Unilateral inflammation of the hindpaw in rats as a model of prolonged noxious stimulation: alterations in behavior and nociceptive thresholds.

Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into one hindpaw of rats led to a localized inflammation that became apparent within 12 hours and reached its peak between 2 and 3 weeks. FCA-treated rats displayed a diminished rate of body weight gain, a reduction of food and water intake and a disruption of circadian temperature regulation, as well as decreased locomotor activity and pronounced scratching behavior in the open field. Paw pressure thresholds were reduced only in inflamed paws. Contralateral, noninflamed paws showed comparable thresholds to those of control animals. Tail-flick and tail-pressure responses were not different from controls. These data suggest that FCA-treated animals experience increased noxious input from the inflamed limb and that changes in thresholds to acutely applied nociceptive stimuli are due to a peripheral hypersensitivity of inflamed tissue. The present condition resembles most closely a state of acute inflammatory pain. The term "chronic pain" in its strict sense is not appropriate in this model.