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

Dose-response study of topical allyl isothiocyanate (mustard oil) as a human surrogate model of pain, hyperalgesia, and neurogenic inflammation

Despite being a ubiquitous animal pain model, the natural TRPA1-agonist allyl isothiocyanate (AITC, also known as "mustard oil") has only been sparsely investigated as a potential human surrogate model of pain, sensitization, and neurogenic inflammation. Its dose-response as an algogenic, sensitizing irritant remains to be elucidated in human skin. Three concentrations of AITC (10%, 50%, and 90%) and vehicle (paraffin) were applied for 5 minutes to 3 × 3 cm areas on the volar forearms in 14 healthy volunteers, and evoked pain intensity (visual analog scale 0-100 mm) and pain quality were assessed. In addition, a comprehensive battery of quantitative sensory tests was conducted, including assessment of mechanical and thermal sensitivity. Neurogenic inflammation was quantified using full-field laser perfusion imaging. Erythema and hyperpigmentation were assessed before, immediately after, and ≈64 hours after AITC exposure. AITC induced significant dose-dependent, moderate-to-severe spontaneous burning pain, mechanical and heat hyperalgesia, and dynamic mechanical allodynia (P < 0.05). No significant differences in induced pain hypersensitivity were observed between the 50% and 90% AITC concentrations. Acute and prolonged inflammation was evoked by all concentrations, and assessments by full-field laser perfusion imaging demonstrated a significant dose-dependent increase with a ceiling effect from 50% to 90%. Topical AITC application produces pain and somatosensory sensitization in a dose-dependent manner with optimal concentrations recommended to be >10% and ≤50%. The model is translatable to humans and could be useful in pharmacological proof-of-concept studies of TRPA1-antagonists, analgesics, and anti-inflammatory compounds or for exploratory clinical purposes, eg, loss- or gain-of-function in peripheral neuropathies.

The human pain system exhibits higher-order plasticity (metaplasticity)

The human pain system can be bidirectionally modulated by high-frequency (HFS; 100 Hz) and low-frequency (LFS; 1 Hz) electrical stimulation of nociceptors leading to long-term potentiation or depression of pain perception (pain-LTP or pain-LTD). Here we show that priming a test site by very low-frequency stimulation (VLFS; 0.05 Hz) prevented pain-LTP probably by elevating the threshold (set point) for pain-LTP induction. Conversely, prior HFS-induced pain-LTP was substantially reversed by subsequent VLFS, suggesting that preceding HFS had primed the human nociceptive system for pain-LTD induction by VLFS. In contrast, the pain elicited by the pain-LTP-precipitating conditioning HFS stimulation remained unaffected. In aggregate these experiments demonstrate that the human pain system expresses two forms of higher-order plasticity (metaplasticity) acting in either direction along the pain-LTD to pain-LTP continuum with similar shifts in thresholds for LTD and LTP as in synaptic plasticity, indicating intriguing new mechanisms for the prevention of pain memory and the erasure of hyperalgesia related to an already established pain memory trace. There were no apparent gender differences in either pain-LTP or metaplasticity of pain-LTP. However, individual subjects appeared to present with an individual balance of pain-LTD to pain-LTP (a pain plasticity "fingerprint").

Depressive Symptoms and Sleep Efficiency Sequentially Mediate Racial Differences in Temporal Summation of Mechanical Pain

BACKGROUND

Racial differences in endogenous pain facilitatory processes have been previously reported. Evidence suggests that psychological and behavioral factors, including depressive symptoms and sleep, can alter endogenous pain facilitatory processes. Whether depressive symptoms and sleep might help explain racial differences in endogenous pain facilitatory processes has yet to be determined.

PURPOSE

This observational, microlongitudinal study examined whether depressive symptoms and sleep were sequential mediators of racial differences in endogenous pain facilitatory processes.

METHODS

A total of 50 (26 African American and 24 non-Hispanic white) community-dwelling adults without chronic pain (mean 49.04 years; range 21-77) completed the Center for Epidemiological Studies Depression Scale prior to seven consecutive nights of sleep monitoring with actigraphy in the home environment. Participants subsequently returned to the laboratory for assessment of endogenous pain facilitation using a mechanical temporal summation protocol.

RESULTS

Findings revealed greater depressive symptoms, poorer sleep efficiency, and greater temporal summation of mechanical pain in African Americans compared to non-Hispanic whites. In a sequential mediation model, greater depressive symptoms predicted poorer sleep efficiency (t = -2.55, p = .014), and poorer sleep efficiency predicted enhanced temporal summation of mechanical pain (t = -4.11, p < .001), particularly for African Americans.

CONCLUSIONS

This study underscores the importance of examining the contribution of psychological and behavioral factors when addressing racial differences in pain processing. Additionally, it lends support for the deleterious impact of depressive symptoms on sleep efficiency, suggesting that both sequentially mediate racial differences in endogenous pain facilitation.

Generalized Pain Sensitization and Endogenous Oxytocin in Individuals With Symptoms of Migraine: A Cross-Sectional Study

OBJECTIVE

The current study examined pain and neurogenic inflammation responses to topical capsaicin during the interictal period (between headache) and their relationship with plasma oxytocin in individuals with migraine.

BACKGROUND

Individuals with migraine can experience generalized (extracephalic) hyperalgesia, which can persist even between headache attacks. Elevated levels of plasma and cerebrospinal fluid oxytocin have been observed during migraine attacks, oxytocin levels being positively associated with the intensity of migraine symptoms. However, whether oxytocin plays a role in the mechanisms of generalized pain sensitization and neurogenic inflammation during the interictal period has not been studied yet. Understanding migraineurs' interictal pain phenotype and endogenous oxytocin might help identify individuals who would benefit from intranasal oxytocin treatment.

METHODS

Thirty-two subjects with migraine and 26 healthy controls underwent pain testing. The current study compared capsaicin-induced pain, central sensitization (areas of secondary mechanical allodynia and hyperalgesia), and neurogenic inflammation (capsaicin-induced flare) responses on the nondominant volar forearm between migraineurs and healthy controls. Additionally, we studied plasma oxytocin levels and their relationship to migraine symptoms, experimental pain and affect.

