Differential sensitivity in the sites of fever production by prostaglandin E1 within the hypothalamus of the rat

Network pharmacology integrated with molecular docking reveals the common experiment-validated antipyretic mechanism of bitter-cold herbs

ETHNOPHARMACOLOGICAL RELEVANCE

Bitter-cold herbs have been used to clearing heat and expelling damp in clinical practice in China for thousands of years.

AIM OF THE STUDY

This study aimed to investigate the common molecular mechanism of bitter-cold herbs through network pharmacology analysis, molecular docking and experimental validation in vivo.

MATERIALS AND METHODS

Network pharmacological analysis integrated with molecular docking was employed to identify the active compounds and core action targets of the bitter-cold herbs. Then, the yeast-induced pathological model was established, and the antipyretic effect of the herbs was evaluated by checking rectal temperatures of the mice hourly. Lastly, the protein expression of core targets was examined to reveal the antipyretic mechanism.

RESULTS

A total of 52 lead compounds from the four bitter-cold herbs, Phellodendri Chinensis Cortex (PCC), Sophorae Flavescentis Radix (SFR), Gentianae Radix Et Rhozima (GRER) and Coptidis Rhizoma (CR), and 248 compounds-related targets were screened out with PTGS2 ranking the first. The results from molecular docking showed that 22 compounds adopted the same orientation as aspirin and had an excellent stability in the active site pocket of PTGS2. Furthermore, these herbs exerted potential therapeutic effects through 38 related pathways. On the other hand, the outcome of animal experiments showed that they could significantly attenuate the yeast-induced mice fever with dose-dependent relationship. Further experimental results demonstrated that administration of yeast suspension raised protein expression of PTGS2 significantly, which was evidently inhibited in the high or low-dose groups of GRER as well as in the low-dose group of SFR (P < 0.01) though a higher expression of PTGS2 was shown in the low-dose group of CR compared with FM group (P < 0.01).

CONCLUSIONS

The bitter-cold herbs can alleviate fever response and their antipyretic effect may mainly be attributed to regulating the expression of PTGS2 after the formation of ligand-receptor/PTGS2 complexes, and their active compounds might be nominated as antipyretic lead-ligand candidates.

Efferent thermoregulatory pathways regulating cutaneous blood flow and sweating

Cutaneous vasoconstrictor nerves regulate heat retention, and are activated by falls in skin or core temperature. The efferent pathways controlling this process originate within the preoptic area. A descending GABAergic pathway, activated by warm skin or core, indirectly inhibits sympathetic premotor neurons in the medullary raphé. Those premotor neurons drive cutaneous vasoconstriction via excitatory glutamatergic and serotonergic connections to spinal preganglionic neurons. Cold skin and/or cold core temperatures activate a direct preoptic-to-raphé excitatory pathway. The balance of inhibitory and excitatory influences reaching the medullary raphé determines cutaneous blood flow. During fever, prostaglandin E₂ inhibits preoptic GABAergic neurons, resulting in disinhibition of the excitatory preoptic-to-raphé pathway, and hence, cutaneous vasoconstriction. A weaker, parallel source of descending excitatory drive reaches cutaneous preganglionic neurons from the rostral ventrolateral medulla. Sweating follows local heating of the preoptic area in cats and monkeys, and heated humans show sweating-related activation of this same region in functional magnetic resonance imaging (fMRI) studies. A descending pathway that drives sweating has been traced in cats from the hypothalamus to putative premotor neurons in the parafacial region at the pontomedullary junction. The homologous parafacial region in humans also shows sweating-related activation in fMRI studies. The central pathways that drive active vasodilatation in human nonacral skin remain unknown.

Control of the Cutaneous Circulation by the Central Nervous System

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

Central 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.

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.

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

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

Thermoregulatory responses elicited by microinjection of L-glutamate and its interaction with thermogenic effects of GABA and prostaglandin E2 in the preoptic area

The aim of the present study was to investigate the thermoregulatory effects of neuronal activation with sodium L-glutamate (glutamate) in the preoptic area (POA) of the hypothalamus and to examine its possible interaction with the thermogenic effects of GABA and prostaglandin E(2) (PGE(2)). Unilateral microinjection of glutamate (5 nmol) into the lateral POA or its vicinity elicited a prompt increase in tail skin temperature and simultaneous decreases in the O(2) consumption rate (VO(2)), heart rate, and colonic temperature in urethane-chloralose-anesthetized rats. A central subpopulation of these sites at around the level of bregma was also responsive to the thermogenic and tachycardic effects of GABA (30 nmol). Although the microinjection of GABA into nearby sites elicited no direct effect, it greatly attenuated the hypothermic effects of glutamate subsequently administered to the same site. These results suggest that activation of the lateral POA elicited heat-loss responses and that its central part provided a tonic inhibitory drive toward heat production and tail vasoconstriction. On the other hand, the microinjection of glutamate elicited initial small decreases and subsequent large increases in VO(2) and heart rate in the rostromedial POA. However, no thermoregulatory response was elicited by the microinjection of glutamate at sites where the microinjection of PGE(2) (35 fmol) elicited thermogenic, tachycardic and hyperthermic responses. These results may suggest that the rostromedial POA contained two glutamate-responsive cell groups that had opposite influences on thermoregulation and that the locus that was highly sensitive to the thermogenic effect of PGE(2) was unreactive to glutamate. Collectively, activation of neurons in the lateral POA and rostromedial POA evoked distinct thermoregulatory responses.

Heat loss responses and blockade of prostaglandin E2-induced thermogenesis elicited by alpha1-adrenergic activation in the rostromedial preoptic area

The unilateral microinjection of noradrenaline (NA), but not vehicle solution, into the rostromedial preoptic area (POA) elicited simultaneous increases in cutaneous temperatures of the tail and sole of the foot and decreases in the whole-body O(2) consumption rate, heart rate, and colonic temperature in urethane-chloralose-anesthetized rats, suggesting a coordinate increase in heat loss and decrease in heat production. The magnitude of these responses increased dose-dependently over the range of 1-100 pmol, except for the metabolic and bradycardic responses. Similar hypothermic responses were elicited by the microinjection of 40 pmol methoxamine (an alpha(1)-adrenergic agonist), but not by that of clonidine (an alpha(2)-agonist) or isoproterenol (a beta-agonist). Sites at which microinjection of NA elicited hypothermic responses were in the vicinity of the organum vasculosum of the lamina terminalis including the median preoptic nucleus, whereas no thermal or metabolic response was elicited when NA was microinjected into the lateral POA or caudal part of the medial POA. The microinjection of 130 fmol prostaglandin (PG) E(2) into the NA-sensitive site always elicited thermogenic, tachycardic, and hyperthermic responses. Furthermore, the PGE(2)-induced febrile responses were greatly attenuated by prior administration of NA at the same site. These results demonstrate that NA in the rostromedial POA exerts alpha(1)-adrenoceptor-mediated hypothermic effects and opposes PGE(2)-induced fever.

