Inhibition of anti-inflammatory drugs of prostaglandin production in cultured macrophages

[The therapy of pain in rheumatic joint-and spine diseases.]

The therapy of pain caused by rheumatic diseases above all must take into consideration the cause of the pain. In rheumatoid arthritis, especially in the early stages, inflammation is the primary cause of the pain. The pain decreases the inflammation subsides following the administration of non-steroidal anti-inflammatory drugs (NSAIDs), or corticosteroids, if necessary. The so-called disease modifying anti-rheumatic drugs do not influence the inflammation or consequently, the pain directly, but rather through mechanisms before the local joint process some of which are not exactly known. In later stages of the progressive joint degeneration the NSAIDs only have a limited effect regarding the inhibition of inflammation. In osteoarthrosis, in which the pain is caused by a secondary inflammation and increasingly by capsular, muscular and tendon involvement, the pain is only treated by NSAIDs in active inflammatory stages; otherwise, the treatment is physical activity and medication. In degenerative and static disorders of the spine the pain is caused predominantly by muscular bracing. Therefore, physical and especially gymnastic therapy play a major role. Whether muscle relaxants have an effect on muscle bracing is doubtful. If there is pressure on the ligaments and, in cases of vertebral dislocation with overstraining of the vertebral joints, therapy with local injections is indicated. The pain in osteoporosis is also predominantly muscular and must be treated accordingly. Above all, high doses of calcium and calcitonin are effective analgesics. Moreover, fluoride also acts as an analgesic once the osteoporosis has stabilized. In most cases fibromyalgia, which is mostly of a psychosomatic nature, cannot be influenced by medical therapy. Instead repeated attempts at treatment help to make the affliction chronic with neurotic fixation. Also, as a rule, myotonolytic and tranquilizing substances are not effective.

Persistence of NSAIDs at effect sites and rapid disappearance from side-effect compartments contributes to tolerability

BACKGROUND

Non-steroidal, anti-inflammatory drugs (NSAIDs) are still the most widely used drugs worldwide. The introduction of selective cyclooxygenase (COX)-2 inhibitors has led to compounds which appear less damaging to the gastrointestinal tract, but possibly more risky to the cardiovascular system than older drugs. None has as yet reached OTC-status.

OBJECTIVE

This situation necessitates an analysis of the characteristics of those older ones which - due to their relative safety - have achieved over-the-counter (OTC) status.

DESIGN

The pharmacodynamic and pharmacokinetic characteristics of non-selective COX inhibitors in OTC use were obtained from the literature by systematic search, examined and used to construct a coherent hypothesis why they achieved OTC status, i.e. effectiveness and relative safety at low doses.

RESULTS

Pharmacodynamic (COX-2 preferential, but not selective inhibition) and, more importantly, pharmacokinetic characteristics of some of the older compounds may make them particularly safe drugs if used at low (OTC) doses with treatment limited to a few days of intake. The reason why some NSAIDs are particularly active while being relatively free from side-effects may be due to their specific biodistribution and metabolism, leading to drug accumulation and persistence in inflamed tissue (effect compartment) together with fast clearance from the central compartment, including blood, vascular wall, heart and kidney, i.e., possible sideeffect compartments.

CONCLUSION

This specific pharmacokinetic behavior of some non-selective COX inhibitors, such as diclofenac and ibuprofen, may explain why these widely used, non-steroidal, anti-inflammatory compounds are relatively well suited for OTC use and why some are more appropriate for the therapy of certain pain conditions than others.

Von der Weidenrinde zu den Coxiben

Antiphlogistic analgesics comprise the most widely used class of drugs worldwide. These compounds derive more or less directly from three prototypes which were discovered about 130 years ago in Central Europe: acetylsalicylic acid (aspirin), acetanilide (the forerunner of acetaminophen), and phenazone. All of them are still available. Attempts to improve their effect/side effect spectrum and enhance their analgesic activity led to the development of animal models of inflammatory pain which allowed for the screening and discovery of the so-called aspirin-like drugs, also termed nonsteroidal antiinflammatory drugs (NSAIDs) or cyclooxygenase inhibitors. This group presently dominates the market despite the fact that all these compounds imply the risk of unwanted drug effects, including gastrointestinal ulcers, renal dysfunction, inhibition of blood coagulation, pseudoallergic reactions, and possibly also accelerated development of atherosclerosis. Attempts to reduce these unwanted drug effects on the basis of molecular pharmacological insights resulted in the development of the so-called selective cyclooxygenase-2 inhibitors which are presently discussed ambiguously. These compounds appear to go along with less gastrointestinal toxicity, they do not inhibit blood coagulation, and have a reduced propensity for causing pseudoallergic asthmatic attacks. They may, on the other hand, cause more unwanted cardiovascular effects than the traditional NSAIDs. Hope for further reduction of unwanted drug effects comes from the recently discovered role of glycinergic spinal pain control. It is hoped that new classes of analgesic compounds may result from these new glycinergic mechanisms.

Mechanism of action of paracetamol

Paracetamol (acetaminophen) is generally considered to be a weak inhibitor of the synthesis of prostaglandins (PGs). However, the in vivo effects of paracetamol are similar to those of the selective cyclooxygenase-2 (COX-2) inhibitors. Paracetamol also decreases PG concentrations in vivo, but, unlike the selective COX-2 inhibitors, paracetamol does not suppress the inflammation of rheumatoid arthritis. It does, however, decrease swelling after oral surgery in humans and suppresses inflammation in rats and mice. Paracetamol is a weak inhibitor of PG synthesis of COX-1 and COX-2 in broken cell systems, but, by contrast, therapeutic concentrations of paracetamol inhibit PG synthesis in intact cells in vitro when the levels of the substrate arachidonic acid are low (less than about 5 mumol/L). When the levels of arachidonic acid are low, PGs are synthesized largely by COX-2 in cells that contain both COX-1 and COX-2. Thus, the apparent selectivity of paracetamol may be due to inhibition of COX-2-dependent pathways that are proceeding at low rates. This hypothesis is consistent with the similar pharmacological effects of paracetamol and the selective COX-2 inhibitors. COX-3, a splice variant of COX-1, has been suggested to be the site of action of paracetamol, but genomic and kinetic analysis indicates that this selective interaction is unlikely to be clinically relevant. There is considerable evidence that the analgesic effect of paracetamol is central and is due to activation of descending serotonergic pathways, but its primary site of action may still be inhibition of PG synthesis. The action of paracetamol at a molecular level is unclear but could be related to the production of reactive metabolites by the peroxidase function of COX-2, which could deplete glutathione, a cofactor of enzymes such as PGE synthase.

