Neuronal basis of Hammel's model for set-point thermoregulation

Effects of lead on avian thermoregulation in the heat: An experimental test with pied crows (Corvus albus)

Many of the negative physiological effects of lead involve the hypothalamus, but the possibility that thermoregulation is affected has received little attention. We tested the hypothesis that lead exposure reduces avian thermoregulatory performance under hot conditions in pied crows (Corvus albus) experimentally exposed to lead in their diet. Crows in our high lead treatment (blood [Pb] = 87.3 ± 44.7 μg dL-1) showed significantly higher air temperature (Tair) inflections for evaporative water loss (EWL) and resting metabolic rate (RMR) compared to control (6.4 ± 1.8 μg dL-1) or intermediate (53.9 ± 23.7 μg dL-1) lead groups, which did not differ. EWL, RMR and body temperature (Tb) all increased more rapidly at Tair > Tb in the high lead treatment. In contrast, neither maximum Tair tolerated by the crows nor maximum Tb varied with treatment. Our data reveal that water and energy balance during hot weather is affected by lead exposure.

Modeling thermoregulatory responses during high-intensity exercise in warm environments

The six cylinder thermoregulatory model (SCTM) has been validated thoroughly for resting humans. This type of modeling is helpful to predict and develop guidance for safe performance of work and recreational activities. In the context of a warming global climate, updating the accuracy of the model for intense exercise in warm environments will help a wide range of individuals in athletic, recreational, and military settings. Three sets of previously collected data were used to determine SCTM accuracy. Dataset 1: two groups [large (LG) 91.5 kg and small (SM) 67.7 kg] of individuals performed 60 min of semirecumbent cycling in temperate conditions (25.1°C) at metabolic rates of 570-700 W. Dataset 2: two LG (100 kg) and SM (65.8 kg) groups performed 60 min of semirecumbent cycling in warm/hot environmental conditions (36.2°C) at metabolic rates of 590-680 W. Dataset 3: seven volunteers completed 8-km track trials (∼30 min) in cool (17°C) and warm (30°C) environments. The volunteers' metabolic rates were estimated to be 1,268 W and 1,166 W, respectively. For all datasets, SCTM-predicted core temperatures were found to be similar to the observed core temperatures. The root mean square deviations (RMSDs) ranged from 0.06 to 0.46°C with an average of 0.2°C deviation, which is less than the acceptance threshold of 0.5°C. Thus, the present validation shows that SCTM predicts core temperatures with acceptable accuracy during intense exercise in warm environments and successfully captures core temperature differences between large and small individuals.NEW & NOTEWORTHY The SCTM has been validated thoroughly for resting humans in warm and cold environments and during water immersion. The present study further demonstrated that SCTM predicts core temperatures with acceptable accuracy during intense exercise up to 1,300 W in temperate and warm environments and captures core temperature differences between large and small individuals. SCTM is potentially useful to develop guidance for safe operation in athletic, military, and occupational settings during exposure to warm or hot environments.

Traumatic brain injury extending to the striatum alters autonomic thermoregulation and hypothalamic monoamines in recovering rats

The brain cortex is the structure that is typically injured in traumatic brain injury (TBI) and is anatomically connected with other brain regions, including the striatum and hypothalamus, which are associated in part with motor function and the regulation of body temperature, respectively. We investigated whether a TBI extending to the striatum could affect peripheral and core temperatures as an indicator of autonomic thermoregulatory function. Moreover, it is unknown whether thermal modulation is accompanied by hypothalamic and cortical monoamine changes in rats with motor function recovery. The animals were allocated into three groups: the sham group (sham), a TBI group with a cortical contusion alone (TBI alone), and a TBI group with an injury extending to the dorsal striatum (TBI + striatal injury). Body temperature and motor deficits were evaluated for 20 days post-injury. On the 3rd and 20th days, rats were euthanized to measure the serotonin (5-HT), noradrenaline (NA), and dopamine (DA) levels using high-performance liquid chromatography (HPLC). We observed that TBI with an injury extending to the dorsal striatum increased core and peripheral temperatures. These changes were accompanied by a sustained motor deficit lasting for 14 days. Furthermore, there were notable increases in NA and 5-HT levels in the brain cortex and hypothalamus both 3 and 20 days after injury. In contrast, rats with TBI alone showed no changes in peripheral temperatures and achieved motor function recovery by the 7th day post-injury. In conclusion, our results suggest that TBI with an injury extending to the dorsal striatum elevates both core and peripheral temperatures, causing a delay in functional recovery and increasing hypothalamic monoamine levels. The aftereffects can be attributed to the injury site and changes to the autonomic thermoregulatory functions.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 1: Foundational principles and theories of regulation

This contribution is the first of a four-part, historical series encompassing foundational principles, mechanistic hypotheses and supported facts concerning human thermoregulation during athletic and occupational pursuits, as understood 100 years ago and now. Herein, the emphasis is upon the physical and physiological principles underlying thermoregulation, the goal of which is thermal homeostasis (homeothermy). As one of many homeostatic processes affected by exercise, thermoregulation shares, and competes for, physiological resources. The impact of that sharing is revealed through the physiological measurements that we take (Part 2), in the physiological responses to the thermal stresses to which we are exposed (Part 3) and in the adaptations that increase our tolerance to those stresses (Part 4). Exercising muscles impose our most-powerful heat stress, and the physiological avenues for redistributing heat, and for balancing heat exchange with the environment, must adhere to the laws of physics. The first principles of internal and external heat exchange were established before 1900, yet their full significance is not always recognised. Those physiological processes are governed by a thermoregulatory centre, which employs feedback and feedforward control, and which functions as far more than a thermostat with a set-point, as once was thought. The hypothalamus, today established firmly as the neural seat of thermoregulation, does not regulate deep-body temperature alone, but an integrated temperature to which thermoreceptors from all over the body contribute, including the skin and probably the muscles. No work factor needs to be invoked to explain how body temperature is stabilised during exercise.

