Immunocytochemical detection of prostaglandin E2 in microvasculature and in neurons of rat brain after administration of bacterial endotoxin

Neuromodulation by the immune system: a focus on cytokines

Interactions between the immune system and the nervous system have been described mostly in the context of diseases. More recent studies have begun to reveal how certain immune cell-derived soluble effectors, the cytokines, can influence host behaviour even in the absence of infection. In this Review, we contemplate how the immune system shapes nervous system function and how it controls the manifestation of host behaviour. Interactions between these two highly complex systems are discussed here also in the context of evolution, as both may have evolved to maximize an organism's ability to respond to environmental threats in order to survive. We describe how the immune system relays information to the nervous system and how cytokine signalling occurs in neurons. We also speculate on how the brain may be hardwired to receive and process information from the immune system. Finally, we propose a unified theory depicting a co-evolution of the immune system and host behaviour in response to the evolutionary pressure of pathogens.

Early Axonal Injury and Delayed Cytotoxic Cerebral Edema are Associated with Microglial Activation in a Mouse Model of Sepsis

Sepsis-induced brain injury is associated with an acute deterioration of mental status resulting in cognitive impairment and acquisition of new functional limitations in sepsis survivors. However, the exact nature of brain injury in this setting is often subtle and remains to be fully characterized both in preclinical studies and at the bedside. Given the translation potential for the use of magnetic resonance imaging (MRI) to define sepsis-induced brain injury, we sought to determine and correlate the cellular changes with neuroradiographic presentations in a classic murine model of sepsis induced by cecal ligation and puncture (CLP). Sepsis was induced in 6-10-week-old male C57/BL6 mice by CLP. We used immunohistochemistry (IHC) to define neuropathology in a mouse model of sepsis along with parallel studies using MRI, focusing on cerebral edema, blood-brain barrier (BBB) disruption, and microglial activation on days 1 and 4 days after CLP. We demonstrate that septic mice had evidence of early axonal injury, inflammation, and robust microglial activation on day 1 followed by cytotoxic edema on day 4 in the cortex, thalamus, and hippocampus in the absence of BBB disruption. We note the superiority of the MRI to detect subtle brain injury and cytotoxic cerebral edema in comparison with the traditional gold standard assessment, i.e., percent brain water (wet-dry weight method). We conclude that inflammatory changes in the septic brain can be detected in real time, and further studies are needed to understand axonal injury and the impact of inhibition of microglial activation on the development of cerebral edema.

Can neuroimmune mechanisms explain the link between ultraviolet light (UV) exposure and addictive behavior?

High ultraviolet (UV) light exposure on the skin acts as a reinforcing stimulus, increasing sun-seeking behavior and even addiction-like sun seeking behavior. However, the physiological mechanisms that underlie this process remain to be defined. Here, we propose a novel hypothesis that neuroimmune signaling, arising from inflammatory responses in UV-damaged skin cells, causes potentiated signaling within the cortico-mesolimbic pathway, leading to increased sun-seeking behaviors. This hypothesized UV-induced, skin-to-brain signaling depends upon cell stress signals, termed alarmins, reaching the circulation, thereby triggering the activation of innate immune receptors, such as toll-like receptors (TLRs). This innate immune response is hypothesized to occur both peripherally and centrally, with the downstream signaling from TLR activation affecting both the endogenous opioid system and the mesolimbic dopamine pathway. As both neurotransmitter systems play a key role in the development of addiction behaviors through their actions at key brain regions, such as the nucleus accumbens (NAc), we hypothesize a novel connection between UV-induced inflammation and the activation of pathways that contribute to the development of addiction. This paper is a review of the existing literature to examine the evidence which suggests that chronic sun tanning resembles a behavioral addiction and proposes a novel pathway by which persistent sun-seeking behavior could affect brain neurochemistry in a manner similar to that of repeated drug use.

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.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Attenuation of prostaglandin E2 elimination across the mouse blood-brain barrier in lipopolysaccharide-induced inflammation and additive inhibitory effect of cefmetazole

Background

Peripheral administration of lipopolysaccharide (LPS) induces inflammation and increases cerebral prostaglandin E₂ (PGE₂) concentration. PGE₂ is eliminated from brain across the blood-brain barrier (BBB) in mice, and this process is inhibited by intracerebral or intravenous pre-administration of anti-inflammatory drugs and antibiotics such as cefmetazole and cefazolin that inhibit multidrug resistance-associated protein 4 (Mrp4/Abcc4)-mediated PGE₂ transport. The purpose of this study was to examine the effect of LPS-induced inflammation on PGE₂ elimination from brain, and whether antibiotics further inhibit PGE₂ elimination in LPS-treated mice.

Methods

[³H]PGE₂ elimination across the BBB of intraperitoneally LPS-treated mice was assessed by the brain efflux index (BEI) method. Transporter protein amounts in brain capillaries were quantified by liquid chromatography-tandem mass spectrometry.

Results

The apparent elimination rate of [³H]PGE₂ from brain was lower by 87%, in LPS-treated mice compared with saline-treated mice. The Mrp4 protein amount was unchanged in brain capillaries of LPS-treated mice compared with saline-treated mice, while the protein amounts of organic anion transporter 3 (Oat3/Slc22a8) and organic anion transporting polypeptide 1a4 (Oatp1a4/Slco1a4) were decreased by 26% and 39%, respectively. Either intracerebral or intravenous pre-administration of cefmetazole further inhibited PGE₂ elimination in LPS-treated mice. However, intracerebral or intravenous pre-administration of cefazolin had little effect on PGE₂ elimination in LPS-treated mice, or in LPS-untreated mice given Oat3 and Oatp1a4 inhibitors. These results indicate that peripheral administration of cefmetazole inhibits PGE₂ elimination across the BBB in LPS-treated mice.

Conclusion

PGE₂ elimination across the BBB is attenuated in an LPS-induced mouse model of inflammation. Peripheral administration of cefmetazole further inhibits PGE₂ elimination in LPS-treated mice.

Sensory circumventricular organs in health and disease

Circumventricular organs (CVOs) are specialized brain structures located around the third and fourth ventricles. They differ from the rest of the brain parenchyma in that they are highly vascularised areas that lack a blood-brain barrier. These neurohaemal organs are classified as "sensory", when they contain neurons that can receive chemical inputs from the bloodstream. This review focuses on the sensory CVOs to describe their unique structure, and their functional roles in the maintenance of body fluid homeostasis and cardiovascular regulation, and in the generation of central acute immune and febrile responses. In doing so, the main neural connections to visceral regulatory centres such as the hypothalamus, the medulla oblongata and the endocrine hypothalamic-pituitary axis, as well as some of the relevant chemical substances involved, are described. The CVOs are vulnerable to circulating pathogens and can be portals for their entry in the brain. This review highlights recent investigations that show that the CVOs and related structures are involved in pathological conditions such as sepsis, stress, trypanosomiasis, autoimmune encephalitis, systemic amyloidosis and prion infections, while detailed information on their role in other neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis is lacking. It is concluded that studies of the CVOs and related structures may help in the early diagnosis and treatment of such disorders.

The impact of IL-1 modulation on the development of lipopolysaccharide-induced cognitive dysfunction

Introduction

The impact of pro-inflammatory cytokines on neuroinflammation and cognitive function after lipopolysaccharide (LPS) challenge remains elusive. Herein we provide evidence that there is a temporal correlation between high-mobility group box 1 (HMGB-1), microglial activation, and cognitive dysfunction. Disabling the interleukin (IL)-1 signaling pathway is sufficient to reduce inflammation and ameliorate the disability.

Methods

Endotoxemia was induced in wild-type and IL-1R^-/- mice by intra peritoneal injection of E. Coli LPS (1 mg/kg). Markers of inflammation were assessed both peripherally and centrally, and correlated to behavioral outcome using trace fear conditioning.

