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

The effects of intrathecal administration of an antagonist for prostaglandin E receptor subtype EP(1) on mechanical and thermal hyperalgesia in a rat model of postoperative pain

Despite substantial advances in understanding acute pain mechanisms and in the treatment of pain, postoperative pain, especially mechanically evoked pain (incident pain), is generally not effectively treated. Tissue injury and inflammation increase the release of prostaglandin E(2) in the spinal cord, contributing to the development of hyperalgesia. We designed the present study to determine whether the intrathecal administration of an antagonist for prostaglandin E(2) receptor subtype EP(1), ONO-8711, has an analgesic effect on incision-induced mechanical and thermal hyperalgesia. A 1-cm longitudinal skin incision was made in the plantar aspect of the rat foot. The withdrawal threshold to mechanical stimulation and the withdrawal latency to thermal stimulation applied adjacent to the wound of the hindpaw were investigated. Both mechanical and thermal hyperalgesia were observed at 2 h and 24 h after the incision had been made. ONO-8711 (50, 80, 100 micro g) or saline was administered intrathecally. ONO-8711 significantly increased the withdrawal thresholds to mechanical stimulation, but not to thermal stimulation, in a dose- and time-dependent manner. We conclude that EP(1) receptor-mediated sensitization of the spinal dorsal horn may contribute to the generation of mechanical, but not thermal, hyperalgesia and that an EP(1) receptor antagonist administered intrathecally is a potential analgesic for postoperative pain, especially mechanically evoked pain (incident pain).

IMPLICATIONS

We examined the effects of an intrathecally administered selective EP(1) receptor antagonist on mechanical and thermal hyperalgesia in a postoperative pain model. The intrathecal EP(1) receptor antagonist inhibited the mechanical, but not thermal, hyperalgesia, indicating the potential for an EP(1) receptor antagonist to be used as an analgesic for postoperative pain, especially incident pain.

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

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

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

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

The spinal phospholipase-cyclooxygenase-prostanoid cascade in nociceptive processing

Intrathecal phospholipase A2 (PLA2) and cyclooxygenase-2 (COX-2), but not COX-1, inhibitors attenuate facilitated pain states generated by peripheral injury/inflammation and by direct activation of spinal glutamate and substance P receptors. These results are consistent with the constitutive expression of PLA2 and COX-2 in spinal cord, the spinal release of prostaglandins by persistent afferent input, and the effects of prostaglandins on spinal excitability. Whereas the acute actions of COX-2 inhibitors are clearly mediated by constitutively expressed spinal COX-2, studies of spinal COX-2 expression indicate that it is upregulated by neural input and circulating cytokines. Given the intrathecal potency of COX-2 inhibitors, the comparable efficacy of intrathecal versus systemic COX-2 inhibitors in hyperalgesic states not associated with inflammation, and the onset of antihyperalgesic activity prior to COX-2 upregulation, it is argued that a principal antihyperalgesic mechanism of COX-2 inhibitors lies with modulation of constitutive COX-2 present at the spinal level.

The role of cyclooxygenase-1 and -2 in the rat formalin test

Prostaglandins are thought to play an important role in nociceptive transmission at peripheral sites and in the spinal cord. Prostaglandins are produced by cyclooxygenase (COX), which catalyzes the conversion of arachidonic acid. Two forms of COX have been identified: COX-1, which is constitutively expressed, and COX-2, which is an inducible enzyme. To define the role of COX-1 in nociceptive transmission, we examined the effect of oral and intrathecal administration of 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC-560), a selective COX-1 inhibitor, on the rat formalin test and compared the effect of SC-560 with that of celecoxib, a COX-2 selective inhibitor, and indomethacin, a nonselective COX-1 and COX-2 inhibitor, on the rat formalin test. Oral and intrathecal administration of SC-560 had no effect on the agitation behavior in the rat formalin test. Oral and intrathecal administration of celecoxib and indomethacin depressed agitation behavior during the rat formalin test. These data suggest that prostaglandins synthesized by COX-1 are not involved in nociceptive transmission during the rat formalin test but that COX-2 does play an important role in the rat formalin test.

IMPLICATIONS

Our data suggest that a COX-2 selective inhibitor, but not a COX-1 selective inhibitor, may produce a good analgesic effect on the inflammatory pain state in a clinical situation.

Central nociceptive role of prostacyclin (IP) receptor induced by peripheral inflammation

Prostacyclin (PGI2) is well known to play crucial roles in induction of edema and pain behavior in the periphery. In the present study, we investigated the central role of PGI2 in inflammatory pain. Intraplantar injection of carrageenan markedly induced the expression of prostacyclin receptor (IP receptor) mRNA with the maximum at 6 h, coincidently induction of the inducible form of cyclooxygenase (COX-2), although IP receptor mRNA was weakly expressed in the spinal cord of naive mice. Intrathecal administration of the IP agonist cicaprost induced mechanical hyperalgesia 6 h after carrageenan injection. These results suggest that PGI2 is involved in pain transmission at the spinal cord following expression of IP receptor mRNA induced by peripheral inflammation.

