Nociceptive effects induced by intrathecal administration of prostaglandin D2, E2, or F2 alpha to conscious mice

Mast cell-sensory neuron crosstalk in allergic diseases

Mast cells (MCs) are tissue-resident immune cells, well-positioned at the host-environment interface for detecting external antigens and playing a critical role in mobilizing innate and adaptive immune responses. Sensory neurons are afferent neurons innervating most areas of the body but especially in the periphery, where they sense external and internal signals and relay information to the brain. The significance of MC-sensory neuron communication is now increasingly becoming recognized, especially because both cell types are in close physical proximity at the host-environment interface and around major organs of the body and produce specific mediators that can activate each other. In this review, we explore the roles of MC-sensory neuron crosstalk in allergic diseases, shedding light on how activated MCs trigger sensory neurons to initiate signaling in pruritus, shock, and potentially abdominal pain in allergy, and how activated sensory neurons regulate MCs in homeostasis and atopic dermatitis associated with contact hypersensitivity and type 2 inflammation. Throughout the review, we also discuss how these 2 sentinel cell types signal each other, potentially resulting in a positive feedback loop that can sustain inflammation. Unraveling the mysteries of MC-sensory neuron crosstalk is likely to unveil their critical roles in various disease conditions and enable the development of new therapeutic approaches to combat these maladies.

Sex-differences in prostaglandin signaling: a semi-systematic review and characterization of PTGDS expression in human sensory neurons

There is increasing evidence of sex differences in underlying mechanisms causing pain in preclinical models, and in clinical populations. There are also important disconnects between clinical pain populations and the way preclinical pain studies are conducted. For instance, osteoarthritis pain more frequently affects women, but most preclinical studies have been conducted using males in animal models. The most widely used painkillers, nonsteroidal anti-inflammatory drugs (NSAIDs), act on the prostaglandin pathway by inhibiting cyclooxygenase (COX) enzymes. The purpose of this study was to analyze the preclinical and clinical literature on the role of prostaglandins and COX in inflammation and pain. We aimed to specifically identify studies that used both sexes and investigate whether any sex-differences in the action of prostaglandins and COX inhibition had been reported, either in clinical or preclinical studies. We conducted a PubMed search and identified 369 preclinical studies and 100 clinical studies that matched our inclusion/exclusion criteria. Our analysis shows that only 17% of preclinical studies on prostaglandins used both sexes and, out of those, only 19% analyzed or reported data separated by sex. In contrast, 79% of the clinical studies analyzed used both sexes. However, only 6% of those reported data separated by sex. Interestingly, 14 out of 15 preclinical studies and 5 out of 6 clinical studies that analyzed data separated by sex have identified sex-differences. This builds on the increasing evidence of sex-differences in prostaglandin signaling and the importance of sex as a biological variable in data analysis. The preclinical literature identifies a sex difference in prostaglandin D2 synthase (PTGDS) expression where it is higher in female than in male rodents in the nervous system. We experimentally validated that PTGDS expression is higher in female human dorsal root ganglia (DRG) neurons recovered from organ donors. Our semi-systematic literature review reveals a need for continued inclusivity of both male and female animals in prostaglandins studies and data analysis separated by sex in preclinical and clinical studies. Our finding of sex-differences in neuronal PTGDS expression in humans exemplifies the need for a more comprehensive understanding of how the prostaglandin system functions in the DRG in rodents and humans.

Prostaglandin-E2 levels over the course of glyceryl trinitrate provoked migraine attacks

Administration of glyceryl trinitrate (GTN), a donor of nitric oxide, can induce migraine-like attacks in subjects with migraine. Provocation with GTN typically follows a biphasic pattern; it induces immediate headache in subjects with migraine, as well as in healthy controls, whereafter only subjects with migraine may develop a migraine-like headache several hours later. Interestingly, intravenous infusion with prostaglandin-E₂ (PGE₂) can also provoke a migraine-like headache, but seems to have a more rapid onset compared to GTN. The aim of the study was to shed light on the mechanistic aspect PGE₂ has in migraine attack development. Therefore, PGE₂ plasma levels were measured towards the (pre)ictal state of an attack, which we provoked with GTN. Blood samples from women with migraine (n = 37) and age-matched female controls (n = 25) were obtained before and ∼ 140 min and ∼ 320 min after GTN infusion. PGE₂ levels were measured using liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. Data was analyzed using a generalized linear mixed-effect model. Immediate headache after GTN infusion occurred in 85 % of migraine participants and in 75 % of controls. A delayed onset migraine-like attack was observed in 82 % of migraine subjects and in none of the controls. PGE₂ levels were not different between the interictal and preictal state (P = 0.527) nor between interictal and ictal state (defined as having migraine-like headache) (P = 0.141). Hence, no evidence was found that a rise in PGE₂ is an essential step in the initiation of GTN-induced migraine-like attacks.

Effect of calcium hydroxide alone or in combination with ibuprofen and ciprofloxacin on postoperative pain and periapical prostaglandin E2 level: A randomized clinical study

AIM

The aim of the present study was to compare pure Ca(OH)2, Ca(OH)2 + ibuprofen and Ca(OH)2 + ciprofloxacin in terms of postoperative pain and prostaglandin E2 (PGE2) level in previously treated teeth with periapical lesions.

MATERIALS AND METHODS

Sixty-six patients were randomly assigned into 3 groups according to the intracanal medication (Ca(OH)2, Ca(OH)2 + ibuprofen and Ca(OH)2 + ciprofloxacin). After removing gutta-percha from the root canals, the PGE2 sample collection was obtained by introducing three sterile paper points into the root canals through the root apex (2 mm). Selected intracanal medicament was placed into the root canal and the participants were told to record postoperative pain levels at 24, 48, and 72 h and on 1 week after treatment using visual analog scale (VAS). At the second appointment, the medicaments were removed and second sampling was performed using the same method. The PGE2 levels measured by enzyme-linked immunosorbent assay kits, and the data were statistically analyzed.

RESULTS

All the tested Ca(OH)2 pastes were found to be significantly effective in lowering the preoperative PGE2 levels. However, intergroup analyses revealed that the Ca(OH)2 + ciprofloxacin group had the highest effectiveness in lowering PGE2 with a significant difference when compared with the pure Ca(OH)2 group. There was no statistically significant difference among the groups in terms of pre- and post-operative pain levels.

CONCLUSION

The Ca(OH)2 + ciprofloxacin medication is more effective than the pure Ca(OH)2 medication in lowering periapical PGE2 level. However, addition of ibuprofen or ciprofloxacin to the Ca(OH)2 paste does not provide extra benefit in terms of post-operative pain relief.

Paracetamol - An old drug with new mechanisms of action

Paracetamol (acetaminophen) is the most commonly used over-the-counter (OTC) drug in the world. Despite its popularity and use for many years, the safety of its application and its mechanism of action are still unclear. Currently, it is believed that paracetamol is a multidirectional drug and at least several metabolic pathways are involved in its analgesic and antipyretic action. The mechanism of paracetamol action consists in inhibition of cyclooxygenases (COX-1, COX-2, and COX-3) and involvement in the endocannabinoid system and serotonergic pathways. Additionally, paracetamol influences transient receptor potential (TRP) channels and voltage-gated Kv7 potassium channels and inhibits T-type Cav3.2 calcium channels. It also exerts an impact on L-arginine in the nitric oxide (NO) synthesis pathway. However, not all of these effects have been clearly confirmed. Therefore, the aim of our paper was to summarize the current state of knowledge of the mechanism of paracetamol action with special attention to its safety concerns.

Sex Differences in Nociceptor Translatomes Contribute to Divergent Prostaglandin Signaling in Male and Female Mice

BACKGROUND

There are clinically relevant sex differences in acute and chronic pain mechanisms, but we are only beginning to understand their mechanistic basis. Transcriptome analyses of rodent whole dorsal root ganglion (DRG) have revealed sex differences, mostly in immune cells. We examined the transcriptome and translatome of the mouse DRG with the goal of identifying sex differences.

