Thermoregulation: some concepts have changed. Functional architecture of the thermoregulatory system

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

Invited review: Influence of climatic conditions on the development, performance, and health of calves

The objective of this review is to provide the reader with an overview of thermoregulatory mechanisms and the influence of climatic conditions in different housing systems on the development, performance, and health of calves. Thermic stress is observed in association with extreme temperatures and large temperature variations, but other variables such as relative humidity and wind speed can also contribute to thermic stress. Thermoregulation in calves is similar to that in adult cattle, but especially dystocial calves are more prone to heat loss. Heat or cold stress results in direct economic losses because of increased calf mortality and morbidity, as well as indirect costs caused by reduced weight gain, performance, and long-term survival. The climatic conditions in a variety of housing systems, associated health problems, and strategies to mitigate thermic stress are discussed in this review. The goal of housing is to alleviate the effect of climate on calves and provide a microclimate. Adequate ventilation with fresh air is essential to reduce respiratory disease. Common practices such as raising calves in individual outdoor enclosures have been challenged lately. Recent research seeks to evaluate the suitability of group housing under practical, economic, and animal welfare considerations. Limited results for reducing thermic stress can be achieved by simple measures such as shades or shelter, but additional heat or cold stress relieving strategies can be required depending on the housing system.

The influence of light on thermal responses

Light is essential for vision and plays an important role in non-visual responses, thus affecting alertness, mood and circadian rhythms. Furthermore, light influences physiological processes, such as thermoregulation, and therefore may be expected to play a role in thermal comfort (TC) as well. A systematic literature search was performed for human studies exploring the relation between ocular light exposure, thermophysiology and TC. Experimental results show that light in the evening can reduce melatonin secretion, delay the natural decline in core body temperature (CBT) and slow down the increase in distal skin temperature. In the morning though, bright light can result in a faster decline in melatonin levels, thus enabling a faster increase in CBT. Moreover, the colour of light can affect temperature perception of the environment. Light with colour tones towards the red end of the visual spectrum leads to a warmer perception compared to more bluish light tones. It should be noted, however, that many results of light on thermal responses are inconclusive, and a theoretical framework is largely lacking. In conclusion, light is capable of evoking thermophysiological responses and visual input can alter perception of the thermal environment. Therefore, lighting conditions should be taken into consideration during thermophysiological research and in the design of indoor climates.

Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports

Exercise heat acclimation induces physiological adaptations that improve thermoregulation, attenuate physiological strain, reduce the risk of serious heat illness, and improve aerobic performance in warm-hot environments and potentially in temperate environments. The adaptations include improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain, improved fluid balance, altered metabolism, and enhanced cellular protection. The magnitudes of adaptations are determined by the intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (i.e., dry or humid heat). Evidence is emerging that controlled hyperthermia regimens where a target core temperature is maintained, enable more rapid and complete adaptations relative to the traditional constant work rate exercise heat acclimation regimens. Furthermore, inducing heat acclimation outdoors in a natural field setting may provide more specific adaptations based on direct exposure to the exact environmental and exercise conditions to be encountered during competition. This review initially examines the physiological adaptations associated with heat acclimation induction regimens, and subsequently emphasizes their application to competitive athletes and sports.

Heat stroke

Heat stroke is a life-threatening condition clinically diagnosed as a severe elevation in body temperature with central nervous system dysfunction that often includes combativeness, delirium, seizures, and coma. Classic heat stroke primarily occurs in immunocompromised individuals during annual heat waves. Exertional heat stroke is observed in young fit individuals performing strenuous physical activity in hot or temperature environments. Long-term consequences of heat stroke are thought to be due to a systemic inflammatory response syndrome. This article provides a comprehensive review of recent advances in the identification of risk factors that predispose to heat stroke, the role of endotoxin and cytokines in mediation of multi-organ damage, the incidence of hypothermia and fever during heat stroke recovery, clinical biomarkers of organ damage severity, and protective cooling strategies. Risk factors include environmental factors, medications, drug use, compromised health status, and genetic conditions. The role of endotoxin and cytokines is discussed in the framework of research conducted over 30 years ago that requires reassessment to more clearly identify the role of these factors in the systemic inflammatory response syndrome. We challenge the notion that hypothalamic damage is responsible for thermoregulatory disturbances during heat stroke recovery and highlight recent advances in our understanding of the regulated nature of these responses. The need for more sensitive clinical biomarkers of organ damage is examined. Conventional and emerging cooling methods are discussed with reference to protection against peripheral organ damage and selective brain cooling.

TRP channels: a missing bond in the entrainment mechanism of peripheral clocks throughout evolution

Circadian rhythm may be understood as a temporal organization that works to orchestrate physiological processes and behavior in a period of approximately 24 h. Because such temporal organization has evolved in the presence of predictable environmental clues, such as day length, tides, seasons, and temperature, the organism has confronted the natural selection in highly precise intervals of opportunities and risks, generating temporal programs and resetting mechanisms, which are well conserved among different taxa of animals. The present review brings some evidence of how these programs may have co-evolved in systems able to deal with 2 or more environmental clues, and how they similarly function in different group of animals, stressing how important temperature and light were to establish the temporal organizations. For example, melanopsin and rhodopsin, photopigments present respectively in circadian and visual photoreceptors, are required for temperature discrimination in Drosophila melanogaster. These pigments may signal light and temperature via activation of cationic membrane channel, named transient-receptor potential channel (TRP). In fact, TRPs have been suggested to function as thermal sensor for various groups of animals. Another example is the clock machinery at the molecular level. A set of very-well conserved proteins, known as clock proteins, function as transcription factors in positive and negative auto-regulatory loops generating circadian changes of their expression, and of clock-controlled genes. Similar molecular machinery is present in organisms as diverse as cyanobacteria (Synechococcus), fungi (Neurospora), insects (Drosophila), and vertebrates including humans.

