Similar Effects of Acetylsalicylic Acid and Morphine on Immediate Responses to Acute Noxious Stimulation

The formalin and writhing tests in mice were employed to investigate a possible delay in the onset of antinociceptive action of acetylsalicylic acid (ASA). Drugs were given 30 min. before the noxious stimulation by either formalin or acetic acid. In the formalin test, the difference between the drug treated groups and the control group reach statistical significance within 30 sec. of noxious stimulation. ASA (400 mg/kg intraperitoneally) and morphine (5 mg/kg intraperitoneally) treated groups were not significantly different in any of the ten periods (30 sec. each) that were analysed during the first 5 min. In the writhing test, the number of writhings in response to intraperitoneal injection of acetic acid was counted during the first 20 min. The difference between the drug treated groups and the control group reached statistical significance after 3 min. both for ASA (400 mg/kg subcutaneously) and morphine (2 mg/kg subcutaneously) and no significant differences between the drug treated groups were found in any of the one min. periods that were analysed. Thus no delay of onset in the action of ASA compared to morphine could be demonstrated, and ASA seems to be antinociceptive also in acute non-inflammatory pain. Actions apart from inhibition of the synthesis of prostaglandins are suggested for this effect of ASA.