Spinal prostaglandins are involved in the development but not the maintenance of inflammation-induced spinal hyperexcitability

Prostaglandins (PGs) are local mediators of several functions in the CNS. Both primary afferent neurons and intrinsic cells in the spinal cord produce PGs, with a marked upregulation during peripheral inflammation. Therefore, the significance of spinal PGs in the neuronal processing of mechanosensory information was herein investigated. In anesthetized rats, the discharges of spinal nociceptive neurons with input from the knee joint were extracellularly recorded. Topical administration of prostaglandin E(2) (PGE(2)) to the spinal cord facilitated the discharges and expanded the receptive field of dorsal horn neurons to innocuous and noxious pressure applied to the knee joint, the ankle, and the paw, thus mimicking inflammation-induced central sensitization. Conversely, topical administration of the PG synthesis inhibitor indomethacin to the spinal cord before and during development of knee joint inflammation attenuated the generation of inflammation-induced spinal neuronal hyperexcitability. However, after development of inflammation, the responses of spinal neurons to mechanical stimuli were only reduced by systemic indomethacin but not by indomethacin applied to the spinal cord. Thus, spinal PG synthesis is important for the induction and initial expression but not for the maintenance of spinal cord hyperexcitability. Spinal PGE(2) application facilitated dorsal horn neuronal firing elicited by ionophoretic delivery of NMDA, suggesting that an interaction of PGs and NMDA receptors may contribute to inflammation-induced central sensitization. However, after development of inflammation, spinal indomethacin failed to reduce responses to ionophoretic delivery of NMDA or AMPA, suggesting that such an interaction is not required for the maintenance of central sensitization.

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.

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.

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.

The acute antihyperalgesic action of nonsteroidal, anti-inflammatory drugs and release of spinal prostaglandin E2 is mediated by the inhibition of constitutive spinal cyclooxygenase-2 (COX-2) but not COX-1

Western blots show the constitutive expression of COX-1 and COX-2 in the rat spinal dorsal and ventral horns and in the dorsal root ganglia. Using selective inhibitors of cyclooxygenase (COX) isozymes, we show that in rats with chronic indwelling intrathecal catheters the acute thermal hyperalgesia evoked by the spinal delivery of substance P (SP; 20 nmol) or NMDA (2 nmol) and the thermal hyperalgesia induced by the injection of carrageenan into the paw are suppressed by intrathecal and systemic COX-2 inhibitors. The intrathecal effects are dose-dependent and stereospecific. In contrast, a COX-1 inhibitor given systemically, but not spinally, reduced carrageenan-evoked thermal hyperalgesia but had no effect by any route with spinal SP hyperalgesia. Using intrathecal loop dialysis catheters, we showed that intrathecal SP would enhance the release of prostaglandin E(2) (PGE(2)). This intrathecally evoked release of spinal PGE(2) was diminished by systemic delivery of nonspecific COX and COX-2-selective inhibitors, but not a COX-1-selective inhibitor. Given at systemic doses that block SP- and carrageenan-evoked hyperalgesia, COX-2, but not COX-1, inhibitors reduced spinal SP-evoked PGE(2) release. Thus, constitutive spinal COX-2, but not COX-1, is an important contributor to the acute antihyperalgesic effects of spinal as well as systemic COX-2 inhibitors.

The effects of capsaicin cream on prostaglandin-induced allodynia

Although intradermal injection of capsaicin produces acute pain and secondary hyperalgesia, long-term topical application of capsaicin cream has been used as a medication for pain relief in various pain conditions. We previously reported that intrathecal administration of prostaglandin (PG) E(2) and PGF(2alpha) into mice induced touch-evoked pain (allodynia) through capsaicin-sensitive and capsaicin-insensitive afferent fibers, respectively. To clarify the mechanism of an analgesic effect by capsaicin cream, here we applied it to the tail and hind paws of mice and investigated its effects on PGE(2)- and PGF(2alpha)-induced allodynia. Twenty-four-hour pretreatment of mice with 0.025% or 0.05% capsaicin cream markedly alleviated allodynia induced by PGE(2), but not by PGF(2alpha). These results suggest that the topical application of capsaicin cream modulates capsaicin-sensitive afferents and ameliorates allodynia evoked by PGE(2) at the spinal level.

IMPLICATIONS

Topical application of capsaicin cream alleviates touch-evoked pain induced by the intrathecal administration of prostaglandin E(2). This study may provide a rationale for the use of capsaicin cream as a therapeutic drug for pain relief.

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.

Prostaglandin EP3 receptor protein in serotonin and catecholamine cell groups: a double immunofluorescence study in the rat brain

Prostaglandin E(2) exerts diverse physiological actions in the central nervous system with unknown mechanisms. We have reported the immunohistochemical localization of the EP3 receptor, one of the prostaglandin E receptor subtypes, in various brain regions including many monoaminergic nuclei. In the present study, a double immunofluorescence technique with an antibody to EP3 receptor and antibodies to markers for monoamine neurons was employed to examine the expression of the receptor in serotonin and catecholamine neurons, and to reveal the distribution of the receptor-expressing monoamine neurons in the rat brain. Almost all serotonergic cells in the medulla oblongata (B1-B4) exhibited EP3 receptor-like immunoreactivity, whereas mesencephalic and pontine serotonergic cell groups (B5-B9) contained relatively small populations of EP3 receptor-immunoreactive cells. In the catecholaminergic cell groups, many of the noradrenergic A7 cells in the subcoeruleus nucleus showed immunoreactivity for the receptor. The locus coeruleus exhibited EP3 receptor-like immunoreactivity densely in the neuropil and occasionally in neuronal cell bodies, all of which were immunopositive for dopamine beta-hydroxylase, as observed by confocal laser microscopy. Many of the other noradrenergic and adrenergic cell groups contained small populations of EP3 receptor-like immunoreactive cells. In contrast, no EP3 receptor-like immunoreactivity was detected in the noradrenergic A2 and A4, the adrenergic C2, and all the dopaminergic cell groups. The expression of EP3 receptor by most of the serotonergic, noradrenergic and adrenergic cell groups suggests that prostaglandin E(2) modulates many physiological processes mediated by widely distributed monoaminergic projections through activation of the EP3 receptor on the monoaminergic neurons; for instance, it may modulate nociceptive and autonomic processes by affecting the descending serotonergic pathway from the raphe magnus nucleus to the spinal cord.