Results

Increase in plasma tumor necrosis factor-α (TNFα) peaked at 30 minutes after LPS challenge. Up-regulation of IL-1β, IL-6 and HMGB-1 was more persistent, with detectable levels up to day three. A 15-fold increase in IL-6 and a 6.5-fold increase in IL-1β mRNA at 6 hours post intervention (P < 0.001 respectively) was found in the hippocampus. Reactive microgliosis was observed both at days one and three, and was associated with elevated HMGB-1 and impaired memory retention (P < 0.005). Preemptive administration of IL-1 receptor antagonist (IL-1Ra) significantly reduced plasma cytokines and hippocampal microgliosis and ameliorated cognitive dysfunction without affecting HMGB-1 levels. Similar results were observed in LPS-challenged mice lacking the IL-1 receptor to those seen in LPS-challenged wild type mice treated with IL-1Ra.

Conclusions

These data suggest that by blocking IL-1 signaling, the inflammatory cascade to LPS is attenuated, thereby reducing microglial activation and preventing the behavioral abnormality.

Preoptic norepinephrine mediates the febrile response of guinea pigs to lipopolysaccharide

Norepinephrine (NE) microdialyzed in the preoptic area (POA) raises core temperature (Tc) via 1) α1-adrenoceptors (AR), quickly and independently of POA PGE2, and 2) α2-AR, after a delay and PGE2 dependently. Since systemic lipopolysaccharide (LPS) activates the central noradrenergic system, we investigated whether preoptic NE mediates LPS fever. We injected LPS (2 μg/kg iv) in guinea pigs prepared with intra-POA microdialysis probes and determined POA cerebrospinal (CSF) NE levels. We similarly microdialyzed prazosin (α1 blocker, 1 μg/μl), yohimbine (α2 blocker, 1 μg/μl), SC-560 [cyclooxygenase (COX)-1 blocker, 5 μg/μl], acetaminophen (presumptive COX-1v blocker, 5 μg/μl), or MK-0663 (COX-2 blocker, 0.5 μg/μl) in other animals before intravenous LPS and measured CSF PGE2. All of the agents were perfused at 2 μg/min for 6 h. Tc was monitored constantly. POA NE peaked within 30 min after LPS and then returned to baseline over the next 90 min. Tc increased within 12 min to a first peak at ∼60 min and to a second at ∼150 min and then declined over the following 2.5 h. POA PGE2 followed a concurrent course. Prazosin pretreatment eliminated the first Tc rise but not the second; PGE2 rose normally. Yohimbine pretreatment did not affect the first Tc rise, which continued unchanged for 6 h; the second rise, however, was absent, and PGE2 levels did not increase. SC-560 and acetaminophen did not alter the LPS-induced PGE2 and Tc rises; MK-0663 prevented both the late PGE2 and Tc rises. These results confirm that POA NE is pivotal in the development of LPS fever.

Brain distribution of cytokine mRNA induced by systemic administration of interleukin-1beta or tumor necrosis factor alpha

Brain cytokine mRNA levels are impacted by systemic cytokines. For example, systemic interleukin-1beta (IL1beta) increases brain IL1beta mRNA; subdiaphragmatic vagotomy blocks this effect. To localize which brain regions respond to intraperitoneal cytokines, we measured mRNA levels in selected brain regions for a variety of cytokines and growth factors, IL1beta, TNFalpha, interleukin-6 (IL-6), interleukin-10 (IL10), nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Relative to saline administration, IL1beta increased IL1beta, TNFalpha and IL6 mRNAs in the nucleus tractus solitarius (NTS), hypothalamus, hippocampus and somatosensory cortex (SSctx), but did not induce any changes in IL10. TNFalpha also increased TNFalpha and IL1beta mRNAs in the hypothalamus, hippocampus and SSctx. TNFalpha increased TNFalpha, IL1beta and IL10 mRNAs in the NTS, but did not induce any changes in IL-6 mRNA. In the amygdala, IL1beta enhanced IL6 mRNA and TNFalpha increased IL1beta mRNAs. In the insular cortex, IL1beta enhanced IL6 mRNA and TNFalpha increased IL1beta mRNA. TNFalpha administration increased NGF mRNA in the SSctx but decreased NGF and BDNF mRNA levels in the insular cortex. Both IL1beta and TNFalpha decreased BDNF mRNA in the amygdala. We also verified the IL1beta-induced increases in TNFalpha mRNA within the NTS using in situ hybridization. These results support the hypothesis that somnogenic doses of IL1beta and TNFalpha enhance their own mRNA levels as well as affect mRNA levels for other sleep-promoting substances.

Endotoxic fever: New concepts of its regulation suggest new approaches to its management

Endotoxic fever is regulated by endogenous factors that provide pro- and anti-pyretic signals at different points along the febrigenic pathway, from the periphery to the brain. Current evidence indicates that the febrile response to invading Gram-negative bacteria and their products is initiated upon their arrival in the liver via the circulation and their uptake by Kupffer cells (Kc). These pathogens activate the complement cascade on contact, hence generating complement component 5a. It, in turn, very rapidly stimulates Kc to release prostaglandin (PG)E2. Pyrogenic cytokines (TNF-alpha, etc.) are produced later and are no longer considered to be the immediate triggers of fever. The Kc-generated PGE2 either (1) may be transported by the bloodstream to the ventromedial preoptic-anterior hypothalamus (POA, the locus of the temperature-regulating center), presumptively diffusing into it and acting on thermoregulatory neurons; PGE2 is thus taken to be the final, central fever mediator. Or (2) it may activate hepatic vagal afferents projecting to the medulla oblongata, thence to the POA via the ventral noradrenergic bundle. Norepinephrine consequently secreted stimulates alpha1-adrenoceptors on thermoregulatory neurons, rapidly evoking an initial rise in core temperature (Tc) not associated with any change in POA PGE2; this neural, PGE2-independent signaling pathway is quicker than the blood-borne route. Elevated POA PGE2 and a secondary Tc rise occur later, consequent to alpha2 stimulation. Endogenous counter-regulatory factors are also elaborated peripherally and centrally at different points during the course of the febrile response; they are, therefore, anti-pyretic. These multiple interacting pathways are the subject of this review.

Cerebrovascular responses in the fetal sheep brain to low-dose endotoxin

Clinical and experimental evidence indicate that infection in pregnancy is associated with fetal brain damage. However, the inflammatory processes that compromise the fetal brain are not fully understood. In this study, we used a single, low dose of lipopolysaccharide (LPS, 0.1 microg/kg i.v.) to provoke an acute-phase response in unanesthetized fetal sheep in utero. COX-2 mRNA was increased in the cortex and cerebellum at 24 and 48 h after LPS, and immunoreactive COX-2 protein was increased in perivascular cells throughout gray and white matter at 24 h after LPS administration. Plasma albumin was observed in the parenchyma of the brain in cortex, thalamus, hypothalamus, corpus callosum, fornix, hippocampus, midbrain, subcallosal bundle, and cerebellar Purkinje cells. Large, rounded, lectin-positive cells with the appearance of macrophages were observed around blood vessels in subventricular white matter. These results indicate that blood-brain barrier permeability is increased in the fetal brain after exposure to endotoxin and suggests that cytotoxic and pro-inflammatory substances could pass from the circulation into the brain after peripheral inflammatory stimulation.

