The human capsaicin model of allodynia and hyperalgesia: sources of variability and methods for reduction

Interleukin-1α links peripheral CaV2.2 channel activation to rapid adaptive increases in heat sensitivity in skin

Neurons have the unique capacity to adapt output in response to changes in their environment. Within seconds, sensory nerve endings can become hypersensitive to stimuli in response to potentially damaging events. The underlying behavioral response is well studied, but several of the key signaling molecules that mediate sensory hypersensitivity remain unknown. We previously discovered that peripheral voltage-gated CaV2.2 channels in nerve endings in skin are essential for the rapid, transient increase in sensitivity to heat, but not to mechanical stimuli, that accompanies intradermal capsaicin. Here we report that the cytokine interleukin-1α (IL-1α), an alarmin, is necessary and sufficient to trigger rapid heat and mechanical hypersensitivity in skin. Of 20 cytokines screened, only IL-1α was consistently detected in hind paw interstitial fluid in response to intradermal capsaicin and, similar to behavioral sensitivity to heat, IL-1α levels were also dependent on peripheral CaV2.2 channel activity. Neutralizing IL-1α in skin significantly reduced capsaicin-induced changes in hind paw sensitivity to radiant heat and mechanical stimulation. Intradermal IL-1α enhances behavioral responses to stimuli and, in culture, IL-1α enhances the responsiveness of Trpv1-expressing sensory neurons. Together, our data suggest that IL-1α is the key cytokine that underlies rapid and reversible neuroinflammatory responses in skin.

Interleukin-1α links peripheral CaV2.2 channel activation to rapid adaptive increases in heat sensitivity in skin

Neurons have the unique capacity to adapt output in response to changes in their environment. Within seconds, sensory nerve endings can become hypersensitive to stimuli in response to potentially damaging events. The underlying behavioral response is well studied, but several of the key signaling molecules that mediate sensory hypersensitivity remain unknown. We previously discovered that peripheral voltage-gated CaV2.2 channels in nerve endings in skin are essential for the rapid, transient increase in sensitivity to heat, but not to mechanical stimuli, that accompanies intradermal capsaicin. Here we report that the cytokine interleukin-1α (IL-1α), an alarmin, is necessary and sufficient to trigger rapid heat and mechanical hypersensitivity in skin. Of 20 cytokines screened, only IL-1α was consistently detected in hind paw interstitial fluid in response to intradermal capsaicin and, similar to behavioral sensitivity to heat, IL-1α levels were also dependent on peripheral CaV2.2 channel activity. Neutralizing IL-1α in skin significantly reduced capsaicin-induced changes in hind paw sensitivity to radiant heat and mechanical stimulation. Intradermal IL-1α enhances behavioral responses to stimuli and, in culture, IL-1α enhances the responsiveness of Trpv1-expressing sensory neurons. Together, our data suggest that IL-1α is the key cytokine that underlies rapid and reversible neuroinflammatory responses in skin.

Responders and nonresponders to topical capsaicin display distinct temporal summation of pain profiles

Introduction

Topical application of capsaicin can produce an ongoing pain state in healthy participants. However, approximately one-third report no pain response (ie, nonresponders), and the reasons for this are poorly understood.

Objectives

In this study, we investigated temporal summation of pain (TSP) profiles, pain ratings and secondary hyperalgesia responses in responders and nonresponders to 1% topical capsaicin cream.

Methods

Assessments were made at baseline and then during an early (ie, 15 minutes) and late (ie, 45 minutes) time points post-capsaicin in 37 healthy participants.

Results

Participants reporting a visual analogue scale (VAS) rating of >50 were defined as responders (n = 24) and those with <50 VAS rating were defined as nonresponders (n = 13). There was a facilitation of TSP during the transition from an early to the late time point post-capsaicin (P<0.001) and the development of secondary hyperalgesia (P<0.05) in the responder group. Nonresponders showed no changes in TSP or secondary hyperalgesia during the early and late time points. There was an association between baseline TSP scores and the later development of a responder or nonresponder phenotype (r = 0.36; P = 0.03). Receiver operating characteristic analysis revealed that baseline TSP works as a good response predictor at an individual level (area under the curve = 0.75).

Conclusion

These data suggest that responders and nonresponders have different facilitatory pain mechanisms. The assessment of TSP may help to identify participants with stronger endogenous pain facilitation who may be more likely to respond to topical capsaicin.

Cryoneurolysis with Injectable Ice Slurry Modulates Mechanical Skin Pain

Cutaneous pain is a common symptom of skin disease, and available therapies are inadequate. We developed a neural selective and injectable method of cryoneurolysis with ice slurry, which leads to a long-lasting decrease in mechanical pain. The aim of this study is to determine whether slurry injection reduces cutaneous pain without inducing the side effects associated with conventional cryoneurolysis. Using the rat sciatic nerve, we examined the effects of slurry on nerve structure and function in comparison with the effects of a Food and Drug Administration‒approved cryoneurolysis device (Iovera). Coherent anti-Stokes Raman scattering microscopy and immunofluorescence staining were used to investigate histological effects on the sciatic nerve and on downstream cutaneous nerve fibers. Complete Freund's Adjuvant model of cutaneous pain was used to study the effect of the slurry on reducing pain. Structural changes in myelin induced by slurry were comparable with those induced by Iovera, which uses much colder temperatures. Compared with that of Iovera, the decrease in mechanical pain due to slurry was less profound but lasted longer without signs of dysesthesia. Slurry did not cause a reduction of epidermal nerve fibers or a change in thermal pain sensitivity. Slurry-treated rats showed reduced cutaneous mechanical pain in response to Complete Freund's Adjuvant. Slurry injection can be used to successfully reduce cutaneous pain without causing dysesthesia.

Modulation Of Experimental Prolonged Pain and Sensitization Using High-Definition Transcranial Direct Current Stimulation: A Double-Blind, Sham-Controlled Study

High definition transcranial direct current stimulation (HD-tDCS) targeting brain areas involved in pain processing has shown analgesic effects in some chronic pain conditions, but less modulatory effect on mechanical and thermal pain thresholds in asymptomatic subjects. This double-blinded study assessed the HD-tDCS effects on experimental pain and hyperalgesia maintained for several days in healthy participants. Hyperalgesia and pain were assessed during three consecutive days following provocation of experimental pain (nerve growth factor injected into the right-hand muscle) and daily HD-tDCS sessions (20-minutes). Forty subjects were randomly assigned to Active-tDCS targeting primary motor cortex and dorsolateral prefrontal cortex simultaneously or Sham-tDCS. Tactile and pressure pain sensitivity were assessed before and after each HD-tDCS session, as well as the experimentally-induced pain intensity scored on a numerical rating scale (NRS). Subjects were effectively blinded to the type of HD-tDCS protocol. The Active-tDCS did not significantly reduce the NGF-induced NRS pain score (3.5±2.4) compared to Sham-tDCS (3.9±2.0, P > .05) on day 3 and both groups showed similarly NGF-decreased pressure pain threshold in the right hand (P < .001). Comparing Active-tDCS with Sham-tDCS, the manifestation of pressure hyperalgesia was delayed on day 1, and an immediate (pre-HD-tDCS to post-HD-tDCS) reduction in pressure hyperalgesia was found across all days (P < .05). PERSPECTIVE: The non-significant differences between Active-tDCS and Sham-tDCS on experimental prolonged pain and hyperalgesia suggest that HD-tDCS has no effect on moderate persistent experimental pain. The intervention may still have a positive effect in more severe pain conditions, with increased intensity, more widespread distribution, or increased duration and/or involving stronger affective components.

