Hypothalamic mechanisms of pain modulatory actions of cytokines and prostaglandin E2

Pharmacological mechanisms of sinomenine in anti-inflammatory immunity and osteoprotection in rheumatoid arthritis: A systematic review

BACKGROUND

Sinomenine (SIN) is the main pharmacologically active component of Sinomenii Caulis and protects against rheumatoid arthritis (RA). In recent years, many studies have been conducted to elucidate the pharmacological mechanisms of SIN in the treatment of RA. However, the molecular mechanism of SIN in RA has not been fully elucidated.

PURPOSE

To summarize the pharmacological effects and molecular mechanisms of SIN in RA and clarify the most valuable regulatory mechanisms of SIN to provide clues and a basis for basic research and clinical applications.

METHODS

We systematically searched SciFinder, Web of Science, PubMed, China National Knowledge Internet (CNKI), the Wanfang Databases, and the Chinese Scientific Journal Database (VIP). We organized our work based on the PRISMA statement and selected studies for review based on predefined selection criteria.

OUTCOME

After screening, we identified 201 relevant studies, including 88 clinical trials and 113 in vivo and in vitro studies on molecular mechanisms. Among these studies, we selected key results for reporting and analysis.

CONCLUSIONS

We found that most of the known pharmacological mechanisms of SIN are indirect effects on certain signaling pathways or proteins. SIN was manifested to reduce the release of inflammatory cytokines such as Tumor necrosis factor-α (TNF-α), Interleukin-6 (IL-6), and IL-1β, thereby reducing the inflammatory response, and apparently blocking the destruction of bone and cartilage. The regulatory effects on inflammation and bone destruction make SIN a promising drug to treat RA. More notably, we believe that the modulation of α7nAChR and the regulation of methylation levels at specific GCG sites in the mPGES-1 promoter by SIN, and its mechanism of directly targeting GBP5, certainly enriches the possibilities and the underlying rationale for SIN in the treatment of inflammatory immune-related diseases.

Systemic Lipopolysaccharide-Induced Pain Sensitivity and Spinal Inflammation Were Reduced by Minocycline in Neonatal Rats

In this study, we investigated the effects of minocycline, a putative suppressor of microglial activation, on systemic lipopolysaccharide (LPS)-induced spinal cord inflammation, allodynia, and hyperalgesia in neonatal rats. Intraperitoneal (i.p.) injection of LPS (2 mg/kg) or sterile saline was performed in postnatal day 5 (P5) rat pups and minocycline (45 mg/kg) or vehicle (phosphate buffer saline; PBS) was administered (i.p.) 5 min after LPS injection. The von Frey filament and tail-flick tests were performed to determine mechanical allodynia (a painful sensation caused by innocuous stimuli, e.g., light touch) and thermal hyperalgesia (a condition of altered perception of temperature), respectively, and spinal cord inflammation was examined 24 h after the administration of drugs. Systemic LPS administration resulted in a reduction of tactile threshold in the von Frey filament tests and pain response latency in the tail-flick test of neonatal rats. The levels of microglia and astrocyte activation, pro-inflammatory cytokine interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), and prostaglandin E2 (PGE2) in the spinal cord of neonatal rats were increased 24 h after the administration of LPS. Treatment with minocycline significantly attenuated LPS-induced allodynia, hyperalgesia, the increase in spinal cord microglia, and astrocyte activation, and elevated levels of IL-1β, COX-2, and PGE2 in neonatal rats. These results suggest that minocycline provides protection against neonatal systemic LPS exposure-induced enhanced pain sensitivity (allodynia and hyperalgesia), and that the protective effects may be associated with its ability to attenuate LPS-induced microglia activation, and the levels of IL-1β, COX-2, and PGE2 in the spinal cord of neonatal rats.

Inhibition of NLRP3 Inflammasome Prevents LPS-Induced Inflammatory Hyperalgesia in Mice: Contribution of NF-κB, Caspase-1/11, ASC, NOX, and NOS Isoforms

The nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP3), an intracellular signaling molecule that senses many environmental- and pathogen/host-derived factors, has been implicated in the pathogenesis of several diseases associated with inflammation. It has been suggested that NLRP3 inflammasome inhibitors may have a therapeutic potential in the treatment of NLRP3-related inflammatory diseases. The aim of this study was to determine whether inhibition of NLRP3 inflammasome prevents inflammatory hyperalgesia induced by lipopolysaccharide (LPS) in mice as well as changes in expression/activity of nuclear factor κB (NF-κB), caspase-1/11, nicotinamide adenine dinucleotide phosphate oxidase (NOX), and endothelial/neuronal/inducible nitric oxide synthase (eNOS/nNOS/iNOS) that may regulate NLRP3/apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)/pro-caspase-1 inflammasome formation and activity by using a selective NLRP3 inflammasome inhibitor, MCC950. Male mice received saline (10 ml/kg; i.p.), LPS (10 mg/kg; i.p.), and/or MCC950 (3 mg/kg; i.p.). Reaction time to thermal stimuli within 1 min was evaluated after 6 h. The mice were killed and the brains, hearts, and lungs were collected for measurement of NF-κB, caspase-1, caspase-11, NLRP3, ASC, NOX subunits (gp91^phox; NOX2), and p47^phox; NOXO2), nitrotyrosine, eNOS, nNOS, iNOS, and β-actin protein expression, NOS activity, and interleukin (IL)-1β levels. LPS-induced hyperalgesia was associated with a decrease in eNOS, nNOS, and iNOS protein expression and activity as well as an increase in expression of NF-κB p65, caspase-1 p20, caspase-11 p20, NLRP3, ASC, gp91^phox, p47^phox, and nitrotyrosine proteins in addition to elevated IL-1β levels. The LPS-induced changes were prevented by MCC950. The results suggest that inhibition of NLRP3/ASC/pro-caspase-1 inflammasome formation and activity prevents inflammatory hyperalgesia induced by LPS in mice as well as changes in NF-κB, caspase-11, NOX2, NOXO2, and eNOS/nNOS/iNOS expression/activity.

