Spinothalamic lamina I neurones selectively responsive to cutaneous warming in cats

Differential modification of ascending spinal outputs in acute and chronic pain states

Pain hypersensitivity arises from the induction of plasticity in peripheral and spinal somatosensory neurons, which modifies nociceptive input to the brain, altering pain perception. We applied longitudinal calcium imaging of spinal dorsal projection neurons to determine whether and how the representation of somatosensory stimuli in the anterolateral tract, the principal pathway transmitting nociceptive signals to the brain, changes between distinct pain states. In healthy mice, we identified stable outputs selective for cooling or warming and a neuronal ensemble activated by noxious thermal and mechanical stimuli. Induction of acute peripheral sensitization by topical capsaicin transiently re-tuned nociceptive output neurons to encode low-intensity stimuli. In contrast, peripheral nerve injury resulted in a persistent suppression of innocuous spinal outputs coupled with persistent activation of a normally silent population of high-threshold neurons. These results demonstrate differential modulation of spinal outputs to the brain during nociceptive and neuropathic pain states.

The Life and Legacy of Arthur D. Craig, Jr.: A Pioneer in Interoception

A.D. (Bud) Craig (1951-2023) redefined the concept of interoception and provided a novel, revolutionary understanding of the neural basis for human awareness. In unsurpassed anatomical-physiological studies in monkeys, Craig showed that the insular cortex is the primary sensory cortex for interoception, or the image of the "material me" that provides a homeostatic representation of the physiological condition of the body. He showed that the insula contains a postero-anteriorly organized somatotopic map of the interoceptive sensations, and that it encodes both the localization and the intensity discrimination of interoceptive sensations. In seminal work in humans, he demonstrated that the interoceptive feelings are re-represented, and multimodally integrated, in anterior portions of the insula in sequence of increasingly homeostatically efficient representations that integrate all salient neural activity. He further showed that subjective awareness is associated with activation of the anterior insular cortex and suggested that this brain region also is critical for fluid intelligence and the perception of time. His work has led to a paradigm shift in our understanding of interoception and how interoceptive sensations underlie consciousness, a topic that long has been considered elusive, or even beyond our comprehension.

Neuroanatomy of spinal nociception and pain in dogs and cats: a practical review for the veterinary clinician

Chronic pain is a prevalent condition in companion animals and poses significant welfare challenges. To address these concerns effectively, veterinary clinicians must have a comprehensive understanding of the neuroanatomy of nociception and the intricate processes underlying pain perception. This knowledge is essential for planning and implementing targeted treatment strategies. However, much of the existing information on pain mechanisms is derived from studies on rodents or humans, highlighting the need for further translational research to bridge this gap for veterinary applications. This review aims to provide veterinary clinicians with an in-depth overview of the spinal nociceptive pathways in the dog and cat, tracing the journey from nociceptor activation to cortical processing in the brain. Additionally, the review explores factors influencing nociceptive signaling and pain perception. By enhancing the understanding of these fundamental physiological processes, this work seeks to lay the groundwork for developing effective therapies to manage the complexities of chronic pain in companion animals.

Selective modification of ascending spinal outputs in acute and neuropathic pain states

Pain hypersensitivity arises from the plasticity of peripheral and spinal somatosensory neurons, which modifies nociceptive input to the brain and alters pain perception. We utilized chronic calcium imaging of spinal dorsal horn neurons to determine how the representation of somatosensory stimuli in the anterolateral tract, the principal pathway transmitting nociceptive signals to the brain, changes between distinct pain states. In healthy conditions, we identify stable, narrowly tuned outputs selective for cooling or warming, and a neuronal ensemble activated by intense/noxious thermal and mechanical stimuli. Induction of an acute peripheral sensitization with capsaicin selectively and transiently retunes nociceptive output neurons to encode low-intensity stimuli. In contrast, peripheral nerve injury-induced neuropathic pain results in a persistent suppression of innocuous spinal outputs coupled with activation of a normally silent population of high-threshold neurons. These results demonstrate the differential modulation of specific spinal outputs to the brain during nociceptive and neuropathic pain states.

A computationally informed distinction of interoception and exteroception

While interoception is of major neuroscientific interest, its precise definition and delineation from exteroception continue to be debated. Here, we propose a functional distinction between interoception and exteroception based on computational concepts of sensor-effector loops. Under this view, the classification of sensory inputs as serving interoception or exteroception depends on the sensor-effector loop they feed into, for the control of either bodily (physiological and biochemical) or environmental states. We explain the utility of this perspective by examining the perception of skin temperature, one of the most challenging cases for distinguishing between interoception and exteroception. Specifically, we propose conceptualising thermoception as inference about the thermal state of the body (including the skin), which is directly coupled to thermoregulatory processes. This functional view emphasises the coupling to regulation (control) as a defining property of perception (inference) and connects the definition of interoception to contemporary computational theories of brain-body interactions.

In vivo imaging of the neuronal response to spinal cord injury: a narrative review

Deciphering the neuronal response to injury in the spinal cord is essential for exploring treatment strategies for spinal cord injury (SCI). However, this subject has been neglected in part because appropriate tools are lacking. Emerging in vivo imaging and labeling methods offer great potential for observing dynamic neural processes in the central nervous system in conditions of health and disease. This review first discusses in vivo imaging of the mouse spinal cord with a focus on the latest imaging techniques, and then analyzes the dynamic biological response of spinal cord sensory and motor neurons to SCI. We then summarize and compare the techniques behind these studies and clarify the advantages of in vivo imaging compared with traditional neuroscience examinations. Finally, we identify the challenges and possible solutions for spinal cord neuron imaging.

Central Mechanisms of Thermoregulation and Fever in Mammals

Thermoregulation is a fundamental homeostatic function in mammals mediated by the central nervous system. The framework of the central circuitry for thermoregulation lies in the hypothalamus and brainstem. The preoptic area (POA) of the hypothalamus integrates cutaneous and central thermosensory information into efferent control signals that regulate excitatory descending pathways through the dorsomedial hypothalamus (DMH) and rostral medullary raphe region (rMR). The cutaneous thermosensory feedforward signals are delivered to the POA by afferent pathways through the lateral parabrachial nucleus, while the central monitoring of body core temperature is primarily mediated by warm-sensitive neurons in the POA for negative feedback regulation. Prostaglandin E2, a pyrogenic mediator produced in response to infection, acts on the POA to trigger fever. Recent studies have revealed that this circuitry also functions for physiological responses to psychological stress and starvation. Master psychological stress signaling from the medial prefrontal cortex to the DMH has been discovered to drive a variety of physiological responses for stress coping, including hyperthermia. During starvation, hunger signaling from the hypothalamus was found to activate medullary reticular neurons, which then suppress thermogenic sympathetic outflows from the rMR for energy saving. This thermoregulatory circuit represents a fundamental mechanism of the central regulation for homeostasis.

