Thermoeffector neuronal pathways in fever: a study in rats showing a new role of the locus coeruleus

Diversity of ancestral brainstem noradrenergic neurons across species and multiple biological factors

The brainstem region, locus coeruleus (LC), has been remarkably conserved across vertebrates. Evolution has woven the LC into wide-ranging neural circuits that influence functions as broad as autonomic systems, the stress response, nociception, sleep, and high-level cognition among others. Given this conservation, there is a strong possibility that LC activity is inherently similar across species, and furthermore that age, sex, and brain state influence LC activity similarly across species. The degree to which LC activity is homogenous across these factors, however, has never been assessed due to the small sample size of individual studies. Here, we pool data from 20 laboratories (1,855 neurons) and show diversity across both intrinsic and extrinsic factors such as species, age, sex and brain state. We use a negative binomial regression model to compare activity from male monkeys, and rats and mice of both sexes that were recorded across brain states from brain slices ex vivo or under different anesthetics or during wakefulness in vivo. LC activity differed due to complex interactions of species, sex, and brain state. The LC became more active during aging, independent of sex. Finally, in contrast to the foundational principle that all species express two distinct LC firing modes ("tonic" or "phasic"), we discovered great diversity within spontaneous LC firing patterns. Different factors were associated with higher incidence of some firing modes. We conclude that the activity of the evolutionarily-ancient LC is not conserved. Inherent differences due to age and species-sex-brain state interactions have implications for understanding the role of LC in species-specific naturalistic behavior, as well as in psychiatric disorders, cardiovascular disease, immunology, and metabolic disorders.

Inhibition of rat locus coeruleus neurons by prostaglandin E2 EP3 receptors: pharmacological characterization ex vivo

Prostaglandin E2 (PGE2) is an inflammatory mediator synthesized by the brain constitutive cyclooxygenase enzyme. PGE2 binds to G protein-coupled EP1-4 receptors (EP1 to Gq, EP2,4 to Gs, and EP3 to Gi/o). EP2, EP3 and EP4 receptors are expressed in the locus coeruleus (LC), the main noradrenergic nucleus in the brain. EP3 receptors have been explored in the central nervous system, although its role regulating the locus coeruleus neuron activity has not been pharmacologically defined. Our aim was to characterize the function of EP3 receptors in neurons of the LC. Thus, we studied the effect of EP3 receptor agonists on the firing activity of LC cells in rat brain slices by single-unit extracellular electrophysiological techniques. The EP3 receptor agonist sulprostone (0.15 nM-1.28 µM), PGE2 (0.31 nM-10.2 µM) and the PGE1 analogue misoprostol (0.31 nM-2.56 µM) inhibited the firing rate of LC neurons in a concentration-dependent manner (EC50 = 15 nM, 110 nM, and 51 nM, respectively). The EP3 receptor antagonist L-798,106 (3-10 µM), but not the EP2 (PF-04418948, 3-10 µM) or EP4 (L-161,982, 3-10 µM) receptor antagonists, caused rightward shifts in the concentration-effect curves for the EP3 receptor agonists. Sulprostone-induced effect was attenuated by the Gi/o protein blocker pertussis toxin (pertussis toxin, 500 ng ml-1) and the inhibitors of inwardly rectifying potassium channels (GIRK) BaCl2 (300 µM) and SCH-23390 (15 µM). In conclusion, LC neuron firing activity is regulated by EP3 receptors, presumably by an inhibitory Gi/o protein- and GIRK-mediated mechanism.

Sleep and Neuroimmunomodulation for Maintenance of Optimum Brain Function: Role of Noradrenaline

Immune function and sleep are two normal physiological processes to protect the living organism from falling sick. There is hardly any disease in which they remain unaffected, though the quantum of effect may differ. Therefore, we propose the existence of a strong correlation between sleep (quality or quantity) and immune response. This may be supported by the fact that sleep loss modulates many of the immunological molecules, which includes interferons; however, not much is known about their mechanism of action. Sleep is divided into rapid eye movement sleep (REMS) and non-REMS. For practical reasons, experimental studies have been conducted mostly by inducing loss of REMS. It has been shown that withdrawal of noradrenaline (NA) is a necessity for generation of REMS. Moreover, NA level increases in the brain upon REMS loss and the elevated NA is responsible for many of the sleep loss-associated symptoms. In this review, we describe how sleep (and its disturbance/loss) modulates the immune system by modulating the NA level in the brain or vice versa to maintain immune functions, physiological homeostasis, and normal healthy living. The increased levels of NA during REMS loss may cause neuroinflammation possibly by glial activation (as NA is a key modulator of microglia). Therefore, maintaining sleep hygiene plays a crucial role for a normal healthy living.

