Relationship of EP(1-4) prostaglandin receptors with rat hypothalamic cell groups involved in lipopolysaccharide fever responses

NK1 receptor mediates cerebral cellular and extracellular morphological changes during the LPS-induced febrile response

Fever elicited by bacterial lypopolyssacharide (LPS) is mediated by pro-inflammatory cytokines, which activate central mediators and regulate the hypothalamic temperature setpoint. This response is often accompanied by morphological changes involving the extracellular matrix, neurons and glial cells, with significant health impacts. The NK1 receptor is involved in the febrile response induced by LPS but its effects over the extracellular matrix in the context of neuroinflammation remain unknown. The present work aims to clarify the extracellular changes associated with NK1 signaling in LPS-induced fever. Male Wistar rats were exposed to LPS intraperitoneally. Experimental groups were pre-treated intracerebroventricularly with the NK1 selective inhibitor SR140333B or saline. Histological changes involving the brain extracellular matrix were evaluated using hematoxylin and eosin, Mason's trichrome, picrosirius, alcian blue, periodic acid Schiff's stains. The expression of matrix metalloproteinase 9 (MMP9) was studied using confocal microscopy. Fever was accompanied by edema, perivascular lymphoplamacytic and neutrophylic infiltration, spongiosis and MMP9 overexpression. SR140333B significantly reduced LPS-induced fever (p < 0.0001), MMP9 overexpression (p < 0.01) and associated histological changes. These results contribute to characterize cerebral extracellular matrix changes associated with LPS-induced fever. Overall, the present work supports a role for NK1 receptor in these neuroinflammatory changes, involving MMP9 overexpression, edema and leukocytic infiltration.

Microbial and Host Metabolites at the Backstage of Fever: Current Knowledge about the Co-Ordinate Action of Receptors and Molecules Underlying Pathophysiology and Clinical Implications

Fever represents an elevation of body temperature, that exerts a protective effect against pathogens. Innate immune cells and neurons are implicated in the regulation of body temperature. Pathogen-associated molecular patterns, i.e., lipopolysaccharides from Gram-negative bacteria and peptidoglycan and lipoteichoic acid from Gram-positive bacteria are exogenous pyrogens, that bind to Toll-like receptors on immune and non-immune cells. The subsequent release of pro-inflammatory cytokines [interleukin-1 (IL-1), IL-6 and Tumor necrosis factor-alpha] and their passage through the brain trigger the febrile response. In fact, neurons of the pre-optic area produce prostaglandin E2 (PGE2), that, in turn, bind to the PGE2 receptors; thus, generating fever. Apart from classical non-steroidal anti-inflammatory drugs, i.e., aspirin and acetaminophen, various botanicals are currently used as antipyretic agents and, therefore, their mechanisms of action will be elucidated.

A preoptic neuronal population controls fever and appetite during sickness

During infection, animals exhibit adaptive changes in physiology and behaviour aimed at increasing survival. Although many causes of infection exist, they trigger similar stereotyped symptoms such as fever, warmth-seeking, loss of appetite and fatigue^1,2. Yet exactly how the nervous system alters body temperature and triggers sickness behaviours to coordinate responses to infection remains unknown. Here we identify a previously uncharacterized population of neurons in the ventral medial preoptic area (VMPO) of the hypothalamus that are activated after sickness induced by lipopolysaccharide (LPS) or polyinosinic:polycytidylic acid. These neurons are crucial for generating a fever response and other sickness symptoms such as warmth-seeking and loss of appetite. Single-nucleus RNA-sequencing and multiplexed error-robust fluorescence in situ hybridization uncovered the identity and distribution of LPS-activated VMPO (VMPO^LPS) neurons and non-neuronal cells. Gene expression and electrophysiological measurements implicate a paracrine mechanism in which the release of immune signals by non-neuronal cells during infection activates nearby VMPO^LPS neurons. Finally, we show that VMPO^LPS neurons exert a broad influence on the activity of brain areas associated with behavioural and homeostatic functions and are synaptically and functionally connected to circuit nodes controlling body temperature and appetite. Together, these results uncover VMPO^LPS neurons as a control hub that integrates immune signals to orchestrate multiple sickness symptoms in response to infection.

