Effects of acetylsalicylic acid and indomethacin on single groups III and IV sensory units from acutely inflamed joints

(-)-Englerin-A Has Analgesic and Anti-Inflammatory Effects Independent of TRPC4 and 5

Recently, we found that the deletion of TRPC5 leads to increased inflammation and pain-related behaviour in two animal models of arthritis. (-)-Englerin A (EA), an extract from the East African plant Phyllanthus engleri has been identified as a TRPC4/5 agonist. Here, we studied whether or not EA has any anti-inflammatory and analgesic properties via TRPC4/5 in the carrageenan model of inflammation. We found that EA treatment in CD1 mice inhibited thermal hyperalgesia and mechanical allodynia in a dose-dependent manner. Furthermore, EA significantly reduced the volume of carrageenan-induced paw oedema and the mass of the treated paws. Additionally, in dorsal root ganglion (DRG) neurons cultured from WT 129S1/SvIm mice, EA induced a dose-dependent cobalt uptake that was surprisingly preserved in cultured DRG neurons from 129S1/SvIm TRPC5 KO mice. Likewise, EA-induced anti-inflammatory and analgesic effects were preserved in the carrageenan model in animals lacking TRPC5 expression or in mice treated with TRPC4/5 antagonist ML204.This study demonstrates that while EA activates a sub-population of DRG neurons, it induces a novel TRPC4/5-independent analgesic and anti-inflammatory effect in vivo. Future studies are needed to elucidate the molecular and cellular mechanisms underlying EA's anti-inflammatory and analgesic effects.

Physiologie der Schmerzentstehung in der Peripherie

Dieser Beitrag gibt einen Überblick über den Kenntnisstand zu den Mechanismen der Schmerzentstehung im Gelenk. Er fokussiert sich auf den Vorgang der Nozizeption in nozizeptiven Nervenfasern des Gelenks und stellt dar, wie Krankheitsprozesse im Gelenk auf Nozizeptoren wirken. Während Nozizeptoren im normalen Gelenk eine hohe Erregungsschwelle besitzen und nur durch hochintensive Reize aktiviert werden, kommt es bei Gelenkerkrankungen häufig zu einer Sensibilisierung dieser Nervenfasern, sodass sie bereits auf leichte Reize (Bewegungen, Palpation) ansprechen und nach zentraler Verarbeitung Schmerzempfindungen auslösen. Eine Sensibilisierung wird meistens durch Entzündungsmediatoren ausgelöst, für die die Nozizeptoren Rezeptoren besitzen. Werden Nervenfasern im Erkrankungsprozess geschädigt, können neuropathische Schmerzmechanismen hinzukommen. Chronische Gelenkerkrankungen sind durch entzündliche und destruktive Prozesse charakterisiert. Sowohl bei primären Arthritiden als auch bei Arthrosen sind entzündliche Prozesse für die Sensibilisierung der Nozizeptoren verantwortlich. Dafür werden neben den Prostaglandinen auch proinflammatorische Zytokine und der Nervenwachstumsfaktor (NGF) verantwortlich gemacht, für die viele Nozizeptoren Rezeptoren exprimieren. Demgemäß sind diese Moleküle auch Target innovativer Schmerztherapien, z. B. die Gabe von Antikörpern gegen NGF bei Arthrose. Besonders für die Neutralisation von TNF ist ein direkt schmerzlindernder Effekt nachgewiesen, der aus der Unterbrechung von nozizeptiven Vorgängen am Nozizeptor resultiert. Der direkte pronozizeptive Effekt der Zytokine und Bindungsstellen für Fc-Fragmente von Antikörpern an Nozizeptoren zeigen, dass Immunmechanismen auch für die Schmerzentstehung große Bedeutung haben. Auch destruktive Gelenkprozesse können Schmerzen verursachen. So kann bereits die Osteoklastenaktivität im präklinischen Stadium einer Arthritis Schmerzen verursachen, und nach Ausbruch der Arthritis tragen Destruktionsprozesse zu Schmerzen bei. Inwieweit die Hemmung der Osteoklastenaktivität Gelenkschmerzen lindert, wird derzeit erforscht. Auch weitere neue Ansätze, peripher wirksame Opioide, Cannabinoide und Ionenkanalblocker werden dargestellt. Schließlich geht der Beitrag auf generelle/systemische Faktoren ein, die Krankheitsprozesse im Gelenk und die Schmerzentstehung beeinflussen. Hier wird in erster Linie die Bedeutung des Diabetes mellitus angesprochen. Diese Stoffwechselerkrankung stellt einen Risikofaktor für die Entwicklung von Arthrosen dar, und sie trägt zur Schmerzintensivierung bei. Dabei können verstärkte Entzündungsprozesse und auch neuropathische Schmerzkomponenten beteiligt sein.

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

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

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.

Excitation and sensitization of nociceptors by bradykinin: what do we know?

