Gaseous Mediators in Temperature Regulation Luiz G.S. Branco,*1 Renato N. Soriano,1 and Alexandre A. Steiner2 ABSTRACT Deep body temperature (Tb ) is kept relatively constant despite a wide range of ambient temperature variation. Nevertheless, in particular situations it is beneficial to decrease or to increase Tb in a regulated manner. Under hypoxia for instance a regulated drop in Tb (anapyrexia) is key to reduce oxygen demand of tissues when oxygen availability is diminished, leading to an increased survival rate in a number of species when experiencing low levels of inspired oxygen. On the other hand, a regulated rise in Tb (fever) assists the healing process. These regulated changes in Tb are mediated by the brain, where afferent signals converge and the most important regions for the control of Tb are found. The brain (particularly some hypothalamic structures located in the preoptic area) modulates efferent activities that cause changes in heat production (modulating brown adipose tissue activity and perfusion, for instance) and heat loss (modulating tail skin vasculature blood flow, for instance). This review highlights key advances about the role of the gaseous neuromodulators nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S) C 2014 American Physiological in thermoregulation, acting both on the brain and the periphery. Society. Compr Physiol 4:1301-1338, 2014.
Introduction Temperature, a critical factor in every biochemical or biophysical process, is among the most important determinants of the transfer of matter and energy in any living cell, organism and ecosystem (112). Therefore, it is not surprising that thermoregulation is almost as old as life itself, being present even in unicellular organisms in a thermal gradient (212). An elaborate thermoregulatory system seems to provide adaptive advantages, with the transition from ectothermy (reliance on external heat source for thermoregulation) to endothermy (availability of internal heat source for thermoregulation) being considered a hallmark in vertebrate evolution (55, 133, 316). Mammals and birds are endothermic and, for most of the time, maintain their body temperature (Tb ) relatively constant despite variations in the ambient temperature (Ta ). However, in the face of adverse conditions such as infection, hypoxia, and starvation, Tb may be taken away from this “constant,” euthermic level by means of regulated thermoregulatory responses. A regulated rise in Tb has long been known as fever (170); a regulated decrease in Tb is often referred to as anapyrexia (332). In our view, there is no difference between the terms anapyrexia and regulated hypothermia, as these terms have been used interchangeably to refer to the same phenomenon over the years (26, 118, 124, 332). However, for consistency, only the term anapyrexia will be employed throughout this review. Although virtually all endothermic species, including humans, are capable of developing fever and anapyrexia, it is important to point out that the origin of these responses precedes the origin of endothermy (48,172,318,360). These evolutionarily conserved thermoregulatory responses have been shown to have biological values (170, 229, 332, 393), and
Volume 4, October 2014
are likely to be relevant to clinical practice, as spontaneous increases and decreases in Tb frequently occur during serious medical conditions, which include sepsis (281), hemorrhage (51), hypoglycemia (368), and intoxication (125). For assessments of the costs and benefits of fever or anapyrexia in these and related conditions see Refs. (81, 128, 191, 192, 200, 370). It is evidently important to understand the mechanisms of Tb regulation, not only to find better ways to manage Tb [see Ref. (6)] under specific diseases, but also to circumvent unwanted thermoregulatory effects of drugs developed for other purposes. As an example, a promising new class of analgesic drugs (TRPV1 antagonists) was recently pulled out of clinical trials due to unforeseen hyperthermic effects (108), an event that was quickly followed by a surge of interest in the thermoregulatory effects of these drugs (103, 344). In this context, gaseous bioactive substances such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S) have emerged as attractive therapeutic targets for a variety of diseases [for reviews, see Refs. (58, 253, 291, 353, 387)]. At the same time, they were found to play key roles in thermoregulation, roles that could open avenues for improving Tb management as much as they could limit the viability of these newly identified targets for therapies not aimed at modulating Tb . Here, we review the current knowledge on * Correspondence 1 Dental
to [email protected] School of Ribeir˜ ao Preto, University of S˜ ao Paulo, S˜ ao Paulo,
Brazil 2 Institute
of Biomedical Sciences, University of S˜ ao Paulo, S˜ ao Paulo, Brazil Published online, October 2014 (comprehensivephysiology.com) DOI: 10.1002/cphy.c130053 C American Physiological Society. Copyright
1301
Gaseous Mediators in Temperature Regulation
the roles of NO, CO, and H2 S in thermoregulation. A brief overview of the thermoregulatory system and of the febrile and anapyrexic responses is followed by an in-depth discussion of the evidence implicating each of the abovementioned gaseous mediators in thermoregulation. Abbreviations can be found in Table 1.
