An investigation of binding sites for paracetamol in the mouse brain and spinal cord

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.

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.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Paracetamol inhibits nitric oxide synthesis in murine spinal cord slices

Paracetamol is an effective analgesic but its mechanism of action is unclear. We investigated the effect of paracetamol and the analgesic adjuvant caffeine on the activity of NO synthase in mouse spinal cord and cerebellar slices in vitro, by measuring the conversion of [(3)H]arginine to [(3)H]citrulline. Paracetamol (100 microM) had no effect on NO synthase activity in cerebellum, but in the spinal cord both paracetamol (100 microM) and caffeine (30 microM) attenuated glutamate (5 mM)-induced [(3)H]citrulline production and in combination they abolished it. In conclusion paracetamol inhibits spinal cord NO synthesis and this may be related to its analgesic effects.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).