Review article: Associations between immune activation, intestinal permeability and the irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS), one of the most common gastrointestinal disorders, markedly impairing patients' quality of life. Drug development for IBS treatment has been hampered by the lack of understanding of IBS aetiology. In recent years, numerous data have emerged that suggest the involvement of immune activation in IBS, at least in a subset of patients.

AIM

To determine whether immune activation and intestinal permeabilisation are more frequently observed in IBS patients compared with healthy controls.

METHODS

The scientific bibliography was searched using the following keywords: irritable bowel syndrome, inflammation, immune activation, permeabilisation, intestine, assay, histology and human. The retrieved studies, including blood, faecal and histological studies, were analysed to provide a comprehensive and structured overview of the available data including the type of assay, type of inflammatory marker investigated or intestinal segment studied.

RESULTS

Immune activation was more frequently observed in IBS patients than in healthy controls. An increase in the number of mast cells and lymphocytes, an alteration in cytokine levels and intestinal permeabilisation were reported in IBS patients. No consistent changes in the numbers of B cells or enterochromaffin cells or in mucosal serotonin production were demonstrated.

CONCLUSIONS

The changes observed were modest and often heterogeneous among the studied population. Only appropriate interventions improving irritable bowel syndrome symptoms could highlight and confirm the role of immune activation in this pathophysiology.

State-dependent properties of a new T-type calcium channel blocker enhance Ca(V)3.2 selectivity and support analgesic effects

T-type calcium channels encoded by the Ca(V)3.2 isoform are expressed in nociceptive primary afferent neurons where they contribute to hyperalgesia and thus are considered as a potential therapeutic target to treat pathological pain. Here we report that the small organic state-dependent T-type channel antagonist TTA-A2 efficiently inhibits recombinant and native Ca(V)3.2 currents. Although TTA-A2 is a pan Ca(V)3 blocker, it demonstrates a higher potency for Ca(V)3.2 compared to Ca(V)3.1. TTA-A2 selectivity for T-type currents was demonstrated in sensory neurons where it lowered cell excitability uniquely on neurons expressing T-type channels. In vivo pharmacology in Ca(V)3.2 knockout and wild type mice reveal that TTA-A2-mediated antinociception critically depends on Ca(V)3.2 expression. The pathophysiology of irritable bowel syndrome (IBS) was recently demonstrated to involve Ca(V)3.2 in a rat model of this disease. Oral administration of TTA-A2 produced a dose-dependent reduction of hypersensitivity in an IBS model, demonstrating its therapeutic potential for the treatment of pathological pain. Overall, our results suggest that the high potency of TTA-A2 in the depolarized state strengthen its analgesic efficacy and selectivity toward pathological pain syndromes. This characteristic would be beneficial for the development of analgesics targeting T-type channels, in particular for the treatment of pain associated with IBS.

Functional dysbiosis within the gut microbiota of patients with constipated-irritable bowel syndrome

BACKGROUND

The role of the gut microbiota in patho-physiology of irritable bowel syndrome (IBS) is suggested by several studies. However, standard cultural and molecular methods used to date have not revealed specific and consistent IBS-related groups of microbes.

AIM

To explore the constipated-IBS (C-IBS) gut microbiota using a function-based approach.

METHODS

The faecal microbiota from 14 C-IBS women and 12 sex-match healthy subjects were examined through a combined strictly anaerobic cultural evaluation of functional groups of microbes and fluorescent in situ hybridisation (16S rDNA gene targeting probes) to quantify main groups of bacteria. Starch fermentation by C-IBS and healthy faecal samples was evaluated in vitro.

RESULTS

In C-IBS, the numbers of lactate-producing and lactate-utilising bacteria and the number of H(2) -consuming populations, methanogens and reductive acetogens, were at least 10-fold lower (P < 0.05) compared with control subjects. Concomitantly, the number of lactate- and H(2) -utilising sulphate-reducing population was 10 to 100 fold increased in C-IBS compared with healthy subjects. The butyrate-producing Roseburia - E. rectale group was in lower number (0.01 < P < 0.05) in C-IBS than in control. C-IBS faecal microbiota produced more sulphides and H(2) and less butyrate from starch fermentation than healthy ones.