Differential expressions of hypothalamic thermosensitive TRP ion channels may underlie the posthatching ontogeny of brain cooling capacity in broiler chickens

Two trials were performed to evaluate the association of hypothalamic abundances of thermosensitive transient receptor potential (TRP) ion channels with thermoregulation in broiler chickens. In trial 1, temporal changes in body temperatures, and hypothalamic expression patterns of TRP channels and thermoregulatory neurotransmitter concentrations were assessed from 3 to 42 d of age. In trial 2, the same variables were compared at 2 age stages between 2 distinct types of birds with high or low rectal temperatures (HRT or LRT). The core-to-brain temperature difference exhibited a rapid increase after hatching, arriving at a steady state in the fourth week (P < 0.01). The hypothalamus saw a progressive decrease of TRPV4 protein expression through 28 d (P < 0.01), followed by a great increase in the abundance of other channels right up to the end (P < 0.05). Compared to LRT birds, a decline in hypothalamic content of TRPV4 (P < 0.05), together with a bigger core-to-brain temperature difference (P < 0.01), was evident in the HRT counterpart at 33 d. In both trials, the core-to-brain and core-to-surface temperature differences were controlled in a synchronous and coordinated manner. These results allow concluding that developmental changes in the thermal sensitivity of hypothalamic neurons, determined by brain cooling capacity, involve a neuro-genomic mechanism, which regulates the ratio between thermosensitive TRP ion channels to attain a lower proportion of TRPV4 in comparison with other channels.

Energy and time optimization during exit from torpor in vertebrate endotherms

Torpor is used in small sized birds and mammals as an energy conservation trait. Considerable effort has been put towards elucidating the mechanisms underlying its entry and maintenance, but little attention has been paid regarding the exit. Firstly, we demonstrate that the arousal phase has a stereotyped dynamic: there is a sharp increase in metabolic rate followed by an increase in body temperature and, then, a damped oscillation in body temperature and metabolism. Moreover, the metabolic peak is around two-fold greater than the corresponding euthermic resting metabolic rate. We then hypothesized that either time or energy could be crucial variables to this event and constructed a model from a collection of first principles of physiology, control engineering and thermodynamics. From the model, we show that the stereotyped pattern of the arousal is a solution to save both time and energy. We extended the analysis to the scaling of the use of torpor by endotherms and show that variables related to the control system of body temperature emerge as relevant to the arousal dynamics. In this sense, the stereotyped dynamics of the arousal phase necessitates a certain profile of these variables which is not maintained as body size increases.

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

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

Arginine vasopressin effects on membrane potentials of preoptic area temperature-sensitive and -insensitive neurons in rat hypothalamic tissue slices

Arginine vasopressin (AVP) plays a hypothermic regulatory role in thermoregulation and is an important endogenous mediator in this mechanism. In the preoptic area (POA), AVP increases the spontaneous firing and thermosensitivity of warm-sensitive neurons and decreases those of cold-sensitive and temperature-insensitive neurons. Because POA neurons play a crucial role in precise thermoregulatory responses, these findings indicate that there is an association between the hypothermia and changes in the firing activity of AVP-induced POA neurons. However, the electrophysiological mechanisms by which AVP controls this firing activity remain unclear. Therefore, in the present study, using in vitro hypothalamic brain slices and whole-cell recordings, we elucidated the membrane potential responses of temperature-sensitive and -insensitive POA neurons to identify the applications of AVP or V_1a vasopressin receptor antagonists. By monitoring changes in the resting potential and membrane potential thermosensitivity of the neurons before and during experimental perfusion, we observed that AVP increased the changes in the resting potential of 50% of temperature-insensitive neurons but reduced them in others. These changes are because AVP enhances the membrane potential thermosensitivity of nearly 50% of the temperature-insensitive neurons. On the other hand, AVP changes both the resting potential and membrane potential thermosensitivity of temperature-sensitive neurons, with no differences between the warm- and cold-sensitive neurons. Before and during AVP or V_1a vasopressin receptor antagonist perfusion, no correlation was observed between changes in the thermosensitivity and membrane potential of all neurons. Furthermore, no correlation was observed between the thermosensitivity and membrane potential thermosensitivity of the neurons during experimental perfusion. In the present study, we found that AVP induction did not result in any changes in resting potential, which is unique to temperature-sensitive neurons. The study results suggest that AVP-induced changes in the firing activity and firing rate thermosensitivity of POA neurons are not controlled by resting potentials.

Hypothalamic warm-sensitive neurons require TRPC4 channel for detecting internal warmth and regulating body temperature in mice

Precise monitoring of internal temperature is vital for thermal homeostasis in mammals. For decades, warm-sensitive neurons (WSNs) within the preoptic area (POA) were thought to sense internal warmth, using this information as feedback to regulate body temperature (T_core). However, the cellular and molecular mechanisms by which WSNs measure temperature remain largely undefined. Via a pilot genetic screen, we found that silencing the TRPC4 channel in mice substantially attenuated hypothermia induced by light-mediated heating of the POA. Loss-of-function studies of TRPC4 confirmed its role in warm sensing in GABAergic WSNs, causing additional defects in basal temperature setting, warm defense, and fever responses. Furthermore, TRPC4 antagonists and agonists bidirectionally regulated T_core. Thus, our data indicate that TRPC4 is essential for sensing internal warmth and that TRPC4-expressing GABAergic WSNs function as a novel cellular sensor for preventing T_core from exceeding set-point temperatures. TRPC4 may represent a potential therapeutic target for managing T_core.

