Temporal and qualitative properties of cold pain and heat pain: a psychophysical study

Ion channels of cold transduction and transmission

Thermosensation requires the activation of a unique collection of ion channels and receptors that work in concert to transmit thermal information. It is widely accepted that transient receptor potential melastatin 8 (TRPM8) activation is required for normal cold sensing; however, recent studies have illuminated major roles for other ion channels in this important somatic sensation. In addition to TRPM8, other TRP channels have been reported to contribute to cold transduction mechanisms in diverse sensory neuron populations, with both leak- and voltage-gated channels being identified for their role in the transmission of cold signals. Whether the same channels that contribute to physiological cold sensing also mediate noxious cold signaling remains unclear; however, recent work has found a conserved role for the kainite receptor, GluK2, in noxious cold sensing across species. Additionally, cold-sensing neurons likely engage in functional crosstalk with nociceptors to give rise to cold pain. This Review will provide an update on our understanding of the relationship between various ion channels in the transduction and transmission of cold and highlight areas where further investigation is required.

Chemotherapy for pain: reversing inflammatory and neuropathic pain with the anticancer agent mithramycin A

ABSTRACT

The persistence of inflammatory and neuropathic pain is poorly understood. We investigated a novel therapeutic paradigm by targeting gene networks that sustain or reverse persistent pain states. Our prior observations found that Sp1-like transcription factors drive the expression of TRPV1, a pain receptor, that is blocked in vitro by mithramycin A (MTM), an inhibitor of Sp1-like factors. Here, we investigate the ability of MTM to reverse in vivo models of inflammatory and chemotherapy-induced peripheral neuropathy (CIPN) pain and explore MTM's underlying mechanisms. Mithramycin reversed inflammatory heat hyperalgesia induced by complete Freund adjuvant and cisplatin-induced heat and mechanical hypersensitivity. In addition, MTM reversed both short-term and long-term (1 month) oxaliplatin-induced mechanical and cold hypersensitivity, without the rescue of intraepidermal nerve fiber loss. Mithramycin reversed oxaliplatin-induced cold hypersensitivity and oxaliplatin-induced TRPM8 overexpression in dorsal root ganglion (DRG). Evidence across multiple transcriptomic profiling approaches suggest that MTM reverses inflammatory and neuropathic pain through broad transcriptional and alternative splicing regulatory actions. Mithramycin-dependent changes in gene expression following oxaliplatin treatment were largely opposite to and rarely overlapped with changes in gene expression induced by oxaliplatin alone. Notably, RNAseq analysis revealed MTM rescue of oxaliplatin-induced dysregulation of mitochondrial electron transport chain genes that correlated with in vivo reversal of excess reactive oxygen species in DRG neurons. This finding suggests that the mechanism(s) driving persistent pain states such as CIPN are not fixed but are sustained by ongoing modifiable transcription-dependent processes.

Differential Effects of Thermal Stimuli in Eliciting Temporal Contrast Enhancement: A Psychophysical Study

Offset analgesia (OA) is observed when pain relief is disproportional to the reduction of noxious input and is based on temporal contrast enhancement (TCE). This phenomenon is believed to reflect the function of the inhibitory pain modulatory system. However, the mechanisms contributing to this phenomenon remain poorly understood, with previous research focusing primarily on painful stimuli and not generalizing to nonpainful stimuli. Therefore, the aim of this study was to investigate whether TCE can be induced by noxious as well as innocuous heat and cold stimuli. Asymptomatic subjects (n = 50) were recruited to participate in 2 consecutive experiments. In the first pilot study (n = 17), the parameters of noxious and innocuous heat and cold stimuli were investigated in order to implement them in the main study. In the second (main) experiment, subjects (n = 33) participated in TCE paradigms consisting of 4 different modalities, including noxious heat (NH), innocuous heat (IH), noxious cold (NC), and innocuous cold (IC). The intensity of the sensations of each thermal modality was assessed using an electronic visual analog scale. TCE was confirmed for NH (P < .001), NC (P = .034), and IC (P = .002). Conversely, TCE could not be shown for IH (P = 1.00). No significant correlation between TCE modalities was found (r < .3, P > .05). The results suggest that TCE can be induced by both painful and nonpainful thermal stimulation but not by innocuous warm temperature. The exact underlying mechanisms need to be clarified. However, among other potential mechanisms, this may be explained by a thermo-specific activation of C-fiber afferents by IH and of A-fiber afferents by IC, suggesting the involvement of A-fibers rather than C-fibers in TCE. More research is needed to confirm a peripheral influence. PERSPECTIVE: This psychophysical study presents the observation of temporal contrast enhancement during NH, NC, and innocuous cold stimuli but not during stimulation with innocuous warm temperatures in healthy volunteers. A better understanding of endogenous pain modulation mechanisms might be helpful in explaining the underlying aspects of pain disorders.

Vigilance to Painful Laser Stimuli is Associated with Increased State Anxiety and Tense Arousal

Introduction

Pain is frequently accompanied by enhanced arousal and hypervigilance to painful sensations. Here, we describe our findings in an experimental vigilance task requiring healthy participants to indicate when randomly timed moderately painful stimuli occur in a long train of mildly painful stimuli.

Methods

During a continuous performance task with painful laser stimuli (CPTpain), 18 participants rated pain intensity, unpleasantness, and salience. We tested for a vigilance decrement over time using classical metrics including correct targets (hits), incorrectly identified non-targets (false alarms), hit reaction time, and false alarm reaction time. We measured state anxiety and tense arousal before and after the task.

Results

We found a vigilance decrement across four 12.5-minute blocks of painful laser stimuli in hits [F3,51=2.91; p=0.043; time block 1>block 4 (t=2.77; p=0.035)]. Both self-report state anxiety (tpaired,17=3.34; p=0.0039) and tense arousal (tpaired,17=3.20; p=0.0053) increased after the task. We found a vigilance decrement during our laser pain vigilance task consistent with vigilance decrements found in other stimulus modalities. Furthermore, state anxiety positively correlated with tense arousal.

