Expression of PGE2 EP3 receptor subtypes in the mouse preoptic region

Prostaglandin E2 production in the brainstem parabrachial nucleus facilitates the febrile response

Our body temperature is normally kept within a narrow range of 1°C. For example, if our body temperature rises, such as in a hot environment or due to strenuous exercise, our thermoregulatory system will trigger a powerful heat defense response with vasodilation, sweating, and lowered metabolism. During fever, which often involves body temperatures of up to 41°C, this heat defense mechanism is apparently inhibited; otherwise, the rising body temperature would be immediately combated, and fever would not be allowed to develop. New evidence suggests how and where this inhibition takes place. In two consecutive studies from Cheng et al. and Xu et al., it has been shown that prostaglandin E2, which generates fever by acting on thermosensory neurons in the preoptic hypothalamus, also acts on neurons in the brainstem parabrachial nucleus, which receive temperature information from temperature-activated spinal cord neurons and relay this information to the thermoregulatory center in the hypothalamus to either induce cold or heat defenses. By acting on the same type of prostaglandin E2 receptor that is critical for fever generation in the preoptic hypothalamus, the EP3 receptor, prostaglandin E2 inhibits the signaling of the heat-responsive parabrachial neurons, while stimulating the cold-responsive neurons. These novel findings thus show that prostaglandin E2, by binding to the same receptor subtype in the parabrachial nucleus as in the preoptic hypothalamus, adjusts the sensitivity of the thermosensory system in a coordinated manner to allow the development of febrile body temperatures.

Thermoregulatory pathway underlying the pyrogenic effects of prostaglandin E2 in the lateral parabrachial nucleus of male rats

It has been shown that prostaglandin (PG) E2 synthesized in the lateral parabrachial nucleus (LPBN) is involved in lipopolysaccharide-induced fever. But the neural mechanisms of how intra-LPBN PGE2 induces fever remain unclear. In this study, we investigated whether the LPBN-preoptic area (POA) pathway, the thermoafferent pathway for feed-forward thermoregulatory responses, mediates fever induced by intra-LPBN PGE2 in male rats. The core temperature (Tcore) was monitored using a temperature radiotelemetry transponder implanted in rat abdomen. We showed that microinjection of PGE2 (0.28 nmol) into the LPBN significantly enhanced the density of c-Fos-positive neurons in the median preoptic area (MnPO). The chemical lesioning of MnPO with ibotenate or selective genetic lesioning or inhibition of the LPBN-MnPO pathway significantly attenuated fever induced by intra-LPBN injection of PGE2. We demonstrated that EP3 receptor was a pivotal receptor for PGE2-induced fever, since microinjection of EP3 receptor agonist sulprostone (0.2 nmol) or EP3 receptor antagonist L-798106 (2 nmol) into the LPBN mimicked or weakened the pyrogenic action of LPBN PGE2, respectively, but this was not the case for EP4 and EP1 receptors. Whole-cell recording from acute LPBN slices revealed that the majority of MnPO-projecting neurons originating from the external lateral (el) and dorsal (d) LPBN were excited and inhibited, respectively, by PGE2 perfusion, initiating heat-gain and heat-loss mechanisms. The amplitude but not the frequency of spontaneous and miniature glutamatergic excitatory postsynaptic currents (sEPSCs and mEPSCs) in MnPO-projecting LPBel neurons increased after perfusion with PGE2; whereas the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) and the A-type potassium (IA) current density did not change. In MnPO-projecting LPBd neurons, neither sEPSCs nor sIPSCs responded to PGE2; however, the IA current density was significantly increased by PGE2 perfusion. These electrophysiological responses and the thermoeffector reactions to intra-LPBN PGE2 injection, including increased brown adipose tissue thermogenesis, shivering, and decreased heat dissipation, were all abolished by L-798106, and mimicked by sulprostone. These results suggest that the pyrogenic effects of intra-LPBN PGE2 are mediated by both the inhibition of the LPBd-POA pathway through the EP3 receptor-mediated activation of IA currents and the activation of the LPBel-POA pathway through the selective enhancement of glutamatergic synaptic transmission via EP3 receptors.