The experienced temperature sensitivity and regulation survey

Individuals differ in thermosensitivity, thermoregulation, and zones of thermoneutrality and thermal comfort. Whereas temperature sensing and -effectuating processes occur in part unconsciously and autonomic, awareness of temperature and thermal preferences can affect thermoregulatory behavior as well. Quantification of trait-like individual differences of thermal preferences and experienced temperature sensitivity and regulation is therefore relevant to obtain a complete understanding of human thermophysiology. Whereas several scales have been developed to assess instantaneous appreciation of heat and cold exposure, a comprehensive scale dedicated to assess subjectively experienced autonomic or behavioral thermoregulatory activity has been lacking so far. We constructed a survey that specifically approaches these domains from a trait-like perspective, sampled 240 volunteers across a wide age range, and analyzed the emergent component structure. Participants were asked to report their thermal experiences, captured in 102 questions, on a 7-point bi-directional Likert scale. In a second set of 32 questions, participants were asked to indicate the relative strength of experiences across different body locations. Principal component analyses extracted 21 meaningful dimensions, which were sensitive to sex-differences and age-related changes. The questions were also assessed in a matched sample of 240 people with probable insomnia to evaluate the sensitivity of these dimensions to detect group differences in a case-control design. The dimensions showed marked mean differences between cases and controls. The survey thus has discriminatory value. It can freely be used by anyone interested in studying individual or group differences in thermosensitivity and thermoregulation.

The rostral ventromedial medulla control of cutaneous vasomotion of paws and tail in the rat: implication for pain studies

Thermal neutrality in rodents is achieved by large cyclic variations of the sympathetic drive of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Given the pivotal functional role of rostral ventromedial medulla (RVM) in nociception and rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, we aimed at circumscribing the brainstem regions that are the source of premotor afferents to sympathetic preganglionic neurons that control the vasomotor tone of the tail and hind paws. A thermometric infrared camera recorded indirectly the vasomotor tone of the tail and hind paws. During the control period, the rat was maintained in vasoconstriction by preserving a stable, homogeneous, and constant surrounding temperature, slightly below the core temperature. The functional blockade of the RVM/rMR by the GABAA receptor agonist muscimol (0.5 nmol, 50 nl) elicited an extensive increase of the temperature of the paws and tail, associated with a slight decrease of blood pressure and heart rate. Both the increased heat loss through vasodilatation and the decrease heart-induced heat production elicited a remarkable reduction of the central temperature. The effective zones were circumscribed to the parts of the RVM/rMR facing the facial nucleus. They match very exactly the brain regions often described as specifically devoted to the control of nociception. Our data support and urge on the highest cautiousness regarding the interpretation of results aimed at studying the effects of any pharmacological manipulations of RVM/rMR with the usual tests of pain.

Ondansetron and promethazine have differential effects on hypothermic responses to lithium chloride administration and to provocative motion in rats

We recently reported that provocative motion (rotation in a home cage) causes hypothermic responses in rats, similar to the hypothermic responses associated with motion sickness in humans. Many stimuli inducing emesis in species with an emetic reflex also provoke hypothermia in the rat, therefore we hypothesized that a fall in body temperature may reflect a “nausea-like” state in these animals. As rats do not possess an emetic reflex, we employed a pharmacological approach to test this hypothesis. In humans, motion- and chemically-induced nausea have differential sensitivity to anti-emetics. We thus tested whether the hypothermia induced in rats by provocative motion (rotation at 0.7 Hz) and by the emetic LiCl (63 mg/kg i.p.) have a similar differential pharmacological sensitivity. Both provocations caused a comparable robust fall in core body temperature (−1.9 ± 0.3°C and −2.0 ± 0.2°C for chemical and motion provocations, respectively). LiCl⁻induced hypothermia was completely prevented by ondansetron (2mg/kg, i.p., a 5-HT₃ receptor antagonist that reduces cancer chemotherapy-induced nausea and vomiting), but was insensitive to promethazine (10 mg/kg, i.p., a predominantly histamine-H₁ and muscarinic receptor antagonist that is commonly used to treat motion sickness). Conversely, motion-induced hypothermia was unaffected by ondansetron but promethazine reduced the rate of temperature decline from 0.20 ± 0.02 to 0.11 ± 0.03°C/min (P < 0.05) with a trend to decrease the magnitude. We conclude that this differential pharmacological sensitivity of the hypothermic responses of vestibular vs. chemical etiology in rats mirrors the observations in other pre-clinical models and humans, and thus supports the idea that a “nausea-like” state in rodents is associated with disturbances in thermoregulation.

