Raphe magnus/pallidus neurons regulate tail but not mesenteric arterial blood flow in rats

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

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

Sleep deprivation soon after recovery from synthetic torpor enhances tau protein dephosphorylation in the rat brain

Neuronal Tau protein hyperphosphorylation (PPtau) is a hallmark of tauopathic neurodegeneration. However, a reversible brain PPtau occurs in mammals during either natural or "synthetic" torpor (ST), a transient deep hypothermic state that can be pharmacologically induced in rats. Since in both conditions a high sleep pressure builds up during the regaining of euthermia, the aim of this work was to assess the possible role of post-ST sleep in PPtau dephosphorylation. Male rats were studied at the hypothermic nadir of ST, and 3-6 h after the recovery of euthermia, after either normal sleep (NS) or total sleep deprivation (SD). The effects of SD were studied by assessing: (i) deep brain temperature (Tb); (ii) immunofluorescent staining for AT8 (phosphorylated Tau) and Tau-1 (non-phosphorylated Tau), assessed in 19 brain structures; (iii) different phosphorylated forms of Tau and the main cellular factors involved in Tau phospho-regulation, including pro- and anti-apoptotic markers, assessed through western blot in the parietal cortex and hippocampus; (iv) systemic factors which are involved in natural torpor; (v) microglia activation state, by considering morphometric variations. Unexpectedly, the reversibility of PPtau was more efficient in SD than in NS animals, and was concomitant with a higher Tb, higher melatonin plasma levels, and a higher frequency of the microglia resting phenotype. Since the reversibility of ST-induced PPtau was previously shown to be driven by a latent physiological molecular mechanism triggered by deep hypothermia, short-term SD soon after the regaining of euthermia seems to boost the possible neuroprotective effects of this mechanism.

Chronic Pain-Associated Cardiovascular Disease: The Role of Sympathetic Nerve Activity

Chronic pain affects many people world-wide, and this number is continuously increasing. There is a clear link between chronic pain and the development of cardiovascular disease through activation of the sympathetic nervous system. The purpose of this review is to provide evidence from the literature that highlights the direct relationship between sympathetic nervous system dysfunction and chronic pain. We hypothesize that maladaptive changes within a common neural network regulating the sympathetic nervous system and pain perception contribute to sympathetic overactivation and cardiovascular disease in the setting of chronic pain. We review clinical evidence and highlight the basic neurocircuitry linking the sympathetic and nociceptive networks and the overlap between the neural networks controlling the two.

Synthetic torpor triggers a regulated mechanism in the rat brain, favoring the reversibility of Tau protein hyperphosphorylation

Introduction: Hyperphosphorylated Tau protein (PPTau) is the hallmark of tauopathic neurodegeneration. During "synthetic torpor" (ST), a transient hypothermic state which can be induced in rats by the local pharmacological inhibition of the Raphe Pallidus, a reversible brain Tau hyperphosphorylation occurs. The aim of the present study was to elucidate the - as yet unknown - molecular mechanisms underlying this process, at both a cellular and systemic level. Methods: Different phosphorylated forms of Tau and the main cellular factors involved in Tau phospho-regulation were assessed by western blot in the parietal cortex and hippocampus of rats induced in ST, at either the hypothermic nadir or after the recovery of euthermia. Pro- and anti-apoptotic markers, as well as different systemic factors which are involved in natural torpor, were also assessed. Finally, the degree of microglia activation was determined through morphometry. Results: Overall, the results show that ST triggers a regulated biochemical process which can dam PPTau formation and favor its reversibility starting, unexpectedly for a non-hibernator, from the hypothermic nadir. In particular, at the nadir, the glycogen synthase kinase-β was largely inhibited in both regions, the melatonin plasma levels were significantly increased and the antiapoptotic factor Akt was significantly activated in the hippocampus early after, while a transient neuroinflammation was observed during the recovery period. Discussion: Together, the present data suggest that ST can trigger a previously undescribed latent and regulated physiological process, that is able to cope with brain PPTau formation.

Mitochondrial respiration in rats during hypothermia resulting from central drug administration

The ability to induce a hypothermia resembling that of natural torpor would be greatly beneficial in medical and non-medical fields. At present, two procedures based on central nervous pharmacological manipulation have been shown to be effective in bringing core body temperature well below 30 °C in the rat, a non-hibernator: the first, based on the inhibition of a key relay in the central thermoregulatory pathway, the other, based on the activation of central adenosine A1 receptors. Although the role of mitochondria in the activation and maintenance of torpor has been extensively studied, no data are available for centrally induced hypothermia in non-hibernators. Thus, in the present work the respiration rate of mitochondria in the liver and in the kidney of rats following the aforementioned hypothermia-inducing treatments was studied. Moreover, to have an internal control, the same parameters were assessed in a well-consolidated model, i.e., mice during fasting-induced torpor. Our results show that state 3 respiration rate, which significantly decreased in the liver of mice, was unchanged in rats. An increase of state 4 respiration rate was observed in both species, although it was not statistically significant in rats under central adenosine stimulation. Also, a significant decrease of the respiratory control ratio was detected in both species. Finally, no effects were detected in kidney mitochondria in both species. Overall, in these hypothermic conditions liver mitochondria of rats remained active and apparently ready to be re-activated to produce energy and warm up the cells. These findings can be interpreted as encouraging in view of the finalization of a translational approach to humans.

