Allodynia evoked by intrathecal administration of prostaglandin E2 to conscious mice

Antinociceptive effect of gelsenicine, principal toxic alkaloids of gelsemium, on prostaglandin E2-induced hyperalgesia in mice: Comparison with gelsemine and koumine

Gelsemium elegans (G.elegans) is a plant of the Loganiaceae family, known for its indole alkaloids, including gelsemine, koumine, and gelsenicine. Gelsemine and koumine are well-studied active alkaloids with low toxicity, valued for their anti-anxiety and analgesic properties. However, gelsenicine, another important alkaloid, remains underexplored due to its high toxicity. This study focuses on evaluating the analgesic properties of gelsenicine and comparing them with gelsemine and koumine. The results indicate that all three alkaloids exhibit robust analgesic properties, with gelsemine, koumine, and gelsenicine showing ED50 values of 0.82 mg/kg, 0.60 mg/kg, and 8.43 μg/kg, respectively, as assessed by the hot plate method. Notably, the therapeutic dose of gelsenicine was significantly lower than its toxic dose (LD50 = 0.185 mg/kg). The study also investigated the mechanism of action by analyzing the expression levels of GlyRα3 and Gephyrin. The PGE2 model group showed decreased expression levels of GlyRα3 and Gephyrin, while groups treated with gelsemine, koumine, and gelsenicine were able to reverse this decrease. These results suggest that gelsenicine effectively alleviates PGE2-induced hyperalgesia by upregulating the expression of GlyRα3 and Gephyrin, which are key targets of the Gly receptor pathway.

Spinal GABAergic disinhibition allows microglial activation mediating the development of nociplastic pain in male mice

Previously we developed a murine model in which postinjury stimulation of an injured area triggers a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. This hypersensitivity was maintained by sex-specific mechanisms; specifically, activated spinal microglia maintained the hypersensitivity only in males. Here we investigated whether spinal microglia drive the transition from acute injury-induced pain to nociplastic pain in males, and if so, how they are activated by normally innocuous stimulation after peripheral injury. Using intraplantar capsaicin injection as an acute peripheral injury and vibration of the injured paw as postinjury stimulation, we found that inhibition of spinal microglia prevents the vibration-induced transition to a nociplastic pain state. The transition was mediated by the ATP-P2X4 pathway, but not BDNF-TrkB signaling. Intrathecally injected GABA receptor agonists after intraplantar capsaicin injection prevented the vibration-induced transition to a nociplastic pain state. Conversely, in the absence of intraplantar capsaicin injection, intrathecally injected GABA receptor antagonists allowed the vibration stimulation of a normal paw to trigger the transition to a spinal microglia-mediated nociplastic pain state only in males. At the spinal level, TNF-α, IL-1β, and IL-6, but not prostaglandins, contributed to the maintenance of the nociplastic pain state in males. These results demonstrate that in males, the transition from acute injury-induced pain to nociplastic pain is driven by spinal microglia causing neuroinflammation and that peripheral injury-induced spinal GABAergic disinhibition is pivotal for normally innocuous stimulation to activate spinal microglia.

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

Prostaglandin D2 contributes to cisplatin-induced neuropathic pain in rats via DP2 receptor in the spinal cord

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is a major reason for stopping or changing anticancer therapy. Among the proposed pathomechanisms underlying CIPN, proinflammatory processes have attracted increasing attention. Here we assessed the role of prostaglandin D² (PGD²) signaling in cisplatin-induced neuropathic pain.

Methods

CIPN was induced by intraperitoneal administration of cisplatin 2 mg/kg for 4 consecutive days using adult male Sprague-Dawley rats. PGD² receptor DP1 and/or DP2 antagonists were administered intrathecally and the paw withdrawal thresholds were measured using von Frey filaments. Spinal expression of DP1, DP2, hematopoietic PGD synthase (H-PGDS), and lipocalin PGD synthase (L-PGDS) proteins were analyzed by western blotting.

Results

The DP1 and DP2 antagonist AMG 853 and the selective DP2 antagonist CAY10471, but not the DP1 antagonist MK0524, significantly increased the paw withdrawal threshold compared to vehicle controls (P = 0.004 and P < 0.001, respectively). Western blotting analyses revealed comparable protein expression levels in DP1 and DP2 in the spinal cord. In the CIPN group the protein expression level of L-PGDS, but not of H-PGDS, was significantly increased compared to the control group (P < 0.001).

Conclusions

The findings presented here indicate that enhanced PGD² signaling, via upregulation of L-PGDS in the spinal cord, contributes to mechanical allodynia via DP2 receptors in a cisplatin-induced neuropathic pain model in rats, and that a blockade of DP2 receptor activation may present a novel therapeutic target for managing CIPN.

Role of Lipocalin-Type Prostaglandin D Synthase in Experimental Osteoarthritis

OBJECTIVE

Lipocalin-type prostaglandin D synthase (L-PGDS) catalyzes the formation of prostaglandin D₂ (PGD₂ ), which has important roles in inflammation and cartilage metabolism. We undertook this study to investigate the role of L-PGDS in the pathogenesis of osteoarthritis (OA) using an experimental mouse model.