Preoptic alpha 1- and alpha 2-noradrenergic agonists induce, respectively, PGE2-independent and PGE2-dependent hyperthermic responses in guinea pigs

We have shown previously that norepinephrine (NE) microdialyzed into the preoptic area (POA) of conscious guinea pigs stimulates local PGE(2) release. To identify the cyclooxygenase (COX) isozyme that catalyzes the production of this PGE(2) and the adrenoceptor (AR) subtype that mediates this effect, we microdialyzed for 6 h NE, cirazoline (alpha(1)-AR agonist), and clonidine (alpha(2)-AR agonist) into the POA of conscious guinea pigs pretreated intrapreoptically (intra-POA) with SC-560 (COX-1 inhibitor) or nimesulide or MK-0663 (COX-2 inhibitors) and measured the animals' core temperature (T(c)) and intra-POA PGE(2) responses. Cirazoline induced T(c) rises promptly after the onset of its dialysis without altering PGE(2) levels. NE and clonidine caused early falls followed by late rises of T(c); intra-POA PGE(2) levels were closely correlated with this thermal course. COX-1 inhibition attenuated the clonidine-induced T(c) and PGE(2) falls but not the NE-elicited hyperthermia, but COX-2 inhibition suppressed both the clonidine- and NE-induced T(c) and PGE(2) rises. Coinfused cirazoline and clonidine reproduced the late T(c) rise of clonidine but not its early fall and also not the early rise produced by cirazoline; on the other hand, the PGE(2) responses were similar to those to NE. Prazosin (alpha(1)-AR antagonist) and yohimbine (alpha(2)-AR antagonist) blocked the effects of their respective agonists. These results indicate that alpha(1)- and alpha(2)-AR agonists microdialyzed into the POA of conscious guinea pigs evoke distinct T(c) responses: alpha(1)-AR activation produces quick, PGE(2)-independent T(c) rises, and alpha(2)-AR stimulation causes an early T(c) fall and a late, COX-2/PGE(2)-dependent T(c) rise.

Interleukin-6 is an afferent signal to the hypothalamo-pituitary-adrenal axis during local inflammation in mice

The cytokines IL-1 and IL-6 are able to induce prostaglandin (PG)-dependent activation of the hypothalamo-pituitary-adrenal axis (HPAA) and are thought to play key roles in immune-neuroendocrine interactions during inflammation. The present study shows that inflammation induced by im injection of turpentine (TPS) in the hind limb of mice causes an increase in the plasma concentration of IL-6, but not that of IL-1 alpha or IL-1 beta, together with a prolonged (>18-h) activation of the HPAA. IL-6 plays a causal role in the TPS-induced elevation in HPAA activity, because the sustained (8-18 h) increases in 1) plasma corticosterone, 2) plasma ACTH, and 3) induction of c-Fos in the hypothalamic paraventricular nucleus are all markedly blunted in IL-6-deficient (IL-6(-/-)) mice. Peripheral administration of a neutralizing IL-6 antiserum inhibited the plasma corticosterone response of normal (C57BL/6) mice to hind limb inflammation to an extent similar to that seen in IL-6(-/-) mice, suggesting that the IL-6 responsible for the increased HPAA activity is produced, or acts, on the blood side of the blood-brain barrier. We also show that IL-6 in the circulation is induced almost exclusively at the local inflammatory site, where IL-1 beta is produced. Induction of IL-6 and activation of the HPAA are dependent upon prior activation of an IL-1 type I receptor, as both are inhibited in type I IL-1 receptor-deficient mice. Furthermore, hind limb inflammation induced cyclooxygenase-2 protein expression around the cerebrovasculature of normal (IL-6(+/+)), but not IL-6(-/-), mice. Based on these data, we propose that IL-6 is produced at the local inflammatory site under the control of IL-1 beta and is the circulating afferent signal that is in part responsible for elevated HPAA activity, possibly acting via eicosanoid production within the cerebrovasculature.

Effects of glossopharyngeal nerve transection on central and peripheral cytokines and serum corticosterone induced by localized inflammation

Bilateral transection of the glossopharyngeal nerves (GLOx) disrupts the immune-to-brain communication from the posterior oral cavity. The current report tested whether this effect is due to the afferent (sensory) or efferent (parasympathetic motor) components of the nerve. Injection of lipopolysaccharide (LPS) into the soft palate (ISP) of GLOx or sham-operated (SHAM) rats increased the circulating levels of interleukin-1beta (IL-1beta), interleukin-1 receptor antagonist (IL-1ra) and corticosterone (CORT), as well the hypothalamic content of IL-1beta; no difference in circulating levels and hypothalamic content was found between GLOx and SHAM at 2 and 4.5 h after LPS injection. These results indicate that glossopharyngeal neural efferents do not mediate the effects of GLOx on the immune-to-brain communication.

Prostaglandin E(2)-synthesizing enzymes in fever: differential transcriptional regulation

The febrile response to lipopolysaccharide (LPS) consists of three phases (phases I-III), all requiring de novo synthesis of prostaglandin (PG) E(2). The major mechanism for activation of PGE(2)-synthesizing enzymes is transcriptional upregulation. The triphasic febrile response of Wistar-Kyoto rats to intravenous LPS (50 microg/kg) was studied. Using real-time RT-PCR, the expression of seven PGE(2)-synthesizing enzymes in the LPS-processing organs (liver and lungs) and the brain "febrigenic center" (hypothalamus) was quantified. Phase I involved transcriptional upregulation of the functionally coupled cyclooxygenase (COX)-2 and microsomal (m) PGE synthase (PGES) in the liver and lungs. Phase II entailed robust upregulation of all enzymes of the major inflammatory pathway, i.e., secretory (s) phospholipase (PL) A(2)-IIA --> COX-2 --> mPGES, in both the periphery and brain. Phase III was accompanied by the induction of cytosolic (c) PLA(2)-alpha in the hypothalamus, further upregulation of sPLA(2)-IIA and mPGES in the hypothalamus and liver, and a decrease in the expression of COX-1 and COX-2 in all tissues studied. Neither sPLA(2)-V nor cPGES was induced by LPS. The high magnitude of upregulation of mPGES and sPLA(2)-IIA (1,257-fold and 133-fold, respectively) makes these enzymes attractive targets for anti-inflammatory therapy.

Induction of microsomal prostaglandin E synthase in the rat brain endothelium and parenchyma in adjuvant-induced arthritis

Although central nervous symptoms such as hyperalgesia, fatigue, malaise, and anorexia constitute major problems in the treatment of patients suffering from chronic inflammatory disease, little has been known about the signaling mechanisms by which the brain is activated during such conditions. Here, in an animal model of rheumatoid arthritis, we show that microsomal prostaglandin E-synthase, the inducible terminal isomerase in the prostaglandin E(2)-synthesizing pathway, is expressed in endothelial cells along the blood-brain barrier and in the parenchyma of the paraventricular hypothalamic nucleus. The endothelial cells but not the paraventricular hypothalamic cells displayed a concomitant induction of cyclooxygenase-2 and expressed interleukin-1 type 1 receptors, which indicates that the induction is due to peripherally released cytokines. In contrast to cyclooxygenase-2, microsomal prostaglandin E synthase had very sparse constitutive expression, suggesting that it could be a target for developing drugs that will carry fewer side effects than the presently available cyclooxygenase inhibitors. These findings, thus, suggest that immune-to-brain communication during chronic inflammatory conditions involves prostaglandin E2-synthesis both along the blood-brain barrier and in the parenchyma of the hypothalamic paraventricular nucleus and point to novel avenues for the treatment of the brain-elicited disease symptoms during these conditions.

Potent analgesic and anti-inflammatory actions of a novel thymulin-related peptide in the rat

1. The present study examines the effect of PAT (peptide analogue of thymulin) in two rat models of inflammatory hyperalgesia induced by either i.pl. (1.25 microg in 50 microl saline) or i.p. (50 microg in 100 microl) injections of endotoxin ET. 2. Pretreatment with PAT (1, 5 or 25 microg in 100 microl saline, i.p.) decreased, in a dose dependent manner, both mechanical hyperalgesia, determined by the paw pressure (PP) test and thermal hyperalgesia determined by the hot plate (HP), the paw immersion (PI) and the tail flick (TF) tests. 3. Compared to the tripeptides K(D)PT and K(D)PV, known to antagonize interleukin (IL)-1beta or IL-1beta and PGE(2) mechanisms, PAT, at lower dosages, exerted stronger anti-hyperalgesic effects. 4. When compared with the effect of a steroidal (dexamethasone) and a non-steroidal (indomethacin) anti-inflammatory drugs (NSAID), PAT demonstrated equal analgesic actions. 5. Pretreatment with PAT, reduced significantly the increased concentration of IL-1beta, IL-6, TNF-alpha and NGF due to i.pl. injection of ET. 6. Injection of i.p. ET produced sickness behaviour characterized by hyperalgesia and fever. Pretreatment with PAT prevented the hyperalgesia and maintained the body temperature within the normal range and was accompanied by a down-regulation of the levels of pro-inflammatory cytokines and PGE(2) in the liver. 7. PAT, in all doses used, did not result in any evident changes in the physiological parameters or in the normal behaviour of the rats. 8. The anti-hyperalgesic and anti-inflammatory effects of PAT can be attributed, at least partially, to the down-regulation of pro-inflammatory mediators.

