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

An Updated Review on the Metabolite (AM404)-Mediated Central Mechanism of Action of Paracetamol (Acetaminophen): Experimental Evidence and Potential Clinical Impact

Paracetamol remains the recommended first-line option for mild-to-moderate acute pain in general population and particularly in vulnerable populations. Despite its wide use, debate exists regarding the analgesic mechanism of action (MoA) of paracetamol. A growing body of evidence challenged the notion that paracetamol exerts its analgesic effect through cyclooxygenase (COX)-dependent inhibitory effect. It is now more evident that paracetamol analgesia has multiple pathways and is mediated by the formation of the bioactive AM404 metabolite in the central nervous system (CNS). AM404 is a potent activator of TRPV₁, a major contributor to neuronal response to pain in the brain and dorsal horn. In the periaqueductal grey, the bioactive metabolite AM404 activated the TRPV₁ channel-mGlu5 receptor-PLC-DAGL-CB1 receptor signaling cascade. The present article provides a comprehensive literature review of the centrally located, COX-independent, analgesic MoA of paracetamol and relates how the current experimental evidence can be translated into clinical practice. The evidence discussed in this review established paracetamol as a central, COX-independent, antinociceptive medication that has a distinct MoA from non-steroidal anti-inflammatory drugs (NSAIDs) and a more tolerable safety profile. With the establishment of the central MoA of paracetamol, we believe that paracetamol remains the preferred first-line option for mild-to-moderate acute pain for healthy adults, children, and patients with health concerns. However, safety concerns remain with the high dose of paracetamol due to the NAPQI-mediated liver necrosis. Centrally acting paracetamol/p-aminophenol derivatives could potentiate the analgesic effect of paracetamol without increasing the risk of hepatoxicity. Moreover, the specific central MoA of paracetamol allows its combination with other analgesics, including NSAIDs, with a different MoA. Future experiments to better explain the central actions of paracetamol could pave the way for discovering new central analgesics with a better benefit-to-risk ratio.

Neonatal and maternal adverse outcomes and exposure to nonsteroidal anti-inflammatory drugs during early pregnancy in South Korea: A nationwide cohort study

BACKGROUND

Existing data on the use of nonsteroidal anti-inflammatory drugs (NSAIDs) during late pregnancy is well established, providing assurance. However, the use of NSAIDs during early pregnancy remains inconclusive owing to conflicting findings on adverse neonatal outcomes as well as the limited data on adverse maternal outcomes. Therefore, we sought to investigate whether early prenatal exposure to NSAIDs was associated with neonatal and maternal adverse outcomes.

METHODS AND FINDINGS

We conducted a nationwide, population-based cohort study using Korea's National Health Insurance Service (NHIS) database with a mother-offspring cohort constructed and validated by the NHIS to include all live births in women aged 18 to 44 years between 2010 and 2018. We defined exposure to NSAIDs as at least two records of NSAID prescriptions during early pregnancy (first 90 days of pregnancy for congenital malformations and first 19 weeks for nonmalformation outcomes) and compared against three distinct referent groups of (1) unexposed, no NSAID prescription during the 3 months before pregnancy start to end of early pregnancy; (2) acetaminophen-exposed, at least two acetaminophen prescriptions during early pregnancy (i.e., active comparator); and (3) past users, at least two NSAID prescriptions before the start of pregnancy but no relevant prescriptions during pregnancy. Outcomes of interest were adverse birth outcomes of major congenital malformations and low birth weight and adverse maternal outcomes of antepartum hemorrhage and oligohydramnios. We estimated relative risks (RRs) with 95% CIs using generalized linear models within a propensity score (PS) fine stratification weighted cohort that accounted for various potential confounders of maternal sociodemographic characteristics, comorbidities, co-medication use, and general markers of burden of illness. Of 1.8 million pregnancies in the PS weighted analyses, exposure to NSAIDs during early pregnancy was associated with slightly increased risks for neonatal outcomes of major congenital malformations (PS-adjusted RR, 1.14 [CI, 1.10 to 1.18]) and low birth weight (1.29 [1.25 to 1.33]), and for maternal outcome of oligohydramnios (1.09 [1.01 to 1.19]) but not antepartum hemorrhage (1.05 [0.99 to 1.12]). The risks of overall congenital malformations, low birth weight, and oligohydramnios remained significantly elevated despite comparing NSAIDs against acetaminophen or past users. Risks of adverse neonatal and maternal outcomes were higher with cyclooxygenase-2 selective inhibitors or use of NSAIDs for more than 10 days, whereas generally similar effects were observed across the three most frequently used individual NSAIDs. Point estimates were largely consistent across all sensitivity analyses, including the sibling-matched analysis. Main limitations of this study are residual confounding by indication and from unmeasured factors.

CONCLUSIONS

This large-scale, nationwide cohort study found that exposure to NSAIDs during early pregnancy was associated with slightly higher risks of neonatal and maternal adverse outcomes. Clinicians should therefore carefully weigh the benefits of prescribing NSAIDs in early pregnancy against its modest, but possible, risk of neonatal and maternal outcomes, where if possible, consider prescribing nonselective NSAIDs for <10 days, along with continued careful monitoring for any safety signals.

Systematic Review of Systemic and Neuraxial Effects of Acetaminophen in Preclinical Models of Nociceptive Processing

Acetaminophen (APAP) in humans has robust effects with a high therapeutic index in altering postoperative and inflammatory pain states in clinical and experimental pain paradigms with no known abuse potential. This review considers the literature reflecting the preclinical actions of acetaminophen in a variety of pain models. Significant observations arising from this review are as follows: 1) acetaminophen has little effect upon acute nociceptive thresholds; 2) acetaminophen robustly reduces facilitated states as generated by mechanical and thermal hyperalgesic end points in mouse and rat models of carrageenan and complete Freund’s adjuvant evoked inflammation; 3) an antihyperalgesic effect is observed in models of facilitated processing with minimal inflammation (eg, phase II intraplantar formalin); and 4) potent anti-hyperpathic effects on the thermal hyperalgesia, mechanical and cold allodynia, allodynic thresholds in rat and mouse models of polyneuropathy and mononeuropathies and bone cancer pain. These results reflect a surprisingly robust drug effect upon a variety of facilitated states that clearly translate into a wide range of efficacy in preclinical models and to important end points in human therapy. The specific systems upon which acetaminophen may act based on targeted delivery suggest both a spinal and a supraspinal action. Review of current targets for this molecule excludes a role of cyclooxygenase inhibitor but includes effects that may be mediated through metabolites acting on the TRPV1 channel, or by effect upon cannabinoid and serotonin signaling. These findings suggest that the mode of action of acetaminophen, a drug with a long therapeutic history of utilization, has surprisingly robust effects on a variety of pain states in clinical patients and in preclinical models with a good therapeutic index, but in spite of its extensive use, its mechanisms of action are yet poorly understood.

Pharmacological hypotheses: Is acetaminophen selective in its cyclooxygenase inhibition?

The precise mechanistic action of acetaminophen (ACT; paracetamol) remains debated. ACT's analgesic and antipyretic actions are attributed to cyclooxygenase (COX) inhibition preventing prostaglandin (PG) synthesis. Two COX isoforms (COX1/2) share 60% sequence structure, yet their functions vary. COX variants have been sequenced among various mammalian species including humans. A COX1 splice variant (often termed COX3) is purported by some as the elusive target of ACT's mechanism of action. Yet a physiologically functional COX3 isoform has not been sequenced in humans, refuting these claims. ACT may selectively inhibit COX2, with evidence of a 4.4-fold greater COX2 inhibition than COX1. However, this is markedly lower than other available selective COX2 inhibitors (up to 433-fold) and tempered by proof of potent COX1 inhibition within intact cells when peroxide tone is low. COX isoform inhibition by ACT may depend on subtle in vivo physiological variations specific to ACT. In vivo ACT efficacy is reliant on intact cells and low peroxide tone while the arachidonic acid concentration state can dictate the COX isoform preferred for PG synthesis. ACT is an effective antipyretic (COX2 preference for PG synthesis) and can reduce afebrile core temperature (likely COX1 preference for PG synthesis). Thus, we suggest with specificity to human in vivo physiology that ACT: (i) does not act on a third COX isoform; (ii) is not selective in its COX inhibition; and (iii) inhibition of COX isoforms are determined by subtle and nuanced physiological variations. Robust research designs are required in humans to objectively confirm these hypotheses.