Current understanding on the neurophysiology of behavioral thermoregulation

Temperature influence on the physiology and biochemistry of living organisms has long been recognized, which propels research in the field of thermoregulation. With the cloning and characterization of the transient receptor potential (TRP) ion channels as the principal temperature sensors of the mammalian somatosensory neurons, the understanding, at a molecular level, of thermosensory and thermoregulatory mechanisms became promising. Because thermal environment can be extremely hostile (temperature range on earth's surface is from ∼ −69°C to 58°C), living organisms developed an array of thermoregulatory strategies to guarantee survival, which include both autonomic mechanisms, which aim at increasing or decreasing heat exchange between body, and ambient and behavioral strategies. The knowledge regarding neural mechanisms involved in autonomic thermoregulatory strategies has progressed immensely compared to the knowledge on behavioral thermoregulation. This review aims at collecting the up-to-date knowledge on the neural basis for behavioral thermoregulation in mammals in order to point out perspectives and deployment of this research field.

Nonthermal sensory input and altered human thermoregulation: effects of visual information depicting hot or cold environments

A recent study showed that thermoregulatory-like cardiovascular responses can be invoked simply by exposure to visual information, even though the thermal environments are neutral and unchanged. However, it was not clear how such responses affect actual human body temperature regulation. We investigated whether such visually invoked physiological responses can substantively affect human core body temperature in a thermally challenging cold environment. Participants comprised 13 graduate or undergraduate students viewing different video images containing hot, cold, or no scenery, while room temperature was gradually lowered from 28 to 16 °C over 80 min. Rectal temperature, mean skin temperature, core to skin temperature gradient, and oxygen consumption were measured during the experiment. Rectal temperature was significantly lower when hot video images were presented compared to when control video images were presented. Oxygen consumption was comparable among all video images, but core to skin temperature gradient was significantly lower when hot video images were presented. This result suggests that visual information, even in the absence of thermal energy, can affect human thermodynamics and core body temperature.

Heat Transfer in Health and Healing

Our bodies depend on an exquisitely sensitive and refined temperature control system to maintain a state of health and homeostasis. The exceptionally broad range of physical activities that humans engage in and the diverse array of environmental conditions we face require remarkable strategies and mechanisms for regulating internal and external heat transfer processes. On the occasions for which the body suffers trauma, therapeutic temperature modulation is often the approach of choice for reversing injury and inflammation and launching a cascade of healing. The focus of human thermoregulation is maintenance of the body core temperature within a tight range of values, even as internal rates of energy generation may vary over an order of magnitude, environmental convection, and radiation heat loads may undergo large changes in the absence of any significant personal control, surface insulation may be added or removed, all occurring while the body's internal thermostat follows a diurnal circadian cycle that may be altered by illness and anesthetic agents. An advanced level of understanding of the complex physiological function and control of the human body may be combined with skill in heat transfer analysis and design to develop life-saving and injury-healing medical devices. This paper will describe some of the challenges and conquests the author has experienced related to the practice of heat transfer for maintenance of health and enhancement of healing processes.

The absorption and metabolism of a single L-menthol oral versus skin administration: Effects on thermogenesis and metabolic rate

We investigated the absorption and metabolism pharmacokinetics of a single L-menthol oral versus skin administration and the effects on human thermogenesis and metabolic rate. Twenty healthy adults were randomly distributed into oral (capsule) and skin (gel) groups and treated with 10 mg kg(-1) L-menthol (ORALMENT; SKINMENT) or control (lactose capsule: ORALCON; water application: SKINCON) in a random order on two different days. Levels of serum L-menthol increased similarly in ORALMENT and SKINMENT (p > 0.05). L-menthol glucuronidation was greater in ORALMENT than SKINMENT (p < 0.05). Cutaneous vasoconstriction, rectal temperature and body heat storage showed greater increase following SKINMENT compared to ORALMENT and control conditions (p < 0.05). Metabolic rate increased from baseline by 18% in SKINMENT and 10% in ORALMENT and respiratory exchange ratio decreased more in ORALMENT (5.4%) than SKINMENT (4.8%) compared to control conditions (p < 0.05). Levels of plasma adiponectin and leptin as well as heart rate variability were similar to control following either treatment (p > 0.05). Participants reported no cold, shivering, discomfort, stress or skin irritation. We conclude that a single L-menthol skin administration increased thermogenesis and metabolic rate in humans. These effects are minor following L-menthol oral administration probably due to faster glucuronidation and greater blood menthol glucuronide levels.

Recent advances in thermoregulation

Thermoregulation is the maintenance of a relatively constant core body temperature. Humans normally maintain a body temperature at 37°C, and maintenance of this relatively high temperature is critical to human survival. This concept is so important that control of thermoregulation is often the principal example cited when teaching physiological homeostasis. A basic understanding of the processes underpinning temperature regulation is necessary for all undergraduate students studying biology and biology-related disciplines, and a thorough understanding is necessary for those students in clinical training. Our aim in this review is to broadly present the thermoregulatory process taking into account current advances in this area. First, we summarize the basic concepts of thermoregulation and subsequently assess the physiological responses to heat and cold stress, including vasodilation and vasoconstriction, sweating, nonshivering thermogenesis, piloerection, shivering, and altered behavior. Current research is presented concerning the body's detection of thermal challenge, peripheral and central thermoregulatory control mechanisms, including brown adipose tissue in adult humans and temperature transduction by the relatively recently discovered transient receptor potential channels. Finally, we present an updated understanding of the neuroanatomic circuitry supporting thermoregulation.

