Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

Comparison of brain functional response to mechanical prickling stimuli to the glabrous and hairy skin

BACKGROUND

A kind of prickle sensation, which is a composite feeling of pain and itch, can be evoked by mechanical stimulation of fiber ends from fabric surface against to human hairy skin, rather than glabrous skin. Now, a functional magnetic resonance imaging (fMRI) study was conducted to investigate the cognitive differences in the brain for mechanical prickling stimuli to the two types of skin.

MATERIALS AND METHODS

A nylon filament with the diameter of 205 μm and the length of 8 mm was used to deliver mechanical prickling stimuli respectively to two skin sites, fingertip (glabrous skin) and volar forearm (hairy skin), of eight healthy male subjects. Simultaneously, the technology of fMRI was adopted to acquire BOLD (Blood Oxygen Level-Dependent) signals of brain functional response of the subjects.

RESULTS

Somatosensory areas, emotional areas, and the posterior parietal cortex (especially the precuneus) are important brain regions that distinguish between the two conditions. The representation of mechanical prickling stimulation to glabrous skin in the brain favors much more the tactile information of the stimulation and contains no itch, while the key brain area, precuneus, involved in itch was activated by the same mechanical prickling stimulation to hairy skin, and brain response for the condition of hairy skin contains more emotional information, which plays an important role in pain processing.

CONCLUSION

Therefore, it can be inferred that a kind of stronger prickle sensation, which contains both pain and itch, was evoked by mechanical stimulation to hairy skin than glabrous skin.

Prevention of Bortezomib-Induced Peripheral Neuropathy in Newly Multiple Myeloma Patients Using Nervonic Acid, Curcuma Rizoma, and L-Arginine Compound: A Pilot Study

INTRODUCTION

This is a phase II pilot study to evaluate the efficacy of a nutraceutical compound composed of nervonic acid, curcuma rizoma, and l-Arginine to prevent the onset of bortezomib-induced peripheral neuropathy (BIPN) in 16 newly diagnosed multiple myeloma (MM) patients treated with bortezomib (BTZ) over 6 months.

MATERIALS AND METHODS

Assessments included neurological examination and electroneurography, Common Terminology Criteria for Adverse Events (NCI-CTCAE), reduced version of Total Neuropathic Score (TNSr), pain evaluation, functional autonomy scales, self-perceived symptoms and quality of life questionnaires at baseline and after 6 months.

RESULTS

No patients were symptomatic at baseline, despite neurophysiological data and TNSr evidence of peripheral neuropathy (PN) in 11 of them. After 6 months, only 9 patients completed the study. All had modifications in neurological examination with 8 out of 9 showing neurophysiological data of PN (2 of which had a NCI-CTCAE grade of neurotoxicity ≥2); 4 patients dropped out due to BIPN, 2 because of MM progression, 1 for scarce compliance.

DISCUSSION

In our study, the compound was not adequate to prevent BIPN. The incidence of subclinical PN in MM patients is a risk factor for the development of severe neurotoxicity during BTZ treatment. For this reason to evaluate the efficacy of any preventive compound, as well as to manage MM patients, it should be mandatory to include neurophysiological study as a standard procedure.

Encoding of cutaneous stimuli by lamina I projection neurons

ABSTRACT

Lamina I of the dorsal horn, together with its main output pathway, lamina I projection neurons, has long been implicated in the processing of nociceptive stimuli, as well as the development of chronic pain conditions. However, the study of lamina I projection neurons is hampered by technical challenges, including the low throughput and selection biases of traditional electrophysiological techniques. Here we report on a technique that uses anatomical labelling strategies and in vivo imaging to simultaneously study a network of lamina I projection neurons in response to electrical and natural stimuli. Although we were able to confirm the nociceptive involvement of this group of cells, we also describe an unexpected preference for innocuous cooling stimuli. We were able to characterize the thermal responsiveness of these cells in detail and found cooling responses decline when exposed to stable cold temperatures maintained for more than a few seconds, as well as to encode the intensity of the end temperature, while heating responses showed an unexpected reliance on adaptation temperatures.

Lasting modulation of human cortical swallowing motor pathways following thermal tongue stimulation

BACKGROUND

Thermal tactile oropharyngeal stimulation has been clinically used to facilitate swallowing initiation in dysphagic patients. We previously demonstrated that thermal stimulation applied to the oral cavity provokes an immediate excitability in pharyngeal motor cortex. The aim of the current study was to investigate whether thermal stimulation can produce longer lasting effects on the corticopharyngeal neural pathway.

METHODS

Healthy volunteers (n = 8/12) underwent baseline pharyngeal motor evoked potential (PMEP) measurements evoked by transcranial magnetic stimulation. In the first experiment, subjects received thermal stimulation alternating 30 seconds of 15 and 36°C applied to the tongue surface for either 10 minutes, 5 minutes, or sham. In the second experiment, one of three intermittent thermal stimulus patterns was delivered: cold (alternating 30 seconds of 15 and 36°C), warm (continuous 36°C), or hot (alternating 30 seconds of 45 and 36°C) for 10 minutes. In both experiments, PMEP were remeasured every 15 minutes up to 60 minutes following thermal stimulation.

KEY RESULTS

Repeated measures ANOVA for each stimulus time in the first experiment showed a significant increased change in PMEP amplitude at 30 minutes following only 10-minute stimulation compared with sham (P < .05). In the second experiment, we found that cold stimulation was more effective than the other stimulation (P < .05) at increasing PMEP amplitudes.