Involvement of postsynaptic EP4 and presynaptic EP3 receptors in actions of prostaglandin E2 in rat supraoptic neurones

We have reported that supraoptic nucleus (SON) neurones are excited by prostaglandin E2 (PGE2) presumably via dual postsynaptic PG receptors, FP receptors and unidentified EP receptors, and that presynaptic EP receptors may also be involved in the excitation. In the present study, to clarify the receptor mechanism of the PGE2-mediated actions on SON neurones, we studied the pre- and postsynaptic effects of four newly developed EP agonists that are selective for each of the four EP receptors, EP1-4, on rat SON neurones using extracellular recording and whole-cell patch-clamp techniques. The EP4 agonist ONO-AE1-329 mimicked the excitatory effects of PGE2, whereas the EP1 agonist ONO-DI-004, the EP2 agonist ONO-AE1-257 and the EP3 agonist ONO-AE-248 had little or no effect. The effects of ONO-AE1-329 were unaffected by the EP1/FP/TP antagonist, ONO-NT-012, which potently suppressed the excitation caused by the FP agonist fluprostenol and PGE2. ONO-AE1-329 caused marked excitation when responses to fluprostenol were desensitized by repeated applications of fluprostenol. Patch-clamp analysis in SON neurones showed that ONO-AE1-329 induced inward currents at a holding potential of -70 mV and the reversal potential of the currents was -35.1 +/- 2.3 mV. On the other hand, the frequency of spontaneous inhibitory postsynaptic currents recorded from SON slice preparations was suppressed by ONO-AE-248, but unaffected by the other three EP agonists. These results suggest that SON neurones possess postsynaptic EP4 receptors and that gamma-aminobutyric acid neurones innervating SON neurones possess presynaptic EP3 receptors in their terminals. Activation of the two EP receptors may be involved in the excitatory regulation of SON neurones by PGE2.

A perfusion technique for the determination of pro-inflammatory mediators induced by intradental application of irritants

INTRODUCTION

Several morphological and functional features contribute to the consideration of the tooth as a separate compartment having special type of innervation and special immune mechanisms. This study describes a new method allowing the intradental perfusion of rat incisors for the in vivo assessment of pulpal reaction to inflammatory agents.

METHODS

Under deep anesthesia, the distal 2-3 mm of each of the rat lower incisors was cut and wrapped in a polyethylene tubing connected to a perfusion chamber made of tigone tubing (ID 1/8 in., volume 100-150 microl). Several groups of rats (n=5 each) were used for intradental application of either saline, capsaicin (100 microg in 100 microl), or endotoxin (ET, 20 microg in 100 microl) for a period of 40 min followed by filling the tooth chamber with saline and collecting the perfusate every 40 min for a period of 8 h. The collected perfusates were stored at -70 degrees C for subsequent determination of the concentration of prostaglandin E(2) (PGE(2)) and nerve growth factor (NGF) by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Dentinal injury produced a moderate increase in the levels of NGF and PGE(2) in incisors perfused with saline. Application of ET or capsaicin, however, produced a highly significant increase in the levels of both mediators. These effects peaked at 1.5-3 h for PGE(2) and at 5 h for NGF. Capsaicin showed the most significant effects.

DISCUSSION

The reported results cannot be attributed to any factor other than the inflammation of the incisor's pulp, because the described chamber does not allow any spread or leak of the applied irritants. Further studies using other reagents can allow the determination of the variation of the levels of the various pro-inflammatory mediators and their modulation by treatment with anti-inflammatory drugs.

Coexpression of microsomal-type prostaglandin E synthase with cyclooxygenase-2 in brain endothelial cells of rats during endotoxin-induced fever

Fever is triggered by an elevation of prostaglandin E(2) (PGE(2)) in the brain. However, the mechanism of its elevation remains unanswered. We herein cloned the rat glutathione-dependent microsomal prostaglandin E synthase (mPGES), the terminal enzyme for PGE(2) biosynthesis, and examined its induction in the rat brain after intraperitoneal injection of pyrogen lipopolysaccharide (LPS). In Northern blot analysis, mPGES mRNA was weakly expressed in the brain under the normal conditions but was markedly induced between 2 and 4 hr after the LPS injection. In situ hybridization study revealed that LPS-induced mPGES mRNA signals were mainly associated with brain blood vessels, especially vein or venular-type ones, in the whole brain area. Immunohistochemical study demonstrated that mPGES-like immunoreactivity was expressed in the perinuclear region of brain endothelial cells, which were identified as von Willebrand factor-positive cells. Furthermore, in the perinuclear region of the endothelial cells, mPGES was colocalized with cyclooxygenase-2 (COX-2), which is the enzyme essential for the production of the mPGES substrate PGH(2). Inhibition of cyclooxygenase-2 activity resulted in suppression of both PGE(2) level in the CSF and fever (Cao et al., 1997), suggesting that the two enzymes were functionally linked and that this link is essential for fever. These results demonstrate that brain endothelial cells play an essential role in the PGE(2) production during fever by expressing COX-2 and mPGES.

The glossopharyngeal nerve as a novel pathway in immune-to-brain communication: relevance to neuroimmune surveillance of the oral cavity

Glossopharyngeal afferents may be the neural channel by which immune challenge of the posterior oral cavity conveys information to the brain. If this is the case, then bilateral transection of the glossopharyngeal nerves (GLOx) should disrupt this communication. Injection of lipopolysaccharide (LPS) or interleukin (IL)-1beta into the soft palate (ISP) of sham-operated rats induced a dose-related febrile response. GLOx significantly attenuated the febrile response induced by ISP injection of both LPS and IL-1beta. In contrast, GLOx did not affect the febrile response when LPS or IL-1beta were injected intraperitoneally, indicating that the effect of GLOx is not systemic. These results provide experimental evidence for a novel neural pathway for immune-to-brain communication.

Differential inhibition by nimesulide of the early and late phases of intravenous- and intracerebroventricular-LPS-induced fever in guinea pigs

OBJECTIVES

The findings that inducible cyclooxygenase (COX)-2, but not constitutive COX-1, is upregulated in the brain of conscious rats approximately 1.5 h after intraperitoneal pyrogen administration, that the systemic administration of COX-2 inhibitors abolishes fever, and that COX-2-deficient mice do not develop fever in response to intraperitoneal lipopolysaccharide (LPS) have strongly implicated COX-2 in the mediation of the febrile response. However, the biosynthesis of COX-2 is significantly slower than the onset of the fever produced by intravenously injected LPS. It consequently seems improbable that inducible COX-2 could play a role in the initiation of this febrile response, but a role for COX-1 has not yet been categorically ruled out; or, alternatively, a constitutive isoform of COX-2 could have such a role. We have studied, therefore, the effects of the non-selective COX inhibitor indomethacin, the COX-1-selective inhibitor SC-560, and the COX-2-selective inhibitor nimesulide on the characteristically biphasic fever induced by intravenous LPS in conscious guinea pigs; it has an onset latency of approximately 10 min.