Mechanisms of action of paracetamol and related analgesics

Paracetamol and salicylate are weak inhibitors of both isolated cyclooxygenase-1 (COX-1) and COX-2 but are potent inhibitors of prostaglandin (PG) synthesis in intact cells if low concentrations of arachidonic acid are available. The effects of both drugs are overcome by increased levels of hydroperoxides. At low concentrations of arachidonic acid, COX-2 is the major isoenzyme involved in PG synthesis when both COX-1 and COX-2 are present in cells. Therefore, paracetamol and salicylate may selectively inhibit PG synthesis involving COX-2 because the lower flux through this pathway produces lesser levels of the hydroperoxide, PGG(2), than the pathway involving COX-1. Apart from the lack of anti-inflammatory effect of paracetamol in rheumatoid arthritis, the clinical effects of paracetamol and salicylate are very similar and resemble those of the selective COX-2 inhibitors. A splice variant of COX-1, termed COX-3, may be a site of action of these drugs but, further work, particularly at low concentrations of arachidonic acid is required. We suggest that paracetamol, salicylate and, possibly, the pyrazolone drugs, such as dipyrone, may represent a distinct class of atypical NSAIDs which could be termed peroxide sensitive analgesic and antipyretic drugs (PSAADs).

The selection of non-steroidal anti-inflammatory agents for dermal delivery

An analysis has been conducted to show how the penetration of a selection of non-steroidal anti-inflammatory agents (NSAIDs) through the skin may be predicted. The calculations are based on physicochemical parameters that can be predicted using commercially available software. Where available the predictions compare favourably with the literature values. The bio-effectiveness of the NSAID will be a function of both its penetration through the skin and its potency. The variation in potency has also been considered. Most NSAIDs are carboxylic acids, therefore the pK(a) will be an important determinant in ionisation and hence permeation. pH partition behaviour into the skin has been considered together with the relative impact of decreased permeation but increased solubility with degree of ionisation.

Pharmacological basis for the therapy of pain and inflammation with nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs (NSAIDs) belong to the most frequently used drugs. The discovery of an inducible isoform of cyclo-oxygenase (COX-2) has led to an intensive worldwide search and the introduction of selective COX-2 inhibitors. In this review, recent advances in understanding the mechanism of action of NSAIDs and, in this context, clinical findings on NSAID-induced gastrointestinal side effects are summarized. This knowledge is important for the effective treatment of pain and inflammation, as well as for preventing serious and sometimes lethal gastrointestinal side effects.

Possible Contribution of Prostaglandin E2 to the antiproliferative effect of phosphodiesterase 4 inhibitors in human mononuclear cells

Phosphodiesterase (PDE) 4, mixed PDE3/4, and non-selective PDE inhibitors have been shown to inhibit the proliferation of human peripheral blood mononuclear cells (HPBM). The aim of the present study was to examine whether endogenous prostaglandins, in particular prostaglandin E2 (PGE2), are involved in mediating the antiproliferative actions of PDE inhibitors, by comparing their effects with drugs which elevate or mimic adenosine 3',5'-cyclic monophosphate (cAMP) through mechanisms other than PDE inhibition. Indomethacin significantly reduced the antiproliferative effects of the PDE4 inhibitors rolipram and CDP840 and the mixed PDE3/4 inhibitor zardaverine, increasing the IC50 values from 2.51 microM to >10 microM, 0.81 microM to 2.82 microM, and 1.58 microM to 4.82 microM, respectively (P < 0.05), but did not alter the effects of theophylline. Forskolin, PGE2, and dibutyryl cAMP also inhibited HPBM proliferation, and in the presence of indomethacin the effects of forskolin and dibutyryl cAMP were reduced (although this was not significant), whereas PGE2 was not affected. Rolipram, CDP840, zardaverine, and dibutyryl cAMP all produced a concentration-related increase in PGE2 production (P < 0.05, ANOVA), but theophylline significantly increased PGE2 production only at the highest concentration examined, 1000 microM. The ability of indomethacin to reduce the antiproliferative effects of rolipram, CDP840, and zardaverine, together with the fact that these drugs can stimulate PGE2 production, suggests that their antiproliferative actions may be mediated in part by stimulation of endogenous PGE2 production. In contrast, it appears that endogenous PGE2 is not critical for the antiproliferative actions of theophylline, forskolin, and dibutyryl cAMP in HPBM. These results establish the importance of co-ordinated regulation of the cAMP phosphodiesterase and cyclooxygenase-PGE2 systems for the regulation of lymphocyte function in man, and have clinical implications for therapeutic approaches to diseases associated with lymphocyte dysregulation.

