Characterization of EP-receptor subtypes involved in allodynia and hyperalgesia induced by intrathecal administration of prostaglandin E2 to mice

An LPAR5-antagonist that reduces nociception and increases pruriception

Introduction

The G-protein coupled receptor LPAR₅ plays a prominent role in LPA-mediated pain and itch signaling. In this study we focus on the LPAR₅-antagonist compound 3 (cpd3) and its ability to affect pain and itch signaling, both in vitro and in vivo.

Methods

Nociceptive behavior in wild type mice was induced by formalin, carrageenan or prostaglandin E2 (PGE₂) injection in the hind paw, and the effect of oral cpd3 administration was measured. Scratch activity was measured after oral administration of cpd3, in mice overexpressing phospholipase A2 (sPLA2tg), in wild type mice (WT) and in TRPA1-deficient mice (Trpa1 KO). In vitro effects of cpd3 were assessed by measuring intracellular calcium release in HMC-1 and HEK-TRPA1 cells.

Results

As expected, nociceptive behavior (induced by formalin, carrageenan or PGE₂) was reduced after treatment with cpd3. Unexpectedly, cpd3 induced scratch activity in mice. In vitro addition of cpd3 to HEK-TRPA1 cells induced an intracellular calcium wave that could be inhibited by the TRPA1-antagonist A-967079. In Trpa1 KO mice, however, the increase in scratch activity after cpd3 administration was not reduced.

Conclusions

Cpd3 has in vivo antinociceptive effects but induces scratch activity in mice, probably by activation of multiple pruriceptors, including TRPA1. These results urge screening of antinociceptive candidate drugs for activity with pruriceptors.

Prostaglandin E2 sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 channels

Background

Cough hypersensitivity is a major characteristic feature associated with several types of cough, including chronic cough, but its underlying mechanisms remain to be fully understood. Inflammatory mediators, such as prostaglandin E₂ (PGE₂), have been implicated in both peripheral induction and sensitization of the cough reflex. In this study, using a conscious guinea pig model of cough, we investigated whether PGE₂ can sensitize the cough reflex via central actions and, if so, via which mechanisms.

Methods

All drugs were administered by intracerebroventricular (i.c.v.) route and whole-body plethysmograph set-up was used for both induction, using aerosolized citric acid (0.2 M), and recording of cough. Immunohistochemistry was performed to confirm the expression of NaV 1.8 channels in the nucleus tractus solitarius (nTS).

Results

We show that both PGE₂ and the non-selective EP1/EP3 agonist, sulprostone, dose-dependently enhanced the citric acid-induced cough (P ≤ 0.001, P ≤ 0.01, respectively). Pretreatment with the EP1 antagonist, ONO-8130, did not affect the sulprostone-induced cough sensitization, whilst the EP3 antagonist, L-798,106, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, treatment with either the EP2 agonist, butaprost or the EP4 agonist, L-902,688, had no effect on cough sensitization. Additionally, pretreatment with either the TRPV1 antagonist, JNJ-17203212 or the TRPA1 antagonist, HC-030031, alone or in combination, nor with the NaV 1.1, 1.2, 1.3, 1.4, 1.6 and 1.7 channel blocker, tetrodotoxin, had any effect on the cough. In contrast, pretreatment with the NaV 1.8 antagonist, A-803467, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, NaV 1.8 channels were shown to be expressed in the nTS.

Conclusion

Collectively, our findings show that PGE₂ sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 but independently of TRPV1,TRPA1 and TTX-sensitive sodium channel activation. These results indicate that PGE₂ plays an important role in central sensitization of the cough reflex and suggest that central EP3 receptors and/or NaVv 1.8 channels may represent novel antitussive molecular targets.

Graphical Abstract

Suppression of TLR4/MyD88/TAK1/NF-κB/COX-2 Signaling Pathway in the Central Nervous System by Bexarotene, a Selective RXR Agonist, Prevents Hyperalgesia in the Lipopolysaccharide-Induced Pain Mouse Model

A selective RXR agonist, bexarotene, has been shown to have anti-inflammatory, anti-nociceptive, and neuroprotective effects in several models of numerous neurological diseases characterized by systemic inflammation. The mechanisms underlying these effects remains unknown. To elucidate these mechanisms, we investigated whether the TLR4/MyD88/TAK1/NF-κB/COX-2 signaling pathway in the CNS mediates the effect of bexarotene to prevent hyperalgesia in the LPS-induced inflammatory pain mouse model. The reaction time to thermal stimuli within 30 s was evaluated by the hot plate test in male mice treated with saline, LPS (10 mg/kg), DMSO, and/or bexarotene (0.1, 1, 3, or 10 mg/kg) after 6 h. The latency to the thermal stimulus (18.11 ± 1.36 s) in the LPS-treated mice was significantly decreased by 30% compared with saline-treated mice (25.84 ± 1.99 s). Treatment with bexarotene only at a dose of 10 mg/kg showed a significant increase in the latency by 22.49 ± 1.00 s compared with LPS-treated mice. Bexarotene also prevented the reduction in RXRα protein expression associated with a rise in the expression of TLR4, MyD88, phosphorylated TAK1, NF-κB p65, phosphorylated NF-κB p65, COX-2, and IL-1β proteins, in addition to COX-2 activity and levels of PGE2 and IL-1β in the brains and spinal cords of the LPS-treated animals. Likely, decreased activity of TLR4/MyD88/TAK1/NF-κB/COX-2 signaling pathway in addition to increased pro-inflammatory cytokine formation in the CNS of mice participates in the protective effect of bexarotene against hyperalgesia induced by LPS.

