Central pathway for spontaneous and prostaglandin E2-evoked cutaneous vasoconstriction

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.

Neural circuits of long-term thermoregulatory adaptations to cold temperatures and metabolic demands

The mammalian brain controls heat generation and heat loss mechanisms that regulate body temperature and energy metabolism. Thermoeffectors include brown adipose tissue, cutaneous blood flow and skeletal muscle, and metabolic energy sources include white adipose tissue. Neural and metabolic pathways modulating the activity and functional plasticity of these mechanisms contribute not only to the optimization of function during acute challenges, such as ambient temperature changes, infection and stress, but also to longitudinal adaptations to environmental and internal changes. Exposure of humans to repeated and seasonal cold ambient conditions leads to adaptations in thermoeffectors such as habituation of cutaneous vasoconstriction and shivering. In animals that undergo hibernation and torpor, neurally regulated metabolic and thermoregulatory adaptations enable survival during periods of significant reduction in metabolic rate. In addition, changes in diet can activate accessory neural pathways that alter thermoeffector activity. This knowledge may be harnessed for therapeutic purposes, including treatments for obesity and improved means of therapeutic hypothermia.

Characteristics of Functional Hyperthermia Detected in an Outpatient Clinic for Fever of Unknown Origin

Background: Functional hyperthermia (FH) is characterized by hyperthermia resulting from sympathetic hyperactivity rather than inflammation, and it is frequently overlooked by medical practitioners due to the absence of abnormalities in a medical examination. Although FH is an important differential diagnosis for fever of unknown origin (FUO), the literature on FUO cases in Japan lacks information on FH. In this study, we aimed to uncover the population of FH patients hidden in FUO cases. Methods: An outpatient clinic for FUO was established at Okayama University Hospital, and 132 patients were examined during the period from May 2019 to February 2022. Results: A diagnosis of FH was made in 31.1% of the FUO cases, and FH predominantly affected individuals in their third and fourth decades of life with a higher incidence in females (68.3%). The frequency of a history of psychiatric illness was higher in patients with FH than in patients with other febrile illnesses. Although the C-reactive protein (CRP) is generally negative in FH cases, some obese patients, with a body mass index ≥ 25 had slightly elevated levels of CRP but were diagnosed with FH. Conclusions: The results showed the importance of identifying FH when encountering patients with FUO without any organic etiology.

Harnessing Hyperthermia: Molecular, Cellular, and Immunological Insights for Enhanced Anticancer Therapies

Hyperthermia, the raising of tumor temperature (≥39°C), holds great promise as an adjuvant treatment for cancer therapy. This review focuses on 2 key aspects of hyperthermia: its molecular and cellular effects and its impact on the immune system. Hyperthermia has profound effects on critical biological processes. Increased temperatures inhibit DNA repair enzymes, making cancer cells more sensitive to chemotherapy and radiation. Elevated temperatures also induce cell cycle arrest and trigger apoptotic pathways. Furthermore, hyperthermia modifies the expression of heat shock proteins, which play vital roles in cancer therapy, including enhancing immune responses. Hyperthermic treatments also have a significant impact on the body's immune response against tumors, potentially improving the efficacy of immune checkpoint inhibitors. Mild systemic hyperthermia (39°C-41°C) mimics fever, activating immune cells and raising metabolic rates. Intense heat above 50°C can release tumor antigens, enhancing immune reactions. Using photothermal nanoparticles for targeted heating and drug delivery can also modulate the immune response. Hyperthermia emerges as a cost-effective and well-tolerated adjuvant therapy when integrated with immunotherapy. This comprehensive review serves as a valuable resource for the selection of patient-specific treatments and the guidance of future experimental studies.

Prostaglandin EP3 receptor-expressing preoptic neurons bidirectionally control body temperature via tonic GABAergic signaling

The bidirectional controller of the thermoregulatory center in the preoptic area (POA) is unknown. Using rats, here, we identify prostaglandin EP3 receptor-expressing POA neurons (POA^EP3R neurons) as a pivotal bidirectional controller in the central thermoregulatory mechanism. POA^EP3R neurons are activated in response to elevated ambient temperature but inhibited by prostaglandin E₂, a pyrogenic mediator. Chemogenetic stimulation of POA^EP3R neurons at room temperature reduces body temperature by enhancing heat dissipation, whereas inhibition of them elicits hyperthermia involving brown fat thermogenesis, mimicking fever. POA^EP3R neurons innervate sympathoexcitatory neurons in the dorsomedial hypothalamus (DMH) via tonic (ceaseless) inhibitory signaling. Although many POA^EP3R neuronal cell bodies express a glutamatergic messenger RNA marker, their axons in the DMH predominantly release γ-aminobutyric acid (GABA), and their GABAergic terminals are increased by chronic heat exposure. These findings demonstrate that tonic GABAergic inhibitory signaling from POA^EP3R neurons is a fundamental determinant of body temperature for thermal homeostasis and fever.

Neural circuits mediating circulating interleukin-1β-evoked fever in the absence of prostaglandin E2 production

Infectious diseases and inflammatory conditions recruit the immune system to mount an appropriate acute response that includes the production of cytokines. Cytokines evoke neurally-mediated responses to fight pathogens, such as the recruitment of thermoeffectors, thereby increasing body temperature and leading to fever. Studies suggest that the cytokine interleukin-1β (IL-1β) depends upon cyclooxygenase (COX)-mediated prostaglandin E₂ production for the induction of neural mechanisms to elicit fever. However, COX inhibitors do not eliminate IL-1β-induced fever, thus suggesting that COX-dependent and COX-independent mechanisms are recruited for increasing body temperature after peripheral administration of IL-1β. In the present study, we aimed to build a foundation for the neural circuit(s) controlling COX-independent, inflammatory fever by determining the involvement of brain areas that are critical for controlling the sympathetic outflow to brown adipose tissue (BAT) and the cutaneous vasculature. In anesthetized rats, pretreatment with indomethacin, a non-selective COX inhibitor, did not prevent BAT thermogenesis or cutaneous vasoconstriction (CVC) induced by intravenous IL-1β (2 µg/kg). BAT and cutaneous vasculature sympathetic premotor neurons in the rostral raphe pallidus area (rRPa) are required for IL-1β-evoked BAT thermogenesis and CVC, with or without pretreatment with indomethacin. Additionally, activation of glutamate receptors in the dorsomedial hypothalamus (DMH) is required for COX-independent, IL-1β-induced BAT thermogenesis. Therefore, our data suggests that COX-independent mechanisms elicit activation of neurons within the DMH and rRPa, which is sufficient to trigger and mount inflammatory fever. These data provide a foundation for elucidating the brain circuits responsible for COX-independent, IL-1β-elicited fevers.