CONCLUSIONS

A major functional dysbiosis was observed in constipated-irritable bowel syndrome gut microbiota, reflecting altered intestinal fermentation. Sulphate-reducing population increased in the gut of C-IBS and were accompanied by alterations in other microbial groups. This could be responsible for changes in the metabolic output and enhancement in toxic sulphide production which could in turn influence gut physiology and contribute to IBS pathogenesis.

Spinal cord plasticity and acid-sensing ion channels involvement in a rodent model of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common functional gastro-intestinal disorder characterized by intractable chronic abdominal pain. In this study, we examined the possible spinal mechanisms underlying colonic hypersensitivity (CHS) using a non-inflammatory rat model of IBS induced by rectal enemas of butyrate, a short-chain fatty acid. We hypothesized that spinal plasticity could be responsible for CHS and that ASIC channels, which are known to support pain-elicited currents in the spinal cord, could contribute to central sensitization in our model of IBS. First, in order to determine if visceral pain relies on changes in spinal activity, we analyzed Fos expression in the spinal cord of rats treated with butyrate following a challenge with repetitive noxious colorectal distension. We found that Fos immunoreactivity was increased in thoracic T10-11-12, lumbar L1-2-6 and sacral S1 spinal segments. In control rats treated with saline, noxious repetitive colorectal distensions evoked Fos expression only in L1-2-6 and S1 spinal segments. Secondly, intrathecal injection of PcTx1, a specific ASIC1A antagonist, in the lumbar spinal cord completely prevented the development of CHS induced by butyrate. ASIC1 and 2 mRNAs, especially ASIC1A, were upregulated in the lumbar spinal cord. ASIC1A could thus contribute to spinal sensitization in our model of IBS, as it is supported by spinal colocalization of ASIC1A and Fos proteins. The whole data pinpoint a potential critical role of thoracic spinal cord in non-inflammatory pain states such as IBS and suggest that ASIC channels are part of the molecular effectors of central sensitization leading to visceral pain.

Abstract 2255: In vivo study of serum cancer biomarkers and pain-induced bone metastasis of human prostate tumor in Nude rats

Bone is one of the most frequent sites of spread for many cancers such as breast, prostate, lung or kidney. Management of metastatic bone disease and associated pain can control the symptoms and prevent further complications such as the decrease of quality of life, pathological fracture or compression of the spinal cord.

Until recently, investigation of novel drugs in the treatment of cancer-induced bone pain was hampered by a lack of suitable models. The aim of this study was to characterize biochemical markers and behavioral pain responses in an experimental animal model of bone metastases induced by intracardiac injection of PC-3 human prostate cancer cells in Nude rats.

Firstly, potential differences of nociceptive responses between healthy Nude rats (classically used in cancer studies) and healthy SD rats (routinely used in pain studies) were assessed in 2 mechanical tests (electronic Von Frey and paw pressure tests) and 2 thermal tests (plantar and acetone tests). Pharmacological response to morphine was also assessed in both strains using the paw pressure test. Secondly, Nude rats were intracardiacally inoculated with PC3 cancer cells or RPMI medium and nociceptive responses were measured between tumor-bearing and non tumor-bearing Nude rats. Behavioral tests were carried out once a week during 4 consecutive weeks. In a satellite group, 15 days after tumor cell injection, 5 Nude rats received 4 intravenous injections of paclitaxel (5 mg/kg every 4 days). Development of bone metastases was monitored by ELISA quantification of serum biomarkers such as CTX-1 and TRACP 5b (bone resorption) and PINP (bone formation).

Nociceptive tests in healthy SD and Nude rats did not demonstrate any major difference of hypersensitivity between both strains. Morphine (3 mg/kg, s.c.) induced a similar and classically observed analgesic effect in both strains. Any significant difference between Nude rats inoculated or not with cancer cells was observed neither in mechanical nor in thermal hyperalgesia assays.

CTX-1 and TRACP-5b levels were significantly higher in tumor-bearing Nude rats than in non tumor-bearing ones. PINP level was lower in Nude rats injected with PC-3 cells than in non injected-rats. Paclitaxel significantly increased the life span of tumor-bearing Nude rats (ILS &gt; 120%) and affected the kinetic of the 3 serum biomarkers. At sacrifice, CTX-1 and TRACP-5b serum levels were decreased by 51% and 95% respectively, and PINP level was increased by 325% compared to the non-treated rats.