EP4 receptor as a novel promising therapeutic target in colon cancer

The most prevalent malignancy that can occur in the gastrointestinal tract is colon cancer. The current treatment options for colon cancer patients include chemotherapy, surgery, radiotherapy, immunotherapy, and targeted therapy. Although the chance of curing the disease in the early stages is high, there is no cure for almost all patients with advanced and metastatic disease. It has been found that over-activation of cyclooxygenase 2 (COX-2), followed by the production of prostaglandin E2 (PGE2) in patients with colon cancer are significantly increased. The tumorigenic function of COX-2 is mainly due to its role in the production of PGE2. PGE2, as a main generated prostanoid, has an essential role in growth and survival of colon cancer cell's. PGE2 exerts various effects in colon cancer cells including enhanced expansion, angiogenesis, survival, invasion, and migration. The signaling of PGE2 via the EP4 receptor has been shown to induce colon tumorigenesis. Moreover, the expression levels of the EP4 receptor significantly affect tumor growth and development. Overexpression of EP4 by various mechanisms increases survival and tumor vasculature in colon cancer cells. It seems that the pathway starting with COX2, continuing with PGE2, and ending with EP4 can promote the spread and growth of colon cancer. Therefore, targeting the COX-2/PGE2/EP4 axis can be considered as a worthy therapeutic approach to treat colon cancer. In this review, we have examined the role and different mechanisms that the EP4 receptor is involved in the development of colon cancer.

C-Phycocyanin Alleviates Bladder Inflammation and Dysfunction in Cyclophosphamide-Induced Cystitis in a Mouse Model by Inhibiting COX-2 and EP4

Objective

To explore the effect of C-phycocyanin (C-PC) on voiding behavior and histological changes in cyclophosphamide- (CYP-) induced cystitis in mice.

Methods

Sixty female mice were included. The mice in the C-PC group received C-PC (25 mg/kg, twice, i.p.) and then CYP (200 mg/kg, i.p.) two hours later, while the mice in the CYP group only received the equivalent CYP. Saline was injected in the mice in the control group. A voided stain on paper (VSOP) test was conducted to analyze the micturition. The bladders were harvested for histological evaluation and measurements of inflammatory factors.

Results

C-PC reduced the micturition frequency in the mice with CYP-induced cystitis. The bladder/body weight ratio and edema were remarkably higher in the CYP group compared to the C-PC group. C-PC suppressed the expressions of COX-2, PGE₂, and EP4 (prostaglandin E receptor 4) according to the ELISA assay. Immunohistochemical staining also indicated that C-PC reduced the expressions of COX-2 in urothelium and EP4 in smooth muscles.

Conclusions

C-PC relieved symptoms associated with CYP-induced cystitis in mice by inhibiting bladder inflammation through COX-2 and EP4 expression.

Prostaglandin Signaling Governs Spike Timing-Dependent Plasticity at Sensory Synapses onto Mouse Spinal Projection Neurons

Highly correlated presynaptic and postsynaptic activity evokes spike timing-dependent long-term potentiation (t-LTP) at primary afferent synapses onto spinal projection neurons. While prior evidence indicates that t-LTP depends upon an elevation in intracellular Ca²⁺ within projection neurons, the downstream signaling pathways that trigger the observed increase in glutamate release from sensory neurons remain poorly understood. Using in vitro patch-clamp recordings from female mouse lamina I spino-parabrachial neurons, the present study demonstrates a critical role for prostaglandin synthesis in the generation of t-LTP. Bath application of the selective phospholipase A₂ (PLA₂) inhibitor arachidonyl trifluoromethyl ketone (AACOCF₃) or the cyclooxygenase 2 (Cox-2) inhibitor nimesulide prevented t-LTP at sensory synapses onto spino-parabrachial neurons. Similar results were observed following the block of the EP2 subtype of prostaglandin E₂ (PGE₂) receptor with PF 04418948. Meanwhile, perfusion with PGE₂ or the EP2 agonist butaprost potentiated the amplitude of monosynaptic primary afferent-evoked EPSCs while decreasing the paired-pulse ratio, suggesting a presynaptic site of action. Cox-2 was constitutively expressed in both spinal microglia and lamina I projection neurons within the superficial dorsal horn (SDH). Suppression of microglial activation with minocycline had no effect on the production of t-LTP, suggesting the possibility that prostaglandins produced within projection neurons could contribute to an enhanced probability of glutamate release at primary afferent synapses. Collectively, the results suggest that the amplification of ascending nociceptive transmission by the spinal SDH network is governed by PLA₂-Cox-2-PGE₂ signaling.SIGNIFICANCE STATEMENT Long-term potentiation (LTP) of primary afferent synapses contributes to the sensitization of spinal nociceptive circuits and has been linked to greater pain sensation in humans. Prior work has implicated elevated glutamate release in the generation of spike timing-dependent LTP (t-LTP) at sensory synapses onto ascending spinal projection neurons, but the underlying mechanisms remain unknown. Here we provide evidence that the activation of EP2 prostaglandin receptors by prostaglandin E₂, occurring downstream of phospholipase A₂ and cyclooxygenase 2 activation, mediates t-LTP at these synapses via changes in presynaptic function. This suggests that prostaglandins can increase the flow of nociceptive information from the spinal cord to the brain independently of their known ability to suppress synaptic inhibition within the dorsal horn.