Discussion

CPTpain acutely increased tense arousal and state anxiety, consistent with previous results implicating the reciprocal interaction of state anxiety and acute painful sensations and the role of pain in augmenting tense arousal. These results may indicate a psychological process which predisposes the hypervigilant to developing greater acute pain, resulting in positive feedback, greater pain and anxiety.

Neural and Psychosocial Mechanisms Underlying Alcohol Use and Pain Interactions: Overview of Current Evidence and Future Directions

Purpose of Review

A growing body of research indicates bidirectional associations between alcohol use and pain. In this review, we highlight common neural and psychosocial mechanisms underlying pain and alcohol use and identify current gaps in the literature regarding alcohol/pain interactions. We also suggest future directions for the field moving forward, including more nuanced conceptualization of alcohol's negative reinforcing effects in the context of pain, broader use of clinically-relevant experimental pain induction modalities, and characterization of age, biological sex, gender, race, and ethnicity as moderators of pain/alcohol interactions.

Recent Findings

Acute alcohol intake has analgesic and negative-reinforcing effects in the context of pain, and chronic heavy alcohol use appears to increase risk for development of chronic pain. At the same time, pain, both acute and chronic, acts as a proximal antecedent for alcohol use and is associated with relapse risk for individuals in recovery from alcohol use disorder.

Summary

Although the links between alcohol use and pain are increasingly appreciated, significant gaps in understanding remain and systematic study of alcohol/pain interactions at all levels, including basic, preclinical, translational, and interventional, is needed.

Quantitative thermal testing as a screening and follow-up tool for diabetic sensorimotor polyneuropathy in patients with type 2 diabetes and prediabetes

Introduction

The diagnosis and assessment of neuropathy severity of diabetic sensorimotor polyneuropathy (DSPN) are mainly based on clinical neuropathy scores and electrophysiologic studies. This study aimed to determine whether quantitative thermal testing (QTT) can be used as a screening and follow-up tool for DSPN of prediabetes and type 2 diabetes at baseline and at 1-year follow-up.

Methods

All patients were assessed using the Toronto Clinical Neuropathy Score (TCNS) and underwent electrophysiological testing, including a nerve conduction study (NCS) and QTT, at baseline and at a 1-year follow-up. The TCNS and the composite scores of nerve conduction were used to assess the severity of DSPN. The DSPN status at the 1-year follow-up was classified as remaining no DSPN, remaining DSPN, regression to no DSPN, or progression to DSPN.

Results

Diabetic sensorimotor polyneuropathy was initially diagnosed in 89 patients with prediabetes and type 2 diabetes (22%). The regressed to no DSPN in 29 patients and progressed to DSPN in 20 patients at the 1-year follow-up. TCNS was significantly correlated with composite scores of nerve conduction, hand cold detection threshold (CDT), hand warm detection threshold (WDT), foot CDT, and foot WDT. Stepwise logistic regression demonstrated that the foot CDT (p &lt; 0.0001) was independently associated with the presence of DSPN. The TCNS, composite scores of the nerve conduction, hand WDT, hand CDT, foot WDT, and foot CDT were all statistically significant among the four different DSPN status groups at two different time periods (baseline and the 1-year follow-up).

Conclusion

The foot CDT can be used as an initial screening tool for DSPN alternatively. The characteristics of nerve damage after 1 year of DSPN can be progressive or reversible, and the neurological functions of large and small fibers have a parallel trend, which can be objectively measured by NCS and QTT.

Upper Body Thermal Referral and Tactile Masking for Localized Feedback

This paper investigates the effects of thermal referral and tactile masking illusions to achieve localized thermal feedback on the upper body. Two experiments are conducted. The first experiment uses a 2D array of sixteen vibrotactile actuators (4 × 4) with four thermal actuators to explore the thermal distribution on the user's back. A combination of thermal and tactile sensations is delivered to establish the distributions of thermal referral illusions with different numbers of vibrotactile cues. The result confirms that localized thermal feedback can be achieved through cross-modal thermo-tactile interaction on the user's back of the body. The second experiment is conducted to validate our approach by comparing it with thermal-only conditions with an equal and higher number of thermal actuators in VR. The results show that our thermal referral with a tactile masking approach with a lesser number of thermal actuators achieves higher response time and better location accuracy than thermal-only conditions. Our findings can contribute to thermal-based wearable design to achieve greater user performance and experiences.

A classification model for continuous responses: Identifying risk perception groups on health-related activities

In the current literature on latent variable models, much effort has been put on the development of dichotomous and polytomous cognitive diagnostic models (CDMs) for assessments. Recently, the possibility of using continuous responses in CDMs has been brought to discussion. But no Bayesian approach has been developed yet for the analysis of CDMs when responses are continuous. Our work is the first Bayesian framework for the continuous deterministic inputs, noisy, and gate (DINA) model. We also propose new interpretations for item parameters in this DINA model, which makes the analysis more interpretable than before. In addition, we have conducted several simulations to evaluate the performance of the continuous DINA model through our Bayesian approach. Then, we have applied the proposed DINA model to a real data example of risk perceptions for individuals over a range of health-related activities. The application results exemplify the high potential of the use of the proposed continuous DINA model to classify individuals in the study.

Acute tryptophan depletion alters affective touch perception

RATIONALE

Affiliative tactile interactions help regulate physiological arousal and confer resilience to acute and chronic stress. C-tactile afferents (CTs) are a population of unmyelinated, low threshold mechanosensitive cutaneous nerve fibres which respond optimally to a low force stimulus, moving at between 1 and 10 cm/s. As CT firing frequencies correlate positively with subjective ratings of touch pleasantness, they are hypothesised to form the first stage of encoding affiliative tactile interactions. Serotonin is a key modulator of social responses with known effects on bonding.

OBJECTIVES

The aim of the present study was to determine the effect of acutely lowering central serotonin levels on perceptions of CT-targeted affective touch.

METHODS

In a double blind, placebo-controlled design, the effect of acute tryptophan depletion (ATD) on 25 female participants' ratings of directly and vicariously experienced touch was investigated. Psychophysical techniques were used to deliver dynamic tactile stimuli; some velocities were targeted to optimally activate CTs (1-10 cm/s), whereas other, faster and slower strokes fell outside the CT optimal range. Discriminative tactile function, cold pain threshold and tolerance were also measured.