METHODS

We injected the inhibitors 30 min before LPS, in various combinations of doses and routes; their respective vehicles were the control solutions. Core temperatures (T(c)) were monitored continuously, and plasma and brain PGE(2) levels were measured before and at 2-hour intervals after LPS administration.

RESULTS

Intraperitoneal indomethacin at 10 mg kg(-1) attenuated both phases of intravenous LPS (2 microg kg(-1)) fever, but the first more so than the second; at 50 mg kg(-1), it inhibited the febrile response completely. Intraperitoneal SC-560 (5 mg kg(-1)) did not affect the febrile response to intravenous LPS (2 microg kg(-1)). Intraperitoneal nimesulide (0.3, 1.0, and 3.0 mg kg(-1)) dose dependently attenuated intravenous LPS (0.1 and 2 microg kg(-1)) fever; the second phase of the biphasic T(c) rise was affected significantly more than the first. Intraperitoneal nimesulide also prevented the associated rises in plasma and brain PGE(2) levels. Intracerebroventricular LPS (150 ng kg(-1)) evoked a monophasic fever with a long onset latency ( approximately 30 min); it was accompanied by a rise in brain PGE(2) only, implying that the febrigenic PGE(2) was generated directly in the brain. This response, however, was completely abolished by intraperitoneal nimesulide (3 mg kg(-1)), indicating that nimesulide crosses the blood-brain barrier. Intracerebroventricular nimesulide at 0.3 mg kg(-1) prevented the rise in plasma PGE(2) after intravenous LPS (2 microg kg(-1)) and again attenuated the second febrile peak significantly more than the first.

CONCLUSIONS

COX-1 is not involved in intravenous LPS fever production, and COX-2 appears to play a greater role in the late than in the early phase of intravenous LPS fever in guinea pigs. The involvement of a constitutive COX-2 is inferred in the early phase.

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

The action of prostaglandin E(2) (PGE(2)) in the preoptic area is thought to play an important role in producing fever. Pharmacologic evidence suggests that, among the four subtypes of E-series prostaglandin (EP) receptors, i.e., EP(1), EP(2), EP(3), and EP(4), the EP(1) receptor mediates fever responses. In contrast, evidence from mice with EP receptor gene deletions indicates that the EP(3) receptor is required for the initial (<1 hour) fever after intravenous (i.v.) lipopolysaccharide (LPS). To investigate which subtypes of EP receptors mediate systemic infection-induced fever, we assessed the coexpression of Fos-like immunoreactivity (Fos-IR) and EP(1-4) receptor mRNA in nuclei in the rat hypothalamus that have been shown to be involved in fever responses. Two hours after the administration of i.v. LPS (5 microg/kg), Fos-IR was observed in the ventromedial preoptic nucleus, the median preoptic nucleus, and the paraventricular hypothalamic nucleus. In these nuclei, EP(4) receptor mRNA was strongly expressed and the Fos-IR intensely colocalized with EP(4) receptor mRNA. Strong EP(3) receptor mRNA expression was only seen within the median preoptic nucleus but Fos-IR showed little coexpression with EP(3) receptor mRNA. EP(2) receptor mRNA was not seen in the PGE(2) sensitive parts of the preoptic area. Although approximately half of the Fos-immunoreactive neurons also expressed EP(1) receptor mRNA, EP(1) mRNA expression was weak and its distribution was so diffuse in the preoptic area that it did not represent a specific relationship. In the paraventricular nucleus, EP(4) mRNA was found in most Fos-immunoreactive neurons and levels of EP(4) receptor expression increased after i.v. LPS. Our findings indicate that neurons expressing EP(4) receptor are activated during LPS-induced fever and suggest the involvement of EP(4) receptors in the production of fever.

Pyrogen sensing and signaling: old views and new concepts

Fever is thought to be caused by endogenous pyrogenic cytokines, which are elaborated and released into the circulation by systemic mononuclear phagocytes that are activated by exogenous inflammatory agents and transported to the preoptic-anterior hypothalamic area (POA) of the brain, where they act. Prostaglandin (PG) E2 is thought to be an essential, proximal mediator in the POA, and induced by these cytokines. It seems unlikely, however, that these factors could directly account for early production of PGE2 following the intravenous administration of bacterial endotoxic lipopolysaccharides (LPS), because PGE2 is generated before the cytokines that induce it are detectable in the blood and the before cyclooxygenase-2, the synthase that they stimulate, is expressed. Hence other, more quickly evoked mediators are presumed to be involved in initiating the febrile response; moreover, their message may be conveyed to the brain by a neural rather than a humoral pathway. This article reviews current conceptions of pyrogen signalling from the periphery to the brain and presents new, developing hypotheses about the mechanism by which LPS initiates fever.

Anorexia of infection: current prospects

The anorexia of infection is part of the host's acute phase response (APR). Despite being beneficial in the beginning, long lasting anorexia delays recovery and is ultimately deleterious. Microbial products such as bacterial cell wall compounds (e.g., lipopolysaccharides and peptidoglycans), microbial nucleic acids (e. g., bacterial DNA and viral double-stranded RNA), and viral glycoproteins trigger the APR and presumably also the anorexia during infections. Microbial products stimulate the production of proinflammatory cytokines (e.g., interleukins [ILs], tumor necrosis factor-alpha, interferons), which serve as endogenous mediators. Several microbial products and cytokines reduce food intake after parenteral administration, suggesting a role of these substances in the anorexia during infection. Microbial products are mainly released and cytokines are produced in the periphery during most infections; they might inhibit feeding through neural and humoral pathways activated by their peripheral actions. Activation of peripheral afferents by locally produced cytokines is involved in several cytokine effects, but is not crucial for the anorectic effect of microbial products and IL-1beta. Cytokines increase leptin expression in the adipose tissue, and leptin may contribute to, but is also not essential for, the anorectic effects of microbial products and cytokines. In addition, a direct action of cytokines and microbial products on the central nervous system (CNS) is presumably involved in the anorexia during infection. Cytokines can reach CNS receptors through circumventricular organs and through active or passive transport mechanisms or they can act through receptors on endothelial cells of the brain vasculature and stimulate the release of subsequent mediators such as eicosanoids. De novo CNS cytokine synthesis occurs in response to peripheral infections, but its role in the accompanying anorexia is still open to discussion. Central mediators of the anorexia during infection appear to be neurochemicals involved in the normal control of feeding, such as serotonin, dopamine, histamine, corticotropin releasing factor, neuropeptide Y, and alpha-melanocyte-stimulating hormone. Reciprocal, synergistic, and antagonistic interactions between various pleiotropic cytokines, and between cytokines and neurochemicals, form a complex network that mediates the anorexia during infection. Current knowledge on the mechanisms involved suggests some therapeutic options for treatment. Substances that block common key steps in cytokine synthesis or cytokine action, or inhibitors of eicosanoid synthesis, may hold more promise than attempts to antagonize specific cytokines. To target the neurochemical mediation of the anorexia during infection may be even more efficient. Future research should address these neurochemical mechanisms and the cytokine actions at the blood-brain barrier. Further unanswered questions concern the modulation of the anorexia during infection by gender and nutritional state.