RESULTS

The results indicated a significant group effect (P = .019): Migraineurs reported greater capsaicin-induced pain unpleasantness (M = 1.2, SD = 1.4) on a 0-10 scale and showed larger areas of flare (LnM = 2.8, SD = 0.4) than healthy controls (M = 0.5, SD = 0.8; LnM = 2.6, SD = 0.4; ps < .032). In a subgroup analysis, enhanced capsaicin-induced pain unpleasantness was found in the chronic (P = .007), but not the episodic (Ps > .200), migraineurs. The oxytocin levels were elevated in migraineurs and accounted for 18% of the group difference in capsaicin-induced pain unpleasantness. Within migraineurs, interictal oxytocin levels were negatively associated with psychological distress (Ps < .030). However, during the interictal period, pain sensitivity in extracephalic regions and plasma oxytocin levels were unrelated to migraine symptom parameters (Ps > .074). Lastly, the results found no group difference in areas of secondary mechanical allodynia and hyperalgesia (Ps >.298).

CONCLUSION

The current study revealed that individuals with migraine exhibit enhanced extracephalic capsaicin-induced pain unpleasantness and flare responses during interictal periods. In addition, migraineurs, especially those with chronic migraine, had slightly elevated interictal oxytocin levels compared to controls, which was associated with their affective component of experimental pain. Therefore, treatment targeting affective pain during the interictal period may help to reduce generalized pain in migraine. Furthermore, endogenous increases in oxytocin may be a compensatory mechanism that may help decrease affective distress in migraineurs. The therapeutic effects of intranasal oxytocin may benefit migraineurs by reducing their affective distress.

Turtle Flexion Reflex Motor Patterns Show Windup, Mediated Partly by L-type Calcium Channels

Windup is a form of multisecond temporal summation in which identical stimuli, delivered seconds apart, trigger increasingly strong neuronal responses. L-type Ca²⁺ channels have been shown to play an important role in the production of windup of spinal cord neuronal responses, initially in studies of turtle spinal cord and later in studies of mammalian spinal cord. L-type Ca²⁺ channels have also been shown to contribute to windup of limb withdrawal reflex (flexion reflex) in rats, but flexion reflex windup has not previously been described in turtles and its cellular mechanisms have not been studied. We studied windup of flexion reflex motor patterns, evoked with weak mechanical and electrical stimulation of the dorsal hindlimb foot skin and assessed via a hip flexor (HF) nerve recording, in spinal cord-transected and immobilized turtles in vivo. We found that an L-type Ca²⁺ channel antagonist, nifedipine, applied at concentrations of 50 μM or 100 μM to the hindlimb enlargement spinal cord, significantly reduced windup of flexion reflex motor patterns, while lower concentrations of nifedipine had no such effect. Nifedipine similarly reduced the amplitude of an individual flexion reflex motor pattern evoked by a stronger mechanical stimulus, in a dose-dependent manner, suggesting that L-type Ca²⁺ channels contribute to each flexion reflex as well as to multisecond summation of flexion reflex responses in turtles. We also found that we could elicit flexion reflex windup consistently using a 4-g von Frey filament, which is not usually considered a nociceptive stimulus. Thus, it may be that windup can be evoked by a wide range of tactile stimuli and that L-type calcium channels contribute to multisecond temporal summation of diverse tactile stimuli across vertebrates.

How stable are quantitative sensory testing measurements over time? Report on 10-week reliability and agreement of results in healthy volunteers

BACKGROUND

Quantitative sensory testing (QST) is a diagnostic tool for the assessment of the somatosensory system. To establish QST as an outcome measure for clinical trials, the question of how similar the measurements are over time is crucial. Therefore, long-term reliability and limits of agreement of the standardized QST protocol of the German Research Network on Neuropathic Pain were tested.

METHODS

QST on the lower back and hand dorsum (dominant hand) were assessed twice in 22 healthy volunteers (10 males and 12 females; mean age: 46.6±13.0 years), with sessions separated by 10.0±2.9 weeks. All measurements were performed by one investigator. To investigate long-term reliability and agreement of QST, differences between the two measurements, correlation coefficients, intraclass correlation coefficients (ICCs), Bland-Altman plots (limits of agreement), and standard error of measurement were used.

RESULTS

Most parameters of the QST were reliable over 10 weeks in healthy volunteers: Almost-perfect ICCs were observed for heat pain threshold (hand) and mechanical pain sensitivity (back). Substantial ICCs were observed for heat pain threshold (back), pressure pain threshold (back), mechanical pain sensitivity (hand), and vibration detection threshold (back and hand). Some QST parameters, such as cold detection threshold, exhibited low ICCs, but also very low variability. Generally, QST measures exhibited narrow limits of agreement in the Bland-Altman plots.

CONCLUSION

The standardized QST protocol of the German Research Network on Neuropathic Pain is feasible to be used in treatment trials. Moreover, defining a statistically meaningful change is possible, which is a prerequisite for the use of QST in clinical trials as well as in long-term investigations of disease progression.

Pain catastrophizing may moderate the association between pain and secondary hyperalgesia

Catastrophizing, a persistent negative mental set characterized by helplessness, rumination, and magnification of pain sensations, has a potent effect on pain report and clinical outcomes. Previous studies have documented an association between cognitive factors and central sensitization. The current analysis sought to test the potential modulating effect of pain catastrophizing on the association between capsaicin pain and the region of secondary hyperalgesia. Thirty-eight healthy individuals (50% women, mean age = 25.7, SD = 5.3) completed the Pain Catastrophizing Scale (PCS), then underwent topical application of 10% capsaicin, which was covered by a thermode maintained at 40°C for 90-min. Following removal of the capsaicin, the region of secondary hyperalgesia was determined. Hayes' PROCESS macro was employed to examine catastrophizing's potential moderating effect, which did not reveal a significant association between capsaicin pain ratings and the region of secondary hyperalgesia (β = 15.1, p = .06). Though PCS was not associated with area of secondary hyperalgesia (β = 23.9, p = .29), a significant interaction was present between PCS and capsaicin pain ratings (β = 3.7, p = .0004). Specifically, those endorsing higher catastrophizing levels and higher pain ratings experienced the greatest areas of secondary hyperalgesia. The Johnson-Neyman technique was used to determine the regional effect of the moderation, which indicated that when PCS scores were ≥10.6, capsaicin pain significantly moderated the association between pain and area of secondary hyperalgesia. These results suggest that catastrophizing plays an important role in the area of secondary hyperalgesia, and potentially central sensitization, warranting further testing in future research.

