Involvement of glutamate receptors in allodynia induced by prostaglandins E2 and F2 alpha injected into conscious mice

Pharmacodynamics assessment of β-carboline from the roots of Psammosilene tunicoides as analgesic compound

ETHNOPHARMACOLOGICAL RELEVANCE

The root of Psammosilene tunicoides (W. C. Wu et C. Y. Wu) is a well-known medicinal herb for the treatment of pain, hemostasia and rheumatoid arthritis among Chinese people.

AIM OF THE STUDY

The present study aimed to investigate the antinociceptive activity and mechanism of β-carboline alkaloids 1-4 which were extracted from the roots of P. tunicoides.

MATERIALS AND METHODS

The analgesic effects were evaluated using peripheral and central pain mouse models of nociception, including the formalin test and the tail flick test. The levels of glutamic acid (Glu) and nitric oxide (NO) in cerebellar cortexes and spinal cords (L4-6) were determined. The anti-inflammatory of all compounds were then assessed on RAW264.7 cells.

RESULTS

Our results showed that compounds 1-4 had significant analgesic effects on both phases of formalin test of mice. Furthermore, all compounds showed suppressive effects on Glu in the brain and NO levels in the brain cortex and the spinal cord. Except for compound 1, the others could extend the pain threshold of hot water tail-flick in mice. In addition, compounds 2 and 3 (60 μmol/kg) could decrease GABAAα1 protein levels in spinal cord. All compounds exhibited anti-inflammatory effects by inhibiting lipopolysaccharide (LPS)-induced NO production in RAW264.7 cells with half-maximal inhibitory concentration (IC₅₀) 1.1-34.9 μM.

CONCLUSION

β-carboline alkaloids from the roots of P. tunicoides had significant analgesic activity by both central and peripheral mechanisms. Our findings suggested that regulating the release of NO or Glu or GABAα1 are some of the mechanisms of analgesic activity of β-carboline alkaloids.

Analgesic effects of prostaglandin E2 receptor subtype EP1 receptor antagonist: experimental study of application of nucleus pulposus

STUDY DESIGN

The effect of an EP1 receptor antagonist on pain-related behavior induced by nucleus pulposus (NP) applied to the dorsal root ganglion (DRG) in rats was investigated.

OBJECTIVE

We investigated pain-related behavior, the amount of prostaglandin E2 (PGE2), and neural damage to the DRG after application of NP to the DRG after administration of an EP1 receptor antagonist.

SUMMARY OF BACKGROUND DATA

PGE2 induces mechanical allodynia and hyperalgesia, which are mediated by PGE2 receptors. EP1 is one of the PGE2 receptor subtypes. An EP1 antagonist reduces hyperalgesia, allodynia, and c-fos expression in the rat chronic nerve constriction model.

METHODS

Sprague-Dawley rats (n = 103) were used. Animals receiving NP were divided into three experimental groups (n = 12 in each group): saline, high-dose (5 mg/kg) EP1 receptor antagonist (RA), and low-dose (2.5 mg/kg) EP1-RA (orally once daily for 5 days). Animals in the sham group did not receive NP. Von Frey tests were used for pain-behavior testing. The amount of PGE2 in DRG and the number of activating transcription factor-3 (ATF3) immunoreactive positive cells were compared among groups.

RESULTS

The mechanical thresholds in the three groups decreased 7 days after surgery (just before treatment). The threshold in both the high- and low-dose EP1-RA groups increased at 11 days (5 days after treatment) and continued for 14 days. The thresholds in both the low- and high-dose EP1-RA groups increased significantly compared with the saline group (P < 0.05). The amount of PGE2 was significantly increased in the NP group compared with the sham and naïve animals after application of NP. ATF3 expression was increased by NP but was not increased after administration of the EP1-RA.

CONCLUSION

An EP1 receptor antagonist improves pain-related behavior in the rat model and might be a potential agent to improve pain-related behavior in patients with lumbar disc herniation.

Stress-induced hyperalgesia is associated with a reduced and delayed GABA inhibitory control that enhances post-synaptic NMDA receptor activation in the spinal cord

GABA and glutamate are both affected by stress and are involved in nociception. Thus, we determined whether stress-induced enhancement of inflammatory hyperalgesia is mediated by an imbalance between glutamate and GABA neurotransmission. Male rats were subjected daily to 10 to 20 minutes per day of either forced swimming (FS) or sham swimming for 3 consecutive days; nonconditioned rats served as controls. Some rats were treated i.p. with ketamine (5 mg/kg), diazepam (2 mg/kg), flumazenil (0.1 mg/kg), or vehicle (0.9% NaCl), 30 to 60 minutes before each conditioning session or nociception assessment. Pain behavior, spinal nociceptive neuronal activation and GABA and glutamate release were respectively evaluated by the formalin test, the expression of c-Fos and in vivo microdialysis of superficial laminae of the lumbar spinal cord, 48 hours after the last conditioning session. Nitric oxide metabolites (NO(x)) were determined as markers of post-synaptic NMDA receptor activation. FS stress enhanced formalin-induced hyperalgesia, increased pain-elicited c-Fos expression, decreased basal and delayed pain-induced GABA release, and increased basal and induced glutamate release. Hyperalgesia and c-Fos overexpression were blocked only by prestress treatment with diazepam and post-stress treatment with ketamine, whereas changes in GABA and glutamate release were reversed by prestress treatment with diazepam. Diazepam effects were blocked by flumazenil. NO(x) increased in lumbar spinal cord of FS rats by a mechanism antagonized by ketamine. Thus, stress-induced hyperalgesia is initiated by a decreased and delayed GABA release and GABA-A receptor activation, whereas it is maintained by increased glutamate release and NMDA glutamate receptor activation at the spinal level.

A novel role of prostaglandin E2 in neuropathic pain: blockade of microglial migration in the spinal cord

Neuropathic pain produced by damage to or dysfunction of the nervous system is a common and severely disabling state that affects millions of people worldwide. Recent evidence indicates that activated microglia are key cellular intermediaries in the pathogenesis of neuropathic pain and that ATP serves as the mediator. However, the in vivo mechanism underlying the retention of activated microglia in the injured region has not yet been completely elucidated. Prostaglandin E(2) (PGE(2)) is the principal proinflammatory prostanoid and plays versatile roles by acting via four PGE receptor subtypes, EP1-EP4. In the present study, we investigated the role of PGE(2) in spinal microglial activation in relation to neuropathic pain by using genetic and pharmacological methods. Mice deficient in microsomal prostaglandin E synthase-1 impaired the activation of microglia and the NMDA-nitric oxide (NO) cascade in spinal neurons in the dorsal horn and did not exhibit mechanical allodynia after peripheral nerve injury. The intrathecal injection of indomethacin, a nonsteroidal anti-inflammatory drug, ONO-8713, a selective EP1 antagonist, or 7-nitroindole, a neuronal NO synthase inhibitor, attenuated mechanical allodynia and the increase in activated microglia observed in the established neuropathic-pain state. We further demonstrated that ATP-induced microglial migration was blocked in vitro by PGE(2) via EP2 and by S-nitrosoglutathione, an NO donor. Taken together, the present study suggests that PGE(2) participated in the maintenance of neuropathic pain in vivo not only by activating spinal neurons, but also by retaining microglia in the central terminals of primary afferent fibers via EP2 subtype and via EP1-mediated NO production.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Involvement of prostaglandin F 2 alpha receptor in ATP-induced mechanical allodynia