METHODS

Experimental OA was induced in wild-type (WT) and L-PGDS-deficient (L-PGDS^-/- ) mice (n = 10 per genotype) by destabilization of the medial meniscus (DMM). Cartilage degradation was evaluated by histology. The expression of matrix metalloproteinase 13 (MMP-13) and ADAMTS-5 was assessed by immunohistochemistry. Bone changes were determined by micro-computed tomography. Cartilage explants from L-PGDS^-/- and WT mice (n = 6 per genotype) were treated with interleukin-1α (IL-1α) ex vivo in order to evaluate proteoglycan degradation. Moreover, the effect of intraarticular injection of a recombinant adeno-associated virus type 2/5 (rAAV2/5) encoding L-PGDS on OA progression was evaluated in WT mice (n = 9 per group).

RESULTS

Compared to WT mice, L-PGDS^-/- mice had exacerbated cartilage degradation and enhanced expression of MMP-13 and ADAMTS-5 (P < 0.05). Furthermore, L-PGDS^-/- mice displayed increased synovitis and subchondral bone changes (P < 0.05). Cartilage explants from L-PGDS^-/- mice showed enhanced proteoglycan degradation following treatment with IL-1α (P < 0.05). Intraarticular injection of rAAV2/5 encoding L-PGDS attenuated the severity of DMM-induced OA-like changes in WT mice (P < 0.05). The L-PGDS level was increased in OA tissues of WT mice (P < 0.05).

CONCLUSION

Collectively, these findings suggest a protective role of L-PGDS in OA, and therefore enhancing levels of L-PGDS may constitute a promising therapeutic strategy.

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.

Intramuscularly injected neurotropin reduced muscular mechanical hyperalgesia induced by repeated cold stress in rats

An extract of rabbit skin inflamed by inoculation with the vaccinia virus, neurotropin [by intravenous, oral, and intramuscular (i.m.) administration], has been used in China and Japan for the treatment of chronic pain. In this study, we investigated the analgesic mechanism of i.m. neurotropin. Rats were exposed to repeated cold stress, and muscular mechanical hyperalgesia was evaluated by measuring the withdrawal threshold of the gastrocnemius muscle using Randall-Selitto apparatus. I.m. but not subcutaneous, neurotropin dose dependently reduced the repeated cold stress-induced muscular mechanical hyperalgesia for 3 h, but it had no effect in normal rats. Injections of neurotropin into the right gastrocnemius, quadriceps femoris, biceps brachii, and trapezius muscles reduced the muscular mechanical hyperalgesia of the gastrocnemius muscle bilaterally. Intrathecal administration of antagonists to GABAergic, serotonergic, and cholinergic receptors, but not α2-adrenergic receptors, and intraperitoneal administration of opioid receptor antagonist inhibited the analgesic effect of neurotropin. These results indicated that an i.m. injection of neurotropin induced long-lasting wide-spread bilateral muscular analgesia by activating spinal serotonergic and GABAergic receptors. As distinct from analgesia by systemic administration, spinal cholinergic and opioidergic, but not adrenergic receptors, are also involved. The present study supports the effectiveness of neurotropin treatment for muscular mechanical hyperalgesia.

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.

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.

Serum protein changes in a rat model of chronic pain show a correlation between animal and humans

In previous works we showed the overexpression of some proteins in biological fluids from patients suffering chronic pain. In this proteomic study we analysed serum from a rat model of neuropathic pain obtained by the chronic constriction injury (CCI) of sciatic nerve, at two time intervals, 2 and 5 weeks after the insult, to find proteins involved in the expression or mediation of pain. Sham-operated and CCI rats were treated with saline or indomethacin. Two weeks after ligation, we identified three serum proteins overexpressed in CCI rats, two of which, alpha-1-macroglobulin and vitamin D-binding protein (VDBP), remained increased 5 weeks post-surgery; at this time interval, we found increased levels of further proteins, namely apolipoprotein A-I (APOA1), apolipoprotein E (APOE), prostaglandin-H2 D-isomerase (PTGDS) and transthyretin (TTR), that overlap the overexpressed proteins found in humans. Indomethacin treatment reversed the effects of ligation. The qPCR analysis showed that transcript levels of APOA1, APOE, PTGDS and VDBP were overexpressed in the lumbar spinal cord (origin of sciatic nerve), but not in the striatum (an unrelated brain region), of CCI rats treated with saline 5 weeks after surgery, demonstrating that the lumbar spinal cord is a possible source of these proteins.

Failure of intrathecal ketorolac to reduce remifentanil-induced postinfusion hyperalgesia in humans

In rodents, acute exposure to opioids results in transient antinociception followed by longer lasting hypersensitivity to tactile or thermal stimuli, a phenomenon termed opioid-induced hyperalgesia. This hypersensitivity can be blocked or reversed by intrathecally administered cyclooxygenase inhibitors, including ketorolac, suggesting a role for spinal prostaglandins. In surgical patients, the dose of intraoperative opioid, particularly the short-acting drug, remifentanil, is directly related to increased pain and opioid requirements for many hours postoperatively. In addition, experimentally induced tactile hypersensitivity in humans is exaggerated after cessation of remifentanil infusions. The degree of this experimental opioid-induced hyperalgesia is reduced by systemic treatment with cyclooxygenase inhibitors, and investigators have speculated that this reduction reflects the actions in the central nervous system, most likely in the spinal cord. To test this hypothesis, we measured cerebrospinal fluid prostaglandin E2 concentrations during and after remifentanil infusion in 30 volunteers. These volunteers received intrathecal ketorolac or saline in a random, blinded manner during intravenous remifentanil infusion after generation of hypersensitivity by topical capsaicin. Remifentanil reduced pain to noxious heat stimuli and reduced areas of capsaicin-induced hypersensitivity similarly in those receiving intrathecal ketorolac or saline. The primary outcome measure, area of capsaicin-induced hypersensitivity after stopping remifentanil, showed a similar increase in those receiving ketorolac as in those receiving saline. Cerebrospinal fluid prostaglandin E2 concentrations did not increase during postinfusion hyperalgesia compared with those during infusion, and they were not increased during infusion compared with those in historical controls. These data fail to support the hypothesis that acute opioid-induced hyperalgesia reflects spinal cyclooxygenase activation causing central sensitization.

