Phenacetin, acetaminophen and dipyrone: analgesic and rewarding effects

Structure-activity relationship of pharmacophores and toxicophores: the need for clinical strategy

OBJECTIVES

Sometimes clinical efficacy and potential risk of therapeutic and toxic agents are difficult to predict over a long period of time. Hence there is need for literature search with a view to assessing cause of toxicity and less efficacy of drugs used in clinical practice.

METHOD

Hence literatures were searched for physicochemical properties, chemical formulas, molecular masses, pH values, ionization, receptor type, agonist and antagonist, therapeutic, toxic and structure-activity relationship of chemical compounds with pharmacophore and toxicophore, with a view to identifying high efficacious and relative low toxic agents. Inclusion criteria were manuscripts published on PubMed, Scopus, Web of Science, PubMed Central, Google Scholar among others, between 1960 and 2023. Keywords such as pharmacophore, toxicophore, structure-activity-relationship and disease where also searched. The exclusion criteria were the chemicals that lack pharmacophore, toxicophore and manuscripts published before 1960.

RESULTS

Findings have shown that pharmacophore and toxicophore functional groups determine clinical efficacy and safety of therapeutics, but if they overlap therapeutic and toxicity effects go concurrently. Hence the functional groups, dose, co-administration and concentration of drugs at receptor, drug-receptor binding and duration of receptor binding are the determining factors of pharmacophore and toxicophore activity. Molecular mass, chemical configuration, pH value, receptor affinity and binding capacity, multiple pharmacophores, hydrophilic/lipophilic nature of the chemical contribute greatly to functionality of pharmacophore and toxicophore.

CONCLUSION

Daily single therapy, avoidance of reversible pharmacology, drugs with covalent adduct, maintenance of therapeutic dose, and the use of multiple pharmacophores for terminal diseases will minimize toxicity and improve efficacy.

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.

Paracetamol: unconventional uses of a well-known drug

OBJECTIVES

To describe and map what is known about unconventional uses of paracetamol through a scoping review of published literature by adopting adopted a PRISMA systematic approach methodology.

KEY FINDINGS

Four themes for unconventional uses of paracetamol emerged: (a) use of paracetamol in sleep (a-1) positive effect of paracetamol on sleep (n = 9) or (a-2) neutral or negative effect of paracetamol on sleep (n = 9); (b) use of paracetamol in sport (n = 13); (c) mixing paracetamol with drinks, waterpipe and illicit drugs (n = 5); and (d) miscellaneous uses (n = 4). Forty records were reviewed and charted. Available literature supports concern around the potential of harmful or non-medical use of paracetamol, especially among patients with a history of substance use, parents of young children or athletes.

SUMMARY

Paracetamol (acetaminophen) is one of the most popular and widely used drugs for the treatment of pain and fever. It is considered remarkably safe if used within instructions. However, there is growing evidence that paracetamol, is sometimes used outside approved indications or abused (i.e. used for non-medical reasons). This review highlights the need for enhanced pharmacovigilance and surveillance of non-medical paracetamol use and raising general public awareness of its potential dangers especially in higher than recommended doses.

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.

Kinetic removal of acetaminophen and phenacetin during LED-UV365 photolysis of persulfate system: Reactive oxygen species generation

Acetaminophen (ACT) and phenacetin (PNT) removal during light-emitting diode (LED)-UV photolysis of persulfate (PS) was evaluated with a typical wavelength of 365 nm. Decay of PNT and ACT in pH ranges of 5.5-8.5 followed pseudo-first order kinetics. Maximum pseudo-first order rate constants (k_obs) of ACT and PNT decomposition of 1.8 × 10^-1 and 1.2 × 10^-1 min^-1, respectively, were obtained at pH 8.5. Hydroxyl radicals (·OH), sulfate radicals (SO₄·^-), superoxide radicals (O₂^-·), and singlet oxygen (¹O₂) were determined in-situ electron paramagnetic resonance (EPR) and alcohol scavenging tests. The average contributions of ·OH and SO₄·^- were 23.5% and 53.0% for PNT removal, and 15.9% and 53.0% for ACT removal at pH ranges of 5.5-8.5. In samples subjected to chlorination after LED-UV₃₆₅/PS pre-oxidation, a relatively small total concentration of five halogenated disinfection by-products (DBPs) was obtained of 90.9 μg L^-1 (pH 5.5) and 126.7 μg L^-1 (pH 7.0), which is 58.5% and 30.2% lower than that in system without LED-UV₃₆₅/PS pre-oxidation. Meanwhile, a higher maximum value of total DBP concentration was obtained at pH 8.5 (445.6 μg L^-1) following LED-UV₃₆₅/PS pre-oxidation. The results of economy evaluation showed that UV₃₆₅ was more cost-effective in application for organic contaminant removal compared with UV₂₅₄.

Nature and nurture of sexual partner preference: Teachings from prenatal administration of acetaminophen in male rats

The organizational-activational hypothesis indicates that activation of adult sexual behavior in males depends on organization of the masculine brain during the perinatal sensitive period. In the medial preoptic area such masculinization depends on a neuroendocrine cascade that includes exposure to testosterone, aromatization to estradiol, activation of estrogen receptors, synthesis of cyclooxygenase (COX), increase of prostaglandins, release of glutamate, and activation of AMPA receptors that result in the formation of more dendritic spines. Thus, in the present study we assessed the sexual partner preference (SPP) of adult male rats prenatally treated with acetaminophen (APAP), an analgesic/antipyretic drug that inhibits COX-2 and is commonly used and prescribed during pregnancy. Female rats received either saline (2 ml/kg s.c.) or APAP (50 mg/kg s.c.) every 12 h, during days 16-20 of pregnancy. At postnatal day PD60 half of the male offspring were exposed to sexual experience with receptive females during 5 trials, and the other half remained sexually naïve. At PD90 all them were tested for SPP with one sexually receptive female and one stud male. The results indicated that only APAP-naïve males failed to display SPP. However, APAP-experienced males displayed SPP for females. We discuss the effects of prenatal APAP in the disruption of unconditioned responses towards females (nature mechanisms), and the effects of sexual experience (nurture mechanisms) in the development of conditioned heterosexual preference.