Genetic identification of preoptic neurons that regulate body temperature in mice

There has been an explosion recently in our understanding of the neuronal populations in the preoptic area involved in thermoregulation of mice. Recent studies have identified several genetically specified populations of neurons predominantly in the median preoptic nucleus (MnPO) but spreading caudolaterally into the preoptic area that regulate body temperature. . These include warm-responsive neurons that express the peptides PACAP, BDNF, or QRFP; and receptors for temperature, leptin, estrogen, or prostaglandin E2 (PGE2). These neurons are predominantly glutamatergic and driving them opto- or chemogenetically can cause profound hypothermia, and in some cases, periods of torpor or a hibernation-like state. Conversely, fever response is likely to depend upon inhibiting the activity of these neurons through the PGE2 receptor EP3. Another cell group, the Brs3-expressing MnPO neurons, are apparently cold-responsive and cause increases in body temperature. MnPO-QRFP neurons cause hypothermia via activation of their terminals in the region of the dorsomedial nucleus of the hypothalamus (DMH). As the MnPO-QRFP neurons are essentially glutamatergic, and the DMH largely uses glutamatergic projections to the raphe pallidus to increase body temperature, this model suggests the existence of local inhibitory interneurons in the DMH region between the MnPO-QRFP glutamatergic neurons that cause hypothermia and the DMH glutamatergic neurons that cause hyperthermia. The new genetically targeted studies in mice provide a way to identify the precise neuronal circuitry that is responsible for our physiological observations in this species, and will suggest critical experiments that can be undertaken to compare these with the thermoregulatory circuitry in other species.

The Role of Prostaglandin E2 Synthesized in Rat Lateral Parabrachial Nucleus in LPS-Induced Fever

INTRODUCTION

The lateral parabrachial nucleus (LPBN) is considered to be a brain site of the pyrogenic action of prostaglandin (PG) E2 outside of the preoptic area. Yet, the role of the LPBN in fever following a systemic immune challenge remains poorly understood.

METHODS

We examined the expression of cyclooxygenase-2 (COX-2) and microsomal PGE synthase-1 (mPGES-1) in the LPBN after the intraperitoneal injection of lipopolysaccharide (LPS). We investigated the effects of LPBN NS-398 (COX-2 inhibitor) on LPS-induced fever, the effects of direct LPBN PGE2 administration on the energy expenditure (EE), brown adipose tissue (BAT) thermogenesis, neck muscle electromyographic activity and tail temperature, and the effects of PGE2 on the spontaneous firing activity and thermosensitivity of in vitro LPBN neurons in a brain slice.

RESULTS

The COX-2 and mPGES-1 enzymes were upregulated at both mRNA and protein levels. The microinjection of NS-398 in the LPBN attenuated the LPS-induced fever. Direct PGE2 administration in the LPBN resulted in a febrile response by a coordinated response of increased EE, BAT thermogenesis, shivering, and possibly decreased heat loss through the tail. The LPBN neurons showed a clear anatomical distinction in the firing rate response to PGE2, with the majority of PGE2-excited or -inhibited neurons being located in the external lateral or dorsal subnucleus of the LPBN, respectively. However, neither the firing rate nor the thermal coefficient response to PGE2 showed any difference between warm-sensitive, cold-sensitive, and temperature-insensitive neurons in the LPBN.

CONCLUSIONS

PGE2 synthesized in the LPBN was at least partially involved in LPS-induced fever via its different modulations of the firing rate of neurons in different LPBN subnuclei.

Insulin sensing by astrocytes is critical for normal thermogenesis and body temperature regulation

The important role of astrocytes in the central control of energy balance and glucose homeostasis has recently been recognized. Changes in thermoregulation can lead to metabolic dysregulation, but the role of astrocytes in this process is not yet clear. Therefore, we generated mice congenitally lacking insulin receptors (Ir) in astrocytes (IrKOGFAP mice) to investigate the involvement of astrocyte insulin signaling. IrKOGFAP mice displayed significantly lower energy expenditure and a strikingly lower basal and fasting body temperature. When exposed to cold, however, they were able to mount a thermogenic response. IrKOGFAP mice displayed sex differences in metabolic function and thermogenesis that may contribute to the development of obesity and type II diabetes as early as 2 months of age. While brown adipose tissue exhibited higher adipocyte size in both sexes, more apoptosis was seen in IrKOGFAP males. Less innervation and lower BAR3 expression levels were also observed in IrKOGFAP brown adipose tissue. These effects have not been reported in models of astrocyte Ir deletion in adulthood. In contrast, body weight and glucose regulatory defects phenocopied such models. These findings identify a novel role for astrocyte insulin signaling in the development of normal body temperature control and sympathetic activation of BAT. Targeting insulin signaling in astrocytes has the potential to serve as a novel target for increasing energy expenditure.