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.

Effect of antioxidant treatment on spinal GABA neurons in a neuropathic pain model in the mouse

One feature of neuropathic pain is a reduced spinal gamma-aminobutyric acid (GABA)-ergic inhibitory function. However, the mechanisms behind this attenuation remain to be elucidated. This study investigated the involvement of reactive oxygen species in the spinal GABA neuron loss and reduced GABA neuron excitability in spinal nerve ligation (SNL) model of neuropathic pain in mice. The importance of spinal GABAergic inhibition in neuropathic pain was tested by examining the effects of intrathecally administered GABA receptor agonists and antagonists in SNL and naïve mice, respectively. The effects of SNL and antioxidant treatment on GABA neuron loss and functional changes were examined in transgenic GAD67-enhanced green fluorescent protein positive (EGFP+) mice. GABA receptor agonists transiently reversed mechanical hypersensitivity of the hind paw in SNL mice. On the other hand, GABA receptor antagonists made naïve mice mechanically hypersensitive. Stereological analysis showed that the numbers of enhanced green fluorescent protein positive (EGFP+) GABA neurons were significantly decreased in the lateral superficial laminae (I-II) on the ipsilateral L5 spinal cord after SNL. Repeated antioxidant treatments significantly reduced the pain behaviors and prevented the reduction in EGFP+ GABA neurons. The response rate of the tonic firing GABA neurons recorded from SNL mice increased with antioxidant treatment, whereas no change was seen in those recorded from naïve mice, which suggested that oxidative stress impaired some spinal GABA neuron activity in the neuropathic pain condition. Together the data suggest that neuropathic pain, at least partially, is attributed to oxidative stress, which induces both a GABA neuron loss and dysfunction of surviving GABA neurons.

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.

Peripheral inflammatory hyperalgesia depends on the COX increase in the dorsal root ganglion

It is well established that dorsal root ganglion (DRG) cells synthesize prostaglandin. However, the role that prostaglandin plays in the inflammatory hyperalgesia of peripheral tissue has not been established. Recently, we have successfully established a technique to inject drugs (3 μL) directly into the L5-DRG of rats, allowing in vivo identification of the role that DRG cell-derived COX-1 and COX-2 play in the development of inflammatory hyperalgesia of peripheral tissue. IL-1β (0.5 pg) or carrageenan (100 ng) was administered in the L5-peripheral field of rat hindpaw and mechanical hyperalgesia was evaluated after 3 h. Administration of a nonselective COX inhibitor (indomethacin), selective COX-1 (valeryl salicylate), or selective COX-2 (SC-236) inhibitors into the L5-DRG prevented the hyperalgesia induced by IL-1β. Similarly, oligodeoxynucleotide-antisense against COX-1 or COX-2, but not oligodeoxynucleotide-mismatch, decreased their respective expressions in the L5-DRG and prevented the hyperalgesia induced by IL-1β in the hindpaw. Immunofluorescence analysis demonstrated that the amount of COX-1 and COX-2, constitutively expressed in TRPV-1(+) cells of the DRG, significantly increased after carrageenan or IL-1β administration. In addition, indomethacin administered into the L5-DRG prevented the increase of PKCε expression in DRG membrane cells induced by carrageenan. Finally, the administration of EP1/EP2 (7.5 ng) or EP4 (10 µg) receptor antagonists into L5-DRG prevented the hyperalgesia induced by IL-1β in the hindpaw. In conclusion, the results of this study suggest that the inflammatory hyperalgesia in peripheral tissue depends on activation of COX-1 and COX-2 in C-fibers, which contribute to the induction and maintenance of sensitization of primary sensory neurons.

Reversal of chronic stress-induced pain by transcranial direct current stimulation (tDCS) in an animal model

Transcranial direct current stimulation (tDCS) has been suggested as a therapeutic tool for pain syndromes. Although initial results in human subjects are encouraging, it still remains unclear whether the effects of tDCS can reverse maladaptive plasticity associated with chronic pain. To investigate this question, we tested whether tDCS can reverse the specific behavioral effects of chronic stress in the pain system, and also those indexed by corticosterone and interleukin-1β levels in serum and TNFα levels in the hippocampus, in a well-controlled rat model of chronic restraint stress (CRS). Forty-one adult male Wistar rats were divided into two groups control and stress. The stress group was exposed to CRS for 11 weeks for the establishment of hyperalgesia and mechanical allodynia as shown by the hot plate and von Frey tests, respectively. Rats were then divided into four groups control, stress, stress+sham tDCS and stress+tDCS. Anodal or sham tDCS was applied for 20min/day over 8 days and the tests were repeated. Then, the animals were killed, blood collected and hippocampus removed for ELISA testing. This model of CRS proved effective to induce chronic pain, as the animals exhibited hyperalgesia and mechanical allodynia. The hot plate test showed an analgesic effect, and the von Frey test, an anti-allodynic effect after the last tDCS session, and there was a significant decrease in hippocampal TNFα levels. These results support the notion that tDCS reverses the detrimental effects of chronic stress on the pain system and decreases TNFα levels in the hippocampus.

