Antinociceptive effect of paracetamol in rats is partly dependent on spinal serotonergic systems

Acetaminophen effects upon formalin-evoked flinching, postformalin, and postincisional allodynia and conditioned place preference

Introduction

We explored in mice, the analgesic, tolerance, dependency, and rewarding effects of systemic acetaminophen (APAP).

Methods

Studies employed adult mice (C57Bl6). (1) Intraplantar formalin flinching + post formalin allodynia. Mice were given intraperitoneal APAP in a DMSO (5%)/Tween 80 (5%) or a water-based formulation before formalin flinching on day 1 and tactile thresholds assessed before and after APAP at day 12. (2) Paw incision. At 24 hours and 8 days after hind paw incision in male mice, effects of intraperitoneal APAP on tactile allodynia were assessed. (3) Repeated delivery. Mice received daily (4 days) analgesic doses of APAP or vehicle and tested upon formalin flinching on day 5. (4) Conditioned place preference. For 3 consecutive days, vehicle was given in the morning in either of 2 chambers and in each afternoon, an analgesic dose of morphine or APAP in the other chamber. On days 5 and 10, animals were allowed to select a "preferred" chamber.

Results

Formalin in male mice resulted in biphasic flinching and an enduring postformalin tactile allodynia. Acetaminophen dose dependently decreased phase 2 flinching, and reversed allodynia was observed postflinching. At a comparable APAP dose, female mice showed similarly reduced phase 2 flinching. Incision allodynia was transiently reversed by APAP. Repeated APAP delivery showed no loss of effect after sequential injections or signs of withdrawal. Morphine, but not APAP or vehicle, resulted in robust place preference.

Conclusions

APAP decreased flinching and allodynia observed following formalin and paw incision and an absence of tolerance, dependence, or rewarding properties.

2-Hydroxybenzohydrazide as a novel potential candidate against nociception, inflammation, and pyrexia: in vitro, in vivo, and computational approaches

The current study was designed to evaluate the 2-hydroxybenzohydrazide (HBH) as a drug having efficacy against pyrexia, inflammation, and nociception. Besides, the therapeutic effects of HBH on oxidative stress and C-reactive proteins were also evaluated. The pharmacological studies on HBH (20-60 mg/kg) were conducted using nociception, inflammation, and pyrexia standard models. Naloxone antagonism was performed to assess the possible involvement of opioidergic mechanisms. The antioxidant study was conducted on ABTS and DPPH assays using gallic acid as a standard. Moreover, the binding capability of HBH with enzymes cyclooxygenase-I/II (COX-I/II) was determined using molecular modeling analysis. The findings indicated that the HBH dose-dependently inhibited pain, inflammation, and pyrexia. The HBH has significant anti-nociceptive and anti-inflammatory activities at 60 mg/kg (***p < 0.001), similar to the lower doses of diclofenac sodium (50 mg/kg) and tramadol (30 mg/kg). The HBH at 60 mg/kg reduced pyrexia as paracetamol (150 mg/kg). The HBH at 20-60 mg/kg doses declined the plasma C-reactive protein concentration. The mechanistic studies showed that the anti-nociceptive effect of HBH was antagonized by naloxone, indicating that the opioidergic mechanisms are involved. Furthermore, computational studies showed that the HBH exhibited an affinity for COX-I/II target receptors. The HBH significantly inhibited ABTS and DPPH radicals (IC50 = 33.81 and 26.74 μg/ml). These results proposed that the HBH has significant antipyretic, anti-inflammatory, and anti-nociceptive activities involving opioidergic mechanism.

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.

GC-MS Analysis and Various In Vitro and In Vivo Pharmacological Potential of Habenaria plantaginea Lindl

Background. The current study aims to give a scientific origin for employing Habenaria plantaginea Lindl. as a potential candidate against nociception, inflammation, and pyrexia. The pharmacological studies were performed on crude extract and subfractions. In the gas chromatography-mass spectroscopy analysis, a total of 21 compounds were identified. The plant samples were displayed for in vitro anti-inflammatory potentials. The observed IC₅₀ for chloroform against cyclooxygenase-2 and 5-lipoxygenase enzymes was 33.81 and 26.74 μg/mL, respectively. The in vivo activities were prerequisites with the acute toxicity studies. In carrageenan-induced inflammation, the chloroform fraction exhibited 46.15% inhibition similar to that of standard drug diclofenac sodium 47.15%. Likewise, in the acetic acid-induced writhing test, the ethyl acetate fraction displayed 71.42% activity, which was dose-dependent as that of standard drug. In Brewer's yeast-induced antipyretic activity, a significant decrease in rectal volume was observed after 30, 60, and 90 minutes. Moreover, the results of this study indicated that the chloroform and ethyl acetate fractions inhibited nociception, inflammation, and pyrexia dose dependently. Likewise, mechanistic insights indicated that naloxone antagonized the antinociceptive effect of chloroform and ethyl acetate fractions, thereby signifying the involvement of opioidergic mechanisms respectively. These results suggest that these molecules present in this plant have synergistically beneficial potential for the cure and management of analgesia, inflammation, and pyrexia.

