Cells and circuits for thermosensation in mammals

Effective brain connectivity related to non-painful thermal stimuli using EEG

Understanding the brain response to thermal stimuli is crucial in the sensory experience. This study focuses on non-painful thermal stimuli, which are sensations induced by temperature changes without causing discomfort. These stimuli are transmitted to the central nervous system through specific nerve fibers and are processed in various regions of the brain, including the insular cortex, the prefrontal cortex, and anterior cingulate cortex. Despite the prevalence of studies on painful stimuli, non-painful thermal stimuli have been less explored. This research aims to bridge this gap by investigating brain functional connectivity during the perception of non-painful warm and cold stimuli using electroencephalography (EEG) and the partial directed coherence technique (PDC). Our results demonstrate a clear contrast in the direction of information flow between warm and cold stimuli, particularly in the theta and alpha frequency bands, mainly in frontal and temporal regions. The use of PDC highlights the complexity of brain connectivity during these stimuli and reinforces the existence of different pathways in the brain to process different types of non-painful warm and cold stimuli.

Transient receptor potential (TRP) channels in Sebastes schlegelii: Genome-wide identification and ThermoTRP expression analysis under high-temperature

Transient Receptor Potential (TRP) channels, essential for sensing environmental stimuli, are widely distributed. Among them, thermosensory TRP channels play a crucial role in temperature sensing and regulation. Sebastes schlegelii, a significant aquatic economic species, exhibits sensitivity to temperature across multiple aspects. In this study, we identified 18 SsTRP proteins using whole-genome scanning. Motif analysis revealed motif 2 in all TRP proteins, with conserved motifs in subfamilies. TRP-related domains, anchored repeats, and ion-transmembrane domains were found. Chromosome analysis showed 18 TRP genes on 11 chromosomes and a scaffold. Phylogenetics classified SsTRPs into four subfamilies: TRPM, TRPA, TRPV, and TRPC. In diverse organisms, four monophyletic subfamilies were identified. Additionally, we identified key TRP genes with significantly upregulated transcription levels under short-term (30 min) and long-term (3 days) exposure at 24 °C (optimal elevated temperature) and 27 °C (critical high temperature). We propose that genes upregulated at 30 min may be involved in the primary response process of temperature sensing, while genes upregulated at 3 days may participate in the secondary response process of temperature perception. This study lays the foundation for understanding the regulatory mechanisms of TRPs responses to environmental stimuli in S. schlegelii and other fishes.

The Role of the Skin in Interoception: A Neglected Organ?

In the past 2 decades, interoception has received increasing attention in the fields of psychology and cognitive science, as well as neuroscience and physiology. A plethora of studies adopted the perception of cardiac signals as a proxy for interoception. However, recent findings have cast doubt on the methodological and intrinsic validity of the tasks used thus far. Therefore, there is an ongoing effort to improve the existing cardiac interoceptive tasks and to identify novel channels to target the perception of the physiological state of the body. Amid such scientific abundancy, one could question whether the field has been partially neglecting one of our widest organs in terms of dimensions and functions: the skin. According to some views grounded on anatomical and physiological evidence, skin-mediated signals such as affective touch, pain, and temperature have been redefined as interoceptive. However, there is no agreement in this regard. Here, we discuss some of the anatomical, physiological, and experimental arguments supporting the scientific study of interoception by means of skin-mediated signals. We argue that more attention should be paid to the skin as a sensory organ that monitors the bodily physiological state and further propose thermosensation as a particularly attractive model of skin-mediated interoception.

TRPV1 drugs alter core body temperature via central projections of primary afferent sensory neurons

TRPV1, a capsaicin- and heat-activated ion channel, is expressed by peripheral nociceptors and has been implicated in various inflammatory and neuropathic pain conditions. Although pharmacological modulation of TRPV1 has attracted therapeutic interest, many TRPV1 agonists and antagonists produce thermomodulatory side effects in animal models and human clinical trials, limiting their utility. These on-target effects may result from the perturbation of TRPV1 receptors on nociceptors, which transduce signals to central thermoregulatory circuits and release proinflammatory factors from their peripheral terminals, most notably the potent vasodilative neuropeptide, calcitonin gene-related peptide (CGRP). Alternatively, these body temperature effects may originate from the modulation of TRPV1 on vascular smooth muscle cells (vSMCs), where channel activation promotes arteriole constriction. Here, we ask which of these pathways is most responsible for the body temperature perturbations elicited by TRPV1 drugs in vivo. We address this question by selectively eliminating TRPV1 expression in sensory neurons or vSMCs and show that only the former abrogates agonist-induced hypothermia and antagonist-induced hyperthermia. Furthermore, lesioning the central projections of TRPV1-positive sensory nerve fibers also abrogates drug-mediated thermomodulation, whereas eliminating CGRP has no effect. Thus, TRPV1 drugs alter core body temperature by modulating sensory input to the central nervous system, rather than through peripheral actions on the vasculature. These findings suggest how mechanistically distinct TRPV1 antagonists may diminish inflammatory pain without affecting core body temperature.