Tonic pain alters functional connectivity of the descending pain modulatory network involving amygdala, periaqueductal gray, parabrachial nucleus and anterior cingulate cortex

INTRODUCTION

Resting state functional connectivity (FC) is widely used to assess functional brain alterations in patients with chronic pain. However, reports of FC accompanying tonic pain in pain-free persons are rare. A network we term the Descending Pain Modulatory Network (DPMN) is implicated in healthy and pathologic pain modulation. Here, we evaluate the effect of tonic pain on FC of specific nodes of this network: anterior cingulate cortex (ACC), amygdala (AMYG), periaqueductal gray (PAG), and parabrachial nuclei (PBN).

METHODS

In 50 pain-free participants (30F), we induced tonic pain using a capsaicin-heat pain model. functional MRI measured resting BOLD signal during pain-free rest with a 32°C thermode and then tonic pain where participants experienced a previously warm temperature combined with capsaicin. We evaluated FC from ACC, AMYG, PAG, and PBN with correlation of self-report pain intensity during both states. We hypothesized tonic pain would diminish FC dyads within the DPMN.

RESULTS

Of all hypothesized FC dyads, only PAG and subgenual ACC was weakly altered during pain (F=3.34; p=0.074; pain-free>pain d=0.25). After pain induction sACC-PAG FC became positively correlated with pain intensity (R=0.38; t=2.81; p=0.007). Right PBN-PAG FC during pain-free rest positively correlated with subsequently experienced pain (R=0.44; t=3.43; p=0.001). During pain, this connection's FC was diminished (paired t=-3.17; p=0.0026). In whole-brain analyses, during pain-free rest, FC between left AMYG and right superior parietal lobule and caudate nucleus were positively correlated with subsequent pain. During pain, FC between left AMYG and right inferior temporal gyrus negatively correlated with pain. Subsequent pain positively correlated with right AMYG FC with right claustrum; right primary visual cortex and right temporo-occipitoparietal junction Conclusion: We demonstrate sACC-PAG tonic pain FC positively correlates with experienced pain and resting right PBN-PAG FC correlates with subsequent pain and is diminished during tonic pain. Finally, we reveal PAG- and right AMYG-anchored networks which correlate with subsequently experienced pain intensity. Our findings suggest specific connectivity patterns within the DPMN at rest are associated with subsequently experienced pain and modulated by tonic pain. These nodes and their functional modulation may reveal new therapeutic targets for neuromodulation or biomarkers to guide interventions.

A systematic review of porcine models in translational pain research

Translating basic pain research from rodents to humans has proven to be a challenging task. Efforts have been made to develop preclinical large animal models of pain, such as the pig. However, no consistent overview and comparison of pig models of pain are currently available. Therefore, in this review, our primary aim was to identify the available pig models in pain research and compare these models in terms of intensity and duration. First, we systematically searched Proquest, Scopus and Web of Science and compared the duration for which the pigs were significantly sensitized as well as the intensity of mechanical sensitization. We searched models within the specific field of pain and adjacent fields in which pain induction or assessment is relevant, such as pig production. Second, we compared assessment methodologies in surrogate pain models in humans and pigs to identify areas of overlap and possible improvement. Based on the literature search, 23 types of porcine pain models were identified; 13 of which could be compared quantitatively. The induced sensitization lasted from hours to months and intensities ranged from insignificant to the maximum attainable. We also found a near to complete overlap of assessment methodologies between human and pig models within the area of peripheral neurophysiology, which allows for direct comparison of results obtained in the two species. In spite of this overlap, further development of pain assessment methodologies is still needed. We suggest that central nervous system electrophysiology, such as electroencephalography, electrocorticography or intracortical recordings, may pave the way for future objective pain assessment.

Chiropractic Spinal Manipulation Prevents Secondary Hyperalgesia Induced by Topical Capsaicin in Healthy Individuals

Background and Aims: Spinal manipulation (SM) is currently recommended for the management of back pain. Experimental studies indicate that the hypoalgesic mechanisms of SM may rely on inhibition of segmental processes related to temporal summation of pain and, possibly, on central sensitization, although this remains unclear. The aim of this study was to determine whether experimental back pain, secondary hyperalgesia, and pain-related brain activity induced by capsaicin are decreased by segmental SM.

Methods: Seventy-three healthy volunteers were randomly allocated to one of four experimental groups: SM at T5 vertebral level (segmental), SM at T9 vertebral level (heterosegmental), placebo intervention at T5 vertebral level, or no intervention. Topical capsaicin was applied to the area of T5 vertebra for 40 min. After 20 min, the interventions were administered. Pressure pain thresholds (PPTs) were assessed outside the area of capsaicin application at 0 and 40 min to examine secondary hyperalgesia. Capsaicin pain intensity and unpleasantness were reported every 4 min. Frontal high-gamma oscillations were also measured with electroencephalography.

Results: Pain ratings and brain activity were not significantly different between groups over time (p > 0.5). However, PPTs were significantly decreased in the placebo and control groups (p < 0.01), indicative of secondary hyperalgesia, while no hyperalgesia was observed for groups receiving SM (p = 1.0). This effect was independent of expectations and greater than placebo for segmental (p < 0.01) but not heterosegmental SM (p = 1.0).

Conclusions: These results indicate that segmental SM can prevent secondary hyperalgesia, independently of expectations. This has implications for the management of back pain, particularly when central sensitization is involved.

During capsaicin-induced central sensitization, brush allodynia is associated with baseline warmth sensitivity, whereas mechanical hyperalgesia is associated with painful mechanical sensibility, anxiety and somatization

BACKGROUND

Mechanical hyperalgesia and allodynia incidence varies considerably amongst neuropathic pain patients. This study explored whether sensory or psychological factors associate with mechanical hyperalgesia and brush allodynia in a human experimental model.

METHODS

Sixty-six healthy volunteers (29 male) completed psychological questionnaires and participated in two quantitative sensory testing (QST) sessions. Warmth detection threshold (WDT), heat pain threshold (HPT) and suprathreshold mechanical pain (STMP) ratings were measured before exposure to a capsaicin-heat pain model (C-HP). After C-HP exposure, brush allodynia and STMP were measured in one session, whilst mechanical hyperalgesia was measured in another session.

RESULTS

WDT and HPT measured in sessions separated by 1 month demonstrated significant but moderate levels of reliability (WDT: ICC = 0.5, 95%CI [0.28, 0.77]; HPT: ICC = 0.62, 95%CI [0.40, 0.77]). Brush allodynia associated with lower WDT (z = -3.06, p = 0.002; ϕ = 0.27). Those with allodynia showed greater hyperalgesia intensity (F = 7.044, p = 0.010, η_p ²  = 0.107) and area (F = 9.319, p = 0.004, η_p ²  = 0.163) than those without allodynia. No psychological self-report measures were significantly different between allodynic and nonallodynic groups. Intensity of hyperalgesia in response to lighter mechanical stimuli was associated with lower HPT, higher STMP ratings and higher Pain Sensitivity Questionnaire scores at baseline. Hyperalgesia to heavier probe stimuli associated with state anxiety and to a lesser extent somatic awareness. Hyperalgesic area associated with lower baseline HPT and higher STMP ratings. Hyperalgesic area was not correlated with allodynic area across individuals.