Oral acetaminophen-induced spinal 5-hydroxytriyptamine release produces analgesic effects in the rat formalin test

The mechanism by which acetaminophen produces its analgesic effects is not fully understood. One possible mechanism is the activation of the spinal 5-hydroxytryptamine (5-HT) receptor, although direct evidence of spinal 5-HT release has not yet been reported. N-arachidonoylphenolamine (AM404), a metabolite of acetaminophen, is believed to be the key substance that contributes to the analgesic effects of acetaminophen. In this study, we examined whether acetaminophen and AM404 induce spinal 5-HT release and the mechanism through which spinal 5-HT receptor activation exerts analgesic effects in a rat formalin test in an inflammatory pain model. Spinal 5-HT release was examined by intrathecal microdialysis in conscious and freely moving rats. Acetaminophen was administered orally, and AM404 was administered intracerebroventricularly. In rat formalin tests, oral acetaminophen and intracerebroventricular AM404 induced significant spinal 5-HT release and produced analgesic effects. The analgesic effect of oral acetaminophen was partially antagonized by intrathecal administration of WAY100135 (a 5-HT_1A receptor antagonist) and SB269970 (a 5-HT₇ receptor antagonist). In contrast, the analgesic effect of intracerebroventricular AM404 was completely antagonized by WAY100135, while SB269970 had no effect. Our data suggest that while oral acetaminophen and intracerebroventricular AM404 activate the spinal 5-HT system, the role of the spinal 5-HT system activated by oral acetaminophen differs from that activated by intracerebroventricular AM404.

Participation of the descending noradrenergic inhibitory system in the anti-hyperalgesic effect of acetaminophen in a rat model of inflammation

AIMS

This study investigated the relationship between the analgesic efficacy of acetaminophen and the descending noradrenergic systems using rodent models of inflammatory pain.

MAIN METHODS

Inflammatory pain models were established by carrageenan injection into rats' paws. The models were defined as acute (4 h after carrageenan injection), subacute (24 h after carrageenan injection), and late (1 week after carrageenan injection) phase. To evaluate intravenous acetaminophen treatment, the withdrawal threshold to mechanical stimuli was assessed simultaneously with in vivo microdialysis assay of noradrenaline levels in the locus coeruleus (LC). Further analyses were performed to observe the effect of yohimbine on the treatment and the impact of AM404 treatment, a metabolite of acetaminophen, on noradrenaline levels in the LC.

KEY FINDINGS

In all phases, intravenous acetaminophen had a significant anti-hyperalgesic effect (p < 0.05). There was a significant time-dependent increase in the noradrenaline concentration within the LC (acetaminophen versus saline treatment; at 30 min, p < 0.001; 60 min, p < 0.01) in the subacute pain model, but not in the acute and late phase pain models. Intrathecal pre-injection of yohimbine attenuated the anti-hyperalgesic effect after acetaminophen injection only in the subacute model (p < 0.05). In the subacute pain model, intracerebroventricular administration of AM404 showed the same trend in noradrenaline levels as acetaminophen administration (AM404 versus vehicle group at 30 min, p < 0.001).

SIGNIFICANCE

We found the descending noradrenergic inhibitory system is involved in the antinociceptive action of acetaminophen in the subacute phase of inflammatory pain.

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.

Acetaminophen and ibuprofen in the treatment of pediatric fever: a narrative review

OBJECTIVE

A narrative review of randomized, blinded, controlled studies assessing the antipyretic effect of ibuprofen versus acetaminophen or combined or alternating treatment in children was conducted.

METHODS

Searches of the PubMed and Embase literature databases were conducted to identify relevant articles. Selected articles were limited to studies published in English that investigated OTC oral tablet and syrup formulations of acetaminophen and ibuprofen; there were no publication date limits. Open-label studies, nonrandomized studies, and those evaluating intravenous or suppository formulations of acetaminophen or ibuprofen were excluded. Variations in designs, endpoints, methods, and patient populations precluded our ability to conduct a formal systematic review.

