Gaseous Mediators in Temperature Regulation

Inhibition of nitric oxide synthase or cystathionine gamma-lyase abolishes leptin-induced fever in male rats

Leptin is an adipokine that regulates energy balance and immune function. Peripheral leptin administration elicits prostaglandin E₂-dependent fever in rats. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H₂S) are also involved in lipopolysaccharide (LPS)-induced fever response. However, there is no data in the literature indicating if these gasotransmitters have a role in leptin-induced fever response. Here, we investigate the inhibition of NO and H₂S enzymes neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase (iNOS), and cystathionine γ-lyase (CSE) in leptin-induced fever response, respectively. Selective nNOS inhibitor 7-nitroindazole (7-NI), selective iNOS inhibitor aminoguanidine (AG), and CSE inhibitor dl-propargylglycine (PAG) were administered intraperitoneally (ip). Body temperature (Tb), food intake, and body mass were recorded in fasted male rats. Leptin (0.5 mg/kg ip) induced a significant increase in Tb, whereas AG (50 mg/kg ip), 7-NI (10 mg/kg ip), or PAG (50 mg/kg ip) caused no changes in Tb. AG, 7-NI, or PAG abolished leptin increase in Tb. Our results highlight the potential involvement of iNOS, nNOS, and CSE in leptin-induced febrile response without affecting anorexic response to leptin in fasted male rats 24 h after leptin injection. Interestingly, all the inhibitors alone had the same anorexic effect induced by leptin. These findings have implications for understanding the role of NO and H₂S in leptin-induced febrile response.

Metal-organic framework for biomimetic nitric oxide generation and anticancer drug delivery

The potential therapeutic implications of nitric oxide (NO) have drawn a great deal of interest for reversing multidrug resistance (MDR) in cancer; however, previous strategies utilized unstable or toxic NO donors often oxidized by the excessive addition of reactive oxygen species, leading to unexpected side effects. Therefore, this study proposed a metal-organic framework (MOF), Porous coordination network (PCN)-223-Fe, to be loaded with a biocompatible NO donor, L-arginine (L-arg; i.e., PCN-223-Fe/L-arg). This specific MOF possesses a ligand of Fe-porphyrin, a biomimetic catalyst. Thus, with PCN-223-Fe/L-arg, L-arg was released in a sustained manner, which generated NO by a catalytic reaction between L-arg and Fe-porphyrin in PCN-223-Fe. Through this biomimetic process, PCN-223-Fe/L-arg could generate sufficient NO to reverse MDR at the expense of hydrogen peroxide already present and highly expressed in cancer environments. For treatment of MDR cancer, this study also proposed PCN-223-Fe loaded with an anticancer drug, irinotecan (CPT-11; i.e., PCN-223-Fe/CPT-11), to be formulated together with PCN-223-Fe/L-arg. Owing to the synergistic effect of reversed MDR by NO generation and sustained release of CPT-11, this combined formulation exhibited a higher anticancer effect on MDR cancer cells (MCF-7/ADR). When intratumorally injected in vivo, coadministration of PCN-223-Fe/L-arg and PCN-223-Fe/CPT-11 greatly suppressed tumor growth in nude mice bearing MDR tumors.

CORM-401, an orally active carbon monoxide-releasing molecule, increases body temperature by activating non-shivering thermogenesis in rats

Thermoregulation is critical in health and disease and is tightly controlled to maintain body temperature homeostasis. Carbon monoxide (CO), an endogenous gasotransmitter produced during heme degradation by heme oxygenases, has been suggested to play a role in body core temperature (Tb) regulation. However, a direct involvement of CO in thermoregulation has not been confirmed and its mechanism(s) of action remain largely unknown. In the present study we characterized the effects of systemic delivery of CO by administration of an orally active CO-releasing molecule (CORM-401) on Tb regulation in conscious freely moving rats. Specifically, we evaluated the main thermo effectors in rats treated with CORM-401 by assessing: (i) non-shivering thermogenesis, i.e. the increased metabolism of brown fat measured through oxygen consumption and (ii) the rate of heat loss from the tail through calculations of heat loss index. We found that oral administration of CORM-401 (30 mg/kg) resulted in augmented CO delivery into the blood circulation as evidenced a by significant increase in carbon monoxy hemoglobin levels(COHb). In addition, treatment with CORM-401 increased Tb, which was caused by an elevated non-shivering thermogenesis indicated by increased oxygen consumption without significant changes in the tail heat loss. On the other hand, CORM-401 did not affect blood pressure, but significantly decreased heart rate. In summary, the findings of the present study reveal that increased circulating CO levels lead to a rise in Tb, which could have important implications in the emerging role of CO in the modulation of energetic metabolism.