Current concepts of the actions of paracetamol (acetaminophen) and NSAIDs

There is much uncertainty about the mechanism of action of paracetamol (acetaminophen). It is commonly stated that, unlike the non-steroidal anti-inflammatory drugs (NSAIDs), it is a weak inhibitor of the synthesis of prostaglandins. This conclusion is made largely from studies in which the synthesis of prostaglandins was measured in homogenized tissues. However, in several cellular systems, paracetamol is an inhibitor of the synthesis of prostaglandins with IC(50) values ranging from approximately 4 microM to 200 microM. Paracetamol is not bound significantly to plasma proteins and therefore the concentrations in plasma can be equated directly with those used in in vitro experiments. After oral doses of 1 g, the peak plasma concentrations of paracetamol are approximately 100 microM and the plasma concentrations are therefore in the range where marked inhibition of the synthesis of prostaglandins should occur in some cells. Paracetamol is metabolized by the peroxidase component of prostaglandin H synthase but the relationship of this to inhibition of the cyclooxygenase or peroxidase activities of the enzyme is unclear. Paracetamol is also metabolized by several other peroxidases, including myeloperoxidase, the enzyme in neutrophils which is responsible for the production of hypochlorous acid (HOCl). The metabolism of paracetamol by myeloperoxidase leads to the decreased total production of HOC1 by both intact neutrophils and isolated myeloperoxidase, even though the initial rate of production of HOC1 is increased. The IC(50) value, derived from inhibition of the total production of HOC1 by isolated myeloperoxidase, is 81 microM. Several NSAIDs inhibit functions of neutrophils in media containing low concentrations of protein but their effects, in contrast to that of paracetamol, are generally produced only at concentrations greater than those of the unbound drug in plasma during treatment with the NSAIDs. However, neutrophils isolated during treatment with NSAIDs, such as piroxicam, ibuprofen and indomethacin show decreased function. Paracetamol has little or no anti-inflammatory activity by itself but may potentiate the clinical activity of NSAIDs in the treatment of rheumatoid arthritis.

Viper venom-induced inflammation and inhibition of free radical formation by pure compound (2-hydroxy-4-methoxy benzoic acid) isolated and purified from anantamul (Hemidesmus indicus R. BR) root extract

The present investigation explored the possible venom neutralizing effect of a pure compound (2-hydroxy-4-methoxy benzoic acid) isolated and purified from the methanolic root extract of Hemidesmus indicus R.Rr. 2-OH-4-MeO benzoic acid possessed potent anti-inflammatory, antipyretic and antioxidant properties. The compound effectively neutralized inflammation induced by Vipera russelli venom in male albino mice and reduced cotton pellet-induced granuloma in rats. The compound produced a significant fall in body temperature in yeast-induced pyrexia in rats but did not change the normothermic body temperature. The compound effectively neutralized viper venom-induced changes in serum phosphatase and transaminase activity in male albino rats. It also neutralized free radical formation as estimated by TBAPS and superoxide dismutase activities. The antisnake venom activity of the pure compound is partly mediated through the above physiological process.

Non-steroidal anti-inflammatory agents, Part 20. Method for testing non-steroidal anti-inflammatories: the modified hen's egg chorioallantoic membrane test (HET-CAM test) compared to other procedures

The delay of onset of irritation phenomena at the chorioallantoic membrane of incubated hen's eggs, a parameter for anti-inflammatory activity, was determined for the pharmaceutical substances diclofenac, flufenamic acid, ibuprofen, indomethacin, ketoprofen, piroxicam, phenylbutazone, salicylic acid, and sodium salicylate. Alongside questions relating to the dose-effect ratio, metabolisation, recovery, and diffusion of the substances to their site of action were investigated. The reproducibility of the procedure and its selectivity with regard to substances with a different mechanism of action is proven. The method allows classification of the substances according to their anti-inflammatory potency. However, correlation with the results of enzyme inhibition or in vivo results is only possible to a limited extent.

hP, the component of log P controlling structure-activity relationships amongst non-steroidal anti-inflammatory drugs

A range of 25 drugs and other compounds selected from published sources by strict criteria, has been used to confirm that their ability to inhibit the production of prostaglandin by mouse peritoneal macrophage does not correlate with log P but with hP, the parameter composed of only the hydrophobic contributions (atoms and groups composed of carbon, hydrogen and halogens). Other heteroatoms and physical properties can usually be ignored. Anti-inflammatory activity does not depend upon pKa or partition phenomena unless extreme, nor does it depend primarily upon the structural types within the range phenols, salicylic acids, mefenamic acids, areneacetic and profenic acids.

Anti-nociception induced by systemic or PAG-microinjected lysine-acetylsalicylate in rats. Effects on tail-flick related activity of medullary off- and on-cells

Previous experiments using metamizol have shown that this non-steroidal anti-inflammatory drug (NSAID) produces a central anti-nociceptive effect probably through neural substrates that also support the analgesic effects of opiates, such as the periaqueductal grey matter (PAG) and the off- and on-cells of the rostral ventromedial medulla (RVM). Off- and on-cells have been postulated to respectively inhibit and facilitate nociceptive transmission, since the heat-elicited tail flick reflex (TF) occurs only after off-cells have decreased (pause), and on-cells, have increased (burst) their activity. The aim of the present study was to examine whether the effect of metamizol upon TF and off- and on-cells responses could be generalized to other NSAIDs such as, in this case, lysine-acetylsalicylate (LASA). Fifty-nine off- and on-cells of the RVM were recorded in lightly anaesthetized rats. Systemic administration (200 and 300 mg/kg) or PAG microinjection (30, 50 and 100 micrograms) of LASA caused retardation of the heat-elicited off-cell pause, on-cell burst and the corresponding TF. Neuronal responses and TF retained their mutual time relationship but shifted simultaneously toward longer latencies. This anti-nociceptive effect of LASA was dose-dependent, present 5 min after administration and reached a maximum in 30 min for both administration methods. These data confirm that analgesics typically defined as peripherally-acting, such as metamizol and LASA in this study, may also have an anti-nociceptive effect by acting directly upon PAG, and suggest that this central effect involves the RVM off- and on-cells.

Nonsteroidal antiinflammatory drugs cause apoptosis and induce cyclooxygenases in chicken embryo fibroblasts

Programmed cell death (apoptosis) is an intrinsic part of organismal development and aging. Here we report that many nonsteroidal antiinflammatory drugs (NSAIDs) cause apoptosis when applied to v-src-transformed chicken embryo fibroblasts (CEFs). Cell death was characterized by morphological changes, the induction of tissue transglutaminase, and autodigestion of DNA. Dexamethasone, a repressor of cyclooxygenase (COX) 2, neither induced apoptosis nor altered the NSAID effect. Prostaglandin E2, the primary eicosanoid made by CEFs, also failed to inhibit apoptosis. Expression of the protooncogene bcl-2 is very low in CEFs and is not altered by NSAID treatment. In contrast, p20, a protein that may protect against apoptosis when fibroblasts enter G0 phase, was strongly repressed. The NSAID concentrations used here transiently inhibit COXs. Nevertheless, COX-1 and COX-2 mRNAs and COX-2 protein were induced. In some cell types, then, chronic NSAID treatment may lead to increased, rather than decreased, COX activity and, thus, exacerbate prostaglandin-mediated inflammatory effects. The COX-2 transcript is a partially spliced and nonfunctional form previously described. Thus, these findings suggest that COXs and their products play key roles in preventing apoptosis in CEFs and perhaps other cell types.

