Effects of acetylsalicylic acid on serotonin brain receptor subtypes

Central effects of acetylsalicylic acid on trigeminal-nociceptive stimuli

BACKGROUND

Acetylsalicylic acid is one of the most used analgesics to treat an acute migraine attack. Next to the inhibitory effects on peripheral prostaglandin synthesis, central mechanisms of action have also been discussed.

METHODS

Using a standardized model for trigeminal-nociceptive stimulation during fMRI scanning, we investigated the effect of acetylsalicylic acid on acute pain compared to saline in 22 healthy volunteers in a double-blind within-subject design. Painful stimulation was applied using gaseous ammonia and presented in a pseudo-randomized order with several control stimuli. All participants were instructed to rate the intensity and unpleasantness of every stimulus on a VAS scale. Based on previous results, we hypothesized to find an effect of ASA on central pain processing structures like the ACC, SI and SII as well as the trigeminal nuclei and the hypothalamus.

RESULTS

Even though we did not find any differences in pain ratings between saline and ASA, we observed decreased BOLD signal changes in response to trigemino-nociceptive stimulation in the ACC and SII after administration of ASA compared to saline. This finding is in line with earlier imaging results investigating the effect of ASA on acute pain. Contrary to earlier findings from animal studies, we could not find an effect of ASA on the trigeminal nuclei in the brainstem or within the hypothalamic area.

CONCLUSION

Taken together our study replicates earlier findings of an attenuating effect of ASA on pain processing structures, which adds further evidence to a possibly central mechanism of action of ASA.

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.

Differential effects of two analgesic drugs, morphine chlorhydrate and acetylsalicylic acid, on thalamic mast cell numbers in rat

Thalamic mast cells (TMCs), the only immunocytes known to infiltrate the brain in physiological conditions, respond to pharmacological agents including sumatriptan - a serotonergic anti-migraine agent - that increases their number. We analysed the effects of two other main analgesics: morphine chlorhydrate, a micro opioid agonist, and acetylsalicylic acid (ASA), a non-steroidal anti-inflammatory drug. All three drugs have specific modes of action, and morphine and ASA, unlike sumatriptan, are also known to interact with peripheral mast cells. Only ASA was effective in promoting TMC number decrease. TMCs, unlike other mast cells, do not express cyclooxygenase (COX) - the key enzyme in the production of prostanoids and the main site of action of ASA - thus dismissing a direct local cellular COX-mediated action. Direct TMC COX-independent mechanisms or effects mediated via distant populations of COX-positive cells such as platelets, leptomeningeal, endothelial and peripheral mast cells are thus probable. ASA, morphine and sumatriptan have distinct TMC effects, suggesting that the TMC number variations they induce are more likely to derive from systemic vasoactive actions than from pharmacological mechanisms devoted to pain relief.

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.

Acetylsalicylic acid potentiates the antinociceptive effect of morphine in the rat: involvement of the central serotonergic system

Acetylsalicylic acid and morphine are the most widely distributed and most frequently used drugs in the relief of pain, but their analgesic activity has adverse side-effects. Mixtures containing these two drugs are frequently used to relieve mild to moderate pain despite the paucity of relevant experimental evidence so far published. We set out to study the possible antinociceptive effect of a combination of subactive doses of the two drugs in rats. A combination of low doses of acetylsalicylic acid (50 mg/kg i.p.) and morphine (3 mg/kg s.c.) was administered and the pain threshold was evaluated in the hot-plate and formalin tests, and 5-HT2 receptor binding capacity, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were measured in the cortex and pontine areas of the brain. The combination of acetylsalicylic acid and morphine had an analgesic effect in both tests that was associated with an increase in 5-HT levels and a decrease in 5-HT2 receptors in the cortex. These effects were either completely abolished or partially prevented by i.p. pretreatment with naloxone (1 mg/kg i.p.). Our results demonstrate that subactive doses of acetylsalicylic acid and morphine can exert analgesic and biochemical effects when given in combination in the rat and suggest an involvement of serotonergic and opiatergic systems.