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

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

Early-life thermal stress mediates long-term alterations in hypothalamic microglia

Stressful environmental events in early life have long-lasting consequences on later stress responses. We previously showed that heat conditioning of 3-day-old chicks during the critical period of heat-response development leads to heat vulnerability later in life. Here we assessed the role of early-life heat stress on the inflammatory response in the chick anterior hypothalamus (AH), focusing on hypothalamic microglia. We identified the microglial cell population in the chick AH using anti-KUL01 and anti-CD45 antibodies. Specific microglial features were also confirmed by expression of their signature genes. Under normal environmental conditions, hypothalamic microglia isolated from lipopolysaccharide (LPS)-injected chicks displayed a classical activated proinflammatory profile accompanied by a decreased homeostatic signature, demonstrating similarity of immune response with mammalian microglial cells. In accordance with our previous observations, conditioning of 3-day-old chicks under high ambient temperature decreased the number of newborn cells in the AH, among them microglial precursors. Although heat exposure did not affect microglial cell viability, it had a long-term impact on LPS-induced inflammatory response. Exposure to harsh heat led to heat vulnerability, and attenuated recruitment of peripheral monocytes and T cells into the AH following LPS challenge. Moreover, heat conditioning altered microglial reactivity, manifested as suppressed microglial activation in response to LPS. Innate immune memory developed by heat conditioning might underlie suppression of the microglial response to LPS challenge. We describe alterations in genome-wide CpG methylation profile of hypothalamic microglia, demonstrating probable epigenetic involvement in the reprogramming of microglial function, leading to heat-induced inflammatory cross-tolerance.

Predicting thermal pleasure experienced in dynamic environments from simulated cutaneous thermoreceptor activity

Research into human thermal perception indoors has focused on "neutrality" under steady-state conditions. Recent interest in thermal alliesthesia has highlighted the hedonic dimension of our thermal world that has been largely overlooked by science. Here, we show the activity of sensory neurons can predict thermal pleasure under dynamic exposures. A numerical model of cutaneous thermoreceptors was applied to skin temperature measurements from 12 human subjects. A random forest model trained on simulated thermoreceptor impulses could classify pleasure responses (F1 score of 67%) with low false positives/negatives (4%). Accuracy increased (83%) when excluding the few extreme (dis)pleasure responses. Validation on an independent dataset confirmed model reliability. This is the first empirical demonstration of the relationship between thermoreceptors and pleasure arising from thermal stimuli. Insights into the neurophysiology of thermal perception can enhance the experience of built environments through designs that promote sensory excitation instead of neutrality.

Slit3 secreted from M2-like macrophages increases sympathetic activity and thermogenesis in adipose tissue

Beiging of white adipose tissue (WAT) is associated with an increase of anti-inflammatory M2-like macrophages in WAT. However, mechanisms through which M2-like macrophages affect beiging are incompletely understood. Here, we show that the macrophage cytokine Slit3 is secreted by adipose tissue macrophages and promotes cold adaptation by stimulating sympathetic innervation and thermogenesis in mice. Analysing the transcriptome of M2-like macrophages in murine inguinal WAT (iWAT) after cold exposure, we identify Slit3 as a secreted cytokine. Slit3 binds to the ROBO1 receptor on sympathetic neurons to stimulate Ca2+/calmodulin-dependent protein kinase II signalling and norepinephrine release, which enhances adipocyte thermogenesis. Adoptive transfer of Slit3-overexpressing M2 macrophages to iWAT promotes beiging and thermogenesis, whereas mice that lack Slit3 in myeloid cells are cold-intolerant and gain more weight. Our findings shed new light on the integral role of M2-like macrophages for adipose tissue homeostasis and uncover the macrophage-Slit3-sympathetic neuron-adipocyte signalling axis as a regulator of long-term cold adaptation.

Post-exercise Body Cooling: Skin Blood Flow, Venous Pooling, and Orthostatic Intolerance

Athletes and certain occupations (e.g., military, firefighters) must navigate unique heat challenges as they perform physical tasks during prolonged heat stress, at times while wearing protective clothing that hinders heat dissipation. Such environments and activities elicit physiological adjustments that prioritize thermoregulatory skin perfusion at the expense of arterial blood pressure and may result in decreases in cerebral blood flow. High levels of skin blood flow combined with an upright body position augment venous pooling and transcapillary fluid shifts in the lower extremities. Combined with sweat-driven reductions in plasma volume, these cardiovascular alterations result in levels of cardiac output that do not meet requirements for brain blood flow, which can lead to orthostatic intolerance and occasionally syncope. Skin surface cooling countermeasures appear to be a promising means of improving orthostatic tolerance via autonomic mechanisms. Increases in transduction of sympathetic activity into vascular resistance, and an increased baroreflex set-point have been shown to be induced by surface cooling implemented after passive heating and other arterial pressure challenges. Considering the further contribution of exercise thermogenesis to orthostatic intolerance risk, our goal in this review is to provide an overview of post-exercise cooling strategies as they are capable of improving autonomic control of the circulation to optimize orthostatic tolerance. We aim to synthesize both basic and applied physiology knowledge available regarding real-world application of cooling strategies to reduce the likelihood of experiencing symptomatic orthostatic intolerance after exercise in the heat.

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.

A thermoregulatory role of the medullary raphe in birds

The brainstem region medullary raphe modulates non-shivering and shivering thermogenesis and cutaneous vasomotion in rodents. Whether the same scenario occurs in the other endothermic group, i.e. birds, is still unknown. Therefore, we hypothesised that the medullary raphe modulates heat gain and loss thermoeffectors in birds. We investigated the effect of glutamatergic and GABAergic inhibitions in this specific region on body temperature (Tb), oxygen consumption (thermogenesis), ventilation (O2 supply in cold, thermal tachypnea in heat) and heat loss index (cutaneous vasomotion) in one-week-old chicken exposed to neutral (31°C), cold (26°C) and heat (36°C) conditions. Intra-medullary raphe antagonism of NMDA glutamate (AP5; 0.5, 5 mM) and GABAA (bicuculline; 0.05, 0.5 mM) receptors reduced Tb of chicks at 31°C and 26oC, due mainly to an O2 consumption decrease. AP5 transiently increased breathing frequency during cold exposure. At 31°C, heat loss index was higher in the bicuculline and AP5 groups (higher doses) than vehicle at the beginning of the Tb reduction. No treatment affected any variable tested at 36oC. The results suggest that glutamatergic and GABAergic excitatory influences on the medullary raphe of chicks modulate thermogenesis and glutamatergic stimulation prevents tachypnea, without having any role in warmth-defence responses. A double excitation influence on the medullary raphe may provide a protective neural mechanism for supporting thermogenesis during early life, when energy expenditure to support growth and homeothermy is high. This novel demonstration of a thermoregulatory role for the raphe in birds suggests a convergent brainstem neurochemical regulation of body temperature in endotherms.