The hyperthermic effect of central cholecystokinin is mediated by the cyclooxygenase-2 pathway

Cholecystokinin (CCK) increases core body temperature via CCK₂ receptors when administered intracerebroventricularly (icv). The mechanisms of CCK-induced hyperthermia are unknown, and it is also unknown whether CCK contributes to the fever response to systemic inflammation. We studied the interaction between central CCK signaling and the cyclooxygenase (COX) pathway. Body temperature was measured in adult male Wistar rats pretreated with intraperitoneal infusion of the nonselective COX enzyme inhibitor metamizol (120 mg/kg) or a selective COX-2 inhibitor, meloxicam, or etoricoxib (10 mg/kg for both) and, 30 min later, treated with intracerebroventricular CCK (1.7 µg/kg). In separate experiments, CCK-induced neuronal activation (with and without COX inhibition) was studied in thermoregulation- and feeding-related nuclei with c-Fos immunohistochemistry. CCK increased body temperature by ∼0.4°C from 10 min postinfusion, which was attenuated by metamizol. CCK reduced the number of c-Fos-positive cells in the median preoptic area (by ∼70%) but increased it in the dorsal hypothalamic area and in the rostral raphe pallidus (by ∼50% in both); all these changes were completely blocked with metamizol. In contrast, CCK-induced satiety and neuronal activation in the ventromedial hypothalamus were not influenced by metamizol. CCK-induced hyperthermia was also completely blocked with both selective COX-2 inhibitors studied. Finally, the CCK₂ receptor antagonist YM022 (10 µg/kg icv) attenuated the late phases of fever induced by bacterial lipopolysaccharide (10 µg/kg; intravenously). We conclude that centrally administered CCK causes hyperthermia through changes in the activity of "classical" thermoeffector pathways and that the activation of COX-2 is required for the development of this response.NEW & NOTEWORTHY An association between central cholecystokinin signaling and the cyclooxygenase-prostaglandin E pathway has been proposed but remained poorly understood. We show that the hyperthermic response to the central administration of cholecystokinin alters the neuronal activity within efferent thermoeffector pathways and that these effects are fully blocked by the inhibition of cyclooxygenase. We also show that the activation of cyclooxygenase-2 is required for the hyperthermic effect of cholecystokinin and that cholecystokinin is a modulator of endotoxin-induced fever.

Central sympathetic network for thermoregulatory responses to psychological stress

In mammals, many types of psychological stressors elicit a variety of sympathoexcitatory responses paralleling the classic fight-or-flight response to a threat to survival, including increased body temperature via brown adipose tissue thermogenesis and cutaneous vasoconstriction, and increased skeletal muscle blood flow via tachycardia and visceral vasoconstriction. Although these responses are usually supportive for stress coping, aberrant sympathetic responses to stress can lead to clinical issues in psychosomatic medicine. Sympathetic stress responses are mediated mostly by sympathetic premotor drives from the rostral medullary raphe region (rMR) and partly by those from the rostral ventrolateral medulla (RVLM). Hypothalamomedullary descending pathways from the dorsomedial hypothalamus (DMH) to the rMR and RVLM mediate important, stress-driven sympathoexcitatory transmission to the premotor neurons to drive the thermal and cardiovascular responses. The DMH also likely sends an excitatory input to the paraventricular hypothalamic nucleus to stimulate stress hormone release. Neurons in the DMH receive a stress-related excitation from the dorsal peduncular cortex and dorsal tenia tecta (DP/DTT) in the ventromedial prefrontal cortex. By connecting the corticolimbic emotion circuit to the central sympathetic and somatic motor systems, the DP/DTT → DMH pathway plays as the primary mediator of the psychosomatic signaling that drives a variety of sympathetic and behavioral stress responses. These brain regions together with other stress-related regions constitute a central neural network for physiological stress responses. This network model is relevant to understanding the central mechanisms by which stress and emotions affect autonomic regulations of homeostasis and to developing new therapeutic strategies for various stress-related disorders.

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.

Pathophysiology of Fever and Application of Infrared Thermography (IRT) in the Detection of Sick Domestic Animals: Recent Advances

Body-temperature elevations are multifactorial in origin and classified as hyperthermia as a rise in temperature due to alterations in the thermoregulation mechanism; the body loses the ability to control or regulate body temperature. In contrast, fever is a controlled state, since the body adjusts its stable temperature range to increase body temperature without losing the thermoregulation capacity. Fever refers to an acute phase response that confers a survival benefit on the body, raising core body temperature during infection or systemic inflammation processes to reduce the survival and proliferation of infectious pathogens by altering temperature, restriction of essential nutrients, and the activation of an immune reaction. However, once the infection resolves, the febrile response must be tightly regulated to avoid excessive tissue damage. During fever, neurological, endocrine, immunological, and metabolic changes occur that cause an increase in the stable temperature range, which allows the core body temperature to be considerably increased to stop the invasion of the offending agent and restrict the damage to the organism. There are different metabolic mechanisms of thermoregulation in the febrile response at the central and peripheral levels and cellular events. In response to cold or heat, the brain triggers thermoregulatory responses to coping with changes in body temperature, including autonomic effectors, such as thermogenesis, vasodilation, sweating, and behavioral mechanisms, that trigger flexible, goal-oriented actions, such as seeking heat or cold, nest building, and postural extension. Infrared thermography (IRT) has proven to be a reliable method for the early detection of pathologies affecting animal health and welfare that represent economic losses for farmers. However, the standardization of protocols for IRT use is still needed. Together with the complete understanding of the physiological and behavioral responses involved in the febrile process, it is possible to have timely solutions to serious problem situations. For this reason, the present review aims to analyze the new findings in pathophysiological mechanisms of the febrile process, the heat-loss mechanisms in an animal with fever, thermoregulation, the adverse effects of fever, and recent scientific findings related to different pathologies in farm animals through the use of IRT.

Physiological and Behavioral Mechanisms of Thermoregulation in Mammals

This review analyzes the main anatomical structures and neural pathways that allow the generation of autonomous and behavioral mechanisms that regulate body heat in mammals. The study of the hypothalamic neuromodulation of thermoregulation offers broad areas of opportunity with practical applications that are currently being strengthened by the availability of efficacious tools like infrared thermography (IRT). These areas could include the following: understanding the effect of climate change on behavior and productivity; analyzing the effects of exercise on animals involved in sporting activities; identifying the microvascular changes that occur in response to fear, pleasure, pain, and other situations that induce stress in animals; and examining thermoregulating behaviors. This research could contribute substantially to understanding the drastic modification of environments that have severe consequences for animals, such as loss of appetite, low productivity, neonatal hypothermia, and thermal shock, among others. Current knowledge of these physiological processes and complex anatomical structures, like the nervous systems and their close relation to mechanisms of thermoregulation, is still limited. The results of studies in fields like evolutionary neuroscience of thermoregulation show that we cannot yet objectively explain even processes that on the surface seem simple, including behavioral changes and the pathways and connections that trigger mechanisms like vasodilatation and panting. In addition, there is a need to clarify the connection between emotions and thermoregulation that increases the chances of survival of some organisms. An increasingly precise understanding of thermoregulation will allow us to design and apply practical methods in fields like animal science and clinical medicine without compromising levels of animal welfare. The results obtained should not only increase the chances of survival but also improve quality of life and animal production.