Impact of Bevacizumab on Experimentally Induced Endometriotic Lesions: Angiogenesis, Invasion, Apoptosis, and Cell Proliferation

Endometriosis is responsible for pain symptoms with great impact on the patient's quality of life. Several medication lines have been studied aiming at its definitive treatment. Among them, angiogenesis inhibitor factors may be effective given that angiogenesis has fundamental role in the establishment and growth of endometriotic lesions. In this study, we investigated the influence of bevacizumab, anti-factor drug of endothelial growth (anti-VEGF), used at two different dosages, in experimental endometriosis induced in rats. After the induction of endometriosis lesions in rats, they were divided in 3 groups: control group, no treatment, and two other groups were treated with different dosages of the same medication for 4 weeks. At the end of the treatment, endometriotic lesions were removed and evaluated regarding area of lesions, presence of endometrial tissue in microscopy, positivity for anti-VEGF antibody in immunohistochemistry, and gene expression of Pcna, Mmp9, Tp63, and Vegfa. Bevacizumab acted by reducing the area of lesions in the groups that received medication (p = 0.002) and reducing gene expression to Tp63 in lesions (p = 0.04). There was no significant result in other evaluations. We observed that there was significant reduction of the area of lesions among groups, suggesting that bevacizumab has a positive effect on disease control. The gene expression of Tp63 was significantly lower in the group that received high dose of the drug when compared with the other two groups; therefore, we concluded that bevacizumab acts by reducing cell proliferation and differentiation in lesions, constituting a real option for treating endometriosis.

Intranasal Acetaminophen Abuse and Nasal, Pharyngeal, and Laryngotracheal Damage

A young adult female originally presented with necrosis of the nasal cavity mucosa and septum after sniffing crushed acetaminophen. She underwent endoscopic sinus surgery and debridement but continued to use acetaminophen intranasally. Four months later, the destruction had extended to include the posterior pharyngeal wall and subglottis. The diagnosis was confirmed by polarizable talc found on biopsy of the subglottis. While nasal insufflation of cocaine and hydrocodone-acetaminophen has been well-documented, intranasal abuse of exclusively acetaminophen is not well understood. This case demonstrates the destructive potential of intranasal acetaminophen use and may help physicians recognize unusual signs and symptoms of intranasal drug abuse.

Topical Anti-Inflammatory and Analgesic Activities of Citrullus colocynthis Extract Cream in Rats

Background and objectives:Citrullus colocynthis (CC), known as bitter apple, is used to treat diabetes in Iranian traditional medicine. The aim of this study is to evaluate the anti-inflammatory and analgesic activities of CC cream in rats. Materials and Methods: The carrageenan-induced edema in a rat hind paw was carried out to evaluate the topical anti-inflammatory effect of the CC fruit extract cream (2–8%) and the tissue levels of IL-6 and TNF-α were estimated by using a commercial ELISA kit. The topical antinociceptive activity of CC cream (2–8%) was evaluated in the rat formalin test. To determine the role of opioid receptors in the local antinociceptive effect of the CC cream, naloxone (20 μg/paw, i.pl.), a non-selective opioid antagonist, was used. Results: The results showed that the CC cream (2–8%) dose-dependently reduced the carrageenan-induced paw edema and reversed the changes in the level of TNF-α and IL-6 due to carrageenan-induced edema (p < 0.01). The anti-inflammatory effect of CC cream 8% was comparable to that of hydrocortisone ointment 1%. Furthermore, the application of CC cream (2–8%) dose-dependently inhibited both first and second phases of the formalin test (p < 0.05). The antinociceptive effect of the CC cream (8%) was comparable to that of methyl salicylate cream 30%. Moreover, the administration of naloxone significantly reversed the topical antinociceptive effect of the CC cream (p < 0.05). Conclusions: For the first time, this study indicated that the topical application of CC cream possesses significant anti-inflammatory and antinociceptive activities in animal models, which were probably mediated by opioid receptors and the suppression of pro-inflammatory cytokines (TNF-α and IL-6). Thus, the CC cream can be used to treat inflammatory pain and inflammatory diseases.

Identification and Quantification of Cocaine and Active Adulterants in Coca-Paste Seized Samples: Useful Scientific Support to Health Care

Adulteration is a common practice in the illicit drugs market, but the psychoactive and toxic effects provided by adulterants are clinically underestimated. Coca-paste (CP) is a smokable form of cocaine which has an extremely high abuse liability. CP seized samples are sold adulterated; however, qualitative and quantitative data of CP adulteration in forensic literature is still scarce. Besides, it is unknown if adulterants remain stable when CP is heated. This study was designed to report the chemical content of an extensive series of CP seized samples and to demonstrate the stability (i.e., chemical integrity) of the adulterants heated. To achieve this goal, the following strategies were applied: (1) a CP adulterated sample was heated and its fume was chemically analyzed; (2) the vapor of isolated adulterants were analyzed after heating; (3) plasma levels of animals exposed to CP and adulterants were measured. Ninety percent of CP seized samples were adulterated. Adulteration was dominated by phenacetin and caffeine and much less by other compounds (i.e., aminopyrine, levamisole, benzocaine). In the majority of CP analyzed samples, both cocaine and caffeine content was 30%, phenacetin 20% and the combination of these three components reached 90%. Typical cocaine pyrolysis compounds (i.e., BA, CMCHTs, and AEME) were observed in the volatilized cocaine and CP sample but no pyrolysis compounds were found after isolated adulterants heating. Cocaine, phenacetin, and caffeine were detected in plasma. We provide current forensic data about CP seized samples and demonstrated the chemical integrity of their adulterants heated.