Central nervous system regulation of brown adipose tissue

Thermogenesis, the production of heat energy, in brown adipose tissue is a significant component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature in many species from mouse to man and plays a key role in elevating body temperature during the febrile response to infection. The sympathetic neural outflow determining brown adipose tissue (BAT) thermogenesis is regulated by neural networks in the CNS which increase BAT sympathetic nerve activity in response to cutaneous and deep body thermoreceptor signals. Many behavioral states, including wakefulness, immunologic responses, and stress, are characterized by elevations in core body temperature to which central command-driven BAT activation makes a significant contribution. Since energy consumption during BAT thermogenesis involves oxidation of lipid and glucose fuel molecules, the CNS network driving cold-defensive and behavioral state-related BAT activation is strongly influenced by signals reflecting the short- and long-term availability of the fuel molecules essential for BAT metabolism and, in turn, the regulation of BAT thermogenesis in response to metabolic signals can contribute to energy balance, regulation of body adipose stores and glucose utilization. This review summarizes our understanding of the functional organization and neurochemical influences within the CNS networks that modulate the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolic alterations in BAT thermogenesis and BAT energy expenditure that contribute to overall energy homeostasis and the autonomic support of behavior.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Maintaining thermogenesis in cold exposed humans: relying on multiple metabolic pathways

In cold exposed humans, increasing thermogenic rate is essential to prevent decreases in core temperature. This review describes the metabolic requirements of thermogenic pathways, mainly shivering thermogenesis, the largest contributor of heat. Research has shown that thermogenesis is sustained from a combination of carbohydrates, lipids, and proteins. The mixture of fuels is influenced by shivering intensity and pattern as well as by modifications in energy reserves and nutritional status. To date, there are no indications that differences in the types of fuel being used can alter shivering and overall heat production. We also bring forth the potential contribution of nonshivering thermogenesis in adult humans via the activation of brown adipose tissue (BAT) and explore some means to stimulate the activity of this highly thermogenic tissue. Clearly, the potential role of BAT, especially in young lean adults, can no longer be ignored. However, much work remains to clearly identify the quantitative nature of this tissue's contribution to total thermogenic rate and influence on shivering thermogenesis. Identifying ways to potentiate the effects of BAT via cold acclimation and/or the ingestion of compounds that stimulate the thermogenic process may have important implications in cold endurance and survival.

Environmental Hyperthermia in Prehospital Patients with Major Traumatic Brain Injury

BACKGROUND

Traumatic brain injury (TBI) results in an estimated 1.7 million emergency department visits each year in the United States. These injuries frequently occur outside, leaving injured individuals exposed to environmental temperature extremes before they are transported to a hospital.

OBJECTIVE

Evaluate the existing literature for evidence that exposure to high temperatures immediately after TBI could result in elevated body temperatures (EBTs), and whether or not EBTs affect patient outcomes.

DISCUSSION

It has been clear since the early 1980s that after brain injury, exposure to environmental temperatures can cause hypothermia, and that this represents a significant contributor to increased morbidity and mortality. Less is known about elevated body temperature. Early evidence from the Iraq and Afghanistan wars indicated that exposure to elevated environmental temperatures in the prehospital setting may result in significant EBTs, however, it is unclear what impact these EBTs might have on outcomes in TBI patients. In the hospital, EBT, or neurogenic fever, is thought to be due to the acute-phase reaction that follows critical injury, and these high body temperatures are associated with poor outcomes after TBI.

CONCLUSION

Hospital data suggest that EBTs are associated with poor outcomes, and some preliminary reports suggest that early EBTs are common after TBI in the prehospital setting. However, it remains unclear whether patients with TBI have an increased risk of EBTs after exposure to high environmental temperatures, or if this very early "hyperthermia" might cause secondary injury after TBI.

How genetically engineered systems are helping to define, and in some cases redefine, the neurobiological basis of sleep and wake

The advent of genetically engineered systems, including transgenic animals and recombinant viral vectors, has facilitated a more detailed understanding of the molecular and cellular substrates regulating brain function. In this review we highlight some of the most recent molecular biology and genetic technologies in the experimental "systems neurosciences," many of which are rapidly becoming a methodological standard, and focus in particular on those tools and techniques that permit the reversible and cell-type specific manipulation of neurons in behaving animals. These newer techniques encompass a wide range of approaches including conditional deletion of genes based on Cre/loxP technology, gene silencing using RNA interference, cell-type specific mapping or ablation and reversible manipulation (silencing and activation) of neurons in vivo. Combining these approaches with viral vector delivery systems, in particular adeno-associated viruses (AAV), has extended, in some instances greatly, the utility of these tools. For example, the spatially- and/or temporally-restricted transduction of specific neuronal cell populations is now routinely achieved using the combination of Cre-driver mice and stereotaxic-based delivery of AAV expressing Cre-dependent cassettes. We predict that the experimental application of these tools, including creative combinatorial approaches and the development of even newer reagents, will prove necessary for a complete understanding of the neuronal circuits subserving most neurobiological functions, including the regulation of sleep and wake.

