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

Metamizol, a non-opioid analgesic, acts via endocannabinoids in the PAG-RVM axis during inflammation in rats

The most commonly used drugs against pain act by inhibiting the cyclooxygenases (COXs). Metamizol (dipyrone) inhibits the COXs and is widely used in Europe and Latin America as a non-opioid analgesic. One target of metamizol and other non-opioid analgesics is the periaqueductal grey matter (PAG), where they trigger descending inhibition of spinal nociceptive transmission. Also, cannabinoids exert an analgesic action at several structures in the peripheral and central nervous system, including the PAG. The present study investigates whether the antinociceptive action of metamizol in the lateral-ventrolateral (LVL) PAG during inflammation is related to endocannabinoids. In anaesthetized rats, unitary action potentials were recorded from spinal nociceptive neurons with receptive fields in the ipsilateral hind paw. Inflammation of the paw induced neuronal hyperexcitability, which was attenuated by intra-LVL-PAG microinjection of metamizol either at the beginning of inflammation or when hyperexcitability was fully established. In both cases, the antinociceptive effect of metamizol was reduced by a microinjection of AM251, an antagonist at the CB1 cannabinoid receptor, either into the LVL-PAG or into the rostral ventromedial medulla (RVM). The RVM is a downstream structure that funnels PAG-derived descending inhibition into the spinal cord. These results show that endocannabinoids and their CB1 receptor (1) contribute at the LVL-PAG to the antinociceptive effects of metamizol, and possibly other non-opioid analgesics; and (2) participate in the PAG-derived activation of RVM descending antinociceptive influences.

Pharmacogenetics of new analgesics

Patient phenotypes in pharmacological pain treatment varies between individuals, which could be partly assigned to their genotypes regarding the targets of classical analgesics (OPRM1, PTGS2) or associated signalling pathways (KCNJ6). Translational and genetic research have identified new targets, for which new analgesics are being developed. This addresses voltage-gated sodium, calcium and potassium channels, for which SCN9A, CACNA1B, KCNQ2 and KCNQ3, respectively, are primary gene candidates because they code for the subunits of the respective channels targeted by analgesics currently in clinical development. Mutations in voltage gated transient receptor potential (TRPV) channels are known from genetic pain research and may modulate the effects of analgesics under development targeting TRPV1 or TRPV3. To this add ligand-gated ion channels including nicotinic acetylcholine receptors, ionotropic glutamate-gated receptors and ATP-gated purinergic P2X receptors with most important subunits coded by CHRNA4, GRIN2B and P2RX7. Among G protein coupled receptors, δ-opioid receptors (coded by OPRD1), cannabinoid receptors (CNR1 and CNR2), metabotropic glutamate receptors (mGluR5 coded by GRM5), bradykinin B(1) (BDKRB1) and 5-HT(1A) (HTR1A) receptors are targeted by new analgesic substances. Finally, nerve growth factor (NGFB), its tyrosine kinase receptor (NTRK1) and the fatty acid amide hydrolase (FAAH) have become targets of interest. For most of these genes, functional variants have been associated with neuro-psychiatric disorders and not yet with analgesia. However, research on the genetic modulation of pain has already identified variants in these genes, relative to pain, which may facilitate the pharmacogenetic assessments of new analgesics. The increased number of candidate pharmacogenetic modulators of analgesic actions may open opportunities for the broader clinical implementation of genotyping information.

EP1 receptor within the ventrolateral periaqueductal grey controls thermonociception and rostral ventromedial medulla cell activity in healthy and neuropathic rat

The aim of this study was to investigate the expression of prostaglandin EP1 receptor within the ventrolateral periaqueductal grey (VL PAG). The role of VL PAG EP1 receptor in controlling thermonociception and rostral ventromedial medulla (RVM) activity in healthy and neuropathic rats was also examined. EP1 receptor was indeed found to be expressed within the VL PAG and co-localized with vesicular GABA transporter. Intra-VL PAG microinjection of ONO-DI-004, a selective EP1 receptor agonist, dose-dependently reduced tail flick latency as well as respectively increasing and decreasing the spontaneous activity of ON and OFF cells. Furthermore, it increased the ON cell burst and OFF cell pause. Intra-VL PAG prostaglandin E2 (PGE2) behaved similarly to ONO-DI-004. The effects of ONO-DI-004 and PGE2 were antagonized by intra-VL PAG L335677, a selective EP1 receptor antagonist. L335677 dose-dependently increased the tail flick latency and ongoing activity of the OFF cells, while reducing the ongoing ON cell activity. It also decreased the ON cell burst and OFF cell pause. In neuropathic rats using spare nerve injury (SNI) of the sciatic nerve model, EP1 receptor expression decreased in the VL PAG. However, ONO-DI-004 and L335677 were able to alter pain responses and ON and OFF cell activity, as they did in healthy animals. Collectively, these data show that within the VL PAG, EP1 receptor has a facilitatory effect on the nociceptive response and consistently affects RVM neuron activity. Thus, the blockade of EP1 receptor in the VL PAG leads to antinociception in neuropathic pain conditions, despite its down-regulation. The expression of EP1 receptor on GABAergic neurons is consistent with an EP1 receptor blockade-induced disinhibition of the antinociceptive descending pathway at VL PAG level.