[Metamizole (dipyrone): mode of action, drug-drug interactions, and risk of agranulocytosis]

Metamizole (dipyrone) is a nonsteroidal compound with strong analgesic as well as antipyretic and spasmolytic properties. Based on a small number of cases of agranulocytosis, metamizole was withdrawn from the market in some countries. Other countries restricted its use. This paper discusses the safety aspects of metamizole and compares it with other compounds used for similar indications.

Metamizole (dipyrone) effects on sevoflurane requirements and postoperative hyperalgesia in rats

Unlike non-steroidal anti-inflammatory drugs (NSAIDs), metamizole has poor anti-inflammatory effects; and is suitable for models where analgesia, but not anti-inflammatory effects, is desirable. Like opioids, these drugs produce perioperative analgesia while reducing anaesthetic requirements, but it remains unclear whether they may develop tolerance or hyperalgesia, and thus decrease in analgesic efficacy. The aim was to determine whether tolerance or hyperalgesia to metamizole occurred in rats, and whether the sevoflurane minimum alveolar concentration (MAC) was affected. In a randomized, prospective, controlled study, male Wistar rats ( n = 8 per group) were administered metamizole (300 mg/kg, day 4). Previously, the following treatments were provided: daily metamizole for four days (0-3), morphine (10 mg/kg; positive control, day 0 only) or saline (negative control). The main outcome measures were mechanical (MNT) and warm thermal (WNT) nociceptive quantitative sensory thresholds. The baseline sevoflurane MAC and the reduction produced by the treatments were also determined. The mean (SD) baseline MAC [2.4(0.2)%vol] was decreased by morphine and metamizole by 45(11)% and 33(7)% ( P = 0.000, both), respectively. Baseline MNT [35.4(4.5) g] and WNT [13.2(2.4) s] were decreased by morphine and metamizole: MNT reduction of 22(6)% ( P = 0.000) and 22(7)% ( P = 0.001), respectively and WNT reduction of 34(14)% ( P = 0.000) and 24(13)% ( P = 0.001). The baseline MAC on day 4 was neither modified by treatments nor the MAC reduction produced by metamizole (days 0 and 4; P > 0.05). In conclusion, repeated metamizole administration may produce hyperalgesia, although it may not modify its anaesthetic sparing effect. The clinical relevance of this effect in painful research models requiring prolonged analgesic therapy warrants further investigation.

Prevention of Withdrawal Movement Associated with the Injection of Rocuronium in Children: Comparison of Paracetamol and Lidocaine

OBJECTIVE

Pain from rocuronium injection is observed in 50%-80 % of patients. This study aimed to compare the effectiveness of pretreatment with paracetamol and lidocaine in preventing pain-induced withdrawal caused by the intravenous injection of rocuronium during the induction of general anaesthesia in paediatric patients.

METHODS

Ninety children were randomized into two groups using a simple drawing from the box method: a paracetamol group (Group P, n=45) and a lidocaine group (Group L, n=45). After anaesthesia induction, venous occlusion was applied by a paediatric cuff inflated to a pressure of 75 mmHg and by 50 mg paracetamol and 0.5 mg kg(-1) lidocaine was injected in Groups P and L, respectively. Venous occlusion was then released, followed by rocuronium injection (0.6 mg kg(-1)). Withdrawal was evaluated using a 4-point scale (1, no response; 2, movement at the wrist only; 3, movement/withdrawal involving arm only (elbow/shoulder) and 4, generalized response, movement/withdrawal in more than one extremity).

RESULTS

The incidence of withdrawal movement was 42% and 26% in the Groups P and L, respectively (p=0.120). Although no significant differences were noted in the number of patients who had no withdrawal movement and mild withdrawal movement in Groups P and L, compared with Group L, the incidences of moderate withdrawal movement were significantly higher in Group P (p<0.05). No patient in either group revealed generalized movement.

CONCLUSION

Using a venous occlusion technique, pretreatment with 50 mg paracetamol can prevent withdrawal movement caused by rocuronium injection in children but is not as effective as lidocaine to prevent moderate withdrawal movement.

Acute Pain Management

Acute pain management is improving steadily over the past few years, but training and professional education are still lacking in many professions. Untreated or undertreated acute pain could have detrimental effects on the patient in terms of comfort and recovery from trauma or surgery. Acute undertreated pain can decrease a patient's vascular perfusion, increase oxygen demand, suppress the immune system, and possibly risk increased incidence of venous thrombosis. Although acute postoperative pain needs to be managed aggressively, patients are most vulnerable during this period for developing adverse effects, and therefore, patient assessment and careful drug therapy evaluation are necessary processes in therapeutic planning. Acute pain management requires careful and thorough initial assessment and follow-up reassessment in addition to frequent dosage adjustments, and managing analgesic induced side effects. Analgesic selection and dosing must be based on the patient's past and recent analgesic exposure. There is no single acute pain management regimen that is suitable for all patients. Analgesics must be tailored to the individual patient.

Antinociceptive and anti-hyperalgesic effects of bis(4-methylbenzoyl) diselenide in mice: evidence for the mechanism of action

CONTEXT

The organoselenium compounds have been described to demonstrate several biological activities, including pain management.

OBJECTIVE

This study investigated the antinociceptive, hyperalgesic, and toxic effects of oral administration of bis(4-methylbenzoyl) diselenide (BMD) in mice.

MATERIALS AND METHODS

The antinociceptive and anti-hyperalgesic effects of BMD (1, 5, 10, 25, and 50 mg/kg, p.o.) were evaluated using models of nociception: formalin, capsaicin, bradykinin (BK), cinnamaldehyde, phorbol myristate acetate (PMA), 8-bromo-cAM, and glutamate-induced nociception; and mechanical hyperalgesia induced by carrageenan (Cg) or complete Freund's adjuvant (CFA). The acute toxicity was evaluated by biochemical markers for hepatic and renal damages.