Sexually dimorphic role of the locus coeruleus PAC1 receptors in regulating acute stress-associated energy metabolism

Severe stress leads to alterations in energy metabolism with sexually dimorphic onset or severity. The locus coeruleus (LC) in the brainstem that mediates fight-or-flight-or-freeze response to stress is sexually dimorphic in morphology, plays a key role in interactions between diet and severe stressors, and has neuronal input to the brown adipose tissue (BAT)-a thermogenic organ important for energy balance. Yet, little is known on how LC coordinates stress-related metabolic adaptations. LC expresses receptors for the neuropeptide PACAP (pituitary adenylate cyclase activating peptide) and PACAP signaling through PAC1 (PACAP receptor) are critical regulators of various types of stressors and energy metabolism. We hypothesized that LC-PAC1 axis is a sex-specific central "gatekeeper" of severe acute stress-driven behavior and energy metabolism. Selective ablation of PAC1 receptors from the LC did not alter stress response in mice of either sex, but enhanced food intake in females and was associated with increased energy expenditure and BAT thermogenesis in male mice. These results show a sexually dimorphic role of the LC-PAC1 in regulating acute stress-related energy metabolism. Thus, by disrupting LC-PAC1 signaling, our studies show a unique and previously unexplored role of LC in adaptive energy metabolism in a sex-dependent manner.

Melanin-concentrating hormone regulates the hypercapnic chemoreflex by acting in the lateral hypothalamic area

NEW FINDINGS

What is the central question of this study? MCH suppresses the hypercapnic chemoreflex but the mechanism by which this effect is produced has not been previously explored. What is the main finding and its importance? MCH acting in the lateral hypothalamic area but not in the locus coeruleus in rats, in the light period, attenuates the hypercapnic chemoreflex. Our data provide new insight regarding the role of MCH in the modulation of the hypercapnic ventilatory response.

ABSTRACT

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide involved in a broad range of homeostatic functions including regulation of the hypercapnic chemoreflex. We evaluated whether MCH modulates the hypercapnic ventilatory response by acting in the lateral hypothalamic area (LHA) and/or in the locus coeruleus (LC). Here, we measured pulmonary ventilation (V_E ), body temperature, electroencephalogram (EEG) and electromyogram (EMG) of unanesthetized adult male Wistar rats before and after microinjection of MCH [0.4 mM] or MCH1-R antagonist (SNAP-94847 [63 mM]) into the LHA and LC, in room air and 7% CO₂ conditions during wakefulness and sleep, in the dark and light periods. MCH intra-LHA caused a decreased CO₂ ventilatory response during wakefulness and sleep in the light period, while SNAP-94847 intra-LHA increased this response, during wakefulness in the light period. In the LC, MCH or the MCH1-R antagonist caused no change in the hypercapnic ventilatory response. Our results suggest that MCH, in the LHA, exerts an inhibitory modulation of the hypercapnic ventilatory response during the light-inactive period in rats. This article is protected by copyright. All rights reserved.

The innate immune system and fever under redox control: A Narrative Review

ABSTRACT:

In living cells, redox potential is vitally important for normal physiological processes that are closely regulated by antioxidants, free amino acids and proteins that either have reactive oxygen and nitrogen species capture capability or can be compartmentalized. Although hundreds of experiments support the regulatory role of free radicals and their derivatives, several authors continue to claim that these perform only harmful and non-regulatory functions. In this paper we show that countless intracellular and extracellular signal pathways are directly or indirectly linked to regulated redox processes. We also briefly discuss how artificial oxidative stress can have important therapeutic potential and the possible negative effects of popular antioxidant supplements.

L-Citrulline Influences the Body Temperature, Heat Shock Response and Nitric Oxide Regeneration of Broilers Under Thermoneutral and Heat Stress Condition

Heat stress (HS) adversely affects several physiological responses in organisms, but the underlying molecular mechanisms involved are yet to be fully understood. L-Citrulline (L-Cit) is a nutraceutical amino acid that is gaining research interest for its role in body temperature regulation and nitric oxide synthesis. This study investigated whether dietary supplementation with L-Cit (1% of basal diet) could ameliorate the effects of acute HS on thermotolerance, redox balance, and inflammatory responses of broilers. Ross 308 broilers (288 chicks) were subjected to two environments; thermoneutral at 24°C (TNZ) or HS at 35°C for 5 h, and fed two diets; control or L-Cit. The results showed that HS increased the ear, rectal (RT), and core body (CBT) temperatures of broilers, along with higher respiratory rate. The RT and CBT readings were intermittently affected with time effect, whereas, L-Cit supplementation lowered the mean CBT than the control diet. Antioxidant assays showed that superoxide dismutase was increased during HS, while, catalase was promoted by L-Cit supplementation. In addition, L-Cit induced glutathione peroxidase activity compared to the control diet during HS. Hypothalamic heat shock protein (HSP)-90 was upregulated by HS, but L-Cit downregulated heat shock factor (HSF)-1, and HSP 60 mRNA expressions. HSF 3 mRNA expression was downregulated by L-Cit under TNZ condition. More so, HS increased the plasma nitric oxide (NO) concentration but lowered the total NO synthase (tNOS) activity. In contrast, L-Cit supplementation limited NO production but increased the tNOS activity. Arginase activity was increased in the control fed group during HS but L-Cit supplementation lowered this effect. The NOS-COX pathway was significantly affected under TNZ condition, since L-Cit supplementation downregulated the mRNA expression of iNOS-COX2 in the hypothalamus, and further reduced the serum PGE2 concentration. Together, these data indicates that L-Cit influenced the antioxidant defense, heat shock response and nitric oxide regeneration both under thermoneutral and HS conditions; and that L-Cit may be directly and/or indirectly involved in the central regulation of body temperature.

