Direct pyrogenic input from prostaglandin EP3 receptor-expressing preoptic neurons to the dorsomedial hypothalamus

Different populations of prostaglandin EP3 receptor-expressing preoptic neurons project to two fever-mediating sympathoexcitatory brain regions

The central mechanism of fever induction is triggered by an action of prostaglandin E(2) (PGE(2)) on neurons in the preoptic area (POA) through the EP3 subtype of prostaglandin E receptor. EP3 receptor (EP3R)-expressing POA neurons project directly to the dorsomedial hypothalamus (DMH) and to the rostral raphe pallidus nucleus (rRPa), key sites for the control of thermoregulatory effectors. Based on physiological findings, we hypothesize that the febrile responses in brown adipose tissue (BAT) and those in cutaneous vasoconstrictors are controlled independently by separate neuronal pathways: PGE(2) pyrogenic signaling is transmitted from EP3R-expressing POA neurons via a projection to the DMH to activate BAT thermogenesis and via another projection to the rRPa to increase cutaneous vasoconstriction. In this case, DMH-projecting and rRPa-projecting neurons would constitute segregated populations within the EP3R-expressing neuronal group in the POA. Here, we sought direct anatomical evidence to test this hypothesis with a double-tracing experiment in which two types of the retrograde tracer, cholera toxin b-subunit (CTb), conjugated with different fluorophores were injected into the DMH and the rRPa of rats and the resulting retrogradely labeled populations of EP3R-immunoreactive neurons in the POA were identified with confocal microscopy. We found substantial numbers of EP3R-immunoreactive neurons in both the DMH-projecting and the rRPa-projecting populations. However, very few EP3R-immunoreactive POA neurons were labeled with both the CTb from the DMH and that from the rRPa, although a substantial number of neurons that were not immunoreactive for EP3R were double-labeled with both CTbs. The paucity of the EP3R-expressing neurons that send collaterals to both the DMH and the rRPa suggests that pyrogenic signals are sent independently to these caudal brain regions from the POA and that such pyrogenic outputs from the POA reflect different control mechanisms for BAT thermogenesis and for cutaneous vasoconstriction by distinct sets of POA neurons.

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.

Single nigrostriatal dopaminergic neurons form widely spread and highly dense axonal arborizations in the neostriatum

The axonal arbors of single nigrostriatal dopaminergic neurons were visualized with a viral vector expressing membrane-targeted green fluorescent protein in rat brain. All eight reconstructed tyrosine hydroxylase-positive dopaminergic neurons possessed widely spread and highly dense axonal arborizations in the neostriatum. All of them emitted very little axon collateral arborization outside of the striatum except for tiny arborization in the external pallidum. The striatal axonal bush of each reconstructed dopaminergic neuron covered 0.45-5.7% (mean +/- SD = 2.7 +/- 1.5%) of the total volume of the neostriatum. Furthermore, all the dopaminergic neurons innervated both striosome and matrix compartments of the neostriatum, although each neuron's arborization tended to favor one of these compartments. Our findings demonstrate that individual dopaminergic neurons of the substantia nigra can broadcast a dopamine signal and exert strong influence over a large number of striatal neurons. This divergent signaling should be a key to the function of the nigrostriatal system in dopamine-based learning and suggests that neurodegeneration of individual nigral neurons can affect multiple neurons in the striatum. Thus, these results would also contribute to understanding the clinicopathology of Parkinson's disease and related syndromes.

Roles of two preoptic cell groups in tonic and febrile control of rat tail sympathetic fibers