Bradykinin is an endogenous nonapeptide known to induce pain and hyperalgesia to heat and mechanical stimulation. Correspondingly, it excites nociceptors in various tissues and sensitizes them to heat, whereas sensitizing effect on the mechanical response of nociceptors is not well established. Protein kinase C and TRPV1 contribute to the sensitizing mechanism of bradykinin to heat. In addition, TRPA1 and other ion channels appear to contribute to excitation caused by bradykinin. Finally, prostaglandins sensitize bradykinin-induced excitation in normal tissues by restoring desensitized responses due to the inhibition of protein kinase A.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Characterization of a method for measuring primary hyperalgesia of deep somatic tissue

Measuring primary hyperalgesia from deep somatic tissue (ie, muscle and joint) is difficult in laboratory animals but clinically important. In this study, we modified a newly developed method to measure primary hyperalgesia of muscle in rats and compared this with primary hyperalgesia from the knee. Compression withdrawal thresholds of the gastrocnemius muscle or the knee joint were measured with a device consisting of strain gauges attached to forceps. Compression of the muscle or joint with the forceps results in hind limb withdrawal, and thresholds were measured before and 4 hours after induction of inflammation by 3% carrageenan injected into the gastrocnemius muscle or 3% kaolin-carrageenan injected into the knee joint. Compression thresholds were significantly decreased 4 hours after induction of inflammation in the muscle or knee joint compared with thresholds before inflammation. Surprisingly, in animals with muscle inflammation, compression thresholds were also significantly decreased on the contralateral hind limb. Systemic morphine (5 mg/kg, intraperitoneal) or lidocaine (2%) injected into the inflamed tissue reversed the decreased compression threshold induced by deep tissue inflammation. However, local anesthetic applied to the skin overlying the muscle or knee joint did not affect the decreased threshold. Thus, we report a consistent and convenient method to measure primary hyperalgesia in deep tissues of rats. The measured hyperalgesia originates in the inflamed tissues and has no measurable contribution from skin.

PERSPECTIVE

The current method measures primary hyperalgesia directly from injured deep somatic tissues. Thus it is relevant to painful clinical conditions that are aggravated by mechanical pressure at the site of injury. As such, it might serve as a model for basic mechanistic studies as well as drug discovery for musculoskeletal pain syndromes.

Pathophysiology of pain

Pain is a major symptom of many different diseases. Modern pain research has uncovered important neuronal mechanisms that are underlying clinically relevant pain states, and research goes on to define different types of pains on the basis of their neuronal and molecular mechanisms. This review will briefly outline neuronal mechanisms of pathophysiological nociceptive pain resulting from inflammation and injury, and neuropathic pain resulting from nerve damage. Pain is the sensation that is specifically evoked by potential or actual noxious (i.e. tissue damaging) stimuli or by tissue injury. Pain research has not only explored the neuronal and molecular basis of the "pain system" of the healthy subject but has also provided insights into the function and plasticity of the "pain system" during clinically relevant pains such as post-injury pain, inflammatory pain, postoperative pain, cancer pain and neuropathic pain. This review will briefly describe the "pain system" and then address neuronal mechanisms that are involved in clinical pain states.

Voltage-gated calcium channels may be involved in the regulation of the mechanosensitivity of slowly conducting knee joint afferents in rat

Voltage-gated Ca(2+) channels play an important role in the central processing of nociceptive information. Recently, it has been shown that L- and N-type voltage-gated Ca(2+) channels are also present on peptidergic, fine afferent nerve fibers in the knee joint capsule. Therefore, the influence of specific blockers for L-type (verapamil) or N-type (omega-conotoxin GVIA) Ca(2+) channels on the mechanosensitivity of slowly conducting afferents was tested in the rat knee joint. Topical application of 100 microM verapamil onto the receptive field reduced the mean response to knee joint rotation to 67+/-8% (SEM, n=12), obtained by outward rotations with a torque of 10 mNm above the mechanical threshold and compared with control movements. In the presence of 50 microM omega-conotoxin GVIA, the mean response decreased to 44+/-5% ( n=12), a reduction that was also observed during rotations of other intensities. Simultaneous application of both substances further reduced the response to 25+/-11% ( n=6). In additional experiments it was shown that L- and N-type voltage-gated Ca(2+) channels do not influence activity-dependent changes of the mechanical excitability. In conclusion, the data of the present study indicate that voltage-gated Ca(2+) channels may also be involved in the regulation of the mechanosensitivity of nociceptive nerve fiber endings.