[Non-steroidal anti-inflammatory drugs (NSAIDs): Antirheumatic, anti-pyretic or analgesic drugs?]

This manuscript reviews some aspects of the unorthodox development of analgesics. Most of them were not discovered by systematic research for analgesics; rather they were developed primarily as antipyretic or antirheumatic drugs that later became analgesics due to clinical observations. One reason for this development may be that the definition and quantification of analgesic effects under clinical circumstances is rather difficult and a clear dose-response relationship almost impossible to obtain. These drawbacks limit the discovery of new analgesics. On the other hand, modern insights into the site of action and the mode of action of antipyretic analgesics are likely to further the discovery of new and better analgesics.

Modulation by stereoselective inhibition of cyclo-oxygenase of electromechanical coupling in the guinea-pig isolated renal pelvis

1. The effects of the (S)- and (R)-enantiomers of the cyclo-oxygenase (COX) inhibitor, ketoprofen, have been investigated on the spontaneous activity of the guinea-pig isolated renal pelvis and on electrical field stimulation-(EFS) induced contractions of the guinea-pig ureter in comparison with the effects of the achiral COX inhibitor, indomethacin. 2. (S)-ketoprofen (0.1-100 microM) produced a concentration- and time-dependent inhibition of the spontaneous myogenic activity of the renal pelvis. The maximal inhibitory effect (% inhibition of motility index) averaged 29, 42, 47 and 56% inhibition of control values at 0.1, 1, 10 and 100 microM. The (R)-enantiomer was ineffective up to 10 microM. 3. Indomethacin (0.1-100 microM) likewise produced a concentration- and time-dependent inhibition of spontaneous motility of the isolated renal pelvis: its maximal inhibitory effect was larger than that produced by (S)-ketoprofen and averaged 21, 40, 69 and 95% inhibition of motility index at 0.1, 1, 10 and 100 microM respectively. In the presence of a maximally effective (100 microM) concentration of (S)-ketoprofen, 100 microM indomethacin produced > 90% inhibition of residual motility. 4. In the guinea-pig isolated ureter, phasic contractions were induced by EFS (5 ms pulse width, 60 V): (S)-ketoprofen (100-500 microM) had no effect on the EFS-evoked contractions. Indomethacin (100-500 microM) produced a concentration-dependent inhibition and/or suppression of the EFS-evoked contractions. When contraction of the ureter was evoked by 80 mM KCl, indomethacin produced about 30 and 80% inhibition at 100 and 300 microM, respectively, while (S)-ketoprofen (300 microM) was ineffective. 5. The effect of (S)-ketoprofen or indomethacin (10 microM each) on the propagation of myogenic impulses along the ureter was determined by use of a three chamber organ bath. The renal end of the ureter was electrically stimulated while recording the mechanical activity of the renal and bladder ends of the ureter: addition of either (S)-ketoprofen or indomethacin (10 microM) did not effect propagation of impulses from the renal to the bladder end of the ureter, while nifedipine (10 microM) promptly blocked the propagated contractions. 6. In sucrose gap experiments, (S)-ketoprofen (10-100 microM) produced a time-dependent shortening of spontaneous action potentials of the guinea-pig renal pelvis and reduced the amplitude and duration of the accompanying phasic contractions. Indomethacin (10 microM) produced comparable effects on the same parameters and significantly reduced the maximal amplitude of depolarization of the pacemaker potential. In the presence of 100 microM (S)-ketoprofen, 100 microM indomethacin promptly suppressed the residual pacemaker potential and contraction.7. Neither (S)-ketoprofen nor indomethacin (10 microM each for 60 min) affected the parameters of action potential and contraction of the guinea-pig ureter evoked by EFS. Both drugs produced a sustained membrane depolarization.8. The present findings demonstrate that stereoselective COX inhibition affects pacemaker potentials and contractility (electromechanical coupling) in the guinea-pig renal pelvis. The modulatory role of endogenous prostanoids involves an amplification of electromechanical coupling in the renal pelvis while excitability, contractility or propagation of impulses along the ureter appear almost independent of prostanoid generation. Previous reports of a total suppression of pyeloureteral motility by indomethacin may reflect a combination of COX inhibition and nonspecific effect on electromechanical coupling.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

Lack of activity of ketorolac in hot-plate test and serotonin binding capacity of brain membranes in rats

An increasing number of observations indicate that prostaglandin synthesis inhibition is not a satisfactory explanation for the antinociceptive activity of the non-steroidal anti-inflammatory drugs. In the hot-plate test performed 1 or 2 h after ketorolac at 40, 70 and 100 mg/kg i.p., the drug does not display any significant antinociceptive activity. Nor, at the two higher doses used, does it affect the cortical and pontine serotonin binding capacity of rat brain membranes 2 h after treatment. The data suggest that this lack of anti-nociceptive activity in the hot-plate test is associated with the drug's inability to affect the central serotoninergic system.