Acetylcholine and norepinephrine mediate GABAergic but not glycinergic transmission enhancement by melittin in adult rat substantia gelatinosa neurons

GABAergic and glycinergic inhibitory synaptic transmissions in substantia gelatinosa (SG; lamina II of Rexed) neurons of the spinal dorsal horn play an important role in regulating nociceptive transmission from the periphery. It has not yet been well known whether each of the inhibitory transmissions plays a distinct role in the regulation. We report an involvement of neurotransmitters in GABAergic but not glycinergic transmission enhancement produced by the PLA2 activator melittin, where the whole-cell patch-clamp technique is applied to the SG neurons of adult rat spinal cord slices. Glycinergic but not GABAergic spontaneous inhibitory postsynaptic current (sIPSC) was increased in frequency and amplitude by melittin in the presence of nicotinic, muscarinic acetylcholine, and α1-adrenergic receptor antagonists (mecamylamine, atropine, and WB-4101, respectively). GABAergic transmission enhancement produced by melittin was unaffected by the 5-hydroxytryptamine 3 receptor and P2X receptor antagonists (ICS-205,930 and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid, respectively). Nicotinic and muscarinic acetylcholine receptor agonists [(−)-nicotine and carbamoylcholine, respectively] and norepinephrine, as well as melittin, increased GABAergic sIPSC frequency and amplitude. A repeated application of (−)-nicotine, carbamoylcholine, and norepinephrine, but not melittin, at an interval of 30 min produced a similar transmission enhancement. These results indicate that melittin produces the release of acetylcholine and norepinephrine, which activate (nicotinic and muscarinic) acetylcholine and α1-adrenergic receptors, respectively, resulting in GABAergic but not glycinergic transmission enhancement in SG neurons. The desensitization of a system leading to the acetylcholine and norepinephrine release is slow in recovery. This distinction in modulation between GABAergic and glycinergic transmissions may play a role in regulating nociceptive transmission.

Anti-inflammatory and antinociceptive activity of ouabain in mice

Ouabain, an inhibitor of the Na⁺/K⁺-ATPase pump, was identified as an endogenous substance of human plasma. Ouabain has been studied for its ability to interfere with various regulatory mechanisms. Despite the studies portraying the ability of ouabain to modulate the immune response, little is known about the effect of this substance on the inflammatory process. The aim of this work was to study the effects triggered by ouabain on inflammation and nociceptive models. Ouabain produced a reduction in the mouse paw edema induced by carrageenan, compound 48/80 and zymosan. This anti-inflammatory potential might be related to the inhibition of prostaglandin E2, bradykinin, and mast-cell degranulation but not to histamine. Ouabain also modulated the inflammation induced by concanavalin A by inhibiting cell migration. Besides that, ouabain presented antinociceptive activity. Taken these data together, this work demonstrated, for the first time, that ouabain presented in vivo analgesic and anti-inflammatory effects.

Enhancement of apamin-sensitive medium afterhyperpolarization current by anandamide and its role in excitability control in cultured hippocampal neurons

Although endocannabinoid anandamide (AEA) plays an important role in synaptic signaling and neuronal survival, the underlying mechanism is not fully understood. Afterhyperpolarization (AHP) is the critical modulator of cell excitability and in turn shapes the neuronal output. Here, we examined the effects of AEA on AHP current and action potential firing in cultured rat hippocampal neurons. In whole-cell patch-clamp recording, AEA applied in the extracellular medium at nanomolar concentration enhanced medium AHP (mAHP) current and spike-frequency adaptation. Activation of apamin-sensitive, small conductance Ca(2+)-activated K(+) (SK) channels, probably SK2 and SK3 as the immunofluorescence analysis indicated, attributed largely to the AEA action on mAHP. Interestingly, AEA-induced potentiation of mAHP current was abolished by inositol 1,4,5-trisphosphate receptors (IP(3)Rs) blockade. However, the potentiation was not affected by inhibiting Ca(2+) influx or Ca(2+) release from internal store through ryanodine receptors. In addition, blockade of CB1, TRPV1 or Gi/o-protein did not attenuate the potentiation. Thus, AEA might enhance the SK mAHP currents mainly in a non-CB1/TRPV1 receptor way. Our study provides the first evidence that a functional cascade might lie among AEA, IP(3)Rs and SK channels, which may keep the membrane excitability stable in a negative-feedback manner.

Indirect role of beta2-adrenergic receptors in the mechanism of analgesic action of nonsteroidal antiinflammatory drugs

OBJECTIVE

Adrenal gland hormones have been shown to have a role in the antiinflammatory effect mechanism of nonsteroidal antiinflammatory drugs. This study investigates whether the analgesic effects of indomethacin, diclofenac sodium, aspirin, and nimesulide (IDAN; upper case letters of the four drugs we used) are also related to adrenal gland hormones.

DESIGN

The analgesic effects of IDAN were studied in the carrageenan-induced inflammatory pain model using both intact and adrenalectomized rats. Paw withdrawal tests were performed in adrenalectomized rats that had been pretreated with phenoxybenzamine, propranolol, and metoprolol.

SETTING

This study was performed in Pharmacology and Biochemistry Laboratories of Faculty of Medicine.

PATIENTS/SUBJECTS

A total of 306 (114 intact and 192 adrenalectomized) male Albino Wistar rats were used.

INTERVENTIONS

Adrernalectomy, drug administrations, pain model induction and pain threshold measurements were performed during the study.

MEASUREMENTS AND MAIN RESULTS

Although the analgesic effects of nonsteroidal antiinflammatory drugs were lost in adrenalectomized rats, they exerted significant analgesia in adrenalectomized rats that had been pretreated with prednisolone and adrenalin. All these drugs were found to decrease serum adrenalin concentration but did not change serum cortisole (corticosterone in rats) concentration. Prednisolone and adrenalin inhibited carrageenan-induced hyperalgesia in adrenalectomized rat groups pretreated with metoprolol or phenoxybenzamine, but not in rats given propranolol. Propranolol also negated the analgesic effects of IDAN in intact rats. The analgesic effects provided by either prednisolone or adrenalin could not be inhibited by the alpha1, alpha2, or beta1 blockers but disappeared when beta2 receptors were blocked.