RESULTS

At physician-directed dosing (acetaminophen 15 mg/kg vs ibuprofen 10 mg/kg), no significant differences in antipyretic effects from 0‒6 h and between 0‒6, ‒12, ‒24, or ‒48 h, with single or multiple-doses, respectively, were observed. Tolerability profiles at physician dosing were similar. In 14 over-the-counter dose comparisons (acetaminophen, 10-15 mg/kg; ibuprofen, 2.5-10 mg/kg), antipyresis favored ibuprofen in 6, was similar between groups in 7, and favored acetaminophen (15 mg/kg vs ibuprofen 5 mg/kg) in 1 comparison. Both medications were well tolerated. Efficacy favored combination over individual components in 3 of 4 studies; alternating use results were mixed. All combination or alternating treatments were well tolerated.

CONCLUSIONS

Antipyretic effects of ibuprofen and acetaminophen are similar at physician-directed doses; ibuprofen may be modestly superior at over-the-counter doses.

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.

Safety and tolerability of fixed-dose combinations of ibuprofen and acetaminophen: pooled analysis of phase 1-3 clinical trials

OBJECTIVES

An ibuprofen (IBU)/acetaminophen (APAP) fixed-dose combination (FDC) for over-the-counter (OTC) use was developed with the goal of providing the same effective analgesic activity as full doses of the individual monocomponents, while reducing individual monocomponent drug exposures. Here, the safety and tolerability of the FDC is characterized using pooled safety data from phase 1-3 clinical trials in the FDC development program.

METHODS

We conducted a pooled safety analysis of data from 7 clinical trials: three phase 1 pharmacokinetic trials, a phase 2 proof-of-concept trial, and three phase 3 trials (a single- and a multiple-dose trial in a dental pain model and a single-dose trial in an induced-fever model). Safety and tolerability of the FDC were assessed by adverse events (AEs) for the total group and subgroups (age, sex, race).

RESULTS

A total of 1,477 participants were enrolled in the 7 trials; 715 were treated with FDC IBU/APAP, 432 with IBU monotherapy, 330 with APAP monotherapy, and 156 with placebo. Most subjects were white (86.5%), and 44% were female. Two trials enrolling 195 adolescents accounted for 13.2% of the overall study population. All-causality treatment-emergent AEs (TEAEs) occurred in 19.7% of the 1477 participants. Nausea (13.5%), vomiting (7.4%), dizziness (4.5%), headache (1.2%), and feeling hot (1.0%) were the only TEAEs reported in ≥1% of subjects. Treatment-related AEs occurred in 1.8% of the subjects in the overall population. The incidence of AEs, including treatment-related AEs, was consistently lower in all active treatment groups than in the placebo group; this also applied to subgroups according to sex, race, and age, including adolescents aged 12-17 years. The higher rate of AEs with placebo was likely due to lack of pain/fever control.

CONCLUSION

Single-dose or short-course FDC IBU/APAP OTC use was well tolerated, with an AE profile similar to its IBU and APAP monocomponents.

CLINICALTRIALS.GOV REGISTRATION

NCT01559259; NCT02912650; NCT02837952; NCT02761980. The pharmacokinetic studies (n = 3) did not require registration.

Antinociceptive Effects of Sinomenine Combined With Ligustrazine or Paracetamol in Animal Models of Incisional and Inflammatory Pain

The management of postoperative and inflammatory pain has been a pressing challenge in clinical settings. Sinomenine (SN) is a morphinan derived alkaloid with remarkable analgesic properties in various kinds of pain models. The aim of the current study is to investigate if SN can enhance the effect of ligustrazine hydrochloride (LGZ) or paracetamol (PCM) in animal models of postoperative and inflammatory pain. And to determine if the combined therapeutic efficacies can be explained by pharmacokinetics changes. Pharmacological studies were performed using a rat model of incisional pain, and a mouse model of carrageenan induced inflammatory pain. Pharmacokinetic studies were performed using a microdialysis sampling and HPLC-MS/MS assay method to quantify SN, LGZ, and PCM levels in blood and extracellular fluid in brain. We found that SN plus LGZ or SN plus PCM produced marked synergistic analgesic effects. However, such synergy was subjected to pain modalities, and differed among pain models. Pharmacological discoveries could be partially linked to pharmacokinetic alterations in SN combinations. Though further evaluation is needed, our findings advocate the potential benefits of SN plus LGZ for postoperative pain management, and SN plus PCM for controlling inflammatory pain.

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.

Eslicarbazepine acetate interacts in a beneficial manner with standard and alternative analgesics to reduce trigeminal nociception

RATIONALE

Acute pain states in the trigeminal region (headaches, dental pain) fall into the most prevalent painful conditions. Standard analgesics (paracetamol/NSAIDs) represent the cornerstone of their treatment, whereas triptans are primarily used in migraine attacks. Due to limited efficacy and/or side effects of current treatments, identifying favorable combinations of available drugs is justified.