In response to cold and in fever, heat dissipation from the skin is reduced by sympathetic vasoconstriction. The preoptic region has been implicated in regulating basal, thermal, and febrile vasoconstriction of cutaneous vessels such as the rat's tail, but the neurons responsible for these functions have not been well localized. We recorded activity from single sympathetic nerve fibers supplying tail vessels in urethane-anesthetized rats, while microinjections of GABA (300 mM, 15–30 nl) were used to inhibit neurons in different parts of the preoptic region. Tail fiber activity increased promptly after GABA injections in two distinct regions: a rostromedial preoptic region (RMPO) centered around the organum vasculosum of the lamina terminalis, and a second region centered ∼1 mm caudolaterally (CLPO). Responses to GABA within each region were similar. The febrile mediator, PGE2 (0.2 or 1 ng in 15 nl) was then microinjected into GABA-sensitive preoptic sites. Injections of PGE2 into the RMPO induced a rapid increase in tail fiber activity followed by a rise in core temperature; injections into the rostromedial part of CLPO gave delayed tail fiber responses; injections into the central and caudal parts of CLPO were without effect. These results indicate that neurons in two distinct preoptic regions provide tonic inhibitory drive to the tail vasoconstrictor supply, but febrile vasoconstriction is mediated by PGE2 selectively inhibiting neurons in the rostromedial region.

The role of interleukin-6 in lipopolysaccharide-induced fever by mechanisms independent of prostaglandin E2

Fever has been shown to be elicited by prostaglandin E(2) (PGE(2)) binding to its receptors on thermoregulatory neurons in the anterior hypothalamus. The signals that trigger PGE(2) production are thought to include proinflammatory cytokines, such as IL-6. However, although the presence of IL-6 is critical for fever, IL-6 by itself is not or only weakly pyrogenic. Here we examined the relationship between IL-6 and PGE(2) in lipopolysaccharide (LPS)-induced fever. Immune-challenged IL-6 knockout mice did not produce fever, in contrast to wild-type mice, but the expression of the inducible PGE(2)-synthesizing enzymes, cyclooxygenase-2 and microsomal prostaglandin E synthase-1, was similarly up-regulated in the hypothalamus of both genotypes, which also displayed similarly elevated PGE(2) levels in the cerebrospinal fluid. Nevertheless, both wild-type and knockout mice displayed a febrile response to graded concentrations of PGE(2) injected into the lateral ventricle. There was no major genotype difference in the expression of IL-1beta and TNFalpha or their receptors, and pretreatment of IL-6 knockout mice with soluble TNFalpha receptor ip or intracerebroventricularly or a cyclooxygenase-2 inhibitor ip did not abolish the LPS unresponsiveness. Hence, although IL-6 knockout mice have both an intact PGE(2) synthesis and an intact fever-generating pathway downstream of PGE(2), endogenously produced PGE(2) is not sufficient to produce fever in the absence of IL-6. The findings suggest that IL-6 controls some factor(s) in the inflammatory cascade, which render(s) IL-6 knockout mice refractory to the pyrogenic action of PGE(2), or that it is involved in the mechanisms that govern release of synthesized PGE(2) onto its target neurons.

Reduced acute brain injury in PGE2 EP3 receptor-deficient mice after cerebral ischemia

Ischemic stroke is one of the leading causes of mortality and morbidity in humans. During brain ischemia and the subsequent reperfusion that occurs with stroke, the generation of the so-called "proinflammatory" prostaglandin E(2) (PGE(2)) increases significantly. Therefore, interest is growing regarding the differential functions of the individual PGE(2) receptors (EP1-4) and their relative contribution to brain damage following ischemic and inflammatory stimuli. Here, we address the contribution of the EP3 receptor in dictating early outcomes after transient cerebral ischemia. An oxygen-glucose deprivation (OGD)-induced in vitro model of brain ischemia was used in mouse hippocampal slice cultures. For transient ischemia, the right middle cerebral artery (MCA) of wildtype (WT) and EP3 knockout (EP3(-/-)) C57BL/6 male mice was occluded for 90 min and reperfused for 48 or 96 h, after which neurobehavioral scores and infarct volumes were determined. Mean arterial blood pressure, pH, blood gases (PaO(2) and PaCO(2)), cerebral blood flow, and body temperature were also determined before and during ischemia and reperfusion. OGD-induced cell death was significantly lower in brain slice cultures of EP3(-/-) mice than in those of WT mice. EP3(-/-) mice that underwent transient ischemia had significantly smaller infarct volumes than did WT mice at 48 h, but this difference was not sustained at 96 h. Neurological score deficits correlated with infarct volume, but no significant differences in the physiological parameters monitored were detected between the two genotypes. The results further support a role for EP3 receptors in contributing to acute ischemic stroke, but EP3 is not likely the sole contributor to the long-term detrimental consequences of PGE(2).

