Effect of acetylsalicylic acid on formalin test and on serotonin system in the rat brain

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.

Roles of serotonergic and adrenergic receptors in the antinociception of selective cyclooxygenase-2 inhibitor in the rat spinal cord

Background

The analgesic mechanisms of cyclooxygenase (COX)-2 inhibitors have been explained mainly on the basis of the inhibition of prostaglandin biosynthesis. However, several lines of evidence suggest that their analgesic effects are mediated through serotonergic or adrenergic transmissions. We investigated the roles of these neurotransmitters in the antinociception of a selective COX-2 inhibitor at the spinal level.

Methods

DUP-697, a selective COX-2 inhibitor, was delivered through an intrathecal catheter to male Sprague-Dawley rats to examine its effect on the flinching responses evoked by formalin injection into the hindpaw. Subsequently, the effects of intrathecal pretreatment with dihydroergocristine, prazosin, and yohimbine, which are serotonergic, α1 adrenergic and α2 adrenergic receptor antagonists, respectively, on the analgesia induced by DUP-697 were assessed.

Results

Intrathecal DUP-697 reduced the flinching response evoked by formalin injection during phase 1 and 2. But, intrathecal dihydroergocristine, prazosin, and yohimbine had little effect on the antinociception of intrathecal DUP-697 during both phases of the formalin test.

Conclusions

Intrathecal DUP-697, a selective COX-2 inhibitor, effectively relieved inflammatory pain in rats. Either the serotonergic or adrenergic transmissions might not be involved in the analgesic activity of COX-2 inhibitors at the spinal level.

Mechanisms of non-opioid analgesics beyond cyclooxygenase enzyme inhibition

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

The antinociceptive effect of acetylsalicylic acid is differently affected by a CB1 agonist or antagonist and involves the serotonergic system in rats

AIMS

Combinations of non-steroidal anti-inflammatory drugs (NSAIDs) and cannabinoids are promising because of their potential synergistic effects in analgesia, resulting in a reduction in dosage and minimizing adverse reactions. The analgesic effect of acetylsalicylic acid (ASA), probably due to a central mechanism, also implicates changes in the central monoaminergic system. Therefore, we decided to evaluate the antinociceptive interaction between the CB(1) receptor agonist, HU210, and ASA in tests involving central pain in rats as well as the implication of the central serotonergic system thereon.

MAIN METHODS

The selective CB(1) antagonist SR141716A and the potent cannabinoid agonist HU210 were evaluated alone and in combination with ASA in both algesimetric tests (hot-plate and formalin tests) and for 5-HT activity and 5-HT(2) receptor density and affinity.

KEY FINDINGS

ASA or HU210 alone showed a dose-dependent effect in both tests. HU210, at an inactive dose, significantly increased the antinociceptive effect of the sub-active dose of ASA. SR141716A (1.5mg/kgi.p.) was ineffective per se and failed to modify antinociception induced by the HU210 plus ASA combination in either test. HU210 plus ASA significantly decreased the 5-HIAA/5-HT ratio and the 5-HT(2) receptor density in the frontal cortex, changes not antagonized by SR141716A.

SIGNIFICANCE

The present study provides evidence that mutual potentiation of the antinociceptive effects of HU210 and ASA may, at least partly, depend on serotonergic mechanisms, with an indirect participation of cannabinodiergic mechanism. In conclusion, combinations of low doses of cannabinoids and NSAIDs may be of interest from the therapeutic point of view.

Shortened onset of action of antidepressants in major depression using acetylsalicylic acid augmentation: a pilot open-label study

Based on our preclinical data showing a potential accelerating effect of acetylsalicylic acid (ASA) in combination with fluoxetine in an animal model of depression, we examined the effect of ASA augmentation therapy on selective reuptake inhibitors (SSRI) in major depressed non-responder patients. Twenty-four non-responder patients having received at least 4 weeks of an adequate SSRI treatment were included in a pilot open-label study. Participants were treated openly during 4 weeks with 160 mg/day ASA in addition to their current antidepressant treatment. The combination SSRI-ASA was associated with a response rate of 52.4%. Remission was achieved in 43% of the total sample and 82% of the responder sample. In the responder group, a significant improvement was observed within week 1 (mean Hamilton Depression Rating Scale-21 items at day 0=29.3+/-4.5, at day 7=14.0+/-4.1; P<0.0001) and remained sustained until day 28. Despite limitations due to the open nature of this study, our preliminary results confirm our preclinical findings and are in favour of an accelerating effect of ASA in combination with SSRIs in the treatment of major depression. Potential physiological and biochemical mechanisms may involve an anti-inflammatory and/or neurotrophic effect.