Comprehensive Physiology
Table 1
Abbreviations
AC: adenilate cyclase aCSF: artificial cerebrospinal fluid ADP: adenosine diphosphate ATP: adenosine triphosphate AVPO : anteroventral preoptic region
The thermoregulatory system: Basic principles
BAT: brown adipose tissue
Thermoregulation is achieved by means of behavioral and autonomic effectors, most of which are shared with other physiological systems. Behavioral effectors are the most ancient, with their most primitive form (selection of a preferred thermal environment) being encountered in virtually all members of the animal kingdom (67,97). Autonomic effectors appeared later in evolution and, along with them, came the ability to fine tune the rate of heat exchange with the environment and, perhaps most importantly, the ability to maintain Tb above or below Ta (133, 358). There are two categories of autonomic effectors: in one category are those effectors that promote heat loss (e.g., skin vasodilation and sweating); in the other category are those that generate heat (e.g., shivering and nonshivering thermogenesis). In small mammals, which are commonly employed in thermoregulation studies, nonshivering thermogenesis is the function of a highly specialized organ, the interscapular brown adipose tissue (BAT) (55). Although larger mammals such as adult humans do not have an interscapular BAT, relatively discrete depots of brown adipocytes have been found in the cervical, clavicular, and paraspinal regions (75, 239, 375). The relevance of brown fat to whole body thermogenesis in adult humans is still under debate (374). For many decades, the regulation of Tb by the brain was modeled by analogy with an engineered thermostat, according to which the input from thermal afferents (external signal) was compared to an integrated internal reference, the so-called thermoregulatory set point. Initially proposed by Hammel (136), this model was refined over the years (34,40), but its basis always laid on the conjecture that a single signal-reference comparator would coordinate the activity of all thermoeffectors. Over the past decade, however, the set-point model of thermoregulation has been questioned by several authors (156, 217, 279) in view of the realization that thermoeffectors can be activated or inhibited independently of each other in the course of thermoregulatory responses (11, 282, 285, 297, 380), a property that cannot be accounted by a single comparator. These lines of evidence reinforce Partridge’s call for caution when employing manmade engineering principles to explain biological control systems that emerged in layers over the course of evolution (258). Moreover, detailed studies on the neuronal pathways that control thermoeffector activity have failed to identify any neuroanatomical substrate for the elusive comparator that is so fundamental to the set-point model (217, 228, 275, 279, 300). On the contrary, the available evidence is consistent with a model in which Tb is regulated by parallel, functionally
BV: biliverdin
1302
cAMP: cyclic adenoside 3 ,5 -monophosphate CBS: cystathionine beta-synthase cGMP: cyclic guanosine 3 ,5 -monophosphate CNS: central nervous system CO: carbon monoxide CO2: carbon dioxide COX-2: cyclooxygenase-2 CSE: cystathionine gamma-lyase eNOS: endothelial isoform of nitric oxide synthase H2 S: hydrogen sulfide HO: heme oxygenase icv: intracerebroventricular iNOS: inducible isoform of nitric oxide synthase KO: knock out LC: locus coeruleus LPS: lipopolysaccharide MPO: medial preoptic area MST: 3-mercaptopyruvate sulfurtransferase NMDA: N-methyl-D-aspartate nNOS: neuronal isoform of nitric oxide synthase NO: nitric oxide NTS: nucleus of the solitary tract OVLT: organum vasculosum laminae terminalis PDE: phosphodiesterase PGE2 : prostaglandin E2 PKA: protein kinase A PKC: protein kinase C PKG: protein kinase G POA: preoptic area of the hypothalamus PVN: paraventricular nucleus of the hypothalamus ROS: reactive oxygen species rRPa: rostral region of the raphe pallidus sGC: soluble guanilate cyclase SNA: S-nitrosoalbumin SNG: S-nitrosoglutathione Ta : ambient temperature Tb : deep body temperature TNZ: thermoneutral zone VMPO: ventromedial preoptic nucleus VO2 : oxygen consumption
Volume 4, October 2014
Comprehensive Physiology
independent feedback loops, each controlling a single thermoeffector and defending a different level of Tb [see, for example, Refs. (238, 247, 400)]. These control loops consist of sensory and effector pathways that, in the case of autonomic control, meet primarily at the level of the preoptic hypothalamic area (POA) (41, 157, 158, 237, 249). The neural pathways for thermoregulatory behavior as much less understood (275), and do not necessary depend on the integrity of the POA (9, 27, 301). Grounded on these lines of evidence, we herein review the thermoregulatory roles of gaseous transmitters on the basis of independent thermoregulatory circuitries. We acknowledge, though, that the debate surrounding the set-point model is still an ongoing matter. While realizing the independent nature of thermoeffector control, it is important to point out that the temperature thresholds for activation of heat-production and heat-loss effectors remain within a few tenths of a degree from each other for most of the time, when adverse conditions such as disease are not present. This mode of thermoregulation (generally referred to as euthermia) is associated with selection of a Ta at which the cost of thermoregulation is kept at a minimum (119, 120, 126, 285), though exceptions might exist (121). Thermoneutral zone is the name given to the Ta range associated with low-cost thermoregulation. Knowledge of this zone is essential not only for thermoregulatory studies (122, 283), but also for studies of processes that share effectors with the thermoregulatory system, such as the body weight regulation and blood pressure regulation (204, 352, 392). Figure 1A depicts the relationship between the Ta and the activity of thermoeffectors in euthermia. Within the thermoneutral zone, Tb is regulated primarily by fluctuations in skin blood flow that alter the rate of nonevaporative heat loss (283). But, of course, a neutral Ta is not always available. Laboratories, for example, are usually maintained at temperatures that are neutral for humans but subneutral (cool) for many experimental species, including rats and mice; see, for example, Refs. (284, 289). When the Ta is subneutral, skin vessels are maximally constricted and Tb is maintained by activation of thermogenesis, with nonshivering thermogenesis receiving priority over shivering in the face of mild cooling without prior cold adaptation (142, 296) as well as in the face of severe cooling following cold adaptation (30, 84, 115). On the other hand, when the Ta is supraneutral, skin vessels are maximally dilated and Tb is maintained by increasing evaporative heat loss via mechanisms that vary from species to species: sweating in humans and horses (109,313); saliva spreading in rats and mice (275); panting in dogs, sheep, and even in some reptiles (276, 358).