Design, synthesis, and biological evaluation of new thalidomide-donepezil hybrids as neuroprotective agents targeting cholinesterases and neuroinflammation

A new series of eight multifunctional thalidomide-donepezil hybrids were synthesized based on the multi-target-directed ligand strategy and evaluated as potential neuroprotective, cholinesterase inhibitors and anti-neuroinflammatory agents against neurodegenerative diseases. A molecular hybridization approach was used for structural design by combining the N-benzylpiperidine pharmacophore of donepezil and the isoindoline-1,3-dione fragment from the thalidomide structure. The most promising compound, PQM-189 (3g), showed good AChE inhibitory activity with an IC₅₀ value of 3.15 μM, which was predicted by docking studies as interacting with the enzyme in the same orientation observed in the AChE-donepezil complex and a similar profile of interaction. Additionally, compound 3g significantly decreased iNOS and IL-1β levels by 43% and 39%, respectively, after 24 h of incubation with lipopolysaccharide. In vivo data confirmed the ability of 3g to prevent locomotor impairment and changes in feeding behavior elicited by lipopolysaccharide. Moreover, the PAMPA assay evidenced adequate blood-brain barrier and gastrointestinal tract permeabilities with an Fa value of 69.8%. Altogether, these biological data suggest that compound 3g can treat the inflammatory process and oxidative stress resulting from the overexpression of iNOS and therefore the increase in reactive nitrogen species, and regulate the release of pro-inflammatory cytokines such as IL-1β. In this regard, compound PQM-189 (3g) was revealed to be a promising neuroprotective and anti-neuroinflammatory agent with an innovative thalidomide-donepezil-based hybrid molecular architecture.

The EP3 and EP4 Receptor Subtypes both Mediate the Fever-producing Effects of Prostaglandin E2 in the Rostral Ventromedial Preoptic Area of the Hypothalamus in Rats

This study aimed to re-examine the receptor subtype that mediates the fever-producing effects of prostaglandin E₂ (PGE₂) in the rostral ventromedial preoptic area (rvmPOA) of the hypothalamus. Among the four subtypes of PGE₂ receptors (EP₁, EP₂, EP₃, and EP₄), EP₃ receptor is crucially involved in the febrile effects of PGE₂. However, it is possible for other subtypes of PGE₂ receptor to contribute in the central mechanism of fever generation. Accordingly, effects of microinjection of PGE₂ receptor subtype-specific agonists or antagonists were examined at the locus where a microinjection of a small amount (420 fmol) of PGE₂ elicited prompt increases in the O₂ consumption rate (VO₂), heart rate, and colonic temperature (T_c) in the rvmPOA of urethane-chloralose-anesthetized rats. The EP₃ agonist sulprostone mimicked, whereas its antagonist L-798,106 reduced, the febrile effects of PGE₂ microinjected into the same site. Similarly, the EP₄ agonist rivenprost mimicked, whereas its antagonist ONO-AE3-208 reduced, the effects of PGE₂ microinjected into the same site. In contrast, microinjection of the EP₁ agonist iloprost induced a very small increase in VO₂ but did not have significant influences on the heart rate and T_c, whereas its antagonist, AH6809, did not affect the PGE₂-induced responses. Microinjection of the EP₂ agonist butaprost had no effects on the VO₂, heart rate, and T_c. The results suggest that the EP₃ and EP₄ receptor subtypes are both involved in the fever generated by PGE₂ in the rvmPOA.

The median preoptic nucleus: A major regulator of fluid, temperature, sleep, and cardiovascular homeostasis

Located in the midline lamina terminalis of the anterior wall of the third ventricle, the median preoptic nucleus is a thin elongated nucleus stretching around the rostral border of the anterior commissure. Its neuronal elements, composed of various types of excitatory glutamatergic and inhibitory GABAergic neurons, receive afferent neural signals from (1) neighboring subfornical organ and organum vasculosum of the lamina terminalis related to plasma osmolality and hormone concentrations, e.g., angiotensin II; (2) from peripheral sensors such as arterial baroreceptors and cutaneous thermosensors. Different sets of these MnPO glutamatergic and GABAergic neurons relay output signals to hypothalamic, midbrain, and medullary regions that drive homeostatic effector responses. Included in the effector responses are (1) thirst, antidiuretic hormone secretion and renal sodium excretion that subserve osmoregulation and body fluid homeostasis; (2) vasoconstriction or dilatation of skin blood vessels, and shivering and brown adipose tissue thermogenesis for core temperature homeostasis; (3) inhibition of hypothalamic and midbrain nuclei that stimulate wakefulness and arousal, thereby promoting both REM and non-REM sleep; and (4) activation of sympathetic pathways that drive vasoconstriction and heart rate to maintain arterial pressure and the perfusion of vital organs. The small size of MnPO belies its massive homeostatic significance.