Central sympathetic network for thermoregulatory responses to psychological stress

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

A hypothalamomedullary network for physiological responses to environmental stresses

Various environmental stressors, such as extreme temperatures (hot and cold), pathogens, predators and insufficient food, can threaten life. Remarkable progress has recently been made in understanding the central circuit mechanisms of physiological responses to such stressors. A hypothalamomedullary neural pathway from the dorsomedial hypothalamus (DMH) to the rostral medullary raphe region (rMR) regulates sympathetic outflows to effector organs for homeostasis. Thermal and infection stress inputs to the preoptic area dynamically alter the DMH → rMR transmission to elicit thermoregulatory, febrile and cardiovascular responses. Psychological stress signalling from a ventromedial prefrontal cortical area to the DMH drives sympathetic and behavioural responses for stress coping, representing a psychosomatic connection from the corticolimbic emotion circuit to the autonomic and somatic motor systems. Under starvation stress, medullary reticular neurons activated by hunger signalling from the hypothalamus suppress thermogenic drive from the rMR for energy saving and prime mastication to promote food intake. This Perspective presents a combined neural network for environmental stress responses, providing insights into the central circuit mechanism for the integrative regulation of systemic organs.

A thermoregulatory role of the medullary raphe in birds

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

Regulation of Body Temperature by the Nervous System

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

Central Mechanisms for Thermoregulation

Maintenance of a homeostatic body core temperature is a critical brain function accomplished by a central neural network. This orchestrates a complex behavioral and autonomic repertoire in response to environmental temperature challenges or declining energy homeostasis and in support of immune responses and many behavioral states. This review summarizes the anatomical, neurotransmitter, and functional relationships within the central neural network that controls the principal thermoeffectors: cutaneous vasoconstriction regulating heat loss and shivering and brown adipose tissue for heat production. The core thermoregulatory network regulating these thermoeffectors consists of parallel but distinct central efferent pathways that share a common peripheral thermal sensory input. Delineating the neural circuit mechanism underlying central thermoregulation provides a useful platform for exploring its functional organization, elucidating the molecular underpinnings of its neuronal interactions, and discovering novel therapeutic approaches to modulating body temperature and energy homeostasis.

Medullary raphe nuclei activate the lumbosacral defecation center through the descending serotonergic pathway to regulate colorectal motility in rats

Colorectal motility is regulated by two defecation centers located in the brain and spinal cord. In previous studies, we have shown that administration of serotonin (5-HT) in the lumbosacral spinal cord causes enhancement of colorectal motility. Because spinal 5-HT is derived from neurons of the medullary raphe nuclei, including the raphe magnus, raphe obscurus, and raphe pallidus, we examined whether stimulation of the medullary raphe nuclei enhances colorectal motility via the lumbosacral defecation center. Colorectal pressure was recorded with a balloon in vivo in anesthetized rats. Electrical stimulation of the medullary raphe nuclei failed to enhance colorectal motility. Because GABAergic neurons can be simultaneously activated by the raphe stimulation and released GABA masks accelerating actions of the raphe nuclei on the lumbosacral defecation center, a GABA_A receptor antagonist was preinjected intrathecally to manifest excitatory responses. When spinal GABA_A receptors were blocked by the antagonist, electrical stimulation of the medullary raphe nuclei increased colorectal contractions. This effect of the raphe nuclei was inhibited by intrathecal injection of 5-hydroxytryptamine type 2 (5-HT₂) and type 3 (5-HT₃) receptor antagonists. In addition, injection of a selective 5-HT reuptake inhibitor in the lumbosacral spinal cord augmented the raphe stimulation-induced enhancement of colorectal motility. Transection of the pelvic nerves, but not transection of the colonic nerves, prevented the effect of the raphe nuclei on colorectal motility. These results demonstrate that activation of the medullary raphe nuclei causes augmented contractions of the colorectum via 5-HT₂ and 5-HT₃ receptors in the lumbosacral defecation center. NEW & NOTEWORTHY We have shown that electrical stimulation of the medullary raphe nuclei causes augmented contractions of the colorectum via pelvic nerves in rats. The effect of the medullary raphe nuclei on colorectal motility is exerted through activation of 5-hydroxytryptamine type 2 and type 3 receptors in the lumbosacral defecation center. The descending serotoninergic raphespinal tract represents new potential therapeutic targets against colorectal dysmotility such as irritable bowel syndrome.