The spinal actions of nonsteroidal anti-inflammatory drugs and the dissociation between their anti-inflammatory and analgesic effects

The traditional classification of nonsteroidal anti-inflammatory drugs (NSAIDs) as exclusively 'peripherally acting' agents is no longer valid. For many of these agents there is a growing body of evidence in favour of an additional central mechanism for their anti-inflammatory and analgesic effects. This view is further supported by the recent discovery that a substantial component of the hyperalgesia and allodynia that characterise postinjury hypersensitivity occurs in the CNS, notably the spinal dorsal horn. An important corollary is that inhibition of central nociceptive processing may represent an important analgesic mode of action for those NSAIDs that are effective in the management of pain after tissue injury. Historically, attempts to group this heterogeneous class of compounds into a single entity are largely derived from the observation that the majority of clinically useful NSAIDs are weak organic acids (pKa 3 to 5), bind extensively to plasma albumin (= 99%), and inhibit (to varying degrees) prostaglandin synthesis. However, the significance of these various unifying features is becoming increasingly obscure. While inhibition of prostaglandin synthesis apparently remains an important analgesic mode of action for NSAIDs both in the periphery and the CNS, other mechanisms should be considered. Some NSAIDs, in addition to their effects on prostaglandin synthesis, also affect the synthesis and activity of other neuroactive substances believed to have key roles in processing nociceptive input within the dorsal horn. It has been argued that these other actions, in conjunction with inhibition of prostaglandin synthesis, may synergistically augment the effects of NSAIDs on spinal nociceptive processing. Despite much effort, it remains a formidable task to assess the significance of these differential mechanisms upon clinical pain states. In the meantime, however, it may be possible, on the basis of in vivo studies, to evaluate the impact of putative spinal analgesic mechanisms that are unrelated to inhibition of prostaglandin synthesis. This approach has recently been extended to include the identification of pharmacokinetic and clinical correlates of these derived in vivo parameters, and in this way attempt to demonstrate clinical relevance.

Central analgesic activity of nonsteroidal antiinflammatory drugs in animal and human pain models

Nonsteroidal antiinflammatory drugs (NSAIDs) often are considered peripherally acting analgesics. This traditional view has been challenged by several authors who have put forward evidence for a central analgesic action for both acidic and nonacidic NSAIDs. The most compelling evidence, reviewed in this report, comes from well-defined animal pain models in which central administration of low doses of NSAIDs provides effective analgesia. In humans, the nociceptive flexion reflex provides a valuable means of dissociating central and peripheral effects. The mechanism of the central action of NSAIDs remains unclear. Several lines of evidence implicate the inhibition of central prostaglandin synthesis, particularly in models with a strong hyperalgesic component to their pathophysiology. However, not all potent inhibitors of prostaglandin synthesis are active by central routes of administration, and it seems likely that other actions may be involved.

Stereoselective cyclooxygenase inhibition in cellular models by the enantiomers of ketoprofen

The pharmacological activity of rac-ketoprofen and its enantiomers was investigated in vitro using different cellular models. The effect of these compounds on arachidonic acid metabolism was assessed by measuring the inhibition of prostanoid generation under the action of several agonists. Thus, we have evaluated the inhibition of (1) thromboxane B2 synthesis in rabbit platelets and human polymorphonuclear leukocytes (PMNs), (2) prostaglandin E2 synthesis in three cultured cells, namely human umbilical vein endothelial cells (HUVEC), human keratinocytes, and mouse macrophage-like P388D1 cells. The IC50 values found for (+)-(S)-ketoprofen were in the range between 0.1 nM and 0.8 microM, being slightly lower in all models than those found for rac-ketoprofen (0.4 nM-3 microM). On the other hand (-)-(R)-ketoprofen showed inhibition of cyclooxygenase only at concentrations two or three orders of magnitude higher than those required for the (+)-(S) enantiomer. These results, obtained with cell types of relevance for inflammatory processes and with compounds of high optical purity, demonstrate that the prostanoid biosynthesis inhibition caused by the drug rac-ketoprofen is exclusively due to its dextrorotatory enantiomer.

In-vitro evaluation of 5-lipoxygenase and cyclo-oxygenase inhibitors using bovine neutrophils and platelets and HPLC

Inhibition of 5-lipoxygenase has been determined by monitoring the formation of leukotriene B4 and 5-hydroxyeicosatetraenoic acid in bovine polymorphonuclear leucocytes. For evaluating the inhibition of cyclo-oxygenase two different test systems are presented: the first uses 12-hydroxyheptadecatrienoic acid produced by bovine platelets as an indicator of the cyclo-oxygenase activity; the second test system monitors the prostaglandin E2 formation by bovine platelets. All arachidonic acid metabolites are quantified by reverse-phase HPLC with UV-detection.

Direct spinal effect of intrathecal acetaminophen on visceral noxious stimulation in rabbits

The aim of this study was to investigate the effect of intrathecal acetaminophen on visceral and somatic noxious stimulation in the intact, non-anesthetized rabbit. Sixteen rabbits had intrathecal catheters implanted surgically. Visceral noxious stimulation was induced by intestinal distension of the distal colon and somatic stimulation with increasing electrical current through skin electrodes placed in either the cervical or the lumbar area. The effect on visceral noxious stimulation was assessed following intrathecal injection of 0.5, 2.5 and 5 mg of acetaminophen and following 10 and 50 mg acetaminophen intravenously. Naloxone 0.2 mg and yohimbine 0.1 mg were administered intrathecally prior to intrathecal injection of acetaminophen 5 mg. A dose-dependent effect of intrathecal acetaminophen against the visceromotor reflex produced by intestinal distension was shown. No effects on thresholds to lumbar or cervical electrical stimulation or intestinal distension were observed following i.v. administration. Thresholds to noxious electrical stimulation were only significantly elevated at the lumbar level following i.t. injection of 5 mg acetaminophen. Naloxone failed to antagonize the effect of intrathecal acetaminophen, whereas intrathecal yohimbine attenuated the effect of intrathecal acetaminophen in both tests. In conclusion, a spinal, dose-dependent, naloxone-irreversible, and yohimbine-reversible effect of intrathecal acetaminophen on electrical and visceral noxious stimulation was demonstrated.