Acetaminophen Exerts an Analgesic Effect on Muscular Hyperalgesia in Repeated Cold-Stressed Rats through the Enhancement of the Descending Pain Inhibitory System Involving Spinal 5-HT3 and Noradrenergic α2 Receptors

Musculoskeletal and psychological complaints have increased with the widespread use of visual display terminals, and musculoskeletal pain is known to be closely related to stress. One method of experimentally inducing persistent muscle pain is repeated cold stress (RCS), and animals exposed to such stress exhibit a dysfunction in the descending pain inhibitory system. Acetaminophen (N-acetyl-p-aminophenol; APAP) is widely used to relieve several types of pain, including musculoskeletal pain, and is available as an OTC drug. However, the mechanism underlying its analgesic action has not yet been fully elucidated. In this study, we compared the analgesic effect of APAP on RCS-induced muscular hyperalgesia with those of other analgesics to identify its mechanism of action. The daily oral administration of APAP significantly suppressed the decrease in the mechanical withdrawal threshold caused by RCS, similar to the results for neurotropin but not for the cyclooxygenase inhibitor ibuprofen (IBP). Moreover, the intrathecal administration of antagonists of the 5-hydroxytryptamine (5-HT)3 receptor or α2-adrenoceptor significantly abolished the analgesic effect of APAP but not of IBP. These results suggest that the analgesic effect of APAP on RCS-induced muscular pain might be exerted due to the activation of the descending pathways involving the spinal 5-HT3 receptor or α2-adrenoceptor.

Evaluation of KP-1199: a novel acetaminophen analog for hemostatic function and antinociceptive effects

BACKGROUND

Acetaminophen (APAP) is a widely self-prescribed analgesic for mild to moderate pain, but overdose or repeat doses can lead to liver injury and death. Kalyra Pharmaceuticals has developed a novel APAP analog, KP-1199, currently in Phase 1 clinical studies, which lacks hepatotoxicity. In this study, the authors evaluated the antinociceptive effect of KP-1199 on thermal injury-induced nociceptive behaviors as well as hemostatic parameters using human blood samples.

METHODS

Full-thickness thermal injury was induced in anesthetized adult male Sprague-Dawley rats. On day 7 post-injury, KP-1199 (30 and 60 mg/kg) or APAP (60 mg/kg) was administered orally. Antinociception of KP-1199 and APAP were assessed at multiple time points using Hargreaves' test. In separate experiments, human whole blood was collected and treated with either KP-1199, APAP, or Vehicle (citrate buffer) at 1× (214 μg/ml) and 10× (2140 μg/ml) concentrations. The treated blood samples were assessed for: clotting function, thrombin generation, and platelet activation.

RESULTS

APAP did not produce antinociceptive activity. KP-1199 treatment significantly increased the nociceptive threshold, and the antinociceptive activity persisted up to 3 h post-treatment. In human samples, 10× APAP caused significantly prolonged clotting times and increased platelet activation, whereas KP-1199 had caused no negative effects on either parameter tested.

CONCLUSION

These results suggest that KP-1199 possesses antinociceptive activity in a rat model of thermal injury. Since KP-1199 does not induce platelet activation or inhibit coagulation, it presents an attractive alternative to APAP for analgesia, especially for battlefield or surgical scenarios where blood loss and blood clotting are of concern.

Paracetamol (acetaminophen): A familiar drug with an unexplained mechanism of action

Paracetamol (acetaminophen) is undoubtedly one of the most widely used drugs worldwide. As an over-the-counter medication, paracetamol is the standard and first-line treatment for fever and acute pain and is believed to remain so for many years to come. Despite being in clinical use for over a century, the precise mechanism of action of this familiar drug remains a mystery. The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to the inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted by paracetamol and on the nature of the molecular interactions with these enzymes. Paracetamol has been proposed to selectively inhibit COX-2 by working as a reducing agent, despite the fact that in vitro screens demonstrate low potency on the inhibition of COX-1 and COX-2. In vivo data from COX-1 transgenic mice suggest that paracetamol works through inhibition of a COX-1 variant enzyme to mediate its analgesic and particularly thermoregulatory actions (antipyresis and hypothermia). A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules. AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems. The current review gives an update and in some cases challenges the different theories on the pharmacology of paracetamol and raises questions on some of the inadequately explored actions of paracetamol. List of Abbreviations: AM404, N-(4-hydroxyphenyl)-arachidonamide; CB1R, Cannabinoid receptor-1; Cmax, Maximum concentration; CNS, Central nervous system; COX, Cyclooxygenase; CSF, Cerebrospinal fluid; ED50, 50% of maximal effective dose; FAAH, Fatty acid amidohydrolase; IC50, 50% of the maximal inhibitor concentration; LPS, Lipopolysaccharide; NSAIDs, Non-steroidal anti-inflammatory drugs; PGE2, Prostaglandin E2; TRPV1, Transient receptor potential vanilloid-1.

Analgesic Effect of Acetaminophen: A Review of Known and Novel Mechanisms of Action

Acetaminophen is one of the most commonly used analgesic agents for treating acute and chronic pain. However, its metabolism is complex, and its analgesic mechanisms have not been completely understood. Previously, it was believed that acetaminophen induces analgesia by inhibiting cyclooxygenase enzymes; however, it has been considered recently that the main analgesic mechanism of acetaminophen is its metabolization to N-acylphenolamine (AM404), which then acts on the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid 1 receptors in the brain. We also recently revealed that the acetaminophen metabolite AM404 directly induces analgesia via TRPV1 receptors on terminals of C-fibers in the spinal dorsal horn. It is known that, similar to the brain, the spinal dorsal horn is critical to pain pathways and modulates nociceptive transmission. Therefore, acetaminophen induces analgesia by acting not only on the brain but also the spinal cord. In addition, acetaminophen is not considered to possess any anti-inflammatory activity because of its weak inhibition of cyclooxygenase (COX). However, we also revealed that AM404 induces analgesia via TRPV1 receptors on the spinal dorsal horn in an inflammatory pain rat model, and these analgesic effects were stronger in the model than in naïve rats. The purpose of this review was to summarize the previous and new issues related to the analgesic mechanisms of acetaminophen. We believe that it will allow clinicians to consider new pain management techniques involving acetaminophen.

Intraoperative Peritoneal Interleukin-6 Concentration Changes in Relation to the High-Mobility Group Protein B1 and Heat Shock Protein 70 Levels in Children Undergoing Cholecystectomy

The aim was the evaluation of IL-6 concentration in peritoneal lavage fluid of children which underwent cholecystectomy to ascertain if there is a difference in early inflammatory response depending on the type of surgical approach (open vs. laparoscopy). The analysis of high-mobility group protein B1 (HMGB1) and heat shock protein 70 (HSP70) was performed to find out if the source of IL-6 was related to tissue damage. IL-6 concentration in peritoneal lavage fluid samples, obtained at the beginning and at the end of the laparoscopic (N = 23) and open cholecystectomy (N = 14), was tested with a routinely used electrochemiluminescence assay. The concentrations of HMGB1 and HSP70 were analyzed with the use of an ELISA method. Statistical analysis was performed using the STATISTICA PL release 12.5 Program. The differences were assessed using the Mann-Whitney U test and Wilcoxon matched pairs test. Correlations were studied by using the Spearman correlation test. Our results demonstrated significant peritoneal lavage fluid IL-6 concentration growth measured at the end of the cholecystectomy as compared to the beginning, regardless of the type of the procedure. IL-6 growth during open cholecystectomy was greater compared to laparoscopic cholecystectomy (62.51-fold vs. 3.19-fold). IL-6 concentration did not correlate with HMGB1 and HSP70, which indicate that the significant growth of this cytokine was not related to mechanical tissue damage due to surgical procedure. A clinical significance of the study could be related to the fact that the evaluation of IL-6 concentration in peritoneal lavage fluid may be useful to assess an early local inflammatory response.

The Contribution of Serotonergic Receptors and Nitric Oxide Systems in the Analgesic Effect of Acetaminophen: An Overview of the Last Decade

Acetaminophen is a widely used analgesic and antipyretic agent. It is also available in over the counter formulations, which has increased its wide use. There have been many studies to date that have aimed to evaluate the mechanism of the analgesic action of acetaminophen. Additional to the inhibition of the cyclooxygenase pathway in the central nervous system, the involvement of opioidergic, cannabinoidergic, dopaminergic, cholinergic, and nitrergic systems as well as the contribution of descending pain inhibitory systems like the bulbospinal serotonergic pathway has been proposed as possible mechanisms of the analgesic action of acetaminophen. In this review, we aimed to collect the data from studies revealing the contribution of the central serotonergic system and the role of central nervous system-located serotonergic receptor subtypes in the analgesic effect of acetaminophen. While doing this, we mainly focused on the research that has been performed in the last ten years and tried to link the previous data with the lately added results. In addition to serotonergic system involvement, we also reviewed the role of nitric oxide in the analgesic action of acetaminophen, especially with the new findings reported over the last decade.