Hypothalamic mechanisms coordinating cardiorespiratory function during exercise and defensive behaviour

Defensive behaviour evoked by mild or moderate psychological stress as well as increased activity and arousal are part of everyday life in humans and other animals. Both defensive behaviour and exercise are associated with marked and often quite stereotyped changes in autonomic and respiratory function. These patterned responses are generated by feed-forward or "central command" mechanisms, and are also modulated by feedback from peripheral receptors. In this review we first describe the pattern of autonomic and respiratory changes associated with defensive behaviour and exercise, and then discuss the central mechanisms that generate these patterned responses in the light of recent studies, with a particular focus on the role of the dorsomedial hypothalamus (DMH). We consider the hypothesis that the cardiorespiratory changes associated with defensive behaviour and exercise may, at least in part, be driven by common central mechanisms. Finally, we discuss the possible role of the DMH in generating circadian rhythms in arterial blood pressure and heart rate, and also in generating longer-term increases in sympathetic activity in some types of hypertension.

Blockade of prostaglandin E2-induced thermogenesis by unilateral microinjection of GABAA receptor antagonist into the preoptic area

Previous studies have demonstrated that pretreatment of rats with a GABA(A) receptor antagonist microinjected bilaterally into the preoptic area (POA) blocked cold- or lipopolysaccharide-induced thermogenesis. Here, the involvement of GABA(A) receptors in prostaglandin (PG)E2-induced fever was examined. Thermogenic, tachycardic, vasoconstrictive, and hyperthermic responses were elicited by the unilateral microinjection of 0.57-1.1 pmol PGE2 into the region adjacent to the organum vasculosum of the lamina terminalis in urethane-chloralose-anesthetized rats. All these responses were blocked 10 min after pretreatment of the rats with a GABA(A) receptor antagonist, bicuculline methiodide or gabazine (50-500 pmol), microinjected unilaterally into the POA; and recovery occurred at approximately 70 min. Though the antagonist treatment alone had no effect on the O2 consumption rate or colonic temperature, it did elicit a bradycardic response. Pretreatment with the vehicle, saline, had no effect on the PGE2-induced responses. However, the blocking action of bicuculline/gabazine was efficacious when the agent was administered unilaterally, but not necessarily bilaterally, into the POA either contralateral or ipsilateral to the PGE2 injection site. These results suggest that the PGE2-induced responses are not simply mediated by the GABAergic transmission from the PGE2-sensitive site to the thermoefferent structure in the POA, although a tonic inhibitory input to POA neurons has a permissive role for the full expression of PGE2-induced fever.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

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.

Preoptic mechanism for cold-defensive responses to skin cooling

We recently identified a somatosensory pathway that transmits temperature information from the skin to a median subregion of the preoptic area (POA), a thermoregulatory centre. Here, we investigated in vivo the local neuronal circuit in the rat POA that processes the thermosensory information and outputs thermoregulatory effector signals. Skin cooling-evoked increases in sympathetic thermogenesis in brown adipose tissue, in metabolism and in heart rate were reversed by inhibition of neurons in the median preoptic nucleus (MnPO). Glutamatergic stimulation or disinhibition of MnPO neurons evoked thermogenic, metabolic and cardiac responses that mimicked the cold-defensive responses to skin cooling and were reversed by antagonizing GABA(A) receptors in the medial preoptic area (MPO), which is thought to contain neurons providing thermoregulatory output to effectors. These results suggest that GABA inhibition of output neurons in the MPO by MnPO neurons that are activated by cool sensory signals from the skin is a core thermoregulatory mechanism within the POA that is essential for the feedforward defence of body temperature against cold challenges in the environment.