RESULTS

BMD significantly inhibited the licking time of the injected paw in the early and late phases of a formalin test with ED50 values of 14.2 and 10.8 mg/kg, respectively. This compound reduced nociception produced by capsaicin (ED50 of 32.5 mg/kg), BK (ED50 of 24.6 mg/kg), glutamate (ED50 of 28.7 mg/kg), cinnamaldehyde (ED50 of 18.9 mg/kg), PMA (ED50 of 9.6 mg/kg), and 8-bromo-cAMP (ED50 of 24.8 mg/kg). In the glutamate test, the pretreatment with nitric oxide (NO) precursor, L-arginine, reversed antinociception caused by BMD or N(ω)-nitro-L-arginine (L-NOARG), but the effect of BMD was not abolished by naloxone. Mechanical hyperalgesia induced by Cg and CFA was attenuated by BMD, 70 ± 4% and 65 ± 4%, respectively. Furthermore, a single oral dose of BMD did not change plasma aspartate (AST) and alanine aminotransferase (ALT) activities or urea and creatinine levels.

CONCLUSION

BMD demonstrated as a promising compound because of the antinociceptive and anti-hyperalgesic properties in mice.

The effect of intravenous paracetamol for the prevention of rocuronium injection pain

Rocuronium is a nondepolarizing neuromuscular blocking agent used in anesthesia induction and is associated with considerable discomfort and burning pain during injection, which is reported to occur in 50-80% of patients. This study was carried out to investigate the effectiveness of intravenous paracetamol pretreatment compared with lidocaine and normal saline to prevent rocuronium injection pain. The study included 150 ASA I-II patients undergoing elective orthopedic, gastrointestinal, and gynecological procedures under general anesthesia. They were allocated into three groups according to pretreatment drugs: lidocaine (40 mg) (n = 50), paracetamol (n = 50), and normal saline group (n = 50). Before anesthesia induction with propofol, all patients were pretreated with rocuronium. The pain caused by the injection was evaluated. Local signs were assessed on the arm at the end of the injection, as well as 24 hours after recovery from anesthesia. There were no patients with blurred speech or vision and there was no respiratory depression in any group after pretreatment with the study drug. The level of pain on injection was statistically lower in those who had received paracetamol compared to normal saline (p = 0.009). There were more patients in the saline group with severe pain (p < 0.001). Paracetamol relieved the rocuronium injection pain better than normal saline but lidocaine was the best of the three drugs (p < 0.001).

Abuse liability of centrally acting non-opioid analgesics and muscle relaxants--a brief update based on a comparison of pharmacovigilance data and evidence from the literature

There is a lack of data regarding the abuse liability of centrally acting non-opioid analgesics (NOA) and muscle relaxants (MR). A comparison of data retrieved from a German pharmacovigilance database (BfArM; accessed May 2013) and data from the literature concerning the abuse liability of NOA and MR approved in Germany was performed. The BfArM-database demonstrated cases of abuse only for clonidine and paracetamol, whereas the literature suggests evidence for an abuse potential of baclofen, clonidine, ketamine, metamizole, methocarbamol, orphenadrine, paracetamol, propyphenazone, and tizanidine. The low number of detected cases in the BfArM-database could be a result of under-reporting.

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.

Acetaminophen modulation of hydrocodone reward in rats

Abuse of prescription opioid analgesics in non-medical context has been on the rise over the past decade. The most commonly abused analgesic in this drug class consists of a combined formulation of hydrocodone and acetaminophen. The present study was aimed to determine the rewarding effects of hydrocodone, acetaminophen, and their combination using the conditioned place preference (CPP) paradigm. Using a 6-day CPP paradigm, rats were paired with hydrocodone (0.5, 1.0 or 5.0 mg/kg) or acetaminophen (50, 100 or 300 mg/kg) to determine whether the drugs given alone would produce a CPP. Rats conditioned with the highest dose of hydrocodone exhibited place preference, whereas rats conditioned with acetaminophen did not demonstrate place preference. In a second experiment, varying doses of hydrocodone and acetaminophen were combined to determine whether acetaminophen would enhance hydrocodone reward. Acetaminophen (100 mg/kg) enhanced the rewarding effects of hydrocodone (1mg/kg), although the effect was unique to this particular dose combination. Higher or lower doses of acetaminophen combined with hydrocodone did not alter hydrocodone CPP. The present findings suggest that acetaminophen has a limited potential of modulating the rewarding properties of hydrocodone in rats.

Antinociceptive and anti-hypernociceptive effects of Se-phenyl thiazolidine-4-carboselenoate in mice

In this study, the antinociceptive, anti-hypernociceptive and toxic effects of orally administered (R)-Se-phenyl thiazolidine-4-carboselenoate (Se-PTC, 1-50 mg/kg) were evaluated in mice. Se-PTC did not change plasma aspartate (AST) and alanine aminotransferase (ALT) activities or urea and creatinine levels. Furthermore, in an open field test, Se-PTC did not alter the number of crossings and rearing. Se-PTC significantly reduced the amount of writhing when assessed by acetic acid-induced visceral nociception and attenuated the licking time of the injected paw in the early and late phases of a formalin test. In addition, Se-PTC reduced nociception produced by intra-plantar (i.pl.) injection of glutamate, capsaicin, cinnalmaldehyde, bradykinin, phorbol myristate acetate and 8-Bromo-cAMP. Se-PTC caused a significant increase in hot plate and tail-immersion response latencies, but the antinociceptive effect of Se-PTC in the tail immersion was not abolished by pretreatment with the non-selective opioid receptor antagonist, naloxone. Se-PTC (25 mg/kg) significantly inhibited nociceptive behavior induced by intrathecal (i.t.) injection of glutamate, N-methyl-D-aspartate (NMDA) and (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), but failed to affect nociception induced by kainate and α-amino-3-hydroxy-5-mehtyl-4-isoxazolepropionic acid (AMPA). Mechanical hypernociception induced by carrageenan and Complete Freund's Adjuvant was attenuated by Se-PTC administration. These results indicate that Se-PTC produces antinociception in several models of nociception.

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.