Nociceptive primary afferents have the capacity to induce a state of increased excitability in the dorsal horn neurons of the spinal cord. It is well accepted that capsaicin-sensitive C-fibers transduce noxious stimulation and acute pain and that capsaicin-insensitive A beta-fibers are responsible for touch and innocuous sensation. It has been reported that the intrathecal (i.t.) administration of prostaglandin F(2 alpha) (PGF(2 alpha)) and ATP induces mechanical allodynia via the capsaicin-insensitive primary afferent pathway. In the present study, we investigated the interaction of purinoceptor P2X and the PGF(2 alpha) receptor (FP) in the induction of allodynia by use of mice lacking FP (FP(-/-)). Both PGF(2 alpha) and the P2X receptor agonist alphabeta-methylene ATP administered i.t. strongly induced allodynia for 50 min by tactile stimuli to the flank of mice. The allodynia induced by alphabeta-methylene ATP, but not that by PGF(2 alpha), was suppressed by simultaneous i.t. administration of P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2,4-disulphonic acid and A-317491. In contrast, the allodynia induced by alphabeta-methylene ATP as well as that by PGF(2 alpha) was not observed in FP(-/-) mice. Immunostaining of beta-galactosidase, a reporter knocked into the endogenous FP locus in FP(-/-) mice, showed that the FP receptor was co-localized with P2X(2) and P2X(3) receptors in neurons of the spinal cord. alphabeta-Methylene ATP evoked a transient or sustained [Ca(2+)](i) increase in most of the PGF(2 alpha)-responsive cells in the deeper layer of the spinal cord, and the alphabeta-methylene ATP-evoked increase was blocked by the FP receptor antagonist AL-8810 in two-thirds of the cells. Neither PGF(2 alpha) nor alphabeta-methylene ATP induced the activation of spinal microglia. The present study demonstrates that the alphabeta-methylene ATP-evoked allodynia is mediated by the FP receptor, possibly via the functional coupling between the activation of P2X(2/3) receptors on the central terminal of capsaicin-insensitive fibers and FP receptors on spinal neurons.

Selective blockade of PGE2 EP1 receptor protects brain against experimental ischemia and excitotoxicity, and hippocampal slice cultures against oxygen-glucose deprivation

Cyclooxygenase-2 (COX-2) enzyme increases abnormally during excitotoxicity and cerebral ischemia and promotes neurotoxicity. Although COX-2 inhibitors could be beneficial, they have significant side effects. We and others have shown that the EP1 receptor is important in mediating PGE2 toxicity. Here, we tested the hypothesis that pretreatment with a highly selectiveEP1 receptor antagonist, ONO-8713, would improve stroke outcome and that post-treatment would attenuate NMDA-induced acute excitotoxicity and protect organotypic brain slices from oxygen-glucose deprivation (OGD)-induced toxicity. Male C57BL/6 mice were injected intracerebroventricularly with ONO-8713 before being subjected to 90-min middle cerebral artery occlusion (MCAO) and 96-h reperfusion.Significant reduction in infarct size was observed in groups given 0.1 (25.9 +/- 4.7%) and 1.0 nmol(27.7 +/- 2.8%) ONO-8713 as compared with the vehicle-treated control group. To determine the effects of ONO-8713 post-treatment on NMDA induced excitotoxicity, mice were given a unilateral intrastriatal NMDA injection followed by one intraperitoneal injection of 10 microg/kg ONO-8713, 1 and 6 h later. Significant attenuation of brain damage (26.6 +/-4.9%) was observed at 48 hin the ONO-8713-treated group. Finally, brain slice cultures were protected (25.5 +/- 2.9%) by the addition of ONO-8713 to the medium after OGD.These findings support the notion that the EP1receptor propagates neurotoxicity and that selective blockade could be considered as a potential preventive and/or therapeutic tool against ischemic/hypoxic neurological conditions.

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.

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.

Protective effect of intrathecal ketorolac in spinal cord ischemia in rats: a microdialysis study

BACKGROUND

The prevention of ischemic paraplegia after thoracoabdominal aortic surgery is challenging for both anesthesiologists and surgeons. In a previous study, we showed that intrathecal ketorolac pre-treatment protects rats against ischemic spinal cord injury. In the present study, using a microdialysis method, we investigated whether this neuroprotective effect was related to changes in the spinal cord release of nitric oxide (NO) or the excitatory amino acids (EAAs) aspartate and glutamate.

METHODS

Rats were randomized to receive either intrathecal saline or ketorolac 60 microg (10 rats per group), 1 h before spinal cord ischemic injury induced by balloon inflation of a 2F Fogarty catheter in the thoracic aorta with maintenance of the proximal arterial blood pressure at 40 mmHg for 11 min, followed by reperfusion. Another 10 animals were used as the sham-operated control group. Ischemic injury was assessed by hind limb motor function. Cerebrospinal fluid dialysates were collected at baseline (before ischemia) and at 1, 2, 3, 4, 6, 12 and 24 h after the start of reperfusion, and were analyzed for EAAs using high-performance liquid chromatography and for NO metabolites using an NO analyzer.

RESULTS

The results showed that intrathecal ketorolac attenuated spinal cord ischemic injury. Dialysate concentrations of NO and EAAs were increased after spinal cord ischemia, and this effect was inhibited by intrathecal administration of ketorolac.

CONCLUSIONS

The results of this study suggest that the neuroprotective effect of intrathecal ketorolac in spinal cord ischemia in rats may be caused by a decrease in the spinal cord release of NO and EAAs.

Altered spinal arachidonic acid turnover after peripheral nerve injury regulates regional glutamate concentration and neuropathic pain behaviors in rats

Spinal glutamate transporters (GT) have been implicated in the mechanisms of neuropathic pain; however, how spinal GT uptake activity is regulated remains unclear. Here we show that alteration of spinal arachidonic acid (AA) turnover after peripheral nerve injury regulated regional GT uptake activity and glutamate homeostasis. Chronic constriction nerve injury (CCI) in rats significantly reduced spinal GT uptake activity ((3)H-glutamate uptake) with an associated increase in extracellular AA and glutamate concentration from spinal microdialysates on postoperative day 8. AACOCF3 (a cytosolic phospholipase A2 inhibitor, 30mug) given intrathecally twice a day for postoperative day 1-7 reversed this CCI-induced spinal AA production, prevented the reduced spinal GT uptake activity and increased extracellular glutamate concentration. Conversely, alteration of spinal AA metabolism by diclofenac (a cyclooxygenase 1/2 inhibitor, 200mug) further reduced spinal GT uptake activity and increased extracellular glutamate concentration in CCI rats. GT uptake activity was also attenuated when AA (10 or 100nM) was directly added into spinal samples of naïve rats in an in vitro(3)H-glutamate uptake assay, indicating a direct inhibitory effect of AA on GT uptake activity. Consistent with these findings, AACOCF3 reduced the development of both thermal hyperalgesia and mechanical allodynia, whereas diclofenac exacerbated thermal hyperalgesia, in CCI rats. Thus, spinal AA turnover may serve as a regulator in CCI-induced changes in regional GT uptake activity, glutamate homeostasis, and neuropathic pain behaviors. These data suggest that regulating spinal AA turnover may be a useful approach to improving the clinical management of neuropathic pain.

