Differential effect of painful heterotopic stimulation on capsaicin-induced pain and allodynia

Conditioned pain modulation-A comprehensive review

Conditioned pain modulation (CPM) is a centrally processed measure of the net effect of the descending pain pathway. This comprises both the facilitatory as well as the inhibitory effect. In the past, CPM or similar effects have been previously described using different terminologies such as diffuse noxious inhibitory control (DNIC), heterotopic noxious conditioning stimulation (HNCS) or endogenous analgesia (EA). A variety of patient-related factors such as age, gender, hormones, race, genetic and psychological factors have been thought to influence the CPM paradigms. CPM paradigms have also been associated with a wide range of methodological variables including the mode of application of the 'test' as well as the 'conditioning' stimuli. Despite all these variabilities, CPM seems to reliably lend itself to the pain modulation profile concept and could in future become one of the phenotypic biomarkers for pain and also a guide for mechanism-based treatment in chronic pain. Future research should focus on establishing consistent methodologies for measuring CPM and thereby enhancing the robustness of this emerging biomarker for pain.

No association of polymorphisms in the serotonin transporter gene with thermal pain sensation in healthy individuals

Background

Recent studies have suggested an association between genotypes affecting the expression of the serotonin transporter and thermal pain perception and the thermal grill. The aim of this study was to investigate differences in thermal and mechanical pain perception and the thermal grill in two groups of healthy volunteers according to their genotype, associated with either high (n = 40) or low (n = 40) expression of the serotonin transporter and according to gender. Cold and warm detection and pain thresholds, pressure pain threshold and cold, warm and pain sensations to single or alternating stimuli with cold (20°C) and warm (40°C) temperatures (known as the thermal grill) were determined. In addition, intensity of ongoing pain and area and intensity of pinprick hyperalgesia in the secondary hyperalgesic area following topical application of capsaicin and vehicle control (ethanol) were determined.

Results

No significant differences in detection and pain thresholds for cold and warm temperatures, presence of paradoxical heat sensation, pressure pain threshold and pain responses to suprathreshold thermal stimuli were observed. There was also no difference in capsaicin-evoked ongoing pain and secondary hyperalgesia between the two genotype groups (p >0.4), also when subdivided by gender (p >0.17). In addition, there were no significant differences in the perception of the thermal grill between the two genotypes (p >0.5), also when subdivided by gender.

Conclusions

Genotypes associated with high or low expression of the serotonin transporter were not associated with thermal pain thresholds, pressure pain threshold, pain after capsaicin application or responses to the thermal grill.

The present results do not support that the investigated genotypes play a major role in thermal pain perception among healthy individuals.

Conditioned pain modulation: a predictor for development and treatment of neuropathic pain

Psychophysical evaluation of endogenous pain inhibition via conditioned pain modulation (CPM) represents a new generation of laboratory tests for pain assessment. In this review we discuss recent findings on CPM in neuropathic pain and refer to psychophysical, neurophysiological, and methodological aspects of its clinical implications. Typically, chronic neuropathic pain patients express less efficient CPM, to the extent that incidence of acquiring neuropathic pain (e.g. post-surgery) and its intensity can be predicted by a pre-surgery CPM assessment. Moreover, pre-treatment CPM evaluation may assist in the correct choice of serotonin-noradrenalin reuptake inhibitor analgesic agents for individual patients. Evaluation of pain modulation capabilities can serve as a step forward in individualizing pain medicine.

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.

Time dependant differences in pain sensitivity during unilateral ischemic pain provocation in healthy volunteers

Plurisegmental endogenous pain inhibitory mechanisms related to diffuse noxious inhibitory controls (DNIC) were demonstrated in animal experiments to act on multireceptive neurons of the entire cord outside the conditioned segment without any side differences. Human experiments have demonstrated altered pain sensitivity to pressure, heat and electrical stimulation during heterotopic noxious conditioning stimulation (HNCS). The purpose of the study was to examine if side and/or time differences in pain thresholds and suprathreshold pain sensitivity for pressure and heat, respectively, could be detected during HNCS. Perception thresholds to pressure and heat pain as well as the sensitivity to suprathreshold pressure (SPP) and heat pain (SHP) were assessed in 18 healthy volunteers bilaterally at the thighs before, during and following ischemia-induced pain of the left forearm (HNCS). The assessments started with either the right (10 subjects) or the left thigh (8 subjects). During HNCS the pressure pain threshold increased significantly (p<0.001) on both sides alike. No significant difference in the magnitude of the altered pressure pain threshold was seen between sides for the first or the lastly assessed side. On the lastly assessed side only SPP and SHP increased significantly on both sides alike (p<0.02 and p<0.03, respectively), without magnitude differences between sides. During unilateral HNCS of the left arm, a time factor was demonstrated only for alterations in suprathreshold pain sensitivity, without any differences in magnitude between sides. Therefore, the results have implications for future design of HNCS-related experimental and clinical studies.