Translational aspects of pharmacological research into anxiety disorders: the stress-induced hyperthermia (SIH) paradigm

In anxiety research, the search for models with sufficient clinical predictive validity to support the translation of animal studies on anxiolytic drugs to clinical research is often challenging. This review describes the stress-induced hyperthermia (SIH) paradigm, a model that studies the activation of the autonomic nervous system in response to stress by measuring body temperature. The reproducible and robust SIH response, combined with ease of testing, make the SIH paradigm very suitable for drug screening. We will review the current knowledge on the neurobiology of the SIH response, discuss the role of GABA(A) and serotonin (5-HT) pharmacology, as well as how the SIH response relates to infectious fever. Furthermore, we will present novel data on the SIH response variance across different mice and their sensitivity to anxiolytic drugs. The SIH response is an autonomic stress response that can be successfully studied at the level of its physiology, pharmacology, neurobiology and genetics and possesses excellent animal-to-human translational properties.

Functional topography of the dorsomedial hypothalamus

The dorsomedial hypothalamus (DMH) has been proposed to play key roles in both the defense reaction to acute stress and in the thermoregulatory response to cold. We reasoned that the autonomic/respiratory motor patterns of these responses would be mediated by at least partly distinct DMH neuron populations. To test this, we made simultaneous recordings of phrenic nerve and plantar cutaneous vasoconstrictor (CVC) activity in 14 vagotomized, ventilated, urethane-anesthetized rats. Microinjections of d,l-homocysteic acid (DLH; 15 nl, 50 mM) were used to cause localized, short-lasting (<1 min) activation of DMH neuron clusters. Cooling the rat's trunk skin by perfusing cold water through a water jacket-activated plantar CVC activity but depressed phrenic burst rate (cold-response pattern). The expected “stress/defense response” pattern would be phrenic activation, with increased blood pressure, heart rate, and possibly CVC activity. DLH microinjections into 76 sites within the DMH region never reduced phrenic activity. They frequently increased phrenic rate and/or plantar CVC activity, but the magnitudes of those two responses were not significantly correlated. Plantar CVC responses were evoked most strongly from the dorsal hypothalamic area and most dorsal part of the dorsomedial nucleus, whereas peak phrenic rate responses were evoked from more caudal sites; their relative magnitudes varied systematically with rostrocaudal position. Tachycardia correlated with plantar CVC responses but not phrenic rate. These findings indicate that localized activation of DMH neurons does not evoke full “cold-response” or stress/defense response patterns, but they demonstrate the existence of significant functional topography within the DMH region.

A thermosensory pathway that controls body temperature

Defending body temperature against environmental thermal challenges is one of the most fundamental homeostatic functions that are governed by the nervous system. Here we describe a somatosensory pathway that essentially constitutes the afferent arm of the thermoregulatory reflex that is triggered by cutaneous sensation of environmental temperature changes. Using in vivo electrophysiological and anatomical approaches in the rat, we found that lateral parabrachial neurons are pivotal in this pathway by glutamatergically transmitting cutaneous thermosensory signals received from spinal somatosensory neurons directly to the thermoregulatory command center, the preoptic area. This feedforward pathway mediates not only sympathetic and shivering thermogenic responses but also metabolic and cardiac responses to skin cooling challenges. Notably, this 'thermoregulatory afferent' pathway exists in parallel with the spinothalamocortical somatosensory pathway that mediates temperature perception. These findings make an important contribution to our understanding of both the somatosensory system and thermal homeostasis -- two mechanisms that are fundamental to the nervous system and to our survival.

Physiology and pathophysiology of prostanoid receptors

Prostanoids, consisting of prostaglandins (PGs) and thromboxanes (TXs), are oxygenated products of C20 unsaturated fatty acids. They include PGD2, PGE2, PGF2 α , PGI2, and TXA2. Given that aspirin-like nonsteroidal anti-inflammatory drugs exert their actions by suppressing prostanoid production, prostanoids have been implicated in processes inhibited by these drugs, including inflammation, fever, and pain. Prostanoids also contribute to vascular homeostasis, reproduction, and regulation of kidney and gastrointestinal functions. How prostanoids exert such a variety of actions had remained unclear, however. Prostanoids are released outside of cells immediately after their synthesis and exert their actions by binding to receptors on target cells. We have identified a family of eight types or subtypes of G protein-coupled receptors that mediate prostanoid actions. Another G protein-coupled receptor was also identified as an additional receptor for PGD2. Genes for these receptors have been individually disrupted in mice, and analyses of these knockout mice have not only elucidated the molecular and cellular mechanisms of known prostanoid actions but also revealed previously unknown actions. In this article, I review the physiological and pathophysiological roles of prostanoids and their receptors revealed by these studies.