Prostaglandins, glutamate and nitric oxide synthase mediate nitroglycerin-induced hyperalgesia in the formalin test

Increasing evidence supports a possible role for nitric oxide (NO) in the transmission of pain signals and in the development of central mechanisms of hyperalgesia. Previously, we have shown that nitroglycerin, an NO donor, is able to induce a long-lasting hyperalgesic state in rats. Nitroglycerin-induced hyperalgesia can be detected as an increase in the nociceptive behavior evoked by the formalin test. In the present study we investigated the possible mediators in the nitroglycerin-induced hyperalgesic state. Male Sprague-Dawley rats were injected with nitroglycerin and pretreated with indomethacin, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) or N(omega)-nitro-L-arginine methyl ester (L-NAME). The results obtained showed that inhibition of prostaglandins or NO synthesis prevents nitroglycerin-induced hyperalgesia in Phase II of the formalin test. A similar inhibitory effect was also observed following pretreatment with the glutamate antagonist MK801. The present findings point to the role of prostaglandins, NO synthesis and glutamate activity in the induction of nitroglycerin-induced hyperalgesia.

Prostaglandin EP1 receptor contributes to excitotoxicity and focal ischemic brain damage

The clinical side effects associated with the inhibition of cyclooxygenase enzymes under pathologic conditions have recently raised concerns. A better understanding of neuroinflammatory mechanisms and neuronal survival requires knowledge of cyclooxygenase downstream pathways, especially PGE2 and its G-protein-coupled receptors. In this study, we postulate that EP1 receptor is one of the mechanisms that propagate neurotoxicity and could be a therapeutic target in brain injury. This hypothesis was tested by pretreating C57BL/6 wildtype mice with the EP1 receptor selective agonist ONO-DI-004 and the selective antagonist ONO-8713, followed by striatal unilateral NMDA injection. Results revealed that ONO-DI-004 increased NMDA-induced lesion volume up to 128.7 +/- 12.0%, while ONO-8713 significantly decreased lesion volume to 71.3 +/- 10.9%, as compared to the NMDA-control group. Neurotoxic EP1 receptor properties were also studied using C57BL/6 EP1 receptor knockout (EP1-/-) mice, which revealed a significant decrease to 74.5 +/- 8.2%, as compared to wildtype controls. The protective effect of the antagonist ONO-8713 was also tested in the EP1-/- mice, revealing no additional protection in these mice. Together, these results support the selectivity of ONO-8713 toward EP1 receptor and suggest the neurotoxic role of EP1 receptor. Furthermore, the EP1 receptor role in ischemic brain damage was investigated using a model of middle cerebral artery occlusion (MCAO) and reperfusion. The infarct volume was significantly reduced to 56.9 +/- 11.5% in EP1-/- mice, as compared to wildtype controls. This is the first study that demonstrates that EP1 receptor aggravates neurotoxicity and that modulation of this receptor can determine the outcomes in both excitotoxic and focal ischemic neuronal damage.

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

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

Differential effects of naproxen and rofecoxib on the development of hypersensitivity following nerve injury in rats

The present study was undertaken to determine the effects of cyclooxygenase (COX) inhibitors on the development of neuropathic pain in rats following chronic constriction injury (CCI). A single intraperitoneal administration of naproxen, a nonselective COX inhibitor (10 or 30 mg/kg), or rofecoxib, a selective COX-2 inhibitor (3 or 10 mg/kg) 2 h before nerve injury did not attenuate the development of neuropathic state for 28 days. However, the administration of naproxen [10 or 30 mg/kg, intraperitonelly (i.p.)], but not rofecoxib (3 or 10 mg/kg, i.p.), on day 7 attenuated hypersensitivity but did not alter its development for 28 days. Furthermore, naproxen significantly reduced hyperalgesia and allodynia for 4 h, but the efficacy was not observed 24 h after the treatment, whereas rofecoxib failed to modify the hypersensitivity following perineural (p.n.) or intrathecal (i.t.) administration on day 7. Chronic administration of naproxen (3, 10 or 30 mg/kg), but not rofecoxib (1, 3 or 10 mg/kg), 2 h before, daily for 7 days, after nerve injury significantly attenuated and further delayed the development of hypersensitivity for 21 days following nerve injury. These results suggest that the development of hypersensitivity in the CCI model is not COX-2 dependent and that the chronic administration of naproxen started early before peripheral nerve injury could attenuate the development of hypersensitivity.

Membrane-associated prostaglandin E synthase-1 is required for neuropathic pain

It is widely accepted that prostaglandin (PG) E2 is the principal pro-inflammatory prostanoid and plays an important role in inflammatory pain. However whether PGE2 is involved in neuropathic pain remains unknown. PGE2 is produced from arachidonic acid via PGH2 by at least three PGE synthases (PGES), cytosolic PGES (cPGES), and membrane-associated PGES (mPGES)-1 and -2. In the present study, to clarify the involvement of PGE2 and identify PGES mediating neuropathic pain, we applied a neuropathic pain model prepared by L5 spinal nerve transection to mPGES-1 knockout (mPGES-1-/-) mice. Whereas they retained normal nociceptive responses, mPGES-1-/- mice did not exhibit mechanical allodynia and thermal hyperalgesia over a week. These results demonstrate that PGE2 produced by mPGES-1 is involved in neuropathic pain.

Long-lasting inflammation and long-term hyperalgesia after subcutaneous formalin injection into the rat hindpaw

Subcutaneous formalin injection is widely used as a nociceptive stimulus in the rat. This procedure evokes overt behaviors that last about 90 minutes. However, little is known about the duration of paw inflammation and alterations in pain sensitivity to noxious stimuli after 2 hours. We studied the nociceptive responses to thermal and mechanical stimuli 2 hours to 4 weeks after formalin injection into the dorsal or plantar side of the hindpaw. Thirty-two adult male Sprague-Dawley rats were divided into 3 groups: In group I, 50 microL of 5% formalin was injected into the plantar side (n = 12); in group II, 50 microL of 5% formalin was injected into the dorsal side (n = 12); in group III, 50 microL saline was injected into the dorsal or plantar side of the hindpaw (n = 8). Nociceptive responses to thermal and mechanical stimuli applied to the dorsal or plantar surfaces of the injected and the contralateral hindpaws were recorded. The injection of formalin into the rat's hindpaw produced a hypoalgesic region around the injection site. In contrast, hyperalgesic responses to thermal and mechanical stimulation were induced on the opposite surface of the injected hindpaw as well as in the contralateral noninjected hindpaw. The hyperalgesic responses, which were observed 2 hours after formalin administration, were enhanced 1 to 3 days after injection and lasted 3 to 4 weeks. These results suggest that peripheral inflammation after subcutaneous formalin injection produces a long-lasting sensitization. Possible mechanisms for these changes in nociception are discussed.

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

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

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

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

Immunocytochemical localization of prostaglandin F synthase II in the rat spinal cord

Prostaglandin F synthase has at least two isozymes, i.e. prostaglandin F synthase I and II. Recently, we demonstrated immunocytochemically that prostaglandin F synthase I was localized in neuronal dendrites and somata, and in endothelial cells of blood vessels in the whole area of rat spinal cord. In the present study, we immunocytochemically localized prostaglandin F synthase II in ependymal cells and tanycytes surrounding the central canal and in endothelial cells of blood vessels, but not in any neuronal elements at all segmental levels of the rat spinal cord. Immunoelectron microscopy and confocal laser scanning microscopy confirmed these findings and further revealed that strong immunoreactivity was found in the basal processes of the tanycytes. Our present and recent studies using antibodies against the two isozymes of prostaglandin F synthase clearly indicated that they were localized differentially in ependymal (prostaglandin F synthase II) and neuronal elements (prostaglandin F synthase I), but were co-localized in blood vessels in the rat spinal cord. The distinct localization of the two isozymes suggests that prostaglandin F(2) has different transcellular biological actions via different cell groups.