A role for endocannabinoids in indomethacin-induced spinal antinociception

Inhibition of prostaglandins synthesis does not completely explain non-steroidal anti-inflammatory drug-induced spinal antinociception. Among other mediators, endocannabinoids are involved in pain modulation. Indomethacin-induced antinociception, in the formalin test performed in spinally microdialysed mice, was reversed by co-administration of the cannabinoid 1 (CB(1)) antagonist, N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1-H-pyrazole-3-carboxamide (AM-251), but not by co-infusion of prostaglandin E(2). Indomethacin was ineffective in CB(1) knockout mice. AM-251 also reversed the indomethacin-induced antinociception in a test of inflammatory hyperalgesia to heat. Furthermore, during the formalin test, indomethacin lowered the levels of spinal nitric oxide (NO), which activates cellular reuptake and thus breakdown of endocannabinoids. The pronociceptive effect of an NO donor, 3-methyl-N-nitroso-sydnone-5-imine (RE-2047), was abolished by co-administration of the endocannabinoid transporter blocker N-(4-hydroxyphenyl) arachidonoyl amide (AM-404). Moreover, the antinociceptive activity of the NO synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME), was reversed by AM-251. Thus we propose that at the spinal level, indomethacin induces a shift of arachidonic acid metabolism towards endocannabinoids synthesis secondary to cyclooxygenase inhibition. In addition, it lowers NO levels with subsequent higher levels of endocannabinoids.

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.

B2 receptor-mediated enhanced bradykinin sensitivity of rat cutaneous C-fiber nociceptors during persistent inflammation

Bradykinin (BK), which has potent algesic and sensitizing effect on nociceptors, is of current interest in understanding the mechanisms of chronic pain. BK response is mediated by B2 receptor in normal conditions; however, findings that B1 receptor blockade alleviated hyperalgesia in inflammation have been highlighting the role of B1 receptor in pathological conditions. It has not yet been clear whether nociceptor activities are modified by B1 receptor agonists or antagonists during inflammation. In addition, previous studies reported the change in BK sensitivity of nociceptors during short-lasting inflammation, and data in persistent inflammation are lacking. Therefore we investigated whether an experimentally induced persistent inflammatory state modulates the BK sensitivity of nociceptors and which receptor subtype plays a more important role in this condition. Complete Freund's adjuvant was injected into the rat-tail and after 2-3 wk, persistent inflammation developed, which was prominent in the ankle joint. Using an in vitro skin-saphenous nerve preparation, single-fiber recordings were made from mechano-heat sensitive C-fiber nociceptors innervating rat hairy hindpaw skin, and their responses were compared with those obtained from C-fibers tested similarly in normal animals. BK at 10(-8) M excited none of the 10 C-fibers in normal animals while it excited 5 of 11 (45%) C-fibers of inflamed animals, and at 10(-6) M BK excited all of the 11 inflamed C-fibers (or 94% of 36 tested C-fibers) but only 4 of 10 (or 45% of 58 tested C-fibers) in normal animals. Thus the concentration-response curves based on the incidence of BK induced excitation, and the total number of impulses evoked in response to BK were significantly shifted to the left. Moreover, an increased percentage of the inflamed C-fibers responded to 10(-6) M BK with bursting or high-frequency discharges. Thirty-percent of inflamed C-fibers had spontaneous activity, and these fibers showed comparatively less tachyphylaxis to consecutive second and third 10(-6) M BK stimulation. A B2 receptor antagonist (D-Arg-[Hyp3, Thi5,8,D-phe7]-BK) completely eliminated BK responses in inflamed rats, while B1 receptor antagonists (B 9958 and Des-Arg9-[Leu8]-BK) had no effect. Selective B1 receptor agonist (Des-Arg10-Kallidin) excited 46% (n = 13) of inflamed C-fibers at 10(-5) M concentration, which is 1,000 times higher than that of BK needed to excite the same percentage of inflamed C-fibers. We conclude that in chronically inflamed tissue, sensitivity of C-fiber nociceptors to BK, which is B2 receptor mediated, is strongly increased and that B1 receptor may not be important to a persistent inflammatory state, at least at the primary afferent level.

Properties of sensory neurons innervating synovial joints

While it is clear that the TMJ joint has a sensory innervation like other synovial joints, there is little specific information about the function of the innervation of the TMJ. In order to provide a template upon which future studies might build, this article briefly reviews the function of sensory neurons in other synovial joints.

Histamine excites groups III and IV afferents from the cat knee joint depending on their resting activity

The effect of histamine on the sensory activity of primary afferents was studied in normal and acutely inflamed cat knee joints. A subpopulation of groups III and IV articular afferents could be activated by close-arterial bolus injections of histamine: units with a high resting activity (about 100/min) were particular sensitive to histamine and were excited even by 3.3 fg histamine. The lower the resting discharges of groups III and IV units both from normal and acutely inflamed joints, the higher the dose of histamine (up to 3.3 or 33 microg) necessary to excite the nerve fibres. Thirty-seven of 39 units without any resting activity were completely insensitive to histamine. In contrast to its clear excitatory effect, histamine caused only minor changes in the responses to joint movements. Movement-evoked activity remained unchanged in 22 of 28 units, 1 unit was sensitized and 5 units showed reduced activity after histamine (3.3 microg). The present results support the notion that histamine may participate in the mediation of pain from injured or inflamed tissue. It is remarkable that histamine has a profound excitatory action on a proportion of both groups III and IV articular afferents without changing their sensitivity to joint movements.