Short-term hot water immersion results in substantial thermal strain and partial heat acclimation; comparisons with heat-exercise exposures

OBJECTIVE

To examine the effectiveness of hot water immersion (HWI) as a heat acclimation strategy in comparison to time and temperature matched, exercise-heat acclimation (EHA).

METHODS

8 males performed heat stress tests (HST) (45 min of cycling at 50% of VO_2max in 40 °C, 40% RH) before and after heat acclimation sessions. Acclimation sessions were either three consecutive bouts of HWI (40 min of submersion at 40 °C) or EHA (40 min of cycling at 50% VO_2max in 40 °C, 40% RH).

RESULTS

Average change in tympanic temperature (T_Tympanic) was significantly higher following HWI (2.1 °C ± 0.4) compared to EHA (1.5 °C ± 0.4) (P < 0.05). Decreases in peak heart rate (HR) (HWI: -10 bpm ± 8; EHA: -6 ± 7), average HR (-7 bpm ± 6; -3 ± 4), and average core temperature (-0.4 °C ± 0.3; -0.2 ± 0.4) were evident following acclimation (P < 0.05), but not different between interventions (P > 0.05). Peak rate of perceived exertion (RPE_Peak) decreased for HWI and EHA (P < 0.05). Peak thermal sensation (TS_Peak) decreased following HWI (P < 0.05) but was not different between interventions (P > 0.05). Plasma volume increased in both intervention groups (HWI: 5.9% ± 5.1; EHA: 5.4% ± 3.7) but was not statistically different (P > 0.05).

CONCLUSION

HWI induced significantly greater thermal strain compared to EHA at equivalent temperatures during time-matched exposures. However, the greater degree of thermal strain did not result in between intervention differences for cardiovascular, thermoregulatory, or perceptual variables. Findings suggest three HWI sessions may be a potential means to lower HR, TCore, and perceptual strain during exercise in the heat.

Activation of Preoptic Arginine Vasopressin Neurons Induces Hyperthermia in Male Mice

Arginine vasopressin (AVP) is a neuropeptide acting as a neuromodulator in the brain and plays multiple roles, including a thermoregulatory one. However, the cellular mechanisms of action are not fully understood. Carried out are patch clamp recordings and calcium imaging combined with pharmacological tools and single-cell RT-PCR to dissect the signaling mechanisms activated by AVP. Optogenetics combined with patch-clamp recordings were used to determine the neurochemical nature of these neurons. Also used is telemetry combined with chemogenetics to study the effect of activation of AVP neurons in thermoregulatory mechanisms. This article reports that AVP neurons in the medial preoptic (MPO) area release GABA and display thermosensitive firing activity. Their optogenetic stimulation results in a decrease of the firing rates of MPO pituitary adenylate cyclase-activating polypeptide (PACAP) neurons. Local application of AVP potently modulates the synaptic inputs of PACAP neurons, by activating neuronal AVPr1a receptors and astrocytic AVPr1b receptors. Chemogenetic activation of MPO AVP neurons induces hyperthermia. Chemogenetic activation of all AVP neurons in the brain similarly induces hyperthermia and, in addition, decreases the endotoxin activated fever as well as the stress-induced hyperthermia.

Cytokines in Febrile Diseases

The human body has a perfect thermoregulatory system to meet the needs of normal life activities. The central regulation of body temperature is mainly explained by the theory of "setting point (setpoint, SP)". Fever is a positive but nonspecific response of the body to infections and other pyrogens, which causes immune cells to release cytokines, leading to a brain protein-mediated rise in body temperature. Cytokines can be roughly divided into 2 categories: proinflammatory cytokines and anti-inflammatory cytokines. IL-1, TNF-α, and IL-6 are proinflammatory cytokines, whereas IL-4 and IL-10 are anti-inflammatory cytokines. IL-2 is a cytokine that can both activate and inhibit immunity. IL-8 is a neutrophil chemotactic factor, and IFN is a cytokine that plays a key role in the proper induction and maintenance of innate and acquired immunity. This article reviews the pathophysiological characteristics of fever and the cytokines related to fever (IL-2, 4, 6, 8, 10, IFN, TNF, etc.).

Physiology and Pathophysiology of the Autonomic Nervous System

PURPOSE OF THE REVIEW

This article reviews the anatomic, functional, and neurochemical organization of the sympathetic and parasympathetic outputs; the effects on target organs; the central mechanisms controlling autonomic function; and the pathophysiologic basis for core symptoms of autonomic failure.

RECENT FINDINGS

Functional neuroimaging studies have elucidated the areas involved in central control of autonomic function in humans. Optogenetic and other novel approaches in animal experiments have provided new insights into the role of these areas in autonomic control across behavioral states, including stress and the sleep-wake cycle.

SUMMARY

Control of the function of the sympathetic, parasympathetic, and enteric nervous system functions depends on complex interactions at all levels of the neuraxis. Peripheral sympathetic outputs are critical for maintenance of blood pressure, thermoregulation, and response to stress. Parasympathetic reflexes control lacrimation, salivation, pupil response to light, beat-to-beat control of the heart rate, gastrointestinal motility, micturition, and erectile function. The insular cortex, anterior and midcingulate cortex, and amygdala generate autonomic responses to behaviorally relevant stimuli. Several nuclei of the hypothalamus generate coordinated patterns of autonomic responses to internal or social stressors. Several brainstem nuclei participate in integrated control of autonomic function in relationship to respiration and the sleep-wake cycle. Disorders affecting the central or peripheral autonomic pathways, or both, manifest with autonomic failure (including orthostatic hypotension, anhidrosis, gastrointestinal dysmotility, and neurogenic bladder or erectile dysfunction) or autonomic hyperactivity, primary hypertension, tachycardia, and hyperhidrosis.