Activation of presynaptic NMDA receptors coupled to NaV1.8-resistant sodium channel C-fibers causes retrograde mechanical nociceptor sensitization

The present study investigated whether activation of presynaptic N-methyl-d-aspartate (NMDA) receptors in the spinal cord produces a retrograde nociceptor sensitization (hypernociception) to mechanical nonnoxious stimulus. By using an electronic version of the von Frey hair test (pressure meter), s.c. intraplantar administration of prostaglandin E(2) (PGE(2)) (50-400 ng per paw) evoked a dose-related ipsilateral paw hypernociception. In contrast, intrathecal (i.t.) administration of NMDA (5-80 ng) and PGE(2) (15-150 ng) evoked dose-related bilateral paw hypernociception. The s.c. intraplantar administration of dipyrone (80-320 microg per paw) or morphine (3 and 9 microg per paw), usually used to antagonize peripheral PGE(2) (100 ng per paw), induced hypernociception and also antagonized the ipsilateral (without affecting the contralateral) paw hypernociception induced by i.t. injections of NMDA (40 ng) or PGE(2) (50 ng). These doses of drugs did not modify the basal mechanical sensitivity of control paws. This result shows that intraspinal NMDA or PGE(2) produces sensitization of the primary sensory neuron in response to mechanical stimulation. In a second series of experiments it was shown that the i.t. treatment with NaV1.8 (SNS/PN3) sodium channel antisense oligodeoxynucleotides, but not mismatch oligodeoxynucleotides, decreased the mRNA expression of sodium tetrodotoxin-resistant channels on the dorsal root ganglia and abolished the mechanical hypernociception induced by i.t. administration of NMDA. Thus, our results support the suggestion that glutamate release in the spinal cord during inflammation causes retrograde hypernociception of nociceptors associated with sodium tetrodotoxin-resistant channels in primary nociceptive sensory neurons.

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.

Phase I safety assessment of intrathecal ketorolac

Spinal prostaglandin synthesis has been implicated in acute pain processes and in generation and maintenance of central sensitization, and intrathecal injection of cyclo-oxygenase (COX) inhibitors produce antinociception and reduce hypersensitivity in animals. We herein report a Phase I safety assessment of intrathecal injection of the COX inhibitor, ketorolac, in healthy volunteers, and demonstrate no serious side effects. Preclinical studies suggest a major site of action of COX inhibitors for analgesia lies in the central nervous system, especially the spinal cord. For example, COX isoenzymes are expressed in the spinal cord, acute noxious stimuli and inflammation increase spinal prostaglandin production, and spinally administered prostaglandins excite dorsal horn projection neurons, induce release of excitatory neurotransmitters, and cause nociceptive behavior. Intrathecal injection of COX inhibitors increases thermal and mechanical withdrawal threshold in animals with inflammation or nerve injury at doses several 100-fold less than those required systemically. Following pre-clinical neurotoxicity screening and regulatory agency approval, we examined the safety of intrathecal injection of a preservative-free formulation of the COX inhibitor, ketorolac. In an open label, dose-escalating design, 20 healthy volunteers received intrathecal ketorolac, 0.25, 0.5, 1, or 2mg (n=5 per group). Ketorolac did not alter blood pressure, although there was small (10-12%), dose-independent reduction in heart rate for the first hour after injection when data from all subjects were pooled. Ketorolac did not affect sensory or motor function or deep tendon reflexes, and there were no subjective sensations, neurologic or otherwise, reported by the volunteers. Ketorolac did not reduce pain report to heat stimuli applied to the lateral calf. One subject had a mild headache 24h after study, resolving the next day. There were no long-term side effects 6 months after study. These data suggest that intrathecal ketorolac does not produce a high incidence of serious adverse events, and they support further investigation for analgesia.

Spinal cord heme oxygenase participates in glutamate-induced pain-related behaviors

Heme oxygenase catalyzes the formation of CO, Fe(2+) and biliverdin from the substrate heme. In these studies, we attempted to define the roles heme oxygenase play in pain-related behaviors induced by intrathecal injection of the spinal neurotransmitter glutamate. The intrathecal injection of glutamate or the more selective agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in C57Bl/6 mice lead to caudally directed pain behaviors which were sensitive to the heme oxygenase inhibitors tin protoporphyrin (Sn-protoporphyrin) and chromium mesoporphyrin (Cr-mesoporphyrin). Intrathecal injections of glutamate in heme oxygenase type 2 (HO-2) null-mutant animals resulted in reduced pain-related behaviors when compared with wild type animals. Glutamate, NMDA and AMPA stimulated cGMP accumulation in mouse spinal cord slices, which was blocked by heme oxygenase inhibitors. Glutamate did not stimulate cGMP production in HO-2 null-mutant animals. Our data are consistent with the hypothesis that pain-related behaviors induced by spinal glutamate rely on the activation of HO-2 and subsequent production of cGMP.

Central and peripheral roles of prostaglandins in pain and their interactions with novel neuropeptides nociceptin and nocistatin

While acute pain has a fundamental role to operate a protective system, chronic pain associated with inflammation and nerve injury often outlasts its biological usefulness. Therefore, there has recently been great interest in the neurochemical mechanisms of hyperalgesia to noxious stimuli and tactile pain (allodynia) to innocuous stimuli with a hope to relieve persistent, intractable pain. Over several decades non-steroidal anti-inflammatory drugs and opioids have been employed for clinical management of pain. The introduction of molecular biology to pain research has enabled us to describe the mechanism of pain at the molecular level and to develop analgesics with selectivity for targets and with less adverse effects. This review focuses on current knowledge concerning mechanisms and pathways for pain induced by prostaglandins and their interactions with novel neuropeptides nociceptin/orphanin FQ and nocistatin derived from the same opioid precursor protein.

Spinorphin, an endogenous inhibitor of enkephalin-degrading enzymes, potentiates leu-enkephalin-induced anti-allodynic and antinociceptive effects in mice

Spinorphin (LVVYPWT) has been isolated from the bovine spinal cord as an endogenous inhibitor of enkephalin-degrading enzymes. It has been reported that spinorphin has an antinociceptive effect, inhibitory effect on contraction of smooth muscle and anti-inflammatory effect. In the present study, the effects of leu-enkephalin and spinorphin on allodynia and mechanical and thermal nociceptions were examined in vivo using mice. Intrathecal (i.t.) administration of leu-enkephalin or spinorphin inhibited the allodynia induced by intrathecal nociceptin in a dose-dependent manner. Furthermore, spinorphin enhanced the inhibitory effect of enkephalin on allodynia induced by nociceptin. Naloxone antagonized both inhibitory effects of leu-enkephalin and spinorphin, suggesting that the endogenous opioidergic system can modulate allodynia. Intracerebroventricular (i.c.v.) administration of leu-enkephalin increased the nociceptive threshold of heat or mechanical stimulation to a mouse. Although i.c.v. administration of spinorphin had no effect on the threshold of heat or mechanical stimulation, spinorphin enhanced and prolonged the antinociceptive effect of leu-enkephalin. The enhancement of spinorphin on the antinociception produced by leu-enkephalin was reversed by pretreatment with naloxone. From these results, it is suggested that the effects of spinorphin on enkephalin-induced anti-allodynic and antinociceptive effects are due to inhibition of enkephalin-degrading enzymes.