Spinal serotonergic system is partially involved in antinociception induced by Trigonella foenum-graecum (TFG) leaf extract

It has been reported that Trigonella foenum-graecum (TFG) extract exerts analgesic, anti-inflammatory and anti-pyretic effects in different experimental models. The major objective of this paper was to investigate the site and mechanism of the analgesia induced by Trigonella foenum-graecum extract. We studied the analgesic effects of different doses of Trigonella foenum-graecum extract after i.p., i.t. and i.c.v. administration in formalin test, using male NMRI rats (200-250 g). Trigonella foenum-graecum extract showed analgesic effects in i.p. (1 g/kg) and i.t. (0.5, 1, and 2 mg/rat) (P < 0.05 in all groups) but not in i.c.v. (1 and 3 mg/rat) administrations. Based on the similarities between the effects of Trigonella foenum-graecum extract with those of nonsteroidal anti-inflammatory drugs (NSAIDs) and the role of 5-HT system in analgesic effects of NSAIDs, we tried to investigate the role of spinal 5-HT system in analgesic effects of Trigonella foenum-graecum extract. After lesioning of spinal 5-HT system by 5,7-dihydroxytryptamine (5,7-DHT), it was shown that the analgesic effect of Trigonella foenum-graecum extract (0.5 and 3 mg/rat) in the second phase of formalin test, was abolished completely and reduced relatively after using a low-dose (0.5 mg/rat) and a high-dose (3 mg/rat), respectively (P < 0.05). So, the antinociception partially remained (P < 0.05) after using the latter dose. Meanwhile, administration of naloxone (2mg/kg, i.p.) had no effect on the Trigonella foenum-graecum extract (1 g/kg, i.p.) analgesia. In conclusion, this study confirms the central action of Trigonella foenum-graecum extract and that spinal 5-HT system is partially involved in the analgesia induced by it in the second phase of formalin test and also indicates for co-existence of other analgesic mechanism(s).

2002 Wolff Award. 5 -HT2A receptor activation and nitric oxide synthesis: a possible mechanism determining migraine attacks

OBJECTIVE

To determine the effect of the 5-HT2A receptor in control of spinal nociception, cerebral circulation, and nitric oxide synthase (nNOS) expression in trigeminovascular neurons.

BACKGROUND

The plasticity of the 5-HT2A receptor is a possible factor determining the course of migraine. Up-regulation of this receptor has been demonstrated to correlate with the increasing frequency of migraine attacks and may underlie the development of chronic daily headache.

METHODS

Adult male Wistar rats were divided into groups receiving the 5-HT2A agonist, 1,2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), nitroglycerin, or normal saline. The tail flick test and chemical nociception-evoked Fos-expression in dorsal horn neurons were used as indicators of nociception. Regional cerebral blood flow was monitored using laser Doppler flowmetry. Expression of Fos and nNOS was studied using immunohistochemical method.

RESULTS

Administration of DOI led to the shortening of tail flick latency (1.3 +/- 0.2 and 7.2 +/- 0.6 seconds for DOI-treated and control groups, respectively). The number of Fos-immunoreactive neurons was also greater in the DOI-treated group compared with the control group. DOI also produced long-lasting cerebral hyperemia (123% of baseline value) associated with the enlargement of perivascular nNOS-immunoreactive nerve fibers and increased nNOS-immunoreactive neurons in trigeminal ganglia and trigeminal nucleus caudalis. These findings resembled those observed in the rats exposed to nitroglycerin.

CONCLUSION

Our results suggest that activation of the 5-HT2A receptor leads to an enhancement of NO production in trigeminovascular pathway. NO may trigger migraine attacks by inducing cerebral vasodilation and sensitizing the perivascular nociceptors and central nociceptive neurons in trigeminovascular system. Up-regulation of this pronociceptive receptor can increase headache attacks and contributes to the development of chronic daily headache.