Integrative cardiovascular control in women: Regulation of blood pressure, body temperature, and cerebrovascular responsiveness

Over the past several decades, it has become increasingly clear that women have distinct cardiovascular profiles compared to men. In this review, our goal is to provide an overview of the literature regarding the influences of female sex and reproductive hormones (primarily estradiol) on mechanisms of cardiovascular control relevant to regulation of blood pressure, body temperature, and cerebral blood flow. Young women tend to have lower resting blood pressure compared with men. This sex difference is reversed at menopause, when women develop higher sympathetic nerve activity and the risk of systemic hypertension increases sharply as postmenopausal women age. Vascular responses to thermal stress, including cutaneous vasodilation and vasoconstriction, are also affected by reproductive hormones in women, where estradiol appears to promote vasodilation and heat dissipation. The influence of reproductive hormones on cerebral blood flow and sex differences in the ability of the cerebral vasculature to increase its blood flow (cerebrovascular reactivity) are relatively new areas of investigation. Sex and hormonal influences on integrative blood flow regulation have further implications during challenges to physiological homeostasis, including exercise. We propose that increasing awareness of these sex-specific mechanisms is important for optimizing health care and promotion of wellness in women across the life span.

Early-Life m6A RNA Demethylation by Fat Mass and Obesity-Associated Protein (FTO) Influences Resilience or Vulnerability to Heat Stress Later in Life

Early life heat stress leads to either resilience or vulnerability to a similar stress later in life. We have previously shown that this tuning of the stress response depends on neural network organization in the preoptic anterior hypothalamus (PO/AH) thermal response center and is regulated by epigenetic mechanisms. Here, we expand our understanding of stress response establishment describing a role for epitranscriptomic regulation of the epigenetic machinery. Specifically, we explore the role of N⁶-methyladenosine (m⁶A) RNA methylation in long-term response to heat stress. Heat conditioning of 3-d-old chicks diminished m⁶A RNA methylation in the hypothalamus, simultaneously with an increase in the mRNA levels of the m⁶A demethylase, fat mass and obesity-associated protein (FTO). Moreover, a week later, methylation of two heat stress-related transcripts, histone 3 lysine 27 (H3K27) methyltransferase, enhancer of zeste homolog 2 (EZH2) and brain-derived neurotrophic factor (BDNF), were downregulated in harsh-heat-conditioned chicks. During heat challenge a week after conditioning, there was a reduction of m⁶A levels in mild-heat-conditioned chicks and an elevation in harsh-heat-conditioned ones. This increase in m⁶A modification was negatively correlated with the expression levels of both BDNF and EZH2. Antisense “knock-down” of FTO caused an elevation of global m⁶A RNA methylation, reduction of EZH2 and BDNF mRNA levels, and decrease in global H3K27 dimethylation as well as dimethyl H3K27 level along BDNF coding region, and, finally, led to heat vulnerability. These findings emphasize the multilevel regulation of gene expression, including both epigenetic and epitranscriptomic regulatory mechanisms, fine-tuning the neural network organization in a response to stress.

Modeling the long term effects of thermoregulation on human sleep

The connection between human sleep and energy exertion has long been regarded as part of the reasoning for the need to sleep. A recent theory proposes that during REM sleep, energy utilized for thermoregulation is diverted to other relevant biological processes. We present a mathematical model of human sleep/wake regulation with thermoregulatory functions to gain quantitative insight into the effects of ambient temperature on sleep quality. Our model extends previous models by incorporating equations for the metabolic processes that control thermoregulation during sleep. We present numerical simulations that provide a quantitative answer for how humans adjust by changing the normal sleep stage progression when it is challenged with ambient temperatures away from thermoneutral. We explore the dynamics for a single night and several nights. Our results indicate that including the effects of temperature is a vital component of modeling sleep.

Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond

The participation of women in physically strenuous athletic and occupational tasks has increased substantially in the past decade. Female sex steroids have influences on thermoregulatory processes that could impact physical performance in the heat. Here, we summarize and evaluate the current literature regarding sex differences in thermoregulation and provide recommendations for heat-illness risk-mitigation strategies.

Depiction of Physiological Homeostasis by Self-Coupled System and Its Significance

The negative feedback system (NFS) was regarded as the basic unit of regulation of physiological homeostasis for more than 70 years. However, NFS-based depiction possesses some limitations. The self-coupled system (SCS), a non-stop system in which the output of the current moment becomes the input of the next moment, can also be utilized to depict homeostasis. In SCS-based depiction, all of the related regulatory mechanisms of a homeostasis are regarded as an entity. Then, homeostatic dynamics can be expressed by simple mathematical language. A new disease group was revealed and some useful inferences were obtained through mathematical deduction. They were supported by published studies. SCS-based depiction of homeostasis should be a requisite supplement to medical knowledge systems based on NFS.

Effect of local skin blood flow during light and medium activities on local skin temperature predictions

The quality of local skin temperature prediction by thermophysiological models depends on the local skin blood flow (SBF) control functions. These equations were derived for low activity levels (0.8-1met) and mostly in sitting or supine position. This study validates and discusses the prediction of foot SBF during activities of 1-3met in male and females, and the effect on the foot skin temperature prediction (ΔT_skin,foot) using the thermophysiological simulation model ThermoSEM. The SBF at the foot was measured for ten male and ten female human subjects at baseline and during three activities (sitting, walking at 1km/h, preferred walking around 3km/h). Additional measurements included the energy expenditure, local skin temperatures (T_skin,loc), environmental conditions and body composition. Measured, normalized foot SBF is 2-8 times higher than the simulated SBF during walking sessions. Also, SBF increases are significantly higher in females vs. males (preferred walking: 4.8±1.5 versus 2.7±1.4, P < 0.05). The quality of ΔT_skin,foot using the simulated foot SBF is poor (median deviation is -4.8°C, maximumumdeviationis-6°C). Using the measured SBF in ThermoSEM results in an improved local skin temperature prediction (new maximum deviation is -3.3°C). From these data a new SBF model was developed that includes the walking activity level and gender, and improves SBF prediction and ΔT_skin,foot of the thermophysiological model. Accurate SBF and local skin temperature predictions are beneficial in optimizing thermal comfort simulations in the built environment, and might also be applied in sport science or patient's temperature management.