A Guide to Targeting the Endocannabinoid System in Drug Design

The endocannabinoid system (ECS) is one of the most crucial systems in the human organism, exhibiting multi-purpose regulatory character. It is engaged in a vast array of physiological processes, including nociception, mood regulation, cognitive functions, neurogenesis and neuroprotection, appetite, lipid metabolism, as well as cell growth and proliferation. Thus, ECS proteins, including cannabinoid receptors and their endogenous ligands’ synthesizing and degrading enzymes, are promising therapeutic targets. Their modulation has been employed in or extensively studied as a treatment of multiple diseases. However, due to a complex nature of ECS and its crosstalk with other biological systems, the development of novel drugs turned out to be a challenging task. In this review, we summarize potential therapeutic applications for ECS-targeting drugs, especially focusing on promising synthetic compounds and preclinical studies. We put emphasis on modulation of specific proteins of ECS in different pathophysiological areas. In addition, we stress possible difficulties and risks and highlight proposed solutions. By presenting this review, we point out information pivotal in the spotlight of ECS-targeting drug design, as well as provide an overview of the current state of knowledge on ECS-related pharmacodynamics and show possible directions for needed research.

Clinical evaluation of postoperative analgesia, cardiorespiratory parameters and changes in liver and renal function tests of paracetamol compared to meloxicam and carprofen in dogs undergoing ovariohysterectomy

Background

In veterinary medicine, the administration of nonsteroidal anti-inflammatory analgesics (NSAIDs) for the control of postsurgical pain in dogs and cats is common given the anti-inflammatory, analgesic, and antipyretic effects of these drugs. This study compared the serum biochemical changes and postoperative analgesic effects of paracetamol, meloxicam, and carprofen in bitches submitted to an ovariohysterectomy using the Dynamic Interactive Visual Analog Scale (DIVAS) and Pain Scale of the University of Melbourne (UMPS) scoring systems.

Methods

Thirty bitches of different breeds underwent elective ovariohysterectomies and were randomly assigned to one of three treatment groups: a paracetamol group [15 mg kg^-1 intravenous (IV)], a carprofen group (4 mg kg^-1 IV), and a meloxicam group (0.2 mg kg^-1 IV). All treatments were administered 30 minutes prior to surgery. Paracetamol was administered every 8 hours postoperatively for 48 hours total, while carprofen and meloxicam were intravenously administered every 24 hours. An evaluation of post-surgical pain was done with the DIVAS and the UMPS. The first post-surgical pain measurement was performed 1 hour after surgery and then 2, 4, 6, 8, 12, 16, 20, 24, 36, and 48 hours after surgery.

Results

All groups exhibited a gradual reduction in pain throughout the postoperative period in both scales; however, neither scale significantly differed between the three treatment groups (P > 0.05) during the 48 postoperative hours.

Conclusions

Paracetamol was as effective as meloxicam and carprofen for post-surgical analgesia in bitches subjected to elective ovariohysterectomy. The present study demonstrates that paracetamol may be considered a tool for the effective treatment of acute perioperative pain in dogs. Furthermore, this drug led to no adverse reactions or changes in the parameters assessed in the present study, indicating its safety.

Paracetamol is a centrally acting analgesic using mechanisms located in the periaqueductal grey

BACKGROUND AND PURPOSE

We previously demonstrated that paracetamol has to be metabolised in the brain by fatty acid amide hydrolase enzyme into AM404 (N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide) to activate CB₁ receptors and TRPV1 channels, which mediate its analgesic effect. However, the brain mechanisms supporting paracetamol-induced analgesia remain unknown.

EXPERIMENTAL APPROACH

The effects of paracetamol on brain function in Sprague-Dawley rats were determined by functional MRI. Levels of neurotransmitters in the periaqueductal grey (PAG) were measured using in vivo ¹ H-NMR and microdialysis. Analgesic effects of paracetamol were assessed by behavioural tests and challenged with different inhibitors, administered systemically or microinjected in the PAG.

KEY RESULTS

Paracetamol decreased the connectivity of major brain structures involved in pain processing (insula, somatosensory cortex, amygdala, hypothalamus, and the PAG). This effect was particularly prominent in the PAG, where paracetamol, after conversion to AM404, (a) modulated neuronal activity and functional connectivity, (b) promoted GABA and glutamate release, and (c) activated a TRPV1 channel-mGlu₅ receptor-PLC-DAGL-CB₁ receptor signalling cascade to exert its analgesic effects.

CONCLUSIONS AND IMPLICATIONS

The elucidation of the mechanism of action of paracetamol as an analgesic paves the way for pharmacological innovations to improve the pharmacopoeia of analgesic agents.

Nonsteroidal anti-inflammatory drugs and acetaminophen ameliorate muscular mechanical hyperalgesia developed after lengthening contractions via cyclooxygenase-2 independent mechanisms in rats

Nonsteroidal anti-inflammatory drugs and acetaminophen are cyclooxygenase inhibitors commonly used as symptomatic medicines for myofascial pain syndrome. Using the selective inhibitors celecoxib and zaltoprofen, cyclooxygenase-2 has been shown to be involved in the initiation, but not the maintenance, of muscular mechanical hyperalgesia induced by lengthening contractions, which serves as a useful model for the study of myofascial pain syndrome. The effect of other cyclooxygenase-2 inhibitors, such as acetylsalicylic acid, ibuprofen, loxoprofen sodium, and acetaminophen, on muscular mechanical hyperalgesia during maintenance has not been studied. Here, we examined the analgesic effects of the nonsteroidal anti-inflammatory drugs and acetaminophen on the model. Consistent with previous studies, mechanical withdrawal threshold of the muscle was significantly decreased and reached its lowest level 24 h after lengthening contractions. Celecoxib had no effect on muscular mechanical hyperalgesia, when orally administered 24 h after lengthening contractions. In contrast, acetylsalicylic acid, ibuprofen, loxoprofen sodium, and acetaminophen increased the withdrawal threshold, which had decreased by lengthening contractions, in a dose-dependent manner. These results demonstrate the analgesic actions of nonsteroidal anti-inflammatory drugs and acetaminophen in the maintenance process of lengthening contraction-induced muscular mechanical hyperalgesia, which may occur through cyclooxygenase-2 independent mechanisms.

Proteasome Activity and C-Reactive Protein Concentration in the Course of Inflammatory Reaction in Relation to the Type of Abdominal Operation and the Surgical Technique Used

Surgical tissue damage and the accompanying inflammatory response lead to proteasome activation, initiation of damaged protein degradation, and induction of acute-phase inflammatory response. The aim of this study was to investigate the rate of change in proteasome chymotrypsin-like (ChT-L) activity and C-reactive protein concentration depending on the degree of tissue damage and their correlation with prealbumin concentrations in children before and after abdominal surgery. This experimental study included children who underwent abdominal surgery between 2015 and 2017. Plasma prealbumin concentrations and C-reactive protein levels (CRP) were determined by standard biochemical laboratory procedures. Proteasome activity was assessed using a Suc-Leu-Leu-Val-Tyr-AMC peptide substrate. Elevation of plasma proteasome activity was noted in children after laparoscopic and open abdominal surgeries. However, 20S proteasome activity in children undergoing conventional open surgery was significantly higher (P < 0.05) than in patients subjected to laparoscopy. At the same time, an increase in the CRP level was observed. However, there was no correlation between C-reactive protein concentrations and the type of abdominal surgery while there was a correlation observed in the case of proteasomes. Proteasome activity correlates with the degree of surgical tissue damage and prealbumin concentrations. More invasive surgery leads to a stronger activation of the proteasome involved in removing proteins that were damaged due to the surgical procedure. Proteasomes are more specific markers because there is a correlation between proteasome activity and the type of abdominal surgery in contrast to C-reactive protein concentrations which are not different in response to surgery performed in regard to ovarian cysts or cholelithiasis.

Involvement of 5-HT1A/1B receptors in the antinociceptive effect of paracetamol in the rat formalin test

The mechanism of analgesic action of paracetamol (acetominophen) remains still unknown. However, a relationship between serotonergic system and the effect of paracetamol has been previously demonstrated. The serotonin activity in the brainstem is primarily under the control of 5-HT_1A somatodendritic receptors, although some data also suggest the involvement of 5-HT_1B receptors. To determine whether the 5-HT_1A and 5-HT_1B receptors are involved in the antinociceptive effect of paracetamol, we evaluated the effect of paracetamol (0.125-1 g/kg i.p.) followed by different antagonists [WAY 100,635 (0.8 mg/kg s.c.) and SB 216,641 (0.8 mg/kg s.c.)] or agonists [8-OH-DPAT (0.125 mg/kg s.c.) and CP 93,129 (0.125 mg/kg s.c.)] of 5-HT_1A and 5-HT_1B receptors, respectively, in the rat model of formalin-induced pain. We demonstrated that paracetamol administration showed a dose-dependent antinociceptive effect in the formalin test. WAY 100,635 (5-HT_1A antagonist) induced an increase in the antinociceptive effect of paracetamol at 250 mg/kg doses. Conversely, 8-OH-DPAT (5-HT_1A agonist) decreased the antinociceptive effect of paracetamol at 500-1000 mg/kg doses. However, SB216641 (5-HT_1B antagonist) modified weakly the antinociceptive effect of paracetamol at 250 mg/kg doses and CP 93,129 (5-HT_1B agonist) not produce a clear effect in the antinociceptive effect of paracetamol. These results suggest that the antinociceptive effect of paracetamol can be enhanced mainly by compounds having 5-HT_1A antagonist properties in the formalin test and maybe by 5-HT_1B receptors antagonists.