EP3R-Expressing Glutamatergic Preoptic Neurons Mediate Inflammatory Fever

Fever is a common phenomenon during infection or inflammatory conditions. This stereotypic rise in body temperature (Tb) in response to inflammatory stimuli is a result of autonomic responses triggered by prostaglandin E2 action on EP3 receptors expressed by neurons in the median preoptic nucleus (MnPO^EP3R neurons). To investigate the identity of MnPO^EP3R neurons, we first used in situ hybridization to show coexpression of EP3R and the VGluT2 transporter in MnPO neurons. Retrograde tracing showed extensive direct projections from MnPO^VGluT2 but few from MnPO^Vgat neurons to a key site for fever production, the raphe pallidus. Ablation of MnPO^VGluT2 but not MnPO^Vgat neurons abolished fever responses but not changes in Tb induced by behavioral stress or thermal challenges. Finally, we crossed EP3R conditional knock-out mice with either VGluT2-IRES-cre or Vgat-IRES-cre mice and used both male and female mice to confirm that the neurons that express EP3R and mediate fever are glutamatergic, not GABAergic. This finding will require rethinking current concepts concerning the central thermoregulatory pathways based on the MnPO^EP3R neurons being GABAergic.SIGNIFICANCE STATEMENT Body temperature is regulated by the CNS. The rise of the body temperature, or fever, is an important brain-orchestrated mechanism for fighting against infectious or inflammatory disease, and is tightly regulated by the neurons located in the median preoptic nucleus (MnPO). Here we demonstrate that excitatory MnPO neurons mediate fever and examine a potential central circuit underlying the development of fever responses.

Neural Mechanisms of Inflammation-Induced Fever

Fever is a common symptom of infectious and inflammatory disease. It is well-established that prostaglandin E2 is the final mediator of fever, which by binding to its EP3 receptor subtype in the preoptic hypothalamus initiates thermogenesis. Here, we review the different hypotheses on how the presence of peripherally released pyrogenic substances can be signaled to the brain to elicit fever. We conclude that there is unequivocal evidence for a humoral signaling pathway by which proinflammatory cytokines, through their binding to receptors on brain endothelial cells, evoke fever by eliciting prostaglandin E2 synthesis in these cells. The evidence for a role for other signaling routes for fever, such as signaling via circumventricular organs and peripheral nerves, as well as transfer into the brain of peripherally synthesized prostaglandin E2 are yet far from conclusive. We also review the efferent limb of the pyrogenic pathways. We conclude that it is well established that prostaglandin E2 binding in the preoptic hypothalamus produces fever by disinhibition of presympathetic neurons in the brain stem, but there is yet little understanding of the mechanisms by which factors such as nutritional status and ambient temperature shape the response to the peripheral immune challenge.

Label-free quantitative proteomics of rat hypothalamus under fever induced by LPS and PGE2

Fever is a brain-mediated increase in body temperature mainly during inflammatory or infectious challenges. Although there is considerable data regarding the inflammation pathways involved in fever, metabolic alterations necessary to orchestrate the complex inflammatory response are not totally understood. We performed proteomic analysis of rat hypothalamus using label-free LC-MS/MS in a model of fever induced by lipopolysaccharide (LPS) or prostaglandin E₂ (PGE₂). In total, 7021 proteins were identified. As far as we know, this is the largest rat hypothalamus proteome dataset available to date. Pathway analysis showed proteins from both stimuli associated with inflammatory and metabolic pathways. Concerning metabolic pathways, rats exposed to LPS or PGE₂ presented lower relative abundance of proteins involved in glycolysis, pentose phosphate pathway and tricarboxylic acid cycle. Mitochondrial function may also be altered by both stimuli because significant downregulation of several proteins was found, mainly in complexes I and IV. LPS was able to induce downregulation of important proteins in the enzymatic antioxidant system, thereby contributing to oxidative stress. The results offered comprehensive information about fever responses and helped to reveal new insights into proteins potentially involved in inflammatory signaling and metabolic changes in the hypothalamus during systemic LPS and central PGE₂ administration.