Effects of selective COX-1 and -2 inhibition on formalin-evoked nociceptive behaviour and prostaglandin E(2) release in the spinal cord

Nociception evoked prostaglandin (PG) release in the spinal cord considerably contributes to the induction of hyperalgesia and allodynia. To evaluate the relative contribution of cyclooxygenase-1 (COX-1) and COX-2 in this process we assessed the effects of the selective COX-1 inhibitor SC560 and the selective COX-2 inhibitor celecoxib on formalin-evoked nociceptive behaviour and spinal PGE(2) release. SC560 (10 and 20 mg/kg) significantly reduced the nociceptive response and completely abolished the formalin-evoked PGE(2) raise. In contrast, celecoxib (10 and 20 mg/kg) was ineffective in both regards, i.e. the flinching behaviour was largely unaltered and the formalin-induced PGE(2) raise as assessed using microdialysis was only slightly, not significantly reduced. This suggests that the formalin-evoked rapid PG release was primarily caused by COX-1 and was independent of COX-2. Mean free spinal cord concentrations of celecoxib during the formalin assay were 32.0 +/- 4.5 nM, thus considerably higher than the reported IC50 for COX-2 (3-7 nM). Therefore, the lack of efficacy of celecoxib is most likely not to be a result of poor tissue distribution. COX-2 mRNA and protein expression in the spinal cord were not affected by microdialysis alone but the mRNA rapidly increased following formalin injection and reached a maximum at 2 h. COX-2 protein was unaltered up to 4 h after formalin injection. The time course of COX-2 up-regulation suggests that the formalin-induced nociceptive response precedes COX-2 protein de novo synthesis and may therefore be unresponsive to COX-2 inhibition. Considering the results obtained with the formalin model it may be hypothesized that the efficacy of celecoxib in early injury evoked pain may be less than that of unselective NSAIDs.

The release of spinal prostaglandin E2 and the effect of nitric oxide synthetase inhibition during strychnine-induced allodynia

The removal of spinal glycinergic inhibition by intrathecal strychnine produces an allodynia-like state in rodents. Our objective was to measure spinal prostaglandin E2 (PGE2) release during strychnine-allodynia and examine the effects of Nomega-nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide synthetase. Under halothane, rats were fitted with intrathecal and spinal microdialysis catheters, and microelectrodes implanted into the locus coeruleus for measurement of catechol oxidation current (CAOC) using voltammetry. Animals were then administered urethane and treated as follows: 1) baseline control 10 min, intrathecal strychnine (40 microg) 10 min, 10 min of hair deflection, and 2) 10-min control followed by intrathecal strychnine (40 microg) with hair deflection for 60 min. Spinal dialysate samples were collected for PGE2 levels determined by using immunoassay. In separate experiments, the effect of intrathecal strychnine (40 microg) followed by hair deflection was studied in rats pretreated with intrathecal l-NOARG (50 nmol). After intrathecal strychnine, hair deflection significantly increased spinal PGE2 release (619% +/- 143%), locus coeruleus CAOC (181% +/- 6%), and mean arterial pressure (123% +/- 2%) P < 0.05. Pretreatment with intrathecal l-NOARG significantly inhibited strychnine-allodynia. In this model, hair deflection evokes spinal PGE2 release, locus coeruleus activation, and an increase in mean arterial pressure. L-NOARG pretreatment attenuated the locus coeruleus CAOC, a biochemical index of strychnine-allodynia, suggesting a mediator role of nitric oxide. A mediator role of nitric oxide is also implicated, helping to explain the pathophysiology of this allodynic pain.

Characterization of EP receptor subtypes responsible for prostaglandin E2-induced pain responses by use of EP1 and EP3 receptor knockout mice

Prostaglandin E2 (PGE2) is known to be the principal pro-inflammatory prostanoid and play an important role in nociception. To identify PGE receptor (EP) subtypes that mediate pain responses to noxious and innocuous stimuli, we studied them by use of EP1 and EP3 knockout (EP1(-/-) and EP3(-/-)) mice. PGE2 could induce mechanical allodynia in EP1(+/+), EP3(+/+) and EP3(-/-) mice, but not in EP1(-/-) mice. N-methyl-D-aspartate (NMDA), the substrate of nitric oxide (NO) synthase L-arginine, or the NO donor sodium nitroprusside administered intrathecal (i.t.) could induce allodynia in EP3(-/-) and EP1(-/-) mice. Activation of EP1 receptors appears to be upstream, rather than downstream, of NMDA receptor activation and NO production in the PGE2-induced allodynia. Although PGE2 produced thermal hyperalgesia over a wide range of dosages from 50 pg to 0.5 microg kg(-1) in EP3(+/+) mice, it showed a monophasic hyperalgesic action at 5 ng kg(-1) or higher doses in EP3(-/-) mice. The selective EP3 agonist, ONO-AE-248, induced hyperalgesia at 500 pg kg(-1) in EP3(+/+) mice, but not in EP3(-/-) mice. Saline-injected EP1(-/-) mice showed hyperalgesia, which was reversed by i.t. PGE2 in a dose-dependent manner. There was no significant difference in the formalin-induced behaviours between EP1(-/-) or EP3(-/-) mice and the cognate wild-type mice. These results demonstrate that spinal EP1 receptors are involved in the PGE2-induced allodynia and that spinal EP3 receptors are involved in the hyperalgesia induced by low doses of PGE2. However, the formalin-induced pain cannot be ascribed to a single EP receptor subtype EP1 or EP3.

Topical bicuculline to the rat spinal cord induces highly localized allodynia that is mediated by spinal prostaglandins

The purpose of this study was to investigate the allodynic effect of bicuculline (BIC) given topically to the dorsal surface of the rat spinal cord, and to determine if spinal prostaglandins (PGs) mediate the allodynic state arising from spinal GABA(A)-receptor blockade. Male Sprague-Dawley rats (325-400 g) were anaesthetized with halothane and maintained with urethane for the continuous monitoring of blood pressure (MAP), heart rate (HR) and cortical electroencephalogram (EEG). A laminectomy was performed to expose the dorsal surface of the spinal cord. Unilateral application of BIC (0.1 microg in 0.1 microl) to the L5 or L6 spinal segment induced a highly localized allodynia (e.g. one or two digits) on the ipsilateral hind paw. Thus, hair deflection (brushing the hair with a cotton-tipped applicator) in the presence, but not absence of BIC, evoked an increase in MAP and HR, abrupt motor responses (MR; e.g. withdrawal of the hind leg, kicking, and/or scratching) on the affected side, and desynchrony of the EEG. BIC-allodynia was dose-dependent, yielding ED(50)'s (95% CI's) of 45 ng (31-65) for MAP; 68 ng (46-101) for HR and 76 ng (60-97) for MR. Allodynia was sustained for up to 2 h with repeated BIC application without any detectable change in the location or area of peripheral sensitization. Pretreatment with either the EP(1)- receptor antagonist, SC-51322, the cyclooxygenase (COX)-2 selective inhibitor, NS-398, or the NMDA-receptor antagonist, AP-7, inhibited BIC-allodynia in a dose-dependent manner. The results demonstrate: (a) BIC, applied to the dorsal surface of the spinal cord, induces highly localized allodynia; (b) this effect can be sustained with repeated BIC application; (c) it is evoked by NMDA-dependent afferent input; (d) spinal PGs are synthesized by constitutive COX-2 during BIC-allodynia; and (e) spinal PGs contribute to the abnormal processing of tactile input via spinal EP1-receptors.