Habituation of the cold shock response: A systematic review and meta-analysis

Cold water immersion (CWI) evokes the life-threatening reflex cold shock response (CSR), inducing hyperventilation, increasing cardiac arrhythmias, and increasing drowning risk by impairing safety behaviour. Repeated CWI induces CSR habituation (i.e., diminishing response with same stimulus magnitude) after ∼4 immersions, with variation between studies. We quantified the magnitude and coefficient of variation (CoV) in the CSR in a systematic review and meta-analysis with search terms entered to Medline, SportDiscus, PsychINFO, Pubmed, and Cochrane Central Register. Random effects meta-analyses, including effect sizes (Cohen's d) from 17 eligible groups (k), were conducted for heart rate (HR, n = 145, k = 17), respiratory frequency (fR, n = 73, k = 12), minute ventilation (Ve, n = 106, k = 10) and tidal volume (Vt, n = 46, k=6). All CSR variables habituated (p < 0.001) with large or moderate pooled effect sizes: ΔHR -14 (10) bt. min-1 (d: -1.19); ΔfR -8 (7) br. min-1 (d: -0.78); ΔVe, -21.3 (9.8) L. min-1 (d: -1.64); ΔVt -0.4 (0.3) L -1. Variation was greatest in Ve (control vs comparator immersion: 32.5&24.7%) compared to Vt (11.8&12.1%). Repeated CWI induces CSR habituation potentially reducing drowning risk. We consider the neurophysiological and behavioural consequences.

Identification of a convergent spinal neuron population that encodes itch

Itch is a protective sensation that drives scratching. Although specific cell types have been proposed to underlie itch, the neural circuit basis for itch remains unclear. Here, we used two-photon Ca2+ imaging of the dorsal horn to visualize the neuronal populations that are activated by itch-inducing agents. We identify a convergent population of spinal neurons that is defined by the expression of GRPR. Moreover, we discover that itch is conveyed to the brain via GRPR-expressing spinal output neurons that target the lateral parabrachial nucleus. Further, we show that nalfurafine, a clinically effective kappa opioid receptor agonist, relieves itch by inhibiting GRPR spinoparabrachial neurons. Finally, we demonstrate that a subset of GRPR spinal neurons show persistent, cell-intrinsic Ca2+ oscillations. These experiments provide the first population-level view of the spinal neurons that respond to pruritic stimuli, pinpoint the output neurons that convey itch to the brain, and identify the cellular target of kappa opioid receptor agonists for the inhibition of itch.

Quantitative Sensory Testing to Characterize Sensory Changes in Hidradenitis Suppurativa Skin Lesions

Importance

Pain is the most impactful symptom in patients with hidradenitis suppurativa (HS). Characterization of sensory profiles may improve understanding of pain mechanisms in HS and facilitate identification of effective pain management strategies.

Objective

To characterize somatosensory profiles in patients with HS at clinically affected and nonaffected sites compared with pain-free reference data.

Design, Setting, and Participants

This cross-sectional study was conducted at the Emory University Dermatology Clinic. It was hypothesized (1) that patients with HS would demonstrate hypersensitivity to pain in HS lesions and (2) that some patients would have sensory profiles consistent with complex pain mechanisms. Therefore, adults with dermatologist-diagnosed HS and at least 1 painful HS lesion at the time of testing were enrolled between September 10, 2020, and March 21, 2022. Patients with other diagnoses contributing to pain or neuropathy were excluded. Data analysis was conducted between March and April 2022.

Exposure

Quantitative sensory testing was performed on HS lesions and control skin according to a standardized protocol.

Main Outcomes and Measures

Quantitative sensory testing outcomes included innocuous thermal and mechanical sensitivity (cold, warmth, and light touch detection thresholds), noxious thermal and mechanical sensitivity (cold, heat, pinprick, and deep pressure pain thresholds and suprathreshold pinprick sensitivity), temporal summation of pinprick, paradoxical thermal sensations, and dynamic mechanical allodynia (pain upon light stroking of the skin). Sensitivity in HS lesions was compared with sensitivity in a control location (the hand) and in pain-free controls using t tests.

Results

This study included 20 participants with a median age of 35.5 (IQR, 30.0-46.5) years, the majority of whom were women (15 [75%]). In terms of race and ethnicity, 2 participants (10%) self-identified as Asian, 11 (55%) as Black, 6 (30%) as White, and 1 (5%) as more than 1 race or ethnicity. Compared with site-specific reference values from healthy, pain-free control participants, HS lesions were insensitive to innocuous cold and warmth, noxious heat, and light touch (t = -5.69, -10.20, -3.84, and 4.46, respectively; all P < .001). In contrast, HS lesions also demonstrated significant hypersensitivity to deep pressure pain (t = 8.36; P < .001) and cutaneous pinprick (t = 2.07; P = .046). Hypersensitivity to deep pressure pain was also observed in the control site (t = 5.85; P < .001). A subset of patients with HS displayed changes in pain processing that are often seen in neuropathic and nociplastic pain conditions, including hypersensitivity to repetitive pinprick (5 [26%]), paradoxical thermal sensations (3 [15%]), and pain upon light stroking of the skin (10 [50%]).

Conclusions and Relevance

The findings of this cross-sectional study suggest that HS involves local changes in the skin or its free nerve endings, possibly leading to peripheral neuropathy and alterations in the transduction of innocuous and noxious thermal and mechanical stimuli. For some patients, central nervous system changes in somatosensory processing may also occur, but confirmatory evidence is needed. Better understanding of neuropathic and nociplastic mechanisms in HS pain could lead to individually tailored treatments.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

The cellular coding of temperature in the mammalian cortex

Temperature is a fundamental sensory modality separate from touch, with dedicated receptor channels and primary afferent neurons for cool and warm1-3. Unlike for other modalities, however, the cortical encoding of temperature remains unknown, with very few cortical neurons reported that respond to non-painful temperature, and the presence of a 'thermal cortex' is debated4-8. Here, using widefield and two-photon calcium imaging in the mouse forepaw system, we identify cortical neurons that respond to cooling and/or warming with distinct spatial and temporal response properties. We observed a representation of cool, but not warm, in the primary somatosensory cortex, but cool and warm in the posterior insular cortex (pIC). The representation of thermal information in pIC is robust and somatotopically arranged, and reversible manipulations show a profound impact on thermal perception. Despite being positioned along the same one-dimensional sensory axis, the encoding of cool and that of warm are distinct, both in highly and broadly tuned neurons. Together, our results show that pIC contains the primary cortical representation of skin temperature and may help explain how the thermal system generates sensations of cool and warm.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