Constitutive cyclo-oxygenase-2 does not contribute to the development of human visceral pain hypersensitivity

BACKGROUND AND AIMS

Central sensitisation (CS), contributes to the development and maintenance of gastrointestinal pain hypersensitivity. Constitutive cyclo-oxygenase-2 (COX-2) contributes to central sensitisation in somatic pain hypersensitivity but its role in mediating visceral pain hypersensitivity is unknown. We therefore conducted a study to determine if COX-2 inhibition with Valdecoxib attenuates the development or early maintenance of CS in a validated human oesophageal pain hypersensitivity model.

METHODS

Healthy volunteers were studied in two randomised, double blind, crossover studies in which pain thresholds (PT) to electrical stimulation were assessed in the proximal oesophagus, chest wall and foot, prior to and following a distal oesophageal acid infusion. Protocol 1: Valdecoxib, (40 mg) or matching placebo was given orally for 4 days prior to oesophageal acid infusion. Protocol 2: IV Parecoxib (40 mg) or saline was given 120 min after oesophageal acid infusion.

RESULTS

Valdecoxib did not prevent the induction of secondary allodynia in the proximal oesophagus nor did it attenuate it following its establishment. Chest wall PT fell following oesophageal acid but foot PT remained unchanged; highlighting the development viscero-somatic convergence due to CS. Valdecoxib had no analgesic or anti-hyperalgesic effect on chest wall or foot PT.

CONCLUSIONS

Neither the induction nor initial maintenance of acid induced oesophageal pain hypersensitivity is prevented by Valdecoxib, suggesting that constitutive spinal COX-2 does not contribute to the development or early maintenance of acute visceral central sensitisation.

Pharmacology of intracisternal or intrathecal glycine, muscimol, and baclofen in strychnine-induced thermal hyperalgesia of mice

Glycine and γ-aminobutyric acid (GABA) are localized and released by the same interneurons in the spinal cord. Although the effects of glycine and GABA on analgesia are well known, little is known about the effect of GABA in strychnine-induced hyperalgesia. To investigate the effect of GABA and the role of the glycine receptor in thermal hyperalgesia, we designed an experiment involving the injection of muscimol (a GABA(A) receptor agonist), baclofen (a GABA(B) receptor agonist) or glycine with strychnine (strychnine sensitive glycine receptor antagonist). Glycine, muscimol, or baclofen with strychnine was injected into the cisterna magna or lumbar subarachnoidal spaces of mice. The effects of treatment on strychnine-induced heat hyperalgesia were observed using the pain threshold index via the hot plate test. The dosages of experimental drugs and strychnine we chose had no effects on motor behavior in conscious mice. Intracisternal or intrathecal administration of strychnine produced thermal hyperalgesia in mice. Glycine antagonize the effects of strychnine, whereas, muscimol or baclofen does not. Our results indicate that glycine has anti-thermal hyperalgesic properties in vivo; and GABA receptor agonists may lack the binding abilities of glycine receptor antagonists with their sites in the central nervous system.

Intracisternal or intrathecal glycine, taurine, or muscimol inhibit bicuculline-induced allodynia and thermal hyperalgesia in mice

AIM

To investigate the effects of GABA and glycine on analgesia in the central nervous system.

METHODS

Glycine, taurine, or muscimol was injected with bicuculline into the cistern magna or the lumbar subarachnoidal space in ICR mice. The effects on bicuculline-induced allodynia in a touch-evoked agitation test and on pain threshold index in a hot-plate test were assessed.

RESULTS

The dosages of the amino acids administered with bicuculline had no effect on motor behavior in conscious mice. Glycine or muscimol reduced bicuculline-induced allodynia regardless of the administration site, whereas intrathecal taurine reduced bicuculline-induced allodynia. Glycine, taurine, and muscimol all antagonized the effects induced by bicuculline in the hot-plate test, regardless of the administration site.

CONCLUSION

Glycine, taurine, and muscimol were found to have anti-allodynic and anti-thermal hyperalgesic properties in vivo. These observations suggest an interaction between glycine and GABA receptors during the regulation of antinociception.

Role of PAF receptor in proinflammatory cytokine expression in the dorsal root ganglion and tactile allodynia in a rodent model of neuropathic pain

BACKGROUND

Neuropathic pain is a highly debilitating chronic pain following damage to peripheral sensory neurons and is often resistant to all treatments currently available, including opioids. We have previously shown that peripheral nerve injury induces activation of cytosolic phospholipase A(2) (cPLA(2)) in injured dorsal root ganglion (DRG) neurons that contribute to tactile allodynia, a hallmark of neuropathic pain. However, lipid mediators downstream of cPLA(2) activation to produce tactile allodynia remain to be determined.