Afferent pathways for autonomic and shivering thermoeffectors

Body core temperature of mammals is regulated by the central nervous system, in which the preoptic area (POA) of the hypothalamus plays a pivotal role. The POA receives peripheral and central thermosensory neural information and provides command signals to effector organs to elicit involuntary thermoregulatory responses, including shivering thermogenesis, nonshivering brown adipose tissue thermogenesis, and cutaneous vasoconstriction. Cool-sensory and warm-sensory signals from cutaneous thermoreceptors, monitoring environmental temperature, are separately transmitted through the spinal-parabrachial-POA neural pathways, distinct from the spinothalamocortical pathway for perception of skin temperature. These cutaneous thermosensory inputs to the POA likely impinge on warm-sensitive POA neurons, which monitor body core (brain) temperature, to alter thermoregulatory command outflows from the POA. The cutaneous thermosensory afferents elicit rapid thermoregulatory responses to environmental thermal challenges before they impact body core temperature. Peripheral humoral signals also act on neurons in the POA to transmit afferent information of systemic infection and energy storage to induce fever and to regulate energy balance, respectively. This chapter describes the thermoregulatory afferent mechanisms that convey cutaneous thermosensory signals to the POA and that integrate the neural and humoral afferent inputs to the POA to provide descending command signals to thermoregulatory effectors.

Skin vasoconstriction as a heat conservation thermoeffector

Cold exposure stimulates heat production and conservation to protect internal temperature. Heat conservation is brought about via reductions in skin blood flow. The focus, here, is an exploration of the mechanisms, particularly in humans, leading to that cutaneous vasoconstriction. Local skin cooling has several effects: (1) reduction of tonic nitric oxide formation by inhibiting nitric oxide synthase and element(s) downstream of the enzyme, which removes tonic vasodilator effects, yielding a relative vasoconstriction; (2) translocation of intracellular alpha-2c adrenoceptors to the vascular smooth-muscle cell membrane, enhancing adrenergic vasoconstriction; (3) increased norepinephrine release from vasoconstrictor nerves; and (4) cold-induced vasodilation, seen more clearly in anastomoses-rich glabrous skin. Cold-induced vasodilation occurs in nonglabrous skin when nitric oxide synthase or sympathetic function is blocked. Reflex responses to general body cooling complement these local effects. Sympathetic excitation leads to the increased release of norepinephrine and its cotransmitter neuropeptide Y, each of which contributes significantly to the vasoconstriction. The contributions of these two transmitters vary with aging, disease and, in women, reproductive hormone status. Interaction between local and reflex mechanisms is in part through effects on baseline and in part through removal of the inhibitory effects of nitric oxide on adrenergic vasoconstriction.

The relevance of kalikrein-kinin system via activation of B2 receptor in LPS-induced fever in rats

PURPOSE

This study evaluated the involvement of endogenous kallikrein-kinin system and the bradykinin (BK) B₁ and B₂ receptors on LPS- induced fever and the POA cells involved in this response.

MATERIAL AND METHODS

Male Wistar rats received either i.v. (1 mg/kg), i.c.v. (20 nmol) or i.h. (2 nmol) injections of icatibant (B₂ receptor antagonist) 30 or 60 min, respectively, before the stimuli. DALBK (B₁ receptor antagonist) was given either 15min before BK (i.c.v.) or 30 min before LPS (i.v.). Captopril (5 mg/kg, sc.,) was given 1 h prior LPS or BK. Concentrations of BK and total kininogenon CSF, plasma and tissue kallikrein were evaluated. Rectal temperatures (rT) were assessed by telethermometry. Ca⁺⁺ signaling in POA cells was performed in rat pup brain tissue microcultures.

RESULTS

Icatibant reduced LPS fever while, captopril exacerbated that response, an effect abolished by icatibant. Icatibant (i.h.) reduced fever to BK (i.h.) but not that induced by LPS (i.v.). BK increased intracellular calcium concentration in neurons and astrocytes. LPS increased levels of bradykinin, tissue kallikrein and total kininogen. BK (i.c.v.) increased rT and decreased tail skin temperature. Captopril potentiated BK-induced fever an effect abolished by icatibant. DALBK reduced the fever induced by BK. BK (i.c.v.) increased the CSF PGE₂concentration. Effect abolished by indomethacin (i.p.).

CONCLUSIONS

LPS activates endogenous kalikrein-kinin system leading to production of BK, which by acting on B₂-receptors of POA cells causes prostaglandin synthesis that in turn produces fever. Thus, a kinin B₂-receptor antagonist that enters into the brain could constitute a new and interesting strategy to treat fever.