Central Neural Circuits Orchestrating Thermogenesis, Sleep-Wakefulness States and General Anesthesia States

Great progress has been made in specifically identifying the central neural circuits (CNCs) of the core body temperature (Tcore), sleep-wakefulness states (SWs), and general anesthesia states (GAs), mainly utilizing optogenetic or chemogenetic manipulations. We summarize the neuronal populations and neural pathways of these three CNCs, which gives evidence for the orchestration within these three CNCs, and the integrative regulation of these three CNCs by different environmental light signals. We also outline some transient receptor potential (TRP) channels that function in the CNCs-Tcore and are modulated by some general anesthetics, which makes TRP channels possible targets for addressing the general-anesthetics-induced-hypothermia (GAIH). We suggest this review will provide new orientations for further consummating these CNCs and elucidating the central mechanisms of GAIH.

Involvement of brainstem noradrenergic system in cutaneous heat loss during exercise

The involvement of brainstem noradrenergic system in thermoregulation during exercise was evaluated by assessing the neuronal activation of A1, A2, locus coeruleus (LC) during exercise. Male Wistar rats weighing 280-330 g were used in the present study. Ninety minutes after exercise bout until fatigue, animals were anaesthesiated and brain removed and processed immunohistochemically for Fos protein and tyrosine hydroxylase in A1, A2 and LC and for Fos in POA subregions. Core and tail temperature were recorded during all running period by telemetry system. Heat storage rate (HSR, cal.min^-1), maximum tail vasoconstriction (°C) and vasodilatation threshold (°C) were calculated and correlated with Fos expression in all nuclei studied. Fos expression in LC correlated inversely with maximum tail skin vasoconstriction (r = -0.787, p < 0.03) and HSR (r = -0.834, p < 0.02) and positively to time to fatigue (r = 0.862, p < 0.01). A1 nucleus showed an inverse correlation with tail skin vasodilatation threshold (r = -0.861, p < 0.01). Fos expression in LC correlated inversely with Fos expression in the median (MnPO, r = -0.909, p < 0.01) and medial preoptic nucleus (MPOM, r = -0.942, p < 0.05). Our results bring further evidences that noradrenergic neurons from LC and A1 nuclei are involved in cutaneous heat loss mechanisms during exercise. LC nucleus probably modulates the sympathetic tonus of tail artery and integrates the central network LC / POA that could represent an important circuitry of temperature regulation during exercise. Also, noradrenergic neurons from A1 nucleus could be involved in cutaneous heat loss during exercise by modulating of vasodilatation threshold.

Molecular and Functional Sex Differences of Noradrenergic Neurons in the Mouse Locus Coeruleus

Preclinical work has long focused on male animals, though biological sex clearly influences risk for certain diseases, including many psychiatric disorders. Such disorders are often treated by drugs targeting the CNS norepinephrine system. Despite roles for noradrenergic neurons in behavior and neuropsychiatric disease models, their molecular characterization has lagged. We profiled mouse noradrenergic neurons in vivo, defining over 3,000 high-confidence transcripts expressed therein, including druggable receptors. We uncovered remarkable sex differences in gene expression, including elevated expression of the EP3 receptor in females-which we leverage to illustrate the behavioral and pharmacologic relevance of these findings-and of Slc6a15 and Lin28b, both major depressive disorder (MDD)-associated genes. Broadly, we present a means of transcriptionally profiling locus coeruleus under baseline and experimental conditions. Our findings underscore the need for preclinical work to include both sexes and suggest that sex differences in noradrenergic neurons may underlie behavioral differences relevant to disease.

Central fractalkine stimulates central prostaglandin E2 production and induces systemic inflammatory responses

Fractalkine (FKN; CX3CL1) belongs to gamma-chemokine family and binds to CX3CR1 receptors. Currently, the mechanisms involving FKN-induced inflammatory mediators are research targets in an attempt to study immune diseases mechanisms. Besides, FKN seems to modulate inflammation in the nervous system by inducing the secretion of pro-inflammatory mediators such as prostaglandin E₂ (PGE₂). PGE₂ is a classic and important mediator of fever that activates warm-responsive neurons in the anteroventral preoptic region of the hypothalamus (AVPO). Here, we tested the hypothesis that central FKN modulates febrigenic signaling both centrally and peripherally. We performed intracerebroventricular (icv) microinjections of saline (1 μL) or FKN (doses of 5, 50, 500 pg/μL) in rats and measured body temperature (Tb) besides assessing tail skin temperature (Tsk) as a thermoeffector indicator used to calculate the heat loss index (HLI). We also measured the time course changes in AVPO PGE₂, besides plasma corticosterone (CORT) and interleukin-6 (IL-6) levels. FKN induced a long lasting febrile response in which the highest dose (500 pg/μL) induced a marked rise on Tb that was accompanied by a reduced Tsk and HLI, consequently. FKN increased AVPO PGE₂ production in a time-dependent manner besides increasing plasma CORT and IL-6 levels. Our data consistently indicate that FKN increases AVPO PGE₂ production and Tb, accompanied by raised plasma IL-6 levels and activation of the hypothalamus-pituitary-adrenal axis.

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).