OBJECTIVES

Eslicarbazepine acetate (ESL) is a novel antiepileptic drug whose effectiveness against trigeminal pain was recently demonstrated. Here, we examined the interactions between ESL and several standard/alternative analgesics (paracetamol, propyphenazone, naproxen, zolmitriptan, and metoclopramide) in a model of trigeminal pain.

METHODS

The antinociceptive effects of orally administered ESL, standard/alternative analgesics, and two-drug ESL-analgesic combinations were examined in the orofacial formalin test in mice. The type of interaction between drugs was determined by isobolographic analysis.

RESULTS

ESL, analgesics, and two-drug ESL-analgesic combinations significantly and dose-dependently reduced nociceptive behaviour in the second, inflammatory phase of the test. Isobolographic analysis revealed that ESL interacted additively with paracetamol/propyphenazone/zolmitriptan and synergistically with naproxen/metoclopramide (with about a 4-fold and 3-fold reduction of doses in the ESL-naproxen and ESL-metoclopramide combination, respectively).

CONCLUSIONS

ESL interacted in a beneficial manner with several analgesics that are used for trigeminal pain treatment, producing synergistic interactions with naproxen/metoclopramide and additive interactions with paracetamol/propyphenazone/zolmitriptan. Our results suggest that combining ESL with analgesics could theoretically enable the use of lower doses of individual drugs for achieving pain relief.

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.

The effect of ondansetron on analgesic efficacy of acetaminophen after hysterectomy: A randomized double blinded placebo controlled trial

OBJECTIVES

To determine that perioperative ondansetron reduces the analgesic efficacy of acetaminophen.

DESIGN

Randomized, double-blinded study.

PATIENTS

120 patients ASA I-II who underwent abdominal hysterectomy.

INTERVENTIONS

All the patients were given 1g acetaminophen at skin closure. Patients were divided into two groups; ondansetron HCl (8mg, 2ml IV) (Group I, N=60) and saline (2ml IV) (Group II, N=60) at the skin closure.

MEASUREMENT

Postoperative pain scores (VAS) while resting in bed and sitting, total opioid consumption were noted.

MAIN RESULTS

Patients randomized to ondansetron had significantly worse pain scores upon arrival to the recovery unit [by 1.7 (99.7% CI: 0.75, 2.59) cm] and at 1h [by 1.3 (0.5, 2.1) cm] while resting in bed. Pain scores while sitting were also significantly greater in ondansetron group at arrival in PACU by 0.6 (99.7% CI: 0.1, 1.0) cm. Thereafter, pain scores did not differ significantly. Median total opioid (tramadol) consumption was 441 [Q1, Q3: 280, 578] mg in the ondansetron group and 412 [309, 574] mg in the placebo group, P=0.95.

CONCLUSIONS

Ondansetron significantly decreased the analgesic effect of acetaminophen during the initial postoperative period. Our results thus confirm that acetaminophen analgesia is partially mediated by serotonin receptors. However, the reduction was of marginal clinical importance and short-lived.

Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla

Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action. Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB₁ receptors by the acetaminophen metabolite and endocannabinoid reuptake inhibitor AM 404. However, the contribution of the cannabinoid system to antihyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB₁ receptors have remained elusive. Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent antihyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their antihyperalgesic activity in CB₁^-/- mice, confirming the involvement of the cannabinoid system. Consistent with a mechanism downstream of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia induced by intrathecal prostaglandin E₂ To distinguish between a peripheral/spinal and a supraspinal action, we administered acetaminophen and AM 404 to hoxB8-CB₁^-/- mice, which lack CB₁ receptors from the peripheral nervous system and the spinal cord. These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action. Accordingly, local injection of the CB₁ receptor antagonist rimonabant into the rostral ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but not in CB₁^-/- mice. Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB₁ receptors reside in the rostral ventromedial medulla.SIGNIFICANCE STATEMENT Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action, including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue-specific CB₁ receptor-deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.

Pharmacogenomics in pain treatment

The experience of chronic pain is one of the commonest reasons for seeking medical attention, being a major issue in clinical practice. While pain is a universal experience, only a small proportion of people who felt pain develop pain syndromes. In addition, painkillers are associated with wide inter-individual variability in the analgesic response. This may be partly explained by the presence of single nucleotide polymorphisms in genes encoding molecular entities involved in pharmacodynamics and pharmacokinetics. However, uptake of this information has been slow due in large part to the lack of robust evidences demonstrating clinical utility. Furthermore, novel therapies, including targeting of epigenetic changes and gene therapy-based approaches are further broadening future options for the treatment of chronic pain. The aim of this article is to review the evidences behind pharmacogenetics (PGx) to individualize therapy (boosting the efficacy and minimizing potential toxicity) and genes implicated in pain medicine, in two parts: (i) genetic variability with pain sensitivity and analgesic response; and (ii) pharmacological concepts applied on PGx.