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.

Immune-mediated febrile response in female rats: Role of central hypothalamic mediators

Lipopolysaccharide (LPS) induces fever through cytokines like receptor-activator of nuclear factor κB ligand (RANKL), triggering mediators like prostaglandins (PG), endothelin-1 (ET-1), corticotrophin-releasing factor (CRF), substance P (SP) and endogenous opioids. LPS-induced fever is reduced in females compared with males except in ovariectomized (OVX) females which show increased fever mediated by PG. The present study aimed to identify the mediators involved in fever in intact and OVX female rats. Fever was induced with LPS (50 μg/kg) intraperitoneally or CRF (2.5 μg), ET-1 (1 pg), morphine (10 μg) and SP (500 ng) intracerebroventricularly in sham-operated and OVX rats. The role of RANKL was evaluated with osteoprotegerin (OPG, 1 μg, intracerebroventricularly). Expression of RANK, CRFI/II, ETB, μ-opioid (MOR) and NK1 receptors was evaluated by confocal microscopy. Besides LPS, only morphine induced fever in OVX rats while all mediators induced fever in sham-operated animals. OPG abolished LPS-induced fever in OVX but not sham-operated animals. Overall, fever involves similar central mediators in cycling females and males but only morphine induced fever in OVX females. Importantly, RANK/RANKL participates in LPS-induced fever in OVX females, as in males but not in cycling females.

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.

Molecular and Functional Sex Differences of Noradrenergic Neurons in the Mouse Locus Coeruleus

Preclinical work has long focused on male animals, though biological sex clearly influences risk for certain diseases, including many psychiatric disorders. Such disorders are often treated by drugs targeting the CNS norepinephrine system. Despite roles for noradrenergic neurons in behavior and neuropsychiatric disease models, their molecular characterization has lagged. We profiled mouse noradrenergic neurons in vivo, defining over 3,000 high-confidence transcripts expressed therein, including druggable receptors. We uncovered remarkable sex differences in gene expression, including elevated expression of the EP3 receptor in females-which we leverage to illustrate the behavioral and pharmacologic relevance of these findings-and of Slc6a15 and Lin28b, both major depressive disorder (MDD)-associated genes. Broadly, we present a means of transcriptionally profiling locus coeruleus under baseline and experimental conditions. Our findings underscore the need for preclinical work to include both sexes and suggest that sex differences in noradrenergic neurons may underlie behavioral differences relevant to disease.

Central nervous system circuits that control body temperature

Maintenance of mammalian core body temperature within a narrow range is a fundamental homeostatic process to optimize cellular and tissue function, and to improve survival in adverse thermal environments. Body temperature is maintained during a broad range of environmental and physiological challenges by central nervous system circuits that process thermal afferent inputs from the skin and the body core to control the activity of thermoeffectors. These include thermoregulatory behaviors, cutaneous vasomotion (vasoconstriction and, in humans, active vasodilation), thermogenesis (shivering and brown adipose tissue), evaporative heat loss (salivary spreading in rodents, and human sweating). This review provides an overview of the central nervous system circuits for thermoregulatory reflex regulation of thermoeffectors.

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.

Reverse Engineering the Febrile System

Fever, the elevation of core body temperature by behavioral or physiological means, is one of the most salient aspects of human sickness, yet there is debate regarding its functional role. In this paper, we demonstrate that the febrile system is an evolved adaptation shaped by natural selection to coordinate the immune system to fight pathogens. First, we show that previous arguments in favor of fever being an adaptation are epistemologically inadequate, and we describe how an adaptationist strategy addresses this issue more effectively. Second, we argue that the mechanisms producing fever provide clear indications of adaptation. Third, we demonstrate that there are many beneficial immune system responses activated during fever and that these responses are not mere byproducts of heat on chemical reactions. Rather, we show that natural selection appears to have modified several immune system effects to be coordinated by fever. Fourth, we argue that there are some adaptations that coordinate the febrile system with other important fitness components, particularly growth and reproduction. Finally, we discuss evidence that the febrile system may also have evolved an antitumor function, providing suggestions for future research into this area. This research informs the debate on the functional value of fever and antipyretic use.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

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.