QPLOT Neurons-Converging on a Thermoregulatory Preoptic Neuronal Population

The preoptic area of the hypothalamus is a homeostatic control center. The heterogeneous neurons in this nucleus function to regulate the sleep/wake cycle, reproduction, thirst and hydration, as well as thermogenesis and other metabolic responses. Several recent studies have analyzed preoptic neuronal populations and demonstrated neuronal subtype-specific roles in suppression of thermogenesis. These studies showed similar thermogenesis responses to chemogenetic modulation, and similar synaptic tracing patterns for neurons that were responsive to cold, to inflammatory stimuli, and to violet light. A reanalysis of single-cell/nucleus RNA-sequencing datasets of the preoptic nucleus indicate that these studies have converged on a common neuronal population that when activated, are sufficient to suppress thermogenesis. Expanding on a previous name for these neurons (Q neurons, which reflect their ability to promote quiescence and expression of Qrfp), we propose a new name: QPLOT neurons, to reflect numerous molecular markers of this population and to capture its broader roles in metabolic regulation. Here, we summarize previous findings on this population and present a unified description of QPLOT neurons, the excitatory preoptic neuronal population that integrate a variety of thermal, metabolic, hormonal and environmental stimuli in order to regulate metabolism and thermogenesis.

Thermoregulation and Sleep: Functional Interaction and Central Nervous Control

Each of the wake-sleep states is characterized by specific changes in autonomic activity and bodily functions. The goal of such changes is not always clear. During non-rapid eye movement (NREM) sleep, the autonomic outflow and the activity of the endocrine system, the respiratory system, the cardiovascular system, and the thermoregulatory system seem to be directed at increasing energy saving. During rapid eye movement (REM) sleep, the goal of the specific autonomic and regulatory changes is unclear, since a large instability of autonomic activity and cardiorespiratory function is observed in concomitance with thermoregulatory changes, which are apparently non-functional to thermal homeostasis. Reciprocally, the activation of thermoregulatory responses under thermal challenges interferes with sleep occurrence. Such a double-edged and reciprocal interaction between sleep and thermoregulation may be favored by the fact that the central network controlling sleep overlaps in several parts with the central network controlling thermoregulation. The understanding of the central mechanism behind the interaction between sleep and thermoregulation may help to understand the functionality of thermoregulatory sleep-related changes and, ultimately, the function(s) of sleep. © 2021 American Physiological Society. Compr Physiol 11:1591-1604, 2021.

Effects of Caffeine on Brown Adipose Tissue Thermogenesis and Metabolic Homeostasis: A Review

The impact of brown adipose tissue (BAT) metabolism on understanding energy balance in humans is a relatively new and exciting field of research. The pathogenesis of obesity can be largely explained by an imbalance between caloric intake and energy expenditure, but the underlying mechanisms are far more complex. Traditional non-selective sympathetic activators have been used to artificially elevate energy utilization, or suppress appetite, however undesirable side effects are apparent with the use of these pharmacological interventions. Understanding the role of BAT, in relation to human energy homeostasis has the potential to dramatically offset the energy imbalance associated with obesity. This review discusses paradoxical effects of caffeine on peripheral adenosine receptors and the possible role of adenosine in increasing metabolism is highlighted, with consideration to the potential of central rather than peripheral mechanisms for caffeine mediated BAT thermogenesis and energy expenditure. Research on the complex physiology of adipose tissue, the embryonic lineage and function of the different types of adipocytes is summarized. In addition, the effect of BAT on overall human metabolism and the extent of the associated increase in energy expenditure are discussed. The controversy surrounding the primary β-adrenoceptor involved in human BAT activation is examined, and suggestions as to the lack of translational findings from animal to human physiology and human in vitro to in vivo models are provided. This review compares and distinguishes human and rodent BAT effects, thus developing an understanding of human BAT thermogenesis to aid lifestyle interventions targeting obesity and metabolic syndrome. The focus of this review is on the effect of BAT thermogenesis on overall metabolism, and the potential therapeutic effects of caffeine in increasing metabolism via its effects on BAT.

Stimulatory, but not anxiogenic, doses of caffeine act centrally to activate interscapular brown adipose tissue thermogenesis in anesthetized male rats

The role of central orexin in the sympathetic control of interscapular brown adipose tissue (iBAT) thermogenesis has been established in rodents. Stimulatory doses of caffeine activate orexin positive neurons in the lateral hypothalamus, a region of the brain implicated in stimulating BAT thermogenesis. This study tests the hypothesis that central administration of caffeine is sufficient to activate BAT. Low doses of caffeine administered either systemically (intravenous [IV]; 10 mg/kg) and centrally (intracerebroventricular [ICV]; 5-10 μg) increases BAT thermogenesis, in anaesthetised (1.5 g/kg urethane, IV) free breathing male rats. Cardiovascular function was monitored via an indwelling intra-arterial cannula and exhibited no response to the caffeine. Core temperature did not significantly differ after administration of caffeine via either route of administration. Caffeine administered both IV and ICV increased neuronal activity, as measured by c-Fos-immunoreactivity within subregions of the hypothalamic area, previously implicated in regulating BAT thermogenesis. Significantly, there appears to be no neural anxiety response to the low dose of caffeine as indicated by no change in activity in the basolateral amygdala. Having measured the physiological correlate of thermogenesis (heat production) we have not measured indirect molecular correlates of BAT activation. Nevertheless, our results demonstrate that caffeine, at stimulatory doses, acting via the central nervous system can increase thermogenesis, without adverse cardio-dynamic impact.

Insulin sensing by astrocytes is critical for normal thermogenesis and body temperature regulation

The important role of astrocytes in the central control of energy balance and glucose homeostasis has recently been recognized. Changes in thermoregulation can lead to metabolic dysregulation, but the role of astrocytes in this process is not yet clear. Therefore, we generated mice congenitally lacking insulin receptors (Ir) in astrocytes (IrKOGFAP mice) to investigate the involvement of astrocyte insulin signaling. IrKOGFAP mice displayed significantly lower energy expenditure and a strikingly lower basal and fasting body temperature. When exposed to cold, however, they were able to mount a thermogenic response. IrKOGFAP mice displayed sex differences in metabolic function and thermogenesis that may contribute to the development of obesity and type II diabetes as early as 2 months of age. While brown adipose tissue exhibited higher adipocyte size in both sexes, more apoptosis was seen in IrKOGFAP males. Less innervation and lower BAR3 expression levels were also observed in IrKOGFAP brown adipose tissue. These effects have not been reported in models of astrocyte Ir deletion in adulthood. In contrast, body weight and glucose regulatory defects phenocopied such models. These findings identify a novel role for astrocyte insulin signaling in the development of normal body temperature control and sympathetic activation of BAT. Targeting insulin signaling in astrocytes has the potential to serve as a novel target for increasing energy expenditure.

