the underlying mechanisms that occur with different frequencies in different pain syndromes. This might be helpful for the future establishment of mechanism-based pharmacotherapy. Since normative data are available, QST also contributes to the individual diagnosis of neuropathy. The present article gives an outline of QST application in diagnosis and its limitations for the evaluation of neuropathic pain.

Sensory changes and loss of intraepidermal nerve fibers in painful unilateral nerve injury

OBJECTIVES

Dysaesthesias is a common symptom in patients with neuropathic pain after peripheral nerve injury (PNI). In contrast to neuropathies with comparable symptoms there is little knowledge of the underlying mechanisms in PNI patients.

METHODS

Quantitative sensory testing according to the German Research Network on Neuropathic Pain protocol, and changes in intraepidermal nerve fiber density were assessed in 15 patients with dysaesthesias after PNI of the lower limb. According to their small-fiber function patients were assigned into 2 subgroups.

RESULTS

The sensory profiles of PNI patients were characterized predominantly by minus symptoms (significantly increased thresholds for perception of cold, warm, touch and vibration, and significantly increased thresholds for heat and mechanical pain) on the affected compared with the unaffected side. The only plus symptom reported was a significantly reduced pressure pain threshold. The sensory profile of patients with a severe loss of small-fiber function (n=7) showed a thermal and tactile hypoaesthesia and hypoalgesia; this was in contrast to patients with a moderate loss of small-fiber function, who showed a mild thermal and tactile hypoaesthesia associated with an increased mechanical pain sensitivity. Mean intraepidermal nerve fiber density was significantly decreased in the affected compared with unaffected skin [3.50 (4.00) vs. 11.10 (7.60) fibers/mm] and correlated with warm and mechanical detection thresholds (both r=-0.60).

DISCUSSION

In conclusion, even though patients presented with comparable clinical symptoms, their sensory profiles differed, supporting the concept of different underlying mechanisms leading to chronic pain in PNI patients. Skin biopsies support the validity of quantitative sensory testing.

Managing migraine associated with sensitization

The majority of migraineurs seeking secondary or tertiary medical care experience throbbing pain and cutaneous allodynia during the course of migraine. Underlying the origin of these symptoms are peripheral and central trigeminovascular neurons, whose cell bodies are located in the trigeminal ganglion and the spinal dorsal horn, respectively. The development of throbbing in the initial phase of migraine is mediated by sensitization of peripheral trigeminovascular neurons, whereas the development of cutaneous allodynia later in the attack is propelled by sensitization of central trigeminovascular neurons which, unfortunately, are not equipped to respond to triptans directly. Triptans appear to act presynaptically in the dorsal horn, such as to inhibit signal transmission from peripheral to central trigeminovascular neurons. Reining in the central neurons using triptan treatment is possible as long as their excitability remains driven by incoming signals from the meninges, but not after they develop autonomous activity. Accordingly, attacks with allodynia can be effectively terminated, provided that the patient vigilantly resorts to triptan therapy before or soon after the onset of allodynia, but not after allodynia has become firmly established. On the other hand, allodynic patients who missed the critical window for effective triptan therapy can still be rendered pain-free using an intravenous infusion of non-steroidal anti-inflammatory drugs.

Reliability of intraoral quantitative sensory testing (QST)

The German Research Network on Neuropathic Pain (DFNS) has recommended a protocol with 13 quantitative sensory testing (QST) measures for detecting somatosensory abnormalities. Reliability is an important scientific property and has been adequately tested for cutaneous QST. This study evaluates intraoral sites for which no reliability trials have yet been published. Inter- and intra-examiner reliability of 13 QST measures at intra- and extraoral trigeminal sites were investigated. Twenty-one healthy volunteers from Malmö University, Malmö, Sweden (13 women and 8 men, mean age 40.4 years, range 24-71) participated. Two independent examiners previously trained in the DFNS QST protocol examined the participants using the entire protocol. Each participant was examined twice on the same day, once by each examiner (inter-examiner reliability). After 1-3 weeks, one examiner re-examined all participants (intra-examiner reliability). The measurements were made on the skin of the right cheek, the tip of the tongue, and bilaterally on the gingival mucosa of the upper premolar region. The intraclass correlation coefficient (ICC) or kappa was used to calculate variations. Most tests had acceptable to excellent inter-examiner (ICC 0.41-0.89) and intra-examiner (ICC 0.43-0.87) reliability. For each test, inter- and intra-examiner reliabilities at intra- and extraoral sites were similar. No significant differences between right and left sides were found intraorally. We conclude that inter- and intra-examiner reliabilities of most QST measures are acceptable for assessing somatosensory function in the orofacial region.

Explicit episodic memory for sensory-discriminative components of capsaicin-induced pain: immediate and delayed ratings

Pain memory is thought to affect future pain sensitivity and thus contribute to clinical pain conditions. Systematic investigations of the human capacity to remember sensory features of experimental pain are sparse. In order to address long-term pain memory, nine healthy male volunteers received intradermal injections of three doses of capsaicin (0.05, 1 and 20 microg, separated by 15 min breaks), each given three times in a balanced design across three sessions at one week intervals. Pain rating was performed using a computerized visual analogue scale (0-100) digitized at 1/s, either immediately online or one hour or one day after injection. Subjects also recalled their pains one week later. Capsaicin injection reliably induced a dose-dependent flare (p<0.001) without any difference within or across sessions. The strong burning pain decayed exponentially within a few minutes. Subjects were able to reliably discriminate pain magnitude and duration across capsaicin doses (both p<0.001), regardless of whether first-time ratings were requested immediately, after one hour or after one day. Pain recall after one week was similarly precise (magnitude: p<0.01, duration: p<0.05). Correlation with rating recall after one week was best when first-time ratings were requested as late as one day after injection (R(2)=0.79) indicating that both rating retrievals utilized similar memory traces. These results indicate a reliable memory for magnitude and duration of experimentally induced pain. The data further suggest that the consolidation of this memory is an important interim stage, and may take up to one day.