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

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

Control of the Cutaneous Circulation by the Central Nervous System

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

Central neural control of thermoregulation and brown adipose tissue

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

The rostral ventromedial medulla control of cutaneous vasomotion of paws and tail in the rat: implication for pain studies

Thermal neutrality in rodents is achieved by large cyclic variations of the sympathetic drive of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Given the pivotal functional role of rostral ventromedial medulla (RVM) in nociception and rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, we aimed at circumscribing the brainstem regions that are the source of premotor afferents to sympathetic preganglionic neurons that control the vasomotor tone of the tail and hind paws. A thermometric infrared camera recorded indirectly the vasomotor tone of the tail and hind paws. During the control period, the rat was maintained in vasoconstriction by preserving a stable, homogeneous, and constant surrounding temperature, slightly below the core temperature. The functional blockade of the RVM/rMR by the GABAA receptor agonist muscimol (0.5 nmol, 50 nl) elicited an extensive increase of the temperature of the paws and tail, associated with a slight decrease of blood pressure and heart rate. Both the increased heat loss through vasodilatation and the decrease heart-induced heat production elicited a remarkable reduction of the central temperature. The effective zones were circumscribed to the parts of the RVM/rMR facing the facial nucleus. They match very exactly the brain regions often described as specifically devoted to the control of nociception. Our data support and urge on the highest cautiousness regarding the interpretation of results aimed at studying the effects of any pharmacological manipulations of RVM/rMR with the usual tests of pain.

Neural circuit for psychological stress-induced hyperthermia

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

Control of cutaneous blood flow by central nervous system

Hairless skin acts as a heat exchanger between body and environment, and thus greatly contributes to body temperature regulation by changing blood flow to the skin (cutaneous) vascular bed during physiological responses such as cold- or warm-defense and fever. Cutaneous blood flow is also affected by alerting state; we 'go pale with fright'. The rabbit ear pinna and the rat tail have hairless skin, and thus provide animal models for investigating central pathway regulating blood flow to cutaneous vascular beds. Cutaneous blood flow is controlled by the centrally regulated sympathetic nervous system. Sympathetic premotor neurons in the medullary raphé in the lower brain stem are labeled at early stage after injection of trans-synaptic viral tracer into skin wall of the rat tail. Inactivation of these neurons abolishes cutaneous vasomotor changes evoked as part of thermoregulatory, febrile or psychological responses, indicating that the medullary raphé is a common final pathway to cutaneous sympathetic outflow, receiving neural inputs from upstream nuclei such as the preoptic area, hypothalamic nuclei and the midbrain. Summarizing evidences from rats and rabbits studies in the last 2 decades, we will review our current understanding of the central pathways mediating cutaneous vasomotor control.

Motion sickness is associated with an increase in vestibular modulation of skin but not muscle sympathetic nerve activity

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered bilaterally at frequencies of 0.08-2.00 Hz, causes a pronounced modulation of muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA), together with robust frequency-dependent illusions of side-to-side motion. At low frequencies of sGVS (≤0.2 Hz), some subjects report nausea, so we tested the hypothesis that vestibular modulation of MSNA and SSNA is augmented in individuals reporting nausea. MSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 22 awake, seated subjects; SSNA was recorded in 14 subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08, 0.13, and 0.18 Hz. Nausea was reported by 21 out of 36 subjects (58 %), but across frequencies of sGVS there was no difference in the magnitude of the vestibular modulation of MSNA in subjects who reported nausea (27.1 ± 1.8 %) and those who did not (30.4 ± 2.9 %). This contrasts with the significantly greater vestibular modulation of SSNA with nausea (41.1 ± 2.0 vs. 28.7 ± 3.1 %) and indicates an organ-specific modulation of sympathetic outflow via the vestibular system during motion sickness.

Reduced brown adipose tissue thermogenesis during environmental interactions in transgenic rats with ataxin-3-mediated ablation of hypothalamic orexin neurons

Thermogenesis in brown adipose tissue (BAT) contributes to substantial increases in body temperature evoked by threatening or emotional stimuli. BAT thermogenesis also contributes to increases in body temperature that occur during active phases of the basic rest-activity cycle (BRAC), as part of normal daily life. Hypothalamic orexin-synthesizing neurons influence many physiological and behavioral variables, including BAT and body temperature. In conscious unrestrained animals maintained for 3 days in a quiet environment (24-26°C) with ad libitum food and water, we compared temperatures in transgenic rats with ablation of orexin neurons induced by expression of ataxin-3 (Orx_Ab) with wild-type (WT) rats. Both baseline BAT temperature and baseline body temperature, measured at the onset of BRAC episodes, were similar in Orx_Ab and WT rats. The time interval between BRAC episodes was also similar in the two groups. However, the initial slopes and amplitudes of BRAC-related increases in BAT and body temperature were reduced in Orx_Ab rats. Similarly, the initial slopes and amplitudes of the increases in BAT temperatures induced by sudden exposure to an intruder rat (freely moving or confined to a small cage) or by sudden exposure to live cockroaches were reduced in resident Orx_Ab rats. Constriction of the tail artery induced by salient alerting stimuli was also reduced in Orx_Ab rats. Our results suggest that orexin-synthesizing neurons contribute to the intensity with which rats interact with the external environment, both when the interaction is "spontaneous" and when the interaction is provoked by threatening or salient environmental events.