CONCLUSIONS

The analgesic effects of nonsteroidal antiinflammatory drugs appear to be related to endogenous adrenalin and cortisole. We have demonstrated that adrenalin and prednisolone play important roles in the analgesic effect mechanism of IDAN. Prednisolone and adrenalin produce analgesic effects through beta2-adrenergic receptors, suggesting an indirect role for beta2-adrenergic receptors in the analgesic effect mechanism of the nonsteroidal antiinflammatory drugs mentioned.

Sustained morphine treatment augments prostaglandin E2-evoked calcitonin gene-related peptide release from primary sensory neurons in a PKA-dependent manner

Tissue damage leads to pain sensitization due to peripheral and central release of excitatory mediators such as prostaglandin E₂ (PGE₂). PGE₂ sensitizes spinal pain neurotransmitter such as calcitonin gene-related peptide (CGRP) release via activation of cyclic AMP (cAMP)/protein kinase A (PKA)-dependent signaling mechanisms. Our previous data demonstrate that sustained morphine pretreatment sensitizes adenylyl cyclase(s) (AC) toward the direct stimulator, forskolin, in cultured primary sensory neurons (AC superactivation). In the present work we investigated the hypothesis that morphine pretreatment also sensitizes ACs toward Gs-protein-coupled excitatory modulators (such as PGE₂), leading to augmented PKA-dependent CGRP release from PGE₂-stimulated primary sensory dorsal root ganglion (DRG) neurons. Our results show that sustained morphine treatment potentiated PGE₂-mediated cAMP formation and augmented PGE₂-evoked CGRP release from cultured primary sensory neurons in a PKA-dependent manner. Our data suggest that attenuation of AC superactivation in primary sensory neurons may prevent the development of opioid-induced hyperalgesia.

Chronic prostaglandin E2 treatment induces the synthesis of the pain-related peptide substance P and calcitonin gene-related peptide in cultured sensory ganglion explants

Prostaglandin E2 (PGE2) is a well known pain and pro-inflammatory mediator abundantly produced in inflamed tissue. It causes pain by directly exciting nociceptive primary sensory neurons (nociceptors) and indirectly stimulating the release of pain-related peptide substance P (SP) and calcitonin gene-related peptide (CGRP). In an ex vivo culture of sensory ganglion explants, we tested the hypothesis that PGE2 could induce the synthesis of SP and CGRP in nociceptors. A stabilized PGE2 analog, 16,16-dimethyl PGE2, in a concentration- and time-dependent manner, significantly increased mRNA and peptide levels of SP and CGRP. The agonists of EP1 and EP4 receptors also significantly increased SP and CGRP levels. Moreover, 16,16-dimethyl PGE2-induced SP and CGRP were blocked by EP1 and EP4 antagonists as well as the inhibitors of both protein kinase A and protein kinase C. Nerve growth factor was partially involved in PGE2-induced SP and CGRP synthesis. Taken together, these results indicate that PGE2 contributes to the synthesis of SP and CGRP in nociceptors, an event mediated by EP1 and EP4 receptors, nerve growth factor and protein kinase A and protein kinase C signalling pathways. We thus conclude that facilitating the synthesis of pain-related peptides in nociceptors is a novel mechanism underlying the role of PGE2 in nociception and chronic pain states.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

NSAIDs, Opioids, Cannabinoids and the Control of Pain by the Central Nervous System

Nonsteroidal anti-inflammatory drugs (NSAIDs) act upon peripheral tissues and upon the central nervous system to produce analgesia. A major central target of NSAIDs is the descending pain control system. The rostral structures of the descending pain control system send impulses towards the spinal cord and regulate the transmission of pain messages. Key structures of the descending pain control system are the periaqueductal gray matter (PAG) and the rostral ventromedial region of the medulla (RVM), both of which are critical targets for endogenous opioids and opiate pharmaceuticals. NSAIDs also act upon PAG and RVM to produce analgesia and, if repeatedly administered, induce tolerance to themselves and cross-tolerance to opioids. Experimental evidence shows that this is due to an interaction of NSAIDs with endogenous opioids along the descending pain control system. Analgesia by NSAIDs along the descending pain control system also requires an activation of the CB1 endocannabinoid receptor. Several experimental approaches suggest that opioids, NSAIDs and cannabinoids in PAG and RVM cooperate to decrease GABAergic inhibition and thus enhance the descending flow of impulses that inhibit pain.

The importance of brain PGE2 inhibition versus paw PGE2 inhibition as a mechanism for the separation of analgesic and antipyretic effects of lornoxicam in rats with paw inflammation

Objectives

Lornoxicam is a non-selective cyclooxygenase inhibitor that exhibits strong analgesic and anti-inflammatory effects but a weak antipyretic effect in rat models. Our aim was to investigate the mechanism of separation of potencies or analgesic and antipyretic effecls of lornoxicam in relatioin to its effect on prostaglandin E2 (PGE2) production in the inflammatory paw and the brain.

Methods

A model of acute or chronic paw inflammation was induced by Freund's complete adjuvant injection into the rat paw. Lornoxicam (0.01–1 mg/kg), celecoxib (0.3–30 mg/kg) or loxoprofen (0.3–30 mg/kg) was administered orally to the rats and the analgesic and antipyretic effects were compared. The paw hyperalgesia was assessed using the Randall–Selitto test or the flexion test. Dorsal subcutaneous body temperature was measured as indicator of pyresis. After the measurement of activities, the rats were sacrificed and the PGE2 content in the paw exudate, cerebrospinal fluid or brain hypothalamus was measured by enzme-immunoassay.