Cyclooxygenase-1 is a marker for a subpopulation of putative nociceptive neurons in rat dorsal root ganglia

Immunocytochemical and morphometric techniques were used to quantify the distribution of cyclooxygenase (cox)-containing neurons in rat L5 dorsal root ganglia (DRG). Cox-1 immunolabelling was almost exclusively restricted to small diameter DRG neurons (< 1000 microm2), and was extensively colocalized with calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4). Cox-1 was present in 65% and 70% of CGRP- and IB4-labelled neurons, respectively. Cox-1 labelling was also found in neurons expressing the sensory neuron-specific (SNS) Na+ channel. Cox-2 labelling was absent in DRG from normal rats. In the Freund's adjuvant model of monoarthritis, the proportion of cox-1-positive DRG neurons was unchanged and no neurons were found to be labelled for cox-2. In primary tissue culture, cox-1 immunolabelling persisted in vitro for up to 9 days and was present in morphologically identical neurons. The selective expression of cox-1 in peripheral ganglia was confirmed by the small number of nodose ganglion neurons and superior cervical ganglion (SCG) neurons labelled for cox-1. These data suggest that cox-1 is a marker for a subpopulation of putative nociceptive neurons in vitro and in vivo, and suggests that the prostaglandins synthesized by these neurons may be important for nociceptor function. These data may have important implications for the mode and mechanism of action of non-steroidal anti-inflammatory drugs (NSAIDs).

The effects of S- and R-flurbiprofen on the inflammation-evoked intraspinal release of immunoreactive substance P--a study with antibody microprobes

Using antibody coated microprobes in anesthetized rats, we studied the intraspinal release of immunoreactive substance P during development of kaolin/carrageenan-induced inflammation in the knee joint, and the effects of S- and R-flurbiprofen on inflammation-evoked intraspinal release of immunoreactive substance P once inflammation was established. During the first 6 h after induction of acute inflammation, the basal release and the release of immunoreactive substance P evoked by innocuous pressure applied to the knee showed increases (n=4 rats). An intravenous dose of 9 mg/kg S-flurbiprofen (a potent inhibitor of cyclooxygenases that is anti-inflammatory and antinociceptive) did not significantly alter the pattern of inflammation-evoked release of immunoreactive substance P within 2 h although this dose reduced the responses of spinal cord neurons to pressure applied to the inflamed knee joint within 15 min to about 15% of the predrug value (Neugebauer et al., J. Pharmacol. Exp. Ther. 275 (1995) 618-628). The subsequent i.v. injection of 27 mg/kg S-flurbiprofen significantly changed the pattern of release of immunoreactive substance P showing a reduction of the level of immunoreactive substance P in the dorsal horn within 1 h (n=4 rats). The release of immunoreactive substance P was also reduced after the i.v. injection of 27 mg/kg R-flurbiprofen that is also antinociceptive but less anti-inflammatory (n=5 rats). These data show that both S- and R-flurbiprofen reduce the inflammation-evoked intraspinal release of immunoreactive substance P within hours. However, the reduction of release of immunoreactive substance P does not seem to be a prerequisite for the initial antinociceptive action of non-steroidal anti-inflammatory drugs. It may be rather important in the long term range.

Analgesic effects of lamotrigine and phenytoin on cold-induced pain: a crossover placebo-controlled study in healthy volunteers

The analgesic activity of a single dose of lamotrigine (300 mg p.o.) and phenytoin (300 mg p.o.) was evaluated in a randomised, double-blind, placebo-controlled study in 12 healthy volunteers. A computerised cold-pressor test (CPT) was used to measure analgesia. Dihydrocodeine (90 mg p.o.) was used to validate the effectiveness of the CPT in measuring analgesia in the volunteers. On each study day the volunteers performed the CPT before study medication and at 1.25, 2.75, 4.25 and 5.75 h post-dose. Psychomotor tests were carried out before each CPT to determine possible drug-induced sedation. These included digit symbol substitution, critical flicker fusion and choice reaction time. Subjective feelings of concentration, vigilance and relaxation were also measured using visual analogue scales. All three active drugs significantly reduced pain scores. Maximum pain relief was achieved at 1.25 h post-dose for both dihydrocodeine and lamotrigine, whereas for phenytoin it occurred at 4.25 h post-dose. There was a significant association between analgesia and plasma concentrations of lamotrigine (P = 0.013) and phenytoin (P = 0.028). There were no significant differences in the sedation produced by any of the active drugs, compared to placebo. The findings of this study suggest that lamotrigine and phenytoin could have a wider clinical use as analgesics.