Post-exercise Hypothermia Varies Between High- and Low-Altitude Populations in the Asiatic Toad (Bufo gargarizans)

Whether and how poikilothermic animals change their thermal performance to cope with global warming are crucial questions to predict the future of biodiversity. Intraspecific comparison among populations that occur in different climatic zones can provide insight into how poikilotherms may alter their thermal performance under a particular climatic event. We compared populations of the Asiatic toad (Bufo gargarizans) from two altitudinal zones (3239 and 926 m above sea level) to explore variations of post-exercise hypothermia, which can lead to lower temperature preference than normal conditions. Common garden experiment was also employed to test plasticity of hypothermic performance in adult toads. As results, exhaustive exercise induced measurable reduction in body temperature for both populations. Furthermore, high-altitude population experienced larger reduction in body temperature than low-altitude conspecifics in both original habitat and common garden conditions. Therefore, low-altitude toads may to enhance their hypothermic reaction if they shift their ranges to higher altitudes to survive warming climate; However, the relatively limited plasticity of hypothermic performance may constraint their adaptative process.

Metabolic trade-offs favor regulated hypothermia and inhibit fever in immune-challenged chicks

The febrile response to resist a pathogen is energetically expensive while regulated hypothermia seems to preserve energy for vital functions. We hypothesized here that immune challenged birds under metabolic trade-offs (reduced energy supply / increased energy demand) favor a regulated hypothermic response at the expense of fever. To test this hypothesis, we compared 5-days old broiler chicks exposed to fasting, cold (25oC), and fasting combined with cold to a control group fed at thermoneutral condition (30oC). The chicks were injected with saline or with a high dose of endotoxin known to induce a biphasic thermal response composed of body temperature (Tb) drop followed by fever. Then Tb, oxygen consumption (metabolic rate), peripheral vasomotion (cutaneous heat exchange), breathing frequency (respiratory heat exchange), and huddling behavior (heat conservation indicator) were analyzed. Irrespective of metabolic trade-offs, chicks presented a transient regulated hypothermia in the first hour, which relied on a suppressed metabolic rate for all groups, increased breathing frequency for chicks fed/fasted at 30oC, and peripheral vasodilation in fed/fasted chicks at 25oC. Fever was observed only in chicks kept at thermoneutrality and was supported by peripheral vasoconstriction and huddling behavior. Fed and fasted chicks at 25oC completely eliminated fever despite the ability to increase metabolic rate for thermogenesis in the phase correspondent to fever when it was pharmacologically induced by 2.4-Dinitrophenol. Our data suggest that increased competing demands affect chicks’ response to an immune challenge favoring regulated hypothermia to preserve energy while the high costs of fever to resist a pathogen are avoided.

Correlation between convection requirement and carotid body responses to hypoxia and hemoglobin affinity: comparison between two rat strains

We tested the hypothesis that differences in ventilatory ([Formula: see text]) or convection requirement ([Formula: see text]/[Formula: see text]O₂₎ response to hypoxia would be correlated with differences in hemoglobin (Hb) oxygen affinity between two strains of rats, as they have been shown to be among several species of mammals, birds and reptiles. Brown Norway (BN) rats reduce metabolism more than they increase ventilation in response to hypoxia and both the ventilatory and convection requirement responses to hypoxia are lower in the BN than the Sprague-Dawley (SD) rat. The lower threshold of the ventilation/convection requirement responses of the BN to hypoxia are associated with a higher affinity Hb than the SD rats, (P₅₀ values of 32.4 (± 0.6) versus 34.4 (± 0.5), respectively (P < 0.05), and P₇₅ values of 46.1 (± 0.5) for BN versus 50.7 (± 0.8) for SD (P < 0.001). This significant difference, particularly near the inflection point of the dissociation curve, supported our hypothesis. A reduced sensitivity of BN compared to SD carotid bodies was found. BN carotid bodies (from 36 20-60-day-olds) had a mean estimated volume of 26.64 ± 1.47 × 10⁶ μm³, significantly (P < 0.0001) smaller than SD carotid bodies (from 46 16-40-day-olds) at 50.66 ± 3.41 × 10⁶ μm³. Both genetic and epigenetic/developmental mechanisms may account for the observed inter-strain differences.