Proteasome and C-reactive protein inflammatory response in children undergoing shorter and longer lasting laparoscopic cholecystectomy

Operations of varying duration cause the release of a number of inflammatory mediators, in particular cytokines which lead to proteasome and acute-phase reactions. The purpose of this novel human study, was to characterize inflammatory response in children undergoing laparoscopic cholecystectomy, by analyzing changes in selected inflammatory mediators: C-reactive protein concentration and circulating 20S proteasome activity following surgical injury and to correlate them with the duration of the surgical procedure. Plasma C-reactive protein concentration (CRP) was determined by standard biochemical laboratory procedures. Proteasome activity in the plasma of children was assessed using Suc-Leu-Leu-Val-Tyr-AMC peptide substrate. Statistically significant increase in the plasma proteasome activity and C-reactive protein concentration, was noted (p < .05) in children after laparoscopic cholecystectomy. We found the correlation between the 20S proteasome activity and the length of the procedure. In children undergoing longer lasting laparoscopic cholecystectomy the proteasome activity was much higher than in patients having shorter surgical procedure. The CRP concentration and 20S proteasome activity significantly increase after surgery, but only 20S proteasome activity correlate with the length of the surgery. This may confirm that CRP is only an indicator of pathological state, while the function of the proteasomes is more complex because of their participation in the processes of repair and wound healing, and in the removal of damaged proteins.

Neonatal paracetamol treatment reduces long-term nociceptive behaviour after neonatal procedural pain in rats

BACKGROUND

Pain from skin penetrating procedures (procedural pain) during infancy in the neonatal intensive care unit (NICU) may result in changes of nociceptive sensitivity in later life. This supports the need for pain management during such vulnerable periods in life. This study, therefore, analyses the short- and long-term consequences of neonatal paracetamol (acetaminophen) treatment on pain behaviour in an experimental rat model of neonatal procedural pain.

METHODS

A repetitive needle-prick model was used, in which neonatal rats received four needle pricks into the left hind paw per day from postnatal day 0 to day 7 (P0-P7). Paracetamol (50 mg/kg/day s.c.) was administered daily (P0-P7), and sensitivity to mechanical stimuli was compared with a needle-prick/saline-treated group and to a tactile control group. At 8 weeks of age, all animals underwent an ipsilateral paw-incision, modelling postoperative pain, and the duration of hypersensitivity was assessed.

RESULTS

Neonatal paracetamol administration had no effect upon short-term mechanical hypersensitivity during the first postnatal week or upon long-term baseline sensitivity from 3 to 8 weeks. However, neonatal paracetamol administration significantly reduced the postoperative mechanical hypersensitivity in young adults, caused by repetitive needle pricking.

CONCLUSION

Paracetamol administration during neonatal procedural pain does not alter short-term or long-term effects on mechanical sensitivity, but does reduce the duration of increased postoperative mechanical hypersensitivity in a clinically relevant neonatal procedural pain model.

WHAT DOES THIS STUDY ADD

Paracetamol can be used safely in neonatal rats. Neonatal paracetamol treatment had no effect upon short-term mechanical hypersensitivity during the first postnatal week, nor upon long-term baseline sensitivity from 3 to 8 weeks. Paracetamol treatment during the first postnatal week significantly reduced the postoperative mechanical hypersensitivity in young adult rats.

Effect of Acetaminophen Alone and in Combination with Morphine and Tramadol on the Minimum Alveolar Concentration of Isoflurane in Rats

Background

It has been observed that acetaminophen potentiates the analgesic effect of morphine and tramadol in postoperative pain management. Its capacity as an analgesic drug or in combinations thereof to reduce the minimum alveolar concentration (MAC) of inhalational anesthetics represents an objective measure of this effect during general anesthesia. In this study, the effect of acetaminophen with and without morphine or tramadol was evaluated on the isoflurane MAC.

Methods

Forty-eight male Wistar rats were anesthetized with isoflurane in oxygen. MAC_ISO was determined from alveolar gas samples at the time of tail clamping without the drug, after administering acetaminophen (300 mg/kg), morphine (3 mg/kg), tramadol (10 mg/kg), acetaminophen (300 mg/kg) + morphine (3 mg/kg), and acetaminophen (300 mg/kg) + tramadol (10 mg/kg).

Results

The control and acetaminophen groups did not present statistically significant differences (p = 0.98). The values determined for MAC_ISO after treatment with acetaminophen + morphine, acetaminophen + tramadol, morphine, and tramadol were 0.98% ± 0.04%, 0.99% ± 0.009%, 0.97% ± 0.02%, and 0.99% ± 0.01%, respectively.

Conclusions

The administration of acetaminophen did not reduce the MAC of isoflurane and did not potentiate the reduction in MAC_ISO by morphine and tramadol in rats, and therefore does not present a sparing effect of morphine or tramadol in rats anesthetized with isoflurane.

Preventive effect of intrathecal paracetamol on spinal cord injury in rats

Background:

Ischemic injury of the spinal cord during the surgical repair of thoracoabdominal aortic aneurysms might lead to paraplegia. Although a number of different mechanisms have been proposed, the exact cause of paraplegia has remained unknown, hampering the development of effective pharmacologic or other strategies for prevention of this condition. A number of studies suggested that cyclooxygenases (COX) contribute to neural breakdown; thus, COX inhibitors might reduce injury.

Objectives:

We aimed to assess the preventive effect of intrathecal (IT) pretreatment with paracetamol on spinal cord injury in a rat model.

Materials and Methods:

This experimental study was performed in Ataturk University Animal Research Laboratory Center, Erzurum, Turkey. Adult male Wistar rats were randomly allocated to three experimental groups (n = 6) to receive IT physiologic saline (controls), 50 µg of paracetamol, or 100 µg paracetamol one hour before induction of spinal cord ischemia. Six other rats were considered as the sham group. For the assessment of ischemic injury, motor functions of the hind limbs and histopathologic changes of the lumbar spinal cord were evaluated. Additional 20 rats were divided into two equal groups for the second part of the study where the survival rates were recorded in controls and in animals receiving 100 µg of paracetamol during the 28-day observation period.

Results:

Pretreatment with 100 µg of paracetamol resulted in a significant improvement in motor functions and histopathologic findings (P < 0.05). Despite a higher rate of survival in 100 µg of paracetamol group (70%) at day 28, the difference was not statistically significant in comparison with controls.

Conclusions:

Our results suggest a protective effect of pretreatment with IT paracetamol on ischemic spinal cord injury during thoracolumbar aortic aneurysm surgery.

Effects of naloxone and flumazenil on antinociceptive action of acetaminophen in rats

BACKGROUND

Studies of acetaminophen suggest that multiple nociceptive pathways are involved in the drug's analgesic action.

OBJECTIVE

The purpose of this study was to determine whether naloxone and flumazenil were able to modify or antagonize the antinociceptive effect of acetaminophen in rats.

METHODS

Adult albino Wistar rats were used in the study and randomly allocated to 1 of 4 groups. The acetaminophen group (A group) was administered IP saline and then 300 mg/kg IP acetaminophen 5 minutes thereafter. The acetaminophen + naloxone group (AN group) was pretreated with 1 mg/kg IP naloxone, followed by 300 mg/kg IP acetaminophen 5 minutes later. The acetaminophen + flumazenil group (AF group) was pretreated with 1 mg/kg IP flumazenil, followed by 300 mg/kg IP acetaminophen 5 minutes later. The control group received 2.5 mL IP saline, followed by an additional 2.5 mL IP injection of saline 5 minutes later. The paw-withdrawal latency period of the rats was assessed by an investigator blinded to treatment using the hot-plate test at 30, 45, 60, and 90 minutes after administration of acetaminophen.