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.

Cold-induced limb pain decreases sensitivity to pressure-pain sensations in the ipsilateral forehead

The aim of this study was to investigate the effect of unilateral limb pain on sensitivity to pain on each side of the forehead. In the first experiment, pressure-pain thresholds and sharpness sensations were assessed on each side of the forehead in 45 healthy volunteers before and after a 10 degrees C cold pressor of the hand and in 18 controls who were not subjected to the cold pressor. In a second experiment, forehead sensitivity was assessed in 32 healthy volunteers before and after a 2 degrees C cold pressor. The assessments were repeated without the cold pressor, and before and after six successive 4 degrees C cold pressor tests. The 10 degrees C cold pressor did not influence forehead sensitivity, whereas the 2 degrees C cold pressor and the 4 degrees C cold pressor tests resulted in bilateral analgesia to sharpness and pressure. The analgesia to pressure was greater in the ipsilateral forehead. Stress-induced analgesia and diffuse noxious inhibitory controls may have contributed to the analgesia to pressure-pain and sharpness sensations bilaterally after the most painful cold pressor tests. The locus coeruleus inhibits ipsilateral nociceptive activity in dorsal horn neurons during limb inflammation, and thus may have mediated the ipsilateral component of analgesia. Pain-evoked changes in forehead sensitivity differed for sharpness and pressure, possibly due to separate thalamic or cortical representations of cutaneous and deep tissue sensibility. These findings suggest that several mechanisms act concurrently to influence pain sensitivity at sites distant from a primary site of painful stimulation.

Noxious counterirritation in patients with advanced osteoarthritis of the knee reduces MCC but not SII pain generators: A combined use of MEG and EEG

Chronic pain is mainly a result of two processes: peripheral and central sensitization, which can result in neuroplastic changes. Previous psychophysical studies suggested a decrease of the so-called pain-inhibiting-pain effect (DNIC) in chronic pain patients. We aimed to study the DNIC effect on the neuronal level using magnetoencephalography and electroencephalography in 12 patients suffering from advanced unilateral knee osteoarthritis (OA). DNIC was induced in patients by provoking the typical OA pain by a slightly hyperextended joint position, while they received short electrical pain stimuli. Although the patients did not report a reduction of electrical pain perception, the cingulate gyrus showed a decrease of activation during provoked OA pain, while activity in the secondary somatosensory cortex did not change. Based on much stronger DNIC induction at comparable intensities of an acute counterirritant pain in healthy subjects this result suggests a deficit of DNIC in OA patients. We suggest that the strength of DNIC is subject to neuronal plasticity of descending inhibitory pain systems and diminishes during the development of a chronic pain condition.

Experimental muscle pain impairs descending inhibition

In chronic musculoskeletal pain conditions, the balance between supraspinal facilitation and inhibition of pain shifts towards an overall decrease in inhibition. Application of a tonic painful stimulus results in activation of diffuse noxious inhibitory controls (DNIC). The aims of the present experimental human study were (1) to compare DNIC, evoked separately, by hypertonic saline (6%)-induced muscle pain (tibialis anterior) or cold pressor pain; (2) to investigate DNIC evoked by concomitant experimental muscle pain and cold pressor pain, and (3) to analyze for gender differences. Ten males and 10 age matched females participated in two sessions. In the first session unilateral muscle pain or unilateral cold pressor pain were induced separately; in the second session unilateral muscle pain and unilateral cold pressor pain were induced concomitantly. Pressure pain thresholds (PPT) were measured around the knee joint before, during, and after DNIC induction. Cold pressor pain increased PPT in both males and females with greater increases in males. Hypertonic saline-evoked muscle pain significantly increased PPT in males but not in females. When cold pressor and muscle pain were applied concomitantly the PPT increases were smaller when compared to the individual sessions. This study showed for the first time that two concurrent conditioning tonic pain stimuli (muscle pain and cold pressor pain) cause less DNIC compared with either of the conditioning stimuli given alone; and males showed greater DNIC than females. This may explain why patients with chronic musculoskeletal pain have impaired DNIC.