Effects of the inhibition of cyclo-oxygenase 1 or 2 or 5-lipoxygenase on the activation of the hypothalamic-pituitary-adrenal axis induced by interleukin-1beta in the male Rat

The limited entry of interleukin-1beta (IL-1beta) into the central nervous system has led to the hypothesis that IL-1beta acts, through IL-1beta receptors located notably on endothelial cells, on the release of prostaglandins which in turn stimulate the hypothalamic-pituitary-adrenal (HPA) axis. We used cyclo-oxygenase-1 (COX-1) and cyclo-oxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX) inhibitors, before the injection of IL-1beta, to explore the role of arachidonic acid metabolic pathways on HPA axis activation. Adult male rats were i.m injected 20 min before i.p injection of IL-1beta, with (i): a COX-1/COX-2 inhibitor (ketoprofen); (ii) a COX-2 selective inhibitor (NS 398); or (iii) a 5-LOX inhibitor (BW A4C). Following this, rats were killed 90 min after i.p. IL-1beta injection and analysis for plasma adrenocorticotropic hormone (ACTH) and corticosterone concentrations and determination of anterior pituitary pro-opio melanocortin (POMC) gene transcription was conducted. Administration of the COX-1/COX-2 inhibitor led to a complete blockage of ACTH and corticosterone secretion and POMC gene transcription. The COX-2 inhibitor led to a complete blockade of ACTH secretion and POMC gene transcription but had no effect on corticosterone secretion. The 5-LOX inhibitor had no significant effect on any parameter. These results demonstrate the crucial role of eicosanoid pathways in mediating the stimulation of the HPA axis induced by IL-1beta. Moreover, we found a clear dissociation of the effect of the blockage of COXs upon ACTH and corticosterone secretion, suggesting that IL-1beta may act at the brain as well as at the adrenal cortex to stimulate the secretion of corticosterone.

Circuits and mechanisms governing hypothalamic responses to stress: a tale of two paradigms

The results of recent studies support a partitioning of stress models into at least two basic classes. While these have been referred to as 'systemic' and 'neurogenic', we would suggest that the terms interoceptive and exteroceptive, respectively, are more apt descriptors. This is based on the similarities in the overall patterns of activational responses seen as a consequence of exposure to a range of perturbations in the internal versus external environments. While stressors of each class may share in common such fundamental features as a capacity to enlist certain PVH effector populations and medullary catecholamine-containing neurons, both the capacity to involve specific output neuron classes and the dependence of hypothalamic effects on the integrity of aminergic afferents in at least some interoceptive and exteroceptive models, are clearly differential. The available evidence suggests that interoceptive stress effects on PVH effector populations may be conceived essentially as simple reflex responses, mediated at a subcortical level by cell groups and associated circumventricular organs that comprise the core of a system involved in the processing of visceral sensory information. Based on the general pattern of acute footshock-induced Fos expression and commonalities of cellular activation profiles seen in this and other acute exteroceptive paradigms, it seems a reasonable assumption that pathways that convey somatosensory/nociceptive information to the PVH are apt to mediate adaptive visceromotor responses in these models. Multiple candidates for such roles have been identified at various levels of what may be viewed as the ascent of the spinothalamic pathway through the brainstem and thalamus, and on through the limbic forebrain and hypothalamus. Dissecting the relative contributions of these in determining PVH output will speak to important conceptual issues concerning the extent to which the affective and visceromotor responses to exteroceptive stressors are organized, and the level(s) at which these different avenues of emotional expression may be integrated.

Prostaglandin E2 inhibits spontaneous inhibitory postsynaptic currents in rat supraoptic neurones via presynaptic EP receptors

Prostaglandin E2 (PGE2) has been implicated in the excitatory regulation of magnocellular neurones in the supraoptic nucleus (SON). We have recently reported that PGE2 excited SON neurones by directly activating postsynaptic PGE2 receptors (EP receptors) of a subclass other than EP1-3, but did not affect excitatory postsynaptic currents (EPSCs). In the present study, we examined presynaptic effects of PGE2 on rat SON neurones by measuring spontaneous inhibitory postsynaptic currents (IPSCs) by a slice patch-clamp technique. PGE2 inhibited spontaneous IPSCs in a dose-dependent and reversible manner. PGE2 selectively suppressed the frequency of IPSCs without affecting the amplitude of IPSCs in the presence of tetrodotoxin, a blocker of Na+ channels, indicating that the effects were presynaptic. The inhibitory effects of PGE2 on the frequency of IPSCs were mimicked by the EP1/EP3 agonists, 17PT-PGE2 and sulprostone, and the EP2/EP3 agonist, misoprostol, whereas the EP2 agonist, butaprost, or the FP agonist, fluprostenol, had little effect. The effects of PGE2 on IPSCs were unaffected by the selective EP1 antagonist, SC-51322. They were unaffected also by antagonists of GABAB and alpha2 adrenergic receptors, which are present at presynaptic terminals of GABA neurones in the SON and cause suppression of spontaneous IPSCs. The inhibitor of PG synthesis, indomethacin, had little effect on spontaneous IPSCs and on the inhibitory effects of PGE2 as well as of the GABAB agonist, baclofen, and noradrenaline. These results suggest that PGE2 inhibits release of GABA from the GABAergic terminals innervating SON neurones by activating presynaptic EP receptors, presumably of the EP3 subclass, and that such a presynaptic mechanism may play a role in the excitatory regulation of SON neurones by PGE2.

Interleukins and tumor necrosis factor as inhibitors of food intake

Cytokines, such as interleukins and tumor necrosis factor-alpha (TNFalpha), are produced in response to immune stimulation and have systemic effects, mediated by the central nervous system (CNS). Interleukins, in particular interleukin [IL]-1beta, and TNFalpha reduce food intake after peripheral and central administration, suggesting that they contribute to the anorexia during various infectious, neoplastic and autoimmune diseases. Because cytokines are mainly produced in the periphery during most of these diseases, IL-1beta and TNFalpha may inhibit feeding indirectly through neural and humoral pathways activated by their peripheral actions. Activation of afferent nerve fibers by locally produced cytokines in the periphery is involved in several cytokine effects, but is not crucial for the anorectic effect of systemic immune stimulation. Cytokines increase OB protein (leptin) expression in the adipose tissue, and leptin may contribute to, but is also not essential for, the anorectic effects of cytokines. Finally, circulating IL-1beta and TNFalpha may act directly on the brain or cytokine synthesis in the brain may contribute to the anorectic effect of systemic immune stimulation. Central mediators of the anorectic effects of cytokines appear to be neurochemicals involved in the normal control of feeding, such as serotonin, corticotropin releasing factor, histamine, alpha-melanocyte stimulating hormone, and neuropeptide Y. The well-documented cytokine production in the gut in relation to feeding and the expression of TNFalpha by adipocytes suggest that IL-1beta and TNFalpha may also play a role in the control of normal feeding and energy balance. All in all, reciprocal, synergistic and antagonistic interactions between various pleiotropic cytokines and between cytokines and neurochemicals form a complex network that mediates the effects of cytokines on feeding and energy balance.

Localization of cytosolic phospholipase A2 messenger RNA mainly in neurons in the rat brain

Ca2(+)-sensitive 85,000 mol. wt cytosolic phospholipase A2 plays an essential role in the selective and stimulus-dependent release of arachidonic acid from membrane phospholipids. Cytosolic phospholipase A2-catalysed lipid mediators including arachidonic acid and its metabolites have been suggested to be involved in a variety of neuronal functions in the CNS. Since the cellular localization of cytosolic phospholipase A2 is still controversial and obscure, we tried an improved method of rapid processing of each specimens and succeeded in obtaining intense signals of cytosolic phospholipase A2 messenger RNA in the normal rat brain by northern blot analysis and in situ hybridization. Northern blot analysis showed the abundant distribution of cytosolic phospholipase A2 messenger RNA in most regions of the brain, with intense signals observed in the pineal gland and pons. Macroautoradiographs prepared after in situ hybridization with three different antisense riboprobes gave essentially similar patterns of localization; significant signals were widely detected in the gray matter of various regions, i.e. the olfactory bulb, cerebral cortex, hippocampus, amygdala, several thalamic and hypothalamic nuclei and cerebellum. Microautoradiographs showed that most of the intense signals were predominant in neurons, and that faint signals were from glial cells and other non-neuronal cells in the choroid plexus, inner surface cells of veins and the leptomeninges. In addition, the cycloheximide treatment increased the cytosolic phospholipase A2 messenger RNA level in the same cell populations originally possessing messenger RNA signals. Predominant expression of cytosolic phospholipase A2 messenger RNA in neurons may provide the basis for the contribution of cytosolic phospholipase A2-catalysed lipid mediators to a variety of neurotransmission and synaptic functions in the CNS.