RESULTS

ATD significantly increased pleasantness ratings of both directly and vicariously experienced affective touch, increasing discrimination of the specific hedonic value of CT targeted velocities. While ATD had no effect on either tactile or cold pain thresholds, there was a trend for reduced tolerance to cold pain.

CONCLUSIONS

These findings are consistent with previous reports that depletion of central serotonin levels modulates neural and behavioural responsiveness to appetitive sensory signals.

Reactive Oxygen Species Are Essential for Vasoconstriction upon Cold Exposure

Purpose

We explored the role of ROS in cold-induced vasoconstriction and corresponding mechanism.

Methods

Three experiments were performed. First, we measured blood flow in human hands before and after cold exposure. Second, 24 mice were randomly divided into 3 groups: 8 mice received saline injection, 8 received subcutaneous Tempol injection, and 8 received intrathecal Tempol injection. After 30 min, we determined blood flow in the skin before and after cold exposure. Finally, we used Tempol, CCG-1423, and Go 6983 to pretreat HAVSMCs and HUVECs for 24 h. Then, cells in the corresponding groups were exposed to cold (6 h, 4°C). After cold exposure, the cytoskeleton was stained. Intracellular Ca²⁺ and ROS levels were measured by flow cytometry and fluorescence microscopy. We measured protein expression via Western blotting.

Results

In the first experiment, after cold exposure, maximum skin blood flow decreased to 118.4 ± 50.97 flux units. Then, Tempol or normal saline pretreatment did not change skin blood flow. Unlike intrathecal Tempol injection, subcutaneous Tempol injection increased skin blood flow after cold exposure. Finally, cold exposure for 6 h shrank the cells, making them narrower, and increased intracellular Ca²⁺ and ROS levels in HUVECs and HAVSMCs. Tempol reduced cell shrinkage and decreased intracellular Ca²⁺ levels. In addition, Tempol decreased intracellular ROS levels. Cold exposure increased RhoA, Rock1, p-MLC-2, ET-1, iNOS, and p-PKC expression and decreased eNOS expression. Tempol or CCG-1423 pretreatment decreased RhoA, Rock1, and p-MLC-2 levels in HAVSMCs. Furthermore, Tempol or Go 6983 pretreatment decreased ET-1, iNOS, and p-PKC expression and increased eNOS expression in HUVECs.

Conclusion

ROS mediate the vasoconstrictor response within the cold-induced vascular response, and ROS in blood vessel tissues rather than nerve fibers are involved in vasoconstriction via the ROS/RhoA/ROCK1 and ROS/PKC/ET-1 pathways in VSMCs and endothelial cells.

A thermal nociceptive patch in the S2 cortex of nonhuman primates: a combined functional magnetic resonance imaging and electrophysiology study

ABSTRACT

Human functional magnetic resonance imaging (fMRI) and behavioral studies have established the roles of cortical areas along the Sylvian fissure in sensing subjective pain. Yet, little is known about how sensory aspects of painful information are represented and processed by neurons in these regions and how their electrophysiological activities are related to fMRI signals. The current study aims to partially address this critical knowledge gap by performing fMRI-guided microelectrode mapping and recording studies in the homologous region of the parietal operculum in squirrel monkeys under light anesthesia. In each animal studied (n = 8), we detected mesoscale mini-networks for heat nociception in cortical regions around the lateral sulcus. Within the network, we discovered a ∼1.5 × 1.5-mm2-sized cortical patch that solely contained heat nociceptive neurons that aligned with the heat fMRI activation locus. These neurons responded slowly to thermal (heat and cold) nociceptive stimuli exclusively, continued firing for several seconds after the succession of stimulation, and exhibited multidigit receptive fields and high spontaneous firing rates. Similar to the fMRI responses, increasing temperatures in the nociceptive range led to a nonlinear increase in firing rates. The finding of a clustering of heat nociceptive neurons provides novel insights into the unique functional organization of thermal nociception in the S2 subregion of the primate brain. With fMRI, it supports the existence of a modality-preferred heat nociceptive patch that is spatially separated and intermingled with touch patches containing neurons with comparable receptive fields and the presence of functionally distinct mini-networks in primate opercular cortex.

Pain sensitivity does not differ between obese and healthy weight individuals

Introduction

There is emerging evidence suggesting a relationship between obesity and chronic pain.

Objectives

The aim of this study was to determine whether pain-free obese individuals display altered pain responses to acute noxious stimuli, thus raising the possibility of greater pain sensitivity and potential susceptibility for chronic pain development.

Methods

Psychophysical and anthropometric data were collected from 38 individuals with an obese body mass index (BMI) classification (BMI ≥ 30) and 41 age/sex-matched individuals of a healthy BMI (BMI < 24.9). Because BMI may be an inaccurate index of obesity, additional anthropometric parameters of central adiposity and percent body fat were examined. Pain responses to suprathreshold noxious heat and cold stimuli were examined. Subjects provided pain intensity and unpleasantness ratings to noxious heat (49°C) applied at varying durations and locations (ventral forearm/lower leg). Cold pain ratings, thresholds, and tolerances were obtained after immersion of the hand in a cold-water bath (0-2°C). Between-group differences in pain responses, as well as relationships between pain responses and obesity parameters, were examined. Importantly, confounds that may influence pain such as anxiety, depression, impulsivity, sleepiness, and quality of life were assessed.

Results

No between-group differences in pain sensitivity to noxious heat and cold stimuli were found. No relationships were found between central adiposity or body fat (percentage or distribution) and pain responses to noxious heat or cold stimuli.

Conclusions

Obesity has minimal influence on pain sensitivity. Accordingly, it is unlikely that obesity alone increases susceptibility for chronic pain development through amplification of nociceptive processes.