Area 3a neuron response to skin nociceptor afferent drive

Area 3a neurons are identified that respond weakly or not at all to skin contact with a 25-38 degrees C probe, but vigorously to skin contact with the probe at > or =49 degrees C. Maximal rate of spike firing associated with 1- to 7-s contact at > or =49 degrees C occurs 1-2 s after probe removal from the skin. The activity evoked by 5-s contact with the probe at 51 degrees C remains above-background for approximately 20 s after probe retraction. After 1-s contact at 55-56 degrees C activity remains above-background for approximately 4 s. Magnitude of spike firing associated with 5-s contact increases linearly as probe temperature is increased from 49-51 degrees C. Intradermal capsaicin injection elicits a larger (approximately 2.5x) and longer-lasting (100x) increase in area 3a neuron firing rate than 5-s contact at 51 degrees C. Area 3a neurons exhibit enhanced or novel responsivity to 25-38 degrees C contact for a prolonged time after intradermal injection of capsaicin or alpha, beta methylene adenosine triphosphate. Their 1) delayed and persisting increase in spike firing in response to contact at > or =49 degrees C, 2) vigorous and prolonged response to intradermal capsaicin, and 3) enhanced and frequently novel response to 25-38 degrees C contact following intradermal algogen injection or noxious skin heating suggest that the area 3a neurons identified in this study contribute to second pain and mechanical hyperalgesia/allodynia.

Depression and changed pain perception: hints for a central disinhibition mechanism

Although patients with a depressive disorder report often of pain, their sensitivity to experimental pain is controversial, probably due to differences in sensory testing methods and to the lack of normal values. Therefore, we used a standardized and validated comprehensive sensory testing paradigm to assess the peripheral and central nervous system performance in depressive patients compared to healthy controls and chronic pain patients with fibromyalgia syndrome (FMS), in which depression is a common comorbidity. Twenty-five depressive psychiatric inpatients (pain-free: n=20), 35 FMS outpatients and 25 healthy controls underwent quantitative sensory testing (QST), including thermal and mechanical detection and pain thresholds, pain sensitivity and responsiveness to repetitive noxious mechanical stimuli (wind-up). In depressive disorder (to a lesser extent also in FMS), significantly decreased cold pain thresholds and an increased wind-up were found, although the mechanical pain thresholds and pain sensitivity were comparable to those of the healthy controls. All the detection thresholds were within the normal range in all the groups. In depressive disorder, there were no significant side differences in the detection and pain thresholds. The results contradict the former assumption of a general insensitivity to experimental pain in depressive disorder. In the mostly pain-free patients signs of an enhanced central hyperexcitability are even more pronounced than usually found in chronic pain patients (e.g. FMS), indicating common mechanisms in depressive disorder and chronic pain in accordance with the assumption of non-pain associated mechanisms in depressive disorder for central hyperexcitability, e.g. by inhibited serotonergic function. Furthermore, this trial demonstrates the feasibility of QST in depressive patients.

Quantitative sensory testing with Von Frey monofilaments in patients with allodynia: what are we quantifying?

The International Association for the Study of Pain defines allodynia as pain due to a stimulus that does not normally provoke pain and hyperalgesia as an increased response to a stimulus, which is normally painful. However, does "normally painful" mean "any stimulation of nociceptors" or "the subjective pain response?" We argue that "normally painful" should not mean "any stimulation of nociceptors," as Von Frey monofilaments may evoke a pricking sensation--which implies the involvement of nociceptors--without necessarily leading to a subjective pain perception. In this paper, we propose that the diagnosis of either allodynia or hyperalgesia should be based on the patient's report, that is, painful versus not painful, rather than on the (sub) type of afferent fiber involved.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

Retigabine, the specific KCNQ channel opener, blocks ectopic discharges in axotomized sensory fibres

The M-current has been proposed as a potential target for analgesia under neuropathic pain conditions. M-currents and/or their molecular correlates, KCNQ proteins, have been demonstrated in key elements of the nociceptive system including spinal and dorsal root ganglion neurons. Here we demonstrate that retigabine, a selective KCNQ channel opener, applied at neuromatose endings modulates the excitability of axotomized fibres inhibiting ectopic discharges. Responses to mechanical and chemical stimulation were obtained from intact and previously axotomized Adelta- and C-fibres using in vitro preparations and extracellular electrophysiological recording techniques. Application of retigabine (10 microM) produced an estimated approximately 80% reduction in the number of discharges produced by mechanical and chemical stimulation of most axotomized fibres tested (24/27). The electrical threshold of stimuli applied to the neuroma was found to increase in the presence of retigabine (+17.5+/-2.3%) and to decrease in the presence of a high potassium medium (-16.5+/-3.7%). This indicates that retigabine produces a hyperpolarization and a subsequent reduction of the excitability in aberrant sensory endings. Application of XE-991 (10 microM), a KCNQ channel blocker, had no effect on responses to stimulation of the neuroma but blocked the effects of retigabine indicating a specific involvement of KCNQ channels. In contrast to the strong effects on ectopic discharges, retigabine did not change responses to stimulation recorded from intact receptors. Results indicate that KCNQ channel opening at axotomized endings may constitute a novel and selective mechanism for modulation of some neuropathic pain symptoms.