METHODS

We used translating ribosome affinity purification sequencing and behavioral pharmacology to test the hypothesis that in Nav1.8-positive neurons, most of which are nociceptors, translatomes would differ by sex.

RESULTS

We found 80 genes with sex differential expression in the whole DRG transcriptome and 66 genes whose messenger RNAs were sex differentially actively translated (translatome). We also identified different motifs in the 3' untranslated region of messenger RNAs that were sex differentially translated. In further validation studies, we focused on Ptgds, which was increased in the translatome of female mice. The messenger RNA encodes the prostaglandin PGD₂ synthesizing enzyme. We observed increased PTGDS protein and PGD₂ in female mouse DRG. The PTGDS inhibitor AT-56 caused intense pain behaviors in male mice but was only effective at high doses in female mice. Conversely, female mice responded more robustly to another major prostaglandin, PGE₂, than did male mice. PTGDS protein expression was also higher in female cortical neurons, suggesting that DRG findings may be generalizable to other nervous system structures.

CONCLUSIONS

Our results demonstrate sex differences in nociceptor-enriched translatomes and reveal unexpected sex differences in one of the oldest known nociceptive signaling molecule families, the prostaglandins.

Exploring the Promise of Flavonoids to Combat Neuropathic Pain: From Molecular Mechanisms to Therapeutic Implications

Neuropathic pain (NP) is the result of irregular processing in the central or peripheral nervous system, which is generally caused by neuronal injury. The management of NP represents a great challenge owing to its heterogeneous profile and the significant undesirable side effects of the frequently prescribed psychoactive agents, including benzodiazepines (BDZ). Currently, several established drugs including antidepressants, anticonvulsants, topical lidocaine, and opioids are used to treat NP, but they exert a wide range of adverse effects. To reduce the burden of adverse effects, we need to investigate alternative therapeutics for the management of NP. Flavonoids are the most common secondary metabolites of plants used in folkloric medicine as tranquilizers, and have been claimed to have a selective affinity to the BDZ binding site. Several studies in animal models have reported that flavonoids can reduce NP. In this paper, we emphasize the potentiality of flavonoids for the management of NP.

6-Methoxyflavanone abates cisplatin-induced neuropathic pain apropos anti-inflammatory mechanisms: A behavioral and molecular simulation study

Cisplatin is used as a first line therapy in treating cancers. However, its use is often accompanied with the development of peripheral neuropathy. 6-Methoxyflavanone (6-MeOF) is a positive allosteric modulator at GABAA receptors and is known for attenuating diabetes-induced neuropathic pain. Neuropathy was induced in male Sprague-Dawley rats (150-250 g), via intraperitoneal injection of cisplatin (3 mg/kg) once a week for four consecutive weeks. 6-MeOF (25, 50 and 75 mg/kg, i.p) and gabapentin (75 mg/kg, i.p) were administered 30 min before each cisplatin injection. Static and dynamic allodynia were assessed using von Frey filaments and cotton buds. The anti-inflammatory activity was analyzed with plethysmometer. Body weights were also measured each week. The binding affinity of 6-MeOF with chloride channel, Cyclooxygenase-1 (COX-1) and Cyclooxygenase-2 (COX-2) was studied using docking approach. The in vitro COX-1 and COX-2 inhibitory effect of 6-MeOF was conducted with COX colorimetric assay. Administration of cisplatin for four consecutive weeks induced static (decreased paw withdrawal threshold; PWT) and dynamic allodynia (decreased paw withdrawal latency; PWL). Co-administration of 6-MeOF for four weeks significantly attenuated the cisplatin-induced expression of nocifensive behaviors observed as significant increase in PWT and PWL. Moreover, it also prevented the body weight loss induced by cisplatin administration. In silico studies depicted a good interaction of 6-MeOF with chloride ion channels and COX-1 and COX-2 enzymes. The in vitro study confirmed the inhibitory activity of 6-MeOF for COX-1 and COX-2. 6-MeOF may be effective in attenuating cisplatin-induced allodynia, probably through interaction with GABAergic receptors and reducing inflammation.

Electroacupuncture for pain relief in labour inhibits spinal p38 MAPK-mediated prostaglandin E2 release and uterine prostaglandin E2 receptor expression in rats

BACKGROUND

p38 mitogen-activated protein kinase (p38 MAPK) activation involves the release of prostaglandin E2 (PGE2) and hyperalgesia. We have previously reported that electroacupuncture (EA) relieves labour pain, but the potential mechanisms remain unclear.

OBJECTIVE

To observe the effects of EA on labour pain intensity, serum PGE2 levels and the p38 MAPK signalling pathway in rats during labour.

METHODS

Female rats copulated with male rats to induce pregnancy, and then received castor oil to trigger labour. During labour, rats remained untreated (Control group, n=30) or were treated with remifentanil (n=30) or EA at Jiaji (n=30) or SP6+LI4 (n=30), respectively. The warm water tail-flick test was used to assess labour pain. Serum PGE2 levels were measured by ELISA. Protein expression of prostaglandin E2 receptor (PGER2), p38 MAPK and phospholipase A2 (PLA2) were analysed by Western blotting, and mRNA levels were measured by real-time PCR.

RESULTS

EA treatment at Jiaji or SP6+LI4 significantly relieved labour pain, decreased serum PGE2 levels and inhibited protein and gene expression of PGER2 in the myometrium. Moreover, EA reduced protein expression of PLA2 and p38 MAPK, and inhibited phosphorylation of p38 MAPK in the lumbar spinal cord but not in the cerebral grey matter. Additionally, EA markedly decreased mRNA levels of p38 MAPK in the lumbar spinal cord and significantly reduced PLA2-IV mRNA levels in both the lumbar spinal cord and cerebral grey matter.

CONCLUSIONS

This study indicates that EA relieves labour pain through, at least in part, inhibition of spinal p38 MAPK-mediated PGE2 release and uterine PGER2 expression in rats.

Celecoxib-mediated reduction of prostanoid release in Hoffa's fat pad from donors with cartilage pathology results in an attenuated inflammatory phenotype

OBJECTIVE

The Hoffa's fat pad (HFP) is an intra-articular adipose tissue which is situated under and behind the patella. It contains immune cells next to adipocytes and secretes inflammatory factors during osteoarthritis (OA). In this study, we compared the release profile of prostanoids, which are involved in inflammation, of HFP from OA patients vs patients with a focal cartilage defect (CD) without evidence for OA on MRI and investigated the prostanoid modulatory anti-inflammatory action of celecoxib on HFP.

DESIGN

Prostanoid release was analyzed in conditioned medium of HFP explant cultures from 17 osteoarthritic patients and 12 CD patients, in the presence or absence of celecoxib. Furthermore, gene expression of COX enzymes and expression of genes indicative of a pro-inflammatory or anti-inflammatory phenotype of HFP was analyzed.

RESULTS

Prostanoid release by HFP from knee OA patients clustered in two subgroups with high and low prostanoid producers. HFP from high prostanoid producers released higher amounts of PGE₂, PGF_2α and PGD₂ compared to HFP from CD patients. PGE₂ release by OA HFP was positively associated with expression of genes known to be expressed by M1 macrophages, indicating a role for macrophages. Celecoxib modulated prostanoid release by HFP, and also modulated the inflammation ratio towards a more favorable anti-inflammatory M2 phenotype, most effectively in patients with higher prostanoid release profiles.

CONCLUSION

In knee OA patients with inflamed HFP's, celecoxib may exert positive effects in the knee joint via decreasing the release of prostanoids produced by the HFP and by favorably modulating the anti-inflammatory marker expression in HFP.

Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla

Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action. Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB₁ receptors by the acetaminophen metabolite and endocannabinoid reuptake inhibitor AM 404. However, the contribution of the cannabinoid system to antihyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB₁ receptors have remained elusive. Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent antihyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their antihyperalgesic activity in CB₁^-/- mice, confirming the involvement of the cannabinoid system. Consistent with a mechanism downstream of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia induced by intrathecal prostaglandin E₂ To distinguish between a peripheral/spinal and a supraspinal action, we administered acetaminophen and AM 404 to hoxB8-CB₁^-/- mice, which lack CB₁ receptors from the peripheral nervous system and the spinal cord. These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action. Accordingly, local injection of the CB₁ receptor antagonist rimonabant into the rostral ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but not in CB₁^-/- mice. Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB₁ receptors reside in the rostral ventromedial medulla.SIGNIFICANCE STATEMENT Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action, including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue-specific CB₁ receptor-deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.

The Synergistic Effect of Intrathecally Administered Dexmedetomidine and Ketorolac on Mechanical Allodynia in Rats with Spinal Nerve Ligation

Objectives This research was carried out to identify the synergistic effect of dexmedetomidine and ketorolac on neuropathic pain alleviation. Methods The anti-allodynic effect of intrathecal dexmedetomidine and ketorolac was investigated in rats after L5 spinal nerve ligation (SNL). Mechanical allodynia was assessed using Von Frey filaments. Every day for 3 consecutive days, beginning on the 10th day after SNL, behavioral tests were carried out at 1, 2, and 4 hr after drug injection. Results Significant increases in ipsilateral paw withdrawal thresholds (PWTs) were observed 1, 2, and 4 hr after drug injection in the groups of rats which received intrathecal injection of either dexmedetomidine (group D) or ketorolac (group K), compared to group S (P< 0.05). And group DK, which received simultaneous intrathecal injection of both dexmedetomidine and ketorolac, showed statistically significantly higher ipsilateral PWTs than groups D and K, which received only one of them (P< 0.05). Conclusions The results of this research demonstrated the anti-allodynic effect of dexmedetomidine and ketorolac on neuropathic pain induced by SNL in rats. They also suggest that synergistic analgesia can be induced by the simultaneous injection of dexmedetomidine and ketorolac, and that combination therapy is an effective approach to treating chronic neuropathic pain syndrome.

Simultaneous Inhibition of PGE2 and PGI2 Signals Is Necessary to Suppress Hyperalgesia in Rat Inflammatory Pain Models

Prostaglandin E2 (PGE2) is well known as a mediator of inflammatory symptoms such as fever, arthritis, and inflammatory pain. In the present study, we evaluated the analgesic effect of our selective PGE2 synthesis inhibitor, compound I, 2-methyl-2-[cis-4-([1-(6-methyl-3-phenylquinolin-2-yl)piperidin-4-yl]carbonyl amino)cyclohexyl] propanoic acid, in rat yeast-induced acute and adjuvant-induced chronic inflammatory pain models. Although this compound suppressed the synthesis of PGE2 selectively, no analgesic effect was shown in both inflammatory pain models. Prostacyclin (PGI2) also plays crucial roles in inflammatory pain, so we evaluated the involvement of PGI2 signaling in rat inflammatory pain models using prostacyclin receptor (IP) antagonist, RO3244019. RO3244019 showed no analgesic effect in inflammatory pain models, but concomitant administration of compound I and RO3244019 showed analgesic effects comparable to celecoxib, a specific cyclooxygenase- (COX-) 2 inhibitor. Furthermore, coadministration of PGE2 receptor 4 (EP4) antagonist, CJ-023423, and RO3244019 also showed an analgesic effect. These findings suggest that both PGE2 signaling, especially through the EP4 receptor, and PGI2 signaling play critical roles in inflammatory pain and concurrent inhibition of both signals is important for suppression of inflammatory hyperalgesia.

Prostaglandins in migraine: update

PURPOSE OF REVIEW

This review presents recent findings on the role of prostaglandins in migraine pathophysiology.

RECENT FINDINGS

Experimental studies have shown that prostaglandins are distributed in the trigeminal-vascular system and its receptors are localized in the trigeminal ganglion and the trigeminal nucleus caudalis. Prostaglandins were found in smooth muscles of cranial arteries, and functional studies in vivo showed that prostaglandins induced dilatation of cranial vessels. Human studies showed that intravenous infusion of vasodilating prostaglandins such as prostaglandin E₂ (PGE₂), prostaglandin I₂ (PGI₂) and prostaglandin D₂ (PGD₂) induced headache and dilatation of intra-cranial and extra-cranial arteries in healthy volunteers. In contrast, infusion of non-dilating prostaglandin F₂α (PGF₂α) caused no headache or any vascular responses in cranial arteries. PGE₂ and PGI₂ triggered migraine-like attacks in migraine patients without aura, accompanied by dilatation of the intra-cerebral and extra-cerebral arteries. A novel EP4 receptor antagonist could not prevent PGE₂-induced headache in healthy volunteers.

SUMMARY

Recent in-vitro/in-vivo data demonstrated presence and action of prostaglandins within the trigeminal pain pathways. Migraine induction after intravenous administration of PGE₂ and PGI₂ suggests a specific blockade of their receptors, EP and IP respectively, as a new potential drug target for the acute treatment of migraine.

The modern pharmacology of paracetamol: therapeutic actions, mechanism of action, metabolism, toxicity and recent pharmacological findings

Paracetamol is used worldwide for its analgesic and antipyretic actions. It has a spectrum of action similar to that of NSAIDs and resembles particularly the COX-2 selective inhibitors. Paracetamol is, on average, a weaker analgesic than NSAIDs or COX-2 selective inhibitors but is often preferred because of its better tolerance. Despite the similarities to NSAIDs, the mode of action of paracetamol has been uncertain, but it is now generally accepted that it inhibits COX-1 and COX-2 through metabolism by the peroxidase function of these isoenzymes. This results in inhibition of phenoxyl radical formation from a critical tyrosine residue essential for the cyclooxygenase activity of COX-1 and COX-2 and prostaglandin (PG) synthesis. Paracetamol shows selectivity for inhibition of the synthesis of PGs and related factors when low levels of arachidonic acid and peroxides are available but conversely, it has little activity at substantial levels of arachidonic acid and peroxides. The result is that paracetamol does not suppress the severe inflammation of rheumatoid arthritis and acute gout but does inhibit the lesser inflammation resulting from extraction of teeth and is also active in a variety of inflammatory tests in experimental animals. Paracetamol often appears to have COX-2 selectivity. The apparent COX-2 selectivity of action of paracetamol is shown by its poor anti-platelet activity and good gastrointestinal tolerance. Unlike both non-selective NSAIDs and selective COX-2 inhibitors, paracetamol inhibits other peroxidase enzymes including myeloperoxidase. Inhibition of myeloperoxidase involves paracetamol oxidation and concomitant decreased formation of halogenating oxidants (e.g. hypochlorous acid, hypobromous acid) that may be associated with multiple inflammatory pathologies including atherosclerosis and rheumatic diseases. Paracetamol may, therefore, slow the development of these diseases. Paracetamol, NSAIDs and selective COX-2 inhibitors all have central and peripheral effects. As is the case with the NSAIDs, including the selective COX-2 inhibitors, the analgesic effects of paracetamol are reduced by inhibitors of many endogenous neurotransmitter systems including serotonergic, opioid and cannabinoid systems. There is considerable debate about the hepatotoxicity of therapeutic doses of paracetamol. Much of the toxicity may result from overuse of combinations of paracetamol with opioids which are widely used, particularly in USA.