CONCLUSIONS AND INFERENCES

Ten-minute cold stimulation on the tongue can induce a delayed (30 minutes) increase in pharyngeal cortical excitability, providing a clinically useful therapeutic window for its application in dysphagic patients.

Comparison of unpleasant and pain thresholds of thermal stimuli in the orofacial regions: a psychophysical study using quantitative sensory testing in healthy young men

Purpose/Aim: To gain a better understanding of the psychophysics of thermal pain perception in a clinical setting, this study investigated whether thermal thresholds of unpleasantness are different from pain thresholds of cold and heat stimuli. Of particular interest was the relationship between unpleasantness and pain thresholds for cold vs heat stimuli.

MATERIAL AND METHODS

Thirty healthy male volunteers (mean age 26.1 years, range 23 to 32 years) participated. Thermal detection, cold pain (CPT) and heat pain (HPT) thresholds were measured at 5 trigeminal sites by the method of limits using quantitative sensory testing (QST), followed by cold unpleasant (CUT) and heat unpleasant (HUT) thresholds.

RESULTS

The temperatures at which individuals first reported thermal sensations as unpleasant or painful substantially differed among subjects. CUT exhibited a higher mean value with less variability than CPT, and HUT presented a lower mean than HPT (p < .001). As with CPT, CUT did not show any significant difference between the test sites. On the other hand, HUT, like HPT, exhibited site differences (p < .001). There was moderate correlation between CUT and CPT, whereas HUT and HPT were strongly correlated. The relationship between unpleasant and pain thresholds of cold vs heat stimuli was significantly different even when controlling for test site variability (p < .001).

CONCLUSION

These findings indicate that unpleasant and pain thresholds to thermal stimuli differ in healthy young men. Of particular note is the distinct relationship of unpleasant and pain thresholds of cold vs heat stimuli, revealing the thermal difference in temperature transition from unpleasantness to pain.

Discrete Modules and Mesoscale Functional Circuits for Thermal Nociception within Primate S1 Cortex

This study addresses one long-standing question of whether functional separations are preserved for somatosensory modalities of touch, heat, and cold nociception within primate primary somatosensory (S1) cortex. This information is critical for understanding how the nature of pain is represented in the primate brain. Using a combination of submillimeter-resolution fMRI and microelectrode local field potential (LFP) and spike recordings, we identified spatially segregated cortical zones for processing touch and nociceptive heat and cold stimuli in somatotopically appropriate areas 3a, 3b, 1, and 2 of S1 in male monkeys. The distances between zones were comparable (∼3.4 mm) across stimulus modalities (heat, cold, and tactile), indicating the existence of uniform, modality-specific modules. Stimulus-evoked LFP maps validated the fMRI maps in areas 3b and 1. Isolation of heat and cold nociceptive neurons from the fMRI zones confirmed the validity of using fMRI to probe nociceptive regions and circuits. Resting-state fMRI analysis revealed distinct intrinsic functional circuits among functionally related zones. We discovered distinct modular structures and networks for thermal nociception within S1 cortex, a finding that has significant implications for studying chronic pain syndromes and guiding the selection of neuromodulation targets for chronic pain management.SIGNIFICANCE STATEMENT Primate S1 subregions contain discrete heat and cold nociceptive modules. Modules with the same properties exhibit strong functional connection. Nociceptive fMRI response coincides with LFP and spike activities of nociceptive neurons. Functional separation of heat and cold pain is retained within primate S1 cortex.

Thermosensory Perceptual Learning Is Associated with Structural Brain Changes in Parietal-Opercular (SII) Cortex

The location of a sensory cortex for temperature perception remains a topic of substantial debate. Both the parietal–opercular (SII) and posterior insula have been consistently implicated in thermosensory processing, but neither region has yet been identified as the locus of fine temperature discrimination. Using a perceptual learning paradigm in male and female humans, we show improvement in discrimination accuracy for subdegree changes in both warmth and cool detection over 5 d of repetitive training. We found that increases in discriminative accuracy were specific to the temperature (cold or warm) being trained. Using structural imaging to look for plastic changes associated with perceptual learning, we identified symmetrical increases in gray matter volume in the SII cortex. Furthermore, we observed distinct, adjacent regions for cold and warm discrimination, with cold discrimination having a more anterior locus than warm. The results suggest that thermosensory discrimination is supported by functionally and anatomically distinct temperature-specific modules in the SII cortex.

SIGNIFICANCE STATEMENT We provide behavioral and neuroanatomical evidence that perceptual learning is possible within the temperature system. We show that structural plasticity localizes to parietal–opercular (SII), and not posterior insula, providing the best evidence to date resolving a longstanding debate about the location of putative “temperature cortex.” Furthermore, we show that cold and warm pathways are behaviorally and anatomically dissociable, suggesting that the temperature system has distinct temperature-dependent processing modules.

Identification of Molecular Fingerprints in Human Heat Pain Thresholds by Use of an Interactive Mixture Model R Toolbox (AdaptGauss)

Biomedical data obtained during cell experiments, laboratory animal research, or human studies often display a complex distribution. Statistical identification of subgroups in research data poses an analytical challenge. Here were introduce an interactive R-based bioinformatics tool, called “AdaptGauss”. It enables a valid identification of a biologically-meaningful multimodal structure in the data by fitting a Gaussian mixture model (GMM) to the data. The interface allows a supervised selection of the number of subgroups. This enables the expectation maximization (EM) algorithm to adapt more complex GMM than usually observed with a noninteractive approach. Interactively fitting a GMM to heat pain threshold data acquired from human volunteers revealed a distribution pattern with four Gaussian modes located at temperatures of 32.3, 37.2, 41.4, and 45.4 °C. Noninteractive fitting was unable to identify a meaningful data structure. Obtained results are compatible with known activity temperatures of different TRP ion channels suggesting the mechanistic contribution of different heat sensors to the perception of thermal pain. Thus, sophisticated analysis of the modal structure of biomedical data provides a basis for the mechanistic interpretation of the observations. As it may reflect the involvement of different TRP thermosensory ion channels, the analysis provides a starting point for hypothesis-driven laboratory experiments.