Direct activation of rat spinal dorsal horn neurons by prostaglandin E2

Whole-cell patch-clamp and intracellular recording techniques have been used to study the action of prostaglandin E2 (PGE2) on neurons in adult rat transverse spinal cord slices. Bath-applied PGE2 (1-20 microm) induced an inward current or membrane depolarization in the majority of deep dorsal horn neurons (laminas III-VI; 83 of 139 cells), but only in a minority of lamina II neurons (6 of 53 cells). PGE2 alone never elicited spontaneous action potentials; however, it did convert subthreshold EPSPs to suprathreshold, leading to action potential generation. PGE2-induced inward currents were unaffected by perfusion with either a Ca(2+)-free/high Mg(2+) (5 mm) solution or tetrodotoxin (1 microm), indicating a direct postsynaptic action. Both 17-phenyl trinor prostaglandin E2 (an EP1 agonist) and sulprostone (an EP3 agonist) had little effect on membrane current, whereas butaprost methyl ester (an EP2 agonist) mimicked the effect of PGE2. Depolarizing responses to PGE2 were associated with a decrease in input resistance, and the amplitude of inward current was decreased as the holding potential was depolarized. PGE2-induced inward currents were reduced by substitution of extracellular Na(+) with N-methyl-d-glucamine and inhibited by flufenamic acid (50-200 microm), which is compatible with activation of a nonselective cation channel. These results suggest that PGE2, acting via an EP2-like receptor, directly depolarizes spinal neurons. Moreover, these findings imply an involvement of spinal cord-generated prostanoids in modulating sensory processing through an alteration in dorsal horn neuronal excitability.

Pharmacologic treatment of neuropathic pain

Neuropathic pain, or pain after nervous system injury, can be very refractory to pharmacologic interventions. Through a better understanding of the pathophysiology of neuropathic pain, it has been suggested that nonopioid agents, such as antidepressants and anticonvulsants, may be more efficacious in the treatment of neuropathic pain than common analgesics, such as opioids or nonsteroidal anti-inflammatory drugs. However, this has not been consistently demonstrated in clinical studies. Conversely, many confounding factors of neuropathic pain make it difficult to interpret clinical studies. Therefore, we must develop a better understanding of the preclinical models of neuropathic pain to better understand the application of new and old drugs to the human neuropathic pain state. This article provides an overview of the commonly used preclinical neuropathic pain models, followed by a summary of the efficacy of currently available agents in preclinical pain models and human correlates.

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.

Distribution of the EP3 prostaglandin E(2) receptor subtype in the rat brain: relationship to sites of interleukin-1-induced cellular responsiveness

The activation of neurosecretory neurons that express corticotropin-releasing hormone (CRH) in response to increased circulating levels of interleukin-1beta (IL-1beta) depends on prostaglandin E(2) (PGE(2)) acting locally within the brain parenchyma. To identify potential central targets for PGE(2) relevant to pituitary-adrenal control, the distribution of mRNA encoding the PGE(2) receptor subtype EP3 (EP3R) was analyzed in rat brain. Hybridization histochemistry revealed prominent labeling of cells in discrete portions of the olfactory system, iso- and hippocampal cortices, and subcortical telencephalic structures in the septal region and amygdala. Labeling over the midline, intralaminar, and anterior thalamic groups was particularly prominent. EP3R expression was enriched in the median preoptic nucleus and adjoining aspects of the medial preoptic area (MPO) implicated in thermoregulatory/febrile responses and sleep induction. EP3R-expressing cells were also prominent in brainstem cell groups involved in nociceptive information processing/modulation (periaqueductal gray, locus coeruleus (LC), parabrachial nucleus (PB), caudal raphé nuclei), arousal and wakefulness (LC, midbrain raphé and tuberomammillary nuclei); and in conveying interoceptive input, including systemic IL-1 signals, to the endocrine hypothalamus (nucleus of the solitary tract (NTS) and rostral ventrolateral medulla [VLM]). Combined hybridization histochemical detection of EP3R mRNA with immunolocalization of IL-1beta-induced Fos protein expression identified cytokine-sensitive, EP3R-positive cells in the medial NTS, rostral VLM, and, to a lesser extent, aspects of the MPO. These findings are consistent with the view that increased circulating IL-1 may stimulate central neural mechanisms, including hypothalamic CRH neurons, through an EP3R-dependent mechanism involving PGE(2)-mediated activation of cells in the caudal medulla and/or preoptic region.