Regulated changes in Tb : Fever and anapyrexia In the latest version of the Glossary of Terms for Thermal Physiology (147), fever (same as pyrexia) is defined as a state of elevated Tb that is due to regulated changes in thermal control associated with an elevated thermoregulatory set point, whereas anapyrexia (from the Greek, ana meaning reverse and pyretos meaning fever) is defined as a state of reduced
Volume 4, October 2014
Gaseous Mediators in Temperature Regulation
Figure 1 Relationship between ambient temperature and the activity of thermoeffectors in euthermia (A), fever (B), and anapyrexia (C). Values for temperature thresholds and preferred ambient temperatures are approximations based on the thermal biology of the laboratory rat (271). Arrows indicate the possible direction (or directions) of the change in thermoeffector activity. TNZ, thermoneutral zone.
Tb that is due to regulated changes in thermal control, presumably associated with a reduction in the thermoregulatory set point. These definitions have been criticized lately, not because they refer to regulated, physiological responses that change Tb in opposite directions, but because they are still tied up to a thermoregulatory set point that arguably does not exist (277). To avoid confusion with regard to this terminology, we have eliminated the set-point component of the definitions for fever and anapyrexia in this review, focusing on the patterns of thermoeffector alterations reported to date. More specifically, we define fever as any thermoregulatory response in which heat-loss effectors are changed in
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Gaseous Mediators in Temperature Regulation
a regulated fashion to permit an “intentional” increase in Tb when coupled with an appropriate thermoregulatory behavior, most commonly preference for a warmer environment; heatproduction effectors may or may not be changed (Fig. 1B). We believe these revised definitions are faithful to the main purpose of the original definitions, which is to make a distinction between those changes in Tb that an organism works to achieve (fever and anapyrexia) from those that are unwanted, but occur when thermoregulatory effectors are unable to compensate for excessive thermal loads (118,169,345). On the other hand, we define anapyrexia as any thermoregulatory response in which heat-production effectors are changed in a regulated fashion to permit an “intentional” decrease in Tb when coupled with an appropriate thermoregulatory behavior, most commonly preference for a cooler environment; heat-loss effectors may or may not be changed (Fig. 1C). Fever is the most common thermoregulatory response to infection, and has been best characterized in rodent models of systemic inflammation induced by low-to-moderate doses of bacterial lipopolysaccharide (LPS). These doses of LPS cause warmth-seeking behavior in both endothermic and ectothermic species [see Refs. (8, 26)]. In endothermic species, LPS also produces consistent increases in the temperature thresholds for activation of skin vasodilation and respiratory heat loss, which then defend a higher level of Tb (138, 146, 379, 380). Therefore, it appears that the most natural way to produce a fever is by means of heat conservation in a warm environment. Nevertheless, there are reports of low doses of LPS also increasing the temperature threshold for activation of thermogenesis (366, 367, 379), a thermoeffector alteration that allows fever to develop even when the environment is subneutral (cool). It is important to point out, though, that this upward shift in the thermogenesis threshold has been reported only in the earliest stages of the systemic inflammatory response. In other stages of the response, an increase in the thermogenesis threshold is not always present (138,146,285,380), which is consistent with the many reports of rodents being unable to develop fever at room (subneutral) temperatures [for a discussion, see Refs. (284, 355)]. The LPS-induced fever model is the most widely accepted model to study the mechanisms involved in the physiological responses during systemic inflammation. This response seems not to be mediated directly by the exogenous pyrogen (LPS), but rather by the endogenously produced protein mediators such as interleukin (IL)-1β, IL-6, interferons, and tumor necrosis factor, and by mediators of lipid origin such as prostaglandins (31). Among these mediators, prostaglandin E2 is thought to be the most proximal mediator of fever, acting directly on the preoptic region (31). However, this LPS model is not the only one that exists to induce fever. Actually, a number of studies have shown that acute exposure to psychological stress, which includes handling, cage switching, exposure to open field, and restraint, increases Tb . For a long time, this increased Tb was believed to be hyperthermia; however, more recent reports have indicated that it is indeed a fever, that is, a regulated rise in Tb , similarly to LPS fever. In agreement
