Evidence for the involvement of peripheral cold-sensitive TRPM8 channels in human cutaneous hygrosensation

An evaluation of the effects of localised skin cooling on microvascular, inflammatory, structural, and perceptual responses to sustained mechanical loading of the sacrum: A study protocol

This study protocol aims to investigate how localised cooling influences the skin's microvascular, inflammatory, structural, and perceptual tolerance to sustained mechanical loading at the sacrum, evaluating factors such as morphology, physiology, and perceptual responses. The protocol will be tested on individuals of different age, sex, skin tone and clinical status, using a repeated-measure design with three participants cohorts: i) young healthy (n = 35); ii) older healthy (n = 35); iii) spinal cord injured (SCI, n = 35). Participants will complete three testing sessions during which their sacrum will be mechanically loaded (60 mmHg; 45 min) and unloaded (20 min) with a custom-built thermal probe, causing pressure-induced ischemia and post-occlusive reactive hyperaemia. Testing sessions will differ by the probe's temperature, which will be set to either 38°C (no cooling), 24°C (mild cooling), or 16°C (strong cooling). We will measure skin blood flow (via Laser Doppler Flowmetry; 40 Hz); pro- and anti-inflammatory biomarkers in skin sebum (Sebutape); structural skin properties (Optical Coherence Tomography); and ratings of thermal sensation, comfort, and acceptance (Likert Scales); throughout the loading and unloading phases. Changes in post-occlusive reactive hyperaemia will be considered as the primary outcome and data will be analysed for the independent and interactive effects of stimuli's temperature and of participant group on within- and between-subject mean differences (and 95% Confidence Intervals) in peak hyperaemia, by means of a 2-way mixed model ANOVA (or Friedman). Regression models will also be developed to assess the relationship between absolute cooling temperatures and peak hyperaemia. Secondary outcomes will be within- and between-subject mean changes in biomarkers' expression, skin structural and perceptual responses. This analysis will help identifying physiological and perceptual thresholds for the protective effects of cooling from mechanically induced damage underlying the development of pressure ulcers in individuals varying in age and clinical status.

Biophysical, thermo-physiological and perceptual determinants of cool-seeking behaviour during exercise in younger and older women

Women continue to be under-represented in thermoregulatory research despite their undergoing unique physiological changes across the lifespan. This study investigated the biophysical, thermo-physiological, and perceptual determinants of cool-seeking behaviour during exercise in younger and older women. Eleven younger (25 ± 5 years; 1.7 ± 0.1 m; 63.1 ± 5.2 kg) and 11 older women (53 ± 6 years; 1.7 ± 0.1 m; 65.4 ± 13.9 kg) performed a 40-min incremental cycling test in a thermoneutral environment (22 ± 1.7°C; 36 ± 4% relative humidity). Throughout the test, participants freely adjusted the temperature of a cooling probe applied to their wrists to offset their thermal discomfort. We continuously recorded the probe-wrist interface temperature to quantify participants' cool-seeking behaviour. We also measured changes in participants' rate of metabolic heat production, core and mean skin temperatures, and skin wetness. Finally, we body-mapped participants' skin heat, cold and wetness sensitivity. Our results indicated that: (1) older and younger women exhibited similar onset and magnitude of cool-seeking behaviour, despite older women presented reduced autonomic heat-dissipation responses (i.e., whole-body sweat losses); (2) older women's thermal behaviour was less determined by changes in core temperature (this being a key driver in younger women), and more by changes in multiple thermo-physiological and biophysical parameters (i.e., physical skin wetness, temperature and heat production); (3) older women did not present lower regional skin thermal and wetness sensitivity than younger women. We conclude that predictions of female cool-seeking behaviours based on thermo-physiological variables should consider the effects of ageing. These findings are relevant for the design of wearable cooling systems and sports garments that meet the thermal needs of women across the lifespan.