The Hypnotic Analgesia Suggestion Mitigated the Effect of the Transcranial Direct Current Stimulation on the Descending Pain Modulatory System: A Proof of Concept Study

Objective

We evaluated whether active(a)-tDCS combined with hypnotic analgesia suggestion (HS) would be more effective than a single active(a)-tDCS, and/or sham-(s)-tDCS and s-tDCS/HS on the following outcomes: function of descending pain modulatory system (DPMS) during the conditioned pain modulation test (CPM-test) (primary outcome), heat pain threshold (HPT), heat pain tolerance (HPTo) and cold pressor test (CPT) (secondary outcomes). We also examined whether their effects are related to neuroplasticity state evaluated by serum brain-derived-neurotropic factor (BDNF).

Materials and Methods

Forty-eight females received one session of one of the four interventions (a-tDCS/HS, s-tDCS/HS, a-tDCS, and s-tDCS) in an incomplete randomized crossover sequence. The a-tDCS or s-tDCS was applied over the left dorsolateral prefrontal cortex (DLPFC) for 30 minutes at 2mA.

Results

A generalized linear model revealed a significant main effect for the intervention group (P <0.032). The delta-(Δ) pain score on the Numerical Pain Scale (NPS0-10) during CPM-test in the a-tDCS/HS group was -0.25 (0.43). The (Δ) pain score on NPS (0-10) during CPM-test in the other three groups was a-tDCS=-0.54 (0.41), HS -0.01 (0.41) and s-tDCS/HS=-0.19 (0.43). A-tDCS/HS intervention increased the CPT substantially compared to all other interventions. Also, higher baseline levels of BDNF were associated with a larger change in CPT and HPTo.

Conclusion

These findings indicate that the HS combined with a-tDCS mitigated the effect of the a-tDCS on the DPMS. The a-tDCS up-regulates the inhibition on DPMS, and the HS improved pain tolerance. And, together they enhanced the reaction time substantially upon the CPT.

Clinical Trial Registration

www.ClinicalTrials.gov, identifier NCT03744897.

Nerve Function Impairment After Acute Vibration Exposure

OBJECTIVE

This study was to investigate the acute effects of hand-arm vibrations on the nerve functions of the hands, and the impact of the grip force applied to the vibrating tool during exposure.

METHODS

Grip strength and perception of vibration, touch, and temperature were evaluated using quantitative sensory testing (QST) before and after vibration exposure in 21 occupationally unexposed individuals. The procedure was performed twice, with a higher grip force being applied during exposure on the second occasion.

RESULTS

Vibration perception was significantly impaired after both exposures. Grip strength, perception of touch, and temperature were only significantly affected after the high grip force exposure.

CONCLUSIONS

Exposure to hand-arm vibrations has acute effects on hand nerve function that are sensitive to the grip force applied during exposure.

Mouse Parabrachial Neurons Signal a Relationship between Bitter Taste and Nociceptive Stimuli

Taste and somatosensation both mediate protective behaviors. Bitter taste guides avoidance of ingestion of toxins while pain sensations, such as noxious heat, signal adverse conditions to ward off harm. Although brain pathways for taste and somatosensation are typically studied independently, prior data suggest that they intersect, potentially reflecting their common protective role. To investigate this, we applied electrophysiologic and optogenetic techniques in anesthetized mice of both sexes to evaluate relationships between oral somatosensory and taste activity in the parabrachial nucleus (PbN), implicated for roles in gustation and pain. Spikes were recorded from taste-active PbN neurons tested with oral delivery of thermal and chemesthetic stimuli, including agonists of nocisensitive transient receptor potential (TRP) ion channels on somatosensory fibers. Gustatory neurons were also tested to follow electrical pulse stimulation of an oral somatosensory region of the spinal trigeminal subnucleus caudalis (Vc), which projects to the PbN. Neurons composed classic taste groups, including sodium, electrolyte, appetitive, or bitter cells. Across groups, most neurons spiked to Vc pulse stimulation, implying that trigeminal projections reach PbN gustatory neurons. Among such cells, a subpopulation responsive to the bitter taste stimuli quinine and cycloheximide, and aversive concentrations of sodium, cofired to agonists of nocisensitive TRP channels, including capsaicin, mustard oil, and noxious heat. Such neurons populated the lateral PbN. Further, nociceptive activity in PbN bitter taste neurons was suppressed during optogenetic-assisted inhibition of the Vc, implying convergent trigeminal input contributed to such activity. Our results reveal a novel role for PbN gustatory cells in cross-system signaling related to protection.SIGNIFICANCE STATEMENT Prior data suggest that gustatory and trigeminal neural pathways intersect and overlap in the parabrachial area. However, no study has directly examined such overlap and why it may exist. Here we found that parabrachial gustatory neurons can receive afferent projections from trigeminal nuclei and fire to oral nociceptive stimuli that excite somatosensory receptors and fibers. Activation to aversive nociceptive stimuli in gustatory cells was associated with responding to behaviorally avoided bitter tastants. We were further able to show that silencing trigeminal projections inhibited nociceptive activity in parabrachial bitter taste neurons. Our results imply that in the parabrachial area, there is predictable overlap between taste and somatosensory processing related to protective coding and that classically defined taste neurons contribute to this process.

Fibromyalgia patients and healthy volunteers express difficulties and variability in rating experimental pain: a qualitative study

Background and aims Despite the enormous body of literature spanning more than 50 years describing results of pain experiments, very few have used qualitative methods to explore subjects' thoughts while scoring experimental painful stimuli, and none in the available literature have used qualitative interviews to do so. The current study examined how participants in experimental pain research delineate pain ratings to better understand the unique influences of the experimental setting on pain scores. An additional aim was to highlight how individuals with fibromyalgia and healthy volunteers are differently influenced by characteristics of the experimental setting. Methods This was an inductive, qualitative study in which individual, semi-structured interviews were performed with 31 fibromyalgia patients and 44 healthy volunteers. Participants had taken part in a pain experiment during which a thermode was used to induce painful heat stimuli on two skin areas. There were two primary interview questions analyzed for this report: (1) "Thinking back to when you were getting the heat pain on your leg, what were you thinking about when deciding on your pain score?" and (2) Participants who said that it was difficult to decide on a pain score were asked to, "Describe what made it difficult to choose a number." Thematic analysis was used to generate conceptual categories from textual data and find common themes. Results Three notable differences were found between fibromyalgia patients and healthy volunteers: (1) using current daily pain as a benchmark was seen more in patients, (2) wanting to appear strong in front of the study investigators was more common in healthy volunteers, and (3) becoming mentally fatigued from rating many stimuli was more common for fibromyalgia patients. Thoughts while scoring pain included: (1) comparing with previous or current pain, (2) self-monitoring of one's ability to endure the pain, (3) focusing on the physical aspects of the pain, (4) knowing the experimental setting is safe, (5) focusing on the pain scale as an anchor, and (6) desire to appear strong. Additionally, five difficulties in scoring experimental pain were identified: (1) falling asleep, (2) mentally fatigued, (3) feeling as though they were guessing, (4) having to make a quick decision, and (5) difficulty in being consistent. Conclusions This study provides insights into the thoughts of participants in experimental pain research studies. Participants were distracted and influenced by the experimental setting and some factors differed for fibromyalgia patients versus healthy volunteers. Implications Understanding the ways in which the experimental setting influences pain ratings may help pain researchers better design and interpret studies. Researchers can use these findings to mitigate difficulties for participants in experimental research to add to its validity.