Imiquimod Treatment Causes Systemic Disease in Mice Resembling Generalized Pustular Psoriasis in an IL-1 and IL-36 Dependent Manner

Generalized pustular psoriasis (GPP) is a severe form of psoriasis that can be caused by missense mutations in the interleukin-36 (IL-36) receptor antagonist. In addition to neutrophil rich skin inflammation, GPP patients typically also experience anorexia, fever, malaise, and pain. The imiquimod-induced skin inflammation mouse model has rapidly become a popular way to study plaque psoriasis, which typically does not involve symptoms of systemic disease. In this model, neutrophil recruitment to the skin is dependent upon the inflammatory mediators IL-1, via its receptor IL-1R1, and IL-36α. Unexpectedly, we observed that mice also exhibited signs of anorexia (weight loss and decreased food intake), general malaise (decreased activity and loss of interest in building nests), and pain (nose bulging and hunched posture). A scoring system allowing quantitative comparisons of test groups was developed. Female mice were found to develop more severe disease than male mice. Furthermore, mice deficient in both IL-1R1 and IL-36α are nearly disease-free, while mice lacking only one of these inflammatory mediators have less severe disease than wild type mice. Hence, the imiquimod-induced skin inflammation mouse model recapitulates not only plaque psoriasis, but also the more severe symptoms, that is, anorexia, malaise, and pain, seen in GPP.

Accounting for the delay in the transition from acute to chronic pain: axonal and nuclear mechanisms

Acute insults produce hyperalgesic priming, a neuroplastic change in nociceptors that markedly prolongs inflammatory mediator-induced hyperalgesia. After an acute initiating insult, there is a 72 h delay to the onset of priming, for which the underlying mechanism is unknown. We hypothesized that the delay is due to the time required for a signal to travel from the peripheral terminal to the cell body followed by a return signal to the peripheral terminal. We report that when an inducer of hyperalgesic priming (monocyte chemotactic protein 1) is administered at the spinal cord of Sprague Dawley rats, priming is detected at the peripheral terminal with a delay significantly shorter than when applied peripherally. Spinally induced priming is detected not only when prostaglandin E2 (PGE2) is presented to the peripheral nociceptor terminals, but also when it is presented intrathecally to the central terminals in the spinal cord. Furthermore, when an inducer of priming is administered in the paw, priming can be detected in spinal cord (as prolonged hyperalgesia induced by intrathecal PGE2), but only when the mechanical stimulus is presented to the paw on the side where the priming inducer was administered. Both spinally and peripherally induced priming is prevented by intrathecal oligodeoxynucleotide antisense to the nuclear transcription factor CREB mRNA. Finally, the inhibitor of protein translation reversed hyperalgesic priming only when injected at the site where PGE2 was administered, suggesting that the signal transmitted from the cell body to the peripheral terminal is not a newly translated protein, but possibly a newly expressed mRNA.

The dorsomedial hypothalamus mediates stress-induced hyperalgesia and is the source of the pronociceptive peptide cholecystokinin in the rostral ventromedial medulla

While intense or highly arousing stressors have long been known to suppress pain, relatively mild or chronic stress can enhance pain. The mechanisms underlying stress-induced hyperalgesia (SIH) are only now being defined. The physiological and neuroendocrine effects of mild stress are mediated by the dorsomedial hypothalamus (DMH), which has documented connections with the rostral ventromedial medulla (RVM), a brainstem region capable of facilitating nociception. We hypothesized that stress engages both the DMH and the RVM to produce hyperalgesia. Direct pharmacological activation of the DMH increased sensitivity to mechanical stimulation in awake animals, confirming that the DMH can mediate behavioral hyperalgesia. A behavioral model of mild stress also produced mechanical hyperalgesia, which was blocked by inactivation of either the DMH or the RVM. The neuropeptide cholecystokinin (CCK) acts in the RVM to enhance nociception and is abundant in the DMH. Using a retrograde tracer and immunohistochemical labeling, we determined that CCK-expressing neurons in the DMH are the only significant supraspinal source of CCK in the RVM. However, not all neurons projecting from the DMH to the RVM contained CCK, and microinjection of the CCK2 receptor antagonist YM022 in the RVM did not interfere with SIH, suggesting that transmitters in addition to CCK play a significant role in this connection during acute stress. While the RVM has a well-established role in facilitation of nociception, the DMH, with its well-documented role in stress, may also be engaged in a number of chronic or abnormal pain states. Taken as a whole, these findings establish an anatomical and functional connection between the DMH and RVM by which stress can facilitate pain.