Central Fibroblast Growth Factor 21 Browns White Fat via Sympathetic Action in Male Mice

Fibroblast growth factor 21 (FGF21) has multiple metabolic actions, including the induction of browning in white adipose tissue. Although FGF21 stimulated browning results from a direct interaction between FGF21 and the adipocyte, browning is typically associated with activation of the sympathetic nervous system through cold exposure. We tested the hypothesis that FGF21 can act via the brain, to increase sympathetic activity and induce browning, independent of cell-autonomous actions. We administered FGF21 into the central nervous system via lateral ventricle infusion into male mice and found that the central treatment increased norepinephrine turnover in target tissues that include the inguinal white adipose tissue and brown adipose tissue. Central FGF21 stimulated browning as assessed by histology, expression of uncoupling protein 1, and the induction of gene expression associated with browning. These effects were markedly attenuated when mice were treated with a β-blocker. Additionally, neither centrally nor peripherally administered FGF21 initiated browning in mice lacking β-adrenoceptors, demonstrating that an intact adrenergic system is necessary for FGF21 action. These data indicate that FGF21 can signal in the brain to activate the sympathetic nervous system and induce adipose tissue thermogenesis.

Control of cutaneous blood flow by central nervous system

Hairless skin acts as a heat exchanger between body and environment, and thus greatly contributes to body temperature regulation by changing blood flow to the skin (cutaneous) vascular bed during physiological responses such as cold- or warm-defense and fever. Cutaneous blood flow is also affected by alerting state; we 'go pale with fright'. The rabbit ear pinna and the rat tail have hairless skin, and thus provide animal models for investigating central pathway regulating blood flow to cutaneous vascular beds. Cutaneous blood flow is controlled by the centrally regulated sympathetic nervous system. Sympathetic premotor neurons in the medullary raphé in the lower brain stem are labeled at early stage after injection of trans-synaptic viral tracer into skin wall of the rat tail. Inactivation of these neurons abolishes cutaneous vasomotor changes evoked as part of thermoregulatory, febrile or psychological responses, indicating that the medullary raphé is a common final pathway to cutaneous sympathetic outflow, receiving neural inputs from upstream nuclei such as the preoptic area, hypothalamic nuclei and the midbrain. Summarizing evidences from rats and rabbits studies in the last 2 decades, we will review our current understanding of the central pathways mediating cutaneous vasomotor control.

Effect of capsaicin on thermoregulation: an update with new aspects

Capsaicin, a selective activator of the chemo- and heat-sensitive transient receptor potential (TRP) V1 cation channel, has characteristic feature of causing long-term functional and structural impairment of neural elements supplied by TRPV1/capsaicin receptor. In mammals, systemic application of capsaicin induces complex heat-loss response characteristic for each species and avoidance of warm environment. Capsaicin activates cutaneous warm receptors and polymodal nociceptors but has no effect on cold receptors or mechanoreceptors. In this review, thermoregulatory features of capsaicin-pretreated rodents and TRPV1-mediated neural elements with innocuous heat sensitivity are summarized. Recent data support a novel hypothesis for the role of visceral warmth sensors in monitoring core body temperature. Furthermore, strong evidence suggests that central presynaptic nerve terminals of TRPV1-expressing cutaneous, thoracic and abdominal visceral receptors are activated by innocuous warmth stimuli and capsaicin. These responses are absent in TRPV1 knockout mice. Thermoregulatory disturbance induced by systemic capsaicin pretreatment lasts for months and is characterized by a normal body temperature at cool environment up to a total dose of 150 mg/kg s.c. Upward differential shift of set points for activation vasodilation, other heat-loss effectors and thermopreference develops. Avoidance of warm ambient temperature (35°C, 40°C) is severely impaired but thermopreference at cool ambient temperatures (Tas) are not altered. TRPV1 knockout or knockdown and genetically altered TRPV1, TRPV2 and TRPM8 knockout mice have normal core temperature in thermoneutral or cool environments, but the combined mutant mice have impaired regulation in warm or cold (4°C) environments. Several lines of evidence support that in the preoptic area warmth sensitive neurons are activated and desensitized by capsaicin, but morphological evidence for it is controversial. It is suggested that these neurons have also integrator function. Fever is enhanced in capsaicin-desensitized rats and the inhibition observed after pretreatment with low i.p. doses does not support in the light of their warmth sensitivity the concept that abdominal TRPV1-expressing nerve terminals serve as nonthermal chemosensors for reference signals in thermoregulation.

TRPV4 activates autonomic and behavioural warmth-defence responses in Wistar rats

AIM

In this study, we aimed at investigating the involvement of the warmth-sensitive channel - TRPV4 (in vitro sensitive to temperatures in the range of approx. 24-34 °C) - on the thermoregulatory mechanisms in rats.

METHODS

We treated rats with a chemical selective agonist (RN-1747) and two antagonists (RN-1734 and HC-067047) of the TRPV4 channel and measured core body temperature, metabolism, heat loss index and preferred ambient temperature.