The febrile response to lipopolysaccharide is blocked in cyclooxygenase-2(-/-), but not in cyclooxygenase-1(-/-) mice

Various lines of evidence have implicated inducible cyclooxygenase-2 (COX-2) in fever production. Thus, its expression is selectively enhanced in brain after peripheral exogenous (e.g., lipopolysaccharide [LPS]) or endogenous (e.g., interleukin-1) pyrogen administration, while selective COX-2 inhibitors suppress the fever induced by these pyrogens. In this study, we assessed the febrile response to LPS of congenitally constitutive COX-1 (COX-1-/-) and COX-2 (COX-2-/-)-deficient C57BL/6J-derived mice. COX-1+/- and COX-2+/- mice were also evaluated; controls were wild-type C57BL/6J mice (Jackson Labs.). All the animals were pretrained daily for two weeks to the experimental procedures. LPS was injected intraperitoneally at 1 microgram/mouse; pyrogen-free saline (PFS) was the vehicle and control solution. Core temperatures (Tcs) were recorded using thermocouples inserted 2 cm into the colon. The presence of the COX isoforms was determined in cerebral blood vessels immunocytochemically after the experiments, without knowledge of the functional results. The data showed that the wild-type, COX-1+/-, and COX-1-/- mice all responded to LPS with a 1 degrees C rise in Tc within 1 h; the fever gradually abated over the next 4 h. By contrast, COX-2+/- and COX-2-/- mice displayed no Tc rise after LPS. PFS did not affect the Tc of any animal. It would appear therefore that COX-2 is necessary for LPS-induced fever production.

Lipopolysaccharide injected into the cerebral ventricle evokes fever through induction of cyclooxygenase-2 in brain endothelial cells

Activation of the arachidonic acid cascade is an essential step for the development of fever during brain inflammation. We investigated the brain sites where this activation takes place by use of a rat model of brain inflammation. Intracerebroventricular administration of lipopolysaccharide but not of its vehicle evoked fever. The fever was markedly suppressed when the rats had been pretreated with a cyclooxygenase-2-specific inhibitor. In situ hybridization and immunohistochemical studies revealed that cyclooxygenase-2 mRNA and its protein were induced by lipopolysaccharide in blood vessels near the cerebral ventricles and in those in the subarachnoidal space. Double immunohistochemical staining revealed that these cyclooxygenase-2-positive cells were mostly endothelial cells. The time course of fever and that of cyclooxygenase-2 induction in the endothelial cells were in parallel. Cyclooxygenase-2 mRNA in a certain type of telencephalic neurons was also upregulated by the intracerebroventricular administration, but this neuronal response occurred both in vehicle-injected rats and in lipopolysaccharide-injected ones to the same extent. Therefore, the neuronal response was not essential to the development of fever. These results suggest that brain endothelial cells play a crucial role in the development of fever during brain inflammation by activating their arachidonic acid cascade.

Behavioral and neurochemical consequences of lipopolysaccharide in mice: anxiogenic-like effects

Systemic administration of lipopolysaccharide (LPS) induces sickness behaviors, as well as alterations of hypothalamic-pituitary-adrenal functioning commonly associated with stressors. In the present investigation, it was demonstrated that systemic LPS treatment induced a sickness-like behavioral profile (reduced active behaviors, soporific effects, piloerection, ptosis), which appeared to be dependent upon the novelty of the environmental context in which animals were tested. As well, LPS induced anxiogenic-like responses, including decreased time spent in the illuminated portion of a light-dark box, reduced open-arm entries in a plus-maze test, and decreased contact with a novel stimulus object in an open-field situation. The behavioral changes were accompanied by increased plasma ACTH and corticosterone levels. As well, LPS induced increased turnover of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the paraventricular nucleus (PVN), median eminence plus arcuate nucleus, hippocampus, as well as NE turnover within the locus coeruleus and DA turnover within the nucleus accumbens. Although these neurochemical variations were reminiscent of those elicited by stressors, LPS was not particularly effective in modifying DA activity within the prefrontal cortex or NE within the amygdala, variations readily induced by stressors. Whether the LPS-induced anxiogenic-like responses were secondary to the illness engendered by the endotoxin remains to be determined. Nevertheless, it ought to be considered that bacterial endotoxin challenge, and the ensuing cytokine changes, may contribute to emotionality and perhaps even anxiety-related behavioral disturbances.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Actions of prostaglandin E2 on rat supraoptic neurones

Prostaglandins (PGs) have been implicated in the regulation of vasopressin (VP) and oxytocin (OT) release in response to various stimuli. To examine the site and mechanism of actions of PGs, we studied effects of PGE2 and PG-receptor agonists on supraoptic nucleus (SON) neurones of rat hypothalamic slice preparations using extracellular recording and whole-cell patch-clamp techniques. PGE2 modulated the electrical activity of more than 80% of the neurones studied. The effects of PGE2 on both phasic and non-phasic neurones were mostly excitatory, and dose-dependent. The effects of PGE2 were mimicked by PGF2alpha or the FP agonist, fluprostenol, whereas PGD2 or the selective EP, IP or TP agonist was less effective or had no effect. The effects of PGE2 were unaffected by the EP1 antagonist, SC-51322, but reduced to 80% of control by the EP1/FP/TP antagonist, ONO-NT-012, which reduced the effects of fluprostenol to 32% of control. Moreover, some neurones responsive to PGE2 did not respond to fluprostenol. Patch-clamp analysis in SON slice preparations revealed that PGE2 at 10(-6) M depolarized the membrane potential by 3.9+/-0.3 mV from the resting membrane potential of -58.4+/-2.2 mV in the current-clamp mode. In the voltage-clamp mode, PGE2 induced inward currents at a holding potential of -70 or -80 mV, while it did not affect spontaneous excitatory postsynaptic currents. PGE2 induced currents also in dissociated SON neurones and the reversal potential of the currents was -35.5+/-0.9 mV, which was similar to that of currents induced by fluprostenol. These results suggest that SON neurones possess at least two types of PG receptors, FP receptors and EP receptors of a subclass different from EP1, EP2, or EP3, and that activation of these receptors leads to the opening of nonselective cation channels, membrane depolarization and increase of the action potential discharge.

Individual variation in hypothalamus-pituitary-adrenal responsiveness of rats to endotoxin and interleukin-1 beta

Intraperitoneal (i.p.) administration of lipopolysaccharide (LPS) or interleukin (IL)-1 beta induces activation of the hypothalamus-pituitary-adrenal (HPA) axis. In some experiments, a marked individual variation has been observed in HPA responses to these stimuli. We reasoned that only parameters that correlate with this variability may reflect signals involved in HPA activation. Although IL-1 beta is found in the peritoneal cavity and has been implicated in the HPA response to i.p. LPS, IL-1 beta levels in peritoneal lavage fluid did not correlate with the variation in HPA responsiveness and neither did IL-1 beta concentrations in plasma. In contrast, IL-6 concentrations in plasma, but not in peritoneal lavage fluid, correlated with this variation to i.p. LPS or IL-1 beta. We conclude that IL-6 in the plasma represents a major determinant of the individual variation in HPA responses to i.p. LPS or IL-1 beta. Because of its positive correlation with Fos expression in various brain-stem nuclei, we suggest that circulating IL-6 may facilitate the generation of signals in vagal afferents or potentiate vagal information transfer to lower brain-stem nuclei.