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.

The location and characteristics of the thermal sudomotor pathways in the human brainstem: A reappraisal

To elucidate location and characteristics of the central thermoregulatory sudomotor pathway in the human brainstem, thermoregulatory sweating (TS) in 91 patients with focal brainstem lesions was studied. TS was symmetric or minimally asymmetric in 40 subjects (Group S), and was apparently asymmetric in 51 patients (Group AS). In Group AS, the main abnormality was ipsilateral segmental hypohidrosis with a varying extent, involving predominantly the upper half of the body. Lesion locations, correlations between thermoregulatory sweat test results, and other autonomic and somatic functions were compared between the groups. The results suggested following: (1) The hypothalamospinal pathway related to TS may pass through the posterior hypothalamus and descend in the dorsolateral part of the brainstem, near the spinal trigeminal and spinothalamic tracts; (2) the pathway may descend together with those related to oculosympathetic and vasoconstrictor systems, but each of these may form distinct fiber groups; (3) the majority of the central TS fibers may reach ipsilateral sudomotor sympathetic neurons of the spinal cord, even though some fibers may cross at various levels; (4) in this descending pathway, somatotopic arrangements corresponding to each of the spinal sympathetic segments must be present; (5) There may be another fiber group passing through the central to dorsal paramedian portions of the brainstem, and lesions of these fibers also result in asymmetric TS, but seldom in oculosympathetic dysfunction. This second pathway probably exerts contralateral inhibitory influence on TS, but its origin, intracerebral course and exact physiological function require further clinical investigations.

STIM1 thermosensitivity defines the optimal preference temperature for warm sensation in mice

Mammals possess a remarkable ability to sense subtle temperature deviations from the thermoneutral skin temperature of ~33 °C, which ensures precise warm sensation. However, the underlying mechanisms remain unclear. Here we show that STIM1, an endoplasmic reticulum (ER) resident transmembrane protein that responds to both ER Ca²⁺ depletion and heat, mediates temperature-induced Ca²⁺ influx in skin keratinocytes via coupling to Orai Ca²⁺ channels in plasma membrane. Behaviorally, the keratinocyte-specific knockout of STIM1 shifts the optimal preference temperature (OPT) of mice from ~32 °C to ~34 °C, resulting in a strikingly reversed preference between 32 °C and 34 °C. Importantly, the thermally inactive STIM1-ΔK knock-in mice show altered OPT and warm preference behaviors as well, demonstrating the requirement of STIM1 thermosensitivity for warm sensation. Furthermore, the wild-type and mutant mice prefer temperatures closer to their respective OPTs, but poorly distinguish temperatures that are equally but oppositely deviated from their OPTs. Mechanistically, keratinocyte STIM1 affects the in vivo warm responses of sensory neurons by likely involving TRPA1 as a downstream transduction channel. Collectively, our data suggest that STIM1 serves as a novel in vivo thermosensor in keratinocytes to define the OPT, which might be utilized as a peripheral reference temperature for precise warm sensation.

Independent Risk Factors for the Shivering Occurrence During Induction Period in Out-of-Hospital Cardiac Arrest Survivors Treated with Targeted Temperature Management

The occurrence of shivering during the induction period of targeted temperature management (TTM) remains a therapeutic obstacle, which delays the achievement of target temperature. The aim of this study was to identify risk factors leading to shivering during the induction period. We analyzed a prospective cohort of adult out-of-hospital cardiac arrest (OHCA) survivors treated with TTM from January 2015 to June 2017. Patients who developed shivering during the induction period were compared to those who did not. Multivariable analysis was performed to determine risk factors of shivering. Among 80 patients treated with TTM, shivering occurred in 22 patients (27.5%). In the shivering group, the time to achieve target temperature was significantly delayed (245 minutes vs. 151 minutes, p = 0.005). Multivariable analysis showed that being underweight (OR, 18.40; 95% CI, 1.89-179.19) or overweight (OR, 8.65; 95% CI, 1.60-46.80), age <65 years (OR, 5.54; 95% CI, 1.25-16.12), and duration of cardiac arrest <20 minutes (OR, 4.50; 95% CI, 1.25-16.12) were predictors for the occurrence of shivering. OHCA patients with abnormal body weight, age <65 years, and duration of cardiac arrest <20 minutes should be monitored thoroughly for early recognition of shivering.

Central thermoreceptors

Homeotherms maintain their core body temperature within a narrow range by employing multiple redundant mechanisms to control heat production and dissipation. Preoptic area/anterior hypothalamic (PO/AH) neurons receive thermal signals from peripheral and deep-body thermoreceptors as well as hormonal and metabolic signals. A population of PO/AH neurons termed warm-sensitive increase their firing temperature with warming and are considered central thermoreceptors. Electrophysiologic and pharmacologic experiments have provided descriptions of their characteristics and signaling mechanisms. These studies have also allowed insights into the mechanisms by which neurochemicals important in thermoregulation exert their influence. Finally, the cellular mechanism involved in the interactions between thermoregulation and other aspects of homeostasis, such as energy metabolism and osmoregulation, have started to be unraveled.