RESULTS

Our data revealed that chemical activation of TRPV4 channels by topical application of RN-1747 on the skin leads to hypothermia and this effect was blocked by the pre-treatment with the selective antagonist of this channel. Intracerebroventricular treatment with RN-1747 did not cause hypothermia, indicating that the observed response was indeed due to activation of TRPV4 channels in the periphery. Intravenous blockade of this channel with HC-067047 caused an increase in core body temperature at ambient temperature of 26 and 30 °C, but not at 22 and 32 °C. At 26 °C, HC-067047-induced hyperthermia was accompanied by increase in oxygen consumption (an index of thermogenesis), while chemical stimulation of TRPV4 increased tail heat loss, indicating that these two autonomic thermoeffectors in the rat are modulated through TRPV4 channels. Furthermore, rats chemically stimulated with TRPV4 agonist choose colder ambient temperatures and cold-seeking behaviour after thermal stimulation (28-31 °C) was inhibited by TRPV4 antagonist.

CONCLUSION

Our results suggest, for the first time, that TRPV4 channel is involved in the recruitment of behavioural and autonomic warmth-defence responses to regulate core body temperature.

Fever and the thermal regulation of immunity: the immune system feels the heat

Fever is a cardinal response to infection that has been conserved in warm-blooded and cold-blooded vertebrates for more than 600 million years of evolution. The fever response is executed by integrated physiological and neuronal circuitry and confers a survival benefit during infection. In this Review, we discuss our current understanding of how the inflammatory cues delivered by the thermal element of fever stimulate innate and adaptive immune responses. We further highlight the unexpected multiplicity of roles of the pyrogenic cytokine interleukin-6 (IL-6), both during fever induction and during the mobilization of lymphocytes to the lymphoid organs that are the staging ground for immune defence. We also discuss the emerging evidence suggesting that the adrenergic signalling pathways associated with thermogenesis shape immune cell function.

Integration of body temperature into the analysis of energy expenditure in the mouse

OBJECTIVES

We quantified the effect of environmental temperature on mouse energy homeostasis and body temperature.

METHODS

The effect of environmental temperature (4-33 °C) on body temperature, energy expenditure, physical activity, and food intake in various mice (chow diet, high-fat diet, Brs3 (-/y) , lipodystrophic) was measured using continuous monitoring.

RESULTS

Body temperature depended most on circadian phase and physical activity, but also on environmental temperature. The amounts of energy expenditure due to basal metabolic rate (calculated via a novel method), thermic effect of food, physical activity, and cold-induced thermogenesis were determined as a function of environmental temperature. The measured resting defended body temperature matched that calculated from the energy expenditure using Fourier's law of heat conduction. Mice defended a higher body temperature during physical activity. The cost of the warmer body temperature during the active phase is 4-16% of total daily energy expenditure. Parameters measured in diet-induced obese and Brs3 (-/y) mice were similar to controls. The high post-mortem heat conductance demonstrates that most insulation in mice is via physiological mechanisms.

CONCLUSIONS

At 22 °C, cold-induced thermogenesis is ∼120% of basal metabolic rate. The higher body temperature during physical activity is due to a higher set point, not simply increased heat generation during exercise. Most insulation in mice is via physiological mechanisms, with little from fur or fat. Our analysis suggests that the definition of the upper limit of the thermoneutral zone should be re-considered. Measuring body temperature informs interpretation of energy expenditure data and improves the predictiveness and utility of the mouse to model human energy homeostasis.

Induction of therapeutic hypothermia by pharmacological modulation of temperature-sensitive TRP channels: theoretical framework and practical considerations

Therapeutic hypothermia has emerged as a remarkably effective method of neuroprotection from ischemia and is being increasingly used in clinics. Accordingly, it is also a subject of considerable attention from a basic scientific research perspective. One of the fundamental problems, with which current studies are concerned, is the optimal method of inducing hypothermia. This review seeks to provide a broad theoretical framework for approaching this problem, and to discuss how a novel promising strategy of pharmacological modulation of the thermosensitive ion channels fits into this framework. Various physical, anatomical, physiological and molecular aspects of thermoregulation, which provide the foundation for this text, have been comprehensively reviewed and will not be discussed exhaustively here. Instead, the first part of the current review, which may be helpful for a broader readership outside of thermoregulation research, will build on this existing knowledge to outline possible opportunities and research directions aimed at controlling body temperature. The second part, aimed at a more specialist audience, will highlight the conceptual advantages and practical limitations of novel molecular agents targeting thermosensitive Transient Receptor Potential (TRP) channels in achieving this goal. Two particularly promising members of this channel family, namely TRP melastatin 8 (TRPM8) and TRP vanilloid 1 (TRPV1), will be discussed in greater detail.

Role of nociceptin/orphanin FQ in thermoregulation

Nociceptin/Orphanin FQ (N/OFQ) is a 17-amino acid peptide that binds to the nociceptin receptor (NOP). N/OFQ and NOP receptors are expressed in numerous brain areas. The generation of specific agonists, antagonists and receptor-deficient mice or rats has enabled progress in elucidating the biological functions of N/OFQ. These tools have been employed to identify the biological significance of the N/OFQ system and how it interacts with other endogenous systems to regulate several body functions. The present review focuses on the role of N/OFQ in the regulation of body temperature and its relationship with energy balance. Critical evaluation of the literature data suggests that N/OFQ, acting through the NOP receptor, may cause hypothermia by influencing the complex thermoregulatory system that operates as a federation of independent thermoeffector loops to control body temperature at the hypothalamic level. Furthermore, N/OFQ counteracts hyperthermia elicited by cannabinoids or µ-opioid agonists. N/OFQ-induced hypothermia is prevented by ω-conotoxin GVIA, an N-type calcium channel blocker. Hypothermia induced by N/OFQ is considered within the framework of the complex action that this neuropeptide exerts on energy balance. Energy stores are regulated through the complex neural controls exerted on both food intake and energy expenditure. In laboratory rodents, N/OFQ stimulates consummatory behavior and decreases energy expenditure. Taken together, these studies support the idea that N/OFQ contributes to the regulation of energy balance by acting as an "anabolic" neuropeptide as it elicits effects similar to those produced in the hypothalamus by other neuropeptides such as orexins and neuropeptide Y.