Central Sensitization of Mechanical Nociceptive Pathways Is Associated with a Long-Lasting Increase of Pinprick-Evoked Brain Potentials

Intense or sustained nociceptor activation, occurring, for example, after skin injury, can induce “central sensitization,” i.e., an increased responsiveness of nociceptive neurons in the central nervous system. A hallmark of central sensitization is increased mechanical pinprick sensitivity in the area surrounding the injured skin. The aim of the present study was to identify changes in brain activity related to this increased pinprick sensitivity. In 20 healthy volunteers, increased pinprick sensitivity was induced using high frequency electrical stimulation of the forearm skin (HFS). Mechanical pinprick stimulation (64 and 90 mN) was used to elicit event-related brain potentials (ERPs). The recordings were performed before, 20 min after and 45 min after applying HFS. The contralateral non-sensitized arm served as control. Pinprick stimulation of 64 mN, but not 90 mN, applied in the area of increased pinprick sensitivity elicited a significant increase of a late-latency positive wave, between 300 and 1100 ms after stimulus onset and was maximal at midline posterior electrodes. Most importantly, this increase in EEG activity followed the time course of the increase in pinprick perception, both being present 20 and 45 min after applying HFS. Our results show that the central sensitization of mechanical nociceptive pathways, manifested behaviorally as increased pinprick sensitivity, is associated with a long-lasting increase in pinprick-evoked brain potentials provided that a 64 mN stimulation intensity is used.

Activation of Peripheral μ-opioid Receptors by Dermorphin [D-Arg2, Lys4] (1-4) Amide Leads to Modality-preferred Inhibition of Neuropathic Pain

BACKGROUND

Opioids have long been regarded as the most effective drugs for the treatment of severe acute and chronic pain. Unfortunately, their therapeutic efficacy and clinical utility have been limited because of central and peripheral side effects.

METHODS

To determine the therapeutic value of peripheral μ-opioid receptors as a target for neuropathic pain treatment, the authors examined the effects of dermorphin [D-Arg2, Lys4] (1-4) amide (DALDA), a hydrophilic, peripherally acting μ-opioid receptor agonist, in male and female rats with spinal nerve ligation-induced neuropathic pain. The authors also utilized behavioral, pharmacologic, electrophysiologic, and molecular biologic tools to characterize DALDA's possible mechanisms of action in male rats.

RESULTS

DALDA, administered subcutaneously, had 70 times greater efficacy for inhibiting thermal (n = 8 to 11/group) than mechanical hypersensitivity (n = 6 to 8/group) in male rats. The pain inhibitory effects of DALDA on mechanical and heat hypersensitivity were abolished in animals pretreated with systemic methylnaltrexone (n = 7 to 9/group), a peripheral μ-opioid receptor antagonist. In the spinal wide-dynamic range neurons, systemic DALDA inhibited C-fiber-mediated, but not A-fiber-mediated, response in neuropathic male rats (n = 13). In primary sensory neurons, DALDA inhibited the capsaicin-induced [Ca2+] increase more than the β-alanine-induced [Ca] increase (n = 300); capsaicin and β-alanine activate subpopulations of neurons involved in the signaling of heat and mechanical pain, respectively. DALDA-treated rats (n = 5 to 8/group) did not exhibit motor deficits and locomotor impairment suggesting that it does not induce central side effects.

CONCLUSIONS

These findings suggest that DALDA may represent a potential alternative to current opioid therapy for the treatment of neuropathic pain and is likely to be associated with minimal adverse effects.

Quantitative Sensory Testing in Cluster Headache: Increased Sensory Thresholds

To determine if recently reported changes in sensory thresholds during migraine attacks can also be seen in cluster headache (CH), we performed quantitative sensory testing (QST) in 10 healthy subjects and in 16 patients with CH. Eight of the patients had an episodic CH and the other eight a chronic CH. The tests were performed on the right and left cheeks and on the right and left side of the back of the hands to determine the subjects' perception and pain thresholds for thermal (use of a thermode) and mechanical (vibration, pressure pain thresholds, pin prick, von Frey hairs) stimuli. Six patients were examined in the attack-free period. Three were also willing to repeat the tests a second time during an acute headache attack, which was elicited with nitroglycerin. The healthy subjects performed the experiments in the morning and evening of the same day to determine if sensory thresholds are independent of the time of day. If they were, this would allow estimation of the influence of the endogenous cortisone concentration on these thresholds. The control group showed no influence of the time of day on the thresholds. There was a significant difference in pain sensitivity between the back of the hands and the cheeks ( P < 0.05): higher thresholds were found on the back of the hands. The thresholds generally exhibited little intersubject variability, indicating that QST is a reliable method. There was also a significant difference between the test areas in the patient group ( P < 0.001): the cheeks were also more sensitive than the back of the hands. In comparison with reference data of healthy volunteers, the detection thresholds were increased in the patients on both test areas. These were statistically significant for warmth, thermal sensory limen (TSL), heat and pressure on the back of the hands ( P < 0.04) and for the warmth and TSL thresholds on the cheeks ( P < 0.05). There were no differences in the thresholds regardless of whether the patients were examined in or outside of a cluster bout. Furthermore, we found no cutaneous allodynia in the three patients tested during an attack. The increased sensory thresholds on the cheeks as well as on the back of the hands are in agreement with an increased activation of the patients' antinociceptive system. The seasonal variation and the temporal regularity of single attacks as well as the findings in imaging studies indicate that the hypothalamus is involved in the pathophysiology of CH. In view of the strong connectivity between the hypothalamus and areas involved in the antinociceptive system in the brainstem, we hypothesize that this connection is the reason for the increased sensory thresholds in CH patients found in our study.

Protein Kinase C γ Interneurons Mediate C-fiber–induced Orofacial Secondary Static Mechanical Allodynia, but Not C-fiber–induced Nociceptive Behavior

Background

Tissue injury enhances pain sensitivity both at the site of tissue damage and in surrounding uninjured skin (secondary hyperalgesia). Secondary hyperalgesia encompasses several pain symptoms including pain to innocuous punctate stimuli or static mechanical allodynia. How injury-induced barrage from C-fiber nociceptors produces secondary static mechanical allodynia has not been elucidated.

Methods

Combining behavioral, immunohistochemical, and Western blot analysis, the authors investigated the cell and molecular mechanisms underlying the secondary static mechanical allodynia in the rat medullary dorsal horn (MDH) using the capsaicin model (n = 4 to 5 per group).

Results

Intradermal injection of capsaicin (25 μg) into the vibrissa pad produces a spontaneous pain and a secondary static mechanical allodynia. This allodynia is associated with the activation of a neuronal network encompassing lamina I–outer lamina III, including interneurons expressing the γ isoform of protein kinase C (PKCγ) within inner lamina II (IIi) of MDH. PKCγ is concomitantly phosphorylated (+351.4 ± 79.2%, mean ± SD; P = 0.0003). Mechanical allodynia and innocuous punctate stimulus–evoked laminae I to III neuronal activation can be replicated after intracisternally applied γ-aminobutyric acid receptor type A (GABAA) antagonist (bicuculline: 0.05 μg) or reactive oxygen species (ROS) donor (tert-butyl hydroperoxide: 50 to 250 ng). Conversely, intracisternal PKCγ antagonist, GABAA receptor agonist, or ROS scavenger prevent capsaicin-induced static mechanical allodynia and neuronal activation.