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.

Critical role of the pore domain in the cold response of TRPM8 channels identified by ortholog functional comparison

In mammals, the main molecular entity involved in innocuous cold transduction is TRPM8. This polymodal ion channel is activated by cold, cooling compounds such as menthol and voltage. Despite its relevance, the molecular determinants involved in its activation by cold remain elusive. In this study we explored the use of TRPM8 orthologs with different cold responses as a strategy to identify new molecular determinants related with their thermosensitivity. We focused on mouse TRPM8 (mTRPM8) and chicken TRPM8 (cTRPM8), which present complementary thermosensitive and chemosensitive phenotypes. Although mTRPM8 displays larger responses to cold than cTRPM8 does, the avian ortholog shows a higher sensitivity to menthol compared with the mouse channel, in both HEK293 cells and primary somatosensory neurons. We took advantage of these differences to build multiple functional chimeras between these orthologs, to identify the regions that account for these discrepancies. Using a combination of calcium imaging and patch clamping, we identified a region encompassing positions 526-556 in the N terminus, whose replacement by the cTRPM8 homolog sequence potentiated its response to agonists. More importantly, we found that the characteristic cold response of these orthologs is due to nonconserved residues located within the pore loop, suggesting that TRPM8 has evolved by increasing the magnitude of its cold response through changes in this region. Our results reveal that these structural domains are critically involved in cold sensitivity and functional modulation of TRPM8, and support the idea that the pore domain is a key molecular determinant in temperature responses of this thermo-transient receptor potential (TRP) channel.

Selective cold pain inhibition by targeted block of TRPM8-expressing neurons with quaternary lidocaine derivative QX-314

Treatment of pain with local anesthetics leads to an unfavorable decrease in general sensory acuity due to their indiscriminate block of both pain sensing (nociceptors) and non-pain sensing nerves. However, the cell impermeant lidocaine derivative QX-314 can be selectively targeted to only nociceptors by permeation through ligand-gated cation channels. Here we show that localized injection of QX-314 with agonists for the menthol receptor TRPM8 specifically blocks cold-evoked behaviors in mice, including cold allodynia and hyperalgesia. Remarkably, cooling stimuli also promotes QX-314-mediated inhibition of cold behaviors, and can be used to block cold allodynia, while retaining relatively normal cold sensation. The effects of both agonist and thermally evoked uptake of QX-314 are TRPM8-dependent, results demonstrating an effective approach to treat localized cold pain without altering general somatosensation.

Serra Ongun, Angela Sarkisian and David McKemy show that localized co-injection of lidocaine derivative QX-314 and receptor agonists is able to block cold sensitivity in mice in a targeted way, with implications for treating cold pain associated with injury and disease.

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.

Molecular basis of peripheral innocuous cold sensitivity

Of somatosensory modalities cold is one of the more ambiguous percepts, evoking the pleasant sensation of cooling, the stinging bite of cold pain, and welcome relief from chronic pain. Moreover, unlike the precipitous thermal thresholds for heat activation of thermosensitive afferent neurons, thresholds for cold fibers are across a range of cool to cold temperatures that spans over 30°C. Until recently, how cold produces this myriad of biologic effects was unknown. However, recent advances in our understanding of cold mechanisms at the behavioral, physiologic, and cellular level have begun to provide insights into this sensory modality. The identification of a number of ion channels that either serve as the principal detectors of a cold stimulus in the peripheral nervous system, or are part of a differential expression pattern of channels that maintain cell excitability in the cold, endows select neurons with properties that are amenable to electric signaling in the cold. This chapter highlights the current understanding of the molecules involved in cold transduction in the mammalian peripheral nervous system, as well as presenting a hypothetic model to account for the broad range of cold thermal thresholds and distinct functions of cold fibers in perception, pain, and analgesia.

Effects of cold temperatures on the excitability of rat trigeminal ganglion neurons that are not for cold sensing

Aside from a small population of primary afferent neurons for sensing cold, which generate sensations of innocuous and noxious cold, it is generally believed that cold temperatures suppress the excitability of primary afferent neurons not responsible for cold sensing. These not-for-cold-sensing neurons include the majority of non-nociceptive and nociceptive afferent neurons. In this study we have found that the not-for-cold-sensing neurons of rat trigeminal ganglia (TG) change their excitability in several ways at cooling temperatures. In nearly 70% of not-for-cold-sensing TG neurons, a cooling temperature of 15°C increases their membrane excitability. We regard these neurons as cold-active neurons. For the remaining 30% of not-for-cold-sensing TG neurons, the cooling temperature of 15°C either has no effect (cold-ineffective neurons) or suppress their membrane excitability (cold-suppressive neurons). For cold-active neurons, the cold temperature of 15°C increases their excitability as is evidenced by increases in action potential (AP) firing numbers and/or the reduction in AP rheobase when these neurons are depolarized electrically. The cold temperature of 15°C significantly inhibits M-currents and increases membrane input resistance of cold-active neurons. Retigabine, an M-current activator, abolishes the effect of cold temperatures on AP firing, but not the effect of cold temperature on AP rheobase levels. The inhibition of M-currents and the increases of membrane input resistance are likely two mechanisms by which cooling temperatures increase the excitability of not-for-cold-sensing TG neurons. This article is part of the special article series "Pain".