Neural circuitry engaged by prostaglandins during the sickness syndrome

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

Do psychological and physiological stressors alter the acute pain response to castration and tail docking in lambs?

OBJECTIVE

To investigate whether events that may be stressful to young lambs, including simulated infection or social isolation, modulate pain experienced by lambs following castration and tail docking (C/D).

STUDY DESIGN

Randomised, controlled, prospective study.

ANIMALS

Fifty male lambs born to 46 second-parity Mule ewes.

METHODS

Lambs were allocated randomly to one of four groups, experiencing either a potential stressor or handling on day 2 after birth, followed by C/D or handling only on day 3. Quantitative sensory testing (QST) data [mechanical nociceptive thresholds (MNT), Semmes Weinstein filaments (SW), response to cold] and serum cortisol concentration were measured at time points after application of treatments to lambs on days 2 and 3 after birth. The treatment groups were LPS, injection of bacterial lipopolysaccharide IV on day 2, C/D on day 3; ISOL, isolation from the dam for 10 minutes on day 2, C/D on day 3; CAST, handling only on day 2, C/D on day 3; CONT, handled only on days 2 and 3.

RESULTS

Castration and tail docking caused transient hypoalgesia as measured by MNT and SW. Simulated infection and isolation caused hyperalgesia 3 hours after application, indicated by a reduction in MNT, however they did not alter the pain response to C/D compared to lambs in the CAST group. Injection of LPS and C/D caused increased serum cortisol concentration. The magnitude of the cortisol response to C/D was not altered by prior exposure to either LPS or isolation.

CONCLUSIONS AND CLINICAL RELEVANCE

LPS and isolation did not modulate the response to C/D but did cause hyperalgesia. This highlights the importance of flock health management and husbandry techniques to reduce the incidence of either systemic infection or psychological stressors in young lambs.

Interleukin-1 receptor antagonist ameliorates neonatal lipopolysaccharide-induced long-lasting hyperalgesia in the adult rats

An increasing amount of data show that central inflammation contributes to many debilitating diseases and produces spontaneous pain and hyperalgesia (an increased sensitivity to painful stimuli), and these processes may be associated with the production of proinflammatory cytokines by activated microglia. In the present study, we demonstrate that neonatal intracerebral injection of lipopolysaccharide (LPS) (1mg/kg) in postnatal day 5 (P5) rats produced hyperalgesia that lasted into adulthood as indicated by decreased latency in the tail-flick test. Neonatal LPS administration resulted in a long-lasting increase in the number of activated microglial in the P70 rat brain. The effects of interleukin-1beta (IL-1β) and IL-1 receptor antagonists on hyperalgesia were determined to examine the possible role of inflammatory cytokines in LPS-induced hyperalgesia. Our data show that neonatal intracerebral injection of IL-1β (1 μg/kg) produced a hyperalgesic tendency similar to that induced by LPS. Neonatal administration of an IL-1 receptor antagonist (0.1mg/kg) significantly attenuated long-lasting hyperalgesia induced by LPS and reduced the number of activated microglia in the adult rat brain. These data reveal that neonatal intracerebral LPS exposure results in long-lasting hyperalgesia and an elevated number of activated microglia in later life. This effect is similar to that induced by IL-1β and can be prevented by an IL-1 receptor antagonist. The present study suggests that an IL-1 receptor antagonist effectively attenuates or blocks long-lasting hyperalgesia and microglia activation produced by LPS exposure in the neonatal period of rats.

Peripheral and central mediators of lipopolysaccharide induced suppression of defensive rage behavior in the cat

Based upon recent findings in our laboratory that cytokines microinjected into the medial hypothalamus or periaqueductal gray (PAG) powerfully modulate defensive rage behavior in cat, the present study determined the effects of peripherally released cytokines following lipopolysaccharide (LPS) challenge upon defensive rage. The study involved initial identification of the effects of peripheral administration of LPS upon defensive rage by electrical stimulation from PAG and subsequent determination of the peripheral and central mechanisms governing this process. The results revealed significant elevation in response latencies for defensive rage from 60 to 300 min, post LPS injection, with no detectable signs of sickness behavior present at 60 min. In contrast, head turning behavior elicited by stimulation of adjoining midbrain sites was not affected by LPS administration, suggesting a specificity of the effects of LPS upon defensive rage. Direct administration of LPS into the medial hypothalamus had no effect on defensive rage, suggesting that the effects of LPS were mediated by peripheral cytokines rather than by any direct actions upon hypothalamic neurons. Complete blockade of the suppressive effects of LPS by peripheral pretreatment with an Anti-tumor necrosis factor-alpha (TNFalpha) antibody but not with an anti- interleukin-1 (IL-1) antibody demonstrated that the effects of LPS were mediated through TNF-alpha rather than through an IL-1 mechanism. A determination of the central mechanisms governing LPS suppression revealed that pretreatment of the medial hypothalamus with PGE(2) or 5-HT(1A) receptor antagonists each completely blocked the suppressive effects of LPS, while microinjections of a TNF-alpha antibody into the medial hypothalamus were ineffective. Microinjections of -Iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) benzamide monohydrochloride (p-MPPI) into lateral hypothalamus (to test for anatomical specificity) had no effect upon LPS induced suppression of defensive rage. The results demonstrate that LPS suppresses defensive rage by acting through peripheral TNF-alpha in periphery and that central effects of LPS suppression of defensive rage are mediated through PGE(2) and 5-HT(1A) receptors in the medial hypothalamus.