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

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

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

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

Chemokine ligand (CCL)-3 promotes an integrated febrile response when injected within pre-optic area (POA) of rats and induces calcium signaling in cells of POA microcultures but not TNF-α or IL-6 synthesis

Although studies have shown that chemokines are pyrogenic when injected into the brain, there are no data indicating which cell types and receptors in the CNS are employed by chemokines such as CCL3 (synonym: MIP-1α) to induce fever in rats. We aimed to study, whether CCL3 induces fever when injected directly into the thermoregulatory center within the pre-optic area (POA). Moreover, we investigated whether CCL3 activates cells from POA microcultures resulting in intracellular Ca++ mobilization and synthesis/release of TNF-α and IL-6. Microinjections of CCL3 into the POA induced a dose-dependent fever, which was accompanied by a decrease in tail skin temperature. The primary microcultures of the POA (from topographically excised rat pup brain tissue) were stimulated by bolus administrations of 100 μl CCL3 (0.1 or 0.01 μg) or sterile PBS as control. We evaluated the responses of 261 (30.89%) neurons, 346 (40.94%) astrocytes and 238 microglia cells (29.17%). Stimulation of rat POA microcultures with CCL3 was capable of inducing Ca++ signaling in 15.31% of all astrocytes and 5.75% of all neurons investigated. No cellular Ca++-signals were observed after overnight incubation of the cultures with antiCCR1 or antiCCR5 antibodies. CCL3 did not alter the release of the pyrogenic cytokines IL-6 or TNF-α into the supernatant of the cultures. In conclusion the present study shows for the first time that CCL-3 injected directly into the rat POA, evoked an integrated febrile response. In parallel this chemokine induces Ca++ signaling in astrocytes and neurons via both CCR1 and CCR5 receptors when administered to POA microcultures without stimulating the synthesis of TNF-α and IL-6. It is a possibility that CCL3-induced fever may occur via CCR1 and CCR5 receptors stimulation of astrocytes and neurons from POA.

The inhibition of neurons in the central nervous pathways for thermoregulatory cold defense induces a suspended animation state in the rat

The possibility of inducing a suspended animation state similar to natural torpor would be greatly beneficial in medical science, since it would avoid the adverse consequence of the powerful autonomic activation evoked by external cooling. Previous attempts to systemically inhibit metabolism were successful in mice, but practically ineffective in nonhibernators. Here we show that the selective pharmacological inhibition of key neurons in the central pathways for thermoregulatory cold defense is sufficient to induce a suspended animation state, resembling natural torpor, in a nonhibernator. In rats kept at an ambient temperature of 15°C and under continuous darkness, the prolonged inhibition (6 h) of the rostral ventromedial medulla, a key area of the central nervous pathways for thermoregulatory cold defense, by means of repeated microinjections (100 nl) of the GABA(A) agonist muscimol (1 mm), induced the following: (1) a massive cutaneous vasodilation; (2) drastic drops in deep brain temperature (reaching a nadir of 22.44 ± 0.74°C), heart rate (from 440 ± 13 to 207 ± 12 bpm), and electroencephalography (EEG) power; (3) a modest decrease in mean arterial pressure; and (4) a progressive shift of the EEG power spectrum toward slow frequencies. After the hypothermic bout, all animals showed a massive increase in NREM sleep Delta power, similarly to that occurring in natural torpor. No behavioral abnormalities were observed in the days following the treatment. Our results strengthen the potential role of the CNS in the induction of hibernation/torpor, since CNS-driven changes in organ physiology have been shown to be sufficient to induce and maintain a suspended animation state.

Location of cat brain stem neurons that drive sweating

The brain stem premotor pathways controlling most noncardiovascular sympathetic outflows are unknown. Here, we mapped the brain stem neurons that drive sweating, by microinjecting excitant amino acid (L-glutamate or D,L-homocysteate: 0.4-3 nmol) into 420 sites over the pons and medulla of eight chloralose-anesthetized cats (70 mg/kg iv). Sweating was recorded by the electrodermal potential at the ipsilateral forepaw pad. Responses were classified as immediate (<5 s latency) or delayed (>10 s latency). Immediate responses were obtained from 16 sites (1-3 per animal) and were accompanied by no change in blood pressure. Those sites were clustered between the facial nucleus and the pyramidal tract in the rostral ventromedial medulla (RVMM). Microinjections into 33 surrounding sites caused delayed electrodermal responses of lesser amplitude, while the remaining 371 sites evoked none. To retrogradely label bulbospinal neurons that may mediate electrodermal responses, fluorescent latex microspheres were injected into the region of the intermediolateral cell column in the fourth thoracic segment in an earlier preparatory procedure on six of the animals. A cluster of retrogradely labeled neurons was identified between the facial nucleus and the pyramidal tract. Neurons in this discrete region of the RVMM, thus, drive sweating in the cat's paw and may do so via direct spinal projections.