Development of a Microsoft Excel tool for one-parameter Rasch model of continuous items: an application to a safety attitude survey

Background

Many continuous item responses (CIRs) are encountered in healthcare settings, but no one uses item response theory’s (IRT) probabilistic modeling to present graphical presentations for interpreting CIR results. A computer module that is programmed to deal with CIRs is required.

To present a computer module, validate it, and verify its usefulness in dealing with CIR data, and then to apply the model to real healthcare data in order to show how the CIR that can be applied to healthcare settings with an example regarding a safety attitude survey.

Methods

Using Microsoft Excel VBA (Visual Basic for Applications), we designed a computer module that minimizes the residuals and calculates model’s expected scores according to person responses across items. Rasch models based on a Wright map and on KIDMAP were demonstrated to interpret results of the safety attitude survey.

Results

The author-made CIR module yielded OUTFIT mean square (MNSQ) and person measures equivalent to those yielded by professional Rasch Winsteps software. The probabilistic modeling of the CIR module provides messages that are much more valuable to users and show the CIR advantage over classic test theory.

Conclusions

Because of advances in computer technology, healthcare users who are familiar to MS Excel can easily apply the study CIR module to deal with continuous variables to benefit comparisons of data with a logistic distribution and model fit statistics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12874-016-0276-2) contains supplementary material, which is available to authorized users.

Sustained Morphine Administration Induces TRPM8-Dependent Cold Hyperalgesia

It is not uncommon for patients chronically treated with opioids to exhibit opioid-induced hyperalgesia, and this has been widely reported clinically and experimentally. The molecular substrate for this hyperalgesia is multifaceted, and associated with a complex neural reorganization even in the periphery. For instance, we have recently shown that chronic morphine-induced heat hyperalgesia is associated with an increased expression of GluN2B containing N-methyl-D-aspartate receptors, as well as of the neuronal excitatory amino acid transporter 3/excitatory amino acid carrier 1, in small-diameter primary sensory neurons only. Cold allodynia is also a common complaint of patients chronically treated with opioids, yet its molecular mechanisms remain to be understood. Here we present evidence that the cold sensor TRPM8 channel is involved in opioid-induced hyperalgesia. After 7 days of morphine administration, we observed an upregulation of TRPM8 channels using patch clamp recording on sensory neurons and Western blot analysis on dorsal root ganglia. The selective TRPM8 antagonist RQ-00203078 blocked cold hyperalgesia in morphine-treated rats. Also, TRPM8 knockout mice failed to develop cold hyperalgesia after chronic administration of morphine. Our results show that chronic morphine upregulates TRPM8 channels, which is in contrast with the previous finding that acute morphine triggers TRPM8 internalization.

PERSPECTIVE

Patients receiving chronic opioid are sensitive to cold. We show in mice and rats that sustained morphine administration induces cold hyperalgesia and an upregulation of TRPM8. Knockout or selectively blocking TRPM8 reduces morphine-induced cold hyperalgesia suggesting TRPM8 is regulated by opioids.

Pain modulation by intranasal oxytocin and emotional picture viewing - a randomized double-blind fMRI study

The hormone oxytocin has been hypothesized to influence the emotional dimension of pain. This randomized, placebo-controlled, double-blind, crossover study explored whether intranasal oxytocin and emotional context can affect heat pain perception in 30 healthy male volunteers. After receiving 36 IU oxytocin or placebo, participants underwent functional Magnetic Resonance Imaging (fMRI) during which noxious and non-noxious thermode heat stimuli were applied. Simultaneously, scenes from the International Affective Pictures System (IAPS) with positive, neutral, and negative emotional valence were shown. Heat intensity and unpleasantness ratings were obtained. The activity of whole-brain correlates of heat processing was quantified via multi-voxel pattern analysis. We observed no appreciable main effects of oxytocin on ratings or neural pain correlates. Effects of emotional picture valence on ratings were smaller than reported in previous studies. Nevertheless, oxytocin was found to significantly enhance the influence of picture valence on unpleasantness ratings at noxious heat levels. No corresponding changes in whole-brain correlates of heat intensity processing were found. Our study provides evidence that intranasal oxytocin increases the effects of emotional context on the subjective unpleasantness of experimental heat pain. Future studies are needed to determine whether this effect can be utilized in clinical settings.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

TRPA1 is essential for the vascular response to environmental cold exposure

The cold-induced vascular response, consisting of vasoconstriction followed by vasodilatation, is critical for protecting the cutaneous tissues against cold injury. Whilst this physiological reflex response is historic knowledge, the mechanisms involved are unclear. Here by using a murine model of local environmental cold exposure, we show that TRPA1 acts as a primary vascular cold sensor, as determined through TRPA1 pharmacological antagonism or gene deletion. The initial cold-induced vasoconstriction is mediated via TRPA1-dependent superoxide production that stimulates α_2C-adrenoceptors and Rho-kinase-mediated MLC phosphorylation, downstream of TRPA1 activation. The subsequent restorative blood flow component is also dependent on TRPA1 activation being mediated by sensory nerve-derived dilator neuropeptides CGRP and substance P, and also nNOS-derived NO. The results allow a new understanding of the importance of TRPA1 in cold exposure and provide impetus for further research into developing therapeutic agents aimed at the local protection of the skin in disease and adverse climates.

Blood flow in the skin of mammals changes in response to cold, but the mechanisms driving this response are unclear. Aubdool et al. show that the non-selective cation channel, TRPA1, is a vascular cold sensor and required for the vascular protective response to local cold exposure.

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.

Rubber hand illusion reduces discomfort caused by cold stimulus

There is a growing interest in body-ownership disruptions and their consequences for subjective experiences such as tactile sensations or pain. Here, we investigated the effect of the rubber hand illusion (RHI) on the perceived discomfort caused by cold stimulus applied to the real hand. The results showed reduced discomfort to cold reflected in behavioural and subjective measures. The stronger the illusion, the later the cold temperature became unpleasant and the less intense the experience was rated. We discuss the link between thermoception and body ownership as well as possible theoretical and methodological implications for studies on pain experience under RHI.