Effect of heterotopic noxious conditioning stimulation on electrical and pressure pain thresholds in two different anatomical regions

OBJECTIVES

The aims of the study were to investigate the influence of heterotopic noxious conditioning stimulation (HNCS) on pain thresholds in the orofacial and spinal regions and to find out whether there are gender differences in this respect.

MATERIAL AND METHODS

Thirty healthy subjects (15 of each sex) with a mean (SD) age of 25.1 (4.4) years participated. Pain thresholds to electrical (EPT) and pressure stimuli (PPT) were recorded in the masseter muscle and 1st upper incisor (tooth), as well as in the fingertip, before, during, and 5 and 15 min after a cold pressor task to the contralateral hand immersed in ice-cold water for a maximum of 5 min.

RESULTS

With the exception of the EPT in the orofacial region, all pain thresholds increased during the HNCS and then returned to baseline during the 15 min follow-up. The significant changes in EPT were greater in the finger than in the tooth, while the changes in PPT were greater in the masseter muscle than in the finger. Electrical stimuli in the finger induced greater significant changes of pain thresholds than pressure. In the orofacial region, pressure induced greater significant changes in pain thresholds during HNCS than electrical stimuli did. The HNCS induced pain of high intensity and unpleasantness, i.e. varying between 5 and 10 on the numeric rating scale (NRS). There were no gender differences in the response to the HNCS.

CONCLUSION

We conclude that, in general, HNCS induced by cold pressor stimulation increases pain thresholds, but the magnitude of the effect differs between the orofacial region and the finger and is influenced by the tissue and type of test stimuli.

Effects of cold stimulation on secondary hyperalgesia (HA) induced by capsaicin in healthy volunteers

Little is known about the sensory characteristics and underlying mechanisms behind secondary hyperalgesia (HA) (2 degrees HA). The aim of the present study was to investigate the relationships between two different noxious stimuli, mechanical and cold on capsaicin-induced 2 degrees HA. Fourteen healthy volunteers were exposed to three different cold stimuli (20, 10, 0 degrees C) 30 s each, on both forearms. The cold stimuli were applied before (baseline) and 8 min after intradermal administration of 50-microg capsaicin to the forearm, distally to the injection site in the inspected area of 2 degrees HA. Pain intensities were assessed immediately after each cold stimulus by means of a visual analogue scale (cold-VAS). Additionally, areas of mechanical HA (cm(2)) were assessed distally and proximally to the injection site at three different time points: 5, 8 (right after the second series of cold stimuli), and 30 min after the injection. No significant differences in cold-VAS were found between pre- and post-capsaicin injection at the tested forearm (P= 0.334), whereas significant reduction from pre- to post-injection was found in cold-VAS in the control forearm (P= 0.024). Further, 8 min after the injection, the cold stimulation led to an expansion of 2 degrees HA area (from 5.1+/-1.38 to 11.4+/-1.72 cm(2)) to punctuate stimuli distally but not proximally to the injection site (P<0.05). It is concluded that there is no HA to cold stimuli within the area of mechanical 2 degrees HA. However, cooling acts as a conditioning stimulus and expands the area of capsaicin-induced punctuate HA.

The Effects of Greater Occipital Nerve Block and Trigger Point Injection on Brush Allodynia and Pain in Migraine

OBJECTIVE

To evaluate the effect of GONB, with or without trigger point injection (TPI), on dynamic mechanical (brush) allodynia (BA) and on head pain in migraine. Background.-Patients with migraine often have cutaneous allodynia that is related to sensitization of central pain neurons. Greater occipital nerve block (GONB) is an effective treatment for migraine headache; however, its effect on cutaneous allodynia in migraine is unknown.

METHODS

We studied patients with migraine and BA who were treated with GONB with or without TPI. Demographic data, migraine history, and headache features were documented. Allodynia was evaluated using a structured questionnaire and by applying a 4 x 4-inch gauze pad to skin areas in the trigeminal and cervical dermatomes. Degree of allodynia (the allodynia score) was measured on a 100-mm visual analog scale (VAS) before treatment and 10 and 20 minutes thereafter. Headache levels were assessed using an 11-point verbal scale. Allodynia scores, as well as headache levels, before and after treatment were compared.