Role of prostaglandin D2 in mast cell activation-induced sensitization of esophageal vagal afferents

Sensitization of esophageal afferents plays an important role in esophageal nociception, but the mechanism is less clear. Our previous studies demonstrated that mast cell (MC) activation releases the preformed mediators histamine and tryptase, which play important roles in sensitization of esophageal vagal nociceptive C fibers. PGD2 is a lipid mediator released by activated MCs. Whether PGD2 plays a role in this sensitization process has yet to be determined. Expression of the PGD2 DP1 and DP2 receptors in nodose ganglion neurons was determined by immunofluorescence staining, Western blotting, and RT-PCR. Extracellular recordings were performed in ex vivo esophageal-vagal preparations. Action potentials evoked by esophageal distension were compared before and after perfusion of PGD2, DP1 and DP2 receptor agonists, and MC activation, with or without pretreatment with antagonists. The effect of PGD2 on 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled esophageal nodose neurons was determined by patch-clamp recording. Our results demonstrate that DP1 and DP2 receptor mRNA and protein were expressed mainly in small- and medium-diameter neurons in nodose ganglia. PGD2 significantly increased esophageal distension-evoked action potential discharges in esophageal nodose C fibers. The DP1 receptor agonist BW 245C mimicked this effect. PGD2 directly sensitized DiI-labeled esophageal nodose neurons by decreasing the action potential threshold. Pretreatment with the DP1 receptor antagonist BW A868C significantly inhibited PGD2 perfusion- or MC activation-induced increases in esophageal distension-evoked action potential discharges in esophageal nodose C fibers. In conclusion, PGD2 plays an important role in MC activation-induced sensitization of esophageal nodose C fibers. This adds a novel mechanism of visceral afferent sensitization.

COX-1-dependent prostaglandin D2 in microglia contributes to neuropathic pain via DP2 receptor in spinal neurons

Cyclooxygenase (COX) enzyme synthesizes prostaglandins (PGs) from arachidonic acid and exists as two major isozymes, COX-1 and COX-2. The crucial role of prostaglandins in the pathogenesis of inflammatory pain in peripheral tissue and the spinal cord has been established; however its expression dynamics after peripheral nerve injury and its role in neuropathic pain are not clear. In this study, we examined the detailed expression patterns of genes for COX, PGD2 and thromboxane A2 synthases and their receptors in the spinal cord. Furthermore, we explored the altered gene expression of these molecules using the spared nerve injury (SNI) model. We also examined whether these molecules have a role in the development or maintenance of neuropathic pain. We found a number of interesting results in this study, the first was that COX-1 was constitutively expressed in the spinal cord and up-regulated in microglia located in laminae I-II after nerve injury. Second, COX-2 mRNA expression was induced in blood vessels after nerve injury. Third, TXA2 synthase and hematopoietic PGD synthase mRNAs were dramatically increased in the microglia after nerve injury. Finally, we found that intrathecal injection of a COX-1 inhibitor and DP2 receptor antagonist significantly attenuated the mechanical allodynia. Our findings indicate that PGD2 produced by microglia is COX-1 dependent, and that neurons in the spinal cord can receive PGD2 from microglia following peripheral nerve injury. We believe that PGD2 signaling via DP2 signaling pathway from microglia to neurons is one of the triggering factors for mechanical allodynia in this neuropathic pain model.

Neuroplastic alteration of TTX-resistant sodium channel with visceral pain and morphine-induced hyperalgesia

The discovery of the tetrodotoxin-resistant (TTX-R) Na⁺ channel in nociceptive neurons has provided a special target for analgesic intervention. In a previous study we found that both morphine tolerance and persistent visceral inflammation resulted in visceral hyperalgesia. It has also been suggested that hyperexcitability of sensory neurons due to altered TTX-R Na⁺ channel properties and expression contributes to hyperalgesia; however, we do not know if some TTX-R Na⁺ channel property changes can be triggered by visceral hyperalgesia and morphine tolerance, or whether there are similar molecular or channel mechanisms in both situations. To evaluate the effects of morphine tolerance and visceral inflammation on the channel, we investigated the dorsal root ganglia (DRG) neuronal change following these chronic treatments. Using whole-cell patch clamp recording, we recorded TTX-R Na⁺ currents in isolated adult rat lumbar and sacral (L6−S2) DRG neurons from normal and pathologic rats with colon inflammatory pain or chronic morphine treatment. We found that the amplitudes of TTX-R Na⁺ currents were significantly increased in small-diameter DRG neurons with either morphine tolerance or visceral inflammatory pain. Meanwhile, the result also showed that those treatments altered the kinetics properties of the electrical current (ie, the activating and inactivating speed of the channel was accelerated). Our current results suggested that in both models, visceral chronic inflammatory pain and morphine tolerance causes electrophysiological changes in voltage-gated Na channels due to the chronic administration of these medications. For the first time, the present investigation explored the adaptations of this channel, which may contribute to the hyperexcitability of primary afferent nerves and hyperalgesia during these pathologic conditions. The results also suggest that neurophysiologic mechanisms of morphine tolerance and visceral hyperalgesia are related at the TTX-R Na⁺ channel.

Effect of intrathecal administration of E-series prostaglandin 1 receptor antagonist in a cyclophosphamide-induced cystitis rat model

OBJECTIVES

To investigate the effect of intrathecal administration of E-series prostaglandin 1 antagonist in cyclophosphamide-induced murine cystitis.

METHODS

Female Wistar rats were used for this experimental study. Intrathecal administration of E-series prostaglandin 1 antagonist (ONO-8711; 0.5, 5 and 50 µg) in sham controls and rats with cystitis induced by a single intraperitoneal injection of cyclophosphamide (300 mg/kg) was assessed by evaluating micturition pressure and intercontraction interval using a conscious-filling cystometry at 48 h after cyclophosphamide or saline injection. In both groups, prostaglandin E2 concentrations and the expression of E-series prostaglandin 1 receptor in the spinal cord were measured by enzyme-linked immunosorbent assay and reverse transcription polymerase chain reaction, respectively.

RESULTS

Rats with cyclophosphamide-induced cystitis showed a shorter intercontraction interval compared with controls, where the cumulative intrathecal administration of ONO-8711 did not significantly change micturition pressure or intercontraction interval compared with the baseline. In rats with cyclophosphamide-induced cystitis, each dose of ONO-8711 significantly increased the intercontraction interval compared with the baseline (46% increase at 50 µg intrathecally). Polymerase chain reaction revealed the expression of E-series prostaglandin 1 receptor in the spinal cord of both sham and cyclophosphamide-induced cystitis rats. In rats with cyclophosphamide-induced cystitis, PGE2 concentration in the dorsal horn of the L5-6 spinal cord was significantly higher than that in controls (3.55 ± 1.24 vs 0.99 ± 0.06 pg/mg tissue).

CONCLUSIONS

In rats with cyclophosphamide-induced cystitis, urinary frequency seems to be caused by prostaglandin E2 acting on E-series prostaglandin 1 receptor at the level of the spinal cord. Blockade of the spinal E-series prostaglandin 1 receptor by ONO-8711 might have a therapeutic potential in the control of interstitial cystitis/bladder pain syndrome.

Prostaglandin E(2) induces immediate migraine-like attack in migraine patients without aura

BACKGROUND

Prostaglandin E(2) (PGE(2)) has been suggested to play an important role in the pathogenesis of migraine. In the present experiment we investigated if an intravenous infusion of PGE(2) would induce migraine-like attacks in patients with migraine.

METHODS

Twelve patients with migraine without aura were randomly allocated to receive 0.4 µg/kg/min PGE(2) (Prostin(®)E2, dinoprostone) or placebo over 25 minutes in a two-way, crossover study. Headache intensity was recorded on a verbal rating scale, middle cerebral artery blood flow velocity (V(MCA)) was measured by transcranial Doppler (TCD) and diameter of the superficial temporal artery (STA) was obtained by c-series scan (Dermascan C).

RESULTS

In total, nine migraine patients (75%) experienced migraine-like attacks after PGE(2) compared to none after placebo (p = 0.004). Seven out of 9 (58%) patients reported the migraine-like attacks during the immediate phase (0-90 min) (p = 0.016). Only two patients experienced the delayed migraine-like attacks several hours after the PGE(2) infusion stop (p = 0.500). The V(MCA) decreased during the PGE(2) infusion (p = 0.005) but there was no significant dilatation of the STA (p = 0.850).

CONCLUSION

The migraine-like attacks during, and immediately after, the PGE(2) infusion contrast with those found in previous provocation studies, in which the other pharmacological compounds triggered the delayed migraine-like attacks several hours after the infusion. We suggest that PGE(2) may be one of the important final products involved in the generation of migraine attacks.