RESULTS

Thirty-two rats were evenly randomized by envelope method into 4 groups of 8 rats each. Baseline values for the A, AN, AF, and control groups were not significantly different (9.1 [2.3], 10.5 [2.7], 9.8 [3.0], and 8.9 [1.4] sec, respectively). In the AF group, flumazenil appeared to antagonize the analgesic effect exerted by the acetaminophen in the hot-plate test (30 min, 10.3 [3.7] sec; 45 min, 11.7 [5.1] sec; 60 min, 12.1 [5.1] sec; and 90 min, 12.2 [4.9] sec) and values were not significantly different from those obtained in the control group (30 min, 9.8 [2.2] sec; 45 min, 9.0 [1.6] sec; 60 min, 9.2 [1.6] sec; and 90 min, 8.5 [2.0] sec). In the AN group, naloxone did not significantly affect the values observed in the hot-plate test (30 min, 18.0 [4.5] sec; 45 min, 21.5 [7.8] sec; 60 min, 20.5 [5.9] sec; and 90 min, 22.3 [7.4] sec) and values at all time points were not significantly different from those obtained in the A group (30 min, 17.8 [7.6] sec; 45 min, 20.9 [6.9] sec; 60 min, 21.5 [7.3] sec; and 90 min, 23.8 [8.6] sec). All postbaseline values in the A and AN groups were significantly increased versus baseline and versus the control group values (all, P < 0.05). All postbaseline values in the A group were significantly greater than those in the AF group (all, P < 0.05).

CONCLUSION

Flumazenil antagonized the analgesic effect exerted by acetaminophen, while naloxone had no significant effect on acetaminophen's antinociceptive action in this pain model in rats.

Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol

To exert its analgesic action, paracetamol requires complex metabolism to produce a brain-specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T-type calcium-channel inhibition (Ca(v)3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Ca(v)3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Ca(v)3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Ca(v)3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Ca(v)3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Ca(v)3.2(-/-) mice. Our study, for the first time, (1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; (2) identifies the relationship between TRPV1 and the Ca(v)3.2 channel; and (3) suggests supraspinal Ca(v)3.2 inhibition as a potential pharmacological strategy to alleviate pain.

The analgesic efficacy of intra-articular acetaminophen in an experimental model of carrageenan-induced arthritis

BACKGROUND

Acetaminophen is one of the most common drugs used for the treatment of pain and fever.

OBJECTIVES

To examine the effects of intra-articular (IA) acetaminophen on carrageenan-induced arthritic pain-related behaviour and spinal c-Fos expression in rats.

METHODS

The present study was performed using 20 Sprague Dawley rats. Forty microlitres of IA 0.9% NaCl was injected in the control group, and 40 µL of IA carrageenan was injected in the carrageenan group. One hour after carrageenan injection, 400 µg of IA acetaminophen was injected in the IA acetaminophen group, and 400 µg of intraperitoneal (IP) acetaminophen was injected in the IP acetaminophen group. One day before injection, and 4 h and 8 h after injection, diameters of both knee joints, motility of the rat, paw loading and joint mobility were assessed. After the rats were euthanized, L3 and L4 spinal segments were excised for c-Fos assessment.

RESULTS

IA acetaminophen decreased both the severity and distribution of c-Fos expression. IP acetaminophen decreased only the distribution of c-Fos expression. IA acetaminophen decreased knee diameter at 8 h. IA and IP acetaminophen increased rat motility and paw loading scores. Joint mobility scores of IP acetaminophen were similar to saline at 8 h.

CONCLUSIONS

Results of the present study indicate an analgesic and⁄or possible anti-inflammatory effect of IA acetaminophen and provide further evidence on the efficacy of systemic acetaminophen injection in reducing arthritic pain.

The antinociceptive effect of reversible monoamine oxidase-A inhibitors in a mouse neuropathic pain model

Neuropathic pain is a debilitating condition that is often resistant to common analgesics, such as opioids, but is sensitive to some antidepressants, an effect that seems to be mediated by spinal cord 5-HT3 receptors. Because the analgesic potential of monoamine oxidase-A (MAO-A) inhibitors is understudied, we evaluated the potential antinociceptive effect of the reversible MAO-A inhibitors moclobemide and 2-(3,4-dimethoxy-phenyl)-4,5-dihydro-1H-imidazole (2-DMPI) in a mouse neuropathic pain model induced by chronic constriction injury (CCI) of the sciatic nerve. Neuropathic mice showed a decreased mechanical paw withdrawal threshold (PWT) 7 days after lesion compared with the baseline PWT, characterizing the development of hyperalgesia. Moclobemide (100-300 μmol/kg, s.c.) and 2-DMPI (30-300 μmol/kg, s.c.) treatments were able to reverse the CCI-induced hyperalgesia, with 50% inhibitory dose (ID50) values of 39 (18-84) and 11 (4-33) μmol/kg, and maximum inhibition (Imax) values of 88±14 and 98±15%, respectively, at the 300 μmol/kg dose. In addition, we observed a significant increase in the MAO-A activity in the lumbar spinal cord of CCI-submitted mice compared with sham-operated animals. Furthermore, the antihyperalgesic effects of both 2-DMPI and moclobemide were largely reversed by intrathecal injection of the 5-HT3 receptor antagonist ondansetron (10 μg/site). These results suggest a possible involvement of MAO-A in the mechanisms of neuropathic pain and a potential utility of the reversible inhibitors of MAO-A in the development of new therapeutic approaches to treat it.

The modern pharmacology of paracetamol: therapeutic actions, mechanism of action, metabolism, toxicity and recent pharmacological findings

Paracetamol is used worldwide for its analgesic and antipyretic actions. It has a spectrum of action similar to that of NSAIDs and resembles particularly the COX-2 selective inhibitors. Paracetamol is, on average, a weaker analgesic than NSAIDs or COX-2 selective inhibitors but is often preferred because of its better tolerance. Despite the similarities to NSAIDs, the mode of action of paracetamol has been uncertain, but it is now generally accepted that it inhibits COX-1 and COX-2 through metabolism by the peroxidase function of these isoenzymes. This results in inhibition of phenoxyl radical formation from a critical tyrosine residue essential for the cyclooxygenase activity of COX-1 and COX-2 and prostaglandin (PG) synthesis. Paracetamol shows selectivity for inhibition of the synthesis of PGs and related factors when low levels of arachidonic acid and peroxides are available but conversely, it has little activity at substantial levels of arachidonic acid and peroxides. The result is that paracetamol does not suppress the severe inflammation of rheumatoid arthritis and acute gout but does inhibit the lesser inflammation resulting from extraction of teeth and is also active in a variety of inflammatory tests in experimental animals. Paracetamol often appears to have COX-2 selectivity. The apparent COX-2 selectivity of action of paracetamol is shown by its poor anti-platelet activity and good gastrointestinal tolerance. Unlike both non-selective NSAIDs and selective COX-2 inhibitors, paracetamol inhibits other peroxidase enzymes including myeloperoxidase. Inhibition of myeloperoxidase involves paracetamol oxidation and concomitant decreased formation of halogenating oxidants (e.g. hypochlorous acid, hypobromous acid) that may be associated with multiple inflammatory pathologies including atherosclerosis and rheumatic diseases. Paracetamol may, therefore, slow the development of these diseases. Paracetamol, NSAIDs and selective COX-2 inhibitors all have central and peripheral effects. As is the case with the NSAIDs, including the selective COX-2 inhibitors, the analgesic effects of paracetamol are reduced by inhibitors of many endogenous neurotransmitter systems including serotonergic, opioid and cannabinoid systems. There is considerable debate about the hepatotoxicity of therapeutic doses of paracetamol. Much of the toxicity may result from overuse of combinations of paracetamol with opioids which are widely used, particularly in USA.

Influence of acetaminophen consumption and exercise on Achilles tendon structural properties in male Wistar rats

Chronic consumption of acetaminophen (APAP) during exercise training leads to a reduction in tendon stiffness and modulus compared with a placebo. We explored whether this effect could be due to a reduction in tendon collagen content or cross-linking. Ten-week-old male Wistar rats ( n = 50) were divided into placebo or APAP groups and into sedentary or treadmill-exercised groups. APAP (200 mg/kg) or saline was administered once daily by oral gavage. Rats in the exercise groups ran on a treadmill 5 days per week for 8 wk with progression to 60 min per day, 20 m/min, and 8° incline. After 8 wk, lyophilized Achilles tendon samples were assayed for the collagen-specific amino acid hydroxyproline and cross-linking [hydroxylyslpyridinoline (HP)] content by high-performance liquid chromatrography. Collagen content was not influenced by exercise or APAP ( P > 0.05). Compared with placebo, tendon water content was 7% ( P = 0.006, main effect) lower in animals consuming APAP (placebo: 54.79 ± 0.8%, APAP: 50.89 ± 1.2%). HP in the Achilles tendon was 36% greater (sedentary: 141 ± 15, exercise: 204 ± 26 mmol/mol collagen) in the exercise-trained rats independent of drug treatment ( P = 0.020, main effect). Independent of exercise, HP content was 33% lower ( P = 0.032, main effect) in the animals consuming APAP (placebo: 195 ± 21, APAP: 140 ± 19 mmol/mol collagen). Our data suggests that chronic consumption of APAP results in a reduction in collagen cross-linking and a loss of tissue water independent of chronic exercise. This reduction in cross-linking and water content could contribute to the decrease in tendon stiffness noted in humans chronically consuming APAP.