Angiotensin AT(2) receptor agonists act as anti-opioids via EP(3) receptor in mice

Novokinin (Arg-Pro-Leu-Lys-Pro-Trp) is a vasorelaxing and hypotensive peptide acting through the angiotensin AT(2) receptor. Centrally administrated novokinin (30nmol/mouse) inhibited the antinociceptive effect of micro agonist morphine in mice, as evaluated by the tail-pinch test. The anti-opioid effect of novokinin was blocked by PD123319, an antagonist of the AT(2) receptor. Angiotensin II (0.01nmol/mouse, i.c.v.) and [p-aminophenylalanine(6)]-angiotensin II [p-NH(2)Phe(6)]-Ang II (0.1nmol/mouse, i.c.v.), a highly selective AT(2) receptor agonist, also inhibited the antinociceptive effect of morphine, and the effects were also blocked by PD123319. Angiotensin II did not suppress the antinociceptive effect induced by kappa or delta agonists. Novokinin, angiotensin II and [p-NH(2)Phe(6)]-Ang did not have affinity for the micro receptor. The anti-opioid effects induced by these peptides were blocked by ONO-AE3-240, an antagonist of the EP(3) receptor. These results suggest that the anti-opioid effects of AT(2) agonists are mediated by the PGE(2)-EP(3) receptor system downstream of the AT(2) receptor.

Immune-to-brain signalling: the role of cerebral CD163-positive macrophages

Systemic inflammation induces cytokine synthesis within the central nervous system. This results in sickness behaviour and may exacerbate ongoing neuroinflammatory disease. The precise mechanisms underlying the relay of signal from the periphery to the central nervous system are not entirely understood. CD163-positive macrophages occupy a unique position at the blood-brain barrier and upregulate prostaglandin-synthesizing enzymes in response to systemic inflammation. This finding suggests that they might play a role in signalling inflammation to the central nervous system. However, here we demonstrate that de novo brain cytokine transcription during systemic endotoxaemia may be prostaglandin-independent. We therefore set out to interrogate more directly the role of CD163-positive macrophages in immune-to-brain signalling. Intracerebroventricular injections of clodronate liposomes were used to selectively deplete CD163-positive macrophages. We show that de novo brain cytokine synthesis during systemic endotoxaemia persists in the absence of CD163-positive macrophages. Cerebral endothelial cells outnumber CD163-positive macrophages and are arguably better situated to signal circulating inflammatory stimuli to the brain.

Stress induces a switch of intracellular signaling in sensory neurons in a model of generalized pain

Stress dramatically exacerbates pain in diseases such as fibromyalgia and rheumatoid arthritis, but the underlying mechanisms are unknown. We tested the hypothesis that stress causes generalized hyperalgesia by enhancing pronociceptive effects of immune mediators. Rats exposed to nonhabituating sound stress exhibited no change in mechanical nociceptive threshold, but showed a marked increase in hyperalgesia evoked by local injections of prostaglandin E(2) or epinephrine. This enhancement, which developed more than a week after exposure to stress, required concerted action of glucocorticoids and catecholamines at receptors located in the periphery on sensory afferents. The altered response to pronociceptive mediators involved a switch in coupling of their receptors from predominantly stimulatory to inhibitory G-proteins (G(s) to G(i)), and for prostaglandin E(2), emergence of novel dependence on protein kinase C epsilon. Thus, an important mechanism in generalized pain syndromes may be stress-induced coactivation of the hypothalamo-pituitary-adrenal and sympathoadrenal axes, causing a long-lasting alteration in intracellular signaling pathways, enabling normally innocuous levels of immune mediators to produce chronic hyperalgesia.

Increased susceptibility of annexin-A1 null mice to nociceptive pain is indicative of a spinal antinociceptive action of annexin-A1

BACKGROUND AND PURPOSE

Annexin-A1 (ANXA1), a glucocorticoid-regulated protein, mediates several of the anti-inflammatory actions of the glucocorticoids. Previous studies demonstrated that ANXA1 is involved in pain modulation. The current study, using ANXA1 knockout mice (ANXA1-/-), is aimed at addressing the site and mechanism of the modulatory action of ANXA1 as well as possible involvement of ANXA1 in mediating the analgesic action of glucocorticoids.

EXPERIMENTAL APPROACH

The acetic acid-induced writhing response was performed in ANXA1-/- and wild-type (ANXA1+/+) mice with spinal and brain levels of prostaglandin E2 (PGE2) examined in both genotypes. The effect of the ANXA1 peptomimetic Ac2-26 as well as methylprednisolone on the writhing response and on spinal cord PGE2 of ANXA1+/+ and ANXA1-/- was compared. The expression of proteins involved in PGE2 synthesis, cytosolic phospholipase A2 (cPLA2) and cyclooxygenases (COXs), in the spinal cord of ANXA1+/+ and ANXA1-/- was also compared.