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Comprehensive Physiology
with this notion, it has been shown that tail skin vasculature responses observed during stress-induced fever is comparable to the rise in Tb during LPS fever; reptiles thermoregulate essentially by means of behavioral mechanisms, choosing a higher preferred Ta when are in a thermal gradient after stress [cf. (299)]; IL-6 and IL-1β plasma concentrations rise during psychological stress (195, 220); antitumor necrosis factor-α antiserum potentiates not only LPS-induced fever but also stress-induced fever (206); inhibition of the enzyme cyclooxygenase and consequently preventing a rise in prostaglandin E2 levels weakens stress fever (171, 226); and intracerebroventricularly administered arginine-vasopressin (which attenuates LPS-induced fever) plays also an important antipyretic role during restraint stress fever (364). It is interesting to note that the gaseous mediators, NO (299) and CO (340) have been reported to participate in stressinduced fever. As to the NO study, nNOS isoform seems to play a major role in restraint stress-induced fever since systemic administration of a selective nNOS inhibitor, weakens the increase in Tc caused by restraint stress. See further details below. As to the CO study, it seems that brain HO-CO pathway also plays a key role during stress fever effecting Tb either through a thermoregulatory mechanism or by means of a proanxiety action of CO, or both of them. Eventually, as far as we are concern, no information exists as to the role of H2 S. As it is well established, peripherally administered LPS is not only capable of stimulating the release of cytokines from activated immune cells in peripheral tissues but can also increase the levels of cytokines and prostaglandins in various brain areas. In the brain, physiological responses to systemic inflammation are coordinated. As to the febrile response, the brain receives the febrigenic signals from the periphery and resets the thermoeffectors activities to generate a regulated increase in Tb . These signals, triggered in the periphery, enter the brain by at least four different routes: (i) via afferent fibers through the vagus nerve that synapse the nucleus of the solitary tract (32); (ii) via circumventricular organs, such as the organum vasculosum laminae terminalis (OVLT) and the subfornical organ, which lack a blood-brain barrier (32, 356); (iii) via interaction of cytokines and/or prostanoids with cells located in the blood-brain interface, that is, endothelial cells located in this interface (56, 365) and perivascular cells (86, 305); and (iv) via active transport of cytokines through the blood-brain barrier (31, 280). Once at least one of these afferent pathways is activated, PGE2 reaches and/or is increased in the brain (339). PGE2 is thought to interact with EP3 receptors (244, 373) causing a decrease in the intracellular level of cyclic AMP in preoptic neurons (328, 334), triggering a thermoeffector response that increases Tb . From the preoptic region, both inhibitory and excitatory pathways project to nuclei located in the hypothalamus, midbrain, and brainstem that are involved in the control of thermoeffector activity (234). There is evidence supporting the involvement of POA noradrenaline in the genesis of fever (91,252), and the major noradrenergic nucleus in the brain, the locus coeruleus (LC)—a well-delineated cluster of noradrenaline-containing
Volume 4, October 2014
Comprehensive Physiology
neurons located adjacent to the fourth ventricle in the pontine brainstem (24)—has been shown to be involved (10). About 50% of all noradrenergic projections in the CNS derive from the LC (24). Therefore, there is solid evidence to support the hypothesis that LC neurons play a major role in the development of fever. Having demonstrated that the POA is not modulated by CO (337), our group investigated the putative role of the HO pathway in the LC during fever induced by bacterial LPS in rats. Interestingly, Ravanelli et al. (270) documented that the endogenous HO-CO-cGMP pathway in the LC plays a prominent antipyretic role during LPS fever. They observed that intra-LC administration of the HO pathway inducer hemelysinate attenuated LPS fever, whereas the microinjection of the ZnDPBG (a competitive inhibitor of the HO) potentiated the febrile response. Moreover, their data also provided evidence that the HO product with an antipyretic action in the LC was the CO since this is the only HO product that acts through a cGMP-dependent way. This suggestion was supported by the fact that icv administration of the sGC inhibitor ODQ abolished the effect of heme-lysinate on LPS fever. While infection is the best known inducer of fever, hypoxia is the best known inducer of anapyrexia. Hypoxia has been shown to decrease Tb in a variety of invertebrate and vertebrate species (229, 332, 393), including man (177). The