Conclusions

Sensitization of lamina IIi PKCγ interneurons is required for the manifestation of secondary static mechanical allodynia but not for spontaneous pain. Such sensitization is driven by ROS and GABAAergic disinhibition. ROS released during intense C-fiber nociceptor activation might produce a GABAAergic disinhibition of PKCγ interneurons. Innocuous punctate inputs carried by Aδ low-threshold mechanoreceptors onto PKCγ interneurons can then gain access to the pain transmission circuitry of superficial MDH, producing pain.

Brain potentials evoked by intraepidermal electrical stimuli reflect the central sensitization of nociceptive pathways

Secondary mechanical punctate hyperalgesia is a cardinal sign of central sensitization (CS), an important mechanism of chronic pain. Our study demonstrates that hyperalgesia from intraepidermal electrical stimulation coexists with mechanical punctate hyperalgesia and elicits electroencephalographic (EEG) potentials that predict the occurrence of punctate hyperalgesia in a human experimental model of CS. These findings inform clinical development of EEG-based biomarkers of CS.

Central sensitization (CS), the increased sensitivity of the central nervous system to somatosensory inputs, accounts for secondary hyperalgesia, a typical sign of several painful clinical conditions. Brain potentials elicited by mechanical punctate stimulation using flat-tip probes can provide neural correlates of CS, but their signal-to-noise ratio is limited by poor synchronization of the afferent nociceptive input. Additionally, mechanical punctate stimulation does not activate nociceptors exclusively. In contrast, low-intensity intraepidermal electrical stimulation (IES) allows selective activation of type II Aδ-mechano-heat nociceptors (II-AMHs) and elicits reproducible brain potentials. However, it is unclear whether hyperalgesia from IES occurs and coexists with secondary mechanical punctate hyperalgesia, and whether the magnitude of the electroencephalographic (EEG) responses evoked by IES within the hyperalgesic area is increased. To address these questions, we explored the modulation of the psychophysical and EEG responses to IES by intraepidermal injection of capsaicin in healthy human subjects. We obtained three main results. First, the intensity of the sensation elicited by IES was significantly increased in participants who developed robust mechanical punctate hyperalgesia after capsaicin injection (i.e., responders), indicating that hyperalgesia from IES coexists with punctate mechanical hyperalgesia. Second, the N2 peak magnitude of the EEG responses elicited by IES was significantly increased after the intraepidermal injection of capsaicin in responders only. Third, a receiver-operator characteristics analysis showed that the N2 peak amplitude is clearly predictive of the presence of CS. These findings suggest that the EEG responses elicited by IES reflect secondary hyperalgesia and therefore represent an objective correlate of CS.

A Comparison of the Sensitivity of Brush Allodynia and Semmes-Weinstein Monofilament Testing in the Detection of Allodynia Within Regions of Secondary Hyperalgesia in Humans

BACKGROUND

Two of the most common Quantitative Sensory Techniques (QST) employed to detect allodynia include mechanical brush allodynia and Semmes-Weinstein monofilaments. However, their relative sensitivity at detecting allodynia is poorly understood. The purpose of this study was to compare the sensitivity of brush allodynia against Semmes-Weinstein monofilament technique for detecting allodynia within regions of secondary hyperalgesia in humans.

METHODS

Twenty subjects (10 males, 10 females; 21.1 ± 0.9 years) were recruited and randomly allocated to allodynia or monofilament groups. Topical capsaicin (Zostrix 0.075%) was applied to a target region defined by C4-C7 dermatomes. Allodynia testing was performed at 0- (baseline) and 10 minutes postcapsaicin. The Semmes-Weinstein group assessed changes in skin sensitivity 8 cm inferior to target region and 2 cm lateral to the spinous process, while brush allodynia was employed to detect the point inferior to the target region where subjects reported changes in skin sensitivity. The distance (cm) from this point to the inferior border of the target region was termed the Allodynia Score.

RESULTS

Statistically significant increases in the Allodynia Score were observed at 10 minutes postcapsaicin compared to baseline (P < 0.001). No differences in monofilament scores were observed between 10 minutes postcapsaicin and baseline (P = 0.125). Brush allodynia also demonstrated superior sensitivity, detecting allodynia in 100% of cases compared to 60% in the Semmes-Weinstein group.

CONCLUSION

Brush allodynia is more sensitive than Semmes-Weinstein monofilaments for detecting mechanical allodynia in regions of secondary hyperalgesia. Brush allodynia may be preferred over Semmes-Weinstein monofilaments for clinical applications requiring reliable detection of allodynia.

Differential contributions of A- and C-nociceptors to primary and secondary inflammatory hypersensitivity in the rat

There is sensitization to thermal A-nociceptor activation in arthritic secondary hyperalgesia, with enhanced activation of spinal lamina I neurons.

Primary hyperalgesia is characterized by increased responsiveness to both heat and mechanical stimulation in the area of injury. By contrast, secondary hyperalgesia is generally associated with increased responses to mechanical but not heat stimuli. We tested the hypothesis that sensitization in secondary hyperalgesia is dependent on the class of peripheral nociceptor (C- or A-nociceptor) rather than the modality of stimulation (mechanical vs heat). A- and C-nociceptors were selectively activated using contact heat ramps applied to the hind paw dorsum in animals with hind paw inflammation (primary hyperalgesia) and knee inflammatory arthritis (secondary hyperalgesia). Sensitization to A- and C-nociceptor activation in primary and secondary hyperalgesia was assessed by reflex withdrawal thresholds and by Fos immunocytochemistry in the dorsal horn of the spinal cord, as an index of neuronal activation. In primary hyperalgesia, only C-nociceptor-evoked withdrawal reflexes were sensitized. This was associated with increased spinal lamina I neuronal activation to both A- and C-nociceptor activation. Fos-like immunoreactivity (FLI) was unchanged in other dorsal horn laminae. In secondary hyperalgesia, only A-nociceptor-evoked withdrawal reflexes were sensitized, and FLI was increased in both superficial and deep dorsal laminae. Neurons in the superficial dorsal horn receive and process nociceptor inputs from the area of primary hyperalgesia, resulting in functional sensitization to C-nociceptive inputs. In inflammatory arthritis, secondary hyperalgesia is evoked by A-nociceptor thermal stimulation, suggesting that secondary hyperalgesia is A-nociceptor, rather than stimulus modality (mechanical vs thermal), dependent. Fos-like immunoreactivity evoked by A-nociceptor stimulation in secondary hyperalgesia suggests that the sensitization is underpinned by spinal neuronal sensitization in laminae I and IV/V.