The generation of immune-induced fever and emotional stress-induced hyperthermia in mice does not involve brown adipose tissue thermogenesis

We examined the role of brown adipose tissue (BAT) for fever and emotional stress-induced hyperthermia. Wild-type and uncoupling protein-1 (UCP-1) knockout mice were injected with lipopolysaccharide intraperitoneally or intravenously, or subjected to cage exchange, and body temperature monitored by telemetry. Both genotypes showed similar febrile responses to immune challenge and both displayed hyperthermia to emotional stress. Neither procedure resulted in the activation of BAT, such as the induction of UCP-1 or peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) mRNA, or reduced BAT weight and triglyceride content. In contrast, in mice injected with a β₃ agonist, UCP-1 and PGC-1α were strongly induced, and BAT weight and triglyceride content reduced. Both lipopolysaccharide and the β₃ agonist, and emotional stress, induced UCP-3 mRNA in skeletal muscle. A β₃ antagonist did not attenuate lipopolysaccharide-induced fever, but augmented body temperature decrease and inhibited BAT activation when mice were exposed to cold. An α₁ /α_2b antagonist or a 5HT_1A agonist, which inhibit vasoconstriction, abolished lipopolysaccharide-induced fever, but had no effect on emotional stress-induced hyperthermia. These findings demonstrate that in mice, UCP-1-mediated BAT thermogenesis does not take part in inflammation-induced fever, which is dependent on peripheral vasoconstriction, nor in stress-induced hyperthermia. However, both phenomena may involve UCP-3-mediated muscle thermogenesis.

Regulation of Body Temperature by the Nervous System

The regulation of body temperature is one of the most critical functions of the nervous system. Here we review our current understanding of thermoregulation in mammals. We outline the molecules and cells that measure body temperature in the periphery, the neural pathways that communicate this information to the brain, and the central circuits that coordinate the homeostatic response. We also discuss some of the key unresolved issues in this field, including the following: the role of temperature sensing in the brain, the molecular identity of the warm sensor, the central representation of the labeled line for cold, and the neural substrates of thermoregulatory behavior. We suggest that approaches for molecularly defined circuit analysis will provide new insight into these topics in the near future.

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.

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.

Central cholinergic activation induces greater thermoregulatory and cardiovascular responses in spontaneously hypertensive than in normotensive rats

There is evidence that central cholinergic stimulation increases heat dissipation in normotensive rats besides causing changes on the cardiovascular system via modulation of baroreceptors activity. However, the contribution of the central cholinergic system on thermoregulatory responses and its relationship with cardiovascular adjustments in spontaneously hypertensive rats (SHRs), an animal model of reduced baroreceptor sensitivity and thermoregulatory deficit, has not been completely clarified. Therefore, the aim of this study was to verify the involvement of the central cholinergic system in cardiovascular and thermoregulatory adjustments in SHRs. Male Wistar rats (n = 17) and SHRs (n = 17) were implanted with an intracerebroventricular cannula for injections of 2 µL of physostigmine (phy) or saline solution. Tail temperature (T_tail), internal body temperature (T_int), systolic arterial pressure (SAP), heart rate (HR) and metabolic rate were registered during 60 min while the animals remained at rest after randomly receiving the injections. The variability of the SAP and the HR was estimated by the fast Fourier transform. Phy treatment began a succession of cardiovascular and thermoregulatory responses that resulted in increased SAP, reduced HR and increased T_tail in both Wistar and SHRs groups. The magnitude of these effects seems to be more intense in SHRs, since the improvement of heat dissipation reflected in T_int. Taken together, these results provide evidence that hypertensive rats present greater cardiovascular and thermoregulatory responses than normotensive rats after central cholinergic stimulation.

Neural Mechanisms of Inflammation-Induced Fever

Fever is a common symptom of infectious and inflammatory disease. It is well-established that prostaglandin E2 is the final mediator of fever, which by binding to its EP3 receptor subtype in the preoptic hypothalamus initiates thermogenesis. Here, we review the different hypotheses on how the presence of peripherally released pyrogenic substances can be signaled to the brain to elicit fever. We conclude that there is unequivocal evidence for a humoral signaling pathway by which proinflammatory cytokines, through their binding to receptors on brain endothelial cells, evoke fever by eliciting prostaglandin E2 synthesis in these cells. The evidence for a role for other signaling routes for fever, such as signaling via circumventricular organs and peripheral nerves, as well as transfer into the brain of peripherally synthesized prostaglandin E2 are yet far from conclusive. We also review the efferent limb of the pyrogenic pathways. We conclude that it is well established that prostaglandin E2 binding in the preoptic hypothalamus produces fever by disinhibition of presympathetic neurons in the brain stem, but there is yet little understanding of the mechanisms by which factors such as nutritional status and ambient temperature shape the response to the peripheral immune challenge.

A Glutamatergic Hypothalamomedullary Circuit Mediates Thermogenesis, but Not Heat Conservation, during Stress-Induced Hyperthermia

Stress elicits a variety of autonomic responses, including hyperthermia (stress fever) in humans and animals. In this present study, we investigated the circuit basis for thermogenesis and heat conservation during this response. We first demonstrated the glutamatergic identity of the dorsal hypothalamic area (DHA^Vglut2) neurons that innervate the raphe pallidus nucleus (RPa) to regulate core temperature (Tc) and mediate stress-induced hyperthermia. Then, using chemogenetic and optogenetic methods to manipulate this hypothalamomedullary circuit, we found that activation of DHA^Vglut2 neurons potently drove an increase in Tc, but surprisingly, stress-induced hyperthermia was only reduced by about one-third when they were inhibited. Further investigation showed that DHA^Vglut2 neurons activate brown adipose tissue (BAT) but do not cause vasoconstriction, instead allowing reflex tail artery vasodilation as a response to BAT-induced hyperthermia. Retrograde rabies virus tracing revealed projections from DHA^Vglut2 neurons to RPa^Vglut3, but not to RPa^GABA neurons, and identified a set of inputs to DHA^Vglut2 → RPa neurons that are likely to mediate BAT activation. The dissociation of the DHA^Vglut2 thermogenic pathway from the thermoregulatory vasoconstriction (heat-conserving) pathway may explain stress flushing (skin vasodilation but a feeling of being too hot) during stressful times.

Efferent thermoregulatory pathways regulating cutaneous blood flow and sweating

Cutaneous vasoconstrictor nerves regulate heat retention, and are activated by falls in skin or core temperature. The efferent pathways controlling this process originate within the preoptic area. A descending GABAergic pathway, activated by warm skin or core, indirectly inhibits sympathetic premotor neurons in the medullary raphé. Those premotor neurons drive cutaneous vasoconstriction via excitatory glutamatergic and serotonergic connections to spinal preganglionic neurons. Cold skin and/or cold core temperatures activate a direct preoptic-to-raphé excitatory pathway. The balance of inhibitory and excitatory influences reaching the medullary raphé determines cutaneous blood flow. During fever, prostaglandin E₂ inhibits preoptic GABAergic neurons, resulting in disinhibition of the excitatory preoptic-to-raphé pathway, and hence, cutaneous vasoconstriction. A weaker, parallel source of descending excitatory drive reaches cutaneous preganglionic neurons from the rostral ventrolateral medulla. Sweating follows local heating of the preoptic area in cats and monkeys, and heated humans show sweating-related activation of this same region in functional magnetic resonance imaging (fMRI) studies. A descending pathway that drives sweating has been traced in cats from the hypothalamus to putative premotor neurons in the parafacial region at the pontomedullary junction. The homologous parafacial region in humans also shows sweating-related activation in fMRI studies. The central pathways that drive active vasodilatation in human nonacral skin remain unknown.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

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.