Spinal cyclooxygenase-2 is involved in development of allodynia after nerve injury in rats

Increased spinal cyclooxygenase activity is associated with nociception induced by tissue inflammation. In the present study, we examined the changes of cyclooxygenase-1 and cyclooxygenase-2 protein expression in several regions of the CNS associated with pain perception, and the role of spinal cyclooxygenase activity in the development of allodynia following nerve injury. Allodynia was induced by ligation of the left L5 and L6 spinal nerves in rats. Using western blot analysis, we found that the cyclooxygenase-2 protein levels in the dorsal spinal cord and thalamus (but not in the ventral spinal cord, cingulate cortex and locus coeruleus) increased significantly one day after nerve ligation, compared with those in the sham animals. The cyclooxygenase-2 protein levels in the above tissues were similar in nerve-injured and sham animals three and 14 days after surgery. In contrast, cyclooxygenase-1 protein was not detectable in any of the neural tissues examined one, three, and 14 days after nerve injury. In the behavioral experiments, we observed that intrathecal injection of 100microg of indomethacin immediately or one day after nerve ligation attenuated the development of tactile allodynia. However, intrathecal injection of indomethacin had no effect on established allodynia two weeks after nerve injury.Collectively, our results suggest that cyclooxygenase-2 is preferentially up-regulated in the dorsal spinal cord and thalamus in response to nerve injury in rats. Spinal cyclooxygenase-2 probably plays an important role in the early development, but not in the maintenance, of tactile allodynia caused by the nerve injury in this rat model of neuropathic pain.

Immunohistochemical localization of prostaglandin EP3 receptor in the rat nervous system

The prostaglandin EP3 receptor (EP3R) subtype is believed to mediate large portions of diverse physiologic actions of prostaglandin E2 in the nervous system. However, the distribution of EP3R protein has not yet been unveiled in the peripheral or central nervous systems. The authors raised a polyclonal antibody against an amino-terminal portion of rat EP3R that recognized specifically the receptor protein. In this study, immunoblotting analysis with this antibody showed several immunoreactive bands with different molecular weights in rat brain extracts and in membrane fractions of recombinant EP3R-expressing culture cells, and treatment with N-glycosidase shifted those immunoreactive bands to an apparently single band with a lower molecular weight, suggesting that EP3R proteins are modified posttranslationally with carbohydrate moieties of various sizes. The authors performed immunohistochemical investigation of EP3R in the rat brain, spinal cord, and peripheral ganglia by using the antibody. EP3R-like immunoreactivity was observed in many and discrete regions of the rostrocaudal axis of the nervous system. The signals were particularly strong in the anterior, intralaminar, and midline thalamic nuclear groups; the median preoptic nucleus; the medial mammillary nucleus; the superior colliculus; the periaqueductal gray; the lateral parabrachial nucleus; the nucleus of the solitary tract; and laminae I and II of the medullary and spinal dorsal horns. Sensory ganglia, such as the trigeminal, dorsal root, and nodose ganglia, contained many immunopositive neurons. Neuronal cells in the locus coeruleus and raphe nuclei exhibited EP3R-like immunoreactivity. This suggests that EP3R plays regulatory roles in the noradrenergic and serotonergic monoamine systems. Autonomic preganglionic nuclei, such as the dorsal motor nucleus of the vagus nerve, the spinal intermediolateral nucleus, and the sacral parasympathetic nucleus, also contained neuronal cell bodies with the immunoreactivity, implying modulatory functions of EP3R in the central autonomic nervous system. The characteristic distribution of EP3R provides valuable information on the mechanisms for various physiologic actions of prostaglandin E2 in the central and peripheral nervous systems.

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.

Prostaglandin D synthase: structure and function

Prostaglandin (PG) D synthase catalyzes the isomerization of PGH2, a common precursor of various prostanoids, to produce PGD2 in the presence of sulfhydryl compounds. PGD2 induces sleep, regulates nociception, inhibits platelet aggregation, acts as an allergic mediator, and is further converted to 9 alpha, 11 beta-PGF2 or the J series of prostanoids, such as PGJ2, delta 12-PGJ2, and 15-deoxy-delta 12,14-PGJ2. We have purified two distinct types of PGD synthase; one is the lipocalin-type enzyme and the other is the hematopoietic enzyme. We isolated the cDNA and the gene for each enzyme and determined the tissue distribution profile and the cellular localization in several animal species. Lipocalin-type PGD synthase is localized in the central nervous system and male genital organs of various mammals and the human heart and is secreted into cerebrospinal fluid, seminal plasma, and plasma, respectively. The human enzyme was identified as beta-trace, which is a major protein in human cerebrospinal fluid. This enzyme is considered to be a dual-function protein; it acts as a PGD2-producing enzyme and also as a lipophilic ligand-binding protein, because the enzyme binds retinoids, thyroids, and bile pigments, with high affinities. Hematopoietic PGD synthase is widely distributed in the peripheral tissues and localized in the antigen-presenting cells, mast cells, and megakaryocytes. The hematopoietic enzyme is the first recognized vertebrate homolog of the sigma class of glutathione S-transferase. X-ray crystallographic analyses and generation of gene-knockout and transgenic mice for each enzyme have been performed.

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.

NSAID-induced cyclooxygenase inhibition differentially depresses long-lasting versus brief synaptically-elicited responses of rat spinal dorsal horn neurons in vivo