Fever and hypothermia in systemic inflammation

Systemic inflammation-associated syndromes (e.g., sepsis and septic shock) often have high mortality and remain a challenge in emergency medicine. Systemic inflammation is usually accompanied by changes in body temperature: fever or hypothermia. In animal studies, systemic inflammation is often modeled by administering bacterial lipopolysaccharide, which triggers autonomic and behavioral thermoeffector responses and causes either fever or hypothermia, depending on the dose and ambient temperature. Fever and hypothermia are regulated changes of body temperature, which correspond to mild and severe forms of systemic inflammation, respectively. Mediators of fever and hypothermia are called endogenous pyrogens and cryogens; they are produced when the innate immune system recognizes an infectious pathogen. Upon an inflammatory challenge, hepatic and pulmonary macrophages (and later brain endothelial cells) start to release lipid mediators, of which prostaglandin (PG) E₂ plays the key role, and cytokines. Blood PGE₂ enters the brain and triggers fever. At later stages of fever, PGE₂ synthesized within the blood-brain barrier maintains fever. In both cases, PGE₂ is synthesized by cyclooxygenase-2 and microsomal PGE₂synthase-1. Mediators of hypothermia are not well established. Both fever and hypothermia are beneficial host defense responses. Based on evidence from studies in laboratory animals and clinical trials in humans, fever is beneficial for fighting mild infection. Based mainly on animal studies, hypothermia is beneficial in severe systemic inflammation and infection.

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 serotonin attenuates LPS-induced systemic inflammation

Serotonin (5-HT) is a neuromodulator involved in several central-mediated mechanisms, such as endocrine processes, behavior, and sleep. Dysfunction of the serotonergic system is mainly linked to psychiatric disorders, but emerging evidence suggests that immune system activation may also alter brain 5-HT signaling. However, whether central 5-HT modulates systemic inflammation (SI) remains unknown. For this purpose, male Wistar rats (280-350g, 8-9weeks) were submitted to the experimental protocols beginning between 9 and 10AM with the performance of injections. The animals were housed at controlled conditions [temperature (25±1°C), light (06:00-18:00) and humidity (60-65%)]. Thus, we measured 5-HT and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA) in the anteroventral preoptic region [(AVPO) - the hierarchically most important region for body temperature (Tb) control] during lipopolysaccharide (LPS)-induced SI. We also combined LPS (100μg/kg) treatment with intracerebroventricular (icv) injection of 5-HT (5, 10 and 40μg/μL) and measured Tb ("hallmark" of SI), AVPO prostaglandin E₂ [(PGE₂) - an essential mediator of fever] and prostaglandin D₂ [(PGD₂) - a cryogenic mediator], plasma corticosterone [(CORT) - a stress marker with an endogenous anti-inflammatory effect] and interleukin-6 [(IL-6) - an immune mediator] levels. Detection limits of PGE₂, PGD₂, CORT and IL-6 assays were 39.1-2500pg/mL, 19.5-2500pg/mL, 0.12-2000μg/dL, and 0.125-8ng/mL, respectively. We also assessed tail skin temperature [used to calculate heat loss index (HLI)] to assess a key thermoeffector mechanism. As expected we observed LPS-induced increases in Tb, AVPO PGE₂ (whereas PGD₂ remained unchanged), plasma CORT and IL-6 levels, as well as a decrease in HLI. These changes were accompanied by reduced levels of AVPO 5-HT and 5-HIAA. Furthermore, we also observed a negative correlation between 5-HT and plasma CORT levels. Moreover, icv 5-HT (5, 10 and 40μg/μL) microinjection caused a U-shaped dose-response curve in LPS fever, in which the intermediate dose reduced the febrile response. Icv 5-HT (10μg/μL) microinjection prevented the LPS-induced increases in AVPO PGE₂ (whereas not altering PGD₂), plasma CORT and IL-6 levels, as well as preventing reduced HLI. Our data are consistent with the notion that AVPO 5-HT synthesis is down-regulated during SI, favoring AVPO PGE₂ synthesis and consequently potentiating the immune response. These results reveal a novel effect of central 5-HT as an anti-inflammatory neuromodulator that may take place during psychiatric disorder treatment with 5-HT reuptake inhibitors as well as suggesting that 5-HT modulation per se is a potential therapeutic approach for inflammatory diseases.

Influence of estrous cycle hormonal fluctuations and gonadal hormones on the ventilatory response to hypoxia in female rats

Sex hormones may influence many physiological processes. Recently, we demonstrated that hormonal fluctuations of cycling female rats do not affect respiratory parameters during hypercapnia. However, it is still unclear whether sex hormones and hormonal fluctuations that occur during the estrous cycle can affect breathing during a hypoxic challenge. Our study aimed to evaluate respiratory, metabolic, and thermal responses to hypoxia in female rats on different days of the estrous cycle (proestrus, estrus, metestrus, and diestrus) and in ovariectomized rats that received replacement with oil (OVX), estradiol (OVX + E₂), or a combination of estradiol and progesterone (OVX + E₂P). Ventilation (V _E), tidal volume (V _T), respiratory frequency (fR), oxygen consumption (VO₂), and V _E/VO₂ were not different during the estrous cycle in normoxia or hypoxia. Body temperature (Tb) was higher during estrus, but decreased similarly in all groups during hypoxia. Compared with intact females in estrus, gonadectomized rats also had lower Tb in normoxia, but not in hypoxia. OVX rats experienced a significant drop in the ventilatory response to hypoxia, but hormonal replacement did not restore values to the levels of an intact animal. Our data demonstrate that the different phases of the estrous cycle do not alter ventilation during normoxia and hypoxia, but OVX animals display lower ventilatory responses to hypoxia compared with ovary-intact rats. Because estradiol and progesterone replacement did not cause significant differences in ventilation, our findings suggest that a yet-to-be-defined non-steroidal ovarian hormone is likely to stimulate the ventilatory responses to hypoxia in females.