Prostanoids synthesized by cyclo-oxygenase isoforms in rat spinal cord and their contribution to the development of neuronal hyperexcitability

1. The responses of wide dynamic range spinal dorsal horn neurones to noxious mechanical stimulation of the ankle or knee joint were tested before and after spinal administration of the non-selective cyclooxygenase (COX) inhibitors, indomethacin and meclofenamic acid. Neither of these drugs altered the responses of these neurones to noxious mechanical stimulation. 2. Wind-up of a spinal nociceptive reflex evoked by electrical stimulation of the sural nerve at C-fibre strength was dose-dependently inhibited by intravenous administration of indomethacin, a non-selective COX inhibitor, and SC58125, a selective COX-2 inhibitor. Intrathecal administration of indomethacin also reduced the wind-up of this nociceptive reflex. 3. Western blot analysis of proteins extracted from normal rat spinal cord revealed the presence of both cyclo-oxygenase (COX)-1 and COX-2 proteins. 4. Immunocytochemistry of sections of normal rat spinal cord with specific COX-1 antiserum revealed little specific COX-1-like immunoreactivity in the grey matter. With the same antiserum, intense COX-1-like immunoreactivity was observed in the cytoplasm, nuclear membrane and axonal processes of small to medium sized (< 1000 microns2) dorsal root ganglion (DRG) cell bodies. 5. Immunocytochemistry of sections of normal rat spinal cord incubated with specific COX-2 antiserum showed intense COX-2-like immunoreactivity (COX-2-li) in the superficial dorsal horn of the spinal cord (laminae I and II) and around the central canal (lamina X). COX-2-li was also observed in some neurones in deep dorsal horn and in individual motor neurones in ventral horn. COX-2-li was not observed in the cell bodies of DRG. 6. Superfusion of the lumbar spinal cord of normal rats with artificial CSF and subsequent radioimmunoassay revealed the presence of prostaglandin D2 (PGD2) < PGE2, but not PGI2 (determined by measurement of the stable metabolite, 6-keto-PGF1 alpha) or PGF2 alpha. 7. These data suggest that eicosanoids synthesized by an active COX pathway in the spinal cord of normal animals may contribute to nociceptive processing, but only when the spinal cord neurones are rendered hyperexcitable following C-fibre stimulation. Selective inhibition of one or both of the COX isoforms in normal animals may represent a novel target for spinal analgesia.

Effect of lamotrigine in the acute and chronic hyperalgesia induced by PGE2 and in the chronic hyperalgesia in rats with streptozotocin-induced diabetes

There is still a need for a new analgesic devoid of the side effects presented by opioids or non-steroidal anti-inflammatory drugs, for the treatment of some acute and chronic pain conditions. Lamotrigine (Lamictal1, 10-100 mg/kg), a new anticonvulsant, showed analgesic effects in the acute model of prostaglandin E2 (PGE2)-induced hyperalgesia when given orally before or after the subplantar injection of PGE2 in the rat. It also inhibited the development of sustained hyperalgesia induced by multiple subplantar injections of PGE2 when administered orally prior to the PGE2 injections. Furthermore, lamotrigine induced analgesia in the model of chronic hyperalgesia in streptozotocin-induced diabetic rats. The effects of carbamazepine and phenytoin are compared to the effects of lamotrigine in this model. The results suggest that lamotrigine could be used in pain conditions where neuronal sensitization may be present and possibly also where it could inhibit the development of this sensitization.

Effects of a carrageenan-induced inflammation in rabbit lumbar facet joint capsule and adjacent tissues

The effects of experimentally induced inflammation of the lumbar facet joint capsule and adjacent tissues were investigated electrophysiologically and histologically. Type II carrageenan was injected into the receptive fields innervated by identified mechanosensitive afferent units. The multi-unit spontaneous background discharge rate showed increases that consisted of two phases over a time period of 150 min: the first phase (0-30 min) and the second phase (45-150 min). The time course of single units, identified as groups II, III and IV, and silent units, was also investigated. The silent unit discharge rates displayed a gradual increase in the first 15 min and persisted beyond 75 min. Histological examination revealed inflammatory changes in carrageenan injected tissues. In contrast, in isotonic saline injected control experiments there were no changes observed in the electrophysiological or histological studies. This study shows the effects of inflammation in rabbit lumbar facet joint capsule and adjacent tissues. The electrophysiological results show that inflammation of the facet joint and deep back muscles causes (1) increases in multi-unit discharge rate, (2) sensitization to mechanical stimuli and (3) recruitment of previously silent units. Inflammatory changes were also demonstrated histologically.