This electrophysiological study examined the effects of NSAID administration on synaptically-elicited responses of rat single spinal dorsal horn neurons to natural stimulation of peripheral receptive fields. Nociceptive responses consisted of a fast initial discharge during the stimulus followed by a slowly-decaying afterdischarge. The cyclooxygenase inhibitor, indomethacin (2.0-8.0 mg/kg, i.v.), was without effect on the on-going rate of discharge but dose-dependently inhibited synaptically-elicited responses to noxious cutaneous mechanical stimulation (fast initial discharge: n = 3/3 with 2 mg/kg, 5/8 with 4 mg/kg, 5/6 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/3 with 2 mg/kg, 6/8 with 4 mg/kg, 6/6 with 8 mg/kg) and thermal (fast initial discharge: n = 7/9 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/4 with 4 mg/kg, n = 7/9 with 8 mg/kg). The inhibitory effect of indomethacin started within 2-4 min and lasted up to 120 min. To eliminate any effect of indomethacin via cutaneous sensory receptors it was tested on the responses of some neurons to high intensity electrical stimulation of the sciatic nerve; indomethacin depressed these evoked responses (fast initial discharge: n = 5/6 with 2 mg/kg, n = 7/7 with 4 mg/kg; slowly-decaying afterdischarge: n = 6/6 with 2 mg/kg, n = 7/7 with 4 mg/kg). The brief excitatory responses to innocuous pressure (fast initial discharge: n = 2/3 with 2 mg/kg, n = 6/8 with 4 mg/kg, n = 4/6 with 8 mg/kg) and hair (n = 2/7 with 2 and 4 mg/kg, respectively) stimulation in both non-nociceptive and wide dynamic range neurons were also depressed but to a lesser extent. However, the prolonged excitation of three wide dynamic range neurons to continuous hair stimulation was almost entirely inhibited by indomethacin. Overall, inhibition of the afterdischarge and the excitatory effect of long-lasting synaptic input were greater than inhibition of the fast synaptic input-evoked initial discharge. The evidence supports the suggestion that systemically-administered indomethacin has an effect in the spinal cord and demonstrates an action specifically in the dorsal horn. The data are interpreted to suggest that sensory inputs are more involved than input-independent excitation of dorsal horn neurons in leading to de novo synthesis of eicosanoids and that the time course of this synthesis brings the levels to a point where COX inhibition can have an observable effect during prolonged excitation. Although the data suggest that COX inhibition differentially inhibits nociceptive versus non-nociceptive mechanisms at the cellular level, irrespective of the modality of the stimulus, this is the first direct demonstration that prolonged activation of synaptic mechanisms are preferentially inhibited. According to this it would be predictable that NSAIDs would be more effective on nociceptive types of pain characterized by time or prolonged inputs of primary afferents.

A novel biological role for prostaglandin D2 is suggested by distribution studies of the rat DP prostanoid receptor

We report the cloning, functional expression and cell-specific localization of the rat homologue of the prostaglandin D2 receptor (DP). In situ hybridization, utilizing multiple digoxigenin-labelled riboprobes and their complementary sense controls, was performed to determine the detailed distribution of DP receptor mRNA in the central nervous system and the gastrointestinal tract. Within the brain, the leptomeninges and choroid plexus expressed DP receptor mRNA. Transcripts detected in the spinal cord were localized to the sensory and motor neurons of the dorsal and ventral horns, respectively, suggesting a role for the DP receptor in the modulation of central nervous system processes, including pain transmission. Within the gastrointestinal tract (stomach, duodenum, ileum and colon) signals were highly localized to the mucous-secreting goblet cells and the columnar epithelium. These findings suggest a novel biological role for prostaglandin D2-mediated activity at the DP receptor, namely mucous secretion. In addition, radioligand binding assays (saturation analyses and equilibrium competition assays) and functional assays (measuring cAMP accumulation) were performed to characterize the recombinant rat DP receptor expressed in human embryonic kidney (HEK) 293(EBNA) cells. A single site of binding (K(D) = 14 nM, Bmax = 115 fmol/mg protein) was measured for prostaglandin D2-specific binding to the rat DP receptor. Prostaglandin D2 proved to be a potent agonist at the rat DP receptor (EC50 = 5 nM). The rank order of efficacy for DP receptor specific agonists [prostaglandin D2 = prostaglandin J2 = BW 245C (5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropylhydantoin)) > L-644,698 ((4-(3-(3-(3-hydroxyoctyl)-4-oxo-2-thiazolidinyl) propyl) benzoic acid) (racemate)] reflected the affinity with which the ligands bound to the receptor.

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.

Intrathecal substance P-induced thermal hyperalgesia and spinal release of prostaglandin E2 and amino acids

Substance P is an important neuromediator in spinal synaptic transmission, particularly in processing nociceptive afferent information. The effects of substance P are mediated by activation of the neurokinin 1 receptor. Evidence has suggested that excitatory amino acids such as glutamate, and prostaglandins including prostaglandin E2 are involved in the enhanced spinal excitability and hyperalgesia produced by spinal substance P. In the present study, we have demonstrated that intrathecal injection of substance P (20 nmol) in rats chronically implanted with intrathecal dialysis catheters induced a decrease in thermal paw withdrawal latency (before: 10.4+/-0.3 s; after 7.6+/-0.6 s), which was accompanied by an increase in prostaglandin E2 (362+/-37% of baseline), glutamate (267+/-84%) and taurine (279+/-57%), but not glycine, glutamine, serine or asparagine. Intrathecal injection of artificial cerebrospinal fluid had no effect upon the behavior or release. Substance P-induced thermal hyperalgesia and prostaglandin E2 release were significantly attenuated by a selective neurokinin 1 receptor antagonist RP67580, but not by an enantiomer RP68651. However, substance P-induced release of glutamate and taurine was not reduced by treatment with RP67580. SR140333, another neurokinin 1 receptor antagonist, displayed the same effects as RP67580 (i.e. block of thermal hyperalgesia and prostaglandin E2 release, but not release of amino acids). These results provide direct evidence suggesting that the spinal substance P-induced thermal hyperalgesia is mediated by an increase in spinal prostaglandin E2 via activation of the neurokinin 1 receptor. These findings define an important linkage between small afferents, sensory neurotransmitter release and spinal prostanoids in the cascade of spinally-mediated hyperalgesia. The evoked release of glutamate is apparently not a result of activation of neurokinin 1 receptors. Accordingly, consistent with other pharmacological data, acute spinal glutamate release does not contribute to the hyperalgesia induced by activation of spinal neurokinin 1 receptors.