Traveling from the hypothalamus to the adipose tissue: The thermogenic pathway

Brown adipose tissue (BAT) is a specialized tissue critical for non-shivering thermogenesis producing heat through mitochondrial uncoupling; whereas white adipose tissue (WAT) is responsible of energy storage in the form of triglycerides. Another type of fat has been described, the beige adipose tissue; this tissue emerges in existing WAT depots but with thermogenic ability, a phenomenon known as browning. Several peripheral signals relaying information about energy status act in the brain, particularly the hypothalamus, to regulate thermogenesis in BAT and browning of WAT. Different hypothalamic areas have the capacity to regulate the thermogenic process in brown and beige adipocytes through the sympathetic nervous system (SNS). This review discusses important concepts and discoveries about the central control of thermogenesis as a trip that starts in the hypothalamus, and taking the sympathetic roads to reach brown and beige fat to modulate thermogenic functions.

Orexins and the cardiovascular events of awakening

This brief review aims to provide an updated account of the cardiovascular events of awakening, proposing a testable conceptual framework that links these events with the neural control of sleep and the autonomic nervous system, with focus on the hypothalamic orexin (hypocretin) neurons. Awakening from non-rapid-eye-movement sleep entails coordinated changes in brain and cardiovascular activity: the neural "flip-flop" switch that governs state transitions becomes biased toward the ascending arousal systems, arterial blood pressure and heart rate rise toward waking values, and distal skin temperature falls. Arterial blood pressure and skin temperature are sensed by baroreceptors and thermoreceptors and may positively feedback on the brain wake-sleep switch, thus contributing to sharpen, coordinate, and stabilize awakening. These effects may be enhanced by the hypothalamic orexin neurons, which may modulate the changes in blood pressure, heart rate, and skin temperature upon awakening, while biasing the wake-sleep switch toward wakefulness through direct neural projections. A deeper understanding of the cardiovascular events of awakening and of their links with skin temperature and the wake-sleep neural switch may lead to better treatments options for patients with narcolepsy type 1, who lack the orexin neurons.

Neuronal Deletion of Ghrelin Receptor Almost Completely Prevents Diet-Induced Obesity

Ghrelin signaling has major effects on energy and glucose homeostasis, but it is unknown whether ghrelin's functions are centrally and/or peripherally mediated. The ghrelin receptor, growth hormone secretagogue receptor (GHS-R), is highly expressed in the brain and detectable in some peripheral tissues. To understand the roles of neuronal GHS-R, we generated a mouse line where Ghsr gene is deleted in all neurons using synapsin 1 (Syn1)-Cre driver. Our data showed that neuronal Ghsr deletion abolishes ghrelin-induced spontaneous food intake but has no effect on total energy intake. Remarkably, neuronal Ghsr deletion almost completely prevented diet-induced obesity (DIO) and significantly improved insulin sensitivity. The neuronal Ghsr-deleted mice also showed improved metabolic flexibility, indicative of better adaption to different fuels. In addition, gene expression analysis suggested that hypothalamus and/or midbrain might be the sites that mediate the effects of GHS-R in thermogenesis and physical activity, respectively. Collectively, our results indicate that neuronal GHS-R is a crucial regulator of energy metabolism and a key mediator of DIO. Neuronal Ghsr deletion protects against DIO by regulating energy expenditure, not by energy intake. These novel findings suggest that suppressing central ghrelin signaling may serve as a unique antiobesity strategy.

Central nervous system regulation of brown adipose tissue

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

Ventilatory, metabolic, and thermal responses to hypercapnia in female rats: effects of estrous cycle, ovariectomy, and hormonal replacement

The aim of this study was to examine how estrous cycle, ovariectomy, and hormonal replacement affect the respiratory [ventilation (V̇e), tidal volume, and respiratory frequency], metabolic (V̇o2), and thermoregulatory (body temperature) responses to hypercapnia (7% CO2) in female Wistar rats. The parameters were measured in rats during different phases of the estrous cycle, and also in ovariectomized (OVX) rats supplemented with 17β-estradiol (OVX+E2), with a combination of E2 and progesterone (OVX+E2P), or with corn oil (OVX+O, vehicle). All experiments were conducted on day 8 after ovariectomy. The intact animals did not present alterations during normocapnia or under hypercapnia in V̇e, tidal volume, respiratory frequency, V̇o2, and V̇e/V̇o2 in the different phases of the estrous cycle. However, body temperature was higher in female rats on estrus. Hormonal replacement did not change the ventilatory, thermoregulatory, or metabolic parameters during hypercapnia, compared with the OVX animals. Nevertheless, OVX+E2, OVX+E2P, and OVX+O presented lower hypercapnic ventilatory responses compared with intact females on the day of estrus. Also, rats in estrus showed higher V̇e and V̇e/V̇o2 during hypercapnia than OVX animals. The data suggest that other gonadal factors, besides E2 and P, are possibly involved in these responses.