Treatment with albumin-hydroxyoleic acid complex restores sensorimotor function in rats with spinal cord injury: Efficacy and gene expression regulation

Sensorimotor dysfunction following incomplete spinal cord injury (SCI) is often characterized by paralysis, spasticity and pain. Previously, we showed that intrathecal (i.t.) administration of the albumin-oleic acid (A-OA) complex in rats with SCI produced partial improvement of these symptoms and that oral 2-hydroxyoleic acid (HOA, a non-hydrolyzable OA analogue), was efficacious in the modulation and treatment of nociception and pain-related anxiety, respectively. Here we observed that intrathecal treatment with the complex albumin-HOA (A-HOA) every 3 days following T9 spinal contusion injury improved locomotor function assessed with the Rotarod and inhibited TA noxious reflex activity in Wistar rats. To investigate the mechanism of action of A-HOA, microarray analysis was carried out in the spinal cord lesion area. Representative genes involved in pain and neuroregeneration were selected to validate the changes observed in the microarray analysis by quantitative real-time RT-PCR. Comparison of the expression between healthy rats, SCI rats, and SCI treated with A-HOA rats revealed relevant changes in the expression of genes associated with neuronal morphogenesis and growth, neuronal survival, pain and inflammation. Thus, treatment with A-HOA not only induced a significant overexpression of growth and differentiation factor 10 (GDF10), tenascin C (TNC), aspirin (ASPN) and sushi-repeat-containing X-linked 2 (SRPX2), but also a significant reduction in the expression of prostaglandin E synthase (PTGES) and phospholipases A1 and A2 (PLA1/2). Currently, SCI has very important unmet clinical needs. A-HOA downregulated genes involved with inflammation and upregulated genes involved in neuronal growth, and may serve to promote recovery of function after experimental SCI.

Analgesic effects of novel lysophosphatidic acid receptor 5 antagonist AS2717638 in rodents

Lysophosphatidic acid (LPA) is a bioactive lipid that acts via at least six G protein-coupled receptors, LPA receptors 1-6 (LPA1-6), for various physiological functions. We examined (1) whether LPA5 is involved in pain signaling in the spinal cord; and (2) the pharmacological effects of a novel LPA5 antagonist on intrathecal prostaglandin (PG)- and (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced allodynia, and neuropathic and inflammatory pain in rodents. Intrathecal injection of a selective LPA5 agonist, geranylgeranyl diphosphate, and a non-selective agonist, LPA, induced allodynia in wild type, but not in LPA5 knockout mice. These novel results suggest that LPA5 is important for pain signal transmission in the spinal cord. AS2717638 (6,7-dimethoxy-2-(5-methyl-1,2-benzoxazol-3-yl)-4-(piperidin-1-ylcarbonyl)isoquinolin-1(2H)-one) bound to the LPA-binding site on LPA5 and selectively inhibited LPA-induced cyclic adenosine monophosphate accumulation in human LPA5-but not LPA1-, 2-, or 3-expressing cells. Further, oral administration of AS2717638 inhibited LPA5 agonist-induced allodynia in mice. AS2717638 also significantly improved PGE₂-, PGF_2α-, and AMPA-induced allodynia, while both pregabalin and duloxetine alleviated only PGE₂-induced allodynia in mice. Similarly, AS2717638 significantly ameliorated static mechanical allodynia and thermal hyperalgesia in rat models of chronic constriction injury (CCI)-induced neuropathic pain. AS2717638 also showed analgesic effects in a rat model of inflammatory pain. These findings suggest that LPA5 antagonists elicit broad analgesic effects against both neuropathic and inflammatory pain. Accordingly, pharmacological LPA5 antagonists are attractive development candidates for potential novel pain therapies.

Genetic deletion of the prostaglandin E2 E prostanoid receptor subtype 3 improves anatomical and functional outcomes after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a stroke subtype associated with high mortality and morbidity. Following ICH, excitotoxicity and inflammation significantly contribute to secondary brain injury and poor outcomes. Prostaglandin E2 (PGE2 ) levels rise locally with insult to the nervous system, and PGE2 is known to modulate these processes mainly through its E prostanoid (EP) receptors, EP1-4. EP receptor subtype 3 (EP3) is the most abundant EP receptor in the brain and we have previously shown that signaling through the PGE2 -EP3 axis exacerbates excitotoxicity and ischemic stroke outcomes. This study aimed to investigate the contribution of this pathway in modulating anatomical outcomes and functional recovery following ICH. Genetic deletion of EP3 resulted in 48.2 ± 7.3% less ICH-induced brain injury (P < 0.005) and improved functional recovery (P < 0.05), as identified by neurological deficit scoring. To start investigating the mechanisms involved in neuroprotection with impaired PGE2 -EP3 signaling, histological staining was performed to evaluate blood and ferric iron accumulation, neuroinflammation, blood-brain barrier dysfunction, and peripheral neutrophil infiltration. After ICH, EP3 knockout mice demonstrated 49.5 ± 8.8% and 42.8 ± 13.1% less blood (P < 0.01) and ferric iron (P < 0.05), respectively. Furthermore, EP3 knockout mice had significantly reduced astrogliosis, microglial activation, blood-brain barrier breakdown, and neutrophil infiltration. Collectively, these results suggest an injurious role for the PGE2 -EP3 signaling axis in modulating brain injury, inflammation, and neurological functional recovery after ICH. Modulation of the PGE2 -EP3 signaling axis may represent a putative therapeutic avenue for the treatment of ICH.

The lipid kinase PIP5K1C regulates pain signaling and sensitization

Numerous pain-producing (pronociceptive) receptors signal via phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis. However, it is currently unknown which lipid kinases generate PIP2 in nociceptive dorsal root ganglia (DRG) neurons and if these kinases regulate pronociceptive receptor signaling. Here, we found that phosphatidylinositol 4-phosphate 5 kinase type 1C (PIP5K1C) is expressed at higher levels than any other PIP5K and, based on experiments with Pip5k1c(+/-) mice, generates at least half of all PIP2 in DRG neurons. Additionally, Pip5k1c haploinsufficiency reduces pronociceptive receptor signaling and TRPV1 sensitization in DRG neurons as well as thermal and mechanical hypersensitivity in mouse models of chronic pain. We identified a small molecule inhibitor of PIP5K1C (UNC3230) in a high-throughput screen. UNC3230 lowered PIP2 levels in DRG neurons and attenuated hypersensitivity when administered intrathecally or into the hindpaw. Our studies reveal that PIP5K1C regulates PIP2-dependent nociceptive signaling and suggest that PIP5K1C is a therapeutic target for chronic pain.