Amplitudes of Pain-Related Evoked Potentials Are Useful to Detect Small Fiber Involvement in Painful Mixed Fiber Neuropathies in Addition to Quantitative Sensory Testing - An Electrophysiological Study

To investigate the usefulness of pain-related evoked potentials (PREP) elicited by electrical stimulation for the identification of small fiber involvement in patients with mixed fiber neuropathy (MFN). Eleven MFN patients with clinical signs of large fiber impairment and neuropathic pain and ten healthy controls underwent clinical and electrophysiological evaluation. Small fiber function, electrical conductivity and morphology were examined by quantitative sensory testing (QST), PREP, and skin punch biopsy. MFN was diagnosed following clinical and electrophysiological examination (chronic inflammatory demyelinating neuropathy: n = 6; vasculitic neuropathy: n = 3; chronic axonal ­neuropathy: n = 2). The majority of patients with MFN characterized their pain by descriptors that mainly represent C-fiber-mediated pain. In QST, patients displayed elevated cold, warm, mechanical, and vibration detection thresholds and cold pain thresholds indicative of MFN. PREP amplitudes in patients correlated with cold (p < 0.05) and warm detection thresholds (p < 0.05). Burning pain and the presence of par-/dysesthesias correlated negatively with PREP amplitudes (p < 0.05). PREP amplitudes correlating with cold and warm detection thresholds, burning pain, and par-/dysesthesias support employing PREP amplitudes as an additional tool in conjunction with QST for detecting small fiber impairment in patients with MFN.

Characterizing pinprick-evoked brain potentials before and after experimentally induced secondary hyperalgesia

Secondary hyperalgesia is believed to be a key feature of "central sensitization" and is characterized by enhanced pain to mechanical nociceptive stimuli. The aim of the present study was to characterize, using EEG, the effects of pinprick stimulation intensity on the magnitude of pinprick-elicited brain potentials [event-related potentials (ERPs)] before and after secondary hyperalgesia induced by intradermal capsaicin in humans. Pinprick-elicited ERPs and pinprick-evoked pain ratings were recorded in 19 healthy volunteers, with mechanical pinprick stimuli of varying intensities (0.25-mm probe applied with a force extending between 16 and 512 mN). The recordings were performed before (T0) and 30 min after (T1) intradermal capsaicin injection. The contralateral noninjected arm served as control. ERPs elicited by stimulation of untreated skin were characterized by 1) an early-latency negative-positive complex peaking between 120 and 250 ms after stimulus onset (N120-P240) and maximal at the vertex and 2) a long-lasting positive wave peaking 400-600 ms after stimulus onset and maximal more posterior (P500), which was correlated to perceived pinprick pain. After capsaicin injection, pinprick stimuli were perceived as more intense in the area of secondary hyperalgesia and this effect was stronger for lower compared with higher stimulus intensities. In addition, there was an enhancement of the P500 elicited by stimuli of intermediate intensity, which was significant for 64 mN. The other components of the ERPs were unaffected by capsaicin. Our results suggest that the increase in P500 magnitude after capsaicin is mediated by facilitated mechanical nociceptive pathways.

Effects of Combined Electrical Stimulation of the Dorsal Column and Dorsal Roots on Wide-Dynamic-Range Neuronal Activity in Nerve-Injured Rats

OBJECTIVES

Electrical stimulation at the dorsal column (DC) and dorsal root (DR) may inhibit spinal wide-dynamic-range (WDR) neuronal activity in nerve-injured rats. The objective of this study was to determine if applying electrical conditioning stimulation (CS) at both sites provides additive or synergistic benefits.

MATERIALS AND METHODS

By conducting in vivo extracellular recordings of WDR neurons in rats that had undergone L5 spinal nerve ligation, we tested whether combining 50 Hz CS at the two sites in either a concurrent (2.5 min) or alternate (5 min) pattern inhibits WDR neuronal activity better than CS at DC alone (5 min). The intensities of CS were determined by recording antidromic compound action potentials to graded stimulation at the DC and DR. We measured the current thresholds that resulted in the first detectable Aα/β waveform (Ab0) and the peak Aα/β waveform (Ab1) to select CS intensity at each site. The same number of electrical pulses and amount of current were delivered in different patterns to allow comparison.

RESULTS

At a moderate intensity of 50% (Ab0 + Ab1), different patterns of CS all attenuated the C-component of WDR neurons in response to graded intracutaneous electrical stimuli (0.1-10 mA, 2 msec) and inhibited windup in response to repetitive noxious stimuli (0.5 Hz). However, the inhibitory effects did not differ significantly between different patterns. At the lower intensity (Ab0), no CS inhibited WDR neurons.

CONCLUSIONS

These findings suggest that combined stimulation of DC and DR may not be superior to DC stimulation alone for inhibition of WDR neurons.

Demarcation of secondary hyperalgesia zones: Punctate stimulation pressure matters

BACKGROUND

Secondary hyperalgesia is increased sensitivity in normal tissue near an injury, and it is a measure of central sensitization reflecting injury-related effects on the CNS. Secondary hyperalgesia areas (SHAs), usually assessed by polyamide monofilaments, are important outcomes in studies of analgesic drug effects in humans. However, since the methods applied in demarcating the secondary hyperalgesia zone seem inconsistent across studies, we examined the effect of a standardized approach upon the measurement of SHA following a first degree burn injury (BI).

NEW METHOD

The study was a two-observer, test-retest study with the two sessions separated by 6wk. An observer-blinded design adjusted to examine day-to-day and observer-to-observer variability in SHA was used. In 23 healthy volunteers (12 females/11 males) a BI was induced by a contact thermode (47.0°C, 420s, 2.5×5.0cm(2)). The SHA, demarcated by polyamide monofilaments (bending force: 0.2, 69 and 2569mN) and a "weighted-pin" stimulator (512mN), were assessed 45 to 75min after each BI.

RESULTS

A random effect, linear mixed model demonstrated a logarithmic correlation between elicited skin pressures (mN/mm(2)) and the SHAs (P<0.0001). No day-to-day or observer-to-observer differences in SHAs were observed. Intraclass correlation coefficients, in the range of 0.51 to 0.84, indicated a moderate to almost perfect reliability between observers.