Immune-Induced Fever Is Dependent on Local But Not Generalized Prostaglandin E2 Synthesis in the Brain

Fever occurs upon binding of prostaglandin E₂ (PGE₂) to EP₃ receptors in the median preoptic nucleus of the hypothalamus, but the origin of the pyrogenic PGE₂ has not been clearly determined. Here, using mice of both sexes, we examined the role of local versus generalized PGE₂ production in the brain for the febrile response. In wild-type mice and in mice with genetic deletion of the prostaglandin synthesizing enzyme cyclooxygenase-2 in the brain endothelium, generated with an inducible CreER^T2 under the Slco1c1 promoter, PGE₂ levels in the CSF were only weakly related to the magnitude of the febrile response, whereas the PGE₂ synthesizing capacity in the hypothalamus, as reflected in the levels of cyclooxygenase-2 mRNA, showed strong correlation with the immune-induced fever. Histological analysis showed that the deletion of cyclooxygenase-2 in brain endothelial cells occurred preferentially in small- and medium-sized vessels deep in the brain parenchyma, such as in the hypothalamus, whereas larger vessels, and particularly those close to the neocortical surface and in the meninges, were left unaffected, hence leaving PGE₂ synthesis largely intact in major parts of the brain while significantly reducing it in the region critical for the febrile response. Furthermore, injection of a virus vector expressing microsomal prostaglandin E synthase-1 (mPGES-1) into the median preoptic nucleus of fever-refractive mPGES-1 knock-out mice, resulted in a temperature elevation in response to LPS. We conclude that the febrile response is dependent on local release of PGE₂ onto its target neurons and not on the overall PGE₂ production in the brain.SIGNIFICANCE STATEMENT By using mice with selective deletion of prostaglandin synthesis in brain endothelial cells, we demonstrate that local prostaglandin E₂ (PGE₂) production in deep brain areas, such as the hypothalamus, which is the site of thermoregulatory neurons, is critical for the febrile response to peripheral inflammation. In contrast, PGE₂ production in other brain areas and the overall PGE₂ level in the brain do not influence the febrile response. Furthermore, partly restoring the PGE₂ synthesizing capacity in the anterior hypothalamus of mice lacking such capacity with a lentiviral vector resulted in a temperature elevation in response to LPS. These data imply that the febrile response is dependent on the local release of PGE₂ onto its target neurons, possibly by a paracrine mechanism.

Thermoregulation as a disease tolerance defense strategy

Physiological responses that occur during infection are most often thought of in terms of effectors of microbial destruction through the execution of resistance mechanisms, due to a direct action of the microbe, or are maladaptive consequences of host-pathogen interplay. However, an examination of the cellular and organ-level consequences of one such response, thermoregulation that leads to fever or hypothermia, reveals that these actions cannot be readily explained within the traditional paradigms of microbial killing or maladaptive consequences of host-pathogen interactions. In this review, the concept of disease tolerance is applied to thermoregulation during infection, inflammation and trauma, and we discuss the physiological consequences of thermoregulation during disease including tissue susceptibility to damage, inflammation, behavior and toxin neutralization.

Unraveling the Molecular Details Involved in the Intimate Link between the Immune and Neuroendocrine Systems

During systemic infections, the immune system can signal the brain and act on different neuronal circuits via soluble molecules, such as proinflammatory cytokines, that act on the cells forming the blood-brain barrier and the circumventricular organs. These activated cells release prostaglandin of the E2 type (PGE2), which is the endogenous ligand that triggers the pathways involved in the control of autonomic functions necessary to restore homeostasis and provide inhibitory feedback to innate immunity. Among these neurophysiological functions, activation of the circuits that control the plasma release of glucocorticoids is probably the most critical to the survival of the host in the presence of pathogens. This review revisits this issue and describes in depth the molecular details (including the emerging role of Toll-like receptors during inflammation) underlying the influence of circulating inflammatory molecules on the cerebral tissue, focusing on their contribution in the synthesis and action PGE2 in the brain. We also provide an innovative view supporting the concept of “fast and delayed response” involving the same ligands but different groups of cells, signal transduction pathways, and target genes.