A hypothalamomedullary network for physiological responses to environmental stresses

Various environmental stressors, such as extreme temperatures (hot and cold), pathogens, predators and insufficient food, can threaten life. Remarkable progress has recently been made in understanding the central circuit mechanisms of physiological responses to such stressors. A hypothalamomedullary neural pathway from the dorsomedial hypothalamus (DMH) to the rostral medullary raphe region (rMR) regulates sympathetic outflows to effector organs for homeostasis. Thermal and infection stress inputs to the preoptic area dynamically alter the DMH → rMR transmission to elicit thermoregulatory, febrile and cardiovascular responses. Psychological stress signalling from a ventromedial prefrontal cortical area to the DMH drives sympathetic and behavioural responses for stress coping, representing a psychosomatic connection from the corticolimbic emotion circuit to the autonomic and somatic motor systems. Under starvation stress, medullary reticular neurons activated by hunger signalling from the hypothalamus suppress thermogenic drive from the rMR for energy saving and prime mastication to promote food intake. This Perspective presents a combined neural network for environmental stress responses, providing insights into the central circuit mechanism for the integrative regulation of systemic organs.

Pancreas-Brain Crosstalk

The neural regulation of glucose homeostasis in normal and challenged conditions involves the modulation of pancreatic islet-cell function. Compromising the pancreas innervation causes islet autoimmunity in type 1 diabetes and islet cell dysfunction in type 2 diabetes. However, despite the richly innervated nature of the pancreas, islet innervation remains ill-defined. Here, we review the neuroanatomical and humoral basis of the cross-talk between the endocrine pancreas and autonomic and sensory neurons. Identifying the neurocircuitry and neurochemistry of the neuro-insular network would provide clues to neuromodulation-based approaches for the prevention and treatment of diabetes and obesity.

Predicting the location of the preoptic and anterior hypothalamic region by visualizing the thermoregulatory center on fMRI in craniopharyngioma using cold and warm stimuli

Hypothalamic nuclei in the preoptic and anterior hypothalamic region (POAH) are critically involved in thermoregulation and neuroendocrine regulation and can be displaced by craniopharyngiomas (CPs). We aimed to locate the POAH by visualizing hypothalamic thermoregulation through task-related functional magnetic resonance imaging (fMRI) to guide hypothalamus protection intraoperatively. Nine adult healthy volunteers (HVs) and thirty-two adult primary CP patients underwent task-related fMRI for POAH localization by warm (60° C) and cold (0° C) cutaneous thermoreceptor stimulation. Approach selection and intraoperative POAH protection were performed based on preoperative POAH localization. In all HVs and patients, significant single positive blood oxygen level-dependent (BOLD) signal changes were located in the POAH. The BOLD activity was significantly greater for cold (P=0.03) and warm (P=0.03) stimuli in patients than in HVs. Gross total resection and near-total resection were achieved in 28 (87.5%) and 4 (12.5%) patients, respectively. New-onset diabetes insipidus and new-onset hypopituitarism occurred in 6 patients (18.8%) and 10 patients (31.3%), respectively. Our findings suggest that cutaneous thermoreceptor stimulation could accurately activate the hypothalamic thermoregulatory center and allow POAH localization through task-related fMRI. Preoperative POAH localization could help neurosurgeons protect hypothalamic function intraoperatively. The CP patients were more sensitive to thermal stimulation.

Alterations of Heartbeat Evoked Magnetic Fields Induced by Sounds of Disgust

The majority of the models of emotional processing attribute subjective emotional feelings to physiological changes in the internal milieu, which are sensed by the interoceptive system. These physiological reactions evoked by emotional phenomena occur via the autonomic nervous system, and give rise to alterations in body-mind interactions that are characterized by heartbeat evoked magnetic fields (HEFs) involving brain regions associated with emotional perception. The current study used magnetoencephalography (MEG) to examine regional cortical activity and connectivity changes in HEFs provoked by the emotion of disgust. MEG results from 39 healthy subjects (22 female) revealed that passively listening to sounds of disgust elicited right insular cortical activity and enhancement of cortical connectivity between the right anterior ventral insular cortex and left ventromedial prefrontal cortex, demonstrated by phase lag indexes in the beta frequency range. Furthermore, inter-trial coherence significantly increased at 19 Hz and 23 Hz, and decreased at 14 Hz, which highlights the involvement of low beta oscillations in emotional processing. As these results were based on spontaneously triggered bioelectrical signals, more indigenous and induced signals were extracted with a block designed experiment. The insular cortices play an important role in emotional regulation and perception as the main cortical target for signals with interoceptive information, providing direct substrates of emotional feelings. The current results provide a novel insight into frequency properties of emotional processing, and suggest that emotional arousal evoked by listening to sounds of disgust partially impact the autonomic nervous system, altering HEFs via connectivity changes in the right anterior ventral insular cortex and left ventromedial prefrontal cortex.

Spinal cord projection neurons: a superficial, and also deep, analysis

Today there are extensive maps of the molecular heterogeneity of primary afferents and dorsal horn interneurons, yet there is a dearth of molecular and functional information regarding the projection neurons that transmit pain and itch information to the brain. Additionally, most contemporary research into the spinal cord and medullary projection neurons focuses on neurons in the superficial dorsal horn; the contribution of deep dorsal horn and even ventral horn projection neurons to pain and itch processing is often overlooked. In the present review we integrate conclusions from classical as well as contemporary studies and provide a more balanced view of the diversity of projection neurons. A major question addressed is the extent to which labeled-lines are maintained in these different populations or whether the brain generates distinct pain and itch percepts by decoding complex convergent inputs that engage projection neurons.

Probing the coding logic of thermosensation using spinal cord calcium imaging

The spinal cord dorsal horn is the first relay station of the neural network for processing somatosensory information. High-throughput optical recording methods facilitate the study of sensory coding in the cortex but have not been successfully applied to study spinal cord circuitry until recently. Here, we review the development of an in vivo two-photon spinal calcium imaging preparation and biological findings from the first systematic characterization of the spinal response to cutaneous thermal stimuli, focusing on the difference between the coding of heat and cold, and the contribution of different peripheral inputs to thermosensory response in the spinal cord. Here we also report that knockout of TRPV1 channel impairs sensation of warmth, and somatostatin- and calbindin2-expressing neurons in the spinal dorsal horn preferentially respond to heat. Future work combining this technology with genetic tools and animal models of chronic pain will further elucidate the role of each neuronal type in the spinal thermosensory coding and their plasticity under pathological condition.