Endocannabinoid Catabolic Enzymes Play Differential Roles in Thermal Homeostasis in Response to Environmental or Immune Challenge

Cannabinoid receptor agonists, such as Δ(9)-THC, the primary active constituent of Cannabis sativa, have anti-pyrogenic effects in a variety of assays. Recently, attention has turned to the endogenous cannabinoid system and how endocannabinoids, including 2-arachidonoylglycerol (2-AG) and anandamide, regulate multiple homeostatic processes, including thermoregulation. Inhibiting endocannabinoid catabolic enzymes, monoacylglycerol lipase (MAGL) or fatty acid amide hydrolase (FAAH), elevates levels of 2-AG or anandamide in vivo, respectively. The purpose of this experiment was to test the hypothesis that endocannabinoid catabolic enzymes function to maintain thermal homeostasis in response to hypothermic challenge. In separate experiments, male C57BL/6J mice were administered a MAGL or FAAH inhibitor, and then challenged with the bacterial endotoxin lipopolysaccharide (LPS; 2 mg/kg ip) or a cold (4 °C) ambient environment. Systemic LPS administration caused a significant decrease in core body temperature after 6 h, and this hypothermia persisted for at least 12 h. Similarly, cold environment induced mild hypothermia that resolved within 30 min. JZL184 exacerbated hypothermia induced by either LPS or cold challenge, both of which effects were blocked by rimonabant, but not SR144528, indicating a CB1 cannabinoid receptor mechanism of action. In contrast, the FAAH inhibitor, PF-3845, had no effect on either LPS-induced or cold-induced hypothermia. These data indicate that unlike direct acting cannabinoid receptor agonists, which elicit profound hypothermic responses on their own, neither MAGL nor FAAH inhibitors affect normal body temperature. However, these endocannabinoid catabolic enzymes play distinct roles in thermoregulation following hypothermic challenges.

TRPM3 in temperature sensing and beyond

TRPM3, also known as melastatin 2 (MLSN2), LTRPC3 (long TRPC3) and KIAA1616, is a member of the TRPM subfamily of transient receptor potential (TRP) ion channels. The channel was originally identified as a volume-regulated ion channel that can be activated upon reduction of the extracellular osmolality. Later, the channel was proposed to be involved in the modulation of insulin release in pancreatic islets. However, new evidence has uncovered a role of TRPM3 as a thermosensitive nociceptor channel implicated in the detection of noxious heat. The channel is functionally expressed in a subset of neurons of the somatosensory system and can be activated by heat. The purpose of the present review is to summarize existing knowledge of the expression, biophysics and pharmacology of TRPM3 and to serve as a guide for future studies aimed at improving the understanding of the mechanism of thermosensation and proposed physiological functions of TRPM3.

Thermoregulatory modeling for cold stress

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

The Effects of Panax ginseng and Panax quinquefolius on Thermoregulation in Animal Models

We devised a study using animal models of hyperthermia and hypothermia and also attempted to accurately assess the effects of Panax ginseng (PG) and Panax quinquefolius (PQ) on body temperature using these models. In addition, we investigated the effects of PG and PQ in our animal models in high and low temperature environments. The results of our experiments show that mice with normothermia, hyperthermia, and hypothermia maintained their body temperatures after a certain period in accordance with the condition of each animal model. In our experiments of body temperature change in models of normal, low, or high room temperature, the hyperthermic model did not show any body temperature change in either the PG- or PQ-administered group. In the normal and low room temperature models, the group administered PG maintained body temperature, while the body temperature of the PQ-administered group was lower than or similar to that of the control group. In conclusion, the fact that PG increases body temperature could not be verified until now. We also showed that the effect of maintaining body temperature in the PG-administered group was superior in a hypothermia-prone low temperature environment.