Nocistatin inhibits pregnant rat uterine contractions in vitro: roles of calcitonin gene-related peptide and calcium-dependent potassium channel

The endogenous neuropeptide nociceptin/orphanin FQ, translated from the prepronociceptin gene, exerts a contraction-inhibitory effect on the rat uterus. As nocistatin has been reported to cause functional antagonism of the pro-nociceptive effects of nociceptin, we set out to investigate its effects on the pregnant rat uterus and to elucidate its signalling pathway. The expression of prepronociceptin mRNA in the uterus and nocistatin levels in the uterus and the plasma were confirmed by RT-PCR and radioimmunoassay. The uterine levels of prepronociceptin mRNA and nocistatin were significantly increased by the last day of pregnancy, while the plasma nocistatin levels remained unchanged. In the isolated organ bath studies nocistatin inhibited the prostaglandin- and the KCl-evoked contractions in the uterus dose-dependently. This latter effect was decreased by preincubation with capsaicin. Incubation with calcitonin gene-related peptide after capsaicin treatment caused an elevation in the contraction-inhibitory effect of nocistatin. The effect of nocistatin was also decreased by the Ca(2+)-dependent K(+) channel inhibitor paxilline, against spontaneous uterine contractions. Nociceptin potentiated the action of nocistatin. Naloxone decreased the effect of nocistatin administered either alone or in combination with nociceptin. In Ca(2+)-poor environment, this effect of naloxone was suspended. Enzyme immunoassay for the uterine intracellular cAMP levels partially confirmed the results of in vitro contractility studies. We conclude that nocistatin, generated locally in the uterus, exerts an inhibitory effect, the mechanism being mediated in part by Ca(2+)-dependent K(+) channels, the elevation of cAMP levels and sensory neuropeptides.

Prostaglandin E(2) mediates acid-induced heartburn in healthy volunteers

Prostaglandin E(2) (PGE(2)) plays a major role in pain processing and hypersensitivity. This study investigated whether PGE(2) levels are increased in the esophageal mucosa after acid infusion and whether increases in PGE(2) are associated with heartburn. Furthermore, expression of the PGE(2) receptor EP1 was investigated in human esophageal mucosa. Fourteen healthy male volunteers were randomized to 30-min lower esophageal acid (1% HCl) or saline perfusion. Before and after acid perfusion, endoscopic biopsies were taken from the distal esophagus. PGE(2) concentration (pg/mg protein) and EP1 mRNA and protein in biopsy samples were measured by ELISA, RT-PCR, and Western blotting. Symptom status of heartburn was evaluated with a validated categorical rating scale with a higher values corresponding to increasing intensity. PGE(2) levels in the esophageal mucosa significantly increased after acid infusion (before vs. after acid infusion: 23.2 ± 8.6 vs. 68.6 ± 18.3, P < 0.05), but not after saline infusion (before vs. after saline infusion: 9.3 ± 2.5 vs. 9.0 ± 3.2, NS). Time to first sensation (min) after acid infusion was less than after saline (saline vs. acid infusion: 22.1 ± 4.1 vs. 5.4 ± 1.5, P < 0.05). Intensity of heartburn in the acid-infusion group was also significantly greater compared with saline (saline vs. acid infusion: 54.3 ± 13.1 vs. 178.5 ± 22.8, P < 0.01). Changes in PGE(2) levels in the esophagus correlated with symptom intensity score (r = 0.80, P = 0.029). EP1 mRNA and protein expression were observed in the normal human esophageal mucosa. Esophageal PGE(2) expression is associated with mucosal acid exposure and heartburn.

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.

Activation of descending pain-facilitatory pathways from the rostral ventromedial medulla by cholecystokinin elicits release of prostaglandin-E₂ in the spinal cord

Cholecystokinin (CCK) has been suggested to be both pro-nociceptive and "anti-opioid" by actions on pain-modulatory cells within the rostral ventromedial medulla (RVM). One consequence of activation of RVM CCK₂ receptors may be enhanced spinal nociceptive transmission; but how this might occur, especially in states of pathological pain, is unknown. Here, in vivo microdialysis was used to demonstrate that levels of RVM CCK increased by approximately 2-fold after ligation of L₅/L₆ spinal nerves (SNL). Microinjection of CCK into the RVM of naïve rats elicited hypersensitivity to tactile stimulation of the hindpaw. In addition, RVM CCK elicited a time-related increase in (prostaglandin-E₂) PGE₂ measured in cerebrospinal fluid from the lumbar spinal cord. The peak increase in spinal PGE₂ was approximately 5-fold and was observed at approximately 80 minutes post-RVM CCK, a time coincident with maximal RVM CCK-induced mechanical hypersensitivity. Spinal administration of naproxen, a nonselective COX-inhibitor, significantly attenuated RVM CCK-induced hindpaw tactile hypersensitivity. RVM-CCK also resulted in a 2-fold increase in spinal 5-hydroxyindoleacetic acid (5-HIAA), a 5-hydoxytryptophan (5-HT) metabolite, as compared with controls, and mechanical hypersensitivity that was attenuated by spinal application of ondansetron, a 5-HT₃ antagonist. The present studies suggest that chronic nerve injury can result in activation of descending facilitatory mechanisms that may promote hyperalgesia via ultimate release of PGE₂ and 5-HT in the spinal cord.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Modulation of bladder function by prostaglandin EP3 receptors in the central nervous system