Discrepancy between strong cephalic arterial dilatation and mild headache caused by prostaglandin D2 (PGD2)

Introduction: Prostaglandins (PGs) are involved in nociception and mast cell degranulation. Prostaglandin D2 (PGD2) is a vasodilatator released during mast cell degranulation. The headache-eliciting effect of PGD2 has not been studied in man.

Subjects and methods: Twelve healthy volunteers were randomly allocated to receive intravenous infusion of 384 ng/kg/min PGD2 over 25 min in a placebo-controlled, double-blind cross-over study. We recorded headache intensity and associated symptoms, velocity in the middle cerebral artery (VMCA) and diameter of the superficial temporal artery (STA) and radial artery (RA) using ultrasonography.

Results: In the period 0–14 h, 11 subjects reported headache on PGD2 compared to one subject on placebo ( P = 0.002). During the in-hospital phase (0–120 min), the area under the headache curve was larger on PGD2 compared to placebo ( P < 0.05). Median peak headache, 1 (0–1), occurred 10 min after start of PGD2 infusion. There was no difference in incidence of headache in the post-hospital phase between PGD2 ( n = 3) and placebo ( n = 1). There was a decrease in VMCA ( P < 0.001), increase in STA ( P < 0.001) and RA ( P < 0.006) diameter during PGD2 infusion compared to placebo. Peak decrease in VMCA was 28.3% after 10 min and peak increase in STA was 55.7% after 20 min on the PGD2 day. Conclusions: The present study shows that PGD2 is a very strong vasodilator of MCA, STA and RA, but causes only mild headache.

Synergy of TLR2 and H1R on Cox-2 Activation in Pulpal Cells

Although pulp fibroblasts are a major cell type in dental pulp, their roles in microbial recognition and pulpal inflammation are not well-understood. Considering the pivotal role of Toll-like receptors (TLRs) in the recognition of micro-organisms, we hypothesized that TLRs on pulp fibroblasts may induce inflammatory signals in dental pulp. In human pulp fibroblasts, TLR2, 3, 4, and 5 were constitutively expressed. Stimulation of TLR2 and 3 induced the expression of pro-inflammatory genes such as CXCL10, CCL5, and/or Cox-2 in pulp fibroblasts. Interestingly, histamine synergistically activated TLR2-mediated Cox-2 expression and PGE(2) production. The synergistic effect of histamine is mediated by histamine receptor-1 (H1R). Studies on the intra-cellular signaling pathways revealed that p38 activation is required for the synergistic activation of Cox-2 by TLR2 and histamine. Analysis of these data suggests that TLR2 on pulp fibroblasts, in concert with H1R, can induce an inflammatory response during microbial infection in dental pulp.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Sensitization of central trigeminovascular neurons: blockade by intravenous naproxen infusion

We have previously observed that migraine attacks impervious to triptan therapy were readily terminated by subsequent i.v. administration of the non-steroidal anti-inflammatory drug (NSAID) ketorolac. Since such attacks were associated with periorbital allodynia--a symptom of central sensitization--we examined whether infusion of the NSAID naproxen can block sensitization of central trigeminovascular neurons in the medullary dorsal horn, using in vivo single-unit recording in the rat. Topical exposure of the cerebral dura to inflammatory soup (IS) for 5 min resulted in a short-term burst of activity (<8 min) and a long-lasting (>120 min) neuronal hyper-responsiveness to stimulation of the dura and periorbital skin (group 1). Infusion of naproxen (1 mg/kg) 2 h after IS (group 1) brought all measures of neuronal responsiveness back to the baseline values recorded prior to IS, and depressed ongoing spontaneous activity well below baseline. When given preemptively 1 h before IS (group 2), naproxen blocked the short-term burst of activity and every long-term measure of neuronal hyper-responsiveness that was studied in the central neurons. The same preemptive treatment, however, failed to block IS-induced short-term bursts of activity in C-unit meningeal nociceptors (group 3). The results suggest that parenteral administration of naproxen, unlike triptan therapy, can exert direct inhibition over central trigeminovascular neurons in the dorsal horn. Though impractical as a routine migraine therapy, parenteral NSAID administration should be useful as a non-narcotic rescue therapy for migraine in the setting of the emergency department.

Prostanoids in nociception and pain

Prostaglandins are lipid mediators produced by cyclooxygenases from arachidonic acid, which serve pivotal functions in inflammation and pain. Inhibition of their production is the major analgesic mechanism of action of non-steroidal anti-inflammatory drugs (NSAIDs)-but also the source of most of their unwanted effects. While the development of selective inhibitors of inducible cyclooxygenase (COX)-2 (so called coxibs) has greatly reduced gastrointestinal side effects, the recent disappointment about a potential cardiovascular toxicity of COX-2-selective inhibitors has boosted interest in alternative targets. The discovery of several prostaglandin synthases and of distinct prostaglandin receptors has unraveled an unforeseen diversity within the prostanoid synthetic pathway. Behavioral and electrophysiological work in particular with genetically engineered mice meanwhile provides new clues to the role of different prostaglandins, prostaglandin synthases and prostaglandin receptors in pain pathways.

Upregulation of cerebrospinal fluid and peripheral prostaglandin E2 in a rat postoperative pain model

Analgesic management of postoperative pain associated with thoracic surgery remains a difficult clinical challenge. In the present study we used a thoracic muscle incision model to characterize pain-related behavior and changes in prostaglandin E2 (PGE2) in both thoracic cerebrospinal fluid (CSF) and incision site tissues. A deep muscle incision was made in the left thoracic region of rats anesthetized with isoflurane, propofol, or spinal bupivacaine. Thoracic CSF and incision site tissue concentrations of PGE2 were monitored for 6 h using microdialysis loop catheters. Postoperative pain-related behavior was assessed by recording exploratory locomotive activity. Thoracic muscle surgery decreased rearing and ambulation. Oral ketorolac or rofecoxib 3 mg/kg restored normal rearing and ambulation. Postoperative CSF PGE2 concentration increased most (threefold) with spinal anesthesia, and not at all with propofol. With surgery under isoflurane or spinal bupivacaine, presurgical oral administration of ketorolac or rofecoxib 3 mg/kg reduced postsurgical CSF PGE2 levels and tissue PGE2 levels. Intrathecal ketorolac (4 microg) reduced CSF PGE2 after surgery without affecting tissue PGE2 levels, whereas intrathecal L-745,337 (80 microg) did not reduce CSF PGE2. Thoracic surgical wounds increase pain-related behavior and CSF and tissue PGE2 levels, all of which can be attenuated by oral cyclooxygenase inhibitors.

Postoperative Modulation of Central Nervous System Prostaglandin E2by Cyclooxygenase Inhibitors after Vascular Surgery: Retracted

BACKGROUND

The clinical availability of injectable cyclooxygenase inhibitors allows examination of the importance of cyclooxygenase 1 and 2 after surgery. The authors hypothesize that spinal prostaglandin E2 increases with lower extremity vascular surgery and that spinal prostaglandin E2 decreases with intravenous postsurgical administration of either a mixed cyclooxygenase 1/2 inhibitor (ketorolac) or a cyclooxygenase 2 selective inhibitor (parecoxib).

METHODS

Thirty patients undergoing elective lower extremity revascularization under continuous spinal anesthesia had cerebrospinal fluid obtained at baseline and then up to 6 h after the start of surgery. Four hours after surgical incision, patients were randomized to receive intravenous parecoxib 40 mg, ketorolac 30 mg, or preservative-free normal saline. Patients were administered intravenous fentanyl in the postanesthesia care unit and acetaminophen/oxycodone on the surgical ward to control pain.

RESULTS

Cerebrospinal fluid prostaglandin E2 concentrations were increased during and after surgery. After surgery, intravenous parecoxib 40 mg rapidly decreased cerebrospinal fluid prostaglandin E2, and intravenous ketorolac 30 mg also reduced cerebrospinal fluid prostaglandin E2 compared with placebo, but not as much as parecoxib. Postanesthesia care unit pain scores were reduced in the two drug groups compared with placebo, and surgical ward pain scores were also decreased for both drug groups, especially with parecoxib. No patient receiving parecoxib required postoperative intravenous fentanyl. Acetaminophen/oxycodone consumption was reduced in both drug groups compared with placebo, more so with parecoxib.