Supraspinal-selective TRPV1 desensitization induced by intracerebroventricular treatment with resiniferatoxin

The transient receptor potential vanilloid type 1 (TRPV1) is a thermosensitive cation channel that triggers heat pain in the periphery. Long-term desensitization of TRPV1, which can be induced by excess amounts of agonists, has been a method for investigating the physiological relevance of TRPV1-containing neuronal circuits, and desensitization induced by various routes of administration, including systemic, intrathecal and intraganglionic, has been demonstrated in rodents. In the present study, we examined the effect of intracerebroventricular (i.c.v.) treatment with an ultrapotent TRPV1 agonist, resiniferatoxin (RTX), on nociception and the analgesic effect of acetaminophen, which is known to mediate the activation of central TRPV1. I.c.v. administration of RTX a week before the test did not affect the licking/biting response to intraplantar injection of RTX (RTX test), suggesting that such i.c.v. treatment spares the function of TRPV1 at the hindpaw. Mice that had been i.c.v.-administered RTX also exhibited normal nociceptive responses in the formalin test and the tail pressure test, but acetaminophen failed to induce analgesia in those mice in any of the tests. These results suggest that i.c.v. administration of RTX leads to brain-selective TRPV1 desensitization in mice.

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.

Pain management following spinal surgeries: An appraisal of the available options

Spinal procedures are generally associated with intense pain in the postoperative period, especially for the initial few days. Adequate pain management in this period has been seen to correlate well with improved functional outcome, early ambulation, early discharge, and preventing the development of chronic pain. A diverse array of pharmacological options exists for the effective amelioration of post spinal surgery pain. Each of these drugs possesses inherent advantages and disadvantages which restricts their universal applicability. Therefore, combination therapy or multimodal analgesia for proper control of pain appears as the best approach in this regard. The current manuscript discussed the pathophysiology of postsurgical pain including its nature, the various tools for assessment, and the various pharmacological agents (both conventional and upcoming) available at our disposal to respond to post spinal surgery pain.

Up-regulation of calcitonin gene-related peptide in trigeminal ganglion following chronic exposure to paracetamol in a CSD migraine animal model

Previously, our group has demonstrated that chronic paracetamol (APAP) treatment induces alterations to the trigeminovascular nociceptive system in the cortical spreading depression (CSD) migraine animal model. The calcitonin gene related peptide (CGRP) is a key neuropeptide involved in the activation of the trigeminovascular nociceptive system. Therefore, this study examined the expression levels of CGRP in the trigeminal ganglion (TG) after chronic APAP exposure (0, 15, and 30 days) using a CSD model. Rats were divided into control, CSD only, APAP only and APAP treatment with CSD groups. A single injection (i.p.) of APAP (200 mg/kg body weight) was given to the 0-day APAP-treated groups, while the other APAP-treated groups received daily injections for 15 and 30 days. CSD was induced by the topical application of KCl to the parietal cortex. The protein expression of CGRP in the TG was evaluated by immunohistochemistry, and the CGRP mRNA level was investigated by real-time quantitative reverse transcription polymerase chain reaction. The results revealed that the induction of CSD significantly increased the level of CGRP protein but had no effect on CGRP mRNA level. Pretreatment with APAP 1 hour before CSD activation significantly reduced CGRP expression induced by CSD. In contrast, chronic treatment with APAP (15 and 30 days) significantly enhanced CGRP expression in both protein and mRNA levels when compared with the control groups. In combination with CSD, the expression of CGRP further increased in the animal with 30 day treatment. These findings indicate that chronic treatment with APAP induces an increase of CGRP expression in the TG. This alteration may be associated with the increased trigeminovascular nociception observed in our previous studies.

A comparison between intravenous paracetamol plus fentanyl and intravenous fentanyl alone for postoperative analgesia during laparoscopic cholecystectomy

PURPOSE

our study compared the effect of fentanyl alone with fentanyl plus intravenous Paracetamol for analgesic efficacy, opioid sparing effects, and opioid-related side effects after laparoscopic cholecystectomy.

MATERIALS AND METHODS

eighty patients undergoing laparoscopic cholecystectomy were randomized into two groups, who were given either an IV placebo or an IV injection of 1g paracetamol just before induction. Both groups received fentanyl during induction and IM diclofenac for pain relief every 8 hourly for 24 h after surgery. The postoperative pain relief was evaluated by a visual analog scale (VAS) and consumption of fentanyl as rescue analgesic in the postoperative period for 24 h after surgery was measured. The incidence of PONV and sedation scores was also measured in the postoperative period.