Prostaglandin E2 facilitates subcellular translocation of the EP4 receptor in neuroectodermal NE-4C stem cells

Prostaglandin E2 (PGE₂) is a lipid mediator released from the phospholipid membranes that mediates important physiological functions in the nervous system via activation of four EP receptors (EP1-4). There is growing evidence for the important role of the PGE₂/EP4 signaling in the nervous system. Previous studies in our lab show that the expression of the EP4 receptor is significantly higher during the neurogenesis period in the mouse. We also showed that in mouse neuroblastoma cells, the PGE₂/EP4 receptor signaling pathway plays a role in regulation of intracellular calcium via a phosphoinositide 3-kinase (PI3K)-dependent mechanism. Recent research indicates that the functional importance of the EP4 receptor depends on its subcellular localization. PGE₂-induced EP4 externalization to the plasma membrane of primary sensory neurons has been shown to play a role in the pain pathway. In the present study, we detected a novel PGE₂–dependent subcellular trafficking of the EP4 receptor in neuroectodermal (NE-4C) stem cells and differentiated NE-4C neuronal cells. We show that PGE₂ induces EP4 externalization from the Golgi apparatus to the plasma membrane in NE-4C stem cells. We also show that the EP4 receptors translocate to growth cones of differentiating NE-4C neuronal cells and that a higher level of PGE₂ enhances its growth cone localization. These results demonstrate that the EP4 receptor relocation to the plasma membrane and growth cones in NE-4C cells is PGE₂ dependent. Thus, the functional role of the PGE₂/EP4 pathway in the developing nervous system may depend on the subcellular localization of the EP4 receptor.

•

Function of the PGE₂/EP4 pathway depends on the localization of the EP4 receptor.

•

PGE₂ induces EP4 trafficking from Golgi to plasma membrane in NE-4C stem cells.

•

EP4 receptors translocate to growth cones in differentiating NE-4C neuronal cells.

•

Higher PGE₂ level enhanced EP4 trafficking to growth cones of NE-4C neuronal cells.

Prostaglandin D2 elicits the reversible neurite retraction in hypothalamic cell line

Prostaglandins (PGs) play important roles in diverse physiological processes in the central nervous system. PGD2 is the most abundant PG in the brain and acts through specific receptors, DP1 and CRTH2. We investigated the effects of PGD2 on the morphology of the hypothalamic cell line mHypoE-N37 (N37). In N37 cells, serum starvation induced neurite outgrowth and PGD2 elicited neurite retraction, although we failed to detect transcripts for DP1 and CRTH2. Such an effect of PGD2 was efficiently mimicked by its metabolite, 15-deoxy-Δ(12,14)-prostaglandin J2. N-acetyl cysteine completely abolished the effect of PGD2, and reactive oxygen species (ROS) were considered to be important. Notably, neurite outgrowth was restored by PGD2 removal. These results suggest that PGD2 induces reversible neurite retraction in a ROS-mediated mechanism that does not involve any known receptor.

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.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

The roles of prostaglandin E2 and D2 in lipopolysaccharide-mediated changes in sleep

When living organisms become sick as a result of a bacterial infection, a suite of brain-mediated responses occur, including fever, anorexia and sleepiness. Systemic administration of lipopolysaccharide (LPS), a common constituent of bacterial cell walls, increases body temperature and non-rapid eye movement (NREM) sleep in animals and induces the production of pro-inflammatory prostaglandins (PGs). PGE2 is the principal mediator of fever, and both PGE2 and PGD2 regulate sleep-wake behavior. The extent to which PGE2 and PGD2 are involved in the effect of LPS on NREM sleep remains to be clarified. Therefore, we examined LPS-induced changes in body temperature and NREM sleep in mice with nervous system-specific knockouts (KO) for the PGE2 receptors type EP3 or EP4, in mice with total body KO of microsomal PGE synthase-1 or the PGD2 receptor type DP, and in mice treated with the cyclooxygenase (COX) inhibitor meloxicam. We observed that LPS-induced NREM sleep was slightly attenuated in mice lacking EP4 receptors in the nervous system, but was not affected in any of the other KO mice or in mice pretreated with the COX inhibitor. These results suggest that the effect of LPS on NREM sleep is partially dependent on PGs and is likely mediated mainly by other pro-inflammatory substances. In addition, our data show that the main effect of LPS on body temperature is hypothermia in the absence of nervous system EP3 receptors or in the presence of a COX inhibitor.

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.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

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.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

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.