Efferent neural pathways for the control of brown adipose tissue thermogenesis and shivering

The fundamental central neural circuits for thermoregulation orchestrate behavioral and autonomic repertoires that maintain body core temperature during thermal challenges that arise from either the ambient or the internal environment. This review summarizes our understanding of the neural pathways within the fundamental thermoregulatory reflex circuitry that comprise the efferent (i.e., beyond thermosensory) control of brown adipose tissue (BAT) and shivering thermogenesis: the motor neuron systems consisting of the BAT sympathetic preganglionic neurons and BAT sympathetic ganglion cells, and the alpha- and gamma-motoneurons; the premotor neurons in the region of the rostral raphe pallidus, and the thermogenesis-promoting neurons in the dorsomedial hypothalamus/dorsal hypothalamic area. Also included are inputs to, and neurochemical modulators of, these efferent neuronal populations that could influence their activity during thermoregulatory responses. Signals of metabolic status can be particularly significant for the energy-hungry thermoeffectors for heat production.

Central regulation of brown adipose tissue thermogenesis and energy homeostasis dependent on food availability

Energy homeostasis of mammals is maintained by balancing energy expenditure within the body and energy intake through feeding. Several lines of evidence indicate that brown adipose tissue (BAT), a sympathetically activated thermogenic organ, turns excess energy into heat to maintain the energy balance in rodents and humans, in addition to its thermoregulatory role for the defense of body core temperature in cold environments. Elucidating the central circuit mechanism controlling BAT thermogenesis dependent on nutritional conditions and food availability in relation to energy homeostasis is essential to understand the etiology of symptoms caused by energy imbalance, such as obesity. The central thermogenic command outflow to BAT descends through an excitatory neural pathway mediated by hypothalamic, medullary and spinal sites. This sympathoexcitatory thermogenic drive is controlled by tonic GABAergic inhibitory signaling from the thermoregulatory center in the preoptic area, whose tone is altered by body core and cutaneous thermosensory inputs. This circuit controlling BAT thermogenesis for cold defense also functions for the development of fever and psychological stress-induced hyperthermia, indicating its important role in the defense from a variety of environmental stressors. When food is unavailable, hunger-driven neural signaling from the hypothalamus activates GABAergic neurons in the medullary reticular formation, which then block the sympathoexcitatory thermogenic outflow to BAT to reduce energy expenditure and simultaneously command the masticatory motor system to promote food intake—effectively commanding responses to survive starvation. This article reviews the central mechanism controlling BAT thermogenesis in relation to the regulation of energy and thermal homeostasis dependent on food availability.

Median preoptic glutamatergic neurons promote thermoregulatory heat loss and water consumption in mice

KEY POINTS

Glutamatergic neurons in the median preoptic area were stimulated using genetically targeted Channelrhodopsin 2 in transgenic mice. Stimulation of glutamatergic median preoptic area neurons produced a profound hypothermia due to cutaneous vasodilatation. Stimulation also produced drinking behaviour that was inhibited as water was ingested, suggesting pre-systemic feedback gating of drinking. Anatomical mapping of the stimulation sites showed that sites associated with hypothermia were more anteroventral than those associated with drinking, although there was substantial overlap.

ABSTRACT

The median preoptic nucleus (MnPO) serves an important role in the integration of water/electrolyte homeostasis and thermoregulation, but we have a limited understanding these functions at a cellular level. Using Cre-Lox genetic targeting of Channelrhodospin 2 in VGluT2 transgenic mice, we examined the effect of glutamatergic MnPO neuron stimulation in freely behaving mice while monitoring drinking behaviour and core temperature. Stimulation produced a strong hypothermic response in 62% (13/21) of mice (core temperature: -4.6 ± 0.5°C, P = 0.001 vs. controls) caused by cutaneous vasodilatation. Stimulating glutamatergic MnPO neurons also produced robust drinking behaviour in 82% (18/22) of mice. Mice that drank during stimulation consumed 912 ± 163 μl (n = 8) during a 20 min trial in the dark phase, but markedly less during the light phase (421 ± 83 μl, P = 0.0025). Also, drinking during stimulation was inhibited as water was ingested, suggesting pre-systemic feedback gating of drinking. Both hypothermia and drinking during stimulation occurred in 50% of mice tested. Anatomical mapping of the stimulation sites showed that sites associated with hypothermia were more anteroventral than those associated with drinking, although there was substantial overlap. Thus, activation of separate but overlapping populations of glutamatergic MnPO neurons produces effects on drinking and autonomic thermoregulatory mechanisms, providing a structural basis for their frequently being coordinated (e.g. during hyperthermia).

Neuromedin U precursor-related peptide (NURP) exerts neuromedin U-like sympathetic nerve action in the rat

It has been suggested that novel peptide that is produced from the neuromedin U (NMU) precursor may exist, as this precursor contains multiple consensus sequences for proteolytic processing. Recently, we identified two mature novel peptides comprising 33 and 36 residues in the rat brain, which were designated neuromedin U precursor-related peptide (NURP) 33 and 36. In the present study, we compared the roles of NURP33 and 36 with that of NMU, as neither activates the NMU receptors. Immunoreactivity for NMU and NURPs was widely present in the central nervous system and showed a similar distribution. Intracerebroventricular (icv) injection of NURP33 in rats increased locomotor activity, energy expenditure, heart rate and back surface temperature (BS-T), similarly to NMU or NURP36. NMU treatment reduced food intake, but NURP33 did not. Pretreatment with the β3 blocker, SR59230A, and the cyclooxygenase blocker, indomethacin, inhibited the NURP33- or NMU-induced increase of BS-T. In addition, icv injection of NURP33 or NMU increased the expression of mRNA for cyclooxygenase 2 in the hypothalamus and for uncoupling protein 1 in the brown adipose tissue. These results suggest that although NURP33 and 36 do not activate the NMU receptors, they might exert NMU-like sympathetic nerve action in the brain.

Cold-Induced Thermogenesis and Inflammation-Associated Cold-Seeking Behavior Are Represented by Different Dorsomedial Hypothalamic Sites: A Three-Dimensional Functional Topography Study in Conscious Rats

In the past, we showed that large electrolytic lesions of the dorsomedial hypothalamus (DMH) promoted hypothermia in cold-exposed restrained rats, but attenuated hypothermia in rats challenged with a high dose of bacterial lipopolysaccharide (LPS) in a thermogradient apparatus. The goal of this study was to identify the thermoeffector mechanisms and DMH representation of the two phenomena and thus to understand how the same lesion could produce two opposite effects on body temperature. We found that the permissive effect of large electrolytic DMH lesions on cold-induced hypothermia was due to suppressed thermogenesis. DMH-lesioned rats also could not develop fever autonomically: they did not increase thermogenesis in response to a low, pyrogenic dose of LPS (10 μg/kg, i.v.). In contrast, changes in thermogenesis were uninvolved in the attenuation of the hypothermic response to a high, shock-inducing dose of LPS (5000 μg/kg, i.v.); this attenuation was due to a blockade of cold-seeking behavior. To compile DMH maps for the autonomic cold defense and for the cold-seeking response to LPS, we studied rats with small thermal lesions in different parts of the DMH. Cold thermogenesis had the highest representation in the dorsal hypothalamic area. Cold seeking was represented by a site at the ventral border of the dorsomedial nucleus. Because LPS causes both fever and hypothermia, we originally thought that the DMH contained a single thermoregulatory site that worked as a fever-hypothermia switch. Instead, we have found two separate sites: one that drives thermogenesis and the other, previously unknown, that drives inflammation-associated cold seeking.SIGNIFICANCE STATEMENT Cold-seeking behavior is a life-saving response that occurs in severe systemic inflammation. We studied this behavior in rats with lesions in the dorsomedial hypothalamus (DMH) challenged with a shock-inducing dose of bacterial endotoxin. We built functional maps of the DMH and found the strongest representation of cold-seeking behavior at the ventral border of the dorsomedial nucleus. We also built maps for cold-induced thermogenesis in unanesthetized rats and found the dorsal hypothalamic area to be its main representation site. Our work identifies the neural substrate of cold-seeking behavior in systemic inflammation and expands the functional topography of the DMH, a structure that modulates autonomic, endocrine, and behavioral responses and is a potential therapeutic target in anxiety and panic disorders.