Neural Control of Energy Expenditure

The continuous rise in obesity is a major concern for future healthcare management. Many strategies to control body weight focus on a permanent modification of food intake with limited success in the long term. Metabolism or energy expenditure is the other side of the coin for the regulation of body weight, and strategies to enhance energy expenditure are a current focus for obesity treatment, especially since the (re)-discovery of the energy depleting brown adipose tissue in adult humans. Conversely, several human illnesses like neurodegenerative diseases, cancer, or autoimmune deficiency syndrome suffer from increased energy expenditure and severe weight loss. Thus, strategies to modulate energy expenditure to target weight gain or loss would improve life expectancies and quality of life in many human patients. The aim of this book chapter is to give an overview of our current understanding and recent progress in energy expenditure control with specific emphasis on central control mechanisms.

Central neural regulation of brown adipose tissue thermogenesis and energy expenditure

Thermogenesis, the production of heat energy, is the specific, neurally regulated, metabolic function of brown adipose tissue (BAT) and contributes to the maintenance of body temperature during cold exposure and to the elevated core temperature during several behavioral states, including wakefulness, the acute phase response (fever), and stress. BAT energy expenditure requires metabolic fuel availability and contributes to energy balance. This review summarizes the functional organization and neurochemical influences within the CNS networks governing the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolically driven alterations in BAT thermogenesis and energy expenditure that contribute to overall energy homeostasis.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Ionotropic but not metabotropic glutamatergic receptors in the locus coeruleus modulate the hypercapnic ventilatory response in unanaesthetized rats

AIM

Central chemoreceptors are important to detect changes of CO2/H(+), and the Locus coeruleus (LC) is one of the many putative central chemoreceptor sites. Here, we studied the contribution of LC glutamatergic receptors on ventilatory, cardiovascular and thermal responses to hypercapnia.

METHODS

To this end, we determined pulmonary ventilation (V(E)), body temperatures (T(b)), mean arterial pressure (MAP) and heart rate (HR) of male Wistar rats before and after unilateral microinjection of kynurenic acid (KY, an ionotropic glutamate receptor antagonist, 10 nmol/0.1 μL) or α-methyl-4-carboxyphenylglycine (MCPG, a metabotropic glutamate receptor antagonist, 10 nmol/0.1 μL) into the LC, followed by 60 min of air breathing or hypercapnia exposure (7% CO2).

RESULTS

Ventilatory response to hypercapnia was higher in animals treated with KY intra-LC (1918.7 ± 275.4) compared with the control group (1057.8 ± 213.9, P < 0.01). However, the MCPG treatment within the LC had no effect on the hypercapnia-induced hyperpnea. The cardiovascular and thermal controls were not affected by hypercapnia or by the injection of KY and MCPG in the LC.

CONCLUSION

These data suggest that glutamate acting on ionotropic, but not metabotropic, receptors in the LC exerts an inhibitory modulation of hypercapnia-induced hyperpnea.

Exogenous ghrelin attenuates endotoxin fever in rats

Ghrelin is a gut-derived peptide that plays a role in energy homeostasis. Recent studies have implicated ghrelin in systemic inflammation, showing increased plasma ghrelin levels after endotoxin (lipopolysaccharide, LPS) administration. The aims of this study were (1) to test the hypothesis that ghrelin administration affects LPS-induced fever; and (2) to assess the putative effects of ghrelin on plasma corticosterone secretion and preoptic region prostaglandin (PG) E(2) levels in euthermic and febrile rats. Rats were implanted with a temperature datalogger capsule in the peritoneal cavity to record body core temperature. One week later, they were challenged with LPS (50 μg/kg, intraperitoneal, i.p.) alone or combined with ghrelin (0.1mg/kg, i.p.). In another group of rats, plasma corticosterone and preoptic region PGE(2) levels were measured 2h after injections. In euthermic animals, systemic administration of ghrelin failed to elicit any thermoregulatory effect, and caused no significant changes in basal plasma corticosterone and preoptic region PGE(2) levels. LPS caused a typical febrile response, accompanied by increased plasma corticosterone and preoptic PGE(2) levels. When LPS administration was combined with ghrelin fever was attenuated, corticosterone secretion further increased, and the elevated preoptic PGE(2) levels were relatively reduced, but a correlation between these two variables (corticosterone and PGE(2)) failed to exist. The present data add ghrelin to the neurochemical milieu controlling the immune/thermoregulatory system acting as an antipyretic molecule. Moreover, our findings also support the notion that ghrelin attenuates fever by means of a direct effect of the peptide reducing PGE(2) production in the preoptic region.

Central circuitries for body temperature regulation and fever

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

Reduced stress fever is accompanied by increased glucocorticoids and reduced PGE2 in adult rats exposed to endotoxin as neonates

Immune challenges during neonatal period may permanently program immune responses later in life, including endotoxin fever. We tested the hypothesis that neonatal endotoxin exposure affects stress fever in adult rats. In control rats (treated with saline as neonates; nSal) body temperature peaked approximately 1.5 degrees C during open-field stress, whereas in rats exposed to endotoxin (lipopolysaccharide, LPS) as neonates (nLPS) stress fever was significantly attenuated. Following stress, plasma corticosterone levels significantly increased from 74.29+/-7.05 ng ml(-1) to 226.29+/-9.87 ng ml(-1) in nSal rats, and from 83.43+/-10.31 ng ml(-1) to 324.7+/-36.87 ng ml(-1) in nLPS rats. Animals treated with LPS as neonates and adrenalectomized one week before experimentation no longer displayed the attenuated febrile response to stress. This attenuated stress fever caused by an increased corticosterone secretion is likely to be linked to an inhibitory effect of glucocorticoids on cyclooxygenase activity/PGE(2) production in preoptic/anteroventral third ventricular region (AV3V) since stress failed to cause a significant increase in PGE(2) in nLPS rats, and this effect was reverted by adrenalectomy. Altogether, the present results indicate that endogenous glucocorticoids are key modulators of the attenuated stress fever in adult rats treated with LPS as neonates, and they act downregulating PGE(2) production in AV3V. Moreover, our findings also support the notion that neonatal immune stimulus affects programming of stress responses during adulthood, despite the fact that inflammation and stress are two distinct processes mediated largely by different neurobiological mechanisms.