Tb fall induced by hypoxia occurs when oxygen supply is still sufficient to support aerobic metabolism (295,362). Therefore, metabolic failure is unlikely to account for the decrease in Tb . For this and other reasons, hypoxia is now thought to elicit a regulated decrease in Tb , that is, anapyrexia, which serves as a preemptive strategy to reduce metabolic demands before severe hypoxia ensues. The most pronounced thermoeffector alteration in anapyrexia is a decrease in the temperature threshold for activation of thermogenesis (18,362), which, in conjunction with cold-seeking behavior (51,127,393), is very efficacious at lowering Tb with minimal cost. Heat-loss effectors may also be activated during hypoxia, at least in some species such as the cat (37) and the golden-mantled ground squirrel (361). However, the only study (37) that measured the temperature threshold for a heatloss effector (thermal polypnea) during hypoxia suggests that the downward shift in this threshold is rather modest (
Introduction Temperature, a critical factor in every biochemical or biophysical process, is among the most important determinants of the transfer of matter and energy in any living cell, organism and ecosystem (112). Therefore, it is not surprising that thermoregulation is almost as old as life itself, being present even in unicellular organisms in a thermal gradient (212). An elaborate thermoregulatory system seems to provide adaptive advantages, with the transition from ectothermy (reliance on external heat source for thermoregulation) to endothermy (availability of internal heat source for thermoregulation) being considered a hallmark in vertebrate evolution (55, 133, 316). Mammals and birds are endothermic and, for most of the time, maintain their body temperature (Tb ) relatively constant despite variations in the ambient temperature (Ta ). However, in the face of adverse conditions such as infection, hypoxia, and starvation, Tb may be taken away from this “constant,” euthermic level by means of regulated thermoregulatory responses. A regulated rise in Tb has long been known as fever (170); a regulated decrease in Tb is often referred to as anapyrexia (332). In our view, there is no difference between the terms anapyrexia and regulated hypothermia, as these terms have been used interchangeably to refer to the same phenomenon over the years (26, 118, 124, 332). However, for consistency, only the term anapyrexia will be employed throughout this review. Although virtually all endothermic species, including humans, are capable of developing fever and anapyrexia, it is important to point out that the origin of these responses precedes the origin of endothermy (48,172,318,360). These evolutionarily conserved thermoregulatory responses have been shown to have biological values (170, 229, 332, 393), and
Volume 4, October 2014
are likely to be relevant to clinical practice, as spontaneous increases and decreases in Tb frequently occur during serious medical conditions, which include sepsis (281), hemorrhage (51), hypoglycemia (368), and intoxication (125). For assessments of the costs and benefits of fever or anapyrexia in these and related conditions see Refs. (81, 128, 191, 192, 200, 370). It is evidently important to understand the mechanisms of Tb regulation, not only to find better ways to manage Tb [see Ref. (6)] under specific diseases, but also to circumvent unwanted thermoregulatory effects of drugs developed for other purposes. As an example, a promising new class of analgesic drugs (TRPV1 antagonists) was recently pulled out of clinical trials due to unforeseen hyperthermic effects (108), an event that was quickly followed by a surge of interest in the thermoregulatory effects of these drugs (103, 344). In this context, gaseous bioactive substances such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S) have emerged as attractive therapeutic targets for a variety of diseases [for reviews, see Refs. (58, 253, 291, 353, 387)]. At the same time, they were found to play key roles in thermoregulation, roles that could open avenues for improving Tb management as much as they could limit the viability of these newly identified targets for therapies not aimed at modulating Tb . Here, we review the current knowledge on * Correspondence 1 Dental
to [email protected] School of Ribeir˜ ao Preto, University of S˜ ao Paulo, S˜ ao Paulo,
Brazil 2 Institute
of Biomedical Sciences, University of S˜ ao Paulo, S˜ ao Paulo, Brazil Published online, October 2014 (comprehensivephysiology.com) DOI: 10.1002/cphy.c130053 C American Physiological Society. Copyright
1301
Gaseous Mediators in Temperature Regulation
the roles of NO, CO, and H2 S in thermoregulation. A brief overview of the thermoregulatory system and of the febrile and anapyrexic responses is followed by an in-depth discussion of the evidence implicating each of the abovementioned gaseous mediators in thermoregulation. Abbreviations can be found in Table 1.