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.

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.

CCK2 receptor nullification attenuates lipopolysaccharide-induced sickness behavior

Systemic infection produces a highly regulated set of responses such as fever, anorexia, adipsia, inactivity, and cachexia, collectively referred to as sickness behavior. Although the expression of sickness behavior requires immune-brain communication, the mechanisms by which peripheral cytokines signal the brain are unclear. Several mechanisms have been proposed for neuroimmune communication, including the interaction of cytokines with peripheral nerves. A critical role has been ascribed to the vagus nerve in mediating sickness behavior after intraperitoneally delivered immune activation, and converging evidence suggests that this communication may involve neurochemical intermediaries afferent and/or efferent to this nerve. Mice lacking functional CCK2/gastrin receptors (CCK2KO) and wild-type (WT) controls were administered LPS (50, 500, or 2,500 μg/kg; serotype 0111:B4; ip). Results indicate a role for CCK2 receptor activation in the initiation and maintenance of LPS-induced sickness behavior. Compared with WT controls, CCK2KO mice were significantly less affected by LPS on measures of body temperature, activity, body weight, and food intake, with the magnitude of effects increasing with increasing LPS dose. Although activation of CCK2 receptors at the level of the vagus nerve cannot be excluded, a possible role for these receptors in nonvagal routes of immune-brain communication is suggested.

Oxyresveratrol dampens neuroimmune responses in vivo: a selective effect on TNF-α

Consumption of nutrients rich in hydroxystilbenes has been promoted because of their health benefits, including dampening of inflammatory responses. However, few studies have examined their effects in vivo. Here, we show that the hydroxystilbene oxyresveratrol (trans-2,3′,4,5′-tetrahydroxystilbene: o-RES) blocked hypothermia but caused no significant effect on the febrile response to the immune stimulus, bacterial LPS in rats. This was associated with a reduction in the LPS-induced plasma cytokine, tumor necrosis factor (TNF)-α, but not IL-6. Both IL-6-stimulated STAT-3 and LPS-induced cycoloxygenase-2 expression in the hypothalamus were not affected by o-RES. These data strongly suggest that the o-RES-induced dampening of neuroimmune responses is largely due to its inhibitory effect on TNF-α production. In contrast to in vitro experiments, o-RES has no direct effect on NF-κB signaling pathway in vivo. The specific inhibitory effect of o-RES on TNF-α opens new avenues for the clinical use of o-RES in pathological conditions where excessive production of TNF-α is deleterious.

A subsidiary fever center in the medullary raphé?

In fever, as in normal thermoregulation, signals from the preoptic area drive both cutaneous vasoconstriction and thermogenesis by brown adipose tissue (BAT). Both of these responses are mediated by sympathetic nerves whose premotor neurons are located in the medullary raphé. EP3 receptors, key prostaglandin E2(PGE2) receptors responsible for fever induction, are expressed in this same medullary raphé region. To investigate whether PGE2in the medullary raphé might contribute to the febrile response, we tested whether direct injections of PGE2into the medullary raphé could drive sympathetic nerve activity (SNA) to BAT and cutaneous (tail) vessels in anesthetized rats. Microinjections of glutamate (50 mM, 60–180 nl) into the medullary raphé activated both tail and BAT SNA, as did cooling the trunk skin. PGE2injections (150–500 ng in 300–1,000 nl) into the medullary raphé had no effect on tail SNA, BAT SNA, body temperature, or heart rate. By contrast, 150 ng PGE2injected into the preoptic area caused large increases in both tail and BAT SNA (+60 ± 17 spikes/15 s and 1,591 ± 150% of control, respectively), increased body temperature (+1.8 ± 0.2°C), blood pressure (+17 ± 2 mmHg), and heart rate (+124 ± 19 beats/min). These results suggest that despite expression of EP3 receptors, neurons in the medullary raphé are unable to drive febrile responses of tail and BAT SNA independently of the preoptic area. Rather, they appear merely to transmit signals for heat production and heat conservation originating from the preoptic area.

Effects of prostaglandin E2 on the electrical properties of thermally classified neurons in the ventromedial preoptic area of the rat hypothalamus

BACKGROUND

Physiological and morphological evidence suggests that activation of the ventromedial preoptic area of the hypothalamus (VMPO) is an essential component of an intravenous LPS-dependent fever. In response to the endogenous pyrogen prostaglandin E2 (PGE2), the majority of temperature insensitive neurons in the VMPO show an increase in firing rate, while warm sensitive neurons are inhibited. We have hypothesized that these PGE2 dependent effects on firing rate are due to changes in the inherent electrical properties of VMPO neurons, which are regulated by the activity of specific ionic currents.

RESULTS

To characterize the electrical properties of VMPO neurons, whole-cell recordings were made in tissue slices from male Sprague-Dawley rats. Our results indicate that PGE2 dependent firing rate responses were not the result of changes in resting membrane potential, action potential amplitude and duration, or local synaptic input. However, PGE2 reduced the input resistance of all VMPO neurons, while increasing the excitability of temperature insensitive neurons and decreasing the excitability of warm sensitive neurons. In addition, the majority of temperature insensitive neurons responded to PGE2 with an increase in the rate of rise of the depolarizing prepotential that precedes each action potential. This response to PGE2 was reversed for warm sensitive neurons, in which the prepotential rate of rise decreased.

CONCLUSION

We would therefore suggest that PGE2 is having an effect on the ionic currents that regulate firing rate by controlling how fast membrane potential rises to threshold during the prepotential phase of the action potential.