Comparative insights into the integration mechanism of neuropeptides to starvation and temperature stress

Stress is known as the process of biological responses evoked by internal or external stimuli. The ability to sense, integrate and respond to stress signals is a requisite for life. Temperature and photoperiod are very important environmental factors for animals. In addition, stress signals can also be inputted from peripheral tissue, such as starvation and inflammation. Through afferent pathways, stress signals input to the central nervous system (CNS), where various signals will integrate, and the integrated information will transmit to the peripheral effectors. As the regulators of neural activity, neuropeptides play important roles in these processes. The present review summarizes recent findings about the integration mechanism of stress signals in the CNS, emphasizing on the role of neuropeptides.

Synchronous and opponent thermosensors use flexible cross-inhibition to orchestrate thermal homeostasis

Body temperature homeostasis is essential and reliant upon the integration of outputs from multiple classes of cooling- and warming-responsive cells. The computations that integrate these outputs are not understood. Here, we discover a set of warming cells (WCs) and show that the outputs of these WCs combine with previously described cooling cells (CCs) in a cross-inhibition computation to drive thermal homeostasis in larval Drosophila WCs and CCs detect temperature changes using overlapping combinations of ionotropic receptors: Ir68a, Ir93a, and Ir25a for WCs and Ir21a, Ir93a, and Ir25a for CCs. WCs mediate avoidance to warming while cross-inhibiting avoidance to cooling, and CCs mediate avoidance to cooling while cross-inhibiting avoidance to warming. Ambient temperature-dependent regulation of the strength of WC- and CC-mediated cross-inhibition keeps larvae near their homeostatic set point. Using neurophysiology, quantitative behavioral analysis, and connectomics, we demonstrate how flexible integration between warming and cooling pathways can orchestrate homeostatic thermoregulation.

An operant temperature sensory assay provides a means to assess thermal discrimination

Mouse behavioral assays have proven useful for the study of thermosensation, helping to identify receptors and circuits responsible for the transduction of thermal stimuli and information relay to the brain. However, these methods typically rely on observation of behavioral responses to various temperature stimuli to infer sensory ability and are often unable to disambiguate innocuous thermosensation from thermal nociception or to study thermosensory circuitry which do not produce easily detectable innate behavioral responses. Here we demonstrate a new testing apparatus capable of delivering small, rapid temperature change stimuli to the mouse’s skin, permitting the use of operant conditioning to train mice to recognize and report temperature change. Using this assay, mice that were trained to detect a large temperature change were found to generalize this learning to distinguish much smaller temperature changes across the entire range of innocuous temperatures tested. Mice with ablated TRPV1 and TRPM8 neuronal populations had reduced ability to discriminate temperature differences in the warm (>35°C) and cool (<30°C) ranges, respectively. Furthermore, mice that were trained to recognize temperature changes in only the cool, TRPM8-mediated temperature range did not generalize this learning in the warm, TRPV1-mediated range (and vice versa), suggesting that thermosensory information from the TRPM8- and TRPV1-neuronal populations are perceptually distinct.

Thermoregulation in fish

Thermoregulation is critical for survival and animals therefore employ strategies to keep their body temperature within a physiological range. As ectotherms, fish exclusively rely on behavioral strategies for thermoregulation. Different species of fish seek out their specific optimal temperatures through thermal navigation by biasing behavioral output based on experienced environmental temperatures. Like other vertebrates, fish sense water temperature using thermoreceptors in trigeminal and dorsal root ganglia neurons that innervate the skin. Recent research in larval zebrafish has revealed how neural circuits subsequently transform this sensation of temperature into thermoregulatory behaviors. Across fish species, thermoregulatory strategies rely on a modulation of swim vigor based on current temperature and a modulation of turning based on temperature change. Interestingly, temperature preferences are not fixed but depend on other environmental cues and internal states. The following review is intended as an overview on the current knowledge as well as open questions in fish thermoregulation.