Prostaglandins and cyclooxygenases [correction of cycloxygenases] in the spinal cord

The spinal cord is one of the sites where non-steroidal anti-inflammatory drugs (NSAIDs) act to produce analgesia and antinociception. Expression of cyclooxygenase(COX)-1 and COX-2 in the spinal cord and primary afferents suggests that NSAIDs act here by inhibiting the synthesis of prostaglandins (PGs). Basal release of PGD(2), PGE(2), PGF(2alpha) and PGI(2) occurs in the spinal cord and dorsal root ganglia. Prostaglandins then bind to G-protein-coupled receptors located in intrinsic spinal neurons (receptor types DP and EP2) and primary afferent neurons (EP1, EP3, EP4 and IP). Acute and chronic peripheral inflammation, interleukins and spinal cord injury increase the expression of COX-2 and release of PGE(2) and PGI(2). By activating the cAMP and protein kinase A pathway, PGs enhance tetrodotoxin-resistant sodium currents, inhibit voltage-dependent potassium currents and increase voltage-dependent calcium inflow in nociceptive afferents. This decreases firing threshold, increases firing rate and induces release of excitatory amino acids, substance P, calcitonin gene-related peptide (CGRP) and nitric oxide. Conversely, glutamate, substance P and CGRP increase PG release. Prostaglandins also facilitate membrane currents and release of substance P and CGRP induced by low pH, bradykinin and capsaicin. All this should enhance elicitation and synaptic transfer of pain signals in the spinal cord. Direct administration of PGs to the spinal cord causes hyperalgesia and allodynia, and some studies have shown an association between induction of COX-2, increased PG release and enhanced nociception. NSAIDs diminish both basal and enhanced PG release in the spinal cord. Correspondingly, spinal application of NSAIDs generally diminishes neuronal and behavioral responses to acute nociceptive stimulation, and always attenuates behavioral responses to persistent nociception. Spinal application of specific COX-2 inhibitors sometimes diminishes behavioral responses to persistent nociception.

Release of glutamate, nitric oxide and prostaglandin E2 and metabolic activity in the spinal cord of rats following peripheral nociceptive stimulation

Peripheral tissue injury and inflammation may result in a facilitated spinal nociceptive transmission and central sensitization. Particularly, nitric oxide (NO) and prostaglandins (PGs) have been shown to be key mediators involved in the induction and maintenance of this state. By means of spinal cord microdialysis we have determined interstitial glutamate, NO (NO2-/NO3-), PGE2, glycerol, glucose and lactate concentrations in the dorsal horns of the spinal cord following peripheral nociceptive stimulation to gain further insight into the link between excitatory neurotransmitters and metabolic functions in the spinal cord during nociception. Formalin and zymosan injection into one hind paw evoked a biphasic release of glutamate and NO with the glutamate peaks preceding those of NO. Moreover, zymosan induced a biphasic increase of interstitial glycerol concentrations accompanied by an increase of interstitial lactate indicating metabolic disturbances. In contrast, formalin injection led to an elevation of dialysate glucose concentrations which may be interpreted as an indication of enhanced metabolic activity. The sequential release of glutamate and NO in the dorsal horns of the spinal cord in response to peripheral nociceptive stimulation supports the theory that NO may act as a retrograde transmitter. The metabolic changes observed after formalin and zymosan injection suggest that an intense peripheral nociceptive stimulation may not only activate but also disturb metabolic activity and possibly membrane integrity in the spinal cord.

Characterization of the glutamatergic system for induction and maintenance of allodynia

Glutamate is the main excitatory neurotransmitter in the central nervous system and has been shown to be involved in spinal nociceptive processing. We previously demonstrated that intrathecal (i.t.) administration of prostaglandin (PG) E(2) and PGF(2 alpha) induced touch-evoked pain (allodynia) through the glutamatergic system by different mechanisms. In the present study, we characterized glutamate receptor subtypes and glutamate transporters involved in induction and maintenance of PGE(2)- and PGF(2 alpha)-evoked allodynia. In addition to PGE(2) and PGF(2 alpha), N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), but not kainate, induced allodynia. PGE(2)- and NMDA-induced allodynia were observed in NMDA receptor epsilon 4 (NR2D) subunit knockout (GluR epsilon 4(-/-)) mice, but not in epsilon 1 (NR2A) subunit knockout (GluR epsilon 1(-/-)) mice. Conversely, PGF(2 alpha)- and AMPA-induced allodynia were observed in GluR epsilon 1(-/-) mice, but not in GluR epsilon 4(-/-) mice. The induction of allodynia by PGE(2) and NMDA was abolished by the NMDA receptor epsilon 2 (NR2B) antagonist CP-101,606 and neonatal capsaicin treatment. PGF(2 alpha)- and AMPA-induced allodynia were not affected by CP-101,606 and by neonatal capsaicin treatment. On the other hand, the glutamate transporter blocker DL-threo-beta-benzyloxyaspartate (DL-TBOA) blocked all the allodynia induced by PGE(2), PGF(2 alpha), NMDA, and AMPA. These results demonstrate that there are two pathways for induction of allodynia mediated by the glutamatergic system and suggest that the glutamate transporter is essential for the induction and maintenance of allodynia.

Localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in mouse spinal cord

Prostaglandins are important mediators in spinal nociceptive processing. They are produced by cyclo-oxygenase isoforms, cyclo-oxygenase-1 and -2, which are both constitutively expressed in the central nervous system. The present immunohistochemical study details localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in lumbar spinal cord before and after induction of a painful paw inflammation in mice. Cyclo-oxygenase-1 immunoreactivity was found in glial cells of the dorsal and ventral horns, but not in neurons. In unstimulated mice, cyclo-oxygenase-2 immunoreactivity was found in motoneurons of the ventral horns and in lamina X, but not in dorsal horn neurons. After induction of a paw inflammation with zymosan, cyclo-oxygenase-2 immunoreactivity increased dramatically in dorsal horn neurons of laminae I-VI and X, paralleled by a significant increase in prostaglandin E(2) release from lumbar spinal cord. Cyclo-oxygenase-2 was co-localized with neuronal nitric oxide synthase immunoreactivity in several neurons in superficial laminae of the dorsal horns and in the area surrounding the central canal. Nitric oxide synthase was distributed in the cytoplasm and extended to processes of some neurons. In contrast, electron microscopy revealed that cyclo-oxygenase-2 immunoreactivity was restricted to the nuclear membrane and rough endoplasmic reticulum. It is shown in the present study that both cyclo-oxygenase isoforms are constitutively expressed in the spinal cord, cyclo-oxygenase-1 in glial cells of the dorsal and ventral horns and cyclo-oxygenase-2 in motoneurons. After induction of a hindpaw inflammation, several dorsal horn neurons express cyclo-oxygenase-2. Some of them are also positive for neuronal nitric oxide synthase, which is also induced following peripheral inflammation. Intracellularly, cyclo-oxygenase-2 is bound to the membranes of the nucleus and endoplasmic reticulum, whereas neuronal nitric oxide synthase is found in the cytoplasm.