Excitatory amino acid receptor activation in the raphe pallidus area mediates prostaglandin-evoked thermogenesis

To investigate the role of excitatory amino acid neurotransmission within the rostral raphe pallidus area (RPa) in thermogenic and cardiovascular responses, changes in sympathetic nerve activity to brown adipose tissue (BAT), BAT temperature, expired CO(2), arterial pressure, and heart rate were recorded after microinjection of excitatory amino acid (EAA) receptor agonists into the RPa in urethan-chloralose-anesthetized, ventilated rats. To determine whether EAA neurotransmission within the RPa is necessary for the responses evoked by disinhibition of the RPa or by prostaglandin E(2) acting within the medial preoptic area, BAT sympathetic nerve activity, BAT temperature, expired CO(2), arterial pressure, and heart rate were measured during these treatments both before and after blockade of EAA receptors within the RPa. Microinjection of EAA receptor agonists into the RPa resulted in significant increases in all measured variables; these increases were attenuated by prior microinjection of the respective EAA receptor antagonists into the RPa. Microinjection of prostaglandin E(2) into the medial preoptic area or microinjection of bicuculline into the RPa resulted in respective significant increases in BAT sympathetic nerve activity (+approximately 190% and +approximately 235% of resting levels), in BAT temperature (approximately 1.8 degrees C and approximately 2 degrees C), in expired CO(2) (approximately 1.1% and approximately 1.1%), and in heart rate (approximately 97 beats per minute (bpm) and approximately 100 bpm). Blockade of ionotropic EAA receptors within the RPa by microinjection of kynurenate completely reversed the prostaglandin E(2) or bicuculline-evoked increases in all of the measured variables. Blockade of either N-methyl-D-aspartate (NMDA) receptors or non-NMDA receptors alone resulted in marked attenuations of the prostaglandin E(2)-evoked effects on all of the measured variables. These data demonstrate that activation of an EAA input to the RPa is necessary for the BAT thermogenic and the cardiovascular effects resulting from the actions of prostaglandin E(2) within the medial preoptic area or from the disinhibition of local neurons in the RPa.

Excitatory amino acid receptors in the dorsomedial hypothalamus mediate prostaglandin-evoked thermogenesis in brown adipose tissue

We determined whether the dorsomedial hypothalamus (DMH) plays a role in the thermogenic, metabolic, and cardiovascular effects evoked by centrally administered PGE2. Microinjection of PGE2 (170 pmol/60 nl) into the medial preoptic area of the hypothalamus in urethane-chloralose-anesthetized, artificially ventilated rats increased brown adipose tissue (BAT) sympathetic nerve activity (SNA; +207 +/- 18% of control), BAT temperature (1.5 +/- 0.2 degrees C), expired CO2 (0.9 +/- 0.1%), heart rate (HR; 106 +/- 12 beats/min), and mean arterial pressure (22 +/- 4 mmHg). Within 5 min of subsequent bilateral microinjections of the GABAA receptor agonist muscimol (120 pmol.60 nl-1.side-1) or the ionotropic excitatory amino acid antagonist kynurenate (6 nmol.60 nl-1.side-1) into the DMH, the PGE2-evoked increases were, respectively, attenuated by 91 +/- 3% and 108 +/- 7% for BAT SNA, by 73 +/- 12% and 102 +/- 28% for BAT temperature, by 100 +/- 4% and 125 +/- 21% for expired CO2, by 72 +/- 11% and 70 +/- 16% for HR, and by 84 +/- 19% and 113 +/- 16% for mean arterial pressure. Microinjections outside the DMH within the dorsal hypothalamic area adjacent to the mamillothalamic tracts or within the ventromedial hypothalamus were less effective for attenuating the PGE2-evoked thermogenic, metabolic, and cardiovascular responses. These results demonstrate that activation of excitatory amino acid receptors within the DMH is necessary for the thermogenic, metabolic, and cardiovascular responses evoked by microinjection of PGE2 into the medial preoptic area.

Characteristics of thermoregulatory and febrile responses in mice deficient in prostaglandin EP1 and EP3 receptors

Previous studies have disagreed about whether prostaglandin EP1 or EP3 receptors are critical for producing febrile responses. We therefore injected lipopolysaccharide (LPS) at a variety doses (1 microg kg(-1)-1 mg kg(-1)) intraperitoneally (i.p.) into wild-type (WT) mice and mice lacking the EP1 or the EP3 receptors and measured changes in core temperature (Tc) by using telemetry. In WT mice, i.p. injection of LPS at 10 microg kg(-1) increased Tc about 1 degrees C, peaking 2 h after injection. At 100 microg kg(-1), LPS increased Tc, peaking 5-8 h after injection. LPS at 1 mg kg(-1) decreased Tc, reaching a nadir at 5-8 h after injection. In EP1 receptor knockout (KO) mice injected with 10 microg kg(-1) LPS, only the initial (< 40 min) increase in Tc was lacking; with 100 microg kg(-1) LPS the mice showed no febrile response. In EP3 receptor KO mice, LPS decreased Tc in a dose- and time-dependent manner. Furthermore, in EP3 receptor KO mice subcutaneous injection of turpentine did not induce fever. Both EP1 and EP3 receptor KO mice showed a normal circadian cycle of Tc and brief hyperthermia following psychological stress (cage-exchange stress and buddy-removal stress). The present study suggests that both the EP1 and the EP3 receptors play a role in fever induced by systemic inflammation but neither EP receptor is involved in the circadian rise in Tc or psychological stress-induced hyperthermia in mice.

Neuroimmune response to endogenous and exogenous pyrogens is differently modulated by sex steroids

The objective of this study was to explore whether and how ovarian hormones interact with the febrile response to pyrogens. Estrogen and progesterone treatment of ovariectomized rats was associated with a reduction in lipopolysaccharide (LPS)-induced fever, compared with ovariectomized controls. LPS-fever reduction was accompanied by reduced levels of the inducible cyclooxygenase-2 (COX-2) protein expression in the hypothalamus as well as reduced plasma levels of IL-1beta. The amount of LPS-induced IL-6 in the plasma was not affected by ovarian hormone replacement. In contrast, hypothalamic COX-2 expression in response to intraperitoneal injection of IL-1beta was potentiated by the ovarian hormone replacement. IL-1beta induced a moderate increase in plasma levels of IL-6 that was suppressed by ovarian hormone replacement. These data suggest that ovarian hormone replacement attenuated the proinflammatory response to LPS by suppressing the LPS-induced IL-1beta production and COX-2 expression in the hypothalamus. The markedly different action of ovarian hormones on IL-1beta and LPS effects suggests that this sex hormone modulation of the immune response is a function of the nature of infection and provides further evidence that LPS actions are different from those of IL-1beta.