SIGNIFICANCE

The evolutionary persistence of fever, despite the elevated cost for maintenance of this response, suggests that elevation in core temperature may represent an interesting strategy for survival. Fever response is achieved through the integrated behavioral, physiological, immunological and biochemical processes that determine the balance between heat generation and elimination. The development of such complex response arouses interest in studying how the cell metabolism responds or even contributes to promote fever. Our results offered comprehensive information about fever responses, including metabolic and inflammatory pathways, providing new insights into candidate proteins potentially involved in inflammatory signaling and metabolic changes in the hypothalamus during fever induced by systemic LPS and central PGE₂ perturbation.

Mapping Molecular Datasets Back to the Brain Regions They are Extracted from: Remembering the Native Countries of Hypothalamic Expatriates and Refugees

This article focuses on approaches to link transcriptomic, proteomic, and peptidomic datasets mined from brain tissue to the original locations within the brain that they are derived from using digital atlas mapping techniques. We use, as an example, the transcriptomic, proteomic and peptidomic analyses conducted in the mammalian hypothalamus. Following a brief historical overview, we highlight studies that have mined biochemical and molecular information from the hypothalamus and then lay out a strategy for how these data can be linked spatially to the mapped locations in a canonical brain atlas where the data come from, thereby allowing researchers to integrate these data with other datasets across multiple scales. A key methodology that enables atlas-based mapping of extracted datasets-laser-capture microdissection-is discussed in detail, with a view of how this technology is a bridge between systems biology and systems neuroscience.

Effect of PGE2-EPs pathway on primary cultured rat neuron injury caused by aluminum

To observe the characteristic changes of PGE₂-EP_s pathway and divergent functions of PGE₂ receptor subtypes on neuronal injury. The primary cultured rat hippocampus neuron injury model was established via aluminum maltolate (100 μM). The aluminum-overload neurons were treated with the agonists of EP1 (17-phenyl trinor Prostaglandin E2 ethyl amide), EP2 (Butaprost), EP3 (Sulprostone) and EP4 (CAY10598) and antagonists of EP1 (SC-19220), EP2 (AH6809) and EP4 (L-161982) at different concentrations, respectively. The neuronal viability, lactate dehydrogenase leakage rate and PGE2 content were detected by MTT assay, lactate dehydrogenase assay kit and enzyme-linked immunosorbent assay, respectively. The mRNA and protein expressions of mPGES-1 and EPs were determined by RT-PCR and western blot, respectively. The pathomorphology was identified by hematoxylin-eosin staining. In the model group, neuronal viability significantly decreased, while lactate dehydrogenase leakage rate and PGE2 content increased. The mPGES-1, EP1, EP2 and EP4 mRNA expression, and the mPGES-1, EP1 and EP2 protein expression increased, while EP₃ level decreased. EP3 agonist exerted protective function in neuronal viability and lactate dehydrogenase leakage rate, while EP1 agonist, EP2 and EP4 antagonist exerted an opposite effect. In conclusion, aluminum-overload caused an imbalance of PGE₂-EP_1-4 pathway and activation of EP receptor may provide a viable therapeutic target in neuronal injury.