Comprehensive Physiology
Table 1
Abbreviations
AC: adenilate cyclase aCSF: artificial cerebrospinal fluid ADP: adenosine diphosphate ATP: adenosine triphosphate AVPO : anteroventral preoptic region
The thermoregulatory system: Basic principles
BAT: brown adipose tissue
Thermoregulation is achieved by means of behavioral and autonomic effectors, most of which are shared with other physiological systems. Behavioral effectors are the most ancient, with their most primitive form (selection of a preferred thermal environment) being encountered in virtually all members of the animal kingdom (67,97). Autonomic effectors appeared later in evolution and, along with them, came the ability to fine tune the rate of heat exchange with the environment and, perhaps most importantly, the ability to maintain Tb above or below Ta (133, 358). There are two categories of autonomic effectors: in one category are those effectors that promote heat loss (e.g., skin vasodilation and sweating); in the other category are those that generate heat (e.g., shivering and nonshivering thermogenesis). In small mammals, which are commonly employed in thermoregulation studies, nonshivering thermogenesis is the function of a highly specialized organ, the interscapular brown adipose tissue (BAT) (55). Although larger mammals such as adult humans do not have an interscapular BAT, relatively discrete depots of brown adipocytes have been found in the cervical, clavicular, and paraspinal regions (75, 239, 375). The relevance of brown fat to whole body thermogenesis in adult humans is still under debate (374). For many decades, the regulation of Tb by the brain was modeled by analogy with an engineered thermostat, according to which the input from thermal afferents (external signal) was compared to an integrated internal reference, the so-called thermoregulatory set point. Initially proposed by Hammel (136), this model was refined over the years (34,40), but its basis always laid on the conjecture that a single signal-reference comparator would coordinate the activity of all thermoeffectors. Over the past decade, however, the set-point model of thermoregulation has been questioned by several authors (156, 217, 279) in view of the realization that thermoeffectors can be activated or inhibited independently of each other in the course of thermoregulatory responses (11, 282, 285, 297, 380), a property that cannot be accounted by a single comparator. These lines of evidence reinforce Partridge’s call for caution when employing manmade engineering principles to explain biological control systems that emerged in layers over the course of evolution (258). Moreover, detailed studies on the neuronal pathways that control thermoeffector activity have failed to identify any neuroanatomical substrate for the elusive comparator that is so fundamental to the set-point model (217, 228, 275, 279, 300). On the contrary, the available evidence is consistent with a model in which Tb is regulated by parallel, functionally
BV: biliverdin
1302
cAMP: cyclic adenoside 3 ,5 -monophosphate CBS: cystathionine beta-synthase cGMP: cyclic guanosine 3 ,5 -monophosphate CNS: central nervous system CO: carbon monoxide CO2: carbon dioxide COX-2: cyclooxygenase-2 CSE: cystathionine gamma-lyase eNOS: endothelial isoform of nitric oxide synthase H2 S: hydrogen sulfide HO: heme oxygenase icv: intracerebroventricular iNOS: inducible isoform of nitric oxide synthase KO: knock out LC: locus coeruleus LPS: lipopolysaccharide MPO: medial preoptic area MST: 3-mercaptopyruvate sulfurtransferase NMDA: N-methyl-D-aspartate nNOS: neuronal isoform of nitric oxide synthase NO: nitric oxide NTS: nucleus of the solitary tract OVLT: organum vasculosum laminae terminalis PDE: phosphodiesterase PGE2 : prostaglandin E2 PKA: protein kinase A PKC: protein kinase C PKG: protein kinase G POA: preoptic area of the hypothalamus PVN: paraventricular nucleus of the hypothalamus ROS: reactive oxygen species rRPa: rostral region of the raphe pallidus sGC: soluble guanilate cyclase SNA: S-nitrosoalbumin SNG: S-nitrosoglutathione Ta : ambient temperature Tb : deep body temperature TNZ: thermoneutral zone VMPO: ventromedial preoptic nucleus VO2 : oxygen consumption
Volume 4, October 2014
Comprehensive Physiology
independent feedback loops, each controlling a single thermoeffector and defending a different level of Tb [see, for example, Refs. (238, 247, 400)]. These control loops consist of sensory and effector pathways that, in the case of autonomic control, meet primarily at the level of the preoptic hypothalamic area (POA) (41, 157, 158, 237, 249). The neural pathways for thermoregulatory behavior as much less understood (275), and do not necessary depend on the integrity of the POA (9, 27, 301). Grounded on these lines of evidence, we herein review the thermoregulatory roles of gaseous transmitters on the basis of independent thermoregulatory circuitries. We acknowledge, though, that the debate surrounding the set-point model is still an ongoing matter. While realizing the independent nature of thermoeffector control, it is important to point out that the temperature thresholds for activation of heat-production and heat-loss effectors remain within a few tenths of a degree from each other for most of the time, when adverse conditions such as disease are not present. This mode of thermoregulation (generally referred to as euthermia) is associated with selection of a Ta at which the cost of thermoregulation is kept at a minimum (119, 120, 126, 285), though exceptions might exist (121). Thermoneutral zone is the name given to the Ta range associated with low-cost thermoregulation. Knowledge of this zone is essential not only for thermoregulatory studies (122, 283), but also for studies of processes that share effectors with the thermoregulatory system, such as the body weight regulation and blood pressure regulation (204, 352, 392). Figure 1A depicts the relationship between the Ta and the activity of thermoeffectors in euthermia. Within the thermoneutral zone, Tb is regulated primarily by fluctuations in skin blood flow that alter the rate of nonevaporative heat loss (283). But, of course, a neutral Ta is not always available. Laboratories, for example, are usually maintained at temperatures that are neutral for humans but subneutral (cool) for many experimental species, including rats and mice; see, for example, Refs. (284, 289). When the Ta is subneutral, skin vessels are maximally constricted and Tb is maintained by activation of thermogenesis, with nonshivering thermogenesis receiving priority over shivering in the face of mild cooling without prior cold adaptation (142, 296) as well as in the face of severe cooling following cold adaptation (30, 84, 115). On the other hand, when the Ta is supraneutral, skin vessels are maximally dilated and Tb is maintained by increasing evaporative heat loss via mechanisms that vary from species to species: sweating in humans and horses (109,313); saliva spreading in rats and mice (275); panting in dogs, sheep, and even in some reptiles (276, 358).