The antinociceptive effect of acetaminophen in a rat model of neuropathic pain

Acetaminophen is one of the most popular and widely used analgesics for the treatment of pain and fever but few studies have evaluated its effects on neuropathic pain. This study examined the effect of acetaminophen on thermal hyperalgesia, mechanical and cold allodynia in a rat model of neuropathic pain. Male Sprague-Dawley rats were prepared by tightly ligating the left L5 and L6 spinal nerves to produce a model of neuropathic pain. Sixty neuropathic rats were assigned randomly into six groups. Normal saline and acetaminophen (25, 50, 100, 200 and 300 mg/kg) were administered intraperitoneally to these individual groups. Thermal hyperalgesia, mechanical and cold allodynia were examined at preadministration and at 15, 30, 60, 90, 120, 180, 240 and 360 min after administering the drug. Mechanical allodynia was quantified by measuring the paw withdrawal threshold to stimuli with von Frey filaments. Cold allodynia was quantified by measuring the frequency of foot lift after applying 100% acetone. Thermal hyperalgesia was quantified by measuring the thermal withdrawal threshold. The rotarod performance was measured to detect any drug-induced adverse effects, such as drowsiness. The hepatic and renal adverse effect was also assessed by measuring the serum levels of aspartate aminotransferase, alanine aminotransferase, blood urea nitrogen and creatinine. The paw withdrawal thresholds to mechanical stimuli and the thermal withdrawal threshold were increased significantly and withdrawal frequencies to cold stimuli were reduced by acetaminophen administration in a dose-dependent manner. Acetaminophen reduces thermal hyperalgesia, mechanical and cold allodynia in a rat model of neuropathic pain, and might be useful for managing neuropathic pain.

Paracetamol and opioid pathways: a pilot randomized clinical trial

Previous studies suggest that the antinociceptive action of paracetamol (acetaminophen, APAP) might involve descending inhibitory pain pathways and the opioidergic system: this study explores this issue in humans with naloxone, the opioid antagonist. After ethical approval, 12 healthy male volunteers were included in this randomized, controlled, double-blind, crossover, four-arm study. They were administered intravenous paracetamol (APAP 1 g) or saline (placebo, pl) followed at 100 min with IV naloxone (Nal 8 mg) or saline, every week for 4 weeks. The amplitude of cerebral potentials evoked by thermal/painful stimuli applied on the arm was recorded nine times over 150 min, witnessing of pain integration at central level. Amplitude changes as well as areas under the curve (AUCs) over 150 min were compared for the four treatments by repeated measures ANOVA (significance 0.05). Amplitude changes were significant for APAP/pl vs. pl/pl at t150: -44% (95%CI -58 to -30) vs. -27% (95%CI -37 to -17; P < 0.05) but not vs. APAP/Nal. AUC (0-150) of APAP/pl is significantly different from pl/pl (-3452%.min (95%CI -4705 to -2199) vs. -933% min (95%CI -2273 to 407; P = 0.015) but not from APAP/Nal (-1731% min (95%CI -3676 to 214; P = 0.08) and other treatments. AUC (90-150) is not significantly different. This pilot study shows for the first time in human volunteers that naloxone does not inhibit paracetamol antinociception, suggesting no significant implication of the opioid system in paracetamol mechanism of action: this needs be confirmed on a larger number of subjects.

Nimesulide inhibits protein kinase C epsilon and substance P in sensory neurons - comparison with paracetamol

In this paper we describe new actions of nimesulide and paracetamol in cultured peripheral neurons isolated from rat dorsal root ganglia (DRG). Both drugs were able to decrease in a dose-dependent fashion the number of cultured DRG neurons showing translocation of protein kinase C epsilon (PKCɛ) caused by exposure to 1 μM bradykinin or 100 nM thrombin. In addition, the level of substance P (SP) released by DRG neurons and the level of preprotachykinin mRNA expression were measured in basal conditions and after 70 minutes or 36 hours of stimulation with nerve growth factor (NGF) or with an inflammatory soup containing bradykinin, thrombin, endothelin-1, and KCl. Nimesulide (10 μM) significantly decreased the mRNA levels of the SP precursor preprotachykinin in basal and in stimulated conditions, and decreased the amount of SP released in the medium during stimulation of neurons with NGF or with the inflammatory soup. The effects of paracetamol (10 μM) on such response was lower. Nimesulide completely inhibited the release of prostaglandin E2 (PGE2) from DRG neurons, either basal or induced by NGF and by inflammatory soup, while paracetamol decreased PGE2 release only partially. Our data demonstrate, for the first time, a direct effect of two drugs largely used as analgesics on DRG neurons. The present results suggest that PKCɛ might be a target for the effect of nimesulide and paracetamol, while inhibition of SP synthesis and release is clearly more relevant for nimesulide than for paracetamol mechanism of action.

What do we (not) know about how paracetamol (acetaminophen) works?

WHAT IS KNOWN AND BACKGROUND

Although paracetamol (acetaminophen), N-(4-Hydroxyphenyl)acetamide, is one of the world's most widely used analgesics, the mechanism by which it produces its analgesic effect is largely unknown. This lack is relevant because: (i) optimal pain treatment matches the analgesic mechanism to the (patho)physiology of the pain and (ii) modern drug discovery relies on an appropriate screening assay.

OBJECTIVE

To review the clinical profile and preclinical studies of paracetamol as means of gaining insight into its mechanism of analgesic action.

METHODS

A literature search was conducted of clinical and preclinical literature and the information obtained was organized and reviewed from the perspective of its contribution to an understanding of the mechanism of analgesic action of paracetamol.

RESULTS

Paracetamol's broad spectrum of analgesic and other pharmacological actions is presented, along with its multiple postulated mechanism(s) of action. No one mechanism has been definitively shown to account for its analgesic activity.

WHAT IS NEW AND CONCLUSION

Further research is needed to uncover the mechanism of analgesic action of paracetamol. The lack of this knowledge affects optimal clinical use and impedes drug discovery efforts.

Antinociceptive effects of combination of tramadol and acetaminophen on painful diabetic neuropathy in streptozotocin-induced diabetic rats

OBJECTIVE

The purpose of this study was to establish the streptozotocin (STZ)-induced diabetic model with rats and investigate the antinociceptive effect of combination of Tramadol (TR) and Acetaminophen (NAPA) on the animal model for the first time.

METHODS

Diabetic model was induced by a single injection of STZ (60 mg/kg, intraperitoneal). Nociceptive thresholds were measured by means of electronic von Frey test, hot-plate test, and tail-flick test. On the 28th day of diabetes induction, diabetic rats with significant hyperalgesia were randomly divided into three groups: TR, NAPA, and TR-NAPA combination group. Each group was randomly divided into four subgroups. Three geometric series of drugs were given to each group respectively. Antinociceptive effects of the drugs were assessed at 15, 30, 60, 120, and 180 minutes after drug administration. 50% Maximum antinociceptive effect of each drug was determined by probit analysis, whereas interaction between TR and NAPA was evaluated by isobolographic analysis.

RESULTS

Hyperalgesia, along with hyperglycemia, developed 4 days after STZ injection and persisted at all tested time points until 28 days. TR, NAPA, and TR-NAPA combination administration all produced dose-dependent antinociceptive effects. Isobolographic analysis showed a significant deviation of TR/NAPA 50% maximum antinociceptive effect (in tail-flick test, but not in von Frey test) from the additive line.

CONCLUSIONS

Combination of the two drugs produces an additive antinociceptive effect in tail-flick test, whereas probable additive antinociceptive effect in von Frey test in painful diabetic neuropathy rats.

TRPV1 in brain is involved in acetaminophen-induced antinociception

BACKGROUND

Acetaminophen, the major active metabolite of acetanilide in man, has become one of the most popular over-the-counter analgesic and antipyretic agents, consumed by millions of people daily. However, its mechanism of action is still a matter of debate. We have previously shown that acetaminophen is further metabolized to N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z -eicosatetraenamide (AM404) by fatty acid amide hydrolase (FAAH) in the rat and mouse brain and that this metabolite is a potent activator of transient receptor potential vanilloid 1 (TRPV(1)) in vitro. Pharmacological activation of TRPV(1) in the midbrain periaqueductal gray elicits antinociception in rats. It is therefore possible that activation of TRPV(1) in the brain contributes to the analgesic effect of acetaminophen.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that the antinociceptive effect of acetaminophen at an oral dose lacking hypolocomotor activity is absent in FAAH and TRPV(1) knockout mice in the formalin, tail immersion and von Frey tests. This dose of acetaminophen did not affect the global brain contents of prostaglandin E(2) (PGE(2)) and endocannabinoids. Intracerebroventricular injection of AM404 produced a TRPV(1)-mediated antinociceptive effect in the mouse formalin test. Pharmacological inhibition of TRPV(1) in the brain by intracerebroventricular capsazepine injection abolished the antinociceptive effect of oral acetaminophen in the same test.