CONCLUSIONS

Cerebrospinal fluid prostaglandin E2 is elevated in patients after lower extremity vascular surgery. Postsurgical intravenous administration of the cyclooxygenase 1/2 inhibitor ketorolac, and especially the cyclooxygenase 2 inhibitor parecoxib, reduces cerebrospinal fluid prostaglandin E2 concentration and postoperative pain.

The opioid peptide nociceptin/orphanin FQ mediates prostaglandin E2-induced allodynia, tactile pain associated with nerve injury

Pain often outlasts its usefulness as warning and aid in wound healing, and becomes chronic and intractable after tissue damage and nerve injury. Many molecules have been implicated as mediators and modulators in persistent pain such as hyperalgesia and tactile pain (allodynia). We previously showed that prostaglandin (PG) E(2), PGF(2alpha) or the neuropeptide nociceptin, also called orphanin FQ (N/OFQ) administered intrathecally (i.t.) produced allodynia in conscious mice. In the present study, we examined the relationship of pain responses between PGs and N/OFQ using the N/OFQ receptor (NOP) antagonist, N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxy-methyl)benzamide monohydrochloride (JTC-801), and in mice lacking the N/OFQ prepropeptide (ppN/OFQ(-/-)) and the NOP receptor (NOP(-/-)). JTC-801 dose-dependently blocked the N/OFQ- and PGE(2)-induced allodynia, but not the PGF(2alpha)-induced one. Neither N/OFQ nor PGE(2) induced allodynia in NOP(-/-) mice. By contrast, the N/OFQ-induced allodynia was not affected by inhibition of PG production by a 60-min pretreatment with the non-steroidal anti-inflammatory drug, indomethacin. Among PGE receptor (EP) subtype-selective agonists, the EP4 agonist, AE1-329, markedly stimulated the release of N/OFQ from spinal slices and induced allodynia. AE1-329 also increased nitric oxide production in spinal slices using fluorescent nitric oxide detection, which was blocked by pretreatment with JTC-801. Conversely, PGE(2)-induced allodynia was not observed in ppN/OFQ(-/-) mice. N/OFQ immunoreactive puncta were colocalized with EP4. Taken together, these results demonstrate that PGE(2) induced allodynia by stimulation of N/OFQ release in the spinal cord via EP4 receptor subtypes.

Phospholipase A2 activation by melittin enhances spontaneous glutamatergic excitatory transmission in rat substantia gelatinosa neurons

In order to know a role of phospholipase A2 in modulating nociceptive transmission, the effect of a secreted phospholipase A2 activator melittin on spontaneous glutamatergic excitatory transmission was investigated in substantia gelatinosa neurons of an adult rat spinal cord slice by using the whole-cell patch-clamp technique. Bath-applied melittin at concentrations higher than 0.5 microM increased both the amplitude and the frequency of spontaneous excitatory postsynaptic current in a manner independent of tetrodotoxin; the latter effect of which was examined in detail. In 80% of the neurons examined (n = 64), melittin superfused for 3 min gradually increased spontaneous excitatory postsynaptic current frequency (by 65+/-6% at 1 microM; n = 51) in a dose-dependent manner (effective concentration for half-maximal effect = 1.1 microM). This effect subsided within 3 min after washout. The spontaneous excitatory postsynaptic current frequency increase produced by melittin was reduced by the phospholipase A2 inhibitor 4-bromophenacryl bromide (10 microM) while being unaffected by the cyclooxygenase inhibitor indomethacin (100 microM) and the lipoxygenase inhibitor nordihydroguaiaretic acid (100 microM). A similar increase in spontaneous excitatory postsynaptic current frequency was produced by exogenous arachidonic acid (50 microM); this effect was also unaffected by the cyclooxygenase or lipoxygenase inhibitor. Melittin failed to increase spontaneous excitatory postsynaptic current frequency in a nominally Ca2+-free or La3+-containing Krebs solution. We conclude that melittin increases the spontaneous release of L-glutamate to substantia gelatinosa neurons by activating secreted phospholipase A2 and increasing Ca2+ influx through voltage-gated Ca2+ channels in nerve terminals, probably with an involvement of arachidonic acid but not its metabolites produced by cyclooxygenase and lipoxygenase. Considering that the substantia gelatinosa plays an important role in regulating nociceptive transmission, it is suggested that this transmission may be positively modulated by secreted phospholipase A2 activation in the substantia gelatinosa.

COX-dependent mechanisms involved in the antinociceptive action of NSAIDs at central and peripheral sites

Despite the diverse chemical structure of aspirin-like drugs, the antinociceptive effect of NSAIDs is mainly due to their common property of inhibiting cyclooxygenases involved in the formation of prostaglandins. Prostaglandins are potent hyperalgesic mediators which modulate multiple sites along the nociceptive pathway and enhance both transduction (peripheral sensitizing effect) and transmission (central sensitizing effect) of nociceptive information. Inhibition of the formation of prostaglandins at peripheral and central sites by NSAIDs thus leads to the normalisation of the increased pain threshold associated with inflammation. The contribution of peripheral and central mechanisms to the overall antinociceptive action of NSAIDs depends on several factors including the location of the targets of drug action, the site of drug delivery and the uptake and distribution to the site of action. The present work reviews the data on the regulation and location of cyclooxygenases at central and peripheral sites of the nociceptive pathway and focuses on the role of COX in the generation and maintenance of pain hypersensitivity. Experimental and clinical evidences are used to evaluate the significance of the peripheral and central antihyperalgesic effects of NSAIDs.

Spinal inflammatory hyperalgesia is mediated by prostaglandin E receptors of the EP2 subtype

Blockade of prostaglandin (PG) production by COX inhibitors is the treatment of choice for inflammatory pain but is also prone to severe side effects. Identification of signaling elements downstream of COX inhibition, particularly of PG receptor subtypes responsible for pain sensitization (hyperalgesia), provides a strategy for better-tolerated analgesics. Here, we have identified PGE2 receptors of the EP2 receptor subtype as key signaling elements in spinal inflammatory hyperalgesia. Mice deficient in EP2 receptors (EP2-/- mice) completely lack spinal PGE2-evoked hyperalgesia. After a peripheral inflammatory stimulus, EP2-/- mice exhibit only short-lasting peripheral hyperalgesia but lack a second sustained hyperalgesic phase of spinal origin. Electrophysiological recordings identify diminished synaptic inhibition of excitatory dorsal horn neurons as the dominant source of EP2 receptor-dependent hyperalgesia. Our results thus demonstrate that inflammatory hyperalgesia can be treated by targeting of a single PG receptor subtype and provide a rational basis for new analgesic strategies going beyond COX inhibition.

Spinal inflammatory hyperalgesia is mediated by prostaglandin E receptors of the EP2 subtype

Blockade of prostaglandin (PG) production by COX inhibitors is the treatment of choice for inflammatory pain but is also prone to severe side effects. Identification of signaling elements downstream of COX inhibition, particularly of PG receptor subtypes responsible for pain sensitization (hyperalgesia), provides a strategy for better-tolerated analgesics. Here, we have identified PGE2 receptors of the EP2 receptor subtype as key signaling elements in spinal inflammatory hyperalgesia. Mice deficient in EP2 receptors (EP2-/- mice) completely lack spinal PGE2-evoked hyperalgesia. After a peripheral inflammatory stimulus, EP2-/- mice exhibit only short-lasting peripheral hyperalgesia but lack a second sustained hyperalgesic phase of spinal origin. Electrophysiological recordings identify diminished synaptic inhibition of excitatory dorsal horn neurons as the dominant source of EP2 receptor-dependent hyperalgesia. Our results thus demonstrate that inflammatory hyperalgesia can be treated by targeting of a single PG receptor subtype and provide a rational basis for new analgesic strategies going beyond COX inhibition.