Effect of pretreatment with acetaminophen on withdrawal movements associated with injection of rocuronium: a prospective, randomized, double-blind, placebo controlled study

BACKGROUND

Withdrawal movement during rocuronium injection is a common, unresolved adverse effect. We aimed to investigate the effect of IV acetaminophen pretreatment on withdrawal movement during rocuronium injection.

METHODS

This study enrolled 120 American Society of Anesthesiologists (ASA) I-II patients undergoing general anesthesia. They were randomly assigned to three treatment groups. After occluding venous drainage using a tourniquet on the upper arm, the saline group received 5 ml of 0.9% sodium chloride solution, the lidocaine group received 40 mg of lidocaine, and the acetaminophen group received 50 mg of acetaminophen. During injection of pretreatment drug, pain was assessed on a four-point scale. The tourniquet was released after 120 seconds and anesthesia was performed using thiopental sodium 5 mg/kg followed by rocuronium 0.6 mg/kg. The withdrawal movement was graded on a four-point scale in a double-blind manner.

RESULTS

The incidence of pain on pretreatment injection in saline, lidocaine, and acetaminophen groups was 7.7%, 5.1%, and 2.5%, respectively. The incidence of withdrawal movements was 77.5% in saline group, 32.5% in lidocaine group, and 37.5% in acetaminophen group (P < 0.05).

CONCLUSIONS

Acetaminophen and lidocaine reduced the incidence of withdrawal movement after rocuronium injection compared with saline.

Endocannabinoids mediate anxiolytic-like effect of acetaminophen via CB1 receptors

Acetaminophen (Paracetamol), a most commonly used antipyretic/analgesic agent, is metabolized to AM404 (N-arachidonoylphenolamine) that inhibits uptake and degradation of anandamide which is reported to mediate the analgesic action of acetaminophen via CB1 receptor. AM404 and anandamide are also reported to produce anxiolytic-like behavior. In view of the implication of endocannabinoids in the effect of acetaminophen, we contemplated that acetaminophen may have anxiolytic-like effect. Therefore, this possibility was tested by observing the effects of various doses of acetaminophen in mice on anxiety-related indices of Vogel conflict test and social interaction test. The results from both the tests indicated that acetaminophen (50, 100, or 200 mg/kg, i.p.) or anandamide (10 or 20 microg/mouse, i.c.v.) dose dependently elicited anxiolytic-like effect, that was comparable to diazepam (2 mg/kg, i.p.). Moreover, co-administration of sub-effective dose of acetaminophen (25 mg/kg, i.p.) and anandamide (5 microg/mouse, i.c.v) produced similar anxiolytic effect. Further, pre-treatment with AM251 (a CB1 receptor antagonist; 1 mg/kg, i.p.) antagonized the effects of acetaminophen and anandamide with no per se effect at 1 mg/kg dose, while anxiogenic effect was evident at a higher dose (5 mg/kg, i.p.). None of the treatment/s was found to induce any antinociceptive or locomotor impairment effects. In conclusion, the findings suggested that acetaminophen (50, 100, or 200 mg/kg, i.p.) exhibited dose dependent anxiolytic effect in mice and probably involved endocannabinoid-mediated mechanism in its effect.

The Link Between Pain Patient and Analgesic Medication is Greater in Migraine than in Rheumatic Disease Patients

Our aim was to measure and compare the link between pain patients and the different kinds of analgesic medications they use by the Leeds Dependence Questionnaire (LDQ). This is a self-completion 10-item instrument to measure the severity of dependence upon a variety of substances. LDQ was administered to 200 episodic migraine patients (EM group), 77 chronic migraine patients (CM group) overusing acute medications, and 114 patients suffering from rheumatic disease (RD group), consecutively attending the Headache Centre or the Rheumatology Clinic of the University Hospital of Modena in the course of the first semester of 2007. The link with analgesics was greater in migraine patients than in patients with rheumatic disease, since the LDQ total score was significantly higher in the EM (6.65 ± 0.32, P < 0.005) and CM groups (9.61 ± 0.59, P < 0.0001) than in the RD group (5.17 ± 0.37) (Kruskal–Wallis and Mann–Whitney U-tests). Migraine patients were significantly more linked to triptans and to combined medications than to non-steroidal anti-inflammatory drugs. The strength of the link between migraine patients and the analgesic medications they take could represent a factor of vulnerability: overusing these medications could develop medication overuse headache.

Differential involvement of opioidergic and serotonergic systems in the antinociceptive activity of N-arachidonoyl-phenolamine (AM404) in the rat: comparison with paracetamol

It is recognized that paracetamol undergoes a metabolic transformation to N-arachydonylphenolamine (AM404), a CB(1) receptor ligand and anandamide uptake inhibitor. Using hot-plate and paw pressure tests, we decided to establish whether AM404 may act through opioidergic and serotonergic mechanisms. Thus, we pretreated rats with opioid mu(1) (naloxonazine) and kappa (MR2266) or 5-HT(1A) (NAN-190), 5-HT(2) (ketanserin), and 5-HT(3) (ondansetron) receptor antagonists. We investigated the possible changes in 5-hydroxyindoleacetic acid/serotonin ratio in the frontal cortex and pons. The antinociceptive effect of AM404 (10 mg/kg, intraperitoneally) or paracetamol (400 mg/kg, intraperitoneally) in either test was abolished by naloxonazine or MR2266. Ondansetron prevented AM404 activity; NAN-190 and ketanserin were ineffective. Ketanserin antagonized paracetamol activity; NAN-190 and ondansetron were inactive. AM404 did not change serotonergic activity, while paracetamol decreased serotonin turnover. The diverse antinociceptive potency of the compounds might be explained by the different influence on the serotonergic system, despite a similar involvement of opioidergic one.