The Hypothermic Effect of Hydrogen Sulfide Is Mediated by the Transient Receptor Potential Ankyrin-1 Channel in Mice

Hydrogen sulfide (H₂S) has been shown in previous studies to cause hypothermia and hypometabolism in mice, and its thermoregulatory effects were subsequently investigated. However, the molecular target through which H₂S triggers its effects on deep body temperature has remained unknown. We investigated the thermoregulatory response to fast-(Na₂S) and slow-releasing (GYY4137) H₂S donors in C57BL/6 mice, and then tested whether their effects depend on the transient receptor potential ankyrin-1 (TRPA1) channel in Trpa1 knockout (Trpa1^−/−) and wild-type (Trpa1^+/+) mice. Intracerebroventricular administration of Na₂S (0.5–1 mg/kg) caused hypothermia in C57BL/6 mice, which was mediated by cutaneous vasodilation and decreased thermogenesis. In contrast, intraperitoneal administration of Na₂S (5 mg/kg) did not cause any thermoregulatory effect. Central administration of GYY4137 (3 mg/kg) also caused hypothermia and hypometabolism. The hypothermic response to both H₂S donors was significantly (p < 0.001) attenuated in Trpa1^−/− mice compared to their Trpa1^+/+ littermates. Trpa1 mRNA transcripts could be detected with RNAscope in hypothalamic and other brain neurons within the autonomic thermoeffector pathways. In conclusion, slow- and fast-releasing H₂S donors induce hypothermia through hypometabolism and cutaneous vasodilation in mice that is mediated by TRPA1 channels located in the brain, presumably in hypothalamic neurons within the autonomic thermoeffector pathways.

Acute intermittent hypoxia evokes ventilatory long-term facilitation and active expiration in unanesthetized rats

Acute intermittent hypoxia (AIH) modifies the functioning of the respiratory network, causing respiratory motor facilitation in anesthetized animals and a compensatory increase in pulmonary ventilation in freely behaving animals. However, it is still unclear whether the ventilatory facilitation induced by AIH in unanesthetized animals is associated with changes in the respiratory pattern. We found that Holtzman male rats (80-150 g) exposed to AIH (10 × 6% O₂ for 30-40 s every 5 min, n = 9) exhibited a prolonged (30 min) increase in baseline minute ventilation (P < 0.05) compared to control animals (n = 13), combined with the occurrence of late expiratory peak flow events, suggesting the presence of active expiration. The increase in ventilation after AIH was also accompanied by reductions in arterial CO₂ and body temperature (n = 5-6, P < 0.05). The systemic treatment with ketanserin (a 5-HT₂ receptor antagonist) before AIH prevented the changes in ventilation and active expiration (n = 11) but potentiated the hypothermic response (n = 5, P < 0.05) when compared to appropriate control rats (n = 13). Our findings indicate that the ventilatory long-term facilitation elicited by AIH exposure in unanesthetized rats is linked to the generation of active expiration by mechanisms that may depend on the activation of serotonin receptors. In contrast, the decrease in body temperature induced by AIH may not require 5-HT₂ receptor activation.

Hypothermic Effect of Acute Citral Treatment during LPS-induced Systemic Inflammation in Obese Mice: Reduction of Serum TNF-α and Leptin Levels