Pain-facilitating medullary neurons contribute to opioid-induced respiratory depression

Respiratory depression is a therapy-limiting side effect of opioid analgesics, yet our understanding of the brain circuits mediating this potentially lethal outcome remains incomplete. Here we studied the contribution of the rostral ventromedial medulla (RVM), a region long implicated in pain modulation and homeostatic regulation, to opioid-induced respiratory depression. Microinjection of the μ-opioid agonist DAMGO in the RVM of lightly anesthetized rats produced both analgesia and respiratory depression, showing that neurons in this region can modulate breathing. Blocking opioid action in the RVM by microinjecting the opioid antagonist naltrexone reversed the analgesic and respiratory effects of systemically administered morphine, showing that this region plays a role in both the analgesic and respiratory-depressant properties of systemically administered morphine. The distribution of neurons directly inhibited by RVM opioid microinjection was determined with a fluorescent opioid peptide, dermorphin-Alexa 594, and found to be concentrated in and around the RVM. The non-opioid analgesic improgan, like DAMGO, produced antinociception but, unlike DAMGO, stimulated breathing when microinjected into the RVM. Concurrent recording of RVM neurons during improgan microinjection showed that this agent activated RVM ON-cells, OFF-cells, and NEUTRAL-cells. Since opioids are known to activate OFF-cells but suppress ON-cell firing, the differential respiratory response to these two analgesic drugs is best explained by their opposing effects on the activity of RVM ON-cells. These findings show that pain relief can be separated pharmacologically from respiratory depression and identify RVM OFF-cells as important central targets for continued development of potent analgesics with fewer side effects.

Low-frequency galvanic vestibular stimulation evokes two peaks of modulation in skin sympathetic nerve activity

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered bilaterally at 0.2-2.0 Hz, evokes a potent entrainment of sympathetic outflow to muscle and skin. Most recently, we showed that stimulation at 0.08-0.18 Hz generates two bursts of modulation of muscle sympathetic nerve activity (MSNA), more pronounced at 0.08 Hz, which we interpreted as reflecting bilateral projections from the vestibular nuclei to the medullary nuclei responsible for the generation of MSNA. Here, we test the hypothesis that these very low frequencies of sGVS modulate skin sympathetic nerve activity (SSNA) in a similar fashion. SSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08, 0.13 and 0.18 Hz. As with MSNA, cross-correlation analysis revealed two bursts of modulation of SSNA for each cycle of stimulation but, unlike MSNA, this modulation was equally pronounced at all frequencies. These results further support our conclusion that bilateral sGVS causes cyclical modulation of the left and right vestibular nerves and a resultant modulation of sympathetic outflow that reflects the summed activity of bilateral projections from the vestibular nuclei onto, in this case, the primary output nuclei responsible for SSNA-the medullary raphé. Furthermore, these findings emphasise the role of the vestibular system in the control of skin sympathetic outflow, and the cutaneous expression of motion sickness: pallor and sweat release. Indeed, vestibular modulation of SSNA was higher in those subjects reporting nausea than in those who did not report nausea during this low-frequency sGVS.

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.

2010 Carl Ludwig Distinguished Lectureship of the APS Neural Control and Autonomic Regulation Section: Central neural pathways for thermoregulatory cold defense

Central neural circuits orchestrate the homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the research leading to a model representing our current understanding of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for control of heat loss, and brown adipose tissue, skeletal muscle, and the heart for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific core efferent pathways within the central nervous system (CNS) that share a common peripheral thermal sensory input. The thermal afferent circuit from cutaneous thermal receptors includes neurons in the spinal dorsal horn projecting to lateral parabrachial nucleus neurons that project to the medial aspect of the preoptic area. Within the preoptic area, warm-sensitive, inhibitory output neurons control heat production by reducing the discharge of thermogenesis-promoting neurons in the dorsomedial hypothalamus. The rostral ventromedial medulla, including the raphe pallidus, receives projections form the dorsomedial hypothalamus and contains spinally projecting premotor neurons that provide the excitatory drive to spinal circuits controlling the activity of thermogenic effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a platform for further understanding of the functional organization of central thermoregulation.