Central nervous system circuits that control body temperature

Maintenance of mammalian core body temperature within a narrow range is a fundamental homeostatic process to optimize cellular and tissue function, and to improve survival in adverse thermal environments. Body temperature is maintained during a broad range of environmental and physiological challenges by central nervous system circuits that process thermal afferent inputs from the skin and the body core to control the activity of thermoeffectors. These include thermoregulatory behaviors, cutaneous vasomotion (vasoconstriction and, in humans, active vasodilation), thermogenesis (shivering and brown adipose tissue), evaporative heat loss (salivary spreading in rodents, and human sweating). This review provides an overview of the central nervous system circuits for thermoregulatory reflex regulation of thermoeffectors.

Physiological properties of the lamina I spinoparabrachial neurons in the mouse

KEY POINTS

Spinal cord lamina I neurons receiving dense input from nociceptors and projecting to the parabrachial area at the ponto-mesencephalic junction form the major ascending pain-related pathway in rodents. Lamina I spinoparabrachial (SPB) neurons have never been characterized in mice, despite the growing and extensive use of this species to understand the contribution of lamina I SPB neurons in chronic pain. The electrophysiological properties of lamina I SPB neurons recorded here in anaesthetized mice are comparable to those of rat or cat, forming a nociceptive and thermoreceptive pathway. It was confirmed 'on line' that lamina I SPB neurons that normally encode noxious stimuli can receive input from low threshold mechanoreceptors in certain conditions. The present work indicates that the study of lamina I SPB neurons in vivo could take advantage of the use of genetically modified mice.

ABSTRACT

Ongoing studies investigating the role of lamina I projection neurons in the generation of chronic pain are mainly based on the use of genetically modified mice. However, lamina I projection neurons have never been physiologically characterized in this species. The present work aimed to fill this gap, and to assess the effect of spinal 'disinhibition' that may occur in chronic pain states on the responses of these neurons to light touch. Seventy lamina I spinoparabrachial (SPB) neurons were characterized in anaesthetized mice. These neurons showed low central conduction velocities (<12.4 m s^-1 ) and wide range of responses. Fifty-six neurons responded equally to noxious mechanical and thermal (heat) stimuli (16% responded consistently to light touch). Modality-specific neurons responded preferentially to thermal (cold) stimuli (n = 10) and pinch (n = 2), or specifically to heat (n = 2). Spinal bicuculline and strychnine application induced responses to brush in half of the neurons tested, confirming directly the potential connection between low threshold mechanoreceptors and nociceptive-specific neurons, responsible for mechanical allodynia. Remarkably, the effect of the treatment was highly variable and apparently independent of the initial profile of the neurons. The present data confirm that mice lamina I SPB neurons have the expected characteristics to form a nociceptive and thermoreceptive pathway, but they constitute a highly heterogeneous group. The differential effect of spinal disinhibition observed here suggests that a subgroup of lamina I SPB neurons might be responsible for abnormal pain in pathological conditions, and emphasizes the importance of in vivo recording, a neglected approach.

The neural circuits of thermal perception

Thermal information about skin surface temperature is a key sense for the perception of object identity and valence. The identification of ion channels involved in the transduction of thermal changes has provided a genetic access point to the thermal system. However, from sensory specific 'labeled-lines' to multimodal interactive pathways, the functional organization and identity of the neural circuits mediating innocuous thermal perception have been debated for over 100 years. Here we highlight points in the system that require further attention and review recent advances using in vivo electrophysiology, cellular resolution calcium imaging, optogenetics and thermal perceptual tasks in behaving mice that have begun to uncover the anatomical principles and neural processing mechanisms underlying innocuous thermal perception.

Central Mechanisms for Thermoregulation

Maintenance of a homeostatic body core temperature is a critical brain function accomplished by a central neural network. This orchestrates a complex behavioral and autonomic repertoire in response to environmental temperature challenges or declining energy homeostasis and in support of immune responses and many behavioral states. This review summarizes the anatomical, neurotransmitter, and functional relationships within the central neural network that controls the principal thermoeffectors: cutaneous vasoconstriction regulating heat loss and shivering and brown adipose tissue for heat production. The core thermoregulatory network regulating these thermoeffectors consists of parallel but distinct central efferent pathways that share a common peripheral thermal sensory input. Delineating the neural circuit mechanism underlying central thermoregulation provides a useful platform for exploring its functional organization, elucidating the molecular underpinnings of its neuronal interactions, and discovering novel therapeutic approaches to modulating body temperature and energy homeostasis.

Increased heart rate after exercise facilitates the processing of fearful but not disgusted faces

Embodied theories of emotion assume that emotional processing is grounded in bodily and affective processes. Accordingly, the perception of an emotion re-enacts congruent sensory and affective states; and conversely, bodily states congruent with a specific emotion facilitate emotional processing. This study tests whether the ability to process facial expressions (faces having a neutral expression, expressing fear, or disgust) can be influenced by making the participants’ body state congruent with the expressed emotion (e.g., high heart rate in the case of faces expressing fear). We designed a task requiring participants to categorize pictures of male and female faces that either had a neutral expression (neutral), or expressed emotions whose linkage with high heart rate is strong (fear) or significantly weaker or absent (disgust). Critically, participants were tested in two conditions: with experimentally induced high heart rate (Exercise) and with normal heart rate (Normal). Participants processed fearful faces (but not disgusted or neutral faces) faster when they were in the Exercise condition than in the Normal condition. These results support the idea that an emotionally congruent body state facilitates the automatic processing of emotionally-charged stimuli and this effect is emotion-specific rather than due to generic factors such as arousal.

Afferent pathways for autonomic and shivering thermoeffectors

Body core temperature of mammals is regulated by the central nervous system, in which the preoptic area (POA) of the hypothalamus plays a pivotal role. The POA receives peripheral and central thermosensory neural information and provides command signals to effector organs to elicit involuntary thermoregulatory responses, including shivering thermogenesis, nonshivering brown adipose tissue thermogenesis, and cutaneous vasoconstriction. Cool-sensory and warm-sensory signals from cutaneous thermoreceptors, monitoring environmental temperature, are separately transmitted through the spinal-parabrachial-POA neural pathways, distinct from the spinothalamocortical pathway for perception of skin temperature. These cutaneous thermosensory inputs to the POA likely impinge on warm-sensitive POA neurons, which monitor body core (brain) temperature, to alter thermoregulatory command outflows from the POA. The cutaneous thermosensory afferents elicit rapid thermoregulatory responses to environmental thermal challenges before they impact body core temperature. Peripheral humoral signals also act on neurons in the POA to transmit afferent information of systemic infection and energy storage to induce fever and to regulate energy balance, respectively. This chapter describes the thermoregulatory afferent mechanisms that convey cutaneous thermosensory signals to the POA and that integrate the neural and humoral afferent inputs to the POA to provide descending command signals to thermoregulatory effectors.