Key findings

In a chronic model of arthritis, lornoxicam, celecoxib and loxoprofen reduced hyperalgesia with an effective dose that provides 50% inhibition (ED50) of 0.083, 3.9 and 4.3 mg/kg respectively, whereas the effective dose of these drugs in pyresis was 0.58, 0.31 and 0.71 mg/kg respectively. These drugs significantly reduced the PGE2 level in paw exudate and the cerebrospinal fluid. In acute oedematous rats, lornoxicam 0.16 mg/kg, celecoxib 4 mg/kg and loxoprofen 2.4 mg/kg significantly reduced hyperalgesia to a similar extent. On the other hand, lornnoxicam did not affect the elevated body temperature, whereas celecoxib and loxoprofen siginificantly reduced the pyrexia to almost the normal level. These drugs significantly reduced the PGE2 level in inflamed paw exudate lo almost the normal level. On the other hand, lornoxicam did not change PGE2 level in the brain hypothalamus, whereas celecoxib and loxoprofen strongly decreased it.

Conclusions

Lornoxicam exhibits strong analgesic but weak antipyretic effects in rats with paw inflammation. Such a separation of effects is related to its efficacy in the reduction of PGE2 levels in the paw and brain hypothalamus.

p38 MAPK inhibition selectively mitigates inflammatory mediators and VEGF production in AF cells co-cultured with activated macrophage-like THP-1 cells

OBJECTIVES

Recent data have suggested that macrophages are involved in the pathogenesis of discogenic back pain and enhance the secretion of inflammatory mediators in co-cultured annulus fibrosus (AF) cells. The purpose of these studies is to determine the role of p38 mitogen-activated protein kinase (p38 MAPK) signaling in the interactions between macrophage and AF cells.

METHODS

Human AF cells were co-cultured with phorbol myristate acetate-stimulated macrophage-like THP-1 cells with and without p38 MAPK inhibition. Conditioned media from co-cultured cells were assayed for interleukin (IL)-6, IL-8, prostaglandin E2 (PGE2), PGF2alpha, and vascular endothelial growth factor (VEGF). Naïve and macrophage-exposed AF cell responses to 10ng/ml tumor necrosis factor-alpha (TNF-alpha) were compared using the same outcome measures.

RESULTS

IL-6, IL-8, PGE2, PGF2alpha, and VEGF were secreted in greater quantities by cells maintained in co-culture compared to macrophages or AF cells cultured alone. SB202190 blunted IL-6, PGE2, and PGF2alpha production in a dose-dependent manner in co-culture. However, it did not suppress IL-8 and VEGF production. TNF-alpha-stimulated AF cell inflammatory mediators were up-regulated by macrophage exposure. SB202190 successfully suppressed IL-6, IL-8, PGE2, and PGF2alpha secretion in macrophage-exposed AF cells in response to TNF-alpha.

CONCLUSIONS

Annular injury can result in macrophage infiltration, and this can cause enhanced inflammatory mediator and VEGF production by AF cells. The p38 MAPK pathway signals are responsible for much of IL-6 and PG secretion from AF cells with macrophage-like cells, suggesting that blockade of this signal may serve as a therapeutic approach to discogenic pain.

Spinal antinociceptive effects of cyclooxygenase inhibition during inflammation: Involvement of prostaglandins and endocannabinoids

Both cyclooxygenase-1 and -2 are expressed in the spinal cord, and the spinal COX product prostaglandin E(2) (PGE(2)) contributes to the generation of central sensitization upon peripheral inflammation. Vice versa spinal COX inhibition is considered an important mechanism of antihyperalgesic pain treatment. Recently, however, COX-2 was shown to be also involved in the metabolism of endocannabinoids. Because endocannabinoids can have analgesic actions it is conceivable that inhibition of spinal COX produces analgesia not only by inhibition of PG synthesis but also by inhibition of endocannabinoid breakdown. In the present study, we recorded from spinal cord neurons with input from the inflamed knee joint and we measured the spinal release of PGE(2) and the endocannabinoid 2-arachidonoyl glycerol (2-AG) in vivo, using the same stimulation procedures. COX inhibitors were applied spinally. Selective COX-1, selective COX-2 and non-selective COX inhibitors attenuated the generation of spinal hyperexcitability when applied before and during development of inflammation but, when inflammation and spinal hyperexcitability were established, only selective COX-2 inhibitors reversed spinal hyperexcitability. During established inflammation all COX inhibitors reduced release of spinal PGE(2) almost equally but only the COX-2 inhibitor prevented breakdown of 2-AG. The reversal of spinal hyperexcitability by COX-2 inhibitors was prevented or partially reversed by AM-251, an antagonist at the cannabinoid-1 receptor. We conclude that inhibition of spinal COX-2 not only reduces PG production but also endocannabinoid breakdown and provide evidence that reversal of inflammation-evoked spinal hyperexcitability by COX-2 inhibitors is more related to endocannabinoidergic mechanisms than to inhibition of spinal PG synthesis.