Central neural pathways for thermoregulation

Central neural circuits orchestrate a homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the functional organization of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for heat loss, the brown adipose tissue, skeletal muscle and heart for thermogenesis and species-dependent mechanisms (sweating, panting and saliva spreading) for evaporative heat loss. These effectors are regulated by parallel but distinct, effector-specific neural pathways that share a common peripheral thermal sensory input. The thermal afferent circuits include cutaneous thermal receptors, spinal dorsal horn neurons and lateral parabrachial nucleus neurons projecting to the preoptic area to influence warm-sensitive, inhibitory output neurons which control thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus neurons controlling cutaneous vasoconstriction.

Vestibular and pulse-related modulation of skin sympathetic nerve activity during sinusoidal galvanic vestibular stimulation in human subjects

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), a means of a selectively modulating vestibular afferent input without affecting other inputs, can cause partial entrainment of muscle sympathetic nerve activity (MSNA). Given that motion sickness causes sweating and pallor, we tested the hypothesis that sGVS also entrains skin sympathetic nerve activity (SSNA), but that the optimal frequencies are closer to those associated with slow postural changes (0.2 Hz). SSNA was recorded via tungsten microelectrodes inserted into the common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sinusoidal GVS (+/-2 mA, 200 cycles) was applied to the mastoid processes at frequencies of 0.2, 0.5, 0.8, 1.1, 1.4, 1.7 and 2.0 Hz. All subjects reported strong postural illusions of 'rocking in a boat' or 'swaying in a hammock'. Sinusoidal GVS caused a marked entrainment of SSNA at all frequencies. Measured as the modulation index, vestibular modulation ranged from 81.5 +/- 4.0% at 0.2 Hz to 76.6 +/- 3.6% at 1.7 Hz; it was significantly weaker at 2.0 Hz (63.2 +/- 5.4%). Interestingly, pulse-related modulation of SSNA, which is normally weak, increased significantly during sGVS but was stronger at 0.8 Hz (86.2 +/- 2.0%) than at 0.2 Hz (69.3 +/- 8.3%), the opposite of the pattern seen with vestibular modulation of MSNA. We conclude that vestibular inputs can entrain the firing of cutaneous sympathetic neurones and increase their normally weak pulse-related rhythmicity.

Cutaneous vasodilation elicited by disinhibition of the caudal portion of the rostral ventromedial medulla of the free-behaving rat

Putative sympathetic premotor neurons controlling cutaneous vasomotion are contained within the rostral ventromedial medulla (RVMM) between levels corresponding, rostrally, to the rostral portion of the nucleus of the facial nerve (RVMM(fn)) and, caudally, to the rostral pole of the inferior olive (RVMM(io)). Cutaneous vasoconstrictor premotor neurons in the RVMM(fn) play a major role in mediating thermoregulatory changes in cutaneous vasomotion that regulate heat loss. To determine the role of neurons in the RVMM(io) in regulating cutaneous blood flow, we examined the changes in the tail and paw skin temperature of free-behaving rats following chemically-evoked changes in the activity of neurons in the RVMM(io). Microinjection of the GABA(A) agonist, muscimol, within either the RVMM(fn) or the RVMM(io) induced a massive peripheral vasodilation; microinjection of the GABA(A) antagonist bicuculline methiodide within the RVMM(fn) reversed the increase in cutaneous blood flow induced by warm exposure and, unexpectedly, disinhibition of RVMM(io) neurons produced a rapid cutaneous vasodilation. We conclude that the tonically-active neurons driving cutaneous vasoconstriction, likely sympathetic premotor neurons previously described in the RVMM(fn), are also located in the RVMM(io). However, in the RVMM(io), these are accompanied by a population of neurons that receives a tonically-active GABAergic inhibition in the conscious animal and that promotes a cutaneous vasodilation upon relief of this inhibition. Whether the vasodilator neurons located in the RVMM(io) play a role in thermoregulation remains to be determined.

Characterization and pharmacological evaluation of febrile response on zymosan-induced arthritis in rats

The present study investigated the febrile response in zymosan-induced arthritis, as well as the increase in PGE2concentration in the cerebrospinal fluid (CSF), along with the effects of antipyretic drugs on these responses in rats. Zymosan intra-articularly injected at the dose of 0.5 mg did not affect the body core temperature (Tc) compared with saline (control), whereas at doses of 1 and 2 mg, zymosan promoted a flattened increase in Tc and declined thereafter. The dose of 4 mg of zymosan was selected for further experiments because it elicited a marked and long-lasting Tc elevation starting at 3 1/2 h, peaking at 5 1/2 h, and remaining until 10 h. This temperature increase was preceded by a decrease in the tail skin temperature, as well as hyperalgesia and edema in the knee joint. No febrile response was observed in the following days. In addition, zymosan-induced fever was not modified by the sciatic nerve excision. Zymosan increased PGE2concentration in the CSF but not in the plasma. Oral pretreatment with ibuprofen (5–20 mg/kg), celecoxib (1–10 mg/kg), dipyrone (60–240 mg/kg), and paracetamol (100–200 mg/kg) or subcutaneous injection of dexamethasone (0.25–1.0 mg/kg) dose-dependently reduced or prevented the fever during the zymosan-induced arthritis. Celecoxib (5 mg/kg), paracetamol (150 mg/kg), and dipyrone (120 mg/kg) decreased CSF PGE2concentration and fever during zymosan-induced arthritis, suggesting the involvement of PGE2in this response.