RESULTS

the mean VAS score in first and second hour after surgery was less in the group receiving IV Paracetamol (3.3±0.4* vs. 5.2±0.9; 3.1±0.4* vs. 4.3±0.3); the fentanyl consumption over first 24 h was also less in the group receiving IV paracetamol (50±14.9 vs. 150±25.8). The time requirement of first dose of rescue analgesic in the postoperative period was also significantly prolonged in the group receiving IV paracetamol (76±24.7 vs. 48±15.8). There was no difference in the sedation scores and in the incidence of PONV in the two groups.

CONCLUSION

The study demonstrates the usefulness of intravenous paracetamol as pre-emptive analgesic in the treatment of postoperative pain after laparoscopic cholecystectomy.

Combining ibuprofen and acetaminophen for acute pain management after third-molar extractions: translating clinical research to dental practice

BACKGROUND

Effective and safe drug therapy for the management of acute postoperative pain has relied on orally administered analgesics such as ibuprofen, naproxen and acetaminophen, or N-acetyl-p-aminophenol (APAP), as well as combination formulations containing opioids such as hydrocodone with APAP. The combination of ibuprofen and APAP has been advocated in the last few years as an alternative therapy for postoperative pain management. The authors conducted a critical analysis to evaluate the scientific evidence for using the ibuprofen-APAP combination and propose clinical treatment recommendations for its use in managing acute postoperative pain in dentistry.

TYPES OF STUDIES REVIEWED

The authors used quantitative evidence-based reviews published by the Cochrane Collaboration to determine the relative analgesic efficacy and safety of combining ibuprofen and APAP. They found additional articles by searching the Ovid MEDLINE, PubMed and ClinicalTrials.gov databases.

CONCLUSIONS

The results of the quantitative systematic reviews indicated that the ibuprofen-APAP combination may be a more effective analgesic, with fewer untoward effects, than are many of the currently available opioid-containing formulations. In addition, the authors found several randomized controlled trials that also indicated that the ibuprofen-APAP combination provided greater pain relief than did ibuprofen or APAP alone after third-molar extractions. The adverse effects associated with the combination were similar to those of the individual component drugs. Practical Implications. Combining ibuprofen with APAP provides dentists with an additional therapeutic strategy for managing acute postoperative dental pain. This combination has been reported to provide greater analgesia without significantly increasing the adverse effects that often are associated with opioid-containing analgesic combinations. When making stepwise recommendations for the management of acute postoperative dental pain, dentists should consider including ibuprofen-APAP combination therapy.

Arsenic decreases antinociceptive activity of paracetamol: possible involvement of serotonergic and endocannabinoid receptors

We assessed whether repeated arsenic exposure can decrease paracetamol-mediated antinociception by modulating serotonergic and endocannabinoid pathways. Rats were preexposed to elemental arsenic (4ppm) as sodium arsenite through drinking water for 28 days. Next day paracetamol's (400mg/kg, oral) antinociceptive activity was assessed through formalin-induced nociception. Serotonin content and gene expression of 5-HT1A, 5-HT2A and CB1 receptors were evaluated in brainstem and frontal cortex. Arsenic decreased paracetamol-mediated analgesia. Paracetamol, but not arsenic, increased serotonin content in these regions. Arsenic attenuated paracetamol-mediated increase in serotonin level. Paracetamol did not alter 5-HT1A expression, but caused down-regulation of 5-HT2A and up-regulation of CB1 receptors. Arsenic down-regulated these receptors. However, paracetamol-mediated down-regulation of 5-HT2A was more pronounced. Arsenic did not modify paracetamol's effect on 5-HT1A expression, but reduced paracetamol-mediated down-regulation of 5-HT2A and reversed up-regulation of CB1 receptors. Results suggest arsenic reduced paracetamol-induced analgesia possibly by interfering with pronociceptive 5-HT2A and antinociceptive CB1 receptors.

Analgesic activity of Eugenia jambolana leave constituent: a dikaempferol rhamnopyranoside from ethyl acetate soluble fraction

CONTEXT

Eugenia jambolana Lam. (Myrtaceae) is a medicinal plant used in folk medicine for the treatment of diabetes, inflammation, and pain.

OBJECTIVE

We investigated the antinociceptive effect of kaempferol-7-O-α-l-rhamnopyranoside]- 4'-O-4'-[kaempferol-7-O-α-l-rhamnopyranoside (EJ-01), isolated from the E. jambolana leaves.

MATERIALS AND METHODS

EJ-01 (3, 10, and 30 mg kg(-1), orally) was assessed for peripheral (formalin-nociception and acetic acid-writhing) and central (hot plate and tail flick test) analgesic activity in mice and the in vitro anti-inflammatory activity (25, 50, and 100 µg mL(-1)) in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells.