Antinociceptive actions of honokiol and magnolol on glutamatergic and inflammatory pain

The antinociceptive effects of honokiol and magnolol, two major bioactive constituents of the bark of Magnolia officinalis, were investigated on animal paw licking responses and thermal hyperalgesia induced by glutamate receptor agonists including glutamate, N-methyl-D-aspartate (NMDA), and metabotropic glutamate 5 receptor (mGluR5) activator (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), as well as inflammatory mediators such as substance P and prostaglandin E₂ (PGE₂) in mice. The actions of honokiol and magnolol on glutamate-induced c-Fos expression in the spinal cord dorsal horn were also examined. Our data showed that honokiol and magnolol blocked glutamate-, substance P- and PGE₂-induced inflammatory pain with similar potency and efficacy. Consistently, honokiol and magnolol significantly decreased glutamate-induced c-Fos protein expression in superficial (I-II) laminae of the L4-L5 lumbar dorsal horn. However, honokiol was more selective than magnolol for inhibition of NMDA-induced licking behavioral and thermal hyperalgesia. In contrast, magnolol was more potent to block CHPG-mediated thermal hyperalgesia. These results demonstrate that honokiol and magnolol effectively decreased the inflammatory pain. Furthermore, their different potency on inhibition of nociception provoked by NMDA receptor and mGluR5 activation should be considered.

Complete Freund's adjuvant-induced intervertebral discitis as an animal model for discogenic low back pain

BACKGROUND

Although numerous animal models for low back pain associated with intervertebral disk (IVD) degeneration have been proposed, insufficient data have been provided to make any conclusions regarding pain. Our aim in this study was to determine the reliability of complete Freund's adjuvant (CFA) injection into the rat spine as an animal model representing human discogenic pain.

METHODS

We studied IVD degenerative changes with pain development after a 10-microL CFA injection into the L5-6 IVD of adult rats using behavioral, histologic, and biochemical studies. Serial histologic changes were analyzed to detect degenerative changes. Expression of calcitonin gene-related peptide (CGRP), prostaglandin E (PGE), and inducible nitric oxide synthase (iNOS) were determined using immunohistochemistry or real-time polymerase chain reaction as support data for pain development. In addition, CGRP immunoreactivity (ir) at the IVD was considered indirect evidence of neural ingrowth into the IVD.

RESULTS

There was a significant increase of the hindpaw withdrawal response in the CFA group until 7 wk postoperatively (P < 0.05). Histologic analyses revealed progressive degenerative changes of the disks without any damage in adjacent structures, including nerve roots. In the CGRP-ir staining study, the bilateral dorsal horns and IVD had positive ir after intradiscal CFA injection. CGRP mRNA expression was increased in the dorsal root ganglion (DRG) at 2 and 4 wk, whereas PGE and iNOS mRNAs were markedly increased at 2 wk. The increment of CGRP expression was higher in allodynic rats compared with nonallodynic rats.

CONCLUSION

Intradiscal CFA injection led to chronic disk degeneration with allodynia, which was suggested by pain behavior and expression of pain-related mediators. The increment of CGRP, PGE, and iNOS also suggest pain-related signal processing between the IVD and the neural pathway in this animal model. This animal model may be useful for future research related to the pathophysiology and development of novel treatment for spine-related pain.

Modulatory effects of probenecid on the nitroglycerin-induced changes in the rat caudal trigeminal nucleus

Four hours after systemic administration of the nitric oxide donor nitroglycerin (10 mg/kg bodyweight, s.c.), the neurons of the rat caudal trigeminal nucleus are activated, the area covered by calcitonin gene-related peptide (CGRP)-immunoreactive fibres is decreased and the neuronal nitric oxide synthase (nNOS)- and the calmodulin-dependent protein kinase II alpha (CamKIIalpha)-immunopositive neurons in the same area are increased. Probenecid is a non-selective inhibitor of multidrug-resistance associated proteins and organic anion transporters thus it can modulate the transport functions in the central nervous system influencing nociception. Accordingly, the aim of the present experiments was to examine the effects of probenecid administration on the nitroglycerin-induced expressions of nNOS, CamKIIalpha and CGRP in the rat caudal trigeminal nucleus. Probenecid (200 mg/kg bodyweight, i.p.) pretreatment proved to mitigate the nitroglycerin-induced changes in expression in the rat caudal trigeminal nucleus. The data suggest that the changes caused by nitroglycerin in the expressions of CGRP, nNOS and CamKIIalpha can be influenced by probenecid modulating the inflammatory functions in the nervous system. These data may be of relevance for the pathogenesis of migraine headache.

Capnellene, a natural marine compound derived from soft coral, attenuates chronic constriction injury-induced neuropathic pain in rats

BACKGROUND AND PURPOSE

Natural compounds obtained from marine organisms have received considerable attention as potential sources of novel drugs for treatment of human inflammatory diseases. Capnellene, isolated from the marine soft coral Capnella imbricate, 4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalene-2,3a-diol (GB9) exhibited anti-inflammatory actions on activated macrophages in vitro. Here we have assessed the anti-neuroinflammatory properties of GB9 and its acetylated derivative, acetic acid 3a-hydroxy-4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalen-2-yl ester (GB10).

EXPERIMENTAL APPROACH

Effects of GB9 or GB10 on the expression of inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) in interferon-gamma (IFN-gamma)-stimulated mouse microglial BV2 cells were measured by Western blot. The in vivo effects of these compounds were examined in the chronic constriction injury (CCI) rat model of neuropathic pain, measuring thermal hyperalgesia, and microglial activation and COX-2 protein in lumbar spinal cord, by immunohistochemistry.