These results indicated that intracardiac injection of PC-3 human prostate cancer cells in Nude rats associated with bone formation/resorption serum biomarkers monitoring is a useful experimental in vivo model for bone metastases study and anti-cancer drug evaluation. Surprisingly, pain induced by experimental bone metastases was not demonstrated in this model with classical painful assays.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2255.

Lactobacillus acidophilus modulates intestinal pain and induces opioid and cannabinoid receptors

Abdominal pain is common in the general population and, in patients with irritable bowel syndrome, is attributed to visceral hypersensitivity. We found that oral administration of specific Lactobacillus strains induced the expression of mu-opioid and cannabinoid receptors in intestinal epithelial cells, and mediated analgesic functions in the gut-similar to the effects of morphine. These results suggest that the microbiology of the intestinal tract influences our visceral perception, and suggest new approaches for the treatment of abdominal pain and irritable bowel syndrome.

Interactive involvement of brain derived neurotrophic factor, nerve growth factor, and calcitonin gene related peptide in colonic hypersensitivity in the rat

BACKGROUND AND AIMS

Neutrophins are involved in somatic and visceral hypersensitivity. The action of nerve growth factor (NGF) on sensory neurones contributes to the development of referred colonic hypersensitivity induced by trinitrobenzene sulfonic acid (TNBS). Based on data on brain derived neurotrophic factor (BDNF) and calcitonin gene related peptide (CGRP) in pain, the aims of the present study were: (1) to investigate the involvement of BDNF and CGRP in this model of referred colonic hypersensitivity, (2) to test the effect of exogenous BDNF and CGRP on the colonic pain threshold, and (3) to investigate the relationship between BDNF, NGF, and CGRP by testing antineurotrophin antibodies or h-CGRP 8-37 (a CGRP antagonist) on bowel hypersensitivity induced by these peptides.

METHODS

Colonic sensitivity was assessed using a colonic distension procedure.

RESULTS

Anti-BDNF antibody and h-CGRP 8-37 reversed the induced decrease in colonic threshold (33.4 (2.1) and 40.3 (4.1) mm Hg, respectively, compared with a vehicle score of approximately 18 mm Hg; p<0.001). BDNF (1-100 ng/rat intraperitoneally) induced a significant dose dependent decrease in colonic reaction threshold in healthy rats. This effect was reversed by an anti-BDNF antibody and an anti-NGF antibody (33.4 (0.6) v 18.7 (0.7) mm Hg (p<0.001), anti-NGF v vehicle). NGF induced colonic hypersensitivity was reversed by h-CGRP 8-37 but not by the anti-BDNF antibody. Finally, antineurotrophin antibody could not reverse CGRP induced colonic hypersensitivity (at a dose of 1 microg/kg intraperitoneally).

CONCLUSION

Systemic BDNF, NGF, and CGRP can induce visceral hypersensitivity alone and interactively. This cascade might be involved in TNBS induced referred colonic hypersensitivity in which each of these peptides is involved.

Antidepressants and pain

Tricyclic antidepressants, together with anticonvulsants, are considered to be first-line drugs for the treatment of neuropathic pain. Antidepressants are analgesic in patients with chronic pain and no concomitant depression, indicating that the analgesic and antidepressant effects occur independently. The analgesia induced by these drugs seems to be centrally mediated but consistent evidence also indicates a peripheral site of action. Several pharmacological mechanisms account for their antinociceptive effect but the inhibition of monoamine transporters (and, consequently, the facilitation of descending inhibition pain systems) is implicated on the basis of mechanistic and knockout-mouse studies. However, pain is a common symptom of depression, and depression is frequent in chronic pain patients, supporting the hypothesis that pain and depression share some common biochemical mechanisms. We suggest that antidepressants have a genuine analgesic effect and that research into their mechanisms of action will help to facilitate the development of new drugs.

Circadian pattern of spontaneous behavior in monarthritic rats: a novel global approach to evaluation of chronic pain and treatment effectiveness

OBJECTIVE

Preclinical evaluation is an essential step in the assessment of new antiinflammatory or analgesic drugs. This study was undertaken to develop a new mode of evaluation of drug effectiveness based on behavior indicating well-being in a rat model of chronic inflammatory pain. We chose to examine the circadian pattern of spontaneous behavior.