RESULTS

Nineteen patients were studied. Mean age was 43.6+/-11.8 years. Twenty minutes after treatment, headache was reduced in 17 patients (89.5%) and did not change in 2 (10.5%). The average headache level was 6.53 before treatment and 3.47, 20 minutes after it. The average allodynia score decreased after 20 minutes in all patients. Average allodynia score per site was reduced by 18.69 mm and 13.74 mm in the trigeminal and cervical areas, respectively. There was a positive correlation between allodynia index, obtained through the questionnaire, and allodynia score, obtained by examination.

CONCLUSION

GONB, with or without TPI, reduced both head pain and brush allodynia in this migraine patient group.

Homotopic and heterotopic effects of endogenous analgesia in healthy volunteers

Although research on DNIC has revealed the inhibitory effect occurring between two remote pain stimuli, the interrelation between two adjacent painful stimuli has not yet been characterized. In the present study, we used a sample of 40 healthy volunteers to examine the effect of 30-s immersion of the fingers in water of 1 degree C, as a conditioning stimulus, on pain intensities produced by conditioned mechanical punctuate stimuli, applied both adjacent and contralateral to the cooled area. There was a significant decrease in mechanical pain intensities from 17.23+/-2.39 at baseline to 12.45+/-2.39 when stimulating immediately after the cold immersion at an adjacent site, and from 20.00+/-2.39 to 15.08+/-2.39 at remote sites (F=20.02, p<0.0001). A significant positive correlation between the extent of pain reduction in the cooled and in the uncooled hand was found (r(s)=0.59, p=0.0001). The extent of pain reduction following cooling in the cooled and in the uncooled hand was also found to be similar for males and for females (p=0.63). It is concluded that under the conditions of this experiment, EA affects heterotopic and homotopic regions similarly and without gender differences.

Reduced cold pressor pain tolerance in non-recovered whiplash patients: a 1-year prospective study

Whiplash injury and chronic whiplash syndrome represent major health problems in certain western communities, pain being the main symptom. Sensitization of the nociceptive system may play a role for non-recovery after whiplash injury.

AIMS

This study examined if tolerance to endure pain stimuli may predict outcome in whiplash injury. In a prospective fashion, 141 acute whiplash patients exposed to rear-end car collision (WAD grade 1-3) and 40 ankle-injured controls were followed and exposed to a cold pressor test, respectively, 1 week, 1, 3, 6 and 12 months after the injury. VAS score of pain and discomfort was obtained before, during and after immersion of the dominant hand into cold water for 2 min. The McGill Pain Questionnaire showed that ankle-injured controls had higher initial pain scores than the corresponding whiplash group, while whiplash-injured subjects had higher scores at 6 months; pain scores being similar at other time points. No difference was found in cold pressor pain between recovered whiplash patients and ankle-injured subjects. Non-recovery was only encountered in whiplash injury. Eleven non-recovered whiplash patients (defined as: handicap after 1 year) showed reduced time to peak pain from 1 week to 3 months (P<0.001), 6 months (P<0.01), but not 12 months after the injury. A larger pain area was seen in non-recovered vs. recovered whiplash-injured subjects during the entire observation period (P<0.001). Non-recovery after whiplash was associated with initially reduced cold pressor pain endurance and increased peak pain, suggesting that dysfunction of central pain modulating control systems plays a role in chronic pain after acute whiplash injury.

Comparative EEG activation to skin pain and muscle pain induced by capsaicin injection

Skin pain differs from muscle pain in quality and affective dimension, but it is unknown how the brain processes the nociceptive inputs from skin and muscle differently. To delineate the differential effects of nociceptive inputs from skin and muscle, the EEG topography and power spectra were analysed on the basis of two databases acquired from two separate studies regarding skin (Neurosci. Lett. 305 (2001b) 49) and muscle pain (Exp. Brain Res. 141 (2001c) 195). The same experimental protocol was applied to the same subject-group in the two separate experiments. In the two independent experiments, skin pain and muscle pain were, respectively, induced by intracutaneous and intramuscular injection of capsaicin in the left forearm. Visual analogue scale (VAS) and EEG data acquired before, during the vehicle and capsaicin injections were quantitatively compared. The results showed that the VAS profiles for skin and muscle pain are highly similar in spite of distinct qualities perceived. Skin pain produced a similar but not identical EEG topographic pattern as muscle evoked. Muscle pain induced a significant increase of beta-2 activity in the extensive frontal, parietal and occipital areas compared to skin pain. No difference was found between the vehicle-induced non-painful sensations in skin and muscle. These results implicate that the nociceptive inputs from muscle and skin are processed differently in the similar neural matrix of the brain.