CONCLUSIONS

These findings support research in neuropathic pain patients and human experimental models that peripheral sensory input and individual sensibility are related to development of mechanical allodynia and hyperalgesia during central sensitization, whilst psychological factors play a lesser role.

SIGNIFICANCE

We evaluated differential relationships of psychological and perceptual sensitivity to the development of capsaicin-induced mechanical allodynia and hyperalgesia. Fifty percent of healthy volunteers failed to develop mechanical allodynia. Baseline pain sensitivity was greater in those developing allodynia and was related to the magnitude and area of hyperalgesia. State psychological factors, whilst unrelated to allodynia, were related to mechanical hyperalgesia. This supports that the intensity of peripheral sensory input and individual sensibility are related to development of mechanical allodynia and hyperalgesia during central sensitization, whilst psychological factors play a lesser role.

Modality-specific facilitation of noninjurious sharp mechanical pain by topical capsaicin

We had previously shown that a "blunt blade" stimulator can mimic the noninjurious strain phase of incisional pain, but not its sustained duration. Here, we tested whether acute sensitization of the skin with topical capsaicin can add the sustained phase to this noninvasive surrogate model of intraoperative pain. Altogether, 110 healthy volunteers (55 male and 55 female; 26 ± 5 years) participated in several experiments using the "blunt blade" (0.25 × 4 mm) on normal skin (n = 36) and on skin pretreated by a high-concentration capsaicin patch (8%, Qutenza; n = 36). These data were compared with an experimental incision (n = 40) using quantitative and qualitative pain ratings by numerical rating scale and SES Pain Perception Scale descriptors. Capsaicin sensitization increased blade-induced pain magnitude and duration significantly (both P < 0.05), but it failed to fully match the sustained duration of incisional pain. In normal skin, the SES pattern of pain qualities elicited by the blade matched incision in pain magnitude and pattern of pain descriptors. In capsaicin-treated skin, the blade acquired a significant facilitation only of the perceived heat pain component (P < 0.001), but not of mechanical pain components. Thus, capsaicin morphed the descriptor pattern of the blade to become more capsaicin-like, which is probably explained best by peripheral sensitization of the TRPV1 receptor. Quantitative sensory testing in capsaicin-sensitized skin revealed hyperalgesia to heat and pressure stimuli, and loss of cold and cold pain sensitivity. These findings support our hypothesis that the blade models the early tissue-strain-related mechanical pain phase of surgical incisions.

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Dermatopharmacokinetic and pharmacodynamic evaluation of a novel nanostructured formulation containing capsaicinoids for treating neuropathic pain

The in vivo skin penetration by dermal microdialysis and the pharmacological efficacy of a chitosan hydrogel containing capsaicinoids-loaded nanocapsules (CHNC_Caps) was evaluated in this study. Such gel has previously been proven to control capsaicinoids release and decrease the drugs side effects in humans. The nanocapsules containing capsaicinoids had an average size around 150 nm, with a low polydispersity index, positive zeta potential, and high encapsulation efficiency of the drugs. The CHNC_Caps showed intact nanocapsules, a slightly acid pH value, and a pseudoplastic behavior suitable for topical application. Microdialysis experiments showed a 1.6-fold increase in the concentration of capsaicinoids in the dermis (after 12 h of its application) when CHNC_Caps was administered compared to a chitosan hydrogel containing capsaicinoids in hydroethanolic solution (CHET_Caps) and the commercial cream. The CHNC_Caps showed antiallodynic and antihyperalgesic effects from 6 h to 96 h after treatment initiation, whereas CHET_Caps and the commercial cream showed antiallodynic and antihyperalgesic effects only at 48 h and 96 h after treatment initiation, respectively. CHNC_Caps and the commercial cream maintained antihyperalgesic activity for 6 days after treatment interruption. For mechanical allodynia, the antinociceptive effect was maintained for 48 h after treatment interruption only with CHNC_Caps. In conclusion, CHNC_Caps is a promising formulation for treating peripheral neuropathic pain.

Sensorimotor Peak Alpha Frequency Is a Reliable Biomarker of Prolonged Pain Sensitivity

Previous research has observed that the speed of alpha band oscillations (8-12 Hz range) recorded during resting electroencephalography is slowed in chronic pain patients. While this slowing may reflect pathological changes that occur during the chronification of pain, an alternative explanation is that healthy individuals with slower alpha oscillations are more sensitive to prolonged pain, and by extension, more susceptible to developing chronic pain. To test this hypothesis, we examined the relationship between the pain-free, resting alpha oscillation speed of healthy individuals and their sensitivity to two models of prolonged pain, Phasic Heat Pain and Capsaicin Heat Pain, at two visits separated by 8 weeks on average (n = 61 Visit 1, n = 46 Visit 2). We observed that the speed of an individual's pain-free alpha oscillations was negatively correlated with sensitivity to both models and that this relationship was reliable across short (minutes) and long (weeks) timescales. Furthermore, the speed of pain-free alpha oscillations can successfully identify the most pain sensitive individuals, which we validated on data from a separate, independent study. These results suggest that alpha oscillation speed is a reliable biomarker of prolonged pain sensitivity with potential for prospectively identifying pain sensitivity in the clinic.

A Randomized, Double-Blind, Placebo- and Active Comparator-Controlled Phase I Study of Analgesic/Antihyperalgesic Properties of ASP8477, a Fatty Acid Amide Hydrolase Inhibitor, in Healthy Female Subjects

Objectives

To evaluate the analgesic/antihyperalgesic effect of ASP8477.

Design

Randomized, double-blind, double-dummy, cross-over, placebo- and active comparator-controlled study.

Setting

HPR Dr. Schaffler GmbH, Munich, Germany.

Subjects

Healthy female subjects aged 18–65 years.

Methods

Eligible subjects were randomly assigned to one of six treatment sequences and received multiple ascending doses of ASP8477, duloxetine, and placebo over three treatment periods (each consisting of 21-day dosing separated by 14-day washout periods). On the last day of each dose level, laser evoked potentials (LEPs) and visual analog scales (VAS pain) on capsaicin-treated skin at baseline and at multiple postdose time points were assessed. The primary end point was the difference in LEP N2-P2 peak-to-peak (PtP) amplitudes for ASP8477 100 mg vs placebo.

Results

Twenty-five subjects were randomized. In all subjects, LEP N2-P2 PtP amplitudes were numerically lower for ASP8477 100 mg vs placebo (P = 0.0721); in subjects who demonstrated positive capsaicin skin effects, a greater mean difference of –2.24 µV (P = 0.0146) was observed. Across all doses, LEP N2-P2 PtP amplitudes were lower for duloxetine compared with ASP8477 (mean difference –3.80 µV; P < 0.0001) or placebo (mean difference –5.21 µV; P < 0.0001). The effect of ASP8477 (all doses) on down-scoring the VAS pain score was significant compared with placebo (mean difference –2.55%; P < 0.0007).

Conclusions

ASP8477 was well tolerated in this study. Analysis of all subjects did not demonstrate a significant difference in LEP for ASP8477 100 mg over placebo but did in subjects who demonstrated positive capsaicin skin effects.