EP₃ receptors mediate PGE₂-induced hypothalamic paraventricular nucleus excitation and sympathetic activation

Prostaglandin E(2) (PGE(2)), an important mediator of the inflammatory response, acts centrally to elicit sympathetic excitation. PGE(2) acts on at least four E-class prostanoid (EP) receptors known as EP(1), EP(2), EP(3), and EP(4). Since PGE(2) production within the brain is ubiquitous, the different functions of PGE(2) depend on the expression of these prostanoid receptors in specific brain areas. The type(s) and location(s) of the EP receptors that mediate sympathetic responses to central PGE(2) remain unknown. We examined this question using PGE(2), the relatively selective EP receptor agonists misoprostol and sulprostone, and the available selective antagonists for EP(1), EP(3), and EP(4). In urethane-anesthetized rats, intracerebroventricular (ICV) administration of PGE(2), sulprostone or misoprostol increased renal sympathetic nerve activity, blood pressure, and heart rate. These responses were significantly reduced by ICV pretreatment with the EP(3) receptor antagonist; the EP(1) and EP(4) receptor antagonists had little or no effect. ICV PGE(2) or misoprostol increased the discharge of neurons in the hypothalamic paraventricular nucleus (PVN). ICV misoprostol increased the c-Fos immunoreactivity of PVN neurons, an effect that was substantially reduced by the EP(3) receptor antagonist. Real-time PCR detected EP(3) receptor mRNA in PVN, and immunohistochemical studies revealed sparsely distributed EP(3) receptors localized in GABAergic terminals and on a few PVN neurons. Direct bilateral PVN microinjections of PGE(2) or sulprostone elicited sympathoexcitatory responses that were significantly reduced by the EP(3) receptor antagonist. These data suggest that EP(3) receptors mediate the central excitatory effects of PGE(2) on PVN neurons and sympathetic discharge.

Neural controls of prostaglandin 2 pyrogenic, tachycardic, and anorexic actions are anatomically distributed

Fever and anorexia are induced by immune system challenges. Because these responses are adaptive when short lasting but deleterious when prolonged, an understanding of the mediating neural circuitry is important. Prostaglandins (PGE) are a critical signaling element for these immune responses. Despite the widespread distribution of PGE receptors throughout the brain, research focuses on the hypothalamic preoptic area as the mediating site of PGE action. Paraventricular nucleus of the hypothalamus (PVH), parabrachial nucleus (PBN), and nucleus tractus solitarius (NTS) neurons also express PGE receptors and are activated during systemic pathogen infection. A role for these neurons in PGE-induced fever, tachycardia, and anorexia is unexplored and is the subject of this report. A range of PGE₂ doses was microinjected into third or fourth ventricles (v), or directly into the dorsal PVH, lateral PBN, and medial NTS, and core and brown adipose tissue temperature, heart rate, locomotor activity, and food intake were measured in awake, behaving rats. PGE₂ delivery to multiple brain sites (third or fourth v, PVH, or PBN) induced a short- latency (< 10 min) fever and tachycardia. By contrast, an anorexic effect was observed only in response to third v and PVH stimulation. NTS PGE₂ stimulation was without effect; locomotor activity was not affected for any of the sites. The data are consistent with a view of PGE₂-induced effects as mediated by anatomically distributed sites rather than a single center. The data also underscore a potential anatomical dissociation of the neural pathways mediating pyrogenic and anorexic effects of PGE₂.

Prostanoid signaling: dual role for prostaglandin E2 in neurotoxicity

The prostanoids, a naturally occurring subclass of eicosanoids, are lipid mediators generated through oxidative pathways from arachidonic acid. These cyclooxygenase metabolites, consisting of the prostaglandins (PG), prostacyclin and tromboxane, are released in response to a variety of physiological and pathological stimuli in almost all organs, including the brain. They are produced by various cell types and act upon targeted cells via specific G protein-coupled receptors. The existence of multiple receptors, cross-reactivity and coupling to different signal transduction pathways for each prostanoid, collectively establish their diverse effects. Notably, these effects can occur in functionally opposing directions within the same cell or organ. Prostaglandin E(2) (PGE(2)) is the most versatile prostanoid because of its receptors, E Prostanoid (EP) receptor subtypes 1 through 4, its biological heterogeneity and its differential expression on neuronal and glial cells throughout the central nervous system. Since PGE(2) plays an important role in processes associated with various neurological diseases, this review focuses on its dual neuroprotective and neurotoxic role in EP receptor subtype signaling pathways in different models of brain injury.