Prostaglandin EP3 receptors in the central nervous system (CNS) may exert an excitatory effect on urinary bladder function via modulation of bladder afferent pathways. We have studied this action, using two EP3 antagonists, (2 E)-3-{1-[(2,4-dichlorophenyl)methyl]-5-fluoro-3-methyl-1 H-indol-7-yl}- N-[(4,5-dichloro-2-thienyl)sulfonyl]-2-propenamide (DG041) and (2 E)- N-{[5-bromo-2-(methyloxy)phenyl] sulfonyl}-3-[2-(2-naphthalenylmethyl)phenyl]-2-propenamide (CM9). DG041 and CM9 were proven to be selective EP3 antagonists with radioligand binding and functional fluorescent imaging plate reader (FLIPR) assays. Their effects on volume-induced rhythmic bladder contraction and the visceromotor reflex (VMR) response to urinary bladder distension (UBD) were evaluated in female rats after intrathecal or intracerebroventricular administration. Both DG041 and CM9 showed a high affinity for EP3 receptors at subnanomolar concentrations without significant selectivity for any splice variants. At the human EP3C receptor, both inhibited calcium influx produced by the nonselective agonist PGE2. After intrathecal or intracerebroventricular administration both CM9 and DG041 dose-dependently reduced the frequency, but not the amplitude, of the bladder rhythmic contraction. With intrathecal administration DG041 and CM9 produced a long-lasting and robust inhibition on the VMR response to UBD, whereas with intracerebroventricular injection both compounds elicited only a transient reduction of the VMR response to bladder distension. These data support the concept that EP3 receptors are involved in bladder micturition at supraspinal and spinal centers and in bladder nociception at the spinal cord. A centrally acting EP3 receptor antagonist may be useful in the control of detrusor overactivity and/or pain associated with bladder disorders.

Transient allodynia pain models in mice for early assessment of analgesic activity

BACKGROUND AND PURPOSE

The most common preclinical models of neuropathic pain involve surgical ligation of sensory nerves, which is especially difficult in mice. Transient models of chemically sensitized allodynia are potentially useful for rapidly characterizing the analgesic profile of compounds and conducting mechanistic studies.

EXPERIMENTAL APPROACH

Increasing doses of NMDA, sulprostone (an EP1/EP3 prostaglandin receptor agonist) or phenylephrine (an alpha (1) adrenoceptor agonist) were injected intrathecally (i.t.) or i.p., and animals were subsequently assessed for allodynia. The effects of receptor antagonists and analgesic compounds on allodynia were also assessed.

KEY RESULTS

A comparison of total body doses that cause allodynia following spinal or systemic administration indicated that NMDA induces allodynia in the spinal cord while sulprostone and phenylephrine act through a peripheral mechanism. Inhibition of the allodynia with receptor antagonists indicated that each agent induces allodynia by a distinct mechanism. The three models were benchmarked using compounds known to be active in neuropathic pain patients and nerve injury animal models, including gabapentin, amitriptyline and clonidine.

CONCLUSIONS AND IMPLICATIONS

These transient allodynia models are a useful addition to the toolbox of preclinical pain models. They are simple, rapid and reproducible, and will be especially useful for characterizing the pain phenotype of knockout mice.

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.

CJ-023,423, a novel, potent and selective prostaglandin EP4 receptor antagonist with antihyperalgesic properties

The prostaglandin (PG) EP(4) receptor subtype is expressed by peripheral sensory neurons. Although a potential role of EP(4) receptor in pain has been suggested, a limited number of selective ligands have made it difficult to explore the physiological functions of EP(4) or its potential as a new analgesic target. Here, we describe the in vitro and in vivo pharmacology of a novel EP(4) receptor antagonist, N-[({2-[4-(2-ethyl-4,6-dimethyl-1H-imidazo [4,5-c] pyridin-1-yl) phenyl]ethyl}amino) carbonyl]-4-methylbenzenesulfonamide (CJ-023,423). In vitro, CJ-023,423 inhibits [(3)H]PGE(2) binding to both human and rat EP(4) receptors with K(i) of 13 +/- 4 and 20 +/- 1 nM, respectively. CJ-023,423 is highly selective for the human EP(4) receptor over other human prostanoid receptor subtypes. It also inhibits PGE(2)-evoked elevation in intracellular cAMP at the human and rat EP(4) receptors with pA(2) of 8.3 +/- 0.03 and 8.2 +/- 0.2 nM, respectively. In vivo, oral administration of CJ-023,423 significantly reduces thermal hyperalgesia induced by intraplantar injection of PGE(2) (ED(50) = 12.8 mg/kg). CJ-023,423 is also effective in models of acute and chronic inflammatory pain. CJ-023,423 significantly reduces mechanical hyperalgesia in the carrageenan model. Furthermore, CJ-023,423 significantly reverses complete Freund's adjuvant-induced chronic inflammatory pain response. Taken together, the present data indicate that CJ-023,423, a highly potent and selective antagonist of both human and rat EP(4) receptors, produces antihyperalgesic effects in animal models of inflammatory pain. Thus, specific blockade of the EP(4) receptor signaling may represent a novel therapeutic approach for the treatment of inflammatory pain.

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.

Prostaglandin E2 receptor EP4 contributes to inflammatory pain hypersensitivity

Prostaglandin E(2) (PGE(2)) is both an inflammatory mediator released at the site of tissue inflammation and a neuromodulator that alters neuronal excitability and synaptic processing. The effects of PGE(2) are mediated by four G-protein-coupled EP receptors (EP1-EP4). Here we show that the EP4 receptor subtype is expressed by a subset of primary sensory dorsal root ganglion (DRG) neurons, and that its levels, but not that of the other EP1-3 subtypes, increase in the DRG after complete Freund' adjuvant-induced peripheral inflammation. Administration of both an EP4 antagonist [AH23848, (4Z)-7-[(rel-1S,2S,5R)-5-((1,1'-biphenyl-4-yl)methoxy)-2-(4-morpholinyl)-3-oxocyclopentyl]-4-heptenoic acid] and EP4 knockdown with intrathecally delivered short hairpin RNA attenuates inflammation-induced thermal and mechanical behavioral hypersensitivity, without changing basal pain sensitivity. AH23848 also reduces the PGE(2)-mediated sensitization of capsaicin-evoked currents in DRG neurons in vitro. These data suggest that EP4 is a potential target for the pharmacological treatment of inflammatory pain.