Electrophysiological evidence for a spinal antinociceptive action of dipyrone

Electrophysiological experiments in anesthetized cats and rats were performed in order to study the effects of dipyrone on single afferent fibers from the knee joint and on spinal cord neurons with knee joint input. The neurons were activated and/or rendered hyperexcitable by an acute inflammation in the knee joint. In the joint nerve in cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity in 10/12 thinly myelinated afferents but only in 1/10 unmyelinated afferents; the responses to movements of the inflamed knee were reduced in 8/10 thinly myelinated but only in 3/10 unmyelinated units. The reduction of activity was significant 20-30 min after application and was maximal at 60-180 min. In the spinal cord of spinalized cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity and/or responses to pressure onto the inflamed knee in 14/16 neurons and in non-spinalized rats similar effects were seen in 10/11 neurons. Effects on spinal cord neurons started 5-10 min after application and were maximal after 20-40 min. These data show pronounced suppression of inflammation-induced nociception by dipyrone and they suggest that the spinal cord is a major site of action of this compound.

Quantitative assessment of joint pain following treatment of rheumatoid arthritis with ibuprofen cream

The antiinflammatory effect of percutaneous application of NSAID (DOLGIT cream) was evaluated quantitatively on 11 patients with symmetric rheumatoid arthritis of the metacarpophalangeal and/or proximal interphalangeal joints. The pressure pain detection threshold (PDT) and pressure pain tolerance threshold (PTT) on the test joints were measured before and on days 3 and 7 after double-blind placebo controlled application of NSAID cream. The clinical pain was assessed by a visual analog scale (VAS) following controlled finger movements. The relative median PDT differences between NSAID and placebo treatment median were 10 kPa (N.S.) on day 3 and 17 kPa (N.S.) on day 7. The corresponding median differences in PTT were -5 kPa (N.S.) and 23 kPa (P < 0.05), respectively. The corresponding median decreases in VAS score were 4.2 mm (N.S.) and 15.5 mm (N.S.), respectively. The experimental joint pressure techniques can assess selectively pain from small joints and is a new useful tool to evaluate antinociceptive and/or antiinflammatory effects.

Activation of normal and inflamed fine articular afferent units by serotonin

In cats anesthetized with alpha-chloralose, extracellular recordings were made from fine afferent units belonging to the medial articular nerve (MAN) of the knee joint. The excitatory and sensitizing effects on articular afferents of serotonin (5-HT) applied intra-arterially close to the joint were examined. The joints were either normal or an experimental arthritis had been induced some hours before the recording session. Bolus injections of 1.35-135 micrograms 5-HT excited about 43% of group III (CV: 2.5-20 m/sec) and 73% of group IV units (CV: less than 2.5 m/sec) from normal joints. The latency was usually between 10 and 30 sec, and the duration and size of the responses were dose-dependent. Fast group III units (CV: greater than 16 m/sec) and group II units (CV: greater than 20 m/sec) were never excited by 5-HT. Repetitive administration led to pronounced tachyphylaxis of the 5-HT response. Inflammation induced an enhanced sensitivity of group III articular afferent units to close intra-arterial application of 5-HT. In particular the total duration of each response was considerably prolonged (4-10 min against 1-2 min under normal conditions). At the same time the tachyphylaxis seen under normal conditions was greatly reduced. In contrast, group IV articular afferent units did not become sensitized to 5-HT in the course of inflammation. In normal joints 5-HT did not sensitize fine afferent units for movement-induced responses. However, after inflammation, a distinct sensitization to such movements by 5-HT application could be observed both in group III and group IV fiber ranges. The sensitization had a short time course not exceeding 7 min. The tonic component of the movement-induced response was more enhanced than the phasic one. The bolus application of 5-HT led to temporary vasoconstriction of the knee joint vessels. This vasoconstriction was especially pronounced in inflamed joints and impeded the access of subsequently applied substances to the terminal regions of the afferent units under observation. It is concluded that the present results support the notion that 5-HT may participate in the mediation of pain from inflamed tissue such as an arthritic joint by exciting and sensitizing fine afferent units. During inflammation group III units are particularly sensitive to 5-HT and, thus, may carry the bulk of the 5-HT-induced nociceptive messages.

Acetylsalicylic acid activates antinociceptive brain-stem reflex activity in headache patients and in healthy subjects

The exteroceptive suppression (ES) of electrical activity in the temporal muscle is an inhibitory antinociceptive brain-stem reflex. We investigated whether aspirin can significantly modulate latencies or durations of the early (ES1) and late (ES2) exteroceptive suppression periods of electrical activity in the temporal muscle. Participating in the randomized double-blind crossover study were 20 patients with migraine without aura, 20 patients with tension-type headache, and 20 healthy subjects. ES1 and ES2 elicited by an electrical stimulus of 20 mA lasting 0.2 msec were recorded during maximal voluntary contraction of the mastication muscles before and 30 min after medication. In a randomized and double-blind fashion half of the subjects were given 1200 mg of aspirin in the form of an effervescent solution and the other half were given an identically tasting solution without aspirin. One week later the experiment was repeated with the substances exchanged in crossover fashion. The administration of placebo as well as aspirin caused a highly significant increase in ES1 duration (P less than or equal to 0.001). While aspirin caused a highly significant increase in ES2 duration (P less than or equal to 0.001) the taking of placebo showed no significant effect on ES2 duration. In giving aspirin as opposed to the placebo, there was a significant interaction between groups and drug effect on the latency of ES1; whereas in migraine patients and in patients with tension-type headache the latency of ES1 was reduced by administration of aspirin, it was increased in healthy subjects (P less than or equal to 0.05). Neither aspirin nor placebo significantly varied the ES2 latency.(ABSTRACT TRUNCATED AT 250 WORDS)