Stress responses: the contribution of prostaglandin E(2) and its receptors

Stress is a state of physiological or psychological strain caused by adverse stimuli; responses to stress include activation of the sympathetic nervous system, glucocorticoid secretion and emotional behaviors. Prostaglandin E(2) (PGE(2)), acting through its four receptor subtypes (EP1, EP2, EP3 and EP4), is involved in these stress responses. Studies of EP-selective drugs and mice lacking specific EPs have identified the neuronal pathways regulated by PGE(2). In animals with febrile illnesses, PGE(2) acts on neurons expressing EP3 in the preoptic hypothalamus. In illness-induced activation of the hypothalamic-pituitary-adrenal axis, EP1 and EP3 regulate distinct neuronal pathways that converge at the paraventricular hypothalamus. During psychological stress, EP1 suppresses impulsive behaviors via the midbrain dopaminergic systems. PGE(2) promotes illness-induced memory impairment, yet also supports hippocampus-dependent memory formation and synaptic plasticity via EP2 in physiological conditions. In response to illness, PGE(2) is synthesized by enzymes induced in various cell types inside and outside the brain, whereas constitutively expressed enzymes in neurons and/or microglia synthesize PGE(2) in response to psychological stress. Dependent on the type of stress stimuli, PGE(2) released from different cell types activates distinct EP receptors, which mobilize multiple neuronal pathways, resulting in stress responses.

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.

A new look on brain mechanisms of acute illness anorexia

Bacterial lipopolysaccharide (LPS) and other microbial substances trigger the organism's acute phase response and cause acute illness anorexia. Pro-inflammatory cytokines are major endogenous mediators of acute illness anorexia, but how LPS or cytokines stimulate the brain to inhibit eating is not fully resolved. One emerging mechanism involves the activation of the enzyme cyclooxygenase-2 (COX-2) in blood-brain barrier endothelial cells and the subsequent release of prostaglandin E2 (PGE2). Serotonin neurons in the midbrain raphe are targets of PGE2, and serotonergic projections from the midbrain raphe to the hypothalamus appear to be crucial for LPS anorexia. That is, raphe projections activate (1) the corticotrophin-releasing hormone neurons in the paraventricular nucleus which then elicit the stress response and (2) the pro-opiomelanocortin neurons in the arcuate nucleus which then release alphaMSH and elicit anorexia. Here we review available data to support a role for this brain mechanism in acute illness anorexia by center staging PGE2 signaling pathways that converge on central neural circuits that control normal eating. In addition, we review interactions between gonadal hormones and immune function that lead to sex differences in acute illness anorexia. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

The dance of the perivascular and endothelial cells: mechanisms of brain response to immune signaling

The mechanisms underlying the brain response to systemic inflammation remain unclear. In this issue of Neuron, Serrats and colleagues demonstrate that two cell types that produce prostaglandins that act on the brain, perivascular and endothelial cells, have an unexpectedly complex interaction in regulating the timing and types of brain responses that occur.

Fever

Measurement of body temperature remains one of the most common ways to assess health. An increase in temperature above what is considered to be a normal value is inevitably regarded as a sure sign of disease and referred to with one simple word: fever. In this review, we summarize how research on fever allowed the identification of the exogenous and endogenous molecules and pathways mediating the fever response. We also show how temperature elevation is common to different pathologies and how the molecular components of the fever-generation pathway represent drug targets for antipyretics, such as acetylsalicylic acid, the first "blockbuster drug". We also show how fever research provided new insights into temperature and energy homeostasis, and into treatment of infection and inflammation.

Neurobiology of inflammation-associated anorexia

Compelling data demonstrate that inflammation-associated anorexia directly results from the action of pro-inflammatory factors, primarily cytokines and prostaglandins E2, on the nervous system. For instance, the aforementioned pro-inflammatory factors can stimulate the activity of peripheral sensory neurons, and induce their own de novo synthesis and release into the brain parenchyma and cerebrospinal fluid. Ultimately, it results in the mobilization of a specific neural circuit that shuts down appetite. The present article describes the different cell groups and neurotransmitters involved in inflammation-associated anorexia and examines how they interact with neural systems regulating feeding such as the melanocortin system. A better understanding of the neurobiological mechanisms underlying inflammation-associated anorexia will help to develop appetite stimulants for cancer and AIDS patients.