Effects of prostaglandin E2 on cells cultured from the rat organum vasculosum laminae terminalis and median preoptic nucleus

The time course of the induction of enzymes responsible for the formation of prostaglandin E2 (PGE2) after an inflammatory insult, in relation to the concomitant febrile response, suggests that peripherally generated PGE2 is involved in the induction of the early phase of fever, while centrally produced PGE2 exerts pyrogenic capacities during the later stages of fever within the hypothalamic median preoptic nucleus (MnPO). The actions of peripherally derived PGE2 on the brain might occur at the level of the organum vasculosum laminae terminalis (OVLT), which lacks a tight blood-brain barrier and is implicated in fever, while the effects of PGE2 within the MnPO might interfere with glutamatergic neurotransmission within a recently characterized central efferent pathway for the activation of cold-defence reactions. Using the fura-2 ratio imaging technique we, therefore, measured changes of the intracellular Ca(2+)-concentration in primary neuroglial microcultures of rat OVLT and MnPO stimulated with PGE2 and/or glutamate. In cultures from the OVLT, as opposed to those derived from the MnPO, substantial numbers of neurons (8% of 385), astrocytes (19% of 645) and microglial cells (28% of 43) directly responded to PGE2 with a transient increase of intracellular Ca(2+). The most pronounced effect of PGE2 on cells from MnPO microcultures was its modulatory influence on the strength of glutamate-induced Ca(2+)-signals. In 72 out of 512 neurons and in 105 out of 715 astrocytes PGE2 significantly augmented glutamate-induced Ca(2+)-signals. About 30% of these neurons were GABAergic. These observations are in agreement with putative roles of peripheral PGE2 as a directly acting circulating agent at the level of the OVLT, and of central MnPO-intrinsic PGE2 as an enhancer of glutamatergic neurotransmission, which causes disinhibition of thermogenic heat production, a crucial component for the manifestation of fever. In microcultures from both brain sites investigated incubation with PGE2 significantly reduced the lipopolysaccharide-induced release of cytokines (tumor necrosis factor-α and interleukin-6) into the supernatant. PGE2, thus, seems to be involved in a negative feed-back loop to limit the strength of the brain inflammatory process and to play a dual role with pro- as well as anti-inflammatory properties.

Role of an excitatory preoptic-raphé pathway in febrile vasoconstriction of the rat's tail

Heat dissipation from the rat's tail is reduced in response to cold and during fever. The sympathetic premotor neurons for this mechanism, located in the medullary raphé, are under tonic inhibitory control from the preoptic area. In parallel with the inhibitory pathway, an excitatory pathway from the rostromedial preoptic region (RMPO) to the medullary raphé mediates the vasoconstrictor response to cold skin. Whether this applies also to the tail vasoconstrictor response in fever is unknown. Single- or a few-unit tail sympathetic nerve activity (SNA) was recorded in urethane-anesthetized, artificially ventilated rats. Experimental fever was induced by PGE2 injected into the lateral cerebral ventricle (50 ng in 1.5 μl icv) or into the RMPO (0.2 ng in 60 nl); in both cases, there was a robust increase in tail SNA and a delayed rise in core temperature. Microinjection of glutamate receptor antagonist kynurenate (50 mM, 120 nl) into the medullary raphé completely reversed the tail SNA response to intracerebroventricular or RMPO PGE2 injection. Inhibiting RMPO neurons by microinjecting glycine (0.5 M, 60 nl) or the GABAA receptor agonist, muscimol (2 mM, 30-60 nl), reduced the tail SNA response to PGE2 injected into the same site by approximately half. Vehicle injections into the medullary raphé or RMPO were without effect. These results suggest that the tail vasoconstrictor response during experimental fever depends on a glutamatergic excitatory synaptic relay in the medullary raphé and that an excitatory output signal from the RMPO contributes to the tail vasoconstrictor response during fever.

Minor changes in gene expression in the mouse preoptic hypothalamic region by inflammation-induced prostaglandin E2

We investigated to what extent inflammation-induced prostaglandin E2 (PGE2 ) regulates gene expression in the central nervous system. Wild-type mice and mice with deletion of the gene encoding microsomal prostaglandin E synthase-1 (mPGES-1), which cannot produce inflammation-induced PGE2 , were subjected to peripheral injection of bacterial wall lipopolysaccharide (LPS) and killed after 5 h. The median and medial preoptic nuclei, which are rich in prostaglandin E receptors, were isolated by laser capture microdissection (LCM), and subjected to whole genome microarray analysis. Although the immune stimulus induced robust transcriptional changes in the brain, as seen by a quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) on selected genes, only small PGE2 -dependent gene expression changes were observed in the gene array analysis and, for only two genes, a pronounced differential expression between LPS-treated wild-type and mPGES-1 knockout mice could be verified by qRT-PCR. These were Hspa1a and Hspa1b, encoding heat shock proteins, which showed a two- to three-fold higher expression in wild-type mice than in knockout mice after immune challenge. However, the induced expression of these genes was found to be secondary to increased body temperature because they were induced also by cage exchange stress, which did not elicit PGE2 synthesis, and thus were not induced per se by PGE2 -elicited transcriptional events. Our findings suggest that inflammation-induced PGE2 has little effect on gene expression in the preoptic region, and that centrally elicited disease symptoms, although PGE2 -dependent, occur as a result of regulation of neuronal excitability that is a consequence of intracellular, transcriptional-independent signalling cascades. Our findings also imply that the profound changes in gene expression in the brain that are elicited by peripheral inflammation occur independently of PGE2 via a yet unidentified mechanism.