Feeling the burn: When it looks like it hurts, and belongs to me, it really does hurt more

We examined changes in pain sensitivity in the rubber hand illusion (RHI). Experiment 1 investigated changes in pain tolerance immediately after a "healthy" and "wounded" RHI when immersing the hand in a cold pressor ice bath. There was 19% increased pain tolerance and increased perception detection threshold after the healthy RHI, but 11% reduction after the wounded RHI. Experiment 2 examined pain experience during the wounded RHI with capsaicin-induced hyperalgesia. Pain intensity and unpleasantness was higher on the illusion arm during the synchronous RHI, compared with asynchronous trials. There was no change in pain experience on the control arm, and both arms had similar pain sensitivity after the experiment. Our results highlight the impact of embodying a substitute limb on pain, with increased tolerance and reduced tactile sensitivity when the fake limb is healthy and apparently pain-free, but increased pain sensitivity when the self-attributed limb appears to be wounded.

Multimodal distribution of human cold pain thresholds

Background

It is assumed that different pain phenotypes are based on varying molecular pathomechanisms. Distinct ion channels seem to be associated with the perception of cold pain, in particular TRPM8 and TRPA1 have been highlighted previously. The present study analyzed the distribution of cold pain thresholds with focus at describing the multimodality based on the hypothesis that it reflects a contribution of distinct ion channels.

Methods

Cold pain thresholds (CPT) were available from 329 healthy volunteers (aged 18 – 37 years; 159 men) enrolled in previous studies. The distribution of the pooled and log-transformed threshold data was described using a kernel density estimation (Pareto Density Estimation (PDE)) and subsequently, the log data was modeled as a mixture of Gaussian distributions using the expectation maximization (EM) algorithm to optimize the fit.

Results

CPTs were clearly multi-modally distributed. Fitting a Gaussian Mixture Model (GMM) to the log-transformed threshold data revealed that the best fit is obtained when applying a three-model distribution pattern. The modes of the identified three Gaussian distributions, retransformed from the log domain to the mean stimulation temperatures at which the subjects had indicated pain thresholds, were obtained at 23.7 °C, 13.2 °C and 1.5 °C for Gaussian #1, #2 and #3, respectively.

Conclusions

The localization of the first and second Gaussians was interpreted as reflecting the contribution of two different cold sensors. From the calculated localization of the modes of the first two Gaussians, the hypothesis of an involvement of TRPM8, sensing temperatures from 25 – 24 °C, and TRPA1, sensing cold from 17 °C can be derived. In that case, subjects belonging to either Gaussian would possess a dominance of the one or the other receptor at the skin area where the cold stimuli had been applied. The findings therefore support a suitability of complex analytical approaches to detect mechanistically determined patterns from pain phenotype data.

Relevance of Water Temperature, Apparatus, and Age to Children's Pain during the Cold Pressor Task

BACKGROUND

Effective use of experimental pain in research depends on a thorough understanding of factors influencing their use. Although studies using the cold pressor task (CPT) have critically advanced our knowledge of pain mechanisms, assessment, and management in adults and children, the impact of identified methodological variability in its use is not known (ie, water temperature and apparatus); furthermore, whether methodological variations differentially impact children across development.

METHODS

Pain outcomes were examined in 113 healthy children from three age groups (8- to 9-, 10- to 11-, and 12- to 14-year-olds) who completed three CPTs at different water temperatures (5, 7, 10°C) in counterbalanced order. Children were randomly assigned to one of two apparatus (ice- vs. electric-cooled) for all CPTs. Children's hand was warmed to its baseline temperature between CPTs.

RESULTS

Colder water (2 to 3°C decreases) was associated with significantly higher pain intensity and unpleasantness, and lower pain tolerance and threshold. Older children (12 to 14 years) reported significantly worse pain intensity and unpleasantness as compared to 8- to 9-year-olds, likely due to longer pain tolerance. Pain outcomes in 10- to 11-year-olds fell between the other age groups, with significant differences for pain unpleasantness and pain tolerance (at 10°C). Higher pain-related fear and pain unpleasantness occurred with the electric-cooled apparatus. Girls had higher pain tolerance and threshold at all temperatures.

CONCLUSIONS

These results provide critically important information about water temperature, apparatus, and child age on CPT pain responding. It informs design of future CPT studies and directs consideration of methodological variability and child age when interpreting study findings.

Bortezomib-induced painful neuropathy in patients with multiple myeloma

Neurotoxicity towards the peripheral nervous system which appears clinically in the form of painful neuropathy is an essential dose-limiting factor during the treatment of multiple myeloma. In this review article different forms of this painful neuropathy are presented together with currently available diagnostic tools which are usually applied to confirm the diagnosis. Also, the most often used neurological scales estimating neurological deficit are presented. Special attention is paid to the management of the reversibility of bortezomib-induced neuropathic pain.