In vitro prostanoid release from spinal cord following peripheral inflammation: effects of substance P, NMDA and capsaicin

1. Spinal prostanoids are implicated in the development of thermal hyperalgesia after peripheral injury, but the specific prostanoid species that are involved are presently unknown. The current study used an in vitro spinal superfusion model to investigate the effect of substance P (SP), N-methyl-d-aspartate (NMDA), and capsaicin on multiple prostanoid release from dorsal spinal cord of naive rats as well as rats that underwent peripheral injury and inflammation (knee joint kaolin/carrageenan). 2. In naive rat spinal cords, PGE2 and 6-keto-PGF1alpha, but not TxB2, levels were increased after inclusion of SP, NMDA, or capsaicin in the perfusion medium. 3. Basal PGE2 levels from spinal cords of animals that underwent 5-72 h of peripheral inflammation were elevated relative to age-matched naive cohorts. The time course of this increase in basal PGE2 levels coincided with peripheral inflammation, as assessed by knee joint circumference. Basal 6-keto-PGF1alpha levels were not elevated after injury. 4. From this inflammation-evoked increase in basal PGE2 levels, SP and capsaicin significantly increased spinal PGE2 release in a dose-dependent fashion. Capsaicin-evoked increases were blocked dose-dependently by inclusion of S(+) ibuprofen in the capsaicin-containing perfusate. 5. These data suggest a role for spinal PGE2 and NK-1 receptor activation in the development of hyperalgesia after injury and demonstrate that this relationship is upregulated in response to peripheral tissue injury and inflammation.

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.

Pain modulatory actions of cytokines and prostaglandin E2 in the brain

Proinflammatory cytokines such as IL-1, IL-6, and TNF alpha are known to enhance nociception at peripheral inflammatory tissues. These cytokines are also produced in the brain. We found that an intracerebroventricular injection of IL-1 beta only at nonpyrogenic doses in rats reduced the paw-withdrawal latency on a hot plate and enhanced the responses of the wide dynamic range neurons in the trigeminal nucleus caudalis to noxious stimuli. This hyperalgesia, as assessed by behavioral and neuronal responses, was blocked by pretreatment with IL-1 receptor antagonist (IL-1Ra), Na salicylate, or alpha melanocyte-stimulating hormone, indicating the involvement of IL-1 receptors and the synthesis of prostanoids. IL-6 and TNF alpha at nonpyrogenic doses also induced hyperalgesia in a prostanoid-dependent way. Furthermore, the preoptic area (POA) was most sensitive to IL-1 beta (5-50 pg/kg) in the induction of behavioral hyperalgesia. The maximal response was obtained 30 min after injection of IL-1 beta at 20 pg/kg. On the other hand, an injection of IL-1 beta (20-50 pg/kg) into the ventromedial hypothalamus (VMH) prolonged the paw-withdrawal latency maximally 10 min after injection. This analgesia, as well as the intraPOA IL-1 beta-induced hyperalgesia, was completely blocked by IL-1Ra or Na salicylate. Our previous study has revealed that i.c.v. injection of PGE2 induces hyperalgesia through EP3 receptors and analgesia through EP1 receptors by its central action. The results, taken together, suggest (1) that IL-1 beta at lower doses in the brain induces hyperalgesia through EP3 receptors in the POA and (2) that the higher doses of brain IL-1 beta produces analgesia through EP1 receptors, probably, in the VMH.

The role of IP prostanoid receptors in inflammatory pain

Prostanoid receptor-mediated sensitization of sensory nerve fibres is a key contributor to the generation of hyperalgesia. It is generally thought that prostaglandin (PG) E2 is the principal pro-inflammatory prostanoid. Consequently, prostanoid EP receptors on sensory neurones have been identified as potential therapeutic targets. However, IP prostanoid receptors are also present on sensory neurones, and recent data from transgenic mice lacking the IP receptor demonstrate its importance in the induction of oedema and pain behaviour. PGI2, the primary endogenous agonist for the IP receptor, is rapidly produced following tissue injury or inflammation; thus, it may be of equal, or greater, importance than PGE2 during episodes of inflammatory pain. In this review, Keith Bley, John Hunter, Richard Eglen and Jacqueline Smith compare the roles of EP and IP receptors in nociception and suggest that the IP receptor constitutes a novel target for anti-nociceptive agents.

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.

Biphasic modulation in the trigeminal nociceptive neuronal responses by the intracerebroventricular prostaglandin E2 may be mediated through different EP receptors subtypes in rats

To determine which prostaglandin E2 (PGE2) receptor subtypes are involved in the brain-derived PGE2-induced changes in nociception, we injected synthetic EP1, EP2 and EP3 receptor agonists (0.01 fmol to 10 nmol) into the lateral cerebroventricle (LCV) of urethane-anesthetized rats and observed the changes in the responses of the wide dynamic range (WDR) neurons in the trigeminal nucleus caudalis to noxious pinching of facial skin. The enhancement and suppression of the nociceptive responses of the WDR neurons were observed after the LCV injection of MB28767 (an EP3 receptor agonist) at a low dose range (1-100 fmol) and 17-phenyl-omega-trinor PGE2 (an EP1 receptor agonist) at high doses (1-10 nmol), respectively. Furthermore, the suppression of nociceptive neuronal responses after the LCV injection of PGE2 (1 nmol) was completely blocked by SC19220 (an EP1 receptor antagonist, 300 nmol). On the other hand, butaprost (an EP2 receptor agonist) at any doses tested (0.1 fmol to 1 nmol) had no effect on the nociceptive responses. The LCV injection of MB28767 (10 fmol) and 17-phenyl-omega-trinor PGE2 (1 nmol), which respectively enhanced and suppressed the nociceptive neuronal responses, did not affect the responses of the low threshold mechanoreceptive neurons to innocuous tactile stimuli. These results provide electrophysiological evidence that brain-derived PGE2 induces mechanical hyperalgesia and hypoalgesia through EP3 and EP1 receptors, respectively, in the rat.