Regulation of the development of allodynia by intrathecally administered P2 purinoceptor agonists and antagonists in mice

Effects of agonists and antagonists of P2X-purinoceptors on the regulation of the development of allodynia were examined in mice; the drugs were administered intrathecally to the spinal cord. Suramin (5, 10 microg) and pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS), antagonists of P2X receptors, inhibited prostaglandin (PG) E(2)-induced allodynia. PPADS did not block glutamate-induced allodynia. alpha,beta-Methylene ATP (alpha, beta-meATP), an agonist of P2X receptor, elicited allodynia. alpha, beta-me ATP-induced allodynia was blocked by co-administration of alpha,beta-meATP with PPADS, MK 801 or N(omega)-nitro-L-arginine methyl ester (L-NAME). Suramin at higher doses (20, 40 microg) induced allodynia, which was inhibited by MK 801 or L-NAME. These results suggest that ATP P2X receptors in the spinal cord are involved in the regulation of tactile allodynia. Glutamate receptor and nitric oxide systems play an important role in the development of allodynia produced by alpha,beta-meATP and suramin.

The role of nitric oxide and prostaglandin signaling pathways in spinal nociceptive processing in chronic inflammation

Both nitric oxide (NO) and prostaglandins (PG) and their associated enzymes nitric oxide synthases (NOS) and cyclooxygenases (COX) (specifically COX-2) have been implicated in the development of hyperalgesia. This study examined the effects of naturally occurring chronic inflammation, chronic mastitis, on spinal nociceptive processing in sheep and focused on potential alterations in spinal PG and NO signaling pathways. Mechanical withdrawal thresholds were significantly lower in animals suffering from chronic inflammation (n=6) compared to control animals (n=6). Hyperalgesia was restricted to the side contralateral to the inflammation (decrease from ipsilateral side: hindlimb 33.2+/-5%, forelimb 19.4+/-5%). Neuronal NOS-immunoreactivity was significantly reduced bilaterally in lumbar and cervical spinal cord throughout laminae I-III (decrease 18.4+/-5% and 16.9+/-4%, respectively) and in lamina X (decrease 29.1+/-6% and 17.1+/-4%, respectively) in mastitic animals relative to control animals. No difference was detected in eNOS or iNOS-immunoreactivity or in NADPH-diaphorase staining, a marker of dynamically active NOS. RT-PCR failed to detect any change in levels of nNOS, eNOS, iNOS, COX-1 or COX-2 mRNAs. However, a marked increase in the PGE receptor, EP(3) (but not EP(2)) mRNA was detected in ipsilateral spinal cord tissue from animals with chronic inflammation. This increase in EP(3) receptor expression indicates that spinal PGs are important in the spinal response to chronic peripheral inflammation. Contralateral mechanical hyperalgesia may not be directly linked to changes in spinal EP(3) receptor mRNA expression, however, the bilateral changes in nNOS suggest that this pathway may contribute to the adaptive behavioural response observed.

cDNA cloning, expression and characterization of human prostaglandin F synthase

A cDNA clone of prostaglandin F synthase (PGFS) was isolated from human lung by using cDNA of bovine lung-type PGFS as a probe and its protein expressed in Escherichia coli was purified to apparent homogeneity. The human PGFS catalyzed the reduction of prostaglandin (PG) D2, PGH2 and phenanthrenequinone (PQ), and the oxidation of 9alpha,11beta-PGF2 to PGD2. The kcat/Km values for PGD2 and 9alpha,11beta-PGF2 were 21000 and 1800 min(-1) mM(-1), respectively, indicating that the catalytic efficiency for PGD2 and 9alpha,11beta-PGF2 was the highest among the various substrates, except for PQ. The PGFS activity in the cytosol of human lung was completely absorbed with antihuman PGFS antiserum. Moreover, mRNA of PGFS was expressed in peripheral blood lymphocytes and the expression in lymphocytes was markedly suppressed by the T cell mitogen concanavalin A. These results support the notion that human PGFS plays an important role in the pathogenesis of allergic diseases such as asthma.

Opposite modulation of capsaicin-evoked substance P release by glutamate receptors

Substance P and glutamate are present in primary afferent C-fibers and play important roles in persistent inflammatory and neuropathic pain. In the present study, we have examined whether activation of different glutamate receptor subtypes modulates the release of substance P evoked by the C-fiber selective stimulant capsaicin (1 microM) from rat trigeminal nucleus slices. The selective NMDA glutamate receptor agonist L-CCG-IV (1-10 microM) enhanced capsaicin-evoked substance P release about 100%. This facilitatory effect was blocked by 0.3 microM MK-801, a selective NMDA receptor antagonist. The metabotropic glutamate receptor agonists L-AP4 (group III) and DHPG (group I) (30-100 microM) inhibited capsaicin-evoked substance P release by approximately 60%. These inhibitory effects were blocked by the selective metabotropic glutamate receptor antagonist (+/-)-MCPG (5 microM). On the other hand, AMPA and kainate (0.1-10 microM), did not significantly affect capsaicin-evoked substance P release. Thus, substance P release from non-myelinated primary afferents, and possibly nociception, may be under the functional antagonistic control of some metabotropic and ionotropic glutamate receptor subtypes.

The intraspinal release of prostaglandin E2 in a model of acute arthritis is accompanied by an up-regulation of cyclo-oxygenase-2 in the spinal cord

In anaesthetized rats, the intraspinal release of immunoreactive prostaglandin E2 was measured using antibody microprobes. We addressed the question of whether the release of immunoreactive prostaglandin E2 is altered during development of acute inflammation in the knee evoked by intra-articular injections of kaolin and carrageenan. We also examined cyclo-oxygenase-1 and cyclo-oxygenase-2 protein levels in the spinal cord during the development of inflammation using the same model of arthritis. Densitometric analysis of microprobes showed that basal release of immunoreactive prostaglandin E2 in the period 175-310 min after kaolin was slightly higher than in the absence of inflammation. A pronounced enhancement of basal release of immunoreactive prostaglandin E2 was observed 430-530 min after kaolin. Enhanced levels of immunoreactive prostaglandin E2 were observed throughout the dorsal and ventral horns. Release of immunoreactive prostaglandin E2 was not altered further by the application of innocuous and noxious pressure onto the inflamed knee. Western blot analysis revealed that cyclo-oxygenase-2 but not cyclo-oxygenase-1 protein levels were elevated in the spinal cords of animals with inflammation compared to normal animals. This effect was evident as early as 3 h after the induction of arthritis. The maximum elevation of cyclo-oxygenase-2 protein levels (six-fold) was observed 12 h after the induction of arthritis. The results show that there is a tonic release of immunoreactive prostaglandin E2 from the spinal cord following the induction of arthritis, which is accompanied by enhanced expression of cyclo-oxygenase-2 protein in the spinal cord. We suggest that intraspinal prostaglandins may play a role in inflammation-evoked central sensitization of spinal cord neurons.

Involvement of primary afferent C-fibres in touch-evoked pain (allodynia) induced by prostaglandin E2

Nociceptive primary afferents have the capacity to induce a state of increased excitability in dorsal horn neurons of the spinal cord or central sensitization causing thermal hyperalgesia and touch-evoked pain (allodynia). It is believed that primary afferent C-fibres become hypersensitive and induce hyperalgesia and that low-threshold Abeta-fibres are responsible for induction of allodynia, the mechanisms of which remain elusive. We previously showed that intrathecal administration of prostaglandin E2 (PGE2) and prostaglandin F2alpha (PGF2alpha) induce allodynia in conscious mice. Here we demonstrated that selective elimination of C-fibres by neonatal capsaicin treatment resulted in the disappearance of allodynia induced by PGE2, but not that by PGF2alpha. PGE2-induced allodynia was not observed in N-methyl-D-aspartate (NMDA) receptor epsilon1 subunit knockout mice and was sensitive to morphine. In contrast, PGF2alpha-induced allodynia was not observed in NMDA epsilon4 subunit knockout mice and was insensitive to morphine. Furthermore, while PGF2alpha showed a capsaicin-insensitive feeble facilitatory action on evoked excitatory postsynaptic currents in dorsal horn neurons, PGE2 induced a long-lasting facilitation of evoked excitatory postsynaptic currents in a capsaicin-sensitive manner. Taken together, the present study demonstrates that there are two pathways for induction of allodynia and that capsaicin-sensitive C-fibres may participate in PGE2-induced allodynia.