KEY RESULTS

In BV2 cells, GB9 and GB10 inhibited the expression of iNOS and COX-2, stimulated by IFN-gamma. Intrathecal administration of GB9 and GB10 inhibited CCI-induced nociceptive sensitization and thermal hyperalgesia in a dose-dependent manner. Intraperitoneal injection of GB9 inhibited CCI-induced thermal hyperalgesia and also inhibited CCI-induced activation of microglial cells and up-regulation of COX-2 in the dorsal horn of the lumbar spinal cord ipsilateral to the injury.

CONCLUSION AND IMPLICATIONS

Taken together, these data indicate that the marine-derived capnellenes, GB9 and GB10, had anti-neuroinflammatory and anti-nociceptive properties in IFN-gamma-stimulated microglial cells and in neuropathic rats respectively. Therefore, capnellene may serve as a useful lead compound in the search for new therapeutic agents for treatment of neuroinflammatory diseases.

Activation of the prostaglandin system in response to sleep loss in healthy humans: potential mediator of increased spontaneous pain

Insufficient duration of sleep is a highly prevalent behavioral pattern in society that has been shown to cause an increase in spontaneous pain and sensitivity to noxious stimuli. Prostaglandins (PGs), in particular PGE2, are key mediators of inflammation and pain, and we investigated whether PGE2 is a potential mediator in sleep-loss-induced changes in nociceptive processing. Twenty-four participants (7 females, age 35.1+/-7.1 years) stayed for 7 days in the Clinical Research Center. After two baseline days, participants were randomly assigned to either 3 days of 88 h of sleep deprivation (TSD, N=15) or 8h of sleep per night (N=9), followed by a night of recovery sleep. Participants rated the intensity of various pain-related symptoms every 2h across waking periods on computerized visual analog scales. PGE2 was measured in 24-h-urine collections during baseline and third sleep deprivation day. Spontaneous pain, including headache, muscle pain, stomach pain, generalized body pain, and physical discomfort significantly increased by 5-14 units on a 100-unit scale during TSD, compared to the sleep condition. Urinary PGE2 metabolite significantly increased by about 30% in TSD over sleep condition. TSD-induced increase in spontaneous pain, in particular headache and muscle pain, was significantly correlated with increase in PGE2 metabolite. Activation of the PGE2 system appears to be a potential mediator of increased spontaneous pain in response to insufficient sleep.

A new paradigm in radioadaptive response developing from microbeam research

A classic paradigm in radiation biology asserts that all radiation effects on cells, tissues and organisms are due to the direct action of radiation on living tissue. Using this model, possible risks from exposure to low dose ionizing radiation (below 100 mSv) are estimated by extrapolating from data obtained after exposure to higher doses of radiation, using a linear non-threshold model (LNT model). However, the validity of using this dose-response model is controversial because evidence accumulated over the past decade has indicated that living organisms, including humans, respond differently to low dose/low dose-rate radiation than they do to high dose/high dose-rate radiation. These important responses to low dose/low dose-rate radiation are the radiation-induced adaptive response, the bystander response, low-dose hypersensitivity, and genomic instability. The mechanisms underlying these responses often involve biochemical and molecular signals generated in response to targeted and non-targeted events. In order to define and understand the bystander response to provide a basis for the understanding of non-targeted events and to elucidate the mechanisms involved, recent sophisticated research has been conducted with X-ray microbeams and charged heavy particle microbeams, and these studies have produced many new observations. Based on these observations, associations have been suggested to exist between the radioadaptive and bystander responses. The present review focuses on these two phenomena, and summarizes observations supporting their existence, and discusses the linkage between them in light of recent results obtained from experiments utilizing microbeams.

COX2 in CNS neural cells mediates mechanical inflammatory pain hypersensitivity in mice

A cardinal feature of peripheral inflammation is pain. The most common way of managing inflammatory pain is to use nonsteroidal antiinflammatory agents (NSAIDs) that reduce prostanoid production, for example, selective inhibitors of COX2. Prostaglandins produced after induction of COX2 in immune cells in inflamed tissue contribute both to the inflammation itself and to pain hypersensitivity, acting on peripheral terminals of nociceptors. COX2 is also induced after peripheral inflammation in neurons in the CNS, where it aids in developing a central component of inflammatory pain hypersensitivity by increasing neuronal excitation and reducing inhibition. We engineered mice with conditional deletion of Cox2 in neurons and glial cells to determine the relative contribution of peripheral and central COX2 to inflammatory pain hypersensitivity. In these mice, basal nociceptive pain was unchanged, as was the extent of peripheral inflammation, inflammatory thermal pain hypersensitivity, and fever induced by lipopolysaccharide. By contrast, peripheral inflammation-induced COX2 expression in the spinal cord was reduced, and mechanical hypersensitivity after both peripheral soft tissue and periarticular inflammation was abolished. Mechanical pain is a major symptom of most inflammatory conditions, such as postoperative pain and arthritis, and induction of COX2 in neural cells in the CNS seems to contribute to this.

Teleantagonism: A pharmacodynamic property of the primary nociceptive neuron

Previous work from our group showed that intrathecal (i.t.) administration of substances such as glutamate, NMDA, or PGE(2) induced sensitization of the primary nociceptive neuron (PNN hypernociception) that was inhibited by a distal intraplantar (i.pl.) injection of either morphine or dipyrone. This pharmacodynamic phenomenon is referred to in the present work as "teleantagonism". We previously observed that the antinociceptive effect of i.t. morphine could be blocked by injecting inhibitors of the NO signaling pathway in the paw (i.pl.), and this effect was used to explain the mechanism of opioid-induced peripheral analgesia by i.t. administration. The objective of the present investigation was to determine whether this teleantagonism phenomenon was specific to this biochemical pathway (NO) or was a general property of the PNNs. Teleantagonism was investigated by administering test substances to the two ends of the PNN (i.e., to distal and proximal terminals; i.pl. plus i.t. or i.t. plus i.pl. injections). We found teleantagonism when: (i) inhibitors of the NO signaling pathway were injected distally during the antinociception induced by opioid agonists; (ii) a nonselective COX inhibitor was tested against PNN sensitization by IL-1beta; (iii) selective opioid-receptor antagonists tested against antinociception induced by corresponding selective agonists. Although the dorsal root ganglion seems to be an important site for drug interactions, the teleantagonism phenomenon suggests that, in PNNs, a local sensitization spreads to the entire cell and constitutes an intriguing and not yet completely understood pharmacodynamic property of this group of neurons.