RESULTS AND DISCUSSION

EJ-01 (10 and 30 mg kg(-1)) significantly inhibited mean writhing counts (37.74 and 36.83) in acetic acid writhing and paw licking time (55.16 and 45.66 s) in the late phase of the formalin test as compared with the respective control (60.66 and 104.33 s). EJ-01 did not show analgesic activity in central pain models. Significant reduction in the tumor necrosis factor (TNF)-α (295.48, 51.20, and 49.47 pg mL(-1)) and interleukin (IL)-1β (59.38, 20.08, and 15.46 pg mL(-1)) levels were observed in EJ-01-treated medium (25, 50, and 100 µg mL(-1)) as compared with vehicle-treated control values (788.67 and 161.77 pg mL(-1)), respectively. Significant reduction in total nitrite plus nitrate (NOx) levels (70.80 nmol) was observed in the EJ-01-treated medium (100 µg mL(-1)) as compared with the vehicle-treated value (110.41 nmol).

CONCLUSION

EJ-01 is a valuable analgesic constituent of E. jambolana leaves and this study supports the pharmacological basis for the use of this plant in traditional medicine for curing inflammatory pain.

An improved model of heat-induced hyperalgesia--repetitive phasic heat pain causing primary hyperalgesia to heat and secondary hyperalgesia to pinprick and light touch

This study tested a modified experimental model of heat-induced hyperalgesia, which improves the efficacy to induce primary and secondary hyperalgesia and the efficacy-to-safety ratio reducing the risk of tissue damage seen in other heat pain models. Quantitative sensory testing was done in eighteen healthy volunteers before and after repetitive heat pain stimuli (60 stimuli of 48°C for 6 s) to assess the impact of repetitive heat on somatosensory function in conditioned skin (primary hyperalgesia area) and in adjacent skin (secondary hyperalgesia area) as compared to an unconditioned mirror image control site. Additionally, areas of flare and secondary hyperalgesia were mapped, and time course of hyperalgesia determined. After repetitive heat pain conditioning we found significant primary hyperalgesia to heat, and primary and secondary hyperalgesia to pinprick and to light touch (dynamic mechanical allodynia). Acetaminophen (800 mg) reduced pain to heat or pinpricks only marginally by 11% and 8%, respectively (n.s.), and had no effect on heat hyperalgesia. In contrast, the areas of flare (−31%) and in particular of secondary hyperalgesia (−59%) as well as the magnitude of hyperalgesia (−59%) were significantly reduced (all p<0.001). Thus, repetitive heat pain induces significant peripheral sensitization (primary hyperalgesia to heat) and central sensitization (punctate hyperalgesia and dynamic mechanical allodynia). These findings are relevant to further studies using this model of experimental heat pain as it combines pronounced peripheral and central sensitization, which makes a convenient model for combined pharmacological testing of analgesia and anti-hyperalgesia mechanisms related to thermal and mechanical input.

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.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

Perioperative pain management in the neurosurgical patient

Perioperative pain management in neurosurgical patients has been inadequately recognized and treated. An increased awareness of pain management and advances in understanding of pain modulation and pathophysiology have led to improved perioperative care of patients. There is a need to assess neurologic function while providing superior analgesia with minimal side effects. Several classes of drugs are currently available or under investigation for use as adjuvants or alternative therapies. There remains a need to determine the best treatment of perioperative pain in this patient population. Improved awareness, assessment, and treatment of pain result in better care and overall patient outcome.

Systemic paracetamol-induced analgesic and antihyperalgesic effects through activation of descending serotonergic pathways involving spinal 5-HT₇ receptors

Although some studies have shown the essential role of descending serotonergic pathways and spinal 5-HT(1A), 5-HT(2A), or 5-HT(3) receptors in the antinociceptive effects of paracetamol, other studies have presented conflicting results, and the particular subtype of spinal 5-HT receptors involved in paracetamol-induced analgesia remains to be clarified. Recent studies have demonstrated the importance of spinal 5-HT(7) receptors in descending serotonergic pain inhibitory pathways. In this study, we investigated the role of descending serotonergic pathways and spinal 5-HT(7) receptors compared with 5-HT(3) and 5-HT(2A) receptors in the antinociceptive and antihyperalgesic effects of paracetamol. Tail-flick, hot plate and plantar incision tests were used to determine nociception in male BALB/c mice. Lesion of serotonergic bulbospinal pathways was performed by intrathecal (i.th.) injection of 5,7-dihydroxytryptamine (5,7-DHT), and spinal 5-HT levels were measured by HPLC. To evaluate the particular subtypes of the spinal 5-HT receptors, the selective 5-HT(7), 5-HT(3) and 5-HT(2A) receptor antagonists SB 269970, ondansetron and ketanserin, respectively, were given i.th. after oral administration of paracetamol. Oral paracetamol (200, 400 and 600 mg/kg) elicits dose-dependent antinociceptive and antihyperalgesic effects. I.th. pretreatment with 5,7-DHT (50 μg) sharply reduced 5-HT levels in the spinal cord. Depletion of spinal 5-HT totally abolished the antinociceptive and antihyperalgesic effects of paracetamol. I.th. injection of SB 2669970 (10 μg) blocked the antinociceptive and antihyperalgesic effects of paracetamol, but ondansetron and ketanserin (10 μg) did not. Our findings suggest that systemic administration of paracetamol may activate descending serotonergic pathways and spinal 5-HT(7) receptors to produce a central antinociceptive and antihyperalgesic effects.