Contrasting effects of E type prostaglandin (EP) receptor agonists on core body temperature in rats

Prostaglandin E2 (PGE2) is thought to be a principal fever mediator. There are four subtypes of PGE (EP) receptors, EP1-EP4. We investigated which EP receptors mediate PGE2-induced hyperthermia by injecting selective EP receptor agonists into the rat lateral cerebral ventricle under unrestrained condition. ONO-DI-004, an EP1 receptor agonist, increased the core temperature (T(c)) in a dose-dependent manner (1.6+/-0.1 degrees C at 20 nmol, with the peak 30 min after injection) with a time course similar to PGE2-induced hyperthermia. ONO-AE1-259-01 (20 nmol), an EP2 receptor agonist, did not change the T(c). ONO-AE-248 (20 nmol), an EP3 receptor agonist, also increased the T(c). However, the peak effect was delayed (1.2+/-0.2 degrees C, 50 min after injection) compared to PGE2. In contrast, ONO-AE1-329, an EP4 receptor agonist, decreased the T(c). These findings suggest that the EP1, EP3, and EP4 receptors all may contribute to the thermoregulatory response to PGE2, but each may have a different role.

The effects of prostaglandin E2 on the firing rate activity of thermosensitive and temperature insensitive neurons in the ventromedial preoptic area of the rat hypothalamus

In response to an immune system challenge with lipopolysaccharide (LPS), recent work has shown that Fos immunoreactivity is displayed by neurons in the ventromedial preoptic area of the hypothalamus (VMPO). In addition, neurons in this region show distinct axonal projections to the anterior perifornical area (APFx) and the paraventricular nucleus (PVN). It has been hypothesized that neurons within the VMPO integrate their local responses to temperature with changes in firing activity that result from LPS induced production of prostaglandin E(2) (PGE(2)). This may be an important mechanism by which the set-point regulation of thermoeffector neurons in the APFx and PVN is altered, resulting in hyperthermia. To characterize the firing rate activity of VMPO neurons, single-unit recordings were made of neuronal extracellular activity in rat hypothalamic tissue slices. Based on the slope of firing rate as a function of tissue temperature, neurons were classified as either warm sensitive or temperature insensitive. Neurons were then treated with PGE(2) (200 nM) while tissue temperature was held at a constant level ( approximately 36 degrees C). The majority of temperature insensitive neurons responded to PGE(2) with an increase in firing rate activity, while warm sensitive neurons showed a reduction in firing rate. This suggests that both warm sensitive and temperature insensitive neurons in the VMPO may play critical and contrasting roles in the production of a fever during an acute phase response to infection.

Fetal exposure to nicotine does not alter the core temperature response of 7- to 8-week-old rats to intracerebroventricular administration of PGE(1)

Prenatal exposure to nicotine attenuates stress-induced hyperthermia in adult male and female rats upon exposure to a novel environment. Given that prostaglandins play an important role in mediating stress-induced hyperthermia, our current experiments were carried out to determine if prenatal exposure to nicotine alters the thermogenic response of adult rats to an E-series prostaglandin. Forty-eight chronically instrumented adults rats (24 males and 24 females) received an intracerebroventricular injection of prostaglandin E(1) (PGE(1); 0.2 microg in 10 microl of artificial cerebrospinal fluid [aCSF]) or vehicle (10 microl aCSF) at 7-8 weeks of postnatal life (i.e. adulthood as defined by the ability to reproduce) following prenatal exposure to nicotine (6 mg of nicotine tartrate per kilogram of maternal body weight per day) or vehicle via a maternally implanted osmotic mini-pump from Day 6 or 7 of gestation to term. In female rats, intracerebroventricular injection of PGE(1) following prenatal exposure to vehicle produced a monophasic fever with a magnitude of approximately 1.5 degrees C and a duration of approximately 66 min. In male rats, however, intracerebroventricular injection of PGE(1) following prenatal exposure to vehicle produced a monophasic fever with a magnitude of only approximately 0.9 degrees C and a duration of approximately 42 min. Prenatal exposure to nicotine did not significantly alter the febrile responses of male or female rats to intracerebroventricular injection of PGE(1) as compared to that observed following prenatal exposure to vehicle. Thus, prenatal exposure to nicotine does not significantly alter the thermogenic response of adult rats to central administration of the pyrogen PGE(1). It is unlikely, therefore, that an altered thermoregulatory effector response to E-series prostaglandins is responsible for mediating the attenuated stress-induced hyperthermia in adult male and female rats upon exposure to a novel environment following prenatal exposure to nicotine.

Suppression of fever at near term is associated with reduced COX-2 protein expression in rat hypothalamus

The fever response is blunted at near term. As the enzyme cyclooxygenase-2 (COX-2) plays a critical role in fever development, we measured its expression in rat hypothalamus during pregnancy and lactation. Western blot analysis revealed a 72-kDa COX-2-immunoreactive band in non-immune-challenged, pregnant rats at day 15 of pregnancy. In contrast, it was almost undetectable at near term and at lactation day 5. COX-2 was significantly induced at the 15th day of pregnancy and at the 5th lactating day after intraperitoneal lipopolysaccharide (50 microg/kg). However, this COX-2 induction was significantly reduced at near term compared with values before and after term. The protein levels of the EP3 receptor in the hypothalamus, one of the prostaglandin E(2) (PGE(2)) receptors suggested to be a key receptor for fever induction, were unaffected throughout the pregnancy and lactation in both non-immune-challenged and lipopolysaccharide-treated rats. These data suggest that suppression of fever at near term is associated with a significantly reduced induction of COX-2 by lipopolysaccharide, resulting in a reduced production of PGE(2). Altered expression of the EP3 receptor does not seem to be involved in this fever refractoriness at near term.

Function of prostanoid receptors: studies on knockout mice

Prostanoids consisting of the prostaglandins (PGs) and the thromboxanes (TXs) are the cyclooxygenase metabolites of arachidonic acid. They exert a range of actions mediated by their respective receptors expressed in the target cells. The receptors include the DP, EP, FP, IP and TP receptors for PGD, PGE, PGF, PGI and TXA, respectively. Furthermore, EP is subdivided into four subtypes, EP1, EP2, EP3 and EP4, which are encoded by different genes and differ in their responses to various agonists and antagonists. Recent developments in the molecular biology of the prostanoid receptors have enabled the investigation of physiological roles of each receptor by disruption of the respective gene. At this point, all the eight types and subtypes of the prostanoid receptors have been individually knocked out in mice, and various phenotypes have been reported for each strain. Here, we review the findings obtained in these studies. The results from these knockout mice studies may be useful in the development of novel therapeutics that can selectively manipulate actions mediated by each receptor.