Afferent pathways of pyrogen signaling

We and others recently showed that fever induced by intravenously or intraperitoneally injected lipopolysaccharide (LPS) may involve brain signaling via hepatic vagal afferents. This suggests that LPS fever may be initiated by mediators released mainly by cells in the liver, presumably macrophages (Kupffer cells, Kc). To verify this possibility, we disabled the Kc of conscious guinea pigs with gadolinium chloride and monitored their core temperature and associated preoptic prostaglandin E2 (PGE2) responses to i.v. LPS. Gadolinium chloride pretreatment significantly attenuated both the febrile and PGE2 rises, thus supporting the hypothesis. Additionally, fluorescein-labeled LPS was detected in Kc 15 minutes after its i.v. administration. Paradoxically, however, the label was also present in gadolinium chloride-pretreated guinea pigs. Thus, either Kc are not the primary source of pyrogenic mediators or LPS does not provide the stimulus for their production. Because the i.v. injection of LPS elicits virtually immediately the production of complement fragments, and Kc express their receptors and produce various mediators on their activation, we hypocomplemented guinea pigs with cobra venom factor. The core temperature rises produced by i.v. LPS were reduced by complement depletions > 60%. LPS i.v. per se decreased complement, that is, complement was consumed by 12% within 10 minutes. Thus, the onset of LPS fever may involve complement system and Kc activation, but their precise roles await clarification.

Brain endothelial cells express cyclooxygenase-2 during lipopolysaccharide-induced fever: light and electron microscopic immunocytochemical studies

Cyclooxygenase-2 (COX-2), a key enzyme in the biosynthesis of prostaglandins, is induced in brain blood vessels by pyrogens, and its essential role in fever has been hypothesized. In this study, we determined (1) the type of cells that express cyclooxygenase-2 in brain blood vessels of lipopolysaccharide-treated rats, and (2) the precise relationship between the time course of fever and that of cyclooxygenase-2 protein expression in these cells. Five hours after the lipopolysaccharide injection (100 microg/kg, i.p.), cyclooxygenase-2-like immunoreactive cells were found in the parenchymal and subarachnoidal blood vessels. In these blood vessels, the cyclooxygenase-2-like immunoreactivity was restricted to the perinuclear region of the endothelial cells as revealed by a laser confocal microscopy, double-immunofluorescence staining with an endothelial marker, and immunoelectron microscopy. On the other hand, the cyclooxygenase-2-like immunoreactive cells were distinct from microglia or perivascular/meningeal macrophages as revealed by double immunostaining with macrophage/microglia-specific antibodies. Cyclooxygenase-2-like immunoreactive cells were first found at 1.5 hr after the lipopolysaccharide injection, at which time the fever had not been developed. After that, the number of cyclooxygenase-2-like immunoreactive cells and fever followed a similar time course, both being highest at 5 hr after the lipopolysaccharide injection and both returning to the baseline by 24 hr. These results demonstrate that brain endothelial cells are the primary sites where the activation of arachidonic acid cascade takes place during fever after intraperitoneal injection of lipopolysaccharide.

Thermogenic and corticosterone responses to intravenous cytokines (IL-1beta and TNF-alpha) are attenuated by subdiaphragmatic vagotomy

The brain orchestrates changes in behavior and physiology as a consequence of peripheral immune activation and infection. These changes require that the brain receives signals from the periphery that an immunological challenge has occurred. Previous research has established that cytokines play a role in signalling the brain. What remains unclear, however, is how peripheral cytokines signal the central nervous system. A recent proposal is that cytokines signal the brain by stimulating peripheral nerves. The hypothesis states that following infection and the release of cytokines such as IL-1beta into local tissue or microvasculature, IL-1beta stimulates IL-1 receptors on vagal afferent terminals, or more likely on cells of vagal paraganglia. Vagal afferents, in turn, signal the brain. Previous work has demonstrated that transection of the vagus below the level of the diaphragm blocks or attenuates many illness consequences of intraperitoneally (i.p.) administered lipopolysaccharide (LPS) or IL-1beta. The present studies extend these findings by examining the effect of subdiaphragmatic vagotomy on illness consequences following intravenously (i.v.) administered IL-1beta and TNF-alpha. Subdiaphragmatic vagotomy attenuated both the fever response and corticosterone response produced by i.v. administered cytokines. This effect was dose dependent. The results add support to the hypothesis that vagal afferents are involved in peripheral cytokine-to-brain communication.

Prostaglandin E2 and 4-aminopyridine prevent the lipopolysaccharide-induced outwardly rectifying potassium current and interleukin-1beta production in cultured rat microglia

Brain inflammation includes microglial activation and enhanced production of diffusible chemical mediators, including prostaglandin E2. Prostaglandin E2 is generally considered a proinflammatory molecule, but it also promotes neuronal survival and down-regulates some aspects of microglial activation. It remains unknown, however, if and how prostaglandin E2 prevents microglial activation. In primary culture, microglial activation is predicted by a characteristic pattern of whole-cell potassium currents and interleukin-1beta production. We investigated if prostaglandin E2 could alter these currents and, if so, whether these currents are necessary for microglial activation. Microglia were isolated from mixed cell cultures prepared from neonatal rat brains and exposed to 0-10 microM prostaglandin E2 and lipopolysaccharide for 24 h. Currents were elicited by using standard patch-clamp technique, and interleukin-1beta production was measured by ELISA. Peak outward current densities in microglia treated with lipopolysaccharide plus prostaglandin E2 (10 nM) were reduced significantly from those of cells treated with lipopolysaccharide alone. Prostaglandin E2 and 4-aminopyridine (a blocker of outward potassium currents) also significantly reduced interleukin-1beta production. Thus, although prostaglandin E2 is classified generally as a proinflammatory chemical, it has complex roles in brain inflammation that include preventing microglial activation, perhaps by reducing the outward potassium current.

Cyclooxygenase-2 is induced in brain blood vessels during fever evoked by peripheral or central administration of tumor necrosis factor

Cyclooxygenase-2 (COX-2) is an inducible type of enzyme that is involved in prostaglandin biosynthesis. In the present study, we examined whether or not COX-2 is involved in fever that is induced by tumor necrosis factor-alpha (TNF-alpha) and, if so, where in the brain COX-2 is induced by this factor. Intraperitoneal (i.p.) injection of TNF-alpha into rats evoked a fever that started 1 h after the TNF injection, peaked 3 h after the injection, and then gradually declined. The fever was suppressed by pretreatment with a COX-2-specific inhibitor. With a time course similar to that of fever, COX-2 mRNA was induced in brain blood vessels. On the other hand, in some of the telencephalic neurons, COX-2 mRNA was constitutively expressed under the normal condition; but its level gradually decreased during the course of fever. Fever was also evoked by an intracerebroventricular (i.c.v.) injection of TNF-alpha. This febrile response was also suppressed by a COX-2 specific inhibitor and was associated with the induction of COX-2 mRNA in the brain blood vessels. On the other hand, the telencephalic neurons did not show consistent change in COX-2 mRNA level after i.c.v. injection of TNF-alpha or saline. COX-2-like immunoreactivity was found in some cells of the brain blood vessels 3 h after the TNF-alpha injection by either i.p. or i.c.v. route. Most of the COX-2-like immunoreactive cells were endothelial cells since COX-2-like immunoreactivity was colocalized with von Willebrand factor, an endothelial cell marker, in the same cells. These results suggest that the brain blood vessels are the major sites where TNF-alpha enhances PG biosynthesis after peripheral as well as after central injection, and provides further evidence supporting the hypothesis that COX-2 induced in the brain blood vessels is involved in fever.

The role of the vagus nerve in cytokine-to-brain communication

Peripheral interleukin-1 beta (IL-beta) and inflammatory stimuli that induce the synthesis and release of IL-1 beta produce a variety of central nervous system responses. Most proposals designed to explain how peripheral IL-1 beta influences the CNS have focused on blood-borne routes of communication. We will review data that indicate that at least some of the CNS response to peripheral IL-1 beta are instead mediated by a neural route of communication between the periphery and the CNS. IL-1 beta activates afferent vagal fibers that terminate in the nucleus tractus solitarius, and communication via the vagus is responsible for much of the hyperalgesia, fever, anorexia, taste aversions, increased levels of plasma corticosteroid, and brain norepinephrine changes produced by intraperitoneal injections of IL-1 beta and LPS. Data extending this analysis to TNF-alpha and intravenous routes will be described.