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.

Cyclooxygenases and the central nervous system

Prostaglandins (PGs) were first described in the brain by Samuelsson over 30 years ago (Samuelsson, 1964). Since then a large number of studies have shown that PGs are formed in regions of the brain and spinal cord in response to a variety of stimuli. The recent identification of two forms of cyclooxygenase (COX; Kujubu et al., 1991; Xie et al., 1991; Smith and DeWitt, 1996), both of which are expressed in the brain, along with superior tools for mapping COX distribution, has spurred a resurgence of interest in the role of PGs in the central nervous system (CNS). In this review we will describe new data in this area, focusing on the distribution and potential role of the COX isoforms in brain function and disease.

Effect of spinal cyclooxygenase inhibitors in rat using the formalin test and in vitro prostaglandin E2 release

Spinally delivery of the non-specific cyclooxygenase inhibitor, S(+)-ibuprofen, reduces the second phase of the formalin test and the evoked release of prostaglandin E2 (prostaglandin E2) from rat spinal cord in vitro. Using two selective cyclooxygenase-2 inhibitors, SC58125 (1-[(4-methysufonyl)phenyl]-3-tri-fluoromethyl-5-(4-fluorophenyl)p yrazole) and SC-236 (4-[5-(4-chlorophenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfon amide), we observed that neither agent at the highest dose/concentration employed altered the second phase of the formalin test after intrathecal delivery or K+-evoked prostaglandin E2 release from spinal cord in vitro, although ibuprofen was effective in both models. These observations suggest that cyclooxygenase-2 may not be associated with spinal prostanoid synthesis acutely or with facilitated nociception which occurs within the limited time frame of the formalin test.

Characterization of nociceptin hyperalgesia and allodynia in conscious mice

1. Intrathecal (i.t.) administration of nociceptin and high doses of morphine induced allodynia in response to innocuous tactile stimuli, and i.t. nociceptin evoked hyperalgesia in response to noxious thermal stimuli in conscious mice. Here we have characterized the nociceptin-induced allodynia and compared it with the morphine-induced allodynia and the nociceptin-evoked hyperalgesia. 2. Nociceptin-induced allodynia was evoked by the first stimulus 5 min after i.t. injection, reached a maximum at 10 min, and continued for a 50 min experimental period. Dose-dependency of the allodynia showed a bell-shaped pattern from 50 pg to 5 ng kg-1, and the maximum effect was observed at 2.5 ng kg-1. 3. Morphine-induced allodynia reached the maximum effect at 15 min and declined progressively until cessation by 40-50 min. The dose-response curve showed a bell-shaped pattern, similar to that induced by nociceptin, with a maximum effect at 0.5 mg kg-1, five orders of magnitude higher than that of nociceptin. 4. The allodynia evoked by nociceptin and morphine were dose-dependently blocked by glycine, D(-)-2-amino-5-phosphonovaleric acid (D-AP5, an N-methyl-D-aspartate (NMDA) receptor antagonist), gamma-D-glutamylaminomethyl sulphonic acid (GAMS, a non-NMDA receptor antagonist) and methylene blue (a soluble guanylate cyclase inhibitor), but were not affected by muscimol (a gamma-aminobutyric acidA (GABAA) receptor agonist) and baclofen (a GABAB receptor agonist). 5. Morphine did not inhibit forskolin-stimulated cyclicAMP formation in cultured cells expressing the nociceptin receptor. 6. Nociceptin-induced hyperalgesia was evoked 10-15 min after i.t. injection. Nociceptin produced a monophasic hyperalgesic action over a wide range of doses from 5 fg to 50 ng kg-1. The nociceptin-induced hyperalgesia was blocked by glycine only among the agents examined. 7. None of the pain responses evoked by nociceptin and morphine were blocked by naloxone. 8. These results demonstrate that, whereas the mechanisms of the nociceptin-induced allodynia and hyperalgesia are evidently distinct, they involve a common neurochemical event beginning with the disinhibition of the inhibitory glycinergic response. Morphine may induce allodynia through a pathway common to nociceptin, but the nociceptin receptor does not mediate the action of high doses of morphine.