Fever Phobia as a Reason for Pediatric Emergency Department Visits: Does the Primary Care Physician Make a Difference?

BACKGROUND

Fever is a source of considerable parental anxiety. Numerous studies have also confirmed similar anxiety among health care workers. This study analyzed caregiver knowledge of fever, and beliefs concerning children with a febrile illness, with an emphasis on the referring physician.

METHODS

This was a cross-sectional study of 100 caregivers of children 3 months to 12 years old, treated at an urban tertiary care pediatric emergency department for fever. Caregiver knowledge was assessed with a questionnaire.

RESULTS

Most caregivers correctly defined the threshold for fever as >38.0-38.3°C. Caregivers commonly believed that fever can cause brain damage and epilepsy; the frequency of this belief was not affected by whether they were referred to the emergency department by their pediatrician/family physician or by another physician or arrived without a referral. For a comfortable-appearing child with a temperature not above 38.0°C, both groups reported that they would give antipyretics in similar proportions (mean 31%). The majority of parents in both groups believed that teething could cause fever (mean 74%).

CONCLUSION

Caregivers in this study had limited knowledge of fever and its management in children, even if referred by their primary care physician. We suggest that there is a need for aggressive educational interventions to reduce parents' fever phobia, in clinics as well as in pediatric emergency departments, and that this need may extend to the education of medical personnel as well.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Control of the Cutaneous Circulation by the Central Nervous System

The central nervous system (CNS), via its control of sympathetic outflow, regulates blood flow to the acral cutaneous beds (containing arteriovenous anastomoses) as part of the homeostatic thermoregulatory process, as part of the febrile response, and as part of cognitive-emotional processes associated with purposeful interactions with the external environment, including those initiated by salient or threatening events (we go pale with fright). Inputs to the CNS for the thermoregulatory process include cutaneous sensory neurons, and neurons in the preoptic area sensitive to the temperature of the blood in the internal carotid artery. Inputs for cognitive-emotional control from the exteroceptive sense organs (touch, vision, sound, smell, etc.) are integrated in forebrain centers including the amygdala. Psychoactive drugs have major effects on the acral cutaneous circulation. Interoceptors, chemoreceptors more than baroreceptors, also influence cutaneous sympathetic outflow. A major advance has been the discovery of a lower brainstem control center in the rostral medullary raphé, regulating outflow to both brown adipose tissue (BAT) and to the acral cutaneous beds. Neurons in the medullary raphé, via their descending axonal projections, increase the discharge of spinal sympathetic preganglionic neurons controlling the cutaneous vasculature, utilizing glutamate, and serotonin as neurotransmitters. Present evidence suggests that both thermoregulatory and cognitive-emotional control of the cutaneous beds from preoptic, hypothalamic, and forebrain centers is channeled via the medullary raphé. Future studies will no doubt further unravel the details of neurotransmitter pathways connecting these rostral control centers with the medullary raphé, and those operative within the raphé itself. © 2016 American Physiological Society. Compr Physiol 6:1161-1197, 2016.

The Effect of Antipyretic Temperature Reduction on Heart Rate in Febrile Infants: a Pilot Study

It is generally accepted that fever is associated with tachycardia and that the relationship, of an approximate increase of 10 beats/minute per °Celsius rise in temperature, is particularly pronounced in infants. It would be useful to determine how closely pulse and fever are associated during pharmacological temperature reduction, and whether it is influenced by the infectious status of the infant. Infants with fever presenting to the pediatric emergency department were prospectively enrolled in a study to determine temperature, heart rate, and the activity at initial assessment and 1 hour after antipyretic administration. Patients were also grouped into those with and without signs of serious bacterial infection for analysis. During pharmacological antipyresis, the relationship between pulse and fever deviated from the accepted norm, established during the febrile state: there was an average pulse rate reduction of 21.1 beats/minute/°Celsius decrement in fever (P = 0.0027; 95% CI, 14.05-28.15). Pulse reduction/°Celsius may also differ in infants with suspected serious bacterial infection. Our findings suggest caution in ascribing pulse changes to fever alone, especially in sick looking infants, and that this is especially true after the administration of antipyretics, which could be related to the mechanism of action of antipyretic drugs. Further research is needed to ascertain the clinical utility and importance of our observations.

Central neural control of thermoregulation and brown adipose tissue

Central neural circuits orchestrate the homeostatic repertoire that maintains body temperature during environmental temperature challenges and alters body temperature during the inflammatory response. This review summarizes the experimental underpinnings of our current model of the CNS pathways controlling the principal thermoeffectors for body temperature regulation: cutaneous vasoconstriction controlling heat loss, and shivering and brown adipose tissue for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific, core efferent pathways within the CNS that share a common peripheral thermal sensory input. Via the lateral parabrachial nucleus, skin thermal afferent input reaches the hypothalamic preoptic area to inhibit warm-sensitive, inhibitory output neurons which control heat production by inhibiting thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to thermogenesis-controlling premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation of spinal circuits necessary to drive thermogenic thermal 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 useful platform for further understanding of the functional organization of central thermoregulation and elucidating the hypothalamic circuitry and neurotransmitters involved in body temperature regulation.

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.

Neural circuit for psychological stress-induced hyperthermia

Psychological stress-induced hyperthermia (PSH) is a basic physiological stress response to increase physical performances to defend homeostasis and life from stressors, such as natural enemies. However, excessive and long-lasting stressors can lead to chronic hyperthermia, particularly recognized in humans as a psychosomatic symptom called “psychogenic fever.” The sympathetic and neuroendocrine responses that can contribute to PSH include brown adipose tissue (BAT) thermogenesis, cutaneous vasoconstriction, tachycardia and glucocorticoid secretion. Research on the central circuits underlying these stress responses has recently revealed several fundamental circuit mechanisms including hypothalamomedullary pathways driving the sympathetic stress responses. Psychological stress activates a monosynaptic glutamatergic excitatory neurotransmission from the dorsomedial hypothalamus (DMH) to sympathetic premotor neurons in the rostral medullary raphe region (rMR) to drive BAT thermogenesis and tachycardia, leading to the development of PSH. This glutamatergic neurotransmission could be potentiated by orexin neurons in the lateral hypothalamus through their projections to the rMR. Psychological stress also activates another monosynaptic pathway from the DMH to the paraventricular hypothalamic nucleus to stimulate the hypothalamo-pituitary-adrenal axis for the secretion of glucocorticoids. PSH is independent from the prostaglandin-mediated trigger mechanism for inflammation-induced fever, and several forebrain regions are considered to provide stress-driven inputs to the DMH to activate the sympathetic- and neuroendocrine-driving neurons. The circuit mechanism of PSH based on animal experiments would be relevant to understandings of the etiology of psychogenic fever in humans. This review describes the current understandings of the central circuit mechanism of PSH with recent important progress in research.