Pirt functions as an endogenous regulator of TRPM8

Pirt is a membrane protein that is specifically expressed in the peripheral nervous system, where it has been shown to increase the sensitivity of the transient receptor potential vanilloid 1 channel and modulate its role in heat pain. The broad expression of Pirt among dorsal root ganglion neurons suggests it may modulate other transient receptor potentials, such as the menthol and cooling sensor TRPM8. The discrepancies in the channel properties of TRPM8 in native neurons versus heterologous cells indicate the existence of endogenous modulators of the channel. Here we show that Pirt regulates the function of TRPM8 and its role in detecting cold. Pirt(-/-) mice exhibit decreased behavioural responses to cold and cool temperatures, and Pirt increases the sensitivity of TRPM8 to menthol and cool temperature. Our data suggest Pirt is an endogenous regulator of TRPM8.

Differential roles of galanin on mechanical and cooling responses at the primary afferent nociceptor

Background

Galanin is expressed in a small percentage of intact small diameter sensory neurons of the dorsal root ganglia and in the afferent terminals of the superficial lamina of the dorsal horn of the spinal cord. The neuropeptide modulates nociception demonstrating dose-dependent pro- and anti-nociceptive actions in the naïve animal. Galanin also plays an important role in chronic pain, with the anti-nociceptive actions enhanced in rodent neuropathic pain models. In this study we compared the role played by galanin and its receptors in mechanical and cold allodynia by identifying individual rat C-fibre nociceptors and characterising their responses to mechanical or acetone stimulation.

Results

Mechanically evoked responses in C-fibre nociceptors from naive rats were sensitised after close intra-arterial infusion of galanin or Gal2-11 (a galanin receptor-2/3 agonist) confirming previous data that galanin modulates nociception via activation of GalR2. In contrast, the same dose and route of administration of galanin, but not Gal2-11, inhibited acetone and menthol cooling evoked responses, demonstrating that this inhibitory mechanism is not mediated by activation of GalR2. We then used the partial saphenous nerve ligation injury model of neuropathic pain (PSNI) and the complete Freund’s adjuvant model of inflammation in the rat and demonstrated that close intra-arterial infusion of galanin, but not Gal2-11, reduced cooling evoked nociceptor activity and cooling allodynia in both paradigms, whilst galanin and Gal2-11 both decreased mechanical activation thresholds. A previously described transgenic mouse line which inducibly over-expresses galanin (Gal-OE) after nerve injury was then used to investigate whether manipulating the levels of endogenous galanin also modulates cooling evoked nociceptive behaviours after PSNI. Acetone withdrawal behaviours in naive mice showed no differences between Gal-OE and wildtype (WT) mice. 7-days after PSNI Gal-OE mice demonstrated a significant reduction in the duration of acetone-induced nociceptive behaviours compared to WT mice.

Conclusions

These data identify a novel galaninergic mechanism that inhibits cooling evoked neuronal activity and nociceptive behaviours via a putative GalR1 mode of action that would also be consistent with a TRP channel-dependent mechanism.

The face as a sensory organ

BACKGROUND

The human face is a highly specialized organ for receiving the sensory information from the environment and for its transmission to the cortex. The advent of facial transplantation has shown that excellent reconstruction of disfiguring defects can be achieved; thus, the expectations are now focused on functional recovery of the transplant. So far, restoration of facial sensation has not received the same attention as the recovery of motor function.

METHODS

A thorough review of the literature was performed to investigate the current knowledge on the sensory pathways of the human face and their functions to evaluate current methods of sensory assessment and the available data on normal sensation.

RESULTS

The presence of Meissner and Ruffini corpuscles, Merkel disks, hair-associated fibers, and intraepidermal free nerve endings was confirmed. Occurrence of extensive cross-communications between trigeminal and facial nerve was substantiated. Two-point discrimination and pressure thresholds represented the most objective measures of facial sensation. Age, sex, and smoker status of the patients were shown to influence normal sensibility values. The most suitable areas for sensory testing based on the tested modality and innervation were inferred. The anatomical course of the nerves and their variations had implications for the harvest of face allografts and repair of the sensory nerves.

CONCLUSIONS

This review has illustrated the complexity of sensory pathways of the face and their influence on somatic and visceral responses. In view of the discussed data, during facial transplantation, it is important to consider different mechanisms of restoration of facial sensation.

The emerging role of TRP channels in mechanisms of temperature and pain sensation

Pain is universal and vital to survival. It is an essential component of our sense of touch; together, touch and pain have evolved to enable our awareness of the intricacies of our environment and to warn us of danger and possible injury. There is a clear link between temperature sensation and pain-painful temperature sensations occur acutely and are a hallmark of inflammatory and chronic pain disorders of the nervous system. Mounting evidence suggests a subset of Transient Receptor Potential (TRP) ion channels activated by temperature (thermoTRPs) are important molecular players in acute, inflammatory and chronic pain states. Varying degrees of heat activate four of these channels (TRPV1-4), while cooling temperatures ranging from pleasant to painful activate two distantly related thermoTRP channels (TRPM8 and TRPA1). ThermoTRP channels are also chemosensitive, being activated and or modulated by plant-derived small molecules and endogenous inflammatory mediators. All thermoTRPs are expressed in tissues essential to cutaneous thermal and pain sensation. This review examines the contribution of thermoTRP channels to our understanding of temperature and pain transduction at the molecular level.