Indomethacin impairs LPS-induced behavioral fever in toads

We tested the hypothesis that PGs mediate lipopolysaccharide (LPS)-induced behavioral fever in the toad Bufo paracnemis. Measurements of preferred body temperature (T(b)) were performed with a thermal gradient. Toads were injected intraperitoneally with the cyclooxygenase inhibitor indomethacin (5 mg/kg), which inhibits PG biosynthesis, or its vehicle (Tris) followed 30 min later by LPS (0.2 and 2 mg/kg) into the lymph sac. LPS at the dose of 0.2 mg/kg caused a significant increase in T(b) from 7 to 10 h after injection, and then T(b) returned toward baseline values. LPS at the dose of 2 mg/kg produced a different pattern of response, with a longer latency to the onset of fever (10th h) and a longer duration (until the end of the experiment at the 15th h). Tris significantly attenuated the fever induced by LPS at 0.2 mg/kg, but not at 2 mg/kg. Moreover, indomethacin completely blocked the fever evoked by LPS (2 mg/kg). These results indicate that the behavioral fever induced by LPS in toads requires the activation of the COX pathway, suggesting that the involvement of PG in fever has an ancient phylogenetic history and that endogenous PGs raise the thermoregulatory set point to produce fever, because behavioral thermoregulation seems to be related to changes in the thermoregulatory set point.

Association of prolonged fever and hypernatremia: rare presentation of hypothalamic/third ventricle tumor in a toddler

The authors describe a 2-year-old boy with a clinical presentation of prolonged fever of unknown origin and severe hypernatremia. This rare association was the result of a hypothalamic/third ventricle tumor. The lesion was removed and was found to be a low-grade neuronal tumor. After surgery, the child did generally well, but hypothalamic thermoregulatory and osmoregulatory functions were not restored. These presenting symptoms, their pathophysiology, and the implications for pediatric practice are discussed.

Effects of heme oxygenase system on the cyclooxygenase in the primary cultured hypothalamic cells

Endogenous carbon monoxide (CO) shares with nitric oxide (NO) a role as a putative neural messenger in the brain. Both gases are believed to modulate CNS function via an increase in cytoplasmic cGMP concentrations secondary to the activation of soluble guanylate cyclase (sGC). Recently CO and NO were proposed as a possible mediator of febrile response in hypothalamus. NO has been reported to activate both the constitutive and inducible isoform of the cyclooxygenase (COX). Thus, we investigated whether CO arising from heme catabolism by heme oxygenase (HO) is involved in the febrile response via the activation of COX in the hypothalamus. PGE2 which is a final mediator of febrile response released from primary cultured hypothalamic cells was taken as a marker of COX activity. PGE2 concentration was measured with EIA kits. Exogenous CO (CO-saturated medium) and hemin (a substrate and potent inducer of HO) evoked an increase in PGE2 release from hypothalamic cells, and these effects were blocked by methylene blue (an inhibitor of sGC). And membrane permeable cGMP analogue, dibutyryl-cGMP elicited significant increases in PGE2 release. These results suggest that there may be a functional link between HO and COX enzymatic activities. The gaseous product of hemin through the HO pathway, CO, might play a role through the modulation of the COX activity in the hypothalamus.

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.

Effects of caffeine and paracetamol alone or in combination with acetylsalicylic acid on prostaglandin E(2) synthesis in rat microglial cells

Paracetamol has mild analgesic and antipyretic properties and is, along with acetylsalicylic acid, one of the most popular "over the counter" analgesic agents. However, the mechanism underlying its clinical effects is unknown. Another drug whose mechanism of action is unknown is caffeine, which is often used in combination with other analgesics, augmenting their effect. We investigated the inhibitory effect of paracetamol and caffeine on lipopolysaccharide (LPS)-induced cyclooxygenase (COX)- and prostaglandin (PG)E(2)-synthesis in primary rat microglial cells and compared it with the effect of acetylsalicylic acid, salicylic acid, and dipyrone. Furthermore, combinations of these drugs were used to investigate a possible synergistic inhibitory effect on PGE(2)-synthesis. Both paracetamol (IC(50)=7.45 microM) and caffeine (IC(50)=42.5 microM) dose-dependently inhibited microglial PGE(2) synthesis. In combination with acetylsalicylic acid (IC(50)=3.12 microM), both substances augmented the inhibitory effect of acetylsalicylic acid on LPS-induced PGE(2)-synthesis. Whereas paracetamol inhibited only COX enzyme activity, caffeine also inhibited COX-2 protein synthesis. These results are compatible with the view that the clinical activity of paracetamol and caffeine is due to inhibition of COX. Furthermore, these results may help explain the clinical experience of an adjuvant analgesic effect of caffeine and paracetamol when combined with acetylsalicylic acid.

Effect of transection of various branches of the vagus nerve on lipopolysaccharide-induced synthesis of corticotropin-releasing hormone mRNA in the paraventricular nuclei of rat hypothalamus

Effects of selective transection of the gastric, celiac, and hepatic branches of the vagus nerve on expression of corticotropin-releasing hormone mRNA in small cell neurons of the hypothalamic paraventricular nuclei in rats administered with bacterial lipopolysaccharide were studied using in situ hybridization technique. Low doses of lipopolysaccharide stimulated expression of corticotropin-releasing hormone mRNA in rats subjected to axotomy of the gastric or celiac branches of the vagus nerve, but did not change the intensity of autoradiographic labeling in animals with transected hepatic branches. High doses of lipopolysaccharide enhanced expression of corticotropin-releasing hormone mRNA in vagotomized rats of all groups, which indicated the existence of a vagus-independent mechanism responsible for activation of paraventricular neurons mediating the effect of this hormone. The data suggest that the inflammation-dependent activation of stress-regulating neurons in the hypothalamus is controlled by several mechanisms, whose activation depends on the severity of inflammatory processes.

Interleukin-6 expression and regulation in astrocytes

The physiological function of interleukin-6 (IL-6) within the central nervous system (CNS) is complex; IL-6 exerts neurotrophic and neuroprotective effects, and yet can also function as a mediator of inflammation, demyelination, and astrogliosis, depending on the cellular context. In the normal brain, IL-6 levels remain low. However, elevated expression occurs in injury, infection, stroke, and inflammation. Given the diverse biological functions of IL-6 and its expression in numerous CNS conditions, it is critical to understand its regulation in the brain in order to control its expression and ultimately its effects. Accumulating data demonstrate that the predominant CNS source of IL-6 is the activated astrocyte. Furthermore, a wide range of factors have been demonstrated to be involved in IL-6 regulation by astrocytes. In this review, we summarize information concerning IL-6 regulation in astrocytes, focusing on the role of proinflammatory factors, neurotransmitters, and second messengers.