The thermoregulation system and how it works

Heat exchange processes between the body and the environment are introduced. The definition of the thermoneutral zone as the ambient temperature range within which body temperature (T_b) regulation is achieved only by nonevaporative processes is explained. Thermoreceptors, thermoregulatory effectors (both physiologic and behavioral), and neural pathways and T_b signals that connect receptors and effectors into a thermoregulation system are reviewed. A classification of thermoeffectors is proposed. A consensus concept is presented, according to which the thermoregulation system is organized as a dynamic federation of independent thermoeffector loops. While the activity of each effector is driven by a unique combination of deep (core) and superficial (shell) T_bs, the regulated variable of the system can be viewed as a spatially distributed T_b with a heavily represented core and a lightly represented shell. Core T_b is the main feedback; it is always negative. Shell T_bs (mostly of the hairy skin) represent the auxiliary feedback, which can be negative or positive, and which decreases the system's response time and load error. Signals from the glabrous (nonhairy) skin about the temperature of objects in the environment serve as feedforward signals for various behaviors. Physiologic effectors do not use feedforward signals. The system interacts with other homeostatic systems by "meshing" with their loops. Coordination between different thermoeffectors is achieved through the common controlled variable, T_b. The term balance point (not set point) is used for a regulated level of T_b. The term interthreshold zone is used for a T_b range in which no effectors are activated. Thermoregulatory states are classified, based on whether: T_b is increased (hyperthermia) or decreased (hypothermia); the interthreshold zone is narrow (homeothermic type of regulation) or wide (poikilothermic type); and the balance point is increased (fever) or decreased (anapyrexia). During fever, thermoregulation can be either homeothermic or poikilothermic; anapyrexia is always a poikilothermic state. The biologic significance of poikilothermic states is discussed. As an example of practical applications of the concept presented, thermopharmacology is reviewed. Thermopharmacology uses drugs to modulate specific temperature signals at the level of a thermoreceptor (transient receptor potential channel).

Arginine vasopressin antagonizes the effects of prostaglandin E2 on the spontaneous activity of warm-sensitive and temperature-insensitive neurons in the medial preoptic area in rats

Arginine vasopressin (AVP) plays an important role in thermoregulation and antipyresis. We have demonstrated that AVP could change the spontaneous activity of thermosensitive and temperature insensitive neurons in the preoptic area. However, whether AVP influences the effects of prostaglandin E₂ (PGE₂) on the spontaneous activity of neurons in the medial preoptic area (MPO) remains unclear. Our experiment showed that PGE₂ decreased the spontaneous activity of warm-sensitive neurons, and increased that of low-slope temperature-insensitive neurons in the MPO. AVP attenuated the inhibitory effect of PGE₂ on warm-sensitive neurons, and reversed the excitatory effect of PGE₂ on low-slope temperature-insensitive neurons, demonstrating that AVP antagonized the effects of PGE₂ on the spontaneous activity of these neurons. The effect of AVP was suppressed by an AVP V_1a receptor antagonist, suggesting that V_1a receptor mediated the action of AVP. We also demonstrated that AVP attenuated the PGE₂-induced decrease in the prepotential's rate of rise in warm-sensitive neurons and the PGE₂-induced increase in that in low-slope temperature-insensitive neurons through the V_1a receptor. Together, these data indicated that AVP antagonized the PGE₂-induced change in the spontaneous activity of warm-sensitive and low-slope temperature-insensitive neurons in the MPO partly by reducing the PGE₂-induced change in the prepotential of these neurons in a V_1a receptor-dependent manner.

Effect of Heat Exposure on Cognition in Persons with Tetraplegia

Individuals with cervical spinal cord injury (SCI) have impaired thermoregulatory mechanisms attributed to interruption of motor, sensory, and autonomic neuropathways. To determine the effects of heat exposure on core body temperature (Tcore) and cognitive performance in persons with tetraplegia, 8 individuals with chronic tetraplegia (C3-C7, American Spinal Cord Injury Association Impairment Scale A-B) and 9 able-bodied controls were acclimated to 27°C at baseline (BL) before being exposed to 35°C for up to 120 min (Heat Challenge). Rectal temperature (Tcore), distal skin temperatures (Tsk_avg), sweat rate (QS_avg), microvascular skin perfusion (LDF_avg), and plasma norepinephrine (NE) were measured. Cognitive performance was assessed using Stroop Color and Word and Wechsler Adult Intelligence Scale-Fourth Edition Digit Span tests at BL and at the end of Heat Challenge. After Heat Challenge, Tcore increased 0.78 ± 0.18°C (p < 0.001) in tetraplegics after an average of 118 ± 5 min. Tcore did not change in controls after 120 min. The increase in QS_avg was larger in controls than in tetraplegics (946 ± 672% vs. 51 ± 12%; p = 0.007, respectively). LDF_avg increased only in controls (109 ± 93%; p = 0.008). Tsk_avg appeared to increase less in tetraplegics than in controls. Plasma NE levels remained lower in tetraplegics compared to controls after Heat Challenge (86 ± 64 vs. 297 ± 84 pg/mL, respectively; p < 0.001). Stroop Color, Interference, and WAIS-IV Sequence scores increased only in tetraplegics (19.4 ± 17.2%; p < 0.05, 8.3 ± 5.9%; p < 0.05, 29.1 ± 27.4%; p < 0.05, respectively). Dysfunctional thermoregulatory mechanisms in the tetraplegic group allowed Tcore to rise from subnormal levels to normothermia during heat exposure. Normothermia was associated with improvements in attention, working memory, and executive function.

Sex difference in thermal preference of adult mice does not depend on presence of the gonads

Background

The thermoneutral zone (TNZ) is a species-specific range of ambient temperature (T_a), at which mammals can maintain a constant body temperature with the lowest metabolic rate. The TNZ for an adult mouse is between 26 and 34 °C. Interestingly, female mice prefer a higher T_a than male mice although the underlying mechanism for this sex difference is unknown. Here, we tested whether gonadal hormones are dominant factors controlling temperature preference in male and female mice.

Methods

We performed a temperature preference test in which 10-week-old gonadectomized and sham-operated male and female C57BL/6J mice were allowed to choose to reside at the thermoneutral cage of 29 °C or an experimental cage of 26, 29, or 32 °C.