COMPARISON WITH EXISTING METHODS

No standardized approach in SHA-assessment has hitherto been presented.

CONCLUSIONS

This is the first study to demonstrate that demarcation of secondary hyperalgesia zones depends on the developed pressure of the punctate stimulator used.

Effects of centrally acting analgesics on spinal segmental reflexes and wind-up

BACKGROUND

The spinal cord is a prime site of action for analgesia. Here we characterize the effects of established analgesics on segmental spinal reflexes. The aim of the study was to look for the pattern of action or signature of analgesic effects on these reflexes.

METHODS

We used a spinal cord in vitro preparation of neonate mice to record ventral root responses to dorsal root stimulation. Pregabalin, clonidine, morphine and duloxetine and an experimental sigma-1 receptor antagonist (S1RA) were applied to the preparation in a cumulative concentration protocol. Drug effects on the wind-up produced by repetitive stimulation of C-fibres and on responses to single A- and C-fibre intensity stimuli were analysed.

RESULTS

All compounds produced a concentration-dependent inhibition of total spikes elicited by repetitive stimulation. Concentrations producing ∼50% reduction in this parameter were (in μM) clonidine (0.01), morphine (0.1), pregabalin (1), duloxetine (10) and S1RA (30). At these concentrations clonidine, pregabalin and S1RA had significant effects on the wind-up index and little depressant effects on responses to single stimuli. Morphine and duloxetine did not depress wind-up index and showed large effects on responses to single stimuli. None of the compounds had strong effects on the amplitude of the non-nociceptive monosynaptic reflex.

CONCLUSIONS

morphine and duloxetine had general depressant effects on spinal reflexes, whereas the effects of clonidine, pregabalin and S1RA appeared to be restricted to signals originated by strong repetitive activation of C-fibres. Results are discussed in the context of reported behavioural effects of the compounds studied.

Dynamic Pain Phenotypes are Associated with Spinal Cord Stimulation-Induced Reduction in Pain: A Repeated Measures Observational Pilot Study

OBJECTIVE

Spinal cord stimulation (SCS) has become a widely used treatment option for a variety of pain conditions. Substantial variability exists in the degree of benefit obtained from SCS and patient selection is a topic of expanding interest and importance. However, few studies have examined the potential benefits of dynamic quantitative sensory testing (QST) to develop objective measures of SCS outcomes or as a predictive tool to help patient selection. Psychological characteristics have been shown to play an important role in shaping individual differences in the pain experience and may aid in predicting responses to SCS. Static laboratory pain-induction measures have also been examined in their capacity for predicting SCS outcomes.

METHODS

The current study evaluated clinical, psychological and laboratory pain measures at baseline, during trial SCS lead placement, as well as 1 month and 3 months following permanent SCS implantation in chronic pain patients who received SCS treatment. Several QST measures were conducted, with specific focus on examination of dynamic models (central sensitization and conditioned pain modulation [CPM]) and their association with pain outcomes 3 months post SCS implantation.

RESULTS

Results suggest few changes in QST over time. However, central sensitization and CPM at baseline were significantly associated with clinical pain at 3 months following SCS implantation, controlling for psycho/behavioral factors and pain at baseline. Specifically, enhanced central sensitization and reduced CPM were associated with less self-reported pain 3 months following SCS implantation.

CONCLUSIONS

These findings suggest a potentially important role for dynamic pain assessment in individuals undergoing SCS, and hint at potential mechanisms through which SCS may impart its benefit.

Transcutaneous spinal DC stimulation reduces pain sensitivity in humans

Non-invasive approaches to pain management are needed to manage patient pain escalation and to providing sufficient pain relief. Here, we evaluate the potential of transcutaneous spinal direct current stimulation (tsDCS) to modulate pain sensitivity to electrical stimuli and mechanical pinpricks in 24 healthy subjects in a sham-controlled, single-blind study. Pain ratings to mechanical pinpricks and electrical stimuli were recorded prior to and at three time points (0, 30, and 60min) following 15min of anodal tsDCS (2.5mA, "active" electrode centered over the T11 spinous process, return electrode on the left posterior shoulder). Pain ratings to the pinpricks of the highest forces tested (128, 256, 512mN) were reduced at 30min and 60min following anodal tsDCS. These findings demonstrate that pain sensitivity in healthy subjects can be suppressed by anodal tsDCS and suggest that tsDCS may provide a non-invasive tool to manage mechanically-induced pain.

Electrical stimulation of dorsal root entry zone attenuates wide-dynamic-range neuronal activity in rats

OBJECTIVES

Recent clinical studies suggest that neurostimulation at the dorsal root entry zone (DREZ) may alleviate neuropathic pain. However, the mechanisms of action for this therapeutic effect are unclear. Here, we examined whether DREZ stimulation inhibits spinal wide-dynamic-range (WDR) neuronal activity in nerve-injured rats.

MATERIALS AND METHODS

We conducted in vivo extracellular single-unit recordings of WDR neurons in rats after an L5 spinal nerve ligation (SNL) or sham surgery. We set bipolar electrical stimulation (50 Hz, 0.2 msec, 5 min) of the DREZ at the intensity that activated only Aα/β-fibers by measuring the lowest current at which DREZ stimulation evoked a peak antidromic sciatic Aα/β-compound action potential without inducing an Aδ/C-compound action potential (i.e., Ab1).

RESULTS

The elevated spontaneous activity rate of WDR neurons in SNL rats (n = 25; data combined from post-SNL groups at days 14-16 [n = 15] and days 45-75 [n = 10]) was significantly decreased from the prestimulation level (p < 0.01) at 0-15 min and 30-45 min post-stimulation. In both sham-operated (n = 8) and nerve-injured rats, DREZ stimulation attenuated the C-component, but not the A-component, of the WDR neuronal response to graded intracutaneous electrical stimuli (0.1-10 mA, 2 msec) applied to the skin receptive field. Further, DREZ stimulation blocked windup (a form of brief neuronal sensitization) to repetitive noxious stimuli (0.5 Hz) at 0-15 min in all groups (p < 0.05).

CONCLUSIONS

Attenuation of WDR neuronal activity may contribute to DREZ stimulation-induced analgesia. This finding supports the notion that DREZ may be a useful target for neuromodulatory control of pain.