PRINCIPAL FINDINGS

Here we provide evidence that platelet-activating factor (PAF) is a potential candidate. Pharmacological blockade of PAF receptors (PAFRs) reduced the development and expression of tactile allodynia following nerve injury. The expression of PAFR mRNA was increased in the DRG ipsilateral to nerve injury, which was seen mainly in macrophages. Furthermore, mice lacking PAFRs showed a reduction of nerve injury-induced tactile allodynia and, interestingly, a marked suppression of upregulation of tumor necrosis factor alpha (TNFalpha) and interleukin-1beta (IL-1beta) expression in the injured DRG, crucial proinflammatory cytokines involved in pain hypersensitivity. Conversely, a single injection of PAF near the DRG of naïve rats caused a decrease in the paw withdrawal threshold to mechanical stimulation in a dose-dependent manner and an increase in the expression of mRNAs for TNFalpha and IL-1beta, both of which were inhibited by pretreatment with a PAFR antagonist.

CONCLUSIONS

Our results indicate that the PAF/PAFR system has an important role in production of TNFalpha and IL-1beta in the DRG and tactile allodynia following peripheral nerve injury and suggest that blocking PAFRs may be a viable therapeutic strategy for treating neuropathic pain.

Blockade of glycine transporter (GlyT) 2, but not GlyT1, ameliorates dynamic and static mechanical allodynia in mice with herpetic or postherpetic pain

Glycine is an inhibitory neurotransmitter in the spinal dorsal horn and its extracellular concentration is regulated by glial glycine transporter (GlyT) 1 and neuronal GlyT2. This study was conducted to elucidate the effects of intrathecal injections of GlyT1 and GlyT2 inhibitors on two distinct types of mechanical allodynia, dynamic and static allodynia, in mice with herpetic or postherpetic pain. The GlyT2 inhibitor ALX1393, but not the GlyT1 inhibitor sarcosine, suppressed dynamic and static allodynia at the herpetic and postherpetic stages. Intrathecal ALX1393 suppressed dynamic allodynia induced by intrathecal strychnine and N-methyl-D-aspartate (NMDA). Intrathecal sarcosine suppressed dynamic allodynia induced by intrathecal strychnine, but not NMDA. Expression level of GlyT1, but not GlyT2, mRNA in the lumbar dorsal horn was decreased at the herpetic and postherpetic stages. Glycine receptor alpha1-subunit mRNA was decreased in the lumbar dorsal horn at the herpetic, but not postherpetic stage, without alteration in alpha3-subunit mRNA. The results suggest that GlyT2 is a potential target for treatment of dynamic and static allodynia in patients with herpes zoster and postherpetic neuralgia. The lack of efficacy of GlyT1 inhibitor may be explained by activation of NMDA receptors and the down-regulation of GlyT1 in the lumbar dorsal horn.

[Postoperative use of nonsteroidal anti-inflammatory agents in orthopedic surgery]

Nonsteroidal antiinflamatory drugs (NSAIDs) lead to satisfactory acute and chronic pain relief. Besides that, they exert potent antiinflamatory effect. Their analgesic potency is dose related and limited. Orthopedic patients are often on these medications preoperatively and experience opioid-sparing effect in the postoperative period. Chronic NSAIDs use is related to higher rate of sistemic adverse effects, but even short time exposure in the postoperative period is not risk-free. Although Coxibs reduce GIT bleeding incidence due to prolonged use of NSAIDs, there has to be judicious decision considering their cardiovascular adverse effects. There is evidence that NSAIDs producing moderate, dose-dependent increased bleeding time within normal values. High risk of bleeding have patients with established coagulopathy, alcohol abuse and on anticoagulant treatment. There is no strong evidence on influence of NSAIDs on bone growth. Nevertheless, there is evidence that NSAIDs do prevent heterotropic ossification. Prostaglandins are vital contributors for maintainig tissue homeostasis and NSAIDs use can lead to many unwanted effects. Those adverse effect are more common with prolonged exposure, are dose-related and risks have to be carefully and individually assesed in the postoperative pain management.

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.

Activation of cytosolic phospholipase A2 in dorsal root ganglion neurons by Ca2+/calmodulin-dependent protein kinase II after peripheral nerve injury

Background

Peripheral nerve injury leads to a persistent neuropathic pain state in which innocuous stimulation elicits pain behavior (tactile allodynia), but the underlying mechanisms have remained largely unknown. We have previously shown that spinal nerve injury induces the activation of cytosolic phospholipase A₂ (cPLA₂) in injured dorsal root ganglion (DRG) neurons that contribute to tactile allodynia. However, little is known about the signaling pathway that activates cPLA₂ after nerve injury. In the present study, we sought to determine the mechanisms underlying cPLA₂ activation in injured DRG neurons in an animal model of neuropathic pain, focusing on mitogen-activated protein kinases (MAPKs) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII).

Results

Pharmacological inhibition of either p38 or extracellular signal-regulated kinase (ERK) in the injured DRG, which led to suppression of the development of tactile allodynia, did not affect cPLA₂ phosphorylation and translocation after nerve injury. By contrast, a CaMKII inhibitor prevented the development and expression of nerve injury-induced tactile allodynia and reduced both the level of cPLA₂ phosphorylation and the number of DRG neurons showing translocated cPLA₂ in response to nerve injury. Applying ATP to cultured DRG neurons increased the level of both phosphorylated cPLA₂ and CaMKII in the vicinity of the plasma membrane and caused physical association of these two proteins. In addition, ATP-stimulated cPLA₂ and CaMKII phosphorylation were inhibited by both a selective P2X₃R/P2X₂₊₃R antagonist and a nonselective voltage-dependent Ca²⁺ channel (VDCC) blocker.