METHODS

The work was performed with a model of chronic monarthritis induced by Freund's complete adjuvant. Variations in behavioral patterns during the time course of arthritis were analyzed. In a second phase, the impact of acetaminophen and 2 nonsteroidal antiinflammatory drugs (aspirin and celecoxib), which are currently used in clinical practice to treat chronic inflammation, was studied after 7 days of treatment.

RESULTS

The nocturnal pattern of activity of healthy rats comprised 3 main bursts. Chronic painful monarthritis altered this spontaneous pattern of nocturnal behavior (normal period of activity). Monarthritic rats showed a decrease in the total time spent in activity during the night, and lost their pattern of activity. These behavioral disturbances were reversed after long-term treatment with acetaminophen or celecoxib, with celecoxib appearing to be more effective. Aspirin was ineffective.

CONCLUSION

These results enabled us to test this new procedure as a means of assessing well-being or ill- being during stages of chronic inflammatory pain in rats, and the effectiveness of repeated pharmacologic treatments.

Rectal instillation of butyrate provides a novel clinically relevant model of noninflammatory colonic hypersensitivity in rats

BACKGROUND & AIMS

The treatment of irritable bowel syndrome (IBS), characterized by abdominal pain and bloating, is empirical and often poorly efficient. Research lacks suitable models for studying the pathophysiologic mechanisms of the colonic hypersensitivity and new pharmacologic targets. The present study aimed to develop a novel model of colonic hypersensitivity possessing several of the characteristics encountered in patients with IBS.

METHODS

Rats received enemas of a butyrate solution (8-1000 mmol/L) twice daily for 3 days. A time course was determined for colonic hypersensitivity (colorectal distention test) and referred cutaneous lumbar hyperalgesia (von Frey hairs). Macroscopic and histologic analyses were performed on colonic mucosa. The efficacy of morphine, U50488H (a kappa opioid agonist), and trimebutine on the 2 pain parameters was determined. Finally, the involvement of peptidergic C-fibers was evaluated using capsaicin-pretreated animals and treatments with calcitonin gene-related peptide (CGRP) and neurokinin 1 receptor antagonists.

RESULTS

Butyrate enemas induced a sustained, concentration-dependent colonic hypersensitivity and, to a lesser extent, a referred cutaneous mechanical hyperalgesia, particularly in female rats, but no macroscopic and histologic modifications of the colonic mucosa, as observed in patients with IBS. Both pain parameters were sensitive to morphine, U50488H, trimebutine, neonatal capsaicin treatment, and the CGRP receptor antagonist but not to the neurokinin 1 receptor antagonist.

CONCLUSIONS

These results present our noninflammatory model of chronic colonic hypersensitivity as a useful novel tool for studying IBS. The CGRP receptor antagonist-induced reduction of colonic hypersensitivity suggests that CGRP receptors may provide a promising target for treatment of IBS.

Decrease in non-selective, non-sustained attention induced by a chronic visceral inflammatory state as a new pain evaluation in rats

The aim of this study was to develop a new behavioral pain test based on the evaluation of cognitive capacity impairments in rats with colitis and to determine the impact of different acute analgesic treatments. Colitis was induced in rats by an enema containing 2,4,6-trinitrobenzen sulfonic acid. Visual non-selective, non-sustained attentional level was assessed by a new behavioral testing procedure. Animals were familiarized on three consecutive days with an open field containing four small, similar, familiar objects. On the day of testing, one of the objects was randomly replaced by a new one. Attentional level was determined by the ability of the rat to perceive this small modification to its familiar environment. The effect of morphine, acetaminophen, aspirin or ibuprofen treatment was assessed on testing day and compared with that observed during a Von Frey test to assess referred tactile hypersensitivity of the skin of the lower back. Rats with colitis had decreased attentional level but no change in their locomotor activity, interest in the environment or memory encoding. Morphine (1 mg/kg, s.c. and 10 microg/rat, i.t.) and acetaminophen (200 mg/kg, p.o.) had a beneficial effect on attentional level and on referred tactile hypersensitivity. Testing for the latter showed that aspirin and ibuprofen (400 mg/kg, p.o.) were ineffective. The decrease in visual non-selective, non-sustained attention induced by chronic inflammatory painful state can be relieved by effective analgesic treatments. This finding could lead to the development of a new behavioral test to assess spontaneous pain in chronic painful subjects.