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Brain sites of action of endogenous interleukin-1 in the febrile response to localized inflammation in the rat

1. Interleukin (IL)-1 is a potent endogenous pyrogen which causes fever when injected into a number of brain sites. However, the brain sites at which endogenous IL-1 acts to influence body temperature remain equivocal. The aim of this study was to determine the effect of local administration of the interleukin-1 receptor antagonist (IL-1ra) into specific sites in the hypothalamus, and other brain regions known to contain receptors for IL-1, on the febrile response of rats to peripheral injection of lipopolysaccharide (LPS) into a subcutaneous air pouch (intrapouch, i.p.o.) that does not lead to LPS appearance in the circulation. 2. Injection of LPS (100 microgram kg-1, i.p.o.) induced a rise in body temperature which commenced 1.5 h after injection and was maximal at 3 h (38.9 +/- 0.2 C, compared with 37.0 +/- 0.1 C at 0 h, n = 6, P < 0.001). Intracerebroventricular (i.c.v.) IL-1ra (500 microgram in 5 microliter) significantly attenuated LPS fever (IL-1ra, 37.7 +/- 0.2 C; saline, 38.9 +/- 0.2 C; n = 6, P < 0.001). Unilateral microinjection of IL-1ra (50 microgram in 0.5 microliter at 0 + 1 h) into the anterior hypothalamus (AH), paraventricular hypothalamic nucleus (PVH), peri-subfornical organ, subfornical organ (SFO) or hippocampus (dentate gyrus and CA3 region) also significantly reduced the fever induced by LPS. 3. The same dose of IL-1ra had no effect on fever when administered into the ventromedial hypothalamus (VMH), organum vasculosum lamina terminalis (OVLT), CA1 field of the hippocampus, striatum or cortex. 4. These data indicate that the action of endogenous IL-1 in the brain during fever is site specific, acting at the AH, PVH, SFO and hippocampus, but not the VMH, OVLT and striatum or cortex.

Deregulation of cyclooxygenase and nitric oxide synthase gene expression in the inflammatory cascade triggered by experimental group B streptococcal meningitis in the newborn brain and cerebral microvessels

Group B Streptococcus (GBS) is the most common cause of neonatal sepsis and meningitis. Despite antibiotics, GBS in the newborn initiates a cascade of molecular and biological events leading to altered cerebral perfusion, blood-brain barrier disruption, cerebral edema, intracranial hypertension, neurological damage, and even death. Having previously shown that GBS infection impairs cerebral blood flow autoregulation and increases prostaglandin (PG) levels, we examined the regulation of some crucial inflammatory mediators (PGs, nitric oxide (NO), tumor necrosis factor-a) in the brain and cerebral microvessels (MVs) from newborn piglets. Cyclooxygenase (COX), the key enzyme in PG biosynthesis, exists in two isoforms, COX-1 and COX-2. Both may be directly induced by NO in a model of renal inflammation. Besides its neurotransmitter role, NO is a potent vasorelaxant whose production is catalyzed by at least three distinct nitric oxide synthases (NOS) (bNOS, ecNOS, iNOS). Western blot analyses showed that the newborn (4 day old) brain expressed lower levels of COX-1 (8-fold), COX-2 (20-fold), bNOS (12-fold), and ecNOS (5-fold) than in the 1 day old. MV showed approximately equal levels of COX-2, lower levels of COX-1 (4-fold), bNOS (5-fold), and higher levels of ecNOS (20-fold) in comparison to 4-day-old cerebral MV. A 4-day-old brain expressed lower levels of bNOS (5-fold), ecNOS (10-fold), and COX-1 (2-fold) than the 6-week-old pig. COX-2 protein was undetected in a 4-day-old pig brain, but present in great excess in MV. Purified MV showed lower ecNOS (14-fold), COX-1 (2-fold), and about equal levels of bNOS and COX-2 in comparison with MV from 6-week-old pigs. Reverse transcription polymerase chain reaction analyses confirmed these results. Treatment with noo-nitro-L-arginine (LNA), a NOS inhibitor, downregulated COX-1 expression in the newborn brain and both COX-1 and COX-2 cerebral MV expression. GBS infection (10(9) colony-forming units, 0.5 mL intracerebroventricular) of sedated newborn piglets induced the expression of tumor necrosis factor-alpha in the cerebrospinal fluid after 2 hours, upregulated bNOS expression in both brain and MVs, upregulated ecNOS in MVs, and downregulated COX-1, COX-2, and ecNOS in the brain. GBS did not trigger the expression of iNOS. Our data suggest that there is a net deficiency of NOS isoforms in the immature brain and microvasculature of the 4-day-old piglet and that the differences in expression lead to the immature control of NO and PG production, rendering newborns particularly susceptible to neurological damage because of the undeveloped nature of their response mechanisms. Moreover, the GBS-induced cascade deregulates the gene expression of interacting inflammatory mediators and may cause a net vasoconstrictor/vasodilator imbalance, leading to cerebral hypertension and edema in the early stages of infection. Pharmacological manipulations of the inflammatory cascade could lead to novel therapeutic approaches for the treatment of GBS meningitis.

Expression of COX-2 by normal and reactive astrocytes in the adult rat central nervous system

We have used a previously characterized antiserum against cycloxygenase-2 (COX-2) together with cold methanol fixation to immunohistochemically locate the protein in astrocytes in rat brain. Although in cerebral cortex most enzyme was located in neuronal perikarya as previously described, a number of glial fibrillary acidic protein (GFAP)-positive astrocytes were also labeled. No COX-2-positive neurons were seen in the cerebellum, but here also a subset of GFAP+ astrocytes was present which contained the enzyme. The number of COX-2-positive astrocytes increased considerably after injection of the neurotoxin kainate into the cerebellum. These immunohistochemical data were supported by semiquantitative RT-PCR results, which were used to assess the levels of COX-2 mRNA relative to the housekeeping gene hypoxanthine phosphoribosyl transferase. PGE2 levels were measured in contralateral and lesioned cerebellum to correlate changes in COX-2 immunoreactivity and mRNA with physiological events. PGE2 levels increased by 230% in the lesioned cerebellar hemispheres in comparison to the contralateral ones. We discuss the possibility that the targets for astrocytic prostaglandins might include both autocrine effects and paracrine responses of neurons, lymphocytes and capillary endothelial cells.

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.