Enhanced brain responses to C-fiber input in the area of secondary hyperalgesia induced by high-frequency electrical stimulation of the skin

High-frequency electrical stimulation (HFS) of the human skin induces an increase in both mechanical and heat pain sensitivity in the surrounding unconditioned skin. The aim of this study was to investigate the effect of HFS on the intensity of perception and brain responses elicited by the selective activation of C fibers. HFS was applied to the ventral forearm of 15 healthy volunteers. Temperature-controlled CO2 laser stimulation was used to activate selectively low-threshold C-fiber afferents without concomitantly activating Aδ-fiber afferents. These stimuli were detected with reaction times compatible with the conduction velocity of C fibers. The intensity of perception and event-related brain potentials (ERPs) elicited by thermal stimuli delivered to the surrounding unconditioned skin were recorded before (T0) and after HFS (T1: 20 min after HFS; T2: 45 min after HFS). The contralateral forearm served as a control. Mechanical hyperalgesia following HFS was confirmed by measuring the change in the intensity of perception elicited by mechanical punctate stimuli. HFS resulted in increased intensity of perception to mechanical punctate stimulation and selective C-fiber thermal stimulation at both time points. In contrast, the N2 wave of the ERP elicited by C-fiber stimulation (679 ± 88 ms; means ± SD) was enhanced at T1 but not at T2. The P2 wave (808 ± 105 ms) was unaffected by HFS. Our results suggest that HFS enhances the sensitivity to thermal C-fiber input in the area of secondary hyperalgesia. However, there was no significant enhancement of the magnitude of the C-fiber ERPs at T2, suggesting that quickly adapting C fibers do not contribute to this enhancement.

An improved model of heat-induced hyperalgesia--repetitive phasic heat pain causing primary hyperalgesia to heat and secondary hyperalgesia to pinprick and light touch

This study tested a modified experimental model of heat-induced hyperalgesia, which improves the efficacy to induce primary and secondary hyperalgesia and the efficacy-to-safety ratio reducing the risk of tissue damage seen in other heat pain models. Quantitative sensory testing was done in eighteen healthy volunteers before and after repetitive heat pain stimuli (60 stimuli of 48°C for 6 s) to assess the impact of repetitive heat on somatosensory function in conditioned skin (primary hyperalgesia area) and in adjacent skin (secondary hyperalgesia area) as compared to an unconditioned mirror image control site. Additionally, areas of flare and secondary hyperalgesia were mapped, and time course of hyperalgesia determined. After repetitive heat pain conditioning we found significant primary hyperalgesia to heat, and primary and secondary hyperalgesia to pinprick and to light touch (dynamic mechanical allodynia). Acetaminophen (800 mg) reduced pain to heat or pinpricks only marginally by 11% and 8%, respectively (n.s.), and had no effect on heat hyperalgesia. In contrast, the areas of flare (−31%) and in particular of secondary hyperalgesia (−59%) as well as the magnitude of hyperalgesia (−59%) were significantly reduced (all p<0.001). Thus, repetitive heat pain induces significant peripheral sensitization (primary hyperalgesia to heat) and central sensitization (punctate hyperalgesia and dynamic mechanical allodynia). These findings are relevant to further studies using this model of experimental heat pain as it combines pronounced peripheral and central sensitization, which makes a convenient model for combined pharmacological testing of analgesia and anti-hyperalgesia mechanisms related to thermal and mechanical input.

Effect of a reversal mirror condition on orofacial mechanical sensitivity

The aim of this study was to investigate whether watching one's own face being touched in a reversal mirror condition modulates orofacial somatosensory sensitivity. A total of 37 healthy volunteers participated in a pilot study, the main study, and a control experiment. In the main experiment, 16 participants received seven different intensities of pinprick stimuli in the right infraorbital region. The perceived stimulus intensity was rated on a 0-50-100 numerical rating scale (NRS). In addition, the pinprick threshold (PiPT) was evaluated in the same region using an electronic von Frey device. During stimuli, participants were watching their own face in two different conditions (normal and reversal mirror) in randomized order. Subjective experiences during each condition were assessed with a questionnaire containing nine statements. The participants rated their level of agreement with the statements using a 7-item Likert scale. There were significant main effects on NRS scores of stimulus forces (p < 0.001) and experimental condition (p < 0.001). Post hoc analyses showed that stimulation with higher force levels induced significantly higher NRS scores (p < 0.001), but interestingly, there were significantly lower NRS scores in the reversal mirror condition than in the normal mirror condition (p < 0.001). There was no significant main effect of experimental condition on PiPT (p = 0.184). The experimental condition influenced the response to several statements significantly (p < 0.001). The somatosensory sensitivity may be impaired when the location of stimulation is not in accordance with the perception. In conclusion, hypoesthetic effects of a reversal mirror were present for fixed force measures but not for threshold measures. Further studies are now needed to describe the potential implications for other somatosensory modalities and orofacial pain conditions.

Evidence for acute central sensitization to prolonged experimental pain in posttraumatic stress disorder

BACKGROUND

Post-traumatic stress disorder (PTSD) and pain have a well-documented high comorbidity; however, the underlying mechanisms of this comorbidity are currently poorly understood. The aim of this psychophysical study was to investigate the behavioral response to a prolonged suprathreshold pain stimulus in subjects with combat-related PTSD and combat controls (CC) for clinical evidence of central sensitization.

METHODS

Ten male subjects with current PTSD related to combat and 11 CC male subjects underwent baseline quantitative sensory testing (QST), temporal pain summation, and psychological profiling followed by an intramuscular injection of capsaicin into the quadriceps muscle.

RESULTS

There was no significant between-group difference for the initial maximal pain response or an initial pain reduction for the first 15 minutes postinjection on QST or pain ratings. However, we observed significantly higher scores in the PTSD group for the second 15 minutes postinjection on both pain intensity and pain unpleasantness ratings. Assessment of temporal summation to repetitive pressure stimuli showed significantly higher subjective pain in the PTSD group.

CONCLUSION

These findings are consistent with a significantly higher degree of acute central sensitization in individuals with PTSD. Increased acute central sensitization may underlie increased vulnerability for developing pain-related conditions following combat trauma.

Can quantitative sensory testing move us closer to mechanism-based pain management?

OBJECTIVE

This review summarizes the scientific literature relating to the use of quantitative sensory testing (QST) for mechanism-based pain management.

DESIGN

A literature search was undertaken using PubMed and search terms including quantitative sensory testing, pain, chronic pain, response to treatment, outcome measure.

SETTINGS AND PATIENTS

Studies including QST in healthy individuals and those with painful disorders were reviewed.

MEASURES

Publications reported on QST methodological issues including associations among measures and reliability. We also included publications on the use of QST measures in case-control studies, their associations with biopsychosocial mechanisms, QST measures predicting clinical pain, as well as predicting and reflecting treatment responses.