KEY RESULTS

ANXA1-/- mice exhibited a significantly greater writhing response and increased spinal cord levels of PGE2 compared with ANXA1+/+ mice. Ac2-26 produced analgesia and reduced spinal PGE2 levels in ANXA1+/+ and ANXA1-/- mice, whereas methylprednisolone reduced the writhing response and spinal PGE2 levels in ANXA1+/+, but not in ANXA1-/- mice. The expression of cPLA2, COX-1, COX-2 and COX-3 in spinal cord tissues was upregulated in ANXA1-/-compared with ANXA1+/+.

CONCLUSIONS AND IMPLICATIONS

We conclude that ANXA1 protein modulates nociceptive processing at the spinal level, by reducing synthesis of PGE2 by modulating cPLA2 and/or COX activity. The analgesic activity of methylprednisolone is mediated by spinal ANXA1.

Interleukin-1 signaling modulates stress-induced analgesia

Exposure to stressful stimuli is often accompanied by reduced pain sensitivity, termed "stress-induced analgesia" (SIA). In the present study, the hypothesis that interleukin-1 (IL-1) may play a modulatory role in SIA was examined. Two genetic mouse models impaired in IL-1-signaling and their wild-type (WT) controls were employed. Another group of C57 mice was acutely administered with IL-1 receptor antagonist (IL-1ra). Mice were exposed to 2min swim stress at one of three water temperatures: 32 degrees C (mild stress), 20-23 degrees C (moderate stress), or 15 degrees C (severe stress); and then tested for pain sensitivity using the hot-plate test. Corticosterone levels were assessed in separate groups of WT and mutant mice following exposure to the three types of stress. Mild stress induced significant analgesia in the two WT strains and saline-treated mice, but not in the mutant strains or the IL-1ra-treated mice. Similarly, mild stress induced significantly elevated corticosterone levels in WT mice, and blunted corticosterone response in mutant mice. In contrast, both WT and mutant strains, as well as IL-1ra-treated mice, displayed analgesic and corticosterone responses following moderate and severe stress. Interestingly, the analgesic response to moderate stress was markedly potentiated in the mutant strains, as compared with their WT controls. The present results support our previous findings that in the absence of IL-1, stress response to mild stress is noticeably diminished. However, the analgesic response to moderate stress is markedly potentiated in mice with impaired IL-1 signaling, corroborating the anti-analgesic role of IL-1 in several pain modulatory conditions, including SIA.

Cyclooxygenase-2 and antagonists in pain management

Non-steroidal anti-inflammatory drugs are widely used for the treatment of pain and inflammation by inhibiting the formation of prostaglandins. However, their use is limited by their side-effects, including gastrointestinal, renal function, cardiovascular and platelet function. Cyclooxygenase activity is the principal target for the action of non-steroidal anti-inflammatory drugs. Two isoforms of cyclooxygenase have been characterized: (i) cyclooxygenase-1, which is found in many tissues and is generally constitutively expressed and synthesizes prostanoids that mediate homeostatic functions; and (ii) cyclooxygenase-2, the inducible isoform, which is mainly expressed at sites of injury or inflammation and synthesizes prostanoids that mediate inflammation, pain and fever. These findings led to the development of selective cyclooxygenase-2 inhibitors, with comparable anti-inflammatory and analgesic properties to traditional non-steroidal anti-inflammatory drugs, but with significantly fewer side-effects. However, these new selective cyclooxygenase-2 inhibitors are not risk free, and care should be taken when using these drugs, especially with elderly patients with multiple medical problems. Finally, the future is bright for the broader usage of these agents in the treatment of diseases other than inflammation and pain, such as Alzheimer's disease, colonic polyp and colon cancer, just to name a few.

The involvement of a cyclooxygenase 1 gene-derived protein in the antinociceptive action of paracetamol in mice

Paracetamol is a widely used analgesic and antipyretic with weak anti-inflammatory properties. Experimental evidence suggests that inhibition of prostaglandin biosynthesis contributes to its pharmacological actions. Three cyclooxygenase (COX) isoenzymes are involved in prostaglandin biosynthesis, COX-1, COX-2 and a recently discovered splice-variant of COX-1, COX-3. Our aim was to identify the relative roles for these enzymes in the antinociceptive action of paracetamol in mice. We compared the antinociceptive action of paracetamol with the non-selective non-steroid anti-inflammatory drug, diclofenac and studied paracetamol antinociception in COX-1 and COX-2 knockout mice. Paracetamol (100-400 mg/kg) inhibited both acetic acid- and iloprost-induced writhing responses. In contrast, diclofenac (10-100 mg/kg) inhibited only acetic acid-induced writhing. Only diclofenac reduced peripheral prostaglandin biosynthesis whereas both drugs reduced central prostaglandin production. Prostaglandin E(2) (PGE(2)) concentrations were reduced in different brain regions by administration of paracetamol. COX-1, COX-2 and COX-3 enzyme proteins were expressed in the same brain regions. The effects of paracetamol on writhing responses and on brain PGE(2) levels were reduced in COX-1, but not COX-2, knockout mice. The selective COX-3 inhibitors, aminopyrine and antipyrine also reduced writhing responses and brain PGE(2) biosynthesis. These results suggest that the antinociceptive action of paracetamol may be mediated by inhibition of COX-3.