2-Deoxy-D-glucose-induced hypothermia in anesthetized rats: Lack of forebrain contribution and critical involvement of the rostral raphe/parapyramidal regions of the medulla oblongata

Systemic or central administration of 2-deoxy-d-glucose (2DG), a competitive inhibitor of glucose utilization, induces hypothermia in awake animals and humans. This response is mediated by the central nervous system, though the neural mechanism involved is largely unknown. In this study, I examined possible involvement of the forebrain, which contains the hypothalamic thermoregulatory center, and the medullary rostral raphe/parapyramidal regions (rRPa/PPy), which mediate hypoxia-induced heat-loss responses, in 2DG-induced hypothermia in urethane-chloralose-anesthetized, neuromuscularly blocked, artificially ventilated rats. The intravenous injection of 2DG (250mgkg(-1)) elicited an increase in tail skin temperature and decreases in body core temperature and the respiratory exchange ratio, though it did not induce any significant change in the metabolic rate. These results indicate that the hypothermic response was caused by an increase in heat loss, but not by a decrease in heat production and that it was accompanied by a decrease in carbohydrate utilization and/or an increase in lipid utilization as energy substrates. Complete surgical transection of the brainstem between the hypothalamus and the midbrain had no effect on the 2DG-induced hypothermic responses, suggesting that the hindbrain, but not the forebrain, was sufficient for the responses. However, pretreatment of the rRPa/PPy with the GABAA receptor blocker bicuculline methiodide, but not with vehicle saline, greatly attenuated the 2DG-induced responses, suggesting that the 2DG-induced hypothermia was mediated, at least in part, by GABAergic neurons in the hindbrain and activation of GABAA receptors on cutaneous sympathetic premotor neurons in the rRPa/PPy.

Psychogenic fever: how psychological stress affects body temperature in the clinical population

Psychogenic fever is a stress-related, psychosomatic disease especially seen in young women. Some patients develop extremely high core body temperature (Tc) (up to 41°C) when they are exposed to emotional events, whereas others show persistent low-grade high Tc (37-38°C) during situations of chronic stress. The mechanism for psychogenic fever is not yet fully understood. However, clinical case reports demonstrate that psychogenic fever is not attenuated by antipyretic drugs, but by psychotropic drugs that display anxiolytic and sedative properties, or by resolving patients' difficulties via natural means or psychotherapy. Animal studies have demonstrated that psychological stress increases Tc via mechanisms distinct from infectious fever (which requires proinflammatory mediators) and that the sympathetic nervous system, particularly β3-adrenoceptor-mediated non-shivering thermogenesis in brown adipose tissue, plays an important role in the development of psychological stress-induced hyperthermia. Acute psychological stress induces a transient, monophasic increase in Tc. In contrast, repeated stress induces anticipatory hyperthermia, reduces diurnal changes in Tc, or slightly increases Tc throughout the day. Chronically stressed animals also display an enhanced hyperthermic response to a novel stress, while past fearful experiences induce conditioned hyperthermia to the fear context. The high Tc that psychogenic fever patients develop may be a complex of these diverse kinds of hyperthermic responses.

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.

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.

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.

"On-" and "off-" cells in the rostral ventromedial medulla of rats held in thermoneutral conditions: are they involved in thermoregulation?

In thermal neutral condition, rats display cyclic variations of the vasomotion of the tail and paws, synchronized with fluctuations of blood pressure, heart rate, and core body temperature. "On-" and "off-" cells located in the rostral ventromedial medulla, a cerebral structure implicated in somatic sympathetic drive, 1) exhibit similar spontaneous cyclic activities in antiphase and 2) are activated and inhibited by thermal nociceptive stimuli, respectively. We aimed at evaluating the implication of such neurons in autonomic regulation by establishing correlations between their firing and blood pressure, heart rate, and skin and core body temperature variations. When, during a cycle, a relative high core body temperature was reached, the on-cells were activated and within half a minute, the off-cells and blood pressure were depressed, followed by heart rate depression within a further minute; vasodilatation of the tail followed invariably within ∼3 min, often completed with vasodilatation of hind paws. The outcome was an increased heat loss that lessened the core body temperature. When the decrease of core body temperature achieved a few tenths of degrees, sympathetic activation switches off and converse variations occurred, providing cycles of three to seven periods/h. On- and off-cell activities were correlated with inhibition and activation of the sympathetic system, respectively. The temporal sequence of events was as follows: core body temperature → on-cell → off-cell ∼ blood pressure → heart rate → skin temperature → core body temperature. The function of on- and off-cells in nociception should be reexamined, taking into account their correlation with autonomic regulations.

Relief learning is distinguished from safety learning by the requirement of the nucleus accumbens

Aversive events induce aversive memories (fear learning) and can also establish appetitive memories. This is the case for cues associated with the cessation of an aversive event (relief learning) or occurring in an explicitly unpaired fashion (safety learning). However, the neural basis of relief and safety learning is poorly understood. In particular, it is not clear whether relief learning and safety learning are neuronally distinct. In the present study, we ask whether the nucleus accumbens is required for the acquisition of relief- and/or safety memory. Temporary inactivation of the nucleus accumbens by local injections of the GABA-A receptor agonist muscimol during the learning session abolished relief learning whereas safety learning was not affected. Thus, the requirement for a functional nucleus accumbens distinguishes relief from safety learning, showing that these two forms of learning are neuronally distinct.

Inhibition of the dorsomedial hypothalamus, but not the medullary raphe pallidus, decreases hyperthermia and mortality from MDMA given in a warm environment

The central mechanisms through which MDMA mediates life-threatening hyperthermia when taken in a warm environment are not well described. It is assumed that MDMA alters normal thermoregulatory circuits resulting in increased heat production through interscapular brown adipose tissue (iBAT) and decreased heat dissipation through cutaneous vasoconstriction. We studied the role of the dorsomedial hypothalamus (DMH) and medullary raphe pallidus (mRPa) in mediating iBAT, tail blood flow, and locomotor effects produced by MDMA. Rats were instrumented with guide cannulas targeting either the DMH or the mRPa-brain regions involved in regulating iBAT and cutaneous vascular beds. In all animals, core temperature and locomotion were recorded with surgically implanted telemetric transmitters; and additionally either iBAT temperature (via telemetric transmitter) or tail artery blood flow (via tail artery Doppler cuff) were also recorded. Animals were placed in an environmental chamber at 32°C and microinjected with either control or the GABA agonist muscimol (80pmol) followed by an intravenous injection of saline or MDMA (7.5 mg kg^-1). To prevent undue suffering, a core temperature of 41°C was chosen as the surrogate marker of mortality. Inhibition of the DMH, but not the mRPa, prevented mortality and attenuated hyperthermia and locomotion. Inhibition of either the DMH or the mRPa did not affect iBAT temperature increases or tail blood flow decreases. While MDMA increases iBAT thermogenesis and decreases heat dissipation through cutaneous vasoconstriction, thermoregulatory brain regions known to mediate these effects are not involved. Rather, the finding that inhibiting the DMH decreases both locomotion and body temperature suggests that locomotion may be a key central contributor to MDMA-evoked hyperthermia.