Central neural substrates involved in temperature discrimination, thermal pain, thermal comfort, and thermoregulatory behavior

A phylogenetically novel pathway that emerged with primate encephalization is described, which conveys high-fidelity cutaneous thermosensory activity in "labeled lines" to a somatotopic map in the dorsal posterior insular cortex. It originates in lamina I of the superficial dorsal horn and ascends by way of the lateral spinothalamic tract and a distinct region in posterolateral thalamus. It evolved from the homeostatic sensory activity that represents the physiologic (interoceptive) condition of the body and drives the central autonomic network, which underlies all affective feelings from the body. Accordingly, human discriminative thermal sensations are accompanied by thermally motivated behaviors and thermal feelings of comfort or discomfort (unless neutral), which evidence suggests are associated with activity in the insular, cingulate, and orbitofrontal cortices, respectively. Yet, the substrates for thermoregulatory behavior have not been established, and several strong candidates (including the hypothalamus and the bed nucleus of the stria terminalis) are discussed. Finally, the neural underpinnings for relationships between thermal affect and social feelings (warm-positive/cold-negative) are addressed, including the association of hyperthermia with clinical depression.

Innocuous warming enhances peripheral serotonergic itch signaling and evokes enhanced responses in serotonin-responsive dorsal horn neurons in the mouse

Itch is often triggered by warming the skin in patients with itchy dermatitis, but the underlying mechanism is largely unknown. We presently investigated if warming the skin enhances histamine- or serotonin (5-HT)-evoked itch behavior or responses of sensory dorsal root ganglion (DRG) cells, and if responses of superficial dorsal horn neurons to innocuous warming are enhanced by these pruritogens. In a temperature-controlled environmental chamber, mice exhibited greater scratching following intradermal injection of 5-HT, but not histamine, SLIGRL, or BAM8-22, when the skin surface temperature was above 36°C. Calcium imaging of DRG cells in a temperature-controlled bath revealed that responses to 5-HT, but not histamine, were significantly greater at a bath temperature of 35°C vs. lower temperatures. Single-unit recordings revealed a subpopulation of superficial dorsal horn neurons responsive to intradermal injection of 5-HT. Of these, 58% responded to innocuous skin warming (37°C) prior to intradermal injection of 5-HT, while 100% responded to warming following intradermal injection of 5-HT. Warming-evoked responses were superimposed on the 5-HT-evoked elevation in firing and were significantly larger compared with responses pre-5-HT, as long as 30 min after the intradermal injection of 5-HT. Five-HT-insensitive units, and units that either did or did not respond to intradermal histamine, did not exhibit any increase in the incidence of warmth sensitivity or in the mean response to warming following intradermal injection of the pruritogen. The results suggest that 5-HT-evoked responses of pruriceptors are enhanced during skin warming, leading to increased firing of 5-HT-sensitive dorsal horn neurons that signal nonhistaminergic itch.

NEW & NOTEWORTHY

Skin warming often exacerbates itch in patients with itchy dermatitis. We demonstrate that warming the skin enhanced serotonin-evoked, but not histamine-evoked, itch behavior and responses of sensory dorsal root ganglion cells. Moreover, serotonin, but not histamine, enhanced responses of superficial dorsal horn neurons to innocuous warming. The results suggest that skin warming selectively enhances the responses of serotonin-sensitive pruriceptors, leading to increased firing of serotonin-sensitive dorsal horn neurons that signal nonhistaminergic itch.

Highway to thermosensation: a traced review, from the proteins to the brain

Temperature maintenance and detection are essential for the survival and perpetuation of any species. This review is focused on thermosensation; thus a detailed and traced explanation of the anatomical and physiological characteristics of each component of this sensation is given. First, the proteins that react to temperature changes are identified; next, the nature of the neurons involved in thermosensation is described; and then, the pathways from the skin through the spinal cord to the brain are outlined. Finally, the areas of the brain and their interconnections where thermoperception arises are explained. Transduction of the external and internal temperature information is essentially mediated by the transient receptor potential ion channels (TRPs). These proteins are embedded in the neurons' membrane and they hyper- or de-polarize neurons in function of the intrinsic voltage and the temperature changes. There are distinct TRP sensors for different temperature ranges. Interestingly, the primary afferent neurons have either cold or hot receptors, so they are dedicated separately to cold or hot sensation. The information is transmitted by different pathways from the skin to the brain, where it either remains separated or is integrated to generate a response. It seems that both the determination of how thermoperception is produced and how we interact with the world are dependent on the particular arrangement and nature of the components, the way of transduction of information and the communication between these elements.

The coding of cutaneous temperature in the spinal cord

The spinal cord is the initial stage that integrates temperature information from peripheral inputs. Here we used molecular genetics and in vivo calcium imaging to investigate the coding of cutaneous temperature in the spinal cord in mice. We found that heating or cooling the skin evoked robust calcium responses in spinal neurons, and their activation threshold temperatures distributed smoothly over the entire range of stimulation temperatures. Once activated, heat-responding neurons encoded the absolute skin temperature without adaptation and received major inputs from transient receptor potential (TRP) channel V1 (TRPV1)-positive dorsal root ganglion (DRG) neurons. By contrast, cold-responding neurons rapidly adapted to ambient temperature and selectively encoded temperature changes. These neurons received TRP channel M8 (TRPM8)-positive DRG inputs as well as novel TRPV1(+) DRG inputs that were selectively activated by intense cooling. Our results provide a comprehensive examination of the temperature representation in the spinal cord and reveal fundamental differences in the coding of heat and cold.

Spinal cord thermosensitivity: An afferent phenomenon?

We review the evidence for thermoregulatory temperature sensors in the mammalian spinal cord and reach the following conclusions. 1) Spinal cord temperature contributes physiologically to temperature regulation. 2) Parallel anterolateral ascending pathways transmit signals from spinal cooling and spinal warming: they overlap with the respective axon pathways of the dorsal horn neurons that are driven by peripheral cold- and warm-sensitive afferents. 3) We hypothesize that these ‘cold’ and ‘warm’ ascending pathways transmit all extracranial thermosensory information to the brain. 4) Cutaneous cold afferents can be activated not only by cooling the skin but also by cooling sites along their axons: we consider that this is functionally insignificant in vivo. 5) By a presynaptic action on their central terminals, local spinal cooling enhances neurotransmission from incoming ‘cold’ afferent action potentials to second order neurons in the dorsal horn; this effect disappears when the spinal cord is warm. 6) Spinal warm sensitivity is due to warm-sensitive miniature vesicular transmitter release from afferent terminals in the dorsal horn: this effect is powerful enough to excite second order neurons in the ‘warm’ pathway independently of any incoming sensory traffic. 7) Distinct but related presynaptic mechanisms at cold- and warm-sensitive afferent terminals can thus account for the thermoregulatory actions of spinal cord temperature.