Role of nitric oxide/cyclic GMP/K(+) channel pathways in the antinociceptive effect caused by 2,3-bis(mesitylseleno)propenol

The present study examined the antinociceptive effects induced by 2,3-bis(mesitylseleno)propenol, a bis-selenide alkene derivate, given orally, in chemical models of pain in rats and mice. Selenide administered orally (p.o.) into the rats caused antinociception against the first and second phases of the formalin test, with mean ID(50) values of 28.17 and 39.68 mg/kg, respectively. The antinociceptive effect caused by selenide (50 mg/kg, p.o.) on the formalin test was reversed by pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, a nitric oxide (NO) synthase inhibitor), methylene blue (a non-specific NO/guanylyl cyclase inhibitor) and glibenclamide (an ATP-sensitive K(+) channel inhibitor), but not by atropine (a muscarinic antagonist). Given orally selenide in mice produced an inhibition of glutamate-, histamine- and compound 48/80-induced nociception with mean ID(50) values of 27.58, 36.18 and 44.53 mg/kg, respectively. Moreover, oral treatment with selenide in mice decreased licking -- induced by serotonin (mean ID(50) value of >50 mg/kg). The data show that selenide exerts pronounced systemic antinociception in chemical (formalin, glutamate, histamine, compound 48/80 and serotonin-induced pain) models of nociception. Taken together, these results suggest that the antinociceptive effect of selenide on the formalin test involves the participation of nitric oxide/cyclic GMP/K(+) channel pathways in rats.

The association between acetaminophen concentrations in the cerebrospinal fluid and temperature decline in febrile infants

The objective of this study consisting of a prospective cohort of febrile infants was to describe the correlation between cerebrospinal fluid (CSF) acetaminophen (paracetamol) concentrations and changes in body temperature in febrile infants. Infants, one week to one year of age, with rectal temperature >or=38.0 degrees C, treated with acetaminophen were studied if they underwent a lumbar puncture (LP). Patients received 15 mg/kg of acetaminophen 30 minutes to 4 hours before lumbar puncture was performed. Rectal temperature was documented before acetaminophen administration and at the time of lumbar puncture. Plasma and CSF acetaminophen levels were determined using high-pressure liquid chromatography. Thirty-one infants were studied. In a nonlinear regression, the relationship among acetaminophen concentrations in the CSF, time, and temperature differences is best described by a Lorentzian distribution. The model suggests that a peak effect on temperature is achieved at CSF concentration of 11.9 microg/mL and 182 minutes after acetaminophen administration (P<0.001 and P<0.001, respectively r=0.9 adjusted r square=0.78). Temperature decrement in young febrile infants, treated with acetaminophen, correlates with time and acetaminophen concentrations in the CSF. High concentrations of acetaminophen in the CSF, exceeding a certain level, are not associated with greater temperature decrement.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Acetaminophen, phenacetin and dipyrone do not modulate pressor responses to arachidonic Acid or to pressor agents

In contrast to nonsteroidal anti-inflammatory drugs (NSAIDs), the nonopioid analgesics phenacetin, acetaminophen and dipyrone exhibit weak anti-inflammatory properties. An explanation for this difference in pharmacologic activity was provided by the recent discovery of a new cyclooxygenase isoform, cyclooxygenase (COX)-3, that is reported to be inhibited by phenacetin, acetaminophen and dipyrone. However, COX-3 was found to be a spliced variant of COX-1 and renamed COX-1b. Although recent studies provide evidence for the existence of this new COX isoform, it is uncertain whether this COX-3 (COX-1b) isoform, or putative acetaminophen-sensitive pathway, plays a role in the generation of vasoactive prostaglandins. NSAIDs increase systemic blood pressure by inhibiting the formation of vasodilator prostanoids. Angiotensin II, norepinephrine and other vasoconstrictor agents have been reported to release prostaglandins. It is possible that this acetaminophen-sensitive pathway also modulates pressor responses to these vasoconstrictor agents. Therefore, the purpose of the present study was to determine whether this acetaminophen-sensitive pathway plays a role in the generation of vasoactive products of arachidonic acid or in the modulation of vasoconstrictor responses in the pulmonary and systemic vascular bed of the intact-chest rat. In the present study, the nonopioid analgesics did not attenuate changes in pulmonary or systemic arterial pressure in response to injections of the prostanoid precursor, arachidonic acid, to the thromboxane A(2) mimic, U46619, or to angiotensin II or norepinephrine. The results of the present study do not provide evidence in support of a role of a functional COX-3 (COX-1b) isoform, or an acetaminophen-sensitive pathway, in the generation of vasoactive prostanoids or in the modulation of responses to vasoconstrictor hormones in the intact-chest rat.

Paracetamol: new vistas of an old drug

Paracetamol (acetaminophen) is one of the most popular and widely used drugs for the treatment of pain and fever. It occupies a unique position among analgesic drugs. Unlike NSAIDs it is almost unanimously considered to have no antiinflammatory activity and does not produce gastrointestinal damage or untoward cardiorenal effects. Unlike opiates it is almost ineffective in intense pain and has no depressant effect on respiration. Although paracetamol has been used clinically for more than a century, its mode of action has been a mystery until about one year ago, when two independent groups (Zygmunt and colleagues and Bertolini and colleagues) produced experimental data unequivocally demonstrating that the analgesic effect of paracetamol is due to the indirect activation of cannabinoid CB(1) receptors. In brain and spinal cord, paracetamol, following deacetylation to its primary amine (p-aminophenol), is conjugated with arachidonic acid to form N-arachidonoylphenolamine, a compound already known (AM404) as an endogenous cannabinoid. The involved enzyme is fatty acid amide hydrolase. N-arachidonoylphenolamine is an agonist at TRPV1 receptors and an inhibitor of cellular anandamide uptake, which leads to increased levels of endogenous cannabinoids; moreover, it inhibits cyclooxygenases in the brain, albeit at concentrations that are probably not attainable with analgesic doses of paracetamol. CB(1) receptor antagonist, at a dose level that completely prevents the analgesic activity of a selective CB(1) receptor agonist, completely prevents the analgesic activity of paracetamol. Thus, paracetamol acts as a pro-drug, the active one being a cannabinoid. These findings finally explain the mechanism of action of paracetamol and the peculiarity of its effects, including the behavioral ones. Curiously, just when the first CB(1) agonists are being introduced for pain treatment, it comes out that an indirect cannabino-mimetic had been extensively used (and sometimes overused) for more than a century.