Pure enantiomers of 2-arylpropionic acids: tools in pain research and improved drugs in rheumatology

The mode of action of aspirinlike drugs in pain is widely referred to as inhibition of prostaglandin synthesis. Salicylic acid, however, at low doses, is an analgesic but not a potent anti-inflammatory agent. This "enigma" may be resolved by recent findings employing 2-arylpropionic acids. Pure enantiomers of these chiral drugs show a different pharmacodynamic and pharmacokinetic profile. Using pure enantiomers of flurbiprofen, ibuprofen, and ketoprofen, we could show that (1) R-enantiomers of these drugs are inverted to S-enantiomers to a different degree in different species, including humans, (2) the pharmacokinetic parameters of both pure enantiomers differ in a drug- and a species-specific manner, and (3) both enantiomers exert differential analgesic effects. It appears particularly interesting that R-flurbiprofen, for instance, which is not or only to a small extent inverted in humans and rats, is practically devoid of prostaglandin synthesis inhibition in vitro. Consequently, in line with current thinking, R-flurbiprofen is not toxic to the gastrointestinal tract and shows no anti-inflammatory effects. In contrast to current concepts, however, this enantiomer does exert analgesic activity in different models of pain and nociception. It is concluded that R-flurbiprofen and, possibly, other R-enantiomers of 2-arylpropionic acids may exert novel analgesic effects independently of peripheral prostaglandin synthesis inhibition in inflamed tissue.

Influence of prostaglandins on DNA and matrix synthesis in growth plate chondrocytes

Prostaglandins are locally produced in a number of tissues in response to a variety of stimuli, including local growth factors and systemic hormones. The present investigation characterizes prostaglandin effects on growth plate chondrocytes. Since cyclic adenosine monophosphate (cAMP) may act as a prostaglandin-stimulated second messenger, the effects of prostaglandins A1, D2, E1, E2, F2 alpha, and I2 (10(-10)-10(-6) M) on cAMP levels and thymidine incorporation were evaluated. The stimulation of cAMP and thymidine incorporation by the various prostaglandin metabolites were dose dependent and highly correlated (r = 0.99, p less than 0.001). The magnitude of the effect varied but was maximal at 10(-6) M for each of the prostaglandins. Prostaglandins of the E series (E1 and E2) were the most potent, causing significant effects at 10(-10) M and with maximal 12- and 13-fold increases in DNA synthesis after a 24 h exposure. Prostaglandins D2 and A1 maximally stimulated thymidine incorporation by 4.7- and 3.1-fold but caused significant increases only at 10(-8) M. Prostaglandins F2 alpha and I2 were the least stimulatory, producing small but significant increases in thymidine incorporation at 10(-6) M (30 and 100% stimulations). A causal relationship between cAMP and thymidine incorporation was further verified by the ability of dibutyryl-cAMP to increase DNA synthesis. Long-term chondrocyte cultures treated continuously with PGE2 demonstrated an increase in cell number, confirming the proliferative effect. Indomethacin did not alter the potent dose-dependent stimulations of chondrocyte DNA synthesis by TGF-beta 1, basic FGF, or PTH, indicating that these known mitogens act independently of prostaglandin metabolism. PGE2 was further examined for its effects of matrix synthesis. PGE2 inhibited collagen synthesis with a maximal 42% decrease but did not alter noncollagen protein synthesis. In contrast, PGE2 maximally increased sulfate incorporation by 35% and caused a small dose-dependent inhibition in alkaline phosphatase activity. Thus, prostaglandins alter DNA and matrix synthesis in growth plate chondrocytes and may have an important role in chondrocyte metabolism in the growth plate, fracture callus, and other areas of endochondral ossification.

Second messengers mediating activation of chloride current by intracellular GTP gamma S in bovine chromaffin cells

1. Intracellular mechanisms and second messengers involved in chloride current activation by intracellular GTP gamma S (guanosine 5'-O-(3-thiotriphosphate] in bovine chromaffin cells were studied using the whole-cell patch-clamp technique combined with measurements of intracellular calcium [Ca2+]i. 2. No correlation between the time of current activation and the appearance of [Ca2+]i transients was observed; intracellular introduction of sufficient EGTA (10 mM) to suppress the [Ca2+]i transients did not affect the current activation by GTP gamma S. 3. The cyclic nucleotides, cyclic AMP or cyclic GMP, did not activate the current when introduced intracellularly (50-250 microM). The ability of GTP gamma S to activate the current decreased when cyclic GMP (250 microM), together with MgATP (2 mM), was added to the perfusate. 4. Neomycin (0.5-1 mM), a presumed inhibitor of phospholipase C effectively prevented the current activation by GTP gamma S but it did not prevent [Ca2+]i transients. 5. Modulation of protein kinase C activity using specific inhibitors (H-7, 300 microM; polymyxin B, 400 U/ml) or activators (phorbol ester PMA, 100 nM, 20-90 min at 37 degrees C) did not affect the current activation by GTP gamma S nor did it cause current activation in the absence of GTP gamma S. 6. Activation of the current by GTP gamma S could be prevented by incubating the cells for 10-15 min with 2.5 microM p-bromophenacyl bromide (p-BPB), an inhibitor of phospholipase A2 activity. Exogenous arachidonic acid (5-10 microM), applied extracellularly or intracellularly, neither activated the current itself nor did it interfere with its activation by GTP gamma S. 7. Activation of the current by GTP gamma S could also be prevented by incubating the cells with 1 microM-nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, but not with indomethacin (2 microM), an inhibitor of cyclo-oxygenase pathway of arachidonic acid metabolism. 8. It is suggested that chloride current activation by GTP gamma S in bovine chromaffin cells involves G protein-mediated stimulation of phospholipase A2 activity and subsequent formation of lipoxygenase product(s) of arachidonic acid metabolism.