Non-invasive Motor Cortex Neuromodulation Reduces Secondary Hyperalgesia and Enhances Activation of the Descending Pain Modulatory Network

Central sensitization is a driving mechanism in many chronic pain patients, and manifests as hyperalgesia and allodynia beyond any apparent injury. Recent studies have demonstrated analgesic effects of motor cortex (M1) stimulation in several chronic pain disorders, yet its neural mechanisms remain uncertain. We evaluated whether anodal M1 transcranial direct current stimulation (tDCS) would mitigate central sensitization as measured by indices of secondary hyperalgesia. We used a capsaicin-heat pain model to elicit secondary mechanical hyperalgesia in 27 healthy subjects. In an assessor and subject-blind randomized, sham-controlled, crossover trial, anodal M1 tDCS decreased the intensity of pinprick hyperalgesia more than cathodal or sham tDCS. To elucidate the mechanism driving analgesia, subjects underwent fMRI of painful mechanical stimuli prior to and following induction of the pain model, after receiving M1 tDCS. We hypothesized that anodal M1 tDCS would enhance engagement of a descending pain modulatory (DPM) network in response to mechanical stimuli. Anodal tDCS normalized the effects of central sensitization on neurophysiological responses to mechanical pain in the medial prefrontal cortex, pregenual anterior cingulate cortex, and periaqueductal gray, important regions in the DPM network. Taken together, these results provide support for the hypothesis that anodal M1-tDCS reduces central sensitization-induced hyperalgesia through the DPM network in humans.

Designing and conducting proof-of-concept chronic pain analgesic clinical trials

Introduction:

The evolution of pain treatment is dependent on successful development and testing of interventions. Proof-of-concept (POC) studies bridge the gap between identification of a novel target and evaluation of the candidate intervention's efficacy within a pain model or the intended clinical pain population.

Methods:

This narrative review describes and evaluates clinical trial phases, specific POC pain trials, and approaches to patient profiling.

Results:

We describe common POC trial designs and their value and challenges, a mechanism-based approach, and statistical issues for consideration.

Conclusion:

Proof-of-concept trials provide initial evidence for target use in a specific population, the most appropriate dosing strategy, and duration of treatment. A significant goal in designing an informative and efficient POC study is to ensure that the study is safe and sufficiently sensitive to detect a preliminary efficacy signal (ie, a potentially valuable therapy). Proof-of-concept studies help avoid resources wasted on targets/molecules that are not likely to succeed. As such, the design of a successful POC trial requires careful consideration of the research objective, patient population, the particular intervention, and outcome(s) of interest. These trials provide the basis for future, larger-scale studies confirming efficacy, tolerability, side effects, and other associated risks.

The Effect of a Combination of Diclofenac and Methadone Applied as Gel in a Human Experimental Pain Model - A Randomized, Placebo-controlled Trial

Pain involves responses in which both peripheral and central mechanisms contribute to the generation of pain. Pre-clinical laboratory data have supported that a topical formulation of combined diclofenac and methadone (Diclometh) may alleviate local pain, and potentially, the side effect profile should be low. We hypothesized that antiallodynic and antihyperalgesic effects of Diclometh could be demonstrated in a human experimental pain model and that Diclometh would be safe to administer. Thus, the aims were as follows: (i) to compare two doses of Diclometh versus placebo; and (ii) to assess the safety profile of Diclometh. The study was a crossover, randomized, double-blind, placebo-controlled comparison of two doses of Diclometh gel (0.1% and 0.2%) administered topically in healthy participants. Nerve growth factor (NGF) and capsaicin intradermal injections were used as human pain models. Pressure stimulation, contact heat stimulation, hyperalgesia (pinprick stimulation) and allodynia (brush stimulation) to mechanical stimulation were performed in the area where capsaicin and NGF were injected. Side effects were recorded on a four-point Likert scale. Twenty-one men completed the study (mean age 26.14 ± 5.3). Diclometh 0.2% reduced the capsaicin-induced dynamic mechanical allodynia compared to placebo (primary end-point, p = 0.03). No other primary or secondary end-points were found significantly different (all p > 0.05). All side effects were reported as mild with no differences between treatments (p = 0.15). Indication of antiallodynic effect of Diclometh 0.2% was found. Additionally, it was demonstrated that Diclometh was safe to use.

Pharmacological validation of a novel nonhuman primate measure of thermal responsivity with utility for predicting analgesic effects

Introduction

The development of novel analgesics to treat acute or chronic pain has been a challenge due to a lack of translatable measurements. Preclinical end points with improved translatability are necessary to more accurately inform clinical testing paradigms, which may help guide selection of viable drug candidates.

Methods

In this study, a nonhuman primate biomarker which is sensitive to standard analgesics at clinically relevant plasma concentrations, can differentiate analgesia from sedation and utilizes a protocol very similar to that which can be employed in human clinical studies is described. Specifically, acute heat stimuli were delivered to the volar forearm using a contact heat thermode in the same manner as the clinical setting.

Results

Clinically efficacious exposures of morphine, fentanyl, and tramadol produced robust analgesic effects, whereas doses of diazepam that produce sedation had no effect.

Conclusion

We propose that this assay has predictive utility that can help improve the probability of success for developing novel analgesics.

Cerebral peak alpha frequency predicts individual differences in pain sensitivity

The identification of neurobiological markers that predict individual predisposition to pain are not only important for development of effective pain treatments, but would also yield a more complete understanding of how pain is implemented in the brain. In the current study using electroencephalography (EEG), we investigated the relationship between the peak frequency of alpha activity over sensorimotor cortex and pain intensity during capsaicin-heat pain (C-HP), a prolonged pain model known to induce spinal central sensitization in primates. We found that peak alpha frequency (PAF) recorded during a pain-free period preceding the induction of prolonged pain correlated with subsequent pain intensity reports: slower peak frequency at pain-free state was associated with higher pain during the prolonged pain condition. Moreover, the degree to which PAF decreased between pain-free and prolonged pain states was correlated with pain intensity. These two metrics were statistically uncorrelated and in combination were able to account for 50% of the variability in pain intensity. Altogether, our findings suggest that pain-free state PAF over relevant sensory systems could serve as a marker of individual predisposition to prolonged pain. Moreover, slowing of PAF in response to prolonged pain could represent an objective marker for subjective pain intensity. Our findings potentially lead the way for investigations in clinical populations in which alpha oscillations and the brain areas contributing to their generation are used in identifying and formulating treatment strategies for patients more likely to develop chronic pain.

Pain perception in healthy volunteers: effect of repeated exposure to experimental systemic inflammation

We aimed to study the relationship between pain perception and cytokine release during systemic inflammation. We present a randomized crossover trial in healthy volunteers (n = 17) in 37 individual trials. Systemic inflammation was induced by an i.v. bolus of Escherichia coli LPS (2 ng/kg) on two separate trial days, with or without a nicotine patch applied 10 h previously. Pain perception at baseline, and 2 and 6 h after LPS was assessed by pressure algometry and tonic heat stimulation at an increasing temperature (45-48℃) during both trials. Compared with baseline, pain pressure threshold was reduced 2 and 6 h after LPS, while heat pain perception was accentuated at all testing temperatures after 2 but not 6 h. The magnitude of changes in pain perception did not correlate to cytokine release. No effect of transdermal nicotine or training status was observed. In conclusion, LPS administration in healthy human volunteers leads to reduction in pain pressure threshold and an increase in pain perception to heat stimuli, supporting a relationship between acute systemic inflammation and pain perception.