Cyclooxygenases, lipoxygenases, and epoxygenases in CNS: Their role and involvement in neurological disorders

Three enzyme systems, cyclooxygenases that generate prostaglandins, lipoxygenases that form hydroxy derivatives and leukotrienes, and epoxygenases that give rise to epoxyeicosatrienoic products, metabolize arachidonic acid after its release from neural membrane phospholipids by the action of phospholipase A(2). Lysophospholipids, the other products of phospholipase A(2) reactions, are either reacylated or metabolized to platelet-activating factor. Under normal conditions, these metabolites play important roles in synaptic function, cerebral blood flow regulation, apoptosis, angiogenesis, and gene expression. Increased activities of cyclooxygenases, lipoxygenases, and epoxygenases under pathological situations such as ischemia, epilepsy, Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and Creutzfeldt-Jakob disease produce neuroinflammation involving vasodilation and vasoconstriction, platelet aggregation, leukocyte chemotaxis and release of cytokines, and oxidative stress. These are closely associated with the neural cell injury which occurs in these neurological conditions. The metabolic products of docosahexaenoic acid, through these enzymes, generate a new class of lipid mediators, namely docosatrienes and resolvins. These metabolites antagonize the effect of metabolites derived from arachidonic acid. Recent studies provide insight into how these arachidonic acid metabolites interact with each other and other bioactive mediators such as platelet-activating factor, endocannabinoids, and docosatrienes under normal and pathological conditions. Here, we review present knowledge of the functions of cyclooxygenases, lipoxygenases, and epoxygenases in brain and their association with neurodegenerative diseases.

Discovery of new chemical leads for selective EP1 receptor antagonists

A series of 4-([2-[alkyl(phenylsulfonyl)amino]phenoxy]methyl)benzoic acids were identified as functional PGE(2) antagonists with selectivity for the EP1 receptor subtype starting from a chemical lead 1, which was found while screening our in-house compound library. Discovery of the optimized analogs 21-23 is presented here and structure-activity relationships (SAR) are also discussed.

Further optimization of sulfonamide analogs as EP1 receptor antagonists: synthesis and evaluation of bioisosteres for the carboxylic acid group

4-{[2-[(2-Furylsulfonyl)(isobutyl)amino]-5-(trifluoromethyl)phenoxy]methyl}benzoic acid analogs 2a and b and a series of the acid analogs, in which the carboxylic acid residue of 2b was replaced with various kinds of carboxylic acid bioisosteres, were synthesized and evaluated as EP1 receptor antagonists. Compound 2b and its monocyclic acid analogs, in which the carboxylic acid residue of 2b was replaced with monocyclic acid bioisosteres, were found to show potent EP1 receptor antagonist activity. Optimization of the linker Y between the phenyl moiety and the carboxylic acid residue of 2b was also carried out (Table 5). Compounds 2b and 16 and 17 possessing conformationally restricted linker Y were found to show the most optimized potency among the tested compounds. Cytochrome P450 inhibition of optimized compounds was also investigated. Details of the structure-activity relationship study are presented.

Role of periaqueductal grey prostaglandin receptors in formalin-induced hyperalgesia

In this study we have investigated the role of periaqueductal grey prostaglandin receptors in formalin-induced hyperalgesia in mice. Glutamate and GABA release changes have been monitored by in vivo microdialysis. Intra-periaqueductal grey microinjections of misoprostol, a non-selective prostaglandin receptor agonist, increased nociceptive responses in the formalin test only during the late phase. Prostanoid EP(1) (L-335677), EP(2) (AH 6809), EP(3) (L-826266) and EP(4) (L-161982) receptor antagonists prevented the nociceptive response induced by misoprostol in formalin-injected mice. Prostanoid EP(1), EP(2), EP(3) and EP(4) antagonists reduced, per se, the late hyperalgesic phase. Intra-periaqueductal grey perfusion with misoprostol increased periaqueductal grey glutamate, whereas it produced an increase followed by a decrease in GABA. Likewise, formalin increased glutamate and produced a biphasic response on GABA. When misoprostol was perfused in combination with the peripheral injection of formalin, we observed an increase of glutamate and an increase followed by a stronger decrease in GABA release. These data show that periaqueductal grey prostaglandin receptor stimulation increased formalin-induced nociceptive response in the late phase by increasing glutamate release and by producing a biphasic change in GABA release.

Prostaglandin E2 deteriorates N-methyl-D-aspartate receptor-mediated cytotoxicity possibly by activating EP2 receptors in cultured cortical neurons

The activation of glutamate receptors, particularly N-methyl-D-aspartate (NMDA) receptors, initiates ischemic cascade in the early stages of cerebral ischemia. Postischemia, cerebral ischemia is also associated with an inflammatory reaction that contributes to tissue damage. The up-regulation of neuronal cyclooxygenase-2 (COX-2) and elevation of prostaglandin E2 (PGE2) have been reported to occur after cerebral ischemic insult. We therefore studied whether the COX-2 reaction product PGE2 affects glutamate receptor-mediated cell death in cultured rat cortical cells. PGE2 was found to augment NMDA-mediated cell death. The transcription of EP1, EP2, EP3 and EP4 PGE2 receptor genes was investigated using reverse transcriptase-polymerase chain reaction (RT-PCR). EP1, EP2 and EP3 receptor genes were found in cortical cells. Butaprost (an EP2 agonist) markedly enhanced NMDA-mediated cell death, whereas 17-phenyl trinor-PGE2 (an EP1 agonist) and sulprostone (an EP3 agonist) had little effect. Both PGE2 and butaprost elevated cAMP intracellular levels in the cortical cells; moreover, forskolin, an activator of adenylate cyclase, enhanced NMDA-mediated cell death. These results suggest that PGE2, acting via EP2 receptors, aggravates excitotoxic neurodegeneration by a cAMP-dependent mechanism.