Conclusion

These results suggest that CaMKII, but not MAPKs, has an important role in cPLA₂ activation following peripheral nerve injury, probably through P2X₃R/P2X₂₊₃R and VDCCs in primary afferent neurons.

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.

Nimesulide 90 mg orally twice daily does not influence postoperative morphine requirements after major chest surgery

BACKGROUND

Cyclooxygenase 2 inhibition has proven analgesic efficacy in a variety of surgical procedures. We postulated that perioperative cyclooxygenase 2 inhibition significantly reduces postoperative morphine requirements after major thoracic surgery and investigated the site of this potential analgesic effect.

METHODS

Ninety-two patients participated in this single-center, double-blind, randomized, placebo-controlled, parallel-group trial. Patients between the ages of 18 and 80 yr undergoing a thoracotomy or median sternotomy were randomized to receive either nimesulide or placebo in combination with a standard analgesic regimen perioperatively. Nimesulide was administered orally the evening before surgery and at 12-h intervals for 5 days postoperatively. The primary efficacy variables were morphine consumption and pain scores for the first 48 h postoperatively. The secondary efficacy variable was the effect of nimesulide on cyclooxygenase activity in cerebrospinal fluid (CSF).

RESULTS

Pain scores at rest or with movement, and total morphine consumption for the first 48 h postoperatively, were not statistically different between the groups. The mean difference in total morphine consumption up to 48 h postoperatively between the nimesulide and placebo group was a 9.0 mg reduction (95% CI: -28.9 to 10.9 mg) (P = 0.37). Adjusted mean (se) CSF 6-keto-PGF1alpha (6-keto-PGF1alpha) concentrations increased by 54.7 (25.7) pg/mL from preoperatively to Day + 2 postoperatively in the placebo group, whereas adjusted mean (se) CSF 6-keto-PGF1alpha concentration decreased by 0.6 pg/mL (18.2 pg/mL) in the nimesulide group. These changes were not statistically different between the groups (P = 0.095).

CONCLUSION

Nimesulide, at a dose of 90 mg twice daily in combination with a standard analgesic regimen, does not influence pain scores, morphine requirements, or CSF prostaglandin levels after major thoracic surgery.

Different effects of spinally applied prostaglandin D2 on responses of dorsal horn neurons with knee input in normal rats and in rats with acute knee inflammation

Prostaglandin D2(PGD2) is the most produced prostanoid in the CNS of mammals, and in behavioral experiments it has been implicated in the modulation of spinal nociception. In the present study we addressed the effects of spinal PGD2 on the discharge properties of nociceptive spinal cord neurons with input from the knee joint using extracellular recordings in vivo, both in normal rats and in rats with acute inflammation in the knee joint. Topical application of PGD2 to the spinal cord of normal rats did not influence responses to mechanical stimulation of the knee and ankle joint except at a high dose. Specific agonists at either the prostaglandin D2 receptor 1 (DP1) or the prostaglandin D2 receptor 2 (DP2) receptor had no effect on responses to mechanical stimulation of the normal knee. By contrast, in rats with inflamed knee joints either PGD2 or a DP1 receptor agonist decreased responses to mechanical stimulation of the inflamed knee and the non-inflamed ankle thus reducing established inflammation-evoked spinal hyperexcitability. Vice versa, spinal application of an antagonist at DP1 receptors increased responses to mechanical stimulation of the inflamed knee joint and the non-inflamed ankle joint suggesting that endogenous PGD2 attenuated central sensitization under inflammatory conditions, through activation of DP1 receptors. Spinal application of a DP2 receptor antagonist had no effect. The conclusion that spinal PGD2 attenuates spinal hyperexcitability under inflammatory conditions is further supported by the finding that spinal coapplication of PGD2 with prostaglandin E2 (PGE2) attenuated the PGE2-induced facilitation of responses to mechanical stimulation of the normal joint.

Analgesic effect of percutaneously absorbed non-steroidal anti-inflammatory drugs: an experimental study in a rat acute inflammation model

Background

External medication that is absorbed percutaneously may be used to reduce inflammation and relieve pain from acute injuries such as ankle sprains and bruises. The plaster method of percutaneous absorption for non-steroidal anti-inflammatory drugs (NSAIDs) was established in Japan in 1988. However, due to the possibility of a placebo effect, the efficacy of this method remains unclear. This experimental study was conducted to control for the placebo effect and to study the efficacy of the plaster method in relieving pain by using a rat model of inflammation.

Methods

Male Wistar-Imamichi rats were used. A yeast suspension was injected into the right hind paw to induce inflammation. A sheet (2.0 × 1.75 cm) containing the drug was adhered to the inflamed paw. Five treatment groups were used, and each sheet contained a single drug: loxoprofen sodium (loxoprofen-Na) (2.5 mg); felbinac (1.75 mg); indomethacin (1.75 mg); ketoprofen (0.75 mg); or base only (control, 0 mg). Mechanical pain threshold, expression of c-Fos in the dorsal horn, and amount of prostaglandin (PG) E₂ in the inflamed paw were evaluated.