Results

All mice preferred a T_a higher than 26 °C, especially in the inactive phase. Choosing between 29 and 32 °C, female mice resided more at 32 °C while male mice had no preference between the temperatures. Hence, the preferred T_a for female mice was significantly higher (0.9 ± 0.2 °C) than that for male mice. However, gonadectomy did not influence the T_a preference.

Conclusions

Female mice prefer a warmer environment than male mice, a difference not affected by gonadectomy. This suggests that thermal-sensing mechanisms may be influenced by sex-specific pathways other than gonadal factors or that the thermoregulatory set point has already been determined prior to puberty.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-017-0145-7) contains supplementary material, which is available to authorized users.

A hypothalamic circuit that controls body temperature

The homeostatic control of body temperature is essential for survival in mammals and is known to be regulated in part by temperature-sensitive neurons in the hypothalamus. However, the specific neural pathways and corresponding neural populations have not been fully elucidated. To identify these pathways, we used cFos staining to identify neurons that are activated by a thermal challenge and found induced expression in subsets of neurons within the ventral part of the lateral preoptic nucleus (vLPO) and the dorsal part of the dorsomedial hypothalamus (DMD). Activation of GABAergic neurons in the vLPO using optogenetics reduced body temperature, along with a decrease in physical activity. Optogenetic inhibition of these neurons resulted in fever-level hyperthermia. These GABAergic neurons project from the vLPO to the DMD and optogenetic stimulation of the nerve terminals in the DMD also reduced body temperature and activity. Electrophysiological recording revealed that the vLPO GABAergic neurons suppressed neural activity in DMD neurons, and fiber photometry of calcium transients revealed that DMD neurons were activated by cold. Accordingly, activation of DMD neurons using designer receptors exclusively activated by designer drugs (DREADDs) or optogenetics increased body temperature with a strong increase in energy expenditure and activity. Finally, optogenetic inhibition of DMD neurons triggered hypothermia, similar to stimulation of the GABAergic neurons in the vLPO. Thus, vLPO GABAergic neurons suppressed the thermogenic effect of DMD neurons. In aggregate, our data identify vLPO→DMD neural pathways that reduce core temperature in response to a thermal challenge, and we show that outputs from the DMD can induce activity-induced thermogenesis.

The Central Control of Energy Expenditure: Exploiting Torpor for Medical Applications

Autonomic thermoregulation is a recently acquired function, as it appears for the first time in mammals and provides the brain with the ability to control energy expenditure. The importance of such control can easily be highlighted by the ability of a heterogeneous group of mammals to actively reduce metabolic rate and enter a condition of regulated hypometabolism known as torpor. The central neural circuits of thermoregulatory cold defense have been recently unraveled and could in theory be exploited to reduce energy expenditure in species that do not normally use torpor, inducing a state called synthetic torpor. This approach may represent the first steps toward the development of a technology to induce a safe and reversible state of hypometabolism in humans, unlocking many applications ranging from new medical procedures to deep space travel.

The TRPM2 channel is a hypothalamic heat sensor that limits fever and can drive hypothermia

Body temperature homeostasis is critical for survival and requires precise regulation by the nervous system. The hypothalamus serves as the principal thermostat that detects and regulates internal temperature. We demonstrate that the ion channel TRPM2 [of the transient receptor potential (TRP) channel family] is a temperature sensor in a subpopulation of hypothalamic neurons. TRPM2 limits the fever response and may detect increased temperatures to prevent overheating. Furthermore, chemogenetic activation and inhibition of hypothalamic TRPM2-expressing neurons in vivo decreased and increased body temperature, respectively. Such manipulation may allow analysis of the beneficial effects of altered body temperature on diverse disease states. Identification of a functional role for TRP channels in monitoring internal body temperature should promote further analysis of molecular mechanisms governing thermoregulation and foster the genetic dissection of hypothalamic circuits involved with temperature homeostasis.

Anorexia is Associated with Stress-Dependent Orexigenic Responses to Exogenous Neuropeptide Y

Chicken lines that have been divergently selected for either low (LWS) or high (HWS) body weight at 56 days of age for more than 57 generations have different feeding behaviours in response to a range of i.c.v. injected neurotransmitters. The LWS have different severities of anorexia, whereas the HWS become obese. Previously, we demonstrated that LWS chicks did not respond, whereas HWS chicks increased food intake, after central injection of neuropeptide Y (NPY). The present study aimed to determine the molecular mechanisms underlying the loss of orexigenic function of NPY in LWS. Chicks were divided into four groups: stressed LWS and HWS on day of hatch, and control LWS and HWS. The stressor was a combination of food deprivation and cold exposure. On day 5 post-hatch, each chick received an i.c.v. injection of vehicle or 0.2 nmol of NPY. Only the LWS stressed group did not increase food intake in response to i.c.v. NPY. Hypothalamic mRNA abundance of appetite-associated factors was measured at 1 h post-injection. Interactions of genetic line, stress and NPY treatment were observed for the mRNA abundance of agouti-related peptide (AgRP) and synaptotagmin 1 (SYT1). Intracerebroventricular injection of NPY decreased and increased AgRP and SYT1 mRNA, respectively, in the stressed LWS and increased AgRP mRNA in stressed HWS chicks. Stress was associated with increased NPY, orexin receptor 2, corticotrophin-releasing factor receptor 1, melanocortin receptor 3 (MC3R) and growth hormone secretagogue receptor expression. In conclusion, the loss of responsiveness to exogenous NPY in stressed LWS chicks may be a result of the decreased and increased hypothalamic expression of AgRP and MC3R, respectively. This may induce an intensification of anorexigenic melanocortin signalling pathways in LWS chicks that block the orexigenic effect of exogenous NPY. These results provide insights onto the anorexic condition across species, and especially for forms of inducible anorexia such as human anorexia nervosa.

Sympathetic regulation during thermal stress in human aging and disease

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.