Hypoxia-induced hypothermia mediated by GABA in the rostral parapyramidal area of the medulla oblongata

Hypoxia evokes a regulated decrease in the body core temperature (Tc) in a variety of animals. The neuronal mechanisms of this response include, at least in part, glutamatergic activation in the lateral preoptic area (LPO) of the hypothalamus. As the sympathetic premotor neurons in the medulla oblongata constitute a cardinal relay station in the descending neuronal pathway from the hypothalamus for thermoregulation, their inhibition can also be critically involved in the mechanisms of the hypoxia-induced hypothermia. Here, I examined the hypothesis that hypoxia-induced hypothermia is mediated by glutamate-responsive neurons in the LPO that activate GABAergic transmission in the rostral raphe pallidus (rRPa) and neighboring parapyramidal region (PPy) of the medulla oblongata in urethane-chloralose-anesthetized, neuromuscularly blocked, artificially ventilated rats. Unilateral microinjection of GABA (15nmol) into the rRPa and PPy regions elicited a prompt increase in tail skin temperature (Ts) and decreases in Tc, oxygen consumption rate (VO2), and heart rate. Next, when the GABAA receptor blocker bicuculline methiodide (bicuculline methiodide (BMI), 10pmol) alone was microinjected into the rRPa, it elicited unexpected contradictory responses: simultaneous increases in Ts, VO2 and heart rate and a decrease in Tc. Then, when BMI was microinjected bilaterally into the PPy, no direct effect on Ts was seen; and thermogenic and tachycardic responses were slight. However, pretreatment of the PPy with BMI, but not vehicle saline, greatly attenuated the hypothermic responses evoked by hypoxic (10%O2-90%N2, 5min) ventilation or bilateral microinjections of glutamate (5nmol, each side) into the LPO. The results suggest that hypoxia-induced hypothermia was mediated, at least in part, by the activation of GABAA receptors in the PPy.

Meth math: modeling temperature responses to methamphetamine

Methamphetamine (Meth) can evoke extreme hyperthermia, which correlates with neurotoxicity and death in laboratory animals and humans. The objective of this study was to uncover the mechanisms of a complex dose dependence of temperature responses to Meth by mathematical modeling of the neuronal circuitry. On the basis of previous studies, we composed an artificial neural network with the core comprising three sequentially connected nodes: excitatory, medullary, and sympathetic preganglionic neuronal (SPN). Meth directly stimulated the excitatory node, an inhibitory drive targeted the medullary node, and, in high doses, an additional excitatory drive affected the SPN node. All model parameters (weights of connections, sensitivities, and time constants) were subject to fitting experimental time series of temperature responses to 1, 3, 5, and 10 mg/kg Meth. Modeling suggested that the temperature response to the lowest dose of Meth, which caused an immediate and short hyperthermia, involves neuronal excitation at a supramedullary level. The delay in response after the intermediate doses of Meth is a result of neuronal inhibition at the medullary level. Finally, the rapid and robust increase in body temperature induced by the highest dose of Meth involves activation of high-dose excitatory drive. The impairment in the inhibitory mechanism can provoke a life-threatening temperature rise and makes it a plausible cause of fatal hyperthermia in Meth users. We expect that studying putative neuronal sites of Meth action and the neuromediators involved in a detailed model of this system may lead to more effective strategies for prevention and treatment of hyperthermia induced by amphetamine-like stimulants.

Role of an excitatory preoptic-raphé pathway in febrile vasoconstriction of the rat's tail

Heat dissipation from the rat's tail is reduced in response to cold and during fever. The sympathetic premotor neurons for this mechanism, located in the medullary raphé, are under tonic inhibitory control from the preoptic area. In parallel with the inhibitory pathway, an excitatory pathway from the rostromedial preoptic region (RMPO) to the medullary raphé mediates the vasoconstrictor response to cold skin. Whether this applies also to the tail vasoconstrictor response in fever is unknown. Single- or a few-unit tail sympathetic nerve activity (SNA) was recorded in urethane-anesthetized, artificially ventilated rats. Experimental fever was induced by PGE2 injected into the lateral cerebral ventricle (50 ng in 1.5 μl icv) or into the RMPO (0.2 ng in 60 nl); in both cases, there was a robust increase in tail SNA and a delayed rise in core temperature. Microinjection of glutamate receptor antagonist kynurenate (50 mM, 120 nl) into the medullary raphé completely reversed the tail SNA response to intracerebroventricular or RMPO PGE2 injection. Inhibiting RMPO neurons by microinjecting glycine (0.5 M, 60 nl) or the GABAA receptor agonist, muscimol (2 mM, 30-60 nl), reduced the tail SNA response to PGE2 injected into the same site by approximately half. Vehicle injections into the medullary raphé or RMPO were without effect. These results suggest that the tail vasoconstrictor response during experimental fever depends on a glutamatergic excitatory synaptic relay in the medullary raphé and that an excitatory output signal from the RMPO contributes to the tail vasoconstrictor response during fever.

Orexin neurons are indispensable for prostaglandin E2-induced fever and defence against environmental cooling in mice

We recently showed using prepro-orexin knockout (ORX-KO) mice and orexin neuron-ablated (ORX-AB) mice that orexin neurons in the hypothalamus, but not orexin peptides per se, are indispensable for stress-induced thermogenesis. To examine whether orexin neurons are more generally involved in central thermoregulatory mechanisms, we applied other forms of thermogenic perturbations, including brain prostaglandin E2 (PGE2) injections which mimic inflammatory fever and environmental cold exposure, to ORX-KO mice, ORX-AB mice and their wild-type (WT) litter mates. ORX-AB mice, but not ORX-KO mice, exhibited a blunted PGE2-induced fever and intolerance to cold (5°C) exposure, and these findings were similar to the results previously obtained with stress-induced thermogenesis. PGE2-induced shivering was also attenuated in ORX-AB mice. Both mutants responded similarly to environmental heating (39°C). In WT and ORX-KO mice, the administration of PGE2 and cold exposure activated orexin neurons, as revealed by increased levels of expression of c-fos. Injection of retrograde tracer into the medullary raphe nucleus revealed direct and indirect projection from the orexin neurons, of which the latter seemed to be preserved in the ORX-AB mice. In addition, we found that glutamate receptor antagonists (D-(-)-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione) but not orexin receptor antagonists (SB334867 and OX2 29) successfully inhibited PGE2-induced fever in WT mice. These results suggest that orexin neurons are important in general thermogenic processes, and their importance is not restricted to stress-induced thermogenesis. In addition, these results indicate the possible involvement of glutamate in orexin neurons implicated in PGE2-induced fever.