Neural changes in acute arthritis in monkeys. I. Parallel enhancement of responses of spinothalamic tract neurons to mechanical stimulation and excitatory amino acids

Somatosensory neurons of the spinal cord, including projection neurons, become hyperexcitable to mechanical stimuli during the development of experimental arthritis in rats and cats and hence are suggested to participate in the generation of arthritic hyperalgesia in humans. The experiments described here show a potentiation of the responses of spinothalamic tract (STT) neurons in monkeys during the development of an acute arthritis. The results demonstrate that the responses of STT neurons to mechanical stimuli and to iontophoretically applied excitatory amino acids (EAAs), particularly those acting at non-N-methyl-D-aspartate (non-NMDA) receptors, become enhanced during the development of inflammation produced by intra-articular injection of kaolin and carrageenan. Since the enhancement of both responses follows a similar time course, the results of this work suggest a role for EAAs in the hyperalgesia associated with arthritis and hence may provide a possible pharmacologic target for alleviation and/or prevention of arthritic pain.

Response properties of thick myelinated group II afferents in the medial articular nerve of normal and inflamed knee joints of the cat

This study investigated the responses to innocuous and noxious mechanical stimuli of joint mechanoreceptors with thick myelinated articular afferents (conduction velocities 21-65 m/sec) in the medial articular nerve of the cat's knee. In nine experiments, we examined whether acute arthritis would modify the discharge properties. The vast majority of the group II afferents were excited by gentle local stimuli and by movements in the working range of the knee. Although they encoded pressure and particular movement stimuli up to the noxious range, their responses were more closely related to the particular type of stimulus (e.g., a movement in a specific direction) than to its intensity (innocuous vs. noxious). In inflamed joints, the response patterns of group II units were similar with regard to local mechanical thresholds, thresholds for passive movements, and patterns of responses to passive movements. In both situations, most units had no resting activity. These results suggest that articular group II afferents do not play a significant role in nociception. Rather, they subserve proprioceptive functions such as deep pressure sensation and kinesthesia in normal as well as inflamed joints.

Central effect of the non-steroid anti-inflammatory agents, indomethacin, ibuprofen, and diclofenac, determined in C fibre-evoked activity in single neurones of the rat thalamus

This study aimed to investigate if the non-steroid anti-inflammatory agents, indomethacin, ibuprofen, and diclofenac, are capable of depressing sensory responses of the nociceptive system by a central action. For this purpose, experiments were carried out on rats under urethane anaesthesia in which activity was elicited by electrical stimulation of afferent C fibres in the sural nerve. Recordings were made ipsi- or contralaterally from single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus. The 3 drugs produced a dose-dependent depression of the evoked activity which amounted to about 60% of the controls at the highest doses employed and lasted longer than 60 min. Their potency ranking, according to the ED50 values (in brackets), is: indomethacin (5 mg/kg) greater than diclofenac (10.9 mg/kg) greater than ibuprofen (15.6 mg/kg). The results suggest that a central action might contribute to the analgesia produced by these non-steroid anti-inflammatory agents.

Excitation and sensitization of fine articular afferents from cat's knee joint by prostaglandin E2

1. In cats anaesthetized with alpha-chloralose extracellular recordings were made from fine afferent units belonging to the medial articular nerve of the knee joint. The excitatory and sensitizing effects on articular afferents of prostaglandin E2 (PGE2) applied intra-arterially close to the joint were examined. 2. Bolus injections of PGE2 doses of 0.03-30 micrograms excited about 60% of both the group III (conduction velocity 2.5-20 m/s) and the group IV units (conduction velocity less than 2.5 m/s). The duration and size of the responses were dose dependent consisting in most cases of low-frequency discharges which lasted up to several minutes. Excitation was found among afferents with low and high mechanosensitivity. 3. Among the group III units PGE2 sensitized 64% for their responses to movements and 50% for their responses to bradykinin (applied intra-arterially close to the joint). Sensitization did not depend on the mechanical threshold previous to chemical stimulation. Among the group IV units PGE2 sensitized only 25% for their responses to movements but 75% for their reactions to bradykinin. In group IV fibres a low mechanical threshold predisposed for sensitization to movements and a higher threshold for sensitization to bradykinin. 4. Some units were sensitized and excited, others were either sensitized or excited and some units were not affected by PGE2. We conclude that PGE2 induces in a large proportion of fine articular afferents of normal joints discharges which are similar to those induced by an experimental inflammation. Thus PGE2 may be an inflammatory mediator which has a major role in the generation of the afferent activity developing in the course of an arthritis.

