Potentiation by caffeine of the analgesic effect of aspirin in the pain-induced functional impairment model in the rat

The ability of caffeine to potentiate the analgesic effect of aspirin was studied in the pain-induced functional impairment model in the rat. Female Wistar rats received an intra-articular injection of 30% uric acid in the right hind limb, inducing its dysfunction. Once the dysfunction was complete, animals received aspirin oral doses of 0, 0.55, 0.98, and 1.74 mmol/kg with and without 0.17 mmol/kg of caffeine, and the recovery of functionality over time was considered as an expression of analgesia. Blood samples were drawn simultaneously with hind limb functionality determinations; and plasma concentrations of aspirin, salicylic acid, and gentisic acid were measured by high-performance liquid chromatography. Aspirin induced a dose-dependent analgesic effect. Caffeine alone was ineffective. However, caffeine significantly increased the analgesic effect of aspirin at all doses, without modifying aspirin, salicylic acid, or gentisic acid plasma levels. It is concluded that caffeine potentiates the analgesic effect of aspirin by a pharmacodynamic, but not by a pharmacokinetic mechanism. Key words: aspirin, caffeine, analgesia, potentiation, experimental pain models.