Citral is a mixture of monoterpenes present in the essential oil of several plants, such as Cymbopogon citratus and Zingiber officinale, possessing anti-inflammatory, anti-ulcerogenic, and antipyretic actions. We investigated the action of citral on body temperature (Tb) and inflammatory signaling in eutrophic and obese mice during Systemic Inflammation (SI) induced by Lipopolysaccharide (LPS). Thus, we assessed the effect of citral (25, 100, and 300 mg/kg) and ibuprofen in LPS-induced SI in Swiss male mice fed a standard diet (SD) or high-fat diet (HFD) for 12 weeks. Following SI induction, we measured Tb and collected the serum, hypothalamus, and gastric mucosa for biochemical measurements. Acute treatment with citral decreased the Tb of both SD and HFD-fed animals. Citral (300 mg/kg) treatment caused a significantly lower Tb variation in HFD-fed animals than in those fed the SD. Citral reduced peripheral levels of tumor necrosis factor (TNF)-α in SD and HFD mice and decreased serum leptin concentration in HFD mice 90 min after the LPS challenge. Furthermore, citral also reduced interleukin (IL)-6 levels in the hypothalamus of obese mice. In summary, citral effectively reduced Tb during SI by reducing inflammatory mediators with a distinct action profile in HFD mice when compared with SD.

Central leukotrienes modulate fever tolerance to LPS in rats

Leukotrienes mediate several inflammatory events such as neutrophil chemoattraction, leukocyte adhesion, and central-release of cytokines and fever. However, there is no information available about their putative role in lipopolysaccharide (LPS) tolerance. The rational of the present study was to find out if central leukotrienes are involved in the development of LPS tolerance. Thus, we inhibited central leukotriene synthesis in tolerant rats using a pharmacological tool, i.e., a selective inhibitor of leukotriene synthesis MK-886 injected into the third ventricle (3V) of rats. Body core temperature (Tb) was measured using a datalogger placed inside the abdominal cavity. A low-dose of LPS (100 μg/kg ip) was given for 4 consecutive days to induce LPS tolerance. At day 4, rats received a microinjection of MK-886 into the 3V immediately before LPS, whereas control groups were treated with vehicle (saline). We observed that LPS failed to induce plasma cytokines surges, increased hypothalamic PGE₂ levels and fever 3 days post LPS treatment, aptly characterizing the tolerance. When MK-886 was given to control rats treated with saline, no significant change in Tb was observed. However, a full LPS-induced fever was observed in tolerant rats pretreated with MK-886, which was associated with an enhancement in the hypothalamic PGE₂ levels, that were not accompanied by plasma cytokines (IL-1β, and IL-6) and PGE₂ surges. These data are consistent with the notion that central leukotrienes play a role in fever tolerance to LPS.

Central serotonin prevents hypotension and hypothermia and reduces plasma and spleen cytokine levels during systemic inflammation

An exceptionally high mortality rate is observed in sepsis and septic shock. Systemic administration of lipopolysaccharide (LPS) has been used as an experimental model for sepsis resulting in an exacerbated immune response, brain neurochemistry adjustments, hypotension, and hypothermia followed by fever. Central serotonergic pathways not only modulate systemic inflammation (SI) but also are affected by SI, including in the anteroventral region of the hypothalamus (AVPO), which is the hierarchically most important region for body temperature (Tb) control. In this study, we sought to determine if central serotonin (5-HT) plays a role in SI induced by intravenous administration of LPS (1.5 mg/kg) in male Wistar rats (280-350 g) by assessing 5-HT levels in the AVPO, mean arterial pressure, heart rate, and Tb up to 300 min after LPS administration, as well as assessing plasma and spleen cytokine levels, nitric oxide (NO) plasma levels, and prostaglandin (PG) E₂ levels in the AVPO at 75 min and 300 min after LPS administration. We observed reduced AVPO 5-HT levels, hypotension, tachycardia, hypothermia followed by fever, as well as observing increased plasma NO, plasma and spleen cytokines and AVPO PGE₂ levels in SI. Intracerebroventricular (icv) administration of 5-HT 30 min before LPS administration prevented hypotension and hypothermia, which were accompanied by reduced plasma NO, as well as plasma TNF-α, IL-1β, IL-6, and IL-10 and spleen TNF-α and IL-10 levels. We suggest that SI reduced 5-HT levels in the AVPO favor an increased pro-inflammatory status both centrally and peripherally that converge to hypotension and hypothermia. Moreover, our results are consistent with the notion that exogenous 5-HT given icv prevents hypotension and hypothermia probably activating the splenic anti-inflammatory pathway.