Injured nerve-derived COX2/PGE2 contributes to the maintenance of neuropathic pain in aged rats

Neuropathic pain (NeP) is a debilitating disease afflicting mostly the aged population. Inflammatory responses in injured nerves play a pivotal role in the pathogenesis of NeP. Injured nerve derived cyclooxygenase 2/prostaglandin E2 (COX2/PGE2) contributes to the genesis of NeP at the early stage in young rats. Here we show that COX2/PGE2 is involved in the maintenance of NeP at a chronic stage in aged rats. Eighteen months after partial sciatic nerve ligation (PSNL), NeP remained prominent in aged rats. COX2 expressing macrophages and PGE2 levels were increased in injured nerves. PGE2 receptors (EP1 and EP4) and pain-related ion channel transient receptor potential vanilloid-1 (TRPV1) were increased in the ipsilateral dorsal root ganglion (DRG) neurons of aged PSNL rats. Perineural injection of a selective COX2 inhibitor NS-398 relieved NeP, reversed PSNL increased expression of EP1, EP4 and TRPV1 and suppressed the levels of pain-related peptide substance P and calcitonin gene-related peptide in DRG neurons. These data suggest that injured nerve-derived PGE2 contributes to the maintenance of NeP at the chronic stage in aged rats. Chronically facilitating the synthesis of pain-related molecules in nociceptive DRG neurons is a novel mechanism underpinning the contribution of PGE2.

Consequences of altered eicosanoid patterns for nociceptive processing in mPGES-1-deficient mice

Cyclooxygenase-2 (COX-2)-dependent prostaglandin (PG) E₂ synthesis in the spinal cord plays a major role in the development of inflammatory hyperalgesia and allodynia. Microsomal PGE₂ synthase-1 (mPGES-1) isomerizes COX-2-derived PGH₂ to PGE₂. Here, we evaluated the effect of mPGES-1-deficiency on the noci-ceptive behavior in various models of nociception that depend on PGE₂ synthesis. Surprisingly, in the COX-2-dependent zymosan-evoked hyperalgesia model, the nociceptive behavior was not reduced in mPGES-1-deficient mice despite a marked decrease of the spinal PGE₂ synthesis. Similarly, the nociceptive behavior was unaltered in mPGES-1-deficient mice in the formalin test. Importantly, spinal cords and primary spinal cord cells derived from mPGES-1-deficient mice showed a redirection of the PGE₂ synthesis to PGD₂, PGF_2α and 6-keto-PGF_1α (stable metabolite of PGI₂). Since the latter prostaglandins serve also as mediators of noci-ception they may compensate the loss of PGE₂ synthesis in mPGES-1-deficient mice.

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

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

Chronic tendinopathy tissue pathology, pain mechanisms, and etiology with a special focus on inflammation

Continuing progress in research in molecular biology and biomechanics has provided considerable new information and has given rise to new hypotheses in chronic tendinopathy. Overloading is still, however, crucial in the development of tendinopathy. Most of the histologic findings in tendinopathy represent chronic degeneration, regeneration, and microtears of the tendinous tissue. The prevailing opinion is that no histological evidence of acute inflammation has been documented, but in newer studies using immunohistochemistry and flow cytometry inflammatory cells have been detected. The existing data indicate that the initiators of the tendinopathic pathway include many proinflammatory agents (e.g. cytokines, prostaglandins, different growth factors, and neuropetides). Because of the complex interaction between the classic proinflammatory agents and the neuropeptides, it seems impossible and somewhat irrelevant to distinguish sharply between chemical and neurogenic inflammation. Furthermore, glucocorticoids are, at the moment, the most effective treatment in tendinopathy with regard to reduction of pain, tendon thickness, and neovascularization. This review indicates - despite a great deal of uncertainty regarding the concepts - that an inflammatory process may be related not only to the development of tendinopathy but also chronic tendinopathy. More attention should be directed towards the "tendinitis myth" in the future.

Effect of 17beta-estradiol on signal transduction pathways and secondary damage in experimental spinal cord trauma

Because studies have shown that 17beta-estradiol (E2) produces anti-inflammatory effects after various adverse circulatory conditions, we examined whether administration of E2 before spinal cord injury (SCI) has any salutary effects in reducing SCI. Spinal cord injury was induced by the application of vascular clips (force of 24 g) to the dura via a four-level T5-T8 laminectomy. To gain a better insight into the mechanism of action of the anti-inflammatory effects of E2, the following end points of the inflammatory process were evaluated: (1) spinal cord inflammation and tissue injury (histological score); (2) neutrophil infiltration (myeloperoxidase activity); (3) expression of iNOS, nitrotyrosine, and COX-2; (4) apoptosis (terminal deoxynucleotidyltransferase-mediated UTP end labeling staining and Bax and Bcl-2 expression); and (5) tissue TNF-alpha, IL-6, IL-1beta, and monocyte chemoattractant protein 1 levels. In another set of experiments, the pretreatment or posttreatment with E2 significantly ameliorates the recovery of limb function (evaluated by motor recovery score). To elucidate whether the protective effects of E2 were mediated via the estrogen receptors, we investigated the effect of an estrogen receptor antagonist, ICI 182,780, on the protective effects of E2. ICI 182,780 (500 microg/kg, s.c., 1 h before treatment with E2) significantly antagonized the effect of the E2 and abolished the protective effect against SCI. Taken together, our results clearly demonstrate that administration of E2 before SCI reduces the development of inflammation and tissue injury associated with spinal cord trauma.