Thermoregulatory correlates of nausea in rats and musk shrews

Nausea is a prominent symptom and major cause of complaint for patients receiving anticancer chemo- or radiation therapy. The arsenal of anti-nausea drugs is limited, and their efficacy is questionable. Currently, the development of new compounds with anti-nausea activity is hampered by the lack of physiological correlates of nausea. Physiological correlates are needed because common laboratory rodents lack the vomiting reflex. Furthermore, nausea does not always lead to vomiting. Here, we report the results of studies conducted in four research centers to investigate whether nausea is associated with any specific thermoregulatory symptoms. Two species were studied: the laboratory rat, which has no vomiting reflex, and the house musk shrew (Suncus murinus), which does have a vomiting reflex. In rats, motion sickness was induced by rotating them in their individual cages in the horizontal plane (0.75 Hz, 40 min) and confirmed by reduced food consumption at the onset of dark (active) phase. In 100% of rats tested at three centers, post-rotational sickness was associated with marked (~1.5°C) hypothermia, which was associated with a short-lasting tail-skin vasodilation (skin temperature increased by ~4°C). Pretreatment with ondansetron, a serotonin 5-HT3 receptor antagonist, which is used to treat nausea in patients in chemo- or radiation therapy, attenuated hypothermia by ~30%. In shrews, motion sickness was induced by a cyclical back-and-forth motion (4 cm, 1 Hz, 15 min) and confirmed by the presence of retching and vomiting. In this model, sickness was also accompanied by marked hypothermia (~2°C). Like in rats, the hypothermic response was preceded by transient tail-skin vasodilation. In conclusion, motion sickness is accompanied by hypothermia that involves both autonomic and thermoeffector mechanisms: tail-skin vasodilation and possibly reduction of the interscapular brown adipose tissue activity. These thermoregulatory symptoms may serve as physiological correlates of nausea.

A case of transient hypothermia after trans-lamina terminalis and third ventricle clipping of an extremely high-position basilar tip aneurysm

Reports on the trans-lamina terminalis and trans-third ventricular approach are rare. The risk associated with this approach is unknown. After an unsuccessful endovascular surgery, we performed direct surgical clipping via the third ventricle on a 78-year-old woman presenting with an extremely high-positioned, ruptured basilar tip aneurysm. She experienced transient hypothermia for 5 days, and it was considered that this was due to hypothalamic dysfunction. It is necessary to recognize that there is the potential for hypothermia after surgery via the lamina terminalis and third ventricle, even though the mechanisms of hypothalamic thermoregulation are still unclear.

Human skin wetness perception: psychophysical and neurophysiological bases

The ability to perceive thermal changes in the surrounding environment is critical for survival. However, sensing temperature is not the only factor among the cutaneous sensations to contribute to thermoregulatory responses in humans. Sensing skin wetness (i.e. hygrosensation) is also critical both for behavioral and autonomic adaptations. Although much has been done to define the biophysical role of skin wetness in contributing to thermal homeostasis, little is known on the neurophysiological mechanisms underpinning the ability to sense skin wetness. Humans are not provided with skin humidity receptors (i.e., hygroreceptors) and psychophysical studies have identified potential sensory cues (i.e. thermal and mechanosensory) which could contribute to sensing wetness. Recently, a neurophysiological model of human wetness sensitivity has been developed. In helping clarifying the peripheral and central neural mechanisms involved in sensing skin wetness, this model has provided evidence for the existence of a specific human hygrosensation strategy, which is underpinned by perceptual learning via sensory experience. Remarkably, this strategy seems to be shared by other hygroreceptor-lacking animals. However, questions remain on whether these sensory mechanisms are underpinned by specific neuromolecular pathways in humans. Although the first study on human wetness perception dates back to more than 100 years, it is surprising that the neurophysiological bases of such an important sensory feature have only recently started to be unveiled. Hence, to provide an overview of the current knowledge on human hygrosensation, along with potential directions for future research, this review will examine the psychophysical and neurophysiological bases of human skin wetness perception.

Regulation of the Motivation to Eat

Although food intake is necessary to provide energy for all bodily activities, considering food intake as a motivated behavior is complex. Rather than being a simple unconditioned reflex to energy need, eating is mediated by diverse factors. These include homeostatic signals such as those related to body fat stores, to food available and being eaten, and to circulating energy-rich compounds like glucose and fatty acids. Eating is also greatly influenced by non-homeostatic signals that convey information related to learning and experience, hedonics, stress, the social situation, opportunity, and many other factors. Recent developments identifying the intricate nature of the relationships between homeostatic and non-homeostatic influences significantly add to the complexity underlying the neural basis of the motivation to eat. The future of research in the field of food intake would seem to lie in the identification of the neural circuitry and interactions between homeostatic and non-homeostatic influences.

Motion sickness, nausea and thermoregulation: The "toxic" hypothesis

Principal symptoms of motion sickness in humans include facial pallor, nausea and vomiting, and sweating. It is less known that motion sickness also affects thermoregulation, and the purpose of this review is to present and discuss existing data related to this subject. Hypothermia during seasickness was firstly noted nearly 150 years ago, but detailed studies of this phenomenon were conducted only during the last 2 decades. Motion sickness-induced hypothermia is philogenetically quite broadly expressed as besides humans, it has been reported in rats, musk shrews and mice. Evidence from human and animal experiments indicates that the physiological mechanisms responsible for the motion sickness-induced hypothermia include cutaneous vasodilation and sweating (leading to an increase of heat loss) and reduced thermogenesis. Together, these results suggest that motion sickness triggers highly coordinated physiological response aiming to reduce body temperature. Finally, we describe potential adaptive role of this response, and describe the benefits of using it as an objective measure of motion sickness-induced nausea.

A unifying theory for the functional architecture of endothermic thermoregulation

Developing a unifying theory for the functional architecture of endothermic thermoregulation has been proven to be a challenging endeavor. Three papers published in this issue of Temperature take a closer look at this problem and add interesting views to our knowledge about the way that endothermic thermoregulation works.