Pain and Weather associations - Action Mechanisms; Personalized profiling

It is a well-known hypothesis that weather can influence human health, including pain sensation. The primary meteorological factors are atmospheric pressure, wind, humidity, precipitation, and temperature, which vary from the climate and seasons, but the parameters of space weather (e.g., geomagnetic and cosmic ray activities) also may affect our body condition. Despite a significant number of experimental studies, reviews, and meta-analyses concerning the potential role of weather in pain sensitivity, the findings are heterogeneous and lack consensus. Therefore, rather than attempting a comprehensive analysis of the entire literature on the effects of weather on different pain types, this study highlights the potential action mechanisms of the meteorological factors, and the possible causes of the controversial results. The few data available about the individual evaluations are discussed in detail to reveal the significance of the personalized analysis of the possible relationships between the most available weather parameters and the pain scores. The use of special algorithms may enable the individual integration of different data for a precise outcome concerning the link between pain sensitivity and weather parameters. It is presumed that despite the high level of interindividual differences in response to meteorological parameters, the patients can be clustered in different groups based on their sensitivity to the weather parameters with a possible disparate treatment design. This information may help patients to control their daily activities and aid physicians to plan more valuable management for patients with pain states when the weather conditions change.

Browning of Adipocytes: A Potential Therapeutic Approach to Obesity

The increasing prevalence of overweight and obesity suggests that current strategies based on diet, exercise, and pharmacological knowledge are not sufficient to tackle this epidemic. Obesity results from a high caloric intake and energy storage, the latter by white adipose tissue (WAT), and when neither are counterbalanced by an equally high energy expenditure. As a matter of fact, current research is focused on developing new strategies to increase energy expenditure. Against this background, brown adipose tissue (BAT), whose importance has recently been re-evaluated via the use of modern positron emission techniques (PET), is receiving a great deal of attention from research institutions worldwide, as its main function is to dissipate energy in the form of heat via a process called thermogenesis. A substantial reduction in BAT occurs during normal growth in humans and hence it is not easily exploitable. In recent years, scientific research has made great strides and investigated strategies that focus on expanding BAT and activating the existing BAT. The present review summarizes current knowledge about the various molecules that can be used to promote white-to-brown adipose tissue conversion and energy expenditure in order to assess the potential role of thermogenic nutraceuticals. This includes tools that could represent, in the future, a valid weapon against the obesity epidemic.

Sex differences in thermal sensitivity and perception: Implications for behavioral and autonomic thermoregulation

Temperature sensitive receptors in the skin and deep body enable the detection of the external and internal environment, including the perception of thermal stimuli. Changes in heat balance require autonomic (e.g., sweating) and behavioral (e.g., seeking shade) thermoeffector initiation to maintain thermal homeostasis. Sex differences in body morphology can largely, but not entirely, account for divergent responses in thermoeffector and perceptual responses to environmental stress between men and women. Thus, it has been suggested that innate differences in thermosensation may exist between men and women. Our goal in this review is to summarize the existing literature that investigates localized and whole-body cold and heat exposure pertaining to sex differences in thermal sensitivity and perception, and the interplay between autonomic and behavioral thermoeffector responses. Overall, it appears that local differences in thermal sensitivity and perception are minimized, yet still apparent, when morphological characteristics are well-controlled. Sex differences in the early vasomotor response to environmental stress and subsequent changes in blood flow likely contribute to the heightened thermal awareness observed in women. However, the contribution of thermoreceptors to observed sex differences in thermal perception and thermoeffector function is unclear, as human studies investigating these questions have not been performed.

A novel method to selectively elicit cold sensations without touch

BACKGROUND

Thermal and tactile stimuli are transduced by different receptor classes. However, mechano- and thermo-sensitive afferents interact at spinal and supraspinal levels. Yet, most studies on responses to cooling stimuli are confounded by mechanical contact, making these interactions difficult to isolate. Methods for precise control of non-mechanical thermal stimulations remain challenging, particularly in the cold range.

NEW METHOD

We developed a non-tactile, focal, temperature-controlled, multi-purpose cooling stimulator. This method controls the exposure of a target skin region to a dry-ice source. Using a thermal camera to monitor skin temperature, and adjusting the source-skin distance accordingly, we could deliver non-tactile cooling stimuli with customisable profiles, for studying different aspects of cold sensation.

RESULTS

To validate our method, we measured absolute and relative thresholds for cold sensation without mechanical contact in 13 human volunteer participants, using the method of limits. We found that the absolute cold detection threshold was 32.71 ^oC ± 0.88 ^oC. This corresponded to a threshold relative to each participant's baseline skin temperature of - 1.08 ^oC ± 0.37 ^oC.