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.

Role of prostaglandin receptor subtype EP1 in prostaglandin E2-induced nociceptive transmission in the rat spinal dorsal horn

It has been indicated that prostaglandin E2 (PGE2) and the receptor for PGE2 (EP receptor) are key factors contributing to the facilitated generation of nociception. This study was designed to investigate the roles of PGE2 and EP1 receptors in the spinal cord in the nociceptive transmission, using behavioral and intracellular calcium ion concentration ([Ca2+]i) assays and in situ hybridization. Experiments were conducted on Sprague-Dawley rats. In behavioral assays, withdrawal thresholds to mechanical stimuli were evaluated using von Frey filament. The effect of an intrathecally administered selective EP1 antagonist, 6-[(2S,3S)-3-(4-chloro-2-methylphenylsulfonylaminomethyl)-bicyclo[2.2.2]octan-2-yl]-5Z-hexenoic acid (ONO-8711), on the intrathecal PGE2-induced hyperalgesia was examined. In [Ca2+]i assays, we measured [Ca2+]i in the dorsal horn of spinal cord slices and examined the effects of PGE2 and ONO-8711 perfusion on the [Ca2+]i changes. In situ hybridization using EP1 digoxigenin probe was performed on the slice sections of the lumbar spinal cord and bilateral L4 and L5 dorsal root ganglions (DRGs). Mechanical hyperalgesia was observed after intrathecal PGE2 administration. Intrathecal administration of ONO-8711 attenuated the PGE2-induced mechanical hyperalgesia in a dose- and time-dependent manner. Perfusion of ONO-8711 markedly suppressed PGE2-induced [Ca2+]i increment in laminae II-VI in dorsal horn of the spinal cord slice. Moreover, in situ hybridization revealed EP1 hybridization signals in the DRG neurons, but not in the spinal cord. The results of this study suggested that spinal PGE2 activates the EP1 receptors existing on the central terminals of primary afferents, subsequently increasing in [Ca2+]i in the spinal dorsal horn, which are involved in the mechanisms of spinal PGE2-induced nociceptive transmission.

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.

Inhibition of neurokinin-1-substance P receptor and prostanoid activity prevents and reverses the development of morphine tolerance in vivo and the morphine-induced increase in CGRP expression in cultured dorsal root ganglion neurons

Chronic treatment with opioid drugs such as morphine leads to the development of tolerance, which manifests as a loss of drug potency. The mechanisms underlying this phenomenon are poorly understood, but recent evidence suggests that increased activity of nociceptive sensory transmitters [calcitonin gene-related peptide (CGRP) and substance P] and other signalling messengers (prostaglandins) contribute to its development. Chronic intrathecal morphine administration to rats for 7 days produced analgesic tolerance. Co-administration of SR140333, a selective substance P receptor (neurokinin-1) antagonist, or nimesulide, a cyclooxygenase-2-selective inhibitor, augmented the acute effects of morphine, prevented morphine tolerance and reversed established tolerance. In cultured adult dorsal root ganglion neurons, exposure to morphine for 5 days increased the number of neurons expressing CGRP immunoreactivity. Co-exposure with the peptide CGRP receptor antagonist CGRP8-37, SR140333 or nimesulide prevented the morphine-induced increase in the expression of CGRP immunoreactivity. Additionally, BIBN4096BS, a nonpeptide CGRP receptor antagonist, stereoselectively produced similar effects. In summary, this investigation demonstrates that activity of CGRP and substance P contributes to both the induction and expression of opioid analgesic tolerance. Additionally, it highlights the involvement of prostaglandins generated by spinal cyclooxygenase-2 activity in the genesis of opioid tolerance. The neuropeptide and prostanoid activity contributing to tolerance is expressed at the level of the primary afferents terminating in the spinal cord. The combination of opioids with agents that block this activity may represent a useful strategy for the prevention as well as the reversal of clinical opioid tolerance.

Functional evidence for interaction between prostaglandin EP3 and kappa-opioid receptor pathways in tactile pain induced by human immunodeficiency virus type-1 (HIV-1) glycoprotein gp120

HIV-1 glycoprotein gp120 administered intrathecally induces tactile pain (allodynia) in animals. In the present study, we investigated the mechanism of gp120-induced allodynia and possible functional connections with factors modulating pain transmission at the spinal level. Gp120 evoked allodynia in a dose-dependent manner with the maximum effect at 1 pg/mouse, and stimulated a rapid increase in intracellular free Ca2+ concentration ([Ca2+]i) in the dorsal horn cells of the spinal cord. These responses evoked by gp120 were blocked by galactocerebroside. The gp120-induced allodynia was also attenuated by the non-steroidal anti-inflammatory drug indomethacin, which inhibits prostaglandin synthesis, and did not develop in mice lacking the EP3 prostaglandin E receptor subtype (EP3(-/-)). Pretreatment of spinal slices with indomethacin dose-dependently decreased the percentage of the cells that showed increased [Ca2+]i in response to gp120, and the decrease was reversed by addition of the selective EP3 agonist ONO-AE-248. The kappa-opioid agonist U-50,488 significantly enhanced the gp120-stimulated increase in [Ca2+]i in spinal slices prepared from EP3(-/-) mice, and the simultaneous addition of U-50,488 with gp120 reproduced the gp120-induced allodynia in EP3(-/-) mice. These results suggest that gp120 induced allodynia by increasing [Ca2+]i, concomitant with activation of prostanoid EP3 and kappa-opioid receptors in the spinal cord.

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.