The locus coeruleus and central chemosensitivity

The locus coeruleus (LC) lies in the dorsal pons and supplies noradrenergic (NA) input to many regions of the brain, including respiratory control areas. The LC may provide tonic input for basal respiratory drive and is involved in central chemosensitivity since focal acidosis of the region stimulates ventilation and ablation reduces CO(2)-induced increased ventilation. The output of LC is modulated by both serotonergic and glutamatergic inputs. A large percentage of LC neurons are intrinsically activated by hypercapnia. This percentage and the magnitude of their response are highest in young neonates and decrease dramatically after postnatal day P10. The cellular bases for intrinsic chemosensitivity of LC neurons are comprised of multiple factors, primary among them being reduced extracellular and intracellular pH, which inhibit inwardly rectifying and voltage-gated K(+) channels, and activate L-type Ca(2+) channels. Activation of K(Ca) channels in LC neurons may limit their ultimate response to hypercapnia. Finally, the LC mediates central chemosensitivity and contains pH-sensitive neurons in amphibians, suggesting that the LC has a long-standing phylogenetic role in respiratory control.

Effects of estradiol on the thermoneutral zone and core temperature in ovariectomized rats

Hot flushes represent a disorder of central thermoregulation characterized by the episodic activation of heat loss mechanisms. Although flushes are associated with estrogen withdrawal, there is little understanding of the effects of estrogen on thermoregulation in any species. It has been proposed that hormone withdrawal increases the sensitivity of hypothalamic neural pathways that control heat dissipation effectors. If so, we predicted that ovariectomized rats without estradiol treatment would activate tail skin vasodilatation (a major heat loss effector) at lower ambient temperatures and thereby lower the thermoneutral zone. The thermoneutral zone, defined as the range of ambient temperatures in which thermoregulation is achieved only by sensible (dry) heat loss, was evaluated based on properties of skin vasomotion. Core and tail skin temperatures were recorded in ovariectomized rats (with and without estradiol-17beta) exposed to ambient temperatures from 13 to 34 C in an environmental chamber. Rats without estradiol exhibited increased skin vasodilatation and a shift in the thermoneutral zone to lower ambient temperatures. Moreover, the ambient temperature threshold for skin vasodilatation was significantly lower in rats without estradiol treatment. At most ambient temperatures, average core temperature was unaffected by estradiol. However, at ambient temperatures of 32.5 C and above, untreated ovariectomized rats exhibited higher core temperatures compared with estradiol-treated rats. Thus, estradiol-17beta treatment enhanced the maintenance of core temperature during heat exposure. These findings support the hypothesis that estrogen withdrawal increases the sensitivity of thermoregulatory neural pathways and modifies the activation of heat loss mechanisms.

Propyretic role of the locus coeruleus nitric oxide pathway

Nitric oxide has been reported to modulate fever in the brain. However, the sites where NO exerts this modulation remain somewhat unclear. Locus coeruleus (LC) neurons express not only nitric oxide synthase (NOS) but also soluble guanylyl cyclase (sGC). In the present study, we evaluated in vivo and ex vivo the putative role of the LC NO-cGMP pathway in fever. To this end, deep body temperature was measured before and after pharmacological modulations of the pathway. Moreover, nitrite/nitrate (NOx) and cGMP levels in the LC were assessed. Conscious rats were microinjected within the LC with a non-selective NOS inhibitor (N(G)-monomethyl-l-arginine acetate), a NO donor (NOC12), a sGC inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) or a cGMP analogue (8-bromo-cGMP) and injected intraperitoneally with endotoxin. Inhibition of NOS or sGC before endotoxin injection significantly increased the latency to the onset of fever. During the course of fever, inhibition of NOS or sGC attenuated the febrile response, whereas microinjection of NOC12 or 8-bromo-cGMP increased the response. These findings indicate that the LC NO-cGMP pathway plays a propyretic role. Furthermore, we observed a significant increase in NOx and cGMP levels, indicating that the febrile response to endotoxin is accompanied by stimulation of the NO-cGMP pathway in the LC.