Intradermal glutamate and capsaicin injections: intra- and interindividual variability of provoked hyperalgesia and allodynia

Intradermal injections of glutamate and capsaicin are attractive to use in human experimental pain models because hyperalgesia and allodynia mimic isolated aspects of clinical pain disorders. The aim of the present study was to investigate the reproducibility of these models. Twenty healthy male volunteers (mean age 24 years; range 18-38 years) received intradermal injections of glutamate and capsaicin in the volar forearm. Magnitudes of secondary pinprick hyperalgesia and brush-evoked allodynia were investigated using von Frey filaments (gauges 10, 15, 60 and 100 g) and brush strokes. Areas of secondary hyperalgesia and allodynia were quantified immediately after injection and after 15, 30 and 60 min. Two identical experiments separated by at least 7 days were performed. Reproducibility across and within volunteers (inter- and intra-individual variation, respectively) was assessed using intraclass correlation coefficient (ICC) and coefficient of variation (CV). Secondary pinprick hyperalgesia was observed as a marked increase in the visual analogue scale (VAS) response to von Frey gauges 60 and 100 g (P < 0.001) after glutamate injection. For capsaicin, secondary pinprick hyperalgesia was detected with all von Frey gauges (P < 0.001). Glutamate evoked reproducible VAS response to all von Frey gauges (ICC > 0.60) and brush strokes (ICC > 0.83). Capsaicin injection was reproducible for secondary hyperalgesia (ICC > 0.70) and allodynia (ICC > 0.71). Intra-individual variability was generally lower for the VAS response to von Frey and brush compared with areas of secondary hyperalgesia and allodynia. In conclusion, glutamate and capsaicin yield reproducible hyperalgesic and allodynic responses, and the present model is well suited for basic research, as well as for assessing the modulation of central phenomena.

Individualized pharmacological treatment of neuropathic pain

Patients with the same disease may suffer from completely different pain symptoms yet receive the same drug treatment. Several studies elucidate neuropathic pain and treatment response in human surrogate pain models. They show promising results toward a patient stratification according to the mechanisms underlying the pain, as reflected in their symptoms. Several promising new drugs produced negative study results in clinical phase III trials. However, retrospective analysis of treatment response based on baseline pain phenotyping could demonstrate positive results for certain subgroups of patients. Thus, a prospective classification of patients according to pain phenotype may play an increasingly important role in personalized treatment of neuropathic pain states. A recent prospective study using stratification based on pain-related sensory abnormalities confirmed the concept of personalized pharmacological treatment of neuropathic pain.

Capsaicin-induced neurogenic inflammation in pig skin: a behavioural study

Topical capsaicin is a well-established model of experimental hyperalgesia. Its application to the study of animals has been limited to few species. The effect of topical capsaicin on hyperalgesia in porcine skin was evaluated as part of a study of inflammatory pain in the pig. Two experiments were carried out on pigs of 27 ± 5 kg (n = 8) and 57 ± 3 kg (n = 16). Thermal and mechanical noxious stimuli were provided (CO2 laser and Pressure Application Measurement device) to assess avoidance behaviours. Capsaicin induced significant thermal hyperalgesia in the smaller pigs (P < 0.05), while no mechanical hyperalgesia was observed in either animal group. The present model of topical capsaicin application may be useful to investigate the mechanisms of primary hyperalgesia in this species, although some experimental conditions, such as the administration route and cutaneous morphology, need to be evaluated.

Development of an experimental model to study trigeminal nerve-mediated vasodilation on the human forehead

BACKGROUND

During migraine, trigeminal sensory nerve terminals release calcitonin gene-related peptide (CGRP), inducing nociception and vasodilation. Applied on the skin, capsaicin activates the transient receptor potential vanilloid type 1 (TRPV1) channel and releases CGRP from sensory nerve terminals, thus increasing dermal blood flow (DBF). Using capsaicin application and electrical stimulation of the forehead skin, a trigeminal nerve-innervated dermatome, we aimed to develop a model to measure trigeminal nerve-mediated vasodilation in humans.

METHODS

Using laser Doppler imaging, forehead DBF responses to application of capsaicin (0.06 mg/ml and 6.0 mg/ml) and saline, with and without iontophoresis, were studied in healthy subjects. The within-subject coefficient of variation (WCV) of repeated DBF measurements was calculated to assess reproducibility.

RESULTS

Maximal DBF responses to 6.0 mg/ml capsaicin with and without iontophoresis did not differ (Emax 459 ± 32 and 424 ± 32 arbitrary units (a.u.), WCV 6 ± 4%). In contrast, DBF responses to 0.06 mg/ml capsaicin were significantly larger with than without iontophoresis (Emax 307 ± 60 versus 187 ± 21 a.u., WCV 21 ± 13%). Saline with iontophoresis significantly increased DBF (Emax: 245 ± 26 a.u, WCV 11 ± 8%), while saline application without iontophoresis did not affect DBF.

CONCLUSION

Topical application of capsaicin and electrical stimulation induce reproducible forehead DBF increases and therefore are suitable to study trigeminal nerve-mediated vasodilation in humans.

Evaluation of the analgesic efficacy and psychoactive effects of AZD1940, a novel peripherally acting cannabinoid agonist, in human capsaicin-induced pain and hyperalgesia

The aim of the present study was to investigate the effects of AZD1940, a novel peripherally acting cannabinoid CB(1) /CB(2) receptor agonist, on capsaicin-induced pain and hyperalgesia, as well as on biomarkers of cannabinoid central nervous system (CNS) effects. The present study was a randomized, double-blind, placebo-controlled, four-sequence, two-period, cross-over study in 44 male healthy volunteers aged 20-45 years. The effects of two single oral doses of AZD1940 (400 and 800 μg) were compared with placebo. Pain intensity after intradermal capsaicin injections in the forearm was assessed on a continuous visual analogue scale (VAS; 0-100 mm). Primary and secondary hyperalgesia induced by application of capsaicin cream on the calf were assessed by measuring heat pain thresholds and the area of mechanical allodynia, respectively. The CNS effects were assessed at baseline and up to 24 h after dosing using a visual analogue mood scales (VAMS) for feeling 'stimulated', 'high', 'anxious', 'sedated' or 'down'. AZD1940 did not significantly attenuate ongoing pain or primary or secondary hyperalgesia compared with placebo. Mild CNS effects for AZD1940were observed on the VAMS for 'high' and 'sedated'. Dose-dependent mild-to-moderate CNS-related and gastrointestinal adverse events were reported following treatment with AZD1940. No evidence of analgesic efficacy was found for a peripherally acting CB(1)/CB(2) receptor agonist in the human capsaicin pain model. The emergence of mild dose-dependent CNS effects suggests that the dose range predicted from preclinical data had been attained.

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.

Somatosensory profiling of intra-oral capsaicin and menthol in healthy subjects

This study was designed to investigate the effect of surrogate orofacial pain models on the quantitative sensory testing (QST) profile in healthy participants. Capsaicin, menthol, or saline (control) were applied topically onto the gingiva of 15 healthy subjects for 15 min. During application, the subjects rated pain intensity on a score of 0-10, on an electronic visual analog scale (VAS). A standardized intra-oral QST protocol was performed before and immediately after application. Data obtained before and after application were compared using rank-sum tests, and QST profiles were made after Z-transformation. Application of capsaicin caused moderate levels of pain (VAS(peak)  = 6.0 ± 0.7), and application of menthol produced mild levels of pain (VAS(peak)  = 1.8 ± 0.6). Capsaicin induced hypersensitivity to warmth, heat pain and cold pain and hyposensitivity to mechanical stimuli. Menthol induced hypersensitivity to cold and warmth. Saline caused hypersensitivity to heat pain and hyposensitivity to mechanical stimuli. However, somatosensory profiles from Z-scores demonstrated sensory gains regarding warmth detection and heat pain only after application of capsaicin. In conclusion, a standardized battery of QST showed somatosensory changes after application of capsaicin, menthol and saline to the gingiva. However, the Z-score-based profiles may only reflect the most prominent somatosensory changes and thus represent a conservative approach for evaluation of data.