Peripheral inflammation modifies the effect of intrathecal IL-1β on spinal PGE2 production mainly through cyclooxygenase-2 activity. A spinal microdialysis study in freely moving rats

Acute inflammation induces upregulation of IL-1beta both at the site of the peripheral inflammation and in the cerebrospinal fluid (CSF). The central increase of IL-1beta mainly contributes to the development of hypersensitivity. However, the spinal mechanisms for the effects of IL-1beta in nociceptive transmission are incompletely understood. It is also unknown whether previous sensitization changes IL-1beta activity. We therefore investigated the dose-effect relationship of intrathecal (i.t.) IL-1beta on spinal PGE(2) production in the absence and presence of peripheral formalin inflammation with spinal microdialysis in freely moving rats. The possible involvement of cyclooxygenase (COX) isoforms in the IL-1beta-mediated spinal PGE(2) production on the background of peripheral formalin inflammation was further evaluated with the selective COX-1 and COX-2 inhibitors. We found that the i.t. administration of IL-1beta, with doses of 1, 2, 8, or 16 ng, increased PGE(2) levels in CSF in a dose-related fashion. This IL-1beta-evoked PGE(2) release occurred within 30min after IL-1beta administration, peaked at 30-60 min interval, and returned gradually to the baseline level within 4h. Peripheral formalin inflammation in the paw induced a more prolonged effect of spinal IL-1beta with larger PGE(2) releases in the CSF compared with the non-inflammatory state, suggesting that peripheral inflammation enhances central sensitization. The COX-2 inhibitor SC58236 (15 mg/kg) reduced the IL-1beta-mediated PGE(2) increase in CSF by 86% while the COX-1 inhibitor SC58560 (15 mg/kg) had less effect (28%). Our study suggests that mainly the COX-2 enzyme mediates the IL-1beta-induced increase in spinal PGE(2) in the presence of peripheral formalin inflammation.

Lipopolysaccharide-induced changes in mesenteric afferent sensitivity of rat jejunum in vitro: role of prostaglandins

Bacterial translocation across the intestinal mucosal barrier leads to a macrophage-mediated inflammatory response, visceral hyperalgesia, and ileus. Our aim was to examine how mediators released into mesenteric lymph following LPS treatment influence intestinal afferent sensitivity and the role played by prostanoids in any sensitization. Intestinal lymph was collected from awake rats following treatment with either saline or LPS (5 mg/kg ip). Extracellular multiunit afferent recordings were made from paravascular mesenteric nerve bundles supplying the rat jejunum in vitro following arterial administration of control lymph, LPS lymph, and LPS. Mesenteric afferent discharge increased significantly after LPS lymph compared with control lymph. Peak discharge occurred within 2 min and remained elevated for 5 to 8 min. This response was attenuated by pretreatment with naproxen (10 microM), and restored upon addition of prostaglandin E(2) (5 microM) in the presence of naproxen, but AH6809 (5 microM), an EP(1)/EP(2) receptor(s) antagonist, failed to decrease the magnitude of LPS lymph-induced response. LPS itself also stimulated mesenteric afferent discharge but was unaffected by naproxen. TNF-alpha was significantly increased in LPS lymph compared with control lymph (1,583 +/- 197 vs. 169 +/- 38 pg/ml, P < 0.01) but exogenous TNF-alpha failed to evoke any afferent nerve discharge. We concluded that inflammatory mediators released from the gut into mesenteric lymph during endotoxemia have a profound effect on afferent discharge. These mediators influence afferent firing via the release of local prostaglandins.

Effects of surgical stress on brain prostaglandin E2 production and on the pituitary–adrenal axis: Attenuation by preemptive analgesia and by central amygdala lesion

Surgical stress is the combined result of tissue injury, anesthesia, and postoperative pain. It is characterized by elevated levels of adrenocorticotropin (ACTH), corticosterone (CS), and elevated levels of prostaglandin E2 (PGE2) in the periphery and in the spinal cord. The present study examined the effects of perioperative pain management in rats undergoing laparotomy on serum levels of ACTH, CS, and on the production of PGE2 in several brain regions, including the amygdala. The amygdala is known to modulate the pituitary-adrenal axis response to stress. We, therefore, also examined the effects of bilateral lesions in the central amygdala (CeA) on laparotomy-induced activation of the pituitary-adrenal axis in rats. In the first experiment, rats either underwent laparotomy or were not operated upon. Half the rats received preemptive analgesia extended postoperatively, the other received saline. ACTH, CS serum levels, and ex vivo brain production of PGE2 were determined. In the second experiment, rats underwent bilateral lesions of the CeA. Ten days later, rats underwent laparotomy, and ACTH and CS serum levels were determined. Laparotomy significantly increased amygdala PGE2 production, and CS and ACTH serum levels. This elevation was markedly attenuated by perioperative analgesia. Bilateral CeA lesions also attenuated the pituitary-adrenal response to surgical stress. The present findings suggest that the amygdala plays a regulatory role in mediating the neuroendocrine response to surgical stress. Effective perioperative analgesia attenuated the surgery-induced activation of pituitary-adrenal axis and PGE2 elevation. The diminished elevation of PGE2 may suggest a mechanism by which pain relief mitigates pituitary-adrenal axis activation.

