Weight bearing as an objective measure of arthritic pain in the rat

This study compares two methods for evaluating pain-related behavior in an animal model with carrageenan-induced monoarthritis. Rats injected with lambda-carrageenan into the right tibio-tarsal joint were videofilmed at various times after injection and later scored regarding their stance. Immediately after each videorecording session the animals were tested in a box constructed to register the weight load exerted by the hindpaws by means of force plates inserted in the floor. Following carrageenan injection (300 micrograms in 50 microL) the load on the injected paw fell from a control value of 39.3% +/- 0.4% of the body weight (mean +/- SEM, n = 6) to a minimum of 5.1% +/- 1.8% at 6 hr and then slowly increased to approach control levels at 72 hr. The weight load on the contralateral paw increased from a control value of 38.9% +/- 0.6% to 52.4% +/- 1.4% at 6 hr, whereafter it gradually decreased. The video-based stance scores also showed a maximal impairment at 4-6 hr, with a gradual return towards control values at 72 hr. However, the results based on the force plate measurements were less variable and more graded. Morphine inhibited the carrageenan-induced effect in a dose-dependent manner in both paradigms. In conclusion, the present results indicate that measurement of weight bearing as described in the present paper is a practical, useful, and objective method to assess the degree of arthritic pain in the rat.