Results

Pain threshold increased after treatment, and was significantly increased in the loxoprofen-Na group compared with the control group (p < 0.05). Amounts of PGE₂ were significantly decreased in the loxoprofen-Na and indomethacin groups compared with the control group (p < 0.05). Expression of c-Fos was significantly decreased in the loxoprofen-Na group compared with the control group (p < 0.05).

Conclusion

Percutaneously absorbed NSAIDs have an analgesic effect, inhibit expression of c-Fos in the dorsal horn, and reduce PGE₂ in inflamed tissue, indicating the efficacy of this method of administration for acute inflammation and localized pain.

P2X receptors-mediated cytosolic phospholipase A2activation in primary afferent sensory neurons contributes to neuropathic pain

Activation of P2X(3) and P2X(2/3) receptors (P2X(3)R/P2X(2/3)R), ionotropic ATP receptor subtypes, in primary sensory neurons is involved in neuropathic pain, a debilitating chronic pain that occurs after peripheral nerve injury. However, the underlying mechanisms remain unknown. We investigated the role of cytosolic phospholipase A(2) (cPLA(2)) as a downstream molecule that mediates the P2X(3)R/P2X(2/3)R-dependent neuropathic pain. We found that applying ATP to cultured dorsal root ganglion (DRG) neurons increased the level of Ser505-phosphorylated cPLA(2) and caused translocation of Ser505-phosphorylated cPLA(2) to the plasma membrane. The ATP-induced cPLA(2) activation was inhibited by a selective antagonist of P2X(3)R/P2X(2/3)R and by a selective inhibitor of cPLA(2). In the DRG in vivo, the number of cPLA(2)-activated neurons was strikingly increased after peripheral nerve injury but not after peripheral inflammation produced by complete Freund's adjuvant. Pharmacological blockade of P2X(3)R/P2X(2/3)R reversed the nerve injury-induced cPLA(2) activation in DRG neurons. Moreover, administering the cPLA(2) inhibitor near the DRG suppressed nerve injury-induced tactile allodynia, a hallmark of neuropathic pain. Our results suggest that P2X(3)R/P2X(2/3)R-dependent cPLA(2) activity in primary sensory neurons is a key event in neuropathic pain and that cPLA(2) might be a potential target for treating neuropathic pain.

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.

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.

Substance P initiates NFAT-dependent gene expression in spinal neurons

Persistent hyperalgesia is associated with increased expression of proteins that contribute to enhanced excitability of spinal neurons, however, little is known about how expression of these proteins is regulated. We tested the hypothesis that Substance P stimulation of neurokinin receptors on spinal neurons activates the transcription factor nuclear factor of activated T cells isoform 4 (NFATc4). The occurrence of NFATc4 in spinal cord was demonstrated with RT-PCR and immunocytochemistry. Substance P activated NFAT-dependent gene transcription in primary cultures of neonatal rat spinal cord transiently transfected with a luciferase DNA reporter construct. The effect of Substance P was mediated by neuronal neurokinin-1 receptors that coupled to activation of protein kinase C, l-type voltage-dependent calcium channels, and calcineurin. Interestingly, Substance P had no effect on cyclic AMP response element (CRE)-dependent gene expression. Conversely, calcitonin gene-related peptide, which activated CRE-dependent gene expression, did not activate NFAT signaling. These data provide evidence that peptides released from primary afferent neurons regulate discrete patterns of gene expression in spinal neurons. Because the release of Substance P and calcitonin gene-related peptide from primary afferent neurons is increased following peripheral injury, these peptides may differentially regulate the expression of proteins that underlie persistent hyperalgesia.

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.

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.

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.

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.

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.

Intrathecal Protease-Activated Receptor Stimulation Produces Thermal Hyperalgesia through Spinal Cyclooxygenase Activity

Activation of protease-activated receptors (PARs) in non-neural tissue results in prostaglandin production. Because PARs are found in the spinal cord and increased prostaglandin release in the spinal cord causes thermal hyperalgesia, we hypothesized that activation of these spinal PARs would stimulate prostaglandin production and cause a cyclooxygenase-dependent thermal hyperalgesia. PARs were activated using either thrombin or peptide agonists derived from the four PAR subtypes, delivered to the lumbar spinal cord. Dialysis experiments were conducted in conscious, unrestrained rats using loop microdialysis probes placed in the lumbar intrathecal space. Intrathecal thrombin stimulated release of prostaglandin E (PGE)(2) but not aspartate or glutamate. Intrathecal delivery of the PAR 1-derived peptide SFLLRN-NH(2) and the PAR 2-derived peptide SLIGRL both stimulated PGE(2) release; PAR 3-derived TFRGAP and PAR 4-derived GYPGQV were inactive. Intrathecal thrombin had no effect upon formalin-induced flinching or tactile sensitivity but resulted in a thermal hyperalgesia. Intrathecal SFLLRN-NH(2) and SLIGRL both produced thermal hyperalgesia. Consistent with their effects on spinal PGE(2), hyperalgesia from these peptides was blocked by pretreatment with the cyclooxygenase inhibitor ibuprofen. SLIGRL-induced hyperalgesia was also blocked by the selective inhibitors SC 58,560 [5-(4-fluorophenyl)-1-[4-(methylsulfonyl)phenyl]-3-(trifluoromethyl)-1H-pyrazole; cyclooxygenase (COX) 1] and SC 58,125 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole; COX 2]. These data indicate that activation of spinal PAR 2 and possibly PAR 1 results in the stimulation of the spinal cyclooxygenase cascade and a prostaglandin-dependent thermal hyperalgesia.