Putative role of prostaglandin receptor in intracerebral hemorrhage

Each year, approximately 795,000 people experience a new or recurrent stroke. Of all strokes, 84% are ischemic, 13% are intracerebral hemorrhage (ICH) strokes, and 3% are subarachnoid hemorrhage strokes. Despite the decreased incidence of ischemic stroke, there has been no change in the incidence of hemorrhagic stroke in the last decade. ICH is a devastating disease 37–38% of patients between the ages of 45 and 64 die within 30 days. In an effort to prevent ischemic and hemorrhagic strokes we and others have been studying the role of prostaglandins and their receptors. Prostaglandins are bioactive lipids derived from the metabolism of arachidonic acid. They sustain homeostatic functions and mediate pathogenic mechanisms, including the inflammatory response. Most prostaglandins are produced from specific enzymes and act upon cells via distinct G-protein coupled receptors. The presence of multiple prostaglandin receptors cross-reactivity and coupling to different signal transduction pathways allow differentiated cells to respond to prostaglandins in a unique manner. Due to the number of prostaglandin receptors, prostaglandin-dependent signaling can function either to promote neuronal survival or injury following acute excitotoxicity, hypoxia, and stress induced by ICH. To better understand the mechanisms of neuronal survival and neurotoxicity mediated by prostaglandin receptors, it is essential to understand downstream signaling. Several groups including ours have discovered unique roles for prostaglandin receptors in rodent models of ischemic stroke, excitotoxicity, and Alzheimer disease, highlighting the emerging role of prostaglandin receptor signaling in hemorrhagic stroke with a focus on cyclic-adenosine monophosphate and calcium (Ca²⁺) signaling. We review current ICH data and discuss future directions notably on prostaglandin receptors, which may lead to the development of unique therapeutic targets against hemorrhagic stroke and brain injuries alike.

Neural circuitry engaged by prostaglandins during the sickness syndrome

During illnesses caused by infectious disease or other sources of inflammation, a suite of brain-mediated responses called the sickness syndrome occurs, which includes fever, anorexia, sleepiness, hyperalgesia and elevated corticosteroid secretion. Much of the sickness syndrome is mediated by prostaglandins acting on the brain and can be prevented by nonsteroidal anti-inflammatory drugs, such as aspirin or ibuprofen, that block prostaglandin synthesis. By examining which prostaglandins are produced at which sites and how they interact with the nervous system, researchers have identified specific neural circuits that underlie the sickness syndrome.

Emerging roles of PGE2 receptors in models of neurological disease

This review presents an overview of the emerging field of prostaglandin signaling in neurological diseases, focusing on PGE(2) signaling through its four E-prostanoid (EP) receptors. A large number of studies have demonstrated a neurotoxic function of the inducible cyclooxygenase COX-2 in a broad spectrum of neurological disease models in the central nervous system (CNS), from models of cerebral ischemia to models of neurodegeneration and inflammation. Since COX-1 and COX-2 catalyze the first committed step in prostaglandin synthesis, an effort is underway to identify the downstream prostaglandin signaling pathways that mediate the toxic effect of COX-2. Recent epidemiologic studies demonstrate that chronic COX-2 inhibition can produce adverse cerebrovascular and cardiovascular effects, indicating that some prostaglandin signaling pathways are beneficial. Consistent with this concept, recent studies demonstrate that in the CNS, specific prostaglandin receptor signaling pathways mediate toxic effects in brain but a larger number appear to mediate paradoxically protective effects. Further complexity is emerging, as exemplified by the PGE(2) EP2 receptor, where cerebroprotective or toxic effects of a particular prostaglandin signaling pathway can differ depending on the context of cerebral injury, for example, in excitotoxicity/hypoxia paradigms versus inflammatory-mediated secondary neurotoxicity. The divergent effects of prostaglandin receptor signaling will likely depend on distinct patterns and dynamics of receptor expression in neurons, endothelial cells, and glia and the specific ways in which these cell types participate in particular models of neurological injury.