COMPARISONS WITH EXISTING METHOD

Our method allows cooling stimulation without the confound of mechanical contact, in a controllable and focal manner.

CONCLUSIONS

We report a non-contact cooling stimulator and accompanying control system. We used this to measure cold thresholds in the absence of confounding touch. Our method enables more targeted studies of both cold sensory pathways, and of cold-touch interactions.

Characteristics of wet perception during the static touch of moist paper by the index fingertip alongside thermal stimulus application

Several factors have been reported to affect the perception of wetness. In the present study, we aimed to examine how wet perception changes when the factors related to thermal and/or wetness stimuli are modulated. First, the percentage of participants experiencing wet perception among filter papers with different water contents (0.00, 3.75, 7.50, 11.25, 15.00, and 18.75 µg/cm², corresponding to 0.00, 0.18, 0.37, 0.55,0.73 and 0.91 µg/mm³) was evaluated during static touch by the right index finger pad. The stimulus temperature was maintained at 30 °C. Second, the wet perception of paper with a water content of 18.75 µg/cm² was evaluated at stimulus temperature of 20 °C, 25 °C, 30 °C, 35 °C, and 40 °C. In the first experiment, the percentage of participants experiencing wet perception elevated with the increasing water content; however, the percentage plateaued at 11.25 µg/cm² of water (68.1 ± 25.5%). In the second experiment, when the stimulus temperature was < 30 °C, the wet perception increased as the stimulation temperature decreased. However, the wet perception reached a plateau at a stimulation temperature ≥30 °C. Participants experienced wet perception more consistently as the water content increased when the stimulus temperature was 30 ˚C. The effect of temperature on wet perception was limited to the stimulus temperature of <30 °C at which cold sensation was induced. However, no clear relationship between stimulus temperature and wet perception was observed when the stimulus temperature was ≥30 ˚C at which warm/hot sensation was induced.

Skin wetness sensitivity across body sites commonly affected by pain in people with migraine

Objective

The objective of this study was to evaluate skin wetness perception and thermal sensitivity in people with migraine and similar healthy controls.

Background

Environmental triggers, such as cold and humidity, are known triggers for pain in people with migraine. Sensory inputs might be implicated in such heightened responses to cold‐humid environments, such that a migraine‐induced hypersensitivity to cold wetness could be present in people with migraine. However, we lack empirical evidence on skin thermal and wetness sensitivity across skin sites commonly associated with reported pain in migraine, such as the forehead.

Methods

This prospective cross‐sectional observational study, conducted in a university hospital setting, evaluated skin wetness perceptions and thermal sensations to wet non‐noxious warm‐wet, neutral‐wet, and cold‐wet stimuli applied to the forehead, the posterior neck, and the index finger pad of 12 patients with migraine (mean and standard deviation for age 44.5 ± 13.2 years, 7/12 [58%] women) and 36 healthy controls (mean and standard deviation for age 39.4 ± 14.6 years, 18/36 [50%] women).

Results

On the forehead, people with migraine reported a significantly higher wetness perception than healthy controls across all thermal stimulus (15.1 mm, 95% confidence interval [CI]: 1.8 to 28.5, p = 0.027, corresponding to ~ 15% difference), whereas no significant differences were found on the posterior neck nor on the index finger pad. We found no differences among groups in overall thermal sensations (−8.3 mm, 95% CI: −24.0 to 7.3, p = 0.291; −7.8 mm, 95% CI: −25.3 to 9.7, p = 0.375; and 12.4 mm, 95% CI: −4.0 to 28.9, p = 0.133; forehead, posterior neck, and index finger, respectively).

Conclusion

These findings indicate that people with migraine have a heightened sensitivity to skin wetness on the forehead area only, which is where pain attacks occur. Future studies should further explore the underlying mechanisms (e.g., TRPM8‐mediated cold‐wet allodynia) that lead to greater perception of wetness in people with migraine to better understand the role of environmental triggers in migraine.