CONCLUSIONS

This study shows that TRPV(1) in brain is involved in the antinociceptive action of acetaminophen and provides a strategy for developing central nervous system active oral analgesics based on the coexpression of FAAH and TRPV(1) in the brain.

Mechanisms of non-opioid analgesics beyond cyclooxygenase enzyme inhibition

Non-opioid analgesics including both selective and non-selective cyclooxygenase (COX) inhibitors and acetaminophen are the most widely used treatments for pain. Inhibition of COX is thought to be largely responsible for both the therapeutic and adverse effects of this class of drugs. Accumulating evidence over the past two decades has demonstrated effects of non-opioids beyond the inhibition of COX and prostaglandin synthesis that might also explain their therapeutic and adverse effects. These include their interaction with endocannabinoids, nitric oxide, monoaminergic, and cholinergic systems. Moreover, the recent development of microarray technology that allows the study of human gene expression suggests multiple pathways that may be related to the analgesic and anti-inflammatory effects of non-opioids. The present review will discuss the multiple actions of non-opioids and their interactions with these systems during inflammation and pain, suggesting that COX inhibition is an incomplete explanation for the actions of non-opioids and proposes the involvement of multiple selective targets for their analgesic, as well as, their adverse effects.

Paracetamol (Acetaminophen): mechanisms of action

Paracetamol has a central analgesic effect that is mediated through activation of descending serotonergic pathways. Debate exists about its primary site of action, which may be inhibition of prostaglandin (PG) synthesis or through an active metabolite influencing cannabinoid receptors. Prostaglandin H(2) synthetase (PGHS) is the enzyme responsible for metabolism of arachidonic acid to the unstable PGH(2). The two major forms of this enzyme are the constitutive PGHS-1 and the inducible PGHS-2. PGHS comprises of two sites: a cyclooxygenase (COX) site and a peroxidase (POX) site. The conversion of arachidonic acid to PGG(2) is dependent on a tyrosine-385 radical at the COX site. Formation of a ferryl protoporphyrin IX radical cation from the reducing agent Fe(3+) at the POX site is essential for conversion of tyrosine-385 to its radical form. Paracetamol acts as a reducing cosubstrate on the POX site and lessens availability of the ferryl protoporphyrin IX radical cation. This effect can be reduced in the presence of hydroperoxide-generating lipoxygenase enzymes within the cell (peroxide tone) or by swamping the POX site with substrate such as PGG(2). Peroxide tone and swamping explain lack of peripheral analgesic effect, platelet effect, and anti-inflammatory effect by paracetamol. Alternatively, paracetamol effects may be mediated by an active metabolite (p-aminophenol). p-Aminophenol is conjugated with arachidonic acid by fatty acid amide hydrolase to form AM404. AM404 exerts effect through cannabinoid receptors. It may also work through PGHS, particularly in areas of the brain with high concentrations of fatty acid amide hydrolase.

Endocannabinoid and serotonergic systems are needed for acetaminophen-induced analgesia

Acetaminophen is the most used analgesic/antipyretic drug. Its unclear mechanism of action could rely on cyclooxygenase inhibition, NO synthesis blockade or reinforcement of the serotonergic system. Here we show that in thermal, mechanical and chemical pain tests, AM-251, a specific CB(1) receptor antagonist, abolished the analgesic action of acetaminophen, which was also lost in CB(1) receptor knockout mice. Moreover, acetaminophen was shown unable to bind to CB(1) receptors demonstrating an indirect involvement of these receptors in the analgesic effect of this compound. Accordingly with these results, we also demonstrated that the inhibition of FAAH, an enzyme involved in the cerebral metabolism of acetaminophen into AM404, known to reinforce the activity of the endocannabinoid system, suppressed the antinociceptive effect of acetaminophen. In addition, similarly to the interaction of acetaminophen with bulbospinal serotonergic pathways and spinal serotonin receptors, we observed that the antinociceptive activity of ACEA, a CB(1) receptor agonist, was inhibited by lesion of bulbospinal serotonergic pathways and antagonists of spinal 5-HT receptors. We therefore propose that acetaminophen-induced analgesia could involve the following sequence: (1) FAAH-dependent metabolism of acetaminophen into AM404; (2) indirect involvement of CB(1) receptors by this metabolite; (3) endocannabinoid-dependent reinforcement of the serotonergic bulbospinal pathways, and (4) involvement of spinal pain-suppressing serotonergic receptors.

Different mechanisms underlie the analgesic actions of paracetamol and dipyrone in a rat model of inflammatory pain

BACKGROUND AND PURPOSE

The analgesics, paracetamol and dipyrone are weak inhibitors of the cyclooxygenase isoforms 1 or 2 (COX-1, COX-2) but more potent on COX-3. Both are also weak anti-inflammatory agents, relative to their analgesic and antipyretic activities. In a model of inflammatory pain mediated by prostaglandins, both compounds were analgesic. We have analysed this shared effect further in order to elucidate the underlying mechanisms.

EXPERIMENTAL APPROACH

Inflammation was induced in one hind paw of rats by intraplantar injection of 250 microg lambda-carrageenan (CG) and the contralateral paw injected with saline. Nociceptive thresholds to mechanical stimulation were measured immediately before and for 6 h after, injection of CG. The analgesics were s.c. or locally (intraplantar) injected either 30 min before or 2 h after CG. In some groups, naltrexone was injected (s.c. or intraplantar), 1 h before CG.

KEY RESULTS

Pretreatment with paracetamol or dipyrone (60-360 mg kg(-1)) reversed hyperalgesia induced by CG and increased nociceptive threshold in the inflamed paw above the basal level (hypoalgesia). Paracetamol, but not dipyrone, also raised nociceptive thresholds in the non-inflamed paw. Subcutaneous, but not local, administration of naltrexone, a specific opioid antagonist, reversed the hypoalgesia induced by paracetamol, but similar naltrexone treatment had no effect on dipyrone-induced analgesia.

CONCLUSIONS AND IMPLICATIONS

Although both paracetamol and dipyrone are inhibitors of COX isoforms and thus of prostaglandin biosynthesis and were analgesic in our model, their analgesic actions were functionally and mechanistically different. Satisfactory mechanisms of action for these analgesics still remain to be established.

Acetaminophen reinforces descending inhibitory pain pathways

The mechanism of the analgesic action of acetaminophen involves the serotonergic system. This study explores how acetaminophen interferes with serotonergic descending pain pathways. Eighteen rapid metabolizers of tropisetron were included in this double-blind cross-over study. After ethical approval, the healthy volunteers took 1 g oral acetaminophen (A) or placebo (p) combined with either the 5-HT3 antagonist tropisetron (T) (5 mg) or saline, intravenously, at weekly intervals. Mechanical pain thresholds, determined before and after a cold pressor test (CPT), were repeated seven times during the three post-dosing hours, and area under the concentration-time curves (AUCs) of the three treatments were compared. After CPT, AUC (%*min) of Ap (1,561+/-429) was larger than before CPT (393+/-382, P<0.05); these effects were totally inhibited by tropisetron. Acetaminophen reinforces descending inhibitory pain pathways; it suggests a supraspinal target for acetaminophen's antinociceptive action. This study also confirmed that there is a central serotonergic mechanism of action for acetaminophen that is not stimulus-dependent.

Acetaminophen safety and hepatotoxicity--where do we go from here?

Acetaminophen has been widely used for > 50 years in the treatment of pain and fever and provides for the safe and effective relief of these symptoms. In a small minority of patients, however, acetaminophen is responsible for life-threatening liver injury and accounts for up to 50% of all adult cases of acute liver failure in the US. Although approximately two-thirds of adult overdoses are associated with suicide attempts, many are inadvertent, often due to the use of multiple acetaminophen formulations over many days. Additionally, some individuals appear to experience acetaminophen toxicity at 'therapeutic' doses of < 4 g/day, for reasons unknown. In pediatric populations, the overwhelming majority of acetaminophen overdoses are due to unintentional overdoses, except for the predominance of suicidal ingestions in the teenage population. This article seeks to review the mechanism and metabolism of acetaminophen and the features of toxicity in adults, pediatric and special populations. Additionally, expert opinion is presented herein to aid in reducing the frequency and severity of liver injury from acetaminophen.

Isobolographic analysis of the antinociceptive interactions between ketoprofen and paracetamol

The present study was undertaken to evaluate the antinociceptive interaction between paracetamol and ketoprofen. The antinociceptive effect of oral administration of the drugs alone or in combination was evaluated using the mouse abdominal constriction test. The data were interpreted by isobolographic analysis to establish the nature of the interaction. The effective dose that produced 50% antinociception (ED(50,mix)) was calculated from the log dose-response curve of fixed-ratio combinations of paracetamol with ketoprofen. This ED(50,mix) was compared to the theoretical additive ED(50,add) by isobolographic analysis. The experimental ED(50,mix) was found to be significantly smaller than the theoretically calculated ED(50,add), indicating a synergistic antinociceptive interaction between ketoprofen and paracetamol. Pharmacokinetic studies were carried out with mice treated with combined ketoprofen (12 mg/kg) and paracetamol (36 mg/kg). Plasma levels of ketoprofen were not changed by concurrent paracetamol treatment, and similarly no statistically significant difference was observed between paracetamol alone and the combination with ketoprofen. The pharmacokinetic analysis revealed that the combination of ketoprofen with paracetamol exerted a synergistic (supra-additive) interaction that was not associated with a pharmacokinetic interaction. The results of this study demonstrate significant synergism between ketoprofen and paracetamol.

