Pain threshold changes in adjuvant-induced inflammation: a possible model of chronic pain in the mouse

A chronic hyperalgesic condition was induced in mice by the injection of Freund's complete adjuvant (FCA) into the lower lumbar region or directly into the hind footpads. Although little or no visible inflammation was observed after a single intradermal injection of FCA into the lower lumbar area of rats or mice, significant alterations in nociceptive thresholds occurred in each species as determined by decreases in response latency in tail-flick and hot-plate assays. Unilateral intraplantar administration of FCA in mice resulted in visible inflammation in the area of the tibiotarsal (ankle) joint. Changes in the response latency to a noxious stimulus in the areas surrounding the inflamed joint were similar to those observed in non-inflamed limbs, suggesting that changes in sensitivity to noxious stimuli were not merely the result of local hypersensitivity of the inflamed tissue, but may also be due to alterations in nociception at the level of the central nervous system (CNS). When the chronic inflammatory condition induced in the mouse tibiotarsal joint was evaluated by histological and morphological techniques, it was found to have the same characteristics as described in the early stages of FCA-induced arthritis in rats. The similarities between the response to FCA in rat and mouse suggest that injection of FCA in mice may prove to be a useful model for the study of chronic pain in mice as well as in rats.