Somatic influences on subjective well-being and affective disorders: the convergence of thermosensory and central serotonergic systems

Current theories suggest that the brain is the sole source of mental illness. However, affective disorders, and major depressive disorder (MDD) in particular, may be better conceptualized as brain-body disorders that involve peripheral systems as well. This perspective emphasizes the embodied, multifaceted physiology of well-being, and suggests that afferent signals from the body may contribute to cognitive and emotional states. In this review, we focus on evidence from preclinical and clinical studies suggesting that afferent thermosensory signals contribute to well-being and depression. Although thermoregulatory systems have traditionally been conceptualized as serving primarily homeostatic functions, increasing evidence suggests neural pathways responsible for regulating body temperature may be linked more closely with emotional states than previously recognized, an affective warmth hypothesis. Human studies indicate that increasing physical warmth activates brain circuits associated with cognitive and affective functions, promotes interpersonal warmth and prosocial behavior, and has antidepressant effects. Consistent with these effects, preclinical studies in rodents demonstrate that physical warmth activates brain serotonergic neurons implicated in antidepressant-like effects. Together, these studies suggest that (1) thermosensory pathways interact with brain systems that control affective function, (2) these pathways are dysregulated in affective disorders, and (3) activating warm thermosensory pathways promotes a sense of well-being and has therapeutic potential in the treatment of affective disorders.

You turn me cold: evidence for temperature contagion

INTRODUCTION

During social interactions, our own physiological responses influence those of others. Synchronization of physiological (and behavioural) responses can facilitate emotional understanding and group coherence through inter-subjectivity. Here we investigate if observing cues indicating a change in another's body temperature results in a corresponding temperature change in the observer.

METHODS

Thirty-six healthy participants (age; 22.9±3.1 yrs) each observed, then rated, eight purpose-made videos (3 min duration) that depicted actors with either their right or left hand in visibly warm (warm videos) or cold water (cold videos). Four control videos with the actors' hand in front of the water were also shown. Temperature of participant observers' right and left hands was concurrently measured using a thermistor within a Wheatstone bridge with a theoretical temperature sensitivity of <0.0001°C. Temperature data were analysed in a repeated measures ANOVA (temperature × actor's hand × observer's hand).

RESULTS

Participants rated the videos showing hands immersed in cold water as being significantly cooler than hands immersed in warm water, F(1,34) = 256.67, p<0.001. Participants' own hands also showed a significant temperature-dependent effect: hands were significantly colder when observing cold vs. warm videos F(1,34) = 13.83, p = 0.001 with post-hoc t-test demonstrating a significant reduction in participants' own left (t(35) = -3.54, p = 0.001) and right (t(35) = -2.33, p = 0.026) hand temperature during observation of cold videos but no change to warm videos (p>0.1). There was however no evidence of left-right mirroring of these temperature effects p>0.1). Sensitivity to temperature contagion was also predicted by inter-individual differences in self-report empathy.

CONCLUSIONS

We illustrate physiological contagion of temperature in healthy individuals, suggesting that empathetic understanding for primary low-level physiological challenges (as well as more complex emotions) are grounded in somatic simulation.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Eugenol and carvacrol excite first- and second-order trigeminal neurons and enhance their heat-evoked responses

Eugenol and carvacrol from clove and oregano, respectively, are agonists of the warmth-sensitive transient receptor potential channel TRPV3 and the irritant-sensitive transient receptor potential ankyrin (TRPA)-1. Eugenol and carvacrol induce oral irritation that rapidly desensitizes, accompanied by brief enhancement of innocuous warmth and heat pain in humans. We presently investigated if eugenol and carvacrol activate nociceptive primary afferent and higher order trigeminal neurons and enhance their heat-evoked responses, using calcium imaging of cultured trigeminal ganglion (TG) and dorsal root ganglion (DRG) neurons, and in vivo single-unit recordings in trigeminal subnucleus caudalis (Vc) of rats. Eugenol and carvacrol activated 20-30% of TG and 7-20% of DRG cells, the majority of which additionally responded to menthol, mustard oil and/or capsaicin. TG cell responses to innocuous (39°) and noxious (42 °C) heating were enhanced by eugenol and carvacrol. We identified dorsomedial Vc neurons responsive to noxious heating of the tongue in pentobarbital-anesthetized rats. Eugenol and carvacrol dose-dependently elicited desensitizing responses in 55% and 73% of heat-sensitive units, respectively. Responses to noxious heat were briefly enhanced by eugenol and carvacrol. Many eugenol- and carvacrol-responsive units also responded to menthol, cinnamaldehyde and capsaicin. These data support a peripheral site for eugenol and carvacrol to enhance warmth- and noxious heat-evoked responses of trigeminal neurons, and are consistent with the observation that these agonists briefly enhance warmth and heat pain on the human tongue.

Disturbed sensory perception of changes in thermoalgesic stimuli in patients with small fiber neuropathies

The assessment of functional deficits in small fibre neuropathies (SFN) requires using ancillary tests other than conventional neurophysiological techniques. One of the tests with most widespread use is thermal threshold determination, as part of quantitative sensory testing. Thermal thresholds typically reflect one point in the whole subjective experience elicited by a thermal stimulus. We reasoned that more information could be obtained by analyzing the subjective description of the ongoing sensation elicited by slow temperature changes (dynamic thermal testing, DTT). Twenty SFN patients and 20 healthy subjects were requested to describe, by using an electronic visual analog scale system, the sensation perceived when the temperature of a thermode was made to slowly change according to a predetermined pattern. The thermode was attached to the left ventral forearm or the distal third of the left leg and the stimulus was either a monophasic heat or cold stimuli that reached 120% of pain threshold and reversed to get back to baseline at a rate of 0.5 °C/s. Abnormalities seen in patients in comparison to healthy subjects were: (1) delayed perception of temperature changes, both at onset and at reversal, (2) longer duration of pain perception at peak temperature, and (3) absence of an overshoot sensation after reversal, ie, a transient perception of the opposite sensation before the temperature reached again baseline. The use of DTT increases the yield of thermal testing for clinical and physiological studies. It adds information that can be discriminant between healthy subjects and SFN patients and shows physiological details about the process of activation and inactivation of temperature receptors that may be abnormal in SFN.