The role of friction on skin wetness perception during dynamic interactions between the human index fingerpad and materials of varying moisture content

Mechanosensory inputs arising from dynamic interactions between the skin and moisture, such as when sliding a finger over a wet substrate, contribute to the perception of skin wetness. Yet, the exact relationship between the mechanical properties of a wet substrate, such as friction, and the resulting wetness perception remains to be established under naturalistic haptic interactions. We modeled the relationship between mechanical and thermal properties of substrates varying in moisture levels (0.49 × 10⁻⁴; 1.10 × 10⁻⁴; and 2.67 × 10⁻⁴ mL·mm⁻²), coefficient of friction (0.783, 0.848, 1.033, 0.839, 0.876, and 0.763), and maximum thermal transfer rate (Q_max, ranging from 511 to 1,260 W·m⁻²·K⁻¹), and wetness perception arising from the index finger pad’s contact with such substrates. Forty young participants (20M/20F) performed dynamic interactions with 21 different stimuli using their index finger pad at a controlled angle, pressure, and speed. Participants rated their wetness perception using a 100-mm visual analog scale (very dry to very wet). Partial least squares regression analysis indicated that coefficient of friction explained only ∼11% of the variance in wetness perception, whereas Q_max and moisture content accounted for ∼22% and 18% of the variance, respectively. These parameters shared positive relationships with wetness perception, such that the greater the Q_max, moisture content, and coefficient of friction, the wetter the perception. We found no differences in wetness perception between males and females. Our findings indicate that although the friction of a wet substrate modulates wetness perception, it is still secondary to thermal parameters such as Q_max.

NEW & NOTEWORTHY Our skin often interacts with wet materials, yet how their physical properties influence our experience of wetness remains poorly understood. We evaluated wetness perception following naturalistic haptic interactions with materials varying in moisture content, friction, optical profiles, and heat transfer rates. We show that although mechanical parameters can influence wetness perception, their role is secondary to that of thermal factors. These findings expand our understanding of multisensory integration and could guide innovation in healthcare product design.

Dyspnea

The clinical term dyspnea (a.k.a. breathlessness or shortness of breath) encompasses at least three qualitatively distinct sensations that warn of threats to breathing: air hunger, effort to breathe, and chest tightness. Air hunger is a primal homeostatic warning signal of insufficient alveolar ventilation that can produce fear and anxiety and severely impacts the lives of patients with cardiopulmonary, neuromuscular, psychological, and end-stage disease. The sense of effort to breathe informs of increased respiratory muscle activity and warns of potential impediments to breathing. Most frequently associated with bronchoconstriction, chest tightness may warn of airway inflammation and constriction through activation of airway sensory nerves. This chapter reviews human and functional brain imaging studies with comparison to pertinent neurorespiratory studies in animals to propose the interoceptive networks underlying each sensation. The neural origins of their distinct sensory and affective dimensions are discussed, and areas for future research are proposed. Despite dyspnea's clinical prevalence and impact, management of dyspnea languishes decades behind the treatment of pain. The neurophysiological bases of current therapeutic approaches are reviewed; however, a better understanding of the neural mechanisms of dyspnea may lead to development of novel therapies and improved patient care.

A patient-centred evaluation of phantom skin wetness as a sensory symptom in people with multiple sclerosis

BACKGROUND

A noticeable but unknown proportion of people with multiple sclerosis (pwMS) report the sudden experience of wetness on a dry skin site, i.e., phantom wetness. Yet, we lack patient-centred investigations on the prevalence and subjective experience of this uncomfortable symptom.

OBJECTIVES

To assess the prevalence of phantom wetness in pwMS, its association with individual factors, and subjective experience.

METHODS

757 pwMS completed an online survey assessing the frequency and subjective experience of phantom wetness. We calculated descriptive statistics and odd ratios and performed a thematic analysis to extract a patient-centred description of phantom wetness.

RESULTS

220 participants reported experiencing phantom wetness (29%). Females and those affected by Relapsing Remitting (RR) MS were 2.17 [1.39, 3.34] (p<0.001) and 1.73 [1.23, 2.40] (p = 0.001) times as likely to experience phantom wetness as males and those not affected by RR MS, respectively. The thematic analysis indicated phantom wetness is more often experienced as water trickling on the skin of the lower limb.

CONCLUSION

Phantom wetness is a paraesthesia occurring in almost a third of the sample surveyed. Clinicians are encouraged to discuss with pwMS to validate their experience as a genuine symptom. Using the patient-generated language we report may help facilitate such conversations.