Effects of centrally administered prostaglandin E(3) and thromboxane A(3) on plasma noradrenaline and adrenaline in rats: comparison with prostaglandin E(2) and thromboxane A(2)

Previously, we reported the involvement of brain omega-6 prostanoids, especially prostaglandin E(2) and thromboxane A(2), in the activation of central sympatho-adrenomedullary outflow in rats. omega-3 Prostanoids, including prostaglandin E(3) and thromboxane A(3), are believed to be less bioactive than omega-6 prostanoids, although studies on the functions of omega-3 prostanoids in the central nervous system have not been reported. In the present study, therefore, we compared the effects of centrally administered omega-3 prostanoids, prostaglandin E(3) and thromboxane A(3), with those of omega-6 prostanoids, prostaglandin E(2) and thromboxane A(2), on the plasma catecholamines in anesthetized rats. Intracerebroventricularly (i.c.v.) administered prostaglandin E(2) (0.15, 0.3 and 1.5 nmol/animal) and prostaglandin E(3) (0.3 and 3 nmol/animal) predominantly elevated plasma noradrenaline but not adrenaline, but the latter was less efficient than the former. On the other hand, U-46619 (an analog of thromboxane A(2)) (30, 100 and 300 nmol/animal, i.c.v.) and Delta(17)-U-46619 (an analog of thromboxane A(3)) (100 and 300 nmol/animal, i.c.v.) both elevated plasma catecholamines (adrenaline>>noradrenaline) to the same degree. These results suggest that centrally administered prostaglandin E(3) is less effective than prostaglandin E(2) to elevate plasma noradrenaline, and that thromboxane A(3) is almost as equipotent as thromboxane A(2) to elevate plasma catecholamines in rats.

Inducible prostaglandin E2 synthesis interacts in a temporally supplementary sequence with constitutive prostaglandin-synthesizing enzymes in creating the hypothalamic-pituitary-adrenal axis response to immune challenge

Inflammation-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis has been suggested to depend on prostaglandins, but the prostaglandin species and the prostaglandin-synthesizing enzymes that are responsible have not been fully identified. Here, we examined HPA axis activation in mice after genetic deletion or pharmacological inhibition of prostaglandin E(2)-synthesizing enzymes, including cyclooxygenase-1 (Cox-1), Cox-2, and microsomal prostaglandin E synthase-1 (mPGES-1). After immune challenge by intraperitoneal injection of lipopolysaccharide, the rapid stress hormone responses were intact after Cox-2 inhibition and unaffected by mPGES-1 deletion, whereas unselective Cox inhibition blunted these responses, implying the involvement of Cox-1. However, mPGES-1-deficient mice showed attenuated transcriptional activation of corticotropin-releasing hormone (CRH) that was followed by attenuated plasma concentrations of adrenocorticotropic hormone and corticosterone. Cox-2 inhibition similarly blunted the delayed corticosterone response and further attenuated corticosterone release in mPGES-1 knock-out mice. The expression of the c-fos gene, an index of synaptic activation, was maintained in the paraventricular hypothalamic nucleus and its brainstem afferents both after unselective and Cox-2 selective inhibition as well as in Cox-1, Cox-2, and mPGES-1 knock-out mice. These findings point to a mechanism by which (1) neuronal afferent signaling via brainstem autonomic relay nuclei and downstream Cox-1-dependent prostaglandin release and (2) humoral, CRH transcription-dependent signaling through induced Cox-2 and mPGES-1 elicited PGE(2) synthesis, shown to occur in brain vascular cells, play distinct, but temporally supplementary roles for the stress hormone response to inflammation.

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.