Central 5-HT receptors in cardiovascular control during stress

Our aim is to consolidate recent data on relationship between central serotonergic neurotransmission and stress-elicited cardiovascular changes. Activation of central of 5-HT1A receptors attenuates tachycardic and pressor changes elicited by a wide range of stressors (airjet, restraint, open field, fear conditioning, social defeat), supporting the previous view of these receptors as "sympathoinhibitory". Their likely location is the medullary raphe. It is still unknown whether 5-TH1A receptors are sympathoinhibitory in physiological condition, as 5-HT1A antagonists do not affect basal or stress-altered cardiovascular parameters. In contrast to the established view that central 5-HT2A receptors are "sympathoexcitatory", experiments with new selective antagonists indicate that these receptors do not mediate stress-induced pressor and tachycardic responses, and are not involved in cardiovascular control at rest. The exception is control of cutaneous vascular bed, both at rest and during stress, likely at the spinal level. 5-HT3 receptors located in the nucleus tractus silitarius (NTS) contribute to stress-induced suppression of the baroreflex. 5-HT3 receptors located in sympathetic ganglia possibly contribute to the development of sustained hypertension in chronically stressed rats.

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.

Fever response to intravenous prostaglandin E2 is mediated by the brain but does not require afferent vagal signaling

PGE2 produced in the periphery triggers the early phase of the febrile response to infection and may contribute to later phases. It can be hypothesized that peripherally synthesized PGE2 transmits febrigenic signals to the brain via vagal afferent nerves. Before testing this hypothesis, we investigated whether the febrigenic effect of intravenously administered PGE2 is mediated by the brain and is not the result of a direct action of PGE2 on thermoeffectors. In anesthetized rats, intravenously injected PGE2 (100 μg/kg) caused an increase in sympathetic discharge to interscapular brown adipose tissue (iBAT), as well as increases in iBAT thermogenesis, end-expired CO2, and colonic temperature (Tc). All these effects were prevented by inhibition of neuronal function in the raphe region of the medulla oblongata using an intra-raphe microinjection of muscimol. We then asked whether the brain-mediated PGE2 fever requires vagal signaling and answered this question by conducting two independent studies in rats. In a study in anesthetized rats, acute bilateral cervical vagotomy did not affect the effects of intravenously injected PGE2 (100 μg/kg) on iBAT sympathetic discharge and Tc. In a study in conscious rats, administration of PGE2 (280 μg/kg) via an indwelling jugular catheter caused tail skin vasoconstriction, tended to increase oxygen consumption, and increased Tc; none of these responses was affected by total truncal subdiaphragmatic vagotomy performed 2 wk before the experiment. We conclude that the febrile response to circulating PGE2 is mediated by the brain, but that it does not require vagal afferent signaling.

Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2

There is evidence that serotonin [5-hydroxytryptamine (5-HT)] is involved in the physiological responses to hypercapnia. Serotonergic neurons represent the major cell type (comprising 15-20% of the neurons) in raphe magnus nucleus (RMg), which is a medullary raphe nucleus. In the present study, we tested the hypothesis 1) that RMg plays a role in the ventilatory and thermal responses to hypercapnia, and 2) that RMg serotonergic neurons are involved in these responses. To this end, we microinjected 1) ibotenic acid to promote nonspecific lesioning of neurons in the RMg, or 2) anti-SERT-SAP (an immunotoxin that utilizes a monoclonal antibody to the third extracellular domain of the serotonin reuptake transporter) to specifically kill the serotonergic neurons in the RMg. Hypercapnia caused hyperventilation and hypothermia in all groups. RMg nonspecific lesions elicited a significant reduction of the ventilatory response to hypercapnia due to lower tidal volume (Vt) and respiratory frequency. Rats submitted to specific killing of RMg serotonergic neurons showed no consistent difference in ventilation during air breathing but had a decreased ventilatory response to CO(2) due to lower Vt. The hypercapnia-induced hypothermia was not affected by specific or nonspecific lesions of RMg serotonergic neurons. These data suggest that RMg serotonergic neurons do not participate in the tonic maintenance of ventilation during air breathing but contribute to the ventilatory response to CO(2). Ultimately, this nucleus may not be involved in the thermal responses to CO(2).