Blockade of supraspinal 5-HT1A receptors potentiates the inhibitory effect of venlafaxine on wind-up activity in mononeuropathic rats

In mononeuropathic rats submitted to a C-fiber reflex responses paradigm, repeated administration (five successive injections every half-life) of 10 mg/kg, s.c. of venlafaxine, but not of 2.5 mg/kg, s.c., a mixed monoamine reuptake inhibitor with preferential inhibitory activity in 5-HT reuptake, induced a progressive reduction of spinal wind-up. Repeated co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 i.c.v. (50 microg/injection) significantly increased the effect of venlafaxine s.c., indicating that venlafaxine-induced inhibition of spinal wind-up in mononeuropathic rats is potentiated by blockade of central 5-HT1A receptors.

Venlafaxine-induced depression of wind-up activity in mononeuropathic rats is potentiated by inhibition of brain 5-HT1A receptor expression in vivo

Antinociceptive effects of inhibiting 5-HT1A receptor expression by intracerebroventricular administration of an antisense oligodeoxynucleotide were studied in mononeuropathic rats. A 7-day period of treatment with the antisense produced: (i) reduction of mechanical hyperalgesia in the neuropathic hindlimb starting from day 5 of treatment, (ii) decrease of the hypothermic effect of 8-OH-DPAT challenge on day 6 of treatment, and (iii) potentiation of the inhibitory effect of velafaxine on spinal wind-up activity on day 7 of treatment. Results suggest a counteracting role of somatodendritic 5-HT1A receptors of raphe nuclei neurons in the antinociceptive efficacy of antidepressants with serotonergic spectrum in neuropathic pain.

Evidence for an involvement of supraspinal delta- and spinal mu-opioid receptors in the antihyperalgesic effect of chronically administered clomipramine in mononeuropathic rats

The mechanisms of involvement of the opioidergic system in the antinociceptive effect of antidepressants remain to be elucidated. The present study was designed to determine what type of opioid receptors may be involved at the spinal and supraspinal levels in the antihyperalgesic effect of clomipramine, a tricyclic antidepressant commonly prescribed in the treatment of neuropathic pain. Its antihyperalgesic effect on mechanical hyperalgesia (paw pressure test) in rats induced by chronic constriction injury of the sciatic nerve was assessed after repeated administrations (five injections every half-life, a regimen close to clinical use). Naloxone administered at a dose of 1 mg/kg i.v., which blocks all opioid receptors, or at a low dose of 1 microg/kg i.v., which selectively blocks the mu-opioid receptor, inhibited the anti-hyperalgesic effect of clomipramine and hence indicated that mu-opioid receptor is involved. Depending on whether they are administered by the intracerebroventricular or intrathecal route, specific antagonists of the various opioid receptor subtypes [D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2 (CTOP), mu; naltrindole (NTI), delta; and nor-binaltorphimine (nor-BNI), kappa] differently modify the antihyperalgesic effect of chronically injected clomipramine. The effect was inhibited by intrathecal administration of CTOP and intracerebroventricular administration of naltrindole, whereas nor-BNI was ineffective whatever the route of injection. These results demonstrate a differential involvement of opioid receptors according to the level of the central nervous system: delta-receptors at the supraspinal level and mu-receptors at the spinal level. Clomipramine could act via a neuronal pathway in which these two receptors are needed.

Evidence for an antihyperalgesic effect of venlafaxine in vincristine-induced neuropathy in rat

Venlafaxine, a new antidepressant with fewer side effects, could be of interest to reduce neuropathic pain following antineoplasic drug treatment. In the present study, we demonstrated that venlafaxine inhibits hyperalgesia in a new rat model of neuropathy induced by the antineoplasic drug vincristine, and exerts its effect preferentially via supraspinal and spinal mechanisms.