Rumination and interoceptive accuracy predict the occurrence of the thermal grill illusion of pain

Background

While the neurophysiological mechanisms underlying the thermal grill illusion of pain (TGI) have been thoroughly studied, psychological determinants largely remain unknown. The present study aimed to investigate whether cognitive and affective personality traits encompassing rumination, interoception, and suggestibility may be identified as characteristics favouring the elicitation of paradoxical pain experiences.

Methods

The dominant hand of 54 healthy volunteers was stimulated with a water-bath driven thermal grill providing an interlaced temperature combination of 15 and 41°C. Pain intensity and pain unpleasantness perceptions were rated on a combined verbal-numerical scale (NRS). Traits were assessed via questionnaires, the heartbeat-tracking task, and warmth suggestions.

Results

Logistic regression analyses uncovered trait rumination and interoceptive accuracy (IA) as major predictors of the likelihood of the illusive pain occurrence (all p < .05). Rumination and suggestibility had an impact on unpleasant pain perceptions.

Conclusion

Our findings allowed identifying psychological factors substantially involved in the individual pre-disposition to reporting painful sensations in the thermal grill paradigm. These psychological characteristics may also be relevant in the context of central neuropathic pain, which to a large extent incorporates the same neural pathways.

Physical factors influencing pleasant touch during passive fingertip stimulation

OBJECTIVE

Tactile explorations with the fingertips provide information regarding the physical properties of surfaces and their relative pleasantness. Previously, we performed an investigation in the active touch domain and linked several surface properties (i.e. frictional force fluctuations and net friction) with their pleasantness levels. The aim of the present study was to investigate physical factors being important for pleasantness perception during passive fingertip stimulation. Specifically we were interested to see whether factors, such as surfaces' topographies or their frictional characteristics could influence pleasantness. Furthermore, we ascertained how the stimulus pleasantness level was impacted by (i) the normal force of stimulus application (FN) and (ii) the stimulus temperature (TS).

METHODS AND RESULTS

The right index fingertips of 22 blindfolded participants were stimulated using 27 different stimuli, which varied in average roughness (Ra) and TS. A 4-axis robot moved the stimuli horizontally under participants' fingertips with three levels of FN. The robot was equipped with force sensors, which recorded the FN and friction force (FT) during stimulation. Participants rated each stimulus according to a three-level pleasantness scale, as very pleasant (scored 0), pleasant (scored 1), or unpleasant (scored 2). These ordinal pleasantness ratings were logarithmically transformed into linear and unidimensional pleasantness measures with the Rasch model. Statistical analyses were conducted to investigate a possible link between the stimulus properties (i.e. Ra, FN, FT, and TS) and their respective pleasantness levels. Only the mean Ra and FT values were negatively correlated with pleasantness. No significant correlation was detected between FN or TS and pleasantness.

CONCLUSION

Pleasantness perception, resulting from passive fingertip stimulation, seems to be influenced by the surfaces' average roughness levels and average FT occurring during fingertip stimulation.

Intimacies and physiological role of the polymodal cold-sensitive ion channel TRPM8

The detection of environmental temperature is critical for the survival of the most diverse organisms. Thermosensitive transient receptor potential (thermoTRP) channels have evolved as a class of ion channels activated by a wide range of temperatures. These molecular thermal sensors are spread through the different TRP channel subfamilies. Among the Melastatin subfamily of TRP channels, the eighth member, TRPM8, is a calcium-permeable cationic ion channel activated by cold, by substances that evoke cold sensation such as menthol, and by voltage. This channel is considered the main molecular entity responsible for the sensitivity to cold of primary sensory neurons of the somatosensory system. Here we present to the readers a summary of some the most relevant biophysical properties, physiological role, and molecular intimacies of this polymodal thermoTRP channel.

The molecular and cellular basis of cold sensation

Of somatosensory modalities, cold is one of the more ambiguous percepts, evoking the pleasant sensation of cooling, the stinging bite of cold pain, and welcome relief from chronic pain. Moreover, unlike the precipitous thermal thresholds for heat activation of thermosensitive afferent neurons, thresholds for cold fibers are across a range of cool to cold temperatures that spans over 30 °C. Until recently, how cold produces this myriad of biological effects has been poorly studied, yet new advances in our understanding of cold mechanisms may portend a better understanding of sensory perception as well as provide novel therapeutic approaches. Chief among these was the identification of a number of ion channels that either serve as the initial detectors of cold as a stimulus in the peripheral nervous system, or are part of rather sophisticated differential expression patterns of channels that conduct electrical signals, thereby endowing select neurons with properties that are amenable to electrical signaling in the cold. This review highlights the current understanding of the channels involved in cold transduction as well as presents a hypothetical model to account for the broad range of cold thermal thresholds and distinct functions of cold fibers in perception, pain, and analgesia.

Sensory discrimination between innocuous and noxious cold by TRPM8-expressing DRG neurons of rats

The TRPM8 channel is a principal cold transducer that is expressed on some primary afferents of the somatic and cranial sensory systems. However, it is uncertain whether TRPM8-expressing afferent neurons have the ability to convey innocuous and noxious cold stimuli with sensory discrimination between the two sub-modalities. Using rat dorsal root ganglion (DRG) neurons and the patch-clamp recording technique, we characterized membrane and action potential properties of TRPM8-expressing DRG neurons at 24°C and 10°C. TRPM8-expressing neurons could be classified into TTX-sensitive (TTXs/TRPM8) and TTX-resistant (TTXr/TRPM8) subtypes based on the sensitivity to tetrodotoxin (TTX) block of their action potentials. These two subtypes of cold-sensing cells displayed different membrane and action potential properties. Voltage-activated inward Na⁺ currents were highly susceptible to cooling temperature and abolished by ~95% at 10°C in TTXs/TRPM8 DRG neurons, but remained substantially large at 10°C in TTXr/TRPM8 cells. In both TTXs/TRPM8 and TTXr/TRPM8 cells, voltage-activated outward K⁺ currents were substantially inhibited at 10°C, and the cooling-sensitive outward currents resembled A-type K⁺ currents. TTXs/TRPM8 neurons and TTXr/TRPM8 neurons were shown to fire action potentials at innocuous and noxious cold temperatures respectively, demonstrating sensory discrimination between innocuous and noxious cold by the two subpopulations of cold-sensing DRG neurons. The effects of cooling temperatures on voltage-gated Na⁺ channels and A-type K⁺ currents are likely to be contributing factors to sensory discrimination of cold by TTXs/TRPM8 and TTXr/TRPM8 afferent neurons.