Carrageenan-induced Paw Edema in Rat Elicits a Predominant Prostaglandin E2 (PGE2) Response in the Central Nervous System Associated with the Induction of Microsomal PGE2 Synthase-1

Peripheral inflammation involves an increase in cyclooxygenase-2 (COX-2)-mediated prostaglandin (PG) synthesis in the central nervous system (CNS), which contributes to allodynia and hyperalgesia. In the present study we have determined the changes in prostanoid tissue levels and in expression of terminal prostanoid synthases in both the CNS and inflamed peripheral tissue during carrageenan-induced paw inflammation in the rat. Prostanoid levels were measured by liquid chromatography-mass spectrometry and enzyme expression at the RNA level by quantitative PCR analysis during both the early (1-6 h) and late (12 and 24 h) phases of the inflammatory response. In the paw, the early phase was associated with increases in PGE(2) and thromboxane (TX)B(2) levels and with a peak of COX-2 expression that preceded that of microsomal prostaglandin-E(2) synthase-1 (mPGES-1). COX-2 and mPGES-1 remained elevated during the late phase, and PGE(2) continued to further increase through 24 h. The cytosolic PGE(2) synthase (cPGES) showed a small transient increase during the early phase, whereas mPGES-2 expression was not affected by inflammation. In the cerebrospinal fluid, elevated levels of PGE(2), 6-keto-PGF(1alpha), PGD(2), and TXB(2) were detected during the early phase. PGE(2) levels also increased in the spinal cord and, to a lesser extent, in the brain and remained elevated in both the cerebrospinal fluid and the spinal cord during the late phase. The expression of mPGES-1 was strongly up-regulated in the brain and spinal cord during inflammation, whereas no change was detected for the expression of cPGES, mPGES-2, COX-1, and terminal PGD, TX, or PGI synthases. The results show that the carrageenan-induced edema in the paw elicits an early phase of COX-2 induction in the CNS leading to an increase synthesis in PGD(2), 6-keto-PGF(1alpha), and TXB(2) in addition to the major PGE(2) response. The data also indicate that the up-regulation of mPGES-1 contributes to COX-2-mediated PGE(2) production in the CNS during peripheral inflammation.

Cyclooxygenase inhibition in nerve-injury- and TNF-induced hyperalgesia in the rat

After nerve injury, cyclooxygenase-2 (COX-2) is upregulated in spinal cord and peripheral nerve, the latter being dependent on tumor necrosis factor-alpha (TNF). Here we asked whether COX inhibitors attenuate pain behavior induced by chronic constrictive sciatic nerve injury (CCI) or intraneural injection of TNF (2.5 pg/ml). Rats received either 0.9% saline, the nonselective COX inhibitor ibuprofen (40 mg/kg) or the selective COX-2 inhibitor celecoxib (10 or 30 mg/kg) twice daily by gavage started 2 days before, 12 h or 7 days after surgery. Mechanical allodynia and thermal hyperalgesia induced by CCI was moderately, but consistently attenuated by early (day -2 or 12 h after CCI), but not late (7 days after CCI) ibuprofen and celecoxib treatment. Mechanical allodynia, but not thermal hyperalgesia induced by intraneural TNF, was reduced by ibuprofen, but not by celecoxib treatment 5 and 7 days after injection. Sciatic nerves, lumbar dorsal root ganglia (DRG) and spinal cords from rats with treatment started 12 h after surgery were analyzed for prostaglandin E2 (PGE2) levels 10 days after CCI. In injured nerves and ipsilateral DRG, PGE2 levels were increased. Ibuprofen treatment reversed PGE2 levels in injured nerves and DRG, whereas celecoxib blocked increased PGE2 levels only in nerves. In spinal cord, no change in PGE2 levels was observed. In contrast to the marked inhibition of nerve-injury-induced upregulation of PGE2 by COX inhibitors, the effect on pain behavior was modest. Nerve-injury- and TNF-induced pain-related behavior seem to be only partly dependent on peripheral prostaglandins.

Systemic, but not intrathecal ketorolac is antinociceptive to uterine cervical distension in rats

Cyclooxygenase inhibitors demonstrate effective antinociception in many clinical and experimental pain models. Acute uterine cervical distension (UCD) forms the basis for obstetric and some gynecologic pain, and acute UCD in rats results in nocifensor reflexes which are inhibited by morphine in animals lacking, but not in animals with circulating estrogen. We studied the antinociceptive effect of intravenous and intrathecal injection of the cyclooxygenase inhibitor, ketorolac in acute UCD rats and its dependency on estrogen. Virgin rats received estrogen or placebo treatment for 1 week following oviarectomy. An intrathecal catheter was inserted for drug administration. Rats were anesthetized, then the electromyographic response in the rectus abdominus muscle and mean arterial blood pressure change to UCD was recorded before and with cumulative dosing of intravenous or intrathecal ketorolac. Intravenous ketorolac produced dose dependent inhibition of the responses to UCD, but intrathecal ketorolac was ineffective at the maximum test dose (300 microg). Estrogen replacement did not affect the stimulus response or maximum efficacy of ketorolac. Unlike morphine, which reduces response to UCD by spinal and supraspinal mechanisms and whose action is blocked by estrogen, the cyclooxygenase inhibitor, ketorolac acts at an estrogen-independent, non spinal site.