Regulated changes in Tb : Fever and anapyrexia In the latest version of the Glossary of Terms for Thermal Physiology (147), fever (same as pyrexia) is defined as a state of elevated Tb that is due to regulated changes in thermal control associated with an elevated thermoregulatory set point, whereas anapyrexia (from the Greek, ana meaning reverse and pyretos meaning fever) is defined as a state of reduced
Volume 4, October 2014
Gaseous Mediators in Temperature Regulation
Figure 1 Relationship between ambient temperature and the activity of thermoeffectors in euthermia (A), fever (B), and anapyrexia (C). Values for temperature thresholds and preferred ambient temperatures are approximations based on the thermal biology of the laboratory rat (271). Arrows indicate the possible direction (or directions) of the change in thermoeffector activity. TNZ, thermoneutral zone.
Tb that is due to regulated changes in thermal control, presumably associated with a reduction in the thermoregulatory set point. These definitions have been criticized lately, not because they refer to regulated, physiological responses that change Tb in opposite directions, but because they are still tied up to a thermoregulatory set point that arguably does not exist (277). To avoid confusion with regard to this terminology, we have eliminated the set-point component of the definitions for fever and anapyrexia in this review, focusing on the patterns of thermoeffector alterations reported to date. More specifically, we define fever as any thermoregulatory response in which heat-loss effectors are changed in
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a regulated fashion to permit an “intentional” increase in Tb when coupled with an appropriate thermoregulatory behavior, most commonly preference for a warmer environment; heatproduction effectors may or may not be changed (Fig. 1B). We believe these revised definitions are faithful to the main purpose of the original definitions, which is to make a distinction between those changes in Tb that an organism works to achieve (fever and anapyrexia) from those that are unwanted, but occur when thermoregulatory effectors are unable to compensate for excessive thermal loads (118,169,345). On the other hand, we define anapyrexia as any thermoregulatory response in which heat-production effectors are changed in a regulated fashion to permit an “intentional” decrease in Tb when coupled with an appropriate thermoregulatory behavior, most commonly preference for a cooler environment; heat-loss effectors may or may not be changed (Fig. 1C). Fever is the most common thermoregulatory response to infection, and has been best characterized in rodent models of systemic inflammation induced by low-to-moderate doses of bacterial lipopolysaccharide (LPS). These doses of LPS cause warmth-seeking behavior in both endothermic and ectothermic species [see Refs. (8, 26)]. In endothermic species, LPS also produces consistent increases in the temperature thresholds for activation of skin vasodilation and respiratory heat loss, which then defend a higher level of Tb (138, 146, 379, 380). Therefore, it appears that the most natural way to produce a fever is by means of heat conservation in a warm environment. Nevertheless, there are reports of low doses of LPS also increasing the temperature threshold for activation of thermogenesis (366, 367, 379), a thermoeffector alteration that allows fever to develop even when the environment is subneutral (cool). It is important to point out, though, that this upward shift in the thermogenesis threshold has been reported only in the earliest stages of the systemic inflammatory response. In other stages of the response, an increase in the thermogenesis threshold is not always present (138,146,285,380), which is consistent with the many reports of rodents being unable to develop fever at room (subneutral) temperatures [for a discussion, see Refs. (284, 355)]. The LPS-induced fever model is the most widely accepted model to study the mechanisms involved in the physiological responses during systemic inflammation. This response seems not to be mediated directly by the exogenous pyrogen (LPS), but rather by the endogenously produced protein mediators such as interleukin (IL)-1β, IL-6, interferons, and tumor necrosis factor, and by mediators of lipid origin such as prostaglandins (31). Among these mediators, prostaglandin E2 is thought to be the most proximal mediator of fever, acting directly on the preoptic region (31). However, this LPS model is not the only one that exists to induce fever. Actually, a number of studies have shown that acute exposure to psychological stress, which includes handling, cage switching, exposure to open field, and restraint, increases Tb . For a long time, this increased Tb was believed to be hyperthermia; however, more recent reports have indicated that it is indeed a fever, that is, a regulated rise in Tb , similarly to LPS fever. In agreement