Thermal sensitivity of voltage-gated Na+ channels and A-type K+ channels contributes to somatosensory neuron excitability at cooling temperatures

Cooling temperatures may modify action potential firing properties to alter sensory modalities. Herein, we investigated how cooling temperatures modify action potential firing properties in two groups of rat dorsal root ganglion (DRG) neurons, tetrodotoxin-sensitive (TTXs) Na(+) channel-expressing neurons and tetrodotoxin-resistant (TTXr) Na(+) channel-expressing neurons. We found that multiple action potential firing in response to membrane depolarization was suppressed in TTXs neurons but maintained or facilitated in TTXr neurons at cooling temperatures. We showed that cooling temperatures strongly inhibited A-type K(+) currents (IA) and TTXs Na(+) channels but had fewer inhibitory effects on TTXr Na(+) channels and non-inactivating K(+) currents (IK). We demonstrated that the sensitivity of A-type K(+) channels and voltage-gated Na(+) channels to cooling temperatures and their interplay determine somatosensory neuron excitability at cooling temperatures. Our results provide a putative mechanism by which cooling temperatures modify different sensory modalities including pain.

The effects of the steroid androstadienone and pleasant odorants on the mood and pain perception of men and women

BACKGROUND AND PURPOSE

We previously found that for both men and women odors influence mood, which in turn influences pain unpleasantness perception. Others showed that the steroid androstadienone modulates mood differently in men and women, improving mood in women and worsening it or leaving it unchanged in men. Based on its dissociable effect on mood, we hypothesized that women exposed to androstadienone would report lower pain unpleasantness than when exposed to the vehicle, while men would show no change or the reverse pattern. Because of the expected beneficial effect of pleasant odors on mood in both men and women, all subjects should report lower pain unpleasantness when exposed to the pleasant odor compared to the unscented air.

METHODS

In one session subjects were exposed to: (1) androstadienone diluted in a vehicle while breathing unscented air and (2) androstadienone in vehicle while breathing a pleasant odorant. In another session they were exposed to: (1) vehicle and unscented air and (2) vehicle and a pleasant odorant. Phasic contact heat was used to induce pain.

RESULTS AND CONCLUSIONS

Planned comparisons confirmed that, in the absence of pain, androstadienone improved mood only in women, while the pleasant odorants improved mood for all subjects. However, these positive mood changes did not persist with the introduction of pain. Consistent with the absence of positive mood changes, pain unpleasantness was not modulated. Unexpectedly, the presence of androstadienone increased perceived pain intensity especially in women, suggesting an effect of androstadienone on pain perception, independent of mood changes. Heightened attentional state may be responsible.

Pharmacological blockade of TRPM8 ion channels alters cold and cold pain responses in mice

TRPM8 (Transient Receptor Potential Melastatin-8) is a cold- and menthol-gated ion channel necessary for the detection of cold temperatures in the mammalian peripheral nervous system. Functioning TRPM8 channels are required for behavioral responses to innocuous cool, noxious cold, injury-evoked cold hypersensitivity, cooling-mediated analgesia, and thermoregulation. Because of these various roles, the ability to pharmacologically manipulate TRPM8 function to alter the excitability of cold-sensing neurons may have broad impact clinically. Here we examined a novel compound, PBMC (1-phenylethyl-4-(benzyloxy)-3-methoxybenzyl(2-aminoethyl)carbamate) which robustly and selectively inhibited TRPM8 channels in vitro with sub-nanomolar affinity, as determined by calcium microfluorimetry and electrophysiology. The actions of PBMC were selective for TRPM8, with no functional effects observed for the sensory ion channels TRPV1 and TRPA1. PBMC altered TRPM8 gating by shifting the voltage-dependence of menthol-evoked currents towards positive membrane potentials. When administered systemically to mice, PBMC treatment produced a dose-dependent hypothermia in wildtype animals while TRPM8-knockout mice remained unaffected. This hypothermic response was reduced at lower doses, whereas responses to evaporative cooling were still significantly attenuated. Lastly, systemic PBMC also diminished cold hypersensitivity in inflammatory and nerve-injury pain models, but was ineffective against oxaliplatin-induced neuropathic cold hypersensitivity, despite our findings that TRPM8 is required for the cold-related symptoms of this pathology. Thus PBMC is an attractive compound that serves as a template for the formulation of highly specific and potent TRPM8 antagonists that will have utility both in vitro and in vivo.

Scraping through the ice: uncovering the role of TRPM8 in cold transduction

The proper detection of environmental temperatures is essential for the optimal growth and survival of organisms of all shapes and phyla, yet only recently have the molecular mechanisms for temperature sensing been elucidated. The discovery of temperature-sensitive ion channels of the transient receptor potential (TRP) superfamily has been pivotal in explaining how temperatures are sensed in vivo, and here we will focus on the lone member of this cohort, TRPM8, which has been unequivocally shown to be cold sensitive. TRPM8 is expressed in somatosensory neurons that innervate peripheral tissues such as the skin and oral cavity, and recent genetic evidence has shown it to be the principal transducer of cool and cold stimuli. It is remarkable that this one channel, unlike other thermosensitive TRP channels, is associated with both innocuous and noxious temperature transduction, as well as cold hypersensitivity during injury and, paradoxically, cold-mediated analgesia. With ongoing research, the field is getting closer to answering a number of fundamental questions regarding this channel, including the cellular mechanisms of TRPM8 modulation, the molecular context of TRPM8 expression, as well as the full extent of the role of TRPM8 in cold signaling in vivo. These findings will further our understanding of basic thermotransduction and sensory coding, and may have important implications for treatments for acute and chronic pain.