A light-independent oscillatory gene mPer3 in mouse SCN and OVLT

A new member of the mammalian period gene family, mPer3, was isolated and its expression pattern characterized in the mouse brain. Like mPer1, mPer2 and Drosophila period, mPer3 has a dimerization PAS domain and a cytoplasmic localization domain. mPer3 transcripts showed a clear circadian rhythm in the suprachiasmatic nucleus (SCN). Expression of mPer3 was not induced by exposure to light at any phase of the clock, distinguishing this gene from mPer1 and mPer2. Cycling expression of mPer3 was also found outside the SCN in the organum vasculosum lamina terminalis (OVLT), a potentially key region regulating rhythmic gonadotropin production and pyrogen-induced febrile phenomena. Thus, mPer3 may contribute to pacemaker functions both inside and outside the SCN.

Brain eicosanoids and LPS fever: species and age differences

The results of the present study, summarized in Table 2, demonstrate that different species and strains of rodents (rats and mice) and birds (chickens) exhibit rather specific fever response. Systemic administration of LPS caused monophasic elevation in Tb of chickens, biphasic changes in Tb of rats (initial drop followed by an increase in Tb), whereas mice failed to develop hyperthermia and responded by a decreased Tb. The LPS-induced alterations in hypothalamic prostanoid synthesis were also rather species-specific and differ markedly even between the two strains of mice. We failed to find a common direct correlation between LPS-induced changes in Tb and hypothalamic prostanoid production in rodents (rats and mice). This observation is supported by our recent study on age-related changes in fever response in rats, where we found that hypothalami of LPS-treated old and young adult rats produced similar amounts of PGE2 and PGI2, in spite of more pronounced and prolonged hypothermia, and a delayed elevation in Tb of old rats, as compared with young (Fraifeld et al., 1995b). Moreover, the hypothalamus of febrile chickens did not display any detectable activation of PGE2 production, suggesting that PGE2 is not a common central mediator of fever in homeotherms (Fraifeld et al., 1995a). Apparently, the actual body temperature not always reflects the functional state of central thermostat, and increased PGE2 production in hypothalamus would not directly, at least in rodents, lead to body temperature elevation. Furthermore, peripheral effects, including PG-mediated ones, of pyrogens can interfere and even overcome their centrally-mediated effects (Morimoto et al., 1991; Burysek et al., 1993). Previously, we have shown that no additional elevation in hypothalamic PGE2 production occurs in response to doses of LPS over 10 micrograms in rats and 25 micrograms in mice, while the increased doses led to further changes in Tb response (Kaplanski et al., 1993). Morimoto et al. (1991) have considered that PGE2 acts centrally to cause fever and peripherally to cause hypothermia, and, hence, these opposing actions, both being induced by LPS, may act together to determine the final thermoregulatory response. Other possibilities could be related to counterbalance of endogenous antipyretics (Kluger, 1991; Kozak et al., 1995), that may occur not only at the level of thermoregulatory center but also outside the CNS (Klir et al., 1995), and to the existence of PG-independent mechanisms of LPS fever. The latter have been shown for IL-8 (Rothwell et al., 1990; Zampronio et al., 1994) and MIP-1 (Davatelis et al., 1989; Minano et al., 1990; Hayashi et al., 1995; Lopez-Valpuesta and Myers, 1995), which are, apparently, mediated via CRF (Strijbos et al., 1992; Zampronio et al., 1994), and INF-alpha, mediated via the opioid receptor mechanisms (Hori et al., 1991, 1992). However, it has been shown recently that in different species the same pyrogenic cytokines (IL-8) may induced fever via different, PG-independent (in rats; Zampronio et al., 1994) or PG-dependent (in rabbits; Zampronio et al., 1995) mechanisms. It should be noted that fever response is not always accompanied by an elevation in Tb. The final effect of pyrogens on body temperature depends upon the balance between heat production and heat loss, which in turn is highly dependent upon body size and ambient temperature, especially in small animals. Perhaps, the hypothermic response observed in our mice and rats at 22 degrees C may be in part attributed to ambient temperature, which was below a thermoneutral zone. The reduced febrile response is considered, at least in part, to contribute to an increased mortality and prolonged recovery from infections (Kluger, 1986). From this point, it is difficult to suggest whether the hypothermia observed in our mice and rats could be of somewhat adaptive significance. It has been shown that at the ambient temperature of 30 degrees C, Swiss Webster mice can re

Inducible cyclooxygenase expression in canine basilar artery after experimental subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Inducible cyclooxygenase (COX-2) has been found to play a pathological role in cerebral insult. We investigated the expression of COX-2 in the basilar artery after experimental subarachnoid hemorrhage (SAH).

METHODS

In a canine "two-hemorrhage" model of SAH, the basilar arteries were obtained on day 2 after a cisternal injection of autologous blood or on days 4, 6, 7, or 9 after the second injection. Basilar arteries also were obtained 12 hours after intracisternal injection a cytokine: interleukin (IL)-1 beta (0.03 microgram), IL-6 (3 micrograms), or IL-8 (10 micrograms). Western blotting with a polyclonal anti-COX-2 antibody was performed in these arteries.

RESULTS

COX-2 protein was not demonstrated in the basilar artery in control animals without SAH. However, it was expressed in the basilar artery on days 2, 4, 6, and 7 after blood injection but not on day 9. Intracisternal injection of IL-1 beta, IL-6, or IL-8 also induced COX-2 in the basilar artery.

CONCLUSIONS

COX-2 expression was detected in basilar arterial tissue in both acute and chronic stages after SAH. Elevation of inflammatory cytokines after SAH may be involved in the induction of COX-2, which may produce sufficient quantities of eicosanoids to affect hemodynamics after SAH.

Mechanisms of CNS response to systemic immune challenge: the febrile response

The acute-phase reaction is the multisystem response to acute inflammation. The central nervous system (CNS) mediates a coordinated set of autonomic, endocrine and behavioral responses that constitute the cerebral component of the acute-phase reaction. However, the mechanisms of immune signaling of the CNS remain controversial. Emerging evidence indicates that different parts of the acute-phase reaction are initiated by distinct mechanisms and in different brain regions. Cytokines produced as a result of local infections (for example, in the abdominal or thoracic cavities) might activate vagal sensory fibers, resulting in sickness behavior and fevers. Additionally, circulating immune stimuli might activate meningeal macrophages and perivascular microglia along the borders of the brain, eliciting the local production of prostaglandins and responses such as fever, anorexia, sleepiness, and activation of the hypothalamo-pituitary-adrenal (HPA) axis. The biological importance of these responses might favor the existence of multiple parallel CNS pathways that are engaged by cytokines.