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.

Continuous multimechanistic postoperative analgesia: a rationale for transitioning from intravenous acetaminophen and opioids to oral formulations

Good surgical outcomes depend in part on good pain relief, allowing for early mobilization, optimal recovery, and patient satisfaction. Postsurgical pain has multiple mechanisms, and multimechanistic approaches to postoperative analgesia are recommended and may be associated with improved pain relief, lowered opioid doses, and sometimes a lower rate of opioid-associated side effects. Acetaminophen (paracetamol) is a familiar agent for treating many types of pain, including postsurgical pain. Oral acetaminophen has been shown to be safe and effective in a variety of acute pain models. Combination products using a fixed-dose of acetaminophen and an opioid have also been effective in treating postsurgical pain. Combination products with acetaminophen have demonstrated an opioid-sparing effect, which inconsistently results in a reduced rate of opioid-associated side effects. Intravenous (IV) acetaminophen and an opioid analgesic administered in the perioperative period may be followed by an oral acetaminophen and opioid combination in the postoperative period. Transitioning from an IV acetaminophen and opioid formulation to a similar but oral formulation of the same drugs appears to be a reasonable step in that both analgesic therapies are known to be safe and effective. For postsurgical analgesia with any acetaminophen product, patient education is necessary to be sure that the patient does not concurrently take any over-the-counter products containing acetaminophen and accidentally exceed dose limits.

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.

Tropisetron blocks analgesic action of acetaminophen: a human pain model study

Because the mechanism underlying the analgesic action of acetaminophen remains unclear, we investigated the possible interaction of acetaminophen with central serotonergic pathways. The effects of acetaminophen, tropisetron, the combination of both drugs, and saline on pain perception and central sensitization in healthy volunteers were compared. Sixteen healthy volunteers were included in this randomized, double-blind, placebo-controlled crossover study. Intracutaneous electrical stimulation (46.1 ± 19.1 mA) induced acute pain (numeric rating scale, 6 of 10) and stable areas of hyperalgesia and allodynia. Pain intensities and areas of hyperalgesia and allodynia were regularly assessed before, during, and after a 15-min infusion of acetaminophen, tropisetron, the combination of both drugs, and saline. Acetaminophen concentrations were measured to rule out any pharmacokinetic interaction. Both acetaminophen and tropisetron led to decreased pain ratings as compared to saline. However, when acetaminophen and tropisetron were administered simultaneously, the pain ratings were not affected. There was no significant difference in the evolution of the hyperalgesic and allodynic areas during the study period between the study groups (P = .06 and P = .33, respectively). Acetaminophen serum levels were not significantly different when associated with tropisetron (P = .063), although we observed a trend toward lower acetaminophen concentrations when both drugs were concurrently administered. In summary, while the combination of acetaminophen and tropisetron showed no analgesic action, each drug administered alone led to decreased pain ratings as compared to saline. In an electrically evoked human pain model, the combination of acetaminophen with tropisetron was free of any analgesic potential. However, when administered on its own, both acetaminophen and tropisetron were mildly analgesic.

Tropisetron and paracetamol association in post-operative patients

Studies in animals and in healthy volunteers have demonstrated the central serotonergic analgesic mechanism of action of paracetamol involving the inhibition of this analgesia by tropisetron, a 5-HT3 antagonist. This randomized, double-blind, controlled study aims at studying this interaction in post-operative patients after ear surgery. Thirty-six patients are included in two parallel groups with intravenous paracetamol (1 g) and either tropisetron (T, 5 mg/mL) or placebo (c, NaCl 0.9%) administered at the end of surgery. Numerical pain evaluations are performed every 30 min, six times after awakening. The difference between the sums of numerical scales of both groups [9 ± 10 (T) vs. 6 ± 7 (c)] is not significant, but the tropisetron group displays higher pain scores despite additional rescue analgesia. The limits of this trial call for a much larger study to investigate further this pharmacodynamic interaction.