Acetaminophen selectively suppresses peripheral prostaglandin E2 release and increases COX-2 gene expression in a clinical model of acute inflammation

Acetaminophen is widely used for pain management as an alternative to NSAIDs and selective COX-2 inhibitors, but its action at a molecular level is still unclear. We evaluated acetaminophen's effect on PG release and the expression patterns of genes related to PG production in a clinical model of tissue injury and acute inflammation. Subjects (119 outpatients) received either 1000 mg acetaminophen, 50 mg rofecoxib (a selective COX-2 inhibitor), 30 mg ketorolac (a dual COX-1/COX-2 inhibitor), or placebo before the surgical removal of two impacted mandibular third molars. Microdialysis was used to collect inflammatory transudate from the surgical site for measurement of PGE2 and TXB2 levels at the site of injury. Biopsies were collected to investigate the expression patterns of genes related to PG production at baseline prior to surgery and at 3 or 24 h following surgery. PGE2 release was suppressed by ketorolac, rofecoxib and acetaminophen compared to placebo at 3 h coincident with increased COX-2 gene expression in biopsies collected from the surgical site. TXB2 release was suppressed only by ketorolac. COX-2 gene expression remained elevated at 24 h with continued ketorolac and acetaminophen treatment. COX-1 gene expression was significantly down-regulated at 24 h by ketorolac, rofecoxib and acetaminophen. Acetaminophen suppression of PGE2 without inhibiting TXB2 release, when COX-2 gene expression is up-regulated, suggests that acetaminophen is a selective COX-2 inhibitor in vivo. The up-regulation of COX-2 gene and down-regulation of COX-1 gene expression suggests that acetaminophen may result in changes in COX-derived prostanoids with repeated doses.

Intrathecally administered COX-2 but not COX-1 or COX-3 inhibitors attenuate streptozotocin-induced mechanical hyperalgesia in rats

Members of the cyclooxygenase (COX) family are known to catalyze the rate-limiting steps of prostaglandins synthesis and reported to be involved in neuropathic pain. Diabetic neuropathy is a type of neuropathic pain, though it is not clear if COX is relevant to the condition. Recently, spinal COX-2 protein was found to be increasing in streptozotocin-induced rats as compared to the constitutive expression. We attempted to determine which cyclooxygenase isoforms are involved in streptozotocin-induced mechanical hyperalgesia, which was induced by a single intraperitoneal injection of 75 mg/kg of streptozotocin. Intrathecal administrations of the COX-2 inhibitors SC-58125 (7-100 microg) and NS-398 (7-60 microg), as well as a high dose (100 microg) of the COX-1 inhibitor SC-560 attenuated hyperalgesia, whereas intrathecal administrations of a low dose (10 microg) of SC-560 and the COX-3 inhibitor acetaminophen (1-7 mg) did not. Further, intrathecal administration of SC-58125 (100 microg) did not produce an analgesic effect in normal rats. These results indicate that intrathecal administration of COX-2 inhibitors has an anti-hyperalgesic effect on streptozotocin-induced mechanical hyperalgesia and we concluded that spinal COX-2 is pivotal in streptozotocin-induced hyperalgesia.

Changes in dorsal root ganglion CGRP expression in a chronic inflammatory model of the rat knee joint: differential modulation by rofecoxib and paracetamol

Neuropeptide-expressing small diameter sensory neurones are thought to be vital in generating inflammatory hyperalgesic responses. Within the dorsal root ganglion (DRG), both the levels of the neuropeptide calcitonin gene-related peptide (CGRP) and the numbers of CGRP-immunoreactive (CGRP-IR) DRG neurones have been shown to increase in a number of acute adjuvant-induced inflammatory pain models. The aim of this study was to look specifically at changes in numbers of CGRP-IR DRG neurones in a chronic model of inflammatory joint pain following complete Freund's adjuvant (CFA) injection into the rat knee. In this model, there were significant increases in the number of ipsilateral CGRP-IR small DRG neurones at days 1, 16 and 35 following intra-articular CFA, compared to saline-injected sham animals. This correlated with the behavioural readouts of hypersensitivity and knee joint inflammation at the same time points. There was also a significant increase in the number of ipsilateral CGRP-IR medium DRG neurones and contralateral CGRP-IR small DRG neurones at day 1. Following dosing of CFA-injected rats with rofecoxib (Vioxx) or paracetamol, there was a significant decrease in the number of ipsilateral CGRP-IR small and medium DRG neurones in rofecoxib- but not paracetamol-treated rats. These data also correlated with behavioural readouts where hypersensitivity and knee joint inflammation were significantly reduced by rofecoxib but not paracetamol treatment. In conclusion, these data show that changes in ipsilateral CGRP expression within small DRG neurones are consistent with behavioural readouts in both time course, rofecoxib and paracetamol studies in this model of chronic inflammatory pain.

Labeling of Acetaminophen with I-131 and Biodistribution in Rats

The aim of the present study was to label acetaminophen (APAP) with I-131 and to determine its radiopharmaceutical potential in rats. Acetaminophen was labeled with I-131 using the iodogen method. The radiochemical purity of (131)I-APAP was determined by RTLC and paper electrophoresis. The labeling yield was 94 +/- 4%. The biodistribution studies of the labeled compound (specific activity; 56.60 GBq/mmol) were performed in male Albino Wistar rats. The uptake of (131)I-APAP in some organs were determined at different time after injection to the rats. The radioactivity in each organ was counted and the percentage of injected activity per gram of tissue weight (%ID/g) for each organ and blood was calculated. (131)I-APAP uptake in the lung, liver, kidneys, pancreas, blood, stomach and some brain region, were observed. Thus, (131)I-APAP may be radiopharmaceutical for the imaging of the brain.

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.

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.

Putative cyclooxygenase-3 expression in rat brain cells

Cyclooxygenase-3 (COX-3), a new acetaminophen-sensitive isoform of the COX family, has recently been cloned from canine tissues. Canine COX-3 apparently is identical to the full-length form of COX-1, with the exception that the COX-3 mRNA retains intron 1. Additionally, COX-3 mRNA expression is high in the brain. We investigated the expression of the putative rat COX-3 mRNA in primary cultures of neurons, astrocytes, endothelial cells, pericytes, and choroidal epithelial cells from the rat brain. Specific RT-PCR primers were designed to detect putative rat COX-3 mRNA, and the RT-PCR products were sequenced and compared to the known sequence of the rat COX-1 gene. Our results demonstrate that the mRNA of the putative COX-3 is expressed in all of the cell types except neurons. Cerebral endothelial cells showed the highest COX-3 expression. Whereas COX-2 expression increased several-fold after lipopolysaccharide (LPS) challenge, COX-1 and COX-3 expression did not change significantly, suggesting that cells constitutively express COX-3. In summary, we report, for the first time to our knowledge, that the putative COX-3 mRNA is detectable in rats and is differentially expressed in various cell types from rat brain, as well as that its expression is not stimulated by LPS.

Effect of the inhibition of serotonin biosynthesis on the antinociception induced by nonsteroidal anti-inflammatory drugs

The antinociceptive activity of nonsteroidal anti-inflammatory drugs (NSAIDs) has been explained mainly on the basis of their inhibition of the enzyme cyclooxygenase (COX); however, this inhibition is not enough to completely explain the analgesic efficacy of these drugs. The modulation exerted by serotonergic systems on antinociception is well known. The purpose of the present work was to further explore the role of serotonin in the antinociceptive activity of NSAIDs using the writhing test and the tail-flick test of the mice after the inhibition of serotonin biosynthesis with intraperitoneal p-chlorophenylalanine (p-CPA). Pretreatment with p-CPA produced a significant decrease in the antinociceptive activity of NSAIDs administered either by the intraperitoneal or intrathecal routes, in both algesiometric tests. These results suggest a complementary mechanism of antinociception for NSAIDs, independent of their ability to inhibit the activity of COX, involving the activation of descending serotonergic pathways. By the pharmacological nature of the study, one limitation was the absence of biochemical measurement of the synthesis of 5-HT, since the reduction of the brain 5-HT synthesis by pretreatment with p-CPA will be expressed as a diminished antinociceptive activity of NSAIDs, which would be a new argument to consider NSAIDs acting as central analgesic agents.