Emerging drugs for neuropathic pain

Although there are many analgesics on the market for the treatment of nociceptive pain, there are none with FDA approval for the treatment of neuropathic pain. With a better understanding of the anatomy and physiology of pain, there is a significant effort in developing new drugs that interact specifically with pain pathways. This higher drug specificity is likely to result in drugs that are more efficacious with fewer side effects. This has led to the development of many drugs for the treatment of neuropathic pain. These drugs are divided into the following therapeutic classes: 1) N-methyl-D-aspartate (NMDA) receptor antagonists, 2) ion channel antagonists, 3) alpha2-agonists, 4) nicotinic receptor agonists, 5) prostaglandin receptor antagonists, 6) adenosine agonists and adenosine kinase inhibitors, 7) neuropeptide antagonists, and 8) prosaposins. The results of preclinical and clinical trials are promising for these new agents. Whether these agents will be efficacious as single agents is yet to be determined; however, preliminary results show that combination therapy may be more beneficial with fewer side effects.

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.

Development of tactile allodynia and thermal hyperalgesia by intrathecally administered platelet-activating factor in mice

Platelet-activating factor (PAF) is a potent inflammatory lipid mediator in peripheral tissues. However, its role in mediation of nociception in central nervous system is unknown. In the present study, whether PAF plays some role in pain transduction in the spinal cord was studied in mice. Intrathecal injection of PAF induced tactile pain, tactile allodynia at as low as 10 fg to 1 pg with a peak response at 100 fg, while lyso-PAF was without effect in the range of doses. Tactile allodynia induced by PAF was blocked by a PAF receptor antagonists, TCV-309, WEB 2086 and BN 50739. The expression of PAF receptor mRNA by RT-PCR was observed in DRG and spinal cord in mice. ATP P2X receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid and 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5-triphosphate, NMDA receptor antagonist, MK 801 and nitric oxide synthetase inhibitor, 7-nitroindazole blocked the PAF-induced tactile allodynia. PAF-induced tactile allodynia and thermal hyperalgesia disappeared in neonatally capsaicin-treated adult mice, while tactile allodynia but not thermal hyperalgesia induced by intrathecally injected alpha,beta-methylene ATP, a P2X receptor agonist, was capsaicin-insensitive. The present study demonstrated that PAF is a potent inducer of tactile allodynia and thermal hyperalgesia at the level of the spinal cord. PAF-evoked tactile allodynia is suggested to be mediated by ATP and the following NMDA and NO cascade through capsaicin-sensitive fiber, different from exogenously injected alpha,beta-methylene ATP which is insensitive to capsaicin treatment.

Changes in the effect of spinal prostaglandin E2 during inflammation: prostaglandin E (EP1-EP4) receptors in spinal nociceptive processing of input from the normal or inflamed knee joint

Inflammatory pain is caused by sensitization of peripheral and central nociceptive neurons. Prostaglandins substantially contribute to neuronal sensitization at both sites. Prostaglandin E2 (PGE2) applied to the spinal cord causes neuronal hyperexcitability similar to peripheral inflammation. Because PGE2 can act through EP1-EP4 receptors, we addressed the role of these receptors in the spinal cord on the development of spinal hyperexcitability. Recordings were made from nociceptive dorsal horn neurons with main input from the knee joint, and responses of the neurons to noxious and innocuous stimulation of the knee, ankle, and paw were studied after spinal application of recently developed specific EP1-EP4 receptor agonists. Under normal conditions, spinal application of agonists at EP1, EP2, and EP4 receptors induced spinal hyperexcitability similar to PGE2. Interestingly, the effect of spinal EP receptor activation changed during joint inflammation. When the knee joint had been inflamed 7-11 hr before the recordings, only activation of the EP1 receptor caused additional facilitation, whereas spinal application of EP2 and EP4 receptor agonists had no effect. Additionally, an EP3alpha receptor agonist reduced responses to mechanical stimulation. The latter also attenuated spinal hyperexcitability induced by spinal PGE2. In isolated DRG neurons, the EP3alpha agonist reduced the facilitatory effect of PGE2 on TTX-resistant sodium currents. Thus pronociceptive effects of spinal PGE2 can be limited, particularly under inflammatory conditions, through activation of an inhibitory splice variant of the EP3 receptor. The latter might be an interesting target for controlling spinal hyperexcitability in inflammatory pain states.

Cyclo-oxygenase-2 contributes to central sensitization in rats with peripheral inflammation

It has been widely accepted that prostaglandins are involved in peripheral mechanisms of hyperalgesia. Several lines of evidence suggest that prostaglandins also contribute to the mechanisms underlying hyperalgesia at the level of the spinal cord. The nociceptive flexor reflex of the hind limb was used to test the hypothesis that products of cyclo-oxygenase contribute to the increased excitability of spinal neurons during hyperalgesia induced by peripheral injection of complete Freund's adjuvant (CFA) into the hind paw. The reflex was evoked by electrical stimulation of the sural nerve at an intensity that activated A- and C-fibers, and muscle potentials were recorded in hamstring muscles in decerebrate, spinalized rats. Intrathecal administration of (S)-ibuprofen (1-100 nmol) dose-dependently attenuated the flexor reflex in CFA treated rats but had no effect in untreated rats. (R)-Ibuprofen had no effect on the reflex in either control or CFA-treated rats at the dose tested (100 nmol). Western blots of lumbar spinal cord extracts showed increased levels of cyclo-oxygenase (COX)-2 protein in the dorsal spinal cord of rats with peripheral inflammation; no change occurred in the level of COX-1. These results indicate that products of COX-2 contribute to the increased excitability of the spinal cord during persistent peripheral inflammation.

Functional characterization of prostaglandin F2alpha receptor in the spinal cord for tactile pain (allodynia)

Prostaglandin F2alpha (PGF2alpha) binds to its receptor (FP) to increase the intracellular-free calcium concentration ([Ca2+]i) by coupling of FP with Gq protein. Spinal intrathecal administration of PGF2alpha to mouse induces touch-evoked pain (mechanical allodynia), in which capsaicin-insensitive primary afferent Abeta-fibres and N-methyl-d-aspartate receptor epsilon 4 subunit are involved. FP in the spinal cord, however, was not well characterized. Here, we showed constitutive expression of FP mRNA in mouse spinal cord, and functionally characterized spinal FP-expressing cells which were involved in PGF2alpha-induced mechanical allodynia. The method for repetitive administration of oligodeoxyribonucleotides through tubing to conscious mice was established for mechanical allodynia evaluation. We identified an antisense oligodeoxyribonucleotide targeting FP mRNA, causing both disappearance of PGF2alpha-induced mechanical allodynia and decrease of FP mRNA. With saline-administered mice, PGF2alpha rapidly increased [Ca2+]i of the cells in the deeper layer of the dorsal horn. In contrast, when the FP antisense oligodeoxyribonucleotide was repeatedly administered, the population of PGF2alpha-responsive cells in the slices reduced, and PGF2alpha-induced [Ca2+]i increase of these cells diminished. These data strongly suggested that, in the dorsal horn of the spinal cord, there are the FP-expressing cells which are involved in PGF2alpha-induced mechanical allodynia.

Unaltered pain-related behavior in mice lacking NMDA receptor GluRepsilon 1 subunit

Noxious afferent input following tissue damage and inflammation triggers a state of neuronal hyperexcitability-a phenomenon of central sensitization-which manifests behaviorally as allodynia and hyperalgesia. At the molecular level, maintenance of central sensitization is largely dependent on the N-methyl-D-aspartate receptor (NMDAR) activation. NMDARs are composed of GluRzeta1 (NR1) and one of four GluRepsilon (NR2) subunits, which determine the functional properties of native NMDARs. Although there is accumulating evidence to implicate GluRepsilon 2-containing NMDARs in pain mechanisms, the functional significance of GluRepsilon 1-containing NMDARs in this setting has not been examined in detail. Here, we used hind paw injection of formalin, complete Freund's adjuvant and a nerve injury model to investigate the effects of GluRepsilon 1 subunit gene deletion on pain-related behavior in mice. In all of the models tested, GluRepsilon 1-deficient mice exhibited responses similar to wild-type controls. These results suggest that GluRepsilon 1 disruption does not result in altered nociceptive behavior in mice. Although the contribution of other nociceptive pathways cannot be ruled out, we speculate that the preserved function of GluRepsilon 2-containing NMDARs could explain unaltered nociceptive behavior in mutant mice.