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with this notion, it has been shown that tail skin vasculature responses observed during stress-induced fever is comparable to the rise in Tb during LPS fever; reptiles thermoregulate essentially by means of behavioral mechanisms, choosing a higher preferred Ta when are in a thermal gradient after stress [cf. (299)]; IL-6 and IL-1β plasma concentrations rise during psychological stress (195, 220); antitumor necrosis factor-α antiserum potentiates not only LPS-induced fever but also stress-induced fever (206); inhibition of the enzyme cyclooxygenase and consequently preventing a rise in prostaglandin E2 levels weakens stress fever (171, 226); and intracerebroventricularly administered arginine-vasopressin (which attenuates LPS-induced fever) plays also an important antipyretic role during restraint stress fever (364). It is interesting to note that the gaseous mediators, NO (299) and CO (340) have been reported to participate in stressinduced fever. As to the NO study, nNOS isoform seems to play a major role in restraint stress-induced fever since systemic administration of a selective nNOS inhibitor, weakens the increase in Tc caused by restraint stress. See further details below. As to the CO study, it seems that brain HO-CO pathway also plays a key role during stress fever effecting Tb either through a thermoregulatory mechanism or by means of a proanxiety action of CO, or both of them. Eventually, as far as we are concern, no information exists as to the role of H2 S. As it is well established, peripherally administered LPS is not only capable of stimulating the release of cytokines from activated immune cells in peripheral tissues but can also increase the levels of cytokines and prostaglandins in various brain areas. In the brain, physiological responses to systemic inflammation are coordinated. As to the febrile response, the brain receives the febrigenic signals from the periphery and resets the thermoeffectors activities to generate a regulated increase in Tb . These signals, triggered in the periphery, enter the brain by at least four different routes: (i) via afferent fibers through the vagus nerve that synapse the nucleus of the solitary tract (32); (ii) via circumventricular organs, such as the organum vasculosum laminae terminalis (OVLT) and the subfornical organ, which lack a blood-brain barrier (32, 356); (iii) via interaction of cytokines and/or prostanoids with cells located in the blood-brain interface, that is, endothelial cells located in this interface (56, 365) and perivascular cells (86, 305); and (iv) via active transport of cytokines through the blood-brain barrier (31, 280). Once at least one of these afferent pathways is activated, PGE2 reaches and/or is increased in the brain (339). PGE2 is thought to interact with EP3 receptors (244, 373) causing a decrease in the intracellular level of cyclic AMP in preoptic neurons (328, 334), triggering a thermoeffector response that increases Tb . From the preoptic region, both inhibitory and excitatory pathways project to nuclei located in the hypothalamus, midbrain, and brainstem that are involved in the control of thermoeffector activity (234). There is evidence supporting the involvement of POA noradrenaline in the genesis of fever (91,252), and the major noradrenergic nucleus in the brain, the locus coeruleus (LC)—a well-delineated cluster of noradrenaline-containing
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Comprehensive Physiology
neurons located adjacent to the fourth ventricle in the pontine brainstem (24)—has been shown to be involved (10). About 50% of all noradrenergic projections in the CNS derive from the LC (24). Therefore, there is solid evidence to support the hypothesis that LC neurons play a major role in the development of fever. Having demonstrated that the POA is not modulated by CO (337), our group investigated the putative role of the HO pathway in the LC during fever induced by bacterial LPS in rats. Interestingly, Ravanelli et al. (270) documented that the endogenous HO-CO-cGMP pathway in the LC plays a prominent antipyretic role during LPS fever. They observed that intra-LC administration of the HO pathway inducer hemelysinate attenuated LPS fever, whereas the microinjection of the ZnDPBG (a competitive inhibitor of the HO) potentiated the febrile response. Moreover, their data also provided evidence that the HO product with an antipyretic action in the LC was the CO since this is the only HO product that acts through a cGMP-dependent way. This suggestion was supported by the fact that icv administration of the sGC inhibitor ODQ abolished the effect of heme-lysinate on LPS fever. While infection is the best known inducer of fever, hypoxia is the best known inducer of anapyrexia. Hypoxia has been shown to decrease Tb in a variety of invertebrate and vertebrate species (229, 332, 393), including man (177). The Tb fall induced by hypoxia occurs when oxygen supply is still sufficient to support aerobic metabolism (295,362). Therefore, metabolic failure is unlikely to account for the decrease in Tb . For this and other reasons, hypoxia is now thought to elicit a regulated decrease in Tb , that is, anapyrexia, which serves as a preemptive strategy to reduce metabolic demands before severe hypoxia ensues. The most pronounced thermoeffector alteration in anapyrexia is a decrease in the temperature threshold for activation of thermogenesis (18,362), which, in conjunction with cold-seeking behavior (51,127,393), is very efficacious at lowering Tb with minimal cost. Heat-loss effectors may also be activated during hypoxia, at least in some species such as the cat (37) and the golden-mantled ground squirrel (361). However, the only study (37) that measured the temperature threshold for a heatloss effector (thermal polypnea) during hypoxia suggests that the downward shift in this threshold is rather modest (
