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 mu g in 50 mu L) 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.