RESULTS

Although evidence suggests that QST may be useful in a mechanism-based classification of pain, there are gaps in our current understanding that need to be addressed including making QST more applicable in clinical settings. There is a need for developing shorter QST protocols that are clinically predictive of various pain subtypes and treatment responses without requiring expensive equipment. Future studies are needed, examining the clinical predictive value of QST including sensitivity and specificity for pain classification or outcome prediction. These findings could enable third-party payers' reimbursement, which would facilitate clinical implementation of QST.

CONCLUSIONS

With some developments, QST could become a cost-effective and clinically useful component of pain assessment and diagnosis, which can further our progress toward the goal of mechanism-based personalized pain management.

Pinprick-evoked brain potentials: a novel tool to assess central sensitization of nociceptive pathways in humans

Although hyperalgesia to mechanical stimuli is a frequent sign in patients with inflammation or neuropathic pain, there is to date no objective electrophysiological measure for its evaluation in the clinical routine. Here we describe a technique for recording the electroencephalographic (EEG) responses elicited by mechanical stimulation with a flat-tip probe (diameter 0.25 mm, force 128 mN). Such probes activate Aδ nociceptors and are widely used to assess the presence of secondary hyperalgesia, a psychophysical correlate of sensitization in the nociceptive system. The corresponding pinprick-evoked potentials (PEPs) were recorded in 10 subjects during stimulation of the right and left hand dorsum before and after intradermal injection of capsaicin into the right hand and in 1 patient with a selective lesion of the right spinothalamic tract. PEPs in response to stimulation of normal skin were characterized by a vertex negative-positive (NP) complex, with N/P latencies and amplitudes of 111/245 ms and 3.5/11 μV, respectively. All subjects developed a robust capsaicin-induced increase in the pain elicited by pinprick stimulation of the secondary hyperalgesic area (+91.5%, P < 0.005). Such stimulation also resulted in a significant increase of the N-wave amplitude (+92.9%, P < 0.005), but not of the P wave (+6.6%, P = 0.61). In the patient, PEPs during stimulation of the hypoalgesic side were reduced. These results indicate that PEPs 1) reflect cortical activities triggered by somatosensory input transmitted in Aδ primary sensory afferents and spinothalamic projection neurons, 2) allow quantification of experimentally induced secondary mechanical hyperalgesia, and 3) have the potential to become a diagnostic tool to substantiate mechanical hyperalgesia in patients with presumed central sensitization.

The major brain endocannabinoid 2-AG controls neuropathic pain and mechanical hyperalgesia in patients with neuromyelitis optica

Recurrent myelitis is one of the predominant characteristics in patients with neuromyelitis optica (NMO). While paresis, visual loss, sensory deficits, and bladder dysfunction are well known symptoms in NMO patients, pain has been recognized only recently as another key symptom of the disease. Although spinal cord inflammation is a defining aspect of neuromyelitis, there is an almost complete lack of data on altered somatosensory function, including pain. Therefore, eleven consecutive patients with NMO were investigated regarding the presence and clinical characteristics of pain. All patients were examined clinically as well as by Quantitative Sensory Testing (QST) following the protocol of the German Research Network on Neuropathic Pain (DFNS). Additionally, plasma endocannabinoid levels and signs of chronic stress and depression were determined. Almost all patients (10/11) suffered from NMO-associated neuropathic pain for the last three months, and 8 out of 11 patients indicated relevant pain at the time of examination. Symptoms of neuropathic pain were reported in the vast majority of patients with NMO. Psychological testing revealed signs of marked depression. Compared to age and gender-matched healthy controls, QST revealed pronounced mechanical and thermal sensory loss, strongly correlated to ongoing pain suggesting the presence of deafferentation-induced neuropathic pain. Thermal hyperalgesia correlated to MRI-verified signs of spinal cord lesion. Heat hyperalgesia was highly correlated to the time since last relapse of NMO. Patients with NMO exhibited significant mechanical and thermal dysesthesia, namely dynamic mechanical allodynia and paradoxical heat sensation. Moreover, they presented frequently with either abnormal mechanical hypoalgesia or hyperalgesia, which depended significantly on plasma levels of the endogenous cannabinoid 2-arachidonoylglycerole (2-AG). These data emphasize the high prevalence of neuropathic pain and hyperalgesia in patients with NMO. The degree of mechanical hyperalgesia reflecting central sensitization of nociceptive pathways seems to be controlled by the major brain endocannabinoid 2-AG.

Central sensitization in spinal cord injured humans assessed by reflex receptive fields

OBJECTIVE

To investigate the effects of central sensitization, elicited by intramuscular injection of capsaicin, by comparing the reflex receptive fields (RRF) of spinally-intact volunteers and spinal cord injured volunteers that present presensitized spinal nociceptive mechanisms.

METHODS

Fifteen volunteers with complete spinal cord injury (SCI) and fourteen non-injured (NI) volunteers participated in the experiment. Repeated electrical stimulation was applied on eight sites on the foot sole to elicit the nociceptive withdrawal reflex (NWR). RRF were assessed before, 1min after and 60min after an intramuscular injection of capsaicin in the foot sole in order to induce central sensitization.

RESULTS

Both groups presented RRF expansion and lowered NWR thresholds immediately after capsaicin injection, reflected by the enlargement of RRF sensitivity areas and RRF probability areas. Moreover, the topography of the RRF sensitivity and probability areas were significantly different in SCI volunteers compared to NI volunteers in terms of size and shape.

CONCLUSIONS

SCI volunteers can develop central sensitization, despite adaptive/maladaptive changes in synaptic plasticity and lack of supraspinal control.

SIGNIFICANCE

Protective plastic mechanisms may still be functional in SCI volunteers.

Naturopathic reflex therapies for the treatment of chronic pain - Part 2: Quantitative sensory testing as a translational tool

Naturopathic reflex therapies such as massage, Gua Sha massage, cupping, wet packs etc. are likely able to influence chronic pain at different levels of the nociceptive system. Since naturopathic reflex therapies have been shown to reduce symptoms of chronic pain and often utilize intense manipulation of the environment of the nociceptor (e.g. Gua Sha massage or cupping), it can be hypothesized that they unfold part of their effect at the level of the peripheral nociceptor and the spinal cord. However, these hypotheses have to date not been tested systematically. Standardized sensory testing, e.g., as performed by 'quantitative sensory testing' (QST), a comprehensive battery of tests for clinical trials, may offer additional information about the mechanisms of naturopathic reflex therapies since it provides a measure for the mechanisms of nociceptive pain on all levels of the pain processing system. This method paper describes the potential role of QST in research on the neurobiological mechanisms of naturopathic reflex therapies.

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.