The transient receptor potential vanilloid-1 channel in thermoregulation: a thermosensor it is not

The development of antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel as pain therapeutics has revealed that these compounds cause hyperthermia in humans. This undesirable on-target side effect has triggered a surge of interest in the role of TRPV1 in thermoregulation and revived the hypothesis that TRPV1 channels serve as thermosensors. We review literature data on the distribution of TRPV1 channels in the body and on thermoregulatory responses to TRPV1 agonists and antagonists. We propose that two principal populations of TRPV1-expressing cells have connections with efferent thermoeffector pathways: 1) first-order sensory (polymodal), glutamatergic dorsal-root (and possibly nodose) ganglia neurons that innervate the abdominal viscera and 2) higher-order sensory, glutamatergic neurons presumably located in the median preoptic hypothalamic nucleus. We further hypothesize that all thermoregulatory responses to TRPV1 agonists and antagonists and thermoregulatory manifestations of TRPV1 desensitization stem from primary actions on these two neuronal populations. Agonists act primarily centrally on population 2; antagonists act primarily peripherally on population 1. We analyze what roles TRPV1 might play in thermoregulation and conclude that this channel does not serve as a thermosensor, at least not under physiological conditions. In the hypothalamus, TRPV1 channels are inactive at common brain temperatures. In the abdomen, TRPV1 channels are tonically activated, but not by temperature. However, tonic activation of visceral TRPV1 by nonthermal factors suppresses autonomic cold-defense effectors and, consequently, body temperature. Blockade of this activation by TRPV1 antagonists disinhibits thermoeffectors and causes hyperthermia. Strategies for creating hyperthermia-free TRPV1 antagonists are outlined. The potential physiological and pathological significance of TRPV1-mediated thermoregulatory effects is discussed.

Autism, fever, epigenetics and the locus coeruleus

Some children with autism spectrum disorders (ASD) exhibit improved behaviors and enhanced communication during febrile episodes. We hypothesize that febrigenesis and the behavioral-state changes associated with fever in autism depend upon selective normalization of key components of a functionally impaired locus coeruleus-noradrenergic (LC-NA) system. We posit that autistic behaviors result from developmental dysregulation of LC-NA system specification and neural network deployment and modulation linked to the core behavioral features of autism. Fever transiently restores the modulatory functions of the LC-NA system and ameliorates autistic behaviors. Fever-induced reversibility of autism suggests preserved functional integrity of widespread neural networks subserving the LC-NA system and specifically the subsystems involved in mediating the cognitive and behavioral repertoires compromised in ASD. Alterations of complex gene-environmental interactions and associated epigenetic mechanisms during seminal developmental critical periods are viewed as instrumental in LC-NA dysregulation as emphasized by the timing and severity of prenatal maternal stressors on autism prevalence. Our hypothesis has implications for a rational approach to further interrogate the interdisciplinary etiology of ASD and for designing novel biological detection systems and therapeutic agents that target the LC-NA system's diverse network of pre- and postsynaptic receptors, intracellular signaling pathways and dynamic epigenetic remodeling processes involved in their regulation and functional plasticity.

Hyperthermia combined with ethanol administration induces c-fos expression in the central amygdaloid nucleus of the mouse brain. A possible mechanism of heatstroke under the influence of ethanol intake

Heatstroke is defined as a core body temperature that rises above 40.6 degrees C and is accompanied by mental status abnormalities such as delirium, convulsions, or coma resulting from exposure to environmental heat. There is fairly wide agreement that ethanol intake is a predisposing factor in heatstroke. This study was performed to identify the brain changes induced by heatstroke, using a mouse hyperthermia model with and without preceding ethanol administration. Exposure to heat of 42 degrees C until the core temperature reached to 43 degrees C followed by exposure to 37 degrees C for 15 min decreased the levels of partial pressures of O(2) in blood. Preceding ethanol administration and heat exposure induced hypotension, severe metabolic acidosis and respiratory failure, and, accordingly, produced heatstroke. Immunohistochemistry of the brains showed that preceding ethanol administration increased the number of c-fos-immunoreactive neurons, as a marker of neuronal activation, in the central amygdaloid nucleus, which is involved in thermoregulation. These results indicate that combined effects of ethanol and heat exposure induce heatstroke that is associated with activation of the central amygdaloid nucleus, implicating the pathophysiology and mechanisms of heatstroke under the influence of ethanol intake.

Central pathway for spontaneous and prostaglandin E2-evoked cutaneous vasoconstriction

A reduction of heat loss to the environment through increased cutaneous vasoconstrictor (CVC) sympathetic outflow contributes to elevated body temperature during fever. We determined the role of neurons in the dorsomedial hypothalamus (DMH) in increases in CVC sympathetic tone evoked by PGE2 into the preoptic area (POA) in chloralose/urethane-anesthetized rats. The frequency of axonal action potentials of CVC sympathetic ganglion cells recorded from the surface of the tail artery was increased by 1.8 Hz following nanoinjections of bicuculline (50 pmol) into the DMH. PGE2 nanoinjection into the POA elicited a similar excitation of tail CVC neurons (+2.1 Hz). Subsequent to PGE2 into the POA, muscimol (400 pmol/side) into the DMH did not alter the activity of tail CVC neurons. Inhibition of neurons in the rostral raphé pallidus (rRPa) eliminated the spontaneous discharge of tail CVC neurons but only reduced the PGE2-evoked activity. Residual activity was abolished by subsequent muscimol into the rostral ventrolateral medulla. Transections through the neuraxis caudal to the POA increased the activity of tail CVC neurons, which were sustained through transections caudal to DMH. We conclude that while activation of neurons in the DMH is sufficient to activate tail CVC neurons, it is not necessary for their PGE2-evoked activity. These results support a CVC component of increased core temperature elicited by PGE2 in POA that arises from relief of a tonic inhibition from neurons in POA of CVC sympathetic premotor neurons in rRPa and is dependent on the excitation of CVC premotor neurons from a site caudal to DMH.