Comparison of the cold pressor test and contact thermode-delivered cold stimuli for the assessment of cold pain sensitivity

Sensitivity to suprathreshold cold pain stimuli constitutes an important part of comprehensive pain sensitivity testing and can be assessed by the cold pressor test or by using a contact thermode-based testing device. One major difference between the 2 methods is the size of the surface area stimulated, which is thought to affect both recruitment of endogenous pain control mechanisms and vasomotor reactions. It is therefore not clear if the 2 methods can be used interchangeably for the assessment of cold pain. Here we applied 60-second-long stimuli at approximately 3 degrees C to the hands of 47 subjects by both methods. Pain intensity ratings (on a scale from 0 to 10) were significantly higher in the cold pressor test than in the thermode cold test (6.3 +/- 1.8 vs 3.9 +/- 2), associated with a higher rate of dropouts within the 60 seconds (64 vs 11%). Nonetheless, pain intensity ratings obtained with both methods were highly correlated (r = .70). However, the thermode cold test shared a larger amount of variance with 1 or more of the other pain intensity rating tests (phasic and tonic heat, pinprick) than the cold pressor test (53% vs 30%) while the cold pressor test contained a larger proportion of unique variance (39 vs 26%).

PERSPECTIVE

This article compares 2 methods of cold pain assessment in humans and analyzes their relationship to heat and pinprick pain. It could help researchers select the appropriate cold pain test for their study. It may also promote our understanding of commonalities and differences between different pain modalities.

Transient receptor potential channel A1 and noxious cold responses in rat cutaneous nociceptors

The role of transient receptor potential channel A1 (TRPA1) in noxious cold sensation remains unclear. Some data support the hypothesis that TRPA1 is a transducer of noxious cold whilst other data contest it. In this study we investigated the role of TRPA1 in cold detection in cutaneous nociceptors in vivo using complementary experimental approaches. We used noxious withdrawal reflex electromyography, and single fibre recordings in vivo, to test the hypothesis that TRPA1-expressing primary afferents mediate noxious cold responses in anaesthetised rats. TRPV1 and TRPM8 agonists sensitise their cognate receptors to heat and cold stimuli respectively. Herein we show that the TRPA1 agonist cinnamaldehyde applied to the skin in anaesthetised rats did not sensitise noxious cold evoked hind limb withdrawal. In contrast, cinnamaldehyde did sensitise the C fibre-mediated noxious heat withdrawal, indicated by a significant drop in the withdrawal temperature. TRPA1 agonist thus sensitised the noxious reflex withdrawal to heat, but not cold. Thermal stimuli also sensitise transient receptor potential (TRP) channels to agonist. Activity evoked by capsaicin in teased primary afferent fibres showed a significant positive correlation with receptive field temperature, in both normal and Freund's complete adjuvant-induced cutaneous inflammation. Altering the temperature of the receptive field did not modulate TRPA1 agonist evoked-activity in cutaneous primary afferents, in either normal or inflamed skin. In addition, block of the TRPA1 channel with Ruthenium Red did not inhibit cold evoked activity in either cinnamaldehyde sensitive or insensitive cold responsive nociceptors. In cinnamaldehyde-sensitive-cold-sensitive afferents, although TRPA1 agonist-evoked activity was totally abolished by Ruthenium Red, cold evoked activity was unaffected by channel blockade. We conclude that these results do not support the hypothesis that TRPA1-expressing cutaneous afferents play an important role in noxious cold responses.

A multi-scale view of skin thermal pain: from nociception to pain sensation

All biological bodies live in a thermal environment, including the human body, where skin is the interface with a protecting function. When the temperature is out of the normal physiological range, skin fails to protect, and the pain sensation is evoked. Furthermore, in medicine, with advances in laser, microwave and similar technologies, various thermal therapeutic methods have been widely used to cure disease/injury involving skin tissue. However, the corresponding problem of pain relief has limited further application and development of these thermal treatments. Skin thermal pain is induced through both direct (i.e. an increase/decrease in temperature) and indirect (e.g. thermomechanical and thermochemical) ways, and is governed by complicated thermomechanical-chemical-neurophysiological responses. However, a complete understanding of the underlying mechanisms is still far from clear. In this article, starting from an engineering perspective, we aim to recast the biological behaviour of skin in engineering system parlance. Then, by coupling the concepts of engineering with established methods in neuroscience, we attempt to establish multi-scale modelling of skin thermal pain through ion channel to pain sensation. The model takes into account skin morphological plausibility, the thermomechanical response of skin tissue and the biophysical and neurological mechanisms of pain sensation.

Modulation of oral heat and cold pain by irritant chemicals

Common food irritants elicit oral heat or cool sensations via actions at thermosensitive transient receptor potential (TRP) channels. We used a half-tongue, 2-alternative forced-choice procedure coupled with bilateral pain intensity ratings to investigate irritant effects on heat and cold pain. The method was validated in a bilateral thermal difference detection task. Capsaicin, mustard oil, and cinnamaldehyde enhanced lingual heat pain elicited by a 49 degrees C stimulus. Mustard oil and cinnamaldehyde weakly enhanced lingual cold pain (9.5 degrees C), whereas capsaicin had no effect. Menthol significantly enhanced cold pain and weakly reduced heat pain. To address if capsaicin's effect was due to summation of perceptually similar thermal and chemical sensations, one-half of the tongue was desensitized by application of capsaicin. Upon reapplication, capsaicin elicited little or no irritant sensation yet still significantly enhanced heat pain on the capsaicin-treated side, ruling out summation. In a third experiment, capsaicin significantly enhanced pain ratings to graded heat stimuli (47 degrees C to 50 degrees C) resulting in an upward shift of the stimulus-response function. Menthol may induce cold hyperalgesia via enhanced thermal gating of TRPM8 in peripheral fibers. Capsaicin, mustard oil, and cinnamaldehyde may induce heat hyperalgesia via enhanced thermal gating of TRPV1 that is coexpressed with TRPA1 in peripheral nociceptors.