Mechanisms and sites of pyrogenic action exerted by staphylococcal enterotoxin A in rabbits

The febrile responses induced by i.v. administrations of staphylococcal enterotoxin A (SEA) was mimicked by direct injection of SEA into the organum vasculosum laminae terminalis (OVLT) in unanesthetized rabbits. Compared with the febrile responses induced by i.v. injection of SEA, the OVLT route of injection required a much lower dose of SEA to produce a similar fever. Furthermore, the fever induced by intra-OVLT or i.v. injection of SEA was significantly attenuated by pretreatment with intra-OVLT injection of anisomycin (a protein synthesis inhibitor), indomethacin or diclofenac (inhibitors of cyclo-oxygenase (COX)), and aminoguanidine or dexamethasone (inhibitors of inducible nitric oxide synthase (iNOS)). These results suggest that COX or iNOS pathway in the OVLT mediate the SEA-induced fever in rabbits.

Prostaglandin E2 may induce hyperthermia through EP1 receptor in the anterior wall of the third ventricle and neighboring preoptic regions

We have previously reported that intracerebroventricular injection of prostaglandin E2 (PGE2) induces hyperthermia possibly through EP1 receptors in the rat. In the present study, to determine the sites in the brain where PGE2 induces hyperthermia through EP1 receptors, we microinjected an EP1 receptor agonist, 17-phenyl-omega-trinor PGE2 (17-Ph-PGE2, 100 ng) into different sites in the rat brain and observed the colonic temperature (Tco) for 2 h in a 23 +/- 1 degrees C environment. Responsive sites where 17-Ph-PGE2 (100 ng) produced a rise in the Tco of more than 1.1 degrees C within 60 min after injection were found in the medial preoptic area, the subchiasmatic portion of the median preoptic nucleus, the anterior wall of the third ventricle (A3V) and the ventral portion of the diagonal band of Broca. Among these sites, the A3V was the most responsive. In contrast, microinjection of neither butaprost (an EP2 agonist, 100 ng) nor M&B28767 (an EP3 agonist, 100 ng) into these four sites had any effect on the Tco. Intracerebroventricular pretreatment with SC-19220 (an EP1 antagonist, 100 microg) inhibited the rise in the Tco which was induced by microinjection of PGE2 (50 ng) into the A3V. These results thus suggest that PGE2 induces hyperthermia by stimulating EP1 receptors in the A3V and the neighboring preoptic region.

Influence of pregnancy on the febrile response to ICV administration of PGE1 in rats studied in a thermocline

Rats near term of pregnancy have an attenuated febrile response to intracerebroventricular (ICV) injection of prostaglandin E1 (PGE1) when they are studied at an ambient temperature below their thermoneutral zone. Given that nonshivering thermogenesis in brown adipose tissue is impaired in rodents near term of pregnancy, it is possible that the attenuated febrile response is forced by impairment of this component of the autonomic thermoregulatory response. If this were the case, then near-term pregnant rats should develop a "normal" fever after PGE1 administration if they were studied in a thermocline where they could utilize behavioral as well as autonomic thermoregulatory effectors to increase their body core temperature (Tbc). Experiments were, therefore, carried out on 13 nonpregnant and 14 pregnant chronically instrumented rats in a thermocline (temperature gradient 10-40 degrees C) to investigate their Tbc responses to ICV injection of PGE1. ICV injection of 0.2 microgram PGE1 produced significant increases in Tbc and fever index in both nonpregnant and pregnant animals (day 19 of gestation); the increases, however, were significantly attenuated in the pregnant compared with the nonpregnant rats. Behavioral (e.g., selected ambient temperature) and autonomic (e.g., oxygen consumption) thermoregulatory effectors were activated to increase Tbc after ICV PGE1 in both groups of animals, but the duration of activation was shortened in pregnant compared with nonpregnant rats. The abbreviated thermoregulatory effector responses and the resulting attenuated febrile response to PGE1 in the pregnant rats may have resulted from a pregnancy-related activation of an endogenous antipyretic system.

Anteroventral third ventricle lesion suppresses fever, but not stress-induced hyperthermia in rats

Body core temperature (Tco) of unrestrained rats was monitored to compare the effects of electrolytic lesion of the anteroventral third ventricle (AV3V) region on stress-induced hyperthermia or lipopolysaccharide (LPS)-induced fever. We found that stress-induced hyperthermia was not significantly reduced by AV3V lesion, whereas pyrogen-induced fever was significantly lower in rats in which the lesion completely ablated the organum vasculosum laminae terminalis (OVLT), located in the AV3V region. The results indicate that although the central neural elements producing both stress- and pyrogen-induced elevations in Tco may be prostaglandin related, stress hyperthermia is not activated by mechanisms in the OVLT region, as is fever induced by LPS.

Long-term elevation of cyclooxygenase-2, but not lipoxygenase, in regions synaptically distant from spreading depression

Eicosanoids, produced from arachidonic acid by cyclooxygenases (COXs) and lipoxygenases (LIPOXs), are involved in numerous brain processes. To explore if brief and noninjurious stimuli chronically alter expression of these enzymes, we examined the induction of COX-2 and LIPOX expression following unilateral neocortical spreading depression (SD). Expression was examined over time and in regions not experiencing SD (hippocampus) but synaptically connected to the site of stimulation (cortex). One hundred six male Wistar rats had SD induced via microinjection of 0.5 M KCl (0.5 M NaCl for sham) into left parietal cortex every 9 minutes for 1 or 3 hours. One hour before SD some animals received dexamethasone (Dex), mepacrine (Mep), indomethacin (Indo), nordihydroguaiaretic acid (Ndga), phenylephrine (Pe), sodium nitroprusside (Snp) with Pe, or N omega-nitro-L-arginine methyl ester (Lnam). Animals survived for 0, 3, or 6 hours, or 1, 2, 3, 7, 14, 21, or 28 days. Brains were processed immunohistochemically for COX-2 and LIPOX, and the optical density (OD) of the left and right cortex, dentate gyrus (DG), CA3, and CA1 immunoreactivity (IR) were measured. Induction was expressed as the log of left divided by right side OD for each region. COX-2 IR in the left cortex was elevated rapidly and was sustained for 21 days following SD. COX-2 IR was also elevated in the ipsilateral hippocampus not experiencing SD, with the rank order of induction as follows: DG > CA3 > CA1. Dex, Snp, and/or Pe significantly reduced the induction of COX-2. No changes in LIPOX IR were observed. These results show that long-term changes in COX-2 expression are induced by SD and these changes decrease with synaptic distance. Benign stimuli increase COX-2 expression and thus may influence brain function for extended periods and at distant locations.