Selective distribution of GABA(A) receptor subtypes in mouse spinal dorsal horn neurons and primary afferents

In the spinal cord dorsal horn, presynaptic GABA(A) receptors (GABA(A)Rs) in the terminals of nociceptors as well as postsynaptic receptors in spinal neurons regulate the transmission of nociceptive and somatosensory signals from the periphery. GABA(A)Rs are heterogeneous and distinguished functionally and pharmacologically by the type of α subunit variant they contain. This heterogeneity raises the possibility that GABA(A)R subtypes differentially regulate specific pain modalities. Here, we characterized the subcellular distribution of GABA(A)R subtypes in nociceptive circuits by using immunohistochemistry with subunit-specific antibodies combined with markers of primary afferents and dorsal horn neurons. Confocal laser scanning microscopy analysis revealed a distinct, partially overlapping laminar distribution of α1-3 and α5 subunit immunoreactivity in laminae I-V. Likewise, a layer-specific pattern was evident for their distribution among glutamatergic, γ-aminobutyric acid (GABA)ergic, and glycinergic neurons (detected in transgenic mice expressing vesicular glutamate transporter 2-enhanced green fluorescent protein [vGluT2-eGFP], glutamic acid decarboxylase [GAD]67-eGFP, and glycine transporter 2 (GlyT2)-eGFP, respectively). Finally, all four subunits could be detected within primary afferent terminals. C-fibers predominantly contained either α2 or α3 subunit immunoreactivity; terminals from myelinated (Aβ/Aδ) fibers were colabeled in roughly equal proportion with each subunit. The presence of axoaxonic GABAergic synapses was determined by costaining with gephyrin and vesicular inhibitory amino acid transporter to label GABAergic postsynaptic densities and terminals, respectively. Colocalization of the α2 or α3 subunit with these markers was observed in a subset of C-fiber synapses. Furthermore, gephyrin mRNA and protein expression was detected in dorsal root ganglia. Collectively, these results show that differential GABA(A)R distribution in primary afferent terminals and dorsal horn neurons allows for multiple, circuit-specific modes of regulation of nociceptive circuits.

Interoception across modalities: on the relationship between cardiac awareness and the sensitivity for gastric functions

The individual sensitivity for ones internal bodily signals ("interoceptive awareness") has been shown to be of relevance for a broad range of cognitive and affective functions. Interoceptive awareness has been primarily assessed via measuring the sensitivity for ones cardiac signals ("cardiac awareness") which can be non-invasively measured by heartbeat perception tasks. It is an open question whether cardiac awareness is related to the sensitivity for other bodily, visceral functions. This study investigated the relationship between cardiac awareness and the sensitivity for gastric functions in healthy female persons by using non-invasive methods. Heartbeat perception as a measure for cardiac awareness was assessed by a heartbeat tracking task and gastric sensitivity was assessed by a water load test. Gastric myoelectrical activity was measured by electrogastrography (EGG) and subjective feelings of fullness, valence, arousal and nausea were assessed. The results show that cardiac awareness was inversely correlated with ingested water volume and with normogastric activity after water load. However, persons with good and poor cardiac awareness did not differ in their subjective ratings of fullness, nausea and affective feelings after drinking. This suggests that good heartbeat perceivers ingested less water because they subjectively felt more intense signals of fullness during this lower amount of water intake compared to poor heartbeat perceivers who ingested more water until feeling the same signs of fullness. These findings demonstrate that cardiac awareness is related to greater sensitivity for gastric functions, suggesting that there is a general sensitivity for interoceptive processes across the gastric and cardiac modality.

Central circuitries for body temperature regulation and fever

Body temperature regulation is a fundamental homeostatic function that is governed by the central nervous system in homeothermic animals, including humans. The central thermoregulatory system also functions for host defense from invading pathogens by elevating body core temperature, a response known as fever. Thermoregulation and fever involve a variety of involuntary effector responses, and this review summarizes the current understandings of the central circuitry mechanisms that underlie nonshivering thermogenesis in brown adipose tissue, shivering thermogenesis in skeletal muscles, thermoregulatory cardiac regulation, heat-loss regulation through cutaneous vasomotion, and ACTH release. To defend thermal homeostasis from environmental thermal challenges, feedforward thermosensory information on environmental temperature sensed by skin thermoreceptors ascends through the spinal cord and lateral parabrachial nucleus to the preoptic area (POA). The POA also receives feedback signals from local thermosensitive neurons, as well as pyrogenic signals of prostaglandin E(2) produced in response to infection. These afferent signals are integrated and affect the activity of GABAergic inhibitory projection neurons descending from the POA to the dorsomedial hypothalamus (DMH) or to the rostral medullary raphe region (rMR). Attenuation of the descending inhibition by cooling or pyrogenic signals leads to disinhibition of thermogenic neurons in the DMH and sympathetic and somatic premotor neurons in the rMR, which then drive spinal motor output mechanisms to elicit thermogenesis, tachycardia, and cutaneous vasoconstriction. Warming signals enhance the descending inhibition from the POA to inhibit the motor outputs, resulting in cutaneous vasodilation and inhibited thermogenesis. This central thermoregulatory mechanism also functions for metabolic regulation and stress-induced hyperthermia.

Preoptic-raphé connections for thermoregulatory vasomotor control

Blood flow to glabrous skin such as the rat's tail determines heat dissipation from the body and is regulated by sympathetic vasoconstrictor nerves. Tail vasoconstrictor activity is tonically inhibited by neurons in two distinct preoptic regions, rostromedial (RMPO) and caudolateral (CLPO) regions, whose actions may be via direct projections to medullary raphé premotor neurons. In urethane-anesthetized rats, we sought single preoptic neurons that were antidromically activated from the medullary raphé and could subserve this function. Nine of 45 raphé-projecting preoptic neurons, predominantly in the CLPO, showed spontaneous activity under warm conditions and were inhibited by cooling the trunk skin (warm-responsive). Unexpectedly, 14 raphé-projecting preoptic neurons (mostly in the RMPO) were activated by skin cooling (cold-responsive), suggesting that an excitatory pathway from this region could contribute to tail vasoconstriction. Supporting this, neuronal disinhibition in the RMPO by microinjecting the GABA(A) receptor antagonist bicuculline (0.5 mm, 15 nl) caused a rapid increase in tail sympathetic nerve activity (SNA). Similar injections into the CLPO were without effect. Electrical stimulation of the RMPO also activated tail SNA, with a latency ∼25 ms longer than to stimulation of the medullary raphé. Injection of the glutamate receptor antagonist kynurenate (50 mm, 120 nl) into the medullary raphé suppressed tail SNA responses to both RMPO bicuculline and skin cooling. These findings suggest that both inhibitory and excitatory descending drives regulate tail vasoconstriction in the cold and that warm- and cold-responsive raphé-projecting preoptic neurons may mediate these actions.