Endogenous peripheral hydrogen sulfide is propyretic: its permissive role in brown adipose tissue thermogenesis in rats

NEW FINDINGS

What is the central question of this study? In fever, the most striking response in the acute phase reaction of systemic inflammation, plasma H₂ S concentration increases. However, the role of endogenous peripheral H₂ S in fever is unknown. What is the main finding and its importance? Endogenous peripheral H₂ S is permissive for increased brown adipose tissue thermogenesis to maintain thermal homeostasis in cold environments as well as to mount fever. This finding expands the physiological role of the gaseous modulator as a key regulator of thermal control in health (thermal homeostasis) and disease (fever in systemic inflammation).

ABSTRACT

In recent years, hydrogen sulfide (H₂ S) has been reported as a gaseous modulator acting in several tissues in health and disease. In animal models of systemic inflammation, the plasma H₂ S concentration increases in response to endotoxin (bacterial lipopolysaccharide, LPS). The most striking response in the acute phase reaction of systemic inflammation is fever, but we found no reports of the peripheral action of H₂ S on this thermoregulatory response. We aimed at investigating whether endogenous systemic H₂ S modulates LPS-induced fever. A temperature datalogger capsule was inserted in the abdominal cavity of male Wistar rats (220-270 g) to record body core temperature. These animals received an i.p. injection of a systemic H₂ S inhibitor (propargylglycine; 50 or 75 mg kg^-1 ), immediately followed by an i.p. injection of LPS (50 or 2500 μg kg^-1 ), and were exposed to different ambient temperatures (16, 22 or 27°C). At 22°C, but not at 27°C, propargylglycine at 75 mg kg^-1 significantly attenuated (P < 0.0001) the fever induced by LPS (50 μg kg^-1 ), indicating a modulatory (permissive) action of endogenous peripheral H₂ S on brown adipose tissue (BAT) thermogenesis. Evidence on the modulatory role of peripheral H₂ S in BAT thermogenesis was strengthened when we discarded (i) the possible influence of the gas on febrigenic signalling (when measuring plasma cytokines), and (ii) its interaction with the nitric oxide pathway, and mainly when (iii) we carried out physiological and pharmacological activations of BAT. Endogenous peripheral H₂ S modulates (permits) BAT activity not only in fever but also during maintenance of thermal homeostasis in cold environments.

The gentle art of saying NO: how nitric oxide gets things done in the hypothalamus

The chemical signalling molecule nitric oxide (NO), which freely diffuses through aqueous and lipid environments, subserves an array of functions in the mammalian central nervous system, such as the regulation of synaptic plasticity, blood flow and neurohormone secretion. In this Review, we consider the cellular and molecular mechanisms by which NO evokes short-term and long-term changes in neuronal activity. We also highlight recent studies showing that discrete populations of neurons that synthesize NO in the hypothalamus constitute integrative systems that support life by relaying metabolic and gonadal signals to the neuroendocrine brain, and thus gate the onset of puberty and adult fertility. The putative involvement and therapeutic potential of NO in the pathophysiology of brain diseases, for which hormonal imbalances during postnatal development could be risk factors, is also discussed.

Involvement of endogenous central hydrogen sulfide (H2S) in hypoxia-induced hypothermia in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) display autonomic imbalance and abnormal body temperature (Tb) adjustments. Hydrogen sulfide (H2S) modulates hypoxia-induced hypothermia, but its role in SHR thermoregulation is unknown. We tested the hypothesis that SHR display peculiar thermoregulatory response to hypoxia and that endogenous H2S overproduced in the caudal nucleus of the solitary tract (NTS) of SHR modulates this response. SHR and Wistar rats were microinjected into the fourth ventricle with aminooxyacetate (AOA, H2S-synthezing enzyme inhibitor) or sodium sulfide (Na2S, H2S donor) and exposed to normoxia (21% inspired O2) or hypoxia (10% inspired O2, 30 min). Tb was continuously measured, and H2S production rate was assessed in caudal NTS homogenates. In both groups, AOA, Na2S, or saline (i.e., control; 1 μL) did not affect euthermia. Hypoxia caused similar decreases in Tb in both groups. AOA presented a longer latency to potentiate hypoxic hypothermia in SHR. Caudal NTS H2S production rate was higher in SHR. We suggest that increased bioavailability of H2S in the caudal NTS of SHR enables the adequate modulation of excitability of peripheral chemoreceptor-activated NTS neurons that ultimately induce suppression of brown adipose tissue thermogenesis, thus accounting for the normal hypoxic hypothermia.