Safety, efficacy and anti-inflammatory activity of rho iso-alpha-acids from hops

A defined mixture of rho iso-alpha-acids (RIAA), a modified hop extract, was evaluated for anti-inflammatory efficacy and safety. RIAA inhibited LPS-stimulated PGE(2) formation with >200-fold selectivity of COX-2 (IC(50)=1.3 microg/ml) over COX-1 (IC(50)>289 microg/ml). This occurred only when RIAA was added prior to, but not post, LPS stimulation. Consistent with this observation, RIAA produced no physiologically relevant, direct inhibition of COX-1 or COX-2 peroxidase activity. This suggests that RIAA inhibits inducible but not constitutive COX-2. In support, we found RIAA showed minimal PGE(2) inhibition (IC(50)=21mug/ml) relative to celecoxib (IC(50)=0.024 microg/ml), aspirin (IC(50)=0.52 microg/ml) or ibuprofen (IC(50)=0.57 microg/ml) in the AGS gastric mucosal model, where COX-1 and -2 are expressed constitutively. Taken together these results predict RIAA may have lower potential for gastrointestinal and cardiovascular toxicity observed with COX enzyme inhibitors. Following confirmation of bioavailable RIAA administered orally, gastrointestinal safety was assessed using the fecal calprotectin biomarker in a 14-day human clinical study; RIAA (900 mg/day) produced no change compared to naproxen (1000 mg/day), which increased fecal calprotectin 200%. Cardiovascular safety was addressed by PGI-M measurements where RIAA (1000 mg) did not reduce PGI-M or affect the urinary PGI-M/TXB(2) ratio. Drug interaction potential was evaluated against six major CYPs; of relevance, RIAA inhibited CYP2C9. Toxicity was assessed in a 21-day oral, mouse subchronic toxicity study where no dose dependent histopathological effects were noted. Clinically, RIAA (1000 mg/day) produced a 54% reduction in WOMAC Global scores in a 6-week, open-label trial of human subjects exhibiting knee osteoarthritis.

Modulation of sensory neuron potassium conductances by anandamide indicates roles for metabolites

BACKGROUND AND PURPOSE

The endogenous cannabinoid anandamide (AEA) acts at cannabinoid (CB(1)) and vanilloid (TRPV(1)) receptors. AEA also shows antinociceptive properties; although the underlying mechanism for this is not fully understood, both CB(1) and TRPV(1) may be involved. Voltage-activated Ca(2+) channels in rat-cultured dorsal root ganglion (DRG) neurons are modulated by AEA. However, AEA in different populations of neurons enhanced or attenuated KCl-evoked Ca(2+) influx; these effects were linked with soma size. The aim of this study was to determine how AEA or its metabolites might produce these variable responses.

EXPERIMENTAL APPROACH

The whole cell patch-clamp technique and fura-2 Ca(2+) imaging were used to characterize the actions of AEA on action potential firing and voltage-activated K(+) currents and to determine whether AEA metabolism plays any role in its effects on cultured DRG neurons.

KEY RESULTS

AEA attenuated multiple action potential firing evoked by 300 ms depolarizing current commands in a subpopulation of DRG neurons. Application of 1 microM AEA attenuated voltage-activated K(+) currents and the recovery of KCl-evoked Ca(2+) transients. The insensitivity of these responses to the CB(1) receptor antagonist rimonabant (100 nM) and preincubation of DRG neurons with pertussis toxin suggested that these actions are not CB(1) receptor-mediated. Preincubating DRG neurons with the fatty acid amide hydrolase (FAAH) inhibitor phenylmethylsulphonyl fluoride (PMSF) attenuated the inhibitory actions of AEA on K(+) currents and Ca(2+) influx.

CONCLUSION AND IMPLICATIONS

These data suggest that the products of AEA metabolism by FAAH contribute to the attenuation of K(+) conductances and altered excitability of cultured sensory neurons.

Spinal dis-inhibition in inflammatory pain

Inflammatory diseases and neuropathic insults trigger signaling cascades, which frequently lead to intense and long-lasting pain syndromes in affected patients. Such pain syndromes are characterized not only by an increased sensitivity to painful stimuli (hyperalgesia), but also by a qualitative change in the sensory perception of other, tactile stimuli (allodynia) and the occurrence of spontaneous pain in the absence of any sensory input. Long-term potentiation (LTP)-like changes in synaptic transmission between nociceptive C-fibers and spino-periaqueductal grey projection neurons as well as a loss of inhibitory control by GABAergic and glycinergic spinal dorsal horn neurons have repeatedly been proposed as underlying principles. While considerable evidence supports a significant contribution of C-fiber LTP to hyperalgesia, such monosynaptic plasticity cannot explain the occurrence of allodynia and spontaneous pain. In this review, we focus on mechanisms of synaptic dis-inhibition in inflammatory pain and propose that pathologically heightened pain sensitivity can be reversed by restoring synaptic inhibition with drugs that target specific spinal GABAA receptor subtypes.