2010 Carl Ludwig Distinguished Lectureship of the APS Neural Control and Autonomic Regulation Section: Central neural pathways for thermoregulatory cold defense

Central neural circuits orchestrate the homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the research leading to a model representing our current understanding of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for control of heat loss, and brown adipose tissue, skeletal muscle, and the heart for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific core efferent pathways within the central nervous system (CNS) that share a common peripheral thermal sensory input. The thermal afferent circuit from cutaneous thermal receptors includes neurons in the spinal dorsal horn projecting to lateral parabrachial nucleus neurons that project to the medial aspect of the preoptic area. Within the preoptic area, warm-sensitive, inhibitory output neurons control heat production by reducing the discharge of thermogenesis-promoting neurons in the dorsomedial hypothalamus. The rostral ventromedial medulla, including the raphe pallidus, receives projections form the dorsomedial hypothalamus and contains spinally projecting premotor neurons that provide the excitatory drive to spinal circuits controlling the activity of thermogenic effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a platform for further understanding of the functional organization of central thermoregulation.

Central neural pathways for thermoregulation

Central neural circuits orchestrate a homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the functional organization of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for heat loss, the brown adipose tissue, skeletal muscle and heart for thermogenesis and species-dependent mechanisms (sweating, panting and saliva spreading) for evaporative heat loss. These effectors are regulated by parallel but distinct, effector-specific neural pathways that share a common peripheral thermal sensory input. The thermal afferent circuits include cutaneous thermal receptors, spinal dorsal horn neurons and lateral parabrachial nucleus neurons projecting to the preoptic area to influence warm-sensitive, inhibitory output neurons which control thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus neurons controlling cutaneous vasoconstriction.

Threshold and rate sensitivity of low-threshold thermal nociception

Previous studies have shown that sensations of burning, stinging or pricking can be evoked by warming or cooling the skin to innocuous temperatures [low-threshold thermal nociception (LTN)] below the thresholds of cold- and heat-sensitive nociceptors. LTN implies that some primary afferent fibers classically defined as warm and cold fibers relay stimulation to the nociceptive system. We addressed this question in humans by determining if different adaptation temperatures (ATs) and rates of temperature change would affect thermal sensation and LTN similarly. In Experiment 1 subjects rated the intensity of warmth, cold and nociceptive sensations produced by increasing steps in temperature (+/-0.5 degrees C increments) from ATs of 35, 33 and 31 degrees C for cooling, and 30, 32 and 34 degrees C for heating. Depending upon the AT, thresholds for nociceptive and thermal sensations estimated from the rating data differed by as little as -1.0 degrees C for cooling and +1.5 degrees C for heating. Thresholds of thermal and nociceptive sensations shifted by similar amounts across the three ATs during cooling, whereas during heating the nociceptive threshold was significantly affected only between ATs of 32 and 34 degrees C. In Experiment 2, increasing the rate of temperature change from 0.5 to 4.0 degrees C/s increased the intensity of thermal and nociceptive sensations significantly but the effect was greatest for nociceptive sensations during heating. The results of both experiments are consistent with the mediation of LTN by low-threshold thermoreceptors, although LTN caused by heating may depend on a subset of fibers that express less sensitive TRP channels than those that serve sensations of warmth at the mildest temperatures.

The embodiment of emotional feelings in the brain

Central to Walter Cannon's challenge to peripheral theories of emotion was that bodily arousal responses are too undifferentiated to account for the wealth of emotional feelings. Despite considerable evidence to the contrary, this remains widely accepted and for nearly a century has left the issue of whether visceral afferent signals are essential for emotional experience unresolved. Here we combine functional magnetic resonance imaging and multiorgan physiological recording to dissect experience of two distinct disgust forms and their relationship to peripheral and central physiological activity. We show that experience of core and body-boundary-violation disgust are dissociable in both peripheral autonomic and central neural responses and also that emotional experience specific to anterior insular activity encodes these different underlying patterns of peripheral physiological responses. These findings demonstrate that organ-specific physiological responses differentiate emotional feeling states and support the hypothesis that central representations of organism physiological homeostasis constitute a critical aspect of the neural basis of feelings.

Review of therapeutic agents for burns pruritus and protocols for management in adult and paediatric patients using the GRADE classification

To review the current evidence on therapeutic agents for burns pruritus and use the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) classification to propose therapeutic protocols for adult and paediatric patients. All published interventions for burns pruritus were analysed by a multidisciplinary panel of burns specialists following the GRADE classification to rate individual agents. Following the collation of results and panel discussion, consensus protocols are presented. Twenty-three studies appraising therapeutic agents in the burns literature were identified. The majority of these studies (16 out of 23) are of an observational nature, making an evidence-based approach to defining optimal therapy not feasible. Our multidisciplinary approach employing the GRADE classification recommends the use of antihistamines (cetirizine and cimetidine) and gabapentin as the first-line pharmacological agents for both adult and paediatric patients. Ondansetron and loratadine are the second-line medications in our protocols. We additionally recommend a variety of non-pharmacological adjuncts for the perusal of clinicians in order to maximise symptomatic relief in patients troubled with postburn itch. Most studies in the subject area lack sufficient statistical power to dictate a 'gold standard' treatment agent for burns itch. We encourage clinicians to employ the GRADE system in order to delineate the most appropriate therapeutic approach for burns pruritus until further research elucidates the most efficacious interventions. This widely adopted classification empowers burns clinicians to tailor therapeutic regimens according to current evidence, patient values, risks and resource considerations in different medical environments.

A thermosensory pathway mediating heat-defense responses

Afferent neural transmission of temperature sensation from skin thermoreceptors to the central thermoregulatory system is important for the defense of body temperature against environmental thermal challenges. Here, we report a thermosensory pathway that triggers physiological heat-defense responses to elevated environmental temperature. Using in vivo electrophysiological and anatomical approaches in the rat, we found that neurons in the dorsal part of the lateral parabrachial nucleus (LPBd) glutamatergically transmit cutaneous warm signals from spinal somatosensory neurons directly to the thermoregulatory command center, the preoptic area (POA). Intriguingly, these LPBd neurons are located adjacent to another group of neurons that mediate cutaneous cool signaling to the POA. Functional experiments revealed that this LPBd-POA warm sensory pathway is required to elicit autonomic heat-defense responses, such as cutaneous vasodilation, to skin-warming challenges. These findings provide a fundamental framework for understanding the neural circuitry maintaining thermal homeostasis, which is critical to survive severe environmental temperatures.