Preference for palatable food is reduced by the gamma-hydroxybutyrate analogue GET73, in rats

Palatability and variety of foods are major reasons for "hedonic" eating, and hence for overeating and obesity. Palatable food and drugs of abuse share a common reward mechanism, and compounds that block the reinforcing effect of drugs of abuse preferentially suppress the intake of palatable foods. This research was aimed at studying the influence of the gamma-hydroxybutyrate analogue N-(4-trifluoromethylbenzyl)-4-methoxybutanamide (GET73) - that inhibits alcohol consumption - on consumption and reinforcing effect of palatable food. Adult male rats were used. For place preference conditioning, sweetened corn flakes were used as the reinforcer, and GET73 (50, 100 and 200mgkg(-1)) or vehicle were orally (p.o.) administered either 30min before each training session and the test session, or only before the test session. To study the influence on consumption, GET73 was given p.o. at the same doses once daily for 12 days to rats given free access to both palatable and varied food (cafeteria diet) or to standard chow. Both acquisition and expression of palatable food-induced conditioned place preference were inhibited by GET73, either administered throughout the conditioning period or only before the test session. GET73 reduced also the consumption of cafeteria food, while that of standard chow was increased. At these doses, GET73 had no detrimental effect on open-field behaviour. GET73 seems to specifically attenuate the gratification produced by varied and palatable food, without affecting the consumption of not particularly palatable chow. Since, overweight and obesity are mostly due to the overeating of palatable and varied foods, drugs like GET73 could represent a somewhat ideal and rational approach to obesity treatment.

Antinociceptive activity of 3-O-β-d-glucopyranosyl-23,24-dihydrocucurbitacin F from Hintonia standleyana (Rubiaceae)

Oral administration of a MeOH-CH(2)Cl(2) (1:1) extract of the stem bark of Hintonia standleyana (HSE) produced a dose-dependent antinociceptive effect when tested in mice using the writhing (150-750 mg/kg) and the hot-plate (150-600 mg/kg) models. From the active extract 3-O-beta-d-glucopyranosyl-23,24-dihydrocucurbitacin F (GDHCF), 5-O-[beta-d-apiofuranosyl-(1-->6)-beta-d-glucopyranosyl]-7-methoxy-3',4'-dihydroxy-4-phenylcoumarin (AG4-PC) and desoxycordifolinic acid (DCA) were isolated. GDHCF (10-100 mg/kg, p.o.) significantly reduced acetic acid-induced abdominal contortions and increased the hot-plate latency in comparison to vehicle-treated mice. Metamizol (50-100 mg/kg) and morphine (2.5-5 mg/kg) were used as positive controls, respectively. GDHCF-induced antinociception was partially blocked by naloxone (1 mg/kg, i.p.), l-NAME (150 mg/kg, i.p.) and glibenclamide (10mg/kg, i.p.) suggesting that its pharmacological effect could be due to the activation of the nitric oxide pathway, followed by the opening of the ATP-sensitive K(+) channels, as well as an activation of the opioid receptors.

Conversion of acetaminophen to the bioactive N-acylphenolamine AM404 via fatty acid amide hydrolase-dependent arachidonic acid conjugation in the nervous system

Acetaminophen (paracetamol) is a popular domestic analgesic and antipyretic agent with a weak anti-inflammatory action and a low incidence of adverse effects as compared with aspirin and other non-steroidal anti-inflammatory drugs. Here we show that acetaminophen, following deacetylation to its primary amine, is conjugated with arachidonic acid in the brain and the spinal cord to form the potent TRPV1 agonist N-arachidonoylphenolamine (AM404). This conjugation is absent in mice lacking the enzyme fatty acid amide hydrolase. AM404 also inhibits purified cyclooxygenase (COX)-1 and COX-2 and prostaglandin synthesis in lipopolysaccharide-stimulated RAW264.7 macrophages. This novel metabolite of acetaminophen also acts on the endogenous cannabinoid system, which, together with TRPV1 and COX, is present in the pain and thermoregulatory pathways. These findings identify fatty acid conjugation as a novel pathway for drug metabolism and provide a molecular mechanism for the occurrence of the analgesic N-acylphenolamine AM404 in the nervous system following treatment with acetaminophen.

The antinociceptive effect of some Egyptian medicinal plant extracts

The antinociceptive effect of methanolic extracts (200 and 400 mg kg(-1)) of eight Egyptian medicinal plants was studied using acetic acid-induced writhing and tail-flick test in mice. Oral administration of 400 mg kg(-1) methanolic extracts of Convolvulus fatmensis, Alhagi maurorum, Plantago major seeds, Conyza dioscaridis significantly (P < 0.05) inhibited the nociception to acetic acid-induced writhes with a protection of 85.5-61.3%. Schouwia thebaica, Diplotaxis acris, Plantago major leaves and Mentha microphylla, in the large dose, showed a protection of 50.8-45.8%, which were significantly different as compared to control. The smaller dose of the tested plant extracts did not protect animals from painful acetic acid stimulation with the exception of Alhagi maurorum. In the tail-flick test, methanolic extracts of Mentha microphylla, Conyza dioscaridis, Alhagi maurorum, Plantago major leaves, Diplotaxis acris and Convolvulus fatmensis in a dose of 400 mg kg(-1) produced significant increase in the latency to response of tail to thermal stimulation. Mild or no effect was observed by the small dose with the exception of Diplotaxis acris that had significant antinociceptive effect at the dose of 200 mg kg(-1). The extracts of all tested plants in doses up to 2 g kg(-1) b.wt. did not cause any deaths or major signs of acute toxicity. Phytochemical screening indicated the presence of unsaturated sterols, triterpenes, tannins, flavonoids and carbohydrates and/or glycosides as major constituents.