Stress- and lipopolysaccharide-induced c-fos expression and nNOS in hypothalamic neurons projecting to medullary raphe in rats: a triple immunofluorescent labeling study

Neurons in the rostral raphe pallidus (rRP) have been proposed to mediate experimental stress-induced tachycardia and fever in rats, and projections from the dorsomedial hypothalamus (DMH) may signal their activation in these settings. Thus, we examined c-fos expression evoked by air jet/restraint stress and restraint stress or by systemic administration of lipopolysaccharide (10 microg/kg and 100 microg/kg) as well as the distribution of the neuronal nitric oxide synthase (nNOS) in neurons retrogradely labeled from the raphe with cholera toxin B in key hypothalamic regions. Many neurons in the medial preoptic area and the dorsal area of the DMH were retrogradely labeled, and approximately half of those in the medial preoptic area and moderate numbers in the dorsal DMH were also positive for nNOS. Either stress paradigm or dose of lipopolysaccharide increased the number of c-fos-positive neurons and nNOS/c-fos double-labeled neurons in all regions examined. However, retrogradely labeled neurons positive for c-fos were increased only in the dorsal DMH and adjoining region in both stressed and lipopolysaccharide-treated groups, and triple-labeled neurons were found only in this area in rats subjected to either stress paradigm. Thus, hypothalamic neurons that project to the rRP and express c-fos in response to either experimental stress or systemic inflammation are found only in the dorsal DMH, and many of those activated by stress contain nNOS, suggesting that nitric oxide may play a role in signaling in this pathway.

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

Lesions of the anteroventral third ventricle region exaggerate neuroendocrine and thermogenic but not behavioral responses to a novel environment

Mild psychological stressors provoke an acute rise in core temperature (TC), stimulate the hypothalmo-pituitary-adrenocortical (HPA) axis, and induce various stress-related behaviors. In the present study, we examined the effect of ablation of the anteroventral third ventricle region (AV3V) on both physiological and behavioral responses to a novel environment. TC was monitored in male Sprague-Dawley rats, with either sham or AV3V lesions, during a 5-h exposure to a novel environment. Trunk blood was collected, in a second group of rats, for the assessment of plasma levels of ACTH and corticosterone. Novelty-induced grooming and rearing behaviors were assessed in a third group of animals. TC was elevated in all animals after 30 min in the novel environment, but the rise was exaggerated in rats with AV3V lesions (∼0.5°C). AV3V-lesion rats maintained a higher core temperature for 2 h before it returned to the same level as the control group. Plasma levels of ACTH and corticosterone were also exaggerated in the AV3V lesion group after 30 min in a novel environment. In contrast to the physiological responses, the behavioral measures of grooming and rearing revealed no differences between the groups. The results from the current study suggest that neurons within the AV3V region exert an inhibitory influence on the HPA axis and fever developed in response to stressful psychological stimuli. They also confirm that the physiological and hormonal components of the stress response are independent of certain behavioral measures of stress.

Neural substrate of cold-seeking behavior in endotoxin shock

Systemic inflammation is a leading cause of hospital death. Mild systemic inflammation is accompanied by warmth-seeking behavior (and fever), whereas severe inflammation is associated with cold-seeking behavior (and hypothermia). Both behaviors are adaptive. Which brain structures mediate which behavior is unknown. The involvement of hypothalamic structures, namely, the preoptic area (POA), paraventricular nucleus (PVH), or dorsomedial nucleus (DMH), in thermoregulatory behaviors associated with endotoxin (lipopolysaccharide [LPS])-induced systemic inflammation was studied in rats. The rats were allowed to select their thermal environment by freely moving in a thermogradient apparatus. A low intravenous dose of Escherichia coli LPS (10 µg/kg) caused warmth-seeking behavior, whereas a high, shock-inducing dose (5,000 µg/kg) caused cold-seeking behavior. Bilateral electrocoagulation of the PVH or DMH, but not of the POA, prevented this cold-seeking response. Lesioning the DMH with ibotenic acid, an excitotoxin that destroys neuronal bodies but spares fibers of passage, also prevented LPS-induced cold-seeking behavior; lesioning the PVH with ibotenate did not affect it. Lesion of no structure affected cold-seeking behavior induced by heat exposure or by pharmacological stimulation of the transient receptor potential (TRP) vanilloid-1 channel (“warmth receptor”). Nor did any lesion affect warmth-seeking behavior induced by a low dose of LPS, cold exposure, or pharmacological stimulation of the TRP melastatin-8 (“cold receptor”). We conclude that LPS-induced cold-seeking response is mediated by neuronal bodies located in the DMH and neural fibers passing through the PVH. These are the first two landmarks on the map of the circuitry of cold-seeking behavior associated with endotoxin shock.