Differential recruitment of endogenous pain inhibitory systems in neuropathic pain patients

Neuronal hyperexcitability is a key finding in patients with neuropathic pain. Contributing to hyperexcitability may be decreased activity in the endogenous pain inhibitory systems. The present study aimed at recruiting descending inhibition, by the use of painful heterotopic stimulation (HTS), in 16 patients with peripheral chronic neuropathic pain and associated brush-evoked allodynia. Two experiments were performed: one examined the effect of HTS on ongoing pain and intensity of brush-evoked allodynia and the other tested the effect of HTS on ongoing pain and area of brush-evoked allodynia. Both experiments consisted of two sessions, one with painful cold HTS (1 degrees C water bath) another with non-painful neutral HTS (32 degrees C water bath). The area of brush-evoked allodynia was significantly reduced (P=0.003) during painful HTS, as compared to non-painful HTS. In contrast, neither the intensity of brush-evoked allodynia nor the ongoing pain was significantly changed. The results indicate that endogenous pain modulating systems can alter some aspects of chronic neuropathic brush-evoked allodynia. The differential effect of painful HTS on ongoing pain and area of brush-evoked allodynia suggest that separate mechanisms are involved.

The pain inhibiting pain effect: an electrophysiological study in humans

This study examines the counterirritation phenomenon of experimental pain in human subjects. Phasic pain induced by intracutaneous electrical stimuli was simultaneously applied with tonic pain induced by ischemic muscle work. Pain ratings, spontaneous EEG and evoked potentials were measured. We found a significant reduction of phasic pain ratings during and 10 min after tonic pain. The late somatosensory evoked potentials as neurophysiological correlates of phasic pain sensation were attenuated until 20 min after tonic pain offset. The extent of phasic pain relief due to concomitant tonic pain was small but significant, comparable to the effect of a regular systemic dose of a narco-analgesic drug in this experimental pain model. On the other hand, no modulations in the late components of the auditory evoked potential and the power spectrum of the spontaneous EEG were observed. These variables reflect the attention and vigilance of the subject and are well-known to be affected by opioids. The only exception was an increase of beta power, which might reflect hyperarousal during tonic pain. These results support the suggestion, that the analgesic effect of heterotopic noxious stimulation in humans is based on the activation of a specific inhibitory pain control system. Systemic release of endogenous opioids is unlikely to be involved, because the typical effects of opioids on the EEG were not observed.

No effect of sympathetic sudomotor activity on capsaicin-evoked ongoing pain and hyperalgesia

BACKGROUND

Complex regional pain syndromes (causalgia and RSD) can be relieved by blockade of the sympathetic efferent activity. The mechanisms of sympathetically maintained pain (SMP) are unclear. So far an adrenergic interaction between sympathetic vasoconstrictor neurons and nociceptors has been proposed. Alternatively, a cholinergic coupling of sympathetic sudomotor neurons and nociceptors is possible.

OBJECTIVE

To determine the effect of cutaneous sympathetic sudomotor activity on pain induced by primary afferent C-nociceptor activation with capsaicin in humans.

METHODS

In 10 healthy volunteers capsaicin was injected into the forearm skin to induce ongoing pain and dynamic and punctate mechanical hyperalgesia. Intensity of pain and hyperalgesia and area of hyperalgesia (planimetry) were assessed. The local skin temperature at the application and measurement sites was kept constant at 35 degrees C. In each individual the analyses were performed during the presence of low and high sympathetic sudomotor skin activity induced by whole-body temperature changes with a thermal suit. By altering whole-body temperature from a moderately warm to an intensely warm state, sympathetic sudomotor activity is modulated selectively in the widest range that can be achieved physiologically while sympathetic vasoconstrictor activity is continuously inhibited. The degree of sudomotor discharge was monitored by measuring cutaneous sweat production at the forearm with the colour indicator ponso-red. The inhibition of vasocontrictor discharge was monitored by measuring cutaneous blood flow at the index finger with laser Doppler flowmetry.

RESULTS

The intensity and spatial distribution of capsaicin-evoked ongoing pain and dynamic and punctate mechanical hyperalgesia were not significantly different during the presence of high and low sympathetic sudomotor discharge.

CONCLUSIONS

Cutaneous sympathetic sudomotor activity does not influence capsaicin induced pain and mechanical hyperalgesia.