Low-dose endotoxin potentiates capsaicin-induced pain in man: evidence for a pain neuroimmune connection

Despite the wealth of evidence in animals that immune activation has a key role in the development and maintenance of chronic pain, evidence to support this in humans is scant. We have sought such evidence by examining the effect of a subtle immunological stimulus, low dose intravenous endotoxin, on the allodynia, hyperalgesia, flare and pain produced by intradermal capsaicin in healthy volunteers. Here we provide evidence of immune priming of this neuropathic-like pain response in humans. Specifically, in 12 healthy volunteers, activation of Toll-Like Receptor 4 by endotoxin (0.4ng/kg IV) caused significant 5.1-fold increase in the 90-min integral of areas of capsaicin-induced allodynia (95% CI 1.3-9.1), 2.2-fold increase in flare (95% CI 1.9-2.6) and 1.8-fold increase in hyperalgesia (95% CI 1.1-2.5) following 50μg intradermal capsaicin injected into the forearm 3.5h after endotoxin. These data demonstrate clinically a significant role for the neuroimmune pain connection in modifying pain, thus providing evidence that immune priming may produce pain enhancement in humans and hence offer a novel range of pharmacological targets for anti-allodynics and/or analgesics. Additionally, the simplicity of the model makes it suitable as a test-bed for novel immune-targeted pain therapeutics.

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.

Central origin of pinprick hyperalgesia adjacent to an UV-B induced inflammatory skin pain model in healthy volunteers

Background and purpose The UV-B model is an established pain model of different types of hyperalgesia in animal and human pain research. Beside the skin region of the sunburn in human volunteers pinprick hyperalgesia has been described in a large zone of non-inflamed skin adjacent to the sunburn. However, there are opposing results on the existence of pinprick hyperalgesia and most notably a controversial discussion is still on-going whether this mechanical hyperalgesia in the undamaged tissue adjacent to and at some distance from the site of inflammation is of peripheral or central origin. We therefore addressed this in our study by hypothesising that pinprick hyperalgesia around a circular spot of UV-B inflamed skin is not reduced by a superficial local anaesthetic block and therefore underlies centrally mediated mechanisms. Methods This exploratory study was conducted in a prospective, controlled, randomised, single-blinded fashion in relation to the study hypothesis in 12 healthy volunteers. Before circular irradiation with UV-B light (3-times the individual minimal erythema dose at both thighs), a strip of continuous intradermal local anaesthetic block with lidocaine 2% was established via two single plasmaphoresis hollow fibres. These were positioned perpendicular to one thigh overlapping on the midline of the leg at the distal part of the planned irradiation site, and compared with the contralateral control side without anaesthetic block. The local anaesthetic block was established and then maintained via a syringe pump. The area of pinprick hyperalgesia was measured by pricking on a large skin surface including 360° around the circular irradiation site. This was done with a slightly painful pin (256 mN) until 8h after irradiation. Primary outcome was the area of pinprick hyperalgesia in the skin adjacent to the sunburn at 8h. Results Large areas of mechanical hyperalgesia to pinprick surrounding the adjacent skin of the sunburn developed on both sides after 8h without any significant difference between the side of the anaesthetic strip showing an area of 72.6±39.7 cm2 (mean±SD) and the control side (59.1±20.1 cm2); p = 0.24. Moreover, mechanical hyperalgesia to various pin stimuli of different strength was unchanged by the anaesthetic block. Conclusion This trial provides evidence that the development of mechanical hyperalgesia surrounding an experimental sunburn was not influenced by continuous peripheral afferent blockade with local anaesthetic at 8h after UV-B irradiation. Our data support the hypothesis that in the UV-B model peripheral nociceptive afferent input of inflamed skin may enhance central hypersensitivity of mechanosensitive nociceptors in a larger receptive field far beyond the inflamed skin. Furthermore, these findings are in line with other pain models demonstrating comparable central hypersensitivity around the site of injury. Implications As for other pain models this finding provides further evidence that the UV-B model offers secondary mechanical hyperalgesia in addition to its known primary hyperalgesia. Consequently, this is a further validation for the utilisation of the UV-B model in human pain research.

Potency and Stability of Intradermal Capsaicin: Implications for Use as a Human Model of Pain in Multicenter Clinical Trials

Intradermally injected capsaicin has been used extensively both as a human pain model and to assess analgesic efficacy. Factors such as dose, formulation, route, and site are known to affect its sensitivity. We determined whether potency and stability of capsaicin solutions were further sources of variability when following strict manufacturing guidelines. Capsaicin solution (1.0 mg/mL) was prepared according to Current Good Manufacturing Practice (cGMP) guidelines and aseptically filled into sterile amber borosilicate vials and stored at 5°C, 25°C, and 30°C. All samples were analyzed at one, three, six, and twelve months. Chemical stability was determined using HPLC and physical stability was evaluated by visual inspection of color changes, clarity, particulate matter, and product/ container closure abnormalities during each sampling time. Capsaicin intradermal injection was found to be sterile and retained 95% of the initial concentration for at least one year, regardless of studied storage temperatures (P<0.0001). Visual inspection indicated no changes in color, clarity, particulate matter, and product/ container closure abnormalities in all samples. These data show that capsaicin solutions (1.0 mg/mL) maintain their potency and stability over one year when manufactured according to cGMP guidelines. These results suggest that in clinical trials manufacturing of capsaicin solutions is recommended over extemporaneous compounding.

Lack of effect of central nervous system-active doses of nabilone on capsaicin-induced pain and hyperalgesia

The aim of the present study was to investigate the effects of nabilone on capsaicin-induced pain and hyperalgesia, as well as on biomarkers of cannabinoid central nervous system (CNS) effects. A randomized, double-blind, placebo-controlled, crossover study was conducted in 30 healthy male volunteers receiving single doses of nabilone (1, 2 or 3 mg). Pain intensity after intradermal capsaicin injections in the forearm was assessed by continuous visual analogue scale (0-100 mm). Capsaicin cream was applied to the calf to induce hyperalgesia. Primary hyperalgesia was assessed by measuring heat pain thresholds, whereas secondary hyperalgesia was assessed by measuring the area where light tactile stimulation was felt to be painful. Pain and hyperalgesia were measured at baseline and 2-3.5 h after dosing. The CNS effects were assessed at baseline and up to 24 h after dosing using visual analogue mood scales for feeling 'stimulated', 'anxious', 'sedated' and 'down'. Plasma samples for pharmacokinetic analysis were obtained up to 24 h after drug administration. Nabilone did not significantly attenuate either ongoing pain or primary or secondary hyperalgesia, whereas dose-dependent CNS effects were observed from 1.5 to 6 h after dosing, being maximal at 4-6 h. Plasma concentrations of nabilone and its metabolite carbinol were maximal 1-2 h after dosing. Adverse events (AE) were common on nabilone treatment. Four subjects withdrew due to pronounced CNS AE (anxiety, agitation, altered perception, impaired consciousness). Although nabilone had marked CNS effects, no analgesic or antihyperalgesic effects were observed.