Cold sensitivity of recombinant TRPA1 channels

TRPM8 and TRPA1, members of the transient receptor potential (TRP) channel family, are candidates for cooling-activated receptors. It is accepted that TRPM8 responds to moderate cooling, although it is controversial whether TRPA1 responds to deep cooling. Here, using Ca(2+) imaging and/or patch-clamp recordings, we examined the thermal sensitivity of primary cultured dorsal root ganglion (DRG) neurons and mouse TRPA1-expressing human embryonic kidney (HEK) 293 cells. In a subset of cultured mouse DRG neurons, deep cooling (5-18 degrees C) and allyl isothiocyanate (AITC, agonist of TRPA1) induced increases in intracellular Ca(2+) level. Most AITC-sensitive (TRPA1-expressing) neurons responded to deep cooling. In TRPA1-expressing HEK293 cells, deep cooling and AITC-induced Ca(2+) responses and whole-cell currents. In inside-out patches excised from TRPA1-expressing HEK293 cells, deep cooling, and AITC activated the same channels, which were inhibited by camphor (antagonist for TRPA1). When temperature was decreased below 18 degrees C, unit conductance of the channel decreased but open probability of it increased. Deep cooling-induced increase of the open probability of TRPA1 may underlie the increase in whole-cell currents induced by deep cooling. It is concluded that TRPA1 is a deep cooling-activated channel, which supports the previous findings that TRPA1 responds to deep cooling.

Contribution of TRPM8 channels to cold transduction in primary sensory neurons and peripheral nerve terminals

Transient receptor potential melastatin 8 (TRPM8) is the best molecular candidate for innocuous cold detection by peripheral thermoreceptor terminals. To dissect out the contribution of this cold- and menthol-gated, nonselective cation channel to cold transduction, we identified BCTC [N-(4-tert-butylphenyl)-4-(3-chloropyridin-2-yl)piperazine-1-carboxamide] as a potent and full blocker of recombinant TRPM8 channels. In cold-sensitive trigeminal ganglion neurons of mice and guinea pig, responses to menthol were abolished by BCTC. In contrast, the effect of BCTC on cold-evoked responses was variable but showed a good correlation with the presence or lack of menthol sensitivity in the same neuron, suggesting a specific blocking action of BCTC on TRPM8 channels. The biophysical properties of native cold-gated currents (I(cold)), and the currents blocked by BCTC were nearly identical, consistent with a role of this channel in cold sensing at the soma. The temperature activation threshold of native TRPM8 channels was significantly warmer than those reported in previous expression studies. The effect of BCTC on native I(cold) was characterized by a dose-dependent shift in the temperature threshold of activation. The role of TRPM8 in transduction was further investigated in the guinea pig cornea, a peripheral territory densely innervated with cold thermoreceptors. All cold-sensitive terminals were activated by menthol, suggesting the functional expression of TRPM8 channels in their membrane. However, the spontaneous activity and firing pattern characteristic of cold thermoreceptors was totally immune to TRPM8 channel blockade with BCTC or SKF96365 (1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy]ethyl-1H-imidazole hydrochloride). Cold-evoked responses in corneal terminals were also essentially unaffected by these drugs, whereas responses to menthol were completely abolished. The minor impairment in the ability to transduce cold stimuli by peripheral corneal thermoreceptors during TRPM8 blockade unveils an overlapping functional role for various thermosensitive mechanisms in these nerve terminals.

Molecular identification and functional characterization of a temperature-sensitive transient receptor potential channel (TRPM8) from canine

TRPM8 belongs to the family of transient receptor potential channels and is activated by cooling and cooling agents, such as icilin and menthol. It is expressed in a subset of sensory neurons and is thought to be involved in thermosensation. Here, we report the cloning and functional characterization of canine TRPM8 (cTRPM8). cTRPM8 shares 95.1%, 94.1%, and 93.9% protein sequence identity with human, rat and mouse TRPM8, respectively. Similar to these mammalian orthologs, cTRPM8 was activated by menthol and icilin with strong outward rectification and little cation selectivity. Menthol and icilin also caused calcium-dependent desensitization. Interestingly, cTRPM8 was activated at <17 degrees C, a temperature threshold lower than that reported for the other orthologs. At 22 degrees C, the EC(50) for activation of cTRPM8 expressed in HEK293 cells by icilin and menthol was 0.06 and 4.3 microM determined by Fluorometric Imaging Plate Reader (FLIPR) and 0.4 and 85 microM by patch clamp, respectively. Mustard oil also activated cTRPM8 (FLIPR EC(50) = 490 microM). Menthol activation was more potent at +60 mV than at -60 mV (EC(50) = 53 and 124 microM, respectively, in Xenopus ooctyes). Icilin-, menthol- and mustard oil-induced intracellular Ca(2+) increases were similarly blocked by N-(4-tertiarybutyl-phenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carboxamide (BCTC) with IC(50) = 2.3, 2.8 and 1.8 microM, respectively. Cooling-activated current was also inhibited by BCTC. Extracellular calcium blocked cTRPM8 in a concentration- and voltage-dependent manner (half maximal blocking [Ca(2+)] = 1.6 mM at -100 mV). These results constitute the first study of cTRPM8 and support the idea that cTRPM8 functions as a transducer of cold stimuli in vivo.