Dissociation between the antinociceptive and anti-inflammatory effects of the nonsteroidal anti-inflammatory drugs. A survey of their analgesic efficacy

The authors challenge the general view that the analgesic effect of the nonsteroidal anti-inflammatory drugs (NSAIDs) can be universally attributed to their inhibitory effects on the synthesis of peripherally formed prostaglandins. Analgesic activity by some of these compounds in the reduction of physiological pain elicited by a single noxious stimulus, or the treatment of acute pain which results from sudden trauma to otherwise healthy tissue, is better described as an antinociceptive effect. Single-dose studies in the dental pain model that have been conducted in double-blind conditions and included a placebo control group have been reviewed; those NSAIDs which are significantly superior to the reference compound aspirin 650mg and those which could represent real alternatives to the use of narcotics in certain situations for the management of acute pain have been identified. Azapropazone, diflunisal, naproxen, oxaprozin and tolmetin are all weak inhibitors of prostaglandin synthesis, yet they have been shown to be more effective than aspirin. In a model of joint pain, azapropazone 600mg has been shown to be as effective as pethidine (meperidine) 100mg despite being the weakest inhibitor of prostaglandin synthesis. Whether the antinociceptive effect of azapropazone acts at a peripheral or a central level, or both, is not clear; evidence for the effects of NSAIDs on the central nervous system (CNS) is discussed. Historically, the antinociceptive character of some NSAIDs is apparent in several studies in both animals and humans. More recently, experimental algesimetry models designed to distinguish the antinociceptive effects of NSAIDs include the use in humans of photoplethysmography and computer-supported infrared thermographic imaging.

Aspirin-like drugs may block pain independently of prostaglandin synthesis inhibition

Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics.

A comparison of synovial fluid concentrations of non-steroidal anti-inflammatory drugs with their in vitro activity

For NSAIDs it has been widely accepted that prostaglandin inhibition is their mechanism of action in clinical use. Yet many other actions have been described, although it is unclear to what extent these may contribute to clinical activity. This review attempts to relate some of the experimental activities of NSAIDs to concentrations of drugs which occur in clinical use. Since it is assumed that to be effective a drug must reach its target site of action, synovial fluid concentrations for NSAIDs are considered. The resulting analysis suggests that prostaglandin inhibition is a viable mode of action for most, if not all, NSAIDs. However, some NSAIDS may rely as much, if not more, on other actions for their anti-inflammatory effect.

Glucocorticoid-induced reduction of prostanoid synthesis in TPA-differentiated U937 cells is mainly due to a reduced cyclooxygenase activity

The molecular mechanism of the glucocorticoid-induced inhibition of prostanoid synthesis was investigated in human monoblastoid U937 tumor cells and phorbol ester (TPA)-differentiated U937 cells. Prostanoid synthesis was inhibited in TPA-differentiated U937 cells by glucocorticoids such as dexamethasone and prednisolone, whereas aldosterone and progesterone showed no inhibitory effect. None of these methods had any influence on prostanoid secretion of undifferentiated U937 cells. Receptor binding studies revealed the presence of glucocorticoid receptors in both undifferentiated and TPA-differentiated U937 cells (Kp approximately 5 x 10(-9)M), however, the number of receptors per cell was increased 10-fold in TPA-differentiated U937 cells. Expression of lipocortin I and II as measured by Western blot analysis was not affected by dexamethasone. In TPA-differentiated cells, dexamethasone decreased the activities of two enzymes essential for prostanoid synthesis, cyclooxygenase and phospholipase A2, by 60-70% and 30%, respectively. Cells pretreated with the translation inhibitor cyclohexmide and dexamethasone showed similar cyclooxygenase and phospholipase A2 activities as cells treated with cycloheximide alone. Western blot analysis demonstrated that the significantly decreased cyclooxygenase activity correlated with an inhibited protein synthesis. In this human macrophage-like model glucocorticoids thus interfere at least at two levels with prostanoid synthesis by inhibiting the activities of phospholipase A2 as well as cyclooxygenase.

Differential inhibitory potencies of non-steroidal antiinflammatory drugs on smooth muscle prostanoid synthesis

In isolated rat aorta and urinary bladder, indomethacin inhibited the synthesis of the prostaglandins (PG) PGI2, PGE2, PGF2 alpha and TXA2 equipotently when PG synthesis was stimulated with excitatory receptor agonists (noradrenaline and carbachol), fluoride (a G protein activator), phorbol ester (a protein kinase C (PKC) activator) and calcium ionophore A23187 (a creator of artificial calcium channels). However, there was a marked right shift (30 fold) in the indomethacin concentration-inhibition curves when PG synthesis was stimulated by arachidonate (PG substrate) and trauma (freeze fracturing and sonication). Although less potent than indomethacin, the NSAIDs tiaprofenic acid and ibuprofen showed a similar disparity between the IC50s with the same PG stimulators. Since PG synthesis stimulated by receptor agonists, fluoride, phorbol ester and A23187 is dependent on calcium channel activation whereas trauma and arachidonate-stimulated PG synthesis bypass calcium channel activation, these data indicate that NSAIDs inhibit not only cyclooxygenase but also (and more potently) the mobilisation of Ca2+ linked to PG synthesis in these tissues.

Gastrointestinal ulcerations induced by anti-inflammatory drugs in rats. Physicochemical and biochemical factors involved

Aspirin, diclofenac, diflunisal, ibuprofen and indomethacin were given orally or intravenously to fasted or fed rats. The resulting gastric and intestinal damage was assessed using standard methods. The same drugs were administered to rats with biliary fistulas, and the fraction of drug excreted in bile was quantified using HPLC methods. We found that gastric damage occurred only in the fasted animals and was found to be dose-dependent and related to the amount (r = 0.871) and solubility (r = 0.909) of the individual drug. As far as acute gastric toxicity is concerned, neither the potency of a drug as an inhibitor of cyclo-oxygenase nor the fraction of unchanged or conjugated agent excreted in bile appeared to be relevant. Secondly, ulcerations of the small intestine occurred in fed animals only. The degree of damage was related to the amount of unchanged or conjugated drug excreted in bile and cyclo-oxygenase inhibitory potency (r = 0.873). The administered dose (within the range investigated) and drug solubility appeared not to contribute to intestinal toxicity. It is concluded that, in the rat, acute gastric and intestinal toxicity of non-steroidal anti-inflammatory drugs are due to different mechanisms. Whereas gastric toxicity is strongly influenced by the amount of drug dissolved under the pH conditions in the stomach, intestinal toxicity appears to depend on biliary excretion and enterohepatic circulation of a drug as well as on its potency as an inhibitor of prostaglandin synthesis.

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.