Interactions between the immune and neuroendocrine systems

The growing spark of interest in research concerning the molecular links between the nervous, endocrine and immune systems has caused an explosion of new knowledge concerning the fine mechanisms that orchestrate the integrated response to an immune challenge. For instance, elevation in plasma glucocorticoid (GC) levels is one of the most powerful and well-controlled feedback mechanisms on the proinflammatory signal transduction machinery taking place across the organism. Circulating inflammatory molecules have the ability to target their cognate receptors at the levels of blood-brain barrier, the latter in return produces specific prostaglandins (PGs). This chapter presents the brain circuits involved in the activation of the hypothalamic-pituitary-adrenal (HPA) axis by endogenously produced prostaglandin E(2) (PGE(2)) during systemic innate immune insults.

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.

Prostaglandins and sickness behavior: old story, new insights

Previous evidence has shown that prostaglandins play a key role in the development of sickness behavior observed during inflammatory states. In particular, prostaglandin E2 (PGE2) is produced in the brain by a variety of inflammatory signals such as endotoxins or cytokines. Its injection has been also shown to induce symptoms of sickness behavior. The role of cyclooxygenase enzymes (COX), the rate-limiting enzymes converting arachidonic acid into prostaglandins, in sickness behavior has been extensively studied, and it has been demonstrated that strategies aiming at inhibiting these enzymes limit anorexia, body weight loss and fever in animals with inflammatory diseases. However, inhibiting COX activity may lead to negative gastric or cardiovascular effects, since COX enzymes play a role in the synthesis of others prostanoids with various and sometimes contrasting properties. Recently, prostaglandin E synthases (PGES), which specifically catalyze the final step of PGE2 biosynthesis, were characterized. Among these enzymes, the microsomal prostaglandin E synthase-1 (mPGES-1) was of a particular interest since it was shown to be up-regulated by inflammatory signals in a variety of cell types. Moreover, mPGES-1 was shown to be crucial for correct immune-to-brain communication and induction of fever and anorexia by pro-inflammatory agents. This review takes stock of previous knowledge and recent advances in understanding the role of prostaglandins and of their specific synthesizing enzymes in the molecular mechanisms underlying sickness behavior. The review concludes with a short summary of key questions that remain to be addressed and points out therapeutic developments in this research field.

Prostaglandin D(2) sustains the pyrogenic effect of prostaglandin E(2)

Prostaglandin D(2) (PGD(2)) is involved in a variety of physiological and pathophysiological processes, but its role in fever is poorly understood. Here we investigated the effects of central PGD(2) administration on body temperature and prostaglandin levels in the cerebrospinal fluid (CSF) of rats. Administration of PGD(2) into the cisterna magna (i.c.m) evoked a delayed fever response that was paralleled by increased levels of prostaglandin E(2) (PGE(2)) in the CSF. The elevated PGE(2) levels were not caused by an increased expression of cyclooxygenase 2 or microsomal prostaglandin E synthase-1 in the hypothalamus. Interestingly, i.c.m. pretreatment of animals with PGD(2) considerably sustained the pyrogenic effects of i.c.m. administered PGE(2). These data indicate that PGD(2) might control the availability of PGE(2) in the CSF and suggest that centrally produced PGD(2) may play a role in the maintenance of fever.

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.

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).

Therapeutic targets in prostaglandin E2 signaling for neurologic disease

Prostaglandins (PGs) are potent autocrine and paracrine oxygenated lipid molecules that contribute appreciably to physiologic and pathophysiologic responses in almost all organs, including brain. Emerging data indicate that the PGs, and more specifically PGE2, play a central role in brain diseases including ischemic injury and several neurodegenerative diseases. Given concerns over the potential toxicity from protracted use of cyclooxygenase inhibitors in the elderly, attention is now focused on blocking PGE2 signaling that is mediated by interactions with four distinct G protein-coupled receptors, EP1-4, which are differentially expressed on neuronal and glial cells throughout the central nervous system. EP1 activation has been shown to mediate Ca2+-dependent neurotoxicity in ischemic injury. EP2 activation has been shown to mediate microglial-induced paracrine neurotoxicity as well as suppress microglia internalization of aggregated neurotoxic peptides. Animal models support the potential efficacy of targeting specific EP receptor subtypes in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and ischemic stroke. However promising these preclinical studies are, they have yet to be followed by clinical trials targeting any EP receptor in neurologic diseases.

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.