Hypometabolism as the ultimate defence in stress response: how the comparative approach helps understanding of medically relevant questions

First conceptualized from breath-hold diving mammals, later recognized as the ultimate cell autonomous survival strategy in anoxia-tolerant vertebrates and burrowing or hibernating rodents, hypometabolism is typically recruited by resilient organisms to withstand and recover from otherwise life-threatening hazards. Through the coordinated down-regulation of biosynthetic, proliferative and electrogenic expenditures at times when little ATP can be generated, a metabolism turned 'down to the pilot light' allows the re-balancing of energy demand with supply at a greatly suppressed level in response to noxious exogenous stimuli or seasonal endogenous cues. A unifying hallmark of stress-tolerant organisms, the adaptation effectively prevents lethal depletion of ATP, thus delineating a marked contrast with susceptible species. Along with disengaged macromolecular syntheses, attenuated transmembrane ion shuttling and PO₂ -conforming respiration rates, the metabolic slowdown in tolerant species usually culminates in a non-cycling, quiescent phenotype. However, such a reprogramming also occurs in leading human pathophysiologies. Ranging from microbial infections through ischaemia-driven infarcts to solid malignancies, cells involved in these disorders may again invoke hypometabolism to endure conditions non-permissive for growth. At the same time, their reduced activities underlie the frequent development of a general resistance to therapeutic interventions. On the other hand, a controlled induction of hypometabolic and/or hypothermic states by pharmacological means has recently stimulated intense research aimed at improved organ preservation and patient survival in situations requiring acutely administered critical care. The current review article therefore presents an up-to-date survey of concepts and applications of a coordinated and reversibly down-regulated metabolic rate as the ultimate defence in stress responses.

Effect of Physical Exercise on the Febrigenic Signaling is Modulated by Preoptic Hydrogen Sulfide Production

We tested the hypothesis that the neuromodulator hydrogen sulfide (H₂S) in the preoptic area (POA) of the hypothalamus modulates the febrigenic signaling differently in sedentary and trained rats. Besides H₂S production rate and protein expressions of H₂S-related synthases cystathionine β-synthase (CBS), 3-mercaptopyruvate sulfurtransferase (3-MPST) and cystathionine γ-lyase (CSE) in the POA, we also measured deep body temperature (Tb), circulating plasma levels of cytokines and corticosterone in an animal model of systemic inflammation. Rats run on a treadmill before receiving an intraperitoneal injection of lipopolysaccharide (LPS, 100 μg/kg) or saline. The magnitude of changes of Tb during the LPS-induced fever was found to be similar between sedentary and trained rats. In sedentary rats, H₂S production was not affected by LPS. Conversely, in trained rats LPS caused a sharp increase in H₂S production rate that was accompanied by an increased CBS expression profile, whereas 3-MPST and CSE expressions were kept relatively constant. Sedentary rats showed a significant LPS-induced release of cytokines (IL-1β, IL-6, and TNF-α) which was virtually abolished in the trained animals. Correlation between POA H₂S and IL-6 as well as TNF-α was observed. Corticosterone levels were augmented after LPS injection in both groups. We found correlations between H₂S and corticosterone, and corticosterone and IL-1β. These data are consistent with the notion that the responses to systemic inflammation are tightly regulated through adjustments in POA H₂S production which may play an anti-inflammatory role downmodulating plasma cytokines levels and upregulating corticosterone release.

Bone repair: Effects of physical exercise and LPS systemic exposition

Bone repair can be facilitated by grafting, biochemical and physical stimulation. Conversely, it may be delayed lipopolysaccharide (LPS). Physical exercise exerts beneficial effects on the bone, but its effect on bone repair is not known. We investigated the effect of exercise on the LPS action on bone healing through bone densitometry, quantitative histological analysis for bone formation rate and immunohistochemical markers in sedentary and exercised animals. Rats ran on the treadmill for four weeks. After training the rats were submitted to a surgical procedure (bone defect in the right tibia) and 24h after the surgery LPS was administered at a dose of 100μg/kg i.p., whereas the control rats received a saline injection (1ml/kg, i.p.). Right tibias were obtained for analysis after 10days during which rats were not submitted to physical training. Physical exercise had a positive effect on bone repair, increasing bone mineral density, bone mineral content, bone formation rate, type I collagen and osteocalcin expression. These parameters were not affected by systemic administration of LPS. Our data indicate that physical exercise has an important osteogenic effect, which is maintained during acute systemic inflammation induced by exposure to a single dose of LPS.