Rapid deterioration of pain sensory-discriminative information in short-term memory

The assessment of pain and analgesic efficacy sometimes relies on the retrospective evaluation of pain felt in the immediate, recent or distant past, yet we have a very limited understanding of the processes involved in the encoding, maintenance and intentional retrieval of pain. We examine the properties of the short-term memory of thermal and pain sensation intensity with a delayed-discrimination task using pairs of heat pain, warm and cool stimulation in healthy volunteers. Performance decreased as a function of the inter-stimulus interval (ISI), indicating a robust deterioration of sensory information over the test period of 4-14 s. As expected, performance also decreased with smaller temperature differences (Delta-T) and shorter stimulus durations (6-2 s). The relation between performance and Delta-T was adequately described by a power function, the exponent of which increased linearly with longer ISI. Importantly, performance declined steadily with increasing ISI (from 6 to 14 s)--but only for pairs of heat pain stimuli that were relatively difficult to discriminate (Delta-T < or = 1.0 degree C; perceptual difference < or = 32/100 pain rating units) while no deterioration in performance was observed for the largest temperature difference tested (Delta T = 1.5 degrees C; perceptual difference of 50 units). These results are consistent with the possibility that short-term memory for pain and temperature sensation intensity relies on a transient analog representation that is quickly degraded and transformed into a more resistant but less precise categorical format. This implies that retrospective pain ratings obtained even after very short delays may be rather inaccurate but relatively reliable.

Neural correlates of prickle sensation: a percept-related fMRI study

The painful sensations produced by a laceration, freeze, burn, muscle strain or internal injury are readily distinguishable because each is characterized by a particular sensory quality such as sharp, aching, burning or prickling. We propose that there are specific neural correlates of each pain quality, and here we used a new functional magnetic resonance imaging (fMRI) method to identify time-locked responses to prickle sensations that were evoked by noxious cold stimuli. With percept-related fMRI, we identified prickle-related brain activations in the anterior cingulate cortex (ACC), insula, secondary somatosensory cortex (S2), prefrontal cortex (PFC), premotor cortex (PMC), caudate nucleus and dorsomedial thalamus, indicating that multiple pain, sensory and motor areas act together to produce the prickle sensation.

Quantitative assessment of cutaneous thermal and vibration sensation and thermal pain detection thresholds in healthy children and adolescents

Quantitative sensory testing (QST) is a noninvasive, computer-assisted method for assessing function in peripheral small and large sensory fibers. In order to use QST for clinical neurological assessment in children, it is necessary: (1) to determine whether children can reliably perform these tests and (2) to characterize normal ranges in healthy children. Values of cold sensation, warm sensation, cold pain, heat pain, and vibration sensation detection thresholds were determined in the hand and foot with the method of limits (MLI) and method of levels (MLE) in 101 healthy children aged 6-17 years using a commercially available device. Both MLI and MLE were well-accepted by children, and there was good reproducibility between two sessions. The MLE takes longer to perform but produces lower thermal detection thresholds than the MLI. In the MLI, vibration and warm sensation showed higher thresholds in the foot than hand, whereas cold pain showed lower thresholds in the foot than hand. Based on these results, QST may be used to document and monitor the clinical course of sensory abnormalities in children with neurological disorders or neuropathic pain.

Quantitative response characteristics of thermoreceptive and nociceptive lamina I spinothalamic neurons in the cat

The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. Based on recordings made prior to any noxious stimulation, the mean spontaneous discharge rates of these classes also differed: NS = 0.5 +/- 0.7 imp/s (n = 47), HPC = 0.9 +/- 0.7 imp/s (n = 59), and COOL = 3.3 +/- 2.6 imp/s (n = 107). Standard, quantitative, thermal stimulus sequences applied with a Peltier thermode were used to characterize the stimulus-response functions of 76 COOL cells, 47 HPC cells, and 37 NS cells. The COOL cells showed a very linear output from 34 degrees C down to approximately 15 degrees C and a maintained plateau thereafter. The HPC cells showed a fairly linear but accelerating response to cold below a median threshold of approximately 24 degrees C and down to 9 degrees C (measured at the skin-thermode interface with a thermode temperature of 2 degrees C). The HPC cells and the NS cells both showed rapidly increasing, sigmoidal response functions to noxious heat with a fairly linear response between 45 and 53 degrees C, but they had significantly different thresholds; half of the HPC cells were activated at ~45.5 degrees C and half of the NS cells at approximately 43 degrees C. The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.

Sensory experiences in humans and single-unit activity in cats evoked by polymodal stimulation of the cornea

1. The cornea of human subjects and of anaesthetised cats was stimulated with a jet of air of controlled flow, temperature and CO(2) concentration delivered by a gas aesthesiometer. 2. In humans, the intensity and magnitude of various components of the sensory experience (intensity of the sensation, degree of irritation, magnitude of burning and stinging pain, magnitude of the cold and warm components of the sensation) were measured using separate visual analog scales. In anaesthetised cats, the impulse response to the same stimuli was recorded from single mechanosensory, polymodal and cold-sensitive corneal fibres in the ciliary nerves. 3. Intensity-response curves for mechanical stimulation showed that all parameters of the sensation experienced by humans increased with the intensity of the stimulus. Mechanical stimuli recruited mainly phasic mechanosensory and polymodal afferents in the cat. 4. Acidic stimulation with gas mixtures of increasing CO(2) concentration evoked irritation, burning and to a lesser extent stinging pain of a magnitude roughly proportional to the intensity of the stimulus in humans. CO(2) primarily recruited polymodal afferents and weakly excited cold-sensitive fibres in the cat's cornea. 5. Heat stimuli evoked in humans a sensation profile similar to CO(2) but accompanied by a warmth component. In the cat's cornea, heat excited only polymodal fibres and silenced cold-sensitive corneal units. 6. Cold stimuli applied to the human cornea elicited a sensation of cooling that became irritant at the lowest temperatures. Corneal cold-sensitive fibres of the cat were activated in a manner proportional to the temperature drop, while polymodal nociceptor fibres were recruited only by the lowest temperatures. Topical menthol (0.2 mM) applied to humans evoked and later eliminated cold sensations produced by cold stimuli while the irritation sensation caused by low temperature stimuli still persisted. 7. Human subjects were able to identify masked mechanical, thermal and chemical stimuli applied to the cornea. 8. Irritation and cold sensations can therefore be evoked separately from the cornea by selective activation of mechanosensory, polymodal and cold corneal sensory afferents. Stimulation with different forms of energy usually leads to combined activation and/or inhibition of the different populations of sensory afferent fibres, evoking blended sensations that include irritation and thermal components in a variable degree.