Intrathecally applied flurbiprofen produces an endocannabinoid-dependent antinociception in the rat formalin test

It is generally accepted that the phospholipase-A2-cyclooxygenase-prostanoids-cascade mediates spinal sensitization and hyperalgesia. However, some observations are not in line with this hypothesis. The aim of the present work was to investigate whether different components of this cascade exhibit nociceptive or antinociceptive effects in the rat formalin test. Intrathecal (i.th.) injection of prostaglandin E2 (PGE2) induced a dose-dependent antinociceptive effect on the formalin-induced nociception. Furthermore, thimerosal, which inhibits the reacylation of arachidonic acid thereby enhancing arachidonic acid levels, had an antinociceptive effect rather than the expected pronociceptive effect when given i.th. While the phospholipase A2 inhibitor methyl arachidonyl fluorophosphonate (MAFP; i.th.) had a significant antinociceptive effect, its analogue palmitoyl trifluoromethyl ketone (PTFMK; i.th.) had no significant effect on the formalin-induced nociception. However, MAFP, but not PTFMK, showed a cannabinoid CB1 agonistic effect as shown by the inhibition of electrically evoked contractions of the vas deferens isolated from CB1 wild-type mice but not of that from CB1 knockout mice. The antinociceptive effect of MAFP was completely reversed by the CB1 receptor antagonist AM-251 (i.th.), thus attributing such effect to its CB1 agonistic effect. Moreover, the antinociceptive effect of the cyclooxygenase inhibitor, flurbiprofen (i.th.) was reversed by the co-administration of AM-251, but not by PGE2. Finally. the combination of phenylmethylsulfonyl fluoride (PMSF; intraperitoneal), which inhibits the degradation of anandamide through the inhibition of fatty acid amidohydrolase, with thimerosal (i.th.) produced a profound CB1-dependent antinociception. The present results show that endocannabinoids play a major role in mediating flurbiprofen-induced antinociception at the spinal level.

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.

Prostanoids and pain: unraveling mechanisms and revealing therapeutic targets

Advances in our understanding of the synthesis, regulation and function of prostanoids have led to a new appreciation of their actions in health and disease. Prostanoid synthesis is essential for the generation of inflammatory pain and this depends not only on prostanoid production at the site of inflammation, but also on the actions of prostanoids synthesized within the central nervous system (CNS). Moreover, central prostanoid synthesis is controlled both by neural and humoral signals, the latter being a novel form of input to the CNS. Diverse compounds that act along the pathway of prostanoid synthesis and action, both in the periphery and in the CNS, might provide increased benefit for treating inflammatory pain hypersensitivity and its associated sickness syndrome, with a reduced risk of adverse effects.

Spinal administration of prostaglandin E(2) or prostaglandin F(2alpha) primarily produces mechanical hyperalgesia that is mediated by nociceptive specific spinal dorsal horn neurons

The effects of intrathecal (i.t.) administration of prostaglandin E2 (PGE2) and prostaglandin F2 (PGF2) on behavioral and spinal neuronal responses to mechanical and thermal stimuli were examined in rats. i.t. Administration of either PGE2 (1-100 nmol) or PGF2 (1-100 nmol) produced a robust, dose-dependent mechanical hyperalgesia, but only a weak thermal hyperalgesia and touch-evoked allodynia. Spinal administration of either PGE2 (100 pmol-100 nmol) or PGF2 (1-100 nmol) produced dose-dependent increases in responses of nociceptive specific (NS) neurons to mechanical stimuli, but only modest increases in wide dynamic range (WDR) neurons to mechanical stimuli. Spinal administration of PGE2 produced a bi-directional, dose-response effect on thermally-evoked responses of both WDR and NS neurons when prostaglandin-induced changes in background discharges were controlled for. Thermally evoked responses of WDR and NS neurons were decreased at lesser doses of PGE2, but this trend reversed with greater doses, such that responses of WDR neurons were significantly increased at the greatest dose tested at some test temperatures. PGF2 generally produced non-significant increases in thermally evoked neuronal responses, and this trend occurred primarily in WDR neurons. Both PGE2 and PGF2 produced increases in background discharges of WDR and NS neurons, although this effect was most consistently observed with WDR neurons and PGE2. These behavioral and electrophysiological data suggest that mechanical hyperalgesia induced by spinal administration of PGE2 and PGF2 is mediated mainly by changes in NS neurons. The weak thermal hyperalgesia may reflect changes in WDR neurons.

The role of spinal neurokinin-1 and glutamate receptors in hyperalgesia and allodynia induced by prostaglandin E(2) or zymosan in the rat

Recent research has focused on prostaglandins in the central nervous system and their contribution to hyperalgesia and allodynia. This study sought to establish whether neurokinin-1 (NK-1) receptors and glutamate receptors are involved in the hyperalgesic and allodynic effects of spinally administered prostaglandin E2 (PGE2) in rats, and also to determine if the same receptors are involved the hyperalgesia induced by intraplantar administration of zymosan, an inflammatory agent which is known to evoke spinal PGE2 release. Spinal application of antagonists of the NK-1 receptor, the -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate or metabotropic glutamate receptor significantly attenuated the decrease in mechanical paw withdrawal response thresholds produced by either spinal administration of PGE2 or intraplantar administration of zymosan. The decrease in thermal paw withdrawal response latencies induced by PGE2, but not by zymosan, was significantly attenuated by spinal administration of an N-methyl--aspartate (NMDA) receptor antagonist, an AMPA/kainate receptor antagonist, or a metabotropic glutamate receptor antagonist. Allodynia induced by PGE2 was significantly alleviated by antagonists of NMDA or AMPA/kainate receptors. These results suggest that both PGE2-induced and zymosan-induced mechanical hyperalgesia are mediated in part through activation of NK-1, AMPA/kainate and metabotropic glutamate receptors. PGE2-induced, but not zymosan-induced, thermal hyperalgesia is mediated in part by activation of NMDA, AMPA/kainate and metabotropic glutamate receptors. Activation of both NMDA and AMPA/kainate receptors contribute to PGE2-induced allodynia.