Pharmacological differences between R(-)- and S(+)-ibuprofen

Ibuprofen (IBU) is a non-steroidal anti-inflammatory drug exhibiting optical isomerism. Only the racemate is in clinical use. In in vitro studies it has been demonstrated that only the S(+)-enantiomer inhibits the PG synthetase system. Nevertheless, it is widely believed that the sole use of the active isomer does not comprise any advantages since the inactive isomer is converted within the human body. In a triple cross-over study (300 mg S(+), 300 mg R(-), 600 mg racemic IBU; n = 8), we could show that the converted R(-)-IBU after racemate administration provides for only one third of the AUC of S(+)-IBU obtained after S(+)-application. Highest S(+)-peak plasma levels were reached after S(+)-IBU, lower ones after racemate. We, therefore, studied 4 patients with classical rheumatoid arthritis treated with 2-3 doses of 500 mg of S(+)-IBU/day over a two week period. Significant clinical recovery (Ritchie-index p less than 0.01; analogue scale pain p less than 0.05, motion p less than 0.01) was reached after one week. The results indicate that a reduction of dose and of metabolic load is possible if the S(+)-enantiomer is administrated.

Comparative effects of azapropazone on cellular events at inflamed sites. Influence on joint pathology in arthritic rats, leucocyte superoxide and eicosanoid production, platelet aggregation, synthesis of cartilage proteoglycans, synovial production and actions of interleukin-1 in cartilage resorption correlated with drug uptake into cartilage in-vitro

Azapropazone (APZ) has been compared with standard NSAIDs in title systems to establish aspects of its mode of action on cellular events at inflamed sites. APZ (150 mg kg-1 day-1) given for 10-13 days exhibited a reduction in joint pathology in established adjuvant arthritis in rats comparable with that of indomethacin (2 mg kg-1 day-1) and clobuzarit (20 mg kg-1 day-1). APZ was shown to be a potent inhibitor of the production of leucocyte superoxide and synovial interleukin-1 (IL-1)-like activity and stimulated articular cartilage proteoglycan synthesis, but was ineffective as an inhibitor of platelet aggregation or IL-1 induced cartilage degradation in-vitro. These in-vitro effects may have relevance to the mode of action of this weak inhibitor of prostaglandin synthesis.

Clinical benefits of early cold therapy in accident and emergency following ankle sprain

One hundred and forty-three patients presenting with ankle sprains within 24 h of injury were entered into a double blind study. Treatment consisted of a standardized regime of high dose non-steroidal anti-inflammatory medication and an elastic support for all patients, who were then randomly allocated to two groups. One group received immediate cold therapy, the other received simulated therapy. Assessments made at 7 days showed a trend in favour of the group receiving cold therapy, although this did not reach significance. It is concluded that cold therapy together with compression may have a beneficial effect but that a single application in the accident and emergency department is not justified when a background therapy of non-steroidal anti-inflammatory medication is given.

Benefits of early anti-inflammatory medication following acute ankle injury

A total of 122 patients presenting with acute ankle injuries within 6 h of injury were entered into a double-blind study. Treatment consisted of a standardized regimen of physiotherapy and elastic support for all patients, who were then randomized into two groups. One group received immediate ibuprofen (2400 mg/day) while the other group received placebo medication for 48 h, followed by ibuprofen (2400 mg/day) from the 3rd day onwards, i.e. delayed antiinflammatory treatment. Assessments were made by means of a daily diary and also by clinical and radiological examination. The immediate treatment group demonstrated more rapid recovery by day 7 in terms of regression of swelling (P less than 0.01) and clinician's impression of severity (P less than 0.05). This group also tended to consider their ankle more able to bear weight at this stage (P = 0.05). In comparison with an earlier study, in which the only active treatment was an elastic support, a greater percentage of patients recovered earlier in the present study. The incidence of side-effects was low. Immediate high-dose ibuprofen is therefore recommended as treatment for moderate to severe acute ankle injuries.

Influence of renal failure, rheumatoid arthritis and old age on the pharmacokinetics of diflunisal

The single-dose plasma kinetics of diflunisal was studied in healthy young and old subjects, in patients with rheumatoid arthritis, and in patients with renal failure. The plasma and urine kinetics of the glucuronidated metabolites of diflunisal were studied in the healthy elderly subjects and in the patients with renal failure. In addition, the multiple-dose plasma kinetics of diflunisal was assessed in healthy volunteers and in patients with rheumatoid arthritis. After a single dose of diflunisal the terminal plasma half-life, mean residence time and apparent volume of distribution were higher in elderly subjects than in young adults. No difference was observed in any pharmacokinetic parameter between age-matched healthy subjects and patients with rheumatoid arthritis. The elimination half-life of unchanged diflunisal was correlated with the creatinine clearance (r = +0.89) and its apparent total body clearance exhibited linear dependence on creatinine clearance (r = +0.78). In patients with renal failure, the terminal plasma half-life and mean residence time of diflunisal were prolonged. The renal and apparent total body clearances were lower, the mean apparent volume of distribution was higher and the mean area under the concentration-time curve extrapolated to infinity (AUC) was greater in the renal failure patients than in controls. The plasma concentration of the glucuronidated metabolites rapidly rose to levels above those of unchanged drug in renal patients, whereas they were lower than those of unchanged diflunisal in controls. The AUC (0-96 h) of diflunisal glucuronides in the patients was four-times that in controls, and the terminal elimination half-life of the glucuronides was prolonged in them. The renal excretion and clearance of diflunisal glucuronides were reduced when renal function was impaired. After multiple dosing, the pre-dose steady-state plasma-concentration increased with decreasing creatinine clearance (r = -0.79). When the plasma concentration exceeded 200 mumols.l-1, the elimination half-life was doubled, due to partial saturation of diflunisal conjugation. This finding suggests that lower doses could be used in long-term treatment. Thus, old age and arthritic disease appear to have little influence on the kinetics of diflunisal in the absence of renal functional impairment. Ordinary doses can be given for short term treatment of elderly patients with or without RA. In patients with renal failure, however, reduced doses of diflunisal are recommended.

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.