alpha-Melanophore-stimulating hormone (alpha-MSH) antagonizes interleukin-1beta-induced hyperalgesia and Fos expression in the paraventricular and arcuate nucleus of the rat

It is known that intracerebroventricular (ICV) administration of a low dose of interleukin-1beta (IL-1beta) induces hyperalgesia and that this effect can be inhibited by alpha-melanophore-stimulating hormone (alpha-MSH). To identify the part of the brain that is affected by hyperalgesia-induced IL-1beta and the possible site of alpha-MSH inhibition, we have examined Fos expression in the rat brain in response to ICV microinjection of alpha-MSH and/or IL-1beta. Following injection of 10 pg IL-1beta, hyperalgesia was induced and Fos became expressed in the paraventricular nucleus (PVN) of the hypothalamus and in the arcuate nucleus (ARC), which contains alpha-MSH-producing neurons. IL-1beta injection did not induce Fos expression in the pars intermedia of the pituitary gland, which contains endocrine melanotrope cells that release alpha-MSH into the systemic circulation. ICV co-injection of IL-1beta with 30 ng alpha-MSH fully inhibited both hyperalgesia and Fos expression in the PVN and the ARC. We conclude that PVN neurons are activated by hyperalgesic IL-1beta and propose that this effect is abolished by alpha-MSH possibly released from the ARC but not from the pituitary gland.

Optimization of multiplexed bead-based cytokine immunoassays for rat serum and brain tissue

The ability to simultaneously quantify multiple signaling molecule protein levels from microscopic neural tissue samples would be of great benefit to deciphering how they affect brain function. This follows from evidence that indicates signaling molecules can be pleiotropic and can have complex interactive behavior that is regionally and cellularly heterogeneous. Multiplexed examination of tissue proteins has been exceedingly difficult because of the absence of available techniques. This void now has been removed by the commercial availability of bead-based immunoassays for targeted proteins that allow analyses of up to 100 (6-150 kDa) proteins from as little as 12 microl. Thus far used only for sera (human and mouse) and culture media, we demonstrate here that sensitive (as low as 2 pg/ml), wide-ranging (up to 2-32 000 pg/ml), accurate (8% intra-assay covariance) and reliable (4-7% inter-assay covariance) measurements can be made of nine exemplary cytokines (e.g., IL-1alpha, IL-1beta, IL-2, IL-4, IL-6, IL-10, GM-CSF, IFN-gamma, TNF-alpha) simultaneously not only from rat serum but, for the first time, also brain tissue. Furthermore, we describe animal handling procedures that minimize stress as determined by serum glucocorticoid levels since they can influence cytokine expression.

Selective amino acids changes in the medial and lateral preoptic area in the formalin test in rats

A combination of microdialysis in freely moving rats and capillary zone electrophoresis coupled to laser induced fluorescence detection was used to measure extracellular concentrations of amino acid neurotransmitters in different hypothalamic areas during noxious stimulation. Arginine, glutamate and aspartate were monitored every 30 s before and after a s.c. injection of formalin (5%, 50 microl) or saline (0.9%) in the right hind paw. In the medial and lateral preoptic area, calcium and nerve impulse dependent increases of arginine, glutamate and aspartate were observed during the first 2 min after formalin injection. However, amino acid changes were not detected in the lateral hypothalamus or in the ventromedial nucleus when compared with pre-injection levels or with the levels from animals injected with saline in the hind paw. Flinching behavior was also scored during the first 10 min following the formalin or saline injection. Flinching frequency was maximum at minute 2 after formalin injection, whereas saline injection did not elicited any flinching behavior. These results show that nociceptive stimulation induces rapid and differential amino acids changes in discrete areas of the hypothalamus that can be associated with pain-related behavior.

EP3 and EP4 receptor mRNA expression in peptidergic cell groups of the rat parabrachial nucleus

This study examines the distribution of prostaglandin E2 receptors of subtype EP3 and EP4 among brain stem parabrachial neurons that were characterized with respect to their neuropeptide expression. By using a dual-labeling in situ hybridization method, we show that preprodynorphin mRNA expressing neurons in the dorsal and central lateral subnuclei express EP3 receptor mRNA. Such receptors are also expressed in preproenkephalin, calcitonin gene related peptide and preprotachykinin mRNA positive neurons in the external lateral subnucleus, whereas preprodynorphin mRNA expressing neurons in this subnucleus are EP receptor negative. In addition, EP3 receptor expression is seen among some enkephalinergic neurons in the Kölliker-Fuse nucleus. Neurons in the central part of the cholecystokininergic population in the regions of the superior lateral subnucleus express EP4 receptor mRNA, whereas those located more peripherally express EP3 receptors. Taken together with previous findings showing that discrete peptidergic cell groups mediate nociceptive and/or visceral afferent information to distinct brain stem and forebrain regions, the present results suggest that the processing of this information in the parabrachial nucleus is influenced by prostaglandin E2. Recent work has shown that prostaglandin E2 is released into the brain following peripheral immune challenge; hence, the parabrachial nucleus may be a region where humoral signaling of peripheral inflammatory events may interact with neuronal signaling elicited by the same peripheral processes.