Noxious hot and cold stimulation produce common patterns of brain activation in humans: a functional magnetic resonance imaging study

We used functional magnetic resonance imaging (fMRI) to determine whether similar brain regions activate during noxious hot and cold stimulation. Six male subjects underwent whole brain fMRI during phasic delivery of noxious hot (46 degrees C) and noxious cold (5 degrees C) stimulation to the dorsum of the left hand. Mid-brain regions activated included thalamus, basal ganglia and insula. Cortical areas activated included cingulate, somatosensory, premotor and motor cortices, as well as prefrontal and inferior parietal cortex. Most regions activated bilaterally but with stronger activation contralateral to the stimulus. Noxious cold stimulation produced significantly increased volumes of activation compared to noxious heat in prefrontal areas only. Our results suggest a similar network of regions activate common to the perception of pain produced by either noxious hot or cold stimulation.

Cold-evoked pain varies with skin type and cooling rate: a psychophysical study in humans

The psychophysical responses to noxious cold stimulation of the skin in normal human subjects are not well understood. Continuous pain ratings with the visual analogue scale is an important method to assess these responses. In this study, we addressed several important issues about the parameters with which stimuli are delivered: the type of skin stimulated, the rate with which the stimulus temperature decreases, and the dimension of the pain rated by subjects. Cold stimuli were delivered to the thenar eminence (glabrous skin) and the dorso-lateral hand (hairy skin) via a 4 cm(2) Peltier-type stimulator. Cold and pain thresholds were determined by the method of limits (MOL). A computerized visual analogue scale (VAS) was used to obtain continuous ratings of pain intensity and affect. The McGill Pain Questionnaire (MPQ) was used to assess the quality of cold-evoked pain. Supra-threshold stimuli (34 degrees C base) were delivered at 0.5, 1 or 2 degrees C/s to 2 degrees C, held for 20s and returned to baseline at 9 degrees C/s. These studies revealed: (1) Cold thresholds, measured with MOL, were lower (i.e. occurred at higher absolute temperatures) for the hairy skin of the dorso-lateral hand compared to the glabrous skin of the thenar eminence. (2) A similar pattern was evident for cold induced pain thresholds with MOL at 1.5 degrees C/s and with intensity and affect VAS scales at 0.5 and 1 degrees C/s. (3) Exponents for supra-threshold ratings fit to power functions were larger for the glabrous skin site than the hairy skin site regardless of cooling rate or dimension of pain measured. (4) All pain indices were higher for slower cooling rates. (5) No significant differences were found in the pain indices for pain ratings of intensity and affect. (6) A substantial proportion of subjects chose words representing paradoxical heat with the MPQ. (7) Painful paradoxical heat sensations occurred most often during cooling, while innocuous warm sensations mainly occurred during the rewarming phase.

Cold-induced pain and prickle in the glabrous and hairy skin

Little is known concerning the mechanisms underlying the perception of cold pain in humans. An appreciation of these mechanisms is important to understand and possibly treat those disorders in which cold stimuli evoke unpleasant sensations. To study cold pain, I have conducted psychophysical experiments on 16 healthy subjects. A peltier-type stimulator (20 x 25 mm) was used to deliver stimuli to sites on the thenar eminence (glabrous skin) and volar forearm (hairy skin) of each arm. Each trial consisted of a 90 s, 2 degrees C stimulus that was preceded and followed by a 35 degrees C stimulus. A computer-based visual analog scale was used to collect continuous pain ratings throughout each trial. In experiment 1, nine subjects rated the overall evoked pain intensity (four trials/skin type) and the prickle component (four trials/skin type). Typically, subjects perceived the cold-evoked pain as prickly, cold/freezing and achy. The pain intensity and quality was similar for glabrous and hairy skin sites within individual subjects. Pain intensity gradually rose to a plateau by approximately 60 s into each trial. The prickle component differed amongst subjects due to its variable time course. Subjects consistently reported an intense, brief jab of prickle at both hairy and glabrous sites during the rewarming phase. In experiment 2, nine subjects rated the pain intensity during the cold stimulus before and during a compression-ischemic block of Abeta/Adelta fiber conduction. The dominant sensation evoked by the cold stimulus in the hairy and glabrous skin during the block was a sharp, hot/burning pain. The block did not consistently affect the total pain at the hairy sites. However, most subjects reported more pain during the block at the glabrous sites. These data suggest that noxious cold stimuli affect a mosaic of primary afferent input and central processing resulting in a complex pain experience which may differ in glabrous and hairy skin.