Glutamate receptors in the raphe pallidus mediate brown adipose tissue thermogenesis evoked by activation of dorsomedial hypothalamic neurons

Disinhibition of DMH neurons with the GABAA receptor antagonist, bicuculline, increases heart rate (HR) and augments both brown adipose tissue sympathetic nerve activity (BAT SNA) and renal SNA (RSNA) contributing to the evoked increases in BAT thermogenesis and arterial pressure (AP). We determined the role of glutamate receptor activation in the rostral raphe pallidus (RPa) in mediating the sympathoexcitatory responses in HR, BAT SNA and RSNA following disinhibition of DMH neurons in urethane/chloralose anesthetized, artificially ventilated rats. Microinjections of either the selective NMDA receptor agonist, NMDA, or the selective non-NMDA receptor agonist, kainic acid (KA), into the RPa produced increases in BAT SNA (peak: + 502% and + 408% of control, respectively) and BAT temperature (peak: + 0.6 degrees C and + 1.0 degrees C) accompanied by rises in HR (peak: + 38 and + 63 bpm), RSNA (peak: + 57% and + 58% of control) and MAP (peak: + 12 and 15 mmHg). These responses were reversed by subsequent microinjection into RPa of the respective selective glutamate receptor antagonists, AP5 and CNQX. Microinjections of the non-selective glutamate receptor antagonist, kynurenic acid (Kyn), the NMDA receptor antagonist, AP5, or the non-NMDA receptor antagonist, CNQX, were effective in reversing the increases in BAT SNA (for Kyn, from peak of + 419% of control to + 9% of control) and BAT temperature, but not those in HR, MAP or RSNA (for Kyn, from peak of + 143% of control to + 124% of control) evoked by unilateral microinjection of bicuculline into the DMH. These results indicate that both NMDA and non-NMDA glutamate receptors in the RPa play a significant role in mediating the excitatory synaptic transmission producing the activation of BAT thermogenesis following disinhibition of DMH neurons. Glutamate receptors in the RPa may not be important for transmitting cardiovascular responses induced by activation of the DMH neurons.

IL-1beta and LPS induce anorexia by distinct mechanisms differentially dependent on microsomal prostaglandin E synthase-1

Recent work demonstrated that the febrile response to peripheral immune stimulation with proinflammatory cytokine IL-1beta or bacterial wall lipopolysaccharide (LPS) is mediated by induced synthesis of prostaglandin E(2) by the terminal enzyme microsomal prostaglandin E synthase-1 (mPGES-1). The present study examined whether a similar mechanism might also mediate the anorexia induced by these inflammatory agents. Transgenic mice with a deletion of the Ptges gene, which encodes mPGES-1, and wild-type controls were injected intraperitoneally with IL-1beta, LPS, or saline. Mice were free fed, and food intake was continuously monitored with an automated system for 12 h. Body weight was recorded every 24 h for 4 days. The IL-1beta induced anorexia in wild-type but not knock-out mice, and so it was almost completely dependent on mPGES-1. In contrast, LPS induced anorexia of the same magnitude in both phenotypes, and hence it was independent of mPGES-1. However, when the mice were prestarved for 22 h, LPS induced anorexia and concomitant body weight loss in the knock-out animals that was attenuated compared with the wild-type controls. These data suggest that IL-1beta and LPS induce anorexia by distinct immune-to-brain signaling pathways and that the anorexia induced by LPS is mediated by a mechanism different from the fever induced by LPS. However, nutritional state and/or motivational factors also seem to influence the pathways for immune signaling to the brain. Furthermore, both IL-1beta and LPS caused reduced meal size but not meal frequency, suggesting that both agents exerted an anhedonic effect during these experimental conditions.

