Central analgesic effects of acetylsalicylic acid in healthy men

Acetylsalicylic acid (CAS 50-78-2) (1000 mg orally) was investigated in a non-inflammatory experimental pain model in healthy male volunteers, selected for maximal homogeneity. Phasic pain was induced by intracutaneously applied electrical pulses of constant current. The nociceptive responses measured were, the pain ratings, the cerebral potentials and the EEG delta power in response to the stimuli. In addition, spontaneous EEG, auditory evoked potentials and reaction times were evaluated to determine effects upon the vigilance system. The study was performed in a placebo-controlled, double-blind repeated measures design. Blood samples were taken to monitor the plasma concentrations of the active agents. Acetylsalicylic acid produced clear analgesic effects in all pain relevant target variables. The effects increased with post-medication time, becoming significantly different from placebo 90 min after medication (p less than 0.01). At this time point the pain ratings were reduced by 4%, the pain related cerebral potentials by 15%, and the stimulus induced delta power of the EEG by 20%. These findings suggest a central action of acetylsalicylic acid by attenuation of experimentally induced nociceptive activity. No influences could be observed upon auditory evoked potentials, spontaneous EEG and reaction times. In other words, acetylsalicylic acid did not change vigilance by unspecific alterations of the CNS. The plasma concentration of acetylsalicylic acid reached mean values of 2.5 +/- 2.4 micrograms/ml within 25 min after oral medication, which remained constant during the entire post-medication period of 105 min. In contrast, the concentration of the metabolite salicylic acid increased steadily reaching mean values of 32.0 +/- 16.8 micrograms/ml at the end of the investigated period.(ABSTRACT TRUNCATED AT 250 WORDS)