Phenacetin and cocaine in a body packer

A case of acute intoxication of cocaine adulterated with phenacetin is reported. Twenty-four packages were found in the stomach and intestine of a 25-year-old male. The identification of phenacetin was performed by gas chromatography-mass spectrometry (GC/MS) analysis.

Profound, non-opioid analgesia produced by the high-efficacy 5-HT(1A) agonist F 13640 in the formalin model of tonic nociceptive pain

Previously, we have reported that in rat models of chronic pain, in particular, the very-high-efficacy 5-HT(1A) agonist F 13640 induces unprecedented pain relief by novel neuroadaptative mechanisms that involve inverse tolerance and cooperation with nociceptive stimulation in producing analgesia. The present studies detailed the actions of F 13640 and other compounds in the formalin model of tonic nociceptive pain. Intraperitoneal injection of F 13640 (0.01-2.5 mg/kg; t -15 min) caused a dose-dependent and complete inhibition of the paw elevation and paw licking that occurred both early (0-5 min) and late (22.5-27.5 min) after the intraplantar injection of diluted formaldehyde (2.5%) in the rat. The extent to which F 13640 and other 5-HT(1A) receptor ligands inhibited these pain behaviors correlated (p < 0.05) with the extent to which they activated 5-HT(1A) receptors. Under similar conditions, some inhibitory effects were also observed with various agents that are known to produce analgesia by different peripheral and/or central mechanisms (e.g., opioids, NA/5-HT reuptake inhibitors, COX-2 inhibitors and other nonsteroidal anti-inflammatory drugs, gabapentin, and ABT-594). However, with the possible exception of morphine, the effects of all of these agents at nontoxic doses were lower than those of F 13640, in particular in inhibition of early paw elevation. The 5-HT(1A) antagonist WAY 100635, but not naloxone, antagonized the actions of F 13640. These results help to establish large-magnitude 5-HT(1A) receptor activation as a new molecular mechanism of profound, central analgesia and suggest that F 13640 may be particularly effective against pain arising from severe tonic nociceptive stimulation.

Lack of sensitization during place conditioning in rats is consistent with the low abuse potential of tramadol

Based on the recent finding that tramadol (TRAM) produces conditioned place preference (CPP) and dopamine release in the nucleus accumbens, it was suggested that the abuse liability of TRAM may be greater than hitherto assumed. We re-evaluated the effects of TRAM in CPP and behavioral sensitization, in comparison with morphine (MOR) and meptazinol (MEPT), an opioid drug with minimal abuse potential. While MOR produced CPP and very strong locomotor sensitization, TRAM and MEPT produced only CPP. It has been suggested that sensitization plays an important role in the development of addiction, hence our results suggest that the abuse potential of TRAM might resemble more that of MEPT than that of MOR, and they are consistent with the clinical picture, in that although TRAM is not completely devoid of positively reinforcing effects, reports on abuse are rare. The low propensity to induce addiction may be related to the lack of changes in the brain circuitry mediating reward and motivation, as evidenced by the lack of sensitization.

Antinociceptive profiles of aspirin and acetaminophen in formalin, substance P and glutamate pain models

Aspirin (ASA) is widely used oral analgesics and acts as an inhibitor of cyclo-oxygenase. Also, acetaminophen (APAP) is effective analgesics and may selectively inhibit brain prostaglandin synthetase. However, their mechanisms of action in CNS are poorly defined, although several authors have shown that the antinociceptive effects of ASA and APAP have different underlying mechanisms and play some possible roles on spinal nociceptive processing, such as inhibition of prostaglandin synthesis. To define and characterize antinociceptive profiles of ASA and APAP on various pain models, we performed intraplantar formalin injection test, intrathecal (i.t.) substance P (0.7 microg)-induced nociceptive response test, and i.t. glutamate (20 microg)-induced nociceptive response test after ASA or APAP (from 10 to 300 mg/kg) administered orally to the mouse. In the formalin test, ASA produced an antinociceptive effect during only the 2nd phase (20-40 min), but not the 1st phase (0-5 min), in a dose-dependent manner. However, APAP showed a dose-dependent antinociceptive effect during both phases of the formalin test. In addition, both ASA and APAP reduced nociceptive behavior induced by glutamate administered i.t. in a dose-dependent manner. In substance P-induced nociceptive response, APAP, but not ASA, showed antinociceptive effect in a dose-dependent manner. Our results suggest that ASA and APAP administered orally may be mediated by different nociceptive processing at the spinal cord level.

The formalin test in the mouse: a parametric analysis of scoring properties

We investigated the scoring properties of the mouse formalin test using the time-sampling method recently developed for infant and adult rats. Formalin was injected under the plantar surface of one rear paw (10 microl, 1-8%), and pain behaviours (paw favouring, lifting and licking) and behavioural state were recorded. Correlational and regression analyses indicated that scores composed of combinations of all three pain behaviours, either summed or weighted, provided less variable indices of pain than licking alone. The maximum percent effect (MPE(50); i.e. pain behaviour 50% of the time) for the log formalin concentration-effect curves was 3-4% in both phases. Habituation to the test environment prior to testing did not alter the MPE(50)s, but slopes were lower in unhabituated mice, dramatically increasing the size of the confidence interval. Formalin dose-dependently reduced locomotion, rearing and sniffing in both the first phase and the early part of the second phase. The combination measures were sensitive to morphine (2-8 mg/kg), amphetamine (1-4 mg/kg), dipyrone (50-200 mg/kg), xylazine (0.25-1 mg/kg), and acepromazine (0.25-1 mg/kg), and resistant to diazepam (0.5-2 mg/kg), pimozide (0.05-0.25 mg/kg), pentobarbital (10 and 15 mg/kg) and indomethacin (2-8 mg/kg). Decreased pain was correlated with increased motor activity for morphine and amphetamine, and with decreased activity for xylazine and acepromazine; dipyrone and indomethacin did not alter activity levels.