Prostaglandin E2 injected into the posterior hypothalamus has no effect on trigeminal nociception in the rat

Craniovascular prostaglandin E2 (PGE2) release is elevated in the headache phase of migraine and in experimental models of headache. PGE2 synthesised in the brain may be involved in modulating trigeminal nociception. We examined whether PGE2 injected into the posterior hypothalamus could modulate trigeminovascular nociception. In seven rats, electrophysiological recordings were made from trigeminal nucleus caudalis neurons responsive to noxious middle meningeal artery stimulation and inhibited by bicuculline activation of the posterior hypothalamus. Microinjection into the posterior hypothalamus of a non-pyrogenic dose of PGE2 (2.5 microg/ml) produced no effect on nociceptive trigeminal nucleus caudalis neurons compared with saline injection (P=0.29). The mean response to PGE2 injection was 97% of baseline. We conclude that PGE2 in the posterior hypothalamus is unlikely to play a significant role in modulating trigeminal nociception.

Impairment of interleukin-1 (IL-1) signaling reduces basal pain sensitivity in mice: genetic, pharmacological and developmental aspects

The cytokine interleukin-1 (IL-1) has been implicated in modulation of pain perception under various inflammatory conditions. The present study examined the hypothesis that IL-1 signaling is also involved in pain sensitivity under normal, non-inflammatory states, using three mouse models of impaired IL-1 signaling: targeted deletion of the IL-1 receptor type I or the IL-1 receptor accessory protein, and transgenic over-expression of IL-1 receptor antagonist within the brain and spinal cord. Thermal and mechanical pain sensitivity was assessed using the paw-flick, hot-plate, and von Frey tests. All mutant strains displayed significantly lower pain sensitivity, compared with their respective wild-type control strains, and with their parent strains (C57BL/6, CBA and 129), in all tests. In contrast, mice with targeted deletion of the p55 or p75 TNF receptor, or of interleukin-18, displayed normal or higher pain sensitivity compared to their respective controls. To differentiate between developmental vs. on-going effects of IL-1, mice were chronically treated with IL-1 receptor antagonist (IL-1ra) via osmotic micropumps, either in adulthood or prenatally (throughout the last 2 weeks of gestation). Adult mice that were treated with IL-1ra either in adulthood or in utero, displayed lower pain sensitivity, similar to mice with impaired IL-1 signaling. These findings suggest that basal pain sensitivity is genetically, developmentally and tonically influenced by IL-1 signaling.

Mechanisms of cytosolic Ca2+ suppression by prostaglandin E2 receptors in rat melanotrophs

We have previously reported that voltage-dependent Ca2+ (VDC) channels of rat melanotrophs are inhibited by prostaglandin E2 (PGE2). In this study, mechanisms involved in the inhibitory actions of PGE2 receptors of rat melanotrophs were analysed using reverse transcriptase-polymerase chain reaction (RT-PCR), Ca2+-imaging and whole-cell, patch-clamp techniques with recently developed EP agonists, each of which is selective for the known four subclasses of EP receptors (EP1-4). PGE2 reversibly suppressed the cytosolic Ca2+ concentration ([Ca2+]i). The maximum reduction in [Ca2+]i by PGE2 was comparable to that by dopamine or to that by extracellular Ca2+ removal. RT-PCR analysis of all four EP receptors revealed that EP3 and EP4 receptor mRNAs were expressed in the intermediate lobe. The effects of PGE2 to suppress [Ca2+]i were mimicked by the selective EP3 agonist, ONO-AE-248, whereas three other EP agonists, ONO-DI-004 (EP1), ONO-AE1-259 (EP2) and ONO-AE1-329 (EP4), had little or no effect on [Ca2+]i. All four G-protein activated inward rectifying K+ (GIRK) channel mRNAs were identified in intermediate lobe tissues by RT-PCR. Dopamine concentration-dependently activated GIRK currents, whereas PGE2 did not activate GIRK currents, even at the concentration causing maximal inhibition of VDC channels. These results suggest that PGE2 acts on EP3 receptors to suppress Ca2+ entry of rat melanotrophs by selectively inhibiting VDC channels of these cells. We have compared the possible cellular and molecular mechanisms of inhibition by dopamine and PGE2.

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.

Neuroimmune alterations in the complex regional pain syndrome

This review focuses on some clinical aspects of the complex regional pain syndrome, such as oedema, local temperature changes and chronic pain, as a result of supposed neurogenic inflammation. Involvement of the immune system could imply the subsequent release of neuropeptides, pro-inflammatory cytokines and eicosanoids, which in turn leads to a complex cross-talk of primary and secondary generated mediators of inflammation. The development and application of drugs that act through selective receptor antagonism or enzymatic synthesis inhibition to prevent further stimulation of this cascade that could inevitably lead to chronicity of this disease are extensively discussed.