Nitric oxide and fever: immune-to-brain signaling vs. thermogenesis in chicks

Nitric oxide (NO) plays a role in thermogenesis but does not mediate immune-to-brain febrigenic signaling in rats. There are suggestions of a different situation in birds, but the underlying evidence is not compelling. The present study was designed to clarify this matter in 5-day-old chicks challenged with a low or high dose of bacterial LPS. The lower LPS dose (2 μg/kg im) induced fever at 3–5 h postinjection, whereas 100 μg/kg im decreased core body temperature (Tc) (at 1 h) followed by fever (at 4 or 5 h). Plasma nitrate levels increased 4 h after LPS injection, but they were not correlated with the magnitude of fever. The NO synthase inhibitor ( NG-nitro-l-arginine methyl ester, l-NAME; 50 mg/kg im) attenuated the fever induced by either dose of LPS and enhanced the magnitude of the Tc reduction induced by the high dose in chicks at 31–32°C. These effects were associated with suppression of metabolic rate, at least in the case of the high LPS dose. Conversely, the effects of l-NAME on Tc disappeared in chicks maintained at 35–36°C, suggesting that febrigenic signaling was essentially unaffected. Accordingly, the LPS-induced rise in the brain level of PGE2 was not affected by l-NAME. Moreover, l-NAME augmented LPS-induced huddling, which is indicative of compensatory mechanisms to run fever in the face of attenuated thermogenesis. Therefore, as in rats, systemic inhibition of NO synthesis attenuates LPS-induced fever in chicks by affecting thermoeffector activity and not by interfering with immune-to-brain signaling. This may constitute a conserved effect of NO in endotherms.

Spontaneous hypothermia in human sepsis is a transient, self-limiting, and nonterminal response

Hypothermia in sepsis is generally perceived as something dysregulated and progressive although there has been no assessment on the natural course of this phenomenon in humans. This was the first study on the dynamics of hypothermia in septic patients not subjected to active rewarming, and the results were surprising. A sample of 50 subjects presenting with spontaneous hypothermia during sepsis was drawn from the 2005-2012 database of an academic hospital. Hypothermia was defined as body temperature below 36.0°C for longer than 2 h, with at least one reading of 35.5°C or less. The patients presented with 138 episodes of hypothermia, 21 at the time of the sepsis diagnosis and 117 with a later onset. However, hypothermia was uncommon in the final 12 h of life of the patients that succumbed. The majority (97.1%) of the hypothermic episodes were transient and self-limited; the median recovery time was 6 h; body temperature rarely fell below 34.0°C. Bidirectional oscillations in body temperature were evident in the course of hypothermia. Nearly half of the hypothermic episodes had onset in the absence of shock or respiratory distress, and the incidence of hypothermia was not increased during either of these conditions. Usage of antipyretic drugs, sedatives, neuroleptics, or other medications did not predict the onset of hypothermia. In conclusion, hypothermia appears to be a predominantly transient, self-limiting, and nonterminal phenomenon that is inherent to human sepsis. These characteristics resemble those of the regulated hypothermia shown to replace fever in animal models of severe systemic inflammation.

Fever and sickness behavior: Friend or foe?

Fever has been recognized as an important symptom of disease since ancient times. For many years, fever was treated as a putative life-threatening phenomenon. More recently, it has been recognized as an important part of the body's defense mechanisms; indeed at times it has even been used as a therapeutic agent. The knowledge of the functional role of the central nervous system in the genesis of fever has greatly improved over the last decade. It is clear that the febrile process, which develops in the sick individual, is just one of many brain-controlled sickness symptoms. Not only will the sick individual appear "feverish" but they may also display a range of behavioral changes, such as anorexia, fatigue, loss of interest in usual daily activities, social withdrawal, listlessness or malaise, hyperalgesia, sleep disturbances and cognitive dysfunction, collectively termed "sickness behavior". In this review we consider the issue of whether fever and sickness behaviors are friend or foe during: a critical illness, the common cold or influenza, in pregnancy and in the newborn. Deciding whether these sickness responses are beneficial or harmful will very much shape our approach to the use of antipyretics during illness.