Lack of tactile pain (allodynia) in lipocalin-type prostaglandin D synthase-deficient mice

Prostaglandin (PG) D2 is the most abundant prostanoid produced in the central nervous system of mammals and has been implicated in the modulation of neural functions such as sleep induction, nociception, regulation of body temperature, and odor responses. We generated gene-knockout mice for lipocalin-type PGD2 synthase (L-PGDS) and found that the intrathecal administration of PGE2, an endogenous pain-producing substance, failed to elicit allodynia (touch-evoked pain), which is one typical phenomenon of neuropathic pain, whereas it evoked thermal hyperalgesia, in L-PGDS-/- mice. We also found that the allodynic response induced by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline was selectively abolished in the L-PGDS-/- mice, among excitatory and inhibitory agents that induced allodynia in wild-type mice. Interestingly, simultaneous injection of a femtogram amount of PGD2 with PGE2 or bicuculline induced allodynia in L-PGDS-/- mice to the same extent as in wild-type mice. The PGE2- or bicuculline-evoked allodynia in wild-type and in PGD2-supplemented L-PGDS-/- mice was blocked by a PGD2 receptor antagonist given in a femtogram amount. These results reveal that endogenous PGD2 is essential for both PGE2- and bicuculline-induced allodynia.

Anti-nociceptive responses produced by human putative counterpart of nocistatin

b-nocistatin is a heptadecapeptide produced from bovine prepronociceptin and blocks the induction of hyperalgesia and touch-evoked pain (allodynia) by intrathecal administration of nociceptin or prostaglandin E2 (PGE2). Human prepronociceptin may generate a 30-amino acid peptide different in length from b-nocistatin. Here, we examine whether the human putative counterpart of nocistatin (h-nocistatin) possessed the same biological activities as b-nocistatin. Simultaneous intrathecal injection of h-nocistatin in mice blocked the induction of allodynia by nociceptin and PGE2 in a dose-dependent manner with ID50 values of 329 pg kg(-1) and 16.6 ng kg(-1), respectively. h-nocistatin was about 10 times less potent than b-nocistatin. h-nocistatin also attenuated the nociceptin- and PGE2-induced hyperalgesia. These results demonstrate that h-nocistatin is biologically active and may be involved in the processing of pain at the spinal level in humans.

Stimulation of nitric oxide release from rat spinal cord by prostaglandin E2

1. We recently demonstrated that intrathecal administration of prostaglandin E2 (PGE2) and PGF2alpha induced allodynia through a pathway that includes the glutamate receptor and nitric oxide (NO)-generating systems from pharmacological studies. In order to clarify the involvement of NO in prostaglandin-induced allodynia, we measured NO released from rat spinal cord slices by a chemiluminescence method. 2. PGE2 stimulated NO release from both dorsal and ventral regions all along the spinal cord. PGE2 stimulated the release within 10 min and increased it in a time-dependent manner. 3. The PGE2-induced NO release was observed at 100 nM-10 microM. PGF2alpha stimulated the release at concentrations higher than 1 microM, but PGD2 (up to 10 microM) did not enhance it. 4. 17-Phenyl-omega-trinor PGE2 (EP1 > EP3) and sulprostone (EP1 < EP3) were as potent as PGE2, but PGE1 was less potent, in stimulating NO release. While M&B 28767 (EP3) did not enhance the release, butaprost (EP2) stimulated it at 1 microM. The PGE2-evoked release was blocked by ONO-NT-012, a bifunctional EP1 antagonist/EP3 agonist. 5. The PGE2-evoked release was Ca2+-dependent and blocked by MK-801 (NMDA receptor antagonist) and L-NAME (NO synthase inhibitor). The release was also inhibited by PGD2 and dibutyryl-cyclic AMP. 6. The present study demonstrated that PGE2 stimulates NO release in the rat spinal cord by activation of NMDA receptors through the EP1 receptor, and supports our previous findings that the NO-generating system is involved in the PGE2-induced allodynia.

Diminished inflammation and nociceptive pain with preservation of neuropathic pain in mice with a targeted mutation of the type I regulatory subunit of cAMP-dependent protein kinase

To assess the contribution of PKA to injury-induced inflammation and pain, we evaluated nociceptive responses in mice that carry a null mutation in the gene that encodes the neuronal-specific isoform of the type I regulatory subunit (RIbeta) of PKA. Acute pain indices did not differ in the RIbeta PKA mutant mice compared with wild-type controls. However, tissue injury-evoked persistent pain behavior, inflammation of the hindpaw, and ipsilateral dorsal horn Fos immunoreactivity was significantly reduced in the mutant mice, as was plasma extravasation induced by intradermal injection of capsaicin into the paw. The enhanced thermal sensitivity observed in wild-type mice after intraplantar or intrathecal (spinal) administration of prostaglandin E2 was also reduced in mutant mice. In contrast, indices of pain behavior produced by nerve injury were not altered in the mutant mice. Thus, RIbeta PKA is necessary for the full expression of tissue injury-evoked (nociceptive) pain but is not required for nerve injury-evoked (neuropathic) pain. Because the RIbeta subunit is only present in the nervous system, including small diameter trkA receptor-positive dorsal root ganglion cells, we suggest that in inflammatory conditions, RIbeta PKA is specifically required for nociceptive processing in the terminals of small-diameter primary afferent fibers.

Inhibition of nociceptin-induced allodynia in conscious mice by prostaglandin D2

1. We recently showed that intrathecal administration of nociceptin induced allodynia by innocuous tactile stimuli and hyperalgesia by noxious thermal stimuli in conscious mice. In the present study, we examined the effect of prostaglandins on nociceptin-induced allodynia and hyperalgesia. 2. Prostaglandin D2 (PGD2) blocked the allodynia induced by nociceptin in a dose-dependent manner with an IC50 of 26 ng kg(-1), but did not affect the nociceptin-induced hyperalgesia at doses up to 500 ng kg(-1). BW 245C (an agonist for PGD (DP) receptor) blocked the allodynia with an IC50 of 83 ng kg(-1). 3. The blockade of nociceptin-induced allodynia by PGD2 was reversed by the potent and selective DP-receptor antagonist BW A868C in a dose-dependent manner with an ED50 of 42.8 ng kg(-1). 4. Glycine (500 ng kg[-1]) almost completely blocked the nociceptin-induced allodynia. A synergistic effect on the inhibition of nociceptin-evoked allodynia was observed between glycine and PGD2 at below effective doses. 5. Dibutyryl cyclic AMP, but not dibutyryl cyclic GMP, blocked the nociceptin-induced allodynia with an IC50 of 2.9 microg kg(-1). 6. PGE2, PGF2alpha, butaprost (an EP2 agonist) and cicaprost (a PGI receptor agonist) did not affect the nociceptin-induced allodynia. 7. These results demonstrate that PGD2 inhibits the nociceptin-evoked allodynia through DP receptors in the spinal cord and that glycine may be involved in this inhibition.