Antihyperalgesic effect of levetiracetam in neuropathic pain models in rats

The purpose of this study was to assess, in rats, the antinociceptive effects of levetiracetam (i.p.), a novel antiepileptic drug, in acute pain tests and in two models of human neuropathic pain. Levetiracetam and carbamazepine contrasted morphine by an absence of effect in the tail flick and hot plate tests. In normal rats, carbamazepine failed to modify the vocalisation thresholds to paw pressure whereas levetiracetam slightly increased this threshold only at the highest dose (540 mg/kg) for 30 min. In the sciatic nerve with chronic constriction injury model, the highest dose of levetiracetam (540 mg/kg) and carbamazepine (30 mg/kg) reversed the hyperalgesia. In streptozocin-induced diabetic rats, levetiracetam dose-dependently increased the vocalization threshold from 17 to 120 mg/kg reaching a similar effect as 10 mg/kg of carbamazepine. These results indicate that levetiracetam induces an antihyperalgesic effect in two models of human neuropathic pain, suggesting a therapeutic potential in neuropathic pain patients.

The antihyperalgesic effect of venlafaxine in diabetic rats does not involve the opioid system

Venlafaxine (VFX) is a structurally novel antidepressant that inhibits reuptake of serotonin and norepinephrine but, unlike tricyclic antidepressants, has few side effects. The present work studies the antihyperalgesic effect of repeated administrations of VFX (five successive injections of 2.5, 5 or 10 mg/kg, s.c., every half-life) in diabetic rats with the paw pressure test and the effect of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) because an opioidergic mechanism is usually considered to be involved in the analgesic effect of antidepressants. VFX induced a significant dose-dependent increase in vocalization thresholds. This effect was not reversed by naloxone. Thus, we demonstrate a clear antinociceptive effect of VFX which, unlike that of most mixed tricyclic antidepressants, does not involve the endogenous opioid system.

Short-chain fatty acid enemas fail to decrease colonic hypersensitivity and inflammation in TNBS-induced colonic inflammation in rats

BACKGROUND AND AIMS

Short-chain fatty acid (SCFA) (especially butyrate) enemas are widely used to reduce symptoms associated with human inflammatory bowel disease. The purpose of this study was to evaluate their real effect on colonic sensitivity in rats.

METHODS

The effects of saline and SCFA enemas (acetate, propionate and particularly butyrate) were studied on visceral pain thresholds following colonic distension in control rats and in rats with colitis (instilled with trinitrobenzene sulfonic acid (TNBS)).

RESULTS

Butyrate enemas (40 mM twice daily for 14 days) decreased colonic pain thresholds in control rats; they did not reduce the TNBS-induced hypersensitivity, but on the contrary increased its duration (without modifying the inflammation score). This pronociceptive effect was confirmed in control rats receiving twice daily enemas of 80 mM for 3 days and two enemas of 240 mM of a butyrate solution. The other SCFA enemas did not modify the hypersensitivity of rats with colitis and induced proinflammatory effects.

CONCLUSIONS

The beneficial effect of SCFA (especially butyrate) enemas on hypersensitivity and inflammation in inflammatory bowel disease is questionable and needs to be thoroughly investigated in humans.

Paracetamol exerts a spinal, tropisetron-reversible, antinociceptive effect in an inflammatory pain model in rats

Experiments were performed in carrageenin-treated rats to study, the antinociceptive and anti-inflammatory effects of paracetamol intravenously (i.v.) or intrathecally (i.t.) injected on rats submitted to a mechanical noxious stimulus. The influence of intrathecal tropisetron, a 5 hydroxytryptamine(3) (5-HT(3)) receptor antagonist, on the antinociceptive effects of paracetamol, was also studied. Paracetamol induced a significant antinociceptive effect after (100, 200 and 300 mg/kg) i.v. and (50, 100 and 200 microg/rat) i.t. injection, but no change occurred on edema volume. The effect of paracetamol was totally inhibited by tropisetron (10 microg/rat, i.t.). The foregoing results demonstrate that, in conditions of inflammatory pain, paracetamol exerts a central antinociceptive effect involving spinal 5-HT(3) receptors, without inducing any anti-inflammatory action. These data, give further arguments to consider paracetamol as a central analgesic drug which must be distinguished from non-steroidal anti-inflammatory drugs (NSAIDs), which justifies the usual combination of paracetamol in post-operative pain.