Central efferent pathways mediating skin cooling-evoked sympathetic thermogenesis in brown adipose tissue

Control of thermoregulatory effectors by the autonomic nervous system is a critical component of rapid cold-defense responses, which are triggered by thermal information from the skin. However, the central autonomic mechanism driving thermoregulatory effector responses to skin thermal signals remains to be determined. Here, we examined the involvement of several autonomic brain regions in sympathetic thermogenic responses in brown adipose tissue (BAT) to skin cooling in urethane-chloralose-anesthetized rats by monitoring thermogenic [BAT sympathetic nerve activity (SNA) and BAT temperature], metabolic (expired CO(2)), and cardiovascular (arterial pressure and heart rate) parameters. Acute skin cooling, which did not reduce either rectal (core) or brain temperature, evoked increases in BAT SNA, BAT temperature, expired CO(2), and heart rate. Skin cooling-evoked thermogenic, metabolic, and heart rate responses were inhibited by bilateral microinjections of bicuculline (GABA(A) receptor antagonist) into the preoptic area (POA), by bilateral microinjections of muscimol (GABA(A) receptor agonist) into the dorsomedial hypothalamic nucleus (DMH), or by microinjection of muscimol, glycine, 8-OH-DPAT (5-HT(1A) receptor agonist), or kynurenate (nonselective antagonist for ionotropic excitatory amino acid receptors) into the rostral raphe pallidus nucleus (rRPa) but not by bilateral muscimol injections into the lateral/dorsolateral part or ventrolateral part of the caudal periaqueductal gray. These results implicate the POA, DMH, and rRPa in the central efferent pathways for thermogenic, metabolic, and cardiac responses to skin cooling, and suggest that these pathways can be modulated by serotonergic inputs to the medullary raphe.

Corticotropin releasing factor increases in brown adipose tissue thermogenesis and heart rate through dorsomedial hypothalamus and medullary raphe pallidus

Corticotropin releasing factor, acting at hypothalamic corticotropin releasing factor receptors, contributes to the neural signaling pathways mediating stress-related responses, as well as those involved in maintaining energy balance homeostasis. Sympathetically-regulated lipid metabolism and heat production in brown adipose tissue contributes to the non-shivering thermogenic component of stress-evoked hyperthermia and to energy expenditure aspects of body weight regulation. To identify potential central pathways through which hypothalamic corticotropin releasing factor influences brown adipose tissue thermogenesis, corticotropin releasing factor was microinjected into the lateral ventricle (i.c.v.) or into hypothalamic sites while recording sympathetic outflow to brown adipose tissue, brown adipose tissue temperature, expired CO2, heart rate and arterial pressure in urethane/chloralose-anesthetized, artificially-ventilated rats. I.c.v. corticotropin releasing factor or corticotropin releasing factor microinjection into the preoptic area or the dorsomedial hypothalamus, but not the paraventricular nucleus of the hypothalamus, elicited sustained increases in brown adipose tissue sympathetic nerve activity, brown adipose tissue temperature, expired CO2 and heart rate. These sympathetic responses to i.c.v. corticotropin releasing factor were eliminated by inhibition of neuronal activity in the dorsomedial hypothalamus or in the raphe pallidus, a putative site of sympathetic premotor neurons for brown adipose tissue, and were markedly reduced by microinjection of ionotropic glutamate receptor antagonists into the dorsomedial hypothalamus. The increases in brown adipose tissue sympathetic outflow, brown adipose tissue temperature and heart rate elicited from corticotropin releasing factor into the preoptic area were reversed by inhibition of neuronal discharge in dorsomedial hypothalamus. These data indicate that corticotropin releasing factor release within the preoptic area activates a sympathoexcitatory pathway to brown adipose tissue and to the heart, perhaps similar to that activated by increased prostaglandin production in the preoptic area, that includes neurons in the dorsomedial hypothalamus and in the raphe pallidus.