Modulation of the cutaneous and cortical silent period in response to local menthol application

In this study, we investigated the effects of menthol application on the cortical and cutaneous silent period (CSP/cutSP). Both the cutSP and CSP were assessed while participants (n = 11, young adults) exerted a light contraction with the right thumb. In the 1st block of trials, SPs were measured after the application of a Neutral gel (Aloe Vera) to the dorsal aspect of the hand. In the 2nd block, the same measures were repeated following a Menthol gel (4%) application. Subjective ratings of cooling sensations were obtained for each block. The Neutral gel was consistently perceived as slightly cool by participants, wheres the Menthol gel elicited sensations from cool to very cold. Paired t-tests showed no difference in the cutSP duration between the two conditions, whereas a significant increase in the CSP was detected with the Menthol condition. No correlation was found between changes in the CSP and those of the cutSP. These results highlight the difference between the cutSP and the CSP, as inhibitory phenomena, and point to a cortical contribution to the soothing effects associated with topical menthol applications.

Variations in the TRPV1 gene are associated to exertional heat stroke

OBJECTIVES

Exertional Heat Stroke (EHS) is one of the top three causes of sudden death in athletes. Extrinsic and intrinsic risk factors have been identified but the genetic causes still remain unclear. Our aim was to identify genes responsible for EHS, which is a necessary step to identify patients at risk and prevent crises.

DESIGN

Genetic and functional laboratory studies METHODS: Whole Exome Sequencing (WES) was performed to search for candidate genes in a cohort of 15 soldiers who had a documented EHS episode. In silico and in vitro functional studies were performed to evaluate the effect of mutations identified in the candidate gene TRPV1.

RESULTS

WES led to the identification of two missense variations in the TRPV1 gene. These variations were very rare or unreported in control databases and located in critical domains of the protein. In vitro functional studies revealed that both variations induce a strong modification of the channel response to one of its natural agonist, the capsaicin.

CONCLUSIONS

We evidenced mutations altering channel properties of the TRPV1 gene and demonstrated that TRPV1, which is involved in thermoregulation and nociception, is a new candidate gene for EHS. Our data provide the bases to explore genetic causes and molecular mechanisms governing the pathophysiology of EHS.

Influence of Alternate Hot and Cold Thermal Stimulation in Cortical Excitability in Healthy Adults: An fMRI Study

Stroke rehabilitation using alternate hot and cold thermal stimulation (altTS) has been reported to improve motor function in hemiplegia; however, the influence of brain excitability induced by altTS remains unclear. This study examined cortical activation induced by altTS in healthy adults, focusing on motor-related areas. This involved a repeated crossover experimental design with two temperature settings (innocuous altTS with alternate heat-pain and cold-pain thermal and noxious altTS with alternate heat and cold thermal) testing both arms (left side and right side). Thirty-one healthy, right-handed participants received four episodes of altTS on four separate days. Functional magnetic resonance imaging scans were performed both before and after each intervention to determine whether altTS intervention affects cortical excitability, while participants performed a finger-tapping task during scanning. The findings revealed greater response intensity of cortical excitability in participants who received noxious altTS in the primary motor cortex, supplementary motor cortex, and somatosensory cortex than in those who received innocuous altTS. Moreover, there was more motor-related excitability in the contra-lateral brain when heat was applied to the dominant arm, and more sensory-associated excitability in the contra-lateral brain when heat was applied to the nondominant arm. The findings highlight the effect of heat on cortical excitability and provide insights into the application of altTS in stroke rehabilitation.

Sorting nexin 11 knockout mice exhibit enhanced thermosensing behaviour

Temperature sensing is an important adaptive mechanism for warm-blooded animals such as humans. ThermoTRP ion channels are activated by distinct but overlapping physiological temperatures. Our previous research demonstrated that sorting nexin 11 (SNX11) regulates lysosomal degradation of plasma membrane TRPV3, one of ThermoTRP ion channel proteins. Here, we found that SNX11, a vesicular trafficking protein, modulates mouse behaviour in response to temperature changes. Snx11-knockout mice exhibit a stronger preference for mild temperatures along with enhanced sensitivity to harmful heat. Mechanistically, keratinocytes from Snx11-knockout mice exhibit a larger temperature-gated TRPV3 membrane current and have enhanced thermoTRPV3 expression in the plasma membrane compared to wild-type keratinocytes. Additionally, Snx11-knockout mice show higher endogenous TRPV3 protein levels in skin tissues than wild-type mice do. Therefore, our results indicate that SNX11 may regulate thermal perception via alteration of functional thermoTRPV3 on the plasma membrane of thermally sensitive cells, which is the first link between vesicular trafficking and thermal transduction.