Chronic daily headache: a scientist's perspective

Certain features of chronic daily headache, namely, increased headache frequency, expansion of headache area, and cutaneous allodynia, may imply sensitization of central nociceptive neurons in the trigeminal pathway. Repetitive activation of the trigeminal nerve can lead to a biologic and functional change in trigeminal nucleus caudalis neurons, characterized by a decrease in nociceptive threshold and receptive field expansion. Suppression of the endogenous pain control system can facilitate the process of central sensitization. Evidence of such suppression in patients with chronic daily headache includes decreased platelet serotonin, up-regulation of 5-HT2A receptors, increased platelet nitric oxide production, and increased levels of substance P and nerve growth factor in the cerebrospinal fluid. Results from a number of animal experiments have indicated that chronic analgesic exposure leads to changes in serotonin content and density of 5-HT2A receptors in the central nervous system. This plasticity of the serotonin-dependent pain control system may accelerate the process of sensitization; a biologic outcome that is expressed clinically by the development of chronic daily headache associated with analgesic overuse.

The effect of a paracetamol and morphine combination on dynorphin A levels in the rat brain

The purpose of this study was to find out whether the combination of inactive doses of paracetamol (PARA) and morphine was able to change dynorphin (DYN) A levels, evaluated by radioimmunoassay, and whether naloxone or [(-)-2-(3 furylmethyl)-normetazocine] (MR 2266), a kappa-opioid antagonist, modifies or prevents the activity of this combination on nociception and on DYN levels. The work was suggested by our previous findings which demonstrated that inactive doses of PARA and morphine, when given in combination, share an antinociceptive effect, and that PARA, at antinociceptive doses, decreases DYN levels in the frontal cortex, thus indicating a selective action within the CNS. Our present results demonstrate that the combination of inactive doses of PARA (100 mg/kg) and morphine (3 mg/kg) is just as effective in decreasing the levels of DYN A as full antinociceptive doses of PARA or morphine alone in the frontal cortex of the rat. The values, expressed in pmol/g tissue, were: control = 2.83 +/- 0.20; paracetamol (100) = 2.60 +/- 0.23; morphine (3) = 2.73 +/- 0.24; paracetamol + morphine = 1.34 + 0.16 (P < 0.05). The decrease was partially antagonised by MR 2266, but not by naloxone, suggesting that the activity of PARA and morphine in combination on DYN A levels could be mediated, at least in part, through kappa-receptors, although other systems may be involved. On the other hand, both naloxone and MR 2266 prevented the antinociceptive effect of the combination in the hot plate test. All our experimental data suggest that PARA and morphine in combination exert their antinociceptive effect through the opioidergic system, which in turn may cause a decrease in DYN levels in the CNS of the rat.

Aspirin attenuates the anxiolytic actions of ethanol

The present study examined the effect of aspirin on the anxiolytic action of ethanol. Previous research has shown that ethanol reliably produces an anxiolytic effect on rodent's plus-maze performance while aspirin has been demonstrated to attenuate several of ethanol's behavioral actions. Female Sprague-Dawley rats were given s.c. aspirin doses of 0 or 150 mg/kg, followed 30 min later by s.c. ethanol doses of 0, 1.0 or 1.6 g/kg. After 5 min, animals were tested in the elevated plus-maze. Although aspirin did not have a significant effect on anxiety-related behavior, it did attenuate the anxiolytic action of ethanol at the dose of 1.0 g/kg, but not at the 1.6 g/kg dose. Thus, aspirin by itself does not appear to possess anxiolytic actions, but does modify the anxiolytic actions of 1.0 g/kg, but not 1.6 g/kg ethanol.

[Aspirin, pain and inflammation]

SUBJECT

Acetylsalicylic acid (ASA) is among the most commonly analgesic, antipyretic and anti-inflammatory used drugs. The anti-inflammatory effects of ASA are mediated by the inhibition of cyclooxygenase enzymes with the subsequent decrease of prostaglandin synthesis.

NEW DATA

However, since this discovery of Vane in 1971, much of other mechanisms of anti-inflammatory action, without relation with cyclooxygenases, have been proposed. ASA has peripheric analgesic properties by reducing prostaglandin biosynthesis. But there is evidence that the analgesic effects could be mediated by central mechanisms with changes in the monoaminergic and opioid systems. ASA is essentially used in moderate pains with an inflammatory component (rheumatological disorders, headaches, dental and postoperative pains).

PERSPECTIVES

The clinical use of ASA at anti-inflammatory dose is less frequent because the other non steroidal anti-inflammatory drugs are as effective as ASA, but they are associated with less side effects. Nevertheless, the synergism of ASA and morphine association and the possible involvement of the central serotonergic and opiatergic systems in the antinociceptive activity of ASA could confer a greater role of ASA in pain management.