The effects of pregabalin and the glial attenuator minocycline on the response to intradermal capsaicin in patients with unilateral sciatica

BACKGROUND

Patients with unilateral sciatica have heightened responses to intradermal capsaicin compared to pain-free volunteers. No studies have investigated whether this pain model can screen for novel anti-neuropathic agents in patients with pre-existing neuropathic pain syndromes.

AIM

This study compared the effects of pregabalin (300 mg) and the tetracycline antibiotic and glial attenuator minocycline (400 mg) on capsaicin-induced spontaneous pain, flare, allodynia and hyperalgesia in patients with unilateral sciatica on both their affected and unaffected leg.

METHODS/RESULTS

Eighteen patients with unilateral sciatica completed this randomised, double-blind, placebo-controlled, three-way cross-over study. Participants received a 10 µg dose of capsaicin into the middle section of their calf on both their affected and unaffected leg, separated by an interval of 75 min. Capsaicin-induced spontaneous pain, flare, allodynia and hyperalgesia were recorded pre-injection and at 5, 20, 40, 60 and 90 min post-injection. Minocycline tended to reduce pre-capsaicin injection values of hyperalgesia in the affected leg by 28% (95% CI 0% to 56%). The area under the effect time curves for capsaicin-induced spontaneous pain, flare, allodynia and hyperalgesia were not affected by either treatment compared to placebo. Significant limb differences were observed for flare (AUC) (-38% in affected leg, 95% CI for difference -19% to -52%). Both hand dominance and sex were significant covariates of response to capsaicin.

CONCLUSIONS

It cannot be concluded that minocycline is unsuitable for further evaluation as an anti-neuropathic pain drug as pregabalin, our positive control, failed to reduce capsaicin-induced neuropathic pain. However, the anti-hyperalgesic effect of minocycline observed pre-capsaicin injection is promising pilot information to support ongoing research into glial-mediated treatments for neuropathic pain. The differences in flare response between limbs may represent a useful biomarker to further investigate neuropathic pain. Inclusion of a positive control is imperative for the assessment of novel therapies for neuropathic pain.

Intradermal capsaicin as a neuropathic pain model in patients with unilateral sciatica

AIM

This study compared the responses between patients with unilateral sciatica and pain-free volunteers following administration of intradermal capsaicin. METHODS Fourteen patients with unilateral sciatica and 12 pain-free volunteers received one injection per hour over 4 h of 1 µg and 10 µg capsaicin, into each calf. For each dose, spontaneous pain (10 cm VAS), area of flare (cm²) and the sum of allodynia and hyperalgesia radii across eight axes (cm) were recorded pre-injection and at 5, 15, 30, 45 and 60 min post injection.

RESULTS

Sciatica subjects experienced higher spontaneous pain and hyperalgesia responses in both legs compared with pain-free volunteers. The largest mean difference in spontaneous pain was 2.8 cm (95% CI 1.6, 3.9) at 5 min in the unaffected leg following 10 µg. The largest mean difference in hyperalgesia was 19.7 cm (95% CI 12.4, 27.0) at 60 min in the unaffected leg following 10 µg. Allodynia was greater in patients than in controls with the largest mean difference of 2.9 cm (95% CI 1, 4.8) at 5 min following 10 µg in the affected leg. Allodynia was also higher in the affected leg compared with the unaffected leg in sciatica patients with the highest mean difference of 3.0 cm (95% CI 1.2, 4.7) at 5 min following 10 µg.

CONCLUSIONS

The responses to intradermal capsaicin are quantitatively and qualitatively different in unilateral sciatica patients compared with pain-free controls.

Functional MRI brain imaging studies using the Contact Heat Evoked Potential Stimulator (CHEPS) in a human volunteer topical capsaicin pain model

Acute application of topical capsaicin produces spontaneous burning and stinging pain similar to that seen in some neuropathic states, with local hyperalgesia. Use of capsaicin applied topically or injected intradermally has been described as a model for neuropathic pain, with patterns of activation in brain regions assessed using functional magnetic resonance imaging (fMRI) and positron emission tomography. The Contact Heat Evoked Potential Stimulator (CHEPS) is a noninvasive clinically practical method of stimulating cutaneous A-delta nociceptors. In this study, topical capsaicin (1%) was applied to the left volar forearm for 15 minutes of twelve adult healthy human volunteers. fMRI scans and a visual analog pain score were recorded during CHEPS stimulation precapsaicin and postcapsaicin application. Following capsaicin application there was a significant increase in visual analog scale (mean ± standard error of the mean; precapsaicin 26.4 ± 5.3; postcapsaicin 48.9 ± 6.0; P < 0.0001). fMRI demonstrated an overall increase in areas of activation, with a significant increase in the contralateral insular signal (mean ± standard error of the mean; precapsaicin 0.434 ± 0.03; postcapsaicin 0.561 ± 0.07; P = 0.047). The authors of this paper recently published a study in which CHEPS-evoked A-delta cerebral potential amplitudes were found to be decreased postcapsaicin application. In patients with neuropathic pain, evoked pain and fMRI brain responses are typically increased, while A-delta evoked potential amplitudes are decreased. The protocol of recording fMRI following CHEPS stimulation after topical application of capsaicin could be combined with recording of evoked potentials to provide a simple, rapid, and robust volunteer model to develop novel drugs for neuropathic pain.

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.

Pharmacokinetic/pharmacodynamic relationships of transdermal buprenorphine and fentanyl in experimental human pain models

Pharmacokinetic/pharmacodynamic (PK/PD) modelling can be used to characterize the relationship between dose regimen of opioids, plasma concentration and effect of opioids, which in turn can lead to more rational treatment regimens of pain. The aim of this study was to investigate the concentration-effect relationship for transdermal buprenorphine and fentanyl in experimentally induced pain. Twenty-two healthy volunteers were randomized to receive transdermal patches with fentanyl (25 μg/hr, 72 hr), buprenorphine (20 μg/hr, 144 hr) or placebo. The experimental pain tests were pressure at the tibial bone, cutaneous thermal stimulation, cold pressor test (conditioning stimulus (3 ± 0.3°C cold water), nerve growth factor-induced muscle soreness and intradermal capsaicin-induced hyperalgesia and allodynia. Experiments were carried out at baseline, 24, 48, 72 and 144 hr after application of patches. Time-course of placebo was described first and was afterwards added to the description of the time-courses of buprenorphine and fentanyl. This was either described by zero (no drug effect), linear or E(max) model concentration-effect relationships. Time-dependent changes in pain measures in the placebo arm were described by linear or quadratic functions. The time-course of fentanyl and buprenorphine plasma concentrations was complex but could be represented by cubic spline interpolation in the models. Buprenorphine significantly attenuated bone-associated pain, heat pain, nerve growth factor-induced soreness and cold pressor pain. Fentanyl significantly attenuated cold pressor pain for the administered dose regimens. Although the PK/PD relationship for both drugs could be described with similar models, tissue-differentiated analgesic effects between buprenorphine and fentanyl was shown.

A comparison of two formulations of intradermal capsaicin as models of neuropathic pain in healthy volunteers

AIMS

To compare the dose-response relationships of two formulations [Tween- or hydroxypropyl-b-cyclodextrin (HP-b-CD)-based] of intradermal capsaicin in healthy volunteers and to assess the effect of potential covariates of response. One, 10, 30 and 100 microg in 10 ml were compared for the outcomes of flare, spontaneous pain, mechanical allodynia and hyperalgesia in eight healthy men and eight healthy women.