Prostaglandin E1 but not corticosteroid increases nerve root blood flow velocity after lumbar diskectomy in surgical patients

The purpose of this study was to clarify whether prostaglandin E1 (PGE(1)) or corticosteroid could increase blood flow in the nerve root because neurologic symptoms in spinal stenosis may be based on the vascular insufficiency in the nerve root. Fifty-seven patients undergoing lumbar diskectomy were randomly assigned to one of three groups. Each group received one of three protocols for intravenous injection: 10 mL of saline solution, group A (n = 19); 10 mL of PGE(1) (20 microg) solution, group B (n = 19); and 10 mL of dexamethasone (8 mg) solution, group C (n = 19). After lumbar diskectomy, a probe for laser Doppler flowmetry was placed directly on the lumbar nerve root. Nerve root blood flow (RBF) velocity and mean arterial pressure (MAP) were measured before injection (T0), 5 minutes after the start of injection (T1), 10 minutes after the start of injection (T2), and 10 minutes after the end of injection (T3). In groups A and C, these did not change throughout the time course. In group B, MAP decreased significantly at T1 (92%; P <.001), T2 (89%; P <.0001), and T3 (91%; P <.0001), while RBF velocity increased significantly at T1 (125%; P <.05), T2 (128%; P <.05), and T3 (121%; P <.05) compared with T0. The values in group B were different from those in group A (P <.05) and group C (P <.05) at T1 and T2. The results show that intravenous injection of low-dose PGE(1), but not corticosteroid, increases RBF velocity after lumbar diskectomy.

Spinal nerve injury activates prostaglandin synthesis in the spinal cord that contributes to early maintenance of tactile allodynia

To determine if spinal prostaglandins (PG) contribute to tactile allodynia, male, Sprague-Dawley rats were fitted with either intrathecal (i.t.) microdialysis or drug delivery catheters 3 days before tight ligation of the left lumber 5/6 spinal nerves. Paw withdrawal thresholds (PWT) were determined using von Frey filaments. Ligated rats developed tactile allodynia within 24h, as evidenced by a decrease in PWT in the affected hindpaw (<4 g vs. >15 g control). Sham-operated controls were unchanged from baseline (>15 g). Allodynia was also characterized by a significant increase in the evoked release of PGE(2). Thus, brushing the plantar surface of the affected hindpaw with a cotton-tipped applicator, 5 days postligation, increased the [PGE(2)](dialysate) to 199+/-34% of the prestimulus control period. In contrast, brushing had no detectable effect on release before surgery or in sham-operated animals. Basal release (no brushing) was similar before and after surgery (sham-operated and ligated rats). In a separate group of rats and beginning 2 days after ligation, the acute i.t. injection of S(+)-ibuprofen, SC-51322, SC-236, or SC-560 significantly reversed allodynia (maximum effect=69+/-9, 66+/-6, 57+/-4, 20+/-5%, respectively). R(-)-ibuprofen or vehicle were without effect. The results of this study suggest that: (a). spinal PG synthesis and allodynia-like behaviour are triggered by normally innocuous brushing after spinal nerve ligation; (b). pharmacological disruption of this cascade significantly reverses allodynia; (c). COX-2 is the relevant isozyme; and (d). the PG effect is mediated by spinal EP receptors.

Central Components of the Analgesic/Antihyperalgesic Effect of Nimesulide

The analgesic action of NSAIDs has been attributed to the peripheral inhibition of prostaglandin synthesis via the blockade of the enzyme cyclo-oxygenase (COX) and prevention of bradykinin and cytokine-induced hyperalgesia via inhibition of the release of tumour necrosis factor-alpha. However, it is becoming increasingly evident that NSAIDs exert their analgesic effect through several mechanisms. Recent data suggest that significant expression of COX-2 is found in the central nervous system, where COX-2 seems to have, together with nitric oxide, an important role in spinal nociceptive transmission. Nitroglycerin is a nitric oxide donor and induces a hyperalgesic state, partially mediated by central mechanisms. Nimesulide is a preferential COX-2 inhibitor widely used to treat pain. In this study, we evaluated the analgesic effect of nimesulide in several animal models of pain, intending to provide additional information on the characteristics of the analgesic effect of nimesulide, with specific focus on a possible central component.

STUDY DESIGN

Nimesulide was compared with vehicle in groups of 4-10 rats that were randomly tested with different models of pain. The experimental design also included study of the effect of nimesulide upon nitroglycerin-induced neuronal activation at central sites. Analysis of variance was used to evaluate the influence of time and treatments. Differences between groups at specific time-points were analysed by post-hoc t-test. A probability level of less than 5% was regarded as significant.

METHODS

The analgesic effect of nimesulide (or vehicle) was evaluated in male Sprague-Dawley rats. The animals underwent tail-flick and formalin tests, both performed in baseline conditions and after nitroglycerin-induced hyperalgesia. Two separate groups of rats were treated with nitroglycerin alone or nimesulide followed by nitroglycerin, and their brains were processed for immunocytochemical detection of Fos protein, a marker of neuronal activation.

RESULTS

Nimesulide showed a significant analgesic effect in both the tail-flick and the formalin tests in baseline conditions. In addition, the drug proved effective in counteracting nitroglycerin-induced hyperalgesia in both tests. Brain mapping of nuclei activated by the administration of nitroglycerin showed that nimesulide pretreatment significantly inhibited neuronal activation in several areas, namely the supraoptic nucleus, ventrolateral column of the periaqueductal grey, locus coeruleus, nucleus tractus solitarius and area postrema. We conclude that nimesulide possesses a strong analgesic and antihyperalgesic activity, the mechanisms of action of which are partly central.

The prostaglandin E2 receptor-1 (EP-1) mediates acid-induced visceral pain hypersensitivity in humans

BACKGROUND & AIMS

Central sensitization, an activity-dependent increase in spinal cord neuronal excitability, has been shown to contribute to esophageal pain hypersensitivity. Prostaglandin E2 (PGE(2)) is a mediator in both peripheral and central sensitization, in part via the prostaglandin E2 receptor-1 (EP-1), and may be a potential target for treating visceral pain. The purpose of this study was to determine whether acid-induced pain hypersensitivity within the non-acid-exposed esophagus (secondary hyperalgesia) is mediated by PGE(2) activation of the EP-1 receptor.

METHODS

Twelve healthy male subjects participated in a randomized, placebo-controlled crossover study. Upper esophageal pain thresholds (PTs) to electrical stimulation were determined, and either the EP-1 antagonist ZD6416 or a placebo was orally administered. One-hour after dosing, acid or saline (0.15 mol/L) was infused into the lower esophagus for 30 minutes. Upper esophageal PT was monitored for 120 minutes after infusion.

RESULTS

Except in 1 subject (who was excluded), the pH in the upper esophagus remained above 5 throughout all studies. In 8 subjects, ZD6416 attenuated the reduction in PT in the upper esophagus normally induced by acid infusion into the lower esophagus (area under curve [AUC]: -11.9 +/- 2.5 and 6.4 +/- 6.7 for placebo and ZD6416, respectively; P < 0.01). After saline infusion, the effects of ZD6416 and placebo were similar (AUC: 9.9 +/- 6 and 4.1 +/- 2, respectively; P = 0.8). Three subjects had no reduction in PT to acid infusion with placebo and were excluded at post hoc analysis.

CONCLUSIONS

The attenuation of secondary esophageal hyperalgesia by ZD6416 suggests that PGE(2), via the EP-1 receptor, contributes to human visceral pain hypersensitivity.