Distribution of calcitonin gene-related peptide immunoreactivity in relation to the rat central somatosensory projection

The distribution of the neuropeptide calcitonin gene-related peptide (CGRP) was studied in relation to the known subcortical somatosensory pathways and contiguous systems in the central nervous system (CNS) of rats by using peroxidase histochemical methods in order to relate zones of immunoreactivity (IR) to cytoarchitecture. CGRP is the most ubiquitous peptide found to date in sensory ganglion cells: principally small and medium-size neurons emitting thin axons inferred to be largely nociceptive in function on the basis of the peripheral distribution of their terminals. Its apparent absence in sympathetic axons provides an especially useful sensory marker. The distribution of CGRP-IR axons displays remarkable selectivity at each level of the CNS. The trigeminal root distributes axons primarily to the pericornual layers (laminae I and II) of spinal V nucleus caudalis and to subnucleus oralis, evading the subnucleus interpolaris and contributing only few axons to principal V. Although there are only a few CGRP-IR somata at each level, heavily labeled axon trajectories can be traced to the nuclei of the solitary tract, the parabrachial nuclei, several sectors of the caudal medial thalamus, and the central nucleus of the amygdala. A sector of labeled neuron somata lies contiguous to each of these axon terminal zones, the largest of which is a thalamic nucleus containing cells of distinctive dendritic architecture extending from the periaqueductal gray across the posterior group nuclei to the peripeduncular nucleus, forming a linear array at the mesodiencephalic junction. The relation of CGRP-IR axonal distribution to spinothalamic, visceral, and gustatory systems is discussed in the context of a specialized "chemosensory" component of the thin-fiber somatosensory system.

Concerning the management of pain associated with herpes zoster and of postherpetic neuralgia

This simple method of achieving substantial pain control in patients with documented herpes zoster and postherpetic neuralgia has been effective in each of the patients in whom it has been used (the most recent 12 cases have been summarized for this report). It has been more effective than narcotic analgesics, oral anti-inflammatory analgesics, sedatives, tranquilizers, TENS, hypnosis and the wide variety of operative measures we have tried in the past. Although it was initially used pragmatically, there is now a reasonable rationale for its effectiveness that can be proposed based on more recent insights into the anatomy and neurophysiology of cutaneous nociceptors and the neuropharmacology of aspirin. In view of the widely held persuasion that the management of pain syndromes associated with herpes zoster (especially severe postherpetic neuralgia) is an unsatisfactory and frustrating venture, it seemed reasonable to report these more favorable clinical observations.

Depression by morphine and the non-opioid analgesic agents, metamizol (dipyrone), lysine acetylsalicylate, and paracetamol, of activity in rat thalamus neurones evoked by electrical stimulation of nociceptive afferents

Pyrazolone and salicylic acid derivatives and the aniline derivative, paracetamol, are often classified as peripherally acting analgesic agents, while morphine is a centrally acting analgesic agent. Since indications exist that the non-opioid analgesic agents can also produce central effects, experiments were carried out on rats under urethane anaesthesia in which activity was recorded from single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus that was elicited by supramaximal electrical stimulation of nociceptive afferents in the sural nerve. In addition, activity was recorded in ascending axons of the spinal cord which was evoked by electrical stimulation of nociceptive afferents in the sural nerve. The substances studied were morphine, the pyrazolone derivatives, metamizol (dipyrone) and aminophenazone ('Pyramidon'), lysine acetylsalicylate, and paracetamol. All drugs were found to depress dose-dependently evoked activity in VDM neurones after intravenous (i.v.) injection. The ED50 of morphine in depressing evoked activity in VDM neurones is 0.05 mg/kg. Morphine also dose-dependently reduced activity in ascending axons of the spinal cord, the ED50 being 1.7 mg/kg. The ED50 of metamizol in depressing evoked activity in VDM neurones is 120 mg/kg, and that of aminophenazone is 22.7 mg/kg. The 2 ED50 values differ significantly. It has been found previously that metamizol increased nociceptive activity in some ascending axons and aminophenazone increased this activity in all ascending axons tested. The ED50 of lysine acetylsalicylate in depressing evoked activity in VDM neurones is 74 mg/kg. The drug did not reduce nociceptive activity in ascending axons of the spinal cord. The ED50 of paracetamol in depressing evoked activity in VDM neurones is 19.0 mg/kg. Paracetamol did not depress nociceptive activity in ascending axons of the spinal cord at a dose as high as 150 mg/kg administered by intraperitoneal injection. Naloxone (0.2 mg/kg i.v.) abolished the depressant effects of morphine but failed to reduce those of the non-opioid analgesic agents even at a high dose (1 mg/kg i.v.). Unlike morphine, the non-opioid analgesic agents did not completely block evoked activity in VDM neurones but only partially blocked their activation. The results suggest that the non-opioid analgesic agents tested can produce a central analgesic effect which, however, is weaker than that of morphine.