RESULTS

The formulations produced comparable responses at doses 1, 10 and 30 microg, but in all parameters the response was less at 100 microg with the Tween formulation.Mean area for hyperalgesia was 9 cm(2) [95% confidence interval (CI) 5, 13] higher with the HP-beta-CD formulation. Flare area was 5 cm(2) (95% CI 8, 13) greater with the HP-beta-CD formulation. There was a significant difference between pain responses from the injection site on the upper forearm compared with the lower forearm on all four pain assessments. In contrast, significant differences were seen in pain response between nondominant and dominant arm for flare, allodynia and hyperalgesia but not for spontaneous pain. A significant difference in sex was seen only for hyperalgesia. The nominal 100-microg dose of the Tween formulation contained only 39% of label strength in the aqueous phase, which may explain the lower pharmacodynamic response.

CONCLUSION

The formulations are comparable over the dose range 1-30 microg. The significantly lower pain response at the 100 microg dose in the Tween compared with the HP-beta-CD formulation is likely to be due to limitations in solubility at the 100 microg level. Given the greater ease of formulation and the superior dose-response relationship, the HP-beta-CD formulation is preferable for use in the model in future studies.

Identifying brain activity specifically related to the maintenance and perceptual consequence of central sensitization in humans

Central sensitization (CS) refers to an increase in the excitability of spinal dorsal horn neurons that results from, and far outlasts the initiating nociceptive input. Here, functional magnetic resonance imaging was used to examine whether supraspinal activity might contribute to the maintenance of CS in humans. A crossover parametric design was used to distinguish and control for brain activity that is related to the consequence of increased pain experienced during CS. When the intensity of pain during CS and normal states were matched, only activity within the brainstem, including the mesencephalic pontine reticular formation, and the anterior thalami remained increased during CS. Further analyses revealed that activity in the isolated brainstem area correlated positively with the force of noxious stimulation only during CS, whereas activity in the isolated thalamic area was not modulated parametrically in either CS or normal states. Additionally, the mean activity in the isolated brainstem area was increased only during CS, whereas the mean activity in the isolated thalamic area was increased in both states, albeit less so in the normal state. Together, these findings suggest a specific role of the brainstem for the maintenance of CS in humans. Regarding brain areas related to the consequence of increased pain perception during CS, we found that only cortical activity, mainly in the primary somatosensory area, was significantly correlated with intensity of pain that was attributable to both the force of noxious stimulation used and state in which noxious stimulation was applied.

Contralateral hyperalgesia and allodynia following intradermal capsaicin injection in man

OBJECTIVES

Contralateral responses to unilateral stimuli have been well described in animal models. These range from central sensitization to peripheral inflammatory responses. Our aim was to test for contralateral responses following unilateral intradermal capsaicin injection in man.

METHODS

Three groups were investigated. A healthy volunteer group (1) was injected with capsaicin into the volar aspect of one forearm. A group of patients with RA (2) was also injected with capsaicin. A control group of healthy volunteers (3) was not injected with capsaicin. All groups were tested for hyperalgesia and allodynia every 10 min for 1 h following the injection using quantitative sensory testing.

RESULTS

A total of 9/14 healthy volunteers (Group 1) and 10/14 patients with RA (Group 2) demonstrated contralateral sensitization that subsided within 1 h following intradermal capsaicin injection. A total of 2/23 control subjects (Group 3) demonstrated positive responses with the monofilaments. The frequency of the contralateral responses in the experimental groups compared with the control group is significant (P < 0.05). The peak hyperalgesia was relatively delayed contralaterally compared with the ipsilateral side (35 min vs 15 min). The area of sensitization, where present, was reduced compared with the ipsilateral side (5-50%).

CONCLUSIONS

This is the first demonstration of a contralateral response following a unilateral stimulus in man. Bilateral neural pathways mediating contralateral responses may have a role in the pathophysiology of chronically painful or inflammatory diseases and a confounding influence on using the contralateral limb as a control experimentally. We did not find that a systemic inflammatory disease sensitized for this phenomenon.

Psychophysics, Flare, and Neurosecretory Function in Human Pain Models: Capsaicin Versus Electrically Evoked Pain

Intradermal capsaicin injection (CAP) and electrical current stimulation (ES) are analyzed in respect to patterns and test-retest reliability of pain as well as sensory and neurosecretory changes. In 10 healthy subjects, 2x CAP (50 microg) and 2x ES (5 to 30 mA) were applied to the volar forearm. The time period between 2 identical stimulations was about 4 months. Pain ratings, areas of mechanical hyperalgesia, and allodynia were assessed. The intensity of sensory changes was quantified by using quantitative sensory testing. Neurogenic flare was assessed by using laser Doppler imaging. Calcitonin gene-related peptide (CGRP) release was quantified by dermal microdialysis in combination with an enzyme immunoassay. Time course and peak pain ratings were different between CAP and ES. Test-retest correlation was high (r > or = 0.73). Both models induced primary heat hyperalgesia and primary plus secondary pin-prick hyperalgesia. Allodynia occurred in about half of the subjects. Maximum flare sizes did not differ between CAP and ES, but flare intensities were higher for ES. Test-retest correlation was higher for flare sizes than for flare intensity. A significant CGRP release could only be measured after CAP. The different time courses of pain stimulation (CAP: rapidly decaying pain versus ES: pain plateau) led to different peripheral neurosecretory effects but induced similar central plasticity and hyperalgesia.

PERSPECTIVE

The present study gives a detailed overview of psychophysical and neurosecretory characteristics induced by noxious stimulation with capsaicin and electrical current. We describe differences, similarities, and reproducibility of these human pain models. These data might help to interpret past and future results of human pain studies using experimental pain.

Intradermal Capsaicin Causes Dose-Dependent Pain, Allodynia, and Hyperalgesia in Humans

BACKGROUND

Intradermal capsaicin is a human pain model that produces reliable pain and sensitization. This model facilitates controlled testing of analgesic efficacy via a crossover design while minimizing confounding variables in clinical pain states and retaining sufficient power with small samples.

METHODS

To determine the lowest dose of capsaicin that produces consistent neurosensory measures, we administered 0.1, 1, 10, or 100 microg to healthy volunteers in a blinded manner (N = 19). Pain scores were recorded at 0, 5, 10, 15, and 60 minutes on a visual analog scale from 0 to 100. Areas and intensities of mechanical allodynia (foam brush stimulus) and pinprick hyperalgesia (von Frey test) were quantified at 15 and 60 minutes, as were flare areas.

RESULTS

Capsaicin produced dose-dependent increases in spontaneous pain (p = .013), the area and intensity of mechanical allodynia (p = .006 and p < .001, respectively), the area and intensity of pinprick hyperalgesia (p = .010 and p = .014, respectively), and the flare area (p = .010). The 10 microg dose produced greater spontaneous pain than the 1 microg dose (p = .017). The 100 microg dose produced greater spontaneous pain than the 10 microg, but the difference was not statistically significant.

CONCLUSION

The 10 and 100 microg capsaicin doses produced robust pain measures across a range of modalities, and lower doses produced minimal effects. Whereas most studies use 100 microg, using a lower dose is reasonable and may facilitate detection of subtle analgesic effects--particularly with nonopioid analgesics--and drugs can be tested in lower doses, minimizing adverse side effects.