The effect of capsaicin and conventional analgesics in two models of monoarthritis in the rat.

Systemic administration of capsaicin has been shown to be antinociceptive in a number of acute nociceptive models. We present here two models of prolonged hyperalgesia in the rat induced by intra-articular injection of uric acid or Freund's complete adjuvant into the knee. The time course of hyperalgesia lasts up to 48 h with uric acid and 5 to 10 days with Freund's adjuvant. The hyperalgesia was maximal in both models after 18 h. At this time, the weight tolerated by the injected leg was reduced by 70-80% in the Freund's-treated and 50-60% in the uric-treated animals, compared to the uninjected leg. The hyperalgesia was reversed in both models by indomethacin and ibuprofen (1-100 mg kg-1 s.c.), paracetamol (50-500 mg kg-1 s.c.) and morphine (2-16 mg kg-1 s.c.) in a dose-dependent manner. The morphine reversal was blocked by coadministration with naloxone (2.5 mg kg-1 s.c.). Capsaicin (3-9 mg kg-1 s.c.) also reversed the hyperalgesia in both models.