TRPM8 Channels: Advances in Structural Studies and Pharmacological Modulation

The transient receptor potential melastatin subtype 8 (TRPM8) is a cold sensor in humans, activated by low temperatures (>10, <28 °C), but also a polymodal ion channel, stimulated by voltage, pressure, cooling compounds (menthol, icilin), and hyperosmolarity. An increased number of experimental results indicate the implication of TRPM8 channels in cold thermal transduction and pain detection, transmission, and maintenance in different tissues and organs. These channels also have a repercussion on different kinds of life-threatening tumors and other pathologies, which include urinary and respiratory tract dysfunctions, dry eye disease, and obesity. This compendium firstly covers newly described papers on the expression of TRPM8 channels and their correlation with pathological states. An overview on the structural knowledge, after cryo-electron microscopy success in solving different TRPM8 structures, as well as some insights obtained from mutagenesis studies, will follow. Most recently described families of TRPM8 modulators are also covered, along with a section of molecules that have reached clinical trials. To finalize, authors provide an outline of the potential prospects in the TRPM8 field.

Nutritional Regulation of Human Brown Adipose Tissue

The recent identification of brown adipose tissue in adult humans offers a new strategy to increase energy expenditure to treat obesity and associated metabolic disease. While white adipose tissue (WAT) is primarily for energy storage, brown adipose tissue (BAT) is a thermogenic organ that increases energy expenditure to generate heat. BAT is activated upon cold exposure and improves insulin sensitivity and lipid clearance, highlighting its beneficial role in metabolic health in humans. This review provides an overview of BAT physiology in conditions of overnutrition (obesity and associated metabolic disease), undernutrition and in conditions of altered fat distribution such as lipodystrophy. We review the impact of exercise, dietary macronutrients and bioactive compounds on BAT activity. Finally, we discuss the therapeutic potential of dietary manipulations or supplementation to increase energy expenditure and BAT thermogenesis. We conclude that chronic nutritional interventions may represent a useful nonpharmacological means to enhance BAT mass and activity to aid weight loss and/or improve metabolic health.

Skin wetness detection thresholds and wetness magnitude estimations of the human index fingerpad and their modulation by moisture temperature

Humans often experience wet stimuli using their hands, yet we know little on how sensitive our fingers are to wetness and the mechanisms underlying this sensory function. We therefore aimed to quantify the minimum amount of water required to detect wetness on the human index fingerpad, the wetness detection threshold, and assess its modulation by temperature. Eight blinded participants (24.0 ± 5.2 yr; 23.3 ± 3.5 body mass index) used their index fingerpad to statically touch stimuli varying in volume (0, 10, 20, 30, 40, or 50 mL) and temperature (25, 29, 33, or 37°C). During and after contact, participants rated wetness and thermal sensations using a modified yes/no task and a visual analog scale. The wetness detection threshold at a moisture temperature akin to human skin (33°C) was 24.7 ± 3.48 mL. This threshold shifted depending on moisture temperature (R = 0.746), with cooler temperatures reducing (18.7 ± 3.94 mL at 29°C) and warmer temperatures increasing (27.0 ± 3.04 mL at 37°C) thresholds. When normalized over contact area, the wetness detection threshold at 33°C corresponded to 1.926 × 10⁻⁴ mL·mm⁻² [95% confidence interval (CI): 1.873 × 10⁻⁴, 1.979 × 10⁻⁴ mL·mm⁻²]. Threshold differences were reflected by magnitude estimation data, which were analyzed using linear regression to show that both volume and moisture temperature can predict magnitude estimations of wetness (R = 0.949; R = 0.179). Our results indicate high sensitivity to wetness in the human index fingerpad, which can be modulated by moisture temperature. These findings are relevant for the design of products with wetness management properties.

NEW & NOTEWORTHY The perception of wetness is a fundamental sensory experience which underpins many aspects of life, from homeostasis to enjoyable experiences. Although previous research has highlighted the importance of cold sensations in human wetness perception, the maximum sensitivity of our wetness sensing system remains to be established. This research presents a novel methodology, which for the first time, has quantified the high sensitivity of the human index fingerpad to wetness and its modulation by moisture temperature.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.