Absence of prostaglandin E2-induced hyperalgesia in NMDA receptor epsilon subunit knockout mice

1. We have previously found that intrathecal administration of prostaglandins E2 (PGE2) and D2 (PGD2) into conscious mice induced hyperalgesia by the hot plate test. The present study investigated the involvement of N-methyl-D-aspartate (NMDA) receptor in the prostaglandin-induced hyperalgesia by use of mice tacking NMDA receptor epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunits. 2. PGE2 induced hyperalgesia over a wide range of doses from 50 pg to 500 ng kg-1 in wild-type mice. But PGE2 could not induce hyperalgesia in epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunit knockout mice. 3. The NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (D-AP5), the non-NMDA receptor antagonist 7-D-glutamylaminomethyl sulphonic acid (GAMS), and the nitric oxide synthase inhibitor N epsilon-nitro-L-arginine methyl ester (L-NAME) inhibited the PGE2-induced hyperalgesia in wild-type mice. 4. PGD2 induced hyperalgesia at doses of 25 ng to 250 ng kg-1 in both wild-type and epsilon 1/epsilon 4 subunit knockout mice. The substance P receptor antagonist OP 96.345 blocked the PGD2-induced hyperalgesia in wild-type and epsilon 1/epsilon 4 subunit knockout mice. 5. These results demonstrate that the pathways leading to hyperalgesia are different between PGD2 and PGE2, and that both epsilon 1 and epsilon 4 subunits of the NMDA receptor are involved in the PGE2-induced hyperalgesia.

Analysis of the effects of cyclooxygenase (COX)-1 and COX-2 in spinal nociceptive transmission using indomethacin, a non-selective COX inhibitor, and NS-398, a COX-2 selective inhibitor

Prostaglandins are now thought to play an important role in nociceptive information transmission in the spinal cord. Prostaglandins are known to be produced by cyclooxygenase (COX), which catalyzes the conversion of arachidonic acid. Two forms of COX have been identified: COX-1, which is constitutively expressed in most tissues and organs, and COX-2, which is an inducible enzyme and is localized primarily in inflammatory cells and tissues. COX-2 mRNA has been reported to be expressed in the brain in normal rats. We investigated the role of COX-1 and COX-2 in the spinal nociceptive transmission during the rat formalin test and the hot plate test using indomethacin, a non-selective COX-1 and COX-2 inhibitor, and NS398, a selective COX-2 inhibitor. In the formalin test, drugs were administered intrathecally or intraperitoneally 10 min before (pre-treatment study) or 7 min after (post-treatment study) the formalin injection. Both intrathecally administered indomethacin and NS398 inhibited the formalin induced flinching behavior in a dose-dependent manner in the pre-treatment study, but not in the post-treatment study. Both indomethacin and NS398 had no effect on the hot plate test at a dose which depressed the formalin induced flinching behavior. These data suggested that COX-2 is expressed in the central nervous system, including the spinal cord, and that COX-2 plays an important role in the spinal nociceptive transmission during the formalin test, but not during the hot plate test.

Characterization of time course of spinal amino acids, citrulline and PGE2 release after carrageenan/kaolin-induced knee joint inflammation: a chronic microdialysis study

Pharmacological studies have implicated the spinal activation of excitatory amino acids, nitric oxide, and prostaglandins systems in the development of tactile and thermal hypersensitivity and central sensitization after peripheral inflammation. In the present study, using a chronically placed loop dialysis catheter, we examined in the unanesthetized rat the effect of carrageenan/kaolin (C/K)-induced knee joint inflammation on the time course of spinal release of several active factors including excitatory amino acids (glutamate, aspartate), citrulline (a marker of nitric oxide formation), and prostaglandin E2 (PGE2) as well as the concomitant development of tactile and thermal hypersensitivity. Infection of C/K in the knee evoked a significant release of glutamate, with an initial peak seen immediately after knee C/K injection (179 +/- 22%) and with a progressive and consistent increase over a period of 24 h (153-186%). Comparable changes in the concentration of aspartate (123-179%) were observed. Citrulline was constantly above baseline for the 24-h period (121-158%). PGE2 was significantly increased at 10 min (146 +/- 11%) with no change observed between 3-5 h. At 24 h, PGE2 was again significantly (143 +/- 18%) increased. Behaviorally, a prominent thermal and tactile allodynia developed after injection with the peak seen by 1-3 h after induction of the inflammation. This hypersensitivity state, while diminished in its intensity, persisted for the entire observation period. These data suggest that increased spinal release of excitatory amino acids (EAA), nitric oxide and/or PGE2 is involved in the maintenance of the pain state initiated by acute peripheral inflammation.

Interaction of BW A868C, a prostanoid DP-receptor antagonist, with two receptor subtypes in the rabbit isolated saphenous vein

In isolated rings of rabbit saphenous vein (RbSV) pre-contracted with 40 mM KCl, Prostaglandin E2 (PGE2), BW245C (a DP-receptor agonist) and PGD2 caused concentration-dependent relaxations with mean potencies (EC50) of 0.5, 38 and 114 nM respectively. The DP-receptor antagonist, BW A868C, antagonized BW245C concentration-effect (E/[A]) curves, although the corresponding Schild plot had a slope less than unity and displayed a clear infection. Analysis of the data yielded two pKB estimates of 8.5 and 4.9, the higher estimate being consistent with antagonism at DP-receptors. The pA2 estimate of 5.1 obtained for BW A868C against PGE2 was not statistically different to the lower pKB of 4.9 obtained against BW245C, and is probably indicative of antagonism at the EP4-receptor. PGD2 mediated responses were also antagonized by BW A868C, however the resultant E/[A] curves were 'bell shaped' in nature. The weak EP4-receptor antagonist AH23848B, also antagonized BW245C and PGD2 responses, yielding pA2 estimates of 5.6 and 5.5 respectively. These results suggest that in the RbSV, BW245C and PGD2 are nonselective agonists mediating relaxations through DP- and EP4-receptors. BW A868C also displayed an affinity for DP-receptors in this preparation, in addition to a second receptor subtype, presumably the EP4-receptor.